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Type I interferon (IFN-I) is critical for antiviral defense, and plasmacytoid dendritic cells (pDCs) are a predominant source of IFN-I during virus infection. pDC-mediated antiviral responses are stimulated upon physical contact with infected cells, during which immunostimulatory viral RNA is transferred to pDCs, leading to IFN production via the nucleic acid sensor TLR7. Using dengue (DENV), hepatitis C (HCV), and zika (ZIKV) viruses, we demonstrate that the contact site of pDCs with infected cells is a specialized platform we term the interferogenic synapse, which enables viral RNA transfer and antiviral responses.; L’interféron de Type I (IFN-I) est un acteur crucial de la réponse antivirale. Dans le cas d'une infection virale, celui-ci est majoritairement produit par les cellules dendritiques plasmacytoides (pDCs). Afin d'assurer leurs fonctions antivirales, les pDCs forment des contacts physiques avec les cellules infectées, une caractéristique observée pour un grand nombre de virus, même distant génétiquement. Cependant, comment et pourquoi ces contacts s'établissent reste énigmatiques. En utilisant les virus de la dengue (DENV), de l'hépatite C (HCV) et de zika (ZIKV), nous démontrons qu'une plateforme spécialisée dans le transfert d'ARN immuno-stimulateurs se met en place au niveau de l'interface de contact entre les pDCs et les cellules infectées.

Cell-autonomous polarity in Metazoans is inherited from ancestral unicellular organisms. We assume that permanent polarity in unicellular eukaryotes is required for cell motion and sensory reception and that the integration of these two activities corresponds to an evolutionary constrained cell function. While conserving the ancestral flagellum, Metazoans have co-opted a primary cilium/centrosome organ from it, ensuring similar functions, but in different cells, or in the same cell at different moments. We propose that the remodeling necessary to reach a new higher-level unit of selection in multi-cellular organisms, has been triggered by conflicts among individual cell polarities to reach an organismic polarity. We shall provisionally conclude that beyond critical consequences for embryo development, the conservation of cell-autonomous polarity in Metazoans has far reaching implications for the evolution of individuality.

Metastasis is the main cause of cancer-related deaths. Recent data suggest that metastatic dissemination often occurs at an early stage during tumour formation. Tumorigenesis is a multi-step process initiated from oncogenic single cells within a normal tissue. How these cancer cell alterations evolve within tightly regulated normal tissue environments remains elusive. Cell extrusion is a potential mechanism that allows for mutant cells to escape from untransformed epithelia, and the cues promoting tumor cell extrusion need to be identified. Polarity proteins may play essential roles in triggering cell extrusion and early tumor cell dissemination as loss of epithelial cell polarity is a hallmark of cancer cells that is correlated with tumor aggressiveness. The polarity protein kinase aPKCi (atypical Protein Kinase C iota) is an oncogene, which overexpression (aPKCi+) is of poor prognosis. Here, we identify aPKCi+ oncogenic basal extrusion from the normal mammary epithelium as a mechanism for early breast tumor cell dissemination in vivo. By combining in vitro biophysical approaches, we show an increase in cell tension at the interface between aPKCi+ and WT cells. This increase in cell tension depends on myosin II activity and is associated with the relocation of vinculin from cell-cell junctions to focal adhesions in aPKCi+ cells. The shift in vinculin localization affects aPKCi+/WT cell junction stability, leading to the acquisition of pro-migratory features in aPKCi+ cells. We propose that a balance between cell contractility and cell-cell adhesion at the interface of normal and oncogenic aPKCi+ cells is crucial for promoting basal cell extrusion. We anticipate that this mechanism may be conserved in other carcinomas, promoting early cancer cell dissemination.; La formation de métastases issues de carcinomes est la principale cause de mortalité des cancers. Certaines cellules cancéreuses ont la capacité de s’échapper de la glande mammaire et former des métastases à un stade très précoce, bien avant la détection d’une tumeur primaire. Les mécanismes impliqués restent à être identifiés. L’extrusion basale pourrait être un des mécanismes impliqués dans la dissémination précoce de cellules cancéreuses. La perte de la polarité cellulaire est une des signatures des cellules cancéreuses. La protéine kinase C atypique iota (aPKCi) est un oncogène surexprimé dans de nombreux cancers, en particulier dans les cancers du sein, où cette surexpression est un facteur de mauvais pronostic pour la survie des patientes. aPKCi est un bon candidat pour étudier la dissémination tumorale précoce. Mon étude révèle que la surexpression d’aPKCi dans quelques cellules épithéliales de la glande mammaire promeut l’extrusion basale in vivo et ex vivo en dégradant la membrane basale de façon dépendante des métalloprotéases. Les cellules surexprimant aPKCi sont exclues de l’épithélium par des mécanismes de ségrégation cellulaire en modifiant la localisation de la vinculine. La surexpression d’aPKCi entraîne une diminution de la vinculine aux jonctions adhérentes, au profit des adhésions focales, conférant aux cellules aPKCi+ des propriétés pro-migratrices. En parallèle, la contractilité, dépendante de l’activité de la myosine II, augmente spécifiquement à l’interface des cellules aPKCi+/saines. Cette balance entre la diminution de la vinculine aux jonctions adhérentes couplée à l’augmentation de la tension jonctionnelle à l’interface aPKCi+/WT entraîne l’extrusion basale des cellules aPKCi+. Par la suite, les cellules en contact avec la membrane basale, la dégradent grâce aux métalloprotéases et sont capables de migrer dans la MEC, via la renforcement de la vinculine aux points focaux d'adhésion permettant une migration efficace. Cette étude met en évidence un nouveau mécanisme, l’extrusion basale oncogénique, dans la dissémination tumorale précoce et révèle l’importance des modifications des propriétés mécaniques des cellules oncogéniques dans ce processus.

International audience; La polarité propre des cellules des métazoaires est héritée des unicellulaires ancestraux. On supposera que la polarité des unicellulaires eucaryotes est nécessaire pour leur locomotion et leur sensorialité et que l’intégration de ces deux activités correspond à une fonction cellulaire évolutivement contrainte. Tout en conservant le flagelle ancestral, les métazoaires ont coopté à partir de ce dernier un nouvel organite, le cil primaire/centrosome, qui assure les mêmes fonctions, mais dans des cellules différentes ou dans la même cellule, mais à des moments différents. On proposera que le remodelage nécessaire à l’obtention d’une nouvelle unité de sélection chez les multicellulaires ait été déclenché par des conflits entre les polarités des cellules individuelles pour l’obtention d’une polarité au niveau de l’organisme. On conclura provisoirement qu’au-delà de conséquences critiques pour le développement de l’embryon, la conservation d’une polarité cellulaire propre chez les métazoaires a des implications de grande portée pour l’évolution de l’individualité.

Metastasis is the main cause of cancer-related deaths. Recent data suggest that metastatic dissemination often occurs at an early stage during tumour formation. Tumorigenesis is a multi-step process initiated from oncogenic single cells within a normal tissue. How these cancer cell alterations evolve within tightly regulated normal tissue environments remains elusive. Cell extrusion is a potential mechanism that allows for mutant cells to escape from untransformed epithelia, and the cues promoting tumor cell extrusion need to be identified. Polarity proteins may play essential roles in triggering cell extrusion and early tumor cell dissemination as loss of epithelial cell polarity is a hallmark of cancer cells that is correlated with tumor aggressiveness. The polarity protein kinase aPKCi (atypical Protein Kinase C iota) is an oncogene, which overexpression (aPKCi+) is of poor prognosis. Here, we identify aPKCi+ oncogenic basal extrusion from the normal mammary epithelium as a mechanism for early breast tumor cell dissemination in vivo. By combining in vitro biophysical approaches, we show an increase in cell tension at the interface between aPKCi+ and WT cells. This increase in cell tension depends on myosin II activity and is associated with the relocation of vinculin from cell-cell junctions to focal adhesions in aPKCi+ cells. The shift in vinculin localization affects aPKCi+/WT cell junction stability, leading to the acquisition of pro-migratory features in aPKCi+ cells. We propose that a balance between cell contractility and cell-cell adhesion at the interface of normal and oncogenic aPKCi+ cells is crucial for promoting basal cell extrusion. We anticipate that this mechanism may be conserved in other carcinomas, promoting early cancer cell dissemination., La formation de métastases issues de carcinomes est la principale cause de mortalité des cancers. Certaines cellules cancéreuses ont la capacité de s’échapper de la glande mammaire et former des métastases à un stade très précoce, bien avant la détection d’une tumeur primaire. Les mécanismes impliqués restent à être identifiés. L’extrusion basale pourrait être un des mécanismes impliqués dans la dissémination précoce de cellules cancéreuses. La perte de la polarité cellulaire est une des signatures des cellules cancéreuses. La protéine kinase C atypique iota (aPKCi) est un oncogène surexprimé dans de nombreux cancers, en particulier dans les cancers du sein, où cette surexpression est un facteur de mauvais pronostic pour la survie des patientes. aPKCi est un bon candidat pour étudier la dissémination tumorale précoce. Mon étude révèle que la surexpression d’aPKCi dans quelques cellules épithéliales de la glande mammaire promeut l’extrusion basale in vivo et ex vivo en dégradant la membrane basale de façon dépendante des métalloprotéases. Les cellules surexprimant aPKCi sont exclues de l’épithélium par des mécanismes de ségrégation cellulaire en modifiant la localisation de la vinculine. La surexpression d’aPKCi entraîne une diminution de la vinculine aux jonctions adhérentes, au profit des adhésions focales, conférant aux cellules aPKCi+ des propriétés pro-migratrices. En parallèle, la contractilité, dépendante de l’activité de la myosine II, augmente spécifiquement à l’interface des cellules aPKCi+/saines. Cette balance entre la diminution de la vinculine aux jonctions adhérentes couplée à l’augmentation de la tension jonctionnelle à l’interface aPKCi+/WT entraîne l’extrusion basale des cellules aPKCi+. Par la suite, les cellules en contact avec la membrane basale, la dégradent grâce aux métalloprotéases et sont capables de migrer dans la MEC, via la renforcement de la vinculine aux points focaux d'adhésion permettant une migration efficace. Cette étude met en évidence un nouveau mécanisme, l’extrusion basale oncogénique, dans la dissémination tumorale précoce et révèle l’importance des modifications des propriétés mécaniques des cellules oncogéniques dans ce processus.

Type I interferon (IFN-I) is critical for antiviral defense, and plasmacytoid dendritic cells (pDCs) are a predominant source of IFN-I during virus infection. pDC-mediated antiviral responses are stimulated upon physical contact with infected cells, during which immunostimulatory viral RNA is transferred to pDCs, leading to IFN production via the nucleic acid sensor TLR7. Using dengue (DENV), hepatitis C (HCV), and zika (ZIKV) viruses, we demonstrate that the contact site of pDCs with infected cells is a specialized platform we term the interferogenic synapse, which enables viral RNA transfer and antiviral responses., L’interféron de Type I (IFN-I) est un acteur crucial de la réponse antivirale. Dans le cas d'une infection virale, celui-ci est majoritairement produit par les cellules dendritiques plasmacytoides (pDCs). Afin d'assurer leurs fonctions antivirales, les pDCs forment des contacts physiques avec les cellules infectées, une caractéristique observée pour un grand nombre de virus, même distant génétiquement. Cependant, comment et pourquoi ces contacts s'établissent reste énigmatiques. En utilisant les virus de la dengue (DENV), de l'hépatite C (HCV) et de zika (ZIKV), nous démontrons qu'une plateforme spécialisée dans le transfert d'ARN immuno-stimulateurs se met en place au niveau de l'interface de contact entre les pDCs et les cellules infectées.

Septins form a family of GTPases conserved in fungi and animal cells [Kinoshita etal., 2003]. During cell division, they localize at cytokinesis sites and are essential for this process in budding yeast, Drosophila embryos and cultured mammalian cells [Fatyet al., 2002].In budding yeasts, septins, composed of parallel networks of filaments [Byers et al.,1976], form a mother-daughter neck ring. This ring is closely associated with the plasma membrane and constitutes a scaFold for the recruitment of myosin II and other cytokinetic factors at the future cleavage site [Longtine et al., 2003]. In addition, the septin ring contributes to the formation of a lateral diffusion barrier on the plasma membrane, which helps maintain the factors of cell polarity in the bud [Barral et al.,2000; Takizawa et al., 2000].In metazoans, septins are also required for compartmentalization of the cellular cortex [Schmidt et al., 2004; Joo et al., 2005] and are involved in a myriad of cellular processes, including assembly and orientation of the polar body of the spindle [Kusch etal., 2002; Spiliotis et al., 2005], exocytosis and vesicular transport [Hsu et al., 1998;Beites et al., 1999], cell migration [Finger et al., 2003], and apoptosis [Larisch et al.,2000; Gottfried et al., 2004].[...]The set of results presented highlights the molecular arrangement of the monomersin the septin complex and suggests that the septin complexes assemble in filaments and higher order structures at the bud neck in Saccharomyces cerevisiae. Thus, wepropose that septins form the fourth component of the cytoskeleton., Les septines forment une famille de GTPases conservée chez les champignons et dans les cellules animales [Kinoshita et al., 2003]. Pendant la division cellulaire, elles se localisent aux sites de cytocinèse et sont essentielles pour ce processus dans la levure bourgeonnante, les embryons de drosophile et les cellules de mammifère en culture [Faty et al., 2002]. Dans les levures bourgeonnantes, les septines, composées de réseaux parallèles de 1laments [Byers et al., 1976], forment un anneau au cou mère-fille. Cet anneau est étroitement associé à la membrane plasmique et constitue un échafaudage pour le recrutement de la myosine II et d'autres facteurs cytocinétiques au futur site de clivage [Longtine et al., 2003]. De plus, l'anneau de septines contribue à la formation d'une barrière de diffusion latérale sur la membrane plasmique, qui aide à maintenir les facteurs de la polarité cellulaire dans le bourgeon [Barral et al., 2000; Takizawa et al., 2000]. Chez les métazoaires, les septines sont aussi requises pour la compartimentation du cortex cellulaire [Schmidt et al., 2004; Joo et al., 2005] et sont impliquées dans une myriade de processus cellulaires, y compris l'assemblage et l'orientation du corps polaire du fuseau [Kusch et al., 2002; Spiliotis et al., 2005], l'exocytose et le transport vésiculaire [Hsu et al., 1998;Beites et al., 1999], la migration cellulaire [Finger et al., 2003], et l'apoptose Larisch et al., 2000; Gottfried et al., 2004].[...]Dans ce mémoire, après avoir passé en revue les fonctions essentielles des septines chez la levure bourgeonnante Saccharomyces cerevisiae, je présente une étude dans laquelle j’ai démontré que UNC-59 et UNC-61 forment un complexe hétérotétramérique capable de s’associer en structures d’ordre supérieur et en filaments dans les cellules et que le complexe hétérotétramérique de septines est assemblé à partir d’hétérodimères UNC-59/UNC-61. De plus, au cours d’un projet visant à développer un outil moléculaire pour l’étude des fonctions des septines in vivo dans des cellules sauvages de Saccharomyces, j’ai mis au point une méthode d’isolement d’intrabodies à partir de la banque d’anticorps recombinants humains en phage display ETH-2 et démontré leur activité d’inhibition de la formation de l’anneau de septines au col du bourgeon dans les cellules de Saccharomyces cerevisiae. L’ensemble des résultats présentés met en lumière l'arrangement moléculaire des monomères dans le complexe de septines et suggère que les complexes de septines s’assemblent dans les filaments et les structures d'ordre supérieur au col du bourgeon chez Saccharomyces cerevisiae. Ainsi, nous proposons que les septines forment la quatrième composante du cytosquelette.

Cells sense and integrate a wealth of mechanical and biochemical signals. Signal integration is part of a process, which ensures that cellular functions are in accordance with the extracellular environment. While these processes are highly regulated by biochemical and mechanical signalling and feedback loops, some of the fundamental processes appear to rely on actin cytoskeleton autoassembly giving raise to modules with defined geometrical and mechanical properties. Thus the actin cytoskeleton is a modular architecture, and the modules co-exist within the cell with spatial and functional specificity. The actin cytoskeleton, notably, is involved in cell/matrice signalling. This interaction relies mainly on mechanical signalling involving the actin cytoskeleton, cell/matrix adhesions and the extracellular matrix. To characterize these mechanisms we took advantage of advanced micropatterning techniques, traction force measurements and laser microdissection. By downregulating the expression of α-actinin, one of the main actin crosslinking proteins, we demonstrated that actin cytoskeleton connectivity is essential for proper integration of cell/matrix signalling. Connectivity is essential for rigidity sensing and haptotaxis by ensuring balanced force distribution through the whole cell. Therefore connectivity might be crucial for cell differentiation processes and cellular polarity. Further, in the context of a collaborative project, we have contributed to the characterization of a novel cell adhesion protein, namely, Kank2. We showed, by traction force measurements, that this protein is essential for rigidity sensing. Globally this study demonstrated the implication of Kank2 in cell adhesion maturation and mecanotransduction., Une cellule reçoit et intègre une multitude de signaux physiques et biochimiques. Elle est capable de sentir et de répondre à ces signaux de sorte que ses fonctions s’accordent avec son environnement. Si l’intégration de ces signaux est hautement régulée par des voies de signalisation et de rétroaction, certaines étapes semblent résulter de processus d’auto-organisation géométrique et mécanique du réseau d’actine. Il est capable de s’auto-assembler et d’adopter différentes architectures. Celles-ci sont autant de modules qui coexistent dans la cellule avec une claire ségrégation spatiale et fonctionnelle. Notamment, le cytosquelette d’actine participe à l’intégration des signaux encodés par la matrice extracellulaire. Cette intégration suppose entre autre, une régulation des forces de tension entre la cellule et son environnement impliquant le cytosquelette d’actine, les adhésions cellulaire et la matrice. Afin d’explorer ces mécanismes, nous avons eu recours à des techniques avancées de micropatterning, de mesure de force de traction cellulaire et de microdissection laser. Nous avons ainsi montré en réprimant l’expression de l’ alpha-actinine, une des principales protéines de réticulation du cytosquelette d’actine, que la connectivité du réseau d’actine était essentielle à l’intégration des signaux émanant de la matrice extracellulaire. Elle participe à l’évaluation de la rigidité de la matrice et au mécanisme de migration dirigé haptotactique. Elle participe donc potentiellement aux mécanismes de différentiation cellulaire et au maintien de la polarité cellulaire. Dans le même esprit nous avons pris part à une étude portant sur l’organisation et la maturation des adhésions cellulaires, en participant à la caractérisation d’une protéine d’adhésion Kank2. Nous avons ainsi pu démontrer le rôle essentiel de cette protéine dans le phénomène de rigidity sensing. L’ensemble de l’étude ayant montré l’implication de cette protéine dans le processus de maturation des adhésions cellulaires et de mécano-transduction.

The role of primary cilia in adult neurons remains elusive, however their developmental functions during brain morphogenesis have been recently highlighted thanks to mouse models. Unmistakably, they are needed for Hedgehog (Hh)-dependent patterning in the forebrain. Not only for Hh reception itself, but most importantly for a downstream event in the Hh transduction pathway, independent of Hh ligand: the Gli3 processing. Indeed, phenotypes due to cilia disruption in the developing brain, such as early patterning, olfactory bulb or corpus callosum formation, can be rescued by reintroducing Gli3-R (the short truncated form of Gli3 working as a transcriptional repressor of Hh target gene). In addition, primary cilia control the proliferation rate in different neural progenitors in the cortex, the hippocampus and the cerebellum; they are required for proper migration of interneurons. And cilia dysfunction is correlated with hydrocephaly, synaptogenesis defects and aberrant axonal tract projections. Most of these neurodevelopmental defects can be related to the various neurological features frequently observed across the ciliopathy spectrum. And thus, understanding the underlying mechanisms of these diverse functions of primary cilia in the brain is a new fundamental challenge.

Since our seminal study in 2003, much has been written about core planar cell polarity (core PCP) signaling and the inner ear. In just a few years, and using the inner ear as a model system, our understanding of the molecular basis of this signaling pathway and how it can influence the development of tissues in mammals has increased considerably. Recently, a number of studies using various animal models of development have uncovered original relationships between the cilia and PCP, and the study of the hair cells of the inner ear has helped elucidating one of these links. In this review, we highlight the differences of PCP signaling between mammals and invertebrates. In the light of recent results, we sum up our current knowledge about PCP signaling in the mammalian cochlear epithelium and we discuss the impact of recent data in the field. We focus our attention on the interrelationship between asymmetric polarity complexes and the position of the cilium, which is essential for the establishment of the overall tissue polarity.

Advances in genomics, bioinformatics and the creation of model organisms have identified many genes associated with polycystic kidney diseases. Historically, these genes were not necessarily associated with ciliopathies, but it appeared that many connections can be made between the cystic kidney disease and function of the primary cilium. Indeed, the proteins encoded by these genes are localized to the cilium itself, to the basal body or are known to regulate the expression and localization of ciliary proteins. The goal of this article is to describe the multiple cellular processes that may lead to the development of renal cysts if they are deregulated. These include changes in proliferation rate, cell polarity or signaling pathways involved in embryonic kidney development. To highlight the role of the primary cilium in cystogenesis, I will discuss several studies investigating the function of ciliary genes and cilia in the kidneys of different model organisms.

Ciliopathies are a large group of human disorders caused by dysfunction of primary or motile cilia and unified by their overlapping clinical features (brain malformations, retinal dystrophy, cystic kidney disease, liver fibrosis and skeletal abnormalities). Ciliopathies are mendelian disorders with prominent genetic heterogeneity and marked allelism between different clinical entities, which are in part explained by the recently identified functional modules and multi-protein complexes formed by ciliopathy-associated gene products. The current review provides an updated snapshot of this complex evolving field, highlighting the key phenotypic features and causative genes for commonly-studied ciliopathies and summarizing our emerging understanding of the correlations between the functions of subgroups of genes and clinical sub-types of ciliopathies. Using the example of Joubert syndrome, a ciliopathy characterized by a distinctive hindbrain malformation and caused by mutations in more than 20 different genes, this work also reviews the principal methods used for new gene identification, including candidate gene approaches, homozygosity mapping as well as high throughput next-generation and exome sequencing.

Eukaryotic epithelial cells form a sheet of contiguous cells, called epithelium, by means of the establishment of well-developed junctional complexes. These junctional complexes ensure the cell cohesion in the tissue and separate the plasma membrane into an apical and a basolateral compartment. This apicobasal polarity, which is crucial for both the architecture and the function of epithelia, is mainly maintained by tight junctions (TJS). Indeed, TJS weakening or loss disrupts the integrity of the epithelium, a process participating to the formation and progression of carcinomas. It has recently been shown that TRAF4, a protein dynamically localized in TJS and commonly overexpressed in carcinomas, plays a variety of functions in tumor progression. Here, we review recent data implicating TRAF4 in carcinogenesis. First, the conserved TRAF proteins family will be presented, and then the molecular mechanism addressing TRAF4 to TJS which involves lipid binding by the TRAF domain will be described. The various roles of TRAF4 in carcinogenesis will be discussed. Finally, we will highlight the ability of all TRAF proteins to bind lipids and discuss its potential functional relevance.

The LKB1 tumor suppressor gene encodes a master kinase that coordinates the regulation of energetic metabolism, cell growth and cell polarity. We now report the identification of a novel isoform of LKB1 named N-LKB1 that is generated through alternative transcription and internal initiation of translation of the LKB1 mRNA. The N-LKB1 protein lacks the N-terminal region and a portion of the kinase domain. Although N-LKB1 is catalytically inactive, it potentiates the stimulating effect of LKB1 on the AMP-activated protein kinase (AMPK) metabolic sensor through a direct interaction with the regulatory auto-inhibitory domain of AMPK. Contrasting, N-LKB1 negatively interferes with the LKB1 polarizing activity. Finally, combining in vitro and in vivo approaches, we showedthat N-LKB1 has an intrinsic oncogenic property. N-LKB1 is expressed solely in the lung cancer cell line, NCI-H460. Silencing of N-LKB1 decreased survival of NCI-H460 cells and inhibited their tumorigenicity when engrafted in nude mice. In conclusion, we have identified a novel LKB1 isoform that enhances the LKB1-controlled AMPK metabolic activity but inhibits LKB1-induced polarizing activity. Both, the LKB1 tumor suppressor and the oncogene, N-LKB1, are expressed from the same locus and this may account for some of the paradoxical effects of LKB1 during tumorigenesis.; LKB1 est un gène suppresseur de tumeur qui code une kinase « maitre » dont l’activité contrôle la polarité et la prolifération cellulaire en les coordonnant avec l’état métabolique de la cellule. Ce travail a abouti à l’identification d’une nouvelle isoforme LKB1 appelée ∆N-LKB1 qui est générée par transcription alternative et initiation interne de la traduction de l'ARNm LKB1. La protéine ∆N-LKB1 est délétée de sa partie N-terminale incluant une partie de son domaine kinase. Bien que la protéine N-LKB1 soit catalytiquement inactive, elle potentialise l'effet activateur de la protéine LKB1 sur sa cible principale l’APMK, senseur énergétique de la cellule, via une interaction directe avec le domaine d'auto-inhibition de l’AMPK. En revanche, ∆N-LKB1 interfère négativement avec la capacité de LKB1 à induire la polarité cellulaire. Enfin, en utilisant des approches in vitro et in vivo, nous avons montré que N-LKB1 possède une propriété oncogénique intrinsèque. N-LKB1 est exprimée seule dans la lignée NCI-H460 issue du cancer du poumon. L’inhibition de l’expression de N-LKB1 dans les cellules NCI-H460 induit une diminution de la survie de ces cellules et inhibe leur pouvoir oncogénique quand elles sont greffées dans la souris nude. Nous avons donc identifié une nouvelle isoforme LKB1 qui stimule l’adaptation métabolique LKB1-dépendante, mais qui inhibe la polarité cellulaire contrôlée par LKB1. Le suppresseur de tumeur LKB1 ainsi que l’oncogène N-LKB1 sont codé par le même gène, ce qui peut expliquer certains des effets paradoxaux de LKB1 durant la tumorigenèse.

The Tau protein (Tubulin Associated Unit) is a phosphoprotein of the microtubule-associated protein family (MAPs). Its role is the regulation of the microtubule polymerization. The Tau protein is naturally present in brain, heart, muscle, lung, kidney, pancreas and liver. An expression of Tau protein and RNA messengers was also highlighted in the testis that is an organ rich in microtubules. The role of microtubules is essential in the stabilization of the cellular shape and in cell divisions. In the testis, Tau protein could be involved in the division process of the spermatogenesis by acting on the microtubular dynamics in the arrangement of the spermatozoon polarity. This review synthesizes the current knowledge, the localization and the main functions of the Tau protein focused on the testis. The localization and the potential roles of the Tau protein during the spermatogenesis are discussed by emphasizing the link with the microtubular structures of seminiferous tubules.

Historically centered on the study of individual cell motility, the field of cell migration has recently moved up one level to look at cooperative behaviour within migratory cell populations. It is now well established that numerous physiological and pathological migration events involve collectively migrating cells rather than solitary cells or concomitantly migrating individual cells. In this review, we first discuss the criteria allowing a given migratory event to be classified as collective cell migration. We then summarize the main concepts that rule collective cell migration in epithelial and mesenchymal tissues with a main focus on mechanisms controlling polarity and directionality in cell collectives.

Nephronophthisis, a hereditary nephropathy characterized by interstitial fibrosis and cyst formation, is caused by mutations in NPHP genes encoding the ciliary proteins called nephrocystins. We investigate the function of nephrocystin-1, -4 and -8, in vitro and in vivo in mammalian kidney cells and in zebrafish respectively. Depletion of either NPHP1 (N1-KD), NPHP4 (N4-KD) or RPGRIP1L (RPGRIP1L-KD) by shRNA-mediated knockdown in MDCK cells led to abnormal ciliogenesis, delay in tight junction formation and disorganized structures in 3D culture. Moreover NPHP4 modulates the Wnt pathways during morphogenesis of the zebrafish pronephros and in mammalian kidney cells in which NPHP4 interacts with inversin and dishevelled, regulating its stability and its subcellular localization. Rpgrip1l is required for dishevelled stabilization at the cilium base and is necessary for polarized positioning of motile cilia of the zebrafish floor plate and sensory hair cells of the mouse cochlea. In either N1-KD or N4-KD cells, we also showed an over activation of Cdc42 and RhoA, downstream targets of dishevelled. This was accompanied by actin cytoskeletal disorganization, enhanced spreading on collagen, over-activation of proteins that regulate focal adhesion structures i.e p130cas-Pyk2 and increased cell migration. Interestingly, the stable expression of dominant negative form of Cdc42 in knockdown cells rescued the migration and the 3D phenotypes. In parallel, we observed that loss of Nphp4 in mice caused cystic tubular dilatation after subtotal nephrectomy correlated with alteration of ciliogenesis and over activation of Cdc42 and RhoA. Our data show a role of nephrocystins in epithelial cell organization and kidney morphogenesis in particular in regulation of focal adhesion, tight junction, ciliogenesis via dishevelled stability., La néphronophtise (NPH) est une néphropathie tubulo-interstitielle chronique de transmission autosomique récessive. Elle représente la cause génétique la plus fréquente des insuffisances rénales terminales de l’enfant et du jeune adulte (5 à 10%). Elle se caractérise au niveau histologique par des anomalies des membranes basales tubulaires, une fibrose interstitielle massive et par l’apparition tardive de kystes à la jonction cortico-médullaire. Dans 40% des cas, la NPH est associée à des atteintes extra-rénales, notamment oculaires, cérébelleuses ou osseuses, définissant de nombreux syndromes (Senior Løken, Joubert, Jeune, etc). Sur la quinzaine de gènes responsables de la maladie, sept ont été identifiés au laboratoire : NPHP1, NPHP4, NPHP8/RPGRIP1L, NPHP11/MKS3, NPHP12/TTC21B, NPHP13/WDR19 et IFT140. Les protéines codées par ces gènes forment des complexes moléculaires principalement localisés au niveau des jonctions cellulaires et du cil primaire des cellules épithéliales rénales, classifiant la NPH et les syndromes associés dans le groupe des "ciliopathies". Mes travaux de thèse se sont intégrés au projet de recherche de l'équipe, centré sur l'étude des mécanismes pathophysiologiques à l'origine des lésions observées dans la NPH. Pour cela, nous avons développé des modèles de cellules tubulaires rénales (MDCK, IMCD et HEK293), et des modèles animaux (souris et poisson zèbre en collaboration avec l'équipe de Sylvie Schneider-Maunoury UMR7622). Je me suis particulièrement intéressé à l'analyse des phénotypes cellulaires et à la caractérisation des voies de signalisation perturbées dans les cellules épithéliales rénales invalidées pour les gènes NPHP1, NPHP4 et NPHP8/RPGRIP1L. Les protéines codées par ces gènes forment un complexe au niveau du cil primaire et des jonctions cellulaires. J'ai participé à définir le rôle crucial de ces protéines dans l’établissement des jonctions serrées par leur interaction avec les protéines de polarité, la morphogénèse épithéliale en culture 3D et la ciliogenèse. De plus, j'ai mis en évidence que l'absence de ces protéines entraîne des anomalies de migration et d'adhésion cellulaires s’accompagnant d’une activation anormale des protéines Rho GTPases (Cdc42, Rac1 et RhoA) et d’une réorganisation du cytosquelette d’actine. J'ai par ailleurs montré que le complexe NPHP4/inversine/RPGRIP1L régule finement l'expression et la localisation de Dishevelled, élément clé des voies Wnt canonique et Wnt/PCP, dans les cellules rénales. Ceci est en accord avec les défauts de polarité planaire observés dans le pronéphros du poisson zèbre et dans le rein de la souris, après invalidation des gènes Nphp4 ou Rpgrip1l. L'ensemble de ces résultats a permis de mieux comprendre le rôle moléculaire et cellulaire des néphrocystines et les mécanismes pathophysiologiques aboutissant aux altérations retrouvées chez les patients telles que la fibrose interstitielle rénale et la formation de kystes.

Nephronophthisis, a hereditary nephropathy characterized by interstitial fibrosis and cyst formation, is caused by mutations in NPHP genes encoding the ciliary proteins called nephrocystins. We investigate the function of nephrocystin-1, -4 and -8, in vitro and in vivo in mammalian kidney cells and in zebrafish respectively. Depletion of either NPHP1 (N1-KD), NPHP4 (N4-KD) or RPGRIP1L (RPGRIP1L-KD) by shRNA-mediated knockdown in MDCK cells led to abnormal ciliogenesis, delay in tight junction formation and disorganized structures in 3D culture. Moreover NPHP4 modulates the Wnt pathways during morphogenesis of the zebrafish pronephros and in mammalian kidney cells in which NPHP4 interacts with inversin and dishevelled, regulating its stability and its subcellular localization. Rpgrip1l is required for dishevelled stabilization at the cilium base and is necessary for polarized positioning of motile cilia of the zebrafish floor plate and sensory hair cells of the mouse cochlea. In either N1-KD or N4-KD cells, we also showed an over activation of Cdc42 and RhoA, downstream targets of dishevelled. This was accompanied by actin cytoskeletal disorganization, enhanced spreading on collagen, over-activation of proteins that regulate focal adhesion structures i.e p130cas-Pyk2 and increased cell migration. Interestingly, the stable expression of dominant negative form of Cdc42 in knockdown cells rescued the migration and the 3D phenotypes. In parallel, we observed that loss of Nphp4 in mice caused cystic tubular dilatation after subtotal nephrectomy correlated with alteration of ciliogenesis and over activation of Cdc42 and RhoA. Our data show a role of nephrocystins in epithelial cell organization and kidney morphogenesis in particular in regulation of focal adhesion, tight junction, ciliogenesis via dishevelled stability.; La néphronophtise (NPH) est une néphropathie tubulo-interstitielle chronique de transmission autosomique récessive. Elle représente la cause génétique la plus fréquente des insuffisances rénales terminales de l’enfant et du jeune adulte (5 à 10%). Elle se caractérise au niveau histologique par des anomalies des membranes basales tubulaires, une fibrose interstitielle massive et par l’apparition tardive de kystes à la jonction cortico-médullaire. Dans 40% des cas, la NPH est associée à des atteintes extra-rénales, notamment oculaires, cérébelleuses ou osseuses, définissant de nombreux syndromes (Senior Løken, Joubert, Jeune, etc). Sur la quinzaine de gènes responsables de la maladie, sept ont été identifiés au laboratoire : NPHP1, NPHP4, NPHP8/RPGRIP1L, NPHP11/MKS3, NPHP12/TTC21B, NPHP13/WDR19 et IFT140. Les protéines codées par ces gènes forment des complexes moléculaires principalement localisés au niveau des jonctions cellulaires et du cil primaire des cellules épithéliales rénales, classifiant la NPH et les syndromes associés dans le groupe des "ciliopathies". Mes travaux de thèse se sont intégrés au projet de recherche de l'équipe, centré sur l'étude des mécanismes pathophysiologiques à l'origine des lésions observées dans la NPH. Pour cela, nous avons développé des modèles de cellules tubulaires rénales (MDCK, IMCD et HEK293), et des modèles animaux (souris et poisson zèbre en collaboration avec l'équipe de Sylvie Schneider-Maunoury UMR7622). Je me suis particulièrement intéressé à l'analyse des phénotypes cellulaires et à la caractérisation des voies de signalisation perturbées dans les cellules épithéliales rénales invalidées pour les gènes NPHP1, NPHP4 et NPHP8/RPGRIP1L. Les protéines codées par ces gènes forment un complexe au niveau du cil primaire et des jonctions cellulaires. J'ai participé à définir le rôle crucial de ces protéines dans l’établissement des jonctions serrées par leur interaction avec les protéines de polarité, la morphogénèse épithéliale en culture 3D et la ciliogenèse. De plus, j'ai mis en évidence que l'absence de ces protéines entraîne des anomalies de migration et d'adhésion cellulaires s’accompagnant d’une activation anormale des protéines Rho GTPases (Cdc42, Rac1 et RhoA) et d’une réorganisation du cytosquelette d’actine. J'ai par ailleurs montré que le complexe NPHP4/inversine/RPGRIP1L régule finement l'expression et la localisation de Dishevelled, élément clé des voies Wnt canonique et Wnt/PCP, dans les cellules rénales. Ceci est en accord avec les défauts de polarité planaire observés dans le pronéphros du poisson zèbre et dans le rein de la souris, après invalidation des gènes Nphp4 ou Rpgrip1l. L'ensemble de ces résultats a permis de mieux comprendre le rôle moléculaire et cellulaire des néphrocystines et les mécanismes pathophysiologiques aboutissant aux altérations retrouvées chez les patients telles que la fibrose interstitielle rénale et la formation de kystes.

Asymmetric cell division is the process by which a single cell gives rise to two different daughter cells. This process is important to generate cell diversity during the development of multicellular organisms, as well as for stem cell self-renewal in adults. Current knowledge on so-called cancer stem cells suggests that a loss of asymmetry during their division could lead to overproliferation and favour tumorigenesis, highlighting the importance of deciphering the mechanisms governing asymmetric cell division. Two mechanisms can lead to an asymmetric cell division: asymmetry can either be governed by proximity to a given cellular environment (or niche), in which case the mechanism is referred to as extrinsic, or the mother cell polarizes itself without external intervention, in which case the mechanism is referred to as intrinsic. In the last 20 years, our understanding of intrinsic mechanisms leading to asymmetric cell division has progressed, largely after studies carried out in model organisms such as the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster. These models allowed the identification of molecular complexes used by nearly all the cells that divide asymmetrically, including human cells. Here we review the main intrinsic mechanisms of asymmetric cell division as described in model organisms and discuss their relevance towards mammalian tumorigenesis.

Nectin and nectin-like (necl) proteins form a family of 9 adhesion molecules that belong to the immunoglobulin superfamily. They play a key role in different biological processes such as cell polarity, proliferation, differentiation and migration in epithelial, endothelial, immune and nervous systems. Besides their role in physiology, they have been involved in different pathological processes in humans. They serve as virus receptors (poliovirus and herpes simplex virus), they are involved in orofacial malformation (CLPED1) and recently they have been described as markers, actors and potential therapeutics targets in cancer. Among them, necl-5, nectin-2 and nectin-4 are overexpressed in tumors, and are associated with a poor prognosis. On the opposite, necl-1, necl-2 and necl-4 act as tumor suppressors and are repressed in cancer. The involvement of nectins and necls molecules in cancer and their potential used in therapy is discussed in this review.

In 1969, Brambell, while studying the long serum half-life of IgG and their ability to cross the materno-foetal barrier, attributed these two properties to the existence of a specific Fc receptor, which was later denominated FcRn for neonatal Fc receptor. The resolution of its structure revealed that it is a MHC class-I-like molecule. FcRn is able to load IgG and albumin in a pH-dependent manner. It acts as an intracellular transport protein and as such is controling the serum half-life of these proteins (apical recycling of IgG and albumin in endothelial cells), IgG biodistribution (apical to basolateral and basolateral to apical transport of IgG in epithelial and endothelial cells) and it may also contribute to phagocytosis. FcRn is thus a key partner in the pharmacokinetics of therapeutic antibodies, opening interesting prospects for optimisation of their use.

The preimplantation embryo development leads to the formation of a blastocyst made of two cell lineages: an outer layer of epithelial cells, the trophectoderm, that will give rise to some embryonic annexes, and a mass of undifferentiated cells, the inner cell mass, that will form the foetus and the remaining embryonic annexes. The trophectoderm encloses the inner cell mass and protects it from the external medium. Moreover, after hatching, the trophectoderm invades the uterine tissue, a crucial step for the implantation of the embryo. Therefore, the divergence between these two lineages is of crucial importance for the emergence of the foetus itself and for the postimplantation development to take place correctly. The setting up of cell polarity during compaction at the eight-cell stage allows asymmetric divisions to take place, thereby leading to lineage divergence. Phenotypic properties of these two cell populations are progressively reinforced through cell-cell interactions, outer cells undergoing epithelial differentiation while inner cells remain undifferentiated. Although cellular mechanisms controlling the divergence of the first two lineages are quite well known, important efforts have been carried out this last decade to identify the molecular machinery involved in this process and will be presented in this review.

Polarity is a fundamental feature of all organisms both during development and in the adult. This reflects the key role of cell polarity during basic fundamental processes such as cell division, cell differentiation and cell migration. The control of cell polarity relies on functionally conserved proteins. Among these, Scribble, initially identified as a tumor suppressor gene in Drosophila, has been first involved in epithelial polarity. More recently Scribble function has been implicated in neuronal polarity and polarized cell migration. Scribble joins the growing family of tumor suppressors that play a key and conserved function in cell polarity. Scribble illustrates the more and more obvious link between regulation of cell polarity, cell transformation and tumor formation.

More than 10 years after its initial discovery, netrin-1 - the first described chimioattractive molecule controlling the guidance of the commissural axons - has recently known a unsuspected wave of interest because of its implication in the development of the nervous system but also, more recently, fot its role in angiogenesis and tumorigenesis. Because, of a series of recent publications on netrin-1 signaling, we propose here to describe the recent insight in netrin-1 signaling via its main receptor DCC (deleted in colorectal cancer), and the recent discovery that netrin controls the assymetric distribution of beta-actin in the growth cone. Thus, it seems that netrin-1, but also the neurotrophic factor BDNF, controls acute growth cone responses such as collapse and turning by the regulation of localized protein translation, such as beta-actin. This process involves both transport of beta-actin mRNA, bound to Vg1RBP, to specific locations, and mRNA translation upon stimulation by local activation of the translation initiation regulator eIF-4E-binding protein 1. Indeed, Netrin-1 induces the movement of Vg1RBP granules into filopodia, and triggers a polarized increase in beta-actin translation on the near side of the growth cone before growth cone turning. The binding of BDNF to its receptor Trk has a similar effect for growth cone attraction, althought it is differentially regulated. Thus, this asymetrically synthesized beta-actin may direct actin polymerization and consequently the migration of the growth cone toward the cue.

Tight junctions (TJ) are specialized plasma membrane microdomains that encircle the apical pole of each epithelial and endothelial cell, separating apical from basolateral side. They form an intercellular diffusion barrier (or gate) regulating the passage of ions, water, and various macromolecules through the paracellular spaces, and a fence restricting the apical/basolateral diffusion of membrane proteins and lipids. This latter function is deeply involved in cancer cell biology, in terms of loss of cell polarity. Several TJ proteins are involved in organizing signal transduction at TJ. For example, the interaction of TJ protein ZO-1 (zonula occludens 1) with the transcription factor ZONAB (ZO-1 associated nucleic acid protein) is important in the regulation of expression of the proto-oncogene Erb-2 as well as epithelial proliferation/differentiation. Recent studies indicate that different proteins involved in membrane trafficking are associated with tight junctions. Among these proteins are the small G-proteins of the Rab family that regulate specific membrane transport events in both endocytic and exocytic pathways. Rab proteins may coordinate the recruitment of protein complexes necessary for the establishment-maintenance of cell polarity. Based on the studies reviewed here, tight junctions emerge as a platform used to coordinate and regulate cell polarity, and proliferation/differentiation.

Several epitheliums exhibit a clear polarity that lies within the plane of the epithelium. This polarity, referred to as planar polarity or tissue polarity, is oriented perpendicular to the apical-basal polarity of the epithelium. Over the last two decades, the genetic and molecular bases of planar polarity have been intensively investigated in Drosophila. Recent studies have shown that establishment of planar polarity relies on the unipolar distribution of a small number of signaling molecules localizing at the apical cortex. Unipolar localization of planar polarity proteins defines two opposite and complementary cortical domains. These domains show a stereotyped orientation at the tissue level. Positioning of these cortical domains is coordinated at the tissue level by a second class of signaling molecules that form an activity gradient across the epithelium. Together these data have led to a general model of planar polarity establishment. Considering that planar polarity genes have been conserved from flies to vertebrates, this model may be useful for our understanding of epithelium biology in mammals.

Embryonic development depends on the establishment of polarities which define the axial characteristics of the body. In a small number of cases such as the embryo of the fly drosophila, developmental axes are established well before fertilization while in other organisms such as the nematode worm C. elegans these axes are set up only after fertilization. In most organisms the egg posesses a primary (A-V, Animal-Vegetal) axis acquired during oogenesis which participates in the establishment of the embryonic axes. Such is the case for the eggs of ascidians or the frog Xenopus whose AV axes are remodelled by sperm entry to yield the embryonic axes. Embryos of different species thus acquire an anterior end and a posterior end (Antero-Posterior, A-P axis), dorsal and ventral sides (D-V axis) and then a left and a right side.

Flagella and cilia are elaborate cytoskeletal structures conserved from protists to mammals, where they fulfil functions related to motility or sensitivity. We demonstrate a novel role for the flagellum in the control of cell morphogenesis and division of Trypanosoma brucei. To investigate flagellum functions, its formation was perturbed by inducible RNA interference silencing of components required for intraflagellar transport (IFT), a dynamic process necessary for flagellum assembly. First, we show that down-regulation of IFT leads to assembly of a shorter flagellum. Strikingly, cells with a shorter flagellum are smaller, with a direct correlation between flagellum length and cell size. Detailed morphogenetic analysis reveals that the tip of the new flagellum defines the point where cytokinesis is initiated. Furthermore, when new flagellum formation is completely blocked, non-flagellated cells are very short, lose their normal shape and polarity and fail to undergo cytokinesis. We show that flagellum elongation controls formation of cytoskeletal structures present in the cell body that act as molecular organisers of the cell.

Polarity is a fundamental characteristic of most eukaryotic cells. The plasma membrane of such cells consists in two structurally and functionally different domains, i.e., the basolateral and the apical membrane, separated by tight junctions. The generation of the distinct molecular identity of both domains and its maintenance in spite of the dynamics of lipids and proteins at either surface requires sophisticated sorting and trafficking mechanisms. Recent progress in the field of polarized trafficking reveals that, for a detailed understanding of its mechanism and regulation, an integrated approach that includes the flow of both lipids and proteins is imperative. In this review, some recent progress in understanding mechanisms involved in protein sorting and trafficking is discussed. We focus on the role of lipid microdomains (Rafts) in trafficking of proteins to the apical surface of polarized cells.

Establishment and maintenance of epithelial cell polarity rely on finely tuned protein networks comprising cell surface molecules, cytoplasmic adaptors, and enzymes connected to the actin cytoskeleton. Oncogenes and tumor suppressors promote cell proliferation and resistance to apoptosis and, in many cases, alter some of these molecular scaffolds, and profoundly affect the epithelial cytoarchitecture. Reciprocally, loss of central actors of epithelial polarity unleashes normally repressed signaling pathways and perturb the shape and functions of epithelial tissues. Among the newcomers impacting on epithelial integrity, Scribble is a scaffold protein of a remarkable importance that furthermore displays a tumor suppressing activity in Drosophila melanogaster. Together with Discs Large (Dlg) and Lethal Giant Larvae (Lgl), two known tumor suppressors, Scribble acts on the correct positioning of epithelial junctions required to organize functional epithelial sheets. Scribble, Dlg and Lgl proteins are well conserved during evolution at the molecular and subcellular level implying their potential role in cell polarity and tumorigenesis in humans. Recent findings on hScrib, the human orthologue of Scribble, are discussed here.

During metazoan development, cell fate diversity is generated in part by asymmetric cell divisions, in which mother cells divide to produce two daughter cells with distinct developmental potentials. Adoption of different cell fates often relies on the polarised distribution and unequal segregation of cell-fate determinants. Unequal segregation of cell-fate determinants requires that the mother cell becomes polarised prior to mitosis. In response to this polarisation, cell-fate determinants localise asymmetrically and the mitotic spindle lines up with the pole to which cell-fate determinants accumulate, thereby leading to their unequal partitioning upon cytokinesis. I review here the regulatory mechanisms that establish cell asymmetry and orient this asymmetry relative to the body axis in the sensory organ lineages of Drosophila.

During early oogenesis, one cell from a cyst of 16 germ cells is selected to become the oocyte. Recent data suggest that the choice of this cell within the cyst is strongly biased as early as the cyst itself forms. However, it was further shown that, although selected, the oocyte fate needs to be maintained. The maintenance of the oocyte identity requires the activity of the Drosophila homologues of the Caenorhabditis elegans par genes. It was shown that the par genes are required for the first polarisation of the oocyte as early as in region 3 of the germarium. This reveals a striking conservation between the polarisation along the antero-posterior axis of the Caenorhabditis elegans one-cell embryo and the Drosophila oocyte.

The study of the establishment of embryonic axes during early development has shown that this process is a very early event (occurRing either during ovogenesis or during fertilization) for invertebrates and for lower vertebrates. In mammals, it was considered that this establishment appears late during development because of the great plasticity of blastomeres. Recent data in the mouse embryon show that the mammalian ovocyte is a polarized cell, the polar body corresponding to the animal pole of this cell. The blastomeres that are generated by the zygote divide asynchronously. The first that divides is the one which inherits the plasma cell membrane where fertilization takes place. This blastomere will preferentially give rise to the cells of the embryonic pole of the blastocyst whereas the other yields the cells of the abembryonic pole. The mammalian ovocyte is thus a polarized cell with an already established animal-vegetal axis. The point of sperm entry will determine the embryonic-abembryonic axis.

Molecular and structural biodiversity characterises dental mineral tissues. Groups of matrix proteins belong specifically to each tissue; amelogenins to enamel, DSPP to dentine and CAP to cementum. A wide group of proteins is also shares with other mineralized tissues such as calcium (calbindins) and phosphate (alkaline phosphatase) handling proteins. Dental tissues organisation is also based on specific cellular programs of morpho-differentiation (polarity) and on expression patterns of proteins implicated in mineralisation. The regulation of gene expression in tooth has been analysed regarding various hormones such as vitamin D in a first step and recently transcription factors (Osf-2/Cbfa1/Aml3). Other molecular families encoded by divergent homeobox genes (Msx and Dlx) are implicated in the determinism of this gene regulation and of early development. Genetic and hormonal abnormalities of dental mineralized tissues should now be interpreted thanks to the recent availability of cellular models and of odontogenic protein promoter structure.

The apical cell of Sphacelaria (Fucophyceae) exhibits a permanent polarized organization throughout asymmetric divisions. The apex organization was studied by immunolocalization of tubulin, vitronectin, alpha-actinin and beta 1 integrin. Microfilaments were stained directly by fluorescein phalloidin. The apex was highly organized around a patch of microfilaments densely packed at the tip, where vitronectin-like and alpha-actinin-like proteins colocalized. In the same area, an actin-dependent targeted secretion of sulfated polysaccharides was shown. The permanent localization of these components throughout cell elongation suggests that a cortical site involving transmembrane connections between the cytoskeleton and the extracellular matrix is required for cell polarity. A model of the organization of the tip is proposed.

Type I phosphodiesterases are differently expressed by different cell types. Three members have been identified, PC-1, B10 and autotaxin. They are between 40 and 50% identical at the amino acid sequence level. Hepatocytes express both B10 and PC-1 at their plasma membrane. However, B10 is exclusively expressed at the apical pole whereas PC-1 is located at the basolateral pole. Studies of the biosynthetic route of B10 in hepatocytes shows that B10 is first transported to the basolateral surface and secondarily reaches the basolateral surface. The transcytotic step between the basolateral and apical surface occurs through a tubular endosomal compartment identical to the transcytotic compartment of the polymeric IgA receptor. Transfected in the polarized cell lines MDCK and Caco-2 of renal and intestinal origin, B10 and PC-1 are expressed at the apical and basolateral poles respectively, as in hepatocytes. However, the biosynthetic transport of B10 occurs directly in MDCK cells and both directly and by transcytosis in Caco-2 cells. Truncation of the cytoplasmic domain of PC-1 generates an apical protein indicating that the basolateral signal of PC-1 is likely to be in the cytoplasmic domain. The nature of the apical targeting signal of B10 is under investigation.

Protein sorting in epithelial cells is the major event that drive the onset and the maintenance of the functional cell polarity. A lot of interdependent steps are involved in protein sorting and targeting. Recent data describing the last results obtained in this field will be reviewed in the first part of this article. Molecular signals harbored by proteins to specify their destination are thought to be the driven force to sort given protein in a given pathway. The basolateral targeting signals so far identified are known for several years and are of the same nature, whereas apical targeting signals are still discussed and are of diverse molecular nature. Dipeptidyl peptidase IV (DPP IV/CD26) targeting signals have not been described so far and it will be interesting to study these signals, since the protein reach the apical membrane of epithelial cells through different pathways that strongly depend on the cell type considered. These different pathways result in DPP IV membrane localizations that may explain the multifunctional properties of DPP IV such as enzymatic digestion, interaction with extracellular matrix proteins, capture and transport of circulating proteins. We have undertaken the study of DPP IV molecular targeting signals and we will described here how the transmembrane domain and the glycosylation of the ectodomain may be involved in DPP IV apical targeting, with a special reference to the cell type specificity.

Two recent developmental genetics studies suggest an important role for transcription factors of the homeodomain protein class in the morphogenesis of cerebral structures and the form and extent of the various domains that constitute the nervous system (Krumlauf, 1994; Rubenstein and Puelles, 1994). Our team recently took a look at the hypothesis that the same transcription parameters have a function in axonal growth and recognition of paths and targets. It has been shown that cortical or spinal medullary neurons are capable of responding, by a change in axonal growth rate, to regulation of the activity of certain transcription factors (Prochiantz and Théodore, 1995). This demonstration was facilitated by a particular property of the DNA binding region of the transcription factors. This property consists in crossing plasmic membranes and addressing cell nuclei directly (Prochiantz, 1996). The homeodomain proteins are expressed not only during development but also in adulthood. It is possible that their continuous expression or reactivation may be associated with the morphological changes in neurons observed in the mature nervous system. The latter hypothesis will be discussed on the basis of our current knowledge of the expression and role of the transcription factors. The authors will draw some conclusions regarding the possibility of considering these nuclear factors as agents of cerebral plasticity and as targets of potential pharmacological value.

Polarized epithelial cells display a cell surface organization well adapted to their specialized functions in transports and exchanges with the external "milieu". The numerous microvilli that cover the apical plasma membrane arc supported by bundles of actin microfilaments and associated actin binding proteins. Three independent experimental approaches have been used to unravel the function(s) of villin: a) overexpression of the villin gene; b)inhibition of villin production with a villin antisense RNA; c) disruption of the villin gene by homologous recombination. The role of ezrine, a protein able to interact both with components of the plasma membrane and with actin microfilaments, in cellular morphogenesis and the signal transduction cascade elicited by the scatter factor (HGF) and its receptor, has also been investigated with molecular cell biology approaches.

Carbonic anhydrases II and IV play an essential role in the synthesis and secretion of HCO3- ions in pancreatic duct cells. Secretion of these ions is regulated by the CFTR (cystic fibrosis transmembrane conductance regulator) chloride channel. In the present study, the expression of carbonic anhydrases IV and their targeting to plasma membranes were examined during the growth of human pancreatic duct cells in vitro. Human cancerous pancreatic duct cells of Capan-1 cell line which polarize during their growth were used. We show that: a) these cells express carbonic anhydrases IV continuously during growth in culture, and the expression depends on the stage of growth and the conformation of the cells; b) carbonic anhydrases IV are seen in the cytoplasm in non-polarized cells, but become progressively anchored to plasma membranes as the cells polarize, being targeted to the apical membranes of polarized cells; c) the subcellular distribution of carbonic anhydrases IV indicates that these enzymes are synthetized in rough endoplasmic reticulum and then transported towards the plasma membrane using the classical secretory pathway through the Golgi apparatus. The results indicated that targeting of carbonic anhydrases IV in Capan-1 cells is linked to cellular polarization.

Several variants of the microtubule-associated tau proteins, are expressed during brain development and in adulthood. These entities are required to define the polarity of the neuron and the architecture of the axon but differ in sequence and in their microtubule polymerizing activity. Here, we describe a new group of high molecular weight tau proteins that contain one or two additional exons of 711 and 198 bp in their middle region and a variable N-terminal domain. These high molecular weight tau variants are preferentially expressed in the peripheral nervous system. Immunohistochemical studies showed that they are also present in the dorsal horn of the spinal cord where they are probably transported by sensory fibers arising in the periphery. However, a minor fraction of these proteins is present in the motor neurons of the ventral horn. Similar studies were performed with the neuroblastoma N115 cell line which can be differentiated in vitro and expresses only high molecular weight tau forms. In the non differentiated cells, tau antibodies label the domain of the cell body localized around the centrosome whereas, after differentiation, the cell process facing this structure is also stained. These data suggest that axonal polarity is predetermined by the localization of tau proteins in the domain of the cell body defined by the centrosome.