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A major challenge in the neuroscience field is the identification of molecules and pathways that control synaptic plasticity and memory. Dendritic spines play a pivotal role in these processes, as the major sites of excitatory synapses in neuronal communication. Previous studies have shown that the scaffold protein p140Cap localizes into dendritic spines and that its knockdown negatively modulates spine shape in culture. However, so far, there is no information on itsin vivorelevance. By using a knock-out mouse model, we here demonstrate that p140Cap is a key element for both learning and synaptic plasticity. Indeed,p140Cap−/−mice are impaired in object recognition test, as well as in LTP and in LTD measurements. Thein vivoeffects of p140Cap loss are presumably attenuated by noncell-autonomous events, since primary neurons obtained fromp140Cap−/−mice show a strong reduction in number of mushroom spines and abnormal organization of synapse-associated F-actin. These phenotypes are most likely caused by a local reduction of the inhibitory control of RhoA and of cortactin toward the actin-depolymerizing factor cofilin. These events can be controlled by p140Cap through its capability to directly inhibit the activation of Src kinase and by its binding to the scaffold protein Citron-N. Altogether, our results provide new insight into how protein associated with dynamic microtubules may regulate spine actin organization through interaction with postsynaptic density components.

Dendritic spines are highly dynamic protuberances that are thought to be crucial for learning and memory. Although it is well known that actin filaments and membrane dynamics regulate spine plasticity, how these two events are linked locally is less clear. Here, we provide evidence that Citron-N (CIT-N), a binding partner of the small GTPase RhoA, is associated with the actin filaments and Golgi compartments of dendritic spines. We also show that CIT-N is required for recruiting F-actin and Golgi membranes at spines of in vitro-grown neurons. Studies in knockout mice show that this protein is essential for the maturation of dendritic spines. We suggest that CIT-N might function as a scaffold protein in spine organization through its ability to bind to Golgi membranes and by affecting actin remodelling.

During spermatogenesis, the first morphological indication of spermatogonia differentiation is incomplete cytokinesis, followed by the assembly of stable intercellular cytoplasmic communications. This distinctive feature of differentiating male germ cells has been highly conserved during evolution,suggesting that regulation of the cytokinesis endgame is a crucial aspect of spermatogenesis. However, the molecular mechanisms underlying testis-specific regulation of cytokinesis are still largely unknown. Citron kinase is a myotonin-related protein acting downstream of the GTPase Rho in cytokinesis control. We previously reported that Citron kinase knockout mice are affected by a complex neurological syndrome caused by cytokinesis block and apoptosis of specific neuronal precursors. In this report we show that, in addition,these mice display a dramatic testicular impairment, with embryonic and postnatal loss of undifferentiated germ cells and complete absence of mature spermatocytes. By contrast, the ovaries of mutant females appear essentially normal. Developmental analysis revealed that the cellular depletion observed in mutant testes is caused by increased apoptosis of undifferentiated and differentiating precursors. The same cells display a severe cytokinesis defect, resulting in the production of multinucleated cells and apoptosis. Our data indicate that Citron kinase is specifically required for cytokinesis of the male germ line.

In this report, we confirm that the RhoA-binding protein citron kinase (CIT-K) is required for midbody abscission in late cytokinesis, while it has little or no role in early cytokinesis. Moreover, we show that CIT-K, despite being commonly considered a RhoA effector, promotes midbody stability through RhoA and anillin during late cytokinesis., The small GTPase RhoA plays a crucial role in the different stages of cytokinesis, including contractile ring formation, cleavage furrow ingression, and midbody abscission. Citron kinase (CIT-K), a protein required for cytokinesis and conserved from insects to mammals, is currently considered a cytokinesis-specific effector of active RhoA. In agreement with previous observations, we show here that, as in Drosophila cells, CIT-K is specifically required for abscission in mammalian cells. However, in contrast with the current view, we provide evidence that CIT-K is an upstream regulator rather than a downstream effector of RhoA during late cytokinesis. In addition, we show that CIT-K is capable of physically and functionally interacting with the actin-binding protein anillin. Active RhoA and anillin are displaced from the midbody in CIT-K-depleted cells, while only anillin, but not CIT-K, is affected if RhoA is inactivated in late cytokinesis. The overexpression of CIT-K and of anillin leads to abscission delay. However, the delay produced by CIT-K overexpression can be reversed by RhoA inactivation, while the delay produced by anillin overexpression is RhoA-independent. Altogether, these results indicate that CIT-K is a crucial abscission regulator that may promote midbody stability through active RhoA and anillin.

Spinal muscular atrophy (SMA) is a human disease caused by reduced levels of the Survival of Motor Neuron (SMN) protein, leading to progressive loss of motor neurons and muscular paralysis. However, it is still not very clear why these cells are specifically sensitive to SMN levels. Therefore, understanding which proteins may functionally interact with SMN in a neuronal context is a very important issue. PPP4R2, a regulatory subunit of the protein phosphatase 4 (PPP4C), was previously identified as a functional interactor of the SMN complex, but has never been studied in neuronal cells. In this report, we show that PPP4R2 displays a very dynamic intracellular localization in mouse and rat neuronal cell lines and in rat primary hippocampal neurons, strongly correlating with differentiation. More importantly, we found that PPP4R2 loss of function impairs the differentiation of the mouse motor-neuronal cell line NSC-34, an effect that can be counteracted by SMN overexpression. In addition, we show that PPP4R2 may specifically protect NSC-34 cells from DNA damage-induced apoptosis and that it is capable to functionally cooperate with SMN in this activity. Our data indicate that PPP4R2 is a SMN partner that may modulate the differentiation and survival of neuronal cells.

Alzheimer disease (AD) is a neurodegenerative disorder characterized by progressive decline of cognitive function that represents one of the most dramatic medical challenges for the aging population. Aβ peptides, generated by processing of the Amyloid Precursor Protein (APP), are thought to play a central role in the pathogenesis of AD. However, the network of physical and functional interactions that may affect their production and deposition is still poorly understood. The use of a bioinformatic approach based on human/mouse conserved coexpression allowed us to identify a group of genes that display an expression profile strongly correlated with APP. Among the most prominent candidates, we investigated whether the collagen chaperone HSP47 could be functionally correlated with APP. We found that HSP47 accumulates in amyloid deposits of two different mouse models and of some AD patients, is capable to physically interact with APP and can be relocalized by APP overexpression. Notably, we found that it is possible to reduce the levels of secreted Aβ peptides by reducing the expression of HSP47 or by interfering with its activity via chemical inhibitors. Our data unveil HSP47 as a new functional interactor of APP and imply it as a potential target for preventing the formation and/or growth amyloid plaques.

The Down syndrome critical region (DSCR) on Chromosome 21 contains many genes whose duplication may lead to the major phenotypic features of Down syndrome and especially the associated mental retardation. However, the functions of DSCR genes are mostly unknown and their possible involvement in key brain developmental events still largely unexplored. In this report we show that the protein TTC3, encoded by one of the main DSCR candidate genes, physically interacts with Citron kinase (CIT-K) and Citron N (CIT-N), two effectors of the RhoA small GTPase that have previously been involved in neuronal proliferation and differentiation. More importantly, we found that TTC3 levels can strongly affect the NGF-induced differentiation of PC12 cells, by a CIT-K-dependent mechanism. Indeed, TTC3 overexpression leads to strong inhibition of neurite extension, which can be reverted by CIT-K RNAi. Conversely, TTC3 knockdown stimulates neurite extension in the same cells. Finally, we find that Rho, but not Rho kinase, is required for TTC3 differentiation-inhibiting activity. Our results suggest that the TTC3–RhoA–CIT-K pathway could be a crucial determinant of in vivo neuronal development, whose hyperactivity may result in detrimental effects on the normal differentiation program.

Citron-K (CIT-K) is a target molecule for activated Rho which is expressed at high levels in the proliferating areas of the CNS from E10.5 to E16. CIT-K -/- mice display severe defects in neurogenesis, due to altered cytokinesis and aptoptosis: these cellular alterations result in severe microencephaly and in death of the animal due to fatal seizures between the 2nd and the 3rd week of age. We have analysed the development of somatosensory cortex in the CIT-K -/- mice, showing i) a decrease in the barrelfield area and in the size of single whisker-related barrels, ii) a decrease in the cortical thickness, especially in supragranular layers, iii) a decrease in the density of myelinated fibers. We also report cellular changes in cortical neurons: i) both pyramidal neurons and interneurons show altered dendritic development and frequent polyploidy, and ii) the distribution of interneurons is affected by CITK deletion. CIT-K -/- mice are a useful tool to study the role of this molecule in the cellular development of cerebral cortex in vivo, and the development and plasticity of cortical areas and connections in a microencephalic animal. Moreover, they represent an interesting model of neonatal epilepsy, in which to study the role of changes in cellular morphology and interneuron distribution in the genesis of epileptic seizures.

The actin cytoskeleton is best known for its role during cellular morphogenesis. However, other evidence suggests that actin is also crucial for the organization and dynamics of membrane organelles such as endosomes and the Golgi complex1,2,3. As in morphogenesis, the Rho family of small GTPases are key mediators of organelle actin-driven events4,5, although it is unclear how these ubiquitously distributed proteins are activated to regulate actin dynamics in an organelle-specific manner. Here we show that the brain-specific Rho-binding protein Citron-N6 is enriched at, and associates with, the Golgi apparatus of hippocampal neurons in culture. Suppression of the whole protein or expression of a mutant form lacking the Rho-binding activity results in dispersion of the Golgi apparatus. In contrast, high intracellular levels induce localized accumulation of RhoA and filamentous actin, protecting the Golgi from the rupture normally produced by actin depolymerization. Biochemical and functional analyses indicate that Citron-N controls actin locally by assembling together the Rho effector ROCK-II and the actin-binding, neuron-specific, protein Profilin-IIa (PIIa). Together with recent data on endosomal dynamics5, our results highlight the importance of organelle-specific Rho modulators for actin-dependent organelle organization and dynamics.

Citron kinase (CIT-K), a ser/thr kinase, is required during neurogenesis for cytokinesis of neuronal precursors. Deletion of the cit-k gene in mice (cit-k(-/-) mice) leads to a severe malformative central nervous system syndrome characterized by microencephaly, ataxia, and epileptic seizures; affected mice die by the third week of postnatal life. We have used NADPH-diaphorase histochemistry, immunostaining for calbindin, calretinin, parvalbumin, and glutamic acid decarboxylase 67 (GAD67), and histological staining to undertake qualitative and quantitative analyses of the morphology and distribution of interneurons in the barrelfield cortex of cit-k(-/-) mice. By postnatal day 13, lack of CIT-K results in profoundly altered cortical cell morphology: the infragranular layers are populated by large, binucleate interneurons bearing many more dendrites than in control mice, an anatomical profile that has also been reported for the cortex of humans with cortical dysplasias and epilepsy. Tessellation analyses reveal that a clustered distribution of interneurons is maintained in cit-k(-/-) mice, but that their nearest neighbor distance is significantly increased, and thus their density is reduced; the overall number of interneurons is more dramatically decreased in the absence of CIT-K than would be predicted on the basis of the reduced brain size of affected mice. This reduction of inhibitory gamma-aminobutyric acid (GABA)ergic neurons likely underlies the occurrence of epileptic seizures in the cit-k(-/-) mice. Furthermore, the altered distribution of NADPH-diaphorase-positive interneurons could be responsible for an impaired coupling of cortical activity to blood flow, also affecting cortical growth and functioning.

SummaryDendritic spines are the major sites of excitatory synaptic input, and their morphological changes have been linked to learning and memory processes. Here, we report that growing microtubule plus ends decorated by the microtubule tip-tracking protein EB3 enter spines and can modulate spine morphology. We describe p140Cap/SNIP, a regulator of Src tyrosine kinase, as an EB3 interacting partner that is predominately localized to spines and enriched in the postsynaptic density. Inhibition of microtubule dynamics, or knockdown of either EB3 or p140Cap, modulates spine shape via regulation of the actin cytoskeleton. Fluorescence recovery after photobleaching revealed that EB3-binding is required for p140Cap accumulation within spines. In addition, we found that p140Cap interacts with Src substrate and F-actin-binding protein cortactin. We propose that EB3-labeled growing microtubule ends regulate the localization of p140Cap, control cortactin function, and modulate actin dynamics within dendritic spines, thus linking dynamic microtubules to spine changes and synaptic plasticity.