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Amyotrophic lateral sclerosis (ALS) is the most common motor neuron (MN) disease in adults with no curative treatment. Neurofilament (NF) level in patient’ fluids have recently emerged as the prime biomarker of ALS disease progression, while NF accumulation in MNs of patients is the oldest and one of the best pathological hallmarks. However, the way NF accumulations could lead to MN degeneration remains unknown. To assess NF accumulations and study the impact on MNs, we compared MNs derived from induced pluripotent stem cells (iPSC) of patients carrying mutations in C9orf72, SOD1 and TARDBP genes, the three main ALS genetic causes. We show that in all mutant MNs, light NF (NF-L) chains rapidly accumulate in MN soma, while the phosphorylated heavy/medium NF (pNF-M/H) chains pile up in axonal proximal regions of only C9orf72 and SOD1 MNs. Excitability abnormalities were also only observed in these latter MNs. We demonstrate that the integrity of the MN axonal initial segment (AIS), the region of action potential initiation and responsible for maintaining axonal integrity, is impaired in the presence of pNF-M/H accumulations in C9orf72 and SOD1 MNs. We establish a strong correlation between these pNF-M/H accumulations, an AIS distal shift, increased axonal calibers and modified repartition of sodium channels. The results expand our understanding of how NF accumulation could dysregulate components of the axonal cytoskeleton and disrupt MN homeostasis. With recent cumulative evidence that AIS alterations are implicated in different brain diseases, preserving AIS integrity could have important therapeutic implications for ALS.

International audience; One of the main obstacles for studying the molecular and cellular mechanisms underlying human neurodevelopment in vivo is the scarcity of experimental models. The discovery that neurons can be generated from human induced pluripotent stem cells (hiPSCs) paves the way for novel approaches that are stem cell-based. Here, we developed a technique to follow the development of transplanted hiPSC-derived neuronal precursors in the cortex of mice over time. Using post-mortem immunohistochemistry we quantified the differentiation and maturation of dendritic patterns of the human neurons over a total of six months. In addition, entirely hiPSC-derived neuronal parenchyma was followed over eight months using two-photon in vivo imaging through a cranial window. We found that transplanted hiPSC-derived neuronal precursors exhibit a "protracted" human developmental programme in different cortical areas. This offers novel possibilities for the sequential in vivo study of human cortical development and its alteration, followed in "real time".

The accumulation of protein aggregates is involved in the onset of many neurodegenerative diseases. Aggrephagy is a selective type of autophagy that counteracts neurodegeneration by degrading such aggregates. In this study, we found that LC3C cooperates with lysosomal TECPR1 to promote the degradation of disease-related protein aggregates in neural stem cells. The N-terminal WD-repeat domain of TECPR1 selectively binds LC3C which decorates matured autophagosomes. The interaction of LC3C and TECPR1 promotes the recruitment of autophagosomes to lysosomes for degradation. Augmented expression of TECPR1 in neural stem cells reduces the number of protein aggregates by promoting their autophagic clearance, whereas knockdown of LC3C inhibits aggrephagy. The PH domain of TECPR1 selectively interacts with PtdIns(4)P to target TECPR1 to PtdIns(4)P containing lysosomes. Exchanging the PH against a tandem-FYVE domain targets TECPR1 ectopically to endosomes. This leads to an accumulation of LC3C autophagosomes at endosomes and prevents their delivery to lysosomes., Many neurodegenerative disorders are characterised by the accumulation of protein aggregates in neurons. Here, the authors show that the lysosomal protein TECPR1 selectively recruits mature autophagosomes via an interaction with LC3C to break down protein aggregates in neural stem cells.

International audience; By providing access to affected neurons, human induced pluripotent stem cells (iPSc) offer a unique opportunity to model human neurodegenerative diseases. We generated human iPSc from the skin fibroblasts of children with mucopolysaccharidosis type IIIB. In this fatal lysosomal storage disease, defective α-N-acetylglucosaminidase interrupts the degradation of heparan sulfate (HS) proteoglycans and induces cell disorders predominating in the central nervous system, causing relentless progression toward severe mental retardation. Partially digested proteoglycans, which affect fibroblast growth factor signaling, accumulated in patient cells. They impaired isolation of emerging iPSc unless exogenous supply of the missing enzyme cleared storage and restored cell proliferation. After several passages, patient iPSc starved of an exogenous enzyme continued to proliferate in the presence of fibroblast growth factor despite HS accumulation. Survival and neural differentiation of patient iPSc were comparable with unaffected controls. Whereas cell pathology was modest in floating neurosphere cultures, undifferentiated patient iPSc and their neuronal progeny expressed cell disorders consisting of storage vesicles and severe disorganization of Golgi ribbons associated with modified expression of the Golgi matrix protein GM130. Gene expression profiling in neural stem cells pointed to alterations of extracellular matrix constituents and cell-matrix interactions, whereas genes associated with lysosome or Golgi apparatus functions were downregulated. Taken together, these results suggest defective responses of patient undifferentiated stem cells and neurons to environmental cues, which possibly affect Golgi organization, cell migration and neuritogenesis. This could have potential consequences on post-natal neurological development, once HS proteoglycan accumulation becomes prominent in the affected child brain.

International audience; OBJECTIVE: Recessive mutations in alsin, a guanine-nucleotide exchange factor for the GTPases Rab5 and Rac1, cause juvenile amyotrophic lateral sclerosis (ALS2) and related motoneuron disorders. Alsin function in motoneurons remained unclear because alsin knock-out mice do not develop overt signs of motoneuron degeneration. METHODS: To generate an alsin loss-of-function model in an ALS-relevant cell type, we developed a new small interfering RNA electroporation technique that allows efficient knock down of alsin in embryonic rat spinal motoneurons. RESULTS: After small interfering RNA-mediated alsin knockdown, cultured motoneurons displayed a reduced apparent size of EEA1-labeled early endosomes and an increased intracellular accumulation of transferrin and L1CAM. Alsin knockdown induced cell death in 32 to 48% of motoneurons and significantly inhibited axon growth in the surviving neurons. Both cellular phenotypes were mimicked by expression of a dominant-negative Rac1 mutant and were completely blocked by expression of a constitutively active Rac1 mutant. Expression of dominant-negative or constitutively active forms of Rab5 had no such effects. INTERPRETATION: Our data demonstrate that alsin controls the growth and survival of motoneurons in a Rac1-dependant manner. The strategy reported here illustrates how small interfering RNA electroporation can be used to generate cellular models of neurodegenerative disease involving a loss-of-function mechanism.

International audience; In contrast to mouse embryonic stem cells and in spite of overlapping gene expression profiles, neural stem cells (NSCs) isolated from the embryonic spinal cord do not respond to physiological morphogenetic stimuli provided by Sonic hedgehog and retinoic acid and do not generate motor neurons upon differentiation. Transcription factors expressed in motor neuron progenitors during embryogenesis include Pax6, Ngn2, Nkx6.1 and Olig2, whose expression precedes that of factors specifying motor neuron fate, including HB9, Islet1 and LIM3. We showed that all these factors were present in neural progenitors derived from mouse ES cells, whereas NSCs derived from the rat embryonic spinal cord expressed neither HB9 nor Islet1 and contained low levels of Nkx6.1 and LIM3. We constructed a lentivirus vector to express HB9 and GFP in NSCs and examined the consequences of HB9 expression on other transcription factors and cell differentiation. Compared to cell expressing GFP alone, NSCs expressing GFP and HB9 cycled less rapidly, downregulated Pax6 and Ngn2 mRNA levels, produced higher proportions of neurons in vitro and lower numbers of neurons after transplantation in the spinal cord of recipient rats. Oligodendrocytic and astrocytic differentiations were not affected. HB9 expressing NSCs did not express Islet1 or upregulate LIM3. They neither responded to Sonic hedgehog and retinoic acid nor produced cholinergic neurons. We concluded that forced HB9 expression affected neurogenesis but was not sufficient to confer motor neuron fate to NSCs.

International audience; By using the yeast two-hybrid technique, we identified a candidate protein ligand of the myosin 1c tail, PHR1, and found that this protein can also bind to the myosin VIIa tail. PHR1 is an integral membrane protein that contains a pleckstrin homology (PH) domain. Myosin 1c and myosin VIIa are two unconventional myosins present in the inner ear sensory cells. We showed that PHR1 immunoprecipitates with either myosin tail by using protein extracts from cotransfected HEK293 cells. In vitro binding assays confirmed that PHR1 directly interacts with these two myosins. In both cases the binding involves the PH domain. In vitro interactions between PHR1 and the myosin tails were not affected by the presence or absence of Ca2+ and calmodulin. Finally, we found that PHR1 is able to dimerise. As PHR1 is expressed in the vestibular and cochlear sensory cells, its direct interactions with the myosin 1c and VIIa tails are likely to play a role in anchoring the actin cytoskeleton to the plasma membrane of these cells. Moreover, as both myosins have been implicated in the mechanotransduction slow adaptation process that takes place in the hair bundles, we propose that PHR1 is also involved in this process.

International audience; The whirler mouse mutant (wi) does not respond to sound stimuli, and detailed ultrastructural analysis of sensory hair cells in the organ of Corti of the inner ear indicates that the whirler gene encodes a protein involved in the elongation and maintenance of stereocilia in both inner hair cells (IHCs) and outer hair cells (OHCs). BAC-mediated transgene correction of the mouse phenotype and mutation analysis identified the causative gene as encoding a novel PDZ protein called whirlin. The gene encoding whirlin also underlies the human autosomal recessive deafness locus DFNB31. In the mouse cochlea, whirlin is expressed in the sensory IHC and OHC stereocilia. Our findings suggest that this novel PDZ domain–containing molecule acts as an organizer of submembranous molecular complexes that control the coordinated actin polymerization and membrane growth of stereocilia.

Kallmann syndrome represents the association of hypogonadotropic hypogonadism with anosmia. This syndrome is from a defect in the embryonic migratory pathway of gonadotropin-releasing hormone synthesizing neurons and olfactory axons. A candidate gene for the X chromosome-linked form of the syndrome was recently isolated by using a positional cloning strategy based on deletion mapping in the Xp22.3 region. With the PCR, two exons of this candidate gene were amplified on the genomic DNAs from 18 unrelated patients affected with the X chromosome-linked Kallmann syndrome. Three different base transitions--all leading to a stop codon--and one single-base deletion responsible for a frameshift were identified. We thus conclude that the candidate gene is the actual KAL gene responsible for the X chromosome-linked Kallmann syndrome. Furthermore, unilateral renal aplasia in two unrelated patients carrying a stop mutation indicates that the KAL gene is itself responsible for this Kallmann syndrome-associated anomaly. The gene is, therefore, also involved in kidney organogenesis. Additional neurologic symptoms in Kallmann patients are also discussed.