Impaired cerebellar development and deficits in motor coordination in mice lacking the neuronal protein BM88/Cend1

Effects of pyrethroids on the cerebellum and related mechanisms: a narrative review

Pyrethroids (PYRs) are a group of synthetic organic chemicals that mimic natural pyrethrins. Due to their low toxicity and persistence in mammals, they are widely used today. PYRs exhibit higher lipophilicity than other insecticides, which allows them to easily penetrate the blood-brain barrier and directly induce toxic effects on the central nervous system. Several studies have shown that the cerebellum appears to be one of the regions with the largest changes in biomarkers. The cerebellum, which is extremely responsive to PYRs, functions as a crucial region for storing motor learning memories. Exposure to low doses of various types of PYRs during rat development resulted in diverse long-term effects on motor activity and coordination functions. Reduced motor activity may result from developmental exposure to PYRs in rats, as indicated by delayed cerebellar morphogenesis and maturation. PYRs also caused adverse histopathological and biochemical changes in the cerebellum of mothers and their offspring. By some studies, PYRs may affect granule cells and Purkinje cells, causing damage to cerebellar structures. Destruction of cerebellar structures and morphological defects in Purkinje cells are known to be directly related to functional impairment of motor coordination. Although numerous data support that PYRs cause damage to cerebellar structures, function and development, the mechanisms are not completely understood and require further in-depth studies. This paper reviews the available evidence on the relationship between the use of PYRs and cerebellar damage and discusses the mechanisms of PYRs.

CEND1 deficiency induces mitochondrial dysfunction and cognitive impairment in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of neurodegenerative disease featured with memory loss and cognitive function impairments. Chronic mitochondrial stress is a vital pathogenic factor for AD and finally leads to massive neuronal death. However, the underlying mechanism is unclear. By proteomic analysis, we identified a new mitochondrial protein, cell-cycle exit and neuronal differentiation 1 (CEND1), which was decreased significantly in the brain of 5xFAD mice. CEND1 is a neuronal specific protein and locates in the presynaptic mitochondria. Depletion of CEND1 leads to increased mitochondrial fission mediated by upregulation of dynamin related protein 1 (Drp1), resulting in abnormal mitochondrial functions. CEND1 deficiency leads to cognitive impairments in mice. Overexpression of CEND1 in the hippocampus of 5xFAD mice rescued cognitive deficits. Moreover, we identified that CDK5/p25 interacted with and phosphorylated CEND1 which promoted its degradation. Our study provides new mechanistic insights in mitochondrial function regulations by CEND1 in Alzheimer's disease.

Cell cycle exit and neuronal differentiation 1-engineered embryonic neural stem cells enhance neuronal differentiation and neurobehavioral recovery after experimental traumatic brain injury

Our previous study showed that cell cycle exit and neuronal differentiation 1 (CEND1) may participate in neural stem cell cycle exit and oriented differentiation. However, whether CEND1-transfected neural stem cells can improve the prognosis of traumatic brain injury remained unclear. In this study, we performed quantitative proteomic analysis and found that after traumatic brain injury, CEND1 expression was downregulated in mouse brain tissue. Three days after traumatic brain injury, we transplanted CEND1-transfected neural stem cells into the area surrounding the injury site. We found that at 5 weeks after traumatic brain injury, transplantation of CEND1-transfected neural stem cells markedly alleviated brain atrophy and greatly improved neurological function. In vivo and in vitro results indicate that CEND1 overexpression inhibited the proliferation of neural stem cells, but significantly promoted their neuronal differentiation. Additionally, CEND1 overexpression reduced protein levels of Notch1 and cyclin D1, but increased levels of p21 in CEND1-transfected neural stem cells. Treatment with CEND1-transfected neural stem cells was superior to similar treatment without CEND1 transfection. These findings suggest that transplantation of CEND1-transfected neural stem cells is a promising cell therapy for traumatic brain injury. This study was approved by the Animal Ethics Committee of the School of Biomedical Engineering of Shanghai Jiao Tong University, China (approval No. 2016034) on November 25, 2016.

Ccdc134 deficiency impairs cerebellar development and motor coordination

Coiled-coil domain containing 134 (CCDC134) has been shown to serve as an immune cytokine to exert antitumor effects and to act as a novel regulator of hADA2a to affect PCAF acetyltransferase activity. While Ccdc134 loss causes abnormal brain development in mice, the significance of CCDC134 in neuronal development in vivo is controversial. Here, we report that CCDC134 is highly expressed in Purkinje cells (PCs) at all developmental stages and regulates mammalian cerebellar development in a cell type-specific manner. Selective deletion of Ccdc134 in mouse neural stem cells (NSCs) caused defects in cerebellar morphogenesis, including a decrease in the number of PCs and impairment of PC dendritic growth, as well as abnormal granule cell development. Moreover, loss of Ccdc134 caused progressive motor dysfunction with deficits in motor coordination and motor learning. Finally, Ccdc134 deficiency inhibited Wnt signaling but increased Ataxin1 levels. Our findings provide evidence that CCDC134 plays an important role in cerebellar development, possibly through regulating Wnt signaling and Ataxin1 expression levels, and in controlling cerebellar function for motor coordination and motor learning, ultimately making it a potential contributor to cerebellar pathogenesis.

Arhgef2 regulates neural differentiation in the cerebral cortex through mRNA m6A-methylation of Npdc1 and Cend1

N⁶-methyladenosine (m⁶A) is emerging as a vital factor regulating neural differentiation. Here, we report that deficiency of Arhgef2, a novel cause of a neurodevelopmental disorder we identified recently, impairs neurogenesis, neurite outgrowth, and synaptic formation by regulating m⁶A methylation. Arhgef2 knockout decreases expression of Mettl14 and total m⁶A level significantly in the cerebral cortex. m⁶A sequencing reveals that loss of Arhgef2 reduces m⁶A methylation of 1,622 mRNAs, including Npdc1 and Cend1, which are both strongly associated with cell cycle exit and terminal neural differentiation. Arhgef2 deficiency decreases m⁶A methylations of the Npdc1 and Cend1 mRNAs via down-regulation of Mettl14, and thereby inhibits the translation of Npdc1 and nuclear export of Cend1 mRNAs. Overexpression of Mettl14, Npdc1, and Cend1 rescue the abnormal phenotypes in Arhgef2 knockout mice, respectively. Our study provides a critical insight into a mechanism by which defective Arhgef2 mediates m⁶A-tagged target mRNAs to impair neural differentiation.

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Arhgef2 mediates total m⁶A level via Mettl14

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Arhgef2 affects m⁶A methylations of the Npdc1 and Cend1 mRNAs

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Decreased m⁶A methylations inhibits translation of Npdc1 and nuclear export of Cend1

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Reduced protein expression of Npdc1 and Cend1 hinders neural differentiation

MEA6 Deficiency Impairs Cerebellar Development and Motor Performance by Tethering Protein Trafficking

Meningioma expressed antigen 6 (MEA6), also called cutaneous T cell lymphoma-associated antigen 5 (cTAGE5), was initially found in tumor tissues. MEA6 is located in endoplasmic reticulum (ER) exit sites and regulates the transport of collagen, very low density lipoprotein, and insulin. It is also reported that MEA6 might be related to Fahr's syndrome, which comprises neurological, movement, and neuropsychiatric disorders. Here, we show that MEA6 is critical to cerebellar development and motor performance. Mice with conditional knockout of MEA6 (Nestin-Cre;MEA6F/F) display smaller sizes of body and brain compared to control animals, and survive maximal 28 days after birth. Immunohistochemical and behavioral studies demonstrate that these mutant mice have defects in cerebellar development and motor performance. In contrast, PC deletion of MEA6 (pCP2-Cre;MEA6F/F) causes milder phenotypes in cerebellar morphology and motor behaviors. While pCP2-Cre;MEA6F/F mice have normal lobular formation and gait, they present the extensive self-crossing of PC dendrites and damaged motor learning. Interestingly, the expression of key molecules that participates in cerebellar development, including Slit2 and brain derived neurotrophic factor (BDNF), is significantly increased in ER, suggesting that MEA6 ablation impairs ER function and thus these proteins are arrested in ER. Our study provides insight into the roles of MEA6 in the brain and the pathogenesis of Fahr's syndrome.

Cend1, a Story with Many Tales: From Regulation of Cell Cycle Progression/Exit of Neural Stem Cells to Brain Structure and Function

Neural stem/precursor cells (NPCs) generate the large variety of neuronal phenotypes comprising the adult brain. The high diversity and complexity of this organ have its origin in embryonic life, during which NPCs undergo symmetric and asymmetric divisions and then exit the cell cycle and differentiate to acquire neuronal identities. During these processes, coordinated regulation of cell cycle progression/exit and differentiation is essential for generation of the appropriate number of neurons and formation of the correct structural and functional neuronal circuits in the adult brain. Cend1 is a neuronal lineage-specific modulator involved in synchronization of cell cycle exit and differentiation of neuronal precursors. It is expressed all along the neuronal lineage, from neural stem/progenitor cells to mature neurons, and is associated with the dynamics of neuron-generating divisions. Functional studies showed that Cend1 has a critical role during neurogenesis in promoting cell cycle exit and neuronal differentiation. Mechanistically, Cend1 acts via the p53-dependent/Cyclin D1/pRb signaling pathway as well as via a p53-independent route involving a tripartite interaction with RanBPM and Dyrk1B. Upon Cend1 function, Notch1 signaling is suppressed and proneural genes such as Mash1 and Neurogenins 1/2 are induced. Due to its neurogenic activity, Cend1 is a promising candidate therapeutic gene for brain repair, while the Cend1 minimal promoter is a valuable tool for neuron-specific gene delivery in the CNS. Mice with Cend1 genetic ablation display increased NPC proliferation, decreased migration, and higher levels of apoptosis during development. As a result, they show in the adult brain deficits in a range of motor and nonmotor behaviors arising from irregularities in cerebellar cortex lamination and impaired Purkinje cell differentiation as well as a paucity in GABAergic interneurons of the cerebral cortex, hippocampus, and amygdala. Taken together, these studies highlight the necessity for Cend1 expression in the formation of a structurally and functionally normal brain.

Increased Anxiety-Related Behavior, Impaired Cognitive Function and Cellular Alterations in the Brain of Cend1-deficient Mice

Cend1 is a neuronal-lineage specific modulator involved in coordination of cell cycle exit and differentiation of neuronal precursors. We have previously shown that Cend1^-/- mice show altered cerebellar layering caused by increased proliferation of granule cell precursors, delayed radial granule cell migration and compromised Purkinje cell differentiation, leading to ataxic gait and deficits in motor coordination. To further characterize the effects of Cend1 genetic ablation we determined herein a range of behaviors, including anxiety and exploratory behavior in the elevated plus maze (EPM), associative learning in fear conditioning, and spatial learning and memory in the Morris water maze (MWM). We observed significant deficits in all tests, suggesting structural and/or functional alterations in brain regions such as the cortex, amygdala and the hippocampus. In agreement with these findings, immunohistochemistry revealed reduced numbers of γ amino butyric acid (GABA) GABAergic interneurons, but not of glutamatergic projection neurons, in the adult cerebral cortex. Reduced GABAergic interneurons were also observed in the amygdala, most notably in the basolateral nucleus. The paucity in GABAergic interneurons in adult Cend1^-/- mice correlated with increased proliferation and apoptosis as well as reduced migration of neuronal progenitors from the embryonic medial ganglionic eminence (MGE), the origin of these cells. Further we noted reduced GABAergic neurons and aberrant neurogenesis in the adult dentate gyrus (DG) of the hippocampus, which has been previously shown to confer spatial learning and memory deficits. Our data highlight the necessity of Cend1 expression in the formation of a structurally and functionally normal brain phenotype.

Exome sequencing and network analysis identifies shared mechanisms underlying spinocerebellar ataxia

The autosomal dominant cerebellar ataxias, referred to as spinocerebellar ataxias in genetic nomenclature, are a rare group of progressive neurodegenerative disorders characterized by loss of balance and coordination. Despite the identification of numerous disease genes, a substantial number of cases still remain without a genetic diagnosis. Here, we report five novel spinocerebellar ataxia genes, FAT2, PLD3, KIF26B, EP300, and FAT1, identified through a combination of exome sequencing in genetically undiagnosed families and targeted resequencing of exome candidates in a cohort of singletons. We validated almost all genes genetically, assessed damaging effects of the gene variants in cell models and further consolidated a role for several of these genes in the aetiology of spinocerebellar ataxia through network analysis. Our work links spinocerebellar ataxia to alterations in synaptic transmission and transcription regulation, and identifies these as the main shared mechanisms underlying the genetically diverse spinocerebellar ataxia types.

Novel insights into SLC25A46-related pathologies in a genetic mouse model

The mitochondrial protein SLC25A46 has been recently identified as a novel pathogenic cause in a wide spectrum of neurological diseases, including inherited optic atrophy, Charcot-Marie-Tooth type 2, Leigh syndrome, progressive myoclonic ataxia and lethal congenital pontocerebellar hypoplasia. SLC25A46 is an outer membrane protein, member of the Solute Carrier 25 (SLC25) family of nuclear genes encoding mitochondrial carriers, with a role in mitochondrial dynamics and cristae maintenance. Here we identified a loss-of-function mutation in the Slc25a46 gene that causes lethal neuropathology in mice. Mutant mice manifest the main clinical features identified in patients, including ataxia, optic atrophy and cerebellar hypoplasia, which were completely rescued by expression of the human ortholog. Histopathological analysis revealed previously unseen lesions, most notably disrupted cytoarchitecture in the cerebellum and retina and prominent abnormalities in the neuromuscular junction. A distinct lymphoid phenotype was also evident. Our mutant mice provide a valid model for understanding the mechanistic basis of the complex SLC25A46-mediated pathologies, as well as for screening potential therapeutic interventions.

Neurodegenerative diseases encompass a wide range of clinical conditions remaining incurable while the genetic and molecular basis of most of these conditions remains unknown due to their heterogeneity and the lack of animal models. Mutations in nuclear genes encoding mitochondrial proteins have recently emerged as novel causalities in diseases affecting the nervous system. SLC25A46, a novel mitochondrial protein, has recently been identified as a pathogenic target in a wide spectrum of rare genetic neurological diseases, including optic atrophy, Charcot-Marie-Tooth type 2, Leigh syndrome, progressive myoclonic ataxia and lethal congenital pontocerebellar hypoplasia. Following a genetic approach, we identified a novel neurological mouse model caused by a functional mutation in the Slc25a46 gene. Our SLC25A46 mutant mice constitute the first genetic animal model for this disease and represent an ideal tool for elucidating the underlying molecular mechanisms. The aim of this study was to characterize the complex phenotype displayed by the mutant mice in order to understand the mechanistic basis of the SLC25A46-mediated pathologies.

Using the shared genetics of dystonia and ataxia to unravel their pathogenesis

In this review we explore the similarities between spinocerebellar ataxias and dystonias, and suggest potentially shared molecular pathways using a gene co-expression network approach. The spinocerebellar ataxias are a group of neurodegenerative disorders characterized by coordination problems caused mainly by atrophy of the cerebellum. The dystonias are another group of neurological movement disorders linked to basal ganglia dysfunction, although evidence is now pointing to cerebellar involvement as well. Our gene co-expression network approach identified 99 shared genes and showed the involvement of two major pathways: synaptic transmission and neurodevelopment. These pathways overlapped in the two disorders, with a large role for GABAergic signaling in both. The overlapping pathways may provide novel targets for disease therapies. We need to prioritize variants obtained by whole exome sequencing in the genes associated with these pathways in the search for new pathogenic variants, which can than be used to help in the genetic counseling of patients and their families.

Dendrite self-avoidance requires cell-autonomous slit/robo signaling in cerebellar purkinje cells

Dendrites from the same neuron usually develop nonoverlapping patterns by self-avoidance, a process requiring contact-dependent recognition and repulsion. Recent studies have implicated homophilic interactions of cell surface molecules, including Dscams and Pcdhgs, in self-recognition, but repulsive molecular mechanisms remain obscure. Here, we report a role for the secreted molecule Slit2 and its receptor Robo2 in self-avoidance of cerebellar Purkinje cells (PCs). Both molecules are highly expressed by PCs, and their deletion leads to excessive dendrite self-crossing without affecting arbor size and shape. This cell-autonomous function is supported by the boundary-establishing activity of Slit in culture and the phenotype rescue by membrane-associated Slit2 activities. Furthermore, genetic studies show that they act independently from Pcdhg-mediated recognition. Finally, PC-specific deletion of Robo2 is associated with motor behavior alterations. Thus, our study uncovers a local repulsive mechanism required for self-avoidance and demonstrates the molecular complexity at the cell surface in dendritic patterning.

Functional Interactions between BM88/Cend1, Ran-binding protein M and Dyrk1B kinase affect cyclin D1 levels and cell cycle progression/exit in mouse neuroblastoma cells

BM88/Cend1 is a neuronal-lineage specific modulator with a pivotal role in coordination of cell cycle exit and differentiation of neuronal precursors. In the current study we identified the signal transduction scaffolding protein Ran-binding protein M (RanBPM) as a BM88/Cend1 binding partner and showed that BM88/Cend1, RanBPM and the dual specificity tyrosine-phosphorylation regulated kinase 1B (Dyrk1B) are expressed in mouse brain as well as in cultured embryonic cortical neurons while RanBPM can form complexes with either of the two other proteins. To elucidate a potential mechanism involving BM88/Cend1, RanBPM and Dyrk1B in cell cycle progression/exit, we transiently co-expressed these proteins in mouse neuroblastoma Neuro 2a cells. We found that the BM88/Cend1-dependent or Dyrk1B-dependent down-regulation of cyclin D1 is reversed following their functional interaction with RanBPM. More specifically, functional interaction of RanBPM with either BM88/Cend1 or Dyrk1B stabilizes cyclin D1 in the nucleus and promotes 5-bromo-2'-deoxyuridine (BrdU) incorporation as a measure of enhanced cell proliferation. However, the RanBPM-dependent Dyrk1B cytosolic retention and degradation is reverted in the presence of Cend1 resulting in cyclin D1 destabilization. Co-expression of RanBPM with either BM88/Cend1 or Dyrk1B also had a negative effect on Neuro 2a cell differentiation. Our results suggest that functional interactions between BM88/Cend1, RanBPM and Dyrk1B affect the balance between cellular proliferation and differentiation in Neuro 2a cells and indicate that a potentially similar mechanism may influence cell cycle progression/exit and differentiation of neuronal precursors.

Loss of Ahi1 affects early development by impairing BM88/Cend1-mediated neuronal differentiation

Mutations in the Abelson helper integration site-1 (AHI1) gene result in N-terminal Ahi1 fragments and cause Joubert syndrome, an autosomal recessive brain malformation disorder associated with delayed development. How AHI1 mutations lead to delayed development remains unclear. Here we report that full-length, but not N-terminal, Ahi1 binds Hap1, a huntingtin-associated protein that is essential for the postnatal survival of mice and that this binding is regulated during neuronal differentiation by nerve growth factor. Nerve growth factor induces dephosphorylation of Hap1A and decreases its association with Ahi1, correlating with increased Hap1A distribution in neurite tips. Consistently, Ahi1 associates with phosphorylated Hap1A in cytosolic, but not in synaptosomal, fractions isolated from mouse brain, suggesting that Ahi1 functions mainly in the soma of neurons. Mass spectrometry analysis of cytosolic Ahi1 immunoprecipitates reveals that Ahi1 also binds Cend1 (cell cycle exit and neuronal differentiation protein 1)/BM88, a neuronal protein that mediates neuronal differentiation and is highly expressed in postnatal mouse brain. Loss of Ahi1 reduces the levels of Cend1 in the hypothalamus of Ahi1 KO mice, which show retarded growth during postnatal days. Overexpressed Ahi1 can stabilize Cend1 in cultured cells. Furthermore, overexpression of Cend1 can rescue the neurite extension defects of hypothalamic neurons from Ahi1 KO mice. Our findings suggest that Cend1 is involved in Ahi1-associated hypothalamic neuronal differentiation in early development, giving us fresh insight into the mechanism behind the delayed development in Joubert syndrome.

Impaired structural and functional development of cerebellum following gestational exposure of deltamethrin in rats: role of reelin

Reelin is an extracellular matrix molecule that is involved in the normal development of the cerebellar lamination, Bergmann glial fibres alignment, Purkinje cell monolayer arrangement and granule cell migration. In this study, we have examined the effects of maternal exposure of deltamethrin (DLT), a type II pyrethroid insecticide, on the structural and functional development of rat cerebellum during postnatal life. DLT (0.75 mg/kg body weight, intraperitoneally dissolved in dimethylsulphoxide) was administered in timed pregnant rats during two different gestational time periods, i.e. gestational days of 7-10 and 11-14, respectively. In DLT exposed rats, a significant overexpression of reelin was observed in the cells of the external granule cell layer (EGL) and internal granule cell layer along with an ectopic expression of reelin in the EGL as well as in the migrating granule cells just below the EGL, revealing an arrest of granule cell migration in this zone. Mis-orientation and hypertrophy of the Bergmann glial fibres further hampered the journey of the granule cells to their final destination. Possibly reelin overexpression also caused misalignment of the Purkinje cells and inhibited the neurite growth leading to a significant decrease in the spine density, main dendritic length and width of the dendritic arbour. Thus, it is proposed that the DLT exerts its neurotoxic effects possibly via the intracellular accumulation and low release of reelin leading to an impaired granule cell and Purkinje cell migration, inhibition of neurite outgrowth and reduced spine density. Such impaired cerebellar development leads to motor coordination deficits.

Purkinje cell loss and motor coordination defects in profilin1 mutant mice

Profilin1 is an actin monomer-binding protein, essential for cytoskeletal dynamics. Based on its broad expression in the brain and the localization at excitatory synapses (hippocampal CA3-CA1 synapse, cerebellar parallel fiber (PF)-Purkinje cell (PC) synapse), an important role for profilin1 in brain development and synapse physiology has been postulated. We recently showed normal physiology of hippocampal CA3-CA1 synapses in the absence of profilin1, but impaired glial cell binding and radial migration of cerebellar granule neurons (CGNs). Consequently, brain-specific inactivation of profilin1 by exploiting conditional mutants and Nestin-mediated cre expression resulted in a cerebellar hypoplasia, aberrant organization of cerebellar cortex layers, and ectopic CGNs. Apart from these findings we noted a loss of PCs and an irregularly shaped PC layer in adult mutants. In this study, we show that PC migration and development are not affected in profilin1 mutants, suggesting cell type-specific functions for profilin1 in PCs and CGNs. PC loss begins during the second postnatal week and progresses until adulthood with no further impairment in aged mutants. In Nestin-cre profilin1 mutants, defects in cerebellar cortex cytoarchitecture are associated with impaired motor coordination. However, in L7-cre mutants, lacking profilin1 specifically in PCs, the cerebellar cortex cytoarchitecture is unchanged. Thereby, our results demonstrate that the loss of PCs is not caused by cell-autonomous defects, but presumably by impaired CGN migration. Finally, we show normal functionality of PF-PC synapses in the absence of profilin1. In summary, we conclude that profilin1 is crucially important for brain development, but dispensable for the physiology of excitatory synapses.

Nuclear factor one X regulates the development of multiple cellular populations in the postnatal cerebellum

Development of the cerebellum involves the coordinated proliferation, differentiation, maturation, and integration of cells from multiple neuronal and glial lineages. In rodent models, much of this occurs in the early postnatal period. However, our understanding of the molecular mechanisms that regulate this phase of cerebellar development remains incomplete. Here, we address the role of the transcription factor nuclear factor one X (NFIX), in postnatal development of the cerebellum. NFIX is expressed by progenitor cells within the external granular layer and by cerebellar granule neurons within the internal granule layer. Using NFIX⁻/⁻ mice, we demonstrate that the development of cerebellar granule neurons and Purkinje cells within the postnatal cerebellum is delayed in the absence of this transcription factor. Furthermore, the differentiation of mature glia within the cerebellum, such as Bergmann glia, is also significantly delayed in the absence of NFIX. Collectively, the expression pattern of NFIX, coupled with the delays in the differentiation of multiple cell populations of the developing cerebellum in NFIX⁻/⁻ mice, suggest a central role for NFIX in the regulation of cerebellar development, highlighting the importance of this gene for the maturation of this key structure.

The all-trans retinoic acid (atRA)-regulated gene Calmin (Clmn) regulates cell cycle exit and neurite outgrowth in murine neuroblastoma (Neuro2a) cells

The vitamin A metabolite all-trans retinoic acid (atRA) functions in nervous system development and regulates cell proliferation and differentiation. Neuroblastoma cells (SH-SY5Y and Neuro2a or N2A) exposed to atRA undergo growth inhibition and neuronal differentiation, both of which are preceded by an increase in Clmn mRNA. Treatment of N2A cells with atRA produces a reduction in phosphohistone 3 immunostaining and BrdU incorporation, both indicators of a reduction in cell proliferation. These effects are nearly eliminated in atRA-treated shClmn knockdown cells. Loss of Clmn in the mouse N2A cell line also results in a significant reduction of atRA-mediated neurite outgrowth, a response that can be rescued by reintroduction of the Clmn sequence. In contrast, ectopic overexpression of Clmn produces an increase in the cyclin dependent kinase inhibitor, p21(Cip1), a decrease in cyclin D1 protein and an increase in hypophosphorylated Rb, showing that Clmn participates in G(1)/S arrest. Clmn overexpression alone is sufficient to inhibit N2A cell proliferation, whereas both Clmn and atRA must be present to induce neurite outgrowth. This study shows that the atRA-responsive gene Clmn promotes exit from the cell cycle, a requisite event for neuronal differentiation.