The hereditary ataxias are a complex group of neurological disorders characterized by the degeneration of the cerebellum and its associated connections. The molecular mechanisms that trigger the loss of Purkinje cells in this group of diseases remain incompletely understood. Here, we report a previously undescribed dominant mouse model of cerebellar ataxia, moonwalker (Mwk), that displays motor and coordination defects and loss of cerebellar Purkinje cells. Mwk mice harbor a gain-of-function mutation (T635A) in the Trpc3 gene encoding the nonselective transient receptor potential cation channel, type C3 (TRPC3), resulting in altered TRPC3 channel gating. TRPC3 is highly expressed in Purkinje cells during the phase of dendritogenesis. Interestingly, growth and differentiation of Purkinje cell dendritic arbors are profoundly impaired in Mwk mice. Our findings define a previously unknown role for TRPC3 in both dendritic development and survival of Purkinje cells, and provide a unique mechanism underlying cerebellar ataxia.cerebellum ͉ dendritogenesis ͉ trp channel ͉ mouse mutant T he inherited cerebellar ataxias are a complex group of neurodegenerative disorders characterized by loss of balance and coordination (1-3). Cerebellar ataxia is caused by the degeneration of Purkinje cells, which form the sole output of the cerebellum. To date, more than 50 different inherited forms of cerebellar ataxia are known (4). Importantly, increasing evidence points to the existence of common pathological pathways in different forms of ataxia, including transcriptional regulation, protein aggregation, and calcium homeostasis, which trigger the degeneration of Purkinje cells in these disorders (1, 5). However, the molecular mechanisms mediating these pathways remain poorly understood.To identify gene products that might be key to cerebellar degeneration, we used a phenotype-driven approach to screen for ataxic behavior in a large cohort of N-ethyl-N-nitrosourea (ENU)-mutagenized mice (6). Here, we report that a point mutation (T635A) in the C3-type transient receptor potential (TRPC3) channel in the mouse results in Purkinje cell degeneration and cerebellar ataxia. We also find that the development of dendrites is severely impaired in mutant Purkinje cells. Notably, the identified dominant gain-of-function mutation in TRPC3 provides insight into the function of TRPC3 that powerfully complements the findings obtained from the recently published TRPC3 knockout mouse (7). Our findings suggest that TRPC3 is a regulator of development and survival of Purkinje cells, and link aberrant TRPC3 function to cerebellar disease.

The recent identification of multiple dominant mutations in the gene encoding β-catenin in both humans and mice has enabled exploration of the molecular and cellular basis of β-catenin function in cognitive impairment. In humans, β-catenin mutations that cause a spectrum of neurodevelopmental disorders have been identified. We identified de novo β-catenin mutations in patients with intellectual disability, carefully characterized their phenotypes, and were able to define a recognizable intellectual disability syndrome. In parallel, characterization of a chemically mutagenized mouse line that displays features similar to those of human patients with β-catenin mutations enabled us to investigate the consequences of β-catenin dysfunction through development and into adulthood. The mouse mutant, designated batface (Bfc), carries a Thr653Lys substitution in the C-terminal armadillo repeat of β-catenin and displayed a reduced affinity for membrane-associated cadherins. In association with this decreased cadherin interaction, we found that the mutation results in decreased intrahemispheric connections, with deficits in dendritic branching, long-term potentiation, and cognitive function. Our study provides in vivo evidence that dominant mutations in β-catenin underlie losses in its adhesion-related functions, which leads to severe consequences, including intellectual disability, childhood hypotonia, progressive spasticity of lower limbs, and abnormal craniofacial features in adults. Introduction β-Catenin (CTNNB1) is a highly conserved protein that implements key cellular functions by interacting with cell-adhesion proteins, signaling molecules, and transcription factors (1). The characteristic structural feature of the β-catenin protein, its 12 central armadillo repeats, forms a long positively charged groove facilitating interaction with multiple protein partners. This central motif is flanked by the N terminus, crucial in mediating degradation of the protein, and the C terminus, containing the Helix-C motif (2), which enables switching between the protein's dual roles in cell adhesion and proliferation. Loss-of-function studies in mammals have implicated β-catenin in embryonic development, while gain-of-function studies have demonstrated its contribution to various forms of human cancers (reviewed in ref.3). Functional investigation has focused on the role of β-catenin in canonical WNT signaling. β-Catenin interacts with transcriptional coactivators to mediate WNT's transcriptional activation, which orchestrates growth and patterning in the developing embryo, and, when constitutively upregulated, dysregulates growth connected to cancer and metastasis.

SUMMARYIn mouse embryos, loss of Dickkopf-1 (DKK1) activity is associated with an ectopic activation of WNT signalling responses in the precursors of the craniofacial structures and leads to a complete truncation of the head at early organogenesis. Here, we show that ENU-induced mutations of genes coding for two WNT canonical pathway factors, the co-receptor LRP6 and the transcriptional co-activator -catenin, also elicit an ectopic signalling response and result in loss of the rostral tissues of the forebrain. Compound mutant embryos harbouring combinations of mutant alleles of Lrp6, Ctnnb1 and Dkk1 recapitulate the partial to complete head truncation phenotype of individual homozygous mutants. The demonstration of a synergistic interaction of Dkk1, Lrp6 and Ctnnb1 provides compelling evidence supporting the concepts that (1) stringent regulation of the level of canonical WNT signalling is necessary for head formation, (2) activity of the canonical pathway is sufficient to account for the phenotypic effects of mutations in three different components of the signal cascade and (3) rostral parts of the brain and the head are differentially more sensitive to canonical WNT signalling and their development is contingent on negative modulation of WNT signalling activity.

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