Clathrin-mediated endocytosis is the endocytic portal into cells through which cargo is packaged into vesicles with the aid of a clathrin coat. It is fundamental to neurotransmission, signal transduction and the regulation of many plasma membrane activities and is thus essential to higher eukaryotic life. Morphological stages of vesicle formation are mirrored by progression through various protein modules (complexes). The process involves the formation of a putative FCH domain only (FCHO) initiation complex, which matures through adaptor protein 2 (AP2)-dependent cargo selection, and subsequent coat building, dynamin-mediated scission and finally auxilin- and heat shock cognate 70 (HSC70)-dependent uncoating. Some modules can be used in other pathways, and additions or substitutions confer cell specificity and adaptability.

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Molecular mechanism and physiological functions of clathrin-mediated endocytosis

A polymorphic CAG repeat was identified in the human α1A voltage-dependent calcium channel subunit. To test the hypothesis that expansion of this CAG repeat could be the cause of an inherited progressive ataxia, we genotyped a large number of unrelated controls and ataxia patients. Eight unrelated patients with late onset ataxia had alleles with larger repeat numbers (21‐27) compared to the number of repeats (4‐16) in 475 non‐ataxia individuals. Analysis of the repeat length in families of the affected individuals revealed that the expansion segregated with the phenotype in every patient. We identified six isoforms of the human α1A calcium channel subunit. The CAG repeat is within the open reading frame and is predicted to encode glutamine in three of the isoforms. We conclude that a small polyglutamine expansion in the human α1A calcium channel is most likely the cause of a newly classified autosomal dominant spinocerebellar ataxia, SCA6.

Autosomal dominant cerebellar ataxia (SCA6) associated with small polyglutamine expansions in the α(1A)-voltage-dependent calcium channel

A growing number of neurodegenerative diseases have been found to result from the expansion of an unstable trinucleotide repeat. Over the past 6 years, researchers have focused on identifying the mechanism by which the expanded polyglutamine tract renders a protein toxic to a subset of vulnerable neurons. In this review, we summarize the clinicopathologic features of these disorders (spinobulbar muscular atrophy, Huntington disease, and the spinocerebellar ataxias, including dentatorubropallidoluysian atrophy), describe the genes involved and what is known about their products, and discuss the model systems that have lent insight into pathogenesis. The review concludes with a model for pathogenesis that illuminates the unifying features of these polyglutamine disorders. This model may prove relevant to other neurodegenerative disorders as well.

Glutamine Repeats and Neurodegeneration

The causes of amyotrophic lateral sclerosis (ALS), a devastating human neurodegenerative disease, are poorly understood, although the protein TDP-43 has been suggested to have a critical role in disease pathogenesis. Here we show that ataxin 2 (ATXN2), a polyglutamine (polyQ) protein mutated in spinocerebellar ataxia type 2, is a potent modifier of TDP-43 toxicity in animal and cellular models. ATXN2 and TDP-43 associate in a complex that depends on RNA. In spinal cord neurons of ALS patients, ATXN2 is abnormally localized; likewise, TDP-43 shows mislocalization in spinocerebellar ataxia type 2. To assess the involvement of ATXN2 in ALS, we analysed the length of the polyQ repeat in the ATXN2 gene in 915 ALS patients. We found that intermediate-length polyQ expansions (27-33 glutamines) in ATXN2 were significantly associated with ALS. These data establish ATXN2 as a relatively common ALS susceptibility gene. Furthermore, these findings indicate that the TDP-43-ATXN2 interaction may be a promising target for therapeutic intervention in ALS and other TDP-43 proteinopathies.

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Ataxin-2 intermediate-length polyglutamine expansions are associated with increased risk for ALS

Genetic factors play a major role in determining a person's risk to develop Alzheimer's disease (AD). Rare mutations transmitted in a Mendelian fashion within affected families, for example, APP, PSEN1, and PSEN2, cause AD. In the absence of mutations in these genes, disease risk is largely determined by common polymorphisms that, in concert with each other and nongenetic risk factors, modestly impact risk for AD (e.g., the e4-allele in APOE). Recent genome-wide screening approaches have revealed several additional AD susceptibility loci and more are likely to be discovered over the coming years. In this chapter, we review the current state of AD genetics research with a particular focus on loci that now can be considered established disease genes. In addition to reviewing the potential pathogenic relevance of these genes, we provide an outlook into the future of AD genetics research based on recent advances in high-throughput sequencing technologies.

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The genetics of Alzheimer's disease.

Two forms of the neurodegenerative disorder spinocerebellar ataxia are known to be caused by the expansion of a CAG (polyglutamine) trinucleotide repeat. By screening cDNA expression libraries, using an antibody specific for polyglutamine repeats, we identified six novel genes containing CAG stretches. One of them is mutated in patients with spinocerebellar ataxia linked to chromosome 12q (SCA2). This gene shows ubiquitous expression and encodes a protein of unknown function. Normal SCA2 alleles (17 to 29 CAG repeats) contain one to three CAAs in the repeat. Mutated alleles (37 to 50 repeats) appear particularly unstable, upon both paternal and maternal transmissions. The sequence of three of them revealed pure CAG stretches. The steep inverse correlation between age of onset and CAG number suggests a higher sensitivity to polyglutamine length than in the other polyglutamine expansion diseases.

Cloning of the gene for spinocerebellar ataxia 2 reveals a locus with high sensitivity to expanded CAG/glutamine repeats

Centronuclear myopathies are characterized by muscle weakness and abnormal centralization of nuclei in muscle fibers not secondary to regeneration. The severe neonatal X-linked form (myotubular myopathy) is due to mutations in the phosphoinositide phosphatase myotubularin (MTM1), whereas mutations in dynamin 2 (DNM2) have been found in some autosomal dominant cases. By direct sequencing of functional candidate genes, we identified homozygous mutations in amphiphysin 2 (BIN1) in three families with autosomal recessive inheritance. Two missense mutations affecting the BAR (Bin1/amphiphysin/RVS167) domain disrupt its membrane tubulation properties in transfected cells, and a partial truncation of the C-terminal SH3 domain abrogates the interaction with DNM2 and its recruitment to the membrane tubules. Our results suggest that mutations in BIN1 cause centronuclear myopathy by interfering with remodeling of T tubules and/or endocytic membranes, and that the functional interaction between BIN1 and DNM2 is necessary for normal muscle function and positioning of nuclei.

Jean-Marie Garnier

Papers

Cloning of the gene for spinocerebellar ataxia 2 reveals a locus with high sensitivity to expanded CAG/glutamine repeats

Mutations in amphiphysin 2 ( BIN1 ) disrupt interaction with dynamin 2 and cause autosomal recessive centronuclear myopathy