Douglas E. Albrecht

Syntrophins and Dystrobrevins: Defining the Dystrophin Scaffold at Synapses

Dystrophin and its associated proteins were originally identified in skeletal muscle, where the complex provides mechanical stabilization to the sarcolemma during contraction. However, the dystrophin complex is also present at membrane specializations in many non-muscle cells, including synaptic sites in neurons. The function of the dystrophin complex at these sites is still unknown, but emerging results suggest that the dystrophin complex can function as a scaffold for signaling proteins. In this review, we examine the growing body of evidence that suggests the dystrophin complex may have a dual function: membrane stabilization and transmembrane signaling. We focus on the role of two dystrophin-associated proteins, syntrophin and dystrobrevin, in the formation of a signaling scaffold and review evidence suggesting a role in synapse formation and maintenance.

Mutations in Contactin-1, a Neural Adhesion and Neuromuscular Junction Protein, Cause a Familial Form of Lethal Congenital Myopathy

We have previously reported a group of patients with congenital onset weakness associated with a deficiency of members of the syntrophin-alpha-dystrobrevin subcomplex and have demonstrated that loss of syntrophin and dystrobrevin from the sarcolemma of skeletal muscle can also be associated with denervation. Here, we have further studied four individuals from a consanguineous Egyptian family with a lethal congenital myopathy inherited in an autosomal-recessive fashion and characterized by a secondary loss of beta2-syntrophin and alpha-dystrobrevin from the muscle sarcolemma, central nervous system involvement, and fetal akinesia. We performed homozygosity mapping and candidate gene analysis and identified a mutation that segregates with disease within CNTN1, the gene encoding for the neural immunoglobulin family adhesion molecule, contactin-1. Contactin-1 transcripts were markedly decreased on gene-expression arrays of muscle from affected family members compared to controls. We demonstrate that contactin-1 is expressed at the neuromuscular junction (NMJ) in mice and man in addition to the previously documented expression in the central and peripheral nervous system. In patients with secondary dystroglycanopathies, we show that contactin-1 is abnormally localized to the sarcolemma instead of exclusively at the NMJ. The cntn1 null mouse presents with ataxia, progressive muscle weakness, and postnatal lethality, similar to the affected members in this family. We propose that loss of contactin-1 from the NMJ impairs communication or adhesion between nerve and muscle resulting in the severe myopathic phenotype. This disorder is part of the continuum in the clinical spectrum of congenital myopathies and congenital myasthenic syndromes.

The ABCA1 cholesterol transporter associates with one of two distinct dystrophin-based scaffolds in Schwann cells

Cytoskeletal scaffolding complexes help organize specialized membrane domains with unique functions on the surface of cells. In this study, we define the scaffolding potential of the Schwann cell dystrophin glycoprotein complex (DGC) by establishing the presence of four syntrophin isoforms, (α1, β1, β2, and γ2), and one dystrobrevin isoform, (α‐dystrobrevin‐1), in the abaxonal membrane. Furthermore, we demonstrate the existence of two separate DGCs in Schwann cells that divide the abaxonal membrane into spatially distinct domains, the DRP2/periaxin rich plaques and the Cajal bands that contain Dp116, utrophin, α‐dystrobrevin‐1 and four syntrophin isoforms. Finally, we show that the two different DGCs can scaffold unique accessory molecules in distinct areas of the Schwann cell membrane. Specifically, the cholesterol transporter ABCA1, associates with the Dp116/syntrophin complex in Cajal bands and is excluded from the DRP2/periaxin rich plaques.

Complete deletion of all α-dystrobrevin isoforms does not reveal new neuromuscular junction phenotype

The dystrophin glycoprotein complex (DGC) is critical for muscle stability, and mutations in DGC proteins lead to muscular dystrophy. The DGC also contributes to the maturation and maintenance of the neuromuscular junction (NMJ). The gene encoding the DGC protein alpha-dystrobrevin undergoes alternative splicing to produce at least five known isoforms. Isoform-specific antibody staining and reverse transcription PCR in mutant mice with a deletion of exon 3 of the alpha-dystrobrevin gene suggested the existence of a remaining synaptic isoform, which might be compensating for alpha-dystrobrevin function. To test this possibility and to more completely understand the synaptic function of alpha-dystrobrevin, we used a two-step homologous recombination strategy combined with in vivo Cre-mediated excision to generate mice with a large deletion of the alpha-dystrobrevin gene to disrupt all isoforms. However, these mice did not exhibit a more severe NMJ phenotype than that observed in the exon 3-deleted mice. Nonetheless, these mice not only eliminate possible compensation by remaining isoforms of alpha-dystrobrevin, but also offer a conditional allele that could be used to identify tissue-specific and developmental functions of alpha-dystrobrevin. This work also demonstrates a successful strategy to achieve deletion of a large genomic sequence, which can be a valuable tool for functional studies of genes encoding multiple isoforms that span a large genomic region.

G.O.3 Mutations in contactin-1, a neuronal cell adhesion molecule expressed at the neuromuscular junction, cause a novel form of congenital lethal myopathy

phia, United States; Monash University, Department of Biochemistry and Molecular Biology, Clayton, Australia; University of Utah, Department of Human Genetics, Salt Lake City, United States; University of Utah, Department of Pathology, Salt Lake City, United States; 5 Imperial College London, Hammersmith Hospital, Dubowitz Neuromuscular Centre, London, United Kingdom; Stella Maris Scientific Institute, Department of Developmental Neuroscience, Pisa, Italy; The Children’s Hospital of Philadelphia, Protein Core Facility, Philadelphia, United States; University of Pennsylvania, Department of Genetics, Philadelphia, United States; The Children’s Hospital of Philadelphia, Division of Neuropathology and Pathology Core Lab, Philadelphia, United States; The Children’s Hospital of Philadelphia, Department of Pathology and Laboratory Medicine, Philadelphia, United States; 11 Johannes Gutenberg University, Department of Neuropathology, Mainz, Germany; 12 Instituto de Investigaciones Neurologicas, Hospital Nacional de Pediatria J.P. Garrahan, Buenos Aires, Argentina; University of Utah, Departments of Neurology, Human Genetics, Pediatrics, Salt Lake City, United States