Recombinant Human Dystroglycan Protein, CF

Dystroglycan, also called DAG‑1 (dystrophin‑associated glycoprotein 1) or DG, is a heterodimeric adhesion molecule that links the extracellular matrix (ECM) to the cell cytoskeleton (1‑4). Human preproDAG‑1 is an 895 amino acid (aa) type I transmembrane protein that contains a 27 aa signal sequence and an 868 aa proform. Autocatalysis of the proform produces two fragments that remain noncovalently‑linked. The first fragment (or alpha‑chain) is 626 aa in length (aa 28‑653) and contains a mucin‑like region, while the second fragment (or beta‑chain) is a 42‑44 kDa, 242 aa N‑glycosylated protein with an extracellular (aa 654‑749), transmembrane, and cytoplasmic domain (5). Over aa 28‑749, human DAG‑1 shares 93% aa sequence identity with mouse DAG‑1. It is widely expressed but differentially O‑glycosylated on skeletal muscle and epithelia (which contain a 160 kDa alpha‑chain) as compared to cardiac muscle, smooth muscle, fibroblasts, keratinocytes, lymphocytes, and hematopoietic stem cells (which contain a 100 ‑ 140 kDa alpha‑chain) (1‑3, 6‑9). DAG‑1 binding of ECM molecules is influenced by its alpha‑chain O‑glycosylation (2, 6‑10). In addition to skeletal muscle and neuromuscular junctions in which DAG‑1 binds several ECM molecules, DAG‑1 is important for neuronal migration (through neurexin interactions), keratinocyte attachment to the ECM (through laminin), and adhesion at the immunological synapse and in the hematopoietic stem cell niche (through agrin) (3, 6‑11). In muscle, the beta‑chain cytoplasmic domain connects with the cytoskeleton via formation of the dystrophin‑glycoprotein complex with isoforms of dystrophin, sarcoglycan, syntrophin, and sarcospan (3). This complex is critical for skeletal muscle viability and regeneration (3, 4, 10, 11). MMP9 cleavage of the 44 kDa beta‑chain creates a 30 kDa transmembrane form that causes dissociation of the heterodimer and a down‑regulation of ECM interactions (6, 12). Dystroglycanopathies, a group of congenital muscular dystrophies affecting the brain, eye and skeletal muscle, are caused by either abnormalities in glycosyltransferases, or their accessory proteins, or rare DAG‑1 polymorphisms. All result in DAG‑1 hypoglycosylation, especially of O‑mannosyl forms, and affect DAG‑1 binding to ECM proteins (2, 3, 10, 13, 14).