Absence of Post-phosphoryl Modification in Dystroglycanopathy Mouse Models and Wild-type Tissues Expressing Non-laminin Binding Form of α-Dystroglycan

alpha-Dystroglycan (alpha-DG) is a membrane-associated glycoprotein that interacts with several extracellular matrix proteins, including laminin and agrin. Aberrant glycosylation of alpha-DG disrupts its interaction with ligands and causes a certain type of muscular dystrophy commonly referred to as dystroglycanopathy. It has been reported that a unique O-mannosyl tetrasaccharide (Neu5Ac-alpha 2,3-Gal-beta 1,4-GlcNAc-beta 1,2-Man) and a phosphodiester-linked modification on O-mannose play important roles in the laminin binding activity of alpha-DG. In this study, we use several dystroglycanopathy mouse models to demonstrate that, in addition to fukutin and LARGE, FKRP (fukutin-related protein) is also involved in the post-phosphoryl modification of O-mannose on alpha-DG. Furthermore, we have found that the glycosylation status of alpha-DG in lung and testis is minimally affected by defects in fukutin, LARGE, or FKRP. alpha-DG prepared from wild-type lung-or testis-derived cells lacks the post-phosphoryl moiety and shows little laminin-binding activity. These results show that FKRP is involved in post-phosphoryl modification rather than in O-mannosyl tetrasaccharide synthesis. Our data also demonstrate that post-phosphoryl modification not only plays critical roles in the pathogenesis of dystroglycanopathy but also is a key determinant of alpha-DG functional expression as a laminin receptor in normal tissues and cells.