Defective glycosylation of α-dystroglycan contributes to podocyte flattening

Abstract

In addition to skeletal muscle and the nervous system, α-dystroglycan is found in the podocyte basal membrane, stabilizing these cells on the glomerular basement membrane. Fukutin, named after the gene responsible for Fukuyama-type congenital muscular dystrophy, is a putative glycosyltransferase required for the post-translational modification of α-dystroglycan. Chimeric mice targeted for both alleles of fukutin develop severe muscular dystrophy; however, these mice do not have proteinuria. Despite the lack of a functional renal defect, we evaluated glomerular structure and found minor abnormalities in the chimeric mice by light microscopy. Electron microscopy revealed flattening of podocyte foot processes, the number of which was significantly lower in the chimeric compared to wild-type mice. A monoclonal antibody against the laminin-binding carbohydrate residues of α-dystroglycan did not detect α-dystroglycan glycosylation in the glomeruli by immunoblotting or immunohistochemistry. In contrast, expression of the core α-dystroglycan protein was preserved. There was no statistical difference in dystroglycan mRNA expression or in the amount of nephrin and α3-integrin protein in the chimeric compared to the wild-type mice as judged by immunohistochemistry and real-time RT-PCR. Thus, our results indicate that appropriate glycosylation of α-dystroglycan has an important role in the maintenance of podocyte architecture.

The role of cell–extracellular matrix interactions in glomerular injury

“…Although these mice do not develop a severe kidney phenotype (i.e. focal segmental glomerulosclerosis, proteinuria), this study indicates that glycosylation of α-dystroglycan is important for the maintenance of podocyte architecture [95]. This finding agrees with the observation that in two different in vivo models of podocyte-mediated injury, the levels of α-dystroglycan on podocytes decrease with concomitant changes in the fibrillar components of the GBM [96].…”

“…Mice lacking fukutin, a glycosyltransferase required for the post-translational modification of α-dystroglycan, show flattening of podocyte foot processes, and decreased number of podocytes compared to wild type controls [95]. Although these mice do not develop a severe kidney phenotype (i.e.…”

Glycosylation of α-Dystroglycan

“…␣-Dystroglycan (␣-DG) 5 is an evolutionarily conserved glycoprotein component of the dystrophin complex, which is responsible for anchoring the cellular cytoskeleton to the extracellular matrix (ECM) (1)(2)(3). ␣-DG is known to be required for proper skeletal muscle cell integrity and attachment to the ECM.…”

“…Loss of ␣-DG function is responsible for certain forms of muscular dystrophy in humans, a number of which are due to mutations in the glycosyltransferases that modify ␣-DG (2, 4). Furthermore, changes in ␣-DG glycosylation are also associated with various neurological and eye defects, and abnormal kidney morphology (3,5), indicating its role across multiple organs and tissues. In addition to being required for proper cell/ECM attachment in diverse organ systems, ␣-DG is a point of engagement in infection by several viruses, including lymphocytic choriomeningitis virus and Lassa virus (6,7).…”

Dystroglycan does not contribute significantly to kidney development or function, in health or after injury

“…While many of the aforementioned studies assigned a function for DG in renal cells, there is significant evidence against an important role for DG in the kidney. There are no reported kidney function abnormalities in either mouse or human mutants with impaired DG function due to glycosylation defects, although a recent report showed GBM thickening and podocyte foot process widening, but without albuminuria, in chimeric mice generated with fukutin-null embryonic stem cells (27). Furthermore, the utrophin knockout mouse, even when combined with the dystrophin knockout, has normal kidneys (45).…”