Alpha-actinin-4-mediated FSGS: an inherited kidney disease caused by an aggregated and rapidly degraded cytoskeletal protein.
Focal segmental glomerulosclerosis (FSGS) is a common pattern of renal injury, seen as both a primary disorder and as a consequence of underlying insults such as diabetes, HIV infection, and hypertension. Point mutations in the alpha-actinin-4 gene ACTN4 cause an autosomal dominant form of human FSGS. We characterized the biological effect of these mutations by biochemical assays, cell-based studies, and the development of a new mouse model. We found that a fraction of the mutant protein forms large aggregates with a high sedimentation coefficient. Localization of mutant alpha-actinin-4 in transfected and injected cells, as well as in situ glomeruli, showed aggregates of the mutant protein. Video microscopy showed the mutant alpha-actinin-4 to be markedly less dynamic than the wild-type protein. We developed a "knockin" mouse model by replacing Actn4 with a copy of the gene bearing an FSGS-associated point mutation. We used cells from these mice to show increased degradation of mutant alpha-actinin-4, mediated, at least in part, by the ubiquitin-proteasome pathway. We correlate these findings with studies of alpha-actinin-4 expression in human samples. "Knockin" mice with a disease-associated Actn4 mutation develop a phenotype similar to that observed in humans. Comparison of the phenotype in wild-type, heterozygous, and homozygous Actn4 "knockin" and "knockout" mice, together with our in vitro data, suggests that the phenotypes in mice and humans involve both gain-of-function and loss-of-function mechanisms.
Yao
June
J
Renal Division, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts, USA.
Le
Tu Cam
TC
Kos
Claudine H
CH
Henderson
Joel M
JM
Allen
Phillip G
PG
Denker
Bradley M
BM
Pollak
Martin R
MR
D000185:Actinin / Q000235:genetics* / Q000378:metabolism*; D000818:Animals; D001483:Base Sequence; D004195:Disease Models, Animal; D040461:Gene Components; D005786:Gene Expression Regulation / Q000235:genetics*; D005923:Glomerulosclerosis, Focal Segmental / Q000235:genetics*; D006801:Humans; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D007678:Kidney Glomerulus / Q000648:ultrastructure; D051379:Mice; D008817:Mice, Mutant Strains; D008840:Microfilament Proteins / Q000235:genetics*; D008854:Microscopy, Electron; D018715:Microscopy, Video; D009154:Mutation / Q000235:genetics; D010641:Phenotype; D014162:Transfection
C111453:ACTN4 protein, human; D008840:Microfilament Proteins; D000185:Actinin