α-Actinin-4-Mediated FSGS: An Inherited Kidney Disease Caused by an Aggregated and Rapidly Degraded Cytoskeletal Protein

Abstract

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 theα-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 α-actinin-4 in transfected and injected cells, as well as in situ glomeruli, showed aggregates of the mutant protein. Video microscopy showed the mutant α-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 α-actinin-4, mediated, at least in part, by the ubiquitin–proteasome pathway. We correlate these findings with studies of α-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.

Patients with ACTN4 Mutations Demonstrate Distinctive Features of Glomerular Injury

“…Strikingly similar aggregated electrondense material is seen in podocytes of mouse models that express mutant Actn4. 5,11 In mice, this aggregated material has been demonstrated to be comelectron-dense cytoplasmic aggregated material is not seen elsewhere in the podocyte cytoplasm. (C and F) Patients with postadaptive FSGS (2Њ FSGS).…”

“…Kidney disease develops through an apparent "gain of function" mechanism that seems to involve primarily the podocyte. 4,5 The disease-causing ACTN4 mutations that have been identified thus far are part of a growing list of "inherited podocytopathies" that includes mutations in NPHS1, NPHS2, CD2AP, and TRPC6. 6 Patients carrying mutations in ACTN4 initially present with proteinuria or nephrotic syndrome in early adulthood and often progress to renal failure.…”

Recent advances of animal model of focal segmental glomerulosclerosis

“…Knock-in-mouse models have been generated to examine the role of different human mutations [45,46]. These studies helped to identify the purpose and physiological role of this cross-linking protein and the consequence and the severity of point mutations [47,48] (Table 2; Fig. 1).…”

Angiotensin II regulates phosphorylation of actin‐associated proteins in human podocytes

“…The protein LCP1 is one of 3 plastin isoforms that are found in mammalian cells that belong to the a-actinin family of actin cross-linking proteins, which play a crucial role in podocyte biology. Indeed, mutations in the protein a-actinin-4, for example, have previously been linked to podocyte injury (36). In contrast to other a-actinin family members, plastins have 2 actin-binding domains in a single polypeptide chain.…”