Glomerular epithelial cell injury associated with mutant α-actinin-4

Abstract

Focal segmental glomerulosclerosis (FSGS) may be associated with glomerular epithelial cell (GEC; podocyte) apoptosis due to acquired injury or mutations in α-actinin-4. This study addresses how FSGS-associated mutant α-actinin-4 may induce GEC injury, focusing on endoplasmic reticulum (ER) stress and metabolism of mutant α-actinin-4 via the ubiquitin-proteasome system. In a model of experimental FSGS induced by expression of an α-actinin-4 K256E transgene in podocytes, we show induction of ER stress, including upregulation of ER chaperones (bip, grp94), phosphorylation of the eukaryotic translation initiation factor-2α subunit, and induction of the proapop totic gene C/EBP homologous protein-10 (CHOP). To address mechanisms of ER stress, we studied signaling in cultured GEC and COS cells expressing α-actinin-4 K256E. Previously, we showed that expression of this α-actinin-4 mutant in GEC increased apoptosis. In the present study, we show that α-actinin-4 K256E upregulates grp94 and CHOP expression in COS cells and significantly exacerbates induction of bip and CHOP in GEC in the presence of tunicamycin. ER stress was associated with aggregation and ubiquitination of α-actinin-4 K256E and impairment of the ubiquitin-proteasome system. In addition, α-actinin-4 K256E exacerbated apoptosis in the context of mild proteasome inhibition. Thus α-actinin-4 K256E triggers several metabolic abnormalities, which may lead to GEC injury and glomerulosclerosis.