Glomerular epithelial cell injury associated with mutant alpha-actinin-4.
Focal segmental glomerulosclerosis (FSGS) may be associated with glomerular epithelial cell (GEC; podocyte) apoptosis due to acquired injury or mutations in alpha-actinin-4. This study addresses how FSGS-associated mutant alpha-actinin-4 may induce GEC injury, focusing on endoplasmic reticulum (ER) stress and metabolism of mutant alpha-actinin-4 via the ubiquitin-proteasome system. In a model of experimental FSGS induced by expression of an alpha-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-2alpha subunit, and induction of the proapoptotic gene C/EBP homologous protein-10 (CHOP). To address mechanisms of ER stress, we studied signaling in cultured GEC and COS cells expressing alpha-actinin-4 K256E. Previously, we showed that expression of this alpha-actinin-4 mutant in GEC increased apoptosis. In the present study, we show that alpha-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 alpha-actinin-4 K256E and impairment of the ubiquitin-proteasome system. In addition, alpha-actinin-4 K256E exacerbated apoptosis in the context of mild proteasome inhibition. Thus alpha-actinin-4 K256E triggers several metabolic abnormalities, which may lead to GEC injury and glomerulosclerosis.
Cybulsky
Andrey V
AV
Div. of Nephrology, Royal Victoria Hospital, 687 Pine Ave. West, Montreal, Quebec, Canada H3A1A1. andrey.cybulsky@mcgill.ca
Takano
Tomoko
T
Papillon
Joan
J
Bijian
Krikor
K
Guillemette
Julie
J
Kennedy
Chris R J
CR
D000185:Actinin / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D017209:Apoptosis; D019556:COS Cells; D002522:Chlorocebus aethiops; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005923:Glomerulosclerosis, Focal Segmental / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D050199:Podocytes / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D051381:Rats; D051742:Transcription Factor CHOP / Q000378:metabolism
C491752:Actn4 protein, mouse; C491751:Actn4 protein, rat; C497250:Ddit3 protein, mouse; C497251:Ddit3 protein, rat; D008562:Membrane Glycoproteins; C053387:endoplasmin; D000185:Actinin; D051742:Transcription Factor CHOP; D046988:Proteasome Endopeptidase Complex