Calcium Mediates Glomerular Filtration through Calcineurin and mTORC2/Akt Signaling

Abstract

Alterations to the structure of the glomerular filtration barrier lead to effacement of podocyte foot processes, leakage of albumin, and the development of proteinuria. To better understand the signaling pathways involved in the response of the glomerular filtration barrier to injury, we studied freshly isolated rat glomeruli, which allows for the monitoring and pharmacologic manipulation of early signaling events. Administration of protamine sulfate rapidly damaged the isolated glomeruli, resulting in foot process effacement and albumin leakage. Inhibition of calcium channels and chelation of extracellular calcium reduced protamine sulfateinduced damage, suggesting that calcium signaling plays a critical role in the initial stages of glomerular injury. Calcineurin inhibitors (FK506 and cyclosporine A) and the cathepsin L inhibitor E64 all inhibited protamine sulfate-mediated barrier changes, which suggests that calcium signaling acts, in part, through calcineurin-and cathepsin L-dependent cleavage of synaptopodin, a regulator of actin dynamics. The mTOR inhibitor rapamycin also protected glomeruli, demonstrating that calcium signaling has additional calcineurinindependent components. Furthermore, activation of Akt through mTOR had a direct role on glomerular barrier integrity, and activation of calcium channels mediated this process, likely independent of phosphoinositide 3-kinase. Taken together, these results demonstrate the importance of calcium and related signaling pathways in the structure and function of the glomerular filtration barrier.

TORCing up the Importance of Calcium Signaling

“…[2][3][4][5] Identification of mutations in a number of genes causing familial FSGS has resulted in significant advances in the understanding of proteinuric kidney disease. 3,6 -10 In vitro studies focused on podocyte signaling 11,12 combined with evidence of increased calcium transients in TRPC6 mutations seen in familial FSGS 9,10 support a role for calcium signaling in podocyte dysfunction and disruption of the glomerular filtration barrier.In this issue of JASN, Vassiliadis et al 13 treated ex vivo rat glomeruli with protamine sulfate (PS) to recapitulate minimal change disease in an animal model. They confirmed podocyte foot process effacement and albumin leak from glomeruli occurring as early as 45 minutes after administration of PS.…”

“…This leads to NFAT-independent alterations in actin polymerization and proteinuria. 16 Vassiliadis et al 13 tested the hypothesis that calcium-mediated proteinuria and foot process effacement in the PS-treated glomeruli are mediated through the calcineurin/synaptopodin axis. This was confirmed by the observation of decreased expression of fulllength (110 kD) synaptopodin, which was prevented by pretreatment with E64, a CATL inhibitor.…”

“…20 The balance of activity of the TOR signaling, which is described in a number of reviews, and the context in which it happens will determine the net effect on the cell. 20,22 Vassiliadis et al 13 investigated whether mTOR signaling was involved in the calcium-dependent albumin leakage in the PS injury model. Indeed, rapamycin ameliorates the effects of PS administration in a dosage-dependent manner.…”

“…In this issue of JASN, Vassiliadis et al 13 treated ex vivo rat glomeruli with protamine sulfate (PS) to recapitulate minimal change disease in an animal model. They confirmed podocyte foot process effacement and albumin leak from glomeruli occurring as early as 45 minutes after administration of PS.…”

“…17 The effect of rapamycin administration in animal models of glomerular disease has been incongruous with proteinuria sometimes seen in humans. The studies by Vassiliadis et al 13 allied to the aforementioned targeted genetic deletion studies, 17,18 and the ongoing clinical study of small-molecule mTOR kinase inhibitors for treatment of solid tumors advance our understanding of TOR signaling. This knowledge can be applied to identify new therapeutic targets for the treatment of glomerular disease.…”