Mizoribine Corrects Defective Nephrin Biogenesis by Restoring Intracellular Energy Balance

Abstract

Proteins are modified and folded within the endoplasmic reticulum (ER). When the influx of proteins exceeds the capacity of the ER to handle the load, the ER is "stressed" and protein biogenesis is affected. We have previously shown that the induction of ER stress by ATP depletion in podocytes leads to mislocalization of nephrin and subsequent injury of podocytes. The aim of the present study was to determine whether ER stress is associated with proteinuria in vivo and whether the immunosuppressant mizoribine may exert its antiproteinuric effect by restoring normal nephrin biogenesis. Induction of nephrotic-range proteinuria with puromycin aminonucleoside in mice increased expression of the ER stress marker GRP78 in podocytes, and led to the mislocalization of nephrin to the cytoplasm. In vitro, mizoribine, through a mechanism likely dependent on the inhibition of inosine 5Ј-monophosphate dehydrogenase (IMPDH) activity in podocytes, restored the intracellular energy balance by increasing levels of ATP and corrected the posttranslational processing of nephrin. Therefore, we speculate that mizoribine may induce remission of proteinuria, at least in part, by restoring the biogenesis of slit diaphragm proteins in injured podocytes. Further understanding of the ER microenvironment may lead to novel approaches to treat diseases in which abnormal handling of proteins plays a role in pathogenesis.

Participation of endoplasmic reticulum stress in the pathogenesis of spontaneous glomerulosclerosis–Role of intra-renal angiotensin system

“…puromycin aminonucleoside-induced nephrosis is associated with upregulation of podocyte GRP78. 16 In addition induction of familial focal segmental glomerulosclerosis (FSGS) in the mice (by expression of a-actinin-4K256E transgene in podocytes) results in expression of ER stress markers and proapoptotic proteins. 17 Moreover, markers of ER stress have been identified in the renal biopsy specimens from patients with various inflammatory and noninflammatory glomerulopathies.…”

Na+/H+ exchanger-1 reduces podocyte injury caused by endoplasmic reticulum stress via autophagy activation

“…[2][3][4] Previous studies have suggested a role for endoplasmic reticulum (ER) stress induction in podocytes using models of membranous nephropathy (Heymann nephritis) 5,6 and minimal-change nephrotic syndrome (puromycin nephrosis). 7 Moderate stress in the ER triggers many rescue responses, including an unfolded protein response and ER-associated degradation. 8 On the other hand, excessive or long-term exposure to ER stress induces impaired formation of the slit diaphragm complex and its associated lipid rafts as well as actin cytoskeleton abnormalities.…”

mTORC1 activation triggers the unfolded protein response in podocytes and leads to nephrotic syndrome

“…7,8 In a previous study, we implicated ER stress in the development of the rodent model of minimalchange disease (MCD). 9 There are a number of studies suggesting that podocyte damage underlies the development of MCD; 10,11 however, the pathophysiological mechanisms of proteinuria have not been fully elucidated. In the rodent puromycin aminonucleoside (PAN) model of MCD, we showed upregulation of GRP78, a molecular chaperone stimulated by ER stress, in podocytes on days 4 and 5 at a heavy proteinuric state; we also showed that this GRP78 upregulation was associated with a change in the cellular localization of nephrin from the plasma membrane to the cytoplasm.…”

“…In the rodent puromycin aminonucleoside (PAN) model of MCD, we showed upregulation of GRP78, a molecular chaperone stimulated by ER stress, in podocytes on days 4 and 5 at a heavy proteinuric state; we also showed that this GRP78 upregulation was associated with a change in the cellular localization of nephrin from the plasma membrane to the cytoplasm. 9 Nephrin is a transmembrane glycoprotein that is produced by podocytes and is responsible for the permselective barrier of the glomerulus. 12, 13 We previously determined that nephrin trafficking to the plasma membrane required N-glycosylation in the ER 14 and showed that a lack of ATP induced the hypoglycosylation of nephrin and its retention in the ER, which coincided with a strong induction of GRP78.…”

“…15 Interestingly, glucocorticoids and the immunosuppressant mizoribine, which are commonly used to treat MCD, normalized the abnormal N-glycosylation and nephrin localization by restoring ATP levels. 9,15 Thus, the balance of the energy regulatory system must have a crucial role in the correct processing of podocyte-producing proteins, and the imbalance of the energy regulatory system may induce ER stress and the UPR, which possibly underlies proteinuria in MCD. Then a question arose as to what events are the triggers to cause the UPR.…”