Rac1 activation in podocytes induces the spectrum of nephrotic syndrome

Hyper-activation of Rac1, a small GTPase, in glomerular podocytes has been implicated in the pathogenesis of familial proteinuric kidney diseases. However, the role of Rac1 in acquired nephrotic syndrome is unknown. To gain direct insights into this, we generated a transgenic mouse model expressing a doxycycline-inducible constitutively active form of Rac1 (CA-Rac1) in podocytes. Regardless of the copy number, proteinuria occurred rapidly within five days, and the histology resembled minimal change disease. The degree and severity of proteinuria were dependent on the transgene copy number. Upon doxycycline withdrawal, proteinuria resolved completely (one copy) or nearly completely (two copy). After one month of doxycycline treatment, two-copy mice developed glomerulosclerosis that resembled focal segmental glomerulosclerosis (FSGS) with urinary shedding of transgene-expressing podocytes. p38 MAPK was activated in podocytes upon CA-Rac1 induction while a p38 inhibitor attenuated proteinuria, podocyte loss, and glomerulosclerosis. Mechanistically, activation of Rac1 in cultured mouse podocytes reduced adhesiveness to laminin and induced redistribution of β1 integrin, and both were partially reversed by the p38 inhibitor. Activation of Rac1 in podocytes was also seen in kidney biopsies from patients with minimal change disease and idiopathic FSGS by immunofluorescence while sera from the same patients activated Rac1 in cultured human podocytes. Thus, activation of Rac1 in podocytes causes a spectrum of disease ranging from minimal change disease to FSGS, due to podocyte detachment from the glomerular basement membrane that is partially dependent on p38 MAPK.