However, Rictor��podocyte mice exposed to stress such as BSA biological activity overload (31, 32) developed significantly higher transient albuminuria than control littermates (Figure (Figure4G),4G), suggesting that mTORC2 might play a role in podocyte adaptation and foot process reorganization in response to stress. Figure 4 Podocyte-specific knockout of the mTORC2 complex results in reduced ability to adapt to stress. Strikingly, the combined podocyte-specific deletion of mTORC1 and mTORC2 (Raptor/Rictor��podocyte) (Figure (Figure5A)5A) caused a pronounced kidney phenotype with massive proteinuria (Figure (Figure5B)5B) and growth retardation (Figure (Figure5,5, C and D). The severe glomerulosclerosis (Figure (Figure5E)5E) presented with circumferential synechia, crescent formation, vacuolization of podocytes, and often complete glomerular obsolescence at 6 weeks of age.

Massive foot process effacement was documented by electron microscopy (Figure (Figure5F).5F). Consistent with the dramatic phenotype causing renal failure as documented by increased serum creatinine levels (Figure (Figure5G),5G), most Raptor/Rictor��podocyte mice died by 8 weeks of age (Figure (Figure5H).5H). These findings reveal that both mTOR complexes are required for podocyte homeostasis and that mTORC2 plays an unexpected role in podocyte function and stress adaptation. Figure 5 Synergistic action of mTORC1 and mTORC2 complexes are required for glomerular homeostasis. mTORC1 activation is a molecular signature of diabetic nephropathy. Recent studies suggest that the mTOR pathway plays an important role in mechanisms underlying the progression of glomerular diseases (10�C14).

Many glomerular diseases such as diabetic nephropathy are preceded by glomerular hypertrophy and enlargement of podocytes (33, 34). Since mTOR controls cell size (2), these findings suggest an involvement of mTOR; however, conclusive evidence demonstrating elevated mTORC1 activity in podocytes of diabetic models is lacking. To test the hypothesis that mTOR activation Drug_discovery is associated with chronic glomerular diseases, we took advantage of the fact that mTORC1 directly phosphorylates and activates transcription factors, thereby directly increasing mRNA levels of some well-described gene targets such as the SREBP, VEGF, and mitochondrial target genes. We first analyzed transcriptional targets of mTOR in microdissected glomeruli from patients with diabetic nephropathy (35).