As expected, the inducible strategy recapitulated the phenotype thing observed after conventional Cre excision when doxycycline was administered to pregnant female animals to initiate early Cre expression in Raptorflox/flox;NPHS2.rtTA;tetO.Cre mice (Figure (Figure3E).3E). However, when Raptor deletion was induced in adult mice, the levels of protein excretion were higher compared with controls, but did not reach statistical significance (albumin/creatinine ratios 2 months after doxycycline induction: induced Raptorflox/flox;NPHS2.rtTA;tetO.Cre: mean = 0.23 �� 0.07; control littermates mean = 0.13 �� 0.06; n = 13 each) (Figure (Figure3E).3E). Histological alteration in the doxycycline-treated Raptorflox/flox;NPHS2.rtTA;tetO.Cre embryonic C57BL/6 mice resembled those of the Raptorflox/flox;NPHS2.

Cre animals, whereas histology of the late-induced (8 weeks, C57BL/6 background) did not depict any glomerulosclerosis, although some milder glomerular lesions such as glomerular synechia could be observed (Figure (Figure3G).3G). In context of the high interindividual variability of proteinuria in patients receiving mTOR inhibitors, we speculated that the genetic background as modifying factor might contribute to the maintenance phenotype of Raptor deficiency in adult mice. To prove this hypothesis, we transferred Raptorflox/flox;NPHS2.rtTA;tetO.Cre mice to an ICR background (IcrTac:ICR; Taconic USA), which is known to be more sensitive toward glomerular diseases than that of C57BL/6 mice. Strikingly, in these mice, Raptor deletion at 8 weeks of life induced significant proteinuria (Figure (Figure3F).

3F). Taken together, these data indicate that mTORC1 is especially important for podocyte development and growth. Additionally mTORC1 is required for glomerular maintenance under homeostatic conditions, whereby this effect is modified by the genetic background of mice. Figure 3 Time-specific deletion of Raptor indicates the importance of mTORC1 activity during glomerular GSK-3 development. Synergistic action of mTORC1 and mTORC2 complexes are required for glomerular homeostasis. mTORC1 and mTORC2 phosphorylate different substrates to regulate distinct cellular functions. In contrast to mTORC1, mTORC2 is largely rapamycin-insensitive and phosphorylates cellular targets such as AKT, SGK1, and PKC to control cell survival and cytoskeletal organization (2). However, these effects vary among different cell types and the function of mTORC2 in the glomerulus has not been defined. In podocyte-specific mTORC2-deficient mice (Rictor��podocyte) (Figure (Figure4A),4A), the level of Rictor and the activation state of PKC��, which is a well-characterized substrate of mTORC2 (30), was significantly reduced (Figure (Figure4B).4B).