International Journal of Nephrology 3 Podocyte TRPC 6 or TRPC5 CsA (antiproteinuric eﬀect) Plasma membrane Synaptopodin Ca2+ Calcineurin NFAT NFAT-dependent TRPC6 expression NFAT-activation-induced podocyte apoptosis and glomerulosclerosis ? Rcan1 Wnt6 Fzd9 Figure 1: The calcineurin-activation-induced podocyte injury. Upon activation by increased intracellular Ca2+, calcineurin dephospho- rylates the NFAT proteins and synaptopodin [5, 6]. NFAT activation induces podocyte apoptosis [11] and glomerulosclerosis [6]. TRPC6- mediated Ca2+ inﬂux stimulates NFAT-dependent TRPC6 expression [7]. Rcan1, Wnt6, and Fzd9 were shown to be upregulated in glomeruli isolated from NFATc1nuc transgenic mice, making them potentially direct targets of NFAT [6]. which is operative in podocytes and contributes to the maintenance of kidney ﬁlter function. Although Faul et al. argued against an antiproteinuric role of CsA through the suppression of T cells, it is possible for the involvement of NFAT proteins downstream of calcineurin in podocyte injury, since the NFAT transcription factors are the most extensively studied calcineurin substrates and the major regulators of transcription in response to Ca2+/calcineurin signals [47, 52]. Ca2+ signaling through ion channels has recently emerged as a potential modulator of podocyte function, and several Ca2+-permeable channels have been identiﬁed inpodocytes [53, 54]. Generally, upon activation by increased intracellular Ca2+, calcineurin dephosphorylates the NFAT proteins that reside in the cytoplasm in resting cells. This dephosphorylation exposes the concealed nuclear localiza- tion signals of the NFAT proteins, leading to the cytoplasm to nucleus translocation of these proteins. In the nucleus, the NFATc proteins form NFAT transcription complex with their nuclear partners to control the transcription of target genes. TRPC6 mutations were found in families with hereditary FSGS, and TRPC5 and TRPC6 channels are now known as the Ca2+ inﬂux pathways for the nonselective, cationic current in podocytes [55]. Mutations in one of these chan- nels,TRPC6, lead to aberrant Ca2+ signaling, podocyte dys- function [21, 22], and Nephrin and Neph1 have been shown to interact with several Ca2+ channels, including TRPC6 [11, 56]. Vassiliadis et al. [57] showed that Ca2+/calcineurin signals mediated podocyte injury. Inhibition of calcium channels and chelation of extracellular calcium reduced protamine sulfate-induced damage, suggesting that calcium signaling plays a critical role in the initial stages of glomerular injury. Calcineurin inhibitors (FK506 and CsA) inhib- ited protamine sulfate-mediated barrier changes, indicating that calcium signaling acts, in part, through calcineurin- dependent cleavage of synaptopodin. Meanwhile, mutations in TRPC6 enhance the amplitude and duration of the Ca2+ channel current which cause NFAT activation, indicating the activation of the calcineurin-NFAT pathway as a potential mediator of FSGS [58]. Importantly, the study of Wang et al. [8] demonstrated that, in parallel to synaptopodin regulation, there may be an additional pathway from calcineurin to podocyte injury and proteinuria that involves NFAT-mediated regulation of known and novel factors important for podocyte function. To study the role of NFAT signaling in glomerular podocytes, Wang et al. [8] created a system for inducible activation of NFAT signaling in podocytes, in which a Podocin-Cre trans- gene was used to induce the removal of the transcriptional stop cassette in a ROSA26-rtTA allele only in podocytes [59, 60]. And when the cassette is deleted, the ROSA26 promoter drives the production of reverse tetracycline- controlled transactivator (rtTA) in podocytes. When treated with doxycycline, the doxycycline-rtTA complex binds to the TetO sequence of the TetO-N FATc1NUC transgene. The mice carrying three alleles (Podocin-Cre, RO-SA26-rtTA, and TetO-N FATc1NUC) were referred as mutants. Thus, results from Wang et al. [8] provided in vivo evidence that NFAT activation, either in utero or postdevelopmentally, can lead to podocyte injury and proteinuria, which suggest that activation of NFAT signaling may be a key pathogenic molecular change in podocyte injury and the development of proteinuria. Using a similar model for conditional NFAT activation in podocytes, Nijenhuis et al. [9] demonstrated that podocyte- speciﬁc inducible expression of a constitutively active NFAT mutant increased TRPC6 expression and induced severe proteinuria, and that calcineurin inhibition by CsA