2 proteinuria in mice. Thus, both studies provided in vivo evidence that NFAT activation in podocytes may be a critical pathogenic molecular event leading to proteinuria or FSGS. Altogether, these advances in podocyte research indicate that calcineurin-NFAT signal or calcineurin-synaptopodin axis has a direct proteinuric eﬀect on podocytes, and these observations raise the possibility of developing speciﬁc antiproteinuric drugs that lack the unwanted eﬀects of calcineurin or NFAT inhibition [10]. 2. Proteinuria and Its Molecular Mechanisms in Podocyte Proteinuria, a cardinal sign and a prognostic marker of kidney disease, aﬀects several hundred million people worldwide [12]. Proteinuria is also an independent risk factor for cardiovascular morbidity and mortality. Podocytes, endothelial cells, and the glomerular basement membrane (GBM) constitute the glomerular ﬁltration barrier, a highly specialized structure for selective ultraﬁltration. The com- mon denominator in a variety of kidney diseases, including MCD and FSGS, is podocyte injury involving a massive loss of protein in the urine (proteinuria) [13, 14]. Several studies showed that the podocyte has a central role in the development of proteinuria and idiopathic nephrotic syndrome [14]. Eﬀacement of the podocyte foot processes is a common feature of proteinuric diseases [15]. Sev- eral pathogenic pathways involved in eﬀacement of the podocyte foot processes and the development of proteinuria have been discovered. Studies in hereditary proteinuric syndromes have uncovered that mutations of podocyte proteins, including α-actinin-4 [16], CD2AP [16, 17], nephrin [18], PLCE1 [19], podocin [20], TRPC 6 [21, 22], formin protein INF2 [23], and MYO1E [24] lead to protein- uria, podocyte foot processes eﬀacement and podocyte actin cytoskeleton disruption [14, 25]. Other proteins regulate the podocyte actin cytoskeleton and are important for the glomerular ﬁltration barrier [25]. These proteins include Rho GDIalpha [26, 27], podocalyxin [28], FAT1 [29], Nck1/2 [30], and synaptopodin [31]. TRPC6 is a member of the large transient receptor potential superfamily of nonselective cation channels [32, 33]. Mutations in the gene that encodes TRPC6 have been identiﬁed in families with autosomal-dominant FSGS [34– 36]. Many calcium-dependent signals, including calcineurin, may be potential targets of TRPC6 activation in podocytes since TRPC6 are involved in the regulation of intracellular calcium concentration in response to the activation of G- protein-coupled receptors and receptor tyrosine kinases. Meanwhile, some TRPC6 mutations found in human beings with FSGS result in increased amplitude and duration of calcium inﬂux into HeK293 cells [21, 22]. TRPC6-deﬁcient mice do not show any obvious renal phenotype [34]. However, transient induction of TRPC6 overexpression by in vivo gene delivery results in proteinuria [33]. The actin-binding protein synaptopodin, which is highly expressed in podocytes [35], is a key regulator of podocyte function since bigenic heterozygosity for synaptopodin and International Journal of Nephrology CD2-associated protein results in proteinuria and FSGS [37]. Synaptopodin induces stress ﬁbers by stabilizing the GTPase RhoA [31] and suppresses ﬁlopodia by disrupting cell-division cycle-42-insulin receptor substrate p53-Mena signaling complexes [38]. Recently, urokinase receptor (uPAR) and its soluble form (suPAR) have been shown to be involved in the pathogenesis of proteinuria and FSGS [39, 40]. uPAR is a glycosylphosphatidylinositol-anchored protein that has been shown to be a proteinase receptor for urokinase but has also been involved in nonproteolytic pathways, mainly through interactions with other plasma membrane proteins such as integrins [41]. uPAR and β3 integrin colocalize in podocytes and thus form a complex with β3 integrin, thereby causing the activation of β3 integrin. In vivo gene delivery of constitutively active β3 integrin is suﬃcient to induce proteinuria in mice; conversely, inhibition of uPAR expression and β3 integrin activation has an antiproteinuric eﬀect [42]. Interestingly, suPAR, a soluble form of uPAR has been identiﬁed as a circulating FSGS factor that is elevated in the serum of approximately two-thirds of primary FSGS patients. suPAR-mediated activation of β3 integrin on podocyte foot processes may be the mechanism of injury caused by high suPAR blood concentrations [40]. 3. The Role of Calcineurin Signal in Podocyte Injury and Proteinuria The calcineurin is ubiquitously expressed in all mam- malian tissues and is a Ca2+-dependent serine/threonine phosphatase composed of a catalytic subunit, CnA, and a regulatory subunit, CnB [43, 44]. Calcineurin has a wide range of roles in organ development and cellular functions [45, 46], including the regulation of transcription in various renal cells [43, 44]. Clinically, CsA is used to reduce proteinuria in kidney diseases, such as idiopathic nephrotic syndrome (especially FSGS). The immunosuppressive eﬀect of CsA results from inhibition of signaling by the transcription factor NFAT in T cells [47], and this action has also been believed to mediate CsA’s antiproteinuric eﬀect. Although the eﬃcacy of CsA was thought to derive from its suppression of NFAT activation in T cells through the inhibition of calcineurin, not all drugs suppressing T-cell activation have protective eﬀects on glomeruli [1]. The study of Faul et al. [7] suggested a novel role for CsA in inhibiting the dephosphorylation of synaptopodin by calcineurin. Faul et al. [7] found that activation of calcineurin in the podocyte is suﬃcient to cause proteinuria via the degradation of synaptopodin and that CsA blocks the calcineurin-mediated dephosphorylation of synaptopodin, thereby preserving the phosphorylation- dependent synaptopodin-14-3-3b interaction. They also identiﬁed synaptopodin as a substrate of PKA and CaMKII. CsA and E64 ameliorate LPS-induced proteinuria by block- ing the CatL-mediated degradation of synaptopodin [48– 51]. These data unveiled a calcineurin signaling pathway,