Focal and segmental glomerulosclerosis (FSGS) is a leading cause of glomerulonephritis and chronic kidney disease (CKD) in children and young adults (1). As much as one fifth of the ESRD population carries this diagnosis, and the proportion of ESRD attributed to FSGS has increased more than 10-fold over the past two decades (2). The glomerular landmarks of FSGS can develop secondary to a variety of systemic conditions, including disorders that cause chronic hypoxemia, processes that augment glomerular blood flow, and diseases that reduce renal mass. The structural features of FSGS also develop in patients who lack recognized systemic diseases, so-called primary (or idiopathic) FSGS. Over the past three decades, the recognition of familial forms of FSGS has grown, and this information has been highlighted in recent years with characterization of several human genes that cause FSGS (3).
Most recently, a mutation in a member of the canonical transient receptor potential (TRP) family of proteins, TRPC6, was identified to cause FSGS in a large kindred of autosomal dominant FSGS (4). The mutation in TRPC6, in contrast to genes that have been implicated in the pathogenesis of FSGS, seems to perturb cytosolic calcium ([Ca2+]i) signaling. The purpose of this review is to examine what is known about TRPC6 mutations in familial nephrotic syndrome and to explore how this new knowledge may be applied to the care of patients with glomerular diseases.
Familial Forms of FSGS
Exciting discoveries in the past 10 years have identified numerous genes that are important for human glomerular diseases. The study of Mendelian forms of FSGS has been the catalyst behind these discoveries, lending insight into the pathophysiologic processes that participate in this disease. Reports of familial forms of FSGS date back as far as 1956, with the observation of an autosomal recessive disease primarily within the Finnish population. The disease process is characterized by massive proteinuria in utero, with up to 20 to 30 g/d protein loss (5). Advances in gene mapping and molecular biology within the past decade helped localize the causative gene to chromosome 19q31.1 (6), with the subsequent identification of NPHS1 (7). NPHS1 encodes a gene product termed “nephrin,” within which numerous mutations, including deletions, insertions, nonsense, missense, and splicing errors, have been described (8). Structurally, nephrin is composed of eight Ig C2 motifs, a fibronectin III–like domain, and a single transmembrane segment (Figure 1). Nephrin localizes to lipid “rafts” within the slit diaphragm of the podocyte (9–11). Lipid rafts are specialized microdomains of the plasma membrane with a unique lipid content and a concentrated assembly of signal transduction molecules (12). Localization to lipid rafts suggests that mutations of the nephrin gene likely affect its role in regulating podocyte signaling pathways (13,14).
Confirmation of the role that nephrin plays in the development of glomerular disease is demonstrated by mouse models for congenital nephrotic syndrome of the Finnish type (Finnish nephropathy). Targeted disruption of the nephrin gene in embryonic stem cells has resulted in mice homozygous for the mutant NPHS1 allele, and the phenotype of these mice mirrors the human disease with the early onset of massive proteinuria and neonatal death (15). Additional mouse models that mimic recurrence of congenital nephrotic syndrome in humans after renal transplant have been developed (16,17). These models have been created by injection of mice with mAb directed toward the extracellular domain of nephrin (18). The biologic effects of the anti-mouse nephrin antibodies highlight the importance of nephrin and the slit diaphragm in the regulation of glomerular permselectivity (18).
Steroid-resistant nephrotic syndrome is another human disorder that is characterized by autosomal recessive nephrotic syndrome. This disorder manifests between 3 mo and 5 yr of age, rapid progression to ESRD, and with few cases of recurrence after renal transplantation. The gene product is podocin (NPHS2), located on 1q25–31 (19). The structure of podocin is characterized by a single membrane domain forming a hairpin-like structure, with both N- and C-terminal domains in the cytosol. Podocin is expressed exclusively in podocytes (20), and specifically localizes to the base of the foot processes on either side of the slit diaphragm as seen on electron microscopy (21). Mice that lack the podocin gene product have a prominent renal phenotype that includes proteinuria and early death from renal failure (22). Podocin most likely functions in the structural organization of the slit diaphragm and regulation of its filtration function (20). It has been shown to interact in vivo with both nephrin and CD2-associated protein (CD2AP), a cytoplasmic binding partner of nephrin (23). Podocin recruits nephrin to lipid rafts in the slit diaphragm (13,23,24). Recruitment of nephrin to lipid rafts, through the direct interaction of podocin with its cytoplasmic tail, is required for the proper initiation of nephrin signaling. Targeted disruptions of podocin inhibits both nephrin trafficking and nephrin-initiated signal transduction (24).
The nephrin-podocin complex seems crucial to nephrin signal transduction and maintenance of normal podocyte biology, but mutations that compromise the structural integrity of the podocyte also seem to result in foot process effacement and lesions that are characteristic of FSGS. Mutations in the α-actinin 4 gene (ACTN4), which localizes to chromosome 19q13, have been associated with autosomal dominant FSGS, characterized by adult-onset disease of variable severity and rate of progression to ESRD (25,26). ACTN4 is one of four actinin genes, which encodes a 100-kD actin cross-linking protein. Although ACTN4 is expressed in a wide range of tissues, it is very highly expressed in podocytes, functioning in the regulation of the podocyte cytoskeleton. Transgenic mice that express mutant ACTN4 as well as ACTN4-deficient mice develop renal disease that resembles FSGS (27,28). Fractions of the mutant protein have been shown to form large aggregates within podocytes, ultimately compromising the function of the normal actin cytoskeleton, through both its abnormal function and its toxic accumulation (29).
The most recently reported disease-causing mutation for hereditary FSGS has been localized to chromosome 11q21-22, with the subsequent identification by Winn et al. (30) of transient receptor potential cation channel, subfamily C, member 6 (TRPC6) as the disease-causing gene. The missense mutation changes a highly conserved proline in the first ankyrin repeat of TRPC6 to a glutamine at position 112 (P112Q). Additional work was reported by Reiser et al. (31), who corroborated findings implicating TRPC6 in the pathogenesis of familial FSGS. Whereas previously reported mutations such as NPHS1, NPHS2, and ACTN4 have emphasized the importance of cytoskeletal and structural proteins in glomerular diseases, TRPC6-related FSGS suggests an additional mechanism for renal disease pathogenesis. Knowledge of TRPC6-mediated calcium entry into cells may offer unique insights into therapeutic options for glomerular diseases.
Biology of TRP Channels
The TRP channels have an extensive family of mammalian homologues (32). These channels have been implicated in diverse biologic functions such as cell growth, ion homeostasis, mechanosensation, and phospholipase C (PLC)-dependent calcium entry into cells. TRP channels were originally identified in the Drosophila visual system (33). It was observed that in Drosophila with the mutated TRP channel, blindness after intense and prolonged light exposure as a result of sustained Ca2+ entry occurred (34). There are common structural motifs in TRP channels. In addition to sequence homology, they are permeable to cations, especially calcium. The common feature among these channels is six transmembrane-spanning domains. The fifth and sixth transmembrane domains form tetramers that line the pore of the ion channel, and both the N- and C-termini are intracellular. Another frequent feature in some TRP channels is ankyrin binding repeats in the N-terminus.
The regulation and biologic functions of TRP channels have not yet been defined clearly. Hereafter, we concentrate on the TRPC family of proteins and, more specifically, TRPC6. The TRPC family of proteins is made of seven different proteins with four subfamilies (TRPC1, TRPC3,6,7, TRPC4,5, and TRPC2) based on sequence homology and operative similarities. The TRPC3,6,7 subfamily are nearly 75% identical via sequence homology. The TRPC family of proteins is widely expressed in human tissues. It is known that most TRP channels are nonselective cation channels and permit Na+ as well as Ca2+ entry into cells; the TRPC3,6,7 subfamily has selectivity on the order of PCa/PNa 1.5 to 6:1, with TRPC6 being the most selective (32). TRPC6 can be activated either via a G protein–coupled receptor (GPCR) pathway or by application of exogenous analogues such as diacylglycerol (DAG) and act independently of [Ca2+]i store depletion (35). All members of the TRPC family have inositol 1,4,5 triphosphate receptor (IP3R) binding sites in the C-terminus, and calmodulin binds to this peptide sequence in a Ca2+-dependent manner (36). As such, a mechanism for channel function that is coupled to intracellular calcium concentration can be inferred from these findings. TRPC3,6,7 seem to co-assemble when heterologously expressed (37). It is now also believed that TRP channels form a signaling complex, or “signalplex,” with other molecules such as the scaffolding protein INAD; in turn, INAD interacts with immunophilins (38). TRPC6-deficient mice display a higher contractility in tracheal and aortic rings after stimulation with agonists such as methacholine and phenylephrine (39). They also have a modest elevation in blood pressure; we speculate that this may be due to a vascular phenotype and may have an impact on the human disease. In addition, constitutively active TRPC3 ion channels are upregulated in TRPC6-deficient smooth muscle cells and do not replace TRPC6 functionally, therefore indicating a nonredundant role (39). Given the previous emphasis on structural mutations as a cause for the glomerular pathology seen in FSGS, a new paradigm that features podocyte GPCR and downstream cellular signaling now can be added to the emerging mechanisms for this disorder.
PLC-Coupled Receptors on Podocytes
Vasoactive hormones are important regulators of glomerular ultrafiltration and may promote glomerular injury in disease states (40). As a result, the receptor systems and effector pathways that are activated by these hormones have been an intense area of investigation. The effects of vasoactive agents are typically mediated by binding to cell surface receptors that belong to the large superfamily of seven transmembrane-spanning receptor proteins (40,41). As shown in Table 1, podocytes express numerous heptahelical receptor systems (43–61). The majority of these receptors activate intracellular effector pathways (Table 1) by coupling to G proteins (41,62). An important exception may be the type 2 angiotensin II receptor (AT2). Cloning of the AT2 receptor (63,64) suggested that it belonged to a unique group of heptahelical receptors that do not couple to G proteins (64).
The precise role of GPCR in regulating podocyte function is incompletely understood. A large body of evidence, however, suggests that vasoactive hormones modulate glomerular filtration by modulating efferent and afferent arterial tone as well as altering the glomerular ultrafiltration coefficient Kf (40). A role for the podocyte in altering Kf has been proposed by several investigators (40,65,66). In this regard, the podocyte foot process contains a contractile apparatus (67) that may respond to vasoactive hormones (40,65,66). Although direct evidence for such a contractile response is lacking, it has been postulated that Kf may be regulated, in part, by modulating the size of the filtration slits through both calcium- and cAMP-dependent signaling pathways (40,65,66). As shown in Table 1, a large number of GPCR in podocytes are coupled to either cAMP or calcium signaling cascades; therefore, these GPCR systems have the potential to modulate podocyte contractility and may play a role in disease pathogenesis. Because many of these GPCR systems are Gq coupled they have the capacity to activate TRPC6 (68). The Km values suggests that GPCR on podocytes have a high affinity for ligand; therefore, the receptors are physiologically relevant binding sites. The Bmax provides an index of receptor density that, in most receptor systems, correlates with the magnitude of physiologic response to agonist.
Vasoactive hormones, such as angiotensin II (Ang II), also have the capacity to modulate the surface charge of the podocyte and, in turn, alter glomerular permselectivity (69). This change in glomerular permselectivity may play a role in disease pathogenesis by enhancing protein excretion and progression of kidney injury in disease states (69,70). Indeed, GPCR systems expressed by podocytes are likely to play an important role in glomerular disease processes as discussed below.
Glomerular podocytes express numerous GPCR that are implicated in the pathogenesis of glomerular diseases, including receptors for Ang II (AT1 receptor), thromboxane, E-series prostaglandins (EP1 receptor), endothelin, and cysteinyl-leukotrienes (71–76). Although the signaling pathways that are activated by these GPCR are diverse, common to all of these receptor systems is activation of PLCβ through G proteins that belong to the Gq family (71,72,77–80). Although the precise mechanisms are incompletely understood, Gq-coupled receptors are potent activators of TRPC6 (4,31,35,62,81). This activation seems to be mediated by stimulation of PLCβ and generation of the second messengers DAG and inositol 1,4,5-trisphosphate (IP3) (82). Both DAG analogs and IP3 enhance calcium conductance by TRPC6 (4,31,35,62,81). Accumulating evidence suggests that the effects of IP3 on TRPC6 activity may be mediated by binding to a complex that is composed of TRPC6 and the IP3 receptor (68,83) and/or by IP3-stimulated release of calcium from the intracellular stores (68,84). This activation process causes redistribution of TRPC6 from the intracellular pool to the plasma membrane and, in turn, produces a sustained increase in [Ca2+]i levels (68,85).
How the TRPC6P112Q Mutation Causes FSGS
Cytoskeletal and structural proteins have previously been recognized as important in hereditary proteinuric kidney diseases. The implication of a calcium channel in the pathogenesis of FSGS suggests an altogether different cause of glomerular disease pathogenesis. Expression of TRPC6 has been determined to be ubiquitous in the kidney, including podocytes and endothelial and tubular cells (4,86). Expression of TRPC6 in glomeruli is particularly noteworthy as abnormal podocyte function seems to be a final common pathway in a variety of proteinuric kidney diseases (87).
In addition, the TRPC6P112Q mutation causes markedly increased and prolonged calcium influx into cells; biotinylation experiments show an altered subcellular localization of the mutant TRPC6 protein in cells that are transfected with either the wild-type or mutant protein (4). The relative distribution of TRPC6P112Q protein in the plasma membrane is significantly greater than wild-type protein. Increased calcium transients as a result of mutations in TRPC6 are in accordance with reports by others (31).
The importance of Ang II as a mediator of kidney injury was enumerated above. The P112Q mutation does indeed affect Ang II–dependent calcium signaling. This mutation causes higher peak intracellular Ca2+ changes in TRPC6P112Q transfected cells in response to stimulation by Ang II. It is interesting that when the analogous mutation in TRPC3 is introduced in HEK 293 cells, the same increase in TRPC3-mediated calcium entry is observed after Ang II stimulation.
Additional work by others has substantiated the above findings. Recently, Reiser et al. also demonstrated evidence that TRPC6 is an important component of the slit diaphragm (31). These authors also found evidence of mutations in TRPC6 in five families with familial FSGS. They likewise found evidence that TRPC6 is expressed throughout the kidney and specifically in podocytes in the kidney. Immunogold labeling revealed TRPC6 in the major and minor foot processes of the podocyte. TRPC6 co-localized with CD2AP, nephrin, and podocin in cultured mouse podocytes and co-immunoprecipitated with nephrin and podocin. TRPC6 seemed to be upregulated in 2-d-old nephrin-deficient mice. Electrophysiology studies confirmed augmented calcium influx in two of five families. That mutations in either the N- or the C-terminus of the TRPC6 protein cause the same functional changes (an increase in calcium transients) suggests that multiple mechanisms involving TRPC6 abnormalities exist, such as dysregulation of the ion channel, or altered interaction with other slit-diaphragm proteins, with the supposition that this results in disrupted glomerular cell function or causes apoptosis.
It is unclear how mutations in TRPC6 cause FSGS. One could posit many different possibilities. Calcium as a second messenger is a potent effector of cellular functions such as contraction, volume regulation, immunologic responses, cell migration, and proliferation. It is known that intracellular calcium concentrations are tightly regulated. Exaggerated calcium signaling conferred by the TRPC6P112Q mutation may disrupt glomerular cell function or may cause apoptosis (31). Injurious signals that are triggered by Ang II and are known to promote kidney injury and proteinuria may be amplified by the mutant protein. Podocytes are known to be highly dynamic as they adjust to glomerular filtration pressures. Perhaps TRPC6 is unable to guide needed protein chaperones such as nephrin or podocin in sealing the filtration barrier in response to mechanical forces (31,88,89). Another plausible mechanism is that the enhanced and sustained increases in intracellular calcium induced by the TRPC6 mutation activate the calcium-dependent phosphatase calcineurin. Calcineurin is linked directly to induction of apoptosis through dephosphorylation of the protein BAD (90,91).
A potentially key observation in these studies is that the TRPC6 mutations cause sustained increases in [Ca2+]i levels only after agonist stimulation without appreciably altering basal [Ca2+]i levels (4). The dependence on agonist stimulation for increased [Ca2+]i levels indicates that therapies that target the relevant GPCR systems may be useful for slowing the development of renal disease in predisposed individuals. Alternatively, strategies that target final common signaling cascades such as TRPC6 might be an attractive strategy for reducing podocyte injury induced by activation of multiple GPCR systems.
These findings, taken together with previously published studies highlight the importance of [Ca2+]i in podocyte biology. The ability of the podocyte to regulate precisely [Ca2+]i levels seems to play a central role in glomerular disease processes. Because ion channels are generally amenable to pharmacologic manipulation, these studies raise the exciting possibility that manipulating [Ca2+]i levels by targeting TRPC6 may be a useful strategy for treating not only familial disease but also patients with other primary and secondary forms of FSGS.
Native and Mutated TRPC6 in Primary FSGS and Other Chronic Glomerular Diseases
The association between TRPC6 mutations and FSGS will improve our understanding of the pathogenesis of familial forms of glomerulonephritis, but this discovery also advances two hypotheses that may pertain to patients with other forms of progressive glomerular diseases: (1) That adaptation of native TRPC6 channels underlies effective therapies for many progressive glomerular diseases and (2) that the cell-signaling cascade that governs [Ca2+]i, including activity of TRPC6 channels, is critical to maintain the health of the podocyte in CKD.
Because some TRPC6 mutations result in FSGS and exaggerated responses in intracellular calcium, it is attractive to consider that the TRPC6 itself serves as the ideal therapeutic target. Electrophysiologic studies define TRPC6 channels as nonselective for cations with relatively low selectivity for Ca2+ over Na+ (81). As previously discussed, TRPC6 activity is stimulated by analogues of DAG, enhanced intracellular calcium levels, and/or activation of a complex of the IP3 receptor and TRPC6 (4,31,35,68,81,83,92). It is interesting that DAG analogues increase calcium conductance by TRPC6 independent of protein kinase C activity, a common target of DAG (81). Additional knowledge of the TRPC6 activation site or its mode of activation by binding of the inositol 1,4,5-trisphosphate receptor may provide new targets to inactivate the channel (93). The enthusiasm to develop specific TRPC6 antagonists must be balanced, however, by a few observations. Although the receptor-activated calcium transients may be dramatically enhanced in some TRPC6 mutants, the TRPC channels admit relatively small amounts of calcium per channel (94). In addition, because members of the TRPC3, 6, and 7 families share electrophysiologic characteristics, the gene products may serve some redundant roles (95). Therefore, any pharmacologic agent that is effective for blocking TRPC6 may also need to block TRPC3 and TRPC7, which are very similar in function to TRPC6. Regardless, development of specific pharmacologic inhibitors for TRPC6 will be eagerly anticipated.
Normal function of the podocyte TRPC6 channel may be responsible for at least part of the renoprotective effects of medications that block the renin-angiotensin system. Benefits of angiotensin-converting enzyme inhibitors (ACEi) and AT1 receptor blockers (ARB) are that they reduce proteinuria and limit progression of renal disease not only in FSGS but also in chronic glomerular diseases such as diabetic nephropathy and IgA nephropathy (92,96–99). The ACEi also reduce protein excretion and potentially limit progression to ESRD in renal diseases that manifest with less dramatic proteinuria, ostensibly when the podocyte is less disrupted, including hypertensive nephrosclerosis and autosomal dominant polycystic disease (100,101). As detailed above, TRPC6 channels and their activation participate in both the amplitude and the duration of [Ca2+]i responses in the podocyte after agonist activation of the AT1 receptor. Effective blockade of this response with ACEi and ARB has been the most effective and consistent pharmacologic interventions to limit progression of many forms of CKD.
Activity of TRPC6 may also pertain to glucocorticoid therapy, long considered the first effective line of therapy in idiopathic FSGS. When used either intravenously or orally, steroids have pleiotropic effects (102,103). Glucocorticoids bind to cytoplasmic receptors that translocate to the nucleus, where the transcription of certain genes is modified, including genes for cytokines, chemokines, eicosanoids, and other pathways that participate in the inflammatory response (104). As addressed above, a number of eicosanoid products activate GPCR on podocytes and are capable of subsequently increasing TRPC6 activity. Steroids also exert responses at the level of the glomerulus by modifying podocyte metabolism. Glucocorticoids promote podocyte growth and migration in culture and may increase expression of the glucocorticoid receptor (105,106). Recently, dexamethasone was demonstrated to protect podocytes in culture when they were treated with puromycin, an exposure that causes apoptosis of glomerular epithelial cells and causes nephrotic syndrome in experimental animals (107,108). Although the importance of TRPC6 for the steroid response has not been characterized, intracellular calcium influx is widely known to be an important step in apoptosis in the injured renal cell (109).
Nearly 60% of patients who have primary FSGS and receive glucocorticoid therapy for as long as 6 months will be resistant to therapy, will continue to exhibit proteinuria, and will carry a disproportionate risk to develop ESRD (110). These at-risk patients warrant other medical interventions. The North American FSGS Trial, performed in a population of patients with steroid-resistant FSGS, suggested that a 6-month course of cyclosporine (CsA) combined with prednisone will reduce the amount of proteinuria and preserve kidney function (compared with prednisone alone) (111). Clinical trials that were composed of smaller patient groups also suggested that tacrolimus (FK506) may have similar clinical effect to CsA, to increase the likelihood of achieving clinical remission in patients who have FSGS and are resistant to steroid therapy (112). The modes of action of FK506 and CsA require that the drugs bind to the intracellular immunophilins FKBP-12 and cyclophilin, respectively, and that the immunophilin complex interact and inhibit the [Ca2+]i-dependent phosphatase calcineurin (113). Effectiveness of these medications to suppress the immune system and to induce remission in FSGS is traditionally attributed to reduction of calcineurin activity and the resulting decrease in cytokine promoter activity (113,114). Alternatively, effectiveness of calcineurin inhibitors in FSGS may rely on modification of native TRP channels in the podocyte. The immunophilin targets for CsA and FK506 are capable of binding with INAD:TRP complexes (described above). Therefore, the clinical benefit of calcineurin inhibitors in patients with steroid-resistant FSGS may be mediated by interactions with the immunophilin FKBP-12 and TRPC6 (93).
The especially poor prognosis of patients with steroid-resistant FSGS has driven clinical investigators to explore other effective treatments. The National Institutes of Health is currently enrolling these high-risk patients in a trial in that will compare outcomes in groups that are treated with a CsA-based regimen (similar to that used in the North American FSGS Trial outlined above) or with high-dose pulse oral steroids and mycophenolate (68). As a component of the ancillary studies of this trial, genotyping for genetic mutations associated with FSGS, including TRPC6, may provide an opportunity to determine the prevalence of known genetic mutations that are associated with steroid-resistant idiopathic FSGS.
Utility of Genetic Typing of Patients with CKD
Discoveries in the past few years have successfully identified TRPC6 and other genes that impart renal risk. Despite these advances, recognition of genes that are responsible for the majority of cases of ESRD remains relatively elusive. This likely results from the fact that renal failure is a polygenic disease that manifests only after specific combinations of environmental exposures (115). In addition, ESRD is the final end point of a wide variety of illnesses, and it is nai[Combining Diaeresis]ve to anticipate that a single causative human gene can be identified. The clinical utility of a mythical “ESRD gene” may also be limited by comorbid conditions that are associated with all stages of CKD. The National Kidney Foundation proposed four categories of risk factors when considering adverse outcomes in CKD: (1) Factors that increase susceptibility to CKD; (2) factors that initiate kidney damage; (3) factors that promote progression of CKD; and (4) factors that predispose to complications of ESRD (116). If information from new “renal failure” genes such as TRPC6 will influence clinical management of patients with CKD and not be limited to those with FSGS, then it may be more realistic to apply this new information to one of these risk categories. Because the penetrance of FSGS in individuals with the TRPC6 mutation is relatively high in the published descriptions to date, it may be that polymorphisms of TRPC6 act as susceptibility or initiation factors for renal disease. However, additional genetic data need to be gathered in the human FSGS population.
The heterogeneous clinical outcome of patients with primary FSGS suggests that knowledge of a genetic basis of renal disease, including TRPC6 status, could have clinical utility. Baseline criteria of patients who reach a diagnosis of primary FSGS, including race, degree of proteinuria, histologic presentation, and baseline GFR, can be used to determine those who are at risk to develop ESRD (117,118). Responses to treatment, most importantly, complete or partial regression of proteinuria, also correlate with long-term renal prognosis (118). Because only part of the risk for attaining ESRD seems explainable by these baseline and treatment variables, characterization of TRPC6 (and perhaps other causative genes) may aid early identification of at-risk patients with FSGS. Previous knowledge of TRPC6 status may also permit tailoring of initial medication choices in primary FSGS. Status of podocin mutations may be able to predict responsiveness to conventional steroid treatment, but it is not yet clear whether TRPC6 will carry similar implications (119). After attainment of ESRD, however, patients who have TRPC6 mutations and receive a renal allograft are less likely to develop recurrent FSGS. Therefore, TRPC6 characterization may be helpful in preoperative screening of potential kidney transplant recipients and also in potential living-related donors in families that carry the disease.
The clinical responses in other chronic glomerular diseases are similarly varied, and genetic factors such as TRPC6 may explain the inconsistent outcomes. Genotyping may prove to be valuable in this large population of patients. For example, the status of ACE gene polymorphisms has been proposed as a way to determine whether a patient with early diabetes may be at disproportionate risk to develop diabetic nephropathy (120). A clinician might use information regarding TRPC6 status to consider relatively innocuous interventions such as lifestyle modifications or more aggressive management such as early ACEi or ARB therapy. Because many TRPC6 mutations described thus far are associated with exaggerated cytosolic calcium responses, particularly to AT1 receptor activation, these patients may also be considered for regimens that include combination therapy with ACEi and ARB (92). These patients might also be considered for antagonists for other podocyte GPCR when they are developed in the future.
Conclusion
Over the past 10 years, familial nephrotic syndromes have shed significant insight into normal podocyte homeostasis. Mutations involving the podocyte proteins nephrin, podocin, CD2AP, and α-actinin 4 have highlighted the importance of the maintenance of podocyte structure in the pathogenesis of glomerular disease. With the surprise implication of TRPC6 in familial nephrotic syndrome, aberrant calcium signaling and the role of GPCR in regulating podocyte function will now be at the forefront of investigation for developing targeted therapies against FSGS, a disease that has been notoriously difficult to treat. Precise regulation of podocyte [Ca2+]i suggests that perhaps targeting TRPC6 agonists or even direct manipulation of TRPC6 channels themselves will prove to be an effective strategy for treating glomerular diseases. Although knowledge regarding the function of podocyte proteins and their role in maintaining normal cell-signaling cascades will undoubtedly progress, further understanding in the clinical arena regarding the polygenic influences and risk factors that are involved in determining rates of disease progression and response to therapy will need to be ascertained in a standardized manner to permit tailoring of therapy in individuals. Since the initial descriptions of familial nephrotic syndrome made decades ago, considerable progress has been made regarding the importance of normal podocyte function on a molecular level. Optimistically, within the next decade, we will see a translation of this knowledge, imparting long-lasting clinical and therapeutic implications.
;)
Figure 1: Proposed diagram of a podocyte foot process and various podocyte proteins with an important role in hereditary nephrotic syndromes. Transient receptor potential cation channel 6 (TRPC6) is thought to interact with CD2-associated protein (CD2AP), podocin and nephrin. ACTN4, α-actinin 4; F-actin, filamentous actin; GPCR, G protein–coupled receptor. Illustration by Josh Gramling—Gramling Medical Illustrations.
Table 1: Heptahelical receptors that may modulate TRPC6 function in podocytesa
Published online ahead of print. Publication date available at www.jasn.org.
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