Gain-of-function mutations in transient receptor potential C6 (TRPC6) activate extracellular signal-regulated kinases 1/2 (ERK1/2)

Lactate modulates iron metabolism by binding soluble adenylyl cyclase

Overproduction of lactate (LA) can occur during exercise and in many diseases such as cancers. Individuals with hyperlactatemia often display anemia, decreased serum iron, and elevated hepcidin, a key regulator of iron metabolism. However, it is unknown whether and how LA regulates hepcidin expression. Here, we show LA binds to soluble adenylyl cyclase (sAC) in normal hepatocytes and affects systemic iron homeostasis in mice by increasing hepcidin expression. Comprehensive in vitro, in vivo, and in silico experiments show that the LA-sAC interaction raises cyclic adenosine monophosphate (cAMP) levels, which activates the PKA-Smad1/5/8 signaling pathway to increase hepcidin transcription. We verified this regulatory axis in wild-type mice and in mice with disordered iron homeostasis. LA also regulates hepcidin in humans at rest and subjected to extensive exercise that produce elevated LA. Our study links hyperlactatemia to iron deficiency, offering a mechanistic explanation for anemias seen in athletes and patients with lactic acidosis.

TRPC6 promotes daunorubicin-induced mitochondrial fission and cell death in rat cardiomyocytes with the involvement of ERK1/2-DRP1 activation

Daunorubicin (DNR-) induced cardiotoxicity seriously restricts its clinical application. Transient receptor potential cation channel subfamily C member 6 (TRPC6) is involved in multiple cardiovascular physiological and pathophysiological processes. However, the role of TRPC6 anthracycline-induced cardiotoxicity (AIC) remains unclear. Mitochondrial fragmentation greatly promotes AIC. TRPC6-mediated ERK1/2 activation has been shown to favor mitochondrial fission in dentate granule cells. The aim of the present study was to elucidate the effects of TRPC6 on daunorubicin- induced cardiotoxicity and identify the mechanisms associated with mitochondrial dynamics. The sparkling results showed that TRPC6 was upregulated in models in vitro and in vivo. TRPC6 knockdown protected cardiomyocytes from DNR-induced cell apoptosis and death. DNR largely facilitated mitochondrial fission, boosted mitochondrial membrane potential collapse and damaged debilitated mitochondrial respiratory function in H9c2 cells，these effects were accompanied by TRPC6 upregulation. siTRPC6 effectively inhibited these mitochondrial adverse aspects showing a positive unexposed effect on mitochondrial morphology and function. Concomitantly, ERK1/2-DRP1 which is related to mitochondrial fission was significantly activated with amplified phosphorylated forms in DNR-treated H9c2 cells. siTRPC6 effectively suppressedERK1/2-DPR1 over activation, hinting at a potential correlation between TRPC6 and ERK1/2-DRP1 by which mitochondrial dynamics are possibly modulated in AIC. TRPC6 knockdown also raised the Bcl-2/Bax ratio, which may help to block mitochondrial fragmentation-related functional impairment and apoptotic signaling. These findings suggested an essential role of TRPC6 in AIC by intensifying mitochondrial fission and cell death via ERK1/2-DPR1, which could be a potential therapeutic target for AIC.

Ion channels and channelopathies in glomeruli

An essential step in renal function entails the formation of an ultrafiltrate that is delivered to the renal tubules for subsequent processing. This process, known as glomerular filtration, is controlled by intrinsic regulatory systems and by paracrine, neuronal, and endocrine signals that converge onto glomerular cells. In addition, the characteristics of glomerular fluid flow, such as the glomerular filtration rate and the glomerular filtration fraction, play an important role in determining blood flow to the rest of the kidney. Consequently, disease processes that initially affect glomeruli are the most likely to lead to end-stage kidney failure. The cells that comprise the glomerular filter, especially podocytes and mesangial cells, express many different types of ion channels that regulate intrinsic aspects of cell function and cellular responses to the local environment, such as changes in glomerular capillary pressure. Dysregulation of glomerular ion channels, such as changes in TRPC6, can lead to devastating glomerular diseases, and a number of channels, including TRPC6, TRPC5, and various ionotropic receptors, are promising targets for drug development. This review discusses glomerular structure and glomerular disease processes. It also describes the types of plasma membrane ion channels that have been identified in glomerular cells, the physiological and pathophysiological contexts in which they operate, and the pathways by which they are regulated and dysregulated. The contributions of these channels to glomerular disease processes, such as focal segmental glomerulosclerosis (FSGS) and diabetic nephropathy, as well as the development of drugs that target these channels are also discussed.

Store-operated Ca2+ channel signaling: Novel mechanism for podocyte injury in kidney disease

Studies over the last decade have markedly broadened our understanding of store-operated Ca2+ channels (SOCs) and their roles in kidney diseases and podocyte dysfunction. Podocytes are terminally differentiated glomerular visceral epithelial cells which are tightly attached to the glomerular capillary basement membrane. Podocytes and their unique foot processes (pedicels) constitute the outer layer of the glomerular filtration membrane and the final barrier preventing filtration of albumin and other plasma proteins. Diabetic nephropathy and other renal diseases are associated with podocyte injury and proteinuria. Recent evidence demonstrates a pivotal role of store-operated Ca2+ entry (SOCE) in maintaining structural and functional integrity of podocytes. This article reviews the current knowledge of SOCE and its contributions to podocyte physiology. Recent studies of the contributions of SOC dysfunction to podocyte injury in both cell culture and animal models are discussed, including work in our laboratory. Several downstream signaling pathways mediating SOC function in podocytes also are examined. Understanding the pivotal roles of SOC in podocyte health and disease is essential, as SOCE-activated signaling pathways are potential treatment targets for podocyte injury-related kidney diseases.

Pharmacological TRPC6 inhibition improves survival and muscle function in mice with Duchenne muscular dystrophy

Gene mutations causing loss of dystrophin result in the severe muscle disease known as Duchenne muscular dystrophy (DMD). Despite efforts at genetic repair, DMD therapy remains largely palliative. Loss of dystrophin destabilizes the sarcolemmal membrane, inducing mechanosensitive cation channels to increase calcium entry and promote cell damage and, eventually, muscle dysfunction. One putative channel is transient receptor potential canonical 6 (TRPC6); we have shown that TRPC6 contributed to abnormal force and calcium stress-responses in cardiomyocytes from mice lacking dystrophin that were haplodeficient for utrophin (mdx/utrn+/- [HET] mice). Here, we show in both the HET mouse and the far more severe homozygous mdx/utrn-/- mouse that TRPC6 gene deletion or its selective pharmacologic inhibition (by BI 749327) prolonged survival 2- to 3-fold, improving skeletal and cardiac muscle and bone defects. Gene pathways reduced by BI 749327 treatment most prominently regulated fat metabolism and TGF-β1 signaling. These results support the testing of TRPC6 inhibitors in human trials for other diseases as a novel DMD therapy.

Gain-of-function, focal segmental glomerulosclerosis Trpc6 mutation minimally affects susceptibility to renal injury in several mouse models

Mutations in TRPC6 are a cause of autosomal dominant focal segmental glomerulosclerosis in humans. Many of these mutations are known to have a gain-of-function effect on the non-specific cation channel function of TRPC6. In vitro studies have suggested these mutations affect several signaling pathways, but in vivo studies have largely compared wild-type and Trpc6-deficient rodents. We developed mice carrying a gain-of-function Trpc6 mutation encoding an E896K amino acid change, corresponding to a known FSGS mutation in TRPC6. Homozygous mutant Trpc6 animals have no appreciable renal pathology, and do not develop albuminuria until very advanced age. The Trpc6E896K mutation does not impart susceptibility to PAN nephrosis. The animals show a slight delay in recovery from the albumin overload model. In response to chronic angiotensin II infusion, Trpc6E896K/E896K mice have slightly greater albuminuria initially compared to wild-type animals, an effect that is lost at later time points, and a statistically non-significant trend toward more glomerular injury. This phenotype is nearly opposite to that of Trpc6-deficient animals previously described. The Trpc6 mutation does not appreciably impact renal interstitial fibrosis in response to either angiotensin II infusion, or folate-induced kidney injury. TRPC6 protein and TRPC6-agonist induced calcium influx could not be detected in glomeruli. In sum, these findings suggest that a gain-of-function Trpc6 mutation confers only a mild susceptibility to glomerular injury in the mouse.

Trpc6 gain-of-function disease mutation enhances phosphatidylserine exposure in murine platelets

Platelets enhance coagulation by exposing phosphatidylserine (PS) on their cell surface in response to strong agonist activation. Transient receptor potential channels, including TRPC6, have been implicated in the calcium influx central to this process. Here, we characterize the effect of a Trpc6 gain-of-function (GOF) disease-associated, and a dominant negative (DN), mutation on murine platelet activation. Platelets from mice harboring Trpc6^E896K/E896K (GOF) and Trpc6^DN/DN mutations were subject to in vitro analysis. Trpc6^E896K/E896K and Trpc6^DN/DN mutant platelets show enhanced and absent calcium influx, respectively, upon addition of the TRPC3/6 agonist GSK1702934A (GSK). GSK was sufficient to induce integrin αIIbβ3 activation, P-selection and PS exposure, talin cleavage, and MLC2 phosphorylation in Trpc6^E896K/E896K, but not in wild-type, platelets. Thrombin-induced calcium influx and PS exposure were enhanced, and clot retraction delayed, by GOF TRPC6, while no differences were noted between wild-type and Trpc6^DN/DN platelets. In contrast, Erk activation upon GSK treatment was absent in Trpc6^DN/DN, and enhanced in Trpc6^E896K/E896K, platelets, compared to wild-type. The positive allosteric modulator, TRPC6-PAM-C20, and fluoxetine maintained their ability to enhance and inhibit, respectively, GSK-mediated calcium influx in Trpc6^E896K/E896K platelets. The data demonstrate that gain-of-function mutant TRPC6 channel can enhance platelet activation, including PS exposure, while confirming that TRPC6 is not necessary for this process. Furthermore, the results suggest that Trpc6 GOF disease mutants do not simply increase wild-type TRPC6 responses, but can affect pathways not usually modulated by TRPC6 channel activity, displaying a true gain-of-function phenotype.

The Zebrafish Model to Understand Epigenetics in Renal Diseases

Epigenetic modifications are able to alter gene expression and include DNA methylation, different histone variants, and post-transcriptional modifications (PTMs), such as acetylation or phosphorylation, and through short/long RNAs, respectively. In this review, we focus on current knowledge concerning epigenetic modifications in gene regulation. We describe different forms of epigenetic modifications and explain how epigenetic changes can be detected. The relevance of epigenetics in renal diseases is highlighted with multiple examples and the use of the zebrafish model to study glomerular diseases in general and epigenetics in renal diseases in particular is discussed. We end with an outlook on how to use epigenetic modifications as a therapeutic target for different diseases. Here, the zebrafish model can be employed as a high-throughput screening tool not only to discover epigenetic alterations contributing to disease, but also to test novel substances that change epigenetic signatures in vivo. Therefore, the zebrafish model harbors the opportunity to find novel pathogenic pathways allowing a pre-selection of potential targets and compounds to be tested for renal diseases.

The glomerular filtration barrier: a structural target for novel kidney therapies

Loss of normal kidney function affects more than 10% of the population and contributes to morbidity and mortality. Kidney diseases are currently treated with immunosuppressive agents, antihypertensives and diuretics with partial but limited success. Most kidney disease is characterized by breakdown of the glomerular filtration barrier (GFB). Specialized podocyte cells maintain the GFB, and structure-function experiments and studies of intercellular communication between the podocytes and other GFB cells, combined with advances from genetics and genomics, have laid the groundwork for a new generation of therapies that directly intervene at the GFB. These include inhibitors of apolipoprotein L1 (APOL1), short transient receptor potential channels (TRPCs), soluble fms-like tyrosine kinase 1 (sFLT1; also known as soluble vascular endothelial growth factor receptor 1), roundabout homologue 2 (ROBO2), endothelin receptor A, soluble urokinase plasminogen activator surface receptor (suPAR) and substrate intermediates for coenzyme Q10 (CoQ₁₀). These molecular targets converge on two key components of GFB biology: mitochondrial function and the actin-myosin contractile machinery. This Review discusses therapies and developments focused on maintaining GFB integrity, and the emerging questions in this evolving field.

The impact of type III sodium-dependent phosphate transporters (Pit 1 and Pit 2) on podocyte and kidney function

The sodium-dependent phosphate transporters Pit 1 and Pit 2 belong to the solute carrier 20 (SLC20) family of membrane proteins. They are ubiquitously distributed in the human body. Their crucial function is the intracellular transport of inorganic phosphate (Pi) in the form of H₂ PO₄ ^- . They are one of the main elements in maintaining physiological phosphate homeostasis. Recent data have emerged that indicate novel roles of Pit 1 and Pit 2 proteins besides the well-known function of Pi transporters. These membrane proteins are believed to be precise phosphate sensors that mediate Pi-dependent intracellular signaling. They are also involved in insulin signaling and influence cellular insulin sensitivity. In diseases that are associated with hyperphosphatemia, such as diabetes and chronic kidney disease (CKD), disturbances in the function of Pit 1 and Pit 2 are observed. Phosphate transporters from the SLC20 family participate in the calcification of soft tissues, mainly blood vessels, during the course of CKD. The glomerulus and podocytes therein can also be a target of pathological calcification that damages these structures. A few studies have demonstrated the development of Pi-dependent podocyte injury that is mediated by Pit 1 and Pit 2. This paper discusses the role of Pit 1 and Pit 2 proteins in podocyte function, mainly in the context of the development of pathological calcification that disrupts permeability of the renal filtration barrier. We also describe the mechanisms that may contribute to podocyte damage by Pit 1 and Pit 2.

Modulation of Transient Receptor Potential Channels 3 and 6 Regulates Osteoclast Function with Impact on Trabecular Bone Loss

Enhanced osteoclast formation and function is a fundamental cause of alterations to bone structure and plays an important role in several diseases impairing bone quality. Recent work revealed that TRP calcium channels 3 and 6 might play a special role in this context. By analyzing the bone phenotype of TRPC6-deficient mice we detected a regulatory effect of TRPC3 on osteoclast function. These mice exhibit a significant decrease in bone volume per tissue volume, trabecular thickness and -number together with an increased number of osteoclasts found on the surface of trabecular bone. Primary bone marrow mononuclear cells from TRPC6-deficient mice showed enhanced osteoclastic differentiation and resorptive activity. This was confirmed in vitro by using TRPC6-deficient RAW 264.7 cells. TRPC6 deficiency led to an increase of TRPC3 in osteoclasts, suggesting that TRPC3 overcompensates for the loss of TRPC6. Raised intracellular calcium levels led to enhanced NFAT-luciferase reporter gene activity in the absence of TRPC6. In line with these findings inhibition of TRPC3 using the specific inhibitor Pyr3 significantly reduced intracellular calcium concentrations and normalized osteoclastic differentiation and resorptive activity of TRPC6-deficient cells. Interestingly, an up-regulation of TRPC3 could be detected in a cohort of patients with low bone mineral density by comparing micro array data sets of circulating human osteoclast precursor cells to those from patients with high bone mineral density, suggesting a noticeable contribution of TRP calcium channels on bone quality. These observations demonstrate a novel regulatory function of TRPC channels in the process of osteoclastic differentiation and bone loss.

Stretch-Induced Biased Signaling in Angiotensin II Type 1 and Apelin Receptors for the Mediation of Cardiac Contractility and Hypertrophy

The myocardium has an intrinsic ability to sense and respond to mechanical load in order to adapt to physiological demands. Primary examples are the augmentation of myocardial contractility in response to increased ventricular filling caused by either increased venous return (Frank-Starling law) or aortic resistance to ejection (the Anrep effect). Sustained mechanical overload, however, can induce pathological hypertrophy and dysfunction, resulting in heart failure and arrhythmias. It has been proposed that angiotensin II type 1 receptor (AT1R) and apelin receptor (APJ) are primary upstream actors in this acute myocardial autoregulation as well as the chronic maladaptive signaling program. These receptors are thought to have mechanosensing capacity through activation of intracellular signaling via G proteins and/or the multifunctional transducer protein, β-arrestin. Importantly, ligand and mechanical stimuli can selectively activate different downstream signaling pathways to promote inotropic, cardioprotective or cardiotoxic signaling. Studies to understand how AT1R and APJ integrate ligand and mechanical stimuli to bias downstream signaling are an important and novel area for the discovery of new therapeutics for heart failure. In this review, we provide an up-to-date understanding of AT1R and APJ signaling pathways activated by ligand versus mechanical stimuli, and their effects on inotropy and adaptive/maladaptive hypertrophy. We also discuss the possibility of targeting these signaling pathways for the development of novel heart failure therapeutics.

Post-Translational Modification and Natural Mutation of TRPC Channels

Transient Receptor Potential Canonical (TRPC) channels are homologues of Drosophila TRP channel first cloned in mammalian cells. TRPC family consists of seven members which are nonselective cation channels with a high Ca²⁺ permeability and are activated by a wide spectrum of stimuli. These channels are ubiquitously expressed in different tissues and organs in mammals and exert a variety of physiological functions. Post-translational modifications (PTMs) including phosphorylation, N-glycosylation, disulfide bond formation, ubiquitination, S-nitrosylation, S-glutathionylation, and acetylation play important roles in the modulation of channel gating, subcellular trafficking, protein-protein interaction, recycling, and protein architecture. PTMs also contribute to the polymodal activation of TRPCs and their subtle regulation in diverse physiological contexts and in pathological situations. Owing to their roles in the motor coordination and regulation of kidney podocyte structure, mutations of TRPCs have been implicated in diseases like cerebellar ataxia (moonwalker mice) and focal and segmental glomerulosclerosis (FSGS). The aim of this review is to comprehensively integrate all reported PTMs of TRPCs, to discuss their physiological/pathophysiological roles if available, and to summarize diseases linked to the natural mutations of TRPCs.

TRPC6-Mediated ERK1/2 Activation Increases Dentate Granule Cell Resistance to Status Epilepticus Via Regulating Lon Protease-1 Expression and Mitochondrial Dynamics

Transient receptor potential canonical channel-6 (TRPC6) is one of the Ca²⁺-permeable non-selective cation channels. TRPC6 is mainly expressed in dentate granule cell (DGC), which is one of the most resistant neuronal populations to various harmful stresses. Although TRPC6 knockdown evokes the massive DGC degeneration induced by status epilepticus (a prolonged seizure activity, SE), the molecular mechanisms underlying the role of TRPC6 in DGC viability in response to SE are still unclear. In the present study, hyperforin (a TRPC6 activator) facilitated mitochondrial fission in DGC concomitant with increases in Lon protease-1 (LONP1, a mitochondrial protease) expression and extracellular-signal-regulated kinase 1/2 (ERK1/2) phosphorylation under physiological conditions, which were abrogated by U0126 (an ERK1/2 inhibitor) co-treatment. TRPC6 knockdown showed the opposite effects on LONP1 expression, ERK1/2 activity, and mitochondrial dynamics. In addition, TRPC6 siRNA and U0126 evoked the massive DGC degeneration accompanied by mitochondrial elongation following SE, independent of seizure severity. However, LONP1 siRNA exacerbated SE-induced DGC death without affecting mitochondrial length. These findings indicate that TRPC6-ERK1/2 activation may increase DGC invulnerability to SE by regulating LONP1 expression as well as mitochondrial dynamics. Therefore, TRPC6-ERK1/2-LONP1 signaling pathway will be an interesting and important therapeutic target for neuroprotection from various neurological diseases.

Transmembrane insertases and N-glycosylation critically determine synthesis, trafficking, and activity of the nonselective cation channel TRPC6

Transient receptor potential cation channel subfamily C member 6 (TRPC6) is a widely expressed ion channel. Gain-of-function mutations in the human TRPC6 channel cause autosomal-dominant focal segmental glomerulosclerosis, but the molecular components involved in disease development remain unclear. Here, we found that overexpression of gain-of-function TRPC6 channel variants is cytotoxic in cultured cells. Exploiting this phenotype in a genome-wide CRISPR/Cas screen for genes whose inactivation rescues cells from TRPC6-associated cytotoxicity, we identified several proteins essential for TRPC6 protein expression, including the endoplasmic reticulum (ER) membrane protein complex transmembrane insertase. We also identified transmembrane protein 208 (TMEM208), a putative component of a signal recognition particle-independent (SND) ER protein-targeting pathway, as being necessary for expression of TRPC6 and several other ion channels and transporters. TRPC6 expression was also diminished by loss of the previously uncharacterized WD repeat domain 83 opposite strand (WDR83OS), which interacted with both TRPC6 and TMEM208. Additionally enriched among the screen hits were genes involved in N-linked protein glycosylation. Deletion of the mannosyl (α-1,3-)-glycoprotein β-1,2-N-acetylglucosaminyltransferase (MGAT1), necessary for the generation of complex N-linked glycans, abrogated TRPC6 gain-of-function variant-mediated Ca2+ influx and extracellular signal-regulated kinase activation in HEK cells, but failed to diminish cytotoxicity in cultured podocytes. However, mutating the two TRPC6 N-glycosylation sites abrogated the cytotoxicity of mutant TRPC6 and reduced its surface expression. These results expand the targets of TMEM208-mediated ER translocation to include multipass transmembrane proteins and suggest that TRPC6 N-glycosylation plays multiple roles in modulating channel trafficking and activity.

TRPC6 Binds to and Activates Calpain, Independent of Its Channel Activity, and Regulates Podocyte Cytoskeleton, Cell Adhesion, and Motility

BACKGROUND

Mutations in the transient receptor potential channel 6 (TRPC6) gene are associated with an inherited form of FSGS. Despite widespread expression, patients with TRPC6 mutations do not present with any other pathologic phenotype, suggesting that this protein has a unique yet unidentified role within the target cell for FSGS, the kidney podocyte.

METHODS

We generated a stable TRPC6 knockout podocyte cell line from TRPC6 knockout mice. These cells were engineered to express wild-type TRPC6, a dominant negative TRPC6 mutation, or either of two disease-causing mutations of TRPC6, G109S or K874*. We extensively characterized these cells using motility, detachment, and calpain activity assays; immunofluorescence; confocal or total internal reflection fluorescence microscopy; and western blotting.

RESULTS

Compared with wild-type cells, TRPC6^-/- podocytes are less motile and more adhesive, with an altered actin cytoskeleton. We found that TRPC6 binds to ERK1/2 and the actin regulatory proteins, caldesmon (a calmodulin- and actin-binding protein) and calpain 1 and 2 (calcium-dependent cysteine proteases that control the podocyte cytoskeleton, cell adhesion, and motility via cleavage of paxillin, focal adhesion kinase, and talin). Knockdown or expression of the truncated K874* mutation (but not expression of the gain-of-function G019S mutation or dominant negative mutant of TRPC6) results in the mislocalization of calpain 1 and 2 and significant downregulation of calpain activity; this leads to altered podocyte cytoskeleton, motility, and adhesion-characteristics of TRPC6 ^-/- podocytes.

CONCLUSIONS

Our data demonstrate that independent of TRPC6 channel activity, the physical interaction between TRPC6 and calpain in the podocyte is important for cell motility and detachment and demonstrates a scaffolding role of the TRPC6 protein in disease.

Contribution of Coiled-Coil Assembly to Ca2+/Calmodulin-Dependent Inactivation of TRPC6 Channel and its Impacts on FSGS-Associated Phenotypes

BACKGROUND

TRPC6 is a nonselective cation channel, and mutations of this gene are associated with FSGS. These mutations are associated with TRPC6 current amplitude amplification and/or delay of the channel inactivation (gain-of-function phenotype). However, the mechanism of the gain-of-function in TRPC6 activity has not yet been clearly solved.

METHODS

We performed electrophysiologic, biochemical, and biophysical experiments to elucidate the molecular mechanism underlying calmodulin (CaM)-mediated Ca²⁺-dependent inactivation (CDI) of TRPC6. To address the pathophysiologic contribution of CDI, we assessed the actin filament organization in cultured mouse podocytes.

RESULTS

Both lobes of CaM helped induce CDI. Moreover, CaM binding to the TRPC6 CaM-binding domain (CBD) was Ca²⁺-dependent and exhibited a 1:2 (CaM/CBD) stoichiometry. The TRPC6 coiled-coil assembly, which brought two CBDs into adequate proximity, was essential for CDI. Deletion of the coiled-coil slowed CDI of TRPC6, indicating that the coiled-coil assembly configures both lobes of CaM binding on two CBDs to induce normal CDI. The FSGS-associated TRPC6 mutations within the coiled-coil severely delayed CDI and often increased TRPC6 current amplitudes. In cultured mouse podocytes, FSGS-associated channels and CaM mutations led to sustained Ca²⁺ elevations and a disorganized cytoskeleton.

CONCLUSIONS

The gain-of-function mechanism found in FSGS-causing mutations in TRPC6 can be explained by impairments of the CDI, caused by disruptions of TRPC's coiled-coil assembly which is essential for CaM binding. The resulting excess Ca²⁺ may contribute to structural damage in the podocytes.

Role of TRPC6 in Progression of Diabetic Kidney Disease

PURPOSE OF REVIEW

The underlining goal of this review is to offer a concise, detailed look into current knowledge surrounding transient receptor potential canonical channel 6 (TRPC6) in the progression of diabetic kidney disease (DKD).

RECENT FINDINGS

Mutations and over-activation in TRPC6 channel activity lead to the development of glomeruli injury. Angiotensin II, reactive oxygen species, and other factors in the setting of DKD stimulate drastic increases in calcium influx through the TRPC6 channel, causing podocyte hypertrophy and foot process effacement. Loss of the podocytes further promote deterioration of the glomerular filtration barrier and play a major role in the development of both albuminuria and the renal injury in DKD. Recent genetic manipulation with TRPC6 channels in various rodent models provide additional knowledge about the role of TRPC6 in DKD and are reviewed here. The TRPC6 channel has a pronounced role in the progression of DKD, with deviations in activity yielding detrimental outcomes. The benefits of targeting TRPC6 or its upstream or downstream signaling pathways in DKD are prominent.

Epidermal growth factor receptor controls glycogen phosphorylase in T cells through small GTPases of the RAS family

We recently uncovered a regulatory pathway of the muscle isoform of glycogen phosphorylase (PYGM) that plays an important role in regulating immune function in T cells. Here, using various enzymatic, pulldown, and immunoprecipitation assays, we describe signaling cross-talk between the small GTPases RAS and RAP1A, member of RAS oncogene family (RAP1) in human Kit 225 lymphoid cells, which, in turn, is regulated by the epidermal growth factor receptor (EGFR). We found that this communication bridge is essential for glycogen phosphorylase (PYG) activation through the canonical pathway because this enzyme is inactive in the absence of adenylyl cyclase type 6 (ADCY6). PYG activation required stimulation of both exchange protein directly activated by cAMP 2 (EPAC2) and RAP1 via RAS and ADCY6 phosphorylation, with the latter being mediated by Raf-1 proto-oncogene, Ser/Thr kinase (RAF1). Consistent with this model, PYG activation was EGFR-dependent and may be initiated by the constitutively active form of RAS. Consequently, PYG activation in Kit 225 T cells could be blocked with specific inhibitors of RAS, EPAC, RAP1, RAF1, ADCY6, and cAMP-dependent protein kinase. Our results establish a new paradigm for the mechanism of PYG activation, which depends on the type of receptor involved.

Methamphetamine modulates the production of interleukin-6 and tumor necrosis factor-alpha via the cAMP/PKA/CREB signaling pathway in lipopolysaccharide-activated microglia

Methamphetamine (METH) elicits neuroinflammatory effects that may implicate its regulatory role on the microglial immune response. However, the mechanism underlying this remains unclear. In the present study, the effects of METH on lipopolysaccharide (LPS)-induced interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α) productions were tested in BV-2 cells and primary microglial cells. Additionally, western blot analysis was used to examine the phosphorylation of mitogenactivated protein kinases (MAPKs). Next, we detected the alterations in cAMP content and the phosphorylation levels of CREB in microglial cells to determine the involvement of the cAMP/CREB signaling pathway. We also used an adenylyl cyclase (AC) agonist (forskolin) and antagonist (MDL-12330A) and a PKA antagonist (H89) to confirm their participation. We observed that METH alone did not affect the production of IL-6 or TNF-α. In contrast, METH augmented the IL-6 production and inhibited the TNF-α production induced by LPS. A similar effect of forskolin was also observed in BV-2 cells. While MAPK activation was not influenced by METH alone, the LPS-induced phosphorylation of p38, JNK and ERK1/2 were all reduced by METH. Both the concentration of cAMP and the phosphorylation of CREB were increased by METH in LPS-activated microglial cells. The effects of METH were altered by MDL-12330A and H89. Moreover, the inhibition of the phosphorylation of ERK1/2 by METH was also reversed. These results suggest that the differential regulation of IL-6 and TNF-α by METH in LPS-activated microglial cells may be attributable to the cAMP/PKA/CREB signaling pathway.

Analysis of 24 genes reveals a monogenic cause in 11.1% of cases with steroid-resistant nephrotic syndrome at a single center

BACKGROUND

Steroid-resistant nephrotic syndrome (SRNS) is the second most frequent cause of end-stage renal disease (ESRD) among patients manifesting at under 25 years of age. We performed mutation analysis using a high-throughput PCR-based microfluidic technology in 24 single-gene causes of SRNS in a cohort of 72 families, who presented with SRNS before the age of 25 years.

METHODS

Within an 18-month interval, we obtained DNA samples, pedigree information, and clinical information from 77 consecutive children with SRNS from 72 different families seen at Boston Children's Hospital (BCH). Mutation analysis was completed by combining high-throughput multiplex PCR with next-generation sequencing. We analyzed the sequences of 18 recessive and 6 dominant genes of SRNS in all 72 families for disease-causing variants.

RESULTS

We identified the disease-causing mutation in 8 out of 72 (11.1%) families. Mutations were detected in the six genes: NPHS1 (2 out of 72), WT1 (2 out of 72), NPHS2, MYO1E, TRPC6, and INF2. Median age at onset was 4.1 years in patients without a mutation (range 0.5-18.8), and 3.2 years in those in whom the causative mutation was detected (range 0.1-14.3). Mutations in dominant genes presented with a median onset of 4.5 years (range 3.2-14.3). Mutations in recessive genes presented with a median onset of 0.5 years (range 0.1-3.2).

CONCLUSION

Our molecular genetic diagnostic study identified underlying monogenic causes of steroid-resistant nephrotic syndrome in ~11% of patients with SRNS using a cost-effective technique. We delineated some of the therapeutic, diagnostic, and prognostic implications. Our study confirms that genetic testing is indicated in pediatric patients with SRNS.

Klotho suppresses the renin-angiotensin system in adriamycin nephropathy

Backgrounds

Klotho protein interacts with the transforming growth factor β (TGF-β) receptor and Wnt, which contribute to the progression of renal disease, inhibiting their signals. Renal and circulating klotho levels are diminished in chronic kidney disease.

Methods

Experiments were performed to assess whether supplementation of klotho protein could have protective effects on the kidney. Rats were injected with adriamycin (5 mg/kg) and divided into three groups: those treated with vehicle, those treated with klotho protein and those treated with klotho plus 4-benzyl-2-methyl-1,2,4-thiadiazolidine-3,5-dione (TDZD). Rats without adriamycin treatment were used as a control.

Results

Adriamycin reduced the serum klotho concentration and renal expression of klotho and E-cadherin. Adriamycin also increased the renal expression of Wnt, TGF-β, and angiotensinogen, as well as the renal abundance of β-catenin and angiotensin II. Klotho supplementation suppressed adriamycin-induced elevations of β-catenin and angiotensin II with sustained Wnt expression. Combined treatment with klotho and TDZD reversed the klotho-induced improvements in the renal abundance of β-catenin and angiotensin II as well as the expression of TGF-β and angiotensinogen without affecting E-cadherin.

Conclusions

Our data indicate that Wnt is involved in the pathogenesis of adriamycin nephropathy. Furthermore, klotho supplementation inhibited Wnt signaling, ameliorating renal angiotensin II. Finally, klotho protein appears to suppress epithelial-mesenchymal transition by inhibiting TGF-β and Wnt signaling.

Deletion of diacylglycerol-responsive TRPC genes attenuates diabetic nephropathy by inhibiting activation of the TGFβ1 signaling pathway

TRPC6 plays a critical role in proteinuric kidney diseases, and TRPC3 is involved in tubulointerstitial damage and renal fibrosis in obstructed kidneys. Podocyte loss is a characteristic event in diabetic nephropathy (DN). The aim of this study was to examine whether deletion of the closely related diacylglycerol (DAG)-responsive TRPCs in mice (TRPC3/6/7^-/-) affects diabetes-induced renal dysfunction and podocyte loss. We compared urine volume, kidney hypertrophy, glomerular enlargement, albuminuria and podocyte loss between wild type (WT) and TRPC3/6/7^-/- diabetic mice. Finally, we examined whether the TGFβ1 signaling pathway is changed in diabetic WT and TRPC3/6/7^-/- mice. TRPC6 protein in the renal cortex was increased in WT diabetic mice. High glucose (HG) treatment increased TRPC6 expression in human podocytes. TRPC3 protein, however, was not altered in either diabetic mice or HG-treated human podocytes. Although diabetic WT and TRPC3/6/7^-/- mice had similar levels of hyperglycemia, the TRPC3/6/7^-/- diabetic mice showed less polyuria, kidney hypertrophy, glomerular enlargement, albuminuria, and had lost less podocytes compared with WT diabetic mice. In addition, we observed decreased expression of anti-apoptotic Bcl2 and increased expression of pro-apoptotic cleaved caspase 3 in WT diabetic mice, but such changes were not significant in TRPC3/6/7^-/- diabetic mice. Western blot and immunohistochemistry revealed that TGFβ1, p-Smad2/3, and fibronectin were upregulated in WT diabetic mice; however, expression of these signaling molecules was not changed in TRPC3/6/7^-/- diabetic mice. In conclusion, deletion of DAG-responsive TRPCs attenuates diabetic renal injury via inhibiting the upregulation of TGFβ1 signaling in diabetic kidneys.

Therapeutic Effects of FK506 on IgA Nephropathy Rat

BACKGROUND/AIMS

FK506 is an immunosuppressive drug and a calcineurin inhibitor that has been widely used in kidney disease in recent years. FK506 shows a wide range of biological and pharmaceutical effects; however, the mechanism of its anti- proliferative effect has not been well elucidated. An IgA nephropathy (IgAN) model was used to generate a mesangial cell proliferation model. This study aims to examine the effect of FK506 on IgAN rats and the underlying mechanisms.

METHODS

Hematuria, proteinuria and renal function were measured. To observe the pathological conditions, we performed HE (hematoxylin - eosin) and PAS (periodic acid - schiff) staining. Transcription and protein expression levels were detected by qRT - PCR (quantitative real-time polymerase chain reaction) and Wb (western blotting). The location and semi-quantitative expression levels of TRPCs, CaN (Calcineurin) and α-SMA were examined by IHC (Immunohistochemical staining).

RESULTS

We found that FK506 could improve hematuria, proteinuria and renal function, especially in the HF (high-dose FK506) groups. Renal pathological changes were ameliorated in the treatment groups. FK506 could significantly decrease TRPCs, CaN, phosphorylation of ERK1/2 and α-SMA expression.

CONCLUSION

Taken together, these results suggest that the therapeutic effect of FK506 on IgAN might be partially associated with the down-regulated expression of TRPC channels, CaN and phosphorylation of ERK1/2.

Regulation of Gene Transcription Following Stimulation of Transient Receptor Potential (TRP) Channels

Transient receptor potential (TRP) channels belong to a heterogeneous superfamily of cation channels that are involved in the regulation of numerous biological functions, including regulation of Ca²⁺ and glucose homeostasis, tumorigenesis, temperature, and pain sensation. To understand the functions of TRP channels, their associated intracellular signaling pathways and molecular targets have to be identified on the cellular level. Stimulation of TRP channels frequently induces an influx of Ca²⁺ ions into the cells and the subsequent activation of protein kinases. These intracellular signal transduction pathways ultimately induce changes in the gene expression pattern of the cells. Here, we review the effects of TRPC6, TRPM3, and TRPV1 channel stimulation on the activation of the stimulus-responsive transcription factors AP-1, CREB, Egr-1, Elk-1, and NFAT. Following activation, these transcription factors induce the transcription of delayed response genes. We propose that many biological functions of TRP channels can be explained by the activation of stimulus-responsive transcription factors and their delayed response genes. The proteins encoded by those delayed response genes may be responsible for the biochemical and physiological changes following TRP channel activation.

Integration of TRPC6 and NADPH oxidase activation in lysophosphatidylcholine-induced TRPC5 externalization

Lipid oxidation products, including lysophosphatidylcholine (lysoPC), activate canonical transient receptor potential 6 (TRPC6) channels, and the subsequent increase in intracellular Ca²⁺ leads to TRPC5 activation. The goal of this study is to elucidate the steps in the pathway between TRPC6 activation and TRPC5 externalization. Following TRPC6 activation by lysoPC, extracellular regulated kinase (ERK) is phosphorylated. This leads to phosphorylation of p47^phox and subsequent NADPH oxidase activation with increased production of reactive oxygen species. ERK activation requires TRPC6 opening and influx of Ca²⁺ as evidenced by the failure of lysoPC to induce ERK phosphorylation in TRPC6^-/- endothelial cells. ERK siRNA blocks the lysoPC-induced activation of NADPH oxidase, demonstrating that ERK activation is upstream of NADPH oxidase. The reactive oxygen species produced by NADPH oxidase promote myosin light chain kinase (MLCK) activation with phosphorylation of MLC and TRPC5 externalization. Downregulation of ERK, NADPH oxidase, or MLCK with the relevant siRNA prevents TRPC5 externalization. Blocking MLCK activation prevents the prolonged rise in intracellular calcium levels and preserves endothelial migration in the presence of lysoPC.

TRPC6-mediated ERK1/2 phosphorylation prevents dentate granule cell degeneration via inhibiting mitochondrial elongation

Transient receptor potential canonical channel-6 (TRPC6) is one of Ca²⁺-permeable non-selective cation channels. In the rat hippocampus, TRPC6 expression is predominantly observed in dentate granule cells (DGC) rather than other hippocampal components. Interestingly, TRPC6 knockdown results in the massive DGC degeneration following status epilepticus (SE), although DGC is one of the resistant neuronal populations to various harmful stresses. However, the molecular events underlying the DGC degeneration induced by TRPC6 knockdown are still unclear. In the present study, TRPC6 knockdown resulted in mitochondrial elongation accompanied by reduction in dynamin-related proteins 1 (DRP1)-S616 phosphorylation. Furthermore, TRPC6 knockdown selectively decreased extracellular-signal-regulated kinase 1/2 (ERK1/2) phosphorylation. Similar to TRPC6 knockdown, ERK1/2 inhibition by U0126 evoked mitochondrial elongation with diminished DRP1-S616 phosphorylation, and facilitated SE-induced DGC degeneration independent of seizure severity. These findings indicate that TRPC6 may regulate mitochondrial dynamics via ERK1/2-mediaed DRP1 activation, which would be involved in DGC invulnerability to SE. Therefore, TRPC6 will be an interesting and important therapeutic target for neurological diseases related to impaired mitochondrial dynamics.

Canonical Transient Receptor Potential 6 Channel: A New Target of Reactive Oxygen Species in Renal Physiology and Pathology

SIGNIFICANCE

Regulation of Ca2+ signaling cascade by reactive oxygen species (ROS) is becoming increasingly evident and this regulation represents a key mechanism for control of many fundamental cellular functions. Canonical transient receptor potential (TRPC) 6, a member of Ca2+-conductive channel in the TRPC family, is widely expressed in kidney cells, including glomerular mesangial cells, podocytes, tubular epithelial cells, and vascular myocytes in renal microvasculature. Both overproduction of ROS and dysfunction of TRPC6 channel are involved in renal injury in animal models and human subjects. Although regulation of TRPC channel function by ROS has been well described in other tissues and cell types, such as vascular smooth muscle, this important cell regulatory mechanism has not been fully reviewed in kidney cells. Recent Advances: Accumulating evidence has shown that TRPC6 is a redox-sensitive channel, and modulation of TRPC6 Ca2+ signaling by altering TRPC6 protein expression or TRPC6 channel activity in kidney cells is a downstream mechanism by which ROS induce renal damage.

CRITICAL ISSUES

This review highlights how recent studies analyzing function and expression of TRPC6 channels in the kidney and their response to ROS improve our mechanistic understanding of oxidative stress-related kidney diseases.

FUTURE DIRECTIONS

Although it is evident that ROS regulate TRPC6-mediated Ca2+ signaling in several types of kidney cells, further study is needed to identify the underlying molecular mechanism. We hope that the newly identified ROS/TRPC6 pathway will pave the way to new, promising therapeutic strategies to target kidney diseases such as diabetic nephropathy. Antioxid. Redox Signal. 25, 732-748.

Calcium-permeable ion channels in the kidney

Calcium ions (Ca(2+)) are crucial for a variety of cellular functions. The extracellular and intracellular Ca(2+) concentrations are thus tightly regulated to maintain Ca(2+) homeostasis. The kidney, one of the major organs of the excretory system, regulates Ca(2+) homeostasis by filtration and reabsorption. Approximately 60% of the Ca(2+) in plasma is filtered, and 99% of that is reabsorbed by the kidney tubules. Ca(2+) is also a critical signaling molecule in kidney development, in all kidney cellular functions, and in the emergence of kidney diseases. Recently, studies using genetic and molecular biological approaches have identified several Ca(2+)-permeable ion channel families as important regulators of Ca(2+) homeostasis in kidney. These ion channel families include transient receptor potential channels (TRP), voltage-gated calcium channels, and others. In this review, we provide a brief and systematic summary of the expression, function, and pathological contribution for each of these Ca(2+)-permeable ion channels. Moreover, we discuss their potential as future therapeutic targets.

Podocytes

Podocytes are highly specialized cells of the kidney glomerulus that wrap around capillaries and that neighbor cells of the Bowman’s capsule. When it comes to glomerular filtration, podocytes play an active role in preventing plasma proteins from entering the urinary ultrafiltrate by providing a barrier comprising filtration slits between foot processes, which in aggregate represent a dynamic network of cellular extensions. Foot processes interdigitate with foot processes from adjacent podocytes and form a network of narrow and rather uniform gaps. The fenestrated endothelial cells retain blood cells but permit passage of small solutes and an overlying basement membrane less permeable to macromolecules, in particular to albumin. The cytoskeletal dynamics and structural plasticity of podocytes as well as the signaling between each of these distinct layers are essential for an efficient glomerular filtration and thus for proper renal function. The genetic or acquired impairment of podocytes may lead to foot process effacement (podocyte fusion or retraction), a morphological hallmark of proteinuric renal diseases. Here, we briefly discuss aspects of a contemporary view of podocytes in glomerular filtration, the patterns of structural changes in podocytes associated with common glomerular diseases, and the current state of basic and clinical research.

IL-17 induces MIP-1α expression in primary mouse astrocytes via TRPC channel

Our previous study demonstrated IL-17-mediated induction of MIP-1α through its binding to the cognate IL-17RA and MIP-1α was involved in astrocyte activation. Transient receptor potential canonical (TRPC) channel was involved in astrocyte activation, however, whether TRPC channel regulates MIP-1α expression in the context of multiple sclerosis (MS) remains largely unknown. In this study we identify the essential role of TRPC channel in IL-17-mediated MIP-1α expression and astrocyte activation. Moreover, treatment of astrocytes with IL-17 activated MAPKs and PI3K/Akt signaling pathways with downstream NF-κB pathways. Interestingly, the TRPC blocker-SKF96365 (10 μM) and Norgestimate (10 μM) significantly inhibited the increased expression of MIP-1α via suppression of IL-17-mediated ERK, p38 and JNK MAPKs and PI3K/Akt pathway activation, thereby underscoring the role of TRPC channel in this process. Together these data underpin the role of TRPC channel as a novel target that regulates MIP-1α expression and cell activation-mediated by IL-17 with implications for therapeutic intervention for reversal of neuroinflammation inflicted by IL-17. Understanding the regulation of MIP-1α expression may provide insights into the development of potential therapeutic targets for neuroinflammation associated with MS.

Calcium, TRPC channels, and regulation of the actin cytoskeleton in podocytes: towards a future of targeted therapies

With more than 6,000 new pediatric patients with treatment-resistant nephrotic syndrome in the US each year alone, the unmet need for novel, podocyte-specific therapies is substantial. Recently, the established therapeutic benefit of angiotensin-converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARB) was used as a starting point to gain insight into the pathomechanism of primary podocytopathies. A calcium (Ca(2+))-mediated pathway has been identified that connects the angiotensin type 1 receptor (AT1R) to podocyte cytoskeletal dynamics, essential for a functioning glomerular filtration barrier. This discovery provided an important missing piece in our understanding of the pathomechanism of filter barrier damage, revealing Ca(2+) signaling as critical for podocyte health and disease. The identification of the two Ca(2+) permeant channels TRPC5 and TRPC6 as mediators of this pathway not only bolstered the importance of podocyte cytoskeleton dynamics but also revealed promising drug targets for treatment-resistant nephrotic syndrome. This review will focus on this novel signaling pathway in primary podocytopathies and its implications for next-generation therapies for glomerular disease.

Second Messenger-Operated Calcium Entry Through TRPC6

Canonical transient receptor potential 6 (TRPC6) proteins assemble into heteromultimeric structures forming non-selective cation channels. In addition, many TRPC6-interacting proteins have been identified like some enzymes, channels, pumps, cytoskeleton-associated proteins, immunophilins, or cholesterol-binding proteins, indicating that TRPC6 are engaged into macromolecular complexes. Depending on the cell type and the experimental conditions used, TRPC6 activity has been reported to be controlled by diverse modalities. For instance, the second messenger diacylglycerol, store-depletion, the plant extract hyperforin or H2O2 have all been shown to trigger the opening of TRPC6 channels. A well-characterized consequence of TRPC6 activation is the elevation of the cytosolic concentration of Ca(2+). This latter response can reflect the entry of Ca(2+) through open TRPC6 channels but it can also be due to the Na(+)/Ca(2+) exchanger (operating in its reverse mode) or voltage-gated Ca(2+) channels (recruited in response to a TRPC6-mediated depolarization). Although TRPC6 controls a diverse array of biological functions in many tissues and cell types, its pathophysiological functions are far from being fully understood. This chapter covers some key features of TRPC6, with a special emphasis on their biological significance in kidney and blood cells.

Transforming Growth Factor β1-induced Apoptosis in Podocytes via the Extracellular Signal-regulated Kinase-Mammalian Target of Rapamycin Complex 1-NADPH Oxidase 4 Axis

TGF-β is a pleiotropic cytokine that accumulates during kidney injuries, resulting in various renal diseases. We have reported previously that TGF-β1 induces the selective up-regulation of mitochondrial Nox4, playing critical roles in podocyte apoptosis. Here we investigated the regulatory mechanism of Nox4 up-regulation by mTORC1 activation on TGF-β1-induced apoptosis in immortalized podocytes. TGF-β1 treatment markedly increased the phosphorylation of mammalian target of rapamycin (mTOR) and its downstream targets p70S6K and 4EBP1. Blocking TGF-β receptor I with SB431542 completely blunted the phosphorylation of mTOR, p70S6K, and 4EBP1. Transient adenoviral overexpression of mTOR-WT and constitutively active mTORΔ augmented TGF-β1-treated Nox4 expression, reactive oxygen species (ROS) generation, and apoptosis, whereas mTOR kinase-dead suppressed the above changes. In addition, knockdown of mTOR mimicked the effect of mTOR-KD. Inhibition of mTORC1 by low-dose rapamycin or knockdown of p70S6K protected podocytes through attenuation of Nox4 expression and subsequent oxidative stress-induced apoptosis by TGF-β1. Pharmacological inhibition of the MEK-ERK cascade, but not the PI3K-Akt-TSC2 pathway, abolished TGF-β1-induced mTOR activation. Inhibition of either ERK1/2 or mTORC1 did not reduce the TGF-β1-stimulated increase in Nox4 mRNA level but significantly inhibited total Nox4 expression, ROS generation, and apoptosis induced by TGF-β1. Moreover, double knockdown of Smad2 and 3 or only Smad4 completely suppressed TGF-β1-induced ERK1/2-mTORactivation. Our data suggest that TGF-β1 increases translation of Nox4 through the Smad-ERK1/2-mTORC1 axis, which is independent of transcriptional regulation. Activation of this pathway plays a crucial role in ROS generation and mitochondrial dysfunction, leading to podocyte apoptosis. Therefore, inhibition of the ERK1/2-mTORC1 pathway could be a potential therapeutic and preventive target in proteinuric and chronic kidney diseases.

Wnt/β-catenin signalling and podocyte dysfunction in proteinuric kidney disease

Podocytes are unique, highly specialized, terminally differentiated cells that are integral components of the kidney glomerular filtration barrier. Podocytes are vulnerable to a variety of injuries and in response they undergo a series of changes ranging from hypertrophy, autophagy, dedifferentiation, mesenchymal transition and detachment to apoptosis, depending on the nature and extent of the insult. Emerging evidence indicates that Wnt/β-catenin signalling has a central role in mediating podocyte dysfunction and proteinuria. Wnts are induced and β-catenin is activated in podocytes in various proteinuric kidney diseases. Genetic or pharmacologic activation of β-catenin is sufficient to impair podocyte integrity and causes proteinuria in healthy mice, whereas podocyte-specific ablation of β-catenin protects against proteinuria after kidney injury. Mechanistically, Wnt/β-catenin controls the expression of several key mediators implicated in podocytopathies, including Snail1, the renin-angiotensin system and matrix metalloproteinase 7. Wnt/β-catenin also negatively regulates Wilms tumour protein, a crucial transcription factor that safeguards podocyte integrity. Targeted inhibition of Wnt/β-catenin signalling preserves podocyte integrity and ameliorates proteinuria in animal models. This Review highlights advances in our understanding of the pathomechanisms of Wnt/β-catenin signalling in mediating podocyte injury, and describes the therapeutic potential of targeting this pathway for the treatment of proteinuric kidney disease.

Familial FSGS

Focal segmental glomerulosclerosis (FSGS) and nephrotic syndrome can be caused by rare highly penetrant mutations in number of genes. FSGS can follow both recessive and dominant inheritance patterns. In general, recessive forms present early, whereas the autosomal dominant forms present in adolescence or adulthood. Many of the genes found to be mutated in FSGS and nephrotic syndrome patients encode proteins essential for normal podocyte structure and/or function. An exception appears to be APOL1, which harbors common variants responsible for the high rate of FSGS and other nephropathies in people of recent African ancestry. Familial FSGS should be regarded as part of a spectrum of inherited glomerulopathies where the precise histologic presentation may depend on the age of onset, function of the responsible gene and gene products, and other factors.

ADAM17 promotes proliferation of collecting duct kidney epithelial cells through ERK activation and increased glycolysis in polycystic kidney disease

Polycystic kidney disease (PKD) is a common genetic disorder leading to cyst formation in the kidneys and other organs that ultimately results in kidney failure and death. Currently, there is no therapy for slowing down or stopping the progression of PKD. In this study, we identified the disintegrin metalloenzyme 17 (ADAM17) as a key regulator of cell proliferation in kidney tissues of conditional knockout Ift88(-/-) mice and collecting duct epithelial cells from Ift88°(rpk) mice, animal models of autosomal recessive polycystic kidney disease (ARPKD). Using Western blotting, an enzyme activity assay, and a growth factor-shedding assay in the presence or absence of the specific ADAM17 inhibitor TMI-005, we show that increased expression and activation of ADAM17 in the cystic kidney and in collecting duct epithelial cells originating from the Ift88°(rpk) mice (designated as PKD cells) lead to constitutive shedding of several growth factors, including heparin-binding EGF-like growth factor (HB-EGF), amphiregulin, and transforming growth factor-α (TGF-α). Increased growth factor shedding induces activation of the EGFR/MAPK/ERK pathway and maintains higher cell proliferation rate in PKD cells compared with control cells. PKD cells also displayed increased lactate formation and extracellular acidification indicative of aerobic glycolysis (Warburg effect), which was blocked by ADAM17 inhibition. We propose that ADAM17 is a key promoter of cellular proliferation in PKD cells by activating the EGFR/ERK axis and a proproliferative glycolytic phenotype.

Podocyte-associated talin1 is critical for glomerular filtration barrier maintenance

Podocytes are specialized actin-rich epithelial cells that line the kidney glomerular filtration barrier. The interface between the podocyte and the glomerular basement membrane requires integrins, and defects in either α3 or β1 integrin, or the α3β1 ligand laminin result in nephrotic syndrome in murine models. The large cytoskeletal protein talin1 is not only pivotal for integrin activation, but also directly links integrins to the actin cytoskeleton. Here, we found that mice lacking talin1 specifically in podocytes display severe proteinuria, foot process effacement, and kidney failure. Loss of talin1 in podocytes caused only a modest reduction in β1 integrin activation, podocyte cell adhesion, and cell spreading; however, the actin cytoskeleton of podocytes was profoundly altered by the loss of talin1. Evaluation of murine models of glomerular injury and patients with nephrotic syndrome revealed that calpain-induced talin1 cleavage in podocytes might promote pathogenesis of nephrotic syndrome. Furthermore, pharmacologic inhibition of calpain activity following glomerular injury substantially reduced talin1 cleavage, albuminuria, and foot process effacement. Collectively, these findings indicate that podocyte talin1 is critical for maintaining the integrity of the glomerular filtration barrier and provide insight into the pathogenesis of nephrotic syndrome.

Hyperactive adverse mechanical stress responses in dystrophic heart are coupled to transient receptor potential canonical 6 and blocked by cGMP-protein kinase G modulation

RATIONALE

The heart is exquisitely sensitive to mechanical stimuli to adapt rapidly to physiological demands. In muscle lacking dystrophin, such as Duchenne muscular dystrophy, increased load during contraction triggers pathological responses thought to worsen the disease. The relevant mechanotransducers and therapies to target them remain unclear.

OBJECTIVES

We tested the role of transient receptor potential canonical (TRPC) channels TRPC3 and TRPC6 and their modulation by protein kinase G (PKG) in controlling cardiac systolic mechanosensing and determined their pathophysiological relevance in an experimental model of Duchenne muscular dystrophy.

METHODS AND RESULTS

Contracting isolated papillary muscles and cardiomyocytes from controls and mice genetically lacking either TRPC3 or TRPC6 were subjected to auxotonic load to induce stress-stimulated contractility (SSC, gradual rise in force and intracellular Ca(2+)). Incubation with cGMP (PKG activator) markedly blunted SSC in controls and Trpc3(-/-); whereas in Trpc6(-/-), the resting SSC response was diminished and cGMP had no effect. In Duchenne muscular dystrophy myocytes (mdx/utrophin deficient), the SSC was excessive and arrhythmogenic. Gene deletion or selective drug blockade of TRPC6 or cGMP/PKG activation reversed this phenotype. Chronic phosphodiesterase 5A inhibition also normalized abnormal mechanosensing while blunting progressive chamber hypertrophy in Duchenne muscular dystrophy mice.

CONCLUSIONS

PKG is a potent negative modulator of cardiac systolic mechanosignaling that requires TRPC6 as the target effector. In dystrophic hearts, excess SSC and arrhythmia are coupled to TRPC6 and are ameliorated by its targeted suppression or PKG activation. These results highlight novel therapeutic targets for this disease.

TRPC6: physiological function and pathophysiological relevance

TRPC6 is a non-selective cation channel 6 times more permeable to Ca(2+) than to Na(+). Channel homotetramers heterologously expressed have a characteristic doubly rectifying current-voltage relationship and are directly activated by the second messenger diacylglycerol (DAG). TRPC6 proteins are also regulated by specific tyrosine or serine phosphorylation and phosphoinositides. Given its specific expression pattern, TRPC6 is likely to play a number of physiological roles which are confirmed by the analysis of a Trpc6 (-/-) mouse model. In smooth muscle Na(+) influx through TRPC6 channels and activation of voltage-gated Ca(2+) channels by membrane depolarisation is the driving force for contraction. Permeability of pulmonary endothelial cells depends on TRPC6 and induces ischaemia-reperfusion oedema formation in the lungs. TRPC6 was also identified as an essential component of the slit diaphragm architecture of kidney podocytes and plays an important role in the protection of neurons after cerebral ischaemia. Other functions especially in immune and blood cells remain elusive. Recently identified TRPC6 blockers may be helpful for therapeutic approaches in diseases with highly activated TRPC6 channel activity.

The physical basis of renal fibrosis: effects of altered hydrodynamic forces on kidney homeostasis

Healthy kidneys are continuously exposed to an array of physical forces as they filter the blood: shear stress along the inner lumen of the tubules, distension of the tubular walls in response to changing fluid pressures, and bending moments along both the cilia and microvilli of individual epithelial cells that comprise the tubules. Dysregulation of kidney homeostasis via underlying medical conditions such as hypertension, diabetes, or glomerulonephritis fundamentally elevates the magnitudes of each principle force in the kidney and leads to fibrotic scarring and eventual loss of organ function. The purpose of this review is to summarize the progress made characterizing the response of kidney cells to pathological levels of mechanical stimuli. In particular, we examine important, mechanically responsive signaling cascades and explore fundamental changes in renal cell homeostasis after cyclic strain or fluid shear stress exposure. Elucidating the effects of these disease-related mechanical imbalances on endogenous signaling events in kidney cells presents a unique opportunity to better understand the fibrotic process.

Transient receptor potential channel 6 (TRPC6) protects podocytes during complement-mediated glomerular disease

Gain-of-function mutations in the calcium channel TRPC6 lead to autosomal dominant focal segmental glomerulosclerosis and podocyte expression of TRPC6 is increased in some acquired human glomerular diseases, particularly in membranous nephropathy. These observations led to the hypothesis that TRPC6 overactivation is deleterious to podocytes through pathological calcium signaling, both in genetic and acquired diseases. Here, we show that the effects of TRPC6 on podocyte function are context-dependent. Overexpression of TRPC6 alone did not directly affect podocyte morphology and cytoskeletal structure. Unexpectedly, however, overexpression of TRPC6 protected podocytes from complement-mediated injury, whereas genetic or pharmacological TRPC6 inactivation increased podocyte susceptibility to complement. Mechanistically, this effect was mediated by Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activation. Podocyte-specific TRPC6 transgenic mice showed stronger CaMKII activation, reduced podocyte foot process effacement and reduced levels of proteinuria during nephrotoxic serum nephritis, whereas TRPC6 null mice exhibited reduced CaMKII activation and higher levels of proteinuria compared with wild type littermates. Human membranous nephropathy biopsy samples showed podocyte staining for active CaMKII, which correlated with the degree of TRPC6 expression. Together, these data suggest a dual and context dependent role of TRPC6 in podocytes where acute activation protects from complement-mediated damage, but chronic overactivation leads to focal segmental glomerulosclerosis.