TRP channels in kidney disease

Identification of cellular senescence-related genes as biomarkers for lupus nephritis based on bioinformatics

Background

Lupus nephritis (LN) is one of the most common and severe complications of systemic lupus erythematosus with unclear pathogenesis. The most accurate diagnosis criterion of LN is still renal biopsy and nowadays treatment strategies of LN are far from satisfactory. Cellular senescence is defined as the permanent cell cycle arrest marked by senescence-associated secretory phenotype (SASP), which has been proved to accelerate the mobility and mortality of patients with LN. The study is aimed to identify cellular senescence-related genes for LN.

Methods

Genes related to cellular senescence and LN were obtained from the MSigDB genetic database and GEO database respectively. Through differential gene analysis, Weighted Gene Go-expression Network Analysis (WGCNA) and machine learning algorithms, hub cellular senescence-related differentially expressed genes (CS-DEGs) were identified. By external validation, hub CS-DEGs were further filtered and the remaining genes were identified as biomarkers. We explored their potential physiopathologic function through GSEA.

Results

We obtained 432 genes related to cellular senescence, 1,208 differentially expressed genes (DEGs) and 840 genes in the key gene module related to LN, which were intersected with each other for CS-DEGs. Subsequent Machine learning algorithms screened out six hub CS-DEGs and finally three hub CS-DEGs, ALOX5, PTGER2 and PRKCB passed through external validation, which were identified as biomarkers. The three biomarkers were enriched in "B Cell receptor signaling pathway" and "NF-kappa B signaling pathway" based on GESA results.

Conclusion

This study explored the potential relationship between cellular senescence and LN, and identified three biomarkers ALOX5, PTGER2, and PRKCB playing key roles in LN, which will provide new insights for the diagnosis and treatment of LN.

Chronic hyperactivation of midbrain dopamine neurons causes preferential dopamine neuron degeneration

Parkinson's disease (PD) is characterized by the death of substantia nigra (SNc) dopamine (DA) neurons, but the pathophysiological mechanisms that precede and drive their death remain unknown. The activity of DA neurons is likely altered in PD, but we understand little about if or how chronic changes in activity may contribute to degeneration. To address this question, we developed a chemogenetic (DREADD) mouse model to chronically increase DA neuron activity, and confirmed this increase using ex vivo electrophysiology. Chronic hyperactivation of DA neurons resulted in prolonged increases in locomotor activity during the light cycle and decreases during the dark cycle, consistent with chronic changes in DA release and circadian disturbances. We also observed early, preferential degeneration of SNc projections, recapitulating the PD hallmarks of selective vulnerability of SNc axons and the comparative resilience of ventral tegmental area axons. This was followed by eventual loss of midbrain DA neurons. Continuous DREADD activation resulted in a sustained increase in baseline calcium levels, supporting a role for increased calcium in the neurodegeneration process. Finally, spatial transcriptomics from DREADD mice examining midbrain DA neurons and striatal targets, and cross-validation with human patient samples, provided insights into potential mechanisms of hyperactivity-induced toxicity and PD. Our results thus reveal the preferential vulnerability of SNc DA neurons to increased neural activity, and support a potential role for increased neural activity in driving degeneration in PD.

Roles for TRPV4 in disease: A discussion of possible mechanisms

The transient receptor potential vanilloid 4 (TRPV4) ion channel is a ubiquitously expressed Ca2+-permeable ion channel that controls intracellular calcium ([Ca2+]i) homeostasis in various types of cells. The physiological roles for TRPV4 are tissue specific and the mechanisms behind this specificity remain mostly unclarified. It is noteworthy that mutations in the TRPV4 channel have been associated to a broad spectrum of congenital diseases, with most of these mutations mainly resulting in gain-of-function. Mutations have been identified in human patients showing a variety of phenotypes and symptoms, mostly related to skeletal and neuromuscular disorders. Since TRPV4 is so widely expressed throughout the body, it comes as no surprise that the literature is growing in evidence linking this protein to malfunction in systems other than the skeletal and neuromuscular. In this review, we summarize the expression patterns of TRPV4 in several tissues and highlight findings of recent studies that address critical structural and functional features of this channel, particularly focusing on its interactions and signaling pathways related to Ca2+ entry. Moreover, we discuss the roles of TRPV4 mutations in some diseases and pinpoint some of the mechanisms underlying pathological states where TRPV4's malfunction is prominent.

Untangling the Uncertain Role of Overactivation of the Renin-Angiotensin-Aldosterone System with the Aging Process Based on Sodium Wasting Human Models

Every individual at some point encounters the progressive biological process of aging, which is considered one of the major risk factors for common diseases. The main drivers of aging are oxidative stress, senescence, and reactive oxygen species (ROS). The renin-angiotensin-aldosterone system (RAAS) includes several systematic processes for the regulation of blood pressure, which is caused by an imbalance of electrolytes. During activation of the RAAS, binding of angiotensin II (ANG II) to angiotensin II type 1 receptor (AGTR1) activates intracellular nicotinamide adenine dinucleotide phosphate (NADPH) oxidase to generate superoxide anions and promote uncoupling of endothelial nitric oxide (NO) synthase, which in turn decreases NO availability and increases ROS production. Promoting oxidative stress and DNA damage mediated by ANG II is tightly regulated. Individuals with sodium deficiency-associated diseases such as Gitelman syndrome (GS) and Bartter syndrome (BS) show downregulation of inflammation-related processes and have reduced oxidative stress and ROS. Additionally, the histone deacetylase sirtuin-1 (SIRT1) has a significant impact on the aging process, with reduced activity with age. However, GS/BS patients generally sustain higher levels of sirtuin-1 (SIRT1) activity than age-matched healthy individuals. SIRT1 expression in GS/BS patients tends to be higher than in healthy age-matched individuals; therefore, it can be assumed that there will be a trend towards healthy aging in these patients. In this review, we highlight the importance of the hallmarks of aging, inflammation, and the RAAS system in GS/BS patients and how this might impact healthy aging. We further propose future research directions for studying the etiology of GS/BS at the molecular level using patient-derived renal stem cells and induced pluripotent stem cells.

Targeting TRP channels: recent advances in structure, ligand binding, and molecular mechanisms

Transient receptor potential (TRP) channels are a large and diverse family of transmembrane ion channels that are widely expressed, have important physiological roles, and are associated with many human diseases. These proteins are actively pursued as promising drug targets, benefitting greatly from advances in structural and mechanistic studies of TRP channels. At the same time, the complex, polymodal activation and regulation of TRP channels have presented formidable challenges. In this short review, we summarize recent progresses toward understanding the structural basis of TRP channel function, as well as potential ligand binding sites that could be targeted for therapeutics. A particular focus is on the current understanding of the molecular mechanisms of TRP channel activation and regulation, where many fundamental questions remain unanswered. We believe that a deeper understanding of the functional mechanisms of TRP channels will be critical and likely transformative toward developing successful therapeutic strategies targeting these exciting proteins. This endeavor will require concerted efforts from computation, structural biology, medicinal chemistry, electrophysiology, pharmacology, drug safety and clinical studies.

Causal and putative pathogenic mutations identified in 39% of children with primary steroid-resistant nephrotic syndrome in South Africa

There is a paucity of data identifying genetic mutations that account for the high rate of steroid-resistant nephrotic syndrome (SRNS) in a South African paediatric population. The aim was to identify causal mutations in genes implicated in SRNS within a South African paediatric population. We enrolled 118 children with primary nephrotic syndrome (NS), 70 SRNS and 48 steroid-sensitive NS. All children with SRNS underwent kidney biopsy. We first genotyped the NPHS2 gene for the p.V260E variant in all NS cases (n = 118) and controls (n = 219). To further identify additional variants, we performed whole-exome sequencing and interrogated ten genes (NPHS1, NPHS2, WT1, LAMB2, ACTN4, TRPC6, INF2, CD2AP, PLCE1, MYO1E) implicated in SRNS with histopathological features of focal segmental glomerulosclerosis (FSGS) in 56 SRNS cases and 29 controls; we also performed exome sequencing on two patients carrying the NPHS2 p.V260E mutation as positive controls. The overall detection rate of causal and putative pathogenic mutations in children with SRNS was 27/70 (39%): 15 (21%) carried the NPHS2 p.V260E causal mutation in the homozygous state, and 12 (17%) SRNS cases carried a putative pathogenic mutation in the heterozygous state in genes (INF2 (n = 8), CD2AP (n = 3) and TRPC6 (n = 1)) known to have autosomal dominant inheritance mode. NPHS2 p.V260E homozygosity was specifically associated with biopsy-proven FSGS, accounting for 24% of children of Black ethnicity (15 of 63) with steroid-resistant FSGS. No causal or putative pathogenic mutations were identified in NPHS1, WT1, LAMB2, PLCE1, MYO1E and ACTN4. We report four novel variants in INF2, PLCE1, ACTN4 and TRPC6.   Conclusion: We report putative missense variants predicted to be pathogenic in INF2, CD2AP and TRPC6 among steroid-resistant-FSGS children. However, the NPHS2 p.V260E mutation is a prevalent cause of steroid-resistant FSGS among Black South African children occurring in 24% of children with SRNS. Screening all Black African children presenting with NS for NPHS2 p.V260E will provide a precision diagnosis of steroid-resistant FSGS and inform clinical management. What is Known: • Limited data is available on the genetic disparity of SNRS in a South African paediatric setting. • The high rate of steroid resistance in Black South African children with FSGS compared to other racial groups is partially explained by the founder variant NPHS2 p.V260E. What is New: • We report putative missense variants predicted to be pathogenic in INF2, CD2AP and TRPC6 among steroid-resistant FSGS children. • NPHS2 p.V260E mutation remains a prevalent cause of steroid-resistant FSGS among Black South African children, demonstrating precision diagnostic utility.

Sodium-Glucose Cotransporter 2 Inhibitors and Management of Refractory Hypomagnesemia Without Overt Urinary Magnesium Wasting: A Report of 2 Cases

Sodium-glucose cotransporter 2 (SGLT2) inhibitor have become widely used in patients with diabetes, heart failure, and kidney disease to improve clinical outcomes and diminish hospitalizations. They have also been associated with increased serum magnesium levels in patients with type 2 diabetes. The use of SGLT2 inhibitors resulted in improved magnesium homeostasis in a series of patients with refractory hypomagnesemia with urinary magnesium wasting. However, the role of SLGT2 inhibitors in patients with hypomagnesemia without urinary magnesium wasting remains unexplored. We report 2 cases with refractory hypomagnesemia without significant urinary magnesium wasting and dramatically improved serum magnesium levels after the initiation of SGLT2 inhibitors. Case 1 achieved independence from weekly intravenous magnesium infusions and reached sustainably greater serum magnesium levels with decreased oral magnesium supplementation and increased urinary fractional excretion of magnesium. Case 2 demonstrated improved serum magnesium levels with reduced oral magnesium supplementation without significant reduction in urinary fractional excretion of magnesium. These findings not only expand the use of SGLT2 inhibitors but also open the door for further studies to better understand the pathophysiology of how magnesium homeostasis is altered with inhibition of SGLT2.

A Review on the Role of TRP Channels and Their Potential as Drug Targets_An Insight Into the TRP Channel Drug Discovery Methodologies

Transient receptor potential (TRP) proteins are a large group of ion channels that control many physiological functions in our body. These channels are considered potential therapeutic drug targets for various diseases such as neurological disorders, cancers, cardiovascular disease, and many more. The Nobel Prize in Physiology/Medicine in the year 2021 was awarded to two scientists for the discovery of TRP and PIEZO ion channels. Improving our knowledge of technologies for their study is essential. In the present study, we reviewed the role of TRP channel types in the control of normal physiological functions as well as disease conditions. Also, we discussed the current and novel technologies that can be used to study these channels successfully. As such, Flux assays for detecting ionic flux through ion channels are among the core and widely used tools for screening drug compounds. Technologies based on these assays are available in fully automated high throughput set-ups and help detect changes in radiolabeled or non-radiolabeled ionic flux. Aurora's Ion Channel Reader (ICR), which works based on label-free technology of flux assay, offers sensitive, accurate, and reproducible measurements to perform drug ranking matching with patch-clamp (gold standard) data. The non-radiolabeled trace-based flux assay coupled with the ICR detects changes in various ion types, including potassium, calcium, sodium, and chloride channels, by using appropriate tracer ions. This technology is now considered one of the very successful approaches for analyzing ion channel activity in modern drug discovery. It could be a successful approach for studying various ion channels and transporters, including the different members of the TRP family of ion channels.

Three uncommon mutations of the SLC12A3 gene in gitelman syndrome: case reports and review of the literature

Background

Gitelman syndrome is a rare autosomal recessive salt-wasting tubulopathy characterized by low potassium and magnesium levels in the blood, decreased excretion of calcium in the urine, and metabolic alkalosis. It is commonly caused by an inactivating mutation in the SLC12A3 gene (16q13), which encodes a thiazide-sensitive sodium chloride cotransporter. Here, we present three cases with the same clinical and laboratory findings that showed different mutations in the SLC12A3 gene.

Case presentation

Three children, a 14-year-old boy, a 7-year-old girl, and an 11-year-old boy, were admitted to our hospital at different times with nausea, weakness, muscle cramps in hands, and failure to thrive complaints. Blood tests showed hypokalemia, hypomagnesemia and metabolic alkalosis. Patients were referred to Pediatric Nephrology Clinic and diagnosed with Gitelman syndrome. Genetic tests of three cases showed homozygous mutations of c.1928C > T, p.Pro643Leu, c.248G > A, p.Arg83Gln, and c.1919A > G, p.N640S in the SLC12A3 gene exists, respectively. Potassium chloride, magnesium replacements, and indomethacin were given for treatment to patients. During follow-up, patients' heights and weights were increased dramatically, and nausea complaints were over.

Conclusion

Different mutations in the SLC12A3 gene in Gitelman syndrome can be detected but clinical, and laboratory findings were generally similar. Treatment with potassium, magnesium supplements, and indomethacin showed significant improvements in symptoms.

Modulation of Tubular pH by Acetazolamide in a Ca2+ Transport Deficient Mice Facilitates Calcium Nephrolithiasis

Proximal tubular (PT) acidosis, which alkalinizes the urinary filtrate, together with Ca²⁺ supersaturation in PT can induce luminal calcium phosphate (CaP) crystal formation. While such CaP crystals are known to act as a nidus for CaP/calcium oxalate (CaOx) mixed stone formation, the regulation of PT luminal Ca²⁺ concentration ([Ca²⁺]) under elevated pH and/or high [Ca²⁺] conditions are unknown. Since we found that transient receptor potential canonical 3 (TRPC3) knockout (KO; -/-) mice could produce mild hypercalciuria with CaP urine crystals, we alkalinized the tubular pH in TRPC3-/- mice by oral acetazolamide (0.08%) to develop mixed urinary crystals akin to clinical signs of calcium nephrolithiasis (CaNL). Our ratiometric (λ340/380) intracellular [Ca²⁺] measurements reveal that such alkalization not only upsurges Ca²⁺ influx into PT cells, but the mode of Ca²⁺ entry switches from receptor-operated to store-operated pathway. Electrophysiological experiments show enhanced bicarbonate related current activity in treated PT cells which may determine the stone-forming phenotypes (CaP or CaP/CaOx). Moreover, such alkalization promotes reactive oxygen species generation, and upregulation of calcification, inflammation, fibrosis, and apoptosis in PT cells, which were exacerbated in absence of TRPC3. Altogether, the pH-induced alteration of the Ca²⁺ signaling signature in PT cells from TRPC3 ablated mice exacerbated the pathophysiology of mixed urinary stone formation, which may aid in uncovering the downstream mechanism of CaNL.

Regulation of Vessel Permeability by TRP Channels

The vascular endothelium constitutes a semi-permeable barrier between blood and interstitial fluids. Since an augmented endothelial permeability is often associated to pathological states, understanding the molecular basis for its regulation is a crucial biomedical and clinical challenge. This review focuses on the processes controlling paracellular permeability that is the permeation of fluids between adjacent endothelial cells (ECs). Cytosolic calcium changes are often detected as early events preceding the alteration of the endothelial barrier (EB) function. For this reason, great interest has been devoted in the last decades to unveil the molecular mechanisms underlying calcium fluxes and their functional relationship with vessel permeability. Beyond the dicotomic classification between store-dependent and independent calcium entry at the plasma membrane level, the search for the molecular components of the related calcium-permeable channels revealed a difficult task for intrinsic and technical limitations. The contribution of redundant channel-forming proteins including members of TRP superfamily and Orai1, together with the very complex intracellular modulatory pathways, displays a huge variability among tissues and along the vascular tree. Moreover, calcium-independent events could significantly concur to the regulation of vascular permeability in an intricate and fascinating multifactorial framework.

Prediction of the mechanisms of action of Shenkang in chronic kidney disease: A network pharmacology study and experimental validation

ETHNOPHARMACOLOGICAL RELEVANCE

Traditional Chinese medicine provides a unique curative treatment of complex chronic diseases, including chronic kidney disease (CKD), which is not effectively treated with the current therapies. The pharmacological mechanisms of Shenkang (SK), a herbal medicine containing rhubarb (Rheum palmatum L. or R. tanguticum Maxim. ex Balf.), red sage (Salvia miltiorrhiza Bunge), safflower (Carthamus tinctorius L.), and astragalus (Astragalus mongholicus Bunge), widely used to treat CKD in China, are still unclear.

AIM OF THE STUDY

In this study, the comprehensive approach used for elucidating the pharmacological mechanisms of SK included the identification of the effective constituents, target prediction and network analysis, by investigating the interacting pathways between these molecules in the context of CKD. These results were validated by performing an in vivo study and by comparison with literature reviews.

MATERIALS AND METHODS

This approach involved the following main steps: first, we constructed a molecular database for SK and screened for active molecules by conducting drug-likeness and drug half-life evaluations; second, we used a weighted ensemble similarity drug-targeting model to accurately identify the direct drug targets of the bioactive constituents; third, we constructed compound-target, target-pathway, and target-disease networks using the Cytoscape 3.2 software and determined the distribution of the targets in tissues and organs according to the BioGPS database. Finally, the resulting drug-target mechanisms were compared with those proposed by previous research on SK and validated in a mouse model of CKD.

RESULTS

By using Network analysis, 88 potential bioactive compounds in the four component herbs of SK and 85 CKD-related targets were identified, including pathways that involve the nuclear factor-κB, mitogen-activated protein kinase, transient receptor potential, and vascular endothelial growth factor, which were categorized as inflammation, proliferation, migration, and permeability modules. The results also included different tissues (kidneys, liver, lungs, and heart) and different disease types (urogenital, metabolic, endocrine, cardiovascular, and immune diseases as well as pathological processes) closely related to CKD. These findings agreed with those reported in the literature. However, our findings with the network pharmacology prediction did not account for all the effects reported for SK found in the literature, such as regulation of the hemodynamics, inhibition of oxidative stress and apoptosis, and the involvement of the transforming growth factor-β/SMAD3, sirtuin/forkhead box protein O (SIRT/FOXO) and B-cell lymphoma-2-associated X protein pathways. The in vivo validation experiment revealed that SK ameliorated CKD through antifibrosis and anti-inflammatory effects, by downregulating the levels of vascular cell adhesion protein 1, vitamin D receptor, cyclooxygenase-2, and matrix metalloproteinase 9 proteins in the unilateral ureteral obstruction mouse model. This was consistent with the predicted target and pathway networks.

CONCLUSIONS

SK exerted a curative effect on CKD and CKD-related diseases by targeting different organs, regulating inflammation and proliferation processes, and inhibiting abnormal extracellular matrix accumulation. Thus, pharmacological network analysis with in vivo validation explained the potential effects and mechanisms of SK in the treatment of CKD. However, these findings need to be further confirmed with clinical studies.

Probing Cilia-Associated Signaling Proteomes in Animal Evolution

Cilia have evolved to function as essential sensory organelles in animals. To understand why cilia are intimately associated with cell signaling, Sigg et al. (2017) develop and apply a comparative proteomics approach, reported in this issue of Developmental Cell, to analyze the evolutionary relationship between cilia and various signaling pathways.

TRP Channel Reconstitution in Lipid Bilayers

The family of the transient receptor potential (TRP) proteins presents a diverse group of polymodal ion channels intertwined in the regulation of various physiological processes. Currently, TRP channels are well established in temperature-sensation, thermoregulation, pain sensation, and mineral homeostasis. Furthermore, new evidence suggests that TRP channels are also implicated in hormonal signaling, where the channels are responsible for propagating hormone-induced signals along the neural circuitry and also regulating cellular processes of nonexcitable cells. Due to this wide assortment of actions, TRP channels have been attracting immense scientific interest in various fields.In this chapter, I describe incorporation and characterization of several TRP channels using an electrophysiological approach known as planar lipid bilayers. This technique features measurements of functional activities of ion channels in a well-defined reconstituted system. The priority of this electrophysiological approach is identifying intrinsic properties of ion channels, which is particularly valuable in appreciating intrinsic temperature sensitivity concerning thermo-TRP channels, but also direct mechanisms of channels agonists, antagonists, cofactors, and other modifiers.

The bovine TRPV3 as a pathway for the uptake of Na+, Ca2+, and NH4+

Absorption of ammonia from the gastrointestinal tract results in problems that range from hepatic encephalopathy in humans to poor nitrogen efficiency of cattle with consequences for the global climate. Previous studies on epithelia and cells from the native ruminal epithelium suggest functional involvement of the bovine homologue of TRPV3 (bTRPV3) in ruminal NH₄⁺ transport. Since the conductance of TRP channels to NH₄⁺ has never been studied, bTRPV3 was overexpressed in HEK-293 cells and investigated using the patch-clamp technique and intracellular calcium imaging. Control cells contained the empty construct. Divalent cations blocked the conductance for monovalent cations in both cell types, with effects higher in cells expressing bTRPV3. In bTRPV3 cells, but not in controls, menthol, thymol, carvacrol, or 2-APB stimulated whole cell currents mediated by Na⁺, Cs⁺, NH₄⁺_, and K⁺, with a rise in intracellular Ca²⁺ observed in response to menthol. While only 25% of control patches showed single-channel events (with a conductance of 40.8 ± 11.9 pS for NH₄⁺ and 25.0 ± 5.8 pS for Na⁺), 90% of bTRPV3 patches showed much larger conductances of 127.8 ± 4.2 pS for Na⁺, 240.1 ± 3.6 pS for NH₄⁺, 34.0 ± 1.7 pS for Ca²⁺, and ~ 36 pS for NMDG⁺. Open probability, but not conductance, rose with time after patch excision. In conjunction with previous research, we suggest that bTRPV3 channels may play a role in the transport of Na⁺, K⁺, Ca²⁺ and NH₄⁺ across the rumen with possible repercussions for understanding the function of TRPV3 in other epithelia.

TRPC Channels in Health and Disease

This chapter offers a brief introduction of the functions of TRPC channels in non-neuronal systems. We focus on three major organs of which the research on TRPC channels have been most focused on: kidney, heart, and lung. The chapter highlights on cellular functions and signaling pathways mediated by TRPC channels. It also summarizes several inherited diseases in humans that are related to or caused by TRPC channel mutations and malfunction. A better understanding of TRPC channels functions and the importance of TRPC channels in health and disease should lead to new insights and discovery of new therapeutic approaches for intractable disease.

TRPC6 G757D Loss-of-Function Mutation Associates with FSGS

FSGS is a CKD with heavy proteinuria that eventually progresses to ESRD. Hereditary forms of FSGS have been linked to mutations in the transient receptor potential cation channel, subfamily C, member 6 (TRPC6) gene encoding a nonselective cation channel. Most of these TRPC6 mutations cause a gain-of-function phenotype, leading to calcium-triggered podocyte cell death, but the underlying molecular mechanisms are unclear. We studied the molecular effect of disease-related mutations using tridimensional in silico modeling of tetrameric TRPC6. Our results indicated that G757 is localized in a domain forming a TRPC6-TRPC6 interface and predicted that the amino acid exchange G757D causes local steric hindrance and disruption of the channel complex. Notably, functional characterization of model interface domain mutants suggested a loss-of-function phenotype. We then characterized 19 human FSGS-related TRPC6 mutations, the majority of which caused gain-of-function mutations. However, five mutations (N125S, L395A, G757D, L780P, and R895L) caused a loss-of-function phenotype. Coexpression of wild-type TRPC6 and TRPC6 G757D, mimicking heterozygosity observed in patients, revealed a dominant negative effect of TRPC6 G757D. Our comprehensive analysis of human disease-causing TRPC6 mutations reveals loss of TRPC6 function as an additional concept of hereditary FSGS and provides molecular insights into the mechanism responsible for the loss-of-function phenotype of TRPC6 G757D in humans.

Identification of TRPM6 and TRPM7 expression changes in response to a diet supplemented with inulin in porcine kidney

Abstract. Magnesium is the fourth most abundant mineral element in vertebrates and the second most common intracellular cation. Recently identified Mg2+-specific channels – TRPM6 and TRPM7 – have been shown to be essential for whole-body and cellular Mg2+ homeostasis. The aim of the study was to determine the effect of inulin on the expression of TRPM6 and TRPM7 in the renal cortex and medulla of growing pigs. The study was carried out on 16 Danbred  ×  Duroc castrated male piglets fed a cereal-based diet without inulin or with 2 % addition of inulin from chicory root from the 10th day of life. In pigs fed a diet with inulin, TRPM6 expression was greater in both the renal cortex and medulla compared to the control group. The expression of TRPM7 in both the renal cortex and medulla in the control group and in piglets fed a diet enriched with inulin was relatively stable. To our knowledge, this is the first study aimed at the identification of TRPM6 and TRPM7 in the kidneys of pig. It is proposed that inulin addition to fodder resulted not only in a magnesium absorption increase, but also, due to prolonged low plasma Mg concentration of examined piglets, renal magnesium retention. Therefore, higher magnesium reabsorption via increased TRPM6 expression in the kidney was probably observed in order to supplement deficiencies of this element. Diet-unresponsive expression of TRPM7 supports the concept that this channel is not involved in the extracellular magnesium homeostasis.

Evidence for the functional involvement of members of the TRP channel family in the uptake of Na(+) and NH4 (+) by the ruminal epithelium

Large quantities of protein are degraded in the fermentative parts of the gut to ammonia, which is absorbed, detoxified to urea, and excreted, leading to formation of nitrogenous compounds such as N2O that are associated with global warming. In ruminants, channel-mediated uptake of NH4 (+) from the rumen predominates. The molecular identity of these channels remains to be clarified. Ruminal cells and epithelia from cows and sheep were investigated using patch clamp, Ussing chamber, microelectrode techniques, and qPCR. In patch clamp experiments, bovine ruminal epithelial cells expressed a conductance for NH4 (+) that could be blocked in a voltage-dependent manner by divalent cations. In the native epithelium, NH4 (+) depolarized the apical potential, acidified the cytosol and induced a rise in short-circuit current (I sc) that persisted after the removal of Na(+), was blocked by verapamil, enhanced by the removal of divalent cations, and was sensitive to certain transient receptor potential (TRP) channel modulators. Menthol or thymol stimulated the I sc in Na(+) or NH4 (+) containing solutions in a dose-dependent manner and modulated transepithelial Ca(2+) fluxes. On the level of messenger RNA (mRNA), ovine and bovine ruminal epithelium expressed TRPA1, TRPV3, TRPV4, TRPM6, and TRPM7, with any expression of TRPV6 marginal. No bands were detected for TRPV1, TRPV5, or TRPM8. Functional and molecular biological data suggest that the transport of NH4 (+), Na(+), and Ca(2+) across the rumen involves TRP channels, with TRPV3 and TRPA1 emerging as prime candidate genes. TRP channels may also contribute to the transport of NH4 (+) across other epithelia.

Molecular Modeling of the Structural and Dynamical Changes in Calcium Channel TRPV5 Induced by the African-Specific A563T Variation

Transient receptor potential cation channels, vanilloid subfamily, member 5 (TRPV5) plays a key role in active Ca(2+) reabsorption in the kidney. Variations in TRPV5 occur at high frequency in African populations and may contribute to their higher efficiency of Ca(2+) reabsorption. One of the African specific variations, A563T, exhibits increased Ca(2+) transport ability. However, it is unclear how this variation influences the channel pore. On the basis of the structure of TRPV1, a TRPV5 model was generated to simulate the structural and dynamical changes induced by the A563T variation. On the basis of this model, amino acid residue 563 interacts with V540, which is one residue away from the key residue, D542, involved in Ca(2+) selectivity and Mg(2+) blockade. The A563T variation increases secondary structure stability and reduces dynamical motion of D542. In addition, the A563T variation alters the electrostatic potential of the outer surface of the pore. Differences in contact between selective filter residues and residue 563 and in electrostatic potential between the two TRPV5 variants were also observed in another model derived from an alternative alignment in the selective filters between TRPV5 and TRPV1. These findings indicate that the A563T variation induces structural, dynamical, and electrostatic changes in the TRPV5 pore, providing structural insight into the functional alterations associated with the A563T variation.

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.

Epigenetic profiles of pre-diabetes transitioning to type 2 diabetes and nephropathy

AIM: To examine DNA methylation profiles in a longitudinal comparison of pre-diabetes mellitus (Pre-DM) subjects who transitioned to type 2 diabetes mellitus (T2DM).

METHODS: We performed DNA methylation study in bisulphite converted DNA from Pre-DM (n = 11) at baseline and at their transition to T2DM using Illumina Infinium HumanMethylation27 BeadChip, that enables the query of 27578 individual cytosines at CpG loci throughout the genome, which are focused on the promoter regions of 14495 genes.

RESULTS: There were 694 CpG sites hypomethylated and 174 CpG sites hypermethylated in progression from Pre-DM to T2DM, representing putative genes involved in glucose and fructose metabolism, inflammation, oxidative and mitochondrial stress, and fatty acid metabolism. These results suggest that this high throughput platform is able to identify hundreds of prospective CpG sites associated with diverse genes that may reflect differences in Pre-DM compared with T2DM. In addition, there were CpG hypomethylation changes associated with a number of genes that may be associated with development of complications of diabetes, such as nephropathy. These hypomethylation changes were observed in all of the subjects.

CONCLUSION: These data suggest that some epigenomic changes that may be involved in the progression of diabetes and/or the development of complications may be apparent at the Pre-DM state or during the transition to diabetes. Hypomethylation of a number of genes related to kidney function may be an early marker for developing diabetic nephropathy.

Distal convoluted tubule

The distal convoluted tubule (DCT) is a short nephron segment, interposed between the macula densa and collecting duct. Even though it is short, it plays a key role in regulating extracellular fluid volume and electrolyte homeostasis. DCT cells are rich in mitochondria, and possess the highest density of Na+/K+-ATPase along the nephron, where it is expressed on the highly amplified basolateral membranes. DCT cells are largely water impermeable, and reabsorb sodium and chloride across the apical membrane via electroneurtral pathways. Prominent among this is the thiazide-sensitive sodium chloride cotransporter, target of widely used diuretic drugs. These cells also play a key role in magnesium reabsorption, which occurs predominantly, via a transient receptor potential channel (TRPM6). Human genetic diseases in which DCT function is perturbed have provided critical insights into the physiological role of the DCT, and how transport is regulated. These include Familial Hyperkalemic Hypertension, the salt-wasting diseases Gitelman syndrome and EAST syndrome, and hereditary hypomagnesemias. The DCT is also established as an important target for the hormones angiotensin II and aldosterone; it also appears to respond to sympathetic-nerve stimulation and changes in plasma potassium. Here, we discuss what is currently known about DCT physiology. Early studies that determined transport rates of ions by the DCT are described, as are the channels and transporters expressed along the DCT with the advent of molecular cloning. Regulation of expression and activity of these channels and transporters is also described; particular emphasis is placed on the contribution of genetic forms of DCT dysregulation to our understanding.

Recombinant human erythropoietin pretreatment attenuates acute renal tubular injury against ischemia-reperfusion by restoring transient receptor potential channel-6 expression and function in collecting ducts

OBJECTIVE

Acute renal tubular injury is a serious complication in the postoperative period, which is associated with high mortality and increased ICU stay. We aimed to demonstrate the protective effect of rhEPO against acute tubular injury induced by ischemia-reperfusion and to explore the mechanism of canonical transient receptor potential channel-6.

DESIGN

Randomized laboratory animal study.

SETTINGS

Animal research laboratory.

INTERVENTIONS

Male Sprague-Dawley rats were randomly divided into three groups: the sham group, the control group, and the rhEPO group. Experimental acute tubular injury was established in rats by bilateral renal arterial occlusion for 30 minutes followed by reperfusion.

MEASUREMENTS AND MAIN RESULTS

Blood samples were obtained for cystatin-C and neutrophil gelatinase-associated lipocalin measurements by enzyme-linked immunosorbance assays. Seventy-two hours after reperfusion, urine samples were collected for osmolality and fractional excretion of sodium (%) assays on a chemistry analyzer. Kidneys were harvested at 24, 48, and 72 hours after reperfusion. Transient receptor potential channel-6, aquaporin-2, and Na,K-ATPase expression in collecting ducts were studied by immunofluorescence and Western blot. Coimmunoprecipitations were also performed to identify the possible signalplex relation between transient receptor potential channel-6 and aquaporin-2 or Na,K-ATPase channels. RhEPO pretreatment significantly inhibited serum cystatin-C (2 hr: 453 ± 64 μg/L vs 337 ± 28 μg/L, p < 0.01), serum neutrophil gelatinase-associated lipocalin (72 hr: 1,175 ± 107 ng/L vs 1,737 ± 402 ng/L, p < 0.05), and urinary fractional excretion of sodium (%) increase (0.9 ± 0.1 vs 2.2 ± 0.8, p < 0.05) and alleviated the decrease of urinary osmolality (1,293 ± 101 mosmol/kg H2O vs 767 ± 91 mosmol/kg H2O, p < 0.05) induced by ischemia-reperfusion injury. Meanwhile, recombinant human erythropoietin greatly improved the ischemia-reperfusion-induced attenuation of transient receptor potential channel-6 expression (48 hr: 42% ± 2% vs 67% ± 2% and 72 hr: 55% ± 2% vs 66% ± 2%), as well as aquaporin-2 and Na,K-ATPase expression in collecting ducts. Transient receptor potential channel-6 functionally interacted with Na,K-ATPase but not aquaporin-2.

CONCLUSIONS

Recombinant human erythropoietin pretreatment at the dose of 5,000 IU/kg potently prevented ischemia-reperfusion-induced acute tubular injury, which might be partly attributed to the restoring the effect of transient receptor potential channel-6 expression and collecting duct function.

Reduced renal calcium excretion in the absence of sclerostin expression: evidence for a novel calcium-regulating bone kidney axis

The kidneys contribute to calcium homeostasis by adjusting the reabsorption and excretion of filtered calcium through processes that are regulated by parathyroid hormone (PTH) and 1α,25-dihydroxyvitamin D3 (1α,25[OH]2D3). Most of the filtered calcium is reabsorbed in the proximal tubule, primarily by paracellular mechanisms that are not sensitive to calcium-regulating hormones in physiologically relevant ways. In the distal tubule, however, calcium is reabsorbed by channels and transporters, the activity or expression of which is highly regulated and increased by PTH and 1α,25(OH)2D3. Recent research suggests that other, heretofore unrecognized factors, such as the osteocyte-specific protein sclerostin, also regulate renal calcium excretion. Clues in this regard have come from the study of humans and mice with inactivating mutations of the sclerostin gene that both have increased skeletal density, which would necessitate an increase in intestinal absorption and/or renal reabsorption of calcium. Deletion of the sclerostin gene in mice significantly diminishes urinary calcium excretion and increases fractional renal calcium reabsorption. This is associated with increased circulating 1α,25(OH)2D3 levels, whereas sclerostin directly suppresses 1α-hydroxylase in immortalized proximal tubular cells. Thus, evidence is accumulating that sclerostin directly or indirectly reduces renal calcium reabsorption, suggesting the presence of a novel calcium-excreting bone-kidney axis.

A novel fluorescent chemosensor allows the assessment of intracellular total magnesium in small samples

Remarkable features of a novel fluorescent Mg dye: high fluorescence intensity and intracellular retention.

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.

Canonical transient receptor potential channels in diabetes

Canonical transient receptor potential (TRPC) channel proteins have been identified as downstream molecules in a G protein-coupled receptor signaling pathway and are involved in a variety of cell functions due to their ability to regulate intracellular calcium signaling. TRPC channel physiology has been an increasingly interesting and relevant topic over the last decade, and the outcomes from various studies have advanced our understanding of TRPC function in the normal state. Recently, attention has turned to whether or not TRPC proteins are implicated in diseases. Emerging evidence suggests a significant contribution of several isoforms of TRPC proteins to cardiovascular as well as renal diseases. This review focuses on the implication of TRPC proteins as they pertain to diabetes. We summarize the recent findings by other investigators as well as ourselves and additionally discuss the important role of TRPC proteins in the development of various diabetic complications, such as diabetic nephropathy and diabetic vasculopathy. The underlying mechanisms which contribute to these complications are also outlined. Lastly, we elaborate on the role of TRPC proteins as a potential therapeutic target for treating diabetes-associated diseases.

Identification of the vitamin D receptor in various cells of the mouse kidney

The kidney is the major, if not sole, site for the production of 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃), the biologically active form of vitamin D that can stimulate calcium reabsorption in the kidney and may provide renoprotective benefits. The biological effects of 1,25(OH)₂D₃ are mediated through a nuclear hormone receptor, known as the vitamin D receptor (VDR). It is well accepted that the VDR is present in the distal renal convoluted tubule cells; however, whether VDR is present in other kidney cell types is uncertain. Using a highly specific and sensitive anti-VDR antibody, we determined its distribution in the mouse kidney by immunohistochemistry. Our results show that the VDR is not only present in the distal but is also found in the proximal tubules, but at 24-fold lower levels. The VDR was also found in the macula densa of the juxtaglomerular apparatus, glomerular parietal epithelial cells, and podocytes. In contrast, the VDR is either very low or absent in interstitial fibroblasts, glomerular mesangial cells, and juxtaglomerular cells. Thus, identification of VDR in the proximal tubule, macula densa, and podocytes suggests that 1,25(OH)₂D₃ plays a direct role in these cells under normal conditions.

TRPC6 mutations in children with steroid-resistant nephrotic syndrome and atypical phenotype

BACKGROUND AND OBJECTIVES

Mutations in the TRPC6 gene have been recently identified as the cause of late-onset autosomal-dominant focal segmental glomerulosclerosis (FSGS). To extend the screening, we analyzed TRPC6 in 33 Italian children with sporadic early-onset SRNS and three Italian families with adult-onset FSGS.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

TRPC6 mutation analysis was performed through PCR and sequencing. The effects of the detected amino acid substitutions were analyzed by bioinformatics tools and functional in vitro studies. The expression levels of TRPC6 and nephrin proteins were evaluated by confocal microscopy.

RESULTS

Three heterozygous missense mutations (c.374A>G_p.N125S, c.653A>T_p.H218L, c.2684G>T_p.R895L) were identified. The first new mutation, p.H218L, was found in a 18-year-old boy who presented a severe form of FSGS at the age of 8 years. The second, p.R895L, a new de novo mutation, was identified in a girl with collapsing glomerulosclerosis at the age of 2 years. The former mutation, p.N125S, was found in two siblings with early-onset steroid-resistant nephrotic syndrome (SRNS) at the ages of 4 and 14 years. Renal immunofluorescence revealed upregulated expression of TRPC6 and loss of nephrin in glomeruli. The intracellular calcium concentrations were significantly higher in the cells expressing all mutant TRPC6 channels compared with cells expressing wild-type TRPC6.

CONCLUSIONS

Our findings suggest that TRPC6 variants can also be detected in children with early-onset and sporadic SRNS (4 of 33 patients). Moreover, in one patient a new de novo TRPC6 mutation was associated with a rare severe form of childhood collapsing glomerulosclerosis with rapid progression to uremia.

Angiotensin II contributes to podocyte injury by increasing TRPC6 expression via an NFAT-mediated positive feedback signaling pathway

The transient receptor potential channel C6 (TRPC6) is a slit diaphragm-associated protein in podocytes involved in regulating glomerular filter function. Gain-of-function mutations in TRPC6 cause hereditary focal segmental glomerulosclerosis (FSGS), and several human acquired proteinuric diseases show increased glomerular TRPC6 expression. Angiotensin II (AngII) is a key contributor to glomerular disease and may regulate TRPC6 expression in nonrenal cells. We demonstrate that AngII regulates TRPC6 mRNA and protein levels in cultured podocytes and that AngII infusion enhances glomerular TRPC6 expression in vivo. In animal models for human FSGS (doxorubicin nephropathy) and increased renin-angiotensin system activity (Ren2 transgenic rats), glomerular TRPC6 expression was increased in an AngII-dependent manner. TRPC6 expression correlated with glomerular damage markers and glomerulosclerosis. We show that the regulation of TRPC6 expression by AngII and doxorubicin requires TRPC6-mediated Ca(2+) influx and the activation of the Ca(2+)-dependent protein phosphatase calcineurin and its substrate nuclear factor of activated T cells (NFAT). Accordingly, calcineurin inhibition by cyclosporine decreased TRPC6 expression and reduced proteinuria in doxorubicin nephropathy, whereas podocyte-specific inducible expression of a constitutively active NFAT mutant increased TRPC6 expression and induced severe proteinuria. Our findings demonstrate that the deleterious effects of AngII on podocytes and its pathogenic role in glomerular disease involve enhanced TRPC6 expression via a calcineurin/NFAT positive feedback signaling pathway.

New evidence of a dihydropyridine-activated cationic channel in the MDCK cell line

Newborn rat distal cells express an apical Ca2+ channel activated by dihydropyridine drugs. Similarly, in Madin-Darby canine kidney (MDCK) cells, nifedipine increased Ca2+i in a concentration-dependent manner (IC50=4 μM) in fura-2-loaded cells. Response to nifedipine was abolished by EGTA, suggesting that it depends on extracellular calcium. Ca2+ channel antagonist isradipine and agonist BayK8644 increased Ca2+i indicating that this effect is related to the dihydropyridine group. Diltiazem (20 μM) and gadolinium (200 μM) decreased the nifedipine effect (62 and 43%, respectively). Lanthanum (100 μM) did not change the response. Valinomycin clamping of the membrane potential did not modify nifedipine-induced increment, indicating that it was unrelated to potassium fluxes. We performed whole cell clamp experiments in MDCK cells maintained at -50 mV with perfusion solution containing 10 mM CaCl2. Nifedipine (20 μM) induced an increase in current (1.2±0.3 nA), which was partially inhibited by Gd3+. No significant current was induced by nifedipine in the presence of 0.5 mM EGTA. To determine the effects of nifedipine on the membrane potential, we performed oxonol fluorescence experiments. The addition of nifedipine or Bay K8644 induced depolarization, highly dependent on external sodium. Nifedipine (20 μM) induced depolarization of 6.9±0.8 mV (n=21). EC50 to nifedipine was in the 10 μM range. We conclude that MDCK cells exhibit a dihydropyridine-activated cationic channel.

TRP channels and their implications in metabolic diseases

The transient receptor potential (TRP) channel superfamily is composed of 28 nonselective cation channels that are ubiquitously expressed in many cell types and have considerable functional diversity. Although changes in TRP channel expression and function have been reported in cardiovascular disease and renal disorders, the pathogenic roles of TRP channels in metabolic diseases have not been systemically reviewed. In this review, we summarised the distribution of TRP channels in several metabolic tissues and discussed their roles in mediating and regulating various physiological and pathophysiological metabolic processes and diseases including diabetes, obesity, dyslipidaemia, metabolic syndrome, atherosclerosis, metabolic bone diseases and electrolyte disturbances. This review provides new insight into the involvement of TRP channels in the pathogenesis of metabolic disorders and implicates these channels as potential therapeutic targets for the management of metabolic diseases.

Renal TRPathies

Many ion channels and transporters are involved in the filtration, secretion, and resorption of electrolytes by the kidney. In recent years, the superfamily of transient receptor potential (TRP) ion channels have received deserved attention because mutated TRP channels are linked to human kidney diseases. This review focuses on two TRP members--TRPC6 and TRPM6--and their functions in the kidney. Gain-of-function mutations in TRPC6 are the cause for progressive kidney failure with urinary protein loss such as FSGS. Thus, TRPC6 is an essential signaling component in a functional slit diaphragm formed by podocytes around the glomerular capillaries. Loss-of-function mutations in TRPM6 are a molecular cause of hypomagnesemia with secondary hypocalcemia, suggesting that TRPM6 is critically involved in transcellular Mg2+ transport in the kidney. Here, we highlight how recent studies analyzing function and expression of these channels in the kidney improve our mechanistic understanding of TRP channel function in general and pave the way to new, promising therapeutic strategies to target kidney diseases such as FSGS and hypomagnesemia with secondary hypocalcemia.

A novel TRPC6 mutation that causes childhood FSGS

BACKGROUND

TRPC6, encoding a member of the transient receptor potential (TRP) superfamily of ion channels, is a calcium-permeable cation channel, which mediates capacitive calcium entry into the cell. Until today, seven different mutations in TRPC6 have been identified as a cause of autosomal-dominant focal segmental glomerulosclerosis (FSGS) in adults.

METHODOLOGY/PRINCIPAL FINDINGS

Here we report a novel TRPC6 mutation that leads to early onset FSGS. We identified one family in whom disease segregated with a novel TRPC6 mutation (M132T), that also affected pediatric individuals as early as nine years of age. Twenty-one pedigrees compatible with an autosomal-dominant mode of inheritance and biopsy-proven FSGS were selected from a worldwide cohort of 550 families with steroid resistant nephrotic syndrome (SRNS). Whole cell current recordings of the mutant TRPC6 channel, compared to the wild-type channel, showed a 3 to 5-fold increase in the average out- and inward TRPC6 current amplitude. The mean inward calcium current of M132T was 10-fold larger than that of wild-type TRPC6. Interestingly, M132T mutants also lacked time-dependent inactivation. Generation of a novel double mutant M132T/N143S did not further augment TRPC6 channel activity.

CONCLUSIONS

In summary, our data shows that TRPC6 mediated FSGS can also be found in children. The large increase in channel currents and impaired channel inactivation caused by the M132T mutant leads to an aggressive phenotype that underlines the importance of calcium dose channeled through TRPC6.

R168H and V165X mutant podocin might induce different degrees of podocyte injury via different molecular mechanisms

A lot of mutations of podocin, a key protein of podocyte slit diaphragm (SD), have been found both in hereditary and sporadic focal segmental glomeruloscleorosis (FSGS). Nevertheless, the mechanisms of podocyte injury induced by mutant podocins are still unclear. A compound heterozygous podocin mutation was identified in our FSGS patient, leading to a truncated (podocin (V165X)) and a missense mutant protein (podocin (R168H)), respectively. Here, it was explored whether and how both mutant podocins induce podocyte injury in the in vitro cultured podocyte cell line. Our results showed that podocin (R168H) induced more significant podocyte apoptosis and expression changes in more podocyte molecules than podocin (V165X). Podocyte injury caused by the normal localized podocin(V165X) was effectively inhibited by TRPC6 knockdown. The abnormal retention of podocin(R168H) in endoplasmic reticulum (ER) resulted in the mis-localizations of other critical SD molecules nephrin, CD2AP and TRPC6, and significantly up-regulated ER stress markers Bip/grp78, p-PERK and caspase-12. These results implicated that podocin (R168H) and podocin (V165X) induced different degrees of podocyte injury, which might be resulted from different molecular mechanisms. Our findings provided some possible clues for further exploring the pharmacological targets to the proteinuria induced by different mutant podocins.

Organellar calcium signalling mechanisms in Drosophilaepithelial function

Calcium signalling and calcium homeostasis are essential for life. Studies of calcium signalling thus constitute a major proportion of research in the life sciences, although the majority of these studies are based in cell lines or isolated cells. Epithelial cells and tissues are essential in the regulation of critical physiological processes, including fluid transport; and so the modulation of such processes in vivo by cell-specific calcium signalling is thus of interest. In this review, we describe the approaches to measuring intracellular calcium in the genetically tractable fluid-transporting tissue, the Drosophila Malpighian tubule by targeting cell-specific protein-based calcium reporters to defined regions,cells and intracellular compartments of the intact Malpighian tubule. We also discuss recent findings on the roles of plasma membrane and intracellular calcium channels; and on organellar stores – including mitochondria,Golgi and peroxisomes – in Malpighian tubule function.

Interplay between TRP channels and the cytoskeleton in health and disease

Transient receptor potential (TRP) channels are a family of cation channels that play a key role in ion homeostasis and cell volume regulation. In addition, TRP channels are considered universal integrators of sensory information required for taste, vision, hearing, touch, temperature, and the detection of mechanical force. Seminal investigations exploring the molecular mechanisms of phototransduction in Drosophila have demonstrated that TRP channels operate within macromolecular complexes closely associated with the cytoskeleton. More recent evidence shows that mammalian TRP channels similarly connect to the cytoskeleton to affect cytoskeletal organization and cell adhesion via ion-transport-dependent and -independent mechanisms. In this review, we discuss new insights into the interplay between TRP channels and the cytoskeleton and provide recent examples of such interactions in different physiological systems.

Calciotropic and Magnesiotropic TRP Channels

Significant progress has been made into our understanding of the molecular mechanisms responsible for Ca2+ and Mg2+ homeostasis. Members of the transient receptor potential channel (TRP) superfamily proved essential to the maintenance of divalent cation levels by regulating their absorption from renal and intestinal lumina. This review highlights the molecular and functional aspects of these new calciotropic and magnesiotropic TRPs in health and disease.

Transient receptor potential melastatin 6 and 7 channels, magnesium transport, and vascular biology: implications in hypertension

Magnesium, an essential intracellular cation, is critically involved in many biochemical reactions involved in the regulation of vascular tone and integrity. Decreased magnesium concentration has been implicated in altered vascular reactivity, endothelial dysfunction, vascular inflammation, and structural remodeling, processes important in vascular changes and target organ damage associated with hypertension. Until recently, very little was known about mechanisms regulating cellular magnesium homeostasis, and processes controlling transmembrane magnesium transport had been demonstrated only at the functional level. Two cation channels of the transient receptor potential melastatin (TRPM) cation channel family have now been identified as magnesium transporters, TRPM6 and TRPM7. These unique proteins, termed chanzymes because they possess a channel and a kinase domain, are differentially expressed, with TRPM6 being found primarily in epithelial cells and TRPM7 occurring ubiquitously. Vascular TRPM7 is modulated by vasoactive agents, pressure, stretch, and osmotic changes and may be a novel mechanotransducer. In addition to its magnesium transporter function, TRPM7 has been implicated as a signaling kinase involved in vascular smooth muscle cell growth, apoptosis, adhesion, contraction, cytoskeletal organization, and migration, important processes involved in vascular remodeling associated with hypertension and other vascular diseases. Emerging evidence suggests that vascular TRPM7 function may be altered in hypertension. This review discusses the importance of magnesium in vascular biology and implications in hypertension and highlights the transport systems, particularly TRPM6 and TRPM7, which may play a role in the control of vascular magnesium homeostasis. Since the recent identification and characterization of Mg2+-selective transporters, there has been enormous interest in the field. However, there is still a paucity of information, and much research is needed to clarify the exact mechanisms of magnesium regulation in the cardiovascular system and the implications of aberrant transmembrane magnesium transport in the pathogenesis of hypertension and other vascular diseases.