Developmental changes in the expression and function of TRPC6 channels related the F-actin organization during differentiation in podocytes

Tension at the gate: sensing mechanical forces at the blood-brain barrier in health and disease

Microvascular brain endothelial cells tightly limit the entry of blood components and peripheral cells into the brain by forming the blood-brain barrier (BBB). The BBB is regulated by a cascade of mechanical and chemical signals including shear stress and elasticity of the adjacent endothelial basement membrane (BM). During physiological aging, but especially in neurological diseases including multiple sclerosis (MS), stroke, small vessel disease, and Alzheimer's disease (AD), the BBB is exposed to inflammation, rigidity changes of the BM, and disturbed cerebral blood flow (CBF). These altered forces lead to increased vascular permeability, reduced endothelial reactivity to vasoactive mediators, and promote leukocyte transmigration. Whereas the molecular players involved in leukocyte infiltration have been described in detail, the importance of mechanical signalling throughout this process has only recently been recognized. Here, we review relevant features of mechanical forces acting on the BBB under healthy and pathological conditions, as well as the endothelial mechanosensory elements detecting and responding to altered forces. We demonstrate the underlying complexity by focussing on the family of transient receptor potential (TRP) ion channels. A better understanding of these processes will provide insights into the pathogenesis of several neurological disorders and new potential leads for treatment.

Cytoskeleton Rearrangements Modulate TRPC6 Channel Activity in Podocytes

The actin cytoskeleton of podocytes plays a central role in the functioning of the filtration barrier in the kidney. Calcium entry into podocytes via TRPC6 (Transient Receptor Potential Canonical 6) channels leads to actin cytoskeleton rearrangement, thereby affecting the filtration barrier. We hypothesized that there is feedback from the cytoskeleton that modulates the activity of TRPC6 channels. Experiments using scanning ion-conductance microscopy demonstrated a change in migration properties in podocyte cell cultures treated with cytochalasin D, a pharmacological agent that disrupts the actin cytoskeleton. Cell-attached patch-clamp experiments revealed that cytochalasin D increases the activity of TRPC6 channels in CHO (Chinese Hamster Ovary) cells overexpressing the channel and in podocytes from freshly isolated glomeruli. Furthermore, it was previously reported that mutation in ACTN4, which encodes α-actinin-4, causes focal segmental glomerulosclerosis and solidifies the actin network in podocytes. Therefore, we tested whether α-actinin-4 regulates the activity of TRPC6 channels. We found that co-expression of mutant α-actinin-4 K255E with TRPC6 in CHO cells decreases TRPC6 channel activity. Therefore, our data demonstrate a direct interaction between the structure of the actin cytoskeleton and TRPC6 activity.

TRPC6-Mediated Ca2+ Entry Essential for the Regulation of Nano-ZnO Induced Autophagy in SH-SY5Y Cells

Recently, possible applications of zinc oxide nanoparticles (nano-ZnO) have been extensively studied owing to their ease of synthesis. However, the effect of nano-ZnO on the nervous system remains unclear. This study investigates the action of nano-ZnO on SH-SY5Y neuroblastoma cells. We found that nano-ZnO (0-50 µg/mL) induced a significant decrease in cell survival rate in a dose-dependent manner, and increased LC3 puncta formation. However, the apoptosis was not affected by nano-ZnO, because the protein levels of cytochrome c, caspase-3, Bcl-xL, and BAX were not varied by the nano-ZnO treatment. Nano-ZnO increased Ca²⁺ entry and the expression of TRPC6.The results suggested that nano-ZnO increased [Ca²⁺] through the TRPC-dependent Ca²⁺ influx, since Ca²⁺ influx can be prevented by the TRPC inhibitor. Furthermore, cells on nano-ZnO-treatment groups displayed loss of F-actin in a dose dependent manner, which also could be prevented by TRPC inhibitor. Herein, we demonstrated that the nano-ZnO activated TRPC6 channel, thereby increasing the Ca²⁺ flux and resulting in increased autophagy. Nano-ZnO could have possible anticancer effects in neuroblastoma by inhibiting the proliferation of tumor cells. However, we should also pay attention toward the biosecurity of nano materials.

TRPC channels: Structure, function, regulation and recent advances in small molecular probes

Transient receptor potential canonical (TRPC) channels constitute a group of receptor-operated calcium-permeable nonselective cation channels of the TRP superfamily. The seven mammalian TRPC members, which can be further divided into four subgroups (TRPC1, TRPC2, TRPC4/5, and TRPC3/6/7) based on their amino acid sequences and functional similarities, contribute to a broad spectrum of cellular functions and physiological roles. Studies have revealed complexity of their regulation involving several components of the phospholipase C pathway, G_i and G_o proteins, and internal Ca²⁺ stores. Recent advances in cryogenic electron microscopy have provided several high-resolution structures of TRPC channels. Growing evidence demonstrates the involvement of TRPC channels in diseases, particularly the link between genetic mutations of TRPC6 and familial focal segmental glomerulosclerosis. Because TRPCs were discovered by the molecular identity first, their pharmacology had lagged behind. This is rapidly changing in recent years owning to great efforts from both academia and industry. A number of potent tool compounds from both synthetic and natural products that selective target different subtypes of TRPC channels have been discovered, including some preclinical drug candidates. This review will cover recent advancements in the understanding of TRPC channel regulation, structure, and discovery of novel TRPC small molecular probes over the past few years, with the goal of facilitating drug discovery for the study of TRPCs and therapeutic development.

Role of Transient Receptor Potential Canonical Channel 6 (TRPC6) in Diabetic Kidney Disease by Regulating Podocyte Actin Cytoskeleton Rearrangement

Podocyte injury is an important pathogenesis step causing proteinuric kidney diseases such as diabetic kidney disease (DKD). Actin cytoskeleton rearrangement in podocyte induced by multiple pathogenic factors is believed to be the key process resulting in glomerular injury. Many studies have recently shown that transient receptor potential canonical channel 6 (TRPC6) in podocyte plays a critical role in the development and progression of proteinuric kidney disease by regulating its actin cytoskeleton rearrangement. This review is aimed at summarizing the role of TRPC6 on DKD by regulating the podocyte actin cytoskeleton rearrangement, thereby help further broaden our views and understanding on the mechanism of DKD and provide a theoretic basis for exploring new therapeutic targets for DKD patients.

Group I metabotropic glutamate receptor activation induces TRPC6-dependent calcium influx and RhoA activation in cultured human kidney podocytes

Researches have shown that mice lacking the metabotropic glutamate receptor 1 (mGluR) showed albuminuria, remodeling of F-actin, with loss of stress fibers. Selective group I mGluRs agonist (S)-3,5-dihydroxyphenylglycine (DHPG) attenuated albuminuria in several rodent models of nephrotic syndrome. However, the molecular mechanism is obscure. Using a human podocyte cell line, we here investigated the molecular mechanisms of group I mGluRs-induced calcium influx and the formation of stress fibers. Our data showed that group I mGluRs activation by DHPG induced a significant calcium influx, and promoted cytoskeletal stress fiber formation and focal adhesions in podocytes. Pre-incubating podocytes with non-selective inhibitor of transient receptor potential channels (TRPC), or the knockdown of TRPC6 attenuated the calcium influx and the stress fiber formation induced by DHPG. Further, DHPG resulted in an increase of active RhoA expression. However, the knockdown of RhoA by siRNA abolished the DHPG-induced increase in stress fibers. Additionally, nonselective inhibitors of TRPC or TRPC6 knockdown clearly inhibited RhoA activation induced by DHPG, as assessed by Glutathione-S-transferase pull-down assay followed by Western blotting. Taken together, our findings suggest TRPC6 regulates actin stress fiber formation and focal adhesions via the RhoA pathway in response to group I mGluRs activation. Our data can potentially explain the mechanism of protective action of group I mGluRs in glomerular podocyte injury.

Protective role of Trpc6 knockout in the progression of diabetic kidney disease

Diabetic kidney disease (DKD) is a chronic kidney pathology that leads to end-stage renal disease. Previous studies from our laboratory indicate that there is an association between the development of DKD and the transient receptor potential canonical 6 (TRPC6) channel. Trpc6 expression and activity were increased in the streptozotocin (STZ)-treated Dahl Salt-sensitive (Dahl SS) rat, an established model of type 1 diabetes. Here, using a Trpc6 knockout created on the Dahl SS rat background (SS^Trpc6-/-), we test the hypothesis that the absence of Trpc6 will protect podocytes and kidney function during the development of DKD. Four groups of animals (control SS^WT, SS^Trpc6-/-, STZ-treated SS^WT, and STZ-SS^Trpc6-/-) were utilized in this study. Diabetes development was monitored for 11 wk after STZ injection with periodic weight, glucose, and urinary output measurements. There was an increase in albuminuria and glomerular injury following STZ treatment, which was not different between Dahl SS and SS^Trpc6-/- groups. Western blot analysis revealed elevated levels of nephrin in urine samples of STZ-SS^WT rats, which was higher compared with STZ-SS^Trpc6-/- rats. Furthermore, pathological increases in basal [Ca²⁺]_i levels and foot process damage of podocytes during the development of DKD was attenuated in the STZ-SS^Trpc6-/- compared with STZ-SS^WT rats. Overall, our data indicate that TRPC6 channel inhibition may have at least partial renoprotective effects, which could lead to the development of new pharmacological tools to treat or prevent the progression of DKD.

"Immuno-Transient Receptor Potential Ion Channels": The Role in Monocyte- and Macrophage-Mediated Inflammatory Responses

Monocytes and macrophages play important roles in health and disease. They have a central role in protecting the host, as they clear pathogens and modulate other immune cell functions through the production of regulatory molecules. Their functions include immune surveillance, bacterial killing, tissue remodeling and repair, clearance of cell debris and more. Macrophages can have beneficial and detrimental effects on the outcome of several diseases depending on the microenvironment and the activation state of cells. Over the past few years, there has been an increasing interest in the expression and functions of ion channels, in particular of transient receptor potential (TRP) channel family in immune cells. The 30 members of mammalian TRP channels are subdivided into TRPC, TRPV, TRPM, TRPML, TRPP, and TRPA superfamily, and several members of TRP subfamily have been found to be functionally expressed in monocytes and macrophages. TRP are cation-selective channels that are weakly voltage-sensitive and diversely gated by temperature, mechanical force, electrophiles, ligands, and internal cues, such as membrane composition and pH, contributing to immune and inflammatory responses. The TRP channels play major roles in controlling several monocyte and macrophage functions such as phagocytosis, production of chemokines and cytokines, cell survival, polarization and so forth. In addition, they can also be potential therapeutic targets in a variety of inflammatory diseases. Thus, the goal of this review is to describe the role of TRP channels in the control of monocyte–macrophage functions in inflammatory and immune-mediated diseases.

Autophagy is involved in mouse kidney development and podocyte differentiation regulated by Notch signalling

Podocyte dysfunction results in glomerular diseases accounted for 90% of end-stage kidney disease. The evolutionarily conserved Notch signalling makes a crucial contribution in podocyte development and function. However, the underlying mechanism of Notch pathway modulating podocyte differentiation remains less obvious. Autophagy, reported to be related with Notch signalling pathways in different animal models, is regarded as a possible participant during podocyte differentiation. Here, we found the dynamic changes of Notch1 were coincided with autophagy: they both increased during kidney development and podocyte differentiation. Intriguingly, when Notch signalling was down-regulated by DAPT, autophagy was greatly diminished, and differentiation was also impaired. Further, to better understand the relationship between Notch signalling and autophagy in podocyte differentiation, rapamycin was added to enhance autophagy levels in DAPT-treated cells, and as a result, nephrin was recovered and DAPT-induced injury was ameliorated. Therefore, we put forward that autophagy is involved in kidney development and podocyte differentiation regulated by Notch signalling.

Renoprotection: focus on TRPV1, TRPV4, TRPC6 and TRPM2

Members of the transient receptor potential (TRP) cation channel receptor family have unique sites of regulatory function in the kidney which enables them to promote regional vasodilatation and controlled Ca²⁺ influx into podocytes and tubular cells. Activated TRP vanilloid 1 receptor channels (TRPV1) have been found to elicit renoprotection in rodent models of acute kidney injury following ischaemia/reperfusion. Transient receptor potential cation channel, subfamily C, member 6 (TRPC6) in podocytes is involved in chronic proteinuric kidney disease, particularly in focal segmental glomerulosclerosis (FSGS). TRP vanilloid 4 receptor channels (TRPV4) are highly expressed in the kidney, where they induce Ca²⁺ influx into endothelial and tubular cells. TRP melastatin (TRPM2) non-selective cation channels are expressed in the cytoplasm and intracellular organelles, where their inhibition ameliorates ischaemic renal pathology. Although some of their basic properties have been recently identified, the renovascular role of TRPV1, TRPV4, TRPC6 and TRPM2 channels in disease states such as obesity, hypertension and diabetes is largely unknown. In this review, we discuss recent evidence for TRPV1, TRPV4, TRPC6 and TRPM2 serving as potential targets for acute and chronic renoprotection in chronic vascular and metabolic disease.