Essential role of Ca2+ release channels in angiotensin II-induced Ca2+ oscillations and mesangial cell contraction

Activation of Pannexin-1 channels causes cell dysfunction and damage in mesangial cells derived from angiotensin II-exposed mice

Chronic kidney disease (CKD) is a prevalent health concern associated with various pathological conditions, including hypertensive nephropathy. Mesangial cells are crucial in maintaining glomerular function, yet their involvement in CKD pathogenesis remains poorly understood. Recent evidence indicates that overactivation of Pannexin-1 (Panx1) channels could contribute to the pathogenesis and progression of various diseases. Although Panx1 is expressed in the kidney, its contribution to the dysfunction of renal cells during pathological conditions remains to be elucidated. This study aimed to investigate the impact of Panx1 channels on mesangial cell function in the context of hypertensive nephropathy. Using an Ang II-infused mouse model and primary mesangial cell cultures, we demonstrated that in vivo exposure to Ang II sensitizes cultured mesangial cells to show increased alterations when they are subjected to subsequent in vitro exposure to Ang II. Particularly, mesangial cell cultures treated with Ang II showed elevated activity of Panx1 channels and increased release of ATP. The latter was associated with enhanced basal intracellular Ca2+ ([Ca2+]i) and increased ATP-mediated [Ca2+]i responses. These effects were accompanied by increased lipid peroxidation and reduced cell viability. Crucially, all the adverse impacts evoked by Ang II were prevented by the blockade of Panx1 channels, underscoring their critical role in mediating cellular dysfunction in mesangial cells. By elucidating the mechanisms by which Ang II negatively impacts mesangial cell function, this study provides valuable insights into the pathogenesis of renal damage in hypertensive nephropathy.

Hypertensive Nephropathy: Unveiling the Possible Involvement of Hemichannels and Pannexons

Hypertension is one of the most common risk factors for developing chronic cardiovascular diseases, including hypertensive nephropathy. Within the glomerulus, hypertension causes damage and activation of mesangial cells (MCs), eliciting the production of large amounts of vasoactive and proinflammatory agents. Accordingly, the activation of AT1 receptors by the vasoactive molecule angiotensin II (AngII) contributes to the pathogenesis of renal damage, which is mediated mostly by the dysfunction of intracellular Ca²⁺ ([Ca²⁺]_i) signaling. Similarly, inflammation entails complex processes, where [Ca²⁺]_i also play crucial roles. Deregulation of this second messenger increases cell damage and promotes fibrosis, reduces renal blood flow, and impairs the glomerular filtration barrier. In vertebrates, [Ca²⁺]_i signaling depends, in part, on the activity of two families of large-pore channels: hemichannels and pannexons. Interestingly, the opening of these channels depends on [Ca²⁺]_i signaling. In this review, we propose that the opening of channels formed by connexins and/or pannexins mediated by AngII induces the ATP release to the extracellular media, with the subsequent activation of purinergic receptors. This process could elicit Ca²⁺ overload and constitute a feed-forward mechanism, leading to kidney damage.

Angiotensin II-Induced Mesangial Cell Damaged Is Preceded by Cell Membrane Permeabilization Due to Upregulation of Non-Selective Channels

Connexin43 (Cx43), pannexin1 (Panx1) and P2X₇ receptor (P2X₇R) are expressed in kidneys and are known to constitute a feedforward mechanism leading to inflammation in other tissues. However, the possible functional relationship between these membrane channels and their role in damaged renal cells remain unknown. In the present work, we found that MES-13 cells, from a cell line derived from mesangial cells, stimulated with angiotensin II (AngII) developed oxidative stress (OS, thiobarbituric acid reactive species (TBARS) and generated pro-inflammatory cytokines (ELISA; IL-1β and TNF-α). The membrane permeability increased progressively several hours before the latter outcome, which was a response prevented by Losartan, indicating the involvement of AT1 receptors. Western blot analysis showed that the amount of phosphorylated MYPT (a substrate of RhoA/ROCK) and Cx43 increased progressively and in parallel in cells treated with AngII, a response followed by an increase in the amount in Panx1 and P2X₇R. Greater membrane permeability was partially explained by opening of Cx43 hemichannels (Cx43 HCs) and Panx1 channels (Panx1 Chs), as well as P2X₇Rs activation by extracellular ATP, which was presumably released via Cx HCs and Panx1 Chs. Additionally, inhibition of RhoA/ROCK blocked the progressive increase in membrane permeability, and the remaining response was explained by the other non-selective channels. The rise of activity in the RhoA/ROCK-dependent pathway, as well as in Cx HCs, P2X₇R, and to a minor extent in Panx1 Chs led to higher amounts of TBARS and pro-inflammatory cytokines. We propose that AngII-induced mesangial cell damage could be effectively inhibited by concomitantly inhibiting the RhoA/ROCK-dependent pathway and one or more non-selective channel(s) activated through this pathway.

The mitochondrial Na(+)/Ca(2+) exchanger may reduce high glucose-induced oxidative stress and nucleotide-binding oligomerization domain receptor 3 inflammasome activation in endothelial cells

Background

The mitochondrial Na⁺/Ca²⁺ exchanger, NCLX, plays an important role in the balance between Ca²⁺ influx and efflux across the mitochondrial inner membrane in endothelial cells. Mitochondrial metabolism is likely to be affected by the activity of NCLX because Ca²⁺ activates several enzymes of the Krebs cycle. It is currently believed that mitochondria are not only centers of energy production but are also important sites of reactive oxygen species (ROS) generation and nucleotide-binding oligomerization domain receptor 3 (NLRP3) inflammasome activation.

Methods & Results

This study focused on NCLX function, in rat aortic endothelial cells (RAECs), induced by glucose. First, we detected an increase in NCLX expression in the endothelia of rats with diabetes mellitus, which was induced by an injection of streptozotocin. Next, colocalization of NCLX expression and mitochondria was detected using confocal analysis. Suppression of NCLX expression, using an siRNA construct (siNCLX), enhanced mitochondrial Ca²⁺ influx and blocked efflux induced by glucose. Unexpectedly, silencing of NCLX expression induced increased ROS generation and NLRP3 inflammasome activation.

Conclusions

These findings suggest that NCLX affects glucose-dependent mitochondrial Ca²⁺ signaling, thereby regulating ROS generation and NLRP3 inflammasome activation in high glucose conditions. In the early stages of high glucose stimulation, NCLX expression increases to compensate in order to self-protect mitochondrial maintenance, stability, and function in endothelial cells.

AngII-induced glomerular mesangial cell proliferation inhibited by losartan via changes in intracellular calcium ion concentration

This study investigated the changes in intracellular [Ca²⁺]ⁱ (intracellular calcium ion concentration) and TRPC6 (transient receptor potential channel 6) expression during angiotensin II (AngII)-induced glomerular mesangial cell (GMC) proliferation, as well as the inhibitory effect of losartan. GMC cultures were split into four groups treated for 24 h: Group N (blank control group), Group A (10⁻⁷ mol/L AngII), Group LT (10⁻⁷ mol/L AngII and 10⁻⁵ mol/L losartan), and Group Pred (10⁻⁷ mol/L AngII and 10⁻⁵ mol/L prednisone). GMCs proliferation was measured by the MTT and trypan blue assays. The distribution of TRPC6 was monitored by immunofluorescence, the expression of TRPC6 was detected by RT-PCR and Western blotting, and [Ca²⁺]ⁱ was measured by laser scanning confocal microscopy. The results showed that the maximal proliferation of GMCs was induced by treatment with 10⁻⁷ mol/L AngII for 24 h. In Group A, the distribution of TRPC6 was not uniform in the cell membrane, there was increased accumulation of this protein within the cytoplasm, and the increased expression of TRPC6 and [Ca²⁺]ⁱ was consistent with the proliferation of cells. In Group LT, losartan inhibited the proliferation of GMCs significantly, the levels of TRPC6 and [Ca²⁺]ⁱ were diminished, and the distribution of TRPC6 was improved. Prednisone also significantly inhibited the proliferation of GMCs and had no effects on the expression of TRPC6 and [Ca²⁺]ⁱ in Group Pred. These findings suggested that AngII could enhance the expression of TRPC6, increase [Ca²⁺]^i, and demonstrate a time–dose–response relationship with the proliferation of GMCs, while losartan reversed the effect of AngII on GMC proliferation.

Activation of Na+/H+ exchanger NHE3 by angiotensin II is mediated by inositol 1,4,5-triphosphate (IP3) receptor-binding protein released with IP3 (IRBIT) and Ca2+/calmodulin-dependent protein kinase II

Angiotensin II (ANG II) stimulates renal tubular reabsorption of NaCl by targeting Na(+)/H(+) exchanger NHE3. We have shown previously that inositol 1,4,5-triphosphate receptor-binding protein released with inositol 1,4,5-triphosphate (IRBIT) plays a critical role in stimulation of NHE3 in response to elevated intracellular Ca(2+) concentration ([Ca(2+)](i)). In this study, we investigated the role of IRBIT in mediating NHE3 activation by ANG II. IRBIT is abundantly expressed in the proximal tubules where NHE3 is located. ANG II at physiological concentrations stimulates NHE3 transport activity in a model proximal tubule cell line. ANG II-induced activation of NHE3 was abrogated by knockdown of IRBIT, whereas overexpression of IRBIT enhanced the effect of ANG II on NHE3. ANG II transiently increased binding of IRBIT to NHE3 at 5 min but became dissociated by 45 min. In comparison, it took at least 15 min of ANG II treatment for an increase in NHE3 activity and NHE3 surface expression. The stimulation of NHE3 by ANG II was dependent on changes in [Ca(2+)](i) and Ca(2+)/calmodulin-dependent protein kinases II. Inhibition of CaMKII completely blocked the ANG II-induced binding of IRBIT to NHE3 and the increase in NHE3 surface abundance. Several serine residues of IRBIT are thought to be important for IRBIT binding. Mutations of Ser-68, Ser-71, and Ser-74 of IRBIT decreased binding of IRBIT to NHE3 and its effect on NHE3 activity. In conclusion, our current findings demonstrate that IRBIT is critically involved in mediating activation of NHE3 by ANG II via a Ca(2+)/calmodulin-dependent protein kinases II-dependent pathway.

Molecular mechanism of ADP-ribosyl cyclase activation in angiotensin II signaling in murine mesangial cells

ADP-ribosyl cyclase (ADPR-cyclase) produces a Ca(2+)-mobilizing second messenger cyclic ADP-ribose (cADPR) from NAD(+). In this study, we investigated the molecular basis of ADPR-cyclase activation and the following cellular events in angiotensin II (ANG II) signaling in mouse mesangial cells (MMCs). Treatment of MMCs with ANG II induced an increase in intracellular Ca(2+) concentrations through a transient Ca(2+) release via an inositol 1,4,5-trisphosphate receptor and a sustained Ca(2+) influx via L-type Ca(2+) channels. The sustained Ca(2+) signal, but not the transient Ca(2+) signal, was blocked by a cADPR antagonistic analog, 8-bromo-cADPR (8-Br-cADPR), and an ADPR-cyclase inhibitor, 4,4'-dihydroxyazobenzene (DHAB). In support of the results, ANG II stimulated cADPR production in a time-dependent manner, and DHAB inhibited ANG II-induced cADPR production. Application of pharmacological inhibitors revealed that activation of ADPR-cyclase by ANG II involved ANG II type 1 receptor, phosphoinositide 3-kinase, protein tyrosine kinase, and phospolipase C-gamma1. Moreover, DHAB as well as 8-Br-cADPR abrogated ANG II-mediated Akt phosphorylation, nuclear translocation of nuclear factor of activated T cell, and uptake of [(3)H]thymidine and [(3)H]leucine in MMCs. These results demonstrate that ADPR-cyclase in MMCs plays a pivotal role in ANG II signaling for cell proliferation and protein synthesis.

Effects on kidney filtration rate by agmatine requires activation of ryanodine channels for nitric oxide generation

Agmatine, decarboxylated arginine, is produced in the kidney and can increase nephron and kidney filtration rate via renal vasodilatation and increases in plasma flow. This increase in filtration rate after agmatine is prevented by administration of nitric oxide synthase (NOS) inhibitors. In endothelial cells, agmatine-stimulated nitrite production is accompanied by induction of cytosolic calcium. NOS activity requires calcium for activation; however, the source of this calcium remains unknown. Ryanodine receptor (RyR) calcium-activated calcium release channels are present in the kidney cortex, and we evaluated if RyR contributes to the agmatine response. Agmatine microperfused into Bowman's space reversibly increases nephron filtration rate (SNGFR) by approximately 30%. cADP-ribose (cADPR) regulates RyR channel activity. Concurrent infusion of agmatine with the cADPR blocker 8-bromo-cADPR (2 microM) prevents the increase in filtration rate. Furthermore, direct activation of the RyR channel with ryanodine at agonist concentrations (5 microM) increases SNGFR, and, like agmatine, this increase is prevented by administration of N(G)-monomethyl-l-arginine, a nonselective NOS blocker. We demonstrate that agmatine does not elicit ADPR cyclase activity in vascular smooth muscle membranes and does not directly affect RyR calcium channel responses using sea urchin egg homogenates. These results imply interplay between endothelial cell cADPR/RyR/Ca(2+)/NO and the cADPR/RyR/Ca(2+) pathways in vascular smooth muscle cells in arterioles in the regulation of kidney filtration rate. In conclusion, we show that agmatine-induced effects require activation of cADPR and RyR calcium release channels for NO generation, vasodilation, and increased filtration rate.

Membrane current oscillations in descending vasa recta pericytes

We investigated the origin of spontaneous transient inward current (STIC) oscillations in descending vasa recta (DVR) pericytes. In cells clamped at -80 mV, angiotensin II (ANG II; 10 nmol/l) induced oscillations with mean amplitude and frequency of -65.5 pA and 1.2 Hz. Simultaneous recording of cytoplasmic calcium ([Ca(2+)](CYT)) and membrane current oscillations verified their synchrony and the correlation of their amplitudes. Confocal recording in fluo-4-loaded DVR showed that ANG II can induce either stable pericyte [Ca(2+)](CYT) elevation or oscillations, while decreasing adjacent endothelial [Ca(2+)](CYT). Oscillating currents reversed sign at -30.2 mV and were blocked by niflumic acid, implicating charge transfer via Cl(-) ion. Removal of extracellular Ca(2+), blockade of Ca(2+) influx with SKF96365 (30 micromol/l), ryanodine (30 micromol/l), or caffeine (10 mmol/l) inhibited oscillations. In contrast, they were insensitive to removal of extracellular Na(+) and exposure to either nifedipine (1 micromol/l) or 2-aminoethoxydiphenyl borate (10 micromol/l). Ouabain (100 nmol/l) increased basal pericyte [Ca(2+)](CYT) and the frequency of resting STICs but did not affect the larger oscillations that followed ANG II stimulation. We conclude that [Ca(2+)](CYT) oscillations stimulate Cl(-) currents. The former are most likely maintained by repetitive cycles of ryanodine-sensitive SR Ca(2+) release and SKF96365-sensitive store refilling.

ADP-ribosyl cyclase and ryanodine receptor activity contribute to basal renal vasomotor tone and agonist-induced renal vasoconstriction in vivo

An important role for the enzyme ADP-ribosyl cyclase (ADPR cyclase) and its downstream targets, the ryanodine receptors (RyR), is emerging for a variety of vascular systems. We hypothesized that the ADPR cyclase/RyR pathway contributes to regulation of renal vasomotor tone in vivo. To test this, we continuously measured renal blood flow (RBF) in anesthetized Sprague-Dawley rats. Infusion of the ADPR cyclase inhibitor nicotinamide intrarenally at low doses inhibits angiotensin II (ANG II)- and norepinephrine (NE)-induced vasoconstriction by 72 and 67%, respectively ( P < 0.001). RBF studies in rats were extended to mice lacking the predominant form of ADPR cyclase (CD38). Acute renal vasoconstrictor responses to ANG II and NE are impaired by 59 and 52%, respectively, in anesthetized CD38−/− mice compared with wild-type controls ( P < 0.05). Intrarenal injection of the RyR activator FK506 decreases RBF by 22% ( P > 0.03). Furthermore, RyR inhibition with ruthenium red attenuates ANG II and NE responses by 50 and 59%, respectively ( P ≤ 0.01). Given at higher doses, nicotinamide increases basal RBF by 22% ( P > 0.001). Non-receptor-mediated renal vasoconstriction by L-type voltage-gated Ca2+channels is also dependent on ADPR cyclase and RyRs. Nicotinamide and ruthenium red inhibit constriction by the L-type channel agonist BAY K 8644 by 59% ( P > 0.02) and 63% ( P > 0.001). We conclude that 1) ADPR cyclase activity contributes to regulation of renal vasomotor tone under resting conditions, 2) renal vasoconstriction induced by G protein-coupled receptor agonists ANG II and NE is mediated in part by ADPR cyclase and RyRs, and 3) ADPR cyclase and RyRs participate in L-type channel-mediated renal vasoconstriction in vivo.