OCaR1 endows exocytic vesicles with autoregulatory competence by preventing uncontrolled Ca2+ release, exocytosis, and pancreatic tissue damage

Regulated exocytosis is initiated by increased Ca2+ concentrations in close spatial proximity to secretory granules, which is effectively prevented when the cell is at rest. Here we showed that exocytosis of zymogen granules in acinar cells was driven by Ca2+ directly released from acidic Ca2+ stores including secretory granules through NAADP-activated two-pore channels (TPCs). We identified OCaR1 (encoded by Tmem63a) as an organellar Ca2+ regulator protein integral to the membrane of secretory granules that controlled Ca2+ release via inhibition of TPC1 and TPC2 currents. Deletion of OCaR1 led to extensive Ca2+ release from NAADP-responsive granules under basal conditions as well as upon stimulation of GPCR receptors. Moreover, OCaR1 deletion exacerbated the disease phenotype in murine models of severe and chronic pancreatitis. Our findings showed OCaR1 as a gatekeeper of Ca2+ release that endows NAADP-sensitive secretory granules with an autoregulatory mechanism preventing uncontrolled exocytosis and pancreatic tissue damage.

The sGC Activator Runcaciguat Has Kidney Protective Effects and Prevents a Decline of Kidney Function in ZSF1 Rats

Chronic kidney disease (CKD) progression is associated with persisting oxidative stress, which impairs the NO-sGC-cGMP signaling cascade through the formation of oxidized and heme-free apo-sGC that cannot be activated by NO. Runcaciguat (BAY 1101042) is a novel, potent, and selective sGC activator that binds and activates oxidized and heme-free sGC and thereby restores NO-sGC-cGMP signaling under oxidative stress. Therefore, runcaciguat might represent a very effective treatment option for CKD/DKD. The potential kidney-protective effects of runcaciguat were investigated in ZSF1 rats as a model of CKD/DKD, characterized by hypertension, hyperglycemia, obesity, and insulin resistance. ZSF1 rats were treated daily orally for up to 12 weeks with runcaciguat (1, 3, 10 mg/kg/bid) or placebo. The study endpoints were proteinuria, kidney histopathology, plasma, urinary biomarkers of kidney damage, and gene expression profiling to gain information about relevant pathways affected by runcaciguat. Furthermore, oxidative stress was compared in the ZSF1 rat kidney with kidney samples from DKD patients. Within the duration of the 12-week treatment study, kidney function was significantly decreased in obese ZSF1 rats, indicated by a 20-fold increase in proteinuria, compared to lean ZSF1 rats. Runcaciguat dose-dependently and significantly attenuated the development of proteinuria in ZSF1 rats with reduced uPCR at the end of the study by -19%, -54%, and -70% at 1, 3, and 10 mg/kg/bid, respectively, compared to placebo treatment. Additionally, average blood glucose levels measured as HbA1C, triglycerides, and cholesterol were increased by five times, twenty times, and four times, respectively, in obese ZSF1 compared to lean rats. In obese ZSF1 rats, runcaciguat reduced HbA1c levels by -8%, -34%, and -76%, triglycerides by -42%, -55%, and -71%, and cholesterol by -16%, -17%, and -34%, at 1, 3, and 10 mg/kg/bid, respectively, compared to placebo. Concomitantly, runcaciguat also reduced kidney weights, morphological kidney damage, and urinary and plasma biomarkers of kidney damage. Beneficial effects were accompanied by changes in gene expression that indicate reduced fibrosis and inflammation and suggest improved endothelial stabilization. In summary, the sGC activator runcaciguat significantly prevented a decline in kidney function in a DKD rat model that mimics common comorbidities and conditions of oxidative stress of CKD patients. Thus, runcaciguat represents a promising treatment option for CKD patients, which is in line with recent phase 2 clinical study data, where runcaciguat showed promising efficacy in CKD patients (NCT04507061).

Treatment effects of soluble guanylate cyclase modulation on diabetic kidney disease at single-cell resolution

Diabetic kidney disease (DKD) is the most common cause of renal failure. Therapeutics development is hampered by our incomplete understanding of animal models on a cellular level. We show that ZSF1 rats recapitulate human DKD on a phenotypic and transcriptomic level. Tensor decomposition prioritizes proximal tubule (PT) and stroma as phenotype-relevant cell types exhibiting a continuous lineage relationship. As DKD features endothelial dysfunction, oxidative stress, and nitric oxide depletion, soluble guanylate cyclase (sGC) is a promising DKD drug target. sGC expression is specifically enriched in PT and stroma. In ZSF1 rats, pharmacological sGC activation confers considerable benefits over stimulation and is mechanistically related to improved oxidative stress regulation, resulting in enhanced downstream cGMP effects. Finally, we define sGC gene co-expression modules, which allow stratification of human kidney samples by DKD prevalence and disease-relevant measures such as kidney function, proteinuria, and fibrosis, underscoring the relevance of the sGC pathway to patients.

Urinary miRNA profiles in chronic kidney injury - Benefits of extracellular vesicle enrichment and miRNAs as potential biomarkers for renal fibrosis, glomerular injury and endothelial dysfunction

Micro-RNAs (miRNAs) are regulators of gene expression and play an important role in physiological homeostasis and disease. In biofluids miRNAs can be found in protein complexes or in extracellular vesicles (EVs). Altered urinary miRNAs are reported as potential biomarkers for chronic kidney disease (CKD). In this context we compared established urinary protein biomarkers for kidney injury with urinary miRNA profiles in obese ZSF1 and hypertensive renin transgenic rats. Additionally, the benefit of urinary EV enrichment was investigated in vivo and the potential association of urinary miRNAs with renal fibrosis in vitro. Kidney damage in both rat models was confirmed by histopathology, proteinuria, and increased levels of urinary protein biomarkers. In total 290 miRNAs were elevated in obese ZSF1 rats compared to lean controls, while 38 miRNAs were altered in obese ZSF1 rats during 14 to 26 weeks of age. These 38 miRNAs correlated better with disease progression than established urinary protein biomarkers. MiRNAs increased in obese ZSF1 rats were associated with renal inflammation, fibrosis, and glomerular injury. Eight miRNAs were also changed in urinary EVs of renin transgenic rats, including one which might play a role in endothelial dysfunction. EV enrichment increased the number and detection level of several miRNAs implicated in renal fibrosis in vitro and in vivo. Our results show the benefit of EV enrichment for miRNA detection and the potential of total urine and urinary EV-associated miRNAs as biomarkers of altered kidney physiology, renal fibrosis and glomerular injury, and disease progression in hypertension and obesity induced CKD.

Runcaciguat, a novel soluble guanylate cyclase activator, shows renoprotection in hypertensive, diabetic, and metabolic preclinical models of chronic kidney disease

Chronic kidney diseaQueryse (CKD) is associated with oxidative stress which can interrupt the nitric oxide (NO)/soluble guanylyl cyclase (sGC) signaling and decrease cyclic guanosine monophosphate (cGMP) production. Low cGMP concentrations can cause kidney damage and progression of CKD. The novel sGC activator runcaciguat targets the oxidized and heme-free form of sGC, restoring cGMP production under oxidative stress. The purpose of this study is to investigate if runcaciguat could provide an effective treatment for CKD. Runcaciguat was used for the treatment not only in rat CKD models with different etiologies and comorbidities, namely of hypertensive rats, the renin transgenic (RenTG) rat, and angiotensin-supplemented (ANG-SD) rat, but also in rats with diabetic and metabolic CKD, the Zucker diabetic fatty (ZDF) rat. The treatment duration was 2 to 42 weeks and runcaciguat was applied orally in doses from 1 to 10 mg/kg/bid. In these different rat CKD models, runcaciguat significantly reduced proteinuria (urinary protein to creatinine ratio; uPCR). These effects were also significant at doses which did not or only moderately decrease systemic blood pressure. Moreover, runcaciguat significantly decreased kidney injury biomarkers and attenuated morphological kidney damages. In RenTG rats, runcaciguat improved survival rates and markers of heart injury. These data demonstrate that the sGC activator runcaciguat exhibits cardio-renal protection at doses which did not reduce blood pressure and was effective in hypertensive as well as diabetic and metabolic CKD models. These data, therefore, suggest that runcaciguat, with its specific mode of action, represents an efficient treatment approach for CKD and associated CV diseases.

Cardiovascular protection by combination of the selective nonsteroidal MR antagonist finerenone and the SGLT2 inhibitor empagliflozin in a preclinical model of hypertension-induced end-organ damage

Background

The nonsteroidal mineralocorticoid receptor (MR) antagonist finerenone and SGLT2 inhibitors have demonstrated clinical benefits in HFrEF and CKD patients with T2D. Cardiovascular protection with finerenone and the SGLT2 inhibitor empagliflozin in combination in hypertensive cardiorenal disease is unknown.

Purpose

To test the hypothesis that the combination of finerenone with empagliflozin provides cardiovascular protection in preclinical hypertension-induced end-organ damage.

Methods

Cardiovascular morbidity and mortality was studied in hypertensive L-NAME (20 mg/L) treated renin-transgenic (mRen2)27 rats. Rats (10–11 weeks old female, n=13–17/group) were treated once daily orally for up to 7 weeks with placebo, finerenone (1 and 3 mg/kg), empagliflozin (3 and 10 mg/kg), or a combination of the respective low doses. Blood pressure (week 1, 3 and 5), urinary (week 2 and 6) and plasma parameters (week 6 and at the end of the study) were determined during the course of the study, while cardiac histology and left ventricular gene expression analysis were performed after study end.

Results

Empagliflozin induced a strong and dose-dependent increase in urinary glucose excretion which was not influenced by finerenone co-administration in the combination arm. Treatment with 3 mg/kg finerenone and the low dose combination significantly decreased systolic blood pressure (SBP) after 3 and 5 weeks as well as plasma uric acid after 6 weeks. SBP was significantly more reduced in the combination arm vs. the individual monotherapies after 3 weeks. Plasma NT-proBNP was reduced by empagliflozin, finerenone and the combination with similar efficacy. There was a dose-dependent protection from cardiac vasculopathy, cardiac and vascular fibrosis with both agents while low dose combination therapy was more efficient than the respective monotherapy dosages on these cardiac histology parameters. Placebo-treated rats demonstrated a ca. 50% survival rate over the course of 7 weeks while low dose combination provided the most prominent survival benefit (93%).

Conclusion

Non-steroidal MR antagonism by finerenone and SGLT2 inhibition by empagliflozin confer cardiovascular protection in preclinical hypertensive-induced cardiorenal disease. Combination of these two modes of action at low dosages revealed efficacious reduction in blood pressure, cardiac lesions and mortality indicating a strong potential for combined clinical use in cardiorenal patient populations.

Funding Acknowledgement

Type of funding sources: Private company. Main funding source(s): BAYER AG

Effects of Finerenone Combined with Empagliflozin in a Model of Hypertension-Induced End-Organ Damage

INTRODUCTION

The nonsteroidal mineralocorticoid receptor (MR) antagonist finerenone and sodium-glucose cotransporter-2 (SGLT2) inhibitors have demonstrated clinical benefits in CKD patients with type 2 diabetes. Clinical data analyzing the potential value of a combination therapy are currently limited. We therefore investigated cardiorenal protection of respective mono- and combination therapy in a preclinical model of hypertension-induced end-organ damage.

METHODS

Cardiovascular (CV) morbidity and mortality were studied in hypertensive, N(ω)-nitro-L-arginine methyl ester-treated, renin-transgenic (mRen2)27 rats. Rats (10- to 11-week-old females, n = 13-17/group) were treated once daily orally for up to 7 weeks with placebo, finerenone (1 and 3 mg/kg), empagliflozin (3 and 10 mg/kg), or a combination of the respective low doses. Key outcome parameters included mortality, proteinuria, plasma creatinine and uric acid, blood pressure, and cardiac and renal histology.

RESULTS

Placebo-treated rats demonstrated a 50% survival rate over the course of 7 weeks. Drug treatment resulted in variable degrees of survival benefit, most prominently in the low-dose combination group with a survival benefit of 93%. Monotherapies of finerenone or empagliflozin dose-dependently reduced proteinuria, while low-dose combination revealed an early, sustained, and over-additive reduction in proteinuria. Empagliflozin induced a strong and dose-dependent increase in urinary glucose excretion which was not influenced by finerenone coadministration in the combination arm. Low-dose combination but not respective low-dose monotherapies significantly reduced plasma creatinine and plasma uric acid after 6 weeks. Treatment with finerenone and the low-dose combination significantly decreased systolic blood pressure after 5 weeks. There was a dose-dependent protection from cardiac and kidney fibrosis and vasculopathy with both agents, while low-dose combination therapy was more efficient than the respective monotherapy dosages on most cardiorenal histology parameters.

DISCUSSION/CONCLUSIONS

Nonsteroidal MR antagonism by finerenone and SGLT2 inhibition by empagliflozin confer CV protection in preclinical hypertension-induced cardiorenal disease. Combination of these 2 independent modes of action at low dosages revealed efficacious reduction in important functional parameters such as proteinuria and blood pressure, plasma markers including creatinine and uric acid, cardiac and renal lesions as determined by histopathology, and mortality indicating a strong potential for combined clinical use in cardiorenal patient populations.

Control of Insulin Release by Transient Receptor Potential Melastatin 3 (TRPM3) Ion Channels

BACKGROUND/AIMS

The release of insulin in response to increased levels of glucose in the blood strongly depends on Ca²⁺ influx into pancreatic beta cells by the opening of voltage-gated Ca²⁺ channels. Transient Receptor Potential Melastatin 3 proteins build Ca²⁺ permeable, non-selective cation channels serving as pain sensors of noxious heat in the peripheral nervous system. TRPM3 channels are also strongly expressed in pancreatic beta cells that respond to the TRPM3 agonist pregnenolone sulfate with Ca²⁺ influx and increased insulin release. Therefore, we hypothesized that in beta cells TRPM3 channels may contribute to pregnenolone sulfate- as well as to glucose-induced insulin release.

METHODS

We used INS-1 cells as a beta cell model in which we analysed the occurrence of TRPM3 isoformes by immunoprecipitation and western blotting and by cloning of RT-PCR amplified cDNA fragments. We applied pharmacological as well as CRISPR/Cas9-based strategies to analyse the interplay of TRPM3 and voltage-gated Ca²⁺ channels in imaging experiments (FMP, Fura-2) and electrophysiological recordings. In immunoassays, we examined the contribution of TRPM3 channels to pregnenolone sulfate- and glucose-induced insulin release. To confirm our findings, we generated beta cell-specific Trpm3-deficient mice and compared their glucose clearance with the wild type in glucose tolerance tests.

RESULTS

TRPM3 channels triggered the activity of voltage-gated Ca²⁺ channels and both channels together contributed to insulin release after TRPM3 activation. Trpm3-deficient INS-1 cells lacked pregnenolone sulfate-induced Ca²⁺ signals just like the pregnenolone sulfate-induced insulin release. Both, glucose-induced Ca²⁺ signals and the glucose-induced insulin release were strongly reduced. Accordingly, Trpm3-deficient mice displayed an impaired decrease of the blood sugar concentration after intraperitoneal or oral administration of glucose.

CONCLUSION

The present study suggests an important role for TRPM3 channels in the control of glucose-dependent insulin release.

Loss of smooth muscle CYB5R3 amplifies angiotensin II-induced hypertension by increasing sGC heme oxidation

Nitric oxide regulates BP by binding the reduced heme iron (Fe2+) in soluble guanylyl cyclase (sGC) and relaxing vascular smooth muscle cells (SMCs). We previously showed that sGC heme iron reduction (Fe3+ → Fe2+) is modulated by cytochrome b5 reductase 3 (CYB5R3). However, the in vivo role of SMC CYB5R3 in BP regulation remains elusive. Here, we generated conditional smooth muscle cell-specific Cyb5r3 KO mice (SMC CYB5R3-KO) to test if SMC CYB5R3 loss affects systemic BP in normotension and hypertension via regulation of the sGC redox state. SMC CYB5R3-KO mice exhibited a 5.84-mmHg increase in BP and impaired acetylcholine-induced vasodilation in mesenteric arteries compared with controls. To drive sGC oxidation and elevate BP, we infused mice with angiotensin II. We found that SMC CYB5R3-KO mice exhibited a 14.75-mmHg BP increase, and mesenteric arteries had diminished nitric oxide-dependent vasodilation but increased responsiveness to sGC heme-independent activator BAY 58-2667 over controls. Furthermore, acute injection of BAY 58-2667 in angiotensin II-treated SMC CYB5R3-KO mice showed greater BP reduction compared with controls. Together, these data provide the first in vivo evidence to our knowledge that SMC CYB5R3 is an sGC heme reductase in resistance arteries and provides resilience against systemic hypertension development.

Nitric Oxide-Independent Soluble Guanylate Cyclase Activation Improves Vascular Function and Cardiac Remodeling in Sickle Cell Disease

Sickle cell disease (SCD) is associated with intravascular hemolysis and oxidative inhibition of nitric oxide (NO) signaling. BAY 54-6544 is a small-molecule activator of oxidized soluble guanylate cyclase (sGC), which, unlike endogenous NO and the sGC stimulator, BAY 41-8543, preferentially binds and activates heme-free, NO-insensitive sGC to restore enzymatic cGMP production. We tested orally delivered sGC activator, BAY 54-6544 (17 mg/kg/d), sGC stimulator, BAY 41-8543, sildenafil, and placebo for 4-12 weeks in the Berkeley transgenic mouse model of SCD (BERK-SCD) and their hemizygous (Hemi) littermate controls (BERK-Hemi). Right ventricular (RV) maximum systolic pressure (RVmaxSP) was measured using micro right-heart catheterization. RV hypertrophy (RVH) was determined using Fulton's index and RV corrected weight (ratio of RV to tibia). Pulmonary artery vasoreactivity was tested for endothelium-dependent and -independent vessel relaxation. Right-heart catheterization revealed higher RVmaxSP and RVH in BERK-SCD versus BERK-Hemi, which worsened with age. Treatment with the sGC activator more effectively lowered RVmaxSP and RVH, with 90-day treatment delivering superior results, when compared with other treatments and placebo groups. In myography experiments, acetylcholine-induced (endothelium-dependent) and sodium-nitroprusside-induced (endothelium-independent NO donor) relaxation of the pulmonary artery harvested from placebo-treated BERK-SCD was impaired relative to BERK-Hemi but improved after therapy with sGC activator. By contrast, no significant effect for sGC stimulator or sildenafil was observed in BERK-SCD. These findings suggest that sGC is oxidized in the pulmonary arteries of transgenic SCD mice, leading to blunted responses to NO, and that the sGC activator, BAY 54-6544, may represent a novel therapy for SCD-associated pulmonary arterial hypertension and cardiac remodeling.

Methylglyoxal evokes acute Ca2+ transients in distinct cell types and increases agonist-evoked Ca2+ entry in endothelial cells via CRAC channels

Methylglyoxal (MG) is a by-product of glucose metabolism and its accumulation has been linked to the development of diabetic complications such as retinopathy and nephropathy by affecting multiple signalling pathways. However, its influence on the intracellular Ca²⁺ homeostasis and particularly Ca²⁺ entry, which has been reported to be mediated via TRPA1 channels in DRG neurons, has not been studied in much detail in other cell types. In this study, we report the consequences of acute and long-term MG application on intracellular Ca²⁺ levels in endothelial cells. We showed that acute MG application doesn't evoke any instantaneous changes in the intracellular Ca²⁺ concentration in immortalized mouse cardiac endothelial cells (MCECs) and murine microvascular endothelial cells (muMECs). In contrast, an MG-induced rise in intracellular Ca²⁺ level was observed in primary mouse mesangial cells within 30 s, indicating that the modulation of Ca²⁺ homeostasis by MG is strictly cell type specific. The formation of the MG-derived advanced glycation end product (AGE) MG-H1 was found to be time and concentration-dependent in MCECs. Likewise, MG pre-incubation for 6 h increased the angiotensin II-evoked Ca²⁺ entry in MCECs and muMECs which was abrogated by inhibition of Calcium release activated calcium (CRAC) channels with GSK-7975A, but unaffected by an inhibitor specific to TRPA1 channels. Quantitative PCR analysis revealed that MG pre-treatment did not affect expression of the genes encoding the angiotensin receptors AT1R (Agtr 1a & Agtr 1b), Trpa1 nor Orai1, Orai2, Orai3, Stim1, Stim2 and Saraf which operate as constituents or regulators of CRAC channels and store-operated Ca²⁺ entry (SOCE) in other cell types. Together, our results show that long-term MG stimulation leads to the formation of glycation end products, which facilitates the agonist-evoked Ca²⁺ entry in endothelial cells, and this could be a new pathway that might lead to MG-evoked vasoregression observed in diabetic vasculopathies.

TRPC4/TRPC5 channels mediate adverse reaction to the cancer cell cytotoxic agent (-)-Englerin A

(-)-Englerin A (EA) is a natural product which has potent cytotoxic effects on renal cell carcinoma cells and other types of cancer cell but not non-cancer cells. Although selectively cytotoxic to cancer cells, adverse reaction in mice and rats has been suggested. EA is a remarkably potent activator of ion channels formed by Transient Receptor Potential Canonical 4 and 5 proteins (TRPC4 and TRPC5) and TRPC4 is essential for EA-mediated cancer cell cytotoxicity. Here we specifically investigated the relevance of TRPC4 and TRPC5 to the adverse reaction. Injection of EA (2 mg.kg^-1 i.p.) adversely affected mice for about 1 hour, manifesting as a marked reduction in locomotor activity, after which they fully recovered. TRPC4 and TRPC5 single knockout mice were partially protected and double knockout mice fully protected. TRPC4/TRPC5 double knockout mice were also protected against intravenous injection of EA. Importance of TRPC4/TRPC5 channels was further suggested by pre-administration of Compound 31 (Pico145), a potent and selective small-molecule inhibitor of TRPC4/TRPC5 channels which did not cause adverse reaction itself but prevented adverse reaction to EA. EA was detected in the plasma but not the brain and so peripheral mechanisms were implicated but not identified. The data confirm the existence of adverse reaction to EA in mice and suggest that it depends on a combination of TRPC4 and TRPC5 which therefore overlaps partially with TRPC4-dependent cancer cell cytotoxicity. The underlying nature of the observed adverse reaction to EA, as a consequence of TRPC4/TRPC5 channel activation, remains unclear and warrants further investigation.

TRPC proteins contribute to development of diabetic retinopathy and regulate glyoxalase 1 activity and methylglyoxal accumulation

OBJECTIVE

Diabetic retinopathy (DR) is induced by an accumulation of reactive metabolites such as ROS, RNS, and RCS species, which were reported to modulate the activity of cation channels of the TRPC family. In this study, we use Trpc1/4/5/6^-/- compound knockout mice to analyze the contribution of these TRPC proteins to diabetic retinopathy.

METHODS

We used Nanostring- and qPCR-based analysis to determine mRNA levels of TRPC channels in control and diabetic retinae and retinal cell types. Chronic hyperglycemia was induced by Streptozotocin (STZ) treatment. To assess the development of diabetic retinopathy, vasoregression, pericyte loss, and thickness of individual retinal layers were analyzed. Plasma and cellular methylglyoxal (MG) levels, as well as Glyoxalase 1 (GLO1) enzyme activity and protein expression, were measured in WT and Trpc1/4/5/6^-/- cells or tissues. MG-evoked toxicity in cells of both genotypes was compared by MTT assay.

RESULTS

We find that Trpc1/4/5/6^-/- mice are protected from hyperglycemia-evoked vasoregression determined by the formation of acellular capillaries and pericyte drop-out. In addition, Trpc1/4/5/6^-/- mice are resistant to the STZ-induced reduction in retinal layer thickness. The RCS metabolite methylglyoxal, which represents a key mediator for the development of diabetic retinopathy, was significantly reduced in plasma and red blood cells (RBCs) of STZ-treated Trpc1/4/5/6^-/- mice compared to controls. GLO1 is the major MG detoxifying enzyme, and its activity and protein expression were significantly elevated in Trpc1/4/5/6-deficient cells, which led to significantly increased resistance to MG toxicity. GLO1 activity was also increased in retinal extracts from Trpc1/4/5/6^-/- mice. The TRPCs investigated here are expressed at different levels in endothelial and glial cells of the retina.

CONCLUSION

The protective phenotype in diabetic retinopathy observed in Trpc1/4/5/6^-/- mice is suggestive of a predominant action of TRPCs in Müller cells and microglia because of their central position in the retention of a proper homoeostasis of the neurovascular unit.

Heteromeric channels formed by TRPC1, TRPC4 and TRPC5 define hippocampal synaptic transmission and working memory

Canonical transient receptor potential (TRPC) channels influence various neuronal functions. Using quantitative high-resolution mass spectrometry, we demonstrate that TRPC1, TRPC4, and TRPC5 assemble into heteromultimers with each other, but not with other TRP family members in the mouse brain and hippocampus. In hippocampal neurons from Trpc1/Trpc4/Trpc5-triple-knockout (Trpc1/4/5^-/-) mice, lacking any TRPC1-, TRPC4-, or TRPC5-containing channels, action potential-triggered excitatory postsynaptic currents (EPSCs) were significantly reduced, whereas frequency, amplitude, and kinetics of quantal miniature EPSC signaling remained unchanged. Likewise, evoked postsynaptic responses in hippocampal slice recordings and transient potentiation after tetanic stimulation were decreased. In vivo, Trpc1/4/5^-/- mice displayed impaired cross-frequency coupling in hippocampal networks and deficits in spatial working memory, while spatial reference memory was unaltered. Trpc1/4/5^-/- animals also exhibited deficiencies in adapting to a new challenge in a relearning task. Our results indicate the contribution of heteromultimeric channels from TRPC1, TRPC4, and TRPC5 subunits to the regulation of mechanisms underlying spatial working memory and flexible relearning by facilitating proper synaptic transmission in hippocampal neurons.

TRPM4-mediated control of FcεRI-evoked Ca(2+) elevation comprises enhanced plasmalemmal trafficking of TRPM4 channels in connective tissue type mast cells

TRPM4 proteins form Ca²⁺-activated non selective cation (CAN) channels that affect transmembrane Ca²⁺-influx by determining the membrane potential. Tight control of the intracellular Ca²⁺ concentration is essential for mast cell responses. In this study, we analyzed the expression of TRPM4 in peritoneal mast cells (PCMC) as a model for connective tissue type mast cells with respect to FcεRI-evoked calcium changes and the subcellular localization of fluorescently labeled TRPM4 using two viral transduction systems before and following antigen stimulation. Our results show that TRPM4 is expressed in PCMCs, is an essential constituent of the endogenous CAN channels in PCMCs and regulates antigen-evoked increases in intracellular calcium that are significantly enhanced in TRPM4-deficient PCMCs. Compared to PCMCs analyzed before antigen stimulation, the cells depict a substantially increased localization of TRPM4 proteins towards the plasma membrane after FcεRI stimulation. Thus, TRPM4 functions as a limiting factor for antigen evoked calcium rise in connective tissue type mast cells and concurrent translocation of TRPM4 into the plasma membrane is part of this mechanism.

Contractility Measurements on Isolated Papillary Muscles for the Investigation of Cardiac Inotropy in Mice

Papillary muscle isolated from adult mouse hearts can be used to study cardiac contractility during different physiological/pathological conditions. The contractile characteristics can be evaluated independently of external influences such as vascular tonus or neurohumoral status. It depicts a scientific approach between single cell measurements with isolated cardiac myocytes and in vivo studies like echocardiography. Thus, papillary muscle preparations serve as an excellent model to study cardiac physiology/pathophysiology and can be used for investigations like the modulation by pharmacological agents or the exploration of transgenic animal models. Here, we describe a method of isolating the murine left anterior papillary muscle to investigate cardiac contractility in an organ bath setup. In contrast to a muscle strip preparation isolated from the ventricular wall, the papillary muscle can be prepared in toto without damaging the muscle tissue severely. The organ bath setup consists of several temperature-controlled, gassed and electrode-equipped organ bath chambers. The isolated papillary muscle is fixed in the organ bath chamber and electrically stimulated. The evoked twitch force is recorded using a pressure transducer and parameters such as twitch force amplitude and twitch kinetics are analyzed. Different experimental protocols can be performed to investigate the calcium- and frequency-dependent contractility as well as dose-response curves of contractile agents such as catecholamines or other pharmaceuticals. Additionally, pathologic conditions like acute ischemia can be simulated.

A background Ca2+ entry pathway mediated by TRPC1/TRPC4 is critical for development of pathological cardiac remodelling

AIMS

Pathological cardiac hypertrophy is a major predictor for the development of cardiac diseases. It is associated with chronic neurohumoral stimulation and with altered cardiac Ca(2+) signalling in cardiomyocytes. TRPC proteins form agonist-induced cation channels, but their functional role for Ca(2+) homeostasis in cardiomyocytes during fast cytosolic Ca(2+) cycling and neurohumoral stimulation leading to hypertrophy is unknown.

METHODS AND RESULTS

In a systematic analysis of multiple knockout mice using fluorescence imaging of electrically paced adult ventricular cardiomyocytes and Mn(2+)-quench microfluorimetry, we identified a background Ca(2+) entry (BGCE) pathway that critically depends on TRPC1/C4 proteins but not others such as TRPC3/C6. Reduction of BGCE in TRPC1/C4-deficient cardiomyocytes lowers diastolic and systolic Ca(2+) concentrations both, under basal conditions and under neurohumoral stimulation without affecting cardiac contractility measured in isolated hearts and in vivo. Neurohumoral-induced cardiac hypertrophy as well as the expression of foetal genes (ANP, BNP) and genes regulated by Ca(2+)-dependent signalling (RCAN1-4, myomaxin) was reduced in TRPC1/C4 knockout (DKO), but not in TRPC1- or TRPC4-single knockout mice. Pressure overload-induced hypertrophy and interstitial fibrosis were both ameliorated in TRPC1/C4-DKO mice, whereas they did not show alterations in other cardiovascular parameters contributing to systemic neurohumoral-induced hypertrophy such as renin secretion and blood pressure.

CONCLUSIONS

The constitutively active TRPC1/C4-dependent BGCE fine-tunes Ca(2+) cycling in beating adult cardiomyocytes. TRPC1/C4-gene inactivation protects against development of maladaptive cardiac remodelling without altering cardiac or extracardiac functions contributing to this pathogenesis.

Adenylyl cyclase-mediated effects contribute to increased Isoprenaline-induced cardiac contractility in TRPM4-deficient mice

TRPM4 and TRPM5 proteins belong to the Transient Receptor Potential (TRP) ion channel family and form Ca(2+)-activated nonselective cation channels. Recently we showed a significant increase of Isoprenaline-induced inotropy in TRPM4-deficient (Trpm4(-/-)) mice. This is caused by increased Ca(2+) entry via L-type calcium channels due to faster action potential repolarization in Trpm4(-/-) ventricular myocytes [Mathar et al., 2013]. Here, we investigated the contribution of various steps of the β-adrenergic signalling cascade to the augmented positive inotropic response in the absence of TRPM4, and whether the closely related TRPM5 additively contributes to this process using TRPM4/TRPM5-double deficient (Trpm4/Trpm5((-/-)2)) mice. We performed contractility measurements on isolated papillary muscles from wild type, Trpm4(-/-) and Trpm4/Trpm5((-/-)2) mice. As shown in Trpm4(-/-) mice, Isoprenaline-induced inotropy in Trpm4/Trpm5((-/-)2) papillary muscles was significantly increased compared to wild type, whereas basal, frequency- and Ca(2+)-dependent contractility was unaltered. Equivalent to Isoprenaline, activation of adenylyl cyclase using Forskolin led to a significantly increased twitch force in Trpm4(-/-) heart preparations whereas the Isoprenaline-mediated increase in cAMP level was comparable to wild type mice. Notably, the positive inotropic response evoked by phosphodiesterase inhibition with 3-isobutyl-1-methylxanthine (IBMX) was unchanged between both genotypes. Furthermore, experiments performed with increasing concentrations of IBMX after prestimulation with Forskolin and vice versa did not provide evidence that the increased β-adrenergic positive inotropic response in TRPM4-deficient papillary muscles is due to differences in accumulation of cAMP. Compared to inhibition of phosphodiesterase, the rise of intracellular cAMP by activating adenylyl cyclase is accompanied by ATP breakdown. To test the relevance of TRPM4 during forced ATP consumption we measured contractility under ischemic conditions. Here, Trpm4(-/-) papillary muscles showed improved contractile function in comparison to wild type. Our results are consistent with the hypothesis that TRPM4 has a limiting effect on cardiac contractility specifically in ATP depleting conditions. The increased positive inotropic response in Trpm4(-/-) papillary muscles evoked by stimulation of adenylyl cyclase activity is not observed without active enhancement of ATP hydrolysis. Furthermore, the contractility of Trpm4(-/-) papillary muscles was also increased during ischemic simulation. These data underscore the potential of TRPM4 inactivation as an approach to increase inotropy in specific conditions associated with increased catecholamine levels, such as heart failure and ischemia.

TRPM4

TRPM4 is a Ca(2+)-activated nonselective cation channel. The channel is activated by an increase of intracellular Ca(2+) and is regulated by several factors including temperature and Pi(4,5)P2. TRPM4 allows Na(+) entry into the cell upon activation, but is completely impermeable to Ca(2+). Unlike TRPM5, its closest relative in the transient receptor potential family, TRPM4 proteins are widely expressed in the body. Currents with properties that are reminiscent of TRPM4 have been described in a variety of tissues since the advent of the patch clamp technology, but their physiological role is only beginning to be clarified with the increasing characterization of knockout mouse models for TRPM4. Furthermore, mutations in the TRPM4 gene have been associated with cardiac conduction disorders in human patients. This review aims to overview the currently available data on the functional properties of TRPM4 and the current understanding of its physiological role in healthy and diseased tissue.

Increased β-adrenergic inotropy in ventricular myocardium from Trpm4-/- mice

RATIONALE

The Trpm4 gene has recently been associated with several disorders, including cardiac conduction diseases and Brugada syndrome. Transient receptor potential member 4 (TRPM4) proteins constitute Ca2+ -activated, but Ca2+ -impermeable, nonselective cation channels and are expressed both in atrial and in ventricular cardiomyocytes. The physiological function of TRPM4 in the heart remains, however, incompletely understood.

OBJECTIVE

To establish the role of TRPM4 in cardiac muscle function.

METHODS AND RESULTS

We used TRPM4 knockout mice and performed patch-clamp experiments, membrane potential measurements, microfluorometry, contractility measurements, and in vivo pressure-volume loop analysis. We demonstrate that TRPM4 proteins are functionally present in mouse ventricular myocytes and are activated on Ca2+ -induced Ca2+ release. In Trpm4(-/-) mice, cardiac muscle displays an increased β-adrenergic inotropic response both in vitro and in vivo. Measurements of action potential duration show a significantly decreased time for 50% and 90% repolarization in Trpm4(-/-) ventricular myocytes. We provide evidence that this change in action potential shape leads to an increased driving force for the L-type Ca2+ current during the action potential, which explains the altered contractility of the heart muscle.

CONCLUSIONS

Our results show that functional TRPM4 proteins are novel determinants of the inotropic effect of β-adrenergic stimulation on the ventricular heart muscle.

Heart rate contributes to the vascular effects of chronic mental stress: effects on endothelial function and ischemic brain injury in mice

BACKGROUND AND PURPOSE

Vascular effects of mental stress are only partially understood. Therefore, we studied effects of chronic stress and heart rate (HR) on endothelial function and cerebral ischemia.

METHODS

129S6/SvEv mice were randomized to the I(f)-channel inhibitor ivabradine (10 mg/kg per day) or vehicle and underwent a chronic stress protocol for 28 days.

RESULTS

Stress increased HR from 514 ± 10 bpm to 570 ± 14 bpm, this was prevented by ivabradine (485 ± 7 bpm). Endothelium-dependent relaxation of aortic rings was impaired in mice exposed to stress. HR reduction restored endothelial function to the level of naive controls. Vascular lipid hydroperoxides were increased to 333% ± 24% and vascular NADPH oxidase activity was upregulated to 223 ± 38% in stressed mice, which was prevented by ivabradine. Stress reduced aortic endothelial nitric oxide synthase mRNA expression to 84% ± 3% and increased AT1 receptor mRNA to 168% ± 18%. Both effects were attenuated by HR reduction. In brain tissue, stress resulted in an upregulation of lipid hydroperoxides to 140% ± 11%, which was attenuated by HR reduction. Ivabradine increased brain capillary density in naive and in stressed mice. Mice exposed to chronic stress before induction of ischemic stroke by transient middle cerebral artery occlusion exhibited increased lesion size (33.7 ± 2.3 mm3 versus 23.9 ± 2.4 mm3). HR reduction led to a marked reduction of the infarct volume to 12.9 ± 3.3 mm3.

CONCLUSIONS

Chronic stress impairs endothelial function and aggravates ischemic brain injury. HR reduction protects from cerebral ischemia via improvement of endothelial function and reduction of oxidative stress. These results identify heart rate as a mediator of vascular effects induced by chronic stress.

Increased catecholamine secretion contributes to hypertension in TRPM4-deficient mice

Hypertension is an underlying risk factor for cardiovascular disease. Despite this, its pathogenesis remains unknown in most cases. Recently, the transient receptor potential (TRP) channel family was associated with the development of several cardiovascular diseases linked to hypertension. The melastatin TRP channels TRPM4 and TRPM5 have distinct properties within the TRP channel family: they form nonselective cation channels activated by intracellular calcium ions. Here we report the identification of TRPM4 proteins in endothelial cells, heart, kidney, and chromaffin cells from the adrenal gland, suggesting that they have a role in the cardiovascular system. Consistent with this hypothesis, Trpm4 gene deletion in mice altered long-term regulation of blood pressure toward hypertensive levels. No changes in locomotor activity, renin-angiotensin system function, electrolyte and fluid balance, vascular contractility, and cardiac contractility under basal conditions were observed. By contrast, inhibition of ganglionic transmission with either hexamethonium or prazosin abolished the difference in blood pressure between Trpm4-/- and wild-type mice. Strikingly, plasma epinephrine concentration as well as urinary excretion of catecholamine metabolites were substantially elevated in Trpm4-/- mice. In freshly isolated chromaffin cells, lack of TRPM4 was shown to cause markedly more acetylcholine-induced exocytotic release events, while neither cytosolic calcium concentration, size, nor density of vesicles were different. We therefore conclude that TRPM4 proteins limit catecholamine release from chromaffin cells and that this contributes to increased sympathetic tone and hypertension.

Transient receptor potential M3 channels are ionotropic steroid receptors in pancreatic beta cells

Transient receptor potential (TRP) cation channels are renowned for their ability to sense diverse chemical stimuli. Still, for many members of this large and heterogeneous protein family it is unclear how their activity is regulated and whether they are influenced by endogenous substances. On the other hand, steroidal compounds are increasingly recognized to have rapid effects on membrane surface receptors that often have not been identified at the molecular level. We show here that TRPM3, a divalent-permeable cation channel, is rapidly and reversibly activated by extracellular pregnenolone sulphate, a neuroactive steroid. We show that pregnenolone sulphate activates endogenous TRPM3 channels in insulin-producing beta cells. Application of pregnenolone sulphate led to a rapid calcium influx and enhanced insulin secretion from pancreatic islets. Our results establish that TRPM3 is an essential component of an ionotropic steroid receptor enabling unanticipated crosstalk between steroidal and insulin-signalling endocrine systems.

Increased IgE-dependent mast cell activation and anaphylactic responses in mice lacking the calcium-activated nonselective cation channel TRPM4

Mast cells are key effector cells in allergic reactions. Aggregation of the receptor FcepsilonRI in mast cells triggers the influx of calcium (Ca(2+)) and the release of inflammatory mediators. Here we show that transient receptor potential TRPM4 proteins acted as calcium-activated nonselective cation channels and critically determined the driving force for Ca(2+) influx in mast cells. Trpm4(-/-) bone marrow-derived mast cells had more Ca(2+) entry than did TRPM4(+/+) cells after FcepsilonRI stimulation. Consequently, Trpm4(-/-) bone marrow-derived mast cells had augmented degranulation and released more histamine, leukotrienes and tumor necrosis factor. Trpm4(-/-) mice had a more severe IgE-mediated acute passive cutaneous anaphylactic response, whereas late-phase passive cutaneous anaphylaxis was not affected. Our results establish the physiological function of TRPM4 channels as critical regulators of Ca(2+) entry in mast cells.