A novel mechanism of mechanical stress-induced angiotensin II type 1–receptor activation without the involvement of angiotensin II

Adaptive and maladaptive roles of different angiotensin receptors in the development of cardiac hypertrophy and heart failure

Angiotensin II (Ang II) is formed by the action of angiotensin-converting enzyme (ACE) in the renin-angiotensin system. This hormone is known to induce cardiac hypertrophy and heart failure and its actions are mediated by the interaction of both pro- and antihypertrophic Ang II receptors (AT1R and AT2R). Ang II is also metabolized by ACE 2 to Ang-(1-7), which elicits the activation of Mas receptors (MasR) for inducing antihypertrophic actions. Since heart failure under different pathophysiological situations is preceded by adaptive and maladaptive cardiac hypertrophy, we have reviewed the existing literature to gain some information regarding the roles of AT1R, AT2R, and MasR in both acute and chronic conditions of cardiac hypertrophy. It appears that the activation of AT1R may be involved in the development of adaptive and maladaptive cardiac hypertrophy as well as subsequent heart failure because both ACE inhibitors and AT1R antagonists exert beneficial effects. On the other hand, the activation of both AT2R and MasR may prevent the occurrence of maladaptive cardiac hypertrophy and delay the progression of heart failure, and thus therapy with different activators of these antihypertrophic receptors under chronic pathological stages may prove beneficial. Accordingly, it is suggested that a great deal of effort should be made to develop appropriate activators of both AT2R and MasR for the treatment of heart failure subjects.

Transcriptomic profiling of Polycystic Kidney Disease identifies paracrine factors in the early cyst microenvironment

Initial cysts that are formed upon Pkd1 loss in mice impose persistent stress on surrounding tissue and trigger a cystic snowball effect, in which local aberrant PKD-related signaling increases the likelihood of new cyst formation, ultimately leading to accelerated disease progression. Although many pathways have been associated with PKD progression, the knowledge of early changes near initial cysts is limited. To perform an unbiased analysis of transcriptomic alterations in the cyst microenvironment, microdomains were collected from kidney sections of iKsp-Pkd1del mice with scattered Pkd1-deletion using Laser Capture Microdissection. These microdomains were defined as F4/80-low cystic, representing early alterations in the cyst microenvironment, F4/80-high cystic, with more advanced alterations, or non-cystic. RNA sequencing and differential gene expression analysis revealed 953 and 8088 dysregulated genes in the F4/80-low and F4/80-high cyst microenvironment, respectively, when compared to non-cystic microdomains. In the early cyst microenvironment, several injury-repair, growth, and tissue remodeling-related pathways were activated, accompanied by mild metabolic changes. In the more advanced F4/80-high microdomains, these pathways were potentiated and the metabolism was highly dysregulated. Upstream regulator analysis revealed a series of paracrine factors with increased activity in the early cyst microenvironment, including TNFSF12 and OSM. In line with the upstream regulator analysis, TWEAK and Oncostatin-M promoted cell proliferation and inflammatory gene expression in renal epithelial cells and fibroblasts in vitro. Collectively, our data provide an overview of molecular alterations that specifically occur in the cyst microenvironment and identify paracrine factors that may mediate early and advanced alterations in the cyst microenvironment.

Losartan alleviates renal fibrosis by inhibiting the biomechanical stress-induced epithelial-mesenchymal transition of renal epithelial cells

Angiotensin receptor blockers (ARBs) have been reported to be beneficial of renal fibrosis, but the molecular and cellular mechanisms are still unclear. In this study, we investigated the effectiveness and relevant mechanism of ARBs in alleviating renal fibrosis, especially by focusing on biomechanical stress-induced epithelial to mesenchymal transition (EMT) of renal epithelial cells. Unilateral ureteral obstruction (UUO) renal fibrosis model was established in mice by ligating the left ureter, and then randomly received losartan at a low dose (1 mg/kg) or a regular dose (3 mg/kg) for 2 weeks. Compared to the control, histological analysis showed that losartan treatment at either a low dose or a regular dose effectively attenuated renal fibrosis in the UUO model. To further understand the mechanism, we ex vivo loaded primary human renal epithelial cells to 50 mmHg hydrostatic pressure. Western blot and immunostaining analyses indicated that the loading to 50 mmHg hydrostatic pressure for 24 h significantly upregulated vimentin, β-catenin and α-SMA, but downregulated E-cadherin in renal epithelial cells, suggesting the EMT. The addition of 10 or 100 nM losartan in medium effectively attenuated the EMT of renal epithelial cells induced by 50 mmHg hydrostatic pressure loading. Our in vivo and ex vivo experimental data suggest that losartan treatment, even at a low dose can effectively alleviate renal fibrosis in mouse UUO model, at least partly by inhibiting the biomechanical stress-induced EMT of renal epithelial cells. A low dose of ARBs may repurpose for renal fibrosis treatment.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Arterial myogenic response and aging

The arterial myogenic response is an inherent property of resistance arteries. Myogenic tone is crucial for maintaining a relatively constant blood flow in response to changes in intraluminal pressure and protects delicate organs from excessive blood flow. Although this fundamental physiological phenomenon has been extensively studied, the underlying molecular mechanisms are largely unknown. Recent studies identified a crucial role of mechano-activated angiotensin II type 1 receptors (AT1R) in this process. The development of myogenic response is affected by aging. In this review, we summarize recent progress made to understand the role of AT1R and other mechanosensors in the control of arterial myogenic response. We discuss age-related alterations in myogenic response and possible underlying mechanisms and implications for healthy aging.

Adrenal angiotensin II type 1 receptor biased signaling: The case for "biased" inverse agonism for effective aldosterone suppression

Angiotensin II (AngII) uses two distinct G protein-coupled receptor (GPCR) types, AT1R and AT2R, to exert a plethora of physiologic effects in the body and to significantly affect cardiovascular homeostasis. Although not much is known about the signaling of the AT2R, AT1R signaling is known to be quite pleiotropic, mobilizing a variety of signal transducers inside cells to produce a biological outcome. When the outcome in question is aldosterone production from the adrenal cortex, the main transducers activated specifically by the adrenocortical AT1R to signal toward that cellular effect are the Gq/11 protein alpha subunits and the β-arrestins (also known as Arrestin-2 and -3). The existence of various downstream pathways the AT1R signal can travel down on has led to the ever-expanding filed of GPCR pharmacology termed "biased" signaling, which refers to a ligand preferentially activating one signaling pathway over others downstream of the same receptor in the same cell. However, "biased" signaling or "biased" agonism is therapeutically desirable only when the downstream pathways lead to different or opposite cellular outcomes, so the pathway promoting the beneficial effect can be selectively activated over the pathway that leads to detrimental consequences. In the case of the adrenal AT1R, both Gq/11 proteins and β-arrestins mediate signaling to the same end-result: aldosterone synthesis and secretion. Therefore, both pathways need to remain inactive in the adrenal cortex to fully suppress the production of aldosterone, which is one of the culprit hormones elevated in chronic heart failure, hypertension, and various other cardiovascular diseases. Variations in the effectiveness of the AT1R antagonists, which constitute the angiotensin receptor blocker (ARB) class of drugs (also known as sartans), at the relative blockade of these two pathways downstream of the adrenal AT1R opens the door to the flip term "biased" inverse agonism at the AT1R. ARBs that are unbiased and equipotent inverse agonists for both G proteins and β-arrestins at this receptor, like candesartan and valsartan, are the most preferred agents with the best efficacy at reducing circulating aldosterone, thereby ameliorating heart failure. In the present review, the biased signaling of the adrenal AT1R, particularly in relation to aldosterone production, is examined and the term "biased" inverse agonism at the AT1R is introduced and explained, as a means of pharmacological categorization of the various agents within the ARB drug class.

Harnessing Mechanosensation in Next Generation Cardiovascular Tissue Engineering

The ability of the cells to sense mechanical cues is an integral component of ”social” cell behavior inside tissues with a complex architecture. Through ”mechanosensation” cells are in fact able to decrypt motion, geometries and physical information of surrounding cells and extracellular matrices by activating intracellular pathways converging onto gene expression circuitries controlling cell and tissue homeostasis. Additionally, only recently cell mechanosensation has been integrated systematically as a crucial element in tissue pathophysiology. In the present review, we highlight some of the current efforts to assess the relevance of mechanical sensing into pathology modeling and manufacturing criteria for a next generation of cardiovascular tissue implants.

δ Opioid Receptor Inverse Agonists and their In Vivo Pharmacological Effects

The discovery of δ opioid receptor inverse agonist activity induced by ICI-174,864, which was previously reported as an δ opioid receptor antagonist, opened the door for the investigation of inverse agonism/constitutive activity of the receptors. Various peptidic or non-peptidic δ opioid receptor inverse agonists have since been developed. Compared with the reports dealing with in vitro inverse agonist activities of novel compounds or known compounds as antagonists, there have been almost no publications describing the in vivo pharmacological effects induced by a δ opioid receptor inverse agonist. After the observation of anorectic effects with the δ opioid receptor antagonism was discussed in the early 2000s, the short-term memory improving effects and antitussive effects have been very recently reported as possible pharmacological effects induced by a δ opioid receptor inverse agonist. In this review, we will survey the developed δ opioid receptor inverse agonists and summarize the possible in vivo pharmacological effects by δ opioid receptor inverse agonists. Moreover, we will discuss important issues involved in the investigation of the in vivo pharmacological effects produced by a δ opioid receptor inverse agonist.

Hang on tight: reprogramming the cell with microstructural cues

Cells interact intimately with complex microdomains in their extracellular matrix (ECM) and maintain a delicate balance of mechanical forces through mechanosensitive cellular components. Tissue injury results in acute degradation of the ECM and disruption of cell-ECM contacts, manifesting in loss of cytoskeletal tension, leading to pathological cell transformation and the onset of disease. Recently, microscale hydrogel constructs have been developed to provide cells with microdomains to form focal adhesion binding sites, which enable restoration of cytoskeletal tension. These synthetic anchors can recapitulate the complex 3D architecture of the native ECM to provide microtopographical cues. The mechanical deformation of proteins at the cell surface can activate signaling cascades to modulate downstream gene-level transcription, making this a unique materials-based approach for reprogramming cell behavior. An overview of the mechanisms underlying these mechanosensitive interactions in fibroblasts, stem and other cell types is provided to review their effects on cellular reprogramming. Recent investigations on the fabrication, functionalization and implementation of these materials and microtopographical features for drug testing and therapeutic applications are discussed.

The Role of β-Arrestin Proteins in Organization of Signaling and Regulation of the AT1 Angiotensin Receptor

AT1 angiotensin receptor plays important physiological and pathophysiological roles in the cardiovascular system. Renin-angiotensin system represents a target system for drugs acting at different levels. The main effects of ATR1 stimulation involve activation of Gq proteins and subsequent IP3, DAG, and calcium signaling. It has become evident in recent years that besides the well-known G protein pathways, AT1R also activates a parallel signaling pathway through β-arrestins. β-arrestins were originally described as proteins that desensitize G protein-coupled receptors, but they can also mediate receptor internalization and G protein-independent signaling. AT1R is one of the most studied receptors, which was used to unravel the newly recognized β-arrestin-mediated pathways. β-arrestin-mediated signaling has become one of the most studied topics in recent years in molecular pharmacology and the modulation of these pathways of the AT1R might offer new therapeutic opportunities in the near future. In this paper, we review the recent advances in the field of β-arrestin signaling of the AT1R, emphasizing its role in cardiovascular regulation and heart failure.

Trabecular meshwork ECM remodeling in glaucoma: could RAS be a target?

INTRODUCTION

Disturbances of extracellular matrix (ECM) homeostasis in trabecular meshwork (TM) cause increased aqueous outflow resistance leading to elevated intraocular pressure (IOP) in glaucomatous eyes. Therefore, restoration of ECM homeostasis is a rational approach to prevent disease progression. Since renin-angiotensin system (RAS) inhibition positively alters ECM homeostasis in cardiovascular pathologies involving pressure and volume overload, it is likely that RAS inhibitors reduce IOP primarily by restoring ECM homeostasis. Areas covered: Current evidence showing the presence of RAS components in ocular tissue and its role in regulating aqueous humor dynamics is briefly summarized. The role of RAS in ECM remodeling is discussed both in terms of its effects on ECM synthesis and its breakdown. The mechanisms of ECM remodeling involving interactions of RAS with transforming growth factor-β, Wnt/β-catenin signaling, bone morphogenic proteins, connective tissue growth factor, and matrix metalloproteinases in ocular tissue are discussed. Expert opinion: Current literature strongly indicates a significant role of RAS in ECM remodeling in TM of hypertensive eyes. Hence, IOP-lowering effect of RAS inhibitors may primarily be attributed to restoration of ECM homeostasis in aqueous outflow pathways rather than its vascular effects. However, the mechanistic targets for RAS inhibitors have much wider distribution and consequences, which remain relatively unexplored in TM.

FOXP4-AS1 participates in the development and progression of osteosarcoma by downregulating LATS1 via binding to LSD1 and EZH2

Osteosarcoma (OS) is the most common malignant bone tumor in children and adolescents. LncRNA has been confirmed to participate in a variety of cancers. The purpose of this study was to explore the effect of FOXP4-AS1 on the development of osteosarcoma (OS) and its underlying mechanism. FOXP4-AS1 expressions in 60 OS tissues and paracancerous tissues were detected by qRT-PCR (quantitative real-time polymerase chain reaction). We confirmed that FOXP4-AS1 was overexpressed in OS tissues than that of paracancerous tissues. The disease-free survival and overall survival of OS patients were not correlated with age, gender and tumor location, but remarkably correlated with FOXP4-AS1 expression, tumor size and lung metastasis. For in vitro experiments, MG63 cells expressed a higher expression of FOXP4-AS1, whereas U2OS cells expressed a lower expression, which were selected for the following studies. Overexpressed FOXP4-AS1 led to enhanced proliferation, migration and invasion, shortened G0/G1 phase, as well as inhibited cell cycle. Knockdown of FOXP4-AS1 in MG63 cells obtained the opposite results. Furthermore, RIP assay indicated that FOXP4-AS1 could inhibit LATS1 expression by binding to LSD1 and EZH2, so as to participate in OS development. In conclusion, these results revealed that FOXP4-AS1 is overexpressed in OS, and is the independent risk factor in OS prognosis. Upregulated FOXP4-AS1 promotes the proliferation, migration and cell cycle, but inhibits apoptosis of OS cells. Furthermore, FOXP4-AS1 participates in the development and progression of OS by downregulating LATS1 via binding to LSD1 and EZH2.

Cardiovascular Adaptations Induced by Resistance Training in Animal Models

In the last 10 years the number of studies showing the benefits of resistance training (RT) to the cardiovascular system, have grown. In comparison to aerobic training, RT-induced favorable adaptations to the cardiovascular system have been ignored for many years, thus the mechanisms of the RT-induced cardiovascular adaptations are still uncovered. The lack of animal models with comparable protocols to the RT performed by humans hampers the knowledge. We have used squat-exercise model, which is widely used by many others laboratories. However, to a lesser extent, other models are also employed to investigate the cardiovascular adaptations. In the subsequent sections we will review the information regarding cardiac morphological adaptations, signaling pathway of the cardiac cell, cardiac function and the vascular adaptation induced by RT using this animal model developed by Tamaki et al. in 1992. Furthermore, we also describe cardiovascular findings observed using other animal models of RT.

Angiotensin Receptor Blockade Improves Cardiac Surgical Outcomes in Patients With Metabolic Syndrome

BACKGROUND

Perioperative use of angiotensin receptor blockers (ARBs) and angiotensin-converting enzyme inhibitors (ACEis) in patients undergoing cardiac operations remains controversial. The current practice of discontinuing renin-angiotensin-system inhibitors preoperatively may negate their beneficial effects in vulnerable populations, including patients with metabolic syndrome, who exhibit elevated renin-angiotensin system activity. We hypothesized that preoperative ARB use is associated with reduced incidence of postoperative complications, compared with ACEi or no drug, in patients with metabolic syndrome undergoing coronary artery bypass grafting.

METHODS

We used propensity matching to derive a cohort of 1,351 patients from 2,998 who underwent coronary artery bypass grafting based on preoperative use of ARBs, ACEis, or no renin-angiotensin-system inhibitors. Our primary end point was a composite of adverse events occurring within 30 days after the operation: new-onset atrial fibrillation/flutter, arrhythmia requiring cardioversion, perioperative myocardial infarction, acute renal failure, need for dialysis, cerebrovascular accidents, acute respiratory failure, or perioperative death.

RESULTS

At least one adverse event occurred in 524 (38.8%) of matched cohort patients (1,184 [39.6% of all patients]). Adjusting for European System for Cardiac Operative Risk Evaluation and metabolic syndrome in the matched cohort, preoperative use of ARBs was associated with a lower incidence of adverse events in patients with metabolic syndrome compared with preoperative use of no renin-angiotensin-system inhibitors (odds ratio, 0.43; 95% confidence interval, 0.19 to 0.99) or ACEis (odds ratio, 0.38; 95% confidence interval, 0.16 to 0.88).

CONCLUSIONS

Preoperative use of ARBs, but not ACEis, confers a benefit within 30 days after cardiac operations in patients with metabolic syndrome, suggesting potential efficacy differences of these drug classes in reducing cardiovascular morbidity and death in ambulatory vs surgical patients.

Exchange of chemical signals between cardiac cells. Fundamental role on cell communication and metabolic cooperation

The exchange of chemical signals between cardiac cells and its relevance for cell communication and metabolic cooperation was reviewed. The role of gap junctions on the transfer of chemical information was discussed as well as the different factors involved in its regulation including changes in cell volume, high glucose, activation of the renin angiotensin aldosterone system including the intracrine effect of renin and angiotensin II on chemical coupling and cardiac energetics. Finally, the possible role of epigenetic changes of the renin angiotensin aldosterone system (RAAS) on the expression of components of the RAAS was discussed. The evidence available leads to the conception of the heart as a metabolic syncytium in which glucose as well nucleotides and hormones can flow from cell-to-cell though gap junctions, providing a new vision of how alterations in metabolic cooperation can induce cardiac diseases. These findings represent a stimulus for future research in this important area of cardiac physiology and pathology.

The role of endothelial mechanosensitive genes in atherosclerosis and omics approaches

Atherosclerosis is the leading cause of morbidity and mortality in the U.S., and is a multifactorial disease that preferentially occurs in regions of the arterial tree exposed to disturbed blood flow. The detailed mechanisms by which d-flow induces atherosclerosis involve changes in the expression of genes, epigenetic patterns, and metabolites of multiple vascular cells, especially endothelial cells. This review presents an overview of endothelial mechanobiology and its relation to the pathogenesis of atherosclerosis with special reference to the anatomy of the artery and the underlying fluid mechanics, followed by a discussion of a variety of experimental models to study the role of fluid mechanics and atherosclerosis. Various in vitro and in vivo models to study the role of flow in endothelial biology and pathobiology are discussed in this review. Furthermore, strategies used for the global profiling of the genome, transcriptome, miR-nome, DNA methylome, and metabolome, as they are important to define the biological and pathophysiological mechanisms of atherosclerosis. These "omics" approaches, especially those which derive data based on a single animal model, provide unprecedented opportunities to not only better understand the pathophysiology of atherosclerosis development in a holistic and integrative manner, but also to identify novel molecular and diagnostic targets.

International Union of Basic and Clinical Pharmacology. XCIX. Angiotensin Receptors: Interpreters of Pathophysiological Angiotensinergic Stimuli [corrected]

The renin angiotensin system (RAS) produced hormone peptides regulate many vital body functions. Dysfunctional signaling by receptors for RAS peptides leads to pathologic states. Nearly half of humanity today would likely benefit from modern drugs targeting these receptors. The receptors for RAS peptides consist of three G-protein-coupled receptors—the angiotensin II type 1 receptor (AT1 receptor), the angiotensin II type 2 receptor (AT2 receptor), the MAS receptor—and a type II trans-membrane zinc protein—the candidate angiotensin IV receptor (AngIV binding site). The prorenin receptor is a relatively new contender for consideration, but is not included here because the role of prorenin receptor as an independent endocrine mediator is presently unclear. The full spectrum of biologic characteristics of these receptors is still evolving, but there is evidence establishing unique roles of each receptor in cardiovascular, hemodynamic, neurologic, renal, and endothelial functions, as well as in cell proliferation, survival, matrix-cell interaction, and inflammation. Therapeutic agents targeted to these receptors are either in active use in clinical intervention of major common diseases or under evaluation for repurposing in many other disorders. Broad-spectrum influence these receptors produce in complex pathophysiological context in our body highlights their role as precise interpreters of distinctive angiotensinergic peptide cues. This review article summarizes findings published in the last 15 years on the structure, pharmacology, signaling, physiology, and disease states related to angiotensin receptors. We also discuss the challenges the pharmacologist presently faces in formally accepting newer members as established angiotensin receptors and emphasize necessary future developments.

Candesartan improves ischemia-induced impairment of the blood-brain barrier in vitro

Candesartan has been reported to have a protective effect on cerebral ischemia in vivo and in human ischemic stroke. We studied the direct effects of candesartan on blood-brain barrier (BBB) function with our in vitro monolayer model generated using rat brain capillary endothelial cells (RBECs). The in vitro BBB model was subjected to normoxia or 6-h oxygen glucose deprivation (OGD)/24-h reoxygenation, with or without candesartan. 6-h OGD/24-h reoxygenation decreased transendothelial electrical resistance and increased the endothelial permeability for sodium fluorescein in RBEC monolayers. Candesartan (10 nM) improved RBEC barrier dysfunction induced by 6-h OGD/24-h reoxygenation. Immunostaining and immunoblotting analysis indicated that the effect of candesartan on barrier function under 6-h OGD/24-h reoxygenation was not related to the expression levels of tight junction proteins. However, candesartan affected RBEC morphological changes induced by 6-h OGD/24-h reoxygenation. We analyzed oxidative stress and cell viability using chemical reagents. Candesartan improved cell viability following 6-h OGD/24-h reoxygenation, whereas candesartan had no effect on oxidative stress. These results show that candesartan directly improves cell function and viability of brain capillary endothelial cells under OGD/reoxygenation, suggesting that the protective effects of candesartan on ischemic stroke are related to protection of the BBB.

Continuous infusion of angiotensin II modulates hypertrophic differentiation and apoptosis of chondrocytes in cartilage formation in a fracture model mouse

Although components of the renin-angiotensin system (RAS) are reported to be expressed in cultured chondrocytes and cartilage, little is known about the precise function of Angiotensin II (Ang II) in chondrocytes. In this study, we employed a rib fracture model mouse to investigate the effect of Ang II on chondrocytes. Ang II type 1 receptor (AT1R) was expressed in chondrocytes in the growth plate of mouse tibia. Continuous infusion of Ang II to rib-fractured mice resulted in a significant increase in the volume of cartilage, suggesting Ang II-induced hypertrophic differentiation of chondrocytes. It was also confirmed by a significant increase in the mRNA expression of Sox9 and runt-related transcription factor 2 (Runx2), which are genes related to chondrocyte differentiation, and type X collagen, matrix metalloproteinase (MMP)-13 and Indian hedgehog (Ihh), which are hypertrophic chondrocyte-specific molecular markers. Chondrocyte hypertrophy with upregulation of these genes was attenuated by administration of olmesartan, an AT1R blocker, but not by hydralazine. Moreover, Ang II infusion significantly suppressed apoptosis of chondrocytes, accompanied by significant induction of mRNA expression of bcl-2 and bcl-xL. Olmesartan, but not hydralazine, significantly attenuated the reduction of apoptotic cells and the increase in anti-apoptotic genes induced by Ang II infusion. Overall, the present study demonstrated that Ang II promoted hypertrophic differentiation of chondrocytes and reduced apoptosis of hypertrophic chondrocytes independently of high blood pressure. The present data indicate the role of Ang II in cartilage, and might provide a new concept for treatment of cartilage diseases.

A possible mechanism for the progression of chronic renal disease and congestive heart failure

Chronic neurologic diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis, as well as various forms of chronic renal disease and systolic congestive heart failure, are among the most common progressive degenerative disorders encountered in medicine. Each disease follows a nearly relentless course, albeit at varying rates, driven by progressive cell dysfunction and drop-out. The neurologic diseases are characterized by the progressive spread of disease-causing proteins (prion-like proteins) from cell to cell. Recent evidence indicates that cell autonomous renin angiotensin systems operate in heart and kidney, and it is known that functional intracrine proteins can also spread between cells. This then suggests that certain progressive degenerative cardiovascular disorders such as forms of chronic renal insufficiency and systolic congestive heart failure result from dysfunctional renin angiotensin system intracrine action spreading in kidney or myocardium.

Src is required for mechanical stretch-induced cardiomyocyte hypertrophy through angiotensin II type 1 receptor-dependent β-arrestin2 pathways

Angiotensin II (AngII) type 1 receptor (AT1-R) can be activated by mechanical stress (MS) without the involvement of AngII during the development of cardiomyocyte hypertrophy, in which G protein-independent pathways are critically involved. Although β-arrestin2-biased signaling has been speculated, little is known about how AT1-R/β-arrestin2 leads to ERK1/2 activation. Here, we present a novel mechanism by which Src kinase mediates AT1-R/β-arrestin2-dependent ERK1/2 phosphorylation in response to MS. Differing from stimulation by AngII, MS-triggered ERK1/2 phosphorylation is neither suppressed by overexpression of RGS4 (the negative regulator of the G-protein coupling signal) nor by inhibition of Gαq downstream protein kinase C (PKC) with GF109203X. The release of inositol 1,4,5-triphosphate (IP₃) is increased by AngII but not by MS. These results collectively suggest that MS-induced ERK1/2 activation through AT1-R might be independent of G-protein coupling. Moreover, either knockdown of β-arrestin2 or overexpression of a dominant negative mutant of β-arrestin2 prevents MS-induced activation of ERK1/2. We further identifies a relationship between Src, a non-receptor tyrosine kinase and β-arrestin2 using analyses of co-immunoprecipitation and immunofluorescence after MS stimulation. Furthermore, MS-, but not AngII-induced ERK1/2 phosphorylation is attenuated by Src inhibition, which also significantly improves pressure overload-induced cardiac hypertrophy and dysfunction in mice lacking AngII. Finally, MS-induced Src activation and hypertrophic response are abolished by candesartan but not by valsartan whereas AngII-induced responses can be abrogated by both blockers. Our results suggest that Src plays a critical role in MS-induced cardiomyocyte hypertrophy through β-arrestin2-associated angiotensin II type 1 receptor signaling.

Functional closure of the ductus arteriosus at birth: evidence against an intermediary role of angiotensin II

The fetal ductus arteriosus (DA) closes postnatally first functionally and then structurally. Normal rise in blood oxygenation is regarded as a prime trigger, but closure may occur more slowly without this stimulus. Here, our aim was to assess the role of angiotensin II (Ang II) in functional closure of DA since its action may not be conditioned by oxygen. Experiments were performed with wild-type fetal and neonatal mice, using whole-body freezing technique to assess DA caliber in vivo. Transcripts for Ang II type 1 (AT1R) and type 2 (AT2R) receptors were also examined. We found that the AT1R antagonist olmesartan had no effect in the fetus, but delayed ductus closure in the neonate. However, this response was short-lived and disappeared upon concomitant treatment with the AT2R antagonist PD123319. Coincidentally, olmesartan promoted the Agtr2 transcript. We conclude that AT1R-based Ang II has no role in the functional closure of DA. Conversely, the compound may modulate this process through AT2R-mediated vasodilatation.

The role of mechanical tension on lipid raft dependent PDGF-induced TRPC6 activation

Canonical transient receptor potential channel 6 (TRPC6) can play an important role in governing how cells perceive the surrounding material environment and regulate Ca(2+) signaling. We have designed a TRPC6 reporter based on fluorescence resonance energy transfer (FRET) to visualize the TRPC6-mediated calcium entry and hence TRPC6 activity in live cells with high spatiotemporal resolutions. In mouse embryonic fibroblasts (MEFs), platelet-derived growth factor BB (PDGF) can activate the TRPC6 reporter, mediated by phospholipase C (PLC). This TRPC6 activation occurred mainly at lipid rafts regions of the plasma membrane because disruption of lipid raft/caveolae by methyl-β-cyclodextrin (MβCD) or the expression of dominant-negative caveolin-1 inhibited the TRPC6 activity. Culturing cells on soft materials or releasing the intracellular tension by ML-7 reduced this PDGF-induced activation of TRPC6 without affecting the PDGF-regulated Src or inositol 1,4,5-trisphosphate (IP3) receptor function, suggesting a specific role of mechanical tension in regulating TRPC6. We further showed that the release of intracellular tension had similar effect on the diffusion coefficients of TRPC6 and a raft marker, confirming a strong coupling between TRPC6 and lipid rafts. Therefore, our results suggest that the TRPC6 activation mainly occurs at lipid rafts, which is regulated by the mechanical cues of surrounding materials.

Constitutive activity in the angiotensin II type 1 receptor: discovery and applications

The pathophysiological actions of the renin-angiotensin system hormone, angiotensin II (AngII), are mainly mediated by the AngII type 1 (AT1) receptor, a GPCR. The intrinsic spontaneous activity of the AT1 receptor in native tissues is difficult to detect due to its low expression levels. However, factors such as the membrane environment, interaction with autoantibodies, and mechanical stretch are known to increase G protein signaling in the absence of AngII. Naturally occurring and disease-causing activating mutations have not been identified in AT1 receptor. Constitutively active mutants (CAMs) of AT1 receptor have been engineered using molecular modeling and site-directed mutagenesis approaches among which substitution of Asn(111) in the transmembrane helix III with glycine or serine results in the highest basal activity of the receptor. Transgenic animal models expressing the CAM AT1 receptors that mimic various in vivo disease conditions have been useful research tools for discovering the pathophysiological role of AT1 receptor and evaluating the therapeutic potential of inverse agonists. This chapter summarizes the studies on the constitutive activity of AT1 receptor in recombinant as well as physiological systems. The impact of the availability of CAM AT1 receptors on our understanding of the molecular mechanisms underlying receptor activation and inverse agonism is described.

Mechanosensing and Regulation of Cardiac Function

The role of mechanical force as an important regulator of structure and function of mammalian cells, tissues, and organs has recently been recognized. However, mechanical overload is a pathogenesis or comorbidity existing in a variety of heart diseases, such as hypertension, aortic regurgitation and myocardial infarction. Physical stimuli sensed by cells are transmitted through intracellular signal transduction pathways resulting in altered physiological responses or pathological conditions. Emerging evidence from experimental studies indicate that β1-integrin and the angiotensin II type I (AT1) receptor play critical roles as mechanosensors in the regulation of heart contraction, growth and leading to heart failure. Integrin link the extracellular matrix and the intracellular cytoskeleton to initiate the mechanical signalling, whereas, the AT1 receptor could be activated by mechanical stress through an angiotensin-II-independent mechanism. Recent studies show that both Integrin and AT1 receptor and their downstream signalling factors including MAPKs, AKT, FAK, ILK and GTPase regulate heart function in cardiac myocytes. In this review we describe the role of mechanical sensors residing within the plasma membrane, mechanical sensor induced downstream signalling factors and its potential roles in cardiac contraction and growth.

Spironolactone enhances the beneficial effect of aliskiren on cardiac structural and electrical remodeling in TGR(mRen2)27 rats

OBJECTIVE

To investigate the influence of simultaneous administration of spironolactone (20 mg/kg per day, intraperitoneal (i.p.)) and aliskiren (50 mg/kg per day, i.p.) for a period of eight weeks on cardiac remodeling in TGR(mRen2)27 rats.

METHODS

Echocardiographic and electrophysiological and histological methods were used to determine the influence of spironolactone and aliskiren on cardiac remodeling.

RESULTS

1) the beneficial effect of aliskiren on SBP was enhanced by simultaneous administration of spironolactone; 2) echocardiographic studies showed that the left ventricle diameter (LVD), the left ventricle end diastolic volume (LVEDV) and the left ventricle posterior wall thickness (LVPW) were significantly reduced by the combination of both drugs when compared with aliskiren alone; 3) the ejection fraction was also increased; 4) histological studies indicated a greater decline in perivascular and interstitial fibrosis when both drugs were used; 5) the decrease of electrical remodeling of the left ventricle caused by aliskiren was further reduced by simultaneous administration of spironolactone; 6) the cardiac refractoriness increased by aliskiren was further incremented by spironolactone. Spironolactone (20 mg/kg per day) alone increased the ejection fraction and reduced LVD, LVEDV and LVPW but its effect was smaller than that achieved with the combination spironolactone plus aliskiren.

CONCLUSION

The combination of an aldosterone inhibitor with a direct renin inhibitor proved to be of greater benefit for cardiac structural and electrical remodeling in this experimental model of hypertension than aliskiren alone.

Differential mechanisms of activation of the Ang peptide receptors AT1, AT2, and MAS: using in silico techniques to differentiate the three receptors

The renin-angiotensin system is involved in multiple conditions ranging from cardiovascular disorders to cancer. Components of the pathway, including ACE, renin and angiotensin receptors are targets for disease treatment. This study addresses three receptors of the pathway: AT1, AT2, and MAS and how the receptors are similar and differ in activation by angiotensin peptides. Combining biochemical and amino acid variation data with multiple species sequence alignments, structural models, and docking site predictions allows for visualization of how angiotensin peptides may bind and activate the receptors; allowing identification of conserved and variant mechanisms in the receptors. MAS differs from AT1 favoring Ang-(1–7) and not Ang II binding, while AT2 recently has been suggested to preferentially bind Ang III. A new model of Ang peptide binding to AT1 and AT2 is proposed that correlates data from site directed mutagenesis and photolabled experiments that were previously considered conflicting. Ang II binds AT1 and AT2 through a conserved initial binding mode involving amino acids 111 (consensus 325) of AT1 (Asn) interacting with Tyr (4) of Ang II and 199 and 256 (consensus 512 and 621, a Lys and His respectively) interacting with Phe (8) of Ang II. In MAS these sites are not conserved, leading to differential binding and activation by Ang-(1–7). In both AT1 and AT2, the Ang II peptide may internalize through Phe (8) of Ang II propagating through the receptors’ conserved aromatic amino acids to the final photolabled positioning relative to either AT1 (amino acid 294, Asn, consensus 725) or AT2 (138, Leu, consensus 336). Understanding receptor activation provides valuable information for drug design and identification of other receptors that can potentially bind Ang peptides.

Expression of Angiotensin II Receptor-1 in Human Articular Chondrocytes

Background. Besides its involvement in the cardiovascular system, the renin-angiotensin-aldosterone (RAS) system has also been suggested to play an important role in inflammation. To explore the role of this system in cartilage damage in arthritis, we investigated the expression of angiotensin II receptors in chondrocytes. Methods. Articular cartilage was obtained from patients with osteoarthritis, rheumatoid arthritis, and traumatic fractures who were undergoing arthroplasty. Chondrocytes were isolated and cultured in vitro with or without interleukin (IL-1). The expression of angiotensin II receptor types 1 (AT1R) and 2 (AT2R) mRNA by the chondrocytes was analyzed using reverse transcription-polymerase chain reaction (RT-PCR). AT1R expression in cartilage tissue was analyzed using immunohistochemistry. The effect of IL-1 on AT1R/AT2R expression in the chondrocytes was analyzed by quantitative PCR and flow cytometry. Results. Chondrocytes from all patient types expressed AT1R/AT2R mRNA, though considerable variation was found between samples. Immunohistochemical analysis confirmed AT1R expression at the protein level. Stimulation with IL-1 enhanced the expression of AT1R/AT2R mRNA in OA and RA chondrocytes. Conclusions. Human articular chondrocytes, at least partially, express angiotensin II receptors, and IL-1 stimulation induced AT1R/AT2R mRNA expression significantly.

The Angiotensin II Type 2 Receptor in Brain Functions: An Update

Angiotensin II (Ang II) is the main active product of the renin-angiotensin system (RAS), mediating its action via two major receptors, namely, the Ang II type 1 (AT₁) receptor and the type 2 (AT₂) receptor. Recent results also implicate several other members of the renin-angiotensin system in various aspects of brain functions. The first aim of this paper is to summarize the current state of knowledge regarding the properties and signaling of the AT₂ receptor, its expression in the brain, and its well-established effects. Secondly, we will highlight the potential role of the AT₂ receptor in cognitive function, neurological disorders and in the regulation of appetite and the possible link with development of metabolic disorders. The potential utility of novel nonpeptide selective AT₂ receptor ligands in clarifying potential roles of this receptor in physiology will also be discussed. If confirmed, these new pharmacological tools should help to improve impaired cognitive performance, not only through its action on brain microcirculation and inflammation, but also through more specific effects on neurons. However, the overall physiological relevance of the AT₂ receptor in the brain must also consider the Ang IV/AT₄ receptor.

Aliskiren, at low doses, reduces the electrical remodeling in the heart of the TGR(mRen2)27 rat independently of blood pressure

METHODS

The influence of chronic administration of low doses of aliskiren (5 mg/kg/day, i.p.) for a period of eight weeks on cardiac electrophysiological and structural remodeling was investigated in transgenic (TGR)(mRen-2)27 rats. Cardiac and plasma angiotensin II (Ang II) levels were determined by ELISA before and after administration of the drug. Moreover, histological, electrophysiological and echocardiographic studies were performed in controls and at the end of eight weeks of aliskiren administration.

RESULTS

1) The cardiac Ang II levels were significantly reduced while the plasma Ang II levels were not significantly decreased in rats treated with low doses of aliskiren; 2) echocardographic studies showed a decrease of left ventricle diameter (LVD), left ventricle posterior wall thickness (LVPW), left ventricle end diastolic volume (LVEDV) and increased ejection fraction (EF); 3) aliskiren improved the impulse propagation, increased the cardiac refractoriness and reduced the incidence of triggered activity; 4) perivascular and interstitial fibrosis were greatly reduced, which explains the increase in conduction velocity. All these effects of aliskiren were found independently of blood pressure, suggesting that the beneficial effect of aliskiren was related to an inhibition of the local cardiac renin angiotensin system; and 5) the effect of mechanical stretch on action potential duration, conduction velocity and spontaneous rhythmicity was changed by aliskiren, supporting the hypothesis presented here that the beneficial effect of the drug on cardiac remodeling is related to a decreased sensitivity of cardiac muscle to mechanical stress.

Molecular changes in the early phase of renin-dependent cardiac hypertrophy in hypertensive cyp1a1ren-2 transgenic rats

An early response to high arterial pressure is the development of cardiac hypertrophy. Functional and transcriptional regulation of ion channels and Ca(2+) handling proteins are involved in this process but the relative contribution of each is unclear. In this study, we investigated the expression of genes involved in action potential generation and Ca(2+) homeostasis of cardiomyocytes in hypertensive cyp1a1ren-2 transgenic rats. In this model, the transgene prorenin was induced by indole-3-carbinol for 2 weeks allowing the induction of hypertension. Electrophysiological recordings from cardiomyocytes of hypertensive rats revealed a slight increase in membrane capacitance consistent with cellular hypertrophy. L-type calcium current density was reduced by 30%. Left ventricles of hypertensive rats showed a significant increase in transcript and protein levels of the cation channel TRPC6 and FK506-binding protein, whereas levels of SERCA2 and voltage-dependent potassium channels K(v)4.2 and K(v)4.3 were found to be decreased. Further, a marked nuclear localization of the transcription factors GATA4 and NFATC4 was observed in cardiac tissue of hypertensive rats. The cyp1a1ren-2 transgenic rat thus appears to be a valid model to investigate early changes in cardiac hypertrophy. This study points to roles for TRPC6, FK506BP, SERCA2, K(v)4.2, and K(v)4.3 in the development of cardiac hypertrophy.

On mechanosensation, acto/myosin interaction, and hypertrophy

Current concepts of mechanosensation are general and applicable to almost every cell type. However, striated muscle cells are distinguished by their ability to generate strong forces via actin/myosin interaction, and this process is fine-tuned for optimum contractility. This aspect, unique for actively contracting cells, may be defined as "sensing of the magnitude and dynamics of contractility," as opposed to the well-known concepts of the "perception of extracellular mechanical stimuli." The acto/myosin interaction, by producing changes in ATP, ADP, Pi, and force on a millisecond timescale, may be regarded as a novel and previously unappreciated mechanosensory mechanism. In addition, sarcomeric mechanosensory structures, such as the Z-disc, are directly linked to autophagy, survival, and cell death-related pathways. One emerging example is telethonin and its ability to interfere with p53 metabolism and hence apoptosis (mechanoptosis). In this article, we introduce contractility per se as an important mechanosensory mechanism, and we differentiate extracellular from intracellular mechanosensory effects.

G protein-mediated stretch reception

Mechanosensation and -transduction are important for physiological processes like the senses of touch, hearing, and balance. The mechanisms underlying the translation of mechanical stimuli into biochemical information by activating various signaling pathways play a fundamental role in physiology and pathophysiology but are only poorly understood. Recently, G protein-coupled receptors (GPCRs), which are essential for the conversion of light, olfactory and gustatory stimuli, as well as of primary messengers like hormones and neurotransmitters into cellular signals and which play distinct roles in inflammation, cell growth, and differentiation, have emerged as potential mechanosensors. The first candidate for a mechanosensitive GPCR was the angiotensin-II type-1 (AT(1)) receptor. Agonist-independent mechanical receptor activation of AT(1) receptors induces an active receptor conformation that appears to differ from agonist-induced receptor conformations and entails the activation of G proteins. Mechanically induced AT(1) receptor activation plays an important role for myogenic vasoconstriction and for the initiation of cardiac hypertrophy. A growing body of evidence suggests that other GPCRs are involved in mechanosensation as well. These findings highlight physiologically relevant, ligand-independent functions of GPCRs and add yet another facet to the polymodal activation spectrum of this ubiquitous protein family.

Interstitial fluid flow and cyclic strain differentially regulate cardiac fibroblast activation via AT1R and TGF-β1

Cardiac fibroblasts are exposed to both cyclic strain and interstitial fluid flow in the myocardium. The balance of these stimuli is affected by fibrotic scarring, during which the fibroblasts transition to a myofibroblast phenotype. The present study investigates the mechanisms by which cardiac fibroblasts seeded in three-dimensional (3D) collagen gels differentiate between strain and fluid flow. Neonatal cardiac fibroblast-seeded 3D collagen gels were exposed to interstitial flow and/or cyclic strain and message levels of collagens type I and III, transforming growth factor β1 (TGF-β1), and α-smooth muscle actin (α-SMA) were assessed. Flow was found to significantly increase and strain to decrease expression of myofibroblast markers. Corresponding immunofluorescence indicated that flow and strain differentially regulated α-SMA protein expression. The effect of flow was inhibited by exposure to losartan, an angiotensin II type 1 receptor (AT1R) blocker, and by introduction of shRNA constructs limiting AT1R expression. Blocking of TGF-β also inhibited the myofibroblast transition, suggesting that flow-mediated cell signaling involved both AT1R and TGF-β1. Reduced smad2 phosphorylation in response to cyclic strain suggested that TGF-β is part of the mechanism by which cardiac fibroblasts differentiate between strain-induced and flow-induced mechanical stress. Our experiments show that fluid flow and mechanical deformation have distinct effects on cardiac fibroblast phenotype. Our data suggest a mechanism in which fluid flow directly acts on AT1R and causes increased TGF-β1 expression, whereas cyclic strain reduces activation of smad proteins. These results have relevance to the pathogenesis and treatment of heart failure.

Inverse agonism and its therapeutic significance

A large number of G-protein-coupled receptors (GPCRs) show varying degrees of basal or constitutive activity. This constitutive activity is usually minimal in natural receptors but is markedly observed in wild type and mutated (naturally or induced) receptors. According to conventional two-state drug receptor interaction model, binding of a ligand may initiate activity (agonist with varying degrees of positive intrinsic activity) or prevent the effect of an agonist (antagonist with zero intrinsic activity). Inverse agonists bind with the constitutively active receptors, stabilize them, and thus reduce the activity (negative intrinsic activity). Receptors of many classes (α-and β-adrenergic, histaminergic, GABAergic, serotoninergic, opiate, and angiotensin receptors) have shown basal activity in suitable in vitro models. Several drugs that have been conventionally classified as antagonists (β-blockers, antihistaminics) have shown inverse agonist effects on corresponding constitutively active receptors. Nearly all H(1) and H(2) antihistaminics (antagonists) have been shown to be inverse agonists. Among the β-blockers, carvedilol and bucindolol demonstrate low level of inverse agonism as compared to propranolol and nadolol. Several antipsychotic drugs (D(2) receptors antagonist), antihypertensive (AT(1) receptor antagonists), antiserotoninergic drugs and opioid antagonists have significant inverse agonistic activity that contributes partly or wholly to their therapeutic value. Inverse agonism may also help explain the underlying mechanism of beneficial effects of carvedilol in congestive failure, naloxone-induced withdrawal syndrome in opioid dependence, clozapine in psychosis, and candesartan in cardiac hypertrophy. Understanding inverse agonisms has paved a way for newer drug development. It is now possible to develop agents, which have only desired therapeutic value and are devoid of unwanted adverse effect. Pimavanserin (ACP-103), a highly selective 5-HT(2A) inverse agonist, attenuates psychosis in patients with Parkinson's disease with psychosis and is devoid of extrapyramidal side effects. This dissociation is also evident from the development of anxioselective benzodiazepines devoid of habit-forming potential. Hemopressin is a peptide ligand that acts as an antagonist as well as inverse agonist. This agent acts as an antinociceptive agent in different in vivo models of pain. Treatment of obesity by drugs having inverse agonist activity at CB(1/2) receptors is also underway. An exciting development is evaluation of β-blockers in chronic bronchial asthma-a condition akin to congestive heart failure where β-blockade has become the standard mode of therapy. Synthesis and evaluation of selective agents is underway. Therefore, inverse agonism is an important aspect of drug-receptor interaction and has immense untapped therapeutic potential.

On the local cardiac renin angiotensin system. Basic and clinical implications

In the present review we reevaluated the experimental and clinical evidence that there is a local renin angiotensin system in the heart as well as the presence of a functional intracrine component which is activated during pathological conditions like heart failure and hypertension. The implications of these findings for cardiology were discussed. The novel finding that cell swelling impairs cell coupling and impulse propagation through activation of ionic channels with consequent generation of cardiac arrhythmias and the evidence that AT1 receptors are mechanosensors able to alter the heart function independently of Ang II were discussed. Particular attention was given to the role of salt loading on the activation of a local cardiac renin angiotensin and its consequences.

Genetics of mechanosensation in the heart

Mechanosensation (the ultimate conversion of a mechanical stimulus into a biochemical signal) as well as mechanotransduction (transmission of mechanically induced signals) belong to the most fundamental processes in biology. These effects, because of their dynamic nature, are particularly important for the cardiovascular system. Therefore, it is not surprising that defects in cardiac mechanosensation, are associated with various types of cardiomyopathy and heart failure. However, our current knowledge regarding the genetic basis of impaired mechanosensation in the cardiovascular system is beginning to shed light on this subject and is at the centre of this brief review.

Mechanosensitive TRP channels in cardiovascular pathophysiology

Transient receptor potential (TRP) proteins constitute a large non-voltage-gated cation channel superfamily, activated polymodally by various physicochemical stimuli, and are implicated in a variety of cellular functions. Known activators for TRP include not only chemical stimuli such as receptor stimulation, increased acidity and pungent/cooling agents, but temperature change and various forms of mechanical stimuli such as osmotic stress, membrane stretch, and shear force. Recent investigations have revealed that at least ten mammalian TRPs exhibit mechanosensitivity (TRPC1, 5, 6; TRPV1, 2, 4; TRPM3, 7; TRPA1; TRPP2), but the mechanisms underlying it appear considerably divergent and complex. The proposed mechanisms are associated with lipid bilayer mechanics, specialized force-transducing structures, biochemical reactions, membrane trafficking and transcriptional regulation. Many of mechanosensitive (MS)-TRP channel likely undergo multiple regulations via these mechanisms. In the cardiovascular system in which hemodynamic forces constantly operate, the impact of mechanical stress may be particularly significant. Extensive morphological and functional studies have indicated that several MS-TRP channels are expressed in cardiac muscle, vascular smooth muscle, endothelium and vasosensory neurons, each differentially contributing to cardiovascular (CV) functions. To further complexity, the recent evidence suggests that mechanical stress may synergize with neurohormonal mechanisms thereby amplifying otherwise marginal responses. Furthermore, the currently available data suggest that MS-TRP channels may be involved in CV pathophysiology such as cardiac arrhythmia, cardiac hypertrophy/myopathy, hypertension and aneurysms. This review will overview currently known mechanisms for mechanical activation/modulation of TRPs and possible connections of MS-TRP channels to CV disorders.

Synergistic activation of vascular TRPC6 channel by receptor and mechanical stimulation via phospholipase C/diacylglycerol and phospholipase A2/omega-hydroxylase/20-HETE pathways

TRPC6 is a non-voltage-gated Ca(2+) entry/depolarization channel associated with vascular tone regulation and remodeling. Expressed TRPC6 channel responds to both neurohormonal and mechanical stimuli, the mechanism for which remains controversial. In this study, we examined the possible interactions of receptor and mechanical stimulations in activating this channel using the patch clamp technique. In HEK293 cells expressing TRPC6, application of mechanical stimuli (hypotonicity, shear, 2,4,6-trinitrophenol) caused, albeit not effective by themselves, a prominent potentiation of cationic currents (I(TRPC6)) induced by a muscarinic receptor agonist carbachol. This effect was insensitive to a tarantula toxin GsMTx-4 (5 mumol/L). A similar extent of mechanical potentiation was observed after activation of I(TRPC6) by GTPgammaS or a diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol (OAG). Single TRPC6 channel activity evoked by carbachol was also enhanced by a negative pressure added in the patch pipette. Mechanical potentiation of carbachol- or OAG-induced I(TRPC6) was abolished by small interfering RNA knockdown of cytosolic phospholipase A(2) or pharmacological inhibition of omega-hydroxylation of arachidonic acid into 20-HETE (20-hydroxyeicosatetraenoic acid). Conversely, direct application of 20-HETE enhanced both OAG-induced macroscopic and single channel TRPC6 currents. Essentially the same results were obtained for TRPC6-like cation channel in A7r5 myocytes, where its activation by noradrenaline or Arg8 vasopressin was greatly enhanced by mechanical stimuli via 20-HETE production. Furthermore, myogenic response of pressurized mesenteric artery was significantly enhanced by weak receptor stimulation dependently on 20-HETE production. These results collectively suggest that simultaneous operation of receptor and mechanical stimulations may synergistically amplify transmembrane Ca(2+) mobilization through TRPC6 activation, thereby enhancing the vascular tone via phospholipase C/diacylglycerol and phospholipase A(2)/omega-hydroxylase/20-HETE pathways.