Gq-coupled receptors as mechanosensors mediating myogenic vasoconstriction

GPCR-mediated regulation of beige adipocyte formation: Implications for obesity and metabolic health

Obesity and its associated complications pose a significant burden on health. The non-shivering thermogenesis (NST) and metabolic capacity properties of brown adipose tissue (BAT), which are distinct from those of white adipose tissue (WAT), in combating obesity and its related metabolic diseases has been well documented. However, beige adipose tissue, the third and relatively novel type of adipose tissue, which emerges in extensive presence of WAT and shares similar favorable metabolic properties with BAT, has garnered considerable attention in recent years. In this review, we focused on the role of G protein-coupled receptors (GPCRs), the largest receptor family and the most successful class of drug targets in humans, in the induction of beige adipocytes. More importantly, we highlight researchers' clinical treatment attempts to ameliorate obesity and other related metabolic diseases through the formation and activation of beige adipose tissue. In summary, this review provides valuable insights into the formation of beige adipose tissue and the involvement of GPCRs, based on the latest advancements in scientific research.

Role of transient receptor potential channels in the regulation of vascular tone

Vascular tone is a major element in the control of hemodynamics. Transient receptor potential (TRP) channels conducting monovalent and/or divalent cations (e.g. Na+ and Ca2+) are expressed in the vasculature. Accumulating evidence suggests that TRP channels participate in regulating vascular tone by regulating intracellular Ca2+ signaling in both vascular smooth muscle cells (VSMCs) and endothelial cells (ECs). Aberrant expression/function of TRP channels in the vasculature is associated with vascular dysfunction in systemic/pulmonary hypertension and metabolic syndromes. This review intends to summarize our current knowledge of TRP-mediated regulation of vascular tone in both physiological and pathophysiological conditions and to discuss potential therapeutic approaches to tackle abnormal vascular tone due to TRP dysfunction.

Vesicle fusion and release in neurons under dynamic mechanical equilibrium

Vesicular fusion plays a pivotal role in cellular processes, involving stages like vesicle trafficking, fusion pore formation, content release, and membrane integration or separation. This dynamic process is regulated by a complex interplay of protein assemblies, osmotic forces, and membrane tension, which together maintain a mechanical equilibrium within the cell. Changes in cellular mechanics or external pressures prompt adjustments in this equilibrium, highlighting the system's adaptability. This review delves into the synergy between intracellular proteins, structural components, and external forces in facilitating vesicular fusion and release. It also explores how cells respond to mechanical stress, maintaining equilibrium and offering insights into vesicle fusion mechanisms and the development of neurological disorders.

The Hippo signalling pathway in bone homeostasis: Under the regulation of mechanics and aging

The Hippo signalling pathway is a conserved kinase cascade that orchestrates diverse cellular processes, such as proliferation, apoptosis, lineage commitment and stemness. With the onset of society ages, research on skeletal aging-mechanics-bone homeostasis has exploded. In recent years, aging and mechanical force in the skeletal system have gained groundbreaking research progress. Under the regulation of mechanics and aging, the Hippo signalling pathway has a crucial role in the development and homeostasis of bone. We synthesize the current knowledge on the role of the Hippo signalling pathway, particularly its downstream effectors yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ), in bone homeostasis. We discuss the regulation of the lineage specification and function of different skeletal cell types by the Hippo signalling pathway. The interactions of the Hippo signalling pathway with other pathways, such as Wnt, transforming growth factor beta and nuclear factor kappa-B, are also mentioned because of their importance for modulating bone homeostasis. Furthermore, YAP/TAZ have been extensively studied as mechanotransducers. Due to space limitations, we focus on reviewing how mechanical forces and aging influence cell fate, communications and homeostasis through a dysregulated Hippo signalling pathway.

Hypertension, Neurodegeneration, and Cognitive Decline

Elevated blood pressure is a well-established risk factor for age-related cognitive decline. Long linked to cognitive impairment on vascular bases, increasing evidence suggests a potential association of hypertension with the neurodegenerative pathology underlying Alzheimer disease. Hypertension is well known to disrupt the structural and functional integrity of the cerebral vasculature. However, the mechanisms by which these alterations lead to brain damage, enhance Alzheimer pathology, and promote cognitive impairment remain to be established. Furthermore, critical questions concerning whether lowering blood pressure by antihypertensive medications prevents cognitive impairment have not been answered. Recent developments in neurovascular biology, brain imaging, and epidemiology, as well as new clinical trials, have provided insights into these critical issues. In particular, clinical and basic findings on the link between neurovascular dysfunction and the pathobiology of neurodegeneration have shed new light on the overlap between vascular and Alzheimer pathology. In this review, we will examine the progress made in the relationship between hypertension and cognitive impairment and, after a critical evaluation of the evidence, attempt to identify remaining knowledge gaps and future research directions that may advance our understanding of one of the leading health challenges of our time.

Pacemaking in the lymphatic system

Lymphatic collecting vessels exhibit spontaneous phasic contractions that are critical for lymph propulsion and tissue fluid homeostasis. This rhythmic activity is driven by action potentials conducted across the lymphatic muscle cell (LMC) layer to produce entrained contractions. The contraction frequency of a lymphatic collecting vessel displays exquisite mechanosensitivity, with a dynamic range from <1 to >20 contractions per minute. A myogenic pacemaker mechanism intrinsic to the LMCs was initially postulated to account for pressure-dependent chronotropy. Further interrogation into the cellular constituents of the lymphatic vessel wall identified non-muscle cell populations that shared some characteristics with interstitial cells of Cajal, which have pacemaker functions in the gastrointestinal and lower urinary tracts, thus raising the possibility of a non-muscle cell pacemaker. However, recent genetic knockout studies in mice support LMCs and a myogenic origin of the pacemaker activity. LMCs exhibit stochastic, but pressure-sensitive, sarcoplasmic reticulum calcium release (puffs and waves) from IP3R1 receptors, which couple to the calcium-activated chloride channel Anoctamin 1, causing depolarisation. The resulting electrical activity integrates across the highly coupled lymphatic muscle electrical syncytia through connexin 45 to modulate diastolic depolarisation. However, multiple other cation channels may also contribute to the ionic pacemaking cycle. Upon reaching threshold, a voltage-gated calcium channel-dependent action potential fires, resulting in a nearly synchronous calcium global calcium flash within the LMC layer to drive an entrained contraction. This review summarizes the key ion channels potentially responsible for the pressure-dependent chronotropy of lymphatic collecting vessels and various mechanisms of IP3R1 regulation that could contribute to frequency tuning.

Protocol for assessing myogenic tone and perfusion pressure in isolated mouse kidneys

The isolated perfused kidney is a classic ex vivo preparation for studying renal physiology in general and vascular function. Here, we present a protocol for assessing myogenic tone in isolated mouse kidneys as well as vasodilatory and vasoconstrictive responses, expressed as perfusion pressure. We describe steps for pre-operative preparation, kidney and renal artery isolation, and connection of renal artery with glass cannula. We then detail how to measure pressure changes in perfused kidneys and the myogenic tone. For complete details on the use and execution of this protocol, please refer to Cui et al.1.

A new model for evaluating pressure-induced vascular tone in small cerebral arteries

The capacity of small cerebral arteries (SCAs) to adapt to pressure fluctuations has a fundamental physiological role and appears to be relevant in different pathological conditions. Here, we present a new computational model for quantifying the link, and its contributors, between luminal pressure and vascular tone generation in SCAs. This is assembled by combining a chemical sub-model, representing pressure-induced smooth muscle cell (SMC) signalling, with a mechanical sub-model for the tone generation and its transduction at tissue level. The devised model can accurately reproduce the impact of luminal pressure on different cytoplasmic components involved in myogenic signalling, both in the control case and when combined with some specific pharmacological interventions. Furthermore, the model is also able to capture and predict experimentally recorded pressure-outer diameter relationships obtained for vessels under control conditions, both in a Ca2 + -free bath and under drug inhibition. The modularity of the proposed framework allows the integration of new components for the study of a broad range of processes involved in the vascular function.

Phosphodiesterases Mediate the Augmentation of Myogenic Constriction by Inhibitory G Protein Signaling and is Negatively Modulated by the Dual Action of RGS2 and 5

G protein regulation by regulators of G protein signaling (RGS) proteins play a key role in vascular tone maintenance. The loss of Gi/o and Gq/11 regulation by RGS2 and RGS5 in non-pregnant mice is implicated in augmented vascular tone and decreased uterine blood flow (UBF). RGS2 and 5 are closely related and co-expressed in uterine arteries (UA). However, whether and how RGS2 and 5 coordinate their regulatory activities to finetune G protein signaling and regulate vascular tone are unclear. Here, we determined how the integrated activity of RGS2 and 5 modulates vascular tone to promote UBF. Using ultrasonography and pressure myography, we examined uterine hemodynamics and myogenic tone (MT) of UA of wild type (WT), Rgs2-/-, Rgs5-/-, and Rgs2/5 dbKO mice. We found that MT was reduced in Rgs5-/- relative to WT or Rgs2-/- UA. Activating Gi/o with dopamine increased, whereas exogenous cAMP decreased MT in Rgs5-/- UA to levels in WT UA. Dual deletion of Rgs2 and 5 abolished the reduced MT due to the absence of Rgs5 and enhanced dopamine-induced Gi/o effects in Rgs2/5 dbKO UA. Conversely, and as in WT UA, Gi/o inhibition with pertussis toxin or exogenous cAMP decreased MT in Rgs2/5 dbKO to levels in Rgs5-/- UA. Inhibition of phosphodiesterases (PDE) concentration-dependently decreased and normalized MT in all genotypes, and blocked dopamine-induced MT augmentation in Rgs2-/-, Rgs5-/-, and Rgs2/5 dbKO UA. We conclude that Gi/o augments UA MT in the absence of RGS2 by a novel mechanism involving PDE-mediated inhibition of cAMP-dependent vasodilatation..

A dual-clock-driven model of lymphatic muscle cell pacemaking to emulate knock-out of Ano1 or IP3R

Lymphatic system defects are involved in a wide range of diseases, including obesity, cardiovascular disease, and neurological disorders, such as Alzheimer's disease. Fluid return through the lymphatic vascular system is primarily provided by contractions of muscle cells in the walls of lymphatic vessels, which are in turn driven by electrochemical oscillations that cause rhythmic action potentials and associated surges in intracellular calcium ion concentration. There is an incomplete understanding of the mechanisms involved in these repeated events, restricting the development of pharmacological treatments for dysfunction. Previously, we proposed a model where autonomous oscillations in the membrane potential (M-clock) drove passive oscillations in the calcium concentration (C-clock). In this paper, to model more accurately what is known about the underlying physiology, we extend this model to the case where the M-clock and the C-clock oscillators are both active but coupled together, thus both driving the action potentials. This extension results from modifications to the model's description of the IP3 receptor, a key C-clock mechanism. The synchronised dual-driving clock behaviour enables the model to match IP3 receptor knock-out data, thus resolving an issue with previous models. We also use phase-plane analysis to explain the mechanisms of coupling of the dual clocks. The model has the potential to help determine mechanisms and find targets for pharmacological treatment of some causes of lymphoedema.

Mechanosensitive channels in the mechanical component of the exercise pressor reflex

The cardiovascular response is appropriately regulated during exercise to meet the metabolic demands of the active muscles. The exercise pressor reflex is a neural feedback mechanism through thin-fiber muscle afferents activated by mechanical and metabolic stimuli in the active skeletal muscles. The mechanical component of this reflex is referred to as skeletal muscle mechanoreflex. Its initial step requires mechanotransduction mediated by mechanosensors, which convert mechanical stimuli into biological signals. Recently, various mechanosensors have been identified, and their contributions to muscle mechanoreflex have been actively investigated. Nevertheless, the mechanosensitive channels responsible for this muscular reflex remain largely unknown. This review discusses progress in our understanding of muscle mechanoreflex under healthy conditions, focusing on mechanosensitive channels.

Mechanosensitive GPCRs and ion channels in shear stress sensing

As a universal mechanical cue, shear stress plays essential roles in many physiological processes, ranging from vascular morphogenesis and remodeling to renal transport and airway barrier function. Disrupted shear stress is commonly regarded as a major contributor to various human diseases such as atherosclerosis, hypertension, and chronic kidney disease. Despite the importance of shear stress in physiology and pathophysiology, our current understanding of mechanosensors that sense shear stress is far from complete. An increasing number of candidate mechanosensors have been proposed to mediate shear stress sensing in distinct cell types, including G protein-coupled receptors (GPCRs), G proteins, receptor tyrosine kinases, ion channels, glycocalyx proteins, and junctional proteins. Although multiple types of mechanosensors might be able to convert shear stress into downstream biochemical signaling events, in this review, we will focus on discussing the mechanosensitive GPCRs (angiotensin II type 1 receptor, GPR68, histamine H1 receptor, adhesion GPCRs) and ion channels (Piezo, TRP) that have been reported to be directly activated by shear stress.

Membrane mediated mechanical stimuli produces distinct active-like states in the AT1 receptor

The Angiotensin II Type 1 (AT1) receptor is one of the most widely studied GPCRs within the context of biased signaling. While the AT1 receptor is activated by agonists such as the peptide AngII, it can also be activated by mechanical stimuli such as membrane stretch or shear in the absence of a ligand. Despite the importance of mechanical activation of the AT1 receptor in biological processes such as vasoconstriction, little is known about the structural changes induced by external physical stimuli mediated by the surrounding lipid membrane. Here, we present a systematic simulation study that characterizes the activation of the AT1 receptor under various membrane environments and mechanical stimuli. We show that stability of the active state is highly sensitive to membrane thickness and tension. Structural comparison of membrane-mediated vs. agonist-induced activation shows that the AT1 receptor has distinct active conformations. This is supported by multi-microsecond free energy calculations that show unique landscapes for the inactive and various active states. Our modeling results provide structural insights into the mechanical activation of the AT1 receptor and how it may produce different functional outcomes within the framework of biased agonism.

Death-associated protein kinase 3 regulates the myogenic reactivity of cerebral arteries

NEW FINDINGS

What is the central question of this study? DAPK3 contributes to the Ca2+ -sensitization of vascular smooth muscle contraction: does this protein kinase participate in the myogenic response of cerebral arteries? What is the main finding and its importance? Small molecule inhibitors of DAPK3 effectively block the myogenic responses of cerebral arteries. HS38-dependent changes to vessel constriction occur independent of LC20 phosphorylation, and therefore DAPK3 appears to operate via the actin cytoskeleton. A role for DAPK3 in the myogenic response was not previously reported, and the results support a potential new therapeutic target in the cerebrovascular system.

ABSTRACT

The vascular smooth muscle (VSM) of resistance blood vessels is a target of intrinsic autoregulatory responses to increased intraluminal pressure, the myogenic response. In the brain, the myogenic reactivity of cerebral arteries is critical to homeostatic blood flow regulation. Here we provide the first evidence to link the death-associated protein kinase 3 (DAPK3) to the myogenic response of rat and human cerebral arteries. DAPK3 is a Ser/Thr kinase involved in Ca2+ -sensitization mechanisms of smooth muscle contraction. Ex vivo administration of a specific DAPK3 inhibitor (i.e., HS38) could attenuate vessel constrictions invoked by serotonin as well as intraluminal pressure elevation. The HS38-dependent dilatation was not associated with any change in myosin light chain (LC20) phosphorylation. The results suggest that DAPK3 does not regulate Ca2+ sensitization pathways during the myogenic response of cerebral vessels but rather operates to control the actin cytoskeleton. A slow return of myogenic tone was observed during the sustained ex vivo exposure of cerebral arteries to HS38. Recovery of tone was associated with greater LC20 phosphorylation that suggests intrinsic signalling compensation in response to attenuation of DAPK3 activity. Additional experiments with VSM cells revealed HS38- and siDAPK-dependent effects on the actin cytoskeleton and focal adhesion kinase phosphorylation status. The translational importance of DAPK3 to the human cerebral vasculature was noted, with robust expression of the protein kinase and significant HS38-dependent attenuation of myogenic reactivity found for human pial vessels.

Identification and validation of G protein-coupled receptors modulating flow-dependent signaling pathways in vascular endothelial cells

Vascular endothelial cells are exposed to mechanical forces due to their presence at the interface between the vessel wall and flowing blood. The patterns of these mechanical forces (laminar vs. turbulent) regulate endothelial cell function and play an important role in determining endothelial phenotype and ultimately cardiovascular health. One of the key transcriptional mediators of the positive effects of laminar flow patterns on endothelial cell phenotype is the zinc-finger transcription factor, krüppel-like factor 2 (KLF2). Given its importance in maintaining a healthy endothelium, we sought to identify endothelial regulators of the KLF2 transcriptional program as potential new therapeutic approaches to treating cardiovascular disease. Using an approach that utilized both bioinformatics and targeted gene knockdown, we identified endothelial GPCRs capable of modulating KLF2 expression. Genetic screening using siRNAs directed to these GPCRs identified 12 potential GPCR targets that could modulate the KLF2 program, including a subset capable of regulating flow-induced KLF2 expression in primary endothelial cells. Among these targets, we describe the ability of several GPCRs (GPR116, SSTR3, GPR101, LGR4) to affect KLF2 transcriptional activation. We also identify these targets as potential validated targets for the development of novel treatments targeting the endothelium. Finally, we highlight the initiation of drug discovery efforts for LGR4 and report the identification of the first known synthetic ligands to this receptor as a proof-of-concept for pathway-directed phenotypic screening to identify novel drug targets.

Angiotensin receptor blockade is associated with increased risk of giant cell arteritis

OBJECTIVES

Angiotensin II is implicated in GCA pathology. We examined whether the use of angiotensin receptor blockers (ARBs) is associated with GCA risk compared with angiotensin-converting enzyme inhibitors (ACEis) or other antihypertensives.

METHODS

We performed a matched cohort study including adults who were initiators of antihypertensives in UK primary care data between 1995 and 2019. Treatment-naïve individuals without prior GCA or PMR were categorized into three groups-ARB initiators, ACEi initiators, or other antihypertensive initiators (beta-blockers, calcium channel blockers, diuretics or alpha-adrenoceptor blockers)-and followed for up to 5 years. Incident GCA was defined using validated Read codes, with age of onset ≥50 years and two or more glucocorticoid prescriptions. Inverse probability-weighted Cox models were used to model outcome risk, adjusting for lifestyle parameters, comorbidities and comedications.

RESULTS

Among >1 million new starters of antihypertensives (81 780 ARBs, 422 940 ACEis and 873 066 other antihypertensives), the incidence rate of GCA per 10 000 patient-years was 2.73 (95% CI 2.12, 3.50) in the ARB group, 1.76 (95% CI 1.25, 2.39) in the ACEi group and 1.90 (95% CI 1.37, 2.56) in the other antihypertensives group. The hazard of GCA was higher in ARB initiators [hazard ratio (HR) 1.55; 95% CI 1.16, 2.06] than initiators of ACEis, but similar between initiators of other antihypertensives and ACEis (HR 1.08; 95% CI 0.87, 1.35).

CONCLUSIONS

Initiation of ARBs is associated with a higher risk of GCA compared with ACEis or other antihypertensives. Mechanistic studies of angiotensin receptor biology will provide further clarity for our findings.

Post-injury Inhibition of Endothelin-1 Dependent Renal Vasoregulation Mitigates Rhabdomyolysis-Induced Acute Kidney Injury

In patients with rhabdomyolysis, the overwhelming release of myoglobin into the circulation is the primary cause of kidney injury. Myoglobin causes direct kidney injury as well as severe renal vasoconstriction. An increase in renal vascular resistance (RVR) results in renal blood flow (RBF) and glomerular filtration rate (GFR) reduction, tubular injury, and acute kidney injury (AKI). The mechanisms that underlie rhabdomyolysis-induced AKI are not fully understood but may involve the local production of vasoactive mediators in the kidney. Studies have shown that myoglobin stimulates endothelin-1 (ET-1) production in glomerular mesangial cells. Circulating ET-1 is also increased in rats subjected to glycerol-induced rhabdomyolysis. However, the upstream mechanisms of ET-1 production and downstream effectors of ET-1 actions in rhabdomyolysis-induced AKI remain unclear. Vasoactive ET-1 is generated by ET converting enzyme 1 (ECE-1)-induced proteolytic processing of inactive big ET to biologically active peptides. The downstream ion channel effectors of ET-1-induced vasoregulation include the transient receptor potential cation channel, subfamily C member 3 (TRPC3). This study demonstrates that glycerol-induced rhabdomyolysis in Wistar rats promotes ECE-1-dependent ET-1 production, RVR increase, GFR decrease, and AKI. Rhabdomyolysis-induced increases in RVR and AKI in the rats were attenuated by post-injury pharmacological inhibition of ECE-1, ET receptors, and TRPC3 channels. CRISPR/Cas9-mediated knockout of TRPC3 channels attenuated ET-1-induced renal vascular reactivity and rhabdomyolysis-induced AKI. These findings suggest that ECE-1-driven ET-1 production and downstream activation of TRPC3-dependent renal vasoconstriction contribute to rhabdomyolysis-induced AKI. Hence, post-injury inhibition of ET-1-mediated renal vasoregulation may provide therapeutic targets for rhabdomyolysis-induced AKI.

Arteries are finely tuned thermosensors regulating myogenic tone and blood flow

In response to changing blood pressure, arteries adjust their caliber to control perfusion. This vital autoregulatory property, termed vascular myogenic tone, stabilizes downstream capillary pressure. We discovered that tissue temperature critically determines myogenic tone. Heating steeply activates tone in skeletal muscle, gut, brain and skin arteries with temperature coefficients ( Q ₁₀ ) of ∼11-20. Further, arterial thermosensitivity is tuned to resting tissue temperatures, making myogenic tone sensitive to small thermal fluctuations. Interestingly, temperature and intraluminal pressure are sensed largely independently and integrated to trigger myogenic tone. We show that TRPV1 and TRPM4 mediate heat-induced tone in skeletal muscle arteries. Variations in tissue temperature are known to alter vascular conductance; remarkably, thermosensitive tone counterbalances this effect, thus protecting capillary integrity and fluid balance. In conclusion, thermosensitive myogenic tone is a fundamental homeostatic mechanism regulating tissue perfusion.

One-Sentence Summary

Arterial blood pressure and temperature are integrated via thermosensitve ion channels to produce myogenic tone.

Apelin/ELABELA-APJ system in cardiac hypertrophy: Regulatory mechanisms and therapeutic potential

Heart failure is one of the most significant public health problems faced by millions of medical researchers worldwide. And pathological cardiac hypertrophy is considered one of the possible factors of increasing the risk of heart failure. Here, we introduce apelin/ELABELA-APJ system as a novel therapeutic target for cardiac hypertrophy, bringing about new directions in clinical treatment. Apelin has been proven to regulate cardiac hypertrophy through various pathways. And an increasing number of studies on ELABELA, the newly discovered endogenous ligand, suggest it can alleviate cardiac hypertrophy through mechanisms similar or different to apelin. In this review, we elaborate on the role that apelin/ELABELA-APJ system plays in cardiac hypertrophy and the intricate mechanisms that apelin/ELABELA-APJ affect cardiac hypertrophy. We also illuminate and make comparisons of the newly designed peptides and small molecules as agonists and antagonists for APJ, updating the breakthroughs in this field.

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.

ANTXR1 as a potential sensor of extracellular mechanical cues

Cell adhesion molecules mediate cell-cell or cell-matrix interactions, some of which are mechanical sensors, such as integrins. Emerging evidence indicates that anthrax toxin receptor 1 (ANTXR1), a newly identified cell adhesion molecule, can also sense extracellular mechanical signals such as hydrostatic pressure and extracellular matrix (ECM) rigidity. ANTXR1 can interact with ECM through connecting intracellular cytoskeleton and ECM molecules (just like integrins) to regulate numerous biological processes, such as cell adhesion, cell migration or ECM homeostasis. Although with high structural similarity to integrins, its functions and downstream signal transduction are independent from those of integrins. In this perspective, based on existing evidence in literature, we analyzed the structural and functional evidence that ANTXR1 can act as a potential sensor for extracellular mechanical cues. To our knowledge, this is the first in-depth overview of ANTXR1 from the perspective of mechanobiology. STATEMENT OF SIGNIFICANCE: An overview of ANTXR1 from the perspective of mechanobiology; An analysis of mechanical sensitivity of ANTXR1 in structure and function; A summary of existing evidence of ANTXR1 as a potential mechanosensor.

Intraluminal pressure elevates intracellular calcium and contracts CNS pericytes: Role of voltage-dependent calcium channels

Arteriolar smooth muscle cells (SMCs) and capillary pericytes dynamically regulate blood flow in the central nervous system in the face of fluctuating perfusion pressures. Pressure-induced depolarization and Ca2+ elevation provide a mechanism for regulation of SMC contraction, but whether pericytes participate in pressure-induced changes in blood flow remains unknown. Here, utilizing a pressurized whole-retina preparation, we found that increases in intraluminal pressure in the physiological range induce contraction of both dynamically contractile pericytes in the arteriole-proximate transition zone and distal pericytes of the capillary bed. We found that the contractile response to pressure elevation was slower in distal pericytes than in transition zone pericytes and arteriolar SMCs. Pressure-evoked elevation of cytosolic Ca2+ and contractile responses in SMCs were dependent on voltage-dependent Ca2+ channel (VDCC) activity. In contrast, Ca2+ elevation and contractile responses were partially dependent on VDCC activity in transition zone pericytes and independent of VDCC activity in distal pericytes. In both transition zone and distal pericytes, membrane potential at low inlet pressure (20 mmHg) was approximately -40 mV and was depolarized to approximately -30 mV by an increase in pressure to 80 mmHg. The magnitude of whole-cell VDCC currents in freshly isolated pericytes was approximately half that measured in isolated SMCs. Collectively, these results indicate a loss of VDCC involvement in pressure-induced constriction along the arteriole-capillary continuum. They further suggest that alternative mechanisms and kinetics of Ca2+ elevation, contractility, and blood flow regulation exist in central nervous system capillary networks, distinguishing them from neighboring arterioles.

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.

Significance of mechanical loading in bone fracture healing, bone regeneration, and vascularization

In 1892, J.L. Wolff proposed that bone could respond to mechanical and biophysical stimuli as a dynamic organ. This theory presents a unique opportunity for investigations on bone and its potential to aid in tissue repair. Routine activities such as exercise or machinery application can exert mechanical loads on bone. Previous research has demonstrated that mechanical loading can affect the differentiation and development of mesenchymal tissue. However, the extent to which mechanical stimulation can help repair or generate bone tissue and the related mechanisms remain unclear. Four key cell types in bone tissue, including osteoblasts, osteoclasts, bone lining cells, and osteocytes, play critical roles in responding to mechanical stimuli, while other cell lineages such as myocytes, platelets, fibroblasts, endothelial cells, and chondrocytes also exhibit mechanosensitivity. Mechanical loading can regulate the biological functions of bone tissue through the mechanosensor of bone cells intraosseously, making it a potential target for fracture healing and bone regeneration. This review aims to clarify these issues and explain bone remodeling, structure dynamics, and mechano-transduction processes in response to mechanical loading. Loading of different magnitudes, frequencies, and types, such as dynamic versus static loads, are analyzed to determine the effects of mechanical stimulation on bone tissue structure and cellular function. Finally, the importance of vascularization in nutrient supply for bone healing and regeneration was further discussed.

TRPC1 channels underlie stretch-modulated sarcoplasmic reticulum calcium leak in cardiomyocytes

Transient receptor potential canonical 1 (TRPC1) channels are Ca2+-permeable ion channels expressed in cardiomyocytes. An involvement of TRPC1 channels in cardiac diseases is widely established. However, the physiological role of TRPC1 channels and the mechanisms through which they contribute to disease development are still under investigation. Our prior work suggested that TRPC1 forms Ca2+ leak channels located in the sarcoplasmic reticulum (SR) membrane. Prior studies suggested that TRPC1 channels in the cell membrane are mechanosensitive, but this was not yet investigated in cardiomyocytes or for SR localized TRPC1 channels. We applied adenoviral transfection to overexpress or suppress TRPC1 expression in neonatal rat ventricular myocytes (NRVMs). Transfections were evaluated with RT-qPCR, western blot, and fluorescent imaging. Single-molecule localization microscopy revealed high colocalization of exogenously expressed TRPC1 and the sarco/endoplasmic reticulum Ca2+ ATPase (SERCA2). To test our hypothesis that TRPC1 channels contribute to mechanosensitive Ca2+ SR leak, we directly measured SR Ca2+ concentration ([Ca2+]SR) using adenoviral transfection with a novel ratiometric genetically encoded SR-targeting Ca2+ sensor. We performed fluorescence imaging to quantitatively assess [Ca2+]SR and leak through TRPC1 channels of NRVMs cultured on stretchable silicone membranes. [Ca2+]SR was increased in cells with suppressed TRPC1 expression vs. control and Transient receptor potential canonical 1-overexpressing cells. We also detected a significant reduction in [Ca2+]SR in cells with Transient receptor potential canonical 1 overexpression when 10% uniaxial stretch was applied. These findings indicate that TRPC1 channels underlie the mechanosensitive modulation of [Ca2+]SR. Our findings are critical for understanding the physiological role of TRPC1 channels and support the development of pharmacological therapies for cardiac diseases.

Oxidative Regulation of Vascular Cav1.2 Channels Triggers Vascular Dysfunction in Hypertension-Related Disorders

Blood pressure is determined by cardiac output and peripheral vascular resistance. The L-type voltage-gated Ca²⁺ (Ca_v1.2) channel in small arteries and arterioles plays an essential role in regulating Ca²⁺ influx, vascular resistance, and blood pressure. Hypertension and preeclampsia are characterized by high blood pressure. In addition, diabetes has a high prevalence of hypertension. The etiology of these disorders remains elusive, involving the complex interplay of environmental and genetic factors. Common to these disorders are oxidative stress and vascular dysfunction. Reactive oxygen species (ROS) derived from NADPH oxidases (NOXs) and mitochondria are primary sources of vascular oxidative stress, whereas dysfunction of the Ca_v1.2 channel confers increased vascular resistance in hypertension. This review will discuss the importance of ROS derived from NOXs and mitochondria in regulating vascular Ca_v1.2 and potential roles of ROS-mediated Ca_v1.2 dysfunction in aberrant vascular function in hypertension, diabetes, and preeclampsia.

The Effects of TRPC6 Knockout in Animal Models of Kidney Disease

Diseases that induce a loss of renal function affect a substantial portion of the world's population and can range from a slight decline in the glomerular filtration rate or microalbuminuria to complete kidney failure. Kidney disorders can be acute or chronic, but any significant reduction in renal function is associated with increased all-cause morbidity and mortality, especially when the conditions become chronic. There is an urgent need for new therapeutic approaches to slow or halt the progression of kidney disease. One potential target of considerable interest is the canonical transient receptor potential-6 (TRPC6) channel. TRCP6 is a cationic channel with a significant permeability to Ca2+. It is expressed in several tissues, including in multiple cell types of the kidney in glomeruli, microvasculature, and tubules. Here, we will describe TRPC6 channels and their roles in signal transduction, with an emphasis on renal cells, and the studies implicating TRPC6 channels in the progression of inherited and acquired kidney diseases. We then describe studies using TRPC6 knockout mice and rats subjected to treatments that model human diseases, including nephrotic syndromes, diabetic nephropathy, autoimmune glomerulonephritis, and acute kidney injuries induced by renal ischemia and by obstruction of the urinary tract. TRPC6 knockout has been shown to reduce glomerular manifestations of disease in several of these models and reduces renal fibrosis caused by urinary tract obstruction. TRPC6 knockout has proven to be less effective at reducing diabetic nephropathy in mouse and rat models. We also summarize the implications of these studies for drug development.

Gq Signaling in Autophagy Control: Between Chemical and Mechanical Cues

All processes in human physiology relies on homeostatic mechanisms which require the activation of specific control circuits to adapt the changes imposed by external stimuli. One of the critical modulators of homeostatic balance is autophagy, a catabolic process that is responsible of the destruction of long-lived proteins and organelles through a lysosome degradative pathway. Identification of the mechanism underlying autophagic flux is considered of great importance as both protective and detrimental functions are linked with deregulated autophagy. At the mechanistic and regulatory levels, autophagy is activated in response to diverse stress conditions (food deprivation, hyperthermia and hypoxia), even a novel perspective highlight the potential role of physical forces in autophagy modulation. To understand the crosstalk between all these controlling mechanisms could give us new clues about the specific contribution of autophagy in a wide range of diseases including vascular disorders, inflammation and cancer. Of note, any homeostatic control critically depends in at least two additional and poorly studied interdependent components: a receptor and its downstream effectors. Addressing the selective receptors involved in autophagy regulation is an open question and represents a new area of research in this field. G-protein coupled receptors (GPCRs) represent one of the largest and druggable targets membrane receptor protein superfamily. By exerting their action through G proteins, GPCRs play fundamental roles in the control of cellular homeostasis. Novel studies have shown Gαq, a subunit of heterotrimeric G proteins, as a core modulator of mTORC1 and autophagy, suggesting a fundamental contribution of Gαq-coupled GPCRs mechanisms in the control of this homeostatic feedback loop. To address how GPCR-G proteins machinery integrates the response to different stresses including oxidative conditions and mechanical stimuli, could provide deeper insight into new signaling pathways and open potential and novel therapeutic strategies in the modulation of different pathological conditions.

Neural crest mechanosensors: Seeing old proteins in a new light

Mechanical forces exerted on neural crest cells control their collective migration and differentiation. This perspective discusses our current understanding of neural crest mechanotransduction during cell migration and differentiation. Additionally, we describe proteins that have mechanosensitive functions in other systems, such as mechanosensitive G-protein-coupled receptors, mechanosensitive ion channels, cell-cell adhesion, and cell-matrix-interacting proteins, and highlight that these same proteins have in the past been studied in neural crest development from a purely signaling point of view. We propose that future studies elucidate the mechanosensitive functions these receptors may play in neural crest development and integrate this with their known molecular role.

Endothelial Cells and the Cerebral Circulation

Endothelial cells form the innermost layer of all blood vessels and are the only vascular component that remains throughout all vascular segments. The cerebral vasculature has several unique properties not found in the peripheral circulation; this requires that the cerebral endothelium be considered as a unique entity. Cerebral endothelial cells perform several functions vital for brain health. The cerebral vasculature is responsible for protecting the brain from external threats carried in the blood. The endothelial cells are central to this requirement as they form the basis of the blood-brain barrier. The endothelium also regulates fibrinolysis, thrombosis, platelet activation, vascular permeability, metabolism, catabolism, inflammation, and white cell trafficking. Endothelial cells regulate the changes in vascular structure caused by angiogenesis and artery remodeling. Further, the endothelium contributes to vascular tone, allowing proper perfusion of the brain which has high energy demands and no energy stores. In this article, we discuss the basic anatomy and physiology of the cerebral endothelium. Where appropriate, we discuss the detrimental effects of high blood pressure on the cerebral endothelium and the contribution of cerebrovascular disease endothelial dysfunction and dementia. © 2022 American Physiological Society. Compr Physiol 12:3449-3508, 2022.

Piezo1 Is a Mechanosensor Channel in Central Nervous System Capillaries

Capillaries are equipped to sense neurovascular coupling agents released onto the outer wall of a capillary, translating these external signals into electrical/Ca2+ changes that play a crucial role in blood flow regulation and ensuring that neuronal demands are met. However, control mechanisms attributable to forces imposed onto the lumen are less clear. Here, we show that Piezo1 channels act as mechanosensors in central nervous system capillaries. Electrophysiological analyses confirmed expression and function of Piezo1 channels in brain cortical and retinal capillaries. Activation of Piezo1 channels evoked currents that were sensitive to endothelial cell-specific Piezo1 deletion. Using genetically encoded Ca2+ indicator mice and an ex vivo pressurized retina preparation, we found that activation of Piezo1 channels by mechanical forces triggered Ca2+ signals in capillary endothelial cells. Collectively, these findings indicate that Piezo1 channels are capillary mechanosensors that initiate crucial Ca2+ signals and could, therefore, have a profound impact on central nervous system blood flow control.

Gut feelings: mechanosensing in the gastrointestinal tract

The primary function of the gut is to procure nutrients. Synchronized mechanical activities underlie nearly all its endeavours. Coordination of mechanical activities depends on sensing of the mechanical forces, in a process called mechanosensation. The gut has a range of mechanosensory cells. They function either as specialized mechanoreceptors, which convert mechanical stimuli into coordinated physiological responses at the organ level, or as non-specialized mechanosensory cells that adjust their function based on the mechanical state of their environment. All major cell types in the gastrointestinal tract contain subpopulations that act as specialized mechanoreceptors: epithelia, smooth muscle, neurons, immune cells, and others. These cells are tuned to the physical properties of the surrounding tissue, so they can discriminate mechanical stimuli from the baseline mechanical state. The importance of gastrointestinal mechanosensation has long been recognized, but the latest discoveries of molecular identities of mechanosensors and technical advances that resolve the relevant circuitry have poised the field to make important intellectual leaps. This Review describes the mechanical factors relevant for normal function, as well as the molecules, cells and circuits involved in gastrointestinal mechanosensing. It concludes by outlining important unanswered questions in gastrointestinal mechanosensing.

Angiotensin receptor blocker alleviates liver fibrosis by altering the mechanotransduction properties of hepatic stellate cells

Angiotensin receptor blockers have been reported to be beneficial to liver fibrosis, but the relevant molecular and cellular mechanisms remain unclear. We herein investigated whether low-dose angiotensin receptor blocker alleviated liver fibrosis through mechanotransduction regulation. Hydrostatic pressure-induced liver fibrosis model was established in mice by ligating partially the inferior vena cava, and then randomly received a very low dose of losartan (0.5 mg/kg) or placebo treatment for 8 weeks. We found that losartan administration interfered the expression of several mechanotransductive molecules, and effectively alleviated liver fibrosis. Using a commercial device, we further confirmed that ex vivo loading of hepatic stellate cells to 50 mmHg hydrostatic pressure for 24 h significantly upregulated RhoA, ROCK, AT1R, and p-MLC2, which was effectively attenuated by adding 10 nM losartan in medium. Our in vivo and ex vivo experimental data suggest that low-dose angiotensin receptor blockers may alleviate hydrostatic pressure-induced liver fibrosis by altering the mechanotransduction properties of hepatic stellate cells.NEW & NOTEWORTHY Our ex vivo and in vivo experiments clearly indicated that low-dose losartan alleviated liver fibrosis, likely by modulating the mechanotransduction properties of HSCs. Uncovering the biomechanical signaling pathway of ARB treatment on liver fibrosis will be helpful to develop novel molecular targeting therapy for liver diseases.

The normalized slope conductance as a tool for quantitative analysis of current-voltage relations

The patch-clamp method which was rewarded with the Nobel Prize in 1991 is a well-established and indispensable method to study ion channels in living cells and to biophysically characterize non-voltage-gated ion channels that comprise about 70% of all ion channels in the human genome. To investigate the biophysical properties of non-voltage-gated ion channels, whole-cell measurements with application of continuous voltage-ramps are routinely conducted to obtain current-voltage (IV) relationships. However, adequate tools for detailed and quantitative analysis of IV curves are still missing. We use the example of the transient receptor potential classical (TRPC) channel family to elucidate whether the normalized slope conductance (NSC) is an appropriate tool for a reliable discrimination of the IV curves of diverse TRPC channels that differ in their individual curve progression. We provide a robust calculation method for the NSC, and by applying this method we find that TRPC channel activators and modulators can evoke different NSC progressions independent from their expression levels which is pointing to distinguishable active channel states. TRPC6 mutations of patients suffering from focal segmental glomerulosclerosis (FSGS) resulted in distinct NSC progressions suggesting that the NSC is suitable to investigate structure-function relations and might help unravel the unknown pathomechanisms leading to FSGS. Altogether, the NCS represents an effective algorithm for extended biophysical characterization of non-voltage-gated ion channels.

Defining a Role of NADPH Oxidase in Myogenic Tone Development

OBJECTIVE

The myogenic response sets the foundation for blood flow control. Recent findings suggest a role for G-protein coupled receptors (GPCR) and signaling pathways tied to the generation of reactive oxygen species (ROS). In this regard, this study ascertained the impact of NADPH oxidase (Nox) on myogenic tone in rat cerebral resistance arteries.

METHODS

The study employed real-time qPCR (RT-qPCR), pressure myography, and immunohistochemistry.

RESULTS

G_q blockade abolished myogenic tone in rat cerebral arteries, linking GPCR to mechanosensation. Subsequent work revealed that general (TEMPOL) and mitochondrial specific (MitoTEMPO) ROS scavengers had little impact on myogenic tone, whereas apocynin, a broad spectrum Nox inhibitor, initiated transient dilation. RT-qPCR revealed Nox1 and Nox2 mRNA expression in smooth muscle cells. Pressure myography defined Nox1 rather than Nox2 is facilitating myogenic tone. We rationalized that Nox1-generated ROS was initiating this response by impairing the ability of the Ca_V 3.2 channel to elicit negative feedback via BK_Ca . This hypothesis was confirmed in functional experiments. The proximity ligation assay further revealed that Nox1 and Ca_V 3.2 colocalize within 40 nm of one another.

CONCLUSIONS

Our data highlight that vascular pressurization augments Nox1 activity and ensuing ROS production facilitates myogenic tone by limiting Ca²⁺ influx via Ca_V 3.2.

Mechanical regulation of bone remodeling

Bone remodeling is a lifelong process that gives rise to a mature, dynamic bone structure via a balance between bone formation by osteoblasts and resorption by osteoclasts. These opposite processes allow the accommodation of bones to dynamic mechanical forces, altering bone mass in response to changing conditions. Mechanical forces are indispensable for bone homeostasis; skeletal formation, resorption, and adaptation are dependent on mechanical signals, and loss of mechanical stimulation can therefore significantly weaken the bone structure, causing disuse osteoporosis and increasing the risk of fracture. The exact mechanisms by which the body senses and transduces mechanical forces to regulate bone remodeling have long been an active area of study among researchers and clinicians. Such research will lead to a deeper understanding of bone disorders and identify new strategies for skeletal rejuvenation. Here, we will discuss the mechanical properties, mechanosensitive cell populations, and mechanotransducive signaling pathways of the skeletal system.

Myogenic Vasoconstriction Requires Canonical Gq/11 Signaling of the Angiotensin II Type 1 Receptor

Background Blood pressure and tissue perfusion are controlled in part by the level of intrinsic (myogenic) arterial tone. However, many of the molecular determinants of this response are unknown. We previously found that mice with targeted disruption of the gene encoding the angiotensin II type 1a receptor (AT1AR) (Agtr1a), the major murine angiotensin II type 1 receptor (AT1R) isoform, showed reduced myogenic tone; however, uncontrolled genetic events (in this case, gene ablation) can lead to phenotypes that are difficult or impossible to interpret. Methods and Results We tested the mechanosensitive function of AT1R using tamoxifen-inducible smooth muscle-specific AT1aR knockout (smooth muscle-Agtr1a^-/-) mice and studied downstream signaling cascades mediated by G_q/11 and/or β-arrestins. FR900359, Sar1Ile4Ile8-angiotensin II (SII), TRV120027 and TRV120055 were used as selective G_q/11 inhibitor and biased agonists to activate noncanonical β-arrestin and canonical G_q/11 signaling of the AT1R, respectively. Myogenic and Ang II-induced constrictions were diminished in the perfused renal vasculature, mesenteric and cerebral arteries of smooth muscle-Agtr1a^-/- mice. Similar effects were observed in arteries of global mutant Agtr1a^-/- but not Agtr1b^-/- mice. FR900359 decreased myogenic tone and angiotensin II-induced constrictions whereas selective biased targeting of AT1R-β-arrestin signaling pathways had no effects. Conclusions This study demonstrates that myogenic arterial constriction requires G_q/11-dependent signaling pathways of mechanoactivated AT1R but not G protein-independent, noncanonical pathways in smooth muscle cells.

TRPV1 in arteries enables a rapid myogenic tone

KEY POINTS

We explored the physiological role of TRPV1 in vascular smooth muscle. TRPV1 antagonists dilated skeletal muscle arterioles both ex vivo and in vivo, increased coronary perfusion and decreased systemic blood pressure. Stretch of arteriolar myocytes and increases in intraluminal pressure in arteries triggered rapid Ca²⁺ signaling and vasoconstriction respectively. Pharmacologic and/or genetic disruption of TRPV1 significantly inhibited the magnitude and rate of these responses. Furthermore, disrupting TRPV1 blunted the rapid vasodilation evoked by arterial constriction. Pharmacological experiments identified key roles for phospholipase C and protein kinase C, combined with temperature, in TRPV1-dependent arterial tone. These results show that TRPV1 in arteriolar myocytes dynamically regulates myogenic tone and blood flow in the heart and skeletal muscle.

ABSTRACT

Arterioles maintain blow flow by adjusting their diameter in response to changes in local blood pressure. In this process called the myogenic response, a vascular smooth muscle mechanosensor controls tone predominantly through altering the membrane potential. In general, myogenic responses occur slowly (minutes). In the heart and skeletal muscle, however, tone is activated rapidly (tens of seconds) and terminated by brief (100 ms) arterial constrictions. Previously, we identified extensive expression of TRPV1 in the smooth muscle of arterioles supplying skeletal muscle, heart and fat. Here we reveal a critical role for TRPV1 in the rapid myogenic tone of these tissues. TRPV1 antagonists dilated skeletal muscle arterioles in vitro and in vivo, increased coronary flow in isolated hearts, and transiently decreased blood pressure. All of these pharmacologic effects were abolished by genetic disruption of TRPV1. Stretch of isolated vascular smooth muscle cells or raised intravascular pressure in arteries triggered Ca²⁺ signaling and vasoconstriction. The majority of these stretch-responses were TRPV1-mediated, with the remaining tone being inhibited by the TRPM4 antagonist, 9-phenantrol. Notably, tone developed more quickly in arteries from wild-type compared with TRPV1-null mice. Furthermore, the immediate vasodilation following brief constriction of arterioles depended on TRPV1, consistent with a rapid deactivation of TRPV1. Pharmacologic experiments revealed that membrane stretch activates phospholipase C/protein kinase C signaling combined with heat to activate TRPV1, and in turn, L-type Ca²⁺ channels. These results suggest a critical role, for TRPV1 in the dynamic regulation of myogenic tone and blood flow in the heart and skeletal muscle. This article is protected by copyright. All rights reserved.

Central regulation of body fluid homeostasis

Extracellular fluids, including blood, lymphatic fluid, and cerebrospinal fluid, are collectively called body fluids. The Na⁺ concentration ([Na⁺]) in body fluids is maintained at 135-145 mM and is broadly conserved among terrestrial animals. Homeostatic osmoregulation by Na⁺ is vital for life because severe hyper- or hypotonicity elicits irreversible organ damage and lethal neurological trauma. To achieve "body fluid homeostasis" or "Na homeostasis", the brain continuously monitors [Na⁺] in body fluids and controls water/salt intake and water/salt excretion by the kidneys. These physiological functions are primarily regulated based on information on [Na⁺] and relevant circulating hormones, such as angiotensin II, aldosterone, and vasopressin. In this review, we discuss sensing mechanisms for [Na⁺] and hormones in the brain that control water/salt intake behaviors, together with the responsible sensors (receptors) and relevant neural pathways. We also describe mechanisms in the brain by which [Na⁺] increases in body fluids activate the sympathetic neural activity leading to hypertension.

Vascular Smooth Muscle Cells Mechanosensitive Regulators and Vascular Remodeling

Blood vessels are subjected to mechanical loads of pressure and flow, inducing smooth muscle circumferential and endothelial shear stresses. The perception and response of vascular tissue and living cells to these stresses and the microenvironment they are exposed to are critical to their function and survival. These mechanical stimuli not only cause morphological changes in cells and vessel walls but also can interfere with biochemical homeostasis, leading to vascular remodeling and dysfunction. However, the mechanisms underlying how these stimuli affect tissue and cellular function, including mechanical stimulation-induced biochemical signaling and mechanical transduction that relies on cytoskeletal integrity, are unclear. This review focuses on signaling pathways that regulate multiple biochemical processes in vascular mesangial smooth muscle cells in response to circumferential stress and are involved in mechanosensitive regulatory molecules in response to mechanotransduction, including ion channels, membrane receptors, integrins, cytoskeletal proteins, nuclear structures, and cascades. Mechanoactivation of these signaling pathways is closely associated with vascular remodeling in physiological or pathophysiological states.

Synergy of epidermal growth factor (EGFR) and angiotensin II (AT1R) receptor determines composition and temporal pattern of transcriptome variation

The tyrosine kinase receptor EGFR and the G-protein-coupled receptor AT1R induce essential cellular responses, in part via receptor crosstalk with an unknown role in nuclear information transfer and transcription regulation. We investigated whether this crosstalk results in linear, EGFR-mediated nuclear signalling or in parallel, synergistic information transfer leading to qualitative and temporal variations, relevant for gene expression and environment interaction. AT1R and EGFR synergistically activate SRF via the ERK1/2-TCF and actin-MRTF pathways. Synergism, comprised of switch-like and graded single cell response, converges on the transcription factors AP1 and EGR, resulting in synergistic transcriptome alterations, in qualitative (over-additive number of genes), quantitative (over-additive expression changes of individual genes) and temporal (more late onset and prolonged expressed genes) terms. Gene ontology and IPA^® pathway analysis indicate prolonged cell stress (e.g. hypoxia-like) and dysregulated vascular biology. Synergism occurs during separate but simultaneous activation of both receptors and during AT1R-induced EGFR transactivation. EGFR and AT1R synergistically regulate gene expression in qualitative, quantitative and temporal terms with (patho)physiological relevance, extending the importance of EGFR-AT1R crosstalk beyond cytoplasmic signalling.

Calcium-Dependent Ion Channels and the Regulation of Arteriolar Myogenic Tone

Arterioles in the peripheral microcirculation regulate blood flow to and within tissues and organs, control capillary blood pressure and microvascular fluid exchange, govern peripheral vascular resistance, and contribute to the regulation of blood pressure. These important microvessels display pressure-dependent myogenic tone, the steady state level of contractile activity of vascular smooth muscle cells (VSMCs) that sets resting arteriolar internal diameter such that arterioles can both dilate and constrict to meet the blood flow and pressure needs of the tissues and organs that they perfuse. This perspective will focus on the Ca²⁺-dependent ion channels in the plasma and endoplasmic reticulum membranes of arteriolar VSMCs and endothelial cells (ECs) that regulate arteriolar tone. In VSMCs, Ca²⁺-dependent negative feedback regulation of myogenic tone is mediated by Ca²⁺-activated K⁺ (BK_Ca) channels and also Ca²⁺-dependent inactivation of voltage-gated Ca²⁺ channels (VGCC). Transient receptor potential subfamily M, member 4 channels (TRPM4); Ca²⁺-activated Cl⁻ channels (CaCCs; TMEM16A/ANO1), Ca²⁺-dependent inhibition of voltage-gated K⁺ (K_V) and ATP-sensitive K⁺ (K_ATP) channels; and Ca²⁺-induced-Ca²⁺ release through inositol 1,4,5-trisphosphate receptors (IP₃Rs) participate in Ca²⁺-dependent positive-feedback regulation of myogenic tone. Calcium release from VSMC ryanodine receptors (RyRs) provide negative-feedback through Ca²⁺-spark-mediated control of BK_Ca channel activity, or positive-feedback regulation in cooperation with IP₃Rs or CaCCs. In some arterioles, VSMC RyRs are silent. In ECs, transient receptor potential vanilloid subfamily, member 4 (TRPV4) channels produce Ca²⁺ sparklets that activate IP₃Rs and intermediate and small conductance Ca²⁺ activated K⁺ (IK_Ca and sK_Ca) channels causing membrane hyperpolarization that is conducted to overlying VSMCs producing endothelium-dependent hyperpolarization and vasodilation. Endothelial IP₃Rs produce Ca²⁺ pulsars, Ca²⁺ wavelets, Ca²⁺ waves and increased global Ca²⁺ levels activating EC sK_Ca and IK_Ca channels and causing Ca²⁺-dependent production of endothelial vasodilator autacoids such as NO, prostaglandin I₂ and epoxides of arachidonic acid that mediate negative-feedback regulation of myogenic tone. Thus, Ca²⁺-dependent ion channels importantly contribute to many aspects of the regulation of myogenic tone in arterioles in the microcirculation.

Constitutive activation of S1P receptors at the trans-Golgi network is required for surface transport carrier formation

The importance of the G-protein βγ subunits in the regulation of cargo transport from the trans-Golgi network (TGN) to the plasma membrane (PM) is well accepted; however, the molecular mechanism underlying the G-protein activation at the TGN remains unclear. We show here that sphingosine 1-phosphate (S1P) receptors at the PM were trafficked to the TGN in response to a surface transport cargo, temperature-sensitive vesicular stomatitis virus glycoprotein tagged with green fluorescent protein accumulation in the Golgi. The receptor internalization occurred in an S1P-independent manner but required phosphorylation by G-protein receptor kinase 2 and β-arrestin association before internalization. Continuously activated S1P receptors in a manner dependent on S1P at the TGN kept transmitting G-protein signals including the βγ subunits supply necessary for transport carrier formation at the TGN destined for the PM.

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S1P receptors traffic from the PM to Golgi in a surface cargo-dependent manner

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S1PR trafficking follows GRK2-dependent phosphorylation and β-arrestin binding

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S1PRs at the Golgi are continuously activated by S1P while sending G-protein signals

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S1PR/Gβγ signals at the Golgi are indispensable for surface transport carrier formation

Mechanotransduction at the Plasma Membrane-Cytoskeleton Interface

Mechanical cues are crucial for survival, adaptation, and normal homeostasis in virtually every cell type. The transduction of mechanical messages into intracellular biochemical messages is termed mechanotransduction. While significant advances in biochemical signaling have been made in the last few decades, the role of mechanotransduction in physiological and pathological processes has been largely overlooked until recently. In this review, the role of interactions between the cytoskeleton and cell-cell/cell-matrix adhesions in transducing mechanical signals is discussed. In addition, mechanosensors that reside in the cell membrane and the transduction of mechanical signals to the nucleus are discussed. Finally, we describe two examples in which mechanotransduction plays a significant role in normal physiology and disease development. The first example is the role of mechanotransduction in the proliferation and metastasis of cancerous cells. In this system, the role of mechanotransduction in cellular processes, including proliferation, differentiation, and motility, is described. In the second example, the role of mechanotransduction in a mechanically active organ, the gastrointestinal tract, is described. In the gut, mechanotransduction contributes to normal physiology and the development of motility disorders.

Cardiovascular mechanosensitive ion channels-Translating physical forces into physiological responses

Cells and tissues are constantly exposed to mechanical stress. In order to respond to alterations in mechanical stimuli, specific cellular machinery must be in place to rapidly convert physical force into chemical signaling to achieve the desired physiological responses. Mechanosensitive ion channels respond to such physical stimuli in the order of microseconds and are therefore essential components to mechanotransduction. Our understanding of how these ion channels contribute to cellular and physiological responses to mechanical force has vastly expanded in the last few decades due to engineering ingenuities accompanying patch clamp electrophysiology, as well as sophisticated molecular and genetic approaches. Such investigations have unveiled major implications for mechanosensitive ion channels in cardiovascular health and disease. Therefore, in this chapter I focus on our present understanding of how biophysical activation of various mechanosensitive ion channels promotes distinct cell signaling events with tissue-specific physiological responses in the cardiovascular system. Specifically, I discuss the roles of mechanosensitive ion channels in mediating (i) endothelial and smooth muscle cell control of vascular tone, (ii) mechano-electric feedback and cell signaling pathways in cardiomyocytes and cardiac fibroblasts, and (iii) the baroreflex.

Regulation of cerebral blood flow in humans: physiology and clinical implications of autoregulation

Brain function critically depends on a close matching between metabolic demands, appropriate delivery of oxygen and nutrients, and removal of cellular waste. This matching requires continuous regulation of cerebral blood flow (CBF), which can be categorized into four broad topics: 1) autoregulation, which describes the response of the cerebrovasculature to changes in perfusion pressure; 2) vascular reactivity to vasoactive stimuli [including carbon dioxide (CO2)]; 3) neurovascular coupling (NVC), i.e., the CBF response to local changes in neural activity (often standardized cognitive stimuli in humans); and 4) endothelium-dependent responses. This review focuses primarily on autoregulation and its clinical implications. To place autoregulation in a more precise context, and to better understand integrated approaches in the cerebral circulation, we also briefly address reactivity to CO2 and NVC. In addition to our focus on effects of perfusion pressure (or blood pressure), we describe the impact of select stimuli on regulation of CBF (i.e., arterial blood gases, cerebral metabolism, neural mechanisms, and specific vascular cells), the interrelationships between these stimuli, and implications for regulation of CBF at the level of large arteries and the microcirculation. We review clinical implications of autoregulation in aging, hypertension, stroke, mild cognitive impairment, anesthesia, and dementias. Finally, we discuss autoregulation in the context of common daily physiological challenges, including changes in posture (e.g., orthostatic hypotension, syncope) and physical activity.

What Evolutionary Evidence Implies About the Identity of the Mechanoelectrical Couplers in Vascular Smooth Muscle Cells

Loss of pressure-induced vasoconstriction increases susceptibility to renal and cerebral vascular injury. Favored paradigms underlying initiation of the response include transient receptor potential channels coupled to G protein-coupled receptors or integrins as transducers. Degenerin channels may also mediate the response. This review addresses the 1) evolutionary role of these molecules in mechanosensing, 2) limitations to identifying mechanosensitive molecules, and 3) paradigm shifting molecular model for a VSMC mechanosensor.

Mechanical and chemical activation of GPR68 probed with a genetically encoded fluorescent reporter

G-protein-coupled receptor (GPCR) 68 (GPR68, or OGR1) couples extracellular acidifications and mechanical stimuli to G-protein signaling and plays important roles in vascular physiology, neuroplasticity and cancer progression. Inspired by previous GPCR-based reporters, here, we inserted a cyclic permuted fluorescent protein into the third intracellular loop of GPR68 to create a genetically encoded fluorescent reporter of GPR68 activation we call 'iGlow'. iGlow responds to known physiological GPR68 activators such as fluid shear stress and extracellular acidifications. In addition, iGlow responds to Ogerin, a synthetic GPR68-selective agonist, but not to a non-active Ogerin analog, showing the specificity of iGlow-mediated fluorescence signals. Flow-induced iGlow activation is not eliminated by pharmacological modulation of downstream G-protein signaling, disruption of actin filaments or application of GsMTx4, an inhibitor of certain mechanosensitive ion channels activated by membrane stretch. Deletion of the conserved helix 8, proposed to mediate mechanosensitivity in certain GPCRs, does not eliminate flow-induced iGlow activation. iGlow could be useful to investigate the contribution of GPR68-dependent signaling in health and disease.

Myogenic Tone in Peripheral Resistance Arteries and Arterioles: The Pressure Is On!

Resistance arteries and downstream arterioles in the peripheral microcirculation contribute substantially to peripheral vascular resistance, control of blood pressure, the distribution of blood flow to and within tissues, capillary pressure, and microvascular fluid exchange. A hall-mark feature of these vessels is myogenic tone. This pressure-induced, steady-state level of vascular smooth muscle activity maintains arteriolar and resistance artery internal diameter at 50–80% of their maximum passive diameter providing these vessels with the ability to dilate, reducing vascular resistance, and increasing blood flow, or constrict to produce the opposite effect. Despite the central importance of resistance artery and arteriolar myogenic tone in cardiovascular physiology and pathophysiology, our understanding of signaling pathways underlying this key microvascular property remains incomplete. This brief review will present our current understanding of the multiple mechanisms that appear to underlie myogenic tone, including the roles played by G-protein-coupled receptors, a variety of ion channels, and several kinases that have been linked to pressure-induced, steady-state activity of vascular smooth muscle cells (VSMCs) in the wall of resistance arteries and arterioles. Emphasis will be placed on the portions of the signaling pathways underlying myogenic tone for which there is lack of consensus in the literature and areas where our understanding is clearly incomplete.

The Endothelial Glycocalyx as a Double-Edged Sword in Microvascular Homeostasis and Pathogenesis

Expressed on the endothelial cell (EC) surface of blood vessels, the glycocalyx (GCX), a mixture of carbohydrates attached to proteins, regulates the access of cells and molecules in the blood to the endothelium. Besides protecting endothelial barrier integrity, the dynamic microstructure of the GCX confers remarkable functions including mechanotransduction and control of vascular tone. Recently, a novel perspective has emerged supporting the pleiotropic roles of the endothelial GCX (eGCX) in cardiovascular health and disease. Because eGCX degradation occurs in certain pathological states, the circulating levels of eGCX degradation products have been recognized to have diagnostic or prognostic values. Beyond their biomarker roles, certain eGCX fragments serve as pathogenic factors in disease progression. Pharmacological interventions that attenuate eGCX degradation or restore its integrity have been sought. This review provides our current understanding of eGCX structure and function across the microvasculature in different organs. We also discuss disease or injury states, such as infection, sepsis and trauma, where eGCX dysfunction contributes to severe inflammatory vasculopathy.