It takes more than two to tango: mechanosignaling of the endothelial surface

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

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

The effect of combined ultrasound stimulation and gastrodin on seizures in mice

Both physiotherapy and medicine play essential roles in the treatment of epilepsy. The purpose of this research was to evaluate the efficacy of the combined therapy with focus ultrasound stimulation (FUS) and gastrodin (GTD) on seizures in a mouse model. Kainic acid-induced seizure mice were divided into five groups randomly: sham, FUS, saline + sham, GTD + sham and GTD + FUS. The results showed that combined therapy with ultrasound stimulation and gastrodin can significantly reduce the number and duration of seizures in GTD + FUS group. 9.4T magnetic resonance imaging and histologic staining results revealed the underlying mechanism of the combined therapy may be that ultrasound stimulation increases cell membrane permeability to increase GTD concentration in brain. In addition, we verified the safety of FUS combined with GTD therapy. This research provides a new strategy for neurological disorders combining treatment of physical neuromodulation and medicine.

Mechanosensory entities and functionality of endothelial cells

The endothelial cells of the blood circulation are exposed to hemodynamic forces, such as cyclic strain, hydrostatic forces, and shear stress caused by the blood fluid's frictional force. Endothelial cells perceive mechanical forces via mechanosensors and thus elicit physiological reactions such as alterations in vessel width. The mechanosensors considered comprise ion channels, structures linked to the plasma membrane, cytoskeletal spectrin scaffold, mechanoreceptors, and junctional proteins. This review focuses on endothelial mechanosensors and how they alter the vascular functions of endothelial cells. The current state of knowledge on the dysregulation of endothelial mechanosensitivity in disease is briefly presented. The interplay in mechanical perception between endothelial cells and vascular smooth muscle cells is briefly outlined. Finally, future research avenues are highlighted, which are necessary to overcome existing limitations.

Integrating molecular and cellular components of endothelial shear stress mechanotransduction

The lining of blood vessels is constantly exposed to mechanical forces exerted by blood flow against the endothelium. Endothelial cells detect these tangential forces (i.e., shear stress), initiating a host of intracellular signaling cascades that regulate vascular physiology. Thus, vascular health is tethered to the endothelial cells' capacity to transduce shear stress. Indeed, the mechanotransduction of shear stress underlies a variety of cardiovascular benefits, including some of those associated with increased physical activity. However, endothelial mechanotransduction is impaired in aging and disease states such as obesity and type 2 diabetes, precipitating the development of vascular disease. Understanding endothelial mechanotransduction of shear stress, and the molecular and cellular mechanisms by which this process becomes defective, is critical for the identification and development of novel therapeutic targets against cardiovascular disease. In this review, we detail the primary mechanosensitive structures that have been implicated in detecting shear stress, including junctional proteins such as platelet endothelial cell adhesion molecule-1 (PECAM-1), the extracellular glycocalyx and its components, and ion channels such as piezo1. We delineate which molecules are truly mechanosensitive and which may simply be indispensable for the downstream transmission of force. Furthermore, we discuss how these mechanosensors interact with other cellular structures, such as the cytoskeleton and membrane lipid rafts, which are implicated in translating shear forces to biochemical signals. Based on findings to date, we also seek to integrate these cellular and molecular mechanisms with a view of deciphering endothelial mechanotransduction of shear stress, a tenet of vascular physiology.

Mechanical strategies to promote vascularization for tissue engineering and regenerative medicine

Vascularization is a major challenge in the field of tissue engineering and regenerative medicine. Mechanical factors have been demonstrated to play a fundamental role in vasculogenesis and angiogenesis and can affect the architecture of the generated vascular network. Through the regulation of mechanical factors in engineered tissues, various mechanical strategies can be used to optimize the preformed vascular network and promote its rapid integration with host vessels. Optimization of the mechanical properties of scaffolds, including controlling scaffold stiffness, increasing surface roughness and anisotropic structure, and designing interconnected, hierarchical pore structures, is beneficial for the in vitro formation of vascular networks and the ingrowth of host blood vessels. The incorporation of hollow channels into scaffolds promotes the formation of patterned vascular networks. Dynamic stretching and perfusion can facilitate the formation and maturation of preformed vascular networks in vitro. Several indirect mechanical strategies provide sustained mechanical stimulation to engineered tissues in vivo, which further promotes the vascularization of implants within the body. Additionally, stiffness gradients, anisotropic substrates and hollow channels in scaffolds, as well as external cyclic stretch, boundary constraints and dynamic flow culture, can effectively regulate the alignment of vascular networks, thereby promoting better integration of prevascularized engineered tissues with host blood vessels. This review summarizes the influence and contribution of both scaffold-based and external stimulus-based mechanical strategies for vascularization in tissue engineering and elucidates the underlying mechanisms involved.

Prolonged Door-to-Balloon Time Leads to Endothelial Glycocalyx Damage and Endothelial Dysfunction in Patients with ST-Elevation Myocardial Infarction

Damage to the endothelial glycocalyx (eGC) has been reported during acute ischemic events like ST-elevation myocardial infarction (STEMI). In STEMI, a door-to-balloon time (D2B) of <60 min was shown to reduce mortality and nonfatal complications. Here, we hypothesize that eGC condition is associated with D2B duration and endothelial function during STEMI. One hundred and twenty-six individuals were analyzed in this study (STEMI patients vs. age-/sex-matched healthy volunteers). After stimulating endothelial cells with patient/control sera, the eGC's nanomechanical properties (i.e., height/stiffness) were analyzed using the atomic force microscopy-based nanoindentation technique. eGC components were determined via ELISA, and measurements of nitric oxide levels (NO) were based on chemiluminescence. eGC height/stiffness (both p < 0.001), as well as NO concentration (p < 0.001), were reduced during STEMI. Notably, the D2B had a strong impact on the endothelial condition: a D2B > 60 min led to significantly higher serum concentrations of eGC components (syndecan-1: p < 0.001/heparan sulfate: p < 0.001/hyaluronic acid: p < 0.0001). A D2B > 60 min led to the pronounced loss of eGC height/stiffness (both, p < 0.001) with reduced NO concentrations (p < 0.01), activated the complement system (p < 0.001), and prolonged the hospital stay (p < 0.01). An increased D2B led to severe eGC shedding, with endothelial dysfunction in a temporal context. eGC components and pro-inflammatory mediators correlated with a prolonged D2B, indicating a time-dependent immune reaction during STEMI, with a decreased NO concentration. Thus, D2B is a crucial factor for eGC damage during STEMI. Clinical evaluation of the eGC condition might serve as an important predictor for the endothelial function of STEMI patients in the future.

Micro- and Macrovascular Effects of Inflammation in Peripheral Artery Disease-Pathophysiology and Translational Therapeutic Approaches

Inflammation has a critical role in the development and progression of atherosclerosis. On the molecular level, inflammatory pathways negatively impact endothelial barrier properties and thus, tissue homeostasis. Conformational changes and destruction of the glycocalyx further promote pro-inflammatory pathways also contributing to pro-coagulability and a prothrombotic state. In addition, changes in the extracellular matrix composition lead to (peri-)vascular remodelling and alterations of the vessel wall, e.g., aneurysm formation. Moreover, progressive fibrosis leads to reduced tissue perfusion due to loss of functional capillaries. The present review aims at discussing the molecular and clinical effects of inflammatory processes on the micro- and macrovasculature with a focus on peripheral artery disease.

Mechanism Analysis of Vascular Calcification Based on Fluid Dynamics

Vascular calcification is the abnormal deposition of calcium phosphate complexes in blood vessels, which is regarded as the pathological basis of multiple cardiovascular diseases. The flowing blood exerts a frictional force called shear stress on the vascular wall. Blood vessels have different hydrodynamic properties due to discrepancies in geometric and mechanical properties. The disturbance of the blood flow in the bending area and the branch point of the arterial tree produces a shear stress lower than the physiological magnitude of the laminar shear stress, which can induce the occurrence of vascular calcification. Endothelial cells sense the fluid dynamics of blood and transmit electrical and chemical signals to the full-thickness of blood vessels. Through crosstalk with endothelial cells, smooth muscle cells trigger osteogenic transformation, involved in mediating vascular intima and media calcification. In addition, based on the detection of fluid dynamics parameters, emerging imaging technologies such as 4D Flow MRI and computational fluid dynamics have greatly improved the early diagnosis ability of cardiovascular diseases, showing extremely high clinical application prospects.

Role of the Renin Angiotensin Aldosterone System in the Pathogenesis of Sepsis-Induced Acute Kidney Injury: A Systematic Review

BACKGROUND

Sepsis is a life-threatening condition responsible for up to 20% of all global deaths. Kidneys are among the most common organs implicated, yet the pathogenesis of sepsis-induced acute kidney injury (S-AKI) is not completely understood, resulting in the treatment being nonspecific and responsive. In situations of stress, the renin angiotensin aldosterone system (RAAS) may play a role. This systematic review focuses on analyzing the impact of the RAAS on the development of S-AKI and discussing the use of RAAS antagonists as an emerging therapeutic option to minimize complications of sepsis.

METHODS

Studies were identified using electronic databases (Medline via PubMed, Google Scholar) published within the past decade, comprised from 2014 to 2023. The search strategy was conducted using the following keywords: sepsis, S-AKI, RAAS, Angiotensin II, and RAAS inhibitors. Studies on human and animal subjects were included if relevant to the keywords.

RESULTS

Our search identified 22 eligible references pertaining to the inclusion criteria. Treatment of sepsis with RAAS inhibitor medications is observed to decrease rates of S-AKI, reduce the severity of S-AKI, and offer an improved prognosis for septic patients.

CONCLUSION

The use of RAAS antagonists as a treatment after the onset of sepsis has promising findings, with evidence of decreased renal tissue damage and rates of S-AKI and improved survival outcomes.

REGISTRATION

INPLASY202360098.

Biomechanical and Mechanobiological Drivers of the Transition From PostCapillary Pulmonary Hypertension to Combined Pre-/PostCapillary Pulmonary Hypertension

Combined pre-/postcapillary pulmonary hypertension (Cpc-PH), a complication of left heart failure, is associated with higher mortality rates than isolated postcapillary pulmonary hypertension alone. Currently, knowledge gaps persist on the mechanisms responsible for the progression of isolated postcapillary pulmonary hypertension (Ipc-PH) to Cpc-PH. Here, we review the biomechanical and mechanobiological impact of left heart failure on pulmonary circulation, including mechanotransduction of these pathological forces, which lead to altered biological signaling and detrimental remodeling, driving the progression to Cpc-PH. We focus on pathologically increased cyclic stretch and decreased wall shear stress; mechanotransduction by endothelial cells, smooth muscle cells, and pulmonary arterial fibroblasts; and signaling-stimulated remodeling of the pulmonary veins, capillaries, and arteries that propel the transition from Ipc-PH to Cpc-PH. Identifying biomechanical and mechanobiological mechanisms of Cpc-PH progression may highlight potential pharmacologic avenues to prevent right heart failure and subsequent mortality.

Organ Crosstalk in Acute Kidney Injury: Evidence and Mechanisms

Acute kidney injury (AKI) is becoming a public health problem worldwide. AKI is usually considered a complication of lung, heart, liver, gut, and brain disease, but recent findings have supported that injured kidney can also cause dysfunction of other organs, suggesting organ crosstalk existence in AKI. However, the organ crosstalk in AKI and the underlying mechanisms have not been broadly reviewed or fully investigated. In this review, we summarize recent clinical and laboratory findings of organ crosstalk in AKI and highlight the related molecular mechanisms. Moreover, their crosstalk involves inflammatory and immune responses, hemodynamic change, fluid homeostasis, hormone secretion, nerve reflex regulation, uremic toxin, and oxidative stress. Our review provides important clues for the intervention for AKI and investigates important therapeutic potential from a new perspective.

The impact of progredient vessel and tissue stiffening for the development of metabolic syndrome

Established risk factors for the metabolic syndrome as diabetes and arterial hypertension are believed to be the cause of arteriosclerosis and subsequently following diseases like coronary heart disease, apoplexy, or chronic renal failure. Based on broad evidence from the already available experimental literature and clinical experience, an alternative hypothesis is presented that puts an increased vessel and organ stiffness to the beginning of the pathophysiological scenario. The stiffness itself is caused by a persistent activation of mechano-sensitive cation channels like the epithelial/endothelial sodium channel. A further enhancement takes place by proteins like JACD and RhoA coupled phospholipase C coupled G-protein receptors and integrins. A self-enhancing positive feedback loop by activation of YAP/TAZ signaling is a further central pillar of this theory. Further investigations are necessary to verify this hypothesis. If this hypothesis could be confirmed fundamental changes regarding the pharmacologic therapy of the diseases that are currently summarizes as metabolic syndrome would be the consequence.

Flow-induced glycocalyx formation and cell alignment of HUVECs compared to iPSC-derived ECs for tissue engineering applications

The relevance of cellular in vitro models highly depends on their ability to mimic the physiological environment of the respective tissue or cell niche. Static culture conditions are often unsuitable, especially for endothelial models, since they completely neglect the physiological surface shear stress and corresponding reactions of endothelial cells (ECs) such as alignment in the direction of flow. Furthermore, formation and maturation of the glycocalyx, the essential polysaccharide layer covering all endothelial surfaces and regulating diverse processes, is highly dependent on applied fluid flow. This fragile but utterly important macromolecular layer is hard to analyze, its importance is often underestimated and accordingly neglected in many endothelial models. Therefore, we exposed human umbilical vein ECs (HUVECs) and human induced pluripotent stem cell-derived ECs (iPSC-ECs) as two relevant EC models in a side-by-side comparison to static and physiological dynamic (6.6 dyn cm-2) culture conditions. Both cell types demonstrated an elongation and alignment along the flow direction, some distinct changes in glycocalyx composition on the surface regarding the main glycosaminoglycan components heparan sulfate, chondroitin sulfate or hyaluronic acid as well as an increased and thereby improved glycocalyx thickness and functionality when cultured under homogeneous fluid flow. Thus, we were able to demonstrate the maturity of the employed iPSC-EC model regarding its ability to sense fluid flow along with the general importance of physiological shear stress for glycocalyx formation. Additionally, we investigated EC monolayer integrity with and without application of surface shear stress, revealing a comparable existence of tight junctions for all conditions and a reorganization of the cytoskeleton upon dynamic culture leading to an increased formation of focal adhesions. We then fabricated cell sheets of EC monolayers after static and dynamic culture via non-enzymatic detachment using thermoresponsive polymer coatings as culture substrates. In a first proof-of-concept we were able to transfer an aligned iPSC-EC sheet to a 3D-printed scaffold thereby making a step in the direction of vascular modelling. We envision these results to be a valuable contribution to improvements of in vitro endothelial models and vascular engineering in the future.

Endothelial Glycocalyx

The glycocalyx is a polysaccharide structure that protrudes from the body of a cell. It is primarily conformed of glycoproteins and proteoglycans, which provide communication, electrostatic charge, ionic buffering, permeability, and mechanosensation-mechanotransduction capabilities to cells. In blood vessels, the endothelial glycocalyx that projects into the vascular lumen separates the vascular wall from the circulating blood. Such a physical location allows a number of its components, including sialic acid, glypican-1, heparan sulfate, and hyaluronan, to participate in the mechanosensation-mechanotransduction of blood flow-dependent shear stress, which results in the synthesis of nitric oxide and flow-mediated vasodilation. The endothelial glycocalyx also participates in the regulation of vascular permeability and the modulation of inflammatory responses, including the processes of leukocyte rolling and extravasation. Its structural architecture and negative charge work to prevent macromolecules greater than approximately 70 kDa and cationic molecules from binding and flowing out of the vasculature. This also prevents the extravasation of pathogens such as bacteria and virus, as well as that of tumor cells. Due to its constant exposure to shear and circulating enzymes such as neuraminidase, heparanase, hyaluronidase, and matrix metalloproteinases, the endothelial glycocalyx is in a continuous process of degradation and renovation. A balance favoring degradation is associated with a variety of pathologies including atherosclerosis, hypertension, vascular aging, metastatic cancer, and diabetic vasculopathies. Consequently, ongoing research efforts are focused on deciphering the mechanisms that promote glycocalyx degradation or limit its syntheses, as well as on therapeutic approaches to improve glycocalyx integrity with the goal of reducing vascular disease. © 2022 American Physiological Society. Compr Physiol 12: 1-31, 2022.

Endothelial shear stress signal transduction and atherogenesis: From mechanisms to therapeutics

Atherosclerotic vascular disease and its complications are among the top causes of mortality worldwide. In the vascular lumen, atherosclerotic plaques are not randomly distributed. Instead, they are preferentially localized at the curvature and bifurcations along the arterial tree, where shear stress is low or disturbed. Numerous studies demonstrate that endothelial cell phenotypic change (e.g., inflammation, oxidative stress, endoplasmic reticulum stress, apoptosis, autophagy, endothelial-mesenchymal transition, endothelial permeability, epigenetic regulation, and endothelial metabolic adaptation) induced by oscillatory shear force play a fundamental role in the initiation and progression of atherosclerosis. Mechano-sensors, adaptor proteins, kinases, and transcriptional factors work closely at different layers to transduce the shear stress force from the plasma membrane to the nucleus in endothelial cells, thereby controlling the expression of genes that determine cell fate and phenotype. An in-depth understanding of these mechano-sensitive signaling cascades shall provide new translational strategies for therapeutic intervention of atherosclerotic vascular disease. This review updates the recent advances in endothelial mechano-transduction and its role in the pathogenesis of atherosclerosis, and highlights the perspective of new anti-atherosclerosis therapies through targeting these mechano-regulated signaling molecules.

AFM-based nanoindentation indicates an impaired cortical stiffness in the AAV-PCSK9DY atherosclerosis mouse model

Investigating atherosclerosis and endothelial dysfunction has mainly become established in genetically modified ApoE^−/− or LDL-R^−/− mice transgenic models. A new AAV-PCSK9DY^DY mouse model with no genetic modification has now been reported as an alternative atherosclerosis model. Here, we aimed to employ this AAV-PCSK9^DY mouse model to quantify the mechanical stiffness of the endothelial surface, an accepted hallmark for endothelial dysfunction and forerunner for atherosclerosis. Ten-week-old male C57BL/6 N mice were injected with AAV-PCSK9^DY (0.5, 1 or 5 × 10¹¹ VG) or saline as controls and fed with Western diet (1.25% cholesterol) for 3 months. Total cholesterol (TC) and triglycerides (TG) were measured after 6 and 12 weeks. Aortic sections were used for atomic force microscopy (AFM) measurements or histological analysis using Oil-Red-O staining. Mechanical properties of in situ endothelial cells derived from ex vivo aorta preparations were quantified using AFM-based nanoindentation. Compared to controls, an increase in plasma TC and TG and extent of atherosclerosis was demonstrated in all groups of mice in a viral load-dependent manner. Cortical stiffness of controls was 1.305 pN/nm and increased (10%) in response to viral load (≥ 0.5 × 10¹¹ VG) and positively correlated with the aortic plaque content and plasma TC and TG. For the first time, we show changes in the mechanical properties of the endothelial surface and thus the development of endothelial dysfunction in the AAV-PCSK9^DY mouse model. Our results demonstrate that this model is highly suitable and represents a good alternative to the commonly used transgenic mouse models for studying atherosclerosis and other vascular pathologies.

Rapid shear stress-dependent ENaC membrane insertion is mediated by the endothelial glycocalyx and the mineralocorticoid receptor

The contribution of the shear stress-sensitive epithelial Na⁺ channel (ENaC) to the mechanical properties of the endothelial cell surface under (patho)physiological conditions is unclear. This issue was addressed in in vivo and in vitro models for endothelial dysfunction. Cultured human umbilical vein endothelial cells (HUVEC) were exposed to laminar (LSS) or non-laminar shear stress (NLSS). ENaC membrane insertion was quantified using Quantum-dot-based immunofluorescence staining and the mechanical properties of the cell surface were probed with the Atomic Force Microscope (AFM) in vitro and ex vivo in isolated aortae of C57BL/6 and ApoE/LDLR^-/- mice. Flow- and acetylcholine-mediated vasodilation was measured in vivo using magnetic resonance imaging. Acute LSS led to a rapid mineralocorticoid receptor (MR)-dependent membrane insertion of ENaC and subsequent stiffening of the endothelial cortex caused by actin polymerization. Of note, NLSS stress further augmented the cortical stiffness of the cells. These effects strongly depend on the presence of the endothelial glycocalyx (eGC) and could be prevented by functional inhibition of ENaC and MR in vitro endothelial cells and ex vivo endothelial cells derived from C57BL/6, but not ApoE/LDLR^-/- vessel. In vivo In C57BL/6 vessels, ENaC- and MR inhibition blunted flow- and acetylcholine-mediated vasodilation, while in the dysfunctional ApoE/LDLR^-/- vessels, this effect was absent. In conclusion, under physiological conditions, endothelial ENaC, together with the glycocalyx, was identified as an important shear stress sensor and mediator of endothelium-dependent vasodilation. In contrast, in pathophysiological conditions, ENaC-mediated mechanotransduction and endothelium-dependent vasodilation were lost, contributing to sustained endothelial stiffening and dysfunction.

Effects of amiloride on acetylcholine-dependent arterial vasodilation evolve over time in mice on a high salt diet

The maintenance of endothelial health is required for normal vascular function and blood pressure regulation. The epithelial Na+ channel (ENaC) in endothelial cells has emerged as a new molecular player in the regulation of endothelial nitric oxide production and vascular stiffness. While ENaC expression in the kidney is negatively regulated by high [Na+ ], ENaC expression in isolated endothelial cells has been shown to increase in response to a high extracellular [Na+ ]. In culture, this increased expression leads to cellular stiffening and decreased nitric oxide release. In vivo, the effects of high salt diet on endothelial ENaC expression and activity have varied depending on the animal model utilized. Our aim in the present study was to examine the role of endothelial ENaC in mediating vasorelaxation in the C57Bl/6 mouse strain. We utilized pressure myography to test the responsiveness of thoracodorsal arteries to acetylcholine in mice with increased sodium consumption both in the presence and absence of increased aldosterone. ENaC's contribution was assessed with the use of the specific inhibitor amiloride. We found that while aldosterone had very little effect on ENaC's contribution to acetylcholine sensitivity, a high salt diet led to an amiloride-dependent shift in the acetylcholine response of vessels. However, the direction of this shift was dependent on the length of high salt diet administration. Overall, our studies reveal that ENaC's role in the endothelium may be more complicated than previously thought. The channel does not simply inhibit nitric oxide generation, but instead helps preserve a homeostatic response.

Cell and Tissue Nanomechanics: From Early Development to Carcinogenesis

Cell and tissue nanomechanics, being inspired by progress in high-resolution physical mapping, has recently burst into biomedical research, discovering not only new characteristics of normal and diseased tissues, but also unveiling previously unknown mechanisms of pathological processes. Some parallels can be drawn between early development and carcinogenesis. Early embryogenesis, up to the blastocyst stage, requires a soft microenvironment and internal mechanical signals induced by the contractility of the cortical actomyosin cytoskeleton, stimulating quick cell divisions. During further development from the blastocyst implantation to placenta formation, decidua stiffness is increased ten-fold when compared to non-pregnant endometrium. Organogenesis is mediated by mechanosignaling inspired by intercellular junction formation with the involvement of mechanotransduction from the extracellular matrix (ECM). Carcinogenesis dramatically changes the mechanical properties of cells and their microenvironment, generally reproducing the structural properties and molecular organization of embryonic tissues, but with a higher stiffness of the ECM and higher cellular softness and fluidity. These changes are associated with the complete rearrangement of the entire tissue skeleton involving the ECM, cytoskeleton, and the nuclear scaffold, all integrated with each other in a joint network. The important changes occur in the cancer stem-cell niche responsible for tumor promotion and metastatic growth. We expect that the promising concept based on the natural selection of cancer cells fixing the most invasive phenotypes and genotypes by reciprocal regulation through ECM-mediated nanomechanical feedback loop can be exploited to create new therapeutic strategies for cancer treatment.

Renal microvascular endothelial cell responses in sepsis-induced acute kidney injury

Microvascular endothelial cells in the kidney have been a neglected cell type in sepsis-induced acute kidney injury (sepsis-AKI) research; yet, they offer tremendous potential as pharmacological targets. As endothelial cells in distinct cortical microvascular segments are highly heterogeneous, this Review focuses on endothelial cells in their anatomical niche. In animal models of sepsis-AKI, reduced glomerular blood flow has been attributed to inhibition of endothelial nitric oxide synthase activation in arterioles and glomeruli, whereas decreased cortex peritubular capillary perfusion is associated with epithelial redox stress. Elevated systemic levels of vascular endothelial growth factor, reduced levels of circulating sphingosine 1-phosphate and loss of components of the glycocalyx from glomerular endothelial cells lead to increased microvascular permeability. Although coagulation disbalance occurs in all microvascular segments, the molecules involved differ between segments. Induction of the expression of adhesion molecules and leukocyte recruitment also occurs in a heterogeneous manner. Evidence of similar endothelial cell responses has been found in kidney and blood samples from patients with sepsis. Comprehensive studies are needed to investigate the relationships between segment-specific changes in the microvasculature and kidney function loss in sepsis-AKI. The application of omics technologies to kidney tissues from animals and patients will be key in identifying these relationships and in developing novel therapeutics for sepsis.

Mechanically-regulated bone repair

Fracture healing is a complex, multistep process that is highly sensitive to mechanical signaling. To optimize repair, surgeons prescribe immediate weight-bearing as-tolerated within 24 hours after surgical fixation; however, this recommendation is based on anecdotal evidence and assessment of bulk healing outcomes (e.g., callus size, bone volume, etc.). Given challenges in accurately characterizing the mechanical environment and the ever-changing properties of the regenerate, the principles governing mechanical regulation of repair, including their cell and molecular basis, are not yet well defined. However, the use of mechanobiological rodent models, and their relatively large genetic toolbox, combined with recent advances in imaging approaches and single-cell analyses is improving our understanding of the bone microenvironment in response to loading. This review describes the identification and characterization of distinct cell populations involved in bone healing and highlights the most recent findings on mechanical regulation of bone homeostasis and repair with an emphasis on osteo-angio coupling. A discussion on aging and its impact on bone mechanoresponsiveness emphasizes the need for novel mechanotherapeutics that can re-sensitize skeletal stem and progenitor cells to physical rehabilitation protocols.

Flow-dependent regulation of endothelial Tie2 by GATA3 in vivo

BACKGROUND

Reduced endothelial Tie2 expression occurs in diverse experimental models of critical illness, and experimental Tie2 suppression is sufficient to increase spontaneous vascular permeability. Looking for a common denominator among different critical illnesses that could drive the same Tie2 suppressive (thereby leak inducing) phenotype, we identified "circulatory shock" as a shared feature and postulated a flow-dependency of Tie2 gene expression in a GATA3 dependent manner. Here, we analyzed if this mechanism of flow-regulation of gene expression exists in vivo in the absence of inflammation.

RESULTS

To experimentally mimic a shock-like situation, we developed a murine model of clonidine-induced hypotension by targeting a reduced mean arterial pressure (MAP) of approximately 50% over 4 h. We found that hypotension-induced reduction of flow in the absence of confounding disease factors (i.e., inflammation, injury, among others) is sufficient to suppress GATA3 and Tie2 transcription. Conditional endothelial-specific GATA3 knockdown (B6-Gata3tm1-Jfz VE-Cadherin(PAC)-cerERT2) led to baseline Tie2 suppression inducing spontaneous vascular leak. On the contrary, the transient overexpression of GATA3 in the pulmonary endothelium (jet-PEI plasmid delivery platform) was sufficient to increase Tie2 at baseline and completely block its hypotension-induced acute drop. On the functional level, the Tie2 protection by GATA3 overexpression abrogated the development of pulmonary capillary leakage.

CONCLUSIONS

The data suggest that the GATA3-Tie2 signaling pathway might play a pivotal role in controlling vascular barrier function and that it is affected in diverse critical illnesses with shock as a consequence of a flow-regulated gene response. Targeting this novel mechanism might offer therapeutic opportunities to treat vascular leakage of diverse etiologies.

Mechanosensitive Piezo1 Channel Evoked-Mechanical Signals in Atherosclerosis

Recently, more and more works have focused and used extensive resources on atherosclerosis research, which is one of the major causes of death globally. Alongside traditional risk factors, such as hyperlipidemia, smoking, hypertension, obesity, and diabetes, mechanical forces, including shear stress, pressure and stretches exerted on endothelial cells by flow, is proved to be crucial in atherosclerosis development. Studies have recognized the mechanosensitive Piezo1 channel as a special sensor and transducer of various mechanical forces into biochemical signals, and recent studies report its role in atherosclerosis through different mechanical forces in pressure, stretching and turbulent shear stress. Based on our expertise in this field and considering the recent advancement of atherosclerosis research, we will be focusing on the function of Piezo1 and its involvement in various cellular mechanisms and consequent involvement in the development of atherosclerosis in this review. Also, we will discuss various functions of Piezo1 involvement in atherosclerosis and come up with new mechanistic insight for future research. Based on the recent findings, we suggest Piezo1 as a valid candidate for novel therapeutic innovations, in which deep exploration and translating its findings into the clinic will be a new therapeutic strategy for cardiovascular diseases, particularly atherosclerosis.

Detrimental or beneficial: Role of endothelial ENaC in vascular function

In the past, it was believed that the expression of the epithelial sodium channel (ENaC) was restricted to epithelial tissues, such as the distal nephron, airway, sweat glands, and colon, where it is critical for sodium homeostasis. Over the past two decades, this paradigm has shifted due to the finding that ENaC is also expressed in various nonepithelial tissues, notably in vascular endothelial cells. In this review, the recent findings of the expression, regulation, and function of the endothelial ENaC (EnNaC) are discussed. The expression of EnNaC subunits is reported in a variety of endothelial cell lines and vasculatures, but this is controversial across different species and vessels and is not a universal finding in all vascular beds. The expression density of EnNaC is very faint compared to ENaC in the epithelium. To date, little is known about the regulatory mechanism of EnNaC. Through it can be regulated by aldosterone, the detailed downstream signaling remains elusive. EnNaC responds to increased extracellular sodium with the feedforward activation mechanism, which is quite different from the Na⁺ self-inhibition mechanism of ENaC. Functionally, EnNaC was shown to be a determinant of cellular mechanics and vascular tone as it can sense shear stress, and its activation or insertion into plasma membrane causes endothelial stiffness and reduced nitric oxide production. However, in some blood vessels, EnNaC is essential for maintaining the integrity of endothelial barrier function. In this context, we discuss the possible reasons for the distinct role of EnNaC in vasculatures.

Function and Regulation of the Epithelial Na+ Channel ENaC

The Epithelial Na⁺ Channel, ENaC, comprised of 3 subunits (αβγ, or sometimes δβγENaC), plays a critical role in regulating salt and fluid homeostasis in the body. It regulates fluid reabsorption into the blood stream from the kidney to control blood volume and pressure, fluid absorption in the lung to control alveolar fluid clearance at birth and maintenance of normal airway surface liquid throughout life, and fluid absorption in the distal colon and other epithelial tissues. Moreover, recent studies have also revealed a role for sodium movement via ENaC in nonepithelial cells/tissues, such as endothelial cells in blood vessels and neurons. Over the past 25 years, major advances have been made in our understanding of ENaC structure, function, regulation, and role in human disease. These include the recently solved three-dimensional structure of ENaC, ENaC function in various tissues, and mutations in ENaC that cause a hereditary form of hypertension (Liddle syndrome), salt-wasting hypotension (PHA1), or polymorphism in ENaC that contributes to other diseases (such as cystic fibrosis). Moreover, great strides have been made in deciphering the regulation of ENaC by hormones (e.g., the mineralocorticoid aldosterone, glucocorticoids, vasopressin), ions (e.g., Na⁺ ), proteins (e.g., the ubiquitin-protein ligase NEDD4-2, the kinases SGK1, AKT, AMPK, WNKs & mTORC2, and proteases), and posttranslational modifications [e.g., (de)ubiquitylation, glycosylation, phosphorylation, acetylation, palmitoylation]. Characterization of ENaC structure, function, regulation, and role in human disease, including using animal models, are described in this article, with a special emphasis on recent advances in the field. © 2021 American Physiological Society. Compr Physiol 11:1-29, 2021.

Upregulation of Piezo1 (Piezo Type Mechanosensitive Ion Channel Component 1) Enhances the Intracellular Free Calcium in Pulmonary Arterial Smooth Muscle Cells From Idiopathic Pulmonary Arterial Hypertension Patients

[Figure: see text].

Placental blood flow sensing and regulation in fetal growth restriction

The mechanical force of blood flow is a fundamental determinant of vascular homeostasis. This frictional stimulation of cells, fluid shear stress (FSS), is increasingly recognised as being essential to placental development and function. Here, we focus on the role of FSS in regulating fetoplacental circulatory flow, both in normal pregnancy and that affected by fetal growth restriction (FGR).

The fetus is reliant on placental perfusion to meet its circulatory and metabolic demands. Failure of normal vascular adaptation and the mechanisms enabling responsive interaction between fetoplacental and maternal circulations can result in FGR. FSS generates vasodilatation at least partly through the release of endothelial nitric oxide, a process thought to be vital for adequate blood flow. Where FGR is caused by placental dysfunction, placental vascular anatomy is altered, alongside endothelial dysfunction and hypoxia, each impacting upon the complex balance of FSS forces.

Identifying specific mechanical sensors and the mechanisms governing how FSS force is converted into biochemical signals is a fast-paced area of research. Here, we raise awareness of Piezo1 proteins, recently discovered to be FSS-sensitive in fetoplacental endothelium, and with emerging roles in NO generation, vascular tone and angiogenesis. We discuss the emerging concept that activating mechanosensors such as Piezo1 ultimately results in the orchestrated processes of placental vascular adaptation. Piecing together the mechanisms governing endothelial responses to FSS in placental insufficiency is an important step towards developing new treatments for FGR.

Flow-mediated vasodilation through mechanosensitive G protein-coupled receptors in endothelial cells

Currently, endothelium-dependent vasodilatation involves three main mechanisms: production of nitric oxide (NO) by endothelial nitric oxide synthase (eNOS), synthesis of prostanoids by cyclooxygenase, and/or opening of calcium-sensitive potassium channels. Researchers have proposed multiple mechanosensors that may be involved in flow-mediated vasodilation (FMD), including G protein-coupled receptors (GPCRs), ion channels, and intercellular junction proteins, among others. However, GPCRs are considered the major mechanosensors that play a pivotal role in shear stress signal transduction. Among mechanosensitive GPCRs, G protein-coupled receptor 68, histamine H1 receptors, sphingosine-1-phosphate receptor 1, and bradykinin B2 receptors have been identified as endothelial sensors of flow shear stress regulating flow-mediated vasodilation. Thus, this review aims to expound on the mechanism whereby flow shear stress promotes vasodilation through the proposed mechanosensitive GPCRs in ECs.