Integrins inhibit angiotensin II-induced contraction in rat aortic rings

Integrins play a role in stress relaxation of perivascular adipose tissue

Perivascular adipose tissue (PVAT) is known for being anti-contractile in healthy tissues. We discovered a new function of PVAT, the ability to stress relax and maintain a tone in response to a stretch. This is of note because stress relaxation has been attributed to smooth muscle, of which PVAT has none that is organized in a functional layer. We test the hypothesis the interactions of integrins with collagen play a role in stress relaxation. Our model is the thoracic aorta of the male Dahl SS rat. The PVAT and aorta were physically separated for most assays. Results from single nuclei RNA sequencing (snRNAseq) experiments, histochemistry and isometric contractility were also used. Masson Trichrome staining made evident the expression of collagen in PVAT. From snRNA seq experiments of the PVAT, mRNA for multiple collagen and integrin isoforms were detected: the α1 and β1 integrin were most highly expressed. Pharmacological inhibition of integrin/collagen interaction was effected by the specific α1β1 distintegrin obtustatin or general integrin inhibitor RGD peptide. RGD peptide but not obtustatin increased the stress relaxation. Cell-cell communication inference identified integrins αv and α5, two major RGD motif containing isoforms, as potential signaling partners of collagens. Collectively, these findings validate that stress relaxation can occur in a non-smooth muscle tissue, doing so in part through integrin-collagen interactions that may not include α1β1 heterodimers. The importance of this lies in considering PVAT as a vascular layer that possesses mechanical functions.

Loss of smooth muscle α-actin effects on mechanosensing and cell-matrix adhesions

Mutations in ACTA2 , encoding smooth muscle α-actin, are a frequent cause of heritable thoracic aortic aneurysm and dissections. These mutations are associated with impaired vascular smooth muscle cell function, which leads to decreased ability of the cell to sense matrix-mediated mechanical stimuli. This study investigates how loss of smooth muscle α-actin affects cytoskeletal tension development and cell adhesion using smooth muscle cells explanted from aorta of mice lacking smooth muscle α-actin. We tested the hypothesis that reduced vascular smooth muscle contractility due to a loss of smooth muscle α-actin decreases cellular mechanosensing by dysregulating cell adhesion to the matrix. Assessment of functional mechanical properties of the aorta by stress relaxation measurements in thoracic aortic rings suggested two functional regimes for Acta2 −/− mice. Lower stress relaxation was recorded in aortic rings from Acta2 −/− mice at tensions below 10 mN compared with wild type, likely driven by cytoskeletal-dependent contractility. However, no differences were recorded between the two groups above the 10 mN threshold, since at higher tension the matrix-dependent contractility may be predominant. In addition, our results showed that at any given level of stretch, transmural pressure is lower in aortic rings from Acta2 −/− mice than wild type mice. In addition, a three-dimensional collagen matrix contractility assay showed that collagen pellets containing Acta2 −/− smooth muscle cells contracted less than the pellets containing the wild type cells. Moreover, second harmonic generation non-linear microscopy revealed that Acta2 −/− cells locally remodeled the collagen matrix fibers to a lesser extent than wild type cells. Quantification of protein fluorescence measurements in cells also showed that in absence of smooth muscle α-actin, there is a compensatory increase in smooth muscle γ-actin. Moreover, specific integrin recruitment at cell–matrix adhesions was reduced in Acta2 −/− cells. Thus, our findings suggest that Acta2 −/− cells are unable to generate external forces to remodel the matrix due to reduced contractility and interaction with the matrix.

Integrin signaling in atherosclerosis

Atherosclerosis, a chronic lipid-driven inflammatory disease affecting large arteries, represents the primary cause of cardiovascular disease in the world. The local remodeling of the vessel intima during atherosclerosis involves the modulation of vascular cell phenotype, alteration of cell migration and proliferation, and propagation of local extracellular matrix remodeling. All of these responses represent targets of the integrin family of cell adhesion receptors. As such, alterations in integrin signaling affect multiple aspects of atherosclerosis, from the earliest induction of inflammation to the development of advanced fibrotic plaques. Integrin signaling has been shown to regulate endothelial phenotype, facilitate leukocyte homing, affect leukocyte function, and drive smooth muscle fibroproliferative remodeling. In addition, integrin signaling in platelets contributes to the thrombotic complications that typically drive the clinical manifestation of cardiovascular disease. In this review, we examine the current literature on integrin regulation of atherosclerotic plaque development and the suitability of integrins as potential therapeutic targets to limit cardiovascular disease and its complications.

A PLCγ1-dependent, force-sensitive signaling network in the myogenic constriction of cerebral arteries

Maintaining constant blood flow in the face of fluctuations in blood pressure is a critical autoregulatory feature of cerebral arteries. An increase in pressure within the artery lumen causes the vessel to constrict through depolarization and contraction of the encircling smooth muscle cells. This pressure-sensing mechanism involves activation of two types of transient receptor potential (TRP) channels: TRPC6 and TRPM4. We provide evidence that the activation of the γ1 isoform of phospholipase C (PLCγ1) is critical for pressure sensing in cerebral arteries. Inositol 1,4,5-trisphosphate (IP3), generated by PLCγ1 in response to pressure, sensitized IP3 receptors (IP3Rs) to Ca(2+) influx mediated by the mechanosensitive TRPC6 channel, synergistically increasing IP3R-mediated Ca(2+) release to activate TRPM4 currents, leading to smooth muscle depolarization and constriction of isolated cerebral arteries. Proximity ligation assays demonstrated colocalization of PLCγ1 and TRPC6 with TRPM4, suggesting the presence of a force-sensitive, local signaling network comprising PLCγ1, TRPC6, TRPM4, and IP3Rs. Src tyrosine kinase activity was necessary for stretch-induced TRPM4 activation and myogenic constriction, consistent with the ability of Src to activate PLCγ isoforms. We conclude that contraction of cerebral artery smooth muscle cells requires the integration of pressure-sensing signaling pathways and their convergence on IP3Rs, which mediate localized Ca(2+)-dependent depolarization through the activation of TRPM4.

The roles of integrins in mediating the effects of mechanical force and growth factors on blood vessels in hypertension

Hypertension is characterized by a sustained increase in vasoconstriction and attenuated vasodilation in the face of elevated mechanical stress in the blood vessel wall. To adapt to the increased stress, the vascular smooth muscle cell and its surrounding environment undergo structural and functional changes known as vascular remodeling. Multiple mechanisms underlie the remodeling process, including increased expression of humoral factors and their receptors as well as adhesion molecules and their receptors, all of which appear to collaborate and interact in the response to pressure elevation. In this review, we focus on the interactions between integrin signaling pathways and the activation of growth factor receptors in the response to the increased mechanical stress experienced by blood vessels in hypertension.

Key role of α1β1-integrin in the activation of PI3-kinase-Akt by flow (shear stress) in resistance arteries

Resistance arteries are the site of the earliest manifestations of many cardiovascular and metabolic diseases. Flow (shear stress) is the main physiological stimulus for the endothelium through the activation of vasodilatory pathways generating flow-mediated dilation (FMD). The role of FMD in local blood flow control and angiogenesis is well established, and alterations in FMD are early markers of cardiovascular disorders. α1-Integrin, which has a role in angiogenesis, could be involved in FMD. FMD was studied in mesenteric resistance arteries (MRA) isolated in arteriographs. The role of α1-integrins in FMD was tested with selective antibodies and mice lacking the gene encoding for α1-integrins. Both anti-α1blocking antibodies and genetic deficiency in α1-integrin in mice (α1−/−) inhibited FMD without affecting receptor-mediated (acetylcholine) endothelium-dependent dilation or endothelium-independent dilation (sodium nitroprusside). Similarly, vasoconstrictor tone (myogenic tone and phenylephrine-induced contraction) was not affected. In MRA phosphorylated Akt and phosphatidylinositol 3-kinase (PI3-kinase) were significantly lower in α1−/−mice than in α1+/+mice, although total Akt and endothelial nitric oxide synthase (eNOS) were not affected. Pharmacological blockade of PI3-kinase-Akt pathway with LY-294002 inhibited FMD. This inhibitory effect of LY-294002 was significantly lower in α1−/−mice than in α1+/+mice. Thus α1-integrin has a key role in flow (shear stress)-dependent vasodilation in resistance arteries by transmitting the signal to eNOS through activation of PI3-kinase and Akt. Because of the central role of flow (shear stress) activation of the endothelium in vascular disorders, this finding opens new perspectives in the pathophysiology of the microcirculation and provides new therapeutic targets.

alphavbeta3- and alpha5beta1-integrin blockade inhibits myogenic constriction of skeletal muscle resistance arterioles

In isolated resistance arterioles with spontaneous tone, ligation of alpha4beta1- and alpha5beta1-integrins induces vasoconstriction whereas ligation of alphavbeta3-integrin induces vasodilation. However, whether integrins directly participate in myogenic constriction to pressure elevation is not known. To answer this question, isolated rat skeletal muscle arterioles were exposed to step increments in pressure in the absence or presence of peptides and function-blocking antibodies known to bind alpha4beta1-, alpha5beta1-, or alphavbeta3-integrins while vessel diameter was continually monitored. Myogenic constriction, as assessed by the ability of isolated arterioles to reduce their diameter in response to two consecutive increments in intraluminal pressure (90-110 and 110-130 cmH2O), was not affected by treatment with any of the control peptides (RAD, LEV), a control antibody (anti-rat major histocompatibility complex), an alpha4beta1-integrin-binding peptide (LDV), or an anti-alpha4-integrin antibody. In contrast, alpha5beta1-integrin blockade with either anti-alpha5- or anti-beta1-integrin antibody caused a significant inhibition of myogenic constriction. Also, both RGD peptide and anti-beta3-integrin antibody inhibited myogenic constriction. These results indicate that alpha5beta1- and alphavbeta3-integrins are necessary for myogenic constriction and further suggest that integrins are part of the mechanosensory apparatus responsible for the ability of vascular smooth muscle cells to detect and/or respond to changes in intraluminal pressure.

Interaction of ACE2 and integrin beta1 in failing human heart

ACE2 purified from failing human heart was found to form a complex with integrin beta1 by immunoprecipitation, Western blotting, activity assay, and ESI tandem mass spectroscopy. The ACE2/integrin complex showed a Km of 6.8 microM and a Vmax of 2.13 pmol/min/microl purified enzyme. Activity was optimal at pH 7.5 with Ang II substrate.

Extracellular matrix controls myosin light chain phosphorylation and cell contractility through modulation of cell shape and cytoskeletal prestress

The mechanism by which vascular smooth muscle (VSM) cells modulate their contractility in response to structural cues from extracellular matrix remains poorly understood. When pulmonary VSM cells were cultured on increasing densities of immobilized fibronectin (FN), cell spreading, myosin light chain (MLC) phosphorylation, cytoskeletal prestress (isometric tension in the cell before vasoagonist stimulation), and the active contractile response to the vasoconstrictor endothelin-1 all increased in parallel. In contrast, MLC phosphorylation did not increase when suspended cells were allowed to bind FN-coated microbeads (4.5-microm diameter) or cultured on micrometer-sized (30 x 30 microm) FN islands surrounded by nonadhesive regions that support integrin binding but prevent cell spreading. Cell spreading and MLC phosphorylation also both decreased in parallel when the mechanical compliance of flexible FN substrates was raised. MLC phosphorylation was inhibited independently of cell shape when cytoskeletal prestress was dissipated using a myosin ATPase inhibitor in fully spread cells, whereas it increased to maximal levels when microtubules were disrupted using nocodazole in cells adherent to FN but not in suspended cells. These data demonstrate that changes in cell-extracellular matrix (ECM) interactions modulate smooth muscle cell contractility at the level of biochemical signal transduction and suggest that the mechanism underlying this regulation may involve physical interplay between ECM and the cytoskeleton, such that cell spreading and generation of cytoskeletal tension feed back to promote MLC phosphorylation and further increase tension generation.

Fibronectin, laminin, and collagen IV as modulators of cell behavior during adrenal gland development in the human fetus

The specific development of the human fetal adrenal gland requires cell proliferation, migration, apoptosis, and zone-specific steroidogenic activity. The present work was designed to determine the physiological significance of the previously identified spatial distribution of extracellular matrix components in the fetal gland. Primary cultures of human fetal adrenal cells grown on collagen IV, laminin, or fibronectin revealed that cell morphology was affected by environmental cues. Matrices also modulated the profile of steroid secretion by the fetal cells. Collagen IV favored cortisol secretion after ACTH or angiotensin II stimulation and increased dehydroepiandrosterone production when the AT(2) receptor of angiotensin II was specifically stimulated. These effects were correlated by changes in the mRNA levels of 3beta-hydroxysteroid dehydrogenase and cytochrome P450C17. In contrast, fibronectin and laminin decreased cell responsiveness to ACTH in terms of cortisol secretion, but enhanced ACTH-stimulated androgen secretion. Finally, extracellular matrices were able to orchestrate cell behavior. Collagen IV and laminin enhanced cell proliferation, and fibronectin increased cell death. This study is the first to demonstrate that the nature of extracellular matrix coordinates specific steroidogenic pathways and cell turnover in the developing human fetal adrenal gland.

Integrin-binding peptides containing RGD produce coronary arteriolar dilation via cyclooxygenase activation

Integrin binding by Arg-Gly-Asp (RGD)-containing peptides has been shown to alter vascular tone in a variety of blood vessels and has been implicated as a mechanism of vasoregulation during tissue injury. However, the effect of these peptides in the coronary circulation has not been examined. Thus the purpose of our study was to test the hypothesis that integrins act as receptors linked to the regulation of coronary vasomotor function. In particular, the ability of RGD-containing peptides to influence vascular tone by interacting with the alpha(v)beta(3)- and alpha(5)beta(1)-integrins was studied in isolated pig coronary arterioles. All vessels developed basal tone and dilated in a concentration-dependent manner to soluble peptides cyclic GPenGRGDSPCA (cyclic RGD), an alpha(v)beta(3)-cyclic-binding peptide (XJ735), DMP7677, an alpha(5)beta(1)-binding peptide, and to protease-generated (neutrophil elastase) fragments of denatured collagen type I (a major RGD-containing extracellular matrix protein). The vasodilations to cyclic RGD, XJ735, and collagen fragments were almost completely blocked by endothelial removal or by the cyclooxygenase inhibitor indomethacin. In contrast, after endothelial removal and incubation with indomethacin, coronary arterioles showed concentration-dependent constriction to the alpha(5)beta(1)-integrin ligand DMP7677 but not to cyclic RGD or XJ735. Collectively, our results indicate that activation of endothelial alpha(v)beta(3)- and alpha(5)beta(1)-integrins mediates coronary arteriolar dilation via the endothelial production of cyclooxygenase-derived prostaglandins. These data support a role for integrins in the regulation of coronary vascular tone that may be particularly important during myocardial injury.

Integrins and mechanotransduction of the vascular myogenic response

This review summarizes what is currently known about the role of integrins in the vascular myogenic response. The myogenic response is the rapid and maintained constriction of a blood vessel in response to pressure elevation. A role for integrins in this process has been suggested because these molecules form an important mechanical link between the extracellular matrix and the vascular smooth muscle cytoskeleton. We briefly summarize evidence for a general role of integrins in mechanotransduction. We then describe the integrin subunit combinations known to exist in smooth muscle and the vascular wall matrix proteins that may interact with these integrins. We then discuss the effects of integrin-specific peptides and antibodies on vascular tone and on calcium entry mechanisms in vascular smooth muscle. Because integrin function is linked to the cytoskeleton, we discuss evidence for the role of the cytoskeleton in determining myogenic responsiveness. Finally, we analyze evidence that integrin-linked signaling pathways, such as those involving protein tyrosine phosphorylation cascades and mitogen-activated protein kinases, are required for myogenic tone.