Actin isoform and alpha 1B-adrenoceptor gene expression in aortic and coronary smooth muscle is influenced by cyclical stretch

Pericyte-mediated vasoconstriction underlies TBI-induced hypoperfusion

OBJECTIVES

Endothelin-1 is a 21-amino acid peptide that together with specific receptors, A (ETrA) and B (ETrB) is induced following traumatic brain injury (TBI) and has been closely linked to regulation of cerebral vasospasm, oxidative stress, and hypoperfusion. Specific endothelin receptor antagonists have been shown to ameliorate early evidence of neuronal cell injury, activation of microglial cells, and hypoperfusion following TBI. The exact mechanism involved in TBI-induced hypoperfusion is still unclear; however, it is thought that endothelin-1 engagement of ETrA is primarily responsible for changes in blood flow. In this study we question the role of the microvascular pericyte in endothelin-1-mediated pathophysiology in TBI.

METHODS

Pericyte expression of endothelin-1, ETrA, and ETrB was examined in primary culture and in sham and impacted rat brain. Adult male rats were also given intracerebroventricular injections of ETrA (BQ-123) before being subjected to TBI using a closed head acceleration impact model.

RESULTS

Primary pericytes express both endothelin-1 and its receptors ETrA and ETrB. Following TBI, the number of alpha-smooth muscle actin (SMA) positive pericytes located in microvessels is significantly increased by 4 hours post-traumatic impact. Increases in pericyte expression of alpha-SMA correlated with evidence of a reduction in both arteriolar and capillary diameter. Capillary endothelin-1, ETrA, and ETrB transcript and protein was also increased. Increased endothelin-1 expression was seen by 2-4 hours post-impact. Upregulation of receptors was observed by 4-8 hours and maximum by 24 hours. ETrA antagonists decreased the number of alpha-SMA(+) pericytes as well as changes in microvascular diameter.

CONCLUSION

These results suggest that decreased vasoconstriction following TBI may be due to an endothelin-1-induced pericyte-mediated regulation of microvessel blood flow following TBI. Furthermore, results suggest that ETrA antagonists ameliorate trauma induced hypoperfusion, in part, by inhibiting endothelin-1-mediated upregulation of alpha-SMA in pericytes.

Mechanical stress-initiated signal transduction in vascular smooth muscle cells in vitro and in vivo

Increasing evidence has been demonstrated that hypertension-initiated abnormal biomechanical stress is strongly associated with cardio-/cerebrovascular diseases e.g. atherosclerosis, stroke, and heart failure, which is main cause of morbidity and mortality. How the cells in the cardiovascular system sense and transduce the extracellular physical stimuli into intracellular biochemical signals is a crucial issue for understanding the mechanisms of the disease development. Recently, collecting data derived from our and other laboratories showed that many kinds of molecules in the cells such as receptors, ion channels, caveolin, G proteins, cell cytoskeleton, kinases and transcriptional factors could serve as mechanoceptors directly or indirectly in response to mechanical stimulation implying that the activation of mechanoceptors represents a non-specific manner. The sensed signals can be further sorted and/or modulated by processing of the molecules both on the cell surface and by the network of intracellular signaling pathways resulting in a sophisticated and dynamic set of cues that enable cardiovascular cell responses. The present review will summarise the data on mechanotransduction in vascular smooth muscle cells and formulate a new hypothesis, i.e. a non-specific activation of mechanoceptors followed by a variety of signal cascade activation. The hypothesis could provide us some clues for exploring new therapeutic targets for the disturbed mechanical stress-initiated diseases such as hypertension and atherosclerosis.

In vitro models to evaluate acute and chronic injury to the heart and vascular systems

Multiple in vitro model systems are currently available to evaluate structure and function relationships in the cardiovascular system as well as the system's response to injury. As the level of molecular sophistication continues to advance, so does the level of complexity of the analysis. One of the most daunting tasks faced by researchers interested in studying cardiovascular function and injury is the selection of the system or systems best suited to answer the particular question at hand. In order to successfully apply any given model system, the researcher must recognize the advantages and limitations in the system of choice. This review provides a listing of the historical and modern techniques used to study cardiovascular function and chemically-induced toxicity. With the growing number of new pharmaceuticals discovered each year, it is imperative to use experimental model systems that allow for identification of targets that participate in or mediate adverse outcomes. Clearly, in vitro analysis cannot replace in vivo experimentation, but the methods currently available allow for a reduction in the number of animals used for experimentation and a better understanding of the complexity associated with the injury response.

Autologous vascular smooth muscle cell-based myocardial gene therapy to induce coronary collateral growth

For therapeutic angiogenesis to achieve clinical relevance, it must be effective, with minimal side effects to other end organ systems. We developed a cardiac-specific gene delivery mechanism by transfecting autologous vascular smooth muscle cells (VSMC) with VEGF and administering these cells via intracoronary injection. We evaluated the efficacy of this protocol by its ability to stimulate angiogenesis in the presence of a subthreshold stimulus for collateralization. A modified canine repetitive coronary occlusion model was utilized in these experiments with left anterior descending coronary artery occlusions for 2 min every 2 h four times per day for 21 days. An intramyocardial catheter in the perfusion territory of the left anterior descending coronary artery measured proteins in the myocardial interstitial fluid. VSMC from jugular vein explants were isolated, amplified in culture for 3 wk, and transfected with a plasmid expressing VEGF-165 and/or enhanced green fluorescent protein. Cells were injected before commencement of occlusions. VEGF levels in myocardial interstitial fluid were significantly higher in VEGF-transfected animals than in sham (repetitive occlusions without cell transplantation) and control (repetitive occlusions with enhanced green fluorescent protein-transfected cells) animals at the onset of occlusions (P < 0.05). In the VEGF group, collateral flow was increased at day 7 and remained higher than in sham and control groups thereafter. We found that intracoronary administration of VEGF-transfected autologous VSMC effectively promotes collateral development. This approach may provide a way to confine delivery of a gene to a specified organ, thus minimizing complications related to gene transfection in nontargeted organ systems.

Sinusoidal length oscillation- and receptor-mediated mRNA expression of myosin isoforms and alpha-SM actin in airway smooth muscle

We tested the hypothesis that sinusoidal length oscillation and receptor activation interactively regulate the abundance of mRNA encoding alpha-smooth muscle (alpha-SM) actin and myosin isoforms in intact bovine tracheal smooth muscle. We found that sinusoidal length oscillation significantly downregulated abundance of mRNA encoding alpha-SM actin mRNA in unstimulated tissues but not in histamine- and carbachol-activated tissues. This observation suggests antagonistic interactions between mechanical stretch and receptor-mediated signal transduction in regulating the abundance of mRNA encoding alpha-SM actin in intact airway smooth muscle. This pattern of antagonistic interaction was also observed in cholinergic receptor activation experiments. Whereas carbachol significantly upregulated myosin heavy chain SMA isoform expression in muscle strips held at slack length, carbachol did not significantly alter SMA expression in muscle strips at sinusoidal length oscillation. Carbachol also significantly upregulated GAPDH expression in bovine tracheal smooth muscle. However, unlike SMA expression, upregulation of GAPDH expression mediated by cholinergic receptor activation appeared to be insensitive to the mechanical state of airway smooth muscle. Unlike carbachol, histamine did not significantly alter the expression of GAPDH, myosin heavy chain SMA and SMB, myosin light chain LC17a and LC17b, and alpha-SM actin in bovine tracheal smooth muscle. U0126 (10 muM) completely inhibited carbachol-induced ERK1/2 MAPK phosphorylation but did not significantly affect carbachol-induced upregulation of GAPDH and SMA expression, suggesting that the ERK1/2 MAPK pathway was not the underlying mechanism. A potential implication of these findings is that periodic stretching of airways during respiratory cycles may modulate mRNA expression by receptor agonists in airway smooth muscle cells in vivo.

Quantification of stretch-induced cytoskeletal remodeling in vascular endothelial cells by image processing

BACKGROUND

Reorientation of the cell axis induced by cyclic stretching is an early response to mechanical forces in vitro. However, quantitative assay for this phenomenon has been difficult due to lack of robust methods. We hypothesized that cell orientation may be redefined by the orientation of actin fibers. We developed image processing methods to quantitate the orientation and density of actin fibers.

METHODS

A convolution filter using Sobel kernels was adapted to determine the orientation and density of actin fibers in human endothelial cells. Unidirectional stretching (10%, 0.5 Hz) was applied to induce cytoskeletal remodeling by varying the duration of stimulation (control, 0.5, 1, 2, 5, 10, and 20 h). Actin fibers were visualized by fluorescent phalloidin. The image processing method was compared with the manual method for reproducibility. Both confluent and subconfluent cells were tested to assess the efficacy of the methods.

RESULTS

Cyclic stretch-induced dense and uninterrupted actin cabling formed across the cell body and, later, the actin fibers became aligned perpendicular to the stretch direction. The variance of actin fiber orientation became smaller after 2 h of stretch (F < 0.01). The actin fiber density index, a derived parameter related to the density of actin fibers, increased as early as 30 min of stretching (P < 0.05) and decreased after 10 h of stretching. Reproducibility of our method was extremely good. Applicability of the method was not compromised by cell density.

CONCLUSIONS

Our method is reliable for quantifying cytoskeletal remodeling induced by mechanical force.

Mechanotransduction via the elastin-laminin receptor (ELR) in resistance arteries

The arterial wall is composed of dynamically interacting cellular and acellular components that are necessary for the maintenance of vessel homeostasis. Two extracellular proteins in the vessel wall, elastin and laminin, play important structural roles. We recently established a role for the elastin-laminin receptor (ELR) in mechanotransduction of stretch in cultured vascular smooth muscle (VSM) (Am. J. Physiol.: Heart Circ. Physiol. 280(3) (2001) H1354). We found stretch-mediated signaling by the ELR decreased the expression of the proto-oncogene, c-fos, and subsequent cellular proliferation. However, the role for the ELR in mediating pressure-induced changes in gene expression in intact, isolated resistance vessels is unknown and the goal of this study was to ascertain this possibility. In this study, isolated rat cerebral (approximately 180 microm) and mesenteric (approximately 280 microm) arteries were pressurized to 65 mmHg (baseline) and this pressure was held for 2 h. After this equilibration, pressures were increased to either 80 mmHg (n=6) or 140 mmHg (n=6) for 30 min and transcript levels of c-fos and the housekeeping gene, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA were assessed by reverse transcriptase-polymerase chain reaction (RT-PCR). Elevation of pressure in the cerebral arteries decreased the c-fos/GAPDH ratio by 72% in the 140 mmHg group compared to the 80 mmHg control. Importantly, the decrease in c-fos expression was blocked by ELR peptide antagonists (VGVAPG or YIGSR, 10 microM, n=6). In contrast, the decrease in c-fos expression was not observed in the mesenteric resistance arteries. In these vessels, pressure (140 mmHg) increased the c-fos/GAPDH ratio (+68% compared to normotensive control, n=6). To account for the difference between the cerebral and mesenteric vessels, histological analysis of elastin fiber content was performed. Cerebral arteries have greater amounts of loose elastin fibers (fibers outside of the organized elastin laminae) in the tunica media compared to mesenteric arteries. This may explain the opposite stretch-induced responses of c-fos expression in these vessels. Stretch-induced ELR signaling may play a prominent role in vascular adaptations to hypertension in specific organ systems. Our data further suggest that ELR activation may represent a larger component of mechanosensitive signaling in the cerebral circulation than in the mesenteric circulation.

Three-dimensional cellular deformation analysis with a two-photon magnetic manipulator workstation

The ability to apply quantifiable mechanical stresses at the microscopic scale is critical for studying cellular responses to mechanical forces. This necessitates the use of force transducers that can apply precisely controlled forces to cells while monitoring the responses noninvasively. This paper describes the development of a micromanipulation workstation integrating two-photon, three-dimensional imaging with a high-force, uniform-gradient magnetic manipulator. The uniform-gradient magnetic field applies nearly uniform forces to a large cell population, permitting statistical quantification of select molecular responses to mechanical stresses. The magnetic transducer design is capable of exerting over 200 pN of force on 4.5-microm-diameter paramagnetic particles and over 800 pN on 5.0-microm ferromagnetic particles. These forces vary within +/-10% over an area 500 x 500 microm2. The compatibility with the use of high numerical aperture (approximately 1.0) objectives is an integral part of the workstation design allowing submicron-resolution, three-dimensional, two-photon imaging. Three-dimensional analyses of cellular deformation under localized mechanical strain are reported. These measurements indicate that the response of cells to large focal stresses may contain three-dimensional global deformations and show the suitability of this workstation to further studying cellular response to mechanical stresses.

The elastin-laminin receptor functions as a mechanotransducer in vascular smooth muscle

Laminin and elastin, two major constituents of the extracellular matrix, bind to cells via the elastin-laminin receptor (ELR), a receptor distinct from integrins. Despite the ubiquitous nature of elastin and laminin in the matrix, the consequences of activation of the ELR are unknown. Because integrins are capable of mechanosensitive transduction, we hypothesized that the ELR would exert a similar function. Accordingly, we examined the effects of cyclical stretch on canine coronary smooth muscle gene expression and proliferation that are mediated by the ELR. Northern blot analyses showed a 31% decrease in serum-induced expression of c-fos when cells were stretched for 30 min on elastin, but no change in expression was observed on collagen. Serum-induced proliferation of stretched cells was markedly attenuated on elastin when compared with collagen. Both the molecular (decreased c-fos expression) and biological (decreased proliferation) responses on elastin were restored after blockade of the ELR with the elastin fragment hexapeptide (valine-glycine-valine-alanine-proline-glycine, VGVAPG). The inhibition was specific for this peptide, as another hydrophobic hexapeptide (valine-serine-leucine-serine-proline-glycine, VSLSPG) did not inhibit the responses. These results demonstrate that cyclic stretch inhibits c-fos expression and proliferation of coronary vascular smooth muscle cells grown on elastin matrixes, a mechanosensitive response that is transduced by the ELR.

Phenotype dictates the growth response of vascular smooth muscle cells to pulse pressure in vitro

The objective of this study was to determine the effect of phenotype on pulse pressure-induced signaling and growth of vascular smooth muscle cells in vitro. Using a perfused transcapillary culture system, cells were exposed to increases in pulsatile flow and hence pulse pressure and maintained for 72 h before cells were harvested. Cell proliferation was determined by cell number, DNA synthesis, and proliferating cell nuclear antigen expression. Mitogen-activated protein kinase (MAPK) levels were determined by immunoblot and kinase activity by phosphorylation of myelin basic protein. Cell phenotype was determined by immunoblot and immunocytofluorescence using antisera specific for the differentiation markers alpha-actin, myosin, calponin, osteopontin, and phospholamban. In cells that highly expressed these differentiation markers, there was a significant increase in cell growth in response to chronic increases in pulse pressure without a significant change in MAPK activity in these cells. In contrast, in cells that weakly expressed SMC differentiation markers, there was a significant decrease in cell growth concomitant with a significant decrease in MAPK signaling in these cells. We conclude that SMC phenotype dictates the growth response of SMC to mechanical force in vitro.

Eliminating arterial pulsatile strain by external banding induces medial but not neointimal atrophy and apoptosis in the rabbit

We have examined the role of vessel pulsation and wall tension on remodeling and intimal proliferation in the rabbit infrarenal abdominal aorta. A rigid perivascular polyethylene cuff was used to reduce vessel systolic diameter by 25%, producing a region of reduced circumferential strain. At 6 weeks postoperatively, reduced circumferential strain caused medial atrophy, with 45% reduction of medial area and 30% loss of medial smooth muscle cells. Apoptotic cell death was indicated by DNA fragmentation, propidium iodide staining, and cell morphology. Cuffing the aorta after balloon denudation produced medial atrophy but did not inhibit neointimal growth. At 1 week postoperatively, intimal thickness was slightly decreased in regions with reduced strain; however, intimal thickening in regions of reduced strain was not different from control segments at 3 weeks postoperatively (intimal area was 0.37 +/- 0.05 mm2 with reduced strain and 0.50 +/- 0.08 for controls, mean +/- SEM). We conclude that circumferential strain is a major factor controlling medial structure and cell number, whereas growth of the neointima after injury is not significantly affected by either reduced strain or extensive medial cell death. Vessel cuffing represents a new model of blood vessel remodeling in vivo that involves extensive smooth muscle cell apoptosis.

Altered reactivity of coronary arteries located distal to a chronic coronary occlusion

The coronary vasculature located distal to a chronic occlusion (collateral-dependent) has been shown to exhibit altered reactivity to vasoactive agonists. Thus we evaluated effects of chronic coronary artery occlusion on vasomotor responsiveness of collateral-dependent arteries isolated from a canine model of Ameroid occlusion of the left circumflex (LCX) coronary artery. We compared in vitro responses of large (approximately 1.3- to 1.4-mm-ID) and small (approximately 0.6-mm-ID) LCX arteries located distal to an occlusion with responses of similar-sized segments of the unoccluded left anterior descending (LAD) coronary artery. Alpha-adrenergic receptor-mediated contractile responses to norepinephrine (10(-9)-10(-4) M) and phenylephine (10(-9)-10(-4) M) in the presence of propranolol were markedly enhanced in large LCX arteries compared with LAD arteries (P < 0.001). Prazosin (1 microM), an alpha1-adrenergic receptor antagonist, abolished contractile responses of LCX and LAD arteries to norepinephrine. Inhibition of nitric oxide synthesis with N(omega)-nitro-L-arginine methyl ester (100 microM) enhanced norepinephrine-induced contractions of LAD arteries to a greater extent than contractions of LCX arteries. We simultaneously measured myoplasmic free Ca2+ (fura 2 fluorescence ratio) and contractile responses in LCX and LAD arteries denuded of endothelium; norepinephrine-induced increases in myoplasmic free Ca2+ and contractile tension were significantly enhanced in LCX arteries compared with LAD arteries. In addition, large and small LCX arteries exhibited impaired relaxation in response to adenosine (10(-8)-10(-3) M) compared with LAD arteries (P < 0.05). In contrast, relaxation in response to the beta-adrenergic agonist isoproterenol (10(-9)-10(-4) M) and sodium nitroprusside (10(-10)-10(-4) M) was not significantly different in LCX and LAD arteries. Thus collateral-dependent coronary arteries exhibit enhanced alpha-adrenergic vasoconstriction and impaired vasorelaxation in response to adenosine. The enhanced alpha-adrenergic contractile responsiveness involves at least two mechanisms: 1) enhanced alpha1-adrenergic reactivity of smooth muscle and 2) decreased alpha-adrenergic-induced synthesis of nitric oxide by the endothelium.

Dietary gamma-linolenic acid enhances mouse macrophage-derived prostaglandin E1 which inhibits vascular smooth muscle cell proliferation

We previously demonstrated that macrophages isolated from mice fed gamma-linolenic acid (GLA)-enriched diets reduce vascular smooth muscle cell (SMC) proliferation in a cyclooxygenase-dependent fashion and may therefore favorably modulate the atherogenic process. The present study was conducted to elucidate the mechanism(s) by which dietary GLA influences the ability of macrophages to modulate SMC growth programs. Resident peritoneal macrophages were isolated from C57BL/6 female mice fed diets containing variable GLA compositions at 10% (wt/wt), treated with various antibodies and co-cultured with cycling naive vascular SMC isolated from nonpurified diet-fed mice. Smooth muscle cell proliferation and intracellular cAMP levels were measured after co-culture. In parallel experiments, cycling naive vascular SMC isolated from nonpurified diet-fed mice were dosed with exogenous prostaglandin E1 (PGE1 ) for various periods and challenged with cycloheximide for 4 h (8-12 h after PGE1 addition), and intracellular cAMP levels were measured at various time points. Macrophages isolated from mice fed GLA-enriched dietary oils significantly reduced SMC proliferation in co-culture compared with controls (macrophages from mice fed a corn oil diet containing no GLA). Anti-PGE1 antiserum treatment (1:50 or 1:100) blocked the ability of GLA-enriched macrophages to down-regulate SMC proliferation, a response reversed by exogenous PGE1 treatment. Macrophages isolated from mice fed GLA-enriched dietary oils elevated SMC intracellular cAMP levels in a biphasic fashion. In addition, exogenous PGE1 (1 nmol/L to 10 micromol/L) exerted a similar biphasic cAMP response in SMC, and the second phase of cAMP elevation was antagonized by cycloheximide. In conclusion, dietary GLA enhances mouse macrophage-derived prostaglandin E1, which inhibits vascular SMC proliferation.

Isolation and biological properties of osteopontin from bovine milk

A procedure for the isolation of osteopontin (OPN) from bovine milk using ion-exchange and hydrophobic chromatography is described. A DEAE-Sephacel column followed by dual phenyl-Sepharose columns yielded approximately 8 mg of purified protein per liter of milk. SDS-PAGE analysis revealed that the protein migrated at M(r) 60,000. NH2-terminal sequence analysis of the first seven amino acids revealed the protein to be identical to that previously reported for bovine OPN. Also, our preparation demonstrated expected biological properties of OPN including adhesion of both endothelial and vascular smooth muscle cells to OPN in a dose- and Arg-Gly-Asp-dependent manner. Furthermore, OPN coupled to Sepharose was capable of binding the alpha v beta 3 integrin from a detergent extract of endothelial cells. Thus, our procedure yielded biologically active OPN from an abundant and natural source.

Coronary microcirculation in health and disease. Summary of an NHLBI workshop

This article summarizes a 2-day workshop on the coronary microcirculation held in Bethesda, Md, in September 1994 and sponsored by the National Heart, Lung, and Blood Institute of the National Institutes of Health. The workshop explored a variety of topics pertaining to coronary microvascular physiology and pathophysiology. The latest methodologies that are being used to investigate the coronary microvasculature, including endoscopic microscopy of the intramural coronary microvasculature and micro-x-ray computerized tomography, were discussed. The most recent advances in the regulation of the coronary microcirculation-for example, myogenic and flow-dependent responses, KATP channels, and regional heterogeneity-were reported. The workshop touched on the relation of the microcirculation to clinically important conditions and offered recommendations for future research in this important area. Comparisons are made to recent advances in the peripheral circulation and current gaps in our knowledge concerning the coronary microcirculation. In recent years, research on the coronary microcirculation has made substantial advances, in part as a result of investigations in the peripheral microcirculation but also because of the application of unique methodologies. This research is providing new ways to investigate abnormalities of myocardial perfusion, an area of inquiry that until recently has been limited to examination of coronary pressure-flow relationships.

Differentiated properties and proliferation of arterial smooth muscle cells in culture

The smooth muscle cell is the sole cell type normally found in the media of mammalian arteries. In the adult, it is a terminally differentiated cell that expresses cytoskeletal marker proteins like smooth muscle alpha-actin and smooth muscle myosin heavy chains, and contracts in response to chemical and mechanical stimuli. However, it is able to revert to a proliferative and secretory active state equivalent to that seen during vasculogenesis in the fetus, and this is a prerequisite for the involvement of the smooth muscle cell in the formation of atherosclerotic and restenotic lesions. A similar transition from a contractile to a synthetic phenotype occurs when smooth muscle cells are established in culture. Accordingly, an in vitro system has been used extensively to study the regulation of differentiated properties and proliferation of these cells. During the first few days after seeding, the cells are reorganized structurally with a loss of myofilaments and formation of a widespread endoplasmic reticulum and a prominent Golgi complex. In parallel, they lose their contractility and instead become competent to divide in response to a large variety of mitogens, including platelet-derived growth factor (PDGF) and basic fibroblast growth factor (bFGF). After entering the cell cycle, they start to produce these and other mitogens on their own, and continue to replicate in the absence of exogenous stimuli for a restricted number of generations. Furthermore, they start to secrete extracellular matrix components such as collagen, elastin, and proteoglycans. The mechanisms that control this change in morphology and function of the smooth muscle cells are still poorly understood. Adhesive proteins such as fibronectin and laminin apparently have an important role in determining the basic phenotypic state of the cells and exert their effects via integrin receptors. The proliferative and secretory activities of the cells are influenced by a multitude of growth factors, cytokines, and other molecules. Although much work remains before an integrated view of this regulatory machinery can be achieved, there is no doubt that the cell culture technique has contributed substantially to our knowledge of smooth muscle differentiation and growth. At the same time, it has been crucial in exploring the role of these cells in vascular disease and developing new therapeutic strategies to cope with major causes of human death and disability.