Smooth muscle biomechanics and plasticity: relevance for vascular calibre and remodelling

Correlation of cerebral microvascular circulation with vital signs in cerebral compression and the validity of three concepts: vasodilation, autoregulation, and terminal rise in arterial pressure

Background

Vasodilation, autoregulation, and rising arterial pressure are three common concepts in cerebral compression, believed to improve cerebral blood flow to maintain the brain's nutrition. However, these concepts are unclear, unproven, and based on assumptions. This study aimed to correlate cerebral circulation with alterations of vital signs and to evaluate the above concepts based on physics and hemodynamics.

Methods

Without new animal experiments, a large amount of data: recording of vital signs, long movies of cerebral circulation, and numerous photos of histological examination and microvessels obstruction in cerebral compression in cats was studied, and only partial and preliminary results were reported in 1970. The experiments were supported by an NIH grant for head injury, done before the 1985 Institutional Animal Care and Use Committee requirement. The advent of digital technology facilitated digitizing and stepwise correlating them and evaluating the validity of the above concepts.

Results

As cerebral compression increased intracranial pressure (ICP), veins dilated, not arteries, and arterial microvessels obstructed, diminished, and stopped cerebral circulation. Simultaneously, vital signs deteriorated, and pupils became fixed and dilated. There was no evidence for what is believed as autoregulation.

Conclusion

In cerebral compression, rising ICP obstructs cerebral arterial microvessels while simultaneously deteriorating vital signs. There is no evidence for dilatation of the arteries; only veins dilate, best-called venodilation. There is no evidence of autoregulation; what occurs is a cerebral compartmental syndrome. The terminal rise of arterial pressure is the hemodynamic result of cerebral circulation cessation, overloading the aorta. None of the concepts benefit the brain's nutrition.

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.

Microvascular reactivity using laser Doppler measurement in type 2 diabetes with subclinical atherosclerosis

Microangiopathy should be noted in diabetes with subclinical vascular diseases. Little is known about whether various surrogate markers of systemic arterial trees exacerbate simultaneously in preclinical atherosclerosis. To clarify the association of skin microvascular reactivity with arterial stiffness is essential to elucidating early atherosclerotic changes. The post-occlusive reactive hyperemia of skin microcirculation was evaluated in 27 control and 65 type 2 diabetic subjects, including 31 microalbuminuria (MAU) and 34 normoalbuminuria (NAU) patients. The laser Doppler skin perfusion signals were transformed into three frequency intervals for the investigation of endothelial, neurogenic, and myogenic effects on basal and reactive flow motion changes. The analysis of spectral intensity and distribution provided insight into potential significance of microvascular regulation in subclinical atherosclerotic diseases. Systemic arterial stiffness was studied by the brachial ankle pulse wave velocity (baPWV). Following occlusive ischemia, the percent change of endothelial flow motion was lower in MAU than in NAU and control groups. The MAU group revealed a relative increase in myogenic activity and a decrease in endothelial activity in normalized spectra. The baPWV showed more significant associations with reactive endothelial change (r = - 0.48, P < 0.01) and normalized myogenic value (r = - 0.37, P < 0.05) than diabetes duration and HbA1c. By multivariate regression analysis, only endothelial vasomotor changes independently contributed to the decreased baPWV (OR 3.47, 95% CI 1.63-7.42, P < 0.05). Impaired microcirculatory control is associated with increased arterial stiffness in preclinical atherosclerosis. To identify the early manifestations is necessary for at-risk patients to prevent from further vascular damage.

Construction of spider silk protein small-caliber tissue engineering vascular grafts based on dynamic culture and its performance evaluation

Tissue engineering is an alternative method for preparing small-caliber (<6 mm) vascular grafts. Dynamic mechanical conditioning is being researched as a method to improve mechanical properties of tissue engineered blood vessels. This method attempts to induce unique reaction in implanted cells that regenerate the matrix around them, thereby improving the overall mechanical stability of the grafts. In this study, we used a bioreactor to seed endothelial cells and smooth muscle cells into the inner and outer layers of the electrospun spider silk protein scaffold respectively to construct vascular grafts. The cell proliferation, mechanical properties, blood compatibility and other indicators of the vascular grafts were characterized in vitro. Furthermore, the vascular grafts were implanted in Sprague Dawley rats, and the vascular grafts' patency, extracellular matrix formation, and inflammatory response were evaluated in vivo. We aimed to construct spider silk protein vascular grafts with the potential for in vivo implantation by using a pulsating flow bioreactor. The results showed that, when compared with the static culture condition, the dynamic culture condition improved cell proliferation on vascular scaffolds and enhanced mechanical function of vascular scaffolds. In vivo experiments also showed that the dynamic culture of vascular grafts was more beneficial for the extracellular matrix deposition and anti-thrombogenesis, as well as reducing the inflammatory response of vascular grafts. In conclusion, dynamic mechanical conditioning aid in the resolution of challenges impeding the application of electrospun scaffolds and have the potential to construct small-caliber blood vessels with regenerative function for cardiovascular tissue repair.

Association of concomitant headache with hypoperfusion in ischemic stroke: A multimodal CT-based study

Previous investigations indicate that vessel wall elasticity may contribute to the occurrence of an ischemic stroke-associated headache. In this prospective study, the association between radiologic parameters of intracranial hemodynamic changes and concomitant headaches during the early phase of ischemic stroke was examined. Consecutive patients with acute ischemic stroke (AIS) from the First Affiliated Hospital of Soochow University were recruited and divided into two groups according to their questionnaire results and the International Classification of Headache Disorder 3 criteria. Baseline data, including stroke sub-types and neurological function, at admission and discharge were collected. Non-contrast computed tomography (CT), CT angiography, and CT perfusion were performed to assess intracranial hemodynamic changes. Multiple adjusted logistic models were used and possible confounding factors were included in sequential models. A total of 190 patients with AIS (93 headaches and 97 non-headache) were recruited. There were significant differences between the two groups in gender, hypertension, Alberta stroke program early CT score, relative cerebral blood flow (rCBF), and relative cerebral blood volume (rCBV). Furthermore, rCBV (adjusted odds ratio [OR] 0.160; 95% confidence interval [CI], 0.055-0.461; p < 0.001) and rCBF (adjusted OR, 0.309; 95% CI, 0.113-0.844; p < 0.05) were significantly associated with concomitant headache during the early phase of AIS in fully adjusted models. After adjusting for sociodemographic characteristics and other confounding factors, p values for the ORs were robust and intensified. Patients with lower rCBV and rCBF tended to experience the concomitant headache during the early phase of AIS. Regional hypoperfusion and microcirculation might play an important role in this separate clinical entity.

Tissue engineering in reconstructive urology-The current status and critical insights to set future directions-critical review

Advanced techniques of reconstructive urology are gradually reaching their limits in terms of their ability to restore urinary tract function and patients' quality of life. A tissue engineering-based approach to urinary tract reconstruction, utilizing cells and biomaterials, offers an opportunity to overcome current limitations. Although tissue engineering studies have been heralding the imminent introduction of this method into clinics for over a decade, tissue engineering is only marginally applied. In this review, we discuss the role of tissue engineering in reconstructive urology and try to answer the question of why such a promising technology has not proven its clinical usability so far.

ATP activation of peritubular cells drives testicular sperm transport

Spermatogenesis, the complex process of male germ cell proliferation, differentiation, and maturation, is the basis of male fertility. In the seminiferous tubules of the testes, spermatozoa are constantly generated from spermatogonial stem cells through a stereotyped sequence of mitotic and meiotic divisions. The basic physiological principles, however, that control both maturation and luminal transport of the still immotile spermatozoa within the seminiferous tubules remain poorly, if at all, defined. Here, we show that coordinated contractions of smooth muscle-like testicular peritubular cells provide the propulsive force for luminal sperm transport toward the rete testis. Using a mouse model for in vivo imaging, we describe and quantify spontaneous tubular contractions and show a causal relationship between peritubular Ca2+ waves and peristaltic transport. Moreover, we identify P2 receptor-dependent purinergic signaling pathways as physiological triggers of tubular contractions both in vitro and in vivo. When challenged with extracellular ATP, transport of luminal content inside the seminiferous tubules displays stage-dependent directionality. We thus suggest that paracrine purinergic signaling coordinates peristaltic recurrent contractions of the mouse seminiferous tubules to propel immotile spermatozoa to the rete testis.

GROWTH AND CHARACTERIZATION OF A TISSUE-ENGINEERED CONSTRUCT FROM HUMAN CORONARY ARTERY SMOOTH MUSCLE CELLS

Objective

To optimize a bioengineered «I-Wire» platform to grow tissue-engineered constructs (TCs) derived from coronary artery smooth muscle cells and characterize the mechano-elastic properties of the grown TCs.

Materials and Methods

A fibrinogen-based cell mixture was pipetted in a casting mold having two parallel titanium anchoring wires inserted in the grooves on opposite ends of the mold to support the TC. The casting mold was 3 mm in depth, 2 mm in width and 12 mm in length. To measure TC deformation, a flexible probe with a diameter of 365 mk and a length of 42 mm was utilized. The deflection of the probe tip at various tensile forces applied to the TC was recorded using an inverted microscope optical recording system. The elasticity modulus was calculated based on a stretch-stress diagram reconstructed for each TC. The mechano-elastic properties of control TCs and TCs under the influence of isoproterenol (Iso), acetylcholine (ACh), blebbistatin (Bb) and cytochalasin D (Cyto-D) were evaluated. Immunohistochemical staining of smooth muscle α-actin, desmin and the cell nucleus was implemented for the structural characterization of the TCs.

Results

The TCs formed on day 5-6 of incubation. Subsequent measurements during the following 7 days did not reveal significant changes in elasticity. Values of the elastic modulus were 7.4 ± 1.5 kPa at the first day, 7.9 ± 1.4 kPa on the third day, and 7.8 ± 1.9 kPa on the seventh day of culturing after TC formation. Changes in the mechano-elastic properties of the TCs in response to the subsequent application of Bb and Cyto-D had a two-phase pattern, indicating a possible separation of active and passive elements of the TC elasticity. The application of 1 μM of Iso led to an increase in the value of the elastic modulus from 7.9 ± 1.5 kPa to 10.2 ± 2.1 kPa (p<0.05, n = 6). ACh did not cause a significant change in elasticity.

Conclusion

The system allows quantification of the mechano-elastic properties of TCs in response to pharmacological stimuli and can be useful to model pathological changes in vascular smooth muscle cells.

Vascular smooth muscle contraction in hypertension

Hypertension is a major risk factor for many common chronic diseases, such as heart failure, myocardial infarction, stroke, vascular dementia, and chronic kidney disease. Pathophysiological mechanisms contributing to the development of hypertension include increased vascular resistance, determined in large part by reduced vascular diameter due to increased vascular contraction and arterial remodelling. These processes are regulated by complex-interacting systems such as the renin-angiotensin-aldosterone system, sympathetic nervous system, immune activation, and oxidative stress, which influence vascular smooth muscle function. Vascular smooth muscle cells are highly plastic and in pathological conditions undergo phenotypic changes from a contractile to a proliferative state. Vascular smooth muscle contraction is triggered by an increase in intracellular free calcium concentration ([Ca²⁺]_i), promoting actin–myosin cross-bridge formation. Growing evidence indicates that contraction is also regulated by calcium-independent mechanisms involving RhoA-Rho kinase, protein Kinase C and mitogen-activated protein kinase signalling, reactive oxygen species, and reorganization of the actin cytoskeleton. Activation of immune/inflammatory pathways and non-coding RNAs are also emerging as important regulators of vascular function. Vascular smooth muscle cell [Ca²⁺]_i not only determines the contractile state but also influences activity of many calcium-dependent transcription factors and proteins thereby impacting the cellular phenotype and function. Perturbations in vascular smooth muscle cell signalling and altered function influence vascular reactivity and tone, important determinants of vascular resistance and blood pressure. Here, we discuss mechanisms regulating vascular reactivity and contraction in physiological and pathophysiological conditions and highlight some new advances in the field, focusing specifically on hypertension.

Cholesterol Efflux: Does It Contribute to Aortic Stiffening?

Aortic stiffness during cardiac contraction is defined by the rigidity of the aorta and the elastic resistance to deformation. Recent studies suggest that aortic stiffness may be associated with changes in cholesterol efflux in endothelial cells. This alteration in cholesterol efflux may directly affect endothelial function, extracellular matrix composition, and vascular smooth muscle cell function and behavior. These pathological changes favor an aortic stiffness phenotype. Among all of the proteins participating in the cholesterol efflux process, ATP binding cassette transporter A1 (ABCA1) appears to be the main contributor to arterial stiffness changes in terms of structural and cellular function. ABCA1 is also associated with vascular inflammation mediators implicated in aortic stiffness. The goal of this mini review is to provide a conceptual hypothesis of the recent advancements in the understanding of ABCA1 in cholesterol efflux and its role and association in the development of aortic stiffness, with a particular emphasis on the potential mechanisms and pathways involved.

Characterization of mechanical properties of lamellar structure of the aortic wall: Effect of aging

Arterial wall tissues are sensitive to their mechanical surroundings and remodel their structure and mechanical properties when subjected to mechanical stimuli such as increased arterial pressure. Such remodeling is evident in hypertension and aging. Aging is characterized by stiffening of the artery wall which is assigned to disturbed elastin function and increased collagen content. To better understand and provide new insight on microstructural changes induced by aging, the lamellar model of the aortic media was utilized to characterize and compare wall structure and mechanical behavior of the young and old human thoracic aortic samples. Such model regards arterial media as two sets of alternating concentric layers, namely sheets of elastin and interlamellar layers. Histological and biaxial tests were performed and microstructural features and stress-strain curves of media were evaluated in young and old age groups. Then using optimization algorithms and hyperelastic constitutive equations the stress-strain curves of layers were evaluated for both age groups. Results indicated slight elevation in the volume fraction of interlamellar layer among old subjects most probably due to age related collagen deposition. Aging indicated substantial stiffening of interlamellar layers accompanied by noticeable softening of elastic lamellae. The general significant stiffening of old samples were attributed to both increase of volume fraction of interlamellar layers and earlier recruitment of collagen fibers during load bearing due to functional loss of elastin within wall lamellae. Mechanical characterization of lamellar structure of wall media is beneficial in study of arterial remodeling in response to alternated mechanical environment in aging and clinical conditions through coupling of wall microstructure and mechanical behavior.

Length adaptation of smooth muscle contractile filaments in response to sustained activation

Airway and bladder smooth muscles are known to undergo length adaptation under sustained contraction. This adaptation process entails a remodelling of the intracellular actin and myosin filaments which shifts the peak of the active force-length curve towards the current length. Smooth muscles are therefore able to generate the maximum force over a wide range of lengths. In contrast, length adaptation of vascular smooth muscle has attracted very little attention and only a handful of studies have been reported. Although their results are conflicting on the existence of a length adaptation process in vascular smooth muscle, it seems that, at least, peripheral arteries and arterioles undergo such adaptation. This is of interest since peripheral vessels are responsible for pressure regulation, and a length adaptation will affect the function of the cardiovascular system. It has, e.g., been suggested that the inward remodelling of resistance vessels associated with hypertension disorders may be related to smooth muscle adaptation. In this study we develop a continuum mechanical model for vascular smooth muscle length adaptation by assuming that the muscle cells remodel the actomyosin network such that the peak of the active stress-stretch curve is shifted towards the operating point. The model is specialised to hamster cheek pouch arterioles and the simulated response to stepwise length changes under contraction. The results show that the model is able to recover the salient features of length adaptation reported in the literature.

Mechanics of Vascular Smooth Muscle

Vascular smooth muscle (VSM; see Table 1 for a list of abbreviations) is a heterogeneous biomaterial comprised of cells and extracellular matrix. By surrounding tubes of endothelial cells, VSM forms a regulated network, the vasculature, through which oxygenated blood supplies specialized organs, permitting the development of large multicellular organisms. VSM cells, the engine of the vasculature, house a set of regulated nanomotors that permit rapid stress-development, sustained stress-maintenance and vessel constriction. Viscoelastic materials within, surrounding and attached to VSM cells, comprised largely of polymeric proteins with complex mechanical characteristics, assist the engine with countering loads imposed by the heart pump, and with control of relengthening after constriction. The complexity of this smart material can be reduced by classical mechanical studies combined with circuit modeling using spring and dashpot elements. Evaluation of the mechanical characteristics of VSM requires a more complete understanding of the mechanics and regulation of its biochemical parts, and ultimately, an understanding of how these parts work together to form the machinery of the vascular tree. Current molecular studies provide detailed mechanical data about single polymeric molecules, revealing viscoelasticity and plasticity at the protein domain level, the unique biological slip-catch bond, and a regulated two-step actomyosin power stroke. At the tissue level, new insight into acutely dynamic stress-strain behavior reveals smooth muscle to exhibit adaptive plasticity. At its core, physiology aims to describe the complex interactions of molecular systems, clarifying structure-function relationships and regulation of biological machines. The intent of this review is to provide a comprehensive presentation of one biomachine, VSM.

Relation of murine thoracic aortic structural and cellular changes with aging to passive and active mechanical properties

Background

Maintenance of the structure and mechanical properties of the thoracic aorta contributes to aortic function and is dependent on the composition of the extracellular matrix and the cellular content within the aortic wall. Age‐related alterations in the aorta include changes in cellular content and composition of the extracellular matrix; however, the precise roles of these age‐related changes in altering aortic mechanical function are not well understood.

Methods and Results

Thoracic aortic rings from the descending segment were harvested from C57BL/6 mice aged 6 and 21 months. Thoracic aortic diameter and wall thickness were higher in the old mice. Cellular density was reduced in the medial layer of aortas from the old mice; concomitantly, collagen content was higher in old mice, but elastin content was similar between young and old mice. Stress relaxation, an index of compliance, was reduced in aortas from old mice and correlated with collagen fraction. Contractility of the aortic rings following potassium stimulation was reduced in old versus young mice. Furthermore, collagen gel contraction by aortic smooth muscle cells was reduced with age.

Conclusions

These results demonstrate that numerous age‐related structural changes occurred in the thoracic aorta and were related to alterations in mechanical properties. Aortic contractility decreased with age, likely because of a reduction in medial cell number in addition to a smooth muscle contractile deficit. Together, these unique findings provide evidence that the age‐related changes in structure and mechanical function coalesce to provide an aortic substrate that may be predisposed to aortopathies.

Integrative modeling of small artery structure and function uncovers critical parameters for diameter regulation

Organ perfusion is regulated by vasoactivity and structural adaptation of small arteries and arterioles. These resistance vessels are sensitive to pressure, flow and a range of vasoactive stimuli. Several strongly interacting control loops exist. As an example, the myogenic response to a change of pressure influences the endothelial shear stress, thereby altering the contribution of shear-dependent dilation to the vascular tone. In addition, acute responses change the stimulus for structural adaptation and vice versa. Such control loops are able to maintain resistance vessels in a functional and stable state, characterized by regulated wall stress, shear stress, matched active and passive biomechanics and presence of vascular reserve. In this modeling study, four adaptation processes are identified that together with biomechanical properties effectuate such integrated regulation: control of tone, smooth muscle cell length adaptation, eutrophic matrix rearrangement and trophic responses. Their combined action maintains arteries in their optimal state, ready to cope with new challenges, allowing continuous long-term vasoregulation. The exclusion of any of these processes results in a poorly regulated state and in some cases instability of vascular structure.

Derivation and maturation of synthetic and contractile vascular smooth muscle cells from human pluripotent stem cells

AIMS

Embryonic vascular smooth muscle cells (vSMCs) have a synthetic phenotype; in adults, they commit to the mature contractile phenotype. Research shows that human pluripotent stem cells (hPSCs) differentiate into vSMCs, but nobody has yet documented their maturation into the synthetic or contractile phenotypes. This study sought to control the fate decisions of hPSC derivatives to guide their maturation towards a desired phenotype.

METHODS AND RESULTS

The long-term differentiation of hPSCs, including the integration-free-induced PSC line, in high serum with platelet-derived growth factor-BB (PDGF-BB) and transforming growth factor-β1, allowed us to induce the synthetic vSMC (Syn-vSMC) phenotype with increased extracellular matrix (ECM) protein expression and reduced expression of contractile proteins. By monitoring the expression of two contractile proteins, smooth muscle myosin heavy chain (SMMHC) and elastin, we show that serum starvation and PDGF-BB deprivation caused maturation towards the contractile vSMC (Con-vSMC) phenotype. Con-vSMCs differ distinctively from Syn-vSMC derivatives in their condensed morphology, prominent filamentous arrangement of cytoskeleton proteins, production and assembly of elastin, low proliferation, numerous and active caveolae, enlarged endoplasmic reticulum, and ample stress fibres and bundles, as well as their high contractility. When transplanted subcutaneously into nude mice, the human Con-vSMCs aligned next to the host's growing functional vasculature, with occasional circumferential wrapping and vascular tube narrowing.

CONCLUSION

We control hPSC differentiation into synthetic or contractile phenotypes by using appropriate concentrations of relevant factors. Deriving Con-vSMCs from an integration-free hiPSC line may prove useful for regenerative therapy involving blood vessel differentiation and stabilization.