Short-Term Angiotensin II Treatment Affects Large Artery Biomechanics and Function in the Absence of Small Artery Alterations in Mice

Increasing pulse pressure ex vivo, mimicking acute physical exercise, induces smooth muscle cell-mediated de-stiffening of murine aortic segments

The mechanisms by which physical activity affects cardiovascular function and physiology are complex and multifactorial. In the present study, cardiac output during rest or acute physical activity was simulated in isolated aortic segments of healthy C57BL/6J wild-type mice. This was performed using the Rodent Oscillatory Tension Set-up to study Arterial Compliance (ROTSAC) by applying cyclic stretch of different amplitude, duration and frequency in well-controlled and manageable experimental conditions. Our data show that vascular smooth muscle cells (VSMCs) of the aorta have the intrinsic ability to "de-stiffen" or "relax" after periods of high cyclic stretch and to "re-stiffen" slowly thereafter upon return to normal distension pressures. Thereby, certain conditions have to be fulfilled: 1) VSMC contraction and repetitive stretching (loading/unloading cycles) are a prerequisite to induce post-exercise de-stiffening; 2) one bout of high cyclic stretch is enough to induce de- and re-stiffening. Aortic de-stiffening was highly dependent on cyclic stretch amplitude and on the manner and timing of contraction with probable involvement of focal adhesion phosphorylation/activation. Results of this study may have implications for the therapeutic potential of regular and acute physical activity and its role in the prevention and/or treatment of cardiovascular disease.

Pyridoxamine treatment ameliorates large artery stiffening and cerebral artery endothelial dysfunction in old mice

Age-related increases in large artery stiffness are associated with cerebrovascular dysfunction and cognitive impairment. Pyridoxamine treatment prevents large artery stiffening with advancing age, but the effects of pyridoxamine treatment on the cerebral vasculature or cognition is unknown. The purpose of this study was to investigate the effects of pyridoxamine on blood pressure, large artery stiffness, cerebral artery function, and cognitive function in old mice. Old male C57BL/6 mice consumed either pyridoxamine (2 g/L) or vehicle control in drinking water for ∼7.5 months and were compared with young male C57BL/6 mice. From pre- to post-treatment, systolic blood pressure increased in old control mice, but was maintained in pyridoxamine treated mice. Large artery stiffness decreased in pyridoxamine-treated mice but was unaffected in control mice. Pyridoxamine-treated mice had greater cerebral artery endothelium-dependent dilation compared with old control mice, and not different from young mice. Old control mice had impaired cognitive function; however, pyridoxamine only partially preserved cognitive function in old mice. In summary, pyridoxamine treatment in old mice prevented age-related increases in blood pressure, reduced large artery stiffness, preserved cerebral artery endothelial function, and partially preserved cognitive function. Taken together, these results suggest that pyridoxamine treatment may limit vascular aging.

Disparate biomechanical properties of the aorta in non-aneurysmal and aneurysmal mice treated with angiotensin II

In vivo angiotensin II (AngII)-treatment is a widely used experimental model to induce cardiovascular disease and results in a high likelihood of abdominal aorta aneurysm (AAA) formation. This involves progressive and irreversible focal dilation of the abdominal aorta and induces adverse aortic connective tissue remodeling contributing to aortic wall stiffening through inflammation, elastin degradation, and collagen restructuring. Hence, the present study aimed to investigate how AAA formation in AngII-treated mice affects aortic function and biomechanics. To this end, C57Bl/6J mice were treated with AngII (1000 ng/[kg.min]) or PBS infusion for 28 days. Peripheral blood pressure, echocardiography, and aortic pulse wave velocity were measured in vivo. Thoracic aorta rings were studied ex vivo in organ chambers, while aortic vascular smooth muscle cell (VSMC) phenotype was investigated histologically. We confirmed peripheral hypertension, cardiac hypertrophy, aortic stiffening, and increased VSMC proliferation and migration after AngII-treatment. Abdominal aorta aneurysm formation was observed in 8/13 AngII-treated mice. Ex vivo thoracic aortic rings of both aneurysmal and non-aneurysmal AngII-treated mice showed high isobaric aortic stiffness, endothelial dysfunction, heightened α1 -adrenergic contractility, and altered VSMC contractile calcium signaling. However, aortic biomechanics were differently affected, with heightened α1 -adrenoreceptor mediated aortic stiffening in non-aneurysmal mice, whereas contraction-dependent stiffening was impaired in aneurysmal mice. In conclusion, although aneurysmal and non-aneurysmal 4-week AngII-treated mice displayed similar changes in aortic physiology, aortic biomechanics were dissimilarly affected.

Progressive aortic stiffness in aging C57Bl/6 mice displays altered contractile behaviour and extracellular matrix changes

Aortic stiffness is a hallmark of cardiovascular disease, but its pathophysiology remains incompletely understood. This study presents an in-dept characterization of aortic aging in male C57Bl/6 mice (2–24 months). Cardiovascular measurements include echocardiography, blood pressure measurement, and ex vivo organ chamber experiments. In vivo and ex vivo aortic stiffness increases with age, and precede the development of cardiac hypertrophy and peripheral blood pressure alterations. Contraction-independent stiffening (due to extracellular matrix changes) is pressure-dependent. Contraction-dependent aortic stiffening develops through heightened α₁-adrenergic contractility, aberrant voltage-gated calcium channel function, and altered vascular smooth muscle cell calcium handling. Endothelial dysfunction is limited to a modest decrease in sensitivity to acetylcholine-induced relaxation with age. Our findings demonstrate that progressive arterial stiffening in C57Bl/6 mice precedes associated cardiovascular disease. Aortic aging is due to changes in extracellular matrix and vascular smooth muscle cell signalling, and not to altered endothelial function.

A 24-month aging study in male C57Bl/6 mice reveals that aortic aging precedes cardiovascular disease and is due to changes in the extracellular matrix and vascular smooth muscle cell signaling.

Aortic Stiffness in L-NAME Treated C57Bl/6 Mice Displays a Shift From Early Endothelial Dysfunction to Late-Term Vascular Smooth Muscle Cell Dysfunction

Introduction and Aims: Endothelial dysfunction is recognized as a cardiovascular aging hallmark. Administration of nitric oxide synthase blocker N-Ω-Nitro-L-arginine methyl ester hydrochloride (L-NAME) constitutes a well-known small animal model of cardiovascular aging. Despite extensive phenotypic characterization, the exact aortic function changes in L-NAME treated mice are largely unknown. Therefore, this study presents a longitudinal characterization of the aortic reactivity and biomechanical alterations in L-NAME treated C57Bl/6 mice.

Methods and Results: Male C57Bl/6 mice were treated with L-NAME (0.5 mg/ml drinking water) for 1, 2, 4, 8, or 16 weeks. Peripheral blood pressure measurement (tail-cuff) and transthoracic echocardiograms were recorded, showing progressive hypertension after 4 weeks of treatment and progressive cardiac hypertrophy after 8–16 weeks of treatment. Aortic stiffness was measured in vivo as aortic pulse wave velocity (aPWV, ultrasound) and ex vivo as Peterson modulus (E_p). Aortic reactivity and biomechanics were investigated ex vivo in thoracic aortic rings, mounted isometrically or dynamically-stretched in organ bath set-ups. Aortic stiffening was heightened in L-NAME treated mice after all treatment durations, thereby preceding the development of hypertension and cardiac aging. L-NAME treatment doubled the rate of arterial stiffening compared to control mice, and displayed an attenuation of the elevated aortic stiffness at high distending pressure, possibly due to late-term reduction of medial collagen types I, III, and IV content. Remarkably, endothelial dysfunction, measured by acetylcholine concentration-response stimulation in precontracted aortic rings, was only observed after short-term (1–4 weeks) treatment, followed by restoration of endothelial function which coincided with increased phosphorylation of endothelial nitric oxide synthase (S¹¹⁷⁷). In the late-disease phase (8–16 weeks), vascular smooth muscle cell (VSMC) dysfunction developed, including increased contribution of voltage-dependent calcium channels (assessed by inhibition with diltiazem), basal VSMC cytoplasmic calcium loading (assessed by removal of extracellular calcium), and heightened intracellular contractile calcium handling (assessed by measurement of sarcoplasmic reticulum-mediated transient contractions).

Conclusion: Arterial stiffness precedes peripheral hypertension and cardiac hypertrophy in chronic L-NAME treated male C57Bl/6 mice. The underlying aortic disease mechanisms underwent a distinct shift from early endothelial dysfunction to late-term VSMC dysfunction, with continued disease progression.

Mouse aortic biomechanics are affected by short-term defective autophagy in vascular smooth muscle cells

The physiology of vascular smooth muscle (VSMC) cells is affected by autophagy, a catabolic cellular mechanism responsible for nutrient recycling. Autophagy-inducing compounds may reverse arterial stiffening, whereas congenital VSMC-specific autophagy deficiency promotes arterial stiffening. The elevated aortic stiffness in 3.5-month-old C57Bl/6 mice, in which the essential autophagy-related gene Atg7 was specifically deleted in the VSMCs (Atg7F/F SM22α-Cre+ mice) was mainly due to passive aortic wall remodeling. The present study investigated whether aortic stiffness was also modulated by a shorter duration of autophagy deficiency. Therefore, aortic segments of 2-month-old Atg7F/F SM22α-Cre+ mice were studied. Similarly to the older mice, autophagy deficiency in VSMCs promoted aortic stiffening by elastin degradation and elastin breaks, and increased the expression of the calcium binding protein S100A4 (+ 157%), the aortic wall thickness (+ 27%), the sensitivity of the VSMCs to depolarization and the contribution of VGCC mediated Ca2+ influx to α1 adrenergic contractions. Hence, all these phenomena occurred before the age of 2 months. When compared to autophagy deficiency in VSMCs at 3.5 months, shorter term autophagy deficiency led to higher segment diameter at 80 mmHg (+ 7% versus - 2%), normal baseline tonus (versus increased), unchanged IP3-mediated phasic contractions (versus enhanced), and enhanced endothelial cell function (versus normal). Overall, and because in vivo cardiac parameters or aortic pulse wave velocity were not affected, these observations indicate that congenital autophagy deficiency in VSMCs of Atg7F/F SM22α-Cre+ mice initiates compensatory mechanisms to maintain circulatory homeostasis.

High Pulsatile Load Decreases Arterial Stiffness: An ex vivo Study

Measuring arterial stiffness has recently gained a lot of interest because it is a strong predictor for cardiovascular events and all-cause mortality. However, assessing blood vessel stiffness is not easy and the in vivo measurements currently used provide only limited information. Ex vivo experiments allow for a more thorough investigation of (altered) arterial biomechanical properties. Such experiments can be performed either statically or dynamically, where the latter better corresponds to physiological conditions. In a dynamic setup, arterial segments oscillate between two predefined forces, mimicking the diastolic and systolic pressures from an in vivo setting. Consequently, these oscillations result in a pulsatile load (i.e., the pulse pressure). The importance of pulse pressure on the ex vivo measurement of arterial stiffness is not completely understood. Here, we demonstrate that pulsatile load modulates the overall stiffness of the aortic tissue in an ex vivo setup. More specifically, increasing pulsatile load softens the aortic tissue. Moreover, vascular smooth muscle cell (VSMC) function was affected by pulse pressure. VSMC contraction and basal tonus showed a dependence on the amplitude of the applied pulse pressure. In addition, two distinct regions of the aorta, namely the thoracic descending aorta (TDA) and the abdominal infrarenal aorta (AIA), responded differently to changes in pulse pressure. Our data indicate that pulse pressure alters ex vivo measurements of arterial stiffness and should be considered as an important variable in future experiments. More research should be conducted in order to determine which biomechanical properties are affected due to changes in pulse pressure. The elucidation of the underlying pulse pressure-sensitive properties would improve our understanding of blood vessel biomechanics and could potentially yield new therapeutic insights.

Inflammation, Nitro-Oxidative Stress, Impaired Autophagy, and Insulin Resistance as a Mechanistic Convergence Between Arterial Stiffness and Alzheimer's Disease

The average age of the world's elderly population is steadily increasing. This unprecedented rise in the aged world population will increase the prevalence of age-related disorders such as cardiovascular disease (CVD) and neurodegeneration. In recent years, there has been an increased interest in the potential interplay between CVDs and neurodegenerative syndromes, as several vascular risk factors have been associated with Alzheimer's disease (AD). Along these lines, arterial stiffness is an independent risk factor for both CVD and AD. In this review, we discuss several inflammaging-related disease mechanisms including acute tissue-specific inflammation, nitro-oxidative stress, impaired autophagy, and insulin resistance which may contribute to the proposed synergism between arterial stiffness and AD.

Large artery stiffness and brain health: insights from animal models

There are no effective treatments available to halt or reverse the progression of age-related cognitive decline and Alzheimer's disease. Thus, there is an urgent need to understand the underlying mechanisms of disease etiology and progression to identify novel therapeutic targets. Age-related changes to the vasculature, particularly increases in stiffness of the large elastic arteries, are now recognized as important contributors to brain aging. There is a growing body of evidence for an association between greater large artery stiffness and cognitive impairment among both healthy older adults and patients with Alzheimer's disease. However, studies in humans are limited to only correlative evidence, whereas animal models allow researchers to explore the causative mechanisms linking arterial stiffness to neurocognitive dysfunction and disease. Recently, several rodent models of direct modulation of large artery stiffness and the consequent effects on the brain have been reported. Common outcomes among these models have emerged, including evidence that greater large artery stiffness causes cerebrovascular dysfunction associated with increased oxidative stress and inflammatory signaling. The purpose of this mini-review is to highlight the recent findings associating large artery stiffness with deleterious brain outcomes, with a specific focus on causative evidence obtained from animal models. We will also discuss the gaps in knowledge that remain in our understanding of how large artery stiffness affects brain function and disease outcomes.

The role of endothelial autocrine NRG1/ERBB4 signaling in cardiac remodeling

Neuregulin-1 (NRG1) is a paracrine growth factor, secreted by cardiac endothelial cells (ECs) in conditions of cardiac overload/injury. The current concept is that the cardiac effects of NRG1 are mediated by activation of erythroblastic leukemia viral oncogene homolog (ERBB)4/ERBB2 receptors on cardiomyocytes. However, recent studies have shown that paracrine effects of NRG1 on fibroblasts and macrophages are equally important. Here, we hypothesize that NRG1 autocrine signaling plays a role in cardiac remodeling. We generated EC-specific Erbb4 knockout mice to eliminate endothelial autocrine ERBB4 signaling without affecting paracrine NRG1/ERBB4 signaling in the heart. We first observed no basal cardiac phenotype in these mice up to 32 wk. We next studied these mice following transverse aortic constriction (TAC), exposure to angiotensin II (ANG II), or myocardial infarction in terms of cardiac performance, myocardial hypertrophy, myocardial fibrosis, and capillary density. In general, no major differences between EC-specific Erbb4 knockout mice and control littermates were observed. However, 8 wk following TAC both myocardial hypertrophy and fibrosis were attenuated by EC-specific Erbb4 deletion, albeit these responses were normalized after 20 wk. Similarly, 4 wk after ANG II treatment, myocardial fibrosis was less pronounced compared with control littermates. These observations were supported by RNA-sequencing experiments on cultured endothelial cells showing that NRG1 controls the expression of various hypertrophic and fibrotic pathways. Overall, this study shows a role of endothelial autocrine NRG1/ERBB4 signaling in the modulation of hypertrophic and fibrotic responses during early cardiac remodeling. This study contributes to understanding the spatiotemporal heterogeneity of myocardial autocrine and paracrine responses following cardiac injury.NEW & NOTEWORTHY The role of NRG1/ERBB signaling in endothelial cells is not completely understood. Our study contributes to the understanding of spatiotemporal heterogeneity of myocardial autocrine and paracrine responses following cardiac injury and shows a role of endothelial autocrine NRG1/ERBB4 signaling in the modulation of hypertrophic and fibrotic responses during early cardiac remodeling.

Defective autophagy in vascular smooth muscle cells increases passive stiffness of the mouse aortic vessel wall

Aging and associated progressive arterial stiffening are both important predictors for the development of cardiovascular diseases. Recent evidence showed that autophagy, a catabolic cellular mechanism responsible for nutrient recycling, plays a major role in the physiology of vascular cells such as endothelial cells and vascular smooth muscle cells (VSMCs). Moreover, several autophagy inducing compounds are effective in treating arterial stiffness. Yet, a direct link between VSMC autophagy and arterial stiffness remains largely unidentified. Therefore, we investigated the effects of a VSMC-specific deletion of the essential autophagy-related gene Atg7 in young mice (3.5 months) (Atg7^F/F SM22α-Cre⁺ mice) on the biomechanical properties of the aorta, using an in-house developed Rodent Oscillatory Tension Set-up to study Arterial Compliance (ROTSAC). Aortic segments of Atg7^F/F SM22α-Cre⁺ mice displayed attenuated compliance and higher arterial stiffness, which was more evident at higher distention pressures. Passive aortic wall remodeling, rather than differences in VSMC tone, is responsible for these phenomena, since differences in compliance and stiffness between Atg7^+/+ SM22α-Cre⁺ and Atg7^F/F SM22α-Cre⁺ aortas were more pronounced when VSMCs were completely relaxed by the addition of exogenous nitric oxide. These observations are supported by histological data showing a 13% increase in medial wall thickness and a 14% decrease in elastin along with elevated elastin fragmentation. In addition, expression of the calcium-binding protein S100A4, which is linked to matrix remodeling, was elevated in aortic segments of Atg7^F/F SM22α-Cre⁺ mice. Overall, these findings illustrate that autophagy exerts a crucial role in defining arterial wall compliance.

Ex vivo aortic stiffness in mice with different eNOS activity

Endothelial function and NO bioavailability are important determinants of aortic biomechanics and function. With a new technique we investigated the ex vivo aortic segment biomechanics of different mouse models with altered NO signaling. Our experiments clearly show that chronic distortion of NO signaling triggered several compensatory mechanisms that reflect the organism’s attempt to maintain optimal central hemodynamics.

Animal Models of Hypertension: A Scientific Statement From the American Heart Association

Hypertension is the most common chronic disease in the world, yet the precise cause of elevated blood pressure often cannot be determined. Animal models have been useful for unraveling the pathogenesis of hypertension and for testing novel therapeutic strategies. The utility of animal models for improving the understanding of the pathogenesis, prevention, and treatment of hypertension and its comorbidities depends on their validity for representing human forms of hypertension, including responses to therapy, and on the quality of studies in those models (such as reproducibility and experimental design). Important unmet needs in this field include the development of models that mimic the discrete hypertensive syndromes that now populate the clinic, resolution of ongoing controversies in the pathogenesis of hypertension, and the development of new avenues for preventing and treating hypertension and its complications. Animal models may indeed be useful for addressing these unmet needs.

Epigenetic regulation of intercellular communication in the heart

The myocardium is a highly structured tissue consisting of different cell types including cardiomyocytes, endothelial cells, fibroblasts, smooth muscle cells, inflammatory cells, and stem cells. Microvascular endothelial cells are the most abundant cell type in the myocardium and play crucial roles during cardiac development, in normal adult myocardium, and during myocardial diseases such as heart failure. In the last decade, epigenetic changes have been described regulating cellular function in almost every cell type in the organism. Here, we review recent evidence on different epigenetic changes that regulate intercellular communication in normal myocardium and during myocardial diseases, including cardiac remodeling. Epigenetic changes influence many intercellular communication signaling systems, including the nitric oxide, angiotensin, and endothelin signaling systems. In this review, we go beyond discussing classic endothelial function (for instance nitric oxide secretion) and will discuss epigenetic regulation of intercellular communication.

Vascular smooth muscle cell contraction and relaxation in the isolated aorta: a critical regulator of large artery compliance

Over the past few decades, isometric contraction studies of isolated thoracic aorta segments have significantly contributed to our overall understanding of the active, contractile properties of aortic vascular smooth muscle cells (VSMCs) and their cross-talk with endothelial cells. However, the physiological role of VSMC contraction or relaxation in the healthy aorta and its contribution to the pulse-smoothening capacity of the aorta is currently unclear. Therefore, we investigated the acute effects of VSMC contraction and relaxation on the isobaric biomechanical properties of healthy mouse aorta. An in-house developed set-up was used to measure isobaric stiffness parameters of periodically stretched (10 Hz) aortic segments at an extended pressure range, while pharmacologically modulating VSMC tone and endothelial cell function. We found that the effects of α1-adrenergic stimulation with phenylephrine on the pressure-stiffness relationship varied in sensitivity, magnitude and direction, with the basal, unstimulated NO production by the endothelium playing a pivotal role. We also investigated how arterial disease affected this system by using the angiotensin-II-treated mouse. Our results show that isobaric stiffness was increased and that the aortic segments demonstrated a reduced capacity for modulating the pressure-stiffness relationship. This suggests that not only increased isobaric stiffness at normal pressure, but also a reduced capacity of the VSMCs to limit the pressure-associated increase in aortic stiffness, may contribute to the pathogenesis of this mouse model. Overall, this study provides more insight in how aortic VSMC tone affects the pressure-dependency of aortic biomechanics at different physiological and pathological conditions.