Aortic Wall Composition Research Articles

Sex differences and role of lysyl oxidase-like 2 in angiotensin II-induced hypertension in mice.

Hypertension, a disease with known sexual dimorphism, accelerates aging-associated arterial stiffening, partly because of the activation of matrix remodeling caused by increased biomechanical load. In this study, we tested the effect of biological sex and the role of the matrix remodeling enzyme lysyl oxidase-like 2 (LOXL2) in hypertension-induced arterial stiffening. Hypertension was induced by angiotensin II (ANG II) infusion via osmotic minipumps in 12- to 14-wk-old male and female mice. Blood pressure and pulse wave velocity (PWV) were measured noninvasively. Wire myography and uniaxial tensile testing were used to test aortic vasoreactivity and mechanical properties. Aortic wall composition was examined by histology and Western blotting. Uniaxial stretch of cultured cells was used to evaluate the effect of biomechanical strain. LOXL2's catalytic function was examined using knockout and inhibition. ANG II infusion-induced hypertension in both genotypes and sexes. Wild-type (WT) males exhibited arterial stiffening in vivo and ex vivo. Aortic remodeling with increased wall thickness, intralamellar distance, higher LOXL2, and collagen I and IV content was noted in WT males. Female mice did not exhibit increased PWV despite the onset of hypertension. LOXL2 depletion improved vascular reactivity and mechanics in hypertensive males. LOXL2 depletion improved aortic mechanics but worsened hypercontractility in females. Hypertensive cyclic strain contributed to LOXL2 upregulation in the cell-derived matrix in vascular smooth muscle cells (VSMCs) but not endothelial cells. LOXL2's catalytic function facilitated VSMC alignment in response to biomechanical strain. In conclusion, in males, arterial stiffening in hypertension is driven both by VSMC response and matrix remodeling. Females are protected from PWV elevation in hypertension. LOXL2 depletion is protective in males with improved mechanical and functional aortic properties. VSMCs are the primary source of LOXL2 in the aorta, and hypertension increases LOXL2 processing and shifts to collagen I accumulation. Overall, LOXL2 depletion offers protection in young hypertensive males and females.NEW & NOTEWORTHY We examined the effect of sex on the evolution of angiotensin II (ANG II)-induced hypertension and the role of lysyl oxidase-like 2 (LOXL2), an enzyme that catalyzes matrix cross linking. While ANG II led to hypertension and worsening vascular reactivity in both sexes, aortic remodeling and stiffening occurred only in males. LOXL2 depletion improved outcomes in males but not females. Thus males and females exhibit a distinct etiology of hypertension and LOXL2 is an effective target in males.

Sex Differences and Role of Lysyl Oxidase Like 2 (LOXL2) in Angiotensin II-Induced Hypertension in Mice.

Hypertension, a disease with known sexual dimorphism, accelerates aging associated arterial stiffening, in part due to the activation of matrix remodeling caused by increased biomechanical load. In this study, we tested the effect of biological sex and the role of the matrix remodeling enzyme lysyl oxidase like 2 (LOXL2) in hypertension induced arterial stiffening. Angiotensin II (Ang II) was delivered using osmotic pumps in Loxl2+/- and WT male and female mice. Blood pressure and pulse wave velocity (PWV) were measured noninvasively to assess hypertension and aortic stiffness. Wire myography and uniaxial tensile testing were used to test aortic vasoreactivity and mechanical properties. Aortic wall composition was examined by histology and Western blotting. The effect of biomechanical strain on LOXL2 expression and secretion by vascular smooth muscle (VSMC) and endothelial cells (EC) was evaluated by uniaxial cyclic stretching of cultured cells. The role of LOXL2 catalytic function on VSMC alignment in response to mechanical loading was determined with LOXL2 inhibition and knockout. Ang II infusion induced hypertension in WT and Loxl2+/- mice of both sexes and increased PWV in WT males but not in Loxl2+/- males, WT females, or Loxl2+/- females. LOXL2 depletion protected males from Ang II mediated potentiation of vasoconstriction but worsened in females and improved aortic mechanical properties in both sexes. Histological analysis showed increased aortic wall thickness in hypertensive WT males but not females and increased intralamellar distance in both sexes, that was ameliorated in Loxl2+/- mice. Western blotting revealed increased collagen I, decreased collagen IV, and increased LOXL2 accumulation and processing in hypertensive mice. Hypertensive cyclic strain contributed to LOXL2 upregulation in the cell-derived matrix in VSMCs but not ECs. LOXL2 catalytic function facilitated VSMC alignment in response to biomechanical strain. In males, arterial stiffening in hypertension is driven both by VSMC response and matrix remodeling. Females exhibit a delayed onset of Ang II-induced hypertension with minimal ECM remodeling but with VSMC dysfunction. LOXL2 depletion ameliorates arterial stiffening and preserves functional contractility and aortic structure in male hypertensive mice. LOXL2 depletion improves aortic mechanics but worsens aortic contractility in hypertensive females. VSMCs are the primary source of LOXL2 in the aorta and hypertension increases LOXL2 processing and shifts to collagen I accumulation. Overall, LOXL2 depletion offers protection in young hypertensive males and females.

Evolving Mural Defects, Dilatation, and Biomechanical Dysfunction in Angiotensin II-Induced Thoracic Aortopathies.

Thoracic aortopathy associates with extracellular matrix remodeling and altered biomechanical properties. We sought to quantify the natural history of thoracic aortopathy in a common mouse model and to correlate measures of wall remodeling such as aortic dilatation or localized mural defects with evolving microstructural composition and biomechanical properties of the wall. We combined a high-resolution multimodality imaging approach (panoramic digital image correlation and optical coherence tomography) with histopathologic examinations and biaxial mechanical testing to correlate spatially, for the first time, macroscopic mural defects and medial degeneration within the ascending aorta with local changes in aortic wall composition and mechanical properties. Findings revealed strong correlations between local decreases in elastic energy storage and increases in circumferential material stiffness with increasing proximal aortic diameter and especially mural defect size. Mural defects tended to exhibit a pronounced biomechanical dysfunction that is driven by an altered organization of collagen and elastic fibers. While aneurysmal dilatation is often observed within particular segments of the aorta, dissection and rupture initiate as highly localized mechanical failures. We show that wall composition and material properties are compromised in regions of local mural defects, which further increases the dilatation and overall structural vulnerability of the wall. Identification of therapies focused on promoting robust collagen accumulation may protect the wall from these vulnerabilities and limit the incidence of dissection and rupture.

THE DIRECTIONAL DEPENDENCY OF ASCENDING AORTA BIOMECHANICS DISAPPEARS WITH SEVERE MEDIAL DEGENERATION

The study of biomechanics can potentially provide insight into the pathogenesis of aortic complications and improve risk assessment. Aortas are described to have anisotropy, the directional dependency of mechanical properties. We hypothesize that disease-related remodelling of the aortic wall microstructure found in ascending aortic aneurysms results in changes in anisotropy. Excised aneurysmal ascending aortic tissue was obtained at the time of surgery and control tissue was obtained at organ donation or autopsy. The tissue was subjected to biaxial tensile testing. Stress-strain relationships were collected in both the circumferential and axial directions; from this data, both the apparent modulus of elasticity and energy loss were derived. Energy loss describes normalized hysteresis between aortic loading and unloading curves. Movat pentachrome histological staining was performed on each sample and aortic wall composition was quantified colorimetrically. Energy loss was greater in the circumferential direction than in the axial direction for both aneurysmal and control aortas (p<0.0001), while the modulus of elasticity was the same in both directions. This directional dependency in energy loss became diminished in larger aortas (r2=0.15, p=0.01), especially when indexed to body surface area (r2=0.29, p=0.002), aortas with greater energy loss (r2=0.44, p<0.0001), and aortas associated with tricuspid valves (p=0.004). An inflection point was noted in disease severity separating two populations: aortas without any directional dependency in energy loss (isotropic), and those with varying degrees of anisotropy. Aortas with indexed aortic size greater than 3.5 cm/m2 or energy loss greater than 35% were uniformly isotropic. Evaluating the microstructure, aortas with elevated collagen-to-elastin ratios (greater than 2) reflecting severe medial degeneration, were also uniformly isotropic. Aneurysmal aortas with higher collagen-to-elastin ratios tended towards isotropy (r2=0.29, p=0.001). Energy loss detects the anisotropic nature of aortic tissue unlike the apparent modulus of elasticity; this is further evidence that it may be a more sensitive and physiologically relevant metric in the evaluation of aneurysmal disease. Isotropy is associated with severe medial degeneration indicating that destruction of microstructure can be captured by global biomechanics, thereby identifying energy loss isotropy as a marker of disease severity.

Changes in Aortic Wall Structure, Composition, and Stiffness With Continuous-Flow Left Ventricular Assist Devices: A Pilot Study.

The effects of nonpulsatile flow on the aorta are unknown. Our aim was to examine the structure of the aorta from patients with continuous-flow left ventricular assist devices (LVADs) and directly measure aortic wall composition and stiffness. Age-matched aortic wall samples were collected from consecutive patients with heart failure (HF) at the time of transplantation and compared with nonfailing donor hearts. An unbiased stereological approach was used to quantify aortic morphometry and composition, and biomechanical testing was performed to determine the stress-strain relationship of the vessel. Data were obtained from 4 patients without a left ventricular assist device (HF group: mean age, 58.3±8.0 years), 7 patients with a continuous-flow LVAD (HF+LVAD group: mean, 57.7±5.6 years), and 3 nonfailing donors (mean, 53.3±12.9 years). Compared with HF, the aortic walls from HF+LVAD had an increase in wall thickness, collagen, and smooth muscle content accompanied by a reduction in elastin and mucinous ground-substance content. Stress-strain curves from the aortas revealed increased vessel stiffness in HF+LVAD compared with HF and nonfailing. The physiological modulus of the aorta progressively stiffened from 74.3±5.5 kPa in the nonfailing to 134.4±35.0 kPa in the HF to 201.7±36.4kPa in the HF+LVAD groups (P<0.001). Among continuous-flow LVAD patients without aortic valve opening, there are changes in the structure and composition of the aorta as well as an increase in aortic wall stiffness compared with age-matched HF patients and nonfailing donors. Further studies examining the role of nonpulsatile blood flow on aortic function and the potential resultant systemic sequelae are needed.

Regional identification of mechanical properties in arteries

Aortic aneurysms are often characterized by localized changes in aortic wall composition and structure that result from the degradation of elastin, loss of smooth muscle cell function, and remodeling of collagen. Correlations between wall composition and local mechanical properties can potentially provide insight into conditions that render a vessel susceptible to failure. Previous work has shown that chronic angiotensin-II infusion in ApoE-/- mice often results in dissecting aneurysms (Bersi et al. 2012). The objective of this work is to establish a new protocol able to characterize in vitro the local mechanical properties of arteries on the murine model.

Estrogen deficiency and low-calcium diet increased bone loss and urinary calcium excretion but did not alter arterial stiffness in young female rats

Many epidemiological studies have reported that the severity of arterial diseases such as arterial calcification and stiffness is inversely related to bone loss, i.e., osteoporosis. However, the nature of this relationship is unclear. The purpose of the present study was to examine the influences of estrogen deficiency and/or low-calcium diet (0.1% Ca) on bone metabolism and calcium balance, as well as aortic wall composition and stiffness in young female rats. Twenty-eight 6-week-old female rats were randomized into four groups: OVX-Low calcium (OL) and OVX-Normal calcium groups (ON) were ovariectomized, and Sham-Low calcium (SL) and Sham-Normal calcium groups (SN) were sham-operated. After 12 weeks, the bone mineral density of the lumbar spine and tibial proximal metaphysis were significantly lower in ON than in SN, and also significantly lower in OL than in ON. Additionally, OL rats had significant higher (vs. SN and SL) urinary deoxypyridinoline, but not urinary calcium, excretion at 4 weeks after ovariectomy. However, at 12 weeks after ovariectomy, urinary calcium excretion was significantly higher in OL than in SL, with corresponding increases in two bone turnover markers, bone-type alkaline phosphatase and tartrate-resistant acid phosphatase. Neither estrogen deficiency nor low-calcium diet affected aortic stiffness or elastin degeneration and calcium deposition over the course of the present study, although changes of bone metabolism occurred rapidly. Taken together, these results show that bone loss and arterial stiffness did not progress simultaneously in the present experimental protocol.

Making Up and Breaking Up

The concept that blood pressure influences vessel wall composition and structure is well established. Any alterations in stretch (caused by changes in blood pressure) solicit counteracting radial and tangential forces, driving transformations in the vessel wall that aim to accommodate the new conditions and to ultimately restore basal levels of tensile stress.1–3 Thus, when tensile stress increases because of a rise in arterial pressure, smooth muscle cell hypertrophy and collagen and elastin contents augment. Inversely, when the circumferential stress falls, the wall undergoes atrophy.4,5 See page 957 Understanding how vessel wall remodeling occurs is valuable because it relies on the delicate balance between restructuralization and overcompensation. For instance, vascular accumulation of type I, III, and IV collagen in hypertensive patients6 and in animal models of hypertension7,8 effectively counteracts the distending force of blood pressure, but at length it also results in increased vascular stiffness. Clinically, vascular stiffness translates to greater pulse wave velocity, which is an independent predictor of mortality in patients with end-stage renal failure, hypertension, and diabetes, as well as in older individuals.9 In this issue of Atherosclerosis, Thrombosis and Vascular Biology , Jackson et al10 show that the opposite holds true for vessels in which axial tension is off-loaded. In rabbit carotid arteries elongated by interposition of the contralateral carotid artery, the authors found that tortuosity caused by loss of axial strain was not rectified in the months that followed the intervention. Smooth muscle cell proliferation was observed, but this was offset by equivalent cell apoptosis. Likewise, elastin and collagen accumulation was countered by matrix metalloproteinase (MMP) upregulation and activation. But on close observation of the extracellular matrix (ECM), it became apparent that the fenestrae of the internal elastic laminae were greatly enlarged, and indeed the authors found that …

Rapid accumulation of elastin and collagen in the aortas of sheep in the immediate perinatal period.

While characterizing developmental changes in aortic wall composition in sheep, we observed very rapid accumulation of elastin and collagen in the immediate perinatal period. Thoracic aortic elastin content increased by 41%, and collagen content increased by 49% in approximately 1 week, between 140 days gestation and 3 days postpartum (term = 145 days). Even larger changes were observed in the abdominal aorta. Elastin content increased by 66%, and collagen increased by 57%. The pronounced increase in wall tissue accumulation near birth preceded a marked postnatal increase in arterial pressure. We propose that this elastin and collagen accumulation is a preadaptive response in preparation for the later increase in pressure. The prenatal and postnatal events that initiate this synthesis and accumulation are not known. We also found that, in the 3 weeks after this initial rapid increase, accumulation of elastin and collagen was markedly reduced in the abdominal, but not the thoracic, aorta. This latter finding may be linked to the dramatic decrease in flow through this vessel that results from the loss of the placental circulation. Finally, we observed that relatively high smooth muscle cell replication rates in the abdominal aorta postpartum resulted in no net DNA accumulation. This finding indicates that cell turnover plays an important role in postnatal arterial growth and development.

Effect of hypotension on atherogenesis and aortic wall composition

Cynomolgus monkeys with a stable, midthoracic aortic coarctation producing at least a pressure gradient of 10 mm Hg were fed an atherogenic diet containing 2% cholesterol and 25% peanut oil for 6 months. Standard, anatomically defined segments of thoracic aorta above and below the coarctation and of abdominal aorta were then compared to corresponding segments from atherogenic diet and normal diet control animals with respect to mural cholesterol, elastin, collagen, and DNA content and percentage surface atherosclerosis. Atherosclerotic lesions occupied 74 ± 8% of the intimal surface above the coarctations and 56 ± 7% of the corresponding surface in atherogenic diet controls, but the difference was not statistically significant. Upper thoracic mural cholesterol content was the same for the two groups. Atherosclerotic lesions were strikingly less prominent below coarctations occupying only 12 ± 4% of the distal thoracic intimal surface compared to 54 ± 9% in atherogenic diet controls ( P < 0.001) and only 10 ± 3% of the abdominal intimal surface compared to 56 ± 14% in atherogenic diet controls ( P < 0.001). Mural cholesterol content below the coarctation was the same as in normal diet control animals. Distal to the coarct, DNA and collagen content were reduced, suggesting that lowered pressure resulted in increased collagen degradation and decreased cellularity. Despite the absence of persistent hypertension above the coarctation, the upper thoracic aorta contained more elastin than controls but DNA content was normal. The results indicate that even a small reduction in blood pressure below normal levels has a marked inhibiting effect on the induction of diet-induced lesions. The associated reduction in cellularity and the low level of mural cholesterol content suggest that mural lipid accumulation, cell proliferation, and lesion formation are dependent on the presence of normal or elevated blood pressure. High levels of circulating serum lipids are a necessary but insufficient condition for the development of experimental atherosclerosis.