Subnormal shear stress-induced intimal thickening requires medial smooth muscle cell proliferation and migration

Mechanical and chemical cues synergistically promote human venous smooth muscle cell osteogenesis through integrin β1-ERK1/2 signaling: A cell model of hemodialysis fistula calcification

Arteriovenous fistula (AVF) is the vascular access of choice for renal replacement therapy. However, AVF is susceptible to calcification with a high prevalence of 40%-65% in chronic hemodialysis patients. Repeated needle puncture for hemodialysis cannulation results in intimal denudation of AVF. We hypothesized that exposure to blood shear stress in the medial layer promotes venous smooth muscle cell (SMC) osteogenesis. While previous studies of shear stress focused on arterial-type SMCs, SMCs isolated from the vein had not been investigated. This study established a venous cell model of AVF using the fluid shear device, combined with a high phosphate medium to mimic the uremic milieu. Osteogenic gene expression of venous SMCs upon mechanical and chemical cues was analyzed in addition to the activated cell signaling pathways. Our findings indicated that upon shear stress and high phosphate environment, mechanical stimulation (shear stress) had an additive effect in up-regulation of an early osteogenic marker, Runx2. We further identified that the integrin β1-ERK1/2 signaling pathway was responsible for the molecular basis of venous SMC osteogenesis upon shear stress exposure. Mitochondrial biogenesis also took part in the early stage of this venopathy pathogenesis, evident by the up-regulated mitochondrial transcription factor A and mitochondrial DNA polymerase γ in venous SMCs. In conclusion, synergistic effects of fluid shear stress and high phosphate induce venous SMC osteogenesis via the ERK1/2 pathway through activating the mechanosensing integrin β1 signaling. The present study identified a promising druggable target for reducing AVF calcification, which deserves further in vivo investigations.

The mechanobiological theory: a unifying hypothesis on the pathogenesis of moyamoya disease based on a systematic review

OBJECTIVE

Moyamoya angiopathy (MMA) affects the distal internal carotid artery and is designated as moyamoya disease (MMD) when predisposing conditions are absent, or moyamoya syndrome (MMS) when it occurs secondary to other causes. The authors aimed to investigate the reason for this anatomical site predilection of MMA. There is compelling evidence to suggest that MMA is a phenomenon that occurs due to stereotyped mechanobiological processes. Literature regarding MMD and MMS was systematically reviewed to decipher a common pattern relating to the development of MMA.

METHODS

A systematic review was conducted to understand the pathogenesis of MMA in accordance with PRISMA guidelines. PubMed MEDLINE and Scopus were searched using "moyamoya" and "pathogenesis" as common keywords and specific keywords related to six identified key factors. Additionally, a literature search was performed for MMS using "moyamoya" and "pathogenesis" combined with reported associations. A progressive search of the literature was also performed using the keywords "matrix metalloprotease," "tissue inhibitor of matrix metalloprotease," "endothelial cell," "smooth muscle cell," "cytokines," "endothelin," and "transforming growth factor" to infer the missing links in molecular pathogenesis of MMA. Studies conforming to the inclusion criteria were reviewed.

RESULTS

The literature search yielded 44 published articles on MMD by using keywords classified under the six key factors, namely arterial tortuosity, vascular angles, wall shear stress, molecular factors, blood rheology/viscosity, and blood vessel wall strength, and 477 published articles on MMS associations. Information obtained from 51 articles that matched the inclusion criteria and additional information derived from the progressive search mentioned above were used to connect the key factors to derive a network pattern of pathogenesis.

CONCLUSIONS

Based on the available literature, the authors have proposed a unifying theory for the pathogenesis of MMA. The moyamoya phenomenon appears to be the culmination of an interplay of vascular anatomy, hemodynamics, rheology, blood vessel wall strength, and a plethora of intricately linked mechanobiological molecular mediators that ultimately results in the mechanical process of occlusion of the blood vessel, stimulating angiogenesis and collateral blood supply in an attempt to perfuse the compromised brain.

Podosomes in endothelial cell--microenvironment interactions

PURPOSE OF REVIEW

The discovery of podosomes in endothelial cells during the process of angiogenesis in vivo opens a new era in vascular biology. Podosomes are actin-based microdomains located at the plasma membrane that have been extensively described but in vitro and in other cells. This review focuses on podosomes in endothelial cells and aims to rise hypotheses about when and how these structures mediate cell--microenvironment interactions.

RECENT FINDINGS

A wealth of new information regarding podosome organization and functioning has been collected in simple 2D models. Characterization of their modular architecture has unravelled their mechanics. However, context matters and podosome characteristics and functioning are shaped by the microenvironment. Although matrix degradation was seen as the typical function of podosomes, mechanosensing now appears equally prominent and involved in setting of the proteolytic machinery. Endothelial podosomes breach the basement membrane, and are thus, involved in vascular remodelling.

SUMMARY

In endothelial cells, podosomes are involved in breaking up the basement membrane, giving the cells the opportunity to invade adjacent tissues and to engage in new cell--cell interactions. Such functions are particularly relevant to vascular biology and the exploration of podosomes in in vivo settings should bring clues to many unanswered questions.

Simple model of atherosclerosis in cylindrical arteries: impact of anisotropic growth on Glagov remodeling

In 1987, Seymour Glagov observed that arteries went through a two-stage remodeling process as a result of plaque growth: first, a compensatory phase where the lumen area remains approximately constant and second, an encroachment phase where the lumen area decreases over time. In this paper, we investigate the effect of growth anisotropy on Glagov remodeling in five different cases: pure radial, pure circumferential, pure axial, isotropic and general anisotropic growth where the elements of the growth tensor are chosen to minimize the total energy. We suggest that the nature of anisotropy is inclined towards the growth direction that requires the least amount of energy. Our framework is the theory of morphoelasticity on an axisymmetric arterial domain. For each case, we explore their specific effect on the Glagov curves. For the latter two cases, we also provide the changes in collagen fiber orientation and length in the intima, media and adventitia. In addition, we compare the total energy produced by growth in radial, circumferential and axial direction and deduce that using a radially dominant anisotropic growth leads to lower strain energy than isotropic growth.

The Anastomotic Angle of Hemodialysis Arteriovenous Fistula Is Associated With Flow Disturbance at the Venous Stenosis Location on Angiography

The juxta-anastomotic stenosis of an arteriovenous fistula (AVF) is a significant clinical problem in hemodialysis patients with no effective treatment. Previous studies of AV anastomotic angles on hemodynamics and vascular wall injury were based on computational fluid dynamics (CFD) simulations using standardized AVF geometry, not the real-world patient images. The present study is the first CFD study to use angiographic images with patient-specific outcome information, i.e., the exact location of the AVF stenotic lesion. We conducted the CFD analysis utilizing patient-specific AVF geometric models to investigate hemodynamic parameters at different locations of an AVF, and the association between hemodynamic parameters and the anastomotic angle, particularly at the stenotic location. We analyzed 27 patients who used radio-cephalic AVF for hemodialysis and received an angiographic examination for juxta-anastomotic stenosis. The three-dimensional geometrical model of each patient's AVF was built using the angiographic images, in which the shape and the anastomotic angle of the AVF were depicted. CFD simulations of AVF hemodynamics were conducted to obtain blood flow parameters at different locations of an AVF. We found that at the location of the stenotic lesion, the AV angle was significantly correlated with access flow disturbance (r = 0.739; p < 0.001) and flow velocity (r = 0.563; p = 0.002). Furthermore, the receiver operating characteristic (ROC) curve analysis revealed that the AV angle determines the lesion's flow disturbance with a high area under the curve value of 0.878. The ROC analysis also identified a cut-off value of the AV angle as 46.5°, above or below which the access flow disturbance was significantly different. By applying CFD analysis to real-world patient images, the present study provides evidence that an anastomotic angle wider than 46.5° might lead to disturbed flow generation, demonstrating a reference angle to adopt during the anastomosis surgery.

Morphological and hemodynamical alterations in brachial artery and cephalic vein. An image-based study for preoperative assessment for vascular access creation

The current study aims to computationally evaluate the effect of right upper arm position on the geometric and hemodynamic characteristics of the brachial artery (BA) and cephalic vein (CV) and, furthermore, to present in detail the methodology to characterise morphological and hemodynamical healthy vessels. Ten healthy volunteers were analysed in two configurations, the supine (S) and the prone (P) position. Lumen 3D surface models were constructed from images acquired from a non-contrast MRI sequence. Then, the models were used to numerically compute the physiological range of geometric (n = 10) and hemodynamic (n = 3) parameters in the BA and CV. Geometric parameters such as curvature and tortuosity, and hemodynamic parameters based on wall shear stress (WSS) metrics were calculated with the use of computational fluid dynamics. Our results highlight that changes in arm position had a greater impact on WSS metrics of the BA by altering the mean and maximum blood flow rate of the vessel. Whereas, curvature and tortuosity were found not to be significantly different between positions. Inter-variability was associated with antegrade and retrograde flow in BA, and antegrade flow in CV. Shear stress was low and oscillatory shear forces were negligible. This data suggests that deviations from this state may contribute to the risk of accelerated intimal hyperplasia of the vein in arteriovenous fistulas. Therefore, preoperative conditions coupled with post-operative longitudinal data will aid the identification of such relationships.

S-shaped versus conventional straight skin incision: Impact on primary functional maturation, stenosis and thrombosis of autogenous radiocephalic arteriovenous fistula: Impact of incision on maturation, stenosis & failure of RCAVF. Study design: Prospective observational comparative

Introduction

The objective of this study is to test the null hypothesis that an S-shaped surgical incision versus conventional (straight) skin incision in the creation of autogenous radiocephalic arteriovenous fistulas (RCAVFs) have no impact on the primary end-point of primary functional maturation and secondary end points of stenosis and thrombosis.

Methods

A prospective observational comparative consecutive study with intention-to-treat on individuals undergoing only radiocephalic arteriovenous fistula (RCAVFs) over a period of 12 months was conducted. Variables on patient's demographics, comorbidities, anesthesia type, mean arterial blood pressure, thrill, laterality, cephalic vein and radial artery diameter were collated. The test of probability was assessed through Chi-Square, Kaplan-Meier survival estimator and Log-Rank analysis.

Results

Total of n = 83 individuals with median age of 67 years (IQR, 20–89) and male predominance 83% during this period were subjected to RCAVF formation. Total of n = 45 patients in straight skin incision were compared to n = 38 individuals in S-shaped group. Despite equal prevalence of demographics, comorbidities, anesthesia type, mean arterial blood pressure (MAP), thrill, laterality, cephalic vein and radial artery diameter (p > 0.05) higher incidence of juxta-anastomotic stenosis was noted in the straight skin incision group (p = 0.029) in comparative and survival analysis (Log-Rank, p = 0.036). The maturation of the entire cohort was 69% (S-shaped 76% vs. straight group 62%) (p > 0.05).

Conclusion

The outcome of this study demonstrates that S-shaped surgical skin incision is associated with a lower incidence of stenosis in comparison to straight incision type in RCAVF formation.

•

S-shaped skin incision is an alternative to the conventional skin incision for creation of radiocephalic arteriovenous fistula (RCAVF).

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This approach permits better exposure for both vessels and minimise the need for extensive mobilisation of cephalic vein.

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S-shaped skin incision is associated with lower incidence of stenosis within the maturation period.

Engineering of the human vessel wall with hair follicle stem cells in vitro

Hair follicle stem cells (HFSCs) are increasingly used as a stem cell paradigm in vascular tissue engineering due to the fact that they are a rich source of easily accessible multipotent adult stem cells. Promising results have been demonstrated with small diameter (less than 6 mm) tissue engineered blood vessels under low blood pressure, however engineering large vessels (>6 mm in diameter) remains a challenge due to the fact it demands a higher number of seed cells and higher quality biomechanical properties. The aim of the current study was to engineer a large vessel (6 mm in diameter) with differentiated smooth muscle cells (SMCs) induced from human (h)HFSCs using transforming growth factor‑β1 and platelet‑derived growth factor BB in combination with low‑serum culture medium. The cells were seeded onto polyglycolic acid and then wrapped around a silicone tube and further cultured in vitro. A round vessel wall was formed subsequent to 8 weeks of culture. Histological examination indicated that layers of smooth muscle‑like cells and collagenous fibres were oriented in the induced group. In contrast, disorganised cells and collagenous fibres were apparent in the undifferentiated group. The approach developed in the current study demonstrated potential for constructing large muscular vessels with differentiated SMCs induced from hHFSCs.

The Role of Shear Stress in Arteriovenous Fistula Maturation and Failure: A Systematic Review

INTRODUCTION

Non-maturation and post-maturation venous stenosis are the primary causes of failure within arteriovenous fistulae (AVFs). Although the exact mechanisms triggering failure remain unclear, abnormal hemodynamic profiles are thought to mediate vascular remodelling and can adversely impact on fistula patency.

AIM

The review aims to clarify the role of shear stress on outward remodelling during maturation and evaluate the evidence supporting theories related to the localisation and development of intimal hyperplasia within AVFs.

METHODS

A systematic review of studies comparing remodelling data with hemodynamic data obtained from computational fluid dynamics of AVFs during and after maturation was conducted.

RESULTS

Outward remodelling occurred to reduce or normalise the level of shear stress over time in fistulae with a large radius of curvature (curved) whereas shear stress was found to augment over time in fistulae with a small radius of curvature (straight) coinciding with minimal to no increases in lumen area. Although this review highlighted that there is a growing body of evidence suggesting low and oscillating shear stress may stimulate the initiation and development of intimal medial thickening within AVFs. Further lines of evidence are needed to support the disturbed flow theory and outward remodelling findings before surgical configurations and treatment strategies are optimised to conform to them. This review highlighted that variation between the time of analysis, classification of IH, resolution of simulations, data processing techniques and omission of various shear stress metrics prevented forming pooling of data amongst studies.

CONCLUSION

Standardised measurements and data processing techniques are needed to comprehensively evaluate the relationship between shear stress and intimal medial thickening. Advances in image acquisition and flow quantifications coupled with the increasing prevalence of longitudinal studies commencing from fistula creation offer viable techniques and strategies to robustly evaluate the relationship between shear stress and remodelling during maturation and thereafter.

Potential contributions of intimal and plaque hypoxia to atherosclerosis

Injury of arterial endothelium by abnormal shear stress and other insults induces migration and proliferation of vascular smooth muscle cells (VSMCs), which in turn leads to intimal thickening, hypoxia, and vasa vasorum angiogenesis. The resultant new blood vessels extend from the tunica media into the outer intima, allowing blood-borne oxidized low-density lipoprotein (oxLDL) particles to accumulate in outer intimal tissues by extravasation through local capillaries. In response to oxLDL accumulation, monocytes infiltrate into arterial wall tissues, where they differentiate into macrophages and subsequently evolve into foam cells by uptaking large quantities of oxLDL particles, the latter process being stimulated by hypoxia. Increased oxygen demand due to expanding macrophage and foam cell populations contributes to persistent hypoxia in plaque lesions, whereas hypoxia further promotes plaque growth by stimulating angiogenesis, monocyte infiltration, and oxLDL uptake into macrophages. Molecularly, the accumulation of hypoxia-inducible factor (HIF)-1α and the expression of its target genes mediate many of the hypoxia-induced processes during plaque initiation and growth. It is hoped that further understanding of the underlying mechanisms may lead to novel therapies for effective intervention of atherosclerosis.

Animal models of surgically manipulated flow velocities to study shear stress-induced atherosclerosis

Atherosclerosis is a chronic inflammatory disease of the arterial tree that develops at predisposed sites, coinciding with locations that are exposed to low or oscillating shear stress. Manipulating flow velocity, and concomitantly shear stress, has proven adequate to promote endothelial activation and subsequent plaque formation in animals. In this article, we will give an overview of the animal models that have been designed to study the causal relationship between shear stress and atherosclerosis by surgically manipulating blood flow velocity profiles. These surgically manipulated models include arteriovenous fistulas, vascular grafts, arterial ligation, and perivascular devices. We review these models of manipulated blood flow velocity from an engineering and biological perspective, focusing on the shear stress profiles they induce and the vascular pathology that is observed.

A PARAMETRIC NUMERICAL INVESTIGATION OF LOCAL HAEMODYNAMICS IN THE END-TO-SIDE ANASTOMOSIS OF CERVICAL-TO-PETROUS BYPASS BASED ON REAL GEOMETRY OF INTERNAL CAROTID ARTERY

Bypass reconstructed from the cervical segment of internal carotid artery (ICA) to its petrous segment allows high-volume bypass flow without any risk of intracranial dissection. The purpose of this study was to investigate the geometric effect on the end-to-side anastomosis of cervical-to-petrous bypass, for its close relationship to local haemodynamic and the long-term performance of bypass. We focused on two controllable geometric parameters: diameter ratio (Φ) and angle (α) between the graft and host arteries. Different models covering a range of Φ (0.75, 1 and 1.25) and α (30°, 45°, 60° and 90°) were constructed based on real geometry of human ICA. Numerical simulations of blood flow were performed in physiological flow condition. The flow patterns, flow distributions, time-average wall shear stress (TAWSS) and oscillatory shear index (OSI) in different models were compared. Our results showed geometric factors have influence on both the local haemodynamic parameters and the flow velocity through downstream branches. Of models with different geometric parameters, the model with Φ ≥ 1 or α = 45° were the most optimized considering haemodynamic performance.

Biomechanical regulation of vascular smooth muscle cell functions: from in vitro to in vivo understanding

Vascular smooth muscle cells (VSMCs) have critical functions in vascular diseases. Haemodynamic factors are important regulators of VSMC functions in vascular pathophysiology. VSMCs are physiologically active in the three-dimensional matrix and interact with the shear stress sensor of endothelial cells (ECs). The purpose of this review is to illustrate how haemodynamic factors regulate VSMC functions under two-dimensional conditions in vitro or three-dimensional co-culture conditions in vivo. Recent advances show that high shear stress induces VSMC apoptosis through endothelial-released nitric oxide and low shear stress upregulates VSMC proliferation and migration through platelet-derived growth factor released by ECs. This differential regulation emphasizes the need to construct more actual environments for future research on vascular diseases (such as atherosclerosis and hypertension) and cardiovascular tissue engineering.

Neovascularization of coronary tunica intima (DIT) is the cause of coronary atherosclerosis. Lipoproteins invade coronary intima via neovascularization from adventitial vasa vasorum, but not from the arterial lumen: a hypothesis

BACKGROUND

An accepted hypothesis states that coronary atherosclerosis (CA) is initiated by endothelial dysfunction due to inflammation and high levels of LDL-C, followed by deposition of lipids and macrophages from the luminal blood into the arterial intima, resulting in plaque formation. The success of statins in preventing CA promised much for extended protection and effective therapeutics. However, stalled progress in pharmaceutical treatment gives a good reason to review logical properties of the hypothesis underlining our efforts, and to reconsider whether our perception of CA is consistent with facts about the normal and diseased coronary artery.

ANALYSIS

To begin with, it must be noted that the normal coronary intima is not a single-layer endothelium covering a thin acellular compartment, as claimed in most publications, but always appears as a multi-layer cellular compartment, or diffuse intimal thickening (DIT), in which cells are arranged in many layers. If low density lipoprotein cholesterol (LDL-C) invades the DIT from the coronary lumen, the initial depositions ought to be most proximal to blood, i.e. in the inner DIT. The facts show that the opposite is true, and lipids are initially deposited in the outer DIT. This contradiction is resolved by observing that the normal DIT is always avascular, receiving nutrients by diffusion from the lumen, whereas in CA the outer DIT is always neovascularized from adventitial vasa vasorum. The proteoglycan biglycan, confined to the outer DIT in both normal and diseased coronary arteries, has high binding capacity for LDL-C. However, the normal DIT is avascular and biglycan-LDL-C interactions are prevented by diffusion distance and LDL-C size (20 nm), whereas in CA, biglycan in the outer DIT can extract lipoproteins by direct contact with the blood. These facts lead to the single simplest explanation of all observations: (1) lipid deposition is initially localized in the outer DIT; (2) CA often develops at high blood LDL-C levels; (3) apparent CA can develop at lowered blood LDL-C levels. This mechanism is not unique to the coronary artery: for instance, the normally avascular cornea accumulates lipoproteins after neovascularization, resulting in lipid keratopathy.

HYPOTHESIS

Neovascularization of the normally avascular coronary DIT by permeable vasculature from the adventitial vasa vasorum is the cause of LDL deposition and CA. DIT enlargement, seen in early CA and aging, causes hypoxia of the outer DIT and induces neovascularization. According to this alternative proposal, coronary atherosclerosis is not related to inflammation and can occur in individuals with normal circulating levels of LDL, consistent with research findings.

Effects of disturbed flow on vascular endothelium: pathophysiological basis and clinical perspectives

Vascular endothelial cells (ECs) are exposed to hemodynamic forces, which modulate EC functions and vascular biology/pathobiology in health and disease. The flow patterns and hemodynamic forces are not uniform in the vascular system. In straight parts of the arterial tree, blood flow is generally laminar and wall shear stress is high and directed; in branches and curvatures, blood flow is disturbed with nonuniform and irregular distribution of low wall shear stress. Sustained laminar flow with high shear stress upregulates expressions of EC genes and proteins that are protective against atherosclerosis, whereas disturbed flow with associated reciprocating, low shear stress generally upregulates the EC genes and proteins that promote atherogenesis. These findings have led to the concept that the disturbed flow pattern in branch points and curvatures causes the preferential localization of atherosclerotic lesions. Disturbed flow also results in postsurgical neointimal hyperplasia and contributes to pathophysiology of clinical conditions such as in-stent restenosis, vein bypass graft failure, and transplant vasculopathy, as well as aortic valve calcification. In the venous system, disturbed flow resulting from reflux, outflow obstruction, and/or stasis leads to venous inflammation and thrombosis, and hence the development of chronic venous diseases. Understanding of the effects of disturbed flow on ECs can provide mechanistic insights into the role of complex flow patterns in pathogenesis of vascular diseases and can help to elucidate the phenotypic and functional differences between quiescent (nonatherogenic/nonthrombogenic) and activated (atherogenic/thrombogenic) ECs. This review summarizes the current knowledge on the role of disturbed flow in EC physiology and pathophysiology, as well as its clinical implications. Such information can contribute to our understanding of the etiology of lesion development in vascular niches with disturbed flow and help to generate new approaches for therapeutic interventions.

Skeletal nutrient vascular adaptation induced by external oscillatory intramedullary fluid pressure intervention

BACKGROUND

Interstitial fluid flow induced by loading has demonstrated to be an important mediator for regulating bone mass and morphology. It is shown that the fluid movement generated by the intramedullary pressure (ImP) provides a source for pressure gradient in bone. Such dynamic ImP may alter the blood flow within nutrient vessel adjacent to bone and directly connected to the marrow cavity, further initiating nutrient vessel adaptation. It is hypothesized that oscillatory ImP can mediate the blood flow in the skeletal nutrient vessels and trigger vasculature remodeling. The objective of this study was then to evaluate the vasculature remodeling induced by dynamic ImP stimulation as a function of ImP frequency.

METHODS

Using an avian model, dynamics physiological fluid ImP (70 mmHg, peak-peak) was applied in the marrow cavity of the left ulna at either 3 Hz or 30 Hz, 10 minutes/day, 5 days/week for 3 or 4 weeks. The histomorphometric measurements of the principal nutrient arteries were done to quantify the arterial wall area, lumen area, wall thickness, and smooth muscle cell layer numbers for comparison.

RESULTS

The preliminary results indicated that the acute cyclic ImP stimuli can significantly enlarge the nutrient arterial wall area up to 50%, wall thickness up to 20%, and smooth muscle cell layer numbers up to 37%. In addition, 3-week of acute stimulation was sufficient to alter the arterial structural properties, i.e., increase of arterial wall area, whereas 4-week of loading showed only minimal changes regardless of the loading frequency.

CONCLUSIONS

These data indicate a potential mechanism in the interrelationship between vasculature adaptation and applied ImP alteration. Acute ImP could possibly initiate the remodeling in the bone nutrient vasculature, which may ultimately alter blood supply to bone.

Weaving hypothesis of cardiomyocyte sarcomeres: discovery of periodic broadening and narrowing of intercalated disk during volume-load change

To investigate how cardiomyocytes change their length, echocardiographic and morphological studies were performed on rabbit hearts that were subjected to volume overload, overload removal, and repeated cycles of overload and overload removal. These conditions were created by arterio-venous fistula between the carotid artery and jugular vein, closure of the fistula, and cycles of repeatedly forming and closing fistula, respectively. After overload, hearts dilated and myocytes elongated. Intercalated disks repeatedly broadened and narrowed with a 2-day cycle, which continued for 8 weeks in many animals. The cycle consisted of shifts between five modes characterized by two interdigitation elongation-and-shortenings as follows: (I) flat with short ( approximately 1/4 to approximately 1/3 sarcomere long) interdigitations; (II) flat with long (one sarcomere long) interdigitations; (III) grooved with short interdigitations; (IV) grooved with long interdigitations; (V) flat with short interdigitations intermingled by sporadic long interdigitations; and return to (I). After overload removal, hearts contracted and myocytes shortened with similar 2-day broadening and narrowing cycle of intercalated disks, in which the five modes were reversed. Repeated overload and overload removal resulted in the repetition of myocyte elongation and shortening. We hypothesize that a single elongation-and-shortening event creates or disposes one sarcomere layer, and the two consecutive elongation-and-shortenings occur complementarily to each other so that the disks return to their original state after each cycle. Our hypothesis predicts that intercalated disks weave and unravel one sarcomere per myocyte per day.

Analysis of arterial intimal hyperplasia: review and hypothesis

BACKGROUND

Despite a prodigious investment of funds, we cannot treat or prevent arteriosclerosis and restenosis, particularly its major pathology, arterial intimal hyperplasia. A cornerstone question lies behind all approaches to the disease: what causes the pathology?

HYPOTHESIS

I argue that the question itself is misplaced because it implies that intimal hyperplasia is a novel pathological phenomenon caused by new mechanisms. A simple inquiry into arterial morphology shows the opposite is true. The normal multi-layer cellular organization of the tunica intima is identical to that of diseased hyperplasia; it is the standard arterial system design in all placentals at least as large as rabbits, including humans. Formed initially as one-layer endothelium lining, this phenotype can either be maintained or differentiate into a normal multi-layer cellular lining, so striking in its resemblance to diseased hyperplasia that we have to name it "benign intimal hyperplasia". However, normal or "benign" intimal hyperplasia, although microscopically identical to pathology, is a controllable phenotype that rarely compromises blood supply. It is remarkable that each human heart has coronary arteries in which a single-layer endothelium differentiates early in life to form a multi-layer intimal hyperplasia and then continues to self-renew in a controlled manner throughout life, relatively rarely compromising the blood supply to the heart, causing complications requiring intervention only in a small fraction of the population, while all humans are carriers of benign hyperplasia. Unfortunately, this fundamental fact has not been widely appreciated in arteriosclerosis research and medical education, which continue to operate on the assumption that the normal arterial intima is always an "ideal" single-layer endothelium. As a result, the disease is perceived and studied as a new pathological event caused by new mechanisms. The discovery that normal coronary arteries are morphologically indistinguishable from deadly coronary arteriosclerosis continues to elicit surprise.

CONCLUSION

Two questions should inform the priorities of our research: (1) what controls switch the single cell-layer intimal phenotype into normal hyperplasia? (2) how is normal (benign) hyperplasia maintained? We would be hard-pressed to gain practical insights without scrutinizing our premises.

Sex hormones and aortic wall remodeling in an arteriovenous fistula

BACKGROUND

An arteriovenous fistula (AVF) creates high blood flow through the artery and fistula. With this high flow, there is flow-induced remodeling and an increase in diameter, but no intimal hyperplasia. Estrogen has been shown to modify vascular remodeling, decreasing intimal hyperplasia after endothelial injury.

OBJECTIVE

These experiments tested the hypothesis that estrogen administration would decrease wall thickness in an AVF model. Because estrogen may decrease wall thickness, we also tested the hypothesis that testosterone would increase wall thickness.

METHODS

A fistula was created between the abdominal aorta and the inferior vena cava in Sprague-Dawley rats to generate high blood flow conditions in the aorta. Four groups of female animals were examined: sham, control with AVF ovariectomized (OVX) with AVF and OVX plus testosterone with AVF Four groups of male animals were also examined: sham, control with AVF castrated with AVF and castrated plus estrogen with AVF Five weeks after creation of the AVF, the aortas were collected and fixed; wall thickness was measured both proximal and distal to the AVF.

RESULTS

Ovariectomy resulted in a significant decrease in estrogen levels (P < 0.01). Testosterone administration tended to increase testosterone levels in the OVX females, but values did not approach levels observed in the control males. No difference was noted in the proximal wall thickness between the control and the OVX animals. The OVX females receiving testosterone exhibited a significant increase in both proximal and distal wall thickness compared with control females (P < 0.001). In the male animals, there was no significant change in aortic wall thickness in the castrated rats compared with the controls. Estrogen administration in the castrated males resulted in a significant decrease in wall thickness in the proximal and distal aorta (P < 0.05).

CONCLUSION

These studies suggest that, in a model of vascular remodeling, estrogen administration decreases wall thickness, and testosterone administration increases wall thickness.

Laminar Shear Stress Inhibits Cathepsin L Activity in Endothelial Cells

OBJECTIVE

The cysteine proteases, cathepsins, have been implicated in vascular remodeling and atherosclerosis, processes known to be regulated by shear stress. It is not known, however, whether shear regulates cathepsins. We examined the hypothesis that shear stress regulates cathepsin activity in endothelial cells.

METHODS AND RESULTS

Mouse aortic endothelial cells (MAECs) exposed to atheroprotective, unidirectional laminar shear (LS) degraded significantly less BODIPY-labeled elastin and gelatin in comparison to static and proatherogenic oscillatory shear (OS). The cathepsin inhibitor E64 also reduced this activity. Gelatin zymography showed that cathepsin activity of MAECs was blunted by LS exposure and by a cathepsin L inhibitor but not by cathepsin B and S inhibitors, whereas a cathepsin K inhibitor had a minor effect. Cathepsin L siRNA knocked down cathepsin L expression, gelatinase, and elastase activity in OS and static MAECs. A partial reduction of cathepsin B protein raised the possibility that the siRNA effect on the matrix protease activity could have been attributable to cathepsin L or B. Cathepsin B activity study using the synthetic peptide showed it was not regulated by shear.

CONCLUSIONS

These results suggest that cathepsin L is a shear-sensitive matrix protease and that it may play an important role in flow-mediated vascular remodeling and atherogenic responses.

Sympathectomy Causes Aggravated Lesions and Dedifferentiation in Large Rabbit Atherosclerotic Arteries without Involving Nitric Oxide

Previously [Histochem J 1997;29:279-286], we found that sympathectomy induced neointima formation in ear but not cerebral arteries of genetically hyperlipidemic rabbits. To clarify the influence of sympathetic nerves in atherosclerosis, and whether their influence involves vascular NO activity, we studied groups of normocholesterolemic intact (NI) and sympathectomized (NS), and hypercholesterolemic intact (HI) and sympathectomized (HS) rabbits (diet/6-hydroxydopamine for 79 days). Segments of basilar (BA) and femoral (FA) arteries were studied histochemically, to evaluate differentiation (anti-desmin, anti-vimentin, anti-h-caldesmon, and nuclear dye), by confocal microscopy, and by in vitro myography. In BAs, staining of NI and NS groups was similar. In hypercholesterolemic groups, a small neointima developed, more frequently in HS segments where smooth muscle cells (SMCs) positive for all antibodies appeared to be migrating into the neointima. In FAs, SMCs stained for the three antibodies in the NI group, but we observed desmin- and h-caldesmon-negative, vimentin-positive cells in some external medial layers of the NS, HI and HS groups, identical to adventitial fibroblasts. Large neointimas of the HS group contained vimentin-positive and largely desmin- and h-caldesmon-negative cells. Relaxation of BA or FA segments to acetylcholine was not decreased by sympathectomy. Sympathectomy increased the contraction of resting FAs to nitro-L-arginine (p = 0.0379). Thus, sympathectomy aggravates the tendency for FA SMCs to migrate and dedifferentiate, increasing atherosclerotic lesions, without decreasing NO activity, but has only minor effects on BAs.

Continuous flow augments reactivity of rabbit carotid artery by reducing bioavailability of NO despite an increase in release of EDHF

Experiments were designed to investigate the influence of steady flow and pressure on endothelial function in the rabbit carotid artery. Increases and decreases in isometric force were compared in static rings and perfused (5 or 50 ml/min) segments of the same arteries in the presence and absence of endothelium. The alpha(1)-adrenoceptor agonist phenylephrine and the muscarinic agonist acetylcholine were applied as vasoconstrictor and vasodilator stimuli, respectively. Continuous flow (5 and 50 ml/min) reduced the cGMP content and shifted the concentration-response curve to phenylephrine to the left compared with nonperfused static rings. Removal of the endothelium abolished the differences in cGMP content and the sensitivity to phenylephrine between static rings and perfused segments. No difference in sensitivity to phenylephrine was observed in tissues treated with N(omega)-nitro-l-arginine methyl ester (l-NAME). Acetylcholine-evoked relaxations were increased in perfused segments. l-NAME nearly abolished the acetylcholine-evoked relaxation in static rings, whereas about one-half of the relaxation remained in segments exposed to flow. This remnant relaxation was blocked by inhibition of endothelial small- and intermediate-conductance calcium-activated potassium channels by apamin plus 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34). These experiments demonstrate that continuous flow increases the constriction evoked by alpha(1)-adrenergic activation in the rabbit carotid artery through a reduced influence of basally released endothelial NO and, furthermore, that luminal flow unmasks an ability of the endothelium to release a non-NO, noncyclooxygenase vasodilator, presumably endothelium-derived hyperpolarizing factor.

Expression of TGF-beta1 in smooth muscle cells regulates endothelial progenitor cells migration and differentiation

OBJECTIVE

Endothelial angiogenesis in the intima of the arterial wall is one of key events in the pathogenesis of arteriosclerosis. The molecular mechanisms by which transforming growth factor beta 1 (TGFbeta1) and endothelial progenitor cells may be responsible for angiogenesis of arteriosclerosis lesions are poorly understood.

MATERIALS AND METHODS

Primary culture smooth muscle cells were transfected with pMAMneoTGFbeta1. ELISA checked VEGF expression in smooth muscle cells. Human EPCs (CD34+ cells) were cultured in pMAMneoTGFbeta1 or pMAMneo transfected smooth muscle cells conditional medium. After 21 days, differentiated endothelial colonies were confirmed by immunofluorescence for von Willebrand factor (vWF) and vascular-endothelial (VE)-cadherin. The VEGFR-1 expression in differentiated endothelial colonies was detected by ELISA. Cells migration and adhesion toward pMAMneoTGFbeta1 and pMAMneo transfected smooth muscle cells were also measured in parallel flow chamber.

RESULTS

Abundant TGFbeta1 stable expressed in smooth muscle cells. TGFbeta1 transfected smooth muscle cells expressed significantly higher level VEGF than pMAMneo group. As judged by positive staining for endothelial markers vWF and VE-cadherin, the combination of TGFbeta1 transfected smooth muscle cells conditional medium produced significantly more endothelial colonies (P<0.05) than did pMAMneo group. The adhesion force between endothelial progenitor cells and smooth muscle cells in TGFbeta1 group was higher than control.

CONCLUSION

TGFbeta1 expressed smooth muscle cells can be helpful for increasing endothelial progenitor cells adhesion and differentiation. It may be responsible for angiogenesis of arteriosclerosis lesions and useful for blood vessel tissue engineering.

Intermittent short-duration exposure to low wall shear stress induces intimal thickening in arteries exposed to chronic high shear stress

We sought to determine whether intermittent short-duration exposure to low wall shear stress could induce intimal thickening in arteries chronically exposed to high shear stress. An arteriovenous fistula (AVF) was created between the left common carotid artery and the corresponding external jugular vein in 20 Japanese white male rabbits. After 4 weeks, blood flow was increased 10-fold to 182 +/- 39 ml/min and shear stress was increased to 33.4 +/- 13 dyn/cm(2). The AVF was then occluded for 1 h by finger compression with an 85% reduction in carotid artery blood flow (27 +/- 7 ml/min) and a reduction in wall shear stress to 4.9 +/- 1.7 dyn/cm(2) (P < 0.0001). Release of finger compression restored flow to the AVF and high shear stress to the carotid artery. This procedure was repeated at weekly intervals with a cumulative total of 4 h of low shear stress exposure. Arteries exposed to intermittent low shear stress developed a layer of intimal thickening which consisted of 3-4 layers of smooth muscle cells lined with thin elastic fibers and medial hyperplasia. Control arteries exposed to 8 weeks of continuous high shear had no intimal thickening. Transient exposure to low shear stress upregulated TGF-beta1, MMP-2, -14, and TIMP-2 gene expression while MMP-9 expression was downregulated. We conclude that repeated, intermittent short-duration exposure to low shear stress in the setting of high flow and high shear stress can induce arterial intimal thickening. Short-duration alterations in hemodynamic forces can induce rapid vascular cell message expression, which may effect arterial remodeling. This experiment suggests that a threshold value of 5 dyn/cm(2) may be needed in order to initiate and sustain the intimal thickening response.

Normalization of high-flow or removal of flow cannot stop high-flow induced endothelial proliferation

Endothelial cells (ECs) are activated in response to high-flow. Our previous studies using arteriovenous fistula (AVF) model have demonstrated that high-flow in blood vessels induces an early and rapid proliferation of ECs before arterial dilatation. Here, we investigated the proliferation of ECs, which had once been stimulated by high-flow loading, in a situation without the influence of high-flow. First, we induced high-flow in the rabbit common carotid artery by using AVF. Then, we removed the influence of high-flow by normalization of high-flow with the closure of AVF or by removal of flow itself with tissue isolation and organ culture or with cell culture of ECs, at the timing considered that ECs began to proliferate. Kinetics of ECs was investigated by a laser scanning confocal microscopy, phase-contrast microscopy and light microscopy using bromodeoxyuridine labeling method. We found that ECs, which had once been stimulated by high-flow, transiently proliferated even after normalization of high-flow or removal of flow. We assume that proliferation of ECs is promised when these cells start to proliferate after high-flow loading.

Intimal Hyperplasia in Mouse Vein Grafts Is Regulated by Flow

Altered flow conditions are presumed to cause stenosis in vein grafts due to exaggerated neointimal formation. The aim of this study was to establish a mouse model of flow-regulated intimal hyperplasia (IH) in vein grafts. The caval vein was grafted into the common carotid artery of 38 mice, followed by modulation of the blood flow, resulting in vein grafts with high (HF) and low flow (LF). The vessel wall thickening was evaluated after 3, 14 and 42 days by morphometric analyses and immunohistochemistry. There was an immediate significant change in flow, which was persistent throughout the time of observation. After 42 days, flow was increased 2.7 times in HF animals compared to LF animals. The vessel wall was composed of two layers where the inner layer was positive for alpha-actin and considered as IH. The area of neointimal formation was 74% larger in the LF group compared to the HF group. The present study demonstrates that flow regulates IH in vein grafts in mice. This model gives the potential to study the effect of shear stress on vascular biology in genetically modified animals.

Hemodynamic forces regulate mural macrophage infiltration in experimental aortic aneurysms

Blood flow (BF) and wall shear stress (WSS) influence reactive oxygen species production and oxidative stress in abdominal aortic aneurysm (AAA) disease. To gain further insight into the mechanisms of hemodynamic influences on AAA inflammation, we examined aneurysm macrophage density, chemotaxis and survival under varying aortic flow conditions. Rat AAAs were created via porcine pancreatic elastase (PPE) infusion. In selected cohorts, AAA flow was increased via left common femoral arteriovenous fistula (AVF) creation (HF-AAA) or decreased by left common iliac ligation (LF-AAA). WSS was highest in HF-AAA (10.4 +/- 2.3 dyn/cm(2) vs. 2.4 +/- 0.4 and 0.5 +/- 0.2 for NF- and LF-AAA, respectively, P < 0.001) 7 days after PPE infusion, with reduced medial macrophage density and increased apoptosis. Adventitial macrophage density was not significantly influenced by flow. Monocyte chemoattractant protein-1 (MCP-1) and granulocyte-macrophage colony-stimulating factor (GM-CSF) gene expression correlated with observed macrophage densities in the media and adventitia. Luminal flow conditions regulate AAA inflammation in part via influences on medial macrophage density. Hemodynamic forces may modulate AAA inflammation and diameter enlargement via direct regulation of intimal macrophage adhesion, transmural migration or survival.

Arterial enlargement, tortuosity, and intimal thickening in response to sequential exposure to high and low wall shear stress

We investigated the effects of sequential and prolonged exposure to high and low wall shear stress on arterial remodeling using a rabbit arteriovenous fistula (AVF) model. Blood flow was increased by approximately 17-fold to 20-fold when the AVF was open, and returned to normal when the AVF was occluded. Repeated opening and closing of the AVF resulted in sequential exposure of the artery to high and low wall shear stress. High flow and high wall shear stress induced arterial dilatation, elongation, and tortuosity, without intimal thickening. The common carotid artery was elongated 37% after 4 weeks of high flow, and was shortened 10% after 6 weeks of normal flow. Subsequent cycles of high flow induced less elongation, with less shortening after return to normal flow. Enlargement of the distal segment was more dramatic than in the proximal segment, despite exposure to the same volume of flow and the same initial high wall shear stress after creation of the AVF. The distal carotid segment enlarged more than did the proximal segment during each exposure to high flow. In segments of carotid artery exposed to low wall shear stress (<5 dynes/cm(2)) intimal thickening developed. These changes were maximal in the distal carotid segment, just before the AVF. Each cycle of low wall shear stress induced intimal thickening accompanied by medial hyperplasia. Intimal thickening was inhibited during periods of high flow when wall shear stress was high. Three cycles of flow alteration induced three layers of intimal thickening in the distal arterial segment, two layers of intimal thickening in the middle segment, and one layer of intimal thickening in the proximal segment. Long-term exposure to low wall shear stress induced severe intimal thickening and medial hyperplasia in different segments. Thus the response of the carotid artery afferent to an AVF varies along the length of the artery, with maximum enlargement, elongation, and tortuosity in the distal segment, just proximal to the AVF. Similarly, intimal thickening in response to low wall shear stress is maximal in the distal carotid artery. It appears that intimal thickening is related to local levels of low wall shear stress, and occurs when wall shear stress chronically falls to less than 5 dynes/cm(2).

High flow drives vascular endothelial cell proliferation during flow-induced arterial remodeling associated with the expression of vascular endothelial growth factor

Endothelial cell activation and proliferation are the essential steps in flow-induced arterial remodeling. We investigated endothelial cell turnover in the early stages of high-flow in the rabbit common carotid arteries using an arteriovenous fistula (AVF) model by kinetic investigation of cell proliferation and cell molecular analysis. BrdU was administrated to label endothelial cells (ECs) in DNA synthetic phase (S-phase) of the cell mitotic cycle. Pulse labeling revealed that ECs entered S-phase at 1.5 days of AVF (0.93 +/- 0.19%). Endothelial cell labeling index (EC-LI) peaked at 2 days of AVF (8.90 +/- 0.87%) with a high index of endothelial cell mitosis (EC-MI, 1.67 +/- 0.47%). Endothelial cell density increased remarkably at 3 days of AVF with a significant decrease in EC-LI (54%) and EC-MI (60%). Study of kinetics of EC proliferation revealed that endothelial cells took 16-24 h to finish one cycle of cell mitosis. Tracking investigation of pulse BrdU-labeled endothelial cells at 1.5 days showed that more than 66% of endothelial cells were BrdU-labeled 1.5 days after labeling. VEGF, integrin alphanubeta3, PECAM-1, and VE-cadherin were upregulated significantly preceding endothelial cell proliferation and kept at high levels during endothelial cell proliferation. These data suggest that endothelial cell proliferation is the initial step in flow-induced arterial remodeling. Hemodynamic forces may drive endothelial cell downstream migration. Expression of VEGF and cell junction molecules contribute to flow-induced arterial remodeling.

Rapid, arteriovenous graft failure due to intimal hyperplasia: a porcine, bilateral, carotid arteriovenous graft model

BACKGROUND

The loss of patency constitutes the major complication of arteriovenous (AV) polytetrafluoroethylene hemodialysis grafts. In most cases, this graft failure is due to intimal hyperplasia at the venous outflow tract, including proliferation of vascular, smooth muscle cells and fibroblasts with deposition of extracellular matrix proteins. Thus far, procedures developed for improving patency have proven unsuccessful, which can be partly explained by the lack of relevant animal models. For this purpose, we developed a porcine model for AV graft failure that will allow the assessment of promising therapeutic strategies in the near future.

MATERIALS AND METHODS

In 14 pigs, AV grafts were created bilaterally between the carotid artery and the jugular vein using expanded polytetrafluoroethylene. Two, 4 or 8 weeks after AV shunting, the grafts and adjacent vessels were excised and underwent histologic analysis.

RESULTS

From 2 weeks onwards, a thick neo-intima developed at the venous anastomosis, predominantly consisting of alpha-actin-positive vascular smooth muscle cells (VSMC). Intimal area increased over time, coinciding with a decreased graft flow. Grafts remained patent for at least 4 weeks. At 8 weeks, patency rates declined to less than 50% due to thrombus formation superimposed on progressive neo-intima formation.

CONCLUSIONS

Implantation of an AV graft between the carotid artery and jugular vein in pigs causes a rapid neo-intimal response, accompanied by a loss of patency of 50% at 8 weeks after surgery. This model offers a suitable tool to study local interventions aimed at the improvement of AV graft patency rates.