Vein graft adaptation and fistula maturation in the arterial environment

Blood Flow in Idealized Vascular Access for Hemodialysis: A Review of Computational Studies

Although our understanding of the failure mechanism of vascular access for hemodialysis has increased substantially, this knowledge has not translated into successful therapies. Despite advances in technology, it is recognized that vascular access is difficult to maintain, due to complications such as intimal hyperplasia. Computational studies have been used to estimate hemodynamic changes induced by vascular access creation. Due to the heterogeneity of patient-specific geometries, and difficulties with obtaining reliable models of access vessels, idealized models were often employed. In this review we analyze the knowledge gained with the use of computational such simplified models. A review of the literature was conducted, considering studies employing a computational fluid dynamics approach to gain insights into the flow field phenotype that develops in idealized models of vascular access. Several important discoveries have originated from idealized model studies, including the detrimental role of disturbed flow and turbulent flow, and the beneficial role of spiral flow in intimal hyperplasia. The general flow phenotype was consistent among studies, but findings were not treated homogeneously since they paralleled achievements in cardiovascular biomechanics which spanned over the last two decades. Computational studies in idealized models are important for studying local blood flow features and evaluating new concepts that may improve the patency of vascular access for hemodialysis. For future studies we strongly recommend numerical modelling targeted at accurately characterizing turbulent flows and multidirectional wall shear disturbances.

Dialysis-Related Parameters Influence Remodeling in the Venous Part of the Native Arteriovenous Fistula

BACKGROUND

The aim of this study was to evaluate the association of hemodynamic parameters related to hemodialysis and antropometric parameters of patients with changes in the venous part of the arteriovenous fistula (AVF) at points of needling.

METHODS

Two hundred forty-two hemodialysis (HD) patients (60.3% men), with median age 65 (interquartile range [IQR] 56-75) years, on HD treatment for a median of 49 (IQR 20-88) months with functioning fistula were recruited for the study. The history of vascular access, comorbidity, antropometric (body mass index, body surface area, and body composition), and dialysis-related parameters were analyzed. The cross-sectional area of upper extremity vessels were measured using ultrasound and included 2 points: A (arterial point for blood aspiration) and V (venous point for returning the blood after purification). The difference between A and V (A-V) was calculated.

RESULTS

The median cross-sectional area of A was larger than V (1.04 [IQR 0.58-1.7] vs. 0.74 cm² [IQR: 0.41-1.39], P <0.0001). The median difference between A and V (A-V) was 0.17 cm² and positively correlated with mean blood flow (Qb), effective Kt/V, and time of AVF use. Other analyzed factors had no influence on A-V. In the multivariate analysis, the independent factor increasing the difference (A-V) was mean blood flow measured during HD sessions.

CONCLUSIONS

The needling and utilization of AVF for hemodialysis may affect vein anatomy, namely causing dilatation at the arterial point and narrowing at venous point of AVF. We suggest that blood pump velocity of the dialysis machine may have an impact on these changes, but practical importance of these findings has to be elucidated. The significance of (A-V) factor in the prognosis of fistula complications should be further studied and confirmed in the prospective trials.

Increased Oxidative Stress and Hypoxia Inducible Factor-1 Expression during Arteriovenous Fistula Maturation

BACKGROUND

The poor clinical results that are frequently reported for arteriovenous fistulae (AVF) for hemodialysis are typically due to failure of AVF maturation. We hypothesized that early AVF maturation is associated with generation of reactive oxygen species and activation of the hypoxia-inducible factor-1 (HIF-1) pathway, potentially promoting neointimal hyperplasia. We tested this hypothesis using a previously reported mouse AVF model that recapitulates human AVF maturation.

METHODS

Aortocaval fistulae were created in C57Bl/6 mice and compared with sham-operated mice. AVFs or inferior vena cavas were analyzed using a microarray, Amplex Red for extracellular H2O2, quantitative polymerase chain reaction, immunohistochemistry, and immunoblotting for HIF-1α and immunofluorescence for NOX-2, nitrotyrosine, heme oxygenase-1 (HO-1), and vascular endothelial growth factor (VEGF)-A.

RESULTS

Oxidative stress was higher in AVF than that in control veins, with more H2O2 (P = 0.007) and enhanced nitrotyrosine immunostaining (P = 0.005). Immunohistochemistry and immunoblot showed increased HIF-1α immunoreactivity in the AVF endothelium; HIF-1 targets NOX-2, HO-1 and VEGF-A were overexpressed in the AVF (P < 0.01). AVF expressed increased numbers of HIF-1α (P < 0.0001) and HO-1 (P < 0.0001) messenger RNA transcripts.

CONCLUSIONS

Oxidative stress increases in mouse AVF during early maturation, with increased expression of HIF-1α and its target genes NOX-2, HO-1, and VEGF-A. These results suggest that clinical strategies to improve AVF maturation could target the HIF-1 pathway.

Dual Function for Mature Vascular Smooth Muscle Cells During Arteriovenous Fistula Remodeling

BACKGROUND

The arteriovenous fistula (AVF) is the preferred form of hemodialysis access for patients with chronic kidney disease. However, AVFs are associated with significant problems including high incidence of both early and late failures, usually attributed to inadequate venous arterialization and neointimal hyperplasia, respectively. Understanding the cellular basis of venous remodeling in the setting of AVF could provide targets for improving AVF patency rates.

METHODS AND RESULTS

A novel vascular smooth muscle cell (VSMC) lineage tracing reporter mouse, Myh11-Cre/ERT2-mTmG, was used to track mature VSMCs in a clinically relevant AVF mouse model created by a jugular vein branch end to carotid artery side anastomosis. Prior to AVF surgery, differentiated medial layer VSMCs were labeled with membrane green fluorescent protein (GFP) following tamoxifen induction. Four weeks after AVF surgery, we observed medial VSMC layer thickening in the middle region of the arterialized vein branch. This thickened medial VSMC layer was solely composed of differentiated VSMCs that were GFP+/MYH11+/Ki67-. Extensive neointimal hyperplasia occurred in the AVF region proximal to the anastomosis site. Dedifferentiated VSMCs (GFP+/MYH11-) were a major cellular component of the neointima. Examination of failed human AVF samples revealed that the processes of VSMC phenotypic modulation and intimal hyperplasia, as well as medial VSMC layer thickening, also occurred in human AVFs.

CONCLUSIONS

We demonstrated a dual function for mature VSMCs in AVF remodeling, with differentiated VSMCs contributing to medial wall thickening towards venous maturation and dedifferentiated VSMCs contributing to neointimal hyperplasia. These results provide valuable insights into the mechanisms underlying venous adaptations during AVF remodeling.

Native arteriovenous fistula placement in three patients after implantation of a left ventricular assist device with non-pulsatile blood flow

Patients with left ventricular assist device have an increased risk for postoperative acute kidney injury and long-term requirement of dialysis. As a chronic access for dialysis, native arteriovenous fistulae are uncommon options due to suspected maturation failure. We present three patients who received successful arteriovenous fistulae placement after left ventricular assist device implantation with assisted long term patency.

Ca2+/Calmodulin-Dependent Protein Kinase II in Vascular Smooth Muscle

Ca²⁺-dependent signaling pathways are central regulators of differentiated vascular smooth muscle (VSM) contractile function. In addition, Ca²⁺ signals regulate VSM gene transcription, proliferation, and migration of dedifferentiated or "synthetic" phenotype VSM cells. Synthetic phenotype VSM growth and hyperplasia are hallmarks of pervasive vascular diseases including hypertension, atherosclerosis, postangioplasty/in-stent restenosis, and vein graft failure. The serine/threonine protein kinase Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is a ubiquitous mediator of intracellular Ca²⁺ signals. Its multifunctional nature, structural complexity, diversity of isoforms, and splice variants all characterize this protein kinase and make study of its activity and function challenging. The kinase has unique autoregulatory mechanisms, and emerging studies suggest that it can function to integrate Ca²⁺ and reactive oxygen/nitrogen species signaling. Differentiated VSM expresses primarily CaMKIIγ and -δ isoforms. CaMKIIγ isoform expression correlates closely with the differentiated phenotype, and some studies link its function to regulation of contractile activity and Ca²⁺ homeostasis. Conversely, synthetic phenotype VSM cells primarily express CaMKIIδ and substantial evidence links it to regulation of gene transcription, proliferation, and migration of VSM in vitro, and vascular hypertrophic and hyperplastic remodeling in vivo. CaMKIIδ and -γ isoforms have opposing functions at the level of cell cycle regulation, proliferation, and VSM hyperplasia in vivo. Isoform switching following vascular injury is a key step in promoting vascular remodeling. Recent availability of genetically engineered mice with smooth muscle deletion of specific isoforms and transgenics expressing an endogenous inhibitor protein (CAMK2N) has enabled a better understanding of CaMKII function in VSM and should facilitate future studies.

Arteriovenous Vascular Access Selection and Evaluation

When making decisions regarding vascular access creation, the clinician and vascular access team must evaluate each patient individually with consideration of life expectancy, timelines for dialysis start, risks and benefits of access creation, referral wait times, as well as the risk for access complications. The role of the multidisciplinary team in facilitating access choice is reviewed, as well as the clinical evaluation of the patient.

Future research directions to improve fistula maturation and reduce access failure

With the increasing prevalence of end-stage renal disease, there is a growing need for hemodialysis. Arteriovenous fistulae (AVF) are the preferred type of vascular access for hemodialysis, but maturation and failure continue to present significant barriers to successful fistula use. AVF maturation integrates outward remodeling with vessel wall thickening in response to drastic hemodynamic changes in the setting of uremia, systemic inflammation, oxidative stress, and pre-existent vascular pathology. AVF can fail due to both failure to mature adequately to support hemodialysis and development of neointimal hyperplasia that narrows the AVF lumen, typically near the fistula anastomosis. Failure due to neointimal hyperplasia involves vascular cell activation and migration and extracellular matrix remodeling with complex interactions of growth factors, adhesion molecules, inflammatory mediators, and chemokines, all of which result in maladaptive remodeling. Different strategies have been proposed to prevent and treat AVF failure based on current understanding of the modes and pathology of access failure; these approaches range from appropriate patient selection and use of alternative surgical strategies for fistula creation, to the use of novel interventional techniques or drugs to treat failing fistulae. Effective treatments to prevent or treat AVF failure require a multidisciplinary approach involving nephrologists, vascular surgeons, and interventional radiologists, careful patient selection, and the use of tailored systemic or localized interventions to improve patient-specific outcomes. This review provides contemporary information on the underlying mechanisms of AVF maturation and failure and discusses the broad spectrum of options that can be tailored for specific therapy.

New Insights into Dialysis Vascular Access: Impact of Preexisting Arterial and Venous Pathology on AVF and AVG Outcomes

Despite significant improvements in preoperative patient evaluation and surgical planning, vascular access failure in patients on hemodialysis remains a frequent and often unforeseeable complication. Our inability to prevent this complication is, in part, because of an incomplete understanding of how preexisting venous and arterial conditions influence the function of newly created arteriovenous fistulas and grafts. This article reviews the relationship between three preexisting vascular pathologies associated with CKD (intimal hyperplasia, vascular calcification, and medial fibrosis) and hemodialysis access outcomes. The published literature indicates that the pathogenesis of vascular access failure is multifactorial and not determined by any of these pathologies individually. Keeping this observation in mind should help focus our research on the true causes responsible for vascular access failure and the much needed therapies to prevent it.

Traditional graft preparation decreases physiologic responses, diminishes viscoelasticity, and reduces cellular viability of the conduit: A porcine saphenous vein model

Traditional methods of intraoperative human saphenous vein preparation for use as bypass grafts can be deleterious to the conduit. The purpose of this study was to characterize acute graft preparation injury, and to mitigate this harm via an improved preparation technique. Porcine saphenous veins were surgically harvested (unprepared controls, UnP) and prepared using traditional (TraP) and improved preparations (ImP). The TraP used unregulated radial distension, marking with a surgical skin marker and preservation in heparinized normal saline. ImP used pressure-regulated distension, brilliant blue FCF-based pen marking and preservation in heparinized Plasma-Lyte A. Rings from each preparation were suspended in a muscle bath for characterization of physiologic responses to vasoactive agents and viscoelasticity. Cellular viability was assessed using the methyl thiazolyl tetrazolium (MTT) assay and the terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assay for apoptosis. Contractile responses to potassium chloride (110 mM) and phenylephrine (10 µM), and endothelial-dependent and independent vasodilatory responses to carbachol (0.5 µM) and sodium nitroprusside (1 µM), respectively, were decreased in TraP tissues compared to both UnP and ImP tissues (p ⩽ 0.05). TraP tissues demonstrated diminished viscoelasticity relative to UnP and ImP tissues (p ⩽ 0.05), and reduced cellular viability relative to UnP control (p ⩽ 0.01) by the MTT assay. On the TUNEL assay, TraP tissues demonstrated a greater degree of apoptosis relative to UnP and ImP tissues (p ⩽ 0.01). In conclusion, an improved preparation technique prevents vascular graft smooth muscle and endothelial injury observed in tissues prepared using a traditional approach.

Vein graft failure: from pathophysiology to clinical outcomes

Occlusive arterial disease is a leading cause of morbidity and mortality worldwide. Aside from balloon angioplasty, bypass graft surgery is the most commonly performed revascularization technique for occlusive arterial disease. Coronary artery bypass graft surgery is performed in patients with left main coronary artery disease and three-vessel coronary disease, whereas peripheral artery bypass graft surgery is used to treat patients with late-stage peripheral artery occlusive disease. The great saphenous veins are commonly used conduits for surgical revascularization; however, they are associated with a high failure rate. Therefore, preservation of vein graft patency is essential for long-term surgical success. With the exception of 'no-touch' techniques and lipid-lowering and antiplatelet (aspirin) therapy, no intervention has hitherto unequivocally proven to be clinically effective in preventing vein graft failure. In this Review, we describe both preclinical and clinical studies evaluating the pathophysiology underlying vein graft failure, and the latest therapeutic options to improve patency for both coronary and peripheral grafts.

Assessment of micro-mechanical variations in experimental arteriovenous fistulae using atomic force microscopy

PURPOSE

This study presents a method to quantify micro-stiffness variations in experimental arteriovenous fistulae (AVF).

METHODS

AVF created by anastomosing the superficial epigastric vein to the femoral artery in Sprague-Dawley rats were allowed to remodel for 21 days before being harvested and preserved in culture medium. A custom atomic force microscope was used to measure microvascular stiffness (Young's modulus) in three areas of the AVF: the inflow artery, the juxta-anastomotic area, and the outflow vein. Morphometric measurements and collagen and elastin contents were also determined.

RESULTS

Atomic force microscopy indentation revealed an increased stiffness in the juxta-anastomotic area of the AVF compared to the outflow vein and inflow artery. The juxta-anastomotic area was also significantly stiffer than the contralateral vein. The lack of elasticity (higher Young's modulus) of the juxta-anastomotic region was associated with a thicker vascular wall that was rich in collagen but poor in elastin.

CONCLUSIONS

This study demonstrates for the first time the feasibility of using atomic force microscopy to measure local stiffness variations in experimental AVF. This technique could be instrumental in advancing our understanding of how micro-spatial organization of the AVF wall determines the overall biomechanical performance of this type of vascular access.

Decellularized skin/adipose tissue flap matrix for engineering vascularized composite soft tissue flaps

Using a perfusion decellularization protocol, we developed a decellularized skin/adipose tissue flap (DSAF) comprising extracellular matrix (ECM) and intact vasculature. Our DSAF had a dominant vascular pedicle, microcirculatory vascularity, and a sensory nerve network and retained three-dimensional (3D) nanofibrous structures well. DSAF, which was composed of collagen and laminin with well-preserved growth factors (e.g., vascular endothelial growth factor, basic fibroblast growth factor), was successfully repopulated with human adipose-derived stem cells (hASCs) and human umbilical vein endothelial cells (HUVECs), which integrated with DSAF and formed 3D aggregates and vessel-like structures in vitro. We used microsurgery techniques to re-anastomose the recellularized DSAF into nude rats. In vivo, the engineered flap construct underwent neovascularization and constructive remodeling, which was characterized by the predominant infiltration of M2 macrophages and significant adipose tissue formation at 3months postoperatively. Our results indicate that DSAF co-cultured with hASCs and HUVECs is a promising platform for vascularized soft tissue flap engineering. This platform is not limited by the flap size, as the entire construct can be immediately perfused by the recellularized vascular network following simple re-integration into the host using conventional microsurgical techniques.

STATEMENT OF SIGNIFICANCE

Significant soft tissue loss resulting from traumatic injury or tumor resection often requires surgical reconstruction using autologous soft tissue flaps. However, the limited availability of qualitative autologous flaps as well as the donor site morbidity significantly limits this approach. Engineered soft tissue flap grafts may offer a clinically relevant alternative to the autologous flap tissue. In this study, we engineered vascularized soft tissue free flap by using skin/adipose flap extracellular matrix scaffold (DSAF) in combination with multiple types of human cells. Following vascular reanastomosis in the recipient site, the engineered products successful regenerated large-scale fat tissue in vivo. This approach may provide a translatable platform for composite soft tissue free flap engineering for microsurgical reconstruction.

Membrane-mediated regulation of vascular identity

Vascular diseases span diverse pathology, but frequently arise from aberrant signaling attributed to specific membrane-associated molecules, particularly the Eph-ephrin family. Originally recognized as markers of embryonic vessel identity, Eph receptors and their membrane-associated ligands, ephrins, are now known to have a range of vital functions in vascular physiology. Interactions of Ephs with ephrins at cell-to-cell interfaces promote a variety of cellular responses such as repulsion, adhesion, attraction, and migration, and frequently occur during organ development, including vessel formation. Elaborate coordination of Eph- and ephrin-related signaling among different cell populations is required for proper formation of the embryonic vessel network. There is growing evidence supporting the idea that Eph and ephrin proteins also have postnatal interactions with a number of other membrane-associated signal transduction pathways, coordinating translation of environmental signals into cells. This article provides an overview of membrane-bound signaling mechanisms that define vascular identity in both the embryo and the adult, focusing on Eph- and ephrin-related signaling. We also discuss the role and clinical significance of this signaling system in normal organ development, neoplasms, and vascular pathologies.

Eph-B4 mediates vein graft adaptation by regulation of endothelial nitric oxide synthase

OBJECTIVE

Vein graft adaptation is characterized by loss of expression of the tyrosine kinase receptor Eph-B4, the embryonic determinant of venous identity, without increased expression of its ligand ephrin-B2, the embryonic determinant of arterial identity. Endothelial nitric oxide synthase (eNOS) is an important mediator of vessel remodeling. We hypothesized that the mechanism of action of Eph-B4 during vein graft adaptation might be through regulation of downstream eNOS activity.

METHODS

Mouse lung endothelial cells were stimulated with ephrin-B2/Fc, without and with preclustering, without and with the eNOS inhibitor N_ω-nitro-l-arginine methyl ester hydrochloride or the Eph-B4 inhibitor NVP-BHG712, and assessed by Western blot and immunofluorescence for eNOS and Eph-B4 phosphorylation. Nitric oxide (NO) production was assessed using an NO-specific chemiluminescence analyzer. Cell migration was assessed using a Transwell assay. Human and mouse vein graft specimens were examined for eNOS activity by Western blot, and vessel remodeling was assessed in vein grafts in wild-type or eNOS knockout mice.

RESULTS

Ephrin-B2/Fc stimulated both Eph-B4 and eNOS phosphorylation in a bimodal temporal distribution (n = 4; P < .05), with preclustered ephrin-B2/Fc causing prolonged peak Eph-B4 and eNOS phosphorylation as well as altered subcellular localization (n = 4; P < .05). Ephrin-B2/Fc increased NO release (n = 3; P < .01) as well as increased endothelial cell migration (n = 6; P < .05) in an eNOS-dependent fashion. Both human and mouse vein grafts showed increased eNOS phosphorylation compared with normal veins (n = 3; P < .05). Vein grafts from eNOS knockout mice showed less dilation and less wall thickening compared with wild-type vein grafts (n = 7; P < .05).

CONCLUSIONS

eNOS is a mediator of vein graft adaptation to the arterial environment. Eph-B4 stimulates eNOS phosphorylation in vitro and may mediate vein graft adaptation by regulation of eNOS activity in vivo.

Disturbed shear stress reduces Klf2 expression in arterial-venous fistulae in vivo

Laminar shear stress (SS) induces an antiproliferative and anti-inflammatory endothelial phenotype and increases Klf2 expression. We altered the diameter of an arteriovenous fistula (AVF) in the mouse model to determine whether increased fistula diameter produces disturbed SS in vivo and if acutely increased disturbed SS results in decreased Klf2 expression. The mouse aortocaval fistula model was performed with 22, 25, or 28 gauge needles to puncture the aorta and the inferior vena cava. Duplex ultrasound was used to examine the AVF and its arterial inflow and venous outflow, and SS was calculated. Arterial samples were examined with western blot, immunohistochemistry, and immunofluorescence analysis for proteins and qPCR for RNA. Mice with larger diameter fistulae had diminished survival but increased AVF patency. Increased SS magnitudes and range of frequencies were directly proportional to the needle diameter in the arterial limb proximal to the fistula but not in the venous limb distal to the fistula, with 22-gauge needles producing the most disturbed SS in vivo. Klf2 mRNA and protein expression was diminished in the artery proximal to the fistula in proportion to increasing SS. Increased fistula diameter produces increased SS magnitude and frequency, consistent with disturbed SS in vivo. Disturbed SS is associated with decreased mRNA and protein expression of Klf2. Disturbed SS and reduced Klf2 expression near the fistula are potential therapeutic targets to improve AVF maturation.

Temporal regulation of venous extracellular matrix components during arteriovenous fistula maturation

PURPOSE

The venous limb of arteriovenous fistulae (AVF) adapts to the arterial environment by dilation and wall thickening; however, the temporal regulation of the expression of extracellular matrix (ECM) components in the venous limb of the maturing AVF has not been well characterized. We used a murine model of AVF maturation that recapitulates human AVF maturation to determine the temporal pattern of expression of these ECM components.

METHODS

Aortocaval fistulae were created in C57BL/6J mice and the venous limb was analyzed on postoperative days 1, 3, 7, 21, and 42. A gene microarray analysis was performed on day 7; results were confirmed by qPCR, histology, and immunohistochemistry. Proteases, protease inhibitors, collagens, glycoproteins, and other non-collagenous proteins were characterized.

RESULTS

The maturing AVF has increased expression of many ECM components, including increased collagen and elastin. Matrix metalloproteinases (MMPs) and tissue inhibitor of metalloproteinase 1 (TIMP1) showed increased mRNA and protein expression during the first 7 days of maturation. Increased collagen and elastin expression was also significant at day 7. Expression of structural proteins was increased later during AVF maturation. Osteopontin (OPN) expression was increased at day 1 and sustained during AVF maturation.

CONCLUSIONS

During AVF maturation, there is significantly increased expression of ECM components, each of which shows distinct temporal patterns during AVF maturation. Increased expression of regulatory proteins such as MMP and TIMP precedes increased expression of structural proteins such as collagen and elastin, potentially mediating a controlled pattern of ECM degradation and vessel remodeling without structural failure.