Neoangiogenesis and the presence of progenitor cells in the venous limb of an arteriovenous fistula in the rat

Periadventitial biomaterials to improve arteriovenous fistula and graft outcomes

Periadventitial biomaterials have been employed for nearly three decades to promote adaptive venous remodeling following hemodialysis vascular access creation in preclinical models and clinical trials. These systems are predicated on the combination of scaffolds, hydrogels, and/or particles with therapeutics (small molecules, proteins, genes, and cells) to prevent venous stenosis and subsequent maturation failure. Periadventitial biomaterial therapies have evolved from simple drug delivery vehicles for traditional drugs to more thoughtful designs tailored to the pathophysiology of access failure. The emergence of tissue engineering strategies and gene therapies are another exciting new direction. Despite favorable results in experimental and preclinical studies, no periadventitial therapy has been clinically approved to improve vascular access outcomes. After conducting an exhaustive review of the literature, we identify the seminal studies and clinical trials that utilize periadventitial biomaterials and discuss the key features of each biomaterial format and their respective shortcomings as they pertain to access maturation. This review provides a foundation from which clinicians, surgeons, biologists, and engineers can refer to and will hopefully inspire thoughtful, translatable treatments to finally address access failure.

The rodent models of arteriovenous fistula

Arteriovenous fistulas (AVFs) have long been used as dialysis access in patients with end-stage renal disease; however, their maturation and long-term patency still fall short of clinical needs. Rodent models are irreplaceable to facilitate the study of mechanisms and provide reliable insights into clinical problems. The ideal rodent AVF model recapitulates the major features and pathology of human disease as closely as possible, and pre-induction of the uremic milieu is an important addition to AVF failure studies. Herein, we review different surgical methods used so far to create AVF in rodents, including surgical suturing, needle puncture, and the cuff technique. We also summarize commonly used evaluations after AVF placement. The aim was to provide recent advances and ideas for better selection and induction of rodent AVF models. At the same time, further improvements in the models and a deeper understanding of AVF failure mechanisms are expected.

Treatment with imatinib was useful to delay the neointimal hyperplasia of aortocaval fistula in adenine-induced renal failure rats

PURPOSE

The study was conducted to investigate the effect of the treatment with imatinib, a c-kit specific inhibitor, on the neointimal hyperplasia (NIH) of aortocaval fistula (ACF) in adenine-induced renal failure rats.

MATERIALS AND METHODS

All rats were randomly assigned to 4 groups: rats were fed on a normal diet (normal group); rats were fed on a 0.75% adenine-rich diet (renal failure group). The remaining rats underwent ACF after receiving a 0.75% adenine-rich diet and received daily saline gavage (model group) or imatinib gavage (imatinib group) for 7 days after surgery. Immunohistochemical method was used to detect c-kit expression, and Elastomeric Verhoeff-Van Gieson (EVG) staining was used to observe morphological changes of the ACF. The Pearson correlation analysis was used to evaluate the correlations of c-kit expression with intimal thickness and the percentage of stenosis, respectively.

RESULTS

The renal failure group showed positive c-kit expression on the intima of the inferior vena cava (IVC), whereas the normal group did not. Compared to the model group, intimal thickness (P = 0.001), the percentage of stenosis (P = 0.006) and c-kit expression (P = 0.04) were decreased in the imatinib group at 8 weeks postoperatively. C-kit expression was positively correlated with both intimal thickness and percentage of stenosis (intimal thickness: R = 0.650, P = 0.003; the percentage of stenosis: R = 0.581, P = 0.011) in both the model and imatinib groups.

CONCLUSION

Treatment with imatinib, a c-kit specific inhibitor, was useful to delay the NIH of ACF in adenine-induced renal failure rats.

Localized Perivascular Therapeutic Approaches to Inhibit Venous Neointimal Hyperplasia in Arteriovenous Fistula Access for Hemodialysis Use

An arteriovenous fistula (AVF) is the preferred vascular access for chronic hemodialysis, but high failure rates restrict its use. Optimizing patients' perioperative status and the surgical technique, among other methods for preventing primary AVF failure, continue to fall short in lowering failure rates in clinical practice. One of the predominant causes of AVF failure is neointimal hyperplasia (NIH), a process that results from the synergistic effects of inflammation, hypoxia, and hemodynamic shear stress on vascular tissue. Although several systemic therapies have aimed at suppressing NIH, none has shown a clear benefit towards this goal. Localized therapeutic approaches may improve rates of AVF maturation by providing direct structural and functional support to the maturating fistula, as well as by delivering higher doses of pharmacologic agents while avoiding the adverse effects associated with systemic administration of therapeutic agents. Novel materials-such as polymeric scaffolds and nanoparticles-have enabled the development of different perivascular therapies, such as supportive mechanical devices, targeted drug delivery, and cell-based therapeutics. In this review, we summarize various perivascular therapeutic approaches, available data on their effectiveness, and the outlook for localized therapies targeting NIH in the setting of AVF for hemodialysis use. Highlights: Most systemic therapies do not improve AVF patency outcomes; therefore, localized therapeutic approaches may be beneficial. Locally delivered drugs and medical devices may improve AVF patency outcomes by providing biological and mechanical support. Cell-based therapies have shown promise in suppressing NIH by delivering a more extensive array of bioactive substances in response to the biochemical changes in the AVF microenvironment.

Development of a murine iliac arteriovenous fistula model for examination of hemodialysis access-related limb pathophysiology

OBJECTIVE

Hemodialysis access-related hand dysfunction is a common clinical feature of patients with chronic kidney disease (CKD) after arteriovenous fistula (AVF) placement. The heterogeneity in symptoms and the lack of a predictive association with changes in hemodynamic alterations precipitated by the AVF suggest that other factors are involved in the mechanisms responsible for causing hand and limb dysfunction postoperatively. To the best of our knowledge, no suitable animal models have provided a platform for performing preclinical experiments designed to elucidate the biologic drivers of access-related hand dysfunction. Therefore, our objective was to develop a novel murine AVF model that could be used to study dialysis access-related limb dysfunction.

METHODS

Male 8-week-old C57BL/6J mice (n = 15/group) were exposed to either an adenine-supplemented diet to induce CKD or casein-based chow (control). Four weeks after the diet intervention, the mice were randomly assigned to receive an iliac AVF (n = 10/group) or sham surgery (n = 5/group) on the left hindlimb. The mice were sacrificed 2 weeks after surgery, and AVF specimens and hindlimb skeletal muscles were collected for further analysis.

RESULTS

Before AVF or sham surgery, the glomerular filtration rates were significantly reduced and the blood urea nitrogen levels were significantly elevated in the CKD groups compared with the controls (P < .05). AVF surgery was associated with an ∼80% patency rate among the survivors (four control and three CKD mice died postoperatively). Patency was verified by changes in hemodynamics using Doppler ultrasound imaging and altered histologic morphology. Compared with sham surgery, AVF surgery reduced ipsilateral hindlimb perfusion to the tibialis anterior muscle (20%-40%) and paw (40%-50%), which remained stable until euthanasia. Analysis of gastrocnemius muscle mitochondrial respiratory function uncovered a significant decrease (40%-50%) in mitochondrial function in the AVF mice. No changes were found in the muscle mass, myofiber cross-sectional area, or centrally nucleated fiber proportion in the extensor digitorum longus and soleus muscles between the sham and AVF mice.

CONCLUSIONS

The results from the present study have demonstrated that iliac AVF formation is a practical animal model that facilitates examination of hemodialysis access-related limb dysfunction. AVF surgery produced the expected hemodynamic changes, and evaluation of the limb muscle revealed a substantial mitochondrial impairment that was present without changes in muscle size.

Identification of the Crucial Gene in Overflow Arteriovenous Fistula by Bioinformatics Analysis

Objective: The aim was to study the preliminary screening of the crucial genes in intimal hyperplasia in the venous segment of arteriovenous (AV) fistula and the underlying potential molecular mechanisms of intimal hyperplasia with bioinformatics analysis. Methods: The gene expression profile data (GSE39488) was analyzed to identify differentially expressed genes (DEGs). We performed Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis of DEGs. Gene set enrichment analysis (GSEA) was used to understand the potential activated signaling pathway. The protein-protein interaction (PPI) network was constructed with the STRING database and Cytoscape software. The Venn diagram between 10 hub genes and gene sets of 4 crucial signaling pathways was used to obtain core genes and relevant potential pathways. Furthermore, GSEAs were performed to understand their biological functions. Results: A total of 185 DEGs were screened in this study. The main biological function of the 111 upregulated genes in AV fistula primarily concentrated on cell proliferation and vascular remodeling, and the 74 downregulated genes in AV fistula were enriched in the biological function mainly relevant to inflammation. GSEA found four signaling pathways crucial for intimal hyperplasia, namely, MAPK, NOD-like, Cell Cycle, and TGF-beta signaling pathway. A total of 10 hub genes were identified, namely, EGR1, EGR2, EGR3, NR4A1, NR4A2, DUSP1, CXCR4, ATF3, CCL4, and CYR61. Particularly, DUSP1 and NR4A1 were identified as core genes that potentially participate in the MAPK signaling pathway. In AV fistula, the biological processes and pathways were primarily involved with MAPK signaling pathway and MAPK-mediated pathway with the high expression of DUSP1 and were highly relevant to cell proliferation and inflammation with the low expression of DUSP1. Besides, the biological processes and pathways in AV fistula with the high expression of NR4A1 similarly included the MAPK signaling pathway and the pathway mediated by MAPK signaling, and it was mainly involved with inflammation in AV fistula with the low expression of NR4A1. Conclusion: We screened four potential signaling pathways relevant to intimal hyperplasia and identified 10 hub genes, including two core genes (i.e., DUSP1 and NR4A1). Two core genes potentially participate in the MAPK signaling pathway and might serve as the therapeutic targets of intimal hyperplasia to prevent stenosis after AV fistula creation.

The anatomical sources of neointimal cells in the arteriovenous fistula

Neointimal cells are an elusive population with ambiguous origins, functions, and states of differentiation. Expansion of the venous intima in arteriovenous fistula (AVF) is one of the most prominent remodeling processes in the wall after access creation. However, most of the current knowledge about neointimal cells in AVFs comes from extrapolations from the arterial neointima in non-AVF systems. Understanding the origin of neointimal cells in fistulas may have important implications for the design and effective delivery of therapies aimed to decrease intimal hyperplasia (IH). In addition, a broader knowledge of cellular dynamics during postoperative remodeling of the AVF may help clarify other transformation processes in the wall that combined with IH determine the successful remodeling or failure of the access. In this review, we discuss the possible anatomical sources of neointimal cells in AVFs and their relative contribution to intimal expansion.

Soluble vascular endothelial growth factor receptor-1 as a novel marker of arteriovenous fistula stenosis in hemodialysis patients

INTRODUCTION

Arteriovenous fistula (AVF) stenosis is one of the most important clinical problems in hemodialysis patients. The histopathological findings of neointimal hyperplasia and impaired angiogenesis have been well established in stenotic AVFs. Soluble vascular endothelial growth factor receptor-1 (sVEGFR-1) has been implicated in pathological angiogenesis. Thus, we aimed to investigate the association between sVEGFR-1 and AVF stenosis in hemodialysis patients.

METHODS

This prospective cohort study included 70 patients with end-stage renal disease. Forty-five patients were included in the final analysis, and the median follow-up period was 36 months. Venous stenosis was detected by physical examination and documented by fistulography. Blood samples were analyzed a day before the fistula operation, and serum levels of sVEGFR-1 were measured.

FINDINGS

The median sVEGFR-1 level was higher in the stenosis group than in the nonstenosis group (17 pg/mL [89.5%] vs. 5 pg/mL [19.2%], respectively; P < 0.001]. According to body mass index (BMI) categories, obese patients (BMI > 30 kg/m² ) had the shortest stenosis-free survival (20 months [9.35-30.65]). Multivariate Cox analysis showed that sVEGFR-1, serum creatinine, and parathyroid hormone levels were associated with AVF stenosis risk. Kaplan-Meier survival curves showed that patients with less than the median value of sVEGFR-1 (<6093.07 pg/mL) had longer cumulative stenosis-free survival than patients with sVEGFR-1 levels above the median value (P < 0.001).

DISCUSSION

Increased levels of sVEGFR-1 and obesity were found to be associated with AVF stenosis in hemodialysis patients.

Relationship of arteriovenous fistula stenosis and thrombosis with the platelet-lymphocyte ratio in hemodialysis patients

BACKGROUND

The platelet-lymphocyte ratio, which was reported to have a strong relationship with chronic inflammation and thrombosis, is a useful biomarker. The purpose of this study was to evaluate the relationship between the platelet-lymphocyte ratio, arteriovenous stenosis, and thrombosis in patients with chronic renal failure.

METHODS

Patients who were referred to our interventional radiology department due to arteriovenous fistula dysfunction from dialysis units between August 2015 and December 2018 were retrospectively reviewed. In the study, 95 patients with arteriovenous fistula access problems were included. Patients were divided into two groups: stenosis (n = 52) and thrombosis (n = 43). Thirty-six subjects with a patent left radiocephalic arteriovenous fistula proven by both color Doppler ultrasonography and clinically were added to the control group. Blood samples were obtained on the same day before the fistulography.

RESULTS

Platelet counts, lymphocyte counts, and platelet-lymphocyte ratio were found to be significantly different between the three groups. After the Bonferroni post hoc analysis, there was a significant difference between the stenosis and control group (p = 0.017), and the thrombosis and control group (p < 0.001) in terms of the platelet-lymphocyte ratio. No significant difference for any parameter was found between stenosis and thrombosis group.

CONCLUSION

High levels of the platelet-lymphocyte ratio may be a supportive finding of arteriovenous fistula stenosis and thrombosis and can be taken into consideration during hemodialysis-dependent patients' follow-up.

c-Kit suppresses atherosclerosis in hyperlipidemic mice

Atherosclerosis is the most common underlying cause of cardiovascular morbidity and mortality worldwide. c-Kit (CD117) is a member of the receptor tyrosine kinase family, which regulates differentiation, proliferation, and survival of multiple cell types. Recent studies have shown that c-Kit and its ligand stem cell factor (SCF) are present in arterial endothelial cells and smooth muscle cells (SMCs). The role of c-Kit in cardiovascular disease remains unclear. The aim of the current study is to determine the role of c-Kit in atherogenesis. For this purpose, atherosclerotic plaques were quantified in c-Kit-deficient mice (KitMut) after they were fed a high-fat diet (HFD) for 16 wk. KitMut mice demonstrated substantially greater atherosclerosis compared with control (KitWT) littermates (P < 0.01). Transplantation of c-Kit-positive bone marrow cells into KitMut mice failed to rescue the atherogenic phenotype, an indication that increased atherosclerosis was associated with reduced arterial c-Kit. To investigate the mechanism, SMC organization and morphology were analyzed in the aorta by histopathology and electron microscopy. SMCs were more abundant, disorganized, and vacuolated in aortas of c-Kit mutant mice compared with controls (P < 0.05). Markers of the "contractile" SMC phenotype (calponin, SM22α) were downregulated with pharmacological and genetic c-Kit inhibition (P < 0.05). The absence of c-Kit increased lipid accumulation and significantly reduced the expression of the ATP-binding cassette transporter G1 (ABCG1) necessary for lipid efflux in SMCs. Reconstitution of c-Kit in cultured KitMut SMCs resulted in increased spindle-shaped morphology, reduced proliferation, and elevated levels of contractile markers, all indicators of their restored contractile phenotype (P < 0.05).NEW & NOTEWORTHY This study describes the novel vasculoprotective role of c-Kit against atherosclerosis and its function in the preservation of the SMC contractile phenotype.

Deficiency of TLR4 homologue RP105 aggravates outward remodeling in a murine model of arteriovenous fistula failure

Arteriovenous access dysfunction is a major cause of morbidity for hemodialysis patients. The pathophysiology of arteriovenous fistula (AVF) maturation failure is associated with inflammation, impaired outward remodeling (OR) and intimal hyperplasia. RP105 is a critical physiologic regulator of TLR4 signaling in numerous cell types. In the present study, we investigated the impact of RP105 on AVF maturation, and defined cell-specific effects of RP105 on macrophages and vascular smooth muscle cells (VSMCs). Overall, RP105^−/− mice displayed a 26% decrease in venous OR. The inflammatory response in RP105^−/− mice was characterized by accumulation of anti-inflammatory macrophages, a 76% decrease in pro- inflammatory macrophages, a 70% reduction in T-cells and a 50% decrease in MMP-activity. In vitro, anti-inflammatory macrophages from RP105^−/− mice displayed increased IL10 production, while MCP1 and IL6 levels secreted by pro-inflammatory macrophages were elevated. VSMC content in RP105^−/− AVFs was markedly decreased. In vitro, RP105^−/− venous VSMCs proliferation was 50% lower, whereas arterial VSMCs displayed a 50% decrease in migration, relative to WT. In conclusion, the impaired venous OR in RP105^−/− mice could result from of a shift in both macrophages and VSMCs towards a regenerative phenotype, identifying a novel relationship between inflammation and VSMC function in AVF maturation.

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.

Responses of adventitial CD34+ vascular wall-resident stem/progenitor cells and medial smooth muscle cells to carotid injury in rats

Cell culture and carotid injury studies with SD rats were performed to investigate the roles of CD34⁺ vascular wall-resident stem/progenitor cells (VRS/Pcs) and vascular smooth muscle cells (SMCs) in neointimal formation. In vitro, the media-isolated SM MHC⁺ SMCs occupied 93.92±8.62% of total BrdU⁺ cells, whereas the CD34⁺ cells, only 2.61±0.82%, indicating that the cell expansion in SMC culture was attributed to SM MHC⁺ SMCs. The adventitia-isolated CD34⁺ VRS/Pcs responded to PDGF-BB by differentiating into SMC-like cells which expressed SM22α (an early stage SMC marker), but seldom SM MHC (a late stage SMC marker). In carotid injury model, the CD34⁺ VRS/Pcs differentiated SMC-like cells migrated in very few numbers into only the outer layer of the media, and this was further confirmed by a cell tracking analysis. While the neointimal cells were consistently SM MHC⁺ and CD34^- SMCs during whole course of the post-injury remodeling. Thus it is speculated that the adventitial CD34⁺ VRS/Pcs, at least in rat model, do not directly participate in neointimal formation, but function to maintain homeostasis of the media during injury-induced vascular wall remodeling.

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: Molecular Targets in Arteriovenous Fistula and Arteriovenous Graft Failure and Their Potential to Improve Vascular Access Outcomes

Vascular access dysfunction remains a major cause of morbidity and mortality in hemodialysis patients. At present there are few effective therapies for this clinical problem. The poor understanding of the pathobiology that leads to arteriovenous fistula (AVF) and graft (AVG) dysfunction remains a critical barrier to development of novel and effective therapies. However, in recent years we have made substantial progress in our understanding of the mechanisms of vascular access dysfunction. This article presents recent advances and new insights into the pathobiology of AVF and AVG dysfunction and highlights potential therapeutic targets to improve vascular access outcomes.

Establishment of a rat autogenous arteriovenous fistula model following 5/6 nephrectomy

The aim of this study was to establish a stable rat model of autogenous arteriovenous fistula (AVF) with chronic renal function insufficiency. Forty Sprague-Dawley rats were randomly divided into an experimental group (n=20) and sham surgery group (n=20) and a 5/6 nephrectomy model was established in the rats. One week later, in the experimental group, the ipsilateral caroid artery was inserted into the external jugular vein by a cuff technique to establish a carotid arteriovenous fistula; in the sham group, the external jugular vein and carotid artery were dissociated. At 7 and 28 days following the establishment of the AVF, the renal functions of the two groups were measured. Hematoxylin and eosin staining and double collagen and elastin staining were conducted to evaluate the area of intimal hyperplasia in the external jugular vein, and the expression of α-smooth muscle actin in the vein was investigated by immunohistochemistry. The creatinine and urea nitrogen levels in the experimental group at each time-point were significantly higher than those in the sham surgery group (P<0.05). The intimal hyperplasia of the external jugular vein of the experimental group was increased significantly compared with that in the sham group at each time-point (P<0.05). The model, which is easy to establish and simple to master, provides a new and feasible experimental method for the study of intimal hyperplasia associated with autogenous AVF in chronic renal insufficiency, and is worthy of wider use.

Migration of smooth muscle cells from the arterial anastomosis of arteriovenous fistulas requires Notch activation to form neointima

A major factor contributing to failure of arteriovenous fistulas (AVFs) is migration of smooth muscle cells into the forming neointima. To identify the source of smooth muscle cells in neointima, we created end-to-end AVFs by anastomosing the common carotid artery to the jugular vein and studied neural crest-derived smooth muscle cells from the carotid artery which are Wnt1-positive during development. In Wnt1-cre-GFP mice, smooth muscle cells in the carotid artery but not the jugular vein are labeled with GFP. About half of the cells were GFP-positive in the neointima indicating their migration from the carotid artery to the jugular vein in AVFs created in these mice. Since fibroblast-specific protein-1 (FSP-1) regulates smooth muscle cell migration, we examined FSP-1 in failed AVFs and polytetrafluoroethylene (PTFE) grafts from patients with ESRD or from AVFs in mice with chronic kidney disease. In smooth muscle cells of AVFs or PTFE grafts, FSP-1 and activation of Notch1 are present. In smooth muscle cells, Notch1 increased RBP-Jκ transcription factor activity and RBP-Jκ stimulated FSP-1 expression. Conditional knockout of RBP-Jκ in smooth muscle cells or general knockout of FSP-1, suppressed neointima formation in AVFs in mice. Thus, the artery of AVFs is the major source of smooth muscle cells during neointima formation. Knockout of RBP-Jκ or FSP-1 ameliorates neointima formation and might improve AVF patency during long-term follow up.

c-Kit signaling determines neointimal hyperplasia in arteriovenous fistulae

Stenosis of arteriovenous (A-V) fistulae secondary to neointimal hyperplasia (NIH) compromises dialysis delivery, which worsens patients' quality of life and increases medical costs associated with the maintenance of vascular accesses. In the present study, we evaluated the role of the receptor tyrosine kinase c-Kit in A-V fistula neointima formation. Initially, c-Kit was found in the neointima and adventitia of human brachiobasilic fistulae, whereas it was barely detectable in control veins harvested at the time of access creation. Using the rat A-V fistula model to study venous vascular remodeling, we analyzed the spatial and temporal pattern of c-Kit expression in the fistula wall. Interestingly, c-Kit immunoreactivity increased with time after anastomosis, which concurred with the accumulation of cells in the venous intima. In addition, c-Kit expression in A-V fistulae was positively altered by chronic kidney failure conditions. Both blockade of c-Kit with imatinib mesylate (Gleevec) and inhibition of stem cell factor production with a specific short hairpin RNA prevented NIH in the outflow vein of experimental fistulae. In agreement with these data, impaired c-Kit activity compromised the development of NIH in A-V fistulae created in c-KitW/Wv mutant mice. These results suggest that targeting of the c-Kit signaling pathway may be an effective approach to prevent postoperative NIH in A-V fistulae.

Smooth muscle cells from the anastomosed artery are the major precursors for neointima formation in both artery and vein grafts

Accumulation of smooth muscle cells (SMC) results in neointima formation in injured vessels. Two graft models consisting of vein and artery grafts were created by anastomosing common carotid arteries to donor vessels. To identify the origin of the neointima cells from anastomosed arteries, we use Wnt1-Cre/reporter mice to label and track SMCs in the common carotid artery. The contribution of SMCs in the neighboring arteries to neointima formation was studied. On evaluating the artery grafts after 1 month, >90 % of the labeled neointima cells were found to have originated from the anastomosing host arteries. Most of the neointima cells were also smooth muscle α-actin positive (SMA-α(+)) and expressed the smooth muscle myosin heavy chain (SMMHC), the SMC terminal differentiation marker. In vein grafts, about 60 % SMA-α-positive cells were from anastomosing arteries. Bone marrow cells did not contribute to neointima SMCs in vein grafts, but did co-stain with markers of inflammatory cells. Wnt1 expression was not detected in the neointima cells in the vein or artery grafts, or the injured femoral arteries. Neointima SMCs showed the synthetic phenotype and were positively labeled with BrdU in vitro and in vivo. Treatment with the IGF-1 receptor inhibitor suppressed SMC proliferation and neointima formation in vein grafts. Our results indicate that SMCs from the neighboring artery are predominantly present in the neointima formed in both vein and artery grafts and that Wnt1-Cre mice can be used to explore the role of SMCs originating from neighboring vessels in vascular remodeling.

Vein graft adaptation and fistula maturation in the arterial environment

Veins are exposed to the arterial environment during two common surgical procedures, creation of vein grafts and arteriovenous fistulae (AVF). In both cases, veins adapt to the arterial environment that is characterized by different hemodynamic conditions and increased oxygen tension compared with the venous environment. Successful venous adaptation to the arterial environment is critical for long-term success of the vein graft or AVF and, in both cases, is generally characterized by venous dilation and wall thickening. However, AVF are exposed to a high flow, high shear stress, low-pressure arterial environment and adapt mainly via outward dilation with less intimal thickening. Vein grafts are exposed to a moderate flow, moderate shear stress, high-pressure arterial environment and adapt mainly via increased wall thickening with less outward dilation. We review the data that describe these differences, as well as the underlying molecular mechanisms that mediate these processes. Despite extensive research, there are few differences in the molecular pathways that regulate cell proliferation and migration or matrix synthesis, secretion, or degradation currently identified between vein graft adaptation and AVF maturation that account for the different types of venous adaptation to arterial environments.

The transcription factor E26 transformation-specific sequence-1 mediates neointima formation in arteriovenous fistula

Hemodialysis vascular access dysfunction contributes to increased morbidity and mortality in hemodialysis patients. Arteriovenous fistula (AVF) is the preferred type of vascular access for hemodialysis but has high rates of dysfunction, in part because of excessive neointima formation. The transcription factor E26 transformation-specific sequence-1 (ETS-1) is a mediator of proinflammatory responses in hypertension and endovascular injury. We examined the role of ETS-1 in the formation of neointima in AVF. Right carotid artery to internal jugular vein fistulas were created in C57BL/6 mice and assigned to treatment with an ETS-1-dominant negative peptide (ETS-DN), an inactive mutant peptide (ETS-MU), or vehicle (n=6 per group). After 7 and 21 days, AVFs or contralateral internal jugular veins were processed for PCR, immunofluorescence, immunohistochemistry, and morphometry. In AVFs, ETS-1 mRNA increased 2.5-fold at 7 days and 4-fold at 21 days. By immunofluorescence, we confirmed increased expression of ETS-1 predominantly in the neointima and overlying endothelium. Similarly, ETS-1 expression increased in human AVFs compared with normal veins. In mice, ETS-DN, but not ETS-MU, reduced neointima formation at days 7 and 21 and reduced the expression of nitric oxide synthase 2, NADPH oxidase (NOX) 2, NOX4, E-selectin, and monocyte chemotactic protein-1. Shear stress increased ETS-1 phosphorylation in human umbilical vein cells in a NOX-dependent manner, demonstrating a role for reactive oxygen species in ETS-1 activation. These results unveil the role of ETS-1 as a mediator of neointima formation in AVF and may result in the development of novel strategies for the treatment of AVF dysfunction.

The mouse aortocaval fistula recapitulates human arteriovenous fistula maturation

Several models of arteriovenous fistula (AVF) have excellent patency and help in understanding the mechanisms of venous adaptation to the arterial environment. However, these models fail to exhibit either maturation failure or fail to develop stenoses, both of which are critical modes of AVF failure in human patients. We used high-resolution Doppler ultrasound to serially follow mice with AVFs created by direct 25-gauge needle puncture. By day 21, 75% of AVFs dilate, thicken, and increase flow, i.e., mature, and 25% fail due to immediate thrombosis or maturation failure. Mature AVF thicken due to increased amounts of smooth muscle cells. By day 42, 67% of mature AVFs remain patent, but 33% of AVFs fail due to perianastomotic thickening. These results show that the mouse aortocaval model has an easily detectable maturation phase in the first 21 days followed by a potential failure phase in the subsequent 21 days. This model is the first animal model of AVF to show a course that recapitulates aspects of human AVF maturation.

Objectives and design of the hemodialysis fistula maturation study

BACKGROUND

A large proportion of newly created arteriovenous fistulas cannot be used for dialysis because they fail to mature adequately to support the hemodialysis blood circuit. The Hemodialysis Fistula Maturation (HFM) Study was designed to elucidate clinical and biological factors associated with fistula maturation outcomes.

STUDY DESIGN

Multicenter prospective cohort study.

SETTING & PARTICIPANTS

Approximately 600 patients undergoing creation of a new hemodialysis fistula will be enrolled at 7 centers in the United States and followed up for as long as 4 years.

PREDICTORS

Clinical, anatomical, biological, and process-of-care attributes identified pre-, intra-, or postoperatively.

OUTCOMES

The primary outcome is unassisted clinical maturation, defined as successful use of the fistula for dialysis for 4 weeks without maturation-enhancing procedures. Secondary outcomes include assisted clinical maturation, ultrasound-based anatomical maturation, fistula procedures, fistula abandonment, and central venous catheter use.

MEASUREMENTS

Preoperative ultrasound arterial and venous mapping, flow-mediated and nitroglycerin-mediated brachial artery dilation, arterial pulse wave velocity, and venous distensibility; intraoperative vein tissue collection for histopathologic and molecular analyses; postoperative ultrasounds at 1 day, 2 weeks, 6 weeks, and prior to fistula intervention and initial cannulation.

RESULTS

Assuming complete data, no covariate adjustment, and unassisted clinical maturation of 50%, there will be 80% power to detect ORs of 1.83 and 1.61 for dichotomous predictor variables with exposure prevalences of 20% and 50%, respectively.

LIMITATIONS

Exclusion of 2-stage transposition fistulas limits generalizability. The requirement for study visits may result in a cohort that is healthier than the overall population of patients undergoing fistula creation.

CONCLUSIONS

The HFM Study will be of sufficient size and scope to: (1) evaluate a broad range of mechanistic hypotheses, (2) identify clinical practices associated with maturation outcomes, (3) assess the predictive utility of early indicators of fistula outcome, and (4) establish targets for novel therapeutic interventions to improve fistula maturation.

Technical aspects of the mouse aortocaval fistula

Technical aspects of creating an arteriovenous fistula in the mouse are discussed. Under general anesthesia, an abdominal incision is made, and the aorta and inferior vena cava (IVC) are exposed. The proximal infrarenal aorta and the distal aorta are dissected for clamp placement and needle puncture, respectively. Special attention is paid to avoid dissection between the aorta and the IVC. After clamping the aorta, a 25 G needle is used to puncture both walls of the aorta into the IVC. The surrounding connective tissue is used for hemostatic compression. Successful creation of the AVF will show pulsatile arterial blood flow in the IVC. Further confirmation of successful AVF can be achieved by post-operative Doppler ultrasound.

Arteriovenous access failure: more than just intimal hyperplasia?

Haemodialysis vascular access patency is severely compromised by fistula non-maturation and access stenosis. Intimal hyperplasia (IH) is considered the culprit lesion in failed fistulas, resulting in luminal narrowing and stenosis. This review focuses on the biology and pathophysiology of fistula failure and highlights not only the classically associated IH but also some relatively neglected but potentially important contributors such as inadequate outward remodelling. In addition, the complex process and fragile balance of successful fistula maturation might be partially hindered by pre-existent chronic kidney disease-mediated vasculopathy. Further unravelling the (patho)physiology of outward remodelling and IH could contribute to novel therapies and enhance fistula patency.

Sulodexide may alleviate neointimal hyperplasia by inhibiting angiopoietin‑2 in an arteriovenous fistula model

The present study was undertaken to confirm whether sulodexide aleviates neointimal hyperplasia by regulating angiopoietin/Tie in a rat femoral arteriovenous fistula (AVF) model. Sprague Dawley rats were divided into four groups: sham, model, treatment and treatment control. An arteriovenous shunt model was created in the model and treatment groups. Sulodexide was subcutaneously administered (10 mg/kg/day) 6 times per week for 8 weeks in the treatment and treatment control groups. Histology and immunofluorescence were analyzed and the protein expression of angiopoietin‑1, angiopoietin‑2, Tie‑2, p‑ERK and total‑ERK were tested by ELISA and/or western blotting after 8 weeks. HE staining revealed that sulodexide was able to partially alleviate intimal hyperplasia of remodeled veins in the AVF model. Additionally, sulodexide was able to decrease angiopoietin‑2 and Tie‑2 expression while increasing angiopoietin‑1 expression in AVF tissue. Sulodexide was also able to decrease ERK phosphorylation which was increased in the model. Serum levels of soluble Tie-2 (sTie‑2) were also significantly decreased by sulodexide compared with the model. Immunofluorescent analysis also confirmed that sulodexide was able to decrease angiopoietin‑2 expression, possibly partially by inhibiting endothelial cell proliferation. Sulodexide may alleviate venous intimal hyperplasia by regulating the angiopoietin/Tie system, which may play a significant role in assisting remodeled veins to cope with their new biomechanical environment, but whether the angiopoietin/Tie system is beneficial or not requires further study.

The peritoneum as a natural scaffold for vascular regeneration

Objective

The peritoneum has the same developmental origin as blood vessels, is highly reactive and poorly thrombogenic. We hypothesize that parietal peritoneum can sustain development and regeneration of new vessels.

Methods and Results

The study comprised two experimental approaches. First, to test surgical feasibility and efficacy of the peritoneal vascular autograft, we set up an autologous transplantation procedure in pigs, where a tubularized parietal peritoneal graft was covered with a metal mesh and anastomosed end-to-end in the infrarenal aorta. Second, to dissect the contribution of graft vs host cells to the newly developed vessel wall, we performed human-to-rat peritoneal patch grafting in the abdominal aorta and examined the origin of endothelial and smooth muscle cells. In pig experiments, the graft remodeled to an apparently normal blood vessel, without thrombosis. Histology confirmed arterialization of the graft with complete endothelial coverage and neointimal hyperplasia in the absence of erosion, inflammation or thrombosis. In rats, immunostaining for human mitochondri revealed that endothelial cells and smooth muscle cells rarely were of human origin. Remodeling of the graft was mainly attributable to local cells with no clear evidence of c-kit+ endothelial progenitor cells or c-kit+ resident perivascular progenitor cells.

Conclusions

The parietal peritoneum can be feasibly used as a scaffold to sustain the regeneration of blood vessels, which appears to occur through the contribution of host-derived resident mature cells.

FSP-1 Silencing in Bone Marrow Cells Suppresses Neointima Formation in Vein Graft

Rationale:

Fibroblast-specific protein 1 (FSP-1) plays multiple roles in promoting cell proliferation and motility. Increased FSP-1 expression in smooth muscle cells (SMCs) has been associated with their enhanced proliferation.

Objective:

To study how FSP-1 contributes to neointima formation of vein grafts.

Methods:

Arteriovenous grafts were created in wild-type or FSP-1–GFP mice (green fluorescent protein expression regulated by FSP-1 promoter). The effects of FSP-1 on bone marrow (BM) cell migration and on SMC proliferation were studied in vivo and in vitro.

Results:

On creation of a vein graft, there was rapid deposition of platelets on the denuded surface leading to secretion of the chemokine stromal cell–derived factor-1α (SDF-1α). This was followed by recruitment of BM-derived cells expressing the SDF-1α receptor CXCR4; homing of FSP-1–positive cells was found to be dependent on platelet-derived SDF-1α. FSP-1 was expressed in 8% of the BM cells, and 20% of these express CD45; 85% of FSP-1–positive cells express CD11b. We found that the FSP-1–positive cells migrated into the vein graft in a Rac-1–dependent fashion. FSP-1 expression was also found to stimulate proliferation of SMCs through a MEK5-ERK5 signaling pathway that can be suppressed by a dominant-negative Rac1. Consequently, knocking down FSP-1 expression in BM cells prevented neointimal formation.

Conclusions:

BM-derived FSP-1 + cells enhance neointima formation through an increase in transendothelial invasion with stimulation of SMC proliferation. The Rac1 and ERK5 signaling cascade mediate FSP-1–induced responses in SMCs and BM cells. This novel pathophysiology suggests a new therapeutic target, FSP-1, for preventing the development of neointima in vein grafts.

A new arteriovenous fistula model to study the development of neointimal hyperplasia

This study describes an alternative arteriovenous fistula (AVF) model in the rat in which the animals develop significant neointimal hyperplasia (NIH) not only at the distal anastomotic site, but also throughout the fistula body. This aortocaval fistula was established by anastomosing the distal end of the renal vein to the abdominal aorta after unilateral nephrectomy. The increased hemodynamic stress resulting from exposing the renal vein to the arterial circulation induced venous NIH as early as 7 days after surgery. This experimental AVF was characterized by the early lack of endothelium, the accumulation of proliferating vascular smooth muscle cells and the neovascularization of the fistula adventitia. In summary, we have described an informative animal model to study the pathobiology of NIH in native AVF.

An early study on the mechanisms that allow tissue-engineered vascular grafts to resist intimal hyperplasia

Intimal hyperplasia is one of the prominent failure mechanisms for arteriovenous fistulas and arteriovenous access grafts. Human tissue-engineered vascular grafts (TEVGs) were implanted as arteriovenous grafts in a novel baboon model. Ultrasound was used to monitor flow rates and vascular diameters throughout the study. Intimal hyperplasia in the outflow vein of TEVGs was assessed at the anastomosis and at juxta-anastomotic regions via histological analysis, and was compared to intimal hyperplasia with polytetrafluoroethylene (PTFE) grafts in the baboon model and in literature reports from other animal models. Less venous intimal hyperplasia was observed in histological sections with arteriovenous TEVGs than with arteriovenous PTFE grafts. TEVGs were associated with a mild, noninflammatory intimal hyperplasia. The extent of intimal tissue that formed with TEVG placement correlated with the rate of blood flow through tissue engineered vascular grafts at 2 weeks postimplant. Outflow vein dilatation was observed with increased flow rate. Both mid-graft flow rates and outflow vein diameters reached a plateau by week 4, which suggested that venous remodeling and intimal hyperplasia largely occurred within the first 4 weeks of implant in the baboon model. Given their compliant and noninflammatory nature, TEVGs appear resistant to triggers for venous intimal hyperplasia that are common for PTFE arteriovenous grafts, including (1) abundant proinflammatory macrophage populations that are associated with PTFE grafts and (2) compliance mismatch between PTFE grafts and the outflow vein. Our findings suggest that arteriovenous TEVGs develop only a mild form of venous intimal hyperplasia, which results from the typical hemodynamic changes that are associated with arteriovenous settings.

Characterization of a model of an arteriovenous fistula in the rat: the effect of L-NAME

Vascular access dysfunction contributes to the mortality of patients undergoing chronic hemodialysis. The present study analyzed the changes that evolve in a femoral arteriovenous fistula in the rat. The venous segment of this model exhibited, at 1 week, activation of pro-inflammatory transcription factors and up-regulation of pro-inflammatory, proliferative, procoagulant, and profibrotic genes; and at 4 weeks, the venous segment displayed neointimal hyperplasia, smooth muscle proliferation, and thrombus formation. These changes were accompanied by endothelial (e) nitric oxide synthase (NOS) and inducible (i) NOS up-regulation. The administration of NG-nitro-L-arginine methyl ester, an inhibitor of NOS activity, increased venous neointimal hyperplasia and pro-inflammatory gene expression (monocyte chemoattractant protein-1 and cytokine-induced neutrophil chemoattractant-1), increased systolic blood pressure, and decreased blood flow through the fistula. In another hypertensive model, the rat subtotal nephrectomy model, venous neointimal hyperplasia in the arteriovenous fistula was also exacerbated. We conclude that this arteriovenous fistula model recapitulates the salient features observed in dysfunctional, hemodialysis arteriovenous fistulas, and that venous neointimal hyperplasia is exacerbated when this model is superimposed in two different models of systemic hypertension. Since the uremic milieu contains increased amounts of asymmetric dimethylarginine, we speculate that such accumulation of this endogenous inhibitor of NOS, by virtue of its pressor or nitric oxide-depleting effects, or a combination thereof, may contribute to the limited longevity of arteriovenous fistulas used for hemodialysis.

Advances and new frontiers in the pathophysiology of venous neointimal hyperplasia and dialysis access stenosis

Hemodialysis vascular access dysfunction is a major cause of morbidity and mortality in hemodialysis patients. The most common cause of this vascular access dysfunction is venous stenosis as a result of venous neointimal hyperplasia within the perianastomotic region (arteriovenous fistula) or at the graft-vein anastomosis (polytetrafluoroethylene, or PTFE, grafts). There have been few effective treatments to date for venous neointimal hyperplasia, in part, because of the poor understanding of the pathogenesis of venous neointimal hyperplasia. Therefore, this article will (1) describe the pathology of hemodialysis access stenosis in arteriovenous fistulas and grafts, (2) review and describe both current and novel concepts in the pathogenesis of neointimal hyperplasia formation, (3) discuss current and future novel therapies for treating venous neointimal hyperplasia, and (4) suggest future research areas in the field of hemodialysis vascular access dysfunction.

Cellular phenotypes in human stenotic lesions from haemodialysis vascular access

BACKGROUND

Haemodialysis vascular access dysfunction (due to venous stenosis and thrombosis) is a leading cause of hospitalization and morbidity. The aim of the current study was to identify the specific cell types present within stenotic tissue samples from patients with AV fistula and graft failure.

METHODS

Discarded tissue segments were collected from the stenotic portions (usually near the graft-vein anastomosis or the AV anastomosis) of 23 dialysis grafts and 20 AV fistulae, and examined for expression of smooth muscle alpha actin, desmin, vimentin and a macrophage marker.

RESULTS

The majority of cells within the venous neointima (both grafts and fistulae) were myofibroblasts, with a smaller number of desmin positive smooth muscle cells. The graft neointima had a similar cellular phenotype, albeit without any desmin positive contractile smooth muscle cells. The majority of cells within the PTFE graft material were macrophages. Analysis of sequential sections revealed the presence of fibroblasts within the venous neointima and intragraft region.

CONCLUSIONS

Our results demonstrate that contractile smooth muscle cells, myofibroblasts, fibroblasts and macrophages all play a role in the pathogenesis of dialysis access dysfunction (grafts and fistulae). Targeting these specific cell types might result in the development of novel therapeutic paradigms for haemodialysis vascular access dysfunction.