New Insights into Dialysis Vascular Access: Molecular

New Insights into Dialysis Vascular Access: Molecular Targets in Arteriovenous Fistula and Arteriovenous Graft Failure and Their Potential to Improve Vascular Access Outcomes Timmy Lee*† and Sanjay Misra‡

Abstract 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. Clin J Am Soc Nephrol 11: 1504–1512, 2016. doi: 10.2215/CJN.02030216

Introduction The vascular access is the lifeline for the hemodialysis patient. Despite technological improvements, vascular access dysfunction remains the Achilles heel of the hemodialysis procedure because of the high rates of arteriovenous fistula (AVF) maturation failure (1) and recurrent arteriovenous graft (AVG) stenosis (2), as well as the paucity of effective therapies for these two major problems. Recently, seminal discoveries from several investigators in this field have shed new insight into our understanding of the pathobiology of vascular access dysfunction. This article (1) reviews the present understanding of the pathophysiology of AVF and AVG dysfunction and discusses novel pathways and molecular mechanisms for these two disease processes; (2) examines in-depth an important population of cells involved in vascular access dysfunction, neointimal cells, which contribute to both AVF and AVG dysfunction; (3) highlights novel molecular targets that have potential for therapeutic translation; and (4) discusses novel technology and techniques that will further advance the fundamental understanding of vascular access dysfunction in AVF and AVG and to identify novel molecular targets for therapeutic development.

Vascular Remodeling and Neointimal Hyperplasia Development Outward Remodeling and AVF Maturation AVF maturation failure remains a significant clinical problem for hemodialysis patients. A large randomized controlled trial from the National Institutes of Health Dialysis Access Consortium study published in 1504

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2008 reported that 60% of AVFs created failed to mature for successful dialysis use at 4–5 months after creation (1). The precise mechanisms that lead to AVF maturation failure remain unclear but involve mediators that regulate outward remodeling and neointimal hyperplasia development (3,4). For an AVF to successfully mature after creation, several functional and structural adaptations to the inflow artery and outflow vein are essential (Figure 1). First, the low-resistance circuit resulting from creation of the AV anastomosis triggers an immediate increase in blood flow and wall shear stress (WSS) through the inflow artery (5–7). Second, these rapid increases in arterial blood flow and WSS stimulate an endothelium-dependent vasodilatory response in the artery and vein, mediated largely by nitric oxide (NO) and activation of matrix metalloproteinases (MMPs), that dilates both the inflow artery and outflow vein, restores WSS toward baseline, and inhibits neointimal hyperplasia development (7–11). Appropriate upregulation of MMPs results in matrix degradation and restructuring of the vascular scaffold, leading to luminal expansion (9,11,12). Finally, successful vascular remodeling restores WSS toward normal levels in a vessel with increased blood flow and helps maintain luminal diameter (6,13), a key hallmark of successful outward AVF remodeling. Additional pathways and mediators that may have a critical role in AVF maturation include heme oxygenase-1 and -2 (HO-1 and HO-2), monocyte chemoattractant protein-1 (MCP-1), Kruppel-like factor-2 (KLF-2), and TGF-b1 (Figure 1) (14). Several of these pathways and mediators are discussed in greater detail in the following sections.

*Department of Medicine and Division of Nephrology, University of Alabama at Birmingham, Birmingham, Alabama; †Veterans Affairs Medical Center, Birmingham, Alabama; and ‡ Vascular and Interventional Radiology Translational Laboratory, Department of Radiology, Mayo Clinic, Rochester, Minnesota Correspondence: Dr. Timmy Lee, Department of Medicine, Division of Nephrology, University of Alabama at Birmingham, 1720 2nd Avenue South, Birmingham, AL 35294-0007. Email: [email protected]

www.cjasn.org Vol 11 August, 2016

Clin J Am Soc Nephrol 11: 1504–1512, August, 2016

Molecular Targets and Vascular Access Dysfunction, Lee and Misra

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Figure 1 | Pathophysiologic events of successful arteriovenous fistula (AVF) maturation and AVF maturation failure. Left panel describes events of successful AVF maturation and right panel describes events of AVF maturation failure. Successful AVF maturation is dependent on outward vascular remodeling and inhibition of neointimal hyperplasia, regulated through nitric oxide production and appropriate regulation of matrix metalloproteinases. Fibroblast, smooth muscle cell, and myofibroblast activation, migration and proliferation play a key role in neointimal hyperplasia development and AVF maturation failure. Mediators such as heme-oxygenase-1 (HO-1), monocyte chemoattractant protein (MCP-1), kruppel-like factor-2 (KLF-2), transforming growth factor beta (TGF-b1), and high levels of local oxidant stress (e.g., peroxynitrite), play essential roles in regulating cellular proliferation and neointimal hyperplasia development.

Seminal studies by Nath and colleagues have provided new insight into AVF dysfunction through their work with HO-1 and HO-2 (15,16) and MCP-1 (17). First, HO-1, a mediator of inflammation and oxidative stress, plays a critical role in the adaptive vascular response to hemodynamic injury following AVF creation, by regulating inflammatory and extracellular matrix mediators, such as MCP-1 (15). HO-1 also provides a vasoprotective effect in states of pathologic shear stress (18). In AVFs created in HO-1 knockout mice, substantial venous neointimal hyperplasia occurs with marked expression of MCP-1, MMP-2, and MMP-9 (15). These investigators have also recently demonstrated that HO-1 upregulation by adenoassociated viral gene delivery improved AVF blood flow and reduced AVF venous wall thickness (19). Moreover, this group has also demonstrated that HO-2 plays an important role in maintaining AVF blood flow and function in murine AVF models (16). Second, MCP-1, an inflammatory mediator, which regulates chemotaxis of monocytes and macrophages, activation of endothelial cells, and proliferation of smooth muscle cells, plays a critical role in

AVF dysfunction (17). In AVFs created in MCP-1 knockout mice, AVF diameter was substantially increased and neointimal hyperplasia reduced (17), suggesting that MCP-1 is a significant mediator in AVF remodeling. Thus, therapies that can locally increase production of HO-1 and HO2 and inhibit pathways of MCP-1 activation may serve as potential therapeutic targets to improve AVF maturation. In a recent study, Misra and colleagues evaluated the role of simvastatin administered before AVF creation and its effect on AVF remodeling in a mouse AVF model (20). They reported that simvastatin reduced neointimal hyperplasia development and increased luminal AVF diameter through mechanisms involving decreased expression of MMP-9, MMP-2, and vascular endothelial growth factorA (VEGF-A) and reduction in smooth muscle cell activation, proliferation, and migration (20). Thus, simvastatin, if it can be delivered locally at the level of the AVF, could potentially have beneficial effects on outward AVF remodeling and neointimal hyperplasia development. The vascular endothelium plays an important role in AVF remodeling and maturation. The endothelium is the

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sensor of shear stress and changes in blood flow within a vessel, and damage to the endothelial cells can impair vessel vasodilation, normalization of WSS, and AVF remodeling (10,21). The endothelium exerts its effects on vascular remodeling by secreting important vasodilatory factors, such as NO, primarily through activation of NO synthase (NOS). A seminal study by Nath and colleagues (22) in rat AVFs administered NG-nitro-L-arginine methyl ester, an inhibitor of NOS, demonstrated that NOS inhibition decreased AVF blood flow, and increased venous neointimal hyperplasia and gene expression of proinflammatory molecules. Another seminal study from Cheng and colleagues have shown that the endothelial barrier is dysfunctional after AVF creation and exacerbated in the setting of CKD, leading to increased neointimal hyperplasia development (23). Furthermore, this group has also demonstrated that signaling of the Notch pathway may play an important role in endothelial barrier dysfunction (24,25). Finally, Nath and colleagues have shown that generation of oxidant stress occurs within the AVF after AVF creation (26). In a rat AVF model, they demonstrated increased production of superoxide anion within the AVF (26), which likely plays a role in impairing NO bioavailability. Thus, local therapies that improve “healing” of the endothelium and increase and restore NO bioavailability could serve as potential therapeutic targets to enhance AVF maturation.

Neointimal Hyperplasia in AVF Maturation Failure and AVG Dysfunction Venous stenosis is the most common angiographic lesion seen in both AVF maturation failure and AVG dysfunction (Figure 2) (3). Whereas outward AVF remodeling plays a

crucial role in AVF maturation and subsequent early failure, AVG failure results most commonly from development of venous stenosis secondary to aggressive neointimal hyperplasia formation. In seminal work in this field, Roy-Chaudhury and colleagues have shown that neointimal hyperplasia is the most common histologic lesion seen in AVF maturation failure (27) and AVG dysfunction (28). In AVFs and AVGs, the predominant cellular phenotype within the venous stenotic lesion are predominately a smooth muscle cell actin (SMA) (1) and vimentin (1) positive myofibroblasts with a minority of SMA (1) and desmin (1) contractile smooth muscle cells (27–29). In addition, in AVGs there is an active macrophage cell layer lining on the polytetrafluoroethylene graft itself (28,30). Several publications have demonstrated that neointimal hyperplasia is related to endothelial cell activation, activation, and infiltration of circulating inflammatory cells, along with production of chemokines and growth factors that regulate migration of smooth muscle cells and myofibroblasts into the intimal layer of the vessel (23,31). Moreover, several perivascularly delivered antiproliferative therapies have been evaluated in AVG (32,33), but none of these studies have advanced past early phases in clinical trials. Thus, future therapeutic targets may include therapies that inhibit smooth muscle cell and myofibroblasts activation, proliferation, and migration. The pattern of WSS acting on the endothelial cell may influence endothelial cell activation, structure, and function and neointimal hyperplasia development. Nonphysiologic WSS patterns have been linked to stenosis and venous neointimal hyperplasia development in AVF (34). Previous studies have demonstrated that endothelial cells exposed to pulsatile unidirectional WSS downregulate expression

Figure 2 | Histologic and angiographic lesions of venous stenosis in arteriovenous fistula (AVF) and arteriovenous graft (AVG). (A) Angiographic and (B) histologic features of AVF nonmaturation. Note aggressive venous neointimal hyperplasia (NH) at vein-artery anastomosis. (C) Angiographic and (D) histologic features of AVG stenosis. Note aggressive neointimal hyperplasia at graft–vein anastomosis. Arrows show the histologic features at the site of the angiographic venous stenosis in both AVF and AVG. Images are alpha-smooth muscle actin stain and magnification is 4X. Adapted and reprinted from reference 3, with permission.


of proinflammatory, pro-oxidant, proapoptotic genes, while low and oscillatory WSS induces expression of proinflammatory, pro-oxidant, and proapoptotic mediators that activate smooth muscle proliferation and migration (35–37). Recent work from Remuzzi and colleagues from an in vitro model of AVF have identified significant expression of KLF-2 in a unidirectional pulsatile flow model (38). In models using reciprocating flow, KLF-2 expression was not increased, but MCP-1 and IL-8 expression were increased with increased smooth muscle cell proliferation (38). Clinical studies have shown that adjusting the angle of the anastomoses at the time of the arteriovenous (AV) access creation may help positively modulate these WSS and hemodynamic changes (39). Moreover, a recent anastomotic connecting device, which allows controlling of the AV anastomotic angle to 30°–45° and possibly promoting a better WSS profile, has shown promising results with respect to improving AVF patency (40). In addition to vascular remodeling and neointimal hyperplasia development following vascular access creation, pre-existing vascular changes in arteries and veins and elasticity may also significantly contribute to vascular access failure. These topics and concepts are discussed in further detail in a separate article in this Moving Points feature (41).

Origins of Neointimal Cells in Vascular Access Dysfunction Several different sources of cells have been hypothesized to cause neointimal hyperplasia in AVF and AVG. These include cells that reside in the vessel wall found in the adventitia and media, cells infiltrating from the artery, and circulating cells arising from the bone marrow or inflammatory cells, such as monocytes. Experiments using animal models of hemodialysis vascular access failure have been used to identify the potential origin of cells contributing to the formation of the neointima. In 2005, in a porcine AVG model, Misra and colleagues demonstrated that the origin of the early cells causing venous stenosis formation after AVG placement resided in the adventitia and media (42). These results were corroborated by Cheung and colleagues, who showed in a porcine AVG model that adventitial fibroblasts begin to differentiate into myofibroblasts and contribute to neointimal cells (43). Roy-Chaudhury and colleagues have also demonstrated that myofibroblasts contribute to venous stenosis formation in a porcine AVF model (44). One recent study has suggested that the cells contributing to the neointima may arise from the adjacent arterial circulation (24). In this study, Cheng and colleagues performed an elegant experiment in a mouse AVF model (end-to-end anastomosis of carotid artery to the jugular vein) (24). Smooth muscle cells in the carotid arteries and not the jugular vein were labeled with green fluorescent protein to identify the source of smooth muscle cells, which contribute to venous stenosis formation (24). Nearly half of the cells were green fluorescent protein positive in the neointima, suggesting that smooth muscle cells migrate from the carotid artery to the jugular vein in AVFs created in these mice (24). However, another recent study has shown contradictory results and suggests that neointimal cells are derived from local resident cells in the


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venous limb of the AVF (45). The issue of cells originating from the bone marrow has also been explored. Castier et al. demonstrated, in chimeric mice receiving bone marrow from transgenic mice expressing the LacZ gene in smooth muscle cells (SM-LacZ), that bone marrow cells do not contribute to neointimal hyperplasia in AVF arteries (46). The role of other circulating cells, such as macrophages and monocytes, remains unknown. In the Misra laboratory, systemic depletion of monocytes has been performed using clodronate in a mouse AVF model (14). This procedure was associated with improved AVF remodeling and reduced circulating monocyte [Ly6C (1)] numbers, implying that these cells may also contribute to stenosis formation (14). Vazquez-Padron and colleagues have demonstrated in a seminal study in rodent AVF that the receptor tyrosine kinase c-Kit plays an important role in neointimal hyperplasia development (47). The presence of c-Kit may reflect progenitor cell activity in the vascular wall because this receptor tyrosine kinase is considered a marker for stem cell identification. The existence of stem cells in the AVF wall has also been suggested by Nath and colleagues (48), who described this type of cell as a component of adventitial microvessels in rodent AVF models. Moreover, blockade of c-Kit with imatinib mesylate (Gleevec, Novartis, Basel, Switzerland) and inhibition of stem cell factor production with a specific short hairpin RNA prevented venous neointimal hyperplasia in the outflow vein in this rodent AVF model by Vazquez-Padron and colleagues (47). Thus, inhibition of c-kit may serve as a potential target for pharmacologic therapy. Continued work to better characterize the origins of these neointimal cells will play a critical role in development and targeting of specific therapies to prevent neointimal hyperplasia development in both AVF and AVG.

Novel Technologies and Strategies to Unravel Vascular Access Dysfunction To elucidate in greater depth molecular mechanisms and the pathobiology of AVF and AVG dysfunction, animal models that accurately represent and recapitulate the disease process in AVF and AVG dysfunction are crucial. Animal models of AVF have been developed in both rodents (mice and rats) and large animals (pigs, rabbits, and sheep) to characterize the pathobiology of AV access failure and to design new therapeutic strategies aimed to improve durability and survival of these vascular conduits (49). Although large animals, such as pigs, have provided novel insight for device and therapeutic studies and for the initial understanding of the pathobiology of AVF and AVG dysfunction, murine models may have the greatest potential to provide more insight into the pathobiology of AVF failure because of the availability of transgenic mice. Several recent published studies from murine AVF models have evaluated specific knockout genes to understand specific mechanistic actions. Genes in knockout mice that have been evaluated include HO-1 and HO-2 (15,16), MCP-1 (17), intermediate early response gene X-1 (IEX-1) (50), and elastin (51). Other recent studies in murine models have evaluated inhibition of specific genes, such as E26 transformation–specific sequence-1 (52) and Notch-1 (25).

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All these studies have focused on novel pathways that lead to AVF dysfunction and neointimal hyperplasia development, and all may serve as potential therapeutic targets. Finally, to truly and faithfully recapitulate the environment where “clinical” vascular access are created, our preclinical models studying AV access dysfunction must be evaluated in the setting of CKD. In rodent and porcine models in which AVFs and AVGs were created in the setting of CKD, mediators of oxidative stress, inflammation, and endothelial dysfunction are exacerbated at the level of the AVF, displaying decreased AVF blood flow and accelerating neointimal hyperplasia development (19,22,23,53–56). “Omic”-based technology could provide important clues to unraveling the molecular mechanisms of AVF nonmaturation. These include genomics, proteomics, metabolomics, and transcriptomics technology. For example, genomic technology from high-throughput RNA sequencing has significantly advanced clinical fields with pathology similar to that of AVF nonmaturation, such as coronary artery disease and peripheral arterial disease. In coronary artery disease, genomic studies of cardiac tissue have provided valuable clues to identifying genes implicated in atherosclerosis, molecular mechanisms underlying this condition, distinct biomarkers for the condition (57–59), and expression signatures for progression of atherosclerosis (60). In peripheral artery disease, wholegene expression profile studies have revolutionized the understanding of the pathogenesis of atherosclerosis and have shown that immune and inflammatory responses and oxidative stress lead to the development of progressive atherosclerotic lesions (61).

In the vascular access field, few studies have used omicbased technology. Hall et al. in a murine model of AVF, performed a microarray analysis of the venous limb from the AVF and discovered significantly increased temporal expression of extracellular matrix components, including collagen and elastin, and regulatory proteins such as MMP and tissue inhibitors of metalloproteinases (62). Misra et al., in a proteomic analysis in a porcine CKD model of AVG, showed that increased expression of lactoferrin and fetuin-A were present in early venous stenosis by day 14 (63). Furthermore, omic-based technology could also be used in the context of studying cell-specific (e.g., endothelial and smooth muscle cells) expression and profiling. To date there are few published studies using omic-based technology in vascular access research, but this technology may serve as a platform to better elucidate pathobiology of AV access dysfunction in both human and animal studies. Finally, in addition to studying molecular mechanisms, omic-based technology may have the potential to personalize therapies for patients receiving a new vascular access. Imaging technology is also another powerful tool to better elucidate pathophysiology of AVF and AVG dysfunction. Magnetic resonance imaging and computed tomography have been used in both large-animal and clinical studies to evaluate neointimal hyperplasia development, lumen geometry, hemodynamics, and computational fluid dynamics (34,64–66). These imaging technologies would be even more powerful if they could be applied in transgenic rodent models where the combination of pathobiology and computational fluid dynamics modeling could be studied in greater depth. High-frequency ultrasound technology has been used in several small studies to evaluate serial

Figure 3 | Potential targets and therapies for hemodialysis vascular access dysfunction. This figure summarizes potential molecular targets and cell-based targets for therapies, and ongoing clinical trials evaluating novel therapies in arteriovenous fistula (AVF) and arteriovenous graft (AVG). HO-1, heme oxygenase-1; HO-2, heme-oxygenase-2; IEX-1, immediate early-response gene X-1; MCP-1, monocyte chemoattractant protein-1; MMP, matrix metalloproteinase; NO, nitric oxide; NOS, nitric oxide synthase; VEGF-A, vascular endothelial growth factor-A.


intima-medial changes and biomechanics in AVF development (67,68) in the clinical setting and has the potential to serve as a technology to evaluate serial neointimal hyperplasia development and elasticity in a developing AVF. Finally, three-dimensional ultrasonography, which has been used in other fields of vascular disease (69,70) (e.g., carotid atherosclerosis), could also provide a noninvasive method to assess hemodynamic and pathologic changes in AVF development and is potentially translatable to clinical care settings. In addition to identifying the appropriate molecular targets for development of therapies, the drug delivery system is critical because dialysis access dysfunction is primarily a local vascular injury at the anastomotic site. For therapies to treat dialysis access dysfunction to be effective, they must be delivered at a high enough concentration at the AV anastomosis. Recently, Jun and colleagues have developed a novel drug delivery system using nanotechnology allowing for attachment of drugs, gases, and molecules (e.g., insulin, islet cells, NO) using a biomimetic hybrid nanomatrix with the capability to control the time-dependent release profile and have incorporated this technology in cardiovascular devices and local delivery of novel therapies (71,72). This may be a potential technology to evaluate in dialysis access models. Future advances in our drug delivery systems will be essential in successfully delivering novel therapies directed at specific molecular targets.


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Potential Future Molecular Targets That May Translate into Therapeutics for Patients in the Near Future Several different potential therapies may improve clinical outcomes in hemodialysis vascular access failure (Figure 3). These can be divided into their mechanisms of action based on inflammation, oxidative stress, fibrosis, and endothelial dysfunction (several potential targets are discussed in preceding sections and below) (Figure 3). Recently, experimental studies have demonstrated that anti– VEGF-A therapy using bevacizumab treatment before the placement of experimental AVF is protective for formation of venous stenosis (Figure 4) (73). In a small group of patients who received intravitreal treatment of bevacizumab for macular degeneration before the placement of a hemodialysis vascular access, the Misra laboratory showed that the median patency was improved more than two-fold compared with control group (paper under review). A recently completed phase II clinical trial enrolled patients undergoing AVG creation who had perivascular biodegradable collagen wraps with sirolimus placed at the artery–graft and vein–graft anastomoses (33). This small study demonstrated safety and feasibility of the sirolimus wrap in AVGs (33). An ongoing phase III randomized controlled trial is assessing the safety, efficacy, and patency outcomes of a perivascular sirolimus-eluting implant placed at the AVF anastomosis in hemodialysis patients receiving new AVFs (NCT02513303). Through the use

Figure 4 | Novel adventitial delivery of vascular endothelial growth factor-A (VEGF-A) via lentivirus system. This figure is a representative example of a novel and local adventitial delivery system focused on inhibiting VEGF-A activity. (A) Anti–VEGF-A inhibitor administered at the adventitia of the arteriovenous fistula (AVF) anastomosis via a lentivirus. (B) therapy reduces tissue VEGF-A activity and matrix metalloproteinase (MMP). (C) Therapy inhibits proliferation and migration of fibroblasts and smooth muscle cells within the adventitia to media and intima layers of AVF. (D) neointimal hyperplasia is reduced in the AVF.

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of a similar concept, adipose-derived mesenchymal stem cells have been shown experimentally to reduce venous stenosis formation, and this technology is being proposed for use in a clinical trial (74). Finally, a small pilot study using adventitial delivery of calcitriol, a 1,25-dihydroxyvitamin D3 and inhibitor of Iex-1, at the time of angioplasty showed reduction in venous stenosis formation (75). Similar results have been observed experimentally in Iex-1 knockout mice with AVF (76). Perivascular elastase, administered directly over the AVF and AVG artery and vein anastomosis, has been studied in early-phase clinical trials. These early-phase studies of this drug to date have demonstrated safety and feasibility and have shown promising results with regard to patency outcomes (77–79). Given the promising results from early phase studies, an ongoing phase III randomized controlled clinical trial is evaluating perivascularly delivered elastase administered at the time of new radiocephalic AVF creation (NCT02110901). Finally, an ongoing randomized controlled clinical trial is evaluating locally administered nitroglycerin ointment as a therapy to enhance local NO bioavailability and improve AVF maturation (NCT02164318). The goal of this therapy is to restore, in patients with CKD and those with ESRD with planned AVF creation, local endothelial function in order to promote outward AVF remodeling and maturation.

Summary and Conclusions In recent years, significant progress has been made in our understanding of the pathophysiology of AVF and AVG dysfunction. Several novel mediators of oxidative stress, inflammation, fibrosis, and endothelial function that may serve as potential therapeutic targets have been identified. However, unfortunately, to date this has not translated into many robust clinical trials that have evaluated innovative and novel therapies (80) or resulted in effective therapies available for our hemodialysis patients. Further advancements that translate our understanding of neointimal hyperplasia development and vascular remodeling in hemodialysis access dysfunction into therapies will require multidisciplinary efforts that use the advancements from animal studies of AVF and AVG (22,53,63,81), use of innovative and cutting edge technology in areas such as genomics, high-resolution imaging, nanomaterials, and bioengineering, and collaborative efforts with the biomedical industry to invest in promising targets to develop these therapies. In a broader context, AVFs and AVGs may be the ideal clinical model to test future novel therapies for neointimal hyperplasia and vascular remodeling in other areas of vascular disease (e.g., coronary artery disease and bypass grafts, peripheral arterial disease, and postangioplasty restenosis) because of the superficial location of the vessels, the frequency of accessing the vessels, the aggressiveness of the vascular lesion, and the fact that hemodialysis patients are a captive audience because they undergo dialysis three times a week (4,82). Finally, although novel therapies may ultimately have the potential to improve patient vascular access outcomes, they should be incorporated in the future with a combination of good processes of care and a “patient-centered” approach for vascular access selection and placement.

Acknowledgments T.L. is supported by an American Society of Nephrology Carl W. Gottschalk Scholar grant, University of Alabama at Birmingham Nephrology Research Center Anderson Innovation Award, and University of Alabama at Birmingham Center for Clinical and Translational Science Multidisciplinary Pilot Award (1UL1TR00141701). S.M. is supported by a grant from the National Heart, Lung, and Blood Institute (HL098967). Disclosures T.L. is a consultant for Proteon Therapeutics, Waltham, MA. References 1. Dember LM, Beck GJ, Allon M, Delmez JA, Dixon BS, Greenberg A, Himmelfarb J, Vazquez MA, Gassman JJ, Greene T, Radeva MK, Braden GL, Ikizler TA, Rocco MV, Davidson IJ, Kaufman JS, Meyers CM, Kusek JW, Feldman HI; Dialysis Access Consortium Study Group: Effect of clopidogrel on early failure of arteriovenous fistulas for hemodialysis: A randomized controlled trial. JAMA 299: 2164–2171, 2008 2. Dixon BS, Beck GJ, Vazquez MA, Greenberg A, Delmez JA, Allon M, Dember LM, Himmelfarb J, Gassman JJ, Greene T, Radeva MK, Davidson IJ, Ikizler TA, Braden GL, Fenves AZ, Kaufman JS, Cotton JR Jr, Martin KJ, McNeil JW, Rahman A, Lawson JH, Whiting JF, Hu B, Meyers CM, Kusek JW, Feldman HI; DAC Study Group: Effect of dipyridamole plus aspirin on hemodialysis graft patency. N Engl J Med 360: 2191–2201, 2009 3. Lee T: Novel paradigms for dialysis vascular access: Downstream vascular biology–is there a final common pathway? Clin J Am Soc Nephrol 8: 2194–2201, 2013 4. Lee T, Roy-Chaudhury P: Advances and new frontiers in the pathophysiology of venous neointimal hyperplasia and dialysis access stenosis. Adv Chronic Kidney Dis 16: 329–338, 2009 5. Kamiya A, Togawa T: Adaptive regulation of wall shear stress to flow change in the canine carotid artery. Am J Physiol 239: H14– H21, 1980 6. Girerd X, London G, Boutouyrie P, Mourad JJ, Safar M, Laurent S: Remodeling of the radial artery in response to a chronic increase in shear stress. Hypertension 27: 799–803, 1996 7. Ben Driss A, Benessiano J, Poitevin P, Levy BI, Michel JB: Arterial expansive remodeling induced by high flow rates. Am J Physiol 272: H851–H858, 1997 8. Miller VM, Burnett JC Jr: Modulation of NO and endothelin by chronic increases in blood flow in canine femoral arteries. Am J Physiol 263: H103–H108, 1992 9. Tronc F, Mallat Z, Lehoux S, Wassef M, Esposito B, Tedgui A: Role of matrix metalloproteinases in blood flow-induced arterial enlargement: Interaction with NO. Arterioscler Thromb Vasc Biol 20: E120–E126, 2000 10. Tronc F, Wassef M, Esposito B, Henrion D, Glagov S, Tedgui A: Role of NO in flow-induced remodeling of the rabbit common carotid artery. Arterioscler Thromb Vasc Biol 16: 1256–1262, 1996 11. Chan CY, Chen YS, Ma MC, Chen CF: Remodeling of experimental arteriovenous fistula with increased matrix metalloproteinase expression in rats. J Vasc Surg 45: 804–811, 2007 12. Chang CJ, Chen CC, Hsu LA, Chang GJ, Ko YH, Chen CF, Chen MY, Yang SH, Pang JH: Degradation of the internal elastic laminae in vein grafts of rats with aortocaval fistulae: Potential impact on graft vasculopathy. Am J Pathol 174: 1837–1846, 2009 13. Dammers R, Tordoir JH, Kooman JP, Welten RJ, Hameleers JM, Kitslaar PJ, Hoeks AP: The effect of flow changes on the arterial system proximal to an arteriovenous fistula for hemodialysis. Ultrasound Med Biol 31: 1327–1333, 2005 14. Brahmbhatt A, Remuzzi A, Franzoni M, Misra S: The molecular mechanisms of hemodialysis vascular access failure. Kidney Int 89: 303–316, 2016 15. Juncos JP, Tracz MJ, Croatt AJ, Grande JP, Ackerman AW, Katusic ZS, Nath KA: Genetic deficiency of heme oxygenase-1 impairs functionality and form of an arteriovenous fistula in the mouse. Kidney Int 74: 47–51, 2008


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