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.

Promoting Tropoelastin Expression in Arterial and Venous Vascular Smooth Muscle Cells and Fibroblasts for Vascular Tissue Engineering.

Elastin, critical for its structural and regulatory functions, is a missing link in vascular tissue engineering. Several elastin-inducting compounds have previously been reported, but their relative efficiency in promoting elastogenesis by adult arterial and venous vascular smooth muscle cells (VSMCs) and fibroblasts, four main vascular and elastogenic cells, has not been described. In addition to elasto-inductive substances, microRNA-29a was recently established as a potent post-transcriptional inhibitor of elastogenesis. Here, we explored if stimulating positive regulators or blocking inhibitors of elastogenesis could maximize elastin production. We tested whether the elasto-inducing compounds IGF-1, TGF-ß1, and minoxidil could indeed augment elastin production, and whether microRNA-29a antagonism could block elastin production in adult arterial and venous fibroblasts and VSMCs. The effects on elastin, lysyl oxidase, and fibrillin-1 mRNA expression levels and tropoelastin protein were determined. IGF-1 and minoxidil exerted little effect on tropoelastin mRNA expression levels in all cell types, while TGF-ß1 predominantly enhanced mRNA tropoelastin levels, but this mRNA increase did not impact tropoelastin protein abundance. In contrast, microRNA29a inhibition resulted in the upregulation of tropoelastin mRNA in all cell types, but most pronounced in venous VSMCs. Importantly, microRNA-29a-antagonism also enhanced lysyl oxidase and fibrillin-1 mRNA expression, and revealed a dose-dependent increase in tropoelastin protein expression in venous VSMCs. Our studies suggest that the elastogenic potential of microRNA-29a inhibition in vascular cells is superior to that of established elastin-stimulating compounds IGF-1, TGF-ß1, and minoxidil. Thus, microRNA-29a antagonism could serve as an attractive means of enhancing elastin synthesis in tissue-engineered blood vessels.

Development and evaluation of in vivo tissue engineered blood vessels in a porcine model.

BACKGROUND: There's a large clinical need for novel vascular grafts. Tissue engineered blood vessels (TEBVs) have great potential to improve the outcome of vascular grafting procedures. Here, we present a novel approach to generate autologous TEBV in vivo. Polymer rods were engineered and implanted, evoking an inflammatory response that culminates in encapsulation by a fibrocellular capsule. We hypothesized that, after extrusion of the rod, the fibrocellular capsule differentiates into an adequate vascular conduit once grafted into the vasculature. METHODS AND RESULTS: Rods were implanted subcutaneously in pigs. After 4 weeks, rods with tissue capsules grown around it were harvested. Tissue capsules were grafted bilaterally as carotid artery interposition. One and 4-week patency were evaluated by angiography whereupon pigs were sacrificed. Tissue capsules before and after grafting were evaluated on tissue remodeling using immunohistochemistry, RNA profiling and mechanical testing. Rods were encapsulated by thick, well-vascularized tissue capsules, composed of circumferentially aligned fibroblasts, collagen and few leukocytes, with adequate mechanical strength. Patency was 100% after 1 week and 87.5% after 4 weeks. After grafting, tissue capsules remodeled towards a vascular phenotype. Gene profiles of TEBVs gained more similarity with carotid artery. Wall thickness and αSMA-positive area significantly increased. Interestingly, a substantial portion of (myo)fibroblasts present before grafting expressed smooth muscle cell markers. While leukocytes were hardly present anymore, the lumen was largely covered with endothelial cells. Burst pressure remained stable after grafting. CONCLUSIONS: Autologous TEBVs were created in vivo with sufficient mechanical strength enabling vascular grafting. Grafts differentiated towards a vascular phenotype upon grafting.

Liposomal prednisolone inhibits vascular inflammation and enhances venous outward remodeling in a murine arteriovenous fistula model.

Arteriovenous fistulas (AVF) for hemodialysis access have a 1-year primary patency rate of only 60%, mainly as a result of maturation failure that is caused by insufficient outward remodeling and intimal hyperplasia. The exact pathophysiology remains unknown, but the inflammatory vascular response is thought to play an important role. In the present study we demonstrate that targeted liposomal delivery of prednisolone increases outward remodeling of the AVF in a murine model. Liposomes accumulate in the post-anastomotic area of the venous outflow tract in which the vascular pathology is most prominent in failed AVFs. On a histological level, we observed a reduction of lymphocytes and granulocytes in the vascular wall. In addition, a strong anti-inflammatory effect of liposomal prednisolone on macrophages was demonstrated in vitro. Therefore, treatment with liposomal prednisolone might be a valuable strategy to improve AVF maturation.

Candidate Gene Analysis of Mortality in Dialysis Patients.

BACKGROUND: Dialysis patients have high cardiovascular mortality risk. This study aimed to investigate the association between SNPs of genes involved in vascular processes and mortality in dialysis patients. METHODS: Forty two SNPs in 25 genes involved in endothelial function, vascular remodeling, cell proliferation, inflammation, coagulation and calcium/phosphate metabolism were genotyped in 1330 incident dialysis patients. The effect of SNPs on 5-years cardiovascular and non-cardiovascular mortality was investigated. RESULTS: The mortality rate was 114/1000 person-years and 49.4% of total mortality was cardiovascular. After correction for multiple testing, VEGF rs699947 was associated with all-cause mortality (HR1.48, 95% CI 1.14-1.92). The other SNPs were not associated with mortality. CONCLUSIONS: This study provides further evidence that a SNP in the VEGF gene may contribute to the comorbid conditions of dialysis patients. Future studies should unravel the underlying mechanisms responsible for the increase in mortality in these patients.

Tailoring the foreign body response for in situ vascular tissue engineering.

This study describes a screening platform for a guided in situ vascular tissue engineering approach. Polymer rods were developed that upon 3 weeks of subcutaneous implantation evoke a controlled inflammatory response culminating in encapsulation by a tube-shaped autologous fibrocellular tissue capsule, which can form a basis for a tissue-engineered blood vessel. Rods of co-polymer were produced using different ratios of poly(ethylene oxide terephthalate) and poly(butylene terephthalate) to create a range of physicochemical properties. In addition, a set of different physical, chemical, and biological surface modifications were tested on their ability to actively steer this tissue capsule formation using a rat model as testing platform. Tissue capsules were mainly composed of circumferentially aligned collagen and myofibroblasts. Different implant material resulted in distinct differences in tissue capsule formation. Compared to its unmodified counterparts, all surface modifications resulted in increased wall thickness, collagen, and myofibroblasts. Oxygen plasma-treated rods resulted in loose tissue arrangement, collagen, and collagen/TGF-ß-coated rods yielded thick, collagen-rich, densely packed tissue capsules, though with a random distribution of myofibroblasts. In contrast, chloroform-etched rods provided homogenous densely packed tissue capsules, completely populated by myofibroblasts. In conclusion, by varying the implant's surface characteristics, tissue capsule composition, cell distribution, and tissue arrangement could be tailored, enabling controlled guidance of the tissue response for in vivo vascular tissue engineering.

Towards an in vitro model mimicking the foreign body response: tailoring the surface properties of biomaterials to modulate extracellular matrix.

Despite various studies to minimize host reaction following a biomaterial implantation, an appealing strategy in regenerative medicine is to actively use such an immune response to trigger and control tissue regeneration. We have developed an in vitro model to modulate the host response by tuning biomaterials' surface properties through surface modifications techniques as a new strategy for tissue regeneration applications. Results showed tunable surface topography, roughness, wettability, and chemistry by varying treatment type and exposure, allowing for the first time to correlate the effect of these surface properties on cell attachment, morphology, strength and proliferation, as well as proinflammatory (IL-1ß, IL-6) and antiinflammatory cytokines (TGF-ß1, IL-10) secreted in medium, and protein expression of collagen and elastin. Surface microstructuring, derived from chloroform partial etching, increased surface roughness and oxygen content. This resulted in enhanced cell adhesion, strength and proliferation as well as a balance of soluble factors for optimum collagen and elastin synthesis for tissue regeneration. By linking surface parameters to cell activity, we could determine the fate of the regenerated tissue to create successful soft tissue-engineered replacement.