Lu, Kang

The arteriovenous fistula (AVF) is the preferred hemodialysis vascular access, but it is complicated by high failure rates and attendant morbidity. This study provides the first description of a murine AVF model that recapitulates two salient features of hemodialysis AVFs, namely, anastomosis of end-vein to side-artery to create the AVF and the presence of chronic kidney disease (CKD). CKD reduced AVF blood flow, observed as early as 3 days after AVF creation, and increased neointimal hyperplasia, venous wall thickness, thrombus formation, and vasculopathic gene expression in the AVF. These adverse effects of CKD could not be ascribed to preexisting alterations in blood pressure or vascular reactivity in this CKD model. In addition to vasculopathic genes, CKD induced potentially vasoprotective genes in the AVF such as heme oxygenase-1 (HO-1) and HO-2. To determine whether prior HO-1 upregulation may protect in this model, we upregulated HO-1 by adeno-associated viral gene delivery, achieving marked venous induction of the HO-1 protein and HO activity. Such HO-1 upregulation improved AVF blood flow and decreased venous wall thickness in the AVF. Finally, we demonstrate that the administration of carbon monoxide, a product of HO, acutely increased AVF blood flow. This study thus demonstrates: 1) the feasibility of a clinically relevant murine AVF model created in the presence of CKD and involving an end-vein to side-artery anastomosis; 2) the exacerbatory effect of CKD on clinically relevant features of this model; and 3) the beneficial effects in this model conferred by HO-1 upregulation by adeno-associated viral gene delivery.

The present study examined the heme oxygenase (HO) system in an in vivo murine model of pathological shear stress induced by partial carotid artery ligation. In this model, along with upregulation of vasculopathic genes, HO-1 is induced in the endothelium and adventitia, whereas HO-2 is mainly upregulated in the endothelium. Within minutes of ligation, NF-κB, a transcription factor that upregulates vasculopathic genes and HO-1, is activated. Failure to express either HO-1 or HO-2 exaggerates the reduction in carotid blood flow and exacerbates vascular injury. After artery ligation, comparable induction of HO-2 occurred in HO-1+/+and HO-1−/−mice, whereas HO-1 induction was exaggerated in HO-2−/−mice compared with HO-2+/+mice. Upregulation of HO-1 by an adeno-associated viral vector increased vascular HO-1 expression and HO activity and augmented blood flow in both ligated and contralateral carotid arteries. Acute inhibition of HO activity decreased flow in the ligated carotid artery, whereas a product of HO, carbon monoxide (CO), delivered by CO-releasing molecule-3, increased carotid blood flow. In conclusion, in the partial carotid artery ligation model of pathological shear stress, this study provides the first demonstration of 1) upregulation and vasoprotective effects of HO-1 and HO-2 and the vasorelaxant effects of CO as well as 2) vascular upregulation of HO-1 in vivo by an adeno-associated viral vector that is attended by a salutary vascular response. Induction of HO-1 may reside in NF-κB activation, and, along with induced HO-2, such upregulation of HO-1 provides a countervailing vasoprotective response in pathological shear stress in vivo.

Heme oxygenase-2 (HO-2), the constitutive isoform of the heme-degrading enzyme heme oxygenase, may serve as an anti-inflammatory vasorelaxant, in part, by generating carbon monoxide. Arteriovenous fistulas (AVFs) are employed as hemodialysis vascular accesses because they provide an accessible, high-blood-flow vascular segment. We examined the role of vascular expression of HO-2 in AVF function. An AVF was created in mice by anastomosing the carotid artery to the jugular vein. HO-2 expression was detected by immunohistochemistry in the intact carotid artery, mainly in endothelial cells and smooth muscle cells; expression of HO-2 protein and mRNA was modestly increased in the artery of the AVF. Creating an AVF in HO-2−/−mice compared with an AVF in HO-2+/+mice led to markedly reduced AVF blood flow and increased numbers of nonfunctioning AVFs. The impairment of AVF function in the setting of HO-2 deficiency could not be ascribed to either preexisting intrinsic abnormalities in endothelium-dependent and endothelium-independent relaxation of the carotid artery in HO-2-deficient mice or to impaired vasorelaxant responses in the intact carotid artery in vivo. HO-1 mRNA was comparably induced in the AVF in HO-2+/+and HO-2−/−mice, whereas the AVF in HO-2−/−mice compared with that in HO-2+/+mice exhibited exaggerated induction of matrix metalloproteinase (MMP)-9 but similar induction of MMP-2. HO-2 deficiency also led to lower AVF blood flow when AVFs were created in uremia, the latter induced by subtotal nephrectomy. We conclude that HO-2 critically contributes to the adequacy of AVF blood flow and function.

The study of hemodynamic alterations following the creation of an arteriovenous fistula (AVF) is relevant to vascular adaptive responses and hemodialysis access dysfunction. This study examined such alterations in a murine AVF created by anastomosing the carotid artery to the jugular vein. AVF blood flow was markedly increased due to reduced AVF vascular resistance. Despite such markedly increased basal blood flow, AVF blood flow further increased in response to acetylcholine. This AVF model exhibited increased cardiac output and decreased systemic vascular resistance; the kidney, in contrast, exhibited decreased blood flow and increased vascular resistance. Augmentation in AVF blood flow was attended by increased arterial heme oxygenase-1 (HO-1) mRNA and protein expression, the latter localized to smooth muscle cells of the AVF artery; AVF blood flow was substantially reduced in HO-1−/− mice compared with HO-1+/+ mice. Finally, in a murine model of a representative disease known to exhibit impaired hemodynamic responses (sickle cell disease), the creation of an AVF was attended by decreased AVF flow and impaired AVF function. We conclude that this AVF model exhibits markedly increased AVF blood flow, a vasodilatory reserve capacity, increased cardiac output, decreased renal blood flow, and a dependency on intact hemodynamic responses, in general, and HO-1 expression, in particular, in achieving and maintaining AVF blood flow. We suggest that these findings support the utility of this model in investigating the basis for and the consequences of hemodynamic stress, including shear stress, and the pathobiology of hemodialysis AVF dysfunction.