Extracellular RNA released due to shear stress controls natural bypass growth by mediating mechanotransduction in mice.

Fluid shear stress in the vasculature is the driving force for natural bypass growth, a fundamental endogenous mechanism to counteract the detrimental consequences of vascular occlusive disease, such as stroke or myocardial infarction. This process, referred to as "arteriogenesis," relies on local recruitment of leukocytes, which supply growth factors to preexisting collateral arterioles enabling them to grow. Although several mechanosensing proteins have been identified, the series of mechanotransduction events resulting in local leukocyte recruitment is not understood. In a mouse model of arteriogenesis (femoral artery ligation), we found that endothelial cells release RNA in response to increased fluid shear stress and that administration of RNase inhibitor blocking plasma RNases improved perfusion recovery. In contrast, treatment with bovine pancreatic RNase A or human recombinant RNase1 interfered with leukocyte recruitment and collateral artery growth. Our results indicated that extracellular RNA (eRNA) regulated leukocyte recruitment by engaging vascular endothelial growth factor receptor 2 (VEGFR2), which was confirmed by intravital microscopic studies in a murine cremaster model of inflammation. Moreover, we found that release of von Willebrand factor (VWF) as a result of shear stress is dependent on VEGFR2. Blocking VEGFR2, RNase application, or VWF deficiency interfered with platelet-neutrophil aggregate formation, which is essential for initiating the inflammatory process in arteriogenesis. Taken together, the results show that eRNA is released from endothelial cells in response to shear stress. We demonstrate this extracellular nucleic acid as a critical mediator of mechanotransduction by inducing the liberation of VWF, thereby initiating the multistep inflammatory process responsible for arteriogenesis.

Midkine Controls Arteriogenesis by Regulating the Bioavailability of Vascular Endothelial Growth Factor A and the Expression of Nitric Oxide Synthase 1 and 3.

Midkine is a pleiotropic factor, which is involved in angiogenesis. However, its mode of action in this process is still ill defined. The function of midkine in arteriogenesis, the growth of natural bypasses from pre-existing collateral arteries, compensating for the loss of an occluded artery has never been investigated. Arteriogenesis is an inflammatory process, which relies on the proliferation of endothelial cells and smooth muscle cells. We show that midkine deficiency strikingly interferes with the proliferation of endothelial cells in arteriogenesis, thereby interfering with the process of collateral artery growth. We identified midkine to be responsible for increased plasma levels of vascular endothelial growth factor A (VEGFA), necessary and sufficient to promote endothelial cell proliferation in growing collaterals. Mechanistically, we demonstrate that leukocyte domiciled midkine mediates increased plasma levels of VEGFA relevant for upregulation of endothelial nitric oxide synthase 1 and 3, necessary for proper endothelial cell proliferation, and that non-leukocyte domiciled midkine additionally improves vasodilation. The data provided on the role of midkine in endothelial proliferation are likely to be relevant for both, the process of arteriogenesis and angiogenesis. Moreover, our data might help to estimate the therapeutic effect of clinically applied VEGFA in patients with vascular occlusive diseases.

Perivascular Mast Cells Govern Shear Stress-Induced Arteriogenesis by Orchestrating Leukocyte Function.

The body has the capacity to compensate for an occluded artery by creating a natural bypass upon increased fluid shear stress. How this mechanical force is translated into collateral artery growth (arteriogenesis) is unresolved. We show that extravasation of neutrophils mediated by the platelet receptor GPIbα and uPA results in Nox2-derived reactive oxygen radicals, which activate perivascular mast cells. These c-kit(+)/CXCR-4(+) cells stimulate arteriogenesis by recruiting additional neutrophils as well as growth-promoting monocytes and T cells. Additionally, mast cells may directly contribute to vascular remodeling and vascular cell proliferation through increased MMP activity and by supplying growth-promoting factors. Boosting mast cell recruitment and activation effectively promotes arteriogenesis, thereby protecting tissue from severe ischemic damage. We thus find that perivascular mast cells are central regulators of shear stress-induced arteriogenesis by orchestrating leukocyte function and growth factor/cytokine release, thus providing a therapeutic target for treatment of vascular occlusive diseases.

Ribonuclease (RNase) Prolongs Survival of Grafts in Experimental Heart Transplantation.

BACKGROUND: Cell damage, tissue and vascular injury are associated with the exposure and release of intracellular components such as RNA, which promote inflammatory reactions and thrombosis. Based on the counteracting anti-inflammatory and cardioprotective functions of ribonuclease A (RNase A) in this context, its role in an experimental model of heart transplantation in rats was studied. METHODS AND RESULTS: Inbred BN/OrlRj rat cardiac allografts were heterotopically transplanted into inbred LEW/OrlRj rats. Recipients were intravenously treated every other day with saline or bovine pancreatic RNase A (50 µg/kg). Toxic side effects were not found (macroscopically and histologically). Heart tissue flow cytometry and quantitative morphological analyses of explanted hearts at postoperative day 1 or postoperative day 4 showed reduced leukocyte infiltration, edema, and thrombus formation in RNase A-treated rats. In allogeneic mixed lymphocyte reactions, RNase A decreased the proliferation of effector T cells. RNase A treatment of rats resulted in prolonged median graft survival up to 10.5 days (interquartile range 1.8) compared to 6.5 days (interquartile range 1.0) in saline treatment (P=0.001). Treatment of rats with a new generated (recombinant) human pancreatic RNase 1 prolonged median graft survival similarly, unlike treatment with (recombinant) inactive human RNase 1 (each 50 µg/kg IV every other day, 11.0 days, interquartile range 0.3, versus 8.0 days, interquartile range 0.5, P=0.007). CONCLUSIONS: Upon heart transplantation, RNase administration appears to present a promising and safe drug to counteract ischemia/reperfusion injury and graft rejection. Furthermore, RNase treatment may be considered in situations of critical reperfusion after percutaneous coronary interventions or in cardiac surgery using the heart-lung machine.

Impaired endothelial shear stress induces podosome assembly via VEGF up-regulation.

Podosomes are dynamic cytoskeletal membrane structures with local adhesive and proteolytic activity. They are critically involved in angiogenesis and vascular adaptive growth. Here, we studied in HUVECs and murine small vessels whether shear stress controls podosome assembly and local proteolytic activity. Podosomes were characterized by immunohistochemistry, and their proteolytic activity was assessed as degradation imprints in fluorescent gelatin that was used as growth substrate. Compared with controls (10 dyn/cm(2)), the number of podosomes formed per time was doubled when cells were exposed to low shear stress (0.3 dyn/cm(2)) or even increased 5-fold under static conditions. This was a result of an enhanced expression of VEGF after reduction of shear stress. Consequently, enhanced podosome formation could be prevented by a VEGF receptor antagonist as well by interruption of VEGF signaling via inhibition of PI3K, Src, or p38. Increase of podosome assembly went along with significantly augmented cell motility. In vivo experiments in mouse arteries confirmed increased endothelial podosome numbers when shear stress was abolished by vessel occlusion. We conclude that shear stress, by reducing VEGF release, inhibits podosome assembly. Hence, endothelial cell-mediated matrix proteolysis and migratory activity are inhibited, thereby stabilizing the structure of the vessel wall.-Fey, T., Schubert, K. M., Schneider, H., Fein, E., Kleinert, E., Pohl, U., Dendorfer, A. Impaired endothelial shear stress induces podosome assembly via VEGF up-regulation.

De novo sirolimus with low-dose tacrolimus versus full-dose tacrolimus with mycophenolate mofetil after heart transplantation--8-year results.

BACKGROUND: Although acute cellular rejection after heart transplantation (HTX) can be controlled by full-dose calcineurin inhibitor (CNI)-based immunosuppressive regimens, cardiac allograft vasculopathy (CAV), nephrotoxicity, and malignancy remain ongoing problems. To evaluate the potential beneficial effects of sirolimus and CNI reduction, we compared de novo low-dose tacrolimus and sirolimus with standard tacrolimus and mycophenolate mofetil (MMF)-based immunosuppression after HTX. METHODS: We analyzed a long-term follow-up cohort of 126 patients who underwent HTX during the period 1998-2005 and received either de novo low-dose tacrolimus/sirolimus (lowTAC/SIR; n = 61) or full-dose tacrolimus/MMF (TAC/MMF; n = 64). RESULTS: Freedom from treatment switch was less in the low TAC/SIR group than in the TAC/MMF group (51.7% vs 73.0%, p = 0.038) 8 years after HTX. Freedom from acute rejection was 90.6% in the low TAC/SIR group vs 80.3% in the TAC/MMF group (p = 0.100). There was no difference in freedom from International Society for Heart and Lung Transplantation CAV grade ≥ 1 (55.4% vs 60.0%, p = 0.922), time until CAV diagnosis (4.2 ± 2.0 years vs 3.2 ± 2.4 years, p = 0.087), and CAV severity (p = 0.618). The benefit of reduced early maximum creatinine for low TAC/SIR treatment (1.8 ± 0.9 mg/dl vs 2.4 ± 1.1 mg/dl in TAC/MMF group, p < 0.001) did not continue 5 years and 8 years after HTX (1.4 ± 0.4 mg/dl vs 1.7 ± 1.2 mg/dl, p = 0.333, and 1.6 ± 1.1 mg/dl vs 1.6 ± 0.8 mg/dl, p = 0.957). The trend for superior survival at 5 years with low TAC/SIR treatment (93.1% vs 81.3% in TAC/MMF group, p = 0.051) could not be confirmed after 8 years (84.7% vs 75.0%, p = 0.138). Multivariate analysis at 8 years did not reveal any benefit of low TAC/SIR treatment. CONCLUSIONS: Reduction of de novo CNI did not result in superior long-term renal function. Low-dose mechanistic target of rapamycin inhibition did not achieve any benefit in CAV prevention compared with full-dose TAC/MMF after HTX.

Critical role of platelet glycoprotein ibα in arterial remodeling.

OBJECTIVE: Arteriogenesis is strongly dependent on the recruitment of leukocytes, especially monocytes, into the perivascular space of growing collateral vessels. On the basis of previous findings that platelets are central players in inflammatory processes and mediate the recruitment of leukocytes, the aim of this study was to assess the role of platelets in a model of arterial remodeling. APPROACH AND RESULTS: C57Bl6 wild-type mice, IL4-R/Iba mice lacking the extracellular domain of the glycoprotein Ibα (GPIbα) receptor, and mice treated with antibodies to block GPIbα or deplete circulating platelets were studied in peripheral arteriogenesis. Using a novel model of intravital 2-photon and epifluorescence imaging, we visualized and quantified the interaction of platelets with leukocytes and the vascular endothelium in vivo. We found that transient platelet adhesion to the endothelium of collateral vessels was a major event during arteriogenesis and depended on GPIbα. Furthermore, leukocyte recruitment was obviously affected in animals with defective platelet GPIbα function. In IL4-R/Iba mice, transient and firm leukocyte adhesion to the endothelium of collateral vessels, as well as leukocyte accumulation in the perivascular space, were significantly reduced. Furthermore, we detected platelet-leukocyte aggregates within the circulation, which were significantly reduced in IL4-R/Iba animals. Finally, platelet depletion and loss of GPIbα function resulted in poor reperfusion recovery as determined by laser Doppler imaging. CONCLUSIONS: Thus, GPIbα-mediated interactions between platelets and endothelial cells, as well as leukocytes, support innate immune cell recruitment and promote arteriogenesis-establishing platelets as critical players in this process.