*Review* **The Extraordinary Role of Extracellular RNA in Arteriogenesis, the Growth of Collateral Arteries**

**Anna-Kristina Kluever 1, Anna Braumandl 1, Silvia Fischer 2, Klaus T. Preissner 2 and Elisabeth Deindl 1,\***

1 Walter-Brendel-Center of Experimental Medicine, University Hospital, Ludwig-Maximilians-University, 81377 Munich, Germany; A.Kluever@campus.lmu.de (A.-K.K.); Anna.Braumandl@med.uni-muenchen.de(A.B.)


Received: 18 November 2019; Accepted: 6 December 2019; Published: 7 December 2019

**Abstract:** Arteriogenesis is an intricate process in which increased shear stress in pre-existing arteriolar collaterals induces blood vessel expansion, mediated via endothelial cell activation, leukocyte recruitment and subsequent endothelial and smooth muscle cell proliferation. Extracellular RNA (eRNA), released from stressed cells or damaged tissue under pathological conditions, has recently been discovered to be liberated from endothelial cells in response to increased shear stress and to promote collateral growth. Until now, eRNA has been shown to enhance coagulation and inflammation by inducing cytokine release, leukocyte recruitment, and endothelial permeability, the latter being mediated by vascular endothelial growth factor (VEGF) signaling. In the context of arteriogenesis, however, eRNA has emerged as a transmitter of shear stress into endothelial activation, mediating the sterile inflammatory process essential for collateral remodeling, whereby the stimulatory e ffects of eRNA on the VEGF signaling axis seem to be pivotal. In addition, eRNA might influence subsequent steps of the arteriogenesis cascade as well. This article provides a comprehensive overview of the beneficial e ffects of eRNA during arteriogenesis, laying the foundation for further exploration of the connection between the damaging and non-damaging e ffects of eRNA in the context of cardiovascular occlusive diseases and of sterile inflammation.

**Keywords:** arteriogenesis; VEGF; extracellular RNA; shear stress; endothelial activation; mast cell degranulation; macrophages; sterile inflammation; collateral artery growth; TACE
