The Absence of Extracellular Cold-Inducible RNA-Binding Protein (eCIRP) Promotes Pro-Angiogenic Microenvironmental Conditions and Angiogenesis in Muscle Tissue Ischemia.

Extracellular Cold-inducible RNA-binding protein (eCIRP), a damage-associated molecular pattern, is released from cells upon hypoxia and cold-stress. The overall absence of extra- and intracellular CIRP is associated with increased angiogenesis, most likely induced through influencing leukocyte accumulation. The aim of the present study was to specifically characterize the role of eCIRP in ischemia-induced angiogenesis together with the associated leukocyte recruitment. For analyzing eCIRPs impact, we induced muscle ischemia via femoral artery ligation (FAL) in mice in the presence or absence of an anti-CIRP antibody and isolated the gastrocnemius muscle for immunohistological analyses. Upon eCIRP-depletion, mice showed increased capillary/muscle fiber ratio and numbers of proliferating endothelial cells (CD31 ⁺ /CD45 ^- /BrdU ⁺ ). This was accompanied by a reduction of total leukocyte count (CD45 ⁺ ), neutrophils (MPO ⁺ ), neutrophil extracellular traps (NETs) (MPO ⁺ CitH3 ⁺ ), apoptotic area (ascertained via TUNEL assay), and pro-inflammatory M1-like polarized macrophages (CD68 ⁺ /MRC1 ^- ) in ischemic muscle tissue. Conversely, the number of regenerative M2-like polarized macrophages (CD68 ⁺ /MRC1 ⁺ ) was elevated. Altogether, we observed that eCIRP depletion similarly affected angiogenesis and leukocyte recruitment as described for the overall absence of CIRP. Thus, we propose that eCIRP is mainly responsible for modulating angiogenesis via promoting pro-angiogenic microenvironmental conditions in muscle ischemia.