**1. Introduction**

Blood vessel development and growth encompasses three distinct biological processes— vasculogenesis, angiogenesis, and arteriogenesis [1]. The term vasculogenesis denotes the formation of the primitive vascular plexus from progenitor cells in embryo and this mode of blood vessel formation is limited to embryonic development. Angiogenesis encompasses the process of new capillary formation from pre-existing capillary beds that involves proliferation, sprouting and, migration of endothelial cells. Finally, arteriogenesis refers to the growth of new arteries and arterioles either de novo or from pre-existing arterial collaterals [2]. Arteriogenesis, the process of new arterial vasculature growth, is critical to the restoration of tissue perfusion following the development of a functionally significant decrease of arterial inflow. It is important to note that compromised arterial inflow results in two distinct events: distal tissue ischemia that leads to local angiogenesis (e.g., angiogenesis along the myocardial infarction border zone) and arteriogenesis that occurs in close proximity to the site of arterial trunk occlusion, a territory that is typically not ischemic [3,4].

While molecular controls of angiogenesis are well understood, events triggering and regulating arteriogenesis are still a matter of intense study and controversy. Vascular endothelial growth factor (VEGF) is the main factor driving angiogenesis in response to tissue hypoxia [5], ye<sup>t</sup> VEGF is equally critical to arteriogenesis [6]. Indeed, disruption of VEGF signaling and, in particular, a reduction in VEGF-induced endothelial ERK1/2 signaling, has been shown to result in decreased arteriogenesis [7].

While angiogenesis involves simple proliferation and sprouting of capillary endothelial cells, arteriogenesis requires a coordinated response that involves multiple cell types that, in addition to endothelial and vascular smooth muscle cells, include a panoply of inflammatory cells including

lymphocytes [8,9], natural killer cells [10], macrophages [11], and mast cells [12]. The presence of inflammatory cells (and a local inflammatory response at an arteriogenic site) is critical as these cells serve as the major source of VEGF in the absence of tissue ischemia [13–17].

Endothelial response to VEGF stimulation encompasses a complex series of events that include activation of various intracellular signaling cascades [18]. Of these, activation of ERK1/2 has been directly linked to the extent of arteriogenesis [7]. Remarkably, little is known about the individual contribution of ERK isoforms to this process. Global deletion of *Erk2* is embryonic lethal [19] whereas a global deletion of *Erk1* has no apparent vascular phenotype [20]. Furthermore, endothelial-specific deletion of *Erk2* on the *Erk1* global knockout background is lethal early on in embryonic development due to impaired vascular development [21]. Another key role played by the two ERKs in the adult endothelium is the regulation of vascular normalcy and integrity [22].

In this paper, we focused on the role of ERK1/2 isoforms in adult arteriogenesis. Induction of acute hindlimb ischemia resulted in excessive but poorly functional arteriogenesis in mice with a global deletion of *Erk1* whereas mice with endothelial-specific deletion of *Erk2* exhibited a decreased arteriogenesis. Since arteriogenesis involves a number of cell types, we generated a floxed *Erk1* mouse line and conditionally deleted the gene in macrophages, endothelial, and smooth muscle cells. While endothelial or macrophage deletions of *Erk1* failed to recapitulate the phenotype of the *Erk1*−/− mice, combined deletion of *Erk1* in endothelial cells and macrophages came close to the phenotype in global *Erk1* null mice. Altogether, these results show that endothelial and macrophage *Erk1* is critical to endothelial/macrophage crosstalk and e ffective adult arteriogenesis.
