Coronary heart disease is the world's leading cause of death[1]. Promoting the development of collateral vessels, an important blood source to maintain myocardial tissue perfusion and function after coronary artery obstruction, will effectively improve the prognosis of patients with coronary heart disease[2]. Vasculogenesis, sprouting angiogenesis, intussusceptive angiogenesis, coalescent angiogenesis, vessel co-option, vasculogenic mimicry and arteriogenesis are the seven main ways of collateral vessel development. Vasculogenesis and sprouting angiogenesis are the process of de novo development of collateral vessels, the lumen of newly formed collateral vessels is small and the development process is slow[3]; intussusceptive angiogenesis and coalescent angiogenesis only reshape the vascular network and can't increase the blood flow in the myocardial infarction area[3]; vessel co-option and vasculogenic mimicry are currently only found in solid tumors[4, 5]; arteriogenesis is the process in which the pre-existing collateral vessels are eventually remodeled into mature large arteries, although it can alleviate the symptoms of myocardial ischemia to a certain extent, the time required for arterial outward remodeling is long[6]. Therefore, none of the seven methods of collateral vessel development is sufficient to timely rescue a large number of dying myocardial cells in the myocardial infarction area.
Hence, we first proposed the hypothesis that solid cell cords of vascular smooth muscle cell (VSMC) are the precursors of collateral vessels. The development of VSMC solid cell cords into collateral vessels can be divided into the following four stages: 1) the first stage is solid cell cord growth mediated by VSMC proliferation and migration, when the coronary lumen is partially obstructed, the disrupted blood flow at the obstructed site stimulates VSMC proliferation and migration by inducing endothelial cells to secrete various cytokines such as platelet derived growth factor (PDGF), fibroblast growth factor-4 (FGF-4), and vascular endothelial growth factor-A (VEGF-A)[7–9]; 2) the second stage is VSMC phenotype regression (transformation from synthetic phenotype to contractile phenotype) mediated by hypoxia, when the coronary lumen is almost completely blocked. Because VSMC with contractile phenotype is sensitive to vasodilators, VSMC phenotype regression is a prerequisite for the rapid opening of the solid cell cord; 3) the third stage is the rapid opening of solid cell cord mediated by vasodilators such as nitric oxide (NO)[10], forming a tubular structure lined with monocytes to timely rescue a large number of dying myocardial cells in the myocardial infarction area[11, 12]; 4) the fourth stage is the maturation of collateral vessels, the tubular structure lined with monocytes gradually develops into coronary collateral vessels lined with endothelial cells[12, 13].
AMP activated protein kinase (AMPK) is a Serine/Threonine kinase, which is considered as an energy sensor to regulate the energy balance of the whole body[14]. AMPK is composed of three subunits (α, β and γ), these subunits comprises several isoforms (so far α1, α2, β1, β2, γ1, γ2, γ3), therefore, there are at least 12 AMPK isoenzymes with tissue distribution specificity and functional specificity[15]. Activated AMPK acts on both gene expression and protein levels to adapt the cellular metabolism by deactivating energy consuming and anabolic processes, activating processes that deliver "cheap" energy such as glycolysis[16].
Previous studies have shown that cells can maintain energy balance under hypoxic conditions by activating AMPK[15], and VSMC solid cell cords are in a hypoxic state during the second stage of solid cord development. Therefore, we speculate that AMPK plays a crucial role in the transition of VSMC from synthetic phenotype to contractile phenotype. The aim of this study is to confirm the existence of VSMC solid cell cords and clarify the role of AMPK in VSMC phenotype regression.