VEGF, shear stress and muscle angiogenesis: a complicated triangle

In this issue of Acta Physiologica, Uchida and colleagues (Uchida et al. 2015) report interesting findings that help clarify the complex relationship between flow-induced shear stress, expression of Vascular Endothelial Growth Factor A (VEGF) and angiogenesis in skeletal muscle; a relationship that has been particularly elusive to elucidate due to the multiple and mutual influences among these factors. Stimulation of muscle angiogenesis is a therapeutic goal to restore blood flow to the ischaemic limbs of patients suffering from peripheral artery disease (PAD). While the delivery of VEGF and other angiogenic factors to the affected muscles is an active area of ongoing research, no therapeutic angiogenesis approach has yet entered clinical practice. On the other hand, exercise training has been shown to very effectively increase vessel density in trained muscles, through stimulation of VEGF expression. Actually, exercise training is currently the only clinically effective non-surgical therapy for PAD, and the most recent consensus guidelines of the American College of Cardiology Foundation and the American Heart Association recommend exercise training as the first line of treatment for patients with symptomatic PAD and intermittent claudication (Rooke et al. 2013). The effects of exercise on muscle angiogenesis are complex and involve both metabolic (reduced tissue levels of oxygen and nutrients) and hemodynamic signals (vasodilation with increased blood flow and shear stress). However, while both factors increase VEGF expression, they have been described to cause vessel growth by very different processes, namely sprouting angiogenesis and vascular splitting or intussusception respectively. Sprouting relies on the specification of endothelial cells into tip and stalk. Tip cells do not have a lumen, extend numerous thin filopodia, through which they can sense VEGF gradients, and migrate towards the VEGF source, thereby guiding the nascent sprout. Tip cells activate Notch signalling in the neighbouring endothelial cells, causing them to become instead stalk cells, which proliferate just behind the tip, form the body of the sprout and start the process of lumen formation (Blanco & Gerhardt 2013). On the other hand, vascular splitting and intussusception represent the conceptual opposite of sprouting, as endothelial cells extend inside the lumen to split it into two new daughter vessels. Hallmark of these processes is the formation of transluminal tissue pillars, which can occur either through a zone of contact between the endothelial cells of opposite capillary walls, with subsequent reorganization of the endothelial junctions and invasion of the pillar core by myofibroblasts (intussusception), or through the extension and fusion of intraluminal protrusions made exclusively of endothelial cells (splitting). Subsequently, transluminal tissue pillars align along the length of the pre-existing vessel, progressively fuse together and divide the affected vascular segment longitudinally (Egginton 2009, Makanya et al. 2009). It is not completely clear, and the subject of ongoing discussion, whether intussusception and vascular splitting are two distinct mechanisms or different aspects of the same general process. For simplicity, here we will use the term intussusception to mean both. The authors investigated how shear stress induces angiogenesis in muscle in the absence of metabolic changes, using a classic model based on systemic treatment with the a1-adrenergic receptor blocker prazosin, which increases blood flow and shear stress in the downstream microvascular networks purely through arterial vasodilation. The widely held view has been that, while metabolic stimuli cause VEGF expression from muscle fibres, intussusception induced by hemodynamic factors depends on VEGF produced by the endothelial cells themselves, which are the only cell type exposed to shear stress and can sense it directly. However, the authors’ findings shed new light on this dogma: by taking advantage of a knockout mouse model lacking VEGF specifically in skeletal myocytes, but not other cell types, they elegantly showed that the angiogenesis caused by a chronic increase in blood flow and shear stress in skeletal muscles unexpectedly requires myocyte-derived VEGF and endothelial VEGF cannot compensate for its absence (Uchida et al. 2015). Further, they found that nitric oxide (NO), which is known to be released by endothelium in response to augmented shear stress (Hudlicka & Brown 2009), could stimulate VEGF expression independently of flow, in ex vivo isolated muscles, as well as in cultured myocytes. These findings are highly suggestive for a potential mechanism whereby shear stress, sensed by endothelium, may stimulate VEGF