Macro‐ and micro‐heterogeneity of lung endothelial cells: they may not be smooth, but they got the moves

It has long been recognized that there is tremendous heterogeneity in endothelial cells (ECs) from different vascular beds and that organ-specific differences in EC phenotype have important physiological and pathophysiological implications. It has also been suggested that within the vasculature, there are ‘‘niches’’ of highly proliferative, progenitor-like ECs that likely play an important role in vascular repair and regeneration. The lung vascular bed is particularly intriguing because of its many unique features. It has the largest endothelial surface area of any organ and is the only vascular bed that needs to accommodate the entire cardiac output. Moreover, it can do this at rest using only a fraction of the available microvasculature area, at arterial pressures that are little more than venous. As cardiac output increases, the lung can recruit additional unused microvascular area to maintain low arterial pressure. Also, there is a paucity of smooth muscle and other supportive mural cells in its small arterioles, possibly because they operate at such low pressures, which makes these structures particularly fragile. Thus, the observation by the Stevens group some years ago that a significant proportion of pulmonary microvascular ECs (PMVECs) showed high proliferation potential may provide insights into how homeostasis of the lung microvasculature may be maintained, namely by endowing the lung with a uniquely high reparative capacity. The importance of being endowed with high regenerative ability is underlined by the evolving understanding of the role of EC injury and apoptosis in the pathogenesis of pulmonary arterial hypertension (PAH). EC apoptosis has been implicated as a critical trigger in the pathogenesis of PAH, leading to both direct and indirect consequences. Although the ‘‘angioproliferative’’ sequelae have received far more attention, it is not hard to imagine that apoptosis of ECs of fragile distal lung arterioles, consisting of little more than endothelial tubes literally ‘‘hanging in the breeze,’’ could lead to loss of these structures. Therefore, an ability to efficiently replace damaged ECs may be an important mechanism by which the lung maintains vascular homeostasis after exposure to endothelial toxins or other environmental stress. The recent work published in this issue of Pulmonary Circulation takes EC heterogeneity to another level by demonstrating differences in proliferative, angiogenic, and bioenergetic profiles of rat pulmonary arterial ECs (PAECs) derived from the main pulmonary artery and its main branches. This suggests a significant degree of microheterogeneity even within an EC population derived from one level (i.e. conductance arteries) of the same bed. One can only speculate about the implication of this observation, though it is consistent with the idea of progenitor cell ‘‘niches’’ within the pulmonary vasculature, presumably for the purpose of effecting rapid repair after endothelial injury. The authors also show that the parental proliferative, angiogenic, and bioenergetic characteristics are preserved in subsequent generation PAEC colonies derived by single-cell cloning. They show that single-cell colonies derived from PAECs maintain parental growth profiles and glycolytic rates, despite a decrease in basal oxygen consumption, indicative of a lower bioenergetic demand. Second-generation single-cell clones, which produced a more morphologically homogeneous population, exhibited a similar bioenergetics profile to first-generation clones, and were otherwise identical to parental and first-generation populations. Of note, second-generation clones, selected from proliferating firstgeneration single-cell colonies, appeared to produce more colonies than first-generation clones. The authors conclude that single-cell cloning might be useful for the expansion of cells with limited proliferation potential by selectively choosing proliferation competent clones, which also produces a more homogeneous culture while maintaining very strong resemblance to the parent population. Another intriguing finding in this report is the difference in relative use of aerobic glycolysis between different pulmonary EC populations. While PAECs use oxidative phosphorylation as their primary metabolic pathway, PMVECs rely largely on glycolysis for adenosine triphosphate (ATP) production. Recent studies have shown that, unlike other cell types with much higher energetic demands (i.e. myocytes), ECs from many different beds primarily use aerobic glycolysis as the primary mechanism for ATP generation.