Ca2+ Signalling in Damaged Endothelium: Do Connexin Hemichannels Aid in Filling the Gap?

Abstract

A monolayer of endothelial cells (ECs) lines the lumen of blood vessels and forms a multifunctional transducing organ that regulates cardiovascular homeostasis. Disruption of endothelial integrity, as a result of either angioplasty or stent deployment, may produce a late in-stent restenosis and limit the beneficial outcome of reconstructive vascular surgery. Restoration of endothelial lining requires spreading, migration and proliferation of ECs nearby the lesion site. Intracellular Ca 2+ signalling plays a major regulatory role in stimulating wound healing, however, the mechanism whereby injury increases Ca 2+ levels at the wound edge is still unclear. The analysis of Ca 2+ signals elicited by scraping an endothelial monolayer in vitro suggested the involvement of intracellular Ca 2+ release from InsP 3 -sensitive stores and Ca 2+ entry through unknown ion channels in the plasma membrane. Recent studies carried out by our group on excised rat aorta highlighted a novel role for connexin hemichannels (ChHcs) in mediating Ca 2+ entry in injured endothelium. This observation sheds new light on the notion that the rate of wound repair is reduced in ECs transfected with dominant negative connexin inhibitors. Understanding the signal transduction pathway leading to EC activation is likely to provide novel targets to design therapeutic applications aiming at restoring endothelial integrity and treating cardiovascular diseases.

The Mechanism of Injury-Induced Intracellular Calcium Concentration Oscillations in the Endothelium of Excised Rat Aorta

“…It has long been known that endothelial proliferation and migration are tightly regulated by an increase in intracellular Ca 2+ concentration ([Ca 2+ ] i ) [2]. Consistent with a role in wound healing, endothelial scraping induces an elevation in [Ca 2+ ] i in nearby survivors, which has been linked to the subsequent covering of the lesioned area [5,6,7,8]. In particular, a dramatic delay in the coverage of denuded area occurs when extracellular Ca 2+ is removed from the bath [8,9,10].…”

“…Three Cx isoforms were expressed by rat aortic endothelium, namely Cx 37, 40, and 43 [11]. The aforementioned results led to the hypothesis that endothelium scraping uncouples Cx hemichannels of ECs freed from their severed neighbors: the hemichannels do not fully close and enable the long-lasting Ca 2+ influx [5,6]. Notably, a minor percentage of activated cells displayed oscillations in [Ca 2+ ] i that overlapped the prolonged decay phase [5].…”

Hydrogen Sulfide and Endothelial Dysfunction: Relationship with Nitric Oxide

“…The heterogeneity in caveolae distribution is exacerbated by the notion that they are far more enriched in lung microvasculature than in microvascular endothelium of brain, retina, and testes [7]. This feature is highly remarkable when considering that caveolae are not only involved in the transport of material across the EC barrier, but serve as a complex signal transduction platform translating extracellular inputs into well defined intracellular responses, owing to their ability to compartmentalize a myriad of signalling molecules [8,9]. Similarly, the vesciculo-vacuolar organelle (VVO), a recently identified network of interconnected vesicles and vacuoles which provides the main transcellular pathway for macromolecule extravasation, spans venular endothelium from the luminal to the basolateral membrane, whereas it is absent in both arterioles and capillaries [10].…”

“…One form of endothelial dysfunctions is EC detachment from the vascular wall, as observed upon balloon catheter angioplasty, intracoronary stent deployment and coronary artery bypass surgery [8,12]. The de-endothelialisation of arterial vessels exposes the thrombogenic sub-endothelial matrix, thereby inducing platelet activation and aggregation at the site of lesion [1].…”

“…However, the wall of arterial conduits contains a complete hierarchy of proliferating cells, ranging from highly replicating clones to non-dividing cells, which are absent in narrower vessels [25]. These cells are those most likely recruited upon a traumatic injury to restore endothelial integrity when the extent of the lesion is limited to a small EC strip [8]. Therefore, endothelium has been described as an input-output device, which utilizes an intricate and non-linear arrays of signal transduction pathways to continuously translate the host of extracellular stimuli detected by its membrane receptors into a specific cellular behaviour (e.g.…”