Background and Purpose The TRPV4 ion channels are Ca 2+ permeable, non‐selective cation channels that mediate large, but highly localized, Ca 2+ signals in the endothelium. The mechanisms that permit highly localized Ca 2+ changes to evoke cell‐wide activity are incompletely understood. Here, we tested the hypothesis that TRPV4‐mediated Ca 2+ influx activates Ca 2+ release from internal Ca 2+ stores to generate widespread effects. Experimental Approach Ca 2+ signals in large numbers (~100) of endothelial cells in intact arteries were imaged and analysed separately. Key Results Responses to the TRPV4 channel agonist GSK1016790A were heterogeneous across the endothelium. In activated cells, Ca 2+ responses comprised localized Ca 2+ changes leading to slow, persistent, global increases in Ca 2+ followed by large propagating Ca 2+ waves that moved within and between cells. To examine the mechanisms underlying each component, we developed methods to separate slow persistent Ca 2+ rise from the propagating Ca 2+ waves in each cell. TRPV4‐mediated Ca 2+ entry was required for the slow persistent global rise and propagating Ca 2+ signals. The propagating waves were inhibited by depleting internal Ca 2+ stores, inhibiting PLC or blocking IP 3 receptors. Ca 2+ release from stores was tightly controlled by TRPV4‐mediated Ca 2+ influx and ceased when influx was terminated. Furthermore, Ca 2+ release from internal stores was essential for TRPV4‐mediated control of vascular tone. Conclusions and Implications Ca 2+ influx via TRPV4 channels is amplified by Ca 2+ ‐induced Ca 2+ release acting at IP 3 receptors to generate propagating Ca 2+ waves and provide a large‐scale endothelial communication system. TRPV4‐mediated control of vascular tone requires Ca 2+ release from the internal store.

The endothelium is the single layer of cells lining all blood vessels, and it is a remarkable cardiovascular control centre. Each endothelial cell has only a small number (on average six) of interconnected neighbours. Yet this arrangement produces a large repertoire of behaviours, capable of controlling numerous cardiovascular functions in a flexible and dynamic way. The endothelium regulates the delivery of nutrients and removal of waste by regulating blood flow and vascular permeability. The endothelium regulates blood clotting, responses to infection and inflammation, the formation of new blood vessels, and remodelling of the blood vessel wall. To carry out these roles, the endothelium autonomously interprets a complex environment crammed with signals from hormones, neurotransmitters, pericytes, smooth muscle cells, various blood cells, viral or bacterial infection and proinflammatory cytokines. It is generally assumed that the endothelium responds to these instructions with coordinated responses in a homogeneous population of endothelial cells. Here, we highlight evidence that shows that neighbouring endothelial cells are highly heterogeneous and display different sensitivities to various activators. Cells with various sensitivities process different extracellular signals into distinct streams of information in parallel, like a vast switchboard. Communication occurs among cells and new ‘emergent’ signals are generated that are non-linear composites of the inputs. Emergent signals cannot be predicted or deduced from the properties of individual cells. Heterogeneity and emergent behaviour bestow capabilities on the endothelial collective that far exceed those of individual cells. The implications of heterogeneity and emergent behaviour for understanding vascular disease and drug discovery are discussed.

Endothelial cells line all blood vessels and are critical regulators of vascular tone. In hypertension, disruption of endothelial function alters the release of endothelial-derived vasoactive factors and results in increased vascular tone. Although the release of endothelial-derived vasodilators occurs in a Ca 2+ -dependent manner, little is known on how Ca 2+ signaling is altered in hypertension. A key element to endothelial control of vascular tone is Ca 2+ signals at specialized regions (myoendothelial projections) that connect endothelial cells and smooth muscle cells. This work describes disruption in the operation of this key Ca 2+ signaling pathway in hypertension. We show that vascular reactivity to phenylephrine is increased in hypertensive (spontaneously hypertensive rat) when compared with normotensive (Wistar Kyoto) rats. Basal endothelial Ca 2+ activity limits vascular contraction, but that Ca 2+ -dependent control is impaired in hypertension. When changes in endothelial Ca 2+ levels are buffered, vascular contraction to phenylephrine increased, resulting in similar responses in normotension and hypertension. Local endothelial IP 3 (inositol trisphosphate)-mediated Ca 2+ signals are smaller in amplitude, shorter in duration, occur less frequently, and arise from fewer sites in hypertension. Spatial control of endothelial Ca 2+ signaling is also disrupted in hypertension: local Ca 2+ signals occur further from myoendothelial projections in hypertension. The results demonstrate that the organization of local Ca 2+ signaling circuits occurring at myoendothelial projections is disrupted in hypertension, giving rise to increased contractile responses.

Endothelial cells are reported to be glycolytic and to minimally rely on mitochondria for ATP generation. Rather than providing energy, mitochondria in endothelial cells may act as signaling organelles that control cytosolic Ca2+ signaling or modify reactive oxygen species (ROS). To control Ca2+ signaling, these organelles are often observed close to influx and release sites and may be tethered near Ca2+ transporters. In this study, we used high-resolution, wide-field fluorescence imaging to investigate the regulation of Ca2+ signaling by mitochondria in large numbers of endothelial cells (∼50 per field) in intact arteries from rats. We observed that mitochondria were mostly spherical or short-rod structures and were distributed widely throughout the cytoplasm. The density of these organelles did not increase near contact sites with smooth muscle cells. However, local inositol trisphosphate (IP3)-mediated Ca2+ signaling predominated near these contact sites and required polarized mitochondria. Of note, mitochondrial control of Ca2+ signals occurred even when mitochondria were far from Ca2+ release sites. Indeed, the endothelial mitochondria were mobile and moved throughout the cytoplasm. Mitochondrial control of Ca2+ signaling was mediated by ATP production, which, when reduced by mitochondrial depolarization or ATP synthase inhibition, eliminated local IP3-mediated Ca2+ release events. ROS buffering did not significantly alter local Ca2+ release events. These results highlight the importance of mitochondrial ATP production in providing long-range control of endothelial signaling via IP3-evoked local Ca2+ release in intact endothelium.

Ciliopathies encompass a group of genetic disorders characterized by defects in the formation, maintenance, or function of cilia. Retinitis pigmentosa (RP) is frequently one of the clinical features presented in diverse ciliopathies. RP is a heterogeneous group of inherited retinal disorders, characterized by the death of photoreceptors and affecting more than one million individuals worldwide. The retinitis pigmentosa GTPase regulator (RPGR) gene is mutated in up to 20% of all RP patients. RPGR protein has different interacting partners to function in ciliary protein trafficking. In this review, we specifically focus on RPGR and its two interacting proteins: RPGRIP1 and RPGRIP1L. We summarize the function of the three proteins and highlight recent studies that provide insight into the cellular function of those proteins.