Tissue Specificity: The Role of Organellar Membrane Nanojunctions in Smooth Muscle Ca²⁺ Signaling

In this chapter we examine the importance of cytoplasmic nanojunctions-nanometer scale appositions between organellar membranes including the molecular transporters therein-to the cell signaling machinery, with specific reference to Ca²⁺ transport and signaling in vascular smooth muscle and endothelial cells. More specifically, we will consider the extent to which quantitative modeling may aid in the development of our understanding of these processes. Testament to the requirement for such approaches lies in the fact that recent studies have provided evermore convincing evidence in support of the view that cytoplasmic nanospaces may be as significant to the process of Ca²⁺ signaling as the Ca²⁺ transporters, release channels, and Ca²⁺-storing organelles themselves. Moreover, the disruption and/or dysfunction of cytoplasmic nanospaces may be central to the origin of certain diseases. By way of introduction, we provide a historical perspective on the identification of smooth muscle cell plasma membrane (PM)-sarcoplasmic reticulum (SR) nanospaces and the early evidence in support of their role in the generation of asynchronous Ca²⁺ waves. We then summarize how stochastic modeling approaches can aid and guide the development of our understanding of two basic functional steps leading to healthy smooth muscle cell contraction. We furthermore outline how more sophisticated and realistic quantitative stochastic modeling may be employed not only to test working hypotheses, but also to lead in their development in a manner that informs further experimental investigation. Finally, we consider more recently defined nanospaces such as the lysosome-SR junction, by way of demonstrating the importance of quantitative stochastic modeling to our understanding of signaling mechanisms.