Tarun K. Ghosh

Calcium signalling mechanisms in endoplasmic reticulum activated by inositol 1,4,5-trisphosphate and GTP

Ca2+ signals are known to mediate an array of cellular functions including secretion, contraction, and conductivity changes. In spite of the obvious role of Ca2+ in signalling, the control of Ca2+ within cells is known to be a complex phenomenon involving a number of distinct active and passive transport systems functioning within different organelles. Inositol 1,4,5-trisphosphate (IP3) is now established as a central mediator of Ca2+ mobilization, and the endoplasmic reticulum (ER) has been considered to be the site of action of IP3. However, this role has been ascribed almost by default to the ER, based on the knowledge that IP3 functions to release Ca2+ from an intracellular, nonmitochondrial, Ca2+-pumping organelle. Our interest has been to ascertain by what mechanism IP3 activates Ca2+ movements, at what intracellular locations it functions, and how the size and replenishment of the IP3-sensitive Ca2+ pool occurs. During the course of such studies, another mechanism inducing profound movements of Ca2+ within cells was identified. This process is activated by a highly sensitive and specific guanine nucleotide regulatory mechanism, which, while inducing fluxes of Ca2+ that resemble the action of IP3 under certain conditions, has now been determined to involve a quite distinct mechanism. The characteristics of this mechanism are described below. Although involving a very different Ca2+ translocation process to that activated by IP3, several important conclusions have been drawn on the relationship between IP3- and GTP-activated Ca2+ movements leading us to believe that the latter may have a regulatory role in controlling the size and/or entry of Ca2+ into the IP3-sensitive Ca2+ pool.

Mechanisms of Intracellular Calcium Movement Activated by Guanine Nucleotides and Inositol-1,4,5-Trisphosphate

It is now well established that the intracellular second messenger inositol-1,4,5-trisphosphate (IP3) is involved in the release of Ca2+ from a Ca2+ -sequestering organelle, widely considered to be the endoplasmic reticulum (ER) (Berridge and Irvine, 1984; Gill, 1985; Majeruset al., 1986). In a series of recent studies, we observed that a highly sensitive and specific guanine nucleotide regulatory process induces a release of Ca2+ in cells that appears very similar to that mediated by IP3(Gillet al., 1986; Uedaet al., 1986; Chueh and Gill, 1986). Our initial studies were conducted using either permeabilized cells or isolated microsomal membrane vesicles derived from the NIE-115 neuronal cell line; GTP-dependent Ca2+ release was observed to be very similar in the two preparations (Gillet al., 1986; Uedaet al., 1986). Recent studies (Henne and Soling, 1986; Jean and Klee, 1986; Chuehet al., 1987) have extended the number of diverse cell types in which the same GTP-activated Ca2+ release process is observed. In each cell type, submicromolar GTP concentrations rapidly effect a substantial release of Ca2+ sequestered via internal Ca2+ -pumping activity within a nonmitochondrial organelle, believed to be the ER. The Ca2+ -accumulating properties of this intracellular organelle have been described in detail in earlier studies with permeabilized cells (Gill and Chueh, 1985).