Calcium-binding Protein 1 Is an Inhibitor of Agonist-evoked, Inositol 1,4,5-Trisphosphate-mediated Calcium Signaling

Abstract

Calcium is a ubiquitous intracellular second messenger in all vertebrate cells (1). Intracellular resting free Ca 2ϩ concentration ([Ca 2ϩ ] i ) is acutely maintained at around 100 nM via a variety of mechanisms, and even small local rises above this base line are able to elicit dramatic cellular responses. Rises in [Ca 2ϩ ] i are generated through two primary routes (2). First, many physiological agonists activate Ca 2ϩ channels that mediate the direct entry of extracellular Ca 2ϩ . Second, [Ca 2ϩ ] i can be elevated through release of sequestered Ca 2ϩ from intracellular stores (3), most notably the endoplasmic reticulum (ER). 1 Ca 2ϩ release from intracellular stores is controlled predominantly by two related Ca 2ϩ channel families, the inositol 1,4,5-trisphosphate (InsP 3 ) receptor (InsP 3 R) and the ryanodine receptor (RyR). InsP 3 Rs couple to extracellular stimuli that drive the activation of phospholipase C and subsequent liberation of the soluble second messenger InsP 3. Activation of ER-localized InsP 3 Rs by InsP 3 in turn elicits Ca 2ϩ release into the cytosol. InsP 3 R activity can be further modulated by direct binding of Ca 2ϩ itself, and this regulation explains a form of calciuminduced calcium release that is responsible for the establishment of complex regenerative Ca 2ϩ signals within cells (2). In neurons, such Ca 2ϩ signals have been intimately linked to processes as divergent as neurodegeneration, neurite outgrowth, alterations in neuronal gene expression, and neurotransmitter release (4 -10). Factors affecting the generation and propagation of intracellular Ca 2ϩ signals therefore represent potential critical integrators of neuronal responses to complex and varied stimuli.Although Ca 2ϩ is capable of eliciting direct cellular responses such as the aforementioned gating of InsP 3 Rs, many of its effects are registered indirectly through specific Ca 2ϩ -binding proteins. Calmodulin (CaM) is a universal Ca 2ϩ -binding protein, homologues of which are present in all eukaryotes (11), and a large number of Ca 2ϩ driven alterations in cell physiology can be mapped to its Ca 2ϩ sensing activity. CaM coordinates Ca 2ϩ ions through specialized EF-hand domains, and a large number of related proteins have now been characterized. In neurons, these include the neuronal calcium sensor (NCS) family of Ca 2ϩ -binding proteins (12). More recently, a group of CaM-like proteins, termed caldendrins (13, 14) or calciumbinding proteins (CaBPs) (15) have been identified that are vertebrate-specific and display a predominantly neuronal/retinal pattern of expression (14 -16). It has since been reported that members of the CaBP family are able to interact with and modulate the activity of InsP 3 Rs (17). Uniquely, it has been suggested that CaBPs may be capable of activating the InsP 3 R independently of the natural ligand ‡ To whom all correspondence should be addressed.

Endogenously Bound Calmodulin Is Essential for the Function of the Inositol 1,4,5-Trisphosphate Receptor

“…Although this is still controversial (24,30), we have envisaged the hypothesis that CaBP1 could activate IP 3 R after complete removal of CaM from the IP 3 R. However, we did not observe any activation by CaBP1 alone or potentiation of IICR after removal of the endogenously bound CaM. This indicates that the N-terminal CaM-binding site that is also used by CaBP1 (24) is not involved in this process.…”

The endoplasmic reticulum gateway to apoptosis by Bcl-XL modulation of the InsP3R

“…Both His-and Flag-tagged rBcl-X L , generated using distinct purification protocols, were equally effective, whereas His-tagged NCS-1, which does not interact with InsP 3 R (ref. 32), had no effect either alone or in combination with 10 nM InsP 3 (data not shown). Supplementary Information, Fig.…”

IP3 Receptors: Toward Understanding Their Activation

“…There are reports of IP 3 -independent activation of IP 3 R by CaBP1 (Yang et al 2002), a member of the neuronal Ca 2þ -sensor family, and by Gbg subunits (Zeng et al 2003), but the physiological relevance is unclear (Haynes et al 2004;Nadif Kasri et al 2004). The current consensus is that binding of IP 3 to the IP 3 R is essential for its activation, but whether all four IP 3 -binding sites of the tetrameric IP 3 R must be occupied is unresolved.…”