Presence of a Nucleoplasmic Complex Composed of the Inositol 1,4,5-Trisphosphate Receptor/Ca²⁺ Channel, Chromogranin B, and Phospholipids

Abstract

Although the inositol 1,4,5-trisphosphate (IP(3)) induced nuclear Ca(2+) releases have been shown to play key roles in nuclear functions, the presence and operation of the IP(3)-dependent Ca(2+) control mechanism in the nucleoplasm have not been shown. Recently, we found the presence of a high-capacity, low-affinity Ca(2+)-storage protein chromogranin B (CGB) and all three IP(3) receptor (IP(3)R) isoforms in the nucleoplasm, localizing widely in both the heterochromatin and euchromatin regions. In view of the essential role of CGB-IP(3)R coupling in IP(3)-dependent Ca(2+) release in the endoplasmic reticulum, the potential coupling between CGB and the IP(3)Rs in the nucleoplasm was investigated. Hence, we found in the present study the presence of a nucleoplasmic complex, which is composed of the IP(3)R, CGB, and phospholipids, with an estimated molecular mass of approximately 2-3 x 10(7) Da, suggesting the possibility of the presence of an IP(3)-sensitive Ca(2+) store in the nucleoplasm. Moreover, double-labeling immunogold electron microscope studies showed the colocalization of all three IP(3)R isoforms with CGB to the extent that the majority of each IP(3)R isoform-labeling gold particles found in the nucleoplasm was literally next to the CGB-labeling gold particles. In line with the potential existence of an IP(3)-dependent vesicular nucleoplasmic Ca(2+) store, our preliminary results indeed showed a sudden release of Ca(2+) from a putative nucleoplasmic Ca(2+) store in response specifically to IP(3) but not to inositol 1,4-bisphosphate or inositol 1,3,4,5-tetrakisphosphate.

Nuclear membrane receptors for ET-1 in cardiovascular function

“…The activation of these nuclear membrane ET-1 receptors by their respective cytosolic or perinucleoplasmatic ligands would cause nuclear second messenger formation, such as inositol trisphosphate (IP 3 ) (59, 63) and PKC activation (73). These ET-1 receptor activation products may directly regulate nuclear membrane ionic transporters (11,42,53,80) as well as nuclear pore complex trafficking of macromolecules (34). The schematic model in Fig.…”

“…5). Activation of these nuclear membrane receptors would further promote transnuclear membrane Ca 2ϩ influx and release of nucleoplasmic Ca 2ϩ via nuclear ryanodine receptor and IP 3 receptor pools (12,59,64,80). Thus it is possible that activation of nuclear membrane ET-1 receptors may follow the activation of plasma membrane ET-1 receptors.…”

Role of the nuclear envelope in calcium signalling

“…Functional experiments suggest that IP 3 can induce the release of Ca 2+ directly through the INM (see below), but to our knowledge there is no immunohistochemical evidence at the EM level for the localisation of IP 3 R on the INM. It should be noted that some studies describe the presence of IP 3 R on small vesicles inside the nucleus that could be responsible for nuclear Ca 2+ transients (Huh, et al, 2006, Yoo, et al, 2005. Whether these vesicles are different from invaginations of the NE is not clear.…”

“…Today, there is some consensus that Ca 2+ can be delivered directly from the lumen of the NE to the nucleoplasm through the INM, more likely through IP 3 R or in some cases through RyR. It should be noted that small vesicles have been described inside the nucleoplasm that could accumulate Ca 2+ and release it in the presence of IP 3 (Yoo, et al, 2005). These putative Ca 2+ stores in the nucleoplasm can also be involved in some cell types in the genesis of nuclear Ca 2+ signals.…”

Cytosolic organelles shape calcium signals and exo–endocytotic responses of chromaffin cells

“…Experiments with alkalinising agents are certainly interesting; but it is difficult to envision the physiological context that they could mimic. The presence of InsP 3 receptors in the chromaffin vesicle membrane [70,71] and InsP 3 -induced vesicular Ca 2+ release [60,72,73] suggest that the InsP 3 pathway may be physiologically relevant. It seems likely that vesicular Ca 2+ release could be involved in slow pre-exocytotic steps aimed at mobilizing vesicles from a reserve pool to a ready-releasable pool, as it is the case for Ca 2+ release from the ER (see Section 3.4).…”