Activation of Metabotropic Glutamate Receptor mGlu5 on Nuclear Membranes Mediates Intranuclear Ca2+ Changes in Heterologous Cell Types and Neurons

Abstract

Nuclear Ca2؉ plays a critical role in many cellular functions although its mode (s) of regulation is unclear. This study shows that the metabotropic glutamate receptor, mGlu5, mobilizes nuclear Ca 2؉ independent of cytosolic Ca 2؉ regulation. Immunocytochemical, ultrastructural, and subcellular fractionation techniques revealed that the metabotropic glutamate receptor, mGlu5, can be localized to nuclear membranes in heterologous cells as well as midbrain and cortical neurons. Changes in nuclear Ca2ϩ play an integral role in cellular functions such as protein import, apoptosis, and gene transcription (1, 2). Nuclear Ca 2ϩ may be generated from a number of sources including diffusion of cytosolic Ca 2ϩ waves through nuclear pore complexes (2). Because the outer nuclear envelope is continuous with the endoplasmic reticulum, which serves as an internal store of Ca 2ϩ , rises in nuclear Ca 2ϩ may also be attributable to a luminal source (3). Recent studies using high speed imaging of intracellular Ca 2ϩ have shown that waves of Ca 2ϩ can invade the nucleus by emptying intracellular stores (4). Calcium release from internal stores is controlled by various channels including the inositol 1,4,5-trisphosphate (IP 3 ) 1 receptor and ryanodine receptor families (5, 6) both of which are present on nuclear membranes (7,8). Calcium itself is an activator of these channels (1) although nuclear IP 3 can stimulate IP 3 receptors located on the inner nuclear membrane and cADP ribose has been shown to activate nuclear ryanodine receptors (7,8). Luminal Ca 2ϩ is refilled at least in part by the nuclear Ca 2ϩ -ATPase (9, 10) located on the outer nuclear membrane. Thus, although signals originating at the plasma membrane may be transmitted to the nucleus (4), the presence of specific Ca 2ϩ transporters on the nuclear envelope argues for a nuclear Ca 2ϩ regulatory system that may be independent of cytosolic Ca 2ϩ regulation. Many components of G protein signaling pathways are also found in the nucleus or associated with nuclear membranes. These include phospholipase C isozymes (11, 12), nuclear inositol phosphates (12, 13), DAG (13), PKC isozymes (14), adenylate cyclase (15), regulators of G protein signaling (RGS proteins; Refs. 16 and 17) as well as heterotrimeric G proteins themselves (18). These observations raise the possibility that plasma membrane-based signaling components may also serve a similar function at nuclear membranes. Indeed, several recent reports are consistent with the notion that nuclear G protein-coupled receptors directly modulate nuclear signal transduction pathways. For example, angiotensin II receptors were found on hepatocyte nuclear membranes (19), opioid binding sites were described on ventricular myocardial nuclei (20) and endothelin-1 receptors were reported on vascular smooth muscle nuclear membranes (21). Direct evidence of nuclear receptor G protein signaling has also been demonstrated for prostaglandin receptors which, when stimulated, cause rapid Ca 2ϩ influx into the nucleus (22,23). Taken togeth...

Intracrine endothelin signaling evokes IP3-dependent increases in nucleoplasmic Ca2+ in adult cardiac myocytes

“…This increase was prevented by a cellpermeable analog of an ETB-selective antagonist, IRL-2500, but not by adding BQ610 and BQ788 to the extracellular medium, thus confirming a direct role of nuclear ETB in regulating nuclear Ca 2+ signalling in intact ventricular myocytes. Other GPCRs found in nuclear membranes have also been show to increase [Ca 2+ ] n including Ang II [18,91], bradykinin B2 [21], prostaglandin E 2 [22][23][24], lysophosphatidic acid type-1 [25,26], and metabotropic glutamate type-5 [92][93][94][95].…”

Positive Allosteric Modulation of Metabotropic Glutamate 5 (mGlu5) Receptors Reverses N-Methyl-D-Aspartate Antagonist-Induced Alteration of Neuronal Firing in Prefrontal Cortex

“…This is interesting given that agonist stimulation of mGlu1/5 receptors leads to rapid desensitization (Contractor et al 1998;Gereau and Heinemann 1998). Another reason for the sustained effects of CDPPB may be that functional mGlu5 receptors have been localized on the nuclear membrane of cortical and midbrain neurons, where their activation increases intranuclear calcium release (Jong et al 2005;O'Malley et al 2003). Such an increase may stimulate signaling pathways and favor changes in gene expression, leading to sustained firing rate activation.…”

Functional Metabotropic Glutamate Receptors on Nuclei from Brain and Primary Cultured Striatal Neurons

“…Similarly, endothelin receptors A and B are also localized to the perinuclear region of cardiac ventricular myocytes where they mediate nuclear Ca 2ϩ levels and activate nuclear protein kinases (6). Finally, we have shown that the metabotropic glutamate receptor, mGlu5, 1 can be expressed on nuclear membranes where it can couple with endogenous signaling components to induce changes in nuclear Ca 2ϩ (7). Because most studies investigating the properties of nuclear receptors have been performed in heterologous cell types with overexpressed receptors, the question arises as to whether such phenomena are physiologically relevant and, in the case of mGlu5, how a ligand such as glutamate has access to this receptor.…”

“…Cells were homogenized, and nuclei and plasma membranes were prepared as described previously (7). Aliquots from each fraction were used for gel electrophoresis as well as membrane binding.…”