Cloning of multiple opioid receptors has presented opportunities to investigate the mechanisms of multiple opioid receptor signaling and the regulation of these signals. The subsequent identification of receptor gene structures has also provided opportunities to study the regulation of receptor gene expression and to manipulate the concentration of the gene products in vivo. Thus, in the current review, we examine recent advances in the delineation basis for the multiple opioid receptor signaling, and their regulation at multiple levels. We discuss the use of receptor knockout animals to investigate the function and the pharmacology of these multiple opioid receptors. The reasons and basis for the multiple opioid receptor are addressed.

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G protein-coupled receptors are key regulators of cellular communication, mediating the efficient coordination of a cell's responses to extracellular stimuli. When stimulated these receptors modulate the activity of a wide range of intracellular signalling pathways that facilitate the ordered development, growth and reproduction of the organism. There is now a growing body of evidence examining the mechanisms by which G protein-coupled receptors are able to regulate the expression, activity, localization and stability of cell cycle regulatory proteins that either promote or inhibit the initiation of DNA synthesis. In this review, we will detail the intracellular pathways that mediate the G protein-coupled receptor regulation of cellular proliferation, specifically the progression from the G1 phase to the S phase of the cell cycle.

Cell-type-specific G protein-coupled receptor (GPCR) signaling regulates distinct neuronal responses to various stimuli and is essential for axon guidance and targeting during development. However, its function in axonal regeneration in the mature CNS remains elusive. We found that subtypes of intrinsically photosensitive retinal ganglion cells (ipRGCs) in mice maintained high mammalian target of rapamycin (mTOR) levels after axotomy and that the light-sensitive GPCR melanopsin mediated this sustained expression. Melanopsin overexpression in the RGCs stimulated axonal regeneration after optic nerve crush by up-regulating mTOR complex 1 (mTORC1). The extent of the regeneration was comparable to that observed after phosphatase and tensin homolog (Pten) knockdown. Both the axon regeneration and mTOR activity that were enhanced by melanopsin required light stimulation and Gq/11 signaling. Specifically, activating Gq in RGCs elevated mTOR activation and promoted axonal regeneration. Melanopsin overexpression in RGCs enhanced the amplitude and duration of their light response, and silencing them with Kir2.1 significantly suppressed the increased mTOR signaling and axon regeneration that were induced by melanopsin. Thus, our results provide a strategy to promote axon regeneration after CNS injury by modulating neuronal activity through GPCR signaling.axon regeneration | neuronal activity | melanopsin | GPCR | mTOR S evered axons in the adult mammalian CNS do not spontaneously regenerate to restore lost functions. The failure of axons to regenerate is mainly attributed to the diminished growth capacity of neurons as well as an inhibitory environment (1-6). Optic nerves have been extensively studied for mechanisms regulating axon regeneration in CNS. When presented with permissive substrates such as a sciatic nerve graft, only axons of small populations of retinal ganglion cells (RGCs) regrow into the graft (7). When the intrinsic growth program is boosted, distinct subtypes of RGCs regenerate their axons (8). These findings indicate that the differential responses of RGCs to axotomy and growth stimulation are related to their intrinsic properties. One of the critical determinants of the intrinsic regenerative abilities of adult RGCs is neuronal mammalian target of rapamycin (mTOR) activity (9). In retinal axons, the loss of the potential to regrow is accompanied by down-regulation of mTOR activity in RGCs with maturation, and further reduction after axotomy. However, a small percentage of RGCs maintain high mTOR activation levels after optic nerve crush (9, 10). One can ask whether specific subsets of RGCs differ in their ability to maintain mTOR activation. Deciphering the physiological mechanism behind the mTOR maintenance could help elucidate the differential responses of neurons to injury signals and develop strategies to promote axon regeneration.Type 1 melanopsin expressing intrinsically photosensitive retinal ganglion cells (M1 ipRGCs) and αRGCs are resistant to axotomy-induced cell death (8, 11). M1 ipRGCs mainl...

B10 cells are an endothelial clonal line derived spontaneously by culture and selection of rat brain microvascular endothelial cells (1), a source of considerable pharmacological potential. The B10 cells respond to extracellular adenine (but not uracil) nucleotides with intracellular Ca 2ϩ mobilization, mediated by a G-protein-coupled P2Y 1 receptor (2). Whereas the known rat P2Y 1 receptor cDNA could be isolated from the B10 cells (2), no transcript for any other then-known P2Y receptor subtype was detectable in them. However, the B10 cells were found to exhibit another second messenger response, namely the inhibition of stimulated adenylyl cyclase, but with a nucleotide agonist specificity very similar to that of the P2Y 1 receptor (2). It was further demonstrated (3) that selective antagonists of the P2Y 1 receptor, such as adenosine 3Ј-phosphate 5Ј-phosphate, were unable to affect that response, although it showed high sensitivity to 2-propylthio-D-␤,␥-difluoromethylene-ATP (AR-C66096), 1 a specific antagonist (4, 5) of the adenylyl cyclase-inhibitory P2Y T or "P2T" receptor for ADP, which was known as an important functional component of blood platelets. That study (3) demonstrated that a second P2Y receptor activity is present in the B10 cell, with some of its functional features in common with the platelet P2Y T receptor.Recently, a cDNA encoding a nucleotide receptor with a novel seven-transmembrane sequence was identified independently by two groups, starting from a human platelet cDNA library or an orphan human DNA sequence, and shown to be distantly but significantly related to the known P2Y receptors (6, 7). This was designated as the P2Y 12 receptor. Its amino acid sequence lies on a previously unrecognized separate branch of the P2Y family (8). It was characterized in both studies to show its adenine nucleotide specificity and its inhibition of forskolin-stimulated adenylyl cyclase, in which it corresponds to the platelet P2Y T receptor (6, 7). That human P2Y 12 receptor cDNA was expressed again in cell line hosts by other groups, confirming the reported series of agonists but with higher potencies in the cAMP decrease (9) and showing their affinities by competition with the binding of radiolabeled 2-MeSADP (9, 10). In the original identification of the human P2Y 12 receptor, a similar cDNA was also derived from a rat platelet library (6), and its RNA was shown to express in Xenopus oocytes, but this receptor was not further characterized.The question, therefore, obviously arises as to whether the cyclase inhibitory receptor for adenine nucleotides of the B10 capillary endothelial cell is in fact the P2Y 12 receptor. However, this cannot necessarily be assumed, because the P2T receptor has commonly been described in the literature as specific to the platelet (11,12). Furthermore, the adenylyl cyclase-inhibitory P2Y receptor activity in the B10 cell differs in an important

Background: CXCR7 is an atypical heptahelical receptor that functions as scavenger for the endogenous chemokine ligand but fails to signal through G-proteins. Results: The CXCR7 C terminus causes uncoupling from G-protein, rapid receptor-mediated ligand uptake, and constitutive receptor degradation. Conclusion: Atypical CXCR7 functions are encoded in C-terminal elements. Significance: Identification of structural determinants regulating atypical and canonical functions of heptahelical receptors.

Activation of G protein-coupled receptors (GPCRs) leads to stimulation of classical G protein signaling pathways. In addition, GPCRs can activate the mitogen-activated protein kinases (MAPKs) such as the extracellular signal-regulated kinases, c-Jun NH₂-terminal kinases (JNKs), and p38 MAPKs, and thereby influence cell proliferation, cell differentiation and mitogenesis. Cross talk between GPCRs and receptor tyrosine kinases (RTKs) is an incredibly complex process, and the exact signaling molecules involved are largely dependent on the cell type and the type of receptor that is activated. In this review we investigate recent advances that have been made in understanding the mechanisms of cross talk between GPCRs and RTKs, with a focus on GPCR-mediated activation of the Ras/MAPK pathway, GPCR-induced transactivation of RTKs, GPCR-mediated activation of JNK, and p38 MAPK, integration of signals by RhoGTPases, and activation of G protein signaling pathways by RTKs.

The hematopoietic-specific G␣ 16 protein has recently been shown to mediate receptor-induced activation of the signal transducer and activator of transcription 3 (STAT3). In the present study, we have delineated the mechanism by which G␣ 16