Protein receptor for activated C kinase 1 is involved in morphine reward in mice

Establishing protein expression profiles involved in tooth development using a proteomic approach

Various growth and transcription factors are involved in tooth development and developmental abnormalities; however, the protein dynamics do not always match the mRNA expression level. Using a proteomic approach, this study comprehensively analyzed protein expression in epithelial and mesenchymal tissues of the tooth germ during development. First molar tooth germs from embryonic day 14 and 16 Crlj:CD1 (ICR) mouse embryos were collected and separated into epithelial and mesenchymal tissues by laser microdissection. Mass spectrometry of the resulting proteins was carried out, and three types of highly expressed proteins [ATP synthase subunit beta (ATP5B), receptor of activated protein C kinase 1 (RACK1), and calreticulin (CALR)] were selected for immunohistochemical analysis. The expression profiles of these proteins were subsequently evaluated during all stages of amelogenesis using the continuously growing incisors of 3-week-old male ICR mice. Interestingly, these three proteins were specifically expressed depending on the stage of amelogenesis. RACK1 was highly expressed in dental epithelial and mesenchymal tissues during the proliferation and differentiation stages of odontogenesis, except for the pigmentation stage, whereas ATP5B and CALR immunoreactivity was weak in the enamel organ during the early stages, but became intense during the maturation and pigmentation stages, although the timing of the increased protein expression was different between the two. Overall, RACK1 plays an important role in maintaining the cell proliferation and differentiation in the apical end of incisors. In contrast, ATP5B and CALR are involved in the transport of minerals and the removal of organic materials as well as matrix deposition for CALR.

Protein phosphatase 2A deficiency in hippocampal CA1 inhibits priming effect of morphine on conditioned place preference in mice

Studies have shown that protein phosphorylation plays an important role in morphine abuse. However, the neurobiological mechanism of protein phosphatase 2A (PP2A) underlying the morphine-priming process is still unclear. Here we constructed T29-2-Cre; PP2Afl/fl conditional knockout mice (KO) and investigated the role of hippocampal PP2A in morphine priming. We observed that the deficit of PP2A inhibited the priming behavior of morphine and blocked the priming-induced long-term potentiation (LTP) in the hippocampus of KO mice. Moreover, the expression levels of Rack1 and the membrane GluN2B were significantly reduced in the nucleus accumbens of KO mice compared with those in the control mice, which may be attributed to the decreased HDAC4 in the hippocampus of KO mice. Consistent with it, the similar inhibited priming effects were also observed in the wild-type mice treated with sodium butyrate (NaB)-a nonspecific inhibitor of histone deacetylases-3 h after morphine administration. Taken together, our results suggest that hippocampal PP2A may be involved in morphine priming through the PP2A/HDAC4/Rack1 pathway.

Structural Plasticity of Dopaminergic Neurons Requires the Activation of the D3R-nAChR Heteromer and the PI3K-ERK1/2/Akt-Induced Expression of c-Fos and p70S6K Signaling Pathway

We have previously shown that the heteromer composed by the dopamine D3 receptor (D3R) and the nicotinic acetylcholine receptor (nAChR) (D3R-nAChR heteromer) is expressed in dopaminergic neurons, activated by nicotine and represents the molecular unit that, in these neurons, contributes to the modulation of critical events such as structural plasticity and neuroprotection. We now extended this study by investigating the D3R-nAChR heteromer properties using various cell models such as transfected HEK293 cells, primary cultures of mouse dopaminergic neurons and human dopaminergic neurons derived from induced pluripotent stem cells.

We found that the D3R-nAChR heteromer is the molecular effector that transduces the remodeling properties not only associated with nicotine but also with D3R agonist stimulation: neither nAChR nor D3R, in fact, when express as monomers, are able to elicit these effects. Moreover, strong and sustained activation of the PI3K-ERK1/2/Akt pathways is coupled with D3R-nAChR heteromer stimulation, leading to the expression of the immediate-early gene c-Fos and to sustained phosphorylation of cytosolic p70 ribosomal S6 kinase (p70S6K), critical for dendritic remodeling. By contrast, while D3R stimulation results in rapid and transient activation of both Erk1/2 and Akt, that is PI3K-dependent, stimulation of nAChR is associated with persistent activation of Erk1/2 and Akt, in a PI3K-independent way. Thus, the D3R-nAChR heteromer and its ability to trigger the PI3K-ERK1/2/Akt signaling pathways may represent a novel target for preserving dopaminergic neurons healthy and for conferring neuronal protection against injuries.

MicroRNA-132 is involved in morphine dependence via modifying the structural plasticity of the dentate gyrus neurons in rats

Repeated morphine exposure has been shown to induce neuronal plasticity in reward-related areas of the brain. miR-132, a CREB-induced and activation-dependent microRNA, has been suggested to be involved in the neuronal plasticity by increasing neuronal dendritic branches and spinogenesis. However, it is still unclear whether miR-132 is related to morphine dependence. Here, we investigate whether miR-132 is involved in morphine dependence and whether it is related to the structural plasticity of the dentate gyrus (DG) neurons. Sprague-Dawley rats are treated with increasing doses of morphine injection for six consecutive days to develop morphine dependence. Our results show that dendritic branching and spinogenesis of the DG neurons of morphine dependent rats are increased. Morphine treatment (24 h) promotes the differentiation of N2a cells stably expressing μ-opioid receptor by up-regulating miR-132 expression. Moreover, inhibiting miR-132 3p (but not 5p) of the DG neurons can reverse the structural plasticity and disrupt the formation of morphine dependence in rats. These findings indicate that miR-132 in the DG neurons is involved in morphine dependence via modifying the neuronal plasticity.

Changes in c-fos and p-CREB signaling following exposure to forced swim stress or exogenous corticosterone during morphine-induced place preference are dependent on glucocorticoid receptor in the basolateral amygdala

Neural circuitry comprising the nucleus accumbens (NAc), prefrontal cortex (PFC), amygdala (AMY), and hippocampus (HIP) are the main components of the reward circuit. Our previous behavioral data showed that forced swim stress (FSS) and corticosterone administration could inhibit the acquisition of morphine-induced conditioned place preference (CPP), and this effect was blocked by intra-basolateral amygdala (BLA) administration of RU38486, glucocorticoid receptor (GR) antagonist. Therefore, we tried to evaluate the effect of intra-BLA administration of the GR antagonist during the conditioning phase on the c-fos and p-CREB/CREB ratio expression in the AMY, NAc, PFC, and HIP of rats that underwent FSS or received exogenous corticosterone (10 mg/kg; i.p.) before morphine injection (5 mg/kg; s.c.) during 3 conditioning days. Our results showed that morphine-induced CPP could increase c-fos level and p-CREB/CREB ratio in all regions (except in the HIP). In addition, c-fos expression was elevated by FSS in all regions and blockade of GR decreased this effect. In the PFC, in addition to FSS, corticosterone could raise c-fos expression, which was blocked by RU38486. In conclusion, it seems that the intra-BLA administration of RU38486 differently modulates the effect of morphine-induced CPP on the expression of c-fos and p-CREB/CREB ratio in animals that underwent FSS or corticosterone administration.

Morphine-induced RACK1-dependent autophagy in immortalized neuronal cell lines

BACKGROUND AND PURPOSE

Autophagy is a critical cellular catabolic process in cell homoeostasis and brain function. Recent studies indicate that receptor for activated C kinase 1 (RACK1) is involved in autophagosome formation in Drosophila and mice, and that it plays an essential role in morphine-associated memory. However, the exact mechanism of the role of RACK1 in morphine-induced autophagy is not fully understood.

EXPERIMENTAL APPROACH

SH-SY5Y cells were cultured and morphine, rapamycin, 3-methyladenine and RACK1 siRNA were used to evaluate the regulation of RACK1 protein in autophagy. Western blotting and immunofluorescence were used to assess protein expression.

KEY RESULTS

Activation of autophagy (i.e. autophagosome accumulation and an increase in the LC3-II/LC3-I ratio) induced by morphine contributes to the maintenance of conditioned place preference (CPP) memory in mice. Moreover, morphine treatment significantly increased Beclin-1 expression and decreased the p-mTOR/mTOR and SQSTM1/p62 levels, whereas knockdown of RACK1 prevented morphine-induced autophagy in vitro. Furthermore, we found that in the mouse hippocampus, knockdown of RACK1 also markedly suppressed morphine-induced autophagy (decreased LC3-II/LC3-I ratio and increased p-mTOR/mTOR ratio). Importantly, morphine-induced autophagy in a RACK1-dependent manner. Conversely, morphine-induced RACK1 upregulation in vitro is partially inhibited by autophagy feedback.

CONCLUSIONS AND IMPLICATIONS

Our findings revealed a critical role for RACK1-dependent autophagy in morphine-promoted maintenance of CPP memory in mice and supported the notion that control of RACK1-dependent autophagic pathways may become an important target for novel therapeutics for morphine-associated memory.

Orexin A-mediated AKT signaling in the dentate gyrus contributes to the acquisition, expression and reinstatement of morphine-induced conditioned place preference

Accumulating evidence indicates that the hippocampal dentate gyrus (DG), a critical brain region contributing to learning and memory, is involved in the addiction and relapse to abused drugs. Emerging studies also suggest the role of orexin signaling in the rewarding behavior induced by repeated exposure to opiates. In the present study, we investigated the dynamic adaptation of orexin signaling in the DG and its functional significance in the acquisition, expression, maintenance of and relapse to rewarding behavior induced by morphine. Repeated place conditioning with morphine significantly increased the orexin A content released from the lateral hypothalamic area projecting neurons into the DG. Local infusions of orexin A into the DG sensitized the acquisition of and relapse to the conditioned place preference induced by morphine. The application of the orexin receptor type 1 (OXR1) antagonist SB334867 significantly abolished the acquisition, expression and maintenance of the conditioned place preference induced by repeated exposure to morphine. Furthermore, the significant increase of the phosphorylation of AKT in the DG was associated with preference for the morphine-paired chamber in rats, which was reversed by the local administration of an OXR1 antagonist. Thus, these findings suggested that the dynamic upregulation of orexin A signaling, via the AKT pathway in the DG, may promote the acquisition and maintenance of opioid-induced craving behaviors and may increase sensitivity to the rewarding effect of subsequent opioids.

RACK1 promotes maintenance of morphine-associated memory via activation of an ERK-CREB dependent pathway in hippocampus

Existence of long-term drug-associated memories may be a crucial factor in drug cravings and relapse. RACK1 plays a critical role in morphine-induced reward. In the present study, we used conditioned place preference (CPP) to assess the acquisition and maintenance of morphine conditioned place preference memory. The hippocampal protein level of RACK1 and synaptic quantitation were evaluated by Western blotting, immunohistochemistry and electron microscopy, respectively. Additionally, shRACK1 (shGnb2l1) was used to silence RACK1 in vivo to evaluate the role and the underlying mechanism of RACK1 in maintenance of morphine CPP memory. We found that morphine induced CPP was maintained for at least 7 days after the last morphine treatment, which indicated a positive correlation with hippocampal RACK1 level, and was accompanied simultaneously by increases in the synapse density and hippocampal expression of synaptophysin (SYP), phosphorylation of extracellular signal-regulated kinase1/2 (pERK1/2) and the phosphorylation of cyclic adenosine monophosphate response element-binding (pCREB). ShGnb2l1 icv injection significantly suppressed the expression of all above proteins, decreased the synapse density in the hippocampus and attenuated the acquisition and maintenance of morphine CPP. Our present study highlights that RACK1 plays an important role in the maintenance of morphine CPP, likely via activation of ERK-CREB pathway in hippocampus.

The region-specific activation of Ca2+/calmodulin dependent protein kinase II and extracellular signal-regulated kinases in hippocampus following chronic alcohol exposure

Previous studies suggest that hippocampal CA1, CA3, and DG regions may have distinct roles in alcohol dependence. Extracellular signal-regulated kinases (ERKs) and Ca(2+)/calmodulin dependent protein kinase II (CaMKII) have been shown to contribute to the molecular mechanism underlying drug dependence and relapse, and there may be an interaction between the activation of ERKs and CaMKII. However, little is known regarding the mechanisms underlying the effects of alcohol exposure, withdrawal, and relapse, particularly with regard to the interaction between CaMKII and ERK1/2 signaling in hippocampal subregions. In the present study, rats were provided water containing 6% alcohol as their only drinking source. We found that alcohol exerted locomotor stimulant and anxiolytic effects on rats in open field behaviors. Following chronic alcohol exposure, phospho-ERK1/2 was significantly decreased in the DG. Alcohol withdrawal was associated with an increase of phospho-ERK1/2 in the CA1 and DG, while alcohol re-exposure induced a decrease of phospho-ERK1/2 in the CA1, CA3, and DG. The activation of CaMKII (Thr286) correlated with the effects of alcohol on phospho-ERK1/2. Our results indicate that region-specific activation CaMKII-ERK1/2 signaling in the hippocampal CA1 and DG may play an important role in alcohol dependence.

Receptor for activated C kinase ortholog of second-generation merozoite in Eimeria tenella: clone, characterization, and diclazuril-induced mRNA expression

The receptor for activated C kinase (RACK) cDNA of second-generation merozoites of Eimeria tenella was cloned using reverse transcriptase polymerase chain reaction and rapid amplification of cDNA ends, compared with other species, and then successfully expressed using the pET-28a vector in Escherichia coli BL21 (DE3) (EtRACK). Nucleotide sequence analysis revealed that the full length of the cloned cDNA (1,264 bp) encompassed a 957-bp open reading frame encoding a polypeptide of 318 residues with an estimated molecular mass of 34.94 kDa and a theoretical isoelectric point of 5.97. Molecular analysis of EtRACK reveals the presence of seven WD40 repeat motifs. EtRACK localizes to the cytoplasm and nucleus in second-generation merozoites of E. tenella. The cDNA sequence has been submitted to the GenBank Database with accession number JQ292804. EtRACK shared 98% homology with the published sequence of a RACK protein from Toxoplasma gondii at the amino acid level (GenBank XP_002370996.1). Recombinant protein expression was induced using 1 mM of isopropyl β-D-1-thiogalactopyranoside in vitro at 30 °C. Sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis showed that the 39.79-kDa fusion protein existed in unsolvable form. Quantitative real-time PCR analysis showed that compared with the control group, the level of EtRACK mRNA expression in the treatment group was downregulated by 81.3% by diclazuril treatment. The high similarity of EtRACK to previously described RACKs of other organisms, as well as its downregulated expression in second-generation merozoites induced by diclazuril, suggests that it could play a key role in the signaling event that precedes protein secretion and parasite invasion. Moreover, the downregulation of EtRACK mRNA expression also enriches studies on the mechanism of action of diclazuril on E. tenella.

RACK1, A multifaceted scaffolding protein: Structure and function

The Receptor for Activated C Kinase 1 (RACK1) is a member of the tryptophan-aspartate repeat (WD-repeat) family of proteins and shares significant homology to the β subunit of G-proteins (Gβ). RACK1 adopts a seven-bladed β-propeller structure which facilitates protein binding. RACK1 has a significant role to play in shuttling proteins around the cell, anchoring proteins at particular locations and in stabilising protein activity. It interacts with the ribosomal machinery, with several cell surface receptors and with proteins in the nucleus. As a result, RACK1 is a key mediator of various pathways and contributes to numerous aspects of cellular function. Here, we discuss RACK1 gene and structure and its role in specific signaling pathways, and address how posttranslational modifications facilitate subcellular location and translocation of RACK1. This review condenses several recent studies suggesting a role for RACK1 in physiological processes such as development, cell migration, central nervous system (CN) function and circadian rhythm as well as reviewing the role of RACK1 in disease.

RACK1 affects morphine reward via BDNF

Chronic morphine addiction may trigger functional changes in the mesolimbic dopamine system, which is believed to be the neurobiological substrate of opiate addiction. Brain derived neurotrophic factor (BDNF) has been implicated in addiction-related pathology in animal studies. Our previous studies have shown that RACK1 is involved in morphine reward in mice. The recent research indicates nuclear RACK1 by localizing at the promoter IV region of the BDNF gene and the subsequent chromatin modifications leads to the activation of the promoter and transcription of BDNF. The present study was designed to investigate if shRACK1 (a short hairpin RNA of RACK1) could reverse the mice's behavioral responses to morphine and BDNF expression in hippocampus and prefrontal cortex. No significant changes were observed in vehicle-infused mice which received no morphine treatment (CONC) and shRACK1-infused mice which received no morphine treatment (CONR), whereas vehicle-infused mice preceded the morphine injection (MIC) showed increased BDNF expression in hippocampus and prefrontal cortex, as compared to vehicle-infused mice which received no morphine treatment (CONC). Intracerebroventricular shRACK1 treatment reversed these, and in fact, ShRACK1-infused mice preceded the morphine injection (MIR) showed reduced BDNF expression in hippocampus and prefrontal cortex, as compared to MIC. In the conditioned place preference (CPP) test, inactivating RACK1 markedly reduces morphine-induced conditioned place preference. Non-specific changes in CPP could not account for these effects since general CPP of shRACK1- and vehicle-infused animals was not different. Combined behavioral and molecular approaches have support the possibility that the RACK1-BDNF system plays an important role in the response to morphine-induced reward.

Structure of the RACK1 dimer from Saccharomyces cerevisiae

Receptor for activated C-kinase 1 (RACK1) serves as a scaffolding protein in numerous signaling pathways involving kinases and membrane-bound receptors from different cellular compartments. It exists simultaneously as a cytosolic free form and as a ribosome-bound protein. As part of the 40S ribosomal subunit, it triggers translational regulation by establishing a direct link between protein kinase C and the protein synthesis machinery. It has been suggested that RACK1 could recruit other signaling molecules onto the ribosome, providing a signal-specific modulation of the translational process. RACK1 is able to dimerize both in vitro and in vivo. This homodimer formation has been observed in several processes including the regulation of the N-methyl-d-aspartate receptor by the Fyn kinase in the brain and the oxygen-independent degradation of hypoxia-inducible factor 1. The functional relevance of this dimerization is, however, still unclear and the question of a possible dimerization of the ribosome-bound protein is still pending. Here, we report the first structure of a RACK1 homodimer, as determined from two independent crystal forms of the Saccharomyces cerevisiae RACK1 protein (also known as Asc1p) at 2.9 and 3.9 Å resolution. The structure reveals an atypical mode of dimerization where monomers intertwine on blade 4, thus exposing a novel surface of the protein to potential interacting partners. We discuss the significance of the dimer structure for RACK1 function.