Mouse kappa-opioid receptor mRNA differential transport in neurons

Dynorphin / kappa-opioid receptor regulation of excitation-inhibition balance toggles afferent control of prefrontal cortical circuits in a pathway-specific manner

The medial prefrontal cortex (mPFC) controls behavior via connections with limbic excitatory afferents that engage various inhibitory motifs to shape mPFC circuit function. The dynorphin (Dyn) / kappa-opioid receptor (KOR) system is highly enriched in the mPFC, and its dysregulation is implicated in neuropsychiatric disorders. However, it is unclear how the Dyn / KOR system modulates excitatory and inhibitory circuits that are integral for mPFC information processing and behavioral control. Here, we provide a circuit-based framework wherein mPFC Dyn / KOR signaling regulates excitation-inhibition balance by toggling which afferents drive mPFC neurons. Dyn / KOR regulation of afferent inputs is pathway-specific. Dyn acting on presynaptic KORs inhibits glutamate release from afferent inputs to the mPFC, including the basolateral amygdala (BLA), paraventricular nucleus of the thalamus, and contralateral cortex. The majority of excitatory synapses to mPFC neurons, including those from the ventral hippocampus (VH), do not express presynaptic KOR, rendering them insensitive to Dyn / KOR modulation. Dyn / KOR signaling also suppresses afferent-driven recruitment of specific inhibitory sub-networks, providing a basis for Dyn to disinhibit mPFC circuits. Specifically, Dyn / KOR signaling preferentially suppresses SST interneuron- relative to PV interneuron-mediated inhibition. Selective KOR action on afferents or within mPFC microcircuits gates how distinct limbic inputs drive spiking in mPFC neurons. Presynaptic Dyn / KOR signaling decreases KOR-positive input-driven (e.g. BLA) spiking of mPFC neurons. In contrast, KOR-negative input recruitment of mPFC neurons is enhanced by Dyn / KOR signaling via suppression of mPFC inhibitory microcircuits. Thus, by acting on distinct circuit elements, Dyn / KOR signaling shifts KOR-positive and negative afferent control of mPFC circuits, providing mechanistic insights into the role of neuropeptides in shaping mPFC function. Together, these findings highlight the utility of targeting the mPFC Dyn / KOR system as a means to treat neuropsychiatric disorders characterized by dysregulation in mPFC integration of long-range afferents with local inhibitory microcircuits.

Blockade of kappa opioid receptors reduces mechanical hyperalgesia and anxiety-like behavior in a rat model of trigeminal neuropathic pain

It has been shown that kappa opioid receptor (KOR) antagonists, such as nor-binaltorphimine (nor-BNI), have antinociceptive effects in some pain models that affect the trigeminal system. Also, its anxiolytic-like effect has been extensively demonstrated in the literature. The present study aimed to investigate the systemic, local, and central effect of nor-BNI on trigeminal neuropathic pain using the infraorbital nerve constriction model (CCI-ION), as well as to evaluate its effect on anxiety-like behavior associated with this model. Animals received nor-BNI systemically; in the trigeminal ganglion (TG); in the subarachnoid space to target the spinal trigeminal nucleus caudalis (Sp5C) or in the central amygdala (CeA) 14 days after CCI-ION surgery. Systemic administration of nor-BNI caused a significant reduction of facial mechanical hyperalgesia and promoted an anxiolytic-like effect, which was detected in the elevated plus-maze and the light-dark transition tests. When administered in the TG or CeA, the KOR antagonist was able to reduce facial mechanical hyperalgesia induced by CCI-ION, but without changing the anxiety-like behavior. Moreover, no change was observed on nociception and anxiety-like behavior after nor-BNI injection into the Sp5C. The present study demonstrated antinociceptive and anxiolytic-like effects of nor-BNI in a model of trigeminal neuropathic pain. The antinociceptive effect seems to be dissociated from the anxiolytic-like effect, at both the sites involved and at the dose need to achieve the effect. In conclusion, the kappa opioid system may represent a promising target to be explored for the control of trigeminal pain and associated anxiety. However, further studies are necessary to better elucidate its functioning and modulatory role in chronic trigeminal pain states.

RNA imaging in living cells - methods and applications

Numerous types of transcripts perform multiple functions in cells, and these functions are mainly facilitated by the interactions of the RNA with various proteins and other RNAs. Insight into the dynamics of RNA biosynthesis, processing and cellular activities is highly desirable because this knowledge will deepen our understanding of cell physiology and help explain the mechanisms of RNA-mediated pathologies. In this review, we discuss the live RNA imaging systems that have been developed to date. We highlight information on the design of these systems, briefly discuss their advantages and limitations and provide examples of their numerous applications in various organisms and cell types. We present a detailed examination of one application of RNA imaging systems: this application aims to explain the role of mutant transcripts in human disease pathogenesis caused by triplet repeat expansions. Thus, this review introduces live RNA imaging systems and provides a glimpse into their various applications.

Insights into the roles of local translation from the axonal transcriptome

Much of our knowledge on the roles of intra-axonal translation derives from the characterization of a small number of individual mRNAs that were found to be localized in axons. However, two recent studies, using large-scale approaches to provide a more comprehensive characterization of the axonal transcriptome, have led to the discovery of thousands of axonal mRNAs. The apparent abundance of mRNAs in axons raises the possibility that local translation has many more functions than previously thought. Here, we review the recent studies that have profiled axonal mRNAs and discuss how the identification of axonal transcripts might point to unappreciated roles for local translation in axons.

Stress-induced epigenetic regulation of κ-opioid receptor gene involves transcription factor c-Myc

Exposure to stress is associated with adverse emotional and behavioral responses. Whereas the κ-opioid receptor (KOR) system is known to mediate some of the effects, it is unclear whether and how stress affects epigenetic regulation of this gene. Because the KOR gene can use two promoters (Pr1 and Pr2) and two polyadenylation signals (PA1 and PA2), it is also interesting whether and how these distinct regulatory mechanisms are differentially modulated by stress. The current study examined the effects of stress on these different regulatory mechanisms of the KOR gene. Results showed that stress selectively increased the expression of KOR mRNA isoforms controlled by Pr1 and terminated at PA1 in specific brain areas including the medial-prefrontal cortex, hippocampus, brainstem, and sensorimotor cortex, but not in the amygdala or hypothalamus. These effects correlated with altered epigenetic state of KOR Pr1 chromatin, as well as elevation and increased recruitment of the principal transcription factor c-Myc, which could activate Pr1. Stress-induced modulation of Pr1 was further validated using glutamate-sensitive murine hippocampal cell line, HT22. The results revealed a common molecular mechanism underlying the effect of stress on selected chromatin regions of this gene at the cellular level and in the context of whole animal and identified a critical role for c-Myc in stress-triggered epigenetic regulation of the KOR gene locus. This study sheds light on the mechanisms of stress-induced epigenetic regulation that targets specific chromatin segments and suggests certain KOR transcripts and its principal transcription factor c-Myc as potential targets for brain-area-specific intervention.

Axonal mRNA localization and local protein synthesis in nervous system assembly, maintenance and repair

mRNAs can be targeted to specific neuronal subcellular domains, which enables rapid changes in the local proteome through local translation. This mRNA-based mechanism links extrinsic signals to spatially restricted cellular responses and can mediate stimulus-driven adaptive responses such as dendritic plasticity. Local mRNA translation also occurs in growing axons where it can mediate directional responses to guidance signals. Recent profiling studies have revealed that both growing and mature axons possess surprisingly complex and dynamic transcriptomes, thereby suggesting that axonal mRNA localization is highly regulated and has a role in a broad range of processes, a view that is increasingly being supported by new experimental evidence. Here, we review current knowledge on the roles and regulatory mechanisms of axonal mRNA translation and discuss emerging links to axon guidance, survival, regeneration and neurological disorders.

The dynorphin/κ-opioid receptor system and its role in psychiatric disorders

The dynorphin/κ-opioid receptor system has been implicated in the pathogenesis and pathophysiology of several psychiatric disorders. In the present review, we present evidence indicating a key role for this system in modulating neurotransmission in brain circuits that subserve mood, motivation, and cognitive function. We overview the pharmacology, signaling, post-translational, post-transcriptional, transcriptional, epigenetic and cis regulation of the dynorphin/κ-opioid receptor system, and critically review functional neuroanatomical, neurochemical, and pharmacological evidence, suggesting that alterations in this system may contribute to affective disorders, drug addiction, and schizophrenia. We also overview the dynorphin/κ-opioid receptor system in the genetics of psychiatric disorders and discuss implications of the reviewed material for therapeutics development.

Small-molecule fluorescent probes for specific RNA targets

A method was developed that uses small molecules as fluorescent probes to detect specific mRNAs. In this approach, the fluorescence of fluorophore-quencher conjugates is restored by the binding of an mRNA aptamer tag to the quencher segment of the molecules. The method allows real-time detection of mRNA transcripts in vitro.

Dysregulation of axonal transport and motorneuron diseases

MNDs (motorneuron diseases) are neurodegenerative disorders in which motorneurons located in the motor cortex, in the brainstem and in the spinal cord are affected. These diseases in their inherited or sporadic forms are mainly characterized by motor dysfunctions, occasionally associated with cognitive and behavioural alterations. Although these diseases show high variability in onset, progression and clinical symptoms, they share common pathological features, and motorneuronal loss invariably leads to muscle weakness and atrophy. One of the most relevant aspect of these disorders is the occurrence of defects in axonal transport, which have been postulated to be either a direct cause, or a consequence, of motorneuron degeneration. In fact, due to their peculiar morphology and high energetic metabolism, motorneurons deeply rely on efficient axonal transport processes. Dysfunction of axonal transport is known to adversely affect motorneuronal metabolism, inducing progressive degeneration and cell death. In this regard, the understanding of the fine mechanisms at the basis of the axonal transport process and of their possible alterations may help shed light on MND pathological processes. In the present review, we will summarize what is currently known about the alterations of axonal transport found to be either causative or a consequence of MNDs.

Local translation of mRNAs in neural development

Growing axons encounter numerous developmental signals to which they must promptly respond in order to properly form complex neural circuitry. In the axons, these signals are often transduced into a local increase or decrease in protein levels. Contrary to the traditional view that the cell bodies are the exclusive source of axonal proteins, it is becoming increasingly clear not only that de novo protein synthesis takes place in axons, but also that it is required for the axons to respond to certain signals. Here we review the current knowledge of local mRNA translation in developing neurons with a special focus on protein synthesis occurring in axons and growth cones.

The RNA superhighway: axonal RNA trafficking of kappa opioid receptor mRNA for neurite growth

Neurons are highly polarized cells with extensive sub-cellular compartmentalization to accommodate diverse local needs. Information flows between the pre-synaptic (axon) and post-synaptic (dendrite) compartments, as well as between the soma and the nerve termini. It is critical that a neuron controls efficient molecular transfer/transport through its axon. But this is extremely challenging to study because of the long distance molecules must travel through axons and the apparent contextual difference in the axons' various local environments, which should not be examined in isolation. Understanding the action in neurons of drug-responsive neurotransmitter receptors such as opioid receptors has been hindered by the lack of information on the control of molecular flow between such various sub-cellular neuron compartments. Recent studies have uncovered new transport systems other than the classical vesicle transport in neurons, particularly those utilizing various granules containing certain RNAs including protein-coding mRNAs. Through integrated approaches exploiting various experimental systems, tools, and methodologies, studies have provided solid evidence for functional roles of specific RNA granules in several biological processes crucial for the survival and function of neurons. These include neurons' transport of molecules/information, stress response, and local axonal translation. By using the kappa opioid receptor (KOR) as a model, studies have also revealed a novel physiological function of KOR in mediating growth factor-stimulated neurite outgrowth during a critical period of development, which requires specific KOR mRNA untranslated sequences that direct spatially and temporally specific expression of KOR.

Kappa opioid receptor contributes to EGF-stimulated neurite extension in development

Epidermal growth factor (EGF), a mitogen, also stimulates neurite extension during development, but the underlying mechanism is elusive. This study reveals a functional role for kappa opioid receptor (KOR) in EGF-stimulated neurite extension, and the underlying mechanism. EGF and activated EGF receptor (EGFR) levels are elevated in embryonic spinal cords during late gestation stages, with concurrent rise in protein levels of KOR and axon extension markers, growth-associated protein 43 (GAP43), and transient axonal glycoprotein-1 (TAG-1). Both GAP43 and TAG-1 levels are significantly lower in KOR-null (KOR(-/-)) spinal cords, and EGFR inhibitors effectively reduce the levels of KOR, GAP43, and TAG-1 in wild-type embryonic spinal cords. For KOR(-/-) or KOR-knockdown dorsal root ganglion (DRG) neurons, EGF can no longer effectively stimulate axon extension, which can be rescued by introducing a constitutive KOR expressing vector but not by a regulated KOR vector carrying its 5' untranslated region, which can be bound and repressed by growth factor receptor-bound protein 7 (Grb7). Furthermore, blocking KOR activation by application of anti-dynorphin, KOR antagonist, or EGFR inhibitor effectively reduces axon extension of DRG neurons. Thus, EGF-stimulated axon extension during development is mediated, at least partially, by specific elevation of KOR protein production at posttranscriptional level, as well as activation of KOR signaling. The result also reveals an action of EGF to augment posttranscriptional regulation of certain mRNAs during developmental stages.

Gonadal hormones decrease temporomandibular joint kappa-mediated antinociception through a down-regulation in the expression of kappa opioid receptors in the trigeminal ganglia

We have previously demonstrated that activation of kappa-opioid receptor located in the temporomandibular joint (TMJ) of rats induces a significantly greater TMJ antinociception in diestrus females than in proestrus females (higher estradiol serum levels than diestrus) and males. These findings indicate that gonadal hormones decrease TMJ kappa-mediated antinociception. The aim of this study was to investigate some of the mechanisms by which gonadal hormones decrease TMJ kappa-mediated antinociception. Western blot analysis demonstrated a significantly lower kappa-opioid receptor expression in the trigeminal ganglia of intact males than in intact and ovariechtomized (OVX) females and orchidectomized (ORX) males. In females, kappa-opioid receptor expression in the trigeminal ganglia was significantly lower in proestrus than in diestrus and OVX females. Taken together these findings suggest that gonadal hormones, especially male gonadal hormones, down-regulate kappa-opioid receptor expression. Co-application of the NOS inhibitor L-NMMA or the NO-sensitive guanylyl cyclase inhibitor ODQ with the kappa-opioid receptor agonist U50,488 blocked TMJ kappa-mediated antinociception in males and females. These findings suggest that antinociception induced by activation of kappa opioid receptors in the TMJ region is mediated by the L-arginine/NO/cGMP pathway in both sexes. Despite the involvement of the L-arginine/NO/cGMP pathway in TMJ kappa-mediated antinociception in both sexes, gonadal hormones do not diminish the activity of this pathway to decrease TMJ kappa-mediated antinociception. Alternatively, they significantly reduce kappa-opioid receptor expression in the trigeminal ganglia.

Effect of chronic intra-peritoneally administered chimeric peptide of met-enkephalin and FMRFa-[D-Ala2]YFa-on antinociception and opioid receptor regulation

The physiological role of NPFF/FMRFa family of peptides is complex and exact mechanism of action of these peptides is not yet completely understood. In same line of scrutiny, previously we reported an enzymatically stable chimeric analog of YGGFMKKKFMRFamide (YFa) i.e., [D-Ala(2)]YAGFMKKKFMRFamide ([D-Ala(2)]YFa) which have a role in antinociception and modulatory effect on opioid analgesia. In continuation, presently we investigated using tail-flick test whether [D-Ala(2)]YFa on systemic administration induced any antinociception in rats and if so then which specific opioid receptor(s) mu, delta or kappa mediated it. Further, the antinociceptive effect of [D-Ala(2)]YFa on 6 days chronic intra-peritoneal (i.p.) treatment in rats was examined and finally, effect of this chronic treatment on the differential expression of opioid receptors was assessed. [D-Ala(2)]YFa on i.p. administration induced dose dependent antinociception which was mainly mediated by delta (DOR) and partially by mu (MOR) and kappa (KOR) opioid receptors. Moreover, its antinociceptive effect remained comparable throughout the chronic treatment even during insufficient availability of DOR1. Importantly, during this treatment the mRNA expression of all three opioid receptors (MOR1, KOR1 and DOR1) was increased as assessed by real-time RTPCR though subsequent western blot analysis revealed a selective increase in the protein level of DOR1, only. Thus, pharmacological behavior of [d-Ala(2)]YFa suggests that competency of an opioid agonist to bind with multiple opioid receptors may enhance its potency to induce tolerance free analgesia.

Dynein motor contributes to stress granule dynamics in primary neurons

Mobilization and translation of mRNAs, two important events believed to involve stress granules (SGs), in neurons are important for their survival and activities. However, the formation and disassembly of SGs in neurons remains unclear. By using an arsenite-induced neuronal stress model of rat primary spinal cord neuron cultures, we demonstrate the formation of SGs that contain common SG components and RNAs in both stressed neuronal cell bodies and their neurites. By employing small interfering RNA (siRNA) knockdown, we discovered that dynein motor subunit localizes in SG, and is important for SG assembly in neurons. Under stress, dynein motor subunit also facilitates translational repression and enhances the formation and integrity of SG in neurons. By blocking the energy source of dynein motor, both the formation and disassembly of SG are attenuated. These findings demonstrate, for the first time, that dynein motor complex plays a critical role in the dynamics of neuronal SGs, as well as translation of certain mRNAs.

YFa, a chimeric opioid peptide, induces kappa-specific antinociception with no tolerance development during 6 days of chronic treatment

Our previous study showed that YGGFMKKKFMRFamide (YFa), a chimeric peptide of Met-enkephalin, and Phe-Met-Arg-Phe-NH2 induced naloxone-reversible antinociception and attenuated the development of tolerance to morphine analgesia. In continuation, the present study investigated which specific opioid receptors-mu, delta or kappa-mediate the observed YFa antinociception pharmacologically using specific antagonists and whether chronic administration of YFa at 26.01 micromol/kg per day induces tolerance and its effect on the expression of mu and kappa opioid receptors from day 4 to day 6, with endomorphine-1 (EM-1) and saline taken as positive and negative controls, respectively. Quantitative differential expression analysis was carried out by real-time reverse-transcriptase polymerase chain reaction, and the corresponding changes in protein levels were assessed by Western blot. A pharmacological investigation revealed that nor-binaltorphimine, a specific kappa opioid receptor-1 (KOR1) antagonist, completely antagonized the antinociception induced by 39.01 micromol/kg of YFa. Importantly, its chronic intraperitoneal administration did not result in significant tolerance over 6 days, whereas EM-1 induced significant tolerance after day 4. Differential expression analysis revealed that EM-1 caused up-regulation of mu opioid receptor-1 on day 4, followed by down-regulation on later days. Interestingly, YFa treatment caused a decrease on day 4, followed by an increase in the expression of KOR1 from day 5 onward. In conclusion, YFa induces kappa-specific antinociception, with no development of tolerance during 6 days of chronic treatment, which further articulates new directions for improved designing of peptide-based analgesics that may be devoid of adverse effects like tolerance.

Epigenetic regulation of kappa opioid receptor gene in neuronal differentiation

The gene of mouse kappa opioid receptor (KOR) utilizes two promoters, P1 and P2. P1 is active in various brain areas and constitutively in P19 mouse embryonal carcinoma cells. P2 is active in limited brain stem areas of adult animals and only in late differentiated cells of P19 induced for neuronal differentiation in the presence of nerve growth factor (NGF). NGF response of P2 was found to be mediated by a specific binding site for transcription factor activation protein 2 (AP2) located in P2. Electrophoretic gel shift assay showed specific binding of this AP2 site by AP2beta, but not AP2alpha. Knockdown of endogenous AP2beta with siRNA abolished the stimulating effect of NGF on the expression of transcripts driven by P2. Binding of endogenous AP2beta on the endogenous KOR P2 chromatin region was also confirmed by chromatin immunoprecipitation. The effect of NGF was inhibited by LY2942002 (phosphatidylinositol 3-kinase, PI3K inhibitor), suggesting that PI3K was involved in signaling pathway mediating the effect of NGF stimulation on KOR P2. The chromatin of P2 in P19 was found to be specifically modified following NGF stimulation, which included demethylation at Lys9 and dimethylation at Lys4 of histone H3 and was consistent with the increased recruitment of RNA polymerase II to this promoter. This study presents the first evidence for epigenetic changes occurred on a specific KOR promoter triggered by NGF in cells undergoing neuronal differentiation. This epigenetic change is mediated by recruited AP2beta to this promoter and involves the PI3K system.

Regulation of stress granule dynamics by Grb7 and FAK signalling pathway

Cells form stress granules (SGs) in response to environmental stresses, which constitute cytoplasmic domains where mRNAs are stored and translation is halted. Although several components are found in SGs, it is poorly understood as to how SGs are formed and dissolved. We identified growth factor receptor-bound protein 7 (Grb7), an RNA-binding, translational regulator, as an integral component of SGs, which directly interacts with Hu antigen R (HuR) and is required for cells to form SGs. When stress is terminated, Grb7 is hyperphosphorylated by focal adhesion kinase (FAK), loses its ability to directly interact with HuR and is dissociated from SG components, thereby disrupting SGs in recovering cells. Consistently, dominant-negative hypophospho mutants of FAK and Grb7 significantly attenuate SG disassembly during recovery. FAK activation followed by its phosphorylating Grb7 constitutes a cell-autonomous signalling pathway that regulates the disassembly of SGs and translational stimulation during recovery. This is the first reported pathway actively regulating the dynamics of SGs.

Imaging real-time gene expression in living systems with single-transcript resolution: construct design and imaging system setup

INTRODUCTIONThe most common way for a cell to respond to internal and external signals is to change its gene expression pattern. This requires the synchronization of regulatory steps along the expression pathway. Biological imaging techniques can be used to visualize and measure such processes in individual live cells in real time. This article discusses the use of a fluorescent RNA-binding protein system that allows real-time analysis of gene expression with single-transcript resolution.

Imaging real-time gene expression in Mammalian cells with single-transcript resolution

INTRODUCTIONThe MS2 system provides optimal sensitivity for single-molecule detection in cells. It requires two genetically encoded moieties: a reporter mRNA that contains MS2 binding site (MBS) stem loops and a fluorescent MS2 coat protein (MCP-xFP) that binds to the stem loops with high affinity, thus tagging the mRNA within the cell. This protocol describes transfection of COS-7 cells with reporter RNA (e.g., pRSV-Z-24 MBS-β-actin) and MCP-xFP (e.g., pPolII-MCP-GFP-NLS) plasmids using calcium phosphate precipitation. The reporter mRNA plasmid must be co-transfected with the MCP-xFP-NLS plasmid for simultaneous expression in a cell. The unbound MCP-xFP-NLS is sequestered in the nucleus, leaving only the MCP-xFP-NLS that is bound to the reporter mRNA in the cytoplasm. This provides a high signal-to-noise ratio (SNR) that permits detection of single mRNA molecules. The Delta T Imaging System is used for image acquisition of fluorescent particles in the cells.

Alternative promoter usage and alternative splicing contribute to mRNA heterogeneity of mouse monocarboxylate transporter 2

Expression patterns of monocarboxylate transporter 2 (MCT2) display mRNA diversity in a tissue-specific fashion. We cloned and characterized multiple mct2 5'-cDNA ends from the mouse and determined the structural organization of the mct2 gene. We found that transcription of this gene was initiated from five independent genomic regions that spanned >80 kb on chromosome 10, resulting in five unique exon 1 variants (exons 1a, 1b, 1c, 1d, and 1e) that were then spliced to the common exon 2. Alternative splicing of four internal exons (exons AS1, AS2, AS3, and exon 3) greatly increased the complexity of mRNA diversity. While exon 1c was relatively commonly used for transcription initiation in various tissues, other exon 1 variants were used in a tissue-specific fashion, especially exons 1b and 1d that were used exclusively for testis-specific expression. Sequence analysis of 5'-flanking regions upstream of exons 1a, 1b, and 1c revealed the presence of numerous potential binding sites for ubiquitous transcription factors in all three regions and for transcription factors implicated in testis-specific or hypoxia-induced gene expression in the 1b region. Transient transfection assays demonstrated that each of the three regions contained a functional promoter and that the in vitro, cell type-specific activities of these promoters were consistent with the tissue-specific expression pattern of the mct2 gene in vivo. These results indicate that tissue-specific expression of the mct2 gene is controlled by multiple alternative promoters and that both alternative promoter usage and alternative splicing contribute to the remarkable mRNA diversity of the gene.

Copb1-facilitated axonal transport and translation of kappa opioid-receptor mRNA

mRNA of kappa opioid receptor (KOR) can be transported to nerve fibers, including axons of dorsal root ganglia (DRG), and can be locally translated. Yeast three-hybrid screening identifies Copb1 as a kor mRNA-associated protein that form complexes with endogenous kor mRNA, which are colocalized in the soma and axons of DRG neurons. Axonal transport of kor mRNA is demonstrated, directly, by observing mobilization of biotin-labeled kor mRNA in Campenot chambers. Efficient transport of kor mRNA into the side chamber requires Copb1 and can be blocked by a drug that disrupts microtubules. The requirement for Copb1 in mobilizing kor mRNA is confirmed by using the MS2-GFP mRNA-tagging system. Furthermore, Copb1 also facilitates the translation of kor mRNA in the soma and axons. This study provides evidence for a microtubule-dependent, active axonal kor mRNA-transport process that involves Copb1 and can stimulate localized translation and suggests coupling of transport and translation of mRNAs destined to the remote areas such as axons.

Function and translational regulation of mRNA in developing axons

The capacity to synthesize proteins in axons is limited to early stages of neuronal development, while axons are undergoing elongation and pathfinding. Although the roles of local protein synthesis are not fully understood, it has been implicated in regulating the morphological plasticity of growth cones. Recent studies have identified specific mRNAs that are translated in growth cones in response to specific extracellular signals. In this review, we discuss the functional relevance of axonal protein translation for developing axons, the differences in translational capacity between developing and mature vertebrate axons, and possible pathways governing the specific translational activation of axonal mRNAs.

Axonal mRNA transport and localized translational regulation of kappa-opioid receptor in primary neurons of dorsal root ganglia

kappa-opioid receptor (KOR) is detected pre- and postsynaptically, but the subcellular localization, translation, and regulation of kor mRNA in presynaptic compartments of sensory neurons remain elusive. In situ hybridization detected axonal distribution of kor mRNA in primary neurons of dorsal root ganglia (DRG). The MS2-fused GFP tracked kor mRNA transport from DRG neuronal soma to axons, requiring its 5' and 3' UTRs. In Campenot chambers, axonal translation of kor mRNA was demonstrated for DRG neurons, which depended on its 5' UTR and was stimulated by KCl depolarization. KCl depolarization of DRG neurons rendered redistribution of kor mRNA from the postpolysomal fraction to the translationally active polysomal fraction. This study provided evidence for mRNA transport and regulation of presynaptic protein synthesis of nonstructural proteins like KOR in primary sensory neurons and demonstrated a mechanism of KCl depolarization-stimulated axonal mRNA redistribution for localized translational regulation.

Netrin-1 signaling regulates de novo protein synthesis of kappa opioid receptor by facilitating polysomal partition of its mRNA

The expression of kappa opioid receptor (KOR) is subjected to both transcriptional and posttranscriptional controls. We report that KOR translation is regulated by netrin-1 in primary neurons of dorsal root ganglion (DRG) and in P19 embryonal carcinoma cells. Without stimulation, a significant portion of KOR mRNA is maintained in a dormant state and partitions in the translationally inactive, post-polysomal fraction. During netrin-1 stimulation, which activates its downstream target focal adhesion kinase (FAK), KOR mRNA rapidly partitions to the translationally active polysomal fraction. Functionally, the newly synthesized KOR proteins in DRG neurons are able to bind to specific ligands. This report describes the first example of netrin-1 signaling in the translational control of a drug receptor KOR, which involves the mediator of netrin-1, FAK, and a novel mechanism that enhances the association of target mRNA with polysomes for translational activation.

Insights into the receptor transcription and signaling: implications in opioid tolerance and dependence

Drug addiction has great social and economical implications. In order to resolve this problem, the molecular and cellular basis for drug addiction must be elucidated. For the past three decades, our research has focused on elucidating the molecular mechanisms behind morphine tolerance and dependence. Although there are many working hypotheses, it is our premise that cellular modulation of the receptor signaling, either via transcriptional or post-translational control of the receptor, is the basis for morphine tolerance and dependence. Thus, in the current review, we will summarize our recent work on the transcriptional and post-translational control of the opioid receptor, with special emphasis on the mu-opioid receptor, which is demonstrated to mediate the in vivo functions of morphine.