Alternative splicing controls G protein-dependent inhibition of N-type calcium channels in nociceptors

Decrypting the genome's alternative messages

Alternative splicing of messenger RNA (mRNA) precursors affects the majority of human genes, has a considerable impact on eukaryotic gene function and offers distinct opportunities for regulation. Alterations in alternative splicing can cause or modify the progression of a significant number of pathologies. Recent high-throughput technologies have uncovered a wealth of transcript diversity generated by alternative splicing, as well as examples for how this diversity can be established and become misregulated. A variety of mechanisms modulate splice site choice coordinately with other cellular processes, from transcription and mRNA editing or decay to miRNA-based regulation and telomerase function. Alternative splicing studies can contribute to our understanding of multiple biological processes, including genetic diversity, speciation, cell/stem cell differentiation, nervous system function, neuromuscular disorders and tumour progression.

A functional link between T-type calcium channels and mu-opioid receptor expression in adult primary sensory neurons

The mu-opioid receptor agonists have a preferential effect on nociception in adults but their analgesic effect is less selective in neonates. Here we presented our finding that the mu-opioid receptor agonists had no effect on high voltage-activated Ca(2+) channels (HVACCs) in adult dorsal root ganglion (DRG) neurons that exhibited a prominent T-type Ca(2+) current. We also determined the mechanisms underlying the mu-opioid agonists' lack of effect on HVACCs in these neurons. The mu-opioid agonist [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (DAMGO), morphine, and morphine 6-beta-D-glucuronide had no effect on either T-type or HVACC currents despite the presence of a large N-type Ca(2+) current in neurons with T-type Ca(2+) currents. DAMGO still had no effect on HVACC currents when T-type Ca(2+) channels were blocked in these neurons. However, intracellular dialysis of GTP-gamma-S to activate G proteins significantly attenuated HVACC currents. DRG neurons with T-type Ca(2+) currents showed little responses to capsaicin. Single-cell RT-PCR analysis revealed that the mu-opioid receptor mRNA was present only in adult DRG neurons lacking prominent T-type Ca(2+) currents. In the neonatal DRG, DAMGO inhibited HVACC currents in 31% neurons with T-type Ca(2+) currents. The mu-opioid receptor mRNA was detected in all neurons without T-type Ca(2+) currents and also in 28.6% of neurons with T-type Ca(2+) currents in the neonatal DRG. Our study provides novel information that adult DRG neurons with prominent T-type Ca(2+) currents do not express mu-opioid receptors. Expression of T-type Ca(2+) (Ca(V)3.2) channels and mu-opioid receptors may be developmentally co-regulated as some DRG neurons differentiate toward becoming nociceptive neurons.

Melatonin: a hormone that modulates pain

AIMS

Melatonin is a hormone synthesized principally in the pineal gland that has been classically associated with endocrine actions. However, several lines of evidence suggest that melatonin plays a role in pain modulation. This paper reviews the available evidence on melatonin's analgesic effects in animals and human beings.

MAIN METHODS

A medline search was performed using the terms "melatonin", "inflammatory pain", "neuropathic pain", "functional pain", "rats", "mice", "human", "receptors", "opioid" and "free radicals" in combinations.

KEY FINDINGS

The antinociceptive effect of melatonin has been evaluated in diverse pain models, and several findings show that melatonin receptors modulate pain mechanisms as activation induces an antinociceptive effect at spinal and supraspinal levels under conditions of acute and inflammatory pain. More recently, melatonin induced-antinociception has been extended to neuropathic pain states. This effect agrees with the localization of melatonin receptors in thalamus, hypothalamus, dorsal horn of the spinal cord, spinal trigeminal tract, and trigeminal nucleus. The effects of melatonin result from activation of MT(1) and MT(2) melatonin receptors, which leads to reduced cyclic AMP formation and reduced nociception. In addition, melatonin is able to activate opioid receptors indirectly, to open several K(+) channels and to inhibit expression of 5-lipoxygenase and cyclooxygenase 2. This hormone also inhibits the production of pro-inflammatory cytokines, modulates GABA(A) receptor function and acts as a free radical scavenger.

SIGNIFICANCE

Melatonin receptors constitute attractive targets for developing analgesic drugs, and their activation may prove to be a useful strategy to generate analgesics with a novel mechanism of action.

Alternative splicing of voltage-gated calcium channels: from molecular biology to disease

Recent developments in the diversification of voltage-gated calcium channel function center on the rapidly emerging role of the posttranscriptional mechanism of alternative splicing. A number of diseases have been found to relate to the dysfunction of alternatively spliced exons arising from either genetic mutations or alterations in the splicing machinery. Mutations in some genes associated with congenital diseases have been detected to reside in alternatively spliced exons. As such, the severity of tissue-selective pathology of the disease will depend on the level of expression of the alternatively spliced exons in that tissue, as well as the extent in the change in channel properties. Importantly, alteration in channel properties is affected by the backbone array of the combinatorial alternatively spliced exons within the channel. In other words, the context by which mutations or alternatively spliced exons are expressed is a great influence on the alteration of channel properties and as such physiology and disease. We reviewed here recent comprehension of alternative splicing of voltage-gated calcium channels and how such structural and functional diversity of voltage-gated calcium channels will aid to clarify the pathophysiology of relevant diseases. Such understandings will further provide guidance for novel treatment.

Role of voltage-gated calcium channels in ascending pain pathways

Voltage gated calcium channels (VGCCs) are well established mediators of pain signals in primary afferent neurons. N-type calcium channels are localized to synaptic nerve terminals in laminae 1 and 2 of the dorsal horn where their opening results in the release of neurotransmitters such as glutamate and substance P. The contribution of N-type channels to the processing of pain signals is regulated by alternate splicing of the N-type channel gene, with unique N-type channel splice variants being expressed in small nociceptive neurons. In contrast, T-type VGCCs of the Ca(v)3.2 subtype are likely localized to nerve endings where they regulate cellular excitability. Consequently, inhibition of N-type and Ca(v)3.2 T-type VGCCs has the propensity to mediate analgesia. T-type channel activity is regulated by redox modulation, and can be inhibited by a novel class of small organic blockers. N-type VGCC activity can be potently inhibited by highly selective peptide toxins that are delivered intrathecally, and the search for small organic blockers with clinical efficacy is ongoing. Here, we provide a brief overview of recent advances in this area, as presented at the Spring Pain Research conference (Grand Cayman, 2008).

Analgesic alpha-conotoxins Vc1.1 and Rg1A inhibit N-type calcium channels in rat sensory neurons via GABAB receptor activation

alpha-Conotoxins Vc1.1 and Rg1A are peptides from the venom of marine Conus snails that are currently in development as a treatment for neuropathic pain. Here we report that the alpha9alpha10 nicotinic acetylcholine receptor-selective conotoxins Vc1.1 and Rg1A potently and selectively inhibit high-voltage-activated (HVA) calcium channel currents in dissociated DRG neurons in a concentration-dependent manner. The post-translationally modified peptides vc1a and [P6O]Vc1.1 were inactive, as were all other alpha-conotoxins tested. Vc1.1 inhibited the omega-conotoxin-sensitive HVA currents in DRG neurons but not those recorded from Xenopus oocytes expressing Ca(V)2.2, Ca(V)2.1, Ca(V)2.3, or Ca(V)1.2 channels. Inhibition of HVA currents by Vc1.1 was not reversed by depolarizing prepulses but was abolished by pertussis toxin (PTX), intracellular GDPbetaS, or a selective inhibitor of pp60c-src tyrosine kinase. These data indicate that Vc1.1 does not interact with N-type calcium channels directly but inhibits them via a voltage-independent mechanism involving a PTX-sensitive, G-protein-coupled receptor. Preincubation with a variety of selective receptor antagonists demonstrated that only the GABA(B) receptor antagonists, [S-(R*,R*)][-3-[[1-(3,4-dichlorophenyl)ethyl]amino]-2-hydroxy propyl]([3,4]-cyclohexylmethyl) phosphinic acid hydrochloride (2S)-3[[(1S)-1-(3,4-dichlorophenyl)-ethyl]amino-2-hydroxypropyl](phenylmethyl) phosphinic acid and phaclofen, blocked the effect of Vc1.1 and Rg1A on Ca2+ channel currents. Together, the results identify Ca(V)2.2 as a target of Vc1.1 and Rg1A, potentially mediating their analgesic actions. We propose a novel mechanism by which alpha-conotoxins Vc1.1 and Rg1A modulate native N-type (Ca(V)2.2) Ca2+ channel currents, namely acting as agonists via G-protein-coupled GABA(B) receptors.

Endogenous opiates and behavior: 2007

This paper is the thirtieth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2007 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Differential Inhibition of Ca2+ channels by alpha2-adrenoceptors in three functional subclasses of rat sympathetic neurons

A comparison of identified sympathetic neurons in the isolated intact superior cervical ganglion revealed that secretomotor, pilomotor, and vasoconstrictor cells differ in their action potential mechanisms and in their postsynaptic alpha(2)-adrenergic responses to 10 microM norepinephrine (NE). In normal saline, the half-width of the spike afterhyperpolarization (AHP) in secretomotor neurons (103.5 +/- 6.2 ms) was twofold that recorded in vasoconstrictor neurons (47.7 +/- 2.9 ms) and 1.5-fold that in pilomotor neurons (71.4 +/- 10.3 ms). Bath-applied NE reversibly inhibited the action potential repolarization shoulder, AHP amplitude, and AHP duration in secretomotor and pilomotor neurons to a similar extent, but had no effect on vasoconstrictor neurons. The insensitivity of vasomotor neurons to NE was not an artifact produced by microelectrode recording because all three cell groups were similar in terms of resting potential and input resistance. Moreover, NE insensitivity was not a natural consequence of briefer AHP duration in vasoconstrictor cells. Adding 10 mM TEA(+) caused marked accentuation of the shoulder and AHP duration in vasoconstrictor neurons and comparable changes in the other two cell types, but did not unmask any sign of NE sensitivity in the vasoconstrictors. However, the spike shoulder and AHP in vasoconstrictors were Cd(2+) sensitive, blocked by omega-conotoxin, an N-type calcium channel antagonist, and inhibited by oxotremorine-M, a muscarinic receptor agonist. These data show that NE can differentially modulate functional subsets of mammalian sympathetic neurons and that NE insensitivity can serve as a practical experimental criterion for identification of vasomotor neurons in the isolated ganglion.

Voltage-gated calcium channels in chronic pain: emerging role of alternative splicing

N- and T-type voltage-gated calcium channels are key established players in chronic pain. Current work suggests that alternative splicing of these channels constitutes an important aspect in the investigation of their roles in the pathogenesis of chronic pain. Recent N-type channel studies describe a nociceptor-enriched alternatively spliced module responsible for voltage-independent G protein modulation and internalization, which is implicated in the control of distinct nociceptive pathways. On the contrary, although a large body of work has demonstrated that peripheral Cav 3.2-encoded T-type currents are involved in several types of chronic pain, little is known with respect to the expression of numerous newly discovered splice variants in specific pain pathways. The elucidation of the new layers of molecular complexity uncovered in N- and T-type channel splice variants and their respective locations and roles in different pain pathways will allow for the development of better therapeutic strategies for the treatment of chronic pain.

Presynaptic signaling by heterotrimeric G-proteins

G-proteins (guanine nucleotide-binding proteins) are membrane-attached proteins composed of three subunits, alpha, beta, and gamma. They transduce signals from G-protein coupled receptors (GPCRs) to target effector proteins. The agonistactivated receptor induces a conformational change in the G-protein trimer so that the alpha-subunit binds GTP in exchange for GDP and alpha-GTP, and betagamma-subunits separate to interact with the target effector. Effector-interaction is terminated by the alpha-subunit GTPase activity, whereby bound GTP is hydrolyzed to GDP. This is accelerated in situ by RGS proteins, acting as GTPase-activating proteins (GAPs). Galpha-GDP and Gbetagamma then reassociate to form the Galphabetagamma trimer. G-proteins primarily involved in the modulation of neurotransmitter release are G(o), G(q) and G(s). G(o) mediates the widespread presynaptic auto-inhibitory effect of many neurotransmitters (e.g., via M2/M4 muscarinic receptors, alpha(2) adrenoreceptors, micro/delta opioid receptors, GABAB receptors). The G(o) betagamma-subunit acts in two ways: first, and most ubiquitously, by direct binding to CaV2 Ca(2+) channels, resulting in a reduced sensitivity to membrane depolarization and reduced Ca(2+) influx during the terminal action potential; and second, through a direct inhibitory effect on the transmitter release machinery, by binding to proteins of the SNARE complex. G(s) and G(q) are mainly responsible for receptor-mediated facilitatory effects, through activation of target enzymes (adenylate cyclase, AC and phospholipase-C, PLC respectively) by the GTP-bound alpha-subunits.

Genome-wide analysis of alternative pre-mRNA splicing

Alternative splicing of mRNA precursors allows the synthesis of multiple mRNAs from a single primary transcript, significantly expanding the information content and regulatory possibilities of higher eukaryotic genomes. High-throughput enabling technologies, particularly large-scale sequencing and splicing-sensitive microarrays, are providing unprecedented opportunities to address key questions in this field. The picture emerging from these pioneering studies is that alternative splicing affects most human genes and a significant fraction of the genes in other multicellular organisms, with the potential to greatly influence the evolution of complex genomes. A combinatorial code of regulatory signals and factors can deploy physiologically coherent programs of alternative splicing that are distinct from those regulated at other steps of gene expression. Pre-mRNA splicing and its regulation play important roles in human pathologies, and genome-wide analyses in this area are paving the way for improved diagnostic tools and for the identification of novel and more specific pharmaceutical targets.

Neuronal regulation of alternative pre-mRNA splicing

Alternative pre-mRNA splicing has an important role in the control of neuronal gene expression. Many neuronal proteins are structurally diversified through the differential inclusion and exclusion of sequences in the final spliced mRNA. Here, we discuss common mechanisms of splicing regulation and provide examples of how alternative splicing has important roles in neuronal development and mature neuron function. Finally, we describe regulatory proteins that control the splicing of some neuronally expressed transcripts.

Modulation of pain transmission by G-protein-coupled receptors

The heterotrimeric G-protein-coupled receptors (GPCR) represent the largest and most diverse family of cell surface receptors and proteins. GPCR are widely distributed in the peripheral and central nervous systems and are one of the most important therapeutic targets in pain medicine. GPCR are present on the plasma membrane of neurons and their terminals along the nociceptive pathways and are closely associated with the modulation of pain transmission. GPCR that can produce analgesia upon activation include opioid, cannabinoid, alpha2-adrenergic, muscarinic acetylcholine, gamma-aminobutyric acidB (GABAB), groups II and III metabotropic glutamate, and somatostatin receptors. Recent studies have led to a better understanding of the role of these GPCR in the regulation of pain transmission. Here, we review the current knowledge about the cellular and molecular mechanisms that underlie the analgesic actions of GPCR agonists, with a focus on their effects on ion channels expressed on nociceptive sensory neurons and on synaptic transmission at the spinal cord level.

Nociceptors--noxious stimulus detectors

In order to deal effectively with danger, it is imperative to know about it. This is what nociceptors do--these primary sensory neurons are specialized to detect intense stimuli and represent, therefore, the first line of defense against any potentially threatening or damaging environmental inputs. By sensing noxious stimuli and contributing to the necessary reactions to avoid them--rapid withdrawal and the experience of an intensely unpleasant or painful sensation, nociceptors are essential for the maintenance of the body's integrity. Although nociceptive pain is clearly an adaptive alarm system, persistent pain is maladaptive, essentially an ongoing false alarm. Here, we highlight the genesis of nociceptors during development and the intrinsic properties of nociceptors that enable them to transduce, conduct, and transmit nociceptive information and also discuss how their phenotypic plasticity contributes to clinical pain.

Differential role of N-type calcium channel splice isoforms in pain

N-type calcium channels are essential mediators of spinal nociceptive transmission. The core subunit of the N-type channel is encoded by a single gene, and multiple N-type channel isoforms can be generated by alternate splicing. In particular, cell-specific inclusion of an alternatively spliced exon 37a generates a novel form of the N-type channel that is highly enriched in nociceptive neurons and, as we show here, downregulated in a neuropathic pain model. Splice isoform-specific small interfering RNA silencing in vivo reveals that channels containing exon 37a are specifically required for mediating basal thermal nociception and for developing thermal and mechanical hyperalgesia during inflammatory and neuropathic pain. In contrast, both N-type channel isoforms (e37a- and e37b-containing) contribute to tactile neuropathic allodynia. Hence, exon 37a acts as a molecular switch that tailors the channels toward specific roles in pain.

Neuronal calcium channels: splicing for optimal performance

Calcium ion channels coordinate an astounding number of cellular functions. Surprisingly, only 10 Ca(V)alpha(1) subunit genes encode the structural cores of all voltage-gated calcium channels. What mechanisms exist to modify the structure of calcium channels and optimize their coupling to the rich spectrum of cellular functions? Growing evidence points to the contribution of post-translational alternative processing of calcium channel RNA as the main mechanism for expanding the functional potential of this important gene family. Alternative splicing of RNA is essential during neuronal development where fine adjustments in protein signaling promote and inhibit cell-cell interactions and underlie axonal guidance. However, attributing a specific functional role to an individual splice isoform or splice site has been difficult. In this regard, studies of ion channels are advantageous because their function can be monitored with precision, allowing even subtle changes in channel activity to be detected. Such studies are especially insightful when coupled with information about isoform expression patterns and cellular localization. In this paper, we focus on two sites of alternative splicing in the N-type calcium channel Ca(V)2.2 gene. We first describe cassette exon 18a that encodes a 21 amino acid segment in the II-III intracellular loop region of Ca(V)2.2. Here, we show that e18a is upregulated in the nervous system during development. We discuss these new data in light of our previous reports showing that e18a protects the N-type channel from cumulative inactivation. Second, we discuss our published data on exons e37a and e37b, which encode 32 amino acids in the intracellular C-terminus of Ca(V)2.2. These exons are expressed in a mutually exclusive manner. Exon e37a-containing Ca(V)2.2 mRNAs and their resultant channels express at higher density in dorsal root ganglia and, as we showed recently, e37a increases N-type channel sensitivity to G-protein-mediated inhibition, as compared to generic e37b-containing N-type channels.

Beta-arrestin2 and c-Src regulate the constitutive activity and recycling of mu opioid receptors in dorsal root ganglion neurons

Beta-arrestins bind to agonist-activated G-protein-coupled receptors regulating signaling events and initiating endocytosis. In beta-arrestin2-/- (beta arr2-/-) mice, a complex phenotype is observed that includes altered sensitivity to morphine. However, little is known of how beta-arrestin2 affects mu receptor signaling. We investigated the coupling of mu receptors to voltage-gated Ca2+ channels (VGCCs) in beta arr2+/+ and beta arr2-/- dorsal root ganglion neurons. A lack of beta-arrestin2 reduced the maximum inhibition of VGCCs by morphine and DAMGO (D-Ala2-N-Me-Phe4-glycol5-enkephalin) without affecting agonist potency, the onset of receptor desensitization, or the functional contribution of N-type VGCCs. The reduction in inhibition was accompanied by increased naltrexone-sensitive constitutive inhibitory coupling of mu receptors to VGCCs. Agonist-independent mu receptor inhibitory coupling was insensitive to CTAP (Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2), a neutral antagonist that inhibited the inverse agonist action of naltrexone. These functional changes were accompanied by diminished constitutive recycling and increased cell-surface mu receptor expression in beta arr2-/- compared with beta arr2+/+ neurons. Such changes could not be explained by the classical role of beta-arrestins in agonist-induced endocytosis. The localization of the nonreceptor tyrosine kinase c-Src appeared disrupted in beta arr2-/- neurons, and there was reduced activation of c-Src by DAMGO. Using the Src inhibitor PP2 [4-amino-5-(4-chlorophenyl)-(t-butyl)pyrazolo[3,4-d]pyrimidine], we demonstrated that defective Src signaling mimics the beta arr2-/- cellular phenotype of reduced mu agonist efficacy, increased constitutive mu receptor activity, and reduced constitutive recycling. We propose that beta-arrestin2 is required to target c-Src to constitutively active mu receptors, resulting in their internalization, providing another dimension to the complex role of beta-arrestin2 and c-Src in G-protein-coupled receptor function.