Transcriptomic analysis of differentially alternative splicing patterns in mice with inflammatory and neuropathic pain

Although the molecular mechanisms of chronic pain have been extensively studied, a global picture of alternatively spliced genes and events in the peripheral and central nervous systems of chronic pain is poorly understood. The current study analyzed the changing pattern of alternative splicing (AS) in mouse brain, dorsal root ganglion, and spinal cord tissue under inflammatory and neuropathic pain. In total, we identified 6495 differentially alternatively spliced (DAS) genes. The molecular functions of shared DAS genes between these two models are mainly enriched in calcium signaling pathways, synapse organization, axon regeneration, and neurodegeneration disease. Additionally, we identified 509 DAS in differentially expressed genes (DEGs) shared by these two models, accounting for a small proportion of total DEGs. Our findings supported the hypothesis that the AS has an independent regulation pattern different from transcriptional regulation. Taken together, these findings indicate that AS is one of the important molecular mechanisms of chronic pain in mammals. This study presents a global description of AS profile changes in the full path of neuropathic and inflammatory pain models, providing new insights into the underlying mechanisms of chronic pain and guiding genomic clinical diagnosis methods and rational medication.

Both gain-of-function and loss-of-function de novo CACNA1A mutations cause severe developmental epileptic encephalopathies in the spectrum of Lennox-Gastaut syndrome

OBJECTIVE

Developmental epileptic encephalopathies (DEEs) are genetically heterogeneous severe childhood-onset epilepsies with developmental delay or cognitive deficits. In this study, we explored the pathogenic mechanisms of DEE-associated de novo mutations in the CACNA1A gene.

METHODS

We studied the functional impact of four de novo DEE-associated CACNA1A mutations, including the previously described p.A713T variant and three novel variants (p.V1396M, p.G230V, and p.I1357S). Mutant cDNAs were expressed in HEK293 cells, and whole-cell voltage-clamp recordings were conducted to test the impacts on Ca_V 2.1 channel function. Channel localization and structure were assessed with immunofluorescence microscopy and three-dimensional (3D) modeling.

RESULTS

We find that the G230V and I1357S mutations result in loss-of-function effects with reduced whole-cell current densities and decreased channel expression at the cell membrane. By contrast, the A713T and V1396M variants resulted in gain-of-function effects with increased whole-cell currents and facilitated current activation (hyperpolarized shift). The A713T variant also resulted in slower current decay. 3D modeling predicts conformational changes favoring channel opening for A713T and V1396M.

SIGNIFICANCE

Our findings suggest that both gain-of-function and loss-of-function CACNA1A mutations are associated with similarly severe DEEs and that functional validation is required to clarify the underlying molecular mechanisms and to guide therapies.

Alternative RNA splicing: contribution to pain and potential therapeutic strategy

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Alternative pre-mRNA splicing generates multiple proteins from a single gene.

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Control of alternative splicing is a likely therapy in cancer and other disorders.

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Key molecules in pain pathways – GPCRs and channels – are alternatively spliced.

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It is proposed that alternative splicing may be a therapeutic target in pain.

Since the sequencing of metazoan genomes began, it has become clear that the number of expressed proteins far exceeds the number of genes. It is now estimated that more than 98% of human genes give rise to multiple proteins through alternative pre-mRNA splicing. In this review, we highlight the known alternative splice variants of many channels, receptors, and growth factors that are important in nociception and pain. Recently, pharmacological control of alternative splicing has been proposed as potential therapy in cancer, wet age-related macular degeneration, retroviral infections, and pain. Thus, we also consider the effects that known splice variants of molecules key to nociception/pain have on nociceptive processing and/or analgesic action, and the potential for control of alternative pre-mRNA splicing as a novel analgesic strategy.

Mechanisms of the gabapentinoids and α 2 δ-1 calcium channel subunit in neuropathic pain

The gabapentinoid drugs gabapentin and pregabalin are key front‐line therapies for various neuropathies of peripheral and central origin. Originally designed as analogs of GABA, the gabapentinoids bind to the α₂δ‐1 and α₂δ‐2 auxiliary subunits of calcium channels, though only the former has been implicated in the development of neuropathy in animal models. Transgenic approaches also identify α₂δ‐1 as key in mediating the analgesic effects of gabapentinoids, however the precise molecular mechanisms remain unclear. Here we review the current understanding of the pathophysiological role of the α₂δ‐1 subunit, the mechanisms of analgesic action of gabapentinoid drugs and implications for efficacy in the clinic. Despite widespread use, the number needed to treat for gabapentin and pregabalin averages from 3 to 8 across neuropathies. The failure to treat large numbers of patients adequately necessitates a novel approach to treatment selection. Stratifying patients by sensory profiles may imply common underlying mechanisms, and a greater understanding of these mechanisms could lead to more direct targeting of gabapentinoids.

Modulation of voltage-gated Ca2+ channels in rat retinal ganglion cells by gabapentin

The α2δ auxiliary subunits of voltage-gated Ca2+ channels (VGCCs) are important modulators of VGCC function. Gabapentin interacts with α2δ1 and α2δ2 subunits and is reported to reduce Ca2+ channel current amplitude (ICa). This study aimed to determine the effects of gabapentin on VGCCs in retinal ganglion cells (RGCs). Whole cell patch clamp was used to record ICa in isolated RGCs, and calcium imaging was used to measure Ca2+ transients from RGCs in situ. Immunohistochemistry was used to detect the presence of α2δ1-containing VGCCs in isolated RGCs in the absence and presence of gabapentin pretreatment. Acute administration of gabapentin reduced ICa and Ca2+ transients compared to control conditions. In isolated RGCs, pretreatment with gabapentin (4-18 h) reduced ICa, and cell surface α2δ1 staining was reduced compared to nonpretreated cells. Acute administration of gabapentin to isolated RGCs that had been pretreated further reduced ICa. These results show that gabapentin has both short-term and long-term mechanisms to reduce ICa in isolated RGCs. Some Ca2+ channel blockers have been shown to protect RGCs in retinal trauma suggesting that modulation of VGCCs by gabapentin may prevent the deleterious effects of elevated Ca2+ levels in RGCs in trauma and disease.

Neuronal voltage-gated calcium channels: structure, function, and dysfunction

Voltage-gated calcium channels are the primary mediators of depolarization-induced calcium entry into neurons. There is great diversity of calcium channel subtypes due to multiple genes that encode calcium channel α1 subunits, coassembly with a variety of ancillary calcium channel subunits, and alternative splicing. This allows these channels to fulfill highly specialized roles in specific neuronal subtypes and at particular subcellular loci. While calcium channels are of critical importance to brain function, their inappropriate expression or dysfunction gives rise to a variety of neurological disorders, including, pain, epilepsy, migraine, and ataxia. This Review discusses salient aspects of voltage-gated calcium channel function, physiology, and pathophysiology.

Spinal morphine but not ziconotide or gabapentin analgesia is affected by alternative splicing of voltage-gated calcium channel CaV2.2 pre-mRNA

Presynaptic voltage-gated calcium Ca_V2.2 channels play a privileged role in spinal level sensitization following peripheral nerve injury. Direct and indirect inhibitors of Ca_V2.2 channel activity in spinal dorsal horn are analgesic in chronic pain states. Ca_V2.2 channels represent a family of splice isoforms that are expressed in different combinations according to cell-type. A pair of mutually exclusive exons in the Ca_V2.2 encoding Cacna1b gene, e37a and e37b, differentially influence morphine analgesia. In mice that lack exon e37a, which is enriched in nociceptors, the analgesic efficacy of intrathecal morphine against noxious thermal stimuli is reduced. Here we ask if sequences unique to e37a influence: the development of abnormal thermal and mechanical sensitivity associated with peripheral nerve injury; and the actions of two other classes of analgesics that owe part or all of their efficacy to Ca_V2.2 channel inhibition. We find that: i) the analgesic efficacy of morphine, but not ziconotide or gabapentin, is reduced in mice lacking e37a, ii) the induction and maintenance of behaviors associated with sensitization that accompany peripheral nerve injury, do not require e37a-specific sequence, iii) intrathecal morphine, but not ziconotide or gabapentin analgesia to thermal stimuli is significantly lower in wild-type mice after peripheral nerve injury, iv) the analgesic efficacy of ziconotide and gabapentin to mechanical stimuli is reduced following nerve injury, and iv) intrathecal morphine analgesia to thermal stimuli in mice lacking e37a is not further reduced by peripheral nerve injury. Our findings show that the analgesic action of morphine, but not ziconotide or gabapentin, to thermal stimuli is linked to which Cacna1b exon, e37a or e37b, is selected during alternative pre-mRNA splicing.

Implications and mechanism of action of gabapentin in neuropathic pain

Gabapentin is an anti-epileptic agent but now it is also recommended as first line agent in neuropathic pain, particularly in diabetic neuropathy and post herpetic neuralgia. α2δ-1, an auxillary subunit of voltage gated calcium channels, has been documented as its main target and its specific binding to this subunit is described to produce different actions responsible for pain attenuation. The binding to α2δ-1 subunits inhibits nerve injury-induced trafficking of α1 pore forming units of calcium channels (particularly N-type) from cytoplasm to plasma membrane (membrane trafficking) of pre-synaptic terminals of dorsal root ganglion (DRG) neurons and dorsal horn neurons. Furthermore, the axoplasmic transport of α2δ-1 subunits from DRG to dorsal horns neurons in the form of anterograde trafficking is also inhibited in response to gabapentin administration. Gabapentin has also been shown to induce modulate other targets including transient receptor potential channels, NMDA receptors, protein kinase C and inflammatory cytokines. It may also act on supra-spinal region to stimulate noradrenaline mediated descending inhibition, which contributes to its anti-hypersensitivity action in neuropathic pain.

Gabapentin inhibits γ-amino butyric acid release in the locus coeruleus but not in the spinal dorsal horn after peripheral nerve injury in rats

BACKGROUND

Gabapentin reduces acute postoperative and chronic neuropathic pain, but its sites and mechanisms of action are unclear. Based on previous electrophysiologic studies, the authors tested whether gabapentin reduced γ-amino butyric acid (GABA) release in the locus coeruleus (LC), a major site of descending inhibition, rather than in the spinal cord.

METHODS

Male Sprague-Dawley rats with or without L5-L6 spinal nerve ligation (SNL) were used. Immunostaining for glutamic acid decarboxylase and GABA release in synaptosomes and microdialysates were examined in the LC and spinal dorsal horn.

RESULTS

Basal GABA release and expression of glutamic acid decarboxylase increased in the LC but decreased in the spinal dorsal horn after SNL. In microdialysates from the LC, intravenously administered gabapentin decreased extracellular GABA concentration in normal and SNL rats. In synaptosomes prepared from the LC, gabapentin and other α2δ ligands inhibited KCl-evoked GABA release in normal and SNL rats. In microdialysates from the spinal dorsal horn, intravenous gabapentin did not alter GABA concentrations in normal rats but slightly increased them in SNL rats. In synaptosomes from the spinal dorsal horn, neither gabapentin nor other α2δ ligands affected KCl-evoked GABA release in normal and SNL rats.

DISCUSSION

These results suggest that peripheral nerve injury induces plasticity of GABAergic neurons differently in the LC and spinal dorsal horn and that gabapentin reduces presynaptic GABA release in the LC but not in the spinal dorsal horn. The current study supports the idea that gabapentin activates descending noradrenergic inhibition via disinhibition of LC neurons.

Gabapentin inhibits catecholamine release from adrenal chromaffin cells

BACKGROUND

Gabapentin is most commonly prescribed for chronic pain, but acute perioperative effects, including preemptive analgesia and hemodynamic stabilization, have been reported. Adrenal chromaffin cells are a widely used model to investigate neurosecretion, and adrenal catecholamines play important physiologic roles and contribute to the acute stress response. However, the effects of gabapentin on adrenal catecholamine release have never been tested.

METHODS

Primary cultures of bovine adrenal chromaffin cells were treated with gabapentin or vehicle for 18-24 h. The authors quantified catecholamine secretion from dishes of cells using high-performance liquid chromatography and resolved exocytosis of individual secretory vesicles from single cells using carbon fiber amperometry. Voltage-gated calcium channel currents were recorded using patch clamp electrophysiology and intracellular [Ca2+] using fluorescent imaging.

RESULTS

Gabapentin produced statistically significant reductions in catecholamine secretion evoked by cholinergic agonists (24 ± 3%, n = 12) or KCl (16 ± 4%, n = 8) (mean ± SEM) but did not inhibit Ca2+ entry or calcium channel currents. Amperometry (n = 51 cells) revealed that gabapentin inhibited the number of vesicles released upon stimulation, with no change in quantal size or kinetics of these unitary events.

CONCLUSIONS

The authors show Ca2+ entry was not inhibited by gabapentin but was less effective at triggering vesicle fusion. The work also demonstrates that chromaffin cells are a useful model for additional investigation of the cellular mechanism(s) by which gabapentin controls neurosecretion. In addition, it identifies altered adrenal catecholamine release as a potential contributor to some of the beneficial perioperative effects of gabapentin.

Drug management of visceral pain: concepts from basic research

Visceral pain is experienced by 40% of the population, and 28% of cancer patients suffer from pain arising from intra- abdominal metastasis or from treatment. Neuroanatomy of visceral nociception and neurotransmitters, receptors, and ion channels that modulate visceral pain are qualitatively or quantitatively different from those that modulate somatic and neuropathic pain. Visceral pain should be recognized as distinct pain phenotype. TRPV1, Na 1.8, and ASIC3 ion channels and peripheral kappa opioid receptors are important mediators of visceral pain. Mu agonists, gabapentinoids, and GABAB agonists reduce pain by binding to central receptors and channels. Combinations of analgesics and adjuvants in animal models have supra-additive antinociception and should be considered in clinical trials. This paper will discuss the neuroanatomy, receptors, ion channels, and neurotransmitters important to visceral pain and provide a basic science rationale for analgesic trials and management.

Alpha 2 Delta (α(2)δ) Ligands, Gabapentin and Pregabalin: What is the Evidence for Potential Use of These Ligands in Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is a complex disorder that is characterized by abdominal pain and altered bowel habit, and often associates with other gastrointestinal symptoms such as feelings of incomplete bowel movement and abdominal bloating, and extra-intestinal symptoms such as headache, dyspareunia, heartburn, muscle pain, and back pain. It also frequently coexists with conditions that may also involve central sensitization processes, such as fibromyalgia, irritable bladder disorder, and chronic cough. This review examines the evidence to date on gabapentin and pregabalin which may support further and continued research and development of the α₂δ ligands in disorders characterized by visceral hypersensitivity, such as IBS. The distribution of the α₂δ subunit of the voltage-gated calcium channel, possible mechanisms of action, pre-clinical data which supports an effect on motor–sensory mechanisms and clinical evidence that points to potential benefits in patients with IBS will be discussed.

Amperometric measurement of glutamate release modulation by gabapentin and pregabalin in rat neocortical slices: role of voltage-sensitive Ca2+ α2δ-1 subunit

Gabapentin (GBP; Neurontin) and pregabalin (PGB; Lyrica, S-(+)-3-isobutylgaba) are used clinically to treat several disorders associated with excessive or inappropriate excitability, including epilepsy; pain from diabetic neuropathy, postherpetic neuralgia, and fibromyalgia; and generalized anxiety disorder. The molecular basis for these drugs' therapeutic effects are believed to involve the interaction with the auxiliary α(2)δ subunit of voltage-sensitive Ca(2+) channel (VSCC) translating into a modulation of pathological neurotransmitter release. Glutamate as the primary excitatory neurotransmitter in the mammalian central nervous system contributes, under conditions of excessive glutamate release, to neurological and psychiatric disorders. This study used enzyme-based microelectrode arrays to directly measure extracellular glutamate release in rat neocortical slices and determine the modulation of this release by GBP and PGB. Both drugs attenuated K(+)-evoked glutamate release without affecting basal glutamate levels. PGB (0.1-100 μM) exhibited concentration-dependent inhibition of K(+)-evoked glutamate release with an IC(50) value of 5.3 μM. R-(-)-3-Isobutylgaba, the enantiomer of PGB, did not significantly reduce K(+)-evoked glutamate release. The decrease of K(+)-evoked glutamate release by PGB was blocked by the l-amino acid l-isoleucine, a potential endogenous ligand of the α(2)δ subunit. In neocortical slices from transgenic mice having a point mutation (i.e., R217A) of the α(2)δ-1 (subtype) subunit of VSCC, PGB did not affect K(+)-evoked glutamate release yet inhibited this release in wild-type mice. The results show that GBP and PGB attenuated stimulus-evoked glutamate release in rodent neocortical slices and that the α(2)δ-1 subunit of VSCC appears to mediate this effect.

Laminitic pain: parallels with pain states in humans and other species

Laminitis poses a threat to all horses, and is widely considered as being one of the most important diseases of horses and a global equine welfare problem. The effects of laminitis lead to debilitation, development of pronounced digital pain, and great suffering in the afflicted animal. The precise pathophysiological processes that result in laminitic pain are poorly defined, and hence the delivery of effective palliative care is clinically challenging. Knowledge and understanding of pain states in other animal species may further aid the elucidation of equine laminitic pain mechanisms, guide the search for treatable causes of this multifactorial problem, and thereby help achieve enhanced therapeutic and palliative care. However, parallels drawn from pain states in other animals must consider species differences in both anatomy and physiology, and the specific nature of the laminitic disease process.

Calcium channel functions in pain processing

Voltage-gated calcium channels (VGCC) play obligatory physiological roles, including modulation of neuronal: functions, synaptic plasticity, neurotransmitter release and gene transcription. Dysregulation and maladaptive changes in VGCC expression and activities may occur in the sensory pathway under various pathological conditions that could contribute to the development of pain. In this review, we summarized the most recent findings on the regulation of VGCC expression and physiological functions in the sensory pathway, and in dysregulation and maladaptive changes of VGCC under pain-inducing conditions. The implications of: these changes in understanding the mechanisms of pain transduction and in new drug design are also discussed.

Opioid inhibition of N-type Ca2+ channels and spinal analgesia couple to alternative splicing

Alternative pre-mRNA splicing predominates in the nervous systems of complex organisms including humans dramatically expanding the potential size of the proteome. Cell-specific alternative pre-mRNA splicing is thought to optimize protein function for specialized cellular tasks, but direct evidence for this is limited. Transmission of noxious thermal stimuli relies on the activity of N-type Ca_V2.2 calcium channels in nociceptors. Using an exon replacement strategy in mice, we show that mutually exclusive splicing in the Ca_V2.2 gene modulates N-type channel function in nociceptors leading to a change in morphine analgesia. Exon 37a enhances μ-opioid receptor mediated inhibition of N-type calcium channels by promoting activity-independent inhibition. In the absence of e37a spinal morphine analgesia is weakened in vivo without influencing the basal response to noxious thermal stimuli. Our data suggest that highly specialized, discrete cellular responsiveness in vivo can be attributed to alternative splicing events regulated at the level of individual neurons.

The alpha(2)delta subunit augments functional expression and modifies the pharmacology of Ca(V)1.3 L-type channels

The auxiliary Ca(V)alpha(2)delta-1 subunit is an important component of voltage-gated Ca(2+) (Ca(V)) channel complexes in many tissues and of great interest as a drug target. Nevertheless, its exact role in specific cell functions is still unknown. This is particularly important in the case of the neuronal L-type Ca(V) channels where these proteins play a key role in the secretion of neurotransmitters and hormones, gene expression, and the activation of other ion channels. Therefore, using a combined approach of patch-clamp recordings and molecular biology, we studied the role of the Ca(V)alpha(2)delta-1 subunit on the functional expression and the pharmacology of recombinant L-type Ca(V)1.3 channels in HEK-293 cells. Co-expression of Ca(V)alpha(2)delta-1 significantly increased macroscopic currents and conferred the Ca(V)1.3alpha(1)/Ca(V)beta(3) channels sensitivity to the antiepileptic/analgesic drugs gabapentin and AdGABA. In contrast, Ca(V)alpha(2)delta-1 subunits harboring point mutations in N-glycosylation consensus sequences or the proteolytic site as well as in conserved cysteines in the transmembrane delta domain of the protein, reduced functionality in terms of enhancement of Ca(V)1.3alpha(1)/Ca(V)beta(3) currents. In addition, co-expression of the delta domain drastically inhibited macroscopic currents through recombinant Ca(V)1.3 channels possibly by affecting channel synthesis. Together these results provide several lines of evidence that the Ca(V)alpha(2)delta-1 auxiliary subunit may interact with Ca(V)1.3 channels and regulate their functional expression.

Interaction between Ca(v)2.1alpha (1) and CaMKII in Ca (v)2.1alpha (1) mutant mice, Rolling Nagoya

It has been reported earlier that interactions between Ca(v)2.1alpha(1) and calcium/calmodulin-dependent protein kinase II (CaMKII) in the presynaptic fraction and between the NMDA receptor subunit NR2B and CaMKII in the postsynaptic density (PSD) fraction are important for neuronal function. Ca(v)2.1alpha(1), CaMKII, and NR2B are predominantly expressed in the hippocampus. To examine the above interactions and CaMKII activity in the hippocampal presynapse and PSD of Rolling Nagoya mice carrying a mutation in Ca(v)2.1alpha(1) subunit, we performed immunoprecipitation and Western blot analyses. In the presynapse, the interaction between Ca(v)2.1alpha(1) and CaMKII and the phosphorylation of CaMKII (at Thr286) and its substrate Synapsin I (at Ser603) were decreased in mutant mice compared to wild-type mice. In the PSD, a similar pattern was observed for the interaction between NR2B and CaMKII and the phosphorylation of CaMKII (at Thr286) and its substrate AMPA receptor subunit glutamate receptor 1 (at Ser831) between mutant and wild-type mice. Our data indicate that disruption of the interaction between Ca(v)2.1alpha(1) and CaMKII may down-regulate presynaptic CaMKII activity and that Rolling Nagoya mice would be a useful model for examining presynaptic function.