Integration of nutritional, microbial and inflammatory events along the gut-brain axis can alter bowel physiology and organism behaviour. Colonic sensory neurons activate reflex pathways and give ...rise to conscious sensation, but the diversity and division of function within these neurons is poorly understood. The identification of signalling pathways contributing to visceral sensation is constrained by a paucity of molecular markers. Here we address this by comprehensive transcriptomic profiling and unsupervised clustering of individual mouse colonic sensory neurons.
Unbiased single-cell RNA-sequencing was performed on retrogradely traced mouse colonic sensory neurons isolated from both thoracolumbar (TL) and lumbosacral (LS) dorsal root ganglia associated with lumbar splanchnic and pelvic spinal pathways, respectively. Identified neuronal subtypes were validated by single-cell qRT-PCR, immunohistochemistry (IHC) and Ca
-imaging.
Transcriptomic profiling and unsupervised clustering of 314 colonic sensory neurons revealed seven neuronal subtypes. Of these, five neuronal subtypes accounted for 99% of TL neurons, with LS neurons almost exclusively populating the remaining two subtypes. We identify and classify neurons based on novel subtype-specific marker genes using single-cell qRT-PCR and IHC to validate subtypes derived from RNA-sequencing. Lastly, functional Ca
-imaging was conducted on colonic sensory neurons to demonstrate subtype-selective differential agonist activation.
We identify seven subtypes of colonic sensory neurons using unbiased single-cell RNA-sequencing and confirm translation of patterning to protein expression, describing sensory diversity encompassing all modalities of colonic neuronal sensitivity. These results provide a pathway to molecular interrogation of colonic sensory innervation in health and disease, together with identifying novel targets for drug development.
Chemotherapy-induced peripheral neuropathy (CIPN) is a common dose-limiting side effect of cancer treatment, often associated with degeneration of sensory axons or their terminal regions. Presence of ...the slow Wallerian degeneration protein (WLDS), or genetic deletion of sterile alpha and TIR motif containing protein 1 (SARM1), which strongly protect axons from degeneration after injury or axonal transport block, alleviate pain in several CIPN models. However, oxaliplatin can cause an acute pain response, suggesting a different mechanism of pain generation. Here, we tested whether the presence of WLDS or absence of SARM1 protects against acute oxaliplatin-induced pain in mice after a single oxaliplatin injection. In BL/6 and WldS mice, oxaliplatin induced significant mechanical and cold hypersensitivities which were absent in Sarm1−/− mice. Despite the presence of hypersensitivity there was no significant loss of intraepidermal nerve fibers (IENFs) in the footpads of any mice after oxaliplatin treatment, suggesting that early stages of pain hypersensitivity could be independent of axon degeneration. To identify other changes that could underlie the pain response, RNA sequencing was carried out in DRGs from treated and control mice of each genotype. Sarm1−/− mice had fewer gene expression changes than either BL/6 or WldS mice. This is consistent with the pain measurements in demonstrating that Sarm1−/− DRGs remain relatively unchanged after oxaliplatin treatment, unlike those in BL/6 and WldS mice. Changes in levels of four transcripts – Alas2, Hba-a1, Hba-a2, and Tfrc – correlated with oxaliplatin-induced pain, or absence thereof, across the three genotypes. Our findings suggest that targeting SARM1 could be a viable therapeutic approach to prevent oxaliplatin-induced acute neuropathic pain.
•In BL/6 and WldS mice oxaliplatin induced mechanical and cold hypersensitivities.•Mechanical and cold hypersensitivities were absent in treated Sarm1−/− mice.•Lack of IENFs loss in mice suggests that pain do not involve axon degeneration.•Alas2, Hba-a1, Hba-a2 and Tfrc genes could play a role in pain pathophysiology.•Targeting SARM1 could prevent oxaliplatin-induced acute neuropathic pain.
The generation of human sensory neurons by directed differentiation of pluripotent stem cells opens new opportunities for investigating the biology of pain. The inability to generate this cell type ...has meant that up until now their study has been reliant on the use of rodent models. Here, we use a combination of population and single-cell techniques to perform a detailed molecular, electrophysiological, and pharmacological phenotyping of sensory neurons derived from human embryonic stem cells. We describe the evolution of cell populations over 6 weeks of directed differentiation; a process that results in the generation of a largely homogeneous population of neurons that are both molecularly and functionally comparable to human sensory neurons derived from mature dorsal root ganglia. This work opens the prospect of using pluripotent stem-cell–derived sensory neurons to study human neuronal physiology and as in vitro models for drug discovery in pain and sensory disorders.
Key points
Voltage‐gated sodium channels play a fundamental role in determining neuronal excitability.
Specifically, voltage‐gated sodium channel subtype NaV1.7 is required for sensing acute and ...inflammatory somatic pain in mice and humans but its significance in pain originating from the viscera is unknown.
Using comparative behavioural models evoking somatic and visceral pain pathways, we identify the requirement for NaV1.7 in regulating somatic (noxious heat pain threshold) but not in visceral pain signalling.
These results enable us to better understand the mechanisms underlying the transduction of noxious stimuli from the viscera, suggest that the investigation of pain pathways should be undertaken in a modality‐specific manner and help to direct drug discovery efforts towards novel visceral analgesics.
Voltage‐gated sodium channel NaV1.7 is required for acute and inflammatory pain in mice and humans but its significance for visceral pain is unknown. Here we examine the role of NaV1.7 in visceral pain processing and the development of referred hyperalgesia using a conditional nociceptor‐specific NaV1.7 knockout mouse (NaV1.7Nav1.8) and selective small‐molecule NaV1.7 antagonist PF‐5198007. NaV1.7Nav1.8 mice showed normal nociceptive behaviours in response to intracolonic application of either capsaicin or mustard oil, stimuli known to evoke sustained nociceptor activity and sensitization following tissue damage, respectively. Normal responses following induction of cystitis by cyclophosphamide were also observed in both NaV1.7Nav1.8 and littermate controls. Loss, or blockade, of NaV1.7 did not affect afferent responses to noxious mechanical and chemical stimuli in nerve–gut preparations in mouse, or following antagonism of NaV1.7 in resected human appendix stimulated by noxious distending pressures. However, expression analysis of voltage‐gated sodium channel α subunits revealed NaV1.7 mRNA transcripts in nearly all retrogradely labelled colonic neurons, suggesting redundancy in function. By contrast, using comparative somatic behavioural models we identify that genetic deletion of NaV1.7 (in NaV1.8‐expressing neurons) regulates noxious heat pain threshold and that this can be recapitulated by the selective NaV1.7 antagonist PF‐5198007. Our data demonstrate that NaV1.7 (in NaV1.8‐expressing neurons) contributes to defined pain pathways in a modality‐dependent manner, modulating somatic noxious heat pain, but is not required for visceral pain processing, and advocate that pharmacological block of NaV1.7 alone in the viscera may be insufficient in targeting chronic visceral pain.
Key points
Voltage‐gated sodium channels play a fundamental role in determining neuronal excitability.
Specifically, voltage‐gated sodium channel subtype NaV1.7 is required for sensing acute and inflammatory somatic pain in mice and humans but its significance in pain originating from the viscera is unknown.
Using comparative behavioural models evoking somatic and visceral pain pathways, we identify the requirement for NaV1.7 in regulating somatic (noxious heat pain threshold) but not in visceral pain signalling.
These results enable us to better understand the mechanisms underlying the transduction of noxious stimuli from the viscera, suggest that the investigation of pain pathways should be undertaken in a modality‐specific manner and help to direct drug discovery efforts towards novel visceral analgesics.
Summary
Despite pain prevalence altering with age, the effects of aging on the properties of nociceptors are not well understood. Nociceptors, whose somas are located in dorsal root ganglia, are ...frequently divided into two groups based on their ability to bind isolectin B4 (IB4). Here, using cultured neurons from 1‐, 3‐, 5‐, 8‐, 12‐, and 18‐month‐old mice, we investigate age‐dependent changes in IB4‐positive and IB4‐negative neurons. Current‐clamp experiments at physiological temperature revealed nonlinear changes in firing frequency of IB4‐positive, but not IB4‐negative neurons, with a peak at 8 months. This was likely due to the presence of proexcitatory conductances activated at depolarized membrane potentials and significantly higher input resistances found in IB4‐positive neurons from 8‐month‐old mice. Repetitive firing in nociceptors is driven primarily by the TTX‐resistant sodium current, and indeed, IB4‐positive neurons from 8‐month‐old mice were found to receive larger contributions from the TTX‐resistant window current around the resting membrane potential. To further address the mechanisms behind these differences, we performed RNA‐seq experiments on IB4‐positive and IB4‐negative neurons from 1‐, 8‐, and 18‐month‐old mice. We found a larger number of genes significantly affected by age within the IB4‐positive than IB4‐negative neurons from 8‐month‐old mice, including known determinants of nociceptor excitability. The above pronounced age‐dependent changes at the cellular and molecular levels in IB4‐positive neurons point to potential mechanisms behind the reported increase in pain sensitivity in middle‐aged rodents and humans, and highlight the possibility of targeting a particular group of neurons in the development of age‐tailored pain treatments.
Endoplasmic reticulum (ER)-associated degradation (ERAD) is a cell-autonomous process that eliminates large quantities of misfolded, newly synthesized protein, and is thus essential for the survival ...of any basic eukaryotic cell. Accordingly, the proteins involved and their interaction partners are well conserved from yeast to mammals, and Saccharomyces cerevisiae is widely used as a model system with which to investigate this fundamental cellular process. For example, valosin-containing protein (VCP) and its yeast homologue cell division cycle protein 48 (Cdc48p), which help to direct polyubiquitinated proteins for proteasome-mediated degradation, interact with an equivalent group of ubiquitin ligases in mouse and in S. cerevisiae. A conserved structural motif for cofactor binding would therefore be expected. We report a VCP-binding motif (VBM) shared by mammalian ubiquitin ligase E4b (Ube4b)-ubiquitin fusion degradation protein 2a (Ufd2a), hydroxymethylglutaryl reductase degradation protein 1 (Hrd1)-synoviolin and ataxin 3, and a related sequence in Mr 78 000 glycoprotein-Amfr with slightly different binding properties, and show that Ube4b and Hrd1 compete for binding to the N-terminal domain of VCP. Each of these proteins is involved in ERAD, but none has an S. cerevisiae homologue containing the VBM. Some other invertebrate model organisms also lack the VBM in one or more of these proteins, in contrast to vertebrates, where the VBM is widely conserved. Thus, consistent with their importance in ERAD, evolution has developed at least two ways to bring these proteins together with VCP-Cdc48p. However, the differing molecular architecture of VCP-Cdc48p complexes indicates a key point of divergence in the molecular details of ERAD mechanisms.
Voltage-gated sodium channels play a fundamental role in determining neuronal excitability. Specifically, voltage-gated sodium channel subtype Na
1.7 is required for sensing acute and inflammatory ...somatic pain in mice and humans but its significance in pain originating from the viscera is unknown. Using comparative behavioural models evoking somatic and visceral pain pathways, we identify the requirement for Na
1.7 in regulating somatic (noxious heat pain threshold) but not in visceral pain signalling. These results enable us to better understand the mechanisms underlying the transduction of noxious stimuli from the viscera, suggest that the investigation of pain pathways should be undertaken in a modality-specific manner and help to direct drug discovery efforts towards novel visceral analgesics.
Voltage-gated sodium channel Na
1.7 is required for acute and inflammatory pain in mice and humans but its significance for visceral pain is unknown. Here we examine the role of Na
1.7 in visceral pain processing and the development of referred hyperalgesia using a conditional nociceptor-specific Na
1.7 knockout mouse (Na
1.7
) and selective small-molecule Na
1.7 antagonist PF-5198007. Na
1.7
mice showed normal nociceptive behaviours in response to intracolonic application of either capsaicin or mustard oil, stimuli known to evoke sustained nociceptor activity and sensitization following tissue damage, respectively. Normal responses following induction of cystitis by cyclophosphamide were also observed in both Na
1.7
and littermate controls. Loss, or blockade, of Na
1.7 did not affect afferent responses to noxious mechanical and chemical stimuli in nerve-gut preparations in mouse, or following antagonism of Na
1.7 in resected human appendix stimulated by noxious distending pressures. However, expression analysis of voltage-gated sodium channel α subunits revealed Na
1.7 mRNA transcripts in nearly all retrogradely labelled colonic neurons, suggesting redundancy in function. By contrast, using comparative somatic behavioural models we identify that genetic deletion of Na
1.7 (in Na
1.8-expressing neurons) regulates noxious heat pain threshold and that this can be recapitulated by the selective Na
1.7 antagonist PF-5198007. Our data demonstrate that Na
1.7 (in Na
1.8-expressing neurons) contributes to defined pain pathways in a modality-dependent manner, modulating somatic noxious heat pain, but is not required for visceral pain processing, and advocate that pharmacological block of Na
1.7 alone in the viscera may be insufficient in targeting chronic visceral pain.
Wallerian degeneration slow (Wld(S)) mice express a chimeric protein that delays axonal degeneration. The N-terminal domain (N70), which is essential for axonal protection in vivo, binds ...valosin-containing protein (VCP) and targets both Wld(S) and VCP to discrete nuclear foci. We characterized the formation, composition and localization of these potentially important foci. Missense mutations show that the N-terminal sixteen residues (N16) of Wld(S) are essential for both VCP binding and targeting Wld(S) to nuclear foci. Removing N16 abolishes foci, and VCP binding sequences from ataxin-3 or HrdI restore them. In vitro, these puncta co-localize with proteasome subunits. In vivo, Wld(S) assumes a range of nuclear distribution patterns, including puncta, and its neuronal expression and intranuclear distribution is region-specific and varies between spontaneous and transgenic Wld(S) models. We conclude that VCP influences Wld(S) intracellular distribution, and thus potentially its function, by binding within the N70 domain required for axon protection.
Key points * Voltage-gated sodium channels play a fundamental role in determining neuronal excitability. * Specifically, voltage-gated sodium channel subtype Na sub(V)1.7 is required for sensing ...acute and inflammatory somatic pain in mice and humans but its significance in pain originating from the viscera is unknown. * Using comparative behavioural models evoking somatic and visceral pain pathways, we identify the requirement for Na sub(V)1.7 in regulating somatic (noxious heat pain threshold) but not in visceral pain signalling. * These results enable us to better understand the mechanisms underlying the transduction of noxious stimuli from the viscera, suggest that the investigation of pain pathways should be undertaken in a modality-specific manner and help to direct drug discovery efforts towards novel visceral analgesics. Voltage-gated sodium channel Na sub(V)1.7 is required for acute and inflammatory pain in mice and humans but its significance for visceral pain is unknown. Here we examine the role of Na sub(V)1.7 in visceral pain processing and the development of referred hyperalgesia using a conditional nociceptor-specific Na sub(V)1.7 knockout mouse (Na sub(V)1.7 super(Nav1.8)) and selective small-molecule Na sub(V)1.7 antagonist PF-5198007. Na sub(V)1.7 super(Nav1.8) mice showed normal nociceptive behaviours in response to intracolonic application of either capsaicin or mustard oil, stimuli known to evoke sustained nociceptor activity and sensitization following tissue damage, respectively. Normal responses following induction of cystitis by cyclophosphamide were also observed in both Na sub(V)1.7 super(Nav1.8) and littermate controls. Loss, or blockade, of Na sub(V)1.7 did not affect afferent responses to noxious mechanical and chemical stimuli in nerve-gut preparations in mouse, or following antagonism of Na sub(V)1.7 in resected human appendix stimulated by noxious distending pressures. However, expression analysis of voltage-gated sodium channel alpha subunits revealed Na sub(V)1.7 mRNA transcripts in nearly all retrogradely labelled colonic neurons, suggesting redundancy in function. By contrast, using comparative somatic behavioural models we identify that genetic deletion of Na sub(V)1.7 (in Na sub(V)1.8-expressing neurons) regulates noxious heat pain threshold and that this can be recapitulated by the selective Na sub(V)1.7 antagonist PF-5198007. Our data demonstrate that Na sub(V)1.7 (in Na sub(V)1.8-expressing neurons) contributes to defined pain pathways in a modality-dependent manner, modulating somatic noxious heat pain, but is not required for visceral pain processing, and advocate that pharmacological block of Na sub(V)1.7 alone in the viscera may be insufficient in targeting chronic visceral pain. Key points * Voltage-gated sodium channels play a fundamental role in determining neuronal excitability. * Specifically, voltage-gated sodium channel subtype Na sub(V)1.7 is required for sensing acute and inflammatory somatic pain in mice and humans but its significance in pain originating from the viscera is unknown. * Using comparative behavioural models evoking somatic and visceral pain pathways, we identify the requirement for Na sub(V)1.7 in regulating somatic (noxious heat pain threshold) but not in visceral pain signalling. * These results enable us to better understand the mechanisms underlying the transduction of noxious stimuli from the viscera, suggest that the investigation of pain pathways should be undertaken in a modality-specific manner and help to direct drug discovery efforts towards novel visceral analgesics.
The generation of human sensory neurons by directed differentiation of pluripotent stem cells opens new opportunities for investigating the biology of pain. The inability to generate this cell type ...has meant that up until now their study has been reliant on the use of rodent models. Here, we use a combination of population and single-cell techniques to perform a detailed molecular, electrophysi ologi cal, and pharmacological phenotyping of sensory neurons derived from human embryonic stem cells. We describe the evolution of cell populations over 6 weeks of directed differentiation; a process that results in the generation of a largely homogeneous population of neurons that are both molecularly and functionally comparable to human sensory neurons derived from mature dorsal root ganglia. This work opens the prospect of using pluripotent stem-cell-derived sensory neurons to study human neuronal physiology and as in vitro models for drug discovery in pain and sensory disorders.
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