Pain modulators regulate the dynamics of PKA-RII phosphorylation in subgroups of sensory neurons

Identification of Sodium Transients Through NaV1.5 Channels as Regulators of Differentiation in Immortalized Dorsal Root Ganglia Neurons

Neuronal differentiation is a complex process through which newborn neurons acquire the morphology of mature neurons and become excitable. We employed a combination of functional and transcriptomic approaches to deconvolute and identify key regulators of the differentiation process of a DRG neuron-derived cell line, and we focused our study on the NaV1.5 ion channel (encoded by Scn5a) as a channel involved in the acquisition of DRG neuronal features. Overexpression of Scn5a enhances the acquisition of neuronal phenotypic features and increases the KCl-elicited hyperexcitability response in a DRG-derived cell line. Moreover, pharmacologic inhibition of the NaV1.5 channel during differentiation hinders the acquisition of phenotypic features of neuronal cells and the hyperexcitability increase in response to changes in the extracellular medium ionic composition. Taken together, these data highlight the relevance of sodium transients in regulating the neuronal differentiation process in a DRG neuron-derived cell line.

Depolarization induces nociceptor sensitization by CaV1.2-mediated PKA-II activation

Depolarization drives neuronal plasticity. However, whether depolarization drives sensitization of peripheral nociceptive neurons remains elusive. By high-content screening (HCS) microscopy, we revealed that depolarization of cultured sensory neurons rapidly activates protein kinase A type II (PKA-II) in nociceptors by calcium influx through Ca_V1.2 channels. This effect was modulated by calpains but insensitive to inhibitors of cAMP formation, including opioids. In turn, PKA-II phosphorylated Ser1928 in the distal C terminus of Ca_V1.2, thereby increasing channel gating, whereas dephosphorylation of Ser1928 involved the phosphatase calcineurin. Patch-clamp and behavioral experiments confirmed that depolarization leads to calcium- and PKA-dependent sensitization of calcium currents ex vivo and local peripheral hyperalgesia in the skin in vivo. Our data suggest a local activity-driven feed-forward mechanism that selectively translates strong depolarization into further activity and thereby facilitates hypersensitivity of nociceptor terminals by a mechanism inaccessible to opioids.

Serotonin enhances depolarizing spontaneous fluctuations, excitability, and ongoing activity in isolated rat DRG neurons via 5-HT4 receptors and cAMP-dependent mechanisms

Ongoing activity in nociceptors, a driver of spontaneous pain, can be generated in dorsal root ganglion neurons in the absence of sensory generator potentials if one or more of three neurophysiological alterations occur - prolonged depolarization of resting membrane potential (RMP), hyperpolarization of action potential (AP) threshold, and/or increased amplitude of depolarizing spontaneous fluctuations of membrane potential (DSFs) to bridge the gap between RMP and AP threshold. Previous work showed that acute, sustained exposure to serotonin (5-HT) hyperpolarized AP threshold and potentiated DSFs, leading to ongoing activity if a separate source of maintained depolarization was present. Cellular signaling pathways that increase DSF amplitude and promote ongoing activity acutely in nociceptors are not known for any neuromodulator. Here, isolated DRG neurons from male rats were used to define the pathway by which low concentrations of 5-HT enhance DSFs, hyperpolarize AP threshold, and promote ongoing activity. A selective 5-HT₄ receptor antagonist blocked these 5-HT-induced hyperexcitable effects, while a selective 5-HT₄ agonist mimicked the effects of 5-HT. Inhibition of cAMP effectors, protein kinase A (PKA) and exchange protein activated by cAMP (EPAC), attenuated 5-HT's hyperexcitable effects, but a blocker of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels had no significant effect. 5-HT₄-dependent PKA activation was specific to DRG neurons that bind isolectin B4 (a nonpeptidergic nociceptor marker). 5-HT's effects on AP threshold, DSFs, and ongoing activity were mimicked by a cAMP analog. Sustained exposure to 5-HT promotes ongoing activity in nonpeptidergic nociceptors through the G_s-coupled 5-HT₄ receptor and downstream cAMP signaling involving both PKA and EPAC.

Scorpion toxin MeuNaTxα-1 sensitizes primary nociceptors by selective modulation of voltage-gated sodium channels

Venoms are a rich source of highly specific toxins, which allow the identification of novel therapeutic targets. We have now applied high content screening (HCS) microscopy to identify toxins that modulate pain sensitization signaling in primary sensory neurons of rat and elucidated the underlying mechanism. A set of venoms and fractions thereof were analyzed for their ability to activate type II protein kinase A (PKA-II) and extracellular signal-regulated kinases (ERK1/2). We identified MeuNaTxα-1, a sodium channel-selective scorpion α-toxin from Mesobuthus eupeus, which affected both PKA-II and ERK1/2. Recombinant MeuNaTxα-1 showed identical activity to the native toxin on mammalian voltage-gated sodium channels expressed in Xenopus laevis oocytes and induced thermal hyperalgesia in adult mice. The effect of MeuNaTxα-1 on sensory neurons was dose-dependent and tetrodotoxin-sensitive. Application of inhibitors and toxin mutants with altered sodium channel selectivity demonstrated that signaling activation in sensory neurons depends on Na_V 1.2 isoform. Accordingly, the toxin was more potent in neurons from newborn rats, where Na_V 1.2 is expressed at a higher level. Our results demonstrate that HCS microscopy-based monitoring of intracellular signaling is a novel and powerful tool to identify and characterize venoms and their toxins affecting sensory neurons.

Oncostatin M induces hyperalgesic priming and amplifies signaling of cAMP to ERK by RapGEF2 and PKA

Hyperalgesic priming is characterized by enhanced nociceptor sensitization by pronociceptive mediators, prototypically PGE₂ . Priming has gained interest as a mechanism underlying the transition to chronic pain. Which stimuli induce priming and what cellular mechanisms are employed remains incompletely understood. In adult male rats, we present the cytokine Oncostatin M (OSM), a member of the IL-6 family, as an inducer of priming by a novel mechanism. We used a high content microscopy based approach to quantify the activation of endogenous PKA-II and ERK of thousands sensory neurons in culture. Incubation with OSM increased and prolonged ERK activation by agents that increase cAMP production such as PGE₂ , forskolin, and cAMP analogs. These changes were specific to IB4/CaMKIIα positive neurons, required protein translation, and increased cAMP-to-ERK signaling. In both, control and OSM-treated neurons, cAMP/ERK signaling involved RapGEF2 and PKA but not Epac. Similar enhancement of cAMP-to-ERK signaling could be induced by GDNF, which acts mostly on IB4/CaMKIIα-positive neurons, but not by NGF, which acts mostly on IB4/CaMKIIα-negative neurons. In vitro, OSM pretreatment rendered baseline TTX-R currents ERK-dependent and switched forskolin-increased currents from partial to full ERK-dependence in small/medium sized neurons. In summary, priming induced by OSM uses a novel mechanism to enhance and prolong coupling of cAMP/PKA to ERK1/2 signaling without changing the overall pathway structure.

Depolarization-Dependent C-Raf Signaling Promotes Hyperexcitability and Reduces Opioid Sensitivity of Isolated Nociceptors after Spinal Cord Injury

Chronic pain caused by spinal cord injury (SCI) is notoriously resistant to treatment, particularly by opioids. After SCI, DRG neurons show hyperactivity and chronic depolarization of resting membrane potential (RMP) that is maintained by cAMP signaling through PKA and EPAC. Importantly, SCI also reduces the negative regulation by Gαi of adenylyl cyclase and its production of cAMP, independent of alterations in G protein-coupled receptors and/or G proteins. Opioid reduction of pain depends on coupling of opioid receptors to Gαi/o family members. Combining high-content imaging and cluster analysis, we show that in male rats SCI decreases opioid responsiveness in vitro within a specific subset of small-diameter nociceptors that bind isolectin B4. This SCI effect is mimicked in nociceptors from naive animals by a modest 5 min depolarization of RMP (15 mm K⁺; -45 mV), reducing inhibition of cAMP signaling by μ-opioid receptor agonists DAMGO and morphine. Disinhibition and activation of C-Raf by depolarization-dependent phosphorylation are central to these effects. Expression of an activated C-Raf reduces sensitivity of adenylyl cyclase to opioids in nonexcitable HEK293 cells, whereas inhibition of C-Raf or treatment with the hyperpolarizing drug retigabine restores opioid responsiveness and blocks spontaneous activity of nociceptors after SCI. Inhibition of ERK downstream of C-Raf also blocks SCI-induced hyperexcitability and depolarization, without direct effects on opioid responsiveness. Thus, depolarization-dependent C-Raf and downstream ERK activity maintain a depolarized RMP and nociceptor hyperactivity after SCI, providing a self-reinforcing mechanism to persistently promote nociceptor hyperexcitability and limit the therapeutic effectiveness of opioids.SIGNIFICANCE STATEMENT Chronic pain induced by spinal cord injury (SCI) is often permanent and debilitating, and usually refractory to treatment with analgesics, including opioids. SCI-induced pain in a rat model has been shown to depend on persistent hyperactivity in primary nociceptors (injury-detecting sensory neurons), associated with a decrease in the sensitivity of adenylyl cyclase production of cAMP to inhibitory Gαi proteins in DRGs. This study shows that SCI and one consequence of SCI (chronic depolarization of resting membrane potential) decrease sensitivity to opioid-mediated inhibition of cAMP and promote hyperactivity of nociceptors by enhancing C-Raf activity. ERK activation downstream of C-Raf is necessary for maintaining ongoing depolarization and hyperactivity, demonstrating an unexpected positive feedback loop to persistently promote pain.

High-Content Imaging of Immunofluorescently Labeled TRPV1-Positive Sensory Neurons

Studying TRP channel expressing nociceptors requires the identification of the respective subpopulations as well as the quantification of dynamic cellular events. However, the heterogeneity of sensory neurons and associated nonneuronal cells demands the analysis of large numbers of cells to reflect the distribution of entire populations. Here we report a detailed workflow how to apply high-content screening (HCS) microscopy to signaling events in TRPV1-positive neurons as well as an approach to use the selective elimination of TRPV1 positive cells from dissociated rat sensory ganglia as base for transcriptomic analysis of TRPV1-positive cells and/or as control for TRPV1 antibody specificity.

Estimation methods for heterogeneous cell population models in systems biology

Heterogeneity among individual cells is a characteristic and relevant feature of living systems. A range of experimental techniques to investigate this heterogeneity is available, and multiple modelling frameworks have been developed to describe and simulate the dynamics of heterogeneous populations. Measurement data are used to adjust computational models, which results in parameter and state estimation problems. Methods to solve these estimation problems need to take the specific properties of data and models into account. The aim of this review is to give an overview on the state of the art in estimation methods for heterogeneous cell population data and models. The focus is on models based on the population balance equation, but stochastic and individual-based models are also discussed. It starts with a brief discussion of common experimental approaches and types of measurement data that can be obtained in this context. The second part describes computational modelling frameworks for heterogeneous populations and the types of estimation problems occurring for these models. The third part starts with a discussion of observability and identifiability properties, after which the computational methods to solve the various estimation problems are described.

A Novel Framework for Parameter and State Estimation of Multicellular Systems Using Gaussian Mixture Approximations

Multicellular systems play an important role in many biotechnological processes. Typically, these exhibit cell-to-cell variability, which has to be monitored closely for process control and optimization. However, some properties may not be measurable due to technical and financial restrictions. To improve the monitoring, model-based online estimators can be designed for their reconstruction. The multicellular dynamics is accounted for in the framework of population balance models (PBMs). These models are based on single cell kinetics, and each cellular state translates directly into an additional dimension of the obtained partial differential equations. As multicellular dynamics often require detailed single cell models and feature a high number of cellular components, the resulting population balance equations are often high-dimensional. Therefore, established state estimation concepts for PBMs based on discrete grids are not recommended due to the large computational effort. In this contribution a novel approach is proposed, which is based on the approximation of the underlying number density functions as the weighted sum of Gaussian distributions. Thus, the distribution is described by the characteristic properties of the individual Gaussians, like the mean and covariance. Thereby, the complex infinite dimensional estimation problem can be reduced to a finite dimension. The characteristic properties are estimated in a recursive approach. The method is evaluated for two academic benchmark examples, and the results indicate its potential for model-based online reconstruction for multicellular systems.

Protein kinase A regulates inflammatory pain sensitization by modulating HCN2 channel activity in nociceptive sensory neurons

Several studies implicated cyclic adenosine monophosphate (cAMP) as an important second messenger for regulating nociceptor sensitization, but downstream targets of this signaling pathway which contribute to neuronal plasticity are not well understood. We used a Cre/loxP-based strategy to disable the function of either HCN2 or PKA selectively in a subset of peripheral nociceptive neurons and analyzed the nociceptive responses in both transgenic lines. A near-complete lack of sensitization was observed in both mutant strains when peripheral inflammation was induced by an intradermal injection of 8br-cAMP. The lack of HCN2 as well as the inhibition of PKA eliminated the cAMP-mediated increase of calcium transients in dorsal root ganglion neurons. Facilitation of Ih via cAMP, a hallmark of the Ih current, was abolished in neurons without PKA activity. Collectively, these results show a significant contribution of both genes to inflammatory pain and suggest that PKA-dependent activation of HCN2 underlies cAMP-triggered neuronal sensitization.

Protein kinase A activation: Something new under the sun?

Smith and Scott discuss work from Isensee et al. addressing the long-standing question of the regulation of protein kinase A–II activity by phosphorylation.

The role of autophosphorylation of the type II regulatory subunit in activation of protein kinase A (PKA) has been a longstanding question. In this issue, Isensee et al. (2018. J. Cell Biol.https://doi.org/10.1083/jcb.201708053) use antibody tools that selectively recognize phosphorylated RII and the catalytic subunit active site to reexamine PKA holoenzyme activation mechanisms in neurons.

PKA-RII subunit phosphorylation precedes activation by cAMP and regulates activity termination

Activity of endogenous protein kinase A (PKA) could never be analyzed directly in the cellular environment. Isensee et al. used antibodies to quantify conformational changes leading to an open conformation of endogenous PKA-II holoenzymes, which allowed them to analyze and model its activation cycle in primary sensory neurons.

Type II isoforms of cyclic adenosine monophosphate (cAMP)–dependent protein kinase A (PKA-II) contain a phosphorylatable epitope within the inhibitory domain of RII subunits (pRII) with still unclear function. In vitro, RII phosphorylation occurs in the absence of cAMP, whereas staining of cells with pRII-specific antibodies revealed a cAMP-dependent pattern. In sensory neurons, we found that increased pRII immunoreactivity reflects increased accessibility of the already phosphorylated RII epitope during cAMP-induced opening of the tetrameric RII₂:C₂ holoenzyme. Accordingly, induction of pRII by cAMP was sensitive to novel inhibitors of dissociation, whereas blocking catalytic activity was ineffective. Also in vitro, cAMP increased the binding of pRII antibodies to RII₂:C₂ holoenzymes. Identification of an antibody specific for the glycine-rich loop of catalytic subunits facing the pRII-epitope confirmed activity-dependent binding with similar kinetics, proving that the reassociation is rapid and precisely controlled. Mechanistic modeling further supported that RII phosphorylation precedes cAMP binding and controls the inactivation by modulating the reassociation involving the coordinated action of phosphodiesterases and phosphatases.

TLR4 receptor expression and function in F11 dorsal root ganglion × neuroblastoma hybrid cells

TLR4 respond to bacterial LPS to produce inflammatory cytokines. TLR4 are expressed in dorsal root ganglia and play a role in pain. F11 dorsal root ganglia × mouse neuroblastoma cells possess many of the properties seen in nociceptive dorsal root ganglia neuronal cells. Here, we investigated the effect of 2 h and 6 h treatment with LPS upon the expression of inflammatory proteins in undifferentiated and differentiated F11 cells. The cells expressed mRNA for TRL4 (mouse, not rat) and proteins involved in TLR4 signaling. TLR4 expression was confirmed using immunohistochemistry. LPS produced modest increases in mouse and rat IL-6 and in mouse cyclooxygenase-2 levels in undifferentiated cells, but did not significantly affect mouse TNF-α expression. This contrasts with the robust effects of LPS upon cyclooxygenase-2 expression in cultured dorsal root ganglia neurons. F11 cells expressed the endocannabinoid metabolizing enzymes fatty acid amide hydrolase and N-acylethanolamine acid amidase (both murine), which were functionally active. These data suggest that F11 cells are not a useful model for the study of LPS-mediated effects but may be useful for the study of endocannabinoid catabolism.

Crosstalk from cAMP to ERK1/2 emerges during postnatal maturation of nociceptive neurons and is maintained during aging

Maturation of nociceptive neurons depends on changes in transcription factors, ion channels and neuropeptides. Mature nociceptors initiate pain in part by drastically reducing the activation threshold via intracellular sensitization signaling. Whether sensitization signaling also changes during development and aging remains so far unknown. Using a novel automated microscopy approach, we quantified changes in intracellular signaling protein expression and in their signaling dynamics, as well as changes in intracellular signaling cascade wiring, in sensory neurons from newborn to senescent (24 months of age) rats. We found that nociceptive subgroups defined by the signaling components protein kinase A (PKA)-RIIβ (also known as PRKAR2B) and CaMKIIα (also known as CAMK2A) developed at around postnatal day 10, the time of nociceptor maturation. The integrative nociceptor marker, PKA-RIIβ, allowed subgroup segregation earlier than could be achieved by assessing the classical markers TRPV1 and Na_v1.8 (also known as SCN10A). Signaling kinetics remained constant over lifetime despite in part strong changes in the expression levels. Strikingly, we found a mechanism important for neuronal memory - i.e. the crosstalk from cAMP and PKA to ERK1 and ERK2 (ERK1/2, also known as MAPK3 and MAPK1, respectively) - to emerge postnatally. Thus, maturation of nociceptors is closely accompanied by altered expression, activation and connectivity of signaling pathways known to be central for pain sensitization and neuronal memory formation.

Ankyrin-rich membrane spanning protein as a novel modulator of transient receptor potential vanilloid 1-function in nociceptive neurons

BACKGROUND

The ion channel TRPV1 is mainly expressed in small diameter dorsal root ganglion (DRG) neurons, which are involved in the sensation of acute noxious thermal and chemical stimuli. Direct modifications of the channel by diverse signalling events have been intensively investigated, but little is known about the composition of modulating macromolecular TRPV1 signalling complexes. Here, we hypothesize that the novel adaptor protein ankyrin-rich membrane spanning protein/kinase D interacting substrate (ARMS) interacts with TRPV1 and modulates its function in rodent DRG neurons.

METHODS

We used immunohistochemistry, electrophysiology, microfluorimetry and immunoprecipitation experiments to investigate TRPV1 and ARMS interactions in DRG neurons and transfected cells.

RESULTS

We found that TRPV1 and ARMS are co-expressed in a subpopulation of DRG neurons. ARMS sensitizes TRPV1 towards capsaicin in transfected HEK 293 cells and in mouse DRG neurons in a PKA-dependent manner. Using a combination of functional imaging and immunocytochemistry, we show that the magnitude of the capsaicin response in DRG neurons depends not only on TRPV1 expression, but on the co-expression of ARMS alongside TRPV1.

CONCLUSION

These data indicate that ARMS is an important component of the signalling complex regulating the sensitivity of TRPV1.

SIGNIFICANCE

The study identifies ARMS as an important component of the signalling complex regulating the sensitivity of excitatory ion channels (TRPV1) in peripheral sensory neurons (DRG neurons) and transfected cells.

Involvement of BDNF/TrkB and ERK/CREB axes in nitroglycerin-induced rat migraine and effects of estrogen on these signals in the migraine

Migraine is a highly prevalent headache disorder, especially in women. Brain-derived neurotrophic factor (BDNF) and its receptor tropomyosin receptor kinases (TrkB), as well as extracellular signal-regulated kinase (ERK) and its downstream target c-AMP-responsive element binding protein (CREB) are strongly associated with the transmission of nociceptive information. However, the involvement of these substances in migraine has rarely been examined. In the present study, intraperitoneal injection of nitroglycerin (NTC) successfully induced rat migraine attack, as evidenced by behavioral testing. The location and abundance of these substances in the migraine model were determined by immunohistochemistry, real-time polymerase chain reaction (RT-PCR), western blot and enzyme-linked immunosorbant assays (ELISA). Results showed that BDNF, TrkB, phosphor(p)-ERK and p-CREB were up-regulated in the brain neurons of both male and female rats with NTG-induced migraine compared to non-migraine control, whereas their expression levels were decreased in headache-free intervals of the migraine compared to migraine attacks. Estrogen is an important contributor to migraine. Female ovariectomized rats showed significant reduction in the expression of BDNF, TrkB, p-CREB and p-ERK in both attacks and intervals of NTG-induced migraine, relative to rats that have their ovaries. But, intraperitoneal administration of exogenous estrogen recovered their expression in ovariectomized rats. Collectively, this study unveiled a positive correlation of BDNF/TrkB and ERK/CREB axes in NTG-induced migraine and promoting effects of estrogen on their signals in the migraine. These findings contribute to further understanding the pathogenesis of migraine in the molecular basis.

Summary: This study unveiled a positive correlation of BDNF/TrkB and CREB/ERK axes in NTG-induced migraine and the promoting effects of estrogen on their signals in the migraine.

Synergistic regulation of serotonin and opioid signaling contributes to pain insensitivity in Nav1.7 knockout mice

Genetic loss of the voltage-gated sodium channel Na_v1.7 (Na_v1.7^-/-) results in lifelong insensitivity to pain in mice and humans. One underlying cause is an increase in the production of endogenous opioids in sensory neurons. We analyzed whether Na_v1.7 deficiency altered nociceptive heterotrimeric guanine nucleotide-binding protein-coupled receptor (GPCR) signaling, such as initiated by GPCRs that respond to serotonin (pronociceptive) or opioids (antinociceptive), in sensory neurons. We found that the nociceptive neurons of Na_v1.7 knockout (Na_v1.7^-/-) mice, but not those of Na_v1.8 knockout (Na_v1.8^-/-) mice, exhibited decreased pronociceptive serotonergic signaling through the 5-HT₄ receptors, which are Gα_s-coupled GPCRs that stimulate the production of cyclic adenosine monophosphate resulting in protein kinase A (PKA) activity, as well as reduced abundance of the RIIβ regulatory subunit of PKA. Simultaneously, the efficacy of antinociceptive opioid signaling mediated by the Gα_i-coupled mu opioid receptors was increased. Consequently, opioids inhibited more efficiently tetrodotoxin-resistant sodium currents, which are important for pain-initiating neuronal activity in nociceptive neurons. Thus, Na_v1.7 controls the efficacy and balance of GPCR-mediated pro- and antinociceptive intracellular signaling, such that without Na_v1.7, the balance is shifted toward antinociception, resulting in lifelong endogenous analgesia.

β2-adrenergic receptor control of endosomal PTH receptor signaling via Gβγ

Cells express several G-protein-coupled receptors (GPCRs) at their surfaces, transmitting simultaneous extracellular hormonal and chemical signals into cells. A comprehensive understanding of mechanisms underlying the integrated signaling response induced by distinct GPCRs is thus required. Here we found that the β₂-adrenergic receptor, which induces a short cAMP response, prolongs nuclear cAMP and protein kinase A (PKA) activation by promoting endosomal cAMP production in parathyroid hormone (PTH) receptor signaling through the stimulatory action of G protein Gβγ subunits on adenylate cyclase type 2.

Long-term exposure to PGE2 causes homologous desensitization of receptor-mediated activation of protein kinase A

Background

Acute exposure to prostaglandin E₂ (PGE₂) activates EP receptors in sensory neurons which triggers the cAMP-dependent protein kinase A (PKA) signaling cascade resulting in enhanced excitability of the neurons. With long-term exposure to PGE₂, however, the activation of PKA does not appear to mediate persistent PGE₂-induced sensitization. Consequently, we examined whether homologous desensitization of PGE₂-mediated PKA activation occurs after long-term exposure of isolated sensory neurons to the eicosanoid.

Methods

Sensory neuronal cultures were harvested from the dorsal root ganglia of adult male Sprague-Dawley rats. The cultures were pretreated with vehicle or PGE₂ and used to examine signaling mechanisms mediating acute versus persistent sensitization by exposure to the eicosanoid using enhanced capsaicin-evoked release of immunoreactive calcitonin gene-related peptide (iCGRP) as an endpoint. Neuronal cultures chronically exposed to vehicle or PGE₂ also were used to study the ability of the eicosanoid and other agonists to activate PKA and whether long-term exposure to the prostanoid alters expression of EP receptor subtypes.

Results

Acute exposure to 1 μM PGE₂ augments the capsaicin-evoked release of iCGRP, and this effect is blocked by the PKA inhibitor H-89. After 5 days of exposure to 1 μM PGE₂, administration of the eicosanoid still augments evoked release of iCGRP, but the effect is not attenuated by inhibition of PKA or by inhibition of PI3 kinases. The sensitizing actions of PGE₂ after acute and long-term exposure were attenuated by EP2, EP3, and EP4 receptor antagonists, but not by an EP1 antagonist. Exposing neuronal cultures to 1 μM PGE₂ for 12 h to 5 days blocks the ability of PGE₂ to activate PKA. The offset of the desensitization occurs within 24 h of removal of PGE₂ from the cultures. Long-term exposure to PGE₂ also results in desensitization of the ability of a selective EP4 receptor agonist, L902688 to activate PKA, but does not alter the ability of cholera toxin, forskolin, or a stable analog of prostacyclin to activate PKA.

Conclusions

Long-term exposure to PGE₂ results in homologous desensitization of EP4 receptor activation of PKA, but not to neuronal sensitization suggesting that activation of PKA does not mediate PGE₂-induced sensitization after chronic exposure to the eicosanoid.

Subgroup-elimination transcriptomics identifies signaling proteins that define subclasses of TRPV1-positive neurons and a novel paracrine circuit

Normal and painful stimuli are detected by specialized subgroups of peripheral sensory neurons. The understanding of the functional differences of each neuronal subgroup would be strongly enhanced by knowledge of the respective subgroup transcriptome. The separation of the subgroup of interest, however, has proven challenging as they can hardly be enriched. Instead of enriching, we now rapidly eliminated the subgroup of neurons expressing the heat-gated cation channel TRPV1 from dissociated rat sensory ganglia. Elimination was accomplished by brief treatment with TRPV1 agonists followed by the removal of compromised TRPV1(+) neurons using density centrifugation. By differential microarray and sequencing (RNA-Seq) based expression profiling we compared the transcriptome of all cells within sensory ganglia versus the same cells lacking TRPV1 expressing neurons, which revealed 240 differentially expressed genes (adj. p<0.05, fold-change>1.5). Corroborating the specificity of the approach, many of these genes have been reported to be involved in noxious heat or pain sensitization. Beyond the expected enrichment of ion channels, we found the TRPV1 transcriptome to be enriched for GPCRs and other signaling proteins involved in adenosine, calcium, and phosphatidylinositol signaling. Quantitative population analysis using a recent High Content Screening (HCS) microscopy approach identified substantial heterogeneity of expressed target proteins even within TRPV1-positive neurons. Signaling components defined distinct further subgroups within the population of TRPV1-positive neurons. Analysis of one such signaling system showed that the pain sensitizing prostaglandin PGD2 activates DP1 receptors expressed predominantly on TRPV1(+) neurons. In contrast, we found the PGD2 producing prostaglandin D synthase to be expressed exclusively in myelinated large-diameter neurons lacking TRPV1, which suggests a novel paracrine neuron-neuron communication. Thus, subgroup analysis based on the elimination rather than enrichment of the subgroup of interest revealed proteins that define subclasses of TRPV1-positive neurons and suggests a novel paracrine circuit.

ODE constrained mixture modelling: a method for unraveling subpopulation structures and dynamics

Functional cell-to-cell variability is ubiquitous in multicellular organisms as well as bacterial populations. Even genetically identical cells of the same cell type can respond differently to identical stimuli. Methods have been developed to analyse heterogeneous populations, e.g., mixture models and stochastic population models. The available methods are, however, either incapable of simultaneously analysing different experimental conditions or are computationally demanding and difficult to apply. Furthermore, they do not account for biological information available in the literature. To overcome disadvantages of existing methods, we combine mixture models and ordinary differential equation (ODE) models. The ODE models provide a mechanistic description of the underlying processes while mixture models provide an easy way to capture variability. In a simulation study, we show that the class of ODE constrained mixture models can unravel the subpopulation structure and determine the sources of cell-to-cell variability. In addition, the method provides reliable estimates for kinetic rates and subpopulation characteristics. We use ODE constrained mixture modelling to study NGF-induced Erk1/2 phosphorylation in primary sensory neurones, a process relevant in inflammatory and neuropathic pain. We propose a mechanistic pathway model for this process and reconstructed static and dynamical subpopulation characteristics across experimental conditions. We validate the model predictions experimentally, which verifies the capabilities of ODE constrained mixture models. These results illustrate that ODE constrained mixture models can reveal novel mechanistic insights and possess a high sensitivity.

In this manuscript, we introduce ODE constrained mixture models for the analysis of population snapshot data of kinetics and dose responses. Population snapshot data can for instance be derived from flow cytometry or single-cell microscopy and provide information about the population structure and the dynamics of subpopulations. Currently available methods enable, however, only the extraction of this information if the subpopulations are very different. By combining pathway-specific ODE and mixture models, a more sensitive method is obtained, which can simultaneously analyse a variety of experimental conditions. ODE constrained mixture models facilitate the reconstruction of subpopulation sizes and dynamics, even in situations where the subpopulations are hardly distinguishable. This is shown for a simulation example as well as for the process of NGF-induced Erk1/2 phosphorylation in primary sensory neurones. We find that the proposed method allows for a simple but pervasive analysis of heterogeneous cell systems and more profound, mechanistic insights.