Swelling-activated calcium signalling in cultured mouse primary sensory neurons

GAP43 stimulates inositol trisphosphate-mediated calcium release in response to hypotonicity

The identification of osmo/mechanosensory proteins in mammalian sensory neurons is still elusive. We have used an expression cloning approach to screen a human dorsal root ganglion cDNA library to look for proteins that respond to hypotonicity by raising the intracellular Ca(2+) concentration ([Ca(2+)](i)). We report the unexpected identification of GAP43 (also known as neuromodulin or B50), a membrane-anchored neuronal protein implicated in axonal growth and synaptic plasticity, as an osmosensory protein that augments [Ca(2+)](i) in response to hypotonicity. Palmitoylation of GAP43 plays an important role in the protein osmosensitivity. Depletion of intracellular stores or inhibition of phospholipase C (PLC) activity abrogates hypotonicity-evoked, GAP43-mediated [Ca(2+)](i) elevations. Notably, hypotonicity promoted the selective association of GAP43 with the PLC-delta(1) isoform, and a concomitant increase in inositol-1,4,5-trisphosphate (IP(3)) formation. Collectively, these findings indicate that hypo-osmotic activation of GAP43 induces Ca(2+) release from IP(3)-sensitive intracellular stores. The osmosensitivity of GAP43 furnishes a mechanistic framework that links axon elongation with phospho inositide metabolism, spontaneous triggering of cytosolic Ca(2+) transients and the regulation of actin dynamics and motility at the growth cone in response to temporal and local mechanical forces.

Impaired activity of volume-sensitive anion channel during lactacidosis-induced swelling in neuronally differentiated NG108-15 cells

Acidosis coupled to lactate accumulation, called lactacidosis, occurs in cerebral ischemia or trauma and is known to cause persistent swelling in neuronal and glial cells. It is therefore possible that mechanisms of cell volume regulation are impaired during lactacidosis. Here we tested this possibility using neuronally differentiated NG108-15 cells. These cells responded to a hypotonic challenge with osmotic swelling followed by a regulatory volume decrease (RVD) under physiological pH conditions in the absence of lactate. Under normotonic conditions, sustained cell swelling without subsequent RVD was induced by exposure to lactate-containing solution with acidic pH (6.4 or 6.2), but not with physiological pH (7.4). Under whole-cell patch-clamp, osmotic swelling was found to activate outwardly rectifying Cl(-) currents in cells exposed to control hypotonic solution. A Cl(-) channel blocker, NPPB, inhibited both RVD and the swelling-activated Cl(-) current. RVD and the volume-sensitive Cl(-) current were also markedly inhibited by lactacidosis (pH 6.4 or 6.2), but neither by application of lactate with physiological pH (7.4) nor by acidification without lactate (pH 6.2). RT-PCR analysis showed mRNA expression of two isoforms of proton-coupled monocarboxylate transporters, MCT1 and MCT8, in differentiated NG108-15 cells. Thus, we conclude that persistence of neuronal cell swelling under lactacidosis is coupled to an impairment of the activity of the volume-sensitive Cl(-) channel and to dysfunction of RVD. It is also suggested that the volume-sensitive Cl(-) channel is inhibited by intracellular acidification induced by MCT-mediated proton influx under lactacidosis.

Elastoviscous substances with analgesic effects on joint pain reduce stretch-activated ion channel activity in vitro

Activation by noxious mechanical stimuli of sensory nerve fibres that signal joint pain takes place through stretch-activated ion channels, which open in response to increased membrane tension. It has been suggested that the analgesic effect of hyaluronan solutions used for intra-articular treatment of joint pain in humans are mediated by a reduction of the sensitivity of mechanosensory ion channels of nociceptive nerve terminals. We have investigated whether cross-linked hyaluronan solutions (hylans) of different elastoviscosities modify the response characteristics of stretch-activated ion channels of Xenopus laevis oocytes. Patch-clamp recordings on intact oocytes and in excised membrane patches (outside-out and inside-out configurations) were performed in Barth's solution (control condition) and after exposure to hylans of different elastoviscosities. For mechanical stimulation, monitored suction was applied through the microelectrode and the activity of stretch-activated channels was recorded. The activity of stretch-activated channels was significantly reduced in the presence of high elastoviscous hylan A (0.8% polymer content, molecular weight 6M) and of a mixture of hylan A (90% by weight) and hylan B (10% by weight), 0.9% total polymer content, a clinically used hylan product. In contrast, solutions of hylan A with the same chemical composition but reduced elastoviscosity (0.8% polymer content, molecular weight 96000) were found ineffective. It is concluded that stretch-activated channels have a decreased mechanical sensitivity in the presence of elastoviscous solutions of hylan, but not in the presence of non-elastoviscous solutions of hylan of the same concentration. These data suggest that the analgesic effects of intra-articular injections of elastoviscous solutions of hylans are due to a reduction of the sensitivity to mechanical forces of stretch-activated channels present in the membrane of joint mechanonociceptors.

Tissue glucose level modulates the mechanical responses of cutaneous nociceptors in streptozotocin-diabetic rats but not normal rats in vitro

The maintenance of normoglycemia has been reported to reduce painful sensations in diabetic subjects. This suggests that lowering the tissue glucose concentration might inhibit the increased cutaneous nociceptor activities seen in a diabetic conditin. To test this hypothesis, we studied the effect of changing the glucose concentration in the superfusate of in vitro preparations (high, HG: 20 mM or normal glucose, NG: 6.7 mM) on the mechanical response of C-fiber polymodal receptors (C-polymodal receptors). Single fiber activities of C-polymodal receptors were recorded from skin-nerve in vitro preparations of streptozotocin-induced diabetic and age-matched control rats. Pressure stimulation was applied to the receptive field by a servo-controlled mechanical stimulator. C-polymodal receptors from diabetic preparations superfused with HG-solution showed increased spontaneous activity, lowered response threshold, increased response magnitude and a less adaptive response pattern to mechanical stimulation compared with those from control preparations superfused with NG-solution. C-polymodal receptors from diabetic preparations superfused with NG-solution showed no such changes. The responsiveness of C-polymodal receptors from control preparations was not different in NG- or HG-conditions. These data demonstrated that normalization of the glucose concentration normalized the responsiveness of C-polymodal receptors in diabetic animals. This response may be associated with the fact that normoglycemia reduces painful sensations in diabetic subjects.

Reducing agent dithiothreitol facilitates activity of the capsaicin receptor VR-1

The vanilloid receptor subtype 1 (VR1) is expressed in a sub-population of small dorsal root ganglion (DRG) neurones in mammals and serves as the common transducer of the pain-producing signals, such as noxious heat, acids and capsaicin [Caterina et al., Nature 389 (1997) 816-824; Tominaga et al., Neuron 21 (1998) 531-543]. On the extracellular side, VR1 has three cysteine residues at positions 616, 621 and 634. Here we report that dithiothreitol (DTT) (2-60 mM), an agent that maintains -SH groups of cysteines in a reduced state, greatly facilitates membrane currents induced by noxious heat or capsaicin (1 microM) in cultured DRG neurones from the rat and in VR1-transfected HEK293 cells. The effects of DTT are concentration-dependent and fully reversible. We suggest that the ratio between free sulfhydryl groups and disulfide bonds of the cysteine residues of VR1 pre-sets sensitivity of primary nociceptors to algogens and may represent a new target for treating some pain states in humans.

Vanilloid and TRP channels: a family of lipid-gated cation channels

The emergence of the TRP (C) and vanilloid (TRPV) receptor family of Ca(2+) permeable channels has started to provide molecular focus to a linked group of ion channels whose common feature is activation primarily by intracellular ligands. These channels have a central role in Ca(2+) homeostasis in virtually all cells and in particular those that lack voltage-gated Ca(2+) channels. We will discuss recent work that is more precisely defining both molecular form and physiological function of this important group of Ca(2+) permeable channels with particular focus on the intracellular ligands that gate and modulate channel activity.

Lowered response threshold and increased responsiveness to mechanical stimulation of cutaneous nociceptive fibers in streptozotocin-diabetic rat skin in vitro--correlates of mechanical allodynia and hyperalgesia observed in the early stage of diabetes

Rats rendered diabetic by streptozotocin (STZ) show allodynia and hyperalgesia and thus, have been offered as a model of pain in diabetic neuropathy. However, recent electrophysiological studies on these rats found that C-fiber nociceptors were not consistently hyperexcitable to mechanical stimulations by von Frey hairs and that there was no change in their response thresholds. In the present study, we used rat skin-saphenous nerve in vitro preparations, in which the receptive fields of identified single C-polymodal receptors (CPRs) can be accurately stimulated with a servo-controlled mechanical stimulator. Single fiber recordings from CPRs were performed in diabetic rats with an increased behavioral nociceptive response 7-19 days after STZ injection. The proportion of units with spontaneous activity and the magnitude of this activity increased in the diabetic preparations. The response thresholds of CPRs were significantly decreased with ramp-pressure stimulation and their response magnitude to the suprathreshold stimulation was significantly increased in diabetic rats. In addition, the response pattern to mechanical stimulation was also changed to a non-adapting type. These findings suggest that changes in CPRs contribute to the enhanced nociception observed in the early stage of diabetic neuropathy.

The diversity in the vanilloid (TRPV) receptor family of ion channels

Following cloning of the vanilloid receptor 1 (VR1) at least four other related proteins have been identified. Together, these form a distinct subgroup of the transient receptor potential (TRP) family of ion channels. Members of the vanilloid receptor family (TRPV) are activated by a diverse range of stimuli, including heat, protons, lipids, phorbols, phosphorylation, changes in extracellular osmolarity and/or pressure, and depletion of intracellular Ca2+ stores. However, VR1 remains the only channel activated by vanilloids such as capsaicin. These channels are excellent molecular candidates to fulfil a range of sensory and/or cellular roles that are well characterized physiologically. Furthermore, as novel pharmacological targets, the vanilloid receptors have potential for the development of many future disease treatments.

Specificity of cold thermotransduction is determined by differential ionic channel expression

Sensations of cold are mediated by specific thermoreceptor nerve endings excited by low temperature and menthol. Here we identify a population of cold-sensitive cultured mouse trigeminal ganglion neurons with a unique set of biophysical properties. Their impulse activity during cooling and menthol application was similar to that of cold thermoreceptor fibers in vivo. We show that cooling closes a background K+ channel, causing depolarization and firing that is limited by the slower reduction of a cationic inward current (Ih). In cold-insensitive neurons, firing is prevented by a slow, transient, 4-AP-sensitive K+ current (IKD) that acts as an excitability brake. In addition, pharmacological blockade of IKD induced thermosensitivity in cold-insensitive neurons, a finding that may explain cold allodynia in neuropathic pain. These results suggest that cold sensitivity is not associated to a specific transduction molecule but instead results from a favorable blend of ionic channels expressed in a small subset of sensory neurons.

Attenuation of thermal nociception and hyperalgesia by VR1 blockers

Vanilloid receptor subunit 1 (VR1) appears to play a critical role in the transduction of noxious chemical and thermal stimuli by sensory nerve endings in peripheral tissues. Thus, VR1 antagonists are useful compounds to unravel the contribution of this receptor to pain perception, as well as to induce analgesia. We have used a combinatorial approach to identify new, nonpeptidic channel blockers of VR1. Screening of a library of trimers of N-alkylglycines resulted in the identification of two molecules referred to as DD161515 [N-[2-(2-(N-methylpyrrolidinyl)ethyl]glycyl]-[N-[2,4-dichlorophenethyl]glycyl]-N-(2,4-dichlorophenethyl)glycinamide] and DD191515 [[N-[3-(N,N-diethylamino)propyl]glycyl]-[N-[2,4-dichlorophenethyl]glycyl]-N-(2,4-dichlorophenethyl)glycinamide] that selectively block VR1 channel activity with micromolar efficacy, rivaling that characteristic of vanilloid-related inhibitors. These compounds appear to be noncompetitive VR1 antagonists that recognize a receptor site distinct from that of capsaicin. Intraperitoneal administration of both trialkylglycines into mice significantly attenuated thermal nociception as measured in the hot plate test. It is noteworthy that these compounds eliminated pain and neurogenic inflammation evoked by intradermal injection of capsaicin into the animal hindpaw, as well as the thermal hyperalgesia induced by tissue irritation with nitrogen mustard. In contrast, responses to mechanical stimuli were not modified by either compound. Modulation of sensory nerve fibers excitability appears to underlie the peptoid analgesic activity. Collectively, these results indicate that blockade of VR1 activity attenuates chemical and thermal nociception and hyperalgesia, supporting the tenet that this ionotropic receptor contributes to chemical and thermal sensitivity and pain perception in vivo. These trialkylglycine-based, noncompetitive VR1 antagonists may likely be developed into analgesics to treat inflammatory pain.

Force transduction by Triton cytoskeletons

Force-initiated signal transduction can occur either via membrane-based ionic mechanisms or through changes in cytoskeletal-matrix linkages. We report here the stretch-dependent binding of cytoplasmic proteins to Triton X-100 cytoskeletons of L-929 cells grown on collagen-coated silicone. Triton X-100-insoluble cytoskeletons were stretched by 10% and incubated with biotinylated cytoplasmic proteins. Analysis with two-dimensional gel electrophoresis showed stretch-dependent binding of more than 10 cytoplasmic protein spots. Bound cytoplasmic proteins were purified by a photocleavable biotin tag and stretch-dependent binding of paxillin, focal adhesion kinase, and p130Cas was found, whereas the binding of vinculin was unchanged and actin binding decreased with stretch. Paxillin binding upon stretch was morphologically and biochemically similar in vitro and in vivo, that is, enhanced in the periphery and inhibited by the tyrosine phosphatase inhibitor, phenylarsine oxide. Thus, we suggest that transduction of matrix forces occurs through force-dependent conformation changes in the integrated cytoskeleton.