Mechanisms underlying mechanical sensitization induced by complement C5a: the roles of macrophages, TRPV1, and calcitonin gene-related peptide receptors

The complement system significantly contributes to the development of inflammatory and neuropathic pain, but the underlying mechanisms are poorly understood. Recently, we identified the signaling pathway responsible for thermal hypersensitivity induced by the complement system component C5a. Here, we examine the mechanisms of another important action of C5a, induction of mechanical hypersensitivity. We found that intraplantar injection of C5a produced a dose-dependent mechanical sensitization and that this effect was blocked by chemogenetic ablation of macrophages in both male and female mice. Knockout of TRPV1 or pretreatment with the TRPV1 antagonists, AMG9810 or 5'-iodoresiniferatoxin (5'-IRTX), significantly reduced C5a-induced mechanical sensitization. Notably, local administration of 5'-IRTX 90 minutes after C5a injection resulted in a slow, but complete, reversal of mechanical sensitization, indicating that TRPV1 activity was required for maintaining C5a-induced mechanical hypersensitivity. This slow reversal suggests that neurogenic inflammation and neuropeptide release may be involved. Indeed, pretreatment with a calcitonin gene-related peptide (CGRP) receptor antagonist (but not an antagonist of the neurokinin 1 receptor) prevented C5a-induced mechanical sensitization. Furthermore, intraplantar injection of CGRP produced significant mechanical sensitization in both wild-type and TRPV1 knockout mice. Taken together, these findings suggest that C5a produces mechanical sensitization by initiating macrophage-to-sensory-neuron signaling cascade that involves activation of TRPV1 and CGRP receptor as critical steps in this process.

Nerve growth factor (NGF) regulates activity of nuclear factor of activated T-cells (NFAT) in neurons via the phosphatidylinositol 3-kinase (PI3K)-Akt-glycogen synthase kinase 3β (GSK3β) pathway

The Ca(2+)/calcineurin-dependent transcription factor nuclear factor of activated T-cells (NFAT) plays an important role in regulating many neuronal functions, including excitability, axonal growth, synaptogenesis, and neuronal survival. NFAT can be activated by action potential firing or depolarization that leads to Ca(2+)/calcineurin-dependent dephosphorylation of NFAT and its translocation to the nucleus. Recent data suggest that NFAT and NFAT-dependent functions in neurons can also be potently regulated by NGF and other neurotrophins. However, the mechanisms of NFAT regulation by neurotrophins are not well understood. Here, we show that in dorsal root ganglion sensory neurons, NGF markedly facilitates NFAT-mediated gene expression induced by mild depolarization. The effects of NGF were not associated with changes in [Ca(2+)]i and were independent of phospholipase C activity. Instead, the facilitatory effect of NGF depended on activation of the PI3K/Akt pathway downstream of the TrkA receptor and on inhibition of glycogen synthase kinase 3β (GSK3β), a protein kinase known to phosphorylate NFAT and promote its nuclear export. Knockdown or knockout of NFATc3 eliminated this facilitatory effect. Simultaneous monitoring of EGFP-NFATc3 nuclear translocation and [Ca(2+)]i changes in dorsal root ganglion neurons indicated that NGF slowed the rate of NFATc3 nuclear export but did not affect its nuclear import rate. Collectively, our data suggest that NGF facilitates depolarization-induced NFAT activation by stimulating PI3K/Akt signaling, inactivating GSK3β, and thereby slowing NFATc3 export from the nucleus. We propose that NFAT serves as an integrator of neurotrophin action and depolarization-driven calcium signaling to regulate neuronal gene expression.

Mitochondria and plasma membrane Ca2+-ATPase control presynaptic Ca2+ clearance in capsaicin-sensitive rat sensory neurons

The central processes of primary nociceptors form synaptic connections with the second-order nociceptive neurons located in the dorsal horn of the spinal cord. These synapses gate the flow of nociceptive information from the periphery to the CNS, and plasticity at these synapses contributes to centrally mediated hyperalgesia and allodynia. Although exocytosis and synaptic plasticity are controlled by Ca(2+) at the release sites, the mechanisms underlying presynaptic Ca(2+) signalling at the nociceptive synapses are not well characterized. We examined the presynaptic mechanisms regulating Ca(2+) clearance following electrical stimulation in capsaicin-sensitive nociceptors using a dorsal root ganglion (DRG)/spinal cord neuron co-culture system. Cytosolic Ca(2+) concentration ([Ca(2+)]i) recovery following electrical stimulation was well approximated by a monoexponential function with a ∼2 s. Inhibition of sarco-endoplasmic reticulum Ca(2+)-ATPase did not affect presynaptic [Ca(2+)]i recovery, and blocking plasmalemmal Na(+)/Ca(2+) exchange produced only a small reduction in the rate of [Ca(2+)]i recovery (∼12%) that was independent of intracellular K(+). However, [Ca(2+)]i recovery in presynaptic boutons strongly depended on the plasma membrane Ca(2+)-ATPase (PMCA) and mitochondria that accounted for ∼47 and 40%, respectively, of presynaptic Ca(2+) clearance. Measurements using a mitochondria-targeted Ca(2+) indicator, mtPericam, demonstrated that presynaptic mitochondria accumulated Ca(2+) in response to electrical stimulation. Quantitative analysis revealed that the mitochondrial Ca(2+) uptake is highly sensitive to presynaptic [Ca(2+)]i elevations, and occurs at [Ca(2+)]i levels as low as ∼200-300 nm. Using RT-PCR, we detected expression of several putative mitochondrial Ca(2+) transporters in DRG, such as MCU, Letm1 and NCLX. Collectively, this work identifies PMCA and mitochondria as the major regulators of presynaptic Ca(2+) signalling at the first sensory synapse, and underlines the high sensitivity of the mitochondrial Ca(2+) uniporter in neurons to cytosolic Ca(2+).

Distinct activation properties of the nuclear factor of activated T-cells (NFAT) isoforms NFATc3 and NFATc4 in neurons

The Ca(2+)/calcineurin-dependent transcription factor NFAT (nuclear factor of activated T-cells) is implicated in regulating dendritic and axonal development, synaptogenesis, and neuronal survival. Despite the increasing appreciation for the importance of NFAT-dependent transcription in the nervous system, the regulation and function of specific NFAT isoforms in neurons are poorly understood. Here, we compare the activation of NFATc3 and NFATc4 in hippocampal and dorsal root ganglion neurons following electrically evoked elevations of intracellular Ca(2+) concentration ([Ca(2+)](i)). We find that NFATc3 undergoes rapid dephosphorylation and nuclear translocation that are essentially complete within 20 min, although NFATc4 remains phosphorylated and localized to the cytosol, only exhibiting nuclear localization following prolonged (1-3 h) depolarization. Knocking down NFATc3, but not NFATc4, strongly diminished NFAT-mediated transcription induced by mild depolarization in neurons. By analyzing NFATc3/NFATc4 chimeras, we find that the region containing the serine-rich region-1 (SRR1) mildly affects initial NFAT translocation, although the region containing the serine-proline repeats is critical for determining the magnitude of NFAT activation and nuclear localization upon depolarization. Knockdown of glycogen synthase kinase 3β (GSK3β) significantly increased the depolarization-induced nuclear localization of NFATc4. In contrast, inhibition of p38 or mammalian target of rapamycin (mTOR) kinases had no significant effect on nuclear import of NFATc4. Thus, electrically evoked [Ca(2+)](i) elevation in neurons rapidly and strongly activates NFATc3, whereas activation of NFATc4 requires a coincident increase in [Ca(2+)](i) and suppression of GSK3β, with differences in the serine-proline-containing region giving rise to these distinct activation properties of NFATc3 and NFATc4.

[Changes in intracellular mechanisms in sensory neurons in experimental diabetes mellitus]

Here we summarises the results of experimental investigation of changes in intracellular calcium homeostasis in sensory neurones of rats with streptozotocin-induced diabetes mellitus. Decrease in the calcium-accumulating function of both inositol-trisphosphate- as well as caffeine-sensitive endoplasmic reticulum has been detected both in primary sensory neurones of dorsal root ganglia and in secondary neurones of the spinal cord dorsal horn. Predominant depression in the functioning of metabotropic receptors of ligand-gated channels compared with those of ionotropic ones has been demonstrated. Changes in the pharmacological sensitivity of potential-operated calcium channels (predominantly of L-type), linked, probably, with alterations of functional connections between membrane channels and endoplasmic reticulum, are described. A predominant role of changes in the functioning of intracellular Ca(2+)-accumulating structures, leading to prolongation of depolarisation-induced Ca2+ transients in primary and secondary sensory neurones and corresponding changes in the transmission of nociceptive signals during diabetic neuropathy are discussed.