Genetic priming of sensory neurons in mice that overexpress PAR2 enhances allergen responsiveness

Pruritus is a common symptom of inflammatory skin conditions, including atopic dermatitis (AD). Although primary sensory neurons that transmit pruritic signals are well-cataloged, little is known about the neuronal alterations that occur as a result of skin disruption in AD. To address this question, we examined the molecular and behavioral consequences of challenging Grhl3 ^PAR2/+ mice, which overexpress PAR2 in suprabasal keratinocytes, with serial topical application of the environmental allergen house dust mite (HDM). We monitored behavior and used RNA sequencing, qPCR, and in situ hybridization to evaluate gene expression in trigeminal ganglia (TG), before and after HDM. We found that neither Grhl3 ^PAR2/+ nor wild-type (WT) mice exhibited spontaneous scratching, and pruritogen-induced acute scratching did not differ. In contrast, HDM exacerbated scratching in Grhl3 ^PAR2/+ mice. Despite the absence of scratching in untreated Grhl3 ^PAR2/+ mice, several TG genes in these mice were up-regulated compared to WT. HDM treatment of the Grhl3 ^PAR2/+ mice enhanced up-regulation of this set of genes and induced additional genes, many within the subset of TG neurons that express TRPV1. The same set of genes was up-regulated in HDM-treated Grhl3 ^PAR2/+ mice that did not scratch, but at lesser magnitude. Finally, we recorded comparable transcriptional changes in IL31Tg mice, demonstrating that a common genetic program is induced in two AD models. Taken together, we conclude that transcriptional changes that occur in primary sensory neurons in dermatitis-susceptible animals underlie a genetic priming that not only sensitizes the animal to chronic allergens but also contributes to pruritus in atopic skin disease.

Effects of GABAB receptor activation on excitability of IB4-positive maxillary trigeminal ganglion neurons: Possible involvement of TREK2 activation

IB4-positive maxillary trigeminal ganglion (TG) neurons are a subtype of afferent neurons involving nociception in orofacial regions, and excitability of these neurons is associated with orofacial nociceptive sensitivity. TREK-2 channel is a member of two-pore domain potassium (K2P) channel family mediating leak K+ currents. It has been shown previously that TREK-2 channel activity can be enhanced following GABAB receptor activation, leading to a reduction of cortical neuron excitability. In the present study, we have characterized TREK-2 channel expression on maxillary TG neurons and investigated the effect of the GABAB agonist baclofen on electrophysiological properties of small-sized maxillary TG neurons of rats. We show with immunohistochemistry that TREK-2 channels are predominantly expressed in small-sized IB4-positive maxillary TG neurons. Patch-clamp recordings on neurons in ex vivo TG preparations show that baclofen hyperpolarizes resting membrane potentials, increases outward leak currents, and decreases input resistances in IB4-positive maxillary TG neurons. Moreover, baclofen significantly reduces action potential (AP) firing in IB4-positive maxillary TG neurons. In contrast, baclofen shows no significant effect on electrophysiological properties of small-sized nociceptive-like and non-nociceptive-like maxillary trigeminal neurons that are IB4-negatve. Our results suggest that TREK-2 channel activity can be enhanced by baclofen, leading to reduced excitability of IB4-positive maxillary TG neurons. This finding provides new insights into the role of TREK-2 and GABAB receptors in controlling nociceptive sensitivity in orofacial regions, which may have therapeutic implications.

Hybrid Electro-Plasmonic Neural Stimulation with Visible-Light-Sensitive Gold Nanoparticles

Biomedical prosthetics utilizing electrical stimulation have limited, effective spatial resolution due to spread of electrical currents to surrounding tissue, causing nonselective stimulation. So, precise spatial resolution is not possible for traditional neural prosthetic devices, such as cochlear implants. More recently, alternative methods utilize optical stimulation, mainly infrared, sometimes paired with nanotechnology for stimulating action potentials. Infrared stimulation has its own drawbacks, as it may cause collateral heating of surrounding tissue. In previous work, we employed a plasmonic method for stimulation of an electrically excitable neuroblastoma cell line, which had limited success. Here, we report the development of a hybrid electro-plasmonic stimulation platform for spatially and temporally precise neural excitation to address the above deficiencies. Primary trigeminal neurons were costimulated in vitro in a whole-cell patch-clamp configuration with subthreshold-level short-duration (1-5 ms) electrical and visible light pulses (1-5 ms). The visible light pulses were aimed at a gold-nanoparticle-coated nanoelectrode placed alongside the neuron, within 2 μm distance. Membrane action potentials were recorded with a 3-fold higher success rate and 5-fold better poststimulation cell recovery rate than with pure optical stimulation alone. Also, electrical stimulus current input was being reduced by up to 40%. The subthreshold levels of electrical stimuli in conjunction with visible light (532 nm) reliably triggered trains of action potentials. This single-cell hybrid activation was reliable and repeatable, without any damage as observed with pure optical stimulation. This work represents an empirical cellular study of the membrane action potential response produced by the cultured primary sensory trigeminal neurons when costimulated with plasmonic and electrical (hybrid) stimulation. Our hybrid neurostimulation method can be used toward development of high-acuity neural modulation prosthetic devices, tunable for individual needs, which would qualify as a preferred alternative over traditional electrical stimulation technologies.

Targeting Local Osteogenic and Ancillary Cells by Mechanobiologically Optimized Magnesium Scaffolds for Orbital Bone Reconstruction in Canines

Large-sized orbital bone defects have serious consequences that destroy orbital integrity and result in maxillofacial deformities and vision loss. The treatment of orbital bone defects is currently palliative and not reparative, suggesting an urgent demand for biomaterials that regenerate orbital bones. In this study, via alloying, extrusion and surface modification, we developed mechanobiologically optimized magnesium (Mg) scaffolds (Ca-P-coated Mg-Zn-Gd scaffolds, referred to as Ca-P-Mg) for the orthotopic reconstruction of large-sized orbital bone defects. At 6 months after transplanting the scaffolds to a clinically relevant canine large animal model, large-sized defects were successfully bridged by an abundance of new bone with normal mechanical properties that corresponded to gradual degradation of the implants. The osteogenic and ancillary cells, including vascular endothelial cells and trigeminal neurons, played important roles in this process. The scaffolds robustly enhanced bone marrow mesenchymal stem cell (BMSC) osteogenic differentiation. In addition, the increased angiogenesis including increased ratio of the specific endothelial subtype CD31^hi endomucin^hi (CD31^hiEmcn^hi) endothelial cells can facilitate osteogenesis. Furthermore, the scaffolds trigger trigeminal neurons via transient receptor potential vanilloid subtype 1 (Trpv1) to produce the neuropeptide calcitonin gene-related peptide (CGRP), which promotes angiogenesis and osteogenesis. Overall, our investigations revealed the efficacy of Ca-P-Mg scaffolds in healing orbital bone defects and warrant further exploration of these scaffolds for clinical applications.

Ameloblastomas Exhibit Stem Cell Potential, Possess Neurotrophic Properties, and Establish Connections with Trigeminal Neurons

Ameloblastomas are locally invasive and aggressive odontogenic tumors treated via surgical resection, which results in facial deformity and significant morbidity. Few studies have addressed the cellular and molecular events of ameloblastoma onset and progression, thus hampering the development of non-invasive therapeutic approaches. Tumorigenesis is driven by a plethora of factors, among which innervation has been long neglected. Recent findings have shown that innervation directly promotes tumor progression. On this basis, we investigated the molecular characteristics and neurotrophic properties of human ameloblastomas. Our results showed that ameloblastomas express dental epithelial stem cell markers, as well as components of the Notch signaling pathway, indicating persistence of stemness. We demonstrated that ameloblastomas express classical stem cell markers, exhibit stem cell potential, and form spheres. These tumors express also molecules of the Notch signaling pathway, fundamental for stem cells and their fate. Additionally, we showed that ameloblastomas express the neurotrophic factors NGF and BDNF, as well as their receptors TRKA, TRKB, and P75/NGFR, which are responsible for their innervation by trigeminal axons in vivo. In vitro studies using microfluidic devices showed that ameloblastoma cells attract and form connections with these nerves. Innervation of ameloblastomas might play a key role in the onset of this malignancy and might represent a promising target for non-invasive pharmacological interventions.

Stereotyped transcriptomic transformation of somatosensory neurons in response to injury

In mice, spared nerve injury replicates symptoms of human neuropathic pain and induces upregulation of many genes in somatosensory neurons. Here we used single cell transcriptomics to probe the effects of partial infraorbital transection of the trigeminal nerve at the cellular level. Uninjured neurons were unaffected by transection of major nerve branches, segregating into many different classes. In marked contrast, axotomy rapidly transformed damaged neurons into just two new and closely-related classes where almost all original identity was lost. Remarkably, sensory neurons also adopted this transcriptomic state following various minor peripheral injuries. By genetically marking injured neurons, we showed that the injury-induced transformation was reversible, with damaged cells slowly reacquiring normal gene expression profiles. Thus, our data expose transcriptomic plasticity, previously thought of as a driver of chronic pain, as a programed response to many types of injury and a potential mechanism for regulating sensation during wound healing.

Ca2+ -dependent and Ca2+ -independent somatic release from trigeminal neurons

Besides the nerve endings, the soma of trigeminal neurons also respond to membrane depolarizations with the release of neurotransmitters and neuromodulators in the extracellular space within the ganglion, a process potentially important for the cross-communication between neighboring sensory neurons. In this study, we addressed the dependence of somatic release on Ca²⁺ influx in trigeminal neurons and the involvement of the different types of voltage-gated Ca²⁺ (Cav) channels in the process. Similar to the closely related dorsal root ganglion neurons, we found two kinetically distinct components of somatic release, a faster component stimulated by voltage but independent of the Ca²⁺ influx, and a slower component triggered by Ca²⁺ influx. The Ca²⁺ -dependent component was inhibited 80% by ω-conotoxin-MVIIC, an inhibitor of both N- and P/Q-type Cav channels, and 55% by the P/Q-type selective inhibitor ω-agatoxin-IVA. The selective L-type Ca²⁺ channel inhibitor nimodipine was instead without effect. These results suggest a major involvement of N- and P/Q-, but not L-type Cav channels in the somatic release of trigeminal neurons. Thus antinociceptive Cav channel antagonists acting on the N- and P/Q-type channels may exert their function by also modulating the somatic release and cross-communication between sensory neurons.

Receptor Mechanisms Mediating the Pro-Nociceptive Action of Hydrogen Sulfide in Rat Trigeminal Neurons and Meningeal Afferents

Hydrogen sulfide (H₂S), a well-established member of the gasotransmitter family, is involved in a variety of physiological functions, including pro-nociceptive action in the sensory system. Although several reports have shown that H₂S activates sensory neurons, the molecular targets of H₂S action in trigeminal (TG) nociception, implicated in migraine, remains controversial. In this study, using suction electrode recordings, we investigate the effect of the H₂S donor, sodium hydrosulfide (NaHS), on nociceptive firing in rat meningeal TG nerve fibers. The effect of NaHS was also explored with patch-clamp and calcium imaging techniques on isolated TG neurons. NaHS dramatically increased the nociceptive firing in TG nerve fibers. This effect was abolished by the TRPV1 inhibitor capsazepine but was partially prevented by the TRPA1 blocker HC 030031. In a fraction of isolated TG neurons, NaHS transiently increased amplitude of capsaicin-induced currents. Moreover, NaHS by itself induced inward currents in sensory neurons, which were abolished by the TRPV1 inhibitor capsazepine suggesting involvement of TRPV1 receptors. In contrast, the inhibitor of TRPA1 receptors HC 030031 did not prevent the NaHS-induced currents. Imaging of a large population of TG neurons revealed that NaHS induced calcium transients in 41% of tested neurons. Interestingly, this effect of NaHS in some neurons was inhibited by the TRPV1 antagonist capsazepine whereas in others it was sensitive to the TRPA1 blocker HC 030031. Our data suggest that both TRPV1 and TRPA1 receptors play a role in the pro-nociceptive action of NaHS in peripheral TG nerve endings in meninges and in somas of TG neurons. We propose that activation of TRPV1 and TRPA1 receptors by H₂S during neuro-inflammation conditions contributes to the nociceptive firing in primary afferents underlying migraine pain.

The Effect of Pungent and Tingling Compounds from Piper nigrum L. on Background K+ Currents

Black peppercorns (Piper nigrum L.) elicit a pungent and tingling oral impression. Their pungency is partially explained by the agonist activity of some of their active principles, especially piperine, on TRP channels. However, we recently showed that piperine, as well as other pungent compounds, also possess a marked effect on two-pore domain (KCNK, K_2P) K⁺ channels. Members of this family play a key role in maintaining the resting membrane potential of excitable cells. Interestingly, tingling compounds have been shown to induce neuronal excitation by inhibiting KCNK channels. We addressed the question of whether it was plausible that KCNK channels could constitute a physiologically relevant target for the sensory active compounds present in black peppercorns. Because previous studies have demonstrated that mouse trigeminal neurons respond to several pungent compounds, to which humans are also sensitive, we used a primary culture of mouse trigeminal neurons to investigate whether the effect of piperine on these cell types could also be mediated by KCNK channels. We observed that even in the presence of classical TRP-antagonists, piperine was still able to activate a fraction of trigeminal neurons. Furthermore, our results showed that piperine is capable of inducing neuronal depolarization by a mechanism that does not require extracellular Na⁺ or Ca²⁺. This depolarization was mediated by the inhibition of a background K⁺ conductance, most likely corresponding to the KCNK channels of the TASK subfamily. We then performed a screening with 12 other pungent and/or tingling chemosensates isolated from black peppercorns. These compounds were evaluated on Xenopus laevis oocytes expressing the human orthologues of KCNK3, KNCK9 and KCNK18, which we previously showed to be inhibited by piperine. Remarkably, almost all of the isolated chemosensates inhibited the basal activity of hKCNK3, with 1-(octadeca-2E,4E,13/12Z-trienoyl)pyrrolidine acting as one of the most potent natural blockers for hKCNK3 found to date. Our results suggest that KCNK channels, especially KCNK3, are likely to play a complementary role to TRP channels in the complex orosensory impression elicited by black peppercorns, while they also help to expand the pharmacological knowledge of KCNK channels.

Divergence in endothelin-1- and bradykinin-activated store-operated calcium entry in afferent sensory neurons

Endothelin-1 (ET-1) and bradykinin (BK) are endogenous peptides that signal through Gαq/11-protein coupled receptors (GPCRs) to produce nociceptor sensitization and pain. Both peptides activate phospholipase C to stimulate Ca²⁺ accumulation, diacylglycerol production, and protein kinase C activation and are rapidly desensitized via a G-protein receptor kinase 2-dependent mechanism. However, ET-1 produces a greater response and longer lasting nocifensive behavior than BK in multiple models, indicating a potentially divergent signaling mechanism in primary afferent sensory neurons. Using cultured sensory neurons, we demonstrate significant differences in both Ca²⁺ influx and Ca²⁺ release from intracellular stores following ET-1 and BK treatments. As intracellular store depletion may contribute to the regulation of other signaling cascades downstream of GPCRs, we concentrated our investigation on store-operated Ca²⁺ channels. Using pharmacological approaches, we identified transient receptor potential canonical channel 3 (TRPC3) as a dominant contributor to Ca²⁺ influx subsequent to ET-1 treatment. On the other hand, BK treatment stimulated Orai1 activation, with only minor input from TRPC3. Taken together, data presented here suggest that ET-1 signaling targets TRPC3, generating a prolonged Ca²⁺ signal that perpetuates nocifensive responses. In contrast, Orai1 dominates as the downstream target of BK receptor activation and results in transient intracellular Ca²⁺ increases and abridged nocifensive responses.

ATP P2X3 receptors and neuronal sensitization

Increasing evidence indicates the importance of extracellular adenosine triphosphate (ATP) in the modulation of neuronal function. In particular, fine control of ATP release and the selective and discrete ATP receptor operation are crucial elements of the crosstalk between neuronal and non-neuronal cells in the peripheral and central nervous systems. In peripheral neurons, ATP signaling gives an important contribution to neuronal sensitization, especially that involved in neuropathic pain. Among other subtypes, P2X3 receptors expressed on sensory neurons are sensitive even to nanomolar concentrations of extracellular ATP, and therefore are important transducers of pain stimuli. P2X3 receptor function is highly sensitive to soluble factors like neuropeptides and neurotrophins, and is controlled by transduction mechanisms, protein-protein interactions and discrete membrane compartmentalization. More recent findings have demonstrated that P2X3 receptors interact with the synaptic scaffold protein calcium/calmodulin-dependent serine protein kinase (CASK) in a state dependent fashion, indicating that CASK plays a crucial role in the modulation of P2X3 receptor stability and efficiency. Activation of P2X3 receptors within CASK/P2X3 complex has important consequences for neuronal plasticity and possibly for the release of neuromodulators and neurotransmitters. Better understanding of the interactome machinery of P2X3 receptors and their integration with other receptors and channels on neuronal surface membranes, is proposed to be essential to unveil the process of neuronal sensitization and related, abnormal pain signaling.

Regulation of calcitonin gene-related peptide secretion by a serotonergic antimigraine drug

We have investigated the regulation of calcitonin gene-related peptide (CGRP) release from trigeminal neurons by the serotonergic antimigraine drug sumatriptan. Serum levels of the neuropeptide CGRP are elevated during migraine. Treatment with the drug sumatriptan returns CGRP levels to normal coincident with the alleviation of headache. However, despite this clinical efficacy, the cellular target and mechanism of sumatriptan action are not well understood beyond the pharmacology of its recognition of the 5-HT1 class of serotonin receptors. We have used cultured trigeminal neurons to demonstrate that sumatriptan can directly repress CGRP secretion from sensory neurons. The stimulated secretion in response to depolarization or inflammatory agents was inhibited, but not the basal secretion rate. Unexpectedly, sumatriptan did not lower cAMP levels, in contrast to the classical role ascribed to the 5-HT1 receptors. Instead, activation of 5-HT1 receptors caused a slow and remarkably prolonged increase in intracellular calcium. The inhibition of CGRP secretion is attenuated by the phosphatase inhibitor okadaic acid, suggesting that sumatriptan action is mediated by calcium-recruited phosphatases. These results suggest that 5-HT1 agonists may block a deleterious feedback loop in migraine at the trigeminal neurons and provide a general mechanism by which this class of drugs can attenuate stimulated neuropeptide release.