Bacteria activate sensory neurons that modulate pain and inflammation.

Nociceptor sensory neurons are specialized to detect potentially damaging stimuli, protecting the organism by initiating the sensation of pain and eliciting defensive behaviours. Bacterial infections produce pain by unknown molecular mechanisms, although they are presumed to be secondary to immune activation. Here we demonstrate that bacteria directly activate nociceptors, and that the immune response mediated through TLR2, MyD88, T cells, B cells, and neutrophils and monocytes is not necessary for Staphylococcus aureus-induced pain in mice. Mechanical and thermal hyperalgesia in mice is correlated with live bacterial load rather than tissue swelling or immune activation. Bacteria induce calcium flux and action potentials in nociceptor neurons, in part via bacterial N-formylated peptides and the pore-forming toxin α-haemolysin, through distinct mechanisms. Specific ablation of Nav1.8-lineage neurons, which include nociceptors, abrogated pain during bacterial infection, but concurrently increased local immune infiltration and lymphadenopathy of the draining lymph node. Thus, bacterial pathogens produce pain by directly activating sensory neurons that modulate inflammation, an unsuspected role for the nervous system in host-pathogen interactions.

TRPA1 is a major oxidant sensor in murine airway sensory neurons.

Sensory neurons in the airways are finely tuned to respond to reactive chemicals threatening airway function and integrity. Nasal trigeminal nerve endings are particularly sensitive to oxidants formed in polluted air and during oxidative stress as well as to chlorine, which is frequently released in industrial and domestic accidents. Oxidant activation of airway neurons induces respiratory depression, nasal obstruction, sneezing, cough, and pain. While normally protective, chemosensory airway reflexes can provoke severe complications in patients affected by inflammatory airway conditions like rhinitis and asthma. Here, we showed that both hypochlorite, the oxidizing mediator of chlorine, and hydrogen peroxide, a reactive oxygen species, activated Ca(2+) influx and membrane currents in an oxidant-sensitive subpopulation of chemosensory neurons. These responses were absent in neurons from mice lacking TRPA1, an ion channel of the transient receptor potential (TRP) gene family. TRPA1 channels were strongly activated by hypochlorite and hydrogen peroxide in primary sensory neurons and heterologous cells. In tests of respiratory function, Trpa1(-/-) mice displayed profound deficiencies in hypochlorite- and hydrogen peroxide-induced respiratory depression as well as decreased oxidant-induced pain behavior. Our results indicate that TRPA1 is an oxidant sensor in sensory neurons, initiating neuronal excitation and subsequent physiological responses in vitro and in vivo.

Transient receptor potential ankyrin 1 antagonists block the noxious effects of toxic industrial isocyanates and tear gases.

The release of methyl isocyanate in Bhopal, India, caused the worst industrial accident in history. Exposures to industrial isocyanates induce lacrimation, pain, airway irritation, and edema. Similar responses are elicited by chemicals used as tear gases. Despite frequent exposures, the biological targets of isocyanates and tear gases in vivo have not been identified, precluding the development of effective countermeasures. We use Ca(2+) imaging and electrophysiology to show that the noxious effects of isocyanates and those of all major tear gas agents are caused by activation of Ca(2+) influx and membrane currents in mustard oil-sensitive sensory neurons. These responses are mediated by transient receptor potential ankyrin 1 (TRPA1), an ion channel serving as a detector for reactive chemicals. In mice, genetic ablation or pharmacological inhibition of TRPA1 dramatically reduces isocyanate- and tear gas-induced nocifensive behavior after both ocular and cutaneous exposures. We conclude that isocyanates and tear gas agents target the same neuronal receptor, TRPA1. Treatment with TRPA1 antagonists may prevent and alleviate chemical irritation of the eyes, skin, and airways and reduce the adverse health effects of exposures to a wide range of toxic noxious chemicals.

Effect of dimethyl fumarate on lymphocytes in RRMS: Implications for clinical practice.

OBJECTIVE: To assess functional changes in lymphocyte repertoire and subsequent clinical implications during delayed-release dimethyl fumarate (DMF) treatment in patients with multiple sclerosis. METHODS: Using peripheral blood from several clinical trials of DMF, immune cell subsets were quantified using flow cytometry. For some patients, lymphocyte counts were assessed after DMF discontinuation. Incidence of adverse events, including serious and opportunistic infections, was assessed. RESULTS: In DMF-treated patients, absolute lymphocyte counts (ALCs) demonstrated a pattern of decline followed by stabilization, which also was reflected in the global reduction in numbers of circulating functional lymphocyte subsets. The relative frequencies of circulating memory T- and B-cell populations declined and naive cells increased. No increased incidence of serious infection or malignancy was observed for patients treated with DMF, even when stratified by ALC or T-cell subset frequencies. For patients who discontinued DMF due to lymphopenia, ALCs increased after DMF discontinuation; recovery time varied by ALC level at discontinuation. T-cell subsets closely correlated with ALCs in both longitudinal and cross-sectional analyses. CONCLUSIONS: DMF shifted the immunophenotype of circulating lymphocyte subsets. ALCs were closely correlated with CD4+ and CD8+ T-cell counts, indicating that lymphocyte subset monitoring is not required for safety vigilance. No increased risk of serious infection was observed in patients with low T-cell subset counts. Monitoring ALC remains the most effective way of identifying patients at risk of subsequently developing prolonged moderate to severe lymphopenia, a risk factor for progressive multifocal leukoencephalopathy in DMF-treated patients. TRIAL REGISTRATION NUMBERS: EUDRA CT 2015-001973-42, NCT00168701, NCT00420212, NCT00451451, and NCT00835770.

Amino-termini isoforms of the Slack K+ channel, regulated by alternative promoters, differentially modulate rhythmic firing and adaptation.

The rates of activation and unitary properties of Na+-activated K+ (K(Na)) currents have been found to vary substantially in different types of neurones. One class of K(Na) channels is encoded by the Slack gene. We have now determined that alternative RNA splicing gives rise to at least five different transcripts for Slack, which produce Slack channels that differ in their predicted cytoplasmic amino-termini and in their kinetic properties. Two of these, termed Slack-A channels, contain an amino-terminus domain closely resembling that of another class of K(Na) channels encoded by the Slick gene. Neuronal expression of Slack-A channels and of the previously described Slack isoform, now called Slack-B, are driven by independent promoters. Slack-A mRNAs were enriched in the brainstem and olfactory bulb and detected at significant levels in four different brain regions. When expressed in CHO cells, Slack-A channels activate rapidly upon depolarization and, in single channel recordings in Xenopus oocytes, are characterized by multiple subconductance states with only brief transient openings to the fully open state. In contrast, Slack-B channels activate slowly over hundreds of milliseconds, with openings to the fully open state that are approximately 6-fold longer than those for Slack-A channels. In numerical simulations, neurones in which outward currents are dominated by a Slack-A-like conductance adapt very rapidly to repeated or maintained stimulation over a wide range of stimulus strengths. In contrast, Slack-B currents promote rhythmic firing during maintained stimulation, and allow adaptation rate to vary with stimulus strength. Using an antibody that recognizes all amino-termini isoforms of Slack, Slack immunoreactivity is present at locations that have no Slack-B-specific staining, including olfactory bulb glomeruli and the dendrites of hippocampal neurones, suggesting that Slack channels with alternate amino-termini such as Slack-A channels are present at these locations. Our data suggest that alternative promoters of the Slack gene differentially modulate the properties of neurones.

Loss of Kv3.1 tonotopicity and alterations in cAMP response element-binding protein signaling in central auditory neurons of hearing impaired mice.

The promoter for the kv3.1 potassium channel gene is regulated by a Ca2+-cAMP responsive element, which binds the transcription factor cAMP response element-binding protein (CREB). Kv3.1 is expressed in a tonotopic gradient within the medial nucleus of the trapezoid body (MNTB) of the auditory brainstem, where Kv3.1 levels are highest at the medial end, which corresponds to high auditory frequencies. We have compared the levels of Kv3.1, CREB, and the phosphorylated form of CREB (pCREB) in a mouse strain that maintains good hearing throughout life, CBA/J (CBA), with one that suffers early cochlear hair cell loss, C57BL/6 (BL/6). A gradient of Kv3.1 immunoreactivity in the MNTB was detected in both young (6 week) and older (8 month) CBA mice. Although no gradient of CREB was detected, pCREB-immunopositive cells were grouped together in distinct clusters along the tonotopic axis. The same pattern of Kv3.1, CREB, and pCREB localization was also found in young BL/6 mice at a time (6 weeks) when hearing is normal. In contrast, at 8 months, when hearing is impaired, the gradient of Kv3.1 was abolished. Moreover, in the older BL/6 mice there was a decrease in CREB expression along the tonotopic axis, and the pattern of pCREB labeling appeared random, with no discrete clusters of pCREB-positive cells along the tonotopic axis. Our findings are consistent with the hypothesis that ongoing activity in auditory brainstem neurons is necessary for the maintenance of Kv3.1 tonotopicity through the CREB pathway.

Modulation of the kv3.1b potassium channel isoform adjusts the fidelity of the firing pattern of auditory neurons.

Neurons of the medial nucleus of the trapezoid body, which transmit auditory information that is used to compute the location of sounds in space, are capable of firing at high frequencies with great temporal precision. We found that elimination of the Kv3.1 gene in mice results in the loss of a high-threshold component of potassium current and failure of the neurons to follow high-frequency stimulation. A partial decrease in Kv3.1 current can be produced in wild-type neurons of the medial nucleus of the trapezoid body by activation of protein kinase C. Paradoxically, activation of protein kinase C increases temporal fidelity and the number of action potentials that are evoked by intermediate frequencies of stimulation. Computer simulations confirm that a partial decrease in Kv3.1 current is sufficient to increase the accuracy of response at intermediate frequencies while impairing responses at high frequencies. We further establish that, of the two isoforms of the Kv3.1 potassium channel that are expressed in these neurons, Kv3.1a and Kv3.1b, the decrease in Kv3.1 current is mediated by selective phosphorylation of the Kv3.1b isoform. Using site-directed mutagenesis, we identify a specific C-terminal phosphorylation site responsible for the observed difference in response of the two isoforms to protein kinase C activation. Our results suggest that modulation of Kv3.1 by phosphorylation allows auditory neurons to tune their responses to different patterns of sensory stimulation.

Localization of the Na+-activated K+ channel Slick in the rat central nervous system.

Na+-activated K+ currents (K(Na)) have been reported in multiple neuronal nuclei and the properties of K(Na) vary in different cell types. We have described previously the distribution of Slack, a Na+-activated K+ channel subunit. Another recently cloned Na+-activated K+ channel is Slick, which differs from Slack in its rapid activation and its sensitivity to intracellular ATP levels. We now report the localization of Slick in the rat central nervous system using in situ and immunohistochemical techniques. As for Slack, we find that Slick is widely distributed in the brain. Specifically, strong hybridization signals and immunoreactivity were found in the brainstem, including auditory neurons such as the medial nucleus of the trapezoid body. As has also been shown for Slack, Slick is expressed in the olfactory bulb, red nucleus, facial nucleus, pontine nucleus, oculomotor nucleus, substantia nigra, deep cerebellar nuclei, vestibular nucleus, and the thalamus. Slick mRNA and protein, however, also are found in certain neurons that do not express Slack. These neurons include those of the hippocampal CA1, CA2, and CA3 regions, the dentate gyrus, supraoptic nucleus, hypothalamus, and cortical layers II, III, and V. These data suggest that Slick may function independently of Slack in these regions. Computer simulations indicate that Slick currents can cause adaptation during prolonged stimuli. Such adaptation allows a neuron to respond to high-frequency stimulation with lower-frequency firing that remains temporally locked to individual stimuli, a property seen in many auditory neurons. Although it is not yet known if Slick and Slack subunits heteromultimerize, the existence of two genes that encode K(Na), that are widely expressed in the nervous system, with both overlapping and nonoverlapping distributions, provides the basis for the reported heterogeneity in the properties of K(Na) from various neurons.

Regulation of the timing of MNTB neurons by short-term and long-term modulation of potassium channels.

The firing patterns of neurons in central auditory pathways encode specific features of sound stimuli, such as frequency, intensity and localization in space. The generation of the appropriate pattern depends, to a major extent, on the properties of the voltage-dependent potassium channels in these neurons. The mammalian auditory pathways that compute the direction of a sound source are located in the brainstem and include the connection from bushy cells in the anteroventral cochlear nucleus (AVCN) to the principal neurons of the medial nucleus of the trapezoid body (MNTB). To preserve the fidelity of timing of action potentials that is required for sound localization, these neurons express several types of potassium channels, including the Kv3 and Kv1 families of voltage-dependent channels and the Slick and Slack sodium-dependent channels. These channels determine the pattern of action potentials and the amount of neurotransmitter released during repeated stimulation. The amplitude of currents carried by one of these channels, the Kv3.1b channel, is regulated in the short term by protein phosphorylation, and in the long term, by changes in gene expression, such that the intrinsic excitability of the neurons is constantly being regulated by the ambient auditory environment.

Deconstructing the neuropathic pain phenotype to reveal neural mechanisms.

After nerve injury maladaptive changes can occur in injured sensory neurons and along the entire nociceptive pathway within the CNS, which may lead to spontaneous pain or pain hypersensitivity. The resulting neuropathic pain syndromes present as a complex combination of negative and positive symptoms, which vary enormously from individual to individual. This variation depends on a diversity of underlying pathophysiological changes resulting from the convergence of etiological, genotypic, and environmental factors. The pain phenotype can serve therefore, as a window on underlying pathophysiological neural mechanisms and as a guide for developing personalized pain medicine.

Neurogenic inflammation and the peripheral nervous system in host defense and immunopathology.

The peripheral nervous and immune systems are traditionally thought of as serving separate functions. The line between them is, however, becoming increasingly blurred by new insights into neurogenic inflammation. Nociceptor neurons possess many of the same molecular recognition pathways for danger as immune cells, and, in response to danger, the peripheral nervous system directly communicates with the immune system, forming an integrated protective mechanism. The dense innervation network of sensory and autonomic fibers in peripheral tissues and high speed of neural transduction allows rapid local and systemic neurogenic modulation of immunity. Peripheral neurons also seem to contribute to immune dysfunction in autoimmune and allergic diseases. Therefore, understanding the coordinated interaction of peripheral neurons with immune cells may advance therapeutic approaches to increase host defense and suppress immunopathology.

TRPA1 mediates the noxious effects of natural sesquiterpene deterrents.

Plants, fungi, and animals generate a diverse array of deterrent natural products that induce avoidance behavior in biological adversaries. The largest known chemical family of deterrents are terpenes characterized by reactive alpha,beta-unsaturated dialdehyde moieties, including the drimane sesquiterpenes and other terpene species. Deterrent sesquiterpenes are potent activators of mammalian peripheral chemosensory neurons, causing pain and neurogenic inflammation. Despite their wide-spread synthesis and medicinal use as desensitizing analgesics, their molecular targets remain unknown. Here we show that isovelleral, a noxious fungal sesquiterpene, excites sensory neurons through activation of TPRA1, an ion channel involved in inflammatory pain signaling. TRPA1 is also activated by polygodial, a drimane sesquiterpene synthesized by plants and animals. TRPA1-deficient mice show greatly reduced nocifensive behavior in response to isovelleral, indicating that TRPA1 is the major receptor for deterrent sesquiterpenes in vivo. Isovelleral and polygodial represent the first fungal and animal small molecule agonists of nociceptive transient receptor potential channels.