Enhancement by TNF-α of TTX-resistant NaV current in muscle sensory neurons after femoral artery occlusion

Amplified P2X3 pathway activity in muscle afferent dorsal root ganglion neurons and exercise pressor reflex regulation in hindlimb ischaemia-reperfusion

Hindlimb ischaemia-reperfusion (IR) is among the most prominent pathophysiological conditions observed in peripheral artery disease (PAD). An exaggerated arterial blood pressure (BP) response during exercise is associated with an elevated risk of cardiovascular events in individuals with PAD. However, the precise mechanisms leading to this exaggerated BP response are poorly elucidated. The P2X3 signalling pathway, which plays a key role in modifying the exercise pressor reflex (EPR), is the focus of the present study. We determined the regulatory role of P2X3 on the EPR in a rat model of hindlimb IR. In vivo and in vitro approaches were used to determine the expression and functions of P2X3 in muscle afferent nerves and EPR in IR rats. We found that in IR rats there was (1) upregulation of P2X3 protein expression in the L4-6 dorsal root ganglia (DRG); (2) amplified P2X currents in isolated isolectin B4 (IB4)-positive muscle DRG neurons; and (3) amplification of the P2X-mediated BP response. We further verified that both A-317491 and siRNA knockdown of P2X3 significantly decreased the activity of P2X currents in isolated muscle DRG neurons. Moreover, inhibition of muscle afferents' P2X3 receptor using A-317491 was observed to alleviate the exaggerated BP response induced by static muscle contraction and P2X-induced BP response by α,β-methylene ATP injection. P2X3 signalling pathway activity is amplified in muscle afferent DRG neurons in regulating the EPR following hindlimb IR.

Emerging Molecular and Synaptic Targets for the Management of Chronic Pain Caused by Systemic Lupus Erythematosus

Patients with systemic lupus erythematosus (SLE) frequently experience chronic pain due to the limited effectiveness and safety profiles of current analgesics. Understanding the molecular and synaptic mechanisms underlying abnormal neuronal activation along the pain signaling pathway is essential for developing new analgesics to address SLE-induced chronic pain. Recent studies, including those conducted by our team and others using the SLE animal model (MRL/lpr lupus-prone mice), have unveiled heightened excitability in nociceptive primary sensory neurons within the dorsal root ganglia and increased glutamatergic synaptic activity in spinal dorsal horn neurons, contributing to the development of chronic pain in mice with SLE. Nociceptive primary sensory neurons in lupus animals exhibit elevated resting membrane potentials, and reduced thresholds and rheobases of action potentials. These changes coincide with the elevated production of TNFα and IL-1β, as well as increased ERK activity in the dorsal root ganglion, coupled with decreased AMPK activity in the same region. Dysregulated AMPK activity is linked to heightened excitability in nociceptive sensory neurons in lupus animals. Additionally, the increased glutamatergic synaptic activity in the spinal dorsal horn in lupus mice with chronic pain is characterized by enhanced presynaptic glutamate release and postsynaptic AMPA receptor activation, alongside the reduced activity of glial glutamate transporters. These alterations are caused by the elevated activities of IL-1β, IL-18, CSF-1, and thrombin, and reduced AMPK activities in the dorsal horn. Furthermore, the pharmacological activation of spinal GPR109A receptors in microglia in lupus mice suppresses chronic pain by inhibiting p38 MAPK activity and the production of both IL-1β and IL-18, as well as reducing glutamatergic synaptic activity in the spinal dorsal horn. These findings collectively unveil crucial signaling molecular and synaptic targets for modulating abnormal neuronal activation in both the periphery and spinal dorsal horn, offering insights into the development of analgesics for managing SLE-induced chronic pain.

NGF contributes to activities of acid-sensing ion channels in dorsal root ganglion neurons of male rats with experimental peripheral artery disease

A feature of peripheral artery diseases (PAD) includes limb ischemia/reperfusion (I/R) and ischemia. Both I/R and ischemia amplify muscle afferent nerve-activated reflex sympathetic nervous and blood pressure responses (termed as exercise pressor reflex). Nevertheless, the underlying mechanisms responsible for the exaggerated autonomic responses in PAD are undetermined. Previous studies suggest that acid-sensing ion channels (ASICs) in muscle dorsal root ganglion (DRG) play a leading role in regulating the exercise pressor reflex in PAD. Thus, we determined if signaling pathways of nerve growth factor (NGF) contribute to the activities of ASICs in muscle DRG neurons of PAD. In particular, we examined ASIC1a and ASIC3 currents in isolectin B4 -negative muscle DRG neurons, a distinct subpopulation depending on NGF for survival. Hindlimb I/R and ischemia were obtained in male rats. In results, femoral artery occlusion increased the levels of NGF and NGF-stimulated TrkA receptor in DRGs, whereas they led to upregulation of ASIC3 but not ASIC1a. In addition, application of NGF onto DRG neurons increased the density of ASIC3 currents and the effect of NGF was significantly attenuated by TrkA antagonist GW441756. Moreover, the enhancing effect of NGF on the density of ASIC3-like currents was decreased by the respective inhibition of intracellular signaling pathways, namely JNK and NF-κB, by antagonists SP600125 and PDTC. Our results suggest contribution of NGF to the activities of ASIC3 currents via JNK and NF-κB signaling pathways in association with the exercise pressor reflex in experimental PAD.

Characteristics of acid-sensing ion channel currents in male rat muscle dorsal root ganglion neurons following ischemia/reperfusion

Peripheral artery diseases (PAD) increases muscle afferent nerve-activated reflex sympathetic nervous and blood pressure responses during exercise (termed as exercise pressor reflex). However, the precise signaling pathways leading to the exaggerated autonomic responses in PAD are undetermined. Considering that limb ischemia/reperfusion (I/R) is a feature of PAD, we determined the characteristics of acid-sensing ion channel (ASIC) currents in muscle dorsal root ganglion (DRG) neurons under the conditions of hindlimb I/R and ischemia of PAD. In particular, we examined ASIC currents in two distinct subpopulations, isolectin B₄ -positive, and B₄ -negative (IB4+ and IB4-) muscle DRG neurons, linking to glial cell line-derived neurotrophic factor and nerve growth factor. In results, ASIC1a- and ASIC3-like currents were observed in IB4- muscle DRG neurons with a greater percentage of ASIC3-like currents. Hindimb I/R and ischemia did not alter the distribution of ASIC1a and ASIC3 currents with activation of pH 6.7 in IB4+ and IB4- muscle DRG neurons; however, I/R altered the distribution of ASIC3 currents in IB4+ muscle DRG neurons with pH 5.5, but not in IB4- neurons. In addition, I/R and ischemia amplified the density of ASIC3-like currents in IB4- muscle DRG neurons. Our results suggest that a selective subpopulation of muscle afferent nerves should be taken into consideration when ASIC signaling pathways are studied to determine the exercise pressor reflex in PAD.

Voltage-gated sodium channels in diabetic sensory neuropathy: Function, modulation, and therapeutic potential

Voltage-gated sodium channels (Na V ) are the main contributors to action potential generation and essential players in establishing neuronal excitability. Na V channels have been widely studied in pain pathologies, including those that develop during diabetes. Diabetic sensory neuropathy (DSN) is one of the most common complications of the disease. DSN is the result of sensory nerve damage by the hyperglycemic state, resulting in a number of debilitating symptoms that have a significant negative impact in the quality of life of diabetic patients. Among those symptoms are tingling and numbness of hands and feet, as well as exacerbated pain responses to noxious and non-noxious stimuli. DSN is also a major contributor to the development of diabetic foot, which may lead to lower limb amputations in long-term diabetic patients. Unfortunately, current treatments fail to reverse or successfully manage DSN. In the current review we provide an updated report on Na V channels including structure/function and contribution to DSN. Furthermore, we summarize current research on the therapeutic potential of targeting Na V channels in pain pathologies, including DSN.

KV4 channels in isolectin B4 muscle dorsal root ganglion neurons of rats with experimental peripheral artery disease: effects of bradykinin B1 and B2 receptors

Muscle afferent nerve-activated reflex sympathetic nervous and blood pressure responses are exaggerated during exercise in peripheral artery diseases (PAD). However, the precise signaling pathways and molecular mediators responsible for these abnormal autonomic responses in PAD are poorly understood. Our previous study suggests that A-type voltage-gated K+ (KV4) channels regulate the excitability in muscle dorsal root ganglion (DRG) neurons of PAD rats; however, it is still lacking regarding the effects of PAD on characteristics of KV4 currents and engagement of bradykinin (BK) subtype receptors. Thus, we examined KV4 currents in two distinct muscle DRG neurons, namely isolectin B4-positive and B4-negative (IB4+ and IB4-) DRG neurons. IB4+ neurons express receptors for glial cell line-derived neurotrophic factor (GDNF), whereas IB4- DRG neurons are depending on nerve growth factors for survival. Our data showed that current density in muscle DRG neurons of PAD rats was decreased and this particularly appeared in IB4+ DRG neurons as compared with IB4- DRG neurons. We also showed that stimulation of BK B1 and B2 receptors led to a greater inhibitory effect on KV4 currents in IB4+ muscle DRG neurons and siRNA knockdown of KV4 subunit KV4.3 decreased the activity of KV4 currents in IB4+ DRG neurons. In conclusion, our data suggest that limb ischemia and/or ischemia-induced BK inhibit activity of KV4 channels in a subpopulation of the thin fiber muscle afferent neurons depending on GDNF, which is likely a part of signaling pathways involved in the exaggerated blood pressure response during activation of muscle afferent nerves in PAD.

Lipopolysaccharide Modifies Sodium Current Kinetics through ROS and PKC Signalling in Induced Pluripotent Stem-Derived Cardiomyocytes from Brugada Syndrome Patient

Studies have suggested a connection between inflammation and arrhythmogenesis of Brugada syndrome (BrS). However, experimental studies regarding the roles of inflammation in the arrhythmogenesis of BrS and its underlying mechanism are still lacking. This study aimed to investigate the influence of inflammation on BrS-phenotype features using human-induced stem cell-derived cardiomyocytes (hiPSC-CMs) from a BrS-patient carrying an SCN10A variant (c.3749G > A). After LPS treatment, the peak sodium current decreased significantly in SCN10A-hiPSC-CMs, but not in healthy donor-hiPSC-CMs. LPS also changed sodium channel gating kinetics, including activation, inactivation, and recovery from inactivation. NAC (N-acetyl-l-cysteine), a blocker of ROS (reactive oxygen species), failed to affect the sodium current, but prevented the LPS-induced reduction of sodium channel currents and changes in gating kinetics, suggesting a contribution of ROS to the LPS effects. Hydrogen peroxide (H2O2), a main form of ROS in cells, mimicked the LPS effects on sodium channel currents and gating kinetics, implying that ROS might mediate LPS-effects on sodium channels. The effects of H2O2 could be attenuated by a PKC blocker chelerythrine, indicating that PKC is a downstream factor of ROS. This study demonstrated that LPS can exacerbate the loss-of-function of sodium channels in BrS cells. Inflammation may play an important role in the pathogenesis of BrS.

Growth Differentiation Factor-15 Produces Analgesia by Inhibiting Tetrodotoxin-Resistant Nav1.8 Sodium Channel Activity in Rat Primary Sensory Neurons

Growth differentiation factor 15 (GDF-15) is a member of the transforming growth factor-β superfamily. It is widely distributed in the central and peripheral nervous systems. Whether and how GDF-15 modulates nociceptive signaling remains unclear. Behaviorally, we found that peripheral GDF-15 significantly elevated nociceptive response thresholds to mechanical and thermal stimuli in naïve and arthritic rats. Electrophysiologically, we demonstrated that GDF-15 decreased the excitability of small-diameter dorsal root ganglia (DRG) neurons. Furthermore, GDF-15 concentration-dependently suppressed tetrodotoxin-resistant sodium channel Nav1.8 currents, and shifted the steady-state inactivation curves of Nav1.8 in a hyperpolarizing direction. GDF-15 also reduced window currents and slowed down the recovery rate of Nav1.8 channels, suggesting that GDF-15 accelerated inactivation and slowed recovery of the channel. Immunohistochemistry results showed that activin receptor-like kinase-2 (ALK2) was widely expressed in DRG medium- and small-diameter neurons, and some of them were Nav1.8-positive. Blockade of ALK2 prevented the GDF-15-induced inhibition of Nav1.8 currents and nociceptive behaviors. Inhibition of PKA and ERK, but not PKC, blocked the inhibitory effect of GDF-15 on Nav1.8 currents. These results suggest a functional link between GDF-15 and Nav1.8 in DRG neurons via ALK2 receptors and PKA associated with MEK/ERK, which mediate the peripheral analgesia of GDF-15.

Soluble tumor necrosis factor-alpha-induced hyperexcitability contributes to retinal ganglion cell apoptosis by enhancing Nav1.6 in experimental glaucoma

BACKGROUND

Neuroinflammation plays an important role in the pathogenesis of glaucoma. Tumor necrosis factor-alpha (TNF-α) is a major pro-inflammatory cytokine released from activated retinal glial cells in glaucoma. Here, we investigated how TNF-α induces retinal ganglion cell (RGC) hyperexcitability and injury.

METHODS

Whole-cell patch-clamp techniques were performed to explore changes in spontaneous firing and evoked action potentials, and Na+ currents in RGCs. Both intravitreal injection of TNF-α and chronic ocular hypertension (COH) models were used. Western blotting, immunofluorescence, quantitative real-time polymerase chain reaction (q-PCR), and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) techniques were employed to investigate the molecular mechanisms of TNF-α effects on RGCs.

RESULTS

Intravitreal injection of soluble TNF-α significantly increased the spontaneous firing frequencies of RGCs in retinal slices. When the synaptic transmissions were blocked, more than 90% of RGCs still showed spontaneous firing; both the percentage of cells and firing frequency were higher than the controls. Furthermore, the frequency of evoked action potentials was also higher than the controls. Co-injection of the TNF-α receptor 1 (TNFR1) inhibitor R7050 eliminated the TNF-α-induced effects, suggesting that TNF-α may directly act on RGCs to induce cell hyperexcitability through activating TNFR1. In RGCs acutely isolated from TNF-α-injected retinas, Na+ current densities were upregulated. Perfusing TNF-α in RGCs of normal rats mimicked this effect, and the activation curve of Na+ currents shifted toward hyperpolarization direction, which was mediated through p38 MAPK and STAT3 signaling pathways. Further analysis revealed that TNF-α selectively upregulated Nav1.6 subtype of Na+ currents in RGCs. Similar to observations in retinas of rats with COH, intravitreal injection of TNF-α upregulated the expression of Nav1.6 proteins in both total cell and membrane components, which was reversed by the NF-κB inhibitor BAY 11-7082. Inhibition of TNFR1 blocked TNF-α-induced RGC apoptosis.

CONCLUSIONS

TNF-α/TNFR1 signaling induces RGC hyperexcitability by selectively upregulating Nav1.6 Na+ channels, thus contributing to RGC apoptosis in glaucoma.

IL-6 signaling pathway contributes to exercise pressor reflex in rats with femoral artery occlusion in association with Kv4 activity in muscle afferent nerves

Interleukin-6 (IL-6) via trans-signaling pathway plays a role in modifying muscle sensory nerve-exaggerated exercise pressor reflex in rats with ligated femoral arteries, but the underlying mechanisms are poorly understood. It is known that voltage-gated potassium channel subfamily member Kv4 channels contribute to the excitabilities of sensory neurons and neuronal signaling transduction. Thus, in this study, we determined that 1) IL-6 regulates the exaggerated exercise pressor reflex in rats with peripheral artery disease (PAD) induced by femoral artery ligation and 2) Kv4 channels in muscle dorsal root ganglion (DRG) neurons are engaged in the role played by IL-6 trans-signaling pathway. We found that the protein levels of IL-6 and its receptor IL-6R expression were increased in the DRGs of PAD rats with 3-day of femoral artery occlusion. Inhibition of muscle afferents' IL-6 trans-signaling pathway (gp130) by intra-arterial administration of SC144, a gp130 inhibitor, into the hindlimb muscles of PAD rats alleviated blood pressure response to static muscle contraction. On the other hand, we found that 3-day femoral occlusion decreased amplitude of Kv4 currents in rat muscle DRG neurons. The homo IL-6/IL-6Rα fusion protein (H. IL-6/6Rα), but not IL-6 alone significantly inhibited Kv4 currents in muscle DRG neurons; and the effect of H. IL-6/6Rα was largely reverted by SC144. In conclusion, our data suggest that via trans-signaling pathway upregulated IL-6 in muscle afferent nerves by ischemic hindlimb muscles inhibits the activity of Kv4 channels and thus likely leads to adjustments of the exercise pressor reflex in PAD.

Effects of bradykinin on voltage-gated KV 4 channels in muscle dorsal root ganglion neurons of rats with experimental peripheral artery disease

KEY POINTS

During exercise, bradykinin (BK), a muscle metabolite in ischaemic muscles, exaggerates autonomic responses to activation of muscle afferent nerves in peripheral artery disease (PAD). We examined whether BK inhibits activity of KV 4 channels in muscle afferent neurons of PAD rats induced by femoral artery occlusion. We demonstrated that: 1) femoral occlusion attenuates KV 4 currents in dorsal root ganglion (DRG) neurons innervating the hindlimb muscles and decreases the threshold of action potential firing; 2) BK has a greater inhibitory effect on KV 4 currents in muscle DRG neurons of PAD rats; and 3) expression of KV 4.3 is downregulated in DRGs of PAD rats and inhibition of KV 4.3 significantly decreases activity of KV 4 currents in muscle DRG neurons. Femoral artery occlusion-induced limb ischaemia and/or ischaemia-induced metabolites (i.e. BK) inhibit activity of KV 4 channels in muscle afferent neurons and this is likely involved in the exaggerated exercise pressor reflex in PAD.

ABSTRACT

Muscle afferent nerve-activated reflex sympathetic nervous and blood pressure responses are exaggerated during exercise in patients with peripheral artery diseases (PAD) and in PAD rats induced by femoral artery occlusion. However, the precise signalling pathways and molecular mediators responsible for these abnormal autonomic responses in PAD are poorly understood. A-type voltage-gated K+ (KV ) channels are quintessential regulators of cellular excitability in the various tissues. Among KV channels, KV 4 (i.e. KV 4.1 and KV 4.3) in primary sensory neurons mainly participate in physiological functions in regulation of mechanical and chemical sensation. However, little is known about the role of KV 4 in regulating neuronal activity in muscle afferent neurons of PAD. In addition, bradykinin (BK) is considered as a muscle metabolite contributing to the exaggerated exercise pressor reflex in PAD rats with femoral artery occlusion. Our data demonstrated that: 1) KV 4 currents are attenuated in dorsal root ganglion (DRG) neurons innervating the hindlimb muscles of PAD rats, along with a decreasing threshold of action potential firing; 2) KV 4 currents are inhibited by application of BK onto muscle DRG neurons of PAD rats to a greater degree; and 3) expression of KV 4.3 is downregulated in the DRGs of PAD rats and KV 4.3 channel is a major contributor to the activity of KV 4 currents in muscle DRG neurons. In conclusion, data suggest that femoral artery occlusion-induced limb ischaemia and/or ischaemia-induced metabolites (i.e. BK) inhibit the activity of KV 4 channels in muscle afferent neurons likely leading to the exaggerated exercise pressor reflex observed in PAD.