Early differences in membrane properties at the neuromuscular junctions of ALS model mice: Effects of 25-hydroxycholesterol

β2-Adrenergic Regulation of the Neuromuscular Transmission and Its Lipid-Dependent Switch

β2-Adrenoceptors (β2-ARs) are the most abundant subtype of adrenergic receptors in skeletal muscles. Their activation via a stabilization of postsynaptic architecture has beneficial effects in certain models of neuromuscular disorders. However, the ability of β2-ARs to regulate neuromuscular transmission at the presynaptic level is poorly understood. Using electrophysiological recordings and fluorescent FM dyes, we found that β2-AR activation with fenoterol enhanced an involvement of synaptic vesicles in exocytosis and neurotransmitter release during intense activity at the neuromuscular junctions of mouse diaphragm. This was accompanied by an improvement of contractile responses to phrenic nerve stimulation (but not direct stimulation of the muscle fibers) at moderate-to-high frequencies. β2-ARs mainly reside in lipid microdomains enriched with cholesterol and sphingomyelin. The latter is hydrolyzed by sphingomyelinases, whose upregulation occurs in many conditions characterized by muscle atrophy and sympathetic nerve hyperactivity. Sphingomyelinase treatment reversed the effects of β2-AR agonist on the neurotransmitter release and synaptic vesicle recruitment to the exocytosis during intense activity. Inhibition of Gi protein with pertussis toxin completely prevented the sphingomyelinase-mediated inversion in the β2-AR agonist action. Note that lipid raft disrupting enzyme cholesterol oxidase had the same effect on β2-AR agonist-mediated changes in neurotransmission as sphingomyelinase. Thus, β2-AR agonist fenoterol augmented recruitment and release of synaptic vesicles during intense activity in the diaphragm neuromuscular junctions. Sphingomyelin hydrolysis inversed the effects of β2-AR agonist on neurotransmission probably via switching to Gi protein-dependent signaling. This phenomenon may reflect a dependence of the β2-AR signaling on lipid raft integrity in the neuromuscular junctions.

Protective properties of melanin from lichen Lobaria pulmonaria (L.) HOFFM. In models of oxidative stress in skeletal muscle

Melanin is a dark pigment from the group of phenolic or indole polymers with inherent biocompatibility and antioxidant capacity. In extremophilic lichen Lobaria pulmonaria, melanin is responsible for protective properties against hostile environments. Herein, the ability of melanin extracted from L. pulmonaria to counteract oxidative stress and related damages was studied in the mouse diaphragm, the main respiratory muscle. Initial in vitro experiments demonstrated ultraviolet (UV)-absorbing, antioxidant and metal chelating activities of melanin. This melanin can form nanoparticles and stabile colloidal system at concentration of 5 μg/ml. Pretreatment of the muscle with melanin (5 μg/ml) markedly reduced UV-induced increase in intracellular and extracellular reactive oxygen species (ROS) as well as antimycin A-mediated enhancement in mitochondrial ROS production accompanied by lipid peroxidation and membrane asymmetry loss. In addition, melanin attenuated suppression of neuromuscular transmission and alterations of contractile responses provoked by hydrogen peroxide. Thus, this study shed the light on the perspectives of the application of a lichen melanin as a protective component for treatment of skeletal muscle disorders, which are accompanied with an increased ROS production.

Mechanism of Purinergic Regulation of Neurotransmission in Mouse Neuromuscular Junction: The Role of Redox Signaling and Lipid Rafts

Acetylcholine is the main neurotransmitter at the vertebrate neuromuscular junctions (NMJs). ACh exocytosis is precisely modulated by co-transmitter ATP and its metabolites. It is assumed that ATP/ADP effects on ACh release rely on activation of presynaptic Gi protein-coupled P2Y13 receptors. However, downstream signaling mechanism of ATP/ADP-mediated modulation of neuromuscular transmission remains elusive. Using microelectrode recording and fluorescent indicators, the mechanism underlying purinergic regulation was studied in the mouse diaphragm NMJs. Pharmacological stimulation of purinoceptors with ADP decreased synaptic vesicle exocytosis evoked by both low and higher frequency stimulation. This inhibitory action was suppressed by antagonists of P2Y13 receptors (MRS 2211), Ca2+ mobilization (TMB8), protein kinase C (chelerythrine) and NADPH oxidase (VAS2870) as well as antioxidants. This suggests the participation of Ca2+ and reactive oxygen species (ROS) in the ADP-triggered signaling. Indeed, ADP caused an increase in cytosolic Ca2+ with subsequent elevation of ROS levels. The elevation of [Ca2+]in was blocked by MRS 2211 and TMB8, whereas upregulation of ROS was prevented by pertussis toxin (inhibitor of Gi protein) and VAS2870. Targeting the main components of lipid rafts, cholesterol and sphingomyelin, suppressed P2Y13 receptor-dependent attenuation of exocytosis and ADP-induced enhancement of ROS production. Inhibition of P2Y13 receptors decreased ROS production and increased the rate of exocytosis during intense activity. Thus, suppression of neuromuscular transmission by exogenous ADP or endogenous ATP can rely on P2Y13 receptor/Gi protein/Ca2+/protein kinase C/NADPH oxidase/ROS signaling, which is coordinated in a lipid raft-dependent manner.

Effects of membrane cholesterol-targeting chemicals on skeletal muscle contractions evoked by direct and indirect stimulation

Cholesterol is one of the major components of plasma membrane, where its distribution is nonhomogeneous and it participates in lipid raft formation. In skeletal muscle cholesterol and lipid rafts seem to be important for excitation-contraction coupling and for neuromuscular transmission, involving cholesterol-rich synaptic vesicles. In the present study, nerve and muscle stimulation-evoked contractions were recorded to assess the role of cholesterol in contractile function of mouse diaphragm. Exposure to cholesterol oxidase (0.2 U/ml) and cholesterol-depleting agent methyl-β-cyclodextrin (1 mM) did not affect markedly contractile responses to both direct and indirect stimulation at low and high frequency. However, methyl-β-cyclodextrin at high concentration (10 mM) strongly decreased the force of both single and tetanus contractions induced by phrenic nerve stimulation. This decline in contractile function was more profoundly expressed when methyl-β-cyclodextrin application was combined with phrenic nerve activation. At the same time, 10 mM methyl-β-cyclodextrin had no effect on contractions upon direct muscle stimulation at low and high frequency. Thus, strong cholesterol depletion suppresses contractile function mainly due to disturbance of the neuromuscular communication, whereas muscle fiber contractility remains resistant to decline.

25-hydroxycholesterol triggers antioxidant signaling in mouse atria

Oxysterol, 25-hydroxycholesterol (25HC), is a potent regulator of immune reactions, its synthesis greatly increases by macrophages during inflammation. We hypothesize that 25HC can have cardioprotective effects by limiting consequences of excessive β-adrenoceptor (βAR) stimulation, particularly reactive oxygen species (ROS) production, in mouse atria. Isoproterenol, a βAR agonist, increased extra- and intracellular levels of ROS. This enhancement of ROS production was suppressed by NADPH oxidase antagonists as well as 25HC. Inhibition of β3ARs, Gi protein and protein kinase Cε prevented the effect of 25HC on isoproterenol-dependent ROS synthesis. Furthermore, 25HC suppressed isoproterenol-induced lipid peroxidation and mitochondrial ROS generation as well as ROS-dependent component of positive inotropic response to isoproterenol. Additionally, 25HC decreased mitochondrial ROS production and lipid peroxidation induced by antimycin A, a mitochondrial poison. Thus, 25HC exerts antioxidant properties alleviating mitochondrial dysfunction-induced and βAR-dependent cardiac oxidative damage. In the latter case, 25HC can act via signaling mechanism engaging β3ARs, Gi protein and protein kinase Cε.

The Mechanism of α2 adrenoreceptor-dependent Modulation of Neurotransmitter Release at the Neuromuscular Junctions

α2-Adrenoreceptors (ARs) are main Gi-protein coupled autoreceptors in sympathetic nerve terminals and targets for dexmedetomidine (DEX), a widely used sedative. We hypothesize that α2-ARs are also potent regulators of neuromuscular transmission via G protein-gated inwardly rectifying potassium (GIRK) channels. Using extracellular microelectrode recording of postsynaptic potentials, we found DEX-induced inhibition of spontaneous and evoked neurotransmitter release as well as desynchronization of evoked exocytotic events in the mouse diaphragm neuromuscular junction. These effects were suppressed by SKF-86,466, a selective α2-AR antagonist. An activator of GIRK channels ML297 had the same effects on neurotransmitter release as DEX. By contrast, inhibition of GIRK channels with tertiapin-Q prevented the action of DEX on evoked neurotransmitter release, but not on spontaneous exocytosis. The synaptic vesicle exocytosis is strongly dependent on Ca2+ influx through voltage-gated Ca2+ channels (VGCCs), which can be negatively regulated via α2-AR - GIRK channel axis. Indeed, inhibition of P/Q-, L-, N- or R-type VGCCs prevented the inhibitory action of DEX on evoked neurotransmitter release; antagonists of P/Q- and N-type channels also suppressed the DEX-mediated desynchronization of evoked exocytotic events. Furthermore, inhibition of P/Q-, L- or N-type VGCCs precluded the frequency decrease of spontaneous exocytosis upon DEX application. Thus, α2-ARs acting via GIRK channels and VGCCs (mainly, P/Q- and N-types) exert inhibitory effect on the neuromuscular communication by attenuating and desynchronizing evoked exocytosis. In addition, α2-ARs can suppress spontaneous exocytosis through GIRK channel-independent, but VGCC-dependent pathway.

Emerging Role of Astrocyte-Derived Extracellular Vesicles as Active Participants in CNS Neuroimmune Responses

Astrocyte-derived extracellular vesicles (ADEVs) have garnered attention as a fundamental mechanism of intercellular communication in health and disease. In the context of neurological diseases, for which prodromal diagnosis would be advantageous, ADEVs are also being explored for their potential utility as biomarkers. In this review, we provide the current state of data supporting our understanding on the manifold roles of ADEVs in several common neurological disorders. We also discuss these findings from a unique emerging perspective that ADEVs represent a means by which the central nervous system may broadcast influence over other systems in the body to affect neuroinflammatory processes, with both dual potential to either propagate illness or restore health and homeostasis.

Oxysterols in Central and Peripheral Synaptic Communication

Cholesterol is a key molecule for synaptic transmission, and both central and peripheral synapses are cholesterol rich. During intense neuronal activity, a substantial portion of synaptic cholesterol can be oxidized by either enzymatic or non-enzymatic pathways to form oxysterols, which in turn modulate the activities of neurotransmitter receptors (e.g., NMDA and adrenergic receptors), signaling molecules (nitric oxide synthases, protein kinase C, liver X receptors), and synaptic vesicle cycling involved in neurotransmitters release. 24-Hydroxycholesterol, produced by neurons in the brain, could directly affect neighboring synapses and change neurotransmission. 27-Hydroxycholesterol, which can cross the blood-brain barrier, can alter both synaptogenesis and synaptic plasticity. Increased generation of 25-hydroxycholesterol by activated microglia and macrophages could link inflammatory processes to learning and neuronal regulation. Amyloids and oxidative stress can lead to an increase in the levels of ring-oxidized sterols and some of these oxysterols (4-cholesten-3-one, 5α-cholestan-3-one, 7β-hydroxycholesterol, 7-ketocholesterol) have a high potency to disturb or modulate neurotransmission at both the presynaptic and postsynaptic levels. Overall, oxysterols could be used as "molecular prototypes" for therapeutic approaches. Analogs of 24-hydroxycholesterol (SGE-301, SGE-550, SAGE718) can be used for correction of NMDA receptor hypofunction-related states, whereas inhibitors of cholesterol 24-hydroxylase, cholestane-3β,5α,6β-triol, and cholest-4-en-3-one oxime (olesoxime) can be utilized as potential anti-epileptic drugs and (or) protectors from excitotoxicity.

Effects of cholesterol oxidase on neurotransmission and acetylcholine levels at the mice neuromuscular junctions

Membrane cholesterol oxidation is a hallmark of redox and metabolic imbalance, and it may accompany neurodegenerative disorders. Using microelectrode recordings of postsynaptic responses as well as fluorescent dyes for monitoring synaptic vesicle cycling and membrane properties, the action of enzymatic cholesterol oxidation on neuromuscular transmission was studied in the mice diaphragms. Cholesterol oxidase (ChO) at low concentration disturbed lipid-ordering specifically in the synaptic membranes, but it did not change markedly spontaneous exocytosis and evoked release in response to single stimuli. At low external Ca2+ conditions, analysis of single exocytotic events revealed a decrease in minimal synaptic delay and the probability of exocytosis upon plasmalemmal cholesterol oxidation. At moderate- and high-frequency activity, ChO treatment enhanced both neurotransmitter and FM-dye release. Furthermore, it precluded a change in exocytotic mode from full-fusion to kiss-and-run during high-frequency stimulation. Accumulation of extracellular acetylcholine (without stimulation) dependent on vesamicol-sensitive transporters was suppressed by ChO. The effects of plasmalemmal cholesterol oxidation on both neurotransmitter/dye release at intense activity and external acetylcholine levels were reversed when synaptic vesicle membranes were also exposed to ChO (i.e., the enzyme treatment was combined with induction of exo-endocytotic cycling). Thus, we suggest that plasmalemmal cholesterol oxidation affects exocytotic machinery functioning, enhances synaptic vesicle recruitment to the exocytosis and decreases extracellular neurotransmitter levels at rest, whereas ChO acting on synaptic vesicle membranes suppresses the participation of the vesicles in the subsequent exocytosis and increases the neurotransmitter leakage. The mechanisms underlying ChO action can be related to the lipid raft disruption.

Early Alterations in Structural and Functional Properties in the Neuromuscular Junctions of Mutant FUS Mice

Amyotrophic lateral sclerosis (ALS) is manifested as skeletal muscle denervation, loss of motor neurons and finally severe respiratory failure. Mutations of RNA-binding protein FUS are one of the common genetic reasons of ALS accompanied by a 'dying back' type of degeneration. Using fluorescent approaches and microelectrode recordings, the early structural and functional alterations in diaphragm neuromuscular junctions (NMJs) were studied in mutant FUS mice at the pre-onset stage. Lipid peroxidation and decreased staining with a lipid raft marker were found in the mutant mice. Despite the preservation of the end-plate structure, immunolabeling revealed an increase in levels of presynaptic proteins, SNAP-25 and synapsin 1. The latter can restrain Ca2+-dependent synaptic vesicle mobilization. Indeed, neurotransmitter release upon intense nerve stimulation and its recovery after tetanus and compensatory synaptic vesicle endocytosis were markedly depressed in FUS mice. There was a trend to attenuation of axonal [Ca2+]in increase upon nerve stimulation at 20 Hz. However, no changes in neurotransmitter release and the intraterminal Ca2+ transient in response to low frequency stimulation or in quantal content and the synchrony of neurotransmitter release at low levels of external Ca2+ were detected. At a later stage, shrinking and fragmentation of end plates together with a decrease in presynaptic protein expression and disturbance of the neurotransmitter release timing occurred. Overall, suppression of synaptic vesicle exo-endocytosis upon intense activity probably due to alterations in membrane properties, synapsin 1 levels and Ca2+ kinetics could be an early sign of nascent NMJ pathology, which leads to neuromuscular contact disorganization.

Up-regulation of cholesterol synthesis pathways and limited neurodegeneration in a knock-in Sod1 mutant mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a severe neurodegenerative disorder affecting brain and spinal cord motor neurons. Mutations in the copper/zinc superoxide dismutase gene ( SOD1 ) are associated with ∼20% of inherited and 1-2% of sporadic ALS cases. Much has been learned from mice expressing transgenic copies of mutant SOD1, which typically involve high-level transgene expression, thereby differing from ALS patients expressing one mutant gene copy. To generate a model that more closely represents patient gene expression, we created a knock-in point mutation (G85R, a human ALS-causing mutation) in the endogenous mouse Sod1 gene, leading to mutant SOD1 G85R protein expression. Heterozygous Sod1 G85R mutant mice resemble wild type, whereas homozygous mutants have reduced body weight and lifespan, a mild neurodegenerative phenotype, and express very low mutant SOD1 protein levels with no detectable SOD1 activity. Homozygous mutants exhibit partial neuromuscular junction denervation at 3-4 months of age. Spinal cord motor neuron transcriptome analyses of homozygous Sod1 G85R mice revealed up-regulation of cholesterol synthesis pathway genes compared to wild type. Transcriptome and phenotypic features of these mice are similar to Sod1 knock-out mice, suggesting the Sod1 G85R phenotype is largely driven by loss of SOD1 function. By contrast, cholesterol synthesis genes are down-regulated in severely affected human TgSOD1 G93A transgenic mice at 4 months. Our analyses implicate dysregulation of cholesterol or related lipid pathway genes in ALS pathogenesis. The Sod1 G85R knock-in mouse is a useful ALS model to examine the importance of SOD1 activity in control of cholesterol homeostasis and motor neuron survival.

SIGNIFICANCE STATEMENT

Amyotrophic lateral sclerosis is a devastating disease involving the progressive loss of motor neurons and motor function for which there is currently no cure. Understanding biological mechanisms leading to motor neuron death is critical for developing new treatments. Using a new knock-in mutant mouse model carrying a Sod1 mutation that causes ALS in patients, and in the mouse, causes a limited neurodegenerative phenotype similar to Sod1 loss-of-function, we show that cholesterol synthesis pathway genes are up-regulated in mutant motor neurons, whereas the same genes are down-regulated in transgenic SOD1 mice with a severe phenotype. Our data implicate dysregulation of cholesterol or other related lipid genes in ALS pathogenesis and provide new insights that could contribute to strategies for disease intervention.

Sphingomyelinase modulates synaptic vesicle mobilization at the mice neuromuscular junctions

AIMS

Sphingomyelin is an abundant component of the presynaptic membrane and an organizer of lipid rafts. In several pathological conditions, sphingomyelin is hydrolyzed due to an upregulation and release of secretory sphingomyelinases (SMases). Herein, the effects of SMase on exocytotic neurotransmitter release were studied in the diaphragm neuromuscular junctions of mice.

MAIN METHODS

Microelectrode recordings of postsynaptic potentials and styryl (FM) dyes were used to estimate neuromuscular transmission. Membrane properties were assessed with fluorescent techniques.

KEY FINDINGS

Application of SMase at a low concentration (0.01 U ml^-1) led to a disruption of lipid-packing in the synaptic membranes. Neither spontaneous exocytosis nor evoked neurotransmitter release (in response to single stimuli) were affected by SMase treatment. However, SMase significantly increased neurotransmitter release and the rate of fluorescent FM-dye loss from the synaptic vesicles at 10, 20 and 70 Hz stimulation of the motor nerve. In addition, SMase treatment prevented a shift of the exocytotic mode from "full-collapse" fusion to "kiss-and-run" during high-frequency (70 Hz) activity. The potentiating effects of SMase on neurotransmitter release and FM-dye unloading were suppressed when synaptic vesicle membranes were also exposed to this enzyme (i.e., stimulation occurred during SMase treatment).

SIGNIFICANCE

Thus, hydrolysis of the plasma membrane sphingomyelin can enhance mobilization of synaptic vesicles and facilitate full fusion mode of exocytosis, but SMase acting on vesicular membrane had a depressant effect on the neurotransmission. Partially, the effects of SMase can be related with the changes in synaptic membrane properties and intracellular signaling.

25-Hydroxycholesterol as a Signaling Molecule of the Nervous System

Cholesterol is an essential component of plasma membrane and precursor of biological active compounds, including hydroxycholesterols (HCs). HCs regulate cellular homeostasis of cholesterol; they can pass across the membrane and vascular barriers and act distantly as para- and endocrine agents. A small amount of 25-hydroxycholesterol (25-HC) is produced in the endoplasmic reticulum of most cells, where it serves as a potent regulator of the synthesis, intracellular transport, and storage of cholesterol. Production of 25-HC is strongly increased in the macrophages, dendrite cells, and microglia at the inflammatory response. The synthesis of 25-HC can be also upregulated in some neurological disorders, such as Alzheimer’s disease, amyotrophic lateral sclerosis, spastic paraplegia type 5, and X-linked adrenoleukodystrophy. However, it is unclear whether 25-HC aggravates these pathologies or has the protective properties. The molecular targets for 25-HC are transcriptional factors (LX receptors, SREBP2, ROR), G protein-coupled receptor (GPR183), ion channels (NMDA receptors, SLO1), adhesive molecules (α5β1 and ανβ3 integrins), and oxysterol-binding proteins. The diversity of 25-HC-binding proteins points to the ability of HC to affect many physiological and pathological processes. In this review, we focused on the regulation of 25-HC production and its universal role in the control of cellular cholesterol homeostasis, as well as the effects of 25-HC as a signaling molecule mediating the influence of inflammation on the processes in the neuromuscular system and brain. Based on the evidence collected, it can be suggested that 25-HC prevents accumulation of cellular cholesterol and serves as a potent modulator of neuroinflammation, synaptic transmission, and myelinization. An increased production of 25-HC in response to a various type of damage can have a protective role and reduce neuronal loss. At the same time, an excess of 25-HC may exert the neurotoxic effects.

Effects of Oxysterols on Immune Cells and Related Diseases

Oxysterols are the products of cholesterol oxidation. They have a wide range of effects on several cells, organs, and systems in the body. Oxysterols also have an influence on the physiology of the immune system, from immune cell maturation and migration to innate and humoral immune responses. In this regard, oxysterols have been involved in several diseases that have an immune component, from autoimmune and neurodegenerative diseases to inflammatory diseases, atherosclerosis, and cancer. Here, we review data on the participation of oxysterols, mainly 25-hydroxycholesterol and 7α,25-dihydroxycholesterol, in the immune system and related diseases. The effects of these oxysterols and main oxysterol receptors, LXR and EBI2, in cells of the immune system (B cells, T cells, macrophages, dendritic cells, oligodendrocytes, and astrocytes), and in immune-related diseases, such as neurodegenerative diseases, intestinal diseases, cancer, respiratory diseases, and atherosclerosis, are discussed.

Restoration and targeting of aberrant neurotransmitters in Parkinson's disease therapeutics

Neurotransmitters are considered as a fundamental regulator in the process of neuronal growth, differentiation and survival. Parkinson's Disease (PD) occurs due to extensive damage of dopamine-producing neurons; this causes dopamine deficits in the midbrain, followed by the alternation of various other neurotransmitters (glutamate, GABA, serotonin, etc.). It has been observed that fluctuation of neurotransmission in the basal ganglia exhibits a great impact on the pathophysiology of PD. Dopamine replacement therapy, such as the use of L-DOPA, can increase the dopamine level, but it majorly ameliorates the motor symptoms and is also associated with long-term complications (for e.g., LID). While the non-dopaminergic system can efficiently target non-motor symptoms, for instance, the noradrenergic system regulates the synthesis of BDNF via the MAPK pathway, which is important in learning and memory. Herein, we briefly discuss the role of different neurotransmitters, implementation of neurotransmitter receptors in PD. We also illustrate the recent advances of neurotransmitter-based drugs, which are currently under in vivo and clinical studies. Reinstating normal neurotransmitter levels has been believed to be advantageous in the treatment of PD. Thus, there is an increasing demand for drugs that can specifically target the neurotransmission system and reinstate the normal levels of neurotransmitters, which might prevent or delay neurodegeneration in PD.

GIRK channel as a versatile regulator of neurotransmitter release via L-type Ca2+ channel-dependent mechanism in the neuromuscular junction

G protein-gated inwardly rectifying potassium (GIRK) channels are one of the main regulators of neuronal excitability. Activation of GIRK channels in the CNS usually leads to postsynaptic inhibition. However, the function of GIRK channels in the presynaptic processes, notably neurotransmitter release form motor nerve terminals, is yet to be comprehensively understood. Here, using electrophysiological and fluorescent approaches, the role of GIRK channels in neurotransmitter release from frog motor nerve terminals was studied. We found that the inhibition of GIRK channels with nanomolar tertiapin-Q synchronized exocytosis events with action potential but suppressed spontaneous and evoked neurotransmitter release, as well as Ca²⁺ transient and membrane permeability for K⁺. The action of GIRK channel inhibition on evoked neurotransmission was prevented by selective antagonist of voltage-gated Ca²⁺ channels of L-type. Furthermore, the effects of muscarinic acetylcholine receptor activation on neurotransmitter release, Ca²⁺ transient and K⁺ channel activity were markedly modulated by inhibition of GIRK channels. Thus, at the motor nerve terminals GIRK channels can regulate timing of neurotransmitter release and be a positive modulator of synaptic vesicle exocytosis acting partially via L-type Ca²⁺ channels. In addition, GIRK channels are key players in a feedback control of neurotransmitter release by muscarinic acetylcholine receptors.

Immune-related oxysterol modulates neuromuscular transmission via non-genomic liver X receptor-dependent mechanism

Inflammatory reactions induce changes in the neuromuscular system. The mechanisms underlying this link are unclear. Besides cytokines and reactive oxygen species (ROS), production of an antiviral oxysterol 25-hydroxycholesterol (25HC) by immune cells is quickly increased in response to inflammation. Hypothetically, 25HC could contribute to regulation of neuromuscular activity as well as redox status. We found that 25HC (0.01-10 μM) can bidirectionally modulate neurotransmission in mice diaphragm, the main respiratory muscle. Low concentrations (≤0.1 μM) of 25HC reduced involvement of synaptic vesicles (SVs) into exocytosis during 20-Hz activity, whereas higher inflammatory-related concentrations (≥1 μM) had a profound potentiating effect on SV mobilization. The latter stimulatory action of 25HC was accompanied by increase in Ca²⁺ release from intracellular stores via IP₃ receptors. Both increase in SV mobilization and [Ca²⁺]_in were suppressed by a specific antagonist of liver X receptors (LXRs). These receptors formed clusters within the synaptic membranes in a lipid raft-dependent manner. Either raft disruption or intracellular Ca²⁺ chelation prevented 25HC-mediated acceleration of the exocytotic rate. The same action had inhibition of estrogen receptor α, Gi-protein, Gβγ, phospholipase C and protein kinase C. Additionally, 1 μM 25HC upregulated ROS production in a Ca²⁺-dependent way and an antioxidant partially decreased the exocytosis-promoting effect of 25HC. Thus, 25HC has prooxidant properties and it is a potent regulator of SV mobilization via activation of lipid raft-associated LXRs which can trigger signaling via estrogen receptor α - Gi-protein - Gβγ - phospholipase C - Ca²⁺ - protein kinase C pathway. 25HC-mediated increase in ROS may modulate this signaling.