Lipopolysaccharides and trophic factors regulate the LPS receptor complex in nodose and trigeminal neurons

Lipopolysaccharide-Mediated Effects of the Microbiota on Sleep and Body Temperature

Background

Recent research suggests that microbial molecules translocated from the intestinal lumen into the host's internal environment may play a role in various physiological functions, including sleep. Previously, we identified that butyrate, a short-chain fatty acid, produced by intestinal bacteria, and lipoteichoic acid, a cell wall component of gram-positive bacteria induce sleep when their naturally occurring translocation is mimicked by direct delivery into the portal vein. Building upon these findings, we aimed to explore the sleep signaling potential of intraportally administered lipopolysaccharide, a primary component of gram-negative bacterial cell walls, in rats.

Results

Low dose of lipopolysaccharide (1 μg/kg) increased sleep duration and prolonged fever, without affecting systemic lipopolysaccharide levels. Interestingly, administering LPS systemically outside the portal region at a dose 20 times higher did not affect sleep, indicating a localized sensitivity within the hepatoportal region, encompassing the portal vein and liver, for the sleep and febrile effects of lipopolysaccharide. Furthermore, both the sleep- and fever-inducing effects of LPS were inhibited by indomethacin, a prostaglandin synthesis inhibitor, and replicated by intraportal administration of prostaglandin E2 or arachidonic acid, suggesting the involvement of the prostaglandin system in mediating these actions.

Conclusions

These findings underscore the dynamic influence of lipopolysaccharide in the hepatoportal region on sleep and fever mechanisms, contributing to a complex microbial molecular assembly that orchestrates communication between the intestinal microbiota and brain. Lipopolysaccharide is a physiological component of plasma in both the portal and extra-portal circulation, with its levels rising in response to everyday challenges like high-fat meals, moderate alcohol intake, sleep loss and psychological stress. The increased translocation of lipopolysaccharide under such conditions may account for their physiological impact in daily life, highlighting the intricate interplay between microbial molecules and host physiology.

Lipopolysaccharide-induced Trigeminal Ganglion Nerve Fiber Damage is Associated with Autophagy Inhibition

OBJECTIVE

This study aimed to determine whether lipopolysaccharide (LPS) induces the loss of corneal nerve fibers in cultured trigeminal ganglion (TG) cells, and the underlying mechanism of LPS-induced TG neurite damage.

METHODS

TG neurons were isolated from C57BL/6 mice, and the cell viability and purity were maintained for up to 7 days. Then, they were treated with LPS (1 µg/mL) or the autophagy regulator (autophibib and rapamycin) alone or in combination for 48 h, and the length of neurites in TG cells was examined by the immunofluorescence staining of the neuron-specific protein β3-tubulin. Afterwards, the molecular mechanisms by which LPS induces TG neuron damage were explored.

RESULTS

The immunofluorescence staining revealed that the average length of neurites in TG cells significantly decreased after LPS treatment. Importantly, LPS induced the impairment of autophagic flux in TG cells, which was evidenced by the increase in the accumulation of LC3 and p62 proteins. The pharmacological inhibition of autophagy by autophinib dramatically reduced the length of TG neurites. However, the rapamycin-induced activation of autophagy significantly lessened the effect of LPS on the degeneration of TG neurites.

CONCLUSION

LPS-induced autophagy inhibition contributes to the loss of TG neurites.

Immunohistochemical Localization of MD2, a Co-Receptor of TLR4, in the Adult Mouse Brain

Myeloid differentiation factor 2 (MD2) is a co-receptor of a classical proinflammatory protein TLR4 whose activation leads to neuroinflammation. It is widely accepted that TLR4 is expressed on the cell surface of microglia and astrocytes, and MD2 is expected to be expressed by these cells as well. However, our previous study showed that neurons from certain nuclei also expressed MD2. Whether MD2 is expressed by other brain nuclei is still unknown. It is the aim of the present study to map the distribution of MD2-positive cells in the adult mouse brain. Immunohistochemical staining against MD2 was completed to localize MD2-positive cells in the mouse brain by comparing the location of positive cells with the mouse brain atlas. MD2-positive cells were found in the majority of mouse brain nuclei with clusters of cells in the olfactory bulb, cortices, the red nucleus, and cranial nuclei. Subcortical nuclei had heterogeneous staining of MD2 with more prominent cells in the basolateral and the central amygdaloid nuclei. The ventral pallidum and the diagonal bands had positive cells with similar density and shape. Prominent cells were present in thalamic nuclei which were nearly homogeneous and in reticular formation of the brainstem where cells were dispersed with similar density. The hypothalamus had fewer outstanding cells compared with the thalamus. The red nucleus, the substantia nigra, and the ventral tegmental area in the pretectum had outstanding cells. Motor cranial nuclei also had outstanding MD2-positive cells, whereas raphe, sensory cranial, and deep cerebellar nuclei had MD2-positive cells with moderate density. The presence of MD2 in these nuclei may suggest the involvement of MD2 in their corresponding physiological functions.

Time-dependent alteration in the chemoreflex post-acute lung injury

Acute lung injury (ALI) induces inflammation that disrupts the normal alveolar-capillary endothelial barrier which impairs gas exchange to induce hypoxemia that reflexively increases respiration. The neural mechanisms underlying the respiratory dysfunction during ALI are not fully understood. The purpose of this study was to investigate the role of the chemoreflex in mediating abnormal ventilation during acute (early) and recovery (late) stages of ALI. We hypothesized that the increase in respiratory rate (fR) during post-ALI is mediated by a sensitized chemoreflex. ALI was induced in male Sprague-Dawley rats using a single intra-tracheal injection of bleomycin (Bleo: low-dose = 1.25 mg/Kg or high-dose = 2.5 mg/Kg) (day 1) and respiratory variables- fR, V_t (Tidal Volume), and V_E (Minute Ventilation) in response to 10% hypoxia (10% O₂, 0% CO₂) and 5% hypercapnia/21% normoxia (21% O₂, 5% CO₂) were measured weekly from W0-W4 using whole-body plethysmography (WBP). Our data indicate sensitization (∆f_R = 93 ± 31 bpm, p < 0.0001) of the chemoreflex at W1 post-ALI in response to hypoxic/hypercapnic gas challenge in the low-dose bleo (moderate ALI) group and a blunted chemoreflex (∆f_R = -0.97 ± 42 bpm, p < 0.0001) at W1 post-ALI in the high-dose bleo (severe ALI) group. During recovery from ALI, at W3-W4, both low-dose and high-dose groups exhibited a sensitized chemoreflex in response to hypoxia and normoxic-hypercapnia. We then hypothesized that the blunted chemoreflex at W1 post-ALI in the high-dose bleo group could be due to near maximal tonic activation of chemoreceptors, called the "ceiling effect". To test this possibility, 90% hyperoxia (90% O₂, 0% CO₂) was given to bleo treated rats to inhibit the chemoreflex. Our results showed no changes in f_R, suggesting absence of the tonic chemoreflex activation in response to hypoxia at W1 post-ALI. These data suggest that during the acute stage of moderate (low-dose bleo) and severe (high-dose bleo) ALI, chemoreflex activity trends to be slightly sensitized and blunted, respectively while it becomes significantly sensitized during the recovery stage. Future studies are required to examine the molecular/cellular mechanisms underlying the time-course changes in chemoreflex sensitivity post-ALI.

Glial-derived neurotrophic factor regulates the expression of TREK2 in rat primary sensory neurons leading to attenuation of axotomy-induced neuropathic pain

TREK2 is a member of the 2-pore domain family of K+ channels (K2P) preferentially expressed by unmyelinated, slow-conducting and non-peptidergic isolectin B4-binding (IB4+) primary sensory neurons of the dorsal root ganglia (DRG). IB4+ neurons depend on the glial-derived neurotrophic factor (GDNF) family of ligands (GFL's) to maintain their phenotype. In our previous work, we demonstrated that 7 days after spinal nerve axotomy (SNA) of the L5 DRG, TREK2 moves away from the cell membrane resulting in a more depolarised resting membrane potential (Em). Given that axotomy deprives DRG neurons from peripherally-derived GFL's, we hypothesized that they might control the expression of TREK2. Using a combination of immunohistochemistry, immunocytochemistry, western blotting, in vivo pharmacological manipulation and behavioral tests we examined the ability of the GFL's (GDNF, neurturin and artemin) and their selective receptors (GFRα1, GFRα2 and GFRα3) to regulate the expression and function of TREK2 in the DRG. We found that TREK2 correlated strongly with the three receptors normally and ipsilaterally for all GFR's after SNA. GDNF, but not NGF, neurturin or artemin up-regulated the expression of TREK2 in cultured DRG neurons. In vivo continuous, subcutaneous administration of GDNF restored the subcellular distribution of TREK2 ipsilaterally and reversed mechanical and cold allodynia 7 days after SNA. This is the first demonstration that GDNF controls the expression of a K2P channel in nociceptors. As TREK2 controls the Em of C-nociceptors affecting their excitability, our finding has therapeutic potential in the treatment of chronic pain.

Roles of neuronal toll-like receptors in neuropathic pain and central nervous system injuries and diseases

Toll-like receptors (TLRs) are innate immune receptors that are expressed in immune cells as well as glia and neurons of the central and peripheral nervous systems. They are best known for their role in the host defense in response to pathogens and for the induction of inflammation in infectious and non-infectious diseases. In the central nervous system (CNS), TLRs modulate glial and neuronal functions as well as innate immunity and neuroinflammation under physiological or pathophysiological conditions. The majority of the studies on TLRs in CNS pathologies investigated their overall contribution without focusing on a particular cell type, or they analyzed TLRs in glia and infiltrating immune cells in the context of neuroinflammation and cellular activation. The role of neuronal TLRs in CNS diseases and injuries has received little attention and remains underappreciated. The primary goal of this review is to summarize findings demonstrating the pivotal and unique roles of neuronal TLRs in neuropathic pain, Alzheimer's disease, Parkinson's disease and CNS injuries. We discuss how the current findings warrant future investigations to better define the specific contributions of neuronal TLRs to these pathologies. We underline the paucity of information regarding the role of neuronal TLRs in other neurodegenerative, demyelinating, and psychiatric diseases. We draw attention to the importance of broadening research on neuronal TLRs in view of emerging evidence demonstrating their distinctive functional properties.

An MD2-perturbing peptide has therapeutic effects in rodent and rhesus monkey models of stroke

Studies have failed to translate more than 1000 experimental treatments from bench to bedside, leaving stroke as the second leading cause of death in the world. Thrombolysis within 4.5 hours is the recommended therapy for stroke and cannot be performed until neuroimaging is used to distinguish ischemic stroke from hemorrhagic stroke. Therefore, finding a common and critical therapeutic target for both ischemic and hemorrhagic stroke is appealing. Here, we report that the expression of myeloid differentiation protein 2 (MD2), which is traditionally regarded to be expressed only in microglia in the normal brain, was markedly increased in cortical neurons after stroke. We synthesized a small peptide, Trans-trans-activating (Tat)-cold-inducible RNA binding protein (Tat-CIRP), which perturbed the function of MD2 and strongly protected neurons against excitotoxic injury in vitro. In addition, systemic administration of Tat-CIRP or genetic deletion of MD2 induced robust neuroprotection against ischemic and hemorrhagic stroke in mice. Tat-CIRP reduced the brain infarct volume and preserved neurological function in rhesus monkeys 30 days after ischemic stroke. Tat-CIRP efficiently crossed the blood-brain barrier and showed a wide therapeutic index for stroke because no toxicity was detected when high doses were administered to the mice. Furthermore, we demonstrated that MD2 elicited neuronal apoptosis and necroptosis via a TLR4-independent, Sam68-related cascade. In summary, Tat-CIRP provides robust neuroprotection against stroke in rodents and gyrencephalic nonhuman primates. Further efforts should be made to translate these findings to treat both ischemic and hemorrhagic stroke in patients.

Macrophage activation in stellate ganglia contributes to lung injury-induced arrhythmogenesis in male rats

AIM

Patients suffering from acute lung injury (ALI) are at high risk of developing cardiac arrhythmias. We hypothesized that stellate ganglia (SG) neural inflammation contributes to ALI-induced arrhythmia.

METHODS

We created an ALI rat model using a single tracheal instillation of bleomycin (2.5 mg/kg), with saline as a sham control. We recorded ECGs by implanted radiotelemetry in male bleomycin and sham rats treated with and without oral minocycline (20 mg/kg/d), an anti-inflammatory drug that inhibits microglia/macrophage activation. The SG neuronal excitability was assessed by electrophysiology experiments.

RESULTS

ECG data showed that bleomycin-exposed rats exhibited significantly more spontaneous premature ventricular contractions (PVCs) from 1- to 3-week post-induction compared with sham rats, which was mitigated by chronic oral administration of minocycline. The bleomycin-exposed rats displayed a robust increase in both the number of Iba1-positive macrophages and protein expression of interferon regulatory factor 8 in the SG starting as early at 1-week post-exposure and lasted for at least 4 weeks, which was largely attenuated by minocycline. Heart rate variability analysis indicated autonomic imbalance during the first 2-week post-bleomycin, which was significantly attenuated by minocycline. Electrical stimulation of the decentralized SG triggered more PVCs in bleomycin-exposed rats than sham and bleomycin + minocycline rats. Patch-clamp data demonstrated enhanced SG neuronal excitability in the bleomycin-exposed rats, which was attenuated by minocycline. Co-culture of lipopolysaccharide (LPS)-pretreated macrophages with normal SG neurons enhanced SG neuronal excitability.

CONCLUSION

Macrophage activation in the SG contributes to arrhythmogenesis in bleomycin-induced ALI in male rats.

TLR4 Signaling Selectively and Directly Promotes CGRP Release from Vagal Afferents in the Mouse

There has been a long-standing debate regarding the role of peripheral afferents in mediating rapid-onset anorexia among other responses elicited by peripheral inflammatory insults. Thus, the current study assessed the sufficiency of peripheral afferents expressing toll-like receptor 4 (TLR4) to the initiation of the anorexia caused by peripheral bacterial lipopolysaccharide (LPS). We generated a Tlr4 null (Tlr4^LoxTB) mouse in which Tlr4 expression is globally disrupted by a loxP-flanked transcription blocking (TB) cassette. This novel mouse model allowed us to restore the endogenous TLR4 expression in specific cell types. Using Zp3-Cre and Na_v1.8-Cre mice, we produced mice that express TLR4 in all cells (Tlr4^LoxTB X Zp3-Cre) and in peripheral afferents (Tlr4^LoxTB X Na_v1.8-Cre), respectively. We validated the Tlr4^LoxTB mice, which were phenotypically identical to previously reported global TLR4 knock-out mice. Contrary to our expectations, the administration of LPS did not cause rapid-onset anorexia in mice with Na_v1.8-restricted TLR4. The later result prompted us to identify Tlr4-expressing vagal afferents using in situ hybridization (ISH). In vivo, we found that Tlr4 mRNA was primarily enriched in vagal Na_v1.8 afferents located in the jugular ganglion that co-expressed calcitonin gene-related peptide (CGRP). In vitro, the application of LPS to cultured Na_v1.8-restricted TLR4 afferents was sufficient to stimulate the release of CGRP. In summary, we demonstrated using a new mouse model that vagally-expressed TLR4 is selectively involved in stimulating the release of CGRP but not in causing anorexia.

Obesity Affects the Microbiota-Gut-Brain Axis and the Regulation Thereof by Endocannabinoids and Related Mediators

The hypothalamus regulates energy homeostasis by integrating environmental and internal signals to produce behavioral responses to start or stop eating. Many satiation signals are mediated by microbiota-derived metabolites coming from the gastrointestinal tract and acting also in the brain through a complex bidirectional communication system, the microbiota–gut–brain axis. In recent years, the intestinal microbiota has emerged as a critical regulator of hypothalamic appetite-related neuronal networks. Obesogenic high-fat diets (HFDs) enhance endocannabinoid levels, both in the brain and peripheral tissues. HFDs change the gut microbiota composition by altering the Firmicutes:Bacteroidetes ratio and causing endotoxemia mainly by rising the levels of lipopolysaccharide (LPS), the most potent immunogenic component of Gram-negative bacteria. Endotoxemia induces the collapse of the gut and brain barriers, interleukin 1β (IL1β)- and tumor necrosis factor α (TNFα)-mediated neuroinflammatory responses and gliosis, which alter the appetite-regulatory circuits of the brain mediobasal hypothalamic area delimited by the median eminence. This review summarizes the emerging state-of-the-art evidence on the function of the “expanded endocannabinoid (eCB) system” or endocannabinoidome at the crossroads between intestinal microbiota, gut-brain communication and host metabolism; and highlights the critical role of this intersection in the onset of obesity.

Cutaneous inflammation differentially regulates the expression and function of Angiotensin-II types 1 and 2 receptors in rat primary sensory neurons

Neuropathic and inflammatory pain results from cellular and molecular changes in dorsal root ganglion (DRG) neurons. The type-2 receptor for Angiotensin-II (AT2R) has been involved in this type of pain. However, the underlying mechanisms are poorly understood, including the role of the type-1 receptor for Angiotensin-II (AT1R). Here, we used a combination of immunohistochemistry and immunocytochemistry, RT-PCR and in vitro and in vivo pharmacological manipulation to examine how cutaneous inflammation affected the expression of AT1R and AT2R in subpopulations of rat DRG neurons and studied their impact on inflammation-induced neuritogenesis. We demonstrated that AT2R-neurons express C- or A-neuron markers, primarily IB4, trkA, and substance-P. AT1R expression was highest in small neurons and co-localized significantly with AT2R. In vitro, an inflammatory soup caused significant elevation of AT2R mRNA, whereas AT1R mRNA levels remained unchanged. In vivo, we found a unique pattern of change in the expression of AT1R and AT2R after cutaneous inflammation. AT2R increased in small neurons at 1 day and in medium size neurons at 4 days. Interestingly, cutaneous inflammation increased AT1R levels only in large neurons at 4 days. We found that in vitro and in vivo AT1R and AT2R acted co-operatively to regulate DRG neurite outgrowth. In vivo, AT2R inhibition impacted more on non-peptidergic C-neurons neuritogenesis, whereas AT1R blockade affected primarily peptidergic nerve terminals. Thus, cutaneous-induced inflammation regulated AT1R and AT2R expression and function in different DRG neuronal subpopulations at different times. These findings must be considered when targeting AT1R and AT2R to treat chronic inflammatory pain. Cover Image for this issue: doi: 10.1111/jnc.14737.

Changes in the expression of the potassium channels TASK1, TASK3 and TRESK in a rat model of oral squamous cell carcinoma and their relation to malignancy

OBJECTIVES

Potassium channels have been proposed to promote cancer cell proliferation and metastases. Thus, we investigated the expression pattern of three 2-pore domain potassium channels (K2Ps) TASK1, TASK3 and TRESK in advanced oral squamous cell carcinoma (OSCC), the commonest oral malignancy.

DESIGN

We used 4-nitroquinoline-1-oxide (4-NQO) to induce high grade OSCC in male adult rats. We then used immunohistochemistry and Western blotting to study the distribution and expression pattern of TASK1, TASK3 and TRESK in normal versus cancerous tissue. We also examined the expression of β-tubulin III (β-tub3), a marker associated with resistance to taxane-based chemotherapy and poor patient prognosis, and its correlation with the K2Ps. Finally, we studied the expression of TASK1, TASK3 and TRESK in human samples of SCC of oral origin.

RESULTS

We found that TASK3 was significantly up-regulated whereas TASK1 and TRESK were both significantly down-regulated in advanced, poorly differentiated OSCC. Both, rat and human SCC showed a significant increase in the expression of β-tub3. Interestingly, the expression of the latter correlated positively and significantly with TASK3 and TRESK but not TASK1 in rat OSCC. Our initial results showed a similar pattern of up and down regulation and correlation with β-tub3 for these three K2Ps in human SCC.

CONCLUSIONS

The changes in expression and the co-localization with a marker of resistance to taxanes like β-tub3 turn TASK1, TASK3 and TRESK into potentially new prognostic tools and possibly new therapeutic targets for OSCC.

Recovery from an acute systemic and central LPS-inflammation challenge is affected by mouse sex and genetic background

Genetic and sexual factors influence the prevalence and the pathogenesis of many inflammatory disorders. In this study their relevance on the peripheral and central inflammatory status induced by a peripheral injection of lipopolysaccharide (LPS) was evaluated. BALB/c and CD-1 male and female mice were intraperitoneally injected with LPS. Spleens and brains were collected 2 and 72 hours later to study the levels of IL-6, TNF-α and IL-1β. Percentage of microglia and astrocytes was determined in the cortex and hippocampus. Locomotor activity was registered before and during the 72 hours after LPS-treatment. Two hours after LPS-injection, a peripheral increase of the three cytokines was found. In brains, LPS increased TNF-α only in males with higher levels in CD-1 than BALB/c. IL-1β increased only in CD-1 males. IL-6 increased in both strains with lower levels in BALB/c females. Peripheral and central levels of cytokines decline 72 hrs after LPS-treatment whilst a significantly increase of Iba-1 expression was detected. A dramatic drop of the locomotor activity was observed immediately after LPS injection. Our results show that acute systemic administration of LPS leads to peripheral and central increase of pro-inflammatory cytokines and microglia activation, in a strain and sex dependent manner.

Nociceptor-like rat dorsal root ganglion neurons express the angiotensin-II AT2 receptor throughout development

AT2 receptor (AT2R) plays a functional role in foetal development. Its expression declines in most tissues soon after birth but stays high in sensory areas of the adult nervous system. In the dorsal root ganglia (DRG) the expression pattern of AT2R during development and the identity of the subpopulation expressing it remain unknown. Using a combination of semi-quantitative PCR, western blotting and immunohistochemistry we examined the expression of AT2R at mRNA and protein levels in rat DRGs from embryonic day 15 (E15) until postnatal day 30 (PN30). We found that both AT2R mRNA and protein levels exhibited only minor (statistically non-significant) fluctuations from E15 to PN30. Detailed quantitative analysis of ABC/DAB AT2R staining showed a) that the receptor was present in most neurons at E15 and E18 and b) that postnatally it was predominantly expressed by small DRG neurons. Given that small neurons are putative C-nociceptors and the proposed role of AT2R in neuropathic pain, we next examined whether these AT2R-positive neurons co-localized with Ret and trkA embryonically and with IB4-binding postnatally. Most AT2R-positive neurons expressed trkA embryonically and bound IB4 postnatally. We found strong positive statistically highly significant correlations between AT2R cytoplasmic%intensities and trkA at E15/E18 and with Ret only at E18. Cytoplasmic AT2R also strongly and positively correlated with IB4-binding at PN3, 15 and 30. Our demonstration that a subpopulation of C-nociceptor-like neurons expresses AT2R during development supports a role for this receptor in neuropathic pain.

An immortalized rat pancreatic stellate cell line RP-2 as a new cell model for evaluating pancreatic fibrosis, inflammation and immunity

BACKGROUND

Pancreatic stellate cells (PSCs) play a critical role in the pathogenesis of pancreatic fibrosis and have emerging functions as progenitor cells, immune cells or intermediaries in pancreatic exocrine secretion. Increasing evidence has shown that desmin as an exclusive cytoskeleton marker of PSC is only expressed in part of these cells. This study was to establish a desmin-positive PSC cell line and evaluate its actions on pancreatic fibrosis, inflammation and immunity.

METHODS

The presence of cytoskeletal proteins, integrin α5β1 or TLR4, was determined by immunocytochemistry while the production of desmin, collagen I, MMP-1, MMP-2, TIMP-2, or CD14 was evaluated by Western blotting. The levels of desmin, collagen I, IL-1 and IL-6 mRNA were determined by real-time quantitative PCR. The secretion of cytokines was detected by ELISA. Cell function was assessed using adhesion, migration, or proliferation assays.

RESULTS

A stable activated rat PSC cell line (designated as RP-2) was established by RSV promoter/enhancer-driven SV40 large T antigen expression. RP-2 cells retained typical PSC properties, exhibited a myofibroblast-like phenotype and persistently produced desmin. The cells produced collagen I protein, matrix metalloproteinases and inhibitors thereof. RP-2 cells demonstrated typical PSC functions, including proliferation, adherence, and migration, the latter two of which occurred in response to fibronectin and were mediated by integrin α5β1. TLR4 and its response genes including proinflammatory cytokines (IL-1, IL-6, TNF-alpha) and chemotactic cytokines (MCP-1, MIP-1α, Rantes) were produced by RP-2 cells and activated by LPS. LPS-induced IL-1 or IL-6 mRNA expression in this cell line was fully blocked with MyD88 inhibitor.

CONCLUSION

RP-2 cells provide a novel tool for analyzing the properties and functions of PSCs in the pathogenesis of fibrosis, inflammation and immunity in the pancreas.