Toll-like receptor 9 deficiency impacts sensory and motor behaviors

A novel role for microtubule affinity-regulating kinases in neuropathic pain

BACKGROUND AND PURPOSE

Neuropathic pain affects millions of patients, but there are currently few viable therapeutic options available. Microtubule affinity-regulating kinases (MARKs) regulate the dynamics of microtubules and participate in synaptic remodelling. It is unclear whether these changes are involved in the central sensitization of neuropathic pain. This study examined the role of MARK1 or MARK2 in regulating neurosynaptic plasticity induced by neuropathic pain.

EXPERIMENTAL APPROACH

A rat spinal nerve ligation (SNL) model was established to induce neuropathic pain. The role of MARKs in nociceptive regulation was assessed by genetically knocking down MARK1 or MARK2 in amygdala and systemic administration of PCC0105003, a novel small molecule MARK inhibitor. Cognitive function, anxiety-like behaviours and motor coordination capability were also examined in SNL rats. Synaptic remodelling-associated signalling changes were detected with electrophysiological recording, Golgi-Cox staining, western blotting and qRT-PCR.

KEY RESULTS

MARK1 and MARK2 expression levels in amygdala and spinal dorsal horn were elevated in SNL rats. MARK1 or MARK2 knockdown in amygdala and PCC0105003 treatment partially attenuated pain-like behaviours along with improving cognitive deficit, anxiogenic-like behaviours and motor coordination in SNL rats. Inhibition of MARKs signalling reversed synaptic plasticity at the functional and structural levels by suppressing NR2B/GluR1 and EB3/Drebrin signalling pathways both in amygdala and spinal dorsal horn.

CONCLUSION AND IMPLICATIONS

These results suggest that MARKs-mediated synaptic remodelling plays a key role in the pathogenesis of neuropathic pain and that pharmacological inhibitors of MARKs such as PCC0105003 could represent a novel therapeutic strategy for the management of neuropathic pain.

From Immunity to Neurogenesis: Toll-like Receptors as Versatile Regulators in the Nervous System

Toll-like receptors (TLRs) are among the main components of the innate immune system. They can detect conserved structures in microorganisms and molecules associated with stress and cellular damage. TLRs are expressed in resident immune cells and both neurons and glial cells of the nervous system. Increasing evidence is emerging on the participation of TLRs not only in the immune response but also in processes of the nervous system, such as neurogenesis and cognition. Below, we present a review of the literature that evaluates the expression and role of TLRs in processes such as neurodevelopment, behavior, cognition, infection, neuroinflammation, and neurodegeneration.

Molecular evolution of Toll-like receptors in rodents

Toll-like receptors (TLRs), the key sensor molecules in vertebrates, trigger the innate immunity and prime the adaptive immune system. The TLR family of rodents, the largest order of mammals, typically contains 13 TLR genes. However, a clear picture of the evolution of the rodent TLR family has not yet emerged and the TLR evolutionary patterns are unclear in rodent clades. Here, we analyzed the natural variation and the evolutionary processes acting on the TLR family in rodents at both the interspecific and population levels. Our results showed that rodent TLRs were dominated by purifying selection, but a series of positively selected sites (PSSs) primarily located in the ligand-binding domain was also identified. The numbers of PSSs differed among TLRs, and nonviral-sensing TLRs had more PSSs than those in viral-sensing TLRs. Gene-conversion events were found between TLR1 and TLR6 in most rodent species. Population genetic analyses showed that TLR2, TLR8, and TLR12 were under positive selection in Rattus norvegicus and R. tanezumi, whereas positive selection also acted on TLR5 and TLR9 in the former species, as well as TLR1 and TLR7 in the latter species. Moreover, we found that the proportion of polymorphisms with potentially functional change was much lower in viral-sensing TLRs than in nonviral-sensing TLRs in both of these rat species. Our findings revealed the first thorough insight into the evolution of the rodent TLR genetic variability and provided important novel insights into the evolutionary history of TLRs over long and short timescales.

Motor, Cognitive, and Behavioral Impairment in TLR3 and TLR9 Deficient Male Mice: Insights into the Non-Immunological Roles of Toll-Like Receptors

BACKGROUND

Toll-like receptors (TLRs) play a critical role in initiating the innate immune response to infection or injury. Recent studies have uncovered their intriguing functions as moonlighting proteins involved in various biological processes, including development, learning, and memory. However, the specific functions of individual TLRs are still largely unknown.

AIMS

We investigated the effects of TLR3 and TLR9 receptor deficiency on motor, cognitive, and behavioral functions during development using genetically modified male mice of different ages.

METHODS

We evaluated the motor coordination, anxiety-like behavior, spatial learning, and working memory of male mice lacking the TLR3 and TLR9 genes at different ages (two, four, six, and eight months) using the rotarod, open field, water maze, and T-maze tests.

RESULTS

We observed that the deletion of either TLR3 or TLR9 resulted in impaired motor performance. Furthermore, young TLR3-deficient mice exhibited reduced anxiety-like behavior and spatial learning deficits; however, their working memory was unaffected. In contrast, young TLR9-knockout mice showed hyperactivity and a tendency toward decreased working memory.

CONCLUSIONS

These findings provide valuable insights into the broader roles of the TLR system beyond the innate immune response, revealing its involvement in pathways associated with the central nervous system. Importantly, our results establish a strong association between the endosomal receptors TLR3 and TLR9 and the performance of motor, cognitive, and behavioral tasks that change over time. This study contributes to the growing body of research on the multifaceted functions of TLRs and enhances our understanding of their participation in non-immune-related processes.

Neuronal toll like receptor 9 contributes to complete Freund’s adjuvant-induced inflammatory pain in mice

Toll like receptor 9 (TLR9) is a critical sensor for danger-associated molecular patterns (DAMPs) and a crucial marker of non-sterile/sterile inflammation among all TLRs. However, the significance of TLR9 in inflammatory pain remains unclear. Here, we subcutaneously injected Complete Freund’s adjuvant (CFA) into the plantar surface of the hind paw, to established a mouse model of inflammatory pain, and we examined expression and distribution of TLR9 in this model. There was a significant increase of TLR9 mRNA and reduction of mechanical paw withdrawal threshold in mice intraplantar injected with CFA. By contrast, mechanical paw withdrawal threshold significantly increased in mice treated with TLR9 antagonist ODN2088. Furthermore, TLR9 is found predominantly distributed in the neurons by immunofluorescence experiment. Accordingly, neuronal TLR9 downregulation in the spinal cord prevented CFA-induced persistent hyperalgesia. Overall, these findings indicate that neuronal TLR9 in the spinal cord is closely related to CFA-induced inflammatory pain. It provides a potential treatment option for CFA-induced inflammatory pain by applying TLR9 antagonist.

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.

Efficient roles of miR-146a in cellular and molecular mechanisms of neuroinflammatory disorders: An effectual review in neuroimmunology

Known as one of the most sophisticated systems of the human body, the nervous system consists of neural cells and controls all parts of the body. It is closely related to the immune system. The effects of inflammation and immune reactions have been observed in the pathogenesis of some neurological disorders. Defined as the gene expression regulators, miRNAs participate in cellular processes. miR-146a is a mediator in the neuroimmune system, leaving substantial effects on the homeostasis of immune and brain cells, neuronal identities acquisition, and immune responses regulation in the nervous system. Its positive efficiency has been proven in modulating inflammatory reactions, hemorrhagic complications, and pain. Moreover, the miR-146a targets play a key role in the pathogenesis of these illnesses. Based on the performance of its targets, miR-146a can have various effects on the disease progress. The abnormal expression/function of miR-146a has been reported in neuroinflammatory disorders. There is research evidence that this molecule qualifies as a desirable biomarker for some disorders and can even be a therapeutic target. This study aims to provide a meticulous review regarding the roles of miR-146a in the pathogenesis and progression of several neuroinflammatory disorders such as multiple sclerosis, amyotrophic lateral sclerosis, Alzheimer's disease, temporal lobe epilepsy, ischemic stroke, etc. The study also considers its eligibility for use as an ideal biomarker and therapeutic target in these diseases. The awareness of these mechanisms can facilitate the disease management/treatment, lead to patients' amelioration, improve the quality of life, and mitigate the risk of death.

Innate immunity at the crossroads of healthy brain maturation and neurodevelopmental disorders

The immune and nervous systems have unique developmental trajectories that individually build intricate networks of cells with highly specialized functions. These two systems have extensive mechanistic overlap and frequently coordinate to accomplish the proper growth and maturation of an organism. Brain resident innate immune cells - microglia - have the capacity to sculpt neural circuitry and coordinate copious and diverse neurodevelopmental processes. Moreover, many immune cells and immune-related signalling molecules are found in the developing nervous system and contribute to healthy neurodevelopment. In particular, many components of the innate immune system, including Toll-like receptors, cytokines, inflammasomes and phagocytic signals, are critical contributors to healthy brain development. Accordingly, dysfunction in innate immune signalling pathways has been functionally linked to many neurodevelopmental disorders, including autism and schizophrenia. This review discusses the essential roles of microglia and innate immune signalling in the assembly and maintenance of a properly functioning nervous system.

Association Between Magnetic Resonance Imaging-Based Spinal Morphometry and Sensorimotor Behavior in a Hemicontusion Model of Incomplete Cervical Spinal Cord Injury in Rats

Aim: Structural connectivity in the reorganizing spinal cord after injury dictates functional connectivity and hence the neurological outcome. As magnetic resonance imaging (MRI)-based structural parameters are mostly accessible across spinal cord injury (SCI) patients, we studied MRI-based spinal morphological changes and their relationship to neurological outcome in the rat model of cervical SCI. Introduction: Functional connectivity assessments on patients with SCI rely heavily on MRI-based approaches to investigate the complete neural axis (both spinal cord and brain). Hence, underlying MRI-based structural and morphometric changes in the reorganizing spinal cord and their relationship to neurological outcomes is crucial for meaningful interpretation of functional connectivity changes across the neural axis. Methods: Young adult rats, aged 1.5 months, underwent a precise mechanical impact hemicontusion incomplete cervical SCI at the C4/C5 level, after which sensorimotor behavioral assessments were tracked during the reorganization period of 1-6 weeks, followed by MRI of the cervical spinal cord at 8 weeks after SCI. Results: A significant ipsilesional forelimb motor debilitation was observed from 1 to 6 weeks after injury. Heat sensitivity testing (Hargreaves) showed ipsilesional forelimb hypersensitivity at 5 and 6 weeks after SCI. MRI of the cervical spine showed ipsilateral T1- and T2-weighted lesions across all SCI rats compared with no significant lesions in sham rats. Morphometric assessments of the lesional and nonlesional changes showed the diverse nature of their interindividual variability in the SCI receiving rats. While the various T1 and T2 MRI lesional volumes associated weakly or moderately with neurological outcome, the nonlesional spinal morphometric changes associated much more strongly. The results have important implications for interpreting functional MRI-based functional connectivity after SCI by providing vital underlying structural changes and their relative neurological impact. Impact statement Functional connectivity assessments on patients with SCI relies heavily upon MRI based approaches. Hence, underlying MRI based structural and morphometric changes in the reorganizing spinal cord and its relationship to neurological outcomes is vital for meaningful interpretation of functional connectivity changes across the complete neural axis (both spinal cord and the brain).

PCC0208009, an indirect IDO1 inhibitor, alleviates neuropathic pain and co-morbidities by regulating synaptic plasticity of ACC and amygdala

BACKGROUND AND PURPOSE

Indoleamine 2, 3-dioxygenase 1 (IDO1) has been linked to neuropathic pain and IDO1 inhibitors have been shown to reduce pain in animals. Some studies have indicated that IDO1 expression increased after neuropathic pain in hippocampus and spinal cord, whether these changes existing in anterior cingulate cortex (ACC) and amygdala remains obscure and how IDO1 inhibition leads to analgesia is largely unknown. Here, we evaluated the antinociceptive effect of PCC0208009, an indirect IDO1 inhibitor, on neuropathic pain and examined the related neurobiological mechanisms.

EXPERIMENTAL APPROACH

The effects of PCC0208009 on pain, cognition and anxiogenic behaviors were evaluated in a rat model of neuropathic pain. Motor disorder, sedation and somnolence were also assessed. Biochemical techniques were used to measure IDO1-mediated signaling changes in ACC and amygdala.

KEY RESULTS

In rats receiving spinal nerve ligation (SNL), IDO1 expression level was increased in ACC and amygdala. PCC0208009 attenuated pain-related behaviors in the formalin test and SNL model and increased cognition and anxiogenic behaviors in SNL rats at doses that did not affect locomotor activity and sleeping. PCC0208009 inhibited IDO1 expression in ACC and amygdala by inhibiting the IL-6-JAK2/STAT3-IDO1-GCN2-IL-6 pathway. In addition, PCC0208009 reversed synaptic plasticity at the functional and structural levels by suppressing NMDA2B receptor and CDK5/MAP2 or CDK5/Tau pathway in ACC and amygdala.

CONCLUSION AND IMPLICATIONS

These results support the role of IDO1-mediated molecular mechanisms in neuropathic pain and suggest that the IDO1 inhibitor PCC0208009 demonstrates selective pain suppression and could be a useful pharmacological therapy for neuropathic pain.

Astroglial TLR9 antagonism promotes chemotaxis and alternative activation of macrophages via modulation of astrocyte-derived signals: implications for spinal cord injury

BACKGROUND

The recruitment of immune system cells into the central nervous system (CNS) has a profound effect on the outcomes of injury and disease. Glia-derived chemoattractants, including chemokines, play a pivotal role in this process. In addition, cytokines and chemokines influence the phenotype of infiltrating immune cells. Depending on the stimuli present in the local milieu, infiltrating macrophages acquire the classically activated M1 or alternatively activated M2 phenotypes. The polarization of macrophages into detrimental M1 versus beneficial M2 phenotypes significantly influences CNS pathophysiology. Earlier studies indicated that a toll-like receptor 9 (TLR9) antagonist modulates astrocyte-derived cytokine and chemokine release. However, it is not known whether these molecular changes affect astrocyte-induced chemotaxis and polarization of macrophages. The present studies were undertaken to address these issues.

METHODS

The chemotaxis and polarization of mouse peritoneal macrophages by spinal cord astrocytes were evaluated in a Transwell co-culture system. Arrays and ELISA were utilized to quantify chemokines in the conditioned medium (CM) of pure astrocyte cultures. Immunostaining for M1- and M2-specific markers characterized the macrophage phenotype. The percentage of M2 macrophages at the glial scar was determined by stereological approaches in mice sustaining a mid-thoracic spinal cord contusion injury (SCI) and intrathecally treated with oligodeoxynucleotide 2088 (ODN 2088), the TLR9 antagonist. Statistical analyses used two-tailed independent-sample t-test and one-way analysis of variance (ANOVA) followed by Tukey's post hoc test. A p value < 0.05 was considered to be statistically significant.

RESULTS

ODN 2088-treated astrocytes significantly increased the chemotaxis of peritoneal macrophages via release of chemokine (C-C motif) ligand 1 (CCL1). Vehicle-treated astrocytes polarized macrophages into the M2 phenotype and ODN 2088-treated astrocytes promoted further M2 polarization. Reduced CCL2 and CCL9 release by astrocytes in response to ODN 2088 facilitated the acquisition of the M2 phenotype, suggesting that CCL2 and CCL9 are negative regulators of M2 polarization. The percentage of M2 macrophages at the glial scar was higher in mice sustaining a SCI and receiving ODN 2088 treatment as compared to vehicle-treated injured controls.

CONCLUSIONS

TLR9 antagonism could create a favorable environment during SCI by supporting M2 macrophage polarization and chemotaxis via modulation of astrocyte-to-macrophage signals.

GABAAR α2-activated neuroimmune signal controls binge drinking and impulsivity through regulation of the CCL2/CX3CL1 balance

BACKGROUND AND PURPOSE

Toll-like receptors (TLRs) are a family of innate immune system receptors that respond to pathogen-derived and tissue damage-related ligands and are increasingly recognized for their impact on homeostasis and its dysregulation in the nervous system. TLR signaling participates in brain injury and addiction, but its role in the alcohol-seeking behavior, which initiates alcohol drinking, is still poorly understood. In this review, we discuss our findings designed to elucidate the potential contribution of the activated TLR4 signal located in neurons, on impulsivity and the predisposition to initiate alcohol drinking (binge drinking).

RESULTS

Our findings indicate that the TLR4 signal is innately activated in neurons from alcohol-preferring subjects, identifying a genetic contribution to the regulation of impulsivity and the alcohol-seeking propensity. Signal activation is through the non-canonical, previously unknown, binding of TLR4 to the α2 subunit of the γ-aminobutyric 2 acid A receptor (GABAAR α2). Activation is sustained by the stress hormone corticotrophin-releasing factor (CRF) and additional still poorly recognized ligand/scaffold proteins. Focus is on the effect of TLR4 signal activation on the balance between pro- and anti-inflammatory chemokines [chemokine (C-C motif) ligand 2 (CCL2)/chemokine (C-X3-C motif) ligand 1 (CX3CL1)] and its effect on binge drinking.

CONCLUSION

The results are discussed within the context of current findings on the distinct activation and functions of TLR signals located in neurons, as opposed to immune cells. They indicate that the balance between pro- and anti-inflammatory TLR4 signaling plays a major role in binge drinking. These findings have major impact on future basic and translational research, including the development of potential therapeutic and preventative strategies.

Role of the Immune System in the Development of the Central Nervous System

The central nervous system (CNS) and the immune system are both intricate and highly organized systems that regulate the entire body, with both sharing certain common features in developmental mechanisms and operational modes. It is known that innate immunity-related molecules, such as cytokines, toll-like receptors, the complement family, and acquired immunity-related molecules, such as the major histocompatibility complex and antibody receptors, are also expressed in the brain and play important roles in brain development. Moreover, although the brain has previously been regarded as an immune-privileged site, it is known to contain lymphatic vessels. Not only microglia but also lymphocytes regulate cognition and play a vital role in the formation of neuronal circuits. This review provides an overview of the function of immune cells and immune molecules in the CNS, with particular emphasis on their effect on neural developmental processes.

Tlr7 deletion alters expression profiles of genes related to neural function and regulates mouse behaviors and contextual memory

The neuronal innate immune system recognizes endogenous danger signals and regulates neuronal development and function. Toll-like receptor 7 (TLR7), one of the TLRs that trigger innate immune responses in neurons, controls neuronal morphology. To further assess the function of TLR7 in the brain, we applied next generation sequencing to investigate the effect of Tlr7 deletion on gene expression in hippocampal and cortical mixed cultures and on mouse behaviors. Since previous in vivo study suggested that TLR7 is more critical for neuronal morphology at earlier developmental stages, we analyzed two time-points (4 and 18 DIV) to represent young and mature neurons, respectively. At 4 DIV, Tlr7 KO neurons exhibited reduced expression of genes involved in neuronal development, synaptic organization and activity and behaviors. Some of these Tlr7-regulated genes are also associated with multiple neurological and neuropsychiatric diseases. TLR7-regulated transcriptomic profiles differed at 18 DIV. Apart from neuronal genes, genes related to glial cell development and differentiation became sensitive to Tlr7 deletion at 18 DIV. Moreover, Tlr7 KO mice exhibited altered behaviors in terms of anxiety, aggression, olfaction and contextual fear memory. Electrophysiological analysis further showed an impairment of long-term potentiation in Tlr7 KO hippocampus. Taken together, these results indicate that TLR7 regulates neural development and brain function, even in the absence of infectious or pathogenic molecules. Our findings strengthen evidence for the role of the neuronal innate immune system in fine-tuning neuronal morphology and activity and implicate it in neuropsychiatric disorders.

Endosomal TLR3, TLR7, and TLR8 control neuronal morphology through different transcriptional programs

Neuroinflammation is associated with diverse neurological disorders. Endosomal Toll-like receptors (TLRs) including TLR3, TLR7, and TLR8 cell-autonomously regulate neuronal differentiation. However, the mechanisms by which these three TLRs affect neuronal morphology are unclear. In this study, we compare these TLRs in mouse neurons. By combining in vitro neuronal cultures, in utero electroporation, and transcriptomic profiling, we show that TLR8, TLR7, and TLR3 promote dendritic pruning via MYD88 signaling. However, they induce different transcriptomic profiles related to innate immunity, signaling, and neuronal development. The temporal expression patterns and the effects on neuronal morphology are not identical upon activation of these endosomal TLRs. Pathway analyses and in vitro studies specifically implicate mitogen-activated protein kinase signaling in TLR8-mediated dendritic pruning. We further show that TLR8 is more critical for dendritic arborization at a late development stage in vivo. The activation of TLR8, TLR7, or TLR3 results in dendritic shortening, and TLR7 and TLR3 but not TLR8 also control axonal growth. In-depth transcriptomic analyses show that TLRs use different downstream pathways to control neuronal morphology, which may contribute to neuronal development and pathological responses.

Reprint of: Microglial toll-like receptors and Alzheimer's disease

Microglial activation represents an important pathological hallmark of Alzheimer's disease (AD), and emerging data highlight the involvement of microglial toll-like receptors (TLRs) in the course of AD. TLRs have been observed to exert both beneficial and detrimental effects on AD-related pathologies, and transgenic animal models have provided direct and credible evidence for an association between TLRs and AD. Moreover, analyses of genetic polymorphisms have suggested interactions between genetic polymorphisms in TLRs and AD risk, further supporting the hypothesis that TLRs are involved in AD. In this review, we summarize the key evidence in this field. Future studies should focus on exploring the mechanisms underlying the potential roles of TLRs in AD.

Synaptic abnormalities of mice lacking toll-like receptor (TLR)-9

Motor, sensory, and autonomic abnormalities are reported for toll-like receptor 9 (TLR9) knock-out (KO) mice. However, a physiological role of TLR9 in the nervous system is largely unknown. Since altered synaptic transmission can contribute to sensory and motor abnormalities, we evaluated neuromuscular junction (NMJ) function and morphology of TLR9 KO mice. Triangularis sterni nerve-muscle preparations were dissected from TLR9 KO and age-matched control mice. Two-electrode voltage clamp of the motor endplate revealed that the amplitude and frequency of miniature end plate currents (mEPCs) for TLR9 KO NMJs were significantly greater than control. In contrast, mean endplate current (EPC, 1Hz) amplitude was equivalent to control. The ratio of mean EPC to mean mEPC amplitude indicated a decline of quantal content (m) for TLR9 KO NMJs. Furthermore, m declined more rapidly than control in response to 50-Hz stimulus trains. A rightward shift of the mEPC amplitude distribution suggested formation of vesicles containing larger amounts of acetylcholine (ACh). Staining with rhodamine α-bungarotoxin revealed a significant decline of endplate size in TLR9 KO mice. This alteration may result from ACh-induced decline of acetylcholine receptor (AChR) expression resulting from increased frequency and amplitude of mEPCs. At the same time, excessive spontaneous vesicular ACh release may initiate retrograde suppression of excitation-secretion coupling. These data suggest a novel role of TLR9 in the development of the NMJ.

Microglial toll-like receptors and Alzheimer's disease

Microglial activation represents an important pathological hallmark of Alzheimer's disease (AD), and emerging data highlight the involvement of microglial toll-like receptors (TLRs) in the course of AD. TLRs have been observed to exert both beneficial and detrimental effects on AD-related pathologies, and transgenic animal models have provided direct and credible evidence for an association between TLRs and AD. Moreover, analyses of genetic polymorphisms have suggested interactions between genetic polymorphisms in TLRs and AD risk, further supporting the hypothesis that TLRs are involved in AD. In this review, we summarize the key evidence in this field. Future studies should focus on exploring the mechanisms underlying the potential roles of TLRs in AD.

Molecular and cellular mechanisms that initiate pain and itch

Somatosensory neurons mediate our sense of touch. They are critically involved in transducing pain and itch sensations under physiological and pathological conditions, along with other skin-resident cells. Tissue damage and inflammation can produce a localized or systemic sensitization of our senses of pain and itch, which can facilitate our detection of threats in the environment. Although acute pain and itch protect us from further damage, persistent pain and itch are debilitating. Recent exciting discoveries have significantly advanced our knowledge of the roles of membrane-bound G protein-coupled receptors and ion channels in the encoding of information leading to pain and itch sensations. This review focuses on molecular and cellular events that are important in early stages of the biological processing that culminates in our senses of pain and itch.

Innate immune responses regulate morphogenesis and degeneration: roles of Toll-like receptors and Sarm1 in neurons

The central nervous system is recognized as an immunoprivileged site because peripheral immune cells do not typically enter it. Microglial cells are thought to be the main immune cells in brain. However, recent reports have indicated that neurons express the key players of innate immunity, including Toll-like receptors (TLRs) and their adaptor proteins (Sarm1, Myd88, and Trif), and may produce cytokines in response to pathogen infection. In the absence of an immune challenge, neuronal TLRs can detect intrinsic danger signals and modulate neuronal morphology and function. In this article, we review the recent findings on the involvement of TLRs and Sarm1 in controlling neuronal morphogenesis and neurodegeneration. Abnormal behaviors in TLR- and Sarm1-deficient mice are also discussed.

The role of immune mechanisms in Tourette syndrome

Tourette syndrome (TS) is a childhood-onset tic disorder associated with abnormal development of brain networks involved in the sensory and motor processing. An involvement of immune mechanisms in its pathophysiology has been proposed. Animal models based on active immunization with bacterial or viral mimics, direct injection of cytokines or patients' serum anti-neuronal antibodies, and transgenic approaches replicated stereotyped behaviors observed in human TS. A crucial role of microglia in the neural-immune crosstalk within TS and related disorders has been proposed by animal models and confirmed by recent post mortem studies. With analogy to autism, genetic and early life environmental factors could foster the involvement of immune mechanisms to the abnormal developmental trajectories postulated in TS, as well as lead to systemic immune dysregulation in this condition. Clinical studies demonstrate an association between TS and immune responses to pathogens like group A Streptococcus (GAS), although their role as risk-modifiers is still undefined. Overactivity of immune responses at a systemic level is suggested by clinical studies exploring cytokine and immunoglobulin levels, immune cell subpopulations, and gene expression profiling of peripheral lymphocytes. The involvement of autoantibodies, on the other hand, remains uncertain and warrants more work using live cell-based approaches. Overall, a body of evidence supports the hypothesis that disease mechanisms in TS, like other neurodevelopmental illnesses (e.g. autism), may involve dysfunctional neural-immune cross-talk, ultimately leading to altered maturation of brain pathways controlling different behavioral domains and, possibly, differences in organising immune and stress responses. This article is part of a Special Issue entitled SI: Neuroimmunology in Health And Disease.