The serotonin 2B receptor is required in neonatal microglia to limit neuroinflammation and sickness behavior in adulthood

Neuro-immune interactions in health and disease: Insights from FENS-Hertie 2022 Winter School

In a great partnership, the Federation of European Neuroscience Societies (FENS) and the Hertie Foundation organized the FENS-Hertie 2022 Winter School on 'Neuro-immune interactions in health and disease'. The school selected 27 PhD students and 13 postdoctoral fellows from 20 countries and involved 14 faculty members experts in the field. The Winter School focused on a rising field of research, the interactions between the nervous and both innate and adaptive immune systems under pathological and physiological conditions. A fine-tuned neuro-immune crosstalk is fundamental for healthy development, while disrupted neuro-immune communication might play a role in neurodegeneration, neuroinflammation and aging. However, much is yet to be understood about the underlying mechanisms of these neuro-immune interactions in the healthy brain and under pathological scenarios. In addition to new findings in this emerging field, novel methodologies and animal models were presented to foment research on neuro-immunology. The FENS-Hertie 2022 Winter School provided an insightful knowledge exchange between students and faculty focusing on the latest discoveries in the biology of neuro-immune interactions while fostering great academic and professional opportunities for early-career neuroscientists from around the world.

Role of Serotonergic System in Regulating Brain Tumor-Associated Neuroinflammatory Responses

Serotonin signaling regulates wide arrays of both neural and extra-neural functions. Serotonin is also found to affect cancer progression directly as well as indirectly by modulating the immune cells. In the brain, serotonin plays a key role in regulating various functions; disturbance of the normal activities of serotonin leads to various mental illnesses, including the neuroinflammatory response in the central nervous system (CNS). The neuroinflammatory response can be initiated in various psychological illnesses and brain cancer. Serotonergic signaling can impact the functions of both glial as well as the immune cells. It can also affect the tumor immune microenvironment and the inflammatory response associated with brain cancers. Apart from this, many drugs used for treatment of psychological illness are known to modulate serotonergic system and can cross the blood-brain barrier. Understanding the role of serotonergic pathways in regulating neuroinflammatory response and brain cancer will provide a new paradigm in modulating the serotonergic components in treating brain cancer and associated inflammation-induced brain damages.

Limited contribution of the of P2X4 receptor to LPS-induced microglial reaction in mice

Sepsis is life-threatening condition that can trigger long-term neurological sequelae, including cognitive impairment in survivors. The pathogenesis of the so-called sickness behavior is poorly understood, but sepsis-driven neuroinflammation is thought to play a key role. Microglia are the central nervous system resident immune cells and play major roles in the induction and the control of neuroinflammatory processes. Accordingly, we recently demonstrated important microglia reaction, characterized by dramatic microglia transcriptome remodeling, in an experimental model of sepsis. Interfering with microglia pathways thus represents an interesting opportunity to tune microglia reaction towards beneficial outcomes. Purinergic signaling is central to microglia biology and controls key microglia functions. In particular, P2X4 receptors, which are highly permeable to calcium and de novo expressed in reactive microglia, seem to be an interesting target to modulate microglia reaction. Here, we investigated the impact of P2X4 receptors on the LPS-driven microglia transcriptome remodeling. Although we used complementary and sensitive biostatistical approaches, we did not measure significant impact of P2X4 deficiency onto microglia transcriptome either in homeostatic nor reactive condition. Overall, our results revealed that microglia reaction elicited by LPS-mediated sepsis is P2X4 independent and highlights the functional diversity of microglia reaction. These results also promote for the search of disease-specific targets to tune microglia reaction towards beneficial outcomes.

Wiring and Volume Transmission: An Overview of the Dual Modality for Serotonin Neurotransmission

Serotonin is a neurotransmitter involved in the modulation of a multitude of physiological and behavioral processes. In spite of the relatively reduced number of serotonin-producing neurons present in the mammalian CNS, a complex long-range projection system provides profuse innervation to the whole brain. Heterogeneity of serotonin receptors, grouped in seven families, and their spatiotemporal expression pattern account for its widespread impact. Although neuronal communication occurs primarily at tiny gaps called synapses, wiring transmission, another mechanism based on extrasynaptic diffusion of neuroactive molecules and referred to as volume transmission, has been described. While wiring transmission is a rapid and specific one-to-one modality of communication, volume transmission is a broader and slower mode in which a single element can simultaneously act on several different targets in a one-to-many mode. Some experimental evidence regarding ultrastructural features, extrasynaptic localization of receptors and transporters, and serotonin-glia interactions collected over the past four decades supports the existence of a serotonergic system of a dual modality of neurotransmission, in which wiring and volume transmission coexist. To date, in spite of the radical difference in the two modalities, limited information is available on the way they are coordinated to mediate the specific activities in which serotonin participates. Understanding how wiring and volume transmission modalities contribute to serotonergic neurotransmission is of utmost relevance for the comprehension of serotonin functions in both physiological and pathological conditions.

TIAM2S-positive microglia enhance inflammation and neurotoxicity through soluble ICAM-1-mediated immune priming

TIAM Rac1-associated GEF 2 short form (TIAM2S) as an oncoprotein alters the immunity of peripheral immune cells to construct an inflammatory tumor microenvironment. However, its role in the activation of microglia, the primary innate immune cells of the brain, and neuroinflammation remains unknown. This study investigated the mechanism underlying TIAM2S shapes immune properties of microglia to facilitate neuron damage. Human microglial clone 3 cell line (HMC3) and human brain samples were applied to determine the presence of TIAM2S in microglia by western blots and double immunostaining. Furthermore, TIAM2S transgenic mice combined with multiple reconstituted primary neuron-glial culture systems and a cytokine array were performed to explore how TIAM2S shaped immune priming of microglia and participated in lipopolysaccharide (LPS)-induced neuron damage. TIAM2S protein was detectable in HMC3 cells and presented in a small portion (~11.1%) of microglia in human brains referred to as TIAM2S-positive microglia. With the property of secreted soluble factor-mediated immune priming, TIAM2S-positive microglia enhanced LPS-induced neuroinflammation and neural damage in vivo and in vitro. The gain- and loss-of-function experiments showed soluble intercellular adhesion molecule-1 (sICAM-1) participated in neurotoxic immune priming of TIAM2S+ microglia. Together, this study demonstrated a novel TIAM2S-positive microglia subpopulation enhances inflammation and neurotoxicity through sICAM-1-mediated immune priming.

Serotonin sensing by microglia conditions the proper development of neuronal circuits and of social and adaptive skills

The proper maturation of emotional and sensory circuits requires fine-tuning of serotonin (5-HT) level during early postnatal development. Consistently, dysfunctions of the serotonergic system have been associated with neurodevelopmental psychiatric diseases, including autism spectrum disorders (ASD). However, the mechanisms underlying the developmental effects of 5-HT remain partially unknown, one obstacle being the action of 5-HT on different cell types. Here, we focused on microglia, which play a role in brain wiring refinement, and we investigated whether the control of these cells by 5-HT is relevant for neurodevelopment and spontaneous behaviors in mice. Since the main 5-HT sensor in microglia is the 5-HT2B receptor subtype, we prevented 5-HT signaling specifically in microglia by conditional invalidation of the Htr2b gene in these cells. We observed that abrogating the serotonergic control of microglia during early postnatal development affects the phagolysosomal compartment of these cells and their proximity to dendritic spines and perturbs neuronal circuits maturation. Furthermore, this early ablation of microglial 5-HT2B receptors leads to adult hyperactivity in a novel environment and behavioral defects in sociability and flexibility. Importantly, we show that these behavioral alterations result from a developmental effect, since they are not observed when microglial Htr2b invalidation is induced later, at P30 onward. Thus, a primary alteration of 5-HT sensing in microglia, during a critical time window between birth and P30, is sufficient to impair social and flexibility skills. This link between 5-HT and microglia may explain the association between serotonergic dysfunctions and behavioral traits like impaired sociability and inadaptability to novelty, which are prominent in psychiatric disorders such as ASD.

The Missing Piece? A Case for Microglia's Prominent Role in the Therapeutic Action of Anesthetics, Ketamine, and Psychedelics

There is much excitement surrounding recent research of promising, mechanistically novel psychotherapeutics - psychedelic, anesthetic, and dissociative agents - as they have demonstrated surprising efficacy in treating central nervous system (CNS) disorders, such as mood disorders and addiction. However, the mechanisms by which these drugs provide such profound psychological benefits are still to be fully elucidated. Microglia, the CNS's resident innate immune cells, are emerging as a cellular target for psychiatric disorders because of their critical role in regulating neuroplasticity and the inflammatory environment of the brain. The following paper is a review of recent literature surrounding these neuropharmacological therapies and their demonstrated or hypothesized interactions with microglia. Through investigating the mechanism of action of psychedelics, such as psilocybin and lysergic acid diethylamide, ketamine, and propofol, we demonstrate a largely under-investigated role for microglia in much of the emerging research surrounding these pharmacological agents. Among others, we detail sigma-1 receptors, serotonergic and γ-aminobutyric acid signalling, and tryptophan metabolism as pathways through which these agents modulate microglial phagocytic activity and inflammatory mediator release, inducing their therapeutic effects. The current review includes a discussion on future directions in the field of microglial pharmacology and covers bidirectional implications of microglia and these novel pharmacological agents in aging and age-related disease, glial cell heterogeneity, and state-of-the-art methodologies in microglial research.

Targeting the brain 5-HT7 receptor to prevent hypomyelination in a rodent model of perinatal white matter injuries

Approximately 15 million babies are born prematurely every year and many will face lifetime motor and/or cognitive deficits. Children born prematurely are at higher risk of developing perinatal brain lesions, especially white matter injuries (WMI). Evidence in humans and rodents demonstrates that systemic inflammation-induced neuroinflammation, including microglial and astrocyte reactivity, is the prominent processes of WMI associated with preterm birth. Thus, a new challenge in the field of perinatal brain injuries is to develop new neuroprotective strategies to target neuroinflammation to prevent WMI. Serotonin (5-HT) and its receptors play an important role in inflammation, and emerging evidence indicates that 5-HT may regulate brain inflammation by the modulation of microglial reactivity and astrocyte functions. The present study is based on a mouse model of WMI induced by intraperitoneal (i.p.) injections of IL-1β during the first 5 days of life. In this model, certain key lesions of preterm brain injuries can be summarized by (i) systemic inflammation, (ii) pro-inflammatory microglial and astrocyte activation, and (iii) inhibition of oligodendrocyte maturation, leading to hypomyelination. We demonstrate that Htr7 mRNA (coding for the HTR7/5-HT7 receptor) is significantly overexpressed in the anterior cortex of IL-1β-exposed animals, suggesting it as a potential therapeutic target. LP-211 is a specific high-affinity HTR7 agonist that crosses the blood-brain barrier (BBB). When co-injected with IL-1β, LP-211 treatment prevented glial reactivity, the down-regulation of myelin-associated proteins, and the apparition of anxiety-like phenotypes. Thus, HTR7 may represent an innovative therapeutic target to protect the developing brain from preterm brain injuries.

The Role of Oxytocin in Abnormal Brain Development: Effect on Glial Cells and Neuroinflammation

The neonatal period is critical for brain development and determinant for long-term brain trajectory. Yet, this time concurs with a sensitivity and risk for numerous brain injuries following perinatal complications such as preterm birth. Brain injury in premature infants leads to a complex amalgam of primary destructive diseases and secondary maturational and trophic disturbances and, as a consequence, to long-term neurocognitive and behavioral problems. Neuroinflammation is an important common factor in these complications, which contributes to the adverse effects on brain development. Mediating this inflammatory response forms a key therapeutic target in protecting the vulnerable developing brain when complications arise. The neuropeptide oxytocin (OT) plays an important role in the perinatal period, and its importance for lactation and social bonding in early life are well-recognized. Yet, novel functions of OT for the developing brain are increasingly emerging. In particular, OT seems able to modulate glial activity in neuroinflammatory states, but the exact mechanisms underlying this connection are largely unknown. The current review provides an overview of the oxytocinergic system and its early life development across rodent and human. Moreover, we cover the most up-to-date understanding of the role of OT in neonatal brain development and the potential neuroprotective effects it holds when adverse neural events arise in association with neuroinflammation. A detailed assessment of the underlying mechanisms between OT treatment and astrocyte and microglia reactivity is given, as well as a focus on the amygdala, a brain region of crucial importance for socio-emotional behavior, particularly in infants born preterm.

Maternal diet disrupts the placenta-brain axis in a sex-specific manner

High maternal weight is associated with detrimental outcomes in offspring, including increased susceptibility to neurological disorders such as anxiety, depression and communicative disorders. Despite widespread acknowledgement of sex biases in the development of these disorders, few studies have investigated potential sex-biased mechanisms underlying disorder susceptibility. Here, we show that a maternal high-fat diet causes endotoxin accumulation in fetal tissue, and subsequent perinatal inflammation contributes to sex-specific behavioural outcomes in offspring. In male offspring exposed to a maternal high-fat diet, increased macrophage Toll-like receptor 4 signalling results in excess microglial phagocytosis of serotonin (5-HT) neurons in the developing dorsal raphe nucleus, decreasing 5-HT bioavailability in the fetal and adult brains. Bulk sequencing from a large cohort of matched first-trimester human samples reveals sex-specific transcriptome-wide changes in placental and brain tissue in response to maternal triglyceride accumulation (a proxy for dietary fat content). Further, fetal brain 5-HT levels decrease as placental triglycerides increase in male mice and male human samples. These findings uncover a microglia-dependent mechanism through which maternal diet can impact offspring susceptibility for neuropsychiatric disorder development in a sex-specific manner.

Fluoxetine inhibited the activation of A1 reactive astrocyte in a mouse model of major depressive disorder through astrocytic 5-HT2BR/β-arrestin2 pathway

Background

Fluoxetine, a selective serotonin reuptake inhibitor, has been reported to directly bind with 5-HT2B receptor (5-HT2BR), but the precise mechanisms, whereby fluoxetine confers the anti-depressive actions via 5-HT2BR is not fully understood. Although neuroinflammation-induced A1 astrocytes are involved in neurodegenerative diseases, the role of A1 astrocyte in the pathogenesis and treatment of major depressive disorder (MDD) remains unclear.

Methods

Mice were subjected to chronic mild stress (CMS) for 6 weeks and subsequently treated with fluoxetine for 4 weeks. The depressive-like and anxiety-like behaviors and the activation of A1 reactive astrocyte in hippocampus and cortex of mice were measured. Primary astrocytes were stimulated with A1 cocktail (tumor necrosis factor (TNF)-α, interleukin (IL)-1α and C1q), activated (LPS) microglia-conditioned medium (MCM) or IL-6 for 24 h and the expression of A1-special and A2-special markers were determined using RT-qPCR and western blot. The role of 5-HT2BR in the effects of fluoxetine on A1 reactive astrocyte was measured using 5-HT2BR inhibitor and siRNA in vitro and AAVs in vivo. The functions of downstream signaling Gq protein and β-arrestins in the effects of fluoxetine on the activation of A1 astrocyte were determined using pharmacological inhibitor and genetic knockout, respectively.

Results

In this study, we found that fluoxetine inhibited the activation of A1 reactive astrocyte and reduced the abnormal behaviors in CMS mice, as well as ameliorated A1 astrocyte reactivity under three different stimulators in primary astrocytes. We also showed that astrocytic 5-HT2BR was required in the inhibitory effects of fluoxetine on A1 reactive astrocyte in MDD in vivo and in vitro. We further found that the functions of fluoxetine in the activation of A1 astrocyte were independent of either Gq protein or β-arrestin1 in vitro. β-arrestin2 pathway was the downstream signaling of astrocytic 5-HT2BR mediated the inhibitory effects of fluoxetine on A1 astrocyte reactivity in primary astrocytes and CMS mice, as well as the improved roles of fluoxetine in behavioral impairments of CMS mice.

Conclusions

These data demonstrate that fluoxetine restricts reactive A1 astrocyte via astrocytic 5-HT2BR/β-arrestin2 pathway in a mouse model of MDD and provide a novel therapeutic avenue for MDD.

The Combined Effects of Amyloidosis and Serotonin Deficiency by Tryptophan Hydroxylase-2 Knockout Impacts Viability of the APP/PS1 Mouse Model of Alzheimer’s Disease

Background: A decline of brain serotonin (5-HT) is held responsible for the changes in mood that can be observed in Alzheimer’s disease (AD). However, 5-HT’ergic signaling is also suggested to reduce the production of pathogenic amyloid-4β (Aβ). Objective: To investigate the effect of targeted inactivation of tryptophan hydroxylase-2 (Tph2), which is essential for neuronal 5-HT synthesis, on amyloidosis in amyloid precursor protein (APP)swe/presenilin 1 (PS1) ΔE9 transgenic mice. Methods: Triple-transgenic (3xTg) APP/PS1 mice with partial (+/-) or complete Tph2 knockout (–/–) were allowed to survive until 6 months old with APP/PS1, Tph2–/–, and wildtype mice. Survival and weight were recorded. Levels of Aβ 42/40/38, soluble APPα (sAβPPα) and sAβPPβ, and cytokines were analyzed by mesoscale, neurotransmitters by mass spectrometry, and gene expression by quantitative PCR. Tph2, microglia, and Aβ were visualized histologically. Results: Tph2 inactivation in APP/PS1 mice significantly reduced viability, without impacting soluble and insoluble Aβ 42 and Aβ 40 in neocortex and hippocampus, and with only mild changes of soluble Aβ 42/Aβ 40. However, sAβPPα and sAβPPβ in hippocampus and Aβ 38 and Aβ 40 in cerebrospinal fluid were reduced. 3xTg–/–mice were devoid of Tph2 immunopositive fibers and 5-HT. Cytokines were unaffected by genotype, as were neocortical TNF, HTR2a and HTR2b mRNA levels in Tph2–/– mice. Microglia clustered around Aβ plaques regardless of genotype. Conclusion: The results suggest that Tph2 inactivation influences AβPP processing, at least in the hippocampus, although levels of Aβ are unchanged. The reduced viability of 3xTg–/–mice could indicate that 5-HT protects against the seizures that can impact the viability of APP/PS1 mice.

5-hydroxytryptamine in migraine: The puzzling role of ionotropic 5-HT3 receptor in the context of established therapeutic effect of metabotropic 5-HT1 subtypes

5-hydroxytryptamine (5-HT; serotonin) is traditionally considered as a key mediator implicated in migraine. Multiple 5-HT receptor subtypes contribute to a variety of region-specific functional effects. The raphé nuclei control nociceptive inputs by releasing 5-HT in the brainstem, whereas dural mast cells provide the humoral source of 5-HT in the meninges. Triptans (5-HT_1B/D agonists) and ditans (5-HT_1F agonists) are the best established 5-HT anti-migraine agents. However, activation of meningeal afferents via ionotropic 5-HT₃ receptors results in long-lasting excitatory drive suggesting a pro-nociceptive role for these receptors in migraine. Nevertheless, clinical data do not clearly support the applicability of currently available 5-HT₃ antagonists to migraine treatment. The reasons for this might be the presence of 5-HT₃ receptors on inhibitory interneurons dampening the excitatory drive, a lack of 5-HT₃ A-E subunit-selective antagonists and gender/age-dependent effects. This review is focusing on the controversial role of 5-HT₃ receptors in migraine pathology and related pharmacological perspectives of 5-HT ligands.

Microglia Function on Precursor Cells in the Adult Hippocampus and Their Responsiveness to Serotonin Signaling

Microglia are the resident immune cells of the adult brain that become activated in response to pathogen- or damage-associated stimuli. The acute inflammatory response to injury, stress, or infection comprises the release of cytokines and phagocytosis of damaged cells. Accumulating evidence indicates chronic microglia-mediated inflammation in diseases of the central nervous system, most notably neurodegenerative disorders, that is associated with disease progression. To understand microglia function in pathology, knowledge of microglia communication with their surroundings during normal state and the release of neurotrophins and growth factors in order to maintain homeostasis of neural circuits is of importance. Recent evidence shows that microglia interact with serotonin, the neurotransmitter crucially involved in adult neurogenesis, and known for its role in antidepressant action. In this chapter, we illustrate how microglia contribute to neuroplasticity of the hippocampus and interact with local factors, e.g., BDNF, and external stimuli that promote neurogenesis. We summarize the recent findings on the role of various receptors in microglia-mediated neurotransmission and particularly focus on microglia’s response to serotonin signaling. We review microglia function in neuroinflammation and neurodegeneration and discuss their novel role in antidepressant mechanisms. This synopsis sheds light on microglia in healthy brain and pathology that involves serotonin and may be a potential therapeutic model by which microglia play a crucial role in the maintenance of mood.

Serotoninergic system targeting in multiple sclerosis: the prospective for pathogenetic therapy

Serotonin (5-hydroxytryptamine) (5-HT) is a neurotransmitter, which mediates neuropsychological functions of the central nervous system (CNS). Recent studies have shown the modulatory effect of 5-HT on gut microbiota functions, which play an essential role in developing CNS inflammatory diseases. Finally, 5-HT is a direct mediator of neuroimmune interaction. The article reviews the literature data on the role of 5-HT in the regulation of neuroinflammation in multiple sclerosis (MS). The influence of 5-HT and selective serotonin reuptake inhibitors (SSRIs) on experimental autoimmune encephalomyelitis (EAE) and MS pathogenesis, as well as the therapeutic potential of serotoninergic drugs as a pathogenetic therapy of MS, are discussed.

Structure, Function, and Pharmaceutical Ligands of 5-Hydroxytryptamine 2B Receptor

Since the first characterization of the 5-hydroxytryptamine 2B receptor (5-HT2BR) in 1992, significant progress has been made in 5-HT2BR research. Herein, we summarize the biological function, structure, and small-molecule pharmaceutical ligands of the 5-HT2BR. Emerging evidence has suggested that the 5-HT2BR is implicated in the regulation of the cardiovascular system, fibrosis disorders, cancer, the gastrointestinal (GI) tract, and the nervous system. Eight crystal complex structures of the 5-HT2BR bound with different ligands provided great insights into ligand recognition, activation mechanism, and biased signaling. Numerous 5-HT2BR antagonists have been discovered and developed, and several of them have advanced to clinical trials. It is expected that the novel 5-HT2BR antagonists with high potency and selectivity will lead to the development of first-in-class drugs in various therapeutic areas.