A dialogue between the immune system and brain, spoken in the language of serotonin

Finding the missing links among metabolites, microbes, and the host

The unexpected diversity of the human microbiome and metabolome far exceeds the complexity of the human genome. Although we now understand microbial taxonomic and genetic repertoires in some populations, we are just beginning to assemble the necessary computational and experimental tools to understand the metabolome in comparable detail. However, even with the limited current state of knowledge, individual connections between microbes and metabolites, between microbes and immune function, and between metabolites and immune function are being established. Here, we provide our perspective on these connections and outline a systematic research program that could turn these individual links into a broader network that allows us to understand how these components interact. This program will enable us to exploit connections among the microbiome, metabolome, and host immune system to maintain health and perhaps help us understand how to reverse the processes that lead to a wide range of immune and other diseases.

Serotonin 2A receptor gene (HTR2A) regulatory variants: possible association with severity of depression symptoms in children with autism spectrum disorder

OBJECTIVE AND BACKGROUND

Our aim was to characterize the association of 2 functional single nucleotide polymorphisms (rs6311 and rs6314) in the serotonin 2A receptor gene (HTR2A) with severity of depression symptoms in children with autism spectrum disorder. These polymorphisms have been shown to be associated with depression symptom severity and response to selective serotonin reuptake inhibitor drugs in adults with diagnosed depressive disorder.

METHODS

Parents of 104 children with autism spectrum disorder rated their children's depressive symptoms using a validated scale based on criteria from the Diagnostic and Statistical Manual of Mental Disorders, 4th edition. We compared severity of depression symptoms across the rs6311 and rs6314 genotypes, measured from the children's genomic DNA.

RESULTS

Children homozygous for the G allele of rs6311 had significantly more severe depression symptoms than those with G/A or A/A genotypes (P=0.025). The effect size (partial eta-squared) was small (ηp=0.047) but was somewhat larger when we controlled for severity of generalized anxiety disorder symptoms (P=0.006, ηp=0.072). When we restricted our analyses to white participants, our results were essentially the same as for the entire sample (P=0.004, ηp=0.086). There was no significant association between rs6314 (C/C versus T carriers) and severity of depression.

CONCLUSIONS

Our findings suggest that the HTR2A functional rs6311 polymorphism, which other studies have associated with differential HTR2A mRNA expression, may modulate the severity of depression symptoms in children with autism spectrum disorder. These tentative, hypothesis-generating findings need replication with larger, independent samples.

A starring role for microglia in brain sex differences

Microglia, the resident innate immune cells in the brain, have long been understood to be crucial to maintenance in the nervous system, by clearing debris, monitoring for infiltration of infectious agents, and mediating the brain's inflammatory and repair response to traumatic injury, stroke, or neurodegeneration. A wave of new research has shown that microglia are also active players in many basic processes in the healthy brain, including cell proliferation, synaptic connectivity, and physiology. Microglia, both in their capacity as phagocytic cells and via secretion of many neuroactive molecules, including cytokines and growth factors, play a central role in early brain development, including sexual differentiation of the brain. In this review, we present the vast roles microglia play in normal brain development and how perturbations in the normal neuroimmune environment during development may contribute to the etiology of brain-based disorders. There are notable differences between microglia and neuroimmune signaling in the male and female brain throughout the life span, and these differences may contribute to the vast differences in the incidence of neuropsychiatric and neurological disorders between males and females.

A platelet-inspired paradigm for nanomedicine targeted to multiple diseases

Platelets are megakaryocyte-derived anucleated cells found in the blood. They are mainly responsible for rendering hemostasis or clotting to prevent bleeding complications. Decreased platelet numbers or deficiencies in platelet functions can lead to various acute or chronic bleeding conditions and hemorrhage. On the other hand, dysregulated hyperactivity of the clotting process can lead to thrombosis and vascular occlusion. There is significant evidence that beyond hemostasis and thrombosis, platelets play crucial mechanistic roles in other disease scenarios such as inflammation, immune response and cancer metastasis by mediating several cell-cell and cell-matrix interactions, as well as aiding the disease microenvironment via secretion of multiple soluble factors. Therefore, elucidating these mechanistic functions of platelets can provide unique avenues for developing platelet-inspired nanomedicine strategies targeted to these diseases. To this end, the current review provides detailed mechanistic insight into platelets' disease-relevant functions and discusses how these mechanisms can be utilized to engineer targeted nanomedicine systems.

Binding-induced fluorescence of serotonin transporter ligands: A spectroscopic and structural study of 4-(4-(dimethylamino)phenyl)-1-methylpyridinium (APP(+)) and APP(+) analogues

The binding-induced fluorescence of 4-(4-(dimethylamino)-phenyl)-1-methylpyridinium (APP(+)) and two new serotonin transporter (SERT)-binding fluorescent analogues, 1-butyl-4-[4-(1-dimethylamino)phenyl]-pyridinium bromide (BPP(+)) and 1-methyl-4-[4-(1-piperidinyl)phenyl]-pyridinium (PPP(+)), has been investigated. Optical spectroscopy reveals that these probes are highly sensitive to their chemical microenvironment, responding to variations in polarity with changes in transition energies and responding to changes in viscosity or rotational freedom with emission enhancements. Molecular docking calculations reveal that the probes are able to access the nonpolar and conformationally restrictive binding pocket of SERT. As a result, the probes exhibit previously not identified binding-induced turn-on emission that is spectroscopically distinct from dyes that have accumulated intracellularly. Thus, binding and transport dynamics of SERT ligands can be resolved both spatially and spectroscopically.

Genetic dissection of Anopheles gambiae gut epithelial responses to Serratia marcescens

Genetic variation in the mosquito Anopheles gambiae profoundly influences its ability to transmit malaria. Mosquito gut bacteria are shown to influence the outcome of infections with Plasmodium parasites and are also thought to exert a strong drive on genetic variation through natural selection; however, a link between antibacterial effects and genetic variation is yet to emerge. Here, we combined SNP genotyping and expression profiling with phenotypic analyses of candidate genes by RNAi-mediated silencing and 454 pyrosequencing to investigate this intricate biological system. We identified 138 An. gambiae genes to be genetically associated with the outcome of Serratia marcescens infection, including the peptidoglycan recognition receptor PGRPLC that triggers activation of the antibacterial IMD/REL2 pathway and the epidermal growth factor receptor EGFR. Silencing of three genes encoding type III fibronectin domain proteins (FN3Ds) increased the Serratia load and altered the gut microbiota composition in favor of Enterobacteriaceae. These data suggest that natural genetic variation in immune-related genes can shape the bacterial population structure of the mosquito gut with high specificity. Importantly, FN3D2 encodes a homolog of the hypervariable pattern recognition receptor Dscam, suggesting that pathogen-specific recognition may involve a broader family of immune factors. Additionally, we showed that silencing the gene encoding the gustatory receptor Gr9 that is also associated with the Serratia infection phenotype drastically increased Serratia levels. The Gr9 antibacterial activity appears to be related to mosquito feeding behavior and to mostly rely on changes of neuropeptide F expression, together suggesting a behavioral immune response following Serratia infection. Our findings reveal that the mosquito response to oral Serratia infection comprises both an epithelial and a behavioral immune component.

In malaria vector mosquitoes, the presence of bacteria and malaria parasites is tightly linked. Bacteria that are part of the mosquito gut ecosystem are critical modulators of the immune response elicited during infection with malaria parasites. Furthermore, responses against oral bacterial infections can affect malaria parasites. Here, we combined mosquito gut infections with the enterobacterium Serratia marcescens with genome-wide discovery and phenotypic analysis of genes involved in antibacterial responses to characterize molecular processes that control gut bacterial infections thus possibly affecting the mosquito susceptibility to infection by malaria parasites. Our data reveal complex genetic networks controlling the gut bacterial infection load and ecosystem homeostasis. These networks appear to exhibit much higher specificity toward specific classes of bacteria than previously thought and include behavioral response circuits involved in antibacterial immunity.

Serotonin modulation of macrophage polarization: inflammation and beyond

Macrophages display a ample plethora of effector functions whose acquisition is promoted by the surrounding cytokine and cellular environment. Depending on the stimulus, macrophages become specialized ("polarized") for either pathogen elimination, tissue repair and wound healing or immunosuppression. This "polarization" versatility allows macrophages to critically contribute to tissue homeostasis, as they promote initiation and resolution of inflammatory responses. As a consequence, deregulation of the tissue macrophage polarization balance is an etiological agent of chronic inflammation, autoimmune diseases, cancer and even obesity and insulin resistance. In the present review we describe current concepts on the molecular basis and the patho-physiological implications of macrophage polarization, and describe its modulation by serotonin (5-HT), a neurotransmitter that regulates inflammation and tissue repair via a large set of receptors (5-HTR1-7). 5-HT modulates the phenotypic and functional polarization of macrophages, and contributes to the maintenance of an anti-inflammatory state mainly via 5-HTR2B and 5-HTR7, whose activation has a great impact on macrophage gene expression profile. The identification of 5-HTR2B and 5-HTR7 as functionally-relevant polarization markers suggests their therapeutic value in inflammatory pathologies as well as their potential involvement in linking the immune and nervous systems.

Animal models of fear relapse

Whereas fear memories are rapidly acquired and enduring over time, extinction memories are slow to form and are susceptible to disruption. Consequently, behavioral therapies that involve extinction learning (e.g., exposure therapy) often produce only temporary suppression of fear and anxiety. This review focuses on the factors that are known to influence the relapse of extinguished fear. Several phenomena associated with the return of fear after extinction are discussed, including renewal, spontaneous recovery, reacquisition, and reinstatement. Additionally, this review describes recent work, which has focused on the role of psychological stress in the relapse of extinguished fear. Recent developments in behavioral and pharmacological research are examined in light of treatment of pathological fear in humans.

Serotonergic chemosensory neurons modify the C. elegans immune response by regulating G-protein signaling in epithelial cells

The nervous and immune systems influence each other, allowing animals to rapidly protect themselves from changes in their internal and external environment. However, the complex nature of these systems in mammals makes it difficult to determine how neuronal signaling influences the immune response. Here we show that serotonin, synthesized in Caenorhabditis elegans chemosensory neurons, modulates the immune response. Serotonin released from these cells acts, directly or indirectly, to regulate G-protein signaling in epithelial cells. Signaling in these cells is required for the immune response to infection by the natural pathogen Microbacterium nematophilum. Here we show that serotonin signaling suppresses the innate immune response and limits the rate of pathogen clearance. We show that C. elegans uses classical neurotransmitters to alter the immune response. Serotonin released from sensory neurons may function to modify the immune system in response to changes in the animal's external environment such as the availability, or quality, of food.

Mood disturbance after infection

OBJECTIVE

An aetiological link between acute infection and major depression has long been hypothesized, and is increasingly gaining recognition within contemporary literature. This review aims to examine the evidence for such a link, specifically between acute, self-limiting infection and major depression, and to summarize the current understanding of pathophysiological mechanisms underlying this link.

METHODS

Relevant articles were sourced via an online search of published literature from Embase, MEDLINE, PsycINFO and PubMed using a variety of search terms including mood disorder, depression, infection and inflammation. Additionally, a search for articles from the bibliographies of retrieved papers was conducted.

RESULTS

Findings from retrospective studies suggest an association between infection and subsequent mood disturbance, including major depression. This association has been confirmed by studies employing prospective observational or experimental challenge designs. The available evidence supports a multifactorial basis of vulnerability towards major depression in the context of acute infection. Genetic, neuroendocrine, autonomic and psychosocial factors may interact to potentiate the likelihood of a severe and prolonged depressive response to an immunological stressor in some individuals.

CONCLUSION

Mood disturbance is likely to have a host-protective role in the context of an acute sickness response to infection. However, this usually adaptive and reversible response may progress in some vulnerable individuals into a more sustained and severe pattern of behavioural and physiological changes of major depression. Further research is needed to delineate the factors that predispose, precipitate and perpetuate depression in the context of acute infective illness. Such insights will inform effective prevention and treatment strategies.

A Bright Light to Reveal Mobility: Single Quantum Dot Tracking Reveals Membrane Dynamics and Cellular Mechanisms

This perspective describes recent progress in single quantum dot techniques, with an emphasis on their applications in exploring membrane dynamics and cellular mechanisms. In these cases, conventional population measurements, such as fluorescence recovery after photobleaching, yield only a mean value on an ensemble or bulk collection of molecules, where the behavior of individual proteins and vehicles is missing. In recent years, the single quantum dot imaging approach has been introduced as a sub-category of single molecule fluorescent techniques to reveal single protein/vehicle dynamics in real-time. One of the major advantages of using single quantum dots is the high signal-to-noise ratio originating from their unique photophysical properties such as extraordinarily high molar extinction coefficients and large effective Stokes shifts. In addition to a brief overview on the principle of single quantum dot imaging techniques, we highlight recent discoveries and discuss future directions in the field.

Role of psychological factors in pathogenesis of ulcerative colitis

Ulcerative colitis (UC) is a chronic inflammatory disease of the colon of unknown etiology. Multiple factors induce the occurrence and development of UC. Among these factors, psychological factors play an important role. As psychoneuroimmunology concept is gradually being accepted, it is currently believed that emotion can affect immune function through the nervous system. Recent studies suggest that psychological stress can be involved in alterations in intestinal inflammation by changing brain-gut axis function, exciting vegetative nerve, releasing neurotransmitters and altering bacterial-mucosal interactions. This paper reviews recent advances in understanding the role of psychological factors in the pathogenesis of UC and emphasizes the ways for the development of therapeutic psychological interventions.

Physiologically relevant changes in serotonin resolved by fast microdialysis

Online microdialysis is a sampling and detection method that enables continuous interrogation of extracellular molecules in freely moving subjects under behaviorally relevant conditions. A majority of recent publications using brain microdialysis in rodents report sample collection times of 20-30 min. These long sampling times are due, in part, to limitations in the detection sensitivity of high performance liquid chromatography (HPLC). By optimizing separation and detection conditions, we decreased the retention time of serotonin to 2.5 min and the detection threshold to 0.8 fmol. Sampling times were consequently reduced from 20 to 3 min per sample for online detection of serotonin (and dopamine) in brain dialysates using a commercial HPLC system. We developed a strategy to collect and to analyze dialysate samples continuously from two animals in tandem using the same instrument. Improvements in temporal resolution enabled elucidation of rapid changes in extracellular serotonin levels associated with mild stress and circadian rhythms. These dynamics would be difficult or impossible to differentiate using conventional microdialysis sampling rates.