Synthesis of serotonin by a second tryptophan hydroxylase isoform

A 5-HT-mediated urethral defense against urinary tract infections

The urethra is considered a passive conduit for urine. Here, we reveal a surprising multicellular signaling pathway guiding the urethra's dynamic response to an invading pathogen. Using a genetic approach in female mice, we deposited uropathogenic Escherichia coli into the distal urethra to establish a model of ascending urinary tract infection that progresses to the bladder within 4 h. We show that urethral neuroendocrine cells (UNECs), and the serotonin they synthesize, protect the bladder from bacterial colonization. We tested the hypothesis that serotonin initiates urethral contraction to expel ascending bacteria. We identified transient receptor potential cation channel subfamily A member 1, a noncanonical lipopolysaccharide receptor, in human and mouse UNECs and localized the serotonin receptors (HTR) 2B and 3, as well as the calcium-activated chloride channel anoctamin 1 (ANO1) to the pacemaker cells of the human and mouse urethra, the interstitial cells of Cajal (ICCs). HTR2B or ANO1 activation is sufficient for urethral contraction and is required for serotonin-induced mouse urethral contraction. Our results support the hypothesis that the urethra actively surveils its environment and responds to an ascending pathogen by evoking UNECs and ICC to induce urethral contraction and pathogen expulsion.

Autism spectrum disorder-like behaviors induced by hyper-glutamatergic NMDA receptor signaling through hypo-serotonergic 5-HT1A receptor signaling in the prefrontal cortex in mice exposed to prenatal valproic acid

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by repetitive behaviors, social deficits, and cognitive impairments. Maternal use of valproic acid (VPA) during pregnancy is associated with an increased risk of ASD in offspring. The prevailing pathophysiological hypothesis for ASD involves excitation/inhibition (E/I) imbalances and serotonergic dysfunction. Here, we investigated the association between glutamatergic-serotonergic neuronal interactions and ASD-like behaviors in mice exposed to prenatal VPA. Prenatal VPA exposure induced excessive repetitive self-grooming behavior and impaired social behavior and object recognition memory in young adult period. Prenatal VPA mice showed hyper-glutamatergic function (increase in basal extracellular glutamate levels and CaMKII phosphorylation) and hypo-serotonergic function (decrease in 5-hydroxyindoleacetic acid and stimulation-induced serotonin [5-HT] release, but an increase in 5-HT transporter expression) in the prefrontal cortex. Treatment with a low-affinity NMDA receptor antagonist (memantine), a selective 5-HT reuptake inhibitor (fluoxetine), and a 5-HT1A receptor agonist (tandospirone) attenuated both the increase in CaMKII phosphorylation and ASD-like behavior of prenatal VPA mice. Opto-genetic activation of the serotonergic neuronal system attenuated impairments in social behavior and object recognition memory in prenatal VPA mice. WAY-100635-a 5-HT1A receptor antagonist-antagonized the effect of fluoxetine on impaired social behavior and object recognition memory. These results suggest that E/I imbalance and ASD-like behavior are associated with hypo-serotonergic receptor signaling through 5-HT1A receptors in prenatal VPA mice.

The C1473G Mutation in the Mouse Tph2 Gene: From Molecular Mechanism to Biological Consequences

Tryptophan hydroxylase 2 (TPH2) hydroxylates L-tryptophan to L-5-hydroxy tryptophan-the key step of 5-HT synthesis in the mammalian brain. Some mutations in the human hTPH2 gene are associated with psychopathologies and resistance to antidepressant therapy. The C1473G polymorphism in the mouse Tph2 gene decreases the TPH2 activity in the mouse brain. In the present paper, B6-1473C and B6-1473G congenic mice that were different only in the C > G substitution were used. The molecular mechanism of decrease in the mutant enzyme activity and some physiological and behavioral traits affected by this mutation were revealed for the first time. Analysis of thermal denaturation curves in vitro revealed that the C > G substitution reduces the free energy of denaturation, stability and lifetime of mutant TPH2. Later, we evaluated the effect of the 1473G allele on the hierarchical state, competition for a sexual partner in adult mice, mouse embryos, hind legs dystonia and the response to LPS treatment in young mice. No effect of this mutation on the hierarchical state and competition for a female was observed in adult males. The C > G substitution does not affect survival, body mass or the TPH activity in the brain of 19-day-old mouse embryos. At the same time, we found that the 1473G allele causes hind legs dystonia in juvenile (3 weeks old) mice, which can affect their escape capability in threatening situations. Moreover, a significant increase in the vulnerability to LPS in juvenile B6-1473G males was shown: a single ip LPS administration killed about 40% of young mutant mice, but not wild-type ones. The body mass of mutant males was lower compared to wild-type ones, which also can indirectly decrease their concurrent and reproductive success.

Central Serotonin Deficiency Impairs Recovery of Sensorimotor Abilities After Spinal Cord Injury in Rats

Spinal cord injury (SCI) affects millions of people worldwide. One of the main challenges of rehabilitation strategies is re-training and enhancing the plasticity of the spinal circuitry that was preserved or rebuilt after the injury. The serotonergic system appears to be crucial in these processes, since recent studies have reported the capability of serotonergic (5-HT) axons for axonal sprouting and regeneration in response to central nervous system (CNS) trauma or neurodegeneration. We took advantage of tryptophan hydroxylase 2 knockout (TPH2 KO) rats, lacking serotonin specifically in the brain and spinal cord, to study the role of the serotonergic system in the recovery of sensorimotor function after SCI. In the present work, we compared the rate of sensorimotor recovery of TPH2 KO and wild-type (WT) female rats after SCI (lateral hemisection at the T8 spinal level). SCI caused severe motor impairments in the ipsilateral left hindlimb, the most pronounced in the first week after the hemisection with gradual functional recovery during the following 3 weeks. The results demonstrate that TPH2 KO rats have less potential to recover motor functions since the degree of sensorimotor deficit in the tapered beam walking test (TBW) and ladder walking test (LW) was significantly higher in the TPH2 KO group in comparison to the WT animals in the 3rd and 4th weeks after SCI. The recovery dynamics of the hindlimb muscle tone and voluntary movements was in agreement with the restoration of motor performance in TBW and LW. Compound muscle action potential analysis in the gastrocnemius (GM) and tibialis (TA) muscles of both hindlimbs after electrical stimulation of the sciatic nerve or lumbar region (L5-L6) of the spinal cord indicated slower recovery of sensorimotor pathways in the TPH2 KO group versus their WT counterparts. In general, the observed results confirm the significance of central serotonergic mechanisms in the recovery of sensorimotor functions in rats and the relevance of the TPH2 KO rat model in studying the role of the 5-HT system in neurorehabilitation.

Multi-omic insights into molecular mechanism and therapeutic targets in spinocerebellar ataxia type 7

Recent advances in molecular science have significantly enlightened our mechanistic understanding of spinocerebellar ataxia type 7. To further close remaining gaps, we performed a multi-omics analysis using SCA7266Q/5Q mice. Entire brain tissue samples were collected from 12-week-old mice, and RNA sequencing, methylation analysis, and proteomic analysis were performed. Results were integrated to identify genes with identical trends in expression across all three analyses. Data from RNA sequencing and methylation analysis revealed 58 significantly hypomethylated-upregulated genes and 62 hypermethylated-downregulated genes, mostly enriched in GO terms of regulation of axonogenesis, channel activity, and monoamine signaling. In the proteomic analysis, 211 upregulated and 281 downregulated DEPs associated mostly with immune response and cellular mobility were identified. Two genes, Fam107b and Tph2, showed differential expressions in both transcriptomic and proteomic analyses. Findings were validated in RT-qPCR as well as open data source analysis. Our study is the first to perform multi-omics analysis in SCA7 mice and will serve as an important reference for future studies.

Parental Serotonin Modulation Alters Monoamine Balance in Identified Neurons and Affects Locomotor Activity in Progeny of Lymnaea stagnalis (Mollusca: Gastropoda)

Monoamine neurotransmitters play a critical role in the development and function of the nervous system. In this study, we investigated the impact of parental serotonin (5-HT) modulation on the monoamine balance in the identified apical neurons of Lymnaea stagnalis embryos and its influence on embryonic locomotor activity. Using immunocytochemical and pharmacological approaches, we detected serotonin in the apical neurons of veliger-stage embryos, observing that the relative 5-HT level within these neurons varied with seasonal conditions. Pharmacological elevation of parental 5-HT levels significantly increased the relative 5-HT level in the oocytes and subsequently in the apical neurons of their offspring. Notably, while the relative dopamine (DA) levels in these neurons remained stable, the increase in the relative 5-HT level significantly enhanced the embryos' rotational locomotion. The expression of tryptophan hydroxylase (TPH), a key enzyme in serotonin synthesis, is a prerequisite for the elevation of the relative 5-HT level in apical neurons and is detected as early as the gastrula stage. Importantly, neither a reduction of 5-HT in the maternal organism by chlorpromazine application nor its pharmacological elevation via serotonin precursor (5-HTP) application at the cleavage stage affected the monoamine balance in apical neurons. These findings provide novel insights into how the parental 5-HT level selectively alters the monoamine phenotype of the identified neurons, offering a model for studying environmentally induced neural plasticity in early development.

Acute Optogenetic Stimulation of Serotonin Neurons Reduces Cell Proliferation in the Dentate Gyrus of Mice

The dentate gyrus of the hippocampus is targeted by axons from serotonin raphe neurons, where the neurotransmitter modulates adult neurogenesis and antidepressant action, and mediates the neurogenic effect of running. Whether running-induced cell proliferation is directly mediated by serotonin remains unknown. Here, we took advantage of Tph2-ChR2-YFP transgenic mice in which the light-sensitive protein channelrhodopsin-2 (ChR2) is specifically expressed in tryptophan hydroxylase 2 (TPH2)-expressing neurons. We selectively activated serotonin neurons via optogenetics and determined the effect on cell proliferation in the dentate gyrus. Our data reveal a significant reduction in the number of newly generated cells upon overnight raphe stimulation. The decrease in cell proliferation was absent when serotonin neurons were light-activated for six consecutive nights. However, we observed an interhemispheric difference in BrdU-positive cell numbers. We conclude that acute network dynamics occur between serotonin raphe neurons and the hippocampus, directly affecting precursor cell proliferation.

TRPA1-Activated Peptides from Saiga Antelope Horn: Screening, Interaction Mechanism, and Bioactivity

Saiga antelope horn (SAH), a rare traditional Chinese medicine, exhibits activities of anti-feverish convulsions and anti-inflammation, whereas its underlying mechanism and specific pharmacological components are still unclear. In the present study, transient receptor potential ankyrin 1 (TRPA1), a major transient receptor potential cation channel was used as a target protein to identified TRPA1 high-affinity peptides (THPs) from SAH digests. Firstly, the SAH was digested under in vitro gastrointestinal conditions. With the method of affinity ultrafiltration and liquid chromatography-mass spectrometry (AUF-LC/MS), about 200 peptides that have a high-affinity interaction with the TRPA1 protein were screened from SAH digests. Subsequently, bioactivity databases and molecular docking were further exploited to identified three THPs, including RCWPDCR, FGFDGDF, and WFCEGSF. Furthermore, RIN-14B cells, characterized by the high expression of TRPA1 on cell surfaces, were used as the cell model to investigate the biological effect of THPs. Immunofluorescence and ELISA were conducted and showed that THPs can increase the intracellular Ca2+ concentration and serotonin (5-HT) secretion in RIN-14B cells by activating TRPA1, which is evidenced by impaired upregulation of intracellular Ca2+ levels and 5-HT secretion after pretreatment with the TRPA1 inhibitor (HC-030031). Moreover, an analysis of Western blots displayed that THPs up-regulated the expression levels of the 5-HT synthesis rate-limiting enzyme (TPH1) and 5-hydroxytryptophan decarboxylase (Ddc), while serotonin reuptake transporter (SERT) levels were down-regulated, suggesting that THPs enhance 5-HT secretion by regulating the 5-HT synthesis pathway. In summary, our findings demonstrate that THPs, which were identified from SAH digest via TRPA1-targeted affinity panning, exhibited the activation of the TRPA1 channel and enhanced 5-HT release in RIN-14B cells.

Fluoxetine promotes IL-10-dependent metabolic defenses to protect from sepsis-induced lethality

Selective serotonin reuptake inhibitors (SSRIs) are some of the most prescribed drugs in the world. While they are used for their ability to increase serotonergic signaling in the brain, SSRIs are also known to have a broad range of effects beyond the brain, including immune and metabolic effects. Recent studies have demonstrated that SSRIs are protective in animal models and humans against several infections, including sepsis and COVID-19; however, the mechanisms underlying this protection are largely unknown. Here, we mechanistically link two previously described effects of the SSRI fluoxetine in mediating protection against sepsis. We show that fluoxetine-mediated protection is independent of peripheral serotonin and instead increases levels of circulating interleukin-10 (IL-10). IL-10 is necessary for protection from sepsis-induced hypertriglyceridemia, preventing cardiac effects including impairment of glucose oxidation, ectopic lipid accumulation, ventricular stretch and possibly cardiac failure. Our work reveals a beneficial "off-target" effect of fluoxetine, and reveals a protective immunometabolic defense mechanism with therapeutic potential.

Comparison of the Effects of Tetrahydrobiopterine, L-Tryptophan, and Iron Ions on the Thermal Stability of Wild Type and P447R Mutant Tryptophan Hydroxylase 2

The enzyme tryptophan hydroxylase 2 (TPH2) hydroxylates L-tryptophan to L-5-hydroxytryptophan (5-HTP) in the presence of tetrahydrobiopterin (BH4), iron ions (Fe2+), and molecular oxygen, the first and rate-limiting step in the synthesis of the neurotransmitter serotonin (5-HT) in the mammalian brain. We compared the effect of L-tryptophan and bi- and trivalent iron ions on the thermal stability of the mutant (mt) and wild type (wt) TPH2 molecules with those of BH4. Mt TPH2 and wt TPH2 molecules were extracted from the midbrain of BALB/c and C57BL/6 mice, respectively. Thermal stability was examined using a thermal denaturation curve and assessed by T50, the temperature at which half of the enzyme molecules are denatured. L-tryptophan at a concentration of 0.2 mM did not stabilize TPH2. BH4 increased T50 of the mt enzyme at concentration of 0.2 mM, but not 0.1 mM. It was shown for the first time that divalent and trivalent iron ions at concentrations of 0.01, 0.05, and 0.2 mM increased T50 of the mt and wt enzyme significantly higher than BH4. The results prompt using iron ions for elucidation of the molecular mechanisms of thermal stability of TPH2 in order to create pharmacological chaperones for correcting mutation-induced disturbances in TPH2 activity and the associated psychopathologies.

Role of 5-HT in the enteric nervous system and enteroendocrine cells

Since the 1950s, considerable circumstantial evidence had been presented that endogenous 5-HT (serotonin) synthesized from within the wall of the gastrointestinal (GI) tract played an important role in GI motility and transit. However, identifying the precise functional role of gut-derived 5-HT has been difficult to ascertain, for a number of reasons. Over the past decade, as recording techniques have advanced significantly and access to new genetically modified animals improved, there have been major new insights and major changes in our understanding of the functional role of endogenous 5-HT in the GI tract. Data from many different laboratories have shown that major patterns of GI motility and transit still occur with minor or no, change when all endogenous 5-HT is pharmacologically or genetically ablated from the gut. Furthermore, antagonists of 5-HT3 receptors are equally, or more potent at inhibiting GI motility in segments of intestine that are completely depleted of endogenous 5-HT. Here, the most recent findings are discussed with regard to the functional role of endogenous 5-HT in enterochromaffin cells and enteric neurons in gut motility and more broadly in some major homeostatic pathways.

Interaction of the Vagus Nerve and Serotonin in the Gut-Brain Axis

The gut-brain axis represents an important bidirectional communication network, with the vagus nerve acting as a central conduit for peripheral signals from the various gut organs to the central nervous system. Among the molecular mediators involved, serotonin (5-HT), synthesized predominantly by enterochromaffin cells in the gut, plays a pivotal role. Gut-derived serotonin activates vagal afferent fibers, transmitting signals to the nucleus tractus solitarius (NTS) and modulating serotonergic neurons in the dorsal raphe nucleus (DRN) as well as the norepinephrinergic neurons in the locus coeruleus (LC). This interaction influences emotional regulation, stress responses, and immune modulation. Emerging evidence also highlights the role of microbial metabolites, particularly short-chain fatty acids (SCFAs), in enhancing serotonin synthesis and vagal activity, thereby shaping gut-brain communication. This review synthesizes the current knowledge on serotonin signaling, vagal nerve pathways, and central autonomic regulation, with an emphasis on their implications for neuropsychiatric and gastrointestinal disorders. By elucidating these pathways, novel therapeutic strategies targeting the gut-brain axis may be developed to improve mental and physical health outcomes.

A two-sample study on the relationship between polygenic risk score of serotonergic polymorphisms and social phobia: Interpersonal adaptability as a mediator

BACKGROUNDS

The influence of serotonin function on social phobia has been well-documented, yet the polygenic risk score of serotonergic polymorphisms for social phobia remains unclear.

METHODS

We assessed two aspects of social phobia (i.e., social interaction anxiety and social phobia scrutiny fear) and created a polygenic risk score of seven serotonergic polymorphisms in two independent samples.

RESULTS

The results from both samples indicated that a greater polygenic risk score, denoting a higher risk of anxiety, was associated with higher levels of social interaction anxiety and social phobia scrutinizing fear. Interestingly, the association between polygenic risk score and social interaction anxiety was mediated by interpersonal adaptability.

CONCLUSION

These findings demonstrate the importance of serotonergic polymorphisms in social phobia and unveil a psychological pathway whereby interpersonal adaptability mediates the effect of serotonergic polymorphisms on social phobia.

Recent advances of electrochemical and optical point-of-care biosensors for detecting neurotransmitter serotonin biomarkers

Since its discovery in 1984, the monoamine serotonin (5-HT) has been recognized for its critical role as a neuromodulator in both the central and peripheral nervous systems. Recent research reveals that serotonin also significantly influences various neuronal activities. Historically, it was believed that peripheral serotonin, produced by tryptophan hydroxylase in intestinal cells, functioned primarily as a hormone. However, new insights have expanded its known roles, necessitating advanced detection methods. Biosensors have emerged as indispensable tools in biomedical diagnostics, enabling the rapid and minimally invasive detection of target analytes with high spatial and temporal resolution. This review summarizes the progress made in the past decade in developing optical and electrochemical biosensors for serotonin detection. We evaluate various sensing strategies that optimize performance in terms of detection limits, sensitivity, and specificity. The study also explores recent innovations in biosensing technologies utilizing surface-modified electrodes with nanomaterials, including gold, graphite, carbon nanotubes, and metal oxide particles. Applications range from in vivo studies to chemical imaging and diagnostics, highlighting future prospects in the field.

Differences in central dopamine, but not serotonin, activity and welfare associated with age but not with preening cup use in commercial grow-out Pekin duck barns

Preening cups are a semi-open water source for Pekin duck enrichment. To evaluate the ducks' affective state, we combined measuring welfare by walking a transect in the barn with mass spectrometry and qRT-PCR to measure brain neurotransmitter levels and gene expression for serotonin (5-HT) and dopamine (DA) synthesis and metabolism. 5-HT and DA have been established as indicators of mental state and emotions. We visited 4 standard commercial barns that housed approximately 6000-9000 ducks (one preening cup per 1500 ducks) and collected samples on d21 prior to preening cup placement, d28 one week after preening cup placement, and d35 one day prior to processing. Litter samples (n = 3/barn/day) were collected and transect walks were conducted to evaluate the welfare of the birds. Brain samples (n = 8/day/barn) were collected from two locations: ducks actively using the preening cups (PC) and ducks across the barn not actively using the preening cups (CON). The brains were hemisected and dissected in three brain areas: caudal mesencephalon (CM), rostral mesencephalon (RM), and diencephalon (DI). Litter samples showed no significant differences between collection dates. The transect showed significant differences in feather quality, feather cleanliness, and eye due to age, but not preening cups. The right hemisphere showed no differences in 5-HT turnover. For DA turnover, there are differences in CM (p < 0.05) and DI (p < 0.001) over time, but no differences between PC and CON. The left hemisphere measured TPH1, TPH2, and TH. CM and DI brain areas are not significantly different. Within the RM, there is a significant increase in TPH1 expression for ducks on d35 when compared to ducks on d28 and d21. These results suggest that 5-HT and DA do not differ due to duck location. However, DA activity increases as these ducks age. DA is an important neurotransmitter and activity increases as an animal grows allowing for behavioral development. Our data shows that commercial preening cups do not negatively impact ducks' welfare or affective state.

Expression of tryptophan hydroxylase in rat adrenal glands: Upregulation of TPH2 by chronic stress

It has been shown that chronic restraint stress (CRS) increases adrenal 5-HT levels and turnover through a mechanism that appears unrelated to tryptophan hydroxylase (TPH). In the present study we re-analyzed the effects of CRS (20 min/day) for 14 days relative to control (CTRL) conditions on TPH expression, distribution, and activity in rat adrenal glands. On day 15, adrenal glands were collected for TPH1 and TPH2 immunohistochemistry, Western blot, and RT-PCR; TPH activity was estimated by quantification of 5-hydroxytryptophan (5-HTP) and, indirectly, through measurement of 5-HT and 5-hydroxindolacetic acid (5-HIAA) levels and turnover (5-HIAA/5-HT ratio) by HPLC. TPH expression and activity in the dorsal raphe nucleus (DRN) were also determined for comparison. TPH1 and TPH2 immunostaining was observed in the adrenal medulla, and measurable levels of TPH1 and TPH2 protein and mRNA were detected in rat adrenal glands from CTRL animals. CRS exposure noticeably increased TPH2- but not THP1-immunostaining in the medulla and the outer adrenocortical areas of left (LAG) but not of right adrenal glands (RAG). In addition, CRS exposure increased TPH2 protein and mRNA levels in LAG; however, both measures decreased in DRN. Finally, CRS treatment produced an increase and a decrease of TPH activity and 5-HT turnover in LAG and DRN, respectively. Results indicate that TPH is indeed expressed in rat adrenal glands. Exposure to CRS upregulates TPH2 in LAG, while inducing downregulation of it in the DRN. Then, the increased levels of 5-HT in LAG from CRS-exposed animals likely results from TPH2-mediated synthesis.

High Prevalence of Severe Depression in Mexican Patients Diagnosed with HIV Treated with Efavirenz and Atazanavir: Clinical Follow-Up at Four Weeks and Analysis of TPH2 SNPs

Efavirenz (EFV) causes neuropsychiatric effects such as anxiety, depression, and suicidal thoughts in people with HIV (PWH). Depressive disorders have been associated with the Tryptophan hydroxylase type 2 (TPH2) gene. Objectives: This study determines the genotypes and allelic frequencies of three TPH2 single nucleotide polymorphisms (SNPs) in a Mexican cohort of HIV-1 treatment-naïve-patients and the severity of depressive symptoms at baseline and after a four-week clinical follow-up of antiretroviral treatment. Methods: In a pilot prospective study, eighty-one antiretroviral treatment-naïve patients were recruited from the Infectious Disease Hospital, National Medical Center "La Raza", in Mexico City. Of these, 39 were treated using a set-dose combination regimen of tenofovir disoproxil fumarate/emtricitabine (TDF/FTC) plus EFV and 42 were treated with TDF/FTC plus atazanavir/ritonavir (ATV/r), and fifty-nine control volunteers. Genomic DNA was obtained from peripheral blood mononuclear cells. All DNA samples underwent qPCR utilizing TaqMan probes for the three TPH2 SNPs studied. All participants underwent evaluation utilizing the Beck Depression Inventory. Results: Of the three SNPs examined, none exhibited any notable differences in the distribution of the alleles between the groups; nevertheless, rs4570625 TT and rs1386493 GG presented a twofold and fivefold greater risk of severe depression in PWH, respectively, independently of the treatment. Among PWH, those treated with EFV experienced severe depression at a higher rate of 90.4% after four weeks, compared to 87.5% in those treated with ATV/r. Conclusions: High rates of severe depression were identified in PWH, who presented the rs4570625 TT and rs1386493 GG polymorphic variants. Depression increased after four weeks of treatment and was higher with EFV than ATV/r. It is crucial to emphasize the necessity of conducting psychiatric monitoring for every patient with HIV and administering prompt antidepressant treatment.

Trace amine-associated receptor 1 agonist reduces aggression in brain serotonin-deficient tryptophan hydroxylase 2 knockout rats

Introduction

Aggression and self-harm disproportionately occur in youths preoccupied with social status tracking. These pathological conditions are linked to a serotonin (5-HT) deficit in the brain. Ablation of 5-HT biosynthesis by tryptophan hydroxylase 2 knockout (TPH2-KO) increases aggression in rodents. Remarkably, deletion of the trace amine-associated receptor 1 (TAAR1) results in the same consequences. Unlike the nuanced dynamics of social status cues in young people, the social ranks of rats mainly advance when they dominate larger opponents in combat.

Methods

This study explored whether the potent TAAR1 agonist RO5263397 reduces aggression caused by 5-HT depletion, and whether social rank advancement motivates this aggression. The resident-intruder paradigm was applied with larger and smaller intruders to evaluate whether social rank advancement motivates aggressive behaviors in TPH2-KO rats.

Results

When a smaller intruder was introduced, 5-HT-deficient rats did not differ from wild type littermates. However, when the intruders were larger, the mutants extended their aggressive efforts, refusing to submit. Importantly, RO5263397 selectively abolished this abnormal form of aggression in TPH2-KO rats.

Discussion

Results supported social rank advancement as the main incentive. These data also suggest that TAAR1 is a promising target for the development of new treatments for aggression; independent data also support this conclusion.

Hyperphenylalaninemia and serotonin deficiency in Dnajc12-deficient mice

Serotonin exerts numerous neurological and physiological actions in the brain and in the periphery. It is generated by two different tryptophan hydroxylase enzymes, TPH1 and TPH2, in the periphery and in the brain, respectively, which are members of the aromatic amino acid hydroxylase (AAAH) family together with phenylalanine hydroxylase (PAH), degrading phenylalanine, and tyrosine hydroxylase (TH), generating dopamine. In this study, we show that the co-chaperone DNAJC12 is downregulated in serotonergic neurons in the brain of mice lacking TPH2 and thereby central serotonin. DNAJC12 has been described to regulate the stability of PAH and mutations in its gene cause hyperphenylalaninemia and neurological symptoms in patients. We show that DNAJC12 also binds and stabilizes TPH1 and TPH2 in transfected cells. In order to clarify the importance of DNAJC12 in the regulation of neurotransmitter synthesis and phenylalanine degradation in vivo, we generated DNAJC12-deficient mice. These mice show reduced levels and activity of PAH, TPH2, and TPH1 in liver, brain, and pineal gland, respectively, and experience hyperphenylalaninemia and central and peripheral serotonin deficiency. These data support a pivotal role of DNAJC12 in the regulation of AAAH and thereby in neurotransmitter synthesis and phenylalanine homeostasis.

Serotonin Signaling in Mouse Preimplantation Development: Insights from Transcriptomic and Structural-Functional Analyses

Serotonin (5-HT), a versatile signaling molecule, plays a variety of roles in both neurotransmission and tissue regulation. The influence of serotonin on early development was first studied in marine invertebrate embryos and has since been documented in a variety of vertebrate species, including mammals. The present study investigates the expression and functional activity of serotonin components in mouse embryos, focusing on key receptors and transporters. Transcriptomic analyses revealed that mRNA transcripts related to serotonin show marked expression during the oogenesis and preimplantation stages. The results of the immunohistochemical studies show the presence of serotonin, the vesicular monoamine transporter VMAT2, and several membrane receptors (5-HT1B, 5-HT1D, 5-HT2B, 5-HT7) in the early stages of development. A functional analysis performed with the VMAT inhibitor reserpine revealed the crucial role of vesicular transport in the maintenance of serotonin signaling. The findings presented here support the hypothesis that serotonin plays a significant role in oocyte maturation and embryonic development, as well as in interblastomere interactions.

The effect of Vagus nerve stimulation (VNS) on seizure control, cognitive function, and quality of life in individuals with drug-resistant epilepsy: A systematic review article

OBJECTIVES

To evaluate the effect of vagus nerve stimulation (VNS) on seizure control, cognitive functions, and quality of life in individuals with drug-resistant epilepsy.

METHODS

An extensive search of electronic databases was carried out in order to carry out this systematic review. The databases Google Scholar, Embase, PubMed, and the Cochrane Library were searched first to carryout gray literature. To reduce the quantity of pointless studies in the advanced search, the search is limited to "human studies" and "English language" publications only. Combining keywords and Medical Subject Headings (MeSH) terms like ("Vagus Nerve Stimulation" OR "VNS") AND ("Epilepsy" OR "Seizure Control") AND ("Cognitive Function" OR "Quality of Life"). Studies that have been published up to November 30/2023 were included.

RESULTS

The search strategy yielded a total of 392 relevant studies. The mean age of participant's ranges from 11 years to 33 years. The duration of follow-up ranging from 6 to 36 months. Eleven studies were included in the review. The mean≥50% response rate after VNS therapy was 56.94% ranged from 48.90% to 83.00%. Four and three studies provided information about Quality of Life in Epilepsy Inventory (QOLIE-31) and The Liverpool Seizure Severity Scale (LSSS) questionnaires respectively.

SIGNIFICANCE

Epilepsy is a chronic disease characterized by sudden abnormal discharge of brain neurons, which leads to transient brain dysfunction and the presence of spontaneous recurrent seizures. Vagus nerve stimulation has recently been proposed as a potential tool in the treatment of seizure, depressive symptoms, and cognitive impairments. There has been variation in the effects of VNS treatment on seizure control, cognitive functions, and quality of life among patients with drug-resistant epilepsy. So, a comprehensive review of exciting literature is important to see the pooled effect. Previous systematic review and meta-analysis papers were mostly randomized control trial type with specific diseases. The use of a wider variety of study designs than only randomized controlled trials is important. So, we included retrospective and prospective cohort studies in addition to randomized control trials. This enables a more thorough assessment of the connection between quality of life, cognitive function, and vagus nerve stimulation. In addition, the paper looks at a wide range of disease kinds and patterns. We have established a uniform and comprehensive approach throughout the selected studies by mandating the inclusion of all three crucial parameters: vagus nerve stimulation, cognitive function, and quality of life.

PLAIN LANGUAGE SUMMARY

This systematic review examined 392 relevant studies on vagus nerve stimulation (VNS) therapy, with participants ranging from 11 to 33 years old and follow-up durations of 6-36 months. Eleven studies were included, and the mean response rate after VNS therapy was 56.94%, ranging from 48.90% to 83.00%. The review also reported on quality of life and cognitive function, and seizure severity frequency result from several studies.

Brain Specific RagA Overexpression Triggers Depressive-Like Behaviors in Mice via Activating ADORA2A Signaling Pathway

Neuroinflammation hallmarks the pathology of depression although the etiological complexity has not yet been resolved. Previous studies demonstrate that bacterial lipopolysaccharide induces depressive-like behaviors by activating RagA-mTOR-p70S6K signaling pathway. The current project aims to investigate whether and how brain-specific RagA overexpression triggers depressive-like behaviors in mice. Full-length RagA cDNA is cloned into the mammalian expression vector under the control of brain specific promoter, and subsequently overexpressed in the brain of mouse embryos. Indeed, RagA transgenic mice exhibit depressive-like behaviors and memory impairments. RNA-seq profiling of the prefrontal cortex (PFC) transcriptome highlights adenosine A2a receptor (ADORA2A) as a key differentially expressed gene (DEG). Western blotting confirms that ADORA2A and phospho-p70S6K are markedly elevated in RagA transgenic mice. Behavioral assessments demonstrate that ADORA2A inhibitor istradefylline markedly attenuates depressive-like behaviors. Further metabolomics reveals that N-acetylserotonin and several depression-related metabolites are downregulated while proteomic profiling showed that OLIG1 and other proteins are significantly regulated in RagA transgenic mice. Collectively, RagA overexpression alters the expression patterns of signaling proteins and the metabolism of depression-associated metabolites. RagA may cause depressive-like behaviors in mice via activating p70S6K/ADORA2A signaling pathway. Thus, RagA-p70S6K-ADORA2A signaling pathway may be a target for the development of new antidepressant therapies.

Oxidative stress in relation to serotonin under general anaesthesia in dogs undergoing ovariectomy

Abdominal surgery such as ovariectomy is a traumatic event that can cause oxidative stress. The aim of the present study was to evaluate the concentration of serotonin in relation to ovariectomy-induced oxidative stress in dogs undergoing general anesthesia. Thirty-two female dogs, under general anesthesia, received meloxicam before surgery (0.2 mgkg-1 SC) and after surgery (0.1 mgkg-1 OS every 24 h). The physiological, hematological, and biochemical parameters: glycemia, aspartate transaminase (AST), alanine aminotransferase (ALT), total protein, albumin and BUN were evaluated. Oxidative stress was determined by malondialdehyde (MDA) assay, catalase (CAT), superoxide dismutase (SOD), myeloperoxidase (MPO) and butyrylcholinesterase (BuChe) at baseline, 36 and 48 h after the last administration of meloxicam. Serotonin (5-HT) concentration was also evaluated at baseline, 36 and 48 h after the last administration of meloxicam. Responses to surgical stimulus were evaluated. Physiological and hematological parameters they fell within the normal ranges for anesthetized dogs. Glycemia increased, albumin levels decreased after surgery. No rescue analgesia was required. MDA and 5-HT concentrations significantly increased from the baseline at 36 and 48 h after surgery (p < .001). 5-HT levels could be used as an indicator for oxidative stress induced by surgery and it might be employed for objectively quantifying the well-being of the surgical patient.

Neuroanatomical evidence and a mouse calcitonin gene-related peptide model in line with human functional magnetic resonance imaging data support the involvement of peptidergic Edinger-Westphal nucleus in migraine

ABSTRACT

The urocortin 1 (UCN1)-expressing centrally projecting Edinger-Westphal (EWcp) nucleus is influenced by circadian rhythms, hormones, stress, and pain, all known migraine triggers. Our study investigated EWcp's potential involvement in migraine. Using RNAscope in situ hybridization and immunostaining, we examined the expression of calcitonin gene-related peptide (CGRP) receptor components in both mouse and human EWcp and dorsal raphe nucleus (DRN). Tracing study examined connection between EWcp and the spinal trigeminal nucleus (STN). The intraperitoneal CGRP injection model of migraine was applied and validated by light-dark box, and von Frey assays in mice, in situ hybridization combined with immunostaining, were used to assess the functional-morphological changes. The functional connectivity matrix of EW was examined using functional magnetic resonance imaging in control humans and interictal migraineurs. We proved the expression of CGRP receptor components in both murine and human DRN and EWcp. We identified a direct urocortinergic projection from EWcp to the STN. Photophobic behavior, periorbital hyperalgesia, increased c-fos gene-encoded protein immunoreactivity in the lateral periaqueductal gray matter and trigeminal ganglia, and phosphorylated c-AMP-responsive element binding protein in the STN supported the efficacy of CGRP-induced migraine-like state. Calcitonin gene-related peptide administration also increased c-fos gene-encoded protein expression, Ucn1 mRNA, and peptide content in EWcp/UCN1 neurons while reducing serotonin and tryptophan hydroxylase-2 levels in the DRN. Targeted ablation of EWcp/UCN1 neurons induced hyperalgesia. A positive functional connectivity between EW and STN as well as DRN has been identified by functional magnetic resonance imaging. The presented data strongly suggest the regulatory role of EWcp/UCN1 neurons in migraine through the STN and DRN with high translational value.

Socio-affective communication in Tph2-deficient rat pups: communal nesting aggravates growth retardation despite ameliorating maternal affiliation deficits

BACKGROUND

A lack of serotonin (also known as 5-hydroxytryptamine, 5-HT) in the brain due to deficiency of the rate-limiting enzyme in 5-HT synthesis, tryptophan hydroxylase 2 (TPH2), was recently reported to result in impaired maternal affiliation across species, including mice, rats, and monkeys. In rodents, this was reflected in a lack of preference for maternal odors and reduced levels of isolation-induced ultrasonic vocalizations (USV), possibly contributing to a severe growth retardation phenotype.

METHODS

Here, we tested whether growth retardation, maternal affiliation deficits, and/or impairments in socio-affective communication caused by Tph2 deficiency can be rescued through early social enrichment in rats. To this aim, we compared male and female Tph2-/- knockout and Tph2+/- heterozygous rat pups to Tph2+/+ wildtype littermate controls, with litters being randomly assigned to standard nesting (SN; one mother with her litter) or communal nesting (CN; two mothers with their two litters).

RESULTS

Our results show that Tph2 deficiency causes severe growth retardation, together with moderate impairments in somatosensory reflexes and thermoregulatory capabilities, partially aggravated by CN. Tph2 deficiency further led to deficits in socio-affective communication, as evidenced by reduced emission of isolation-induced USV, associated with changes in acoustic features, clustering of subtypes, and temporal organization. Although CN did not rescue the impairments in socio-affective communication, CN ameliorated the maternal affiliation deficit caused by Tph2 deficiency in the homing test. To close the communicative loop between mother and pup, we assessed maternal preference and showed that mothers display a preference for Tph2+/+ controls over Tph2-/- pups, particularly under CN conditions. This is consistent with the aggravated growth phenotype in Tph2-/- pups exposed to the more competitive CN environment.

CONCLUSION

Together, this indicates that CN aggravates growth retardation despite ameliorating maternal affiliation deficits in Tph2-deficient rat pups, possibly due to reduced and acoustically altered isolation-induced USV, hindering efficient socio-affective communication between mother and pup.

Molecular mechanisms and therapeutic significance of Tryptophan Metabolism and signaling in cancer

Tryptophan (Trp) metabolism involves three primary pathways: the kynurenine (Kyn) pathway (KP), the 5-hydroxytryptamine (serotonin, 5-HT) pathway, and the indole pathway. Under normal physiological conditions, Trp metabolism plays crucial roles in regulating inflammation, immunity, and neuronal function. Key rate-limiting enzymes such as indoleamine-2,3-dioxygenase (IDO), Trp-2,3-dioxygenase (TDO), and kynurenine monooxygenase (KMO) drive these metabolic processes. Imbalances in Trp metabolism are linked to various cancers and often correlate with poor prognosis and adverse clinical characteristics. Dysregulated Trp metabolism fosters tumor growth and immune evasion primarily by creating an immunosuppressive tumor microenvironment (TME). Activation of the KP results in the production of immunosuppressive metabolites like Kyn, which modulate immune responses and promote oncogenesis mainly through interaction with the aryl hydrocarbon receptor (AHR). Targeting Trp metabolism therapeutically has shown significant potential, especially with the development of small-molecule inhibitors for IDO1, TDO, and other key enzymes. These inhibitors disrupt the immunosuppressive signals within the TME, potentially restoring effective anti-tumor immune responses. Recently, IDO1 inhibitors have been tested in clinical trials, showing the potential to enhance the effects of existing cancer therapies. However, mixed results in later-stage trials underscore the need for a deeper understanding of Trp metabolism and its complex role in cancer. Recent advancements have also explored combining Trp metabolism inhibitors with other treatments, such as immune checkpoint inhibitors, chemotherapy, and radiotherapy, to enhance therapeutic efficacy and overcome resistance mechanisms. This review summarizes the current understanding of Trp metabolism and signaling in cancer, detailing the oncogenic mechanisms and clinical significance of dysregulated Trp metabolism. Additionally, it provides insights into the challenges in developing Trp-targeted therapies and future research directions aimed at optimizing these therapeutic strategies and improving patient outcomes.

Mouse enteric neurons control intestinal plasmacytoid dendritic cell function via serotonin-HTR7 signaling

Serotonergic neurons in the central nervous system control behavior and mood, but knowledge of the roles of serotonergic circuits in the regulation of immune homeostasis is limited. Here, we employ mouse genetics to investigate the functions of enteric serotonergic neurons in the control of immune responses and find that these circuits regulate IgA induction and boost host defense against oral, but not systemic Salmonella Typhimurium infection. Enteric serotonergic neurons promote gut-homing, retention and activation of intestinal plasmacytoid dendritic cells (pDC). Mechanistically, this neuro-immune crosstalk is achieved through a serotonin-5-HT receptor 7 (HTR7) signaling axis that ultimately facilitates the pDC-mediated differentiation of IgA+ B cells from IgD+ precursors in the gut. Single-cell RNA-seq data further reveal novel patterns of bidirectional communication between specific subsets of enteric neurons and lamina propria DC. Our findings thus reveal a close interplay between enteric serotonergic neurons and gut immune homeostasis that enhances mucosal defense.

Gut microbial metabolism in Alzheimer's disease and related dementias

Multiple studies over the last decade have established that Alzheimer's disease and related dementias (ADRD) are associated with changes in the gut microbiome. These alterations in organismal composition result in changes in the abundances of functions encoded by the microbial community, including metabolic capabilities, which likely impact host disease mechanisms. Gut microbes access dietary components and other molecules made by the host and produce metabolites that can enter circulation and cross the blood-brain barrier (BBB). In recent years, several microbial metabolites have been associated with or have been shown to influence host pathways relevant to ADRD pathology. These include short chain fatty acids, secondary bile acids, tryptophan derivatives (such as kynurenine, serotonin, tryptamine, and indoles), and trimethylamine/trimethylamine N-oxide. Notably, some of these metabolites cross the BBB and can have various effects on the brain, including modulating the release of neurotransmitters and neuronal function, inducing oxidative stress and inflammation, and impacting synaptic function. Microbial metabolites can also impact the central nervous system through immune, enteroendocrine, and enteric nervous system pathways, these perturbations in turn impact the gut barrier function and peripheral immune responses, as well as the BBB integrity, neuronal homeostasis and neurogenesis, and glial cell maturation and activation. This review examines the evidence supporting the notion that ADRD is influenced by gut microbiota and its metabolites. The potential therapeutic advantages of microbial metabolites for preventing and treating ADRD are also discussed, highlighting their potential role in developing new treatments.

Leveraging DNA methylation to predict treatment response in major depressive disorder: A critical review

Major depressive disorder (MDD) is a debilitating and prevalent mental disorder with a high disease burden. Despite a wide array of different treatment options, many patients do not respond to initial treatment attempts. Selection of the most appropriate treatment remains a significant clinical challenge in psychiatry, highlighting the need for the development of biomarkers with predictive utility. Recently, the epigenetic modification DNA methylation (DNAm) has emerged to be of great interest as a potential predictor of MDD treatment outcomes. Here, we review efforts to date that seek to identify DNAm signatures associated with treatment response in individuals with MDD. Searches were conducted in the databases PubMed, Scopus, and Web of Science with the concepts and keywords MDD, DNAm, antidepressants, psychotherapy, cognitive behavior therapy, electroconvulsive therapy, transcranial magnetic stimulation, and brain stimulation therapies. We identified 32 studies implicating DNAm patterns associated with MDD treatment outcomes. The majority of studies (N = 25) are focused on selected target genes exploring treatment outcomes in pharmacological treatments (N = 22) with a few studies assessing treatment response to electroconvulsive therapy (N = 3). Additionally, there are few genome-scale efforts (N = 7) to characterize DNAm patterns associated with treatment outcomes. There is a relative dearth of studies investigating DNAm patterns in relation to psychotherapy, electroconvulsive therapy, or transcranial magnetic stimulation; importantly, most existing studies have limited sample sizes. Given the heterogeneity in both methods and results of studies to date, there is a need for additional studies before existing findings can inform clinical decisions.

Histamine synthesis and transport are coupled in axon terminals via a dual quality control system

Monoamine neurotransmitters generated by de novo synthesis are rapidly transported and stored into synaptic vesicles at axon terminals. This transport is essential both for sustaining synaptic transmission and for limiting the toxic effects of monoamines. Here, synthesis of the monoamine histamine by histidine decarboxylase (HDC) and subsequent loading of histamine into synaptic vesicles are shown to be physically and functionally coupled within Drosophila photoreceptor terminals. This process requires HDC anchoring to synaptic vesicles via interactions with N-ethylmaleimide-sensitive fusion protein 1 (NSF1). Disassociating HDC from synaptic vesicles disrupts visual synaptic transmission and causes somatic accumulation of histamine, which leads to retinal degeneration. We further identified a proteasome degradation system mediated by the E3 ubiquitin ligase, purity of essence (POE), which clears mislocalized HDC from the soma, thus eliminating the cytotoxic effects of histamine. Taken together, our results reveal a dual mechanism for translocation and degradation of HDC that ensures restriction of histamine synthesis to axonal terminals and at the same time rapid loading into synaptic vesicles. This is crucial for sustaining neurotransmission and protecting against cytotoxic monoamines.

Boosting Serotonin Synthesis Is Not Sufficient to Improve Motor Coordination of Mecp2 Heterozygous Mouse Model of Rett Syndrome

Motor deficit is a core symptom of Rett syndrome, a rare neurological disease caused in most cases by mutations of the methyl-CpG-binding protein2 (MECP2) gene. Serotonin reuptake inhibitors improve motor coordination in Mecp2 heterozygous (Het) mice and serotonin depletion prevented this effect. Here, we assess alterations in indole levels in various brain regions and whether boosting brain serotonin synthesis with the serotonin precursors tryptophan, 5-hydroxytryptophan and α-lactalbumin rescued motor coordination deficit of Mecp2 Het mice. Motor coordination was assessed in the accelerated rotarod during and after systemic administration of serotonin precursors for 2-3 weeks. Since no data are available, the effect of α-lactalbumin on tryptophan, serotonin and 5-hydroxyindoleacetic acid levels was evaluated in various brain regions in order to identify the dose of ALAC to evaluate on motor coordination. As compared to WT, Mecp2 Het mice show reduced levels of serotonin in the whole brain, hippocampus, brainstem and cerebral cortex, but not the striatum. Reduced levels of 5-hydroxyindoleacetic acid were observed in the hippocampus and brainstem. Doses of serotonin precursors increasing brain tryptophan and/or serotonin production and metabolism had no effect on motor coordination. The results indicate that boosting serotonin synthesis is not sufficient to improve motor coordination of Mecp2 Het mice.

Enhancing pig growth and gut health with fermented Jatropha curcas cake: Impacts on microbiota, metabolites, and neurotransmitters

Given the escalating global crisis in feed protein availability, Jatropha curcas L. cake has attracted significant interest as a viable alternative protein source in animal feed. This experiment was conducted to investigate the effects of fermented Jatropha curcas L. cake (FJCC) as a protein feed in the diet of pigs. A total of 96 growing pigs with an average weight of 27.60 ± 1.59 kg were divided into three dietary groups with varying FJCC inclusion levels (0, 2.5, and 5%) for a 28 d trial. Results showed that the diet with 5% FJCC (FJCC5) demonstrated significant improvements in average daily gain (p = 0.009), feed-to-gain ratio (p = 0.036), nutrient digestibility, and intestinal morphology. Furthermore, the FJCC5 diet resulted in a decrease in pH values in different gut sections (jejunum p = 0.045, cecum p = 0.001, colon p = 0.012), and favorably altered the profile of short-chain fatty acids (SCFAs) with increased butyric acid content (p = 0.005) and total SCFAs (p = 0.019). Additionally, this diet notably decreased IL-6 levels in the jejunum (p = 0.008) and colon (=0.047), significantly reduced IL-1 levels in the hypothalamus (p < 0.001), and lowered IL-1, IL-6, and IL-10 levels in plasma (p < 0.05). Microbiota and metabolite profile analysis revealed an elevated abundance of beneficial microbes (p < 0.05) and key metabolites such as 4-aminobutyric acid (GABA) (p = 0.003) and serotonin (5-HT) (p = 0.022), linked to neuroactive ligand-receptor interaction. Moreover, FJCC5 significantly boosted circulating neurotransmitter levels of 5-HT (p = 0.006) and GABA (p = 0.002) in plasma and hypothalamus, with corresponding increases in precursor amino acids (p < 0.05). These findings suggest that FJCC, particularly at a 5% inclusion rate, can be an effective substitute for traditional protein sources like soybean meal, offering benefits beyond growth enhancement to gut health and potentially impacting the gut-brain axis. This research underscores FJCC's potential as a valuable component in sustainable animal nutrition strategies.

The role of serotonin in depression-A historical roundup and future directions

Depression is one of the most common psychiatric disorders worldwide, affecting approximately 280 million people, with probably much higher unrecorded cases. Depression is associated with symptoms such as anhedonia, feelings of hopelessness, sleep disturbances, and even suicidal thoughts. Tragically, more than 700 000 people commit suicide each year. Although depression has been studied for many decades, the exact mechanisms that lead to depression are still unknown, and available treatments only help a fraction of patients. In the late 1960s, the serotonin hypothesis was published, suggesting that serotonin is the key player in depressive disorders. However, this hypothesis is being increasingly doubted as there is evidence for the influence of other neurotransmitters, such as noradrenaline, glutamate, and dopamine, as well as larger systemic causes such as altered activity in the limbic network or inflammatory processes. In this narrative review, we aim to contribute to the ongoing debate on the involvement of serotonin in depression. We will review the evolution of antidepressant treatments, systemic research on depression over the years, and future research applications that will help to bridge the gap between systemic research and neurotransmitter dynamics using biosensors. These new tools in combination with systemic applications, will in the future provide a deeper understanding of the serotonergic dynamics in depression.

Cell-cell communication: new insights and clinical implications

Multicellular organisms are composed of diverse cell types that must coordinate their behaviors through communication. Cell-cell communication (CCC) is essential for growth, development, differentiation, tissue and organ formation, maintenance, and physiological regulation. Cells communicate through direct contact or at a distance using ligand-receptor interactions. So cellular communication encompasses two essential processes: cell signal conduction for generation and intercellular transmission of signals, and cell signal transduction for reception and procession of signals. Deciphering intercellular communication networks is critical for understanding cell differentiation, development, and metabolism. First, we comprehensively review the historical milestones in CCC studies, followed by a detailed description of the mechanisms of signal molecule transmission and the importance of the main signaling pathways they mediate in maintaining biological functions. Then we systematically introduce a series of human diseases caused by abnormalities in cell communication and their progress in clinical applications. Finally, we summarize various methods for monitoring cell interactions, including cell imaging, proximity-based chemical labeling, mechanical force analysis, downstream analysis strategies, and single-cell technologies. These methods aim to illustrate how biological functions depend on these interactions and the complexity of their regulatory signaling pathways to regulate crucial physiological processes, including tissue homeostasis, cell development, and immune responses in diseases. In addition, this review enhances our understanding of the biological processes that occur after cell-cell binding, highlighting its application in discovering new therapeutic targets and biomarkers related to precision medicine. This collective understanding provides a foundation for developing new targeted drugs and personalized treatments.

Assessment of Probiotics' Impact on Neurodevelopmental and Behavioral Responses in Zebrafish Models: Implications for Autism Spectrum Disorder Therapy

Autism spectrum disorder (ASD) is a neurodevelopmental disorder; the prevalence of which has been on the rise with unknown causes. Alterations in the gut-brain axis have been widely recognized in ASD patients, and probiotics are considered to potentially benefit the rescuing of autism-like behaviors. However, the effectiveness and mechanisms of multiple probiotics on zebrafish models are still not clearly revealed. This study aims to use the germ-free (GF) and conventionally raised (CR) AB wild-type zebrafish and the mutant Tbr1b-/- and Katnal2-/- lines as human-linked ASD animal models to evaluate the effects of multiple probiotics on mitigating developmental and behavioral defects. Results showed that the addition of probiotics increased the basic important developmental indexes, such as body length, weight, and survival rate of treated zebrafish. Moreover, the Lactobacillus plantarum and Lactobacillus rhamnosus affected the behavior of CR zebrafish by increasing their mobility, lowering the GF zebrafish manic, and mitigating transgenic zebrafish abnormal behavior. Moreover, the expression levels of key genes related to gamma-aminobutyric acid (GABA), dopamine (DA), and serotonin (5-HT) as important neuropathways to influence the appearance and development of autism-related disorders, including gad1b, tph1a, htr3a, th, and slc6a3, were significantly activated by some of the probiotics' treatment at some extent. Taken together, this study indicates the beneficial effects of different probiotics, which may provide a novel understanding of probiotic function in related diseases' therapy.

Harnessing intestinal tryptophan catabolism to relieve atherosclerosis in mice

Tryptophan (Trp) is an essential amino acid, whose metabolism is a key gatekeeper of intestinal homeostasis. Yet, its systemic effects, particularly on atherosclerosis, remain unknown. Here we show that high-fat diet (HFD) increases the activity of intestinal indoleamine 2, 3-dioxygenase 1 (IDO), which shifts Trp metabolism from the production of microbiota-derived indole metabolites towards kynurenine production. Under HFD, the specific deletion of IDO in intestinal epithelial cells leads to intestinal inflammation, impaired intestinal barrier, augmented lesional T lymphocytes and atherosclerosis. This is associated with an increase in serotonin production and a decrease in indole metabolites, thus hijacking Trp for the serotonin pathway. Inhibition of intestinal serotonin production or supplementation with indole derivatives alleviates plaque inflammation and atherosclerosis. In summary, we uncover a pivotal role of intestinal IDO in the fine-tuning of Trp metabolism with systemic effects on atherosclerosis, paving the way for new therapeutic strategies to relieve gut-associated inflammatory diseases.

New insights into the effects of serotonin on Parkinson's disease and depression through its role in the gastrointestinal tract

Neuropsychiatric and neurodegenerative disorders are frequently associated with gastrointestinal (GI) co-pathologies. Although the central and enteric nervous systems (CNS and ENS, respectively) have been studied separately, there is increasing interest in factors that may contribute to conditions affecting both systems. There is compelling evidence that serotonin (5-HT) may play an important role in several gut-brain disorders. It is well known that 5-HT is essential for the development and functioning of the CNS. However, most of the body's 5-HT is produced in the GI tract. A deeper understanding of the specific effects of enteric 5-HT on gut-brain disorders may provide the basis for the development of new therapeutic targets. This review summarizes current data focusing on the important role of 5-HT in ENS development and motility, with particular emphasis on novel aspects of 5-HT signaling in conditions where CNS and ENS comorbidities are common, such as Parkinson's disease and depressive disorders.

Linking serotonin homeostasis to gut function: Nutrition, gut microbiota and beyond

Serotonin (5-HT) produced by enterochromaffin (EC) cells in the digestive tract is crucial for maintaining gut function and homeostasis. Nutritional and non-nutritional stimuli in the gut lumen can modulate the ability of EC cells to produce 5-HT in a temporal- and spatial-specific manner that toning gut physiology and immune response. Of particular interest, the interactions between dietary factors and the gut microbiota exert distinct impacts on gut 5-HT homeostasis and signaling in metabolism and the gut immune response. However, the underlying mechanisms need to be unraveled. This review aims to summarize and discuss the importance of gut 5-HT homeostasis and its regulation in maintaining gut metabolism and immune function in health and disease with special emphasis on different types of nutrients, dietary supplements, processing, and gut microbiota. Cutting-edge discoveries in this area will provide the basis for the development of new nutritional and pharmaceutical strategies for the prevention and treatment of serotonin homeostasis-related gut and systematic disorders and diseases.

Novel Tryptophan Hydroxylase Inhibitor TPT-001 Reverses PAH, Vascular Remodeling, and Proliferative-Proinflammatory Gene Expression

The serotonin pathway has long been proposed as a promising target for pulmonary arterial hypertension (PAH)-a progressive and uncurable disease. We developed a highly specific inhibitor of the serotonin synthesizing enzyme tryptophan hydroxylase 1 (TPH1), TPT-001 (TPHi). In this study, the authors sought to treat severe PAH in the Sugen/hypoxia (SuHx) rat model with the oral TPHi TPT-001. Male Sprague Dawley rats were divided into 3 groups: 1) ConNx, control animals; 2) SuHx, injected subcutaneously with SU5416 and exposed to chronic hypoxia for 3 weeks, followed by 6 weeks in room air; and 3) SuHx+TPHi, SuHx animals treated orally with TPHi for 5 weeks. Closed-chest right- and left heart catheterization and echocardiography were performed. Lungs were subject to histologic and mRNA sequencing analyses. Compared with SuHx-exposed rats, which developed severe PAH and right ventricular (RV) dysfunction, TPHi-treated SuHx rats had greatly lowered RV systolic (mean ± SEM: 41 ± 2.3 mm Hg vs 86 ± 6.5 mm Hg; P < 0.001) and end-diastolic (mean ± SEM: 4 ± 0.7 mm Hg vs 14 ± 1.7 mm Hg; P < 0.001) pressures, decreased RV hypertrophy and dilation (all not significantly different from control rats), and reversed pulmonary vascular remodeling. We identified perivascular infiltration of CD3+ T cells and proinflammatory F4/80+ and CD68+ macrophages and proliferating cell nuclear antigen-positive alveolar epithelial cells all suppressed by TPHi treatment. Whole-lung mRNA sequencing in SuHx rats showed distinct gene expression patterns related to pulmonary arterial smooth muscle cell proliferation (Rpph1, Lgals3, Gata4), reactive oxygen species, inflammation (Tnfsrf17, iNOS), and vasodilation (Pde1b, Kng1), which reversed expression with TPHi treatment. Inhibition of TPH1 with a new class of drugs (here, TPT-001) has the potential to attenuate or even reverse severe PAH and associated RV dysfunction in vivo by blocking the serotonin pathway.

Serotonin signalling in cancer: Emerging mechanisms and therapeutic opportunities

BACKGROUND

Serotonin (5-hydroxytryptamine) is a multifunctional bioamine serving as a neurotransmitter, peripheral hormone and mitogen in the vertebrate system. It has pleiotropic activities in central nervous system and gastrointestinal function via an orchestrated action of serotonergic elements, particularly serotonin receptor-mediated signalling cascades. The mitogenic properties of serotonin have garnered recognition for years and have been exploited for repurposing serotonergic-targeted drugs in cancer therapy. However, emerging conflicting findings necessitate a more comprehensive elucidation of serotonin's role in cancer pathogenesis.

MAIN BODY AND CONCLUSION

Here, we provide an overview of the biosynthesis, metabolism and action modes of serotonin. We summarise our current knowledge regarding the effects of the peripheral serotonergic system on tumourigenesis, with a specific emphasis on its immunomodulatory activities in human cancers. We also discuss the dual roles of serotonin in tumour pathogenesis and elucidate the potential of serotonergic drugs, some of which display favourable safety profiles and impressive efficacy in clinical trials, as a promising avenue in cancer treatment.

KEY POINTS

Primary synthesis and metabolic routes of peripheral 5-hydroxytryptamine in the gastrointestinal tract. Advanced research has established a strong association between the serotonergic components and carcinogenic mechanisms. The interplay between serotonergic signalling and the immune system within the tumour microenvironment orchestrates antitumour immune responses. Serotonergic-targeted drugs offer valuable clinical options for cancer therapy.

Genetic polymorphisms and their association with neurobiological and psychological factors in anorexia nervosa: a systematic review

Background and aims

Anorexia nervosa (AN) is a complex neuropsychiatric disorder. This systematic review synthesizes evidence from diverse studies to assess and investigate the association between gene polymorphisms and psychological and neurobiological factors in patients with AN.

Methods

A systematic search across PubMed, PsycINFO, Scopus, and Web of Science databases, along with manual searching, was conducted. The review protocol was approved by PROSPERO (CRD42023452548). Out of 1,250 articles, 11 met the inclusion criteria. The quality of eligible articles was assessed using the Newcastle-Ottawa Scale (NOS) tool. The systematic review followed the PRISMA guidelines.

Results

The serotoninergic system, particularly the 5-HTTLPR polymorphism, is consistently linked to altered connectivity in the ventral attention network, impaired inhibitory control, and increased susceptibility to AN. The 5-HTTLPR polymorphism affects reward processing, motivation, reasoning, working memory, inhibition, and outcome prediction in patients with AN. The dopaminergic system, involving genes like COMT, DRD2, DRD3, and DAT1, regulates reward, motivation, and decision-making. Genetic variations in these dopaminergic genes are associated with psychological manifestations and clinical severity in patients with AN. Across populations, the Val66Met polymorphism in the BDNF gene influences personality traits, eating behaviors, and emotional responses. Genes like OXTR, TFAP2B, and KCTD15 are linked to social cognition, emotional processing, body image concerns, and personality dimensions in patients with AN.

Conclusion

There was an association linking multiple genes to the susceptibly and/or severity of AN. This genetic factor contributes to the complexity of AN and leads to higher diversity of its clinical presentation. Therefore, conducting more extensive research to elucidate the underlying mechanisms of anorexia nervosa pathology is imperative for advancing our understanding and potentially developing targeted therapeutic interventions for the disorder.Systematic review registration: [https://clinicaltrials.gov/], identifier [CRD42023452548].

Exploring the contribution of mammary-derived serotonin on liver and pancreas metabolism during lactation

During lactation, the murine mammary gland is responsible for a significant increase in circulating serotonin. However, the role of mammary-derived serotonin in energy homeostasis during lactation is unclear. To investigate this, we utilized C57/BL6J mice with a lactation and mammary-specific deletion of the gene coding for the rate-limiting enzyme in serotonin synthesis (TPH1, Wap-Cre x TPH1FL/FL) to understand the metabolic contributions of mammary-derived serotonin during lactation. Circulating serotonin was reduced by approximately 50% throughout lactation in Wap-Cre x TPH1FL/FL mice compared to wild-type mice (TPH1FL/FL), with mammary gland and liver serotonin content reduced on L21. The Wap-Cre x TPH1FL/FL mice had less serotonin and insulin immunostaining in the pancreatic islets on L21, resulting in reduced circulating insulin but no changes in glucose. The mammary glands of Wap-Cre x TPH1FL/FL mice had larger mammary alveolar areas, with fewer and smaller intra-lobular adipocytes, and increased expression of milk protein genes (e.g., WAP, CSN2, LALBA) compared to TPH1FL/FL mice. No changes in feed intake, body composition, or estimated milk yield were observed between groups. Taken together, mammary-derived serotonin appears to contribute to the pancreas-mammary cross-talk during lactation with potential implications in the regulation of insulin homeostasis.

Impact of menopause-associated frailty on traumatic brain injury

Navigating menopause involves traversing a complex terrain of hormonal changes that extend far beyond reproductive consequences. Menopausal transition is characterized by a decrease in estradiol-17β (E2), and the impact of menopause resonates not only in the reproductive system but also through the central nervous system, musculoskeletal, and gastrointestinal domains. As women undergo menopausal transition, they become more susceptible to frailty, amplifying the risk and severity of injuries, including traumatic brain injury (TBI). Menopause triggers a cascade of changes leading to a decline in muscle mass, accompanied by diminished tone and excitability, thereby restricting the availability of irisin, a crucial hormone derived from muscles. Concurrently, bone mass undergoes reduction, culminating in the onset of osteoporosis and altering the dynamics of osteocalcin, a hormone originating from bones. The diminishing levels of E2 during menopause extend their influence on the gut microbiota, resulting in a reduction in the availability of tyrosine, tryptophan, and serotonin metabolites, affecting neurotransmitter synthesis and function. Understanding the interplay between menopause, frailty, E2 decline, and the intricate metabolisms of bone, gut, and muscle is imperative when unraveling the nuances of TBI after menopause. The current review underscores the significance of accounting for menopause-associated frailty in the incidence and consequences of TBI. The review also explores potential mechanisms to enhance gut, bone, and muscle health in menopausal women, aiming to mitigate frailty and improve TBI outcomes.

Alterations in the brain serotonin system and serotonin-regulated behavior during aging in zebrafish males and females

The brain serotonin (5-HT) system performs a neurotrophic function and supports the plasticity of the nervous system, while its age-related changes can increase the risk of senile neurodegeneration. Zebrafish brain is highly resistant to damage and neurodegeneration due to its high regeneration potential and it is a promising model object in searching for molecular factors preventing age-related neurodegeneration. In the present study alterations in 5-HT-related behavior in the home tank and the novel tank diving test, as well as 5-HT, 5-HIAA levels, tryptophan hydroxylase (TPH), monoamine oxidase (MAO) activity and the expression of genes encoding TPH, MAO, 5-HT transporter and 5-HT receptors in the brain of 6, 12, 24 and 36 month old zebrafish males and females are investigated. Marked sexual dimorphism in the locomotor activity in the novel tank test is revealed: females of all ages move slower than males. No sexual dimorphism in 5-HT-related traits is observed. No changes in 5-HT and 5-HIAA levels in zebrafish brain during aging is observed. At the same time, the aging is accompanied by a decrease in the locomotor activity, TPH activity, tph2 and htr1aa genes expression as well as an increase in the MAO activity and slc6a4a gene expression in their brain. These results indicate that the brain 5-HT system in zebrafish is resistant to age-related alterations.

No association between peripheral serotonin-gene-related DNA methylation and brain serotonin neurotransmission in the healthy and depressed state

BACKGROUND

Methylation of serotonin-related genes has been proposed as a plausible gene-by-environment link which may mediate environmental stress, depressive and anxiety symptoms. DNA methylation is often measured in blood cells, but little is known about the association between this peripheral epigenetic modification and brain serotonergic architecture. Here, we evaluated the association between whole-blood-derived methylation of four CpG sites in the serotonin transporter (SLC6A4) and six CpG sites of the tryptophan hydroxylase 2 (TPH2) gene and in-vivo brain levels of serotonin transporter (5-HTT) and serotonin 4 receptor (5-HT4) in a cohort of healthy individuals (N = 254) and, for 5-HT4, in a cohort of unmedicated patients with depression (N = 90). To do so, we quantified SLC6A4/TPH2 methylation using bisulfite pyrosequencing and estimated brain 5-HT4 and 5-HTT levels using positron emission tomography. In addition, we explored the association between SLC6A4 and TPH2 methylation and measures of early life and recent stress, depressive and anxiety symptoms on 297 healthy individuals.

RESULTS

We found no statistically significant association between peripheral DNA methylation and brain markers of serotonergic neurotransmission in patients with depression or in healthy individuals. In addition, although SLC6A4 CpG2 (chr17:30,236,083) methylation was marginally associated with the parental bonding inventory overprotection score in the healthy cohort, statistical significance did not remain after accounting for blood cell heterogeneity.

CONCLUSIONS

We suggest that findings on peripheral DNA methylation in the context of brain serotonin-related features should be interpreted with caution. More studies are needed to rule out a role of SLC6A4 and TPH2 methylation as biomarkers for environmental stress, depressive or anxiety symptoms.

The Imperative for Innovative Enteric Nervous System-Intestinal Organoid Co-Culture Models: Transforming GI Disease Modeling and Treatment

This review addresses the need for innovative co-culture systems integrating the enteric nervous system (ENS) with intestinal organoids. The breakthroughs achieved through these techniques will pave the way for a transformative era in gastrointestinal (GI) disease modeling and treatment strategies. This review serves as an introduction to the companion protocol paper featured in this journal. The protocol outlines the isolation and co-culture of myenteric and submucosal neurons with small intestinal organoids. This review provides an overview of the intestinal organoid culture field to establish a solid foundation for effective protocol application. Remarkably, the ENS surpasses the number of neurons in the spinal cord. Referred to as the "second brain", the ENS orchestrates pivotal roles in GI functions, including motility, blood flow, and secretion. The ENS is organized into myenteric and submucosal plexuses. These plexuses house diverse subtypes of neurons. Due to its proximity to the gut musculature and its cell type complexity, there are methodological intricacies in studying the ENS. Diverse approaches such as primary cell cultures, three-dimensional (3D) neurospheres, and induced ENS cells offer diverse insights into the multifaceted functionality of the ENS. The ENS exhibits dynamic interactions with the intestinal epithelium, the muscle layer, and the immune system, influencing epithelial physiology, motility, immune responses, and the microbiome. Neurotransmitters, including acetylcholine (ACh), serotonin (5-HT), and vasoactive intestinal peptide (VIP), play pivotal roles in these intricate interactions. Understanding these dynamics is imperative, as the ENS is implicated in various diseases, ranging from neuropathies to GI disorders and neurodegenerative diseases. The emergence of organoid technology presents an unprecedented opportunity to study ENS interactions within the complex milieu of the small and large intestines. This manuscript underscores the urgent need for standardized protocols and advanced techniques to unravel the complexities of the ENS and its dynamic relationship with the gut ecosystem. The insights gleaned from such endeavors hold the potential to revolutionize GI disease modeling and treatment paradigms.

Serotonin regulation of mitochondria in kidney diseases

Serotonin, while conventionally recognized as a neurotransmitter in the CNS, has recently gained attention for its role in the kidney. Specifically, serotonin is not only synthesized in the kidney, but it also regulates glomerular function, vascular resistance, and mitochondrial homeostasis. Because of serotonin's importance to mitochondrial health, this review is focused on the role of serotonin and its receptors in mitochondrial function in the context of acute kidney injury, chronic kidney disease, and diabetic kidney disease, all of which are characterized by mitochondrial dysfunction and none of which has approved pharmacological treatments. Evidence indicates that activation of certain serotonin receptors can stimulate mitochondrial biogenesis (MB) and restore mitochondrial homeostasis, resulting in improved renal function. Serotonin receptor agonists that induce MB are therefore of interest as potential therapeutic strategies for renal injury and disease. SIGNIFICANCE STATEMENT: Mitochondrial dysfunction is associated with many human renal diseases such as acute kidney injury, chronic kidney disease, and diabetic kidney disease, which are associated with increased morbidity and mortality. Unfortunately, none of these pathologies has an FDA-approved pharmacological intervention, underscoring the urgency of identifying new therapeutics for such disorders. Studies show that induction of mitochondrial biogenesis via serotonin (5-hydroxytryptamine, 5-HT) receptors reduces kidney injury markers, restores mitochondrial and renal function after kidney injury, and decreases mortality, suggesting that targeting 5-HT receptors may be a promising therapeutic avenue for mitochondrial dysfunction in kidney diseases. While numerous reviews describe the importance of mitochondria and mitochondrial quality control mechanisms in kidney disease, the relevance of 5-HT receptor-mediated mitochondrial metabolic modulation in the kidney has yet to be thoroughly explored.

Tryptophan in the mouse diet is essential for embryo implantation and decidualization

Introduction

Nutritional deficiency occurs frequently during pregnancy and breastfeeding. Tryptophan (Trp), an essential amino acid which is critical for protein synthesis, serves as the precursor for serotonin, melatonin, and kynurenine (Kyn). The imbalance between serotonin and kynurenine pathways in Trp metabolism is closely related to inflammation and depression. This study assessed the effects of Trp deficiency on mouse early pregnancy.

Methods

Embryo implantation and decidualization were analyzed after female mice had been fed diets containing 0.2% Trp (for the control group), 0.062% Trp (for the low Trp group) and 0% Trp (for the Trp-free group) for two months. The uteri of the mice were collected on days 4, 5, and 8 of pregnancy for further analysis.

Results

On day 8 of pregnancy, the number of implantation sites were found to be similar between the control and the low Trp groups. However, no implantation sites were detected in the Trp-free group. On day 5 of pregnancy, plane polarity- and decidualization-related molecules showed abnormal expression pattern in the Trp-free group. On day 4 of pregnancy, there was no significant difference in uterine receptivity molecules between the low-Trp group and the control group, but uterine receptivity was abnormal in the Trp-free group. At implantation sites of the Trp-free group, IDO and AHR levels were markedly elevated. This potentially increased levels of Kyn, 2-hydroxy estradiol, and 4-hydroxy estradiol to affect decidualization.

Conclusions

Trp-free diet may impair decidualization via the IDO-KYN-AHR pathway.

Serotonin Signaling through Lipid Membranes

Serotonin (5-HT) is a vital modulatory neurotransmitter responsible for regulating most behaviors in the brain. An inefficient 5-HT synaptic function is often linked to various mental disorders. Primarily, membrane proteins controlling the expression and activity of 5-HT synthesis, storage, release, receptor activation, and inactivation are critical to 5-HT signaling in synaptic and extra-synaptic sites. Moreover, these signals represent information transmission across membranes. Although the lipid membrane environment is often viewed as fairly stable, emerging research suggests significant functional lipid-protein interactions with many synaptic 5-HT proteins. These protein-lipid interactions extend to almost all the primary lipid classes that form the plasma membrane. Collectively, these lipid classes and lipid-protein interactions affect 5-HT synaptic efficacy at the synapse. The highly dynamic lipid composition of synaptic membranes suggests that these lipids and their interactions with proteins may contribute to the plasticity of the 5-HT synapse. Therefore, this broader protein-lipid model of the 5-HT synapse necessitates a reconsideration of 5-HT's role in various associated mental disorders.