Transglutaminase Activity Determines Nuclear Localization of Serotonin Immunoreactivity in the Early Embryos of Invertebrates and Vertebrates

Role of Serotonylation and SERT Posttranslational Modifications in Alzheimer's Disease Pathogenesis

The neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) is implicated mainly in Alzheimer's disease (AD) and reported to be responsible for several processes and roles in the human body, such as regulating sleep, food intake, sexual behavior, anxiety, and drug abuse. It is synthesized from the amino acid tryptophan. Serotonin also functions as a signal between neurons to mature, survive, and differentiate. It plays a crucial role in neuronal plasticity, including cell migration and cell contact formation. Various psychiatric disorders, such as depression, schizophrenia, autism, and Alzheimer's disease, have been linked to an increase in serotonin-dependent signaling during the development of the nervous system. Recent studies have found 5-HT and other monoamines embedded in the nuclei of various cells, including immune cells, the peritoneal mast, and the adrenal medulla. Evidence suggests these monoamines to be involved in widespread intracellular regulation by posttranslational modifications (PTMs) of proteins. Serotonylation is the calcium-dependent process in which 5-HT forms a long-lasting covalent bond to small cytoplasmic G-proteins by endogenous transglutaminase 2 (TGM2). Serotonylation plays a role in various biological processes. The purpose of our article is to summarize historical developments and recent advances in serotonin research and serotonylation in depression, aging, AD, and other age-related neurological diseases. We also discussed several of the latest developments with Serotonin, including biological functions, pathophysiological implications and therapeutic strategies to treat patients with depression, dementia, and other age-related conditions.

A GAPDH serotonylation system couples CD8+ T cell glycolytic metabolism to antitumor immunity

Apart from the canonical serotonin (5-hydroxytryptamine [5-HT])-receptor signaling transduction pattern, 5-HT-involved post-translational serotonylation has recently been noted. Here, we report a glyceraldehyde-3-phosphate dehydrogenase (GAPDH) serotonylation system that promotes the glycolytic metabolism and antitumor immune activity of CD8+ T cells. Tissue transglutaminase 2 (TGM2) transfers 5-HT to GAPDH glutamine 262 and catalyzes the serotonylation reaction. Serotonylation supports the cytoplasmic localization of GAPDH, which induces a glycolytic metabolic shift in CD8+ T cells and contributes to antitumor immunity. CD8+ T cells accumulate intracellular 5-HT for serotonylation through both synthesis by tryptophan hydroxylase 1 (TPH1) and uptake from the extracellular compartment via serotonin transporter (SERT). Monoamine oxidase A (MAOA) degrades 5-HT and acts as an intrinsic negative regulator of CD8+ T cells. The adoptive transfer of 5-HT-producing TPH1-overexpressing chimeric antigen receptor T (CAR-T) cells induced a robust antitumor response. Our findings expand the known range of neuroimmune interaction patterns by providing evidence of receptor-independent serotonylation post-translational modification.

Biochemical and Functional Characterization of the Three Zebrafish Transglutaminases 2

Transglutaminase 2 (TG2) is a multifunctional protein widely distributed in various tissues and involved in many physiological and pathological processes. However, its actual role in biological processes is often controversial as TG2 shows different effects in these processes depending on its localization, cell type, or experimental conditions. We characterized the enzymatic and functional properties of TG2 proteins expressed in Danio rerio (zebrafish) to provide the basis for using this established animal model as a reliable tool to characterize TG2 functions in vivo. We confirmed the existence of three genes orthologous to human TG2 (zTGs2) in the zebrafish genome and their expression and function during embryonic development. We produced and purified the zTGs2s as recombinant proteins and showed that, like the human enzyme, zTGs2 catalyzes a Ca2+ dependent transamidation reaction that can be inhibited with TG2-specific inhibitors. In a cell model of human fibroblasts, we also demonstrated that zTGs2 can mediate RGD-independent cell adhesion in the extracellular environment. Finally, we transfected and selected zTGs2-overexpressing HEK293 cells and demonstrated that intracellular zTGs2 plays a very comparable protective/damaging role in the apoptotic process, as hTG2. Overall, our results suggest that zTGs2 proteins behave very similarly to the human ortholog and pave the way for future in vivo studies of TG2 functions in zebrafish.

Post-translational modifications of histone proteins by monoamine neurotransmitters

Protein monoaminylation is a biochemical process through which biogenic monoamines (e.g., serotonin, dopamine, histamine, etc.) are covalently bonded to certain protein substrates via Transglutaminase 2, an enzyme that catalyzes the transamidation of primary amines to the γ-carboxamides of glutamine residues. Since their initial discovery, these unusual post-translational modifications have been implicated in a wide variety of biological processes, ranging from protein coagulation to platelet activation and G-protein signaling. More recently, histone proteins - specifically histone H3 at glutamine 5 (H3Q5) - have been added to the growing list of monoaminyl substrates in vivo, with H3Q5 monoaminylation demonstrated to regulate permissive gene expression in cells. Such phenomena have further been shown to contribute critically to various aspects of (mal)adaptive neuronal plasticity and behavior. In this short review, we examine the evolution of our understanding of protein monoaminylation events, highlighting recent advances in the elucidation of their roles as important chromatin regulators.

Direct and Inherited Epigenetic Changes in the Nervous System Caused by Intensive Locomotion: Possible Adaptive Significance

Abstract

This review is devoted to the analysis of works that investigated the long-term effects of species-specific forms of intensive locomotion on the cognitive functions of animals and humans, which can be transmitted to the next generation. To date, the anxiolytic and cognitive-enhancing long-term effects of intensive locomotion have been demonstrated in humans, rodents, fish, insects, mollusks, and nematodes. In rodents, changes in the central nervous system caused by intense locomotion can be transmitted through the maternal and paternal line to the descendants of the first generation. These include reduced anxiety, improved spatial learning and memory, increased levels of brain neurotrophic factor and vascular endothelial growth factor in the hippocampus and frontal cortex. The shift of the balance of histone acetylation in the hippocampus of rodents towards hyperacetylation, and the balance of DNA methylation towards demethylation manifests itself both as a direct and as a first-generation inherited effect of motor activity. The question about the mechanisms that link locomotion with an increase in the plasticity of a genome in the brain of descendants remains poorly understood, and invertebrate model organisms can be an ideal object for its study. Currently, there is a lack of a theoretical model explaining why motor activity leads to long-term improvement of some cognitive functions that can be transmitted to the next generation and why such an influence could have appeared in evolution. The answer to these questions is not only of fundamental interest, but it is necessary for predicting therapeutic and possible side effects of motor activity in humans. In this regard, the article pays special attention to the review of ideas on the evolutionary aspects of the problem. We propose our own hypothesis, according to which the activating effect of intensive locomotion on the function of the nervous system could have been formed in evolution as a preadaptation to a possible entry into a new environment.

Maternal Serotonin: Shaping Developmental Patterns and Behavioral Strategy on Progeny in Molluscs

Serotonin is a well-known neurotransmitter and neurohormone regulating mood, sleep, feeding, and learning in high organisms. Serotonin also affects the embryonic events related to neurogenesis and maturation of hormonal systems, the underlying organism adaptation to a changing environment. Such serotonin-based mother-to-embryo signaling is realized via direct interactions in case of internal fertilization and embryonic development inside the mother body. However, the possibility of such signaling is less obvious in organisms with the ancestral type of embryogenesis and embryo development within the egg, outside the mother body. Our data, based on the investigation of freshwater gastropod molluscs (Lymnaea and Helisoma), demonstrated a correlation between seasonal variations of serotonin content within the female reproductive system, and developmental patterns and the behavioral characteristics of progeny. The direct action of serotonin via posttranslational protein modification—serotonylation—during early development, as well as classical receptor-mediated effects, underlies such serotonin-modulated developmental changes. In the present paper, I will shortly overview our results on freshwater molluscs and parallel the experimental data with the living strategy of these species occupying almost all Holarctic regions.

Differential localization of serotoninergic system elements in human amniotic epithelial cells†

Serotonin or 5-hydroxytryptamine (5-HT) is a biogenic amine involved in regulating several functions, including development. However, its impact on human embryo development has been poorly studied. The present work investigated the expression and distribution of the main components of the serotoninergic system in human amniotic tissue and human amniotic epithelial cells (hAEC) in vitro, as an alternative model of early human embryo development. Amniotic membranes from full-term healthy pregnancies were used. Human amnion tissue or hAEC isolated from the amnion was processed for reverse transcription-polymerase chain reaction and immunofluorescence analyses of the main components of the serotoninergic system. We found the expression of tryptophan hydroxylase type 1 (TPH1), type 2 (TPH2), serotonin transporter (SERT), monoamine oxidase-A (MAOA), as well as HTR1D and HTR7 receptors at mRNA level in amnion tissue as well in hAEC. Interestingly, we found the presence of 5-HT in the nucleus of the cells in amnion tissue, whereas it was located in the cytoplasm of isolated hAEC. We detected TPH1, TPH2, and HTR1D receptor in both the nucleus and cytoplasm. SERT, MAOA, and HTR7 receptor were only observed in the cytoplasm. The results presented herein show, for the first time, the presence of the serotoninergic system in human amnion in vivo and in vitro.

Identification and characterization of a novel 5-hydroxytryptamine receptor in the sea cucumber Apostichopus japonicus (Selenka)

Serotonin (5-hydroxytryptamine [5-HT]) receptors (5-HTRs) mediate neuroendocrine signaling via interactions with the ligand serotonin (5-HT). The 5-HT signaling system has been well studied in vertebrates, but rarely known in invertebrate animals, especially in the marine invertebrates. In this study, we identified and characterized a novel 5-HTR from the sea cucumber Apostichopus japonicus (Aj5-HT_4/6 ). The cloned Aj5-HT_4/6 open reading frame comprised 1290 bp and encoded 429 amino acids. Bioinformatic analysis of the receptor indicated that it was a member of the class A of the G protein-coupled receptor family. Further experiments using Aj5-HT_4/6 -transfected HEK293 cells demonstrated that treatment with 5-HT could induce rapid internalization of Aj5-HT_4/6 fused with enhanced green fluorescent protein from the cell surface into the cytoplasm and triggered a significant increase in levels of the second messenger cAMP as well as mitogen-activated protein kinase phosphorylation in a 5-HT dose-dependent manner. Quantitative real time-polymerase chain reaction demonstrated that Aj5-HT_4/6 was predominantly expressed in the muscle and respiratory tree, and its expression was significantly decreased during estivation. Taken together, these results imply that Aj5-HT_4/6 is potentially involved in the movement and metabolism of the sea cucumber.

Role of serotonin in vertebrate embryo development

Since its discovery in 1937, serotonin (5-HT) has become one of the most studied biogenic amines due to its predominant role in regulating several physiological processes such as mood, sleep, and food intake. This amine and the main components of the serotoninergic system are in almost all cells of the body. The presence of 5-HT and the serotoninergic system has been observed in oocytes and in different embryo development stages of fish, amphibians, birds, and mammals. In several classes of vertebrates, the change in the concentration of 5-HT or the alteration of the serotoninergic system, interfere with early embryo development. These data suggest that 5-HT participates in embryo development of vertebrates.

Serotonylation: Serotonin Signaling and Epigenetics

Serotonylation, the covalent linkage of serotonin to proteins has been discovered more than 60 years ago but only recently the mechanisms and first functions have been elucidated. It has been found that transglutaminases (TG) such as TG2 and the blood coagulation factor XIIIa are the enzymes which catalyze the linkage of serotonin and other monoamines to distinct glutamine (Gln) residues of target proteins. The first target proteins, small G-proteins and extracellular matrix constituents, were found in platelets and are pivotally involved in platelet aggregation and the formation of thrombi. The serotonylation of the same proteins is also involved in insulin secretion and in the proliferation of pulmonary vascular smooth muscle cells and thereby in the pathogenesis of pulmonary arterial hypertension (PAH). Recently histones have been described as targets of serotonylation opening the area of transcriptional control to this posttranslational protein modification. Future studies will certainly reveal further target proteins, signaling pathways, cellular processes, and diseases, in which serotonylation or, more general, monoaminylation is important.