Distribution of exogenous [125I]-3-iodothyronamine in mouse in vivo: relationship with trace amine-associated receptors

An injectable refrigerated hydrogel for inducing local hypothermia and neuroprotection against traumatic brain injury in mice

BACKGROUND

Hypothermia is a promising therapy for traumatic brain injury (TBI) in the clinic. However, the neuroprotective outcomes of hypothermia-treated TBI patients in clinical studies are inconsistent due to several severe side effects. Here, an injectable refrigerated hydrogel was designed to deliver 3-iodothyronamine (T1AM) to achieve a longer period of local hypothermia for TBI treatment. Hydrogel has four advantages: (1) It can be injected into injured sites after TBI, where it forms a hydrogel and avoids the side effects of whole-body cooling. (2) Hydrogels can biodegrade and be used for controlled drug release. (3) Released T1AM can induce hypothermia. (4) This hydrogel has increased medical value given its simple operation and ability to achieve timely treatment.

METHODS

Pol/T hydrogels were prepared by a low-temperature mixing method and characterized. The effect of the Pol/T hydrogel on traumatic brain injury in mice was studied. The degradation of the hydrogel at the body level was observed with a small animal imager. Brain temperature and body temperature were measured by brain thermometer and body thermometer, respectively. The apoptosis of peripheral nerve cells was detected by immunohistochemical staining. The protective effect of the hydrogels on the blood-brain barrier (BBB) after TBI was evaluated by the Evans blue penetration test. The protective effect of hydrogel on brain edema after injury in mice was detected by Magnetic resonance (MR) in small animals. The enzyme linked immunosorbent assay (ELISA) method was used to measure the levels of inflammatory factors. The effects of behavioral tests on the learning ability and exercise ability of mice after injury were evaluated.

RESULTS

This hydrogel was able to cool the brain to hypothermia for 12 h while maintaining body temperature within the normal range after TBI in mice. More importantly, hypothermia induced by this hydrogel leads to the maintenance of BBB integrity, the prevention of cell death, the reduction of the inflammatory response and brain edema, and the promotion of functional recovery after TBI in mice. This cooling method could be developed as a new approach for hypothermia treatment in TBI patients.

CONCLUSION

Our study showed that injectable and biodegradable frozen Pol/T hydrogels to induce local hypothermia in TBI mice can be used for the treatment of traumatic brain injury.

A Pilot Study on a Possible Mechanism behind Olfactory Dysfunction in Parkinson's Disease: The Association of TAAR1 Downregulation with Neuronal Loss and Inflammation along Olfactory Pathway

Parkinson's disease (PD) is characterized not only by motor symptoms but also by non-motor dysfunctions, such as olfactory impairment; the cause is not fully understood. Our study suggests that neuronal loss and inflammation in brain regions along the olfactory pathway, such as the olfactory bulb (OB) and the piriform cortex (PC), may contribute to olfactory dysfunction in PD mice, which might be related to the downregulation of the trace amine-associated receptor 1 (TAAR1) in these areas. In the striatum, although only a decrease in mRNA level, but not in protein level, of TAAR1 was detected, bioinformatic analyses substantiated its correlation with PD. Moreover, we discovered that neuronal death and inflammation in the OB and the PC in PD mice might be regulated by TAAR through the Bcl-2/caspase3 pathway. This manifested as a decrease of anti-apoptotic protein Bcl-2 and an increase of the pro-apoptotic protein cleaved caspase3, or through regulating astrocytes activity, manifested as the increase of TAAR1 in astrocytes, which might lead to the decreased clearance of glutamate and consequent neurotoxicity. In summary, we have identified a possible mechanism to elucidate the olfactory dysfunction in PD, positing neuronal damage and inflammation due to apoptosis and astrocyte activity along the olfactory pathway in conjunction with the downregulation of TAAR1.

Effect of Combined Levothyroxine (L-T4) and 3-Iodothyronamine (T1AM) Supplementation on Memory and Adult Hippocampal Neurogenesis in a Mouse Model of Hypothyroidism

Mood alterations, anxiety, and cognitive impairments associated with adult-onset hypothyroidism often persist despite replacement treatment. In rodent models of hypothyroidism, replacement does not bring 3-iodothyronamine (T1AM) brain levels back to normal. T1AM is a thyroid hormone derivative with cognitive effects. Using a pharmacological hypothyroid mouse model, we investigated whether augmenting levothyroxine (L-T4) with T1AM improves behavioural correlates of depression, anxiety, and memory and has an effect on hippocampal neurogenesis. Hypothyroid mice showed impaired performance in the novel object recognition test as compared to euthyroid mice (discrimination index (DI): 0.02 ± 0.09 vs. 0.29 ± 0.06; t = 2.515, p = 0.02). L-T4 and L-T4+T1AM rescued memory (DI: 0.27 ± 0.08 and 0.34 ± 0.08, respectively), while T1AM had no effect (DI: -0.01 ± 0.10). Hypothyroidism reduced the number of neuroprogenitors in hippocampal neurogenic niches by 20%. L-T4 rescued the number of neuroprogenitors (mean diff = 106.9 ± 21.40, t = 4.99, pcorr = 0.003), while L-T4+T1AM produced a 30.61% rebound relative to euthyroid state (mean diff = 141.6 ± 31.91, t = 4.44, pcorr = 0.004). We performed qPCR analysis of 88 genes involved in neurotrophic signalling pathways and found an effect of treatment on the expression of Ngf, Kdr, Kit, L1cam, Ntf3, Mapk3, and Neurog2. Our data confirm that L-T4 is necessary and sufficient for recovering memory and hippocampal neurogenesis deficits associated with hypothyroidism, while we found no evidence to support the role of non-canonical TH signalling.

Nongenomic roles of thyroid hormones and their derivatives in adult brain: are these compounds putative neurotransmitters?

We review the evidence regarding the nongenomic (or non-canonical) actions of thyroid hormones (thyronines) and their derivatives (including thyronamines and thyroacetic acids) in the adult brain. The paper seeks to evaluate these compounds for consideration as candidate neurotransmitters. Neurotransmitters are defined by their (a) presence in the neural tissue, (b) release from neural tissue or cell, (c) binding to high-affinity and saturable recognition sites, (d) triggering of a specific effector mechanism and (e) inactivation mechanism. Thyronines and thyronamines are concentrated in brain tissue and show distinctive patterns of distribution within the brain. Nerve terminals accumulate a large amount of thyroid hormones in mature brain, suggesting a synaptic function. However, surprisingly little is known about the potential release of thyroid hormones at synapses. There are specific binding sites for thyroid hormones in nerve-terminal fractions (synaptosomes). A notable cell-membrane binding site for thyroid hormones is integrin αvβ3. Furthermore, thyronines bind specifically to other defined neurotransmitter receptors, including GABAergic, catecholaminergic, glutamatergic, serotonergic and cholinergic systems. Here, the thyronines tend to bind to sites other than the primary sites and have allosteric effects. Thyronamines also bind to specific membrane receptors, including the trace amine associated receptors (TAARs), especially TAAR1. The thyronines and thyronamines activate specific effector mechanisms that are short in latency and often occur in subcellular fractions lacking nuclei, suggesting nongenomic actions. Some of the effector mechanisms for thyronines include effects on protein phosphorylation, Na+/K+ ATPase, and behavioral measures such as sleep regulation and measures of memory retention. Thyronamines promptly regulate body temperature. Lastly, there are numerous inactivation mechanisms for the hormones, including decarboxylation, deiodination, oxidative deamination, glucuronidation, sulfation and acetylation. Therefore, at the current state of the research field, thyroid hormones and their derivatives satisfy most, but not all, of the criteria for definition as neurotransmitters.

Psychosocial Stress Induces Orofacial Mechanical Allodynia Due to the Enhancement of Transient Receptor Potential Ankyrin 1 Expression in Trigeminal Ganglion Neurons via the Increment of the Trace Amine-Associated Receptor 7f Expression

(1) Background: Chronic psychosocial stress can lead to oral dysesthesia with tongue pain. We examined whether psychosocial stress causes orofacial pain, and analyzed the comprehensive gene expression patterns of circulating cells and transient receptor potential ankyrin 1 (TRPA1) expression in trigeminal ganglion (TG) neurons in a mouse model of psychosocial stress. (2) Methods: Mice were divided into two groups: one group was kept in confrontational housing, and the other group was kept in single housing. Blood, adrenal gland, and tongue were collected. The head withdrawal threshold (HWT) of mechanical stimulation to the whisker pad skin was measured. TRPA1-positive TG neurons were immunohistochemically examined. DNA microarray analysis and quantitative reverse transcription polymerase chain reaction analysis were performed. (3) Results: The HWT was significantly lower in mice under the psychosocial stress condition compared to non-stressed mice. In stress-loaded mice, the number of TRPA1-positive TG neurons was significantly increased. Moreover, we showed that trace amine-associated receptor 7f expression was upregulated in circulating cells in blood and downregulated in the tongue. (4) Conclusions: Our results indicated that chronic psychosocial stress induced the orofacial mechanical allodynia through enhancement of TRPA1 expression in TG neurons with changes in the levels of trace amine-associated receptor 7f.

Modelling of p-tyramine transport across human intestinal epithelial cells predicts the presence of additional transporters

p-Tyramine (TYR) is an endogenous trace amine, which can also be synthesized by intestinal microbiota, and is present in commonly consumed diets. TYR is an agonist for the intracellular trace amine-associated receptor 1, which has been implicated in psychiatric, metabolic, and immune-related disorders. We have previously demonstrated TYR readily diffuses across lipid bilayers, while transport across Caco-2 cell membranes involves Organic Cation Transporter 2 (OCT2) and a Na⁺-dependent active transporter. Here we developed mathematical models to determine whether known kinetics for these processes are sufficient to explain observed transcellular TYR passage. Ordinary differential equations were developed for known TYR transport processes to predict concentration-time relationships. Michaelis-Menten kinetics were assumed for all transporter-mediated processes and a one phase exponential function used for simple diffusion. Modelled concentration-time plots were compared to published experimental results. Additional transporter functions were sequentially added to models to improve consistency, and a least squares error minimization approach utilized to determine added transporter kinetics. Finally, possible TYR compartmentalization was also modelled. Following apical loading, transport across the apical, but not the basolateral, membrane was modelled without additional transporters, suggesting a basolateral transporter was missing. Consistent with this, models of basolateral compartment loading did not match experimental observations, indicating missing basolateral transporters were bidirectional. Addition of a transporter with the kinetic characteristics of OCT2 did not improve models. Varying the kinetic parameters of the added transporter improved models of basolateral, but worsened apical, loading models, suggesting the need for either a directional preference in transporters, or intracellular TYR compartmentalization. Experimental parameters were recapitulated by introducing asymmetry into the apical OCT2 (K_{t_OCT2_apicaltocell} = 110.4 nM, K_{t_OCT2_celltoapical} = 1,227.9 nM), and a symmetric basolateral facilitated diffusion transporter (V_max = 6.0 nM/s, K_t = 628.3 nM). The apparent directionality of OCT2 may reflect altered TYR ionization due to known pH differences between compartments. Models for asymmetry and compartmentalization were compared by root mean square deviation from experimental data, and it was found that TYR compartmentalization could only partially replace the need for asymmetry of OCT2. In conclusion, modelling indicates that known TYR transport processes are insufficient to explain experimental concentration-time profiles and that asymmetry of the apical membrane OCT2 combined with additional, low affinity, basolateral membrane facilitated diffusion transporters are required.

Therapeutic Potential of TAAR1 Agonists in Schizophrenia: Evidence from Preclinical Models and Clinical Studies

Trace amine-associated receptor 1 (TAAR1) has emerged as a promising therapeutic target for neuropsychiatric disorders due to its ability to modulate monoaminergic and glutamatergic neurotransmission. In particular, agonist compounds have generated interest as potential treatments for schizophrenia and other psychoses due to TAAR1-mediated regulation of dopaminergic tone. Here, we review unmet needs in schizophrenia, the current state of knowledge in TAAR1 circuit biology and neuropharmacology, including preclinical behavioral, imaging, and cellular evidence in glutamatergic, dopaminergic and genetic models linked to the pathophysiology of psychotic, negative and cognitive symptoms. Clinical trial data for TAAR1 drug candidates are reviewed and contrasted with antipsychotics. The identification of endogenous TAAR1 ligands and subsequent development of small-molecule agonists has revealed antipsychotic-, anxiolytic-, and antidepressant-like properties, as well as pro-cognitive and REM-sleep suppressing effects of TAAR1 activation in rodents and non-human primates. Ulotaront, the first TAAR1 agonist to progress to randomized controlled clinical trials, has demonstrated efficacy in the treatment of schizophrenia, while another, ralmitaront, is currently being evaluated in clinical trials in schizophrenia. Coupled with the preclinical findings, this provides a rationale for further investigation and development of this new pharmacological class for the treatment of schizophrenia and other psychiatric disorders.

Potential of Ligands for Trace Amine-Associated Receptor 1 (TAAR1) in the Management of Substance Use Disorders

Trace amines, including β-phenylethylamine (β-PEA), p-tyramine (TYR), tryptamine (TRP), and p-octopamine (OCT), represent a group of amines expressed at low levels in the mammalian brain. Given the close structural similarities to traditional monoamines, links between trace amines and the monoaminergic system have long been suspected. Trace amine-associated receptor 1 (TAAR1), the most well characterized receptor in the TAAR family, has been shown to be potently activated by trace amines such as TYR and PEA. Further, catecholamine metabolites and amphetamine analogs are also potent agonists of TAAR1, implicating the receptor in mediating the monoaminergic system and in substance use disorders. In the central nervous system, TAAR1 is expressed in brain regions involved in dopaminergic, serotonergic, and glutamatergic transmission, and genetic animal models and electrophysiological studies have revealed that TAAR1 is a potent modulator of the monoaminergic system. Selective and potent engineered TAAR1 ligands, including full (RO5166017 and RO5256390) and partial (RO5203648, RO5263397 and RO5073012) agonists and the antagonist EPPTB (N-(3-ethoxyphenyl)-4-(1-pyrrolidinyl)-3-(trifluoromethyl) benzamide, RO5212773), serve as invaluable tools for the investigation of TAAR1 functions and display significant potential for the development of TAAR1-based pharmacotherapies for the treatment of substance use disorders. Despite a number of advances that have been made, more clinical studies are warranted in order to test the potential and efficacy of TAAR1 ligands in the treatment of psychiatric disorders, including substance use disorders.

Role of the trace amine associated receptor 5 (TAAR5) in the sensorimotor functions

Classical monoamines are well-known modulators of sensorimotor neural networks. However, the role of trace amines and their receptors in sensorimotor function remains unexplored. Using trace amine-associated receptor 5 knockout (TAAR5-KO) mice, that express beta-galactosidase mapping its localization, we observed TAAR5 expression in the Purkinje cells of the cerebellum and the medial vestibular nucleus, suggesting that TAAR5 might be involved in the vestibular and motor control. Accordingly, in various behavioral tests, TAAR5-KO mice demonstrated lower endurance, but better coordination and balance compared to wild-type controls. Furthermore, we found specific changes in striatal local field potentials and motor cortex electrocorticogram, such as a decrease in delta and an increase in theta oscillations of power spectra, respectively. The obtained data indicate that TAAR5 plays a considerable role in regulation postural stability, muscle force, balance, and motor coordination during active movements, likely via modulation of monoaminergic systems at different levels of sensorimotor control involving critical brain areas such as the brainstem, cerebellum, and forebrain.

Differential effects of β-methylphenylethylamine and octopamine on contractile parameters of the rat gastrointestinal tract

This study tested the effects of β-methylphenylethylamine (β-MPEA) and octopamine on contractile parameters of the gastrointestinal tract in rats. We hypothesized that some of their effects result from interactions with trace amine (TA)-associated receptors or serotoninergic 5-hydroxytryptamine (5-HT) receptors. β-MPEA-induced contractions in rat gastric fundus strips under resting tonus conditions, but induced relaxation in preparations that were previously contracted with carbachol. Octopamine relaxed gastric fundus strips maintained at resting tonus or contracted with carbachol. The contractile effect of β-MPEA was reduced by cyproheptadine and methiothepin, antagonists of excitatory 5-HT receptors. The relaxing effect of β-MPEA on gastric fundus was insensitive to pretreatment with N-(3-ethoxyphenyl)-4-(1-pyrrolidinyl)-3-(trifluoromethyl)benzamide (EPPTB) and tropisetron, antagonists of TA1 and 5-HT4 receptors, respectively. Both EPPTB and tropisetron inhibited the relaxant effects of octopamine on carbachol-contracted preparations. Contrarily, EPPTB did not reduce the relaxant effects of RO5263397 (TA1 agonist) or zacopride (5-HT4 agonist). Octopamine, but not β-MPEA, delayed the gastrointestinal transit of a liquid test meal in awaken rats. In isolated preparations of the small intestine under resting conditions, β-MPEA did not alter the basal tonus, but octopamine relaxed it. Intestinal preparations previously contracted with carbachol relaxed after the addition of octopamine and decreased the magnitude of their spontaneous rhythmic contractions in a tropisetron-dependent manner. Thus, β-MPEA and octopamine exerted pharmacological actions on the rat gastrointestinal tract. The excitatory effects of β-MPEA involved 5-HT receptors. Octopamine inhibited the rat gut contractility through the likely involvement of 5-HT4 and TA receptors. Overall, octopamine effectively inhibited rat gastrointestinal transit.

Spasmolytic Effects of Aphanizomenon Flos Aquae (AFA) Extract on the Human Colon Contractility

The blue-green algae Aphanizomenon flos aquae (AFA), rich in beneficial nutrients, exerts various beneficial effects, acting in different organs including the gut. Klamin^® is an AFA extract particularly rich in β-PEA, a trace-amine considered a neuromodulator in the central nervous system. To date, it is not clear if β-PEA exerts a role in the enteric nervous system. The aims of the present study were to investigate the effects induced by Klamin^® on the human distal colon mechanical activity, to analyze the mechanism of action, and to verify a β-PEA involvement. The organ bath technique, RT-PCR, and immunohistochemistry (IHC) were used. Klamin^® reduced, in a concentration-dependent manner, the amplitude of the spontaneous contractions. EPPTB, a trace-amine receptor (TAAR1) antagonist, significantly antagonized the inhibitory effects of both Klamin^® and exogenous β-PEA, suggesting a trace-amine involvement in the Klamin^® effects. Accordingly, AphaMax^®, an AFA extract containing lesser amount of β-PEA, failed to modify colon contractility. Moreover, the Klamin^® effects were abolished by tetrodotoxin, a neural blocker, but not by L-NAME, a nitric oxide-synthase inhibitor. On the contrary methysergide, a serotonin receptor antagonist, significantly antagonized the Klamin^® effects, as well as the contractility reduction induced by 5-HT. The RT-PCR analysis revealed TAAR1 gene expression in the colon and the IHC experiments showed that 5-HT-positive neurons are co-expressed with TAAR1 positive neurons. In conclusion, the results of this study suggest that Klamin^® exerts spasmolytic effects in human colon contractility through β-PEA, that, by activating neural TAAR1, induce serotonin release from serotoninergic neurons of the myenteric plexus.

Delivery of Thyronamines (TAMs) to the Brain: A Preliminary Study

Recent reports highlighted the significant neuroprotective effects of thyronamines (TAMs), a class of endogenous thyroid hormone derivatives. In particular, 3-iodothyronamine (T1AM) has been shown to play a pleiotropic role in neurodegeneration by modulating energy metabolism and neurological functions in mice. However, the pharmacological response to T1AM might be influenced by tissue metabolism, which is known to convert T1AM into its catabolite 3-iodothyroacetic acid (TA1). Currently, several research groups are investigating the pharmacological effects of T1AM systemic administration in the search of novel therapeutic approaches for the treatment of interlinked pathologies, such as metabolic and neurodegenerative diseases (NDDs). A critical aspect in the development of new drugs for NDDs is to know their distribution in the brain, which is fundamentally related to their ability to cross the blood–brain barrier (BBB). To this end, in the present study we used the immortalized mouse brain endothelial cell line bEnd.3 to develop an in vitro model of BBB and evaluate T1AM and TA1 permeability. Both drugs, administered at 1 µM dose, were assayed by high-performance liquid chromatography coupled to mass spectrometry. Our results indicate that T1AM is able to efficiently cross the BBB, whereas TA1 is almost completely devoid of this property.

The trace aminergic system: a gender-sensitive therapeutic target for IBS?

Due to a lack of specific or sensitive biomarkers, drug discovery advances have been limited for individuals suffering from irritable bowel syndrome (IBS). While current therapies provide symptomatic relief, inflammation itself is relatively neglected, despite the presence of chronic immune activation and innate immune system dysfunction. Moreover, considering the microgenderome concept, gender is a significant aetiological risk factor. We believe that we have pinpointed a "missing link" that connects gender, dysbiosis, diet, and inflammation in the context of IBS, which may be manipulated as therapeutic target. The trace aminergic system is conveniently positioned at the interface of the gut microbiome, dietary nutrients and by-products, and mucosal immunity. Almost all leukocyte populations express trace amine associated receptors and significant amounts of trace amines originate from both food and the gut microbiota. Additionally, although IBS-specific data are sparse, existing data supports an interpretation in favour of a gender dependence in trace aminergic signalling. As such, trace aminergic signalling may be altered by fluctuations of especially female reproductive hormones. Utilizing a multidisciplinary approach, this review discusses potential mechanisms of actions, which include hyperreactivity of the immune system and aberrant serotonin signalling, and links outcomes to the symptomology clinically prevalent in IBS. Taken together, it is feasible that the additional level of regulation by the trace aminergic system in IBS has been overlooked, until now. As such, we suggest that components of the trace aminergic system be considered targets for future therapeutic action, with the specific focus of reducing oxidative stress and inflammation.

Involvement of Organic Cation Transporter 2 and a Na+-dependent active transporter in p-tyramine transport across Caco-2 intestinal cells

AIMS

We have previously demonstrated that p-tyramine (TYR), an endogenous trace amine-associated receptor 1 agonist, passage across neuronal membranes involves a transporter exhibiting the pharmacological profile of Organic Cation Transporter 2 (OCT2). Since TYR is also a constituent of foodstuffs and produced by the intestinal microbiota, here we have investigated whether similar processes are involved in the passage of 100 nM TYR across apical and basolateral membranes of the Caco-2 human intestinal epithelial cell line.

MATERIALS AND METHODS

[³H]TYR transport across apical and basolateral membranes of Caco-2 cell monolayers was measured in the presence of inhibitors of TYR metabolizing enzymes. Cellular, apical, and basolateral compartments were collected at various timepoints, TYR concentrations calculated, and transport properties pharmacologically characterized.

KEY FINDINGS

Apical transport resulted in equimolar accumulation of TYR within cells. Pentamidine (OCT1/OCT2 inhibitor) decreased apical transport (P = 0.001) while atropine (OCT1 inhibitor) had no effect, suggesting apical transport involved OCT2. In contrast, basolateral transport resulted in 500-1000 nM cellular concentrations (P < 0.0001) indicating the presence of an active transporter. Replacement of Na⁺ on an equimolar basis with choline resulted in loss of TYR transport (P = 0.017). Unexpectedly, this active transport was also atropine-sensitive (P = 0.020). Kinetic analysis of the active transporter revealed V_max = 43.0 nM/s with a K_t = 33.1 nM.

SIGNIFICANCE

We have demonstrated for the first time that TYR is transported across Caco-2 apical membranes via facilitated diffusion by OCT2, whereas transport across basolateral membranes is by a Na⁺-dependent, atropine-sensitive, active transporter.

Genomic and Non-Genomic Mechanisms of Action of Thyroid Hormones and Their Catabolite 3,5-Diiodo-L-Thyronine in Mammals

Since the realization that the cellular homologs of a gene found in the retrovirus that contributes to erythroblastosis in birds (v-erbA), i.e. the proto-oncogene c-erbA encodes the nuclear receptors for thyroid hormones (THs), most of the interest for THs focalized on their ability to control gene transcription. It was found, indeed, that, by regulating gene expression in many tissues, these hormones could mediate critical events both in development and in adult organisms. Among their effects, much attention was given to their ability to increase energy expenditure, and they were early proposed as anti-obesity drugs. However, their clinical use has been strongly challenged by the concomitant onset of toxic effects, especially on the heart. Notably, it has been clearly demonstrated that, besides their direct action on transcription (genomic effects), THs also have non-genomic effects, mediated by cell membrane and/or mitochondrial binding sites, and sometimes triggered by their endogenous catabolites. Among these latter molecules, 3,5-diiodo-L-thyronine (3,5-T2) has been attracting increasing interest because some of its metabolic effects are similar to those induced by T3, but it seems to be safer. The main target of 3,5-T2 appears to be the mitochondria, and it has been hypothesized that, by acting mainly on mitochondrial function and oxidative stress, 3,5-T2 might prevent and revert tissue damages and hepatic steatosis induced by a hyper-lipid diet, while concomitantly reducing the circulating levels of low density lipoproteins (LDL) and triglycerides. Besides a summary concerning general metabolism of THs, as well as their genomic and non-genomic effects, herein we will discuss resistance to THs and the possible mechanisms of action of 3,5-T2, also in relation to its possible clinical use as a drug.

3-Iodothyronamine Affects Thermogenic Substrates' Mobilization in Brown Adipocytes

We investigated the effect of 3-iodothyronamine (T1AM) on thermogenic substrates in brown adipocytes (BAs). BAs isolated from the stromal fraction of rat brown adipose tissue were exposed to an adipogenic medium containing insulin in the absence (M) or in the presence of 20 nM T1AM (M+T1AM) for 6 days. At the end of the treatment, the expression of p-PKA/PKA, p-AKT/AKT, p-AMPK/AMPK, p-CREB/CREB, p-P38/P38, type 1 and 3 beta adrenergic receptors (β1–β3AR), GLUT4, type 2 deiodinase (DIO2), and uncoupling protein 1 (UCP-1) were evaluated. The effects of cell conditioning with T1AM on fatty acid mobilization (basal and adrenergic-mediated), glucose uptake (basal and insulin-mediated), and ATP cell content were also analyzed in both cell populations. When compared to cells not exposed, M+T1AM cells showed increased p-PKA/PKA, p-AKT/AKT, p-CREB/CREB, p-P38/P38, and p-AMPK/AMPK, downregulation of DIO2 and β1AR, and upregulation of glycosylated β3AR, GLUT4, and adiponectin. At basal conditions, glycerol release was higher for M+T1AM cells than M cells, without any significant differences in basal glucose uptake. Notably, in M+T1AM cells, adrenergic agonists failed to activate PKA and lipolysis and to increase ATP level, but the glucose uptake in response to insulin exposure was more pronounced than in M cells. In conclusion, our results suggest that BAs conditioning with T1AM promote a catabolic condition promising to fight obesity and insulin resistance.

3-Iodothyronamine and Derivatives: New Allies Against Metabolic Syndrome?

In the two decades since its discovery, a large body of evidence has amassed to highlight the potential of 3-iodothyronamine (T1AM) as an antiobesity drug, whose pleiotropic signaling actions profoundly impact energy metabolism. In the present review, we recapitulate the most relevant properties of T1AM, including its structural and functional relationship to thyroid hormone, its endogenous levels, molecular targets, as well as its genomic and non-genomic effects on metabolism elicited in experimental models after exogenous administration. The physiological and pathophysiological relevance of T1AM in the regulation of energy homeostasis and metabolism is also discussed, along with its potential therapeutic applications in metabolic disturbances. Finally, we examine a number of T1AM analogs that have been recently developed with the aim of designing novel pharmacological agents for the treatment of interlinked diseases, such as metabolic and neurodegenerative disorders, as well as additional synthetic tools that can be exploited to further explore T1AM-dependent mechanisms and the physiological roles of trace amine-associated receptor 1 (TAAR1)-mediated effects.

TAAR Agonists

Trace amine-associated receptors (TAARs) are a family of G protein-coupled receptors (GPCRs) that are evolutionarily conserved in vertebrates. The first discovered TAAR1 is mainly expressed in the brain, and is able to detect low abundant trace amines. TAAR1 is also activated by several synthetic compounds and psychostimulant drugs like amphetamine. Activation of TAAR1 by specific agonists can regulate the classical monoaminergic systems in the brain. Further studies have revealed that other TAAR family members are highly expressed in the olfactory system which are termed olfactory TAARs. In vertebrates, olfactory TAARs can specifically recognize volatile or water-soluble amines. Some of these TAAR agonists are produced by decarboxylation of amino acids. In addition, some TAAR agonists are ethological odors that mediate animal innate behaviors. In this study, we provide a comprehensive review of TAAR agonists, including their structures, biosynthesis pathways, and functions.

Novel thyroid hormones

The field of thyroid hormone signaling has grown more complex in recent years. In particular, it has been suggested that some thyroid hormone derivatives, tentatively named "novel thyroid hormones" or "active thyroid hormone metabolites", may act as independent chemical messengers. They include 3,5-diiodothyronine (T2), 3-iodothyronamine (T1AM), and several iodothyroacetic acids, i.e., 3,5,3',5'-thyroacetic acid (TA4), 3,5,3'-thyroacetic acid (TA3), and 3-thyroacetic acid (TA1). We summarize the present knowledge on these compounds, namely their biosynthetic pathways, endogenous levels, molecular targets, and the functional effects elicited in experimental preparations or intact animals after exogenous administration. Their physiological and pathophysiological role is discussed, and potential therapeutic applications are outlined. The requirements needed to qualify these substances as chemical messengers must still be validated, although promising evidence has been collected. At present, the best candidate to the role of independent chemical messenger appears to be T1AM, and its most interesting effects concern metabolism and brain function. The responses elicited in experimental animals have suggested potential therapeutic applications. TA3 has an established role in thyroid hormone resistance syndromes, and is under investigation in Allen-Herndon-Dudley syndrome. Other potential targets are represented by obesity and dyslipidemia (for T2 and T1AM); dementia and degenerative brain disease (for T1AM and TA1); cancer (for T1AM and TA4). Another intriguing and unexplored question is the potential relevance of these metabolites in the clinical picture of hypothyroidism and in the response to replacement therapy.

Lipolytic Effects of 3-Iodothyronamine (T1AM) and a Novel Thyronamine-Like Analog SG-2 through the AMPK Pathway

3-Iodothyronamine (T1AM) and its synthetic analog SG-2 are rapidly emerging as promising drivers of cellular metabolic reprogramming. Our recent research indicates that in obese mice a sub-chronic low dose T1AM treatment increased lipolysis, associated with significant weight loss independent of food consumption. The specific cellular mechanism of T1AM's lipolytic effect and its site of action remains unknown. First, to study the mechanism used by T1AM to gain entry into cells, we synthesized a fluoro-labeled version of T1AM (FL-T1AM) by conjugating it to rhodamine (TRITC) and analyzed its cellular uptake and localization in 3T3-L1 mouse adipocytes. Cell imaging using confocal microscopy revealed a rapid intercellular uptake of FL-T1AM into mitochondria without localization to the lipid droplet or nucleus of mature adipocytes. Treatment of 3T3-L1 adipocytes with T1AM and SG-2 resulted in decreased lipid accumulation, the latter showing a significantly higher potency than T1AM (10 µM vs. 20 µM, respectively). We further examined the effects of T1AM and SG-2 on liver HepG2 cells. A significant decrease in lipid accumulation was observed in HepG2 cells treated with T1AM or SG-2, due to increased lipolytic activity. This was confirmed by accumulation of glycerol in the culture media and through activation of the AMPK/ACC signaling pathways.

Brain Histamine Modulates the Antidepressant-Like Effect of the 3-Iodothyroacetic Acid (TA1)

3-iodothyroacetic acid (TA1), an end metabolite of thyroid hormone, has been shown to produce behavioral effects in mice that are dependent on brain histamine. We now aim to verify whether pharmacologically administered TA1 has brain bioavailability and is able to induce histamine-dependent antidepressant-like behaviors. TA1 brain, liver and plasma levels were measured by LC/MS-MS in male CD1 mice, sacrificed 15 min after receiving a high TA1 dose (330 μgkg^-1). The hypothalamic mTOR/AKT/GSK-β cascade activation was evaluated in mice treated with 0.4, 1.32, 4 μgkg^-1 TA1 by Western-blot. Mast cells were visualized by immuno-histochemistry in brain slices obtained from mice treated with 4 μgkg^-1 TA1. Histamine release triggered by TA1 (20-1000 nM) was also evaluated in mouse peritoneal mast cells. After receiving TA1 (1.32, 4 or 11 μgkg^-1; i.p.) CD1 male mice were subjected to the forced swim (FST) and the tail suspension tests (TST). Spontaneous locomotor and exploratory activities, motor incoordination, and anxiolytic or anxiogenic effects, were evaluated. Parallel behavioral tests were also carried out in mice that, prior to receiving TA1, were pre-treated with pyrilamine (10 mgkg^-1; PYR) or zolantidine (5 mgkg^-1; ZOL), histamine type 1 and type 2 receptor antagonists, respectively, or with p-chloro-phenylalanine (100 mgkg^-1; PCPA), an inhibitor of serotonin synthesis. TA1 given i.p. to mice rapidly distributes in the brain, activates the hypothalamic mTOR/AKT and GSK-3β cascade and triggers mast cells degranulation. Furthermore, TA1 induces antidepressant effects and stimulates locomotion with a mechanism that appears to depend on the histaminergic system. TA1 antidepressant effect depends on brain histamine, thus highlighting a relationship between the immune system, brain inflammation and the thyroid.

TAAR1 levels and sub-cellular distribution are cell line but not breast cancer subtype specific

Trace amine-associated receptors are G protein-coupled receptors of which TAAR1 is the most well-studied. Recently, Vattai et al. (J Cancer Res Clin Oncol 143:1637-1647 https://doi.org/10.1007/s00432-017-2420-8 , 2017) reported that expression of TAAR1 may be a marker of breast cancer (BC) survival, with a positive correlation also suggested between TAAR1 expression and HER2 positivity. Neither a role for TAAR1 in breast tissue, nor in cancer, had previously been suspected. We, therefore, sought to provide independent validation and to further examine these putative relationships. First, a bioinformatic analysis on 58 total samples including normal breast tissue, BC-related cell lines, and tumour samples representing different BC sub-types found no clear correlation between TAAR1 mRNA levels and any BC subtype, including HER2 + . We next confirmed the bioinformatics data correlated to protein expression using a well validated anti-human TAAR1 antibody. TAAR1 mRNA levels correlated with the relative intensity of immunofluorescence staining in six BC cell lines (MCF-7, T47D, MDA-MB-231, SKBR3, MDA-MB-468, BT-474), but not in the MCF-10A immortalized mammary gland line, which had high mRNA but low protein levels. As expected, TAAR1 protein was intracellular in all cell lines. Surprisingly MCF-7, SKBR3, and MDA-MB-468 showed pronounced nuclear localization. The relative protein expression in MCF-7, MDA-MB-231, and MCF-10A lines was further confirmed by semi-quantitative flow cytometry. Finally, we demonstrate that the commercially available anti-TAAR1 antibody has poor selectivity, which likely explains the lack of correlation with the previous study. Therefore, while we clearly demonstrate variable expression and sub-cellular localization of TAAR1 across BC cell lines, we find no evidence for association with BC subtype.

3-Iodothyronamine-A Thyroid Hormone Metabolite With Distinct Target Profiles and Mode of Action

The rediscovery of the group of thyronamines (TAMs), especially the first detailed description of their most prominent congener 3-iodothyronamine (3T1AM) 14 years ago, boosted research on this thyroid hormone metabolite tremendously. TAMs exert actions partly opposite to and distinct from known functions of thyroid hormones. These fascinating metabolic, anapyrexic, cytoprotective, and brain effects quickly evoked the hope to use hormone-derived TAMs as a therapeutic option. The G protein-coupled receptor (GPCR) TAAR1, a member of the trace amine-associated receptor (TAAR) family, was identified as the first target and effector of TAM action. The initial enthusiasm on pharmacological actions of exogenous TAMs elicited many questions, such as sites of biosynthesis, analytics, modes of action, inactivation, and role of TAMs in (patho)physiology. Meanwhile, it became clear that TAMs not only interact with TAAR1 or other TAAR family members but also with several aminergic receptors and non-GPCR targets such as transient receptor potential channels, mitochondrial proteins, and the serum TAM-binding protein apolipoprotein B100, thus classifying 3T1AM as a multitarget ligand. The physiological mode of action of TAMs is still controversial because regulation of endogenous TAM production and the sites of its biosynthesis are not fully elucidated. Methods for 3T1AM analytics need further validation, as they revealed different blood and tissue concentrations depending on detection principles used such as monoclonal antibody-based immunoassay vs liquid chromatography- matrix-assisted laser desorption/ionization mass spectrometry or time-of-flight mass spectrometry. In this review, we comprehensively summarize and critically evaluate current basic, translational, and clinical knowledge on 3T1AM and its main metabolite 3-iodothyroacetic acid, focusing on endocrine-relevant aspects and open but highly challenging issues.

Thyroid Hormone, Thyroid Hormone Metabolites and Mast Cells: A Less Explored Issue

Mast cells are primary players in immune and inflammatory diseases. In the brain, mast cells are located at the brain side of the blood brain barrier (BBB) exerting a crucial role in protecting the brain from xenobiotic invasion. Furthermore, recent advances in neuroscience indicate mast cells may play an important role in glial cell-neuron communication through the release of mediators, including histamine. Interestingly, brain mast cells contain not only 50% of the brain histamine but also hormones, proteases and lipids or amine mediators; and cell degranulation may be triggered by different stimuli activating membrane bound receptors including the four types of histaminergic receptors. Among hormones, mast cells can store thyroid hormone (T3) and express membrane-bound thyroid stimulating hormone receptors (TSHRs), thus suggesting from one side that thyroid function may affect mast cells function, from the other that mast cell degranulation may impact on thyroid function. In this respect, the research on hormones in mast cells is scarce. Recent pharmacological evidence indicates the existence of a non-genomic portion of the thyroid secretion including thyroid hormone metabolites. Among which the 3,5 diiodothyronine (3,5-T2), 3-iodothyroanamine (T1AM) and 3-iodothyroacetic acid (TA1) are the most studied. All these compounds are endogenously occurring and found to be increased in inflammatory-based diseases involving mast cells. T1AM and TA1 induce, as T3, neuroprotective effects and itch but also hyperalgesia in rodents with a mechanism largely unknown but mediated by the release of histamine. Due to the rapid onset of their effectiveness they may trigger histamine release from a cell where it is “ready-to-be released,” i.e., mast cells. Following a very thin path which passes through old experimental and clinical evidence, at the light of novel acquisitions on endogenous T3 metabolites, we aim to stimulate the attention on the possibility that mast cell histamine may be the connector of a novel (neuro) endocrine pathway linking the thyroid with mast cells.

Trace Amines and Their Receptors

Trace amines are endogenous compounds classically regarded as comprising β-phenylethyalmine, p-tyramine, tryptamine, p-octopamine, and some of their metabolites. They are also abundant in common foodstuffs and can be produced and degraded by the constitutive microbiota. The ability to use trace amines has arisen at least twice during evolution, with distinct receptor families present in invertebrates and vertebrates. The term "trace amine" was coined to reflect the low tissue levels in mammals; however, invertebrates have relatively high levels where they function like mammalian adrenergic systems, involved in "fight-or-flight" responses. Vertebrates express a family of receptors termed trace amine-associated receptors (TAARs). Humans possess six functional isoforms (TAAR1, TAAR2, TAAR5, TAAR6, TAAR8, and TAAR9), whereas some fish species express over 100. With the exception of TAAR1, TAARs are expressed in olfactory epithelium neurons, where they detect diverse ethological signals including predators, spoiled food, migratory cues, and pheromones. Outside the olfactory system, TAAR1 is the most thoroughly studied and has both central and peripheral roles. In the brain, TAAR1 acts as a rheostat of dopaminergic, glutamatergic, and serotonergic neurotransmission and has been identified as a novel therapeutic target for schizophrenia, depression, and addiction. In the periphery, TAAR1 regulates nutrient-induced hormone secretion, suggesting its potential as a novel therapeutic target for diabetes and obesity. TAAR1 may also regulate immune responses by regulating leukocyte differentiation and activation. This article provides a comprehensive review of the current state of knowledge of the evolution, physiologic functions, pharmacology, molecular mechanisms, and therapeutic potential of trace amines and their receptors in vertebrates and invertebrates.

3-Iodothyronamine Activates a Set of Membrane Proteins in Murine Hypothalamic Cell Lines

3-Iodothyronamine (3-T1AM) is an endogenous thyroid hormone metabolite. The profound pharmacological effects of 3-T1AM on energy metabolism and thermal homeostasis have raised interest to elucidate its signaling properties in tissues that pertain to metabolic regulation and thermogenesis. Previous studies identified G protein-coupled receptors (GPCRs) and transient receptor potential channels (TRPs) as targets of 3-T1AM in different cell types. These two superfamilies of membrane proteins are largely expressed in tissue which influences energy balance and metabolism. As the first indication that 3-T1AM virtually modulates the function of the neurons in hypothalamus, we observed that intraperitoneal administration of 50 mg/kg bodyweight of 3-T1AM significantly increased the c-FOS activation in the paraventricular nucleus (PVN) of C57BL/6 mice. To elucidate the underlying mechanism behind this 3-T1AM-induced signalosome, we used three different murine hypothalamic cell lines, which are all known to express PVN markers, GT1-7, mHypoE-N39 (N39) and mHypoE-N41 (N41). Various aminergic GPCRs, which are the known targets of 3-T1AM, as well as numerous members of TRP channel superfamily, are expressed in these cell lines. Effects of 3-T1AM on activation of GPCRs were tested for the two major signaling pathways, the action of Gαs/adenylyl cyclase and Gi/o. Here, we demonstrated that this thyroid hormone metabolite has no significant effect on Gi/o signaling and only a minor effect on the Gαs/adenylyl cyclase pathway, despite the expression of known GPCR targets of 3-T1AM. Next, to test for other potential mechanisms involved in 3-T1AM-induced c-FOS activation in PVN, we evaluated the effect of 3-T1AM on the intracellular Ca2+ concentration and whole-cell currents. The fluorescence-optic measurements showed a significant increase of intracellular Ca2+ concentration in the three cell lines in the presence of 10 μM 3-T1AM. Furthermore, this thyroid hormone metabolite led to an increase of whole-cell currents in N41 cells. Interestingly, the TRPM8 selective inhibitor (10 μM AMTB) reduced the 3-T1AM stimulatory effects on cytosolic Ca2+ and whole-cell currents. Our results suggest that the profound pharmacological effects of 3-T1AM on selected brain nuclei of murine hypothalamus, which are known to be involved in energy metabolism and thermoregulation, might be partially attributable to TRP channel activation in hypothalamic cells.

Trace Amine-Associated Receptors as Novel Therapeutic Targets for Immunomodulatory Disorders

Trace amines and their receptors (trace amine-associated receptors; TAARs) are an emerging pharmacological target for the treatment of human disorders. While most studies have focused on their therapeutic potential for neurologic and psychiatric disorders, TAARs are also expressed throughout the periphery, including prominent expression in human leukocytes. Furthermore, recent independent, unbiased metabolomic studies have consistently identified one or more TAAR ligands as potential etiologic factors in inflammatory bowel disease (IBD). The putative role of TAARs in diseases such as IBD that are associated with hyperactive immune responses has not, however, previously been systematically addressed. Here, we review the current state of the knowledge of the effects of TAARs on leukocyte function, in particular in the context of mucosal epithelial cells that interface with the environment; developing a model whereby TAARs may be considered as a novel therapeutic target for disorders associated with dysregulated immune responses to environmental factors. In this model, we hypothesize that altered trace amine homeostasis results in hyperactivity of the immune system. Such loss of homeostasis can occur through many different mechanisms including TAAR polymorphisms and altered trace amine load due to changes in host synthesis and/or degradative enzymes, diet, or microbial dysbiosis. The resulting alterations in TAAR functioning can then lead to a loss of homeostasis of leukocyte chemotaxis, differentiation, and activation, as well as an altered ability of members of the microbiota to adhere to and penetrate the epithelial cell layers. Such changes would generate a pro-inflammatory state at mucosal epithelial barrier layers that can manifest as clinical symptomatology such as that seen in IBD. These alterations may also have the potential to induce systemic effects, which could possibly contribute to immunomodulatory disorders in other systems, including neurological diseases.

Metabolic Reprogramming by 3-Iodothyronamine (T1AM): A New Perspective to Reverse Obesity through Co-Regulation of Sirtuin 4 and 6 Expression

Obesity is a complex disease associated with environmental and genetic factors. 3-Iodothyronamine (T1AM) has revealed great potential as an effective weight loss drug. We used metabolomics and associated transcriptional gene and protein expression analysis to investigate the tissue specific metabolic reprogramming effects of subchronic T1AM treatment at two pharmacological daily doses (10 and 25 mg/kg) on targeted metabolic pathways. Multi-analytical results indicated that T1AM at 25 mg/kg can act as a novel master regulator of both glucose and lipid metabolism in mice through sirtuin-mediated pathways. In liver, we observed an increased gene and protein expression of Sirt6 (a master gene regulator of glucose) and Gck (glucose kinase) and a decreased expression of Sirt4 (a negative regulator of fatty acids oxidation (FAO)), whereas in white adipose tissue only Sirt6 was increased. Metabolomics analysis supported physiological changes at both doses with most increases in FAO, glycolysis indicators and the mitochondrial substrate, at the highest dose of T1AM. Together our results suggest that T1AM acts through sirtuin-mediated pathways to metabolically reprogram fatty acid and glucose metabolism possibly through small molecules signaling. Our novel mechanistic findings indicate that T1AM has a great potential as a drug for the treatment of obesity and possibly diabetes.

Do Gametes Woo? Evidence for Their Nonrandom Union at Fertilization

A fundamental tenet of inheritance in sexually reproducing organisms such as humans and laboratory mice is that gametes combine randomly at fertilization, thereby ensuring a balanced and statistically predictable representation of inherited variants in each generation. This principle is encapsulated in Mendel's First Law. But exceptions are known. With transmission ratio distortion, particular alleles are preferentially transmitted to offspring. Preferential transmission usually occurs in one sex but not both, and is not known to require interactions between gametes at fertilization. A reanalysis of our published work in mice and of data in other published reports revealed instances where any of 12 mutant genes biases fertilization, with either too many or too few heterozygotes and homozygotes, depending on the mutant gene and on dietary conditions. Although such deviations are usually attributed to embryonic lethality of the underrepresented genotypes, the evidence is more consistent with genetically-determined preferences for specific combinations of egg and sperm at fertilization that result in genotype bias without embryo loss. This unexpected discovery of genetically-biased fertilization could yield insights about the molecular and cellular interactions between sperm and egg at fertilization, with implications for our understanding of inheritance, reproduction, population genetics, and medical genetics.

3-Iodothyronamine reduces insulin secretion in vitro via a mitochondrial mechanism

PURPOSE

3-iodothyronamine (3-T₁AM), a decarboxylated and deiodinated thyroid hormone metabolite, leads at pharmacological doses to hypoinsulinemia, hyperglucagonemia and hyperglycemia in vivo. As the pancreatic Langerhans islets express thyroid hormone transmembrane transporters (THTT), we tested the hypothesis that not only plasma membrane-mediated 3-T₁AM binding to and activation of G-protein coupled receptors, but also 3-T₁AM metabolite(s) generated by 3-T₁AM uptake and metabolism might decrease glucose-stimulated insulin secretion (GSIS).

METHODS

Murine pancreatic β-cells MIN6 were characterized for gene expression of THTT, deiodinases and monoamine oxidases. 3-T₁AM uptake and intracellular metabolism to the corresponding 3-iodothyroacetic acid were analysed by liquid-chromatography tandem mass spectrometry (LC-MS/MS) at different time points in cells as well as the conditioned medium. Mitochondrial activity, especially ATP-production, was monitored real-time after 3-T₁AM application using Seahorse Bioanalyzer technique. Effect of 3-T₁AM on GSIS into the culture medium was assayed by ELISA.

RESULTS

MIN6 cells express classical THTT, proposed to transport 3-T₁AM, as well as 3-T₁AM metabolizing enzymes comparable to murine primary pancreatic islets. 3-T₁AM accumulates in MIN6 cells and is metabolized by intracellular MaoB to 3-iodothyroacetic, which in turn is rapidly exported. 3-T₁AM decreases mitochondrial ATP-production concentration dependently. GSIS is diminished by 3-T₁AM treatment. Using LC-MS/MS, no further 3-T₁AM metabolites except 3-iodothyroacetic were detectable.

CONCLUSIONS

This data provides a first link between cellular 3-T₁AM uptake and regulation of mitochondrial energy metabolism in ß-cells, resulting in reduced insulin secretion. We conclude that MIN6 is an appropriate cell model to study 3-T₁AM-dependent (intra-)cellular biochemical mechanisms affecting insulin production in vitro.

The Case for TAAR1 as a Modulator of Central Nervous System Function

TAAR1 is widely expressed across the mammalian brain, particularly in limbic and monoaminergic areas, allegedly involved in mood, attention, memory, fear, and addiction. However, the subcellular distribution of TAAR1 is still unclear, since TAAR1 signal is largely intracellular. In vitro, TAAR1 is activated with nanomolar to micromolar affinity by some endogenous amines, particularly p-tyramine, beta-phenylethylamine, and 3-iodothyronamine (T1AM), the latter representing a novel branch of thyroid hormone signaling. In addition, TAAR1 responds to a number of psychoactive drugs, i.e., amphetamines, ergoline derivatives, bromocriptine and lisuride. Trace amines have been identified as neurotransmitters in invertebrates, and they are considered as potential neuromodulators. In particular, beta-phenylethylamine and p-tyramine have been reported to modify the release and/or the response to dopamine, norepinephrine, acetylcholine and GABA, while evidence of cross-talk between TAAR1 and other aminergic receptors has been provided. Systemic or intracerebroventricular injection of exogenous T1AM produced prolearning and antiamnestic effects, reduced pain threshold, decreased non-REM sleep, and modulated the firing rate of adrenergic neurons in locus coeruleus. However each of these substances may have additional molecular targets, and it is unclear whether their endogenous levels are sufficient to produce significant TAAR1 activation in vivo. TAAR1 knock out mice show a worse performance in anxiety and working memory tests, and they are more prone to develop ethanol addiction. They also show increased locomotor response to amphetamine, and decreased stereotypical responses induced by apomorphine. Notably, human genes for TAARs cluster on chromosome 6 at q23, within a region whose mutations have been reported to confer susceptibility to schizophrenia and bipolar disorder. For human TAAR1, around 200 non-synonymous and 400 synonymous single nucleotide polymorphisms have been identified, but their functional consequences have not been extensively investigated yet. In conclusion, the bulk of evidence points to a significant physiological role of TAAR1 in the modulation of central nervous system function and a potential pharmacological role of TAAR1 agonists in neurology and/or psychiatry. However, the specific effects of TAAR1 stimulation are still controversial, and many crucial issues require further investigation.

Central Effects of 3-Iodothyronamine Reveal a Novel Role for Mitochondrial Monoamine Oxidases

3-Iodothyronamine (T1AM) is the last iodinated thyronamine generated from thyroid hormone alternative metabolism found circulating in rodents and in humans. So far, the physiopathological meaning of T1AM tissue levels is unknown. Much is instead known on T1AM pharmacological effects in rodents. Such evidence indicates that T1AM acutely modifies, with high potency and effectiveness, rodents' metabolism and behavior, often showing inverted U-shaped dose-response curves. Although several possible targets for T1AM were identified, the mechanism underlying T1AM behavioral effects remains still elusive. T1AM pharmacokinetic features clearly indicate the central nervous system is not a preferential site for T1AM distribution but it is a site where T1AM levels are critically regulated, as it occurs for neuromodulators or neurotransmitters. We here summarize and discuss evidence supporting the hypothesis that central effects of T1AM derive from activation of intracellular and possibly extracellular pathways. In this respect, consisting evidence indicates the intracellular pathway is mediated by the product of T1AM phase-I non-microsomal oxidation, the 3-iodothryoacetic acid, while other data indicate a role for the trace amine-associated receptor, isoform 1, as membrane target of T1AM (extracellular pathway). Overall, these evidence might sustain the non-linear dose-effect curves typically observed when increasing T1AM doses are administered and reveal an interesting and yet unexplored link between thyroid, monoamine oxidases activity and histamine.

Directional thyroid hormone distribution via the blood stream to target sites

Thyroid hormones are bound to three major serum transport proteins, thyroxin-binding globulin (TBG), transthyretin (TTR) and human serum albumin (HSA). TBG has the strongest affinity for thyroid hormones, TTR is also found in the cerebrospinal fluid and HSA is the most abundant protein in plasma. Combination defects of either a high affinity TTR or HSA variant do not compensate TBG deficiency, underscoring the dominant role of TBG among the thyroid hormone transport proteins. On the other hand, coexistence of raised affinity TTR and HSA variants causes an augmented hyperthyroxinemia. Variations in thyroid hormone transport proteins may alter thyroid function tests to mimic hypo- or hyperthyroidism. As affected individuals are clinically euthyroid and do not require treatment, identification of thyroid hormone transport protein defects is important to avoid unnecessary diagnostic and therapeutic interventions. Mammals share the multilayered system of thyroid hormone binding proteins with humans. Some of them, especially carnivores, do not express TBG. In dogs, this defect has been shown to be caused by a defective hepatocyte nuclear factor-1 binding site in the TBG promoter, preventing TBG synthesis in the liver. The major endogenous thyroid hormone metabolite 3-iodothyronamine (3-T1AM) exerts marked cryogenic, metabolic, cardiac and central nervous system actions. It is bound to apolipoproteinB-100 (ApoB100), possibly facilitating its cellular uptake via interaction with the low density lipoprotein-receptor. This review summarizes the handling of hydrophobic charged thyroid hormone signaling molecules and their metabolite 3-T1AM in aqueous body fluids and the advantages and limits of their serum distributor proteins.

3-Iodothyronamine Induces Tail Vasodilation Through Central Action in Male Mice

3-Iodothyronamine (3-T1AM) is an endogenous thyroid hormone (TH)-derived metabolite that induces severe hypothermia in mice after systemic administration; however, the underlying mechanisms have remained enigmatic. We show here that the rapid 3-T1AM-induced loss in body temperature is a consequence of peripheral vasodilation and subsequent heat loss (e.g., over the tail surface). The condition is subsequently intensified by hypomotility and a lack of brown adipose tissue activation. Although the possible 3-T1AM targets trace amine-associated receptor 1 or α2a-adrenergic receptor were detected in tail artery and aorta respectively, myograph studies did not show any direct effect of 3-T1AM on vasodilation, suggesting that its actions are likely indirect. Intracerebroventricular application of 3-T1AM, however, replicated the phenotype of tail vasodilation and body temperature decline and led to neuronal activation in the hypothalamus, suggesting that the metabolite causes tail vasodilation through a hypothalamic signaling pathway. Consequently, the 3-T1AM response constitutes anapyrexia rather than hypothermia and closely resembles the heat-stress response mediated by hypothalamic temperature-sensitive neurons. Our results thus underline the well-known role of the hypothalamus as the body's thermostat and suggest an additional molecular link between TH signaling and the central control of body temperature.

Modulatory effect of intestinal polyamines and trace amines on the spontaneous phasic contractions of the isolated ileum and colon rings of mice

Background: Gastrointestinal motility modulatory factors include substances of the intestinal content, such as polyamines and trace amines (TAs), the focus of this study.

Methods: The amines of food, intestinal content and from faecal bacteria of Swiss mice were determined by HPLC and functionally characterised in isolated distal ileum and medial colon rings.

Results: Mouse food and intestinal content contain polyamines (spermidine>putrescine>spermine) and TAs (isoamylamine>cadaverine). Intestinal bacteria mainly produce putrescine and cadaverine. The amines inhibited the spontaneous motility of the ileum (0.1-3 mM) and colon rings (0.01-3 mM, with lower IC₅₀), with: spermine~isoamylamine~spermidine. Spermine inhibition was tetrodotoxin (TTX)-insensitive, while isoamylamine was TTX-sensitive, suggesting neural control. Mainly in the ileum, isoamylamine (3 mM) elicited acute effects modified by TTX, atropine and propranolol, and suppressed by spermine (3 mM), not being localized at the smooth muscle level. The amines assayed (3 mM), except putrescine and cadaverine in the ileum and isoamylamine in the colon, antagonised acetylcholine (ACh, 0.1 mM)-elicited phasic contractions. Isoamylamine and spermine in colon relaxed KCl (100 mM)-elicited tonic contractions, suggesting an effect on smooth muscle, but did not justify the suppression of motility caused by spermine and isoamylamine.

Conclusions: Polyamines and TAs of the intestinal content might act on chemosensors and modulate intestinal peristalsis.

Tyramine and β-phenylethylamine, from fermented food products, as agonists for the human trace amine-associated receptor 1 (hTAAR1) in the stomach

The aromatic amines tyramine and β-phenylethylamine are abundant in fermented foods. Recently, a family of human trace amine-associated receptors (hTAARs) was discovered that responds to these compounds. This study examined the expression of hTAAR genes in five human organs. Among them, the stomach expressed hTAAR1 and hTAAR9. Interestingly, more hTAAR1 was expressed in the pylorus than in the other stomach regions. The CRE-SEAP reporter assay revealed that only hTAAR1 functioned as a G_s-coupled receptor in response to tyramine and β-phenylethylamine stimulation. The β-phenylethylamine-mediated hTAAR1 activity could be potentiated using 3-isobutyl-1-methylxanthine. These data suggest that tyramine and β-phenylethylamine in fermented foods act at hTAAR1 as agonists in the pylorus of stomach.

Thyronamines and Derivatives: Physiological Relevance, Pharmacological Actions, and Future Research Directions

Thyronamines (3-T₁AM, T₀AM) are endogenous compounds probably derived from L-thyroxine or its intermediate metabolites. Combined activities of intestinal deiodinases and ornithine decarboxylase generate 3-T₁AM in vitro. Alternatively, 3-T₁AM might be formed by the thyroid gland and secreted into the blood. 3-T₁AM and T₀AM concentrations have been determined by liquid chromatography-tandem mass spectrometry analysis (LC-MS/MS) in tissues, serum, and cell lines. However, large variations of 3-T₁AM concentrations in human serum were reported by LC-MS/MS compared with a monoclonal antibody-based immunoassay. These differences might be caused by strong binding of the highly hydrophobic 3-T₁AM to apolipoprotein B100. Pharmacological administration of 3-T₁AM results in dose-dependent reversible effects on body temperature, cardiac function, energy metabolism, and neurological functions. The physiological relevance of these actions is unclear, but may occur at tissue concentrations close to the estimated endogenous concentrations of 3-T₁AM or its metabolites T₀AM or thyroacetic acid (TA₁). A number of putative receptors, binding sites, and cellular target molecules mediating actions of the multi-target ligand 3-T₁AM have been proposed. Among those are members of the trace amine associated receptor family, the adrenergic receptor ADRα2a, and the thermosensitive transient receptor potential melastatin 8 channel. Preclinical studies employing various animal experimental models are in progress, and more stable receptor-selective agonistic and antagonistic analogues of 3-T₁AM are now available for testing. The potent endogenous thyroid hormone-derived biogenic amine 3-T₁AM exerts marked cryogenic, metabolic, cardiac and central actions and represents a valuable lead compound linking endocrine, metabolic, and neuroscience research to advance development of new drugs.

A Transcriptome-Led Exploration of Molecular Mechanisms Regulating Somatostatin-Producing D-Cells in the Gastric Epithelium

The stomach epithelium contains a myriad of enteroendocrine cells that modulate a range of physiological functions, including postprandial secretion of regulatory peptides, gastric motility, and nutrient absorption. Somatostatin (SST)-producing D-cells are present in the oxyntic and pyloric regions of the stomach, and provide a tonic inhibitory tone that regulates activity of neighboring enteroendocrine cells and gastric acid secretion. Cellular mechanisms underlying the effects of regulatory factors on gastric D-cells are poorly defined due to problems in identifying primary D-cells, and uncertainty remains about which stimuli influence D-cells directly. In this study, we introduce a transgenic mouse line, SST-Cre, which upon crossing with Cre reporter strains, facilitates the identification and purification of gastric D-cells, or cell-specific expression of genetically encoded calcium indicators. Populations of D-cells from the gastric antrum and corpus were isolated and analyzed by RNA sequencing and quantitative RT-PCR. The expression of hormones, hormone receptors, neurotransmitter receptors, and nutrient receptors was quantified. Pyy, Gipr, Chrm4, Calcrl, Taar1, and Casr were identified as genes that are highly enriched in D-cells compared with SST-negative cells. Hormone secretion assays performed in mixed gastric epithelial cultures confirmed that SST secretion is regulated by incretin hormones, cholecystokinin, acetylcholine, vasoactive intestinal polypeptide, calcitonin gene-related polypeptide, oligopetides, and trace amines. Cholecystokinin and oligopeptides elicited increases in intracellular calcium in single-cell imaging experiments performed using cultured D-cells. Our data provide the first transcriptomic analysis and functional characterization of gastric D-cells, and identify regulatory pathways that underlie the direct detection of stimuli by this cell type.

Trace Amine-Associated Receptor 1 Localization at the Apical Plasma Membrane Domain of Fisher Rat Thyroid Epithelial Cells Is Confined to Cilia

BACKGROUND

The trace amine-associated receptor 1 (Taar1) is one member of the Taar family of G-protein-coupled receptors (GPCR) accepting various biogenic amines as ligands. It has been proposed that Taar1 mediates rapid, membrane-initiated effects of thyronamines, the endogenous decarboxylated and deiodinated relatives of the classical thyroid hormones T4 and T3.

OBJECTIVES

Although the physiological actions of thyronamines in general and 3-iodothyronamine (T1AM) in particular are incompletely understood, studies published to date suggest that synthetic T1AM-activated Taar1 signaling antagonizes thyromimetic effects exerted by T3. However, the location of Taar1 is currently unknown.

METHODS

To fill this gap in our knowledge we employed immunofluorescence microscopy and a polyclonal antibody to detect Taar1 protein expression in thyroid tissue from Fisher rats, wild-type and taar1-deficient mice, and in the polarized FRT cells.

RESULTS

With this approach we found that Taar1 is expressed in the membranes of subcellular compartments of the secretory pathway and on the apical plasma membrane of FRT cells. Three-dimensional analyses further revealed Taar1 immunoreactivity in cilial extensions of postconfluent FRT cell cultures that had formed follicle-like structures.

CONCLUSIONS

The results suggest Taar1 transport along the secretory pathway and its accumulation in the primary cilium of thyrocytes. These findings are of significance considering the increasing interest in the role of cilia in harboring functional GPCR. We hypothesize that thyronamines can reach and activate Taar1 in thyroid follicular epithelia by acting from within the thyroid follicle lumen, their potential site of synthesis, as part of a nonclassical mechanism of thyroid autoregulation.

3-Iodothyroacetic acid lacks thermoregulatory and cardiovascular effects in vivo

BACKGROUND AND PURPOSE

3-Iodothyronamine (3-T1 AM) is an endogenous thyroid hormone derivative reported to induce strong hypothermia and bradycardia within minutes upon injection in rodents. Although 3-T1 AM is rapidly converted to several other metabolites in vivo, these strong pharmacological responses were solely attributed to 3-T1 AM, leaving potential contributions of downstream products untested. We therefore examined the cardiometabolic effects of 3-iodothyroacetic acid (TA1 ), the main degradation product of 3-T1 AM.

EXPERIMENTAL APPROACH

We used a sensitive implantable radiotelemetry system in C57/Bl6J mice to study the effects of TA1 on body temperature and heart rate, as well as other metabolic parameters.

KEY RESULTS

Interestingly, despite using pharmacological TA1 doses, we observed no effects on heart rate or body temperature after a single TA1 injection (50 mg·kg(-1) , i.p.) compared to sham-injected controls. Repeated administration of TA1 (5 mg·kg(-1) , i.p. for 7 days) likewise did not alter body weight, food and water intake, heart rate, blood pressure, brown adipose tissue (BAT) thermogenesis or body temperature. Moreover, mRNA expression of tissue specific genes in heart, kidney, liver, BAT and lung was also not altered by TA1 compared to sham-injected controls.

CONCLUSIONS AND IMPLICATIONS

Our data therefore conclusively demonstrate that TA1 does not contribute to the cardiovascular or thermoregulatory effects observed after 3-T1 AM administration in mice, suggesting that the oxidative deamination constitutes an important deactivation mechanism for 3-T1 AM with possible implications for cardiovascular and thermoregulatory functions.

Analysis of human TAAR8 and murine Taar8b mediated signaling pathways and expression profile

The thyroid hormone derivative 3-iodothyronamine (3-T₁AM) exerts metabolic effects in vivo that contradict known effects of thyroid hormones. 3-T₁AM acts as a trace amine-associated receptor 1 (TAAR1) agonist and activates G_s signaling in vitro. Interestingly, 3-T₁AM-meditated in vivo effects persist in Taar1 knockout-mice indicating that further targets of 3-T₁AM might exist. Here, we investigated another member of the TAAR family, the only scarcely studied mouse and human trace-amine-associated receptor 8 (Taar8b, TAAR8). By RT-qPCR and locked-nucleic-acid (LNA) in situ hybridization, Taar8b expression in different mouse tissues was analyzed. Functionally, we characterized TAAR8 and Taar8b with regard to cell surface expression and signaling via different G-protein-mediated pathways. Cell surface expression was verified by ELISA, and cAMP accumulation was quantified by AlphaScreen for detection of G_s and/or G_i/o signaling. Activation of G-proteins G_q/11 and G_12/13 was analyzed by reporter gene assays. Expression analyses revealed at most marginal Taar8b expression and no gender differences for almost all analyzed tissues. In heart, LNA-in situ hybridization demonstrated the absence of Taar8b expression. We could not identify 3-T₁AM as a ligand for TAAR8 and Taar8b, but both receptors were characterized by a basal G_i/o signaling activity, a so far unknown signaling pathway for TAARs.

Update on 3-iodothyronamine and its neurological and metabolic actions

3-iodothyronamine (T1AM) is an endogenous amine, that has been detected in many rodent tissues, and in human blood. It has been hypothesized to derive from thyroid hormone metabolism, but this hypothesis still requires validation. T1AM is not a ligand for nuclear thyroid hormone receptors, but stimulates with nanomolar affinity trace amine-associated receptor 1 (TAAR1), a G protein-coupled membrane receptor. With a lower affinity it interacts with alpha2A adrenergic receptors. Additional targets are represented by apolipoprotein B100, mitochondrial ATP synthase, and membrane monoamine transporters, but the functional relevance of these interactions is still uncertain. Among the effects reported after administration of exogenous T1AM to experimental animals, metabolic and neurological responses deserve special attention, because they were obtained at low dosages, which increased endogenous tissue concentration by about one order of magnitude. Systemic T1AM administration favored fatty acid over glucose catabolism, increased ketogenesis and increased blood glucose. Similar responses were elicited by intracerebral infusion, which inhibited insulin secretion and stimulated glucagon secretion. However, T1AM administration increased ketogenesis and gluconeogenesis also in hepatic cell lines and in perfused liver preparations, providing evidence for a peripheral action, as well. In the central nervous system, T1AM behaved as a neuromodulator, affecting adrenergic and/or histaminergic neurons. Intracerebral T1AM administration favored learning and memory, modulated sleep and feeding, and decreased the pain threshold. In conclusion T1AM should be considered as a component of thyroid hormone signaling and might play a significant physiological and/or pathophysiological role. T1AM analogs have already been synthetized and their therapeutical potential is currently under investigation. 3-iodothyronamine (T1AM) is a biogenic amine whose structure is closely related to that of thyroid hormone (3,5,3′-triiodothyronine, or T3). The differences with T3 are the absence of the carboxylate group and the substitution of iodine with hydrogen in 5 and 3′ positions (Figure 1). In this paper we will review the evidence supporting the hypothesis that T1AM is a chemical messenger, namely that it is an endogenous substance able to interact with specific receptors producing significant functional effects. Special emphasis will be placed on neurological and metabolic effects, which are likely to have physiological and pathophysiological importance.

Uptake and metabolic effects of 3-iodothyronamine in hepatocytes

3-Iodothyronamine (T1AM) is an endogenous relative of thyroid hormone with profound metabolic effects. In different experimental models, T1AM increased blood glucose, and it is not clear whether this effect is entirely accounted by changes in insulin and/or glucagone secretion. Thus, in the present work, we investigated the uptake of T1AM by hepatocytes, which was compared with the uptake of thyroid hormones, and the effects of T1AM on hepatic glucose and ketone body production. Two different experimental models were used: HepG2 cells and perfused rat liver. Thyronines and thyronamines (T0AMs) were significantly taken up by hepatocytes. In HepG2 cells exposed to 1 μM T1AM, at the steady state, the cellular concentration of T1AM exceeded the medium concentration by six- to eightfold. Similar accumulation occurred with 3,5,3'-triiodothyronine and thyroxine. Liver experiments confirmed significant T1AM uptake. T1AM was partly catabolized and the major catabolites were 3-iodothyroacetic acid (TA1) (in HepG2 cells) and T0AM (in liver). In both preparations, infusion with 1 μM T1AM produced a significant increase in glucose production, if adequate gluconeogenetic substrates were provided. This effect was dampened at higher concentration (10 μM) or in the presence of the amine oxidase inhibitor iproniazid, while TA1 was ineffective, suggesting that T1AM may have a direct gluconeogenetic effect. Ketone body release was significantly increased in liver, while variable results were obtained in HepG2 cells incubated with gluconeogenetic substrates. These findings are consistent with the stimulation of fatty acid catabolism, and a shift of pyruvate toward gluconeogenesis. Notably, these effects are independent from hormonal changes and might have physiological and pathophysiological importance.

Optical imaging of mitochondrial redox state in rodent models with 3-iodothyronamine

This study used an optical technique to measure the effects of treating low (10 mg/kg) and high (25 mg/kg) doses of 3-iodothyronamine (T₁AM) on the metabolism in the kidney and heart of mice. The ratio of two intrinsic fluorophores in tissue, (NADH/FAD), called the NADH redox ratio (NADH RR), is a marker of the metabolic state of the tissue. A cryofluorescence imaging instrument was used to provide a quantitative assessment of NADH RR in both kidneys and hearts in mice treated with 3-iodothyronamine. We compared those results to corresponding tissues in control mice. In the kidneys of mice treated with a high dose T₁AM, the mean values of the maximum projection of NADH RR were 2.6 ± 0.6 compared to 3.20 ± 0.03 in control mice, indicating a 19% (± 0.4) significant increase in oxidative stress (OS) in the high dose-treated kidneys (P = 0.047). However, kidneys treated with a low dose of T₁AM showed no difference in NADH RR compared to the kidneys of control mice. Furthermore, low versus high dose treatment of T₁AM showed different responses in the heart than in the kidneys. The mean value of the maximum projection of NADH RR in the heart changed from 3.0 ± 0.3 to 3.2 ± 0.6 for the low dose and the high dose T₁AM-treated mice, respectively, as compared to 2.8 ± 0.7 in control mice. These values correspond to a 9% (±0.5) (P = 0.045) and 14% (±0.5) (P = 0.008) significant increase in NADH RR in the T₁AM-treated hearts, indicating that the high dose T₁AM-treated tissues have reduced OS compared to the low dose-treated tissues or the control tissues. These results suggest that while T₁AM at a high dose increases oxidative response in kidneys, it has a protective effect in the heart and may exert its effect through alternative pathways at different doses and at tissue specific levels.