Localization of organic cation transporter 2 (OCT2) in monoaminergic and cholinergic axon terminals of the mouse brain

A neurotransmitter atlas of C. elegans males and hermaphrodites

Mapping neurotransmitter identities to neurons is key to understanding information flow in a nervous system. It also provides valuable entry points for studying the development and plasticity of neuronal identity features. In the C. elegans nervous system, neurotransmitter identities have been largely assigned by expression pattern analysis of neurotransmitter pathway genes that encode neurotransmitter biosynthetic enzymes or transporters. However, many of these assignments have relied on multicopy reporter transgenes that may lack relevant cis-regulatory information and therefore may not provide an accurate picture of neurotransmitter usage. We analyzed the expression patterns of 16 CRISPR/Cas9-engineered knock-in reporter strains for all main types of neurotransmitters in C. elegans (glutamate, acetylcholine, GABA, serotonin, dopamine, tyramine, and octopamine) in both the hermaphrodite and the male. Our analysis reveals novel sites of expression of these neurotransmitter systems within both neurons and glia, as well as non-neural cells. The resulting expression atlas defines neurons that may be exclusively neuropeptidergic, substantially expands the repertoire of neurons capable of co-transmitting multiple neurotransmitters, and identifies novel neurons that uptake monoaminergic neurotransmitters. Furthermore, we also observed unusual co-expression patterns of monoaminergic synthesis pathway genes, suggesting the existence of novel monoaminergic transmitters. Our analysis results in what constitutes the most extensive whole-animal-wide map of neurotransmitter usage to date, paving the way for a better understanding of neuronal communication and neuronal identity specification in C. elegans.

Dolutegravir Discontinuation for Neuropsychiatric Symptoms in People Living with HIV and Their Outcomes after Treatment Change: A Pharmacogenetic Study

Neuropsychiatric symptoms have been reported in patients receiving dolutegravir, a known inhibitor of the renal and neuronal-expressed organic anion transporter 2 (encoded by SLC22A2 gene). The effect of the genetic variant SLC22A2 808C>A on dolutegravir discontinuation was assessed and analyzed by real-time PCR. We enrolled 627 participants: CA/AA carriers showed a higher prevalence of pre-existing psychiatric comorbidities and use of antidepressants. After 27.9 months, 108 participants discontinued dolutegravir, 64 for neuropsychiatric symptoms. Patients with pre-existing psychiatric comorbidities were at higher risk of dolutegravir discontinuation, while patients carrying the SLC22A2 CA/AA genotype were not. Combining the two variables, an opposite effect of SLC22A2 variants according to pre-existing psychiatric disorders was observed. Using multivariate Cox models, the combined variable pre-existing psychiatric comorbidities/SLC22A2 variants and the use of non-tenofovir alafenamide containing antiretroviral regimens were predictors of dolutegravir discontinuation for neuropsychiatric symptoms. Within 30 days, the majority of participants had a complete resolution of symptoms (61.8%), while 32.7% and 5.5% had partial or no change after dolutegravir discontinuation, respectively. Discontinuation of dolutegravir for neuropsychiatric symptoms was not uncommon and more frequent in participants with pre-existing psychiatric disorders. We described an interaction between SLC22A2 genetic variant and psychiatric comorbidities. In 38.2% of patients, a complete neuropsychiatric symptoms resolution was not observed after dolutegravir discontinuation suggesting the involvement of additional factors.

Metformin in nucleus accumbens core reduces cue-induced cocaine seeking in male and female rats

This study investigated the potential therapeutic effects of the FDA‐approved drug metformin on cue‐induced reinstatement of cocaine seeking. Metformin (dimethyl‐biguanide) is a first‐line treatment for type II diabetes that, among other mechanisms, is involved in the activation of adenosine monophosphate activated protein kinase (AMPK). Cocaine self‐administration and extinction is associated with decreased levels of phosphorylated AMPK within the nucleus accumbens core (NAcore). Previously, it was shown that increasing AMPK activity in the NAcore decreased cue‐induced reinstatement of cocaine seeking. Decreasing AMPK activity produced the opposite effect. The goal of the present study was to determine if metformin in the NAcore reduces cue‐induced cocaine seeking in adult male and female Sprague Dawley rats. Rats were trained to self‐administer cocaine followed by extinction prior to cue‐induced reinstatement trials. Metformin microinjected in the NAcore attenuated cue‐induced reinstatement in male and female rats. Importantly, metformin's effects on cocaine seeking were not due to a general depression of spontaneous locomotor activity. In female rats, metformin's effects did generalize to a reduction in cue‐induced reinstatement of sucrose seeking. These data support a potential role for metformin as a pharmacotherapy for cocaine use disorder but warrant caution given the potential for metformin's effects to generalize to a natural reward in female rats.

The organic cation Transporter 2 regulates dopamine D1 receptor signaling at the Golgi apparatus

Dopamine is a key catecholamine in the brain and kidney, where it is involved in a number of physiological functions such as locomotion, cognition, emotion, endocrine regulation, and renal function. As a membrane-impermeant hormone and neurotransmitter, dopamine is thought to signal by binding and activating dopamine receptors, members of the G protein coupled receptor (GPCR) family, only on the plasma membrane. Here, using novel nanobody-based biosensors, we demonstrate for the first time that the dopamine D1 receptor (D1DR), the primary mediator of dopaminergic signaling in the brain and kidney, not only functions on the plasma membrane but becomes activated at the Golgi apparatus in the presence of its ligand. We present evidence that activation of the Golgi pool of D1DR is dependent on organic cation transporter 2 (OCT2), a dopamine transporter, providing an explanation for how the membrane-impermeant dopamine accesses subcellular pools of D1DR. We further demonstrate that dopamine activates Golgi-D1DR in murine striatal medium spiny neurons, and this activity depends on OCT2 function. We also introduce a new approach to selectively interrogate compartmentalized D1DR signaling by inhibiting Gαs coupling using a nanobody-based chemical recruitment system. Using this strategy, we show that Golgi-localized D1DRs regulate cAMP production and mediate local protein kinase A activation. Together, our data suggest that spatially compartmentalized signaling hubs are previously unappreciated regulatory aspects of D1DR signaling. Our data provide further evidence for the role of transporters in regulating subcellular GPCR activity.

The Organic Cation Transporter 2 Inhibitor Quinidine Modulates the Neuroprotective Effect of Nerve Growth Factor and Memantine on Cholinergic Neurons of the Basal Nucleus of Meynert in Organotypic Brain Slices

INTRODUCTION

Alzheimer's disease (AD) is a severe neurodegenerative disorder of the brain characterized by degeneration of cholinergic neurons which is directly linked to cognitive decline. Nerve growth factor (NGF) is the most potent protective factor for cholinergic neurons, additionally the NMDA antagonist memantine blocks glutamate-mediated excitotoxic activity. Quinidine is an inhibitor of organic cation transporter 2 (OCT2). OCT2 is located on cholinergic neurons and plays a role in presynaptic reuptake and recycling of acetylcholine in the brain. We hypothesize that quinidine can modulate the protective effects of NGF and memantine on cholinergic neurons in organotypic brain slices of the nucleus basalis of Meynert (nBM).

METHODS

Organotypic brain slices of nBM were incubated with 100 ng/mL NGF, 10 µM memantine, 10 µM quinidine, and combinations of these treatments for 2 weeks. Cholinergic neurons were immunohistochemically stained for choline acetyltransferase (ChAT).

RESULTS

Our data show that NGF as well as memantine counteracted the cell death of cholinergic nBM neurons. Quinidine alone had no toxic effect on cholinergic neurons but inhibited the protective effect of NGF and memantine when applied simultaneously.

DISCUSSION/CONCLUSION

Our data provide evidence that quinidine modulates the survival of cholinergic nBM neurons via OCT2.

Neurochemical characterization of mouse dorsal root ganglion neurons expressing organic cation transporter 2

Organic cation transporters (OCTs) are poly-specific carriers for endogenous and exogenous cationic compounds. These are widely distributed in the nervous system and mediate neuronal activities. As antineoplastic cationic drugs accumulate in the dorsal root ganglion (DRG), OCT function has been studied mainly in cultured DRG neurons. However, the histological distribution of OCTs in the DRG is unclear. This study investigated the localization of OCT2 (a member of OCTs) in mouse DRG neurons and determined their histochemical properties. OCT2 expression was found in about 20% of DRG neurons, which were small to medium size. OCT2-expressing neurons were labeled with markers for peptidergic nociceptive (substance P or calcitonin gene-related peptide) and tactile/proprioceptive (neurofilament 200 or tropomyosin receptor kinase B or C) neurons. OCT2 was also expressed in cholinergic DRG neurons identified by choline acetyltransferase promoter-derived Cre expression. In the spinal dorsal horn, OCT2 was distributed in superficial to deep laminae. OCT2 immunoreactivity was punctate in appearance and localized in the nerve terminals of sensory afferents with labeling of neurochemical markers. Our findings suggest that OCT2 as a low-affinity, high-capacity carrier may take up substrates including cationic neurotransmitters and drugs from the extracellular space around cell bodies in DRG neurons.

Altered hippocampal gene expression, glial cell population, and neuronal excitability in aminopeptidase P1 deficiency

Inborn errors of metabolism are often associated with neurodevelopmental disorders and brain injury. A deficiency of aminopeptidase P1, a proline-specific endopeptidase encoded by the Xpnpep1 gene, causes neurological complications in both humans and mice. In addition, aminopeptidase P1-deficient mice exhibit hippocampal neurodegeneration and impaired hippocampus-dependent learning and memory. However, the molecular and cellular changes associated with hippocampal pathology in aminopeptidase P1 deficiency are unclear. We show here that a deficiency of aminopeptidase P1 modifies the glial population and neuronal excitability in the hippocampus. Microarray and real-time quantitative reverse transcription-polymerase chain reaction analyses identified 14 differentially expressed genes (Casp1, Ccnd1, Myoc, Opalin, Aldh1a2, Aspa, Spp1, Gstm6, Serpinb1a, Pdlim1, Dsp, Tnfaip6, Slc6a20a, Slc22a2) in the Xpnpep1^−/− hippocampus. In the hippocampus, aminopeptidase P1-expression signals were mainly detected in neurons. However, deficiency of aminopeptidase P1 resulted in fewer hippocampal astrocytes and increased density of microglia in the hippocampal CA3 area. In addition, Xpnpep1^−/− CA3b pyramidal neurons were more excitable than wild-type neurons. These results indicate that insufficient astrocytic neuroprotection and enhanced neuronal excitability may underlie neurodegeneration and hippocampal dysfunction in aminopeptidase P1 deficiency.

Regulation of Neurogenesis by Organic Cation Transporters: Potential Therapeutic Implications

Neurogenesis is the process by which new neurons are generated from neural stem cells (NSCs), which are cells that have the ability to proliferate and differentiate into neurons, astrocytes, and oligodendrocytes. The process is essential for homeostatic tissue regeneration and the coordination of neural plasticity throughout life, as neurons cannot regenerate once injured. Therefore, defects in neurogenesis are related to the onset and exacerbation of several neuropsychiatric disorders, and therefore, the regulation of neurogenesis is considered to be a novel strategy for treatment. Neurogenesis is regulated not only by NSCs themselves, but also by the functional microenvironment surrounding the NSCs, known as the "neurogenic niche." The neurogenic niche consists of several types of neural cells, including neurons, glial cells, and vascular cells. To allow communication with these cells, transporters may be involved in the secretion and uptake of substrates that are essential for signal transduction. This chapter will focus on the involvement of polyspecific solute carriers transporting organic cations in the possible regulation of neurogenesis by controlling the concentration of several organic cation substrates in NSCs and the neurogenic niche. The potential therapeutic implications of neurogenesis regulation by these transporters will also be discussed.

Genetic and Epigenetic Regulation of Organic Cation Transporters

Organic cation transporters (OCTs) of the solute carrier family (SLC) 22 are the subject of intensive research because they mediate the transport of many clinically-relevant drugs such as the antidiabetic agent metformin, the opioid tramadol, and the antimigraine agent sumatriptan. OCT1 (SLC22A1) and OCT2 (SLC22A2) are highly expressed in human liver and kidney, respectively, while OCT3 (SLC22A3) shows a broader tissue distribution. As suggested from studies using knockout mice, particularly OCT2 and OCT3 appear to be of relevance for brain physiological function and drug response. The knowledge of genetic factors and epigenetic modifications affecting function and expression of OCTs is important for a better understanding of disease mechanisms and for personalized treatment of patients. This review briefly summarizes the impact of genetic variants and epigenetic regulation of OCTs in general. A comprehensive overview is given on the consequences of OCT2 and OCT3 knockout in mice and the implications of genetic OCT2 and OCT3 variants on central nervous system function in humans.

SLC22A2 variants and dolutegravir levels correlate with psychiatric symptoms in persons with HIV

BACKGROUND

Neuropsychiatric symptoms (NPs) have been reported with dolutegravir use. We hypothesized that increasing dolutegravir trough concentrations (Ctrough) and/or polymorphism in the SLC22A2 gene, encoding the organic cation transporter-2 (OCT2), which is involved in monoamine clearance in the CNS and is inhibited by dolutegravir, might be associated with NPs.

METHODS

A cross-sectional cohort of HIV-positive patients treated with a dolutegravir-containing regimen underwent determination of allelic discrimination for SLC22A2 808 C → A polymorphism and dolutegravir Ctrough. The Symptom Checklist-90-R [investigating 10 psychiatric dimensions and reporting a general severity index (GSI)], a self-reported questionnaire and the Mini-International Neuropsychiatric Interview were offered to investigate current NPs. The effects of dolutegravir Ctrough and the SLC22A2 gene variant on NPs were explored by multivariable logistic regression.

RESULTS

A cohort of 203 patients was analysed: 71.4% were male, with median age 51 years and 11 years of ART exposure. Median time on dolutegravir was 18 months. Dolutegravir was associated with different antiretroviral combinations (mainly lamivudine, 38.9%, and abacavir/lamivudine, 35.5%). SLC22A2 CA genotype was independently associated with an abnormal GSI [adjusted OR (aOR) 2.43; P = 0.072], anxiety (aOR 2.61; P = 0.044), hostility (aOR 3.76; P = 0.012) and with moderate to severe headache (aOR 5.55; P = 0.037), and dolutegravir Ctrough was associated with hostility (fourth versus first quartile aOR 6.70; P = 0.007) and psychoticism (fourth versus first quartile aOR 19.01; P = 0.008). Other NPs were not associated with SLC22A2 polymorphism or dolutegravir Ctrough.

CONCLUSIONS

A variant of the OCT2-encoding gene, in addition to or in synergy with higher dolutegravir Ctrough, is associated with a set of NPs observed during dolutegravir therapy.

Re-exploring the core genes and modules in the human frontal cortex during chronological aging: insights from network-based analysis of transcriptomic studies

Frontal cortical dysfunction is a fundamental pathology contributing to age-associated behavioral and cognitive deficits that predispose older adults to neurodegenerative diseases. It is established that aging increases the risk of frontal cortical dysfunction; however, the underlying molecular mechanism remains elusive. Here, we used an integrative meta-analysis to combine five frontal cortex microarray studies with a combined sample population of 161 younger and 155 older individuals. A network-based analysis was used to describe an outline of human frontal cortical aging to identify core genes whose expression changes with age and to reveal the interrelationships among these genes. We found that histone deacetylase 1 (HDAC1) and YES proto-oncogene 1 (YES1) are the two most upregulated genes, while cell division cycle 42 (CDC42) is the central regulatory gene decreased in the aged human frontal cortex. Quantitative PCR assays revealed corresponding changes in frontal cortical Hdac1, Yes1 and Cdc42 mRNA levels in an established aging mouse model. Moreover, analysis of the GSE48350 dataset confirmed similar changes in HDAC1, CDC42 and YES1 expression in Alzheimer's disease, thereby providing a molecular connection between aging and Alzheimer's disease (AD). This framework of network-based analysis could provide novel strategies for detecting and monitoring aging in the brain.

Social Defeat Stress Decreases mRNA for Monoamine Oxidase A and Increases 5-HT Turnover in the Brain of Male Nile Tilapia (Oreochromis niloticus)

Stress induces various neurobiological responses and causes psychiatric disorders, including depression. Monoamine oxidase A (MAO-A) plays an important role in various functions of the brain, such as regulation of mood, anxiety and aggression, and dysregulation of MAO-A is observed in stress-related psychiatric disorders. This study addressed the question whether acute social stress induces changes to transcriptional and/or post-transcriptional regulation of MAO-A expression in the brain. Using male Nile tilapia (Oreochromis niloticus), we investigated whether acute social stress, induced by the presence of a dominant male fish, changes the expression of MAO-A. We measured gene expression of MAO-A by quantitative PCR, enzymatic activity of MAO-A by the luminescent method, and 5-HT and 5-HIAA levels by liquid chromatography–mass spectrometry in the brain of socially stressed and control fish. Socially stressed males showed decreased MAO-A mRNA levels, consistent MAO-A enzymatic activity, increased 5-HT turnover in the brain, and elevated plasma cortisol levels, compared to controls. Our results suggest that acute social stress suppresses the transcription of MAO-A gene, enhances 5-HT metabolism but does not affect the production of MAO-A protein.

Pharmacological characterization of a high-affinity p-tyramine transporter in rat brain synaptosomes

p-Tyramine is an archetypal member of the endogenous family of monoamines known as trace amines, and is one of the endogenous agonists for trace amine-associated receptor (TAAR)1. While much work has focused on the function of TAAR1, very little is known about the regulation of the endogenous agonists. We have previously reported that p-tyramine readily crosses lipid bilayers and that its release from synaptosomes is non-exocytotic. Such release, however, showed characteristics of modification by one or more transporters. Here we provide the first characterization of such a transporter. Using frontal cortical and striatal synaptosomes we show that p-tyramine passage across synaptosome membranes is not modified by selective inhibition of either the dopamine, noradrenaline or 5-HT transporters. In contrast, inhibition of uptake-2 transporters significantly slowed p-tyramine re-uptake. Using inhibitors of varying selectivity, we identify Organic Cation Transporter 2 (OCT2; SLC22A2) as mediating high affinity uptake of p-tyramine at physiologically relevant concentrations. Further, we confirm the presence of OCT2 protein in synaptosomes. These results provide the first identification of a high affinity neuronal transporter for p-tyramine, and also confirm the recently described localization of OCT2 in pre-synaptic terminals.