Targeted mutagenesis of negatively charged amino acids outlining the substrate translocation path within the human organic cation transporter 3

Recently published cryo-EM structures of human organic cation transporters of the SLC22 family revealed seven, sequentially arranged glutamic and aspartic acid residues, which may be relevant for interactions with positively charged substrates. We analyzed the functional consequences of removing those negative charges by creating D155N, E232Q, D382N, E390Q, E451Q, E459Q, and D478N mutants of OCT3. E232Q, E459Q, and D478N resulted in a lack of localization in the outer cell membrane and no relevant uptake activity. However, D155N and E451Q showed a substrate-specific loss of transport activity, whereas E390Q had no remaining activity despite correct membrane localization. In contrast, D382N showed almost wild-type-like uptake. D155 is located at the entrance to the substrate binding pocket and could, therefore be involved in guiding cationic substrates towards the inside of the binding pocket. For E390, we confirm its critical function for transporter function as it was recently shown for the corresponding position in OCT1. Interestingly, E451 seems to be located at the bottom of the binding pocket in the outward-open confirmation of the transporter. Substrate-specific loss of transport activity of the E451Q variant suggests an essential role in the transport cycle of specific substances as part of an opportunistic binding site. In general, our study highlights the impact of the cryo-EM structures in guiding mutagenesis studies to understand the molecular level of transporter-ligand interactions, and it also confirms the importance of testing multiple substrates in mutagenesis studies of polyspecific OCTs.

Monoamine oxidase A and organic cation transporter 3 coordinate intracellular β1AR signaling to calibrate cardiac contractile function

We have recently identified a pool of intracellular β1 adrenergic receptors (β1ARs) at the sarcoplasmic reticulum (SR) crucial for cardiac function. Here, we aim to characterize the integrative control of intracellular catecholamine for subcellular β1AR signaling and cardiac function. Using anchored Förster resonance energy transfer (FRET) biosensors and transgenic mice, we determined the regulation of compartmentalized β1AR-PKA signaling at the SR and plasma membrane (PM) microdomains by organic cation transporter 3 (OCT3) and monoamine oxidase A (MAO-A), two critical modulators of catecholamine uptake and homeostasis. Additionally, we examined local PKA substrate phosphorylation and excitation-contraction coupling in cardiomyocyte. Cardiac-specific deletion of MAO-A (MAO-A-CKO) elevates catecholamines and cAMP levels in the myocardium, baseline cardiac function, and adrenergic responses. Both MAO-A deletion and inhibitor (MAOi) selectively enhance the local β1AR-PKA activity at the SR but not PM, and augment phosphorylation of phospholamban, Ca2+ cycling, and myocyte contractile response. Overexpression of MAO-A suppresses the SR-β1AR-PKA activity and PKA phosphorylation. However, deletion or inhibition of OCT3 by corticosterone prevents the effects induced by MAOi and MAO-A deletion in cardiomyocytes. Deletion or inhibition of OCT3 also negates the effects of MAOi and MAO-A deficiency in cardiac function and adrenergic responses in vivo. Our data show that MAO-A and OCT3 act in concert to fine-tune the intracellular SR-β1AR-PKA signaling and cardiac fight-or-flight response. We reveal a drug contraindication between anti-inflammatory corticosterone and anti-depressant MAOi in modulating adrenergic regulation in the heart, providing novel perspectives of these drugs with cardiac implications.

Organic Cation Transporters and Nongenomic Glucocorticoid Action

Corticosteroid hormones exert powerful influences on neuronal physiology and behavior by activating intracellular glucocorticoid receptors (GR) and mineralocorticoid receptors (MR), which act as ligand-gated transcription factors, altering gene expression. In addition to these genomic effects on physiology and behavior, which are usually delayed by minutes to hours, corticosteroid hormones also initiate rapid effects through diverse nongenomic mechanisms. One such mechanism involves the direct inhibition by corticosteroid hormones of monoamine transport mediated by the "uptake₂" transporter, organic cation transporter 3 (OCT3), a high-capacity, low-affinity transporter for norepinephrine, epinephrine, dopamine, serotonin, and histamine. In this review we describe studies that demonstrate OCT3 expression and corticosterone-sensitive monoamine transport in the brain and present evidence supporting the hypothesis that corticosterone exerts rapid, nongenomic actions on glia and neurons, ultimately modulating physiology and behavior, by inhibiting OCT3-mediated monoamine clearance. We also describe the corticosteroid sensitivity of the other members of the uptake₂ family and examine their potential contributions to nongenomic effects of corticosteroids in the brain.

Evidence that preimmunization with a heat-killed preparation of Mycobacterium vaccae reduces corticotropin-releasing hormone mRNA expression in the extended amygdala in a fear-potentiated startle paradigm

Posttraumatic stress disorder (PTSD) is a trauma and stressor-related disorder that is characterized by dysregulation of glucocorticoid signaling, chronic low-grade inflammation, and impairment in the ability to extinguish learned fear. Corticotropin-releasing hormone (Crh) is a stress- and immune-responsive neuropeptide secreted from the paraventricular nucleus of the hypothalamus (PVN) to stimulate the hypothalamic-pituitary-adrenal (HPA) axis; however, extra-hypothalamic sources of Crh from the central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST) govern specific fear- and anxiety-related defensive behavioral responses. We previously reported that preimmunization with a heat-killed preparation of the immunoregulatory environmental bacterium Mycobacterium vaccae NCTC 11659 enhances fear extinction in a fear-potentiated startle (FPS) paradigm. In this follow-up study, we utilized an in situ hybridization histochemistry technique to investigate Crh, Crhr1, and Crhr2 mRNA expression in the CeA, BNST, and PVN of the same rats from the original study [Fox et al., 2017, Brain, Behavior, and Immunity, 66: 70-84]. Here, we demonstrate that preimmunization with M. vaccae NCTC 11659 decreases Crh mRNA expression in the CeA and BNST of rats exposed to the FPS paradigm, and, further, that Crh mRNA expression in these regions is correlated with fear behavior during extinction training. These data are consistent with the hypothesis that M. vaccae promotes stress-resilience by attenuating Crh production in fear- and anxiety-related circuits. These data suggest that immunization with M. vaccae may be an effective strategy for prevention of fear- and anxiety-related disorders.

The Impact of Stressor Exposure and Glucocorticoids on Anxiety and Fear

Anxiety disorders and trauma- and stressor-related disorders, such as posttraumatic stress disorder (PTSD), are common and are associated with significant economic and social burdens. Although trauma and stressor exposure are recognized as a risk factors for development of anxiety disorders and trauma or stressor exposure is recognized as essential for diagnosis of PTSD, the mechanisms through which trauma and stressor exposure lead to these disorders are not well characterized. An improved understanding of the mechanisms through which trauma or stressor exposure leads to development and persistence of anxiety disorders or PTSD may result in novel therapeutic approaches for the treatment of these disorders. Here, we review the current state-of-the-art theories, with respect to mechanisms through which stressor exposure leads to acute or chronic exaggeration of avoidance or anxiety-like defensive behavioral responses and fear, endophenotypes in both anxiety disorders and trauma- and stressor-related psychiatric disorders. In this chapter, we will explore physiological responses and neural circuits involved in the development of acute and chronic exaggeration of anxiety-like defensive behavioral responses and fear states, focusing on the role of the hypothalamic-pituitary-adrenal (HPA) axis and glucocorticoid hormones.

Organic cation transporter 3: A cellular mechanism underlying rapid, non-genomic glucocorticoid regulation of monoaminergic neurotransmission, physiology, and behavior

Contribution to Special Issue on Fast effects of steroids. Corticosteroid hormones act at intracellular glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) to alter gene expression, leading to diverse physiological and behavioral responses. In addition to these classical genomic effects, corticosteroid hormones also exert rapid actions on physiology and behavior through a variety of non-genomic mechanisms, some of which involve GR or MR, and others of which are independent of these receptors. One such GR-independent mechanism involves corticosteroid-induced inhibition of monoamine transport mediated by "uptake₂" transporters, including organic cation transporter 3 (OCT3), a low-affinity, high-capacity transporter for norepinephrine, epinephrine, dopamine, serotonin and histamine. Corticosterone directly and acutely inhibits OCT3-mediated transport. This review describes the studies that initially characterized uptake₂ processes in peripheral tissues, and outlines studies that demonstrated OCT3 expression and corticosterone-sensitive monoamine transport in the brain. Evidence is presented supporting the hypothesis that corticosterone can exert rapid, GR-independent actions on neuronal physiology and behavior by inhibiting OCT3-mediated monoamine clearance. Implications of this mechanism for glucocorticoid-monoamine interactions in the context-dependent regulation of behavior are discussed.

Preimmunization with a heat-killed preparation of Mycobacterium vaccae enhances fear extinction in the fear-potentiated startle paradigm

The hygiene hypothesis or "Old Friends" hypothesis proposes that inflammatory diseases are increasing in modern urban societies, due in part to reduced exposure to microorganisms that drive immunoregulatory circuits, and a failure to terminate inappropriate inflammatory responses. Inappropriate inflammation is also emerging as a risk factor for trauma-related, anxiety, and affective disorders, including posttraumatic stress disorder (PTSD), which is characterized as persistent re-experiencing of the trauma after a traumatic experience. Traumatic experiences can lead to long-lasting fear memories and exaggerated fear potentiation of the acoustic startle reflex. The acoustic startle reflex is an ethologically relevant reflex and can be potentiated in both humans and rats through Pavlovian conditioning. Mycobacterium vaccae NCTC 11659 is a soil-derived bacterium with immunoregulatory and anti-inflammatory properties that has been demonstrated to confer stress resilience in mice. Here we immunized adult male Sprague Dawley rats 3×, once per week, with a heat-killed preparation of M. vaccae NCTC 11659 (0.1mg, s.c., in 100µl borate-buffered saline) or vehicle, and, then, 3weeks following the final immunization, tested them in the fear-potentiated startle paradigm; controls were maintained under home cage control conditions throughout the experiment (n=11-12 per group). Rats were tested on days 1 and 2 for baseline acoustic startle, received fear conditioning on days 3 and 4, and underwent fear extinction training on days 5-10. Rats were euthanized on day 11 and brain tissue was sectioned for analysis of mRNA expression for genes important in control of brain serotonergic signaling, including tph2, htr1a, slc6a4, and slc22a3, throughout the brainstem dorsal and median raphe nuclei. Immunization with M. vaccae had no effect on baseline acoustic startle or fear expression on day 5. However, M. vaccae-immunized rats showed enhanced between-session and within-session extinction on day 6, relative to vehicle-immunized controls. Immunization with M. vaccae and fear-potentiated startle altered serotonergic gene expression in a gene- and subregion-specific manner. These data are consistent with the hypothesis that immunoregulatory strategies, such as preimmunization with M. vaccae, have potential for prevention of stress- and trauma-related psychiatric disorders.

Implications for extraneuronal norepinephrine uptake

Brown adipose tissue (BAT) is activated by extracellular norepinephrine (NE) released by the sympathetic nervous system. The extracellular concentration of NE is additionally regulated by the disappearance/degradation of NE. Recent studies have introduced the organic cation transporter 3 (OCT3) as a possible contributor in the regulation of NE in adipose tissue. In the present study we set out to investigate the presence of OCT3 in human neck adipose tissue (AT), which is the primary localization of BAT in humans. Moreover, we wanted to assess the possible function and correlation of the transporter with known markers of thermogenic function, e.g. UCP1. When examining neck AT biopsies from 57 individuals we found that OCT3 was expressed at 2.5 ± 0.16 fold higher level in the deep-neck AT compared with subcutaneous AT. UCP1 was found extensively expressed in the deep-neck AT depot and the correlation between UCP1 and OCT3 within the deep-neck AT was found highly significant (r² = 0.4012, P-value < 0.0001). Lastly, we were able to reduce NE uptake in isolated brown adipocytes in an in vitro culture by adding corticosterone which is a known OCT3-blocker. In conclusion, we found that OCT3 may be a regulator of the concentration of NE in AT and by this mechanism a possible regulator of BAT function and a potential target for pharmacological intervention.

Association between the synonymous variant organic cation transporter 3 (OCT3)-1233G>A and the glycemic response following metformin therapy in patients with type 2 diabetes

OBJECTIVES

Organic cation transporter 3 (OCT3) as a high-capacity transporter contribute to the metabolism of metformin. The present study was conducted to determine the genotype frequencies of the variant OCT3-1233G>A (rs2292334) in patients with newly diagnosed type 2 diabetes (T2D) and its relationship with response to metformin.

MATERIALS AND METHODS

This study included 150 patients with T2D who were classified into two groups following three months of metformin therapy: responders (by more than 1% reduction in HbA1c from baseline) and nonresponders (less than 1% reduction in HbA1c from baseline). PCR-based restriction fragment length polymorphism (RFLP) served to genotype OCT3-564G>A variant.

RESULTS

The parameters such as HbA1c (P<0.001) and BMI (P<0.001) in both patients with GA + AA genotype and GG genotype decreased significantly following 3 months of metformin therapy compared with baseline. The mean reduction in HbA1c levels following 3 months was higher in patients with the A allele (0.77% reduction from baseline) than in those with the homozygous G allele (0.54% reduction from baseline). Also, in GA + AA genotypes compared with GG genotypes, the mean reduction in HbA1c values from baseline was 0.34% for responders and 0.14% for non-responders.

CONCLUSION

Considering the roles of genetic variations in the function of metformin transporters, the effect of variations such as 1233G>A in the OCT3, which is a high-capacity transporter widely expressed in various tissues cannot be ignored. Comparing the allele frequencies of OCT3-1233G>A variant in our study and different ethnic populations confirm that the variant is a highly polymorphic variant.

Histamine Clearance Through Polyspecific Transporters in the Brain

Histamine plays an important role as a neurotransmitter in diverse brain functions, and clearance of histamine is essential to avoid excessive histaminergic neuronal activity. Histamine N-methyltransferase, which is an enzyme in the central nervous system that metabolizes histamine, is localized to the cytosol. This suggests that a histamine transport process is essential to inactivate histamine. Previous reports have shown the importance of astrocytes for histamine transport, although neuronal histamine transport could not be ruled out. High-affinity and selective histamine transporters have not yet been discovered, although it has been reported that the following three polyspecific transporters transport histamine: organic cation transporter (OCT) 2, OCT3, and plasma membrane monoamine transporter (PMAT). The K _m values of human OCT2, OCT3, and PMAT are 0.54, 0.64, and 4.4 mM, respectively. The three transporters are expressed in the brain, and their regional distribution is different. Recent studies revealed the contribution of OCT3 and PMAT to histamine transport by primary human astrocytes. Several investigations using mice supported the importance of OCT3 for histamine clearance in the brain. However, further studies are required to elucidate the detailed mechanism of histamine transport in the brain.

Inhibition of organic cation transporter 2 and 3 may be involved in the mechanism of the antidepressant-like action of berberine

Organic cation transporter 2 (OCT2) and 3 (OCT3) are low-affinity, high-capacity transporters (uptake-2) expressed in the central nervous system (CNS) and other major organs. Proven to be essential components in the CNS functions, OCT2 and OCT3 are suggested as potential targets of antidepressant therapeutics recently. Berberine, an active constituent derived from many medicinal plants, such as Coptis chinensis, has been reported to possess antidepressant-like action in the tail suspension test and forced swim test with elevated serotonin/norepinephrine/dopamine (5-HT/NE/DA) level in mouse brain; however the mechanism has not been elucidated. In consideration of the relation between OCT2/3 and antidepressant action, and the characteristic of berberine as an organic cation, we investigated the potential involvement of OCT2 and OCT3 in the antidepressant-like action of berberine in the present study. The results in mouse brain synaptosomes demonstrated that uptake-2 inhibition might play a notable role in enhanced serotonergic and noradrenergic effects induced by berberine. The inhibitory study in transfected MDCK cells displayed that berberine is a potent inhibitor of human OCT2 and OCT3, and its IC50 values for inhibition of transporter-mediated 5-HT/NE uptake are between 0.1 and 1μM. In addition, berberine was identified as a substrate of hOCT2 and hOCT3. In conclusion, berberine is a substrate and an inhibitor of hOCT2 and hOCT3, and its inhibition on OCT2- and OCT3-mediated 5-HT and NE uptake may contribute to the enhanced monoamine neurotransmission in mouse brain. It was deduced that the inhibition of OCT2 and OCT3 probably be implicated in the mechanism of antidepressant-like action.