Uptake of L-3,4-dihydroxyphenylalanine and dopamine formation in cultured renal epithelial cells

Perfused rat term placenta as a preclinical model to investigate placental dopamine and norepinephrine transport

The placenta represents a non-neuronal organ capable of transporting and metabolizing monoamines. Since these bioactive molecules participate in numerous processes essential for placental and fetal physiology, any imbalance in their levels during pregnancy may affect brain development, projecting a higher risk of behavioral disorders in childhood or adulthood. Notably, the monoamine system in the placenta is a target of various psychoactive drugs and can be disrupted in several pregnancy pathologies. As research in pregnant women poses significant ethical restrictions, animal models are widely employed to study monoamine homeostasis as a mechanism involved in fetal programming. However, detailed knowledge of monoamine transport in the rat placenta is still lacking. Moreover, relatability to the human placental monoamine system is not examined. The present study provides insights into the transplacental monoamine dynamics between maternal and fetal circulation. We show that norepinephrine maternal-to-fetal transport is <4% due to high metabolism within the trophoblast. In contrast, dopamine maternal-to-fetal transport exceeds 25%, likely through passive transport across the membrane. In addition, we show high clearance of norepinephrine and dopamine from the fetal circulation mediated by the organic cation transporter 3 (OCT3). Altogether, we present transcriptional and functional evidence that the in situ rat placenta perfusion represents a suitable model for (patho)physiological investigation of dopamine and norepinephrine homeostasis in the fetoplacental unit. With the rapid advancements in drug discovery and environmental toxicity, the use of rat placenta as a preclinical model could facilitate screening of possible xenobiotic effects on monoamine homeostasis in the placenta.

Functional reorganization of monoamine transport systems during villous trophoblast differentiation: evidence of distinct differences between primary human trophoblasts and BeWo cells

BACKGROUND

Three primary monoamines-serotonin, norepinephrine, and dopamine-play major roles in the placenta-fetal brain axis. Analogously to the brain, the placenta has transport mechanisms that actively take up these monoamines into trophoblast cells. These transporters are known to play important roles in the differentiated syncytiotrophoblast layer, but their status and activities in the undifferentiated, progenitor cytotrophoblast cells are not well understood. Thus, we have explored the cellular handling and regulation of monoamine transporters during the phenotypic transitioning of cytotrophoblasts along the villous pathway.

METHODS

Experiments were conducted with two cellular models of syncytium development: primary trophoblast cells isolated from the human term placenta (PHT), and the choriocarcinoma-derived BeWo cell line. The gene and protein expression of membrane transporters for serotonin (SERT), norepinephrine (NET), dopamine (DAT), and organic cation transporter 3 (OCT3) was determined by quantitative PCR and Western blot analysis, respectively. Subsequently, the effect of trophoblast differentiation on transporter activity was analyzed by monoamine uptake into cells.

RESULTS

We present multiple lines of evidence of changes in the transcriptional and functional regulation of monoamine transporters associated with trophoblast differentiation. These include enhancement of SERT and DAT gene and protein expression in BeWo cells. On the other hand, in PHT cells we report negative modulation of SERT, NET, and OCT3 protein expression. We show that OCT3 is the dominant monoamine transporter in PHT cells, and its main functional impact is on serotonin uptake, while passive transport strongly contributes to norepinephrine and dopamine uptake. Further, we show that a wide range of selective serotonin reuptake inhibitors affect serotonin cellular accumulation, at pharmacologically relevant drug concentrations, via their action on both OCT3 and SERT. Finally, we demonstrate that BeWo cells do not well reflect the molecular mechanisms and properties of healthy human trophoblast cells.

CONCLUSIONS

Collectively, our findings provide insights into the regulation of monoamine transport during trophoblast differentiation and present important considerations regarding appropriate in vitro models for studying monoamine regulation in the placenta.

Deciphering Spinal Endogenous Dopaminergic Mechanisms That Modulate Micturition Reflexes in Rats with Spinal Cord Injury

Spinal neuronal mechanisms regulate recovered involuntary micturition after spinal cord injury (SCI). It was recently discovered that dopamine (DA) is synthesized in the rat injured spinal cord and is involved in lower urinary tract (LUT) activity. To fully understand the role of spinal DAergic machinery in micturition, we examined urodynamic responses in female rats during pharmacological modulation of the DA pathway. Three to four weeks after complete thoracic SCI, the DA precursor L-DOPA administered intravenously during bladder cystometrogram (CMG) and external urethral sphincter (EUS) electromyography (EMG) reduced bladder overactivity and increased the duration of EUS bursting, leading to remarkably improved voiding efficiency. Apomorphine (APO), a non-selective DA receptor (DR) agonist, or quinpirole, a selective DR₂ agonist, induced similar responses, whereas a specific DR₂ antagonist remoxipride alone had only minimal effects. Meanwhile, administration of SCH 23390, a DR₁ antagonist, reduced voiding efficiency by increasing tonic EUS activity and shortening the EUS bursting period. Unexpectedly, SKF 38393, a selective DR₁ agonist, increased EUS tonic activity, implying a complicated role of DR₁ in LUT function. In metabolic cage assays, subcutaneous administration of quinpirole decreased spontaneous voiding frequency and increased voiding volume; L-DOPA and APO were inactive possibly because of slow entry into the CNS. Collectively, tonically active DR₁ in SCI rats inhibit urine storage and enhance voiding by differentially modulating EUS tonic and bursting patterns, respectively, while pharmacologic activation of DR₂, which are normally silent, improves voiding by enhancing EUS bursting. Thus, enhancing DA signaling achieves better detrusor-sphincter coordination to facilitate micturition function in SCI rats.

L-Tyr-Induced Phosphorylation of Tyrosine Hydroxylase at Ser40: An Alternative Route for Dopamine Synthesis and Modulation of Na+/K+-ATPase in Kidney Cells

BACKGROUND/AIMS

Dopamine (DA) is a natriuretic hormone that inhibits renal sodium reabsorption, being Angiotensin II (Ang II) its powerful counterpart. These two systems work together to maintain sodium homeostasis and consequently, the blood pressure (BP) within normal limits. We hypothesized that L-tyrosine (L-tyr) or L-dihydroxyphenylalanine (L-dopa) could inhibit the Na+/K+-ATPase activity. We also evaluated whether L-tyr treatment modulates Tyrosine Hydroxylase (TH).

METHODS

Experiments involved cultured LLCPK1 cells treated with L-tyr or L-dopa for 30 minutes a 37°C. In experiments on the effect of Dopa Descarboxylase (DDC) inhibition, cells were pre incubated for 15 minutes with 3-Hydroxybenzylhydrazine dihydrochloride (HBH), and them L-dopa was added for 30 minutes. Na+/K+-ATPase activity was quantified colorimetrically. We used immunoblotting and immunocytochemistry to identify the enzymes TH, DDC and the dopamine receptor D1R in LLCPK1 cells. TH activity was accessed by immunoblotting (increase in the phosphorylation). TH and DDC activities were also evaluated by the modulation of the Na+/K+-ATPase activity, which can be ascribed to the synthesis of dopamine.

RESULTS

LLCPK1 cells express the required machinery for DA synthesis: the enzymes TH, and (DDC) as well as its receptor D1R, were detected in control steady state cells. Cells treated with L-tyr or L-dopa showed an inhibition of the basolateral Na+/K+-ATPase activity. We can assume that DA formed in the cytoplasm from L-tyr or L-dopa led to inhibition of the Na+/K+-ATPase activity compared to control. L-tyr treatment increases TH phosphorylation at Ser40 by 100%. HBH, a specific DDC inhibitor; BCH, a LAT2 inhibitor; and Sch 23397, a specific D1R antagonist, totally suppressed the inhibition of Na+/K+-ATPase activity due to L-dopa or L-tyr administration, as indicated in the figures.

CONCLUSION

The results indicate that DA formed mainly from luminal L-tyr or L-dopa uptake by LAT2, can inhibit the Na+/K+-ATPase. In addition, our results showed for the very first time that TH activity is also significantly increased when the cells were exposed to L-tyr.

The paradox of hyperdopaminuria in aromatic L-amino Acid deficiency explained

Aromatic L-amino acid decarboxylase (AADC) decarboxylates 3,4-L-dihydroxylphenylalanine (L-dopa) to dopamine, and 5-hydroxytryptophan to serotonin. In AADC deficiency, dopamine and serotonin deficiency leads to a severe clinical picture with mental retardation, oculogyric crises, hypotonia, dystonia, and autonomic dysregulation. However, despite dopamine deficiency in the central nervous system, urinary dopamine excretion in AADC-deficient patients is normal to high.In human, renal AADC-activity is very high compared to other tissues including brain tissue. Plasma L-dopa levels are increased in AADC deficiency. In this study, the hypothesis that in AADC deficiency relatively high-residual renal AADC-activity combined with high substrate availability of L-dopa leads to normal or elevated levels of urinary dopamine is tested and verified using 24-h urine collection of two AADC-deficient patients.Renal dopamine is a major regulator of natriuresis and plays a crucial role in the maintenance of sodium homeostasis. Therefore, the preservation of sufficient renal AADC-activity in AADC deficiency might be crucial for survival of AADC-deficient patients.In this study, we underpinned an empirical finding with theory, thereby putting a clinical observation into its physiological context. Our study stresses the difference - not qualitatively but quantitatively - between dopamine production in the central nervous system and peripheral organs. Furthermore, this study clarifies the so far unexplained observation that neurotransmitter profiles in urine should be interpreted with extreme caution in the diagnostic work-up of patients suspected to suffer from neurometabolic disorders.

Uptake and Decarboxylation of l-3,4-Dihydroxyphenylalanine in Cultured Monkey Placenta Amniotic Epithelial Cells

In this study we tested the ability of monkey amniotic epithelial cells (MAEC) to take up and decarboxylate l-3,4-dihydroxyphenylalanine (l-DOPA) by incubating the cells in buffer containing l-DOPA under different experimental conditions followed by assaying cellular dopamine (DA) content using high performance liquid chromatography with electrochemical detection. Cellular contents of DA were significantly increased in a time- and l-DOPA-concentration-dependent manner, suggesting the uptake of l-DOPA by MAEC and indicating the presence of aromatic l-amino acid decarboxylase (AADC). This was confirmed by the decreased DA content in the presence of benserazide, an AADC inhibitor. Neither d-DOPA nor DA uptake blockers such as mazindol and GBR 12935 significantly affected l-DOPA uptake and hence DA levels. Further, synthesis of DA from l-DOPA was decreased in the presence of the amino acids tyrosine, phenylalanine and tryptophan, whereas the amino acids glycine and proline were without any significant effect. These findings suggest that MAEC have the capacity to selectively take up and decarboxylate l-DOPA with subsequent production of DA.

Expression and function of sodium transporters in two opossum kidney cell clonal sublines

The present study describes characteristic features of two clonal subpopulations of opossum kidney (OK) cells (OK(LC) and OK(HC)) that are functionally different but morphologically identical. The most impressive differences between OK(HC) and OK(LC) cells are the overexpression of Na+-K+-ATPase and type 3 Na+/H+ exchanger by the former, accompanied by an increased Na+-K+-ATPase activity (57.6 +/- 5.6 vs. 30.0 +/- 0.1 nmol P(i). mg protein(-1). min(-1)); the increased ability to translocate Na+ from the apical to the basolateral surface; and the increased Na+-dependent pH(i) recovery (0.254 +/- 0.016 vs. 0.094 +/- 0.011 pH units/s). Vmax values (in pH units/s) for Na+-dependent pHi recovery in OK(HC) cells (0.00521 +/- 0.0004) were twice (P < 0.05) those in OK(LC) (0.00202 +/- 0.0001), with similar Km values (in mM) for Na+ (OK(LC), 21.0 +/- 5.5; OK(HC), 14.0 +/- 5.6). In addition, we measured the activities of transporters (organic ions, alpha-methyl-D-glucoside, L-type amino acids, and Na+ and enzymes (adenylyl cyclase, aromatic L-amino acid decarboxylase, and catechol-O-methyltransferase). The cells were also characterized morphologically by optical and scanning electron microscopy and karyotyped. It is suggested that OK(LC) and OK(HC) cells constitute an interesting cell model for the study of renal epithelial physiology and pathophysiology, namely, hypertension.

P-glycoprotein phosphorylation/dephosphorylation and cellular accumulation of L-DOPA in LLC-GA5 Col300 cells

1. The present work was aimed to study the effect of PKC activation and protein-serine/threonine phosphatase (PP1/PP2 A) inhibition on P-glycoprotein (P-gp) mediated transport of L-DOPA in LLC-GA5 Col300 cells, a renal cell line expressing the human P-glycoprotein in the apical membrane. 2. L-DOPA accumulation was a time-and concentration-dependent process with the following kinetic characteristics: kin, 57.3 +/- 1.2 pmol mg protein(-1) min(-1); k(out), 3.3 +/- 0.1 pmol mg(-1) protein min(-1); Amax, 10.6 +/- 0.8; Kn, 198 +/- 64 microM; Vmax, 5.2 +/- 0.7 nmol mg protein(-1). 3. Verapamil (25 microM), a P-glycoprotein inhibitor, markedly increased (approximately 40% increase) the accumulation of a non-saturating concentration of L-DOPA (2.5 microM) at both initial rate of uptake (IRU, 6 min incubation) and at steady-state (SS, 30 min incubation). 4. PKC activation with phorbol 12,13-dibutyrate (PDBu, 1, 3 and 10 nM) produced a concentration-dependent decrease in L-DOPA accumulation at SS, but not at IRU. The inactive phorbol ester, 4alpha-phorbol 12,13-didecanoate (100 nM), produced no change in L-DOPA accumulation. The effect of PDBu was completely reverted by staurosporine (100 nM). The phosphatase inhibitor okadaic acid (100 nM) reduced by 20% the accumulation of L-DOPA at IRU, but not at SS. 5. It is suggested that P-glycoprotein plays a role in regulation of intracellular availability of L-DOPA in renal epithelial cells, and phosphorylation/dephosphorylation of P-glycoprotein may be involved in the regulation of the transporter.

A cognate dopamine transporter-like activity endogenously expressed in a COS-7 kidney-derived cell line

The activity of the dopamine transporter is an important mechanism for the maintenance of normal dopaminergic homeostasis by rapidly removing dopamine from the synaptic cleft. In kidney-derived COS-7, COS-1 and HEK-293 but not in other mammalian cell lines (CHO, Y1, Ltk-), we have characterized a putative functional dopamine transporter displaying a high affinity (Km approximately 250 nM) and a low capacity (approximately 0.1 pmol/10(5) cells/min) for [3H]dopamine uptake. Uptake displayed a pharmacological profile clearly indicative of the neuronal dopamine transporter. Estimated Ki values of numerous substrates and inhibitors for the COS-dopamine transporter and the cloned human neuronal transporter (human dopamine transporter) correlate well with the exception of a few notable compounds, including the endogenous neurotransmitter dopamine, the dopamine transporter inhibitor GBR 12,909 and the dopaminergic agonist apomorphine. As with native neuronal and cloned dopamine transporters, the uptake velocity was sodium-sensitive and reduced by phorbol ester pre-treatment. Two mRNA species of 3.8 and 4.0 kb in COS-7 cells were revealed by Northern blot analysis similar in size to that seen in native neuronal tissue. A reverse-transcribed PCR analysis confirmed the existence of a processed dopamine transporter. However, no immunoreactive proteins of expected dopamine transporter molecular size or [3H]WIN 35,428 binding activity were detected. A partial cDNA of 1.3 kb, isolated from a COS-1 cDNA library and encoding transmembrane domains 1-6, displayed a deduced amino acid sequence homology of approximately 96% to the human dopamine transporter. Taken together, the data suggest the existence of a non-neuronal endogenous high affinity dopamine uptake system sharing strong functional and molecular homology to that of the cloned neuronal dopamine transporter.