Abstract
To determine whether organic cation transporter (OCT) family members might mediate choline transport in choroid plexus (CP), the handling of choline by cloned transporters and by intact CP isolated from the adult rat was investigated. Expression of OCT1 and OCT2 in Xenopus oocytes increased hemicholinium-3-sensitive choline uptake. In contrast, OCT3 did not mediate choline transport. Estimated K(m) values for choline in rOCT1-, rOCT2-, and hOCT2-expressing oocytes were 346 +/- 50, 441 +/- 67, and 102 +/- 80 microm, respectively. Membrane potential was the major driving force for choline uptake in rat and human OCT2-expressing oocytes and in intact CP in vitro. Lowering of medium pH (6 versus 7.4) was equally effective at inhibiting choline uptake in CP, suggesting that there might be a non-OCT component of choline uptake that is responsive to an H(+) gradient. However, choline efflux from CP was not stimulated by a trans-applied H(+) gradient. Choline uptake by CP was Na(+)-independent with an estimated K(m) of 183 microm. Reverse transcriptase-polymerase chain reaction detected OCT2 and OCT3, but not OCT1, mRNA expression in CP. Transfection of intact CP with a rOCT2/green fluorescent protein fusion construct resulted in strong apical membrane fluorescence with no detectable signal in the basal and lateral plasma membranes. These data indicate that OCT2 mediates choline transport across the ventricular membrane of CP.
Highlights
From the Laboratory of Pharmacology and Chemistry, NIEHS, National Institutes of Health, Research Triangle Park, North Carolina 27709
Characterization of Choline Transport by organic cation transporter (OCT)—Oocytes expressing rat OCT1 (rOCT1), rOCT2, and hOCT2 exhibited substantial choline uptake that was completely blocked by the organic cation transport inhibitor quinine and by the prototypical choline transport inhibitor hemicholinium-3 (HC-3) (Fig. 1A)
Choline uptake mediated by rOCT1 and rOCT2 was relatively insensitive to nicotinamide, an effective modulator of brain choline levels (Fig. 1B)
Summary
Villalobos et al [11], using primary cultures of the choroidal epithelium, found that the characteristics of apical choline uptake in CP cells are strikingly similar to those identified for the renal basolateral organic cation (OC) transport system Such a reversal of functional polarity is conceivable because the movement of OCs from CSF to blood requires the membrane potential-sensitive entry step to take place across the apical membrane of the cells of the CP. We present evidence indicating that OCT2 and OCT3 are expressed in rat CP and that choline uptake by intact rat choroid plexus has all the characteristics of OCT2-mediated transport, suggesting that this carrier plays an important role in central nervous system choline homeostasis
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