Mathematical models to predict exchange and facilitated transport across the microvillous plasma membrane (MVM) of the human placenta

Abstract

Introduction: The Na+-independent uptake of serine into microvillous plasma membrane (MVM) vesicles is thought to be mediated by the exchange transporter LAT2. Exchangers are predicted to transport one amino acid (aa) in exchange for another in an obligatory 1:1 manner. However, the observation that serine is transported across MVM under zero-trans conditions, in the absence of internal exchangeable aa, questions whether exchange is indeed obligatory. We revisit the exchange-for-nothing phenomenon by developing mathematical models to examine obligate versus non-obligate exchange, by introducing a hypothesised facilitated component. Methods: Mathematical models were developed based on carrier-mediated transport via a) obligatory exchange, or b) non-obligatory exchange, corresponding to facilitative diffusion. Models were tested by measuring uptake of 7.5 �M 14C-serine into MVM vesicles isolated from normal term placentas, preloaded with either 0 �M or 250 �M (equal to extravesicular serine concentration) or 1 mM serine (outwardly directed substrate gradient to drive facilitated transport). Results: Trans-stimulation of tracer uptake was observed with increasing intra-vesicular serine concentration indicating an exchange component. However, if exchange were obligatory, the model predicted that equilibrium values would be reflected by ratios of intra- and extra-vesicular concentrations, which was not observed. Crucially, the model predicted that for a facilitated transport component, an outwardly directed aa gradient would cause an overshoot of tracer uptake, which was indeed observed.Conclusion: Models were consistent with serine uptake into MVM vesicles by non-obligatory exchange, or exchange together with an as yet uncharacterised facilitated transporter in parallel. This study emphasizes the utility of mathematical modelling in developing an improved quantitative understanding of aa transporter mechanisms. The physiological role for a facilitated transport component on the MVM is at present unclear and warrants further investigation.