title: Computational modelling of amino acid exchange and facilitated transport in placental membrane vesicles.
abstract: Placental amino acid transport is required for fetal development and impaired transport has been associated with poor fetal growth. It is well known that placental amino acid transport is mediated by a broad array of specific membrane transporters with overlapping substrate specificity. However, it is not fully understood how these transporters function, both individually and as an integrated system. We propose that mathematical modelling could help in further elucidating the underlying mechanisms of how these transporters mediate placental amino acid transport. The aim of this work is to model the sodium independent transport of serine, which has been assumed to follow an obligatory exchange mechanism. However, previous amino acid uptake experiments in human placental microvillous plasma membrane vesicles have persistently produced results that are seemingly incompatible with such a mechanism; i.e. transport has been observed under zero-trans conditions, in the absence of internal substrates inside the vesicles to drive exchange. This observation raises two alternative hypotheses; (i) either exchange is not fully obligatory, or (ii) exchange is indeed obligatory, but an unforeseen initial concentration of amino acid substrate is present within the vesicle which could drive exchange. To investigate these possibilities, a mathematical model for tracer uptake was developed based on carrier mediated transport, which can represent either facilitated diffusion or obligatory exchange (also referred to as uniport and antiport mechanisms, respectively). In vitro measurements of serine uptake by placental microvillous membrane vesicles were carried out and the model applied to interpret the results based on the measured apparent Michaelis-Menten parameters Km and Vmax. In addition, based on model predictions, a new time series experiment was implemented to distinguish the hypothesised transporter mechanisms. Analysis of the results indicated the presence of a facilitated transport component, while based on the model no evidence for substantial levels of endogenous amino acids within the vesicle was found.
authors.affiliation: Bioengineering Science Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, UK.
authors.forename: N
authors.initials: N
authors.lastname: Panitchob
authors.affiliation: Maternal & Fetal Health Research Centre, Institute of Human Development, University of Manchester, Manchester, UK; St. Mary's Hospital & Central Manchester University Hospitals NHS Foundation Trust, Manchester Academic Health Science Centre, Manchester, UK.
authors.forename: K L
authors.initials: KL
authors.lastname: Widdows
authors.affiliation: Maternal & Fetal Health Research Centre, Institute of Human Development, University of Manchester, Manchester, UK; St. Mary's Hospital & Central Manchester University Hospitals NHS Foundation Trust, Manchester Academic Health Science Centre, Manchester, UK.
authors.forename: I P
authors.initials: IP
authors.lastname: Crocker
authors.affiliation: Faculty of Medicine, University of Southampton, Southampton, UK.
authors.forename: M A
authors.initials: MA
authors.lastname: Hanson
authors.affiliation: Maternal & Fetal Health Research Centre, Institute of Human Development, University of Manchester, Manchester, UK; St. Mary's Hospital & Central Manchester University Hospitals NHS Foundation Trust, Manchester Academic Health Science Centre, Manchester, UK.
authors.forename: E D
authors.initials: ED
authors.lastname: Johnstone
authors.affiliation: Mathematical Institute, Oxford University, Oxford, UK.
authors.forename: C P
authors.initials: CP
authors.lastname: Please
authors.affiliation: Maternal & Fetal Health Research Centre, Institute of Human Development, University of Manchester, Manchester, UK; St. Mary's Hospital & Central Manchester University Hospitals NHS Foundation Trust, Manchester Academic Health Science Centre, Manchester, UK.
authors.forename: C P
authors.initials: CP
authors.lastname: Sibley
authors.affiliation: Maternal & Fetal Health Research Centre, Institute of Human Development, University of Manchester, Manchester, UK; St. Mary's Hospital & Central Manchester University Hospitals NHS Foundation Trust, Manchester Academic Health Science Centre, Manchester, UK.
authors.forename: J D
authors.initials: JD
authors.lastname: Glazier
authors.affiliation: Faculty of Medicine, University of Southampton, Southampton, UK; Institute for Life Sciences, University of Southampton, Southampton, UK.
authors.forename: R M
authors.initials: RM
authors.lastname: Lewis
authors.affiliation: Bioengineering Science Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton, UK; Institute for Life Sciences, University of Southampton, Southampton, UK. Electronic address: B.G.Sengers@soton.ac.uk.
authors.forename: B G
authors.initials: BG
authors.lastname: Sengers
mesh_terms: D000596:Amino Acids / Q000378:metabolism*; D055459:Facilitated Diffusion; D005260:Female; D006801:Humans; D007700:Kinetics; D008431:Maternal-Fetal Exchange; D008566:Membranes / Q000378:metabolism; D008954:Models, Biological; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D012694:Serine / Q000378:metabolism; D013997:Time Factors; D022161:Transport Vesicles / Q000378:metabolism*
chemical_list: D000596:Amino Acids; D012694:Serine
keywords: Amino acids; Antiport; Carrier model; Membrane transport