J Physiol 587.1 (2009) pp 61–72 61 The SNAT4 isoform of the system A amino acid transporter is functional in human placental microvillous plasma membrane M. Desforges, K. J. Mynett, R. L. Jones, S. L. Greenwood, M. Westwood, C. P. Sibley and J. D. Glazier Maternal and Fetal Health Research Group, Research School of Clinical and Laboratory Sciences, University of Manchester, St Mary’s Hospital, Hathersage Road, Manchester M13 0JH, UK Placental system A activity is important for the supply of neutral amino acids needed for fetal growth. There are three system A isoforms: SNAT1, SNAT2 and SNAT4, but the contribution of each to system A-mediated transport is unknown. Here, we have used immunohistochemistry to demonstrate that all three isoforms are present in the syncytiotrophoblast suggesting each plays a role in amino acid transport across the placenta. We next tested the hypothesis that the SNAT4 isoform is functional in microvillous plasma membrane vesicles (MVM) from normal human placenta using a method which exploits the unique property of SNAT4 to transport both cationic amino acids as well as the system A-speciﬁc substrate MeAIB. The data show that SNAT4 contribution to system A-speciﬁc amino acid transport across MVM is higher in ﬁrst trimester placenta compared to term (approx. 70% and 33%, respectively, P < 0.01). Further experiments performed under more physiological conditions using intact placental villous fragments suggest a contribution of SNAT4 to system A activity in ﬁrst trimester placenta but minimal contribution at term. In agreement, Western blotting revealed that SNAT4 protein expression is higher in ﬁrst trimester MVM compared to term (P < 0.05). This study provides the ﬁrst evidence of SNAT4 activity in human placenta and demonstrates the contribution of SNAT4 to system A-mediated transport decreases between ﬁrst trimester and term: our data lead us to speculate that at later stages of gestation SNAT1 and/or SNAT2 are more important for the supply of amino acids required for normal fetal growth. (Received 12 August 2008; accepted after revision 11 November 2008; ﬁrst published online 17 November 2008) Corresponding author M. Desforges: Maternal and Fetal Health Research Group, University of Manchester, St Mary’s Hospital, Hathersage Road, Manchester M13 0JH, UK. Email: michelle.desforges@manchester.ac.uk During intrauterine life an adequate supply of amino acids from the maternal circulation is essential for normal fetal growth and development. In humans the maternal and fetal circulations are separated by a polarized epithelium in the placenta known as the syncytiotrophoblast. Amino acid transfer across the syncytiotrophoblast is mediated by a number of transporters found in the maternal-facing microvillous plasma membrane (MVM) and fetal-facing basal plasma membrane (BM) (Moe, 1995; Jansson, 2001). Each class of transporter is speciﬁc for a particular group of amino acids and, based on competitive interactions between amino acid substrates, at least 15 transport systems for neutral, basic and acidic amino acids have been identiﬁed in the syncytiotrophoblast analogous to those described for other epithelia (Moe, 1995; Jansson, 2001). System A is a Na+-dependent transporter that actively transports small, zwitterionic, neutral amino acids with short, unbranched side chains such as alanine, serine, and glutamine (Johnson & Smith, 1988). It can also transport N -methylated amino acids such as α-(methylamino)isobutyric acid (MeAIB) (Christensen et al. 1965). This non-metabolizable amino acid analogue has been used extensively to study system A activity in the placenta (Mahendran et al. 1993; Novak et al. 1996; Jozwik et al. 1998; Jansson et al. 2003). Reduced system A activity has been demonstrated in MVM vesicles isolated from placentas in which the fetus was growth restricted (Dicke & Henderson, 1988; Mahendran et al. 1993; Glazier et al. 1997; Jansson et al. 2002). Whether this represents a primary event in cases of intrauterine growth restriction (IUGR) or is secondary to the growth restriction is unknown. However, evidence from three separate animal studies suggests reduced system A activity may be a cause of, rather than merely be associated with, altered fetal growth. Inhibition of system A in pregnant rats leads to fetal growth restriction (Cramer et al. 2002); in a mouse model of IUGR (Constancia et al. 2002) and C 2009 The Authors. Journal compilation C 2009 The Physiological Society DOI: 10.1113/jphysiol.2008.161331