SNAT4 isoform of system A amino acid transporter is expressed in human placenta

Am J Physiol Cell Physiol. 2006 Jan;290(1):C305-12. doi: 10.1152/ajpcell.00258.2005. Epub 2005 Sep 7.


The system A amino acid transporter is encoded by three members of the Slc38 gene family, giving rise to three subtypes: Na+-coupled neutral amino acid transporter (SNAT)1, SNAT2, and SNAT4. SNAT2 is expressed ubiquitously in mammalian tissues; SNAT1 is predominantly expressed in heart, brain, and placenta; and SNAT4 is reported to be expressed solely by the liver. In the placenta, system A has an essential role in the supply of neutral amino acids needed for fetal growth. In the present study, we examined expression and localization of SNAT1, SNAT2, and SNAT4 in human placenta during gestation. Real-time quantitative PCR was used to examine steady-state levels of system A subtype mRNA in early (6-10 wk) and late (10-13 wk) first-trimester and full-term (38-40 wk) placentas. We detected mRNA for all three isoforms from early gestation onward. There were no differences in SNAT1 and SNAT2 mRNA expression with gestation. However, SNAT4 mRNA expression was significantly higher early in the first trimester compared with the full-term placenta (P < 0.01). We next investigated SNAT4 protein expression in human placenta. In contrast to the observation for gene expression, Western blot analysis revealed that SNAT4 protein expression was significantly higher at term compared with the first trimester (P < 0.05). Immunohistochemistry and Western blot analysis showed that SNAT4 is localized to the microvillous and basal plasma membranes of the syncytiotrophoblast, suggesting a role for this isoform of system A in amino acid transport across the placenta. This study therefore provides the first evidence of SNAT4 mRNA and protein expression in the human placenta, both at the first trimester and at full term.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Transport System A / genetics*
  • Amino Acid Transport System A / metabolism
  • Female
  • Gene Expression / physiology
  • Humans
  • Organ Specificity
  • Placenta / physiology*
  • Pregnancy
  • Pregnancy Trimester, First
  • Pregnancy Trimester, Third
  • RNA, Messenger / analysis


  • Amino Acid Transport System A
  • RNA, Messenger
  • SLC38A1 protein, human
  • SLC38A2 protein, human
  • SLC38A4 protein, human