. SNAT4 isoform of system A amino acid transporter is expressed in human placenta. Am J Physiol Cell Physiol 290: C305-C312, 2006. First published September 7, 2005 doi:10.1152/ajpcell.00258.2005.-The system A amino acid transporter is encoded by three members of the Slc38 gene family, giving rise to three subtypes: Na ϩ -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 Ͻ 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 Ͻ 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. SNAT1; SNAT2; gestational expression; syncytiotrophoblast SYSTEM A IS A UBIQUITOUS Na ϩ -dependent transporter that actively transports small, zwitterionic, neutral amino acids with short, unbranched side chains such as alanine, serine, and glutamine (24). It has a unique ability to transport N-methylated amino acids such as ␣-(methylamino)isobutyric acid (MeAIB) (7). This nonmetabolized amino acid analog has been used extensively to study system A in the placenta (21,27,37).Recent data show that system A is encoded by three different members of the SLC38 gene family (Slc38a1, Slc38a2, and Slc38a4), giving rise to the three subtypes of this Na ϩ -coupled neutral amino acid transporter (SNAT): SNAT1, SNAT2, and SNAT4 (previously referred to as ATA1, ATA2, and ATA3, respectively) (31). SNAT1 was the first isoform to be cloned from rat brain and initially was designated GlnT because of its preference for glutamine as a substrate (49). Cloning of the human homolog as well as SNAT2 and SNAT4 followed (18,19,44,45,50,51). These previous studies showed the three isoforms to be highly homologous: amino acid sequences for SNAT1 and S...

The role of parathyroid hormone-related protein (PTHrP) in fetal calcium homeostasis and placental calcium transport was examined in mice homozygous for the deletion of the PTHrP gene (PTHrP −/− null; NL) compared to PTHrP +/+ (wild-type; WT) and PTHrP +/− (heterozygous; HZ) littermates. Fetal blood ionized calcium was significantly reduced in NL fetuses compared to WT and HZ groups at 18 days of pregnancy (dp) with abolition of the fetomaternal calcium gradient. In situ placental perfusion of the umbilical circulation at 18 dp was used to measure unidirectional clearance of 45 Ca across the placenta in maternofetal ( Ca K mf ) and fetoplacental ( Ca K fp ) directions; Ca K fp was < 5% of Ca K mf for all genotypes. At 18 dp, Ca K mf across perfused placenta and intact placenta ( Ca K mf(intact) ) were similar and concordant with net calcium accretion rates in vivo.Ca K mf was significantly raised in NL fetuses compared to WT and HZ littermates. Calcium accretion was significantly elevated in NL fetuses by 19 dp. Placental calbindin-D 9K expression in NL fetuses was marginally enhanced (P < 0.07) but expression of TRPV6/ECaC2 and plasma membrane Ca 2+ -ATPase (PMCA) isoforms 1 and 4 were unaltered. We conclude that PTHrP is an important regulator of fetal calcium homeostasis with its predominant effect being on unidirectional maternofetal transfer, probably mediated by modifying placental calbindin-D 9K expression. In situ perfusion of mouse placenta is a robust methodology for allowing detailed dissection of placental transfer mechanisms in genetically modified mice.

IntroductionSyncytial nuclear aggregates (SNAs) are increased in pregnancy complications; however, little is known about their origin or function. This study aimed to characterise SNAs in more detail than has been reported previously.MethodsImmunohistochemistry and morphological examination at the light and ultrastructural level were used to determine the nature and structure of SNAs.ResultsSNAs comprising bridges and syncytial knots had similar frequency with 974 per mm3 of villous tissue (IQR 717–1193) and 833 per mm3 (IQR 766–1190), respectively while there were approximately four times as many sectioning artefacts than knots and bridges combined. SNAs had increased proportions of condensed nuclei compared to the remaining syncytiotrophoblast (33.3% vs. 8.9%) and decreased proportions of euchromatic nuclei (0.0% vs. 16.2%), as assessed by examination of an electron micrograph archive. SNAs showed little evidence of apoptosis, with weak positivity for the apoptosis markers M30-neoepitope at 16.6% and TUNEL at 10.0%; strong staining was rarely seen for either marker. Immunofluorescence demonstrated rare association of actin (α, β or γ) with SNAs, whereas tubulin was in close proximity to SNAs and cytokeratin was seen within and surrounding SNAs.DiscussionM30-positive SNAs traced through serial sections were significantly more likely to be syncytial knots or sectioning artefacts than bridges. Nuclei within SNAs showed signs consistent with degeneration; however, this is unlikely to be an apoptotic process. There are few changes in configuration of cytoskeletal proteins around SNAs.ConclusionsThese data suggest that the biogenesis and functional significance of SNAs still require resolution.