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The SNAT4 isoform of the system A amino acid transporter is functional in human placental microvillous plasma membrane.

Desforges M, Mynett KJ, Jones RL, Greenwood SL, Westwood M, Sibley CP, Glazier JD - J. Physiol. (Lond.) (2008)

Bottom Line: 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).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.

View Article: PubMed Central - PubMed

Affiliation: Maternal and Fetal Health Research Group, University of Manchester, St Mary's Hospital, Hathersage Road, Manchester M13 0JH, UK. michelle.desforges@manchester.ac.uk

ABSTRACT
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.

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[14C]MeAIB uptake by MVM vesiclesA, the effect of arginine on [14C]MeAIB uptake by first trimester (FT) MVM and B, term MVM vesicles (n= 6 in each group). Data presented are mean ±s.e.m. and statistical analysis by 2-way ANOVA. Symbols shown are; ▪, control; ▴, + 10 mm arginine; ▾, + 20 mm arginine; ∂, + 30 mm arginine; •, + 30 mm MeAIB. **P < 0.01, ****P < 0.0001 versus control. [14C]MeAIB uptake in the presence of 30 mm MeAIB was significantly lower than [14C]MeAIB uptake in all other conditions (P < 0.0001) and was considered non-system A-mediated due to saturation of all isoforms by this high concentration of substrate. In all conditions [14C]MeAIB uptake significantly increased with time (P < 0.0001). C, comparison of system A-mediated [14C]MeAIB uptake (calculated by subtracting [14C]MeAIB uptake in the presence of 30 mm MeAIB from total [14C]MeAIB uptake) by FT and term MVM after 30 s. Data presented are median and interquartile range, n= 6 in each group, *P < 0.05 (Mann–Whitney U-test). D, arginine-inhibitable component of system A activity (expressed as a percentage of system A-mediated [14C]MeAIB uptake) by FT and term MVM. Data presented are median and interquartile range, n= 6 in each group, **P < 0.01 (Mann–Whitney U-test).
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fig03: [14C]MeAIB uptake by MVM vesiclesA, the effect of arginine on [14C]MeAIB uptake by first trimester (FT) MVM and B, term MVM vesicles (n= 6 in each group). Data presented are mean ±s.e.m. and statistical analysis by 2-way ANOVA. Symbols shown are; ▪, control; ▴, + 10 mm arginine; ▾, + 20 mm arginine; ∂, + 30 mm arginine; •, + 30 mm MeAIB. **P < 0.01, ****P < 0.0001 versus control. [14C]MeAIB uptake in the presence of 30 mm MeAIB was significantly lower than [14C]MeAIB uptake in all other conditions (P < 0.0001) and was considered non-system A-mediated due to saturation of all isoforms by this high concentration of substrate. In all conditions [14C]MeAIB uptake significantly increased with time (P < 0.0001). C, comparison of system A-mediated [14C]MeAIB uptake (calculated by subtracting [14C]MeAIB uptake in the presence of 30 mm MeAIB from total [14C]MeAIB uptake) by FT and term MVM after 30 s. Data presented are median and interquartile range, n= 6 in each group, *P < 0.05 (Mann–Whitney U-test). D, arginine-inhibitable component of system A activity (expressed as a percentage of system A-mediated [14C]MeAIB uptake) by FT and term MVM. Data presented are median and interquartile range, n= 6 in each group, **P < 0.01 (Mann–Whitney U-test).

Mentions: Total [14C]MeAIB uptake by first trimester and term MVM vesicles was linear (P < 0.0001 for both, linear regression) over 1 min (see control uptake in Fig. 3A and B) indicating this measurement approximated to initial rate. Figure 3C shows system A-specific [14C]MeAIB uptake (calculated by subtracting uptake in the presence of 30 mm MeAIB from control uptake) by term MVM vesicles was significantly higher than uptake by first trimester vesicles at 30 s.


The SNAT4 isoform of the system A amino acid transporter is functional in human placental microvillous plasma membrane.

Desforges M, Mynett KJ, Jones RL, Greenwood SL, Westwood M, Sibley CP, Glazier JD - J. Physiol. (Lond.) (2008)

[14C]MeAIB uptake by MVM vesiclesA, the effect of arginine on [14C]MeAIB uptake by first trimester (FT) MVM and B, term MVM vesicles (n= 6 in each group). Data presented are mean ±s.e.m. and statistical analysis by 2-way ANOVA. Symbols shown are; ▪, control; ▴, + 10 mm arginine; ▾, + 20 mm arginine; ∂, + 30 mm arginine; •, + 30 mm MeAIB. **P < 0.01, ****P < 0.0001 versus control. [14C]MeAIB uptake in the presence of 30 mm MeAIB was significantly lower than [14C]MeAIB uptake in all other conditions (P < 0.0001) and was considered non-system A-mediated due to saturation of all isoforms by this high concentration of substrate. In all conditions [14C]MeAIB uptake significantly increased with time (P < 0.0001). C, comparison of system A-mediated [14C]MeAIB uptake (calculated by subtracting [14C]MeAIB uptake in the presence of 30 mm MeAIB from total [14C]MeAIB uptake) by FT and term MVM after 30 s. Data presented are median and interquartile range, n= 6 in each group, *P < 0.05 (Mann–Whitney U-test). D, arginine-inhibitable component of system A activity (expressed as a percentage of system A-mediated [14C]MeAIB uptake) by FT and term MVM. Data presented are median and interquartile range, n= 6 in each group, **P < 0.01 (Mann–Whitney U-test).
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fig03: [14C]MeAIB uptake by MVM vesiclesA, the effect of arginine on [14C]MeAIB uptake by first trimester (FT) MVM and B, term MVM vesicles (n= 6 in each group). Data presented are mean ±s.e.m. and statistical analysis by 2-way ANOVA. Symbols shown are; ▪, control; ▴, + 10 mm arginine; ▾, + 20 mm arginine; ∂, + 30 mm arginine; •, + 30 mm MeAIB. **P < 0.01, ****P < 0.0001 versus control. [14C]MeAIB uptake in the presence of 30 mm MeAIB was significantly lower than [14C]MeAIB uptake in all other conditions (P < 0.0001) and was considered non-system A-mediated due to saturation of all isoforms by this high concentration of substrate. In all conditions [14C]MeAIB uptake significantly increased with time (P < 0.0001). C, comparison of system A-mediated [14C]MeAIB uptake (calculated by subtracting [14C]MeAIB uptake in the presence of 30 mm MeAIB from total [14C]MeAIB uptake) by FT and term MVM after 30 s. Data presented are median and interquartile range, n= 6 in each group, *P < 0.05 (Mann–Whitney U-test). D, arginine-inhibitable component of system A activity (expressed as a percentage of system A-mediated [14C]MeAIB uptake) by FT and term MVM. Data presented are median and interquartile range, n= 6 in each group, **P < 0.01 (Mann–Whitney U-test).
Mentions: Total [14C]MeAIB uptake by first trimester and term MVM vesicles was linear (P < 0.0001 for both, linear regression) over 1 min (see control uptake in Fig. 3A and B) indicating this measurement approximated to initial rate. Figure 3C shows system A-specific [14C]MeAIB uptake (calculated by subtracting uptake in the presence of 30 mm MeAIB from control uptake) by term MVM vesicles was significantly higher than uptake by first trimester vesicles at 30 s.

Bottom Line: 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).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.

View Article: PubMed Central - PubMed

Affiliation: Maternal and Fetal Health Research Group, University of Manchester, St Mary's Hospital, Hathersage Road, Manchester M13 0JH, UK. michelle.desforges@manchester.ac.uk

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus