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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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Immunolocalization of SNAT1, SNAT2 and SNAT4 in first trimester and term placentasRepresentative photomicrographs for SNAT1 in first trimester (A–C) and term placenta (D–E). Immunostaining is predominantly in trophoblast cells (cytotrophoblast cells: CT and the syncytiotrophoblast: ST), with heterogeneous intense staining on the microvillous membrane (MVM) (A and D). Additional staining was present in stromal Hofbauer cells (asterisks) and vascular endothelium (V) in first trimester (B and C), but not term (E inset), placenta. SNAT2 exhibited a similar immunostaining pattern in first trimester (G–I) and term (J–L) placenta, but with endothelial staining present in vessels throughout pregnancy (H and L). Representative images for SNAT4 immunohistochemistry in first trimester (M–O) and term (P–R) placenta, illustrate a similar localization. Immunostaining is predominantly syncytial, with MVM staining more pronounced in first trimester (M) than term (P). Negative controls are shown in F, I and R. Scale bars represent 50 μm; scale bar on image A refers to all images, except image O.
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fig01: Immunolocalization of SNAT1, SNAT2 and SNAT4 in first trimester and term placentasRepresentative photomicrographs for SNAT1 in first trimester (A–C) and term placenta (D–E). Immunostaining is predominantly in trophoblast cells (cytotrophoblast cells: CT and the syncytiotrophoblast: ST), with heterogeneous intense staining on the microvillous membrane (MVM) (A and D). Additional staining was present in stromal Hofbauer cells (asterisks) and vascular endothelium (V) in first trimester (B and C), but not term (E inset), placenta. SNAT2 exhibited a similar immunostaining pattern in first trimester (G–I) and term (J–L) placenta, but with endothelial staining present in vessels throughout pregnancy (H and L). Representative images for SNAT4 immunohistochemistry in first trimester (M–O) and term (P–R) placenta, illustrate a similar localization. Immunostaining is predominantly syncytial, with MVM staining more pronounced in first trimester (M) than term (P). Negative controls are shown in F, I and R. Scale bars represent 50 μm; scale bar on image A refers to all images, except image O.

Mentions: Figure 1 shows representative images from immunohistochemical analysis of SNAT1 (Fig. 1A–F), SNAT2 (Fig. 1G–L), and SNAT4 (Fig. 1M–R) localization in first trimester (n= 7) and term (n= 6) placental tissue. Immunostaining was predominantly localized to the syncytiotrophoblast for all SNAT proteins. Cytoplasmic staining was observed in all samples with heterogeneous but intense MVM staining apparent for all three isoforms (Fig. 1A, G and M). In first trimester placenta, the intensity of staining for SNAT1 was less in cytotrophoblasts than syncytiotrophoblast (Fig. 1A and B) but the two cell types had similar levels of staining for SNAT2 (Fig. 1G and H) and SNAT4 (Fig. 1O). SNAT1 staining was also detected in stromal Hofbauer cells and the vascular endothelium of first trimester placenta (Fig. 1B and C), whilst at term, this isoform of system A was exclusively localized to syncytiotrophoblast (Fig. 1D and E). SNAT 2 and 4 were also present in endothelial cells in first trimester placenta (Fig. 1H and N), with faint staining persisting in larger intermediate and stem villous vessels at term (Fig. 1L and Q). Subpopulations of stromal cells were positive for SNAT2 and 4 at both stages of gestation (Fig. 1H, O and Q). Spatial immunostaining patterns were consistent in all samples examined, but considerable variation was detected in staining intensity. Negative control sections lacked detectable stain (Fig. 1F, I and R).


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)

Immunolocalization of SNAT1, SNAT2 and SNAT4 in first trimester and term placentasRepresentative photomicrographs for SNAT1 in first trimester (A–C) and term placenta (D–E). Immunostaining is predominantly in trophoblast cells (cytotrophoblast cells: CT and the syncytiotrophoblast: ST), with heterogeneous intense staining on the microvillous membrane (MVM) (A and D). Additional staining was present in stromal Hofbauer cells (asterisks) and vascular endothelium (V) in first trimester (B and C), but not term (E inset), placenta. SNAT2 exhibited a similar immunostaining pattern in first trimester (G–I) and term (J–L) placenta, but with endothelial staining present in vessels throughout pregnancy (H and L). Representative images for SNAT4 immunohistochemistry in first trimester (M–O) and term (P–R) placenta, illustrate a similar localization. Immunostaining is predominantly syncytial, with MVM staining more pronounced in first trimester (M) than term (P). Negative controls are shown in F, I and R. Scale bars represent 50 μm; scale bar on image A refers to all images, except image O.
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fig01: Immunolocalization of SNAT1, SNAT2 and SNAT4 in first trimester and term placentasRepresentative photomicrographs for SNAT1 in first trimester (A–C) and term placenta (D–E). Immunostaining is predominantly in trophoblast cells (cytotrophoblast cells: CT and the syncytiotrophoblast: ST), with heterogeneous intense staining on the microvillous membrane (MVM) (A and D). Additional staining was present in stromal Hofbauer cells (asterisks) and vascular endothelium (V) in first trimester (B and C), but not term (E inset), placenta. SNAT2 exhibited a similar immunostaining pattern in first trimester (G–I) and term (J–L) placenta, but with endothelial staining present in vessels throughout pregnancy (H and L). Representative images for SNAT4 immunohistochemistry in first trimester (M–O) and term (P–R) placenta, illustrate a similar localization. Immunostaining is predominantly syncytial, with MVM staining more pronounced in first trimester (M) than term (P). Negative controls are shown in F, I and R. Scale bars represent 50 μm; scale bar on image A refers to all images, except image O.
Mentions: Figure 1 shows representative images from immunohistochemical analysis of SNAT1 (Fig. 1A–F), SNAT2 (Fig. 1G–L), and SNAT4 (Fig. 1M–R) localization in first trimester (n= 7) and term (n= 6) placental tissue. Immunostaining was predominantly localized to the syncytiotrophoblast for all SNAT proteins. Cytoplasmic staining was observed in all samples with heterogeneous but intense MVM staining apparent for all three isoforms (Fig. 1A, G and M). In first trimester placenta, the intensity of staining for SNAT1 was less in cytotrophoblasts than syncytiotrophoblast (Fig. 1A and B) but the two cell types had similar levels of staining for SNAT2 (Fig. 1G and H) and SNAT4 (Fig. 1O). SNAT1 staining was also detected in stromal Hofbauer cells and the vascular endothelium of first trimester placenta (Fig. 1B and C), whilst at term, this isoform of system A was exclusively localized to syncytiotrophoblast (Fig. 1D and E). SNAT 2 and 4 were also present in endothelial cells in first trimester placenta (Fig. 1H and N), with faint staining persisting in larger intermediate and stem villous vessels at term (Fig. 1L and Q). Subpopulations of stromal cells were positive for SNAT2 and 4 at both stages of gestation (Fig. 1H, O and Q). Spatial immunostaining patterns were consistent in all samples examined, but considerable variation was detected in staining intensity. Negative control sections lacked detectable stain (Fig. 1F, I and R).

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