Localisation of ABCA1 in First Trimester and Term Placental Tissues
Article Outline
Keywords: ABCA1, Localisation, Immunohistochemistry
To the Editors of Placenta
The recently published study of Bhattacharjee et al. on the localisation of the lipid transporter ABCA1 [1] essentially reflects our immunohistochemical findings in first (n
=5) and third trimester (n=5) human placental tissues. However, some cell types that we have consistently identified as moderately to strongly positive for ABCA1 are apparently lacking in the above-mentioned study. In complement to the results described in Ref. [1], we have evidence that in early gestation (A–D) epithelium of secreting endometrial glands (A, white short arrows), decidual cells (not shown), and possible Hofbauer cells in villous stroma (B, C, arrowheads) are strongly positive for ABCA1 (Fig. 1). Based on our data, villous syncytiotrophoblasts (black long arrows) show variable but unequivocal staining of ABCA1 – ranging from strong positivity of the entire cytoplasm (B), through strong staining of most of the cytoplasm but sparing of the microvillous surface (C), to weak staining or complete negativity (D). This is in partial contrast to Bhattacharjee et al. [1] who observed villous syncytiotrophoblast staining only with immunofluorescence in few villi in the apical membrane, but not with immunohistochemistry. Unlike the villous syncytiotrophoblast, the villous cytotrophoblast (white long arrows) is consistently strongly positive for ABCA1 (B–D). In term placentas (E and F), the cytoplasm of the villous syncytiotrophoblast is negative (black long arrows), while occasionally a weak staining of the apical cell membrane is apparent (E, arrowheads). Of note, villous syncytiotrophoblast staining is weaker than in our previous experiments [2], which may be due to the use of different antibodies and/or visualization systems. Like in early gestation, the villous cytotrophoblast stains strongly positive (E, white long arrows). Similar to first trimester placentas, decidual cells show strong staining of the cytoplasm (F, black short arrows).
Fig. 1
Representative images of immunohistochemical stainings for ABCA1 in first trimester (A–D; 7 6/7–13 0/7 weeks of gestation) and term placentas (E–F; 38 0/7–39 6/7 weeks of gestation). The stainings were carried out with UltraVision LP Value Detection System kit (Labvision) according to the manufacturer’s instructions. Antigen retrieval was performed by treatment with citrate buffer pH 5.5 for 10
min. Endogenous peroxidase was blocked with 3% H2O2 for 12min. UV-blocking solution supplemented with 10% AB-human serum was applied followed by primary antibodies incubation. Primary polyclonal anti-human ABCA1 antibody (Novus Biologicals) was applied at a concentration of 10μg/mL. Rabbit immunoglobulins (Sigma–Aldrich) were used as isotype control at the same concentration. The slides were exposed to labeled polymer horseradish-peroxidase (HRP). The peroxidase was developed with 3-amino-9-ethyl-carbazole/dimethylformamide (AEC). See text for detailed description of the results.
Per se, the distribution of ABCA1 to cytosolic compartments is not unexpected. Localisation of ABCA1 in intracellular vesicles such as early and late endosomes or lysosomes has been previously demonstrated in other cell types [3], [4]. It seems that a complex intracellular trafficking pathway for ABCA1 exists that may play a role in modulating ABCA1 surface expression and transporter activity. It is also known that ABCA1 is endocytosed and rapidly recycled back to the cell surface [4]. This retroendocytosis pathway of ABCA1/apoA1 seems to contribute to HDL formation when excess lipoprotein-derived cholesterol has accumulated in cells. These mechanisms could also occur in placental cells, where cholesterol plays an important role not only as an essential substance for the fetus, but also as a substrate for steroid hormone synthesis.
Whether the discrepancies between Bhattacharjee’s [1] and our current unpublished and previous [2] localisation data are only attributable to the use of different antibodies is debatable. However, our data on the differential staining of villous syncytiotrophoblast between first trimester and term placentas and the strong positivity of decidual cells and endometrial glands may indicate additional yet unidentified roles of ABCA1 in materno–fetal exchange which need to be further investigated.
Acknowledgments
This work was funded by the Swiss National Foundation (grant no. 320030-119984), the SwissLife Jubiläumsstiftung, the Novartis Foundation and the Wolfermann Nägeli Stiftung. The authors have no conflict of interest to declare.
References
- Expression and localization of ATP binding cassette transporter A1 (ABCA1) in first trimester and term human placenta. Placenta. 2010;31:423–430
- Placental ABCA1 expression is reduced in primary antiphospholipid syndrome compared to pre-eclampsia and controls. Placenta. 2007;28:701–708
- Cellular localization and trafficking of the human ABCA1 transporter. J Biol Chem. 2001;276:27584–27590
- . Retroendocytosis pathway of ABCA1/apoA-I contributes to HDL formation. Genes Cells. 2009;14:191–204
PII: S0143-4004(10)00206-7
doi:10.1016/j.placenta.2010.05.010
© 2010 Elsevier Ltd. All rights reserved.
