Abstract
The syncytiotrophoblast separates the maternal and fetal blood and constitutes the primary barrier for maternal-fetal transport. The Maxi-chloride channel from the apical membrane of the syncytiotrophoblast plays a role in the chloride conductance. Annexins can play an important role in the regulation of membrane events. In this study we evaluate the role of annexin 6 in the Maxichloride channel properties. The results showed that annexin 6 is bound in the apical placenta membranes in a calcium-dependent phospholipid-binding manner but also in a calcium-independent fashion. The neutralization of annexin 6 decreased the total current by 39 +/- 1.9% in the range of +/-80 mV, and the currents decrease with the time. The single-channel slope conductance was decreased from 253 +/- 7.4 pS (control) to 105 +/- 13 pS, and the amplitude decreased by 50%. The open probability was also affected when higher voltage steps were used, changes in either the positive or negative direction induced the channel to close, and the open probability (P(o)) did not decrease. In channels with neutralized annexin 6, it was maintained at 1 at +/-40 mV and at +/-80 mV. These results suggest that endogenous annexin 6 could regulate the Maxi-chloride channel. The results obtained with normal placentae, in which annexin 6 was neutralized, are similar to those described for the Maxi-chloride channel isolated from pre-eclamptic placenta. Together these data suggest that annexin 6 could play an important role in ion transport of the placenta.
Highlights
The human placental syncytiotrophoblast is a polarized epithelial structure that results from the fusion of precursor cytotrophoblast cells producing a syncytium
Cellular Localization of Annexin 6 in Normal Human Placenta—Our results show that every section of placental villous tissue from normal pregnant subjects at term exhibited immunostaining for annexin 6 in the apical and basal syncytiotrophoblast membranes (Fig. 1)
Annexin 6 in microvillous membrane (MVM) Isolated from Human Placenta—To demonstrate that annexin 6 is present in the purified MVM vesicles used in the electrophysiological studies, in the presence of the contaminating calcium and in the absence of EGTA, we performed Western blot analysis using a specific annexin 6 polyclonal antibody
Summary
The human placental syncytiotrophoblast is a polarized epithelial structure that results from the fusion of precursor cytotrophoblast cells producing a syncytium. Maxi-chloride channels have been identified in secreting and absorbing epithelia [4], in non-epithelial cell types [5,6,7,8], and in the apical membrane from human placenta using electrophysiological methods by our group and by others (9 – 11). This channel may play a role in complex cellular regulation involving inhibition through phosphorylation by protein kinase C, activation by a Ca2ϩ-dependent process, and in volume regulation. These results suggest that annexin 6 has the ability to regulate the ClϪ conductance in human placenta
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