HtrA4 is required for human trophoblast stem cell differentiation into syncytiotrophoblast

Culture of cryopreserved first trimester placental tissues to study syncytial renewal

Ex vivo studies with first trimester placental tissues are crucial for understanding human placental development, and culturing placental villi in floating conditions is vital to mimic the natural setting. Moreover, being able to cryopreserve these scarce materials for ex vivo studies would open unprecedented avenues, as they are very limited to research and the need to culture freshly adds further hurdles while limiting efficient use. Here we showed that hanging drop method is a simple yet effective approach to culture placental floating villi. We also revealed that these functional units of the early-stage human placenta can be cryopreserved for culturing, and that frozen-thawed villi can regenerate the syncytial layer following denudation. We further illustrated the utility of frozen-thawed tissues to study syncytialization, while validating the importance of HtrA4 in the process which was shown previously in cell models. These represent significant new knowledge to the field of placental biology.

HtrA4 is well conserved only in higher primates and functionally important for EVT differentiation

INTRODUCTION

The placenta differs greatly among species, and deep extra-villous trophoblast (EVT) invasion is a unique feature of placentation of higher primates including humans. We reported serine protease HtrA4 being found predominantly in human placentas with aberrant expression linked to preeclampsia. However, it remains unclear where HtrA4 is produced in the placenta, how it is expressed in other species, and whether it is essential for human placentation.

METHODS

We first compared HtrA4 protein sequences of over 100 species, then scrutinized the key characteristics of HtrA4 in the human, rhesus macaque and mouse, and determined cellular localization in the placenta. We next investigated functional significance of HtrA4 in EVT differentiation using human trophoblast stem cells (TSCs).

RESULTS

Across broader species HtrA4 is well conserved only in higher primates. In humans, only the placenta expressed HtrA4, localising to trophoblasts of villous as well as extra-villous lineages. Rhesus macaques produced HtrA4 but again only in placentas, whereas mice showed no abundant HtrA4 expression anywhere including the placenta, yet it was an active protease if produced. The functional importance of HtrA4 in human EVT was demonstrated using TSCs, which expressed low levels of HtrA4 but significantly up-regulated it during EVT differentiation, and knockdown of HtrA4 severely inhibited the differentiation process.

DISCUSSION

HtrA4 is expressed in placentas of humans and macaques but not mice; it is critical for human EVT differentiation. Together with previous reports showing HtrA4 is also indispensable for syncytialization, this study further revealed HtrA4 as a functionally important protease for human placentation.