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

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.

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).

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.

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.