Trophectoderm development and function: the roles of Na+/K+-ATPase subunit isoforms

Abstract

Preimplantation development is a period of cell division, cell shape change, and cell differentiation leading to the formation of an epithelium, the trophectoderm. The trophectoderm is the part of the conceptus that initiates uterine contact and, after transformation to become the trophoblast, uterine invasion. Thus, trophectoderm development during preimplantation stages is a necessary antecedent to the events of implantation. The preimplantation trophectoderm is a transporting epithelium with distinct apical and basolateral membrane domains that facilitate transepithelial Na+and fluid transport for blastocoel formation. That transport is driven by Na+/K+-ATPase localized in basolateral membranes of the trophectoderm. Preimplantation embryos express multiple α and β subunit isoforms of Na+/K+-ATPase, potentially constituting multiple isozymes, but the basolaterally located α1β1isozyme uniquely functions to drive fluid transport. They also express the γ subunit, which is a modulator of Na+/K+-ATPase activity. In the mouse, two splice variants of the γ subunit, γa and γb, are expressed in the trophectoderm. Antisense knockdown of γ subunit accumulation caused a delay of cavitation, implying an important role in trophectoderm function. The preimplantation trophectoderm offers a unique model for understanding the roles of Na+/K+-ATPase subunit isoforms in transepithelial transport.Key words: preimplantation development, trophectoderm, fluid transport, Na+/K+-ATPase, α subunit, β subunit, γ subunit.