Different combinations of regulatory elements may explain why placenta-specific expression of the glycoprotein hormone alpha-subunit gene occurs only in primates and horses.

This research article explores why the expression of the glycoprotein hormone alpha-subunit gene, which generally takes place in the pituitary of all mammals, is specifically observed in the placenta of only primates and horses.

Introduction

Certain mammals, particularly primates and horses, exhibit a unique trait where the glycoprotein hormone alpha-subunit gene (α-subunit gene) is expressed in the placenta. While the α-subunit gene is typically expressed in the pituitary of all mammals, this placenta-specific expression is exclusive to these animal groups. The study aims at understanding the reason for this selective gene expression.

Regulatory Elements and their Roles

• Two regulatory elements, namely the Upstream Regulatory Element (URE) and the cAMP Response Element (CRE), join forces in humans for placenta-specific expression of the α-subunit gene.
• The URE plays a crucial part as it binds a protein that is exclusive to the placenta.
• Although the CRE, on the other hand, binds a protein that is ubiquitously present, it nevertheless contributes to this specific gene expression.

Functional differences among mammals

• While the α-subunit gene is expressed in the horse’s placenta, the promoter-regulatory region lacks a functional CRE but seems to retain a functional URE.
• This may suggest that a different accessory element and related protein interact with the horse URE for placenta-specific expression or perhaps an altogether different set of regulatory elements is required for the process in horses.
• On the contrary, rodents and cattle, despite possessing the URE-binding protein, do not express the α-subunit gene in their placentae. This is traced to a single nucleotide change, which makes the CRE-like sequence of these genes incapable of binding the protein responsible for making the genes responsive to cAMP.

Concluding Remarks

By examining the alpha-subunit gene from various mammals, the study suggests that a functional URE has been retained, implying the possibility of placenta-specific gene expression. Indirect evidence also points towards the conservation of the URE-binding protein across species, further reinforcing the critical role these regulatory elements play in placenta-specific gene expression in certain mammals.