Detection of steroidogenic acute regulatory protein in equine ovaries.

The research investigates the presence and role of steroidogenic acute regulatory (StAR) protein in horse ovaries, namely its involvement in the movement of cholesterol – significant for steroid production – within mitochondria. The focus lies on how equine chorionic gonadotrophin (eCG), a hormone associated with early pregnancy, affects this process.

Understanding Steroidogenesis and StAR Protein

• The study’s context revolves around steroidogenesis, which is the biological process of steroid hormone production. It involves the conversion of cholesterol into steroid hormones.
• Central to this process is a protein called the Steroidogenic acute regulatory (StAR) protein.
• The primary function of the StAR protein is to transfer cholesterol from the outer mitochondrial membrane to the inner mitochondrial membrane – a crucial step for steroid hormone production.

Role of Equine Chorionic Gonadotrophin (eCG)

• The researchers note that eCG is known to promote steroidogenesis in the corpora lutea (pregnancy supporting structures in ovaries) of early pregnant mares (female horses).
• Earlier studies have indicated that eCG also enhances the activity of the StAR protein in bovine ovaries.
• The researchers, therefore, wanted to explore if a similar effect occurred in horse ovaries.

Methodology

• Ovarian tissue from cyclic and early pregnant mares was examined using immunostaining – a technique used to visualize proteins in cells – to locate the StAR protein.
• They further conducted a Western blot analysis, a method used to determine protein levels, along with phosphor imaging to confirm if eCG secretion during pregnancy corresponded with augmented StAR protein expression.

Key Findings

• The StAR protein was located primarily in the theca interna (an area of the follicle that plays a key role in steroid hormone production) of developing and preovulatory follicles. However, after treatment with hCG (a hormone that helps to mature and release the egg during ovulation), some granulosa cells (cells that form the structure of the ovarian follicle) also showed staining for the protein.
• StAR protein expression was particularly marked in the large luteal cells (cells in the corpus luteum that produce progesterone) in the horse corpus luteum (a temporary endocrine structure involved in ovulation and early pregnancy)
• No noticeable difference was found in the distribution of StAR before or after the commencement of eCG secretion in pregnant mares. However, higher amounts of StAR were found in the corpora lutea of mares at day 40 or day 41 of pregnancy compared to non-pregnant mares and mares at days 20-30 of pregnancy.
• This suggests that StAR protein levels increase within the horse ovaries around the 40th day of pregnancy, aiding in the enhancement of steroid hormone production necessary for the maintenance of pregnancy.