Cell surface binding sites for progesterone on human spermatozoa

Area of Science:

• Reproductive Biology
• Molecular Endocrinology
• Sperm Cell Biology

Background:

• Progesterone plays a critical role in mammalian reproduction, influencing sperm function.
• The presence and localization of progesterone receptors on human spermatozoa have been debated.
• Understanding progesterone interaction with sperm is crucial for fertility research.

Purpose of the Study:

• To identify and characterize progesterone binding proteins on the surface of human spermatozoa.
• To investigate the accessibility and ligand specificity of these binding sites.
• To explore the potential relationship between sperm and intracellular progesterone receptors.

Main Methods:

• Treatment of human spermatozoa and cell membrane fractions with digitonin to expose binding sites.
• Competitive binding assays using radiolabeled progesterone ([3H]-progesterone) and various steroid competitors.
• Immunological characterization using monoclonal antibodies against uterine progesterone receptor.
• Fluorescence microscopy with labeled progesterone-bovine serum albumin to determine binding localization.
• Determination of molecular weight via biochemical methods.

Main Results:

• A masked progesterone binding protein was identified on the human sperm cell surface, exposed by digitonin treatment.
• Specific binding of [3H]-progesterone was observed, with partial competition from R5020, 17alpha-hydroxyprogesterone, and testosterone.
• Binding sites were localized to the acrosomal region of spermatozoa.
• The sperm binding protein cross-reacted with antibodies to the uterine progesterone receptor and had an estimated molecular weight of 85 kDa.
• Follicular fluid was found to facilitate progesterone binding to spermatozoa.

Conclusions:

• Human spermatozoa possess cell-surface progesterone binding sites that are masked and can be exposed by surfactants.
• These sites exhibit structural homology to intracellular progesterone receptors but differ in ligand specificity and location.
• The findings suggest a novel role for cell-surface progesterone receptors in human sperm function and fertilization.
• Follicular fluid enhances progesterone binding, potentially playing a role in sperm capacitation or acrosome reaction.