Amyloid is defined as any protein that self-aggregates and forms a specific cross beta sheet fibrillar structure.  Although in mammals amyloids are typically associated with neurodegenerative diseases, recent work from several labs, including our lab working in the epididymis, has contributed to a paradigm shift in our understanding of amyloid by showing that amyloids normally exist in some cells and carry out biological functions in the absence of pathology.  Because a characteristic of amyloid is its extreme stability with some protease and SDS resistance, amyloids may function as stable scaffolds in a variety of cellular processes.

The acrosomal matrix (AM) is an insoluble structure within the sperm acrosome that serves as a scaffold controlling the release of AM associated proteins during the sperm acrosome reaction.  The AM also interacts with the zona pellucida (ZP) that surrounds the oocyte suggesting a remarkable stability to survive despite being surrounded by proteolytic and hydrolytic enzymes released during the acrosome reaction.  To date, the mechanism for the stability of the AM is not known.  Our studies demonstrate that amyloids are present within the mouse sperm AM and contribute to the formation of an SDS resistant core.  The AM core contained several known amyloidogenic proteins as well as many proteins predicted to form amyloid including several ZP binding proteins suggesting a functional role for the amyloid core in sperm-ZP interactions.  While stable at pH 3, at pH 7 the sperm AM rapidly destabilized.  The pH-dependent dispersion of the AM correlated with a change in amyloid structure leading to a loss of mature and a gain in immature forms suggesting that the reversal of amyloid is integral to AM dispersion.  From these studies we propose that functional amyloids are present within the mouse sperm AM, and contribute to the formation of a stable core infrastructure that plays roles in the sequential dispersion of proteins during the acrosome reaction as well as in downstream interactions with the zona pellucida. 

The zona pellucida that surrounds the oocyte is composed in the mouse of three proteins including ZP1, ZP2, and ZP3 which are arranged in a complex fibrillar network.  Using conformation-dependent antibodies, PAD pulldown, and negative stain electron microscopy, we show that the mouse ZP is composed of amyloid. Taken together, we propose that interactions between sperm AM amyloid and ZP amyloid are necessary for fertilization.  Indeed, it is possible that it is the sperm AM amyloid structure itself that functions as a nonenzymatic “lysin” allowing zona penetration similar to that which occurs in sea urchins and ascidians. Supported by NIH RO1HD056182, R21HD078715, and the CH Foundation (to GAC).