Identifying the regulatory mechanisms underlying cholesterol efflux from the sperm plasma membrane during capacitation

Abstract

Before mammalian spermatozoa gain the capacity to fertilise, they must undergo capacitation, a maturation event involving a series of intensely regulated mechanisms that are induced upon exposure to factors within the female reproductive tract or defined in vitro media. The loss of cholesterol from the sperm plasma membrane, also known as cholesterol efflux, is one such mechanism that occurs during capacitation and is required for successful fertilisation. Unlike the majority of mammalian species, traditional in vitro media like Tyrodes with albumin, lactate and pyruvate (TALP) is unable to effectively stimulate capacitation in ram spermatozoa. Only when spermatozoa from this species are incubated with a combination of factors that upregulate cAMP production (collectively termed cAMP up-regulators), these being the metabolic inert cAMP analog dibutyryl cAMP (db-cAMP) and two phosphodiesterase (PDE) inhibitors, caffeine and theophylline, is there evidence of successful capacitation. Though this is established in literature, there is limited knowledge of the capacitation-related processes that are supported under these elevated cAMP conditions, specifically cholesterol efflux. The research within this thesis investigates the requirements necessary to facilitate cholesterol efflux and explores the mechanisms that may be responsible for regulating this process in ram spermatozoa.