Isoforms of the Mitochondrial Capsule Protein: INTRODUCTION
After exiting the testis, mammalian spermatozoa undergo morphological and biochemical changes as they travel through the epididymis. As a result of these modifications, spermatozoa acquire progressive motility and fertilizing potential; however, they are still fertilization incompetent. To achieve fertilizing capacity, spermatozoa require residence in the microenvironment of the female tract for a finite period of time. This acquisition of functional competence is termed capacitation and was first described independently by Chang and Austin. During ca-pacitation, molecular changes occur in the head that enable spermatozoa to undergo the acrosome reaction in response to zona pellucida or other physiological stimuli, such as progesterone, and in the flagellum, where a hyperactivated motility pattern develops (a whiplash-like flagellar movement with large-amplitude, asymmetric bends). These two physiological processes are considered as the benchmark endpoints of capacitation.
In vitro capacitation can be accomplished by incubating cauda epididymal or ejaculated spermatozoa in a defined medium; serum albumin, bicarbonate, and calcium play an important regulatory role in promoting capacitation. Albumin serves as a sink for the removal of plasma membrane cholesterol that leads to the alteration of membrane fluidity observed during capacitation. The increase of intracellular pH could be due to the transmembrane movement of bicarbonate anions. The role of calcium during capacitation is controversial. Some investigators have described an increase in intracellular calcium during capacitation, whereas others have reported that no changes in calcium levels occur during this physiological event. Because calcium acts as an effector of enzymes involved in signaling cascade(s), it is assumed that calcium may have a critical role in capacitation. Recently, Baker et al. demonstrated that extracellular calcium suppresses tyrosine phosphorylation in both human and mouse spermatozoa by decreasing the availability of intracellular ATP.
Protein tyrosine phosphorylation, a key biochemical event accompanying sperm capacitation, is dependent on the presence of calcium, bicarbonate, and albumin. Sperm protein tyrosine phosphorylation is regulated by a signal transduction pathway involving c-AMP, protein kinase A (PKA), and tyrosine kinases. It has been shown that reactive oxygen species upregulate protein tyrosine phosphorylation of several proteins. Although this signaling cascade generates an array of tyrosine-phos-phorylated polypeptides, their molecular characterization is still limited. Only CABYR, a novel calcium-binding fibrous sheath protein, VCP, a valosin-containing protein, and two members of the A kinase-anchoring protein (AKAP) family are ascribed putative functions. The role of the tyrosine-phosphorylated polypeptides in sperm capacitation remains to be elucidated.
We have demonstrated capacitation-dependent tyrosine phosphorylated polypeptides (M 19 000-99 000) in the hamster sperm flagellum. The objective of the present study was to identify and to characterize a 19-kDa tyrosine-phos-phorylated protein and to elucidate its role in the events associated with capacitation. The results presented here demonstrate that the 19-kDa tyrosine-phosphorylated polypeptide is phospholipid hydroperoxide glutathione peroxidase (PHGPx) and the generation of PHGPx isoforms occurs during in vitro capacitation. The selenoprotein PHGPx is a disulfide-stabilized insoluble structural protein of the sperm mitochondrial capsule. The potential role of PHGPx as related to capacitation is discussed.