Expression of full-length and truncated Fyn tyrosine kinase transcripts and encoded proteins during spermatogenesis and localization during acrosome biogenesis and fertilization.

We report that full-length and truncated transcripts of Fyn tyrosine protein kinase are expressed during testicular development. Truncated Fyn (tr-Fyn) transcripts encode a 24 kDa protein with a N-terminal (NT) domain, a complete Src homology (SH) 3 domain and an incomplete SH2 domain. The kinase domain is missing in tr-Fyn. In contrast, full-length Fyn transcripts encode a 59-55 kDa protein. Fractionated spermatids by centrifugal elutriation express tr-Fyn transcripts and protein, but not full-length Fyn transcripts and protein. Neither full-length Fyn nor tr-Fyn transcripts and encoded proteins are detected in elutriated pachytene spermatocytes. Sertoli cells express full-length and truncated Fyn throughout testicular development. In contrast, sperm contain full-length Fyn transcripts and protein but not the truncated form. tr-Fyn protein is visualized at the cytosolic side of Golgi membranes, derived proacrosomal vesicles, along the outer acrosome membrane and the inner acrosome membrane-acroplaxome complex anchoring the acrosome to the spermatid nuclear envelope. Fyn and phosphotyrosine immunoreactivity coexist in the tail of capacitated sperm. During fertilization, the Fyn-containing acroplaxome seen in the egg-bound and egg-fused sperm is no longer detected upon decondensation of the sperm nucleus. tr-Fyn expands the catalog of truncated tyrosine protein kinases expressed during spermiogenesis. We suggest that the NT and SH3 domains of tr-Fyn may recruit adaptor and effector proteins, in particular GTPase activating proteins, required for acrosome-acroplaxome biogenesis, acroplaxome F-actin dynamics and Sertoli cell function. During fertilization, full-length Fyn in the acroplaxome may contribute to a transient local signaling burst during the early events of sperm-egg interaction.

Signalling in mammalian egg activation: role of protein kinases.

Embryonic development is initiated after the fertilizing spermatozoon enters the egg and triggers a process known as 'egg activation'. Activation results in an increase in intracellular calcium concentration, cortical granule exocytosis (CGE), cell cycle resumption and recruitment of maternal mRNA. Various treatments can induce parthenogenetic activation characterized by the same manifestations. Signal transduction pathways similar to those known for somatic cells mediate the mammalian egg activation. This review focuses on the signal transduction pathways that occur during mammalian fertilization and during parthenogenetic egg activation. We discuss the possibility that members of the protein tyrosine kinase (PTKs) families, the Src family PTKs in particular, operate during egg activation and that protein kinase C can induce CGE.

Signaling through protein kinases during egg activation.

Following penetration of the zona pellucida, the mammalian spermatozoon binds and fuses with the egg plasma membrane, thereby fertilizing the egg that is still arrested at the second metaphase. Fertilization initiates in the egg a sequence of events referred to as 'egg activation'. An initial increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) appears to be the very early cellular event observed which leads to the cortical granules exocytosis and resumption of meiosis. Various treatments can induce parthenogenetic activation mimicking at least part of the fertilization events. Similar to somatic cells, studies in mammalian eggs suggest that signal transduction pathways mediate egg activation. The initial increase in [Ca(2+)](i) appears to be critical for egg activation. However, other messengers such as protein kinase C (PKC) and protein tyrosine kinases (PTKs), were suggested as possible inducers of some aspects of egg activation. In the present work, studies concerning the involvement of protein kinases during egg activation in our laboratory and in others are summarized.