The earliest visible indications for the transition to embryos in mammalian eggs, known as egg activation, are cortical granules exocytosis (CGE) and resumption of meiosis (RM); these events are triggered by the fertilizing spermatozoon through a series

Prior to fertilization, the spindle of vertebrate eggs must remain stable and well organized during the second meiotic metaphase arrest (MII). In a previous study we have determined that the completion of meiosis is a Src family kinase (SFK)-dependent event. In the current study we have used the SFK inhibitors, SU6656 and PP2, and demonstrated that inhibition of SFKs caused the formation of a disorganized spindle. The observation that proper organization of an MII spindle is an SFK-dependent process, combined with our previous finding that Fyn kinase is localized at the microtubules (MTs), prompted us to examine the potential role of Fyn in MT signaling. Our results show an association between Fyn and tubulin, the ability of Fyn to phosphorylate tubulin in vitro and stimulation of meiosis completion by injection of a constitutively active form of Fyn (CAF). We suggested that SFKs mediate significant functions during the organization of the MII spindle. In view of CAF injection experiments, and of the pronounced concentration of Fyn kinase at the spindle, we propose that Fyn may play an important role in some aspects of the spindle functions, possibly those involving the MTs.

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.

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