The origin and assembly of a zona pellucida binding protein, IAM38, during spermiogenesis

IAM38 is the most prominent protein of the inner acrosomal membrane extracellular coat (IAMC) of spermatozoa. It is retained by the inner acrosomal membrane after the acrosome reaction and is implicated in secondary sperm-zona binding (Yu et al., Dev Biol2006;290:32-43). Its gene knockout, Zpbp1(-/-), prevents acrosome compaction and leads to acrosome fragmentation and sterility in male mice (Lin et al., Mol Cell Biol2007;27:6794-6805). Because of this protein's important role in acrosomal formation and fertilization, the authors were interested in tracing its origin and expression during spermiogenesis. The authors reasoned that a detailed immunocytochemical investigation, utilizing anti-IAM38 antibodies as probes at the ultrastructural level, could reveal the processes involved in assembly of the IAMC and the compaction of the acrosome. High level of the IAM38 transcript was already detectable at postnatal day 20, corresponding to the time in mouse testis development when round spermatids first appear and acrosomal formation is initiated. IAM38 protein was first seen during this time (Golgi phase of spermiogenesis) within the dense core of the proacrosomal granules (PG) of round spermatids. After PG fusion to form the acrosomic vesicle (AV) the immunoreactivity was concentrated in the centrally located acrosomic granule (AG) of the AV, until acrosomal-nuclear docking occurred. Subsequently, in the cap phase, IAM38 migrated from the AG to the extracellular side of the acrosomal membrane, eventually disappearing altogether from the AG. During the elongation phase IAM38 labeling disappeared from the outer acrosomal membrane (OAM) in the apical and principal segments of the acrosome but remained lining the OAM in the equatorial segment (ES) as well as the entire IAM of the acrosome. Compaction of the ES in the maturation phase of spermiogenesis coincided with the IAM38-labeled extracellular coat of the inner and outer acrosomal membranes coming together in close proximity of each other to form a "zipper-like" structure found in the ES of mature spermatozoa. In summary, the results set a precedent, showing for the first time that glycoproteins, which originate in the acrosomic granule can be transferred to the acrosomal membrane during acrosome formation. Furthermore, the localization of IAM38 on the extracellular side of both the inner and outer acrosomal membrane during the cap phase of acrosomal development supports the role of this prominent protein in acrosomal compaction.