Proper biogenesis of a sperm-specific organelle, the acrosome, is essential for gamete interaction. An acrosomal matrix protein, ACRBP, is known as a proacrosin-binding protein. In mice, two forms of ACRBP, wild-type ACRBP-W and variant ACRBP-V5, are generated by pre-mRNA alternative splicing of Acrbp. Here, we demonstrate the functional roles of these two ACRBP proteins. ACRBP-null male mice lacking both proteins showed a severely reduced fertility, because of malformation of the acrosome. Notably, ACRBP-null spermatids failed to form a large acrosomal granule, leading to the fragmented structure of the acrosome. The acrosome malformation was rescued by transgenic expression of ACRBP-V5 in ACRBP-null spermatids. Moreover, exogenously expressed ACRBP-W blocked autoactivation of proacrosin in the acrosome. Thus, ACRBP-V5 functions in the formation and configuration of the acrosomal granule during early spermiogenesis. The major function of ACRBP-W is to retain the inactive status of proacrosin in the acrosome until acrosomal exocytosis.

ACRBP/sp32 is a binding protein specific for the precursor (pro-ACR) and intermediate forms of sperm serine protease ACR. In this study, we examined the expression pattern, localization, and possible role of mouse ACRBP in spermatogenic cells and epididymal sperm. Unlike other mammalian ACRBPs, two forms of Acrbp mRNA-wild-type Acrbp-W and variant Acrbp-V5 mRNAs-were generated by alternative splicing of Acrbp in the mouse. ACRBP-W was synthesized in pachytene spermatocytes and haploid spermatids and immediately processed into a mature protein, ACRBP-C, by removal of the N-terminal half. The intron 5-retaining splice variant mRNA produced a predominant form of ACRBP, ACRBP-V5, that was present in pachytene spermatocytes and round spermatids, but was absent in elongating spermatids. ACRBP-W and ACRBP-V5 were both colocalized with pro-ACR in the acrosomal granules of early round spermatids, whereas the sperm acrosome contained only ACRBP-C. Glutathione S-transferase pull-down assays revealed that ACRBP-V5 and ACRBP-C possess a different domain capable of binding each of two segments in the C-terminal region of pro-ACR. Moreover, autoactivation of pro-ACR was remarkably accelerated by the presence of ACRBP-C. These results suggest that ACRBP-V5 and ACRBP-C may function in the transport/packaging of pro-ACR into acrosomal granules during spermiogenesis and in the promotion of ACR release from the acrosome during acrosomal exocytosis, respectively.

Muscular recovery after exercise is an important topic in sports medicine, and accurate and quantitative measurements of changes in muscle are required to assess muscular recovery. In the present study, we report a new analytical method to measure muscular changes quantitatively. The technique consists of three independent methods: image processing of two-dimensional MR images, morphological analysis using three-dimensional MR images, and diffusion tensor MRI. Using this method, we investigated changes in the quadriceps and biceps femoris and gluteus maximus muscles and surrounding tissues before and after 1 mo of exercise wearing training equipment. The subjects were 21 healthy adult female volunteers, 14 of whom wore training equipment and 7 who wore normal equipment. The percentage of adipose tissue in muscle after exercise in subjects who wore training equipment was on average 4.4% (P < 0.001) lower than that before exercise, and the peak point of the dorsal hip after exercise with use of the equipment was on average 10.8 mm higher than that before exercise. Further, the fractional anisotropy of water diffusion in muscles increased by an average of 0.039 (P < 0.001) after exercise with use of training equipment. In contrast, there was no significant difference before and after exercise in subjects who wore normal equipment. These results show that walking exercise while wearing training equipment thickens and tightens the muscular fiber tissues. This noninvasive measurement approach may allow quantitation of the athletic ability of the muscles, which is not measured conventionally, and is an effective method for analyzing skeletal muscles.