Possible involvement of the membrane-bound form of peroxiredoxin 4 in acrosome formation during spermiogenesis of rats

Peroxiredoxins are novel peroxidases that exhibit divergent biological functions. The fourth member, Prx4, is synthesized with a signal sequence and, after processing, secreted as a 27-kDa form in most tissues. A 31-kDa Prx4 which corresponds to the unprocessed, membrane-bound form is found only in testis. This study was undertaken in order to investigate the role of the membrane-bound Prx4 during spermiogenesis in rats. The Prx4 transcript was observed in testes at 14 days of age, the age before puberty, and its level increased only slightly during aging. Only the secretable form of Prx4 was present before 30 days of age, but the membrane-bound Prx4 became detectable in adult testes, although sum of the two forms appeared to be the same after 21 days old. While weak immunoreactivity to the Prx4 antibody was detected in spermatogenic cells for all ages, a strong immunoreactivity was observed in the elongating spermatid and residual body of adult testis. Prx4 was present in the lumen of the endoplasmic reticulum, Golgi bodies, and the perinuclear space in young rat testes. However, the localization of Prx4 was restricted to membranes of the acrosomal vesicle of the elongated spermatid and was not detected in spermatozoon. Once spermiogenesis is accomplished, vesicles containing the membrane-bound Prx4 were released into the residual body, along with residual membranous components. Thus the conversion of the soluble form to the membrane-bound form may have a role in the acrosome formation during vesicular reorganization during spermiogenesis.