Timing of gene expression and oolemma localization of mouse alpha6 and beta1 integrin subunits during oogenesis

The sperm antigen fertilin alpha/beta and the integrin complex alpha6beta1 present on the oolemma are two of the most promising candidates to mediate gamete interaction. During growth, the plasma membrane of both hamster and mouse zona-free oocytes acquires the capacity to fuse with acrosome-reacted sperm when oocytes reach the size of 25-30 microm in diameter, suggesting changes in the membrane molecular composition. The present study has two aims: to determine the timing of (1) gene expression of alpha6 and beta1 integrins and (2) localization of these integrin subunits on the plasma membrane in primordial germ cells and in oocytes during oogenesis. We found that both alpha6 and beta1 genes are expressed in female germ cells during all the stages of development analyzed, from 10.5 to 18.5 d.p. c., during oocyte growth, and in ovulated eggs. The alternatively spliced isoform alpha6B is expressed from 10.5 d.p.c., whereas alpha6A begins to be expressed at 12.5 d.p.c., suggesting a different role for the two variants. In situ immunodetection of alpha6 or beta1 shows a ring of fluorescence on the female germ cell plasma membrane for both integrins at 10.5 d.p.c., then the fluorescent signal becomes undetectable at 12.5 d.p.c. to reappear again, this time with a patchy distribution, at 18.5 d.p.c. This pattern of localization is maintained in oocytes isolated from newborn individuals and only when oocytes during growth reach the size of about 25-30 microm in diameter does the fluorescence become homogenous all around the whole oocyte surface. These data, although not conclusive, support the hypothesis of an involvement of alpha6 and beta1 integrins in sperm-egg fusion.