Microtubule dynamics are essential for the production of a normal meiotic spindle and fertile oocyte. Microtubule severing by the Katanin protein family has been identified as a means of regulating microtubule function. Katanin is an evolutionarily conserved microtubule-severing complex consisting of Katna1, a severing enzyme and Katnb1, a regulatory subunit. The aim of this study was to investigate the role of katanin-mediated microtubule severing in meiotic and mitotic divisions of the oocyte and early mouse embryo.
We developed two conditional knockout mouse models by crossing Katna1 or Katnb1-floxed mice with Zp3-Cre mice. Breeding experiments show infertility in the Katnb1(-/-), and subfertility in the Katna1(-/-) mice. Focusing on the Katnb1(-/-), we saw no difference in the first meiotic spindles compared to controls but at metaphase II, there was a significant increase in spindle abnormalities (20.0% vs 37.0%; p<0.05). After IVF with wildtype sperm to create heterozygous embryos, there was no significant effect on the rate of 2-cell or blastocyst formation, however blastocysts were significantly smaller, with fewer cells (49.5 vs 17.6; p<0.0001) which had a 100% increase in nuclear volume. To examine the role of Katnb1 in the absence of the paternal allele, diploid homozygous Katnb1(-/-) embryos were generated by parthenogenetic activation. The complete absence of Katnb1 resulted in only18.3% of 1-cell embryos reaching the blastocyst stage compared to 79.2% of controls (p<0.01), and the blastocysts generated were morphologically abnormal, with very low cell numbers (71.7 vs 11.5; p<0.0001) and a 100% increase in average nuclear volume. Time-lapse imaging revealed a high incidence of mitotic failure, asymmetric cell division in the first few mitotic cell divisions of Katnb1(-/-)parthenogenetic embryos.
In summary, Katnb1(-/-) is essential for fertility and plays critical roles in spindle formation and function during MII and the early embryonic cell divisions.