Dynamic distribution of NuMA and microtubules in human fetal fibroblasts, developing oocytes and somatic cell nuclear transferred embryos.

Background: The nuclear mitotic apparatus (NuMA) plays a central role in the assembly and maintenance of spindle poles. Somatic cell nuclear transfer (SCNT) studies on non-human primates have shown that meiotic spindle removal during enucleation causes depletion of NuMA and the minus-end-directed motor protein (HSET) from the ooplasm, and this in turn leads to failure of embryo development. To determine whether NuMA from somatic cells could compensate for NuMA loss during enucleation, the distribution of NuMA and microtubule organization were investigated in human fibroblasts, developing oocytes and SCNT embryos.

Methods: Human fetal fibroblasts, oocytes at various maturation stages and human embryos reconstructed by different SCNT methods were analyzed for NuMA and α-tubulin using immunofluorescent confocal microscopy.

Results: NuMA was detected in interphase nuclei of fibroblasts and oocytes. During mitosis and meiosis, NuMA relocated to the domain surrounding the two spindle poles. During the enucleation process, NuMA was removed along with the meiotic spindle. At 2 h after injection into a donor cell, transitory bipolar spindles were organized and NuMA was detected in the reformed poles. NuMA could be detected spreading uniformly across the nucleoplasm of one pseudo-pronucleus in SCNT embryos but was excluded from the nucleolus. Regardless of the method used for SCNT (enucleation-injection or injection-pronuclei enucleation), NuMA aggregated and relocated to the reformed spindle poles at metaphase of the first mitotic event. At interphase, NuMA relocated throughout the nucleus in developmentally arrested SCNT embryos.

Conclusions: Our results show that donor cell nuclei contain NuMA, which might contribute to the maintenance of spindle morphology in SCNT embryos. Normal spindle and NuMA expression were found in human SCNT embryos at different developmental stages.