Updating the zona-free method for mouse cloning using hm1 embryonic stem cells

The zona-free method of SCNT designed for bovine and pig cloning (Booth et al. 2001; Vajta et al. 2001; Oback et al. 2003) was successfully used for horse (Galli et al. 2003). Although simple and efficient in farm animals, its application in the mouse met several problems (Ribas et al. 2005, 2006). The aim of our work was to produce cloned mice using HM1 embryonic stem (ES)cells adapting a zona-free method. Seven- to 24-week-old superovulated B6D2F1 female mice were used as oocytes donors. Cumulus cells were removed by 0.3% hyaluronidase and the zona pellucida by 0.5% pronase in KSOM-HEPES (KSOM-H) 1 h later (Ribas et al. 2006) or immediately after hyaluronidase treatment at 37°C. The HM1 ES cells were cultured in KnockOut DMEM supplemented with leukemia inhibitory factor and 15% fetal bovine serum with or without 2i (Ying et al. 2008) and were synchronized at M phase by 3 ng mL–1 nocodazole for 3 h before fusion. Only spherical cells were selected for NT. Metaphase II chromosome spindle complexes were removed by micromanipulation in KSOM-H medium with 5 μg mL–1 cytochalasin B. Lectin-treated enucleated oocytes were attached to the donor cells in KSOM-H with nocodazole and fused by 2 pulses of 1.3 kV cm–1 DC for 30 μs in 0.3 M mannitol medium. Following 10- to 15-min incubation in KSOM-H, the fusion was assessed and repeated if the constructs were nonfused. Cloned embryos were activated in 1 mM SrCl2 in Ca2+-free KSOM medium for 2 to 2.5 or 5 to 6 h and cultured in 20-μL KSOM droplets using the well-of-the-well (WOW) method (Vajta et al. 2000) under mineral oil at 37°C and 5% CO2. Day 4 compacted morulae and blastocysts were surgically transferred into the uterus of Day-2.5 pseudopregnant recipients that were sacrificed on Day 19.5 to examine fetal development. The donor mice age was important for oocyte survival: ~16% of oocytes of 7- to 10-week-old mice lysed before or during fusion in 33% of experiments (n experiments = 15), whereas oocytes of older mice were not sensitive to enzymatic treatment and electric impulses even after 3 fusion rounds (n = 19). The time of pronase treatment did not affect oocyte survival, whereas extending the time between hyaluronidase treatment and enucleation revealed self-activation in ~25% of oocytes. The fusion efficiency of ES cells was significantly lower compared with serum-starved fibroblasts (61%, n = 623 v. 100%, n = 80). The duration of SrCl2 treatment did not affect embryo development (cleavage: 82% v. 84%; Day 4 blastocysts: 49% v. 52%). ES cell culture with 2i increased Day 4 blastocyst development (60.7% v. 50.4%; P = 0.07), and their ability to implant (52.6% v. 38.2%; P = 0.06). Moreover, only NT embryos derived from 2i-ES cells developed to term (8.2%, n = 5; P = 0.08), and produced live fetuses (4.9%, n = 3). In light of these results, the fusion of ES cells remains the critical step in the mouse zona-free protocol.