Developmental efficiency of somatic cell-reconstructed embryos depends on extensive remodeling of chromatin structural components. Due to their importance for maintaining the high-order chromatin structure and controlling DNA functions, including replication, transcription, repair, and recombination, histones and other chromatin-binding proteins represent leading choice markers to investigate nuclear remodeling in reconstructed embryos. The main objective of this study was to investigate whether or not the exposure of cell membrane permeabilized nuclear donor cells to sodium chloride (salt-extraction) would facilitate the displacement of chromatin-binding proteins in reconstructed porcine embryos. Both linker histone H1 (H1) and high-mobility group (HMG) proteins are known to affect gene expression through the modulation of the high-order chromatin structure. Standard methods of oocyte enucleation and electrofusion were applied for embryo reconstruction using in vitro-matured oocytes and control or salt-extracted fetal fibroblast cells. For salt-extraction, confluent cell cultures were washed with Ca²⁺/Mg²⁺-free Hank's balanced salt solution (HBSS); cells were permeabilized by incubation with 1 µg/mL of streptolysin O at 37°C for 30 min in HBSS, and then maintained in Tris-NaCl buffer (10 mM Tris-HCl, 0.5 mM MgCl₂, 0.7 M NaCl, 1 M sucrose) for 5 min. After salt-extraction, cells were rinsed and cultured in DMEM supplemented with 10% fetal bovine serum (FBS) and 2 mM CaCl₂ for 1 h at 37.5°C for membrane resealing prior to nuclear transfer. Reconstructed embryos were activated using ionomycin (15 µM/5 min) and strontium chloride (Sr²⁺; 10 mM/4 h), and then cultured in PZM-3 medium. Immunostaining for H1 and HMG-17 was performed in nuclear donor cells and embryos at different stages after reconstruction. The time required for H1 displacement in transplanted nuclei was reduced by the salt-extraction treatment (Table 1). Salt-extracted cells showed a stronger HMG-17 cytoplasmic signal compared to control cells. The proportion of HMG-17-positive reconstructed embryos at 1, 3, and 6 h was 54 vs. 19, 57 vs. 44, and 75 vs. 62, for control and salt-extracted cells, respectively. These data suggest that salt-extraction prior to nuclear transplantation enhances the remodeling of chromatin structure in embryos reconstructed with somatic cell nuclei.

This work was supported by a NSERC Discovery Grant to VB.