4 GENE EXPRESSION IN CULTURES OF INNER CELL MASSES ISOLATED FROM IN VITRO-PRODUCED AND IN VIVO-DERIVED BOVINE BLASTOCYSTS

Abstract

Genetic modification of embryonic stem (ES) cells derived from domestic species could be exploited to produce transgenic animals; however, fully validated ES have not been obtained in domestic species. Recent findings regarding key transcription factors and regulation of pluripotency and self-renewal in murine ES cells may provide keys to enable the derivation of ES in domestic species. The aim of this study was to identify and monitor the expression of candidate genes, which are known to be involved in the maintenance of self-renewal and pluripotency in mouse and human ES cells, during the critical first steps in establishment of primary cultures. Inner cell masses (ICMs) were isolated via manual dissection of 25 to 30 commercial in vitro-produced (IVP) blastocysts (Bomed, Inc., Madison, WI, USA) in each of three separate replicates and from 10 in vivo-derived Day 7-8 bovine blastocysts. On the day of ICM isolation (Day 0), 4-5 ICM clumps were collected for RT-PCR analysis. The remaining isolated ICMs were cultured (4-5 ICM clumps per well) on mitomycin C (Sigma-Aldrich, St. Louis, MO, USA)-inactivated mouse embryonic fibroblasts (STO, ATCC, Manassas, VA, USA). The ICM clumps were cultured in 12-well tissue culture dishes in ES medium consisting of Knockout DMEM (Invitrogen, Carlsbad, CA, USA) supplemented with 15% FCS (Hyclone, Logan, UT, USA), 2 mM l-glutamine (Invitrogen), 0.1 mM 2-mercaptoethanol (MP Biomedicals, Irvine, CA, USA), and non-essential amino acids (Sigma). Two to four cultured ICM clumps were collected for RT-PCR analysis on Days 1-4 from IVP embryos and on Days 2, 4, and 6 from in vivo-derived embryos. Total RNA was extracted from the collected samples using the Absolutely RNA Nanoprep Kit (Strategene, La Jolla, CA, USA). First-strand DNAs were synthesized using Superscript III (Invitrogen) and cDNAs were amplified with PfuUltra hotstart PCR mastermix (Stratagene). Primers were designed based on homology between human and mouse sequences and were validated using bovine tissues. In experiments spanning these critical first few days of culture, the pluripotency-related genes (Nanog, Oct-4, Sox-2) and components of the LIF (LIFR, Gp130), BMP (Bmpr1a, Id-1), and Wnt (Beta-catenin, Frizzled) pathways were expressed in the ICM cultures over the 4 days of IVP-ICM cultures and the 6 days of in vivo-derived ICM cultures. These results indicate that the markers of pluripotency and the components of signaling pathways implicated in the maintenance of murine embryonic stem cells are present in ICMs of Day 7-8 bovine blastocysts and continue to be expressed at least during the initial days of culture. Genes (NCAM, Lef1) associated with early differentiation, however, were also expressed. Whether their expression is an indicator of ICM differentiation or of residual contamination with trophectoderm remains to be determined. Further studies will determine whether stimulation of these pathways can facilitate efficient derivation and maintenance ruminant ES cells.