Changes in the transcriptome of morula-stage bovine embryos caused by heat shock: relationship to developmental acquisition of thermotolerance.

Miki Sakatani,Manabu Ozawa,Luciano Bonilla,Jiqiang Yao,Peter J Hansen,Kyle B Dobbs,Jeremy Block,Savita Shanker

doi:10.1186/1477-7827-11-3

Abstract

BackgroundWhile initially sensitive to heat shock, the bovine embryo gains thermal resistance as it progresses through development so that physiological heat shock has little effect on development to the blastocyst stage by Day 5 after insemination. Here, experiments using 3’ tag digital gene expression (3’DGE) and real-time PCR were conducted to determine changes in the transcriptome of morula-stage bovine embryos in response to heat shock (40 degrees C for 8 h) that could be associated with thermotolerance.ResultsUsing 3’DGE, expression of 173 genes were modified by heat shock, with 94 genes upregulated by heat shock and 79 genes downregulated by heat shock. A total of 38 differentially-regulated genes were associated with the ubiquitin protein, UBC. Heat shock increased expression of one heat shock protein gene, HSPB11, and one heat shock protein binding protein, HSPBP1, tended to increase expression of HSPA1A and HSPB1, but did not affect expression of 64 other genes encoding heat shock proteins, heat shock transcription factors or proteins interacting with heat shock proteins. Moreover, heat shock increased expression of five genes associated with oxidative stress (AKR7A2, CBR1, GGH, GSTA4, and MAP2K5), decreased expression of HIF3A, but did not affect expression of 42 other genes related to free radical metabolism. Heat shock also had little effect on genes involved in embryonic development. Effects of heat shock for 2, 4 and 8 h on selected heat shock protein and antioxidant genes were also evaluated by real-time PCR. Heat shock increased steady-state amounts of mRNA for HSPA1A (P<0.05) and tended to increase expression of HSP90AA1 (P<0.07) but had no effect on expression of SOD1 or CAT.ConclusionsChanges in the transcriptome of the heat-shocked bovine morula indicate that the embryo is largely resistant to effects of heat shock. As a result, transcription of genes involved in thermal protection is muted and there is little disruption of gene networks involved in embryonic development. It is likely that the increased resistance of morula-stage embryos to heat shock as compared to embryos at earlier stages of development is due in part to developmental acquisition of mechanisms to prevent accumulation of denatured proteins and free radical damage.

Highlights

While initially sensitive to heat shock, the bovine embryo gains thermal resistance as it progresses through development so that physiological heat shock has little effect on development to the blastocyst stage by Day 5 after insemination
Genes involved in cellular stress and survival Gene networks and gene relationships identified using Ingenuity Pathways Analysis (IPA) were initially scrutinized for effects of heat shock on genes and gene networks involved in cell death and survival
Using Database for Annotation (DAVID), the positive regulation of apoptosis ontology was found to contain overrepresentation of differentially expressed genes, with BOK, NGFR, STK4, and SERINC3 increased by heat shock and PDIA3 decreased by heat shock

Summary

Introduction

While initially sensitive to heat shock, the bovine embryo gains thermal resistance as it progresses through development so that physiological heat shock has little effect on development to the blastocyst stage by Day 5 after insemination. In the mouse [1] and cow [2], exposure to a temperature that is only 1.5-2.0°C above normal body temperature can disrupt development of 1-cell embryos. While initially sensitive to heat shock, the embryo gains thermal resistance as it progresses through development. Heat shock at the pronuclear stage reduced development of mouse embryos more than heat shock at the 2-cell stage [1]. Heat shock was more detrimental to development when applied at the 8-cell stage or earlier than when applied later in development [3,4,5,6]. Adverse effects of exposure of females to heat stress on embryonic survival become reduced after a few days post-fertilization in sheep [7], pigs [8] and cattle [9]

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