3 SEX INFLUENCES REGULATION OF GENE EXPRESSION BY DICKKOPF 1 IN THE BOVINE MORULA

Abstract

Successful embryonic development depends upon molecules secreted by the reproductive tract. Among such molecules is the protein dickkopf 1 (DKK1), an antagonist of canonical WNT signalling that can also activate the noncanonical, planar cell polarity (PCP) pathway. DKK1 increases the proportion of cells that are trophectoderm and hypoblast in the blastocyst and increases competence of embryos to establish pregnancy after transfer to recipients. The objective was to determine whether DKK1 affects cell fate by regulating expression of genes that promote differentiation at the morula stage, possibly by activating the PCP pathway. A second objective was to determine if actions of DKK1 on the embryonic transcriptome were dependent on embryo sex. Bovine oocytes were fertilized in vitro with pools of 3 bulls for which X- and Y-sorted sperm was available. Embryos were treated with 100 ng mL^–1 DKK1 or vehicle at Day 5 of development and harvested 24 h later. Embryos were pooled in 5 replicates of 20 embryos each. Following RNA reverse-transcription and amplification, cDNA was used for microarray analysis of global gene expression using the Affymetrix^® Bovine Gene 1.0 ST array (Affymetrix, Santa, Clara, CA, USA). Statistical analysis was performed by ANOVA using JMP^® Genomics (SAS Institute, Cary, NC, USA). A total of 9931 transcripts were identified as being expressed. Differentially expressed genes (DEG) were considered as those associated with P < 0.05 and fold change ≥1.5 or <0.66. There were 124 DEG between females and males (91 up-regulated in females and 33 up-regulated in males). A total of 68% of the genes up-regulated in females were located in the X chromosome. Treatment with DKK1 resulted in 132 DEG in females (68 up-regulated and 64 down-regulated) and 136 DEG in males (90 up-regulated and 46 down-regulated). Of these, 34 genes were regulated by DKK1 in both sexes: 14 in the same direction and 20 in opposite directions. Analysis by Ingenuity^® Pathway software indicated that changes in gene expression caused by DKK1 would increase formation of actin fibers in females and inhibit formation in males. DKK1 inhibited expression of AMOT in male embryos, indicating that DKK1 may inhibit Hippo signalling at this stage of development. Evidence for regulation of the PCP pathway by DKK1 was the finding that DKK1 regulated expression of genes involved in cell polarization and differentiation in both females and males. In both sexes, DKK1 regulated expression of many genes associated with HNF4A, a marker of hypoblast cells that promotes formation of cell junctions. In conclusion, female and male embryos developing in vitro have different transcriptomic profiles at the morula stage. DKK1 regulates cell differentiation and embryonic development in a sex-dependent manner and effects may be mediated, at least in part, by activation of the PCP pathway.

Supported by USDA AFRI 2011-67015-30688 and NIH R03 HD080855.