Identification of Transcriptional Regulators and Signaling Pathways Mediating Postnatal Rumen Growth and Functional Maturation in Cattle
The rumen plays an essential role in ruminant digestion, absorption, metabolism, and health. The rumen undergoes substantial changes in size and function from birth to adulthood. The molecular mechanisms underlying these changes are not fully understood. This study was aimed to identify the transcription factors and signaling pathways mediating these changes in cattle. We used 6 neonatal bull calves and 6 adult steers, all Angus-crossbred, in this study. We found that the ratios of the emptied rumen, reticulum, omasum, and abomasum to total body weight in adult steers were 4.8 (P < 0.01), 3.1 (P < 0.01), 6.0 (P < 0.01), and 0.8 (P = 0.9) times those in neonatal calves, respectively. Histological examinations showed a 7.4-, 2.0-, 3.0-, 2.9-, and 4.6-fold increase (P < 0.01 for all) in the length of the rumen papillae and the thickness of the rumen epithelium, tunica mucosa and submucosa, tunica muscularis, and tunica serosa, respectively, from neonatal calves to adult steers. Papilla density was lower in adult steers than in neonatal calves (P < 0.05). The size of the rumen epithelial cells was not different between neonatal calves and adult steers (P = 0.57). RNA sequencing identified 2,906 genes differentially expressed (adjusted P < 0.05 and |log2fold change| = 1) in the rumen between neonatal calves and adult steers, of which 1,586 were downregulated and 1,320 upregulated in adult steers. Bioinformatic analyses indicated organ development and morphogenesis, neuronal differentiation, blood vessel development, Ras signaling, and Wnt signaling were among the functional terms enriched in genes downregulated in adult steers, while fatty acid metabolism, ketogenesis, immune responses, PPAR signaling, and Rap1 signaling were among those enriched in genes upregulated in adult steers. Bioinformatic analyses also indicated that SRF, IRF4, SPI1, ETS1, and PURA were the major transcription factors mediating gene upregulation and that TCF4, MESP1, TCF3, ID4, and SNAI2 were the major transcription factors mediating gene downregulation in the rumen from neonatal calves to adult steers. Taken together, these results suggest that the rumen grows by increasing the number, not by the size, of individual cells from birth to adulthood, that the absorptive, metabolic, immune, and motility functions of the rumen are attained or enhanced during the postnatal life, and that the changes in rumen size and function from birth to adulthood are mediated by many transcription factors including SRF and TCF4 and many signaling pathways including the PPAR and Wnt signaling pathways. Overall design: To investigate the gene expression profile, signalling pathways and transcription factors that are responsible for mediating the postnatal rumen growth in cattle We performed RNA-seq on rumen tissues from four newborn calves and four adult steers to analyze their comparative gene expression profiles. We then identified the signaling pathways and transcription factors responsible for the differentially expressed genes based on the gene expression data.