Gene expression in the digestive tissues of ruminants and their relationships with feeding and digestive processes

The gastrointestinal tract (GIT) has multiple functions including digestion, nutrient absorption, secretion of hormones and excretion of wastes. In the ruminant animal, development of this organ system is more complex than that of the monogastric animal due to the necessity to establish a fully functional and differentiated rumen, in which a diverse microbial population of bacteria, fungi and protozoa support fermentation and digestion of dietary fiber. Central to the goal of animal scientists to enhance nutrient uptake and production efficiency of ruminants is the need for a comprehensive understanding of GIT development, as well as conditions that alter the digestion process. The relatively recent availability of genome sequence information has permitted physiological investigations related to the process of digestion for many agriculturally important species at the gene transcript level. For instance, numerous studies have evaluated the expression of ruminant GIT genes to gain insight into mechanisms involved in normal function, physiology and development, such as nutrient uptake and transport across the epithelial cell barrier throughout the alimentary canal, maintenance of rumen pH, and regulation of GIT motility and cell proliferation. Further, multiple studies have examined the effects of dietary modification, including feeding of supplemental fat, starch and protein, or a forage- v. concentrate-based diet on expression of critical gene pathways in the gut. In addition, the expression of genes in the GIT in response to disease, such as infection with gastrointestinal parasites, has been investigated. This review will summarize some of the recent scientific literature related to the gene expression in the GIT of ruminants, primarily cattle, sheep and goats, as it pertains to normal physiology, and dietary, developmental, and disease effects to provide an overview of critical proteins participating in the overall digestive processes, and their physiological functions. Recent findings from our laboratory will be highlighted also related to expression of the glucagon-like peptide two-hormone pathway in the GIT of dairy cattle during in various stages of the development and lactation, alterations in gene pathways associated with the rumen development and differentiation in the weaning calf, and genes of the GIT responding to Ostertagia, a common nematode infection of the cattle. Finally, prospective areas of investigation will be highlighted.