Contribuição para o estudo da adaptação genética da glândula mamária de bovinos da raça Holstein em condições tropicais de exploração

Abstract

The Holstein dairy cattle is known as the world’s highest production dairy animal. The introduction of this breed in tropical areas has been carried out, in an attempt to improve milk production in these regions, where environmental factors, such as temperature and relative humidity, and metabolic heat, associated with the animal maintenance and production processes, contribute to heat stress of dairy cows, causing economic losses to production. The aim of this work was to identify changes in mammary gland genes as an adaptation of Holstein dairy cattle to tropical environmental conditions, enhancing its productive potential. With this purpose we used a transcriptomic technique, the cDNA microarrays. To ensure that only abiotic factors were influencing the animals under study, we used two experimental groups of multiparous cows with similar levels of production, being in the 3rd or 4th month of gestation and fed with similar nutrition level. Both groups are maintained in intensive system of milk production, one adapted to Brazil (HB), tropical environment, and the other in Portugal (HP), temperate environment. Transcriptomics analysis with cDNA microarrays revealed differential expression of 65 transcripts (FC ≥ 1.5 and FDR ≤ 0.1) of which, 34 showed an increased expression in the mammary gland of Holstein dairy cattle from the HB group and 31 were more expressed in the mammary gland of dairy cows from the HP group. These transcripts are classified into 17 functional categories, with higher expression of transcripts related to the categories of protein synthesis, metabolism and cellular communication. For an approach to the aim of this work, we have selected the transcripts that were related to genes of major importance, such as related to milk production, development and cell proliferation, milk components and response to heat stress. The results showed 5 transcripts with a significant role in the chemistry of milk, especially with its lipid component: the transcripts GPAM, RPL4, stearoyl-CoA or -9-desaturase with over-expression in the mammary gland of the HB group, and the transcripts beta-1,4-Galactosyltransferase and LPL with over-expression in the HP group. GPAM encodes the enzyme glycerol-3-phosphate acyltransferase 1, which catalyzes the synthesis of triglycerides. In this gene were also mapped QTL’s associated with milk production, and their lipid and protein composition. The transcript RPL4 is related to the metabolism of triglycerides, encoding the enzyme Monogalactosyldiacylglycerol synthase. The -9-desaturase promotes the biosynthesis of unsaturated fatty acids. Beta-1,4-Galactosyltransferase is an enzyme related to the metabolism of triglycerides, which belongs to the family of glycosyltransferases and participates in the biosynthesis of lactose. The LPL v transcript, encodes the Lipoprotein lipase, mainly found in endothelial cells, that helps in the regulation of triglycerids blood levels. This mammary gland gene expression suggests an increased synthesis of milk fat in the HB group and an increased lipolytic activity and synthesis of lactose in the HP group. CD97, CRDGF and GH in the HB group, and endoglin and CCAR1 in the HP group, were the transcripts over-expressed in mammary gland that revealed functions related to the mammary gland structure. CRDGF is a protein similar to Amphiregulin, which is a growth factor EGF-like and activates a EGF-receptor. Amphiregulin through enzymatic action triggers a set of responses promoting the development of the mammary epithelium. CD97, a glycoprotein present in leukocytes, is associated with the synthesis of the terminal region of EGF, which plays a role in the regulation of calcium, which is essential for the maintenance of protein-protein interactions of the EGF-TM7 signal receptors family. The GH transcript was also differentially expressed in mammary gland. This hormone is vital in galactopoiesis and milk yield, as well as the development and growth of the mammary gland. Endoglin is a glycoprotein expressed on the surface of endothelial cells that belongs to the TGF-beta receptor complex, which controls several processes including proliferation, differentiation, cell growth and apoptosis. The endoglin plays an important role in vascular development. The CCAR1 is a regulator of apoptosis signaling through EGF-receptors, promoting apoptosis processes. The expression of these transcripts related to the structure of the mammary gland suggests that in the HB group there are a major numbers of genes involved in the development of the mammary gland, and a higher cell renovation in the HP group. Two transcripts with possible association to heat stress, were overexpressed in the HB group (Bos taurus chr11 and RPL35) and one in the HP group (v-fos). The transcript Chr11 refers to a chromosome with mapped QTL’s associated to heat stress adaptation features, and transcript RPL35 is a protein, with scarce information about its function, encoded by chromosome 11, which presents QTL’s associated with heat stress. The v-fos is a protein associated with a lower tolerance to heat stress, causing changes in cell survival and expression of proteins related to stress. The mammary gland gene expression analysis suggests that the Holstein dairy cattle animals produced in Brazil may suffered an adaptation process to tropical conditions