Molecular characterization and expression analysis of osteopontin cDNA from lactating mammary gland in yak (Bos grunniens)

The Phylogeny of Osteopontin-Analysis of the Protein Sequence

Osteopontin (OPN) is important for tissue remodeling, cellular immune responses, and calcium homeostasis in milk and urine. In pathophysiology, the biomolecule contributes to the progression of multiple cancers. Phylogenetic analysis of 202 osteopontin protein sequences identifies a core block of integrin-binding sites in the center of the protein, which is well conserved. Remarkably, the length of this block varies among species, resulting in differing distances between motifs within. The amino acid sequence SSEE is a candidate phosphorylation site. Two copies of it reside in the far N-terminus and are variably affected by alternative splicing in humans. Between those motifs, birds and reptiles have a histidine-rich domain, which is absent from other species. Just downstream from the thrombin cleavage site, the common motif (Q/I)(Y/S/V)(P/H/Y)D(A/V)(T/S)EED(L/E)(-/S)T has been hitherto unrecognized. While well preserved, it is yet without assigned function. The far C-terminus, although very different between Reptilia/Aves on the one hand and Mammals on the other, is highly conserved within each group of species, suggesting important functional roles that remain to be mapped. Taxonomic variations in the osteopontin sequence include a lack of about 20 amino acids in the downstream portion, a small unique sequence stretch C-terminally, a lack of six amino acids just upstream of the RGD motifs, and variable length insertions far C-terminally.

Technical note: Selection of suitable reference genes for studying gene expression in milk somatic cell of yak (Bos grunniens) during the lactation cycle

Quantitative real-time PCR is the most sensitive technique for gene expression analysis. Data normalization is essential to correct for potential errors incurred in all steps from RNA isolation to PCR amplification. The commonly accepted approach for normalization is the use of reference gene. Until now, no suitable reference genes have been available for data normalization of gene expression in milk somatic cells of lactating yaks across lactation. In the present study, we evaluated the transcriptional stability of 10 candidate reference genes in milk somatic cells of lactating yak, including ACTB, B2M, GAPDH, GTP, MRPL39, PPP1R11, RPS9, RPS15, UXT, and RN18S1. Four genes, RPS9, PPP1R11, UXT, and MRPL39, were identified as being the most stable genes in milk somatic cells of lactating yak. Using the combination of RPS9, PPP1R11, UXT, and MRPL39 as reference genes, we further assessed the relative expression of 4 genes of interest in milk somatic cells of yak across lactation, including ELF5, ABCG2, SREBF2, and DGAT1. Compared with expression in colostrum, the overall transcription levels of ELF5, ABCG2, and SREBF2 in milk were found to be significantly upregulated in early, peak, and late lactation, and significantly downregulated thereafter, before the dry period. A similar pattern was observed in the relative expression of DGAT1, but no significant difference was revealed in its expression in milk from late lactation compared with colostrum. Based on these results, we suggest that the geometric mean of RPS9, PPP1R11, UXT, and MRPL39 can be used for normalization of real-time PCR data in milk somatic cells of lactating yak, if similar experiments are performed.

Technical note: identification of suitable normalizers for microRNA expression analysis in milk somatic cells of the yak (Bos grunniens)

MicroRNA are approximately 18- to 22-nucleotide nonprotein coding molecules that play important roles in the regulation of gene expression at the posttranscriptional level. In the present study, we assessed the suitability of 8 noncoding small RNA as normalizers for microRNA (miR) quantitative analysis in milk somatic cells of lactating yaks, including 3 small nuclear RNA (snRNA; RNU1A, RNU5A, and RNU6B), 3 small nucleolar RNA (snoRNA; SNORA73A, Z30, and SNORA74A), 1 rRNA (5S), and 1 transfer RNA (Met-tRNA). The snRNA RNU1A, RNU5A, and SNORA73A were identified as the most stable references in milk somatic cells of lactating yaks. Also, a minimum of 3 reference RNA (RNU1A, RNU5A, and SNORA73A) were required for the normalization of microRNA expression data in milk somatic cells of the lactating yak. We further evaluated the suitability of the combination of RNU1A, RNU5A, and SNORA73A as reference RNA in milk somatic cells of lactating yaks via detecting the relative expression of miR 16b, miR 21-5p, miR 145, and miR 155 as microRNA of putative interest. In comparison to the colostrum period, on the whole, the expressions of the 4 microRNA were found to be upregulated at an early period and, thereafter, a declining pattern was exhibited from early to final periods in all microRNA investigated. Based on the results from this study, we recommend that the combination of RNU1A, RNU5A, and SNORA73A can be used as normalizers for microRNA quantitative analysis in future longitudinal studies on milk somatic cells of lactating yaks in relation to lactation.