Establishment of artifically induced lactogenesis and its application to improvement of milk production in dairy cattle

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In order to establish the methods for inducing lactogenesis artificially and efficiently, in the present experiments, we examined pregnancy-and lactation-dependent changes in the mammary gland functions : (1) one being the expression of mitochondrial inner membrane protein(Tim23)mRNA, (2) the other being the post-transcriptional regulation of casein mRNA, and(3) the last one being the milk compositions of colostrum and normal milk. (1). We cloned and sequenced four clones of the Tim23 cDNA.The sequence of the longest mouse Tim23 cDNA clone has been registered as GDB accession No : AB020987(Mus musculus plasmid pCR(R)II mRNA). Among four clones, 5' untranslated and translated regions were the same but 3' untranslated were different in length. Depending on their lengths, it is expected that one or two loops at the 3' untranslated region will be formed. The level of Tim23 mRNA was low during pregnancy but elevated about 5 folds at the end of pregnancy. The level was high at the early stage of lactation and the highest level was observed at mid lactation. (2). The poly(A)tail length of casein mRNA was shortened after weaning but elongated reversibly after the removal of gland's milk. The other part of casein mRNA was the same. Casein mRNA with the short poly(A)tail showed a short half-life in vitro, while that with the longest one had about 2 times longer half-life. The stability of casein mRNA is regulated at the post-transcriptional level in the lactating mammary gland. (3). The micelle structure of casein in colostrum is different from that in normal milk. This is particularly due to the low expression of g-casein mRNA at lactogenesis. The caseins in colostrum was easily digested by gastric enzyme in new-born neonates. Moreover, colostrum-specific new peptide appeared from colostral caseins.