Early events in secretion of main milk proteins: occurrence of precursors

Expression of the cytoplasmic domain of E-cadherin induces precocious mammary epithelial alveolar formation and affects cell polarity and cell-matrix integrity

Cadherins are cell adhesion molecules involved in cell-cell adhesion, signalling, and cellular proliferation and differentiation. E-cadherin is required for the formation of epithelium in vivo. We investigated the contribution of the cytoplasmic domain of E-cadherin to adhesion, signalling, and differentiation during murine mammary gland development, by in vivo expression of a gene encoding a truncated form of E-cadherin lacking the extracellular domain. The expression of this gene in mammary epithelial cells during pregnancy induced precocious lobular epithelial morphogenesis associated with morphological differentiation and the early synthesis of various molecules (advanced milk fat globule appearance and milk protein production). After delivery, when a fully differentiated and secretory epithelium is required for lactation, the cytoplasmic domain of E-cadherin had a dominant-negative effect on cell-cell adhesion and affected the structure and function of the epithelium. This also led to the partial loss of epithelial polarisation and changes in the basement membrane, both important in malignancy. Thus, the cytoplasmic domain of E-cadherin induces epithelial morphogenesis, but also alters the cohesiveness of the fully differentiated epithelium.

Metabolizable energy and absorbed protein requirements for milk component production

Metabolic pathways of milk component synthesis were used to estimate metabolizable energy and absorbed protein requirements for lactation. Amounts of ATP and AA used for synthesis of each component from absorbed substrates were determined. Coefficients were adjusted to account for additional inefficiencies and to define requirements in terms of dietary supply based on NRC energy requirements and N balance data. Assuming that 10% of glucose required was derived from AA, metabolizable energy and absorbed protein requirements were 6.02 Mcal and .136 kg/kg of lactose, 13.43 Mcal and .127 kg/kg of fat, and 7.57 Mcal and 1.069 kg/kg of protein, respectively; an additional .144 Mcal/kg of milk was required for milk volume. For production of milk containing 4.8% lactose, 3.5% fat, and 3.3% protein, absorbed protein required for lactose and fat may account for 14.1 and 9.6%, respectively, of total absorbed protein required for milk production. Efficiency of protein utilization for milk protein synthesis may be as high as 90% when 10% of glucose requirements must be supplied by AA. Expressing nutrient requirements for lactation on a component basis enables calculation of requirements for milk production of any composition and does not rely on correlations between major milk components.

Protein synthesis and degradation in the mammary gland of lactating goats

Lactating goats were given a close arterial infusion of [1-14C]leucine and [4,5-3H]4-methyl-2-oxopentanoic acid into one half of the mammary gland at 2-3 weeks and 34-39 weeks after kidding. Rates of protein synthesis, degradation and net output were determined from measurements of arteriovenous difference and blood flow using a model of leucine metabolism previously developed for muscle (Oddy & Lindsay, 1986). Protein leucine output in milk (Y mumol/min) correlated well with the difference between synthesis and degradation (X mumol/min) derived from the model: Y = 1.30 + 1.24X (r2 = 0.9; n = 9, P less than 0.01). There was substantial synthesis and degradation of protein within the mammary gland. Although only an approximate value could be obtained for the partitioning of protein synthesis and degradation between tissue and milk proteins, there was evidence of appreciable turnover of both. There was no significant difference between mammary leucine and protein metabolism in early and late lactation other than that imparted by a greater mass of mammary tissue in early lactation, although there was a tendency for greater oxidation of leucine in late lactation.

Cloning and sequence analysis of bovine beta-casein cDNA

A bovine beta-casein cDNA clone was isolated from a cDNA library prepared from mammary gland mRNA. Sequence analysis revealed 25 nucleotides (nt) of the 5' noncoding region, 672 nt of the complete sequence coding and a 3' region of approximately 500 nt. When the nucleotide sequence of bovine beta-casein cDNA is compared to rat beta-casein cDNA (5), a high degree of homology is observed in the first 100 nt corresponding to the signal peptide of the pre-beta-caseins.