Submitted to: Journal of Biological Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/9/2004
Publication Date: 10/8/2004
Citation: Reinhardt, T.A., Lippolis, J.D., Shull, G.E., Horst, R.L. 2004. Null mutation in the gene encoding plasma membrane CA2+-ATPase isoform 2 impairs calcium transport into milk. Journal of Biological Chemistry. 279(41):42369-42373. Interpretive Summary: Most metabolic and infectious diseases in dairy cows are problems whose genesis starts with the stresses of the initiation of lactation. Calcium flows rapidly into the mammary gland, prior to the start of lactation, into as yet unidentified storage sites. The rapidity of this calcium loss contributes to milk fever and the complications that result from milk fever and subclinical hypocalcemia. Fine control of mammary milk cell calcium also is key to the health, hormone responsiveness and the quality of milk produced by the cow. The well-being of the cow and her profitability could be greatly enhanced by understanding those factors that regulate the mammary glands transition to milk production and the concomitant excretion of calcium. This paper represents the fourth in a series of studies examining factors controlling calcium flow and storage in the mammary at and around calving. We have previously studied the synthesis of 4 calcium pumps in the milk producing mammary gland. All pumps increased when lactation started but one pump increased prior to the start of milk production. Two pumps stand out as candidates for controlling most of the calcium movement in the mammary gland and thus the hypocalcemia (milk fever) that causes a disease complex in the transition cow. Their scientific names are plasma membrane calcium ATPase 2bw and the secretory pathway calcium ATPase. This paper examines specific role plasma membrane calcium ATPase 2bw in milk calcium transport. The data show that this pump is the primary controller of milk calcium and controls milk protein too. This data aids us in our understanding of Milk Fever in cows. Initially, these data benefit the scientific community of lactation and cell biologists. Ultimately, the dairy farmer will be the beneficiary of this work.
Technical Abstract: The means by which calcium is transported into the milk produced by mammary glands is a poorly understood process. One hypothesis is that it occurs during exocytosis of secretory products via the Golgi pathway, consistent with the observation that the SPCA1 Ca2+-ATPase, which is expressed in the Golgi, is induced in lactating mammary tissue. However, massive upregulation of the PMCA2bw plasma membrane Ca2+-ATPase also occurs during lactation and is more strongly correlated with increases in milk calcium, suggesting that calcium may be secreted directly via this pump. To examine the physiological role of PMCA2bw in lactation we compared lactating PMCA2-null mice to heterozygous and wild-type mice. Relative expression levels of individual milk proteins were unaffected by genotype. However, milk from PMCA2-null mice had 60% less calcium than milk from heterozygous and wild-type mice, the total milk protein concentration was lower, and an indirect measure of milk production (litter weights) suggested that the PMCA2-null mice produce significantly less milk. In contrast, lactose was higher in milk from PMCA2-null mice during early lactation, but by day 12 of lactation, there were no differences in milk lactose between the 3 genotypes. These data demonstrate that the activity of PMCA2bw is required for secretion of much of the calcium in milk. This major secretory function represents a novel biological role for the plasma membrane Ca2+-ATPases, which are generally regarded as premier regulators of intracellular Ca2+.