Milk biosynthesis requires the Golgi cation exchanger TMEM165

Nathan A Snyder,Mitchell V Palmer,Timothy A Reinhardt,Kyle W Cunningham

doi:10.1074/jbc.ra118.006270

Abstract

Milk is a hallmark of mammals that is critical for normal growth and development of offspring. During biosynthesis of lactose in the Golgi complex, H+ is produced as a by-product, and there is no known mechanism for maintaining luminal pH within the physiological range. Here, using conditional, tissue-specific knockout mice, immunostaining, and biochemical assays, we test whether the putative H+/Ca2+/Mn2+ exchanger known as TMEM165 (transmembrane protein 165) participates in normal milk production. We find TMEM165 is crucial in the lactating mammary gland for normal biosynthesis of lactose and for normal growth rates of nursing pups. The milk of TMEM165-deficient mice contained elevated concentrations of fat, protein, iron, and zinc, which are likely caused by decreased osmosis-mediated dilution of the milk caused by the decreased biosynthesis of lactose. When normalized to total protein levels, only calcium and manganese levels were significantly lower in the milk from TMEM165-deficient dams than control dams. These findings suggest that TMEM165 supplies Ca2+ and Mn2+ to the Golgi complex in exchange for H+ to sustain the functions of lactose synthase and potentially other glycosyl-transferases. Our findings highlight the importance of cation and pH homeostasis in the Golgi complex of professional secretory cells and the critical role of TMEM165 in this process.

Highlights

Milk is a hallmark of mammals that is critical for normal growth and development of offspring
These findings suggest that TMEM165 supplies Ca2؉ and Mn2؉ to the Golgi complex in exchange for H؉ to sustain the functions of lactose synthase and potentially other glycosyl-transferases
There is no obvious requirement for a Hϩ/Ca2ϩ/ Mn2ϩ exchanger in this scheme of milk production in Golgi complex, Hϩ is produced as a by-product of all glycosylation reactions including lactose biosynthesis, and there is no known mechanism for eliminating excess Hϩ from the Golgi and maintaining pH within a physiological range

Summary

To whom correspondence may be addressed

We show that the catalytic subunit of lactose synthase, in vitro, is highly sensitive to acidic pH, and we propose that the reduced quality of milk in TMEM165-deficient animals is due to a failure to remove the Hϩ by-product of lactose biosynthesis, a diminished supply of Mn2ϩ in the Golgi, or both, which diminishes lactose synthase activity and osmosis. These findings highlight the importance of ion homeostasis in the Golgi complex of professional secretory cells and the unique role of TMEM165 in exchanging Hϩ, Ca2ϩ, and Mn2ϩ

Results

Discussion

Experimental procedures