The housekeeping gene xanthine oxidoreductase is necessary for milk fat droplet enveloping and secretion: gene sharing in the lactating mammary gland

Abstract

Xanthine oxidoreductase (XOR) is the rate-limiting enzyme in purine catabolism occurring in most cell types. However, this housekeeping gene is expressed at very high levels in a number of mammalian tissues including the lactating mammary epithelium, suggesting additional roles for XOR in these tissues. Mice with targeted disruption of XOR were generated to assess these potential additional roles. XOR−/− mice are runted and do not live beyond 6 wk of age. Strikingly, however, XOR+/− females, although of healthy appearance and normal fertility, are unable to maintain lactation and their pups die of starvation 2 wk postpartum. Histological and whole-mount analyses showed that in XOR+/− females the mammary epithelium collapses, resulting in premature involution of the mammary gland. Electron microscopy showed that XOR is specifically required for enveloping milk fat droplets with the apical plasma membrane prior to secretion from the lactating mammary gland. We present evidence that XOR may have primarily a structural role, as a membrane-associated protein, in milk fat droplet secretion and thus XOR provides another example of "gene sharing". About 5% of women experience primary lactation insufficiency. The above observations suggest that human females suffering from xanthinuria, a deficiency in XOR, are potential candidates for lactation problems.

High-protein diet during gestation and lactation affects mammary gland mRNA abundance, milk composition and pre-weaning litter growth in mice

“…They are also the most abundantly expressed genes in the MG epithelium and responsible for milk protein content (Robinson et al, 1995;Anderson et al, 2007). Knock-out of b-casein (Kumar et al, 1994), WAP (Triplett et al, 2005), a-lactalbumin (Stacey et al, 1995) or XOR (Vorbach et al, 2002) all resulted in disturbances of MG parenchymal differentiation. This can lead to impaired lactational performance and changes in milk composition that may be responsible for poor offspring growth.…”

“…The XOR is an enzyme associated with milk fat globule membranes and plays a role in fat droplet transport into the alveolar lumen (McManaman et al, 2002;Vorbach et al, 2002). Female mice heterozygous for a XOR loss-of-function mutation were unable to maintain lactation due to a failure to secrete fat droplets into the milk and pups started to die on lactation day 12 due to malnourishment (Vorbach et al, 2002).…”

“…Female mice heterozygous for a XOR loss-of-function mutation were unable to maintain lactation due to a failure to secrete fat droplets into the milk and pups started to die on lactation day 12 due to malnourishment (Vorbach et al, 2002). Although, HPC and HPHP lactating dams showed a decreased XOR expression in addition to a decreased parenchyma proportion no change in milk fat content was found.…”

The evolution of milk secretion and its ancient origins

“…In particular, two proteins, butyrophilin and xanthine oxidoreductase (XOR), play an obligatory structural role in MFGM synthesis, and if they are reduced or eliminated from mouse mammary cells via knockout of the genes that code them, mice fail to produce normal milk; the triacylglycerols within the secretory cells fail to be secreted into milk fat droplets, but rather accumulate in the cytoplasm or leak into the alveolar lumen as unstructured, amorphic lipid masses (Vorbach et al, 2002;Ogg et al, 2004). Although the details of protein-protein interactions during formation of the MFGM are not fully understood (Mather, 2011), these two proteins have apparently been co-opted from other cellular functions during the evolution of the mammary gland.…”

“…The MFEs are believed to have evolved as an ancestral XOR in prokaryotes (Garattini et al, 2003). Although the XOR gene is sometimes considered to code for a housekeeping protein (Vorbach et al, 2002), this is debatable, as XOR is unequally expressed in cells (Garattini et al, 2003). In mammals, XOR is initially synthesized as the enzyme xanthine dehydrogenase, but it is readily converted to xanthine oxidase, which is the form typically recovered from milk (Enroth et al, 2000;Nishino et al, 2008).…”