ZnT4 (SLC30A4)-null (“lethal milk”) mice have defects in mammary gland secretion and hallmarks of precocious involution during lactation

Abstract

McCormick NH, Lee S, Hennigar SR, Kelleher SL. ZnT4 (SLC30A4)-null ("lethal milk") mice have defects in mammary gland secretion and hallmarks of precocious involution during lactation. Am J Physiol Regul Integr Comp Physiol 310: R33-R40, 2016. First published November 4, 2015 doi:10.1152/ajpregu.00315.2014.-During lactation, highly specialized secretory mammary epithelial cells (MECs) produce and secrete huge quantities of nutrients and nonnutritive factors into breast milk. The zinc (Zn) transporter ZnT4 (SLC30A4) transports Zn into the trans-Golgi apparatus for lactose synthesis, and across the apical cell membrane for efflux from MECs into milk. This is consistent with observations in "lethal milk" (lm/lm) mice, which have a truncation mutation in SLC30A4, and present with not only low milk Zn concentration, but also smaller mammary glands, decreased milk volume, and lactation failure by lactation day 2. However, the molecular underpinnings of these defects are not understood. Here, we used lactating C57BL/6J lm/lm (ZnT4-null) mice to explore the consequences of a ZnT4-null phenotype on mammary gland function during early lactation. Lactating C57BL/6J lm/lm mice had significantly fewer, smaller, and collapsed alveoli comprising swollen, lipid-filled MECs during early lactation. These defects were associated with decreased Akt expression and STAT5 activation, indicative of defects in MEC secretion. In addition, increased expression of ZnT2, TNF-␣, and cleaved e-cadherin concomitant with increased activation of STAT3 implicated the loss of ZnT4 in precocious activation of involution. Collectively, our study indicates that the loss of ZnT4 has profound consequences on MEC secretion and may promote tissue remodeling in the mammary gland during early lactation.zinc; lactation; mammary gland; SLC30A4; ZnT4 ZNT4 (SLC30A4) IS A ZINC (Zn) transporter that is expressed in numerous tissues, including the liver (13), lung (13), small intestine (28), and mammary gland (13). ZnT4 is particularly important for mammary gland function, as ZnT4 expression increases dramatically during lactation (23). Previous studies in vitro established that ZnT4 transports Zn into the transGolgi apparatus of the mammary epithelial cell (MEC) and delivers Zn to critical Zn-dependent proteins, including galactosyltransferase (a resident Golgi protein that is vital for lactose production) and carbonic anhydrase VI (a secreted milk enzyme that is vital for infant alimentary tract pH balance) (15). The "lethal milk" mouse (C57BL/6J lm/lm ) carries a spontaneous truncation mutation in SLC30A4, resulting in non-sense-mediated decay of ZnT4 mRNA (29). During lactation, ZnT4-null mice have ϳ35% lower milk Zn concentration than their wild-type littermates (37). Although this implicates ZnT4 in Zn secretion into milk, studies in breastfeeding women have yet to identify defects in ZnT4 in women with low milk Zn levels or find an association with transient neonatal Zn deficiency in exclusively breastfed infants (4,14,20). Moreover, ZnT4-null mice have s...

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“…With time, low zinc-concentrations were lethal for suckling pups. ZnT4 has profound consequences on mammary epithelial cell function and negatively influences nutrition secretion into the milk, and additionally promotes tissue remodeling in the mammary gland during early lactation [ 315 ]. Animal studies in mice bearing a malfunction of ZnT4, showed these defects to be associated with decreased Akt expression, and Signal transducer and activator of transcription 5 (STAT5) activation.…”

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“…A spontaneous Znt4 mutant mouse produces milk with reduced zinc levels, so-called lethal milk because pups nursed by these dams die before weaning [ 132 ]. Recently, the Znt4 mutant mouse was shown to have defects in mammary gland secretion and hallmarks of precocious involution during lactation [ 133 ] . Znt5 KO mice display poor growth, osteopenia, and male-specific sudden cardiac death [ 134 ], and also show cytokine production defects in mast cells, which is mediated by the high-affinity immunoglobulin E receptor [ 135 ].…”

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“…These vesicles differ from intracellular vesicular/organellar stores of zinc, as ZIP transporters do not seem to counteract the action of the ZnT transporters (ZnT2, 3, and 8) loading these vesicles with zinc ions. The “lethal milk” mouse has a truncated form of ZnT4, and presents with low zinc concentration in the milk and developmental defects of the mammary gland [ 17 ]. Zinc transporters ZnT4–7 are involved in loading zinc-requiring ectoenzymes with zinc at the trans Golgi network (TGN)/early secretory pathway [ 18 ].…”