Zinc transporter 2 interacts with vacuolar ATPase and is required for polarization, vesicle acidification, and secretion in mammary epithelial cells

Abstract

An important feature of the mammary gland is its ability to undergo profound morphological, physiological, and intracellular changes to establish and maintain secretory function. During this process, key polarity proteins and receptors are recruited to the surface of mammary epithelial cells (MECs), and the vesicle transport system develops and matures. However, the intracellular mechanisms responsible for the development of secretory function in these cells are unclear. The vesicular zinc (Zn) transporter ZnT2 is critical for appropriate mammary gland architecture, and ZnT2 deletion is associated with cytoplasmic Zn accumulation, loss of secretory function and lactation failure. The underlying mechanisms are important to understand as numerous mutations and non-synonymous genetic variation in ZnT2 have been detected in women that result in severe Zn deficiency in exclusively breastfed infants. Here we found that ZnT2 deletion in lactating mice and cultured MECs resulted in Zn-mediated degradation of phosphatase and tensin homolog (PTEN), which impaired intercellular junction formation, prolactin receptor trafficking, and alveolar lumen development. Moreover, ZnT2 directly interacted with vacuolar H-ATPase (V-ATPase), and ZnT2 deletion impaired vesicle biogenesis, acidification, trafficking, and secretion. In summary, our findings indicate that ZnT2 and V-ATPase interact and that this interaction critically mediates polarity establishment, alveolar development, and secretory function in the lactating mammary gland. Our observations implicate disruption in ZnT2 function as a modifier of secretory capacity and lactation performance.

ZnT2 is an electroneutral proton-coupled vesicular antiporter displaying an apparent stoichiometry of two protons per zinc ion

“…This finding highlights the fact that during lactation in which ZnT2 expression is markedly induced, an intricate balance between the overexpression of vesicular ZnT2 and V-ATPase must be maintained in order to ensure that the acidic pH is maintained by V-ATPase as otherwise, alkalization will result in loss of the proton-motive force and consequent disruption of multiple vesicular secretory functions. In support of this hypothesis is a recent paper by Lee et al, [58] which showed that during lactation in a mouse model, the protein levels of V-ATPase are markedly elevated in mammary gland epithelial cells. This of course highlights the obligatory balance between overexpressed ZnT2 (as well as other transporters) which translocates protons to the cytoplasm during vesicular accumulation of zinc (and other physiological substrates) and proper V-ATPase levels.…”

Mechanism of Zinc Excitotoxicity: A Focus on AMPK

“…Like Zinc, AMPK functions as a double-edged sword in the axis of cell survival-death ( Ronnett et al, 2009 ). In case of chronic neurodegenerative diseases such as Alzheimer’s or Parkinson’s disease, physiological levels of zinc or AMPK activity may promote cell survival through the enhancement of lysosomal function and the resultant reduction of protein aggregates accumulation ( Park et al, 2011 ; Lee et al, 2017 ; Jang et al, 2018 ). However, in cases of acute brain injury, excessive zinc influx, and the resultant pathological AMPK activation may trigger cell death ( Eom et al, 2016 ).…”

“…An increase of free zinc in the lysosome induces lysosomal acidification and activates lysosomal enzymes such as cathepsins. In most cases, these changes promote cell survival (Park et al, 2011;Seo et al, 2015;Lee et al, 2017). However, a high concentration of extracellular zinc enters the cytosol through voltage-gated calcium channel (VGCC), calcium-permeable AMPA receptor (AMPA-R), or NMDA-R, and then lysosomes or mitochondria likely via zinc transporters (Sensi and Jeng, 2004).…”

Biogenesis of zinc storage granules inDrosophila melanogaster

“…Two-way ANOVA indicated statistically significant differences both by tissue and by genotype; asterisks denote difference between genotypes for any given tissue from a Tukey's post hoc analysis. Overend et al, 2016;Tognon et al, 2016) does not provide a thermodynamic explanation how ZnT35C accumulates zinc in storage granules as would be the current thinking in the field (Jeong and Eide, 2013;Kambe et al, 2017;Lee et al, 2017b), the alternative hypothesis would be that the ATPase activity of the ABCG2 homologues white and scarlet are implicated in zinc transport, perhaps working in a similar way to K ATP channels (Lee et al, 2017a), i.e. by forming a pump in complex with ZnT35C.…”