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Ablation of Btn1a1 leads to concurrent cell death and renewal in the mammary epithelium during lactation

ABSTRACT: Butyrophilin 1A1 (BTN1A1) was first identified as a major integral protein of both the apical surface of mammary epithelial cells during lactation and the surface membrane surrounding lipid droplets in milk. Current evidence supports a role for BTN1A1 in the secretion of milk lipids as a membrane receptor, which forms a secretion complex with the redox enzyme, xanthine oxidoreductase (XDH). Capping of cytoplasmic lipid droplets with BTN1A1 and XDH serves to envelop the droplets with cellular membranes at the cell apex as an initial step in their subsequent release from the cell. The first evidence that BTN1A1 functions in this process was the generation of Btn1a1-/- mice, in which lipid secretion was disrupted and large unstable droplets were released into alveolar spaces with fragmented surface membranes. We have revisited this mutant mouse line using RNAseq and proteomic analysis to fully assess the consequences of ablating the Btn1a1 gene on the expression of other genes and proteins. Disruption of Btn1a1 expression led to a large build-up of Xdh (6-fold over wild-type levels) in the cytoplasm, induction of acute phase response genes and Lif-activation of STAT3 phosphorylation. At peak lactation, approx. 10% of the cells were dying, as assessed by TUNEL-analysis of nuclear DNA. Cell death appeared to proceed through expression of caspase 8 and activated caspase 3 and not via lysosomal lysis, which is the principal route for elimination of cells during mammary involution in wild-type mice. Other potential death pathways include autophagy and Slc5a8-mediated inactivation of survivin (Birc5). Milk secretion was prolonged by renewal of the secretory epithelium, as evidenced by the expression of cyclins, Fos/Jun and upregulation of Ki67 in approx. 10% of secretory cell nuclei. Thus a 21-day lactation cycle can be maintained, despite significant cell death by compensatory cell division, provided a milking stimulus is maintained. These data highlight the importance of BTN1A1 expression for the maintenance of terminally differentiated mammary epithelial cells and optimal milk production throughout lactation. From a practical standpoint, the complimentary RNAseq and proteomics data should provide a useful reference database for gene and protein expression in C57/Bl6 mice at peak lactation.

INSTRUMENT(S): Orbitrap Fusion

ORGANISM(S): Mus Musculus (mouse)

TISSUE(S): Mammary Epithelium

SUBMITTER: Lisa Jenkins

LAB HEAD: Lisa M. Jenkins

PROVIDER: PXD021249 | Pride | 2021-09-20

REPOSITORIES: Pride

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Project description:During lactation, mammary epithelial secretory cells secrete huge amounts of milk from their apical side. The major milk proteins, the caseins, are secreted by exocytosis, while milk fat globules are released by budding, enwrapped by the plasma membrane. Due to the number and large size of milk fat globules, the membrane surface needed for their secretion might exceed that of the apical plasma membrane. In order to identify the cellular compartments that may provide membrane during the budding of milk fat globules, a large-scale proteomics analysis of both cytoplasmic lipid droplets and secreted milk fat globules membranes was performed in mouse cells. The differential analysis of the protein profiles of these two organelles strongly suggest that, in addition to the apical plasma membrane, at least the endoplasmic reticulum, the secretory vesicles containing caseins, and potentially the mitochondria contribute to the formation of the milk fat globule membrane. Moreover, the specific analysis of the membrane-associated as well as the raft-associated proteins reinforces this possibility and points to a role for lipid rafts in milk product secretion. The subcellular localization of several major proteins of the cytoplasmic lipid droplets or of the milk fat globule membrane was investigated by immunofluorescence in lactating mammary epithelial cells. Furthermore, the localization of GM1 ganglioside, a known marker of lipid rafts and of free cholesterol, showed a clear association with both cytoplasmic lipid droplets and secreted milk fat globules. Additionally, the presence of some SNARE proteins that may be involved in casein exocytosis and of two Rab GTPases on both cytoplasmic lipid droplets and milk fat globules was demonstrated by immunofluorescence. Altogether, our results provide evidence for a pivotal role of the endoplasmic reticulum as membrane contributor to milk fat globule budding, and suggest that some SNARE proteins may spatio-temporally coordinate the secretion of milk products.

Project description:BACKGROUND: The lactating mammary gland responds to changes in milking frequency by modulating milk production. This response is locally regulated and, in dairy cows, the udder is particularly sensitive during early lactation. Relative to cows milked twice-daily throughout lactation, those milked four-times-daily for just the first 3 weeks of lactation produce more milk throughout that lactation. We hypothesized that the milk yield response would be associated with increased mammary cell turnover and changes in gene expression during frequent milking and persisting thereafter. Cows were assigned to unilateral frequent milking (UFM; left udder halves milked twice-daily; right udder halves milked four-times daily) on days 1 to 21 of lactation, followed by twice-daily milking for the remainder of lactation. Relative to udder halves milked twice-daily, those milked four-times produced more milk during UFM; the difference in milk yield declined acutely upon cessation of UFM after day 21, but remained significantly elevated thereafter. We obtained mammary biopsies from both udder halves on days 21, 23, and 40 of lactation. RESULTS: Mammary cell proliferation and apoptosis were not affected by milking frequency. We identified 75 genes that were differentially expressed between paired udder halves on day 21 but exhibited a reversal of differential expression on day 23. Among those genes, we identified four clusters characterized by similar temporal patterns of differential expression. Two clusters (11 genes) were positively correlated with changes in milk yield and were differentially expressed on day 21 of lactation only, indicating involvement in the initial milk yield response. Two other clusters (64 genes) were negatively correlated with changes in milk yield. Twenty-nine of the 75 genes were also differentially expressed on day 40 of lactation. CONCLUSIONS: Changes in milking frequency during early lactation did not alter mammary cell population dynamics, but were associated with coordinated changes in mammary expression of at least 75 genes. Twenty-nine of those genes were differentially expressed 19 days after cessation of treatment, implicating them in the persistent milk yield response. We conclude that we have identified a novel transcriptional signature that may mediate the adaptive response to changes in milking frequency.

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Ablation of Btn1a1 leads to concurrent cell death and renewal in the mammary epithelium during lactation

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