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Prototheca zopfii Genotype II induces mitochondrial apoptosis in models of bovine mastitis

By Muhammad Shahid, Eduardo R. Cobo, Liben Chen, Paloma A. Cavalcante, Herman W. Barkema, Jian Gao, Siyu Xu, Yang Liu, Cameron G. Knight, John P. Kastelic, Bo Han

Posted 27 Feb 2019
bioRxiv DOI: 10.1101/562983 (published DOI: 10.1038/s41598-020-57645-z)

Prototheca zopfii is an alga increasingly isolated from bovine mastitis. Of the two genotypes of P. zopfii (genotype I and II (GT-I and II)), P. zopfii GT-II is the genotype associated with acute mastitis and decreased milk production by unknown mechanisms. The objective was to determine inflammatory and apoptotic roles of P. zopfii GT-II in cultured mammary epithelial cells (from cattle and mice) and murine macrophages and using a murine model of mastitis. Prototheca zopfii GT-II (but not GT-I) invaded bovine and murine mammary epithelial cells (MECs) and induced apoptosis, as determined by the terminal deoxynucleotidyl transferase mediated deoxyuridine triphosphate nick end labeling assay. This P. zopfii GT-II driven apoptosis corresponded to mitochondrial pathways; mitochondrial transmembrane resistance (ΔΨm) was altered and modulation of mitochondrion-mediated apoptosis regulating genes changed (increased transcriptional Bax, cytochrome-c and Apaf-1 and downregulated Bcl-2), whereas caspase-9 and -3 expression increased. Apoptotic effects by P. zopfii GT-II were more pronounced in macrophages compared to MECs. In a murine mammary infection model, P. zopfii GT-II replicated in the mammary gland and caused severe inflammation with infiltration of macrophages and neutrophils and upregulation of pro-inflammatory genes (TNF-α, IL-1β and Cxcl-1) and also apoptosis of epithelial cells. Thus, we concluded P. zopfii GT-II is a mastitis-causing pathogen that triggers severe inflammation and also mitochondrial apoptosis.

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