Researchers at the Institute of Science and Technology Austria (IST Austria) identified a new gene in the fruit fly Drosophila that is required for regulating the addition of T-antigen onto a subset of proteins in immune cells. As the gene mutation causes immune cells to get stuck in the fly’s head, the team called the new gene “Minerva,” after the Roman goddess of wisdom who burst from her father Jupiter’s head.

The study (“A conserved major facilitator superfamily member orchestrates a subset of O-glycosylation to aid macrophage tissue invasion”), published in eLife, is a first step towards understanding new ways in which sugar modifications may be involved in metastasis, according to the scientists.

“Aberrant display of the truncated core1 O-glycan T-antigen is a common feature of human cancer cells that correlates with metastasis. Here we show that T-antigen in Drosophila melanogaster macrophages is involved in their developmentally programmed tissue invasion. Higher macrophage T-antigen levels require an atypical major facilitator superfamily (MFS) member that we named Minerva which enables macrophage dissemination and invasion,” the investigators wrote.

“We characterize for the first time the T and Tn glycoform O-glycoproteome of the Drosophila melanogaster embryo, and determine that Minerva increases the presence of T-antigen on proteins in pathways previously linked to cancer, most strongly on the sulfhydryl oxidase Qsox1 which we show is required for macrophage tissue entry. Minerva’s vertebrate ortholog, MFSD1, rescues the Minerva mutant’s migration and T-antigen glycosylation defects. We thus identify a key conserved regulator that orchestrates O-glycosylation on a protein subset to activate a program governing migration steps important for both development and cancer metastasis.”

Metastasizing cells squeeze between other cells to leave blood vessels and enter tissues to form metastases. T-antigen, a combination of specific sugars, is detected on proteins on metastasizing human cancer cells, but is not normally found in most adult tissues. As macrophages also carry T-antigens and squeeze through other cells to enter tissues, Daria Siekhaus, PhD, an assistant professor at IST Austria, and her group decided to use the fly and its immune cells as a model to study how the appearance of T-antigen on proteins is regulated and which proteins are affected by this.

“The fruit fly is the best place to identify new pathways, as it is fast to do complex genetic experiments in the fly,” she says. “Here again, we used the fly as an initial discovery engine for a problem that is also pertinent to us humans,” she said.

Initially, the team cast a wide net to identify potential genes.

“We looked through the gene database for flies to find a gene that is active at the right time and in the right place: a gene that is active in macrophages when T-antigen is added, and that works in the Golgi apparatus, where sugars like the T-antigen are added to proteins before they get transported to the cell surface,” continued Siekhaus.

Once the group had found such a candidate gene, known then only by the placeholder “CG8602,” they tested its role in the appearance of the T-antigen. When CG8602 is mutated, the researchers found that the level of T-antigen on the macrophages is reduced. The macrophages also do not, as they usually do, exit the head region of the fly embryo and enter other tissues.

The researchers investigated on which proteins Minerva affects T-antigen. By collaborating with a team at the University of Copenhagen, the group had access to a mass-spectrometry technique that let them find which proteins carry the T-antigen sugars when Minerva is present but show reduced levels when Minerva is mutated. They thus identified a set of proteins which need Minerva to display T-antigen. The large majority of the proteins identified that have a version in vertebrates, a so-called ortholog, are involved in cancer. “Nine of the proteins we found have an ortholog in humans, six of these are linked to cancer in some way. This is intriguing,” Siekhaus said.

When the scientists put back the vertebrate ortholog of Minerva, MFSD1, into flies that did not have Minerva anymore, the otherwise stuck macrophages left the head and showed restored levels of T-antigen. This suggests an exciting possibility, according to Siekhaus: “This argues that the function of Minerva is maintained in vertebrates, and that the ortholog of Minerva in vertebrates, MFSD1, could be involved in regulating sugars and metastasis. We are now looking at vertebrate cancer models to see whether and how MFSD1 regulates the migration and invasion of cancer cells.”

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