Rescue of secretion of a rare-disease associated mis-folded mutant glycoprotein in UGGT1 knock-out mammalian cells

Abstract

Endoplasmic reticulum (ER) retention of mis-folded glycoproteins is mediated by the ER- localised eukaryotic glycoprotein secretion checkpoint, UDP-glucose glycoprotein glucosyl-transferase (UGGT). The enzyme recognises a mis-folded glycoprotein and flags it for ER retention by reglucosylating one of its N-linked glycans. In the background of a congenital mutation in a secreted glycoprotein gene, UGGT-mediated ER retention can cause rare disease even if the mutant glycoprotein retains activity ("responsive mutant"). Here, we investigated the subcellular localisation of the human Trop-2 Q118E variant, which causes gelatinous drop- like corneal dystrophy (GDLD). Compared with the wild type Trop-2, which is correctly localised at the plasma membrane, the Trop-2-Q118E variant is found to be heavily retained in the ER. Using Trop-2-Q118E, we tested UGGT modulation as a rescue-of-secretion therapeutic strategy for congenital rare disease caused by responsive mutations in genes encoding secreted glycoproteins. We investigated secretion of a EYFP-fusion of Trop-2-Q118E by confocal laser scanning microscopy. As a limiting case of UGGT inhibition, mammalian cells harbouring CRISPR/Cas9-mediated inhibition of the UGGT1 and/or UGGT2 gene expressions were used. The membrane localisation of the Trop-2-Q118E-EYFP mutant was successfully rescued in UGGT1-/-and UGGT1/2-/- cells. UGGT1 also efficiently reglucosylated Trop-2-Q118E-EYFP in cellula. The study supports the hypothesis that UGGT1 modulation constitutes a novel therapeutic strategy for the treatment of Trop-2-Q118E associated GDLD, and it encourages the testing of modulators of ER glycoprotein folding Quality Control (ERQC) as broad-spectrum rescue- of-secretion drugs in rare diseases caused by responsive secreted glycoprotein mutants. SynopsisDeletion of the UGGT1 and UGGT1/2 genes in HEK 293T cells rescues secretion of an EYFP-fusion of the human Trop-2-Q118E glycoprotein mutant. The mutant is retained in the secretory pathway in wild type cells and it localises to the cell membrane in UGGT1-/- single and UGGT1/2-/- double knock-out cells. The Trop-2-Q118E glycoprotein disease mutant is efficiently glucosylated by UGGT1 in human cells demonstrating that it is a bona fide cellular UGGT1 substrate. O_FIG O_LINKSMALLFIG WIDTH=120 HEIGHT=200 SRC="FIGDIR/small/542711v1_ufig1.gif" ALT="Figure 1"> View larger version (63K): org.highwire.dtl.DTLVardef@ac29f1org.highwire.dtl.DTLVardef@f5bcbforg.highwire.dtl.DTLVardef@13ab5a1org.highwire.dtl.DTLVardef@16a9406_HPS_FORMAT_FIGEXP M_FIG C_FIG