Novel mannosidase inhibitors probe glycoprotein degradation pathways in cells

Multiple isoforms of mammalian alpha-mannosidases are active in the pathways of N-linked glycoprotein synthesis and catabolism. They differ in specificity, function and location within the cell and can be selectively inhibited by imino sugar monosaccharide mimics. Previously, a series of structurally related novel 7-membered iminocyclitols were synthesised and found to be inhibitors of alpha-mannosidase using in vitro assays. The present study aimed to delineate alpha-mannosidases hydrolytic pathways in azepane inhibitor treated cells by the analysis of free oligosaccharides (FOS) as markers of endoplasmic reticulum (ER), Golgi, lysosomal and cytosolic alpha-mannosidase activities. Two compounds were identified as potent and selective cytosolic alpha-mannosidase inhibitors. Two related compounds were shown to be potent inhibitors of lysosomal alpha-mannosidase with different potencies towards alpha 1,6 mannosidase. The specificities of these novel 7-membered imino sugars are related to differences in their structure and d-mannose-like stereochemistry. Specific ER-mannosidase inhibition by kifunensine also reveals significant non-proteasomal degradation following FOS analysis and appears to be cell line dependent. The availability of more selective inhibitors allows the pathways of N-linked oligosaccharide metabolism to be dissected.