Functional characterization of TMEM86A and TMEM86B mutants by a novel lysoplasmalogenase assay

Plasmalogens are an abundant class of glycero-phospholipids with a characteristic 1-O-alk-1'-enyl double bond. While their synthesis has been extensively investigated, their degradation remains understudied. Plasmalogen deficiencies are associated with severe disorders in humans and interfering with their degradation would be a treatment option, but it remains out of reach due to limited knowledge. The plasmalogen double bond is degraded either directly by a plasmalogenase or by conversion to the 2' lyso forms by phospholipase and subsequent cleavage by lysoplasmalogenase (E.C. 3.3.2.2). Two lysoplasmalogenases are known so far, TMEM86A and TMEM86B. While TMEM86B has been expressed in bacteria, purified, and shown to encode lysoplasmalogenase activity by a coupled optical assay, the closely related protein TMEM86A has not yet been purified, but its activity was shown indirectly by a lipidomics approach. Here, we present a novel assay for lysoplasmalogenase activity based on incubation with lysoplasmenylethanolamine or lysoplasmenylcholine, derivatization of the aldehyde product with dansylhydrazine, and hydrazone quantification by reversed-phase HPLC with fluorescence detection. The method was sensitive enough to robustly detect lysoplasmalogenase activity in human embryonic kidney cells following transient expression of TMEM86A or TMEM86B and also suitable for the determination of lysoplasmalogenase activity in mouse tissues where highest activities were found in liver and duodenum. We introduced point mutations at positions proposed to be catalytically relevant and provided experimental evidence that all but one affected lysoplasmalogenase activity. Our novel assay allows direct and fast measurement of lysoplasmalogenase activity, thereby providing a tool to advance research in the field of plasmalogen degradation.