Mannose-6-phosphate attenuates acute lung injury by competitive release of acid sphingomyelinase from the mannose-6-phosphate receptor in endothelial caveolae

Background Platelet-activating factor (PAF)-induced pulmonary endothelial barrier failure is mediated by acid sphingomyelinase (ASM) translocation to caveolae. ASM, however, lacks a transmembrane domain for anchoring inside caveolae. We hypothesised that ASM anchors to cation-independent mannose-6-phosphate (M6P) receptor (CI-M6PR) in caveolae, from where it can be competitively released by M6P. Methods We explored ASM–CI-M6PR interactions using co-immunoprecipitation and proximity ligation assay in isolated lungs and human pulmonary microvascular endothelial cells. ASM release by M6P was determined in human pulmonary microvascular endothelial cells, isolated lungs and in vivo . The effects of M6P on 1) PAF-induced lung oedema formation and endothelial Ca 2+ concentration and 2) lung injury in acid instilled, overventilated mouse lungs were examined. ASM levels were measured in serum and bronchoalveolar lavage fluid of patients with acute respiratory distress syndrome. The TriNetX database was probed for associations of ASM-inhibiting tricyclic antidepressants with outcomes. Results Co-immunoprecipitation and proximity ligation assay revealed an ASM interaction with CI-M6PR in endothelial caveolae, which was further increased by PAF. M6P, but not glucose-6-phosphate, caused ASM release, thereby decreasing ASM content and activity in caveolae in vitro , in situ and in vivo . Analogously, M6P, yet not glucose-6-phosphate, attenuated PAF-induced endothelial Ca 2+ influx and lung oedema in situ , and acute lung injury in vivo . ASM levels were increased in serum but not bronchoalveolar lavage fluid in patients with acute respiratory distress syndrome. Use of tricyclic antidepressants was associated with better outcomes in patients with severe respiratory infections. Conclusions CI-M6PR anchors ASM in caveolae. M6P may hence present a promising target in ASM-related lung injury and oedema.