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 hypothesized that ASM may anchor to cation-independent mannose-6-phosphate receptor (CI-M6PR) in caveolae from where it can be competitively released by M6P.

Methods: We probed for ASM-CI-M6PR interaction by co-immunoprecipitation (Co-IP) and proximity ligation assay (PLA) in isolated lungs and human pulmonary microvascular endothelial cells (hPMVECs). ASM release by M6P was determined in hPMVECs, isolated lungs and in vivo. Effects of M6P i) on PAF-induced lung edema formation and endothelial Ca²⁺ concentration ([Ca²⁺]_i), and ii) lung injury in acid instilled, overventilated mouse lungs were tested. ASM levels were measured in serum and bronchoalveolar lavage fluid (BALF) of ARDS patients. The TriNetX database was probed for the association of ASM-inhibiting tricyclic antidepressants (TCA) with outcome.

Results: Co-IP and PLA revealed ASM interaction with CI-M6PR in endothelial caveolae, which was further increased by PAF. M6P, but not glucose-6-phosphate (Glu6P) caused ASM release, thereby decreasing ASM content and activity in caveolae in vitro, in situ and in vivo. Analogously, M6P, yet not Glu6P attenuated PAF-induced endothelial [Ca²⁺]_i signaling and lung edema in situ, and acute lung injury in vivo. ASM levels were increased in serum, yet not BALF in ARDS patients. Use of TCA was associated with better outcome in patients with severe respiratory infections.

Conclusions: CI-M6PR anchors ASM in caveolae. M6P may hence present a promising strategy against ASM-related lung injury and edema.