Modulation of TNAP activity and apatite formation in biomimetic matrix vesicles studied by 31P solid-state NMR

Skeletal and dental mineralization relies on a precisely regulated sequence of events culminating in apatite deposition onto collagen fibrils. Matrix vesicles (MVs), extracellular vesicles released by mineralization-competent cells, play a pivotal role in this process through the catalytic activity of alkaline phosphatase (TNAP). The lipid composition of MVs, particularly phosphatidylserine (PS)-calcium complexes, facilitates the nucleation of amorphous calcium phosphate and apatite formation. However, the interplay between the TNAP structure, the lipid membrane environment, and its enzymatic activity remains incompletely understood.

Biomimetic models of MVs, as proteoliposomes made with dipalmitoylphosphatidylcholine (DPPC) and various TNAP mutants, were used to investigate the TNAP's activity and mineralization potential. Molecular docking and site-directed mutagenesis revealed that specific cysteine substitutions near TNAP's catalytic and anchoring sites influence structural stability, enzymatic activity, and incorporation into lipid bilayers. Notably, TNAP mutants S221C and P307C exhibited enhanced catalytic efficiency in DPPC liposomes, while A420C showed reduced activity due to steric hindrance near the catalytic site. Solid-state NMR and cryo-TEM analyses confirmed hydroxyapatite formation, with significant contributions from lipid-anchored TNAP to the mineralization process.

These findings highlight the critical influence of the lipid environment on TNAP's functional properties and provide insights into the mechanisms governing biomineralization and related pathologies, including hypophosphatasia associated with various TNAP mutations. The study underscores the importance of ATP and pyrophosphate hydrolysis by TNAP in modulating apatite formation and reveals the role of specific TNAP mutations in regulating enzymatic activity, stability, and mineral propagation. Understanding these interactions could lead to alternate therapeutic strategies in treatment and regenerative medicine.