Osteoclasts are giant bone-digesting cells that harbour specialised lysosome-related organelles termed secretory lysosomes, which give rise to the ruffled border, the osteoclast's bone-resorbing apparatus. Using a proteomic approach, we recently resolved the molecular landscape of osteoclast secretory lysosomes, identifying the sugar transporter Slc37a2 as a novel physiologically important regulator of osteoclast function and skeletal bone mass. Here we report on another identified lysosome-resident transporter, major facilitator superfamily domain containing 12 (MFSD12), a little-characterised twelve transmembrane domain protein implicated in lysosomal import of the amino acid cysteine.

Cysteine is a critical substrate in osteoclasts, serving both as the rate-limiting precursor for glutathione synthesis, the cell's primary antioxidant buffer against the substantial ROS burden generated during bone resorption, and as an essential cofactor for lysosomal cysteine cathepsins that mediate organic matrix degradation. MFSD12-mediated import of cysteine into the lysosomal lumen sits at the intersection of two competing cellular demands, supporting cathepsin activity within the lysosome while potentially drawing on the cytosolic cysteine pool required for glutathione-mediated redox buffering. However, the potential physiological importance of Mfsd12 in osteoclasts and bone homeostasis remains unknown.

Here we show that MFSD12/Mfsd12 is robustly expressed both in human and in mouse osteoclasts during RANKL-induced differentiation, reflecting the cell’s high intracellular demand for cysteine. To investigate the potential physiological role of Mfsd12 in osteoclasts and bone homeostasis, we generated global Mfsd12 knockout (Mfsd12^KO) mice using a CRISPR-Cas9 genome editing strategy. Although born at sub-Mendelian ratios and reduced in size, Mfsd12^KO mice were viable to 30 weeks. At 15 weeks, microCT analysis revealed a dramatic low bone mass phenotype consistent with osteoporosis. Strikingly, gross inspection of Mfsd12^KO viscera uncovered a multisystem disorder, including pale, reduced kidneys harbouring renal calculi, and abnormally enlarged livers. Ultrastructural analyses further revealed a profound lysosomal storage disorder characterised by morphologically distorted lysosomes, some bearing crystals. Altogether, our findings unmask MFSD12 as an essential regulator of lysosome function whose deficiency produces a multisystem lysosomal storage disorder that affects bone homeostasis, and extends to kidneys and the liver, phenotypes that bear resemblance to clinical features observed in patients afflicted with the rare lysosomal storage disorder cystinosis.