Role of lysosomal channel protein TPC2 in osteoclast differentiation and bone remodeling under normal and low-magnesium conditions

Abstract

The bone is the main storage site for Ca²⁺ and Mg²⁺ ions in the mammalian body. Although investigations into Ca²⁺ signaling have progressed rapidly and led to better understanding of bone biology, the Mg²⁺ signaling pathway and associated molecules remain to be elucidated. Here, we investigated the role of a potential Mg²⁺ signaling-related lysosomal molecule, two-pore channel subtype 2 (TPC2), in osteoclast differentiation and bone remodeling. Previously, we found that under normal Mg²⁺ conditions, TPC2 promotes osteoclastogenesis. We observed that under low-Mg²⁺ conditions, TPC2 inhibited, rather than promoted, the osteoclast differentiation and that the phosphatidylinositol 3,5-bisphosphate (PI(3,5)P₂) signaling pathway played a role in the TPC2 activation under low-Mg²⁺ conditions. Furthermore, PI(3,5)P₂ depolarized the membrane potential by increasing the intracellular Na⁺ levels. To investigate how membrane depolarization affects osteoclast differentiation, we generated a light-sensitive cell line and developed a system for the light-stimulated depolarization of the membrane potential. The light-induced depolarization inhibited the osteoclast differentiation. We then tested the effect of myo-inositol supplementation, which increased the PI(3,5)P₂ levels in mice fed a low-Mg²⁺ diet. The myo-inositol supplementation rescued the low-Mg²⁺ diet-induced trabecular bone loss, which was accompanied by the inhibition of osteoclastogenesis. These results indicate that low-Mg²⁺-induced osteoclastogenesis involves changes in the role of TPC2, which are mediated through the PI(3,5)P₂ pathway. Our findings also suggest that myo-inositol consumption might provide beneficial effects in Mg²⁺ deficiency-induced skeletal diseases.

Keywords: ion channel; lysosome; magnesium; optogenetics; osteoclast; phosphatidylinositol phosphatase; plasma membrane.