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 Ca and Mg ions in the mammalian body. Although investigations into Ca signaling have progressed rapidly and led to better understanding of bone biology, the Mg signaling pathway and associated molecules remain to be elucidated. Here, we investigated the role of a potential Mg signaling-related lysosomal molecule, two-pore channel subtype 2 (TPC2), in osteoclast differentiation and bone remodeling. Previously, we found that under normal Mg conditions, TPC2 promotes osteoclastogenesis. We observed that under low-Mg conditions, TPC2 inhibited, rather than promoted, the osteoclast differentiation and that the phosphatidylinositol 3,5-bisphosphate (PI(3,5)P) signaling pathway played a role in the TPC2 activation under low-Mg conditions. Furthermore, PI(3,5)P 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 -inositol supplementation, which increased the PI(3,5)P levels in mice fed a low-Mg diet. The -inositol supplementation rescued the low-Mg diet-induced trabecular bone loss, which was accompanied by the inhibition of osteoclastogenesis. These results indicate that low-Mg-induced osteoclastogenesis involves changes in the role of TPC2, which are mediated through the PI(3,5)P pathway. Our findings also suggest that -inositol consumption might provide beneficial effects in Mg deficiency-induced skeletal diseases.

Targeting Two-Pore Channels: Current Progress and Future Challenges

“…Similarly, knockdown of TPCs in sea star oocytes showed defected embryo development due to debilitated Ca 2+ signalling [ 17 ]. Another study reported that TPC2 mediates osteoclastogenesis [ 18 ]. This finding is consistent with previous studies that identified a role for NAADP/TPC2-induced Ca 2+ signals in neuronal and skeletal muscle differentiation [ 16 , 19 , 20 ].…”

Human genome diversity data reveal that L564P is the predominant TPC2 variant and a prerequisite for the blond hair associated M484L gain-of-function effect

“…Given the many physiological and pathophysiological roles that TPC2 plays as outlined above, our data suggest that individuals with different TPC2 variations will likely show different susceptibilities to various diseases. Indeed, certain TPC2 polymorphisms appear to be correlated with bone mineral density (G734E, loss-of-inhibition) [35] and inversely correlate with type 2 diabetes mellitus (M484L, gain-of-function) [36], conditions previously associated with TPC2 function [23][24][25]37]. Beyond its association with diseases, the functional spectra of TPC2 also appear of interest pharmacologically.…”

Membrane Transport Proteins in Osteoclasts: The Ins and Outs

“…At steady-state Mg 2+ levels, siRNA-mediated knockdown of Tpcn2 has been shown to impair OC differentiation and bone resorption in vitro. Conversely, under low Mg 2+ culture conditions, OC formation was accelerated suggesting that Tpcn2 functions to regulates OC differentiation and activity in response to circulating Mg 2+ levels (Notomi et al, 2012(Notomi et al, , 2017.…”