OR24-2 A RANKL-PKCβ-TFEB Signaling Cascade is Necessary for Lysosomal Biogenesis in Differentiated Osteoclasts

Program: Abstracts - Orals, Featured Poster Presentations, and Posters
Session: OR24-New Mechanisms in Bone Biology: From Cells to Genetics, & Mice to Men
Sunday, June 16, 2013: 11:15 AM-12:45 PM
Presentation Start Time: 11:30 AM
Room 121 (Moscone Center)
Mathieu Ferron*1, Carmine Settembre2, Junko Shimazu1, Julie Lacombe3, Andrea Ballabio2 and Gerard Karsenty1
1Columbia University Medical Center, New York, NY, 2TIGEM, Naple, Italy, 3Albert Einstein College of Medicine, Bronx, NY
Bone resorption by osteoclasts requires the generation of a large number of lysosomes that release proteases in the resorption lacuna, as a result, the resorption lacuna of the osteoclast is often viewed as a giant extracellular lysosome. This feature underscores the importance of lysosomal biogenesis for the function of this cell type. Yet it is not known whether lysosomal biogenesis is merely a consequence of the action of known transcriptional regulators of osteoclast differentiation or if it is under the control of a different and specific transcriptional program. In addressing this question we observed that RANKL treatment of osteoclasts, once fully differentiated, promotes lysosomal biogenesis by increasing expression of Acp5, Atp6v1c1, Clcn7, Ctsk and Tcirg1, five lysosomal genes that play a critical role during bone resorption in mice and humans. In view of this result and in order to understand how RANKL signaling in differentiated osteoclasts promotes lysosomal biogenesis we next studied the function in osteoclasts of TFEB, a member of the MITF/TFE family of transcription factors that has been proposed based on forced expression and knockdown experiments to be a regulator of lysosomal biogenesis. Consistent with previous experiments in other cell types, gain- and loss-of-function experiments demonstrated that this factor is necessary for lysosomal biogenesis in cultured osteoclasts. To demonstrate unambiguously that TFEB is a determinant of lysosomal biogenesis in vivo in osteoclasts we generated mice lacking this transcription factor only in osteoclasts. Mice lacking TFEB specifically in osteoclasts were characterized by a high bone mass phenotype caused by an isolated decrease in osteoclast activity itself secondary to a reduced number of lysosomes. Biochemical and cell-based assays established that indeed RANKL recruits TFEB to promote lysosomal biogenesis. This occurs through a novel pathway including recruitment by RANKL of PKCβ that in turn phosphorylates TFEB on three serine residues located in its last 15 amino acids. This post-translational modification has the consequence to stabilize and to increase the activity of TFEB. In agreement with these biochemical observations, mice lacking PKCβ in osteoclasts show decreased lysosomal gene expression, reduced lysosomal biogenesis and increased bone mass. Altogether, these results demonstrate for the first time in vivo and through loss-of-function experiment that TFEB is a determinant of lysosomal biogenesis. They also uncover a novel RANKL-dependent signaling pathway, taking place in differentiated osteoclasts and culminating in lysosomal biogenesis, a necessary step for proper bone resorption.

Nothing to Disclose: MF, CS, JS, JL, AB, GK

*Please take note of The Endocrine Society's News Embargo Policy at http://www.endo-society.org/endo2013/media.cfm

Sources of Research Support: MF: NIDDK (K99/R00)