Investigating HDAC-MEF2 Roles in Osteoclastogenesis

Abstract

Bone remodeling is the process of removing damaged or old bone and replacing it with new bone. This process occurs in the adult skeleton in order to maintain structural integrity and accomplish other necessary functions. Osteoclasts degrade bone in a localized and controlled manner, and osteoblasts deposit new bone in response to osteoclast activity. The activity of each cell type can be controlled using multiple mechanisms, including osteoclast-mediated regulation of bone formation. In order to determine how osteoclast activity can be regulated in order to manage pathological bone loss, it is necessary to understand transcriptional mechanisms that control their development. With this goal, I expanded on previous research showing histone deacetylase 7 (HDAC7) repressed osteoclast differentiation. I found that overexpression of a fragment of HDAC7 that interacts with the myocyte enhancer factor 2 (MEF2) family of transcription factors repressed osteoclast differentiation to a similar degree as the full-length HDAC7. This led me to investigate the roles of MEF2 in osteoclast differentiation. Mef2a and Mef2d were more highly expressed than Mef2b and Mef2c. Osteoclast-specific deletion of Mef2a (A-KO) or Mef2d (D-KO) in mice compromised in vitro osteoclast differentiation and activity. A-KO and D-KO male mice and D-KO female mice presented no overt skeletal phenotype, but female A-KO mice were osteopetrotic due to increased trabecular number. To determine whether MEF2A compensated for loss of MEF2D and vice versa, I generated mice with osteoclast-specific deletion of both Mef2a and Mef2d (AD-KO). In vitro assays demonstrated a complete block in osteoclast development prior to fusion. However, AD-KO mice presented with osteopenia from reduced cortical and trabecular bone thickness relative to WT mice but unchanged levels of osteoclast activity markers. These experiments suggest specific signals present in vivo but not in vitro rescue osteoclast differentiation of AD-KO osteoclasts. Additionally, MEF2A/D potentially regulates expression of an osteoclastderived factor that regulates osteoblast activity.