Histone deacetylases (Hdacs) are enzymes that epigenetically repress gene transcription. Hdac inhibitors are used in the clinic to treat several diseases including cancer and epilepsy, but unfortunately these drugs are not specific in that they can target many of the 11 Hdacs expressed in cells. Notably, epileptic patients treated with antiepileptic Hdac inhibitors (valproate) have decreased bone mass, increased bone turnover and increased fracture risk. Furthermore, the use of Hdac inhibitors during pregnancy results in teratogenic malformations on the long bones of the developing fetus. A better understanding of the roles of individual Hdacs will lead to the development of better therapies that have fewer side effects on the skeleton. The proposed research will examine the importance of Hdac3 in endochondral ossification, which is the process by which long bones form and many fractures heal. In two focused aims, this proposed study will further define the role of Hdac3 in controlling the synchronized steps of chondrocyte maturation and coupling to ossification. Preliminary data suggests Hdac3 mediates the timely expression of matrix degradation enzyme, matrix metalloproteinase (Mmp)13, through Erk1/2 phosphorylation to modulate vascular invasion and subsequent ossification. In Aim 1, the mechanism(s) responsible for Erk1/2 phosphorylation in Hdac3-deficient chondrocytes will be defined. In Aim 2, the consequences of Hdac3 deletion on the chromatin landscape of Mmp13, as well as other genes, will be evaluated in chondrocytes. These results will establish the epigenomic landscape of Hdac3 in chondrocytes. This study will provide new insight into the molecular pathways and targets regulated by Hdac3 during chondrocyte maturation. A better understanding of Hdac3 activity in chondrocytes will advance the current knowledge of how gene expression and signaling cascades, such as Erk, are coordinated during bone formation. Such information will be essential in the development of more specific Hdac inhibitor therapies.