The diffe rentiation of osteoblasts from mesenchymal precursors requires a series of cell fa te decis ions controlled by a hi erarchy of transcription factors Among these are RUNX2, osterix (OSX) , ATF4 and a la rge number of nuclear co-regulators. During bone development, initial RUNX2 expression coincides with the formation of mesenchyma l con densations well before the branching of chondrogenic and osteogenic lineages. Runx2 is s ubject to a number 0 1' post - tran scriptional controls including regulation by nuclear accessory factors such as ATF4 and DLX5 and post-trans lational mod ificati on, especially phosphOl‘ylation. We previously showed that Runx2-dependent transcription is acti vated by the ex tracellular signal-regulated/MAP kinase pathway in response to ECM/integrin and FGF2 stimulation. To identify and a5sess the function of ERK/MAPK phosphorylation s ites in RUNX2 and esta bli sh the role of MAPK s ignaling in bone fo 1'mation Approaches: A deletion/mutagenesis approach wa5 used to ide ntify 1'egions 0 1' RUNX2 necessary fo1' MAPK responsiveness and phosphorylation. To evaluate the in vivo function of the ERK/MAPK pa thway‘ transgen ic mice were developed wi th osteoblast- specific expression of either dominant-negative 0 1' cons titutively-active MEKl in osteoblasts and crossed with Runx2 heterozygous-• null ammals RUNX2 is phosphorylated on two critical serine res idues in the P/S/T domain. RUNX2 conta ining S/A mutations in these sites is refractory to MAPK stimula ti on while S/E muta tions cause cons titutive activation MAPK activation of RUNX2 was also found to occur in vivo 까'a n sge ni c expression of cons titutively ac tive MEKl in osteo • blas ts accelerated skeletal development while a dominant-negati ve MEKl reta rd ed bone formation in a RUNX2- dependent manner As shown by these studies‘ the ERK/MAPK pathway controls Runx2 tra nscript iona l activ ity by phosphorylation at specific serine residues. This may be a major pathway for controlling osteoblast activity in response to extracellul ar matrix signals. mechanical loads and hormonal stimulation

• Renny T. Francesch(University of Michigan School of dentistry, Ann Arbor, MI)
• Chunxi Ge(University of Michigan School of dentistry, Ann Arbor, MI)
• Guozhi Xiao(University of Pittsburg School of Medicine, Pittsburg, PA.)
• Hernan Roca(University of Michigan School of dentistry, Ann Arbor, MI)
• Oi jiang(University of Michigan School of dentistry, Ann Arbor, MI)