Parameters of subnuclear organization are critical for skeletal development and osteoblastdifferentiation. Our program initially identified Runx2 as the osteoblast-specific nuclear matrixprotein NMP2. We developed the concept that the molecular function of Runx2 is to operate as ascaffolding protein that integrates regulatory signals from multiple osteogenic signaling pathways bymodifying chromatin structure and supporting the organization and assembly of regulatorymachinery for skeletal gene expression at strategic sites on target gene promoters and in focalnuclear microenvironments. We identified and structurally as well as functionally characterized a C-terminal intranuclear trafficking signal that directs Runx factors to nuclear domains where genes areactivated or suppressed. Recently, we have established that Runx2 is bound to mitoticchromosomes to modulate both osteoblast specific gene expression and ribosomal RNA synthesisin osteoblasts immediately following mitosis. In collaborative studies with Projects 2 and 3,Project 1 will now pursue a highly integrated set of experimental approaches to examine thefunction of Runx2 as an epigenetic gene regulator that can control cell fate, lineage commitmentand protein synthesis during both interphase and mitosis. We will determine the physiologicalconsequences of abrogating the ability of Runx2 to integrate osteogenic signals at Runx2subnuclear domains to control skeletogenesis in vivo, as well as osteoblast maturation and bone-specific gene expression ex vivo (Specific Aim 1). In addition, we will examine the osteoblast-related gene regulatory programs that depend on (i) subnuclear targeting of Runx2 duringinterphase and (ii)the epigenetic function of Runx2 that maintains a phenotypic regulatory memoryduring mitosis to control bone phenotypic genes as well as genes for cell cycle and growth controlin progeny cells (Specific Aim 2). Furthermore, we will assess the bone specific molecularmechanisms by which Runx2 controls the anabolic activity of osteoblasts through regulation ofosteoblast-specific genes (transcribed by RNA polymerase II) and ribosomal RNA genes(transcribed by RNA polymerase I) (Specific Aim 3).Relevance: Mechanisms that mediate the intranuclear organization of skeletal gene regulatorymachinery provide necessary and novel options for targeted therapy.
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