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Our lab is interested in two areas of bone biology. The first area is related to understanding the role of the bone morphogenetic proteins in bone and tooth development and in postnatal bone function. Our focus is on transgenic and tissue specific knock-out models of BMP4 and BMP2, directed to the osteoblast, odontoblast, and osteocyte cell lineages. At present we do not know the relative importance of either of these BMP ligands in bone function in vivo.

Our second area of focus is understanding how mechanical signals in bone cells, in particular with a focus on osteocytes, are translated to the genome. By dissecting out the signaling pathways from the load signal and cell deformation, we hope to understand the mechanism of specific gene activation in osteocytes after mechanical stimulation. We now have two excellent genes that show 1) osteocyte specific expression and 2) defined responses to mechanical loading in vivo. These genes belong to the class of extracellular proteins, called SIBLINGs. The two genes are Dentin Matrix Protein 1 or DMP1 and Matrix Extracellular Phosphoprotein or MEPE. Both genes appear to regulate mineralization processes in late osteoblasts and potentially in the osteocyte lacuna-canalicular system. Both genes show high levels of expression in osteocytes relative to osteoblasts and are mechanical responsive in osteocytes to a wide variety. We are actively testing various cis-regulatory regions for both genes to identify the osteocyte enhancers and regions responsive to mechanical signaling.

Bone morphogenetic protein 4 and 2 (BMP4 and BMP2) are thought to be critical components of a wide variety of developmental and postnatal differentiation processes. BMP2 and BMP4 are most likely critical in bone development in embryonic life and in bone formation postnatal, but the roles of these BMPs remain unclear because BMP4 and BMP2 null mutant mice are early embryonic lethals(1,2,3). However, both BMP4 and BMP2 have specific, unique, and non-overlapping roles during these early stages of development. Also the cis-regulatory DNA for BMP2 and BMP4 are very different, suggesting the two genes respond in their own unique way to a variety of signaling molecules and have their own unique roles in bone biology(4,5). Experiments with BMP2 and BMP4 recombinant proteins indicate they have similar capacity to induce mineralization in a variety of cell culture models, as well as ectopic mineralization studies(6,7). It is now possible to define the functional and unique significance of molecules such as BMP4 and BMP2, which affect many developmental and postnatal functions in a variety of tissues, in addition to bone(8). Using tissue specific deletion techniques, our focus will be on understanding the role of BMP4 and BMP2 in bone formation during both embryonic development and in postnatal life(9). Our results suggests that BMP4 plays a unique role in vivo during osteoblast differentiation and additionally, BMP4 is also important in normal bone homeostasis and normal bone structure and function in the adult animal. BMP4 is also necessary for bone formations and coupling to osteoclast function. BMP4 is not sufficient on its own to account for all bone formation, but is clearly an important gene in bone homeostasis.

We have also developed a BMP2 floxed animals model in which BMP2 can be selectively and temporally deleted in a variety of tissues. Using both Collagen-Cre, osteocalcin-cre, and other Cre lines for osteoblasts and for inducible Cre activity, we are now in a position to explore the roles of both BMP2 and BMP4 in osteoblast biology in vivo.

Using the DMP1 and MEPE readout in osteocytes, we can begin to understand the relationship of global strain to local strain readout. Using both cell models, such as 2T3 mineralizing osteocyte-osteoblast model in which osteocytes form in a mineralizing matrix and the osteocyte –like cell model MLO-Y4 in a less complex paradigm, we are using microarray technology to uncover the candidate gene networks and pathways involved in defined levels of mechanical stimulation, applied as fluid flow over the cells and matrix. In collaboration with Charles Turner and Alex Robling at U. of Indiana School of Medicine, we are using the mouse ulnae loading model, in conjunction with microarray analysis and in situ hybridization, to discover some the major pathways involved in mechanical stimulation. This information will then allow us to explore various mutant mice (or where altered pathways are inhibited) where mechanical loading signals fail to induce bone formation or other biological endpoints.

Introduction