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P&F Grant Awards

Year 2

Grant # 3

MAGP1: An ECM regulator of Bone Remodeling

PI: Robert Mecham, Ph.D.,


Specific Aims

In bone, the collagens have been the traditional focus of ECM studies and, as a consequence, the contribution of other ECM elements is less well understood. One such non-collagenous ECM constituent is the fibrillin-containing microfibril. Though microfibrils are less abundant than their collagen counterparts, their significance has been demonstrated clinically. Disruption of fibrillin microfibrils leads to severe musculoskeletal disease affecting the axial skeleton, appendicular skeleton, muscularity and adiposity. Microfibrils are known to regulate cellular activities by controlling the bioavailability of numerous growth factors, particularly those of the TGF-β family. The core components of the microfibril are the fibrillins and microfibril associated glycoproteins (MAGPs), both of which are capable of binding growth factors. Inactivation of the MAGP1 gene in mice (MAGP1Δ) results in osteopenia, slight long bone overgrowth, significantly less trabecular and cortical bone, and altered trabecular microarchitecture. Our preliminary data suggests that MAGP1 antagonizes osteoclast-mediated bone resorption by influencing cellular differentiation, survival and function. The objective of work outlined in this proposal is to obtain more preliminary data to support this hypothesis. The experimental models that will be developed will provide a foundation for future mechanistic studies of the role MAGP1 plays in bone cell biology. Our specific aims are:

  1. Determine the temporal and spatial appearance of MAGP1 in developing bone. To fully understand what role MAGP1 might play in the bone requires knowing when and where it appears in bone differentiation. This aim will utilize in situ hybridization, immunohistochemistry, and RNA quantitation to define the spatial and temporal expression profile of the MAGPs and fibrillins during bone development.

  2. Explore MAGP1-mediated coupling of osteoblasts and osteoclasts in bone remodeling. Bone marrow macrophages produce insignificant amounts of MAGP1 relative to osteoblasts. The importance of osteoblast-derived MAGP1 for osteoclast differentiation and function will be studied using co-cultures of WT (or KO) BMMs with WT and KO osteoblasts. In vivo experiments using radiation chimeras (RC) will be used to complement the ex vivo co-culture studies. RCs will be generated by engrafting γ-irradiated WT mice with marrow from KO mice (and vice versus). The RCs will then be challenged and bone loss assessed. Finally, we will test whether MAGP1’s presence in the matrix influences homing of hematopoietic stem cells (or more specifically monocytes) to the bone by extracting and labeling marrow cells from donor mice, i.v. injection of labeled cells into both WT and MAGP1Δ mice, and tracking the localization of these cells.

  3. Develop a MAGP1 conditional knockout mouse. To confirm that the bone phenotype in the MAGP1-null mouse is due to the absence of MAGP1 from bone and not influenced by a factor extrinsic to bone, our lab has begun the process of generating a conditional MAGP1 mouse with the MAGP1 gene floxed by LoxP sites. Crossing these mice with strains expressing a bone cell-specific Cre will enable us to inactivate MAGP1 only in bone and at different times during bone development.