Adaptive response of bone to mechanical strain in a mouse model of premature aging
Final Report Abstract
Bone adapts to mechanical loading, but there is a reduced adaptive response of the skeleton with aging. The capacity to respond and adapt appropriately to changes in mechanical loading is key to biogenically generate effective material and structural tissue properties during healing and growth. The goal of our project was to better understand the altered response of the aged and progeroid skeleton to mechanical strain. We showed that the reduced response to loading is already present at skeletal maturation. Mechanical loading has a much stronger effect on formation than on resorption in cancellous and cortical bone and this effect is mainly due to an increase in mineralizing surface, which is diminished with skeletal aging. Under the same external load, strains engendered in bone are reduced with increasing age, which could mainly be attributed to age-related changes in the bone morphology and less to alterations in tissue material properties. We also showed that independent of age, formation and resorption are mechano-regulated, preferentially at high strains. However, with increasing age mechanically induced (re)modeling becomes dysregulated, apparent in an inability to inhibit resorption or initiate formation. Molecular mechanisms underlying these age-specific changes remain unclear and are the subject of continued investigation. Regarding the progeroid skeleton, we specifically investigated the role of the Golgi protein GORAB. At the beginning of this project knowledge on the pathomechanism of gerodermia osteodysplastica was restricted to loss of function mutations of GORAB, which was found to reside in the Golgi apparatus, leading to elevated levels of apoptosis and cellular senescence in vitro. We have now shown that the conditional knockout mouse (Gorab^Prx1) have severely abnormal bones with strong cortical porosity. Collagen fibrils are irregularly aligned and there is a reduced proteoglycan content of the ground substance. Possibly as a result of these abnormalities, osteocytes have abnormal morphology, unusual accumulations and reduced number of processes. Gene expression analysis of cortical bone showed that several osteoblast marker genes are dysregulated in the mutant bone, indicating a block in terminal osteoblast differentiation. Most notable was a persistence of osterix expression in the abnormally shaped osteocytes. Since we had indications that GORAB deficient skin fibroblasts from gerodermia osteodysplastica patients tend to undergo premature cellular senescence due to elevated DNA damage levels we also looked for these signs in the Gorab^Prx1 bones. Indeed, the same constellation of elevated DNA damage and higher numbers of senescent cells were detected. Our results show that inactivation of the GORAB gene in mesenchymal stem cells (Gorab^Prx1) leads to increased oxidative stress, cellular senescence, and abnormal differentiation of cells of the osteoblast lineage leading to highly abnormal osteocytes. Therefore, further studies will focus on the role of cellular senescence in relation to reduced mechano-responsiveness and the increased fracture susceptibility of the progeroid disorder gerodermia osteodysplastica. We believe that findings from our present and future studies will help to provide a framework for understanding how regenerative medicine approaches might be combined with mechanical loading or exercise to most effectively combat age-related bone loss or treat bone defects in aged patients.
Publications
- Diminished response to in vivo mechanical loading in trabecular and not cortical bone in adulthood of female C57Bl/6 mice coincides with a reduction in deformation to load, Bone, 55(2):335-346, 2013
Willie BM, Birkhold A, Razi H, Thiele T, Aido M, Kruck B, Schill A, Checa S, Main RP, Duda GN
(See online at https://doi.org/10.1016/j.bone.2013.04.023) - Mineralizing surface is the main target of mechanical stimulation independent of age: 3D Dynamic in Vivo Morphometry, Bone, 66C:15-25, 2014
Birkhold A, Razi H, Duda GN, Weinkamer R, Checa S, Willie BM
(See online at https://doi.org/10.1016/j.bone.2014.05.013) - The influence of age on adaptive bone formation and bone resorption, Biomaterials, 35(34): 9290-9301, 2014
Birkhold A, Razi H, Duda GN, Weinkamer R, Checa S, Willie BM
(See online at https://doi.org/10.1016/j.biomaterials.2014.07.051) - Age-related changes in the mouse tibia morphology lead to reduced mechanical strain under external loading, Acta Biomaterialia, 13:301- 10, 2015
Razi H, Birkhold A, Zaslansky P, Weinkamer R, Duda G, Willie BM, Checa S
- Aging leads to a loss in the mechanical regulation of bone formation and resorption, Journal of Bone and Mineral Research, Apr 9, 2015
Razi H, Birkhold A, Weinkamer R, Duda G, Willie BM, Checa S
(See online at https://doi.org/10.1002/jbmr.2528) - Effect of in vivo loading on bone composition varies with animal age, Experimental Gerontology, 63:48-58, 2015
Aido M, Kerschnitzsky M, Hoerth R, Checa S, Spevak L, Boskey A, Fratzl P, Duda GN, Wagermaier W, Willie BM
(See online at https://doi.org/10.1016/j.exger.2015.01.048) - Maturation substantially affects elastic tissue properties in the endosteal and periosteal regions of loaded mice tibias, Acta Biomaterialia, 21:154-64, 2015
Checa S, Hesse B, Roschger P, Aido M, Raum K, Duda GN, Willie BM
(See online at https://doi.org/10.1016/j.actbio.2015.04.020) - Monitoring in vivo (re)modeling: A computational approach using 4D microCT data to quantify bone surface movements, Bone, 4(75):210-221, 2015
Birkhold A, Razi H, Duda GN, Weinkamer R, Checa S, Willie BM
(See online at https://doi.org/10.1016/j.bone.2015.02.027)
