Nanomechanics of Knockout Mouse Bones
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Nanomechanics of Knockout Mouse Bones N. Beril Kavukcuoglu1, and Adrian B. Mann1,2 1 Materials Science and Engineering, Rutgers University, Piscataway, NJ, 08854 2 Biomedical Engineering, Rutgers University, Piscataway, NJ, 08854
ABSTRACT Osteocalcin (OC) and osteopontin (OPN) are among the most abundant non-collagenous bone matrix proteins. Both have drawn interest from investigators studying their function in osteoporosis, and it is known that mutations of these proteins can also have dramatic effects on the properties of bone. Other proteins including fibrillin 1 and 2 (FBN2) have been less widely studied, but can be mutated in some individuals resulting in connective tissue disorders. It has been reported that abnormal fibrillin may play a role in decreased bone mass. In this study, bones from osteopontin (OPN), osteocalcin (OC) and fibrillin-2 (FBN2) knockout mice have been investigated. The study has identified how these proteins affect the bone’s nanomechanical properties (hardness and elastic modulus). Nanoindentation tests were performed on the radial axis of cortical femora bones from the knockout mice and their wildtype controls. The results showed that young (age< 12 weeks) OPN knock-out bones have significantly lower mechanical properties than wild-type bones, indicating a crucial role for OPN in early bone mineralization. After 12 weeks of age, the OPN knockout and wild-type control bones did not show any statistical difference. In OC deficient mice the mechanical properties were found to increase in the cortical mid-shaft of femora from 1 year old mice, suggesting an increase in bone mineralization, but 3 month old FBN2 deficient mice bones showed a decrease in mechanical properties across the cortical radial axis of the mid- femora. INTRODUCTION Osteoporosis is a bone disease in which bone mineral density (BMD) is reduced and the bone’s microarchitecture deteriorates, leading to an increased fragility [1]. The essential feature of osteoporosis is an increase in the ratio of osteoclastic bone resorption to osteoblastic bone formation. Simultaneously, there is a change in the amount and variety of non-collagenous proteins in the bone, which suggests a link between bone mineralization/resorption and these proteins [2]. Osteopontin (OPN), a phosphorylated glycoprotein and Osteocalcin (OC), a γcarboxyglutamic acid-containing protein (BGP), are among the most abundant non-collagenous bone matrix proteins. Because of their possible role in bone mineralization and crystal maturity, these proteins have drawn interest from many investigators [3-9]. Particularly, it has been indicated that OC functions as a signal in the recruitment of osteoclast pre-cursors to resorption sites and facilitates their differentiation to bone resorbing osteoclasts [2, 3]. Absence of OPN has been linked to a reduction in the effects of osteoporosis in small mammals [7, 9].
Moreover, bones from osteocalcin-deficient (OC-/-) mice were shown to have increased bone formation [4] and increased mineral to matrix ratio (f
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