Differential turnover of cortical and trabecular bone in transgenic mice overexpressing cathepsin K Jukka Morko, Riku Kiviranta, Sara Hurme, Juho Rantakokko, Eero Vuorio T Department of Medical Biochemistry and Molecular Biology, University of Turku, FI-20520 Turku, Finland Received 1 October 2004; revised 18 January 2005; accepted 8 February 2005 Abstract Cathepsin K is a major osteoclastic protease. We have recently shown that overexpression of mouse cathepsin K gene in transgenic UTU17 mouse model results in high turnover osteopenia of metaphyseal trabecular bone at the age of 7 months. The present report extends these studies to a systematic analysis of cortical bone in growing and adult mice overexpressing cathepsin K. Mice homozygous for the transgene locus (UTU17 +/+ ) and their control littermates were studied at the age of 1, 3, 7, and 12 months. Bone properties were analyzed using peripheral quantitative computed tomography (pQCT), histomorphometry, histochemistry, radiography, and biomechanical testing. In addition, the levels of biochemical markers of bone turnover were measured in the sera. Unexpectedly, cortical thickness and cortical bone mineral density were increased in the diaphyseal region of growing and adult UTU17 +/+ mice. This was associated with an increased number of vascular canals leading to increased cortical porosity in UTU17 +/+ mice without changes in the ultimate bending force or stiffness of the bone. In UTU17 +/+ mice, osteopenia of metaphyseal trabecular bone was observed already at the age of 1 month. In sera of 1-month-old UTU17 +/+ mice, the activity of tartrate-resistant acid phosphatase 5b was decreased and the levels of osteocalcin increased. Our results support the role of cathepsin K as a major proteinase in osteoclastic bone resorption. Excessive production of cathepsin K induced osteopenia of metaphyseal trabecular bone and increased the porosity of diaphyseal cortical bone. The increased cortical thickness and bone mineral density observed in diaphyses of UTU17 +/+ mice demonstrate the different nature and reactivity of trabecular and cortical bone in mice. These results suggest that the biomechanical properties of cortical bone are preserved through adaptation as outlined in Wolff’s law. D 2005 Elsevier Inc. All rights reserved. Keywords: Biochemical markers; Bone turnover; Cathepsin K; Computed tomography; Histomorphometry – animal Introduction Bone modeling/remodeling is a complex process con- sisting of differential bone resorption and formation to which biomechanical forces contribute through an interplay of different cell types [1,2]. Bone resorption by osteoclasts consists of several steps, e.g., cell attachment, polarization, demineralization of bone matrix, degradation of organic bone matrix, and cell detachment, followed by osteoclast apoptosis or by initiation of a new resorption cycle [3]. Several lines of evidence suggest that cathepsin K, a lysosomal cysteine proteinase, is a key enzyme in the degradation of organic bone matrix. It is highly and quite specifically expressed by osteoclasts and secreted into the resorption lacunae [4–6] where it can efficiently degrade matrix proteins, including native type I collagen [7–9]. Expression of the cathepsin K gene (Ctsk ) is highly up- regulated under conditions of enhanced bone resorption such as mouse immobilization osteopenia [10]. Inhibition of cathepsin K activity in vitro and in vivo reduces bone resorption [11,12]. Ctsk -deficient mice develop an osteopet- rosis-like phenotype due to their inability to degrade organic bone matrix [13–15]. In humans, mutations in the Ctsk gene cause pycnodysostosis, an osteopetrotic disease character- ized by increased bone mass, short stature, and increased bone fragility [16]. We have recently produced a transgenic UTU17 mouse model harboring extra copies of the Ctsk gene that result in overexpression of cathepsin K [17]. Histomorphometric 8756-3282/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.bone.2005.02.006 T Corresponding author. Fax: +358 2 333 7229. E-mail address: eero.vuorio@utu.fi (E. Vuorio). Bone 36 (2005) 854 – 865 www.elsevier.com/locate/bone