Although our approach to the clinical management of osteoporosis (OP) and degenerative joint diseases (DJD)-major causes of disability and morbidity in the elderly-has greatly advanced in the past decades, curative treatments that could bring ultimate solutions have yet to be found or developed. Effective and timely development of candidate drugs is a critical function of the availability of sensitive and accurate methodological arsenal enabling the recognition and quantification of pharmacodynamic effects. The established concept that both OP and DJD arise from an imbalance in processes of tissue formation and degradation draws attention to need of establishing in vitro, ex vivo, and in vivo experimental settings, which allow obtaining insights into the mechanisms driving increased bone and cartilage degradation at cellular, organ, and organism levels. When addressing changes in bone or cartilage turnover at the organ or organism level, monitoring tools adequately reflecting the outcome of tissue homeostasis become particularly critical. In this context, bioassays targeting the quantification of various degradation and formation products of bone and cartilage matrix elements represent a useful approach. In this review, a comprehensive overview of widely used and recently established in vitro, ex vivo, and in vivo set-ups is provided, which in many cases effectively take advantage of the potentials of biomarkers. In addition to describing and discussing the advantages and limitations of each assay and their methods of evaluation, we added experimental and clinical data illustrating the utility of biomarkers for these methodological approaches.

Objective. To investigate how the time of initiation influences the effects of estrogen therapy on type II collagen (CII) turnover and the structural integrity of articular cartilage in ovariectomized rats and to determine whether estrogen exerts direct effects on the catabolic function of chondrocytes ex vivo.Methods. A total of 46 Sprague-Dawley rats were distributed into 1 of the following treatment groups: 1) ovariectomy, 2) ovariectomy plus early estrogen therapy, 3) ovariectomy plus delayed estrogen therapy, or 4) sham operation. Cartilage turnover was estimated by measuring the serum levels of C-telopeptide of type II collagen (CTX-II). Cartilage lesions at week 9 were quantified using a published scoring technique. The presence of the CTX-II epitope in articular cartilage was assessed by immunohistochemistry. The effects of estrogen (1-100 nM) on chondrocytes were investigated in bovine cartilage explants subjected to catabolic cyto- kines (tumor necrosis factor ␣ [TNF␣] and oncostatin M [OSM]).Results. In ovariectomized rats, estrogen therapy evoked significant decreases in serum CTX-II independently of the time of initiation; yet, delayed initiation resulted in diminished efficacy in terms of preventing cartilage lesions. CTX-II fragments were present in articular cartilage, colocalizing with early lesions at the cartilage surface. In untreated animals, the early relative increases in serum CTX-II were proportional to the severity of cartilage lesions at week 9 (r ‫؍‬ 0.73, P < 0.01). Estrogen significantly and dose-dependently countered CTX-II release from TNF␣ plus OSMstimulated cartilage explants ex vivo.Conclusion. Our results suggest that estrogen counters the acceleration of CII degradation and related structural alterations, and these benefits can be maximized by early initiation after menopause. The protective effect of estrogen seems to involve direct inhibition of the catabolic function of chondrocytes.Menopause is a major event in a woman's life. A deficiency in endogenous estrogen is believed to contribute to a number of health changes, such as menopausal symptoms, acceleration of bone loss, and atherosclerosis (1). Recent observations suggest that the prevalence and incidence of osteoarthritis (OA) increase more pronouncedly in women, beginning in their early 50s (2). Furthermore, degradation products of type II collagen (CII) are significantly higher in postmenopausal women compared with age-and body mass index-matched premenopausal women (3). These observations thus support the notion that estrogen might play a role in the maintenance of the structural integrity of articular cartilage.

This study is the first to demonstrate that a SERM suppresses cartilage degradation in both rodents and humans, suggesting potential therapeutical benefits in the prevention of destructive joint diseases such as osteoarthritis.

IntroductionOsteoarthritis (OA) is a major cause of functional impairment and disability among the elderly [1], yet current therapies predominantly target symptoms rather than providing prevention or curative treatment. Animal models of OA have been used extensively for studying the pathogenesis of cartilage degradation as well as the efficacy of potential therapeutic interventions [2]. However, most of the currently available models only approximate the mechanisms underlying the human disease. Although several animal species -such as mice, Syrian hamsters, guinea pigs, and nonhuman primates -can develop spontaneous OA, the development of disease in these models is slow; typically, more than 9 to 12 months is required for significant cartilage erosion to occur [2]. Consequently, these spontaneous models are cumbersome and time-consuming to use in arthritis research and drug development. Transgenic mice models have been of great help in clarifying the role of numerous pathogenic factors (matrix metalloproteinases, transforming growth factor β, nitric oxide) in the development of OA, yet these models may not be applicable for studies testing the therapeutic potentials of chondroprotective agents [3,4]. Surgically induced joint damage has also been used extensively as a model of OA, though this condition more nearly approximates a traumatic form of OA than it does the natural, spontaneously CTX-I = collagen type I fragments; CTX-II = collagen type II degradation products; ELISA = enzyme-linked immunosorbent assay; OA = osteoarthritis; OVX = ovariectomized; SD = standard deviation; SEM = standard error of the mean; SERM = selective estrogen receptor modulator. AbstractWe aimed to assess the effect of ovariectomy on cartilage turnover and degradation, to evaluate whether ovariectomized (OVX) rats could form an experimental model of postmenopausal osteoarthritis. The effect of ovariectomy on cartilage was studied using two cohorts of female Sprague-Dawley rats, aged 5 and 7 months. In a third cohort, the effect of exogenous estrogen and a selective estrogen receptor modulator was analyzed. Knee joints were assessed by histological analysis of the articular cartilage after 9 weeks. Cartilage turnover was measured in urine by an immunoassay specific for collagen type II degradation products (CTX-II), and bone resorption was quantified in serum using an assay for bone collagen type I fragments (CTX-I). Surface erosion in the cartilage of the knee was more severe in OVX rats than in sham-operated animals, particularly in the 7-month-old cohort (P = 0.008). Ovariectomy also significant increased CTX-I and CTX-II. Both the absolute levels of CTX-II and the relative changes from baseline seen at week 4 correlated strongly with the severity of cartilage surface erosion at termination (r = 0.74, P < 0.01). Both estrogen and the selective estrogen receptor modulator inhibited the ovariectomyinduced acceleration of cartilage and bone turnover and significantly suppressed cartilage degradation and erosion seen in vehicle-treated...

Autosomal dominant osteopetrosis type I (ADOI) is presumably caused by gain-of-function mutations in the LRP5 gene. Patients with a T253I mutation in LRP5 have a high bone mass phenotype, characterized by increased mineralizing surface index but abnormally low numbers of small osteoclasts. To investigate the effect of the T253I mutation in LRP5 on osteoclasts, we isolated CD14؉ monocytes from ADOI patients and assessed their ability to generate osteoclasts when treated with RANKL and M-CSF compared to that of age-and sex-matched control osteoclasts. We found normal osteoclastogenesis, expression of osteoclast markers, morphology, and localization of proteins involved in bone resorption, such as ClC-7 and cathepsin K. The ability to resorb bone was also normal. In vivo, we compared the bone resorption and bone formation response to T 3 in ADOI patients and age-and sex-matched controls. We found attenuated resorptive response to T 3 stimulation, despite a normal bone formation response, in alignment with the reduced number of osteoclasts in vivo. These data demonstrate that ADOI osteoclasts are normal with respect to all aspects investigated in vitro. We speculate that the mutations causing ADOI alter the osteo- Bone remodeling is performed mainly by two cell types, the osteoclasts that resorb bone and the osteoblasts that form bone. Autosomal dominant osteopetrosis type I (ADOI) is a fully penetrant disease, with a benign pathological appearance, 1 characterized by bone and back pain, as well as complications such as cranial nerve compression.1 The bone phenotype is characterized by a generalized osteosclerosis, which is most clearly seen in the cranial vault. Histological and metabolic studies of the ADOI patients, demonstrated increased trabecular and cortical thickness and a decrease in both osteoclast number and size. [1][2][3][4][5][6][7] The mutation causing ADOI was localized to chromosome 11q12-13 8 and later shown to be caused by gainof-function mutations in the low-density lipoprotein receptor-related protein 5 (LRP5).9 Several other mutations in LRP5 were shown to lead to various osteopetrosis-like phenotypes, such as the high bone mass phenotype in G171V patients 10,11 and endosteal hyperostosis in A242T mutations.9 Accordingly, loss of function mutations in LRP5 lead to osteoporosis-pseudoglioma syndrome, 12 which is a disease characterized by a very low bone mass, but with normal bone cell number, in contrast to the high bone mass observed in the ADOI patients. Furthermore, polymorphisms in LRP5 gene were shown to influence the bone mineral density in both mice and man, and influence the fracture rates, [13][14][15][16] implicating LRP5 as an important regulator of bone turnover and strength.