A feline assay using osteoclasts generated in vitro from peripheral blood for screening anti-resorptive agents

Identifying early osteoclastic resorptive lesions in feline teeth: a model for understanding the origin of multiple idiopathic root resorption

BACKGROUND AND OBJECTIVE

Domestic cats commonly suffer from external osteoclastic tooth resorption, a disease with many similarities to human multiple idiopathic root resorption. In both diseases, it is unclear whether anatomical features of the tooth surface are associated with a predisposition for resorptive lesions. The aim of the present study was to investigate the origin and progression of early feline osteoclastic resorptive lesions in teeth exhibiting no clinical signs of disease.

MATERIAL AND METHODS

The entire surfaces of 138 teeth from 13 adult cats were analysed using back-scattered electron microscopy. The distribution of lesions was assessed by tooth type, location and between individuals.

RESULTS

Seventy-three (53%) teeth showed at least one resorptive lesion. Eleven (85%) cats had lesions, and there was a significant association between increasing age and incidence of resorptive lesions. The highest frequency occurred in mandibular molars (82%). On average, there were 3.5 lesions per tooth. Fifty-two (38%) teeth featured resorptive lesions at the cemento-enamel junction. Twenty-three per cent of teeth with resorptive lesions showed evidence of repair of lesions that was limited to the root surface. There was no evidence of repair of resorptive lesions at the cemento-enamel junction.

CONCLUSION

Resorption is prevalent without evidence of clinical disease, and occurred at younger ages than previously reported. It can initiate anywhere on the root surface, but lack of repair of lesions at the cemento-enamel junction indicates that mechanisms of replacement are absent or compromised in this region. Whereas resorption of the root may undergo repair, resorption at the cervix may progress to clinically evident lesions.

The in vitro effect of pH on osteoclasts and bone resorption in the cat: implications for the pathogenesis of FORL

Dental disease due to osteoclast over‐activity reaches epidemic proportions in older domestic cats and has also been reported in wild cats. Feline osteoclastic resorptive lesions (FORL) involve extensive resorption of the tooth leaving it liable to root fracture and subsequent tooth loss. The aetio‐pathogenesis of FORL is not known. Recent work has shown that systemic acidosis causes increased osteoclast activation and that loci of infection or inflammation in cat mouth are likely to be acidotic. To investigate this, we generated osteoclasts from cat blood and found that they formed in large numbers (∼400) in cultures on bovine cortical bone slices. Acidosis caused an increase in the size of cells—in cultures maintained up to 14 days at basal pH 7.25, mean osteoclast area was 0.01 ± 0.003 mm², whereas an 8.6‐fold increase was observed in cells cultured between 11 and 14 days at pH 7.15 (0.086 ± 0.004 mm²). Acidosis caused a modest increase in the number of osteoclasts. Exposure to pH 6.92 exhibited a 5‐fold increase in the area of bone slices covered by resorption lacunae (∼70% bone slice resorbed). In line with this finding, significant increases were observed in the expression of cathepsin K and proton pump enzymes (both approximately 3‐fold) that are key enzymes reflective of resorptive activity in osteoclasts. These results demonstrate that acidosis is a major regulator of osteoclast formation and functional activation in the cat, and suggest that local pH changes may play a significant role in the pathogenesis of FORL. J. Cell. Physiol. 213: 144–150, 2007. © 2007 Wiley‐Liss, Inc.

Hypoxia induces giant osteoclast formation and extensive bone resorption in the cat

Dental disease due to osteoclast (OC) overactivity reaches epidemic proportions in older domestic cats and has also been reported in wild cats. Feline odontoclastic resorptive lesions (FORL) involve extensive resorption of the tooth, leaving it liable to root fracture and subsequent loss. The etiopathogenesis of FORL remains unclear. Here, we explore the hypothesis that FORL is associated with hypoxia in the oral microenvironment, leading to increased OC activity. To investigate this, we developed a method of generating OCs from cat blood. Reducing O2 from 20% to 2% increased the mean area of OC eightfold from 0.01 to 0.08 mm2. In hypoxic cultures, very large OCs containing several hundred nuclei were evident (reaching a maximum size of approximately 14 mm2). Cultures exposed to 2% O2 exhibited an increase of approximately 13-fold in the area of bone slices covered by resorption lacunae. In line with this finding, there was a significant increase in cells differentiating under hypoxic conditions, reflected in increased expression of cathepsin K and proton pump enzymes. In conclusion, these results demonstrate that oxygen tension is a major regulator of OC formation in the cat. However, in this species, hypoxia induces the formation of "giant" OCs, which can be so large as to be visible with the naked eye and yet also actively resorb. This suggests that local hypoxia is likely to play a key role in the pathogenesis of FORL and other inflammatory conditions that are associated with bone resorption in cats.

Quantitative and reliable in vitro method combining scanning electron microscopy and image analysis for the screening of osteotropic modulators

The increased generation and up-regulated activity of bone resorbing cells (osteoclasts) play a part in the impairment of bone remodeling in many bone diseases. Numerous drugs (bisphosphonates, calcitonin, selective estrogen receptor modulators) have been proposed to inhibit this increased osteoclastic activity. In this report, we describe a pit resorption assay quantified by scanning electron microscopy coupled with image analysis. Total rabbit bone cells with large numbers of osteoclasts were cultured on dentin slices. The whole surface of the dentin slice was scanned and both the number of resorption pits and the total resorbed surface area were measured. Resorption pits appeared at 48 h and increased gradually up to 96 h. Despite the observation of a strong correlation between the total resorption area and the number of pits, we suggest that area measurement is the most relevant marker for osteoclastic activity. Osteotropic factors stimulating or inhibiting osteoclastic activity were used to test the variations in resorption activity as measured with our method. This reproducible and sensitive quantitative method is a valuable tool for screening for osteoclastic inhibitors and, more generally, for investigating bone modulators.

In Vitro, Ex Vivo, andIn VivoMethodological Approaches for Studying Therapeutic Targets of Osteoporosis and Degenerative Joint Diseases: How Biomarkers Can Assist?

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.

A Scanning Electron Microscopy Study of Idiopathic External Tooth Resorption in the Cat

BACKGROUND

Multiple idiopathic root resorption (MIRR) is a rare condition in man characterized by cervical resorption leading to significant tooth loss. A similar condition, feline osteoclastic resorptive lesions (FORL), affects up to 70% of domestic cats and thus provides a valuable model for investigating the etiopathogenesis of MIRR. The aim of the present study was to establish changes in the surface microanatomy of the tooth in late stage FORL and to identify whether its location has a surface bias.

METHODS

Scanning electron microscopy (SEM) was used to analyze the surface features of enamel and cementum of feline teeth affected with advanced FORL.

RESULTS

Resorption involved the coronal root at the cementoenamel junction (CEJ) in 95% of teeth and focal resorption of intact enamel was observed in 14% of teeth. In 55% of teeth, the main lesion was on the buccal surface and a distinct circumferential resorption "front" was present at the apical margin of resorption. The root surfaces of most affected teeth either lacked extrinsic fibers or cellular lacunae or featured evidence of cementum remodeling. Woven bone-like tissue was found within lesions, on resorbed dentin, or on the root surface in 27% of teeth.

CONCLUSIONS

This study demonstrates that most FORL involve the CEJ, and the presence of focal lesions at this site suggests that this is where resorption is initiated. This implies that local factors in the oral microenvironment play a role in the etiopathogenesis of this condition. The study also shows that FORL are more likely to occur on buccal surfaces and are associated with changes in the microarchitecture of the root surface consistent with destruction of the normal periodontal attachment and stimulation of a reparative response. These findings may be relevant to understanding the etiopathogenesis of multiple idiopathic resorption areas in man.