RESUMO
Chondrocyte proliferation and differentiation requires their attachment to the collagen type II-rich matrix of developing bone. This interaction is mediated by integrins and their cytoplasmic effectors, such as the integrin-linked kinase (ILK). To elucidate the molecular mechanisms whereby integrins control these processes, we have specifically inactivated the ILK gene in growth plate chondrocytes using the Cre-lox methodology. Mice carrying an ILK allele flanked by loxP sites (ILK-fl) were crossed to transgenic mice expressing the Cre recombinase under the control of the collagen type II promoter. Inactivation of both copies of the ILK-fl allele lead to a chondrodysplasia characterized by a disorganized growth plate and to dwarfism. Expression of chondrocyte differentiation markers such as collagen type II, collagen type X, Indian hedgehog and the PTH-PTHrP receptor was normal in ILK-deficient growth plates. In contrast, chondrocyte proliferation, assessed by BrdU or proliferating cell nuclear antigen labeling, was markedly reduced in the mutant growth plates. Cell-based assays showed that integrin-mediated adhesion of primary cultures of chondrocytes from mutant animals to collagen type II was impaired. ILK inactivation in chondrocytes resulted in reduced cyclin D1 expression, and this most likely explains the defect in chondrocyte proliferation observed when ILK is inactivated in growth plate cells.
Assuntos
Cartilagem/anormalidades , Cartilagem/enzimologia , Condrócitos/enzimologia , Exostose Múltipla Hereditária/enzimologia , Lâmina de Crescimento/anormalidades , Proteínas Serina-Treonina Quinases/deficiência , Animais , Apoptose/genética , Cartilagem/fisiopatologia , Adesão Celular/genética , Diferenciação Celular/genética , Divisão Celular/genética , Condrócitos/patologia , Condrogênese/genética , Colágeno Tipo II/metabolismo , Ciclina D1/metabolismo , Modelos Animais de Doenças , Regulação para Baixo/genética , Nanismo/enzimologia , Nanismo/genética , Nanismo/fisiopatologia , Exostose Múltipla Hereditária/genética , Feminino , Regulação da Expressão Gênica no Desenvolvimento/genética , Lâmina de Crescimento/enzimologia , Lâmina de Crescimento/patologia , Integrases/genética , Integrinas/metabolismo , Masculino , Camundongos , Camundongos Knockout , Osteogênese/genética , Proteínas Serina-Treonina Quinases/genética , Transgenes/genética , Proteínas Virais/genéticaRESUMO
Fibrous dysplasia (FD) of bone can be complicated by renal phosphate wasting. The effect of hypophosphatemia on normal and dysplastic bone of FD patients has not been well characterized. In this study, we compared serum phosphorus (sPi) levels to histomorphometric findings in 27 iliac bone samples from 23 children and adolescents (aged 4.2-16.4 years) with polyostotic FD. The samples were separated into two groups, based on the presence (n = 10) or absence (n = 17) of a dysplastic lesion within the specimen. Histomorphometric results were compared with those from 18 age-matched control subjects without metabolic bone disease. In dysplastic lesions, trabeculae were clearly thinner and increased in number. Osteoid indices, osteoblast surface per bone surface, and mineralization lag time were elevated in dysplastic areas, but there was no detectable effect of sPi concentrations on these indices. In nondysplastic bone tissue, low sPi levels were associated with mildly increased osteoid thickness and prolonged mineralization lag time. None of the 13 patients in whom hand X-rays were available at the time of biopsy had radiological signs of rickets. In conclusion, low sPi can cause a mild systemic mineralization defect in FD, but the more severe mineralization defect seen in dysplastic lesions is independent of sPi levels. It is debatable whether the mild systemic mineralization defect warrants treatment with oral phosphorus supplementation if signs of rickets are absent.