Growth plate chondrocyte vitamin D receptor number and affinity are reduced in avian tibial dyschondroplastic lesions

Tibial dyschondroplasia (TD) is a condition of rapidly growing poultry in which a mass of unmineralized cartilage extends distally from the tibiotarsal growth plate, leading to deformity and lameness. The lesion is characterized by the accumulation of prehypertrophic chondrocytes, probably because the maturing chondrocytes are unable to differentiate fully. The condition can be prevented by feeding 1,25-(OH)(2)D-3. We have investigated the possibility that vitamin D receptors (VDR), through which 1,25-(OH)(2)D-3 exerts its differentiating effects on chondrocytes, may be defective in TD birds. Chondrocytes were isolated from the proliferating and hypertrophic zones of normal tibiotarsi and from the proliferating zone and lesion of affected birds and receptors were characterized by Scatchard analysis. Results showed that, while cells from the proliferating zone in TD birds had normal receptors, those from the TD lesion had significantly lower numbers and affinity for 1,25-(OH)(2)D-3 compared to all other zones, Lesion VDR had low affinity; K-d 83.9 +/- 20.6 pM compared to 30.0 +/- 2.8, 37.8 +/- 3.1, and 33.0 +/- 4.0 pM (p < 0.001), and low receptor number per cell, 920 +/- 74, compared to 1329 +/- 151, 1664 +/- 167, and 1360 +/- 104 (p < 0.01) in the normal proliferating, normal hypertrophic, and TD proliferating cells, respectively. These findings were confirmed by immunohistochemical localization of VDR in sections of normal and TD growth plates using monoclonal antibody 9A7 gamma. In normal growth plate, most cells were VDR positive with intense staining in the mature hypertrophic chondrocytes; in TD growth plates, proliferating zone cells stained well but signal was largely absent from chondrocytes in the lesion. Image analysis showed integrated nuclear staining density per cell of 168.2 +/- 36.9 arbitrary units in normal hypertrophic cartilage compared to 98.8 +/- 60.2 units in the top of the lesion and 2.2 +/- 2.0 units in the midlesion. We conclude that both numbers and affinity of VDR are reduced in TD and this may explain the failure of chondrocytes to differentiate to the mature form. The adverse consequences of defective receptors may be partly overcome by treatment with 1,25-(OH)(2)D-3.