Nanomelic chondrocytes synthesize, but fail to translocate, a truncated aggrecan precursor

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Abstract
Cartilage extracellular matrix (ECM) is composed primarily of type II collagen and large, link stabilized aggregates of hyaluronic acid and chondroitin sulfate proteoglycan (aggrecan). Maturation and function of these complex macromolecules are dependent upon sequential processing events which occur during their movements through specific subcellular compartments in the constituitive secretory pathway. Failure to complete these events successfully results in assembly of a defective ECM and may produce skeletal abnormalities. Nanomelia is a lethal genetic mutation of chickens characterized by shortened and malformed limbs. Previous biochemical studies have shown that cultured nanomelic chondrocytes synthesize a truncated aggrecan core protein precursor that disappears with time; however, the protein does not appear to be processed by the Golgi or secreted. The present study investigates the intracellular trafficking of the defective aggrecan precursor using immunofluorescence, immunoelectron microscopy and several inhibitors. Results indicate that nanomelic chondrocytes assemble an ECM that contains type II collagen, but lacks aggrecan. Instead, aggrecan precursor was localized intracellularly, within small cytoplasmic structures corresponding to extensions of the endoplasmic reticulum (ER). At no time were precursor molecules observed in the Golgi. In contrast, normal and nanomelic chondrocytes exhibited no difference in the intracellular or extracellular distribution of type II procollagen. Therefore, retention of the aggrecan precursor appears to be selective. Incubation of chondrocytes at 15°C resulted in the retention and accumulation of product in the ER. After a return to 37°C, translocation of the product to the Golgi was observed for normal, but not for nanomelic, chondrocytes, although the precursors disappeared with time. Ammonium chloride, an inhibitor of lysosomal function, had no effect on protein loss, suggesting that the precursor was removed by a non-lysosomal mechanism, possibly by ER-associated degradation. Based on these studies, we suggest that nanomelic chondrocytes are a useful model for examining cellular trafficking and sorting events and the processes by which abnormal products are targeted for retention or degradation. Further investigations should provide insight into the mechanisms underlying chondrodystrophies and other related diseases.

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