Visual deprivation upregulates extracellular matrix synthesis by chick scleral chondrocytes

To characterize the cellular events responsible for the exaggerated ocular growth associated with experimental myopia in chicks, the accumulation and synthesis of proteoglycans and collagen were measured in the posterior sclera of control and form vision-deprived chick eyes.

Buttons (10 mm) from the posterior sclera of control and deprived eyes were used for biochemical measurements of glycosaminoglycans and hydroxyproline to estimate proteoglycan and collagen accumulation, respectively. The synthesis of proteoglycan, collagen, total protein, and RNA were measured in cultures of scleral chondrocytes isolated from posterior scleral buttons of control and deprived eyes by measuring the specific incorporation of 35SO4, [3H]proline, [35S]methionine, and [5-3H]uridine, respectively. The relative rate of aggrecan precursor protein synthesis was measured in cultures of control and deprived chondrocytes using immunoprecipitation assays.

Form deprivation resulted in increased accumulation of proteoglycans but not collagen within the posterior sclera. In contrast, chondrocytes isolated from the posterior sclera of form-deprived eyes maintained elevated rates compared with controls of proteoglycan synthesis (+143%) and collagen synthesis (155%), as well as total protein synthesis (115%) and total RNA synthesis (44%). Because total protein synthesis was higher in cultures of deprived chondrocytes, the rate of aggrecan precursor protein synthesis, relative to total protein synthesis, was similar for both populations of cells. Pretreatment of scleral chondrocytes with actinomycin D, an inhibitor of RNA synthesis, resulted in a 112% increase in the rate of proteoglycan synthesis by control chondrocytes, but had no significant effect on the rate of proteoglycan synthesis by chondrocytes isolated from form-deprived eyes.

Because proteoglycans accumulate within the posterior sclera of deprived eyes to a greater extent than collagen, yet form deprivation stimulates the synthesis of collagen and total protein as well as proteoglycans, these data suggest that collagen, and perhaps other scleral components, are selectively remodeled within the posterior sclera during the process of ocular elongation. Furthermore, experiments with actinomycin D suggest that the general upregulation observed in form-deprived chondrocytes may be due to the absence of a inhibitor normally present under conditions of form vision.