Inhibition of scleral proteoglycan synthesis blocks deprivation-induced axial elongation in chicks

A specific inhibitor of proteoglycan synthesis was administered to chicks undergoing the development of form deprivation myopia in order to test the hypothesis that increases in proteoglycan synthesis are responsible for normal and/or deprivation-induced ocular elongation in chicks. Chicks undergoing monocular form deprivation were treated with p-nitrophenyl-beta-D-xylopyranoside (beta-xyloside) via i.p. injection every 8 hr for 5-11 days. Ocular measurements were made at the end of the experiment using high frequency A-scan ultrasound in conjunction with a LabView (v. 5.0) analysis program. Following ultrasound measurements, sclera were isolated and proteoglycans characterized by Sepharose CL-2B and Western blot analyses. Preliminary studies indicated that i.p. administration of beta-xyloside maximally inhibited sulfate incorporation into proteoglycans 8 hr after administration. Beta-xyloside treatment resulted in a significant reduction in the axial length, vitreous chamber depth, and rate of axial elongation of form deprived eyes as compared with form deprived eyes from vehicle treated chicks (P < 0.01, P < 0.05, P < 0.05, respectively). No significant differences were detected in anterior chamber depth, lens thickness, choroid thickness or retina thickness in form deprived eyes of beta-xyloside treated chicks as compared with that of vehicle controls. No significant differences were detected in contralateral non-deprived fellow eyes between beta-xyloside treated and vehicle treated chicks for any ocular measurement. Analysis of proteoglycans indicated that the xyloside treatment resulted in the accumulation of smaller proteoglycans due, in part, to the presence of underglycosylated aggrecan within the scleral matrix. These results indicate that interruption of normal scleral proteoglycan synthesis inhibits form deprivation-induced ocular elongation, supporting the hypothesis that scleral proteoglycan synthesis and accumulation are largely responsible for increases in axial length in form deprived chick eyes.

Identification of choroidal ovotransferrin as a potential ocular growth regulator

PURPOSE

In an effort to identify choroidal factors potentially involved in the regulation of ocular growth, proteins released into culture medium of organ-cultured choroids were compared between control eyes and eyes recovering from form deprivation myopia.

METHODS

The choroids were obtained from the posterior poles of control and recovering chick eyes, and placed into organ culture containing ( 35)S-methionine/(35)S-cysteine. Culture medium was collected after 24 hours and proteins were separated and identified by SDS-polyacrylamide gel electrophoresis (SDS-PAGE), fluorography, immunoprecipitation, western blot analysis and by amino acid sequencing. Choroidal proteins were tested for their effect on scleral proteoglycan synthesis by measuring (35)SO( 4) incorporation into scleral glycosaminoglycans (GAG) in vitro. Choroidal thickness and axial elongation were measured in control and recovering eyes using high frequency A-scan ultrasound.

RESULTS

The synthesis of an 80 kD protein was greatly increased in the choroids of recovering eyes compared with those of control eyes. Amino acid sequencing and immunoprecipitation indicated that the newly synthesized 80 kD protein was ovotransferrin (transferrin, conalbumin). Ovotransferrin release into the culture medium by isolated recovering choroids was associated with a decrease in the rate of axial elongation in recovering eyes. When tested in vitro, ovotransferrin (500 ng/micro) inhibited scleral proteoglycan synthesis in the sclera by 62% in a dose-dependent manner.

CONCLUSIONS

Chick choroids of recovering eyes synthesize and release ovotransferrin during the recovery from form deprivation myopia. Ovotransferrin significantly inhibited proteoglycan synthesis by the sclera, indicating that ovotransferrin may play a role in slowing the rate of vitreous chamber elongation and facilitating the recovery from induced myopia.

Decreased proteoglycan synthesis associated with form deprivation myopia in mature primate eyes

PURPOSE

The rate of proteoglycan synthesis was measured in the scleras of adolescent marmosets that had undergone monocular form deprivation to characterize the scleral extracellular matrix changes associated with the development of myopia in a mature primate.

METHODS

Form deprivation myopia was induced in adolescent marmosets by unilateral lid suture for an average of 108 days. After the lids were reopened, the axial lengths and refractions were measured at intervals for up to 39 weeks. At the end of the study period, sclera were isolated and immediately radiolabeled with 35SO4 in organ culture. Proteoglycan synthesis rates were determined by measurement of 35SO4 incorporation into cetylpyridinium chloride-precipitable glycosaminoglycans after digestion of the scleral samples with proteinase K. Collagen content was determined by measurement of total hydroxyproline in scleral digests. Newly synthesized proteoglycans were separated on a Sepharose CL-4B molecular sieve column and identified by their core proteins by Western blot analyses.

RESULTS

Lid suture resulted in myopia due to a significant increase in vitreous chamber depth. After Sepharose CL-4B chromatography, newly synthesized scleral proteoglycans isolated from normal, form-deprived, and contralateral control eyes, resolved into one major peak that eluted in the position of decorin, a small chondroitin-dermatan sulfate proteoglycan. After digestion of the major peak with chondroitinase ABC, an approximately 45-kDa core protein was detected by Western blot analyses, confirming the presence of decorin. Form deprivation resulted in a significant reduction in the rate of proteoglycan synthesis in the posterior sclera (-43.55%, P < or = 0.001). Proteoglycan synthesis was also significantly reduced in the posterior sclera of form-deprived eyes relative to total collagen content (-36.19%, P < or = 0.01) and was negatively correlated with the rate of vitreous chamber elongation in the deprived eye (r2 = 0.779, P < or = 0.05).

CONCLUSIONS

Significant extracellular matrix remodeling occurs in the posterior sclera of the adolescent primate eye during vitreous chamber elongation and myopia development. The negative correlation between vitreous chamber elongation rates and the synthesis rates of decorin in form-deprived eyes suggests that proteoglycan synthesis within the posterior sclera plays a role in the regulation of ocular size and refraction in the adolescent marmoset.

Proteoglycan composition in the human sclera during growth and aging

PURPOSE

Scleral proteoglycans were characterized from human donor eyes aged 2 months to 94 years to identify age-related changes in the synthesis and/or accumulation of these extracellular matrix components.

METHODS

Newly synthesized proteoglycans (previously radiolabeled with 35SO4) and total accumulated scleral proteoglycans were extracted with 4 M guanidine hydrochloride and separated by molecular sieve chromatography on a Sepharose CL-4B column. The elution positions of newly synthesized and total accumulated proteoglycans were determined by assaying each fraction for radioactivity and glycosaminoglycans, respectively. Regression analyses were performed on the three major proteoglycan peaks to identify age-related changes in scleral proteoglycan composition. Scleral proteoglycans were further purified by anion-exchange chromatography and characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analyses.

RESULTS

Human scleral proteoglycans were apparent as three major peaks after chromatography on Sepharose CL-4B. The two faster eluting peaks contained alternative forms of the cartilage proteoglycan, aggrecan, whereas the third peak contained the small proteoglycans biglycan and decorin. The relative percentage of newly synthesized and total accumulated aggrecan increased approximately two- to sixfold from infancy to 94 years. In contrast, the relative percentage of newly synthesized and total accumulated biglycan and decorin decreased by approximately 25%. Chromatography and Western blot results indicated that the absolute amounts of all three proteoglycans significantly increased in concentration within the sclera from birth to the fourth decade. Beyond the fourth decade, decorin and biglycan decreased in all scleral regions and were present in lowest concentrations by the ninth decade. In contrast, aggrecan, which was present in highest concentration in the posterior sclera, was not significantly reduced with increasing age.

CONCLUSIONS

The age-related changes in scleral proteoglycan composition observed in the present study are likely to contribute to the regional alterations in biomechanical properties of the sclera associated with growth and aging.

Gelatinase A and TIMP-2 expression in the fibrous sclera of myopic and recovering chick eyes

PURPOSE

Myopia, or nearsightedness, is characterized by excessive lengthening of the ocular globe and is associated with extracellular matrix remodeling in the posterior sclera. The activity of gelatinase A, a member of the matrix metalloproteinase family, has been shown to increase in the posterior sclera during the development of induced myopia in several species. In the present study, the distribution and relative expression of gelatinase A and its associated inhibitor, tissue inhibitor of metalloproteinases (TIMP)-2, were measured within the fibrous scleras of experimentally myopic (form-deprived) eyes, control eyes, and eyes recovering from form deprivation to better understand the mechanisms that regulate scleral remodeling and the rate of ocular elongation.

METHODS

Total RNA was extracted from the posterior scleras of form-deprived chick eyes, eyes recovering from deprivation myopia, and paired contralateral control eyes, and subjected to northern blot analysis analyses using cDNA probes to chicken gelatinase A and TIMP-2. The distribution of gelatinase A and TIMP-2 mRNAs was evaluated by in situ hybridization on frozen sections of chick scleras using 33P-labeled RNA probes. Gelatinase A activity within the fibrous scleras of form-deprived eyes and paired contralateral recovering eyes was evaluated by gelatin zymography.

RESULTS

Northern blot analysis indicated that the relative expression of gelatinase A was increased by 128% in deprived eyes (P = 0.009), whereas after 1 day of recovery, levels were decreased by 80% in scleras from recovering eyes (P = 0.005). In contrast, TIMP-2 expression was significantly decreased (-53%, P = 0.027) in the posterior scleras of form-deprived eyes. No significant differences were detected in levels of TIMP-2 expression between recovering eyes and paired control eyes. In situ hybridization indicated that most of the gelatinase A transcripts were present in the fibrous layer of the posterior scleras from form-deprived and recovering eyes.

CONCLUSIONS

Changes in the steady state levels of gelatinase A and TIMP-2 mRNA lead to changes in gelatinase activity within the fibrous sclera and mediate, at least in part, the process of visually regulated ocular growth and scleral remodeling.

Isolated chick sclera shows a circadian rhythm in proteoglycan synthesis perhaps associated with the rhythm in ocular elongation

In the growing chick, ocular elongation is rhythmic, increasing during the day and decreasing at night. Because experimentally induced changes in the rate of ocular elongation are associated with changes in the rate of synthesis of scleral proteoglycans, we asked whether there is a diurnal rhythm in scleral proteoglycan synthesis, whether the rhythm is endogenous, and whether scleras from normal eyes differed from those of faster growing form-deprived eyes. To assess proteoglycan synthesis, we measured the incorporation of labeled sulfate into glycosaminoglycans using two paradigms: (1) punches of sclera were cultured for either 2 or 10 h at various times of day, and (2) punches were cultured in a perifusion system for up to 80 h, and samples of the medium were collected for analysis at 2-h intervals. Synthesis of scleral proteoglycans is higher during the day than during the night. This rhythm persists for at least three cycles in vitro with a period of approximately 24 h. There are no significant differences between rhythms in scleras from normal and form-deprived eyes. Finally, biochemical analyses show the labeled molecule to be similar to aggrecan, the cartilage proteoglycan. We conclude that the synthesis of proteoglycans by scleral chondrocytes is circadian, and we speculate that this rhythm may influence the rhythm in ocular elongation.

Proteoglycan turnover in the sclera of normal and experimentally myopic chick eyes

PURPOSE

The turnover of chick scleral proteoglycans from control and form-vision deprived (myopic) eyes was compared in vivo and in explant cultures to determine whether proteoglycan degradation is altered during the development of myopia and to characterize the mechanism of proteoglycan turnover in the sclera.

METHODS

Seven-day-old chicks were radiolabeled via an intraperitoneal injection of 35SO4, and monocular form deprivation was induced 48 hours later. After 1, 2, and 3 weeks of form deprivation, birds were killed, and the amount of 35SO4-proteoglycans remaining in different scleral regions was measured in control and deprived eyes. Posterior sclera were also radiolabeled in organ culture containing 35SO4, and radiolabeled scleral proteoglycans were chased into unlabeled medium for 0 to 11 days. 35SO4-labeled proteoglycans within the scleral matrix and those released into the medium were characterized by Sepharose CL-2B chromatography and western blot analysis.

RESULTS

The biological half-life of scleral proteoglycans was significantly shorter within the posterior pole of form-deprived eyes (t1/2 = 7.212 days) compared with the same region of control eyes (t1/2 = 9.619 days; P < 0.001), whereas no differences in turnover rates were seen in the anterior sclera or equatorial sclera. When posterior scleral punches were placed in organ culture, 35SO4-labeled proteoglycan turnover rates were similar for control and form-deprived eyes. Chromatographic and western blot analyses indicated that approximately 80% of the total 35SO4 within the posterior sclera is incorporated into the aggrecan. Western blot analyses of aggrecan core protein released into the medium by control and form-deprived scleral punches indicated that the core protein was degraded into a series of smaller fragments of Mr = 102 to 220 kDa. A specific antiserum (anti-FVDIPEN) detected the presence of a 50-kDa C-terminal aggrecan fragment released into the medium, which was generated by the action of the matrix metalloproteinase gelatinase A and/or stromelysin.

CONCLUSIONS

The turnover rate of 35SO4-labeled scleral proteoglycans is vision dependent and is accelerated in the posterior sclera of chick eyes during the development of experimental myopia. The loss of proteoglycans from the scleral matrix involves proteolytic cleavage at various sites along the aggrecan core protein through the action, at least in part, of gelatinase A and/or stromelysin.

Proteoglycans in the human sclera. Evidence for the presence of aggrecan

PURPOSE

The proteoglycans synthesized and accumulated within the adult human sclera (aged 50 to 80 years) were identified by their size, glycosaminoglycan side chains, and core proteins in an effort to characterize the proteoglycan content of the human sclera.

METHODS

Sclerae, unlabeled, or radiolabeled in organ culture with 35SO4 or 3H-proline, were extracted in 4M guanidine-HCl and separated by Sepharose CL-2B and Superose 6 forced-pressure liquid chromatography. Peak fractions, identified by glycosaminoglycan content or radioactivity, were pooled and subjected to G-50 chromatography or sodium dodecyl sulfate-polyacrylamide gel electrophoresis before and after digestion with specific glycosidases. Scleral proteoglycan core proteins were identified in Western blot analysis using specific antisera to decorin, biglycan, and aggrecan. Reverse transcription-polymerase chain reaction analyses were carried out on human scleral fibroblast RNA to confirm the transcription of one scleral proteoglycan. Proteoglycans were localized on sections of scleral tissue using specific antisera.

RESULTS

After chromatography on CL-2B, scleral proteoglycans could be resolved into three major peaks, PG-1, PG-2, and PG-3. The largest scleral proteoglycan, PG-1, contained chondroitin sulfate and keratan sulfate glycosaminoglycan side chains. Results of Western blot analyses indicated that the core protein of PG-1 is the aggrecan core protein, migrating at approximately 350 kDa. Reverse transcription-polymerase chain reaction analyses confirmed that human scleral fibroblasts transcribe aggrecan in vitro and in vivo. PG-2 and PG-3 were identified as biglycan and decorin in Western blot analyses using antibiglycan and antidecorin antibodies, respectively. Immunostaining results indicated that aggrecan, biglycan, and decorin are distributed throughout the thickness of the human sclera.

CONCLUSIONS

The adult human sclera contains three major proteoglycans; aggrecan, biglycan, and decorin. It is likely that these proteoglycans contribute to the structural properties of the sclera and that the ratios of these proteoglycans will change with age, specific region, and condition of the sclera.

Regionalized growth patterns of young chicken corneas

PURPOSE

Extracellular matrix (ECM) accumulation, synthesis, and degradation were compared in central and peripheral corneal regions of 3-week-old chicks to identify regional differences within the cornea.

METHODS

For collagen accumulation experiments, corneas were isolated at the scleral junction, the epithelium was removed, and the central corneal region was isolated from the peripheral region with a 3 mm trephine. After corneal stromas were digested with proteinase K, aliquots were used to estimate total collagen from hydroxyproline and cell number from DNA measurements. For biosynthesis experiments, corneas were placed in organ culture containing either in 3H-thymidine, 3H-proline, or 35SO4. After radiolabeling, the epithelium was removed, central and peripheral regions were isolated, and each corneal sample was analyzed for incorporated radioactivity. Gelatinolytic species present in each corneal region were compared qualitatively by gelatin zymography.

RESULTS

Measurement of hydroxyproline content indicated that the central corneal stroma contained significantly more collagen per DNA than the peripheral corneal region (+40%, P = 0.013) and incorporated significantly higher levels of 3H-thymidine/ng DNA (+92%, P < 0.001), 3H-proline/ng DNA (+980%, P = 0.004), and 35SO4/ng DNA (+650%, P = 0.01) than the peripheral corneal region. Results of gelatin zymography indicated that the central cornea contained the proenzyme form of gelatinase A, whereas the peripheral cornea contained both the proenzyme form and the active form of gelatinase A.

CONCLUSIONS

Fibroblasts located in the central cornea have a significantly higher rate of proliferation and ECM production than those of the peripheral cornea, whereas the presence of active gelatinase only in the peripheral cornea suggests higher gelatinolytic activity and ECM turnover in the periphery.

Increased latent gelatinase activity in the sclera of visually deprived chicks

PURPOSE

Gelatinase activity was measured in the normal chick sclera and in sclera of form-deprived (myopic) eyes to assess the role of this metalloproteinase in ocular elongation associated with experimental myopia.

METHODS

Gelatinases were extracted from anterior and posterior regions of normal chick sclera and sclera from eyes that had been form-vision deprived for 11 days. Gelatinase activity in the extracts was determined by measuring the digestion of 3H-gelatin after incubation with the extracts in the absence or presence of 1 mM aminophenylmercuric acetate (APMA) to activate latent gelatinases. Scleral gelatinases were characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis gelatin zymography and immunoprecipitation analyses.

RESULTS

No significant differences were detected in gelatinase activity between normal and deprived posterior sclera in the absence of APMA. However, when scleral extracts were incubated with APMA, extracts from the posterior sclera of deprived eyes contained significantly more gelatinase activity than paired controls (+127%, P = 0.0105). In contrast, no differences in active or latent gelatinase activity were detected in extracts from the anterior sclera. Removal of tissue inhibitors of metalloproteinases (TIMP) from control scleral extracts by reduction and alkylation resulted in a 222% increase in gelatinolytic activity after APMA-activation (P < or = 0.001), whereas similar treatment of deprived scleral extracts resulted in only a 76% increase in gelatinolytic activity (P < or = 0.001). A 65/58-kd doublet was the major gelatinolytic species from control and deprived posterior sclera that represent the proenzyme and active forms of the 72-kd gelatinase (MMP-2).

CONCLUSIONS

These data indicate that visual deprivation is associated with an increased amount of the 72-kd progelatinase and a decreased amount of TIMP within the posterior sclera. Therefore, an imbalance between the levels of 72-kd progelatinase and its inhibitor may play a role in the remodeling processes of the posterior sclera during the development of form-deprivation myopia.

Reduced extracellular matrix in mammalian sclera with induced myopia

The purpose of this study was to learn whether visual form deprivation, which produces myopia in the deprived eye, alters the scleral extracellular matrix in tree shrew, a mammal closely related to primates. Axial myopia was induced in 10 tree shrews by monocular deprivation imposed with a translucent diffuser. The other eye in each animal was an untreated control. After 21 days of deprivation the refractive state and axial component dimensions were measured and the eyes were assayed for levels of DNA, hydroxyproline, and sulfated glycosaminoglycans (GAGs) in samples of the sclera and the cornea. In comparison to the open control eye, the deprived eyes became myopic and elongated. In the sclera, DNA levels were not significantly changed from the control eye. Sulfated GAG levels were significantly lower in the deprived eyes, as compared to the control eyes, at the posterior pole (-15.6%), at the nasal equatorial region (-18.1%), and in the rest of the sclera (-11.6%). The hydroxyproline level was significantly lower only at the posterior pole (-11.8%). Levels of sulfated GAGs were significantly reduced relative to DNA and relative to hydroxyproline in the total sclera. No significant changes were found in the cornea. The lower level of sulfated GAGs throughout the sclera of the deprived eyes, as compared with the control eyes, suggests that the deprived sclera contained less proteoglycan, or that the proteoglycans were less glycosylated or less sulfated. In contrast, the regional reduction of hydroxyproline suggests that collagen accumulation was specifically reduced only at the posterior pole of deprived eyes. These results suggest that form deprivation slows or reverses the normal process of extracellular matrix accumulation in the sclera of this mammal. This may allow the sclera to be more distensible, permitting the vitreous chamber elongation and resultant myopia.

Regional proteoglycan synthesis in the sclera of experimentally myopic chicks

Proteoglycan distribution and synthesis were compared in the sclera of normal and 10-day-form-vision-deprived (myopic) chick eyes using immunocytochemical, biochemical and autoradiographic techniques. Immunostaining with specific antibodies indicated that decorin is present in both the fibrous and cartilaginous layers of chick sclera, while aggrecan localizes only to the cartilaginous layer. For biochemical analyses of proteoglycan synthesis, sclera were isolated from control and 10-day-form-vision-deprived eyes and radiolabeled in organ culture with 35SO4. Proteoglycan synthesis was significantly increased only within a 6.5-mm-diameter button from the posterior pole of deprived eyes (+113%, P = 0.04), while no significant differences were detected in anterior and equatorial regions of control and deprived eyes. Chromatographic analyses of newly synthesized proteoglycans indicated that form-deprivation stimulates the synthesis of a large chondroitin/keratan sulfate proteoglycan (+77.47%), eluting at the position of aggrecan, as well as smaller chondroitin sulfate and keratan sulfate proteoglycans (+91.05%), which coelute with decorin. Autoradiographic analysis of incorporated sulfate indicated that the increase in proteoglycan synthesis observed in the posterior pole of deprived eyes occurs only in the cartilaginous scleral layer. The distribution of incorporated 35SO4, present over the cartilaginous layer of deprived sclera indicates that proteoglycan synthesis is lowest in scleral cartilage adjacent to the choroid and higher in interstitial regions of posterior cartilaginous sclera as well as in regions near the outer fibrous perichondrium. These results suggest that form-deprivation induced scleral growth in chicks can be attributed to growth and differentiation of scleral cartilage in the posterior pole.

Visual deprivation upregulates extracellular matrix synthesis by chick scleral chondrocytes

PURPOSE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Regulation of corneal collagen fibrillogenesis in vitro by corneal proteoglycan (lumican and decorin) core proteins

Corneal transparency is dependent on the size and arrangement of collagen fibrils within the corneal stroma. The corneal stroma is composed primarily of collagen type 1 fibrils and two proteoglycans: one with chondroitin/dermatan sulfate side-chains (decorin) and one with keratan sulfate side-chains (lumican). We investigated the effects of the corneal proteoglycans on corneal collagen fibrillogenesis, utilizing an in vitro assay for fibril formation. Collagen was extracted from bovine corneal stromas with 0.1 M acetic acid and monomers purified by NaCl precipitation. Decorin and lumican were extracted from bovine corneal stroma with either 0.7 M NaCl or 4 M guanidine HCl and purified by DEAE and Sepharose CL-4B chromatography. Decorin and lumican from both extracts inhibited the rate of collagen fibrillogenesis and the development of turbidity in fibrillogenesis samples. Furthermore, the core proteins of decorin and lumican were shown to be as effective as the intact proteoglycans in inhibiting fibrillogenesis. The decorin core protein isolated from the 0.7 M NaCl extract was determined to be a 20 kDa fragment which lacks the C-terminal half of the core protein. This fragment was approximately 1/36 as effective in inhibiting fibrillogenesis as intact decorin isolated from guanidine extracts. This suggests that the C-terminal half of the decorin core plays an important role in the interaction of this proteoglycan with collagen. Lumican extracted with 0.7 M NaCl was slightly smaller and was only one-sixth as effective in inhibiting collagen fibril formation as 4 M guanidine extracted lumican. Furthermore reduction and alkylation of lumican core protein abolished the inhibitory activity of the core protein on collagen fibrillogenesis. Electron microscopic examination indicated that fibrils formed in the presence of lumican and lumican core protein were significantly thinner than fibrils formed in the absence of proteoglycans. The results of these studies indicate that in addition to decorin, lumican retards corneal collagen fibrillogenesis and results in the formation of collagen fibrils which are significantly thinner than those formed in the absence of any proteoglycan. The inhibitory activity of lumican or decorin on collagen fibrillogenesis resides in he core proteins of these proteoglycans, not the glycosaminoglycan side chains, and that interaction of the lumican core protein with collagen appears to be dependent on the presence of disulfide bridges within the protein core.

Proteoglycan synthesis by scleral chondrocytes is modulated by a vision dependent mechanism

Proteoglycan synthesis was measured in chick sclera at the onset of form-deprivation myopia, as well as in the period immediately following removal of the occluder. Two day-old chicks were monocularly form vision deprived for periods from one to ten days and proteoglycan synthesis was determined after placing posterior scleral buttons in organ culture and measuring 35SO4 incorporation into glycosaminoglycans. Following 24 hrs of form-deprivation, proteoglycan synthesis was 33% higher in myopic eyes as compared with paired control eyes. The rate of proteoglycan synthesis further increased to levels 83% higher than controls after four days of form-deprivation and remained elevated throughout the ten day period of deprivation. Removal of the occluder after 10 days of form-deprivation resulted in a rapid drop in the rate of proteoglycan synthesis to control levels within 24 hrs. Proteoglycan synthesis was also measured in scleral chondrocytes isolated from control and myopic eyes after 10 days of form-deprivation. Proteoglycan synthesis by chondrocytes from myopic eyes did not return to control levels until 48 hrs after plating. Since the rate of proteoglycan synthesis returns to control levels more quickly during the recovery period ex vivo than when scleral chondrocytes from myopic eyes are placed in cell culture, we suggest that a mechanism is present within the eye which rapidly lowers the rate of proteoglycan synthesis in response to form vision.

Biosynthesis of stromal matrix proteoglycans and basement membrane components by human corneal fibroblasts

The proteoglycans produced by intact human corneas and corneal cells in culture were compared by characterizing the biosynthetically radiolabeled proteoglycans and by using antibodies to detect their core proteins. Organ cultures of corneas primarily produce a keratan sulfate proteoglycan (KSPG) and a chondroitin and dermatan sulfate proteoglycan (decorin). Immunostaining with antibodies specific for the core proteins of KSPG and decorin showed that these proteoglycans are localized to the corneal stroma. The stroma also contained trace amounts of matrix that stained with antibodies to basement membrane heparan sulfate proteoglycan (perlecan) and laminin. Corneal fibroblasts in culture produced decorin, but the synthesis of KSPG appeared to be blocked at the level of core protein synthesis. Corneal fibroblasts in culture, however, produced perlecan in greater amounts than they did in organ cultures, and they synthesized both perlecan and laminin in greater amounts than did corneal epithelial cells in culture. These results indicate that the synthesis of proteoglycans by human corneal fibroblasts in culture is altered, resulting in increased production of basement membrane-associated proteoglycans and decreased synthesis of corneal stroma-associated proteoglycans.

Increased aggrecan (cartilage proteoglycan) production in the sclera of myopic chicks

A previously characterized chick model of myopia was used to evaluate biochemical changes in the sclera which are associated with ocular enlargement and myopia. Chicks were monocularly occluded for 10 days and the DNA, hydroxyproline, and glycosaminoglycan contents of the sclera were compared between the normal and the myopic eyes. No significant differences could be detected in total DNA or hydroxyproline content. There was, however, a 34% increase in glycosaminoglycans and a 20.7% decrease in cell density within the posterior sclera of myopic eyes. The biosynthesis of scleral proteoglycans was determined by measuring 35SO4 incorporation in the sclera of chicks visually occluded for 5, 10, and 15 days. No differences could be detected in 35SO4 incorporation into the cornea or the anterior sclera. However, 35SO4 incorporation was significantly increased in the posterior sclera of myopic eyes by 64% at Day 5, 39% at Day 10, and 49% at Day 15. When fractionated on Sepharose CL-4B, scleral proteoglycans were resolved into two peaks which were identified by Western blot analysis as aggrecan (cartilage proteoglycan) and decorin. Furthermore, Western blot and dot blot analyses indicated that significantly more aggrecan core protein was present in the sclera of myopic eyes compared with equivalent amounts of sclera from control eyes. These results indicate that increased synthesis and accumulation of aggrecan, which increases the volume of extracellular matrix in the posterior sclera, are responsible for the ocular enlargement observed in this model of myopia.

Anionic site and immunogold quantitation of heparan sulfate proteoglycans in glomerular basement membranes of puromycin aminonucleoside nephrotic rats

Renal glomerular basement membranes (GBMs) exhibit a charge-selective barrier, consisting of heparan sulfate proteoglycan (HSPG) that restricts the passage of anionic molecules into the urine. Previous efforts to localize the HSPG core protein within various layers of the GBM have been contradictory. Furthermore, attempts to correlate proteinuria in several disease states with a decrease in anionic sites of HSPG core protein have yielded conflicting results. When antibodies to HSPG from the EHS tumor matrix [anti-(EHS) HSPG] and GBMs [anti-(GBM) HSPG] were used together with immunogold to label renal tissues from puromycin aminonucleoside nephrotic (PAN) rats, immunolabeling results indicated that a portion of the protein core recognized by anti-(EHS) HSPG was significantly reduced, while immunolabeling with anti-(GBM) HSPG was only slightly reduced in early PAN. Anionic sites (stained with the cationic probe, polyethyleneimine) within the lamina rara externa of the GBM remained unaltered throughout the course of PAN.

Electron microscopic histochemical and immunochemical analyses of heparan sulfate proteoglycan distribution in renal glomerular basement membranes

Renal glomerular basement membranes (GBMs) exhibit a charge-selective barrier, comprised of anionic sites, that restrict the passage of anionic molecules into the urine. These sites are located primarily in the laminae rarae interna (LRI) and externa (LRE) of the GBM and consist of heparan sulfate proteoglycan (HSPG). Previous efforts to localize HSPG core protein within various layers of the GBM have been contradictory. In the present study when rat renal cortex blocks were treated by immersion with the cationic probe, polyethyleneimine (PEI), GBMs exhibited anionic sites concentrated primarily in the LRE and more irregularly within the LRI and lamina densa. All sites were heparitinase sensitive indicating that PEI positive sites represent negatively charged groups associated with heparan sulfate. In order to gain information on the distribution of the HSPG protein core, antibodies to HSPG from the EHS tumor matrix [anti-(EHS) HSPG] and GBMs [anti-(GBM) HSPG] were used together with immunogold to label thin sections of Lowicryl embedded kidney cortex. Depending upon the antisera used, markedly different distributions of HSPG were obtained. Immunolabelling with anti-(GBM) HSPG suggested a distribution of HSPG which was restricted to the laminae rarae, whereas labelling with anti-(EHS) HSPG indicated that the protein core penetrates through all layers of the GBM.