Collagen type I and type V are present in the same fibril in the avian corneal stroma.

The distribution, supramolecular form, and arrangement of collagen types I and V in the chicken embryo corneal stroma were studied using electron microscopy, collagen type-specific monoclonal antibodies, and a preembedding immunogold method. Double-label immunoelectron microscopy with colloidal gold-tagged monoclonal antibodies was used to simultaneously localize collagen type I and type V within the chick corneal stroma. The results definitively demonstrate, for the first time, that both collagens are codistributed within the same fibril. Type I collagen was localized to striated fibrils throughout the corneal stroma homogeneously. Type V collagen could be localized only after pretreatment of the tissue to partially disrupt collagen fibril structure. After such pretreatments the type V collagen was found in regions where fibrils were partially dissociated and not in regions where fibril structure was intact. When pretreated tissues were double labeled with antibodies against types I and V collagen coupled to different size gold particles, the two collagens colocalized in areas where fibril structure was partially disrupted. Antibodies against type IV collagen were used as a control and were nonreactive with fibrils. These results indicate that collagen types I and V are assembled together within single fibrils in the corneal stroma such that the interaction of these collagen types within heterotypic fibrils masks the epitopes on the type V collagen molecule. One consequence of the formation of such heterotypic fibrils may be the regulation of corneal fibril diameter, a condition essential for corneal transparency.

Embryonic avian cornea contains layers of collagen with greater than average stability.

A unique morphological feature of the embryonic avian cornea is the uniformity of its complement of striated collagen fibrils, each of which has a diameter of 25 nm. We have asked whether this apparent morphological uniformity also reflects an inherent uniformity of the structural and physical properties of these fibrils. For this we have examined the in situ thermal stability of the type I collagen within these fibrils. Corneal tissue sections were reacted at progressively higher temperatures with conformation-dependent monoclonal antibodies directed against the triple-helical domain of the type I collagen molecule. These studies show that the cornea contains layers of collagen fibrils with greater than average stability. The two most prominent of these extend uninterrupted across the entire width of the cornea, and then appear to insert into thick bundles of scleral collagen, which in turn appear to insert into the scleral ossicles, a ring of bony plates which circumscribe the sclera of the avian eye. Once formed, the bands may act to stabilize the shape of the cornea or, conversely, to alter it during accommodation.

Location of procollagen in chick corneal and tendon fibroblasts with ferritin-conjugated antibodies.

Three distinct antiprocollagen preparations were characterized and used in immunocytochemical staining of chick embryo corneal and tendon cells. The several ferritin-conjugated antibody preparations permitted similar location of procollagen in the cisternae of the rough endoplasmic reticulum and in Golgi elements in both cell types. The ability to demonstrate and interpret specific ferritin staining was dependent on the extent of membrane breakage in each of those organelles, coupled with adequate retention of cell morphology. Corneal fibroblasts appeared to suffer more extensive intracellular membrane damage under controlled conditions of homogenization than tendon fibroblasts, facilitating the identification of procollagen in Golgi vacuoles of these cells. None of the labeled material appeared to by cytoplasmic in origin since ferritin was observed in the cytoplasm only in the vicinity of Golgi elements that were extensively broken. This study extends previous immunological evidence for the presence of procollagen in the Golgi complex and calls attention to the problems to be encountered in locating the antigen in small Golgi vesicles and lamellae.

Type VIII collagen has a restricted distribution in specialized extracellular matrices.

A pepsin-resistant triple helical domain (chain 50,000 Mr) of type VIII collagen was isolated from bovine corneal Descemet's membrane and used as an immunogen for the production of mAbs. An antibody was selected for biochemical and tissue immunofluorescence studies which reacted both with Descemet's membrane and with type VIII collagen 50,000-Mr polypeptides by competition ELISA and immunoblotting. This antibody exhibited no crossreactivity with collagen types I-VI by competition ELISA. The mAb specifically precipitated a high molecular mass component of type VIII collagen (EC2, of chain 125,000 Mr) from the culture medium of subconfluent bovine corneal endothelial cells metabolically labeled for 24 h. In contrast, confluent cells in the presence of FCS and isotope for 7 d secreted a collagenous component of chain 60,000 Mr that did not react with the anti-type VIII collagen IgG. Type VIII collagen therefore appears to be synthesized as a discontinuous triple helical molecule with a predominant chain 125,000 Mr by subconfluent, proliferating cells in culture. Immunofluorescence studies with the mAb showed that type VIII collagen was deposited as fibrils in the extracellular matrix of corneal endothelial cells. In the fetal calf, type VIII collagen was absent from basement membranes and was found in a limited number of tissues. In addition to the linear staining pattern observed in the Descemet's membrane, type VIII collagen was found in highly fibrillar arrays in the ocular sclera, in the meninges surrounding brain, spinal cord, and optic nerve, and in periosteum and perichondrium. Fine fibrils were evident in the white matter of spinal cord, whereas a more generalized staining was apparent in the matrices of cartilage and bone. Despite attempts to unmask the epitope, type VIII collagen was not found in aorta, kidney, lung, liver, skin, and ligament. We conclude that this unusual collagen is a component of certain specialized extracellular matrices, several of which are derived from the neural crest.

Morphogenesis of the collagenous stroma in the chick cornea.

The embryonic chick corneal epithelium produces a highly structured acellular matrix beneath its basal surface during early development. This matrix, which contains collagen, serves as a morphogenetic template for subsequent stromal development in that the three-dimensional architecture of the adult corneal stroma is initially established, in miniature, in this epithelially derived connective tissue. Examination of the early corneal epithelium and matrix in both the light and electron microscope suggests that self assembly of the matrix may be one of several important factors in the morphogenesis of this early connective tissue.

Monoclonal antibodies against chicken type V collagen: production, specificity, and use for immunocytochemical localization in embryonic cornea and other organs.

Two monoclonal antibodies have been produced against chick type V collagen and shown to be highly specific for separate, conformational dependent determinants within this molecule. When used for immunocytochemical tissue localization, these antibodies show that a major site for the in situ deposition of type V is within the extracellular matrices of many dense connective tissues. In these, however, it is largely in a form unavailable to the antibodies, thus requiring a specific "unmasking" treatment to obtain successful immunocytochemical staining. The specificity of these two IgG antibodies was determined by inhibition ELISA, in which only type V and no other known collagen shows inhibition. In ELISA, mixtures of the two antibodies give an additive binding reaction to the collagen, suggesting that each is against a different antigenic determinant. That both antigenic determinants are conformational dependent, being either in, or closely associated with, the collagen helix is demonstrated by the loss of antibody binding to molecules that have been thermally denatured. The temperature at which this occurs, as assayed by inhibition ELISA, is very similar to that at which the collagen helix melts, as determined by optical rotation. This gives strong additional evidence that the antibodies are directed against the collagen. The antibodies were used for indirect immunofluorescence analyses of cryostat sections of corneas and other organs from 17 to 18-day-old chick embryos. Of all tissues examined only Bowman's membrane gave a strong staining reaction with cryostat sections of unfixed material. Staining in other areas of the cornea and in other tissues was very light or nonexistent. When, however, sections were pretreated with pepsin dissolved in dilute HAc or, surprisingly, with the dilute HAc itself dramatic new staining by the antibodies was observed in most tissues examined. The staining, which was specific for the anti-type V collagen antibodies, was largely confined to extracellular matrices of dense connective tissues. Experiments using protease inhibitors suggested that the "unmasking" did not involve proteolysis. We do not yet know the mechanism of this unmasking; however, one possibility is that the dilute acid causes swelling or conformational changes in a type-V collagen-containing supramolecular structure. Further studies should allow us to determine whether this is the case.

Mammalian eyes and associated tissues contain molecules that are immunologically related to cartilage proteoglycan and link protein.

Monospecific antibodies to bovine nasal cartilage proteoglycan monomer and link protein were used to demonstrate that immunologically related molecules are present in the bovine eye and associated tissues. With immunofluorescence microscopy, reactions for both proteoglycan and link protein were observed in the sclera, the anterior uveal tract, and the endoneurium of the optic nerve of the central nervous system. Antibody to bovine nasal cartilage proteoglycan also reacted with some connective tissue sheaths of rectus muscle and the perineurium of the optic nerve of the central nervous system. Antibody to proteoglycan purified from rat brain cross-reacted with bovine nasal cartilage proteoglycan, indicating structural similarities between these proteoglycans. ELISA studies and crossed immunoelectrophoresis demonstrated that purified dermatan sulphate proteoglycans isolated from bovine sclera did not react with these antibodies but that the antibody to cartilage proteoglycan reacted with other molecules extracted from sclera. Two molecular species resembling bovine nasal link protein in size and reactivity with antibody were also demonstrated in scleral extracts: the larger molecule was more common. Antibody to link protein reacted with the media of arterial vessels demonstrating the localization of arterial link protein described earlier. Tissues that were unstained for either molecule included the connective tissue stroma of the iris, retina, vitreous body, cornea, and the remainder of the uveal tract. These observations clearly demonstrate that tissues other than cartilage contain molecules that are immunologically related to cartilage-derived proteoglycans and link proteins.

Role of prostaglandin E1 and copper in angiogenesis.

The interstitial fluid of MTW9A and Walker carcinomas and their ethanol extract induced strong angiogenic response in the rabbit (New Zealand White) corneal test. The fluid collected in vivo was rich in E-type prostaglandins, and prostaglandin E1 (PGE1) in particular was strongly angiogenic at the lowest dose as compared with the angiogenic responses of prostaglandins E2, I2, and F2 alpha. Neoplastic fibroblasts also induced a strong angiogenic response, but in indomethacin-treated rabbits neovascularization failed to occur. Copper was concentrated in the cornea during PGE1-induced neovascularization, and copper-deficient rabbits were unable to mount an angiogenic response in the corneal test. Ceruloplasmin, the copper carrier of plasma, was found to be angiogenic at high doses. In indomethacin-treated rabbits, however, ceruloplasmin at the same high doses failed to induce angiogenesis. The experiments are interpreted to indicate that angiogenesis is the end result of a sequence of events, two of which are PGE1 production and copper mobilization in the tissue where neovascularization occurs.

Raman spectroscopy of intact feline corneal collagen.

The Raman Spectrum of Collagen is presented from feline corneas which were fresh and intact, heat denatured, and incubated in 2H20. Two bands in the amide I region at approx. 1630 cm-1 and approx. 1660 cm-1 and two bands at ca. 1270 cm-1 and 1247 cm-1 in the amide III region appear in the Raman spectrum of fresh and heat denatured corneal collagen. The two amide III bands have been assigned to amide III vibrations in the polar and non-polar regions of the protein. Only one small amide I band at approx. 1650 cm-1 appears when corneas are treated with 2H2O suggesting that some portion of the Raman peaks in the amide I region for corneas in water is associated with water vibrations. Feline corneal collagen fibrils do not appear to dissociate appreciably upon heating to 70 degrees C. In fact, heated corneas appear structurally similar to corneas aged 30 h at 50 degrees C. We suggest that the swelling induced by heating and aging is predominantly caused by water being absorbed and remaining between the collagen fibrils, causing a slightly more disordered collagen matrix.

The carbohydrate-protein binding region in keratan sulfate from bovine cornea, structure of a major binding region oligosaccharide.

Peptido-keratan sulfate from bovine cornea was degraded by a combination of desulfation, hydrazinolysis, nitrous acid deamination and NaB3H4 reduction. The tetrasaccharide fraction obtained by gel filtration was studied by degradation with specific exoglycosidases and methylation analysis. The existence of two different binding region oligosaccharide structures was established: The first structure contains one terminal fucose, two mannosine residues and N-acetylglucosamine at the reducing end. In the second structure one N-acetylglucosamine is bound to the protein backbone and substituted with branched 3,6-di-O-alpha-mannosyl-beta-mannose. Both terminal alpha-mannosine residues bear keratan sulfate chains in the 2-position.

The hydration of proteoglycans of bovine cornea.

The water sorptive and retentive capacities of three corneal proteoglycans with different keratan sulfate/chondroitin-4-sulfate compositions were investigated. The calcium salt of a predominantly keratan sulfate containing proteoglycan had hydration properties similar to that of calcium keratan sulfate. The proteoglycan containing predominantly calcium chondroitin-4-sulfate side chains sorbed water to a greater extent than pure calcium chondroitin-4-sulfate but its retentive power was somewhat less. The proteoglycan containing about twice as much keratan sulfate as chondroitin-4-sulfate, on a dissaccharidic molar basis and had hydration properties which were closer to the behavior of chondroitin-4-sulfate than keratan sulfate. The results are discussed in terms of structure and polymer interaction in the proteoglycan matrices.

Bovine type I collagen: A study of cross-linking in various mature tissues.

The cyanogen bromide peptides from insoluble and pepsin solubilised type I collagen of bovine bone, dentine, meniscus, tendon, skin and cornea were compared by SDS-polyacrylamide gel electrophoresis. In each case alpha 1CB6 was shown to be the only peptide of molecular weight greater than 10 000 involved in cross-linking. The major helical peptides alpha 1CB3, alpha 1CB8, alpha 1CB7 and alpha 2CB4 were not implicated in cross-linking in any tissue either by end overlap or helix-helix interaction. The C-terminal alpha 2 chain peptide alpha 2CB3,5, which contains a large helical region, was not involved in cross-linking to any large peptides, although a slight increase in molecular weight in all tissues examined did suggest a possible interaction(s) with a very small peptide of molecular weight 4--5000.

Type VI collagen is a major component of the human cornea.

Collagen type VI is shown to be present in the human cornea. This finding is based on comparative peptide mapping relative to type VI collagen isolated from placenta and on immunoblotting using antibodies specific for human type VI collagen. Scanning of polyacrylamide gels indicates that type VI collagen comprises as much as one quarter of the dry weight of the cornea. Indirect immunofluorescence shows this collagen to be distributed throughout the corneal stroma. Thus, type VI collagen must be considered a major component of the extracellular matrix of the human cornea.

Multiple-reaction cycling: a method for enhancement of the immunochemical signal of monoclonal antibodies.

Most current studies using immunochemical and immunohistochemical procedures to detect antigen-antibody complexes employ some type of indirect method. Such procedures afford signal amplification because several marker-conjugate molecules can bind to each primary antibody molecule. We have observed that for monoclonal antibodies an even greater amplification can be afforded simply by performing two (or more) reaction cycles (i.e., primary antibody, secondary antibody-primary antibody, secondary antibody-etc). In the present report, we demonstrate the utility of this method for immunohistochemical (immunofluorescence) and immunochemical (ELISA: enzyme-linked immunosorbent assay) procedures employing well-characterized monoclonal antibodies directed against avian type IV (basement membrane) collagen.

Microfluorometric analysis of protein thiol groups with a coumarinylphenylmaleimide.

Characteristics of thiol adducts formed by the fluorogenic maleimide, N-(4-(7-diethylamino-4-methylcoumarin-3-yl) phenyl) maleimide (CPM), (Sippel: J Histochem Cytochem 29:314, 1981) were determined in solution, on the epithelia in beef cornea paraffin sections, and with an egg white model. Absorption was found maximal at 385-390 nm with a molar absorbancy of not less than 30,000 cm2/mmol, while emission peaked at 465 nm; the spectra and output were hardly affected by hydrolytic opening of the succinimide ring. Fluorescence measured in an epi-illuminating microfluorometer faded rapidly at high magnifications, but the initial output from sections of increasing thickness under a 10 x objective was proportional to the thiol density (concentration x thickness) up to a limit equivalent to an absorbance of nearly 0.5 at 387 nm. The staining included less than 5% nonspecific fluorescence, as determined on duplicate sections blocked by 2,2'-dithiopyridine. Factors affecting the use of CPM for quantitation of both thiols and disulfides are discussed.

Ocular distribution of keratan sulfates during pre- and postnatal development in rabbits.

Twenty-six monoclonal antibodies (MAbs) developed against rabbit corneal proteokeratan sulfate (PKS), were used to evaluate immunohistochemically the ocular distribution of PKS during prenatal and early postnatal development in rabbits. These MAbs were directed against epitopes located in the keratan sulfate (KS) chains of the proteoglycan (SundarRaj et al., 1985). Staining of cryostat sections of the eyes was carried out using an indirect peroxidase-conjugated technique. Only one of the MAbs reacted with the presumptive corneal region at day 13 or 16 of fetal development. By day 20, more MAbs reacted with the corneal stroma. There were distinct differences, however, in the distribution of the epitopes recognized by the various MAbs. A few of them stained only the posterior region of the cornea, whereas others showed a decreasing staining gradient from the posterior to the anterior region. By day 24, all of the MAbs reacted with the corneal stroma, but some reacted also with the limbal region and with the conjunctival stromal matrix. One MAb also reacted with the conjunctival epithelial layer, but only at this stage of development. Conjunctival staining was more intense at day 28 of fetal development and at day 2 postnatally. KS was not detectable in the conjunctiva of adult rabbits with any of the MABs. These results suggest that although KS synthesis starts at very early stages of fetal development, there are progressive changes in its antigenic structure in specific regions of the cornea and conjunctiva during corneal development.

Glycoconjugates on corneal epithelial surface: effect of neuraminidase treatment.

The purpose of this study was to develop a procedure to quantitatively examine corneal epithelial apical cell membrane-associated glycoconjugates. Saccharide moieties on young, mature, and aged corneal epithelial cells were detected and localized in corneas of immature and adult mice by using colloidal gold-labeled lectins and transmission electron microscopy (TEM). In general, dense binding to the corneal epithelial apical surface cell membranes with wheat germ agglutinin (WGA) was seen in the adult, whereas the immature cornea bound less WGA-gold. Neuraminidase digestion decreased binding of the conjugate on epithelial plasma membranes of young and mature cells in adult cornea. Lectin-gold binding was decreased in the immature cornea on mature and aged cells. WGA-gold binding after neuraminidase was elevated on young cells of immature and on aged cells of adult animals. No binding of peanut agglutinin (PNA) or horse gram agglutinin (DBA) to the corneal epithelial surface was seen in animals of either age. After neuraminidase digestion, PNA binding sites were exposed only on the adult corneal surface. These data suggest that a terminal trisaccharide sequence, sialic acid-galactose beta(1----3)-N-acetylgalactosamine, is present at the adult corneal surface but is absent or at undetectable levels at the corneal surface of the immature animal. These data may be of significance in light of the dissimilar pattern of P. aeruginosa recognition and binding to the immature vs adult corneal epithelium.

Oxygen levels beneath the closed eyelid.

The level of oxygen at the anterior corneal surface beneath the closed eyelid is shown to be equivalent to an atmosphere of 7.7% oxygen. This finding is in good agreement with assumptions which have been based on the oxygen level at the palpebral conjunctiva. However, in some instances a significant amount of oxygen is derived not only from the palpebral conjunctiva but also from the atmosphere as a result of an imperfect palpebral aperture seal.

Corneal epithelial cell--derived thymocyte-activating factor (CETAF).

Supernatants of a primary rabbit corneal epithelial cell culture and an established corneal cell line (SIRC) were assayed for their ability to enhance mitogen-induced C3H/HeJ mouse thymocyte proliferation. Significant levels of thymocyte-enhancing activity were detected in supernatants from both primary cultures and SIRC. Maximal levels of activity were found after 48 to 72 hr of culture in serum-free medium with 1 X 10(5) cells/ml. When monolayers of SIRC were disrupted. supernatants of these cultures consistently contained levels of activity higher than those of undisrupted control cultures. When supernatants from SIRC cultures (both serum-free and containing greater than 10% fetal calf serum) were subjected to gel filtration on AcA 54 and Sephacryl S-200, corneal epithelial cell-derived thymocyte-activating factor was eluted as two major peaks, between mol. wt 95,000 and 55,000 and mol. wt. 30,000 and 15,000. These results indicate that corneal epithelial cells, similar to keratinocytes, produce an Interleukin 1-like activity lacking species specificity, which enhances the proliferative capacity of thymocytes. Therefore corneal epithelial cells may interact with the immune system through the production of this cytokine.

Glycogen and DNA content of corneal epithelium: comparison of preparation methods.

Three methods of corneal epithelial preparation for subsequent analysis of epithelial glycogen and DNA content were compared: scraping, lamellar stripping, and in situ freezing. All three methods gave good recovery of these substances. However, there was incomplete recovery of the sample using the in situ freezing method and significant contamination of epithelial samples by stroma in the lamellar stripping method. Thus, scraping is the most reliable method of collecting pure epithelial samples for glycogen or DNA analysis.