Developmental acquisition of basement membrane heterogeneity: type IV collagen in the avian lens capsule

Elasticity of the porcine lens capsule as measured by osmotic swelling

As an alternative to purely mechanical methods, optical tracking of passive osmotic swelling was used to assess mechanical properties of the porcine lens capsule. A simple model was developed accounting for the permeability of the lens fiber cells and capsule to water, the concentration of fixed charges in the fiber cells, and the capsule's resistance to the swelling of fiber cells. Fitting the model solution to experimental data provided an estimate of the elastic modulus of the lens capsule under the assumption of linear isotropic elasticity. The calculated elastic modulus at a fixed charge density of 20 mol m(-3) was 2.0+/-0.5 MPa (mean+/-95% confidence interval; n=15) for 0.1% saline solution, 0.64+/-0.3 MPa (n=10) for 0.2% saline solution, and 0.28+/-0.5 MPa (n=6) for 0.5% saline solution. These values are comparable to previously reported moduli of elasticity for the porcine lens capsule at small strains (<10%), and the slight increase with hypotonicity is consistent with the nonlinear mechanical behavior of the lens capsule. Although limited by being a single measurement on a heterogeneous tissue, osmotic swelling provides a quantitative assessment of the stiffness of the lens capsule without requiring dissection or manipulation of the lens. Thus, the new method could be useful for small animal models.

Integrins in lens development and disease

Integrins are the major cell surface receptors for proteins in the extracellular matrix. These receptors form major cell signaling centers that are bidirectional, communicating messages between the cell and its environment. They are a large receptor family, with members well-known to regulate cellular processes essential to both development and disease. In this review we examine the literature regarding integrins in the lens. Here we cover integrin function in lens cell differentiation, in the development of the lens and in protection of the lens epithelial cell phenotype. In addition, we analyze the role of integrins in the progression of lens fibrotic diseases, focusing particularly on integrin regulation of TGFbeta signaling pathways in posterior capsule opacification (PCO) and anterior subcapsular cataract (ASC).

Extracellular matrix and integrin signaling in lens development and cataract

During development of the vertebrate lens there are dynamic interactions between the extracellular matrix (ECM) of the lens capsule and lens cells. Disruption of the ECM causes perturbation of lens development and cataract. Similarly, changes in cell signaling can result in abnormal ECM and cataract. Integrins are key mediators of ECM signals and recent studies have documented distinct repertoires of integrin expression during lens development, and in anterior subcapsular cataract (ASC) and posterior caspsule opacification (PCO). Increasingly, studies are being directed to investigating the signaling pathways that integrins modulate and have identified Src, focal adhesion kinase (FAK) and integrin-linked kinase (ILK) as downstream kinases that mediate proliferation, differentiation and morphological changes in the lens during development and cataract formation.

Characterization of new d-β-aspartate-containing proteins in a lens-derived cell line

Although proteins are generally composed of l-alpha-amino acids, biologically uncommon D-beta-aspartic acid (Asp)-containing proteins have been reported in various tissues from elderly individuals. Our previous study indicated that the N/N1003A cell line, derived from rabbit lens, includes D-beta-Asp-containing proteins of approximately 50 kDa by Western blot analysis of a 2D-gel using a polyclonal antibody that is highly specific for D-beta-Asp-containing proteins. In this study, we identified the D-beta-Asp-containing proteins by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and the Mascot online database searching algorithm. The results indicate that one of these 50 kDa proteins is an enolase showing homology with tau-crystallin. Other D-beta-Asp-containing proteins, which we have recently discovered include lamin A/C, cytoplasmic NADP+-dependent isocitrate dehydrogenase, fructose-bisphosphate aldolase A, aldose reductase, L-lactate dehydrogenase A or calponin H2, phosphoglycerate mutase 1, phosphatidylethanolamine-binding protein, alpha-B-crystallin, and peptidyl-prolyl cis-trans isomerase A (PPlase).

Alterations in the lens capsule contribute to cataractogenesis in SPARC-null mice

The lens capsule, which is also called the lens basement membrane, is a specialized extracellular matrix produced anteriorly by the lens epithelium and posteriorly by newly differentiated fiber cells. SPARC (secreted protein,acidic and rich in cysteine) is a matricellular glycoprotein that regulates cell-cell and cell-matrix interactions, cellular proliferation and differentiation, and the expression of genes encoding extracellular matrix components. SPARC-null mice exhibit lens opacity 1 month after birth and mature cataract and capsular rupture at 5-7 months. In this study, we report disruption of the structural integrity of the lens capsule in mice lacking SPARC. The major structural protein of basement membrane, collagen type IV, in the lens capsule was substantially altered in the absence of SPARC. The lens cells immediately beneath the capsule showed aberrant morphology, with numerous protrusions into the lens basement membrane. SPARC-null lenses at 1 month of age exhibited an increased penetration of dye or radioactive tracer through the capsule, as well as a higher content of water than their wild-type counterparts. Moreover, SPARC-null fibers exhibited swelling as early as 1 month of age; by 3 months, all the fiber cells appeared swollen to a marked degree. By contrast, the absence of SPARC had no apparent morphological effect on the early stages of lens formation, cell proliferation or fiber cell differentiation. Degradation of crystallins and MIP 26, or changes in the levels of these proteins, were not detected. These results underscore the importance of the capsular extracellular matrix in the maintenance of lens transparency and indicate that SPARC participates in the synthesis, assembly and/or stabilization of the lens basement membrane.

alpha6 Integrin is regulated with lens cell differentiation by linkage to the cytoskeleton and isoform switching

The developing chicken embryo lens provides a unique model for examining the relationship between alpha6 integrin expression and cell differentiation, since multiple stages of differentiation are expressed concurrently at one stage of development. We demonstrate that alpha6 integrin is likely to mediate the inductive effects of laminin on lens differentiation as well as to function in a matrix-independent manner along the cell-cell interfaces of the differentiating cortical lens fiber cells. Both alpha6 isoform expression and its linkage to the cytoskeleton were regulated in a differentiation-specific manner. The association of alpha6 integrin with the Triton-insoluble cytoskeleton increased as the lens cells differentiated, reaching its highest levels in the cortical fiber region where the lens fiber cells are formed. In this region of the lens alpha6 integrin was uniquely localized along the cell-cell borders of the differentiating fiber cells, similar to beta1. alpha6beta4, the primary transmembrane protein of hemidesmosomes, is also expressed in the lens, but in the absence of hemidesmosomes. Differential expression of alpha6A and alpha6B isoforms with lens cell differentiation was seen at both the mRNA and the protein levels. RT-PCR studies demonstrated that alpha6B was the predominant isoform expressed both early in development, embryonic day 4, and in the epithelial regions of the day 10 embryonic lens. Isoform switching, with alpha6A now the predominant isoform, occurred in the fiber cell zones. Immunoprecipitation studies showed that alpha6B, which is characteristic of undifferentiated cells, was expressed by the lens epithelial cells but was dramatically reduced in the lens fiber zones. Expression of alpha6B began to drop as the cells initiated their differentiation and then dropped precipitously in the cortical fiber zone. In contrast, expression of the alpha6A isoform remained high until the cells became terminally differentiated. alpha6A was the predominant isoform expressed in the cortical fiber region. The down-regulation of alpha6B relative to alpha6A provides a developmental switch in the process of lens fiber cell differentiation.

TGF-beta elicits fibronectin secretion and proliferation in cultured chick lens epithelial cells

PURPOSE

To determine if the cataract forming influence of TGF-beta on lens cells is due to its effects on the ECM.

METHODS

Primary cultures of chick lens annular pad cells were exposed to TGF-beta and various exogenously supplied components of the lens capsule. Proliferative response were measured through tritiated thymidine incorporation into DNA. Cell spreading accompanying increased matrix interactions and growth was monitored with phase contrast microscopy. ECM proteins were detected in culture media and as deposited matrices with Western blotting and silver staining. TGF-beta receptors were identified with Western blotting.

RESULTS

Chick lens cells were shown to express type I and II TGF-beta receptors. TGF-beta stimulated cell growth and ECM production particularly with regard to fibronectin. Fibronectin was secreted into the culture medium and deposited onto plastic substrates. Plating cells on ECM components found in the lens capsule further increased their growth in response to TGF-beta.

CONCLUSIONS

These results indicate that TGF-beta may have a normal function in the lens regulating capsular protein production. The potent stimulation of lens cell growth by TGF-beta may be due to mis-regulated production of lens capsular proteins not normally found in great abundance.

Integrins and development: how might these receptors regulate differentiation of the lens

Integrins transduce both internal signals and signals from the matrix. These interactions between integrins, their extracellular matrix ligands, and their cytoskeletal partners play an important role in the regulation of cellular differentiation. We have shown them to be important in lens cell differentiation. In the lens capsule there is a compartmentalization of matrix components with fibronectin, primarily localized to the anterior capsule, and tenascin in the posterior capsule. Integrins are developmentally regulated in the lens. alpha 5 beta 1 integrin, like fibronectin, is primarily associated with the lens epithelial cells, where together they are likely to be important in regulation of adhesion and proliferation. alpha 6A beta 1, the integrin laminin receptor, is expressed at its highest levels in the equatorial epithelium and the peripheral fiber cells, both migratory populations. Because laminin is uniformly distributed in the lens capsule, such changes in alpha 6A integrin expression are likely critical to the cell's ability to regulate its response to laminin in the matrix. The organization of cytoskeletal molecules associated with the integrin cytoplasmic face also changes with development. In the epithelial regions of the lens, where the initiation of lens cell differentiation occurs, expression of the cytoskeletal proteins involved in cell-substrate interactions, talin, alpha-actinin, and the signaling proteins, are high. In the fiber cell region of the lens, where the cells establish stable cell-cell contacts, vinculin predominates and becomes highly associated with the cytoskeletal fraction. The role of integrins in lens development is not only regulated by changes in the expression of different integrin receptors but is also closely correlated with the expression and organization of the molecules with which they associate.

The expression of the genes for entactin, laminin A, laminin B1 and laminin B2 in murine lens morphogenesis and eye development

The temporal expression of the genes for the excellular matrix proteins entactin and the A, B1, and B2 chains of laminin was examined in the eye of the developing mouse embryo by in situ hybridization of their messenger RNAs. Entactin messenger RNA was found in abundance in specific cells. In the 25 somite embryo entactin message was synthesized by mesenchymal cells and, at later stages, by hyalocytes and lens cells in addition. The message was not detectable in corneal epithelium at embryonic stages E15 and E18.5 and at birth but was present in adjacent stromal cells. At the 28 and 38 somite stages, before pigment granules interfered with the detection of silver grains, no entactin message was detected in pigmented epithelial cells, in contrast to the messages for laminin B1 and B2. Entactin was not found in the neural epithelium at any time during development. The distribution of the laminin B1, B2 and A chain messenger RNAs was distinctly different from that of entactin. In particular, during the early stages of development B1 and B2 messages were synthesized by ectodermal, lens, corneal, pigment epithelial and hyaloid cells. In the older embryos cells in the ganglion layer of the retina synthesized B1 and B2 messages but undetectable amounts of entactin or the A chain messages. In general the A chain message was in lower abundance throughout development. The distribution of laminin and entactin messages suggested that the extracellular matrices, which contained both proteins, can be derived either from a single cell type or from the contributions of multiple cell types. The data demonstrate the complexity of extracellular matrix synthesis and assembly in the diverse structures of the developing eye where the temporal expression of specific molecules are tailored to the specific developmental requirements of particular structures.

A novel chain of basement membrane-associated collagen as revealed by biochemical and immunohistochemical characterizations of the epitope recognized by a monoclonal antibody against human placenta basement membrane collagen

Biochemical and immunohistochemical characterizations of the epitope recognized by a monoclonal antibody, JK-132, originally produced against human type IV collagen showed that it was distinct from the previously reported monoclonal antibody, JK-199 (Kino et al, J Biochem 1988, 103:829-835). The bound fraction of a crude pepsin extract of human placenta on JK-132 antibody-coupled resin showed close similarity to type IV collagen in a triple-helical conformation in terms of the amino acid composition and circular dichroism spectrum. Sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis of the fraction showed six peptide bands with molecular weights of 50,000 or below, both before and after reduction. Four of the peptides reacted with JK-132 on immunoelectroblotting, but none reacted with JK-199. JK-132 reacted with two additional bands with molecular weights of 100,000 and 120,000, which were not visible on direct staining with Coomassie Brilliant Blue R-250. Two peptides (molecular weights 40,000 and 15,000) bound on a JK-199 antibody affinity column were sequenced, and both contained the same amino-terminal sequences as alpha 1(IV) chain. Conversely the sequences of three of the peptides (molecular weights 50,000, 32,000, and 23,000) eluted from a JK-132 antibody affinity column did not match either the alpha 1(IV) or the alpha 2(IV) sequence reported. Immunohistochemically, JK-132 reacted strongly with basement membranes of blood capillaries in skeletal muscle tissues but not with the basement membranes of muscle fibers in frozen sections of periodate-lysine-paraformaldehyde-fixed tissue, suggesting heterogeneity or tissue specificity of basement membrane collagen. By immunoelectron microscopy, the reaction products were found on the basal laminae of endothelium and of smooth muscle cells around blood vessels. These findings suggest the presence of a new collagen chain associated with basal laminae.

Analysis of lens protein synthesis in a cataractous mutant mouse: the Cat Fraser

The cataract produced by the dominant Cat Fraser gene in mouse is associated with quantitative changes in lens proteins (crystallin) and with capsule abnormalities. We have analyzed and compared the protein synthesis in control and mutant lenses using [3H]leucine and [3H]proline incorporation. The specific activities of free [3H]leucine in the intracellular pools of the two mouse strains were identical, while the incorporation of both labelled amino acids in proteins was largely increased in Cat Fraser lens. These data indicate that the higher labelling of Cat Fraser lens proteins reflects a true change in the cellular synthesis activity by Cat Fraser lens cells. Despite the enhanced type IV collagen synthesis by Cat Fraser epithelial cells, the amount of type IV collagen in Cat Fraser capsule is lower than in control. This altered type-IV collagen metabolism may disturb the structure of Cat Fraser capsule which becomes thicker.

Influence of epithelial-mesenchymal interaction on the viability of facial mesenchyme. II: Synthesis of basement-membrane components during tissue recombination

The presence of basement-membrane components during tissue separation procedures was determined employing monoclonal antibodies to laminin and type IV collagen. In addition, the reconstitution of basement-membrane components and the formation of the basement-membrane were examined in isolated epithelium and mesenchyme and in tissue recombination. Epithelium and mesenchyme of maxillary processes of chick embryos were separated by a variety of protocols, including those employed in a prior study (Saber et al: Anat. Rec. 225:56-66, 1989). Results indicated that the protocol previously employed did not remove basement-membrane components after enzymatic tissue separation. A revised protocol in which the basement-membrane components (i.e., laminin and type IV collagen) were removed from isolated tissues prior to recombination revealed that a developmental compartment and a gradient of cell viability, comparable in size and dimensions to that observed in the study of Saber et al. (ibid.) was present in the mesenchyme of recombined explants. Type IV collagen and laminin, therefore, do not appear to be required initially during tissue recombination in order for subsequent growth-sustaining effects to be expressed. Additional studies revealed, however, that synthesis of basement-membrane components occurred not only in isolated tissues but was altered markedly by tissue recombination. Culture of isolated tissues demonstrated induction of laminin synthesis in separated epithelium by 24 hours and induction of collagen synthesis in isolated mesenchyme by 24 hours. Recombination of epithelium and mesenchyme, however, resulted in rapid induction of laminin synthesis within 1 hour. Recombination of epithelium and mesenchyme after 24 hours resulted in the presence of laminin not only in epithelium but in mesenchyme as well. Both tissues were required for basement-membrane formation which appeared to be fully reconstituted by 24 hours in culture. These observations indicate that recombination in culture alters the pattern of synthetic activity of these basement-membrane components. These can be characterized as "early" (temporal) and "late" spatial) responses by the recombined tissues.

The spatial organization of Descemet's membrane-associated type IV collagen in the avian cornea

The organization of type IV collagen in the unconventional basement membrane of the corneal endothelium (Descemet's membrane) was investigated in developing chicken embryos using anti-collagen mAbs. Both immunofluorescence histochemistry and immunoelectron microscopy were performed. In mature embryos (greater than 15 d of development), the type IV collagen of Descemet's membrane was present as an array of discrete aggregates of amorphous material at the interface between Descemet's membrane and the posterior corneal stroma. Immunoreactivity for type IV collagen was also observed in the posterior corneal stroma as irregular plaques of material with a morphology similar to that of the Descemet's membrane-associated aggregates. This arrangement of Descemet's membrane-associated type IV collagen developed from a subendothelial mat of type IV collagen-containing material. This mat, in which type IV collagen-specific immunoreactivity was always discontinuous, first appeared at the time a confluent endothelium was established, well before the onset of Descemet's membrane formation. Immunoelectron microscopy of mature corneas revealed that the characteristic nodal matrix of Descemet's membrane itself was unreactive for type IV collagen, but was penetrated at intervals by projections of type IV collagen-containing material. These projections frequently appeared to contact cell processes from the underlying corneal endothelium. This spatial arrangement of type IV collagen suggests that it serves to suture the corneal endothelium/Descemet's membrane to the dense interfacial matrix of the posterior stroma.

Distribution of type IV collagen, laminin, and fibronectin during maxillary process formation in the chick embryo

The presence and distribution of laminin, type IV collagen, and fibronectin were analyzed in the facial primordia and developing primary palates of chick embryos from stages of development corresponding to maxillary process formation and primary palate closure. Frozen sections through the maxillary process and roof of the stomodeum were prepared for indirect immunofluorescence employing a biotin-avidin system using monoclonal antibodies against laminin, type IV collagen, and fibronectin. Light microscopic examination of sections stained with antibodies against type IV collagen revealed a much stronger fluorescent signal in the roof of the stomodeum than in the maxillary process at all stages examined. Regional differences in signal intensity and staining patterns were noted within the maxillary process; for example, the lateral surface of the maxillary process displayed a much less intense signal at most stages examined than the inferior and medial surfaces. The signal from sections of the maxillary process stained with laminin was much stronger than the signal from the same tissues stained with collagen. Regional differences in signal intensity within the maxillary process were minimal in sections stained with antibodies to laminin, in contrast to the differences seen in sections stained with antibodies to type IV collagen. Differences in signal intensity between the maxillary process and the roof of the stomodeum with laminin were slight. Sections stained with antibody to fibronectin displayed intense staining throughout the mesenchyme in both the maxillary process and the roof of the stomodeum. From comparison of the data of type IV collagen and laminin, the following hypothesis is proposed. In structures which undergo rapid change in form, such as the facial primordia, collagen distribution and/or organization is altered to a much greater extent than laminin, which is more uniformly distributed and which may be required for structural support of other developmentally regulated macromolecules. Where tissue morphology must be maintained, such as the roof of the stomodeum, the concentration and organization of type IV collagen is maintained in a manner that confers stability to these regions.

Early morphogenesis of the canine lens capsule, tunica vasculosa lentis posterior, and anterior vitreous body. A transmission electron microscopic study

This study provides transmission electron microscopic observations on the early morphogenesis (from days 25-35 post coitum) of the canine posterior lens capsule, the tunica vasculosa lentis (TVL) posterior and the anterior part of the vitreous body. The presence of an anlage of the posterior lens capsule as early as day 25, recently described histologically, was confirmed by this study. In the period from day 25 to day 35, the polar part of the posterior lens capsule develops 2-29 continuous and parallel lamellae, matching 50 nm and 1.74 microns, respectively. At these early stages, the TVL consists of capillaries that are simple endothelial tubes. From day 28 onward, these can be classified as A-1-alpha capillaries according to the classification of Bennett et al. [3]. In direct proximity to the lens capsule, the vitreous body contains fibrillar material with a morphological appearance similar to that of the lens capsule. This material probably derives from both the capillary endothelial cells' basal lamina and the lens capsule. Only few cellular components were observed in the anterior vitreous body. The development of the described structures is grossly in accordance with that observed in other mammalian species. The observations presented serve as a reference for studies on the pathogenesis of persistent hyperplastic tunica vasculosa lentis/persistent hyperplastic primary vitreous (PHTVL/PHPV), which is an important cause of leucocoria in children and in some dog breeds.

Basement membrane heterogeneity and variation in corneal epithelial differentiation

We have previously shown that the expression of a major 64-Kda keratin (K3) in corneal epithelium is site-related. It is found suprabasally in limbal epithelium, but uniformly (basal cells included) in central corneal epithelium. In the present study, we used a panel of antibodies against various components of corneal epithelial basement membrane to investigate a possible correlation between basement membrane heterogeneity and differential (basal vs. suprabasal) K3 keratin expression. One of these antibodies, AE27, stains human conjunctival basement membrane weakly, limbal basement membrane heterogeneously, and central corneal basement membrane strongly. Basal cells resting on basement membrane that stains strongly with AE27 tend to stain with monoclonal antibody AE5, which recognizes keratin K3. Basal cells on basement membrane staining weakly with AE27 tend not to stain with AE5. No such correlation exists between AE5 staining and type IV collagen, which is detectable immunohistochemically in conjunctival and limbal basement membrane, but not in corneal basement membrane overlying Bowman's layer. These results suggest that basement membrane of human corneal/conjunctival epithelium can be divided into at least three domains: the conjunctival basement membrane (type IV collagen-positive, AE27-weak), the limbal basement membrane (type IV collagen-positive, AE27-strong), and corneal basement membrane (type IV collagen-negative, AE27-strong). The results also raise the possibility that basement membrane heterogeneity may play a functional role in regulating keratin expression and other aspects of differentiation of corneal epithelium; more experiments are needed to test this hypothesis.

Synthesis of lens capsule and plasma membrane glycoproteins by lens epithelial cells and fibers in the rat

The lens of the eye possesses a capsule which is a greatly hypertrophied basement membrane. To investigate the synthesis of glycoproteins destined for this capsule, 3H-fucose was injected into the vitreous body of intact rats weighing approximately 200 gm. The animals were killed from 10 min to 14.5 months later, and their lenses were processed for electron microscope radioautography. At 10 min after injection, more than 58% of the silver grains were localized to the Golgi apparatus of the lens epithelial cells. By day 1, the heaviest sites of reaction were the plasma membrane (more than 50% of total label), the basal cytoplasm, and the adjacent lens capsule, where a heavy band of reaction was seen. The remainder of the capsule exhibited a lighter diffuse reaction. In the lens fibers, the label was at first localized to clusters of vesicles but then migrated to the plasma membrane and to the region of the capsule adjacent to the basal surface of these fibers. Light microscope radioautographs of the lens capsule at later time intervals revealed that by 1 month after injection the diffuse reaction had disappeared, and only the strongly labeled band remained. By 14.5 months after injection, this band had migrated partially across the lens capsule, but the capsule itself had increased considerably in thickness. On the other hand, the distance between the labeled band and the free edge of the capsule had decreased from that seen at the time of injection.

Immunochemical analysis of extracellular matrix during embryonic lens development of the Cat Fraser mouse

We have characterized the extracellular matrix present during early mouse-lens morphogenesis in Swiss and Cat Fraser mutant mice which produces a thicker capsule. In the two mouse strains, laminin was first detected when the optic vesicle and the head ectoderm are closely associated. At day 10, staining for laminin and fibronectin is especially concentrated at the border of the lens pit. At this stage, type IV collagen and proteoheparan sulphate have a similar distribution to laminin and fibronectin. In the two mouse strains, no major differences were observed in the intensity and the distribution of fluorescent basement-membrane components. This suggests that the overall increase in capsule thickness of the Cat Fraser mutant is more related to an increased cellular synthesis of capsule than to an abnormal distribution of one or more basement-membrane macromolecules.

Heterogeneous distribution of a basement membrane heparan sulfate proteoglycan in rat tissues

A heparan sulfate proteoglycan (HSPG) synthesized by murine parietal yolk sac (PYS-2) cells has been characterized and purified from culture supernatants. A monospecific polyclonal antiserum was raised against it which showed activity against the HSPG core protein and basement membrane specificity in immunohistochemical studies on frozen tissue sections from many rat organs. However, there was no reactivity with some basement membranes, notably those of several smooth muscle types and cardiac muscle. In addition, it was found that pancreatic acinar basement membranes also lacked the HSPG type recognized by this antiserum. Those basement membranes that lacked the HSPG strongly stained with antisera against laminin and type IV collagen. The striking distribution pattern is possibly indicative of multiple species of basement membrane HSPGs of which one type is recognized by this antiserum. Further evidence for multiple HSPGs was derived from the finding that skeletal neuromuscular junction and liver epithelia also did not contain this type of HSPG, though previous reports have indicated the presence of HSPGs at these sites. The PYS-2 HSPG was shown to be antigenically related to the large, low buoyant density HSPG from the murine Engelbreth-Holm swarm tumor. It was, however, confirmed that only a single population of antibodies was present in the serum. Despite the presence of similar epitopes on these two proteoglycans of different hydrodynamic properties, it was apparent that the PYS-2 HSPG represents a basement membrane proteoglycan of distinct properties reflected in its restricted distribution in vivo.

Accumulation and distribution of sulfated materials in the maturing mouse lens capsule

Lenses of late gestational and postnatal normal-eyed mice were tested for accumulated sulfated materials by using Spicer's high-iron-diamine staining method and also for newly incorporated sulfate autoradiographically following administration of 35SO4 either in vivo or in isolated and organ-cultured lenses. Accumulated and newly incorporated sulfate was observed in all lenses for each age group tested. Discrete regional differences were seen in histochemical staining patterns for sulfate on the lens capsule in specimens of all ages, and distinct laminar zonations were seen in the various regions of the capsule in older specimens. Typically, the anterior and equatorial regions of the capsule demonstrated three histochemically distinct laminar zones while the posterior capsule usually demonstrated two laminar zones. Autoradiographic results indicated that sulfate was indeed being incorporated into these regions, and in the same general pattern as seen with histochemistry. The materials were largely insensitive to testicular hyaluronidase but were preferentially sensitive to nitrous acid digestion, indicating the presence of capsular heparan sulfates. Autoradiographic results from organ-cultured lenses indicated that this tissue itself is a primary source of these materials.

Collagen synthesis by long-lived mRNA in embryonic chicken lens

Lens capsule collagen synthesis by epithelial and fiber cells was examined by immunoprecipitation and collagenase digestion in embryonic and posthatch chicken eye lens. Epithelial cells and lens fibers in the process of terminal differentiation produce alpha 1 and alpha 2 type IV collagen chains. At 6 days of embryonic development in addition to the alpha 1 (IV) and alpha 2 (IV) collagen chains, lens cells produce high molecular weight collagenase-sensitive proteins not immunologically related to type IV collagen. Lens capsule collagen components have been identified in central and outer fibers isolated from 18-day embryos and from 10-day posthatch chicken eyes. At these stages, fibers which have an increasing number of picnotic nuclei still show collagen synthesis due to long-lived mRNA. Analysis of collagen synthesis by lens cells incubated with actinomycin D suggests that stabilization of collagen mRNA occurs in lens fiber cells and to a lesser extent in epithelial cells as early as 6 days of embryonic development.

Distribution of basement membrane antigens in cryopreserved early embryonic hearts

The early embryonic heart is composed of two cylindrical epithelial layers, an inner endothelium and an outer myocardium. The cardiac jelly (CJ), an acellular accumulation of extracellular matrix (ECM), fills the space between the two epithelia. During development of the heart, a portion of the endothelial cells of the atrioventricular (AV) region differentiate into mesenchyme cells in a temporally and spacially specific manner. Although contiguous with those in the AV region, endothelial cells lining the ventricle never form mesenchyme in situ. At present, the mechanisms controlling the biphasic differentiation of the endothelium and the subsequent migration of cardiac mesenchymal cells are poorly understood. Although the CJ lies between two epithelial and is spatially equivalent to a basement membrane (BM), it has not traditionally been considered to be organized into a BM-like structure. The potential significance of this observation to developmental biology lies in the possibility that BM or their individual components (i.e., fibronectin (FN), laminin (LM), type IV collagen, and heparin sulfate proteoglycan (HSPG] may function as the regulatory site of epithelial differentiation and morphogenesis. A cryofixation technique was developed in order to determine the in situ immunohistochemical distribution of the BM components in the CJ. Results indicated that the CJ exists as the fusion between a larger myocardially derived BM having a lamina densa and an extended reticular lamina and an attenuated, endothelial-associated BM composed only of a lamina densa. Except for FN, the individual BM components were not all present during early stages, but instead appeared in a sequential manner, suggesting that all components of an adult-type BM are not required to initiate the assembly of a structural and functional BM during development. In the AV canal and outflow tract (OT), FN appeared as a progressively expanding gradient of material with the greatest density nearer the myocardium.

Monoclonal antibodies to human type IV collagen: useful reagents to demonstrate the heterotrimeric nature of the molecule

Monoclonal antibodies (mAbs) have been prepared against type IV collagen isolated from human kidney. Two mAbs, designated CIV 22 and CIV 16, were extensively characterized. CIV 22 reacted only with native type IV collagen, whereas CIV 16 also bound to fragments derived from the alpha 1(IV) chain after reduction and alkylation of the molecule. Therefore, CIV 22 recognizes a conformational epitope on the triple helical type IV collagen, whereas CIV 16 binds to a sequential determinant in the carboxyl-terminal half of the alpha 1(IV) chain. By immunofluorescence, typical basement membrane structures were stained with both mAbs on frozen sections of different human organs. The mAbs were used to investigate the chain composition of type IV collagen. Radiolabeled type IV collagen bound to CIV 22, proving its triple helical configuration. These native probes, containing both the alpha 1(IV) and the alpha 2(IV) chains, also bound to CIV 16. Since CIV 16 does not react with the isolated alpha 2(IV) chain, both chains must be arranged in a single triple helical molecule (heterotrimer).

Thermal stability of the helical structure of type IV collagen within basement membranes in situ: determination with a conformation-dependent monoclonal antibody

To examine the thermal stability of the helical structure of type IV collagen within basement membranes in situ, we have employed indirect immunofluorescence histochemistry performed at progressively higher temperatures using a conformation-dependent antibody, IV-IA8. We previously observed by competition enzyme-linked immunosorbent assay that, in neutral solution, the helical epitope to which this antibody binds undergoes thermal denaturation over the range of 37-40 degrees C. In the present study, we have reacted unfixed cryostat tissue sections with this antibody at successively higher temperatures. We have operationally defined denaturation as the point at which type IV-specific fluorescence is no longer detectable. Under these conditions, the in situ denaturation temperature of this epitope in most basement membranes is 50-55 degrees C. In capillaries and some other small blood vessels the fluorescent signal is still clearly detectable at 60 degrees C, the highest temperature at which we can confidently use this technique. We conclude that the stability of the helical structure of type IV collagen within a basement membrane is considerably greater than it is in solution, and that conformation-dependent monoclonal antibodies can be useful probes for investigations of molecular structure in situ.

Growth, synthesis and regional specialization of the embryonic chicken lens capsule

The growth, synthesis and regional specialization of the lens capsule has been studied in chicken embryos and compared to adult chickens and mammals. During the final 15 days of embryonic development the surface area of the capsule increased 11-fold. This represents the minimum estimate of capsule growth, since it also increased in thickness during this period. Autoradiographic examination of [3H]proline- or 35SO4-labeled, detergent-cleaned, flat-mounted capsules suggested that all lens cells participated in capsule synthesis. This was supported by the observation that both lens epithelial and fiber cells incorporated [35S]-methionine into collagenase-sensitive proteins with molecular weights similar to type IV collagen. Histochemical staining of detergent-cleaned capsule explants (DCCEs) revealed regional differences in the carbohydrate composition of the capsule. Differences in collagenous proteins were also seen between the anterior and posterior regions of the capsule of the embryonic lenses. Following SDS-PAGE and silver staining a total of six collagenase-sensitive proteins were seen with molecular weights between 150 and 180 K. Three of these polypeptides were common to both anterior and posterior capsules, one was found only in the anterior capsule and the other two were found predominantly in the posterior capsule. No regional differences were seen in the collagenous proteins of capsules from adult chickens, rats or hamsters. The possibility is discussed that the unique pattern of collagenous polypeptides seen in embryonic capsules is related to the rapid growth of the embryonic lens.

Monoclonal antibodies against chicken type IV and V collagens: electron microscopic mapping of the epitopes after rotary shadowing

The location of the epitopes for monoclonal antibodies against chicken type IV and type V collagens were directly determined in the electron microscope after rotary shadowing of antibody/collagen mixtures. Three monoclonal antibodies against type IV collagen were examined, each one of which was previously demonstrated to be specific for only one of the three pepsin-resistant fragments of the molecule. The three native fragments were designated (F1)2F2, F3, and 7S, and the antibodies that specifically recognize each fragment were called, respectively, IA8 , IIB12 , and ID2 . By electron microscopy, monoclonal antibody IA8 recognized an epitope located in the center of fragment (F1)2F2 and in tetramers of type IV collagen at a distance of 288 nm from the 7S domain, the region of overlap of four type IV molecules. Monoclonal antibody IIB12 , in contrast, recognized an epitope located only 73 nm from the 7S domain. This result therefore provides direct visual evidence that the F3 fragment is located closest to the 7S domain and the order of the fragments must be 7S-F3-(F1)2F2. The epitope for antibody ID2 was located in the overlap region of the 7S domain, and often several antibody molecules were observed to binding to a single 7S domain. The high frequency with which antibody molecules were observed to bind to fragments of type IV collagen suggests that there is a single population of type IV molecules of chain organization [alpha 1(IV)]2 alpha 2(IV), and that four identical molecules must form a tetramer that is joined in an antiparallel manner at the 7S domain. The monoclonal antibodies against type V collagen, called AB12 and DH2 , were both found to recognize epitopes close to one another, the epitopes being located 45-48 nm from one end of the type V collagen molecule. The significance of this result still remains uncertain, but suggests that this site is probably highly immunoreactive. It may also be related to the specific cleavage site of type V collagen by selected metalloproteinases and by alpha-thrombin. This cleavage site is also known to be located close to one end of the type V molecule.

Monoclonal antibodies against developmentally regulated corneal antigens

The chick cornea is comprised of three cellular layers, each associated with a discrete extracellular matrix. The absence of specific markers for these cellular and acellular components has made it difficult to investigate the cell-cell and cell-matrix interactions which occur during development of this organ. We have approached this problem by producing monoclonal antibodies to species-specific, developmentally regulated antigens of the chick cornea. By immunofluorescence staining patterns the antibodies fall into three distinct groups. One group is directed against the corneal extracellular matrix. At 9 days of embryonic development staining by these antibodies is detected at the endothelial surface (in Descemet's membrane), and in the posterior part of the stroma. During development it progresses anteriorly throughout the entire width of the corneal stroma and Bowman's membrane until, by 14 days, it is found in all three specialized extracellular matrices of the cornea. Throughout most of development these antibodies do not recognize any other ocular or nonocular tissue examined. Late in development they begin to lightly stain nerve bundles. A second group of antibodies is highly selective for the corneal epithelial cell layer. These begin to stain at 12 to 13 days of development and cause very bright fluorescence by 14 days. A third group stains the extracellular matrix of the cornea in a manner spatially and temporally identical to that of the first group, but in addition recognizes certain basement membranes. The possible relationship of the antigens recognized by these groups of antibodies to developmental events occurring at the time of their appearance, and the potential use of all three antibody groups in studying corneal development are discussed.