Basement membrane collagen--evidence for a novel molecular packing

Morphologic alterations in the anterior lens capsule of canine eyes with cataracts

OBJECTIVE

To examine the morphologic changes in the anterior lens capsule and lens epithelium of canine eyes with cataracts.

SAMPLE POPULATION

Anterior lens capsules from the eyes of 25 dogs with cataracts and from an additional 10 canine globes with lenses subjectively assessed to be normal.

PROCEDURE

Thickness of each anterior lens capsule was measured by use of a digital microscopic camera and imaging software. All 25 capsules from eyes with cataracts were submitted for light microscopy; 4 were also submitted for electron microscopy.

RESULTS

Thickness of the anterior lens capsule increased with age for the normal lenses and the lenses with cataracts; the change with age was similar for both groups. Light microscopy revealed fibrous metaplasia of lens epithelial cells in 7 of 25 anterior lens capsules with focal thickenings of the posterior aspect of the capsule. Electron microscopy revealed deposition of collagen and basement membrane-like material by fibroblast-like cells.

CONCLUSIONS

Results indicate that thickness of the anterior lens capsule in dogs increases with age and that this increase in thickness is not significantly different between normal lenses and lenses with cataracts. In addition, epithelial cells from lenses with cataracts may undergo metaplasia to form plaques composed of fibrous tissue and ectopic basement membrane produced by epithelial cells.

Types of collagen synthesised by the lens epithelial cells of human cataracts

AIMS/BACKGROUND

Residual lens epithelial cells (LECs) undergo fibrous proliferation after cataract surgery, resulting in capsular fibrosis. The purpose of this study was to determine the types of collagen produced in cultured LECs derived from human cataract LECs.

METHODS

A circular section of the anterior capsule, about 5 mm in diameter, with LECs attached was obtained by anterior capsulotomy during cataract surgery and cultured directly without dispersion of the cells in a well, on the bottom of which a disc-shaped, thin plate of poly(methyl methacrylate) had been placed. At 5 to 6 weeks of culture, the proliferated cells of the culture were stained immunohistochemically with antibodies against human collagens I-VI by the avidin-biotin-peroxidase complex method.

RESULTS

Collagens I, IV, V, and VI were positive in the cultured cells. Types IV and V were strongly present in almost all the cells whereas types I and VI were only observed in a few cells. Collagens II and III were negative.

CONCLUSIONS

Since the lens capsule is known to be comprised of collagen IV, collagens I, V, and VI seem to be produced newly in culture. The capsular fibrosis seen after cataract surgery in vivo as a wound healing process of the lens capsule, may contain these types of collagens. The present culture model is useful for studying secondary cataract formation.

High resolution platinum-carbon replication of freeze-dried basement membrane

High angle platinum/carbon (Pt/C) replication has proved to be a valuable tool in analyzing basement membrane structure in human amnion, bovine lens capsule, and the Engelbreth-Holm-Swarm (EHS) tumor. High resolution replicas for transmission electron microscopy (TEM) have been achieved by depositing 1.0 +/- 0.1 nm thick Pt/C films backed with rotary deposited 12.5 +/- 2.5 nm thick carbon films. The basement membrane collagen IV network was observed to consist of fine branching filaments containing globular domains intrinsic to the filaments. A second quasi-regular network is formed by laminin. Unidirectional 45 degree angle Pt/C replication was used for most of this work. The merits and deficiencies of unidirectional vertical replication (80 degree angle), unidirectional 45 degree angle, and 20 degrees low angle rotary replication are discussed. Vertical replication produces the highest resolution replicas and has the potential for revealing the overall pattern of basement membrane structural assembly if basement membrane preparations freeze-dried in low salt can faithfully maintain their in vivo structure.

Chemistry of collagen cross-links: glucose-mediated covalent cross-linking of type-IV collagen in lens capsules

The incubation of lens capsules with glucose in vitro resulted in changes in the mechanical and thermal properties of type-IV collagen consistent with increased cross-linking. Differential scanning calorimetry (d.s.c.) of fresh lens capsules showed two major peaks at melting temperatures Tm 1 and Tm 2 at approx. 54 degrees C and 90 degrees C, which can be attributed to the denaturation of the triple helix and 7S domains respectively. Glycosylation of lens capsules in vitro for 24 weeks caused an increase in Tm 1 from 54 degrees C to 61 degrees C, while non-glycosylated, control incubated capsules increased to a Tm 1 of 57 degrees C. The higher temperature required to denature the type-IV collagen after incubation in vitro suggested increased intermolecular cross-linking. Glycosylated lens capsules were more brittle than fresh samples, breaking at a maximum strain of 36.8 +/- 1.8% compared with 75.6 +/- 6.3% for the fresh samples. The stress at maximum strain (or 'strength') was dramatically reduced from 12.0 to 4.7 N.mm.mg-1 after glycosylation in vitro. The increased constraints within the system leading to loss of strength and increased brittleness suggested not only the presence of more cross-links but a difference in the location of these cross-links compared with the natural lysyl-aldehyde-derived cross-links. The chemical nature of the fluorescent glucose-derived cross-link following glycosylation was determined as pentosidine, at a concentration of 1 pentosidine molecule per 600 collagen molecules after 24 weeks incubation. Pentosidine was also determined in the lens capsules obtained from uncontrolled diabetics at a level of about 1 per 100 collagen molecules. The concentration of these pentosidine cross-links is far too small to account for the observed changes in the thermal and mechanical properties following incubation in vitro, clearly indicating that another as yet undefined, but apparently more important cross-linking mechanism mediated by glucose is taking place.

Molecular bending and networks in a basement membrane-like collagen: packing in dogfish egg capsule collagen

Low-angle X-ray diffraction data have been obtained from three mutually perpendicular axes through sheets of the collagenous egg capsule of the dogfish Scyliorhinus caniculus, a collagen that resembles type IV collagen. The data are interpreted in the light of the body of knowledge of the structure derived from transmission electron microscopy by Knight and Hunt. A model to account for the X-ray data is proposed incorporating the main dimensions of the Knight and Hunt model which are confirmed by the diffraction data. Several features of the diffraction patterns are not explained by the existing model however, and a new model is proposed to account for these features. This consists of antiparallel packed pairs of two mutually parallel molecules, each kinked and rotated so as to produce a four-fold helix resembling a crankshaft. This has the advantage of conferring intermolecular linkage in three dimensions throughout the structure with tetragonal symmetry and unit dimensions a = b = 22.6 nm, c (fibre axis direction) = 39.3 nm. The result is a fairly rigid open polygonal network or sponge-like architecture that is capable of accommodating large quantities of water and other molecules.

An immunoelectron microscope study of the aged human lens capsule

The distribution of types I-IV collagen and laminin was studied in seven aged human lens capsules using the immunogold EM technique on LR White embedded tissue. Samples were taken from the anterior, equatorial and posterior regions. Labelling for type II collagen was not observed. Type IV collagen was evenly distributed throughout the thickness of the capsule but was absent from the zonules. However, an unexpected finding was strong labelling for types I and III collagen, again evenly distributed throughout the capsule. The presence of type III collagen makes the lens capsule unique among ocular basement membranes. Laminin was present in linear densities, zonular lamellae and zonular fibres, suggesting that linear densities are an integral part of the zonular apparatus.

Basement membrane proteins: structure, assembly, and cellular interactions

Basement membranes are thin layers of a specialized extracellular matrix that form the supporting structure on which epithelial and endothelial cells grow, and that surround muscle and fat cells and the Schwann cells of peripheral nerves. One common denominator is that they are always in close apposition to cells, and it has been well demonstrated that basement membranes do not only provide a mechanical support and divide tissues into compartments, but also influence cellular behavior. The major molecular constituents of basement membranes are collagen IV, laminin-entactin/nidogen complexes, and proteoglycans. Collagen IV provides a scaffold for the other structural macromolecules by forming a network via interactions between specialized N- and C-terminal domains. Laminin-entactin/nidogen complexes self-associate into less-ordered aggregates. These two molecular assemblies appear to be interconnected, presumably via binding sites on the entactin/nidogen molecule. In addition, proteoglycans are anchored into the membrane by an unknown mechanism, providing clusters of negatively charged groups. Specialization of different basement membranes is achieved through the presence of tissue-specific isoforms of laminin and collagen IV and of particular proteoglycan populations, by differences in assembly between different membranes, and by the presence of accessory proteins in some specialized basement membranes. Many cellular responses to basement membrane proteins are mediated by members of the integrin class of transmembrane receptors. On the intracellular side some of these signals are transmitted to the cytoskeleton, and result in an influence on cellular behavior with respect to adhesion, shape, migration, proliferation, and differentiation. Phosphorylation of integrins plays a role in modulating their activity, and they may therefore be a part of a more complex signaling system.

Three-dimensional structure of type IV collagen in the mammalian lens capsule

The anterior lens capsule provides a thick, easily handled model system for the study of the organization of type IV collagen, the main component of basement membranes. We have used the technique of rapid freezing, deep-etch, and rotary replication to study the three-dimensional organization of the collagen skeleton in mammalian lens capsule after a variety of extraction procedures. In all cases the collagen appeared as a densely packed three-dimensional branching network of fine microfibrils. The organization of the microfibrils appears to show some regularity, with branch points approximately 40 nm apart. Most junctions are three-way and the network forms predominantly five-sided figures. This closely resembles the collagenous network described by Yurchenco and Ruben (1987, 1988) in human amniotic basement membrane and EHS tumor matrix, but extends their findings to another system for which X-ray diffraction data are available. The three-dimensional network is discussed in terms of molecular packing of type IV collagen in light of the information available from the diffraction data.

Short and long range order in basement membrane type IV collagen revealed by enzymic and chemical extraction

Oriented bovine lens capsules give X-ray diffraction patterns suggesting a considerable degree of order in the collagenous components, predominantly type IV collagen. Here we report the effects of preliminary treatment of lens capsules before orientation. Extraction with 4 M guanidinium hydrochloride or with heparinase/hyaluronidase reveals the same collagenous diffraction patterns previously seen after extraction with 1 M NaCl. There is a four-point pattern of d-spacing 3.9 nm, indicating liquid crystal cybotactic nematic organization, along with sharp streaked meridional reflections which index as orders of 21 nm. This suggests that the removal of basement membrane proteoglycans results in a reduction in diffuse scatter and clarification of the pattern. Extraction of the lens capsules with trypsin or dithiothreitol greatly reduces the intensity of the four-point pattern while leaving the meridional pattern unaffected. This strengthens the evidence that the 21 nm period has its origins in the collagen IV helix. Reduction in the four-point pattern could arise if disruption of non-helical NC1 domains or 7S overlap regions allows slippage of the collagen molecules on orientation, weakening the proposed 1 nm intermolecular stagger. Ultra-low angle diffraction patterns of extracted lens capsules show meridional reflections which index as a long-range axial repeat of approximately 95 nm. This is consistent with a model of microfibrils of type IV collagen in which the NC1 domains bind to the collagen helix at approximately 100 nm intervals, as has been previously suggested.

Molecular organization of type IV collagen: polymer liquid crystal-like aspects

A new X-ray diffraction pattern from type IV collagen is described, which can be interpreted on the basis of crystalline and liquid crystalline origins of the reflections. Bovine anterior lens capsules extracted with 1 M NaCl and oriented by extension of 60% under constant load gave medium angle X-ray diffraction patterns showing many of the characteristics typical of liquid crystals. Prominent features, apart from those wide angle features attributable to the collagen triple helix, are (1) a four-point pattern of broad reflections at d-spacing 3.9 nm, and layer line spacing near 5 nm. (2) A broad intense equatorial peak centred at 1.24 nm, indicative of liquid-like lateral molecular associations. (3) A set of five sharp, streaked meridional reflections (previously obscured by the broad peak near 5 nm in unextracted capsules). (4) A further six higher angle reflections of a diffuse, arced and broad appearance on the meridian. The sharp streaked meridional reflections emanate from a long-range periodicity of units 8-9 nm in diameter. These features form a self-consistent system if interpreted on the basis of a staggered liquid crystal-like array of collagen molecules, in which case the first five meridionals and remaining broad reflections, sampled on the meridian, can all be indexed as orders of 21 nm.

The ultrastructural organization of the basement membrane of Bowman's capsule in the rat renal corpuscle

The basement membrane of Bowman's capsule (BCBM) of the rat was studied by means of a modified tissue-preservation technique for transmission electron microscopy, which avoids the usual thorough fixation in OsO4 and applies tannic acid and uranyl acetate for staining (Sakai et al. 1986). At most sites the BCBM is multilayered, consisting of one to seven dense layers separated by electron-lucent layers. The latter, which can be termed laminae rarae, contain fine filaments which connect the dense layers to each other and the innermost dense layer to the basal cell membrane of the parietal epithelium. The laminae densae are basically composed of fine filaments arranged in an anastomosing pattern. Individual filaments ranging from 5 to 15 nm in diameter, combine to form filament bundles up to 100 nm in thickness and 1 to 2 micron in length. Within a dense layer, filaments and filamentous bundles are oriented mainly in the same direction. Often the inner dense layers do not form a continuous sheet, and the filamentous bundles are arranged in anastomosing or spiral patterns to form a ribbon-like structure that we call a "microligament". These microligaments are often embedded in basal furrows of the parietal epithelium and are best developed around the vascular pole. Intracellular actin bundles of the parietal cells are regularly associated with these extracellular ribbon-like structures of the basement membrane. In conclusion, the BCBM has an unusual structure: the laminae densae are characterized by their filamentous nature and are arranged in different patterns, i.e. as a multilayered mat and as microligaments.

Basement membrane structure in situ: evidence for lateral associations in the type IV collagen network

To determine molecular architecture of the type IV collagen network in situ, the human amniotic basement membrane has been studied en face in stereo relief by high resolution unidirectional metal shadow casting aided by antibody decoration and morphometry. The appearance of the intact basement membrane is that of a thin sheet in which there are regions of branching strands. Salt extraction further exposes these strands to reveal an extensive irregular polygonal network that can be specifically decorated with gold-conjugated anti-type IV collagen antibody. At high magnification one sees that the network, which contains integral (9-11 nm net diameter) globular domains, is formed in great part by lateral association of monomolecular filaments to form branching strands of variable but narrow diameters. Branch points are variably spaced apart by an average of 45 nm with 4.4 globular domains per micron of strand length. Monomolecular filaments (1.7-nm net diameter) often appear to twist around each other along the strand axis; we propose that super helix formation is an inherent characteristic of lateral assembly. A previous study (Yurchenco, P. D., and H. Furthmayr. 1984. Biochemistry. 23:1839) presented evidence that purified murine type IV collagen dimers polymerize to form polygonal arrays of laterally as well as end-domain-associated molecules. The architecture of this polymer is similar to the network seen in the amnion, with lateral binding a major contributor to each. Thus, to a first approximation, isolated type IV collagen can reconstitute in vitro the polymeric molecular architecture it assumes in vivo.