X-ray diffraction study of bovine lens capsule collagen

Method for measurement of collagen monomer orientation in fluorescence microscopy

SIGNIFICANCE

Collagen is the most abundant protein in vertebrates and is found in tissues that regularly experience tension, compression, and shear forces. However, the underlying mechanism of collagen fibril formation and remodeling is poorly understood.

AIM

We explore how a collagen monomer is visualized using fluorescence microscopy and how its spatial orientation is determined. Defining the orientation of collagen monomers is not a trivial problem, as the monomer has a weak contrast and is relatively small. It is possible to attach fluorescence tags for contrast, but the size is still a problem for detecting orientation using fluorescence microscopy.

APPROACH

We present two methods for detecting a monomer and classifying its orientation. A modified Gabor filter set and an automatic classifier trained by convolutional neural network based on a synthetic dataset were used.

RESULTS

By evaluating the performance of these two approaches with synthetic and experimental data, our results show that it is possible to determine the location and orientation with an error of ∼37  deg of a single monomer with fluorescence microscopy.

CONCLUSIONS

These findings can contribute to our understanding of collagen monomers interaction with collagen fibrils surface during fibril formation and remodeling.

Impact of heparan sulfate chains and sulfur-mediated bonds on the mechanical properties of bovine lens capsule

We assessed the importance of glycosaminoglycans and sulfur-mediated bonds for the mechanical properties of lens capsules by comparing the stress-strain responses from control and treated pairs of bovine source. No significant change in mechanical properties was observed upon reduction of disulfide bonds. However, removal of glycosaminoglycan chains resulted in a significantly stiffer lens capsule, whereas high concentrations of reducing agent, which is expected to reduce the recently reported sulfilimine bond of collagen IV, resulted in a significantly less stiff lens capsule. A comparison of the diffraction patterns of the control and strongly reduced lens capsules indicated structural rearrangements on a nanometer scale.

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.

A study of piezoelectric and mechanical anisotropies of the human cornea

The piezoelectric and dynamic mechanical properties of human cornea have been investigated as a function of drying time. As expected, the piezoelectric coefficient, d(31), and the Young's modulus, Y, were found to be extremely sensitive to water content. d(31) decreased with dehydration of the corneal tissue and Y increased with dehydration. While these results are significant, the discovery of the unprecedented mechanical and electromechanical anisotropy exhibited by the cornea are the major findings of this study and indicate that the collagen fibrils comprising the cornea are highly oriented. The piezoelectric responses of corneas observed in this study are: diagonally cut samples starting at an average piezoelectric coefficient value of 2250 pC/N, followed by the vertically cut samples, with an average starting value of about 600 pC/N and finally the horizontally cut samples with an average starting value of about 200 pC/N.

Basement membrane collagen--evidence for a novel molecular packing

Type IV collagen is the major structural protein of basement membranes but very little is known about its molecular organisation in vivo. We have used X-ray diffraction of a thick basement membrane, bovine lens capsule, to provide information. Under constant load, lens capsule gave a collagen diffraction pattern of a similar quality to unstretched rat rail tendon. In addition there were clear meridional reflections which indexed as orders of 10 nm, and equatorial reflections at 2.1 and 3.8 nm. These results suggest the ordering of type IV collagen molecules in fibrils, with a 10 nm periodicity along the length of the fibrils.

Evidence for collagen molecular orientation in basement membranes

The following basement membranes (BMs) from representative species of the main vertebrate classes were studied by the Picrosirius-polarization method: lens capsule, Reichert's membrane and glomerular BMs. A distinct birefringence was consistently observed in all BMs from all species studied by this method. The results reported provide a strong evidence for collagen macromolecular orientation in BMs. Heparitin sulphate was the only glycosaminoglycan detected in dogs lens capsules.

Model of the accommodative mechanism in the human eye

The crystalline lens of the age 11 human eye has been modelled mathematically, using simplified assumptions about lens curvature, internal organization and elasticity. From this representation, expressions for description of strain and stress during accommodation have been obtained. Solution of these equations indicates that the lens capsule acts as a force distributor, spreading tension applied by the suspensory apparatus evenly over the surface of the underlying lens material. It also becomes clear that the vitreous body provides an essential support function during the accommodative process. Finally, the relative contribution of lens-associated structures has been determined for five different values of the Poisson ratio. In order for accommodation to occur by relaxation of zonular tension, this value must be greater than 0.38; with an additional constraint of the net axial force equalling zero during a small accommodative change, the Poisson ratio equals 0.46.

A network model for the organization of type IV collagen molecules in basement membranes

Type IV collagen was solubilized from a tumor basement membrane either by acid extraction or by limited digestion with pepsin. The two forms were similar in composition and the size of the constituent chains but differed when examined by electron microscopy and in the fragment pattern produced by bacterial collagenase. The acid-soluble form showed after rotary shadowing strands mainly of a length of 320 nm which terminated in a globule, or two strands connected by a similar globule. The globule was identified as a non-collagenous domain (NC1) which under dissociating conditions could be separated into two peptides showing a monomer-dimer relationship. Higher aggregates of NC1 were visualized under non-dissociating conditions. Some of the acid-extracted molecules have retained the previously 7-S collagen domain. The pepsin-solubilized form lacked domain NC1 and consisted mainly of four triple-helical strands (length 356 nm) joined together at the 7-S domain (length 30 nm). Common to both forms of type IV collagen was a small collagenase-resistant domain NC2 which was composed of collagenous and non-collagenous elements and located between the 7-S domain and the major triple helix. These data indicate that the collagenous matrix of basement membranes consists of a regular network of type IV collagen molecules which is generated by two different interacting sites located at opposite ends of each molecule. The 7-S collagen domain connects four molecules while the NC1 domain connects two molecules. The maximal distance between identical cross-linking sites (7-S or NC1) was estimated to be about 800 nm comprising the length of two molecules.