Researchers from ELZA, the ETH Zurich, the University of Zurich and the University of South Florida investigate corneal biomechanics in a mouse model for Ehlers-Danlos syndrome
Having a weak cornea can be bad for your sight – and can ultimately lead to blindness, which is why there are intense research efforts into understanding why some corneas become weak, and others not. But there’s a problem. In most other diseases, transgenic mouse models can be used to understand diseases better but assessing corneal strength – or “biomechanics” – in mouse eyes is a big challenge. Methods that measure corneal strength, such as extensometry, which involves cutting a strip from the cornea and stretching it in one dimension, and assessing the rate at which it stretches, and how much force is required to break the strip are much harder to perform accurately and reproducibly in mouse eyes than in eyes from larger animals like pigs.
But researchers from both the ETH Zurich (www.ethz.ch), the University of Zurich (www.cabmm.uzh.ch), the University of South Florida, and clinicians from the ELZA Institute, Zurich (www.elza-institute.com), previously adapted this approach for extra-small eye dimensions1, and now added one innovative technique: optical coherence elastography (OCE), and one advanced imaging method, Second Harmonic Generation (SHG) microscopy, in order to compare mice with a genetic collagen V deficiency 2, a model for Ehlers-Danlos syndrome. Collagen V is an important component of the structural part of the cornea, called the stroma, but transgenic mice where both copies of the Col5a1 gene are knocked out fail to develop beyond embryonic day 10-11. However, heterozygotic mice (Col5a1+/-) do survive, with the corneas of these mice having fewer collagen fibrils than their wild-type (WT) littermates.
With the applied experimental techniques, the investigators could assess the strain in the cornea – in other words, how much and how fast the cornea changes in shape and size in response to a force applied to it. The small-dimension extensometry experiments showed differences between the two groups. Compared with WT corneas, heterozygote corneas were 25–30% more likely to rupture during the extensometry experiments – something that is seen in the clinic, where approximately 35% of patients with Ehlers–Danlos syndrome experience corneal ruptures either spontaneously 3 or in response to minimal trauma, a condition termed “oculus fragilis” 4. Furthermore, the corneas became significantly more relaxed after a load was applied. For elastography, the entire experimental set-up was contained in a sealed pressure chamber, and the force applied was a change in chamber pressure. The OCE experiments showed that the heterozygote corneas were significantly thinner than WT corneas, yet able to carry the same amount of axial strain as wild-type corneas. After adjusting for corneal thickness, this meant that the heterozygote corneas had a higher mechanical stiffness than the wild-type corneas. SHG imaging indicated that the heterozygotic corneas displayed “fibril disorganization, increased fibril density and abnormalities in the hierarchical organization”, compared to wild-type corneas.
The study’s first author, Dr. Sabine Kling of the ETH Zurich explained the significance of their work “We have demonstrated that tissues can show a distinct deformation behavior depending on the type and speed a load is applied. Therefore, a single characterisation test could not have provided these insights on the mechanical impact of col5 on corneal tissue as we obtained here.”
Farhad Hafezi, MD PhD states that “our findings provide more insights and might help develop therapeutic approaches in brittle corneas”.
The full text of the article is available to read here: https://rdcu.be/cwlU1
- Hammer A, Kling S, Boldi M-O, et al. Establishing corneal cross-linking with riboflavin and uv-a in the mouse cornea in vivo: biomechanical analysis. Invest Ophthalmol Vis Sci. 56(11):6581-6590 (2015).
- Kling S, Torres-Netto EA, Abdshahzadeh H, Espana EM, Hafezi F. Collagen V insufficiency in a mouse model for Ehlers Danlos‑syndrome affects viscoelastic biomechanical properties explaining thin and brittle corneas. Sci Rep 11(1):17362. (2021).
- Cameron, J. A. Corneal abnormalities in Ehlers–Danlos syndrome type VI. Cornea 12, 54–59 (1993).