National Glaucoma Research
Thao Nguyen, Ph.D.
Johns Hopkins University
Title: The Biomechanics of the Mouse Sclera: Effects of Strain, Age, and Glaucoma
Non-Technical Title: Biomechanics of Glaucoma in Mouse Models
Duration: April 1, 2010 - March 31, 2013
Award Type: Standard
Award Amount: $99,182
Numerous studies have suggested an association between scleral biomechanical alterations and optic nerve dysfunction. Our ultimate goal is to determine how the mechanical properties of the sclera, the white portion of the eye, influence the development of glaucoma optic nerve damage and how the development of glaucoma damage in turn alters the mechanical properties of the sclera. Towards this goal, we are developing an inflation test to characterize and compare the mechanical behavior of the sclera of a glaucoma mouse model and its genetically matched control.
This research is part of a long-term program to study the role of scleral biomechanics in glaucoma. While it is now proposed that differences in the mechanical behavior of the sclera increase the susceptibility to glaucoma, the magnitude of this effect is unknown. In this study, we modify the biomechanical properties (e.g., stiffness) of living eyes, through chemical treatments and gene knockout techniques. These approaches will shed light on the relationship between connective tissue structure, mechanical properties, and disease progression. We will develop a new experimental method to inflate eyes, in vitro, and measure the mechanical behavior of the sclera. The study examines animals at different ages, including a strain of animals that spontaneously develops glaucoma. The results will be compared statistically to identify differences in their mechanical behavior due to strain, age, and glaucoma. The findings of the proposed research could lead to the development of non-invasive testing procedures for glaucoma that measure the properties of the sclera near the transition zone between the cornea and sclera, and identify individuals at higher risk for the initial development and rapid progression of the disease. Moreover it can lead to the development of new drug therapies to slow the progression of the disease by altering the mechanical behavior of the sclera.
M. R. Steinhart, F.E. Cone, C, Nguyen, T. D. Nguyen, M.E. Pease, O. Puk, J. Graw, E.N. Oglesby, H.A. Quigley, (2012) "Mice with an induced mutation in collagen 8A2 develop larger eyes and are resistant to retinal ganglion cell damage in an experimental glaucoma model", Molecular Vision, 18, 1093-1106;
Nguyen C, Cone FE, Nguyen TD, Coudrillier B, Pease ME, Steinhart MR, Oglesby EN, Jefferys JL, Quigley HA. Invest Ophthalmol Vis Sci. 2013 Mar 11;54(3):1767-80. doi: 10.1167/iovs.12-10952.
Dr. Nguyen’s team developed an experimental method using digital image analysis to measure the amount of deformation of the sclera (the “whites of the eyes”) that can happen with glaucoma. The team is currently applying the method to measure and compare the inflation response of the sclera in eyes harvested from three types of mice engineered to have glaucoma: (1) C57BL6, (2) CD1 albino, and (3) Aca23 mice with a mutation in the collagen 8A2 gene and their wild type (healthy) control.
Experimental glaucoma was induced by injecting a thick slurry (viscoelastic solution) into the front of the eye (anterior chamber) to block the aqueous outflow channels. This caused an immediate and sustained increase in the intraocular pressure (IOP), a lengthening or pushing back (called permanent axial elongation) of the eyes, and damage to the axons of the retinal ganglion (optic nerve) cells. By 6 weeks, the eyes of all experimental animals suffered nerve cell axonal loss (called optic nerve hypoplasia) at the pressure points, were statistically larger in axial length and diameters, and exhibited a thinner peripapillary sclera (i.e. the sclera was thinner in the area surrounding the optic nerve head). However, the degree of structural changes and axon loss varied greatly among mouse types. For example, Aca23 mice suffered less axon loss (only 0.57+/-17%) while their healthy controls showed much more axon loss (21+/-31%). Inflation test results showed that the sclera of all animals had a different pressure-strain response, and all the experimental glaucoma eyes developed a stiffer pressure-strain response than the contralateral (opposite eye) controls. The degree of stiffening also varied with the type of mouse. Comparing the collagen 8A2 mutants to their wild type, the eyes with a stiffer pressure-strain response at baseline developed less axonal damage.
The team is currently developing a method to calculate the regionally varying stress response of the sclera from the imposed pressure and measured displacement to determine material stiffness (of the stress-strain response). This will allow them to determine whether the glaucoma-induced stiffening response is caused primarily by changes in anatomical dimensions or material properties of the sclera. A fundamental understanding of the relationship between scleral biomechanics and the severity of glaucoma damage could one day lead to the development of non-invasive screening procedures to identify individuals at higher risk for glaucoma. In addition, Dr. Nguyen’s discoveries could lead to new drug therapies to slow the progression of the disease by altering the mechanical behavior of the sclera.