Measuring the effects of postmortem time and age on mouse lens elasticity using atomic force microscopy

Comparative analysis of rodent lens morphometrics and biomechanical properties

Introduction

Proper ocular lens function requires biomechanical flexibility, which is reduced during aging. As increasing lens size has been shown to correlate with lens biomechanical stiffness in aging, we tested the hypothesis that whole lens size determines gross biomechanical stiffness by comparing lenses of varying sizes from three rodent species (mice, rats, and guinea pigs).

Methods

Coverslip compression assay was performed to measure whole lens biomechanics. Whole mount staining on fixed lenses, followed by confocal microscopy, was conducted to measure lens microstructures.

Results

Among the three species, guinea pig lenses are the largest, rat lenses are smaller than guinea pig lenses, and mouse lenses are the smallest of the three. We found that rat and guinea pig lenses are stiffer than the much smaller mouse lenses. However, despite guinea pig lenses being larger than rat lenses, whole lens stiffness between guinea pigs and rats is not different. This refutes our hypothesis and indicates that lens size does not solely determine lens stiffness. We next compared lens microstructures, including nuclear size, capsule thickness, epithelial cell area, fiber cell widths, and suture organization between mice, rats, and guinea pigs. The lens nucleus is the largest in guinea pigs, followed by rats, and mice. However, the rat nucleus occupies a larger fraction of the lens. Both lens capsule thickness and fiber cell widths are the largest in guinea pigs, followed by mice and then rats. Epithelial cells are the largest in guinea pigs, and there are no differences between mice and rats. In addition, the lens suture shape appears similar across all three species.

Discussion

Overall, our data indicates that whole lens size and microstructure morphometrics do not correlate with lens stiffness, indicating that factors contributing to lens biomechanics are complex and likely multifactorial.

Mechanical Properties of Eye Lens Cortical and Nuclear Membranes and the Whole Lens

Purpose

To elucidate the mechanical properties of the bovine lens cortical membrane (CM), the nuclear membrane (NM) containing cholesterol bilayer domains (CBDs), and whole bovine lenses.

Methods

The total lipids (lipids plus cholesterol) from the cortex and nucleus of a single bovine lens were isolated using the monophasic methanol extraction method. Supported CMs and NMs were prepared from total lipids extracted from the cortex and nucleus, respectively, using a rapid solvent exchange method and probe-tip sonication, followed by the fusion of unilamellar vesicles on a flat, freshly cleaved mica surface. Topographical images and force curves for the CMs and NMs were obtained via atomic force microscopy (AFM) in a fluid cell. Whole bovine lenses were affixed to custom-built glass Petri dishes, and an AFM was used to obtain force curves. Force curves were analyzed to estimate the breakthrough force, membrane stiffness (KA and Em), and lens stiffness (EL).

Results

The NMs containing CBDs exhibited significantly lower breakthrough force, KA, and Em than the CMs without CBDs. The Em values for CMs and NMs were significantly higher than the EL for the whole lens.

Conclusions

The significantly higher stiffness of the CM and NM compared to the stiffness of the whole lens suggests that slight modulation in CM and NM composition may play a crucial role in altering the overall lens stiffness. Furthermore, the NMs containing CBDs were less stiff than CMs without CBDs, suggesting that CBDs decrease lens membrane stiffness and possibly protect against lens hardening and presbyopia.

Effect of compound treatments on mouse lens viscoelasticity

Previous studies have shown that pharmaceutical agents such as lipoic acid have the ability to soften the lens, presenting a promising avenue for treating presbyopia. One obstacle encountered in the preclinical stage of such agents is the need for precise measurements of lens elasticity in experimental models. This study aimed to evaluate the effects of 25-hydroxycholesterol, lipoic acid, and obeticholic acid on the viscoelastic properties of mouse lenses using a custom-built elastometer system. Data were acquired on lenses from C57BL/6J female mice from two age groups: young (age: 8-10 weeks) and old (age: 32-43 weeks). OD lenses were used as the control and OS lenses were treated. Control lenses were immersed in Dulbecco's Modified Eagle Medium (DMEM) and treatment lenses were immersed in a compound solution containing 25-hydroxycholesterol (5 young and 5 old), lipoic acid at 2.35 mM (5 young and 5 old), lipoic acid at 0.66 mM (5 old), or obeticholic acid (5 old) at 37 °C for 18 h. After treatment, the mouse lenses were placed in a DMEM-filled chamber within a custom-built elastometer system that recorded the load and lens shape as the lens was compressed by 600 μm at a speed of 50 μm/s. The load was continuously recorded during compression and during stress-relaxation. The compression phase was fit with a linear function to quantify lens stiffness. The stress-relaxation phase was fit with a 3-term exponential relaxation model providing relaxation time constants (t1, t2, t3), and equilibrium load. The lens stiffness, time constants and equilibrium load were compared for the control and treated groups. Results revealed an increase in stiffness with age for the control group (young: 1.16 ± 0.11 g/mm, old: 1.29 ± 0.14 g/mm) and relaxation time constants decreased with age (young: t1 = 221.9 ± 29.0 s, t2 = 24.7 ± 3.8 s, t3 = 3.12 ± 0.87 s, old: t1 = 183.0 ± 22.0 s, t2 = 20.6 ± 2.6 s and t3 = 2.24 ± 0.43 s). Among the compounds tested, only 25-hydroxycholesterol produced statistically significant changes in the lens stiffness, relaxation time constants, and equilibrium load. In conclusion, older mouse lenses are stiffer and less viscous than young mouse lenses. Notably, no significant change in lens stiffness was observed following treatment with lipoic acid, contrary to previous findings.

Collagen Mimetic Peptides Promote Repair of MMP-1-Damaged Collagen in the Rodent Sclera and Optic Nerve Head

The structural and biomechanical properties of collagen-rich ocular tissues, such as the sclera, are integral to ocular function. The degradation of collagen in such tissues is associated with debilitating ophthalmic diseases such as glaucoma and myopia, which often lead to visual impairment. Collagen mimetic peptides (CMPs) have emerged as an effective treatment to repair damaged collagen in tissues of the optic projection, such as the retina and optic nerve. In this study, we used atomic force microscopy (AFM) to assess the potential of CMPs in restoring tissue stiffness in the optic nerve head (ONH), including the peripapillary sclera (PPS) and the glial lamina. Using rat ONH tissue sections, we induced collagen damage with MMP-1, followed by treatment with CMP-3 or vehicle. MMP-1 significantly reduced the Young's modulus of both the PPS and the glial lamina, indicating tissue softening. Subsequent CMP-3 treatment partially restored tissue stiffness in both the PPS and the glial lamina. Immunohistochemical analyses revealed reduced collagen fragmentation after MMP-1 digestion in CMP-3-treated tissues compared to vehicle controls. In summary, these results demonstrate the potential of CMPs to restore collagen stiffness and structure in ONH tissues following enzymatic damage. CMPs may offer a promising therapeutic avenue for preserving vision in ocular disorders involving collagen remodeling and degradation.