Abstract:
The majority of cases of age-related vision degeneration in the world today are associated with
the lens pathologies presbyopia and cataract. Although effective treatments for presbyopia and
cataract exist, the pathophysiological mechanisms responsible for these two lens pathologies
are not fully understood due to the lack of effective non-invasively methods to study the physiological
optics of the lens under in-vivo conditions. The main parameters that link the cellular
physiology of the lens to its optics are its geometry (shape) and gradient of refractive index
(water to protein ratio). In a series of previous studies, the Molecular Vision Laboratory (MVL)
has used an MRI-based optical modelling approach to show in organ cultured bovine lenses,
that the lens actively maintains its optical properties by regulating lens water transport
(Donaldson et al., 2017). In this thesis, I have optimised the MRI-based optical modelling approach
developed to study the lens in-vitro, to enable it to be successively applied in-vivo to
human subjects across a range of ages, and transgenic mouse models of lens cataract.
I initially developed and validated in-vivo MRI protocols to extract parameters (T2 and geometry)
from the young human lens, which when combined with biometric measures of the eye
and optical modelling (ZEMAX), enabled the physiological optics of the human lens to be
investigated (Pan et al., 2019). Quantitative measurements of lens geometry and T2 showed
high inter-day repeatability, and the resultant optical models produced accurate refractive error
and lens power estimations. To extend this platform to older subjects, I optimised the modelling
platform by implementing equations (Navarro et al., 2007) to capture the GRIN profile change
that develops with age more effectively. Then, I incorporated an additional “age parameter”
into the T2-n calibration to improve the ability of the model to predict each subject’s refractive
error. These improvements to the modelling platform we able to correctly reproduce the decrease
in lens power that is observed clinically with advancing age, known as the lens paradox
(Lie et al., 2020). To measure lens water, I also optimised in-vivo imaging protocols to perform
T1 & PD mapping of the lens by developing a scan protocol that allowed free water (T1) and
total water (PD) content to be rapidly and reliably estimated. The validity of this protocol was
first established using a phantom, before demonstrating its feasibility for in-vivo assessment of
relative contributions of free and total water in the human lens using a cohort of young subjects.
In this cohort, I found that while the T1 profiles of all subjects had the same parabolic shape,
the magnitudes of their T1 values were highly variable. In contrast, the water content of all participants was relatively constant across all regions of the lens. Since this finding was repeatable,
I concluded that the observed subject to subject variability of T1 was not due to technical
issues, but inherent biological variability in free water content between subjects.
In a second project, I had also developed and optimised in-vivo MRI protocols to study the
physiological optics of the mouse lens. This project was conducted at SUNY, Stony Brook,
New York, using a pre-clinical 7T MRI scanner equipped with a customised coil for the mouse
eye. Experiments were performed under the guidance of Dr Eric Muir and utilised transgenic
mouse lines supplied by Professor Thomas White. I helped collect the images and subsequently
developed the post-processing protocols to extract parameters from the acquired images and
perform quantitative analysis. To convert the acquired T2 data to refractive index (n), I utilised
published X-ray interferometry measurements of n to generate a unique T2-n calibration for
the mouse lens (Muir et al., 2020). By combining MRI measurements of ocular biometry and
lens geometry and GRIN into ZEMAX, I created customised eye models for mouse, from
which the optical parameters of the lens can be extracted. This platform was applied to wildtype
C57BL/6 mice and two mouse lines in which the gap junction coupling was either decreased
(Cx46Het) or increased (Cx50KI46) by genetically modifying the expression of Cx46.
These proof of principles experiments showed that reducing Cx46 mediated gap junction coupling
had no significant effects on in either water content, GRIN or geometry in Cx46Het mice
relative to the WT mice. In contrast, increasing gap junction coupling led to a significant increase
in water content and decrease in refractive index, respectively, in the nucleus of
Cx50KI46 mice, which together reduced the optical power of the lens.
In summary, the MRI-based optical modelling protocols I have developed in this thesis have
the potential to allow non-invasive imaging of the human lens in-vivo, which can facilitate
longitudinal studies into how age affects the physiological optics of the lens. In addition, the
application of this platform to study a variety of transgenic mice models will increase our understanding
of the link between the molecular and cellular physiology of the lens and the overall
optical properties of the lens, which will provide new insights into the mechanisms of lens
ageing that result in presbyopia in middle age and cataract in the elderly.