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Recent Studies on Myopia in the Mouse Model: Some Examples
Magnitudes of experimentally induced refractive errors in wild-type mice
The success of experiments to induce “deprivation myopia” (the type of myopia that develops “by default” when the retinal image clarity is experimentally degraded) was surprisingly variable across studies. Schaeffel and Burkhardt,1 using frosted hemispherical diffusers (Fig. 3) obtained a small myopic shift only 3–4 D after two weeks, starting at P24, which reached a significance level of p < 0.00036 in eight mice after one outlyer was excluded. Tejedor and de la Villa,2 using lid suture for up to 20 days, starting at P10-11, induced little more than 6 D of myopia and found an impressively smooth correlation between axial length changes and refraction changes. Barathi et al.,21 using early lid suture in a large number of albino mice (n = 80), starting at P10 and continuing until P56, induced up to 14 D of myopia and an axial elongation of about 200 m. In experiments by Pardue et al.,25 it took wild-type mice eight weeks, starting at P28, to develop a myopic shift of about 5.5 D. Tkatchenko and Tkatchenko30 induced about 45 m increase in vitreous chamber depth by treating C57BL/6J mice with frosted diffusers from P24 for a duration of 21 days. These changes were measured with a demanding, small animal MRI. Biometric changes were accompanied by refraction shifts of about 4 D, but the significance levels were low due to the low number of animals (n = 4).
The large variability in the diffuser experiments cannot be explained by poor measurement resolution (see description of the resolution of the techniques used above); it must result from low sensitivity of the emmetropization mechanism to changes in visual experience, and the fact that axial eye growth is rather slow in mice (Fig. 1A). Somehow, one might expect poor responsiveness due to the large depth of focus (more than +10 D). But then it remains unclear why the variability of the refractions
among untreated animals is considerably less than the depth of focus (about +3 D22; or even less ±1.14 D).10 This poses the question as to what
keeps the refractions in such close range when emmetropization is slightly sensitive to visual input?
Only a few studies are available in which mice were treated with eyeglass lenses. Barathi et al.21 treated 100 albino mice with –10 D lenses for up to
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46 days, and induced an increase in axial length of up to about 0.37 mm and myopia of 14 D — the largest effect ever observed in mice. These lenses had even stronger effects than lid suture over the same treatment period, but non-visual effects of lid suture on the geometry of the anterior segment of the eye are always possible. Faulkner et al.65 used –10 D lenses in nob mice and induced similar amounts of myopia (3–4 D) as with diffusers.
Burkhardt and Schaeffel failed to induce compensatory growth inhibition in response to treatment with positive lenses (unpublished observations, 2006). It remains uncertain whether the mouse eye can distinguish between image diffuse, image degradation and defocus. Further studies, in particular with positive lenses, are necessary.
In summary, the literature agrees that deprivation myopia can be induced in mice but that the treatment duration should be several weeks and the goggles need to be applied as early as possible (soon after eye opening). The effects are small in most studies (few diopters and less than 50 m axial length changes). The few studies also show that –10 D lenses can induce myopia, but it remains to be discovered whether diffuse image blur and image defocus are, in fact, distinguished by the mouse retina.
Refractive development in mutant mice
A few studies have already appeared in which the effects of permanent know-out of a gene on refractive development were studied. Pardue et al.25 found that the susceptibility to deprivation myopia was greatly enhanced in nob mice with a mutation in the Nyx gene (lacking the ERG b-wave due to a defect in the rod ON pathway), which is linked to congenital stationary night blindness (CSNB) in humans: about 5.5 D of myopia could be induced in two weeks, starting at P28, compared to only about 1 D in the respective wild-type. Nob mice had significantly lower retinal dopamine and DOPAC levels than the wild-type and — in contrast to the wild-type — no changes could be induced by diffuser wear. Schippert et al.27 found that mice lacking a functional gene for the transcription factor Egr-1 were more myopic than their heterozygous and wild-type littermates. The homozygous knock-out animals also had significantly longer eyes between P42 and P56, but approached wild-type dimensions later in development. They showed no differences in corneal curvature or anterior chamber depth. They also had normal optomotor responses. This suggests that the effect of Egr-1 knock-out is surprisingly selective for axial eye growth. That these knock-out animals were more myopic fits the idea that
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the EGR-1 protein appears to be associated with an inhibition signal for axial eye growth: in chickens, the protein is up-regulated when hyperopia is induced and down-regulated when myopia is induced.66,67 More recently, Schippert et al.68 presented an analysis of the retinal gene expression patterns in Egr-1 knock-out mice. Similar to Schippert et al.,27 Zhou et al.69 found that an adenosine A2a receptor knock-out mouse went through a phase of longer axial length and relative myopia, similar to what was observed in the Egr-1 knock-out mice (between P42 and 56), but returned to normal axial lengths later in development. Furthermore, these authors found that myopia was accompanied by reduced collagen fibril diameters in the sclera.69 A more extended screening for refractive errors in mutant mice was presented by Faulkner et al.70 Significant refraction differences were detected between C57Bl/6J mice (which had refractive errors from 6.9 to 8.5 D), and nob and rd1 mice, which were about 2 D more hyperopic, and GABAC null mice, which were about 5 D more myopic than C57 mice. A next step would be to find out which morphological changes determine the changes in refraction. It could even be that retinal thickness changes, related to the degenerations, underlie the changes in measured refractions.
Pharmacological studies to inhibit axial eye growth in mice
Atropine is known as a potent inhibitor of myopia in humans and animal models. A problem that arises if atropine is unilaterally applied in mice is that the fellow “control” eye may also be contaminated with atropine due to the cleaning behavior of the mice. Barathi et al.71 have therefore used the light-induced pupil response to probe the transfer of atropine to the fellow eye. They found that the contralateral light-induced pupil response was also affected by ipsilateral atropine application, but only to a small extent, making an inter-ocular comparison of atropine effects possible. They also found that daily application of a single drop of 1% atropine induced more hyperopic refractions and shorter axial lengths over time — despite visual experience being normal. This is different from the chicken where the effects of atropine are largely confined to a suppression of myopia that would be induced by diffusers or negative lenses (e.g. Ref. 72). Barathi et al.73 have also studied the expression of muscarinic receptors in both human and mouse sclera and their role in the regulation of scleral fibroblasts. Further studies will show whether the eye growth inhibition exerted by atropine is mediated through these receptors.