L-opsin expression in chickens is similarly reduced with diffusers and negative lenses

Diurnal gene expression patterns in retina and choroid distinguish myopia progression from myopia onset

The world-wide prevalence of myopia (nearsightedness) is increasing, but its pathogenesis is incompletely understood. Among many putative mechanisms, laboratory and clinical findings have implicated circadian biology in the etiology of myopia. Consistent with a circadian hypothesis, we recently reported a marked variability in diurnal patterns of gene expression in two crucial tissues controlling post-natal refractive development - the retina and choroid-at the onset of form-deprivation myopia in chick, a widely studied and validated model. To extend these observations, we assayed gene expression by RNA-Seq in retina and choroid during the progression of established unilateral form-deprivation myopia of chick. We assayed gene expression every 4 hours during a single day from myopic and contralateral control eyes. Retinal and choroidal gene expression in myopic vs. control eyes during myopia progression differed strikingly at discrete times during the day. Very few differentially expressed genes occurred at more than one time in either tissue during progressing myopia. Similarly, Gene Set Enrichment Analysis pathways varied markedly by time during the day. Some of the differentially expressed genes in progressing myopia coincided with candidate genes for human myopia, but only partially corresponded with genes previously identified at myopia onset. Considering other laboratory findings and human genetics and epidemiology, these results further link circadian biology to the pathogenesis of myopia; but they also point to important mechanistic differences between the onset of myopia and the progression of established myopia. Future laboratory and clinical investigations should systematically incorporate circadian mechanisms in studying the etiology of myopia and in seeking more effective treatments to normalize eye growth in children.

Mechanisms of emmetropization and what might go wrong in myopia

Studies in animal models and humans have shown that refractive state is optimized during postnatal development by a closed-loop negative feedback system that uses retinal image defocus as an error signal, a mechanism called emmetropization. The sensor to detect defocus and its sign resides in the retina itself. The retina and/or the retinal pigment epithelium (RPE) presumably releases biochemical messengers to change choroidal thickness and modulate the growth rates of the underlying sclera. A central question arises: if emmetropization operates as a closed-loop system, why does it not stop myopia development? Recent experiments in young human subjects have shown that (1) the emmetropic retina can perfectly distinguish between real positive defocus and simulated defocus, and trigger transient axial eye shortening or elongation, respectively. (2) Strikingly, the myopic retina has reduced ability to inhibit eye growth when positive defocus is imposed. (3) The bi-directional response of the emmetropic retina is elicited with low spatial frequency information below 8 cyc/deg, which makes it unlikely that optical higher-order aberrations play a role. (4) The retinal mechanism for the detection of the sign of defocus involves a comparison of defocus blur in the blue (S-cone) and red end of the spectrum (L + M-cones) but, again, the myopic retina is not responsive, at least not in short-term experiments. This suggests that it cannot fully trigger the inhibitory arm of the emmetropization feedback loop. As a result, with an open feedback loop, myopia development becomes "open-loop".

Diurnal retinal and choroidal gene expression patterns support a role for circadian biology in myopia pathogenesis

The prevalence of myopia (nearsightedness) is increasing to alarming levels, but its etiology remains poorly understood. Because both laboratory and clinical findings suggest an etiologic role for circadian rhythms in myopia development, we assayed gene expression by RNA-Seq in retina and choroid at the onset of unilateral experimental myopia in chick, isolating tissues every 4 h during a single 24-h period from myopic and contralateral control eyes. Occluded versus open eye gene expression differences varied considerably over the 24-h sampling period, with some occurring at multiple times of day but with others showing differences at only a single investigated timepoint. Some of the genes identified in retina or choroid of chick myopia were previously identified as candidate genes for common human myopia. Like differentially expressed genes, pathways identified by Gene Set Enrichment Analysis also varied dramatically by sampling time. Considered with other laboratory data, human genetic and epidemiology data, these findings further implicate circadian events in myopia pathogenesis. The present results emphasize a need to include time of day in mechanistic studies of myopia and to assess circadian biology directly in trying to understand better the origin of myopia and to develop more effective therapies.