Conversations with Ray Guillery on albinism: linking Siamese cat visual pathway connectivity to mouse retinal development

Three-dimensional topography of eye-specific domains in the lateral geniculate nucleus of pigmented and albino rats

We previously revealed the presence of ocular dominance columns (ODCs) in the primary visual cortex (V1) of pigmented rats. On the other hand, previous studies have shown that the ipsilateral-eye domains of the dorsal lateral geniculate nucleus (dLGN) are segregated into a handful of patches in pigmented rats. To investigate the three-dimensional (3D) topography of the eye-specific patches of the dLGN and its relationship with ODCs, we injected different tracers into the right and left eyes and examined strain difference, development, and plasticity of the patches. Furthermore, we applied the tissue clearing technique to reveal the 3D morphology of the LGN and were able to observe entire retinotopic map of the rat dLGN at a certain angle. Our results show that the ipsilateral domains of the dLGN appear mesh-like at any angle and are developed at around time of eye-opening. Their development was moderately affected by abnormal visual experience, but the patch formation was not disrupted. In albino Wistar rats, ipsilateral patches were observed in the dLGN, but they were much fewer, especially near the central visual field. These results provide insights into how ipsilateral patches of the dLGN arise, and how the geniculo-cortical arrangement is different between rodents and primates.

Pigmented Long-Evans rats demonstrate better visual ability than albino Wistar rats in slow angles-descent forepaw grasping test

Albino people are known to have vision deficit. Albino animals are shown to have abnormal connectivity and malformation of the visual system. However, not many studies have revealed visual impairment of albino animals in the level of perception. To link anatomical abnormality and perceptual visual impairment of albinism, we compared the perceptual vision between the pigmented Long-Evans and the albino Wistar rats. We used the slow angled-descent forepaw grasping (SLAG) test. We hanged the rats in the air by their tails and slowly moved them around a safety bar so that they could see it. When the rats recognized the bar and try to grab it to escape, we counted the trial as 'positive', and we measured positive rates. We also measured the distance between the bar and their whiskers during the rats' initial grasping action, and evaluated type of action at the first contact to the bar. The positive-action rate in the Long-Evans rat group showed significantly higher than the Wistar rat group (0.85 ± 0.047, n = 10, vs. 0.29 ± 0.043, n = 10; P < 0.0001). Besides, when the action was positive, the distance between the bar and their whiskers was longer in the Long-Evans rat group than that in the Wistar rat group (117 ± 5.3 mm vs. 58.8 ± 4.6 mm; P < 0.0001). The Long-Evans rats grasped the bar more precisely than the Wistar rats. The pigmented Long-Evans rats have much better visual perception than the albino Wistar rats.

The contribution of common regulatory and protein-coding TYR variants to the genetic architecture of albinism

Genetic diseases have been historically segregated into rare Mendelian disorders and common complex conditions. Large-scale studies using genome sequencing are eroding this distinction and are gradually unmasking the underlying complexity of human traits. Here, we analysed data from the Genomics England 100,000 Genomes Project and from a cohort of 1313 individuals with albinism aiming to gain insights into the genetic architecture of this archetypal rare disorder. We investigated the contribution of protein-coding and regulatory variants both rare and common. We focused on TYR, the gene encoding tyrosinase, and found that a high-frequency promoter variant, TYR c.-301C>T [rs4547091], modulates the penetrance of a prevalent, albinism-associated missense change, TYR c.1205G>A (p.Arg402Gln) [rs1126809]. We also found that homozygosity for a haplotype formed by three common, functionally-relevant variants, TYR c.[-301C;575C>A;1205G>A], is associated with a high probability of receiving an albinism diagnosis (OR>82). This genotype is also associated with reduced visual acuity and with increased central retinal thickness in UK Biobank participants. Finally, we report how the combined analysis of rare and common variants can increase diagnostic yield and can help inform genetic counselling in families with albinism.

The Dct−/− Mouse Model to Unravel Retinogenesis Misregulation in Patients with Albinism

We have recently identified DCT encoding dopachrome tautomerase (DCT) as the eighth gene for oculocutaneous albinism (OCA). Patients with loss of function of DCT suffer from eye hypopigmentation and retinal dystrophy. Here we investigate the eye phenotype in Dct^−/− mice. We show that their retinal pigmented epithelium (RPE) is severely hypopigmented from early stages, contrasting with the darker melanocytic tissues. Multimodal imaging reveals specific RPE cellular defects. Melanosomes are fewer with correct subcellular localization but disrupted melanization. RPE cell size is globally increased and heterogeneous. P-cadherin labeling of Dct^−/− newborn RPE reveals a defect in adherens junctions similar to what has been described in tyrosinase-deficient Tyr^c/c embryos. The first intermediate of melanin biosynthesis, dihydroxyphenylalanine (L-Dopa), which is thought to control retinogenesis, is detected in substantial yet significantly reduced amounts in Dct^−/− postnatal mouse eyecups. L-Dopa synthesis in the RPE alone remains to be evaluated during the critical period of retinogenesis. The Dct^−/− mouse should prove useful in understanding the molecular regulation of retinal development and aging of the hypopigmented eye. This may guide therapeutic strategies to prevent vision deficits in patients with albinism.

Retinal Ganglion Cell Axon Wiring Establishing the Binocular Circuit

Binocular vision depends on retinal ganglion cell (RGC) axon projection either to the same side or to the opposite side of the brain. In this article, we review the molecular mechanisms for decussation of RGC axons, with a focus on axon guidance signaling at the optic chiasm and ipsi- and contralateral axon organization in the optic tract prior to and during targeting. The spatial and temporal features of RGC neurogenesis that give rise to ipsilateral and contralateral identity are described. The albino visual system is highlighted as an apt comparative model for understanding RGC decussation, as albinos have a reduced ipsilateral projection and altered RGC neurogenesis associated with perturbed melanogenesis in the retinal pigment epithelium. Understanding the steps for RGC specification into ipsi- and contralateral subtypes will facilitate differentiation of stem cells into RGCs with proper navigational abilities for effective axon regeneration and correct targeting of higher-order visual centers.