Establishing the ground squirrel as a superb model for retinal ganglion cell disorders and optic neuropathies

Abstract

Retinal ganglion cell (RGC) death occurs after optic nerve injury due to acute trauma or chronic degenerative conditions such as optic neuropathies (e.g. glaucoma). Currently, there are no effective therapies to prevent permanent vision loss resulting from RGC death, underlining the need for research on the pathogenesis of RGC disorders. Modeling human RGC/optic nerve diseases in non-human primates is ideal because of their similarity to humans, but has practical limitations including high cost and ethical considerations. In addition, many retinal degenerative disorders are age-related making the study in primate models prohibitively slow. For these reasons, mice and rats are commonly used to model RGC injuries. However, as nocturnal mammals, these rodents have retinal structures that differ from primates - possessing less than one-tenth of the RGCs found in the primate retina. Here we report the diurnal thirteen-lined ground squirrel (TLGS) as an alternative model. Compared to other rodent models, the number and distribution of RGCs in the TLGS retina are closer to primates. The TLGS retina possesses ~600,000 RGCs with the highest density along the equatorial retina matching the location of the highest cone density (visual streak). TLGS and primate retinas also share a similar interlocking pattern between RGC axons and astrocyte processes in the retina nerve fiber layer (RNFL). In addition, using TLGS we establish a new partial optic nerve injury model that precisely controls the extent of injury while sparing a portion of the retina as an ideal internal control for investigating the pathophysiology of axon degeneration and RGC death. Moreover, in vivo optical coherence tomography (OCT) imaging and ex vivo microscopic examinations of the retina in optic nerve injured TLGS confirm RGC loss precedes proximal axon degeneration, recapitulating human pathology. Thus, the TLGS retina is an excellent model, for translational research in neurodegeneration and therapeutic neuroprotection.

Developmental errors in the common marmoset retina

“…Given the inter-animal variability we observed, we therefore consider the numerous displaced RGCs preferentially located near the fovea to be misplaced somata and perhaps a reflection of packing density constraints. A similar concentration of displaced RGCs near the visual streak has been documented in the ground squirrel retina (Xiao et al, 2021). We confirmed the intentionally-displaced population of ON SACs, but noted misplaced starburst-like ACs in the outer INL in several retinas with a very unusual extension of their dendrites into the OPL.…”

Pan-retinal ganglion cell markers in mice, rats, and rhesus macaques

“…Methodologically, nuclear BRN3A signals do not overlap with RGC somas, thus enabling easier manual and automated quantification ( Galindo-Romero et al., 2011 ; Geeraerts et al., 2016 ; Masin et al., 2021 ; Miesfeld et al., 2020 ; Nadal-Nicolás et al., 2009 ; Sánchez-Migallón et al., 2011 ). Automatic algorithms of RBPMS are reliable and efficient in mouse retinas ( Dordea et al., 2016 ; Guymer et al., 2020 ; Masin et al., 2021 ; Zhang et al., 2022 ), but in other species, e.g., ground squirrels, high-density RGC areas impair soma discrimination and accurate quantification ( Xiao et al., 2021 ).…”

“…In his work “La retina de los vertebrados” (1893), Nobel Prize winner Santiago Ramón y Cajal summarized why the retina is an excellent model for studying neurodegeneration: “its accessibility, its orderly organization in alternate layers of cell bodies and intercellular contacts, and the easy identification of the main direction of the nervous message flow” ( Cuenca & de la Villa, 2021 ). Indeed, the retina offers several advantages over other parts of the central nervous system (CNS), notably: (i) ocular surfaces are transparent, which allows longitudinal studies in vivo with non-invasive techniques to track morphological and functional changes in the retina; (ii) treatments, whether cells or drugs, can be administered intravitreally or subretinally, thus avoiding possible systemic adverse effects; (iii) systemic treatments are feasible if the drug crosses the blood-retina barrier; (iv) behavioral analyses are available to test the reach of functional and anatomical improvement; and (v) well-established models of retinal degeneration and disease are available in rats and mice ( García-Ayuso et al., 2019a , 2019b ; Parrilla-Reverter et al., 2009 ; Vidal-Sanz et al., 2012 , 2017 ), as well as in other species such as pigs, ground squirrels, and rabbits ( Sasaoka et al., 2006 ; Völgyi & Bloomfield, 2002 ; Xiao et al., 2021 ); and vi: new therapies may be extrapolated to the rest of the CNS.…”

“…BRN3A expression in adult RGCs has been described in fish ( Sato et al., 2007 ), birds ( chickens and pigeons, Galindo-Romero et al., 2016 ; Sheng et al., 2021 ), and mammals ( mice, rats, cats, ground squirrels, humans, monkeys, pigs, dogs, alpacas, and blind moles, Ávila-García et al., 2012 ; Bouskila et al., 2020 ; Esquiva et al., 2016 ; Galindo-Romero et al., 2011 ; Kim et al., 2006 ; Nadal-Nicolás et al., 2009 ; Ruzafa et al., 2017 ; Wang et al., 2015 ; Xiang et al., 1995 ; Xiao et al., 2021 ; Zhang et al., 2010 ).…”

“…; 2010 ). Subsequent studies investigated RGC degeneration models ( Kwong et al., 2011 ) and characterized RBPMS in other mammalian species, such as guinea pigs, monkeys, mice ( Rodriguez et al., 2014 ), cats ( Wang et al., 2016 ), blind moles ( Esquiva et al., 2016 ), pigs, humans ( Pereiro et al., 2020 ), and ground squirrels ( Xiao et al., 2021 ).…”