Novel Rodent Models for Macular Research

BACKGROUND: Many disabling human retinal disorders involve the central retina, particularly the macula. However, the commonly used rodent models in research, mouse and rat, do not possess a macula. The purpose of this study was to identify small laboratory rodents with a significant central region as potential new models for macular research. METHODOLOGY/PRINCIPAL FINDINGS: Gerbillus perpallidus, Meriones unguiculatus and Phodopus campbelli, laboratory rodents less commonly used in retinal research, were subjected to confocal scanning laser ophthalmoscopy (cSLO), fluorescein and indocyanine green angiography, and spectral-domain optical coherence tomography (SD-OCT) using standard equipment (Heidelberg Engineering HRA1 and Spectralis™) adapted to small rodent eyes. The existence of a visual streak-like pattern was assessed on the basis of vascular topography, retinal thickness, and the topography of retinal ganglion cells and cone photoreceptors. All three species examined showed evidence of a significant horizontal streak-like specialization. cSLO angiography and retinal wholemounts revealed that superficial retinal blood vessels typically ramify and narrow into a sparse capillary net at the border of the respective area located dorsal to the optic nerve. Similar to the macular region, there was an absence of larger blood vessels in the streak region. Furthermore, the thickness of the photoreceptor layer and the population density of neurons in the ganglion cell layer were markedly increased in the visual streak region. CONCLUSIONS/SIGNIFICANCE: The retinal specializations of Gerbillus perpallidus, Meriones unguiculatus and Phodopus campbelli resemble features of the primate macula. Hence, the rodents reported here may serve to study aspects of macular development and diseases like age-related macular degeneration and diabetic macular edema, and the preclinical assessment of therapeutic strategies

Immunofluorescence labeling of retinal sections revealed distinct differences in cone cell distribution and vessel density.

<p>(<b>A–F</b>) Areas within the visual streak region are shown in the right panel, areas from dorsal peripheral retina in the left panel. (<b>A</b>) Isolectin IB4-FITC staining of G. perpallidus retina reveals the sparse vessel distribution within the streak region; (<b>B</b>) Peanut agglutinin staining of P. campbelli retina illustrates increased cone densities within the streak region. (<b>C</b>) SWS cone staining of M. unguiculatus retina reveals enhanced SWS cone density within the streak area compared to dorsal periphery. Rods and MWS cones are evenly distributed across the retina as shown by (<b>D</b>) rod transducin (GNAT1) staining in M. unguiculatus, (<b>E</b>) rod opsin staining, and (F) MWS cone opsin staining in G. perpallidus. (<b>A–F</b>) Nuclei were contrasted with DAPI (in blue).</p

Detailed retinal analysis <i>in vitro</i>.

<p>(<b>A</b>, <b>B</b>) G. perpallidus, (<b>C</b>, <b>D</b>) M. unguiculatus, (E, F) P. campbelli. Methylen blue-stained vertical semithin sections, RPE-aligned, confirming the increase in outer retinal thickness. (<b>A</b>, <b>C</b>, <b>E</b>) outside the visual streak, (<b>B</b>, <b>D</b>, <b>F</b>) within the visual streak region. (<b>G</b>, <b>H</b>) Representative electron microscopy of G. perpallidus, (<b>G</b>) outside the visual streak, (H) within the visual streak region, showing the slanted longer outer segments. (<b>I</b>) Ganglion cell layer in the streak region of <i>G. perpallidus</i> demonstrated by cresyl violet-staining of a retinal flatmount. Shown is the dorsal edge of the visual streak (arrowheads) with higher neuron densities inside (bottom) than outside (top). (<b>J</b>) Cone photoreceptors in a retinal flatmount of <i>G. perpallidus</i>, immunolabeled for MWS cone opsin (PAP/DAB). Note the dorsal edge of the streak (arrowheads) with higher cone densities inside (bottom) than outside (top). As the few SWS cones also co-express MWS opsin, the staining shows all cones. (<b>K</b>) Cones in a retinal flatmount of <i>P. campbelli</i>, double-immunoflourescence labeled for MWS opsin (red) and SWS opsin (green). Shown is the SWS opsin expression edge at the streak, dorsal of the edge only MWS opsin is expressed (top). Scale bar in E = 50 µm for A–F, scale bar in J = 200 µm for I and J, scale bar in K = 100 µm.</p

<i>In vivo</i> fundus imaging.

<p>cSLO fundus images of (<b>A</b>) human, (<b>B</b>) mouse, and (<b>C</b>) Meriones unguiculatus. (<b>A–C</b>) D = dorsal, V = ventral. (<b>A</b>) Native infrared (λ = 830 nm) SLO image of one researcher's right eye illustrates the sparse vascularization of the macular region. (<b>B</b>) Fluorescein angiography (FLA) (488 nm, barrier filter at 500 nm) reveals the evenly distributed retinal blood vessels in mice. (<b>C</b>) FLA image of Meriones unguiculatus visualizes the horizontal sparse vascular band denoting the specialized retinal region dorsal to the optic disc.</p

Detailed analysis of the retinal blood vessel distribution <i>in vivo</i> using cSLO and <i>in vitro</i> using collagen IV staining.

<p>(<b>A–C</b>) G. perpallidus, (<b>D–F</b>) M. unguiculatus, and (<b>G–I</b>) P. campbelli. Collagen IV stained retinal wholemounts (<b>A</b>, <b>D</b>, <b>G</b>) illustrate the entire visual streak region. Magnifications of wholemounts (<b>B</b>, <b>E</b>, <b>H</b>) and corresponding in vivo cSLO data (<b>C</b>, <b>F</b>, <b>I</b>) show that retinal vessels ramify and narrow into a sparse capillary net at the border of the respective area.</p

<i>In vivo</i> morphological analysis and retinal layering with cSLO angiography and SD-OCT imaging.

<p>(<b>A–C</b>) Gerbillus perpallidus, (<b>D–F</b>) Meriones unguiculatus, and (<b>G–I</b>) Phodopus campbelli. cSLO en-face imaging of the vascular pattern using (<b>A</b>, <b>D</b>, <b>G</b>) ICG angiography mode (795 nm with barrier filter at 800 nm) depicts both retinal and choroidal structures. Black line depicts OCT scan direction. (<b>B</b>, <b>E</b>, <b>H</b>) FLA displays large retinal vessels and capillaries. Besides, choroidal vessels are visible in the visual steak region as different retinal layering allows light at λ = 488 nm to better penetrate the RPE/choriocapillary complex. (<b>C</b>, <b>F</b>, <b>I</b>) In vivo cross-sectional SD-OCT images show an increased retinal thickness in the visual streak region, indicating a specialized retinal region (arrows). ONL, outer nuclear layer; RPE, retinal pigment epithilium.</p