Two Pairs of ON and OFF Retinal Ganglion Cells Are Defined by Intersectional Patterns of Transcription Factor Expression

Abstract

Visual information is conveyed to the brain by axons of >30 retinal ganglion cell (RGC) types. Characterization of these types is a prerequisite to understanding visual perception. Here, we identify a family of RGCs that we call F-RGCs on the basis of expression of the transcription factor Foxp2. Intersectional expression of Foxp1 and Brn3 transcription factors divides F-RGCs into four types, comprising two pairs, each composed of closely related cells. One pair, F-mini(ON) and F-mini(OFF), shows robust direction selectivity. They are among the smallest RGCs in the mouse retina. The other pair, F-midi(ON) and F-midi(OFF), is larger and not direction selective. Together, F-RGCs comprise >20% of RGCs in the mouse retina, halving the number that remain to be classified and doubling the number of known direction-selective cells. Co-expression of Foxp and Brn3 genes also marks subsets of RGCs in macaques that could be primate homologs of F-RGCs.

Figure 1. Foxp2 expression distinguishes F-RGCs from currently known types

(A) Model showing the major neuronal classes in the retina. R, rods; C, cones; HC, horizontal cells; BC, bipolar cells; AC, amacrine cells; RGC, retinal ganglion cells; inl, inner nuclear layer; ipl, inner plexiform layer; gcl, ganglion cell layer. (B, C) Antibody staining analysis for Foxp2 plus molecular markers for RGCs, RBPMS (B), and amacrine cells, Ap2 (C), in the adult mouse retina. Arrows point to the same cells in each panel. (D) Immunostaining for Foxp2 combined with antibodies to all three Brn3-transcription factors (a, b, c; “pan-Brn3”). (E) Density of Foxp2 RGCs as a group compared to that of molecularly defined RGC types (from Sanes and Masland, 2015). (F) Analysis of Foxp2 RGCs combined with markers for the following RGC types: Osteopontin, alpha-RGCs (top); Melanopsin, ip-RGCs (middle); and Cart, ooDSGCs (bottom). All images are taken from the central/ventral region of the retina. Scale bars in (B, C) = 50 μm; (D, F) = 100 μm. See also Figure S1.

Figure 2. Combinatorial expression of transcription factors divide F-RGCs into 4 types

(A) Retinal mosaics modeled as close-packed hexagonal arrays with positional jitter. Dots represent single arrays (red or green) of the same side-length as defined by Zhang et al. (2012). (B) DRP on a single array (A → A; black dashed line) produces a slope that scales with side-length. DRP on a mixture of 2 arrays (A+B → A+B; grey dashed line) of the same side-length produces a slope mid-way between a random distribution and a single array, allowing for calculation of the fraction f of an array labeled (f < 1, partial array; f > 1, mixture of arrays). Random distribution is represented by the dotted line. (C, D) Immunolabeling of Foxp2 in adult mouse retina (C). DRP of total Foxp2 RGCs is that of a mixed array with f = 2.2 (D). (E, F) Foxp1 divides Foxp2 RGCs into 2 groups (E). DRP on Foxp1⁺ F-RGCs (orange line) and Foxp1⁻ F-RGCs (red line) are compared to that of a mixture of 2 arrays (gray dashed line) and a single array of the same side length (black dashed line) (F). (G, H) Brn3b divides Foxp1⁺ F-RGCs into 2 types, one abundant (Foxp1⁺/Foxp2⁺/Brn3b⁻; asterisks), and one sparse (Foxp1⁺/Foxp2⁺/Brn3b⁺; dashed circles) (G). DRP of abundant (orange line) and sparse (peach line) Foxp1⁺ F-RGCs resemble matched single arrays of similar side-lengths (dashed lines), indicating they each form a single array, with f ≈ 1 (H). (I, J) Brn3c divides Foxp1⁻ F-RGCs into two types, one abundant (Foxp1⁻/Foxp2⁺/Brn3c⁻; asterisks) and one sparse (Foxp1⁻/Foxp2⁺/Brn3c⁺; dashed circles) (I). DRP of abundant (red line) and sparse (pink line) Foxp1⁻ F-RGCs resemble matched single arrays of similar side-lengths (dashed lines), indicating they each form a single array, with f ≈ 1 (J). (K, L) Quadruple immunolabeling with Foxp1, Foxp2, Brn3b, and Brn3c marks the four F-RGC types simultaneously (K). Colored asterisks represent the relative position and identity of molecularly defined F-RGC types (L). Red, F-mini^ON; yellow, F-mini^OFF; purple, F-midi^ON; cyan, F-midi^OFF. Empty dotted circles indicate dim cells which were not categorized. (M) Contribution of each F-RGC type to the total Foxp2⁺ RGC population. n = 4 retinas from 4 animals per type. Scale bar = 50 μm. (N) Dendogram showing the four molecularly defined F-RGC types and their combinatorial TFs. See also Figure S2.

Figure 3. Morphological characterization of F-RGCs

(A) Schematic of the Foxp2-ires-Cre:GFP (Foxp2^Cre) allele. (B) Immunostaining for Foxp2, Foxp1, and GFP in adult Foxp2^Cre retinas following intravitreal injection of high-titer AAV2/9^flexGFP. Only infected Foxp2⁺ RGCs are labeled. (C–F) Injection of low-titer AAV2/9^flexGFP into Foxp2-Cre mice reveals 4 morphologically distinct types that correspond to discrete molecular identities: F-midi^OFF, Foxp2⁺/Foxp1⁺/Brnb⁺ (C) F-mini^OFF, Foxp2⁺/Foxp1⁺/Brnb⁻ (D) F-mini^ON, Foxp2⁺/Foxp1⁻/Brn3c⁻ (E) F-midi^ON, Foxp2⁺/Foxp1⁻/Brn3c⁺ (F). (G) Dendritic stratification depth of individually segmented F-RGC types. n = 5–7 cells per type. (H) Box plot of dendritic field areas for indicated RGC types. W3B and F-mini^ON RGCs are similar in size whereas F-mini^OFF RGCs tended to be smaller with a trend toward significance (p = 0.07). (I) Analysis of dendritic asymmetry calculated for indicated RGC types (0 = perfect symmetry; 1 = perfect asymmetry. (J) Dendritic coverage factor (CF) for F-RGCs. Data for J-, W3B, and alpha- RGC types from Kim et al., 2008, . n = 7–15 cells per type. *, p<0.05, two-tailed t-test. Scale bar in (C–F, bottom row) = 50 μm. See also Figure S3.

Figure 4. F-RGC axons selectively innervate image-forming visual targets in the brain

(A–I) F-RGC central projections revealed by intravitreal injection of AAV2/9^flexGFP and fluorophore-conjugated cholera toxin b (CTB) in adult Foxp2^Cre mice (A). Sections are from dorsal and ventral lateral geniculate nuclei and intergeniculate leaflet of the thalamus (dLGN, vLGN, IGL; B,C); stratum griseum superficiale of the superior colliculus (SGS; D, E); suprachiasmatic nucleus (SCN; F,G); medial terminal nucleus (MTN, top) and olivary pretectal nucleus (OPN, bottom) (H, I). Images are representative from n = 4 animals. (J) Schematic of F-RGC central projections.

Figure 5. Visual responses of F-RGCs

(A–D) Representative responses to a spot flashing ON (white) and OFF (grey) over the receptive field center for each cell. Raster plots of spikes from 14–20 repeats. Histograms (right) show frequency of spikes over time. (E–H) Responses to flashing spots of different radii. The number of spikes are plotted during the optimal response period (ON or OFF) for each type, normalized to the maximum response. Each plot shows a single cell and data points averaged across 10 trials for each spot radius (mean ± SE). (I–L) Responses to bars moving across the receptive field center in different directions. Polar plots represent the number of spikes fired for bars moving in each of the eight directions. (M) Receptive field size calculated as the optimal spot radius. (N) Direction selectivity index (DSI) was calculated as the length of the vector sum in the preferred direction divided by the sum of responses to all directions. F-mini RGCs show directional tuning (DSI > 0.25), whereas F-midi cells are direction non-selective (DSI < 0.1). An alternate method for calculating DSI, preferred minus null response, gives DSI of >0.4 for F-mini and <0.2 for F-midi RGCs. n = 10, 8, 3, and 2 cells for F-mini^ON, F-mini^OFF, F-midi^ON, and F-midi^OFF types. Open circles or squares indicate cells tested for direction selectivity at different speeds. Comparison to other RGC types from Kim et al., 2008, . (O) Speed tuning curves for F-mini RGCs. See also Figure S5.

Figure 6. F-RGCs are organized anisotropically along the DV-axis of the retina

(A–E) Whole mount of retina stained for Foxp2 (A). Density heat maps for all F-RGCs (inset in A) and each F-RGC type (B–E) from retinas stained with Foxp2 plus Foxp1 and Brn3 isoforms. D, dorsal; V, ventral; asterisks, optic disk. (F–I) Scatter plots of dendritic field area versus local density and coverage factor. Density and area co-vary while dendritic coverage stays relatively constant (CF = 2.41 ± 0.10, F-mini^OFF; 2.90 ± 0.11, F-mini^ON; 1.49 ± 0.10, F-midi^OFF; 1.54 ± 0.08, F-midi^ON; mean ± SE). (J–M) Spatial analysis (nearest-neighbor distance and spatial regularity) at different axial positions shows that mosaic spacing is globally maintained for each F-RGC type despite local changes in density. (N) GFP-labeled F-mini^ON RGCs from dorsal and ventral retina. (O) Position and dendritic orientation of F-mini^ON (red) and F-mini^OFF (purple) RGCs. All F-mini^OFF RGCs point ventrally whereas F-mini^ON RGCs point to the opsin transition zone (OTZ; localized as shown in Figure S6C). The dendritic orientation of F-mini^ON RGCs located in the ventral retina are inverted with respect to mini^ON RGCs located in the dorsal retina. In contrast, F-mini^OFF RGCs maintain ventral orientation independent of retinal position. N = 30, 29, 12, and 7 cells for F-mini^ON, F-mini^OFF, F-midi^ON, and F-midi^OFF types, respectively. Scale bar in (A) = 1 mm; (N) = 50 μm. See also Figure S6.

Figure 7. Foxp and Brn3 proteins distinguish RGC types in primate retina

(A, B) Immunostaining analysis of Foxp2, Foxp1, and Brn3 proteins in adult macaque retina. Brn3a is selectively expressed by RGCs localized to the ganglion cell layer (A). Double-staining for Foxp and Brn3 shows their co-localization within subsets (B). Arrows indicate double-positive cells. Neurotrace (NT) was used to label all somata. (C) Whole mount retinas stained for Foxp1, Foxp2, and Brn3b or Brn3c proteins. Foxp1 and Foxp2 combinatorially distinguish three RGC groups (Foxp1⁺/Foxp2⁻, Foxp1⁺/Foxp2⁺, and Foxp2⁺/Foxp1⁻). Brn3b is co-expressed by the Foxp1⁺/Foxp2⁺ group (asterisks in left panel). Brn3c is co-expressed by a subset of the Foxp2⁺/Foxp1⁻ group (asterisks in right panel). (D) Table summarizing five molecularly distinct RGC types marked by Foxp and Brn3 proteins, provisionally called F1-5. (E) DRP analysis of total Foxp2⁺ RGCs suggests that they are a mixture of RGC types. DRP of F3 (red) and F4 (pink) RGCs reveals their non-random mosaic organization, indicating they are a single RGC type. (F) Density map of Foxp2 RGCs, showing their enrichment around the fovea. (G) Quantification of total Brn3a⁺ RGCs and the fraction that are Foxp2⁺ at different distances from the fovea. Foxp2 RGC density drops by more than half while overall RGC density remains stable. Images and plots are representative from n = 2 retinas. Scale bars in (A–C) = 50 μm; (F) = 1 mm. See also Figure S7.