Architecture and activity-mediated refinement of axonal projections from a mosaic of genetically identified retinal ganglion cells

Abstract

Our understanding of how mammalian sensory circuits are organized and develop has long been hindered by the lack of genetic markers of neurons with discrete functions. Here, we report a transgenic mouse selectively expressing GFP in a complete mosaic of transient OFF-alpha retinal ganglion cells (tOFF-alphaRGCs). This enabled us to relate the mosaic spacing, dendritic anatomy, and electrophysiology of these RGCs to their complete map of projections in the brain. We find that tOFF-alphaRGCs project exclusively to the superior colliculus (SC) and dorsal lateral geniculate nucleus and are restricted to a specific laminar depth within each of these targets. The axons of tOFF-alphaRGC are also organized into columns in the SC. Both laminar and columnar specificity develop through axon refinement. Disruption of cholinergic retinal waves prevents the emergence of columnar- but not laminar-specific tOFF-alphaRGC connections. Our findings reveal that in a genetically identified sensory map, spontaneous activity promotes synaptic specificity by segregating axons arising from RGCs of the same subtype.

Figure 1.. A mosaic of GFP⁺ OFF-αRGCs in CB2-GFP mice.

(A) Whole-mount CB2-GFP retina, showing GFP⁺ RGCs. Along the relieving cuts, GFP⁺ amacrine cells are also evident (see E). D/V/N/T: dorsal, ventral, nasal, temporal axes. Scale is 500 μm. (B-D) High magnification of the boxed region in (A). (B) GFP⁺ RGCs (arrows). Inset shows a single GFP⁺ RGC (arrow) with its axon (arrowhead). (C, D) Every GFP⁺ RGC is immunopositive for SMI-32 (SMI-32⁺). Scale in B-D is 125 μm. Scale in B inset is 50 μm. (E) Vertically-sectioned retina. Blue, DAPI staining indicates cellular layers. GFP⁺ cells are amacrine cells in the inner nuclear layer (INL), and regularly spaced RGCs in the GCL (arrows). Scale is 100 μm. (F) Density profile of GFP⁺ RGC cells as a function of distance from other GFP⁺ RGCs (Rodieck, 1991). (G) Four neighboring GFP⁺ RGCs filled with DiI to reveal their entire dendritic arbors. (H) Dendritic field boundaries of the same four GFP⁺ cells. (G, H) Scale is 75 μm. (I) Retinal section stained for DAPI, GFP and the vesicle-acetylcholine transporter (VAChT) which labels the middle of the ON and OFF sublaminae in the IPL (horizontal lines denote boundaries of IPL). GFP⁺ RGC dendrites ramify within the OFF sublaminae at the ~30–35% depth of the IPL. Arrow indicates the GFP⁺ RGC’s axon. Arrowheads indicate axons from other GFP⁺ RGCs whose somata lie outside the field of view. Scale is 75 μm.

Figure 2.. Visual responses of GFP⁺ tOFF-αRGCs.

A) Membrane potential response of a GFP⁺ tOFF-αRGC in response to a 1 s periodic flash. Action potentials are truncated. B) Average subthreshold flash response of 7 cells (gray trace), compared to the average flash response of two example cells (colored traces). Spikes were digitally removed prior to averaging. (C-D) Spatio-temporal receptive field of a GFP⁺ tOFF-αRGC computed from the membrane potential response of the cell to a randomly flickering checkerboard. C) Temporal filter averaged across the spatial receptive field center. D) Two spatial slices of the receptive field taken at the times indicated by the colored lines in C, indicating the biphasic nature of the receptive field center.

Figure 3.. Axonal projections of GFP⁺ tOFF-αRGCs.

(Left margin) Schematics of RGC projections in the mouse. (Top left) Dorsal view. The retinas and optic pathways are in red. Superior colliculus (SC), dorsal lateral geniculate nucleus (dLGN) and olivary pretectal nucleus (OPN). Here and in (A-G) R: rostral, C: caudal, L: lateral, M: medial. Schematics of coronal and sagittal view of the brain at the level of the SC (Middle panels) and dLGN (bottom panel). (A-D) Laminar- and columnar-specific tOFF-αRGC axonal projections to the SC. (A) CTb-594 labeling (red) of all RGC axons. CTb-594 weakly labels RGC axon shafts but densely labels RGC axon terminals and thereby completely labels all retinorecipient nuclei. SO, stratum opticum; lSGS, lower stratum griseum superficialis; uSGS, upper stratum griseum superficialis; SZ, stratum zonale. (A’) High magnification view of box in (A). (B) GFP⁺ tOFF-αRGC axons in the SC. The axons enter the SC through the SO, and selectively arborize in the lSGS. The GFP⁺ arbors are also organized into columns. Small arrow indicates a GFP⁺ cell in the deeper SC; such cells do not contribute to the GFP⁺ axons in the retinorecipient SC (see supplemental figure 2). (B’) High magnification view of box in (B). (C) Merged view of (A) and (B). (C’) High magnification view of box in (C). (A-C) Scale is 250 μm. Coronal plane is shown; D: dorsal, L: lateral. A-C. Scale in A’-C’ is 125μm. (D) Sagittal view of CTb-594⁺ and GFP⁺ tOFF-αRGC axons in the SC. Scale is 250 μm. (E-G) Retino-dLGN projections viewed in the coronal plane. (E) CTb-594 labeled RGC axons fill the entire dLGN, IGL and vLGN. OT, optic tract. (F) GFP⁺ tOFF-αRGC axons bypass the vLGN and IGL, turn medially into the dLGN and project through the lateral third of the dLGN (asterisks) to selectively arborize in the inner dLGN, forming a ”layer”. (G) Merged view of (E) and (F). Scale is 250 μm.

Figure 4.. Development of laminar- and columnar-specific tOFF-αRGC projections involves axonal refinement.

(A-C’) Targeting of tOFF-αRGC axons to the SC is imprecise early in development. (A, B) Coronal view of the SC in a P4 CB2-GFP mouse. tOFF-αRGC axons exhibit rudimentary columnar- and laminar-specificity, but unlike in adult CB2-GFP mice (Figure 3) many GFP⁺ axons project into the uSGS and SZ, and up to the pial margin (arrows). (B’) Higher magnification view of the boxed region in (A, B). Arrows indicate GFP⁺ axons projecting to the uppermost SC. (C, C’) Sagittal view of the SC on P4. GFP⁺ axons are distributed across the retinorecipient depth of the SC. Arrows indicate GFP⁺ axons projecting to the pial margin (arrows). Asterisks in (A) indicate GFP⁺ glia at the pial margin and a few GFP⁺ neurons in the deeper, non-retinorecipient SC (see Figure 3). GFP expression in these glia disappears by ~P8. Neither the GFP⁺ glia nor GFP⁺ SC neurons contribute to the GFP⁺ axons in the young or adult SC (supplemental figures 2 and 3). (D-F’) Targeting of tOFF-αRGC axons to the SC is adult-like by P10; GFP⁺ axons are restricted to the lSGS and segregated into distinct columns. (A-F) Scale is 250 μm. (B’,C’,E’,F’) Scale is 125 μm. (G) Quantification of laminar-specificity as a function of age (see experimental procedures). The number of GFP⁺ axons/mm projecting to the 25–50% (upper third) and 0–25% (upper quarter) of the SC depth is significantly greater on P4 compared to P10 or adult (p<0.0001). P10 versus adult were not significantly different (25–50% depth, p=0.74; 0–25% depth, p=0.23; t-test, ±SEM) (n=4 adult mice, n=6 P10 mice, n=7 P4 mice). (H) Columnar segregation index (see experimental protocols). There is significantly less columnar-specificity on P4, as compared to P10 or adult (p<0.0001), whereas columnar-specificity is indistinguishable between P10 and adult (p=0.53) (n=3 mice per age). (I) The depth (retinorecipient thickness) of the SC is not different between P4, P10, or adult CB2-GFP mice (P4 vs. P10, p=0.3; P4 vs. adult, p=0.48; t-test) (n=5 P4 mice, n=4 P10 mice, n=4 adult mice). (J) The number of GFP⁺ RGCs per retina is not different on P4 compared to P10 and adult (P10+) (p=0.96; t-test; ±SEM; n=5 P4 mice, n=4 P10 mice, and n=4 adult mice).

Figure 5.. Genetic removal of β2nAChRs does not alter the initial targeting of tOFF-αRGC axons.

(A-D) On P4, targeting of tOFF-αRGC axons to the SC is normal in CB2-GFP:β2nAChR+/− mice and CB2-GFP:β2nAChR−/− mice. (A-D) Coronal view of the SC in a P4 CB2-GFP:β2nAChR+/− mouse (A, B), and a P4 CB2-GFP:β2nAChR−/− mouse (C, D). As in WT P4 mice (Figure 4) laminar- and columnar-specificity are not mature at this age. (A-D) Arrows indicate GFP⁺ axons that project up near the pial margin. (A-D) Scale is 250 μm. (E) Quantification of laminar specificity for each genotype on P4/5; laminar specificity is indistinguishable between WT and β2nAChR+/− mice (25–50% depth, p=0.74; 0–25% depth, p=0.54), between β2nAChR+/− and β2nAChR−/− mice (25–50% depth, p=0.82; 0–25% depth, p=0.45), and between WT and β2nAChR−/− mice (25–50% depth, p=0.73; 0–25% depth, p=0.60), indicating that all genotypes start out with similar patterns of axon targeting on P4/5. (F) Quantification of columnar-specificity across genotypes on P4/5 (WT vs. β2nAChR+/−, p=0.67; β2nAChR+/− vs. β2nAChR−/−, p=0.61; WT vs. β2nAChR−/−, p=0.97).

Figure 6.. Cholinergic spontaneous activity mediates columnar- but not laminar-specific refinement of tOFF-αRGC projections to the SC.

(A-B) Coronal view of tOFF-αRGC projections to the SC in a P10 CB2-GFP:β2nAChR+/− mouse. Laminar-specificity and columnar specificity are similar to WT P10 mice (see Figure 4). (C, D) Coronal view of tOFF-αRGC projections to the SC in a P10 CB2-GFP:β2nAChR−/− mouse. Laminar-specificity is normal for this age but columnar specificity is severely perturbed. (A-D) Scale is 250 μm. (E) Quantification of laminar specificity according to genotype. No significant differences were present between WT, β2nAChR+/−, or β2nAChR−/− CB2-GFP mice on P10 (p=0.3–0.7; ±SEM; n=3 mice per condition). (F) Quantification of columnar specificity in the P10 and adult SC, according to genotype. At P10 and at adulthood, CB2-GFP:β2nAChR−/− mice exhibit significantly less columnar-specificity than β2nAChR+/− mice or WT (β2nAChR+/+) mice (**P<0.001; n=3 mice per genotype; ±SEM; students t-test). (G) The thickness of the retinorecipient SC depth is not different between WT, β2nAChR+/− or β2nAChR−/− mice at P10 or in adulthood (p=0.2–0.5, t-test, n=3 mice per genotype). (H) The number of GFP⁺ RGCs per retina is not different between WT and β2nAChR−/− CB2-GFP mice (p=0.75; t-test; ±SEM, n=4 mice per genotype). (I) GFP⁺ tOFF-αRGCs projections to the SC of an adult CB2-GFP:β2nAChR−/− mouse. Columnar specificity is severely reduced (quantified in panel F). Scale is 250 μm.

Figure 7.. Disruption of cholinergic spontaneous retinal activity does not prevent laminar-specific targeting of tOFF-αRGC axons to the dLGN.

(A-C) Retino-dLGN projections in a P10 CB2-GFP:β2nAChR−/− mouse. (A) CTb-594 (red) labeled RGC retino-dLGN projections. (B) GFP⁺ tOFF-αRGC axons avoid the outer third (asterisks) of the dLGN and selectively terminate within the inner portion of the dLGN, forming a distinct layer just as in WT CB2-GFP mice (see Figure 3). Note also that the GFP⁺ tOFF-αRGC axons avoid the vLGN and IGL, just as in WT CB2-GFP mice. (C) Merged view of (A) and (B). Scale is 150 μm.

Figure 8.. Columnar-specific refinement is mediated by cholinergic spontaneous activity in the retina.

(A, B) RGC projections to the SC of a P10 WT CB2-GFP mouse that received epibatidine (epi) injections into the right eye to disrupt spontaneous retinal activity and saline (control) injections into the left eye, from P4-P10. Normal laminar-specific targeting of GFP⁺ tOFF-αRGC axons to the lSGS is evident in both SC hemispheres. Columnar segregation is normal in the saline hemisphere but is perturbed in the SC hemisphere contralateral to the epi-injected eye. Scale is 250 μm. (C) Quantification of columnar specificity in the SC hemisphere receiving input from control, saline- or epi-injected retina (epi vs. saline **P<0.001; saline versus control p=0.67; ±SEM; t-test; n=2 mice).