Abnormal SAC Differentiation Independent of Cell Cycle and Survival Defects
ERGs primarily assess photoreceptor and bipolar cell function, but may miss differentiation defects in other cells. To test for subtle differences we stained the Rb/E2f1 DKO retina with 43 markers (Table S1). Thirty-two proteins displayed identical patterns in WT, E2f1 KO, and Rb/E2f1 DKO retina (Table S1). The other 11 markers revealed a cell-cycle– and apoptosis-independent differentiation defect in SACs. We first studied Calb2 (calretinin), which marks a subset of amacrine and ganglion cell bodies as well as three tracks corresponding to their processes in the IPL (Figure 3A). Normal Calb2 staining was seen in the E2f1 KO IPL (data not shown). However, only one Calb2+ track was evident in the Rb KO IPL, and this defect was not rescued in the Rb/E2f1 DKO retina (Figure 3A). We quantified Calb2+ cell bodies in the Rb KO INL (corresponding to amacrine cell staining only) and observed a reduction from P8 onwards (Figures 3C and S6).
Figure 3  Differentiation Defects in Rb KO SACs
(A) P18 horizontal sections of WT, Rb KO, and Rb/E2f1 DKO retina were stained for nuclei (DAPI, blue), Calb2 (green), and Slc18a3 (red).
(B) Confocal images of P30 horizontal sections of WT and Rb KO retina were stained for nuclei (DAPI, blue), Chat and Slc18a3 (both red), and Camk2a (green). In the Rb KO section, the red stain is Chat only, as Slc18a3 is missing (see [A]).
(C) Quantification of dense Calb2+ cell bodies in the INL, total Slc18a3+ cell bodies, and Camk2a+ cell bodies in the INL. Error bars represent SD of measurements from three animals, and asterisks indicate significant differences between retinas of WT and the indicated genotypes (*, p <0.05; **; p <0.01; ANOVA and Tukey HSD test).
Scale bars are 50 μm in (A) and 20 μm in (B). Of the three Calb2+ tracks in the IPL, the two outer tracks are from SACs, named after their extensive dendritic-tree-like morphology [37]. SACs are cholinergic, represent ~5.2% of amacrine neurons [38], and are critical for both direction selectivity [34,35] and spontaneous rhythmic activity that occurs during normal retinal development [36]. SACs in the INL synapse in the OFF layer of the IPL that responds to decreasing light, while displaced SACs in the GCL have processes that synapse in the nearby ON layer of the IPL that responds to increasing light (reviewed in [39]). Mature SAC processes stain specifically for Slc18a3 (vesicular acetyl choline transporter, VAChT) [37], and, significantly, this marker was absent in the peripheral Rb KO or Rb/E2f1 DKO P18 retina (Figures 3A and S7B). Chat, expressed from the same locus, is also SAC specific, but marks both cell bodies and processes of mature SACs [37]. Chat was seen in fewer cells in the mature Rb KO retina, and was present in the soma but absent from processes (Figure 3B). We obtained similar results for Sv2c, a synaptic vesicle protein found in SACs [40]; Kcnc1b and Kcnc2, potassium channels expressed on SAC soma and dendrites as well as a very small number of ganglion cells [41]; gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter present in about half of amacrine cells including SACs, as well as horizontal and some bipolar neurons [37]; and Calb1 (calbindin), which is expressed in many amacrine cells and labels SAC process faintly (Figure S7A and S7B; Table S1; and data not shown). Finally, we also examined the effect of Rb deletion on SAC differentiation using a Chx10-Cre transgene that is expressed in a mosaic pattern across the retina, generating patches of Cre-expressing cells [42]. Consistent with the mosaic deletion pattern, we observed markedly reduced Chat/Slc18a3 staining in the IPL of Chx10-Cre;RbloxP/loxP retina compared to WT (Figure S7C). Together, these results suggest a role for Rb in SAC differentiation.
The above findings could indicate a defect in SAC specification, SAC survival, or the levels and/or transport of the markers described above. Camk2a marks both SACs and ganglion cells [37], but because ganglion cells are eliminated in the Rb KO retina, Camk2a is a specific SAC marker in this context. Importantly, Camk2a+ tracks and dendrites were present in both the WT and Rb KO retina (Figure 3B), and the number of Camk2a+ soma was similar in WT and Rb KO retina at P30 and beyond, although fewer cells stained in Rb KO retina at P18, suggesting a delay in its appearance (Figures 3C and S6B). Thus, Rb is not required for SAC survival or for process outgrowth, but rather it seems to regulate the expression and/or stability of Calb2, Calb1, Chat, Slc18a3, Sv2c, Kcnc1b, Kcnc2, and GABA in SACs, but leaves Camk2a expression virtually unaffected. The presence of Chat in some cell bodies but never in processes (Figure 3B) also suggests a transport defect. The developmental pattern of Slc18a3 expression also supported this notion. In mature WT SACs Slc18a3 is only in processes, but in early postnatal SACs, it is found in the cell body, and moves into emerging processes at approximately P4–P6. As noted above, Slc18a3 was absent at P18 in the Rb KO retina (Figure 3A); at P4 or P5 it was in cell bodies, yet was rarely present in Rb KO processes (Figures 4A and S6). Slc18a3 became virtually undetectable in Rb KO SACs by P8 (Figures 3C and S6C). These data suggest that Rb affects both the synthesis/stability and transport of SAC markers.
Figure 4  E2f3 Loss Rescues Differentiation of Rb KO SACs
(A) Horizontal retinal sections of the indicated ages and genotypes were stained for nuclei (DAPI, blue), mitosis marker PH3 (green), and Slc18a3 (red), which marks SAC soma at early stages and processes from ~P5 onwards. Arrows show mitotic PH3+ nuclei in Rb KO, Rb/E2f2 DKO, and Rb/E2f3 DKO retinas. E2f1 loss rescues the ectopic mitosis and cell death defects, but not the SAC defect. E2f2 loss has no effect. E2f3 loss does not rescue the ectopic mitosis and cell loss defects, but rescues the SAC defect. Inactivating E2f1 and E2f3 together rescues the ectopic mitosis, cell death, and SAC defects.
(B) The fraction of Camk2a+ cells that are Chat + and Slc18a3+ in the P30 retina.
(C) Horizontal P5 retinal sections were stained for nuclei (DAPI, blue), Slc18a3 (green), and Isl1 (red). Arrows show double-labelled Isl1+/Slc18a3+ cells in the inner INL.
(D) Horizontal P0 retinal sections of the indicated genotypes were stained for nuclei (DAPI, blue), cell division marker Mki67 (green), and Isl1 (red). Arrows show double-labelled dividing SACs.
(E) The fraction of Isl1+ cells in the inner NBL (INBL) of P0 retinas that are dividing (Mki67+).
Error bars represent SD of measurements from three animals, and asterisks indicate significant differences between retinas of WT and the indicated genotypes (*, p <0.05; **; p <0.01; ANOVA and Tukey HSD test). Scale bars in (A), (C), and (D) are 50 μm.