Figure 10.

Vsx1 function in bipolar cells. Vsx1 is expressed in OFF Types 1 and 2 (Chow et al., 2001; Chow et al., 2004) and in ON Type 7 bipolar cells (this study). In Vsx1-null mice, the expression of six cell type-specific markers in OFF bipolar cells (recoverin, Neto1, NK3R, Cabp5, Syt2 and HCN4) is downregulated in Types 1, 2 and 3 OFF bipolar cells (Chow et al., 2004) (Z. Shi, R.L. Chow, submitted for publication). In contrast, the expression of three bipolar markers (Chx10, Cabp5 and GUS8.4GFP) is upregulated in Type 7 bipolar cells (this study). These data reveal a trend in which Vsx1 functions downstream of cell specification as an activator of gene expression in OFF bipolar cells and as a repressor in Type 7 bipolar cells. In Vsx1-null mice, the OFF visual signaling circuitry appears to be intact; however, OFF visual signaling and OFF contrast adaptation are both reduced (Chow et al., 2004; Kerschensteiner et al., 2008). In the present study, we show that Vsx1-expressing Type 7 bipolar cells participate in a directional selective retinal circuit. While this circuit remains intact in Vsx1-null mice, a role for Vsx1 in Type 7 bipolar cell signaling is inferred by defects in directional selective ganglion cell ON response kinetics observed in Vsx1-null mice.