As in zebrafish,
Thrb is an early marker of cones in mice and chicks.
45 Through discovery of an enhancer that regulates
Thrb in these species, along with the cognate TFs that regulate the enhancer, a TF that is important for cone determination, Onecut 1 (
Oc1), was discovered.
44 An understanding of the role of
Oc1 has aided in the definition of a GRN for cones versus rods.
Otx2, which was previously shown to be important for rod and cone genesis,
27 and
Oc1 were shown to combinatorially regulate the
Thrb gene via direct binding to the ThrbCRM1 enhancer, which is active in an RPC that generates horizontal cells and photoreceptors.
Oc1 was found to be expressed in chick and mouse RPCs during the period when cones are generated, but not in the postnatal mouse retina, when only rods, and not cones, are produced (
Fig. 2A). Misexpression of
Oc1 in the postnatal mouse retina, where
Otx2 is expressed, induced the formation of immature cones, along with horizontal cells. This induction was dependent upon
Otx2, as removal of a conditional allele of
Otx2 prevented this induction. These data suggest that
Otx2 and
Oc1 together promote the fates of cones and horizontal cells. A model for
Oc1 and
Otx2 action in the retina was also proposed.
44 The CRM1-active RPCs divide to give rise to cones and horizontal cells. In cone precursor cells, the level of
Oc1 declines and
Otx2 is maintained, while in horizontal cell precursors, the level of
Oc1 increases and
Otx2 decreases. More interestingly,
Oc1 probably plays an important role in cone versus rod fate determination, as the repression of
Oc1 led to increased rod genesis. Specifically, electroporation of the chick retina with a construct in which a transcriptional repressor domain was fused to
Oc1 led to a reduction in
Thrb expression. This construct also led to an upregulation of
MafA, the chick homologue of
Nrl,
46 a key gene in rod differentiation.
47 The
Oc1-repressor domain fusion also led to premature expression of rhodopsin, in keeping with an increase in the production of rods. Moreover, in
Oc1 knockout mice, a reduction in
Thrb mRNA and an upregulation in
Nrl mRNA were seen. These data all point to a role of the
Oc1 gene (and possibly
Oc2, which has high homology to
Oc1) in regulating the cone versus rod fate decision. In summary, this study indicates that coexpression of
Otx2 and
Oc1 may be able to drive early events in cone genesis, leading to cone induction from stem cells.