Previous analysis of mutant mice has revealed that the bHLH genes Mash1 and
Math3, and the homeobox gene Chx10 are essential for generation of bipolar
cells, the interneurons present in the inner nuclear layer of the retina.
Thus, a combination of the bHLH and homeobox genes should be important for
bipolar cell genesis, but the exact functions of each gene remain largely u
nknown. We have found that in Mash1-Math3 double-mutant retina, which exhib
its a complete loss of bipolar cells, Chx10 expression did not disappear bu
t remained in Muller glial cells, suggesting that Chx10 expression per se i
s compatible with gliogenesis. In agreement with this, misexpression of Chx
10 alone with retrovirus in the retinal explant cultures induced generation
of the inner nuclear layer cells, including Muller glia, but few of them w
ere mature bipolar cells. Misexpression of Mash1 or Math3 alone did not pro
mote bipolar cell genesis either, but inhibited Muller gliogenesis. In cont
rast, misexpression of Mash1 or Math3 together with Chx10 increased the pop
ulation of mature bipolar cells and decreased that of Muller glia. Thus, th
e homeobox gene provides the inner nuclear layer-specific identity while th
e bHLH genes regulate the neuronal versus glial fate determination, and the
se two classes of genes together specify the bipolar cell fate. Moreover, M
ash1 and Math3 promoted the bipolar cell fate, but not the other inner nucl
ear layer-specific neuronal subtypes in the presence of Chx10, raising the
possibility that the bHLH genes may be involved in neuronal subtype specifi
cation, in addition to simply making the neuronal versus glial fate choice.