As a first step, we analyzed the localization of MEF2C in the retina. Anti–MEF2C antibody decorated cells most strongly in the GCL of the rat retina and, to some degree, in the plexiform layers (
Fig. 7A, left). The staining pattern for MEF2C protein was similar in wild-type mouse retina but with less prominent plexiform labeling (
Fig. 7A, center). As expected, retinas of conditional
Mef2c knockout mice
41 were devoid of MEF2C immunofluorescence (
Fig. 7A, right), ensuring the specificity of the antibody. Additionally, activation of NMDA receptors on cerebrocortical neurons is known to activate caspase cleavage of MEF2C.
30 To assess NMDA receptor localization in conjunction with MEF2C, we stained for the NR1 subunit, which is present in all functional NMDA receptors. Interestingly, transcription of this NMDA receptor subunit is regulated by MEF2C.
51 We found that NR1-expressing cells in the GCL displayed prominent MEF2C immunoreactivity (
Fig. 7B). At higher magnification, the MAP2-positive neurons in the GCL clearly exhibit MEF2C immunoreactivity, with the strongest signal in their nuclei (
Fig. 7C, arrowheads). Moreover, immunostaining for the RGC-specific marker Brn-3b
42 revealed that MEF2C immunoreactivity was especially strong in RGCs (
Fig. 7D). Hence, we found that in the retina MEF2C predominantly localizes to RGCs.