M1 Type of Intrinsically-Photosensitive Retinal Ganglion Cells Express Brn3 Transcription Factors in rd1 Mouse

Abstract

Purpose: : We have previously found in wild-type retina that the non-M1, but not the M1 type of intrinsically-photosensitive retinal ganglion cells (ipRGCs) express brn3 transcription factors. We asked whether similar pattern of brn3 expression exists after photoreceptor loss in rd1 mouse retina.

Methods: : Retinas of two adult rd1 mice (3 months and 2 years) were flat-mounted and immunofluorescently labeled for brn3 and melanoposin. The staining intensity and soma diameter of melanopsin cells were measured using ImageJ software to differentiate between M1 and non-M1 cells.

Results: : We found 1537 melanopsin cells (density: 146 cells per mm²) in the 3-month old rd1 mouse, while the number was 459 (55 cells per mm²) in the 2-year old rd1 mouse. In both retinas, approximately 30% of the melanopsin cells were two- to three-fold more brightly labeled than the remaining melanopsin cells. The soma diameter of the brightly labeled cells (15.3±0.2 µm) was significantly smaller than the faintly-labeled melanopsin cells (18.1 ± 1.4 µm). These data suggested that the relatively small, brightly-labeled cells were M1 type of ipRGCs. Although the absolute number of melanopsin cells in rd1 retina was different from that in wild-type mouse, the ratio of M1 to non-M1 cells was unaltered. Compared to the wild-type mouse where 95% of the non-M1 cells but only 14% of the M1 cells expressed brn3, in rd1 mice (both 3 months and 2 years old) approximately 98% of the non-M1 cells and >80% of the M1 cells expressed brn3. Overall, approximately 93% of all melanopsin cells in rd1 retina expressed brn3, whereas this value for the wild-type mouse was ~65%.

Conclusions: : These results clearly demonstrate that the M1 cells, which do not express brn3 transcription factors in a wild-type mouse, express brn3 after photoreceptor loss in rd1 mouse. In a wild-type mouse retina, the expression of brn3 transcription factors in the M1 type of ipRGCs may be inhibited by photoreceptors.

Keywords: ganglion cells • transcription factors • retinal degenerations: cell biology