Characterization of Tbr2‐expressing retinal ganglion cells

The mammalian retina contains more than 40 retinal ganglion cell (RGC) subtypes based on their unique morphologies, functions, and molecular profiles. Among them, intrinsically photosensitive RGCs (ipRGCs) are the first specified RGC type emerging from a common retinal progenitor pool during development. Previous work has shown that T‐box transcription factor T‐brain 2 (Tbr2) is essential for the formation and maintenance of ipRGCs, and that Tbr2‐expressing RGCs activate Opn4 expression upon native ipRGC ablation, suggesting that Tbr2+ RGCs contain a reservoir for ipRGCs. However, the identity of Tbr2+ RGCs has not been fully vetted. Here, using genetic sparse labeling and single cell recording, we showed that Tbr2‐expressing retinal neurons include RGCs and a subset of GABAergic displaced amacrine cells (dACs). Most Tbr2+ RGCs are intrinsically photosensitive and morphologically resemble native ipRGCs with identical retinofugal projections. Tbr2+ RGCs also include a unique and rare Pou4f1‐expressing OFF RGC subtype. Using a loss‐of‐function strategy, we have further demonstrated that Tbr2 is essential for the survival of these RGCs and dACs, as well as maintaining the expression of Opn4. These data set a strong foundation to study how Tbr2 regulates ipRGC development and survival, as well as the expression of molecular machinery regulating intrinsic photosensitivity. Using genetic sparse labeling and recording on a newly engineered T‐brain 2 (Tbr2)CreERT2 mouse line paired with various reporter lines, we identified seven types of Tbr2‐expressing retina ganglion cells, including M1–M6 types of intrinsically photosensitive retinal ganglion cells (RGCs) and a Pou4f1+ RGC with bushy and small dendritic arbors stratifying into the OFF sublaminae (in red). Additionally, we identified three types of Tbr2‐expressing amacrine cells, including two widefield and a mediumfield displaced amacrine cell (AC) types, all with dendritic arbors stratifying into the ON sublaminae (in black). Furthermore, we have demonstrated that Tbr2 is essential for the survival of these RGCs and displaced ACs, as well as maintaining the expression of Opn4.