Isl1 plays a critical role in Starburst Amacrine Cell development in a temporal manner

Abstract

Purpose : Neuronal subtype differentiation is regulated by transcription factors (TF’s) that control the initial neuronal specification and the maturation of subtype-specific features. Within the murine retina, there are ~63 distinct Amacrine Cell (AC) subtypes. However, the mechanism by which these distinct subtypes are generated during development is poorly understood. Within the AC population, the TF Isl1 is exclusively expressed starburst amacrine cells (SAC), suggesting it may play a critical role in their development. Using conditional knock-out mice, we deleted Isl1 at distinct stages of development to uncover its cell-autonomous role in SAC development.

Methods : We developed two genetic approaches to conditionally delete Isl1 at distinct stages of SAC development. We used Ptf1a^Cre to delete Isl1 from AC precursors to examine its role in the initial specification of SACs, and ChAT^Cre to delete Isl1 after SACs are specified, but before the completion of dendritic growth to examine how Isl1 regulates SAC-specific features. These conditional knockout lines were crossed with Cre-dependent reporter lines for fate mapping or HB9^GFP mice to label a subset of bi-stratified on-off Direction Selective Retinal Ganglion Cells (ooDSGCs) that are post-synaptic partners of SACs.

Results : Ptf1a^Cre;Isl1^cKO mice showed a lack of SAC specification as indicated by an absence of traditional SAC markers. Fate mapping indicated that half of would-be SACs likely died while the other half adopted a distinct AC fate. The horizontal cell (HC) population remained unchanged in Ptf1a^Cre;Isl1^cKO mice indicating that SACs were not switching to a HC fate. In Ptf1a^Cre;Isl1^cKO retinas that lacked SACs, HB9-GFP+ ooDSGCs, were mono-stratified in S2 of the Inner Plexiform Layer (IPL). In ChAT^Cre;Isl1^cKO retinas, SACs were specified, but lacked the SAC markers ChAT and Calretinin. Dendrites in ChAT^Cre;Isl1^cKO mutant SACs also displayed an aberrant morphology. HB9-GFP+ ooDSGCs in ChAT^Cre;Isl1^cKO retinas remained bi-stratified and maintained contact with mistratified SACs.

Conclusions : Isl1 plays a critical role in SAC development in a temporal manner: Isl1 is required for the initial specification of SACs, and is also required for the subsequent morphological and molecular maturation of SACs. Future directions will identification of the effector genes downstream of Isl1 that are required for SAC development and function.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.