Identification of amacrine subtypes that express the atypical cadherin celsr3

• Normal inhibitory circuit development in retina requires celsr3.
• We made a BAC transgene and categorized amacrine subtypes that express celsr3.
• Ten amacrine cell types with varicose (knobby) dendrites express celsr3.
• Multistratified and a few types of monostratified amacrines express celsr3.
• The select expression pattern is likely key to celsr3 function in circuitry formation.

We previously identified Celsr3, an atypical cadherin, as essential for normal inhibitory circuit formation in the inner retina. Its absence during retinal development leads to increases in GABA receptor numbers on ON-bipolar cell terminals and consequent changes in retinal physiology, but does not cause obvious cell spacing or synaptic lamination defects. This study focuses on defining the subset of amacrine cells that express celsr3 during development of the wild type zebrafish retina. We have developed a BAC transgene expressing EGFP under the control of celsr3 promoter, Tg(celsr3:EGFP). Similar to the retinal expression of the endogenous gene, the transgene is expressed in amacrine, ganglion and bipolar, but not horizontal or photoreceptor cells. We transiently expressed the BAC in zebrafish larvae and categorized 104 celsr3 expressing amacrine cells based on their shape, arborization and lamination. Ten different amacrine cell types express Tg(celsr3:EGFP). These include narrow, medium and wide-field types of varicose cells. There are many multistratified cells, including one not previously identified and a few specific types of monostratified amacrine cells. Non-varicose amacrine cells do not express the transgene. We propose that celsr3 expression in varicose amacrine cells is key to this molecule's function in circuitry formation during retinal development. The BAC transgene we have developed provides a useful tool to study Celsr3 function.