Phototactic T-maze Behavioral Assay for Comparing the Functionality of Color-sensitive Photoreceptor Subtypes in the Drosophila Visual System.

The Drosophila retina contains light-sensitive photoreceptors (R cells) with distinct spectral sensitivities that allow them to distinguish light by its spectral composition. R7 and R8 photoreceptors are important for color vision, and can be further classified into pale (p) or yellow (y) subtypes depending on the rhodopsin expressed. While both R7y and R7p are sensitive to UV light, R8y and R8p detect light in the green and blue spectrum, respectively. The ability of R7 and R8 photoreceptors to distinguish different spectral sensitivities and the natural preference for Drosophila towards light sources (phototaxis), allow for the development of a phototactic T-maze assay that compares the functionality of different R7 and R8 subtypes. A "UV vs. blue" choice can compare the functionalities of R7p and R8p photoreceptors, while a "UV vs. green" choice can compare the functionalities of R7y and R8y photoreceptors. Additionally, a "blue vs. green" choice could be used to compare R8p and R8y photoreceptors, while a "dark vs. light" choice could be used to determine overall vision functionality. Although electrophysiological recordings and calcium imaging have been used to examine functionality of R7 and R8 photoreceptors, these approaches require expensive equipment and are technically challenging. The phototactic T-maze assay we present here is a robust, straight-forward and an inexpensive method to study genetic and developmental factors that contribute to the individual functionality of R7 and R8 photoreceptors, and is especially useful when performing large-scale genetic screens.

The Conserved IgSF9 Protein Borderless Regulates Axonal Transport of Presynaptic Components and Color Vision in Drosophila.

Normal brain function requires proper targeting of synaptic-vesicle (SV) and active-zone components for presynaptic assembly and function. Whether and how synaptogenic signals (e.g., adhesion) at axo-dendritic contact sites promote axonal transport of presynaptic components for synapse formation, however, remain unclear. In this study, we show that Borderless (Bdl), a member of the conserved IgSF9-family trans-synaptic cell adhesion molecules, plays a novel and specific role in regulating axonal transport of SV components. Loss of bdl disrupts axonal transport of SV components in photoreceptor R8 axons, but does not affect the transport of mitochondria. Genetic mosaic analysis, transgene rescue and cell-type-specific knockdown indicate that Bdl is required both presynaptically and postsynaptically for delivering SV components in R8 axons. Consistent with a role for Bdl in R8 axons, loss of bdl causes a failure of R8-dependent phototaxis response to green light. bdl interacts genetically with imac encoding for a member of the UNC-104/Imac/KIF1A-family motor proteins, and is required for proper localization of Imac in R8 presynaptic terminals. Our results support a model in which Bdl mediates specific axo-dendritic interactions in a homophilic manner, which upregulates the Imac motor in promoting axonal transport of SV components for R8 presynaptic assembly and function.SIGNIFICANCE STATEMENT Whether and how synaptogenic adhesion at axo-dendritic contact sites regulates axonal transport of presynaptic components remain unknown. Here we show for the first time that a trans-synaptic adhesion molecule mediates specific interactions at axo-dendritic contact sites, which is required for upregulating the UNC-104/Imac/KIF1A motor in promoting axonal transport of synaptic-vesicle components for presynaptic assembly and function.