Assessing the Role of Cell-Surface Molecules in Central Synaptogenesis in the Drosophila Visual System

Abstract

A hallmark of the central nervous system is its spatial and functional organization in synaptic layers. During neuronal development, axons form transient contacts with potential post-synaptic elements and establish synapses with appropriate partners at specific layers. These processes are regulated by synaptic cell-adhesion molecules. In the Drosophila visual system, R7 and R8 photoreceptor subtypes target distinct layers and form en passant pre-synaptic terminals at stereotypic loci of the axonal shaft. A leucine-rich repeat transmembrane protein, Capricious (Caps), is known to be selectively expressed in R8 axons and their recipient layer, which led to the attractive hypothesis that Caps mediates R8 synaptic specificity by homophilic adhesion. Contradicting this assumption, our results indicate that Caps does not have a prominent role in synaptic-layer targeting and synapse formation in Drosophila photoreceptors, and that the specific recognition of the R8 target layer does not involve Caps homophilic axon-target interactions. We generated flies that express a tagged synaptic marker to evaluate the presence and localization of synapses in R7 and R8 photoreceptors. These genetic tools were used to assess how the synaptic profile is affected when axons are forced to target abnormal layers by expressing axon guidance molecules. When R7 axons were mistargeted to the R8-recipient layer, R7s either maintained an R7-like synaptic profile or acquired a similar profile to r8s depending on the overexpressed protein. When R7 axons were redirected to a more superficial medulla layer, the number of presynaptic terminals was reduced. These results indicate that cell-surface molecules are able to dictate synapse loci by changing the axon terminal identity in a partially cell-autonomous manner, but that presynapse formation at specific sites also requires complex interactions between pre- and post-synaptic elements.

Control of Synaptic Connectivity by a Network of Drosophila IgSF Cell Surface Proteins

“…To visualize yR7 terminals, we used the active zone reporter line Rh4-LexA ::p65 , 8XLexAop2-Brp-short mCherry ( Rh4>Brp-short mCherry )(Berger-Muller et al, 2013). Bruchpilot (Brp) is an obligate active zone component at most or all synapses, and Brp-short mCherry is a truncated form of Brp that labels existing active zones by associating with full-length Brp but cannot create new active zones when overexpressed (Berger-Muller et al, 2013; Fouquet et al, 2009). In adults heterozygous for dpr11 MiMIC or DIP-γ MiMIC , yR7 terminals labeled by Brp-short mCherry were bulb-shaped and regularly arranged in M6 (Figures 7A, C).…”

“…To examine whether formation of synapses between yR7s and Dm8s involves interactions between Dpr11 and DIP-γ, we expressed a marker for existing active zones, Brp-short mCherry , in yR7s (Berger-Muller et al, 2013; Fouquet et al, 2009). In control animals, yR7 terminals are bulb-shaped and regularly arranged in M6.…”

Cell surface molecule, Klingon, mediates the refinement of synaptic specificity in the Drosophila visual system

“…R7 terminates in the deeper M6 layer, whereas R8 terminates in the M3 layer. The synapse can be visualized by a molecular marker fusing active zone component Bruchpilot (Brp) and mCherry (Berger-Muller et al, 2013;Fouquet et al, 2009;Sugie et al, 2015) or GFP (Chen et al, 2014).…”

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

“…To knock down bdl in postsynaptic targets of R8, ort C1-3 -GAL4 (Gao et al, 2008) was used to drive the expression of UAS-bdl-RNAi. The lexA/lexAop system was used to label SV components in bdl knockdown flies; the SV marker lexAop-nSyb-spGFP1-10 (Macpherson et al, 2015), was expressed under control of the greensensitive-R8-specific driver Rh6-lexA::p65 (Berger-Müller et al, 2013). We found that knocking down bdl in postsynaptic targets of R8 caused a similar SV mislocalization phenotype in all individuals examined (100%, n ϭ 4 animals; (Fig.…”