Complex Formation of Immunoglobulin Superfamily Molecules Side-IV and Beat-IIb Regulates Synaptic Specificity in theDrosophilaVisual System

Abstract

Neurons express many cell surface proteins as mutually binding key-lock molecules that can create synapses. However, the molecular mechanisms of how neurons make synapses only with preferred targets are not completely understood. Here we identified Side-IV and Beat-IIb, belonging to the Drosophila immunoglobulin superfamily, as a new key-lock combination capable of inducing synapse formation. Side-IV interaction with Beat-IIb transduces bifurcated signaling to Side-IV's co-receptor, Kirre, and a synaptic scaffold protein, Dsyd-1. Localization and genetic interaction analyses revealed that Side-IV localizes subcellularly at synapse formations defined by Beat-IIa/b and anchors Dsyd-1.Our data demonstrate that a complex made up of Side-IV, Beat-IIb, Kirre, and Dsyd-1 not only narrows neuronal binding specificity but also recruits synapse formation factors Kirre and Dsyd-1 to restrict synapse formation loci and inhibit miswiring. We propose a mechanism by which key-lock molecules set a hierarchy of preference among neuronal pairs in a complex circuit in vivo.

Synaptic promiscuity in brain development

“…For example, loss of DIP-β in visual interneuron L4 does not lead to a loss of synapses at the place of normal DIP-β localization, but instead to ectopic synapse formation in an adjacent region where DIP-β is not normally localized ( Figure 4 A) 71 . Similarly, loss of Side-IV in L2, which forms reciprocal synapses with L4, does not lead to a loss of synapses where Side-IV could induce synapse formation, but instead to ectopic synapse formation in the same distal region where both Side-IV and DIP-β are not normally localized in L4 ( Figure 4 A) 69 . Likewise, loss of the DIP-α–Dpr6/10 interaction between Dm12 and its synaptic partner neurons in the medulla does not prevent synapse formation in medulla layer 3 where branches normally occur but leads to ectopic branching and synapse formation in layer 8 where neuronal branches are not normally localized ( Figure 4 B) 53 .…”

“…single synapses with four distinct, albeit necessarily incorrect, partners. In fact, mutant analyses on some of the best candidates for trans-synaptic contact specification at the moment of choice in Drosophila do not cause loss of synapses, but instead ectopic synapse formation elsewhere 53 , 69 , 70 , 71 , as discussed in detail in the following section on the moment of choice.…”

“…While a local increase of synaptic competency can explain ectopic synapse formation and synapse constancy in loss of function mutants, the missing link in this picture is the actual induction of synapse formation. In the case of Side-IV in L2 neurons, such a link has been proposed to be another cell surface receptor: loss of the immunoglobulin superfamily protein Kirre, in contrast to Side-IV or DIP-β, actually leads to a loss of reciprocal L4–L2 synapses 87 and the Kirre protein directly interacts with Side-IV as co-receptor in L2 69 . Kirre is a member of a smaller family of cell surface proteins that have been suggested to act both before and at the moment of choice 88 .…”

“…Hence, a cell surface receptor that both creates intercellular adjacency with a synaptic partner and recruits intracellular building material may partner with a co-receptor that is required for actual synapse induction. Recruitment of a co-receptor might also explain why overexpression of several of the DIPs and Sides in particular have been shown to be sufficient for ectopic synapse formation 53 , 69 , 71 . Irrespective of whether the details of this mechanism are correct, these data suggest a molecular mechanistic separation of three functions prior to and at the moment of choice: creating intercellular adjacency, increasing local synaptic competency by recruiting intracellular synaptic building material, and induction of synapse formation.…”