A Screen of Cell-Surface Molecules Identifies Leucine-Rich Repeat Proteins as Key Mediators of Synaptic Target Selection

Abstract

Summary In Drosophila embryos and larvae, a small number of identified motor neurons innervate body wall muscles in a highly stereotyped pattern. Although genetic screens have identified many proteins that are required for axon guidance and synaptogenesis in this system, little is known about the mechanisms by which muscle fibers are defined as targets for specific motor axons. To identify potential target labels, we screened 410 genes encoding cell-surface and secreted proteins, searching for those whose overexpression on all muscle fibers causes motor axons to make targeting errors. Thirty such genes were identified, and a number of these were members of a large gene family encoding proteins whose extracellular domains contain leucine-rich repeat (LRR) sequences, which are protein interaction modules. By manipulating gene expression in muscle 12, we showed that four LRR proteins participate in the selection of this muscle as the appropriate synaptic target for the RP5 motor neuron.

Extracellular leucine-rich repeat proteins are required to organize the apical extracellular matrix and maintain epithelial junction integrity inC. elegans

“…In Drosophila, the eLRRon protein convoluted is required in tracheal epithelial cells for proper apical ECM organization and tube length (Swanson et al, 2009). Several transmembrane eLRRon proteins, including mammalian LRRTM1-3 and Drosophila capricious and tartan, promote synaptic junction formation or maintenance (Kurusu et al, 2008;ShinzaKameda et al, 2006;Shishido et al, 1998;Taniguchi et al, 2000). Although synaptic junctions can be influenced by the ECM (Ackley et al, 2003;Fox et al, 2008) C. elegans eLRRon proteins reveal a link between apical ECM organization and epithelial junction maintenance We showed here that the TM eLRRon proteins LET-4 and EGG-6 and their secreted paralog SYM-1 are required for both apical ECM organization and epithelial junction stability in the C. elegans epidermis and excretory duct and pore tubes.…”

Protein coadaptation and the design of novel approaches to identify protein–protein interactions

“…There are abundant interactions among nervous system CSS described to date, however, there is a vast number of orphan CSS without known interacting partners. Different screens have highlighted the relevance of LRR family of proteins in the developing and adult nervous system (46)(47)(48). Among them, several proteins or families are still orphan such as the Elron or the Elfns (46).…”

Fine tuning cellular recognition

“…13 LRT belongs to a family of LRR proteins discovered recently in Drosophila. 14,15 These proteins were found to be essential for correct recognition between muscles and their innervating motor axons. They are expressed by the target muscle tissue, while the identity of their interacting receptor on the axon surface has not yet been characterized.…”

“…15,18 In addition, a number of LRR proteins including Tartan, Capricious, Haf and others are expressed by distinct muscle types. 15 These receptors contribute to the targeting specificity of peripheral axons to distinct types of a trans-membrane domain (thus called LRRTM proteins) are important for trans-synaptic signaling and synapse formation.…”

“…15,18 In addition, a number of LRR proteins including Tartan, Capricious, Haf and others are expressed by distinct muscle types. 15 These receptors contribute to the targeting specificity of peripheral axons to distinct types of a trans-membrane domain (thus called LRRTM proteins) are important for trans-synaptic signaling and synapse formation. 19,20 These include previously characterized proteins such as the Netrin-G-ligand (NGL) proteins NGL-2 and NGL-3, who have been shown to interact with Netrin-G and the LAR receptor protein phosphatase, respectively, 21,22 and members of the Fibronectin Leucine Rich Transmembrane (FLRT) family of proteins.…”