Systematic analysis of the Frazzled receptor interactome establishes previously unreported regulators of axon guidance

The Netrin receptor Dcc and its Drosophila homolog Frazzled play crucial roles in diverse developmental process, including axon guidance. In Drosophila, Fra regulates midline axon guidance through a Netrin-dependent and a Netrin-independent pathway. However, what molecules regulate these distinct signaling pathways remain unclear. To identify Fra-interacting proteins, we performed affinity purification mass spectrometry to establish a neuronal-specific Fra interactome. In addition to known interactors of Fra and Dcc, including Netrin and Robo1, our screen identified 85 candidate proteins, the majority of which are conserved in humans. Many of these proteins are expressed in the ventral nerve cord, and gene ontology, pathway analysis and biochemical validation identified several previously unreported pathways, including the receptor tyrosine phosphatase Lar, subunits of the COP9 signalosome and Rho-5, a regulator of the metalloprotease Tace. Finally, genetic analysis demonstrates that these genes regulate axon guidance and may define as yet unknown signaling mechanisms for Fra and its vertebrate homolog Dcc. Thus, the Fra interactome represents a resource to guide future functional studies.

Frazzled/Dcc acts independently of Netrin to promote germline survival during Drosophila oogenesis

The Netrin receptor Frazzled/Dcc (Fra in Drosophila) functions in diverse tissue contexts to regulate cell migration, axon guidance and cell survival. Fra signals in response to Netrin to regulate the cytoskeleton and also acts independently of Netrin to directly regulate transcription during axon guidance in Drosophila. In other contexts, Dcc acts as a tumor suppressor by directly promoting apoptosis. In this study, we report that Fra is required in the Drosophila female germline for the progression of egg chambers through mid-oogenesis. Loss of Fra in the germline, but not the somatic cells of the ovary, results in the degeneration of egg chambers. Although a failure in nutrient sensing and disruptions in egg chamber polarity can result in degeneration at mid-oogenesis, these factors do not appear to be affected in fra germline mutants. However, similar to the degeneration that occurs in those contexts, the cell death effector Dcp-1 is activated in fra germline mutants. The function of Fra in the female germline is independent of Netrin and requires the transcriptional activation domain of Fra. In contrast to the role of Dcc in promoting cell death, our observations reveal a role for Fra in regulating germline survival by inhibiting apoptosis.

The Role of Apoptotic Signaling in Axon Guidance

Navigating growth cones are exposed to multiple signals simultaneously and have to integrate competing cues into a coherent navigational response. Integration of guidance cues is traditionally thought to occur at the level of cytoskeletal dynamics. Drosophila studies indicate that cells exhibit a low level of continuous caspase protease activation, and that axon guidance cues can activate or suppress caspase activity. We base a model for axon guidance on these observations. By analogy with other systems in which caspase signaling has non-apoptotic functions, we propose that caspase signaling can either reinforce repulsion or negate attraction in response to external guidance cues by cleaving cytoskeletal proteins. Over the course of an entire trajectory, incorrectly navigating axons may pass the threshold for apoptosis and be eliminated, whereas axons making correct decisions will survive. These observations would also explain why neurotrophic factors can act as axon guidance cues and why axon guidance systems such as Slit/Robo signaling may act as tumor suppressors in cancer.

Midline axon guidance in the Drosophila embryonic central nervous system

Studies in the fruit fly Drosophila melanogaster have provided many fundamental insights into the genetic regulation of neural development, including the identification and characterization of evolutionarily conserved axon guidance pathways and their roles in important guidance decisions. Due to its highly organized and fast-developing embryonic nervous system, relatively small number of neurons, and molecular and genetic tools for identifying, labeling, and manipulating individual neurons or small neuronal subsets, studies of axon guidance in the Drosophila embryonic CNS have allowed researchers to dissect these genetic mechanisms with a high degree of precision. In this review, we discuss the major axon guidance pathways that regulate midline crossing of axons and the formation and guidance of longitudinal axon tracts, two processes that contribute to the development of the precise three-dimensional structure of the insect nerve cord. We focus particularly on recent insights into the roles and regulation of canonical midline axon guidance pathways, and on additional factors and pathways that have recently been shown to contribute to axon guidance decisions at and near the midline.

Motor axon guidance in Drosophila

The Drosophila motor system starts to assemble during embryonic development. It is composed of 30 muscles per abdominal hemisegment and 36 motor neurons assembling into nerve branches to exit the CNS, navigate within the muscle field and finally establish specific connections with their target muscles. Several families of guidance molecules that play a role controlling this process as well as transcriptional regulators that program the behavior of specific motor neuron have been identified. In this review we summarize the role of both groups of molecules in the motor system as well as their relationship where known. It is apparent that partially redundant guidance protein families and membrane molecules with different functional output direct guidance decisions cooperatively. Some distinct transcriptional regulators seem to control guidance of specific nerve branches globally directing the expression of groups of pathfinding molecules in all motor neurons within the same motor branch.

Netrins and Frazzled/DCC promote the migration and mesenchymal to epithelial transition of Drosophila midgut cells

Mesenchymal-epithelial transitions (METs) are important in both development and the growth of secondary tumours. Although the molecular basis for epithelial polarity is well studied, less is known about the cues that induce MET. Here we show that Netrins, well known as chemotropic guidance factors, provide a basal polarising cue during the Drosophila midgut MET. Both netrinA and netrinB are expressed in the visceral mesoderm, the substrate upon which midgut cells migrate, while their receptor frazzled (fra) is expressed in midgut cells. Netrins are required to polarise Fra to the basal surface, and Netrins and Fra undergo mutually-dependent endocytosis, with Fra subsequently trafficking to late endosomes. Mutations to fra and netrins affect both migration and MET but to different degrees. Loss of fra strongly delays migration, midgut cells fail to extend protrusions, and apico-basal polarisation of proteins and epithelium formation is inhibited. In netrin mutants, the migration phenotype is weaker and cells still extend protrusions. However, apico-basal polarisation of proteins, including Fra, and FActin is greatly disrupted and a monolayer fails to form. Delocalised accumulations of FActin are prevalent in netrin mutants but not fra mutants suggesting delocalised Fra may disrupt the MET. βPS localisation is also affected in netrin mutants in that a basal gradient is reduced while localisation to the midgut/VM interface is increased. Since a similar effect is seen when endocytosis is inhibited, Netrin and Fra may regulate Integrin turnover. The results suggest Netrin-dependent basal polarisation of Fra is critical for the formation of an epithelium.

Netrin and Frazzled regulate presynaptic gap junctions at a Drosophila giant synapse

Netrin and its receptor, Frazzled, dictate the strength of synaptic connections in the giant fiber system (GFS) of Drosophila melanogaster by regulating gap junction localization in the presynaptic terminal. In Netrin mutant animals, the synaptic coupling between a giant interneuron and the "jump" motor neuron was weakened and dye coupling between these two neurons was severely compromised or absent. In cases in which Netrin mutants displayed apparently normal synaptic anatomy, half of the specimens exhibited physiologically defective synapses and dye coupling between the giant fiber (GF) and the motor neuron was reduced or eliminated, suggesting that gap junctions were disrupted in the Netrin mutants. When we examined the gap junctions with antibodies to Shaking-B (ShakB) Innexin, they were significantly decreased or absent in the presynaptic terminal of the mutant GF. Frazzled loss of function mutants exhibited similar defects in synaptic transmission, dye coupling, and gap junction localization. These data are the first to show that Netrin and Frazzled regulate the placement of gap junctions presynaptically at a synapse.

Distinct functional domains of the Abelson tyrosine kinase control axon guidance responses to Netrin and Slit to regulate the assembly of neural circuits

To develop a functional nervous system, axons must initially navigate through a complex environment, directed by guidance ligands and receptors. These receptors must link to intracellular signaling cascades to direct axon pathfinding decisions. The Abelson tyrosine kinase (Abl) plays a crucial role in multiple Drosophila axon guidance pathways during development, though the mechanism by which Abl elicits a diverse set of guidance outputs is currently unknown. We identified Abl in a genetic screen for genes that contribute to Netrin-dependent axon guidance in midline-crossing (commissural) neurons. We find that Abl interacts both physically and genetically with the Netrin receptor Frazzled, and that disrupting this interaction prevents Abl from promoting midline axon crossing. Moreover, we find that Abl exerts its diverse activities through at least two different mechanisms: (1) a partly kinase-independent, structural function in midline attraction through its C-terminal F-actin binding domain (FABD) and (2) a kinase-dependent inhibition of repulsive guidance pathways that does not require the Abl C terminus. Abl also regulates motor axon pathfinding through a non-overlapping set of functional domains. These results highlight how a multifunctional kinase can trigger diverse axon guidance outcomes through the use of distinct structural motifs.

Frazzled/DCC facilitates cardiac cell outgrowth and attachment during Drosophila dorsal vessel formation

Drosophila embryonic dorsal vessel (DV) morphogenesis is a highly stereotyped process that involves the migration and morphogenesis of 52 pairs of cardioblasts (CBs) in order to form a linear tube. This process requires spatiotemporally-regulated localization of signaling and adhesive proteins in order to coordinate the formation of a central lumen while maintaining simultaneous adhesion between CBs. Previous studies have shown that the Slit/Roundabout and Netrin/Unc5 repulsive signaling pathways facilitate site-specific loss of adhesion between contralateral CBs in order to form a luminal space. However, the concomitant mechanism by which attraction initiates CB outgrowth and discrete localization of adhesive proteins remains poorly understood. Here we provide genetic evidence that Netrin signals through DCC (Deleted in Colorectal Carcinoma)/UNC-40/Frazzled (Fra) to mediate CB outgrowth and attachment and that this function occurs prior to and independently of Netrin/UNC-5 signaling. fra mRNA is expressed in the CBs prior to and during DV morphogenesis. Loss-of-fra-function results in significant defects in cell shape and alignment between contralateral CB rows. In addition, CB outgrowth and attachment is impaired in both fra loss- and gain-of-function mutants. Deletion of both Netrin genes (NetA and NetB) results in CB attachment phenotypes similar to fra mutants. Similar defects are also seen when both fra and unc5 are deleted. Finally we show that Fra accumulates at dorsal and ventral leading edges of paired CBs, and this localization is dependent upon Netrin. We propose that while repulsive guidance mechanisms contribute to lumen formation by preventing luminal domains from coming together, site-specific Netrin/Frazzled signaling mediates CB attachment.