Molecular Mechanisms Underlying Motor Axon Guidance in Drosophila

Neuroligin 2 governs synaptic morphology and function through RACK1-cofilin signaling in Drosophila

Neuroligins are transmembrane cell adhesion proteins well-known for their genetic links to autism spectrum disorders. Neuroligins can function by regulating the actin cytoskeleton, however the factors and mechanisms involved are still largely unknown. Here, using the Drosophila neuromuscular junction as a model, we reveal that F-Actin assembly at the Drosophila NMJ is controlled through Cofilin signaling mediated by an interaction between DNlg2 and RACK1, factors not previously known to work together. The deletion of DNlg2 displays disrupted RACK1-Cofilin signaling pathway with diminished actin cytoskeleton proteo-stasis at the terminal of the NMJ, aberrant NMJ structure, reduced synaptic transmission, and abnormal locomotion at the third-instar larval stage. Overexpression of wildtype and activated Cofilin in muscles are sufficient to rescue the morphological and physiological defects in dnlg2 mutants, while inactivated Cofilin is not. Since the DNlg2 paralog DNlg1 is known to regulate F-actin assembly mainly via a specific interaction with WAVE complex, our present work suggests that the orchestration of F-actin by Neuroligins is a diverse and complex process critical for neural connectivity.

Chemical and mechanical control of axon fasciculation and defasciculation

Neural networks are constructed through the development of robust axonal projections from individual neurons, which ultimately establish connections with their targets. In most animals, developing axons assemble in bundles to navigate collectively across various areas within the central nervous system or the periphery, before they separate from these bundles in order to find their specific targets. These processes, called fasciculation and defasciculation respectively, were thought for many years to be controlled chemically: while guidance cues may attract or repulse axonal growth cones, adhesion molecules expressed at the surface of axons mediate their fasciculation. Recently, an additional non-chemical parameter, the mechanical longitudinal tension of axons, turned out to play a role in axon fasciculation and defasciculation, through zippering and unzippering of axon shafts. In this review, we present an integrated view of the currently known chemical and mechanical control of axon:axon dynamic interactions. We highlight the facts that the decision to cross or not to cross another axon depends on a combination of chemical, mechanical and geometrical parameters, and that the decision to fasciculate/defasciculate through zippering/unzippering relies on the balance between axon:axon adhesion and their mechanical tension. Finally, we speculate about possible functional implications of zippering-dependent axon shaft fasciculation, in the collective migration of axons, and in the sorting of subpopulations of axons.

Regulation of Off-track bidirectional signaling by Semaphorin-1a and Wnt signaling in the Drosophila motor axon guidance

Off-track receptor tyrosine kinase (OTK) has been shown to play an important role in the Drosophila motor axon pathfinding. The results of biochemical and genetic interactions previously suggested that OTK acts as a component of Semaphorin-1a/Plexin A (Sema-1a/PlexA) signaling during embryonic motor axon guidance and further showed that OTK binds to Wnt family members Wnt2 and Wnt4 and their common receptor Frizzled (Fz). However, the molecular mechanisms underlying the motor axon guidance function of OTK remain elusive. Here, we conclude that OTK mediates the forward and reverse signaling required for intersegmental nerve b (ISNb) motor axon pathfinding and we also demonstrate that the loss of two copies of Sema-1a synergistically enhances the bypass phenotype observed in otk mutants. Furthermore, the amorphic wnt2 mutation resulted in increased premature branching phenotypes, and the loss of fz function caused a frequent inability of ISNb motor axons to defasciculate at specific choice points. Consistent with a previous study, wnt4 mutant axons were often defective in recognizing target muscles. Interestingly, the bypass phenotype of otk mutants was robustly suppressed by loss of function mutations in wnt2, wnt4, or fz. In contrast, total ISNb defects of otk were increased by the loss-of-function alleles in wnt2 and wnt4, but not fz. These findings indicate that OTK may participate in the crosstalk between the Sema-1a/PlexA and Wnt signaling pathways, thereby contributing to ISNb motor axon pathfinding and target recognition.

Dpr10 and Nocte are required for Drosophila motor axon pathfinding

The paths axons travel to reach their targets and the subsequent synaptic connections they form are highly stereotyped. How cell surface proteins (CSPs) mediate these processes is not completely understood. The Drosophila neuromuscular junction (NMJ) is an ideal system to study how pathfinding and target specificity are accomplished, as the axon trajectories and innervation patterns are known and easily visualized. Dpr10 is a CSP required for synaptic partner choice in the neuromuscular and visual circuits and for axon pathfinding in olfactory neuron organization. In this study, we show that Dpr10 is also required for motor axon pathfinding. To uncover how Dpr10 mediates this process, we used immunoprecipitation followed by mass spectrometry to identify Dpr10 associated proteins. One of these, Nocte, is an unstructured, intracellular protein implicated in circadian rhythm entrainment. We mapped nocte expression in larvae and found it widely expressed in neurons, muscles, and glia. Cell-specific knockdown suggests nocte is required presynaptically to mediate motor axon pathfinding. Additionally, we found that nocte and dpr10 genetically interact to control NMJ assembly, suggesting that they function in the same molecular pathway. Overall, these data reveal novel roles for Dpr10 and its newly identified interactor, Nocte, in motor axon pathfinding and provide insight into how CSPs regulate circuit assembly.