Maintaining Muscle Mitochondria via Transsynaptic Signaling

Dominant VAPB mutations are implicated in neurodegenerative disease, including amyotrophic lateral sclerosis and spinal muscular atrophy. In the current issue, Han et al. (2012) uncover a mechanism through which the secreted VAPB MSP domain regulates actin organization and mitochondrial function in muscle cells through LAR and Robo receptor activation

The guanine nucleotide exchange factor trio mediates axonal development in the Drosophila embryo

Recent analysis of Rho subfamily GTPases in Drosophila revealed roles for Rac and Cdc42 during axonogenesis. Here, we describe the identification and characterization of the Drosophila counterpart of Trio, a guanine nucleotide exchange factor (GEF) that associates with the receptor phosphatase LAR and regulates GTPase activation in vertebrate cells. Mutants deficient in trio activity display defects in both central and peripheral axon pathways reminiscent of pheno-types observed in embryos deficient in small GTPase function. Double mutant analysis shows that trio interacts with Rac in a dose-sensitive manner but not with Rho. Moreover, reduction of trio activity potentiates the phenotype of mutations in the LAR homolog Dlar, suggesting that these proteins collaborate in orchestrating the cytoskeletal events that underlie normal axonogenesis

Drosophila semaphorin2b is Required for the Axon Guidance of a Subset of Embryonic Neurons

Background: The process of axon guidance is important in establishing functional neural circuits. The differential expression of cell-autonomous axon guidance factors is crucial for allowing axons of different neurons to take unique trajectories in response to spatially and temporally restricted cell non-autonomous axon guidance factors. A key motivation in the field is to provide adequate explanations for axon behavior with respect to the differential expression of these factors. Results: We report the characterization of a predicted secreted semaphorin family member, semaphorin2b (Sema-2b) in Drosophila embryonic axon guidance. Misexpression of Sema-2b in neurons causes highly penetrant axon guidance phenotypes in specific longitudinal and motoneuron pathways; however, expression of Sema-2b in muscles traversed by these motoneurons has no effect on axon guidance. In Sema-2b loss-of-function embryos, specific motoneuron and interneuron axon pathways display guidance defects. Specific visualization of the neurons that normally express Sema-2b reveals that this neuronal cohort is strongly affected by Sema-2b loss-of-function alleles. Conclusions: While secreted semaphorins have been implicated as cell non-autonomous chemorepellants in a variety of contexts, here we report previously undescribed Sema-2b loss-of-function and misexpression phenotypes that are consistent with a cell-autonomous role for Sema-2b. Developmental Dynamics 242:861–873, 2013. © 2013 Wiley Periodicals, Inc. Key Findings Misexpression of the secreted semaphorin Sema-2b in neurons results in specific axon guidance phenotypes. Both Sema-2b loss-of-function and misexpression phenotypes are congruent with a cell-autonomous role for Sema-2b. Novel axon guidance phenotypes caused by Sema-2b loss-of-function mutations are characterized

The receptor tyrosine phosphatase Dlar and integrins organize actin filaments in the Drosophila follicular epithelium

Background: Regulation of actin structures is instrumental in maintaining proper cytoarchitecture in many tissues. In the follicular epithelium of Drosophila ovaries, a system of actin filaments is coordinated across the basal surface of cells encircling the oocyte. These filaments have been postulated to regulate oocyte elongation; however, the molecular components that control this cytoskeletal array are not yet understood. Results: We find that the receptor tyrosine phosphatase (RPTP) Dlar and integrins are involved in organizing basal actin filaments in follicle cells. Mutations in Dlar and the common β-integrin subunit mys cause a failure in oocyte elongation, which is correlated with a loss of proper actin filament organization. Immunolocalization shows that early in oogenesis Dlar is polarized to membranes where filaments terminate but becomes generally distributed late in development, at which time β-integrin and Enabled specifically associate with actin filament terminals. Rescue experiments point to the early period of polar Dlar localization as critical for its function. Furthermore, clonal analysis shows that loss of Dlar or mys influences actin filament polarity in wild-type cells that surround mutant tissues, suggesting that communication between neighboring cells regulates cytoskeletal organization. Finally, we find that two integrin α subunits encoded by mew and if are required for proper oocyte elongation, implying that multiple components of the ECM are instructive in coordinating actin fiber polarity. Conclusions: Dlar cooperates with integrins to coordinate actin filaments at the basal surface of the follicular epithelium. To our knowledge, this is the first direct demonstration of an RPTP\u27s influence on the actin cytoskeleton

Presynaptic morphogenesis, active zone organization and structural plasticity in Drosophila

Effective adaptation of neural circuit function to a changing environment requires many forms of plasticity. Among these, structural plasticity is one of the most durable, and is also an intrinsic part of the developmental logic for the formation and refinement of synaptic connectivity. Structural plasticity of presynaptic sites can involve the addition, remodeling, or removal of pre- and post-synaptic elements. However, this requires coordination of morphogenesis and assembly of the subcellular machinery for neurotransmitter release within the presynaptic neuron, as well as coordination of these events with the postsynaptic cell. While much progress has been made in revealing the cell biological mechanisms of postsynaptic structural plasticity, our understanding of presynaptic mechanisms is less complete

MicroRNA-276a Functions in Ellipsoid Body and Mushroom Body Neurons for Naive and Conditioned Olfactory Avoidance in Drosophila

MicroRNA (miRNA)-mediated gene regulation plays a key role in brain development and function. But there are few cases in which the roles of individual miRNAs have been elucidated in behaving animals. We report a miR-276a::DopR regulatory module in Drosophila that functions in distinct circuits for naive odor responses and conditioned odor memory. Drosophila olfactory aversive memory involves convergence of the odors (conditioned stimulus) and the electric shock (unconditioned stimulus) in mushroom body (MB) neurons. Dopamine receptor DopR mediates the unconditioned stimulus inputs onto MB. Distinct dopaminergic neurons also innervate ellipsoid body (EB), where DopR function modulates arousal to external stimuli. We demonstrate that miR-276a is required in MB neurons for memory formation and in EB for naive responses to odors. Both roles of miR-276a are mediated by tuning DopR expression. The dual role of this miR-276a::DopR genetic module in these two neural circuits highlights the importance of miRNA-mediated gene regulation within distinct circuits underlying both naive behavioral responses and memory

MicroRNA-8 promotes robust motor axon targeting by coordinate regulation of cell adhesion molecules during synapse development

Neuronal connectivity and specificity rely upon precise coordinated deployment of multiple cell-surface and secreted molecules. MicroRNAs have tremendous potential for shaping neural circuitry by fine-tuning the spatio-temporal expression of key synaptic effector molecules. The highly conserved microRNA miR-8 is required during late stages of neuromuscular synapse development in Drosophila. However, its role in initial synapse formation was previously unknown. Detailed analysis of synaptogenesis in this system now reveals that miR-8 is required at the earliest stages of muscle target contact by RP3 motor axons. We find that the localization of multiple synaptic cell adhesion molecules (CAMs) is dependent on the expression of miR-8, suggesting that miR-8 regulates the initial assembly of synaptic sites. Using stable isotope labelling in vivo and comparative mass spectrometry, we find that miR-8 is required for normal expression of multiple proteins, including the CAMs Fasciclin III (FasIII) and Neuroglian (Nrg). Genetic analysis suggests that Nrg and FasIII collaborate downstream of miR-8 to promote accurate target recognition. Unlike the function of miR-8 at mature larval neuromuscular junctions, at the embryonic stage we find that miR-8 controls key effectors on both sides of the synapse. MiR-8 controls multiple stages of synapse formation through the coordinate regulation of both pre- and postsynaptic cell adhesion proteins

The Heparan Sulfate Proteoglycans Dally-like and Syndecan Have Distinct Functions in Axon Guidance and Visual-System Assembly in Drosophila

SummaryHeparan sulfate proteoglycans (HSPGs), a class of glycosaminoglycan-modified proteins, control diverse patterning events via their regulation of growth-factor signaling and morphogen distribution [1]. In C. elegans, zebrafish, and the mouse, heparan sulfate (HS) biosynthesis is required for normal axon guidance [2–4], and mutations affecting Syndecan (Sdc), a transmembrane HSPG, disrupt axon guidance in Drosophila embryos [5, 6]. Glypicans, a family of glycosylphosphatidylinositol (GPI)-linked HSPGs, are expressed on axons and growth cones in vertebrates, but their role in axon guidance has not been determined [7, 8]. We demonstrate here that the Drosophila glypican Dally-like protein (Dlp) is required for proper axon guidance and visual-system function. Mosaic studies revealed that Dlp is necessary in both the retina and the brain for different aspects of visual-system assembly. Sdc mutants also showed axon guidance and visual-system defects, some that overlap with dlp and others that are unique. dlp+ transgenes were able to rescue some sdc visual-system phenotypes, but sdc+ transgenes were ineffective in rescuing dlp abnormalities. Together, these findings suggest that in some contexts HS chains provide the biologically critical component, whereas in others the structure of the protein core is also essential

Growth cone-specific functions of XMAP215 in restricting microtubule dynamics and promoting axonal outgrowth

Background: Microtubule (MT) regulators play essential roles in multiple aspects of neural development. In vitro reconstitution assays have established that the XMAP215/Dis1/TOG family of MT regulators function as MT ‘plus-end-tracking proteins’ (+TIPs) that act as processive polymerases to drive MT growth in all eukaryotes, but few studies have examined their functions in vivo. In this study, we use quantitative analysis of high-resolution live imaging to examine the function of XMAP215 in embryonic Xenopus laevis neurons. Results: Here, we show that XMAP215 is required for persistent axon outgrowth in vivo and ex vivo by preventing actomyosin-mediated axon retraction. Moreover, we discover that the effect of XMAP215 function on MT behavior depends on cell type and context. While partial knockdown leads to slower MT plus-end velocities in most cell types, it results in a surprising increase in MT plus-end velocities selective to growth cones. We investigate this further by using MT speckle microscopy to determine that differences in overall MT translocation are a major contributor of the velocity change within the growth cone. We also find that growth cone MT trajectories in the XMAP215 knockdown (KD) lack the constrained co-linearity that normally results from MT-F-actin interactions. Conclusions: Collectively, our findings reveal unexpected functions for XMAP215 in axon outgrowth and growth cone MT dynamics. Not only does XMAP215 balance actomyosin-mediated axon retraction, but it also affects growth cone MT translocation rates and MT trajectory colinearity, all of which depend on regulated linkages to F-actin. Thus, our analysis suggests that XMAP215 functions as more than a simple MT polymerase, and that in both axon and growth cone, XMAP215 contributes to the coupling between MTs and F-actin. This indicates that the function and regulation of XMAP215 may be significantly more complicated than previously appreciated, and points to the importance of future investigations of XMAP215 function during MT and F-actin interactions