A role for Drosophila Drac1 in neurite outgrowth and synaptogenesis in the giant fiber system.
ABSTRACT Recent studies have shown the small GTPases, Rac1, Rho, and CDC42, to have a role in axon guidance. To assess their participation in synapse assembly and function we have expressed various forms of Drac1 in the giant fiber system of Drosophila. Overexpression of wild-type Drac1 in the giant fiber (GF) lead to a disruption in axonal morphology; axons often terminate prematurely in a large swelling in the target area but lack the normal lateral bend where the synapse with the jump motor neuron would normally be found. Electrophysiological assays revealed longer latencies and lowering following frequencies indicating defects in the synapse between the GF and the tergotrochanteral motor neuron (TTMn). Thickened abnormal GF dendrites were also observed in the brain. Overexpression of the dominant-negative form of Drac1, (N17), resulted in axons that produced extra branches in the second thoracic neuromere (T2); however, the synaptic connection to the TTMn was present and functioned normally. Conversely, expression of the constitutively active form, Drac1(V12), resulted in a complete lack of neurite outgrowth and this was also seen with overexpression of Dcdc42(V12). In the absence of a GF, these flies showed no response in the jump (TTM) or flight (DLM) muscles upon brain stimulation. Taken together these results show that the balance of actin polymerization and depolymerization determines local process outgrowth and thereby synapse structure and function.
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- "Additional studies have used GAL4 enhancer traps to target expression of genes to the GFS. Using targeted expression in the GFS, the role of dynein–dynactin in synaptogenesis has been explored (Allen et al. 1999), and the cytoskeletal structure of the GF axon is controlled in part by the Ras-–Rac signalling pathways (Allen et al. 2000). In addition, the roles of the three Robo homologues and semaphorin in axon guidance, dendrite formation and synaptic function of GFS components have been partially dissected (Godenschwege et al. 2002a, b). "
ABSTRACT: The giant fibre system (GFS) of Drosophila is a simple neural circuit that mediates escape responses in adult flies. Here we report the initial characterization of two genes that are preferentially expressed in the GFS. Two P-element insertion lines, carrying the GAL4 transcriptional activator, were identified that exhibited pronounced expression in elements of the GFS and relatively low levels elsewhere within the adult central nervous system. Genomic DNA flanking the P-element insertion site was recovered from each of these lines, sequenced, and nearby transcripts identified and confirmed to exhibit GFS expression by in situ hybridization. This analysis revealed that these P-elements were in previously characterized genes. Line P[GAL4]-A307 has an insert in the gene short stop for which we have identified a novel transcript, while line P[GAL4]-141 has an insert in the transcription factor ken and barbie. Here we show that ken and barbie mutants have defects in escape behaviour, behavioural responses to visual stimuli and synaptic functions in the GFS. We have therefore revealed a neural role for a transcription factor that previously had no implicated neural function.Genes Brain and Behavior 07/2007; 6(4):347-58. DOI:10.1111/j.1601-183X.2006.00263.x · 3.51 Impact Factor
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- "Of note, there is some contradictory data regarding the absolute requirement for activated Rac and Cdc42 in neurite extension. Studies have shown that constitutively active Rac can inhibit total neurite length  and inhibit neurite extension in Drosophila . While activated Rac, and dominant negative and activated Cdc42 have no effect on DRG neurite outgrowth . "
ABSTRACT: The Cas family proteins are a family of adhesion docking molecules that mediate protein-protein interactions and contribute to a number of signal transduction pathways. Recent studies of two family members, p130Cas and Sin, have suggested that they may play a role in neurite formation. The current study demonstrates that the third family member, HEF1, can also stimulate the formation of neurite-like processes, in the presence of Rho kinase inhibitors. The HEF1-promoted processes actively extend from the cell body and resemble neurites both in the manner of process extension and in the distribution of adhesion-associated molecules. The HEF1-promoted processes are dependent on the presence of an intact microtubule system and can be inhibited by co-expression of either constitutively active Rac or Cdc42 GTPase. Together, our data support a role for the Cas proteins in regulating cellular morphologies that contribute to tissue specialization.Biochimica et Biophysica Acta 01/2006; 1746(2):143-54. DOI:10.1016/j.bbamcr.2005.10.008 · 4.66 Impact Factor
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- "Conversely , expression of wild-type or CA Rac1 can cause the premature termination or stalling of axons in the Drosophila giant fiber system (Allen et al. 2000) and in Purkinje cells of CA Rac1 transgenic mice (Luo et al. 1996a). Expression of wild-type or DN Rac1 in the Drosophila giant fiber system (Allen et al. 2000), and CA Rac1 in the Drosophila CNS (Fritz and VanBerkum 2002), also causes inappropriate midline crossing. All in all, Rac activity appears to be required for the proper growth and guidance of axons, with loss of Rac function resulting in an overshoot of axons and increased Rac activity causing axons to stall. "
ABSTRACT: Our brain serves as a center for cognitive function and neurons within the brain relay and store information about our surroundings and experiences. Modulation of this complex neuronal circuitry allows us to process that information and respond appropriately. Proper development of neurons is therefore vital to the mental health of an individual, and perturbations in their signaling or morphology are likely to result in cognitive impairment. The development of a neuron requires a series of steps that begins with migration from its birth place and initiation of process outgrowth, and ultimately leads to differentiation and the formation of connections that allow it to communicate with appropriate targets. Over the past several years, it has become clear that the Rho family of GTPases and related molecules play an important role in various aspects of neuronal development, including neurite outgrowth and differentiation, axon pathfinding, and dendritic spine formation and maintenance. Given the importance of these molecules in these processes, it is therefore not surprising that mutations in genes encoding a number of regulators and effectors of the Rho GTPases have been associated with human neurological diseases. This review will focus on the role of the Rho GTPases and their associated signaling molecules throughout neuronal development and discuss how perturbations in Rho GTPase signaling may lead to cognitive disorders.Genes & Development 02/2005; 19(1):1-49. DOI:10.1101/gad.1256405 · 12.64 Impact Factor