A Role for Drosophila Drac1 in Neurite Outgrowth and Synaptogenesis in the Giant Fiber System

Department of Biology, Morrill Science Center, University of Massachusetts, Amherst, Massachusetts 01003, USA.
Molecular and Cellular Neuroscience (Impact Factor: 3.84). 01/2001; 16(6):754-65. DOI: 10.1006/mcne.2000.0903
Source: PubMed


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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    • "Several studies using over-expression of dominant-negative transgenes, or homozygous adult viable mutations, have recently shed light on signaling mechanisms during the formation of the GF-TTMn synapse. These include the receptors Semaphorin 1a and Roundabout [4,5]; the L-1 type cell-adhesion molecule Neuroglian [6]; the endocytotic and ubiquitin machinery [7-10]; the small GTPase DRac1 [11], and the transcription factor Ken [12]. However, the precise mechanisms by which these integrate during synaptogenesis are yet to emerge. "
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    • "Cdc42 regulates various aspects of neuronal differentiation including neurite outgrowth and extension (for review see Govek et al., 2005). However, paradoxically, both active and inactive mutants of Cdc42 have been reported to inhibit neurite outgrowth and neuronal differentiation (Luo et al., 1994; Allen et al., 2000; Aoki et al., 2004). NGF-driven differentiation of PC12 cells normally proceeds through a two-step process where cells initially spread and produce lamellipodia and unstable filopodia and then extend stable neurites (Greene and Tischler, 1976; Aoki et al., 2004). "
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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). "
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