The Ras1–Mitogen-Activated Protein Kinase Signal Transduction Pathway Regulates Synaptic Plasticity through Fasciclin II-Mediated Cell Adhesion

Abstract

Ras proteins are small GTPases with well known functions in cell proliferation and differentiation. In these processes, they play key roles as molecular switches that can trigger distinct signal transduction pathways, such as the mitogen-activated protein kinase (MAPK) pathway, the phosphoinositide-3 kinase pathway, and the Ral-guanine nucleotide dissociation stimulator pathway. Several studies have implicated Ras proteins in the development and function of synapses, but the molecular mechanisms for this regulation are poorly understood. Here, we demonstrate that the Ras-MAPK pathway is involved in synaptic plasticity at the Drosophila larval neuromuscular junction. Both Ras1 and MAPK are expressed at the neuromuscular junction, and modification of their activity levels results in an altered number of synaptic boutons. Gain- or loss-of-function mutations in Ras1 and MAPK reveal that regulation of synapse structure by this signal transduction pathway is dependent on fasciclin II localization at synaptic boutons. These results provide evidence for a Ras-dependent signaling cascade that regulates fasciclin II-mediated cell adhesion at synaptic terminals during synapse growth.

Plasticity and Second Messengers During Synapse Development

“…This change was indistinguishable from the increase in boutons observed in a Ras variant that selectively activates the MAPKinase pathway, and by constitutively active Raf F179 , suggesting that these changes were induced by activation of the MAPKinase pathway. Consistent with these results, a hypomorphic mutation in ras1 had the opposite phenotype, a decrease in bouton number, and a gain of function mutation in the fly MAPKinase gene rolled (rl) led to an increase in bouton number (Koh et al, 2002). These results are in agreement with the studies in Aplysia dissociated neurons, which show that ApMAPKinase is involved in the internalization of ApCAM Martin et al, 1997).…”

“…Further support that the changes in bouton number elicited by alterations in Ras1 and MAPKinase activity are mediated by alterations in FasII levels was demonstrated by examining the overall expression of FasII in MAPKinase gain or loss of function alleles, by examining the distribution of FasII within single synaptic boutons in relationship to active MAPKinase, and by using hypomorphic fasII mutants (Koh et al, 2002). The studies with rl mutants demonstrated that there was an inverse relationship between levels of synaptic FasII and MAPKinase activity.…”

“…Both Ras1 and MAPKinase are expressed at the NMJ, and modification in their activity levels results in an altered number of synaptic boutons (Koh et al, 2002). As discussed above, synapse stability and synapse expansion during muscle growth at the NMJ are regulated by changes in FasII expression at pre-and postsynaptic membranes, and FasII expression is in part controlled by electrical activity (Schuster et al, 1996b;Ashley et al, 2005).…”

“…3B). The structural effects of hyperexcitability mutants such as eagSh are mediated by FasII (Schuster et al, 1996a) as are the effects of alterations in the Ras/MAPK cascade (Koh et al, 2002) and CaMKII (Koh et al, 1999). The trafficking of FasII to and from the postsynaptic cell surface is accomplished by SNARE-mediated fusion and requires the Drosophila homolog of Amphiphysin (Mathew et al, 2003) and it is localized in the postsynaptic membrane by binding to Dlg, a member of the MAGUK family of proteins which includes PSD95 (Thomas et al, 1997).…”

“…One mechanism through which electrical activity alters FasII levels is by regulating its synaptic clustering via CaMKII-dependent phosphorylation of Dlg (Thomas et al, 1997;Koh et al, 1999). An additional mechanism by which the levels of FasII at the presynaptic terminal are modified is by the activation of the Ras/MAPKinase pathway (Koh et al, 2002). This redundant mechanism may serve the differential regulation of FasII localization at the pre-and postsynaptic site, or may represent FasII regulation in response to different signals.…”