␦-Catenin is a synaptic adherens junction protein pivotally positioned to serve as a signaling sensor and integrator. Expression of ␦-catenin induces filopodia-like protrusions in neurons. Here we show that the small GTPases of the Rho family act coordinately as downstream effectors of ␦-catenin. A dominant negative Rac prevented ␦-catenin-induced protrusions, and Cdc42 activity was dramatically increased by ␦-catenin expression. A kinase dead LIMK (LIM kinase) and a mutant Cofilin also prevented ␦-catenin-induced protrusions. To link the effects of ␦-catenin to a physiological pathway, we noted that (S)-3,5-dihydroxyphenylglycine (DHPG) activation of metabotropic glutamate receptors induced dendritic protrusions that are very similar to those induced by ␦-catenin. Furthermore, ␦-catenin RNA-mediated interference can block the induction of dendritic protrusions by DHPG. Interestingly, DHPG dissociated PSD-95 and N-cadherin from the ␦-catenin complex, increased the association of ␦-catenin with Cortactin, and induced the phosphorylation of ␦-catenin within the sites that bind to these protein partners.␦-Catenin is a component of the synaptic adherens junction that is necessary for normal learning and memory (1). In the absence of ␦-catenin, mice have severe deficits in several types of memory as well as synaptic plasticity. However, the functional basis for these deficits is not obvious, particularly because the morphological changes in ␦-catenin null mice are minimal. ␦-Catenin contains 10 Armadillo repeats (a 42-amino acid motif, originally described in the Drosophila segment polarity gene, armadillo) spaced in the characteristic arrangement of all members of this gene family which includes the prototypical member, p120 ctn , as well as p0071, ARVCF (Armadillo Repeat gene deleted in Velo-Cardio-Facial syndrome) (2), and the plakophilins, both components of the desmosome (3-6). The core functions of this protein family are stabilization of cadherins by binding to a highly conserved sequence in the juxtamembrane region and regulatory coordination over Rho GTPases (7). ␦-Catenin is localized to the post-synaptic adherens junction, collaborates with Rho GTPases to set a balance between neurite elongation and branching, and robustly induces dendritic protrusions (8). Among the cadherin binding family members, ␦-catenin is the only one that is a neural-specific protein. However, ␦-catenin null mice develop normally, whereas p120 ctn can regulate synapse and spine development (9).Because both p120 ctn and ␦-catenin are expressed in neurons, an important question is the added functionality provided by co-expression of these paralogs. ⌱n contrast to p120 ctn , ␦-catenin contains a short carboxyl-terminal motif that corresponds to a ligand sequence for PDZ (postsynaptic density-95 (PSD-95) 6 /discs large/zona occludens-1) domain-containing proteins. Through the versatility of this domain, the multiple complex interactions of ␦-catenin with the synapse arise. ␦-Catenin binds to the synaptic scaffolding molecule (S-SCAM) (...

In this study, we have examined the effects of rotenone in primary cultures of hippocampal and dopaminergic neurons in order to obtain insights into the possible mechanisms underlying the neurotoxic effects of this pesticide. The results obtained indicate that a 48-h exposure to rotenone (0.1 microM) produces a complete and selective suppression of axon formation. This effect was dose dependent, not accompanied by changes in microtubule organization, and reversible after washout of the agrochemical from the tissue culture medium. Interestingly, pull-down assays revealed that rotenone decreases Cdc42 and Rac activities, whereas increasing that of Rho. In accordance with this, treatment of neuronal cultures with cytochalasin D, an actin-depolymerizing drug, or with the Rho-kinase inhibitor Y27632, or overexpression of Tiam1, a guanosine nucleotide exchange factor for Rac, reverts the inhibitory effect of rotenone on axon formation. Taken together, our data suggest that at least some of the neurotoxic effects of rotenone are associated with an inhibition of actin dynamics through modifications of Rho-GTPase activity.

Rotenone, a broad-spectrum insecticide, piscicide and pesticide, produces a complete and selective suppression of axonogenesis in cultured hippocampal neurons. This effect is associated with an inhibition of actin dynamics through activation of Ras homology member A (RhoA) activity. However, the upstream signaling mechanisms involved in rotenone-induced RhoA activation were unknown. We hypothesized that rotenone might inhibit axon growth by the activation of RhoA/ROCK pathway because of the changes in microtubule (MT) dynamics and the concomitant release of Lfc, a MT-associated Guanine Nucleotide Exchange Factor (GEF) for RhoA. In this study, we demonstrate that rotenone decreases MT stability in morphologically unpolarized neurons. Taxol (3 nM), a drug that stabilizes MT, attenuates the inhibitory effect of rotenone (0.1 μM) on axon formation. Radiometric Forster Resonance Energy Transfer, revealed that this effect is associated with inhibition of rotenone-induced RhoA and ROCK activation. Interestingly, silencing of Lfc, but not of the RhoA GEF ArhGEF1, prevents the inhibitory effect of rotenone on axon formation. Our results suggest that rotenone-induced MT de-stabilization releases Lfc from MT thereby promoting RhoA and ROCK activities and the consequent inhibition of axon growth. Open Science: This manuscript was awarded with the Open Materials Badge. For more information see: https://cos.io/our-services/open-science-badges/.