Tyrosine phosphorylation of the Rho guanine nucleotide exchange factor Trio regulates netrin-1/DCC-mediated cortical axon outgrowth.

The chemotropic guidance cue netrin-1 mediates attraction of migrating axons during central nervous system development through the receptor Deleted in Colorectal Cancer (DCC). Downstream of netrin-1, activated Rho GTPases Rac1 and Cdc42 induce cytoskeletal rearrangements within the growth cone. The Rho guanine nucleotide exchange factor (GEF) Trio is essential for Rac1 activation downstream of netrin-1/DCC, but the molecular mechanisms governing Trio activity remain elusive. Here, we demonstrate that Trio is phosphorylated by Src family kinases in the embryonic rat cortex in response to netrin-1. In vitro, Trio was predominantly phosphorylated at Tyr(2622) by the Src kinase Fyn. Though the phospho-null mutant Trio(Y2622F) retained GEF activity toward Rac1, its expression impaired netrin-1-induced Rac1 activation and DCC-mediated neurite outgrowth in N1E-115 neuroblastoma cells. Trio(Y2622F) impaired netrin-1-induced axonal extension in cultured cortical neurons and was unable to colocalize with DCC in growth cones, in contrast to wild-type Trio. Furthermore, depletion of Trio in cortical neurons reduced the level of cell surface DCC in growth cones, which could be restored by expression of wild-type Trio but not Trio(Y2622F). Together, these findings demonstrate that Trio(Y2622) phosphorylation is essential for the regulation of the DCC/Trio signaling complex in cortical neurons during netrin-1-mediated axon outgrowth.

The activation of ezrin-radixin-moesin proteins is regulated by netrin-1 through Src kinase and RhoA/Rho kinase activities and mediates netrin-1-induced axon outgrowth.

The receptor Deleted in Colorectal Cancer (DCC) mediates the attractive response of axons to the guidance cue netrin-1 during development. On netrin-1 stimulation, DCC is phosphorylated and induces the assembly of signaling complexes within the growth cone, leading to activation of cytoskeleton regulators, namely the GTPases Rac1 and Cdc42. The molecular mechanisms that link netrin-1/DCC to the actin machinery remain unclear. In this study we seek to demonstrate that the actin-binding proteins ezrin-radixin-moesin (ERM) are effectors of netrin-1/DCC signaling in embryonic cortical neurons. We show that ezrin associates with DCC in a netrin-1-dependent manner. We demonstrate that netrin-1/DCC induces ERM phosphorylation and activation and that the phosphorylation of DCC is required in that context. Moreover, Src kinases and RhoA/Rho kinase activities mediate netrin-1-induced ERM phosphorylation in neurons. We also observed that phosphorylated ERM proteins accumulate in growth cone filopodia, where they colocalize with DCC upon netrin-1 stimulation. Finally, we show that loss of ezrin expression in cortical neurons significantly decreases axon outgrowth induced by netrin-1. Together, our findings demonstrate that netrin-1 induces the formation of an activated ERM/DCC complex in growth cone filopodia, which is required for netrin-1-dependent cortical axon outgrowth.

Novel α1 and γ2 GABAA receptor subunit mutations in families with idiopathic generalized epilepsy.

Epilepsy is a heterogeneous neurological disease affecting approximately 50 million people worldwide. Genetic factors play an important role in both the onset and severity of the condition, with mutations in several ion-channel genes being implicated, including those encoding the GABA(A) receptor. Here, we evaluated the frequency of additional mutations in the GABA(A) receptor by direct sequencing of the complete open reading frame of the GABRA1 and GABRG2 genes from a cohort of French Canadian families with idiopathic generalized epilepsy (IGE). Using this approach, we have identified three novel mutations that were absent in over 400 control chromosomes. In GABRA1, two mutations were found, with the first being a 25-bp insertion that was associated with intron retention (i.e. K353delins18X) and the second corresponding to a single point mutation that replaced the aspartate 219 residue with an asparagine (i.e. D219N). Electrophysiological analysis revealed that K353delins18X and D219N altered GABA(A) receptor function by reducing the total surface expression of mature protein and/or by curtailing neurotransmitter effectiveness. Both defects would be expected to have a detrimental effect on inhibitory control of neuronal circuits. In contrast, the single point mutation identified in the GABRG2 gene, namely P83S, was indistinguishable from the wildtype subunit in terms of surface expression and functionality. This finding was all the more intriguing as the mutation exhibited a high degree of penetrance in three generations of one French Canadian family. Further experimentation will be required to understand how this mutation contributes to the occurrence of IGE in these individuals.

Compartmentalized DCC signalling is distinct from DCC localized to lipid rafts.

BACKGROUND INFORMATION: Netrin-1 is a bi-functional cue that attracts or repels different classes of neurons during development. The netrin-1 receptor DCC (deleted in colorectal cancer) acts as a tyrosine kinase-associated receptor to mediate the attractive response towards netrin-1. The lipid raft-localized Src family kinase Fyn is required for DCC-mediated axon guidance. DCC functions are also dependent on lipid rafts, membrane microdomains corresponding to a low-density, detergent-resistant membrane fraction. However, it remains unclear how the association of DCC with lipid rafts controls netrin-1 signalling. RESULTS: DCC targeted to lipid rafts represented a minor proportion of total DCC inside the cell, but predominated on the cell surface of both IMR-32 human neuroblastoma cells and embryonic cortical neurons. Netrin-1 accumulated in lipid rafts, but had no effect on the targeting of DCC to that compartment, with DCC remaining on the cell surface in lipid rafts through 60 min post-treatment. However, DCC was able to interact with Fyn, both in the lipid rafts and soluble compartments isolated from embryonic E19 rat brains, whereas early downstream signalling components such as Nck-1, and total and active focal adhesion kinase were mainly localized to the non-lipid raft compartment. CONCLUSIONS: Together, these results suggest that DCC can be found in raft and non-raft portions of the plasma membrane, with early signalling events propagated by non-raft associated DCC.

Identification of a novel di-leucine motif mediating K(+)/Cl(-) cotransporter KCC2 constitutive endocytosis.

The neuron-specific potassium-chloride cotransporter 2 (KCC2) plays a crucial role, by controlling chloride extrusion, in the development and maintenance of inhibitory neurotransmission. Although it is now well established that activity-dependent mechanisms can down regulate KCC2 gene expression, the role of post-translational mechanisms in controlling KCC2 expression, specifically at the cell-surface, are poorly understood. We therefore set out to identify the mechanisms and motifs regulating KCC2 endocytosis, one important pathway that may control KCC2 membrane expression. Using a fluorescence-based assay, we show KCC2 when expressed in HEK293 cells is constitutively internalized via a dynamin- and clathrin-dependent pathway. Consistent with this, we demonstrate KCC2 from adult mouse brain associates in vivo with the clathrin-binding adaptor protein-2 (AP-2) complex. Using an endocytosis reporter system, we identify the presence of an autonomous endocytosis motif in the carboxyl cytoplasmic terminus of KCC2. By site-directed mutagenesis we define this novel KCC2 endocytic motif as a non-canonical di-leucine motif, (657)LLXXEE(662). Finally by mutating this motif in the context of full-length KCC2 we demonstrate that this novel KCC2 endocytic motif is essential for the constitutive internalization of KCC2 and for binding to the AP-2 complex. Subsequent sequence analysis reveals this motif is highly conserved between the closely related K(+)/Cl(-) family members that mediate chloride efflux, but absent from the more distant related cotransporters controlling chloride influx. In conclusion, our results indicate constitutive internalization of KCC2 is clathrin-mediated and dependent on the binding of AP-2 to this novel endocytic motif. Furthermore, that this process appears to be an evolutionarily conserved mechanism amongst functionally homologous cotransporters.

The UBL domain of PLIC-1 regulates aggresome formation.

Defects in protein folding and the proteasomal pathway have been linked with many neurodegenerative diseases. PLIC-1 (protein linking IAP to the cytoskeleton) is a ubiquitin-like protein that binds to the ubiquitin-interacting motif (UIM) of the proteasomal subunit S5a. Here, we show that PLIC-1 also binds to the UIM proteins ataxin 3--a deubiquitinating enzyme--HSJ1a--a co-chaperone--and EPS15 (epidermal growth factor substrate 15)--an endocytic protein. Using a polyglutamine (polyQ) disease model, we found that both endogenous PLIC-1 and EPS15 localize to perinuclear aggresomes, and that polyQ enhances their in vivo interaction. We show that knockdown of PLIC-1 and EPS15 by RNA interference reduces aggresome formation. In addition, PLIC-1(DeltaUBL) functions as a dominant-negative mutant, blocking both polyQ transport to aggresomes and the association of EPS15 with dispersed aggregates. We also show that PLIC-1 is upregulated by arsenite-induced protein misfolding. These results indicate a role for PLIC-1 in the protein aggregation-stress pathway, and we propose a novel function for the ubiquitin-like (UBL) domain--by means of UBL-UIM interactions--in transport to aggresomes.

The amino terminus of the glial glutamate transporter GLT-1 interacts with the LIM protein Ajuba.

We have identified a cytoplasmic LIM protein, Ajuba, which interacts with the amino terminus of GLT-1, the most abundant plasma membrane glutamate transporter in the brain. Ajuba has a cytoplasmic location when expressed alone in COS cells, but translocates to colocalize with GLT-1 at the plasma membrane when GLT-1 is coexpressed. Ajuba is expressed in cerebellum, cortex, hippocampus, and retina and also in organs outside the CNS. Ajuba is found with GLT-1 in astrocytes, cerebellar Bergmann glia and retinal neurons, and antibodies to Ajuba coimmunoprecipitate GLT-1 from brain. For GLT-1 expressed in COS cells, coexpression of Ajuba did not affect the transporter's K(m) or V(max) for glutamate. Since Ajuba is known to activate MAP kinase enzymes, and its homologue Zyxin binds to cytoskeletal proteins, we propose that Ajuba is a scaffolding protein allowing GLT-1 to regulate intracellular signaling or interact with the cytoskeleton.