L1-Mediated Branching Is Regulated by Two Ezrin-Radixin-Moesin (ERM)-Binding Sites, the RSLE Region and a Novel Juxtamembrane ERM-Binding Region

Abstract

We investigated how the neural cell adhesion molecule L1 mediates neurite outgrowth through L1-L1 homophilic interactions. Wild-type L1 and L1 with mutations in the cytoplasmic domain (CD) were introduced into L1 knock-out neurons, and transfected neurons were grown on an L1 substrate. Neurite length and branching were compared between wild-type L1 and L1CD mutations. Surprisingly, the L1CD is not required for L1-mediated neurite outgrowth but plays a critical role in neurite branching, through both the juxtamembrane region and the RSLE region. We demonstrate that both regions serve as ezrin-moesin-radixin-binding sites. A truncation mutant that deletes 110 of 114 amino acids of the L1CD still supports neurite outgrowth on an L1 substrate, suggesting that a coreceptor binds to L1 in cis and mediates neurite outgrowth and that L1-ankyrin interactions are not essential for neurite initiation or outgrowth. These data are consistent with a model in which L1 can influence L1-mediated neurite outgrowth and branching through both the L1CD and a coreceptor.

MAP Kinase Pathway–dependent Phosphorylation of the L1-CAM Ankyrin Binding Site Regulates Neuronal Growth

“…Both ankyrin-dependent and ankyrin-independent pathways are likely to operate in parallel, a conclusion supported by the incomplete rescue of neuronal growth by the AP-YF peptide after U0126 treatment ( Figure 6C). Previous work has shown that ankyrin binding to L1-CAM inhibits L1-mediated traction-force generation and neuronal growth (Gil et al, 2003;Cheng et al, 2005). The identification of the MAP kinase pathway as a regulator of L1-FIGQY phosphorylation, L1-ankyrin B interactions and L1-mediated neuronal growth reinforces the central role of ankyrin B binding in the regulated growth of neurons on L1-CAM ligands.…”

“…However, ankyrin binding in the growing neurite plays an inhibitory role; reagents that block L1-CAM-ankyrin interactions increase the L1-CAM-dependent growth of neurons in culture (Gil et al, 2003). Additionally, neurons expressing a truncated form of L1 that lacks the ankyrin binding site produce longer axons than neurons expressing full-length receptor, again supporting an inhibitory role for ankyrin binding in L1-mediated nerve growth (Cheng et al, 2005). Finally, the binding of ankyrin G to the L1-family member neurofascin promotes neurofascin-mediated cell adhesion .…”

“…L1-CAM binds to components of the cytoskeleton, including members of the ankyrin family of adaptor proteins (Davis and Bennett, 1994), members of the ezrin-radixin-moesin (ERM) family (Dickson et al, 2002) and components of the AP-2 clathrin complex (Kamiguchi et al, 1998). L1-CAM interactions with ERM proteins regulate axon branching on L1-CAM substrates (Dickson et al, 2002;Cheng et al, 2005), whereas binding to AP-2 is necessary for L1-CAM endocytosis and some aspects of L1-CAM-mediated signaling (Schaefer et al, 2002). In contrast, the binding of ankyrin to the L1-CAM cytoplasmic tail appears to regulate both adhesion and axon growth.…”

Interplay between adhesion turnover and cytoskeleton dynamics in the control of growth cone migration

“…α-and β-catenin in the case of N-cadherin, 10 ankyrin and ezrin in the case of IgCAMs such as L1. [11][12][13][14] These purely mechanical connections can also be accompanied by signalling events such as Rac-1 activation by N-cadherin liganding 15 and phosphorylation of the L1 intracellular tail that regulates binding to ankyrin. 11,12 When only few molecular bonds are formed, e.g.…”

“…31 Neurite growth on L1 was estimated from references. 7,14,30 (Grey) Interaction between endogenous SynCAM1 molecules expressed on growth cones and SynCAM-Fc ligands coated on microspheres or flat glass. 16 The turnover of SynCAM homophilic interactions was estimated from SynCAM-coated Quantum dots detaching from neurons transfected with SynCAM1.…”