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

Abstract

The growth of neuronal processes depends critically on the function of adhesion proteins that link extracellular ligands to the cytoskeleton. The neuronal adhesion protein L1-CAM serves as a receptor for nerve growth-promoting proteins, a process that is inhibited by the interaction between L1-CAM and the cytoskeleton adaptor ankyrin. Using a novel reporter based on intramolecular bioluminescence resonance energy transfer, we have determined that the MAP kinase pathway regulates the phosphorylation of the FIGQY motif in the adhesion protein L1-CAM and its interaction with ankyrin B. MAP kinase pathway inhibitors block L1-CAM-mediated neuronal growth. However, this blockade is partially rescued by inhibitors of L1-CAM-ankyrin binding. These results demonstrate that the MAP kinase pathway regulates L1-CAMmediated nerve growth by modulating ankyrin binding, suggesting that nerve growth can be regulated at the level of individual receptors.

Ankyrin-Dependent and -Independent Mechanisms Orchestrate Axonal Compartmentalization of L1 Family Members Neurofascin and L1/Neuron–Glia Cell Adhesion Molecule

“…During early axon outgrowth, L1 is not stably associated with ankB along axons and shows higher levels of phosphorylation of the FIGQY motif. This observation supports the notion that axon outgrowth requires dynamic association of L1 with cytoskeletal elements, especially dynamic actin filaments (Gil et al, 2003;Whittard et al, 2006). Ankyrin binding stabilizes L1 in the membrane and reduces engagement of the dynamic actin flow (Gil et al, 2003) (see also Nishimura et al, 2003).…”

“…Regulation of FIGQY phosphorylation therefore can modulate the extent to which L1 engages either F-actin or ankyrin. Growth factors as well as ligand binding of L1 are potentially important regulators of ankyrin binding (Cheng et al, 2005;Whittard et al, 2006). It is surprising that our results show low/nondetectable levels of phospho-FIGQY staining in growth cones and filopodia in which most of the dynamic actin resides.…”

“…For L1, we propose that it also is targeted to axons by an ankyrinindependent mechanism and that its ankyrin-binding capacity is temporally and spatially regulated by phosphorylation. The ability to regulate the ankyrin-binding capacity of L1 is likely important for controlling the dynamic functions of L1 in different places and at different times Tuvia et al, 1997;Kizhatil et al, 2002;Godenschwege et al, 2006;Whittard et al, 2006). More work is needed to fully dissect when and where L1 binds ankyrin and to what effect.…”

“…Ankyrin binding modulates L1-mediated neurite outgrowth (Gil et al, 2003;Nishimura et al, 2003;Whittard et al, 2006) and NF adhesion in culture assays . We show that NF binds strongly to ankG at the IS at all times of axonogenesis.…”

L1 and NCAM adhesion molecules as signaling coreceptors in neuronal migration and process outgrowth

“…All L1-CAMs reversibly engage the actin cytoskeleton through a conserved motif FIGQ/AY in the cytoplasmic domain, which contains a critical tyrosine residue required for binding the spectrin-actin adaptor ankyrin [7]. ERK, a serine/threonine protein kinase, was shown to indirectly induce tyrosine phosphorylation in the FIGQY motif in the L1 intracellular domain [8], thereby dissociating ankyrin and uncoupling L1 from the actin cytoskeleton. The tyrosine kinase responsible for FIGQ/AY phosphorylation remains elusive.…”