Ankyrin-dependent and -independent mechanisms orchestrate axonal compartmentalization of L1 family members neurofascin and L1/neuron-glia cell adhesion molecule.
Axonal initial segments (IS) and nodes of Ranvier are functionally important membrane subdomains in which the clustering of electrogenic channels enables action potential initiation and propagation. In addition, the initial segment contributes to neuronal polarity by serving as a diffusion barrier. To study the mechanisms of axonal compartmentalization, we focused on two L1 family of cell adhesion molecules (L1-CAMs) [L1/neuron-glia cell adhesion molecule (L1/NgCAM) and neurofascin (NF)] and two neuronal ankyrins (ankB and ankG). NF and ankG accumulate specifically at the initial segment, whereas L1/NgCAM and ankB are expressed along the entire lengths of axons. We find that L1/NgCAM and NF show distinct modes of steady-state accumulation during axon outgrowth in cultured hippocampal neurons. Despite their different steady-state localizations, both L1/NgCAM and NF show slow diffusion and low detergent extractability specifically in the initial segment but fast diffusion and high detergent extractability in the distal axon. We propose that L1-CAMs do not strongly bind ankB in the distal axon because of spatial regulation of ankyrin affinity by phosphorylation. NF, conversely, is initially enriched in an ankyrin-independent manner in the axon generally and accumulates progressively in the initial segment attributable to preferential binding to ankG. Our results suggest that NF and L1/NgCAM accumulate in the axon by an ankyrin-independent pathway, but retention at the IS requires ankyrin binding.
Boiko
Tatiana
T
Department of Neuroscience, University of Virginia, Charlottesville, Virginia 22908, USA.
Vakulenko
Max
M
Ewers
Helge
H
Yap
Chan Choo
CC
Norden
Caren
C
Winckler
Bettina
B
D000818:Animals; D017487:Ankyrins / Q000096:biosynthesis / Q000378:metabolism / Q000502:physiology*; D001369:Axons / Q000737:chemistry* / Q000378:metabolism; D015815:Cell Adhesion Molecules / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D019062:Cell Adhesion Molecules, Neuron-Glia / Q000096:biosynthesis / Q000378:metabolism / Q000502:physiology*; D002460:Cell Line; D002478:Cells, Cultured; D006801:Humans; D005810:Multigene Family; D009414:Nerve Growth Factors / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D039842:Neural Cell Adhesion Molecule L1 / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D009474:Neurons / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding / Q000235:genetics; D051381:Rats
D017487:Ankyrins; D015815:Cell Adhesion Molecules; D019062:Cell Adhesion Molecules, Neuron-Glia; D009414:Nerve Growth Factors; D039842:Neural Cell Adhesion Molecule L1; C504594:Nfasc protein, rat