L1 Trafficking

Over the past 5 years we have done a series of experiments on how L1 trafficking is regulated and what its functions include. This line of research came about due to a collision between three projects. The first, started by H. Kamiguchi, was based on his identification of a potential tyrosine based sorting motif in the L1 cytoplasmic domain. This sequence is present in neuronal L1 due to alternative splicing. The second was experiments by Andy Schaefer suggesting that changes in L1 phosphorylation around the RSLE is influenced by cell contact. The third were experiments by G. Cheng that revealed that RSLE minus L1 is more adhesive than RSLE plus L1.

A tyrosine followed 3 amino acids later by a hydrophobic amino acid creates a motif that can bind adaptors like AP-1 and AP-2. These adaptors are involved in sorting of proteins to different membrane compartments. H. Kamiguchi and other members of the lab showed that the YRSLE sequence in L1 is important in sorting of L1 into axons and in retrieval of L1 from the plasma membrane. Of particular interest was the observations that L1 is internalized in the central regions of growth cones and then traffics inside the growth cone to the peripheral domain where it is reinserted into the membrane. This occurs on L1 substrates but not on laminin. We suspect that the regulation of this process is very important for L1 based motility since internalization of L1 provides a mechanism to reduce adhesion at the rear of the growth cones, permitting growth cone advancement. It also reduces the demands on the growth cone by allowing L1 to be reused numerous times as a growth cone moves forward. Dr. Kamiguchi continues to study L1 trafficking in his new laboratory at RIKEN in Tokyo, Japan.

It is well know that the amount of an adhesion molecule on the cell surface has a profound effect on adhesion and that this is often non-linear, i.e. relatively small increases in CAM number can lead to large increases in adhesion. There are different ways to alter CAM expression. These include transcriptional regulation to increase or decrease CAM expression, although this is a relatively slow process. In the immune system there are examples of regulated insertion of CAMs in the cell surface to increase CAM number. During cell migration, there are examples of proteolysis of CAMs on the surface to permit attachment release at the rear of cells. We hypothesized that regulation of L1 internalization might be a way to dynamically alter adhesion. Kristin Long, an MD/PhD student in the lab, found that the RSLE minus form of L1 was significantly (2 fold) more adhesive than the RSLE plus form. The RSLE plus form is internalized more rapidly (2-3 times) than the RSLE minus form. If we inhibited internalization then the RSLE plus form became equally adhesive to the RSLE minus form.

L1 trafficking is regulated by phosphorylation and dephosphorylation. If the tyrosine (Y1176) in the sorting motif described above is phosphorylated, L1 internalization is inhibited. Therefore, tyrosine 1176 is a critical control point for regulating L1 trafficking. We have found that L1 is phosphorylated in growth cones but it is dephosphorylated at the rear of growth cones. This permits internalization and recycling. L1-L1 homophilic binding can induce the dephosphorylation of L1, showing that outside-in signaling regulated L1 phosphorylation (Schaefer et al, 2002). Similarly, changes in phosphorylation of the Y1176 alters L1 adhesion, a form of indise-out signalling, by altering L1 internalization rates (Long et all, 2001).

Related Publications

Kamiguchi, H., Lemmon, V. A neuronal form of the cell adhesion molecule L1 contains a tyrosine-based signal required for sorting to the axonal growth cone. J. Neurosci., 18:3749-3756, 1998. Download PDF

Kamiguchi, H., Long, K.E., Pendergast, M., Schaefer, A.W., Rapoport, I, Kirchhausen, T., Lemmon, V. The neural cell adhesion molecule L1 interacts with the AP-2 adaptor and is endocytosed via the clathrin-mediated pathway. J. Neurosci. 18:5311-5321, 1998. Download PDF

Schaefer, A.W., Kamiguchi, H., Wong, E.V., Beach, C.M., Landreth, G., and Lemmon, V. Activation of the MAPK Signal Cascade by the Neural Cell Adhesion Molecule L1 Requires L1 Internalization. J. Biol. Chem. 274:37965-37973., 1999. Download PDF

Kamiguchi, H., Lemmon, V. Recycling of the Cell Adhesion Molecule L1 in Axonal Growth Cones J. Neurosci., 15:3676-3686, 2000. Download PDF

Long, K.E. and Lemmon, V. Dynamic regulation of cell adhesion molecules during axon outgrowth. J. Neurobiol. 44:230-245, 2000. Download PDF

Kamiguchi, H., and Lemmon, V. IgCAMs: Bi-directional signals underlying neurite growth. Current Opinion in Cell Biology, 12:598-605, 2000. Download PDF

Long, K.E., Asou, H., Snider, M.D., Lemmon, V., The role of endocytosis in regulating L1 mediate adhesion. J. Biol. Chem., 276:1285-1290, 2001. Download PDF

A.W. Schaefer, Y. Kamei, H. Kamiguchi, E.V. Wong, I. Rapoport, T. Kirchhausen, C.M. Beach, G. Landreth, S.K. Lemmon, V. Lemmon. L1 endocytosis is controlled by a phosphorylation-dephosphorylation cycle stimulated by outside-in signaling by L1. J. Cell Biology, 157: 1223-1232, 2002. Download PDF