The role of endocytosis in regulating L1-mediated adhesion

L1 is a neural cell adhesion molecule critical for neural development. Full-length L1 (L1_FL) contains an alternatively spliced cytoplasmic sequence, RSLE, which is absent in L1 expressed in nonneuronal cells. The RSLE sequence follows a tyrosine, creating an endocytic motif that allows rapid internalization via clathrin-mediated endocytosis. We hypothesized that L1_FL would internalize more rapidly than L1 lacking the RSLE sequence (L1_ΔRSLE) and that internalization might regulate L1- mediated adhesion. L1 internalization was measured by immunofluorescence microscopy and by uptake of ¹²⁵I-anti-rat-L1 antibody, demonstrating that L1_FL is internalized 2-3 times faster than L1_ΔRSLE. Inhibition of clathrin-mediated endocytosis slowed internalization of L1_FL but did not affect initial uptake of L1_ΔRSLE. To test whether L1 endocytosis regulates L1 adhesion, cell aggregation rates were tested. L1_ΔRSLE cells aggregated two times faster than L1_FL cells. Inhibition of clathrin-mediated endocytosis increases the aggregation rate of the L1_FL cells to that of L1_ΔRSLE cells. Our results demonstrate that rapid internalization of L1 dramatically affects L1 adhesion.