Plasma membrane recycling and flow in growing neurites

Axonal growth requires insertion of newly synthesized membrane components i nto the plasmalemma. Imbalance between exocytotic membrane addition and end ocytic retrieval at specific axonal sites may lead to the bulk plasma membr ane flow along the axon and, thus, contribute to the renewal of plasma memb rane components. By using fluorescent lipid analogs incorporated into the p lasma membrane, we determined the sites of membrane internalization in grow ing Xenopus embryo neurons. Vectorial traffic of endocytic membranes from t he distal axon to the cell body was observed, suggesting bulk retrieval of plasma membrane at the growth cone. No long-range axonal transport of membr ane material internalized at the cell body or along the axon was detected. In addition, we measured the rate of plasma membrane flow in Xenopus neurit es. Axonal plasma membrane was found to flow anterogradely with the rate eq ual to approximately 30% of the rate of neurite elongation. Our results suggest that the "growth from the tip" pattern of neurite elong ation may coexist with transport of new membrane components along plasmalem ma by anterograde membrane flow. (C) 2000 IBRO. published by Elsevier Scien ce Ltd.