Plasma Membrane Recycling and Flow in Growing Neurites

Axonal growth requires insertion of newly synthesized membrane components into the plasmalemma. Imbalance between exocytotic membrane addition and endocytic retrieval at specific axonal sites may lead to the bulk plasma membrane flow along the axon and, thus, contribute to the renewal of plasma membrane components. By using fluorescent lipid analogs incorporated into the plasma membrane, we determined the sites of membrane internalization in growing Xenopus embryo neurons. Vectorial traffic of endocytic membranes from the distal axon to the cell body was observed, suggesting bulk retrieval of plasma membrane at the growth cone. No long-range axonal transport of membrane material internalized at the cell body or along the axon was detected. In addition, we measured the rate of plasma membrane flow in Xenopus neurites. Axonal plasma membrane was found to flow anterogradely with the rate equal to approximately 30% of the rate of neurite elongation. Our results suggest that the "growth from the tip" pattern of neurite elongation may coexist with transport of new membrane components along plasmalemma by anterograde membrane flow.