Nerve growth occurs by activity of the growth cone, coordinated with addition of new membrane material to the growing axon. Although the dynamics of the neuronal cytoskeleton has been a subject of intensive investigation in the last years, very little is known about the mechanism of membrane addition to the growing axon. It is generally believed that new membrane necessary for nerve growth is delivered along axonal microtubules to the tip of the growing axon where it is inserted into the plasmalemma. Based on our preliminary studies we suggest that at the initial stages of neuronal process formation, axonal growth occurs by a fundamentally different mechanism. Namely, new membrane material is added not at the growth cone, but along the axon and/or at the soma, resulting in the generation of the forward flow of the plasma membrane components. We suggest that at this initial stage of axonal development both plasma membrane flow and recently described microtubule translocation are important components of axonal growth. We hypothesize that at the later stages of axonal growth new membrane material is preferentially inserted into the growth cone region leading to termination of plasma membrane flow. To test this hypothesis, we propose to conduct a detailed study of phenomena of plasma membrane flow and microtubule translocation in growing nerve processes. Experiments will be carried out in neuronal cell cultures. A major component of the studies will rely on imaging approaches to the detection of plasma membrane flow. Particular emphasis will be placed on detecting sites of membrane insertion and internalization in growing axons and on examining the relationship between plasma membrane flow and vectorial translocation of axonal microtubules. Photobleaching and photoactivation techniques will be used to characterize the phenomenon of vectorial microtubule translocation during axonal growth. By inhibiting plasma membrane or microtubule flow we will examine how one of them is influenced by the other and how this inhibition affects neurite growth. This work will lead to a better understanding of the fundamental mechanisms of axonal growth. Information about the mechanisms of axonal growth is a necessary prerequisite to understanding the neuronal development and nerve regeneration. This, in turn, should permit the rational development of better therapeutic regiments for treatment of nervous system injuries.
