Calcium, protease activation, and cytoskeleton remodeling underlie growth cone formation and neuronal regeneration

The cytoarchitecture, synaptic connectivity, and physiological properties of neurons are determined during their development by the interactions between the intrinsic properties of the neurons and signals provided by the microenvironment through which they grow. Many of these interactions are mediated and translated to specific growth patterns and connectivity by specialized compartments at the tips of the extending neurites: the growth cones (GCs). The mechanisms underlying GC formation at a specific time and location during development, regeneration, and some forms of learning processes, are therefore the subject of intense investigation. Using cultured Aplysia neurons we studied the cellular mechanisms that lead to the transformation of a differentiated axonal segment into a motile GC. We found that localized and transient elevation of the free intracellular calcium concentration ([Ca²⁺]_i) to 200-300 μM induces GC formation in the form of a large lamellipodium that branches up into growing neurites. By using simultaneous on-line imaging of [Ca²⁺]_i and of intraaxonal proteolytic activity, we found that the elevated [Ca²⁺]_i activate proteases in the region in which a GC is formed. Inhibition of the calcium-activated proteases prior to the local elevation of the [Ca²⁺]_i blocks the formation of GCs. Using retrospective immunofluorescent methods we imaged the proteolysis of the submembrane spectrin network, and the restructuring of the cytoskeleton at the site of GC formation. The restructuring of the actin and microtubule network leads to local accumulation of transported vesicles, which then fuse with the plasma membrane in support of the GC expansion.