Modification of axonal branching and transport by a low molecular-weight GTP-binding protein involved in associative conditioning.

摘要

Associative conditioning of the clinging response of the marine snail Hermissenda crassicornis causes a long-term transfer of the (unconditioned) response to rotation, to the (conditioned) response to light. A simple, well defined neuronal network mediates the acquisition and long-term storage of the conditioned response. A crucial storage site, the type B photoreceptor, undergoes a long-term increase of excitability through a protein kinase C mediated phosphorylation of K{dollar}sp+{dollar} channels. Structural changes in the terminal axonal branches of the photoreceptor have also been described. Recently, a monomeric G-protein, cp20, a member of the Ras superfamily of G-proteins, has been purified from the Hermissenda nervous system, and shown to be phosphorylated after associative conditioning. Since cp20 also is a powerful regulator of the same K{dollar}sp+{dollar} channels that encode associative memory, the present study explores the possibility that structural changes similar to the ones caused by associative conditioning may be caused through regulation of axonal transport by cp20. This work also explores the functional similarity of cp20 with the ras oncogene products in the induction of the structural changes.; Microinjection of the snail photoreceptor's axons with cp20 caused a reduction of about 50% of the volume enclosed by the axonal branches when compared with that of control neurons. The changes were observed in Lucifer yellow stained photoreceptors with confocal microscopy. This change resembles that observed four days after associative conditioning. The ras gene product c-Ras, and the oncogenic mutant v-Ras caused very similar changes in photoreceptor branch morphology. Movements of medium and large sized membranous organelles were visualized within segments of crab (Pachygrapsus crassipes) walking leg nerve. Perfusion of axons with cp20, but not control solutions, reduced the number of organelles moving in the retrograde direction per time unit, but not their velocity.; Results of this study suggest that cp20 is a regulatory factor in the endocytic pathway of the snail photoreceptor, probably at the late endosome stage; hence it is able to cause structural changes. cp20 and Ras proteins may, therefore, play a role in regulating long-term changes of neuronal function and cytoarchitecture that encode associative memory in a variety of species, including Hermissenda.