NEUROFILAMENT PHOSPHORYLATION AND AXON DIAMETER IN THE SQUID GIANT FIBER SYSTEM

Electron energy loss spectroscopic analysis of squid giant axons in a phosphorus energy window yielded bright signals, which were shown to o riginate from highly phosphorylated neurofilaments. The frequency and distribution of these signals were analysed at defined intervals in cr oss-sections of the giant axon, starting from its origin in the stella te ganglion and extending distally along the stellar nerve. The analys is revealed a proximodistal gradient of increasing neurofilament phosp horylation. Within the stellate ganglion and for some distance beyond, the increase in frequency of signals correlated with the widening of the neurofilament meshwork and the radial growth of the axon. This agr ees with the hypothesis that neurofilament phosphorylation regulates a xon calibre by affecting interfilament spacing. In distal axon domains where the axon diameter diminished, contrary to expectations, the spa cing of signals increased and the signals were significantly larger. H yperphosphorylation apparently compensated for a diminishing supply of neurofilament protein. Contrary to predictions, the presynaptic termi nal of the giant synapse contained a distinct and highly phosphorylate d neurofilament meshwork. We conclude that the growth of the axon diam eter is a function of neurofilament phosphorylation, interfilament spa cing and neurofilament density. A mature and highly phosphorylated neu rofilament cytoskeleton completely filled the presynaptic terminal of the giant synapse. (C) 1998 IBRO. Published by Elsevier Science Ltd.