Neurofilament proteins are known to be transported along axons by slow axonal transport, but the form in which they move is controversial. In previous studies on cultured rat sympathetic neurons, we found that green fluorescent protein-tagged neurofilament proteins move predominantly in the form of filamentous structures, and we proposed that these structures are single neurofilament polymers. In the present study, we have tested this hypothesis by using a rapid perfusion technique to capture these structures as they move through naturally occurring gaps in the axonal neurofilament array. Because the gaps lack neurofilaments, they permit unambiguous identification of the captured structure. Using quantitative immunofluorescence microscopy and correlative light and electron microscopy, we show that the captured structures are single continuous neurofilament polymers. Thus, neurofilament polymers are one of the cargo structures of slow axonal transport.

Studies on the axonal transport of neurofilament proteins in cultured neurons have shown they move at fast rates, but their overall rate of movement is slow because they spend most of their time not moving. Using correlative light and electron microscopy, we have shown that these proteins move in the form of assembled neurofilament polymers. However, the polypeptide composition of these moving polymers is not known. To address this, we visualized neurofilaments in cultured neonatal mouse sympathetic neurons using GFP-tagged neurofilament protein M and performed time-lapse fluorescence microscopy of naturally occurring gaps in the axonal neurofilament array. When neurofilaments entered the gaps, we stopped them in their tracks using a rapid perfusion and permeabilization technique and then processed them for immunofluorescence microscopy. To compare moving neurofilaments to the total neurofilament population, most of which are stationary at any point in time, we also performed immunofluorescence microscopy on neurofilaments in detergent-splayed axonal cytoskeletons. All neurofilaments, both moving and stationary, contained NFL, NFM, peripherin and α-internexin along >85% of their length. NFH was absent due to low expression levels in these neonatal neurons. These data indicate that peripherin and α-internexin are integral and abundant components of neurofilament polymers in these neurons and that both moving and stationary neurofilaments in these neurons are complex heteropolymers of at least four different neuronal intermediate filament proteins.

Disrupted microtubule dynamics in neuronal synapses has been suggested as an underlying cause for several devastating neurological diseases, including Hereditary Spastic Paraplegia (HSP) and Fragile X Syndrome (FXS). However, previous studies have been restricted to indirect assays of synaptic microtubules, i.e. immunocytochemistry of microtubule-associated proteins and posttranslationally modified tubulins characteristic of microtubules with different stabilities. Very little is known about synaptic microtubule dynamics in vivo, or how microtubule dynamics may be disrupted in disease states. In this study, we develop methods to analyze microtubule dynamics directly in living synaptic boutons in situ. We use fluorescence recovery after photobleaching (FRAP) of transgenic green fluorescent protein (GFP) tagged tubulin at the well-characterized Drosophila neuromuscular junction (NMJ) synapse. FRAP measurements of tubulin-GFP demonstrate biphasic recovery kinetics. Treatment with taxol to stabilize microtubules and promote microtubule assembly reduces both recovery phases. Treatment with vinblastine to disassemble microtubules increases the fast recovery phase and decreases the slow recovery phase. These data indicate that the fast recovery phase is generated by rapid diffusion of tubulin subunits and the slow phase is generated by the relatively slow turnover of microtubules. This study demonstrates that tubulin-GFP fluorescence recovery after photobleaching can be used to assay microtubule dynamics directly in living synapses.