Axonal transport of ribonucleoprotein particles (vaults)

RNA was previously shown to be transported into both dendritic and axonal c ompartments of nerve cells, presumably involving a ribonucleoprotein partic le. In order to reveal potential mechanisms of transport we investigated th e axonal transport of the major vault protein of the electric ray Torpedo m armorata. This protein is the major protein component of a ribonucleoprotei n particle (vault) carrying a non-translatable RNA and has a wide distribut ion in the animal kingdom. It is highly enriched in the cholinergic electro motor neurons and similar in size to synaptic vesicles. The axonal transpor t of vaults was investigated by immunofluorescence, using the anti-vault pr otein antibody as marker, and cytofluorimetric scanning, and was compared t o that of the synaptic vesicle membrane protein SV2 and of the beta-subunit of the F1-ATPase as a marker for mitochondria. Following a crush significa nt axonal accumulation of SV2 proximal to the crush could first be observed after 1 h, that of mitochondria after 3 h and that of vaults after 6 h, al though weekly fluorescent traces of accumulations of vault protein were obs erved in the confocal microscope as early as 3 h. Within the time-period in vestigated (up to 72 h) the accumulation of all markers increased continuou sly. Retrograde accumulations also occurred, and the immunofluorescence for the retrograde component, indicating recycling, was weaker than that for t he anterograde component, suggesting that more than half of the vaults are degraded within the nerve terminal. High resolution immunofluorescence reve aled a granular structure-in accordance with the biochemical characteristic s of vaults. Of interest was the observation that the increase of vault imm unoreactivity proximal to the crush accelerated with time after crushing, w hile that of SV2-containing particles appeared to decelerate, indicating th at the crush procedure with time may have induced perikaryal alterations in the production and subsequent export to the axon of synaptic vesicles and vault protein. Our data show that ribonucleoprotein-immunoreactive particle s can be actively transported within axons in situ from the soma to the ner ve terminal and back. The results suggest that the transport of vaults is d riven by fast axonal transport motors like the SV2-containing vesicles and mitochondria. Vaults exhibit an anterograde and a retrograde transport comp onent, similar to that observed for the vesicular organelles carrying SV2 a nd for mitochondria. Although the function of vaults is still unknown studi es of the axonal transport of this organelle may reveal insights into the m echanisms of cellular transport of ribonucleoprotein particles in general. (C) 1999 IBRO. Published by Elsevier Science Ltd.