TRANSLOCATION OF RNA GRANULES IN LIVING NEURONS

Sorting of RNAs to specific subcellular loci occurs in diverse setting s from fly oocytes to mammalian neurons. Using the membrane-permeable nucleic acid stain SYTO 14, we directly visualized the translocation o f endogenous RNA in living cells. Labeled RNA was distributed nonrando mly as discrete granules in neuronal processes, The labeled granules c olocalized with poly(A(+)) mRNA, with the 60S ribosomal subunit, and w ith elongation factor 1 alpha, suggesting that granules represent a tr anslational unit, A subset of labeled granules colocalized with beta-a ctin mRNA. Correlative light and electron microscopy indicated that th e fluorescent granules corresponded to clusters of ribosomes at the ul trastructural level. Poststaining of sections with heavy metals confir med the presence of ribosomes within these granules. In living neurons , a subpopulation of RNA granules was motile during the observation pe riod. They moved at an average rate of 0.1 mu m/sec. In young cultures their movements were exclusively anterograde, but after 7 d in cultur e, one-half of the motile granules moved in the retrograde direction. Granules in neurites were delocalized after treatment with microtubule -disrupting drugs. These results raise the possibility of a cellular t rafficking system for the targeting of RNA in neurons.