TRANSPORT AND LOCALIZATION OF EXOGENOUS MYELIN BASIC-PROTEIN MESSENGER-RNA MICROINJECTED INTO OLIGODENDROCYTES

We have studied transport and localization of MBP mRNA in oligodendroc ytes in culture by microinjecting labeled mRNA into living cells and a nalyzing the intracellular distribution of the injected RNA by confoca l microscopy. Injected mRNA initially appears dispersed in the perikar yon. Within minutes, the RNA forms granules which, in the case of MBP mRNA, are transported down the processes to the periphery of the cell where the distribution again becomes dispersed. In situ hybridization shows that endogenous MBP mRNA in oligodendrocytes also appears as gra nules in the perikaryon and processes and dispersed in the peripheral membranes. The granules are not released by extraction with non-ionic detergent, indicating that they are associated with the cytoskeletal m atrix. Three dimensional visualization indicates that MBP mRNA granule s are often aligned in tracks along microtubules traversing the cytopl asm and processes. Several distinct patterns of granule movement are o bserved. Granules in the processes undergo sustained directional movem ent with a velocity of approximately 0.2 mum/s. Granules at branch poi nts undergo oscillatory motion with a mean displacement of 0.1 mum/s. Granules in the periphery of the cell circulate randomly with a mean d isplacement of approximately 1 mum/s. The results are discussed in ter ms of a multi-step pathway for transport and localization of MBP mRNA in oligodendrocytes. This work represents the first characterization o f intracellular movement of mRNA in living cells, and the first descri ption of the role of RNA granules in transport and localization of mRN A in cells.