LOCALIZATION AND ACTIVE-TRANSPORT OF MESSENGER-RNA IN AXONS OF SYMPATHETIC NEURONS IN CULTURE

In neurons, the establishment and maintenance of distinct somatic, den dritic, and axonal domains has long been known to rely on regulated tr affic of organelles and proteins. More recently, the local targeting o f specific mRNAs has also been demonstrated, at least in dendrites, to provide a local supply of specific proteins. Here we set out to test directly for the presence of mRNA in axons of cultured chick sympathet ic neurons, to examine their distribution during axonal outgrowth, to determine the reliance of this distribution on specific cytoskeletal e lements, and to assess whether the axonal and somatic mRNA complements differ. Using fluorescent in situ hybridization, we found that sympat hetic axons do contain poly(A(+)) mRNA along their length in a pattern that changes gradually as axons elongate, from an evenly dispersed pu nctate distribution with strong growth cone staining to a distribution focused at branch points, varicosities, and some growth cones. Select ive perturbations of the cytoskeleton revealed that the presence of ax onal mRNA was dependent on microtubules (MTs), but not actin filaments , indicating that mRNA transport and/or anchoring within the axon are active processes involving MTs. Finally, reverse transcription-PCR amp lification of RNAs from the axonal and somatic compartments showed tha t p-actin mRNA was present in both compartments, whereas mRNA encoding a-tubulin was restricted to the somatic compartment and entirely abse nt from the axons. Thus, the mRNA populations in the soma versus the a xon are both quantitatively and qualitatively different, and these neu rons are able to direct specific mRNAs to the axon.