SORTING OF BETA-ACTIN MESSENGER-RNA AND PROTEIN TO NEURITES AND GROWTH CONES IN CULTURE

The transport of mRNAs into, developing dendrites and axons may be a b asic mechanism to localize cytoskeletal proteins to growth cones and i nfluence microfilament organization. Using isoform-specific antibodies and probes for in situ hybridization, we observed distinct localizati on patterns for beta- and gamma-actin within cultured cerebrocortical neurons. beta-Actin protein was highly enriched within growth cones an d filopodia, in contrast to gamma-actin protein, which was distributed uniformly throughout the cell. beta-Actin protein also was shown to b e peripherally localized after transfection of beta-actin cDNA bearing an epitope tag. beta-Actin mRNAs were localized more frequently to ne uronal processes and growth cones, unlike gamma-actin mRNAs, which wer e restricted to the cell body, The rapid localization of beta-actin mR NA, but not gamma-actin mRNA, into processes and growth cones could be induced by dibutyryl cAMP treatment. Using high-resolution in situ hy bridization and image-processing methods, we showed that the distribut ion of beta-actin mRNA within growth cones was statistically nonrandom and demonstrated an association with microtubules. beta-Actin mRNAs w ere detected within minor neurites, axonal processes, and growth cones in the form of spatially distinct granules that colocalized with tran slational components. Ultrastructural analysis revealed polyribosomes within growth cones that colocalized with cytoskeletal filaments. The transport of beta-actin mRNA into developing neurites may be a sequenc e-specific mechanism to synthesize cytoskeletal proteins directly with in processes and growth cones and would provide an additional means to deliver cytoskeletal proteins over long distances.