Dynamics of bidirectional transport of Arc mRNA in neuronal dendrites

The mRNA for Arc (activity-regulated cytoskeletal protein) is delivered into dendrites and localizes selectively at active synapses. Here we use a green fluorescent protein-based labeling system and confocal microscopy to define the transport kinetics of exogenously expressed mRNA from chimaeric Arc constructs (Arc/MS2 mRNA) in the dendrites of living rat neurons in culture. Arc/MS2 mRNA assembles into particles that move independently, bidirectionally, and intermittently in a fashion indicative of transport. Transport velocities range from below 6 to 65 mu m/minute, which is consistent with actin-based and microtubule-based transport, respectively. In general, orthograde translocations are longer than retrograde translocations. Rapidly translocating Arc/MS2 mRNA particles sometimes reverse direction and decrease velocity just before stopping, suggesting that local signals regulate Arc mRNA targeting movements. These observations identify several phases of Arc mRNA movement that serve as potential points for regulating Arc mRNA localization.