Cytoskeletal microdifferentiation: A mechanism for organizing morphological plasticity in dendrites

Experimental evidence suggests that microfilaments and microtubules play co ntrasting roles in regulating the balance between motility and stability in neuronal structures. Actin-containing microfilaments are associated with s tructural plasticity, both during development when their dynamic activity d rives the exploratory activity of growth cones and after circuit formation when the actin-rich dendritic spines of excitatory synapses retain a capaci ty for rapid changes in morphology. By contrast, microtubules predominate i n axonal and dendritic processes, which appear to be morphologically relati vely more stable. To compare the cytoplasmic distributions and dynamics of microfilaments and microtubules we made time-lapse recordings of actin or t he microtubule-associated protein 2 tagged with green fluorescent protein i n neurons growing in dispersed culture or in tissue slices from transgenic: mice. The results complement existing evidence indicating that the high co ncentrations of actin present in dendritic spines is a specialization for m orphological plasticity. By contrast, microtubule-associated protein 2 is l imited to the shafts of dendrites where time-lapse recordings show little e vidence for dynamic activity. A parallel exists between the partitioning of microfilaments and microtubules in motile and stable domains of growing pr ocesses during development and between dendrite shafts and spines at excita tory synapses in established neuronal circuits. These data thus suggest a m echanism, conserved through development and adulthood, in which the differe ntial dynamics of actin and microtubules determine the plasticity of neuron al structures.