The role of the cytoskeleton in dendritic development

We are interested in understanding how "the dynamic skeleton" of the neuron contributes to dendritic development and plasticity. Microtubules are cytoskeletal polymers found in cells that give them their shape, help to make them motile, and form the railroad tracks for intracellular transport of materials. In a report they have published in the open-access journal Neural Development, Withers' team observed microtubule polymerization in living neurons as they were developing, using a fluorescently-tagged protein called EB-1 that sits at the tip of a microtubule only when it is actively polymerizing. title image When they compared the microtubules in dendrites, the receiving part of the neuron, with those in the axon, the part of the neuron that transmits signals, they found that the microtubules in the dendrites were more dynamic than those in the axon. These differences could help to explain not only the different patterns of growth between axons and dendrites. Higher levels of dynamic microtubules could also contribute to the capacity of dendrites to grow and add branches throughout life. This capacity for remodeling neuron structure is thought to be important for learning, memory, and our ability to change our behavior in response to our surrounding environment. This image shows a neuron growing in culture for 4 days, and stained to show the distribution of stable (red) and dynamic (green) microtubules. The axon (arrow) has a high concentration of stable microtubules, whereas the dendrites (all other extensions from the cell body) contain high levels of dynamic (i.e. actively polymerizing) microtubules.

From Kollins, K.M., Bell, R.L.*, Butts, M.*, Withers, G.S. 2009. Dendrites differ from axons in patterns of microtubule stability and polymerization during development. Neural Development 4(1):26 (epub ahead of print, 14 July 4:26, doi:10.1186/1749-8104-4-26). (* = undergraduate co-author).

A role for dynamic actin in the development and maintenance of the dendritic arbor Dieter D. Brandner*, Gabriella R. Sterne*, Ginger S. Withers, presented at the International Brain Research Organization Meeting, Florence, IT, July, 2011 Top image, typical actin (red) and microtubule (green) organization after 3 days in vitro; bottom, development after interfering with actin polymerization with the drug Latrunculin. title image Axon formation appears to require local shifts in the polymerization state of actin, but the extent to which actin instability contributes to the relatively protracted development of the dendritic phenotype is less well understood. To test this, we treated cultured hippocampal neurons with drugs that interfere with actin stability at two key stages of neuronal development: at 1 day in vitro (DIV) as polarity is being established (i.e. axon forms) and at 3 DIV, a major phase of dendritic morphogenesis that follows axon formation. Both morphological and molecular benchmarks of dendritic maturation were examined 48 hours later. As shown previously, neurons treated at 1DIV with the actin depolymerizing agents cytochalasin D (cytoD), or latrunculin A (latA), developed multiple long axon-like processes. The number of immature dendrites also increased significantly, but the mean length of the dendrites was not affected. When actin was destabilized at 3 DIV, there was no change in the number of primary dendrites, but there was a modest increase in dendrite length. Treatment with jasplakinolide, a drug that stabilizes actin, resulted in a significant retraction and net loss of primary dendrites regardless of when it was administered. To index molecular polarization of axon and dendrite markers, we measured the intensity of MAP2 (which is localized to dendrites) and tau-1 (which is concentrated in the axon), comparing levels of fluorescence in the dendrites and axons of each cell. By 5DIV, untreated control cells had the highest polarity index for both tau-1 and MAP2, and drug treatments interfering with actin stability resulted in reduced molecular segregation. Thus, in dendritic development, destabilization early in the development of neuron polarity results in supernumerary dendrites. Destabilization later appears to facilitate outgrowth, but does not lead to additional primary dendrites. It may, however, interfere with the maturation of molecular phenotype.