Determine how dynamic microtubules regulate dendritic spine structure and function
The actin and microtubule (MT) cytoskeleton play important roles in dendritic spine structure and function. Actin filaments are essential in the formation, maintenance and plasticity of dendritic spine structure. Prominent in dendrite shafts, MTs function as railways for intracellular transport of organelles, lipids, proteins and mRNA. Until recently, the role of MTs in spine maintenance and plasticity was not studied because spines were thought to be devoid of MTs. However, we recently pioneered ways to study dynamic cytoskeletal rearrangements over hours in mature cortical and hippocampal cultures. In doing so we discovered that MTs in dendrites remain dynamic throughout neuronal development and are capable of rapidly extending into and out of dendritic spines in an activity-dependent fashion. Importantly, we discovered that MT invasion of spines is directly associated with long term potentiation (LTP), requiring calcium influx through NMDA receptors and subsequent actin polymerization in spines. Moreover, BDNF treatment increases the percent of spines targeted by MTs and the time they spend in spines. These findings suggest that dynamic MTs play an important role in spine structure and function. Indeed, many of the components that are either transported on MTs or are associated with their growing tips (+TIP proteins) are capable of entering spines directly on MTs (Fig. 4).
Figure 4. Three models for entry of cargo by microtubules (MTs) into spines. (A) Direct deposit model where material is directly trafficked into individual spines along MTs that have polymerized into the spine from the dendrite shaft. (B) Diffusion model where substances diffuse into spine. Shown is cytoplasmic diffusion but membrane-based diffusion also occurs for membrane associated proteins. (C) Hand off model where substances are trafficked along MTs in the dendrite shaft, but contain myosin motors, and are handed off to actin filaments for transport into spines. Figure from E.W. Dent 2017.