Neuronal dendritic spines comprise a set of spatially segregated functional subdomains, notably the synapse and the endocytic zone, each of which is strongly influenced by the spine actin cytoskeleton. During learning and behavior, rapid regulation of ongoing actin polymerization underlies diverse forms of synaptic plasticity, notably the control of receptor numbers in the synapse and the morphology of the spine itself.
We hypothesize that spine function is maintained by an internal, dynamic microarchitecture such that: (1) the spine cytoskeleton is organized to allow independent regulation of actin polymerization at functional subdomains; (2) plasticity of healthy synapses involves modification of the dynamic relationship between actin and the postsynaptic density; and (3) disease processes such as Aβ-induced synaptic hypofunction involve disruption of this micro-organization.
However, the submicron dimensions of synapses and spines have made it difficult to measure whether dynamic actin structures exist at the synapse itself, or whether polymerization is regulated at synapses independently from other points in the spine.
We have developed live-cell, super-resolution imaging methods based on photoactivated localization microscopy (PALM) to generate the nanometer spatial resolution necessary to test these hypotheses about the internal organization of living spines. By tracking single molecules of actin as they move within spines, we can now map the distribution of actin polymerization in the spine with respect to the functional landmarks that define spine organizational substructure. This approach will allow us to determine whether plasticity of healthy synapses involves modification of their internal microarchitecture, particularly the dynamic relationship between actin and the postsynaptic density.
Based on this highly resolved view of normal spine function, we will proceed to examine whether toxic species of misprocessed amyloid precursor protein, thought to underlie Alzheimer’s disease pathology, induce changes of spine internal organization that are unique or shared with other forms of plasticity.
These results will provide fundamental insight to synapse dysfunction in Alzheimer’s disease. By forging methods to track cytosolic protein position and movement in living neurons with unprecedented precision, they offer the hope of new assays of pathology in numerous psychiatric and neurological diseases.
