Calcium-triggered exit of F-actin and IP₃ 3-kinase A from dendritic spines is rapid and reversible

The structure of the actin cytoskeleton in dendritic spines is thought to underlie some forms of synaptic plasticity. We have used fixed and live-cell imaging in rat primary hippocampal cultures to characterize the synaptic dynamics of the F-actin binding protein inositol trisphosphate 3-kinase A (IP3K), which is localized in the spines of pyramidal neurons derived from the CA1 region. IP3K was intensely concentrated as puncta in spine heads when Ca²⁺ influx was low, but rapidly and reversibly redistributed to a striated morphology in the main dendrite when Ca²⁺ influx was high. Glutamate stimulated the exit of IP3K from spines within 10 s, and re-entry following blockage of Ca²⁺ influx commenced within a minute; IP3K appeared to remain associated with F-actin throughout this process. Ca ²⁺-triggered F-actin relocalization occurred in about 90% of the cells expressing IP3K endogenously, and was modulated by the synaptic activity of the cultures, suggesting that it is a physiological process. F-actin relocalization was blocked by cytochalasins, jasplakinolide and by the over-expression of actin fused to green fluorescent protein. We also used deconvolution microscopy to visualize the relationship between F-actin and endoplasmic reticulum inside dendritic spines, revealing a delicate microorganization of IP3K near the Ca²⁺ stores. We conclude that Ca²⁺ influx into the spines of CA1 pyramidal neurons triggers the rapid and reversible retraction of F-actin from the dendritic spine head. This process contributes to changes in spine F-actin shape and content during synaptic activity, and might also regulate spine IP3 signals.