In response to stimulation, signaling molecules move through cells to newly available binding sites at a rate consistent with random diffusion. In round cells, such diffusible signals reach different regions of the membrane at nearly the same time. To find out what happens in cells with complex morphologies, such as neurons that have thick and thin branches and spines, Craske et al. employed confocal video microscopy of YFP-labeled protein kinase C (PKC) movement following glutamate stimulation.PKC moved rapidly to the plasma membrane of the soma and thick branches, but arrived at the membrane of thin branches and spines several seconds later. Moreover, PKC remained in the membrane of thin branches and spines well after it had retreated from the membrane around the soma.
Mathematical modeling indicated that diffusion could account for the pattern of PKC localization when cell shape was taken into account. The bulk of the cytosol, and thus the majority of PKC, is in the cell body before cell stimulation. Following stimulation, PKC movement was unimpeded in the soma and thick branches, but the larger surface area to volume ratio of thin branches meant that there was not enough PKC close by to fill the binding sites in the membrane. Thus, PKC continued to drift in from other parts of the cell after binding in the membrane of the soma and thick branches was complete. In the case of the spines, modeling showed that the narrow necks constrained PKC movement and slowed accumulation and dispersal.
The differential rate of accumulation of PKC means that some regions of the cell could remain semi-active for five to ten seconds after signaling was complete in the soma. These cell regions would retain a memory of recent signaling events and would be primed for subsequent ones. The team expects such signaling memory will be found in other membrane-targeted proteins.