SUMMARY The structural modification of dendritic spines plays a critical role in synaptic plasticity. CaMKII is a pivotal molecule involved in this process through both kinase-dependent and independent structural functions, but the respective contributions of these two functions to the synaptic plasticity remain unclear. We demonstrate that the transient interplay between the kinase and structural functions of CaMKII during the induction of synaptic plasticity temporally gates the activity-dependent modification of the actin cytoskeleton. Inactive CaMKII binds F-actin, thereby limiting access of actin regulating proteins to F-actin and stabilizing spine structure. CaMKII-activating stimuli trigger dissociation of CaMKII from F-actin through specific autophosphorylation reactions within the F-actin binding region and permits F-actin remodeling by regulatory proteins followed by reassociation and restabilization. Blocking the autophosphorylation impairs both functional and structural plasticity without affecting kinase activity. These results underpin the importance of the interplay between the kinase and structural functions of CaMKII in defining a time window permissive for synaptic plasticity.

Calcium/calmodulin-dependent protein kinase II (CaMKII) is essential for synaptic plasticity underlying memory formation. Some functions of CaMKII are mediated by interactions with synaptic proteins, and activity-triggered translocation of CaMKII to synapses has been heavily studied. However, CaMKII actions away from the postsynaptic density (PSD) remain poorly understood, in part because of the difficulty in discerning where CaMKII binds in live cells. We used photoactivated localization microscopy (PALM) in rat hippocampal neurons to track single molecules of CaMKII␣, mapping its spatial and kinetic heterogeneity at high resolution. We found that CaMKII␣ exhibits at least three kinetic subpopulations, even within individual spines. Latrunculin application or coexpression of CaMKII␤ carrying its actin-binding domain strongly modulated CaMKII diffusion, indicating that a major subpopulation is regulated by the actin cytoskeleton. CaMKII in spines was typically more slowly mobile than in dendrites, consistent with presence of a higher density of binding partners or obstacles. Importantly, NMDA receptor stimulation that triggered CaMKII activation prompted the immobilization and presumed binding of CaMKII in spines not only at PSDs but also at other points up to several hundred nanometers away, suggesting that activated kinase does not target only the PSD. Consistent with this, single endogenous activated CaMKII molecules detected via STORM immunocytochemistry were concentrated in spines both at the PSD and at points quite distant from the synapse. Together, these results indicate that CaMKII mobility within spines is determined by association with multiple interacting proteins, even outside the PSD, suggesting diverse mechanisms by which CaMKII may regulate synaptic transmission.

In the original publication, we stated that ''Lolicato et al. concluded that the TM4 conformational change of TRAAK hinges on the conserved glycine (G124) that resides on the pore helix (P1) that contacts the top of TM4 (Lolicato et al., 2014).'' We incorrectly identified Glycine (G124) as the hinge site for movement of TM4. This residue (G124) is the site of a gain-of-function mutation. The conserved Glycine residue in TM4, G268, is the hinge site for movement of the lower portion of TM4. We also misspelled ''Shear Stress'' in Figure 2A as ''Sheer Stress.'' This has now been corrected in the article online.

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In neurons, secretory organelles within the cell body are complemented by the dendritic endoplasmic reticulum (ER) and Golgi outposts (GOPs), whose role in neurotransmitter receptor trafficking is poorly understood. c-aminobutyric acid (GABA) type B metabotropic receptors (GABA B Rs) regulate the efficacy of synaptic transmission throughout the brain. Their plasma membrane availability is controlled by mechanisms involving an ER retention motif and assembly-dependent ER export. Thus, they constitute an ideal molecular model to study ER trafficking, but the extent to which the dendritic ER participates in GABA B R biosynthesis has not been thoroughly explored. Here, we show that GABA B1 localizes preferentially to the ER in dendrites and moves long distances within this compartment. Not only diffusion but also microtubule and dyneindependent mechanisms control dendritic ER transport. GABA B Rs insert throughout the somatodendritic plasma membrane but dendritic post-ER carriers containing GABA B Rs do not fuse selectively with GOPs. This study furthers our understanding of the spatial selectivity of neurotransmitter receptors for dendritic organelles.