Role of densin and α-actinin in regulating CaMKII
The ability to learn, memorize and recall information is a prerequisite for the full and comprehensive life of any individual. Understanding molecular mechanisms underlying memory is fundamental to neuroscience research. Excitatory glutamatergic synapses are the sites of changes in the strength of neuronal connectivity that are believed to underlie memory. Synaptic transmission is often modulated by calcium signals through N-methyl D-Aspartate-type glutamate receptors (NMDARs) and/or voltage-gated Ca2+ channels. These signals are detected by calmodulin (CaM), which regulates effector proteins such as calcium-calmodulin-dependent protein kinase II (CaMKII). Activation of CaMKII elicits multiple responses at excitatory synapses. Neuronal postsynaptic densities contain several CaMKII-associated proteins, such as NMDAR GluN2B subunits, α-actinin, and densin, that may collaborate to dynamically regulate CaMKII targeting and/or activity during synaptic activation. The CaMKII holoenzyme can simultaneously interact with GluN2B, densin and α-actinin in vitro and the complex can be isolated from mouse brains. My dissertation has explored the roles of α-actinin and densin in modulating the localization and/or activity of CaMKII. I showed that a novel domain in densin is a potent CaMKII inhibitor when using GluA1 glutamate receptor subunits as the substrate, but is ineffective when using GluN2B as the substrate, both in vitro and in cells. I also found that α-actinin mimics CaM in binding to the CaMKII regulatory domain and targets CaMKIIα to F-actin in cells. The interaction with α-actinin and CaM is differentially regulated by CaMKII autophosphorylation in the regulatory domain. In vitro, α-actinin inhibits Ca2+/CaM-dependent phosphorylation of certain substrates by CaMKII, but can activate phosphorylation of certain substrates. Similarly, in intact cells, α-actinin inhibits CaMKII phosphorylation of glutamate receptor GluA1 subunits, but stabilizes CaMKII association with GluN2B-containing NMDARs and enhances phosphorylation of GluN2B. Taken together these data show that α-actinin and densin can collaborate to inhibit the phosphorylation of GluA1-containing AMPARs, but precisely target CaMKII to phosphorylate GluN2B-containing NMDARs. We postulate that these interactions fine-tune the downstream actions of CaMKII in response to synaptic activity.