CaMKII: claiming center stage in postsynaptic function and organization

Abstract

While CaMKII has long been known to be essential for synaptic plasticity and learning, recent work points to new dimensions of CaMKII function in the nervous system, revealing that CaMKII also plays an important role in synaptic organization. Ca(2+)-triggered autophosphorylation of CaMKII not only provides molecular memory by prolonging CaMKII activity during long-term plasticity (LTP) and learning but also represents a mechanism for autoactivation of CaMKII's multifaceted protein-docking functions. New details are also emerging about the distinct roles of CaMKIIα and CaMKIIβ in synaptic homeostasis, further illustrating the multilayered and complex nature of CaMKII's involvement in synaptic regulation. Here, I review novel molecular and functional insight into how CaMKII supports synaptic function.

Figure 1. CaMKII Structure

(A) Linear depiction of one CaMKII subunit. It shows kinase domain (blue; numbering according to mouse CaMKIIα), autoinhibitory segment (yellow; formed by R1, which includes T286, R2, which binds to S site, and R3 which includes most of the CaM binding site; red), linker region (green) and association domain (grey). (B) Autoinhibition and autophosphorylation of CaMKII subunits. The diagram illustrates schematically two neighboring subunits with the inhibitory segment in black. The T-site (grey half moon) accommodates T286 under resting conditions, fostering the interaction of the pseudosubstrate region immediately downstream of T286 with the catalytic site (S site; grey surface on right side of each subunit). The S-site is formed by the cleft between N and C domains and is in close proximity to the T site. Upon binding of Ca²⁺/CaM to the region defined by T305/306, the inhibitory segments are displaced from the S and T sites (red dashed lines). If two neighboring subunits simultaneously bind Ca²⁺/CaM, T286 from one subunit can reach the catalytic site of the other (red arrow) and becomes phosphorylated. (C–E) CaMKII dodecamer. Each model depicts the two stacked hexameric rings. (C) Schematic of a structural model of CaMKII dodecamer. According to the model, the CaMKII dodecamer can exist in three main conformations: a) a closed inhibited/inactive conformation with the linker folded into the association domain, rendering it inaccessible for Ca²⁺/CaM binding and activation; b) an open inhibited/inactive conformation with the linker extended outward; c) a fully extended active conformation with Ca²⁺/CaM bound to the regulatory segment. The kidney shaped segments represent the catalytic domains. They consist of a smaller globular N and a larger globular C domain. The catalytic cleft is nested in the cleft between the two domains. Adopted with permission from (Stratton et al., 2013). (D,E) Space filling atomic model of the crystal structure of the CaMKII dodecamer in the closed, inhibited conformation. Shown are views from side (D) and top (E). The individual catalytic domains of each subunit are alternating light and dark blue for clarity. The association domains, which form the central hub, are grey. The positions of the linkers are depicted by red spheres. Adopted with permission from (Chao et al., 2011).

Figure 2. Domains that Mediate Interactions between CaMKII,α-actinin, F-actin, Densin, and NMDARs

(A) Linear structures of CaMKII and its most prevalent and functionally important binding proteins in spines. The CaMKII diagram (red) shows the N domain (aa 1–96; numbering according to mammalian CaMKIIα), C domain (aa 97–274), catalytic site nested between N and C domains (includes substrate binding site, S site), autoinhibitory segment (aa 275–340) consisting of R1 (aa 275–291; contains T286 for interact with the T site under resting conditions), R2 (aa 291–297; binds to the S site under resting conditions), and R3 (aa 297–314; includes the Ca²⁺/CaM binding site, which overlaps with R2; dark red oval; T305/T306 inhibit Ca²⁺/CaM binding if phosphorylated), linker segment L (aa 314–340), and association domain (aa 341–478). α-Actinin (blue) consists of two calponin homology domains (CH1: aa 1–132; CH2: aa 148–249; numbering according to mammalian α-Actinin-1), four spectrin repeat domains (SR1: aa 269–384; SR2: aa 395–499; SR3: aa 509–600; SR4: 610–738) and four EF hands (aa 745–894). The linker between CH2 and SR1 (aa 250–268) is an important attachment site for the EF hands in the antiparallel dimer (second protomer is not depicted). Densin (green) is formed by a leucine-rich repeat domain (LRR; consists of 16 leucine-rich repeats; aa 1–420), a central domain of less certain structural identity (aa 421–1404) and a C-terminal PDZ domain (aa 1405–1492). The NMDAR GluN2B subunit (orange) consists of an extracellular N-terminal domain (NTD) and glutamate binding domain (GBD), which is formed by the N-terminus and the extracellular loop between transmembrane segments 2 and 3, three transmembrane segments, which form, together with a membrane-reentry loop, the pore, and an intracellular C-terminus (aa 838–1482). (B) Depiction of CaMKII interaction sites. The linker of CaMKIIβ (horizontal stripes), which is of variable length and different from the linker of CaMKIIα, is important for binding to F-actin (yellow beaded double string crossing underneath the linker). The exact binding site on CaMKIIβ for F-actin is unclear. F-actin also binds to CH1 of α-actinin (crossing underneath CH1). Other connections of CaMKII are depicted by red arrows. EF3 and EF4 hands of α-actinin (aa 819–194; horizontal stripes) and Ca²⁺/CaM compete for binding to R2/R3 on CaMKII (dark red oval). The association domain of CaMKII (aa 341–478) binds to aa 1335–1382 of densin (horizontal stripes) independent of CaMKII activation. The T site of CaMKII binds to aa 1290–1309 of GluN2B (including S1303, which is involved in CaMKII binding and phosphorylated by CaMKII) and aa 793–824 of densin (horizontal stripes) upon addition of Ca²⁺/CaM or phosphorylation of T286. α-actinin binds with its C-terminal ESDL motif to the PDZ domain of densin and with its C-terminal portion of SR4 to the C-terminal portion of GluN2B (blue arrows).

Figure 2. Domains that Mediate Interactions between CaMKII,α-actinin, F-actin, Densin, and NMDARs

(A) Linear structures of CaMKII and its most prevalent and functionally important binding proteins in spines. The CaMKII diagram (red) shows the N domain (aa 1–96; numbering according to mammalian CaMKIIα), C domain (aa 97–274), catalytic site nested between N and C domains (includes substrate binding site, S site), autoinhibitory segment (aa 275–340) consisting of R1 (aa 275–291; contains T286 for interact with the T site under resting conditions), R2 (aa 291–297; binds to the S site under resting conditions), and R3 (aa 297–314; includes the Ca²⁺/CaM binding site, which overlaps with R2; dark red oval; T305/T306 inhibit Ca²⁺/CaM binding if phosphorylated), linker segment L (aa 314–340), and association domain (aa 341–478). α-Actinin (blue) consists of two calponin homology domains (CH1: aa 1–132; CH2: aa 148–249; numbering according to mammalian α-Actinin-1), four spectrin repeat domains (SR1: aa 269–384; SR2: aa 395–499; SR3: aa 509–600; SR4: 610–738) and four EF hands (aa 745–894). The linker between CH2 and SR1 (aa 250–268) is an important attachment site for the EF hands in the antiparallel dimer (second protomer is not depicted). Densin (green) is formed by a leucine-rich repeat domain (LRR; consists of 16 leucine-rich repeats; aa 1–420), a central domain of less certain structural identity (aa 421–1404) and a C-terminal PDZ domain (aa 1405–1492). The NMDAR GluN2B subunit (orange) consists of an extracellular N-terminal domain (NTD) and glutamate binding domain (GBD), which is formed by the N-terminus and the extracellular loop between transmembrane segments 2 and 3, three transmembrane segments, which form, together with a membrane-reentry loop, the pore, and an intracellular C-terminus (aa 838–1482). (B) Depiction of CaMKII interaction sites. The linker of CaMKIIβ (horizontal stripes), which is of variable length and different from the linker of CaMKIIα, is important for binding to F-actin (yellow beaded double string crossing underneath the linker). The exact binding site on CaMKIIβ for F-actin is unclear. F-actin also binds to CH1 of α-actinin (crossing underneath CH1). Other connections of CaMKII are depicted by red arrows. EF3 and EF4 hands of α-actinin (aa 819–194; horizontal stripes) and Ca²⁺/CaM compete for binding to R2/R3 on CaMKII (dark red oval). The association domain of CaMKII (aa 341–478) binds to aa 1335–1382 of densin (horizontal stripes) independent of CaMKII activation. The T site of CaMKII binds to aa 1290–1309 of GluN2B (including S1303, which is involved in CaMKII binding and phosphorylated by CaMKII) and aa 793–824 of densin (horizontal stripes) upon addition of Ca²⁺/CaM or phosphorylation of T286. α-actinin binds with its C-terminal ESDL motif to the PDZ domain of densin and with its C-terminal portion of SR4 to the C-terminal portion of GluN2B (blue arrows).

Figure 3. Interactions of CaMKII with F-actin, α -actinin, and NMDARs in Spines

Under basal conditions CaMKII (pink) is mostly associated with F-actin (black lines). This interaction might also localize CaMKII to an area 50–100 nm underneath the center of the PSD with high CaMKII concentrations also present at the lateral edges of the PSD (Ding et al., 2013). The figure envisions that this association with F-actin occurs in conjunction with α-actinin (blue) as CaMKIIβ subunits within the dodecameric CaMKII complexes as well as α-actinin directly bind to F-actin and to each other. The resulting ‘triades’ (magnified area) are predicted to foster a highly branched F-actin cytoskeleton rather than the parallel fiber arrangement induced by α-actinin alone in the absence of CaMKII. Ca²⁺ influx via NMDARs, which consist of GluN1 (yellow) and GluN2 subunits (orange), triggers release of CaMKII from F-actin as Ca²⁺/CaM will displace CaMKIIβ from F-actin (red arrow in insert) and CaMKIIα and β from α-actinin (blue arrow in insert). CaMKII will then bind to the NMDAR subunit GluN2B (top left area of PSD), which requires either Ca²⁺/CaM or the more lasting T286/287 autophosphorylation. After removal of Ca²⁺/CaM, α-actinin will re-associate with CaMKII possibly forming a trimeric complex with GluN2B (top middle area; right area depicts an α-actinin – NMDAR complex without CaMKII).

Figure 4. Binding of α -actinin to F-actin Makes EF3 and EF4 Accessible for CaMKII Binding

(A) Native structure of α-actinin dimer (PDB ID: 1SJJ) with both actin binding domains (CH1, CH2) in a closed conformation (insert on top of the full length α-actinin structural model). In this structure EF3 and EF4 from the C-terminus of one protomer interacts with the neck region, which connects CH2 with SR1 in the other protomer (Travers et al., 2013). CH1 is quasi folded back like a hook towards the SR region. (B) Actin dimers as present in F-actin (PDB ID: 3G37) are docked onto the α-actinin structure depicting the F-actin cross-linking activity of α-actinin. CH1 binds to F-actin (Travers et al., 2013). The CH1 and CH2 domains are in an open conformation in this F-actin bound state (insert on top of the full length α-actinin – F-actin structural model) as observed (Galkin et al., 2010). EF3 and EF4 of the second antiparallel α-actinin protomer are displaced from the neck region as predicted rendering them accessible for CaMKII binding. Scale bars are 20 Angstrom for main figure and 10 Angstroms for enlarged inserts.

Figure 5. Role of CaMKII Binding to GluN2B in AMPAR Phosphorylation

Ca²⁺ influx during LTP will induce association of CaMKII with the NMDAR. From there, CaMKII can reach and phosphorylate neighboring AMPARs. Phosphorylation of GluA1 on S831 will immediately increase conductance through AMPARs (dark purple). The Ca²⁺ influx will also facilitate detachment of the cytosolic C-termini of TARPs (dark magenta), which have multiple positively charged Arg and Lys residues. The detachment will make 9 phosphorylation sites (red circles) available for CaMKII. The ensuing phosphorylations will reduce the net positive charge and thereby re-association with the plasma membrane. As a result the number of TARPs whose C-termini are available for binding to PDZ domains of PSD-95 (blue) will be increased for enhanced trapping of AMPAR-TARP complexes at postsynaptic sites (Opazo et al., 2010).