Mechanisms of CaMKII action in long-term potentiation

Abstract

Long-term potentiation (LTP) of synaptic strength occurs during learning and can last for long periods, making it a probable mechanism for memory storage. LTP induction results in calcium entry, which activates calcium/calmodulin-dependent protein kinase II (CaMKII). CaMKII subsequently translocates to the synapse, where it binds to NMDA-type glutamate receptors and produces potentiation by phosphorylating principal and auxiliary subunits of AMPA-type glutamate receptors. These processes are all localized to stimulated spines and account for the synapse-specificity of LTP. In the later stages of LTP, CaMKII has a structural role in enlarging and strengthening the synapse.

Figure 1. Synapse-specificity of the processes involved in LTP induction

a A transient elevation of calcium (detected by the calcium-sensitive dye Fluo4-FF; green signal) in a spine can be observed in response to glutamate uncaging at that spine (arrowhead). The calcium-insensitive dye Alexa-594 outlines the spine volume (red). . b Local glutamate uncaging can potentiate AMPA receptor-mediated excitatory postsynaptic currents (EPSCs) in the stimulated spine but not in an adjacent spine. c After LTP induction by local glutamate uncaging (arrowhead), transient activation of CaMKII can be observed (red denotes high activation; level of activation measured by the FRET sensor Camui). . d Stimulation of a spine (arrowhead) in this fashion can cause a persistent increase in spine volume and translocation of CaMKII (here shown by translocation of green fluorescent protein-labelled αCaMKII; red denotes high levels of this kinase)… From .

Figure 2. Role of CaMKII–NMDAR complex in LTP maintenance

Electron microscopy immunolabelling has been used to measure CaMKII levels in postsynaptic densities (PSDs) a before and b one hour after chemically induced LTP. c The histogram shows that there is more CaMKII per unit length of the PSD after LTP induction than before induction of LTD . d The CaMKII–NMDAR complex can be immunoprecipitated with an antibody that detects CaMKII. Synaptic stimulation with NMDA increases the amount of NR2B and NR1 in complexes with CaMKII. These increases can be blocked by the NMDAR antagonist MK801 or by the CaMK inhibitor KN62 . e Overexpression of a mutant form of NR2B (RS/QD) blocks LTP (LTP induction denoted by the arrow) . f LTP (induced at arrow) can be reversed by tatCN21, a peptide that interferes with CaMKII binding to NR2B. In this example, LTP was induced by 4 tetani (T), and an additional tetanus had no effect, indicating that LTP saturation had been reached (left-hand side). Subsequent application of tatCN21 reversed LTP. After 30 mins of tatCN21 application, the peptide was removed and only partial recovery of LTP was observed (note that there was an acute inhibitory effect that was reversed when the peptide was removed). Additional LTP could then be induced using a 4T protocol (right-hand panel).

Figure 3. Role of CaMKII is in AMPA receptor-mediated transmission during early LTP

Calcium entry through the NMDAR activates calmodulin both in the bulk cytoplasm and in the channel nanodomain. Calmodulin activates CaMKII, which then translocates to the PSD where it enhances AMPAR-mediated transmission in two ways: 1) by phosphorylation of GluR1 and S831, which leads to increased average conductance of the channel and 2) phosphorylation of stargazin, which leads to its binding to PSD-95, thereby increasing the number of AMPAR at the synapse. CaMKII, perhaps with other calcium dependent processes, stimulates fusion of vesicles containing AMPAR with the plasma membrane, increasing the extrasynaptic channel concentration. This increase is not necessary for the earliest stage of LTP, but is necessary for subsequent stages.

Figure 4. Mechanisms of expression processes by which CaMKII enhances AMPA receptor-mediated transmission

a CaMKII enhances AMPA receptor (AMPAR) conductance in isolated CA1 pyramidal cells of wild-type mice but not in knockin mice in which phosphorylation of the AMPAR subunit glutamate receptor 1 (GluR1) on S831 or both S831 and S845 is prevented by mutation of the serine residues to alanine residues. Pseudophosphorylation of these sites (that is, expression of S831D or S845D GluR1) enhances AMPAR conductance and prevents further enhancement by CaMKII . b In addition to affecting AMPAR conductance, CaMKII also affects the mobility of these receptors. Single-particle tracking experiments have shown that in neurons under control conditions (left-hand panel), GluR2 subunits exhibit rapid diffusion that follows a broad path (three examples of particle paths are shown at the top and a histogram of diffusion constants for GluR2-labelled particles is shown at the bottom). In neurons overexpressing an active (truncated) form of CaMKII (tCaMKII; right-hand panel), GluR2 molecules show limited diffusion paths (top) and a lower average diffusion constant (bottom), as AMPARs are captured at the synapse c The influence of CaMKII on AMPA receptor-mediated transmission can be observed in LTP experiments. LTP can be induced by a pairing protocol (arrow denotes the point of protocol initiation) in wild-type CA1 pyramidal cells but is prevented in cells expressing a mutant variant of CaMKII (T286A) that cannot become persistently active . d Block of exocytosis of AMPAR containing vesicles by postsynaptic injection of botulinum toxin (left-hand panel) did not affect earliest phase of LTP; control is shown after injection of inactivated botulinum toxin (right-hand panel) . e Stargazin has an important role in CaMKII-enhanced AMPA receptor-mediated transmission. LTP is blocked in CA1 pyramidal cells expressing a mutant variant of stargazin in which the nine potential CaMKII/PKC serine phosphorylation sites are mutated to alanine residues (S9A). LTP can be induced in cells expressing pseudophosphorylated stargazing (S9D). The control condition is without LTP induction, and the arrow denotes the start of the induction protocol .