AKAP150-Anchored Calcineurin Regulates Synaptic Plasticity by Limiting Synaptic Incorporation of Ca²⁺-Permeable AMPA Receptors

Abstract

AMPA receptors (AMPARs) are tetrameric ion channels assembled from GluA1-GluA4 subunits that mediate the majority of fast excitatory synaptic transmission in the brain. In the hippocampus, most synaptic AMPARs are composed of GluA1/2 or GluA2/3 with the GluA2 subunit preventing Ca2+ influx. However, a small number of Ca2+-permeable GluA1 homomeric receptors reside in extrasynaptic locations where they can be rapidly recruited to synapses during synaptic plasticity. Phosphorylation of GluA1 S845 by the cAMP-dependent protein kinase (PKA) primes extrasynaptic receptors for synaptic insertion in response to NMDA receptor (NMDAR) Ca2+ signaling during long-term potentiation (LTP), while phosphatases dephosphorylate S845 and remove synaptic and extrasynaptic GluA1 during long-term depression (LTD). PKA and the Ca2+-activated phosphatase calcineurin (CaN) are targeted to GluA1 through binding to A-kinase anchoring protein (AKAP) 150 in a complex with PSD-95, but we do not understand how the opposing activities of these enzymes are balanced to control plasticity. Here, we generated AKAP150ΔPIX knock-in mice to selectively disrupt CaN anchoring in vivo. We found that AKAP150ΔPIX mice lack LTD but express enhanced LTP at CA1 synapses. Accordingly, basal GluA1 S845 phosphorylation is elevated in AKAP150ΔPIX hippocampus, and LTD-induced dephosphorylation and removal of GluA1, AKAP150, and PSD-95 from synapses is impaired. In addition, basal synaptic activity of GluA2-lacking AMPARs is increased in AKAP150ΔPIX mice and pharmacologic antagonism of these receptors restores normal LTD and inhibits the enhanced LTP. Thus, AKAP150-anchored CaN opposes PKA phosphorylation of GluA1 to restrict synaptic incorporation of Ca2+-permeable AMPARs both basally and during LTP and LTD.

Network compensation of cyclic GMP-dependent protein kinase II knockout in the hippocampus by Ca 2+ -permeable AMPA receptors

“…APV treatment was sufficient to inhibit both WT and KO LTP (WT, 210.4 ± 18.3%, and WT + APV, 107.3 ± 9.5%, P < 0.0001; and KO, 176.4 ± 21.9%, and KO+APV, 100.9 ± 8.3%, P = 0.0001) (Fig. 4B), suggesting that CPAR-mediated LTP in the cGKII KO required NMDARs, consistent with the previous role of NMDARs in CPAR-mediated LTP in mice expressing GluA2 (16,26). Ca 2+ /calmodulin-dependent protein kinase II (CaMKII) also phosphorylates GluA1 during LTP induction, on serine 831, but is not required for CPARmediated LTP in the GluA2 KO hippocampus (30).…”

“…Several studies suggest that LTP is increased when CPARs are present in the hippocampus compared with WT (16,(24)(25)(26). Because we found synaptic expression of CPARs in KO hippocampal neurons, we hypothesized that CPAR expression could compensate in the KO for LTP impairment found when cGKII is acutely inhibited.…”

“…CPAR-mediated hippocampal LTP has been found in several conditions, including in the GluA2 KO (24,25,30) and following calcineurin inhibition (16,23) and acute stress (26). The CPARmediated LTP is functionally distinct from classical NMDARmediated LTP (24)(25)(26)30).…”

“…GluA1, which enables GluA1-containing AMPARs to be endocytosed from the plasma membrane during LTD (15,16). This removes synaptic AMPARs, leading to reduction of receptor function during LTD. Taken together, the activity-dependent trafficking of synaptic GluA1 is regulated by the status of phosphorylation in the CTD, which provides a critical mechanism underlying LTP and LTD.…”

Synaptic AMPA receptor composition in development, plasticity and disease

“…Calcineurin and PKA are anchored at synapses by the protein AKAP150 (the rodent orthologue of human AKAP79), and knock-in of an AKAP150 mutant defective in calcineurin binding leads to enhanced GluA1-S845 phosphorylation and increases synaptic expression of CP-AMPARs 138 . Consistent with increased GluA1-S845 phosphorylation, these mice do not exhibit LTD at CA3-CA1 synapses, but show enhanced LTP 138 . A role for GluA1-S845 phosphorylation has also been proposed in the synaptic incorporation of CP-AMPARs during synaptic scaling.…”