Mechanism underlying hippocampal long-term potentiation and depression based on competition between endocytosis and exocytosis of AMPA receptors

Abstract

N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP) and long-term depression (LTD) of signal transmission form neural circuits and thus are thought to underlie learning and memory. These mechanisms are mediated by AMPA receptor (AMPAR) trafficking in postsynaptic neurons. However, the regulatory mechanism of bidirectional plasticity at excitatory synapses remains unclear. We present a network model of AMPAR trafficking for adult hippocampal pyramidal neurons, which reproduces both LTP and LTD. We show that the induction of both LTP and LTD is regulated by the competition between exocytosis and endocytosis of AMPARs, which are mediated by the calcium-sensors synaptotagmin 1/7 (Syt1/7) and protein interacting with C-kinase 1 (PICK1), respectively. Our result indicates that recycling endosomes containing AMPAR are always ready for Syt1/7-dependent exocytosis of AMPAR at peri-synaptic/synaptic membranes. This is because molecular motor myosin V_b constitutively transports the recycling endosome toward the membrane in a Ca²⁺-independent manner.

Figure 1

AMPAR trafficking model at hippocampal postsynaptic neurons. (a) AMPAR which consists of two GluA1 (GluR1) and two GluA2 (GluR2) subunits is the most predominant AMPAR subtype in hippocampal neurons. The serine-845 site (S845) of GluA1 is phosphorylated and dephosphorylated by protein kinase A (PKA) and protein phosphatase 2B (PP2B, or Calcineurin, CaN), respectively. The serine-880 site (S880) of GluA2 is phosphorylated and dephosphorylated by protein kinase C (PKC) and protein phosphatase 2 (PP2A), respectively. (b) A-kinase anchoring protein 150 (AKAP150)^,, is an anchoring protein that organizes PKA, PP2B, and PKC for phosphoregulation of AMPARs at the synaptic membrane, and thus acts as the AKAP signaling complex. The AKAP signaling complex forms the dimer as shown in (b) (though, for simplification, not shown in (d–e)). (c–h) Experimentally characterized elementary processes involved in the AMPAR trafficking cycle at a hippocampal postsynaptic neuron. (c) A model of tethering the GluA1 and GluA2 subunits to the AKAP150 signaling complex at the synaptic membrane through SAP97 and GRIP1, respectively^,,. (d) A model of phosphorylation and dephosphorylation reactions of the AMPAR due to Ca²⁺ signaling. Dephosphorylation of the S845 site of GluA1 is caused by PP2B, and SAP97 dissociates from GluA1. The S880 site of GluA2 is phosphorylated by PKC, and PICK1 binds to the GluA2 subunit instead of GRIP1^,. (e,f) An endocytic model for synaptic vesicles containing AMPAR mediated by the calcium-sensor PICK1^,,. (g) Active transport of the recycling endosomes by molecular motor myosin V_b^–. In the recycling endosome, PP2A causes the dephosphorylation of S880 of GluA2, and GRIP1 instead of PICK1 binds to GluA2. (h) An exocytic model of the recycling endosome triggered by Ca²⁺-sensor synaptic vesicle protein synaptotagmin 1 (Syt1) together with synaptotagmin 7 (not shown) and synaptobrevin-2 (Syb2)/VAMP2, complexin (not shown), amongst others^,,–,.

Figure 2

The network model of AMPAR trafficking that mediates hippocampal LTP and LTD. For simplification, the AMPAR is schematically depicted as two particles corresponding to the GluA1 and GluA2 subunits. This network model is based on the experimental observations that are summarized in Fig. 1. Here, the reaction network on the phosphorylation/dephosphorylation dynamics of GluA1 and GluA2 is also displayed schematically. The recycling endosomes containing AMPAR are actively transported by myosin V_b toward the peri-synaptic/synaptic membrane. The lateral diffusion relocation of AMPAR is assumed to occur during the phosphorylation and dephosphorylation of AMPAR at the synaptic membrane, in addition to the local diffusional relocation movement of the exocytic AMPAR from the peri-synaptic to synaptic membrane. The phosphorylation state of GluA1 and GluA2 regulates localization of the AMPARs at the synaptic membrane via interactions with various AMPAR interacting proteins (SAP97, GRIP1, PICK1).

Figure 3

Hippocampal LTP and LTD are regulated by the activation of Ca²⁺-sensors Syt1 and PICK1 in response to Ca²⁺ influx. (a) Time course of the membrane AMPAR population, indicating induction of LTP and LTD. Here, 100% represents the basal AMPAR population at the membrane. (b) Ca²⁺-pulse concentrations corresponding to the LTP and LTD stimulation are shown on the left and right axis, respectively. (c–f) The concentrations of Ca²⁺-binding species of PICK1 (c,e) and Syt1 (d,f) as a function of time (t) during LTP (c and d) and LTD induction (e,f). In (f), the concentration of multiple Ca²⁺-binding species other than (Ca)_ASyt1 is too small to see at this scale, indicating that Syt1 is mostly not activated. Therefore, Ca²⁺-dependent exocytosis mediated by Syt1 occurs during LTD.

Figure 4

Competition between exocytosis and endocytosis of AMPARs yields LTP and LTD. (a,c) Total excess fluxes of exocytosis, endocytosis, and myosin V_b transport of recycling endosomes during (a) LTP stimulation and (c) LTD stimulation. (b,d) The time course of concentrations for predominant components during (b) LTP and (d) LTD.

Figure 5

Myosin V_b transport predominantly governs the long-term behavior of LTP induction. (a) Time courses of the membrane AMPAR population obtained for the wild-type and genetically inhibited model of myosin V_b transport. (b) Normalized endocytic fluxes defined as the endocytic flux divided by the basal [AMPAR] at the membrane. (c,d) The populations of predominant components in the cytosol during LTP induction for (c) the wild-type model and (d) a model with inhibited myosin V_b transport, shown as functions of time (t). Myosin V_b-binding recycling endosome in the cytosol, [R1R2endo(GRIP₂)MyoV], is increased under basal conditions by the inhibition of myosin V_b transport, and furthermore is increased during impaired LTP induction, as seen in (d).

Figure 6

Dephosphorylation of GluA1 S845 by protein phosphatase 2B (PP2B, Calcineurin) regulates the induction of LTD. (a) Time courses of the membrane AMPAR population for wild-type and PP2B-anchoring deficient AKAP150ΔPIX mice. (b) Normalized endocytic fluxes, defined as the endocytic flux divided by the basal [AMPAR] at the membrane. (c,d) The populations of the main (top five) components at the membrane during induction of LTD for (c) the wild-type model and (d) the AKAP150ΔPIX model are shown as functions of time (t). PICK1-binding AMPARs at the membrane, which have been prepared for PICK1-mediated endocytosis, such as R1R2(pS2-PICK1) and R1R2(pS2-CaPICK1) (c), are decreased by disrupting the PP2B-dependent dephosphorylation of GluA1 S845. It is noted that R1R2(pS₂-PICK1) shown in (c) is not displayed in (d) because it is present at levels lower than the other components shown here. On the other hand, the AMPARs that are bound to the membrane through tethering of GluA1 to AKAP150 via SAP97, such as R1(pS-SAP)R2(pS₂) and R1(pS₂-SAP₂)R2(pS₂), are increased by the inhibition of the PP2B-dependent dephosphorylation of GluA1 S845.

Figure 7

Schematic model of a hippocampal postsynaptic membrane. Recycling endosomes are localized on peri-synaptic/synaptic membrane surface under basal conditions, thus they are already prepared for Ca²⁺-dependent Syt1-mediated exocytosis resulting in the prompt incorporation of AMPARs into the membranes. The graph shows the time courses of the total concentrations for cytoplasmic components, namely, pre-exocytic recycling endosomes that are localized on the membrane surface and newly internalized endosomes, during LTP induction. The pre-exocytic recycling endosomes are steeply decreased by Syt1-mediated exocytosis, then are gradually increased by the myosin-V_b active transport of newly internalized recycling endosomes by PICK1-mediated endocytosis.