AMPA receptor trafficking and LTP: Carboxy-termini, amino-termini and TARPs

Biology of AMPA receptor interacting proteins - From biogenesis to synaptic plasticity.

The X-linked GRIA3 gene encodes the GLUA3 subunit of AMPA-type glutamate receptors. Pathogenic variants in this gene were previously reported in neurodevelopmental diseases, mostly in male patients but rarely in females. Here we report a de novo pathogenic missense variant in GRIA3 (c.1979G>C; p. R660T) identified in a 1-year-old female patient with severe epilepsy and global developmental delay. When exogenously expressed in human embryonic kidney (HEK) cells, GLUA3_R660T showed slower desensitization and deactivation kinetics compared to wildtype (wt) GLUA3 receptors. Substantial non-desensitized currents were observed with the mutant but not for wt GLUA3 with prolonged exposure to glutamate. When co-expressed with GLUA2, the decay kinetics were similarly slowed in GLUA2/A3_R660T with non-desensitized steady state currents. In cultured cerebellar granule neurons, miniature excitatory postsynaptic currents (mEPSCs) were significantly slower in R660T transfected cells than those expressing wt GLUA3. When overexpressed in hippocampal CA1 neurons by in utero electroporation, the evoked EPSCs and mEPSCs were slower in neurons expressing R660T mutant compared to those expressing wt GLUA3. Therefore our study provides functional evidence that a gain of function (GoF) variant in GRIA3 may cause epileptic encephalopathy and global developmental delay in a female subject by enhancing synaptic transmission.

/pdf/x-linked-neonatal-onset-epileptic-encephalopathy-associated-3ze6v4sxdd.pdf

X-linked neonatal-onset epileptic encephalopathy associated with a gain-of-function variant p.R660T in GRIA3.

AMPA receptor (AMPAR) abundance and positioning at excitatory synapses regulates the strength of transmission. Changes in AMPAR localisation can enact synaptic plasticity, allowing long-term information storage, and is therefore tightly controlled. Multiple mechanisms regulating AMPAR synaptic anchoring have been described, but with limited coherence or comparison between reports, our understanding of this process is unclear. Here, combining synaptic recordings from mouse hippocampal slices and super-resolution imaging in dissociated cultures, we compare the contributions of three AMPAR interaction domains controlling transmission at hippocampal CA1 synapses. We show that the AMPAR C-termini play only a modulatory role, whereas the extracellular N-terminal domain (NTD) and PDZ interactions of the auxiliary subunit TARP γ8 are both crucial, and each is sufficient to maintain transmission. Our data support a model in which γ8 accumulates AMPARs at the postsynaptic density, where the NTD further tunes their positioning. This interplay between cytosolic (TARP γ8) and synaptic cleft (NTD) interactions provides versatility to regulate synaptic transmission and plasticity.

/pdf/ampa-receptor-anchoring-at-ca1-synapses-is-determined-by-n-3wz7holp0c.pdf

AMPA receptor anchoring at CA1 synapses is determined by N-terminal domain and TARP γ8 interactions.

Ca2+ signaling in postsynaptic neurons: Neuroplastin-65 regulates the interplay between plasma membrane Ca2+ ATPases and ionotropic glutamate receptors.

Long-term potentiation (LTP) has long been considered as an important cellular mechanism for learning and memory. LTP expression involves NMDA receptor-dependent synaptic insertion of AMPA receptors (AMPARs). However, how AMPARs are recruited and anchored at the postsynaptic membrane during LTP remains largely unknown. In this study, using CRISPR/Cas9 to delete the endogenous AMPARs and replace them with the mutant forms in single neurons, we have found that the amino-terminal domain (ATD) of GluA1 is required for LTP maintenance. Moreover, we show that GluA1 ATD directly interacts with the cell adhesion molecule neuroplastin-65 (Np65). Neurons lacking Np65 exhibit severely impaired LTP maintenance, and Np65 deletion prevents GluA1 from rescuing LTP in AMPARs-deleted neurons. Thus, our study reveals an essential role for GluA1/Np65 binding in anchoring AMPARs at the postsynaptic membrane during LTP.

https://www.pnas.org/content/pnas/118/9/e2019194118.full.pdf

Chao-Hua Jiang

Papers

The amino-terminal domain of GluA1 mediates LTP maintenance via interaction with neuroplastin-65.