Cornichon2 (CNIH2), an integral component of AMPA receptor (AMPAR) complexes in the mammalian brain, slows deactivation and desensitization of heterologously reconstituted receptor channels. Its significance in neuronal signal transduction, however, has remained elusive. Here we show by paired recordings that CNIH2-containing AMPARs dictate the slow decay of excitatory postsynaptic currents (EPSCs) elicited in hilar mossy cells of the hippocampus by single action potentials in mossy fiber boutons (MFB). Selective knockdown of CNIH2 markedly accelerated EPSCs in individual MFB-mossy cell synapses without altering the EPSC amplitude. In contrast, the rapidly decaying EPSCs in synapses between MFBs and aspiny interneurons that lack expression of CNIH2 were unaffected by the protein knockdown but were slowed by virus-directed expression of CNIH2. These results identify CNIH2 as the molecular distinction between slow and fast EPSC phenotypes and show that CNIH2 influences the time course and, hence, the efficacy of excitatory synaptic transmission.

AMPA-type glutamate receptors (AMPARs) are responsible for a variety of processes in the mammalian brain including fast excitatory neurotransmission, postsynaptic plasticity, or synapse development. Here, with comprehensive and quantitative proteomic analyses, we demonstrate that native AMPARs are macromolecular complexes with a large molecular diversity. This diversity results from coassembly of the known AMPAR subunits, pore-forming GluA and three types of auxiliary proteins, with 21 additional constituents, mostly secreted proteins or transmembrane proteins of different classes. Their integration at distinct abundance and stability establishes the heteromultimeric architecture of native AMPAR complexes: a defined core with a variable periphery resulting in an apparent molecular mass between 0.6 and 1 MDa. The additional constituents change the gating properties of AMPARs and provide links to the protein dynamics fundamental for the complex role of AMPARs in formation and operation of glutamatergic synapses.

Excitatory neurotransmission and its activity-dependent plasticity are largely determined by AMPA-receptors (AMPARs), ion channel complexes whose cell physiology is encoded by their interactome. Here, we delineate the assembly of AMPARs in the endoplasmic reticulum (ER) of native neurons as multi-state production line controlled by distinct interactome constituents: ABHD6 together with porcupine stabilizes pore-forming GluA monomers, and the intellectual-disability-related FRRS1l-CPT1c complexes promote GluA oligomerization and co-assembly of GluA tetramers with cornichon and transmembrane AMPA-regulatory proteins (TARP) to render receptor channels ready for ER exit. Disruption of the assembly line by FRRS1l deletion largely reduces AMPARs in the plasma membrane, impairs synapse formation, and abolishes activity-dependent synaptic plasticity, while FRRS1l overexpression has the opposite effect. As a consequence, FRSS1l knockout mice display severe deficits in learning tasks and behavior. Our results provide mechanistic insight into the stepwise biogenesis of AMPARs in native ER membranes and establish FRRS1l as a powerful regulator of synaptic signaling and plasticity.

Information processing and storage in the brain rely on AMPA-receptors (AMPARs) and their context-dependent dynamics in synapses and extra-synaptic sites. We found that distribution and dynamics of AMPARs in the plasma membrane are controlled by Noelins, a three-member family of conserved secreted proteins expressed throughout the brain in a cell-type-specific manner. Noelin tetramers tightly assemble with the extracellular domains of AMPARs and interconnect them in a network-like configuration with a variety of secreted and membrane-anchored proteins including Neurexin1, Neuritin1, and Seizure 6-like. Knock out of Noelins1-3 profoundly reduced AMPARs in synapses onto excitatory and inhibitory (inter)neurons, decreased their density and clustering in dendrites, and abolished activity-dependent synaptic plasticity. Our results uncover an endogenous mechanism for extracellular anchoring of AMPARs and establish Noelin-organized networks as versatile determinants of constitutive and context-dependent neurotransmission.

AMPA-type glutamate receptors (AMPARs), key elements in excitatory neurotransmission in the brain, are macromolecular complexes whose properties and cellular functions are determined by the co-assembled constituents of their proteome. Here we identify AMPAR complexes that transiently form in the endoplasmic reticulum (ER) and lack the core-subunits typical for AMPARs in the plasma membrane. Central components of these ER AMPARs are the proteome constituents FRRS1l (C9orf4) and CPT1c that specifically and cooperatively bind to the pore-forming GluA1-4 proteins of AMPARs. Bi-allelic mutations in the human FRRS1L gene are shown to cause severe intellectual disability with cognitive impairment, speech delay and epileptic activity. Virus-directed deletion or overexpression of FRRS1l strongly impact synaptic transmission in adult rat brain by decreasing or increasing the number of AMPARs in synapses and extra-synaptic sites. Our results provide insight into the early biogenesis of AMPARs and demonstrate its pronounced impact on synaptic transmission and brain function.