γ-2 and GSG1L bind with comparable affinities to the tetrameric GluA1 core.

BACKGROUND: The AMPA-type ionotropic glutamate receptor mediates fast excitatory neurotransmission in the brain. A variety of auxiliary subunits regulate its gating properties, assembly, and trafficking, but it is unknown if the binding of these auxiliary subunits to the receptor core is dynamically regulated. Here we investigate the interplay of the two auxiliary subunits γ-2 and GSG1L when binding to the AMPA receptor composed of four GluA1 subunits. METHODS: We use a three-color single-molecule imaging approach in living cells, which allows the direct observation of the receptors and both auxiliary subunits. Colocalization of different colors can be interpreted as interaction of the respective receptor subunits. RESULTS: Depending on the relative expression levels of γ-2 and GSG1L, the occupancy of binding sites shifts from one auxiliary subunit to the other, supporting the idea that they compete for binding to the receptor. Based on a model where each of the four binding sites at the receptor core can be either occupied by γ-2 or GSG1L, our experiments yield apparent dissociation constants for γ-2 and GSG1L in the range of 2.0-2.5/µm2. CONCLUSIONS: The result that both binding affinities are in the same range is a prerequisite for dynamic changes of receptor composition under native conditions.

Native GABA(B) receptors are heteromultimers with a family of auxiliary subunits.

GABA(B) receptors are the G-protein-coupled receptors for gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the brain. They are expressed in almost all neurons of the brain, where they regulate synaptic transmission and signal propagation by controlling the activity of voltage-gated calcium (Ca(v)) and inward-rectifier potassium (K(ir)) channels. Molecular cloning revealed that functional GABA(B) receptors are formed by the heteromeric assembly of GABA(B1) with GABA(B2) subunits. However, cloned GABA(B(1,2)) receptors failed to reproduce the functional diversity observed with native GABA(B) receptors. Here we show by functional proteomics that GABA(B) receptors in the brain are high-molecular-mass complexes of GABA(B1), GABA(B2) and members of a subfamily of the KCTD (potassium channel tetramerization domain-containing) proteins. KCTD proteins 8, 12, 12b and 16 show distinct expression profiles in the brain and associate tightly with the carboxy terminus of GABA(B2) as tetramers. This co-assembly changes the properties of the GABA(B(1,2)) core receptor: the KCTD proteins increase agonist potency and markedly alter the G-protein signalling of the receptors by accelerating onset and promoting desensitization in a KCTD-subtype-specific manner. Taken together, our results establish the KCTD proteins as auxiliary subunits of GABA(B) receptors that determine the pharmacology and kinetics of the receptor response.

A Noelin-organized extracellular network of proteins required for constitutive and context-dependent anchoring of AMPA-receptors.

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.

Subunit composition, molecular environment, and activation of native TRPC channels encoded by their interactomes.

In the mammalian brain TRPC channels, a family of Ca2+-permeable cation channels, are involved in a variety of processes from neuronal growth and synapse formation to transmitter release, synaptic transmission and plasticity. The molecular appearance and operation of native TRPC channels, however, remained poorly understood. Here, we used high-resolution proteomics to show that TRPC channels in the rodent brain are macro-molecular complexes of more than 1 MDa in size that result from the co-assembly of the tetrameric channel core with an ensemble of interacting proteins (interactome). The core(s) of TRPC1-, C4-, and C5-containing channels are mostly heteromers with defined stoichiometries for each subtype, whereas TRPC3, C6, and C7 preferentially form homomers. In addition, TRPC1/C4/C5 channels may co-assemble with the metabotropic glutamate receptor mGluR1, thus guaranteeing both specificity and reliability of channel activation via the phospholipase-Ca2+ pathway. Our results unveil the subunit composition of native TRPC channels and resolve the molecular details underlying their activation.

Pacemaking by HCN channels requires interaction with phosphoinositides.

Hyperpolarization-activated, cyclic-nucleotide-gated (HCN) channels mediate the depolarizing cation current (termed I(h) or I(f)) that initiates spontaneous rhythmic activity in heart and brain. This function critically depends on the reliable opening of HCN channels in the subthreshold voltage-range. Here we show that activation of HCN channels at physiologically relevant voltages requires interaction with phosphoinositides such as phosphatidylinositol-4,5-bisphosphate (PIP(2)). PIP(2) acts as a ligand that allosterically opens HCN channels by shifting voltage-dependent channel activation approximately 20 mV toward depolarized potentials. Allosteric gating by PIP(2) occurs in all HCN subtypes and is independent of the action of cyclic nucleotides. In CNS neurons and cardiomyocytes, enzymatic degradation of phospholipids results in reduced channel activation and slowing of the spontaneous firing rate. These results demonstrate that gating by phospholipids is essential for the pacemaking activity of HCN channels in cardiac and neuronal rhythmogenesis.

Functional expression of nicotinic acetylcholine receptors in rat neocortical layer 5 pyramidal cells.

Neuronal nicotinic acetylcholine receptors (nAChRs) expressed by neurons of the neocortex are known to play a role in higher brain functions. Electrophysiological studies of neocortical neurons provided evidence that functional nAChRs are present on the axonal presynaptic terminals, on the somata and on dendrites of gamma-aminobutyric acid (GABA)ergic inhibitory interneurons. However, it is not clear if pyramidal neurons express functional postsynaptic nAChRs. Therefore, we investigated the action of locally applied acetylcholine (ACh) on layer 5 pyramidal neurons in the rat neocortex in vitro. In the presence of atropine, tetrodotoxin, glutamate receptor antagonists, and GABAA receptor antagonists, ACh induced membrane depolarizations which were generated by membrane inward currents consisting of a fast and a slow component. Analysis of the electrophysiological properties, the pharmacological characteristics, and the desensitization behavior of the 2 current components revealed that they were mediated by at least 2 different subtypes of the nAChR, most likely the alpha7-like and the alpha4beta2-like subtype. The expression of nAChRs in neocortical pyramidal cells raises the possibility that these neurons generate nicotinic excitatory postsynaptic potentials, thereby influencing cell excitability. Furthermore, because most nAChRs are permeable to calcium, they may modulate synaptic transmission and neuronal plasticity via a calcium-dependent postsynaptic mechanism.

Recycling endosomes supply cardiac pacemaker channels for regulated surface expression.

AIMS: Cellular excitability is not only determined by the type but also by the number of ion channels in the plasma membrane. Recent evidence indicates that cell surface expression of cardiac pacemaker channels might be controlled beyond the level of biosynthesis by regulating their surface transport. However, neither the underlying trafficking pathways nor their molecular control have yet been investigated. METHODS AND RESULTS: We have studied endocytic trafficking of hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels expressed as fusions with green fluorescent protein or tagged with an extracellular haemagglutinin epitope in opossum kidney cells, dissociated rat hippocampal neurons, and ventricular cardiomyocytes. After being internalized from the plasma membrane, HCN2 and HCN4 are sorted to the Rab11-positive endocytic recycling compartment (ERC). From there, they are transported back to the cell surface depending on active phospholipase D2 (PLD2). The peptide hormone angiotensin II, which is upregulated in a number of cardiac pathologies and a known activator of PLD2, stimulates ERC trafficking of HCN4 channels. It significantly increases HCN surface expression independent of their biosynthesis. CONCLUSION: Recycling endosomes serve as an intracellular storage compartment for the cardiac pacemaker channels HCN2 and HCN4. They are not only crucial for maintaining a homeostatic surface expression but also supply channels for rapid adaptation of their surface expression in response to extracellular stimuli.

A Monoallelic Two-Hit Mechanism in PLCD1 Explains the Genetic Pathogenesis of Hereditary Trichilemmal Cyst Formation.

Trichilemmal cysts are common hair follicle-derived intradermal cysts. The trait shows an autosomal dominant mode of transmission with incomplete penetrance. Here, we describe the pathogenetic mechanism for the development of hereditary trichilemmal cysts. By whole-exome sequencing of DNA from the blood samples of 5 affected individuals and subsequent Sanger sequencing of a family cohort including 35 affected individuals, this study identified a combination of the Phospholipase C Delta 1 germline variants c.903A>G, p.(Pro301Pro) and c.1379C>T, p.(Ser460Leu) as a high-risk factor for trichilemmal cyst development. Allele-specific PCRs and cloning experiments showed that these two variants are present on the same allele. The analysis of tissue from several cysts revealed that an additional somatic Phospholipase C Delta 1 mutation on the same allele is required for cyst formation. In two different functional in vitro assays, this study showed that the protein function of the cyst-specific 1-phosphatidylinositol 4, 5-bisphosphate phosphodiesterase delta-1 protein variant is modified. This pathologic mechanism defines a monoallelic model of the two-hit mechanism proposed for tumor development and other hereditary cyst diseases.

An ER Assembly Line of AMPA-Receptors Controls Excitatory Neurotransmission and Its Plasticity.

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.

AMPA-receptor specific biogenesis complexes control synaptic transmission and intellectual ability.

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.

Auxiliary GABAB receptor subunits uncouple G protein ßγ subunits from effector channels to induce desensitization.

Activation of K(+) channels by the G protein ßγ subunits is an important signaling mechanism of G-protein-coupled receptors. Typically, receptor-activated K(+) currents desensitize in the sustained presence of agonists to avoid excessive effects on cellular activity. The auxiliary GABAB receptor subunit KCTD12 induces fast and pronounced desensitization of the K(+) current response. Using proteomic and electrophysiological approaches, we now show that KCTD12-induced desensitization results from a dual interaction with the G protein: constitutive binding stabilizes the heterotrimeric G protein at the receptor, whereas dynamic binding to the receptor-activated Gßγ subunits induces desensitization by uncoupling Gßγ from the effector K(+) channel. While receptor-free KCTD12 desensitizes K(+) currents activated by other GPCRs in vitro, native KCTD12 is exclusively associated with GABAB receptors. Accordingly, genetic ablation of KCTD12 specifically alters GABAB responses in the brain. Our results show that GABAB receptors are endowed with fast and reversible desensitization by harnessing KCTD12 that intercepts Gßγ signaling.

AMPA receptors commandeer an ancient cargo exporter for use as an auxiliary subunit for signaling.

Fast excitatory neurotransmission in the mammalian central nervous system is mainly mediated by ionotropic glutamate receptors of the AMPA subtype (AMPARs). AMPARs are protein complexes of the pore-lining α-subunits GluA1-4 and auxiliary ß-subunits modulating their trafficking and gating. By a proteomic approach, two homologues of the cargo exporter cornichon, CNIH-2 and CNIH-3, have recently been identified as constituents of native AMPARs in mammalian brain. In heterologous reconstitution experiments, CNIH-2 promotes surface expression of GluAs and modulates their biophysical properties. However, its relevance in native AMPAR physiology remains controversial. Here, we have studied the role of CNIH-2 in GluA processing both in heterologous cells and primary rat neurons. Our data demonstrate that CNIH-2 serves an evolutionarily conserved role as a cargo exporter from the endoplasmic reticulum (ER). CNIH-2 cycles continuously between ER and Golgi complex to pick up cargo protein in the ER and then to mediate its preferential export in a coat protein complex (COP) II dependent manner. Interaction with GluA subunits breaks with this ancestral role of CNIH-2 confined to the early secretory pathway. While still taking advantage of being exported preferentially from the ER, GluAs recruit CNIH-2 to the cell surface. Thus, mammalian AMPARs commandeer CNIH-2 for use as a bona fide auxiliary subunit that is able to modify receptor signaling.

High-resolution proteomics unravel architecture and molecular diversity of native AMPA receptor complexes.

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.

Functional proteomics identify cornichon proteins as auxiliary subunits of AMPA receptors.

Glutamate receptors of the AMPA-subtype (AMPARs), together with the transmembrane AMPAR regulatory proteins (TARPs), mediate fast excitatory synaptic transmission in the mammalian brain. Here, we show by proteomic analysis that the majority of AMPARs in the rat brain are coassembled with two members of the cornichon family of transmembrane proteins, rather than with the TARPs. Coassembly with cornichon homologs 2 and 3 affects AMPARs in two ways: Cornichons increase surface expression of AMPARs, and they alter channel gating by markedly slowing deactivation and desensitization kinetics. These results demonstrate that cornichons are intrinsic auxiliary subunits of native AMPARs and provide previously unknown molecular determinants for glutamatergic neurotransmission in the central nervous system.

Association with the auxiliary subunit PEX5R/Trip8b controls responsiveness of HCN channels to cAMP and adrenergic stimulation.

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are key modulators of neuronal activity by providing the depolarizing cation current I(h) involved in rhythmogenesis, dendritic integration, and synaptic transmission. These tasks critically depend on the availability of HCN channels, which is dynamically regulated by intracellular cAMP; the range of this regulation, however, largely differs among neurons in the mammalian brain. Using affinity purification and high-resolution mass spectrometry, we identify the PEX5R/Trip8b protein as the beta subunit of HCN channels in the mammalian brain. Coassembly of PEX5R/Trip8b affects HCN channel gating in a subtype-dependent and mode-specific way: activation of HCN2 and HCN4 by cAMP is largely impaired, while gating by phosphoinositides and basal voltage-dependence remain unaffected. De novo expression of PEX5R/Trip8b in cardiomyocytes abolishes beta-adrenergic stimulation of HCN channels. These results demonstrate that PEX5R/Trip8b is an intrinsic auxiliary subunit of brain HCN channels and establish HCN-PEX5R/Trip8b coassembly as a mechanism to control the channels' responsiveness to cyclic nucleotide signaling.

NMR analysis of KChIP4a reveals structural basis for control of surface expression of Kv4 channel complexes.

Potassium channel-interacting proteins (KChIPs) are EF-hand calcium-binding proteins of the recoverin/neuronal calcium sensor 1 family that co-assemble with the pore-forming Kv4 alpha-subunits and thus control surface trafficking of the voltage-gated potassium channels mediating the neuronal I(A) and cardiac I(to) currents. Different from the other KChIPs, KChIP4a largely reduces surface expression of the Kv4 channel complexes. Using solution NMR we show that the unique N terminus of KChIP4a forms a 6-turn alpha-helix that is connected to the highly conserved core of the KChIP protein via a solvent-exposed linker. As identified by chemical shift changes, N-terminal alpha-helix and core domain of KChIP4a interact with each other through the same hydrophobic surface pocket that is involved in intermolecular interaction between the N-terminal helix of Kv4alpha and KChIP in Kv4-KChIP complexes. Electrophysiological recordings and biochemical interaction assays of complexes formed by wild-type and mutant Kv4alpha and KChIP4a proteins suggest that competition of these two helical domains for the surface groove is responsible for the reduced trafficking of Kv4-KChIP4a complexes to the plasma membrane. Surface expression of Kv4 complexes may thus be controlled by an auto-inhibitory domain in the KChIP subunit.