Mechanisms of CPT1C-Dependent AMPAR Trafficking Enhancement.

Esther Gratacòs-Batlle,Natalia Yefimenko,Nuria Sánchez-Fernández,Mireia Olivella,Federico Miguez-Cabello,Santiago Ambrosio,Núria Casals,Rut Fadó,Xavier Gasull,David Soto

doi:10.3389/fnmol.2018.00275

Abstract

In neurons, AMPA receptor (AMPAR) function depends essentially on their constituent components:the ion channel forming subunits and ion channel associated proteins. On the other hand, AMPAR trafficking is tightly regulated by a vast number of intracellular neuronal proteins that bind to AMPAR subunits. It has been recently shown that the interaction between the GluA1 subunit of AMPARs and carnitine palmitoyltransferase 1C (CPT1C), a novel protein partner of AMPARs, is important in modulating surface expression of these ionotropic glutamate receptors. Indeed, synaptic transmission in CPT1C knockout (KO) mice is diminished supporting a positive trafficking role for that protein. However, the molecular mechanisms of such modulation remain unknown although a putative role of CPT1C in depalmitoylating GluA1 has been hypothesized. Here, we explore that possibility and show that CPT1C effect on AMPARs is likely due to changes in the palmitoylation state of GluA1. Based on in silico analysis, Ser 252, His 470 and Asp 474 are predicted to be the catalytic triad responsible for CPT1C palmitoyl thioesterase (PTE) activity. When these residues are mutated or when PTE activity is inhibited, the CPT1C effect on AMPAR trafficking is abolished, validating the CPT1C catalytic triad as being responsible for PTE activity on AMPAR. Moreover, the histidine residue (His 470) of CPT1C is crucial for the increase in GluA1 surface expression in neurons and the H470A mutation impairs the depalmitoylating catalytic activity of CPT1C. Finally, we show that CPT1C effect seems to be specific for this CPT1 isoform and it takes place solely at endoplasmic reticulum (ER). This work adds another facet to the impressive degree of molecular mechanisms regulating AMPAR physiology.

Highlights

Amongst ionotropic glutamate receptors, α-amino-3hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are considered the ‘‘work-horses’’ of fast excitatory neurotransmission in the brain since they mediate nearly 90% of synaptic transmission
In hippocampal neurons (HPNs) GluA1/GluA2 is the dominant AMPAR arrangement (Pellegrini-Giampietro et al, 1994; Gold et al, 1996; Sans et al, 2003) and in carnitine palmitoyltransferase 1C (CPT1C) KO animals the number of AMPARs at hippocampal neuron synapses seems to be reduced (Fadó et al, 2015)
Since GluA1 is importantly expressed in other brain areas, we first decided to study the ability of CPT1C to interact with GluA1 subunits in different brain tissues

Summary

Introduction

Α-amino-3hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are considered the ‘‘work-horses’’ of fast excitatory neurotransmission in the brain since they mediate nearly 90% of synaptic transmission. The presence or absence within the receptor of a given AMPAR subunit is an important determinant of its properties, which will translate into differential integration of the signals at postsynaptic sites. In the last decade, the AMPAR field has experienced a fascinating step forward due to the discovery of AMPAR auxiliary subunits, which are important modulators of AMPAR function (Yan and Tomita, 2012; Greger et al, 2017). There are several transmembrane proteins belonging to different families that have been described to command AMPAR function: transmembrane AMPAR regulatory proteins (TARPs), Cornichon homologs (CNIHs), Cystine-knot AMPAR modulating proteins (CKAMPs) and the more recently discovered GSG1L protein (Straub and Tomita, 2012; Farrow et al, 2015; McGee et al, 2015)

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