Glutamate, glutamate receptors, and downstream signaling pathways

Abstract

Glutamate is a nonessential amino acid, a major bioenergetic substrate for proliferating normal and neoplastic cells, and an excitatory neurotransmitter that is actively involved in biosynthetic, bioenergetic, metabolic, and oncogenic signaling pathways. Glutamate signaling activates a family of receptors consisting of metabotropic glutamate receptors (mGluRs) and ionotropic glutamate receptors (iGluRs), both of which have been implicated in chronic disabling brain disorders such as Schizophrenia and neurodegenerative diseases like Alzheimer's, Parkinson's, and multiple sclerosis. In this review, we discuss the structural and functional relationship of mGluRs and iGluRs and their downstream signaling pathways. The three groups of mGluRs, the associated second messenger systems, and subsequent activation of PI3K/Akt, MAPK, NFkB, PLC, and Ca/CaM signaling systems will be discussed in detail. The current state of human mGluR1a as one of the most important isoforms of Group I-mGluRs will be highlighted. The lack of studies on the human orthologues of mGluRs family will be outlined. We conclude that upon further study, human glutamate-initiated signaling pathways may provide novel therapeutic opportunities for a variety of non-malignant and malignant human diseases.

Keywords: GRM1a; Glutamate; iGluR; mGluR; signaling..

Fig 1

iGluR structure and activation. iGluRs share the same basic structure: Beginning at the N terminus (N), the amino terminal domain (ATD) is followed by the ligand binding domain (LBD), the four transmembrane domains (TMD; numbered 1-4) and the C-terminal domain (CTD). Panel A illustrates the native resting conformation of iGluR. Only two subunits are shown, but the entire molecule consists of four subunits which are arranged in such a way as to create a channel through the plasma membrane. Upon Glu binding (Panel B), a conformational change in TMD domain 1 causes opening of the channel where Ca²⁺ passes into the cell by diffusion.

Fig 2

The amino acid sequence homology among various metabotropic glutamate receptors. The phenogram in panel A illustrates the relationship between the three mGluR subgroups (Group I, II and III) and the individual receptors within each group. Branch length is indicative of amino acid identity between the protein sequences. The table in panel B illustrates the percent amino acid identity between all receptor pairwise combinations. Note that the highest similarity is observed between members of the same group (i.e. mGluR1 vs. mGluR5). The phenogram in panel A and the table in panel B were generated using the DNAStar software suite (DNASTAR, Inc., Madison, WI, USA) and amino acid sequences obtained from NCBI. Accession numbers: mGluR1a = NP_000829.2, mGluR2 = EAW65150.1, mGluR3 = NP_000831.2, mGluR4 = NP_000832.1, mGluR5a = NP_001137303.1, mGluR6 = AAB82068.1, mGluR7a = EAW63933.1, and mGluR8a = EAW83621.1.

Fig 3

Amino acid sequence alignment of the eight mGluR genes. In this analysis, the “a” isoform of genes encoding multiple isoforms was used for simplicity. This figure was generated using the DNAStar software suite (DNASTAR, Inc., Madison, WI, USA) using the NCBI data base. NCBI accession numbers: mGluR1a = NP_000829.2, mGluR2 = EAW65150.1, mGluR3 = NP_000831.2, mGluR4 = NP_000832.1, mGluR5a = NP_001137303.1, mGluR6 = AAB82068.1, mGluR7a = EAW63933.1, and mGluR8a = EAW83621.1. Colored blocks indicate the following domains: Green, amino-terminal domain (ATD); blue, cysteine-rich domain (CRD); yellow, transmembrane-spanning domains (TMD); red, carboxy-terminal domain (CTD).

Fig 4

mGluR structure and conformational activation. mGluR family members share the same basic structure: Beginning at the N terminus (N), the amino terminal domain (ATD) is followed by the cysteine rich domain (CRD), the seven transmembrane domains (TMD; numbered) and the C-terminal domain (CTD). i1, i2, etc., indicate intracellular loops between membrane-spanning regions while e1, e2, etc., indicate extracellular loops. The star at loop e2 denotes the cysteine residue that is important in transmitting activation information to the docked G protein complex (see text for detail). Panel A illustrates the native resting conformation of mGluR1a. Cysteine residues in the CRD aid in dimerization, as indicated by the S-S bond. Upon Glu binding (Panel B), a conformational change in the ATD involving the CRD (stars indicate disulfide bridges created by Glu binding) causes activation of bound G proteins (see text for details).

Fig 5

mGluR1 isoforms. This schematic represents the 5 human mGluR1 isoforms including relevant protein domains and amino acid (aa) lengths. Information on isoforms c, d and e were obtained from the literature -, -, while domain information for isoforms “a” and “b” was obtained using the NCBI “conserved domain search” function at: http://www.ncbi.nlm.nih.gov/Structure/cdd/wrpsb.cgi. NCBI accession numbers: mGluR1a = NP_000829.2 and mGluR1b = NP_001107801.1. Domain names are as follows: LBD/ATD = ligand binding domain/amino terminal domain, C = cysteine rich domain, TMD = transmembrane domain, CTD = C-terminal domain, H = Homer 1-binding motif.

Fig 6

A comparison of rodent and human mGluR1a at the amino acid level. The table in panel A illustrates the results of an lalign analysis of mGluR1a from Mus musculus (mouse) and Rattus norveticus (Norway rat). William Pearson's lalign program was used with default settings, which can be found at http://www.ch.embnet.org/software/LALIGN_form.html. The percent amino acid identity is shown. The highest amino acid identity is found in the transmembrane domain (TMD) and the lowest in the cytoplasmic tail domain (CTD). CRD = cysteine rich domain, ATD = amino terminal domain. Panels B and C display the lalign result of a comparison between human and rat CRD and human and mouse CRD, respectively. Note that all cysteine amino acids are conserved, but that a number of differences are found in surrounding amino acids. Between the two proteins, a “:” indicates completely identical while “.” represents a different amino acid.

Fig 7

G-protein signaling pathways activated by mGluR1a. This drawing represents an overview of G-protein dependent signaling pathways that are activated upon Glu binding to mGluR1a. Following receptor activation and downstream of G-protein dissociation, the following second messenger systems are activated in the nervous system: PKB, PLCγ, PI3K/AKT/mTorC1 and 2, IP3/DAG, NFκB and CaM. Please see the text for detailed description on these signaling pathways.