Phylogenetic analysis of ionotropic L-glutamate receptor genes in the Bilateria, with special notes on Aplysia californica

Abstract

BackgroundThe neurotransmitter L-Glutamate (L-Glu) acting at ionotropic L-Glu receptors (iGluR) conveys fast excitatory signal transmission in the nervous systems of all animals. iGluR-dependent neurotransmission is a key component of the synaptic plasticity that underlies learning and memory. During learning, two subtypes of iGluR, α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR) and N-methyl-D-aspartate receptors (NMDAR), are dynamically regulated postsynaptically in vertebrates. Invertebrate organisms such as Aplysia californica (Aplysia) are well-studied models for iGluR-mediated function, yet no studies to date have analyzed the evolutionary relationships between iGluR genes in these species and those in vertebrates, to identify genes that may mediate plasticity. We conducted a thorough phylogenetic analysis spanning Bilateria to elucidate these relationships. The expression status of iGluR genes in the Aplysia nervous system was also examined.ResultsOur analysis shows that ancestral genes for both NMDAR and AMPAR subtypes were present in the common bilaterian ancestor. NMDAR genes show very high conservation in motifs responsible for forming the conductance pore of the ion channel. The number of NMDAR subunits is greater in vertebrates due to an increased number of splice variants and an increased number of genes, likely due to gene duplication events. AMPAR subunits form an orthologous group, and there is high variability in the number of AMPAR genes in each species due to extensive taxon specific gene gain and loss. qPCR results show that all 12 Aplysia iGluR subunits are expressed in all nervous system ganglia.ConclusionsOrthologous NMDAR subunits in all species studied suggests conserved function across Bilateria, and potentially a conserved mechanism of neuroplasticity and learning. Vertebrates display an increased number of NMDAR genes and splice variants, which may play a role in their greater diversity of physiological responses. Extensive gene gain and loss of AMPAR genes may result in different physiological properties that are taxon specific. Our results suggest a significant role for L-Glu mediated responses throughout the Aplysia nervous system, consistent with L-Glu’s role as the primary excitatory neurotransmitter.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0871-1) contains supplementary material, which is available to authorized users.

Whole-transcriptome changes in gene expression accompany aging of sensory neurons in Aplysia californica

“…The NMDA receptor subunit Grin1accounted for the majority of expression and was not down regulated in AII, nor was Grin2 (Figure 5 ). GluR2 and GluR3, belonging to the AMPA subtype, and KA2, a kainate subunit [ 27 ], were significantly down regulated in AII. GluR4 and KA1 subunits were not analyzed due to a low number of counts for these two genes.…”

Metazoan evolution of glutamate receptors reveals unreported phylogenetic groups and divergent lineage-specific events

“…While the phylogeny of the two families of GluRs is well characterized in vertebrates, that of the entire animal kingdom is only poorly understood. The few studies on iGluR evolution outside vertebrates concentrate on a few phyla, leaving many proteins unclassified (Greer et al, 2017; Brockie et al, 2001; Janovjak et al, 2011; Kenny and Dearden, 2013). Similarly, the vast majority of mGluRs described so far fall into the three classes described in vertebrates (Krishnan et al, 2013; Kucharski et al, 2007; Dillon et al, 2006).…”

Altered expression of ionotropic L-Glutamate receptors in aged sensory neurons of Aplysia californica

“…Functional iGluR are formed as heterotetramers, consisting of iGluR subunits belonging to the same subtype. There is evidence that analogues of each of these vertebrate subtypes exists in Aplysia , with varying degrees of relatedness to their vertebrate counterparts [16]. The composition of Aplysia iGluR tetramers, if indeed its functional iGluR are formed as in vertebrates, is unknown.…”

“…NMDAR are the most well-conserved iGluR subtype across bilaterian species, including Aplysia [16], thus their function may be the most conserved across the bilaterian clades. NMDA elicits small amplitude excitatory currents in cultured Aplysia sensory neurons [17, 18] and application of the NMDAR antagonist 2-amino-5-phosphonopentanoic acid (APV) blocks only a minor component of L-Glu responses [19], making the contribution of NMDAR to L-Glu physiology unclear.…”