Evolution of glutamatergic signaling and synapses

Highlights

• Glutamate (Glu) signaling might be one of the most ancient means of cellular communications.

• Glu signaling can be traced to the early injury/stress responses in the first cells.

• >20 classes of iGluRs have been identified in the major eukaryotic lineages.

• The complexity of Glu receptors and signaling correlates with multiple origins of multicellularity and neural organization.

• Glutamatergic synapses might have evolved more than once.

Abstract

Glutamate (Glu) is the primary excitatory transmitter in the mammalian brain. But, we know little about the evolutionary history of this adaptation, including the selection of l-glutamate as a signaling molecule in the first place. Here, we used comparative metabolomics and genomic data to reconstruct the genealogy of glutamatergic signaling. The origin of Glu-mediated communications might be traced to primordial nitrogen and carbon metabolic pathways. The versatile chemistry of L-Glu placed this molecule at the crossroad of cellular biochemistry as one of the most abundant metabolites. From there, innovations multiplied. Many stress factors or injuries could increase extracellular glutamate concentration, which led to the development of modular molecular systems for its rapid sensing in bacteria and archaea. More than 20 evolutionarily distinct families of ionotropic glutamate receptors (iGluRs) have been identified in eukaryotes. The domain compositions of iGluRs correlate with the origins of multicellularity in eukaryotes. Although L-Glu was recruited as a neuro-muscular transmitter in the early-branching metazoans, it was predominantly a non-neuronal messenger, with a possibility that glutamatergic synapses evolved more than once. Furthermore, the molecular secretory complexity of glutamatergic synapses in invertebrates (e.g., Aplysia) can exceed their vertebrate counterparts. Comparative genomics also revealed 15+ subfamilies of iGluRs across Metazoa. However, most of this ancestral diversity had been lost in the vertebrate lineage, preserving AMPA, Kainate, Delta, and NMDA receptors. The widespread expansion of glutamate synapses in the cortical areas might be associated with the enhanced metabolic demands of the complex brain and compartmentalization of Glu signaling within modular neuronal ensembles.

This article is part of the Neuropharmacology Special Issue on ‘Glutamate Receptors - The Glutamatergic Synapse’.

Introduction

It is estimated that over 99 % of all synapses in the mammalian brain use chemical transmission vs. electrical coupling by gap junctions (Greengard, 2001). There are more than 20 low molecular weight neurotransmitters. But, it is surprising that most mammalian brain synapses use l-glutamate (Glu) as an excitatory neurotransmitter (Micheva et al., 2010). Why did Glu ‘conquer’ such a special place in our brain? Understandably, about half of the neuroscientists today work with different aspects of Glu signaling.

Forty years ago, the ‘glutamate’ revolution in neuroscience was underway with the landmark review summarizing synaptic functions of amino acids (Watkins and Evans, 1981). By May 2021, PubMed contained >170,000 papers describing different aspects of glutamate chemistry and biology. ~95,000 reports related to neural functions of glutamate, and ~55,000 are related to glutamatergic (Glu-mediated) neurotransmission. Nonetheless, most of these efforts were focused on mammals, with a smaller fraction of studies using other vertebrates and two basal chordate lineages (tunicates and cephalochordates).

In contrast, there are ‘only’ ~650 papers directly or indirectly associated with the transmitter functions of Glu in invertebrates. Most of these researchers are using Drosophila (~250 articles), C. elegans (~150), Aplysia, and its kin (~150) as models; plus ~50 papers are related to other worms. These model species represent just four (out of 35) extant animal phyla: Arthropoda, Nematoda, Mollusca, Annelida.

Even these limited comparative examples revealed non-canonical solutions for the use of Glu, which are remarkably different from what is described in classical neuroscience textbooks. In invertebrates, Glu often acts as an inhibitory neurotransmitter (Cleland, 1996; Kehoe et al., 2009; Kehoe, 2000) using pentameric cys-loop receptor channels (Cymes and Grosman, 2021; Jaiteh et al., 2016; Kehoe et al., 2009; Lynagh et al., 2015). l-glutamate operates as the neuromuscular transmitter in most animals on the planet (e.g., in arthropods (Jan and Jan, 1976) and nematodes (Dent et al., 1997); fewer neuro-muscular glutamatergic synapses were identified in molluscs (Fox and Lloyd, 1999). But, how and why did we (mammals, vertebrates) arrive at using acetylcholine as a neurotransmitter for our somatic muscles? At least some glutamatergic synapses in molluscs are molecularly more complex than in our brain (see below). Finally, d-glutamate might also act as a signaling molecule, in addition to d-aspartate (Moroz et al., 2020b).

We know virtually nothing about Glu-signaling in the remaining 30 animal phyla. But these phyla represent the largest and the most incredibly diverse forms of neuronal organization and functions (Bracken-Grissom et al., 2014; Brusca and Brusca, 2003; Bullock and Horridge, 1965; Mackie, 1990; Moroz, 2015a, 2018; Nielsen, 2012).

The growing comparative genomic and other ‘omics’ data only recently started to uncover multiple paths of recruiting this ancestral molecule in signaling functions, which possibly has over 3.5 billion years of history. This manuscript emphasizes comparative aspects of Glu-signaling. We will start with a brief history and later focus on molecular components of Glu-mediated transmission in animals and other eukaryotes. The universal core of cellular metabolism and nitrogen utilization in living systems (Smith and Morowitz, 2004) paved the way for selecting Glu as a ubiquitous extracellular messenger in all domains of life. The recruitment of Glu into different forms of neural organization (from chemosensation to cognitive functions) might also be based upon constraints impeded by the energetic demands of metabolism and the chemical properties of this versatile signal molecule. It would be impossible to cover the multifaced evolution of the versatile Glu signalings across time and species in a single manuscript. Thus, we will emphasize the major transitions reflecting the long recruitment history of Glu into the transmitter and integrative functions.