AMPA glutamate receptors are required for sensory-organ formation and morphogenesis in the basal chordate

Abstract

AMPA-type glutamate receptors (GluAs) mediate fast excitatory transmission in the vertebrate central nervous system (CNS), and their function has been extensively studied in the mature mammalian brain. However, GluA expression begins very early in developing embryos, suggesting that they may also have unidentified developmental roles. Here, we identify developmental roles for GluAs in the ascidian Ciona intestinalis. Mammals express Ca2+-permeable GluAs (Ca-P GluAs) and Ca2+-impermeable GluAs (Ca-I GluAs) by combining subunits derived from four genes. In contrast, ascidians have a single gluA gene. Taking advantage of the simple genomic GluA organization in ascidians, we knocked down (KD) GluAs in Ciona and observed severe impairments in formation of the ocellus, a photoreceptive organ used during the swimming stage, and in resorption of the tail and body axis rotation during metamorphosis to the adult stage. These defects could be rescued by injection of KD-resistant GluAs. GluA KD phenotypes could also be reproduced by expressing a GluA mutant that dominantly inhibits glutamate-evoked currents. These results suggest that, in addition to their role in synaptic communication in mature animals, GluAs also have critical developmental functions.

Parallel Visual Circuitry in a Basal Chordate

“…Finally, we also observed that the touch response was not inhibited by perampanel (data not shown), despite the presence of VGLUTpositive epidermal sensory neurons [22]. This would appear to agree with the observation that primary RNs for the PNS, the eminens cells and the AMGs do not express the AMPAR ( [35]; and our observations). In addition to the AMPAR, the Ciona genome contains several other glutamate receptors including one kainate and one NMDA [34], although their expression has not been characterized.…”

“…Our results indicate that the PR-Is, with the exception of two cells, are glutamatergic, while the PR-IIs are a mixture of GABAergic and GABA/glutamatergic. The Ciona genome contains a single glutamate AMPA receptor (AMPAR) [34] that is expressed in larvae in the two antenna cells, and in a small cluster of neurons in the pBV [35]. Published results show that most of the pBV group of AMPAR-positive neurons are clustered at the ends of Arrestin-labeled photoreceptor axons, and that they extend their axons to the MG, suggesting they are photoreceptor RNs (see Figure 2B" in [35]).…”

“…We find that this pBV group is composed of ~6 cells ( Figure S5). To investigate this further, we co-expressed an pAMPAR>GFP construct [35] with pVACHT>CFP and pVGAT>nuclear-RFP constructs. We observed coexpression of the AMPAR reporter in a subset of the VACHT-postive RNs, but never in the VGAT-expressing RNs (Figure 5a).…”

Developmental Roles and Evolutionary Significance of AMPA‐Type Glutamate Receptors

“…First, GluAs are either Ca 2+ ‐permeable or ‐impermeable according to their subunit composition in mammals, while all GluAs are Ca 2+ ‐permeable in Ciona because they consist of only Q‐type subunits, which form Ca 2+ ‐permeable channels . Second, we found that GluAs are expressed in a limited subset of cells, including those in the photoreceptive anlage, during early development in Ciona , whereas they are expressed in most neurons in the mammalian CNS . Ascidian tadpoles have an otolith and ocellus to sense gravity and light, respectively.…”

“…On the other hand, Okamura et al estimated that only one GluA subunit is expressed in the ascidian Ciona , indicating that it is easier to artificially control GluAs in these organisms than in mammals. Taking advantage of the simplicity of ascidians as model animals, Hirai et al discovered the significance of GluAs in development (Figure ). Ascidians have a notochord and exhibit neural induction, and are therefore a prototype of vertebrate embryos .…”