Re-defining a GABAergic neuron – GABAtransporter

The balance between excitation and inhibition is critical to the proper function of neural circuits. Aberrant activity is characteristic of numerous neurological disorders, including autism spectrum disorder, Rett syndrome, schizophrenia, and epilepsy. Inhibitory interneurons are embedded in almost all central neuronal networks where they act to influence cell excitability, spike timing, synchrony, and oscillatory activity.
GABA, the main inhibitory amino-acid neurotransmitter in mature neurons, is a remarkably multi-functional neurotransmitter: it can bind to either ionotropic GABAA (mediating fast neurotransmission) or metabotropic GABAB receptors (mediating slow neurotransmission) that may be localized extra-, peri-, pre- and postsynaptically.
The GABAergic phenotype in vertebrates and invertebrates has been defined classically by the presence of three key players in the presynaptic neurons: (i)glutamic acid decarboxylase (GAD), the enzyme needed to synthetize GABA from glutamate, (ii)the H+-coupled transporter (VGAT) that packages GABA in synaptic vesicles, and (iii)the Na+-coupled transporter (GAT) that recaptures GABA at the nerve terminal after its release in the synaptic cleft. The Caenorhabditis elegans nervous system can be considered as a “microcosm” of the GABA universe as it is much smaller, simpler, and experimentally more accessible than a vertebrate nervous system. For over 20 years, the C. elegans GABAergic nervous system was thought to be composed of only 26 out of the total 302 neurons. However, during my post-doc, I have performed an in-depth revision of the GABAergic nervous system in C. elegans. After optimizing immunohistochemistry techniques in C. elegans for GABA staining and generating several fluorescent reporters, I have significantly given new perspectives on what really define a GABAergic neuron in this model organism. In particular, my work has shown that additional neurons contain GABA but do not always express GAD/unc-25, VGAT/unc-47 and GAT/snf-11, the landmark gene portfolio for classical GABAergic neurons. Indeed, I have identified 22 new GABA-positive cells that do not conform to this classical definition and can be categorized into 4 different types of neurons expressing different combinations of these factors. Two of these types show evidence of alternative modes of GABA transport because they lack expression of known GABA transporters, VGAT/unc-47 and/or GAT/ snf-11, and they do not synthetize GABA. Moreover, in vertebrate dopaminergic neurons, similar observations hint towards the presence of alternative mechanisms for GABA transport too Deciphering these new mechanisms of GABA transport will shed light into the regulation of neural circuits through inhibition. I propose to first take advantage of C. elegans, a powerful genetic model organism, to identify and characterize new presynaptic determinants of the GABAergic neurotransmission, focusing mainly on putative and known transporters. Then, we will test their vertebrate orthologues given that the already known components are very well conserved between mammals and worms. New function for already characterized vertebrate transporters could be uncovered as it happened for the glutamate vesicular transporter 1 (VGLUT1) alias BNPI.
To achieve this goal my research project will be organized around two aims: AIM1-Novel actor(s) for GABA packaging (Identification and characterization of alternative GABA transporters for packaging GABA in synaptic vesicles) and AIM2-Novel actor(s) for GABA reuptake (Identfication and characterization of new GABA transporters at the plasma membrane, that provide an alternative supply mechanisms for GABA)
Altogether, this project aims to extend our knowledge of the cellular mechanisms underlying inhibitory neurotransmission.