Programme Contents

Functional consequences of clustering GABA_A receptors

Inhibitory signals in mammalian brains are mediated primarily by γ-aminobutyric acid type A receptors (GABA_AR). Different subtypes of these receptors, distinguished by their subunit composition, are either concentrated at postsynaptic sites where they mediate phasic inhibition or found at non-synaptic (extrasynaptic) sites where they mediate tonic inhibition. Neurons, therefore, require discrete trafficking mechanisms to regulate the subcellular distribution of GABA_AR subtypes. Although its precise role in vivo is yet to be clearly defined, the GABA_A receptor-associated protein GABARAP has been shown to participate in trafficking and membrane fusion events that underlie organisational processes at GABAergic synapses (Kittler et al., 2001; Kneussel, 2002). Co-expression of GABARAP has been shown to cluster recombinant GABA_A receptors (Chen et al., 2000; Everitt et al., 2004) and, as a consequence of this ordered packing arrangement, the recombinant GABA_A receptors function differently. At the single-channel level we have shown that GABA_A receptors co-expressed with recombinant GABARAP in L929 cells may display high conductances (>40 pS) (Everitt et al., 2004) which is in stark contrast to the conductance of αβγ receptors expressed without GABARAP (<40 pS) (Luu et al., 2005). Single-channel amplitude distribution histograms and open probabilities were analysed to examine the effects of drugs such as GABA and diazepam. The presence of the soluble intracellular protein GABARAP influences recombinant GABA_A channels such that they may open to multiple discrete states and the maximum single-channel conductance is dependent on the effective GABA concentration. It remains a fundamental issue as to what these multiple conductance states represent. Is it a single channel or multiple channels opening in concert? An important conclusion is that recombinantly expressed αβγ receptors behave more like native receptors when cotransfected with GABARAP.

Chen, L., Wang, H.B., Vicini, S. & Olsen, R.W. (2000) Proceedings of the National Academy of Sciences of the United States of America, 97, 11557-11562.

Everitt, A.B., Luu, T., Cromer, B., Tierney, M.L., Birnir, B., Olsen, R.W. & Gage, P.W. (2004) Journal of Biological Chemistry, 279(21), 21701-21706.

Kittler, J.T., Rostaing, P., Schiavo, G., Fritschy, J.M., Olsen, R., Triller, A. & Moss, S.J. (2001) Molecular and Cellular Neurosciences, 18, 13-25.

Luu, T., Cromer, B.C., Gage, P.W. & Tierney, M.L. (2005) Journal of Membrane Biology, 205, in press.

Kneussel, M. (2002) Brain Research - Brain Research Reviews, 39, 74-83.