Neurophysiological process - GABA-A receptor life cycle

GABA-A receptor life cycle

Gamma-aminobutyric acid (GABA) receptors (GABA-A receptors) are ligand-gated ion channels that mediate fast synaptic inhibition in brain and spinal cord. These receptors are heteropentamers that can be assembled from seven subunit classes [1]. Most native GABA-A receptor subtypes consist of two alpha, two beta and one gamma subunits, e.g. GABA-A receptor alpha-1/beta-2/gamma-2 [2].

GABA-A receptors are synthesized and assembled in the Endoplasmic Reticulum (EPR). Then, GABA-A receptors undergo maturation in the Golgi and are transported to the plasma membrane through the secretory path.

Within EPR, the ubiquitin-like protein Ubiquilin-1 directly interacts with GABA-A receptor alpha and beta subunits (e.g., GABA-A receptor alpha-1 subunit and GABA-A receptor beta-2 subunit) and increases their stability. As a result, Ubiquilin-1 facilitates the insertion of GABA-A receptor into the neuronal cell surface [3], [4].

Likewise, the GABA(A) receptor-associated protein (GABARAP) binds directly to GABA-A receptor gamma-2 subunit regulating the delivery of the gamma-2-containing GABA-A receptors to the cell surface membrane. GABARAP interacts with GABA-A receptors in the EPR and Golgi structures, and, to a lesser extent, in the secretory vesicles, participating in the intracellular transport of the receptors [2], [5].

In the Golgi apparatus, GABARAP binds directly to N-ethylmaleimide-sensitive factor (NSF), a critical regulator of vesicular dependent protein trafficking [6]. NSF is a chaperone that activates SNARE proteins in membrane fusion events [1].

NSF plays an important role in regulating GABA-A receptor cell surface expression. NSF can be targeted to GABA-A receptor via 2 distinct mechanisms: indirectly, via interaction with GABARAP, which specifically binds to theGABA-A receptor gamma-2 subunit, and directly, via interaction with the receptor beta subunits (e.g., GABA-A receptor beta-2 subunit). Interaction of GABA-A receptor with GABARAP/ NSF complex can regulate the trafficking of GABA-A receptor from the Golgi to the cell surface. Direct interaction of GABA-A receptor gamma-2 subunit with NSF can regulate GABA-A receptor insertion at the plasma membrane [7].

Phospholipase C-like 1 (PLCL1) and Phospholipase C-like 2 (PLCL2) bind to GABARAP and regulate its association with GABA-A receptors in the Golgi. The formation of complexes among GABA-A receptor beta subunits (e.g., GABA-A receptor beta-2 subunit), GABARAP, and PLCL1 / PLCL2 can facilitate the association of GABARAP with the GABA-A receptor gamma-2 subunit, and thus increase the cell surface expression of the GABA-A receptor; however, the precise molecular mechanisms remain to be elucidated [8].

Synaptic GABA-A receptors reach their destination through the lateral movement in the plasma membrane, where they mediate inhibitory GABAergic neurotransmission.

Gephyrin is a Tubulin-binding protein concentrated in the postsynaptic membrane at many inhibitory synapses. Gephyrin is needed for clustering GABA-A receptors that contain theGABA-A receptor gamma-2 subunit. GABARAP can promote the recruitment of Gephyrin to the plasma membrane and organize postsynaptic GABA-A receptors by linking them to Gephyrin [2].

Endocytosis regulates the cell surface expression of GABA-A receptors. This is one of the mechanisms of the modulation of GABAergic transmission. Synaptic GABA-A receptors undergo constitutive Dynamin-dependent, Clathrin-mediated endocytosis underlying receptor recycling or degradation processes. Endocytosis process is facilitated by the adaptor protein 2 complex (AP complex 2) [8].

The unphosphorylated forms of GABA-A receptor beta subunits (e.g., GABA-A receptor beta-2 subunit) associate with the AP complex 2 musubunit (AP complex 2 medium (mu) chain), therefore GABA-A receptors are constitutively endocytosed. PLCL1 enhances the de-phosphorylation of GABA-A receptor beta subunits by acting as a scaffold protein of the protein phosphatases (e.g., PP1-cat) and regulates the phospho-dependent Clathrin/ AP complex 2 -mediated receptor endocytosis [9].

From the endosomal system GABA-A receptors are either recycled to the cell surface or degraded in the lysosomes. Another degradation system works through the proteasome after ubiquitination [8], [10].

References

1. Kneussel M
   Dynamic regulation of GABA(A) receptors at synaptic sites. Brain research. Brain research reviews 2002 Jun;39(1):74-83
2. Coyle JE, Nikolov DB
   GABARAP: lessons for synaptogenesis. The Neuroscientist : a review journal bringing neurobiology, neurology and psychiatry 2003 Jun;9(3):205-16
3. Bedford FK, Kittler JT, Muller E, Thomas P, Uren JM, Merlo D, Wisden W, Triller A, Smart TG, Moss SJ
   GABA(A) receptor cell surface number and subunit stability are regulated by the ubiquitin-like protein Plic-1. Nature neuroscience 2001 Sep;4(9):908-16
4. Saliba RS, Pangalos M, Moss SJ
   The ubiquitin-like protein Plic-1 enhances the membrane insertion of GABAA receptors by increasing their stability within the endoplasmic reticulum. The Journal of biological chemistry 2008 Jul 4;283(27):18538-44
5. Chen ZW, Olsen RW
   GABAA receptor associated proteins: a key factor regulating GABAA receptor function. Journal of neurochemistry 2007 Jan;100(2):279-94
6. Kittler JT, Rostaing P, Schiavo G, Fritschy JM, Olsen R, Triller A, Moss SJ
   The subcellular distribution of GABARAP and its ability to interact with NSF suggest a role for this protein in the intracellular transport of GABA(A) receptors. Molecular and cellular neurosciences 2001 Jul;18(1):13-25
7. Goto H, Terunuma M, Kanematsu T, Misumi Y, Moss SJ, Hirata M
   Direct interaction of N-ethylmaleimide-sensitive factor with GABA(A) receptor beta subunits. Molecular and cellular neurosciences 2005 Oct;30(2):197-206
8. Kanematsu T, Mizokami A, Watanabe K, Hirata M
   Regulation of GABA(A)-receptor surface expression with special reference to the involvement of GABARAP (GABA(A) receptor-associated protein) and PRIP (phospholipase C-related, but catalytically inactive protein). Journal of pharmacological sciences 2007 Aug;104(4):285-92
9. Kittler JT, Chen G, Honing S, Bogdanov Y, McAinsh K, Arancibia-Carcamo IL, Jovanovic JN, Pangalos MN, Haucke V, Yan Z, Moss SJ
   Phospho-dependent binding of the clathrin AP2 adaptor complex to GABAA receptors regulates the efficacy of inhibitory synaptic transmission. Proceedings of the National Academy of Sciences of the United States of America 2005 Oct 11;102(41):14871-6
10. Barnes EM Jr
    Intracellular trafficking of GABA(A) receptors. Life sciences 2000 Feb 11;66(12):1063-70