Autoimmune Responses to the GABA Receptor Katherine Saunders

Autoimmune Responses to the GABA Receptor Katherine Saunders, Markus Nitka, Matt Jokel, Madison Mc. Avoy

GABA Receptor • GABA is the primary inhibitory neurotransmitter in the adult brain • GABA binds to post-synaptic GABA receptors and inhibits the transmission of an action potential • Medications that bind to the GABA receptor are classified as benzodiazepines • Recent studies have shown that the immune system may synthesize and release GABA to exude effects on immune cells

GABA in the Immune System • Found in lymph nodes, islets of Langerhans, blood • Also encountered when immune cells enter brain • Enzymes for synthesis found in T cells, macrophages, dendritic cells • Receptors + transporters also expressed • Paracrine and/or autocrine effect • Implicated in MS, T 1 D, rheumatoid arthritis, etc. • Main effects related to interaction of GABA with GABA-A receptor on cell surface

GABA receptor types: GABA-A receptor: • A pentameric chloride ion channel • Five subunits: • 2α, 2β and the fifth subunit varies • Subtypes expressed in different regions of the brain • Hyperpolarization of cell from Cl- influx • Identified on CD 4+ and CD 8+ T cells, as well as macrophages • Inhibit cytokine secretion, cell proliferation • Impaired phagocytic activity and chemotaxis Graphic from: Jacob, T. C. , et al. (2009) GABAA receptor structure and neuronal localization. Nature Review Neuroscience. 9(5): 331 -343

GABA receptor types (cont. ): GABA-B receptor: • G-protein coupled receptor • No indication in immune response GABA co-transporter (GAT): • Present on GABAergic neurons • Four isoforms: GAT 1 -4 • Only GAT 1 and GAT 2 identified on T lymphocytes • Responsible for the reuptake of GABA to balance neurotransmitter levels within the synaptic cleft Graphic from: Rowley, N. M. et al. , (2012) Cartoon summarizing the synthesis, packaging, release, transport, and metabolism of glutamate and GABA. Neurochemistry International. 61(4): 546 -558

Mechanism of GABA on GABA receptors located on immune cells: • Glutamate is decarboxylated by glutamic acid decarboxylase 65 and 67 (GAD 65/67) to produce GABA in the GABAnergic neuron • Newly formed GABA is packaged into synaptic vesicles and released into the synapse • GABA enters immune cells via GABA-A receptor • Leftover synaptic GABA taken back via GAT • Maintain lowconcentration extracellular GABA Graphic from: Jin, Z. et al. , (2013) Schematic figure showing the various components of the GABA signalling system. Amino Acids. 45(1): 87 -94

GABA Downregulates Inflammatory Response • GABA-A-R agonism decreases cytokine secretion and T cell proliferation • Reduces transient increase in lymphocyte [Ca] associated with cell activation • Cl influx affects T cell polarization – effects on activity and proliferation • GABA inhibits TCR-mediated cell cycling • T cells remain in G 0/G 1 phase • GAT-1 knockout results in increased T cell proliferation and IFN-γ production • GABAergic agents act directly on APCs to decrease MAPK signals • reduces cytokine production in macrophages, subsequent inflammation • Also reduces cytotoxic immune response, T cell autoimmunity, DTH reactions

GABA Enhances Antimicrobial Response in Macrophages • Intracellular bacterial infection decreases GABA levels within macrophages • GABA treatment promotes autophagy activation and phagosome maturation • GABA-A-R agonism: intracellular calcium increase, AMPK signaling • Intracellular calcium release (not influx) • AMPK signaling upregulates transcription of GABA type A receptor-associated protein-like 1 (GABARAPL 1) • autophagosome formation, autolysosome maturation Graphic from: Kim, J. K. et al. (2018) Schematic figure showing GABA signaling pathway in macrophages. Nature Communications, 9(1): 1– 17

GABARAPL 1 Associates with autophagosomes and lysosomes to facilitate autolysosome formation and autophagy Graphic from: Kim, J. K. et al. (2018) Schematic figure showing GABA signaling pathway in macrophages. Nature Communications, 9(1): 1– 17

Role of GABA in Autoimmune Disease • Immune system is capable of synthesizing and releasing GABA • Inhibitory neurotransmitter in the brain and has parallel inhibitory role in the immune system • GABA treatment decreases inflammatory cytokine production in peripheral macrophages • GABA and GABA-A receptor agonists decrease cytotoxic immune responses and cutaneous delayed-type hypersensitivity reactions

GABA in Multiple Sclerosis (MS) • MS is a T cell mediated autoimmune disorder • GABA does not pass through BBB therefore cannot inhibit spreading of autoreactivity in the CNS • Need for BBB-permeable GABA receptor agonists • Homotaurine (BBB-permeable amino acid)

Future Directions • Potential for GABA as a target for therapeutic treatment of chronic bacterial infections • Important to determine the specific effects and mechanisms GABA exerts on different immune cells • May be clinically significant to identify GABA-A channel subtypes expressed in the immune cells

Summary • GABA is the primary inhibitory neurotransmitter that works by binding to post-synaptic GABA receptors and inhibiting the transmission of an action potential • GABA affects the immune system via interaction with the GABA-A receptor on immune cell surfaces • In T cells, this reduces cytokine secretion and cell proliferation via intracellular [Ca] decrease and entrance into G 0/G 1 • In macrophages, this enhances antimicrobial responses by upregulating expression of GABARAPL 1, a protein required for autophagosome formation

Works Cited • Kim, J. K. , Kim, Y. S. , Lee, H. M. , Jin, H. S. , Neupane, C. , Kim, S. , … Jo, E. K. (2018). GABAergic signaling linked to autophagy enhances host protection against intracellular bacterial infections. Nature Communications, 9(1), 1– 17. https: //doi. org/10. 1038/s 41467 -018 -06487 -5 • Bhat, R. , Axtell, R. , Mitra, A. , Miranda, M. , Lock, C. , Tsien, R. W. , & Steinman, L. (2010). Inhibitory role for GABA in autoimmune inflammation. Proceedings of the National Academy of Sciences of the United States of America, 107(6), 2580– 2585. https: //doi. org/10. 1073/pnas. 0915139107 • Tian, J. , Lu, Y. , Zhang, H. , Chau, C. H. , Dang, H. N. , & Kaufman, D. L. (2004). γ-Aminobutyric Acid Inhibits T Cell Autoimmunity and the Development of Inflammatory Responses in a Mouse Type 1 Diabetes Model. The Journal of Immunology, 173(8), 5298– 5304. https: //doi. org/10. 4049/jimmunol. 173. 8. 5298 • Jin, Z. , Mendu, S. K. , & Birnir, B. (2013). GABA is an effective immunomodulatory molecule. Amino Acids. Springer. https: //doi. org/10. 1007/s 00726 -011 -1193 -7 • Tian, J. , Dang, H. , Wallner, M. , Olsen, R. , and Kaufman, D. L. (2018). Homotaurine, a safe blood-brain barrier permeable GABAA-R-specific agonist, ameliorates disease in mouse models of multiple sclerosis. Sci Rep. 8; 8(1): 16555. • Buddhala C, Hsu CC, Wu JY. A novel mechanism for GABA synthesis and packaging into synaptic vesicles. Neurochem Int. 2009 Jul-Aug; 55(13): 9 -12. doi: 10. 1016/j. neuint. 2009. 01. 020. Epub 2009 Feb 6. PMID: 19428801. • Jacob, T. C. , Moss, S. J. , & Jurd, R. (2008). GABA(A) receptor trafficking and its role in the dynamic modulation of neuronal inhibition. Nature reviews. Neuroscience, 9(5), 331– 343. https: //doi. org/10. 1038/nrn 2370 •