Bryndis Birnir

Metabolic hormones regulate GABA signaling in the brain

It is becoming clear that metabolic hormones like insulin, glucagon-like peptide 1 (GLP-1) and IAPP (islet amyloid polypeptide) facilitate a number of brain functions including motivation and cognition. We have shown that insulin and GLP-1 regulate GABA signaling at synapses and extrasynaptic sites in the brain. And further, that insulin modulates the changing brain during development and in neurodegenerative disease such as Alzheimer’s disease (AD), by regulating the GABA signaling system in neurons. Insulin dysfunction is the principal hallmark of type 2 diabetes mellitus and metabolic dysregulation is a risk factor for cognitive decline, vascular dementia and AD. Our current and previous results on the GABA signaling system in the hippocampus, the center for memory and learning in the brain, support the concept that the effects of metabolic hormones are a critical part of normal brain development, plasticity and homeostasis in health and disease. Whether insulin/GLP-1/IAPP have a role in other brain diseases, e.g. psychiatric disorders or epilepsy, remains to be explored. But considering the wide distribution of e.g. the insulin receptor in the brain, it seems likely that normal signaling by metabolic hormones in the brain is of importance for healthy brain function and may serve to normalize imbalances in a range of brain pathologies. We have a number of on-going projects in this area.

Effects of metabolic hormones on GABAergic inhibition in mouse hippocampus. A) Schematic diagram of the experimental design (Created with Biorender.com). B) Representative GABA-activated current traces from dentate gyrus granule cells from dorsal hippocampal slices from wild-type and tg-APPSwe mice under control conditions (ACSF) and after exposure to 1 nM insulin. Upward shift of the baseline under the application of picrotoxin (PTX, 100 µM) identifies the tonic current amplitude (as the difference between the dashed lines). C) Insulin normalizes increased tonic GABA-activated extrasynaptic current density in dentate gyrus granule cells in 10-12 months old tg-APPSwe mice. The tonic extrasynaptic current density was significantly higher in tg-APPSwe mice when compared with the wild-type littermates. Significance levels are *P < 0.05, ‡P < 0.001. D) Thioflavin S staining showed the presence of neuritic plaques (bright green) in the hippocampus of tg-APPSwe AD but not wild-type mice. Scale bar = 500 µm.
DOI: 10.1111/apha.13623

Selected articles

• Insulin differentially modulates GABA signalling in hippocampal neurons and, in an age-dependent manner, normalizes GABA-activated currents in the tg-APPSwe mouse model of Alzheimer's disease. DOI: 10.1111/apha.13623
• GLP-1 and exendin-4 transiently enhance GABAA receptor-mediated synaptic and tonic currents in rat hippocampal CA3 pyramidal neurons. DOI: 10.2337/db14-0668
• GABA-activated single-channel and tonic currents in rat brain slices. DOI: 10.3791/2858

GABA channels in α-, β-, δ-pancreatic islets cells

We use intact human islets to study how GABA regulates function of the different islet cell-types. GABA is secreted from the β-cells and is in the sub-micromolar range in the interstitial fluids. GABA activates GABA channels in the cells. In type 2 diabetes the GABA channels in b-cells become supersensitive to GABA. We have further showed that a significant proportion of the cells in the human pancreatic islets express transcripts for more than one hormone e.g. insulin, glucagon, somatostatin. Now our focus is on the a cells and the mixed transcripts cells in human islets and how GABA regulates hormone secretion from these cells.

Study of GABA signaling in human pancreatic islet cells. A) Experimental design of the process of single-channel current recording from intact human islet cells followed by single-cell RT-PCR for further identification of the cell-type. The electrophysiological recordings to the right show interstitial GABA-activated currents in α and β cells with representative single-channel conductances (in pS). B) Percentage distribution of single and multiple hormone transcript-expressing cells from non-diabetes (ND) and type 2 diabetes (T2D) donors.
DOI: 10.1016/j.ebiom.2018.03.014
DOI: 10.3390/ijms21020600

Selected articles

• Functional Characterization of Native, High-Affinity GABA A Receptors in Human Pancreatic β Cells. DOI: 10.1016/j.ebiom.2018.03.014

• GABA A Receptor-Mediated Currents and Hormone mRNAs in Cells Expressing More Than One Hormone Transcript in Intact Human Pancreatic Islets. DOI: 10.3390/ijms21020600
• γ-Aminobutyric acid (GABA) signaling in human pancreatic islets is altered in type 2 diabetes. DOI: 10.1007/s00125-012-2548-7

GABA regulates T cells proliferation and cytokine secretion

It was in 2008 that the Birnir Lab demonstrated, for the first time, that high-affinity GABA channels regulate T cells and in 2018, we showed that GABA regulates both proliferation and cytokine secretion from immune cells. We showed that GABA at physiological concentrations is a potent regulator of secretion of both pro- and anti-inflammatory cytokines from peripheral blood mononuclear cells (PBMCs) and T cells, from both healthy and Type 1 diabetes (T1D) donors. Importantly, we identified GABA responder and non-responder cells. Non-responder cells are not inhibited by GABA in the proliferation assay and often already secrete substantial levels of cytokines. Responder cells are those that show decreased proliferation in the presence of GABA. We revealed that the low sub-micromolar GABA concentrations are present in blood from normal donors (ND) and T1D, T2D and psychiatric patients. Nevertheless, the molecular mechanism of GABA modulation of the immune cells e.g. how the GABA regulation of proliferation and cytokine secretion comes about, how the regulation varies among subtypes of T cells and what determines if a cell is a responder or non-responder is not known today and this is what we are focusing on in the Lab.

GABA regulates T cells proliferation and cytokine secretion. A) Schematic diagram of the experimental design and time line (Created with Biorender.com). B) Effect of GABA on proliferation of CD4+ T cells from ND donors identifying GABA non-responder and responder populations of CD4+ T cells. Significance level is ***P < 0.001. C) PEA data summary of cytokine profile regulated by GABA and T1D. DOI: 10.1016/j.ebiom.2018.03.019

Selected articles

• GABA Regulates Release of Inflammatory Cytokines from Peripheral Blood Mononuclear Cells and CD4 +T Cells and Is Immunosuppressive in Type 1 Diabetes. DOI: 10.1016/j.ebiom.2018.03.019
• Depression, GABA, and Age Correlate with Plasma Levels of Inflammatory Markers. DOI: 10.3390/ijms20246172
• GABA, a natural immunomodulator of T lymphocytes. DOI: 10.1016/j.jneuroim.2008.08.017

Cancer and relevance of GABA signaling

GABA and GABA receptors (GABAA and GABAB) have been identified in different types of tumor tissues including glioma, pancreatic cancer and breast cancer. The GABA signaling is implicated in cancer cell proliferation and migration as well as immune response within the tumor microenvironment. We have shown that the expression of GABAA r2 subunit is associated with patient survival in diffuse astrocytomas. In addition, we have detected functional GABAA receptors in a human glioblastoma derived cell line that are modulated by GABAergic agents such as diazepam, propofol and etomidate. This suggest GABA signaling is a potential therapeutic target and we are continuing to investigate the mechanism underlying GABA modulation of cancer cells and infiltrated immune cells.

Selected articles

GABA-A channel subunit expression in human glioma correlates with tumor histology and clinical outcome. DOI: 10.1371/journal.pone.0037041

• Etomidate, propofol and diazepam potentiate GABA-evoked GABAA currents in a cell line derived from human glioblastoma. DOI: 10.1016/j.ejphar.2014.12.001

ENABLE

We are part of the in vivo/safety platform -forming the ion channel platform.

Effects of test compounds on hERG, Nav1.5, Cav1.2 and GABAAR-mediated currents measured by whole-cell patch clamp recordings.

In vitro pharmacological profiling involves the screening of compounds against a broad range of targets such as ion channels, transporters and receptors that are different from the intended therapeutic target (or targets). Ion channels have prominent role in the physiology of all cell-types and drug interactions with ion channels have long been associated with undesired effects. Ion channels of high pharmacological relevance for drug developments are: voltage-gated potassium channel subfamily H member 2 (KCNH2; also known as hERG), voltage-gated sodium (Nav1.5), calcium (Cav1.2), potassium (KCNQ1) channels and ligand-gated receptors like GABAA receptors that are ion channels opened by the neurotransmitter GABA. These different ion channels are very important for i.e. cardiac, neuronal and endocrine functions. Within the consortium we have tested if newly developed compounds interact with these ion channels.

http://nd4bb-enable.eu/consortium

ENABLE: an engine for European antibacterial drug discovery and development https://www.nature.com/articles/d41573-021-00074-y

Methods

More info on established methods, instruments and techniques available in the group.