The role of GABA in islet function

Abstract

Gamma aminobutyric acid (GABA) is a non-proteinogenic amino acid and neurotransmitter that is produced in the islet at levels as high as in the brain. GABA is synthesized by the enzyme glutamic acid decarboxylase (GAD), of which the 65 kDa isoform (GAD65) is a major autoantigen in type 1 diabetes. Originally described to be released via synaptic-like microvesicles or from insulin secretory vesicles, beta cells are now understood to release substantial quantities of GABA directly from the cytosol via volume-regulated anion channels (VRAC). Once released, GABA influences the activity of multiple islet cell types through ionotropic GABA_A receptors and metabotropic GABA_B receptors. GABA also interfaces with cellular metabolism and ATP production via the GABA shunt pathway. Beta cells become depleted of GABA in type 1 diabetes (in remaining beta cells) and type 2 diabetes, suggesting that loss or reduction of islet GABA correlates with diabetes pathogenesis and may contribute to dysfunction of alpha, beta, and delta cells in diabetic individuals. While the function of GABA in the nervous system is well-understood, the description of the islet GABA system is clouded by differing reports describing multiple secretion pathways and effector functions. This review will discuss and attempt to unify the major experimental results from over 40 years of literature characterizing the role of GABA in the islet.

Keywords: beta cell; insulin; islet; pancreas; receptor; signaling; γ-Aminobutyric acid (GABA).

Figure 1

GABA in the whole islet. Application of exogenous GABA has various effects on the islet including stimulation of beta cell regeneration, inhibition of insulin secretion, and negative regulation of immune cells. Endogenous GABA levels are highly enriched in the islet, as high as in the brain, and GABA is synthesized in and secreted from the beta cells. Immunofluorescence image depicts a human islet. GABA is secreted via multiple pathways that are both regulated and unregulated by glucose and with pulsatile, tonic, or phasic dynamics. Once secreted, GABA acts via GABA_AR ligand-gated chloride channels and GABA_AR inhibitory G protein coupled receptors. Set by the chloride equilibrium potential, in beta cells GABA_AR signaling can be excitatory in low glucose and inhibitory in high glucose, while in alpha cells GABA_AR signaling is inhibitory. GABA_BR signaling is also inhibitory but may only be active in mouse and not human beta cells under typical physiological conditions. Created with BioRender.com.

Figure 2

Intracellular trafficking pattern of GAD in beta cells. The two GABA-synthesizing enzymes, GAD65 and GAD67, are synthesized as cytosolic proteins. GAD65 becomes associated with the cytosolic face of ER and Golgi membranes and subsequently modified by double palmitoylation which allows forward trafficking onto post-Golgi intracellular vesicles. The palmitoylation step is reversible and serves to regulate localization of GABA synthesis between cytosolic and membrane-anchored pools of enzyme. GAD67 is the predominant isoform expressed in mouse beta cells while only GAD65 is expressed in human beta cells. In human beta cells GAD65 localizes to Golgi membranes and cytosolic vesicles that are distinct from insulin containing vesicles. Created with BioRender.com.

Figure 3

Tonic and phasic islet GABA. There is evidence favoring both a constant background secretion of GABA that is unregulated by glucose and a vesicular secretion of GABA from the insulin secretory vesicles. In analogy to synaptic and tonic GABA in neurons, vesicle released GABA in beta cells is proposed to trigger phasic Cl^- currents via low affinity GABA_ARs. Tonic Cl^-currents from higher affinity GABA_AR subtypes provide a constant current in response to continuously replenished interstitial GABA. Example GABA_AR current traces in human beta cells from Braun et al. 2010 (76) (with permission) where both tonic and phasic currents were shown to be sensitive to the GABA_AR antagonist SR-95531. Created with BioRender.com.

Figure 4

Effect of GABA_AR chloride channels on beta cell membrane potential. When membrane potential is below the Cl^- equilibrium potential (E_Cl-), endogenous GABA_AR activation contributes to beta cell depolarization and can trigger action potentials in threshold-activated beta cells. Once the beta cell membrane becomes depolarized past E_Cl- during an active phase, GABA_AR activation results in repolarization of the membrane, causing an inhibitory effect on insulin secretion. Because beta cells have a high [Cl^-]i, its sets the E_Cl- at a higher level than for mature neurons where GABA_AR is generally inhibitory. Created with BioRender.com.

Figure 5

Major components of the beta cell GABA system. GABA is synthesized in the cytosol from glutamic acid (Glu) by the enzyme glutamic acid decarboxylase (GAD), of which there are two isoforms, GAD65 and GAD67. GABA is transaminated with alpha ketoglutarate (αKG) via GABA transaminase (GABA-T) to form glutamic acid (Glu) and succinate semialdehyde (SSA). SSA is in turn oxidized to form succinate, which enters the TCA cycle, thus connecting the GABA pool to beta cell metabolism. GABA is secreted to the extracellular space via volume-regulated anion channels (VRAC), which are a mechanism for non-vesicular secretion of osmolytes GABA and taurine from the cytosolic pool in response to changes in osmolarity or other triggers. This non-vesicular form of GABA release does not appear to be regulated by glucose. GABA is also be secreted from a sub-population of large dense core vesicles (LDCV) together with insulin. Vesicular packaging of GABA depends on the presence of the vesicular GABA transporter (VGAT), which is expressed in only a subpopulation of beta cells. Alternatively, we speculate that VMAT2 could serve as a vesicular GABA transporter in the absence of VGAT, although experimental evidence has not yet been generated to support this concept. Once secreted, interstitial islet GABA ligates GABA_A receptors (GABA_AR), which are Cl^- channels. Chloride currents through opened GABA_ARs modulate membrane potential (V_m) and thus control beta cell excitability. The direction that GABA_AR pushes V_m depends on whether the V_m is presently above or below the equilibrium chloride potential (E_Cl-). GABA_AR activation can inhibit insulin secretion by clamping V_m to the E_Cl- or hyperpolarizing the membrane back toward E_Cl- when V_m is more electropositive in excited beta cells. GABA_AR can contribute to beta cell depolarization when glucose concentrations are low and V_m is negative of E_Cl-. GABA also ligates GABA_B receptors (GABA_BR), which are inhibitory G protein (Gi)-coupled receptors that stimulate the opening of G protein-coupled inwardly-rectifying potassium channels (GIRKs) and inhibit adenylyl cyclase. Gi is also known to inhibit P/Q-type and N-type and Ca²⁺ channels but it is not yet confirmed whether this mechanism occurs in beta cells. Overall, the pancreatic islet integrates metabolic, ionotropic, and metabotropic signals together with the paracrine effects of GABA via responses invoked in alpha and delta cells, to result in a net effect of GABA on islet function.