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Human ?1?3?2L gamma-aminobutyric acid type A receptors: High-level production and purification in a functional state.

ABSTRACT: Gamma-aminobutyric acid type A receptors (GABA(A)Rs) are the most important inhibitory chloride ion channels in the central nervous system and are major targets for a wide variety of drugs. The subunit compositions of GABA(A)Rs determine their function and pharmacological profile. GABAA Rs are heteropentamers of subunits, and (?1)2 (?3)2 (?2L)1 is a common subtype. Biochemical and biophysical studies of GABA(A)Rs require larger quantities of receptors of defined subunit composition than are currently available. We previously reported high-level production of active human ?1?3 GABA(A)R using tetracycline-inducible stable HEK293 cells. Here we extend the strategy to receptors containing three different subunits. We constructed a stable tetracycline-inducible HEK293-TetR cell line expressing human (N)-FLAG-?1?3?2L-(C)-(GGS)3 GK-1D4 GABA(A)R. These cells achieved expression levels of 70-90 pmol [(3)H]muscimol binding sites/15-cm plate at a specific activity of 15-30 pmol/mg of membrane protein. Incorporation of the ?2 subunit was confirmed by the ratio of [(3)H]flunitrazepam to [(3)H]muscimol binding sites and sensitivity of GABA-induced currents to benzodiazepines and zinc. The ?1?3?2L GABA(A)Rs were solubilized in dodecyl-D-maltoside, purified by anti-FLAG affinity chromatography and reconstituted in CHAPS/asolectin at an overall yield of ? 30%. Typical purifications yielded 1.0-1.5 nmoles of [(3)H]muscimol binding sites/60 plates. Receptors with similar properties could be purified by 1D4 affinity chromatography with lower overall yield. The composition of the purified, reconstituted receptors was confirmed by ligand binding, Western blot, and proteomics. Allosteric interactions between etomidate and [(3)H]muscimol binding were maintained in the purified state.

SUBMITTER: Dostalova Z

PROVIDER: S-EPMC3926741 | biostudies-literature | 2014 Feb

REPOSITORIES: biostudies-literature

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Human α1β3γ2L gamma-aminobutyric acid type A receptors: High-level production and purification in a functional state.

Dostalova Zuzana Z Zhou Xiaojuan X Liu Aiping A Zhang Xi X Zhang Yinghui Y Desai Rooma R Forman Stuart A SA Miller Keith W KW

Protein science : a publication of the Protein Society 20131216 2

Gamma-aminobutyric acid type A receptors (GABA(A)Rs) are the most important inhibitory chloride ion channels in the central nervous system and are major targets for a wide variety of drugs. The subunit compositions of GABA(A)Rs determine their function and pharmacological profile. GABAA Rs are heteropentamers of subunits, and (α1)2 (β3)2 (γ2L)1 is a common subtype. Biochemical and biophysical studies of GABA(A)Rs require larger quantities of receptors of defined subunit composition than are curr ...[more]

PMID: 24288268

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Project description:ObjectiveHuman hypothalamic hamartomas (HHs) are highly associated with treatment-resistant gelastic seizures. HHs are intrinsically epileptogenic, although the basic cellular mechanisms responsible for seizure activity are unknown. Altered gamma-aminobutyric acid (GABA) function can contribute to epileptogenesis in humans and animal models. Recently, functional GABA(A) receptor (GABA(A) R) rundown has been described in surgically resected human temporal lobe epilepsy tissue. We asked whether functional GABA(A) R rundown also occurs in human HH neurons.MethodsGABA(A) R-mediated currents were measured using perforated patch-clamp recordings in single neurons acutely dissociated from surgically resected HH tissue. In addition, functional GABA(A) Rs were expressed in Xenopus oocytes after microinjection with membrane fractions from either HH or control hypothalamus, and were studied with 2-electrode voltage-clamp recordings.ResultsPerforated patch-clamp recordings in dissociated HH neurons showed that repetitive exposure to GABA (5 consecutive exposures to 0.1 mM GABA with 1-second duration and at 20-second intervals) induced a time-dependent rundown of whole-cell currents in small HH neurons, whereas large HH neurons showed much less rundown using the same protocol. Functional rundown was not observed in HH neurons with repetitive exposure to glycine or glutamate. Two-electrode voltage-clamp recordings (6 consecutive exposures to 1 mM GABA with 10-second duration and at 40-second intervals) induced GABA current rundown in Xenopus oocytes microinjected with HH membrane proteins, but not in the oocytes expressing hypothalamic membrane proteins derived from human autopsy controls. Functional rundown of GABA currents was significantly attenuated by intracellular application of adenosine triphosphate or the nonspecific phosphatase inhibitor, okadaic acid.InterpretationNeurons from surgically resected human HH demonstrate functional rundown of GABA(A) R-mediated transmembrane currents in response to GABA agonist exposure. Rundown may be a marker for impaired GABAergic function and a contributing mechanism for seizure genesis within HH tissue.

Project description:γ-aminobutyric acid type A receptors (GABAARs), the major inhibitory neurotransmitter receptors in the mammalian central nervous system, are arguably the most challenging member of the pentameric Cys-loop receptors to study due to their heteromeric structure. When two or more subunits are expressed together in heterologous systems, receptors of variable subunit type, ratio, and orientation can form, precluding accurate interpretation of data from functional studies. Subunit concatenation is a technique that involves the linking of individual subunits and in theory allows the precise control of the uniformity of expressed receptors. In reality, the resulting concatemers from widely used constructs are flexible in their orientation and may therefore assemble with themselves or free GABAAR subunits in unexpected ways. In this study, we examine functional responses of receptors from existing concatenated constructs and describe refinements necessary to allow expression of uniform receptor populations. We find that dimers from two commonly used concatenated constructs, β-23-α and α-10-β, assemble readily in both the clockwise and the counterclockwise orientations when coexpressed with free subunits. Furthermore, we show that concatemers formed from new tetrameric α-10-β-α-β and α-10-β-α-γ constructs also assemble in both orientations with free subunits to give canonical αβγ receptors. To restrict linker flexibility, we systematically shorten linker lengths of dimeric and pentameric constructs and find optimized constructs that direct the assembly of GABAARs only in one orientation, thus eliminating the ambiguity associated with previously described concatemers. Based on our data, we revisit some noncanonical GABAAR configurations proposed in recent years and explain how the use of some concatenated constructs may have led to wrong conclusions. Our results help clarify current contradictions in the literature regarding GABAAR subunit stoichiometry and arrangement. The lessons learned from this study may guide future efforts in understanding other related heteromeric receptors.

Dataset Information

Human ?1?3?2L gamma-aminobutyric acid type A receptors: High-level production and purification in a functional state.

Publications

Human α1β3γ2L gamma-aminobutyric acid type A receptors: High-level production and purification in a functional state.

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