Photoaffinity labeling with a neuroactive steroid analogue. 6-azi-pregnanolone labels voltage-dependent anion channel-1 in rat brain

GPR161 structure uncovers the redundant role of sterol-regulated ciliary cAMP signaling in the Hedgehog pathway

The orphan G protein-coupled receptor (GPCR) GPR161 plays a central role in development by suppressing Hedgehog signaling. The fundamental basis of how GPR161 is activated remains unclear. Here, we determined a cryogenic-electron microscopy structure of active human GPR161 bound to heterotrimeric Gs. This structure revealed an extracellular loop 2 that occupies the canonical GPCR orthosteric ligand pocket. Furthermore, a sterol that binds adjacent to transmembrane helices 6 and 7 stabilizes a GPR161 conformation required for Gs coupling. Mutations that prevent sterol binding to GPR161 suppress Gs-mediated signaling. These mutants retain the ability to suppress GLI2 transcription factor accumulation in primary cilia, a key function of ciliary GPR161. By contrast, a protein kinase A-binding site in the GPR161 C terminus is critical in suppressing GLI2 ciliary accumulation. Our work highlights how structural features of GPR161 interface with the Hedgehog pathway and sets a foundation to understand the role of GPR161 function in other signaling pathways.

GPR161 structure uncovers the redundant role of sterol-regulated ciliary cAMP signaling in the Hedgehog pathway

The orphan G protein-coupled receptor (GPCR) GPR161 is enriched in primary cilia, where it plays a central role in suppressing Hedgehog signaling1. GPR161 mutations lead to developmental defects and cancers2,3,4. The fundamental basis of how GPR161 is activated, including potential endogenous activators and pathway-relevant signal transducers, remains unclear. To elucidate GPR161 function, we determined a cryogenic-electron microscopy structure of active GPR161 bound to the heterotrimeric G protein complex Gs. This structure revealed an extracellular loop 2 that occupies the canonical GPCR orthosteric ligand pocket. Furthermore, we identify a sterol that binds to a conserved extrahelical site adjacent to transmembrane helices 6 and 7 and stabilizes a GPR161 conformation required for Gs coupling. Mutations that prevent sterol binding to GPR161 suppress cAMP pathway activation. Surprisingly, these mutants retain the ability to suppress GLI2 transcription factor accumulation in cilia, a key function of ciliary GPR161 in Hedgehog pathway suppression. By contrast, a protein kinase A-binding site in the GPR161 C-terminus is critical in suppressing GLI2 ciliary accumulation. Our work highlights how unique structural features of GPR161 interface with the Hedgehog pathway and sets a foundation to understand the broader role of GPR161 function in other signaling pathways.

Neurosteroids as stress modulators and neurotherapeutics: lessons from the retina

Neurosteroids are rapidly emerging as important new therapies in neuropsychiatry, with one such agent, brexanolone, already approved for treatment of postpartum depression, and others on the horizon. These steroids have unique properties, including neuroprotective effects that could benefit a wide range of brain illnesses including depression, anxiety, epilepsy, and neurodegeneration. Over the past 25 years, our group has developed ex vivo rodent models to examine factors contributing to several forms of neurodegeneration in the retina. In the course of this work, we have developed a model of acute closed angle glaucoma that involves incubation of ex vivo retinas under hyperbaric conditions and results in neuronal and axonal changes that mimic glaucoma. We have used this model to determine neuroprotective mechanisms that could have therapeutic implications. In particular, we have focused on the role of both endogenous and exogenous neurosteroids in modulating the effects of acute high pressure. Endogenous allopregnanolone, a major stress-activated neurosteroid in the brain and retina, helps to prevent severe pressure-induced retinal excitotoxicity but is unable to protect against degenerative changes in ganglion cells and their axons under hyperbaric conditions. However, exogenous allopregnanolone, at a pharmacological concentration, completely preserves retinal structure and does so by combined effects on gamma-aminobutyric acid type A receptors and stimulation of the cellular process of macroautophagy. Surprisingly, the enantiomer of allopregnanolone, which is inactive at gamma-aminobutyric acid type A receptors, is equally retinoprotective and acts primarily via autophagy. Both enantiomers are also equally effective in preserving retinal structure and function in an in vivo glaucoma model. These studies in the retina have important implications for the ongoing development of allopregnanolone and other neurosteroids as therapeutics for neuropsychiatric illnesses.

The Mechanism of Enantioselective Neurosteroid Actions on GABAA Receptors

The neurosteroid allopregnanolone (ALLO) and pregnanolone (PREG), are equally effective positive allosteric modulators (PAMs) of GABAA receptors. Interestingly, the PAM effects of ALLO are strongly enantioselective, whereas those of PREG are not. This study was aimed at determining the basis for this difference in enantioselectivity. The oocyte electrophysiology studies showed that ent-ALLO potentiates GABA-elicited currents in α1β3 GABAA receptors with lower potency and efficacy than ALLO, PREG or ent-PREG. The small PAM effect of ent-ALLO was prevented by the α1(Q242L) mutation in the intersubunit neurosteroid binding site between the β3 and α1 subunits. Consistent with this result, neurosteroid analogue photolabeling with mass spectrometric readout, showed that ent-ALLO binds weakly to the β3-α1 intersubunit binding site in comparison to ALLO, PREG and ent-PREG. Rigid body docking predicted that ent-ALLO binds in the intersubunit site with a preferred orientation 180° different than ALLO, PREG or ent-PREG, potentially explaining its weak binding and effect. Photolabeling studies did not identify differences between ALLO and ent-ALLO binding to the α1 or β3 intrasubunit binding sites that also mediate neurosteroid modulation of GABAA receptors. The results demonstrate that differential binding of ent-ALLO and ent-PREG to the β3-α1 intersubunit site accounts for the difference in enantioselectivity between ALLO and PREG.

Lysosomal GPCR-like protein LYCHOS signals cholesterol sufficiency to mTORC1

Lysosomes coordinate cellular metabolism and growth upon sensing of essential nutrients, including cholesterol. Through bioinformatic analysis of lysosomal proteomes, we identified lysosomal cholesterol signaling (LYCHOS, previously annotated as G protein-coupled receptor 155), a multidomain transmembrane protein that enables cholesterol-dependent activation of the master growth regulator, the protein kinase mechanistic target of rapamycin complex 1 (mTORC1). Cholesterol bound to the amino-terminal permease-like region of LYCHOS, and mutating this site impaired mTORC1 activation. At high cholesterol concentrations, LYCHOS bound to the GATOR1 complex, a guanosine triphosphatase (GTPase)-activating protein for the Rag GTPases, through a conserved cytoplasm-facing loop. By sequestering GATOR1, LYCHOS promotes cholesterol- and Rag-dependent recruitment of mTORC1 to lysosomes. Thus, LYCHOS functions in a lysosomal pathway for cholesterol sensing and couples cholesterol concentrations to mTORC1-dependent anabolic signaling.

Neurosteroids as novel antidepressants and anxiolytics: GABA-A receptors and beyond

The recent FDA approval of the neurosteroid, brexanolone (allopregnanolone), as a treatment for women with postpartum depression, and successful trials of a related neuroactive steroid, SGE-217, for men and women with major depressive disorder offer the hope of a new era in treating mood and anxiety disorders based on the potential of neurosteroids as modulators of brain function. This review considers potential mechanisms contributing to antidepressant and anxiolytic effects of allopregnanolone and other GABAergic neurosteroids focusing on their actions as positive allosteric modulators of GABAA receptors. We also consider their roles as endogenous "stress" modulators and possible additional mechanisms contributing to their therapeutic effects. We argue that further understanding of the molecular, cellular, network and psychiatric effects of neurosteroids offers the hope of further advances in the treatment of mood and anxiety disorders.

Multiple neurosteroid and cholesterol binding sites in voltage-dependent anion channel-1 determined by photo-affinity labeling

Voltage-dependent anion channel-1 (VDAC1) is a mitochondrial porin that is implicated in cellular metabolism and apoptosis, and modulated by numerous small molecules including lipids. VDAC1 binds sterols, including cholesterol and neurosteroids such as allopregnanolone. Biochemical and computational studies suggest that VDAC1 binds multiple cholesterol molecules, but photolabeling studies have identified only a single cholesterol and neurosteroid binding site at E73. To identify all the binding sites of neurosteroids in VDAC1, we apply photo-affinity labeling using two sterol-based photolabeling reagents with complementary photochemistry: 5α-6-AziP which contains an aliphatic diazirine, and KK200 which contains a trifluoromethyl-phenyldiazirine (TPD) group. 5α-6-AziP and KK200 photolabel multiple residues within an E73 pocket confirming the presence of this site and mapping sterol orientation within this pocket. In addition, KK200 photolabels four other sites consistent with the finding that VDAC1 co-purifies with five cholesterol molecules. Both allopregnanolone and cholesterol competitively prevent photolabeling at E73 and three other sites indicating that these are common sterol binding sites shared by both neurosteroids and cholesterol. Binding at the functionally important residue E73 suggests a possible role for sterols in regulating VDAC1 signaling and interaction with partner proteins.

Multiple functional neurosteroid binding sites on GABAA receptors

Neurosteroids are endogenous modulators of neuronal excitability and nervous system development and are being developed as anesthetic agents and treatments for psychiatric diseases. While gamma amino-butyric acid Type A (GABA_A) receptors are the primary molecular targets of neurosteroid action, the structural details of neurosteroid binding to these proteins remain ill defined. We synthesized neurosteroid analogue photolabeling reagents in which the photolabeling groups were placed at three positions around the neurosteroid ring structure, enabling identification of binding sites and mapping of neurosteroid orientation within these sites. Using middle-down mass spectrometry (MS), we identified three clusters of photolabeled residues representing three distinct neurosteroid binding sites in the human α₁β₃ GABA_A receptor. Novel intrasubunit binding sites were identified within the transmembrane helical bundles of both the α₁ (labeled residues α₁-N⁴⁰⁸, Y⁴¹⁵) and β₃ (labeled residue β₃-Y⁴⁴²) subunits, adjacent to the extracellular domains (ECDs). An intersubunit site (labeled residues β₃-L²⁹⁴ and G³⁰⁸) in the interface between the β₃(+) and α₁(−) subunits of the GABA_A receptor pentamer was also identified. Computational docking studies of neurosteroid to the three sites predicted critical residues contributing to neurosteroid interaction with the GABA_A receptors. Electrophysiological studies of receptors with mutations based on these predictions (α₁-V²²⁷W, N⁴⁰⁸A/Y⁴¹¹F, and Q²⁴²L) indicate that both the α₁ intrasubunit and β₃-α₁ intersubunit sites are critical for neurosteroid action.

Novel neurosteroid analogue photolabeling reagents identify three specific neurosteroid binding sites on α1β3 GABAA receptors, showing that a site between the α and β subunits, as well as a site within the α-subunit, contribute to neurosteroid-mediated enhancement of GABAA currents.

Neurosteroids are cholesterol metabolites produced by neurons and glial cells that participate in central nervous system (CNS) development, regulate neuronal excitability, and modulate complex behaviors such as mood. Exogenously administered neurosteroid analogues are effective sedative hypnotics and are being developed as antidepressants and anticonvulsants. Gamma amino-butyric acid Type A (GABA_A) receptors, the principal ionotropic inhibitory neurotransmitter receptors in the brain, are the primary functional target of neurosteroids. Understanding the molecular details of neurosteroid interactions with GABA_A receptors is critical to understanding their mechanism of action and developing specific and effective therapeutic agents. In the current study, we developed a suite of neurosteroid analogue affinity labeling reagents, which we used to identify three distinct binding sites on GABA_A receptors and to determine the orientation of neurosteroid binding in each site. Electrophysiological studies performed on receptors with mutations designed to disrupt the identified binding sites showed that two of the three sites contribute to neurosteroid modulation of GABA_A currents. The distinct patterns of neurosteroid affinity, binding orientation, and effect provide the potential for the development of isoform-specific agonists, partial agonists, and antagonists with targeted therapeutic effects.

Inhibitable photolabeling by neurosteroid diazirine analog in the β3-Subunit of human hetereopentameric type A GABA receptors

Pregnanolone and allopregnanolone-type ligands exert general anesthetic, anticonvulsant and anxiolytic effects due to their positive modulatory interactions with the GABA_A receptors in the brain. Binding sites for these neurosteroids have been recently identified at subunit interfaces in the transmembrane domain (TMD) of homomeric β3 GABA_A receptors using photoaffinity labeling techniques, and in homomeric chimeric receptors containing GABA_A receptor α subunit TMDs by crystallography. Steroid binding sites have yet to be determined in human, heteromeric, functionally reconstituted, full-length, glycosylated GABA_A receptors. Here, we report on the synthesis and pharmacological characterization of several photoaffinity analogs of pregnanolone and allopregnanolone, of which 21-[4-(3-(trifluoromethyl)-3H-diazirin-3-yl)benzoxy]allopregnanolone (21-pTFDBzox-AP) was the most potent ligand. It is a partial positive modulator of the human α1β3 and α1β3γ2L GABA_A receptors at sub-micromolar concentrations. [³H]21-pTFDBzox-AP photoincorporated in a pharmacologically specific manner into the α and β subunits of those receptors, with the β3 subunit photolabeled most efficiently. Importantly, photolabeling by [³H]21-pTFDBzox-AP was inhibited by the positive steroid modulators alphaxalone, pregnanolone and allopregnanolone, but not by inhibitory neurosteroid pregnenolone sulfate or by two potent general anesthetics and GABA_AR positive allosteric modulators, etomidate and an anesthetic barbiturate. The latter two ligands bind to sites at subunit interfaces in the GABA_AR that are different from those interacting with neurosteroids. 21-pTFDBzox-AP's potency and pharmacological specificity of photolabeling indicate its suitability for characterizing neurosteroid binding sites in native GABA_A receptors.

Mapping two neurosteroid-modulatory sites in the prototypic pentameric ligand-gated ion channel GLIC

Neurosteroids are endogenous sterols that potentiate or inhibit pentameric ligand-gated ion channels (pLGICs) and can be effective anesthetics, analgesics, or anti-epileptic drugs. The complex effects of neurosteroids on pLGICs suggest the presence of multiple binding sites in these receptors. Here, using a series of novel neurosteroid-photolabeling reagents combined with top-down and middle-down mass spectrometry, we have determined the stoichiometry, sites, and orientation of binding for 3α,5α-pregnane neurosteroids in the Gloeobacter ligand-gated ion channel (GLIC), a prototypic pLGIC. The neurosteroid-based reagents photolabeled two sites per GLIC subunit, both within the transmembrane domain; one site was an intrasubunit site, and the other was located in the interface between subunits. By using reagents with photoreactive groups positioned throughout the neurosteroid backbone, we precisely map the orientation of neurosteroid binding within each site. Amino acid substitutions introduced at either site altered neurosteroid modulation of GLIC channel activity, demonstrating the functional role of both sites. These results provide a detailed molecular model of multisite neurosteroid modulation of GLIC, which may be applicable to other mammalian pLGICs.

Current advances of carbene-mediated photoaffinity labeling in medicinal chemistry

Photoaffinity labeling (PAL) in combination with a chemical probe to covalently bind its target upon UV irradiation has demonstrated considerable promise in drug discovery for identifying new drug targets and binding sites. In particular, carbene-mediated photoaffinity labeling (cmPAL) has been widely used in drug target identification owing to its excellent photolabeling efficiency, minimal steric interference and longer excitation wavelength. Specifically, diazirines, which are among the precursors of carbenes and have higher carbene yields and greater chemical stability than diazo compounds, have proved to be valuable photolabile reagents in a diverse range of biological systems. This review highlights current advances of cmPAL in medicinal chemistry, with a focus on structures and applications for identifying small molecule–protein and macromolecule–protein interactions and ligand-gated ion channels, coupled with advances in the discovery of targets and inhibitors using carbene precursor-based biological probes developed in recent decades.

Photoaffinity labeling (PAL) in combination with a chemical probe to covalently bind its target upon UV irradiation has demonstrated considerable promise in drug discovery for identifying new drug targets and binding sites.

Shedding Light on Anesthetic Mechanisms: Application of Photoaffinity Ligands

Anesthetic photoaffinity ligands have had an increasing presence within anesthesiology research. These ligands mimic parent general anesthetics and allow investigators to study anesthetic interactions with receptors and enzymes; identify novel targets; and determine distribution within biological systems. To date, nearly all general anesthetics used in medicine have a corresponding photoaffinity ligand represented in the literature. In this review, we examine all aspects of the current methodologies, including ligand design, characterization, and deployment. Finally we offer points of consideration and highlight the future outlook as more photoaffinity ligands emerge within the field.

Mapping General Anesthetic Sites in Heteromeric γ-Aminobutyric Acid Type A Receptors Reveals a Potential For Targeting Receptor Subtypes

IV general anesthetics, including propofol, etomidate, alphaxalone, and barbiturates, produce important actions by enhancing γ-aminobutyric acid type A (GABAA) receptor activation. In this article, we review scientific studies that have located and mapped IV anesthetic sites using photoaffinity labeling and substituted cysteine modification protection. These anesthetics bind in transmembrane pockets between subunits of typical synaptic GABAA receptors, and drugs that display stereoselectivity also show remarkably selective interactions with distinct interfacial sites. These results suggest strategies for developing new drugs that selectively modulate distinct GABAA receptor subtypes.

Synthesis and pharmacological evaluation of neurosteroid photoaffinity ligands

Neuroactive steroids are potent positive allosteric modulators of GABA_A receptors (GABA_AR), but the locations of their GABA_AR binding sites remain poorly defined. To discover these sites, we synthesized two photoreactive analogs of alphaxalone, an anesthetic neurosteroid targeting GABA_AR, 11β-(4-azido-2,3,5,6-tetrafluorobenzoyloxy)allopregnanolone, (F4N3Bzoxy-AP) and 11-aziallopregnanolone (11-AziAP). Both photoprobes acted with equal or higher potency than alphaxalone as general anesthetics and potentiators of GABA_AR responses, left-shifting the GABA concentration - response curve for human α1β3γ2 GABA_ARs expressed in Xenopus oocytes, and enhancing [³H]muscimol binding to α1β3γ2 GABA_ARs expressed in HEK293 cells. With EC₅₀ of 110 nM, 11-AziAP is one the most potent general anesthetics reported. [³H]F4N3Bzoxy-AP and [³H]11-AziAP, at anesthetic concentrations, photoincorporated into α- and β-subunits of purified α1β3γ2 GABA_ARs, but labeling at the subunit level was not inhibited by alphaxalone (30 μM). The enhancement of photolabeling by ³H-azietomidate and ³H-mTFD-MPAB in the presence of either of the two steroid photoprobes indicates the neurosteroid binding site is different from, but allosterically related to, the etomidate and barbiturate sites. Our observations are consistent with two hypotheses. First, F4N3Bzoxy-AP and 11-aziAP bind to a high affinity site in such a pose that the 11-photoactivatable moiety, that is rigidly attached to the steroid backbone, points away from the protein. Second, F4N3Bzoxy-AP, 11-aziAP and other steroid anesthetics, which are present at very high concentration at the lipid-protein interface due to their high lipophilicity, act via low affinity sites, as proposed by Akk et al. (Psychoneuroendocrinology2009, 34S1, S59-S66).

Photoaffinity labeling with cholesterol analogues precisely maps a cholesterol-binding site in voltage-dependent anion channel-1

Voltage-dependent anion channel-1 (VDAC1) is a highly regulated β-barrel membrane protein that mediates transport of ions and metabolites between the mitochondria and cytosol of the cell. VDAC1 co-purifies with cholesterol and is functionally regulated by cholesterol, among other endogenous lipids. Molecular modeling studies based on NMR observations have suggested five cholesterol-binding sites in VDAC1, but direct experimental evidence for these sites is lacking. Here, to determine the sites of cholesterol binding, we photolabeled purified mouse VDAC1 (mVDAC1) with photoactivatable cholesterol analogues and analyzed the photolabeled sites with both top-down mass spectrometry (MS), and bottom-up MS paired with a clickable, stable isotope-labeled tag, FLI-tag. Using cholesterol analogues with a diazirine in either the 7 position of the steroid ring (LKM38) or the aliphatic tail (KK174), we mapped a binding pocket in mVDAC1 localized to Thr⁸³ and Glu⁷³, respectively. When Glu⁷³ was mutated to a glutamine, KK174 no longer photolabeled this residue, but instead labeled the nearby Tyr⁶² within this same binding pocket. The combination of analytical strategies employed in this work permits detailed molecular mapping of a cholesterol-binding site in a protein, including an orientation of the sterol within the site. Our work raises the interesting possibility that cholesterol-mediated regulation of VDAC1 may be facilitated through a specific binding site at the functionally important Glu⁷³ residue.

Cell-free production of VDAC directly into liposomes for integration with biomimetic membrane systems

The mitochondrial voltage-dependent anion channel (VDAC) is a pivotal protein since it provides the major transport pathway between the cytosol and the mitochondrial intermembrane space and it is implicated in cell apoptosis by functioning as a gatekeeper for the trafficking of mitochondrial death molecules. VDAC is a beta-barrel channel with a large conductance, and we use it as a model transport protein for the design of biomimetic systems. To overcome the limitations of classical overexpression methods for producing and purifying membrane proteins (MPs) we describe here the use of an optimized cell-free system. In a one-step reaction VDAC is obtained directly integrated into liposomes and purified by ultracentrifugation. We then combine proteoliposomes with different bilayers models in order to validate VDAC insertion and functionality. This VDAC biomimetic model is the first example validating the use of a cell-free expression system for production of MPs into liposomes and tethered bilayers as a toolbox to build a wide range of biomimetic devices.

Propofol inhibits SIRT2 deacetylase through a conformation-specific, allosteric site

meta-Azi-propofol (AziPm) is a photoactive analog of the general anesthetic propofol. We photolabeled a myelin-enriched fraction from rat brain with [(3)H]AziPm and identified the sirtuin deacetylase SIRT2 as a target of the anesthetic. AziPm photolabeled three SIRT2 residues (Tyr(139), Phe(190), and Met(206)) that are located in a single allosteric protein site, and propofol inhibited [(3)H]AziPm photolabeling of this site in myelin SIRT2. Structural modeling and in vitro experiments with recombinant human SIRT2 determined that propofol and [(3)H]AziPm only bind specifically and competitively to the enzyme when co-equilibrated with other substrates, which suggests that the anesthetic site is either created or stabilized in enzymatic conformations that are induced by substrate binding. In contrast to SIRT2, specific binding of [(3)H]AziPm or propofol to recombinant human SIRT1 was not observed. Residues that line the propofol binding site on SIRT2 contact the sirtuin co-substrate NAD(+) during enzymatic catalysis, and assays that measured SIRT2 deacetylation of acetylated α-tubulin revealed that propofol inhibits enzymatic function. We conclude that propofol inhibits the mammalian deacetylase SIRT2 through a conformation-specific, allosteric protein site that is unique from the previously described binding sites of other inhibitors. This suggests that propofol might influence cellular events that are regulated by protein acetylation state.

The mitochondrial voltage-dependent anion channel 1 in tumor cells

VDAC1 is found at the crossroads of metabolic and survival pathways. VDAC1 controls metabolic cross-talk between mitochondria and the rest of the cell by allowing the influx and efflux of metabolites, ions, nucleotides, Ca2+ and more. The location of VDAC1 at the outer mitochondrial membrane also enables its interaction with proteins that mediate and regulate the integration of mitochondrial functions with cellular activities. As a transporter of metabolites, VDAC1 contributes to the metabolic phenotype of cancer cells. Indeed, this protein is over-expressed in many cancer types, and silencing of VDAC1 expression induces an inhibition of tumor development. At the same time, along with regulating cellular energy production and metabolism, VDAC1 is involved in the process of mitochondria-mediated apoptosis by mediating the release of apoptotic proteins and interacting with anti-apoptotic proteins. The engagement of VDAC1 in the release of apoptotic proteins located in the inter-membranal space involves VDAC1 oligomerization that mediates the release of cytochrome c and AIF to the cytosol, subsequently leading to apoptotic cell death. Apoptosis can also be regulated by VDAC1, serving as an anchor point for mitochondria-interacting proteins, such as hexokinase (HK), Bcl2 and Bcl-xL, some of which are also highly expressed in many cancers. By binding to VDAC1, HK provides both a metabolic benefit and apoptosis-suppressive capacity that offer the cell a proliferative advantage and increase its resistance to chemotherapy. Thus, these and other functions point to VDAC1 as an excellent target for impairing the re-programed metabolism of cancer cells and their ability to evade apoptosis. Here, we review current evidence pointing to the function of VDAC1 in cell life and death, and highlight these functions in relation to both cancer development and therapy. In addressing the recently solved 3D structures of VDAC1, this review will point to structure-function relationships of VDAC as critical for deciphering how this channel can perform such a variety of roles, all of which are important for cell life and death. Finally, this review will also provide insight into VDAC function in Ca2+ homeostasis, protection against oxidative stress, regulation of apoptosis and involvement in several diseases, as well as its role in the action of different drugs. We will discuss the use of VDAC1-based strategies to attack the altered metabolism and apoptosis of cancer cells. These strategies include specific siRNA able to impair energy and metabolic homeostasis, leading to arrested cancer cell growth and tumor development, as well VDAC1-based peptides that interact with anti-apoptotic proteins to induce apoptosis, thereby overcoming the resistance of cancer cell to chemotherapy. Finally, small molecules targeting VDAC1 can induce apoptosis. VDAC1 can thus be considered as standing at the crossroads between mitochondrial metabolite transport and apoptosis and hence represents an emerging cancer drug target. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Sites and functional consequence of VDAC-alkylphenol anesthetic interactions

General anesthetics have previously been shown to bind mitochondrial VDAC. Here, using a photoactive analog of the anesthetic propofol, we determined that alkylphenol anesthetics bind to Gly56 and Val184 on rat VDAC1. By reconstituting rat VDAC into planar bilayers, we determined that propofol potentiates VDAC gating with asymmetry at the voltage polarities; in contrast, propofol does not affect the conductance of open VDAC. Additional experiments showed that propofol also does not affect gramicidin A properties that are sensitive to lipid bilayer mechanics. Together, this suggests propofol affects VDAC function through direct protein binding, likely at the lipid-exposed channel surface, and that gating can be modulated by ligand binding to the distal ends of VDAC β-strands where Gly56 and Val184 are located.

11-trifluoromethyl-phenyldiazirinyl neurosteroid analogues: potent general anesthetics and photolabeling reagents for GABAA receptors

RATIONALE

While neurosteroids are well-described positive allosteric modulators of gamma-aminobutyric acid type A (GABAA) receptors, the binding sites that mediate these actions have not been definitively identified.

OBJECTIVES

This study was conducted to synthesize neurosteroid analogue photolabeling reagents that closely mimic the biological effects of endogenous neurosteroids and have photochemical properties that will facilitate their use as tools for identifying the binding sites for neurosteroids on GABAA receptors.

RESULTS

Two neurosteroid analogues containing a trifluromethyl-phenyldiazirine group linked to the steroid C11 position were synthesized. These reagents, CW12 and CW14, are analogues of allopregnanolone (5α-reduced steroid) and pregnanolone (5β-reduced steroid), respectively. Both reagents were shown to have favorable photochemical properties with efficient insertion into the C-H bonds of cyclohexane. They also effectively replicated the actions of allopregnanolone and pregnanolone on GABAA receptor functions: they potentiated GABA-induced currents in Xenopus laevis oocytes transfected with α1β2γ2L subunits, modulated [(35)S]t-butylbicyclophosphorothionate binding in rat brain membranes, and were effective anesthetics in Xenopus tadpoles. Studies using [(3)H]CW12 and [(3)H]CW14 showed that these reagents covalently label GABAA receptors in both rat brain membranes and in a transformed human embryonal kidney (TSA) cells expressing either α1 and β2 subunits or β3 subunits of the GABAA receptor. Photolabeling of rat brain GABAA receptors was shown to be both concentration-dependent and stereospecific.

CONCLUSIONS

CW12 and CW14 have the appropriate photochemical and pharmacological properties for use as photolabeling reagents to identify specific neurosteroid-binding sites on GABAA receptors.

Computational investigation of cholesterol binding sites on mitochondrial VDAC

The mitochondrial voltage-dependent anion channel (VDAC) allows passage of ions and metabolites across the mitochondrial outer membrane. Cholesterol binds mammalian VDAC, and we investigated the effects of binding to human VDAC1 with atomistic molecular dynamics simulations that totaled 1.4 μs. We docked cholesterol to specific sites on VDAC that were previously identified with NMR, and we tested the reliability of multiple docking results in each site with simulations. The most favorable binding modes were used to build a VDAC model with cholesterol occupying five unique sites, and during multiple 100 ns simulations, cholesterol stably and reproducibly remained bound to the protein. For comparison, VDAC was simulated in systems with identical components but with cholesterol initially unbound. The dynamics of loops that connect adjacent β-strands were most affected by bound cholesterol, with the averaged root-mean-square fluctuation (RMSF) of multiple residues altered by 20–30%. Cholesterol binding also stabilized charged residues inside the channel and localized the surrounding electrostatic potentials. Despite this, ion diffusion through the channel was not significantly affected by bound cholesterol, as evidenced by multi-ion potential of mean force measurements. Although we observed modest effects of cholesterol on the open channel, our model will be particularly useful in experiments that investigate how cholesterol affects VDAC function under applied electrochemical forces and also how other ligands and proteins interact with the channel.

The functions of voltage-dependent anion channels in plants

Voltage-dependent anion channels (VDACs), known as outer mitochondrial membrane proteins, are present in all eukaryotic cells. In mammals, they are now recognized to play crucial roles in the regulation of metabolic and energetic functions of mitochondria as well as in mitochondria-mediated apoptosis, in association with various proteins and non-protein modulators. Although there is much less information available for plant than for animal VDACs, their similar electrophysiological and topological properties suggest that some common functions are conserved among eukaryotic VDACs. Recently, it has been revealed that plant VDACs also have various important physiological functions not only in developmental and reproductive processes, but also in biotic and abiotic stress responses, including programmed cell death. In this review, we summarize recent findings about the sequence motifs, localization, and function of plant VDACs and discuss these results in the light of recent advances in research on animal VDACs.

Mechanisms revealed through general anesthetic photolabeling

General anesthetic photolabels are used to reveal molecular targets and molecular binding sites of anesthetic ligands. After identification, the relevance of anesthetic substrates or binding sites can be tested in biological systems. Halothane and photoactive analogs of isoflurane, propofol, etomidate, neurosteroids, anthracene, and long chain alcohols have been used in anesthetic photolabeling experiments. Interrogated protein targets include the nicotinic acetylcholine receptor, GABA_A receptor, tubulin, leukocyte function-associated antigen-1, and protein kinase C. In this review, we summarize insights revealed by photolabeling these targets, as well as general features of anesthetics, such as their propensity to partition to mitochondria and bind voltage-dependent anion channels. The theory of anesthetic photolabel design and the experimental application of photoactive ligands are also discussed.

A propofol binding site on mammalian GABAA receptors identified by photolabeling

Propofol is the most important intravenous general anesthetic in current clinical use. It acts by potentiating GABAA (γ-aminobutyric acid type A) receptors, but where it binds to this receptor is not known and has been a matter of some debate. We synthesized a new propofol analog photolabeling reagent whose biological activity is very similar to that of propofol. We confirmed that this reagent labeled known propofol binding sites in human serum albumin that have been identified using X-ray crystallography. Using a combination of protiated and deuterated versions of the reagent to label mammalian receptors in intact membranes, we identified a new binding site for propofol in GABAA receptors consisting of both β3 homopentamers and α1β3 heteropentamers. The binding site is located within the β subunit at the interface between the transmembrane domains and the extracellular domain and lies close to known determinants of anesthetic sensitivity in the transmembrane segments TM1 and TM2.

Real-time monitoring of superoxide accumulation and antioxidant activity in a brain slice model using an electrochemical cytochrome c biosensor

The overproduction of reactive oxygen species and the resulting damage are central to the pathology of many diseases. The study of the temporal and spatial accumulation of reactive oxygen species has been limited because of the lack of specific probes and techniques capable of continuous measurement. We demonstrate the use of a miniaturized electrochemical cytochrome c (Cyt c) biosensor for real-time measurements and quantitative assessment of superoxide production and inactivation by natural and engineered antioxidants in acutely prepared brain slices from mice. Under control conditions, superoxide radicals produced from the hippocampal region of the brain in 400-μm-thick sections were well within the range of detection of the electrode. Exposure of the slices to ischemic conditions increased the superoxide production twofold and measurements from the slices were stable over a 3- to 4-h period. The stilbene derivative and anion channel inhibitor 4,4'-diisothiocyano-2,2'-disulfonic stilbene markedly reduced the extracellular superoxide signal under control conditions, suggesting that a transmembrane flux of superoxide into the extracellular space may occur as part of normal redox signaling. The specificity of the electrode for superoxide released by cells in the hippocampus was verified by the exogenous addition of superoxide dismutase (SOD), which decreased the superoxide signal in a dose-dependent manner. Similar results were seen with the addition of the SOD mimetic cerium oxide nanoparticles (nanoceria), in that the superoxide anion radical scavenging activity of nanoceria with an average diameter of 15 nm was equivalent to 527 U of SOD for each 1 μg/ml of nanoceria added. This study demonstrates the potential of electrochemical biosensors for studying real-time dynamics of reactive oxygen species in a biological model and the utility of these measurements in defining the relative contribution of superoxide to oxidative injury.

In vivo activation of azipropofol prolongs anesthesia and reveals synaptic targets

General anesthetic photolabels have been instrumental in discovering and confirming protein binding partners and binding sites of these promiscuous ligands. We report the in vivo photoactivation of meta-azipropofol, a potent analog of propofol, in Xenopus laevis tadpoles. Covalent adduction of meta-azipropofol in vivo prolongs the primary pharmacologic effect of general anesthetics in a behavioral phenotype we termed "optoanesthesia." Coupling this behavior with a tritiated probe, we performed unbiased, time-resolved gel proteomics to identify neuronal targets of meta-azipropofol in vivo. We have identified synaptic binding partners, such as synaptosomal-associated protein 25, as well as voltage-dependent anion channels as potential facilitators of the general anesthetic state. Pairing behavioral phenotypes elicited by the activation of efficacious photolabels in vivo with time-resolved proteomics provides a novel approach to investigate molecular mechanisms of general anesthetics.

Neurosteroids, stress and depression: potential therapeutic opportunities

Neurosteroids are potent and effective neuromodulators that are synthesized from cholesterol in the brain. These agents and their synthetic derivatives influence the function of multiple signaling pathways including receptors for γ-aminobutyric acid (GABA) and glutamate, the major inhibitory and excitatory neurotransmitters in the central nervous system (CNS). Increasing evidence indicates that dysregulation of neurosteroid production plays a role in the pathophysiology of stress and stress-related psychiatric disorders, including mood and anxiety disorders. In this paper, we review the mechanisms of neurosteroid action in brain with an emphasis on those neurosteroids that potently modulate the function of GABA(A) receptors. We then discuss evidence indicating a role for GABA and neurosteroids in stress and depression, and focus on potential strategies that can be used to manipulate CNS neurosteroid synthesis and function for therapeutic purposes.

Neuroactive steroids with perfluorobenzoyl group

During an initial study in searching for the alternative derivatives suitable for photolabeling of neuroactive steroids, perfluorobenzoates and perfluorobenzamides in position 17 of 5β-androstan-3α-ol were synthesized from the corresponding 17-hydroxy and 17-amino derivatives. After transformation into glutamates or sulfates, 17α-epimers had comparable inhibitory activity at NMDA receptors to the natural neurosteroid (20-oxo-5β-pregnan-3β-yl sulfate), however, were more potent (2- to 36-fold) than their 17β-substituted analogs. In one case, fluorine in position 4' of perfluorobenzoate group was substituted with azide and activity of the final glutamate was retained comparing with the corresponding perfluorobenzoate. The series was expanded with perfluorobenzoyl derivatives of pregnanolone: Perfluorobenzamide of glutamate and perfluorobenzoate of 11α-hydroxy pregnanolone were prepared and tested. From nine tested compounds, four of them exhibit very good inhibition activity and can serve as promising leads for photolabeling experiments.

A neurosteroid analogue photolabeling reagent labels the colchicine-binding site on tubulin: a mass spectrometric analysis

Previous studies have shown that the neurosteroid analogue, 6-Azi-pregnanolone (6-AziP), photolabels voltage-dependent anion channels and proteins of approximately 55 kDa in rat brain membranes. The present study used two-dimensional electrophoresis and nanoelectrospray ionization ion-trap mass spectrometry (nano-ESI-MS) to identify the 55 kDa proteins (isoelectric point 4.8) as isoforms of β-tubulin. This identification was confirmed by immunoblot and immunoprecipitation of photolabeled protein with anti-β-tubulin antibody and by the demonstration that 6-AziP photolabels purified bovine brain tubulin in a concentration-dependent pattern. To identify the photolabeling sites, purified bovine brain tubulin was photolabeled with 6-AziP, digested with trypsin, and analyzed by matrix-assisted laser desorption/ionization MS (MALDI). A 6-AziP adduct of TAVCDIPPR(m/z = 1287.77), a β-tubulin specific peptide, was detected by MALDI. High-resolution liquid chromatography-MS/MS analysis identified that 6-AziP was covalently bound to cysteine 354 (Cys-354), previously identified as a colchicine-binding site. 6-AziP photolabeling was inhibited by 2-methoxyestradiol, an endogenous derivative of estradiol thought to bind to the colchicine site. Structural modeling predicted that neurosteroids could dock in this colchicine site at the interface between α- and β-tubulin with the photolabeling group of 6-AziP positioned proximate to Cys-354.

Neurosteroid analog photolabeling of a site in the third transmembrane domain of the β3 subunit of the GABA(A) receptor

Accumulated evidence suggests that neurosteroids modulate GABA(A) receptors through binding interactions with transmembrane domains. To identify these neurosteroid binding sites directly, a neurosteroid-analog photolabeling reagent, (3α,5β)-6-azi-pregnanolone (6-AziP), was used to photolabel membranes from Sf9 cells expressing high-density, recombinant, His(8)-β3 homomeric GABA(A) receptors. 6-AziP inhibited (35)S-labeled t-butylbicyclophosphorothionate binding to the His(8)-β3 homomeric GABA(A) receptors in a concentration-dependent manner (IC(50) = 9 ± 1 μM), with a pattern consistent with a single class of neurosteroid binding sites. [(3)H]6-AziP photolabeled proteins of 30, 55, 110, and 150 kDa, in a concentration-dependent manner. The 55-, 110-, and 150-kDa proteins were identified as His(8)-β3 subunits through immunoblotting and through enrichment on a nickel affinity column. Photolabeling of the β3 subunits was stereoselective, with [(3)H]6-AziP producing substantially greater labeling than an equal concentration of its diastereomer [(3)H](3β,5β)-6-AziP. High-resolution mass spectrometric analysis of affinity-purified, 6-AziP-labeled His(8)-β3 subunits identified a single photolabeled peptide, ALLEYAF-6-AziP, in the third transmembrane domain. The identity of this peptide and the site of incorporation on Phe301 were confirmed through high-resolution tandem mass spectrometry. No other sites of photoincorporation were observed despite 90% sequence coverage of the whole β3 subunit protein, including 84% of the transmembrane domains. This study identifies a novel neurosteroid binding site and demonstrates the feasibility of identifying neurosteroid photolabeling sites by using mass spectrometry.

Voltage-dependent-anion-channels (VDACs) in Arabidopsis have a dual localization in the cell but show a distinct role in mitochondria

In mammals, the Voltage-dependent anion channels (VDACs) are predominant proteins of the outer mitochondrial membrane (OMM) where they contribute to the exchange of small metabolites essential for respiration. They were shown to be as well associated with the plasma membrane (PM) and act as redox enzyme or are involved in ATP release for example. In Arabidopsis, we show that four out of six genomic sequences encode AtVDAC proteins. All four AtVDACs are ubiquitously expressed in the plant but each of them displays a specific expression pattern in root cell types. Using two complementary approaches, we demonstrate conclusively that the four expressed AtVDACs are targeted to both mitochondria and plasma membrane but in differential abundance, AtVDAC3 being the most abundant in PM, and conversely, AtVDAC4 almost exclusively associated with mitochondria. These are the first plant proteins to be shown to reside in both these two membranes. To investigate a putative function of AtVDACs, we analyzed T-DNA insertion lines in each of the corresponding genes. Knock-out mutants for AtVDAC1, AtVDAC2 and AtVDAC4 present slow growth, reduced fertility and yellow spots in leaves when atvdac3 does not show any visible difference compared to wildtype plants. Analyses of atvdac1 and atvdac4 reveal that yellow areas correspond to necrosis and the mitochondria are swollen in these two mutants. All these results suggest that, in spite of a localization in plasma membrane for three of them, AtVDAC1, AtVDAC2 and AtVDAC4 have a main function in mitochondria.

Mapping general anesthetic binding site(s) in human α1β3 γ-aminobutyric acid type A receptors with [³H]TDBzl-etomidate, a photoreactive etomidate analogue

The γ-aminobutyric acid type A receptor (GABA(A)R) is a target for general anesthetics of diverse chemical structures, which act as positive allosteric modulators at clinical doses. Previously, in a heterogeneous mixture of GABA(A)Rs purified from bovine brain, [³H]azietomidate photolabeling of αMet-236 and βMet-286 in the αM1 and βM3 transmembrane helices identified an etomidate binding site in the GABA(A)R transmembrane domain at the interface between the β and α subunits [Li, G. D., et.al. (2006) J. Neurosci. 26, 11599-11605]. To further define GABA(A)R etomidate binding sites, we now use [³H]TDBzl-etomidate, an aryl diazirine with broader amino acid side chain reactivity than azietomidate, to photolabel purified human FLAG-α1β3 GABA(A)Rs and more extensively identify photolabeled GABA(A)R amino acids. [³H]TDBzl-etomidate photolabeled in an etomidate-inhibitable manner β3Val-290, in the β3M3 transmembrane helix, as well as α1Met-236 in α1M1, a residue photolabeled by [³H]azietomidate, while no photolabeling of amino acids in the αM2 and βM2 helices that also border the etomidate binding site was detected. The location of these photolabeled amino acids in GABA(A)R homology models derived from the recently determined structures of prokaryote (GLIC) or invertebrate (GluCl) homologues and the results of computational docking studies predict the orientation of [³H]TDBzl-etomidate bound in that site and the other amino acids contributing to this GABA(A)R intersubunit etomidate binding site. Etomidate-inhibitable photolabeling of β3Met-227 in βM1 by [³H]TDBzl-etomidate and [³H]azietomidate also provides evidence of a homologous etomidate binding site at the β3-β3 subunit interface in the α1β3 GABA(A)R.

Plasmalemmal VDAC controversies and maxi-anion channel puzzle

The maxi-anion channel has been observed in many cell types from the very beginning of the patch-clamp era. The channel is highly conductive for chloride and thus can modulate the resting membrane potential and play a role in fluid secretion/absorption and cell volume regulation. A wide nanoscopic pore of the maxi-anion channel permits passage of excitatory amino acids and nucleotides. The channel-mediated release of these signaling molecules is associated with kidney tubuloglomerular feedback, cardiac ischemia/hypoxia, as well as brain ischemia/hypoxia and excitotoxic neurodegeneration. Despite the ubiquitous expression and physiological/pathophysiological significance, the molecular identity of the maxi-anion channel is still obscure. VDAC is primarily a mitochondrial protein; however several groups detected it on the cellular surface. VDAC in lipid bilayers reproduced the most important biophysical properties of the maxi-anion channel, such as a wide nano-sized pore, closure in response to moderately high voltages, ATP-block and ATP-permeability. However, these similarities turned out to be superficial, and the hypothesis of plasmalemmal VDAC as the maxi-anion channel did not withstand the test by genetic manipulations of VDAC protein expression. VDAC on the cellular surface could also function as a ferricyanide reductase or a receptor for plasminogen kringle 5 and for neuroactive steroids. These ideas, as well as the very presence of VDAC on plasmalemma, remain to be scrutinized by genetic manipulations of the VDAC protein expression. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

VDAC, a multi-functional mitochondrial protein regulating cell life and death

Research over the past decade has extended the prevailing view of the mitochondrion to include functions well beyond the generation of cellular energy. It is now recognized that mitochondria play a crucial role in cell signaling events, inter-organellar communication, aging, cell proliferation, diseases and cell death. Thus, mitochondria play a central role in the regulation of apoptosis (programmed cell death) and serve as the venue for cellular decisions leading to cell life or death. One of the mitochondrial proteins controlling cell life and death is the voltage-dependent anion channel (VDAC), also known as mitochondrial porin. VDAC, located in the mitochondrial outer membrane, functions as gatekeeper for the entry and exit of mitochondrial metabolites, thereby controlling cross-talk between mitochondria and the rest of the cell. VDAC is also a key player in mitochondria-mediated apoptosis. Thus, in addition to regulating the metabolic and energetic functions of mitochondria, VDAC appears to be a convergence point for a variety of cell survival and cell death signals mediated by its association with various ligands and proteins. In this article, we review what is known about the VDAC channel in terms of its structure, relevance to ATP rationing, Ca(2+) homeostasis, protection against oxidative stress, regulation of apoptosis, involvement in several diseases and its role in the action of different drugs. In light of our recent findings and the recently solved NMR- and crystallography-based 3D structures of VDAC1, the focus of this review will be on the central role of VDAC in cell life and death, addressing VDAC function in the regulation of mitochondria-mediated apoptosis with an emphasis on structure-function relations. Understanding structure-function relationships of VDAC is critical for deciphering how this channel can perform such a variety of functions, all important for cell life and death. This review also provides insight into the potential of VDAC1 as a rational target for new therapeutics.

A synthetic 18-norsteroid distinguishes between two neuroactive steroid binding sites on GABAA receptors

In the absence of GABA, neuroactive steroids that enhance GABA-mediated currents modulate binding of [35S]t-butylbicyclophosphorothionate in a biphasic manner, with enhancement of binding at low concentrations (site NS1) and inhibition at higher concentrations (site NS2). In the current study, compound (3alpha,5beta,17beta)-3-hydroxy-18-norandrostane-17-carbonitrile (3alpha5beta-18-norACN), an 18-norsteroid, is shown to be a full agonist at site NS1 and a weak partial agonist at site NS2 in both rat brain membranes and heterologously expressed GABAA receptors. 3alpha5beta-18-norACN also inhibits the action of a full neurosteroid agonist, (3alpha,5alpha,17beta)-3-hydroxy-17-carbonitrile (3alpha5alphaACN), at site NS2. Structure-activity studies demonstrate that absence of the C18 methyl group and the 5beta-reduced configuration both contribute to the weak agonist effect at the NS2 site. Electrophysiological studies using heterologously expressed GABAA receptors show that 3alpha5beta-18-norACN potently and efficaciously potentiates the GABA currents elicited by low concentrations of GABA but that it has low efficacy as a direct activator of GABAA receptors. 3alpha5beta-18-norACN also inhibits direct activation of GABAA receptors by 3alpha5alphaACN. 3alpha5beta-18-norACN also produces loss of righting reflex in tadpoles and mice, indicating that action at NS1 is sufficient to mediate the sedative effects of neurosteroids. These data provide insight into the pharmacophore required for neurosteroid efficacy at the NS2 site and may prove useful in the development of selective agonists and antagonists for neurosteroid sites on the GABAA receptor.

Mitochondrial membrane cholesterol, the voltage dependent anion channel (VDAC), and the Warburg effect

Normal cells of aerobic organisms synthesize the energy they require in the form of ATP via the process of oxidative phosphorylation. This complex system resides in the mitochondria of cells and utilizes oxygen to produce a majority of cellular ATP. However, in most tumors, especially those with elevated cholesterogenesis, there is an increased reliance on glycolysis for energy, even in conditions where oxygen is available. This aerobic glycolysis (the Warburg effect) has far reaching ramifications on the tumor itself and the cells that surround it. In this brief review, we will discuss how abnormally high membrane cholesterol levels can result in a subsequent deficiency of oxidative energy production in mitochondria from cultured Morris hepatoma cells (MH-7777). We have identified the voltage dependent anion channel (VDAC) as a necessary component of a protein complex involved in mitochondrial membrane cholesterol distribution and transport. Work in our laboratory demonstrates that the ability of VDAC to influence mitochondrial membrane cholesterol distribution may have implications on mitochondrial characteristics such as oxidative phosphorylation and induction of apoptosis, as well as the propensity of cancer cells to exhibit a glycolytic phenotype.

Multiple modes for conferring surface expression of homomeric beta1 GABAA receptors

The gamma-aminobutyric acid type A (GABA(A)) receptor assembles from individual subunits to form ligand-gated ion channels. Human (h) beta3 subunits assemble to form homomeric surface receptors in somatic cells, but hbeta1 subunits do not. We have identified three distinct sets of amino acid residues in the N-terminal extracellular domain of the hbeta1 subunit, which when mutated to the homologous residue in hbeta3 allow expression as a functional homomeric receptor. The three sets likely result in three modes of assembly. Mode 1 expression results from a single amino acid change at residue hbeta1 Asp-37. Mode 2 expression results from mutations of residues between positions 44 and 73 together with residues between positions 169 and 173. Finally, mode 3 results from the mutations A45V and K196R. Examination of homology-based structural models indicates that many of the residues are unlikely to be involved in physical inter-subunit interactions, suggesting that a major alteration is stabilization of an assembly competent form of the subunit. These mutations do not, however, have a major effect on the surface expression of heteromeric receptors which include the alpha1 subunit.

General anaesthesia: from molecular targets to neuronal pathways of sleep and arousal

The mechanisms through which general anaesthetics, an extremely diverse group of drugs, cause reversible loss of consciousness have been a long-standing mystery. Gradually, a relatively small number of important molecular targets have emerged, and how these drugs act at the molecular level is becoming clearer. Finding the link between these molecular studies and anaesthetic-induced loss of consciousness presents an enormous challenge, but comparisons with the features of natural sleep are helping us to understand how these drugs work and the neuronal pathways that they affect. Recent work suggests that the thalamus and the neuronal networks that regulate its activity are the key to understanding how anaesthetics cause loss of consciousness.

Novel perspectives for progesterone in hormone replacement therapy, with special reference to the nervous system

The utility and safety of postmenopausal hormone replacement therapy has recently been put into question by large clinical trials. Their outcome has been extensively commented upon, but discussions have mainly been limited to the effects of estrogens. In fact, progestagens are generally only considered with respect to their usefulness in preventing estrogen stimulation of uterine hyperplasia and malignancy. In addition, various risks have been attributed to progestagens and their omission from hormone replacement therapy has been considered, but this may underestimate their potential benefits and therapeutic promises. A major reason for the controversial reputation of progestagens is that they are generally considered as a single class. Moreover, the term progesterone is often used as a generic one for the different types of both natural and synthetic progestagens. This is not appropriate because natural progesterone has properties very distinct from the synthetic progestins. Within the nervous system, the neuroprotective and promyelinating effects of progesterone are promising, not only for preventing but also for reversing age-dependent changes and dysfunctions. There is indeed strong evidence that the aging nervous system remains at least to some extent sensitive to these beneficial effects of progesterone. The actions of progesterone in peripheral target tissues including breast, blood vessels, and bones are less well understood, but there is evidence for the beneficial effects of progesterone. The variety of signaling mechanisms of progesterone offers exciting possibilities for the development of more selective, efficient, and safe progestagens. The recognition that progesterone is synthesized by neurons and glial cells requires a reevaluation of hormonal aging.

Identification and characterization of cholest-4-en-3-one, oxime (TRO19622), a novel drug candidate for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by progressive death of cortical and spinal motor neurons, for which there is no effective treatment. Using a cell-based assay for compounds capable of preventing motor neuron cell death in vitro, a collection of approximately 40,000 low-molecular-weight compounds was screened to identify potential small-molecule therapeutics. We report the identification of cholest-4-en-3-one, oxime (TRO19622) as a potential drug candidate for the treatment of ALS. In vitro, TRO19622 promoted motor neuron survival in the absence of trophic support in a dose-dependent manner. In vivo, TRO19622 rescued motor neurons from axotomy-induced cell death in neonatal rats and promoted nerve regeneration following sciatic nerve crush in mice. In SOD1(G93A) transgenic mice, a model of familial ALS, TRO19622 treatment improved motor performance, delayed the onset of the clinical disease, and extended survival. TRO19622 bound directly to two components of the mitochondrial permeability transition pore: the voltage-dependent anion channel and the translocator protein 18 kDa (or peripheral benzodiazepine receptor), suggesting a potential mechanism for its neuroprotective activity. TRO19622 may have therapeutic potential for ALS and other motor neuron and neurodegenerative diseases.

Neurosteroid binding sites on GABA(A) receptors

Controlling neuronal excitability is vitally important for maintaining a healthy central nervous system (CNS) and this relies on the activity of type A gamma-aminobutyric acid (GABA(A)) neurotransmitter receptors. Given this role, it is therefore important to understand how these receptors are regulated by endogenous modulators in the brain and determine where they bind to the receptor. One of the most potent groups of modulators is the neurosteroids which regulate the activity of synaptic and extrasynaptic GABA(A) receptors. This level of regulation is thought to be physiologically important and its dysfunction may be relevant to numerous neurological conditions. The aim of this review is to summarise those studies that over the last 20 years have focussed upon finding the binding sites for neurosteroids on GABA(A) receptors. We consider the nature of steroid binding sites in other proteins where this has been determined at atomic resolution and how their generic features were mapped onto GABA(A) receptors to help locate 2 putative steroid binding sites. Altogether, the findings strongly suggest that neurosteroids do bind to discrete sites on the GABA(A) receptor and that these are located within the transmembrane domains of alpha and beta receptor subunits. The implications for neurosteroid binding to other inhibitory receptors such as glycine and GABA(C) receptors are also considered. Identifying neurosteroid binding sites may enable the precise pathophysiological role(s) of neurosteroids in the CNS to be established for the first time, as well as providing opportunities for the design of novel drug entities.

Mechanisms of neurosteroid interactions with GABA(A) receptors

Neuroactive steroids have some of their most potent actions by augmenting the function of GABA(A) receptors. Endogenous steroid actions on GABA(A) receptors may underlie important effects on mood and behavior. Exogenous neuroactive steroids have potential as anesthetics, anticonvulsants, and neuroprotectants. We have taken multiple approaches to understand more completely the interaction of neuroactive steroids with GABA(A) receptors. We have developed many novel steroid analogues in this effort. Recent work has resulted in synthesis of new enantiomer analogue pairs, novel ligands that probe various properties of the steroid pharmacophore, fluorescent neuroactive steroid analogues, and photoaffinity labels. Using these tools, combined with receptor binding and electrophysiological assays, we have begun to untangle the complexity of steroid actions at this important class of ligand-gated ion channel.

Cellular distribution of the GABAA receptor-modulating 3alpha-hydroxy, 5alpha-reduced pregnane steroids in the adult rat brain

The 3alpha-hydroxy,5alpha-reduced pregnane steroids, allopregnanolone and allotetrahydrodeoxycorticosterone, are the most potent endogenous positive modulators of GABA(A) receptor-mediated inhibition. This study presents the first immunohistochemical examination of the cellular distribution of 3alpha-hydroxy,5alpha-reduced pregnane steroids across the brain. We found a widespread distribution in the adult rat, with dense immunolabelling in the olfactory bulb, striatum and cerebral cortex, and lower density labelling in the brainstem reticular formation. In general terms, this distribution accords with the regional concentrations of 3alpha-hydroxy,5alpha-reduced steroids determined, in other laboratories, by brain region sampling and either gas chromatography-mass fragmentography or radioimmunoassay. However, immunohistochemistry allowed for a more detailed examination of regional distribution and cellular specificity. All immunoreactivity was confined to the cell bodies and thick dendrites of neurones; no identifiable glia were labelled. In most brain areas, the location and morphology of labelled cells identified them as excitatory neurones. In addition, cell populations known to be projecting GABAergic neurones (e.g. cerebellar Purkinje cells) were immunoreactive, whereas local inhibitory neurones generally were not. The cellular distribution of 3alpha-hydroxy,5alpha-reduced steroids suggests that sensory, motor, limbic and homeostatic systems can be influenced by neurosteroids at multiple stages of processing.

Identification of anesthetic binding sites on human serum albumin using a novel etomidate photolabel

We have synthesized a novel analog of the general anesthetic etomidate in which the ethoxy group has been replaced by an azide group, and which can be used as a photolabel to identify etomidate binding sites. This acyl azide analog is a potent general anesthetic in both rats and tadpoles and, as with etomidate, is stereoselective in its actions, with the R(+) enantiomer being significantly more potent than the S(-) enantiomer. Its effects on alpha1beta2gamma2s GABA(A) receptors expressed in HEK-293 cells are virtually indistinguishable from the parent compound etomidate, showing stereoselective potentiation of GABA-induced currents, as well as direct mimetic effects at higher concentrations. In addition, a point mutation (beta2 N265M), which is known to attenuate the potentiating actions of etomidate, also blocks the effects of the acyl azide analog. We have investigated the utility of the analog to identify etomidate binding sites by using it to photolabel human serum albumin, a protein that binds approximately 75% of etomidate in human plasma and which is thought to play a major role in its pharmacokinetics. Using HPLC/mass spectrometry we have identified two anesthetic binding sites on HSA. One site is the well-characterized drug binding site I, located in HSA subdomain IIA, and the second site is also an established drug binding site located in subdomain IIIB, which also binds propofol. The acyl azide etomidate may prove to be a useful new photolabel to identify anesthetic binding sites on the GABA(A) receptor or other putative targets.

Cysteine 98 in CYP3A4 contributes to conformational integrity required for P450 interaction with CYP reductase

Previously human cytochrome P450 3A4 was efficiently and specifically photolabeled by the photoaffinity ligand lapachenole. One of the modification sites was identified as cysteine 98 in the B-C loop region of the protein [B. Wen, C.E. Doneanu, C.A. Gartner, A.G. Roberts, W.M. Atkins, S.D. Nelson, Biochemistry 44 (2005) 1833-1845]. Loss of CO binding capacity and subsequent decrease of catalytic activity were observed in the labeled CYP3A4, which suggested that aromatic substitution on residue 98 triggered a critical conformational change and subsequent loss of enzyme activity. To test this hypothesis, C98A, C98S, C98F, and C98W mutants were generated by site-directed mutagenesis and expressed functionally as oligohistidine-tagged proteins. Unlike the mono-adduction observed in the wild-type protein, simultaneous multiple adductions occurred when C98F and C98W were photolabeled under the same conditions as the wild-type enzyme, indicating a substantial conformational change in these two mutants compared with the wild-type protein. Kinetic analysis revealed that the C98W mutant had a drastic 16-fold decrease in catalytic efficiency (V(max)/K(m)) for 1'-OH midazolam formation, and about an 8-fold decrease in catalytic efficiency (V(max)/K(m)) for 4-OH midazolam formation, while the C98A and C98S mutants retained the same enzyme activity as the wild-type enzyme. Photolabeling of C98A and C98S with lapachenole resulted in monoadduction of only Cys-468, in contrast to the labeling of Cys-98 in wild-type CYP3A4, demonstrating the marked selectivity of this photoaffinity ligand for cysteine residues. The slight increases in the midazolam binding constants (K(s)) in these mutants suggested negligible perturbation of the heme environment. Further activity studies using different P450:reductase ratios suggested that the affinity of P450 to reductase was significantly decreased in the C98W mutant, but not in the C98A and C98S mutants. In addition, the C98W mutant exhibited a 41% decrease in the maximum electron flow rate between P450 and reductase as measured by reduced nicotinamide adenine dinucleotide phosphate consumption at a saturating reductase concentration. In conclusion, our data strongly suggest that cysteine 98 in the B-C loop region significantly contributes to conformational integrity and catalytic activity of CYP3A4, and that this residue or residues nearby might be involved in an interaction with P450 reductase.

Protein expression changes in the nucleus accumbens and amygdala of inbred alcohol-preferring rats given either continuous or scheduled access to ethanol

Chronic ethanol (EtOH) drinking produces neuronal alterations within the limbic system. To investigate changes in protein expression levels associated with EtOH drinking, inbred alcohol-preferring (iP) rats were given one of three EtOH access conditions in their home-cages: continuous ethanol (CE: 24h/day, 7days/week access to EtOH), multiple scheduled access (MSA: four 1-h sessions during the dark cycle/day, 5 days/week) to EtOH, or remained EtOH-naïve. Both MSA and CE groups consumed between 6 and 6.5g of EtOH/kg/day after the 3rd week of access. On the first day of EtOH access for the seventh week, access was terminated at the end of the fourth MSA session for MSA rats and the corresponding time point (2300h) for CE rats. Ten h later, the rats were decapitated, brains extracted, the nucleus accumbens (NAcc) and amygdala (AMYG) microdissected, and protein isolated for 2-dimensional gel electrophoretic analyses. In the NAcc, MSA altered expression levels for 12 of the 14 identified proteins, compared with controls, with six of these proteins altered by CE access, as well. In the AMYG, CE access changed expression levels for 22 of the 27 identified proteins, compared with controls, with 8 of these proteins altered by MSA, as well. The proteins could be grouped into functional categories of chaperones, cytoskeleton, intracellular communication, membrane transport, metabolism, energy production, or neurotransmission. Overall, it appears that EtOH drinking and the conditions under which EtOH is consumed, differentially affect protein expression levels between the NAcc and AMYG. This may reflect differences in neuroanatomical and/or functional characteristics associated with EtOH self-administration and possibly withdrawal, between these two brain structures.