The promiscuous role of the epsilon subunit in GABA(A) receptor biogenesis

The formation of alpha1beta2gamma2epsilon receptors suggests that the epsilon subunit does not displace the single gamma2 subunit in alpha1beta2gamma2 receptors. Thus, epsilon must replace alpha and/or beta subunit(s) if the pentameric receptor structure is to be preserved. To assess the potential for which subunit is replaced in alphabetaepsilon and alphabetagammaepsilon receptors we analyzed the assembly and functional expression of the epsilon subunit with respect to alpha1, beta2 and gamma2 subunits. Using concatenated subunits, we have determined that epsilon is capable of substituting for either (but not both) of the alpha subunits, one of the beta subunits, and possibly the gamma2 subunit. However, the most likely sites at which the epsilon subunit may contribute to receptor function appears to be at position 1 (replaces alpha1) in alphabetagammaepsilon (varepsilon-beta2-alpha1-beta2-gamma2) receptors, or at position 4 (replaces beta2) in alphabetaepsilon (alpha1-beta2-alpha1-varepsilon-beta2) receptors. In both cases, it appears that only a single GABA binding site is present.

Replacement of histidine in position 105 in the alpha(5) subunit by cysteine stimulates zolpidem sensitivity of alpha(5)beta(2)gamma(2) GABA(A) receptors

Zolpidem is a positive allosteric modulator of GABA(A) receptors with sensitivity to subunit composition. While it acts with high affinity and efficacy at GABA(A) receptors containing the alpha(1) subunit, it has a lower affinity to GABA(A) receptors containing alpha(2), alpha(3), or alpha(5) subunits and has a very weak efficacy at receptors containing the alpha(5) subunit. Here, we show that replacing histidine in position 105 in the alpha(5) subunit by cysteine strongly stimulates the effect of zolpidem in receptors containing the alpha(5) subunit. The side chain volume of the amino acid residue in this position does not correlate with the modulation by zolpidem. Interestingly, serine is not able to promote the potentiation by zolpidem. The homologous residues to alpha(5)H105 in alpha(1), alpha(2), and alpha(3) are well-known determinants of the action of classical benzodiazepines. Other studies have shown that replacement of these histidines alpha(1)H101, alpha(2)H101, and alpha(3)H126 by arginine, as naturally present in alpha(4) and alpha(6), leads to benzodiazepine insensitivity of these receptors. Thus, the nature of the amino acid residue in this position is not only crucial for the action of classical benzodiazepines but in alpha(5) containing receptors also for the action of zolpidem.

Readjusting the glucocorticoid balance: an opportunity for modulators of 11beta-hydroxysteroid dehydrogenase type 1 activity?

Glucocorticoids play a pivotal role in the regulation of most essential physiological processes, including energy metabolism, maintenance of electrolyte balance and blood pressure, immune-modulation and stress responses, cell proliferation and differentiation, as well as regulation of memory and cognitive functions. There are several levels at which glucocorticoid action can be modulated. On a tissue-specific level, glucocorticoid action is tightly controlled by 11beta-hydroxysteroid dehydrogenase (11beta-HSD) enzymes. The conversion of inactive 11-ketoglucocorticoids (cortisone and 11-dehydrocorticosterone) into active 11beta-hydroxyglucocorticoids (cortisol and corticosterone) is catalyzed by 11beta-HSD1, which is expressed in many tissues and plays an important role in metabolically relevant tissues such as the liver, adipose tissue and skeletal muscles. Chronically elevated local glucocorticoid action as a result of increased 11beta-HSD1 activity rather than elevated systemic glucocorticoid levels has been associated with metabolic syndrome, which is characterized by obesity, insulin resistance, type 2 diabetes and cardiovascular complications. Recent studies indicate that compounds inhibiting 11beta-HSD1 activity ameliorate the adverse effects of excessive glucocorticoid concentrations on metabolic processes, providing promising opportunities for the development of therapeutic interventions. This review addresses recent findings relevant for the development and application of therapeutically useful compounds that modulate 11beta-HSD1 function.

Cathepsins: key modulators of cell death and inflammatory responses

Apoptosis is a key mechanism in the build up and maintenance of both innate and adaptive immunity as well as in the regulation of cellular homeostasis in almost every organ and tissue. Central to the apoptotic process is a family of intracellular cysteine proteases with aspartate-specificity, called caspases. Nevertheless, there is growing evidence that other non-caspase proteases, in particular lysosomal cathepsins, can play an important role in the regulation of apoptosis. In this review, the players and the molecular mechanisms involved in the lysosomal apoptotic pathways will be discussed as well as the importance of these pathways in the immune system and the pathogenesis of diseases.

Relative positioning of diazepam in the benzodiazepine-binding-pocket of GABA receptors

GABA(A) receptors are the major inhibitory neurotransmitter receptors in the brain. Some of them are targets of benzodiazepines that are widely used in clinical practice for their sedative/hypnotic, anxiolytic, muscle relaxant and anticonvulsant effects. In order to rationally separate these different drug actions, we need to understand the interaction of such compounds with the benzodiazepine-binding pocket. With this aim, we mutated residues located in the benzodiazepine-binding site individually to cysteine. These mutated receptors were combined with benzodiazepine site ligands carrying a cysteine reactive group in a defined position. Proximal apposition of reaction partners will lead to a covalent reaction. We describe here such proximity-accelerated chemical coupling reactions of alpha(1)S205C and alpha(1)T206C with a diazepam derivative modified at the C-3 position with a reactive isothiocyanate group (-NCS). We also provide new data that identify alpha(1)H101C and alpha(1)N102C as exclusive sites of the reaction of a diazepam derivative where the -Cl atom is replaced by a -NCS group. Based on these observations we propose a relative positioning of diazepam within the benzodiazepine-binding site of alpha(1)beta(2)gamma(2) receptors.

Structure of alpha6 beta3 delta GABA(A) receptors and their lack of ethanol sensitivity

Delta (delta) subunit containing GABA(A) receptors are expressed extra-synaptically and mediate tonic inhibition. In cerebellar granule cells, they often form a receptor together with alpha(6) subunits. We were interested to determine the architecture of these receptors. We predefined the subunit arrangement of 24 different GABA(A) receptor pentamers by subunit concatenation. These receptors (composed of alpha(6), beta(3) and delta subunits) were expressed in Xenopus oocytes and their electrophysiological properties analyzed. Currents elicited in response to GABA were determined in presence and absence of 3alpha, 21-dihydroxy-5alpha-pregnan-20-one and to 4,5,6,7-tetrahydroisoxazolo[5,4-c]-pyridin-3-ol. alpha(6)-beta(3)-alpha(6)/delta receptors showed a substantial response to GABA alone. Three receptors, beta(3)-alpha(6)-delta/alpha(6)-beta(3), alpha(6)-beta(3)-alpha(6)/beta(3)-delta and beta(3)-delta-beta(3)/alpha(6)-beta(3), were only uncovered in the combined presence of the neurosteroid 3alpha, 21-dihydroxy-5alpha-pregnan-20-one with GABA. All four receptors were activated by 4,5,6,7-tetrahydroisoxazolo[5,4-c]-pyridin-3-ol. None of the functional receptors was modulated by physiological concentrations (up to 30 mM) of ethanol. GABA concentration response curves indicated that the delta subunit can contribute to the formation of an agonist site. We conclude from the investigated receptors that the delta subunit can assume multiple positions in a receptor pentamer composed of alpha(6), beta(3) and delta subunits.

Wear particles and surface topographies are modulators of osteoclastogenesis in vitro

Prosthetic and osteosynthetic implants from metal alloys will be indispensable in orthopedic surgery, as long as tissue engineering and biodegradable bone substitutes do not lead to products that will be applied in clinical routine for the repair of bone, cartilage, and joint defects. Therefore, the elucidation of the interactions between the periprosthetic tissues and the implant remains of clinical relevance and several factors are known to affect the longevity of implants. Within this study, the effects of metal particles and surface topography on the recruitment of osteoclasts was investigated in vitro in a coculture of osteoblasts and bone marrow cells. The cells were grown in the presence of particles of different sizes and chemical composition or on metal discs with polished or sandblasted surfaces, respectively. At the end of the culture, newly formed osteoclasts were counted. Osteoclastogenesis was reduced when particles were added directly to the coculture. The effect depended on the size of the particles, small particles exerting stronger effects than larger ones. The chemical composition of the particles, however, did not affect the development of osteoclasts. In cocultures grown on sandblasted surfaces, osteoclasts developed at higher rates than they did in cultures on polished surfaces. The data demonstrate that wear particles and implant surfaces affect osteoclastogenesis and thus may be involved in the induction of local bone resorption and the formation of osteolytic lesions, leading eventually to the loosening of orthopedic implants.

Do N-arachidonyl-glycine (NA-glycine) and 2-arachidonoyl glycerol (2-AG) share mode of action and the binding site on the β 2 subunit of GABA A receptors?

NA-glycine is an endogenous lipid molecule with analgesic properties, which is structurally similar to the endocannabinoids 2-AG and anandamide but does not interact with cannabinoid receptors. NA-glycine has been suggested to act at the G-protein coupled receptors GPR18 and GPR92. Recently, we have described that NA-glycine can also modulate recombinant α1β2γ2 GABAA receptors. Here we characterize in more detail this modulation and investigate the relationship of its binding site with that of the endocannabinoid 2-AG.

Molecular analysis of the site for 2-arachidonylglycerol (2-AG) on the β 2 subunit of GABA A receptors

2-arachidonyl glycerol (2-AG) allosterically potentiates GABAA receptors via a binding site located in transmembrane segment M4 of the β2 subunit. Two amino acid residues have been described that are essential for this effect. With the aim to further describe this potential drug target, we performed a cysteine scanning of the entire M4 and part of M3. All four residues in M4 affecting the potentiation here and the two already identified residues locate to the same side of the α-helix. This side is exposed to M3, where further residues were identified. From the fact that the important residues span > 18 Å, we conclude that the hydrophobic tail of the bound 2-AG molecule must be near linear and that the site mainly locates to the inner leaflet but stretches far into the membrane. The influence of the structure of the head group of the ligand molecule on the activity of the molecule was also investigated. We present a model of 2-AG docked to the GABAA receptor.

Prefrontal GABA and glutathione imbalance in posttraumatic stress disorder: Preliminary findings.

Although posttraumatic stress disorder (PTSD) is associated with a variety of structural and functional brain changes, the molecular pathophysiological mechanisms underlying these macroscopic alterations are unknown. Recent studies support the existence of an altered excitation-inhibition balance in PTSD. Further, there is preliminary evidence from blood-sample studies suggesting heightened oxidative stress in PTSD, potentially leading to neural damage through excessive brain levels of free radicals. In this study we investigated PTSD (n=12) and non-PTSD participants (n=17) using single-voxel proton magnetic resonance spectroscopy (MRS) in dorsolateral prefrontal cortex (DLPFC) and anterior cingulate cortex (ACC). We found significantly higher levels of γ-amino butyric acid (GABA) (a primary inhibitory neurotransmitter) and glutathione (a marker for neuronal oxidative stress) in PTSD participants. Atypically high prefrontal inhibition as well as oxidative stress may be involved in the pathogenesis of PTSD.

Detection and functional portrayal of a novel class of dihydrotestosterone derived selective progesterone receptor modulators (SPRM).

In early pregnancy, abortion can be induced by blocking the actions of progesterone receptors (PR). However, the PR antagonist, mifepristone (RU38486), is rather unselective in clinical use because it also cross-reacts with other nuclear receptors. Since the ligand-binding domain of human progesterone receptor (hPR) and androgen receptor (hAR) share 54% identity, we hypothesized that derivatives of dihydrotestosterone (DHT), the cognate ligand for hAR, might also regulate the hPR. Compounds designed and synthesized in our laboratory were investigated for their affinities for hPRB, hAR, glucocorticoid receptor (hGRα) and mineralocorticoid receptor (hMR), using whole cell receptor competitive binding assays. Agonistic and antagonistic activities were characterized by reporter assays. Nuclear translocation was monitored using cherry-hPRB and GFP-hAR chimeric receptors. Cytostatic properties and apoptosis were tested on breast cancer cells (MCF7, T-47D). One compound presented a favorable profile with an apparent neutral hPRB antagonistic function, a selective cherry-hPRB nuclear translocation and a cytostatic effect. 3D models of human PR and AR with this ligand were constructed to investigate the molecular basis of selectivity. Our data suggest that these novel DHT-derivatives provide suitable templates for the development of new selective steroidal hPR antagonists.

The transporter GAT1 plays an important role in GABA-mediated carbon-nitrogen interactions in Arabidopsis

Glutamate derived γ-aminobutyric acid (GABA) is synthetized in the cytosol prior to delivery to the mitochondria where it is catabolized via the TCA cycle. GABA accumulates under various environmental conditions, but an increasing number of studies show its involvement at the crossroad between C and N metabolism. To assess the role of GABA in modulating cellular metabolism, we exposed seedlings of A. thaliana GABA transporter gat1 mutant to full nutrition medium and media deficient in C and N combined with feeding of different concentrations (0.5 and 1 mM) of exogenous GABA. GC-MS based metabolite profiling showed an expected effect of medium composition on the seedlings metabolism of mutant and wild type alike. That being said, a significant interaction between GAT1 deficiency and medium composition was determined with respect to magnitude of change in relative amino acid levels. The effect of exogenous GABA treatment on metabolism was contingent on both the medium and the genotype, leading for instance to a drop in asparagine under full nutrition and low C conditions and glucose under all tested media, but not to changes in GABA content. We additionally assessed the effect of GAT1 deficiency on the expression of glutamate metabolism related genes and genes involved in abiotic stress responses. These results suggest a role for GAT1 in GABA-mediated metabolic alterations in the context of the C-N equilibrium of plant cells.