Structure-Activity Studies of 3,9-Diazaspiro[5.5]undecane-Based γ-Aminobutyric Acid Type A Receptor Antagonists with Immunomodulatory Effect

The 3,9-diazaspiro[5.5]undecane-based compounds 2027 and 018 have previously been reported to be potent competitive γ-aminobutyric acid type A receptor (GABAAR) antagonists showing low cellular membrane permeability. Given the emerging peripheral application of GABAAR ligands, we hypothesize 2027 analogs as promising lead structures for peripheral GABAAR inhibition. We herein report a study on the structural determinants of 2027 in order to suggest a potential binding mode as a basis for rational design. The study identified the importance of the spirocyclic benzamide, compensating for the conventional acidic moiety, for GABAAR ligands. The structurally simplified m-methylphenyl analog 1e displayed binding affinity in the high-nanomolar range (Ki = 180 nM) and was superior to 2027 and 018 regarding selectivity for the extrasynaptic α4βδ subtype versus the α1- and α2- containing subtypes. Importantly, 1e was shown to efficiently rescue inhibition of T cell proliferation, providing a platform to explore the immunomodulatory potential for this class of compounds.

Tamoxifen induces cellular stress in the nervous system by inhibiting cholesterol synthesis

Background

Tamoxifen (TAM) is an important cancer therapeutic and an experimental tool for effecting genetic recombination using the inducible Cre-Lox technique. Despite its widespread use in the clinic and laboratory, we know little about its effects on the nervous system. This is of significant concern because TAM, via unknown mechanisms, induces cognitive impairment in humans. A hallmark of cellular stress is induction of Activating Transcription Factor 3 (Atf3), and so to determine whether TAM induces cellular stress in the adult nervous system, we generated a knock-in mouse in which Atf3 promoter activity drives transcription of TAM-dependent Cre recombinase (Cre-ERT2); when crossed with tdtomato reporter mice, Atf3 induction results in robust and permanent genetic labeling of cells in which it is up-regulated even transiently.

Results

We found that granular neurons of the olfactory bulb and dentate gyrus, vascular cells and ependymal cells throughout the brain, and peripheral sensory neurons expressed tdtomato in response to TAM treatment. We also show that TAM induced Atf3 up-regulation through inhibition of cholesterol epoxide hydrolase (ChEH): reporter expression was mitigated by delivery in vitamin E-rich wheat germ oil (vitamin E depletes ChEH substrates), and was partially mimicked by a ChEH-specific inhibitor.

Conclusions

This work demonstrates that TAM stresses cells of the adult central and peripheral nervous systems and highlights concerns about clinical and experimental use of TAM. We propose TAM administration in vitamin E-rich vehicles such as wheat germ oil as a simple remedy.

HDAC4 is required for inflammation-associated thermal hypersensitivity

Transcriptional alterations are characteristic of persistent pain states, but the key regulators remain elusive. HDAC4 is a transcriptional corepressor that has been linked to synaptic plasticity and neuronal excitability, mechanisms that may be involved in peripheral and central sensitization. Using a conditional knockout (cKO) strategy in mice, we sought to determine whether the loss of HDAC4 would have implications for sensory neuron transcription and nociception. HDAC4 was found to be largely unnecessary for transcriptional regulation of naïve sensory neurons but was essential for appropriate transcriptional responses after injury, with Calca and Trpv1 expression consistently down-regulated in HDAC4 cKO compared to levels in the littermate controls (0.2-0.44-fold change, n = 4 in 2 separate experiments). This down-regulation corresponded to reduced sensitivity to 100 nM capsaicin in vitro (IC50 = 230 ± 20 nM, 76 ± 4.4% wild-type capsaicin responders vs. 56.9 ± 4.7% HDAC4 cKO responders) and to reduced thermal hypersensitivity in the complete Freund's adjuvant (CFA) model of inflammatory pain (1.3-1.4-fold improvement over wild-type controls; n = 5-12, in 2 separate experiments). These data indicate that HDAC4 is a novel inflammatory pain mediator and may be a good therapeutic target, capable of orchestrating the regulation of multiple downstream effectors.

Synaptic GABAA receptors are directly recruited from their extrasynaptic counterparts

GABAA receptors mediate the majority of fast synaptic inhibition in the brain. The accumulation of these ligand-gated ion channels at synaptic sites is a prerequisite for neuronal inhibition, but the molecular mechanisms underlying this phenomenon remain obscure. To further understand these processes, we have examined the cellular origins of synaptic GABAA receptors. To do so, we have created fluorescent GABAA receptors that are capable of binding -bungarotoxin (Bgt), facilitating the visualization of receptor endocytosis, exocytosis and delivery to synaptic sites. Imaging with Bgt in hippocampal neurons revealed that GABAA receptor endocytosis occurred exclusively at extrasynaptic sites, consistent with the preferential colocalization of extrasynaptic receptors with the AP2 adaptin. Receptor insertion into the plasma membrane was also predominantly extrasynaptic, and pulse-chase analysis revealed that these newly inserted receptors were then able to access directly synaptic sites. Therefore, our results demonstrate that synaptic GABAA receptors are directly recruited from their extrasynaptic counterparts. Moreover, they illustrate a dynamic mechanism for neurons to modulate GABAA receptor number at inhibitory synapses by controlling the stability of extrasynaptic receptors.

Phospho-dependent binding of the clathrin AP2 adaptor complex to GABAA receptors regulates the efficacy of inhibitory synaptic transmission

The efficacy of synaptic inhibition depends on the number of gamma-aminobutyric acid type A receptors (GABA(A)Rs) expressed on the cell surface of neurons. The clathrin adaptor protein 2 (AP2) complex is a critical regulator of GABA(A)R endocytosis and, hence, surface receptor number. Here, we identify a previously uncharacterized atypical AP2 binding motif conserved within the intracellular domains of all GABA(A)R beta subunit isoforms. This AP2 binding motif (KTHLRRRSSQLK in the beta3 subunit) incorporates the major sites of serine phosphorylation within receptor beta subunits, and phosphorylation within this site inhibits AP2 binding. Furthermore, by using surface plasmon resonance, we establish that a peptide (pepbeta3) corresponding to the AP2 binding motif in the GABA(A)R beta3 subunit binds to AP2 with high affinity only when dephosphorylated. Moreover, the pepbeta3 peptide, but not its phosphorylated equivalent (pepbeta3-phos), enhanced the amplitude of miniature inhibitory synaptic current and whole cell GABA(A)R current. These effects of pepbeta3 on GABA(A)R current were occluded by inhibitors of dynamin-dependent endocytosis supporting an action of pepbeta3 on GABA(A)R endocytosis. Therefore phospho-dependent regulation of AP2 binding to GABA(A)Rs provides a mechanism to specify receptor cell surface number and the efficacy of inhibitory synaptic transmission.

Dominant-negative synthesis suppression of voltage-gated calcium channel Cav2.2 induced by truncated constructs

Voltage-gated calcium channel alpha1 subunits consist of four domains (I-IV), each with six transmembrane segments. A number of truncated isoforms have been identified to occur as a result of alternative splicing or mutation. We have examined the functional consequences for expression of full-length Ca(v)2.2 (alpha1B) of its coexpression with truncated constructs of Ca(v)2.2. Domains I-II or domains III-IV, when expressed individually, together with the accessory subunits beta1b and alpha2delta-1, did not form functional channels. When they were coexpressed, low-density whole-cell currents and functional channels with properties similar to wild-type channels were observed. However, when domain I-II, domain III-IV, or domain I alone were coexpressed with full-length Ca(v)2.2, they markedly suppressed its functional expression, although at the single channel level, when channels were recorded, there were no differences in their biophysical properties. Furthermore, when it was coexpressed with either domain I-II or domain I, the fluorescence of green fluorescent protein (GFP)-Ca(v)2.2 and expression of Ca(v)2.2 protein was almost abolished. Suppression does not involve sequestration of the Ca(v)beta subunit, because loss of GFP-Ca(v)2.2 expression also occurred in the absence of beta subunit, and the effect of domain I-II or domain I could not be mimicked by the cytoplasmic I-II loop of Ca(v)2.2. It requires transmembrane segments, because the isolated Ca(v)2.2 N terminus did not have any effect. Our results indicate that the mechanism of suppression of Ca(v)2.2 by truncated constructs containing domain I involves inhibition of channel synthesis, which may represent a role of endogenously expressed truncated Ca(v) isoforms.

Interaction between G proteins and accessory subunits in the regulation of 1B calcium channels in Xenopus oocytes

The accessory beta subunits of voltage-dependent Ca2+ channels (VDCCs) have been shown to regulate their biophysical properties and have also been suggested to antagonise the G protein inhibition of N-type (alpha1B), P/Q-type (alpha1A) and alpha1E channels. Here we have examined the voltage-dependent involvement of the four neuronal isoforms (beta1b, beta2a, beta3 and beta4) in the process of G protein modulation of alpha1B Ca2+ channels. All beta subunits hyperpolarized alpha1B current activation, and all antagonised the G protein-mediated depolarisation of current activation. However, except in the case of beta2a, there was no generalised reduction by beta subunits in the maximal extent of receptor-mediated inhibition of alpha1B current. In addition, all VDCC beta subunits enhanced the rate of current facilitation at +100 mV, for both receptor-mediated and tonic modulation. The rank order for enhancement of facilitation rate was beta3 > beta4 > beta1b > beta2a. In contrast, the amount of voltage-dependent facilitation during tonic modulation was reduced by beta subunit co-expression, despite the fact that the apparent Gbetagamma dissociation rate at +100 mV was enhanced by beta subunits to a similar level as for agonist-induced modulation. Our data provide evidence that G protein activation antagonises Ca2+-channel beta subunit-induced hyperpolarisation of current activation. Conversely, co-expression of all beta subunits increases the apparent Gbetagamma dimer dissociation rate during a depolarising prepulse. This latter feature suggests the co-existence of bound Ca2+-channel beta subunits and Gbetagamma dimers on the alpha1B subunits. Future work will determine how the interaction between Gbetagamma dimers and Ca2+-channel beta subunits with alpha1B results in a functional antagonism at the molecular level.

The alpha1B Ca2+ channel amino terminus contributes determinants for beta subunit-mediated voltage-dependent inactivation properties

Co-expression of auxiliary beta subunits with the alpha1B Ca2+ channel subunit in COS-7 cells resulted in an increase in current density and a hyperpolarising shift in the mid-point of activation. Amongst the beta subunits, beta2a in particular, but also beta4 and beta1b caused a significant retardation of the voltage-dependent inactivation compared to currents with alpha1B alone, whilst no significant changes in inactivation properties were seen for the beta3 subunit in this system. Prevention of beta2a palmitoylation, by introducing cysteine to serine mutations (beta2a(C3,4S)), greatly reduced the ability of beta2a to retard voltage-dependent inactivation. Deletion of the proximal half of the alpha1B cytoplasmic amino terminus (alpha1BDelta1-55) differentially affected beta subunit-mediated voltage-dependent inactivation properties. These effects were prominent with the beta2a subunit and, to a lesser extent, with beta1b. For beta2a, the major effects of this deletion were a partial reversal of beta2a-mediated retardation of inactivation and the introduction of a fast component of inactivation, not seen with full-length alpha1B. Deletion of the amino terminus had no other major effects on the measured biophysical properties of alpha1B when co-expressed with beta subunits. Transfer of the whole alpha1B amino terminus into alpha1C (alpha1bCCCC) conferred a similar retardation of inactivation on alpha1C when co-expressed with beta2a to that seen in parental alpha1B. Individual (alpha1B(Q47A) and alpha1B(R52A)) and double (alpha1B(R52,54A)) point mutations within the amino terminus of alpha1B also opposed the beta2a-mediated retardation of alpha1B inactivation kinetics. These results indicate that the alpha1B amino terminus contains determinants for beta subunit-mediated voltage-dependent inactivation properties. Furthermore, effects were beta subunit selective. As deletion of the alpha1B amino terminus only partially opposed beta subunit-mediated changes in inactivation properties, the amino terminus is likely to contribute to a complex site necessary for complete beta subunit function.

Acidic motif responsible for plasma membrane association of the voltage-dependent calcium channel beta1b subunit

Voltage-dependent calcium channels consist of a pore-forming transmembrane alpha1-subunit, which is known to associate with a number of accessory subunits, including alpha2-delta- and beta-subunits. The beta-subunits, of which four have been identified (beta1-4), are intracellular proteins that have marked effects on calcium channel trafficking and function. In a previous study, we observed that the beta1b-subunit showed selective plasma membrane association when expressed alone in COS7 cells, whereas beta3 and beta4 did not. In this study, we have examined the basis for this, and have identified, by making chimeric beta-subunits, that the C-terminal region, which shows most diversity between beta-subunits, of beta1b is responsible for its plasma membrane association. Furthermore we have identified, by deletion mutations, an 11-amino acid motif present in the C terminus of beta1b but not in beta3 (amino acids 547-556 of beta1b, WEEEEDYEEE), which when deleted, reduces membrane association of beta1b. Future research aims to identify what is binding to this sequence in beta1b to promote membrane association of this calcium channel subunit. It is possible that such membrane association is important for the selective localization or clustering of particular calcium channels with which beta1b is associated.

Coexpression of mRNAs for P2X1, P2X2 and P2X4 receptors in rat vascular smooth muscle: an in situ hybridization and RT-PCR study

The expression of mRNAs for three P2X receptor subtypes (PX21, P2X2, P2X4) in the rat vascular system was studied by in situ hybridization and RT-PCR. In heart sections mRNAs transcripts for all three receptors were colocalized in smooth muscle cells of coronary vessels, while no specific positivity was apparent in myocardium. Coexpression of P2X receptor mRNA transcripts were also observed in other peripheral vessels, including aorta, pulmonary artery, internal and external iliac arteries, renal artery and femoral artery. By contrast, no mRNA transcripts of the above receptors were found in the superior mesenteric artery. RT-PCR performed on microdissected tissues (coronary arteries, aorta and myocardium from various heart areas) confirmed the presence of P2X1, P2X2 and P2X4 receptor mRNAs. Furthermore, in the same tissues two splice variants of the P2X2 receptor were identified. These results reveal an important molecular heterogeneity of P2X receptors, thus substantiating the possibility of a heteropolymeric assembly of ATP-gated ion channels in the cardiovascular system.

Characterisation of subtypes of the P2X and P2Y families of ATP receptors in the foetal human heart

ATP exerts a variety of actions within the myocardium, including the regulation of coronary vascular tone and modulation of the autonomic control of the heart. In order to characterise the ATP receptor subtypes involved in these effects, degenerate oligonucleotides were used to clone receptors of both P2X and P2Y families from the human foetal heart. About 1 ng of "Quick-Clone cDNA" from foetal human heart was subjected to amplification with two pairs of degenerate oligonucleotides designed to amplify subtypes of the P2X and P2Y receptor families by means of PCR reactions. The sequence analysis of 34 and 29 clones of the P2X and P2Y receptor families, respectively, demonstrated that P2X1, P2X3 and P2X4 subtypes are present in the human foetal heart together with P2Y6, P2Y2 and P2Y4 receptors. P2X1 and P2Y4 receptor subtypes were here characterised for the first time in the human foetal heart. The present study provides the first molecular characterisation of ATP receptors in the foetal human heart. The results show that many P2 receptor subtypes are expressed in the foetal human heart, perhaps contributing to developmental processes as well as to the activity of the foetal heart.