Functional characterization of two 5-HT3 receptor splice variants isolated from a mouse hippocampal cell line

Feature Article: Selective modulation of tonically active GABAA receptor functional subgroups by G-proteins and protein kinase C

IMPACT STATEMENT

Here we study intracellular mechanisms which regulate inhibitory signaling delivered through continuously (tonically) open ionotropic receptors of γ-aminobutyric acid (GABA) of dentate gyrus granule cells (DGCs). We found that, apart of classical GABA-A receptors (GABA_ARs) which can be activated by GABA binding, a significant part of tonic inhibitory current is delivered by newly discovered spontaneously opening GABA_ARs (s-GABA_ARs), which enter active state without binding of GABA. We have also found that conventional GABA_ARs and s-GABA_ARs are regulated by different intracellular mechanisms, which may overlap and thus induce various signaling repercussions. Our results demonstrate that s-GABA_ARs play a key role in the mechanism that implements DGCs functional role in the brain. On top of that, since regulatory mechanisms under study are affected in a number of pathological states, our results may have broad implications for treatment of neurological disorders.

Expression and function of 5-HT3 receptors in the enteric neurons of mice lacking the serotonin transporter

The actions of enteric 5-HT are terminated by 5-HT transporter (SERT)-mediated uptake, and gastrointestinal motility is abnormal in SERT -/- mice. We tested the hypothesis that adaptive changes in enteric 5-HT(3) receptors help SERT -/- mice survive despite inefficient 5-HT inactivation. Expression of mRNA encoding enteric 5-HT(3A) subunits was similar in SERT +/+ and -/- mice, but that of 5-HT(3B) subunits was fourfold less in SERT -/- mice. 5-HT(3B) mRNA was found, by in situ hybridization, in epithelial cells and enteric neurons. 5-HT evoked a fast inward current in myenteric neurons that was pharmacologically identified as 5-HT(3) mediated. The EC(50) of the 5-HT response was lower in SERT +/+ (18 microM) than in SERT -/- (36 microM) mice and desensitized rapidly in a greater proportion of SERT -/- neurons; however, peak amplitudes, steady-state current, and decay time constants were not different. Adaptive changes thus occur in the subunit composition of enteric 5-HT(3) receptors of SERT -/- mice that are reflected in 5-HT(3) receptor affinity and desensitization.

Direct noncompetitive inhibition of 5-HT(3) receptor-mediated responses by forskolin and steroids

5-HT(3) receptors cloned from NCB-20 cells were expressed in Xenopus oocytes, and the effects of forskolin and steroids on the function of the receptors were investigated using the two-electrode voltage-clamp technique. Forskolin, 17-beta-estradiol, and progesterone inhibited the currents activated by 1 microM 5-HT in a reversible and concentration-dependent manner, with IC(50) values of 12, 33, and 89 microM, respectively. The inhibitory effects of forskolin and 17-beta-estradiol were independent of the membrane potential. Forskolin and 17-beta-estradiol significantly reduced the maximal amplitude of the 5-HT concentration-response curve (E(max)) without significantly affecting the EC(50), indicating that these compounds act as noncompetitive inhibitors of the 5-HT(3) receptor. The cAMP analogue, 8-Br-cAMP (0.2 mM), and the protein kinase A activator, Sp-cAMP (0.1 mM), did not affect the amplitude of 5-HT(3) receptor-mediated currents. The membrane-permeable protein kinase A inhibitor Rp-cAMP (0.1 mM) and the estrogen-receptor antagonist tamoxifen (1 microM) did not affect the inhibition of 5-HT-activated current. In addition, 5-HT(3) receptor-mediated currents were inhibited by both 1,9-dideoxy forskolin (30 microM), which does not activate adenylyl cyclase, and wForskolin (30 microM), a charged hydrophilic analogue of forskolin that is membrane impermeable. These results indicate that both forskolin and 17-beta-estradiol inhibit the function of the 5-HT(3) receptor in a noncompetitive manner and that this inhibition is independent of cAMP levels.

The role of the agonist binding site in Ca(2+) inhibition of the recombinant 5-HT(3A) receptor

The mechanism and site of action of Ca(2+) at the recombinant murine 5-hydroxytryptamine (5-HT)(3A) receptor were investigated using whole-cell voltage clamp, radioligand binding and single-cell Ca(2+) imaging. Inhibition of the 5-HT (3 microM)-induced response by 10 mM Ca(2+) reached a plateau at 68.5% inhibition, with half-maximal effect at 2.6 mM. This was due to an increase in EC(50) from 2.35 to 3.87 microM and a 30% reduction in I(max). Ca(2+) also resulted in the inhibition of binding of both 5-HT(3) receptor agonist [3H]m-chlorophenylbiguanide and antagonist [3H]granisetron due to an increase in K(d), with no change in B(max). An increase in EC(50) from 2.6 (1 mM Ca(2+)) to 4.7 microM (10 mM Ca(2+)), with no change in maximal [Ca(2+)](i), was observed from Ca(2+) imaging studies. Largely similar effects were observed with Mg(2+). The combined data suggest that Ca(2+) acting at a site that directly or indirectly influences the agonist binding site plays a significant role in its inhibitory effect at the 5-HT(3) receptor.

Open probability of homomeric murine 5-HT3A serotonin receptors depends on subunit occupancy

1. The time course of macroscopic current responses of homomeric murine serotonin 5-HT3A receptors was studied in whole cells and excised membrane patches under voltage clamp in response to rapid application of serotonin. 2. Serotonin activated whole cell currents with an EC(50) value for the peak response of 2 microM and a Hill slope of 3.0 (n = 12), suggesting that the binding of at least three agonist molecules is required to open the channel. 3. Homomeric 5-HT3A receptors in excised membrane patches had a slow activation time course (mean +/- S.E.M. 10-90 % rise time 12.5 +/- 1.6 ms; n = 9 patches) for 100 microM serotonin. The apparent activation rate was estimated by fitting an exponential function to the rising phase of responses to supramaximal serotonin to be 136 s(-1). 4. The 5-HT3A receptor response to 100 microM serotonin in outside-out patches (n = 19) and whole cells (n = 41) desensitized with a variable rate that accelerated throughout the experiment. The time course for desensitization was described by two exponential components (for patches tau(slow) 1006 +/- 139 ms, amplitude 31 %; tau(fast) 176 +/- 25 ms, amplitude 69 %). 5. Deactivation of the response following serotonin removal from excised membrane patches (n = 8) and whole cells (n = 29) was described by a dual exponential time course with time constants similar to those for desensitization (for patches tau(slow) 838 +/- 217 ms, 55 % amplitude; tau(fast) 213 +/- 44 ms, 45 % amplitude). 6. In most patches (6 of 8), the deactivation time course in response to a brief 1-5 ms pulse of serotonin was similar to or slower than desensitization. This suggests that the continued presence of agonist can induce desensitization with a similar or more rapid time course than agonist unbinding. The difference between the time course for deactivation and desensitization was voltage independent over the range -100 to -40 mV in patches (n = 4) and -100 to +50 mV in whole cells (n = 4), suggesting desensitization of these receptors in the presence of serotonin does not reflect a voltage-dependent block of the channel by agonist. 7. Simultaneously fitting the macroscopic 5-HT3A receptor responses in patches to submaximal (2 microM) and maximal (100 microM) concentrations of serotonin to a variety of state models suggests that homomeric 5-HT3A receptors require the binding of three agonists to open and possess a peak open probability greater than 0.8. Our modelling also suggests that channel open probability varies with the number of serotonin molecules bound to the receptor, with a reduced open probability for fully liganded receptors. Increasing the desensitization rate constants in this model can generate desensitization that is more rapid than deactivation, as observed in a subpopulation of our patches.

Functional differences between splice variants of the murine 5-HT(3A) receptor: possible role for phosphorylation

The murine 5-HT(3A) receptor subunit is expressed as either of two splice variants which are differentially regulated in vivo. The difference resides in a six-amino acid sequence within the cytoplasmic loop between transmembrane regions 3 and 4, which is present in the long form but not the short form. No physiological roles have yet been ascribed to the two splice variants. Whole cell patch clamp recording from transfected HEK 293 cells stably expressing either long or short form receptors showed very similar responses under control conditions. However, inclusion of 1 mM cAMP (activator of protein kinase A) in the patch pipette caused an initial increase in the desensitization rate of the long form, but a decrease in the short form. With the addition of 100 nM phorbol 12-myristate 13-acetate (PMA; activator of protein kinase C) to the pipette solution, responses elicited with 1 microM 5-HT revealed an increase in the current amplitude in the long but not the short form of the receptor. Over a longer time period, inclusion of PMA in the patch-pipette caused a faster run down of peak current amplitude in response to 30 microM 5-HT in the long form but did not affect the short form; there was no observed long-term effects of cAMP. We conclude that the long and short forms of the 5-HT(3) receptor are differentially modulated by agents that activate PKA and PKC. These different patterns of modulation could have markedly divergent consequences on receptor function.

Glycinergic potentiation by some 5-HT(3) receptor antagonists: insight into selectivity

The ability of various 5-HT(3) receptor antagonists to potentiate spinal glycine responses was investigated. Whereas (3-alpha-tropanyl)-1H-indole-3-carboxylate (ICS 205930), (3-alpha-tropanyl)-3,5-dichlorobenzoate (MDL 72222) and 1-methyl-N-(3-alpha-tropanyl)-1H-indazole-3-carboxamide (LY 278584) exhibited this property, even in identified motoneurones, several other chemically similar 5-HT(3) receptor antagonists did not. Introducing a methyl group on the nitrogen of the azabicyclo moiety of ICS 205930 greatly reduced the ability to potentiate glycine responses. Neither endo-1-methyl-N-(9-methyl-9-azabicyclo[3.3. 1]non-3-yl)-indazole-3-carboxamide (granisetron), differing from LY 278584 by an additional carbon in this cycle, nor 2beta-carbomethoxy-3beta-benzoyloxytropane (cocaine), 1,2,3, 9-tetrahydro-9-methyl-3-[(2-methyl-1H-imidazol-1-yl)-methyl]-4H-carba zol-4-one (ondansetron) and (S)-4-amino-N-(1-azabicyclo[2.2. 2]oct-3-yl)-5-chloro-2-methoxy-benzamide ((S)-zacopride) could potentiate glycine responses. A pharmacophore model of the glycinergic potentiators was generated by molecular modelling using MDL 72222 as a template. According to this model, an aromatic ring, a carbonyl group and a tropane nitrogen atom are required for glycinergic potentiation, as previously described for 5-HT(3) receptor antagonism. However, the steric allowance at the glycine receptor site and the tridimensional arrangement of the pharmacophoric elements appear to be more restricted.

The role of tryptophan residues in the 5-Hydroxytryptamine(3) receptor ligand binding domain

Aromatic amino acids are important components of the ligand binding site in the Cys loop family of ligand-gated ion channels. To examine the role of tryptophan residues in the ligand binding domain of the 5-hydroxytryptamine(3) (5-HT(3)) receptor, we used site-directed mutagenesis to change each of the eight N-terminal tryptophan residues in the 5-HT(3A) receptor subunit to tyrosine or serine. The mutants were expressed as homomeric 5-HT(3A) receptors in HEK293 cells and analyzed with radioligand binding, electrophysiology, and immunocytochemistry. Mutation of Trp(90), Trp(183), and Trp(195) to tyrosine resulted in functional receptors, although with increased EC(50) values (2-92-fold) to 5-HT(3) receptor agonists. Changing these residues to serine either ablated function (Trp(90) and Trp(183)) or resulted in a further increase in EC(50) (Trp(195)). Mutation of residue Trp(60) had no effect on ligand binding or receptor function, whereas mutation of Trp(95), Trp(102), Trp(121), and Trp(214) ablated ligand binding and receptor function, and all but one of the receptors containing these mutations were not expressed at the plasma membrane. We propose that Trp(90), Trp(183), and Trp(195) are intimately involved in ligand binding, whereas Trp(95), Trp(102), Trp(121), and Trp(214) have a critical role in receptor structure or assembly.

Promiscuous coassembly of serotonin 5-HT3 and nicotinic alpha4 receptor subunits into Ca(2+)-permeable ion channels

Serotonin (5-hydroxytryptamine) type 3 receptors (5-HT3R) and nicotinic acetylcholine receptors are structurally and functionally related proteins, yet distinct members of the family of ligand-gated ion channels. For most members of this family a diversity of heteromeric receptors is known at present. In contrast, known 5-HT3R subunits are all homologs of the same 5-HT3R-A subunit and form homopentameric receptors. Here we show, by heterologous expression followed by immunoprecipitation, that 5-HT3R and nicotinic acetylcholine receptor alpha4 subunits coassemble into a novel type of heteromeric ligand-gated ion channel, which is activated by 5-HT. The Ca2+ permeability of this heteromeric ion channel is enhanced as compared with that of the homomeric 5-HT3R channel. Heteromeric 5-HT3/alpha4 and homomeric 5-HT3Rs have similar pharmacological profiles, but distinct sensitivities to block by the antagonist d-tubocurarine. Coassembly of subunits beyond the boundaries of ligand-gated ion channel families may constitute an important mechanism contributing to the diverse properties and functions of native neurotransmitter receptors.

Ionotropic and metabotropic activation of a neuronal chloride channel by serotonin and dopamine in the leech Hirudo medicinalis

1. Cl- channels on the pressure-sensitive (P) neuron in the leech are directly activated by synaptic release of serotonin (5-HT) and are indirectly stimulated by the cAMP second messenger pathway, suggesting an unusual dual regulation of the channels. We have investigated the mode of action of 5-HT and dopamine (DA) on a Cl- channel in adult P cells in culture by recording from cell-attached patches. 2. 5-HT increased Cl- channel activity only when included in the recording pipette and not when applied in the bath. 3. Pipette or, more effectively, bath application of DA led to an increase in Cl- channel activity. This effect was blocked by the potent and specific dopaminergic (DA1) receptor blocker, SCH-23390. 4. The stimulation by DA, but not by 5-HT, was also blocked by the cAMP-dependent protein kinase A (PKA) inhibitor Rp-cAMP and was mimicked by the membrane-permeant cAMP analogue dibutyryl cAMP (db-cAMP). 5. Our results show that 5-HT directly gates a Cl- channel that is also activated by DA via the cAMP pathway. This study demonstrates that a ligand-gated channel can be independently operated by another transmitter acting via a second messenger pathway.

Ion permeation and conduction in a human recombinant 5-HT3 receptor subunit (h5-HT3A)

1. A human recombinant homo-oligomeric 5-HT3 receptor (h5-HT3A) expressed in a human embryonic kidney cell line (HEK 293) was characterized using the whole-cell recording configuration of the patch clamp technique. 2. 5-HT evoked transient inward currents (EC50 = 3.4 microM; Hill coefficient = 1.8) that were blocked by the 5-HT3 receptor antagonist ondansetron (IC50 = 103 pM) and by the non-selective agents metoclopramide (IC50 = 69 nM), cocaine (IC50 = 459 nM) and (+)-tubocurarine (IC50 = 2.8 microM). 3. 5-HT-induced currents rectified inwardly and reversed in sign (E5-HT) at a potential of -2.2 mV. N-Methyl-D-glucamine was finitely permeant. Permeability ratios PNa/PCs and PNMDG/PCs were 0.90 and 0.083, respectively. 4. Permeability towards divalent cations was assessed from measurements of E5-HT in media where Ca2+ and Mg2+ replaced Na+. PCa/PCs and PMg/PCs were calculated to be 1.00 and 0.61, respectively. 5. Single channel chord conductance (gamma) estimated from fluctuation analysis of macroscopic currents increased with membrane hyperpolarization from 243 fS at -40 mV to 742 fS at -100 mV. 6. Reducing [Ca2+]o from 2 to 0.1 mM caused an increase in the whole-cell current evoked by 5-HT. A concomitant reduction in [Mg2+]o produced further potentiation. Fluctuation analysis indicates that a voltage-independent augmentation of gamma contributes to this phenomenon. 7. The data indicate that homo-oligomeric receptors composed of h5-HT3A subunits form inwardly rectifying cation-selective ion channels of low conductance that are permeable to Ca2+ and Mg2+.

Different efficacy of specific agonists at 5-HT3 receptor splice variants: the role of the extra six amino acid segment

1. Whole cell voltage clamp electrophysiology and radioligand binding were used to examine the agonist characteristics of the two splice variants of the 5-HT3 receptor which have been cloned from neuronal cell lines. Homo-oligomeric 5-HT3 receptors were examined in HEK 293 cells stably transfected with either long (5-HT3-L) or short (5-HT3-S) receptor subunit DNAs. 2. Functional homo-oligomeric receptors were formed from both subunits, and responses to 5-HT3 receptor agonists (5-hydroxytryptamine (5-HT), 2-methyl 5-HT and m-chlorophenylbiguanide) were qualitatively similar. 3. Maximum currents (Rmax) elicited by the 5-HT3 receptor agonists m-chlorophenylbiguanide (mCPBG) and 2-methyl-5-HT (2-Me-5-HT), as compared to 5-HT, differed in the two splice variants: Rmax mCPBG/Rmax 5-HT values were 0.68+/-0.04 and 0.91+/-0.01 in 5-HT3-L and 5-HT3-S receptors, respectively. Comparable values for 2-Me-5-HT were 0.30+/-0.02 and 0.23+/-0.02. 4. Radioligand binding data showed no difference in affinity of agonist or antagonist binding sites; thus the six amino acid deletion appears to cause differences in agonist efficacy. 5. The role of the 6 amino acid insertion was further investigated by use of site-directed mutagenesis to create two mutant receptors, one where serine 286 was replaced with alanine, and the second where all 6 amino acids were replaced with alanines. 6. Examination of the mutant receptors when stably expressed in HEK 293 cells revealed agonist properties resembling long and not short 5-HT3 receptors. Thus specific amino acids in this region are not responsible for the observed differences. 7. The data show intracellular structure can have significant effects on ligand-gated ion channel function, and suggest that minor changes in structure may be responsible for differences in function observed when ligand-gated ion channel proteins are modulated intracellularly.

A novel slow hyperpolarization-activated potassium current (IK(SHA)) from a mouse hippocampal cell line

1. A slow hyperpolarization-activated inwardly rectifying K+ current (IK(SHA)) with novel characteristics was identified from the mouse embryonic hippocampus x neuroblastoma cell line HN9.10e. 2. The non-inactivating current activated negative to a membrane potential of -80 mV with slow and complex activation kinetics (tau act approximately 1-7 s) and a characteristic delay of 1-10 s (-80 to -140 mV) that was linearly dependent on the membrane potential. 3. Tail currents and instantaneous open channel currents determined through fast voltage ramps reversed at the K+ equilibrium potential (EK) indicating that primarily K+, but not Na+, permeated the channels. 4. IK(SHA) was unaffected by altering the intracellular Ca2+ concentration between approximately 0 and 10 microM, but was susceptible to block by 5 mM extracellular Ca2+, Ba2+ (Ki = 0.42 mM), and Cs+ (Ki = 2.77 mM) 5. In cells stably transformed with M2 muscarinic receptors, IK(SHA) was rapidly, but reversibly, suppressed by application of micromolar concentrations of muscarine. 6. At the single channel level K(SHA) channel openings were observed with the characteristic delay upon membrane hyperpolarization. Analysis of unitary currents revealed an inwardly rectifying I-V profile and a channel slope conductance of 7 pS. Channel activity persisted in the inside-out configuration for many minutes. 7. It is concluded that IK(SHA) in HN9.10e cells represents a novel K+ current, which is activated upon membrane hyperpolarization. It is functionally different from both classic inwardly rectifying IKir currents and other cationic hyperpolarization-activated IH currents that have been previously described in neuronal or glial cells.