β-Lactam TRPM8 Antagonists Derived from Phe-Phenylalaninol Conjugates: Structure-Activity Relationships and Antiallodynic Activity

The protein transient receptor potential melastatin type 8 (TRPM8), a non-selective, calcium (Ca2+)-permeable ion channel is implicated in several pathological conditions, including neuropathic pain states. In our previous research endeavors, we have identified β-lactam derivatives with high hydrophobic character that exhibit potent and selective TRPM8 antagonist activity. This work describes the synthesis of novel derivatives featuring C-terminal amides and diversely substituted N'-terminal monobenzyl groups in an attempt to increase the total polar surface area (TPSA) in this family of compounds. The primary goal was to assess the influence of these substituents on the inhibition of menthol-induced cellular Ca2+ entry, thereby establishing critical structure-activity relationships. While the substitution of the tert-butyl ester by isobutyl amide moieties improved the antagonist activity, none of the N'-monobencyl derivatives, regardless of the substituent on the phenyl ring, achieved the activity of the model dibenzyl compound. The antagonist potency of the most effective compounds was subsequently verified using Patch-Clamp electrophysiology experiments. Furthermore, we evaluated the selectivity of one of these compounds against other members of the transient receptor potential (TRP) ion channel family and some receptors connected to peripheral pain pathways. This compound demonstrated specificity for TRPM8 channels. To better comprehend the potential mode of interaction, we conducted docking experiments to uncover plausible binding sites on the functionally active tetrameric protein. While the four main populated poses are located by the pore zone, a similar location to that described for the N-(3-aminopropyl)-2-[(3-methylphenyl)methoxy]-N-(2-thienylmethyl)-benzamide (AMTB) antagonist cannot be discarded. Finally, in vivo experiments, involving a couple of selected compounds, revealed significant antinociceptive activity within a mice model of cold allodynia induced by oxaliplatin (OXA).

Comprehensive Survey of Consensus Docking for High-Throughput Virtual Screening

The rapid advances of 3D techniques for the structural determination of proteins and the development of numerous computational methods and strategies have led to identifying highly active compounds in computer drug design. Molecular docking is a method widely used in high-throughput virtual screening campaigns to filter potential ligands targeted to proteins. A great variety of docking programs are currently available, which differ in the algorithms and approaches used to predict the binding mode and the affinity of the ligand. All programs heavily rely on scoring functions to accurately predict ligand binding affinity, and despite differences in performance, none of these docking programs is preferable to the others. To overcome this problem, consensus scoring methods improve the outcome of virtual screening by averaging the rank or score of individual molecules obtained from different docking programs. The successful application of consensus docking in high-throughput virtual screening highlights the need to optimize the predictive power of molecular docking methods.

ThermoTRP channels in pain sexual dimorphism: new insights for drug intervention

Chronic pain is a major burden for the society and remains more prevalent and severe in females. The presence of chronic pain is linked to persistent alterations in the peripheral and the central nervous system. One of the main types of peripheral pain transducers are the transient receptor potential channels (TRP), also known as thermoTRP channels, which intervene in the perception of hot and cold external stimuli. These channels, and especially TRPV1, TRPA1 and TRPM8, have been subjected to profound investigation because of their role as thermosensors and also because of their implication in acute and chronic pain. Surprisingly, their sensitivity to endogenous signaling has been far less studied. Cumulative evidence suggests that the function of these channels may be differently modulated in males and females, in part through sexual hormones, and this could constitute a significant contributor to the sex differences in chronic pain. Here, we review the exciting advances in thermoTRP pharmacology for males and females in two paradigmatic types of chronic pain with a strong peripheral component: chronic migraine and chemotherapy-induced peripheral neuropathy (CIPN). The possibilities of peripheral druggability offered by these channels and the differential exploitation for men and women represent a development opportunity that will lead to a significant increment of the armamentarium of analgesic medicines for personalized chronic pain treatment.

TRPV1 in chronic pruritus and pain: Soft modulation as a therapeutic strategy

Chronic pain and pruritus are highly disabling pathologies that still lack appropriate therapeutic intervention. At cellular level the transduction and transmission of pain and pruritogenic signals are closely intertwined, negatively modulating each other. The molecular and cellular pathways involved are multifactorial and complex, including peripheral and central components. Peripherally, pain and itch are produced by subpopulations of specialized nociceptors that recognize and transduce algesic and pruritogenic signals. Although still under intense investigation, cumulative evidence is pointing to the thermosensory channel TRPV1 as a hub for a large number of pro-algesic and itchy agents. TRPV1 appears metabolically coupled to most neural receptors that recognize algesic and pruritic molecules. Thus, targeting TRPV1 function appears as a valuable and reasonable therapeutic strategy. In support of this tenet, capsaicin, a desensitizing TRPV1 agonist, has been shown to exhibit clinically relevant analgesic, anti-inflammatory, and anti-pruritic activities. However, potent TRPV1 antagonists have been questioned due to an hyperthermic secondary effect that prevented their clinical development. Thus, softer strategies directed to modulate peripheral TRPV1 function appear warranted to alleviate chronic pain and itch. In this regard, soft, deactivatable TRPV1 antagonists for topical or local application appear as an innovative approach for improving the distressing painful and itchy symptoms of patients suffering chronic pain or pruritus. Here, we review the data on these compounds and propose that this strategy could be used to target other peripheral therapeutic targets.

β-Lactam TRPM8 Antagonist RGM8-51 Displays Antinociceptive Activity in Different Animal Models

Transient receptor potential melastatin subtype 8 (TRPM8) is a cation channel extensively expressed in sensory neurons and implicated in different painful states. However, the effectiveness of TRPM8 modulators for pain relief is still a matter of discussion, since structurally diverse modulators lead to different results, depending on the animal pain model. In this work, we described the antinociceptive activity of a β–lactam derivative, RGM8-51, showing good TRPM8 antagonist activity, and selectivity against related thermoTRP channels and other pain-mediating receptors. In primary cultures of rat dorsal root ganglion (DRG) neurons, RGM8-51 potently reduced menthol-evoked neuronal firing without affecting the major ion conductances responsible for action potential generation. This compound has in vivo antinociceptive activity in response to cold, in a mouse model of oxaliplatin-induced peripheral neuropathy. In addition, it reduces cold, mechanical and heat hypersensitivity in a rat model of neuropathic pain arising after chronic constriction of the sciatic nerve. Furthermore, RGM8-51 exhibits mechanical hypersensitivity-relieving activity, in a mouse model of NTG-induced hyperesthesia. Taken together, these preclinical results substantiate that this TRPM8 antagonist is a promising pharmacological tool to study TRPM8-related diseases.

Peimine, an Anti-Inflammatory Compound from Chinese Herbal Extracts, Modulates Muscle-Type Nicotinic Receptors

Fritillaria bulbs are used in Traditional Chinese Medicine to treat several illnesses. Peimine (Pm), an anti-inflammatory compound from Fritillaria, is known to inhibit some voltage-dependent ion channels and muscarinic receptors, but its interaction with ligand-gated ion channels remains unexplored. We have studied if Pm affects nicotinic acetylcholine receptors (nAChRs), since they play broad functional roles, both in the nervous system and non-neuronal tissues. Muscle-type nAChRs were incorporated to Xenopus oocytes and the action of Pm on the membrane currents elicited by ACh (I_AChs) was assessed. Functional studies were combined with virtual docking and molecular dynamics assays. Co-application of ACh and Pm reversibly blocked I_ACh, with an IC₅₀ in the low micromolar range. Pm inhibited nAChR by: (i) open-channel blockade, evidenced by the voltage-dependent inhibition of I_Ach, (ii) enhancement of nAChR desensitization, revealed by both an accelerated I_ACh decay and a decelerated I_ACh deactivation, and (iii) resting-nAChR blockade, deduced from the I_ACh inhibition elicited by Pm when applied before ACh superfusion. In good concordance, virtual docking and molecular dynamics assays demonstrated that Pm binds to different sites at the nAChR, mostly at the transmembrane domain. Thus, Pm from Fritillaria bulbs, considered therapeutic herbs, targets nAChRs with high affinity, which might account for its anti-inflammatory actions.

Investigational drugs in early phase clinical trials targeting thermotransient receptor potential (thermoTRP) channels

INTRODUCTION

Thermo transient receptor potential (thermoTRP) channels are some of the most intensely pursued therapeutic targets of the past decade. They are considered promising targets of numerous diseases including chronic pain and cancer. Modulators of these proteins, in particular TRPV1-4, TRPM8 and TRPA1, have reached clinical development, but none has been approved for clinical practice yet.

AREAS COVERED

The therapeutic potential of targeting thermoTRP channels is discussed. The discussion is centered on our experience and on available data found in SciFinder, PubMed, and ClinicalTrials.gov database from the past decade. This review focuses on the therapeutic progress concerning this family of channels, including strategies to improve their therapeutic index for overcoming adverse effects.

EXPERT OPINION

Although thermoTRPs are pivotal drug targets, translation to the clinic has faced two key problems, (i) unforeseen side effects in Phase I trials and, (ii) poor clinical efficacy in Phase II trials. Thus, there is a need for (i) an enhanced understanding of the physiological role of these channels in tissues and organs and (ii) the development of human-based pre-clinical models with higher clinical translation. Furthermore, progress in nanotechnology-based delivery strategies will positively impact thermoTRP human pharmacology.

Targeting thermoTRP ion channels: in silico preclinical approaches and opportunities

INTRODUCTION

A myriad of cellular pathophysiological responses are mediated by polymodal ion channels that respond to chemical and physical stimuli such as thermoTRP channels. Intriguingly, these channels are pivotal therapeutic targets with limited clinical pharmacology. In silico methods offer an unprecedented opportunity for discovering new lead compounds targeting thermoTRP channels with improved pharmacological activity and therapeutic index.

AREAS COVERED

This article reviews the progress on thermoTRP channel pharmacology because of (i) advances in solving their atomic structure using cryo-electron microscopy and, (ii) progress on computational techniques including homology modeling, molecular docking, virtual screening, molecular dynamics, ADME/Tox and artificial intelligence. Together, they have increased the number of lead compounds with clinical potential to treat a variety of pathologies. We used original and review articles from Pubmed (1997-2020), as well as the clinicaltrials.gov database, containing the terms thermoTRP, artificial intelligence, docking, and molecular dynamics.

EXPERT OPINION

The atomic structure of thermoTRP channels along with computational methods constitute a realistic first line strategy for designing drug candidates with improved pharmacology and clinical translation. In silico approaches can also help predict potential side-effects that can limit clinical development of drug candidates. Together, they should provide drug candidates with upgraded therapeutic properties.

Highly functionalized β-lactams and 2-ketopiperazines as TRPM8 antagonists with antiallodynic activity

The cool sensor transient receptor potential melastatin channel 8 (TRPM8) is highly expressed in trigeminal and dorsal root ganglia, playing a key role in cold hypersensitivity associated to different peripheral neuropathies. Moreover, these channels are aberrantly expressed in different cancers, and seem to participate in tumor progression, survival and invasion. Accordingly, the search for potent and selective TRPM8 modulators attracted great interest in recent years. We describe new heterocyclic TRPM8 antagonist chemotypes derived from N-cloroalkyl phenylalaninol-Phe conjugates. The cyclization of these conjugates afforded highly substituted β-lactams and/or 2-ketopiperazine (KP) derivatives, with regioselectivity depending on the N-chloroalkyl group and the configuration. These derivatives behave as TRPM8 antagonists in the Ca2+ microfluorometry assay, and confirmed electrophysiologically for the best enantiopure β-lactams 24a and 29a (IC50, 1.4 and 0.8 µM). Two putative binding sites by the pore zone, different from those found for typical agonists and antagonists, were identified by in silico studies for both β-lactams and KPs. β-Lactams 24a and 29a display antitumor activity in different human tumor cell lines (micromolar potencies, A549, HT29, PSN1), but correlation with TRPM8 expression could not be established. Additionally, compound 24a significantly reduced cold allodynia in a mice model of oxaliplatin-induced peripheral neuropathy.

Structure-Based Design of Novel Biphenyl Amide Antagonists of Human Transient Receptor Potential Cation Channel Subfamily M Member 8 Channels with Potential Implications in the Treatment of Sensory Neuropathies

Structure-activity relationship studies of a reported menthol-based transient receptor potential cation channel subfamily M member 8 channel (TRPM8) antagonist, guided by computational simulations and structure-based design, uncovers a novel series of TRPM8 antagonists with >10-fold selectivity versus related TRP subtypes. Spiro[4.5]decan-8-yl analogue 14 inhibits icilin-evoked Ca²⁺ entry in HEK-293 cells stably expressing human TRPM8 (hTRPM8) with an IC₅₀ of 2.4 ± 1.0 nM, while in whole-cell patch-clamp recordings this analogue inhibits menthol-evoked currents with a hTRPM8 IC₅₀ of 64 ± 2 nM. Molecular dynamics (MD) simulations of compound 14 in our homology model of hTRPM8 suggest that this antagonist forms extensive hydrophobic contacts within the orthosteric site. In the wet dog shakes (WDS) assay, compound 14 dose-dependently blocks icilin-triggered shaking behaviors in mice. Upon local administration, compound 14 dose dependently inhibits cold allodynia evoked by the chemotherapy oxaliplatin in a murine model of peripheral neuropathy at microgram doses. Our findings suggest that 14 and other biphenyl amide analogues within our series can find utility as potent antagonist chemical probes derived from (-)-menthol as well as small molecule therapeutic scaffolds for chemotherapy-induced peripheral neuropathy (CIPN) and other sensory neuropathies.

Mechanisms of Blockade of the Muscle-Type Nicotinic Receptor by Benzocaine, a Permanently Uncharged Local Anesthetic

Most local anesthetics (LAs) are amine compounds bearing one or several phenolic rings. Many of them are protonated at physiological pH, but benzocaine (Bzc) is permanently uncharged, which is relevant because the effects of LAs on nicotinic acetylcholine (ACh) receptors (nAChRs) depend on their presence as uncharged or protonated species. The aims of this study were to assess the effects of Bzc on nAChRs and to correlate them with its binding to putative interacting sites on this receptor. nAChRs from Torpedo electroplaques were microtransplanted to Xenopus oocytes and currents elicited by ACh (I_AChs), either alone or together with Bzc, were recorded at different potentials. Co-application of ACh with increasing concentrations of Bzc showed that Bzc reversibly blocked nAChRs. I_ACh inhibition by Bzc was voltage-independent, but the I_ACh rebound elicited when rinsing Bzc suggests an open-channel blockade. Besides, ACh and Bzc co-application enhanced nAChR desensitization. When Bzc was just pre-applied it also inhibited I_ACh, by blocking closed (resting) nAChRs. This blockade slowed down the kinetics of both the I_ACh activation and the recovery from blockade. The electrophysiological results indicate that Bzc effects on nAChRs are similar to those of 2,6-dimethylaniline, an analogue of the hydrophobic moiety of lidocaine. Furthermore, docking assays on models of the nAChR revealed that Bzc and DMA binding sites on nAChRs overlap fairly well. These results demonstrate that Bzc inhibits nAChRs by multiple mechanisms and contribute to better understanding both the modulation of nAChRs and how LAs elicit some of their clinical side effects. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

The calmodulin-binding tetraleucine motif of KCNE4 is responsible for association with Kv1.3

The voltage-dependent potassium (Kv) channel Kv1.3 regulates leukocyte proliferation, activation, and apoptosis, and altered expression of this channel is linked to autoimmune diseases. Thus, the fine-tuning of Kv1.3 function is crucial for the immune system response. The Kv1.3 accessory protein, potassium voltage-gated channel subfamily E (KCNE) subunit 4, acts as a dominant negative regulatory subunit to both enhance inactivation and induce intracellular retention of Kv1.3. Mutations in KCNE4 also cause immune system dysfunction. Although the formation of Kv1.3-KCNE4 complexes has profound consequences for leukocyte physiology, the molecular determinants involved in the Kv1.3-KCNE4 association are unknown. We now show that KCNE4 associates with Kv1.3 via a tetraleucine motif situated within the carboxy-terminal domain of this accessory protein. This motif would function as an interaction platform, in which Kv1.3 and Ca²⁺/calmodulin compete for the KCNE4 interaction. Finally, we propose a structural model of the Kv1.3-KCNE4 complex. Our experimental data and the in silico structure suggest that the KCNE4 interaction hides a forward-trafficking motif within Kv1.3 in addition to adding a strong endoplasmic reticulum retention signature to the Kv1.3-KCNE4 complex. Thus, the oligomeric composition of the Kv1.3 channelosome fine-tunes the precise balance between anterograde and intracellular retention elements that control the cell surface expression of Kv1.3 and immune system physiology.-Solé, L., Roig, S. R., Sastre, D., Vallejo-Gracia, A., Serrano-Albarrás, A., Ferrer-Montiel, A., Fernández-Ballester, G., Tamkun, M. M., Felipe, A. The calmodulin-binding tetraleucine motif of KCNE4 is responsible for association with Kv1.3.

Mechanisms Underlying the Strong Inhibition of Muscle-Type Nicotinic Receptors by Tetracaine

Nicotinic acetylcholine (ACh) receptors (nAChRs) are included among the targets of a variety of local anesthetics, although the molecular mechanisms of blockade are still poorly understood. Some local anesthetics, such as lidocaine, act on nAChRs by different means through their ability to present as both charged and uncharged molecules. Thus, we explored the mechanisms of nAChR blockade by tetracaine, which at physiological pH is almost exclusively present as a positively charged local anesthetic. The nAChRs from Torpedo electroplaques were transplanted to Xenopus oocytes and the currents elicited by ACh (I_ACh s), either alone or co-applied with tetracaine, were recorded. Tetracaine reversibly blocked I_ACh , with an IC₅₀ (i.e., the concentration required to inhibit half the maximum I_ACh ) in the submicromolar range. Notably, at very low concentrations (0.1 μM), tetracaine reduced I_ACh in a voltage-dependent manner, the more negative potentials produced greater inhibition, indicating open-channel blockade. When the tetracaine concentration was increased to 0.7 μM or above, voltage-independent inhibition was also observed, indicating closed-channel blockade. The I_ACh inhibition by pre-application of just 0.7 μM tetracaine before superfusion of ACh also corroborated the notion of tetracaine blockade of resting nAChRs. Furthermore, tetracaine markedly increased nAChR desensitization, mainly at concentrations equal or higher than 0.5 μM. Interestingly, tetracaine did not modify desensitization when its binding within the channel pore was prevented by holding the membrane at positive potentials. Tetracaine-nAChR interactions were assessed by virtual docking assays, using nAChR models in the closed and open states. These assays revealed that tetracaine binds at different sites of the nAChR located at the extracellular and transmembrane domains, in both open and closed conformations. Extracellular binding sites seem to be associated with closed-channel blockade; whereas two sites within the pore, with different affinities for tetracaine, contribute to open-channel blockade and the enhancement of desensitization, respectively. These results demonstrate a concentration-dependent heterogeneity of tetracaine actions on nAChRs, and contribute to a better understanding of the complex modulation of muscle-type nAChRs by local anesthetics. Furthermore, the combination of functional and virtual assays to decipher nAChR-tetracaine interactions has allowed us to tentatively assign the main nAChR residues involved in these modulating actions.

Muscle-Type Nicotinic Receptor Modulation by 2,6-Dimethylaniline, a Molecule Resembling the Hydrophobic Moiety of Lidocaine

To identify the molecular determinants responsible for lidocaine blockade of muscle-type nAChRs, we have studied the effects on this receptor of 2,6-dimethylaniline (DMA), which resembles lidocaine’s hydrophobic moiety. Torpedo marmorata nAChRs were microtransplanted to Xenopus oocytes and currents elicited by ACh (I_ACh), either alone or co-applied with DMA, were recorded. DMA reversibly blocked I_ACh and, similarly to lidocaine, exerted a closed-channel blockade, as evidenced by the enhancement of I_ACh blockade when DMA was pre-applied before its co-application with ACh, and hastened I_ACh decay. However, there were marked differences among its mechanisms of nAChR inhibition and those mediated by either the entire lidocaine molecule or diethylamine (DEA), a small amine resembling lidocaine’s hydrophilic moiety. Thereby, the IC₅₀ for DMA, estimated from the dose-inhibition curve, was in the millimolar range, which is one order of magnitude higher than that for either DEA or lidocaine. Besides, nAChR blockade by DMA was voltage-independent in contrast to the increase of I_ACh inhibition at negative potentials caused by the more polar lidocaine or DEA molecules. Accordingly, virtual docking assays of DMA on nAChRs showed that this molecule binds predominantly at intersubunit crevices of the transmembrane-spanning domain, but also at the extracellular domain. Furthermore, DMA interacted with residues inside the channel pore, although only in the open-channel conformation. Interestingly, co-application of ACh with DEA and DMA, at their IC₅₀s, had additive inhibitory effects on I_ACh and the extent of blockade was similar to that predicted by the allotopic model of interaction, suggesting that DEA and DMA bind to nAChRs at different loci. These results indicate that DMA mainly mimics the low potency and non-competitive actions of lidocaine on nAChRs, as opposed to the high potency and voltage-dependent block by lidocaine, which is emulated by the hydrophilic DEA. Furthermore, it is pointed out that the hydrophobic (DMA) and hydrophilic (DEA) moieties of the lidocaine molecule act differently on nAChRs and that their separate actions taken together account for most of the inhibitory effects of the whole lidocaine molecule on nAChRs.

The Integrity of the TRP Domain Is Pivotal for Correct TRPV1 Channel Gating

Transient receptor potential vanilloid subtype I (TRPV1) is a thermosensory ion channel that is also gated by chemical substances such as vanilloids. Adjacent to the channel gate, this polymodal thermoTRP channel displays a TRP domain, referred to as AD1, that plays a role in subunit association and channel gating. Previous studies have shown that swapping the AD1 in TRPV1 with the cognate from the TRPV2 channel (AD2) reduces protein expression and produces a nonfunctional chimeric channel (TRPV1-AD2). Here, we used a stepwise, sequential, cumulative site-directed mutagenesis approach, based on rebuilding the AD1 domain in the TRPV1-AD2 chimera, to unveil the minimum number of amino acids needed to restore protein expression and polymodal channel activity. Unexpectedly, we found that virtually full restitution of the AD1 sequence is required to reinstate channel expression and responses to capsaicin, temperature, and voltage. This strategy identified E692, R701, and T704 in the TRP domain as important for TRPV1 activity. Even conservative mutagenesis at these sites (E692D/R701K/T704S) impaired channel expression and abolished TRPV1 activity. However, the sole mutation of these positions in the TRPV1-AD2 chimera (D692E/K701R/S704T) was not sufficient to rescue channel gating, implying that other residues in the TRP domain are necessary to endow activity to TRPV1-AD2. A biophysical analysis of a functional chimera suggested that mutations in the TRP domain raised the energetics of channel gating by altering the coupling of stimuli sensing and pore opening. These findings indicate that inter- and/or intrasubunit interactions in the TRP domain are essential for correct TRPV1 gating.

Muscle-Type Nicotinic Receptor Blockade by Diethylamine, the Hydrophilic Moiety of Lidocaine

Lidocaine bears in its structure both an aromatic ring and a terminal amine, which can be protonated at physiological pH, linked by an amide group. Since lidocaine causes multiple inhibitory actions on nicotinic acetylcholine receptors (nAChRs), this work was aimed to determine the inhibitory effects of diethylamine (DEA), a small molecule resembling the hydrophilic moiety of lidocaine, on Torpedo marmorata nAChRs microtransplanted to Xenopus oocytes. Similarly to lidocaine, DEA reversibly blocked acetylcholine-elicited currents (I_ACh) in a dose-dependent manner (IC₅₀ close to 70 μM), but unlike lidocaine, DEA did not affect I_ACh desensitization. I_ACh inhibition by DEA was more pronounced at negative potentials, suggesting an open-channel blockade of nAChRs, although roughly 30% inhibition persisted at positive potentials, indicating additional binding sites outside the pore. DEA block of nAChRs in the resting state (closed channel) was confirmed by the enhanced I_ACh inhibition when pre-applying DEA before its co-application with ACh, as compared with solely DEA and ACh co-application. Virtual docking assays provide a plausible explanation to the experimental observations in terms of the involvement of different sets of drug binding sites. So, at the nAChR transmembrane (TM) domain, DEA and lidocaine shared binding sites within the channel pore, giving support to their open-channel blockade; besides, lidocaine, but not DEA, interacted with residues at cavities among the M1, M2, M3, and M4 segments of each subunit and also at intersubunit crevices. At the extracellular (EC) domain, DEA and lidocaine binding sites were broadly distributed, which aids to explain the closed channel blockade observed. Interestingly, some DEA clusters were located at the α-γ interphase of the EC domain, in a cavity near the orthosteric binding site pocket; by contrast, lidocaine contacted with all α-subunit loops conforming the ACh binding site, both in α-γ and α-δ and interphases, likely because of its larger size. Together, these results indicate that DEA mimics some, but not all, inhibitory actions of lidocaine on nAChRs and that even this small polar molecule acts by different mechanisms on this receptor. The presented results contribute to a better understanding of the structural determinants of nAChR modulation.

In silico approach for the discovery of new PPARγ modulators among plant-derived polyphenols

Peroxisome proliferator-activated receptor gamma (PPARγ) is a well-characterized member of the PPAR family that is predominantly expressed in adipose tissue and plays a significant role in lipid metabolism, adipogenesis, glucose homeostasis, and insulin sensitization. Full agonists of synthetic thiazolidinediones (TZDs) have been therapeutically used in clinical practice to treat type 2 diabetes for many years. Although it can effectively lower blood glucose levels and improve insulin sensitivity, the administration of TZDs has been associated with severe side effects. Based on recent evidence obtained with plant-derived polyphenols, the present in silico study aimed at finding new selective human PPARγ (hPPARγ) modulators that are able to improve glucose homeostasis with reduced side effects compared with TZDs. Docking experiments have been used to select compounds with strong binding affinity (ΔG values ranging from −10.0±0.9 to −11.4±0.9 kcal/mol) by docking against the binding site of several X-ray structures of hPPARγ. These putative modulators present several molecular interactions with the binding site of the protein. Additionally, most of the selected compounds have favorable druggability and good ADMET properties. These results aim to pave the way for further bench-scale analysis for the discovery of new modulators of hPPARγ that do not induce any side effects.

Competing Lipid-Protein and Protein-Protein Interactions Determine Clustering and Gating Patterns in the Potassium Channel from Streptomyces lividans (KcsA)

There is increasing evidence to support the notion that membrane proteins, instead of being isolated components floating in a fluid lipid environment, can be assembled into supramolecular complexes that take part in a variety of cooperative cellular functions. The interplay between lipid-protein and protein-protein interactions is expected to be a determinant factor in the assembly and dynamics of such membrane complexes. Here we report on a role of anionic phospholipids in determining the extent of clustering of KcsA, a model potassium channel. Assembly/disassembly of channel clusters occurs, at least partly, as a consequence of competing lipid-protein and protein-protein interactions at nonannular lipid binding sites on the channel surface and brings about profound changes in the gating properties of the channel. Our results suggest that these latter effects of anionic lipids are mediated via the Trp(67)-Glu(71)-Asp(80) inactivation triad within the channel structure and its bearing on the selectivity filter.

TRP channels interaction with lipids and its implications in disease

Transient receptor potential (TRP) proteins are a family of ion channels central for sensory signaling. These receptors and, in particular, those involved in thermal sensing are also involved in pain signaling. Noteworthy, thermosensory receptors are polymodal ion channels that respond to both physical and chemical stimuli, thus integrating different environmental clues. In addition, their activity is modulated by algesic agents and lipidergic substances that are primarily released in pathological states. Lipids and lipid-like molecules have been found that can directly activate some thermosensory channels or modulate their activity by either potentiating or inhibiting it. To date, more than 50 endogenous lipids that can regulate TRP channel activity in sensory neurons have been described, thus representing the majority of known endogenous TRP channel modulators. Lipid modulators of TRP channels comprise lipids from a variety of metabolic pathways, including metabolites of the cyclooxygenase, lipoxygenase and cytochrome-P450 pathways, phospholipids and lysophospholipids. Therefore, TRP-channels are able to integrate and interpret incoming signals from the different metabolic lipid pathways. Taken together, the large number of lipids that can activate, sensitize or inhibit neuronal TRP-channels highlights the pivotal role of these molecules in sensory biology as well as in pain transduction and perception. This article is part of a Special Issue entitled: Lipid-protein interactions. Guest Editors: Amitabha Chattopadhyay and Jean-Marie Ruysschaert.

Mutation of I696 and W697 in the TRP box of vanilloid receptor subtype I modulates allosteric channel activation

Residues I696 and W697 are crucial to coupling between the TRPV1 ligand- and voltage-sensing domains and the channel pore.

The transient receptor potential vanilloid receptor subtype I (TRPV1) channel acts as a polymodal sensory receptor gated by chemical and physical stimuli. Like other TRP channels, TRPV1 contains in its C terminus a short, conserved domain called the TRP box, which is necessary for channel gating. Substitution of two TRP box residues—I696 and W697—with Ala markedly affects TRPV1’s response to all activating stimuli, which indicates that these two residues play a crucial role in channel gating. We systematically replaced I696 and W697 with 18 native l-amino acids (excluding cysteine) and evaluated the effect on voltage- and capsaicin-dependent gating. Mutation of I696 decreased channel activation by either voltage or capsaicin; furthermore, gating was only observed with substitution of hydrophobic amino acids. Substitution of W697 with any of the 18 amino acids abolished gating in response to depolarization alone, shifting the threshold to unreachable voltages, but not capsaicin-mediated gating. Moreover, vanilloid-activated responses of W697X mutants showed voltage-dependent gating along with a strong voltage-independent component. Analysis of the data using an allosteric model of activation indicates that mutation of I696 and W697 primarily affects the allosteric coupling constants of the ligand and voltage sensors to the channel pore. Together, our findings substantiate the notion that inter- and/or intrasubunit interactions at the level of the TRP box are critical for efficient coupling of stimulus sensing and gate opening. Perturbation of these interactions markedly reduces the efficacy and potency of the activating stimuli. Furthermore, our results identify these interactions as potential sites for pharmacological intervention.

The region adjacent to the C-end of the inner gate in transient receptor potential melastatin 8 (TRPM8) channels plays a central role in allosteric channel activation

The ability of transient receptor potential (TRP) channels to sense and respond to environmental and endogenous cues is crucial in animal sensory physiology. The molecular mechanism of channel gating is yet elusive. The TRP box, a conserved region in the N-end of the C terminus domain, has been signaled as pivotal for allosteric activation in TRP channels. Here, we have examined the role of the linker region between the TRPM8 inner gate and the TRP box (referred to as the S6-TRP box linker) to identify structural determinants of channel gating. Stepwise substitutions of segments in the S6-TRP box linker of TRPM8 channel with the cognate TRPV1 channel sequences produced functional chimeric channels, and identified Tyr(981) as a central molecular determinant of channel function. Additionally, mutations in the 986-990 region had a profound impact on channel gating by voltage and menthol, as evidenced by the modulation of the conductance-to-voltage (G-V) relationships. Simulation of G-V curves using an allosteric model for channel activation revealed that these mutations altered the allosteric constants that couple stimuli sensing to pore opening. A molecular model of TRPM8, based on the recently reported TRPV1 structural model, showed that Tyr(981) may lie in a hydrophobic pocket at the end of the S6 transmembrane segment and is involved in inter-subunit interactions with residues from neighbor subunits. The 986-990 region holds intrasubunit interactions between the TRP domain and the S4-S5 linker. These findings substantiate a gating mechanism whereby the TRP domain acts as a coupling domain for efficient channel opening. Furthermore, they imply that protein-protein interactions of the TRP domain may be targets for channel modulation and drug intervention.

A non-canonical di-acidic signal at the C-terminus of Kv1.3 determines anterograde trafficking and surface expression

Impairment of Kv1.3 expression at the cell membrane in leukocytes and sensory neuron contributes to the pathophysiology of autoimmune diseases and sensory syndromes. Molecular mechanisms underlying Kv1.3 channel trafficking to the plasma membrane remain elusive. We report a novel non-canonical di-acidic signal (E483/484) at the C-terminus of Kv1.3 essential for anterograde transport and surface expression. Notably, homologous motifs are conserved in neuronal Kv1 and Shaker channels. Biochemical analysis revealed interactions with the Sec24 subunit of the coat protein complex II. Disruption of this complex retains the channel at the endoplasmic reticulum. A molecular model of the Kv1.3-Sec24a complex suggests salt-bridges between the di-acidic E483/484 motif in Kv1.3 and the di-basic R750/752 sequence in Sec24. These findings identify a previously unrecognized motif of Kv channels essential for their expression on the cell surface. Our results contribute to our understanding of how Kv1 channels target to the cell membrane, and provide new therapeutic strategies for the treatment of pathological conditions.

Advances in modulating thermosensory TRP channels

INTRODUCTION

Thermosensory channels are a subfamily of the transient receptor potential (TRP) channel family that are activated by changes in the environmental temperature. These channels, known as thermoTRPs, cover the entire spectrum of temperatures, from noxious cold (< 15°C) to injurious heat (> 42°C). In addition, dysfunction of these channels contributes to the thermal hypersensitivity that accompanies painful conditions. Moreover, because of their wide tissue and cellular distribution, thermoTRPs are also involved in the pathophysiology of several diseases, from inflammation to cancer.

AREAS COVERED

Although the number of thermoTRPs is increasing with the identification of novel members such as TRPM3, we will cover the recent advances in the pharmacology of the classical thermosensory channels, namely TRPV1, TRPV2, TRPV3, TRPV4, TRPM8 and TRPA1. This review will focus on the therapeutic progress carried out for all these channels and will highlight the tenet that TRPV1, TRPM8 and TRPA1 are the most exploited channels, and that the interest on TRPV3 and TRPV4 is growing with the first TRPV3 antagonist that moves into Phase-II clinical trials. In contrast, the pharmacology of TRPV2 is yet in its infancy.

EXPERT OPINION

Despite the tremendous academic and industrial investment to develop therapeutic modulators of thermoTRPs, it apparently seems that we are still far from the first successful product, although hope is maintained high for all compounds currently in clinical trials. A major concern has been the appearance of side effects. A better knowledge of the thermosensory protein networks (signal-plexes), along with the application of system biology approaches may provide novel strategies to modulate thermoTRPs activity with improved therapeutic index. A case in point is TRPV1, where acting on interacting proteins is providing new therapeutic opportunities.

Ionic channels as targets for drug design: a review on computational methods

Ion channels are involved in a broad range of physiological and pathological processes. The implications of ion channels in a variety of diseases, including diabetes, epilepsy, hypertension, cancer and even chronic pain, have signaled them as pivotal drug targets. Thus far, drugs targeting ion channels were developed without detailed knowledge of the molecular interactions between the lead compounds and the target channels. In recent years, however, the emergence of high-resolution structures for a plethora of ion channels paves the way for computer-assisted drug design. Currently, available functional and structural data provide an attractive platform to generate models that combine substrate-based and protein-based approaches. In silico approaches include homology modeling, quantitative structure-activity relationships, virtual ligand screening, similarity and pharmacophore searching, data mining, and data analysis tools. These strategies have been frequently used in the discovery and optimization of novel molecules with enhanced affinity and specificity for the selected therapeutic targets. In this review we summarize recent applications of in silico methods that are being used for the development of ion channel drugs.

Role of the transient receptor potential vanilloid 1 in inflammation and sepsis

The transient receptor potential vanilloid 1 (TRPV1) is a thermoreceptor that responds to noxious temperatures, as well as to chemical agonists, such as vanilloids and protons. In addition, its channel activity is notably potentiated by proinflammatory mediators released upon tissue damage. The TRPV1 contribution to sensory neuron sensitization by proalgesic agents has signaled this receptor as a prime target for analgesic and anti-inflammatory drug intervention. However, TRPV1 antagonists have notably failed in clinical and preclinical studies because of their unwanted side effects. Recent reports have unveiled previously unrecognized anti-inflammatory and protective functions of TRPV1 in several diseases. For instance, this channel has been suggested to play an anti-inflammatory role in sepsis. Therefore, the use of potent TRPV1 antagonists as a general strategy to treat inflammation must be cautiously considered, given the deleterious effects that may arise from inhibiting the population of channels that have a protective function. The use of TRPV1 antagonists may be limited to treating those pathologies where enhanced receptor activity contributes to the inflamed state. Alternatively, therapeutic paradigms, such as reduction of inflammatory-mediated increase of receptor expression in the cell surface, may be a better strategy to prevent abrogation of the TRPV1 subpopulation involved in anti-inflammatory and protective processes.

Estrogen destabilizes microtubules through an ion-conductivity-independent TRPV1 pathway

Recently, we described estrogen and agonists of the G-protein coupled estrogen receptor GPR30 to induce protein kinase C (PKC)ε-dependent pain sensitization. PKCε phosphorylates the ion channel transient receptor potential, vanilloid subclass I (TRPV1) close to a novel microtubule-TRPV1 binding site. We now modeled the binding of tubulin to the TRPV1 C-terminus. The model suggests PKCε phosphorylation of TRPV1-S800 to abolish the tubulin-TRPV1 interaction. Indeed, in vitro PKCε phosphorylation of TRPV1 hindered tubulin-binding to TRPV1. In vivo, treatment of sensory neurons and F-11 cells with estrogen and the GPR30 agonist, G-1, resulted in microtubule destabilization and retraction of microtubules from filopodial structures. We found estrogen and G-1 to regulate the stability of the microtubular network via PKC phosphorylation of the PKCε-phosphorylation site TRPV1-S800. Microtubule disassembly was not, however, dependent on TRPV1 ion conductivity. TRPV1 knock-down in rats inverted the effect of the microtubule-modulating drugs, Taxol and Nocodazole, on estrogen-induced and PKCε-dependent mechanical pain sensitization. Thus, we suggest the C-terminus of TRPV1 to be a signaling intermediate downstream of estrogen and PKCε, regulating microtubule-stability and microtubule-dependent pain sensitization.

Membrane-tethered peptides patterned after the TRP domain (TRPducins) selectively inhibit TRPV1 channel activity

The transient receptor potential vanilloid 1 (TRPV1) channel is a thermosensory receptor implicated in diverse physiological and pathological processes. The TRP domain, a highly conserved region in the C terminus adjacent to the internal channel gate, is critical for subunit tetramerization and channel gating. Here, we show that cell-penetrating, membrane-anchored peptides patterned after this protein domain are moderate and selective TRPV1 antagonists both in vitro and in vivo, blocking receptor activity in intact rat primary sensory neurons and their peripheral axons with mean decline time of 30 min. The most potent lipopeptide, TRP-p5, blocked all modes of TRPV1 gating with micromolar efficacy (IC(50)<10 μM), without significantly affecting other thermoTRP channels. In contrast, its retrosequence or the corresponding sequences of other TRPV channels did not alter TRPV1 channel activity (IC(50)>100 μM). TRP-p5 did not affect the capsaicin sensitivity of the vanilloid receptor. Our data suggest that TRP-p5 interferes with protein-protein interactions at the level of the TRP domain that are essential for the "conformational" change that leads to gate opening. Therefore, these palmitoylated peptides, which we termed TRPducins, are noncompetitive, voltage-independent, sequence-specific TRPV1 blockers. Our findings indicate that TRPducin-like peptides may embody a novel molecular strategy that can be exploited to generate a selective pharmacological arsenal for the TRP superfamily of ion channels.

Occupancy of nonannular lipid binding sites on KcsA greatly increases the stability of the tetrameric protein

KcsA, a homotetrameric potassium channel from prokaryotes, contains noncovalently bound lipids appearing in the X-ray crystallographic structure of the protein. The binding sites for such high-affinity lipids are referred to as "nonannular" sites, correspond to intersubunit protein domains, and bind preferentially anionic phospholipids. Here we used a thermal denaturation assay and detergent-phospholipid mixed micelles containing KcsA to study the effects of different phospholipids on protein stability. We found that anionic phospholipids stabilize greatly the tetrameric protein against irreversible, heat-induced unfolding and dissociation into subunits. This occurs in a phospholipid concentration-dependent manner, and phosphatidic acid species with acyl chain lengths ranging 14 to 18 carbon atoms are more efficient than similar phosphatidylglycerols in protecting the protein. A docking model of the KcsA-phospholipid complex suggests that the increased protein stability originates from the intersubunit nature of the binding sites and, thus, interaction of the phospholipid with such sites holds together adjacent subunits within the tetrameric protein. We also found that simpler amphiphiles, such as alkyl sulfates longer than 10 carbon atoms, also increase the protein stability to the same extent as anionic phospholipids, although at higher concentrations than the latter. Modeling the interaction of these simpler amphiphiles with KcsA and comparing it with that of anionic phospholipids serve to delineate the features of a hydrophobic pocket in the nonannular sites. Such pocket is predicted to comprise residues from the M2 transmembrane segment of a subunit and from the pore helix of the adjacent subunit and seems most relevant to protein stabilization.

Differential role of the menthol-binding residue Y745 in the antagonism of thermally gated TRPM8 channels

Background

TRPM8 is a non-selective cation channel that belongs to the melastatin subfamily of the transient receptor potential (TRP) ion channels. TRPM8 is activated by voltage, cold and cooling compounds such as menthol. Despite its essential role for cold temperature sensing in mammals, the pharmacology of TRPM8 is still in its infancy. Recently, tyrosine 745 (Y745) was identified as a critical residue for menthol sensitivity of the channel. In this report, we study the effect of mutating this residue on the action of several known TRPM8 antagonists: BCTC, capsazepine, SKF96365, and clotrimazole as well as two new inhibitor candidates, econazole and imidazole.

Results

We show that Y745 at the menthol binding site is critical for inhibition mediated by SKF96365 of cold- and voltage-activated TRPM8 currents. In contrast, the inhibition by other antagonists was unaffected by the mutation (BCTC) or only partially reduced (capsazepine, clotrimazole, econazole), suggesting that additional binding sites exist on the TRPM8 channel from where the inhibitors exert their negative modulation. Indeed, a molecular docking model implies that menthol and SKF96365 interact readily with Y745, while BCTC is unable to bind to this residue.

Conclusion

In summary, we identify structural elements on the TRPM8 channel that are critical for the action of channel antagonists, providing valuable information for the future design of new, specific modulator compounds.

N-type inactivation of the potassium channel KcsA by the Shaker B "ball" peptide: mapping the inactivating peptide-binding epitope

The effects of the inactivating peptide from the eukaryotic Shaker BK(+) channel (the ShB peptide) on the prokaryotic KcsA channel have been studied using patch clamp methods. The data show that the peptide induces rapid, N-type inactivation in KcsA through a process that includes functional uncoupling of channel gating. We have also employed saturation transfer difference (STD) NMR methods to map the molecular interactions between the inactivating peptide and its channel target. The results indicate that binding of the ShB peptide to KcsA involves the ortho and meta protons of Tyr(8), which exhibit the strongest STD effects; the C4H in the imidazole ring of His(16); the methyl protons of Val(4), Leu(7), and Leu(10) and the side chain amine protons of one, if not both, the Lys(18) and Lys(19) residues. When a noninactivating ShB-L7E mutant is used in the studies, binding to KcsA is still observed but involves different amino acids. Thus, the strongest STD effects are now seen on the methyl protons of Val(4) and Leu(10), whereas His(16) seems similarly affected as before. Conversely, STD effects on Tyr(8) are strongly diminished, and those on Lys(18) and/or Lys(19) are abolished. Additionally, Fourier transform infrared spectroscopy of KcsA in presence of (13)C-labeled peptide derivatives suggests that the ShB peptide, but not the ShB-L7E mutant, adopts a beta-hairpin structure when bound to the KcsA channel. Indeed, docking such a beta-hairpin structure into an open pore model for K(+) channels to simulate the inactivating peptide/channel complex predicts interactions well in agreement with the experimental observations.

Dynamics of Tryptophan in the Histidine-Containing Phosphocarrier Protein of Streptomyces coelicolor: Evidence of Multistate Equilibrium Unfolding

The nanosecond dynamics of the single tryptophan, Trp10, of HPr from Streptomyces coelicolor, HPrsc, has been monitored at different pHs. Time-resolved fluorescence methods and DOSY measurements have been used to map the compactness of the protein. At low pHs, where a molten globule-like species has been described, the correlation times from fluorescence showed an abrupt change as the pH was increased. When the protein was folded (above pH 4), two correlation times were observed, which remained practically constant up to pH 9.5. The long correlation time, around 7.5 ns, corresponds to the global rotational motion of the protein, since this value is in agreement with that determined theoretically from hydrodynamic measurements. The short correlation time, around 1.4 ns, must report on fast movements of the protein segment containing the tryptophan residue. On the other hand, fluorescence lifetimes showed the same abrupt change as the correlation times at low pH, but, in addition, a sigmoidal change with a pKa approximately 4.3 was also observed. On the basis of the modeled structure of HPrsc, this last transition could be due to the proximity of Glu12 to Trp10. The changes monitored by the fluorescence lifetimes agree with those observed previously by steady-state fluorescence, CD, and ANS binding experiments. Taken together, these data suggest a multistate equilibrium during folding of HPrsc starting from low pHs.

Biophysical characterization of the enzyme I of the Streptomyces coelicolor phosphoenolpyruvate:sugar phosphotransferase system

The first protein in the bacterial phosphoenolpyruvate (PEP):sugar phosphotransferase system is the homodimeric 60-kDa enzyme I (EI), which autophosphorylates in the presence of PEP and Mg2+. The conformational stability and structure of the EI from Streptomyces coelicolor, EI(sc), were explored in the absence and in the presence of its effectors by using several biophysical probes (namely, fluorescence, far-ultraviolet circular dichroism, Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry) and computational approaches. The structure of EI(sc) was obtained by homology modeling of the isolated N- and C-terminal domains of other EI proteins. The experimental results indicate that at physiological pH, the dimeric EI(sc) had a well-folded structure; however, at low pH, EI(sc) showed a partially unfolded state with the features of a molten globule, as suggested by fluorescence, far-ultraviolet circular dichroism, FTIR, and 8-anilino-1-naphthalene-sulfonic acid binding. The thermal stability of EI(sc), in the absence of PEP and Mg2+, was maximal at pH 7. The presence of PEP and Mg2+ did not change substantially the secondary structure of the protein, as indicated by FTIR measurements. However, quenching experiments and proteolysis patterns suggest conformational changes in the presence of PEP; furthermore, the thermal stability of EI(sc) was modified depending on the effector added. Our approach suggests that thermodynamical analysis might reveal subtle conformational changes.

Modulation of botulinum neurotoxin A catalytic domain stability by tyrosine phosphorylation

Botulinum neurotoxin A (BoNT A) is a substrate of the Src family of tyrosine kinases. Here, we report that the BoNT A light chain (LC) is phosphorylated in the tyrosine-71 located at N-terminus. Covalent modification of this residue notably increases the thermal stability of the endopeptidase activity, without affecting its catalytic efficacy. Similarly, mutation of this residue specifically affected the protein stability but not its endopeptidase function. Fusion of the Tat-translocating domain to the N-terminus of the enzyme produced a cell permeable, functional enzyme, as evidenced by immunocytochemistry and by the cleavage of cytosolic SNAP25 in intact PC12 cells. Noteworthy, truncation of cellular SNAP25 was reduced in cells when the Src kinase activity was inhibited with a specific antagonist, implying that tyrosine phosphorylation of BoNT A LC modulates the in vivo proteolytic activity of the neurotoxin. Taken together, these findings substantiate the tenet that tyrosine phosphorylation of BoNT A LC could be an important modulatory strategy of the neurotoxin stability and suggest that the phosphorylated neurotoxin may be a relevant molecule in vivo.

Influence of C-terminal protein domains and protein-lipid interactions on tetramerization and stability of the potassium channel KcsA

KcsA is a prokaryotic potassium channel formed by the assembly of four identical subunits around a central aqueous pore. Although the high-resolution X-ray structure of the transmembrane portion of KcsA is known [Doyle, D. A., Morais, C. J., Pfuetzner, R. A., Kuo, A., Gulbis, J. M., Cohen, S. L., Chait, B. T., and MacKinnon, R. (1998) Science 280, 69-77], the identification of the molecular determinant(s) involved in promoting subunit tetramerization remains to be determined. Here, C-terminal deletion channel mutants, KcsA Delta125-160 and Delta120-160, as well as 1-125 KcsA obtained from chymotrypsin cleavage of full-length 1-160 KcsA, have been used to evaluate the role of the C-terminal segment on the stability and tetrameric assembly of the channel protein. We found that the lack of the cytoplasmic C-terminal domain of KcsA, and most critically the 120-124 sequence stretch, impairs tetrameric assembly of channel subunits in a heterologous E. coli expression system. Molecular modeling of KcsA predicts that, indeed, such sequence stretch provides intersubunit interaction sites by hydrogen bonding to amino acid residues in N- and C-terminal segments of adjacent subunits. However, once the KcsA tetramer is assembled, its remarkable in vitro stability to detergent or to heat-induced dissociation into subunits is not greatly influenced by whether the entire C-terminal domain continues being part of the protein. Finally and most interestingly, it is observed that, even in the absence of the C-terminal domain involved in tetramerization, reconstitution into membrane lipids promotes in vitro KcsA tetramerization very efficiently, an event which is likely mediated by allowing proper hydrophobic interactions involving intramembrane protein domains.

Influence of C-terminal protein domains and protein-lipid interactions on tetramerization and stability of the potassium channel KcsA

KcsA is a prokaryotic potassium channel formed by the assembly of four identical subunits around a central aqueous pore. Although the high-resolution X-ray structure of the transmembrane portion of KcsA is known [Doyle, D. A., Morais, C. J., Pfuetzner, R. A., Kuo, A., Gulbis, J. M., Cohen, S. L., Chait, B. T., and MacKinnon, R. (1998) Science 280, 69-77], the identification of the molecular determinant(s) involved in promoting subunit tetramerization remains to be determined. Here, C-terminal deletion channel mutants, KcsA Delta125-160 and Delta120-160, as well as 1-125 KcsA obtained from chymotrypsin cleavage of full-length 1-160 KcsA, have been used to evaluate the role of the C-terminal segment on the stability and tetrameric assembly of the channel protein. We found that the lack of the cytoplasmic C-terminal domain of KcsA, and most critically the 120-124 sequence stretch, impairs tetrameric assembly of channel subunits in a heterologous E. coli expression system. Molecular modeling of KcsA predicts that, indeed, such sequence stretch provides intersubunit interaction sites by hydrogen bonding to amino acid residues in N- and C-terminal segments of adjacent subunits. However, once the KcsA tetramer is assembled, its remarkable in vitro stability to detergent or to heat-induced dissociation into subunits is not greatly influenced by whether the entire C-terminal domain continues being part of the protein. Finally and most interestingly, it is observed that, even in the absence of the C-terminal domain involved in tetramerization, reconstitution into membrane lipids promotes in vitro KcsA tetramerization very efficiently, an event which is likely mediated by allowing proper hydrophobic interactions involving intramembrane protein domains.

Identification of a tetramerization domain in the C terminus of the vanilloid receptor

TRPV1 (transient receptor potential vanilloid receptor subtype 1) is a member of the TRP channel family gated by vanilloids, protons, and heat. Structurally, TRPV1 appears to be a tetramer formed by the assembly of four identical subunits around a central aqueous pore. The molecular determinants that govern its subunit oligomerization remain elusive. Here, we report the identification of a segment comprising 684Glu-721Arg (referred to as the TRP-like domain) in the C terminus of TRPV1 as an association domain (AD) of the protein. Purified recombinant C terminus of TRPV1 (TRPV1-C) formed discrete and stable multimers in vitro. Yeast two-hybrid and pull-down assays showed that self-association of the TRPV1-C is blocked when segment 684Glu-721Arg is deleted. Biochemical and immunological analysis indicate that removal of the AD from full-length TRPV1 monomers blocks the formation of stable heteromeric assemblies with wild-type TRPV1 subunits. Deletion of the AD in a poreless TRPV1 subunit suppressed its robust dominant-negative phenotype. Together, these findings are consistent with the tenet that the TRP-like domain in TRPV1 is a molecular determinant of the tetramerization of receptor subunits into functional channels. Our observations suggest that the homologous TRP domain in the TRP protein family may function as a general, evolutionary conserved AD involved in subunit multimerization.

Molecular architecture of the vanilloid receptor. Insights for drug design

The transient receptor potential channel vanilloid receptor subunit 1 (TRPV1) is a molecular integrator of physical and chemical stimuli in the peripheral nociceptor terminals. TRPV1 is an ionotropic channel that plays a critical role in both thermal nociception and inflammatory hyperalgesia. Structure-function relationships are providing fundamental insights of the modular architecture of this neuronal receptor. As a result, the molecular determinants that endow TRPV1 with its physiological properties, namely activation by heat, potentiation by extracellular acidic pH, and interaction with vanilloid-like compounds, as well as its permeation properties are being unveiled. This information can now be used to build up molecular models for the protein which, upon experimental validation, could be used as tools to thrust the target-oriented design of druggable TRPV1 ligands.

Small peptides patterned after the N-terminus domain of SNAP25 inhibit SNARE complex assembly and regulated exocytosis

Synthetic peptides patterned after the C-terminus of synaptosomal associated protein of 25 kDa (SNAP25) efficiently abrogate regulated exocytosis. In contrast, the use of SNAP25 N-terminal-derived peptides to modulate SNAP receptors (SNARE) complex assembly and neurosecretion has not been explored. Here, we show that the N-terminus of SNAP25, specially the segment that encompasses 22Ala-44Ile, is essential for the formation of the SNARE complex. Peptides patterned after this protein domain are potent inhibitors of SNARE complex formation. The inhibitory activity correlated with their propensity to adopt an alpha-helical secondary structure. These peptides abrogated SNARE complex formation only when added previous to the onset of aggregate assembly. Analysis of the mechanism of action revealed that these peptides disrupted the binary complex formed by SNAP25 and syntaxin. The identified peptides inhibited Ca2+-dependent exocytosis from detergent-permeabilized excitable cells. Noteworthy, these amino acid sequences markedly protected intact hippocampal neurones against hypoglycaemia-induced, glutamate-mediated excitotoxicity with a potency that rivalled that displayed by botulinum neurotoxins. Our findings indicate that peptides patterned after the N-terminus of SNAP25 are potent inhibitors of SNARE complex formation and neuronal exocytosis. Because of their activity in intact neurones, these cell permeable peptides may be hits for antispasmodic and analgesic drug development.

Identification of SNARE complex modulators that inhibit exocytosis from an alpha-helix-constrained combinatorial library

Synthetic peptides patterned after the proteins involved in vesicle fusion [the so-called SNARE (soluble N -ethylmaleimide-sensitive fusion protein attachment protein receptor) proteins] are potent inhibitors of SNARE complex assembly and neuronal exocytosis. It is noteworthy that the identification of peptide sequences not related to the SNARE proteins has not been accomplished yet; this is due, in part, to the structural constraints and the specificity of the protein interactions that govern the formation of the SNARE complex. Here we have addressed this question and used a combinatorial approach to identify peptides that modulate the assembly of the SNARE core complex and inhibit neuronal exocytosis. An alpha-helix-constrained, mixture-based, 17-mer combinatorial peptide library composed of 137180 sequences was synthesized in a positional scanning format. Peptide mixtures were assayed for their ability to prevent the formation of the in vitro -reconstituted SDS-resistant SNARE core complex. Library deconvolution identified eight peptides that inhibited the assembly of the SNARE core complex. Notably, the most potent 17-mer peptide (acetyl-SAAEAFAKLYAEAFAKG-NH2) abolished both Ca2+-evoked catecholamine secretion from detergent-permeabilized chromaffin cells and L-glutamate release from intact hippocampal primary cultures. Collectively, these findings indicate that amino acid sequences that prevent SNARE complex formation are not restricted to those that mimic domains of SNARE proteins, thus expanding the diversity of molecules that target neuronal exocytosis. Because of the implication of neurosecretion in the aetiology of several human neurological disorders, these newly identified peptides may be considered hits for the development of novel anti-spasmodic drugs.

Ligand screening by exoproteolysis and mass spectrometry in combination with computer modelling

Here, we present a new approach for protein ligand screening based on the use of limited exoproteolysis coupled to MALDI-TOF mass spectrometry, combined with computational modelling and prediction of binding energies. As a test for this combined approach, we have screened a combinatorial library containing 8000 peptides (organized in 60 peptide samples) based on positional scanning format. This library is attached to a poly-Pro framework, and screened against the Abl-SH3 domain. The results obtained demonstrated the validity of the experimental and theoretical approaches in identifying better ligands and in rationalizing the changes in affinity. Exoproteolysis coupled to MALDI-TOF mass spectrometry could be used to screen complex libraries in a fast and efficient way.

The histidine-phosphocarrier protein of Streptomyces coelicolor folds by a partially folded species at low pH

The folding of a 93-residue protein, the histidine-phosphocarrier protein of Streptomyces coelicolor, HPr, has been studied using several biophysical techniques, namely fluorescence, 8-anilinonaphthalene-1-sulfate binding, circular dichroism, Fourier transform infrared spectroscopy, gel filtration chromatography and differential scanning calorimetry. The chemical-denaturation behaviour of HPr, followed by fluorescence, CD and gel filtration, at pH 7.5 and 25 degrees C, is described as a two-state process, which does not involve the accumulation of thermodynamically stable intermediates. Its conformational stability under those conditions is deltaG = 4.0 +/- 0.2 kcal x mol-1 (1 kcal = 4.18 kJ), which makes the HPr from S. coelicolor the most unstable member of the HPr family described so far. The stability of the protein does not change significantly from pH 7-9, as concluded from the differential scanning calorimetry and thermal CD experiments. Conformational studies at low pH (pH 2.5-4) suggest that, in the absence of cosmotropic agents, HPr does not unfold completely; rather, it accumulates partially folded species. The transition from those species to other states with native-like secondary and tertiary structure, occurs with a pKa = 3.3 +/- 0.3, as measured by the averaged measurements obtained by CD and fluorescence. However, this transition does not agree either with: (a) that measured by burial of hydrophobic patches (8-anilinonaphthalene-1-sulfate binding experiments); or (b) that measured by acquisition of native-like compactness (gel-filtration studies). It seems that acquisition of native-like features occurs in a wide pH range and it cannot be ascribed to a unique side-chain titration. These series of intermediates have not been reported previously in any member of the HPr family.

Tolerance of citrus rootstock seedlings to saline stress based on their ability to regulate ion uptake and transport

Forty-five-day-old seedlings of sour orange (Citrus aurantium L.) and Citrus macrophylla Wester, the most commonly used rootstocks in lemon orchards, were grown in nutrient solutions containing 1 (control), 10, 20, 30 or 60 mM NaCl for 14 days. The effects of salinity on growth, uptake, transport and accumulation of Cl- and Na+ ions in leaves, stem and four root segments were studied. The 60 mM NaCl treatment reduced leaf dry mass more in C. macrophylla (40%) than in sour orange (20%), whereas it reduced root dry mass more in sour orange (36%) than in C. macrophylla (20%). In C. macrophylla, Cl- and Na+ uptake rates were high at the beginning of the saline treatments, but low at the end of the 14-day experiment. In contrast, sour orange showed high uptake rates at the beginning and end of the experiment. In response to increasing salinity, root and shoot concentrations of Cl- and Na+ increased in sour orange, but not in C. macrophylla. Different loading characteristics of Cl- and Na+ were observed between young and old segments of the root system. In general, old root segments reached quasi-steady-states later than young root segments. These results suggest that sour orange and C. macrophylla have different regulatory mechanisms for uptake and transport of Cl- and Na+.

Thermal stabilization of the catalytic domain of botulinum neurotoxin E by phosphorylation of a single tyrosine residue

The catalytic domain of clostridial neurotoxins is a substrate of tyrosine-specific protein kinases. The functional role of tyrosine phosphorylation and also the number and location of its (their) phosphorylation site(s) are yet elusive. We have used the recombinant catalytic domain of botulinum neurotoxin E (BoNT E) to examine these issues. Bacterially expressed and purified BoNT E catalytic domain was fully active, and was phosphorylated in vitro by the tyrosine-specific kinase Src. Tyrosine phosphorylation of the catalytic domain increased the protein thermal stability without affecting its proteolytic activity. Covalent modification of the endopeptidase promoted a disorder-to-order transition, as evidenced by the 35% increment of the alpha-helical content, which resulted in a 4 degrees C increase of its denaturation temperature. Site-directed replacement of tyrosine at position 67 completely abolished phosphate incorporation by Src. Constitutively unphosphorylated endopeptidase mutants exhibited functional properties virtually identical to those displayed by the nonphosphorylated wild-type catalytic domain. These findings indicate the presence of a single phosphorylation site in the catalytic domain of clostridial neurotoxins, and that its covalent modification primarily modulates the protein thermostability.

Thermal unfolding of the cytotoxin alpha-sarcin: phospholipid binding induces destabilization of the protein structure

The effect of membrane binding on the structure and stability of the cytotoxin alpha-sarcin has been studied by differential scanning calorimetry, Fourier-transform infrared and fluorescence spectroscopic techniques. The thermal unfolding of alpha-sarcin in aqueous solution fits into a two-state transition characterized by a transition temperature (Tm) of 52.6 degrees C and a calorimetric enthalpy (delta Hcal) of 136 kcal/mol. Upon interaction with phosphatidylglycerol vesicles, alpha-sarcin undergoes conformational changes, as deduced from the FTIR and fluorescence emission spectra. These changes result in a decreased Tm and delta Hcal values for the thermal unfolding of phospholipid-bound alpha-sarcin. The lower Tm value for lipid-bound alpha-sarcin is also observed at the level of secondary and tertiary structures, based on analyses of both the amide I' infrared spectrum and the tryptophan emission of the protein as a function of temperature, respectively. The results obtained indicate a protein destabilization promoted by the phospholipid interaction.