Structure, Function and Physiology of 5-Hydroxytryptamine Receptors Subtype 3

Functional assessment and influencing factors after staged functional training in patients with ankle fractures

BACKGROUND

The recovery of limb function after ankle fracture surgery is a gradual process. The main purpose of implementing early functional exercise, joint mobility, muscle contraction function, passive ankle flexion and extension exercises, or physical factor therapy techniques is to achieve the rapid recovery of normal physiological limb function. However, currently the most effective rehabilitation training method is staged limb functional exercise, which promotes rapid recovery of limb function while preventing adverse consequences caused by overwork or insufficient training. Staged limb functional exercise divides the rehabilitation process into multiple stages, each of which has specific training objectives and contents. This method helps patients gradually restore limb function. Nevertheless, some patients still exhibit poor limb function after standardized exercise. Therefore, a functional evaluation should be performed to analyze the impact of staged functional training after ankle fracture surgery.

AIM

To perform a functional evaluation and determine the influencing factors of staged functional training in patients with ankle fracture.

METHODS

A retrospective study enrolled 150 patients who underwent surgical treatment for ankle fracture from May 2020 to May 2022 at our hospital. Univariate and multivariate linear regression analyses were performed on general data, functional exercise compliance scale for orthopedic patients, Social Support Rating Scale (SSRS), American Orthopedic Foot and Ankle Score (AOFAS) Ankle-Hindfoot Score, and pain factors [serum bradykinin (BK), prostaglandin E2 (PGE2), 5-hydroxytryptamine (5-HT)].

RESULTS

Based on the AOFAS Ankle-Hindfoot Scale, the cases were divided into the excellent function (n = 111) and ordinary function (n = 39) groups. Univariate analysis revealed that monthly family income, education level, diabetes mellitus, functional exercise compliance scale of orthopedic patients score, SSRS, BK, PGE2, and 5-HT significantly influenced limb function after ankle fracture (P < 0.05); Multiple linear regression analysis showed that the functional exercise compliance scale score, SSRS, BK, PGE2, and 5-HT were independent risk factors affecting functional performance after staged functional exercise (P < 0.05).

CONCLUSION

Exercise compliance, SSRS, and pain level are the independent risk factors affecting functional performance after staged functional training following ankle surgery. Clinical nursing care after ankle surgery should include analgesic and health education measures to ensure optimal recovery of limb function.

A comparative study of acupuncture combined with rehabilitation gymnastics on postoperative anal function of lower rectal cancer

BACKGROUND

From the anal function, inflammatory response and other indicators, acupuncture combined with rehabilitation gymnastics was applied to patients with cancer undergoing low resection, aiming to improve the prognosis of patients.

AIM

To explore the effects of acupuncture combined with rehabilitation gymnastics on anal function after lower rectal cancer surgery.

METHODS

From January 2020 to December 2022, 128 patients who underwent rectal cancer surgery in the Department of Oncology of Hebei Provincial Hospital of Traditional Chinese Medicine Hospital were selected and divided into two groups using the random number table method, with 64 patients in each group. Patients in the control group were not treated with acupuncture or rehabilitation gymnastics and served as blank controls. Patients in the study group were treated with acupuncture and rehabilitation gymnastics from the 7th postoperative day. The anal incontinence scores, changes in serum interleukin-4, interleukin-6, and interleukin-10 Levels, and serum motilin, 5-hydroxytryptamine, and vasoactive intestinal peptide levels were compared.

RESULTS

There were no significant differences in serum interleukin-4, interleukin-6, and interleukin-10 Levels between the groups before treatment (P > 0.05). After treatment, these levels were better than those of the control group (P < 0.05). There was no significant difference in the anal incontinence scores between the groups before and 7 d after surgery (P > 0.05). Anal incontinence scores in the study group were lower than those in the control group at 14 d, 21 d, and 28 d postoperatively (P < 0.05). There were no significant differences in serum motilin, 5-hydroxytryptamine, or vasoactive intestinal peptide levels between the groups before treatment (P > 0.05). After treatment, these levels were higher in the study group than in the control group, and vasoactive intestinal peptide level was lower in the study group than in the control group (P < 0.05).

CONCLUSION

Acupuncture combined with rehabilitation gymnastics can promote the recovery of anal function and reduce the inflammatory response in patients with lower rectal cancer after surgery.

Mapping the molecular motions of 5-HT3 serotonin-gated channel by voltage-clamp fluorometry

The serotonin-gated ion channel (5-HT3R) mediates excitatory neuronal communication in the gut and the brain. It is the target for setrons, a class of competitive antagonists widely used as antiemetics, and is involved in several neurological diseases. Cryo-electron microscopy (cryo-EM) of the 5-HT3R in complex with serotonin or setrons revealed that the protein has access to a wide conformational landscape. However, assigning known high-resolution structures to actual states contributing to the physiological response remains a challenge. In the present study, we used voltage-clamp fluorometry (VCF) to measure simultaneously, for 5-HT3R expressed at a cell membrane, conformational changes by fluorescence and channel opening by electrophysiology. Four positions identified by mutational screening report motions around and outside the serotonin-binding site through incorporation of cysteine-tethered rhodamine dyes with or without a nearby quenching tryptophan. VCF recordings show that the 5-HT3R has access to four families of conformations endowed with distinct fluorescence signatures: 'resting-like' without ligand, 'inhibited-like' with setrons, 'pre-active-like' with partial agonists, and 'active-like' (open channel) with partial and strong agonists. Data are remarkably consistent with cryo-EM structures, the fluorescence partners matching respectively apo, setron-bound, 5-HT bound-closed, and 5-HT-bound-open conformations. Data show that strong agonists promote a concerted motion of all fluorescently labeled sensors during activation, while partial agonists, especially when loss-of-function mutations are engineered, stabilize both active and pre-active conformations. In conclusion, VCF, though the monitoring of electrophysiologically silent conformational changes, illuminates allosteric mechanisms contributing to signal transduction and their differential regulation by important classes of physiological and clinical effectors.

Structural basis for partial agonism in 5-HT3A receptors

Hyperactivity of serotonin 3 receptors (5-HT3R) underlies pathologies associated with irritable bowel syndrome and chemotherapy-induced nausea and vomiting. Setrons, a class of high-affinity competitive antagonists, are used in the treatment of these conditions. Although generally effective for chemotherapy-induced nausea and vomiting, the use of setrons for treating irritable bowel syndrome has been impaired by adverse side effects. Partial agonists are now being considered as an alternative strategy, with potentially less severe side effects than full antagonists. However, a structural understanding of how these ligands work is lacking. Here, we present high-resolution cryogenic electron microscopy structures of the mouse 5-HT3AR in complex with partial agonists (SMP-100 and ALB-148471) captured in pre-activated and open-like conformational states. Molecular dynamics simulations were used to assess the stability of drug-binding poses and interactions with the receptor over time. Together, these studies reveal mechanisms for the functional differences between orthosteric partial agonists, full agonists and antagonists of the 5-HT3AR.

Discovery of a Novel Class of Benzimidazole-Based Nicotinic Acetylcholine Receptor Modulators: Positive and Negative Modulation Arising from Overlapping Allosteric Sites

Here, we present the discovery of a novel class of benzimidazole-based allosteric modulators of nicotinic acetylcholine receptors (nAChRs). The modulators were developed based on a compound (1) exhibiting positive modulatory activity at α4β2 nAChR in a compound library screening by functional characterization of 100 analogues of 1 at nAChRs. Two distinct series of positive and negative allosteric modulators (PAMs and NAMs, respectively) comprising benzimidazole as a shared structural moiety emerged from this SAR study. The PAMs mediated weak modulation of α4β2 and α6β2β3, whereas the NAMs exhibited essentially equipotent inhibition of α4β2, α6β2β3, α6β4β3, and α3β4 nAChRs, with analogue 9j [2-(2,4-dichlorophenoxy)-1,3-dimethyl-1-H-benzo[d]imidazole-3-ium] displaying high-nanomolar and low-micromolar IC50 values at the β2- and β4-containing receptor subtypes, respectively. We propose that the PAMs and NAMs act through overlapping sites in the nAChR, and these findings thus underline the heterogenous modes of modulation that can arise from a shared allosteric site in the receptor.

Evolutionary conservation of Zinc-Activated Channel (ZAC) functionality in mammals: a range of mammalian ZACs assemble into cell surface-expressed functional receptors

In contrast to the other pentameric ligand-gated ion channels in the Cys-loop receptor superfamily, the ZACN gene encoding for the Zinc-Activated Channel (ZAC) is exclusively found in the mammalian genome. Human ZAC assembles into homomeric cation-selective channels gated by Zn2+, Cu2+ and H+, but the function of the receptor in human physiology is presently poorly understood. In this study, the degree of evolutionary conservation of a functional ZAC in mammals was probed by investigating the abilities of a selection of ZACs from 10 other mammalian species than human to be expressed at the protein level and assemble into cell surface-expressed functional receptors in mammalian cells and in Xenopus oocytes. In an enzyme-linked immunosorbent assay, transient transfections of tsA201 cells with cDNAs of hemagglutinin (HA)-epitope-tagged versions of these 10 ZACs resulted in robust total expression and cell surface expression levels of all proteins. Moreover, injection of cRNAs for 6 of these ZACs in oocytes resulted in the formation of functional receptors in two-electrode voltage-clamp recordings. The ZACs exhibited robust current amplitudes in response to Zn2+ (10 mM) and H+ (pH 4.0), and the concentration-response relationships displayed by Zn2+ at these channels were largely comparable to that at human ZAC. In conclusion, the findings suggest that the functionality of ZAC at the molecular level may be conserved throughout mammalian species, and that the channel thus may govern physiological functions in mammals, including humans.

The diverse family of Cys-loop receptors in Caenorhabditis elegans: insights from electrophysiological studies

Cys-loop receptors integrate a large family of pentameric ligand-gated ion channels that mediate fast ionotropic responses in vertebrates and invertebrates. Their vital role in converting neurotransmitter recognition into an electrical impulse makes these receptors essential for a great variety of physiological processes. In vertebrates, the Cys-loop receptor family includes the cation-selective channels, nicotinic acetylcholine and 5-hydroxytryptamine type 3 receptors, and the anion-selective channels, GABAA and glycine receptors, whereas in invertebrates, the repertoire is significantly larger. The free-living nematode Caenorhabditis elegans has the largest known Cys-loop receptor family as well as unique receptors that are absent in vertebrates and constitute attractive targets for anthelmintic drugs. Given the large number and variety of Cys-loop receptor subunits and the multiple possible ways of subunit assembly, C. elegans offers a large diversity of receptors although only a limited number of them have been characterized to date. C. elegans has emerged as a powerful model for the study of the nervous system and human diseases as well as a model for antiparasitic drug discovery. This nematode has also shown promise in the pharmaceutical industry search for new therapeutic compounds. C. elegans is therefore a powerful model organism to explore the biology and pharmacology of Cys-loop receptors and their potential as targets for novel therapeutic interventions. In this review, we provide a comprehensive overview of what is known about the function of C. elegans Cys-loop receptors from an electrophysiological perspective.

Binding motif for RIC-3 chaperon protein in serotonin type 3A receptors

Serotonin or 5-hydroxytryptamine type 3 (5-HT3) receptors belong to the family of pentameric ligand-gated ion channels (pLGICs) that are therapeutic targets for psychiatric disorders and neurological diseases. Due to structural conservation and significant sequence similarities of pLGICs' extracellular and transmembrane domains, clinical trials for drug candidates targeting these two domains have been hampered by off-subunit modulation. With the present study, we explore the interaction interface of the 5-HT3A subunit intracellular domain (ICD) with the resistance to inhibitors of choline esterase (RIC-3) protein. Previously, we have shown that RIC-3 interacts with the L1-MX segment of the ICD fused to maltose-binding protein. In the present study, synthetic L1-MX-based peptides and Ala-scanning identify positions W347, R349, and L353 as critical for binding to RIC-3. Complementary studies using full-length 5-HT3A subunits confirm that the identified Ala substitutions reduce the RIC-3-mediated modulation of functional surface expression. Additionally, we find and characterize a duplication of the binding motif, DWLR…VLDR, present in both the MX-helix and the transition between the ICD MA-helix and transmembrane segment M4. Analogous Ala substitutions at W447, R449, and L454 disrupt MAM4-peptide RIC-3 interactions and reduce modulation of functional surface expression. In summary, we identify the binding motif for RIC-3 in 5-HT3A subunits at two locations in the ICD, one in the MX-helix and one at the MAM4-helix transition.

Phylogenetic analyses of 5-hydroxytryptamine 3 (5-HT3) receptors in Metazoa

The 5-hydroxytrptamine 3 (5-HT3) receptor is a member of the 'Cys-loop' family and the only pentameric ligand gated ion channel among the serotonin receptors. 5-HT3 receptors play an important role in controlling growth, development, and behaviour in animals. Several 5-HT3 receptor antagonists are used to treat diseases (e.g., irritable bowel syndrome, nausea and emesis). Humans express five different subunits (A-E) enabling a variety of heteromeric receptors to form but all contain 5HT3A subunits. However, the information available about the 5-HT3 receptor subunit occurrence among the metazoan lineages is minimal. In the present article we searched for 5-HT3 receptor subunit homologs from different phyla in Metazoa. We identified more than 1000 5-HT3 receptor subunits in Metazoa in different phyla and undertook simultaneous phylogenetic analysis of 526 5HT3A, 358 5HT3B, 239 5HT3C, 70 5HT3D, and 173 5HT3E sequences. 5-HT3 receptor subunits were present in species belonging to 11 phyla: Annelida, Arthropoda, Chordata, Cnidaria, Echinodermata, Mollusca, Nematoda, Orthonectida, Platyhelminthes, Rotifera and Tardigrada. All subunits were most often identified in Chordata phylum which was strongly represented in searches. Using multiple sequence alignment, we investigated variations in the ligand binding region of the 5HT3A subunit protein sequences in the metazoan lineage. Several critical amino acid residues important for ligand binding (common structural features) are commonly present in species from Nematoda and Platyhelminth gut parasites through to Chordata. Collectively, this better understanding of the 5-HT3 receptor evolutionary patterns raises possibilities of future pharmacological challenges facing Metazoa including effects on parasitic and other species in ecosystems that contain 5-HT3 receptor ligands.

Tapping into 5-HT3 Receptors to Modify Metabolic and Immune Responses

5-hydroxytryptamine type 3 (5-HT₃) receptors are ligand gated ion channels, which clearly distinguish their mode of action from the other G-protein coupled 5-HT or serotonin receptors. 5-HT₃ receptors are well established targets for emesis and gastrointestinal mobility and are used as adjunct targets in treating schizophrenia. However, the distribution of these receptors is wider than the nervous system and there is potential that these additional sites can be targeted to modulate inflammatory and/or metabolic conditions. Recent progress in structural biology and pharmacology of 5-HT₃ receptors have provided profound insights into mechanisms of their action. These advances, combined with insights into clinical relevance of mutations in genes encoding 5-HT₃ subunits and increasing understanding of their implications in patient's predisposition to diseases and response to the treatment, open new avenues for personalized precision medicine. In this review, we recap on the current status of 5-HT₃ receptor-based therapies using a biochemical and physiological perspective. We assess the potential for targeting 5-HT₃ receptors in conditions involving metabolic or inflammatory disorders based on recent findings, underscoring the challenges and limitations of this approach.

Direct Structural Insights into GABAA Receptor Pharmacology

GABA_A receptors are pentameric ligand-gated ion channels that mediate most fast neuronal inhibition in the brain. In addition to their important physiological roles, they are noteworthy in their rich pharmacology; prominent drugs used for anxiety, insomnia, and general anesthesia act through positive modulation of GABA_A receptors. Direct structural information for how these drugs work was absent until recently. Efforts in structural biology over the past few years have revealed how important drug classes and natural products interact with the GABA_A receptor, providing a foundation for studies in dynamics and structure-guided drug design. Here, we review recent developments in GABA_A receptor structural pharmacology, focusing on subunit assemblies of the receptor found at synapses.

Recombinant expression and purification of pentameric ligand-gated ion channels for Cryo-EM structural studies

Pentameric ligand-gated ion channels (pLGICs) are central players in synaptic neurotransmission and are targets to a range of drugs used to treat neurological disorders and pain. pLGICs are intrinsically dynamic membrane proteins that upon stimulation by neurotransmitters, undergo global conformational changes across multiple domains spanning a distance of over 165Å. The inter-domain flexibility, a feature crucial for their function as signal transducers in chemical synapses, has been problematic in the efforts toward determining high-resolution structures. Earlier structural studies tackled this issue with a variety of strategies that included partial truncation of flexible domains and the use of antibodies and small-molecule inhibitors to restrict domain movement. With the recent advances in cryo-electron microscopy and single-particle analysis, many of these limitations have been overcome. Here, we describe the methods used in the recombinant expression and purification of full-length constructs of two members of the pentameric ligand-gated ion channel family and the approaches used for capturing multiple conformations in cryo-EM imaging.