Disease-associated mutations in the human TRPM3 render the channel overactive via two distinct mechanisms

Neurodevelopmental disorders caused by variants in TRPM3

Developmental and epileptic encephalopathies (DEE) are a broad and varied group of disorders that affect the brain and are characterized by epilepsy and comorbid intellectual disability (ID). These conditions have a broad spectrum of symptoms and can be caused by various underlying factors, including genetic mutations, infections, and other medical conditions. The exact cause of DEE remains largely unknown in the majority of cases. However, in around 25 % of patients, rare nonsynonymous coding variants in genes encoding ion channels, cell-surface receptors, and other neuronally expressed proteins are identified. This review focuses on a subgroup of DEE patients carrying variations in the gene encoding the Transient Receptor Potential Melastatin 3 (TRPM3) ion channel, where recent data indicate that gain-of-function of TRPM3 channel activity underlies a spectrum of dominant neurodevelopmental disorders.

Phosphoinositide Regulation of TRP Channels: A Functional Overview in the Structural Era

Transient receptor potential (TRP) ion channels have diverse activation mechanisms including physical stimuli, such as high or low temperatures, and a variety of intracellular signaling molecules. Regulation by phosphoinositides and their derivatives is their only known common regulatory feature. For most TRP channels, phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] serves as a cofactor required for activity. Such dependence on PI(4,5)P2 has been demonstrated for members of the TRPM subfamily and for the epithelial TRPV5 and TRPV6 channels. Intracellular TRPML channels show specific activation by PI(3,5)P2. Structural studies uncovered the PI(4,5)P2 and PI(3,5)P2 binding sites for these channels and shed light on the mechanism of channel opening. PI(4,5)P2 regulation of TRPV1-4 as well as some TRPC channels is more complex, involving both positive and negative effects. This review discusses the functional roles of phosphoinositides in TRP channel regulation and molecular insights gained from recent cryo-electron microscopy structures.

Targeting TRP channels: recent advances in structure, ligand binding, and molecular mechanisms

Transient receptor potential (TRP) channels are a large and diverse family of transmembrane ion channels that are widely expressed, have important physiological roles, and are associated with many human diseases. These proteins are actively pursued as promising drug targets, benefitting greatly from advances in structural and mechanistic studies of TRP channels. At the same time, the complex, polymodal activation and regulation of TRP channels have presented formidable challenges. In this short review, we summarize recent progresses toward understanding the structural basis of TRP channel function, as well as potential ligand binding sites that could be targeted for therapeutics. A particular focus is on the current understanding of the molecular mechanisms of TRP channel activation and regulation, where many fundamental questions remain unanswered. We believe that a deeper understanding of the functional mechanisms of TRP channels will be critical and likely transformative toward developing successful therapeutic strategies targeting these exciting proteins. This endeavor will require concerted efforts from computation, structural biology, medicinal chemistry, electrophysiology, pharmacology, drug safety and clinical studies.

Pharmacologically targeting transient receptor potential channels for seizures and epilepsy: Emerging preclinical evidence of druggability

As one of the most prevalent and disabling brain disorders, epilepsy is characterized by spontaneous seizures that result from aberrant, excessive hyperactivity of a group of highly synchronized brain neurons. Remarkable progress in epilepsy research and treatment over the first two decades of this century led to a dramatical expansion in the third-generation antiseizure drugs (ASDs). However, there are still over 30% of patients suffering from seizures resistant to the current medications, and the broad unbearable adversative effects of ASDs significantly impair the quality of life in about 40% of individuals affected by the disease. Prevention of epilepsy in those who are at high risks is another major unmet medical need, given that up to 40% of epilepsy patients are believed to have acquired causes. Therefore, it is important to identify novel drug targets that can facilitate the discovery and development of new therapies engaging unprecedented mechanisms of action that might overcome these significant limitations. Also over the last two decades, calcium signaling has been increasingly recognized as a key contributory factor in epileptogenesis of many aspects. The intracellular calcium homeostasis involves a variety of calcium-permeable cation channels, the most important of which perhaps are the transient receptor potential (TRP) ion channels. This review focuses on recent exciting advances in understanding of TRP channels in preclinical models of seizure disorders. We also provide emerging insights into the molecular and cellular mechanisms of TRP channels-engaged epileptogenesis that might lead to new antiseizure therapies, epilepsy prevention and modification, and even a cure.

Gain-of-function variants in the ion channel gene TRPM3 underlie a spectrum of neurodevelopmental disorders

TRPM3 is a temperature- and neurosteroid-sensitive plasma membrane cation channel expressed in a variety of neuronal and non-neuronal cells. Recently, rare de novo variants in TRPM3 were identified in individuals with developmental and epileptic encephalopathy, but the link between TRPM3 activity and neuronal disease remains poorly understood. We previously reported that two disease-associated variants in TRPM3 lead to a gain of channel function . Here, we report a further 10 patients carrying one of seven additional heterozygous TRPM3 missense variants. These patients present with a broad spectrum of neurodevelopmental symptoms, including global developmental delay, intellectual disability, epilepsy, musculo-skeletal anomalies, and altered pain perception. We describe a cerebellar phenotype with ataxia or severe hypotonia, nystagmus, and cerebellar atrophy in more than half of the patients. All disease-associated variants exhibited a robust gain-of-function phenotype, characterized by increased basal activity leading to cellular calcium overload and by enhanced responses to the neurosteroid ligand pregnenolone sulfate when co-expressed with wild-type TRPM3 in mammalian cells. The antiseizure medication primidone, a known TRPM3 antagonist, reduced the increased basal activity of all mutant channels. These findings establish gain-of-function of TRPM3 as the cause of a spectrum of autosomal dominant neurodevelopmental disorders with frequent cerebellar involvement in humans and provide support for the evaluation of TRPM3 antagonists as a potential therapy.

Structural and functional analyses of a GPCR-inhibited ion channel TRPM3

G-protein coupled receptors (GPCRs) govern the physiological response to stimuli by modulating the activity of downstream effectors, including ion channels. TRPM3 is an ion channel inhibited by GPCRs through direct interaction with G protein (Gβγ) released upon their activation. This GPCR-TRPM3 signaling pathway contributes to the analgesic effect of morphine. Here, we characterized Gβγ inhibition of TRPM3 using electrophysiology and single particle cryo-electron microscopy (cryo-EM). From electrophysiology, we obtained a half inhibition constant (IC50) of ∼240 nM. Using cryo-EM, we determined structures of mouse TRPM3 expressed in human cells with and without Gβγ and with and without PIP₂, a lipid required for TRPM3 activity, at resolutions of 2.7-4.7 Å. Gβγ-TRPM3 interfaces vary depending on PIP₂ occupancy; however, in all cases, Gβγ appears loosely attached to TRPM3. The IC50 in electrophysiology experiments raises the possibility that additional unknown factors may stabilize the TRPM3-Gβγ complex.

Developmental and epileptic encephalopathies - therapeutic consequences of genetic testing

Developmental and epileptic encephalopathies comprise a heterogeneous group of monogenic neurodevelopmental disorders characterized by early-onset seizures, marked epileptic activity and abnormal neurocognitive development. The identification of an increasing number of underlying genetic alterations and their pathophysiological roles in cellular signaling drives the way toward novel precision therapies. The implementation of novel treatments that target the underlying mechanisms gives hope for disease modification that will improve not only the seizure burden but also the neurodevelopmental outcome of affected children. So far, beneficial effects are mostly reported in individual trials and small numbers of patients. There is a need for international collaborative studies to define the natural history and relevant outcome measures and to test novel pharmacological approaches.

Computational and functional studies of the PI(4,5)P2 binding site of the TRPM3 ion channel reveal interactions with other regulators

Transient Receptor Potential Melastatin 3 (TRPM3) is a heat-activated ion channel expressed in peripheral sensory neurons and the central nervous system. TRPM3 activity depends on the membrane phospholipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂], but the molecular mechanism of activation by PI(4,5)P₂ is not known. As no experimental structure of TRPM3 is available, we built a homology model of the channel in complex with PI(4,5)P₂via molecular modeling. We identified putative contact residues for PI(4,5)P₂ in the pre-S1 segment, the S4-S5 linker, and the proximal C-terminal TRP-domain. Mutating these residues increased sensitivity to inhibition of TRPM3 by decreasing PI(4,5)P₂ levels. Changes in ligand-binding affinities via Molecular Mechanics/Generalized Born Surface Area (MM/GBSA) showed reduced PI(4,5)P₂ affinity for the mutants. Mutating PI(4,5)P₂ interacting residues also reduced sensitivity for activation by the endogenous ligand pregnenolone sulfate (PregS), pointing to an allosteric interaction between PI(4,5)P₂ and PregS. Similarly, mutating residues in the PI(4,5)P₂ binding site in TRPM8 resulted in increased sensitivity to PI(4,5)P₂ depletion, and reduced sensitivity to menthol. Mutations of most PI(4,5)P₂-interacting residues in TRPM3 also increased sensitivity to inhibition by Gβγ, indicating allosteric interaction between Gβγ and PI(4,5)P₂. Disease-associated gain-of-function TRPM3 mutations on the other hand, resulted in no change of PI(4,5)P₂ sensitivity, indicating that mutations did not increase channel activity via increasing PI(4,5)P₂ interactions. Our data provide insight into the mechanism of regulation of TRPM3 by PI(4,5)P₂, its relationship to endogenous activators and inhibitors, as well as identify similarities and differences between PI(4,5)P₂ regulation of TRPM3 and TRPM8.

Network-Based Data Analysis Reveals Ion Channel-Related Gene Features in COVID-19: A Bioinformatic Approach

Coronavirus disease 2019 (COVID-19) seriously threatens human health and has been disseminated worldwide. Although there are several treatments for COVID-19, its control is currently suboptimal. Therefore, the development of novel strategies to treat COVID-19 is necessary. Ion channels are located on the membranes of all excitable cells and many intracellular organelles and are key components involved in various biological processes. They are a target of interest when searching for drug targets. This study aimed to reveal the relevant molecular features of ion channel genes in COVID-19 based on bioinformatic analyses. The RNA-sequencing data of patients with COVID-19 and healthy subjects (GSE152418 and GSE171110 datasets) were obtained from the Gene Expression Omnibus (GEO) database. Ion channel genes were selected from the Hugo Gene Nomenclature Committee (HGNC) database. The RStudio software was used to process the data based on the corresponding R language package to identify ion channel-associated differentially expressed genes (DEGs). Based on the DEGs, Gene Ontology (GO) functional and pathway enrichment analyses were performed using the Enrichr web tool. The STRING database was used to generate a protein-protein interaction (PPI) network, and the Cytoscape software was used to screen for hub genes in the PPI network based on the cytoHubba plug-in. Transcription factors (TF)-DEG, DEG-microRNA (miRNA) and DEG-disease association networks were constructed using the NetworkAnalyst web tool. Finally, the screened hub genes as drug targets were subjected to enrichment analysis based on the DSigDB using the Enrichr web tool to identify potential therapeutic agents for COVID-19. A total of 29 ion channel-associated DEGs were identified. GO functional analysis showed that the DEGs were integral components of the plasma membrane and were mainly involved in inorganic cation transmembrane transport and ion channel activity functions. Pathway analysis showed that the DEGs were mainly involved in nicotine addiction, calcium regulation in the cardiac cell and neuronal system pathways. The top 10 hub genes screened based on the PPI network included KCNA2, KCNJ4, CACNA1A, CACNA1E, NALCN, KCNA5, CACNA2D1, TRPC1, TRPM3 and KCNN3. The TF-DEG and DEG-miRNA networks revealed significant TFs (FOXC1, GATA2, HINFP, USF2, JUN and NFKB1) and miRNAs (hsa-mir-146a-5p, hsa-mir-27a-3p, hsa-mir-335-5p, hsa-let-7b-5p and hsa-mir-129-2-3p). Gene-disease association network analysis revealed that the DEGs were closely associated with intellectual disability and cerebellar ataxia. Drug-target enrichment analysis showed that the relevant drugs targeting the hub genes CACNA2D1, CACNA1A, CACNA1E, KCNA2 and KCNA5 were gabapentin, gabapentin enacarbil, pregabalin, guanidine hydrochloride and 4-aminopyridine. The results of this study provide a valuable basis for exploring the mechanisms of ion channel genes in COVID-19 and clues for developing therapeutic strategies for COVID-19.

TRPM3 in the eye and in the nervous system - from new findings to novel mechanisms

The calcium-permeable cation channel TRPM3 can be activated by heat and the endogenous steroid pregnenolone sulfate. TRPM3's best understood function is its role as a peripheral noxious heat sensor in mice. However, the channel is expressed in various tissues and cell types including neurons as well as glial and epithelial cells. TRPM3 expression patterns differ between species and change during development. Furthermore, a plethora of TRPM3 variants that result from alternative splicing have been identified and the majority of these isoforms are yet to be characterized. Moreover, the mechanisms underlying regulation of TRPM3 are largely unexplored. In addition, a micro-RNA gene (miR-204) is located within the TRPM3 gene. This complexity makes it difficult to obtain a clear picture of TRPM3 characteristics. However, a clear picture is needed to unravel TRPM3's full potential as experimental tool, diagnostic marker and therapeutic target. Therefore, the newest data related to TRPM3 have to be discussed and to be put in context as soon as possible to be up-to-date and to accelerate the translation from bench to bedside. The aim of this review is to highlight recent results and developments with particular focus on findings from studies involving ocular tissues and cells or peripheral neurons of rodents and humans.

Impaired TRPM3-dependent calcium influx and restoration using Naltrexone in natural killer cells of myalgic encephalomyelitis/chronic fatigue syndrome patients

Background

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a serious disorder of unknown aetiology. While the pathomechanism of ME/CFS remains elusive, reduced natural killer (NK) cell cytotoxic function is a consistent immunological feature. NK cell effector functions rely on long-term sustained calcium (Ca²⁺) influx. In recent years evidence of transient receptor potential melastatin 3 (TRPM3) dysfunction supports the hypothesis that ME/CFS is potentially an ion channel disorder. Specifically, reports of single nucleotide polymorphisms, low surface expression and impaired function of TRPM3 have been reported in NK cells of ME/CFS patients. It has been reported that mu (µ)-opioid receptor (µOR) agonists, known collectively as opioids, inhibit TRPM3. Naltrexone hydrochloride (NTX), a µOR antagonist, negates the inhibitory action of µOR on TRPM3 function. Importantly, it has recently been reported that NTX restores impaired TRPM3 function in NK cells of ME/CFS patients.

Methods

Live cell immunofluorescent imaging was used to measure TRPM3-dependent Ca²⁺ influx in NK cells isolated from n = 10 ME/CFS patients and n = 10 age- and sex-matched healthy controls (HC) following modulation with TRPM3-agonist, pregnenolone sulfate (PregS) and TRPM3-antaognist, ononetin. The effect of overnight (24 h) NTX in vitro treatment on TRPM3-dependent Ca²⁺ influx was determined.

Results

The amplitude (p < 0.0001) and half-time of Ca²⁺ response (p < 0.0001) was significantly reduced at baseline in NK cells of ME/CFS patients compared with HC. Overnight treatment of NK cells with NTX significantly improved TRPM3-dependent Ca²⁺ influx in ME/CFS patients. Specifically, there was no significance between HC and ME/CFS patients for half-time response, and the amplitude of Ca²⁺ influx was significantly increased in ME/CFS patients (p < 0.0001).

Conclusion

TRPM3-dependent Ca²⁺ influx was restored in ME/CFS patients following overnight treatment of isolated NK cells with NTX in vitro. Collectively, these findings validate that TRPM3 loss of function results in altered Ca²⁺ influx supporting the growing evidence that ME/CFS is a TRP ion channel disorder and that NTX provides a potential therapeutic intervention for ME/CFS.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12967-022-03297-8.

Phenotypic spectrum of the recurrent TRPM3 p.(Val837Met) substitution in seven individuals with global developmental delay and hypotonia

TRPM3 encodes a transient receptor potential cation channel of the melastatin family, expressed in the central nervous system and in peripheral sensory neurons of the dorsal root ganglia. The recurrent substitution in TRPM3: c.2509G>A, p.(Val837Met) has been associated with syndromic intellectual disability and seizures. In this report, we present the clinical and molecular features of seven previously unreported individuals, identified by exome sequencing, with the recurrent p.(Val837Met) variant and global developmental delay. Other shared clinical features included congenital hypotonia, dysmorphic facial features (broad forehead, deep-set eyes, and down turned mouth), exotropia, and musculoskeletal issues (hip dysplasia, hip dislocation, scoliosis). Seizures were observed in two of seven individuals (febrile seizure in one and generalized tonic-clonic seizures with atonic drops in another), and epileptiform activity was observed in an additional two individuals. This report extends the number of affected individuals to 16 who are heterozygous for the de novo recurrent substitution p.(Val837Met). In contrast with the initial report, epilepsy was not a mandatory feature observed in this series. TRPM3 pathogenic variation should be considered in individuals with global developmental delays, moderate-severe intellectual disability with, or without, childhood-onset epilepsy.

Partial Agonistic Actions of Sex Hormone Steroids on TRPM3 Function

Sex hormone steroidal drugs were reported to have modulating actions on the ion channel TRPM3. Pregnenolone sulphate (PS) presents the most potent known endogenous chemical agonist of TRPM3 and affects several gating modes of the channel. These includes a synergistic action of PS and high temperatures on channel opening and the PS-induced opening of a noncanonical pore in the presence of other TRPM3 modulators. Moreover, human TRPM3 variants associated with neurodevelopmental disease exhibit an increased sensitivity for PS. However, other steroidal sex hormones were reported to influence TRPM3 functions with activating or inhibiting capacity. Here, we aimed to answer how DHEAS, estradiol, progesterone and testosterone act on the various modes of TRPM3 function in the wild-type channel and two-channel variants associated with human disease. By means of calcium imaging and whole-cell patch clamp experiments, we revealed that all four drugs are weak TRPM3 agonists that share a common steroidal interaction site. Furthermore, they exhibit increased activity on TRPM3 at physiological temperatures and in channels that carry disease-associated mutations. Finally, all steroids are able to open the noncanonical pore in wild-type and DHEAS also in mutant TRPM3. Collectively, our data provide new valuable insights in TRPM3 gating, structure-function relationships and ligand sensitivity.

Local Ancestry Adjusted Allelic Association Analysis Robustly Captures Tuberculosis Susceptibility Loci

Pulmonary tuberculosis (TB), caused by Mycobacterium tuberculosis, is a complex disease. The risk of developing active TB is in part determined by host genetic factors. Most genetic studies investigating TB susceptibility fail to replicate association signals particularly across diverse populations. South African populations arose because of multi-wave genetic admixture from the indigenous KhoeSan, Bantu-speaking Africans, Europeans, Southeast Asian-and East Asian populations. This has led to complex genetic admixture with heterogenous patterns of linkage disequilibrium and associated traits. As a result, precise estimation of both global and local ancestry is required to prevent both false positive and false-negative associations. Here, 820 individuals from South Africa were genotyped on the SNP-dense Illumina Multi-Ethnic Genotyping Array (∼1.7M SNPs) followed by local and global ancestry inference using RFMix. Local ancestry adjusted allelic association (LAAA) models were utilized owing to the extensive genetic heterogeneity present in this population. Hence, an interaction term, comprising the identification of the minor allele that corresponds to the ancestry present at the specific locus under investigation, was included as a covariate. One SNP (rs28647531) located on chromosome 4q22 was significantly associated with TB susceptibility and displayed a SNP minor allelic effect (G allele, frequency = 0.204) whilst correcting for local ancestry for Bantu-speaking African ancestry (p-value = 5.518 × 10^-7; OR = 3.065; SE = 0.224). Although no other variants passed the significant threshold, clear differences were observed between the lead variants identified for each ancestry. Furthermore, the LAAA model robustly captured the source of association signals in multi-way admixed individuals from South Africa and allowed the identification of ancestry-specific disease risk alleles associated with TB susceptibility that have previously been missed.

Toward a methodology for evaluating DNA variants in nuclear families

The genetic underpinnings of most pediatric-cancer cases are unknown. Population-based studies use large sample sizes but have accounted for only a small proportion of the estimated heritability of pediatric cancers. Pedigree-based studies are infeasible for most human populations. One alternative is to collect genetic data from a single nuclear family and use inheritance patterns within the family to filter candidate variants. This approach can be applied to common and rare variants, including those that are private to a given family or to an affected individual. We evaluated this approach using genetic data from three nuclear families with 5, 4, and 7 children, respectively. Only one child in each nuclear family had been diagnosed with cancer, and neither parent had been affected. Diagnoses for the affected children were benign low-grade astrocytoma, Wilms tumor (stage 2), and Burkitt’s lymphoma, respectively. We used whole-genome sequencing to profile normal cells from each family member and a linked-read technology for genomic phasing. For initial variant filtering, we used global minor allele frequencies, deleteriousness scores, and functional-impact annotations. Next, we used genetic variation in the unaffected siblings as a guide to filter the remaining variants. As a way to evaluate our ability to detect variant(s) that may be relevant to disease status, the corresponding author blinded the primary author to affected status; the primary author then assigned a risk score to each child. Based on this evidence, the primary author predicted which child had been affected in each family. The primary author’s prediction was correct for the child who had been diagnosed with a Wilms tumor; the child with Burkitt’s lymphoma had the second-highest risk score among the seven children in that family. This study demonstrates a methodology for filtering and evaluating candidate genomic variants and genes within nuclear families that may merit further exploration.

Contribution of non-selective membrane channels and receptors in epilepsy

Overcoming refractory epilepsy's resistance to the combination of antiepileptic drugs (AED), mitigating side effects, and preventing sudden unexpected death in epilepsy are critical goals for therapy of this disorder. Current therapeutic strategies are based primarily on neurocentric mechanisms, overlooking the participation of astrocytes and microglia in the pathophysiology of epilepsy. This review is focused on a set of non-selective membrane channels (permeable to ions and small molecules), including channels and ionotropic receptors of neurons, astrocytes, and microglia, such as: the hemichannels formed by Cx43 and Panx1; the purinergic P2X7 receptors; the transient receptor potential vanilloid (TRPV1 and TRPV4) channels; calcium homeostasis modulators (CALHMs); transient receptor potential canonical (TRPC) channels; transient receptor potential melastatin (TRPM) channels; voltage-dependent anion channels (VDACs) and volume-regulated anion channels (VRACs), which all have in common being activated by epileptic activity and the capacity to exacerbate seizure intensity. Specifically, we highlight evidence for the activation of these channels/receptors during epilepsy including neuroinflammation and oxidative stress, discuss signaling pathways and feedback mechanisms, and propose the functions of each of them in acute and chronic epilepsy. Studying the role of these non-selective membrane channels in epilepsy and identifying appropriate blockers for one or more of them could provide complementary therapies to better alleviate the disease.

Description of a novel patient with the TRPM3 recurrent p.Val837Met variant

De novo heterozygous missense mutations in TRPM3 have been shown to cause developmental and epileptic encephalopathies (DEE). It is a very rare condition, as only 9 patients have been described to date. We report here a novel patient carrying the recurrent p.Val837Met variant and presenting new clinical features, such as trigonocephaly, expanding the phenotypical spectrum of the disease.

Mutation of the TRPM3 cation channel underlies progressive cataract development and lens calcification associated with pro-fibrotic and immune cell responses

Transient-receptor-potential cation channel, subfamily M, member 3 (TRPM3) serves as a polymodal calcium sensor in diverse mammalian cell-types. Mutation of the human TRPM3 gene (TRPM3) has been linked with inherited forms of early-onset cataract with or without other eye abnormalities. Here, we have characterized the ocular phenotypes of germline "knock-in" mice that harbor a human cataract-associated isoleucine-to-methionine mutation (p.I65M) in TRPM3 (Trpm3-mutant) compared with germline "knock-out" mice that functionally lack TRPM3 (Trpm3-null). Despite strong expression of Trpm3 in lens epithelial cells, neither heterozygous (Trpm3^+/- ) nor homozygous (Trpm3^-/- ) Trpm3-null mice developed cataract; however, the latter exhibited a mild impairment of lens growth. In contrast, homozygous Trpm3-M/M mutants developed severe, progressive, anterior pyramid-like cataract with microphthalmia, whereas heterozygous Trpm3-I/M and hemizygous Trpm3-M/- mutants developed anterior pyramidal cataract with delayed onset and progression-consistent with a semi-dominant lens phenotype. Histochemical staining revealed abnormal accumulation of calcium phosphate-like deposits and collagen fibrils in Trpm3-mutant lenses and immunoblotting detected increased αII-spectrin cleavage products consistent with calpain hyper-activation. Immunofluorescent confocal microscopy of Trpm3-M/M mutant lenses revealed fiber cell membrane degeneration that was accompanied by accumulation of alpha-smooth muscle actin positive (α-SMA+ve) myofibroblast-like cells and macrosialin positive (CD68+ve) macrophage-like cells. Collectively, our mouse model data support an ocular disease association for TRPM3 in humans and suggest that (1) Trpm3 deficiency impaired lens growth but not lens transparency and (2) Trpm3 dysfunction resulted in progressive lens degeneration and calcification coupled with pro-fibrotic (α-SMA+ve) and immune (CD68+ve) cell responses.

A Chinese patient with developmental and epileptic encephalopathies (DEE) carrying a TRPM3 gene mutation: a paediatric case report

Background

Developmental and epileptic encephalopathies (DEEs) are a heterogeneous group of chronic encephalopathies characterized by epilepsy with comorbid intellectual disability that are frequently associated with de novo nonsynonymous coding variants in ion channels, cell-surface receptors, and other neuronally expressed genes. Mutations in TRPM3 were identified as the cause of DEE. We report a novel patient with DEE carrying a de novo missense mutation in TRPM3, p.(S1202T); this missense mutation has never been reported.

Case presentation

A 7-year and 2-month-old Chinese patient who had recurrent polymorphic seizures was clinically diagnosed with DEE. A de novo missense mutation in TRPM3, which has not yet been reported, was identified in this case. The patient had a clinical phenotype consistent with previous reports.

Conclusions

These findings could expand the spectrum of TRPM3 mutations and might also support that de novo substitutions of TRPM3 are a cause of DEE.

The newest TRP channelopathy: Gain of function TRPM3 mutations cause epilepsy and intellectual disability

Transient Receptor Potential Melastatin 3 (TRPM3) is a Ca²⁺ permeable nonselective cation channel, activated by heat and chemical agonists, such as the endogenous neuro-steroid Pregnenolone Sulfate (PregS) and the chemical compound CIM0216. TRPM3 is expressed in peripheral sensory neurons of the dorsal root ganglia (DRG), and its role in noxious heat sensation in mice is well established. TRPM3 is also expressed in a number of other tissues, including the brain, but its role there has been largely unexplored. Recent reports showed that two mutations in TRPM3 are associated with a developmental and epileptic encephalopathy, pointing to an important role of TRPM3 in the human brain. Subsequent reports found that the two disease-associated mutations increased basal channel activity, and sensitivity of the channel to activation by heat and chemical agonists. This review will discuss these mutations in the context of human diseases caused by mutations in other TRP channels, and in the context of the biophysical properties and physiological functions of TRPM3.

Role of TRPM7 kinase in cancer

Cancer is the second leading cause of death worldwide and accounted for an estimated 9.6 million deaths, or 1 in 6 deaths, in 2018. Despite recent advances in cancer prevention, diagnosis, and treatment strategies, the burden of this disease continues to grow with each year, with dire physical, emotional, and economic consequences for all levels of society. Classic characteristics of cancer include rapid, uncontrolled cell proliferation and spread of cancerous cells to other parts of the body, a process known as metastasis. Transient receptor potential melastatin 7 (TRPM7), a Ca²⁺- and Mg²⁺-permeable nonselective divalent cation channel defined by the atypical presence of an α-kinase within its C-terminal domain, has been implicated, due to its modulation of Ca²⁺ and Mg²⁺ influx, in a wide variety of physiological and pathological processes, including cancer. TRPM7 is overexpressed in several cancer types and has been shown to variably increase cellular proliferation, migration, and invasion of tumour cells. However, the relative contribution of TRPM7 kinase domain activity to cancer as opposed to ion flux through its channel pore remains an area of active discovery. In this review, we describe the specific role of the TRPM7 kinase domain in cancer processes as well as mechanisms of regulation and inhibition of the kinase domain.

TRPM3 in Brain (Patho)Physiology

Already for centuries, humankind is driven to understand the physiological and pathological mechanisms that occur in our brains. Today, we know that ion channels play an essential role in the regulation of neural processes and control many functions of the central nervous system. Ion channels present a diverse group of membrane-spanning proteins that allow ions to penetrate the insulating cell membrane upon opening of their channel pores. This regulated ion permeation results in different electrical and chemical signals that are necessary to maintain physiological excitatory and inhibitory processes in the brain. Therefore, it is no surprise that disturbances in the functions of cerebral ion channels can result in a plethora of neurological disorders, which present a tremendous health care burden for our current society. The identification of ion channel-related brain disorders also fuel the research into the roles of ion channel proteins in various brain states. In the last decade, mounting evidence has been collected that indicates a pivotal role for transient receptor potential (TRP) ion channels in the development and various physiological functions of the central nervous system. For instance, TRP channels modulate neurite growth, synaptic plasticity and integration, and are required for neuronal survival. Moreover, TRP channels are involved in numerous neurological disorders. TRPM3 belongs to the melastatin subfamily of TRP channels and represents a non-selective cation channel that can be activated by several different stimuli, including the neurosteroid pregnenolone sulfate, osmotic pressures and heat. The channel is best known as a peripheral nociceptive ion channel that participates in heat sensation. However, recent research identifies TRPM3 as an emerging new player in the brain. In this review, we summarize the available data regarding the roles of TRPM3 in the brain, and correlate these data with the neuropathological processes in which this ion channel may be involved.

TRPM3 Channels Play Roles in Heat Hypersensitivity and Spontaneous Pain after Nerve Injury

Transient receptor potential melastatin 3 (TRPM3) is a heat-activated ion channel in primary sensory neurons of the dorsal root ganglia (DRGs). Pharmacological and genetic studies implicated TRPM3 in various pain modalities, but TRPM3 inhibitors were not validated in TRPM3^-/- mice. Here we tested two inhibitors of TRPM3 in male and female wild-type and TRPM3^-/- mice in nerve injury-induced neuropathic pain. We found that intraperitoneal injection of either isosakuranetin or primidone reduced heat hypersensitivity induced by chronic constriction injury (CCI) of the sciatic nerve in wild-type, but not in TRPM3^-/- mice. Primidone was also effective when injected locally in the hindpaw or intrathecally. Consistently, intrathecal injection of the TRPM3 agonist CIM0216 reduced paw withdrawal latency to radiant heat in wild-type, but not in TRPM3^-/- mice. Intraperitoneal injection of 2 mg/kg, but not 0.5 mg/kg isosakuranetin, inhibited cold and mechanical hypersensitivity in CCI, both in wild-type and TRPM3^-/- mice, indicating a dose-dependent off-target effect. Primidone had no effect on cold sensitivity, and only a marginal effect on mechanical hypersensitivity. Genetic deletion or inhibitors of TRPM3 reduced the increase in the levels of the early genes c-Fos and pERK in the spinal cord and DRGs in CCI mice, suggesting spontaneous activity of the channel. Intraperitoneal isosakuranetin also inhibited spontaneous pain related behavior in CCI in the conditioned place preference assay, and this effect was eliminated in TRPM3^-/- mice. Overall, our data indicate a role of TRPM3 in heat hypersensitivity and in spontaneous pain after nerve injury.SIGNIFICANCE STATEMENT Neuropathic pain is a major unsolved medical problem. The heat-activated TRPM3 ion channel is a potential target for novel pain medications, but the pain modalities in which it plays a role are not clear. Here we used a combination of genetic and pharmacological tools to assess the role of this channel in spontaneous pain, heat, cold, and mechanical hypersensitivity in a nerve injury model of neuropathic pain in mice. Our findings indicate a role for TRPM3 in heat hyperalgesia, and spontaneous pain, but not in cold and mechanical hypersensitivity. We also find that not only TRPM3 located in the peripheral nerve termini, but also TRPM3 in the spinal cord or proximal segments of DRG neurons are important for heat hypersensitivity.

A new molecular risk pathway for postpartum mood disorders: clues from steroid sulfatase-deficient individuals

Postpartum mood disorders develop shortly after childbirth in a significant proportion of women. These conditions are associated with a range of symptoms including abnormally high or low mood, irritability, cognitive disorganisation, disrupted sleep, hallucinations/delusions, and occasionally suicidal or infanticidal ideation; if not treated promptly, they can substantially impact upon the mother's health, mother-infant bonding, and family dynamics. The biological precipitants of such disorders remain unclear, although large changes in maternal immune and hormonal physiology following childbirth are likely to play a role. Pharmacological therapies for postpartum mood disorders can be effective, but may be associated with side effects, concerns relating to breastfeeding, and teratogenicity risks when used prophylactically. Furthermore, most of the drugs that are used to treat postpartum mood disorders are the same ones that are used to treat mood episodes during non-postpartum periods. A better understanding of the biological factors predisposing to postpartum mood disorders would allow for rational drug development, and the identification of predictive biomarkers to ensure that 'at risk' mothers receive earlier and more effective clinical management. We describe new findings relating to the role of the enzyme steroid sulfatase in maternal postpartum behavioural processes, and discuss how these point to a novel molecular risk pathway underlying postpartum mood disorders. Specifically, we suggest that aberrant steroid hormone-dependent regulation of neuronal calcium influx via extracellular matrix proteins and membrane receptors involved in responding to the cell's microenvironment might be important. Testing of this hypothesis might identify novel therapeutic targets and predictive biomarkers.

TRPM Channels in Human Diseases

The transient receptor potential melastatin (TRPM) subfamily belongs to the TRP cation channels family. Since the first cloning of TRPM1 in 1989, tremendous progress has been made in identifying novel members of the TRPM subfamily and their functions. The TRPM subfamily is composed of eight members consisting of four six-transmembrane domain subunits, resulting in homomeric or heteromeric channels. From a structural point of view, based on the homology sequence of the coiled-coil in the C-terminus, the eight TRPM members are clustered into four groups: TRPM1/M3, M2/M8, M4/M5 and M6/M7. TRPM subfamily members have been involved in several physiological functions. However, they are also linked to diverse pathophysiological human processes. Alterations in the expression and function of TRPM subfamily ion channels might generate several human diseases including cardiovascular and neurodegenerative alterations, organ dysfunction, cancer and many other channelopathies. These effects position them as remarkable putative targets for novel diagnostic strategies, drug design and therapeutic approaches. Here, we review the current knowledge about the main characteristics of all members of the TRPM family, focusing on their actions in human diseases.

TRP Channel Cooperation for Nociception: Therapeutic Opportunities

Chronic pain treatment remains a sore challenge, and in our aging society, the number of patients reporting inadequate pain relief continues to grow. Current treatment options all have their drawbacks, including limited efficacy and the propensity of abuse and addiction; the latter is exemplified by the ongoing opioid crisis. Extensive research in the last few decades has focused on mechanisms underlying chronic pain states, thereby producing attractive opportunities for novel, effective and safe pharmaceutical interventions. Members of the transient receptor potential (TRP) ion channel family represent innovative targets to tackle pain sensation at the root. Three TRP channels, TRPV1, TRPM3, and TRPA1, are of particular interest, as they were identified as sensors of chemical- and heat-induced pain in nociceptor neurons. This review summarizes the knowledge regarding TRP channel-based pain therapies, including the bumpy road of the clinical development of TRPV1 antagonists, the current status of TRPA1 antagonists, and the future potential of targeting TRPM3.