TRPV4 mutations and cytotoxic hypercalcemia in axonal Charcot-Marie-Tooth neuropathies

Clinical Characteristics and Whole Exome Sequencing Analysis in Serbian Cases of Clubfoot Deformity-Single Center Study

BACKGROUND

Recognized as one of the most serious musculoskeletal deformities, occurring in 1-2 per 1000 newborns, 80% of clubfeet are idiopathic while 20% present with associated malformations. The etiopathogenesis of clubfoot is described as multifactorial, including both genetic and environmental risk factors. The aim of this study was to analyze possible genetic causes of isolated and syndromic clubfoot in Serbian children, as well as to correlate clinical and genetic characteristics that would provide insight into clubfoot etiopathogenesis and possibly contribute to global knowledge about clinical features of different genetically defined disorders.

METHODS

We evaluated 50 randomly selected, eligible children with clubfoot aged 3 to 16 years that were initially hospitalized and treated at University Children's Hospital between November 2006 and November 2022. The tested parameters were gender, age, dominant foot, affected foot, degree of deformity, treatment, neuromuscular disorders, positive family history, and maternal smoking. According to the presence of defined genetic mutation/s by whole exome sequencing (WES), patients were separated into two groups: positive (with genetic mutation/s) and negative (without genetic mutation/s).

RESULTS

Seven patients were found to be positive, i.e., with genetic mutation/s. A statistically significant difference between categorical variables was found for families with a history of clubfoot, where more than half (57.14%) of patients with confirmed genetic mutation/s also had a family history of genetic mutation/s (p = 0.023).

CONCLUSIONS

The results from this study further expand the genetic epidemiology of clubfoot. This study contributes to the establishment of genetic diagnostic strategies in pediatric patients with this condition, which can lead to more efficient genetic diagnosis.

Gain-of-function mutations of TRPV4 acting in endothelial cells drive blood-CNS barrier breakdown and motor neuron degeneration in mice

Blood-CNS barrier disruption is a hallmark of numerous neurological disorders, yet whether barrier breakdown is sufficient to trigger neurodegenerative disease remains unresolved. Therapeutic strategies to mitigate barrier hyperpermeability are also limited. Dominant missense mutations of the cation channel transient receptor potential vanilloid 4 (TRPV4) cause forms of hereditary motor neuron disease. To gain insights into the cellular basis of these disorders, we generated knock-in mouse models of TRPV4 channelopathy by introducing two disease-causing mutations (R269C and R232C) into the endogenous mouse Trpv4 gene. TRPV4 mutant mice exhibited weakness, early lethality, and regional motor neuron loss. Genetic deletion of the mutant Trpv4 allele from endothelial cells (but not neurons, glia, or muscle) rescued these phenotypes. Symptomatic mutant mice exhibited focal disruptions of blood-spinal cord barrier (BSCB) integrity, associated with a gain of function of mutant TRPV4 channel activity in neural vascular endothelial cells (NVECs) and alterations of NVEC tight junction structure. Systemic administration of a TRPV4-specific antagonist abrogated channel-mediated BSCB impairments and provided a marked phenotypic rescue of symptomatic mutant mice. Together, our findings show that mutant TRPV4 channels can drive motor neuron degeneration in a non-cell autonomous manner by precipitating focal breakdown of the BSCB. Further, these data highlight the reversibility of TRPV4-mediated BSCB impairments and identify a potential therapeutic strategy for patients with TRPV4 mutations.

Remediation of Perceptual Deficits in Progressive Auditory Neuropathy: A Case Study

BACKGROUND

Auditory neuropathy (AN) is a hearing disorder that affects neural activity in the VIIIth cranial nerve and central auditory pathways. Progressive forms have been reported in a number of neurodegenerative diseases and may occur as a result of both the deafferentiation and desynchronisation of neuronal processes. The purpose of this study was to describe changes in auditory function over time in a patient with axonal neuropathy and to explore the effect of auditory intervention.

METHODS

We tracked auditory function in a child with progressive AN associated with Charcot-Marie-Tooth (Type 2C) disease, evaluating hearing levels, auditory-evoked potentials, and perceptual abilities over a 3-year period. Furthermore, we explored the effect of auditory intervention on everyday listening and neuroplastic development.

RESULTS

While sound detection thresholds remained constant throughout, both electrophysiologic and behavioural evidence suggested auditory neural degeneration over the course of the study. Auditory brainstem response amplitudes were reduced, and perception of auditory timing cues worsened over time. Functional hearing ability (speech perception in noise) also deteriorated through the first 1.5 years of study until the child was fitted with a "remote-microphone" listening device, which subsequently improved binaural processing and restored speech perception ability to normal levels.

CONCLUSIONS

Despite the deterioration of auditory neural function consistent with peripheral axonopathy, sustained experience with the remote-microphone listening system appeared to produce neuroplastic changes, which improved the patient's everyday listening ability-even when not wearing the device.

Case Report: TRPV4 gene mutation causing neuronopathy, distal hereditary motor, type VIII

Neuronopathy, distal hereditary motor, type VIII is an exceedingly rare autosomal dominant genetic disorder, also known as congenital non-progressive distal spinal muscular atrophy. It is characterized by progressive weakness in distal motor function and atrophy of muscles, without accompanying sensory impairment. Presently, there is limited literature on this condition, and accurate epidemiological data regarding its incidence remains unavailable. We report a paediatric case of distal hereditary motor, type VIII that is caused by a heterozygous missense mutation in the TRPV4 gene (NM_021625): c.805C>T. The proband is a 7-year-old male child. During pregnancy, his mother had prenatal ultrasound revealing "inward turning of the feet", a condition persisting after birth. The proband is currently unable to stand independently, exhibiting bilateral clubfoot deformity. Although possessing normal cognitive function, he cannot walk unaided. Computed radiography findings reveal pelvic tilt, bilateral knee joint valgus, and bilateral clubfoot. The patient underwent familial exome sequencing, revealing a mutation in the TRPV4 gene (NM_021625): c.805C>T (p.Arg269Cys). Considering the patient's medical history, clinical manifestations, imaging studies, and genetic test results, the diagnosis for this individual is Neuronopathy, distal hereditary motor, type VIII. This report documents a case involving the TRPV4 gene mutation associated with Neuronopathy, distal hereditary motor, type VIII, contributing valuable case reference for the early diagnosis of this condition.

The implementation and utility of clinical exome sequencing in a South African infant cohort

Genetic disorders are significant contributors to infant hospitalization and mortality globally. The early diagnosis of these conditions in infants remains a considerable challenge. Clinical exome sequencing (CES) has shown to be a successful tool for the early diagnosis of genetic conditions, however, its utility in African infant populations has not been investigated. The impact of the under-representation of African genomic data, the cost of testing, and genomic workforce shortages, need to be investigated and evidence-based implementation strategies accounting for locally available genetics expertise and diagnostic infrastructure need to be developed. We evaluated the diagnostic utility of singleton CES in a cohort of 32 ill, South African infants from two State hospitals in Johannesburg, South Africa. We analysed the data using a series of filtering approaches, including a curated virtual gene panel consisting of genes implicated in neonatal-and early childhood-onset conditions and genes with known founder and common variants in African populations. We reported a diagnostic yield of 22% and identified seven pathogenic variants in the NPHS1, COL2A1, OCRL, SHOC2, TPRV4, MTM1 and STAC3 genes. This study demonstrates the utility value of CES in the South African State healthcare setting, providing a diagnosis to patients who would otherwise not receive one and allowing for directed management. We anticipate an increase in the diagnostic yield of our workflow with further refinement of the study inclusion criteria. This study highlights important considerations for the implementation of genomic medicine in under-resourced settings and in under-represented African populations where variant interpretation remains a challenge.

Ion Channel Genes in Painful Neuropathies

Neuropathic pain (NP) is a typical symptom of peripheral nerve disorders, including painful neuropathy. The biological mechanisms that control ion channels are important for many cell activities and are also therapeutic targets. Disruption of the cellular mechanisms that govern ion channel activity can contribute to pain pathophysiology. The voltage-gated sodium channel (VGSC) is the most researched ion channel in terms of NP; however, VGSC impairment is detected in only <20% of painful neuropathy patients. Here, we discuss the potential role of the other peripheral ion channels involved in sensory signaling (transient receptor potential cation channels), neuronal excitation regulation (potassium channels), involuntary action potential generation (hyperpolarization-activated cyclic nucleotide-gated channels), thermal pain (anoctamins), pH modulation (acid sensing ion channels), and neurotransmitter release (calcium channels) related to pain and their prospective role as therapeutic targets for painful neuropathy.

A TRPV4 mutation caused Charcot-Marie-Tooth disease type 2C with scapuloperoneal muscular atrophy overlap syndrome and scapuloperoneal spinal muscular atrophy in one family: a case report and literature review

BACKGROUND

Charcot-Marie-Tooth disease 2C (CMT2C) and scapuloperoneal spinal muscular atrophy (SPSMA) are different clinical phenotypes of TRPV4 mutation. The mutation of p.R316C has been reported to cause CMT2C and SPSMA separately.

CASE PRESENTATION

Here, we reported a Chinese family harboring the same p.R316C variant, but with an overlap syndrome and different clinical manifestations. A 58-year-old man presented with severe scapula muscle atrophy, resulting in sloping shoulders. He also exhibited distinct muscle atrophy in his four limbs, particularly in the lower limbs. The sural nerve biopsy revealed severe loss of myelinated nerve fibers with scattered regenerating clusters and pseudo-onion bulbs. Nerve conduction study showed axon damage in both motor and sensory nerves. Sensory nerve action potentials could not be evoked in bilateral sural or superficial peroneal nerves. He was diagnosed with Charcot-Marie-Tooth disease type 2C and scapuloperoneal muscular atrophy overlap syndrome, whereas his 27-year-old son was born with clubfoot and clinodactyly. Electromyogram examination indicated chronic neurogenic changes and anterior horn cells involvement. Although there was no obvious weakness or sensory symptoms, early SPSMA could be considered for him.

CONCLUSIONS

A literature review of the clinical characteristics in CMT2C and SPSMA patients with TRPV4 mutation suggested that our case was distinct due to the overlap syndrome and phenotype variation. Altogether, this case broadened the phenotype spectrum and provided the nerve biopsy pathological details of TRPV4-related neuropathies.

Disruption of axonal transport in neurodegeneration

Neurons are markedly compartmentalized, which makes them reliant on axonal transport to maintain their health. Axonal transport is important for anterograde delivery of newly synthesized macromolecules and organelles from the cell body to the synapse and for the retrograde delivery of signaling endosomes and autophagosomes for degradation. Dysregulation of axonal transport occurs early in neurodegenerative diseases and plays a key role in axonal degeneration. Here, we provide an overview of mechanisms for regulation of axonal transport; discuss how these mechanisms are disrupted in neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, hereditary spastic paraplegia, amyotrophic lateral sclerosis, and Charcot-Marie-Tooth disease; and discuss therapeutic approaches targeting axonal transport.

Exploring and expanding the phenotype and genotype diversity in seven Chinese families with spondylo-epi-metaphyseal dysplasia

Spondylo-epi-metaphyseal dysplasia (SEMD) is a heterogeneous group of disorders with different modes of inheritance and is characterized by disproportionate or proportionate short stature. To date, more than 30 disease-causing genes have been identified, and different types of SEMD exhibit greatly overlapping clinical features, which usually complicate the diagnosis. This study was performed to expand the clinical and molecular spectrum of SEMD among Chinese subjects and to explore their potential phenotype–genotype relations. We enrolled seven families including 11 affected patients with SEMD, and their clinical, radiographic, and genetic data were carefully analyzed. All the seven probands showed different degrees of short stature, and each of them exhibited additional specific skeletal manifestations; four probands had extraosseous manifestations. X-rays of the seven probands showed common features of SEMD, including vertebral deformities, irregular shape of the epiphysis, and disorganization of the metaphysis. Seven variants were identified in TRPV4 (c.694C&gt; T, p.Arg232Cys), COL2A1 (c.654 + 1G &gt; C; c.3266_3268del, p.Gly1089del), CCN6 (c.396 T&gt; G, p.Cys132Trp; c.721 T&gt;C, p.Cys241Arg), SBDS (c.258 + 2T&gt; C), and ACAN (c.1508C&gt; A, p.Thr503Lys) genes, and two of them were novel. Two families with TRPV4 variants showed considerable intrafamily and interfamily heterogeneities. In addition, we reported one case of SEMD with a severe phenotype caused by ACAN gene mutation. Our study expands the phenotype and genetic spectrum of SEMD and provides evidence for the phenotype–genotype relations, aiding future molecular and clinical diagnosis as well as procreative management of SEMD.

TRPV4 mutations causing mixed neuropathy and skeletal phenotypes result in severe gain of function

OBJECTIVE

Distinct dominant mutations in the calcium-permeable ion channel TRPV4 (transient receptor potential vanilloid 4) typically cause nonoverlapping diseases of either the neuromuscular or skeletal systems. However, accumulating evidence suggests that some patients develop mixed phenotypes that include elements of both neuromuscular and skeletal disease. We sought to define the genetic and clinical features of these patients.

METHODS

We report a 2-year-old with a novel R616G mutation in TRPV4 with a severe neuropathy phenotype and bilateral vocal cord paralysis. Interestingly, a different substitution at the same residue, R616Q, has been reported in families with isolated skeletal dysplasia. To gain insight into clinical features and potential genetic determinants of mixed phenotypes, we perform in-depth analysis of previously reported patients along with functional and structural assessment of selected mutations.

RESULTS

We describe a wide range of neuromuscular and skeletal manifestations and highlight specific mutations that are more frequently associated with overlap syndromes. We find that mutations causing severe, mixed phenotypes have an earlier age of onset and result in more marked elevations of intracellular calcium, increased cytotoxicity, and reduced sensitivity to TRPV4 antagonism. Structural analysis of the two mutations with the most dramatic gain of ion channel function suggests that these mutants likely cause constitutive channel opening through disruption of the TRPV4 S5 transmembrane domain.

INTERPRETATION

These findings demonstrate that the degree of baseline calcium elevation correlates with development of mixed phenotypes and sensitivity to pharmacologic channel inhibition, observations that will be critical for the design of future clinical trials for TRPV4 channelopathies.

Identification and functional analysis of a biflavone as a novel inhibitor of transient receptor potential vanilloid 4-dependent atherogenic processes

Atherosclerosis, a chronic inflammatory disease of large arteries, is the major contributor to the growing burden of cardiovascular disease-related mortality and morbidity. During early atherogenesis, as a result of inflammation and endothelial dysfunction, monocytes transmigrate into the aortic intimal areas, and differentiate into lipid-laden foam cells, a critical process in atherosclerosis. Numerous natural compounds such as flavonoids and polyphenols are known to have anti-inflammatory and anti-atherogenic properties. Herein, using a fluorometric imaging plate reader-supported Ca2+ influx assay, we report semi high-throughput screening-based identification of ginkgetin, a biflavone, as a novel inhibitor of transient receptor potential vanilloid 4 (TRPV4)-dependent proatherogenic and inflammatory processes in macrophages. We found that ginkgetin (1) blocks TRPV4-elicited Ca2+ influx into macrophages, (2) inhibits oxidized low-density lipoprotein (oxLDL)-induced foam cell formation by suppressing the uptake but not the binding of oxLDL in macrophages, and (3) attenuates oxLDL-induced phosphorylation of JNK2, expression of TRPV4 proteins, and induction of inflammatory mRNAs. Considered all together, the results of this study show that ginkgetin inhibits proatherogenic/inflammatory macrophage function in a TRPV4-dependent manner, thus strengthening the rationale for the use of natural compounds for developing therapeutic and/or chemopreventive molecules.

Interorganelle communication, aging, and neurodegeneration

Our cells are comprised of billions of proteins, lipids, and other small molecules packed into their respective subcellular organelles, with the daunting task of maintaining cellular homeostasis over a lifetime. However, it is becoming increasingly evident that organelles do not act as autonomous discrete units but rather as interconnected hubs that engage in extensive communication through membrane contacts. In the last few years, our understanding of how these contacts coordinate organelle function has redefined our view of the cell. This review aims to present novel findings on the cellular interorganelle communication network and how its dysfunction may contribute to aging and neurodegeneration. The consequences of disturbed interorganellar communication are intimately linked with age-related pathologies. Given that both aging and neurodegenerative diseases are characterized by the concomitant failure of multiple cellular pathways, coordination of organelle communication and function could represent an emerging regulatory mechanism critical for long-term cellular homeostasis. We anticipate that defining the relationships between interorganelle communication, aging, and neurodegeneration will open new avenues for therapeutics.

De novo TRPV4 Leu619Pro variant causes a new channelopathy characterised by giant cell lesions of the jaws and skull, skeletal abnormalities and polyneuropathy

BACKGROUND

Pathogenic germline variants in Transient Receptor Potential Vanilloid 4 Cation Channel (TRPV4) lead to channelopathies, which are phenotypically diverse and heterogeneous disorders grossly divided in neuromuscular disorders and skeletal dysplasia. We recently reported in sporadic giant cell lesions of the jaws (GCLJs) novel, somatic, heterozygous, gain-of-function mutations in TRPV4, at Met713.

METHODS

Here we report two unrelated women with a de novo germline p.Leu619Pro TRPV4 variant and an overlapping systemic disorder affecting all organs individually described in TRPV4 channelopathies.

RESULTS

From an early age, both patients had several lesions of the nervous system including progressive polyneuropathy, and multiple aggressive giant cell-rich lesions of the jaws and craniofacial/skull bones, and other skeletal lesions. One patient had a relatively milder disease phenotype possibly due to postzygotic somatic mosaicism. Indeed, the TRPV4 p.Leu619Pro variant was present at a lower frequency (variant allele frequency (VAF)=21.6%) than expected for a heterozygous variant as seen in the other proband, and showed variable regional frequency in the GCLJ (VAF ranging from 42% to 10%). In silico structural analysis suggests that the gain-of-function p.Leu619Pro alters the ion channel activity leading to constitutive ion leakage.

CONCLUSION

Our findings define a novel polysystemic syndrome due to germline TRPV4 p.Leu619Pro and further extend the spectrum of TRPV4 channelopathies. They further highlight the convergence of TRPV4 mutations on different organ systems leading to complex phenotypes which are further mitigated by possible post-zygotic mosaicism. Treatment of this disorder is challenging, and surgical intervention of the GCLJ worsens the lesions, suggesting the future use of MEK inhibitors and TRPV4 antagonists as therapeutic modalities for unmet clinical needs.

Neuropathy-causing TRPV4 mutations disrupt TRPV4-RhoA interactions and impair neurite extension

TRPV4 is a cell surface-expressed calcium-permeable cation channel that mediates cell-specific effects on cellular morphology and function. Dominant missense mutations of TRPV4 cause distinct, tissue-specific diseases, but the pathogenic mechanisms are unknown. Mutations causing peripheral neuropathy localize to the intracellular N-terminal domain whereas skeletal dysplasia mutations are in multiple domains. Using an unbiased screen, we identified the cytoskeletal remodeling GTPase RhoA as a TRPV4 interactor. TRPV4-RhoA binding occurs via the TRPV4 N-terminal domain, resulting in suppression of TRPV4 channel activity, inhibition of RhoA activation, and extension of neurites in vitro. Neuropathy but not skeletal dysplasia mutations disrupt TRPV4-RhoA binding and cytoskeletal outgrowth. However, inhibition of RhoA restores neurite length in vitro and in a fly model of TRPV4 neuropathy. Together these results identify RhoA as a critical mediator of TRPV4-induced cell structure changes and suggest that disruption of TRPV4-RhoA binding may contribute to tissue-specific toxicity of TRPV4 neuropathy mutations.

TRPV4 dominant mutations cause neuropathy. Here, the authors show that TRPV4 binds and interacts with RhoA, modulating the actin cytoskeleton. Neuropathy-causing mutations of TRPV4 disrupt this complex, leading to RhoA activation and impairment of neurite extension in cultured cells and flies.

The expanding genetic landscape of hereditary motor neuropathies

Hereditary motor neuropathies are clinically and genetically diverse disorders characterized by length-dependent axonal degeneration of lower motor neurons. Although currently as many as 26 causal genes are known, there is considerable missing heritability compared to other inherited neuropathies such as Charcot-Marie-Tooth disease. Intriguingly, this genetic landscape spans a discrete number of key biological processes within the peripheral nerve. Also, in terms of underlying pathophysiology, hereditary motor neuropathies show striking overlap with several other neuromuscular and neurological disorders. In this review, we provide a current overview of the genetic spectrum of hereditary motor neuropathies highlighting recent reports of novel genes and mutations or recent discoveries in the underlying disease mechanisms. In addition, we link hereditary motor neuropathies with various related disorders by addressing the main affected pathways of disease divided into five major processes: axonal transport, tRNA aminoacylation, RNA metabolism and DNA integrity, ion channels and transporters and endoplasmic reticulum.

The Role of TRP Channels and PMCA in Brain Disorders: Intracellular Calcium and pH Homeostasis

Brain disorders include neurodegenerative diseases (NDs) with different conditions that primarily affect the neurons and glia in the brain. However, the risk factors and pathophysiological mechanisms of NDs have not been fully elucidated. Homeostasis of intracellular Ca²⁺ concentration and intracellular pH (pH_i) is crucial for cell function. The regulatory processes of these ionic mechanisms may be absent or excessive in pathological conditions, leading to a loss of cell death in distinct regions of ND patients. Herein, we review the potential involvement of transient receptor potential (TRP) channels in NDs, where disrupted Ca²⁺ homeostasis leads to cell death. The capability of TRP channels to restore or excite the cell through Ca²⁺ regulation depending on the level of plasma membrane Ca²⁺ ATPase (PMCA) activity is discussed in detail. As PMCA simultaneously affects intracellular Ca²⁺ regulation as well as pH_i, TRP channels and PMCA thus play vital roles in modulating ionic homeostasis in various cell types or specific regions of the brain where the TRP channels and PMCA are expressed. For this reason, the dysfunction of TRP channels and/or PMCA under pathological conditions disrupts neuronal homeostasis due to abnormal Ca²⁺ and pH levels in the brain, resulting in various NDs. This review addresses the function of TRP channels and PMCA in controlling intracellular Ca²⁺ and pH, which may provide novel targets for treating NDs.

Increased TRPV4 expression in non-myelinating Schwann cells is associated with demyelination after sciatic nerve injury

Transient receptor potential vanilloid 4 (TRPV4) is a non-selective calcium-permeable cation channel that is widely expressed and activated in various neurons and glial cells in the nervous system. Schwann cells (SCs) are primary glia cells that wrap around axons to form the myelin sheath in the peripheral nervous system. However, whether TRPV4 is expressed and functions in SCs is unclear. Here, we demonstrate functional expression of TRPV4 in mouse SCs and investigated its physiological significance. Deletion of TRPV4 did not affect normal myelin development for SCs in sciatic nerves in mice. However, after sciatic nerve cut injury, TRPV4 expression levels were remarkably increased in SCs following nerve demyelination. Ablation of TRPV4 expression impaired the demyelinating process after nerve injury, resulting in delayed remyelination and functional recovery of sciatic nerves. These results suggest that local activation of TRPV4 could be an attractive pharmacological target for therapeutic intervention after peripheral nerve injury.

Feng et al. report that TRPV4 plays an important role in Schwann cells (SCs) during nerve demyelination and remyelination in mice. Using sciatic nerve cut injury mouse models, they find that TRPV4 expression is remarkably increased in demyelinating SCs during sciatic nerve injury; and ablation of TRPV4 expression impairs the demyelinating process after nerve injury, resulting in their delayed remyelination and functional recovery.

TRPV4 disrupts mitochondrial transport and causes axonal degeneration via a CaMKII-dependent elevation of intracellular Ca2

The cation channel transient receptor potential vanilloid 4 (TRPV4) is one of the few identified ion channels that can directly cause inherited neurodegeneration syndromes, but the molecular mechanisms are unknown. Here, we show that in vivo expression of a neuropathy-causing TRPV4 mutant (TRPV4^R269C) causes dose-dependent neuronal dysfunction and axonal degeneration, which are rescued by genetic or pharmacological blockade of TRPV4 channel activity. TRPV4^R269C triggers increased intracellular Ca²⁺ through a Ca²⁺/calmodulin-dependent protein kinase II (CaMKII)-mediated mechanism, and CaMKII inhibition prevents both increased intracellular Ca²⁺ and neurotoxicity in Drosophila and cultured primary mouse neurons. Importantly, TRPV4 activity impairs axonal mitochondrial transport, and TRPV4-mediated neurotoxicity is modulated by the Ca²⁺-binding mitochondrial GTPase Miro. Our data highlight an integral role for CaMKII in neuronal TRPV4-associated Ca²⁺ responses, the importance of tightly regulated Ca²⁺ dynamics for mitochondrial axonal transport, and the therapeutic promise of TRPV4 antagonists for patients with TRPV4-related neurodegenerative diseases.

Mutations in the TRPV4 channel cause inherited neurodegeneration syndromes, but the molecular mechanisms are unknown. Here the authors reveal that TRPV4 activation causes dose-dependent, CaMKII-mediated neuronal dysfunction and axonal degeneration via disruption of mitochondrial axonal transport.

Yield of next-generation neuropathy gene panels in axonal neuropathies

The use and utility of targeted gene panels for diagnosing the type of Charcot-Marie-Tooth have grown rapidly because commercial gene panels that contain most of the relevant genes are available and affordable for many patients. We used a targeted gene panel to analyze 175 patients who had an unexplained axonal polyneuropathy affecting large myelinated axons, 86 of whom reported a family history of neuropathy, and 89 of whom did not. In patients reporting a family history, the panel identified a pathogenic variant causing the neuropathy in six cases (7%); in patients not reporting a family history, the gene panel identified pathogenic variants causing neuropathy in two patients (2%). Interpretation in a tertiary referral setting, current gene panels identify the genetic cause of neuropathy in a small minority of patients who have an unexplained axonal neuropathy, even in those reporting a family history.

Incidence and Clinical Features of TRPV4-Linked Axonal Neuropathies in a USA Cohort of Charcot-Marie-Tooth Disease Type 2

Mutations in TRPV4 are linked to a group of clinically distinct, but also overlapping axonal neuropathies, including Charcot-Marie-Tooth disease type 2C (CMT2C), scapuloperoneal spinal muscular atrophy, and congenital distal spinal muscular atrophy. The incidence of TRPV4-linked cases ranges from 0 to 7% in overall axonal neuropathy cohorts from European countries and Australia. However, the data from other areas remain largely unknown. In this study, we screened for TRPV4 mutations in a well-characterized USA cohort of 62 unrelated CMT2 patients without mutations in MFN2, GARS, NEFL, and GDAP1. All 15 coding exons of TRPV4 were analyzed by Sanger-sequencing. Clinical features of TRPV4-linked patients were compared with those lacking TRPV4 mutations. We identified two TRPV4 mutations in two patients. A TRPV4-R316C was identified in a patient with family history, while a TRPV4-R269C in an apparently sporadic case. Marked clinical variations were observed in the patients with TRPV4 mutations. Our data suggest that TRPV4-linked CMT2C accounts for a sizable fraction in this USA cohort of CMT2; it has a wide phenotypic spectrum, and vocal cord paralysis, scapular weakness and wasting, skeletal dysplasia, and hearing loss are suggestive signs for TRPV4-linked CMT2C.

The First Case of Riboflavin Transporter Deficiency in sub-Saharan Africa

This report describes the first case of a child with genetically confirmed Brown-Vialetto-van Laere syndrome in sub-Saharan Africa. This is an extremely rare clinical condition that presents with an auditory neuropathy, bulbar palsy, stridor, muscle weakness, and respiratory compromise that manifests with diaphragmatic and vocal cord paralysis. It is an autosomal recessive condition for which the genetic mutation has only recently been linked to a riboflavin transporter deficiency. We describe an 11-month-old affected male infant. He has required long-term respiratory support and a gastrostomy tube to support feeding. With high-dose riboflavin supplementation, he had limited recovery of motor function. His respiratory chain enzyme studies were abnormal suggestive of mitochondrial (mt) dysfunction. In the setting of limited resources, recognition of this striking clinical phenotype is important to highlight, specifically regarding the genetic implications of the condition and the potentially remedial response to vitamin supplementation.

Human pharmacological approaches to TRP-ion-channel-based analgesic drug development

The discovery of novel analgesic drug targets is an active research topic owing to insufficient treatment options for persisting pain. Modulators of temperature-sensing transient receptor potential ion channels (thermoTRPs), in particular TRPV1, TRPV2, TRPM8 and TRPA1, have reached clinical development. This requires access for TRP channels and the effects of specific modulators in humans. This is currently possible via (i) the study of TRP channel function in human-derived cell lines, (ii) immunohistochemical visualization of TRP channel expression in human tissues, (iii) human experimental pain models employing sensitization by means of topical application of TRP channel activators including capsaicin (TRPV1), menthol (TRPM8), mustard oil and cinnamaldehyde (TRPA1), and (iv) the study of phenotypic consequences of human TRP gene variants.

TRPV4: a Sensor for Homeostasis and Pathological Events in the CNS

Transient receptor potential vanilloid type 4 (TRPV4) was originally described as a calcium-permeable nonselective cation channel. TRPV4 is now recognized as a polymodal ionotropic receptor: it is a broadly expressed, nonselective cation channel (permeable to calcium, potassium, magnesium, and sodium) that plays an important role in a multitude of physiological processes. TRPV4 is involved in maintaining homeostasis, serves as an osmosensor and thermosensor, can be activated directly by endogenous or exogenous chemical stimuli, and can be activated or sensitized indirectly via intracellular signaling pathways. Additionally, TRPV4 is upregulated in a variety of pathological conditions. In this review, we focus on the role of TRPV4 in mediating homeostasis and pathological events in the central nervous system (CNS). This review is composed of three parts. Section 1 describes the role of TRPV4 in maintaining homeostatic processes, including the volume of body water, ionic concentrations, volume, and the temperature. Section 2 describes the effects of activation and inhibition of TRPV4 in the CNS. Section 3 focuses on the role of TRPV4 during pathological events in CNS.

A New Mutation in FIG4 Causes a Severe Form of CMT4J Involving TRPV4 in the Pathogenic Cascade

Mutations in FIG4, coding for a phosphoinositol(3,5) bisphosphate 5' phosphatase and involved in vesicular trafficking and fusion, have been shown causing a recessive form of Charcot-Marie-Tooth (CMT). We have identified a novel intronic mutation in the FIG4 in a wheel-chair bound patient presenting with a severe form of CMT4J and provide a longitudinal study. Investigations indicated a demyelinating sensorimotor polyneuropathy with diffuse active denervation and severe axonal loss. Genetic testing revealed that the patient is heterozygous for 2 FIG4 mutations, p.I41T and a T > G transversion at IVS17-10, the latter predicted to cause a splicing defect. FIG4 was severely diminished in patient's fibroblasts indicating loss-of-function. Consistent with FIG4's function in phosphoinositol homeostasis and vesicular trafficking, fibroblasts contained multiple large vacuoles and vesicular organelles were abnormally dispersed. FIG4 deficiency has implications for turnover of membrane proteins. The transient receptor cation channel, TRPV4, accumulated at the plasma membrane of patient's fibroblasts due to slow turnover. Knocking down Fig4 in murine cultured motor neurons resulted in vacuolation and cell death. Inhibiting TRPV4 activity significantly preserved viability, although not correcting vesicular trafficking. In conclusion, we demonstrate a new FIG4 intronic mutation and, importantly, a functional interaction between FIG4 and TRPV4.

TRPV4: Molecular Conductor of a Diverse Orchestra

Transient receptor potential vanilloid type 4 (TRPV4) is a calcium-permeable nonselective cation channel, originally described in 2000 by research teams led by Schultz (Nat Cell Biol 2: 695-702, 2000) and Liedtke (Cell 103: 525-535, 2000). TRPV4 is now recognized as being a polymodal ionotropic receptor that is activated by a disparate array of stimuli, ranging from hypotonicity to heat and acidic pH. Importantly, this ion channel is constitutively expressed and capable of spontaneous activity in the absence of agonist stimulation, which suggests that it serves important physiological functions, as does its widespread dissemination throughout the body and its capacity to interact with other proteins. Not surprisingly, therefore, it has emerged more recently that TRPV4 fulfills a great number of important physiological roles and that various disease states are attributable to the absence, or abnormal functioning, of this ion channel. Here, we review the known characteristics of this ion channel's structure, localization and function, including its activators, and examine its functional importance in health and disease.

Calcium Entry Through Thermosensory Channels

ThermoTRPs are unique channels that mediate Na(+) and Ca(2+) currents in response to changes in ambient temperature. In combination with their activation by other physical and chemical stimuli, they are considered key integrators of environmental cues into neuronal excitability. Furthermore, roles of thermoTRPs in non-neuronal tissues are currently emerging such as insulin secretion in pancreatic β-cells, and links to cancer. Calcium permeability through thermoTRPs appears a central hallmark for their physiological and pathological activities. Moreover, it is currently being proposed that beyond working as a second messenger, Ca(2+) can function locally by acting on protein complexes near the membrane. Interestingly, thermoTRPs can enhance and expand the inherent plasticity of signalplexes by conferring them temperature, pH and lipid regulation through Ca(2+) signalling. Thus, unveiling the local role of Ca(2+) fluxes induced by thermoTRPs on the dynamics of membrane-attached signalling complexes as well as their significance in cellular processes, are central issues that will expand the opportunities for therapeutic intervention in disorders involving dysfunction of thermoTRP channels.

Mitochondrial Fission in Human Diseases

Mitochondria are an essential component of multicellular life - from primitive organisms, to highly complex entities like mammals. The importance of mitochondria is underlined by their plethora of well-characterized essential functions such as energy production through oxidative phosphorylation (OX-PHOS), calcium and reactive oxygen species (ROS) signaling, and regulation of apoptosis. In addition, novel roles and attributes of mitochondria are coming into focus through the recent years of mitochondrial research. In particular, over the past decade the study of mitochondrial shape and dynamics has achieved special significance, as they are found to impact mitochondrial function. Recent advances indicate that mitochondrial function and dynamics are inter-connected, and maintain the balance between health and disease at a cellular and an organismal level. For example, excessive mitochondrial division (fission) is associated with functional defects, and is implicated in multiple human diseases from neurodegenerative diseases to cancer. In this chapter we examine the recent literature on the mitochondrial dynamics-function relationship, and explore how it impacts on the development and progression of human diseases. We will also highlight the implications of therapeutic manipulation of mitochondrial dynamics in treating various human pathologies.

Novel mutations highlight the key role of the ankyrin repeat domain in TRPV4-mediated neuropathy

Objective:

To characterize 2 novel TRPV4 mutations in 2 unrelated families exhibiting the Charcot-Marie-Tooth disease type 2C (CMT2C) phenotype.

Methods:

Direct CMT gene testing was performed on 2 unrelated families with CMT2C. A 4-fold symmetric tetramer model of human TRPV4 was generated to map the locations of novel TRPV4 mutations in these families relative to previously identified disease-causing mutations (neuropathy, skeletal dysplasia, and osteoarthropathy). Effects of the mutations on TRPV4 expression, localization, and channel activity were determined by immunocytochemical, immunoblotting, Ca²⁺ imaging, and cytotoxicity assays.

Results:

Previous studies suggest that neuropathy-causing mutations occur primarily at arginine residues on the convex face of the TRPV4 ankyrin repeat domain (ARD). Further highlighting the key role of this domain in TRPV4-mediated hereditary neuropathy, we report 2 novel heterozygous missense mutations in the TRPV4-ARD convex face (p.Arg237Gly and p.Arg237Leu). Generation of a model of the TRPV4 homotetramer revealed that while ARD residues mutated in neuropathy (including Arg237) are likely accessible for intermolecular interactions, skeletal dysplasia–causing TRPV4 mutations occur at sites suggesting disruption of intramolecular and/or intersubunit interactions. Like previously described neuropathy-causing mutations, the p.Arg237Gly and p.Arg237Leu substitutions do not alter TRPV4 subcellular localization in transfected cells but cause elevations of cytosolic Ca²⁺ levels and marked cytotoxicity.

Conclusions:

These findings expand the number of ARD residues mutated in TRPV4-mediated neuropathy, providing further evidence of the central importance of this domain to TRPV4 function in peripheral nerve.

Activation of transient receptor potential vanilloid 4 induces apoptosis in hippocampus through downregulating PI3K/Akt and upregulating p38 MAPK signaling pathways

Transient receptor potential vanilloid 4 (TRPV4) is a calcium-permeable cation channel that is sensitive to cell swelling, arachidonic acid and its metabolites, epoxyeicosatrienoic acids, which are associated with cerebral ischemia. The activation of TRPV4 induces cytotoxicity in many types of cells, accompanied by an increase in the intracellular free calcium concentration. TRPV4 activation modulates the mitogen-activated protein kinase (MAPK) and phosphatidyl inositol 3 kinase (PI3K)/ protein kinase B (Akt) signaling pathways that regulate cell death and survival. Herein, we examined TRPV4-induced neuronal apoptosis by intracerebroventricular (ICV) injection of a TRPV4 agonist (GSK1016790A) and assessed its involvement in cerebral ischemic injury. ICV injection of GSK1016790A dose-dependently induced apoptosis in the mouse hippocampi (GSK-injected mice). The protein level of phosphorylated p38 MAPK (p-p38 MAPK) was markedly increased and that of phosphorylated c-Jun N-terminal protein kinase (p-JNK) was virtually unchanged. TRPV4 activation also decreased Bcl-2/Bax protein ratio and increased the cleaved caspase-3 protein level, and these effects were blocked by a PI3K agonist and a p38 MAPK antagonist, but were unaffected by a JNK antagonist. ICV injection of the TRPV4 antagonist HC-067047 reduced brain infarction after reperfusion for 48 h in mice with middle cerebral artery occlusion (MCAO). In addition, HC-067047 treatment attenuated the decrease in the phosphorylated Akt protein level and the increase in p-p38 MAPK protein level at 48 h after MCAO, while the increase in p-JNK protein level remained unchanged. Finally, the decreased Bcl-2/Bax protein ratio and the increased cleaved caspase-3 protein level at 48 h after MCAO were markedly attenuated by HC-067047. We conclude that activation of TRPV4 induces apoptosis by downregulating PI3K/Akt and upregulating p38 MAPK signaling pathways, which is involved in cerebral ischemic injury.

Brain transient receptor potential channels and stroke

Transient receptor potential (TRP) channels have been increasingly implicated in the pathological mechanisms of CNS disorders. TRP expression has been detected in neurons, astrocytes, oligodendrocytes, microglia, and ependymal cells as well as in the cerebral vascular endothelium and smooth muscle. In stroke, TRPC3/4/6, TRPM2/4/7, and TRPV1/3/4 channels have been found to participate in ischemia-induced cell death, whereas other TRP channels, in particular those expressed in nonneuronal cells, have been less well studied. This review summarizes the current knowledge on the expression and functions of the TRP channels in various cell types in the brain and our current understanding of TRP channels in stroke pathophysiology. In an aging society, the occurrence of stroke is expected to increase steadily, and there is an urgent requirement to improve the current stroke management strategy. Therefore, elucidating the roles of TRP channels in stroke could shed light on the development of novel therapeutic strategies and ultimately improve stroke outcome.

TRPV4 channel activity is modulated by direct interaction of the ankyrin domain to PI(4,5)P₂

Mutations in the ankyrin repeat domain (ARD) of TRPV4 are responsible for several channelopathies, including Charcot-Marie-Tooth disease type 2C and congenital distal and scapuloperoneal spinal muscular atrophy. However, the molecular pathogenesis mediated by these mutations remains elusive, mainly due to limited understanding of the TRPV4 ARD function. Here we show that phosphoinositide binding to the TRPV4 ARD leads to suppression of the channel activity. Among the phosphoinositides, phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) most potently binds to the TRPV4 ARD. The crystal structure of the TRPV4 ARD in complex with inositol-1,4,5-trisphosphate, the head-group of PI(4,5)P2, and the molecular-dynamics simulations revealed the PI(4,5)P2-binding amino-acid residues. The TRPV4 channel activities were increased by titration or hydrolysis of membrane PI(4,5)P2. Notably, disease-associated TRPV4 mutations that cause a gain-of-function phenotype abolished PI(4,5)P2 binding and PI(4,5)P2 sensitivity. These findings identify TRPV4 ARD as a lipid-binding domain in which interactions with PI(4,5)P2 normalize the channel activity in TRPV4.

Junctophilin-1 is a modifier gene of GDAP1-related Charcot-Marie-Tooth disease

Mutations in the GDAP1 gene cause different forms of Charcot-Marie-Tooth (CMT) disease, and the primary clinical expression of this disease is markedly variable in the dominant inheritance form (CMT type 2K; CMT2K), in which carriers of the GDAP1 p.R120W mutation can display a wide range of clinical severity. We investigated the JPH1 gene as a genetic modifier of clinical expression variability because junctophilin-1 (JPH1) is a good positional and functional candidate. We demonstrated that the JPH1-GDAP1 cluster forms a paralogon and is conserved in vertebrates. Moreover, both proteins play a role in Ca(2+) homeostasis, and we demonstrated that JPH1 is able to restore the store-operated Ca(2+) entry (SOCE) activity in GDAP1-silenced cells. After the mutational screening of JPH1 in a series of 24 CMT2K subjects who harbour the GDAP1 p.R120W mutation, we characterized the JPH1 p.R213P mutation in one patient with a more severe clinical picture. JPH1(p.R213P) cannot rescue the SOCE response in GDAP1-silenced cells. We observed that JPH1 colocalizes with STIM1, which is the activator of SOCE, in endoplasmic reticulum-plasma membrane puncta structures during Ca(2+) release in a GDAP1-dependent manner. However, when GDAP1(p.R120W) is expressed, JPH1 seems to be retained in mitochondria. We also established that the combination of GDAP1(p.R120W) and JPH1(p.R213P) dramatically reduces SOCE activity, mimicking the effect observed in GDAP1 knock-down cells. In summary, we conclude that JPH1 and GDAP1 share a common pathway and depend on each other; therefore, JPH1 can contribute to the phenotypical consequences of GDAP1 mutations.

Transient receptor potential channels and regulation of lung endothelial permeability

This review highlights our current knowledge regarding expression of transient receptor potential (TRP) cation channels in lung endothelium and evidence for their involvement in regulation of lung endothelial permeability. Six mammalian TRP families have been identified and organized on the basis of sequence homology: TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPML (mucolipin), TRPP (polycystin), and TRPA (ankyrin). To date, only TRPC1/4, TRPC6, TRPV4, and TRPM2 have been extensively studied in lung endothelium. Calcium influx through each of these channels has been documented to increase lung endothelial permeability, although their channel-gating mechanisms, downstream signaling mechanisms, and impact on endothelial structure and barrier integrity differ. While other members of the TRPC, TRPV, and TRPM families may be expressed in lung endothelium, we have little or no evidence linking these to regulation of lung endothelial permeability. Further, neither the expression nor functional role(s) of any TRPML, TRPP, and TRPA family members has been studied in lung endothelium. In addition to this assessment organized by TRP channel family, we also discuss TRP channels and lung endothelial permeability from the perspective of lung endothelial heterogeneity, using outcomes of studies focused on TRPC1/4 and TRPV4 channels. The diversity within the TRP channel family and the relative paucity of information regarding roles of a number of these channels in lung endothelium make this field ripe for continued investigation.

Progressive degeneration of dopaminergic neurons through TRP channel-induced cell death

Progressive neurodegenerative diseases are among the most frequently occurring aging-associated human pathologies. In a screen for Caenorhabditis elegans mutant animals that lack their normal complement of dopaminergic neurons, we identified two strains with progressive loss of dopaminergic neurons during postembryonic life. Through whole-genome sequencing we show that both strains harbor dominant (d), gain-of-function mutations in the Transient Receptor Potential (TRP) mechanosensory channel trp-4, a member of the invertebrate and vertebrate TRPN-type of the TRP family channels. Gain-of-function mutations in TRP channels have not been previously implicated in dopaminergic neuronal degeneration. We show that trp-4(d) induces cell death in dopamine neurons through a defined, calcium-related downstream pathway.

Inherited neuropathies: clinical overview and update

Inherited neuropathy is a group of common neurologic disorders with heterogeneous clinical presentations and genetic causes. Detailed neuromuscular evaluations, including nerve conduction studies, laboratory testing, and histopathologic examination, can assist in identification of the inherited component beyond family history. Genetic testing increasingly enables definitive diagnosis of specific inherited neuropathies. Diagnosis, however, is often complex, and neurologic disability may have both genetic and acquired components in individual patients. The decision of which genetic test to order or whether to order genetic tests is often complicated, and the strategies to maximize the value of testing are evolving. Apart from rare inherited metabolic neuropathies, treatment approaches remain largely supportive. We provide a clinical update of the various types of inherited neuropathies, their differential diagnoses, and distinguishing clinical features (where available). A framework is provided for clinical evaluations, including the inheritance assessment, electrophysiologic examinations, and specific genetic tests.

The puzzle of TRPV4 channelopathies

Hereditary channelopathies, that is, mutations in channel genes that alter channel function and are causal for the pathogenesis of the disease, have been described for several members of the transient receptor potential channel family. Mutations in the TRPV4 gene, encoding a polymodal Ca(2+) permeable channel, are causative for several human diseases, which affect the skeletal system and the peripheral nervous system, with highly variable phenotypes. In this review, we describe the phenotypes of TRPV4 channelopathies and overlapping symptoms. Putative mechanisms to explain the puzzle, and how mutations in the same region of the channel cause different diseases, are discussed and experimental approaches to tackle this surprising problem are suggested.

Dominant Charcot-Marie-Tooth syndrome and cognate disorders

Charcot-Marie-Tooth neuropathy (CMT) is a group of genetically heterogeneous disorders sharing a similar phenotype, characterized by wasting and weakness mainly involving the distal muscles of lower and upper limbs, variably associated with distal sensory loss and skeletal deformities. This chapter deals with dominantly transmitted CMT and related disorders, namely hereditary neuropathy with liability to pressure palsies (HNPP) and hereditary neuralgic amyotrophy (HNA). During the last 20 years, several genes have been uncovered associated with CMT and our understanding of the underlying molecular mechanisms has greatly improved. Consequently, a precise genetic diagnosis is now possible in the majority of cases, thus allowing proper genetic counseling. Although, unfortunately, treatment is still unavailable for all types of CMT, several cellular and animal models have been developed and some compounds have proved effective in these models. The first trials with ascorbic acid in CMT type 1A have been completed and, although negative, are providing relevant information on disease course and on how to prepare for future trials.

Molecular genetics of charcot-marie-tooth disease: from genes to genomes

Charcot-Marie-Tooth disease (CMT) is a heterogeneous group of disorders of the peripheral nervous system, mainly characterized by distal muscle weakness and atrophy leading to motor handicap. With an estimated prevalence of 1 in 2,500, this condition is one of the most commonly inherited neurological disorders. Mutations in more than 30 genes affecting glial and/or neuronal functions have been associated with different forms of CMT leading to a substantial improvement in diagnostics of the disease and in the understanding of implicated pathophysiological mechanisms. However, recent data from systematic genetic screening performed in large cohorts of CMT patients indicated that molecular diagnosis could be established only in ∼50-70% of them, suggesting that additional genes are involved in this disease. In addition to providing an overview of genetic and functional data concerning various CMT forms, this review focuses on recent data generated through the use of highly parallel genetic technologies (SNP chips, sequence capture and next-generation DNA sequencing) in CMT families, and the current and future impact of these technologies on gene discovery and diagnostics of CMTs.

Molecular genetics of charcot-marie-tooth disease: from genes to genomes

Charcot-Marie-Tooth disease (CMT) is a heterogeneous group of disorders of the peripheral nervous system, mainly characterized by distal muscle weakness and atrophy leading to motor handicap. With an estimated prevalence of 1 in 2,500, this condition is one of the most commonly inherited neurological disorders. Mutations in more than 30 genes affecting glial and/or neuronal functions have been associated with different forms of CMT leading to a substantial improvement in diagnostics of the disease and in the understanding of implicated pathophysiological mechanisms. However, recent data from systematic genetic screening performed in large cohorts of CMT patients indicated that molecular diagnosis could be established only in ∼50-70% of them, suggesting that additional genes are involved in this disease. In addition to providing an overview of genetic and functional data concerning various CMT forms, this review focuses on recent data generated through the use of highly parallel genetic technologies (SNP chips, sequence capture and next-generation DNA sequencing) in CMT families, and the current and future impact of these technologies on gene discovery and diagnostics of CMTs.

Structural and biochemical consequences of disease-causing mutations in the ankyrin repeat domain of the human TRPV4 channel

The TRPV4 calcium-permeable cation channel plays important physiological roles in osmosensation, mechanosensation, cell barrier formation, and bone homeostasis. Recent studies reported that mutations in TRPV4, including some in its ankyrin repeat domain (ARD), are associated with human inherited diseases, including neuropathies and skeletal dysplasias, probably because of the increased constitutive activity of the channel. TRPV4 activity is regulated by the binding of calmodulin and small molecules such as ATP to the ARD at its cytoplasmic N-terminus. We determined structures of ATP-free and -bound forms of human TRPV4-ARD and compared them with available TRPV-ARD structures. The third inter-repeat loop region (Finger 3 loop) is flexible and may act as a switch to regulate channel activity. Comparisons of TRPV-ARD structures also suggest an evolutionary link between ARD structure and ATP binding ability. Thermal stability analyses and molecular dynamics simulations suggest that ATP increases stability in TRPV-ARDs that can bind ATP. Biochemical analyses of a large panel of TRPV4-ARD mutations associated with human inherited diseases showed that some impaired thermal stability while others weakened ATP binding ability, suggesting molecular mechanisms for the diseases.

TRPV4 mutations in children with congenital distal spinal muscular atrophy

Inherited disorders characterized by motor neuron loss and muscle weakness are genetically heterogeneous. The recent identification of mutations in the gene encoding transient receptor potential vanilloid 4 (TRPV4) in distal spinal muscular atrophy (dSMA) prompted us to screen for TRPV4 mutations in a small group of children with compatible phenotype. In a girl with dSMA and vocal cord paralysis, we detected a new variant (p.P97R) localized in the cytosolic N-terminus of the TRPV4 protein, upstream of the ankyrin-repeat domain, where the great majority of disease-associated mutations reside. In another child with congenital dSMA, in this case associated with bone abnormalities, we detected a previously reported mutation (p.R232C). Functional analysis of the novel p.P97R mutation in a heterologous system demonstrated a loss-of-function mechanism. Protein localization studies in muscle, skin, and cultured skin fibroblasts from both patients showed normal protein expression. No TRPV4 mutations were detected in four children with dSMA without bone or vocal cord involvement. Adding to the clinical and molecular heterogeneity of TRPV4-associated diseases, our results suggest that molecular testing of the TRPV4 gene is warranted in cases of congenital dSMA with bone abnormalities and vocal cord paralysis.

TRPV4-pathy manifesting both skeletal dysplasia and peripheral neuropathy: a report of three patients

Heterozygous missense mutations of transient receptor potential vanilloid 4 channel (TRPV4) cause a spectrum of skeletal disorders, including brachyolmia, spondylometaphyseal dysplasia Kozlowski type, metatropic dysplasia, parastremmatic dysplasia, and spondyloepimetaphyseal dysplasia Maroteaux type. Similarly, heterozygous missense mutations of TRPV4 cause a spectrum of peripheral neuropathy, including hereditary motor and sensory neuropathy type IIC, congenital spinal muscular atrophy, and scapuloperoneal spinal muscular atrophy. There are no apparent differences in the amino acid positions affected or type of change predicted by the TRPV4 mutations responsible for the two disease spectrums; nevertheless, no fundamental phenotypic overlap has been shown between the two spectrums. Here, we report on three patients who had both skeletal dysplasia and peripheral neuropathy caused by heterozygous TRPV4 missense mutations. The skeletal and neurologic phenotypes of these patients covered the wide spectrum of reported TRPV4-pathies (disease caused by TRPV4 mutations). The molecular data are complementary, proving that "neuropathic" mutations can cause skeletal dysplasia but also the "skeletopathic" mutations can lead to neuropathies. Our findings suggest that pathogenic mechanisms of TRPV4-pathies in skeletal and nervous systems are not always mutually exclusive and provide further evidence that there is no clear genotype-phenotype correlation for either spectrum. Co-occurrence of skeletal dysplasia and degenerative neuropathy should be kept in mind in clinical practice including diagnostic testing, surgical evaluation, and genetic counseling.