Mutation of FIG4 causes a rapidly progressive, asymmetric neuronal degeneration

Clinical and genetic features of patients suffering from CMT4J

Mutations in the FIG4 gene have been identified in various diseases, including amyotrophic lateral sclerosis, Parkinson's disease, and Charcot-Marie-Tooth 4 J (CMT4J), with a wide range of phenotypic manifestations. We present eight cases of CMT4J patients carrying the p.Ile41Thr mutation of FIG4. The patients were categorized according to their phenotype. Six patients had a pure CMT; whereas, two patients had a CMT associated with parkinsonism. Three patients had an early onset and exhibited more severe forms of the disease. Three others experienced symptoms in their teenage years and had milder forms. Two patients had a late onset in adulthood. Four patients showed electrophysiological evidence of conduction blocks, typically associated with acquired neuropathies. Consequently, two of them received intravenous immunoglobulin treatment without a significant objective response. Interestingly, two heterozygous patients with the same mutations exhibited contrasting phenotypes, one having a severe early-onset form and the other experiencing a slow disease progression starting at the age of 49. Notably, although 7 out of 8 patients in this study were compound heterozygous for the p.Ile41Thr mutation, only one individual was found to be homozygous for this genetic variant and exhibited an early-onset, severe form of the disease. Additionally, one patient who developed the disease in his youth was also diagnosed with hereditary neuropathy with pressure palsies. Our findings provide insights into the CMT4J subtype by reporting on eight heterogeneous patient cases and highlight the potential for misdiagnosis when conduction blocks or asymmetrical nerve conduction study results are observed in patients with FIG4 mutations.

The endolysosomal pathway and ALS/FTD

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are considered to be part of a disease spectrum that is associated with causative mutations and risk variants in a wide range of genes. Mounting evidence indicates that several of these genes are linked to the endolysosomal system, highlighting the importance of this pathway in ALS/FTD. Although many studies have focused on how disruption of this pathway impacts on autophagy, recent findings reveal that this may not be the whole picture: specifically, disrupting autophagy may not be sufficient to induce disease, whereas disrupting the endolysosomal system could represent a crucial pathogenic driver. In this review we discuss the connections between ALS/FTD and the endolysosomal system, including a breakdown of how disease-associated genes are implicated in this pathway. We also explore the potential downstream consequences of disrupting endolysosomal activity in the brain, outside of an effect on autophagy.

Mitf is a Schwann cell sensor of axonal integrity that drives nerve repair

Schwann cells respond to acute axon damage by transiently transdifferentiating into specialized repair cells that restore sensorimotor function. However, the molecular systems controlling repair cell formation and function are not well defined, and consequently, it is unclear whether this form of cellular plasticity has a role in peripheral neuropathies. Here, we identify Mitf as a transcriptional sensor of axon damage under the control of Nrg-ErbB-PI3K-PI5K-mTorc2 signaling. Mitf regulates a core transcriptional program for generating functional repair Schwann cells following injury and during peripheral neuropathies caused by CMT4J and CMT4D. In the absence of Mitf, core genes for epithelial-to-mesenchymal transition, metabolism, and dedifferentiation are misexpressed, and nerve repair is disrupted. Our findings demonstrate that Schwann cells monitor axonal health using a phosphoinositide signaling system that controls Mitf nuclear localization, which is critical for activating cellular plasticity and counteracting neural disease.

PI4KA and PIKfyve: Essential phosphoinositide signaling enzymes involved in myriad human diseases

Lipid phosphoinositides are master regulators of multiple cellular functions. Misregulation of the activity of the lipid kinases that generate phosphoinositides is causative of human diseases, including cancer, neurodegeneration, developmental disorders, immunodeficiencies, and inflammatory disease. This review will present a summary of recent discoveries on the roles of two phosphoinositide kinases (PI4KA and PIKfyve), which have emerged as targets for therapeutic intervention. Phosphatidylinositol 4-kinase alpha (PI4KA) generates PI4P at the plasma membrane and PIKfyve generates PI(3,5)P2 at endo-lysosomal membranes. Both of these enzymes exist as multi-protein mega complexes that are under myriad levels of regulation. Human disease can be caused by either loss or gain-of-function of these complexes, so understanding how they are regulated will be essential in the design of therapeutics. We will summarize insight into how these enzymes are regulated by their protein-binding partners, with a major focus on the unanswered questions of how their activity is controlled.

The synthetic TRPML1 agonist ML-SA1 rescues Alzheimer-related alterations of the endosomal-autophagic-lysosomal system

Abnormalities in the endosomal-autophagic-lysosomal (EAL) system are an early event in Alzheimer's disease (AD) pathogenesis. However, the mechanisms underlying these abnormalities are unclear. The transient receptor potential channel mucolipin 1(TRPML1, also known as MCOLN1), a vital endosomal-lysosomal Ca2+ channel whose loss of function leads to neurodegeneration, has not been investigated with respect to EAL pathogenesis in late-onset AD (LOAD). Here, we identify pathological hallmarks of TRPML1 dysregulation in LOAD neurons, including increased perinuclear clustering and vacuolation of endolysosomes. We reveal that induced pluripotent stem cell (iPSC)-derived human cortical neurons expressing APOE ε4, the strongest genetic risk factor for LOAD, have significantly diminished TRPML1-induced endolysosomal Ca2+ release. Furthermore, we found that blocking TRPML1 function in primary neurons by depleting the TRPML1 agonist PI(3,5)P2 via PIKfyve inhibition, recreated multiple features of EAL neuropathology evident in LOAD. This included increased endolysosomal Ca2+ content, enlargement and perinuclear clustering of endolysosomes, autophagic vesicle accumulation and early endosomal enlargement. Strikingly, these AD-like neuronal EAL defects were rescued by TRPML1 reactivation using its synthetic agonist ML-SA1. These findings implicate defects in TRPML1 in LOAD EAL pathogenesis and present TRPML1 as a potential therapeutic target.

T2 olivary nuclei hyperintensities: A characteristic neuroimaging finding in FIG4-related leukoencephalopathy

FIG4 related leukoencephalopathy has recently been considered as an expanded spectrum of FIG4 related disorders characterized by upper and lower motor neuron involvement, dystonia, intellectual disability, bulbar symptoms with cerebellar atrophy. We report a 7-year-old girl who presented with classic clinical features of FIG4 related leukoencephalopathy and neuroimaging showed characteristic T2 olivary nuclei hyperintensities in addition to bilateral parietal lobe and thalamic hyperintensities and mild cerebellar atrophy. Trio exome sequencing with Sanger confirmation revealed a novel variant c.504C>G in the FIG4 gene. Phase contrast microscopy of skin fibroblast cultures detect enlarged vacuoles in 50% of patient's fibroblasts as opposed to 18.6% vacuolation in cultured control fibroblasts (p < 0.00001), a feature characteristic of fibroblasts with deleterious variants of FIG4. In addition, we have reviewed and compared the phenotypic features of published cases of FIG4 related leukoencephalopathy from literature. This case adds to the delineation of FIG4 related leukoencephalopathy phenotype. The radiological finding of T2 inferior olivary nuclei hyperintensities appear to be characteristic for the phenotype or at least for the cases due to variants in and around the 168th codon and active effort should be made to detect the same as it can add to the genotype phenotype spectrum.

Prospects for gene replacement therapies in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating and incurable neurodegenerative disease characterized by the progressive loss of upper and lower motor neurons. ALS causes death, usually within 2-5 years of diagnosis. Riluzole, the only drug currently approved in Europe for the treatment of this condition, offers only a modest benefit, increasing survival by 3 months on average. Recent advances in our understanding of causative or disease-modifying genetic variants and in the development of genetic therapy strategies present exciting new therapeutic opportunities for ALS. In addition, the approval of adeno-associated virus-mediated delivery of functional copies of the SMN1 gene to treat spinal muscular atrophy represents an important therapeutic milestone and demonstrates the potential of gene replacement therapies for motor neuron disorders. In this Review, we describe the current landscape of genetic therapies in ALS, highlighting achievements and critical challenges. In particular, we discuss opportunities for gene replacement therapy in subgroups of people with ALS, and we describe loss-of-function mutations that are known to contribute to the pathophysiology of ALS and could represent novel targets for gene replacement therapies.

Chloroquine corrects enlarged lysosomes in FIG4 null cells and reduces neurodegeneration in Fig4 null mice

Loss-of-function mutations of FIG4 impair the biosynthesis of PI(3,5)P₂ and are responsible for rare genetic disorders including Yunis-Varón Syndrome and Charcot-Marie-Tooth Disease Type 4 J. Cultured cells deficient in FIG4 accumulate enlarged lysosomes with hyperacidic pH, due in part to impaired regulation of lysosomal ion channels and elevated intra-lysosomal osmotic pressure. We evaluated the effects of the FDA approved drug chloroquine, which is known to reduce lysosome acidity, on FIG4 deficient cell culture and on a mouse model. Chloroquine corrected the enlarged lysosomes in FIG4 null cells. In null mice, addition of chloroquine to the drinking water slowed progression of the disorder. Growth and mobility were dramatically improved during the first month of life, and spongiform degeneration of the nervous system was reduced. The median survival of Fig4 null mice was increased from 4 weeks for untreated mutants to 8 weeks with chloroquine treatment (p < 0.009). Chloroquine thus corrects the lysosomal swelling in cultured cells and ameliorates Fig4 deficiency in vivo. The improved phenotype of mice with complete loss of Fig4 suggests that chloroquine could be beneficial FIG2 in partial loss-of-function disorders such as Charcot-Marie-Tooth Type 4 J.

Beyond PI3Ks: targeting phosphoinositide kinases in disease

Lipid phosphoinositides are master regulators of almost all aspects of a cell's life and death and are generated by the tightly regulated activity of phosphoinositide kinases. Although extensive efforts have focused on drugging class I phosphoinositide 3-kinases (PI3Ks), recent years have revealed opportunities for targeting almost all phosphoinositide kinases in human diseases, including cancer, immunodeficiencies, viral infection and neurodegenerative disease. This has led to widespread efforts in the clinical development of potent and selective inhibitors of phosphoinositide kinases. This Review summarizes our current understanding of the molecular basis for the involvement of phosphoinositide kinases in disease and assesses the preclinical and clinical development of phosphoinositide kinase inhibitors.

Endosome maturation links PI3Kα signaling to lysosome repopulation during basal autophagy

Autophagy depends on the repopulation of lysosomes to degrade intracellular components and recycle nutrients. How cells co‐ordinate lysosome repopulation during basal autophagy, which occurs constitutively under nutrient‐rich conditions, is unknown. Here, we identify an endosome‐dependent phosphoinositide pathway that links PI3Kα signaling to lysosome repopulation during basal autophagy. We show that PI3Kα‐derived PI(3)P generated by INPP4B on late endosomes was required for basal but not starvation‐induced autophagic degradation. PI(3)P signals were maintained as late endosomes matured into endolysosomes, and served as the substrate for the 5‐kinase, PIKfyve, to generate PI(3,5)P₂. The SNX‐BAR protein, SNX2, was recruited to endolysosomes by PI(3,5)P₂ and promoted lysosome reformation. Inhibition of INPP4B/PIKfyve‐dependent lysosome reformation reduced autophagic clearance of protein aggregates during proteotoxic stress leading to increased cytotoxicity. Therefore under nutrient‐rich conditions, PI3Kα, INPP4B, and PIKfyve sequentially contribute to basal autophagic degradation and protection from proteotoxic stress via PI(3,5)P₂‐dependent lysosome reformation from endolysosomes. These findings reveal that endosome maturation couples PI3Kα signaling to lysosome reformation during basal autophagy.

Defective axonal transport of endo-lysosomes and dense core vesicles in a Drosophila model of C9-ALS/FTD

A GGGGCC (G₄ C₂ ) repeat expansion in the C9orf72 gene is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Although disruptions in axonal transport are implicated in the pathogenesis of multiple neurodegenerative diseases, the underlying mechanisms causing these defects remain unclear. Here, we performed live imaging of Drosophila motor neurons expressing expanded G₄ C₂ repeats in third-instar larvae and investigated the axonal transport of multiple organelles in vivo. Expression of expanded G₄ C₂ repeats causes an increase in static axonal lysosomes, while it impairs trafficking of late endosomes (LEs) and dense core vesicles (DCVs). Surprisingly, however, axonal transport of mitochondria is unaffected in motor axons expressing expanded G₄ C₂ repeats. Thus, our data indicate that expanded G₄ C₂ repeat expression differentially impacts axonal transport of vesicular organelles and mitochondria in Drosophila models of C9orf72-associated ALS/FTD. This article is protected by copyright. All rights reserved.

C698R mutation in Lrsam1 gene impairs nerve regeneration in a CMT2P mouse model

Missense mutation C694R in the RING domain of the LRSAM1 gene results in a dominantly inherited polyneuropathy, Charcot-Marie-Tooth disease type 2P (CMT2P). We have generated and characterized a Lrsam1^C698R knock-in mouse model produced through CRISPR/Cas9 technology. Both heterozygous (Lrsam1^+/C698R) and homozygous (Lrsam1^C698/C698R) knock-in mice exhibited normal motor functions on behavioral tests as well as normal on nerve conduction studies. Axonal density and myelin thickness were not significantly different between mutants and wild-type mice by sciatic nerve morphometric analysis up to 17 months of age. In line with these normal findings, protein–protein interactions between mutant LRSAM1 and RNA-binding proteins (such as FUS and G3BP1) were still present in mouse cells, which differs from the disrupted interactions between these proteins in human CMT2P cells. However, after crush nerve injury, Lrsam1^+/C698R mice had a mild, but statistically significant, reduced compound nerve action potential and conduction velocity during recovery. Therefore, C698R mutation results in a mild impaired nerve regeneration in mice. We speculate that repetitive nerve injuries may, at least partially, contribute to the slowly progressive axonal loss in CMT2P.

Unclassified white matter disorders: A diagnostic journey requiring close collaboration between clinical and laboratory services

BACKGROUND

Next generation sequencing studies have revealed an ever-increasing number of causes for genetic disorders of central nervous system white matter. A substantial number of disorders are identifiable from their specific pattern of biochemical and/or imaging findings for which single gene testing may be indicated. Beyond this group, the causes of genetic white matter disorders are unclear and a broader approach to genomic testing is recommended.

AIM

This study aimed to identify the genetic causes for a group of individuals with unclassified white matter disorders with suspected genetic aetiology and highlight the investigations required when the initial testing is non-diagnostic.

METHODS

Twenty-six individuals from 22 families with unclassified white matter disorders underwent deep phenotyping and genome sequencing performed on trio, or larger, family groups. Functional studies and transcriptomics were used to resolve variants of uncertain significance with potential clinical relevance.

RESULTS

Causative or candidate variants were identified in 15/22 (68.2%) families. Six of the 15 implicated genes had been previously associated with white matter disease (COL4A1, NDUFV1, SLC17A5, TUBB4A, BOLA3, DARS2). Patients with variants in the latter two presented with an atypical phenotype. The other nine genes had not been specifically associated with white matter disease at the time of diagnosis and included genes associated with monogenic syndromes, developmental disorders, and developmental and epileptic encephalopathies (STAG2, LSS, FIG4, GLS, PMPCA, SPTBN1, AGO2, SCN2A, SCN8A). Consequently, only 46% of the diagnoses would have been made via a current leukodystrophy gene panel test.

DISCUSSION

These results confirm the importance of broad genomic testing for patients with white matter disorders. The high diagnostic yield reflects the integration of deep phenotyping, whole genome sequencing, trio analysis, functional studies, and transcriptomic analyses.

CONCLUSIONS

Genetic white matter disorders are genetically and phenotypically heterogeneous. Deep phenotyping together with a range of genomic technologies underpin the identification of causes of unclassified white matter disease. A molecular diagnosis is essential for prognostication, appropriate management, and accurate reproductive counseling.

The vacuolar morphology protein VAC14 plays an important role in sexual development in the filamentous ascomycete Sordaria macrospora

The multiprotein Fab1p/PIKfyve-complex regulating the abundance of the phospholipid phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P₂) is highly conserved among eukaryotes. In yeast/mammals, it is composed of the phosphatidylinositol 3-phosphate 5-kinase Fab1p/PIKfyve, the PtdIns(3,5)P₂ phosphatase Fig4p/Sac3 and the scaffolding subunit Vac14p/ArPIKfyve. The complex is located to vacuolar membranes in yeast and to endosomal membranes in mammals, where it controls the synthesis and turnover of PtdIns(3,5)P₂. In this study, we analyzed the role and function of the Fab1p/PIKfyve-complex scaffold protein SmVAC14 in the filamentous ascomycete Sordaria macrospora (Sm). We generated the Smvac14 deletion strain ∆vac14 and performed phenotypic analysis of the mutant. Furthermore, we conducted fluorescence microscopic localization studies of fluorescently labeled SmVAC14 with vacuolar and late endosomal marker proteins. Our results revealed that SmVAC14 is important for maintaining vacuolar size and appearance as well as proper sexual development in S. macrospora. In addition, SmVAC14 plays an important role in starvation stress response. Accordingly, our results propose that the turnover of PtdIns(3,5)P₂ is of great significance for developmental processes in filamentous fungi.

Roles of PIKfyve in multiple cellular pathways

Phosphoinositide signaling lipids are crucial for eukaryotes and regulate many aspects of cell function. These signaling molecules are difficult to study because they are extremely low abundance. Here, we focus on two of the lowest abundance phosphoinositides, PI(3,5)P2 and PI(5)P, which play critical roles in cellular homeostasis, membrane trafficking and transcription. Their levels are tightly regulated by a protein complex that includes PIKfyve, Fig4 and Vac14. Importantly, mutations in this complex that decrease PI(3,5)P2 and PI(5)P are linked to human diseases, especially those of the nervous system. Paradoxically, PIKfyve inhibitors which decrease PI(3,5)P2 and PI(5)P, are currently being tested for some neurodegenerative diseases, as well as other diverse diseases including some cancers, and as a treatment for SARS-CoV2 infection. A more comprehensive picture of the pathways that are regulated by PIKfyve will be critical to understand the roles of PI(3,5)P2 and PI(5)P in normal human physiology and in disease.

PMP2/FABP8 induces PI(4,5)P2-dependent transbilayer reorganization of sphingomyelin in the plasma membrane

Sphingomyelin (SM) is a mammalian lipid mainly distributed in the outer leaflet of the plasma membrane (PM). We show that peripheral myelin protein 2 (PMP2), a member of the fatty-acid-binding protein (FABP) family, can localize at the PM and controls the transbilayer distribution of SM. Genetic screening with genome-wide small hairpin RNA libraries identifies PMP2 as a protein involved in the transbilayer movement of SM. A biochemical assay demonstrates that PMP2 is a phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂)-binding protein. PMP2 induces the tubulation of model membranes in a PI(4,5)P₂-dependent manner, accompanied by the modification of the transbilayer membrane distribution of lipids. In the PM of PMP2-overexpressing cells, inner-leaflet SM is increased whereas outer-leaflet SM is reduced. PMP2 is a causative protein of Charcot-Marie-Tooth disease (CMT). A mutation in PMP2 associated with CMT increases its affinity for PI(4,5)P₂, inducing membrane tubulation and the subsequent transbilayer movement of lipids.

Mechanisms and Treatments in Demyelinating CMT

Demyelinating forms of Charcot-Marie-Tooth disease (CMT) are genetically and phenotypically heterogeneous and result from highly diverse biological mechanisms including gain of function (including dominant negative effects) and loss of function. While no definitive treatment is currently available, rapid advances in defining the pathomechanisms of demyelinating CMT have led to promising pre-clinical studies, as well as emerging clinical trials. Especially promising are the recently completed pre-clinical genetic therapy studies in PMP-22, GJB1, and SH3TC2-associated neuropathies, particularly given the success of similar approaches in humans with spinal muscular atrophy and transthyretin familial polyneuropathy. This article focuses on neuropathies related to mutations in PMP-22, MPZ, and GJB1, which together comprise the most common forms of demyelinating CMT, as well as on select rarer forms for which promising treatment targets have been identified. Clinical characteristics and pathomechanisms are reviewed in detail, with emphasis on therapeutically targetable biological pathways. Also discussed are the challenges facing the CMT research community in its efforts to advance the rapidly evolving biological insights to effective clinical trials. These considerations include the limitations of currently available animal models, the need for personalized medicine approaches/allele-specific interventions for select forms of demyelinating CMT, and the increasing demand for optimal clinical outcome assessments and objective biomarkers.

Animal Models as a Tool to Design Therapeutical Strategies for CMT-like Hereditary Neuropathies

Since ancient times, animal models have provided fundamental information in medical knowledge. This also applies for discoveries in the field of inherited peripheral neuropathies (IPNs), where they have been instrumental for our understanding of nerve development, pathogenesis of neuropathy, molecules and pathways involved and to design potential therapies. In this review, we briefly describe how animal models have been used in ancient medicine until the use of rodents as the prevalent model in present times. We then travel along different examples of how rodents have been used to improve our understanding of IPNs. We do not intend to describe all discoveries and animal models developed for IPNs, but just to touch on a few arbitrary and paradigmatic examples, taken from our direct experience or from literature. The idea is to show how strategies have been developed to finally arrive to possible treatments for IPNs.

Pmp22 super-enhancer deletion causes tomacula formation and conduction block in peripheral nerves

Copy number variation of the peripheral nerve myelin gene Peripheral Myelin Protein 22 (PMP22) causes multiple forms of inherited peripheral neuropathy. The duplication of a 1.4 Mb segment surrounding this gene in chromosome 17p12 (c17p12) causes the most common form of Charcot-Marie-Tooth disease type 1A, whereas the reciprocal deletion of this gene causes a separate neuropathy termed hereditary neuropathy with liability to pressure palsies (HNPP). PMP22 is robustly induced in Schwann cells in early postnatal development, and several transcription factors and their cognate regulatory elements have been implicated in coordinating the gene's proper expression. We previously found that a distal super-enhancer domain was important for Pmp22 expression in vitro, with particular impact on a Schwann cell-specific alternative promoter. Here, we investigate the consequences of deleting this super-enhancer in vivo. We find that loss of the super-enhancer in mice reduces Pmp22 expression throughout development and into adulthood, with greater impact on the Schwann cell-specific promoter. Additionally, these mice display tomacula formed by excessive myelin folding, a pathological hallmark of HNPP, as have been previously observed in heterozygous Pmp22 mice as well as sural biopsies from patients with HNPP. Our findings demonstrate a mechanism by which smaller copy number variations, not including the Pmp22 gene, are sufficient to reduce gene expression and phenocopy a peripheral neuropathy caused by the HNPP-associated deletion encompassing PMP22.

AAV9-mediated FIG4 delivery prolongs life span in Charcot-Marie-Tooth disease type 4J mouse model

Charcot-Marie-Tooth disease type 4J (CMT4J) is caused by recessive, loss-of-function mutations in FIG4, encoding a phosphoinositol(3,5)P2-phosphatase. CMT4J patients have both neuron loss and demyelination in the peripheral nervous system, with vacuolization indicative of endosome/lysosome trafficking defects. Although the disease is highly variable, the onset is often in childhood and FIG4 mutations can dramatically shorten life span. There is currently no treatment for CMT4J. Here, we present the results of preclinical studies testing a gene-therapy approach to restoring FIG4 expression. A mouse model of CMT4J, the Fig4-pale tremor (plt) allele, was dosed with a single-stranded adeno-associated virus serotype 9 (AAV9) to deliver a codon-optimized human FIG4 sequence. Untreated, Fig4plt/plt mice have a median survival of approximately 5 weeks. When treated with the AAV9-FIG4 vector at P1 or P4, mice survived at least 1 year, with largely normal gross motor performance and little sign of neuropathy by neurophysiological or histopathological evaluation. When mice were treated at P7 or P11, life span was still significantly prolonged and peripheral nerve function was improved, but rescue was less complete. No unanticipated adverse effects were observed. Therefore, AAV9-mediated delivery of FIG4 is a well-tolerated and efficacious strategy in a mouse model of CMT4J.

Identification of Rpd3 as a novel epigenetic regulator of Drosophila FIG 4, a Charcot-Marie-Tooth disease-causing gene

Mutations in the factor-induced-gene 4 (FIG 4) gene are associated with multiple disorders, including Charcot-Marie-Tooth disease (CMT), epilepsy with polymicrogyria, Yunis-Varón syndrome and amyotrophic lateral sclerosis. The wide spectrum of disorders associated with FIG 4 may be related to the dysregulated epigenetics. Using Gene Expression Omnibus, we found that HDAC1 binds to the FIG 4 gene locus in the genome of human CD4+ T cells. Rpd3 is a well-known Drosophila homolog of human HDAC1. We previously established Drosophila models targeting Drosophila FIG 4 (dFIG 4) that exhibited defective locomotive ability, abnormal synapse morphology at neuromuscular junctions, enlarged vacuoles in the fat body and aberrant compound eye morphology. Genetic crossing experiments followed by physiological and immunocytochemical analyses revealed that Rpd3 mutations suppressed these defects induced by dFIG 4 knockdown. This demonstrated Rpd3 to be an important epigenetic regulator of dFIG 4, suggesting that the inhibition of HDAC1 represses the pathogenesis of FIG 4-associated disorders, including CMT. Defects in epigenetic regulators, such as HDAC1, may also explain the diverse symptoms of FIG 4-associated disorders.

The Lysosomal Storage Disorder Due to fig4a Mutation Causes Robust Liver Vacuolation in Zebrafish

The phospholipid phosphatase FIG4/Fig4 is a subunit of PIKFYVE/Pikfyve kinase complex that synthesizes phosphatidylinositol 3,5-bisphosphate (PI(3,5)P₂), a key regulator of endolysosomal trafficking and function. Loss of FIG4/Fig4 leads to intracellular deficiency of PI(3,5)P₂ signaling and multiple endolysosomal defects. Previous works were focused on the effects of FIG4/Fig4 mutations in the nervous and musculoskeletal systems in human clinical and animal studies. In this study, we describe a zebrafish recessive mutant cq35 showing robust liver vacuolation and lethality, with a predicted truncating mutation in fig4a gene. The liver vacuolation progress in fig4a mutant was reversible after regaining normal fig4a transcripts. The hepatic vacuolation pathology was identified as abnormal lysosomal storage with numerous accumulated cargoes, including autophagy intermediates, and caused progressive degeneration of bile canaliculi in mutant liver. These hepatic pathological details of fig4a mutant were repeated in zebrafish pikfyve mutant. Thus, zebrafish possess the conserved structural and functional mechanisms in Pikfyve kinase complex, based on which, pikfyve mutant phenotype covered fig4a mutant phenotype in their double mutant. Our findings represent the first description of the in vivo defects caused by FIG4/Fig4 mutation or PI(3,5)P₂ deficiency in liver, and reveal the conserved complex mechanisms associated with FIG4/Fig4-deficient disorders in zebrafish.

Charcot-Marie-Tooth disease type 4J with spastic quadriplegia, epilepsy and global developmental delay: a tale of three siblings

Charcot-Marie-Tooth (CMT) disease is mainly a disease of peripheral nervous system and patients typically present with features of demyelinating neuropathy or axonal neuropathy or both. Rarely patients present with features of central nervous system involvement. Parkinsonism, aphemia and familial epilepsy syndrome have previously come up as case reports in association with CMT type 4 J.We hereby describe a family with 3 siblings affected with CMT4J with homozygous FIG4 mutation who presented with global developmental delay, epilepsy and spastic quadriparesis.

A dysfunctional endolysosomal pathway common to two sub-types of demyelinating Charcot-Marie-Tooth disease

Autosomal dominant mutations in LITAF are responsible for the rare demyelinating peripheral neuropathy, Charcot-Marie-Tooth disease type 1C (CMT1C). The LITAF protein is expressed in many human cell types and we have investigated the consequences of two different LITAF mutations in primary fibroblasts from CMT1C patients using confocal and electron microscopy. We observed the appearance of vacuolation/enlargement of late endocytic compartments (late endosomes and lysosomes). This vacuolation was also observed after knocking out LITAF from either control human fibroblasts or from the CMT1C patient-derived cells, consistent with it being the result of loss-of-function mutations in the CMT1C fibroblasts. The vacuolation was similar to that previously observed in fibroblasts from CMT4J patients, which have autosomal recessive mutations in FIG4. The FIG4 protein is a component of a phosphoinositide kinase complex that synthesises phosphatidylinositol 3,5-bisphosphate on the limiting membrane of late endosomes. Phosphatidylinositol 3,5-bisphosphate activates the release of lysosomal Ca2+ through the cation channel TRPML1, which is required to maintain the homeostasis of endosomes and lysosomes in mammalian cells. We observed that a small molecule activator of TRPML1, ML-SA1, was able to rescue the vacuolation phenotype of LITAF knockout, FIG4 knockout and CMT1C patient fibroblasts. Our data describe the first cellular phenotype common to two different subtypes of demyelinating CMT and are consistent with LITAF and FIG4 functioning on a common endolysosomal pathway that is required to maintain the homeostasis of late endosomes and lysosomes. Although our experiments were on human fibroblasts, they have implications for our understanding of the molecular pathogenesis and approaches to therapy in two subtypes of demyelinating Charcot-Marie-Tooth disease.

Mitochondria-lysosome contacts regulate mitochondrial Ca2+ dynamics via lysosomal TRPML1

Mitochondria and lysosomes are critical for cellular homeostasis, and dysfunction of both organelles has been implicated in numerous diseases. Recently, interorganelle contacts between mitochondria and lysosomes were identified and found to regulate mitochondrial dynamics. However, whether mitochondria-lysosome contacts serve additional functions by facilitating the direct transfer of metabolites or ions between the two organelles has not been elucidated. Here, using high spatial and temporal resolution live-cell microscopy, we identified a role for mitochondria-lysosome contacts in regulating mitochondrial calcium dynamics through the lysosomal calcium efflux channel, transient receptor potential mucolipin 1 (TRPML1). Lysosomal calcium release by TRPML1 promotes calcium transfer to mitochondria, which was mediated by tethering of mitochondria-lysosome contact sites. Moreover, mitochondrial calcium uptake at mitochondria-lysosome contact sites was modulated by the outer and inner mitochondrial membrane channels, voltage-dependent anion channel 1 and the mitochondrial calcium uniporter, respectively. Since loss of TRPML1 function results in the lysosomal storage disorder mucolipidosis type IV (MLIV), we examined MLIV patient fibroblasts and found both altered mitochondria-lysosome contact dynamics and defective contact-dependent mitochondrial calcium uptake. Thus, our work highlights mitochondria-lysosome contacts as key contributors to interorganelle calcium dynamics and their potential role in the pathophysiology of disorders characterized by dysfunctional mitochondria or lysosomes.

Clinical and radiological characterization of novel FIG4-related combined system disease with neuropathy

Variants in the FIG4 gene, which encodes a phosphatidylinositol-3,5-bisphosphatase lead to obstruction of endocytic trafficking, causing accumulation of enlarged vesicles in murine peripheral neurons and fibroblasts. Bi-allelic pathogenic variants in FIG4 are associated with neurological disorders including Charcot-Marie-Tooth disease type-4J (CMT4J) and Yunis-Varón syndrome (YVS). We present four probands from three unrelated families, all homozygous for a recurrent FIG4 missense variant c.506A>C p.(Tyr169Ser), with a novel phenotype involving features of both CMT4J and YVS. Three presented with infant-onset dystonia and one with hypotonia. All have depressed lower limb reflexes and distal muscle weakness, two have nerve conduction studies (NCS) consistent with severe sensorimotor demyelinating peripheral neuropathy and one had NCS showing patchy intermediate/mildly reduced motor conduction velocities. All have cognitive impairment and three have swallowing difficulties. MRI showed cerebellar atrophy and bilateral T2 hyperintense medullary swellings in all patients. These children represent a novel clinicoradiological phenotype and suggest that phenotypes associated with FIG4 missense variants do not neatly fall into previously described diagnoses but can present with variable features. Analysis of this gene should be considered in patients with central and peripheral neurological signs and medullary radiological changes, providing earlier diagnosis and informing reproductive choices.

Lysosomal Biology and Function: Modern View of Cellular Debris Bin

Lysosomes are the main proteolytic compartments of mammalian cells comprising of a battery of hydrolases. Lysosomes dispose and recycle extracellular or intracellular macromolecules by fusing with endosomes or autophagosomes through specific waste clearance processes such as chaperone-mediated autophagy or microautophagy. The proteolytic end product is transported out of lysosomes via transporters or vesicular membrane trafficking. Recent studies have demonstrated lysosomes as a signaling node which sense, adapt and respond to changes in substrate metabolism to maintain cellular function. Lysosomal dysfunction not only influence pathways mediating membrane trafficking that culminate in the lysosome but also govern metabolic and signaling processes regulating protein sorting and targeting. In this review, we describe the current knowledge of lysosome in influencing sorting and nutrient signaling. We further present a mechanistic overview of intra-lysosomal processes, along with extra-lysosomal processes, governing lysosomal fusion and fission, exocytosis, positioning and membrane contact site formation. This review compiles existing knowledge in the field of lysosomal biology by describing various lysosomal events necessary to maintain cellular homeostasis facilitating development of therapies maintaining lysosomal function.

Membrane trafficking in health and disease

Membrane trafficking pathways are essential for the viability and growth of cells, and play a major role in the interaction of cells with their environment. In this At a Glance article and accompanying poster, we outline the major cellular trafficking pathways and discuss how defects in the function of the molecular machinery that mediates this transport lead to various diseases in humans. We also briefly discuss possible therapeutic approaches that may be used in the future treatment of trafficking-based disorders.

Summary: This At a Glance article and poster summarise the major intracellular membrane trafficking pathways and associated molecular machineries, and describe how defects in these give rise to disease in humans.

Whole exome sequencing establishes diagnosis of Charcot-Marie-Tooth 4J, 1C, and X1 subtypes

BACKGROUND

Charcot-Marie-Tooth (CMT) hereditary polyneuropathies pose a diagnostic challenge. Our aim here is to describe CMT patients diagnosed by whole exome sequencing (WES) following years of fruitless testing.

METHODS/RESULTS

Three patients with polyneuropathy suspected to be genetic in origin, but not harboring PMP22 gene deletion/duplication, were offered WES. The first patient, a 66-year-old man, had been suffering from progressive weakness and atrophies in the lower and upper extremities for 20 years. Due to ambiguous electrophysiological findings, immune therapies were administered to no avail. Twelve years after PMP22 deletion/duplication testing, WES revealed two pathogenic variants in the FIG4 gene (p.Ile41Thr and p.Phe598fs, respectively), as a cause of CMT 4J. The second patient, a 19-year-old man, had been suffering from hearing and gait impairment since at least his infancy, and recently presented with weakness and dystonia of the lower extremities. In this patient, WES identified the p.Leu122Val LITAF gene variant in heterozygous state, suggesting the diagnosis of CMT 1C, several years after initial genetic analyses. The third patient, a 44-year-old man, presented with progressive weakness and atrophies of the lower and upper extremities since the age of 17 years old. In this patient, WES identified the hemizygous p.Arg164Gln pathogenic variant in the GJB1 gene, establishing the diagnosis of CMT X1, 8 years after testing for PMP22 deletion/duplication.

CONCLUSION

Novel diagnostic techniques, such as WES, offer the possibility to decipher the cause of CMT subtypes, ending the diagnostic Odyssey of the patients and sparing them from unnecessary and potentially harmful treatments.

Lysosomal size matters

Lysosomes are key cellular catabolic centers that also perform fundamental metabolic, signaling and quality control functions. Lysosomes are not static and they respond dynamically to intra‐ and extracellular stimuli triggering changes in organelle numbers, size and position. Such physical changes have a strong impact on lysosomal activity ultimately influencing cellular homeostasis. In this review, we summarize the current knowledge on lysosomal size regulation, on its physiological role(s) and association to several disease conditions.

Severe Consequences of SAC3/FIG4 Phosphatase Deficiency to Phosphoinositides in Patients with Charcot-Marie-Tooth Disease Type-4J

Charcot-Marie-Tooth disease type-4J (CMT4J), an autosomal recessively inherited peripheral neuropathy characterized by neuronal degeneration, segmental demyelination, and limb muscle weakness, is caused by compound heterozygous mutations in the SAC3/FIG4 gene, resulting in SAC3/FIG4 protein deficiency. SAC3/FIG4 is a phosphatase that not only turns over PtdIns(3,5)P₂ to PtdIns3P but also promotes PtdIns(3,5)P₂ synthesis by activating the PIKFYVE kinase that also makes PtdIns5P. Whether CMT4J patients have alterations in PtdIns(3,5)P₂, PtdIns5P or in other phosphoinositides (PIs), and if yes, in what direction these changes might be, has never been examined. We performed PI profiling in primary fibroblasts from a cohort of CMT4J patients. Subsequent to myo-[2-³H]inositol cell labeling to equilibrium, steady-state levels of PIs were quantified by HPLC under conditions concurrently detecting PtdIns5P, PtdIns(3,5)P₂, and the other PIs. Immunoblotting verified SAC3/FIG4 depletion in CMT4J fibroblasts. Compared to normal human controls (n = 9), both PtdIns(3,5)P₂ and PtdIns5P levels were significantly decreased in CMT4J fibroblasts (n = 13) by 36.4 ± 3.6% and 43.1 ± 4.4%, respectively (p < 0.0001). These reductions were independent of patients' gender or disease onset. Although mean values for PtdIns3P in the CMT4J cohort remained unchanged, there were high variations in PtdIns3P among individual patients. Aberrant endolysosomal vacuoles, typically seen under PtdIns(3,5)P₂ reduction, were apparent but not in fibroblasts from all patients. The subset of patients without aberrant vacuoles exhibited especially low PtdIns3P levels. Concomitant decreases in PtdIns5P and PtdIns(3,5)P₂ and the link between PtdIns3P levels and cellular vacuolization are novel insights shedding further light into the molecular determinants in CMT4J polyneuropathy.

Identification of CR43467 encoding a long non-coding RNA as a novel genetic interactant with dFIG4, a CMT-causing gene

The eye imaginal disc-specific knockdown of dFIG4, a Drosophila homolog of FIG4 that is one of the Charcot-Marie-Tooth disease (CMT)-causing genes, induces an aberrant adult compound eye morphology, the so-called rough eye phenotype. We previously performed modifier screening on the dFIG4 knockdown-induced rough eye phenotype and identified several genes, including CR18854, encoding a long non-coding RNA (lncRNA) as genetic interactants with dFIG4. In the present study, in more extensive genetic screening, we found that the deletion of a gene locus encoding both Odorant rector 46a (Or46a) and lncRNA CR43467 effectively suppressed the rough eye phenotype induced by the knockdown of dFIG4. Both genes were located on the same locus, but oriented in opposite directions. In order to identify which of these genes is responsible for the suppression of the rough eye phenotype, we established a CR43467-specific knockdown line using the CRISPR-dCas9 system. By using this system, we demonstrated that the CR43467 gene, but not the Or46a gene, genetically interacted with the dFIG4 gene. The knockdown of CR43467 rescued the reductions in the length of synaptic branches and number of boutons at neuromuscular junctions induced by the knockdown of dFIG4. The vacuole enlargement phenotype induced by the fat body-specific dFIG4 knockdown was also effectively suppressed by the knockdown of CR43467. The knockdown of CR43467 also suppressed the rough eye phenotype induced by other peripheral neuropathy-related genes, such as dCOA7, dHADHB, and dPDHB. We herein identified another gene encoding lncRNA, CR43467 as a genetic interactant with the CMT-causing gene.

Phosphoinositides: Regulators of Nervous System Function in Health and Disease

Phosphoinositides, the seven phosphorylated derivatives of phosphatidylinositol have emerged as regulators of key sub-cellular processes such as membrane transport, cytoskeletal function and plasma membrane signaling in eukaryotic cells. All of these processes are also present in the cells that constitute the nervous system of animals and in this setting too, these are likely to tune key aspects of cell biology in relation to the unique structure and function of neurons. Phosphoinositides metabolism and function are mediated by enzymes and proteins that are conserved in evolution, and analysis of knockouts of these in animal models implicate this signaling system in neural function. Most recently, with the advent of human genome analysis, mutations in genes encoding components of the phosphoinositide signaling pathway have been implicated in human diseases although the cell biological basis of disease phenotypes in many cases remains unclear. In this review we evaluate existing evidence for the involvement of phosphoinositide signaling in human nervous system diseases and discuss ways of enhancing our understanding of the role of this pathway in the human nervous system's function in health and disease.

Protective role of the lipid phosphatase Fig4 in the adult nervous system

The signaling lipid phosphatidylinositol 3,5-bisphosphate, PI(3,5)P2, functions in vesicular trafficking through the endo-lysosomal compartment. Cellular levels of PI(3,5)P2 are regulated by an enzyme complex comprised of the kinase PIKFYVE, the phosphatase FIG4, and the scaffold protein VAC14. Mutations of human FIG4 cause inherited disorders including Charcot-Marie-Tooth disease type 4J, polymicrogyria with epilepsy, and Yunis-Varón syndrome. Constitutive Fig4-/- mice exhibit intention tremor, spongiform degeneration of neural tissue, hypomyelination, and juvenile lethality. To determine whether PI(3,5)P2 is required in the adult, we generated Fig4flox/-; CAG-creER mice and carried out tamoxifen-induced gene ablation. Global ablation in adulthood leads to wasting, tremor, and motor impairment. Death follows within 2 months of tamoxifen treatment, demonstrating a life-long requirement for Fig4. Histological examinations of the sciatic nerve revealed profound Wallerian degeneration of myelinated fibers, but not C-fiber axons in Remak bundles. In optic nerve sections, myelinated fibers appear morphologically intact and carry compound action potentials at normal velocity and amplitude. However, when iKO mice are challenged with a chemical white matter lesion, repair of damaged CNS myelin is significantly delayed, demonstrating a novel role for Fig4 in remyelination. Thus, in the adult PNS Fig4 is required to protect myelinated axons from Wallerian degeneration. In the adult CNS, Fig4 is dispensable for fiber stability and nerve conduction, but is required for the timely repair of damaged white matter. The greater vulnerability of the PNS to Fig4 deficiency in the mouse is consistent with clinical observations in patients with Charcot-Marie-Tooth disease.

PIKfyve/Fab1 is required for efficient V-ATPase and hydrolase delivery to phagosomes, phagosomal killing, and restriction of Legionella infection

By engulfing potentially harmful microbes, professional phagocytes are continually at risk from intracellular pathogens. To avoid becoming infected, the host must kill pathogens in the phagosome before they can escape or establish a survival niche. Here, we analyse the role of the phosphoinositide (PI) 5-kinase PIKfyve in phagosome maturation and killing, using the amoeba and model phagocyte Dictyostelium discoideum. PIKfyve plays important but poorly understood roles in vesicular trafficking by catalysing formation of the lipids phosphatidylinositol (3,5)-bisphosphate (PI(3,5)2) and phosphatidylinositol-5-phosphate (PI(5)P). Here we show that its activity is essential during early phagosome maturation in Dictyostelium. Disruption of PIKfyve inhibited delivery of both the vacuolar V-ATPase and proteases, dramatically reducing the ability of cells to acidify newly formed phagosomes and digest their contents. Consequently, PIKfyve- cells were unable to generate an effective antimicrobial environment and efficiently kill captured bacteria. Moreover, we demonstrate that cells lacking PIKfyve are more susceptible to infection by the intracellular pathogen Legionella pneumophila. We conclude that PIKfyve-catalysed phosphoinositide production plays a crucial and general role in ensuring early phagosomal maturation, protecting host cells from diverse pathogenic microbes.

Genetic screening of the genes interacting with Drosophila FIG4 identified a novel link between CMT-causing gene and long noncoding RNAs

Neuron-specific knockdown of the dFIG4 gene, a Drosophila homologue of human FIG4 and one of the causative genes for Charcot-Marie-Tooth disease (CMT), reduces the locomotive abilities of adult flies, as well as causing defects at neuromuscular junctions, such as reduced synaptic branch length in presynaptic terminals of the motor neurons in third instar larvae. Eye imaginal disc-specific knockdown of dFIG4 induces abnormal morphology of the adult compound eye, the rough eye phenotype. In this study, we carried out modifier screening of the dFIG4 knockdown-induced rough eye phenotype using a set of chromosomal deficiency lines on the second chromosome. By genetic screening, we detected 9 and 15 chromosomal regions whose deletions either suppressed or enhanced the rough eye phenotype induced by the dFIG4 knockdown. By further genetic screening with mutants of individual genes in one of these chromosomal regions, we identified the gene CR18854 that suppressed the rough eye phenotype and the loss-of-cone cell phenotype. The CR18854 gene encodes a long non-coding RNA (lncRNA) consisting of 2566 bases. Mutation and knockdown of CR18854 patially suppressed the enlarged lysosome phenotype induced by Fat body-specific knockdown of dFIG4. Further characterization of CR18854, and a few other lncRNAs in relation to dFIG4 in neuron, using neuron-specific dFIG4 knockdown flies indicated a genetic link between the dFIG4 gene and lncRNAs including CR18854 and hsrω. We also obtained data indicating genetic interaction between CR18854 and Cabeza, a Drosophila homologue of human FUS, which is one of the causing genes for amyotrophic lateral sclerosis (ALS). These results suggest that lncRNAs such as CR18854 and hsrω are involved in a common pathway in CMT and ALS pathogenesis.

Myelin abnormality in Charcot-Marie-Tooth type 4J recapitulates features of acquired demyelination

OBJECTIVE

Charcot-Marie-Tooth type 4J (CMT4J) is a rare autosomal recessive neuropathy caused by mutations in FIG4 that result in loss of FIG4 protein. This study investigates the natural history and mechanisms of segmental demyelination in CMT4J.

METHODS

Over the past 9 years, we have enrolled and studied a cohort of 12 CMT4J patients, including 6 novel FIG4 mutations. We evaluated these patients and related mouse models using morphological, electrophysiological, and biochemical approaches.

RESULTS

We found sensory motor demyelinating polyneuropathy consistently in all patients. This underlying myelin pathology was associated with nonuniform slowing of conduction velocities, conduction block, and temporal dispersion on nerve conduction studies, which resemble those features in acquired demyelinating peripheral nerve diseases. Segmental demyelination was also confirmed in mice without Fig4 (Fig4-/- ). The demyelination was associated with an increase of Schwann cell dedifferentiation and macrophages in spinal roots where nerve-blood barriers are weak. Schwann cell dedifferentiation was induced by the increasing intracellular Ca2+ . Suppression of Ca2+ level by a chelator reduced dedifferentiation and demyelination of Schwann cells in vitro and in vivo. Interestingly, cell-specific knockout of Fig4 in mouse Schwann cells or neurons failed to cause segmental demyelination.

INTERPRETATION

Myelin change in CMT4J recapitulates the features of acquired demyelinating neuropathies. This pathology is not Schwann cell autonomous. Instead, it relates to systemic processes involving interactions of multiple cell types and abnormally elevated intracellular Ca2+ . Injection of a Ca2+ chelator into Fig4-/- mice improved segmental demyelination, thereby providing a therapeutic strategy against demyelination. Ann Neurol 2018;83:756-770.

Charcot Marie Tooth disease type 4J with complex central nervous system features

We describe a family with Charcot Marie Tooth disease type 4J presenting with features of Charcot Marie Tooth disease plus parkinsonism and aphemia. Genetic testing found two variants in the FIG4 gene: c.122T>C (p.I41T) – the most common Charcot Marie Tooth disease type 4J variant – and c.1949‐10T>G (intronic). Proband fibroblasts showed absent FIG4 protein on western blot, and skipping of exon 18 by RT‐PCR. As most patients with Charcot Marie Tooth disease type 4J do not have central nervous system deficits, we postulate the intronic variant and I41T mutation together are causing loss of FIG4 protein and subsequently the central nervous system findings in our family.

Motor neuron disease in inherited neurometabolic disorders

Inherited neurometabolic disorders represent a growing group of inborn errors of metabolism that present with major neurological symptoms or a complex spectrum of symptoms dominated by central or peripheral nervous system dysfunction. Many neurological presentations may arise from the same metabolic defect, especially in autosomal-recessive inherited disorders. Motor neuron disease (MND), mainly represented by amyotrophic lateral sclerosis, may also result from various inborn errors of metabolism, some of which may represent potentially treatable conditions, thereby emphasizing the importance of recognizing such diseases. The present review discusses the most important neurometabolic disorders presenting with motor neuron (lower and/or upper) dysfunction as the key clinical and neuropathological feature.

Neuropathology of childhood-onset basal ganglia degeneration caused by mutation of VAC14

Objective

To characterize the clinical features and neuropathology associated with recessive VAC14 mutations.

Methods

Whole-exome sequencing was used to identify the genetic etiology of a rapidly progressive neurological disease presenting in early childhood in two deceased siblings with distinct neuropathological features on post mortem examination.

Results

We identified compound heterozygous variants in VAC14 in two deceased siblings with early childhood onset of severe, progressive dystonia, and neurodegeneration. Their clinical phenotype is consistent with the VAC14-related childhood-onset, striatonigral degeneration recently described in two unrelated children. Post mortem examination demonstrated prominent vacuolation associated with degenerating neurons in the caudate nucleus, putamen, and globus pallidus, similar to previously reported ex vivo vacuoles seen in the late-endosome/lysosome of VAC14-deficient neurons. We identified upregulation of ubiquitinated granules within the cell cytoplasm and lysosomal-associated membrane protein (LAMP2) around the vacuole edge to suggest a process of vacuolation of lysosomal structures associated with active autophagocytic-associated neuronal degeneration.

Interpretation

Our findings reveal a distinct clinicopathological phenotype associated with recessive VAC14 mutations.

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.

PI5P and PI(3,5)P2: Minor, but Essential Phosphoinositides

In most eukaryotes, phosphoinositides (PIs) have crucial roles in multiple cellular functions. Although the cellular levels of phosphatidylinositol 5-phosphate (PI5P) and phosphatidylinositol 3,5-bisphosphate (PI(3,5)P₂) are extremely low relative to some other PIs, emerging evidence demonstrates that both lipids are crucial for the endocytic pathway, intracellular signaling, and adaptation to stress. Mutations that causes defects in the biosynthesis of PI5P and PI(3,5)P₂ are linked to human diseases including neurodegenerative disorders. Here, we review recent findings on cellular roles of PI5P and PI(3,5)P₂, as well as the pathophysiological importance of these lipids.Key words: Phosphoinositides, Membrane trafficking, Endocytosis, Vacuoles/Lysosomes, Fab1/PIKfyve.

Hereditary and inflammatory neuropathies: a review of reported associations, mimics and misdiagnoses

Distinguishing between hereditary and inflammatory neuropathy is usually straightforward on clinical grounds with the help of a family history. There are nevertheless cases where the distinction is less clear. The advent of molecular genetics has in the past several years aided confirmatory diagnosis for an increasing proportion of patients with genetic neuropathy. Various reports have described associations of Charcot-Marie-Tooth disease with a suspected or confirmed inflammatory neuropathy occasionally responding to immunotherapy. Possible predisposition to an inflammatory component was suggested in a subset of patients. Such reports have, however, been relatively few in number, suggesting the rarity of such associations and of such a predisposition if it exists. There have been a number of publications detailing clinical presentations suggestive of inflammatory neuropathy in patients with a known or later proven genetic aetiology, and subsequently felt to be part of the phenotype rather than representing an association. A number of genetically mediated multisystemic diseases with neuropathy have otherwise been reported as mimicking chronic inflammatory demyelinating polyneuropathy (CIDP). The most common example is that of familial amyloid polyneuropathy, of particular concern for the clinician when misdiagnosed as CIDP, in view of the therapeutic implications. We review the literature on reported associations, mimics and misdiagnoses of hereditary and inflammatory neuropathy and attempt to determine a practical approach to the problem in clinical practice using clinical features, electrophysiology, histopathology and targeted early genetic testing. The issue of attempting immunomodulatory therapy is discussed in view of the published literature.

Tuning PAK Activity to Rescue Abnormal Myelin Permeability in HNPP

Schwann cells in the peripheral nervous systems extend their membranes to wrap axons concentrically and form the insulating sheath, called myelin. The spaces between layers of myelin are sealed by myelin junctions. This tight insulation enables rapid conduction of electric impulses (action potentials) through axons. Demyelination (stripping off the insulating sheath) has been widely regarded as one of the most important mechanisms altering the action potential propagation in many neurological diseases. However, the effective nerve conduction is also thought to require a proper myelin seal through myelin junctions such as tight junctions and adherens junctions. In the present study, we have demonstrated the disruption of myelin junctions in a mouse model (Pmp22+/-) of hereditary neuropathy with liability to pressure palsies (HNPP) with heterozygous deletion of Pmp22 gene. We observed a robust increase of F-actin in Pmp22+/- nerve regions where myelin junctions were disrupted, leading to increased myelin permeability. These abnormalities were present long before segmental demyelination at the late phase of Pmp22+/- mice. Moreover, the increase of F-actin levels correlated with an enhanced activity of p21-activated kinase (PAK1), a molecule known to regulate actin polymerization. Pharmacological inhibition of PAK normalized levels of F-actin, and completely prevented the progression of the myelin junction disruption and nerve conduction failure in Pmp22+/- mice. Our findings explain how abnormal myelin permeability is caused in HNPP, leading to impaired action potential propagation in the absence of demyelination. We call it "functional demyelination", a novel mechanism upstream to the actual stripping of myelin that is relevant to many demyelinating diseases. This observation also provides a potential therapeutic approach for HNPP.

Biallelic Mutations of VAC14 in Pediatric-Onset Neurological Disease

In the PI(3,5)P2 biosynthetic complex, the lipid kinase PIKFYVE and the phosphatase FIG4 are bound to the dimeric scaffold protein VAC14, which is composed of multiple heat-repeat domains. Mutations of FIG4 result in the inherited disorders Charcot-Marie-Tooth disease type 4J, Yunis-Varón syndrome, and polymicrogyria with seizures. We here describe inherited variants of VAC14 in two unrelated children with sudden onset of a progressive neurological disorder and regression of developmental milestones. Both children developed impaired movement with dystonia, became nonambulatory and nonverbal, and exhibited striatal abnormalities on MRI. A diagnosis of Leigh syndrome was rejected due to normal lactate profiles. Exome sequencing identified biallelic variants of VAC14 that were inherited from unaffected heterozygous parents in both families. Proband 1 inherited a splice-site variant that results in skipping of exon 13, p.Ile459Profs(∗)4 (not reported in public databases), and the missense variant p.Trp424Leu (reported in the ExAC database in a single heterozygote). Proband 2 inherited two missense variants in the dimerization domain of VAC14, p.Ala582Ser and p.Ser583Leu, that have not been previously reported. Cultured skin fibroblasts exhibited the accumulation of vacuoles that is characteristic of PI(3,5)P2 deficiency. Vacuolization of fibroblasts was rescued by transfection of wild-type VAC14 cDNA. The similar age of onset and neurological decline in the two unrelated children define a recessive disorder resulting from compound heterozygosity for deleterious variants of VAC14.