Production of Phosphatidylinositol 5-Phosphate by the Phosphoinositide 3-Phosphatase Myotubularin in Mammalian Cells

MTM1-mediated production of phosphatidylinositol 5-phosphate fuels the formation of podosome-like protrusions regulating myoblast fusion

Myogenesis is a multistep process that requires a spatiotemporal regulation of cell events resulting finally in myoblast fusion into multinucleated myotubes. Most major insights into the mechanisms underlying fusion seem to be conserved from insects to mammals and include the formation of podosome-like protrusions (PLPs) that exert a driving force toward the founder cell. However, the machinery that governs this process remains poorly understood. In this study, we demonstrate that MTM1 is the main enzyme responsible for the production of phosphatidylinositol 5-phosphate, which in turn fuels PI5P 4-kinase α to produce a minor and functional pool of phosphatidylinositol 4,5-bisphosphate that concentrates in PLPs containing the scaffolding protein Tks5, Dynamin-2, and the fusogenic protein Myomaker. Collectively, our data reveal a functional crosstalk between a PI-phosphatase and a PI-kinase in the regulation of PLP formation.

25 Years of PI5P

The accidental discovery of PI5P (phosphatidylinositol-5-phosphate) was published 25 years ago, when PIP5K type II (phosphoinositide-4-phosphate 5-kinase) was shown to actually be a 4-kinase that uses PI5P as a substrate to generate PI(4,5)P2. Consequently, PIP5K type II was renamed to PI5P4K, or PIP4K for short, and PI5P became the last of the 7 signaling phosphoinositides to be discovered. Much of what we know about PI5P comes from genetic studies of PIP4K, as the pathways for PI5P synthesis, the downstream targets of PI5P and how PI5P affects cellular function all remain largely enigmatic. Nevertheless, PI5P and PI5P-dependent PI(4,5)P2 synthesis have been clearly implicated in metabolic homeostasis and in diseases such as cancer. Here, we review the past 25 years of PI5P research, with particular emphasis on the impact this small signaling lipid has on human health.

Structural rationale to understand the effect of disease-associated mutations on Myotubularin

Myotubularin or MTM1 is a lipid phosphatase that regulates vesicular trafficking in the cell. The MTM1 gene is mutated in a severe form of muscular disease, X-linked myotubular myopathy or XLMTM, affecting 1 in 50,000 newborn males worldwide. There have been several studies on the disease pathology of XLMTM, but the structural effects of missense mutations of MTM1 are underexplored due to the unavailability of a crystal structure. MTM1 consists of three domains-a lipid-binding N-terminal GRAM domain, the phosphatase domain and a coiled-coil domain which aids dimerisation of Myotubularin homologs. While most mutations reported to date map to the phosphatase domain of MTM1, the other two domains on the sequence are also frequently mutated in XLMTM. To understand the overall structural and functional effects of missense mutations on MTM1, we curated several missense mutations and performed in silico and in vitro studies. Apart from significantly impaired binding to substrate, abrogation of phosphatase activity was observed for a few mutants. Possible long-range effects of mutations from non-catalytic domains on phosphatase activity were observed as well. Coiled-coil domain mutants have been characterised here for the first time in XLMTM literature.

Antagonistic control of active surface integrins by myotubularin and phosphatidylinositol 3-kinase C2β in a myotubular myopathy model

X-linked centronuclear myopathy (XLCNM) is a severe human disease without existing therapies caused by mutations in the phosphoinositide 3-phosphatase MTM1. Loss of MTM1 function is associated with muscle fiber defects characterized by impaired localization of β-integrins and other components of focal adhesions. Here we show that defective focal adhesions and reduced active β-integrin surface levels in a cellular model of XLCNM are rescued by loss of phosphatidylinositiol 3-kinase C2β (PI3KC2β) function. Inactivation of the Mtm1 gene impaired myoblast differentiation into myotubes and resulted in reduced surface levels of active β1-integrins as well as corresponding defects in focal adhesions. These phenotypes were rescued by concomitant genetic loss of Pik3c2b or pharmacological inhibition of PI3KC2β activity. We further demonstrate that a hitherto unknown role of PI3KC2β in the endocytic trafficking of active β1-integrins rather than rescue of phosphatidylinositol 3-phosphate levels underlies the ability of Pik3c2b to act as a genetic modifier of cellular XLCNM phenotypes. Our findings reveal a crucial antagonistic function of MTM1 and PI3KC2β in the control of active β-integrin surface levels, thereby providing a molecular mechanism for the adhesion and myofiber defects observed in XLCNM. They further suggest specific pharmacological inhibition of PI3KC2β catalysis as a viable treatment option for XLCNM patients.

X-linked myotubular myopathy is associated with epigenetic alterations and is ameliorated by HDAC inhibition

X-linked myotubular myopathy (XLMTM) is a fatal neuromuscular disorder caused by loss of function mutations in MTM1. At present, there are no directed therapies for XLMTM, and incomplete understanding of disease pathomechanisms. To address these knowledge gaps, we performed a drug screen in mtm1 mutant zebrafish and identified four positive hits, including valproic acid, which functions as a potent suppressor of the mtm1 zebrafish phenotype via HDAC inhibition. We translated these findings to a mouse XLMTM model, and showed that valproic acid ameliorates the murine phenotype. These observations led us to interrogate the epigenome in Mtm1 knockout mice; we found increased DNA methylation, which is normalized with valproic acid, and likely mediated through aberrant 1-carbon metabolism. Finally, we made the unexpected observation that XLMTM patients share a distinct DNA methylation signature, suggesting that epigenetic alteration is a conserved disease feature amenable to therapeutic intervention.

Dynamin-2 reduction rescues the skeletal myopathy of SPEG-deficient mouse model

Striated preferentially expressed protein kinase (SPEG), a myosin light chain kinase, is mutated in centronuclear myopathy (CNM) and/or dilated cardiomyopathy. No precise therapies are available for this disorder, and gene replacement therapy is not a feasible option due to the large size of SPEG. We evaluated the potential of dynamin-2 (DNM2) reduction as a potential therapeutic strategy because it has been shown to revert muscle phenotypes in mouse models of CNM caused by MTM1, DNM2, and BIN1 mutations. We determined that SPEG-β interacted with DNM2, and SPEG deficiency caused an increase in DNM2 levels. The DNM2 reduction strategy in Speg-KO mice was associated with an increase in life span, body weight, and motor performance. Additionally, it normalized the distribution of triadic proteins, triad ultrastructure, and triad number and restored phosphatidylinositol-3-phosphate levels in SPEG-deficient skeletal muscles. Although DNM2 reduction rescued the myopathy phenotype, it did not improve cardiac dysfunction, indicating a differential tissue-specific function. Combining DNM2 reduction with other strategies may be needed to target both the cardiac and skeletal defects associated with SPEG deficiency. DNM2 reduction should be explored as a therapeutic strategy against other genetic myopathies (and dystrophies) associated with a high level of DNM2.

Gradients of PI(4,5)P2 and PI(3,5)P2 Jointly Participate in Shaping the Back State of Dictyostelium Cells

Polarity, which refers to the molecular or structural asymmetry in cells, is essential for diverse cellular functions. Dictyostelium has proven to be a valuable system for dissecting the molecular mechanisms of cell polarity. Previous studies in Dictyostelium have revealed a range of signaling and cytoskeletal proteins that function at the leading edge to promote pseudopod extension and migration. In contrast, how proteins are localized to the trailing edge is not well understood. By screening for asymmetrically localized proteins, we identified a novel trailing-edge protein we named Teep1. We show that a charged surface formed by two pleckstrin homology (PH) domains in Teep1 is necessary and sufficient for targeting it to the rear of cells. Combining biochemical and imaging analyses, we demonstrate that Teep1 interacts preferentially with PI(4,5)P2 and PI(3,5)P2in vitro and simultaneous elimination of these lipid species in cells blocks the membrane association of Teep1. Furthermore, a leading-edge localized myotubularin phosphatase likely mediates the removal of PI(3,5)P2 from the front, as well as the formation of a back-to-front gradient of PI(3,5)P2. Together our data indicate that PI(4,5)P2 and PI(3,5)P2 on the plasma membrane jointly participate in shaping the back state of Dictyostelium cells.

X-linked myotubular myopathy

X-linked myotubular myopathy (XLMTM) is a severe congenital muscle disease caused by mutation in the MTM1 gene. MTM1 encodes myotubularin (MTM1), an endosomal phosphatase that acts to dephosphorylate key second messenger lipids PI3P and PI3,5P2. XLMTM is clinically characterized by profound muscle weakness and associated with multiple disabilities (including ventilator and wheelchair dependence) and early death in most affected individuals. The disease is classically defined by characteristic changes observed on muscle biopsy, including centrally located nuclei, myofiber hypotrophy, and organelle disorganization. In this review, we highlight the clinical and pathologic features of the disease, present concepts related to disease pathomechanisms, and present recent advances in therapy development.

Common Pathogenic Mechanisms in Centronuclear and Myotubular Myopathies and Latest Treatment Advances

Centronuclear myopathies (CNM) are rare congenital disorders characterized by muscle weakness and structural defects including fiber hypotrophy and organelle mispositioning. The main CNM forms are caused by mutations in: the MTM1 gene encoding the phosphoinositide phosphatase myotubularin (myotubular myopathy), the DNM2 gene encoding the mechanoenzyme dynamin 2, the BIN1 gene encoding the membrane curvature sensing amphiphysin 2, and the RYR1 gene encoding the skeletal muscle calcium release channel/ryanodine receptor. MTM1, BIN1, and DNM2 proteins are involved in membrane remodeling and trafficking, while RyR1 directly regulates excitation-contraction coupling (ECC). Several CNM animal models have been generated or identified, which confirm shared pathological anomalies in T-tubule remodeling, ECC, organelle mispositioning, protein homeostasis, neuromuscular junction, and muscle regeneration. Dynamin 2 plays a crucial role in CNM physiopathology and has been validated as a common therapeutic target for three CNM forms. Indeed, the promising results in preclinical models set up the basis for ongoing clinical trials. Another two clinical trials to treat myotubular myopathy by MTM1 gene therapy or tamoxifen repurposing are also ongoing. Here, we review the contribution of the different CNM models to understanding physiopathology and therapy development with a focus on the commonly dysregulated pathways and current therapeutic targets.

Ionization properties of monophosphoinositides in mixed model membranes

Phosphoinositides are found in low concentration in cellular membranes but perform numerous functions such as signaling, membrane trafficking, protein recruitment and modulation of protein activity. Spatiotemporal regulation by enzymes that phosphorylate and dephosphorylate the inositol ring results in the production of seven distinct and functionally diverse derivatives. Ionization properties of the phosphorylated headgroups of anionic lipids have been shown to impact how they interact with proteins and lipids in the membrane. While the ionization properties of the three bis and one tris phosphorylated forms have been studied in physiologically relevant model membranes, that of the monophosphorylated forms (i.e., phosphatidylinositol-3-phosphate (PI3P), phosphatidylinositol-4-phosphate (PI4P), phosphatidylinositol-5-phosphate (PI5P)) has received less attention. Here, we used ³¹P MAS NMR to determine the charge of 5 mol% of the monophosphorylated derivatives in pure dioleoylphosphatidylcholine (DOPC) and DOPC/dioleoylphosphatidylethanolamine (DOPE) bilayers as a function of pH. We find that PI3P, PI4P and PI5P each have unique pKa2 values in a DOPC bilayer, and each is reduced in DOPC/DOPE model membranes through the interaction of their headgroups with DOPE according to the electrostatic-hydrogen bond switch model. In this study, using model membranes mimicking the plasma membrane (inner leaflet), Golgi, nuclear membrane, and endosome (outer leaflet), we show that PI3P, PI4P or PI5P maximize their charge at neutral pH. Our results shed light on the electrostatics of the monophosphorylated headgroups of PI3P, PI4P, and PI5P and form the basis of their intracellular functions.

Striated Preferentially Expressed Protein Kinase (SPEG) in Muscle Development, Function, and Disease

Mutations in striated preferentially expressed protein kinase (SPEG), a member of the myosin light chain kinase protein family, are associated with centronuclear myopathy (CNM), cardiomyopathy, or a combination of both. Burgeoning evidence suggests that SPEG plays critical roles in the development, maintenance, and function of skeletal and cardiac muscles. Here we review the genotype-phenotype relationships and the molecular mechanisms of SPEG-related diseases. This review will focus on the progress made toward characterizing SPEG and its interacting partners, and its multifaceted functions in muscle regeneration, triad development and maintenance, and excitation-contraction coupling. We will also discuss future directions that are yet to be investigated including understanding of its tissue-specific roles, finding additional interacting proteins and their relationships. Understanding the basic mechanisms by which SPEG regulates muscle development and function will provide critical insights into these essential processes and help identify therapeutic targets in SPEG-related disorders.

Genome-wide survey of tyrosine phosphatases in thirty mammalian genomes

The age of genomics has given us a wealth of information and the tools to study whole genomes. This, in turn, has facilitated genome-wide studies among organisms that were relatively less studied in the pre-genomic era or are non-model organisms. This paves the way to the discovery of interesting evolutionary patterns, which are brought to light by genome-wide surveys of protein superfamilies. Phosphorylation is a post-translational modification that is utilised across all clades of life, and acts as an important signalling switch, regulating several cellular processes. Tyrosine phosphatases, which are found predominantly in eukaryotes, act on phosphorylated tyrosine residues and sometimes on other substrates. Extending on our previous effort to look for tyrosine phosphatases in the human genome, we have looked for sequences of the cysteine-based tyrosine phosphatase superfamily in thirty mammalian genomes from all across Mammalia and validated the sequences with the presence of the signature catalytic motif. Domain architecture annotation, followed by in-depth analysis, revealed interesting taxon-specific patterns such as subtle differences between the protein families in marsupials and early mammals versus placental mammals. Finally, we discuss an interesting case of loss of the tyrosine phosphatase domain from a gene product in the course of eutherian evolution.

From Mice to Humans: An Overview of the Potentials and Limitations of Current Transgenic Mouse Models of Major Muscular Dystrophies and Congenital Myopathies

Muscular dystrophies are a group of more than 160 different human neuromuscular disorders characterized by a progressive deterioration of muscle mass and strength. The causes, symptoms, age of onset, severity, and progression vary depending on the exact time point of diagnosis and the entity. Congenital myopathies are rare muscle diseases mostly present at birth that result from genetic defects. There are no known cures for congenital myopathies; however, recent advances in gene therapy are promising tools in providing treatment. This review gives an overview of the mouse models used to investigate the most common muscular dystrophies and congenital myopathies with emphasis on their potentials and limitations in respect to human applications.

Phospholipids: Pulling Back the Actin Curtain for Granule Delivery to the Immune Synapse

Phosphoinositides, together with the phospholipids phosphatidylserine and phosphatidic acid, are important components of the plasma membrane acting as second messengers that, with diacylglycerol, regulate a diverse range of signaling events converting extracellular changes into cellular responses. Local changes in their distribution and membrane charge on the inner leaflet of the plasma membrane play important roles in immune cell function. Here we discuss their distribution and regulators highlighting the importance of membrane changes across the immune synapse on the cytoskeleton and the impact on the function of cytotoxic T lymphocytes.

Ca2+-induced sarcoplasmic reticulum Ca2+ release in myotubularin-deficient muscle fibers

Skeletal muscle deficiency in the 3-phosphoinositide (PtdInsP) phosphatase myotubularin (MTM1) causes myotubular myopathy which is associated with severe depression of voltage-activated sarcoplasmic reticulum Ca²⁺ release through ryanodine receptors. In the present study we aimed at further understanding how Ca²⁺ release is altered in MTM1-deficient muscle fibers, at rest and during activation. While in wild-type muscle fibers, SR Ca²⁺ release exhibits fast stereotyped kinetics of activation and decay throughout the voltage range of activation, Ca²⁺ release in MTM1-deficient muscle fibers exhibits slow and unconventional kinetics at intermediate voltages, suggestive of partial loss of the normal control of ryanodine receptor Ca²⁺ channel activity. In addition, the diseased muscle fibers at rest exhibit spontaneous elementary Ca²⁺ release events at a frequency 30 times greater than that of control fibers. Eighty percent of the events have spatiotemporal properties of archetypal Ca²⁺ sparks while the rest take either the form of lower amplitude, longer duration Ca²⁺ release events or of a combination thereof. The events occur at preferred locations in the fibers, indicating spatially uneven distribution of the parameters determining spontaneous ryanodine receptor 1 opening. Spatially large Ca²⁺ release sources were obviously involved in some of these events, suggesting that opening of ryanodine receptors in one cluster can activate opening of ryanodine receptors in a neighboring one. Overall results demonstrate that opening of Ca²⁺-activated ryanodine receptors is promoted both at rest and during excitation-contraction coupling in MTM1-deficient muscle fibers. Because access to this activation mode is denied to ryanodine receptors in healthy skeletal muscle, this may play an important role in the associated disease situation.

Recent advances in understanding phosphoinositide signaling in the nervous system

Polyphosphoinositides (PPIn) are essential signaling phospholipids that make remarkable contributions to the identity of all cellular membranes and signaling cascades in mammalian cells. They exert regulatory control over membrane homeostasis via selective interactions with cellular proteins at the membrane–cytoplasm interface. This review article briefly summarizes our current understanding of the key roles that PPIn play in orchestrating and regulating crucial electrical and chemical signaling events in mammalian neurons and the significant neuro-pathophysiological conditions that arise following alterations in their metabolism.

Centronuclear myopathies under attack: A plethora of therapeutic targets

Centronuclear myopathies are a group of congenital myopathies characterized by severe muscle weakness, genetic heterogeneity, and defects in the structural organization of muscle fibers. Their names are derived from the central position of nuclei on biopsies, while they are at the fiber periphery under normal conditions. No specific therapy exists yet for these debilitating diseases. Mutations in the myotubularin phosphoinositides phosphatase, the GTPase dynamin 2, or amphiphysin 2 have been identified to cause respectively X-linked centronuclear myopathies (also called myotubular myopathy) or autosomal dominant and recessive forms. Mutations in additional genes, as RYR1, TTN, SPEG or CACNA1S, were linked to phenotypes that can overlap with centronuclear myopathies. Numerous animal models of centronuclear myopathies have been studied over the last 15 years, ranging from invertebrate to large mammalian models. Their characterization led to a partial understanding of the pathomechanisms of these diseases and allowed the recent validation of therapeutic proof-of-concepts. Here, we review the different therapeutic strategies that have been tested so far for centronuclear myopathies, some of which may be translated to patients.

Tamoxifen prolongs survival and alleviates symptoms in mice with fatal X-linked myotubular myopathy

X-linked myotubular myopathy (XLMTM, also known as XLCNM) is a severe congenital muscular disorder due to mutations in the myotubularin gene, MTM1. It is characterized by generalized hypotonia, leading to neonatal death of most patients. No specific treatment exists. Here, we show that tamoxifen, a well-known drug used against breast cancer, rescues the phenotype of Mtm1-deficient mice. Tamoxifen increases lifespan several-fold while improving overall motor function and preventing disease progression including lower limb paralysis. Tamoxifen corrects functional, histological and molecular hallmarks of XLMTM, with improved force output, myonuclei positioning, myofibrillar structure, triad number, and excitation-contraction coupling. Tamoxifen normalizes the expression level of the XLMTM disease modifiers DNM2 and PI3KC2B, likely contributing to the phenotypic rescue. Our findings demonstrate that tamoxifen is a promising candidate for clinical evaluation in XLMTM patients.

Intravenous Administration of a MTMR2-Encoding AAV Vector Ameliorates the Phenotype of Myotubular Myopathy in Mice

X-linked myotubular myopathy (XLMTM) is a severe congenital disorder in male infants that leads to generalized skeletal muscle weakness and is frequently associated with fatal respiratory failure. XLMTM is caused by loss-of-function mutations in the MTM1 gene, which encodes myotubularin, the founder member of a family of 15 homologous proteins in mammals. We recently demonstrated the therapeutic efficacy of intravenous delivery of rAAV vectors expressing MTM1 in animal models of myotubular myopathy. Here, we tested whether the closest homologues of MTM1, MTMR1, and MTMR2 (the latter being implicated in Charcot-Marie-Tooth neuropathy type 4B1) are functionally redundant and could represent a therapeutic target for XLMTM. Serotype 9 recombinant AAV vectors encoding either MTM1, MTMR1, or MTMR2 were injected into the tibialis anterior muscle of Mtm1-deficient knockout mice. Two weeks after vector delivery, a therapeutic effect was observed with Mtm1 and Mtmr2, but not Mtmr1; with Mtm1 being the most efficacious transgene. Furthermore, intravenous administration of a single dose of the rAAV9-Mtmr2 vector in XLMTM mice improved the motor activity and muscle strength and prolonged survival throughout a 3-month study. These results indicate that strategies aiming at increasing MTMR2 expression levels in skeletal muscle may be beneficial in the treatment of myotubular myopathy.

Expression of the neuropathy-associated MTMR2 gene rescues MTM1-associated myopathy

Myotubularins (MTMs) are active or dead phosphoinositides phosphatases defining a large protein family conserved through evolution and implicated in different neuromuscular diseases. Loss-of-function mutations in MTM1 cause the severe congenital myopathy called myotubular myopathy (or X-linked centronuclear myopathy) while mutations in the MTM1-related protein MTMR2 cause a recessive Charcot-Marie-Tooth peripheral neuropathy. Here we aimed to determine the functional specificity and redundancy of MTM1 and MTMR2, and to assess their abilities to compensate for a potential therapeutic strategy. Using molecular investigations and heterologous expression of human MTMs in yeast cells and in Mtm1 knockout mice, we characterized several naturally occurring MTMR2 isoforms with different activities. We identified the N-terminal domain as responsible for functional differences between MTM1 and MTMR2. An N-terminal extension observed in MTMR2 is absent in MTM1, and only the short MTMR2 isoform lacking this N-terminal extension behaved similarly to MTM1 in yeast and mice. Moreover, adeno-associated virus-mediated exogenous expression of several MTMR2 isoforms ameliorates the myopathic phenotype owing to MTM1 loss, with increased muscle force, reduced myofiber atrophy, and reduction of the intracellular disorganization hallmarks associated with myotubular myopathy. Noteworthy, the short MTMR2 isoform provided a better rescue when compared with the long MTMR2 isoform. In conclusion, these results point to the molecular basis for MTMs functional specificity. They also provide the proof-of-concept that expression of the neuropathy-associated MTMR2 gene improves the MTM1-associated myopathy, thus identifying MTMR2 as a novel therapeutic target for myotubular myopathy.

Required hydrophobicity of fluorescent reporters for phosphatidylinositol family of lipid enzymes

The phosphatidylinositol (PtdIns) family of lipids plays important roles in cell differentiation, proliferation, and migration. Abnormal expression, mutation, or regulation of their metabolic enzymes has been associated with various human diseases such as cancer, diabetes, and bipolar disorder. Recently, fluorescent derivatives have increasingly been used as chemical probes to monitor either lipid localization or enzymatic activity. However, the requirements of a good probe have not been well defined, particularly modifications on the diacylglycerol side chain partly due to challenges in generating PtdIns lipids. We have synthesized a series of fluorescent PtdIns(4,5)P₂ (PIP₂) and PtdIns (PI) derivatives with various lengths of side chains and tested their capacity as substrates for PI3KIα and PI4KIIα, respectively. Both capillary electrophoresis and thin-layer chromatography were used to analyze enzymatic reactions. For both enzymes, the fluorescent probe with a longer side chain functions as a better substrate than that with a shorter chain and works well in the presence of the endogenous lipid, highlighting the importance of hydrophobicity of side chains in fluorescent phosphoinositide reporters. This comparison is consistent with their interactions with lipid vesicles, suggesting that the binding of a fluorescent lipid with liposome serves as a standard for assessing its utility as a chemical probe for the corresponding endogenous lipid. These findings are likely applicable to other lipid enzymes where the catalysis takes place at the lipid-water interface.

TRPML1: The Ca(2+)retaker of the lysosome

Efficient functioning of lysosome is necessary to ensure the correct performance of a variety of intracellular processes such as degradation of cargoes coming from the endocytic and autophagic pathways, recycling of organelles, and signaling mechanisms involved in cellular adaptation to nutrient availability. Mutations in lysosomal genes lead to more than 50 lysosomal storage disorders (LSDs). Among them, mutations in the gene encoding TRPML1 (MCOLN1) cause Mucolipidosis type IV (MLIV), a recessive LSD characterized by neurodegeneration, psychomotor retardation, ophthalmologic defects and achlorhydria. At the cellular level, MLIV patient fibroblasts show enlargement and engulfment of the late endo-lysosomal compartment, autophagy impairment, and accumulation of lipids and glycosaminoglycans. TRPML1 is the most extensively studied member of a small family of genes that also includes TRPML2 and TRPML3, and it has been found to participate in vesicular trafficking, lipid and ion homeostasis, and autophagy. In this review we will provide an update on the latest and more novel findings related to the functions of TRPMLs, with particular focus on the emerging role of TRPML1 and lysosomal calcium signaling in autophagy. Moreover, we will also discuss new potential therapeutic approaches for MLIV and LSDs based on the modulation of TRPML1-mediated signaling.

Phosphoinositides, Major Actors in Membrane Trafficking and Lipid Signaling Pathways

Phosphoinositides are lipids involved in the vesicular transport of proteins and lipids between the different compartments of eukaryotic cells. They act by recruiting and/or activating effector proteins and thus are involved in regulating various cellular functions, such as vesicular budding, membrane fusion and cytoskeleton dynamics. Although detected in small concentrations in membranes, their role is essential to cell function, since imbalance in their concentrations is a hallmark of many cancers. Their synthesis involves phosphorylating/dephosphorylating positions D3, D4 and/or D5 of their inositol ring by specific lipid kinases and phosphatases. This process is tightly regulated and specific to the different intracellular membranes. Most enzymes involved in phosphoinositide synthesis are conserved between yeast and human, and their loss of function leads to severe diseases (cancer, myopathy, neuropathy and ciliopathy).

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.

Phosphatidylinositol 3-kinase inhibition restores Ca2+ release defects and prolongs survival in myotubularin-deficient mice

Mutations in the gene encoding the phosphoinositide 3-phosphatase myotubularin (MTM1) are responsible for a pediatric disease of skeletal muscle named myotubular myopathy (XLMTM). Muscle fibers from MTM1-deficient mice present defects in excitation-contraction (EC) coupling likely responsible for the disease-associated fatal muscle weakness. However, the mechanism leading to EC coupling failure remains unclear. During normal skeletal muscle EC coupling, transverse (t) tubule depolarization triggers sarcoplasmic reticulum (SR) Ca²⁺ release through ryanodine receptor channels gated by conformational coupling with the t-tubule voltage-sensing dihydropyridine receptors. We report that MTM1 deficiency is associated with a 60% depression of global SR Ca²⁺ release over the full range of voltage sensitivity of EC coupling. SR Ca²⁺ release in the diseased fibers is also slower than in normal fibers, or delayed following voltage activation, consistent with the contribution of Ca²⁺-gated ryanodine receptors to EC coupling. In addition, we found that SR Ca²⁺ release is spatially heterogeneous within myotubularin-deficient muscle fibers, with focally defective areas recapitulating the global alterations. Importantly, we found that pharmacological inhibition of phosphatidylinositol 3-kinase (PtdIns 3-kinase) activity rescues the Ca²⁺ release defects in isolated muscle fibers and increases the lifespan and mobility of XLMTM mice, providing proof of concept for the use of PtdIns 3-kinase inhibitors in myotubular myopathy and suggesting that unbalanced PtdIns 3-kinase activity plays a critical role in the pathological process.

WANTED - Dead or alive: Myotubularins, a large disease-associated protein family

Myotubularins define a large family of proteins conserved through evolution. Several members are mutated in different neuromuscular diseases including centronuclear myopathies and Charcot-Marie-Tooth (CMT) neuropathies, or are linked to a predisposition to obesity and cancer. While some members have phosphatase activity against the 3-phosphate of phosphoinositides, regulating the phosphorylation status of PtdIns3P and PtdIns(3,5)P₂ implicated in membrane trafficking and autophagy, and producing PtdIns5P, others lack key residues in the catalytic site and are classified as dead-phosphatases. However, these dead phosphatases regulate phosphoinositide-dependent cellular pathways by binding to catalytically active myotubularins. Here we review previous studies on the molecular regulation and physiological roles of myotubularins. We also used the recent myotubularins three-dimensional structures to underline key residues that are mutated in neuromuscular diseases and required for enzymatic activity. In addition, through database mining and analysis, expression profile and specific isoforms of the different myotubularins are described in depth, as well as a revisited protein interaction network. Comparison of the interactome and expression data for each myotubularin highlights specific protein complexes and tissues where myotubularins should have a key regulatory role.

Subcellular localizations of Arabidopsis myotubularins MTM1 and MTM2 suggest possible functions in vesicular trafficking between ER and cis-Golgi

The two Arabidopsis genes AtMTM1 and AtMTM2 encode highly similar phosphoinositide 3-phosphatases from the myotubularin family. Despite the high-level conservation of structure and biochemical activities, their physiological roles have significantly diverged. The nature of a membrane and the concentrations of their membrane-anchored substrates (PtdIns3P or PtdIns3,5P2) and/or products (PtdIns5P and PtdIns) are considered critical for determining the functional specificity of myotubularins. We have performed comprehensive analyses of the subcellular localization of AtMTM1 and AtMTM2 using a variety of specific constructs transiently expressed in Nicotiana benthamiana leaf epidermal cells under the control of 35S promoter. AtMTM1 co-localized preferentially with cis-Golgi membranes, while AtMTM2 associated predominantly with ER membranes. In a stark contrast with animal/human MTMs, neither AtMTM1 nor AtMTM2 co-localizes with early or late endosomes or with TGN/EE compartments, making them unlikely participants in the endosomal trafficking system. Localization of the AtMTM2 is sensitive to cold and osmotic stress challenges. In contrast to animal myotubularins, Arabidopsis myotubularins do not associate with endosomes. Our results suggest that Arabidopsis myotubularins play a role in the vesicular trafficking between ER exit sites and cis-Golgi elements. The significance of these results is discussed also in the context of stress biology and plant autophagy.

Crystal Structure of Human Myotubularin-Related Protein 1 Provides Insight into the Structural Basis of Substrate Specificity

Myotubularin-related protein 1 (MTMR1) is a phosphatase that belongs to the tyrosine/dual-specificity phosphatase superfamily. MTMR1 has been shown to use phosphatidylinositol 3-monophosphate (PI(3)P) and/or phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2) as substrates. Here, we determined the crystal structure of human MTMR1. The refined model consists of the Pleckstrin homology (PH)-GRAM and phosphatase (PTP) domains. The overall structure was highly similar to the previously reported MTMR2 structure. Interestingly, two phosphate molecules were coordinated by strictly conserved residues located in the C(X)5R motif of the active site. Additionally, our biochemical studies confirmed the substrate specificity of MTMR1 for PI(3)P and PI(3,5)P2 over other phosphatidylinositol phosphates. Our structural and enzymatic analyses provide insight into the catalytic mechanism and biochemical properties of MTMR1.

Phosphoinositides in Ca(2+) signaling and excitation-contraction coupling in skeletal muscle: an old player and newcomers

Since the postulate, 30 years ago, that phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P 2) as the precursor of inositol 1,4,5-trisphosphate (Ins(1,4,5)P 3) would be critical for skeletal muscle excitation-contraction (EC) coupling, the issue of whether phosphoinositides (PtdInsPs) may have something to do with Ca(2+) signaling in muscle raised limited interest, if any. In recent years however, the PtdInsP world has expanded considerably with new functions for PtdIns(4,5)P 2 but also with functions for the other members of the PtdInsP family. In this context, the discovery that genetic deficiency in a PtdInsP phosphatase has dramatic consequences on Ca(2+) homeostasis in skeletal muscle came unanticipated and opened up new perspectives in regards to how PtdInsPs modulate muscle Ca(2+) signaling under normal and disease conditions. This review intends to make an update of the established, the questioned, and the unknown regarding the role of PtdInsPs in skeletal muscle Ca(2+) homeostasis and EC coupling, with very specific emphasis given to Ca(2+) signals in differentiated skeletal muscle fibers.

Phosphatidylinositolphosphate phosphatase activities and cancer

Signaling through the phosphoinositide 3-kinase pathways mediates the actions of a plethora of hormones, growth factors, cytokines, and neurotransmitters upon their target cells following receptor occupation. Overactivation of these pathways has been implicated in a number of pathologies, in particular a range of malignancies. The tight regulation of signaling pathways necessitates the involvement of both stimulatory and terminating enzymes; inappropriate activation of a pathway can thus result from activation or inhibition of the two signaling arms. The focus of this review is to discuss, in detail, the activities of the identified families of phosphoinositide phosphatase expressed in humans, and how they regulate the levels of phosphoinositides implicated in promoting malignancy.

TOM1 is a PI5P effector involved in the regulation of endosomal maturation

Phosphoinositides represent a major class of lipids specifically involved in the organisation of signaling cascades, maintenance of the identity of organelles and regulation of multiple intracellular trafficking steps. We previously described that phosphatidylinositol 5-monophosphate (PI5P), produced by the Shigella flexneri phosphatase IpgD, is implicated in the endosomal sorting of the EGFR. Here, we show that the adaptor protein TOM1 is a new PI5P direct binding partner. We identify the domain of TOM1 involved in this interaction and characterize the binding motif. Finally, we demonstrate that the recruitment of TOM1 by PI5P on signaling endosomes is responsible for the delay in EGFR degradation and fluid-phase bulk endocytosis. Taken together, our data strongly suggest that PI5P-enrichment in signaling endosomes prevents endosomal maturation through the recruitment of TOM1, and point out to a new function of PI5P in regulating discrete maturation steps in the endosomal system.

Pathogenic mechanisms in centronuclear myopathies

Centronuclear myopathies (CNMs) are a genetically heterogeneous group of inherited neuromuscular disorders characterized by clinical features of a congenital myopathy and abundant central nuclei as the most prominent histopathological feature. The most common forms of congenital myopathies with central nuclei have been attributed to X-linked recessive mutations in the MTM1 gene encoding myotubularin (“X-linked myotubular myopathy”), autosomal-dominant mutations in the DNM2 gene encoding dynamin-2 and the BIN1 gene encoding amphiphysin-2 (also named bridging integrator-1, BIN1, or SH3P9), and autosomal-recessive mutations in BIN1, the RYR1 gene encoding the skeletal muscle ryanodine receptor, and the TTN gene encoding titin. Models to study and rescue the affected cellular pathways are now available in yeast, C. elegans, drosophila, zebrafish, mouse, and dog. Defects in membrane trafficking have emerged as a key pathogenic mechanisms, with aberrant T-tubule formation, abnormalities of triadic assembly, and disturbance of the excitation–contraction machinery the main downstream effects studied to date. Abnormal autophagy has recently been recognized as another important collateral of defective membrane trafficking in different genetic forms of CNM, suggesting an intriguing link to primary disorders of defective autophagy with overlapping histopathological features. The following review will provide an overview of clinical, histopathological, and genetic aspects of the CNMs in the context of the key pathogenic mechanism, outline unresolved questions, and indicate promising future lines of enquiry.

RNAi-based functional selection identifies novel cell migration determinants dependent on PI3K and AKT pathways

Lentiviral short hairpin RNA (shRNA)-mediated genetic screening is a powerful tool for identifying loss-of-function phenotype in mammalian cells. Here, we report the identification of 91 cell migration-regulating genes using unbiased genome-wide functional genetic selection. Individual knockdown or cDNA overexpression of a set of 10 candidates reveals that most of these cell migration determinants are strongly dependent on the PI3K/PTEN/AKT pathway and on their downstream signals, such as FOXO1 and p70S6K1. ALK, one of the cell migration promoting genes, uniquely uses p55γ regulatory subunit of PI3K, rather than more common p85 subunit, to trigger the activation of the PI3K-AKT pathway. Our method enables the rapid and cost-effective genome-wide selection of cell migration regulators. Our results emphasize the importance of the PI3K/PTEN/AKT pathway as a point of convergence for multiple regulators of cell migration.

Inositol lipid phosphatases in membrane trafficking and human disease

The specific interaction of phosphoinositides with proteins is critical for a plethora of cellular processes, including cytoskeleton remodelling, mitogenic signalling, ion channel regulation and membrane traffic. The spatiotemporal restriction of different phosphoinositide species helps to define compartments within the cell, and this is particularly important for membrane trafficking within both the secretory and endocytic pathways. Phosphoinositide homoeostasis is tightly regulated by a large number of inositol kinases and phosphatases, which respectively phosphorylate and dephosphorylate distinct phosphoinositide species. Many of these enzymes have been implicated in regulating membrane trafficking and, accordingly, their dysregulation has been linked to a number of human diseases. In the present review, we focus on the inositol phosphatases, concentrating on their roles in membrane trafficking and the human diseases with which they have been associated.

Phosphatidylinositol 3,5-bisphosphate: low abundance, high significance

Recent studies of the low abundant signaling lipid, phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2 ), reveal an intriguingly diverse list of downstream pathways, the intertwined relationship between PI(3,5)P2 and PI5P, as well as links to neurodegenerative diseases. Derived from the structural lipid phosphatidylinositol, PI(3,5)P2 is dynamically generated on multiple cellular compartments where interactions with an increasing list of effectors regulate many cellular pathways. A complex of proteins that includes Fab1/PIKfyve, Vac14, and Fig4/Sac3 mediates the biosynthesis of PI(3,5)P2 , and mutations that disrupt complex function and/or formation cause profound consequences in cells. Surprisingly, mutations in this pathway are linked with neurological diseases, including Charcot-Marie-Tooth syndrome and amyotrophic lateral sclerosis. Future studies of PI(3,5)P2 and PI5P are likely to expand the roles of these lipids in regulation of cellular functions, as well as provide new approaches for treatment of some neurological diseases.

The myotubularin-amphiphysin 2 complex in membrane tubulation and centronuclear myopathies

Myotubularin (MTM1) and amphiphysin 2 (BIN1) are two proteins mutated in different forms of centronuclear myopathy, but the functional and pathological relationship between these two proteins was unknown. Here, we identified MTM1 as a novel binding partner of BIN1, both in vitro and endogenously in skeletal muscle. Moreover, MTM1 enhances BIN1-mediated membrane tubulation, depending on binding and phosphoinositide phosphatase activity. BIN1 patient mutations induce a conformational change in BIN1 and alter its binding and regulation by MTM1. In conclusion, we identified the first molecular and functional link between MTM1 and BIN1, supporting a common pathological mechanism in different forms of centronuclear myopathy.

PtdIns5P: news and views of its appearance, disappearance and deeds

Accumulated evidence indicates that PtdIns5P, one of the seven phosphoinositides, found now to be constitutively present in yeast, plants and metazoa, serves as a signaling molecule to modulate pleiotropic cellular functions in both the nucleus and the cytoplasm. The enzymatic routes in biogenesis of basal PtdIns5P have remained incompletely understood. The role for candidate kinase PIKfyve that is principally involved in PtdIns(3,5)P2 production, has been questioned. In this review article we scrutinize the past obstacles that prevented the definitive implication of PIKfyve in PtdIns5P biosynthesis from PtdIns and focus on the recent pharmacological and genetic advancements that now make this conclusion well supported. We further summarize our current knowledge of the diverse stimuli modulating PtdIns5P levels, binding partners and regulated cellular process, with particular reference to the available mechanistic insights for the relevant signaling pathways.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

An Emerging Role for PI5P in T Cell Biology

Phosphoinositides are critical regulators in cell biology. Phosphatidylinositol 4,5-bisphosphate, also known as PI(4,5)P₂ or PIP₂, was the first variety of phosphoinositide to enter in the T cell signaling scene. Phosphatidylinositol bis-phosphates are the substrates for different types of enzymes such as phospholipases C (β and γ isoforms) and phosphoinositide 3-kinases (PI3K class IA and IB) that are largely involved in signal transduction. However until recently, only a few studies highlighted phosphatidylinositol monophosphates as signaling molecules. This was mostly due to the difficulty of detection of some of these phosphoinositides, such as phosphatidylinositol 5-phosphate, also known as PI5P. Some compelling evidence argues for a role of PI5P in cell signaling and/or cell trafficking. Recently, we reported the detection of a PI5P increase upon TCR triggering. Here, we describe the current knowledge of the role of PI5P in T cell signaling. The future challenges that will be important to achieve in order to fully characterize the role of PI5P in T cell biology, will be discussed.

Myotubularin and PtdIns3P remodel the sarcoplasmic reticulum in muscle in vivo

The sarcoplasmic reticulum (SR) is a specialized form of endoplasmic reticulum (ER) in skeletal muscle and is essential for calcium homeostasis. The mechanisms involved in SR remodeling and maintenance of SR subdomains are elusive. In this study, we identified myotubularin (MTM1), a phosphoinositide phosphatase mutated in X-linked centronuclear myopathy (XLCNM, or myotubular myopathy), as a key regulator of phosphatidylinositol 3-monophosphate (PtdIns3P) levels at the SR. MTM1 is predominantly located at the SR cisternae of the muscle triads, and Mtm1-deficient mouse muscles and myoblasts from XLCNM patients exhibit abnormal SR/ER networks. In vivo modulation of MTM1 enzymatic activity in skeletal muscle using ectopic expression of wild-type or a dead-phosphatase MTM1 protein leads to differential SR remodeling. Active MTM1 is associated with flat membrane stacks, whereas dead-phosphatase MTM1 mutant promotes highly curved cubic membranes originating from the SR and enriched in PtdIns3P. Overexpression of a tandem FYVE domain with high affinity for PtdIns3P alters the shape of the SR cisternae at the triad. Our findings, supported by the parallel analysis of the Mtm1-null mouse and an in vivo study, reveal a direct function of MTM1 enzymatic activity in SR remodeling and a key role for PtdIns3P in promoting SR membrane curvature in skeletal muscle. We propose that alteration in SR remodeling is a primary cause of X-linked centronuclear myopathy. The tight regulation of PtdIns3P on specific membrane subdomains may be a general mechanism to control membrane curvature.

Enzyme replacement therapy rescues weakness and improves muscle pathology in mice with X-linked myotubular myopathy

No effective treatment exists for patients with X-linked myotubular myopathy (XLMTM), a fatal congenital muscle disease caused by deficiency of the lipid phosphatase, myotubularin. The Mtm1δ4 and Mtm1 p.R69C mice model severely and moderately symptomatic XLMTM, respectively, due to differences in the degree of myotubularin deficiency. Contractile function of intact extensor digitorum longus (EDL) and soleus muscles from Mtm1δ4 mice, which produce no myotubularin, is markedly impaired. Contractile forces generated by chemically skinned single fiber preparations from Mtm1δ4 muscle were largely preserved, indicating that weakness was largely due to impaired excitation contraction coupling. Mtm1 p.R69C mice, which produce small amounts of myotubularin, showed impaired contractile function only in EDL muscles. Short-term replacement of myotubularin with a prototypical targeted protein replacement agent (3E10Fv-MTM1) in Mtm1δ4 mice improved contractile function and muscle pathology. These promising findings suggest that even low levels of myotubularin protein replacement can improve the muscle weakness and reverse the pathology that characterizes XLMTM.

Role of Phospholipids in Endocytosis, Phagocytosis, and Macropinocytosis

Endocytosis, phagocytosis, and macropinocytosis are fundamental processes that enable cells to sample their environment, eliminate pathogens and apoptotic bodies, and regulate the expression of surface components. While a great deal of effort has been devoted over many years to understanding the proteins involved in these processes, the important contribution of phospholipids has only recently been appreciated. This review is an attempt to collate and analyze the rapidly emerging evidence documenting the role of phospholipids in clathrin-mediated endocytosis, phagocytosis, and macropinocytosis. A primer on phospholipid biosynthesis, catabolism, subcellular distribution, and transport is presented initially, for reference, together with general considerations of the effects of phospholipids on membrane curvature and charge. This is followed by a detailed analysis of the critical functions of phospholipids in the internalization processes and in the maturation of the resulting vesicles and vacuoles as they progress along the endo-lysosomal pathway.

PtdIns5P is an oxidative stress-induced second messenger that regulates PKB activation

Oxidative stress initiates signaling pathways, which protect from stress-induced cellular damage, initiate apoptosis, or drive cells into senescence or into tumorigenesis. Oxidative stress regulates the activity of the cell survival factor PKB, through the regulation of PtdIns(3,4,5)P₃ synthesis. Whether oxidative stress regulates other phosphoinositides to control PKB activation is not clear. Here we show that PtdIns5P is a redox-regulated second messenger. In response to hydrogen peroxide (H₂O₂), we measured an increase in PtdIns5P in cells derived from human osteosarcoma, U2OS (5-fold); breast tumors, MDA-MB-468 (2-fold); and fibrosarcoma, HT1080 (3-fold); and in p53-null murine embryonic fibroblasts (8-fold). In U2OS cells, the increase in H₂O₂-dependent PtdIns5P did not require mTOR, PDK1, PKB, ERK, and p38 signaling or PIKfyve, a lipid kinase that increases PtdIns5P in response to osmotic and oncogenic signaling. A reduction in H₂O₂-induced PtdIns5P levels by the overexpression of PIP4K revealed its role in PKB activation. Suppression of H₂O₂-induced PtdIns5P generation reduced PKB activation and, surprisingly, reduced cell sensitivity to growth inhibition by H₂O₂. These data suggest that inhibition of PIP4K signaling might be useful as a novel strategy to increase the susceptibility of tumor cells to therapeutics that function through increased oxidative stress.

Regulation of phosphatidylinositol-5-phosphate signaling by Pin1 determines sensitivity to oxidative stress

Oxidative signaling and oxidative stress contribute to aging, cancer, and diseases resulting from neurodegeneration. Pin1 is a proline isomerase that recognizes phosphorylated substrates and regulates the localization and conformation of its targets. Pin1(-/-) mice show phenotypes associated with premature aging, yet mouse embryonic fibroblasts (MEFs) from these mice are resistant to hydrogen peroxide (H(2)O(2))-induced cell death. We found that the abundance of phosphatidylinositol-5-phosphate (PtdIns5P) was increased in response to H(2)O(2), an effect that was enhanced in Pin1(-/-) MEFs. Reduction of H(2)O(2)-induced PtdIns5P compromised cell viability in response to oxidative stress, suggesting that PtdIns5P contributed to the enhanced cell viability of Pin1(-/-) MEFs exposed to oxidative stress. The increased PtdIns5P in the Pin1(-/-) MEFs stimulated the expression of genes involved in defense against oxidative stress and reduced the accumulation of reactive oxygen species. Pin1 and PtdIns5P 4-kinases (PIP4Ks), enzymes that phosphorylate and thereby reduce the amount of PtdIns5P, interacted in a manner dependent on the phosphorylation of PIP4K. Although reintroduction of Pin1 into the Pin1(-/-) MEFs reduced the amount of PtdIns5P produced in response to H(2)O(2), in vitro assays indicated that the isomerase activity of Pin1 inhibited PIP4K activity. Whether this isomerise-mediated inhibition of PIP4K occurs in cells remains an open question, but the data suggest that the regulation of PIP4K by Pin1 may be complex.

Phosphatase-dead myotubularin ameliorates X-linked centronuclear myopathy phenotypes in mice

Myotubularin MTM1 is a phosphoinositide (PPIn) 3-phosphatase mutated in X-linked centronuclear myopathy (XLCNM; myotubular myopathy). We investigated the involvement of MTM1 enzymatic activity on XLCNM phenotypes. Exogenous expression of human MTM1 in yeast resulted in vacuolar enlargement, as a consequence of its phosphatase activity. Expression of mutants from patients with different clinical progression and determination of PtdIns3P and PtdIns5P cellular levels confirmed the link between vacuolar morphology and MTM1 phosphatase activity, and showed that some disease mutants retain phosphatase activity. Viral gene transfer of phosphatase-dead myotubularin mutants (MTM1(C375S) and MTM1(S376N)) significantly improved most histological signs of XLCNM displayed by a Mtm1-null mouse, at similar levels as wild-type MTM1. Moreover, the MTM1(C375S) mutant improved muscle performance and restored the localization of nuclei, triad alignment, and the desmin intermediate filament network, while it did not normalize PtdIns3P levels, supporting phosphatase-independent roles of MTM1 in maintaining normal muscle performance and organelle positioning in skeletal muscle. Among the different XLCNM signs investigated, we identified only triad shape and fiber size distribution as being partially dependent on MTM1 phosphatase activity. In conclusion, this work uncovers MTM1 roles in the structural organization of muscle fibers that are independent of its enzymatic activity. This underlines that removal of enzymes should be used with care to conclude on the physiological importance of their activity.

Modulation of synaptic function by VAC14, a protein that regulates the phosphoinositides PI(3,5)P₂ and PI(5)P

Mice deficient for VAC14, a scaffolding protein required for PIP2 biosynthesis and linked to human neuropathies, show increased postsynaptic function due to altered AMPA receptor trafficking.

Normal steady-state levels of the signalling lipids PI(3,5)P₂ and PI(5)P require the lipid kinase FAB1/PIKfyve and its regulators, VAC14 and FIG4. Mutations in the PIKfyve/VAC14/FIG4 pathway are associated with Charcot-Marie-Tooth syndrome and amyotrophic lateral sclerosis in humans, and profound neurodegeneration in mice. Hence, tight regulation of this pathway is critical for neural function. Here, we examine the localization and physiological role of VAC14 in neurons. We report that endogenous VAC14 localizes to endocytic organelles in fibroblasts and neurons. Unexpectedly, VAC14 exhibits a pronounced synaptic localization in hippocampal neurons, suggesting a role in regulating synaptic function. Indeed, the amplitude of miniature excitatory postsynaptic currents is enhanced in both Vac14^−/− and Fig4^−/− neurons. Re-introduction of VAC14 in postsynaptic Vac14^−/− cells reverses this effect. These changes in synaptic strength in Vac14^−/− neurons are associated with enhanced surface levels of the AMPA-type glutamate receptor subunit GluA2, an effect that is due to diminished regulated endocytosis of AMPA receptors. Thus, VAC14, PI(3,5)P₂ and/or PI(5)P play a role in controlling postsynaptic function via regulation of endocytic cycling of AMPA receptors.