Role of dynamin in the formation of transport vesicles from the trans-Golgi network

Dynamins in human diseases: differential requirement of dynamin activity in distinct tissues

Dynamin, a 100-kDa GTPase, is one of the most-characterized membrane fission machineries catalyzing vesicle release from plasma membrane during endocytosis. The human genome encodes three dynamins: DNM1, DNM2 and DNM3, with high amino acid similarity but distinct expression patterns. Ever since the discoveries of dynamin mutations associated with human diseases in 2005, dynamin has become a paradigm for studying pathogenic mechanisms of mutant proteins from the aspects of structural biology, cell biology, model organisms as well as therapeutic strategy development. Here, we review the diseases and pathogenic mechanisms caused by mutations of DNM1 and DNM2, focusing on the activity requirement and regulation of dynamins in different tissues.

Putative biosynthesis mechanism of the neurotoxin β-N-methylamino-L-alanine in marine diatoms based on a transcriptomics approach

The neurotoxin β-N-methylamino-L-alanine (BMAA) has been presumed as an environmental cause of human neurodegenerative disorders, such as Alzheimer's disease. Marine diatoms Thalassiosira minima are demonstrated here to produce BMAA-containing proteins in axenic culture while the isomer diaminobutyric acid was bacterially produced. In the co-culture with Cyanobacterium aponinum, diatom growth was inhibited but the biosynthesis of BMAA-containing proteins was stimulated up to seven times higher than that of the control group by cell-cell interactions. The stimulation effect was not caused by the cyanobacterial filtrate. Nitrogen deprivation also doubled the BMAA content of T. minima cells. Transcriptome analysis of the diatom in mixed culture revealed that pathways involved in T. minima metabolism and cellular functions were mainly influenced, including KEGG pathways valine and leucine/isoleucine degradation, endocytosis, pantothenate and CoA biosynthesis, and SNARE interactions in vesicular transport. Based on the expression changes of genes related to protein biosynthesis, it was hypothesized that ubiquitination and autophagy suppression, and limited COPII vesicles transport accuracy and efficiency were responsible for biosynthesis of BMAA-containing proteins in T. minima. This study represents a first application of transcriptomics to investigate the biological processes associated with BMAA biosynthesis in diatoms.

Differential impact of ubiquitous and muscle dynamin 2 isoforms in muscle physiology and centronuclear myopathy

Dynamin 2 mechanoenzyme is a key regulator of membrane remodeling and gain-of-function mutations in its gene cause centronuclear myopathies. Here, we investigate the functions of dynamin 2 isoforms and their associated phenotypes and, specifically, the ubiquitous and muscle-specific dynamin 2 isoforms expressed in skeletal muscle. In cell-based assays, we show that a centronuclear myopathy-related mutation in the ubiquitous but not the muscle-specific dynamin 2 isoform causes increased membrane fission. In vivo, overexpressing the ubiquitous dynamin 2 isoform correlates with severe forms of centronuclear myopathy, while overexpressing the muscle-specific isoform leads to hallmarks seen in milder cases of the disease. Previous mouse studies suggested that reduction of the total dynamin 2 pool could be therapeutic for centronuclear myopathies. Here, dynamin 2 splice switching from muscle-specific to ubiquitous dynamin 2 aggravated the phenotype of a severe X-linked form of centronuclear myopathy caused by loss-of-function of the MTM1 phosphatase, supporting the importance of targeting the ubiquitous isoform for efficient therapy in muscle. Our results highlight that the ubiquitous and not the muscle-specific dynamin 2 isoform is the main modifier contributing to centronuclear myopathy pathology.

Convergence of secretory, endosomal, and autophagic routes in trans-Golgi-associated lysosomes

At the trans-Golgi, complex traffic connections exist to the endolysosomal system additional to the main Golgi-to-plasma membrane secretory route. Here, we investigated three hits in a Drosophila screen displaying secretory cargo accumulation in autophagic vesicles: ESCRT-III component Vps20, SNARE-binding Rop, and lysosomal pump subunit VhaPPA1-1. We found that Vps20, Rop, and lysosomal markers localize near the trans-Golgi. Furthermore, we document that the vicinity of the trans-Golgi is the main cellular location for lysosomes and that early, late, and recycling endosomes associate as well with a trans-Golgi-associated degradative compartment where basal microautophagy of secretory cargo and other materials occurs. Disruption of this compartment causes cargo accumulation in our hits, including Munc18 homolog Rop, required with Syx1 and Syx4 for Rab11-mediated endosomal recycling. Finally, besides basal microautophagy, we show that the trans-Golgi-associated degradative compartment contributes to the growth of autophagic vesicles in developmental and starvation-induced macroautophagy. Our results argue that the fly trans-Golgi is the gravitational center of the whole endomembrane system.

Overexpression of cytoplasmic dynamin 2 is associated with worse outcomes in patients with clear cell renal cell carcinoma

BACKGROUND: Dynamin 2 (DNM2) involved in tumor progression in various malignancies. OBJECTIVE: For the first time, we evaluated DNM2 expression pattern, its association with clinicopathological characteristics and survival outcomes in RCC subtypes. METHODS: We evaluated the DNM2 expression pattern in RCC tissues as well as adjacent normal tissue using immunohistochemistry on tissue microarray (TMA) slides. RESULTS: Our findings revealed increased DNM2 expression in RCC samples rather than in adjacent normal tissues. The results indicated that there was a statistically significant difference between cytoplasmic expression of DNM2 among subtypes of RCC in terms of intensity of staining, percentage of positive tumor cells, and H-score (P= 0.024, 0.049, and 0.009, respectively). The analysis revealed that increased cytoplasmic expression of DNM2 in ccRCC is associated with worse OS (log rank: P= 0.045), DSS (P= 0.049), and PFS (P= 0.041). Furthermore, cytoplasmic expression of DNM2 was found as an independent prognostic factor affecting DSS and PFS in multivariate analysis. CONCLUSIONS: Our results indicated that DNM2 cytoplasmic expression is associated with tumor aggressiveness and poor outcomes. DNM2 could serve as a promising prognostic biomarker and therapeutic target in patients with ccRCC.

Crucial Players for Inter-Organelle Communication: PI5P4Ks and Their Lipid Product PI-4,5-P2 Come to the Surface

While organelles are individual compartments with specialized functions, it is becoming clear that organellar communication is essential for maintaining cellular homeostasis. This cooperation is carried out by various interactions taking place on the membranes of organelles. The membranes themselves contain a multitude of proteins and lipids that mediate these connections and one such class of molecules facilitating these relations are the phospholipids. There are several phospholipids, but the focus of this perspective is on a minor group called the phosphoinositides and specifically, phosphatidylinositol 4,5-bisphosphate (PI-4,5-P₂). This phosphoinositide, on intracellular membranes, is largely generated by the non-canonical Type II PIPKs, namely, Phosphotidylinositol-5-phosphate-4-kinases (PI5P4Ks). These evolutionarily conserved enzymes are emerging as key stress response players in cells. Further, PI5P4Ks have been shown to modulate pathways by regulating organelle crosstalk, revealing roles in preserving metabolic homeostasis. Here we will attempt to summarize the functions of the PI5P4Ks and their product PI-4,5-P₂ in facilitating inter-organelle communication and how they impact cellular health as well as their relevance to human diseases.

Intracellular Dynamin Elastin-like Polypeptides Assemble into Rodlike, Spherical, and Reticular Dynasomes

Dynamin (DNM) is a family of large GTPases possessing a unique mechanical ability to "pinch" off vesicles entering cells. DNM2 is the most ubiquitously expressed member of the DNM family. We developed a novel tool based on elastin-like polypeptide (ELP) technology to quickly, precisely, and reversibly modulate the structure of DNM2. ELPs are temperature-sensitive biopolymers that self-assemble into microdomains above sharp transition temperatures. When linked together, DNM2 and a temperature-sensitive ELP fusion organize into a range of distinct temperature-dependent structures above a sharp transition temperature, which were not observed with wild-type DNM2 or a temperature-insensitive ELP fusion control. The structures comprised three different morphologies, which were prevalent at different temperature ranges. The size of these structures was influenced by an inhibitor of the DNM2 GTPase activity, dynasore; furthermore, they appear to entrap co-expressed cytosolic ELPs. Having demonstrated an unexpected diversity of morphologically distinct structures, DNM2-ELP fusions may have applications in the exploration of dynamin-dependent biology.

Dynamin Inhibitors Prevent the Establishment of the Cytomegalovirus Assembly Compartment in the Early Phase of Infection

Cytomegalovirus (CMV) infection initiates massive rearrangement of cytoplasmic organelles to generate assembly compartment (AC). The earliest events, the establishment of the preAC, are initiated in the early phase as an extensive reorganization of early endosomes (EEs), endosomal recycling compartment (ERC), trans-Golgi network (TGN), and the Golgi. Here, we demonstrate that dynamin inhibitors (Dynasore, Dyngo-4a, MiTMAB, and Dynole-34-2) block the establishment of the preAC in murine CMV (MCMV) infected cells. In this study, we extensively analyzed the effect of Dynasore on the Golgi reorganization sequence into the outer preAC. We also monitored the development of the inner preAC using a set of markers that define EEs (Rab5, Vps34, EEA1, and Hrs), the EE-ERC interface (Rab10), the ERC (Rab11, Arf6), three layers of the Golgi (GRASP65, GM130, Golgin97), and late endosomes (Lamp1). Dynasore inhibited the pericentriolar accumulation of all markers that display EE-ERC-TGN interface in the inner preAC and prevented Golgi unlinking and dislocation to the outer preAC. Furthermore, in pulse-chase experiments, we demonstrated that the presence of dynasore only during the early phase of MCMV infection (4-14 hpi) is sufficient to prevent not only AC formation but also the synthesis of late-phase proteins and virion production. Therefore, our results indicate that dynamin-2 acts as a part of the machinery required for AC generation and rearrangement of EE/ERC/Golgi membranes in the early phase of CMV infection.

A review of Dynamin 2 involvement in cancers highlights a promising therapeutic target

Dynamin 2 (DNM2) is an ubiquitously expressed large GTPase well known for its role in vesicle formation in endocytosis and intracellular membrane trafficking also acting as a regulator of cytoskeletons. During the last two decades, DNM2 involvement, through mutations or overexpression, emerged in an increasing number of cancers and often associated with poor prognosis. A wide panel of DNM2-dependent processes was described in cancer cells which explains DNM2 contribution to cancer pathomechanisms. First, DNM2 dysfunction may promote cell migration, invasion and metastasis. Second, DNM2 acts on intracellular signaling pathways fostering tumor cell proliferation and survival. Relative to these roles, DNM2 was demonstrated as a therapeutic target able to reduce cell proliferation, induce apoptosis, and reduce the invasive phenotype in a wide range of cancer cells in vitro. Moreover, proofs of concept of therapy by modulation of DNM2 expression was also achieved in vivo in several animal models. Consequently, DNM2 appears as a promising molecular target for the development of anti-invasive agents and the already provided proofs of concept in animal models represent an important step of preclinical development.

Oxygen sensing, mitochondrial biology and experimental therapeutics for pulmonary hypertension and cancer

The homeostatic oxygen sensing system (HOSS) optimizes systemic oxygen delivery. Specialized tissues utilize a conserved mitochondrial sensor, often involving NDUFS2 in complex I of the mitochondrial electron transport chain, as a site of pO₂-responsive production of reactive oxygen species (ROS). These ROS are converted to a diffusible signaling molecule, hydrogen peroxide (H₂O₂), by superoxide dismutase (SOD2). H₂O₂ exits the mitochondria and regulates ion channels and enzymes, altering plasma membrane potential, intracellular Ca²⁺ and Ca²⁺-sensitization and controlling acute, adaptive, responses to hypoxia that involve changes in ventilation, vascular tone and neurotransmitter release. Subversion of this O₂-sensing pathway creates a pseudohypoxic state that promotes disease progression in pulmonary arterial hypertension (PAH) and cancer. Pseudohypoxia is a state in which biochemical changes, normally associated with hypoxia, occur despite normal pO₂. Epigenetic silencing of SOD2 by DNA methylation alters H₂O₂ production, activating hypoxia-inducible factor 1α, thereby disrupting mitochondrial metabolism and dynamics, accelerating cell proliferation and inhibiting apoptosis. Other epigenetic mechanisms, including dysregulation of microRNAs (miR), increase pyruvate dehydrogenase kinase and pyruvate kinase muscle isoform 2 expression in both diseases, favoring uncoupled aerobic glycolysis. This Warburg metabolic shift also accelerates cell proliferation and impairs apoptosis. Disordered mitochondrial dynamics, usually increased mitotic fission and impaired fusion, promotes disease progression in PAH and cancer. Epigenetic upregulation of dynamin-related protein 1 (Drp1) and its binding partners, MiD49 and MiD51, contributes to the pathogenesis of PAH and cancer. Finally, dysregulation of intramitochondrial Ca²⁺, resulting from impaired mitochondrial calcium uniporter complex (MCUC) function, links abnormal mitochondrial metabolism and dynamics. MiR-mediated decreases in MCUC function reduce intramitochondrial Ca²⁺, promoting Warburg metabolism, whilst increasing cytosolic Ca²⁺, promoting fission. Epigenetically disordered mitochondrial O₂-sensing, metabolism, dynamics, and Ca²⁺ homeostasis offer new therapeutic targets for PAH and cancer. Promoting glucose oxidation, restoring the fission/fusion balance, and restoring mitochondrial calcium regulation are promising experimental therapeutic strategies.

Multiple roles for actin in secretory and endocytic pathways

Actin filaments play multiple roles in the secretory pathway and in endosome dynamics in mammals, including maintenance of Golgi structure, release of membrane cargo from the trans-Golgi network (TGN), endocytosis, and endosomal sorting dynamics. In addition, TGN carrier transport and endocytosis both occur by multiple mechanisms in mammals. Actin likely plays a role in at least four mammalian endocytic pathways, five pathways for membrane release from the TGN, and three processes involving endosomes. Also, the mammalian Golgi structure is highly dynamic, and actin is likely important for these dynamics. One challenge for many of these processes is the need to deal with other membrane-associated structures, such as the cortical actin network at the plasma membrane or the matrix that surrounds the Golgi. Arp2/3 complex is a major actin assembly factor in most of the processes mentioned, but roles for formins and tandem WH2-motif-containing assembly factors are being elucidated and are anticipated to grow with further study. The specific role for actin has not been defined for most of these processes, but is likely to involve the generation of force for membrane dynamics, either by actin polymerization itself or by myosin motor activity. Defining these processes mechanistically is necessary for understanding membrane dynamics in general, as well as pathways that utilize these processes, such as autophagy.

Endocytic pathway inhibition attenuates extracellular vesicle-induced reduction of chemosensitivity to bortezomib in multiple myeloma cells

Extracellular vesicles (EVs), including exosomes and microvesicles, derived from bone marrow stromal cells (BMSCs) have been demonstrated as key factors in the progression and drug resistance of multiple myeloma (MM). EV uptake involves a variety of mechanisms which largely depend on the vesicle origin and recipient cell type. The aim of the present study was to identify the mechanisms involved in the uptake of BMSC-derived small EVs (sEVs) by MM cells, and to evaluate the anti-MM effect of targeting this process. Methods: Human BMSC-derived sEVs were identified by transmission electron microscopy, nanoparticle tracking analysis, and western blot. The effects of chemical inhibitors and shRNA-mediated knockdown of endocytosis-associated genes on sEV uptake and cell apoptosis were analyzed by flow cytometry. The anti-MM effect of blocking sEV uptake was evaluated in vitro and in a xenograft MM mouse model. Results: sEVs derived from BMSC were taken up by MM cells in a time- and dose-dependent manner, and subsequently promoted MM cell cycling and reduced their chemosensitivity to bortezomib. Chemical endocytosis inhibitors targeting heparin sulphate proteoglycans, actin, tyrosine kinase, dynamin-2, sodium/proton exchangers, or phosphoinositide 3-kinases significantly reduced MM cell internalization of BMSC-derived sEVs. Moreover, shRNA-mediated knockdown of endocytosis-associated proteins, including caveolin-1, flotillin-1, clathrin heavy chain, and dynamin-2 in MM cells suppressed sEV uptake. Furthermore, an endocytosis inhibitor targeting dynamin-2 preferentially suppressed the uptake of sEV by primary MM cells ex vivo and enhanced the anti-MM effects of bortezomib in vitro and in a mouse model. Conclusion: Clathrin- and caveolin-dependent endocytosis and macropinocytosis are the predominant routes of sEV-mediated communication between BMSCs and MM cells, and inhibiting endocytosis attenuates sEV-induced reduction of chemosensitivity to bortezomib, and thus enhances its anti-MM properties.

Dynamin Superfamily at Pre- and Postsynapses: Master Regulators of Synaptic Transmission and Plasticity in Health and Disease

Dynamin superfamily proteins (DSPs) comprise a large group of GTP-ases that orchestrate membrane fusion and fission, and cytoskeleton remodeling in different cell-types. At the central nervous system, they regulate synaptic vesicle recycling and signaling-receptor turnover, allowing the maintenance of synaptic transmission. In the presynapses, these GTP-ases control the recycling of synaptic vesicles influencing the size of the ready-releasable pool and the release of neurotransmitters from nerve terminals, whereas in the postsynapses, they are involved in AMPA-receptor trafficking to and from postsynaptic densities, supporting excitatory synaptic plasticity, and consequently learning and memory formation. In agreement with these relevant roles, an important number of neurological disorders are associated with mutations and/or dysfunction of these GTP-ases. Along the present review we discuss the importance of DSPs at synapses and their implication in different neuropathological contexts.

Dynamin-2 R465W mutation induces long range perturbation in highly ordered oligomeric structures

High order oligomers are crucial for normal cell physiology, and protein function perturbed by missense mutations underlies several autosomal dominant diseases. Dynamin-2 is one of such protein forming helical oligomers that catalyze membrane fission. Mutations in this protein, where R465W is the most frequent, cause dominant centronuclear myopathy, but the molecular mechanisms underpinning the functional modifications remain to be investigated. To unveil the structural impact of this mutation in dynamin-2, we used full-atom molecular dynamics simulations and coarse-grained models and built dimers and helices of wild-type (WT) monomers, mutant monomers, or both WT and mutant monomers combined. Our results show that the mutation R465W causes changes in the interactions with neighbor amino acids that propagate through the oligomer. These new interactions perturb the contact between monomers and favor an extended conformation of the bundle signaling element (BSE), a dynamin region that transmits the conformational changes from the GTPase domain to the rest of the protein. This extended configuration of the BSE that is only relevant in the helices illustrates how a small change in the microenvironment surrounding a single residue can propagate through the oligomer structures of dynamin explaining how dominance emerges in large protein complexes.

Hsv-1 Endocytic Entry into a Human Oligodendrocytic Cell Line is Mediated by Clathrin and Dynamin but Not Caveolin

Endocytosis is a pathway used by viruses to enter cells that can be classified based on the proteins involved, such as dynamin, clathrin or caveolin. Although the entry of herpes simplex type 1 (HSV-1) by endocytosis has been documented in different cell types, its dependence on clathrin has not been described whereas its dependence on dynamin has been shown according to the cell line used. The present work shows how clathrin-mediated endocytosis (CME) is one way that HSV-1 infects the human oligodendroglial (HOG) cell line. Partial dynamin inhibition using dynasore revealed a relationship between decrease of infection and dynamin inhibition, measured by viral titration and immunoblot. Co-localization between dynamin and HSV-1 was verified by immunofluorescence at the moment of viral entry into the cell. Inhibition by chlorpromazine revealed that viral progeny also decreased when clathrin was partially inhibited in our cell line. RT-qPCR of immediately early viral genes, specific entry assays and electron microscopy all confirmed clathrin's participation in HSV-1 entry into HOG cells. In contrast, caveolin entry assays showed no effect on the entry of this virus. Therefore, our results suggest the participation of dynamin and clathrin during endocytosis of HSV-1 in HOG cells.

β-catenin-mediated Wnt signal transduction proceeds through an endocytosis-independent mechanism

The Wnt pathway is a key intercellular signaling cascade that regulates development, tissue homeostasis, and regeneration. However, gaps remain in our understanding of the molecular events that take place between ligand-receptor binding and target gene transcription. We used a novel tool for quantitative, real-time assessment of endogenous pathway activation, measured in single cells, to answer an unresolved question in the field-whether receptor endocytosis is required for Wnt signal transduction. We combined knockdown or knockout of essential components of clathrin-mediated endocytosis with quantitative assessment of Wnt signal transduction in mouse embryonic stem cells (mESCs). Disruption of clathrin-mediated endocytosis did not affect accumulation and nuclear translocation of β-catenin, as measured by single-cell live imaging of endogenous β-catenin, and subsequent target gene transcription. Disruption of another receptor endocytosis pathway, caveolin-mediated endocytosis, did not affect Wnt pathway activation in mESCs. Additional results in multiple cell lines support that endocytosis is not a requirement for Wnt signal transduction. We show that off-target effects of a drug used to inhibit endocytosis may be one source of the discrepancy among reports on the role of endocytosis in Wnt signaling.

The anti-viral dynamin family member MxB participates in mitochondrial integrity

The membrane deforming dynamin family members MxA and MxB are large GTPases that convey resistance to a variety of infectious viruses. During viral infection, Mx proteins are known to show markedly increased expression via an interferon-responsive promoter to associate with nuclear pores. In this study we report that MxB is an inner mitochondrial membrane GTPase that plays an important role in the morphology and function of this organelle. Expression of mutant MxB or siRNA knockdown of MxB leads to fragmented mitochondria with disrupted inner membranes that are unable to maintain a proton gradient, while expelling their nucleoid-based genome into the cytoplasm. These findings implicate a dynamin family member in mitochondrial-based changes frequently observed during an interferon-based, anti-viral response.

Mx proteins belong to the dynamin family of large GTPases and are highly induced by interferon in virally infected cells. The authors show that uninfected immune cells and hepatocytes also express MxB protein that associates with mitochondria to alter the morphology and genome of this organelle.

Abnormal Excitation-Contraction Coupling and Calcium Homeostasis in Myopathies and Cardiomyopathies

Muscle contraction requires specialized membrane structures with precise geometry and relies on the concerted interplay of electrical stimulation and Ca2+ release, known as excitation-contraction coupling (ECC). The membrane structure hosting ECC is called triad in skeletal muscle and dyad in cardiac muscle, and structural or functional defects of triads and dyads have been observed in a variety of myopathies and cardiomyopathies. Based on their function, the proteins localized at the triad/dyad can be classified into three molecular pathways: the Ca2+ release complex (CRC), store-operated Ca2+ entry (SOCE), and membrane remodeling. All three are mechanistically linked, and consequently, aberrations in any of these pathways cause similar disease entities. This review provides an overview of the clinical and genetic spectrum of triad and dyad defects with a main focus of attention on the underlying pathomechanisms.

Differential sorting behavior for soluble and transmembrane cargoes at the trans-Golgi network in endocrine cells

Regulated secretion of neuropeptides and peptide hormones by secretory granules (SGs) is central to physiology. Formation of SGs occurs at the trans-Golgi network (TGN) where their soluble cargo aggregates to form a dense core, but the mechanisms controlling the sorting of regulated secretory cargoes (soluble and transmembrane) away from constitutively secreted proteins remain unclear. Optimizing the use of the retention using selective hooks method in (neuro-)endocrine cells, we now quantify TGN budding kinetics of constitutive and regulated secretory cargoes. We further show that, by monitoring two cargoes simultaneously, it becomes possible to visualize sorting to the constitutive and regulated secretory pathways in real time. Further analysis of the localization of SG cargoes immediately after budding from the TGN revealed that, surprisingly, the bulk of two studied transmembrane SG cargoes (phogrin and VMAT2) does not sort directly onto SGs during budding, but rather exit the TGN into nonregulated vesicles to get incorporated to SGs at a later step. This differential behavior of soluble and transmembrane cargoes suggests a more complex model of SG biogenesis than anticipated.

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.

Dynamin Is Required for Efficient Cytomegalovirus Maturation and Envelopment

Cytomegalovirus secondary envelopment occurs in a virus-induced cytoplasmic assembly compartment (vAC) generated via a drastic reorganization of the membranes of the secretory and endocytic systems. Dynamin is a eukaryotic GTPase that is implicated in membrane remodeling and endocytic membrane fission events; however, the role of dynamin in cellular trafficking of viruses beyond virus entry is only partially understood. Mouse embryonic fibroblasts (MEF) engineered to excise all three isoforms of dynamin were infected with mouse cytomegalovirus (MCMV-K181). Immediate-early (IE1; m123) viral protein was detected in these triple dynamin knockout (TKO) cells, as well as in mock-induced parental MEF, at early times postinfection, although levels were reduced in TKO cells, indicating that virus entry was affected but not eliminated. Levels of IE1 protein and another viral early protein (m04) were normalized by 48 h postinfection; however, late protein (m55; gB) expression was reduced in infected TKO cells compared to parental MEF. Ultrastructural analysis revealed intact stages of nuclear virus maturation in both cases with equivalent numbers of nucleocapsids containing packaged viral DNA (C-capsids), indicating successful viral DNA replication, capsid assembly, and genome packaging. Most importantly, severe defects in virus envelopment were visualized in TKO cells but not in parental cells. Dynamin inhibitor (dynasore)-treated MEF showed a phenotype similar to TKO cells upon mouse cytomegalovirus infection, confirming the role of dynamin in late maturation processes. In summary, dynamin-mediated endocytic pathways are critical for the completion of cytoplasmic stages of cytomegalovirus maturation.IMPORTANCE Viruses are known to exploit specific cellular functions at different stages of their life cycle in order to replicate, avoid immune recognition by the host and to establish a successful infection. Cytomegalovirus (CMV)-infected cells are characterized by a prominent cytoplasmic inclusion (virus assembly compartment [vAC]) that is the site of virus maturation and envelopment. While endocytic membranes are known to be the functional components of vAC, knowledge of specific endocytic pathways implicated in CMV maturation and envelopment is lacking. We show here that dynamin, which is an integral part of host endocytic machinery, is largely dispensable for early stages of CMV infection but is required at a late stage of CMV maturation. Studies on dynamin function in CMV infection will help us understand the host-virus interaction pathways amenable to targeting by conventional small molecules, as well as by newer generation nucleotide-based therapeutics (e.g., small interfering RNA, CRISPR/CAS gRNA, etc.).

The Calcineurin-Binding, Activity-Dependent Splice Variant Dynamin1xb Is Highly Enriched in Synapses in Various Regions of the Central Nervous System

In the present study, we generated and characterized a splice site-specific monoclonal antibody that selectively detects the calcineurin-binding dynamin1 splice variant dynamin1xb. Calcineurin is a Ca²⁺-regulated phosphatase that enhances dynamin1 activity and is an important Ca²⁺-sensing mediator of homeostatic synaptic plasticity in neurons. Using this dynamin1xb-specific antibody, we found dynamin1xb highly enriched in synapses of all analyzed brain regions. In photoreceptor ribbon synapses, dynamin1xb was enriched in close vicinity to the synaptic ribbon in a manner indicative of a peri-active zone immunolabeling. Interestingly, in dark-adapted mice we observed an enhanced and selective enrichment of dynamin1xb in both synaptic layers of the retina in comparison to light-adapted mice. This could be due to an illumination-dependent recruitment of dynamin1xb to retinal synapses and/or due to a darkness-induced increase of dynamin1xb biosynthesis. These latter findings indicate that dynamin1xb is part of a versatile and highly adjustable, activity-regulated endocytic synaptic machinery.

Hypothermia Identifies Dynamin as a Potential Therapeutic Target in Experimental Stroke

Apoptosis is a cell death pathway that is activated in ischemic stroke. The interaction between Fas and its ligand (FasL) initiates a complex pattern of intracellular events involving the recruitment of specific adaptor proteins and the development of apoptosis. We recently reported that dynamin is increased after experimental stroke, and its inhibition improves neurological outcome. Dynamin has been shown to transport Fas from the endoplasmic reticulum to the cell surface where it can be bound by its ligand, FasL. Hypothermia has been shown to improve outcome in numerous stroke models, and this protection is associated with reduced apoptosis and Fas expression. To explore the contribution of dynamin to hypothermic neuroprotection, we subjected mice to distal middle cerebral artery occlusion (dMCAO) and applied one of two cooling paradigms: one where cooling began at the onset of dMCAO (early hypothermia) and another where cooling began 1 hour later (delayed hypothermia), compared with normothermia (Norm). Both cooling paradigms reduced numbers of apoptotic cells, as well as Fas and dynamin compared with Norm. Fas and dynamin were co-expressed in neurons. Neuronal cultures were exposed to oxygen glucose deprivation. Hypothermia decreased dynamin as well as surface expression of Fas, and this correlated to reduced cell death. The results of this study suggest that dynamin may participate in the Fas-mediated apoptotic pathway, and its reduction may be linked to hypothermic neuroprotection.

Ca2+-dependent Focal Exocytosis of Golgi-derived Vesicles Helps Phagocytic Uptake in Macrophages

The role of Golgi apparatus during phagocytic uptake by macrophages has been ruled out in the past. Notably, all such reports were limited to Fcγ receptor-mediated phagocytosis. Here, we unravel a highly devolved mechanism for recruitment of Golgi-derived secretory vesicles during phagosome biogenesis, which was important for uptake of most cargos, except the IgG-coated ones. We report recruitment of mannosidase-II-positive Golgi-derived vesicles during uptake of diverse targets, including latex beads, Escherichia coli, Salmonella typhimurium, and Mycobacterium tuberculosis in human and mouse macrophages. The recruitment of mannosidase-II vesicles was an early event mediated by focal exocytosis and coincided with the recruitment of transferrin receptor, VAMP3, and dynamin-2. Brefeldin A treatment inhibited mannosidase-II recruitment and phagocytic uptake of serum-coated or -uncoated latex beads and E. coli However, consistent with previous studies, brefeldin A treatment did not affect uptake of IgG-coated latex beads. Mechanistically, recruitment of mannosidase-II vesicles during phagocytic uptake required Ca²⁺ from both extra- and intracellular sources apart from PI3K, microtubules, and dynamin-2. Extracellular Ca²⁺ via voltage-gated Ca²⁺ channels established a Ca²⁺-dependent local phosphatidylinositol 1,4,5-trisphosphate gradient, which guides the focal movement of Golgi-derived vesicles to the site of uptake. We confirmed Golgi-derived vesicles recruited during phagocytosis were secretory vesicles as their recruitment was sensitive to depletion of VAMP2 or NCS1, whereas recruitment of the recycling endosome marker VAMP3 was unaffected. Depletion of both VAMP2 and NCS1 individually resulted in the reduced uptake by macrophages. Together, the study provides a previously unprecedented role of Golgi-derived secretory vesicles in phagocytic uptake, the key innate defense function.

Clinical, Electrophysiology, and Pathology Features of Dynamin Centronuclear Myopathy: A Case Report and Review of Literature

Dynamin (DNM2) centronuclear myopathy (CNM) has variable age of onset, distal greater than proximal muscle weakness, ptosis with or without extraocular muscle weakness, and a characteristic muscle biopsy with radial sarcoplasmic strands giving spoke like appearance. The following case report highlights clinical, electrophysiology, and pathology features of a genetic confirmed DNM2 CNM subject. In addition, a review of literature on all genetic confirmed DNM2 CNM cases published in English literature from 2006 to 2016 is presented.

Reprogramming the Dynamin 2 mRNA by Spliceosome-mediated RNA Trans-splicing

Dynamin 2 (DNM2) is a large GTPase, ubiquitously expressed, involved in membrane trafficking and regulation of actin and microtubule cytoskeletons. DNM2 mutations cause autosomal dominant centronuclear myopathy which is a rare congenital myopathy characterized by skeletal muscle weakness and histopathological features including nuclear centralization in absence of regeneration. No curative treatment is currently available for the DNM2-related autosomal dominant centronuclear myopathy. In order to develop therapeutic strategy, we evaluated here the potential of Spliceosome-Mediated RNA Trans-splicing technology to reprogram the Dnm2-mRNA in vitro and in vivo in mice. We show that classical 3′-trans-splicing strategy cannot be considered as accurate therapeutic strategy regarding toxicity of the pre-trans-splicing molecules leading to low rate of trans-splicing in vivo. Thus, we tested alternative strategies devoted to prevent this toxicity and enhance frequency of trans-splicing events. We succeeded to overcome the toxicity through a 5′-trans-splicing strategy which also allows detection of trans-splicing events at mRNA and protein levels in vitro and in vivo. These results suggest that the Spliceosome-Mediated RNA Trans-splicing strategy may be used to reprogram mutated Dnm2-mRNA but highlight the potential toxicity linked to the molecular tools which have to be carefully investigated during preclinical development.

Shear stress mediates exocytosis of functional TRPV4 channels in endothelial cells

Mechanosensitive ion channels are implicated in the biology of touch, pain, hearing and vascular reactivity; however, the identity of these ion channels and the molecular basis of their activation is poorly understood. We previously found that transient receptor potential vanilloid 4 (TRPV4) is a receptor operated ion channel that is sensitised and activated by mechanical stress. Here, we investigated the effects of mechanical stimulation on TRPV4 localisation and activation in native and recombinant TRPV4-expressing cells. We used a combination of total internal reflection fluorescence microscopy, cell surface biotinylation assay and Ca(2+) imaging with laser scanning confocal microscope to show that TRPV4 is expressed in primary vascular endothelial cells and that shear stress sensitises the response of TRPV4 to its agonist, GSK1016790A. The sensitisation was attributed to the recruitment of intracellular pools of TRPV4 to the plasma membrane, through the clathrin and dynamin-mediated exocytosis. The translocation was dependent on ILK/Akt signalling pathway, release of Ca(2+) from intracellular stores and we demonstrated that shear stress stimulated phosphorylation of TRPV4 at tyrosine Y110. Our findings implicate calcium-sensitive TRPV4 translocation in the regulation of endothelial responses to mechanical stimulation.

Regulation of Notch Signaling Through Intracellular Transport

The highly conserved Notch-signaling pathway performs a central role in cell differentiation, survival, and proliferation. A major mechanism by which cells modulate signaling is by controlling the intracellular transport itinerary of Notch. Indeed, Notch removal from the cell surface and its targeting to the lysosome for degradation is one way in which Notch activity is downregulated since it limits receptor exposure to ligand. In contrast, Notch-signaling capacity is maintained through repeated rounds of receptor recycling and redelivery of Notch to the cell surface from endosomal stores. This review discusses the molecular mechanisms by which Notch transit through the endosome is controlled and how various intracellular sorting decisions are thought to impact signaling activity.

HSV-1 Glycoproteins Are Delivered to Virus Assembly Sites Through Dynamin-Dependent Endocytosis

Herpes simplex virus‐1 (HSV‐1) is a large enveloped DNA virus that belongs to the family of Herpesviridae. It has been recently shown that the cytoplasmic membranes that wrap the newly assembled capsids are endocytic compartments derived from the plasma membrane. Here, we show that dynamin‐dependent endocytosis plays a major role in this process. Dominant‐negative dynamin and clathrin adaptor AP180 significantly decrease virus production. Moreover, inhibitors targeting dynamin and clathrin lead to a decreased transport of glycoproteins to cytoplasmic capsids, confirming that glycoproteins are delivered to assembly sites via endocytosis. We also show that certain combinations of glycoproteins colocalize with each other and with the components of clathrin‐dependent and ‐independent endocytosis pathways. Importantly, we demonstrate that the uptake of neutralizing antibodies that bind to glycoproteins when they become exposed on the cell surface during virus particle assembly leads to the production of non‐infectious HSV‐1. Our results demonstrate that transport of viral glycoproteins to the plasma membrane prior to endocytosis is the major route by which these proteins are localized to the cytoplasmic virus assembly compartments. This highlights the importance of endocytosis as a major protein‐sorting event during HSV‐1 envelopment.

Astrocytes spatially restrict VEGF signaling by polarized secretion and incorporation of VEGF into the actively assembling extracellular matrix

The spatial organization of vascular endothelial growth factor (VEGF) signaling is a key determinant of vascular patterning during development and tissue repair. How VEGF signaling becomes spatially restricted and the role of VEGF secreting astrocytes in this process remains poorly understood. Using a VEGF-GFP fusion protein and confocal time-lapse microscopy, we observed the intracellular routing, secretion and immobilization of VEGF in scratch-activated living astrocytes. We found VEGF to be directly transported to cell-extracellular matrix attachments where it is incorporated into fibronectin fibrils. VEGF accumulated at β1 integrin containing fibrillar adhesions and was translocated along the cell surface prior to internalization and degradation. We also found that only the astrocyte-derived, matrix-bound, and not soluble VEGF decreases β1 integrin turnover in fibrillar adhesions. We suggest that polarized VEGF release and ECM remodeling by VEGF secreting cells is key to control the local concentration and signaling of VEGF. Our findings highlight the importance of astrocytes in directing VEGF functions and identify these mechanisms as promising target for angiogenic approaches.

Temporal cAMP Signaling Selectivity by Natural and Synthetic MC4R Agonists

The melanocortin-4 receptor (MC4R) is a G protein-coupled receptor expressed in the brain, where it controls energy balance through pathways including α-melanocyte-stimulating hormone (α-MSH)-dependent signaling. We have reported that the MC4R can exist in an active conformation that signals constitutively by increasing cAMP levels in the absence of receptor desensitization. We asked whether synthetic MC4R agonists differ in their ability to increase intracellular cAMP over time in Neuro2A cells expressing endogenous MC4R and exogenous, epitope-tagged hemagglutinin-MC4R-green fluorescent protein. By analyzing intracellular cAMP in a temporally resolved Förster resonance energy transfer assay, we show that withdrawal of α-MSH leads to a quick reversal of cAMP induction. By contrast, the synthetic agonist melanotan II (MTII) induces a cAMP signal that persists for at least 1 hour after removal of MTII from the medium and cannot be antagonized by agouti related protein. Similarly, in mHypoE-42 immortalized hypothalamic neurons, MTII, but not α-MSH, induced persistent AMP kinase signal, which occurs downstream of increased cAMP. By using a fluorescence recovery after photobleaching assay, it appears that the receptor exposed to MTII continues to signal after being internalized. Similar to MTII, the synthetic MC4R agonists, THIQ and BIM-22511, but not LY2112688, induced prolonged cAMP signaling after agonist withdrawal. However, agonist-exposed MC4R desensitized to the same extent, regardless of the ligand used and regardless of differences in receptor intracellular retention kinetics. In conclusion, α-MSH and LY2112688, when compared with MTII, THIQ, and BIM-22511, vary in the duration of the acute cAMP response, showing distinct temporal signaling selectivity, possibly linked to specific cell compartments from which cAMP signals may originate.

Dynamin 2 regulates biphasic insulin secretion and plasma glucose homeostasis

Alterations in insulin granule exocytosis and endocytosis are paramount to pancreatic β cell dysfunction in diabetes mellitus. Here, using temporally controlled gene ablation specifically in β cells in mice, we identified an essential role of dynamin 2 GTPase in preserving normal biphasic insulin secretion and blood glucose homeostasis. Dynamin 2 deletion in β cells caused glucose intolerance and substantial reduction of the second phase of glucose-stimulated insulin secretion (GSIS); however, mutant β cells still maintained abundant insulin granules, with no signs of cell surface expansion. Compared with control β cells, real-time capacitance measurements demonstrated that exocytosis-endocytosis coupling was less efficient but not abolished; clathrin-mediated endocytosis (CME) was severely impaired at the step of membrane fission, which resulted in accumulation of clathrin-coated endocytic intermediates on the plasma membrane. Moreover, dynamin 2 ablation in β cells led to striking reorganization and enhancement of actin filaments, and insulin granule recruitment and mobilization were impaired at the later stage of GSIS. Together, our results demonstrate that dynamin 2 regulates insulin secretory capacity and dynamics in vivo through a mechanism depending on CME and F-actin remodeling. Moreover, this study indicates a potential pathophysiological link between endocytosis and diabetes mellitus.

Extensive in silico analysis of Mimivirus coded Rab GTPase homolog suggests a possible role in virion membrane biogenesis

Rab GTPases are the key regulators of intracellular membrane trafficking in eukaryotes. Many viruses and intracellular bacterial pathogens have evolved to hijack the host Rab GTPase functions, mainly through activators and effector proteins, for their benefit. Acanthamoeba polyphaga mimivirus (APMV) is one of the largest viruses and belongs to the monophyletic clade of nucleo-cytoplasmic large DNA viruses (NCLDV). The inner membrane lining is integral to the APMV virion structure. APMV assembly involves extensive host membrane modifications, like vesicle budding and fusion, leading to the formation of a membrane sheet that is incorporated into the virion. Intriguingly, APMV and all group I members of the Mimiviridae family code for a putative Rab GTPase protein. APMV is the first reported virus to code for a Rab GTPase (encoded by R214 gene). Our thorough in silico analysis of the subfamily specific (SF) region of Mimiviridae Rab GTPase sequences suggests that they are related to Rab5, a member of the group II Rab GTPases, of lower eukaryotes. Because of their high divergence from the existing three isoforms, A, B, and C of the Rab5-family, we suggest that Mimiviridae Rabs constitute a new isoform, Rab5D. Phylogenetic analysis indicated probable horizontal acquisition from a lower eukaryotic ancestor followed by selection and divergence. Furthermore, interaction network analysis suggests that vps34 (a Class III PI3K homolog, coded by APMV L615), Atg-8 and dynamin (host proteins) are recruited by APMV Rab GTPase during capsid assembly. Based on these observations, we hypothesize that APMV Rab plays a role in the acquisition of inner membrane during virion assembly.

The miR-199-dynamin regulatory axis controls receptor-mediated endocytosis

Small non-coding RNAs (microRNAs) are important regulators of gene expression that modulate many physiological processes; however, their role in regulating intracellular transport remains largely unknown. Intriguingly, we found that the dynamin (DNM) genes, a GTPase family of proteins responsible for endocytosis in eukaryotic cells, encode the conserved miR-199a and miR-199b family of miRNAs within their intronic sequences. Here, we demonstrate that miR-199a and miR-199b regulate endocytic transport by controlling the expression of important mediators of endocytosis such as clathrin heavy chain (CLTC), Rab5A, low-density lipoprotein receptor (LDLR) and caveolin-1 (Cav-1). Importantly, miR-199a-5p and miR-199b-5p overexpression markedly inhibits CLTC, Rab5A, LDLR and Cav-1 expression, thus preventing receptor-mediated endocytosis in human cell lines (Huh7 and HeLa). Of note, miR-199a-5p inhibition increases target gene expression and receptor-mediated endocytosis. Taken together, our work identifies a new mechanism by which microRNAs regulate intracellular trafficking. In particular, we demonstrate that the DNM, miR-199a-5p and miR-199b-5p genes act as a bifunctional locus that regulates endocytosis, thus adding an unexpected layer of complexity in the regulation of intracellular trafficking.

Dynasore - not just a dynamin inhibitor

Dynamin is a GTPase protein that is essential for membrane fission during clathrin-mediated endocytosis in eukaryotic cells. Dynasore is a GTPase inhibitor that rapidly and reversibly inhibits dynamin activity, which prevents endocytosis. However, comparison between cells treated with dynasore and RNA interference of genes encoding dynamin, reveals evidence that dynasore reduces labile cholesterol in the plasma membrane, and disrupts lipid raft organization, in a dynamin-independent manner. To explore the role of dynamin it is important to use multiple dynamin inhibitors, alongside the use of dynamin mutants and RNA interference targeting genes encoding dynamin. On the other hand, dynasore provides an interesting tool to explore the regulation of cholesterol in plasma membranes.

Journey to the cell surface--the central role of the trans-Golgi network in plants

The secretion of proteins, lipids, and carbohydrates to the cell surface is essential for plant development and adaptation. Secreted substances synthesized at the endoplasmic reticulum pass through the Golgi apparatus and trans-Golgi network (TGN) en route to the plasma membrane via the conventional secretion pathway. The TGN is morphologically and functionally distinct from the Golgi apparatus. The TGN is located at the crossroads of many trafficking pathways and regulates a range of crucial processes including secretion to the cell surface, transport to the vacuole, and the reception of endocytic cargo. This review outlines the TGN's central role in cargo secretion, showing that its behavior is more complex and controlled than the bulk-flow hypothesis suggests. Its formation, structure, and maintenance are discussed along with the formation and release of secretory vesicles.

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.

Dynamin 2-dependent endocytosis is required for normal megakaryocyte development in mice

Dynamins are highly conserved large GTPases (enzymes that hydrolyze guanosine triphosphate) involved in endocytosis and vesicle transport, and mutations in the ubiquitous and housekeeping dynamin 2 (DNM2) have been associated with thrombocytopenia in humans. To determine the role of DNM2 in thrombopoiesis, we generated Dnm2(fl/fl) Pf4-Cre mice specifically lacking DNM2 in the megakaryocyte (MK) lineage. Dnm2(fl/fl) Pf4-Cre mice had severe macrothrombocytopenia with moderately accelerated platelet clearance. Dnm2-null bone marrow MKs had altered demarcation membrane system formation in vivo due to defective endocytic pathway, and fetal liver-derived Dnm2-null MKs formed proplatelets poorly in vitro, showing that DNM2-dependent endocytosis plays a major role in MK membrane formation and thrombopoiesis. Endocytosis of the thrombopoietin receptor Mpl was impaired in Dnm2-null platelets, causing constitutive phosphorylation of the tyrosine kinase JAK2 and elevated circulating thrombopoietin levels. MK-specific DNM2 deletion severely disrupted bone marrow homeostasis, as reflected by marked expansion of hematopoietic stem and progenitor cells, MK hyperplasia, myelofibrosis, and consequent extramedullary hematopoiesis and splenomegaly. Taken together, our data demonstrate that unrestrained MK growth and proliferation results in rapid myelofibrosis and establishes a previously unrecognized role for DNM2-dependent endocytosis in megakaryopoiesis, thrombopoiesis, and bone marrow homeostasis.

Pharmacological inhibition of dynamin II reduces constitutive protein secretion from primary human macrophages

Dynamins are fission proteins that mediate endocytic and exocytic membrane events and are pharmacological therapeutic targets. These studies investigate whether dynamin II regulates constitutive protein secretion and show for the first time that pharmacological inhibition of dynamin decreases secretion of apolipoprotein E (apoE) and several other proteins constitutively secreted from primary human macrophages. Inhibitors that target recruitment of dynamin to membranes (MiTMABs) or directly target the GTPase domain (Dyngo or Dynole series), dose- and time- dependently reduced the secretion of apoE. SiRNA oligo’s targeting all isoforms of dynamin II confirmed the involvement of dynamin II in apoE secretion. Inhibition of secretion was not mediated via effects on mRNA or protein synthesis. 2D-gel electrophoresis showed that inhibition occurred after apoE was processed and glycosylated in the Golgi and live cell imaging showed that inhibited secretion was associated with reduced post-Golgi movement of apoE-GFP-containing vesicles. The effect was not restricted to macrophages, and was not mediated by the effects of the inhibitors on microtubules. Inhibition of dynamin also altered the constitutive secretion of other proteins, decreasing the secretion of fibronectin, matrix metalloproteinase 9, Chitinase-3-like protein 1 and lysozyme but unexpectedly increasing the secretion of the inflammatory mediator cyclophilin A. We conclude that pharmacological inhibitors of dynamin II modulate the constitutive secretion of macrophage apoE as a class effect, and that their capacity to modulate protein secretion may affect a range of biological processes.

Dynamin-mediated lipid acquisition is essential for Chlamydia trachomatis development

Chlamydia trachomatis is an obligate intracellular pathogen responsible for a high burden of human disease. Here, a loss-of-function screen using a set of lentivirally transduced shRNAs identified 14 human host cell factors that modulate C. trachomatis infectivity. Notably, knockdown of dynamin, a host GTPase, decreased C. trachomatis infectivity. Dynamin functions in multiple cytoplasmic locations, including vesicle formation at the plasma membrane and the trans-Golgi network. However, its role in C. trachomatis infection remains unclear. Here we report that dynamin is essential for homotypic fusion of C. trachomatis inclusions but not for C. trachomatis internalization into the host cell. Further, dynamin activity is necessary for lipid transport into C. trachomatis inclusions and for normal re-differentiation from reticulate to elementary bodies. Fragmentation of the Golgi apparatus is proposed to be an important strategy used by C. trachomatis for efficient lipid acquisition and replication within the host. Here we show that a subset of C. trachomatis-infected cells displayed Golgi fragmentation, which was concurrent with increased mitotic accumulation. Golgi fragmentation was dispensable for dynamin-mediated lipid acquisition into C. trachomatis inclusions, irrespective of the cell cycle phase. Thus, our study reveals a critical role of dynamin in host-derived lipid acquisition for C. trachomatis development.

Four-dimensional live imaging of apical biosynthetic trafficking reveals a post-Golgi sorting role of apical endosomal intermediates

Emerging data suggest that in polarized epithelial cells newly synthesized apical and basolateral plasma membrane proteins traffic through different endosomal compartments en route to the respective cell surface. However, direct evidence for trans-endosomal pathways of plasma membrane proteins is still missing and the mechanisms involved are poorly understood. Here, we imaged the entire biosynthetic route of rhodopsin-GFP, an apical marker in epithelial cells, synchronized through recombinant conditional aggregation domains, in live Madin-Darby canine kidney cells using spinning disk confocal microscopy. Our experiments directly demonstrate that rhodopsin-GFP traffics through apical recycling endosomes (AREs) that bear the small GTPase Rab11a before arriving at the apical membrane. Expression of dominant-negative Rab11a drastically reduced apical delivery of rhodopsin-GFP and caused its missorting to the basolateral membrane. Surprisingly, functional inhibition of dynamin-2 trapped rhodopsin-GFP at AREs and caused aberrant accumulation of coated vesicles on AREs, suggesting a previously unrecognized role for dynamin-2 in the scission of apical carrier vesicles from AREs. A second set of experiments, using a unique method to carry out total internal reflection fluorescence microscopy (TIRFM) from the apical side, allowed us to visualize the fusion of rhodopsin-GFP carrier vesicles, which occurred randomly all over the apical plasma membrane. Furthermore, two-color TIRFM showed that Rab11a-mCherry was present in rhodopsin-GFP carrier vesicles and was rapidly released upon fusion onset. Our results provide direct evidence for a role of AREs as a post-Golgi sorting hub in the biosynthetic route of polarized epithelia, with Rab11a regulating cargo sorting at AREs and carrier vesicle docking at the apical membrane.

Reducing dynamin 2 expression rescues X-linked centronuclear myopathy

Centronuclear myopathies (CNM) are congenital disorders associated with muscle weakness and abnormally located nuclei in skeletal muscle. An autosomal dominant form of CNM results from mutations in the gene encoding dynamin 2 (DNM2), and loss-of-function mutations in the gene encoding myotubularin (MTM1) result in X-linked CNM (XLCNM, also called myotubular myopathy), which promotes severe neonatal hypotonia and early death. Currently, no effective treatments exist for XLCNM. Here, we found increased DNM2 levels in XLCNM patients and a mouse model of XLCNM (Mtm1(-/y)). Generation of Mtm1(-/y) mice that were heterozygous for Dnm2 revealed that reduction of DNM2 in XLCNM mice restored life span, whole-body strength, and diaphragm function and increased muscle strength. Additionally, classic CNM-associated histological features, including fiber atrophy and nuclei mispositioning, were absent or reduced. Ultrastructural analysis revealed improvement of sarcomere organization and triad structures. Skeletal muscle-specific decrease of Dnm2 during embryogenesis or in young mice after disease onset revealed that the rescue associated with downregulation of Dnm2 is cell autonomous and is able to stop and potentially revert XLCNM progression. These data indicate that MTM1 and DNM2 regulate muscle organization and force through a common pathway. Furthermore, despite DNM2 being a key mechanoenzyme, its reduction is beneficial for XLCNM and represents a potential therapeutic approach for patients.

Lipid droplet breakdown requires dynamin 2 for vesiculation of autolysosomal tubules in hepatocytes

Dynamin 2 is required for starvation-mediated breakdown of lipid droplets in hepatocytes by promoting vesiculation of autolysosomal tubules to release protolysosomes.

Lipid droplets (LDs) are lipid storage organelles that in hepatocytes may be catabolized by autophagy for use as an energy source, but the membrane-trafficking machinery regulating such a process is poorly characterized. We hypothesized that the large GTPase Dynamin 2 (Dyn2), well known for its involvement in membrane deformation and cellular protein trafficking, could orchestrate autophagy-mediated LD breakdown. Accordingly, depletion or pharmacologic inhibition of Dyn2 led to a substantial accumulation of LDs in hepatocytes. Strikingly, the targeted disruption of Dyn2 induced a dramatic four- to fivefold increase in the size of autolysosomes. Chronic or acute Dyn2 inhibition combined with nutrient deprivation stimulated the excessive tubulation of these autolysosomal compartments. Importantly, Dyn2 associated with these tubules along their length, and the tubules vesiculated and fragmented in the presence of functional Dyn2. These findings provide new evidence for the participation of the autolysosome in LD metabolism and demonstrate a novel role for dynamin in the function and maturation of an autophagic compartment.

The myopathy-causing mutation DNM2-S619L leads to defective tubulation in vitro and in developing zebrafish

DNM2 is a ubiquitously expressed GTPase that regulates multiple subcellular processes. Mutations in DNM2 are a common cause of centronuclear myopathy, a severe disorder characterized by altered skeletal muscle structure and function. The precise mechanisms underlying disease-associated DNM2 mutations are unresolved. We examined the common DNM2-S619L mutation using both in vitro and in vivo approaches. Expression of DNM2-S619L in zebrafish led to the accumulation of aberrant vesicular structures and to defective excitation-contraction coupling. Expression of DNM2-S619L in COS7 cells resulted in defective BIN1-dependent tubule formation. These data suggest that DNM2-S619L causes disease, in part, by interfering with membrane tubulation.

Dynamin-2 function and dysfunction along the secretory pathway

Dynamin-2 is a ubiquitously expressed mechano-GTPase involved in different stages of the secretory pathway. Its most well-known function relates to the scission of nascent vesicles from the plasma membrane during endocytosis; however, it also participates in the formation of new vesicles from the Golgi network, vesicle trafficking, fusion processes and in the regulation of microtubule, and actin cytoskeleton dynamics. Over the last 8 years, more than 20 mutations in the dynamin-2 gene have been associated to two hereditary neuromuscular disorders: Charcot-Marie-Tooth neuropathy and centronuclear myopathy. Most of these mutations are grouped in the pleckstrin homology domain; however, there are no common mutations associated with both disorders, suggesting that they differently impact on dynamin-2 function in diverse tissues. In this review, we discuss the impact of these disease-related mutations on dynamin-2 function during vesicle trafficking and endocytotic processes.