Carcinogenic Effects in a Phenylketonuria Mouse Model

Phenylketonuria (PKU) is a metabolic disorder caused by impaired phenylalanine hydroxylase (PAH). This condition results in hyperphenylalaninemia and elevated levels of abnormal phenylalanine metabolites, among which is phenylacetic acid/phenylacetate (PA). In recent years, PA and its analogs were found to have anticancer activity against a variety of malignancies suggesting the possibility that PKU may offer protection against cancer through chronically elevated levels of PA. We tested this hypothesis in a genetic mouse model of PKU (PAH^enu2) which has a biochemical profile that closely resembles that of human PKU. Plasma levels of phenylalanine in homozygous (HMZ) PAH^enu2 mice were >12-fold those of heterozygous (HTZ) littermates while tyrosine levels were reduced. Phenylketones, including PA, were also markedly elevated to the range seen in the human disease. Mice were subjected to 7,12 dimethylbenz[a]anthracene (DMBA) carcinogenesis, a model which is sensitive to the anticancer effects of the PA derivative 4-chlorophenylacetate (4-CPA). Tumor induction by DMBA was not significantly different between the HTZ and HMZ mice, either in total tumor development or in the type of cancers that arose. HMZ mice were then treated with 4-CPA as positive controls for the anticancer effects of PA and to evaluate its possible effects on phenylalanine metabolism in PKU mice. 4-CPA had no effect on the plasma concentrations of phenylalanine, phenylketones, or tyrosine. Surprisingly, the HMZ mice treated with 4-CPA developed an unexplained neuromuscular syndrome which precluded its use in these animals as an anticancer agent. Together, these studies support the use of PAH^enu2 mice as a model for studying human PKU. Chronically elevated levels of PA in the PAH^enu2 mice were not protective against cancer.     

Independent Neuronal Origin of Seizures and Behavioral Comorbidities in an Animal Model of a Severe Childhood Genetic Epileptic Encephalopathy

The childhood epileptic encephalopathies (EE’s) are seizure disorders that broadly impact development including cognitive, sensory and motor progress with severe consequences and comorbidities. Recently, mutations in DNM1 (dynamin 1) have been implicated in two EE syndromes, Lennox-Gastaut Syndrome and Infantile Spasms. Dnm1 encodes dynamin 1, a large multimeric GTPase necessary for activity-dependent membrane recycling in neurons, including synaptic vesicle endocytosis. Dnm1^Ftfl or “fitful” mice carry a spontaneous mutation in the mouse ortholog of DNM1 and recapitulate many of the disease features associated with human DNM1 patients, providing a relevant disease model of human EE’s. In order to examine the cellular etiology of seizures and behavioral and neurological comorbidities, we engineered a conditional Dnm1^Ftfl mouse model of DNM1 EE. Observations of Dnm1 ^Ftfl/flox mice in combination with various neuronal subpopulation specific cre strains demonstrate unique seizure phenotypes and clear separation of major neurobehavioral comorbidities from severe seizures associated with the germline model. This demonstration of pleiotropy suggests that treating seizures per se may not prevent severe comorbidity observed in EE associated with dynamin-1 mutations, and is likely to have implications for other genetic forms of EE.     

Two Dynamin-2 Genes Are Required for Normal Zebrafish Development

Dynamin-2 (DNM2) is a large GTPase involved in clathrin-mediated endocytosis and related trafficking pathways. Mutations in human DNM2 cause two distinct neuromuscular disorders: centronuclear myopathy and Charcot-Marie-Tooth disease. Zebrafish have been shown to be an excellent animal model for many neurologic disorders, and this system has the potential to inform our understanding of DNM2-related disease. Currently, little is known about the endogenous zebrafish orthologs to human DNM2. In this study, we characterize two zebrafish dynamin-2 genes, dnm2 and dnm2-like. Both orthologs are structurally similar to human DNM2 at the gene and protein levels. They are expressed throughout early development and in all adult tissues examined. Knockdown of dnm2 and dnm2-like gene products resulted in extensive morphological abnormalities during development, and expression of human DNM2 RNA rescued these phenotypes. Our findings suggest that dnm2 and dnm2-like are orthologs to human DNM2, and that they are required for normal zebrafish development.     

Peroxisome proliferation in Hansenula polymorpha requires Dnm1p which mediates fission but not de novo formation

We show that the dynamin-like proteins Dnm1p and Vps1p are not required for re-introduction of peroxisomes in Hansenula polymorpha pex3 cells upon complementation with PEX3-GFP. Instead, Dnm1p, but not Vps1p, plays a crucial role in organelle proliferation via fission. In H. polymorpha DNM1 deletion cells (dnm1) a single peroxisome is present that forms long extensions, which protrude into developing buds and divide during cytokinesis. Budding pex11.dnm1 double deletion cells lack these peroxisomal extensions, suggesting that the peroxisomal membrane protein Pex11p is required for their formation. Life cell imaging revealed that fluorescent Dnm1p-GFP spots fluctuate between peroxisomes and mitochondria. On the other hand Pex11p is present over the entire organelle surface, but concentrates during fission at the basis of the organelle extension in dnm1 cells.Our data indicate that peroxisome fission is the major pathway for peroxisome multiplication in H. polymorpha.     

Disease model: LAMP-2 enlightens Danon disease.

Danon disease ('lysosomal glycogen storage disease with normal acid maltase') is characterized by a cardiomyopathy, myopathy and variable mental retardation. Mutations in the coding sequence of the lysosomal-associated membrane protein 2 (LAMP-2) were shown to cause a LAMP-2 deficiency in patients with Danon disease. LAMP-2 deficient mice manifest a similar vacuolar cardioskeletal myopathy. In addition to the patient reports LAMP-2 deficiency in mice causes pancreatic, hepatocytic, endothelial and leucocyte vacuolation. LAMP-2 deficient mice represent a valuable animal model of Danon disease. They will further be used to study the exact role of LAMP-2 in autophagy and to analyse the consequences of an impaired autophagic pathway in various tissues.     

Defective Autophagy and mTORC1 Signaling in Myotubularin Null Mice

Autophagy is a vesicular trafficking pathway that regulates the degradation of aggregated proteins and damaged organelles. Initiation of autophagy requires several multiprotein signaling complexes, such as the ULK1 kinase complex and the Vps34 lipid kinase complex, which generates phosphatidylinositol 3-phosphate [PtdIns(3)P] on the forming autophagosomal membrane. Alterations in autophagy have been reported for various diseases, including myopathies. Here we show that skeletal muscle autophagy is compromised in mice deficient in the X-linked myotubular myopathy (XLMTM)-associated PtdIns(3)P phosphatase myotubularin (MTM1). Mtm1-deficient muscle displays several cellular abnormalities, including a profound increase in ubiquitin aggregates and abnormal mitochondria. Further, we show that Mtm1 deficiency is accompanied by activation of mTORC1 signaling, which persists even following starvation. In vivo pharmacological inhibition of mTOR is sufficient to normalize aberrant autophagy and improve muscle phenotypes in Mtm1 null mice. These results suggest that aberrant mTORC1 signaling and impaired autophagy are consequences of the loss of Mtm1 and may play a primary role in disease pathogenesis.     

Gag3p, an outer membrane protein required for fission of mitochondrial tubules

Mitochondrial morphology and function depend on MGM1, a Saccharomyces cerevisiae gene encoding a dynamin-like protein of the mitochondrial outer membrane. Here, we show that mitochondrial fragmentation and mitochondrial genome loss caused by lesions in MGM1 are suppressed by three novel mutations, gag1, gag2, and gag3 (for glycerol-adapted growth). Cells with any of the gag mutations displayed aberrant mitochondrial morphology characterized by elongated, unbranched tubes and highly fenestrated structures. Additionally, each of the gag mutations prevented mitochondrial fragmentation caused by loss of the mitochondrial fusion factor, Fzo1p, or by treatment of cells with sodium azide. The gag1 mutation mapped to DNM1 that encodes a dynamin-related protein required for mitochondrial fission. GAG3 encodes a novel WD40-repeat protein previously found to interact with Dnm1p in a two-hybrid assay. Gag3p was localized to mitochondria where it was found to associate as a peripheral protein on the cytosolic face of the outer membrane. This association requires neither the DNM1 nor GAG2 gene products. However, the localization of Dnm1p to the mitochondrial outer membrane is substantially reduced by the gag2 mutation, but unaffected by loss of Gag3p. These results indicate that Gag3p plays a distinct role on the mitochondrial surface to mediate the fission of mitochondrial tubules.     

Inhibition of DNM1L and mitochondrial fission attenuates inflammatory response in fibroblast‐like synoviocytes of rheumatoid arthritis

Mitochondrial fission and fusion are important for mitochondrial function, and dynamin 1‐like protein (DNM1L) is a key regulator of mitochondrial fission. We investigated the effect of mitochondrial fission on mitochondrial function and inflammation in fibroblast‐like synoviocytes (FLSs) during rheumatoid arthritis (RA). DNM1L expression was determined in synovial tissues (STs) from RA and non‐RA patients. FLSs were isolated from STs and treated with a DNM1L inhibitor (mdivi‐1, mitochondrial division inhibitor 1) or transfected with DNM1L‐specific siRNA. Mitochondrial morphology, DNM1L expression, cell viability, mitochondrial membrane potential, reactive oxygen species (ROS), apoptosis, inflammatory cytokine expression and autophagy were examined. The impact of mdivi‐1 treatment on development and severity of collagen‐induced arthritis (CIA) was determined in mice. Up‐regulated DNM1L expression was associated with reduced mitochondrial length in STs from patients with RA and increased RA severity. Inhibition of DNM1L in FLSs triggered mitochondrial depolarization, mitochondrial elongation, decreased cell viability, production of ROS, IL‐8 and COX‐2, and increased apoptosis. DNM1L deficiency inhibited IL‐1β–mediated AKT/IKK activation, NF‐κBp65 nuclear translocation and LC3B‐related autophagy, but enhanced NFKBIA expression. Treatment of CIA mice with mdivi‐1 decreased disease severity by modulating inflammatory cytokine and ROS production. Our major results are that up‐regulated DNM1L and mitochondrial fission promoted survival, LC3B‐related autophagy and ROS production in FLSs, factors that lead to inflammation by regulating AKT/IKK/NFKBIA/NF‐κB signalling. Thus, inhibition of DNM1L may be a new strategy for treatment of RA.     

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.KEY WORDS: Dynamin-2, Excitation-contraction coupling, Myopathy     

Aberrant dynamin 2‐dependent Na+/H+ exchanger‐1 trafficking contributes to cardiomyocyte apoptosis

Sarcolemmal Na⁺/H⁺ exchanger 1 (NHE1) activity is essential for the intracellular pH (pH_i) homeostasis in cardiac myocytes. Emerging evidence indicates that sarcolemmal NHE1 dysfunction was closely related to cardiomyocyte death, but it remains unclear whether defective trafficking of NHE1 plays a role in the vital cellular signalling processes. Dynamin (DNM), a large guanosine triphosphatase (GTPase), is best known for its roles in membrane trafficking events. Herein, using co‐immunoprecipitation, cell surface biotinylation and confocal microscopy techniques, we investigated the potential regulation on cardiac NHE1 activity by DNM. We identified that DNM2, a cardiac isoform of DNM, directly binds to NHE1. Overexpression of a wild‐type DNM2 or a dominant‐negative DNM2 mutant with defective GTPase activity in adult rat ventricular myocytes (ARVMs) facilitated or retarded the internalization of sarcolemmal NHE1, whereby reducing or increasing its activity respectively. Importantly, the increased NHE1 activity associated with DNM2 deficiency led to ARVMs apoptosis, as demonstrated by cell viability, terminal deoxynucleotidyl transferase–mediated dUTP nick‐end labelling assay, Bcl‐1/Bax expression and caspase‐3 activity, which were effectively rescued by pharmacological inhibition of NHE1 with zoniporide. Thus, our results demonstrate that disruption of the DNM2‐dependent retrograde trafficking of NHE1 contributes to cardiomyocyte apoptosis.     

DNM1, a dynamin-related gene, participates in endosomal trafficking in yeast

We identified DNM1, a novel dynamin-related gene in Saccharomyces cerevisiae. Molecular and genetic mapping showed that DNM1 is the most proximal gene to the right of centromere 12, and is predicted to encode a protein of 85 kD, designated Dnm1p. The protein exhibits 41% overall identity with full-length dynamin I and 55% identity with the most highly conserved 400-amino acid GTPase region. Our findings show that like mammalian dynamin, Dnm1p participates in endocytosis; however, it is unlikely to be a cognate homologue. Cells with a disruption in the DNM1 gene showed mating response defects consistent with a delay in receptor-mediated endocytosis. The half-life of the Ste3p pheromone receptor was increased two- to threefold in the dnm1 mutant, demonstrating that Dnm1p participates in the constitutive turnover of the receptor. To define the step in the endocytic pathway at which Dnm1p acts, we analyzed mutant strains at both early and late steps of the process. Initial internalization of epitope-tagged pheromone receptor or of labeled pheromone proceeded with wild-type kinetics. However, delivery of the internalized receptor to the vacuole was greatly impeded during ligand-induced endocytosis. These data suggest that during receptor-mediated endocytosis, Dnm1p acts after internalization, but before fusion with the vacuole. The dnm1 mutant was not defective for sorting of vacuolar proteins, indicating that Dnm1p is not required for transport from the late endosome to the vacuole. Therefore, we suggest that Dnm1p participates at a novel step before fusion with the late endosome.     

PERK EIF2AK3 control of pancreatic beta cell differentiation and proliferation is required for postnatal glucose homeostasis

Mutations in PERK (EIF2AK3) result in permanent neonatal diabetes as well as several other anomalies that underlie the human Wolcott-Rallison syndrome, and these anomalies are mirrored in Perk knockout mice. To identify the cause of diabetes in PERK-deficient mice, we generated a series of tissue- and cell-specific knockouts of the Perk gene and performed a developmental analysis of the progression to overt diabetes. We discovered that PERK is specifically required in the insulin-secreting β cells during the fetal and early neonatal period as a prerequisite for postnatal glucose homeostasis. However, PERK expression in β cells is not required at the adult stage to maintain β cell functions and glucose homeostasis. We show that PERK-deficient mice exhibit severe defects in fetal/neonatal β cell proliferation and differentiation, resulting in low β cell mass, defects in proinsulin trafficking, and abrogation of insulin secretion that culminate in permanent neonatal diabetes.     

Scission,a critical step in autophagosome formation

ABSTRACT

A key feature of macroautophagy (hereafter autophagy) is the formation of the phagophore, a double-membrane compartment sequestering cargos and finally maturing into a vesicle termed an autophagosome; however, where these membranes originate from is not clear. In a previous study, researchers from the Rubinsztein lab proposed a model in which the autophagosome can evolve from the RAB11A-positive recycling endosome. In their recent paper, they determine that DNM2 (dynamin 2) functions in scission of the recycling endosome, and the release of the autophagosome precursor. These findings explain how the centronuclear myopathy (CNM) mutation in DNM2 results in the accumulation of immature autophagic structures.     

Regulation of ER-phagy by a Ypt/Rab GTPase module

Accumulation of misfolded proteins on intracellular membranes has been implicated in neurodegenerative diseases. One cellular pathway that clears such aggregates is endoplasmic reticulum autophagy (ER-phagy), a selective autophagy pathway that delivers excess ER to the lysosome for degradation. Not much is known about the regulation of ER-phagy. The conserved Ypt/Rab GTPases regulate all membrane trafficking events in eukaryotic cells. We recently showed that a Ypt module, consisting of Ypt1 and autophagy-specific upstream activator and downstream effector, regulates the onset of selective autophagy in yeast. Here we show that this module acts at the ER. Autophagy-specific mutations in its components cause accumulation of excess membrane proteins on aberrant ER structures and induction of ER stress. This accumulation is due to a block in transport of these membranes to the lysosome, where they are normally cleared. These findings establish a role for an autophagy-specific Ypt1 module in the regulation of ER-phagy. Moreover, because Ypt1 is a known key regulator of ER-to-Golgi transport, these findings establish a second role for Ypt1 at the ER. We therefore propose that individual Ypt/Rabs, in the context of distinct modules, can coordinate alternative trafficking steps from one cellular compartment to different destinations.     

The chlamydial deubiquitinase Cdu1 supports recruitment of Golgi vesicles to the inclusion

Chlamydia trachomatis is the main cause of sexually transmitted diseases worldwide. As obligate intracellular bacteria Chlamydia replicate in a membrane bound vacuole called inclusion and acquire nutrients for growth and replication from their host cells. However, like all intracellular bacteria, Chlamydia have to prevent eradication by the host's cell autonomous system. The chlamydial deubiquitinase Cdu1 is secreted into the inclusion membrane, facing the host cell cytosol where it deubiquitinates cellular proteins. Here we show that inactivation of Cdu1 causes a growth defect of C. trachomatis in primary cells. Moreover, ubiquitin and several autophagy receptors are recruited to the inclusion membrane of Cdu1‐deficient Chlamydia. Interestingly, the growth defect of cdu1 mutants is not rescued when autophagy is prevented. We find reduced recruitment of Golgi vesicles to the inclusion of Cdu1 mutants indicating that vesicular trafficking is altered in bacteria without active deubiquitinase (DUB). Our work elucidates an important role of Cdu1 in the functional preservation of the chlamydial inclusion surface.     

The Dynamin-related GTPase,Dnm1p,Controls Mitochondrial Morphology in Yeast

The Saccharomyces cerevisiae Dnm1 protein is structurally related to dynamin, a GTPase required for membrane scission during endocytosis. Here we show that Dnm1p is essential for the maintenance of mitochondrial morphology. Disruption of the DNM1 gene causes the wild-type network of tubular mitochondrial membranes to collapse to one side of the cell but does not affect the morphology or distribution of other cytoplasmic organelles. Dnm1 proteins containing point mutations in the predicted GTP-binding domain or completely lacking the GTP-binding domain fail to rescue mitochondrial morphology defects in a dnm1 mutant and induce dominant mitochondrial morphology defects in wild-type cells. Indirect immunofluorescence reveals that Dnm1p is distributed in punctate structures at the cell cortex that colocalize with the mitochondrial compartment. These Dnm1p-containing structures remain associated with the spherical mitochondria found in an mdm10 mutant strain. In addition, a portion of Dnm1p cofractionates with mitochondrial membranes during differential sedimentation and sucrose gradient fractionation of wild-type cells. Our results demonstrate that Dnm1p is required for the cortical distribution of the mitochondrial network in yeast, a novel function for a dynamin-related protein.     

Phosphoinositide regulation of integrin trafficking required for muscle attachment and maintenance

Muscles must maintain cell compartmentalization when remodeled during development and use. How spatially restricted adhesions are regulated with muscle remodeling is largely unexplored. We show that the myotubularin (mtm) phosphoinositide phosphatase is required for integrin-mediated myofiber attachments in Drosophila melanogaster, and that mtm-depleted myofibers exhibit hallmarks of human XLMTM myopathy. Depletion of mtm leads to increased integrin turnover at the sarcolemma and an accumulation of integrin with PI(3)P on endosomal-related membrane inclusions, indicating a role for Mtm phosphatase activity in endocytic trafficking. The depletion of Class II, but not Class III, PI3-kinase rescued mtm-dependent defects, identifying an important pathway that regulates integrin recycling. Importantly, similar integrin localization defects found in human XLMTM myofibers signify conserved MTM1 function in muscle membrane trafficking. Our results indicate that regulation of distinct phosphoinositide pools plays a central role in maintaining cell compartmentalization and attachments during muscle remodeling, and they suggest involvement of Class II PI3-kinase in MTM-related disease.     

Exocytotic "constipation" is a mechanism of tubulin/lysosomal interaction in colchicine myopathy

Colchicine, a known microtubule disrupting agent, produces a human myopathy, characterized by accumulation of lysosomes. We have created a reliable animal model of colchicine myopathy that replicates the subacute myopathy seen in humans, reproducing the chronic proximal weakness and vacuolar changes in nonnecrotic myofibers. If a microtubule network plays a role in lysosomal function in muscle, disturbance of it could alter degradation of intrinsic membrane receptors, presumably at some intracellular processing site or at exocytosis. Thus, we examined, as a possible cellular pathogenesis of colchicine myopathy, how the muscle cytoskeleton affects the degradation of membrane proteins, which are processed through the endosomal/lysosomal pathway. We used the acetylcholine receptor as a model membrane component in cultured myotubes allowed to preincubate with colchicine. We tested at which step colchicine interferes with receptor trafficking by accounting for internalization, delivery to lysosomes, hydrolysis, or exocytotic release of debris. We report that colchicine significantly decreases the exocytosis of AChRs but does not affect receptor internalization, lysosomal hydrolysis, or the number of surface membrane receptors. Further, our immunofluorescence observations revealed a morphologic tubulin network in rat skeletal muscle that is more densely distributed in white (mitochondria-poor) muscle fibers than in red (mitochondria-rich) fibers but is present in both. Ultrastructurally, immunogold labeling localized tubulin in the intermyofibrillar region in a long and linear fashion, unassociated with myofibers or mitochondria. Taken together, our findings suggest the following: (1) Microtubules likely play a functional role in the pathway of lysosomal degradation in normal adult skeletal muscle; (2) The observed decrease in overall apparent degradation of membrane receptors by colchicine must be due primarily to inhibition of exocytosis. These data indicate that lysosomal "constipation" underlies colchicine myopathy. (3) An animal model faithful to the human disorder will allow further pathogenetic studies.