The Caenorhabditis elegans polarity gene ooc-5 encodes a Torsin-related protein of the AAA ATPase superfamily

DYT-TOR1A dystonia: an update on pathogenesis and treatment

DYT-TOR1A dystonia is a neurological disorder characterized by involuntary muscle contractions and abnormal movements. It is a severe genetic form of dystonia caused by mutations in the TOR1A gene. TorsinA is a member of the AAA + family of adenosine triphosphatases (ATPases) involved in a variety of cellular functions, including protein folding, lipid metabolism, cytoskeletal organization, and nucleocytoskeletal coupling. Almost all patients with TOR1A-related dystonia harbor the same mutation, an in-frame GAG deletion (ΔGAG) in the last of its 5 exons. This recurrent variant results in the deletion of one of two tandem glutamic acid residues (i.e., E302/303) in a protein named torsinA [torsinA(△E)]. Although the mutation is hereditary, not all carriers will develop DYT-TOR1A dystonia, indicating the involvement of other factors in the disease process. The current understanding of the pathophysiology of DYT-TOR1A dystonia involves multiple factors, including abnormal protein folding, signaling between neurons and glial cells, and dysfunction of the protein quality control system. As there are currently no curative treatments for DYT-TOR1A dystonia, progress in research provides insight into its pathogenesis, leading to potential therapeutic and preventative strategies. This review summarizes the latest research advances in the pathogenesis, diagnosis, and treatment of DYT-TOR1A dystonia.

Decreased mechanotransduction prevents nuclear collapse in a Caenorhabditis elegans laminopathy

Nuclear envelopathies are a complex group of human diseases caused by mutations in nuclear envelope proteins, including progeria, myopathy, and dystonia. Here, we used the Caenorhabditis elegans germline as a model system to investigate the function of the OOC-5/torsinA AAA+ ATPase, which localizes to the nuclear envelope and is mutated in early-onset DYT1 dystonia in humans. We show that OOC-5/torsinA promotes the function of the LINC complex, which spans the nuclear envelope and transmits forces to the nuclear lamina. Remarkably, decreasing the function of OOC-5/torsinA or the LINC complex prevents nuclear collapse in the absence of a functional nuclear lamina. Therapeutics targeting torsinA or the LINC complex might prevent nuclear damage from endogenous forces in certain nuclear envelopathies.

The function of the nucleus depends on the integrity of the nuclear lamina, an intermediate filament network associated with the linker of nucleoskeleton and cytoskeleton (LINC) complex. The LINC complex spans the nuclear envelope and mediates nuclear mechanotransduction, the process by which mechanical signals and forces are transmitted across the nuclear envelope. In turn, the AAA+ ATPase torsinA is thought to regulate force transmission from the cytoskeleton to the nucleus. In humans, mutations affecting nuclear envelope-associated proteins cause laminopathies, including progeria, myopathy, and dystonia, though the extent to which endogenous mechanical stresses contribute to these pathologies is unclear. Here, we use the Caenorhabditis elegans germline as a model to investigate mechanisms that maintain nuclear integrity as germ cell nuclei progress through meiotic development and migrate for gametogenesis—processes that require LINC complex function. We report that decreasing the function of the C. elegans torsinA homolog, OOC-5, rescues the sterility and premature aging caused by a null mutation in the single worm lamin homolog. We show that decreasing OOC-5/torsinA activity prevents nuclear collapse in lamin mutants by disrupting the function of the LINC complex. At a mechanistic level, OOC-5/torsinA promotes the assembly or maintenance of the lamin-associated LINC complex and this activity is also important for interphase nuclear pore complex insertion into growing germline nuclei. These results demonstrate that LINC complex-transmitted forces damage nuclei with a compromised nuclear lamina. Thus, the torsinA–LINC complex nexus might comprise a therapeutic target for certain laminopathies by preventing damage from endogenous cellular forces.

TorsinA Is Functionally Associated with Spermatogenesis

TorsinA is a member of the AAA+ superfamily of adenosine triphosphatases. These AAA+ proteins have numerous biological functions, including vesicle fusion, cytoskeleton dynamics, intracellular trafficking, protein folding, and degradation as well as organelle biogenesis. Of particular interest is torsinA, which is mainly located in the endoplasmic reticulum (ER) and nuclear envelope (NE). Interestingly, mutations in the TOR1A gene (the gene encoding torsinA) are associated with DYT1 dystonia and with the preferential localization of mutated torsinA at the NE, where it is associated with lamina-associated polypeptide 1. A bioinformatics study of the torsinA interactome revealed reproductive processes to be highly relevant, as proteins in this class were found to interact with the former. Interestingly, the torsin protein family had never been previously described to be associated with the mammalian spermatogenic process. Histological staining of torsinA in human testis tissue revealed a granular cytoplasmic localization in mid- and late spermatocytes. We further sought to understand this newly discovered expression of torsinA in the meiotic phase of human spermatogenesis by studying its specific subcellular distribution. TorsinA is not present in the ER as commonly described. The proposal that torsinA might relocate to the pro-acrosomal vesicles in the Golgi apparatus is discussed.

RNAi Screen Identifies Novel Regulators of RNP Granules in the Caenorhabditis elegans Germ Line

Complexes of RNA and RNA binding proteins form large-scale supramolecular structures under many cellular contexts. In Caenorhabditis elegans, small germ granules are present in the germ line that share characteristics with liquid droplets that undergo phase transitions. In meiotically-arrested oocytes of middle-aged hermaphrodites, the germ granules appear to aggregate or condense into large assemblies of RNA-binding proteins and maternal mRNAs. Prior characterization of the assembly of large-scale RNP structures via candidate approaches has identified a small number of regulators of phase transitions in the C. elegans germ line; however, the assembly, function, and regulation of these large RNP assemblies remain incompletely understood. To identify genes that promote remodeling and assembly of large RNP granules in meiotically-arrested oocytes, we performed a targeted, functional RNAi screen and identified over 300 genes that regulate the assembly of the RNA-binding protein MEX-3 into large granules. Among the most common GO classes are several categories related to RNA biology, as well as novel categories such as cell cortex, ER, and chromosome segregation. We found that arrested oocytes that fail to localize MEX-3 into cortical granules display reduced oocyte quality, consistent with the idea that the larger RNP assemblies promote oocyte quality when fertilization is delayed. Interestingly, a relatively small number of genes overlap with the regulators of germ granule assembly during normal development, or with the regulators of solid RNP granules in cgh-1 oocytes, suggesting fundamental differences in the regulation of RNP granule phase transitions during meiotic arrest.

Torsins Are Essential Regulators of Cellular Lipid Metabolism

Torsins are developmentally essential AAA+ proteins, and mutation of human torsinA causes the neurological disease DYT1 dystonia. They localize in the ER membranes, but their cellular function remains unclear. We now show that dTorsin is required in Drosophila adipose tissue, where it suppresses triglyceride levels, promotes cell growth, and elevates membrane lipid content. We also see that human torsinA at the inner nuclear membrane is associated with membrane expansion and elevated cellular lipid content. Furthermore, the key lipid metabolizing enzyme, lipin, is mislocalized in dTorsin-KO cells, and dTorsin increases levels of the lipin substrate, phosphatidate, and reduces the product, diacylglycerol. Finally, genetic suppression of dLipin rescues dTorsin-KO defects, including adipose cell size, animal growth, and survival. These findings identify that torsins are essential regulators of cellular lipid metabolism and implicate disturbed lipid biology in childhood-onset DYT1 dystonia.

Torsins: not your typical AAA+ ATPases

Torsin ATPases (Torsins) belong to the widespread AAA+ (ATPases associated with a variety of cellular activities) family of ATPases, which share structural similarity but have diverse cellular functions. Torsins are outliers in this family because they lack many characteristics of typical AAA+ proteins, and they are the only members of the AAA+ family located in the endoplasmic reticulum and contiguous perinuclear space. While it is clear that Torsins have essential roles in many, if not all metazoans, their precise cellular functions remain elusive. Studying Torsins has significant medical relevance since mutations in Torsins or Torsin-associated proteins result in a variety of congenital human disorders, the most frequent of which is early-onset torsion (DYT1) dystonia, a severe movement disorder. A better understanding of the Torsin system is needed to define the molecular etiology of these diseases, potentially enabling corrective therapy. Here, we provide a comprehensive overview of the Torsin system in metazoans, discuss functional clues obtained from various model systems and organisms and provide a phylogenetic and structural analysis of Torsins and their regulatory cofactors in relation to disease-causative mutations. Moreover, we review recent data that have led to a dramatically improved understanding of these machines at a molecular level, providing a foundation for investigating the molecular defects underlying the associated movement disorders. Lastly, we discuss our ideas on how recent progress may be utilized to inform future studies aimed at determining the cellular role(s) of these atypical molecular machines and their implications for dystonia treatment options.

Access of torsinA to the inner nuclear membrane is activity dependent and regulated in the endoplasmic reticulum

TorsinA (also known as torsin-1A) is a membrane-embedded AAA+ ATPase that has an important role in the nuclear envelope lumen. However, most torsinA is localized in the peripheral endoplasmic reticulum (ER) lumen where it has a slow mobility that is incompatible with free equilibration between ER subdomains. We now find that nuclear-envelope-localized torsinA is present on the inner nuclear membrane (INM) and ask how torsinA reaches this subdomain. The ER system contains two transmembrane proteins, LAP1 and LULL1 (also known as TOR1AIP1 and TOR1AIP2, respectively), that reversibly co-assemble with and activate torsinA. Whereas LAP1 localizes on the INM, we show that LULL1 is in the peripheral ER and does not enter the INM. Paradoxically, interaction between torsinA and LULL1 in the ER targets torsinA to the INM. Native gel electrophoresis reveals torsinA oligomeric complexes that are destabilized by LULL1. Mutations in torsinA or LULL1 that inhibit ATPase activity reduce the access of torsinA to the INM. Furthermore, although LULL1 binds torsinA in the ER lumen, its effect on torsinA localization requires cytosolic-domain-mediated oligomerization. These data suggest that LULL1 oligomerizes to engage and transiently disassemble torsinA oligomers, and is thereby positioned to transduce cytoplasmic signals to the INM through torsinA.

A novel function for the Caenorhabditis elegans torsin OOC-5 in nucleoporin localization and nuclear import

Mutation in the human AAA+ protein torsinA leads to DYT1 dystonia. Loss of a Caenorhabditis elegans torsin, OOC-5, leads to defects in nucleoporin localization and nuclear import, a novel phenotype for a torsin mutant. NE ultrastructural defects similar to those in mouse and fly torsin mutants are also found, showing conservation of function.

Torsin proteins are AAA+ ATPases that localize to the endoplasmic reticular/nuclear envelope (ER/NE) lumen. A mutation that markedly impairs torsinA function causes the CNS disorder DYT1 dystonia. Abnormalities of NE membranes have been linked to torsinA loss of function and the pathogenesis of DYT1 dystonia, leading us to investigate the role of the Caenorhabditis elegans torsinA homologue OOC-5 at the NE. We report a novel role for torsin in nuclear pore biology. In ooc-5–mutant germ cell nuclei, nucleoporins (Nups) were mislocalized in large plaques beginning at meiotic entry and persisted throughout meiosis. Moreover, the KASH protein ZYG-12 was mislocalized in ooc-5 gonads. Nups were mislocalized in adult intestinal nuclei and in embryos from mutant mothers. EM analysis revealed vesicle-like structures in the perinuclear space of intestinal and germ cell nuclei, similar to defects reported in torsin-mutant flies and mice. Consistent with a functional disruption of Nups, ooc-5–mutant embryos displayed impaired nuclear import kinetics, although the nuclear pore-size exclusion barrier was maintained. Our data are the first to demonstrate a requirement for a torsin for normal Nup localization and function and suggest that these functions are likely conserved.

Emerging common molecular pathways for primary dystonia

The dystonias are a group of hyperkinetic movement disorders whose principal cause is neuron dysfunction at 1 or more interconnected nodes of the motor system. The study of genes and proteins that cause familial dystonia provides critical information about the cellular pathways involved in this dysfunction, which disrupts the motor pathways at the systems level. In recent years study of the increasing number of DYT genes has implicated a number of cell functions that appear to be involved in the pathogenesis of dystonia. A review of the literature published in English-language publications available on PubMed relating to the genetics and cellular pathology of dystonia was performed. Numerous potential pathogenetic mechanisms have been identified. We describe those that fall into 3 emerging thematic groups: cell-cycle and transcriptional regulation in the nucleus, endoplasmic reticulum and nuclear envelope function, and control of synaptic function. © 2013 Movement Disorder Society.

Engineering animal models of dystonia

Dystonia is a neurological disorder characterized by abnormal involuntary movements that are prolonged and often cause twisting and turning. Several genetically modified worms, fruit flies, and rodents have been generated as models of genetic dystonias, in particular DYT1, DYT11, and DYT12 dystonias. Although these models do not show overt dystonic symptoms, the rodent models exhibit motor deficits in specialized behavioral tasks, such as the rotarod and beam-walking tests. For example, in a rodent model of DYT12 dystonia, which is generally stress triggered, motor deficits are observed only after the animal is stressed. Moreover, in a rodent model of DYT1 dystonia, the motor and electrophysiological deficits can be rescued by trihexyphenidyl, a common anticholinergic medication used to treat dystonic symptoms in human patients. Biochemically, the DYT1 and DYT11 animal models also share some similarities to patients, such as a reduction in striatal D2 dopamine receptor and binding activities. In addition, conditional knockout mouse models for DYT1 and DYT11 dystonia demonstrate that loss of the causal dystonia-related proteins in the striatum leads to motor deficits. Interestingly, loss of the DYT1 dystonia causal protein in Purkinje cells shows an improvement in motor performance, suggesting that gene therapy targeting of the cerebellum or intervention in its downstream pathways may be useful. Finally, recent studies using DYT1 dystonia worm and mouse models led to a potential novel therapeutic agent, which is currently undergoing clinical trials. These results indicate that genetic animal models are powerful tools to elucidate the pathophysiology and to further develop new therapeutics for dystonia.

NLF-1 delivers a sodium leak channel to regulate neuronal excitability and modulate rhythmic locomotion

A cation channel NCA/UNC-79/UNC-80 affects neuronal activity. We report here the identification of a conserved endoplasmic reticulum protein NLF-1 (NCA localization factor-1) that regulates neuronal excitability and locomotion through the NCA channel. In C. elegans, the loss of either NLF-1 or NCA leads to a reduced sodium leak current, and a hyperpolarized resting membrane potential in premotor interneurons. This results in a decreased premotor interneuron activity that reduces the initiation and sustainability of rhythmic locomotion. NLF-1 promotes axonal localization of all NCA reporters. Its mouse homolog mNLF-1 functionally substitutes for NLF-1 in C. elegans, interacts with the mammalian sodium leak channel NALCN in vitro, and potentiates sodium leak currents in primary cortical neuron cultures. Taken together, an ER protein NLF-1 delivers a sodium leak channel to maintain neuronal excitability and potentiates a premotor interneuron network critical for C. elegans rhythmic locomotion.

The genetics of dystonias

Dystonia has been defined as a syndrome of involuntary, sustained muscle contractions affecting one or more sites of the body, frequently causing twisting and repetitive movements or abnormal postures. Dystonia is also a clinical sign that can be the presenting or prominent manifestation of many neurodegenerative and neurometabolic disorders. Etiological categories include primary dystonia, secondary dystonia, heredodegenerative diseases with dystonia, and dystonia plus. Primary dystonia includes syndromes in which dystonia is the sole phenotypic manifestation with the exception that tremor can be present as well. Most primary dystonia begins in adults, and approximately 10% of probands report one or more affected family members. Many cases of childhood- and adolescent-onset dystonia are due to mutations in TOR1A and THAP1. Mutations in THAP1 and CIZ1 have been associated with sporadic and familial adult-onset dystonia. Although significant recent progress had been made in defining the genetic basis for most of the dystonia-plus and heredodegenerative diseases with dystonia, a major gap remains in understanding the genetic etiologies for most cases of adult-onset primary dystonia. Common themes in the cellular biology of dystonia include G1/S cell cycle control, monoaminergic neurotransmission, mitochondrial dysfunction, and the neuronal stress response.

Untethering the nuclear envelope and cytoskeleton: biologically distinct dystonias arising from a common cellular dysfunction

Most cases of early onset DYT1 dystonia in humans are caused by a GAG deletion in the TOR1A gene leading to loss of a glutamic acid (ΔE) in the torsinA protein, which underlies a movement disorder associated with neuronal dysfunction without apparent neurodegeneration. Mutation/deletion of the gene (Dst) encoding dystonin in mice results in a dystonic movement disorder termed dystonia musculorum, which resembles aspects of dystonia in humans. While torsinA and dystonin proteins do not share modular domain architecture, they participate in a similar function by modulating a structural link between the nuclear envelope and the cytoskeleton in neuronal cells. We suggest that through a shared interaction with the nuclear envelope protein nesprin-3α, torsinA and the neuronal dystonin-a2 isoform comprise a bridge complex between the outer nuclear membrane and the cytoskeleton, which is critical for some aspects of neuronal development and function. Elucidation of the overlapping roles of torsinA and dystonin-a2 in nuclear/endoplasmic reticulum dynamics should provide insights into the cellular mechanisms underlying the dystonic phenotype.

A predictable worm: application of Caenorhabditis elegans for mechanistic investigation of movement disorders

Ongoing investigations into causes and cures for human movement disorders are important toward the elucidation of diseases such as Parkinson's disease (PD) and dystonia. The use of animal model systems can provide links to susceptibility factors, as well as therapeutic interventions. In this regard, the nematode roundworm, Caenorhabditis elegans, is ideal for examining age-dependent neurodegenerative disease studies. It is genetically tractable, has a short lifespan, and a well-defined nervous system. Green fluorescent protein is readily visualized in C. elegans because it is a transparent organism, thus the nervous system and factors that alter the viability of neurons can be directly examined in vivo. Through expression of the human PD-associated protein (α-synuclein in the worm dopamine neurons), neurodegeneration is observed in an age-dependent manner. Furthermore, expression of the early-onset dystonia-related protein torsinA increases vulnerability to endoplasmic reticulum (ER) stress in C. elegans, because torsinA is located in the ER. Here we provide an overview of collaborative studies we have conducted that collectively demonstrate the usefulness of the nematode model to discern functional effectors of dopaminergic neurodegeneration and ER stress that translate to mammalian data in the fields of PD and dystonia. Taken together, the application of C. elegans toward the evaluation of genetic modifiers for movement disorders research has predictive value and serves to accelerate the path forward for therapeutic interventions.

Exploring the influence of torsinA expression on protein quality control

DYT1 dystonia is caused by a glutamic acid deletion (ΔE) in the endoplasmic reticulum (ER) protein torsinA. Previous studies suggest that torsinA modulates the aggregation of cytosolic misfolded proteins and ER stress responses, although the mechanisms underlying those effects remain unclear. In order to investigate the bases of these observations, we analyzed the interaction between torsinA expression, protein aggregation and ER stress in PC6.3 cells. Unexpectedly, we found that expression of torsinA(wt) or (ΔE) does not influence the inclusion formation by an expanded polyglutamine reporter protein in this cellular model. Furthermore, torsinA does not prevent the activation of ER stress induced by thapsigargin or the reducing agent DTT. Interestingly, DTT induces post-translational changes in torsinA, more prominently for torsinA(wt) than (ΔE). This work highlights the importance of model system selection for the study of torsinA function. Furthermore, it provides additional evidence suggesting that torsinA is sensitive to changes in the cellular redox potential.

A unique redox-sensing sensor II motif in TorsinA plays a critical role in nucleotide and partner binding

Early onset dystonia is commonly associated with the deletion of one of a pair of glutamate residues (ΔE302/303) near the C terminus of torsinA, a member of the AAA+ protein family (ATPases associated with a variety of cellular activities) located in the endoplasmic reticulum lumen. The functional consequences of the disease-causing mutation, ΔE, are not currently understood. By contrast to other AAA+ proteins, torsin proteins contain two conserved cysteine residues in the C-terminal domain, one of which is located in the nucleotide sensor II motif. Depending on redox status, an ATP hydrolysis mutant of torsinA interacts with lamina-associated polypeptide 1 (LAP1) and lumenal domain like LAP1 (LULL1). Substitution of the cysteine in sensor II diminishes the redox-regulated interaction of torsinA with these substrates. Significantly, the dystonia-causing mutation, ΔE, alters the ability of torsinA to mediate the redox-regulated interactions with LAP1 and LULL1. Limited proteolysis experiments reveal redox- and mutation-dependent changes in the local conformation of torsinA as a function of nucleotide binding. These results indicate that the cysteine-containing sensor II plays a critical role in redox sensing and the nucleotide and partner binding functions of torsinA and suggest that loss of this function of torsinA contributes to the development of DYT1 dystonia.

TorsinA and DYT1 dystonia: a synaptopathy?

DYT1 dystonia is an autosomal dominant movement disorder, characterized by early onset of involuntary sustained muscle contractions. It is caused by a 3-bp deletion in the DYT1 gene, which results in the deletion of a single glutamate residue in the C-terminus of the protein TA (torsinA). TA is a member of the AAA+ (ATPase associated with various cellular activities) family of chaperones with multiple functions in the cell. There is no evidence of neurodegeneration in DYT1 dystonia, which suggests that mutant TA leads to functional neuronal abnormalities, leading to dystonic movements. In recent years, different functional roles have been attributed to TA, including being a component of the cytoskeleton and the NE (nuclear envelope), and involvement in the secretory pathway and SV (synaptic vesicle) machinery. The aim of the present review is to summarize these findings and the different models proposed, which have contributed to our current understanding of the function of TA, and also to discuss the evidence implicating TA in SV function.

Chemical enhancement of torsinA function in cell and animal models of torsion dystonia

Movement disorders represent a significant societal burden for which therapeutic options are limited and focused on treating disease symptomality. Early-onset torsion dystonia (EOTD) is one such disorder characterized by sustained and involuntary muscle contractions that frequently cause repetitive movements or abnormal postures. Transmitted in an autosomal dominant manner with reduced penetrance, EOTD is caused in most cases by the deletion of a glutamic acid (DeltaE) in the DYT1 (also known as TOR1A) gene product, torsinA. Although some patients respond well to anticholingerics, therapy is primarily limited to either neurosurgery or chemodenervation. As mutant torsinA (DeltaE) expression results in decreased torsinA function, therapeutic strategies directed toward enhancement of wild-type (WT) torsinA activity in patients who are heterozygous for mutant DYT1 may restore normal cellular functionality. Here, we report results from the first-ever screen for candidate small molecule therapeutics for EOTD, using multiple activity-based readouts for torsinA function in Caenorhabditis elegans, subsequent validation in human DYT1 patient fibroblasts, and behavioral rescue in a mouse model of DYT1 dystonia. We exploited the nematode to rapidly discern chemical effectors of torsinA and identified two classes of antibiotics, quinolones and aminopenicillins, which enhance WT torsinA activity in two separate in vivo assays. Representative molecules were assayed in EOTD patient fibroblasts for improvements in torsinA-dependent secretory function, which was improved significantly by ampicillin. Furthermore, a behavioral defect associated with an EOTD mouse knock-in model was also rescued following administration of ampicillin. These combined data indicate that specific small molecules that enhance torsinA activity represent a promising new approach toward therapeutic development for EOTD, and potentially for other diseases involving the processing of mutant proteins.

Expression profiling in peripheral blood reveals signature for penetrance in DYT1 dystonia

DYT1 dystonia is an autosomal-dominantly inherited movement disorder, which is usually caused by a GAG deletion in the TOR1A gene. Due to the reduced penetrance of approximately 30-40%, the determination of the mutation in a subject is of limited use with regard to actual manifestation of symptoms. In the present study, we used Affymetrix oligonucleotide microarrays to analyze global gene expression in blood samples of 15 manifesting and 15 non-manifesting mutation carriers in order to identify a susceptibility profile beyond the GAG deletion which is associated with the manifestation of symptoms in DYT1 dystonia. We identified a genetic signature which distinguished between asymptomatic mutation carriers and symptomatic DYT1 patients with 86.7% sensitivity and 100% specificity. This genetic signature could correctly predict the disease state in an independent test set with a sensitivity of 87.5% and a specificity of 85.7%. Conclusively, this genetic signature might provide a possibility to distinguish DYT1 patients from asymptomatic mutation carriers.

Kinesin-dependent transport results in polarized migration of the nucleus in oocytes and inward movement of yolk granules in meiotic embryos

During female meiosis, meiotic spindles are positioned at the oocyte cortex to allow expulsion of chromosomes into polar bodies. In C. elegans, kinesin-dependent translocation of the entire spindle to the cortex precedes dynein-dependent rotation of one spindle pole toward the cortex. To elucidate the role of kinesin-1 in spindle translocation, we examined the localization of kinesin subunits in meiotic embryos. Surprisingly, kinesin-1 was not associated with the spindle and instead was restricted to the cytoplasm in the middle of the embryo. Yolk granules moved on linear tracks, in a kinesin-dependent manner, away from the cortex, resulting in their concentration in the middle of the embryo where the kinesin was concentrated. These results suggest that cytoplasmic microtubules might be arranged with plus ends extending inward, away from the cortex. This microtubule arrangement would not be consistent with direct transport of the meiotic spindle toward the cortex by kinesin-1. In maturing oocytes, the nucleus underwent kinesin-dependent migration to the future site of spindle attachment at the anterior cortex. Thus the spindle translocation defect observed in kinesin-1 mutants may be a result of failed nuclear migration, which places the spindle too far from the cortex for the spindle translocation mechanism to function.

TorsinA and dystonia: from nuclear envelope to synapse

A GAG deletion in the DYT1 gene is responsible for the autosomal dominant movement disorder, early onset primary torsion dystonia, which is characterised by involuntary sustained muscle contractions and abnormal posturing of the limbs. The mutation leads to deletion of a single glutamate residue in the C-terminus of the protein torsinA, a member of the AAA+ ATPase family of proteins with multiple functions. Since no evidence of neurodegeneration has been found in DYT1 patients, the dystonic phenotype is likely to be the result of neuronal functional defect(s), the nature of which is only partially understood. Biochemical, structural and cell biological studies have been performed in order to characterise torsinA. These studies, together with the generation of several animal models, have contributed to identify cellular compartments and pathways, including the cytoskeleton and the nuclear envelope, the secretory pathway and the synaptic vesicle machinery where torsinA function may be crucial. However, the role of torsinA and the correlation between the dysfunction caused by the mutation and the dystonic phenotype remain unclear. This review provides an overview of the findings of the last ten years of research on torsinA, a critical evaluation of the different models proposed and insights towards future avenues of research.

Centrosome attachment to the C. elegans male pronucleus is dependent on the surface area of the nuclear envelope

A close association must be maintained between the male pronucleus and the centrosomes during pronuclear migration. In C. elegans, simultaneous depletion of inner nuclear membrane LEM proteins EMR-1 and LEM-2, depletion of the nuclear lamina proteins LMN-1 or BAF-1, or the depletion of nuclear import components leads to embryonic lethality with small pronuclei. Here, a novel centrosome detachment phenotype in C. elegans zygotes is described. Zygotes with defects in the nuclear envelope had small pronuclei with a single centrosome detached from the male pronucleus. ZYG-12, SUN-1, and LIS-1, which function at the nuclear envelope with dynein to attach centrosomes, were observed at normal concentrations on the nuclear envelope of pronuclei with detached centrosomes. Analysis of time-lapse images showed that as mutant pronuclei grew in surface area, they captured detached centrosomes. Larger tetraploid or smaller histone::mCherry pronuclei suppressed or enhanced the centrosome detachment phenotype respectively. In embryos fertilized with anucleated sperm, only one centrosome was captured by small female pronuclei, suggesting the mechanism of capture is dependent on the surface area of the outer nuclear membrane available to interact with aster microtubules. We propose that the limiting factor for centrosome attachment to the surface of abnormally small pronuclei is dynein.

The torsin-family AAA+ protein OOC-5 contains a critical disulfide adjacent to Sensor-II that couples redox state to nucleotide binding

A subgroup of the AAA+ proteins that reside in the endoplasmic reticulum and the nuclear envelope including human torsinA, a protein mutated in hereditary dystonia, is called the torsin family of AAA+ proteins. A multiple-sequence alignment of this family with Hsp100 proteins of known structure reveals a conserved cysteine in the C-terminus of torsin proteins within the Sensor-II motif. A structural model predicts this cysteine to be a part of an intramolecular disulfide bond, suggesting that it may function as a redox sensor to regulate ATPase activity. In vitro experiments with OOC-5, a torsinA homolog from Caenorhabditis elegans, demonstrate that redox changes that reduce this disulfide bond affect the binding of ATP and ADP and cause an attendant local conformational change detected by limited proteolysis. Transgenic worms expressing an ooc-5 gene with cysteine-to-serine mutations that disrupt the disulfide bond have a very low embryo hatch rate compared with wild-type controls, indicating these two cysteines are essential for OOC-5 function. We propose that the Sensor-II in torsin family proteins is a redox-regulated sensor. This regulatory mechanism may be central to the function of OOC-5 and human torsinA.

Glial elements contribute to stress-induced torsinA expression in the CNS and peripheral nervous system

DYT1 dystonia is caused by a single GAG deletion in exon 5 of TOR1A, the gene encoding torsinA, a putative chaperone protein. In this study, central and peripheral nervous system perturbations (transient forebrain ischemia and sciatic nerve transection, respectively) were used to examine the systems biology of torsinA in rats. After forebrain ischemia, quantitative real-time reverse transcriptase-polymerase chain reaction identified increased torsinA transcript levels in hippocampus, cerebral cortex, thalamus, striatum, and cerebellum at 24 h and 7 days. Expression declined toward sham values by 14 days in striatum, thalamus and cortex, and by 21 days in cerebellum and hippocampus. TorsinA transcripts were localized to dentate granule cells and pyramidal neurons in control hippocampus and were moderately elevated in these cell populations at 24 h after ischemia, after which CA1 expression was reduced, consistent with the loss of this vulnerable neuronal population. Increased in situ hybridization signal in CA1 stratum radiatum, stratum lacunosum-moleculare, and stratum oriens at 7 days after ischemia was correlated with the detection of torsinA immunoreactivity in interneurons and reactive astrocytes at 7 and 14 days. Sciatic nerve transection increased torsinA transcript levels between 24 h and 7 days in both ipsilateral and contralateral dorsal root ganglia (DRG). However, increased torsinA immunoreactivity was localized to both ganglion cells and satellite cells in ipsilateral DRG but was restricted to satellite cells contralaterally. These results suggest that torsinA participates in the response of neural tissue to central and peripheral insults and its sustained up-regulation indicates that torsinA may contribute to remodeling of neuronal circuitry. The striking induction of torsinA in astrocytes and satellite cells points to the potential involvement of glial elements in the pathobiology of DYT1 dystonia.

RAB-11 permissively regulates spindle alignment by modulating metaphase microtubule dynamics in Caenorhabditis elegans early embryos

Alignment of the mitotic spindle along a preformed axis of polarity is crucial for generating cell diversity in many organisms, yet little is known about the role of the endomembrane system in this process. RAB-11 is a small GTPase enriched in recycling endosomes. When we depleted RAB-11 by RNAi in Caenorhabditis elegans, the spindle of the one-cell embryo failed to align along the axis of polarity in metaphase and underwent violent movements in anaphase. The distance between astral microtubules ends and the anterior cortex was significantly increased in rab-11(RNAi) embryos specifically during metaphase, possibly accounting for the observed spindle alignment defects. Additionally, we found that normal ER morphology requires functional RAB-11, particularly during metaphase. We hypothesize that RAB-11, in conjunction with the ER, acts to regulate cell cycle-specific changes in astral microtubule length to ensure proper spindle alignment in Caenorhabditis elegans early embryos.

TorsinA and DYT1 early-onset dystonia

Early-onset torsion dystonia is a severe generalized form of primary dystonia, with most cases caused by a specific mutation (ΔGAG) in the DYT1 gene encoding torsinA. This mutation is autosomal dominant and is thought to result in reduced torsinA activity. TorsinA is an AAA protein located in the lumen of the endoplasmic reticulum and nuclear envelope of most cells (with high levels in some brain neurons). It is thought to serve as a chaperone protein and/or a link between these membranes and the cytoskeleton. Other sequence variations in DYT1 can affect penetrance of the ΔGAG mutation and may be associated with more common, late-onset focal forms of dystonia. Animal models of DYT1 dystonia are emerging that will allow preclinical evaluation of drugs that can be used to prevent or treat this non-neurodegenerative neurologic disease.

Mutant torsinA interferes with protein processing through the secretory pathway in DYT1 dystonia cells

TorsinA is an AAA(+) protein located predominantly in the lumen of the endoplasmic reticulum (ER) and nuclear envelope responsible for early onset torsion dystonia (DYT1). Most cases of this dominantly inherited movement disorder are caused by deletion of a glutamic acid in the carboxyl terminal region of torsinA. We used a sensitive reporter, Gaussia luciferase (Gluc) to evaluate the role of torsinA in processing proteins through the ER. In primary fibroblasts from controls and DYT1 patients most Gluc activity (95%) was released into the media and processed through the secretory pathway, as confirmed by inhibition with brefeldinA and nocodazole. Fusion of Gluc to a fluorescent protein revealed coalignment and fractionation with ER proteins and association of Gluc with torsinA. Notably, fibroblasts from DYT1 patients were found to secrete markedly less Gluc activity as compared with control fibroblasts. This decrease in processing of Gluc in DYT1 cells appear to arise, at least in part, from a loss of torsinA activity, because mouse embryonic fibroblasts lacking torsinA also had reduced secretion as compared with control cells. These studies demonstrate the exquisite sensitivity of this reporter system for quantitation of processing through the secretory pathway and support a role for torsinA as an ER chaperone protein.

Down-regulation of torp4a, encoding the Drosophila homologue of torsinA, results in increased neuronal degeneration

Early-onset torsion dystonia is a dominant motor disorder linked to mutations in torsinA. TorsinA is weakly related to a superfamily of chaperone-like proteins. The function of the torsin group remains largely unknown. Here we use RNAi and over-expression to analyze the function of torp4a, the only Drosophila torsin. Targeted down-regulation in the eye causes progressive degeneration of the retina. Conversely, over-expression of torp4a protects from age-related degeneration. In the retinas of young animals, a correlation with the lysosome-related organelle, the pigment granule, is also observed. Lowering torp4a causes an increase in pigment granules, while over-expression causes loss of granules. We have performed a screen for genetic interactors of torp4a identifying a number mutants, including two members of the AP-3 complex. Other genetic interactors found included genes related to actin and myosin function. Our findings implicate the Drosophila torsin, torp4a, to function with molecules consistent with already predicted roles in the endoplasmic reticulum/nuclear envelope compartment, and have identified potential new interactions with AP-3 like components.

Effects of genetic variations in the dystonia protein torsinA: identification of polymorphism at residue 216 as protein modifier

Four naturally occurring sequence variations have been found in the coding region of the DYT1 gene encoding torsinA. One of these, a 3 bp (DeltaGAG) deletion, underlies dominantly inherited cases of early-onset torsion dystonia. Others, including a single nucleotide polymorphism that replaces aspartic acid (D) at residue 216 with histidine (H) in 12% of normal alleles and two other rare deletions, have not been clearly associated with disease. To gain insight into how these sequence variations affect torsinA, we used the structure of the related protein ClpB to provide a model of torsinA's AAA+ domain. Motifs important for ATP hydrolysis-sensor 1 and sensor 2-were identified, mutagenized and used to validate predictions of this model. Inspection revealed that the DeltaGAG deletion associated with dystonia removes one residue from an alpha-helix in the C-terminal portion of the AAA+ domain. The resulting distortion in torsinA structure may underlie this mutant's known tendency to produce ER-derived inclusions as well as its proposed loss of function. The D/H polymorphism at residue 216 falls in the N-terminal portion of the AAA+ domain near the sensor 1 motif. Surprisingly, cells expressing torsinA with the polymorphic histidine developed inclusions similar to those associated with DeltaGAG-torsinA, indicating that this change may also affect torsinA structure. Introducing H216 into DeltaGAG-torsinA reduced its tendency to form inclusions, suggesting that the two changes offset each other. Our findings point to a structural basis for the defects associated with the disease-linked DeltaGAG deletion in torsinA. They also suggest possible connections between the allelic polymorphism at residue 216 and the penetrance of DYT1 dystonia, as well as a possible role for this polymorphism in related disease states.

Dystonia-causing mutant torsinA inhibits cell adhesion and neurite extension through interference with cytoskeletal dynamics

Early onset torsion dystonia is a movement disorder inherited as an autosomal dominant syndrome with reduced penetrance. Symptoms appear to result from altered neuronal circuitry within the brain with no evidence of neuronal loss. Most cases are caused by loss of a glutamic acid residue in the AAA+ chaperone protein, torsinA, encoded in the DYT1 gene. In this study, torsinA was found to move in conjunction with vimentin in three cell culture paradigms-recovery from microtubule depolymerization, expression of a dominant-negative form of kinesin light chain and respreading after trypsinization. Co-immune precipitation studies revealed association between vimentin and torsinA in a complex including other cytoskeletal elements, actin and tubulin, as well as two proteins previously shown to interact with torsinA-the motor protein, kinesin light chain 1, and the nuclear envelope protein, LAP1. Morphologic and functional differences related to vimentin were noted in primary fibroblasts from patients carrying this DYT1 mutation as compared with controls, including an increased perinuclear concentration of vimentin and a delayed rate of adhesion to the substratum. Overexpression of mutant torsinA inhibited neurite extension in human neuroblastoma cells, with torsinA and vimentin immunoreactivity enriched in the perinuclear region and in cytoplasmic inclusions. Collectively, these studies suggest that mutant torsinA interferes with cytoskeletal events involving vimentin, possibly by restricting movement of these particles/filaments, and hence may affect development of neuronal pathways in the brain.

Nucleoporins NPP-1, NPP-3, NPP-4, NPP-11 and NPP-13 are required for proper spindle orientation in C. elegans

Nucleoporins are components of the nuclear pore, which is required for nucleo-cytoplasmic transport. We report a role for a subclass of nucleoporins in orienting the mitotic spindle in C. elegans embryos. RNAi-mediated depletion of any of five putative nucleoporins npp-1, npp-3, npp-4, npp-11, and npp-13 leads to indistinguishable spindle orientation defects. Transgenic worms expressing NPP-1::GFP or NPP-11::GFP show GFP localization at the nuclear envelope, consistent with their predicted function. NPP-1 interacts with the other nucleoporins in yeast two-hybrid assays, suggesting that the proteins affect spindle orientation by a common process. The failed orientation phenotype of npp-1(RNAi) is at least partially epistatic to the ectopic spindle rotation in the AB blastomere of par-3 mutant embryos. This suggests that NPP-1 contributes to the mechanics of spindle orientation. However, NPP-1 is also required for PAR-6 asymmetry at the two-cell stage, indicating that nucleoporins may be required to define cortical domains in the germ line blastomere P1. Nuclear envelope structure is abnormal in npp-1(RNAi) embryos, but the envelope maintains its integrity, and most nuclear proteins we assayed accumulate normally. These findings raise the possibility that these nucleoporins may have direct roles in orienting the mitotic spindle and the maintenance of cell polarity.

Nuclear envelope, nuclear lamina, and inherited disease

The nuclear envelope is composed of the nuclear membranes, nuclear lamina, and nuclear pore complexes. In recent years, mutations in nuclear-envelope proteins have been shown to cause a surprisingly wide array of inherited diseases. While the mutant proteins are generally expressed in most or all differentiated somatic cells, many mutations cause fairly tissue-specific disorders. Perhaps the most dramatic case is that of mutations in A-type lamins, intermediate filament proteins associated with the inner nuclear membrane. Different mutations in the same lamin proteins have been shown to cause striated muscle diseases, partial lipodystrophy syndromes, a peripheral neuropathy, and disorders with features of severe premature aging. In this review, we summarize fundamental aspects of nuclear envelope structure and function, the inherited diseases caused by mutations in lamins and other nuclear envelope proteins, and possible pathogenic mechanisms.

Evolutionary history and higher order classification of AAA+ ATPases

The AAA+ ATPases are enzymes containing a P-loop NTPase domain, and function as molecular chaperones, ATPase subunits of proteases, helicases or nucleic-acid-stimulated ATPases. All available sequences and structures of AAA+ protein domains were compared with the aim of identifying the definitive sequence and structure features of these domains and inferring the principal events in their evolution. An evolutionary classification of the AAA+ class was developed using standard phylogenetic methods, analysis of shared sequence and structural signatures, and similarity-based clustering. This analysis resulted in the identification of 26 major families within the AAA+ ATPase class. We also describe the position of the AAA+ ATPases with respect to the RecA/F1, helicase superfamilies I/II, PilT, and ABC classes of P-loop NTPases. The AAA+ class appears to have undergone an early radiation into the clamp-loader, DnaA/Orc/Cdc6, classic AAA, and "pre-sensor 1 beta-hairpin" (PS1BH) clades. Within the PS1BH clade, chelatases, MoxR, YifB, McrB, Dynein-midasin, NtrC, and MCMs form a monophyletic assembly defined by a distinct insert in helix-2 of the conserved ATPase core, and additional helical segment between the core ATPase domain and the C-terminal alpha-helical bundle. At least 6 distinct AAA+ proteins, which represent the different major clades, are traceable to the last universal common ancestor (LUCA) of extant cellular life. Additionally, superfamily III helicases, which belong to the PS1BH assemblage, were probably present at this stage in virus-like "selfish" replicons. The next major radiation, at the base of the two prokaryotic kingdoms, bacteria and archaea, gave rise to several distinct chaperones, ATPase subunits of proteases, DNA helicases, and transcription factors. The third major radiation, at the outset of eukaryotic evolution, contributed to the origin of several eukaryote-specific adaptations related to nuclear and cytoskeletal functions. The new relationships and previously undetected domains reported here might provide new leads for investigating the biology of AAA+ ATPases.

TorsinA negatively controls neurite outgrowth of SH-SY5Y human neuronal cell line

Early onset generalized dystonia is a severe form of primary dystonia linked to a mutation of the DYT1(TOR1A) gene on chromosome 9q34. DYT1 gene codifies for human torsinA, an AAA+ ATPase associated with the membranes of endoplasmic reticulum (ER) and the synaptic vesicles and proposed to be involved in trafficking of tubular-vesicular membrane through neuronal processes. In this study, the presence and the intracellular distribution of torsinA protein in SH-SY5Y neuroblastoma cells were evaluated by immunofluorescence and Western blot analysis following differentiation with retinoic acid and BDNF. Protein expression was then inhibited by transient antisense transfection and the possible effect on neurite outgrowth was observed. In SH-SY5Y cells torsinA, with an apparent MW of 38 kDa, is endogenously present and distributed, with a punctate pattern, in the cytosol and along the neurites. The protein showed high intensity of immunoreactivity in the neurite varicosities and was partially co-localized with vesicles markers. Terminally differentiated cells showed an increase of protein expression. Oligonucleotide antisense treatment induced a significant response to differentiating stimuli, lead to sprouting of longer neurites and increase in growth cone areas. A relationship between torsinA and tau protein, which is involved in axon elongation and establishment of neuronal polarity, was demonstrated by co-immunoprecipitation experiments. These findings suggest that torsinA, throughout the interaction with microtubule associated proteins, may contribute to control neurite outgrowth and could be involved in maintaining cell polarity.

Anterior-posterior polarity in C. elegans and Drosophila--PARallels and differences

The eggs of Caenorhabditis elegans and Drosophila bear little similarity to each other, yet both depend on the par genes for control of anterior-posterior polarity. Here we explore possible common roles for the par genes (pars) in converting transient asymmetries into stably polarized axes. Although clear mechanistic parallels remain to be established, par-dependent regulation of microtubule dynamics and protein stability emerge as common themes.

Heads or tails: cell polarity and axis formation in the early Caenorhabditis elegans embryo

In C. elegans, the first embryonic axis is established shortly after fertilization and requires both the microtubule and microfilament cytoskeleton. Cues from sperm-donated centrosomes result in a cascade of events that polarize the distribution of widely conserved PAR proteins at the cell cortex. The PAR proteins in turn polarize the cytoplasm and position mitotic spindles. Lessons learned from C. elegans should improve our understanding of how cells become polarized and divide asymmetrically during development.