Dystonia and the nuclear envelope

DYT1 Dystonia Patient-Derived Fibroblasts Have Increased Deformability and Susceptibility to Damage by Mechanical Forces

DYT1 dystonia is a neurological movement disorder that is caused by a loss-of-function mutation in the DYT1/TOR1A gene, which encodes torsinA, a conserved luminal ATPases-associated with various cellular activities (AAA+) protein. TorsinA is required for the assembly of functional linker of nucleoskeleton and cytoskeleton (LINC) complexes, and consequently the mechanical integration of the nucleus and the cytoskeleton. Despite the potential implications of altered mechanobiology in dystonia pathogenesis, the role of torsinA in regulating cellular mechanical phenotype, or mechanotype, in DYT1 dystonia remains unknown. Here, we define the deformability of mouse fibroblasts lacking functional torsinA as well as human fibroblasts isolated from DYT1 dystonia patients. We find that the deletion of torsinA or the expression of torsinA containing the DYT1 dystonia-causing ΔE302/303 (ΔE) mutation results in more deformable cells. We observe a similar increased deformability of mouse fibroblasts that lack lamina-associated polypeptide 1 (LAP1), which interacts with and stimulates the ATPase activity of torsinA in vitro, as well as with the absence of the LINC complex proteins, Sad1/UNC-84 1 (SUN1) and SUN2, lamin A/C, or lamin B1. Consistent with these findings, we also determine that DYT1 dystonia patient-derived fibroblasts are more compliant than fibroblasts isolated from unafflicted individuals. DYT1 dystonia patient-derived fibroblasts also exhibit increased nuclear strain and decreased viability following mechanical stretch. Taken together, our results establish the foundation for future mechanistic studies of the role of cellular mechanotype and LINC-dependent nuclear-cytoskeletal coupling in regulating cell survival following exposure to mechanical stresses.

TOR1A variants cause a severe arthrogryposis with developmental delay, strabismus and tremor

See Ginevrino and Valente (doi:10.1093/brain/awx260) for a scientific commentary on this article. Autosomal dominant torsion dystonia-1 is a disease with incomplete penetrance most often caused by an in-frame GAG deletion (p.Glu303del) in the endoplasmic reticulum luminal protein torsinA encoded by TOR1A. We report an association of the homozygous dominant disease-causing TOR1A p.Glu303del mutation, and a novel homozygous missense variant (p.Gly318Ser) with a severe arthrogryposis phenotype with developmental delay, strabismus and tremor in three unrelated Iranian families. All parents who were carriers of the TOR1A variant showed no evidence of neurological symptoms or signs, indicating decreased penetrance similar to families with autosomal dominant torsion dystonia-1. The results from cell assays demonstrate that the p.Gly318Ser substitution causes a redistribution of torsinA from the endoplasmic reticulum to the nuclear envelope, similar to the hallmark of the p.Glu303del mutation. Our study highlights that TOR1A mutations should be considered in patients with severe arthrogryposis and further expands the phenotypic spectrum associated with TOR1A mutations.

Structure of the Golgi apparatus is not influenced by a GAG deletion mutation in the dystonia-associated gene Tor1a

Autosomal-dominant, early-onset DYT1 dystonia is associated with an in-frame deletion of a glutamic acid codon (ΔE) in the TOR1A gene. The gene product, torsinA, is an evolutionarily conserved AAA+ ATPase. The fact that constitutive secretion from patient fibroblasts is suppressed indicates that the ΔE-torsinA protein influences the cellular secretory machinery. However, which component is affected remains unclear. Prompted by recent reports that abnormal protein trafficking through the Golgi apparatus, the major protein-sorting center of the secretory pathway, is sometimes associated with a morphological change in the Golgi, we evaluated the influence of ΔE-torsinA on this organelle. Specifically, we examined its structure by confocal microscopy, in cultures of striatal, cerebral cortical and hippocampal neurons obtained from wild-type, heterozygous and homozygous ΔE-torsinA knock-in mice. In live neurons, the Golgi was assessed following uptake of a fluorescent ceramide analog, and in fixed neurons it was analyzed by immuno-fluorescence staining for the Golgi-marker GM130. Neither staining method indicated genotype-specific differences in the size, staining intensity, shape or localization of the Golgi. Moreover, no genotype-specific difference was observed as the neurons matured in vitro. These results were supported by a lack of genotype-specific differences in GM130 expression levels, as assessed by Western blotting. The Golgi was also disrupted by treatment with brefeldin A, but no genotype-specific differences were found in the immuno-fluorescence staining intensity of GM130. Overall, our results demonstrate that the ΔE-torsinA protein does not drastically influence Golgi morphology in neurons, irrespective of genotype, brain region (among those tested), or maturation stage in culture. While it remains possible that functional changes in the Golgi exist, our findings imply that any such changes are not severe enough to influence its morphology to a degree detectable by light microscopy.

Behavioral and electrophysiological characterization of Dyt1 heterozygous knockout mice

DYT1 dystonia is an inherited movement disorder caused by mutations in DYT1 (TOR1A), which codes for torsinA. Most of the patients have a trinucleotide deletion (ΔGAG) corresponding to a glutamic acid in the C-terminal region (torsinA(ΔE)). Dyt1 ΔGAG heterozygous knock-in (KI) mice, which mimic ΔGAG mutation in the endogenous gene, exhibit motor deficits and deceased frequency of spontaneous excitatory post-synaptic currents (sEPSCs) and normal theta-burst-induced long-term potentiation (LTP) in the hippocampal CA1 region. Although Dyt1 KI mice show decreased hippocampal torsinA levels, it is not clear whether the decreased torsinA level itself affects the synaptic plasticity or torsinA(ΔE) does it. To analyze the effect of partial torsinA loss on motor behaviors and synaptic transmission, Dyt1 heterozygous knock-out (KO) mice were examined as a model of a frame-shift DYT1 mutation in patients. Consistent with Dyt1 KI mice, Dyt1 heterozygous KO mice showed motor deficits in the beam-walking test. Dyt1 heterozygous KO mice showed decreased hippocampal torsinA levels lower than those in Dyt1 KI mice. Reduced sEPSCs and normal miniature excitatory post-synaptic currents (mEPSCs) were also observed in the acute hippocampal brain slices from Dyt1 heterozygous KO mice, suggesting that the partial loss of torsinA function in Dyt1 KI mice causes action potential-dependent neurotransmitter release deficits. On the other hand, Dyt1 heterozygous KO mice showed enhanced hippocampal LTP, normal input-output relations and paired pulse ratios in the extracellular field recordings. The results suggest that maintaining an appropriate torsinA level is important to sustain normal motor performance, synaptic transmission and plasticity. Developing therapeutics to restore a normal torsinA level may help to prevent and treat the symptoms in DYT1 dystonia.

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.

Functional architecture of the cell's nucleus in development, aging, and disease

In eukaryotes, the function of the cell's nucleus has primarily been considered to be the repository for the organism's genome. However, this rather simplistic view is undergoing a major shift, as it is increasingly apparent that the nucleus has functions extending beyond being a mere genome container. Recent findings have revealed that the structural composition of the nucleus changes during development and that many of these components exhibit cell- and tissue-specific differences. Increasing evidence is pointing to the nucleus being integral to the function of the interphase cytoskeleton, with changes to nuclear structural proteins having ramifications affecting cytoskeletal organization and the cell's interactions with the extracellular environment. Many of these functions originate at the nuclear periphery, comprising the nuclear envelope (NE) and underlying lamina. Together, they may act as a "hub" in integrating cellular functions including chromatin organization, transcriptional regulation, mechanosignaling, cytoskeletal organization, and signaling pathways. Interest in such an integral role has been largely stimulated by the discovery that many diseases and anomalies are caused by defects in proteins of the NE/lamina, the nuclear envelopathies, many of which, though rare, are providing insights into their more common variants that are some of the major issues of the twenty-first century public health. Here, we review the contributions that mouse mutants have made to our current understanding of the NE/lamina, their respective roles in disease and the use of mice in developing potential therapies for treating the diseases.

Generation of a novel rodent model for DYT1 dystonia

A mutation in the coding region of the Tor1A gene, resulting in a deletion of a glutamic acid residue in the torsinA protein (∆ETorA), is the major cause of the inherited autosomal-dominant early onset torsion dystonia (DYT1). The pathophysiological consequences of this amino acid loss are still not understood. Currently available animal models for DYT1 dystonia provided important insights into the disease; however, they differ with respect to key features of torsinA associated pathology. We developed transgenic rat models harboring the full length human mutant and wildtype Tor1A gene. A complex phenotyping approach including classical behavioral tests, electrophysiology and neuropathology revealed a progressive neurological phenotype in ∆ETorA expressing rats. Furthermore, we were able to replicate key pathological features of torsinA associated pathology in a second species, such as nuclear envelope pathology, behavioral abnormalities and plasticity changes. We therefore suggest that this rat model represents an appropriate new model suitable to further investigate the pathophysiology of ∆ETorA and to test for therapeutic approaches.

Genetic background modulates the phenotype of a mouse model of DYT1 dystonia

DYT1 dystonia is a debilitating neurological disease characterized by involuntary twisting movements. The disease is caused by an in-frame deletion (GAG, "ΔE") mutation in the TOR1A gene that encodes the torsinA protein. Intriguingly, only 30% of mutation carriers exhibit motor symptoms despite the fact that functional brain imaging studies show abnormal brain metabolism in all carriers. Because genetic modifiers may be a determinant of this reduced penetrance, we examined the genetic contribution of three different inbred strains of mice on the DYT1 mutation in animals that are homozygous (Tor1a(ΔE/ΔE)) or heterozygous (Tor1a(ΔE/+); disease state) for the disease-causing ΔE mutation. We find that the DBA/2J, C57BL/6J, and CD1-ICR contribution of genes significantly alter lifespan in Tor1a(ΔE/ΔE) mice, which die during the first few days of life on the 129S6/SvEvTac (129) background. The C57BL/6J (B6) strain significantly decreases life expectancy of Tor1a(ΔE/ΔE) animals but, like 129S6/SvEvTac Tor1a(ΔE/+) mice, congenic C57BL/6J Tor1a(ΔE/+) mice do not exhibit any motor abnormalities. In contrast, the DBA/2J (D2) strain significantly increases life expectancy. This effect was not present in congenic DBA/2J Tor1a(ΔE/ΔE) mice, indicating that the extended lifespan of F2 129/D2 mice was due to a combination of homozygous and heterozygous allelic effects. Our observations suggest that genetic modifiers may alter the penetrance of the ΔE mutation, and that mapping these modifiers may provide fresh insight into the torsinA molecular pathway.

Fraying at the edge mouse models of diseases resulting from defects at the nuclear periphery

Eukaryotic cells compartmentalize their genetic material within the nucleus. The boundary separating the genetic material from the cytoplasm is the nuclear envelope (NE) and lamina. Historically, the NE was perceived as functioning primarily as a barrier regulating the entry and exit of macromolecules between the nucleus and cytoplasm via the nuclear pore complexes (NPCs) that traverse the nuclear membranes. However, recent findings have caused a fundamental reassessment with regard to NE and lamina functions. Evidence now points to the NE and lamina functioning as a "hub" in regulating and perhaps integrating critical cellular functions that include chromatin organization, transcriptional regulation, mechanical integrity of the cell, signaling pathways, as well as acting as a key component of the cytoskeleton. Such an integral role for the nuclear boundary has emerged from increased interest into the functions of the NE/lamina, which has been largely stimulated by the discovery that some 24 different diseases and anomalies are caused by defects in proteins of the NE and lamina.

The dystonia-associated protein torsinA modulates synaptic vesicle recycling

The loss of a glutamic acid residue in the AAA-ATPase (ATPases associated with diverse cellular activities) torsinA is responsible for most cases of early onset autosomal dominant primary dystonia. In this study, we found that snapin, which binds SNAP-25 (synaptosome-associated protein of 25,000 Da) and enhances the association of the SNARE complex with synaptotagmin, is an interacting partner for both wild type and mutant torsinA. Snapin co-localized with endogenous torsinA on dense core granules in PC12 cells and was recruited to perinuclear inclusions containing mutant DeltaE-torsinA in neuroblastoma SH-SY5Y cells. In view of these observations, synaptic vesicle recycling was analyzed using the lipophilic dye FM1-43 and an antibody directed against an intravesicular epitope of synaptotagmin I. We found that overexpression of wild type torsinA negatively affects synaptic vesicle endocytosis. Conversely, overexpression of DeltaE-torsinA in neuroblastoma cells increases FM1-43 uptake. Knockdown of snapin and/or torsinA using small interfering RNAs had a similar inhibitory effect on the exo-endocytic process. In addition, down-regulation of torsinA causes the persistence of synaptotagmin I on the plasma membrane, which closely resembles the effect observed by the overexpression of the DeltaE-torsinA mutant. Altogether, these findings suggest that torsinA plays a role together with snapin in regulated exocytosis and that DeltaE-torsinA exerts its pathological effects through a loss of function mechanism. This may affect neuronal uptake of neurotransmitters, such as dopamine, playing a role in the development of dystonic movements.

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.