Anne Messer

Msh2 deficiency prevents in vivo somatic instability of the CAG repeat in huntington disease transgenic mice

Huntington disease (HD), an autosomal dominant, progressive neurodegenerative disorder, is caused by an expanded CAG repeat sequence leading to an increase in the number of glutamine residues in the encoded protein. The normal CAG repeat range is 5–36, whereas 38 or more repeats are found in the diseased state; the severity of disease is roughly proportional to the number of CAG repeats. HD shows anticipation, in which subsequent generations display earlier disease onsets due to intergenerational repeat expansion. For longer repeat lengths, somatic instability of the repeat size has been observed both in human cases at autopsy and in transgenic mouse models containing either a genomic fragment of human HD exon 1 (ref. 9) or an expanded repeat inserted into the endogenous mouse gene Hdh (ref. 10). With increasing repeat number, the protein changes conformation and becomes increasingly prone to aggregation, suggesting important functional correlations between repeat length and pathology. Because dinucleotide repeat instability is known to increase when the mismatch repair enzyme MSH2 is missing, we examined instability of the HD CAG repeat by crossing transgenic mice carrying exon 1 of human HD (ref. 16) with Msh2–/– mice. Our results show that Msh2 is required for somatic instability of the CAG repeat.

Human single-chain Fv intrabodies counteract in situ huntingtin aggregation in cellular models of Huntington's disease

This investigation was pursued to test the use of intracellular antibodies (intrabodies) as a means of blocking the pathogenesis of Huntingtons disease (HD). HD is characterized by abnormally elongated polyglutamine near the N terminus of the huntingtin protein, which induces pathological protein–protein interactions and aggregate formation by huntingtin or its exon 1-containing fragments. Selection from a large human phage display library yielded a single-chain Fv (sFv) antibody specific for the 17 N-terminal residues of huntingtin, adjacent to the polyglutamine in HD exon 1. This anti-huntingtin sFv intrabody was tested in a cellular model of the disease in which huntingtin exon 1 had been fused to green fluorescent protein (GFP). Expression of expanded repeat HD-polyQ-GFP in transfected cells shows perinuclear aggregation similar to human HD pathology, which worsens with increasing polyglutamine length; the number of aggregates in these transfected cells provided a quantifiable model of HD for this study. Coexpression of anti-huntingtin sFv intrabodies with the abnormal huntingtin-GFP fusion protein dramatically reduced the number of aggregates, compared with controls lacking the intrabody. Anti-huntingtin sFv fused with a nuclear localization signal retargeted huntingtin analogues to cell nuclei, providing further evidence of the anti-huntingtin sFv specificity and of its capacity to redirect the subcellular localization of exon 1. This study suggests that intrabody-mediated modulation of abnormal neuronal proteins may contribute to the treatment of neurodegenerative diseases such as HD, Alzheimers, Parkinsons, prion disease, and the spinocerebellar ataxias.

Dystonin Is Essential for Maintaining Neuronal Cytoskeleton Organization.

The mouse neurological mutant dystonia musculorum (dt) suffers from a hereditary sensory neuropathy. We have previously described the cloning and characterization of the dt gene, which we named dystonin (Dst). We had shown that dystonin is a neural isoform of bullous pemphigoid antigen 1 (Bpag1) with an N-terminal actin-binding domain. It has been shown previously that dystonin is a cytoskeletal linker protein, forming a bridge between F-actin and intermediate filaments. Here, we have used two different antibody preparations against dystonin and detected a high-molecular-weight protein in immunoblot analysis of spinal cord extracts. We also show that this high-molecular-weight protein was not detectable in the nervous system of all dt alleles tested. Immunohistochemical analysis revealed that dystonin was present in different compartments of neurons-cell bodies, dendrites, and axons, regions which are rich in the three elements of the cytoskeleton (F-actin, neurofilaments, and microtubules). Ultrastructural analysis of dt dorsal root axons revealed disorganization of the neurofilament network and surprisingly also of the microtubule network. In this context it is of interest that we observed altered levels of the microtubule-associated proteins MAP2 and tau in spinal cord neurons of different dt alleles. Finally, dt dorsal root ganglion neurons formed neurites in culture, but the cytoskeleton was disorganized within these neurites. Our results demonstrate that dystonin is essential for maintaining neuronal cytoskeleton integrity but is not required for establishing neuronal morphology. Copyright 1998 Academic Press.

Autosomal dominance in a late-onset motor neuron disease in the mouse

A late-onset neurological disease has been identified in a substrain of C57Bl/6 mice. The disorder is characterized by hindlimb weakness and ataxia starting at 5-11 months of age, progressing to severe spastic paralysis of all limbs, with premature death. Histopathology reveals degeneration of upper and lower motoneurons. Both sexes are affected; the mice are fertile, although breeding efficiency is reduced. In outcrosses to wild-type, symptoms have been observed in all obligate heterozygotes, with a similar age range for onset to that of homozygotes. We have designated this autosomal dominant disorder Motor neuron degeneration (Mnd).

An scFv Intrabody against the Nonamyloid Component of α-Synuclein Reduces Intracellular Aggregation and Toxicity

Prevention of abnormal misfolding and aggregation of alpha synuclein (syn) protein in vulnerable neurons should be viable therapeutic strategies for reducing pathogenesis in Parkinsons disease. The nonamyloid component (NAC) region of alpha-syn shows strong tendencies to form beta-sheet structures, and deletion of this region has been shown to reduce aggregation and toxicity in vitro and in vivo. The binding of a molecular species to this region may mimic the effects of such deletions. Single-chain variable fragment (scFv) antibodies retain the binding specificity of antibodies and, when genetically manipulated to create high-diversity libraries, allow in vitro selection against peptides. Accordingly, we used a yeast surface display library of an entire naive repertoire of human scFv antibodies to select for binding to a NAC peptide. Candidate scFv antibodies (after transfer to mammalian expression vectors) were screened for viability in a neuronal cell line by transient cotransfection with A53T mutant alpha-syn. This provided a ranking of the protective efficacies of the initial panel of intracellular antibodies (intrabodies). High steady-state expression levels and apparent conformational epitope binding appeared more important than in vitro affinity in these assays. None of the scFv antibodies selected matched the sequences of previously reported anti-alpha-syn scFv antibodies. A stable cell line expressing the most effective intrabody, NAC32, showed highly significant reductions in abnormal aggregation in two separate models. Recently, intrabodies have shown promising antiaggregation and neuroprotective effects against misfolded mutant huntingtin protein. The NAC32 study extends such work significantly by utilizing information about the pathogenic capacity of a specific alpha-syn region to offer a new generation of in vitro-derived antibody fragments, both for further engineering as direct therapeutics and as a tool for rational drug design for Parkinsons disease.

Histopathology of the late-onset motor neuron degeneration (Mnd) mutant in the mouse

The motor neuron degeneration (Mnd) is characterized by a progressive deterioration of motor function (stiff-legged gait, abnormal limb placements and grasping, and finally paralysis; moving from rear to forelimbs). There is a dramatic degeneration of spinal cord motor neurons, more severe in the lumbosacral than in the other regions, as well as variable pathology in the lower cranial nerves. Upper motor neurons of the red nucleus, reticular formation of the pons and medulla, and restricted areas of the cerebral cortex are also affected. Degenerating motor neurons share many characteristics seen in the human disease amyotropic lateral sclerosis, including loss of Nissl substance, increases in lipofuscin and abnormal cytoplasmic inclusions. Additionally, Mnd, like ALS, is a disease of later life.

Mapping of the motor neuron degeneration (Mnd) gene, a mouse model of amyotrophic lateral sclerosis (ALS).

The motor neuron degeneration mutation (Mnd) causes a late-onset, progressive degeneration of upper and lower motor neurons in mice. After establishing genetic and environmental conditions that distinguish the phenotypes of Mnd/Mnd from +/Mnd mice, Mnd was mapped to proximal Chr 8, using endogenous retroviruses as markers. The map location was confirmed with additional linked polymorphic markers. The outcross/intercross matings to the strain AKR/J, which were used to follow the segregation of the retroviral markers with respect to Mnd, also revealed the existence of a timing effect. Approximately one-fourth of the affected Mnd/Mnd F2 progeny showed accelerated disease. The Mnd mouse model should allow study of mechanisms affecting onset and progression of specific neuronal degeneration in both animal and human neurological disease.

MSH3 polymorphisms and protein levels affect CAG repeat instability in Huntington's disease mice.

Expansions of trinucleotide CAG/CTG repeats in somatic tissues are thought to contribute to ongoing disease progression through an affected individuals life with Huntingtons disease or myotonic dystrophy. Broad ranges of repeat instability arise between individuals with expanded repeats, suggesting the existence of modifiers of repeat instability. Mice with expanded CAG/CTG repeats show variable levels of instability depending upon mouse strain. However, to date the genetic modifiers underlying these differences have not been identified. We show that in liver and striatum the R6/1 Huntingtons disease (HD) (CAG)∼100 transgene, when present in a congenic C57BL/6J (B6) background, incurred expansion-biased repeat mutations, whereas the repeat was stable in a congenic BALB/cByJ (CBy) background. Reciprocal congenic mice revealed the Msh3 gene as the determinant for the differences in repeat instability. Expansion bias was observed in congenic mice homozygous for the B6 Msh3 gene on a CBy background, while the CAG tract was stabilized in congenics homozygous for the CBy Msh3 gene on a B6 background. The CAG stabilization was as dramatic as genetic deficiency of Msh2. The B6 and CBy Msh3 genes had identical promoters but differed in coding regions and showed strikingly different protein levels. B6 MSH3 variant protein is highly expressed and associated with CAG expansions, while the CBy MSH3 variant protein is expressed at barely detectable levels, associating with CAG stability. The DHFR protein, which is divergently transcribed from a promoter shared by the Msh3 gene, did not show varied levels between mouse strains. Thus, naturally occurring MSH3 protein polymorphisms are modifiers of CAG repeat instability, likely through variable MSH3 protein stability. Since evidence supports that somatic CAG instability is a modifier and predictor of disease, our data are consistent with the hypothesis that variable levels of CAG instability associated with polymorphisms of DNA repair genes may have prognostic implications for various repeat-associated diseases.

Mini-review: polybrominated diphenyl ether (PBDE) flame retardants as potential autism risk factors.

Brominated flame retardants, including Polybrominated diphenyl ethers (PBDEs) have been used at increasing levels in home furnishings and electronics over the past 25 years. They have also become widespread environmental pollutants. High PBDE levels have been detected in food, household dust, and indoor air, with subsequent appearance in animal and human tissues. This minireview summarizes studies on the extent to which these compounds can act as potent thyroid hormone mimetics, and emerging studies on long-term neurological effects of acute administration of PBDEs during development. When these data are considered in combination with the extensive literature on stage-dependent effects of thyroid hormone on aspects of brain development that are also implicated in autistic brains, a hypothesis that PBDEs might also serve as autism risk factors emerges. Studies designed to explicitly test this hypothesis will require chronic exposure paradigms, and specific body burden and behavioral monitoring in animal models. Such testing may help to prioritize extensive human epidemiological studies, as well as offer protocols for evaluation of future compounds.

Early exploratory behavior abnormalities in R6/1 Huntington's disease transgenic mice

The Huntingtons disease (HD) R6/1 transgenic mouse model, containing a human huntington gene exon-1 with approximately 115 CAG repeats, has multiple biochemical and neuroanatomical abnormalities. Overt neurological symptoms have a relatively late onset (15-21 weeks of age). In this paper, we report exploratory behavior abnormalities that appear well before the onset of obvious pathology. The first differences in exploratory behaviors were evident by 4 weeks of age, when R6/1 mice were hyperactive relative to wild-type controls. However, by 6-7 weeks of age, R6/1 mice were less active than controls. R6/1 mice traveled less in the activity monitor, engaged in fewer stereotypic movements, spent more time resting, and traveled less distance per movement than did wild-type controls. R6/1 mice also displayed intersession habituation abnormalities over the 3 days of testing. These behavioral abnormalities precede the earliest neurochemical and molecular changes reported in the literature to date, and thus indicate subtle early pathology that has not yet been documented. These behavioral abnormalities also occur prior to weight loss in the transgenic mice. Since we were able to detect an abnormal phenotype at an early age in R6/1 mice, this assay may be a useful tool for evaluating therapeutic agents.