Pathogenesis of neurodegenerative diseases associated with expanded glutamine repeats: new answers, new questions

Spinocerebellar Ataxia Type 17 (SCA17)

In 1999, a polyglutamine expansion was identified in the transcription factor TATA-binding protein (TBP) in a patient with ataxia with negative family history. Subsequently, CAG/CAA repeat expansions in the TBP gene were identified in families with spinocerebellar ataxia (SCA), establishing this repeat expansion as the underlying mutation in SCA type 17 (SCA17). There are several characteristic differences between SCA17 and other polyglutamine diseases. First, SCA17 shows a complex and variable clinical phenotype, in some cases overlapping that of Huntington's disease. Second, compared to the other SCA subtypes caused by expanded trinucleotide repeats, anticipation in SCA17 kindreds is rare because of the characteristic structure of the TBP gene. And thirdly, SCA17 patients often have diagnostic problems that may arise from non-penetrance. Because the gap between normal and abnormal repeat numbers is very narrow, it is difficult to determine a cutoff value for pathologic CAG repeat number in SCA17. Herein, we review the clinical, genetic and pathologic features of SCA17.

Limited Effect of Chronic Valproic Acid Treatment in a Mouse Model of Machado-Joseph Disease

Machado-Joseph disease (MJD) is an inherited neurodegenerative disease, caused by a CAG repeat expansion within the coding region of ATXN3 gene, and which currently lacks effective treatment. In this work we tested the therapeutic efficacy of chronic treatment with valproic acid (VPA) (200mg/kg), a compound with known neuroprotection activity, and previously shown to be effective in cell, fly and nematode models of MJD. We show that chronic VPA treatment in the CMVMJD135 mouse model had limited effects in the motor deficits of these mice, seen mostly at late stages in the motor swimming, beam walk, rotarod and spontaneous locomotor activity tests, and did not modify the ATXN3 inclusion load and astrogliosis in affected brain regions. However, VPA chronic treatment was able to increase GRP78 protein levels at 30 weeks of age, one of its known neuroprotective effects, confirming target engagement. In spite of limited results, the use of another dosage of VPA or of VPA in a combined therapy with molecules targeting other pathways, cannot be excluded as potential strategies for MJD therapeutics.

Serotonergic signalling suppresses ataxin 3 aggregation and neurotoxicity in animal models of Machado-Joseph disease

Polyglutamine diseases are a class of dominantly inherited neurodegenerative disorders for which there is no effective treatment. Here we provide evidence that activation of serotonergic signalling is beneficial in animal models of Machado-Joseph disease. We identified citalopram, a selective serotonin reuptake inhibitor, in a small molecule screen of FDA-approved drugs that rescued neuronal dysfunction and reduced aggregation using a Caenorhabditis elegans model of mutant ataxin 3-induced neurotoxicity. MOD-5, the C. elegans orthologue of the serotonin transporter and cellular target of citalopram, and the serotonin receptors SER-1 and SER-4 were strong genetic modifiers of ataxin 3 neurotoxicity and necessary for therapeutic efficacy. Moreover, chronic treatment of CMVMJD135 mice with citalopram significantly reduced ataxin 3 neuronal inclusions and astrogliosis, rescued diminished body weight and strikingly ameliorated motor symptoms. These results suggest that small molecule modulation of serotonergic signalling represents a promising therapeutic target for Machado-Joseph disease.

Deletion of membrane-associated Asp23 leads to upregulation of cell wall stress genes in Staphylococcus aureus

With about 25 000 molecules per cell, Asp23 is one of the most abundant proteins in Staphylococcus aureus. Asp23 has been characterized as a protein that, following an alkaline shock, accumulates in the soluble protein fraction. Transcription of the asp23 gene is exclusively regulated by the alternative sigma factor σ(B) , which controls the response of the bacterium to environmental stress. Sequence analysis identified Asp23 as a member of the widely distributed Pfam DUF322 family, precluding functional predictions based on its sequence. Using fluorescence microscopy we found that Asp23 colocalized with the cell membrane of Staphylococcus aureus. Since Asp23 has no recognizable transmembrane spanning domains, we initiated a search for proteins that link Asp23 to the cell membrane. We identified SAOUHSC_02443 as the Asp23 membrane anchor and have renamed it AmaP (Asp23 membrane anchoring protein). Deletion of the asp23 gene led to an upregulation of the cell wall stress response. In summary, we have identified Asp23 as a membrane-associated protein and we suggest a function for Asp23 in cell envelope homoeostasis.

Mutation analysis of the NRXN1 gene in a Chinese autism cohort

Autism is a brain developmental disorder characterized by impaired social interaction and communication, as well as restricted and repetitive behaviors. The neurexin-1(NRXN1) gene mapped on chromosome 2p16.3 encodes neurexin, a cell adhesion molecule and receptor in the vertebrate nervous system. Rare de novo alterations and copy number variations (CNVs) suggested neurexin-1 as a candidate gene for the pathogenesis of autism, but data on the gene mutation of neurexin-1 in Chinese Han population with autism are limited. By direct sequencing, we analyzed the entire coding regions and associated splice junctions of neurexin-1 in 313 Chinese autism patients. For exons in which non-synonymous variants were identified, sequencing was performed in 500 healthy controls. We identified 22 variants in the neurexin-1 coding regions, including 7 missense variants, 3 deletions, and 12 synonymous mutations. Among them, 3 missense and 3 synonymous variants were not reported in the dbSNP database and absent in 500 control subjects; whereas 4 missense variants, 3 deletions and 3 synonymous mutations were not reported in the dbSNP database but were identified in the control subjects. However, there is no significant association of these mutations with autism risk. Interestingly, there was a statistically significant association of neurexin-1 SNP P300P (rs2303298) with risk of autism (26.2% vs. 13.8%; χ(2) = 22.487; p = 3.45E-006; OR = 2.152 (1.559-2.970)). Our data suggest a possible association of neurexin-1 with autism risk in Chinese Han population, warranting further large-scale study on this gene.

Nuclear Inclusions and Pseudoinclusions: Friends or Foes of the Surgical Pathologist?

Abnormal substances in the nuclei that can be observed by light microscopy are often broadly referred to as nuclear inclusions. Although their recognition in the appropriate clinicopathological settings can aid in the diagnosis of some disease entities and tumor types, they can also be a source of error. There are 2 morphologically distinct types of inclusions with different mechanisms of formation and diagnostic significance, including bona fide nuclear inclusions and nuclear pseudoinclusions. Bona fide nuclear inclusions result from accumulation in the nuclei of viral particles, cytoplasmic materials (such as surfactant, immunoglobulin, and glycogen), biotin, nuclear lamins, or polyglutamine. Some of them are diagnostically helpful, such as surfactant inclusion, which can support the pulmonary origin of an adenocarcinoma, whereas others may be misleading, such as biotin inclusion, which can be mistaken for herpes infection. Nuclear pseudoinclusions, which represent invaginations of cytoplasm into the nucleus, are delimited by the nuclear membrane. Although not totally specific, they are particularly common in papillary thyroid carcinoma, meningioma, and usual ductal hyperplasia of the breast and hence may aid in the diagnosis of these entities. Nuclear pseudo-pseudoinclusions, which are artefactual bubbles in the nuclei that mimic nuclear pseudoinclusions or clear nuclei, can lead to misdiagnosis of follicular adenoma or hyperplastic nodule as papillary thyroid carcinoma.

C-terminal deletion of the atrophin-1 protein results in growth retardation but not neurodegeneration in mice

Dentatorubral-pallidoluysian atrophy (DRPLA) is a dominant hereditary neurodegenerative disorder caused by the expansion of a poly-glutamine (poly-Q) repeat in Atrophin-1 protein. Ectopic expression of a poly-Q expanded human Atrophin-1 is sufficient to induce DRPLA phenotypes in mice. However, it is still unclear whether the dominant effect of poly-Q expansion is due to the functional interference with wild-type Atrophin-1 proteins, which exist in both patients and transgenic mice. Here we report the generation and analysis of an Atrophin-1 targeting allele that expresses a truncated protein lacking both the poly-Q repeat and following C-terminal peptides. Homozygous mutants exhibit growth retardation and progressive male infertility, but no obvious signs of neurodegeneration. Moreover, the mutant allele neither blocked nor enhanced the neurodegenerative phenotypes caused by a poly-Q expanded transgene. These results support the model that poly-Q expanded Atrophin-1 proteins cause DRPLA in a manner independent of any functional interaction with wild-type Atrophin-1 proteins.

Striatal expression of a calmodulin fragment improved motor function, weight loss, and neuropathology in the R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder, caused by a polyglutamine expansion in the huntingtin protein (htt). Increasing evidence suggests that transglutaminase (TGase) plays a critical role in the pathophysiology of HD possibly by stabilizing monomeric, polymeric and aggregated htt. We previously reported that in HEK293 and SH-SY5Y cells expression of a calmodulin (CaM)-fragment, consisting of amino acids 76-121 of CaM, decreased binding of CaM to mutant htt, TGase-modified htt and cytotoxicity associated with mutant htt and normalized intracellular calcium release. In this study, an adeno-associated virus (AAV) that expresses the CaM-fragment was injected into the striatum of HD transgenic R6/2 mice. The CaM-fragment significantly reduced body weight loss and improved motor function as indicated by improved rotarod performance, longer stride length, lower stride frequency, fewer low mobility bouts and longer travel distance than HD controls. A small but insignificant increase in survival was observed in R6/2 mice with CaM-fragment expression. Immunoprecipitation studies show that expression of the CaM-fragment reduced TGase-modified htt in the striatum of R6/2 mice. The percentage of htt-positive nuclei and the size of intranuclear htt aggregates were reduced by the CaM-fragment without striatal volume changes. The effects of CaM-fragment appear to be selective, as activity of another CaM-dependent enzyme, CaM-dependent kinase II, was not altered. Moreover, inhibition of TGase-modified htt was substrate-specific since overall TGase activity in the striatum was not altered by treatment with the CaM-fragment. Together, these results suggest that disrupting CaM-htt interaction may provide a new therapeutic strategy for HD.

Polyglutamine expansion in Drosophila: thermal stress and Hsp70 as selective agents

Repetitive DNA sequences that encode polyglutamine tracts are prone to expansion and cause highly deleterious phenotypes of neurodegeneration. Despite this tendency,polyglutamine tracts ("polyQs") are conserved features of eukaryotic genomes. PolyQs are the most frequent protein-coding homotypic repeat in insect genomes, and are found predominantly in genes encoding transcription factors conserved from Drosophila through human. Although highly conserved across species, polyQ lengths vary widely within species. In D. melanogaster, polyQs in 25 genes have more alleles and higher heterozygosity than all other poly-amino acid tracts. The heat shock protein Hsp70 is a principal suppressor of polyQ expansions and may play a key role in modulating the phenotypes of the alleles that encode them. Hsp70 also promotes tolerance of natural thermal stress in Drosophila and diverse organisms,a role which may deplete the chaperone from buffering against polyQ toxicity. Thus in stressful environments, natural selection against long polyQ alleles more prone to expansion and deleterious phenotypes may be more effective. This hypothesis can be tested by measuring the phenotypic interactions between Hsp70 and polyQ transgenes in D. melanogaster undergoing natural thermal stress, an approach which integrates comparative genomics with experimental and ecological genetics.

Transglutaminase is linked to neurodegenerative diseases

Transglutaminase catalyzes a covalent bond between peptide-bound glutamine residues and either lysine-bound peptide residues or mono- or polyamines. Multiple lines of evidence suggest that transglutaminase is involved in neurodegenerative diseases including Alzheimer disease, progressive supranuclear palsy, Huntington disease (HD), and Parkinson disease. In all of the neurodegenerative diseases examined to date, transglutaminase enzyme activity is upregulated in selectively vulnerable brain regions, transglutaminase proteins are associated with inclusion bodies characteristic of the diseases, and prominent proteins in the inclusion bodies are modified by transglutaminase enzymes. These prominent proteins in the inclusion bodies, including tau, alpha-synuclein, and huntingtin protein, are modified by transglutaminase in vitro and alpha-synuclein and huntingtin protein are modified in cells in culture. Similar changes in transglutaminase and transglutaminase-modified proteins are replicated in transgenic mouse models of the neurodegenerative diseases, including Huntington disease and progressive supranuclear palsy. Lastly, inhibition of transglutaminase either via drug treatments or molecular approaches is beneficial for the treatment of HD transgenic mice but has yet to be explored for the other neurodegenerative diseases. Further research is needed to determine the specific role(s) that transglutaminase plays in the pathophysiology of neurodegenerative diseases with possible implications for transglutaminase as a therapeutic target.

Sodium Butyrate Ameliorates Histone Hypoacetylation and Neurodegenerative Phenotypes in a Mouse Model for DRPLA

Dentatorubral-pallidoluysian atrophy (DRPLA) is a progressive neurodegenerative disease caused by polyglutamine expansion within the Atrophin-1 protein. To study the mechanism of this disease and to test potential therapeutic methods, we established Atro-118Q transgenic mice, which express in neurons a mutant human Atrophin-1 protein that contains an expanded stretch of 118 glutamines. Consistent with the results from previous studies on transgenic mice that expressed mutant Atrophin-1 with 65 glutamines, Atro-118Q mice exhibited several neurodegenerative phenotypes that are commonly seen in DRPLA patients, including ataxia, tremors, and other motor defects. Overexpression of wild-type human Atrophin-1 could not rescue the motor and survival defects in Atro-118Q mice, indicating that the mutant protein with polyglutamine expansion does not simply function in a dominant negative manner. Biochemical analysis of Atro-118Q mice revealed hypoacetylation of histone H3 in brain tissues and thus suggested that global gene repression is an underlying mechanism for neurodegeneration in this mouse model. We further show that intraperitoneal administration of sodium butyrate, a histone deacetylase inhibitor, ameliorated the histone acetylation defects, significantly improved motor performance, and extended the average life span of Atro-118Q mice. These results support the hypothesis that transcription deregulation plays an important role in the pathogenesis of polyglutamine expansion diseases and suggest that reversion of transcription repression with small molecules such as sodium butyrate is a feasible approach to treating DRPLA symptoms.

Transcriptional repression and cell death induced by nuclear aggregates of non-polyglutamine protein

Nuclear aggregates of polyglutamine (polyQ)-expanded proteins are associated with a number of neurodegenerative diseases including Huntington's disease (HD) and spinocerebellar ataxias (SCAs). The nuclear deposition of polyQ proteins correlates with rearrangements of nuclear matrix, transcriptional dysregulation, and cell death. To explore the requirement for polyQ tracks in educing such cellular responses, we examined whether a non-polyQ protein can deposit as nuclear aggregates and elicit similar responses. We report that a protein chimera (GFP170*) composed of the green fluorescent protein (GFP) fused to an internal fragment of the Golgi Complex Protein (GCP-170) forms nuclear aggregates analogous to those formed by polyQ proteins. Like the polyQ nuclear aggregates, GFP170* inclusions recruit molecular chaperones and proteasomal components, alter nuclear structures containing the promyelocytic leukemia protein (PML), recruit transcriptional factors such as CREB-binding protein (CBP) and p53, repress p53 transcriptional activity, and induce cell death. Our results indicate that nuclear aggregation and transcriptional effects are not unique to polyQ-containing proteins and may represent a general response to misfolded proteins in the nucleus.

Satellog: a database for the identification and prioritization of satellite repeats in disease association studies

Background

To date, 35 human diseases, some of which also exhibit anticipation, have been associated with unstable repeats. Anticipation has been reported in a number of diseases in which repeat expansion may have a role in etiology. Despite the growing importance of unstable repeats in disease, currently no resource exists for the prioritization of repeats. Here we present Satellog, a database that catalogs all pure 1–16 repeat unit satellite repeats in the human genome along with supplementary data. Satellog analyzes each pure repeat in UniGene clusters for evidence of repeat polymorphism.

Results

A total of 5,546 such repeats were identified, providing the first indication of many novel polymorphic sites in the genome. Overall, polymorphic repeats were over-represented within 3'-UTR sequence relative to 5'-UTR and coding sequence. Interestingly, we observed that repeat polymorphism within coding sequence is restricted to trinucleotide repeats whereas UTR sequence tolerated a wider range of repeat period polymorphisms. For each pure repeat we also calculate its repeat length percentile rank, its location either within or adjacent to EnsEMBL genes, and its expression profile in normal tissues according to the GeneNote database.

Conclusion

Satellog provides the ability to dynamically prioritize repeats based on any of their characteristics (i.e. repeat unit, class, period, length, repeat length percentile rank, genomic co-ordinates), polymorphism profile within UniGene, proximity to or presence within gene regions (i.e. cds, UTR, 15 kb upstream etc.), metadata of the genes they are detected within and gene expression profiles within normal human tissues. Unstable repeats associated with 31 diseases were analyzed in Satellog to evaluate their common repeat properties. The utility of Satellog was highlighted by prioritizing repeats for Huntington's disease and schizophrenia. Satellog is available online at .

Neuronal inclusions in degenerative disorders

This brief paper analyzes a few degenerative diseases expressing as movement disorders and featuring at sub-cellular level the presence of neuronal inclusions in selective brain regions. We will first draw a short draft of representative neurological diseases featuring inclusion bodies by describing the type of inclusions occurring in various disorders and analyzing both common features and distinctive aspects. As a further step, we move from the bed to the bench side discussing recent developments obtained from experimental models of these disorders which shed new light into the cause and progression of neuronal inclusions, thus helping to understand the pathophysiology of neuronal degeneration underlying movement disorders. In line with this, we will focus on recent studies which led to reproduce neuronal inclusions in vivo and in vitro by manipulating selective cellular structures/enzymatic pathways. In this way, we will try to encompass the dynamics of inclusion formation based on their fine ultrastructure and the analysis of the molecular components as well as their subcellular compartmentalization trying to relate the dynamics of inclusion formation and the pathophysiology of the disease process. An emphasis will be made on the ubiquitin proteasome system and Parkinson's disease where the analysis of neuronal inclusions enlightened potential therapeutic strategies to occlude the progression of this neuronal degeneration featured by movement disorders.

Mutant Huntingtin Protein: A Substrate for Transglutaminase 1, 2, and 3

The most prominent neuropathologic hallmarks of Huntington disease (HD) are cortical and striatal perinuclear cytoplasmic aggregates and intranuclear inclusions of mutant huntingtin. Our laboratory previously demonstrated that huntingtin protein colocalizes with transglutaminase 2 and its product, the epsilon-(gamma-glutamyl)lysine bond in intranuclear inclusions in HD frontal cortex. We also found that transglutaminase 2 cross-links N-terminal fragments of mutant huntingtin (htt-N63-148Q-myc) in cells in culture. We now report a significant increase in transglutaminase 2 mRNA in HD cortex (225% of controls) and striatum (399% of controls). Expression of the short transglutaminase 2 mRNA splice variant was not detectable in HD, although previous studies demonstrated upregulation in Alzheimer disease and progressive supranuclear palsy. Cells co-transfected with GFP-tagged transglutaminase 1, 2, or 3 and htt-N63-148Q-myc exhibit increased cross-linked huntingtin in the insoluble fraction of cell lysates. Treatment of cells with cystamine, a chemical inhibitor of transglutaminase, decreased aggregated and cross-linked huntingtin and increased viability of cells that were transfected with transglutaminase 2 and htt-N63-148Q-myc. These data suggest that transglutaminase 1, 2, and 3 could be involved in cross-linking of huntingtin into intranuclear inclusions in HD and that inhibiting transglutaminase should be explored as a potential treatment strategy for HD.

Calmodulin regulates transglutaminase 2 cross-linking of huntingtin

Striatal and cortical intranuclear inclusions and cytoplasmic aggregates of mutant huntingtin are prominent neuropathological hallmarks of Huntington's disease (HD). We demonstrated previously that transglutaminase 2 cross-links mutant huntingtin in cells in culture and demonstrated the presence of transglutaminase-catalyzed cross-links in the HD cortex that colocalize with transglutaminase 2 and huntingtin. Because calmodulin regulates transglutaminase activity in erythrocytes, platelets, and the gizzard, we hypothesized that calmodulin increases cross-linking of huntingtin in the HD brain. We found that calmodulin colocalizes at the confocal level with transglutaminase 2 and with huntingtin in HD intranuclear inclusions. Calmodulin coimmunoprecipitates with transglutaminase 2 and huntingtin in cells transfected with myc-tagged N-terminal huntingtin fragments containing 148 polyglutamine repeats (htt-N63-148Q-myc) and transglutaminase 2 but not in cells transfected with myc-tagged N-terminal huntingtin fragments containing 18 polyglutamine repeats. Our previous studies demonstrated that transfection with both htt-N63-148Q-myc and transglutaminase 2 resulted in cross-linking of mutant huntingtin protein fragments and the formation of insoluble high-molecular-weight aggregates of huntingtin protein fragments. Transfection with transglutaminase 2 and htt-N63-148Q-myc followed by treatment of cells with N-(6-aminohexyl)-1-naphthalenesulfonamide, a calmodulin inhibitor, resulted in a decrease in cross-linked huntingtin. Inhibiting the interaction of calmodulin with transglutaminase and huntingtin protein could decrease cross-linking and diminish huntingtin aggregate formation in the HD brain.

Searching for needles in haystacks-the genetics of multiple sclerosis and other common neurological diseases

Recent years have witnessed considerable advances in our understanding of monogenic neurodegenerative diseases, such as hereditary motor sensory neuropathy and Huntington's Chorea. Progress has been slower in the genetic dissection of other more common neurological diseases with a complex mode of inheritance. The identification of relevant genes in some, such as Alzheimer's disease (AD) or Parkinson's disease (PD), has been facilitated by characteristic pathological findings and autosomal dominant inheritance in a proportion of early onset families. Attempts to identify relevant genes for multiple sclerosis have highlighted the role of the major histocompatibility complex, but so far failed to unequivocally implicate other immunologic or structural candidate genes. Six linkage-based whole genome screens have been completed in multiple sclerosis and several regions of interest have been identified. As technology and progress in the human genome project advance, it has become clear that future studies of common neurological diseases will depend critically on the availability of large sample sizes and will have to address issues of disease heterogeneity.

PML nuclear bodies and neuronal intranuclear inclusion in polyglutamine diseases

In polyglutamine diseases, accumulation in the nucleus of mutant proteins induces the formation of neuronal intranuclear inclusions (NIIs). The nucleus is compartmentalized into structural and functional domains, which are involved in NII formation. Promyelocytic leukemia protein (PML), a major component of nuclear bodies, and mSin3A, a component of the transcription co-repressor complex, were used to investigate how the intranuclear domains/sites relate to NII formation in SCA2, SCA3, SCA7, SCA17 and DRPLA brains. We demonstrate that the size of PML-positive intranuclear structures was larger in pathological brains than in control ones and that these structures contained mutant proteins. PML colocalized only with small NIIs, which maintained the ring-like structure of normal nuclear bodies. Enlarged ring-like PML-positive structures, devoid of mutant proteins, were also found and might represent structures where mutant polyglutamine proteins have been successfully processed. These data suggest that NIIs originate from nuclear bodies, where mutant proteins accumulate for degradation.

Parkin facilitates the elimination of expanded polyglutamine proteins and leads to preservation of proteasome function

Parkin, the most commonly mutated gene in familial Parkinson's disease, encodes an E3 ubiquitin ligase. A number of candidate substrates have been identified for parkin ubiquitin ligase action including CDCrel-1, o-glycosylated alpha-synuclein, Pael-R, and synphilin-1. We now show that parkin promotes the ubiquitination and degradation of an expanded polyglutamine protein. Overexpression of parkin reduces aggregation and cytotoxicity of an expanded polyglutamine ataxin-3 fragment. Using a cellular proteasome indicator system based on a destabilized form of green fluorescent protein, we demonstrate that parkin reduces proteasome impairment and caspase-12 activation induced by an expanded polyglutamine protein. Parkin forms a complex with the expanded polyglutamine protein, heat shock protein 70 (Hsp70) and the proteasome, which may be important for the elimination of the expanded polyglutamine protein. Hsp70 enhances parkin binding and ubiquitination of expanded polyglutamine protein in vitro suggesting that Hsp70 may help to recruit misfolded proteins as substrates for parkin E3 ubiquitin ligase activity. We speculate that parkin may function to relieve endoplasmic reticulum stress by preserving proteasome activity in the presence of misfolded proteins. Loss of parkin function and the resulting proteasomal impairment may contribute to the accumulation of toxic aberrant proteins in neurodegenerative diseases including Parkinson's disease.

Transglutaminase cross-links in intranuclear inclusions in Huntington disease

Cortical and striatal perinuclear cytoplasmic aggregates and intranuclear inclusions of mutant huntingtin are neuropathological hallmarks of Huntington disease (HD). Although the mechanisms involved in the formation of these aggregates are unclear, a recent hypothesis implicates cross-linking of mutant huntingtin protein into aggregates by transglutaminase. This study explores the hypothesis that transglutaminase catalyzes cross-linking of huntingtin into intranuclear inclusions. Using immunofluorescence and confocal microscopy we demonstrate 99% colocalization of transglutaminase-catalyzed epsilon-(gamma-glutamyl) lysine covalent cross-links with nuclear aggregates of huntingtin protein in the frontal cortex of postmortem HD brain tissue. Furthermore, the transglutaminase 2 isoform colocalizes with both huntingtin protein and epsilon-(gamma-glutamyl) lysine covalent cross-links in HD intranuclear inclusions. Transient transfection of N-terminally truncated huntingtin with an expanded glutamine domain (htt-N63-148Q-myc) with and without and transglutaminase 2 into HEK 293T cells resulted in an increase in cross-linked huntingtin in the insoluble formic acid-treated pellet in comparison to transfection of N-terminally truncated huntingtin with normal length glutamine domain (htt-N63-18Q-myc). Transfection with both htt-N63-148Q-myc and transglutaminase 2 resulted in high molecular weight huntingtin in the insoluble fraction. These data support the hypothesis that transglutaminase catalyzed cross-linking of mutant huntingtin is involved in the formation and/or stabilization of huntingtin protein aggregates in HD. Based on these and other studies, modulation of transglutaminase activity could be explored as a treatment for HD.

Comparison of huntingtin proteolytic fragments in human lymphoblast cell lines and human brain

Proteolytic fragments of huntingtin (htt) in human lymphoblast cell lines from HD and control cases were compared to those in human HD striatal and cortical brain regions, by western blots with epitope-specific antibodies. HD lymphoblast cell lines were heterozygous and homozygous for the expanded CAG triplet repeat mutations, which represented adult onset and juvenile HD. Lymphoblasts contained NH(2)- and COOH-terminal htt fragments of 20-100 kDa, with many similar htt fragments in HD compared to control lymphoblast cell lines. Detection of htt fragments in a homozygous HD lymphoblast cell line demonstrated proteolysis of mutant htt. It was of interest that adult HD lymphoblasts showed a 63-64 kDa htt fragment detected by the NH(2)-domain antibody, which was not found in controls. In addition, control and HD heterozygous cells showed a common 60-61 kDa band (detected by the NH(2)-domain antibody), which was absent in homozygous HD lymphoblast cells. These results suggest that the 63-64 kDa and 60-61 kDa NH(2)-domain htt fragments may be associated with mutant and normal htt, respectively. In juvenile HD lymphoblasts, the presence of a 66-kDa, instead of the 63-64 kDa N-domain htt fragment, may be consistent with the larger polyglutamine expansion of mutant htt in the juvenile case of HD. Lymphoblasts and striatal or cortical regions from HD brains showed similarities and differences in NH(2)- and COOH-terminal htt fragments. HD striatum showed elevated levels of 50 and 45 kDa NH(2)-terminal htt fragments [detected with anti(1-17) serum] compared to controls. Cortex from HD and control brains showed similar NH(2)-terminal htt fragments of 50, 43, 40, and 20 kDa; lymphoblasts also showed NH(2)-terminal htt fragments of 50, 43, 40, and 20 kDa. In addition, a 48-kDa COOH-terminal htt band was elevated in HD striatum, which was also detected in lymphoblasts. Overall, results demonstrate that mutant and normal htt undergo extensive proteolysis in lymphoblast cell lines, with similarities and differences compared to htt fragments observed in HD striatal and cortical brain regions. These data for in vivo proteolysis of htt are consistent with the observed neurotoxicity of recombinant NH(2)-terminal mutant htt fragments expressed in transgenic mice and in transfected cell lines that may be related to the pathogenesis of HD.

Creatine increase survival and delays motor symptoms in a transgenic animal model of Huntington's disease

There is substantial evidence for bioenergetic defects in Huntington's disease (HD). Creatine administration increases brain phosphocreatine levels and it stabilizes the mitochondrial permeability transition. We examined the effects of creatine administration in a transgenic mouse model of HD produced by 82 polyglutamine repeats in a 171 amino acid N-terminal fragment of huntingtin (N171-82Q). Dietary supplementation of 2% creatine significantly improved survival, slowed the development of motor symptoms, and delayed the onset of weight loss. Creatine lessened brain atrophy and the formation of intranuclear inclusions, attenuated reductions in striatal N-acetylaspartate as assessed by NMR spectroscopy, and delayed the development of hyperglycemia. These results are similar to those observed using dietary creatine supplementation in the R6/2 transgenic mouse model of HD and provide further evidence that creatine may exert therapeutic effects in HD.

Anticipation, imprinting, trinucleotide repeat expansions and psychoses

1. Since 1991, approximately 20 trinucleotide repeat expansion type neurodegenerative disorders have been reported. They are clinically characterized by anticipation, i.e., worsening severity or earlier age at onset with each succeeding generation for an inherited disease, and imprinting, i.e., a process whereby specific genes are differentially marked during parental gametogenesis, resulting in the differential expression of these genes in the embryo and adult. 2. The phenomenon of anticipation in psychoses has been pointed out since the 19th century; however, it was ignored because no one knew the genetic mechanism underlying this type of inheritance pattern at the time, and because of several possible biases. 3. The discovery of trinucleotide repeat expansion diseases has reawakened interest in the phenomenon of anticipation in psychiatric diseases. Anticipation has been confirmed in schizophrenia, mood disorders, and anxiety disorders in much more sophisticated manners, although still not perfectly. 4. Molecular approaches as well as clinical ones have been taken to reveal the involvement of trinucleotide repeat expansion mechanism in psychoses by means of direct analyses of candidate genes, RED and DIRECT. Most efforts have been made for CAG type trinucleotide repeats. So far, direct analyses have failed to reveal pathogenic gene(s). There were several positive RED data at first, however, nowadays there seems to be a tendency of much more negative results. The DIRECT results did not support trinucleotide repeat expansions mechanism in psychoses either. One plausable explanation for the 'false positive' result is the presence of CAG trinucleotide repeats which are highly polymorphic but not associated with an obvious abnormal phenotype. Screening for trinucleotide repeats other than ones of the CAG type remained to be performed.

A census of glutamine/asparagine-rich regions: implications for their conserved function and the prediction of novel prions

Glutamine/asparagine (Q/N)-rich domains have a high propensity to form self-propagating amyloid fibrils. This phenomenon underlies both prion-based inheritance in yeast and aggregation of a number of proteins involved in human neurodegenerative diseases. To examine the prevalence of this phenomenon, complete proteomic sequences of 31 organisms and several incomplete proteomic sequences were examined for Q/N-rich regions. We found that Q/N-rich regions are essentially absent from the thermophilic bacterial and archaeal proteomes. Moreover, the average Q/N content of the proteins in these organisms is markedly lower than in mesophilic bacteria and eukaryotes. Mesophilic bacterial proteomes contain a small number (0-4) of proteins with Q/N-rich regions. Remarkably, Q/N-rich domains are found in a much larger number of eukaryotic proteins (107-472 per proteome) with diverse biochemical functions. Analyses of these regions argue they have been evolutionarily selected perhaps as modular "polar zipper" protein-protein interaction domains. These data also provide a large pool of potential novel prion-forming proteins, two of which have recently been shown to behave as prions in yeast, thus suggesting that aggregation or prion-like regulation of protein function may be a normal regulatory process for many eukaryotic proteins with a wide variety of functions.

Neural development and neurodegeneration: two faces of neuropathy target esterase

Neuropathy target esterase (NTE) is an integral membrane protein in vertebrate neurons. Recent evidence suggests that NTE plays an important role in neural development, possibly via involvement in a signalling pathway between neurons and glial cells. NTE is a member of a novel protein family, represented in organisms from bacteria to man. NTE comprises an N-terminal regulatory domain (with some sequence similarity to cyclic nucleotide-binding proteins) and a C-terminal catalytic domain: the latter has three predicted transmembrane segments and requires membrane-association for activity. In vitro, NTE potently catalyses hydrolysis of phenyl valerate: however, its physiological substrate is likely to be a metabolite of a much longer chain carboxylic acid, possibly associated with cell membranes. NTE was discovered originally as the primary target for those organophosphorus esters (OPs) which cause a delayed neuropathy with degeneration of long axons in peripheral nerves and spinal cord. Paradoxically, NTE's catalytic activity appears redundant in adult vertebrates. Neuropathic OPs react covalently with NTE in a rapid two-step process which not only inhibits catalytic activity but also leaves a negatively-charged OP group attached to the active site serine. The latter event is proposed to induce a toxic gain of function in NTE. OP-modified NTE somehow engenders a "chemical transection of the axon". In turn, this leads to calcium entry, elevation of axonal calpain activity and Wallerian-type degeneration. The net damage to peripheral nerve axons is a balance between ongoing degenerative and repair processes: the latter involve serine hydrolases which can be inhibited by the same OPs used to modify NTE.

Enhanced association of mutant triosephosphate isomerase to red cell membranes and to brain microtubules

In a Hungarian family with triosephosphate isomerase (TPI; D-glyceraldehyde-3-phosphate keto-isomerase, EC 5.3.1.1) deficiency, two germ-line identical, but phenotypically differing compound heterozygote brothers (one of them with neurological disorder) have been identified with the same very low (<5%) TPI activity and 20- or 40-fold higher erythrocyte dihydroxyacetone phosphate levels as compared with normal controls. Our present studies with purified TPI and hemolysates revealed the binding of TPI, and the binding of human wild-type and mutant TPIs in hemolysate, to the red cell membrane, and the interference of binding with other hemolysate proteins. The binding of the mutant TPI is enhanced as compared with the wild-type enzyme. The increased binding is influenced by both the altered structure of the mutant and the changes in the red cell membrane. Compared with binding of glyceraldehyde-3-phosphate dehydrogenase, the isomerase binding is much less sensitive to ionic strength or blocking of the N-terminal tail of the band-3 transmembrane protein. The binding of TPIs to the membrane decreases the isomerase activity, resulting in extremely high dihydroxyacetone phosphate levels in deficient cells. In cell-free brain extract, tubulin copolymerizes with TPI and with other cytosolic proteins forming highly decorated microtubules as shown by immunoblot analysis with anti-TPI antibody and by electron microscopic images. The efficacy order of TPI binding to microtubules is propositus > brother without neurological disorder > normal control. This distinct microcompartmentation of mutant proteins may be relevant in the development of the neurodegenerative process in TPI deficiency and in other, more common neurological diseases.

Saccharomyces cerevisiae pol30 (proliferating cell nuclear antigen) mutations impair replication fidelity and mismatch repair

To understand the role of POL30 in mutation suppression, 11 Saccharomyces cerevisiae pol30 mutator mutants were characterized. These mutants were grouped based on their mutagenic defects. Many pol30 mutants harbor multiple mutagenic defects and were placed in more than one group. Group A mutations (pol30-52, -104, -108, and -126) caused defects in mismatch repair (MMR). These mutants exhibited mutation rates and spectra reminiscent of MMR-defective mutants and were defective in an in vivo MMR assay. The mutation rates of group A mutants were enhanced by a msh2 or a msh6 mutation, indicating that MMR deficiency is not the only mutagenic defect present. Group B mutants (pol30-45, -103, -105, -126, and -114) exhibited increased accumulation of either deletions alone or a combination of deletions and duplications (4 to 60 bp). All deletion and duplication breakpoints were flanked by 3 to 7 bp of imperfect direct repeats. Genetic analysis of one representative group B mutant, pol30-126, suggested polymerase slippage as the likely mutagenic mechanism. Group C mutants (pol30-100, -103, -105, -108, and -114) accumulated base substitutions and exhibited synergistic increases in mutation rate when combined with msh6 mutations, suggesting increased DNA polymerase misincorporation as a mutagenic defect. The synthetic lethality between a group A mutant, pol30-104, and rad52 was almost completely suppressed by the inactivation of MSH2. Moreover, pol30-104 caused a hyperrecombination phenotype that was partially suppressed by a msh2 mutation. These results suggest that pol30-104 strains accumulate DNA breaks in a MSH2-dependent manner.

Neuronal apoptosis induced by beta-amyloid is mediated by caspase-8

The Alzheimer disease-associated beta-amyloid peptide has been shown to induce apoptotic neuronal death. In the present study, we test the hypothesis that the apoptotic pathway activated by beta-amyloid is similar to the pathway activated by the Fas/TNFR family of death receptors, which requires caspase-8 activity and adaptor proteins such as FADD. We demonstrate that the selective caspase-8 inhibitor IETD-fmk blocks neuronal death induced by beta-amyloid. Furthermore, using viral-mediated gene delivery, we show that neurons expressing dominant-negative FADD are protected from apoptosis induced by beta-amyloid. Together these results indicate that the apoptotic pathway activated by beta-amyloid requires both caspase-8 activity and FADD. These findings further support the hypothesis that beta-amyloid might initiate apoptosis by cross-linking death receptors of the Fas/TNFR family.