Exon 1 of the HD gene with an expanded CAG repeat is sufficient to cause a progressive neurological phenotype in transgenic mice

A YAC mouse model for Huntington's disease with full-length mutant huntingtin, cytoplasmic toxicity, and selective striatal neurodegeneration

We have produced yeast artificial chromosome (YAC) transgenic mice expressing normal (YAC18) and mutant (YAC46 and YAC72) huntingtin (htt) in a developmental and tissue-specific manner identical to that observed in Huntington's disease (HD). YAC46 and YAC72 mice show early electrophysiological abnormalities, indicating cytoplasmic dysfunction prior to observed nuclear inclusions or neurodegeneration. By 12 months of age, YAC72 mice have a selective degeneration of medium spiny neurons in the lateral striatum associated with the translocation of N-terminal htt fragments to the nucleus. Neurodegeneration can be present in the absence of macro- or microaggregates, clearly showing that aggregates are not essential to initiation of neuronal death. These mice demonstrate that initial neuronal cytoplasmic toxicity is followed by cleavage of htt, nuclear translocation of htt N-terminal fragments, and selective neurodegeneration.

Characterization of progressive motor deficits in mice transgenic for the human Huntington's disease mutation

Transgenic mice expressing exon 1 of the human Huntington's disease (HD) gene carrying a 141-157 CAG repeat (line R6/2) develop a progressive neurological phenotype with motor symptoms resembling those seen in HD. We have characterized the motor deficits in R6/2 mice using a battery of behavioral tests selected to measure motor aspects of swimming, fore- and hindlimb coordination, balance, and sensorimotor gating [swimming tank, rotarod, raised beam, fore- and hindpaw footprinting, and acoustic startle/prepulse inhibition (PPI)]. Behavioral testing was performed on female hemizygotic R6/2 transgenic mice (n = 9) and female wild-type littermates (n = 22) between 5 and 14 weeks of age. Transgenic mice did not show an overt behavioral phenotype until around 8 weeks of age. However, as early as 5-6 weeks of age they had significant difficulty swimming, traversing the narrowest square (5 mm) raised beam, and maintaining balance on the rotarod at rotation speeds of 33-44 rpm. Furthermore, they showed significant impairment in prepulse inhibition (an impairment also seen in patients with HD). Between 8 and 15 weeks, R6/2 transgenic mice showed a progressive deterioration in performance on all of the motor tests. Thus R6/2 mice show measurable deficits in motor behavior that begin subtly and increase progressively until death. Our data support the use of R6/2 mice as a model of HD and indicate that they may be useful for evaluating therapeutic strategies for HD, particularly those aimed at reducing the severity of motor symptoms or slowing the course of the disease.

Nuclear and neuropil aggregates in Huntington's disease: relationship to neuropathology

The data we report in this study concern the types, location, numbers, forms, and composition of microscopic huntingtin aggregates in brain tissues from humans with different grades of Huntington's disease (HD). We have developed a fusion protein antibody against the first 256 amino acids that preferentially recognizes aggregated huntingtin and labels many more aggregates in neuronal nuclei, perikarya, and processes in human brain than have been described previously. Using this antibody and human brain tissue ranging from presymptomatic to grade 4, we have compared the numbers and locations of nuclear and neuropil aggregates with the known patterns of neuronal death in HD. We show that neuropil aggregates are much more common than nuclear aggregates and can be present in large numbers before the onset of clinical symptoms. There are also many more aggregates in cortex than in striatum, where they are actually uncommon. Although the striatum is the most affected region in HD, only 1-4% of striatal neurons in all grades of HD have nuclear aggregates. Neuropil aggregates, which we have identified by electron microscopy to occur in dendrites and dendritic spines, could play a role in the known dendritic pathology that occurs in HD. Aggregates increase in size in advanced grades, suggesting that they may persist in neurons that are more likely to survive. Ubiquitination is apparent in only a subset of aggregates, suggesting that ubiquitin-mediated proteolysis of aggregates may be late or variable.

Brain neurotransmitter deficits in mice transgenic for the Huntington's disease mutation

Huntington's disease (HD) is associated with an expansion in the CAG repeat sequence of a gene on chromosome 4, resulting in a neurodegenerative process particularly affecting the striatum and with profound but selective changes in content of various neurotransmitters. Recently, transgenic mice expressing a fragment of the human HD gene containing a large CAG expansion have been generated; these mice exhibit a progressive neurological phenotype that includes motor disturbances, as well as neuronal deficits. To investigate their underlying neurotransmitter pathology, we have determined concentrations of GABA, glutamate, and the monoamine neurotransmitters in several brain regions in these mice and control animals at times before and after the emergence of the behavioural phenotype. In contrast to the findings in HD, striatal GABA was unaffected, although a deficit was observed in the cerebellum, consistent with a dysfunction of Purkinje cells. Losses of the monoamine transmitters were observed, some of which are not seen in HD. Thus, 5-hydroxytryptamine and, to a greater extent, 5-hydroxyindoleacetic acid levels were diminished in all brain regions studied, and noradrenaline was particularly affected in the hippocampus. Dopamine was decreased in the striatum in older animals, parallelling evidence for diminished dopaminergic activity in HD.

The role of excitotoxicity in neurodegenerative disease: implications for therapy

Glutamic acid is the principal excitatory neurotransmitter in the mammalian central nervous system. Glutamic acid binds to a variety of excitatory amino acid receptors, which are ligand-gated ion channels. It is activation of these receptors that leads to depolarisation and neuronal excitation. In normal synaptic functioning, activation of excitatory amino acid receptors is transitory. However, if, for any reason, receptor activation becomes excessive or prolonged, the target neurones become damaged and eventually die. This process of neuronal death is called excitotoxicity and appears to involve sustained elevations of intracellular calcium levels. Impairment of neuronal energy metabolism may sensitise neurones to excitotoxic cell death. The principle of excitotoxicity has been well-established experimentally, both in in vitro systems and in vivo, following administration of excitatory amino acids into the nervous system. A role for excitotoxicity in the aetiology or progression of several human neurodegenerative diseases has been proposed, which has stimulated much research recently. This has led to the hope that compounds that interfere with glutamatergic neurotransmission may be of clinical benefit in treating such diseases. However, except in the case of a few very rare conditions, direct evidence for a pathogenic role for excitotoxicity in neurological disease is missing. Much attention has been directed at obtaining evidence for a role for excitotoxicity in the neurological sequelae of stroke, and there now seems to be little doubt that such a process is indeed a determining factor in the extent of the lesions observed. Several clinical trials have evaluated the potential of antiglutamate drugs to improve outcome following acute ischaemic stroke, but to date, the results of these have been disappointing. In amyotrophic lateral sclerosis, neurolathyrism, and human immunodeficiency virus dementia complex, several lines of circumstantial evidence suggest that excitotoxicity may contribute to the pathogenic process. An antiglutamate drug, riluzole, recently has been shown to provide some therapeutic benefit in the treatment of amyotrophic lateral sclerosis. Parkinson's disease and Huntington's disease are examples of neurodegenerative diseases where mitochondrial dysfunction may sensitise specific populations of neurones to excitotoxicity from synaptic glutamic acid. The first clinical trials aimed at providing neuroprotection with antiglutamate drugs are currently in progress for these two diseases.

Pathogenesis of inclusion bodies in (CAG)n/Qn-expansion diseases with special reference to the role of tissue transglutaminase and to selective vulnerability

At least eight neurodegenerative diseases, including Huntington disease, are caused by expansions in (CAG)n repeats in the affected gene and by an increase in the size of the corresponding polyglutamine domain in the expressed protein. A hallmark of several of these diseases is the presence of aberrant, proteinaceous aggregates in the nuclei and cytosol of affected neurons. Recent studies have shown that expanded polyglutamine (Qn) repeats are excellent glutaminyl-donor substrates of tissue transglutaminase, and that the substrate activity increases with increasing size of the polyglutamine domain. Tissue transglutaminase is present in the cytosol and nuclear fractions of brain tissue. Thus, the nuclear and cytosolic inclusions in Huntington disease may contain tissue transglutaminase-catalyzed covalent aggregates. The (CAG)n/Qn-expansion diseases are classic examples of selective vulnerability in the nervous system, in which certain cells/structures are particularly susceptible to toxic insults. Quantitative differences in the distribution of the brain transglutaminase(s) and its substrates, and in the activation mechanism of the brain transglutaminase(s), may explain in part selective vulnerability in a subset of neurons in (CAG)n-expansion diseases, and possibly in other neurodegenerative disease. If tissue transglutaminase is found to be essential for development of pathogenesis, then inhibitors of this enzyme may be of therapeutic benefit.

Developing mouse models of aging: a consideration of strain differences in age-related behavioral and neural parameters

Increased interest is emerging for using mouse models to assess the genetics of brain aging and age-related neurodegenerative diseases. Despite this demand, relatively little information is available on aging in behavioral or neuromorphological parameters in various mouse strains that are being used to create transgenic and null mutant mice. We review several issues regarding selection of appropriate strains as follows: (1) Does the behavioral parameter exhibit a significant age by strain interaction? (2) Do the strains differ in lifespan? (3) Are there potential intervening variables, such as strain-specific performance strategies or disease, in the behavioral task being investigated that would confound the desired conclusions? (4) Does the behavioral difference have an underlying neural correlate? In this context we present a conceptual model pertaining to the selection of mouse strains and behavioral parameters for genetic analyses. We also review the importance of applying stereological techniques for determining age-related structural changes in the mouse brain as well as the potential value of a database that would catalog this information. Thus, our intention is to underscore the growing importance of mouse models of brain aging and the concomitant need for additional information about mouse aging in general.

Pathogenesis of polyglutamine-induced disease: A model for SCA1

During the past 7 years several inheritable neurological disorders have been found to be due to the expansion of an unstable CAG trinucleotide repeat that leads to an increase in the length of a polyglutamine tract within a disease-specific protein. Based on pathological evidence obtained from the brains of affected individuals and transgenic mice expressing a mutant human gene, it was proposed that the formation of nuclear aggregates of the polyglutamine protein plays a critical role in pathogenesis. However, recent evidence indicates that this may not be the case. This review focuses on our results for one of these disorders, spinocerebellar ataxia type 1 (SCA1), and presents a model for SCA1 pathogenesis.

Cellular localization of huntingtin in striatal and cortical neurons in rats: lack of correlation with neuronal vulnerability in Huntington's disease

Immunohistochemistry and single-cell RT-PCR were used to characterize the localization of huntingtin and/or its mRNA in the major types of striatal neurons and in corticostriatal projection neurons in rats. Single-label immunohistochemical studies revealed that striatum contains scattered large neurons rich in huntingtin and more numerous medium-sized neurons moderate in huntingtin. Double-label immunohistochemical studies showed that the large huntingtin-rich striatal neurons include nearly all cholinergic interneurons and some parvalbuminergic interneurons. Somatostatinergic striatal interneurons, which are medium in size, rarely contained huntingtin. Calbindin immunolabeling showed that the vast majority of the medium-sized striatal neurons that contain huntingtin are projection neurons, but only approximately 65% of calbindin-labeled projection neurons (localized to the matrix compartment of striatum) were labeled for huntingtin. Calbindin-containing projection neurons of the matrix compartment and calbindin-negative projection neurons of the striatal patch compartment contained huntingtin with comparable frequency. Single-cell RT-PCR confirmed that striatal cholinergic interneurons contain huntingtin, but only approximately 65% of projection neurons contained detectable huntingtin message. The finding that huntingtin is not consistently found in striatal projection neurons [which die in Huntington's disease (HD)] but is abundant in striatal cholinergic interneurons (which survive in Huntington's disease) suggests that the mutation in huntingtin that causes HD may not directly kill neurons. In contrast to the heterogeneous expression of huntingtin in the different striatal neuron types, we found all corticostriatal neurons to be rich in huntingtin protein and mRNA. One possibility raised by our findings is that the HD mutation may render corticostriatal neurons destructive rather than render striatal neurons vulnerable.

Neuronal degeneration in the basal ganglia and loss of pallido-subthalamic synapses in mice with targeted disruption of the Huntington's disease gene

Huntington's disease (HD) is a progressive neurodegenerative disorder associated with CAG repeat expansion within a novel gene (IT15). We have previously created a targeted disruption in exon 5 of Hdh (Hdhex5), the murine homologue of the HD gene. Homozygotes for the Hdhex5 mutation exhibit embryolethality before embryonic day 8.5, while heterozygotes survive to adulthood and display increased motor activity and cognitive deficits. Detailed morphometric and stereological analyses of the basal ganglia in adult heterozygous mice were performed by light and electron microscopy. Morphometric analyses demonstrated a significant loss of neurons from both the globus pallidus (29%) and the subthalamic nucleus (51%), with a normal complement of neurons in the caudate-putamen and substantia nigra. The ultrastructural appearance of sporadic degenerating neurons in these regions indicated apoptosis. The highest frequency of apoptotic neurons was observed in the globus pallidus and subthalamic nucleus. Stereological analyses in the subthalamic nucleus revealed a significant decrease in the numerical density of symmetric synapses (43%), suggesting a relatively selective loss of inhibitory pallido-subthalamic afferents. Immunohistochemistry using antibodies against enkephalin and substance-P was unremarkable in heterozygotes, indicating a normal complement of enkephalin-immunoreactive striatopallidal afferents and substance-P-immunoreactive striatopeduncular and striatonigral afferents in these animals. These findings show that loss of an intact huntingtin protein is associated with significant morphological alterations in the basal ganglia of adult mice, indicating an important role for this protein during development of the central nervous system.

Mitochondrial involvement in Parkinson's disease, Huntington's disease, hereditary spastic paraplegia and Friedreich's ataxia

Respiratory chain dysfunction has been identified in several neurodegenerative disorders. In Friedreich's ataxia (FA) and Huntington's disease (HD), where the respective mutations are in nuclear genes encoding non-respiratory chain mitochondrial proteins, the defects in oxidative phosphorylation are clearly secondary. In Parkinson's disease (PD) the situation is less clear, with some evidence for a primary role of mitochondrial DNA in at least a proportion of patients. The pattern of the respiratory chain defect may provide some clue to its cause; in PD there appears to be a selective complex I deficiency; in HD and FA the deficiencies are most severe in complex II/III with a less severe defect in complex IV. Aconitase activity in HD and FA is severely decreased in brain and muscle, respectively, but appears to be normal in PD brain. Free radical generation is thought to be of importance in both HD and FA, via excitotoxicity in HD and abnormal iron handling in FA. The oxidative damage observed in PD may be secondary to the mitochondrial defect. Whatever the cause(s) and sequence of events, respiratory chain deficiencies appear to play an important role in the pathogenesis of neurodegeneration. The mitochondrial abnormalities induced may converge on the function of the mitochondrion in apoptosis. This mode of cell death is thought to play an important role in neurodegenerative diseases and it is tempting to speculate that the observed mitochondrial defects in PD, HD and FA result directly in apoptotic cell death, or in the lowering of a cell's threshold to undergo apoptosis. Clarifying the role of mitochondria in pathogenesis may provide opportunities for the development of treatments designed to reverse or prevent neurodegeneration.

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

Eight diseases are now known to be caused by an expansion mutation of the trinucleotide repeat CAG encoding glutamine. Each disease is caused by a CAG expansion in a different gene, and the genes bear no similarity to each other except for the presence of the repeat. Nonetheless, the essential feature of all of these disorders is neurodegeneration in a set of overlapping cortical and subcortical regions. Disease age of onset, and in some cases severity, is correlated with repeat length. These and other observations have led to the hypothesis that CAG expansion causes disease by a toxic gain-of-function of the encoded stretch of polyglutamine residues. Expansion-induced abnormalities of cytoskeletal function or neuronal signalling processes may contribute to the pathogenic process. In addition, theoretical and experimental analysis of the chemistry of uninterrupted stretches of glutamine residues suggest that polyglutamine-containing proteins or protein fragments may aggregate, via a "polar zipper", into beta pleated sheets. Recent findings have now established the presence of such aggregates in selected regions of brain from affected individuals, in transgenic mice expressing expanded repeats, and in isolated cells transfected with expanded repeats. The aggregates are most prominently manifest as neuronal intranuclear inclusion bodies. As the investigation of the link between these inclusions and cell dysfunction and death continues, it is possible that new avenues for therapeutic intervention will emerge.

Identification of the physiological promoter for spinocerebellar ataxia 2 gene reveals a CpG island for promoter activity situated into the exon 1 of this gene and provides data about the origin of the nonmethylated state of these types of islands

In order to further use the spinocerebellar ataxia 2 (SCA2) promoter for transgenic mice models of "CAG repeat" neurodegeneration, different fragments of this 5' end were ligated into pGL3-Luc plasmid to obtain the better promoter-activity of the physiological promoter for SCA2. Base-par composition of the SCA2-5' region, and promoter prediction algorithms such as TSSW and TSSG, together with the high firefly luciferase expression after 48 hours of transient transfection in mammalian cells lines, showed a typical CpG island for promoter-activity. The promoter activity was specifically localized into the exon 1 of the SCA2 gene. The higher expression of firefly luciferase in the embryonal F9 cells by the use of SCA2 promoter, rather than by the use of CMV promoter may be related with the origin of the nonmethylated CpG island during the early embryogenesis. Analysis of the 5' region from HD gene revealed to a CpG island, which could be containing the physiological promoter for this gene.

Polyglutamine-mediated dysfunction and apoptotic death of a Caenorhabditis elegans sensory neuron

The effect of expressing human huntingtin fragments containing polyglutamine (polyQ) tracts of varying lengths was assessed in Caenorhabditis elegans ASH sensory neurons in young and old animals. Expression of a huntingtin fragment containing a polyQ tract of 150 residues (Htn-Q150) led to progressive ASH neurodegeneration but did not cause cell death. Progressive cell death and enhanced neurodegeneration were observed in ASH neurons that coexpressed Htn-Q150 and a subthreshold dose of a toxic OSM-10::green fluorescent protein (OSM-10::GFP) fusion protein. Htn-Q150 huntingtin protein fragments formed protein aggregates in ASH neurons, and the number of ASH neurons containing aggregates increased as animals aged. ASH neuronal cell death required ced-3 caspase function, indicating that the observed cell death is apoptotic. Of interest, ced-3 played a critical role in Htn-Q150-mediated neurodegeneration but not in OSM10::GFP-mediated ASH neurodegeneration. ced-3 function was important but not essential for the formation of protein aggregates. Finally, behavioral assays indicated that ASH neurons, coexpressing Htn-Q150 and OSM10::GFP, were functionally impaired at 3 days before the detection of neurodegeneration, cell death, and protein aggregates.

Recruitment and the role of nuclear localization in polyglutamine-mediated aggregation

The inherited neurodegenerative diseases caused by an expanded glutamine repeat share the pathologic feature of intranuclear aggregates or inclusions (NI). Here in cell-based studies of the spinocerebellar ataxia type-3 disease protein, ataxin-3, we address two issues central to aggregation: the role of polyglutamine in recruiting proteins into NI and the role of nuclear localization in promoting aggregation. We demonstrate that full-length ataxin-3 is readily recruited from the cytoplasm into NI seeded either by a pathologic ataxin-3 fragment or by a second unrelated glutamine-repeat disease protein, ataxin-1. Experiments with green fluorescence protein/polyglutamine fusion proteins show that a glutamine repeat is sufficient to recruit an otherwise irrelevant protein into NI, and studies of human disease tissue and a Drosophila transgenic model provide evidence that specific glutamine-repeat-containing proteins, including TATA-binding protein and Eyes Absent protein, are recruited into NI in vivo. Finally, we show that nuclear localization promotes aggregation: an ataxin-3 fragment containing a nonpathologic repeat of 27 glutamines forms inclusions only when targeted to the nucleus. Our findings establish the importance of the polyglutamine domain in mediating recruitment and suggest that pathogenesis may be linked in part to the sequestering of glutamine-containing cellular proteins. In addition, we demonstrate that the nuclear environment may be critical for seeding polyglutamine aggregates.

Genetic neurodegenerative diseases: the human illness and transgenic models

Review The neurodegenerative disorders, a heterogeneous group of chronic progressive diseases, are among the most puzzling and devastating illnesses in medicine. Some of these disorders, such as Alzheimer's disease, amyotrophic lateral sclerosis, the prion diseases, and Parkinson's disease, can occur sporadically and, in some instances, are caused by inheritance of gene mutations. Huntington's disease is acquired in an entirely genetic manner. Transgenic mice that express disease-causing genes recapitulate many features of these diseases. This review provides an overview of transgenic mouse models of familial amyotrophic lateral sclerosis, familial Alzheimer's disease, and Huntington's disease and the emerging insights relevant to the underlying molecular mechanisms of these diseases.

Modeling dopamine system dysfunction in experimental animals

Quite a substantial number of human disorders have been associated with a primary or a secondary impairment of one or several of the dopaminergic pathways. Among disorders associated with a primary impairment of dopaminergic transmission are Parkinson's disease, striatonigral degeneration, progressive supranuclear palsy, and possibly schizophrenia. Diseases of secondary dopamine dysfunction are chiefly represented by Huntington's disease in which dopaminergic transmission is being interrupted by progressive loss of the striatal neurons bearing the postsynaptic D1- and D2-dopamine receptors. Central dopaminergic systems have anatomical as well as organizational properties that render them unique by comparison to other neurotransmission systems, making them able to play a pivotal role in the modulation of various important brain functions such as locomotor activity, attention, and some cognitive abilities. These properties of dopamine neurons have obviously several implications in the clinical expression of human disorders involving dopamine neuron dysfunction. In addition, they can greatly influence the clinical/behavioral consequences of experimental lesions in animal models of dopamine dysfunctions.

Striatal transplantation in a transgenic mouse model of Huntington's disease

Striatal grafts have been proposed as a potential strategy for striatal repair in Huntington's disease, but it is unknown whether the diseased brain will compromise graft survival. A transgenic mouse line has recently been described in which hemizygotes with an expanded CAG repeat in exon 1 of the HD gene exhibit a progressive neurological phenotype similar to the motor symptoms of Huntington's disease. We have therefore evaluated the effects of the transgenic brain environment on the survival, differentiation, and function of intrastriatal striatal grafts and undertaken a preliminary analysis of the effects of the grafts on the development of neurological deficits in the host mice. Hemizygote transgenic and wild-type littermate female mice received striatal grafts at 10 weeks of age and were allowed to survive 6 weeks. Normal healthy grafts were seen to survive and differentiate within the striatum of transgenic mice in a manner comparable to that seen in control mice. The transgenic mice exhibited a progressive decline in body weight from 9 weeks of age and a progressive hypoactivity in an open field test of general locomotor behavior. Although striatal grafts exerted a statistically significant influence on several indices of this impairment, all behavioral effects were small and did not exert any clinically relevant effect on the profound neurological deficiency of the transgenic mice.

Comparative analysis of mammalian stanniocalcin genes

The recent discovery of mammalian stanniocalcin (STC) prompted an investigation of its gene structure and expression pattern to study its function and regulation. We show that both the human and mouse genes are composed of four exons spanning about 13 kb, with 85% nucleotide sequence identity in coding regions. Remarkably high sequence conservation between species also exists in the approximately 3-kb 3'-untranslated region. Comparative analysis of the 5'-untranslated region and flanking DNA from the rat and human STC genes showed long stretches of CAG trinucleotide repeats and an additional (CA)25 dinucleotide repeat unique to the rat promoter. An analysis of STC expression in the mouse showed that ovary contained the highest level of messenger RNA, with lower, but detectable, levels in most tissues. In situ hybridization revealed strong, specific hybridization over the thecal-interstitial cells of the ovarian stroma, whereas immunohistochemical analysis indicated that STC was present not only in the stroma, but also in the corpora lutea and oocyte of the developing follicle. Consequently, STC may act as a signaling molecule between the thecal-interstitial cell compartment and the corpus luteum and oocyte, thereby regulating the activity of these structures in some way. These findings suggest that in addition to its role in mineral metabolism, STC has acquired an important function in reproduction during its evolution to mammals.

Molecular aspects of Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative disease striking principally medium spiny GABAergic neurons of the caudate nucleus of the basal ganglia. It affects about one in 10,000 individuals and is transmitted in an autosomal dominant fashion. The molecular basis of the disease is expansion of the trinucleotide CAG in the first exon of a gene on chromosome four. The CAG repeats are translated to polyglutamine repeats in the expressed protein, huntingtin. The normal function of huntingtin remains incompletely characterized, but based upon recently defined protein-protein interactions, it appears to be associated with the cytoskeleton and required for neurogenesis. Huntingtin has been demonstrated to interact with such proteins as HAP1, HIP1, microtubules, GADPH, calmodulin, and an ubiquitin-conjugating enzyme. Polyglutamine expansion alters many of these interactions and leads to huntingtin aggregation and the formation of neuronal nuclear inclusions, ultimately culminating in cell death. In this review, we discuss the molecular aspects of HD, including the present understanding of huntingtin-protein interactions, studies with transgenic mice, and postulated mechanisms of huntingtin aggregation.

Progressive ataxia, myoclonic epilepsy and cerebellar apoptosis in cystatin B-deficient mice

Loss-of-function mutations in the gene (CSTB) encoding human cystatin B, a widely expressed cysteine protease inhibitor, are responsible for a severe neurological disorder known as Unverricht-Lundborg disease (EPM1). The primary cellular events and mechanisms underlying the disease are unknown. We found that mice lacking cystatin B develop myoclonic seizures and ataxia, similar to symptoms seen in the human disease. The principal cytopathology appears to be a loss of cerebellar granule cells, which frequently display condensed nuclei, fragmented DNA and other cellular changes characteristic of apoptosis. This mouse model of EPM1 provides evidence that cystatin B, a non-caspase cysteine protease inhibitor, has a role in preventing cerebellar apoptosis.

Huntingtin acts in the nucleus to induce apoptosis but death does not correlate with the formation of intranuclear inclusions

The mechanisms by which mutant huntingtin induces neurodegeneration were investigated using a cellular model that recapitulates features of neurodegeneration seen in Huntington's disease. When transfected into cultured striatal neurons, mutant huntingtin induces neurodegeneration by an apoptotic mechanism. Antiapoptotic compounds or neurotrophic factors protected neurons against mutant huntingtin. Blocking nuclear localization of mutant huntingtin suppressed its ability to form intranuclear inclusions and to induce neurodegeneration. However, the presence of inclusions did not correlate with huntingtin-induced death. The exposure of mutant huntingtin-transfected striatal neurons to conditions that suppress the formation of inclusions resulted in an increase in mutant huntingtin-induced death. These findings suggest that mutant huntingtin acts within the nucleus to induce neurodegeneration. However, intranuclear inclusions may reflect a cellular mechanism to protect against huntingtin-induced cell death.

Ataxin-1 nuclear localization and aggregation: role in polyglutamine-induced disease in SCA1 transgenic mice

Transgenic mice carrying the spinocerebellar ataxia type 1 (SCA1) gene, a polyglutamine neurodegenerative disorder, develop ataxia with ataxin-1 localized to aggregates within cerebellar Purkinje cells nuclei. To examine the importance of nuclear localization and aggregation in pathogenesis, mice expressing ataxin-1[82] with a mutated NLS were established. These mice did not develop disease, demonstrating that nuclear localization is critical for pathogenesis. In a second series of transgenic mice, ataxin-1[77] containing a deletion within the self-association region was expressed within Purkinje cells nuclei. These mice developed ataxia and Purkinje cell pathology similar to the original SCA1 mice. However, no evidence of nuclear ataxin-1 aggregates was found. Thus, although nuclear localization of ataxin-1 is necessary, nuclear aggregation of ataxin-1 is not required to initiate pathogenesis in transgenic mice.

Protein precipitation: a common etiology in neurodegenerative disorders?

Molecular genetic analyses have elucidated a class of inherited neurodegenerative disorders caused by expanded CAG repeats encoding polyglutamines (e.g. Huntington disease and Machado-Joseph disease). Proteins containing expanded polyglutamine repeats appear to precipitate by self-aggregation and, as a result, produce a core disease-related phenotype: neuronal cell death or degeneration. In other neurodegenerative disorders, such as Alzheimer disease, prion disease, Parkinson disease and amyotrophic lateral sclerosis, precipitation of abnormal proteins is also now considered to play a key role. These observations might lead to the elucidation of universal mechanisms for neurodegeneration and to effective treatments for many neurodegenerative disorders.

SH3GL3 associates with the Huntingtin exon 1 protein and promotes the formation of polygln-containing protein aggregates

The mechanism by which aggregated polygins cause the selective neurodegeneration in Huntington's disease (HD) is unknown. Here, we show that the SH3GL3 protein, which is preferentially expressed in brain and testis, selectively interacts with the HD exon 1 protein (HDex1p) containing a glutamine repeat in the pathological range and promotes the formation of insoluble polyglutamine-containing aggregates in vivo. The C-terminal SH3 domain in SH3GL3 and the proline-rich region in HDex1p are essential for the interaction. Coimmunoprecipitations and immunofluorescence studies revealed that SH3GL3 and HDex1p colocalize in transfected COS cells. Additionally, an anti-SH3GL3 antibody was also able to coimmunoprecipitate the full-length huntingtin from an HD human brain extract. The characteristics of the interaction between SH3GL3 and huntingtin and the colocalization of the two proteins suggest that SH3GL3 could be involved in the selective neuronal cell death in HD.

Filamentous nerve cell inclusions in neurodegenerative diseases

Recent work has shown that abnormal filamentous inclusions within some nerve cells is a characteristic shared by Alzheimer's disease, some frontotemporal dementias, Parkinson's disease, dementia with Lewy bodies, multiple system atrophy, as well as Huntington's disease and other trinucleotide repeat disorders. This suggests that in each of these disorders, the affected nerve cells degenerate as a result of these abnormal inclusions. Except for trinucleotide repeat disorders, the filaments involved have been shown to consist of either the microtubule-associated protein tau or alpha-synuclein. Over the past year, mutations in the genes for tau and alpha-synuclein have been identified as the genetic causes of some familial forms of frontotemporal dementia and Parkinson's disease, respectively. The discovery last year of neuronal intranuclear inclusions in Huntington's disease and other disorders with expanded glutamine repeats has suggested a unifying mechanism underlying the pathogenesis of this class of neurodegenerative diseases.

Behavioural abnormalities and selective neuronal loss in HD transgenic mice expressing mutated full-length HD cDNA

Huntington disease (HD) is an adult-onset, autosomal dominant inherited human neurodegenerative disorder characterized by hyperkinetic involuntary movements, including motor restlessness and chorea, slowing of voluntary movements and cognitive impairment. Selective regional neuron loss and gliosis in striatum, cerebral cortex, thalamus, subthalamus and hippocampus are well recognized as neuropathological correlates for the clinical manifestations of HD. The underlying genetic mutation is the expansion of CAG trinucleotide repeats (coding for polyglutamines) to 36-121 copies in exon 1 of the HD gene. The HD mRNA and protein product (huntingtin) show widespread distribution, and thus much remains to be understood about the selective and progressive neurodegeneration in HD. To create an experimental animal model for HD, transgenic mice were generated showing widespread expression of full-length human HD cDNA with either 16, 48 or 89 CAG repeats. Only mice with 48 or 89 CAG repeats manifested progressive behavioural and motor dysfunction with neuron loss and gliosis in striatum, cerebral cortex, thalamus and hippocampus. These animals represent clinically relevant models for HD pathogenesis, and may provide insights into the underlying pathophysiological mechanisms of other triplet repeat disorders.

Characterization of the Huntington's disease (HD) gene homologue in the zebrafish Danio rerio

The Huntington's disease (HD) gene contains a trinucleotide repeat that is expanded and unstable in patients with the disease (HDCRG, 1993). As the first step toward investigating a potential role for this gene in early vertebrate development, we isolated the homolog of the Huntington's disease (ZHD) cDNA in zebrafish. This cDNA encodes a predicted protein product of 3121 amino acids with 70% identity to human huntingtin. The first exon is predicted to encode four glutamines, followed by only one proline, demonstrating that the polymorphic polyproline stretch found in mammalian HD sequences is absent in the fish. We sequenced approximately 900bp upstream from the predicted start codon and found that it lacks a TATA box, CCAAT box, or Sp1 binding sites. Western blot analysis revealed that the protein is expressed at a high level in late embryonic development and at moderate levels in the adult head.

Striking changes in anxiety in Huntington's disease transgenic mice

Huntington's disease transgenic mice were tested in the elevated plus-maze test of anxiety at 6, 8, 10 and 12 weeks of age. At all ages, they showed significant and striking increases in the percentages of open arm entries and time spent on the open arms, compared with their normal littermates, indicating reduced anxiety. These increases were not secondary to a non-specific stimulant effect, since the transgenic mice made fewer closed arm entries, significantly so from 10 weeks of age. The mice were also tested in the holeboard, which provides measures of locomotor activity and directed exploration. From 8 weeks of age, the Huntington's mice were significantly less active than their normal littermates and made fewer exploratory head-dips. The increased open arm activity in the elevated plus-maze cannot therefore be secondary to increased exploration in the transgenic mice. In order to determine whether the reduced anxiety was due to differences in benzodiazepine receptor function, the mice were challenged with the benzodiazepine receptor antagonist, flumazenil. The results indicated that some of the reduced anxiety could be attributed to the presence of an endogenous anxiolytic ligand.

Polyglutamine-expanded human huntingtin transgenes induce degeneration of Drosophila photoreceptor neurons

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder. Disease alleles contain a trinucleotide repeat expansion of variable length, which encodes polyglutamine tracts near the amino terminus of the HD protein, huntingtin. Polyglutamine-expanded huntingtin, but not normal huntingtin, forms nuclear inclusions. We describe a Drosophila model for HD. Amino-terminal fragments of human huntingtin containing tracts of 2, 75, and 120 glutamine residues were expressed in photoreceptor neurons in the compound eye. As in human neurons, polyglutamine-expanded huntingtin induced neuronal degeneration. The age of onset and severity of neuronal degeneration correlated with repeat length, and nuclear localization of huntingtin presaged neuronal degeneration. In contrast to other cell death paradigms in Drosophila, coexpression of the viral antiapoptotic protein, P35, did not rescue the cell death phenotype induced by polyglutamine-expanded huntingtin.

Mitochondrial dysfunction in neurodegenerative diseases

A potential pivotal role for mitochondrial dysfunction in neurodegenerative diseases is gaining increasing acceptance. Mitochondrial dysfunction leads to a number of deleterious consequences including impaired calcium buffering, generation of free radicals, activation of the mitochondrial permeability transition and secondary excitotoxicity. Neurodegenerative diseases of widely disparate genetic etiologies may share mitochondrial dysfunction as a final common pathway. Recent studies using cybrid cell lines suggest that sporadic Alzheimer's disease is associated with a deficiency of cytochrome oxidase. Friedreich's ataxia is caused by an expanded GAA repeat resulting in dysfunction of frataxin, a nuclear encoded mitochondrial protein involved in mitochondrial iron transport. This results in increased mitochondrial iron and oxidative damage. Familial amyotrophic lateral sclerosis is associated with point mutations in superoxide dismutase, which may lead to increased generation of free radicals and thereby contribute to mitochondrial dysfunction. Huntington's disease (HD) is caused by an expanded CAG repeat in an unknown protein termed huntingtin. The means by which this leads to energy impairment is unclear, however studies in both HD patients and a transgenic mouse model show evidence of bioenergetic defects. Mitochondrial dysfunction leads to oxidative damage which is well documented in several neurodegenerative diseases. Therapeutic approaches include methods to buffer intracellular ATP and to scavenge free radicals.

Mitochondrial dysfunction in neurodegenerative disorders

Mutations of mitochondrial DNA (mtDNA) are associated with a wide spectrum of disorders encompassing the myopathies, encephalopathies and cardiomyopathies, in addition to organ specific presentations such as diabetes mellitus and deafness. The pathogenesis of mtDNA mutations is not fully understood although it is assumed that their final common pathway involves impaired oxidative phosphorylation. The identification of a specific respiratory chain defect (complex I deficiency) in Parkinson's disease (PD) 10 years ago focused attention on the aetiological and pathogenetic roles that mitochondria may play in neurodegenerative diseases. There is evidence now emerging that mtDNA abnormalities may determine the complex I defect in a proportion of PD patients and it may prove possible to use biochemical analysis of platelet and cybrid complex I function to identify those that lie within this group. Respiratory chain defects of a different pattern have been identified in Huntington's disease (HD) (complex II/III deficiency) and Friedreich's ataxia (FA) complex I-III deficiency). In both these disorders, the mitochondrial abnormality is secondary to the primary nuclear mutation:CAG repeat in the huntingtin gene in HD, and GAA repeat in the frataxin gene in FA. Nevertheless, it appears that the mitochondrion may be the target of the biochemical defects that are the consequence of these mutations. There is a close and reciprocal relationship between respiratory chain dysfunction and free radical generation, and there is evidence for oxidative stress and damage in PD, HD and FA, which together with the mitochondrial defect may result in cell damage. Impaired oxidative phosphorylation and free radical generation may independently adversely affect the maintenance of mitochondrial transmembrane potential (Deltapsim). A fall in Deltapsim is an early event (preceding nuclear fragmentation) in the apoptotic pathway. It is possible therefore that mitochondrial dysfunction in the neurodegenerative disorders may result in a fall in the apoptotic threshold of neurones which, in some, may be sufficient to induce cell death whilst, in others, additional factors may be required. In any event, mitochondria present an important target for future strategies for 'neuroprotection' to prevent or retard neurodegeneration.

Advances in Huntington's disease: implications for experimental therapeutics

The gene mutation causing Huntington's disease was identified in 1993 as an expanded trinucleotide repeat within the coding region for a 348-kd protein called huntingtin. The mechanism by which this cytosine-adenosine-guanosine repeat produces the progressive signs and symptoms of Huntington's disease remains uncertain, but recent advances have begun to provide insights into this process. Promising developments include transgenic mouse models of Huntington's disease with neuronal intranuclear inclusions, the identification of differential neuronal features which might account for the selective vulnerability of neurons seen in Huntington's disease and further evidence for the role of excitotoxicity and impaired mitochondrial energy production. These observations have suggested new therapeutic strategies, and have lent further support for experimental therapeutics aimed at improving mitochondrial function and reducing excitotoxic injury.

A human HAP1 homologue. Cloning, expression, and interaction with huntingtin

Huntington's disease (HD) is caused by the expansion of a glutamine repeat in the protein huntingtin. The expanded glutamine repeat is thought to mediate a gain of function by causing huntingtin to abnormally interact with other proteins. We previously identified a rat huntingtin-associated protein (HAP1) that binds to huntingtin; HAP1 binds more tightly to huntingtin with an expanded glutamine repeat than to wild type huntingtin. Identification of the human homologue of HAP1 is necessary for investigation of the potential role of HAP1 in HD pathology. Here, we report the cloning of a human HAP1 homologue (hHAP) that shares 62% identity with rat HAP1 over its entire sequence and 82% amino acid identity in the putative huntingtin-binding region. The hHAP gene encodes a 4.1-kilobase transcript and a 75-kDa protein which are specifically expressed in human brain tissues. Its expression in Huntington's disease brains is reduced in parallel with a decreased expression of huntingtin. While two isoforms of rat HAP1 are expressed at similar levels in rat brain, only a single major form of hHAP is found in primate brains. In vitro binding, immunoprecipitation, and coexpression studies confirm the interaction of hHAP with huntingtin. The in vitro binding of hHAP to huntingtin is enhanced by lengthening the glutamine repeat. Despite similar binding properties of rat HAP1 and hHAP, differences in the sequences and expression of hHAP may contribute to a specific role for its interaction with huntingtin in humans.

Apoptosis in Degenerative Diseases of the Basal Ganglia

Degenerative disorders of the basal ganglia are characterized by disturbances of motor control. Prototypic examples are Parkinson's disease, which is caused by degeneration of dopamine neurons of the substantia nigra, and Huntington's disease, which is caused by degeneration of neurons of the striatum. In recent years, it has been postulated that some of these disorders may be caused by programmed cell death or apoptosis, a genetically regulated form of cell death. There is clear evidence that apoptosis occurs in neurons of the basal ganglia during normal development, that it can be regulated, and that it can be induced in some animal models of these disorders. Although there is some suggestive direct evidence that apoptosis may occur in the human brain in these disorders, the evidence to date is partial and not yet compelling. Nevertheless, programmed cell death is an important new hypothesis for the pathogenesis of these disorders and warrants vigorous further investigation, particularly with molecular markers in addition to classic morphological markers. The concept of programmed cell death is relevant not only to the pathogenesis of these diseases but also to therapeutic issues, such as transplantation approaches.

Expanded polyglutamine protein forms nuclear inclusions and causes neural degeneration in Drosophila

Spinocerebellar ataxia type 3 (SCA3/MJD) is one of at least eight human neurodegenerative diseases caused by glutamine-repeat expansion. We have recreated glutamine-repeat disease in Drosophila using a segment of the SCA3/MJD protein. Targeted expression of the protein with an expanded polyglutamine repeat led to nuclear inclusion (NI) formation and late-onset cell degeneration. Differential sensitivity to the mutant transgene was observed among different cell types, with neurons being particularly susceptible; NI formation alone was not sufficient for degeneration. The viral antiapoptotic gene P35 mitigated polyglutamine-induced degeneration in vivo. Our results demonstrate that cellular mechanisms of human glutamine-repeat disease are conserved in invertebrates. This fly model will aid in identifying additional factors that modulate neurodegeneration.

Short-sequence DNA repeats in prokaryotic genomes

Short-sequence DNA repeat (SSR) loci can be identified in all eukaryotic and many prokaryotic genomes. These loci harbor short or long stretches of repeated nucleotide sequence motifs. DNA sequence motifs in a single locus can be identical and/or heterogeneous. SSRs are encountered in many different branches of the prokaryote kingdom. They are found in genes encoding products as diverse as microbial surface components recognizing adhesive matrix molecules and specific bacterial virulence factors such as lipopolysaccharide-modifying enzymes or adhesins. SSRs enable genetic and consequently phenotypic flexibility. SSRs function at various levels of gene expression regulation. Variations in the number of repeat units per locus or changes in the nature of the individual repeat sequences may result from recombination processes or polymerase inadequacy such as slipped-strand mispairing (SSM), either alone or in combination with DNA repair deficiencies. These rather complex phenomena can occur with relative ease, with SSM approaching a frequency of 10(-4) per bacterial cell division and allowing high-frequency genetic switching. Bacteria use this random strategy to adapt their genetic repertoire in response to selective environmental pressure. SSR-mediated variation has important implications for bacterial pathogenesis and evolutionary fitness. Molecular analysis of changes in SSRs allows epidemiological studies on the spread of pathogenic bacteria. The occurrence, evolution and function of SSRs, and the molecular methods used to analyze them are discussed in the context of responsiveness to environmental factors, bacterial pathogenicity, epidemiology, and the availability of full-genome sequences for increasing numbers of microorganisms, especially those that are medically relevant.

Altered brain neurotransmitter receptors in transgenic mice expressing a portion of an abnormal human huntington disease gene

Loss of neurotransmitter receptors, especially glutamate and dopamine receptors, is one of the pathologic hallmarks of brains of patients with Huntington disease (HD). Transgenic mice that express exon 1 of an abnormal human HD gene (line R6/2) develop neurologic symptoms at 9-11 weeks of age through an unknown mechanism. Analysis of glutamate receptors (GluRs) in symptomatic 12-week-old R6/2 mice revealed decreases compared with age-matched littermate controls in the type 1 metabotropic GluR (mGluR1), mGluR2, mGluR3, but not the mGluR5 subtype of G protein-linked mGluR, as determined by [3H]glutamate receptor binding, protein immunoblotting, and in situ hybridization. Ionotropic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and kainate receptors were also decreased, while N-methyl-D-aspartic acid receptors were not different compared with controls. Other neurotransmitter receptors known to be affected in HD were also decreased in R6/2 mice, including dopamine and muscarinic cholinergic, but not gamma-aminobutyric acid receptors. D1-like and D2-like dopamine receptor binding was drastically reduced to one-third of control in the brains of 8- and 12-week-old R6/2 mice. In situ hybridization indicated that mGluR and D1 dopamine receptor mRNA were altered as early as 4 weeks of age, long prior to the onset of clinical symptoms. Thus, altered expression of neurotransmitter receptors precedes clinical symptoms in R6/2 mice and may contribute to subsequent pathology.

The expression of Huntingtin-associated protein (HAP1) mRNA in developing, adult and ageing rat CNS: implications for Huntington's disease neuropathology

Using radioactive in situ hybridization, we have mapped the expression of Huntingtin-associated protein (HAP1) mRNA in rat brain at developmental stages (E12-E19, PO-P21), in adult rats (3 months) and in 'aged' (19-21 months) rats. Using two pairs of 45mer oligonucleotide probes specific for HAP1A and a probe which recognizes regions of both the HAP1A and HAP1B mRNA sequences (panHAP1), we find that the expression of HAP1 mRNA is specific to the CNS and restricted predominantly to anatomically connected limbic structures, particularly the amygdala (medial and corticomedial nuclei), the hypothalamus (arcuate, preoptic, paraventricular and lateral hypothalamic area), bed nucleus of the stria terminalis (BNST) and the lateral septal nuclei. HAP1 mRNA was detected in embryos at E12 and displayed a prevalent distribution in the developing limbic structures by E15. In aged, 19-21-months-old, rats there is a downregulation of HAP1 mRNA expression across all CNS loci where HAP1 was previously abundant. The lowest levels of HAP1 mRNA expression corresponded with the areas of greatest pathological cell loss in Huntington's disease (HD); the caudate putamen, globus pallidus and neocortex. These observations support the suggestion that HAP1 plays an important role in the neuropathology of HD.

Borna disease virus-induced neurological disorder in mice: infection of neonates results in immunopathology

Borna disease virus (BDV) is a neurotropic nonsegmented negative-stranded RNA virus that persistently infects warm-blooded animals. In horses and other natural animal hosts, infections with BDV cause meningoencephalitis and behavioral disturbances. Experimental infection of adult mice takes a nonsymptomatic course, an observation previously believed to indicate that this animal species is not suitable for pathogenesis studies. We now demonstrate that BDV frequently induces severe neurological disease in infected newborn mice. Signs of neurological disease were first observed 4 to 6 weeks after intracerebral infection. They included a characteristic nonphysiological position of the hind limbs at an early stage of the disease and paraparesis at a later stage. Histological examination revealed large numbers of perivascular and meningeal inflammatory cells in brains of diseased mice and, unexpectedly, no increase in immunoreactivity to glial fibrillar acidic protein. The incidence and severity of BDV-induced disease varied dramatically among mouse strains. While only 13% of the infected C57BL/6 mice showed disease symptoms, which were mostly transient, more than 80% of the infected MRL mice developed severe neurological disorder. In spite of these differences in susceptibility to disease, BDV replicated to comparable levels in the brains of mice of the various strains used. Intracerebral infections of newborn beta2-microglobulin-deficient C57BL/6 and MRL mice, which both lack CD8+ T cells, did not result in meningoencephalitis or neurological disease, indicating that the BDV-induced neurological disorder in mice is a cytotoxic T-cell-mediated immunopathological process. With this new animal model it should now be possible to characterize the disease-inducing immune response to BDV in more detail.

HAP1-huntingtin interactions do not contribute to the molecular pathology in Huntington's disease transgenic mice

HAP1 (huntingtin associated protein) has previously been found to interact with huntingtin (htt) in a glutamine length dependent manner and has been proposed to play a role in the cell specific neurodegeneration observed in Huntington's disease (HD). We have isolated mouse HAP1 (hap1) and have shown that expression is not enriched in areas specifically affected in HD. We have used the yeast two hybrid system to demonstrate that htt amino acids 171-230 are necessary for the hap1-htt binding and that hapl does not interact with the transgene exon 1 protein in a transgenic model of HD.

Mouse mutant embryos lacking huntingtin are rescued from lethality by wild-type extraembryonic tissues

Mouse embryos nullizygous for a targeted disruption of the Huntington's disease gene homologue (Hdh), which encodes a protein (huntingtin) of unknown biochemical function, become developmentally retarded and disorganized, and die early in development. Using chimeric analysis, we demonstrate that extensively chimeric embryos derived by injection of Hdh null ES cells into wild-type host blastocysts are rescued from lethality. In contrast, when wild-type ES cells are injected into Hdh null blastocysts, the chimeric embryos are morphologically indistinguishable from Hdh null mutants derived from natural matings, and die shortly after gastrulation. Therefore, the primary defect in the absence of huntingtin lies in extraembryonic tissues, whereas the epiblast and its derivatives are affected secondarily. It is likely that the mutation results in impairment of the nutritive functions of the visceral endoderm, which otherwise appears to differentiate normally, as evidenced by the expression of several specific marker genes. Consistent with preliminary histochemical analysis indicating that at least the transport of ferric ions is defective in Hdh mutants and in conjunction with the known localization of huntingtin in the membranes of vesicles associated with microtubules, we hypothesize that this protein is involved in the intracellular trafficking of nutrients in early embryos.

Tissue transglutaminase-catalyzed formation of high-molecular-weight aggregates in vitro is favored with long polyglutamine domains: a possible mechanism contributing to CAG-triplet diseases

To investigate possible biochemical mechanisms underlying the "toxic gain of function" associated with polyglutamine expansions, the ability of guinea pig liver tissue transglutaminase to catalyze covalent attachments of various polyamines to polyglutamine peptides was examined. Of the polyamines tested, spermine is the most active substrate, followed by spermidine and putrescine. Formation of covalent cross links between polyglutamine peptides and polyamines yields high-M(r) aggregates--a process that is favored with longer polyglutamines. In the presence of tissue transglutaminase, purified glyceraldehyde-3-phosphate dehydrogenase (a key glycolytic enzyme that binds tightly to the polyglutamine domains of both huntingtin and dentatorubral-pallidoluysian atrophy proteins) is covalently attached to polyglutamine peptides in vitro, resulting in the formation of high-M(r) aggregates. In addition, endogenous glyceraldehyde-3-phosphate dehydrogenase of a Balb-c 3T3 fibroblast cell line overexpressing human tissue transglutaminase forms cross-links with a Q60 polypeptide added to the cell homogenate. Possibly, expansion of polyglutamine domains (thus far known to occur in the gene products associated with at least seven neurodegenerative diseases) leads to increased/aberrant tissue transglutaminase-catalyzed cross-linking reactions with both polyamines and susceptible proteins, such as glyceraldehyde-3-phosphate dehydrogenase. Formation of cross-linked heteropolymers may lead to deposition of high-M(r) protein aggregates, thereby contributing to cell death.

Caspase cleavage of gene products associated with triplet expansion disorders generates truncated fragments containing the polyglutamine tract

The neurodegenerative diseases Huntington disease, dentatorubropallidoluysian atrophy, spinocerebellar atrophy type 3, and spinal bulbar muscular atrophy are caused by expansion of a polyglutamine tract within their respective gene products. There is increasing evidence that generation of truncated proteins containing an expanded polyglutamine tract may be a key step in the pathogenesis of these disorders. We now report that, similar to huntingtin, atrophin-1, ataxin-3, and the androgen receptor are cleaved in apoptotic extracts. Furthermore, each of these proteins is cleaved by one or more purified caspases, cysteine proteases involved in apoptotic death. The CAG length does not modulate susceptibility to cleavage of any of the full-length proteins. Our results suggest that by generation of truncated polyglutamine-containing proteins, caspase cleavage may represent a common step in the pathogenesis of each of these neurodegenerative diseases.