Identification and localization of huntingtin in brain and human lymphoblastoid cell lines with anti-fusion protein antibodies

Subtype-specific enhancement of NMDA receptor currents by mutant huntingtin

Evidence suggests that NMDA receptor-mediated neurotoxicity plays a role in the selective neurodegeneration underlying Huntington's disease (HD). The gene mutation that causes HD encodes an expanded polyglutamine tract of >35 in huntingtin, a protein of unknown function. Both huntingtin and NMDA receptors interact with cytoskeletal proteins, and, for NMDA receptors, such interactions regulate surface expression and channel activity. To determine whether mutant huntingtin alters NMDA receptor expression or function, we coexpressed mutant or normal huntingtin, containing 138 or 15 glutamine repeats, respectively, with NMDA receptors in a cell line and then assessed receptor channel function by patch-clamp recording and surface expression by western blot analysis. It is interesting that receptors composed of NR1 and NR2B subunits exhibited significantly larger currents when coexpressed with mutant compared with normal huntingtin. Moreover, this effect was selective for NR1/NR2B, as NR1/NR2A showed similar currents when coexpressed with mutant versus normal huntingtin. However, ion channel properties and total surface expression of the NR1 subunit were unchanged in cells cotransfected with NR1/NR2B and mutant huntingtin. Our results suggest that mutant huntingtin may increase numbers of functional NR1/NR2B-type receptors at the cell surface. Because NR1/NR2B is the predominant NMDA receptor subtype expressed in medium spiny neostriatal neurons, our findings may help explain the selective vulnerability of these neurons in HD.

Impaired synaptic plasticity in mice carrying the Huntington's disease mutation

Cognitive impairment is an early symptom of Huntington's disease (HD). Mice engineered to carry the HD mutation in the endogenous huntingtin gene showed a significant reduction in long-term potentiation (LTP), a measure of synaptic plasticity often thought to be involved in memory. However, LTP could be induced in mutant slices by an 'enhanced' tetanic stimulus, implying that the LTP-producing mechanism is intact in mutant mice, but that their synapses are less able to reach the threshold for LTP induction. Mutant mice showed less post-tetanic potentiation than wild-type animals, and also showed decreased paired pulse facilitation, suggesting that excitatory synapses in HD mutant mice are impaired in their ability to sustain transmission during repetitive stimulation. We show that mutants, while normal in their ability to transmit at low frequencies, released significantly less glutamate during higher frequency synaptic activation. Thus, a reduced ability of Huntington synapses to respond to repetitive synaptic demand of even moderate frequency could result not only in a functional impairment of LTP induction, but could also serve as a substrate for the cognitive symptoms that comprise the early-stage pathology of HD.

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.

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.

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.

Rapid aggregate formation of the huntingtin N-terminal fragment carrying an expanded polyglutamine tract

Huntington's disease (HD) is caused by an expansion of the CAG repeat in the HD gene. The repeat is translated to the polyglutamine tract as huntingtin, the product of HD gene. Several studies showed that the expansion of polyglutamine tract leads to formation of cytoplasminc and/or intranuclear aggregates in vivo or in vitro. To understand the molecular mechanism of the aggregate formation, we studied the transient expression of HD exon 1-GFP fusion proteins in COS-7 cells. The fusion protein carrying 77 glutamine repeats aggregated in a time-dependent manner, while the fusion protein carrying 25 glutamine tract remained to be distributed diffusely in the cytoplasm even 72 hours after transfection. Initially, fluorescent signals were diffusely distributed in the COS-7 cells that were transfected with the construct containing the 77 CAG repeats. Approximately 40 hours later after the transfection, large aggregates grew very rapidly in those cells and the diffuse cytoplasmic fluorescence faded out. This process was completed within 40 minutes from the appearance of small aggregates in the perinuclear regions. The addition of cycloheximide reduced the frequencies of aggregate formation. A possibility was discussed that the aggregate formation was via nucleation. The focal concentration of mutated proteins in neurons may trigger the aggregate formation.

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.

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.

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 'tissue' transglutaminase binding proteins in neural cells committed to apoptosis

Overexpression of 'tissue' transglutaminase (tTG) in the human neuroblastoma cells increases spontaneous apoptosis and renders these cells highly susceptible to death induced by various stimuli. We used immunoprecipitation to identify cellular proteins that interact specifically with tTG in SK-N-BE(2) -derived stable transfectants. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis showed that tTG binding proteins have molecular masses of 110, 50, 22, 14, and 12 kDa. Microsequencing and computer search analyses allowed us to identify these polypeptides as the beta-tubulin (50 kDa), the histone H2B (14 kDa), and two GST P1-1-truncated forms (22 and 12 kDa). The specificity of the interaction between tTG and these proteins was confirmed by competing tTG binding with purified enzyme and by detecting tTG in immunoprecipitates obtained using beta-tubulin or GST P1-1 mAb's. Here we demonstrate that the GST P1-1 acts as an efficient acyl donor as well as acceptor tTG substrate both in cells and in vitro. The tTG-catalyzed polymerization of GST P1-1 leads to its functional inactivation and is competitively inhibited by GSH. By contrast, the tTG-beta-tubulin interaction does not result in the cross-linking of this cytoskeletal protein, which suggests that microtubules act as the anchorage site for tTG and GST P1-1 interaction.

Oxidative stress in Huntington's disease

It has been five years since the elucidation of the genetic mutation underlying the pathogenesis of Huntington's disease (HD) (97), however the precise mechanism of the selective neuronal death it propagates still remains an enigma. Several different etiological processes may play roles, and strong evidence from studies in both humans and animal models suggests the involvement of energy metabolism dysfunction, excitotoxic processes, and oxidative stress. Importantly, the recent development of transgenic mouse models of HD led to the identification of neuronal intranuclear inclusion bodies in affected brain regions in both mouse models and in HD brain, consisting of protein aggregates containing fragments of mutant huntingtin protein. These observations opened new avenues of investigation into possible huntingtin protein interactions and their putative pathogenetic sequelae. Amongst these studies, findings of elevated levels of oxidative damage products such as malondialdehyde, 8-hydroxydeoxyguanosine, 3-nitrotyrosine and heme oxygenase in areas of degeneration in HD brain, and of increased free radical production in animal models, indicate the involvement of oxidative stress either as a causative event, or as a secondary constituent of the cell death cascade in the disease. Here we review the evidence for oxidative damage and potential mechanisms of neuronal death in HD.

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.

Heterogeneous intracellular localization and expression of ataxin-3

Spinocerebellar ataxia type 3 or Machado-Joseph disease (SCA3/MJD) is an autosomal dominant neurodegenerative disorder caused by an unstable and expanded CAG trinucleotide repeat that leads to the expansion of a polyglutamine tract in a protein of unknown function, ataxin-3. We have generated and characterized a panel of monoclonal and polyclonal antibodies raised against ataxin-3 and used them to analyze its expression and localization. In Hela cells, multiple isoforms are expressed besides the major 55-kDa form. While the majority of ataxin-3 is cytosolic, both immunocytofluorescence and subcellular fractionation studies indicate the presence of ataxin-3, in particular, of some of the minor isoforms, in the nuclear and mitochodrial compartments. We also show that ataxin-3 can be phosphorylated. In the brain, only one ataxin-3 isoform containing the polyglutamine stretch was detected, and normal and mutated proteins were found equally expressed in all patient brain regions analyzed. In most neurons, ataxin-3 had a cytoplasmic, dendritic, and axonal localization. Some neurons presented an additional nuclear localization. Ataxin-3 is widely expressed throughout the brain, with a variable intensity specific for subpopulations of neurons. Its expression is, however, not restricted to regions that show intranuclear inclusions and neurodegeneration in SCA3/MJD.

The cellular and subcellular localization of huntingtin-associated protein 1 (HAP1): comparison with huntingtin in rat and human

The cellular and subcellular distribution of HAP1 was examined in rat brain by light and electron microscopic immunocytochemistry and subcellular fractionation. HAP1 localization was also determined in human postmortem tissue from control and Huntington's disease (HD) cases by light microscopic immunocytochemistry. At the cellular level, the heterogeneity of HAP1 expression was similar to that of huntingtin; however, HAP1 immunoreactivity was more widespread. The subcellular distribution of HAP1 was examined using immunogold electron microscopy. Like huntingtin, HAP1 is a cytoplasmic protein that associates with microtubules and many types of membranous organelles, including mitochondria, endoplasmic reticulum, tubulovesicles, endosomal and lysosomal organelles, and synaptic vesicles. A quantitative comparison of the organelle associations of HAP1 and huntingtin showed them to be almost identical. Within HAP1-immunoreactive neurons in rat and human brain, populations of large and small immunoreactive puncta were visible by light microscopy. The large puncta, which were especially evident in the ventral forebrain, were intensely HAP1 immunoreactive. Electron microscopic analysis revealed them to be a type of nucleolus-like body, which has been named a stigmoid body, that may play a role in protein synthesis. The small puncta, less intensely labeled, were primarily mitochondria. These results indicate that the localization of HAP1 and huntingtin is more similar than previously appreciated and provide further evidence that HAP1 and huntingtin have localizations consistent with roles in intracellular transport. Our data also suggest, however, that HAP1 is not present in the abnormal intranuclear and neuritic aggregates containing the N-terminal fragment of mutant huntingtin that are found in HD brains.

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.

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.

The cellular and subcellular localization of huntingtin-associated protein 1 (HAP1): comparison with huntingtin in rat and human

The cellular and subcellular distribution of HAP1 was examined in rat brain by light and electron microscopic immunocytochemistry and subcellular fractionation. HAP1 localization was also determined in human postmortem tissue from control and Huntington's disease (HD) cases by light microscopic immunocytochemistry. At the cellular level, the heterogeneity of HAP1 expression was similar to that of huntingtin; however, HAP1 immunoreactivity was more widespread. The subcellular distribution of HAP1 was examined using immunogold electron microscopy. Like huntingtin, HAP1 is a cytoplasmic protein that associates with microtubules and many types of membranous organelles, including mitochondria, endoplasmic reticulum, tubulovesicles, endosomal and lysosomal organelles, and synaptic vesicles. A quantitative comparison of the organelle associations of HAP1 and huntingtin showed them to be almost identical. Within HAP1-immunoreactive neurons in rat and human brain, populations of large and small immunoreactive puncta were visible by light microscopy. The large puncta, which were especially evident in the ventral forebrain, were intensely HAP1 immunoreactive. Electron microscopic analysis revealed them to be a type of nucleolus-like body, which has been named a stigmoid body, that may play a role in protein synthesis. The small puncta, less intensely labeled, were primarily mitochondria. These results indicate that the localization of HAP1 and huntingtin is more similar than previously appreciated and provide further evidence that HAP1 and huntingtin have localizations consistent with roles in intracellular transport. Our data also suggest, however, that HAP1 is not present in the abnormal intranuclear and neuritic aggregates containing the N-terminal fragment of mutant huntingtin that are found in HD brains.

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.

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.

Wild-type and mutant huntingtins function in vesicle trafficking in the secretory and endocytic pathways

Huntingtin is a cytoplasmic protein that is found in neurons and somatic cells. In patients with Huntington's disease (HD), the NH2-terminal region of huntingtin has an expanded polyglutamine tract. An abnormal protein interaction by mutant huntingtin has been proposed as a mechanism for HD pathogenesis. Huntingtin associates with vesicle membranes and interacts with proteins involved in vesicle trafficking. It is unclear where along vesicle transport pathways wild-type and mutant huntingtins are found and whether polyglutamine expansion affects this localization. To distinguish wild-type and mutant huntingtin, fibroblasts from normals and HD patients with two mutant alleles (homozygotes) were examined. Immunofluorescence confocal microscopy showed that mutant huntingtin localized with clathrin in membranes of the trans Golgi network and in clathrin-coated and noncoated endosomal vesicles in the cytoplasm and along plasma membranes. Separation of organelles in Nycodenz gradients showed that in normal and HD homozygote patient cells, huntingtin was present in membrane fractions enriched in clathrin. Similar results were obtained in fibroblasts from heterozyote juvenile HD patients who had a highly expanded polyglutamine tract in the HD allele. Western blot analysis of membrane fractions from rat brain showed that wild-type huntingtin was present in fractions that contained purified clathrin-coated membranes or a mixture of clathrin-coated and noncoated membranes. Electron microscopy of huntingtin immunoreactivity in rat brain revealed labeling along dendritic plasma membranes in association with clathrin-coated pits and clusters of noncoated endosomal vesicles 40-60 nm in diameter. These data suggest that wild-type and mutant huntingtin can influence vesicle transport in the secretory and endocytic pathways through associations with clathrin-coated vesicles.

Huntingtin: a single bait hooks many species

Cloning of the Huntington's disease gene uncovered huntingtin, which is remarkable for its lack of similarity with known proteins despite its large size, approximately 350 kDa. Subsequent experiments established that huntingtin has an as yet unknown function, crucial for embryonic development and neurogenesis. Recent protein trapping to identify huntingtin interactors now reveals that many different prey fall victim to huntingtin bait.

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.

Huntingtin protein colocalizes with lesions of neurodegenerative diseases: An investigation in Huntington's, Alzheimer's, and Pick's diseases

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease associated with a CAG trinucleotide repeat expansion in a large gene on chromosome 4. The gene encodes the protein huntingtin with a polyglutamine tract encoded by the CAG repeat at the N-terminus. The number of CAG repeats in HD are significantly increased (36 to 120+) compared with the normal population (8-39). The pathological mechanism associated with the expanded CAG repeat in HD is not clear but there is evidence that polyglutamine is directly neurotoxic. We have immunolocalized huntingtin with an in-house, well-characterised, polyclonal antibody in HD, Alzheimer's disease (AD), and Picks disease (PiD) brains. Control brain tissue sections were from head injured and cerebral ischaemia cases. In HD, huntingtin was immunopositive in the surviving but damaged neurons and reactive astrocytes of the caudate and putamen. However, in AD and PiD the immunostaining was largely restricted to the characteristic intracellular inclusion bodies associated with the disease process in each case. In AD, huntingtin was localized only in the intracellular neurofibrillary tangles and dystrophic neurites within the neuritic amyloid plaques but not with the amyloid. In PiD, strongly positive huntingtin immunostaining was present within cytoplasmic Pick bodies. Our findings suggest huntingtin selectively accumulates in association with abnormal intracytoplasmic and cytoskeletal filaments of neurons and glia in neurodegenerative diseases such as HD, AD, and PiD. Cells in the CNS appear sensitive to damage by the aggregated, toxic levels of huntingtin and evidence of its interaction with neurofilaments could provide information about its potential role in the aetiology of HD.

Intranuclear neuronal inclusions in Huntington's disease and dentatorubral and pallidoluysian atrophy: correlation between the density of inclusions and IT15 CAG triplet repeat length

Huntington's disease (HD) is caused by CAG triplet repeat expansion in IT15 which leads to polyglutamine stretches in the HD protein product, huntingtin. The pathological hallmark of HD is the degeneration of subsets of neurons, primarily those in the striatum and neocortex. Specific morphological markers of affected cells have not been identified in patients with HD, although a unique itranuclear inclusion was recently reported in neurons of transgenic animals expressing a construct encoding the N-terminal part (including the glutamine repeat) of huntingtin (Davies et al., 1997). In order to understand the importance of this finding, we sought for comparable nuclear abnormalities in autopsy material from patients with HD. In all 20 HD cases examined, anti-ubiquitin and N-terminal huntingtin antibodies identified itranuclear inclusions in neurons and the frequency of these lesions correlated with the length of the CAG repeat in IT15. In addition, examination of material from the related HD-like triplet repeat disorder, dentatorubral and pallidoluysian atrophy, also revealed intranuclear neuronal inclusions. These findings suggest that intranuclear inclusions containing protein aggregates may be common feature of the pathogenesis of glutamine repeat neurodegenerative disorders.

Interaction of huntingtin-associated protein with dynactin P150Glued

Huntingtin is the protein product of the gene for Huntington's disease (HD) and carries a polyglutamine repeat that is expanded in HD (>36 units). Huntingtin-associated protein (HAP1) is a neuronal protein and binds to huntingtin in association with the polyglutamine repeat. Like huntingtin, HAP1 has been found to be a cytoplasmic protein associated with membranous organelles, suggesting the existence of a protein complex including HAP1, huntingtin, and other proteins. Using the yeast two-hybrid system, we found that HAP1 also binds to dynactin P150(Glued) (P150), an accessory protein for cytoplasmic dynein that participates in microtubule-dependent retrograde transport of membranous organelles. An in vitro binding assay showed that both huntingtin and P150 selectively bound to a glutathione transferase (GST)-HAP1 fusion protein. An immunoprecipitation assay demonstrated that P150 and huntingtin coprecipitated with HAP1 from rat brain cytosol. Western blot analysis revealed that HAP1 was enriched in rat brain microtubules and comigrated with P150 and huntingtin in sucrose gradients. Immunofluorescence showed that transfected HAP1 colocalized with P150 and huntingtin in human embryonic kidney (HEK) 293 cells. We propose that HAP1, P150, and huntingtin are present in a protein complex that may participate in dynein-dynactin-associated intracellular transport.

Complex I defect in muscle from patients with Huntington's disease

We found a variable defect of complex I of the mitochondrial respiratory chain, ranging in severity from 25% to 63% of control values, in muscle of patients with Huntington's disease (HD). The most severe defect was observed in the patient with the greatest expansion of CAG triplets. Muscle morphology showed myopathic changes such as moth-eaten fibers, angulated fibers, increased subsarcolemmal oxidative activities, or an increased number of enlarged mitochondria with abnormal cristae. Multiple mitochondrial DNA deletions were found by polymerase chain reaction (PCR) analysis in muscle of the patient with the most severe defect of complex I. Our data further support the involvement of energetic defects and oxidative damage in muscle of patients with HD.

Interaction of huntingtin-associated protein with dynactin P150Glued

Huntingtin is the protein product of the gene for Huntington's disease (HD) and carries a polyglutamine repeat that is expanded in HD (>36 units). Huntingtin-associated protein (HAP1) is a neuronal protein and binds to huntingtin in association with the polyglutamine repeat. Like huntingtin, HAP1 has been found to be a cytoplasmic protein associated with membranous organelles, suggesting the existence of a protein complex including HAP1, huntingtin, and other proteins. Using the yeast two-hybrid system, we found that HAP1 also binds to dynactin P150(Glued) (P150), an accessory protein for cytoplasmic dynein that participates in microtubule-dependent retrograde transport of membranous organelles. An in vitro binding assay showed that both huntingtin and P150 selectively bound to a glutathione transferase (GST)-HAP1 fusion protein. An immunoprecipitation assay demonstrated that P150 and huntingtin coprecipitated with HAP1 from rat brain cytosol. Western blot analysis revealed that HAP1 was enriched in rat brain microtubules and comigrated with P150 and huntingtin in sucrose gradients. Immunofluorescence showed that transfected HAP1 colocalized with P150 and huntingtin in human embryonic kidney (HEK) 293 cells. We propose that HAP1, P150, and huntingtin are present in a protein complex that may participate in dynein-dynactin-associated intracellular transport.

Huntingtin is required for neurogenesis and is not impaired by the Huntington's disease CAG expansion

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disorder caused by a CAG repeat expansion that lengthens a glutamine segment in the novel huntingtin protein. To elucidate the molecular basis of HD, we extended the polyglutamine tract of the mouse homologue, Hdh, by targetted introduction of an expanded human HD CAG repeat, creating mutant HdhneoQ50 and HdhQ50 alleles that express reduced and wild-type levels of altered huntingtin, respectively. Mice homozygous for reduced levels displayed characteristic aberrant brain development and perinatal lethality, indicating a critical function for Hdh in neurogenesis. However, mice with normal levels of mutant huntingtin did not display these abnormalities, indicating that the expanded CAG repeat does not eliminate or detectably impair huntingtin's neurogenic function. Thus, the HD defect in man does not mimic complete or partial Hdh inactivation and appears to cause neurodegenerative disease by a gain-of-function mechanism.

Transglutaminase-catalyzed inactivation of glyceraldehyde 3-phosphate dehydrogenase and alpha-ketoglutarate dehydrogenase complex by polyglutamine domains of pathological length

Several adult-onset neurodegenerative diseases are caused by genes with expanded CAG triplet repeats within their coding regions and extended polyglutamine (Qn) domains within the expressed proteins. Generally, in clinically affected individuals n >/= 40. Glyceraldehyde 3-phosphate dehydrogenase binds tightly to four Qn disease proteins, but the significance of this interaction is unknown. We now report that purified glyceraldehyde 3-phosphate dehydrogenase is inactivated by tissue transglutaminase in the presence of glutathione S-transferase constructs containing a Qn domain of pathological length (n = 62 or 81). The dehydrogenase is less strongly inhibited by tissue transglutaminase in the presence of constructs containing shorter Qn domains (n = 0 or 10). Purified alpha-ketoglutarate dehydrogenase complex also is inactivated by tissue transglutaminase plus glutathione S-transferase constructs containing pathological-length Qn domains (n = 62 or 81). The results suggest that tissue transglutaminase-catalyzed covalent linkages involving the larger poly-Q domains may disrupt cerebral energy metabolism in CAG/Qn expansion diseases.

Huntington's disease gene product, huntingtin, associates with microtubules in vitro

The gene responsible for Huntington's disease produces a large protein with a molecular weight of approximately 350 k, designated huntingtin. Here, we report that the protein can associate in vitro with the microtubules. Through the process of assembly and disassembly of microtubules, both wild-type and mutant huntingtin associate with microtubules to almost the same degree. Huntingtin does not bind to the tubulin-affinity column directly. Huntingtin appears to interact with polymerized tubulin. These results suggest that huntingtin may have a role in intracellular organelle transport or axonal transport by its association with microtubules.

Differential distribution of the normal and mutated forms of huntingtin in the human brain

Huntington's disease is an inherited disorder caused by expansion of a CAG trinucleotide repeat in the IT15 gene, which leads to expansion of a polyglutamine tract within the protein called huntingtin. Despite the characterization of the IT15 gene and the mutation involved in the disease, the normal function of huntingtin and the effects of the mutation on its function and on its neuronal location remain unknown. To study whether mutated huntingtin has the same neuronal distribution and intracellular location as normal huntingtin, we analyzed immunohistochemically both forms of this protein in the brain of 5 controls and 5 patients with Huntington's disease. We show that the distribution of mutated huntingtin is, like that of the normal form, heterogeneous throughout the brain, but is not limited to vulnerable neurons in Huntington's disease, supporting the hypothesis that the presence of the mutated huntingtin in a neuron is not in itself sufficient to lead to neuronal death. Moreover, whereas normal huntingtin is detected in some neuronal perikarya, nerve fibers, and nerve endings, the mutated form is observed in some neuronal perikarya and proximal nerve processes but is not detectable in nerve endings. Our results suggest that the expression or processing of the mutated huntingtin in perikarya and nerve endings differs quantitatively or qualitatively from the expression of the normal form in the same neuronal compartments.

Huntingtin localization in brains of normal and Huntington's disease patients

The immunohistochemical localization of huntingtin was examined in the Huntington's disease (HD) brain with an antibody that recognizes the wild-type and mutant proteins. Neuronal staining was reduced in areas of the HD striatum depleted of medium-sized neurons; large striatal neurons, which are spared in HD, retained normal levels of huntingtin expression. Neuronal labeling was markedly reduced in both segments of the globus pallidus including in brains with minimal loss of pallidal neurons. In some HD cortical and striatal neurons with normal looking morphology, huntingtin was associated with punctate cytoplasmic granules that at the ultrastructural level resembled the multivesicular body, an organelle involved in retrograde transport and protein degradation. Some immunoreactive processes showed blebbing and segmentation similar to that induced experimentally by hypoxic-ischemic or excitotoxic injury. Huntingtin staining was more concentrated in the perinuclear cytoplasm and reduced or absent in processes of atrophic cortical neurons. Nuclear staining was also evident. Fibers in the subcortical white matter of HD patients had significantly increased huntingtin immunoreactivity compared with those of controls. Results suggest that there may be changes in the neuronal expression and transport of wild-type and/or mutant huntingtin at early and late stages of neuronal degeneration in affected areas of the HD brain.

Oligomerization of expanded-polyglutamine domain fluorescent fusion proteins in cultured mammalian cells

Six inherited neurologic diseases, including Huntington's disease, result from the expansion of a CAG domain of the disease genes to produce a domain of more than 40 glutamines in the expressed protein. The mechanism by which expansion of this polyglutamine domain causes disease is unknown. Recent studies demonstrated oligomerization of polyglutamine-domain proteins in mammalian neurons. To study oligomerization of polyglutamine proteins and to identify heterologous protein interactions, varying length polyglutamine-green fluorescent protein fusion proteins were expressed in cultured COS-7 cells. The 19- and 35-glutamine fusion proteins (non-pathologic length) distributed diffusely throughout the cytoplasm. In contrast, 56- and 80-glutamine fusion proteins (pathologic length) formed fibrillar arrays resembling those previously observed in neurons in Huntington's disease and in a transgenic mouse model. These aggregates were intranuclear and intracytoplasmic. Intracytoplasmic aggregates were surrounded by collapsed intermediate filaments. The intermediate filament protein vimentin co-immunoisolated with expanded polyglutamine fusion proteins. This cellular model will expedite investigations into oligomerization of polyglutamine proteins and their interactions with other proteins.

Formation of neuronal intranuclear inclusions underlies the neurological dysfunction in mice transgenic for the HD mutation

Huntington's disease (HD) is one of an increasing number of human neurodegenerative disorders caused by a CAG/polyglutamine-repeat expansion. The mutation occurs in a gene of unknown function that is expressed in a wide range of tissues. The molecular mechanism responsible for the delayed onset, selective pattern of neuropathology, and cell death observed in HD has not been described. We have observed that mice transgenic for exon 1 of the human HD gene carrying (CAG)115 to (CAG)156 repeat expansions develop pronounced neuronal intranuclear inclusions, containing the proteins huntingtin and ubiquitin, prior to developing a neurological phenotype. The appearance in transgenic mice of these inclusions, followed by characteristic morphological change within neuronal nuclei, is strikingly similar to nuclear abnormalities observed in biopsy material from HD patients.

Huntingtin-encoded polyglutamine expansions form amyloid-like protein aggregates in vitro and in vivo

The mechanism by which an elongated polyglutamine sequence causes neurodegeneration in Huntington's disease (HD) is unknown. In this study, we show that the proteolytic cleavage of a GST-huntingtin fusion protein leads to the formation of insoluble high molecular weight protein aggregates only when the polyglutamine expansion is in the pathogenic range. Electron micrographs of these aggregates revealed a fibrillar or ribbon-like morphology, reminiscent of scrapie prions and beta-amyloid fibrils in Alzheimer's disease. Subcellular fractionation and ultrastructural techniques showed the in vivo presence of these structures in the brains of mice transgenic for the HD mutation. Our in vitro model will aid in an eventual understanding of the molecular pathology of HD and the development of preventative strategies.

Intranuclear inclusions of expanded polyglutamine protein in spinocerebellar ataxia type 3

The mechanism of neurodegeneration in CAG/polyglutamine repeat expansion diseases is unknown but is thought to occur at the protein level. Here, in studies of spinocerebellar ataxia type 3, also known as Machado-Joseph disease (SCA3/MJD), we show that the disease protein ataxin-3 accumulates in ubiquitinated intranuclear inclusions selectively in neurons of affected brain regions. We further provide evidence in vitro for a model of disease in which an expanded polyglutamine-containing fragment recruits full-length protein into insoluble aggregates. Together with recent findings from transgenic models, our results suggest that intranuclear aggregation of the expanded protein is a unifying feature of CAG/polyglutamine diseases and may be initiated or catalyzed by a glutamine-containing fragment of the disease protein.

The neuropathology of CAG repeat diseases: review and update of genetic and molecular features

Classification of inherited neurodegenerative diseases is increasingly based on their genetic features, which supplement, clarify, and sometimes replace the older clinical and pathologic schemata. This change has been particularly rapid and impressive for the CAG repeat disorders. In Huntington's disease, X-linked spinobulbar muscular atrophy, dentatorubropallidoluysian atrophy, and a series of autosomal dominant cerebellar atrophies, genetic advances have resolved many nosologic issues, and opened new avenues for exploration of pathogenesis. In this review, we summarize classic and current concepts in neuropathology of these CAG repeat diseases.

Dentatorubral and pallidoluysian atrophy (DRPLA). Clinical and neuropathological findings in genetically confirmed North American and European pedigrees

Dentatorubral and pallidoluysian atrophy (DRPLA) is an autosomal dominant disorder that clinically overlaps with Huntington's disease (HD) and manifests combinations of chorea, myoclonus, seizures, ataxia, and dementia. DRPLA is caused by a CAG triplet repeat (CTG-B37) expansion coding for polyglutamine on chromosome 12 and exhibits the genetic phenomenon of anticipation. This neurodegenerative disease has only rarely been reported in non-Japanese pedigrees, and there are only a few neuropathological studies in genetically confirmed patients. We report 10 cases of DRPLA from two North American and two British pedigrees in which CTG-B37 expansions have been demonstrated within each kindred (54-83 repeats), individually in 8 of the 10 cases, and describe the neuropathological findings in 4 cases. Members of DRPLA kindreds have a wide range of clinical phenotypes and markedly variable ages at onset. The neuropathological spectrum is centered around the cerebellifugal and pallidofugal systems, but neurodegenerative changes can be found in many nuclei, tracts, and systems. Evidence of CTG-B37 triplet repeat expansion should be sought in HD-like cases that are negative for expanded triplet repeats within the HD IT15 gene or in autopsy cases with degeneration of the dentatorubral or pallidoluysian systems.

Huntington disease: advances in molecular and cell biology

Huntington disease is an inherited neurodegeneration, for which the associated mutation was isolated in 1993. The mutation is an expansion of a CAG trinucleotide repeat, which translates to give a polyglutamine tract at the N-terminus of a large protein, huntingtin. Neither the normal nor the pathogenic functions of this protein have been identified, but it is clear that pathogenesis is mediated through the expanded polyglutamine tract within the protein, and that polyglutamine is toxic to cells. A number of proteins which interact with the N-terminal region of huntingtin have been isolated, but this has not, so far, yielded a rationale for pathogenesis. Huntingtin is found in areas of the brain that degenerate in this disease but is also associated with pathogenic inclusions in Alzheimer disease and Pick disease. It is possible that Huntington disease has pathogenic mechanisms in common with these other neurodegenerative diseases, and that the mechanism may relate to the formation of abnormal, cytoskeletal-associated, inclusions within cells.

Heterogeneous topographic and cellular distribution of huntingtin expression in the normal human neostriatum

A striking heterogeneous distribution of topographic and cellular huntingtin immunoreactivity was observed within the human neostriatum using three distinct huntingtin antibodies. Patchy areas of low huntingtin immunoreactivity were present in both the caudate nucleus and putamen, surrounded by an intervening area of greater immunoreactivity. Comparison of huntingtin immunoreactivity with contiguous serial sections stained for enkephalin and calbindin D28k immunoreactivities showed that the topographic heterogeneity of huntingtin immunostaining corresponded to the patch (striosome) and matrix compartments within the striatum. Huntingtin immunoreactivity was confined primarily to neurons and neuropil within the matrix compartment, whereas little or no neuronal or neuropil huntingtin immunostaining was observed within the patch compartment. There was marked variability in the intensity of huntingtin immunolabel among medium-sized striatal neurons, whereas a majority of large striatal neurons were only faintly positive or without any immunoreactivity. Combined techniques for NADPH-diaphorase enzyme histochemistry and huntingtin immunocytochemistry, as well as double immunofluorescence for either nitric oxide synthase or calbindin D28k in comparison with huntingtin expression, revealed a striking correspondence between calbindin D28k and huntingtin immunoreactivities, with little or no colocalization between NADPH-diaphorase or nitric oxide synthase neurons and huntingtin expression. These observations suggest that the selective vulnerability of spiny striatal neurons and the matrix compartment observed in Huntington's disease is associated with higher levels of huntingtin expression, whereas the relative resistance of large and medium-sized aspiny neurons and the patch compartments to degeneration is associated with low levels of huntingtin expression.

Heterogeneous topographic and cellular distribution of huntingtin expression in the normal human neostriatum

A striking heterogeneous distribution of topographic and cellular huntingtin immunoreactivity was observed within the human neostriatum using three distinct huntingtin antibodies. Patchy areas of low huntingtin immunoreactivity were present in both the caudate nucleus and putamen, surrounded by an intervening area of greater immunoreactivity. Comparison of huntingtin immunoreactivity with contiguous serial sections stained for enkephalin and calbindin D28k immunoreactivities showed that the topographic heterogeneity of huntingtin immunostaining corresponded to the patch (striosome) and matrix compartments within the striatum. Huntingtin immunoreactivity was confined primarily to neurons and neuropil within the matrix compartment, whereas little or no neuronal or neuropil huntingtin immunostaining was observed within the patch compartment. There was marked variability in the intensity of huntingtin immunolabel among medium-sized striatal neurons, whereas a majority of large striatal neurons were only faintly positive or without any immunoreactivity. Combined techniques for NADPH-diaphorase enzyme histochemistry and huntingtin immunocytochemistry, as well as double immunofluorescence for either nitric oxide synthase or calbindin D28k in comparison with huntingtin expression, revealed a striking correspondence between calbindin D28k and huntingtin immunoreactivities, with little or no colocalization between NADPH-diaphorase or nitric oxide synthase neurons and huntingtin expression. These observations suggest that the selective vulnerability of spiny striatal neurons and the matrix compartment observed in Huntington's disease is associated with higher levels of huntingtin expression, whereas the relative resistance of large and medium-sized aspiny neurons and the patch compartments to degeneration is associated with low levels of huntingtin expression.

Machado-Joseph disease gene product is a cytoplasmic protein widely expressed in brain

Machado-Joseph disease (MJD) is one of at least six neurodegenerative diseases caused by expansion of a CAG repeat encoding a polyglutamine tract in the disease protein. To study the molecular mechanism of disease, we isolated both normal and expanded repeat MJD1 cDNAs, and generated antiserum against the recombinant gene product, called ataxin-3. Using this antiserum, we demonstrate that in disease tissue, both the normal and mutant ataxin-3 protein are expressed throughout the body and in all regions of the brain examined, including areas generally spared by disease. In brain, certain regions (the striatum, for example) express ataxin-3 in only a limited subset of neurons. Immunolocalization studies in normal and disease brain, and in transfected cells, indicate that ataxin-3 is predominantly a cytoplasmic protein that localizes to neuronal processes as well. We conclude that in MJD, as in other polyglutamine repeat diseases, cellular expression of the disease gene is not itself sufficient to cause neuronal degeneration; other cell-specific factors must be invoked to explain the restricted neuropathology seen in MJD. The restricted expression of ataxin-3 in certain regions, however, may influence the pattern of neurodegeneration and provide clues to the protein's function.