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

Huntingtin: A Protein with a Peculiar Solvent Accessible Surface

Taking advantage of the last cryogenic electron microscopy structure of human huntingtin, we explored with computational methods its physicochemical properties, focusing on the solvent accessible surface of the protein and highlighting a quite interesting mix of hydrophobic and hydrophilic patterns, with the prevalence of the latter ones. We then evaluated the probability of exposed residues to be in contact with other proteins, discovering that they tend to cluster in specific regions of the protein. We then found that the remaining portions of the protein surface can contain calcium-binding sites that we propose here as putative mediators for the protein to interact with membranes. Our findings are justified in relation to the present knowledge of huntingtin functional annotation.

Quantitative Proteomic Analysis Reveals Similarities between Huntington's Disease (HD) and Huntington's Disease-Like 2 (HDL2) Human Brains

The pathogenesis of HD and HDL2, similar progressive neurodegenerative disorders caused by expansion mutations, remains incompletely understood. No systematic quantitative proteomics studies, assessing global changes in HD or HDL2 human brain, were reported. To address this deficit, we used a stable isotope labeling-based approach to quantify the changes in protein abundances in the cortex of 12 HD and 12 control cases and, separately, of 6 HDL2 and 6 control cases. The quality of the tissues was assessed to minimize variability due to post mortem autolysis. We applied a robust median sweep algorithm to quantify protein abundance and performed statistical inference using moderated test statistics. 1211 proteins showed statistically significant fold changes between HD and control tissues; the differences in selected proteins were verified by Western blotting. Differentially abundant proteins were enriched in cellular pathways previously implicated in HD, including Rho-mediated, actin cytoskeleton and integrin signaling, mitochondrial dysfunction, endocytosis, axonal guidance, DNA/RNA processing, and protein transport. The abundance of 717 proteins significantly differed between control and HDL2 brain. Comparative analysis of the disease-associated changes in the HD and HDL2 proteomes revealed that similar pathways were altered, suggesting the commonality of pathogenesis between the two disorders.

The Potential Regulatory Mechanisms of miR-196a in Huntington's Disease through Bioinformatic Analyses

High throughput screening is a powerful tool to identify the potential candidate molecules involved during disease progression. However, analysis of complicated data is one of the most challenging steps on the way to obtaining useful results from this approach. Previously, we showed that a specific miRNA, miR-196a, could ameliorate the pathological phenotypes of Huntington’s disease (HD) in different models, and performed high throughput screening by using the striatum of transgenic mice. In this study, we further tried to identify the potential regulatory mechanisms using different bioinformatic tools, including Database for Annotation, Visualization and Integrated Discovery (DAVID), Molecular Signatures Database (MSigDB), TargetScan and MetaCore. The results showed that miR-196a dominantly altered “ABC transporters”, “RIG-I-like receptor signaling pathway”, immune system”, “adaptive immune system”,“tissue remodeling and wound repair” and “cytoskeleton remodeling”. In addition, miR-196a also changed the expression of several well-defined pathways of HD, such as apoptosis and cell adhesion. Since these analyses showed the regulatory pathways are highly related to the modification of the cytoskeleton, we further confirmed that miR-196a could enhance the neurite outgrowth in neuroblastoma cells, suggesting miR-196a might provide beneficial functions through the alteration of cytoskeleton structures. Since impairment of the cytoskeleton has been reported in several neuronal diseases, this study will provide not only the potential working mechanisms of miR-196a but also insights for therapeutic strategies for use with different neuronal diseases.

Calretinin interacts with huntingtin and reduces mutant huntingtin-caused cytotoxicity

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by an expansion of CAG trinucleotide repeats encoding for polyglutamine (polyQ) in the huntingtin (Htt) gene. Despite considerable effort, the mechanisms underlying the toxicity of the mutated Htt protein remains largely uncertain. To identify novel therapeutic targets, we recently employed the approach of tandem affinity purification and discovered that calretinin (Cr), a member of the EF-hand family of calcium-binding proteins, is preferentially associated with mHtt, although it also interacts with wild-type Htt. These observations were supported by coimmunoprecipitation and by colocalization of Cr with mHtt in neuronal cultures. Over- expression of Cr reduced mHtt-caused cytotoxicity in both non-neuronal and neuronal cell models of HD, whereas knockdown of Cr expression in the cells enhanced mHtt-caused neuronal cell death. In addition, over-expression of Cr was also associated with reduction of intracellular free calcium and activation of Akt. These results suggest that Cr may be a potential therapeutic target for treatment of HD.

Comparative studies of microtubule mechanics with two competing models suggest functional roles of alternative tubulin lateral interactions

The dynamic assembly and disassembly of microtubules and the mechanical properties of these polymers are essential for many key cellular processes. Mathematical and computational modeling, especially coupled mechanochemical modeling, has contributed significantly to our understanding of microtubule dynamics. However, critical discrepancies exist between experimental observations and modeling results that need to be resolved before further progress toward a complete model can be made. Open sheet structures ranging in length from several hundred nanometers to one micron have often been observed at the growing ends of microtubules in in vitro studies. Existing modeling studies predict these sheet structures to be short and rare intermediates of microtubule disassembly rather than important components of the assembly process. Atomic force microscopy (AFM) studies also reveal interesting step-like gaps of the force-indentation curve that cannot yet be explained by existing theoretical models. We have carried out computational studies to compare the mechanical properties of two alternative models: a more conventional model where tubulin dimers are added directly into a microtubule lattice, and one that considers an additional type of tubulin lateral interaction proposed to exist in intermediate sheet structures during the microtubule assembly process. The first model involves a single type of lateral interactions between tubulin subunits, whereas the latter considers a second type that can convert to the canonical lateral contact during microtubule closure into a cylinder. Our analysis shows that only the second model can reproduce the AFM results over a broad parameter range. We propose additional studies using different sizes of AFM tips that would allow to unambiguously distinguish the relative validity of the two models.

Molecular biology of Huntington's disease

It has been more than 17 years since the causative mutation for Huntington's disease was discovered as the expansion of the triplet repeat in the N-terminal portion of the Huntingtin (HTT) gene. In the intervening time, researchers have discovered a great deal about Huntingtin's involvement in a number of cellular processes. However, the role of Huntingtin in the key pathogenic mechanism leading to neurodegeneration in the disease process has yet to be discovered. Here, we review the body of knowledge that has been uncovered since gene discovery and include discussions of the HTT gene, CAG triplet repeat expansion, HTT expression, protein features, posttranslational modifications, and many of its known protein functions and interactions. We also highlight potential pathogenic mechanisms that have come to light in recent years.

Tricyclic pyrone compounds prevent aggregation and reverse cellular phenotypes caused by expression of mutant huntingtin protein in striatal neurons

Background

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a CAG repeat expansion mutation in the coding region of a novel gene. The mechanism of HD is unknown. Most data suggest that polyglutamine-mediated aggregation associated with expression of mutant huntingtin protein (mhtt) contributes to the pathology. However, recent studies have identified early cellular dysfunctions that preclude aggregate formation. Suppression of aggregation is accepted as one of the markers of successful therapeutic approaches. Previously, we demonstrated that tricyclic pyrone (TP) compounds efficiently inhibited formation of amyloid-β (Aβ) aggregates in cell and mouse models representing Alzheimer's Disease (AD). In the present study, we aimed to determine whether TP compounds could prevent aggregation and restore early cellular defects in primary embryonic striatal neurons from animal model representing HD.

Results

TP compounds effectively inhibit aggregation caused by mhtt in neurons and glial cells. Treatment with TP compounds also alleviated cholesterol accumulation and restored clathrin-independent endocytosis in HD neurons.

Conclusion

We have found that TP compounds not only blocked mhtt-induced aggregation, but also alleviated early cellular dysfunctions that preclude aggregate formation. Our data suggest TP molecules may be used as lead compounds for prevention or treatment of multiple neurodegenerative diseases including HD and AD.

Detection of alpha-rod protein repeats using a neural network and application to huntingtin

A growing number of solved protein structures display an elongated structural domain, denoted here as alpha-rod, composed of stacked pairs of anti-parallel alpha-helices. Alpha-rods are flexible and expose a large surface, which makes them suitable for protein interaction. Although most likely originating by tandem duplication of a two-helix unit, their detection using sequence similarity between repeats is poor. Here, we show that alpha-rod repeats can be detected using a neural network. The network detects more repeats than are identified by domain databases using multiple profiles, with a low level of false positives (<10%). We identify alpha-rod repeats in approximately 0.4% of proteins in eukaryotic genomes. We then investigate the results for all human proteins, identifying alpha-rod repeats for the first time in six protein families, including proteins STAG1-3, SERAC1, and PSMD1-2 & 5. We also characterize a short version of these repeats in eight protein families of Archaeal, Bacterial, and Fungal species. Finally, we demonstrate the utility of these predictions in directing experimental work to demarcate three alpha-rods in huntingtin, a protein mutated in Huntington's disease. Using yeast two hybrid analysis and an immunoprecipitation technique, we show that the huntingtin fragments containing alpha-rods associate with each other. This is the first definition of domains in huntingtin and the first validation of predicted interactions between fragments of huntingtin, which sets up directions toward functional characterization of this protein. An implementation of the repeat detection algorithm is available as a Web server with a simple graphical output: http://www.ogic.ca/projects/ard. This can be further visualized using BiasViz, a graphic tool for representation of multiple sequence alignments.

Activated caspase-6 and caspase-6-cleaved fragments of huntingtin specifically colocalize in the nucleus

Proteolysis of mutant huntingtin is crucial to the development of Huntington disease (HD). Specifically preventing proteolysis at the capase-6 (C6) consensus sequence at amino acid 586 of mutant huntingtin prevents the development of behavioural, motor and neuropathological features in a mouse model of HD. However, the mechanism underlying the selective toxicity of the 586 amino acid cleavage event is currently unknown. We have examined the subcellular localization of different caspase proteolytic fragments of huntingtin using neo-epitope antibodies. Our data suggest that the nucleus is the primary site of htt cleavage at amino acid 586. Endogenously cleaved 586 amino acid fragments are enriched in the nucleus of immortalized striatal cells and primary striatal neurons where they co-localize with active C6. Cell stress induced by staurosporine results in the nuclear translocation and activation of C6 and an increase in 586 amino acid fragments of huntingtin in the nucleus. In comparison, endogenous caspase-2/3-generated huntingtin 552 amino acid fragments localize to the perinuclear region. The different cellular itineraries of endogenously generated caspase products of huntingtin may provide an explanation for the selective toxicity of huntingtin fragments cleaved at amino acid 586.

Oxidative stress and mitochondrial dysfunction in neurodegenerative diseases

In recent years, it has become increasingly clear that mitochondrial dysfunction and oxidative damage are major contributors to neuronal loss. Free radicals, typically generated from mitochondrial respiration, cause oxidative damage of nucleic acids, lipids, carbohydrates and proteins. Despite enormous amount of effort, however, the mechanism by which oxidative damage causes neuronal death is not well understood. Emerging data from a number of neurodegenerative diseases suggest that there may be common features of toxicity that are related to oxidative damage. In this review, while focusing on Huntington's disease (HD), we discuss similarities among HD, Friedreich ataxia and xeroderma pigmentosum, which provide insight into shared mechanisms of neuronal death.

Huntingtin is cleaved by caspases in the cytoplasm and translocated to the nucleus via perinuclear sites in Huntington's disease patient lymphoblasts

Accumulation of mutant Huntingtin (Htt), especially the N-terminal-cleaved Htt, participates in the pathophysiology of Huntington's disease (HD). It is difficult to elucidate temporal properties of the translocation of "endogenous" Htt using autopsy HD patient brains. Thus, we examined the cell biology of "endogenous" Htt cleavage and nuclear translocation in cultured lymphoblasts of HD patients and controls. Apoptotic stimulation of lymphoblasts elicits caspase-dependent cleavage and selective nuclear translocation of N-terminal portions of Htt. Discrete clusters of the N-terminal Htt accumulate at unique perinuclear sites prior to nuclear translocation. Our findings suggest that caspase cleavage of Htt is cytoplasmic and precedes sorting to specific perinuclear sites followed by nuclear translocation in HD patient tissue.

Early changes in Huntington's disease patient brains involve alterations in cytoskeletal and synaptic elements

Huntington's disease (HD) is caused by a polyglutamine repeat expansion in the N-terminus of the huntingtin protein. Huntingtin is normally present in the cytoplasm where it may interact with structural and synaptic elements. The mechanism of HD pathogenesis remains unknown but studies indicate a toxic gain-of-function possibly through aberrant protein interactions. To investigate whether early degenerative changes in HD involve alterations of cytoskeletal and vesicular components, we examined early cellular changes in the frontal cortex of HD presymptomatic (PS), early pathological grade (grade 1) and late-stage (grade 3 and 4) patients as compared to age-matched controls. Morphologic analysis using silver impregnation revealed a progressive decrease in neuronal fiber density and organization in pyramidal cell layers beginning in presymptomatic HD cases. Immunocytochemical analyses for the cytoskeletal markers alpha -tubulin, microtubule-associated protein 2, and phosphorylated neurofilament demonstrated a concomitant loss of staining in early grade cases. Immunoblotting for synaptic proteins revealed a reduction in complexin 2, which was marked in some grade 1 HD cases and significantly reduced in all late stage cases. Interestingly, we demonstrate that two synaptic proteins, dynamin and PACSIN 1, which were unchanged by immunoblotting, showed a striking loss by immunocytochemistry beginning in early stage HD tissue suggesting abnormal distribution of these proteins. We propose that mutant huntingtin affects proteins involved in synaptic function and cytoskeletal integrity before symptoms develop which may influence early disease onset and/or progression.

End4/Sla2 is involved in establishment of a new growth zone inSchizosaccharomyces pombe

The rod-shaped Schizosaccharomyces pombe cell grows in a polarized fashion from opposing ends. Correct positioning of the growth zones is directed by the polarity marker Tea1 located at the cell ends where actin patches accumulate and cell growth takes place. We show that the S. pombe homologue of Saccharomyces cerevisiae SLA2, a protein involved in cortical actin organization and endocytosis, provides a link between the polarity marker and the growth machinery. In wild-type fission yeast cells, this homologue End4/Sla2 is enriched at cell ends during interphase and localizes to a medial ring at cell division, mirroring the actin localization pattern throughout the cell cycle. Proper localization relies on membrane trafficking and is independent of both the actin and microtubule cytoskeletons. End4/Sla2 is required for the establishment of new polarised growth zones, and deletion of its C-terminal talin-like domain prevents the establishment of a new growth zone after cell fission. We propose that End4/Sla2 acts downstream of the polarity marker Tea1 and is implicated in the recruitment of the actin cytoskeleton to bring about polarised cell growth.

Mutant huntingtin impairs axonal trafficking in mammalian neurons in vivo and in vitro

Recent data in invertebrates demonstrated that huntingtin (htt) is essential for fast axonal trafficking. Here, we provide direct and functional evidence that htt is involved in fast axonal trafficking in mammals. Moreover, expression of full-length mutant htt (mhtt) impairs vesicular and mitochondrial trafficking in mammalian neurons in vitro and in whole animals in vivo. Particularly, mitochondria become progressively immobilized and stop more frequently in neurons from transgenic animals. These defects occurred early in development prior to the onset of measurable neurological or mitochondrial abnormalities. Consistent with a progressive loss of function, wild-type htt, trafficking motors, and mitochondrial components were selectively sequestered by mhtt in human Huntington's disease-affected brain. Data provide a model for how loss of htt function causes toxicity; mhtt-mediated aggregation sequesters htt and components of trafficking machinery leading to loss of mitochondrial motility and eventual mitochondrial dysfunction.

Huntingtin Controls Neurotrophic Support and Survival of Neurons by Enhancing BDNF Vesicular Transport along Microtubules

Polyglutamine expansion (polyQ) in the protein huntingtin is pathogenic and responsible for the neuronal toxicity associated with Huntington's disease (HD). Although wild-type huntingtin possesses antiapoptotic properties, the relationship between the neuroprotective functions of huntingtin and pathogenesis of HD remains unclear. Here, we show that huntingtin specifically enhances vesicular transport of brain-derived neurotrophic factor (BDNF) along microtubules. Huntingtin-mediated transport involves huntingtin-associated protein-1 (HAP1) and the p150(Glued) subunit of dynactin, an essential component of molecular motors. BDNF transport is attenuated both in the disease context and by reducing the levels of wild-type huntingtin. The alteration of the huntingtin/HAP1/p150(Glued) complex correlates with reduced association of motor proteins with microtubules. Finally, we find that the polyQ-huntingtin-induced transport deficit results in the loss of neurotrophic support and neuronal toxicity. Our findings indicate that a key role of huntingtin is to promote BDNF transport and suggest that loss of this function might contribute to pathogenesis.

Genetic and environmental factors in the pathogenesis of Huntington's disease

Huntington's disease is a fatal inherited disorder in which there is progressive neurodegeneration in specific brain areas, mainly the striatum and cerebral cortex, producing motor, cognitive, and psychiatric symptoms. The trinucleotide repeat mutation involved is common to many other brain diseases, which may therefore involve similar mechanisms of pathogenesis. We are beginning to understand how a CAG trinucleotide repeat expansion in the disease gene, encoding an expanded polyglutamine tract, induces neuronal dysfunction and symptomatology in Huntington's disease. Recent evidence that environmental factors modify the onset and progression of neurodegeneration has shed new light on Huntington's disease and other devastating brain diseases. This review focuses on genetic mediators, environmental modulators, and associated gene-environment interactions in the pathogenesis of Huntington's disease.

Microtubule destabilization and nuclear entry are sequential steps leading to toxicity in Huntington's disease

There has been a longstanding debate regarding the role of proteolysis in Huntington's disease. The toxic peptide theory posits that N-terminal cleavage fragments of mutant Huntington's disease protein [mutant huntingtin (mhtt)] enter the nucleus to cause transcriptional dysfunction. However, recent data suggest a second model in which proteolysis of full-length mhtt is inhibited. Importantly, the two competing theories differ with respect to subcellular distribution of mhtt at initiation of toxicity: nuclear if cleaved and cytoplasmic in the absence of cleavage. Using quantitative single-cell analysis and time-lapse imaging, we show here that transcriptional dysfunction is "downstream" of cytoplasmic dysfunction. Primary and reversible toxic events involve destabilization of microtubules mediated by full-length mhtt before cleavage. Restoration of microtubule structure by taxol inhibits nuclear entry and increases cell survival.

Huntingtin-interacting protein-1-related protein of rat (rHIP1R) is localized in the postsynaptic regions

We cloned a rHIP1R (GenBank Accession No., AB005052) encoding a Sla2/huntingtin-interacting protein (HIP1) family protein from a rat brain cDNA library. Localization of rHIP1R was investigated in the rat brain using an antibody specific to the HIP1R antibody. The rHIP1R protein was enriched in the synaptic plasma membrane fraction along with huntingtin, a synaptic protein and a causal protein for Huntington's disease. The electron microscopic examination revealed that HIP1R was localized at postsynaptic spines. Localization of HIP1R in the small vesicular structures in the spine, possible sites of vesicular transport of synaptic proteins, together with the structure-based analysis, suggested a role of HIP1R for vesicle trafficking through interaction with F-actin and working together with huntingtin and HIP1 at the synaptic sites.

Cargo-carrying motor vehicles on the neuronal highway: Transport pathways and neurodegenerative disease

Within axons vital cargoes must be transported over great distances along microtubule tracks to maintain neuronal viability. Essential to this system are the molecular motors, kinesin and dynein, which transport a variety of neuronal cargoes. Elucidating the transport pathways, the identity of the cargoes transported, and the regulation of motor-cargo complexes are areas of intense investigation. Evidence suggests that essential components, including signaling proteins, neuroprotective and repair molecules, and vesicular and cytoskeletal components are all transported. In addition newly emerging data indicate that defects in axonal transport pathways may contribute to the initiation or progression of chronic neuronal dysfunction. In this review we concentrate on microtubule-based motor proteins, their linkers, and cargoes and discuss how factors in the axonal transport pathway contribute to disease states. As additional cargo complexes and transport pathways are identified, an understanding of the role these pathways play in the development of human disease will hopefully lead to new diagnostic and treatment strategies.

Evidence for dysfunction of the nigrostriatal pathway in the R6/1 line of transgenic Huntington's disease mice

The present multidisciplinary study examined nigrostriatal dopamine and striatal amino acid transmission in the R6/1 line of transgenic Huntington's disease (HD) mice expressing exon 1 of the HD gene with 115 CAG repeats. Although the number of tyrosine hydroxylase-positive neurons was not reduced and nigrostriatal connectivity remained intact in 16-week-old R6/1 mice, the size of tyrosine hydroxylase-positive neurons in the substantia nigra was reduced by 15%, and approximately 30% of these cells exhibited aggregated huntingtin. In addition, using in vivo microdialysis, we found that basal extracellular striatal dopamine levels were reduced by 70% in R6/1 mice compared to their wild-type littermates. Intrastriatal perfusion with malonate in R6/1 mice resulted in a short-lasting, attenuated increase in local dopamine release compared to wild-type mice. Furthermore, the size of the malonate-induced striatal lesion was 80% smaller in these animals. Taken together, these findings suggest that a functional deficit in nigrostriatal dopamine transmission may contribute to the behavioral phenotype and the resistance to malonate-induced neurotoxicity characteristic of R6/1 HD mice.

HIP1 and HIP12 display differential binding to F-actin, AP2, and clathrin. Identification of a novel interaction with clathrin light chain

Huntingtin-interacting protein 1 (HIP1) and HIP12 are orthologues of Sla2p, a yeast protein with essential functions in endocytosis and regulation of the actin cytoskeleton. We now report that HIP1 and HIP12 are major components of the clathrin coat that interact but differ in their ability to bind clathrin and the clathrin adaptor AP2. HIP1 contains a clathrin-box and AP2 consensus-binding sites that display high affinity binding to the terminal domain of the clathrin heavy chain and the ear domain of the AP2 alpha subunit, respectively. These consensus sites are poorly conserved in HIP12 and correspondingly, HIP12 does not bind to AP2 nor does it demonstrate high affinity clathrin binding. Moreover, HIP12 co-sediments with F-actin in contrast to HIP1, which exhibits no interaction with actin in vitro. Despite these differences, both proteins efficiently stimulate clathrin assembly through their central helical domain. Interestingly, in both HIP1 and HIP12, this domain binds directly to the clathrin light chain. Our data suggest that HIP1 and HIP12 play related yet distinct functional roles in clathrin-mediated endocytosis.

Alteration of nuclear glyceraldehyde-3-phosphate dehydrogenase structure in Huntington's disease fibroblasts

The expression of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) may be involved in neuronal disease and in programmed cell death. Recent investigations indicate an in vitro physical association between GAPDH and huntingtin, the mutated protein in Huntington's disease (HD). Previous studies reveal the functional diversity of GAPDH as a membrane, cytoplasmic and nuclear protein. These activities are independent of its classical glycolytic function. Thus, huntingtin-GAPDH interactions could affect not only energy production but also result in pleiotropic effects involving various biochemical pathways in HD cells. We now report the identification of a nuclear high molecular weight (HMW) GAPDH species in Huntington's disease cells. In contrast, nuclei from age-matched control normal human cells did not contain the HMW GAPDH species. Further, this GAPDH structure was not observed in HD whole cell sonicates which are characterized by normal GAPDH activity. The disruption of intracellular structure is implicit in the preparation of whole cell sonicates. Therefore, these results suggest that the dissociation of the GAPDH protein from its high molecular weight structure results in the recovery of its function. These findings reveal a singular, new subcellular phenotype in HD cells. As such, they indicate an interrelationship between nuclear GAPDH function and huntingtin localization in this CAG expansion neuronal disease.

Requirement of an intact microtubule cytoskeleton for aggregation and inclusion body formation by a mutant huntingtin fragment

Huntington's disease is caused by the expansion of CAG repeats coding for a polyglutamine tract in the huntingtin protein. The major pathological feature found in Huntington's disease neurons is the presence of detergent-insoluble ubiquitinated inclusion bodies composed of the huntingtin protein. However, the mechanisms that underlie inclusion body formation, and the precise relationship between inclusion bodies and events that initiate toxicity, remain unclear. Here, we analyzed the effects of drugs or genetic mutations that disrupt the microtubule cytoskeleton in a Saccharomyces cerevisiae model of the aggregation of an amino-terminal polyglutamine-containing fragment of huntingtin exon 1 (HtEx1). Treatment of yeast with drugs that disrupt microtubules resulted in less than 2% of the detergent-insoluble HtEx1 observed in mock-treated cells and prevented the formation of large juxtanuclear inclusion bodies. Disruption of microtubules also unmasked a potent glutamine length-dependent toxicity of HtEx1 under conditions where HtEx1 exists in an entirely detergent-soluble nonaggregated form. Results from the yeast model paralleled those from neuronal pheochromocytoma cells, where disruption of microtubules eliminated the formation of juxtanuclear and intranuclear inclusion bodies by HtEx1. Our results suggest that active transport along microtubules may be required for inclusion body formation by HtEx1 and that inclusion body formation may have evolved as a cellular mechanism to promote the sequestration or clearance of soluble species of HtEx1 that are otherwise toxic to cells.

Identification of tissue transglutaminase-reactive lysine residues in glyceraldehyde-3-phosphate dehydrogenase

Polyglutamine domains are excellent substrates for tissue transglutaminase resulting in the formation of cross-links with polypeptides containing lysyl residues. This finding suggests that tissue transglutaminase may play a role in the pathology of neurodegenerative diseases associated with polyglutamine expansion. The glycolytic enzyme GAPDH previously was shown to tightly bind several proteins involved in such diseases. The present study confirms that GAPDH is an in vitro lysyl donor substrate of tissue transglutaminase. A dansylated glutamine-containing peptide was used as probe for labeling the amino-donor sites. SDS gel electrophoresis of a time-course reaction mixture revealed the presence of both fluorescent GAPDH monomers and high molecular weight polymers. Western blot analysis performed using antitransglutaminase antibodies reveals that tissue transglutaminase takes part in the formation of heteropolymers. The reactive amino-donor sites were identified using mass spectrometry. Here, we report that of the 26 lysines present in GAPDH, K191, K268, and K331 were the only amino-donor residues modified by tissue transglutaminase.

Nuclear relocation of normal huntingtin

In Huntington's Disease (HD), the huntingtin protein (Htt) includes an expanded polyglutamine domain. Since mutant Htt concentrates in the nucleus of affected neurons, we have inquired whether normal Htt (Q16--23) is also able to access the nucleus. We observe that a major pool of normal full-length Htt of HeLa cells is anchored to endosomes and also detect RNase-sensitive nuclear foci which include a 70-kDa N-terminal Htt fragment. Agents which damage DNA trigger caspase-3-dependent cleavage of Htt and dramatically relocate the 70 kDa fragment to the nucleoplasm. Considering that polyglutamine tracts stimulate caspase activation, mutant Htt is therefore poised to enter the nucleus. These considerations help rationalize the nuclear accumulation of Htt which is characteristic of HD and provide a first example of involvement of caspase cleavage in release of membrane-bound proteins which subsequently enter the nucleus.

The selective vulnerability of nerve cells in Huntington's disease

It is now more than 7 years since the genetic mutation causing Huntington's disease (HD) was first identified. Unstable CAG expansion in the IT15 gene, responsible for disease, is translated into an abnormally long polyglutamine (polyQ) tract near the N-terminus of the huntingtin protein. The presence of expanded polyQ in the mutant protein leads to its abnormal proteolytic cleavage with liberation of toxic N-terminal fragments that tend to aggregate, probably first in the cytoplasm. Subsequent nuclear translocation of the cleaved mutant huntingtin is associated with formation of intranuclear protein aggregates and neurotoxicity, probably involving apoptotic cascades. These processes, which can be experimentally modelled in cultured neuronal and non-neuronal cells, seem to underlie neurodegeneration in HD, and also other polyQ disorders, such as dentatorubro-pallidoluysian degeneration, spinal and bulbar muscular atrophy and the spinocerebellar ataxias. They do not, however, explain why within the corpus striatum and cerebral cortex certain nerve cells are susceptible to disease and others are not. In the human HD brain, vulnerable pyramidal neurones within the deeper layers of the cerebral cortex frequently contain large intranuclear inclusions composed of N-terminal fragments of huntingtin. Such inclusions are, however, rare within neurones of the striatum, even in the medium spiny neurones preferentially lost from this region. While inclusions per se do not seem to be neurotoxic, they may provide a surrogate marker of molecular pathology. Recent studies indicate that the nuclear accumulation of mutant huntingtin interferes with transcriptional events. Of particular importance may be the effect on the genes encoding neurotransmitter receptor proteins, especially those involved with glutamatergic neurotransmission. Such changes may trigger or facilitate a low-grade, chronic excitotoxicity of the glutamatergic cortical projection neurones on their target cells in the striatum, already partly compromised by the toxic effects of the HD mutation. This combination of insults, for anatomical reasons experienced predominantly by striatal projection neurones, would eventually cause their selective demise.

Neuroimmunophilin ligands: evaluation of their therapeutic potential for the treatment of neurological disorders

Neuroimmunophilin ligands are a class of compounds that hold great promise for the treatment of nerve injuries and neurological disease. In contrast to neurotrophins (e.g., nerve growth factor), these compounds readily cross the blood-brain barrier, being orally effective in a variety of animal models of ischaemia, traumatic nerve injury and human neurodegenerative disorders. A further distinction is that neuroimmunophilin ligands act via unique receptors that are unrelated to the classical neurotrophic receptors (e.g., trk), making it unlikely that clinical trials will encounter the same difficulties found with the neurotrophins. Another advantage is that two neuroimmunophilin ligands (cyclosporin A and FK-506) have already been used in humans (as immunosuppressant drugs). Whereas both cyclosporin A and FK-506 demonstrate neuroprotective actions, only FK-506 and its derivatives have been clearly shown to exhibit significant neuroregenerative activity. Accordingly, the neuroprotective and neuroregenerative properties seem to arise via different mechanisms. Furthermore, the neuroregenerative property does not involve calcineurin inhibition (essential for immunosuppression). This is important since most of the limiting side effects produced by these drugs arise via calcineurin inhibition. A major breakthrough for the development of this class of compounds for the treatment of human neurological disorders was the ability to separate the neuroregenerative property of FK-506 from its immunosuppressant action via the development of non-immunosuppressant (non-calcineurin inhibiting) derivatives. Further studies revealed that different receptor subtypes, or FK-506-binding proteins (FKBPs), mediate immunosuppression and nerve regeneration (FKBP-12 and FKBP-52, respectively, the latter being a component of steroid receptor complexes). Thus, steroid receptor chaperone proteins represent novel targets for future drug development of novel classes of compounds for the treatment of a variety of human neurological disorders, including traumatic injury (e.g., peripheral nerve and spinal cord), chemical exposure (e.g., vinca alkaloids, Taxol) and neurodegenerative disease (e.g. , diabetic neuropathy and Parkinson's disease).

Participation of par-4 in the degeneration of striatal neurons induced by metabolic compromise with 3-nitropropionic acid

Huntington's disease (HD) is a progressive neurodegenerative disorder characterized by chorea, psychiatric disturbances, and dementia. It is caused by a polyglutamine repeat expansion in the huntingtin protein. The striatum is a major site of neuronal loss in HD, but the mechanisms underlying the neurodegenerative process have not been established. Systemic administration of the succinate dehydrogenase inhibitor 3-nitropropionic acid (3NP) to rodents results in motor dysfunction and degeneration of striatal neurons with features similar to those of HD. Here we report that levels of prostate apoptosis response-4 (Par-4; a protein recently linked to neuronal apoptosis) increase in striatum, and to a lesser extent in cortex and hippocampus, after systemic administration of 3NP to adult rats. The increase in Par-4 levels occurred within 6 h of 3NP administration and was followed by an increase in caspase activation which preceded neuronal loss. Exposure of cultured primary striatal neurons to 3NP induced a rapid increase of Par-4 levels and caspase activation. Treatment of striatal neurons with a Par-4 antisense oligonucleotide blocked Par-4 induction by 3NP, suppressed caspase activation, and attenuated neuronal apoptosis. The caspase-3 inhibitor DEVD suppressed 3NP-induced apoptosis of striatal neurons, but did not prevent induction of Par-4, indicating that Par-4 acts upstream of caspase-3 activation in the cell death pathway. Our results suggest that Par-4 plays an important role in the degeneration of striatal neurons in an experimental model of HD.

Alpha-synuclein immunoreactivity of huntingtin polyglutamine aggregates in striatum and cortex of Huntington's disease patients and transgenic mouse models

Polyglutamine expansions in proteins are implicated in at least eight inherited neurodegenerative disorders, including Huntington's disease. These mutant proteins can form aggregates within the nucleus and processes of neurons possibly due to misfolding of the proteins. Polyglutamine aggregates are ubiquitinated and sequester molecular chaperone proteins and proteasome components. To investigate other protein components of polyglutamine aggregates, cerebral cortex and striata from patients with Huntington's disease and full-length cDNA transgenic mouse models for this disease were examined immunohistochemically for alpha-synuclein reactivity. Our findings demonstrate that alpha-synuclein can be used as a marker for huntingtin polyglutamine aggregates in both human and mice. Moreover in the HD transgenic mice, the intensity of immunoreactivity increases with age. The significance of recruitment of alpha-synuclein into huntingtin aggregates and its translocation away from the synapses remains to be determined. We propose that aberrant interaction of mutant huntingtin with other proteins, including alpha-synuclein, may influence disease progression.

Tubulin immunoreactive neuronal intranuclear inclusions in the human brain

Intranuclear filamentous and crystalline inclusion bodies have been described in the nuclei of a variety of cells in both normal and pathological states. The functional significance of these structures remains to be elucidated. Moreover, although the proteinaceous nature of these inclusions has been inferred in some histochemical studies, the identity of their constituent proteins remains to be determined. In the present study, immunohistochemistry was used to investigate the presence of intranuclear inclusions in neurones of the human brain which are intensely immunoreactive for the neuronal cytoskeletal protein class III beta tubulin. The ability to label these structures immunohistochemically was exploited to investigate the topographic pattern of distribution of these inclusions in the human brain. Intranuclear inclusions were rod-shaped, polygonal, or irregular in shape. They were present in neurones and ependymal cells. Intranuclear inclusion-bearing neurones were distributed in an anatomically heterogeneous pattern in the brain. Areas exhibiting relatively high densities of inclusions included the substantia inominata and anterior olfactory nucleus, dentate gyrus, substantia nigra, inferior olivary nucleus, and dentate nucleus of the cerebellum. In addition, intranuclear inclusions were prevalent in neurones in layers II, V, and VI of the cerebral cortex. They were particularly prevalent in the mesial basal temporal neocortex. The relationship of these structures to the intranuclear rods and sheets of the classical microscopists is uncertain. The demonstration that they are composed, at least in part, of tubulin, a major cytoskeletal protein, provides important clues regarding the mechanisms underlying their formation and provides a springboard for developing hypotheses regarding their functional significance. Furthermore, the ability to demonstrate these inclusions immunohistochemically provides an avenue for further studies directed at elucidating the potential involvement of these inclusions in various pathological settings.

Are there multiple pathways in the pathogenesis of Huntington's disease?

Studies of huntingtin localization in human post-mortem brain offer insights and a framework for basic experiments in the pathogenesis of Huntington's disease. In neurons of cortex and striatum, we identified changes in the cytoplasmic localization of huntingtin including a marked perinuclear accumulation of huntingtin and formation of multivesicular bodies, changes conceivably pointing to an altered handling of huntingtin in neurons. In Huntington's disease, huntingtin also accumulates in aberrant subcellular compartments such as nuclear and neuritic aggregates co-localized with ubiquitin. The site of protein aggregation is polyglutamine-dependent, both in juvenile-onset patients having more aggregates in the nucleus and in adult-onset patients presenting more neuritic aggregates. Studies in vitro reveal that the genesis of these aggregates and cell death are tied to cleavage of mutant huntingtin. However, we found that the aggregation of mutant huntingtin can be dissociated from the extent of cell death. Thus properties of mutant huntingtin more subtle than its aggregation, such as its proteolysis and protein interactions that affect vesicle trafficking and nuclear transport, might suffice to cause neurodegeneration in the striatum and cortex. We propose that mutant huntingtin engages multiple pathogenic pathways leading to neuronal death.

Recent advances on the pathogenesis of Huntington's disease

We review recent advances regarding the pathogenesis of Huntington's disease (HD). This genetic neurodegenerative disorder is caused by an expanded CAG repeat in a gene coding for a protein, with unknown function, called huntingtin. There is selective death of striatal and cortical neurons. Both in patients and a transgenic mouse model of the disease, neuronal intranuclear inclusions, immunoreactive for huntingtin and ubiquitin, develop. Huntingtin interacts with the proteins GAPDH, HAP-1, HIP1, HIP2, and calmodulin, and a mutant huntingtin is specifically cleaved by the proapoptotic enzyme caspase 3. The pathogenetic mechanism is not known, but it is presumed that there is a toxic gain of function of the mutant huntingtin. Circumstantial evidence suggests that excitotoxicity, oxidative stress, impaired energy metabolism, and apoptosis play a role.

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.

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.

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.

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.

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.

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.

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.