Analysis of the subcellular localization of huntingtin with a set of rabbit polyclonal antibodies in cultured mammalian cells of neuronal origin: comparison with the distribution of huntingtin in Huntington's disease autopsy brain

Polyclonal Antibody Generation against PvTRAg for the Development of a Diagnostic Assay for Plasmodium vivax

The World Health Organization (WHO) has set forth a global call for eradicating malaria, caused majorly by the protozoan parasites Plasmodium falciparum and Plasmodium vivax. The lack of diagnostic biomarkers for P. vivax, especially those that differentiate the parasite from P. falciparum, significantly hinders P. vivax elimination. Here, we show that P. vivax tryptophan-rich antigen (PvTRAg) can be a diagnostic biomarker for diagnosing P. vivax in malaria patients. We report that polyclonal antibodies against purified PvTRAg protein show interactions with purified PvTRAg and native PvTRAg using Western blots and indirect enzyme-linked immunosorbent assay (ELISA). We also developed an antibody-antigen-based qualitative assay using biolayer interferometry (BLI) to detect vivax infection using plasma samples from patients with different febrile diseases and healthy controls. The polyclonal anti-PvTRAg antibodies were used to capture free native PvTRAg from the patient plasma samples using BLI, providing a new expansion range to make the assay quick, accurate, sensitive, and high-throughput. The data presented in this report provides a proof of concept for PvTRAg, a new antigen, for developing a diagnostic assay for P. vivax identification and differentiation from the rest of the Plasmodium species and, at a later stage, translating the BLI assay into affordable, point-of-care formats to make it more accessible.

Huntingtin Subcellular Localisation Is Regulated by Kinase Signalling Activity in the StHdhQ111 Model of HD

Huntington's disease is a neurodegenerative disorder characterised primarily by motor abnormalities, and is caused by an expanded polyglutamine repeat in the huntingtin protein. Huntingtin dynamically shuttles between subcellular compartments, and the mutant huntingtin protein is mislocalised to cell nuclei, where it may interfere with nuclear functions, such as transcription. However, the mechanism by which mislocalisation of mutant huntingtin occurs is currently unknown. An immortalised embryonic striatal cell model of HD (StHdhQ111) was stimulated with epidermal growth factor in order to determine whether the subcellular localisation of huntingtin is dependent on kinase signalling pathway activation. Aberrant phosphorylation of AKT and MEK signalling pathways was identified in cells carrying mutant huntingtin. Activity within these pathways was found to contribute to the regulation of huntingtin and mutant huntingtin localisation, as well as to the expression of immediate-early genes. We propose that altered kinase signalling is a phenotype of Huntington's disease that occurs prior to cell death; specifically, that altered kinase signalling may influence huntingtin localisation, which in turn may impact upon nuclear processes such as transcriptional regulation. Aiming to restore the balance of activity between kinase signalling networks may therefore prove to be an effective approach to delaying Huntington's disease symptom development and progression.

Huntingtin is required for ciliogenesis and neurogenesis during early Xenopus development

Huntington's Disease (HD) is a neurodegenerative disorder that results from the abnormal expansion of poly-glutamine (polyQ) repeats in the Huntingtin (HTT) gene. Although HTT has been linked to a variety of cellular events, it is still not clear what the physiological functions of the protein are. Because of its critical role during mouse embryonic mouse development, we investigated the functions of Htt during early Xenopus embryogenesis. We find that reduction of Htt levels affects cilia polarity and function and causes whole body paralysis. Moreover, Htt loss of function leads to abnormal development of trigeminal and motor neurons. Interestingly, these phenotypes are partially rescued by either wild-type or expanded HTT. These results show that the Htt activity is required for normal embryonic development, and highlight the usefulness of the Xenopus system for investigating proteins involved in human diseases.

The Drosophila Huntington's disease gene ortholog dhtt influences chromatin regulation during development

Huntington's disease is an autosomal dominant neurodegenerative disorder caused by a CAG expansion mutation in HTT, the gene encoding huntingtin. Evidence from both human genotype-phenotype relationships and mouse model systems suggests that the mutation acts by dysregulating some normal activity of huntingtin. Recent work in the mouse has revealed a role for huntingtin in epigenetic regulation during development. Here, we examine the role of the Drosophila huntingtin ortholog (dhtt) in chromatin regulation in the development of the fly. Although null dhtt mutants display no overt phenotype, we found that dhtt acts as a suppressor of position-effect variegation (PEV), suggesting that it influences chromatin organization. We demonstrate that dhtt affects heterochromatin spreading in a PEV model by modulating histone H3K9 methylation levels at the heterochromatin-euchromatin boundary. To gain mechanistic insights into how dhtt influences chromatin function, we conducted a candidate genetic screen using RNAi lines targeting known PEV modifier genes. We found that dhtt modifies phenotypes caused by knockdown of a number of key epigenetic regulators, including chromatin-associated proteins, histone demethylases (HDMs) and methyltransferases. Notably, dhtt strongly modifies phenotypes resulting from loss of the HDM dLsd1, in both the ovary and wing, and we demonstrate that dhtt appears to act as a facilitator of dLsd1 function in regulating global histone H3K4 methylation levels. These findings suggest that a fundamental aspect of huntingtin function in heterochromatin/euchromatin organization is evolutionarily conserved across phyla.

Pathological implications of nucleic acid interactions with proteins associated with neurodegenerative diseases

Protein misfolding disorders (PMDs) refer to a group of diseases related to the misfolding of particular proteins that aggregate and deposit in the cells and tissues of humans and other mammals. The mechanisms that trigger protein misfolding and aggregation are still not fully understood. Increasing experimental evidence indicates that abnormal interactions between PMD-related proteins and nucleic acids (NAs) can induce conformational changes. Here, we discuss these protein-NA interactions and address the role of deoxyribonucleic (DNA) and ribonucleic (RNA) acid molecules in the conformational conversion of different proteins that aggregate in PMDs, such as Alzheimer's, Parkinson's, and prion diseases. Studies on the affinity, stability, and specificity of proteins involved in neurodegenerative diseases and NAs are specifically addressed. A landscape of reciprocal effects resulting from the binding of prion proteins, amyloid-β peptides, tau proteins, huntingtin, and α-synuclein are presented here to clarify the possible role of NAs, not only as encoders of genetic information but also in triggering PMDs.

Huntingtin localisation studies - a technical review

It is well recognised that there are pitfalls when defining the subcellular localisation of a protein with immunocytochemistry. Accurate protein localisation to particular cellular micro-architecture is crucial in defining its role within the cell. Huntingtin (HTT), the protein mutated in the neurodegenerative disorder Huntington’s disease (HD) is a large protein of ill-defined function. Bearing little resemblance to other proteins, its function has been difficult to assign, therefore localising this protein with precision within the cell may provide further clues as to its normal and pathological function. Lack of consistency between methods employed in different studies has resulted in varying conclusions as to its subcellular localisation. This technical review investigates the effects that different immunocytological methods can have upon the apparent subcellular localisation of the huntingtin protein, and discusses the implications this may have.

Genetics and neuropathology of Huntington's disease

Huntington's disease (HD) is an autosomal dominant progressive neurodegenerative disorder that prominently affects the basal ganglia, leading to affective, cognitive, behavioral and motor decline. The basis of HD is a CAG repeat expansion to >35 CAG in a gene that codes for a ubiquitous protein known as huntingtin, resulting in an expanded N-terminal polyglutamine tract. The size of the expansion is correlated with disease severity, with increasing CAG accelerating the age of onset. A variety of possibilities have been proposed as to the mechanism by which the mutation causes preferential injury to the basal ganglia. The present chapter provides a basic overview of the genetics and pathology of HD.

Huntingtin modulates transcription, occupies gene promoters in vivo, and binds directly to DNA in a polyglutamine-dependent manner

Transcriptional dysregulation is a central pathogenic mechanism in Huntington's disease, a fatal neurodegenerative disorder associated with polyglutamine (polyQ) expansion in the huntingtin (Htt) protein. In this study, we show that mutant Htt alters the normal expression of specific mRNA species at least partly by disrupting the binding activities of many transcription factors which govern the expression of the dysregulated mRNA species. Chromatin immunoprecipitation (ChIP) demonstrates Htt occupation of gene promoters in vivo in a polyQ-dependent manner, and furthermore, ChIP-on-chip and ChIP subcloning reveal that wild-type and mutant Htt exhibit differential genomic distributions. Exon 1 Htt binds DNA directly in the absence of other proteins and alters DNA conformation. PolyQ expansion increases Htt-DNA interactions, with binding to recognition elements of transcription factors whose function is altered in HD. Together, these findings suggest mutant Htt modulates gene expression through abnormal interactions with genomic DNA, altering DNA conformation and transcription factor binding.

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.

Selective degeneration in YAC mouse models of Huntington disease

Huntington disease (HD) is one of at least nine polyglutamine disorders caused by a CAG expansion in the coding region of a disease-causing gene. These disorders are characterized by selective degeneration of different regions of the brain, which is not explained by the expression pattern of the mutant protein. In HD, degeneration primarily occurs in the striatum and cortex. To examine the mechanisms responsible for the selective neuronal loss in HD, we have generated yeast artificial chromosome (YAC) transgenic models of HD that express full length mutant huntingtin (htt) from a YAC. These mice have appropriate tissue-specific and temporal expression of mutant htt and accordingly recapitulate the motor deficits, cognitive impairment and selective degeneration of HD. As in human patients, mutant htt expression is not increased in the affected regions of the brain. In contrast, detection of mutant htt in the nucleus is earliest and greatest in the striatum, the region most affected in HD, suggesting that selective nuclear localization of mutant htt may contribute to the region specific atrophy in these mice. Selective phosphorylation of mutant htt on serine 421 may also contribute, as phosphorylation of mutant htt reduces its toxicity and is decreased in the striatum compared to other regions of the brain. Finally, the fact that mutant htt expression increases the susceptibility of striatal neurons to excitotoxicity but not neurons from the cerebellum, suggests that altered sensitization to excitotoxic death may also contribute to selective degeneration in YAC mice. Overall, YAC mice recapitulate the region specific damage that occurs in HD and provide a suitable model for examining the mechanisms underlying of selective degeneration.

Selective neuronal degeneration in Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative disorder that generally begins in middle age with abnormalities of movement, cognition, personality, and mood. Neuronal loss is most marked among the medium-sized projection neurons of the dorsal striatum. HD is an autosomal dominant genetic disorder caused by a CAG expansion in exon 1 of the HD gene, encoding an expanded polyglutamine (polyQ) tract near the N-terminus of the protein huntingtin. Despite identification of the gene mutation more than a decade ago, the normal function of this ubiquitously expressed protein is still under investigation and the mechanisms underlying selective neurodegeneration in HD remain poorly understood. Detailed postmortem analyses of brains of HD patients have provided important clues, and HD transgenic and knock-in mouse models have facilitated investigations into potential pathogenic mechanisms. Subcellular fractionation and immunolocalization studies suggest a role for huntingtin in organelle transport, protein trafficking, and regulation of energy metabolism. Consistent with this, evidence from vertebrate and invertebrate models of HD indicates that expression of the polyQ-expanded form of huntingtin results in early impairment of axonal transport and mitochondrial function. As well, alteration in activity of the N-methyl-d-aspartate (NMDA) type glutamate receptor, which has been implicated as a main mediator of excitotoxic neuronal death, especially in the striatum, is an early effect of mutant huntingtin. Proteolysis and nuclear localization of huntingtin also occur relatively early, while formation of ubiquitinated aggregates of huntingtin and transcriptional dysregulation occur as late effects of the gene mutation. Although each of these processes may contribute to neuronal loss in HD, here we review the data to support a strong role for NMDA receptor (NMDAR)-mediated excitotoxicity and mitochondrial dysfunction in conferring selective neuronal vulnerability in HD.

Small N-terminal mutant huntingtin fragments, but not wild type, are mainly present in monomeric form: Implications for pathogenesis

N-terminal fragments of huntingtin containing an expanded polyglutamine stretch play an important role in the molecular pathogenesis of Huntington's disease. Their ultimate accumulation in insoluble protein aggregates constitutes an important pathological hallmark of Huntington's disease. We report on systematic biochemical comparison studies of soluble wild type and mutant N-terminal huntingtin fragments. The results show that soluble wild type exon 1 fragments are predominantly present in higher molecular weight complexes with a molecular size of approximately 300 kDa, while their mutant counterparts are mainly present in their monomeric form. In contrast, longer N-terminal fragments corresponding to peptides produced by caspase cleavage do not display these differential properties. These findings suggest that especially an increased amount of monomeric form of small N-terminal mutant huntingtin fragments may facilitate aberrant interactions both with itself via the polyglutamine stretch and with other proteins and thereby contribute to molecular pathogenesis.

Huntingtin is localized in the nucleus during preimplanatation embryo development in mice

Huntington's disease (HD) is a dominant neurodegenerative disorder caused by the expansion of a CAG repeat in the gene encoding huntingtin. Moreover, the nuclear targeting of mutant huntingtin increases cellular toxicity, whereas normal huntingtin resides mainly in the cytoplasm, and is associated with membranes or microtubules. Huntingtin is enriched in neurons and its expression is increased during neural development. The inactivation of the HD gene results in embryonic lethality before nervous system development. Thus, huntingtin is critical during early embryonic development. Nevertheless, the function of huntingtin at this stage is unknown, even the distribution of the protein has not been described. The present study was undertaken to elucidate the distribution of huntingtin during the early developmental period in the mouse embryo. At the preimplantation stage, huntingtin was detected in nuclei up to 2.5 days post coitum (dpc), but disappeared from nuclei during the blastocyst stage (3.5 dpc). Following this stage, huntingtin was mainly localized in the cytoplasm and co-localized with mitotic spindles. These data suggest that the nuclear targeting of normal huntingtin is required during early embryo development in mice.

Selective degeneration and nuclear localization of mutant huntingtin in the YAC128 mouse model of Huntington disease

Huntington disease (HD) is an adult onset neurodegenerative disorder that predominantly affects the striatum and cortex despite ubiquitous expression of mutant huntingtin (htt). Here we demonstrate that this pattern of selective degeneration is present in the YAC128 mouse model of HD. At 12 months, YAC128 mice show significant atrophy in the striatum, globus pallidus and cortex with relative sparing of the hippocampus and cerebellum (striatum: -10.4%, P<0.001; globus pallidus: -10.8%, P=0.04; cortex: -8.6%, P=0.001; hippocampus: +0.3%, P=0.9; cerebellum: +2.9%, P=0.6). Similarly, neuronal loss at this age is present in the striatum (-9.1%, P<0.001) and cortex of YAC128 mice (-8.3%, P=0.02) but is not detected in the hippocampus (+1.5%, P=0.72). Mutant htt expression levels are similar throughout the brain and fail to explain the selective neuronal degeneration. In contrast, nuclear detection of mutant htt occurs earliest and to the greatest extent in the striatum-the region most affected in HD. The appearance of EM48-reactive mutant htt in the nucleus in the striatum at 2 months coincides with the onset of behavioral abnormalities in YAC128 mice. In contrast to YAC128 mice, the R6/1 mouse model of HD, which expresses exon 1 of mutant htt, exhibits non-selective, widespread atrophy along with non-selective nuclear detection of mutant htt at 10 months of age. Our findings suggest that selective nuclear localization of mutant htt may contribute to the selective degeneration in HD and that appropriately regulated expression of full-length mutant htt in YAC128 mice results in a pattern of degeneration remarkably similar to human HD.

Wild-type huntingtin plays a role in brain development and neuronal survival

While the role of the mutated Huntington's disease (HD) protein in the pathogenesis of HD has been the focus of intensive investigation, the normal protein has received less attention. Nonetheless, the wild-type HD protein appears to be essential for embryogenesis, since deletion of the HD gene in mice results in early embryonic lethality. This early lethality is due to a critical role the HD protein, called huntingtin (Htt), plays in extraembryonic membrane function, presumably in vesicular transport of nutrients. Studies of mutant mice expressing low levels of Htt and of chimeric mice generated by blastocyst injection of Hdh-/- embryonic stem cells show that wildtype Htt plays an important role later in development as well, specifically in forebrain formation. Moreover, various lines of study suggest that normal Htt is also critical for survival of neurons in the adult forebrain. The observation that Htt plays its key developmental and survival roles in those brain areas most affected in HD raises the possibility that a subtle loss of function on the part of the mutant protein or a sequestering of wild-type Htt by mutant Htt may contribute to HD pathogenesis. Regardless of whether this is so, the prosurvival role of Htt suggests that HD therapies that block production of both wild-type and mutant Htt may themselves be harmful.

Apoptosis in Huntington's disease

Huntington's disease (HD) is an autosomal dominant, fatal disorder. Patients display increasing motor, psychiatric and cognitive impairment and at autopsy, late-stage patient brains show extensive striatal (caudate and putamen), pallidal and cortical atrophy. The initial and primary target of degeneration in HD is the striatal medium spiny GABAergic neuron, and by end stages of the disease up to 95% of these neurons are lost [J. Neuropathol. Exp. Neurol. 57 (1998) 369]. The disease is caused by an elongation of a polyglutamine tract in the N-terminal of the huntingtin gene, but it is not known how this mutation leads to such extensive, but selective, cell death [Cell 72 (1993) 971]. There is substantial evidence from in vitro studies that connects apoptotic pathways and apoptosis with the mutant protein, and theories linking apoptosis to neuronal death in HD have existed for several years. Despite this, evidence of apoptotic neuronal death in HD is scarce. It may be that the processes involved in apoptosis, rather than apoptosis per se, are more important for HD pathogenesis. Upregulation of the proapoptotic proteins could lead to cleavage of huntingtin and as recent data has shown, the consequent toxic fragment may itself elicit toxic effects on the cell by disrupting transcription. In addition, the increased levels of proapoptotic proteins could contribute to slowly developing cell death in HD, selective for the striatal medium spiny GABAergic neurons and later spreading to other areas. Here we review the evidence supporting these mechanisms of pathogenesis in HD.

Proteases acting on mutant huntingtin generate cleaved products that differentially build up cytoplasmic and nuclear inclusions

Proteolytic processing of mutant huntingtin (mhtt) is regarded as a key event in the pathogenesis of Huntington's disease (HD). Mhtt fragments containing a polyglutamine expansion form intracellular inclusions and are more cytotoxic than full-length mhtt. Here, we report that two distinct mhtt fragments, termed cp-A and cp-B, differentially build up nuclear and cytoplasmic inclusions in HD brain and in a cellular model for HD. Cp-A is released by cleavage of htt in a 10 amino acid domain and is the major fragment that aggregates in the nucleus. Furthermore, we provide evidence that cp-A and cp-B are most likely generated by aspartic endopeptidases acting in concert with the proteasome to ensure the normal turnover of htt. These proteolytic processes are thus potential targets for therapeutic intervention in HD.

Cellular localization and development of neuronal intranuclear inclusions in striatal and cortical neurons in R6/2 transgenic mice

The cellular localization and development of neuronal intranuclear inclusions (NIIs) in cortex and striatum of R6/2 HD transgenic mice were studied to ascertain the relationship of NIIs to symptom formation in these mice and gain clues regarding the possible relationship of NII formation to neuropathology in Huntington's disease (HD). All NIIs observed in R6/2 mice were ubiquitinated, and no evidence was observed for a contribution to them from wild-type huntingtin; they were first observed in cortex and striatum at 3.5 weeks of age. In cortex, NIIs increased rapidly in size and prevalence after their appearance. Generally, cortical projection neurons developed NIIs more rapidly than cortical interneurons containing calbindin or parvalbumin. Few cortical somatostatinergic interneurons, however, formed NIIs. In striatum, calbindinergic projection neurons and parvalbuminergic interneurons rapidly formed NIIs, but they formed more gradually in cholinergic interneurons, and few somatostatinergic interneurons developed NIIs. Striatal NIIs tended to be smaller than those in cortex. The early accumulation of NIIs in cortex and striatum in R6/2 mice is consistent with the early appearance of motor and learning abnormalities in these mice, and the eventual pervasiveness of NIIs at ages at which severe abnormalities are evident is consistent with their contribution to a neuronal dysfunction underlying the abnormalities. That cortex develops larger NIIs than striatum, however, is inconsistent with the preferential loss of striatal neurons in HD but is consistent with recent evidence of early morphological abnormalities in cortical neurons in HD. That calbindinergic and parvalbuminergic striatal neurons develop large NIIs is consistent with a contribution of nuclear aggregate formation to their high degree of vulnerability in HD.

Arfaptin 2 regulates the aggregation of mutant huntingtin protein

Huntington's disease (HD) is an inherited neurodegenerative disorder. Here we demonstrate that expression of arfaptin 2/POR1 (partner of Rac1) in cultured cells induces the formation of pericentriolar and nuclear aggregates, which morphologically resemble mutant huntingtin aggregates characteristic of HD. Endogenous arfaptin 2 localizes to aggregates induced by expression of an abnormal amino-terminal fragment of huntingtin that contains polyglutamine (polyQ) expansions. A dominant inhibitory mutant of arfaptin 2 inhibits aggregation of mutant huntingtin, but not in the presence of proteasome inhibitors. Using cell-free biochemical assays, we show that arfaptin 2 inhibits proteasome activity. Finally, we show that expression of arfaptin 2 is increased at sites of neurodegeneration and the protein localizes to huntingtin aggregates in HD transgenic mouse brains. Our data suggest that arfaptin 2 is involved in regulating huntingtin protein aggregation, possibly by impairing proteasome function.

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.