N-terminal Fragments Of Mutant Htt Research Articles

Mutant huntingtin affects p53 function in a context‐dependent manner in a cell model of Huntington's disease

Huntington's disease (HD) is an adult‐onset neurodegenerative disease caused by expansion of a polyglutamine (polyQ) repeat in the N‐terminal region of the huntingtin (htt) protein. In HD, multiple N‐terminal mutant htt fragments, smaller than the first 500 amino acids, have been found to accumulate in disease tissue. These N‐terminal htt fragments associate abnormally with many cellular proteins, disturbing their function. The p53 tumor suppressor protein has been shown to abnormally interact with mutant htt and is associated with cellular dysfunction in HD. However, it is unknown whether different htt fragments affect p53 function differentially. Here, we investigated the affect of different length N‐terminal mutant htt fragments on p53 localization and function in a human cell model of HD. We found that shorter htt fragments cause more accumulation of p53 protein in the nucleus which is associated with decreased cell viability. Our results demonstrate the importance of mutant htt protein context in HD. While the p53 pathway is implicated as a potential target to ameliorate Huntington disease progression, the therapeutic benefit may be limited by the relative abundance of shorter N‐terminal htt fragments in target cells.Support or Funding InformationThis work was supported by the National Science Foundation under Grant No. 1458212. JC was supported in part by a faculty fellowship from the Appalachian College Association.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Soluble N-terminal fragment of mutant Huntingtin protein impairs mitochondrial axonal transport in cultured hippocampal neurons

Huntington's disease (HD) is an autosomal dominant, progressive, neurodegenerative disorder caused by an unstable expansion of CAG repeats (>35 repeats) within exon 1 of the interesting transcript 15 (IT15) gene. This gene encodes a protein called Huntingtin (Htt), and mutation of the gene results in a polyglutamine (polyQ) near the N-terminus of Htt. The N-terminal fragments of mutant Htt (mHtt), which tend to aggregate, are sufficient to cause HD. Whether these aggregates are causal or protective for HD remains hotly debated. Dysfunctional mitochondrial axonal transport is associated with HD. It remains unknown whether the soluble or aggregated form of mHtt is the primary cause of the impaired mitochondrial axonal transport in HD pathology. Here, we investigated the impact of soluble and aggregated N-terminal fragments of mHtt on mitochondrial axonal transport in cultured hippocampal neurons. We found that the N-terminal fragment of mHtt formed aggregates in almost half of the transfected neurons. Overexpression of the N-terminal fragment of mHtt decreased the velocity of mitochondrial axonal transport and mitochondrial mobility in neurons regardless of whether aggregates were formed. However, the impairment of mitochondrial axonal transport in neurons expressing the soluble and aggregated N-terminal fragments of mHtt did not differ. Our findings indicate that both the soluble and aggregated N-terminal fragments of mHtt impair mitochondrial axonal transport in cultured hippocampal neurons. We predict that dysfunction of mitochondrial axonal transport is an early-stage event in the progression of HD, even before mHtt aggregates are formed.

A role of mitochondrial complex II defects in genetic models of Huntington's disease expressing N-terminal fragments of mutant huntingtin

Huntington's disease (HD) is a neurodegenerative disorder caused by an abnormal expansion of a CAG repeat encoding a polyglutamine tract in the huntingtin (Htt) protein. The mutation leads to neuronal death through mechanisms which are still unknown. One hypothesis is that mitochondrial defects may play a key role. In support of this, the activity of mitochondrial complex II (C-II) is preferentially reduced in the striatum of HD patients. Here, we studied C-II expression in different genetic models of HD expressing N-terminal fragments of mutant Htt (mHtt). Western blot analysis showed that the expression of the 30 kDa Iron–Sulfur (Ip) subunit of C-II was significantly reduced in the striatum of the R6/1 transgenic mice, while the levels of the FAD containing catalytic 70 kDa subunit (Fp) were not significantly changed. Blue native gel analysis showed that the assembly of C-II in mitochondria was altered early in N171-82Q transgenic mice. Early loco-regional reduction in C-II activity and Ip protein expression was also demonstrated in a rat model of HD using intrastriatal injection of lentiviral vectors encoding mHtt. Infection of the rat striatum with a lentiviral vector coding the C-II Ip or Fp subunits induced a significant overexpression of these proteins that led to significant neuroprotection of striatal neurons against mHtt neurotoxicity. These results obtained in vivo support the hypothesis that structural and functional alterations of C-II induced by mHtt may play a critical role in the degeneration of striatal neurons in HD and that mitochondrial-targeted therapies may be useful in its treatment.

Degradation of mutant huntingtin via the ubiquitin/proteasome system is modulated by FE65

An unstable expansion of the polyglutamine repeat within exon 1 of the protein Htt (huntingtin) causes HD (Huntington's disease). Mounting evidence shows that accumulation of N-terminal mutant Htt fragments is the source of disruption of normal cellular processes which ultimately leads to neuronal cell death. Understanding the degradation mechanism of mutant Htt and improving its clearance has emerged as a new direction in developing therapeutic approaches to treat HD. In the present study we show that the brain-enriched adaptor protein FE65 is a novel interacting partner of Htt. The binding is mediated through WW-polyproline interaction and is dependent on the length of the polyglutamine tract. Interestingly, a reduction in mutant Htt protein level was observed in FE65-knockdown cells, and the process requires the UPS (ubiquitin/proteasome system). Moreover, the ubiquitination level of mutant Htt was found to be enhanced when FE65 is knocked down. Immunofluroescence staining revealed that FE65 associates with mutant Htt aggregates. Additionally, we demonstrated that overexpression of FE65 increases mutant Htt-induced cell death both in vitro and in vivo. These results suggest that FE65 facilitates the accumulation of mutant Htt in cells by preventing its degradation via the UPS, and thereby enhances the toxicity of mutant Htt.

Huntingtin with an expanded polyglutamine repeat affects the Jab1-p27(Kip1) pathway

Expansion of polyglutamine repeats is the cause of at least nine inherited human neurodegenerative disorders, including Huntington's disease (HD). It is widely accepted that deregulation of the transcriptional coactivator CBP by expanded huntingtin (htt) plays an important role in HD molecular pathogenesis. In this study, we report on a novel target of expanded polyglutamine stretches, the transcriptional coactivator Jun activation domain-binding protein 1 (Jab1), which shares DNA-sequence-specific transcription factor targets with CBP. Jab1 also plays a major role in the degradation of the cyclin-dependent-kinase inhibitor and putative transcription cofactor p27(Kip1). We found that Jab1 accumulates in aggregates when co-expressed with either expanded polyglutamine stretches or N-terminal fragments of mutant htt. In addition, the coactivator function of Jab1 was suppressed both by aggregated expanded polyglutamine solely and by mutant htt. Inhibition by mutant htt even preceded the appearance of microscopic aggregation. In an exon 1 HD cell model, we found that endogenous Jab1 could be recruited into aggregates and that this was accompanied by the accumulation of p27(Kip1). Accumulation of p27(Kip1) was also found in brains derived from HD patients. The repression of Jab1 by various mechanisms coupled with an increase of p27(Kip1) at late stages may have important transcriptional effects. In addition, the interference with the Jab1-p27(Kip1) pathway may contribute to the observed lower incidence of cancer in HD patients and may also be relevant for the understanding of the molecular pathogenesis of polyglutamine disorders in general.

Inhibiting the ubiquitin–proteasome system leads to preferential accumulation of toxic N-terminal mutant huntingtin fragments

An expanded polyglutamine (polyQ) domain in the N-terminal region of huntingtin (htt) causes misfolding and accumulation of htt in neuronal cells and the subsequent neurodegeneration of Huntington's disease (HD). Clearing the misfolded htt is critical for preventing neuropathology, and this process is mediated primarily by both the ubiquitin-proteasome system (UPS) and autophagy. Although overexpression of mutant htt can inhibit UPS activity in cultured cells, mutant htt does not inhibit global UPS activity in the brains of HD transgenic mice. These findings underscore the importance of investigating the function of the UPS and autophagy in the brain when mutant proteins are not overexpressed. When cultured PC12 cells were treated with either UPS or autophagy inhibitors, more N-terminal mutant htt fragments accumulated via inhibition of the UPS. Furthermore, in HD CAG repeat knock-in mouse brain, inhibiting the UPS also resulted in a greater accumulation of N-terminal, but not full-length, mutant htt than inhibiting autophagy did. Our findings suggest that impairment of the UPS may be more important for the accumulation of N-terminal mutant htt and might therefore make an attractive therapeutic target.

Accumulation of N-terminal mutant huntingtin in mouse and monkey models implicated as a pathogenic mechanism in Huntington's disease

A number of mouse models expressing mutant huntingtin (htt) with an expanded polyglutamine (polyQ) domain are useful for studying the pathogenesis of Huntington's disease (HD) and identifying appropriate therapies. However, these models exhibit neurological phenotypes that differ in their severity and nature. Understanding how transgenic htt leads to variable neuropathology in animal models would shed light on the pathogenesis of HD and help us to choose HD models for investigation. By comparing the expression of mutant htt at the transcriptional and protein levels in transgenic mice expressing N-terminal or full-length mutant htt, we found that the accumulation and aggregation of mutant htt in the brain is determined by htt context. HD mouse models demonstrating more severe phenotypes show earlier accumulation of N-terminal mutant htt fragments, which leads to the formation of htt aggregates that are primarily present in neuronal nuclei and processes, as well as glial cells. Similarly, transgenic monkeys expressing exon-1 htt with a 147-glutamine repeat (147Q) died early and showed abundant neuropil aggregates in swelling neuronal processes. Fractionation of HD150Q knock-in mice brains revealed an age-dependent accumulation of N-terminal mutant htt fragments in the nucleus and synaptosomes, and this accumulation was most pronounced in the striatum due to decreased proteasomal activity. Our findings suggest that the neuropathological phenotypes of HD stem largely from the accumulation of N-terminal mutant htt fragments and that this accumulation is determined by htt context and cell-type-dependent clearance of mutant htt.

N-Terminal Mutant Huntingtin Associates with Mitochondria and Impairs Mitochondrial Trafficking

Huntington's disease (HD) is caused by polyglutamine (polyQ) expansion in huntingtin (htt), a large (350 kDa) protein that localizes predominantly to the cytoplasm. Proteolytic cleavage of mutant htt yields polyQ-containing N-terminal fragments that are prone to misfolding and aggregation. Disease progression in HD transgenic models correlates with age-related accumulation of soluble and aggregated forms of N-terminal mutant htt fragments, suggesting that multiple forms of mutant htt are involved in the selective neurodegeneration in HD. Although mitochondrial dysfunction is implicated in the pathogenesis of HD, it remains unclear which forms of cytoplasmic mutant htt associate with mitochondria to affect their function. Here we demonstrate that specific N-terminal mutant htt fragments associate with mitochondria in Hdh(CAG)150 knock-in mouse brain and that this association increases with age. The interaction between soluble N-terminal mutant htt and mitochondria interferes with the in vitro association of microtubule-based transport proteins with mitochondria. Mutant htt reduces the distribution and transport rate of mitochondria in the processes of cultured neuronal cells. Reduced ATP level was also found in the synaptosomal fraction isolated from Hdh(CAG)150 knock-in mouse brain. These findings suggest that specific N-terminal mutant htt fragments, before the formation of aggregates, can impair mitochondrial function directly and that this interaction may be a novel target for therapeutic strategies in HD.

Inducing huntingtin inclusion formation in primary neuronal cell culture and in vivo by high‐capacity adenoviral vectors expressing truncated and full‐length huntingtin with polyglutamine expansion

Huntington's disease (HD) is an inherited autosomal dominant neurodegenerative disease caused by the expansion of a CAG trinucleotide repeat in exon 1 of the huntingtin (htt) gene. Vector-mediated delivery of N-terminal fragments of mutant htt has been used to study htt function in vitro and to establish HD models in rats. Due to the large size of the htt cDNA vector-mediated delivery of full-length htt has not been achieved so far. High-capacity adenoviral (HC-Ad) vectors were generated expressing mutant and wild-type versions of N-terminal truncated and full-length htt either in vitro in primary neuronal cells or in the striatum of mice. In vitro these vectors were used for transduction of primary neuronal cells isolated from E17 mouse embryos. Expression of mutant htt resulted in the formation of htt inclusions, a surrogate marker of the HD pathology. Kinetics of generation and localization of htt inclusions differed between truncated and full-length htt carrying identical mutations. Following injection into the striatum vector-mediated expression of mutant truncated htt led to prominent accumulation of htt inclusions in cell nuclei, while inclusions formed upon expression of mutant full-length htt localized to the cytoplasm. These results indicate that HC-Ad vector-mediated in vitro and in vivo delivery of truncated and full-length mutant htt results in prominent inclusion formation in neuronal cells but in different cell compartments. These vectors will be useful tools for studying HD and may be used to generate large animal HD models.

Context-dependent Dysregulation of Transcription by Mutant Huntingtin

Huntington disease (HD) is an adult-onset neurodegenerative disease caused by expansion of a polyglutamine (poly(Q) tract in the N-terminal region of huntingtin (htt). Although the precise mechanisms leading to neurodegeneration in HD have not been fully elucidated, transcriptional dysregulation has been implicated in disease pathogenesis. In HD, multiple N-terminal mutant htt fragments smaller than the first 500 amino acids have been found to accumulate in the nucleus and adversely affect gene transcription. It is unknown whether different htt fragments in the nucleus can differentially bind transcription factors and affect transcription. Here, we report that shorter N-terminal htt fragments, which are more prone to misfolding and aggregation, are more competent to bind Sp1 and inhibit its activity. These effects can be reversed by Hsp40, a molecular chaperone that reduces the misfolding of mutant htt. Our results provide insight into the beneficial effects of molecular chaperones and suggest that context dependent transcriptional dysregulation may contribute to differential toxicity of various N-terminal htt fragments.

Progressive phenotype and nuclear accumulation of an amino-terminal cleavage fragment in a transgenic mouse model with inducible expression of full-length mutant huntingtin

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized behaviorally by chorea, incoordination, and shortened lifespan and neuropathologically by huntingtin inclusions and neuronal degeneration. In order to facilitate studies of pathogenesis and therapeutics, we have generated a new inducible mouse model of HD expressing full-length huntingtin (Htt) using a tetracycline-regulated promoter. In double transgenic mice Htt was expressed widely in the brain under the control of the tet-transactivator (tTA) driven by the prion promoter PrP (in the absence of doxycycline). Mice expressing full-length mutant Htt, but not full-length normal Htt, displayed a progressive behavioral phenotype, consisting of slowed and irregular voluntary movements, gait ataxia, tremor and jerky movements, incoordination, and weight loss, with a shortened lifespan. Neuropathology included prominent intranuclear inclusions in cortex and striatum as well as cytoplasmic aggregates. This phenotype is very similar to the phenotypes of previous transgenic mice expressing N-terminal fragments of mutant Htt. The current HD-transgenic mice had nuclear accumulation of Htt, particularly an approximately 60-kDa fragment, which appears to represent an N-terminal cleavage product. This fragment is smaller than calpain or caspase-derived cleavage products of Htt, but it is comparable to a product, termed cp-A, which accumulates in nuclei of cells in a previously described cell model. This new mouse model may be useful in the future for pathogenic and preclinical therapeutic studies related to HD. The data suggest that proteolytic processing could be a part of the pathogenesis of HD, potentially representing an attractive therapeutic target.

A structure-based analysis of huntingtin mutant polyglutamine aggregation and toxicity: evidence for a compact beta-sheet structure

Huntington's disease (HD) arises from an expanded polyglutamine (polyQ) in the N-terminus of the huntingtin (htt) protein. Neuronal degeneration and inclusions containing N-terminal fragments of mutant htt are present in the cortex and striatum of HD brain. Recently, a model of polyQ aggregate structure has been proposed on the basis of studies with synthetic polyQ peptides and includes an alternating beta-strand/beta-turn structure with seven glutamine residues per beta-strand. We tested this model in the context of the htt exon-1 N-terminal fragment in both mammalian cell culture and cultured primary cortical neurons. We found our data support this model in the htt protein and provide a better understanding of the structural basis of polyQ aggregation in toxicity in HD.

Polyglutamine expansion of huntingtin impairs its nuclear export

Proteins with polyglutamine (polyQ) expansions accumulate in the nucleus and affect gene expression. The mechanism by which mutant huntingtin (htt) accumulates intranuclearly is not known; wild-type htt, a 350-kDa protein of unknown function, is normally found in the cytoplasm. N-terminal fragments of mutant htt, which contain a polyQ expansion (>37 glutamines), have no conserved nuclear localization sequences or nuclear export sequences but can accumulate in the nucleus and cause neurological problems in transgenic mice. Here we report that N-terminal htt shuttles between the cytoplasm and nucleus in a Ran GTPase-independent manner. Small N-terminal htt fragments interact with the nuclear pore protein translocated promoter region (Tpr), which is involved in nuclear export. PolyQ expansion and aggregation decrease this interaction and increase the nuclear accumulation of htt. Reducing the expression of Tpr by RNA interference or deletion of ten amino acids of N-terminal htt, which are essential for the interaction of htt with Tpr, increased the nuclear accumulation of htt. These results suggest that Tpr has a role in the nuclear export of N-terminal htt and that polyQ expansion reduces this nuclear export to cause the nuclear accumulation of htt.

Huntington and its Role in Neuronal Degeneration

Huntington’s disease results from a polyglutamine expansion in the N-terminal region of huntingtin (htt). This abnormality causes protein aggregation and leads to neurotoxicity. Despite its widespread expression in the brain and body, mutant htt causes selective neurodegeneration in Huntington’s disease patient brains. However, Huntington’s disease mouse models expressing mutant htt do not have obvious neurodegeneration despite significant neurological symptoms. Most Huntington’s disease mouse models display the accumulation of toxic N-terminal mutant htt fragments in both the nucleus and neuronal processes, suggesting that these subcellular sites are hotspots for the early neuropathology of Huntington’s disease. Intranuclear htt affects gene expression and may cause neuronal dysfunction. Mutant htt in neuronal processes affects axonal transport and induces degeneration, and these effects may be more relevant to the selective neurodegeneration in Huntington’s disease. Growing evidence has also suggested that mutant htt mediates multiple pathological pathways. This review discusses the early pathological changes identified in Huntington’s disease cellular and animal models. These changes may be the causes of neurode-generation.

Nuclear-targeting of mutant huntingtin fragments produces Huntington's disease-like phenotypes in transgenic mice.

Huntington's disease (HD) results from the expansion of a glutamine repeat near the N-terminus of huntingtin (htt). At post-mortem, neurons in the central nervous system of patients have been found to accumulate N-terminal fragments of mutant htt in nuclear and cytoplasmic inclusions. This pathology has been reproduced in transgenic mice expressing the first 171 amino acids of htt with 82 glutamines along with losses of motoric function, hypoactivity and abbreviated life-span. The relative contributions of nuclear versus cytoplasmic mutant htt to the pathogenesis of disease have not been clarified. To examine whether pathogenic processes in the nucleus disproportionately contribute to disease features in vivo, we fused a nuclear localization signal (NLS) derived from atrophin-1 to the N-terminus of an N171-82Q construct. Two lines of mice (lines 8A and 61) that were identified expressed NLS-N171-82Q at comparable levels and developed phenotypes identical to our previously described HD-N171-82Q mice. Western blot and immunohistochemical analyses revealed that NLS-N171-82Q fragments accumulate in nuclear, but not cytoplasmic, compartments. These data suggest that disruption of nuclear processes may account for many of the disease phenotypes displayed in the mouse models generated by expressing mutant N-terminal fragments of htt.