Development of a method for the high-throughput quantification of cellular proteins

Transglutaminase 6 Is Colocalized and Interacts with Mutant Huntingtin in Huntington Disease Rodent Animal Models

Mammalian transglutaminases (TGs) catalyze calcium-dependent irreversible posttranslational modifications of proteins and their enzymatic activities contribute to the pathogenesis of several human neurodegenerative diseases. Although different transglutaminases are found in many different tissues, the TG6 isoform is mostly expressed in the CNS. The present study was embarked on/undertaken to investigate expression, distribution and activity of transglutaminases in Huntington disease transgenic rodent models, with a focus on analyzing the involvement of TG6 in the age- and genotype-specific pathological features relating to disease progression in HD transgenic mice and a tgHD transgenic rat model using biochemical, histological and functional assays. Our results demonstrate the physical interaction between TG6 and (mutant) huntingtin by co-immunoprecipitation analysis and the contribution of its enzymatic activity for the total aggregate load in SH-SY5Y cells. In addition, we identify that TG6 expression and activity are especially abundant in the olfactory tubercle and piriform cortex, the regions displaying the highest amount of mHTT aggregates in transgenic rodent models of HD. Furthermore, mHTT aggregates were colocalized within TG6-positive cells. These findings point towards a role of TG6 in disease pathogenesis via mHTT aggregate formation.

Targeting Gpr52 lowers mutant HTT levels and rescues Huntington's disease-associated phenotypes

See Huang and Gitler (doi:10.1093/brain/awy112) for a scientific commentary on this article.

Lowering the levels of disease-causing proteins is an attractive treatment strategy for neurodegenerative disorders, among which Huntington’s disease is an appealing disease for testing this strategy because of its monogenetic nature. Huntington’s disease is mainly caused by cytotoxicity of the mutant HTT protein with an expanded polyglutamine repeat tract. Lowering the soluble mutant HTT may reduce its downstream toxicity and provide potential treatment for Huntington’s disease. This is hard to achieve by small-molecule compound drugs because of a lack of effective targets. Here we demonstrate Gpr52, an orphan G protein-coupled receptor, as a potential Huntington’s disease drug target. Knocking-out Gpr52 significantly reduces mutant HTT levels in the striatum and rescues Huntington’s disease-associated behavioural phenotypes in a knock-in Huntington’s disease mouse model expressing endogenous mutant Htt. Importantly, a novel Gpr52 antagonist E7 reduces mutant HTT levels and rescues Huntington’s disease-associated phenotypes in cellular and mouse models. Our study provides an entry point for Huntington’s disease drug discovery by targeting Gpr52.

Biofluid Biomarkers in Huntington's Disease

Huntington's disease (HD) is a chronic progressive neurodegenerative condition where new markers of disease progression are needed. So far no disease-modifying interventions have been found, and few interventions have been proven to alleviate symptoms. This may be partially explained by the lack of reliable indicators of disease severity, progression, and phenotype.Biofluid biomarkers may bring advantages in addition to clinical measures, such as reliability, reproducibility, price, accuracy, and direct quantification of pathobiological processes at the molecular level; and in addition to empowering clinical trials, they have the potential to generate useful hypotheses for new drug development.In this chapter we review biofluid biomarker reports in HD, emphasizing those we feel are likely to be closest to clinical applicability.

Suppression of MAPK11 or HIPK3 reduces mutant Huntingtin levels in Huntington's disease models

Most neurodegenerative disorders are associated with accumulation of disease-relevant proteins. Among them, Huntington disease (HD) is of particular interest because of its monogenetic nature. HD is mainly caused by cytotoxicity of the defective protein encoded by the mutant Huntingtin gene (HTT). Thus, lowering mutant HTT protein (mHTT) levels would be a promising treatment strategy for HD. Here we report two kinases HIPK3 and MAPK11 as positive modulators of mHTT levels both in cells and in vivo. Both kinases regulate mHTT via their kinase activities, suggesting that inhibiting these kinases may have therapeutic values. Interestingly, their effects on HTT levels are mHTT-dependent, providing a feedback mechanism in which mHTT enhances its own level thus contributing to mHTT accumulation and disease progression. Importantly, knockout of MAPK11 significantly rescues disease-relevant behavioral phenotypes in a knockin HD mouse model. Collectively, our data reveal new therapeutic entry points for HD and target-discovery approaches for similar diseases.

Phosphorylation of huntingtin at residue T3 is decreased in Huntington's disease and modulates mutant huntingtin protein conformation

Posttranslational modifications can have profound effects on the biological and biophysical properties of proteins associated with misfolding and aggregation. However, their detection and quantification in clinical samples and an understanding of the mechanisms underlying the pathological properties of misfolding- and aggregation-prone proteins remain a challenge for diagnostics and therapeutics development. We have applied an ultrasensitive immunoassay platform to develop and validate a quantitative assay for detecting a posttranslational modification (phosphorylation at residue T3) of a protein associated with polyglutamine repeat expansion, namely Huntingtin, and characterized its presence in a variety of preclinical and clinical samples. We find that T3 phosphorylation is greatly reduced in samples from Huntington's disease models and in Huntington's disease patients, and we provide evidence that bona-fide T3 phosphorylation alters Huntingtin exon 1 protein conformation and aggregation properties. These findings have significant implications for both mechanisms of disease pathogenesis and the development of therapeutics and diagnostics for Huntington's disease.

Polyglutamine expansion affects huntingtin conformation in multiple Huntington's disease models

Conformational changes in disease-associated or mutant proteins represent a key pathological aspect of Huntington’s disease (HD) and other protein misfolding diseases. Using immunoassays and biophysical approaches, we and others have recently reported that polyglutamine expansion in purified or recombinantly expressed huntingtin (HTT) proteins affects their conformational properties in a manner dependent on both polyglutamine repeat length and temperature but independent of HTT protein fragment length. These findings are consistent with the HD mutation affecting structural aspects of the amino-terminal region of the protein, and support the concept that modulating mutant HTT conformation might provide novel therapeutic and diagnostic opportunities. We now report that the same conformational TR-FRET based immunoassay detects polyglutamine- and temperature-dependent changes on the endogenously expressed HTT protein in peripheral tissues and post-mortem HD brain tissue, as well as in tissues from HD animal models. We also find that these temperature- and polyglutamine-dependent conformational changes are sensitive to bona-fide phosphorylation on S13 and S16 within the N17 domain of HTT. These findings provide key clinical and preclinical relevance to the conformational immunoassay, and provide supportive evidence for its application in the development of therapeutics aimed at correcting the conformation of polyglutamine-expanded proteins as well as the pharmacodynamics readouts to monitor their efficacy in preclinical models and in HD patients.

Discovery of Therapeutic Approaches for Polyglutamine Diseases: A Summary of Recent Efforts

Polyglutamine (PolyQ) diseases are a group of neurodegenerative disorders caused by the expansion of cytosine-adenine-guanine (CAG) trinucleotide repeats in the coding region of specific genes. This leads to the production of pathogenic proteins containing critically expanded tracts of glutamines. Although polyQ diseases are individually rare, the fact that these nine diseases are irreversibly progressive over 10 to 30 years, severely impairing and ultimately fatal, usually implicating the full-time patient support by a caregiver for long time periods, makes their economic and social impact quite significant. This has led several researchers worldwide to investigate the pathogenic mechanism(s) and therapeutic strategies for polyQ diseases. Although research in the field has grown notably in the last decades, we are still far from having an effective treatment to offer patients, and the decision of which compounds should be translated to the clinics may be very challenging. In this review, we provide a comprehensive and critical overview of the most recent drug discovery efforts in the field of polyQ diseases, including the most relevant findings emerging from two different types of approaches-hypothesis-based candidate molecule testing and hypothesis-free unbiased drug screenings. We hereby summarize and reflect on the preclinical studies as well as all the clinical trials performed to date, aiming to provide a useful framework for increasingly successful future drug discovery and development efforts.

Conformational modulation mediated by polyglutamine expansion in CAG repeat expansion disease-associated proteins

We have previously reported TR-FRET based immunoassays to detect a conformational change imparted on huntingtin protein by the polyglutamine expansion, which we confirmed using biophysical methodologies. Using these immunoassays, we now report that polyglutamine expansion influences the conformational properties of other polyglutamine disease proteins, exemplified by the androgen receptor (associated with spinal bulbar muscular atrophy) and TATA binding protein (associated with spinocerebellar ataxia 17). Using artificial constructs bearing short or long polyglutamine expansions or a multimerized, unrelated epitope (mimicking the increase in anti-polyglutamine antibody epitopes present in polyglutamine repeats of increasing length) we confirmed that the conformational TR-FRET based immunoassay detects an intrinsic conformational property of polyglutamine repeats. The TR-FRET based conformational immunoassay may represent a rapid, scalable tool to identify modulators of polyglutamine-mediated conformational change in different proteins associated with CAG triplet repeat disorders.

iPSC-based drug screening for Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder, caused by an expansion of the CAG repeat in exon 1 of the huntingtin gene. The disease generally manifests in middle age with both physical and mental symptoms. There are no effective treatments or cures and death usually occurs 10-20 years after initial symptoms. Since the original identification of the Huntington disease associated gene, in 1993, a variety of models have been created and used to advance our understanding of HD. The most recent advances have utilized stem cell models derived from HD-patient induced pluripotent stem cells (iPSCs) offering a variety of screening and model options that were not previously available. The discovery and advancement of technology to make human iPSCs has allowed for a more thorough characterization of human HD on a cellular and developmental level. The interaction between the genome editing and the stem cell fields promises to further expand the variety of HD cellular models available for researchers. In this review, we will discuss the history of Huntington's disease models, common screening assays, currently available models and future directions for modeling HD using iPSCs-derived from HD patients. This article is part of a Special Issue entitled SI: PSC and the brain.

Polyglutamine- and temperature-dependent conformational rigidity in mutant huntingtin revealed by immunoassays and circular dichroism spectroscopy

BACKGROUND

In Huntington's disease, expansion of a CAG triplet repeat occurs in exon 1 of the huntingtin gene (HTT), resulting in a protein bearing>35 polyglutamine residues whose N-terminal fragments display a high propensity to misfold and aggregate. Recent data demonstrate that polyglutamine expansion results in conformational changes in the huntingtin protein (HTT), which likely influence its biological and biophysical properties. Developing assays to characterize and measure these conformational changes in isolated proteins and biological samples would advance the testing of novel therapeutic approaches aimed at correcting mutant HTT misfolding. Time-resolved Förster energy transfer (TR-FRET)-based assays represent high-throughput, homogeneous, sensitive immunoassays widely employed for the quantification of proteins of interest. TR-FRET is extremely sensitive to small distances and can therefore provide conformational information based on detection of exposure and relative position of epitopes present on the target protein as recognized by selective antibodies. We have previously reported TR-FRET assays to quantify HTT proteins based on the use of antibodies specific for different amino-terminal HTT epitopes. Here, we investigate the possibility of interrogating HTT protein conformation using these assays.

METHODOLOGY/PRINCIPAL FINDINGS

By performing TR-FRET measurements on the same samples (purified recombinant proteins or lysates from cells expressing HTT fragments or full length protein) at different temperatures, we have discovered a temperature-dependent, reversible, polyglutamine-dependent conformational change of wild type and expanded mutant HTT proteins. Circular dichroism spectroscopy confirms the temperature and polyglutamine-dependent change in HTT structure, revealing an effect of polyglutamine length and of temperature on the alpha-helical content of the protein.

CONCLUSIONS/SIGNIFICANCE

The temperature- and polyglutamine-dependent effects observed with TR-FRET on HTT proteins represent a simple, scalable, quantitative and sensitive assay to identify genetic and pharmacological modulators of mutant HTT conformation, and potentially to assess the relevance of conformational changes during onset and progression of Huntington's disease.

TR-FRET assays of Huntingtin protein fragments reveal temperature and polyQ length-dependent conformational changes

Time-Resolved Fluorescence Resonance Energy Transfer (TR-FRET) technology is a widely used immunoassay that enables high-throughput quantitative measurements of proteins of interest. One of the well established examples is the TR-FRET assay for mutant huntingtin protein (HTT), which is the major cause of the neurodegenerative Huntington's disease (HD). To measure the mutant HTT protein, the published assays utilize a polyQ antibody, MW1, paired with HTT N-terminal antibodies. MW1 has much higher apparent affinity to mutant HTT with expanded polyQ stretch than to wild-type HTT with shorter polyQ, and thus the assays detect mutant HTT preferentially. Here we report a reversible temperature dependent change of TR-FRET signals for HTT N-terminal fragments: the signals become higher when the temperature is lowered from room temperature to 4°C. Interestingly, the temperature sensitivity of the TR-FRET signals is much higher for the Q25 (wild-type) than for the Q72 (mutant) protein. We further revealed that it is likely due to a temperature and polyQ length-dependent structural or spatial change of HTT, which is potentially useful for understanding polyQ structure and toxicity.

Simple assay for proteases based on aggregation of stimulus-responsive polypeptides

Unregulated changes in protease activity are linked to many diseases including cancer. Fast, accurate, and low-cost assays for detection of these changes are being explored for early diagnosis and monitoring of these diseases and can also be used as platforms for the discovery of new drugs. We report a new methodology for the simple detection and quantification of protease activity in buffer and human serum. The assay is based on recombinant diblock polypeptides that undergo temperature- or salt-triggered micellization in water. The coronae of the micelles are linked to the water-insoluble cores by a peptide substrate that is cleaved in the presence of the target protease. Protease cleavage of the diblock polypeptide triggers the aggregation of the core-forming segment, leading to a change in solution optical density, which can be used to detect the presence of, and to quantify the concentration of, protease. We used matrix metalloproteinase-1 (MMP-1) as a model protease and found peptide aggregation time to be proportional to enzyme concentration over a range from endogenous MMP-1 level in human serum (∼3 ng/mL) to 100 ng/mL (0.15-5 nM) in 40% human serum and 1-100 ng/mL in buffer. The assay does not require any intermediate steps or sophisticated data analysis, and the modular design of the assay system is amenable to straightforward adaptation for the detection of a wide range of proteases.

Quantification assays for total and polyglutamine-expanded huntingtin proteins

The expansion of a CAG trinucleotide repeat in the huntingtin gene, which produces huntingtin protein with an expanded polyglutamine tract, is the cause of Huntington's disease (HD). Recent studies have reported that RNAi suppression of polyglutamine-expanded huntingtin (mutant HTT) in HD animal models can ameliorate disease phenotypes. A key requirement for such preclinical studies, as well as eventual clinical trials, aimed to reduce mutant HTT exposure is a robust method to measure HTT protein levels in select tissues. We have developed several sensitive and selective assays that measure either total human HTT or polyglutamine-expanded human HTT proteins on the electrochemiluminescence Meso Scale Discovery detection platform with an increased dynamic range over other methods. In addition, we have developed an assay to detect endogenous mouse and rat HTT proteins in pre-clinical models of HD to monitor effects on the wild type protein of both allele selective and non-selective interventions. We demonstrate the application of these assays to measure HTT protein in several HD in vitro cellular and in vivo animal model systems as well as in HD patient biosamples. Furthermore, we used purified recombinant HTT proteins as standards to quantitate the absolute amount of HTT protein in such biosamples.

Experimental models for identifying modifiers of polyglutamine-induced aggregation and neurodegeneration

Huntington's disease (HD) typifies a class of inherited neurodegenerative disorders in which a CAG expansion in a single gene leads to an extended polyglutamine tract and misfolding of the expressed protein, driving cumulative neural dysfunction and degeneration. HD is invariably fatal with symptoms that include progressive neuropsychiatric and cognitive impairments, and eventual motor disability. No curative therapies yet exist for HD and related polyglutamine diseases; therefore, substantial efforts have been made in the drug discovery field to identify potential drug and drug target candidates for disease-modifying treatment. In this context, we review here a range of early-stage screening approaches based in in vitro, cellular, and invertebrate models to identify pharmacological and genetic modifiers of polyglutamine aggregation and induced neurodegeneration. In addition, emerging technologies, including high-content analysis, three-dimensional culture models, and induced pluripotent stem cells are increasingly being incorporated into drug discovery screening pipelines for protein misfolding disorders. Together, these diverse screening strategies are generating novel and exciting new probes for understanding the disease process and for furthering development of therapeutic candidates for eventual testing in the clinical setting.

Fragments of HdhQ150 mutant huntingtin form a soluble oligomer pool that declines with aggregate deposition upon aging

Cleavage of the full-length mutant huntingtin (mhtt) protein into smaller, soluble aggregation-prone mhtt fragments appears to be a key process in the neuropathophysiology of Huntington’s Disease (HD). Recent quantification studies using TR-FRET-based immunoassays showed decreasing levels of soluble mhtt correlating with an increased load of aggregated mhtt in the aging HdhQ150 mouse brain. To better characterize the nature of these changes at the level of native mhtt species, we developed a detection method that combines size exclusion chromatography (SEC) and time-resolved fluorescence resonance energy transfer (TR-FRET) that allowed us to resolve and define the formation, aggregation and temporal dynamics of native soluble mhtt species and insoluble aggregates in the brain of the HdhQ150 knock-in mouse. We found that mhtt fragments and not full-length mhtt form oligomers in the brains of one month-old mice long before disease phenotypes and mhtt aggregate histopathology occur. As the HdhQ150 mice age, brain levels of soluble full-length mhtt protein remain similar. In contrast, the soluble oligomeric pool of mhtt fragments slightly increases during the first two months before it declines between 3 and 8 months of age. This decline inversely correlates with the formation of insoluble mhtt aggregates. We also found that the pool-size of soluble mhtt oligomers is similar in age-matched heterozygous and homozygous HdhQ150 mouse brains whereas insoluble aggregate formation is greatly accelerated in the homozygous mutant brain. The capacity of the soluble mhtt oligomer pool therefore seems exhausted already in the heterozygous state and likely kept constant by changes in flux and, as a consequence, increased rate of insoluble aggregate formation. We demonstrate that our novel findings in mice translate to human HD brain but not HD patient fibroblasts.

A screen for enhancers of clearance identifies huntingtin as a heat shock protein 90 (Hsp90) client protein

Mechanisms to reduce the cellular levels of mutant huntingtin (mHtt) provide promising strategies for treating Huntington disease (HD). To identify compounds enhancing the degradation of mHtt, we performed a high throughput screen using a hippocampal HN10 cell line expressing a 573-amino acid mHtt fragment. Several hit structures were identified as heat shock protein 90 (Hsp90) inhibitors. Cell treatment with these compounds reduced levels of mHtt without overt toxic effects as measured by time-resolved Förster resonance energy transfer assays and Western blots. To characterize the mechanism of mHtt degradation, we used the potent and selective Hsp90 inhibitor NVP-AUY922. In HdhQ150 embryonic stem (ES) cells and in ES cell-derived neurons, NVP-AUY922 treatment substantially reduced soluble full-length mHtt levels. In HN10 cells, Hsp90 inhibition by NVP-AUY922 enhanced mHtt clearance in the absence of any detectable Hsp70 induction. Furthermore, inhibition of protein synthesis with cycloheximide or overexpression of dominant negative heat shock factor 1 (Hsf1) in HdhQ150 ES cells attenuated Hsp70 induction but did not affect NVP-AUY922-mediated mHtt clearance. Together, these data provided evidence that direct inhibition of Hsp90 chaperone function was crucial for mHtt degradation rather than heat shock response induction and Hsp70 up-regulation. Co-immunoprecipitation experiments revealed a physical interaction of mutant and wild-type Htt with the Hsp90 chaperone. Hsp90 inhibition disrupted the interaction and induced clearance of Htt through the ubiquitin-proteasome system. Our data suggest that Htt is an Hsp90 client protein and that Hsp90 inhibition may provide a means to reduce mHtt in HD.

Multiplexed reporter gene assays: monitoring the cell viability and the compound kinetics on luciferase activity

High-throughput screening assays with multiple readouts enable one to monitor multiple assay parameters. By capturing as much information about the underlying biology as possible, the detection of true actives can be improved. This report describes an extension to standard luciferase reporter gene assays that enables multiple parameters to be monitored from each sample. The report describes multiplexing luciferase assays with an orthogonal readout monitoring cell viability using reduction of resazurin. In addition, this technical note shows that by using the luciferin substrate in live cells, an assay time course can be recorded. This enables the identification of nonactive or unspecific compounds that act by inhibiting luciferase, as well as compounds altering gene expression or cell growth.

Proteolysis of mutant huntingtin produces an exon 1 fragment that accumulates as an aggregated protein in neuronal nuclei in Huntington disease

Huntingtin proteolysis has been implicated in the molecular pathogenesis of Huntington disease (HD). Despite an intense effort, the identity of the pathogenic smallest N-terminal fragment has not been determined. Using a panel of anti-huntingtin antibodies, we employed an unbiased approach to generate proteolytic cleavage maps of mutant and wild-type huntingtin in the HdhQ150 knock-in mouse model of HD. We identified 14 prominent N-terminal fragments, which, in addition to the full-length protein, can be readily detected in cytoplasmic but not nuclear fractions. These fragments were detected at all ages and are not a consequence of the pathogenic process. We demonstrated that the smallest fragment is an exon 1 huntingtin protein, known to contain a potent nuclear export signal. Prior to the onset of behavioral phenotypes, the exon 1 protein, and possibly other small fragments, accumulate in neuronal nuclei in the form of a detergent insoluble complex, visualized as diffuse granular nuclear staining in tissue sections. This methodology can be used to validate the inhibition of specific proteases as therapeutic targets for HD by pharmacological or genetic approaches.

Single-step detection of mutant huntingtin in animal and human tissues: a bioassay for Huntington's disease

The genetic mutation causing Huntington's disease is a polyglutamine expansion in the huntingtin protein where more than 37 glutamines cause disease by formation of toxic intracellular fragments, aggregates, and cell death. Despite a clear pathogenic role for mutant huntingtin, understanding huntingtin expression during the presymptomatic phase of the disease or during disease progression has remained obscure. Central to clarifying the role in the pathomechanism of disease is the ability to easily and accurately measure mutant huntingtin in accessible human tissue samples as well as cell and animal models. Here we describe a highly sensitive time-resolved Förster resonance energy transfer (FRET) assay for quantification of soluble mutant huntingtin in brain, plasma, and cerebrospinal fluid. Surprisingly, in mice, soluble huntingtin levels decrease during disease progression, inversely correlating with brain aggregate load. Mutant huntingtin is easily detected in human brain and blood-derived fractions, providing a utility to assess mutant huntingtin expression during disease course as well as a pharmacodynamic marker for disease-modifying therapeutics targeting expression, cleavage, or degradation of mutant huntingtin. The design of the homogeneous one-step method for huntingtin detection is such that it can be easily applied to measure other proteins of interest.