The localization and interactions of huntingtin

Insights into the promising prospect of medicinal chemistry studies against neurodegenerative disorders

Medicinal chemistry is an interdisciplinary field that incorporates organic chemistry, biochemistry, physical chemistry, pharmacology, informatics, molecular biology, structural biology, cell biology, and other disciplines. Additionally, it considers molecular factors such as the mode of action of the drugs, their chemical structure-activity relationship (SAR), and pharmacokinetic aspects like absorption, distribution, metabolism, elimination, and toxicity. Neurodegenerative disorders (NDs), which are defined by the breakdown of neurons over time, are affecting an increasing number of people. Oxidative stress, particularly the increased production of Reactive Oxygen Species (ROS), plays a crucial role in the growth of various disorders, as indicated by the identification of protein, lipid, and Deoxyribonucleic acid (DNA) oxidation products in vivo. Because of their inherent nature, most biological molecules are vulnerable to ROS, even if they play a role in metabolic parameters and cell signaling. Due to their high polyunsaturated fatty acid content, low antioxidant barrier, and high oxygen uptake, neurons are particularly vulnerable to oxidation by nature. As a result, excessive ROS generation in neurons looks especially harmful, and the mechanisms associated with biomolecule oxidative destruction are several and complex. This review focuses on the formation and management of ROS, as well as their chemical characteristics (both thermodynamic and kinetic), interactions, and implications in NDs.

Conformational studies of pathogenic expanded polyglutamine protein deposits from Huntington's disease

Huntington’s disease, like other neurodegenerative diseases, continues to lack an effective cure. Current treatments that address early symptoms ultimately fail Huntington’s disease patients and their families, with the disease typically being fatal within 10–15 years from onset. Huntington’s disease is an inherited disorder with motor and mental impairment, and is associated with the genetic expansion of a CAG codon repeat encoding a polyglutamine-segment-containing protein called huntingtin. These Huntington’s disease mutations cause misfolding and aggregation of fragments of the mutant huntingtin protein, thereby likely contributing to disease toxicity through a combination of gain-of-toxic-function for the misfolded aggregates and a loss of function from sequestration of huntingtin and other proteins. As with other amyloid diseases, the mutant protein forms non-native fibrillar structures, which in Huntington’s disease are found within patients’ neurons. The intracellular deposits are associated with dysregulation of vital processes, and inter-neuronal transport of aggregates may contribute to disease progression. However, a molecular understanding of these aggregates and their detrimental effects has been frustrated by insufficient structural data on the misfolded protein state. In this review, we examine recent developments in the structural biology of polyglutamine-expanded huntingtin fragments, and especially the contributions enabled by advances in solid-state nuclear magnetic resonance spectroscopy. We summarize and discuss our current structural understanding of the huntingtin deposits and how this information furthers our understanding of the misfolding mechanism and disease toxicity mechanisms.

Genetics Modulate Gray Matter Variation Beyond Disease Burden in Prodromal Huntington's Disease

Huntington’s disease (HD) is a neurodegenerative disorder caused by an expansion mutation of the cytosine–adenine–guanine (CAG) trinucleotide in the HTT gene. Decline in cognitive and motor functioning during the prodromal phase has been reported, and understanding genetic influences on prodromal disease progression beyond CAG will benefit intervention therapies. From a prodromal HD cohort (N = 715), we extracted gray matter (GM) components through independent component analysis and tested them for associations with cognitive and motor functioning that cannot be accounted for by CAG-induced disease burden (cumulative effects of CAG expansion and age). Furthermore, we examined genetic associations (at the genomic, HD pathway, and candidate region levels) with the GM components that were related to functional decline. After accounting for disease burden, GM in a component containing cuneus, lingual, and middle occipital regions was positively associated with attention and working memory performance, and the effect size was about a tenth of that of disease burden. Prodromal participants with at least one dystonia sign also had significantly lower GM volume in a bilateral inferior parietal component than participants without dystonia, after controlling for the disease burden. Two single-nucleotide polymorphisms (SNPs: rs71358386 in NCOR1 and rs71358386 in ADORA2B) in the HD pathway were significantly associated with GM volume in the cuneus component, with minor alleles being linked to reduced GM volume. Additionally, homozygous minor allele carriers of SNPs in a candidate region of ch15q13.3 had significantly higher GM volume in the inferior parietal component, and one minor allele copy was associated with a total motor score decrease of 0.14 U. Our findings depict an early genetical GM reduction in prodromal HD that occurs irrespective of disease burden and affects regions important for cognitive and motor functioning.

An ovine transgenic Huntington's disease model

Huntington's disease (HD) is an inherited autosomal dominant neurodegenerative disorder caused by an expansion of a CAG trinucleotide repeat in the huntingtin (HTT) gene [Huntington's Disease Collaborative Research Group (1993) A novel gene containing a trinucleotide repeat that is expanded and unstable on Huntington's disease chromosomes. The Huntington's Disease Collaborative Research Group. Cell, 72, 971-983]. Despite identification of the gene in 1993, the underlying life-long disease process and effective treatments to prevent or delay it remain elusive. In an effort to fast-track treatment strategies for HD into clinical trials, we have developed a new large-animal HD transgenic ovine model. Sheep, Ovis aries L., were selected because the developmental pattern of the ovine basal ganglia and cortex (the regions primarily affected in HD) is similar to the analogous regions of the human brain. Microinjection of a full-length human HTT cDNA containing 73 polyglutamine repeats under the control of the human promotor resulted in six transgenic founders varying in copy number of the transgene. Analysis of offspring (at 1 and 7 months of age) from one of the founders showed robust expression of the full-length human HTT protein in both CNS and non-CNS tissue. Further, preliminary immunohistochemical analysis demonstrated the organization of the caudate nucleus and putamen and revealed decreased expression of medium size spiny neuron marker DARPP-32 at 7 months of age. It is anticipated that this novel transgenic animal will represent a practical model for drug/clinical trials and surgical interventions especially aimed at delaying or preventing HD initiation. New sequence accession number for ovine HTT mRNA: FJ457100.

Nucleocytoplasmic trafficking and transcription effects of huntingtin in Huntington's disease

There are nine genetic neurodegenerative diseases caused by a similar genetic defect, a CAG DNA triplet-repeat expansion in the disease gene's open reading frame resulting in a polyglutamine expansion in the disease proteins. Despite the commonality of polyglutamine expansion, each of the polyglutamine diseases manifest as unique diseases, with some similarities, but important differences. These differences suggest that the context of the polyglutamine expansion is important to the mechanism of pathology of the disease proteins. Therefore, it is becoming increasingly paramount to understand the normal functions of these polyglutamine disease proteins, which include huntingtin, the polyglutamine-expanded protein in Huntington's disease (HD). Transcriptional dysregulation is seen in HD. Here we discuss the role of normal huntingtin in transcriptional regulation and misregulation in Huntington's disease in relation to potentially analogous model systems, and to other polyglutamine disease proteins. Huntingtin has functional roles in both the cytoplasm and the nucleus. One commonality of activity of polyglutamine disease proteins is at the level of protein dynamics and ability to import and export to and from the nucleus. Knowing the temporal location of huntingtin protein in response to signaling and neuronal communication could lead to valuable insights into an important trigger of HD pathology.

Mutant huntingtin increases nuclear corepressor function and enhances ligand-dependent nuclear hormone receptor activation

There is increasing evidence that transcriptional dysregulation is important in Huntington's disease pathogenesis. The transcriptional protein, nuclear corepressor (NCoR), acts to repress transcription of nuclear hormone receptors, such as the thyroid hormone receptor (TR) and retinoic acid receptor, in the absence of their appropriate ligand. NCoR has been shown to bind to the mutated huntingtin protein in a yeast two-hybrid screen. This aberrant interaction may have profound effects on both the function of the NCoR protein and on its control of nuclear hormone receptor-mediated transcription. To test this hypothesis, reporter gene assays were performed in inducible PC12 cell lines expressing exon 1 of the human huntingtin protein (Htt) with either a 25 or 103 polyglutamine (Q) repeat. Expression of mutant 103Q protein appears to enhance the ability of NCoR to repress TR-mediated transcription in the absence of ligand. Western analyses indicated that the expression of the mutant 103Q Htt protein did not change the endogenous NCoR levels in the HD103Q PC12 cells when compared to uninduced cells. Interestingly, using GST pull-down assays we found that a mutant Htt exon 1 construct with 53 polyglutamine (HD53Q) did not bind to NCoR in a polyglutamine-dependent fashion. These findings suggest that an aberrant NCoR-Htt interaction does not exist in vitro. Expression of the mutant 103Q protein was also found to enhance ligand-dependent activation of TR and retinoic acid receptor. In vitro binding data shows that TR binds to HD53Q in the presence of ligand. Taken together these data suggest that Htt may function as a transcriptional coactivator of nuclear hormone receptors.

Characterization of NO binding to human cystathionine beta-synthase: possible implications of the effects of CO and NO binding to the human enzyme

Homocysteine is a key junction metabolite that can be converted to cystathionine in a reaction catalyzed by the heme and pyridoxal phosphate-dependent cystathionine beta-synthase. The heme has unusual spectroscopic properties and the axial ligands have been assigned as histidine and cysteine, respectively. Its role in the protein is not obvious from the chemistry of the beta-replacement reaction that is catalyzed. We have characterized the binding of the gaseous signaling molecule, NO, to cystathionine beta-synthase and examined its effect on the reactions catalyzed by the truncated dimeric form of the enzyme, W409X, which is a natural variant. Binding of NO appears to result in the formation of a five-coordinate ferrous nitrosyl species in which both endogenous ligands have been lost. This is in contrast to CO binding which is reported to displace the thiolate ligand and form a six-coordinate species. NO binds to the full-length enzyme with a K(d) of 281+/-50 microM and to the truncated enzyme with a K(d) of 350+/-44 microM. Binding of NO to the full-length enzyme inhibits activity with a K(i) of 320+/-60 microM. These studies demonstrate that as with CO, perturbation of the heme environment by NO is communicated to the active site with concomitant inhibition of enzyme activity, and suggests a regulatory role for heme in cystathionine beta-synthase.

Developmental mechanisms in the pathogenesis of neurodegenerative diseases

Cellular genes that are mutated in neurodegenerative diseases code for proteins that are expressed throughout neural development. Genetic analysis suggests that these genes are essential for a broad range of normal neurodevelopmental processes. The proteins they code for interact with numerous other cellular proteins that are components of signaling pathways involved in patterning of the neural tube and in regional specification of neuronal subtypes. Further, pathogenetic mutations of these genes can cause progressive, sublethal alterations in the cellular homeostasis of evolving regional neuronal subpopulations, culminating in late-onset cell death. Therefore, as a consequence of the disease mutations, targeted cell populations may retain molecular traces of abnormal interactions with disease-associated proteins by exhibiting changes in a spectrum of normal cellular functions and enhanced vulnerability to a host of environmental stressors. These observations suggest that the normal functions of these disease-associated proteins are to ensure the fidelity and integration of developmental events associated with the progressive elaboration of neuronal subtypes as well as the maintenance of mature neuronal populations during adult life. The ability to identify alterations within vulnerable neuronal precursors present in pre-symptomatic individuals prior to the onset of irrevocable cellular injury may help foster the development of effective therapeutic interventions using evolving pharmacologic, gene and stem cell technologies.

Transcriptional dysregulation in Huntington's disease

Although the gene responsible for Huntington's disease was discovered in 1993, the pathogenic mechanisms by which mutant huntingtin causes neuronal dysfunction and death remain unclear. However, increasing evidence suggests that mutant huntingtin disrupts the normal transcriptional program of susceptible neurons. Thus, transcriptional dysregulation might be an important pathogenic mechanism in Huntington's disease.