Huntingtin in health and disease

Dysregulation of choline metabolism and therapeutic potential of citicoline in Huntington's disease

Huntington's disease (HD) is associated with dysregulated choline metabolism, but the underlying mechanisms remain unclear. This study investigated the expression of key enzymes in this pathway in R6/2 HD mice and human HD postmortem brain tissues. We further explored the therapeutic potential of modulating choline metabolism for HD. Both R6/2 mice and HD patients exhibited reduced expression of glycerophosphocholine phosphodiesterase 1 (GPCPD1), a key enzyme in choline metabolism, in the striatum and cortex. The striatum of R6/2 mice also showed decreased choline and phosphorylcholine, and increased glycerophosphocholine, suggesting disruption in choline metabolism due to GPCPD1 deficiency. Treatment with citicoline significantly improved motor performance, upregulated anti-apoptotic Bcl2 expression, and reduced oxidative stress marker malondialdehyde in both brain regions. Metabolomic analysis revealed partial restoration of disrupted metabolic patterns in the striatum and cortex following citicoline treatment. These findings strongly suggest the role of GPCPD1 deficiency in choline metabolism dysregulation in HD. The therapeutic potential of citicoline in R6/2 mice highlights the choline metabolic pathway as a promising target for future HD therapies.

Modeling of age-related neurological disease: utility of zebrafish

Many age-related neurological diseases still lack effective treatments, making their understanding a critical and urgent issue in the globally aging society. To overcome this challenge, an animal model that accurately mimics these diseases is essential. To date, many mouse models have been developed to induce age-related neurological diseases through genetic manipulation or drug administration. These models help in understanding disease mechanisms and finding potential therapeutic targets. However, some age-related neurological diseases cannot be fully replicated in human pathology due to the different aspects between humans and mice. Although zebrafish has recently come into focus as a promising model for studying aging, there are few genetic zebrafish models of the age-related neurological disease. This review compares the aging phenotypes of humans, mice, and zebrafish, and provides an overview of age-related neurological diseases that can be mimicked in mouse models and those that cannot. We presented the possibility that reproducing human cerebral small vessel diseases during aging might be difficult in mice, and zebrafish has potential to be another animal model of such diseases due to their similarity of aging phenotype to humans.

Age-Associated Changes of Sirtuin 2 Expression in CNS and the Periphery

Sirtuin 2 (SIRT2), one of the seven members of the sirtuin family, has emerged as a potential regulator of aging and age-related pathologies since several studies have demonstrated that it shows age-related changes in humans and different animal models. A detailed analysis of the relevant works published to date addressing this topic shows that the changes that occur in SIRT2 with aging seem to be opposite in the brain and in the periphery. On the one hand, aging induces an increase in SIRT2 levels in the brain, which supports the notion that its pharmacological inhibition is beneficial in different neurodegenerative diseases. However, on the other hand, in the periphery, SIRT2 levels are reduced with aging while keeping its expression is protective against age-related peripheral inflammation, insulin resistance, and cardiovascular diseases. Thus, systemic administration of any known modulator of this enzyme would have conflicting outcomes. This review summarizes the currently available information on changes in SIRT2 expression in aging and the underlying mechanisms affected, with the aim of providing evidence to determine whether its pharmacological modulation could be an effective and safe pharmacological strategy for the treatment of age-related diseases.

Small-Molecule Disruptors of Mutant Huntingtin-Calmodulin Protein-Protein Interaction Attenuate Deleterious Effects of Mutant Huntingtin

Huntington's disease is a progressive and lethal neurodegenerative disease caused by an increased CAG repeat mutation in exon 1 of the huntingtin gene (mutant huntingtin). Current drug treatments provide only limited symptomatic relief without impacting disease progression. Previous studies in our lab and others identified the abnormal binding of mutant huntingtin protein with calmodulin, a key regulator of calcium signaling. Disrupting the abnormal binding of mutant huntingtin to calmodulin reduces perturbations caused by mutant huntingtin in cell and mouse models of Huntington's disease and importantly normalizes receptor-stimulated calcium release. Using a series of high-throughput in vitro and cell-based screening assays, we identified numerous small-molecule hits that disrupt the binding of mutant huntingtin to calmodulin and demonstrate protective effects. Iterative optimization of one hit resulted in nontoxic, selective compounds that are protective against mutant huntingtin cytotoxicity and normalized receptor-stimulated intracellular calcium release in PC12 cell models of Huntington's disease. Importantly, the compounds do not work by reducing the levels of mutant huntingtin, allowing this strategy to complement future molecular approaches to reduce mutant huntingtin expression. Our novel scaffold will serve as a prototype for further drug development in Huntington's disease. These studies indicate that the development of small-molecule compounds that disrupt the binding of mutant huntingtin to calmodulin is a promising approach for the advancement of therapeutics to treat Huntington's disease.

Computational insights into missense mutations in HTT gene causing Huntington's disease and its interactome networks

BACKGROUND

Huntington's disease is a rare neurodegenerative illness of the central nervous system that is inherited in an autosomal dominant pattern. Mutant huntingtin protein is produced as a result of enlargement of CAG repeat in the N-terminal of the polyglutamine tract.

AIM OF THE STUDY

Herein, we aim to investigate the mutations and their effects on the HTT gene and its genetic variants. Additionally, the protein-protein interaction of HTT with other proteins and receptor-ligand interaction with the three-dimensional structure of huntingtin protein were identified.

METHODS

A comprehensive analysis of the HTT interactome and protein-ligand interaction has been carried out to provide a global picture of structure-function analysis of huntingtin protein. Mutations were analyzed and mutation verification tools were used to check the effect of mutation on protein function.

RESULTS

The results showed, mutations in a single gene are not only responsible for causing a particular disease but may also cause other hereditary disorders as well. Moreover, the modification at the nucleotide level also cause the change in the specific amino acid which may disrupt the function of HTT and its interacting proteins contributing in disease pathogenesis. Furthermore, the interaction between MECP2 and BDNF lowers the rate of transcriptional activity. Molecular docking further confirmed the strong interaction between MECP2 and BDNF with highest affinity. Amino acid residues of the HTT protein, involved in the interaction with tetrabenazine were N912, Y890, G2385, and V2320. These findings proved, tetrabenazine as one of the potential therapeutic agent for treatment of Huntington's disease.

CONCLUSION

These results give further insights into the genetics of Huntington's disease for a better understanding of disease models which will be beneficial for the future therapeutic studies.

Antisense Oligonucleotide Therapy: From Design to the Huntington Disease Clinic

Huntington disease (HD) is a fatal progressive neurodegenerative disorder caused by an inherited mutation in the huntingtin (HTT) gene, which encodes mutant HTT protein. Though HD remains incurable, various preclinical studies have reported a favorable response to HTT suppression, emphasizing HTT lowering strategies as prospective disease-modifying treatments. Antisense oligonucleotides (ASOs) lower HTT by targeting transcripts and are well suited for treating neurodegenerative disorders as they distribute broadly throughout the central nervous system (CNS) and are freely taken up by neurons, glia, and ependymal cells. With the FDA approval of an ASO therapy for another disease of the CNS, spinal muscular atrophy, ASOs have become a particularly attractive therapeutic option for HD. However, two types of ASOs were recently assessed in human clinical trials for the treatment of HD, and both were halted early. In this review, we will explore the differences in chemistry, targeting, and specificity of these HTT ASOs as well as preliminary clinical findings and potential reasons for and implications of these halted trials.

Modulation of SETDB1 activity by APQ ameliorates heterochromatin condensation, motor function, and neuropathology in a Huntington's disease mouse model

The present study describes evaluation of epigenetic regulation by a small molecule as the therapeutic potential for treatment of Huntington's disease (HD). We identified 5-allyloxy-2-(pyrrolidin-1-yl)quinoline (APQ) as a novel SETDB1/ESET inhibitor using a combined in silico and in vitro cell based screening system. APQ reduced SETDB1 activity and H3K9me3 levels in a HD cell line model. In particular, not only APQ reduced H3K9me3 levels in the striatum but it also improved motor function and neuropathological symptoms such as neuronal size and activity in HD transgenic (YAC128) mice with minimal toxicity. Using H3K9me3-ChIP and genome-wide sequencing, we also confirmed that APQ modulates H3K9me3-landscaped epigenomes in YAC128 mice. These data provide that APQ, a novel small molecule SETDB1 inhibitor, coordinates H3K9me-dependent heterochromatin remodelling and can be an epigenetic drug for treating HD, leading with hope in clinical trials of HD.

Distinct effect of prenatal and postnatal brain expression across 20 brain disorders and anthropometric social traits: a systematic study of spatiotemporal modularity

Different spatiotemporal abnormalities have been implicated in different neuropsychiatric disorders and anthropometric social traits, yet an investigation in the temporal network modularity with brain tissue transcriptomics has been lacking. We developed a supervised network approach to investigate the genome-wide association study (GWAS) results in the spatial and temporal contexts and demonstrated it in 20 brain disorders and anthropometric social traits. BrainSpan transcriptome profiles were used to discover significant modules enriched with trait susceptibility genes in a developmental stage-stratified manner. We investigated whether, and in which developmental stages, GWAS-implicated genes are coordinately expressed in brain transcriptome. We identified significant network modules for each disorder and trait at different developmental stages, providing a systematic view of network modularity at specific developmental stages for a myriad of brain disorders and traits. Specifically, we observed a strong pattern of the fetal origin for most psychiatric disorders and traits [such as schizophrenia (SCZ), bipolar disorder, obsessive-compulsive disorder and neuroticism], whereas increased co-expression activities of genes were more strongly associated with neurological diseases [such as Alzheimer's disease (AD) and amyotrophic lateral sclerosis] and anthropometric traits (such as college completion, education and subjective well-being) in postnatal brains. Further analyses revealed enriched cell types and functional features that were supported and corroborated prior knowledge in specific brain disorders, such as clathrin-mediated endocytosis in AD, myelin sheath in multiple sclerosis and regulation of synaptic plasticity in both college completion and education. Our study provides a landscape view of the spatiotemporal features in a myriad of brain-related disorders and traits.

N-Methyl-d-Aspartate (NMDA) Receptor Blockade Prevents Neuronal Death Induced by Zika Virus Infection

Zika virus (ZIKV) infection is a global health emergency that causes significant neurodegeneration. Neurodegenerative processes may be exacerbated by N-methyl-d-aspartate receptor (NMDAR)-dependent neuronal excitoxicity. Here, we have exploited the hypothesis that ZIKV-induced neurodegeneration can be rescued by blocking NMDA overstimulation with memantine. Our results show that ZIKV actively replicates in primary neurons and that virus replication is directly associated with massive neuronal cell death. Interestingly, treatment with memantine or other NMDAR blockers, including dizocilpine (MK-801), agmatine sulfate, or ifenprodil, prevents neuronal death without interfering with the ability of ZIKV to replicate in these cells. Moreover, in vivo experiments demonstrate that therapeutic memantine treatment prevents the increase of intraocular pressure (IOP) induced by infection and massively reduces neurodegeneration and microgliosis in the brain of infected mice. Our results indicate that the blockade of NMDARs by memantine provides potent neuroprotective effects against ZIKV-induced neuronal damage, suggesting it could be a viable treatment for patients at risk for ZIKV infection-induced neurodegeneration.

Zika virus (ZIKV) infection is a global health emergency associated with serious neurological complications, including microcephaly and Guillain-Barré syndrome. Infection of experimental animals with ZIKV causes significant neuronal damage and microgliosis. Treatment with drugs that block NMDARs prevented neuronal damage both in vitro and in vivo. These results suggest that overactivation of NMDARs contributes significantly to the neuronal damage induced by ZIKV infection, and this is amenable to inhibition by drug treatment.

IP3 receptor mutations and brain diseases in human and rodents

The inositol 1,4,5-trisphosphate receptor (IP₃ R) is a huge Ca²⁺ channel that is localized at the endoplasmic reticulum. The IP₃ R releases Ca²⁺ from the endoplasmic reticulum upon binding to IP₃ , which is produced by various extracellular stimuli through phospholipase C activation. All vertebrate organisms have three subtypes of IP₃ R genes, which have distinct properties of IP₃ -binding and Ca²⁺ sensitivity, and are differently regulated by phosphorylation and by their associated proteins. Each cell type expresses the three subtypes of IP₃ R in a distinct proportion, which is important for creating and maintaining spatially and temporally appropriate intracellular Ca²⁺ level patterns for the regulation of specific physiological phenomena. Of the three types of IP₃ Rs, the type 1 receptor (IP₃ R1) is dominantly expressed in the brain and is important for brain function. Recent emerging evidence suggests that abnormal Ca²⁺ signals from the IP₃ R1 are closely associated with human brain pathology. In this review, we focus on the recent advances in our knowledge of the regulation of IP₃ R1 and its functional implication in human brain diseases, as revealed by IP₃ R mutation studies and analysis of human disease-associated genes. This article is part of the mini review series "60th Anniversary of the Japanese Society for Neurochemistry".

Mechanisms underlying neurodegeneration in Huntington disease: applications to novel disease-modifying therapies

The CAG repeat expansion mutation that causes Huntington Disease (HD) was discovered more than 20 years ago, yet no treatment has yet been developed to stop the relentless course of the disease. Nonetheless, substantial progress has been made in understanding HD pathogenesis. We review insights that have been gleaned from HD genetics, metabolism, and pathology; HD mouse and cell models; the structure, function and post-translational modification of normal and mutant huntingtin (htt) protein; gene expression profiles in HD cells and tissue; the neurotoxicy of mutant htt RNA; and the expression of an antisense transcript from the HD locus. We conclude that rationale therapeutics for HD is within sight, though many questions remain to be answered.

Functional impairment of precerebral arteries in Huntington disease

BACKGROUND

Cardiovascular pathology of Huntington disease (HD) appears to be complex; while microvascular dysfunction seems to appear early, deaths from cardiomyopathy and stroke might occur in the late phase of HD.

METHODS

Our study evaluated global risk factors for coronary heart disease (CHD), structure and function of precerebral arteries in 41 HD subjects and 41 matched controls. HD subjects were divided into groups by the United Huntington disease rating scale (presymptomatic-PHD, early-EHD, midstage-MHD and late-LHD). CHD risk factors assessment and Doppler examination of precerebral arteries were performed, including measurements of the carotid artery intima-media thickness (IMT), and parameters indicating local carotid artery distensibility (stiffness index β, pulse wave velocity, pressure strain elasticity module and carotid artery compliance).

RESULTS

In the HD and controls we identified a comparable number of non-obstructive plaques (<50% lumen narrowing). No obstructive plaques (>50% lumen narrowing) were found. There was significantly increased IMT in MHD. In PHD and EHD the parameters of arterial stiffness were significantly higher and the carotid artery compliance was significantly lower.

CONCLUSIONS

Our results reveal functional vascular pathology in PHD, EHD, and MHD. Precerebral arteries dysfunction in HD therefore appears to be mostly functional and in agreement with recently described autonomic nervous system changes in HD.

Neural Differentiation of Human Pluripotent Stem Cells for Nontherapeutic Applications: Toxicology, Pharmacology, and In Vitro Disease Modeling

Human pluripotent stem cells (hPSCs) derived from either blastocyst stage embryos (hESCs) or reprogrammed somatic cells (iPSCs) can provide an abundant source of human neuronal lineages that were previously sourced from human cadavers, abortuses, and discarded surgical waste. In addition to the well-known potential therapeutic application of these cells in regenerative medicine, these are also various promising nontherapeutic applications in toxicological and pharmacological screening of neuroactive compounds, as well as for in vitro modeling of neurodegenerative and neurodevelopmental disorders. Compared to alternative research models based on laboratory animals and immortalized cancer-derived human neural cell lines, neuronal cells differentiated from hPSCs possess the advantages of species specificity together with genetic and physiological normality, which could more closely recapitulate in vivo conditions within the human central nervous system. This review critically examines the various potential nontherapeutic applications of hPSC-derived neuronal lineages and gives a brief overview of differentiation protocols utilized to generate these cells from hESCs and iPSCs.

Metabotropic glutamate receptor 5 positive allosteric modulators are neuroprotective in a mouse model of Huntington's disease

Background and Purpose

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a polyglutamine expansion in the huntingtin protein. We have previously demonstrated that the cell signalling of the metabotropic glutamate receptor 5 (mGluR5) is altered in a mouse model of HD. Although mGluR5-dependent protective pathways are more activated in HD neurons, intracellular Ca²⁺ release is also more pronounced, which could contribute to excitotoxicity. In the present study, we aim to investigate whether mGluR5 positive allosteric modulators (PAMs) could activate protective pathways without triggering high levels of Ca²⁺ release and be neuroprotective in HD.

Experimental Approach

We performed a neuronal cell death assay to determine which drugs are neuroprotective, Western blot and Ca²⁺ release experiments to investigate the molecular mechanisms involved in this neuroprotection, and object recognition task to determine whether the tested drugs could ameliorate HD memory deficit.

Key Results

We find that mGluR5 PAMs can protect striatal neurons from the excitotoxic neuronal cell death promoted by elevated concentrations of glutamate and NMDA. mGluR5 PAMs are capable of activating Akt without triggering increased intracellular Ca²⁺ concentration ([Ca²⁺]_i); and Akt blockage leads to loss of PAM-mediated neuroprotection. Importantly, PAMs' potential as drugs that may be used to treat neurodegenerative diseases is highlighted by the neuroprotection exerted by mGluR5 PAMs on striatal neurons from a mouse model of HD, BACHD. Moreover, mGluR5 PAMs can activate neuroprotective pathways more robustly in BACHD mice and ameliorate HD memory deficit.

Conclusions and Implications

mGluR5 PAMs are potential drugs that may be used to treat neurodegenerative diseases, especially HD.

The mGluR5 positive allosteric modulator, CDPPB, ameliorates pathology and phenotypic signs of a mouse model of Huntington's disease

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disorder caused by a polyglutamine expansion in the amino-terminal region of the huntingtin protein (htt), leading to motor dysfunction, cognitive decline, psychiatric alterations, and death. The metabotropic glutamate receptor 5 (mGluR5) has been implicated in HD and we have recently demonstrated that mGluR5 positive allosteric modulators (PAMs) are neuroprotective in vitro. In the present study we demonstrate that the mGluR5 PAM, CDPPB, is a potent neuroprotective drug, in vitro and in vivo, capable of delaying HD-related symptoms. The HD mouse model, BACHD, exhibits many HD features, including neuronal cell loss, htt aggregates, motor incoordination and memory impairment. However, chronic treatment of BACHD mice with CDPPB 1.5 mg/kg s.c. for 18 weeks increased the activation of cell signaling pathways important for neuronal survival, including increased AKT and ERK1/2 phosphorylation and augmented the BDNF mRNA expression. CDPPB chronic treatment was also able to prevent the neuronal cell loss that takes place in the striatum of BACHD mice and decrease htt aggregate formation. Moreover, CDPPB chronic treatment was efficient to partially ameliorate motor incoordination and to rescue the memory deficit exhibited by BACHD mice. Importantly, no toxic effects or stereotypical behavior were observed upon CDPPB chronic treatment. Thus, CDPPB is a potential drug to treat HD, preventing neuronal cell loss and htt aggregate formation and delaying HD symptoms.

Animal models of neurodegenerative diseases

The prevalence of neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD), increases with age, and the number of affected patients is expected to increase worldwide in the next decades. Accurately understanding the etiopathogenic mechanisms of these diseases is a crucial step for developing disease-modifying drugs able to preclude their emergence or at least slow their progression. Animal models contribute to increase the knowledge on the pathophysiology of neurodegenerative diseases. These models reproduce different aspects of a given disease, as well as the histopathological lesions and its main symptoms. The purpose of this review is to present the main animal models for AD, PD, and Huntington's disease.

Noninvasive quantification of metabotropic glutamate receptor type 1 with [¹¹C]ITDM: a small-animal PET study

Because of its role in multiple central nervous system (CNS) pathways, metabotropic glutamate receptor type 1 (mGluR1) is a crucial target in the development of pharmaceuticals for CNS disorders. N-[4-[6-(isopropylamino)-pyrimidin-4-yl]-1,3-thiazol-2-yl]-N-methyl-4-[(11)C]-methylbenzamide ([(11)C]ITDM) was recently developed as a positron emission tomography (PET) ligand for mGluR1. To devise a method for measurement of the binding potential (BPND) of [(11)C]ITDM to mGluR1, reference tissue methods aimed at replacing measurement of the arterial input function are desirable. In this study, we evaluated a noninvasive quantification method of mGluR1 with [(11)C]ITDM, demonstrating its accuracy using Huntington disease model R6/2 mice. The BPND measurements based on the Logan reference (Logan Ref) method have closely approximated that based on the arterial input method. We performed PET analysis with Logan Ref to assess its accuracy in quantifying the decline of mGluR1 expression in R6/2 mice. Significant decreases in BPND values in R6/2 mice were detected in cerebellum, thalamus, striatum, and cingulate cortex. We compared autoradiographs of R6/2 mouse brain sections with immunohistochemical images, and found a close correlation between changes in radioactive signal intensity and degree of mGluR1 expression. In conclusion, [(11)C]ITDM-PET is a promising tool for in vivo quantification of mGluR1 expression.

Adenosine receptor control of cognition in normal and disease

Adenosine and adenosine receptors (ARs) are increasingly recognized as important therapeutic targets for controlling cognition under normal and disease conditions for its dual roles of neuromodulation as well as of homeostatic function in the brain. This chapter first presents the unique ability of adenosine, by acting on the inhibitory A1 and facilitating A2A receptor, to integrate dopamine, glutamate, and BNDF signaling and to modulate synaptic plasticity (e.g., long-term potentiation and long-term depression) in brain regions relevant to learning and memory, providing the molecular and cellular bases for adenosine receptor (AR) control of cognition. This led to the demonstration of AR modulation of social recognition memory, working memory, reference memory, reversal learning, goal-directed behavior/habit formation, Pavlovian fear conditioning, and effort-related behavior. Furthermore, human and animal studies support that AR activity can also, through cognitive enhancement and neuroprotection, reverse cognitive impairments in animal models of Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease, and schizophrenia. Lastly, epidemiological evidence indicates that regular human consumption of caffeine, the most widely used psychoactive drug and nonselective AR antagonists, is associated with the reduced cognitive decline in aging and AD patients, and with the reduced risk in developing PD. Thus, there is a convergence of the molecular studies revealing AR as molecular targets for integrating neurotransmitter signaling and controlling synaptic plasticity, with animal studies demonstrating the strong procognitive impact upon AR antagonism in normal and disease brains and with epidemiological and clinical evidences in support of caffeine and AR drugs for therapeutic modulation of cognition. Since some of adenosine A2A receptor antagonists are already in phase III clinical trials for motor benefits in PD patients with remarkable safety profiles, additional animal and human studies to better understand the mechanism underlying the AR-mediated control of cognition under normal and disease conditions will provide the required rationale to stimulate the necessary clinical investigation to rapidly translate adenosine and AR drug as a novel strategy to control memory impairment in neuropsychiatric disorders.

Vaccinia-related kinase 2 mediates accumulation of polyglutamine aggregates via negative regulation of the chaperonin TRiC

Misfolding of proteins containing abnormal expansions of polyglutamine (polyQ) repeats is associated with cytotoxicity in several neurodegenerative disorders, including Huntington's disease. Recently, the eukaryotic chaperonin TRiC hetero-oligomeric complex has been shown to play an important role in protecting cells against the accumulation of misfolded polyQ protein aggregates. It is essential to elucidate how TRiC function is regulated to better understand the pathological mechanism of polyQ aggregation. Here, we propose that vaccinia-related kinase 2 (VRK2) is a critical enzyme that negatively regulates TRiC. In mammalian cells, overexpression of wild-type VRK2 decreased endogenous TRiC protein levels by promoting TRiC ubiquitination, but a VRK2 kinase-dead mutant did not. Interestingly, VRK2-mediated downregulation of TRiC increased aggregate formation of a polyQ-expanded huntingtin fragment. This effect was ameliorated by rescue of TRiC protein levels. Notably, small interference RNA-mediated knockdown of VRK2 enhanced TRiC protein stability and decreased polyQ aggregation. The VRK2-mediated reduction of TRiC protein levels was subsequent to the recruitment of COP1 E3 ligase. Among the members of the COP1 E3 ligase complex, VRK2 interacted with RBX1 and increased E3 ligase activity on TRiC in vitro. Taken together, these results demonstrate that VRK2 is crucial to regulate the ubiquitination-proteosomal degradation of TRiC, which controls folding of polyglutamine proteins involved in Huntington's disease.

Metabotropic glutamate receptor 5 knockout promotes motor and biochemical alterations in a mouse model of Huntington's disease

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disorder caused by a polyglutamine expansion in the amino-terminal region of the huntingtin protein, which promotes progressive neuronal cell loss, neurological symptoms and death. In the present study, we show that blockade of mGluR5 with MTEP promotes increased locomotor activity in both control (Hdh(Q20/Q20)) and mutant HD (Hdh(Q111/Q111)) mice. Although acute injection of MTEP increases locomotor activity in both control and mutant HD mice, locomotor activity is increased in only control mice, not mutant HD mice, following the genetic deletion of mGluR5. Interestingly, treatment of mGluR5 knockout mice with either D1 or D2 dopamine antagonists eliminates the increased locomotor activity of mGluR5 knockout mice. Amphetamine treatment increases locomotor activity in control mice, but not mGluR5 null mutant HD mice. However, the loss of mGluR5 expression improves rotarod performance and decreases the number of huntingtin intranuclear inclusions in mutant HD mice. These adaptations may be due to mutant huntingtin-dependent alterations in gene expression, as microarray studies have identified several genes that are altered in mutant, but not wild-type HD mice lacking mGluR5 expression. qPCR experiments confirm that the mRNA transcript levels of dynein heavy chain, dynactin 3 and dynein light chain-6 are altered following the genetic deletion of mGluR5 in mutant HD mice, as compared with wild-type mutant HD mice. Thus, our data suggest that mutant huntingtin protein and mGluR5 exhibit a functional interaction that may be important for HD-mediated alterations in locomotor behavior and the development of intranuclear inclusions.

Genome-wide loss of 5-hmC is a novel epigenetic feature of Huntington's disease

5-Hydroxymethylcytosine (5-hmC) may represent a new epigenetic modification of cytosine. While the dynamics of 5-hmC during neurodevelopment have recently been reported, little is known about its genomic distribution and function(s) in neurodegenerative diseases such as Huntington's disease (HD). We here observed a marked reduction of the 5-hmC signal in YAC128 (yeast artificial chromosome transgene with 128 CAG repeats) HD mouse brain tissues when compared with age-matched wild-type (WT) mice, suggesting a deficiency of 5-hmC reconstruction in HD brains during postnatal development. Genome-wide distribution analysis of 5-hmC further confirmed the diminishment of the 5-hmC signal in striatum and cortex in YAC128 HD mice. General genomic features of 5-hmC are highly conserved, not being affected by either disease or brain regions. Intriguingly, we have identified disease-specific (YAC128 versus WT) differentially hydroxymethylated regions (DhMRs), and found that acquisition of DhmRs in gene body is a positive epigenetic regulator for gene expression. Ingenuity pathway analysis (IPA) of genotype-specific DhMR-annotated genes revealed that alternation of a number of canonical pathways involving neuronal development/differentiation (Wnt/β-catenin/Sox pathway, axonal guidance signaling pathway) and neuronal function/survival (glutamate receptor/calcium/CREB, GABA receptor signaling, dopamine-DARPP32 feedback pathway, etc.) could be important for the onset of HD. Our results indicate that loss of the 5-hmC marker is a novel epigenetic feature in HD, and that this aberrant epigenetic regulation may impair the neurogenesis, neuronal function and survival in HD brain. Our study also opens a new avenue for HD treatment; re-establishing the native 5-hmC landscape may have the potential to slow/halt the progression of HD.

Adenovirus vector-based in vitro neuronal cell model for Huntington's disease with human disease-like differential aggregation and degeneration

BACKGROUND

Neuronal degeneration, in particular in the striatum, and the formation of nuclear and cytoplasmic inclusions are characteristics of Huntington's disease (HD) as a result of the expansion of a polyglutamine tract located close to the N-terminus of huntingtin (htt). Because of the large (10-kb) size of the htt cDNA, expression of full-length htt in primary neurons has proved difficult in the past.

METHODS

We generated a new chronic in vitro model that is based on high-capacity adenovirus vector-mediated transduction of primary murine striatal and cortical neurons. Because the vector has a large capacity for transport of foreign DNA, it was possible to quantitatively express in these primary cells normal and mutant full-length htt (designed as fusion proteins with enhanced green fluorescent protein) in addition to its truncated versions. Pathological changes caused by mutant htt were characterized.

RESULTS

The model mimicked several features observed in HD patients: prominent nuclear inclusions in cortical but not in striatal neurons, preferential neuronal degeneration of striatal neurons and neurofilament fragmentation in this cell type. Compared with expressed truncated mutant htt, the expression of full-length mutant htt in neurons resulted in a much slower appearance of pathological changes. Different from cortical neurons, the vast majority of nuclei in striatal cells contained only diffusely distributed N-terminal htt fragments. Cytoplasmic inclusions in both cell types contained full-length mutant htt.

CONCLUSIONS

This model and the adenovirus vectors used will be valuable for studying the function of htt and the pathogenesis of HD at molecular and cellular levels in different neuronal cell types.

The sirtuin 2 inhibitor AK-7 is neuroprotective in Huntington's disease mouse models

Inhibition of sirtuin 2 (SIRT2) deacetylase mediates protective effects in cell and invertebrate models of Parkinson's disease and Huntington's disease (HD). Here we report the in vivo efficacy of a brain-permeable SIRT2 inhibitor in two genetic mouse models of HD. Compound treatment resulted in improved motor function, extended survival, and reduced brain atrophy and is associated with marked reduction of aggregated mutant huntingtin, a hallmark of HD pathology. Our results provide preclinical validation of SIRT2 inhibition as a potential therapeutic target for HD and support the further development of SIRT2 inhibitors for testing in humans.

Biophysical and computational fragment-based approaches to targeting protein-protein interactions: applications in structure-guided drug discovery

Drug discovery has classically targeted the active sites of enzymes or ligand-binding sites of receptors and ion channels. In an attempt to improve selectivity of drug candidates, modulation of protein-protein interfaces (PPIs) of multiprotein complexes that mediate conformation or colocation of components of cell-regulatory pathways has become a focus of interest. However, PPIs in multiprotein systems continue to pose significant challenges, as they are generally large, flat and poor in distinguishing features, making the design of small molecule antagonists a difficult task. Nevertheless, encouragement has come from the recognition that a few amino acids - so-called hotspots - may contribute the majority of interaction-free energy. The challenges posed by protein-protein interactions have led to a wellspring of creative approaches, including proteomimetics, stapled α-helical peptides and a plethora of antibody inspired molecular designs. Here, we review a more generic approach: fragment-based drug discovery. Fragments allow novel areas of chemical space to be explored more efficiently, but the initial hits have low affinity. This means that they will not normally disrupt PPIs, unless they are tethered, an approach that has been pioneered by Wells and co-workers. An alternative fragment-based approach is to stabilise the uncomplexed components of the multiprotein system in solution and employ conventional fragment-based screening. Here, we describe the current knowledge of the structures and properties of protein-protein interactions and the small molecules that can modulate them. We then describe the use of sensitive biophysical methods - nuclear magnetic resonance, X-ray crystallography, surface plasmon resonance, differential scanning fluorimetry or isothermal calorimetry - to screen and validate fragment binding. Fragment hits can subsequently be evolved into larger molecules with higher affinity and potency. These may provide new leads for drug candidates that target protein-protein interactions and have therapeutic value.

Engineered antibody therapies to counteract mutant huntingtin and related toxic intracellular proteins

The engineered antibody approach to Huntington's disease (HD) therapeutics is based on the premise that significantly lowering the levels of the primary misfolded mutant protein will reduce abnormal protein interactions and direct toxic effects of the misfolded huntingtin (HTT). This will in turn reduce the pathologic stress on cells, and normalize intrinsic proteostasis. Intracellular antibodies (intrabodies) are single-chain (scFv) and single-domain (dAb; nanobody) variable fragments that can retain the affinity and specificity of full-length antibodies, but can be selected and engineered as genes. Functionally, they represent a protein-based approach to the problem of aberrant mutant protein folding, post-translational modifications, protein-protein interactions, and aggregation. Several intrabodies that bind on either side of the expanded polyglutamine tract of mutant HTT have been reported to improve the mutant phenotype in cell and organotypic cultures, fruit flies, and mice. Further refinements to the difficult challenges of intraneuronal delivery, cytoplasmic folding, and long-term efficacy are in progress. This review covers published studies and emerging approaches on the choice of targets, selection and engineering methods, gene and protein delivery options, and testing of candidates in cell and animal models. The resultant antibody fragments can be used as direct therapeutics and as target validation/drug discovery tools for HD, while the technology is also applicable to a wide range of neurodegenerative and other diseases that are triggered by toxic proteins.

Constitutive upregulation of chaperone-mediated autophagy in Huntington's disease

Autophagy contributes to the removal of prone-to-aggregate proteins, but in several instances these pathogenic proteins have been shown to interfere with autophagic activity. In the case of Huntington's disease (HD), a congenital neurodegenerative disorder resulting from mutation in the huntingtin protein, we have previously described that the mutant protein interferes with the ability of autophagic vacuoles to recognize cytosolic cargo. Growing evidence supports the existence of cross talk among autophagic pathways, suggesting the possibility of functional compensation when one of them is compromised. In this study, we have identified a compensatory upregulation of chaperone-mediated autophagy (CMA) in different cellular and mouse models of HD. Components of CMA, namely the lysosome-associated membrane protein type 2A (LAMP-2A) and lysosomal-hsc70, are markedly increased in HD models. The increase in LAMP-2A is achieved through both an increase in the stability of this protein at the lysosomal membrane and transcriptional upregulation of this splice variant of the lamp-2 gene. We propose that CMA activity increases in response to macroautophagic dysfunction in the early stages of HD, but that the efficiency of this compensatory mechanism may decrease with age and so contribute to cellular failure and the onset of pathological manifestations.

The role of induced pluripotent stem cells in regenerative medicine: neurodegenerative diseases

Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and Friedreich's ataxia are the most common human neurodegenerative diseases pathologically characterized by a progressive and specific loss of certain neuronal populations. The exact mechanisms of neuronal cell death in these diseases are unclear, although some forms of the diseases are inherited and genes causing these diseases have been identified. Currently there are no effective clinical therapies for many of these diseases. The recently acquired ability to reprogram human adult somatic cells to induced pluripotent stem cells (iPSCs) in culture may provide a powerful tool for in vitro neurodegenerative disease modeling and an unlimited source for cell replacement therapy. In the present review, we summarize recent progress on iPSC generation and differentiation into neuronal cell types and discuss the potential application for in vitro disease mechanism study and in vivo cell replacement therapy.

Modeling pathogenesis of Huntington's disease with inducible neuroprogenitor cells

Huntington's disease (HD) is caused by an abnormal expansion of CAG trinucleotide repeats encoding polyglutamine (polyQ) in the first exon of the huntingtin (htt) gene. Despite considerable efforts, the pathogenesis of HD remains largely unclear due to a paucity of models that can reliably reproduce the pathological characteristics of HD. Here, we report a neuronal cell model of HD using the previously established tetracycline regulated rat neuroprogenitor cell line, HC2S2. Stable expression of enhanced green fluorescence protein tagged htt exon 1 (referred to as 28Q and 74Q, respectively) in the HC2S2 cells did not affect rapid neuronal differentiation. However, compared to the cells expressing wild type htt, the cell line expressing mutant htt showed an increase in time-dependent cell death and neuritic degeneration, and displayed increased vulnerability to oxidative stress. Increased protein aggregation during the process of neuronal aging or when the cells were exposed to oxidative stress reagents was detected in the cell line expressing 74Q but not in its counterpart. These results suggest that the neuroprogenitor cell lines mimic the major neuropathological characteristics of HD and may provide a useful tool for studying the neuropathogenesis of HD and for high throughput screening of therapeutic compounds.

Huntington's disease: from molecular pathogenesis to clinical treatment

Huntington's disease is a progressive, fatal, neurodegenerative disorder caused by an expanded CAG repeat in the huntingtin gene, which encodes an abnormally long polyglutamine repeat in the huntingtin protein. Huntington's disease has served as a model for the study of other more common neurodegenerative disorders, such as Alzheimer's disease and Parkinson's disease. These disorders all share features including: delayed onset; selective neuronal vulnerability, despite widespread expression of disease-related proteins during the whole lifetime; abnormal protein processing and aggregation; and cellular toxic effects involving both cell autonomous and cell-cell interaction mechanisms. Pathogenic pathways of Huntington's disease are beginning to be unravelled, offering targets for treatments. Additionally, predictive genetic testing and findings of neuroimaging studies show that, as in some other neurodegenerative disorders, neurodegeneration in affected individuals begins many years before onset of diagnosable signs and symptoms of Huntington's disease, and it is accompanied by subtle cognitive, motor, and psychiatric changes (so-called prodromal disease). Thus, Huntington's disease is also emerging as a model for strategies to develop therapeutic interventions, not only to slow progression of manifest disease but also to delay, or ideally prevent, its onset.

Huntington's disease: clinical presentation and treatment

Huntington's disease (HD) is a devastating inherited neurodegenerative disease characterized primarily by progressive motor, cognitive, and psychiatric symptoms. It is caused by autosomal dominant inheritance of an expanded CAG repeat within the Huntington's gene on chromosome 4. In this chapter, we characterize the typical and variant motor phenotypes of the disease and then proceed to describe the cognitive and psychiatric profile. We then give an overview of a suggested multidisciplinary approach to the management of HD, emphasizing the fact that it is a disease which impacts on entire families rather than affecting individuals in isolation. We then describe the pharmacological and nonpharmacological options available for management of specific symptoms.

Huntington's disease and Group I metabotropic glutamate receptors

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by involuntary body movement, cognitive impairment and psychiatric disturbance. A polyglutamine expansion in the amino-terminal region of the huntingtin (htt) protein is the genetic cause of HD. Htt protein interacts with a wide variety of proteins, and htt mutation causes cell signaling alterations in various neurotransmitter systems, including dopaminergic, glutamatergic, and cannabinoid systems, as well as trophic factor systems. This review will overview recent findings concerning htt-promoted alterations in cell signaling that involve different neurotransmitters and trophic factor systems, especially involving mGluR1/5, as glutamate plays a crucial role in neuronal cell death. The neuronal cell death that takes place in the striatum and cortex of HD patients is the most important factor underlying HD progression. Metabotropic glutamate receptors (mGluR1 and mGluR5) have a very controversial role in neuronal cell death and it is not clear whether mGluR1/5 activation either protects or exacerbates neuronal death. Thus, understanding how mutant htt protein affects glutamatergic receptor signaling will be essential to further establish a role for glutamate receptors in HD and develop therapeutic strategies to treat HD.

Mutant huntingtin-impaired degradation of beta-catenin causes neurotoxicity in Huntington's disease

Huntington's disease (HD) is a fatal neurodegenerative disorder causing selective neuronal death in the brain. Dysfunction of the ubiquitin-proteasome system may contribute to the disease; however, the exact mechanisms are still unknown. We report here a new pathological mechanism by which mutant huntingtin specifically interferes with the degradation of beta-catenin. Huntingtin associates with the beta-catenin destruction complex that ensures its equilibrated degradation. The binding of beta-catenin to the destruction complex is altered in HD, leading to the toxic stabilization of beta-catenin. As a consequence, the beta-transducin repeat-containing protein (beta-TrCP) rescues polyglutamine (polyQ)-huntingtin-induced toxicity in striatal neurons and in a Drosophila model of HD, through the specific degradation of beta-catenin. Finally, the non-steroidal anti-inflammatory drug indomethacin that decreases beta-catenin levels has a neuroprotective effect in a neuronal model of HD and in Drosophila and increases the lifespan of HD flies. We thus suggest that restoring beta-catenin homeostasis in HD is of therapeutic interest.

Inositol trisphosphate receptor Ca2+ release channels in neurological diseases

The modulation of cytoplasmic Ca2+ concentration by release from internal stores through the inositol trisphosphate receptor (InsP3R) Ca2+ release channel is a ubiquitous signaling system involved in the regulation of numerous processes. Because of its ubiquitous expression and roles in regulating diverse cell physiological processes, it is not surprising that the InsP3R has been implicated in a number of disease states. However, relatively few mutations in InsP3R genes have been identified to date. Here, I will discuss mutations in the type 1 InsP3R that have been discovered by analyses of human patients and mice with neurological disorders. In addition, I will highlight diseases caused by mutations in other genes, including Huntington's and Alzheimer's diseases and some spinocerebellar ataxias, where the mutant proteins have been found to exert strong influences on InsP3R function that may link InsP3R to disease pathogenesis.

Metabotropic glutamate receptor-mediated cell signaling pathways are altered in a mouse model of Huntington's disease

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disorder caused by a polyglutamine expansion in the huntingtin protein (Htt). Group I metabotropic glutamate receptors (mGluRs) are coupled to G(alphaq) and play an important role in neuronal survival. We have previously demonstrated that mGluRs interact with Htt. Here we used striatal neuronal primary cultures and acute striatal slices to demonstrate that mGluR-mediated signaling pathways are altered in a presymptomatic mouse model of HD (Hdh(Q111/Q111)), as compared to those of control mice (Hdh(Q20/Q20)). mGluR1/5-mediated inositol phosphate (InsP) formation is desensitized in striatal slices from Hdh(Q111/Q111) mice and this desensitization is PKC-mediated. Despite of decreased InsP formation, (S)-3,5-dihydroxylphenylglycine (DHPG)-mediated Ca(2+) release is higher in Hdh(Q111/Q111) than in Hdh(Q20/Q20) neurons. Furthermore, mGluR1/5-stimulated AKT and extracellular signal-regulated kinase (ERK) activation is altered in Hdh(Q111/Q111) mice. Basal AKT activation is higher in Hdh(Q111/Q111) neurons and this increase is mGluR5 dependent. Moreover, mGluR5 activation leads to higher levels of ERK activation in Hdh(Q111/Q111) than in Hdh(Q20/Q20) striatum. PKC inhibition not only brings Hdh(Q111/Q111) DHPG-stimulated InsP formation to Hdh(Q20/Q20) levels, but also causes an increase in neuronal cell death in Hdh(Q111/Q111) neurons. However, PKC inhibition does not modify neuronal cell death in Hdh(Q20/Q20) neurons, suggesting that PKC-mediated desensitization of mGluR1/5 in Hdh(Q111/Q111) mice might be protective in HD. Together, these data indicate that group I mGluR-mediated signaling pathways are altered in HD and that these cell signaling adaptations could be important for striatal neurons survival.

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

The quantification of cellular proteins is essential for the study of many different biological processes. This study describes an assay for the detection of the intracellular mutant huntingtin, the causative agent of Huntington's disease, with a method that may be generally applicable to other cellular proteins. A small recombinant protein tag that is recognized by a pair of readily available, high-affinity monoclonal antibodies was designed. This tag was then added to an inducible fragment of the mutant huntingtin protein by genetic engineering. We show that it is possible to use time-resolved FRET to detect low intracellular levels of huntingtin by a simple lysis and detection procedure. This assay was then adapted into a homogeneous, miniaturized format suitable for screening in 1536-well plates. The use of time-resolved FRET also permits the assay to be multiplexed with a standard readout of cell toxicity, thus allowing the identification of conditions causing reduction of protein levels simply due to cytotoxicity. The screening results demonstrated that the assay is able to identify compounds that modulate the levels of huntingtin both positively and negatively and that represent valuable starting points for drug discovery programs.

Diminished hippocalcin expression in Huntington's disease brain does not account for increased striatal neuron vulnerability as assessed in primary neurons

Hippocalcin is a neuronal calcium sensor protein previously implicated in regulating neuronal viability and plasticity. Hippocalcin is the most highly expressed neuronal calcium sensor in the medium spiny striatal output neurons that degenerate selectively in Huntington's disease (HD). We have previously shown that decreased hippocalcin expression occurs in parallel with the onset of disease phenotype in mouse models of HD. Here we show by in situ hybridization histochemistry that hippocalcin RNA is also diminished by 63% in human HD brain. These findings lead us to hypothesize that diminished hippocalcin expression might contribute to striatal neurodegeneration in HD. We tested this hypothesis by assessing whether restoration of hippocalcin expression would decrease striatal neurodegeneration in cellular models of HD comprising primary striatal neurons exposed to mutant huntingtin, the mitochondrial toxin 3-nitropropionic acid or an excitotoxic concentration of glutamate. Counter to our hypothesis, hippocalcin expression did not improve the survival of striatal neurons under these conditions. Likewise, expression of hippocalcin together with interactor proteins including the neuronal apoptosis inhibitory protein did not increase the survival of striatal cells in cellular models of HD. These results indicate that diminished hippocalcin expression does not contribute to HD-related neurodegeneration.

Effective expression of recombinant cytotoxic protein via its attachment to a polyglutamine domain

Inadvertent cytotoxicity may hinder the expression of many recombinant proteins that are of industrial or medicinal importance. Here, we show that covalent binding of the influenza A cytotoxic protein M2 to a polyglutamine domain (polyQ-M2; QM2) results in significant delay of its cytotoxic effects when compared to wild-type protein (M2wt). We also show that while expression of recombinant M2wt from A/WSN/1933 strain could not be attained in vaccinia virus (VV), polyQ-M2 was successfully expressed in this system. Moreover, we demonstrate that in cell culture, the polyQ domain is cleaved off following 48 h of expression, thus releasing free and active M2. Similarly, we show the spontaneous cleavage and polyQ release from fusion with another distinct polypeptide, green fluorescent protein (GFP). Expression of M2 from QM2 construct was more prolonged than one based on M2wt-expressing construct, markedly exceeding it at the later time points. Therefore, cell death caused by a toxic polypeptide may be suppressed via genetic fusion with polyQ, resulting in its enhanced expression, followed by slow release of the free polypeptide from the fusion. Collectively, covalent fusion with polyQ or other aggregate-forming domains presents a novel approach for industrial production of cytotoxic proteins and also holds promise for gene therapy applications.

Age-dependent alterations of corticostriatal activity in the YAC128 mouse model of Huntington disease

Huntington disease is a genetic neurodegenerative disorder that produces motor, neuropsychiatric, and cognitive deficits and is caused by an abnormal expansion of the CAG tract in the huntingtin (htt) gene. In humans, mutated htt induces a preferential loss of medium spiny neurons in the striatum and, to a lesser extent, a loss of cortical neurons as the disease progresses. The mechanisms causing these degenerative changes remain unclear, but they may involve synaptic dysregulation. We examined the activity of the corticostriatal pathway using a combination of electrophysiological and optical imaging approaches in brain slices and acutely dissociated neurons from the YAC128 mouse model of Huntington disease. The results demonstrated biphasic age-dependent changes in corticostriatal function. At 1 month, before the behavioral phenotype develops, synaptic currents and glutamate release were increased. At 7 and 12 months, after the development of the behavioral phenotype, evoked synaptic currents were reduced. Glutamate release was decreased by 7 months and was markedly reduced by 12 months. These age-dependent alterations in corticostriatal activity were paralleled by a decrease in dopamine D(2) receptor modulation of the presynaptic terminal. Together, these findings point to dynamic alterations at the corticostriatal pathway and emphasize that therapies directed toward preventing or alleviating symptoms need to be specifically designed depending on the stage of disease progression.

Neuroprotective effects of inositol 1,4,5-trisphosphate receptor C-terminal fragment in a Huntington's disease mouse model

Huntington's disease (HD) is a dominantly inherited, progressive neurodegenerative disease caused by an expanded polyglutamine tract in huntingtin protein (Htt). Medium spiny striatal neurons (MSNs) are primarily affected in HD. Mutant huntingtin protein (Htt(exp)) specifically binds to and activates type 1 inositol 1,4,5-trisphosphate receptor (InsP(3)R1), an intracellular Ca(2+) release channel. Htt(exp)-InsP(3)R1 association is mediated by a cytosolic C-terminal tail of InsP(3)R1 (a 122-aa-long IC10 fragment). To evaluate an importance of Htt(exp) association with InsP(3)R1 for HD pathology, we generated lentiviral and adeno-associated viruses expressing GFP-IC10 fusion protein and performed a series of experiments with YAC128 HD transgenic mouse. Infection with Lenti-GFP-IC10 virus stabilized Ca(2+) signaling in cultured YAC128 MSNs and protected YAC128 MSNs from glutamate-induced apoptosis. Intrastriatal injections of AAV1-GFP-IC10 significantly alleviated motor deficits and reduced MSN loss and shrinkage in YAC128 mice. Our results demonstrate an importance of InsP(3)R1-Htt(exp) association for HD pathogenesis and suggested that InsP(3)R1 is a potential therapeutic target for HD. Our data also support potential use of IC10 peptide as a novel HD therapeutic agent.

Calpain-mediated signaling mechanisms in neuronal injury and neurodegeneration

Calpain is a ubiquitous calcium-sensitive protease that is essential for normal physiologic neuronal function. However, alterations in calcium homeostasis lead to persistent, pathologic activation of calpain in a number of neurodegenerative diseases. Pathologic activation of calpain results in the cleavage of a number of neuronal substrates that negatively affect neuronal structure and function, leading to inhibition of essential neuronal survival mechanisms. In this review, we examine the mechanistic underpinnings of calcium dysregulation resulting in calpain activation in the acute neurodegenerative diseases such as cerebral ischemia and in the chronic neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, prion-related encephalopathy, and amylotrophic lateral sclerosis. The premise of this paper is that analysis of the signaling and transcriptional consequences of calpain-mediated cleavage of its various substrates for any neurodegenerative disease can be extrapolated to all of the neurodegenerative diseases vulnerable to calcium dysregulation.

Adjuvant potential of aggregate-forming polyglutamine domains

Aggregation may significantly affect the fate of a polypeptide, including its susceptibility to proteasome-dependent or autophagic degradation, its interaction with chaperones, etc. Since all these factors may affect the antigenicity of a polypeptide, we hypothesized that stimulating aggregation of an antigenic protein by its fusion to polyQ domain may enhance its antigenic potential. This hypothesis was tested with the weakly immunogenic model antigen GFP, which was fused to either long polyQ domain that triggers protein aggregation (103Q), or short polyQ domain that does not promote aggregation (25Q). Plasmids encoding control pGFP or soluble 25Q-GFP generated a very weak antibody response, while a significant increase in anti-GFP antibody titer was seen in groups immunized with DNA encoding aggregating 103Q-GFP. Similarly, fusion with 103Q strongly enhanced anti-GFP CTL activity, compared to fusion with 25Q. No apparent toxicity was observed after immunization with polyQ-GFP fusions. These data suggest that fusion of an antigen with expanded polyQ domains could have a significant adjuvant potential.

Autophagy and neurodegeneration

All cellular components are subjected to continuous surveillance by intracellular quality control systems. The major players involved in this quality control are molecular chaperones, which detect the abnormal components, and proteases, which eliminate them from the cell. Malfunctioning of the cellular surveillance systems inexorably leads to cell toxicity, and often cell death, due to the accumulation of unwanted nonfunctional components inside cells. In this work, we review the contribution of the autophagic system to cellular quality control and the consequences that autophagy malfunction has on cellular function. Special emphasis is made on the recently identified role of this system in maintenance of neuronal homeostasis and in the links currently established between alterations in the autophagic system and major neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, and Huntington's disease.

Phosphorylation of huntingtin by cyclin-dependent kinase 5 is induced by DNA damage and regulates wild-type and mutant huntingtin toxicity in neurons

Huntingtin is an antiapoptotic protein that becomes toxic when its polyglutamine stretch is expanded, resulting in Huntington's disease (HD). Protein context and posttranslational modifications regulate huntingtin toxicity. Identifying signaling pathways that act on huntingtin is, therefore, key to understanding huntingtin function in normal and pathological conditions. We show here that huntingtin is phosphorylated by the cyclin-dependent kinase 5 (Cdk5) at serines 1181 and 1201. Phosphorylation can be induced by DNA damage in vitro and in vivo. The state of huntingtin phosphorylation is a crucial regulator of neuronal cell death. Absence of phosphorylation of huntingtin at serines 1181 and 1201 confers toxic properties to wild-type huntingtin in a p53-dependent manner in striatal neurons and accelerates neuronal death induced by DNA damage. In contrast, phosphorylation at serines 1181 and 1201 protects against polyQ-induced toxicity. Finally, we show in late stages of HD a sustained DNA damage that is associated with a decrease in Cdk5/p35 levels. We propose that wild-type huntingtin is a component of the DNA damage response signal in neurons and that the Cdk5/DNA damage pathway is dysregulated in HD.