p53 mediates cellular dysfunction and behavioral abnormalities in Huntington's disease

Cancer incidence in patients with polyglutamine diseases: a population-based study in Sweden

BACKGROUND

Polyglutamine (polyQ) diseases are characterised by the expansion of CAG triplet repeats in specific genes. The accumulated encoded proteins affect the transcription of numerous transcription factors. We investigated whether polyQ diseases reduce the risk of cancer development.

METHODS

Data on patients with the polyQ diseases Huntington's disease (HD), spinobulbar muscular atrophy (SBMA), and hereditary ataxia (HA) in Sweden were linked to the Swedish Cancer Registry. We calculated standardised incidence ratios for cancers at specific sites or of specific types and the risks were compared with those in the general population. We also analysed risks in the unaffected parents of patients.

FINDINGS

In the period January, 1969, to December, 2008, we identified 1510 patients with HD, 471 with SBMA, and 3425 with HA. Cancer was diagnosed in 91 (6·0%) HD patients, 34 (7·2%) SBMA patients, and 421 (12·3%) HA patients. The standardised incidence ratios were 0·47 (95% CI 0·38-0·58), 0·65 (0·45-0·91), and 0·77 (0·70-0·85), respectively. Before diagnosis of polyQ disease, the risk of cancer was even lower. Cancer incidence and risk in the unaffected parents of patients with polyQ diseases were similar to those in the general population.

INTERPRETATION

The consistently decreased incidence of cancer in patients with polyQ diseases suggests that a common mechanism protects against the development of cancer. This feature could be related to the polyQ-tract expansion seen in these diseases. Further studies are warranted to investigate the underlying mechanisms linking cancer and polyQ diseases.

FUNDING

Swedish Cancer Society, Swedish Council for Working Life and Social Research.

N-Acetylcysteine reverses mitochondrial dysfunctions and behavioral abnormalities in 3-nitropropionic acid-induced Huntington's disease

Mitochondrial dysfunction is a major event involved in the pathogenesis of Huntington's disease (HD). The present study evaluates the role of N-acetyl-L-cysteine (NAC) in preventing mitochondrial dysfunctions in a 3-nitropropionic acid (3-NP)-induced model of HD. Administration of 3-NP to rats (Wistar strain) resulted in significant inhibition of mitochondrial complexes II, IV and V in the striatum. However, no significant effect on complex I was observed. Increased generation of reactive oxygen species and lipid peroxidation was observed in mitochondria of 3-NP-treated animals. Endogenous antioxidants (thiols and manganese-superoxide dismutase) were lowered in mitochondria of 3-NP-treated animals. 3-NP-treated animals showed increased cytosolic cytochrome c levels and mitochondrial swelling. Increased expressions of caspase-3 and p53 were also observed in 3-NP-treated animals. Histopathological examination of the striata of 3-NP-treated animals revealed increased neural space, neurodegeneration and gliosis. This was accompanied by cognitive and motor deficits. NAC treatment, on the other hand, was found to be effective in reversing 3-NP-induced mitochondrial dysfunctions and neurobehavioral deficits. Our findings suggest a beneficial effect of NAC in HD.

Genomic analysis of wig-1 pathways

Background

Wig-1 is a transcription factor regulated by p53 that can interact with hnRNP A2/B1, RNA Helicase A, and dsRNAs, which plays an important role in RNA and protein stabilization. in vitro studies have shown that wig-1 binds p53 mRNA and stabilizes it by protecting it from deadenylation. Furthermore, p53 has been implicated as a causal factor in neurodegenerative diseases based in part on its selective regulatory function on gene expression, including genes which, in turn, also possess regulatory functions on gene expression. In this study we focused on the wig-1 transcription factor as a downstream p53 regulated gene and characterized the effects of wig-1 down regulation on gene expression in mouse liver and brain.

Methods and Results

Antisense oligonucleotides (ASOs) were identified that specifically target mouse wig-1 mRNA and produce a dose-dependent reduction in wig-1 mRNA levels in cell culture. These wig-1 ASOs produced marked reductions in wig-1 levels in liver following intraperitoneal administration and in brain tissue following ASO administration through a single striatal bolus injection in FVB and BACHD mice. Wig-1 suppression was well tolerated and resulted in the reduction of mutant Htt protein levels in BACHD mouse brain but had no effect on normal Htt protein levels nor p53 mRNA or protein levels. Expression microarray analysis was employed to determine the effects of wig-1 suppression on genome-wide expression in mouse liver and brain. Reduction of wig-1 caused both down regulation and up regulation of several genes, and a number of wig-1 regulated genes were identified that potentially links wig-1 various signaling pathways and diseases.

Conclusion

Antisense oligonucleotides can effectively reduce wig-1 levels in mouse liver and brain, which results in specific changes in gene expression for pathways relevant to both the nervous system and cancer.

Role of noncoding RNAs in trinucleotide repeat neurodegenerative disorders

Increasingly complex networks of noncoding RNAs are being found to play important and diverse roles in the regulation of gene expression throughout the genome. Many lines of evidence are linking mutations and dysregulations of noncoding RNAs to a host of human diseases, and noncoding RNAs have been implicated in the molecular pathogenesis of some neurodegenerative disorders. The expansion of trinucleotide repeats is now recognized as a major cause of neurological disorders. Here we will review our current knowledge of the proposed mechanisms behind the involvement of noncoding RNAs in the molecular pathogenesis of neurodegenerative disorders, particularly the sequestration of specific RNA-binding proteins, the regulation of antisense transcripts, and the role of the microRNA pathway in the context of known neurodegenerative disorders caused by the expansion of trinucleotide repeats.

Nucleic Acid-Based Therapy Approaches for Huntington's Disease

Huntington's disease (HD) is caused by a dominant mutation that results in an unstable expansion of a CAG repeat in the huntingtin gene leading to a toxic gain of function in huntingtin protein which causes massive neurodegeneration mainly in the striatum and clinical symptoms associated with the disease. Since the mutation has multiple effects in the cell and the precise mechanism of the disease remains to be elucidated, gene therapy approaches have been developed that intervene in different aspects of the condition. These approaches include increasing expression of growth factors, decreasing levels of mutant huntingtin, and restoring cell metabolism and transcriptional balance. The aim of this paper is to outline the nucleic acid-based therapeutic strategies that have been tested to date.

Reactive oxygen/nitrogen species and their functional correlations in neurodegenerative diseases

The continuous production and efflux of reactive oxygen/nitrogen species from endogenous and exogenous sources can damage biological molecules and initiate a cascade of events. Mitochondria are pivotal in controlling cell survival and death. Cumulative oxidative stress, disrupted mitochondrial respiration, and mitochondrial damage are related with various neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, and others. Biochemical cascades of apoptosis are mediated in signaling molecules, including protein kinases and transcription factors. The expressions in the pro-apoptotic signal transduction networks may indeed promote cell death and degeneration in brain cells. The regulation of that protein phosphorylation by kinases and phosphatases is emerging as a prerequisite mechanism in the control of the apoptotic cell death program. In this review, we attempt to put forth the evidence for possible mechanistic explanations for involvement of free radicals in the pathogenesis of neurodegenerative diseases.

Mitochondria in neurodegeneration

Many neurodegenerative diseases demonstrate abnormal mitochondrial morphology and biochemical dysfunction. Alterations are often systemic rather than brain-limited. Mitochondrial dysfunction may arise as a consequence of abnormal mitochondrial DNA, mutated nuclear proteins that interact directly or indirectly with mitochondria, or through unknown causes. In most cases it is unclear where mitochondria sit in relation to the overall disease cascades that ultimately causes neuronal dysfunction and death, and there is still controversy regarding the question of whether mitochondrial dysfunction is a necessary step in neurodegeneration. In this chapter we highlight and catalogue mitochondrial perturbations in some of the major neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD). We consider data that suggest mitochondria may be critically involved in neurodegenerative disease neurodegeneration cascades.

Polyglutamine tracts as modulators of transcriptional activation from yeast to mammals

Microsatellite repeats are genetically unstable and subject to expansion and shrinkage. A subset of them, triplet repeats, can occur within the coding region and specify homomeric tracts of amino acids. Polyglutamine (polyQ) tracts are enriched in eukaryotic regulatory proteins, notably transcription factors, and we had shown before that they can contribute to transcriptional activation in mammalian cells. Here we generalize this finding by also including evolutionarily divergent organisms, namely, Drosophila and baker’s yeast. In all three systems, Gal4-based model transcription factors were more active if they harbored a polyQ tract, and the activity depended on the length of the tract. By contrast, a polyserine tract was inactive. PolyQs acted from either an internal or a C-terminal position, thus ruling out a merely structural ‘linker’ effect. Finally, a two-hybrid assay in mammalian cells showed that polyQ tracts can interact with each other, supporting the concept that a polyQ-containing transcription factor can recruit other factors with polyQ tracts or glutamine-rich activation domains. The widespread occurrence of polyQ repeats in regu­latory proteins suggests a beneficial role; in addition to the contribution to transcriptional activity, their genetic instability might help a species to adapt to changing environmental conditions in a potentially reversible manner.

Long non-coding RNAs in Huntington's disease neurodegeneration

Neurodegeneration in the brains of Huntington's disease patients is accompanied by widespread changes in gene regulatory networks. Recent studies have found that these changes are not restricted to protein-coding genes, but also include non-coding RNAs (ncRNAs). One particularly abundant but poorly understood class of ncRNAs is the long non-coding RNAs (lncRNAs), of which at least ten thousand have been identified in the human genome. Although we presently know little about their function, lncRNAs are widely expressed in the mammalian nervous system, and many are likely to play critical roles in neuronal development and activity. LncRNAs are now being implicated in neurodegenerative processes, including Alzheimer's (AD) and Huntington's disease (HD). In the present study, I discuss the potential significance of lncRNAs in HD. To support this, I have mined existing microarray data to discover seven new lncRNAs that are dysregulated in HD brains. Interestingly, several of these contain genomic binding sites for the transcriptional repressor REST, a key mediator of transcriptional changes in HD, including the known REST target lncRNA, DGCR5. Previously described lncRNAs TUG1 (necessary for retinal development) and NEAT1 (a structural component of nuclear paraspeckles) are upregulated in HD caudate, while the brain-specific tumour-suppressor MEG3 is downregulated. Three other lncRNAs of unknown function are also significantly changed in HD brains. Many lncRNAs regulate gene expression through formation of epigenetic ribonucleoprotein complexes, including TUG1 and MEG3. These findings lead me to propose that lncRNA expression changes in HD are widespread, that many of these result in altered epigenetic gene regulation in diseased neurons, and that contributes to neurodegeneration. Therefore, elucidating lncRNA network changes in HD may be important in understanding and treating this and other neurodegenerative processes.

Mitochondrial and metabolic-based protective strategies in Huntington's disease: the case of creatine and coenzyme Q

Huntington's disease (HD) is a neurodegenerative genetic disorder caused by an expansion of CAG repeats in the HD gene encoding for huntingtin (Htt), resulting in progressive death of striatal neurons, with clinical symptoms of chorea, dementia and dramatic weight loss. Metabolic and mitochondrial dysfunction caused by the expanded polyglutamine sequence have been described along with other mechanisms of neurodegeneration previously described in human tissues and animal models of HD. In this review, we focus on mitochondrial and metabolic disturbances affecting both the central nervous system and peripheral cells, including mitochondrial DNA damage, mitochondrial complexes defects, loss of calcium homeostasis and transcriptional deregulation. Glucose abnormalities have also been described in peripheral tissues of HD patients and in HD animal and cellular models. Moreover, there are no effective neuroprotective treatments available in HD. Thus, we briefly discuss the role of creatine and coenzyme Q10 that target mitochondrial dysfunction and impaired bioenergetics and have been previously used in HD clinical trials.

Emerging roles of p53 in glial cell function in health and disease

Emerging evidence suggests that p53, a tumor suppressor protein primarily involved in cancer biology, coordinates a wide range of novel functions in the CNS including the mediation of pathways underlying neurodegenerative disease pathogenesis. Moreover, an evolving concept in cell and molecular neuroscience is that glial cells are far more fundamental to disease progression than previously thought, which may occur via a noncell-autonomous mechanism that is heavily dependent on p53 activities. As a crucial hub connecting many intracellular control pathways, including cell-cycle control and apoptosis, p53 is ideally placed to coordinate the cellular response to a range of stresses. Although neurodegenerative diseases each display a distinct and diverse molecular pathology, apoptosis is a widespread hallmark feature and the multimodal capacity of the p53 system to orchestrate apoptosis and glial cell behavior highlights p53 as a potential unifying target for therapeutic intervention in neurodegeneration.

Suppression of aggregate formation of mutant huntingtin potentiates CREB-binding protein sequestration and apoptotic cell death

Although aggregates of mutant huntingtin are a pathological hallmark of Huntington's disease (HD), the role of inclusions in the pathogenesis remains inconclusive. Sequestration of CBP into mutant huntingtin has been reported to play a significant role in the pathogenesis of HD. However, whether aggregate formation of mutant huntingtin is necessary for the sequestration of CBP is not fully elucidated. In the present study, YFP was linked into either N- or C-terminus of exon 1 huntingtin to modulate the aggregation propensity of huntingtin. Efficient aggregation was observed with C-terminally YFP-tagged huntingtin (MT-YFP) whereas N-terminally YFP-tagged mutant huntingtin (YFP-MT) exhibited significantly attenuated aggregation frequency. The sequestration of CBP and apoptosis were significantly increased with YFP-MT. Microarray study showed transcriptional changes favoring apoptosis. Furthermore, expression of PGC1-α was significantly decreased with YFP-MT. The data strongly demonstrate that microscopically non-aggregate form of mutant huntingtin might exert essential pathogenic role of mutant huntingtin in HD.

Role of p53 in neurodegenerative diseases

BACKGROUND

p53 plays an important role in many areas of cellular physiology and biology, ranging from cellular development and differentiation to cell cycle arrest and apoptosis. Many of its functions are attributed to its role in assuring proper cellular division. However, since the establishment of its role in cell cycle arrest, damage repair, and apoptosis (thus also establishing its importance in cancer development), numerous reports have demonstrated additional functions of p53 in various cells. In particular, p53 appears to have important functions as it relates to neurodegeneration and synaptic plasticity.

OBJECTIVE

In this review, we will address p53 functions as it relates to various neurodegenerative diseases, mainly its implications in the development of HIV-associated neurocognitive disorders.

CONCLUSION

p53 plays a pivotal role in the development of neurodegenerative diseases through its interaction with cellular factors, viral factors, and/or small RNAs that have the ability to promote the development of these diseases. Hence, inhibition of p53 may present an ideal target to restore neuronal functions.

Ser46 phosphorylation and prolyl-isomerase Pin1-mediated isomerization of p53 are key events in p53-dependent apoptosis induced by mutant huntingtin

Huntington disease (HD) is a neurodegenerative disorder caused by a CAG repeat expansion in the gene coding for huntingtin protein. Several mechanisms have been proposed by which mutant huntingtin (mHtt) may trigger striatal neurodegeneration, including mitochondrial dysfunction, oxidative stress, and apoptosis. Furthermore, mHtt induces DNA damage and activates a stress response. In this context, p53 plays a crucial role in mediating mHtt toxic effects. Here we have dissected the pathway of p53 activation by mHtt in human neuronal cells and in HD mice, with the aim of highlighting critical nodes that may be pharmacologically manipulated for therapeutic intervention. We demonstrate that expression of mHtt causes increased phosphorylation of p53 on Ser46, leading to its interaction with phosphorylation-dependent prolyl isomerase Pin1 and consequent dissociation from the apoptosis inhibitor iASPP, thereby inducing the expression of apoptotic target genes. Inhibition of Ser46 phosphorylation by targeting homeodomain-interacting protein kinase 2 (HIPK2), PKCδ, or ataxia telangiectasia mutated kinase, as well as inhibition of the prolyl isomerase Pin1, prevents mHtt-dependent apoptosis of neuronal cells. These results provide a rationale for the use of small-molecule inhibitors of stress-responsive protein kinases and Pin1 as a potential therapeutic strategy for HD treatment.

Mass spectrometric identification of novel lysine acetylation sites in huntingtin

Huntingtin (Htt) is a protein with a polyglutamine stretch in the N-terminus and expansion of the polyglutamine stretch causes Huntington's disease (HD). Htt is a multiple domain protein whose function has not been well characterized. Previous reports have shown, however, that post-translational modifications of Htt such as phosphorylation and acetylation modulate mutant Htt toxicity, localization, and vesicular trafficking. Lysine acetylation of Htt is of particular importance in HD as this modification regulates disease progression and toxicity. Treatment of mouse models with histone deacetylase inhibitors ameliorates HD-like symptoms and alterations in acetylation of Htt promotes clearance of the protein. Given the importance of acetylation in HD and other diseases, we focused on the systematic identification of lysine acetylation sites in Htt23Q (1-612) in a cell culture model using mass spectrometry. Myc-tagged Htt23Q (1-612) overexpressed in the HEK 293T cell line was immunoprecipitated, separated by SDS-PAGE, digested and subjected to high performance liquid chromatography tandem MS analysis. Five lysine acetylation sites were identified, including three novel sites Lys-178, Lys-236, Lys-345 and two previously described sites Lys-9 and Lys-444. Antibodies specific to three of the Htt acetylation sites were produced and confirmed the acetylation sites in Htt. A multiple reaction monitoring MS assay was developed to compare quantitatively the Lys-178 acetylation level between wild-type Htt23Q and mutant Htt148Q (1-612). This report represents the first comprehensive mapping of lysine acetylation sites in N-terminal region of Htt.

Targeted disruption of neuronal 19S proteasome subunits induces the formation of ubiquitinated inclusions in the absence of cell death

Proteasome-mediated proteolysis is a major protein degradation mechanism in cells and its dysfunction has been implicated in the pathogenesis of several neurodegenerative diseases, each with the common features of neuronal death and formation of ubiquitinated inclusions found within neurites, the cell body, or nucleus. Previous models of proteasome dysfunction have employed pharmacological inhibition of the catalytic subunits of the 20S proteasome core, or the genetic manipulation of specific subunits resulting in altered proteasome assembly. In this study, we report the use of dominant negative subunits of the 19S regulatory proteasome complex that mediate the recognition of ubiquitinated substrates as well as the removal of the poly-ubiquitin chain. Interestingly, while each mutant subunit-induced inclusion formation, like that seen with pharmacological inhibition of the 20S proteasome, none was able to induce apoptotic death, or trigger activation of macroautophagy, in either dopaminergic cell lines or primary cortical neurons. This finding highlights the dissociation between the mechanisms of neuronal inclusion formation and the induction of cell death, and represents a novel cellular model for Lewy body-like inclusion formation in neurons.

p53 pro-oxidant activity in the central nervous system: implication in aging and neurodegenerative diseases

Recent advances in delineating the biological functions of p53 had shed the light on its key role in the multifacets of cellular homeostasis. After its activation, via DNA damage, oxidative stress, or aberrant expression of oncogenes, p53 transduces its classical effect through several mechanisms comprising activation of the DNA repair machinery, cell cycle arrest, and initiation of apoptosis or senescence. In the mammalian brain, p53 plays critical functions in normal development, tumor suppression, neurodegenerative diseases, and aging. Herein, we focus on the constitutive pro-oxidant activity of p53 in neurons and discuss the potential implication of this finding in the context of neurodegenerative diseases and normal brain aging.

Regulation of miR-146a by RelA/NFkB and p53 in STHdh(Q111)/Hdh(Q111) cells, a cell model of Huntington's disease

Huntington's disease (HD) is caused by the expansion of N-terminal polymorphic poly Q stretch of the protein huntingtin (HTT). Deregulated microRNAs and loss of function of transcription factors recruited to mutant HTT aggregates could cause characteristic transcriptional deregulation associated with HD. We observed earlier that expressions of miR-125b, miR-146a and miR-150 are decreased in STHdh^Q111/Hdh^Q111 cells, a model for HD in comparison to those of wild type STHdh^Q7/Hdh^Q7 cells. In the present manuscript, we show by luciferase reporter assays and real time PCR that decreased miR-146a expression in STHdh^Q111/Hdh^Q111 cells is due to decreased expression and activity of p65 subunit of NFkB (RelA/NFkB). By reporter luciferase assay, RT-PCR and western blot analysis, we also show that both miR-150 and miR-125b target p53. This partially explains the up regulation of p53 observed in HD. Elevated p53 interacts with RelA/NFkB, reduces its expression and activity and decreases the expression of miR-146a, while knocking down p53 increases RelA/NFkB and miR-146a expressions. We also demonstrate that expression of p53 is increased and levels of RelA/NFkB, miR-146a, miR-150 and miR-125b are decreased in striatum of R6/2 mice, a mouse model of HD and in cell models of HD. In a cell model, this effect could be reversed by exogenous expression of chaperone like proteins HYPK and Hsp70. We conclude that (i) miR-125b and miR-150 target p53, which in turn regulates RelA/NFkB and miR-146a expressions; (ii) reduced miR-125b and miR-150 expressions, increased p53 level and decreased RelA/NFkB and miR-146a expressions originate from mutant HTT (iii) p53 directly or indirectly regulates the expression of miR-146a. Our observation of interplay between transcription factors and miRNAs using HD cell model provides an important platform upon which further work is to be done to establish if such regulation plays any role in HD pathogenesis.

Huntington's Disease and Striatal Signaling

Huntington’s Disease (HD) is the most frequent neurodegenerative disease caused by an expansion of polyglutamines (CAG). The main clinical manifestations of HD are chorea, cognitive impairment, and psychiatric disorders. The transmission of HD is autosomal dominant with a complete penetrance. HD has a single genetic cause, a well-defined neuropathology, and informative pre-manifest genetic testing of the disease is available. Striatal atrophy begins as early as 15 years before disease onset and continues throughout the period of manifest illness. Therefore, patients could theoretically benefit from therapy at early stages of the disease. One important characteristic of HD is the striatal vulnerability to neurodegeneration, despite similar expression of the protein in other brain areas. Aggregation of the mutated Huntingtin (HTT), impaired axonal transport, excitotoxicity, transcriptional dysregulation as well as mitochondrial dysfunction, and energy deficits, are all part of the cellular events that underlie neuronal dysfunction and striatal death. Among these non-exclusive mechanisms, an alteration of striatal signaling is thought to orchestrate the downstream events involved in the cascade of striatal dysfunction.

Mutant Huntingtin induces activation of the Bcl-2/adenovirus E1B 19-kDa interacting protein (BNip3)

Huntington's disease (HD) is a neurodegenerative disorder characterized by progressive neuronal death in the basal ganglia and cortex. Although increasing evidence supports a pivotal role of mitochondrial dysfunction in the death of patients' neurons, the molecular bases for mitochondrial impairment have not been elucidated. We provide the first evidence of an abnormal activation of the Bcl-2/adenovirus E1B 19-kDa interacting protein 3 (BNip3) in cells expressing mutant Huntingtin. In this study, we show an abnormal accumulation and dimerization of BNip3 in the mitochondria extracted from human HD muscle cells, HD model cell cultures and brain tissues from HD model mice. Importantly, we have shown that blocking BNip3 expression and dimerization restores normal mitochondrial potential in human HD muscle cells. Our data shed light on the molecular mechanisms underlying mitochondrial dysfunction in HD and point to BNip3 as a new potential target for neuroprotective therapy in HD.

Forebrain striatal-specific expression of mutant huntingtin protein in vivo induces cell-autonomous age-dependent alterations in sensitivity to excitotoxicity and mitochondrial function

HD (Huntington's disease) is characterized by dysfunction and death of striatal MSNs (medium-sized spiny neurons). Excitotoxicity, transcriptional dysregulation and mitochondrial abnormalities are among the mechanisms that are proposed to play roles in HD pathogenesis. To determine the extent of cell-autonomous effects of mhtt (mutant huntingtin) protein on vulnerability to excitotoxic insult in MSNs in vivo, we measured the number of degenerating neurons in response to intrastriatal injection of QA (quinolinic acid) in presymptomatic and symptomatic transgenic (D9-N171-98Q, also known as DE5) mice that express mhtt in MSNs but not in cortex. After QA, the number of degenerating neurons in presymptomatic DE5 mice was not significantly different from the number in WT (wild-type) controls, suggesting the early, increased vulnerability to excitotoxicity demonstrated in other HD mouse models has a largely non-cell-autonomous component. Conversely, symptomatic DE5 mice showed significantly fewer degenerating neurons relative to WT, implying the resistance to excitotoxicity observed at later ages has a primarily cell-autonomous origin. Interestingly, mitochondrial complex II respiration was enhanced in striatum of symptomatic mice, whereas it was reduced in presymptomatic mice, both relative to their age-matched controls. Consistent with the QA data, MSNs from symptomatic mice showed decreased NMDA (N-methyl-d-aspartate) currents compared with age-matched controls, suggesting that in addition to aging, cell-autonomous mechanisms mitigate susceptibility to excitotoxicity in the symptomatic stage. Also, symptomatic DE5 mice did not display some of the electrophysiological alterations present in other HD models, suggesting that blocking the expression of mhtt in cortical neurons may restore corticostriatal function in HD.

Transcription factor p53 influences microglial activation phenotype

Several neurodegenerative diseases are influenced by the innate immune response in the central nervous system (CNS). Microglia have proinflammatory and subsequently neurotoxic actions as well as anti-inflammatory functions that promote recovery and repair. Very little is known about the transcriptional control of these specific microglial behaviors. We have previously shown that in HIV-associated neurocognitive disorders (HAND), the transcription factor p53 accumulates in microglia and that microglial p53 expression is required for the in vitro neurotoxicity of the HIV coat glycoprotein gp120. These findings suggested a novel function for p53 in regulating microglial activation. Here, we report that in the absence of p53, microglia demonstrate a blunted response to interferon-γ, failing to increase expression of genes associated with classical macrophage activation or secrete proinflammatory cytokines. Microarray analysis of global gene expression profiles revealed increased expression of genes associated with anti-inflammatory functions, phagocytosis, and tissue repair in p53 knockout (p53(-/-)) microglia compared with those cultured from strain matched p53 expressing (p53(+/+)) mice. We further observed that p53(-/-) microglia demonstrate increased phagocytic activity in vitro and expression of markers for alternative macrophage activation both in vitro and in vivo. In HAND brain tissue, the alternative activation marker CD163 was expressed in a separate subset of microglia than those demonstrating p53 accumulation. These data suggest that p53 influences microglial behavior, supporting the adoption of a proinflammatory phenotype, while p53 deficiency promotes phagocytosis and gene expression associated with alternative activation and anti-inflammatory functions.

The role of IκB kinase complex in the neurobiology of Huntington's disease

The IκB kinase β (IKKβ) is a prominent regulator of neuroinflammation, which is implicated in the pathogenesis of Huntington's disease (HD). Inflammatory mediators accumulate in the serum and CNS of premanifest and manifest HD patients, and cytokine levels correlate with disease progression. IKKβ may also directly regulate the neurotoxicity of huntingtin (Htt). Activation of IKKβ by DNA damage triggers caspase-dependent cleavage of WT and mutant Htt and enhances the accumulation of oligomeric fragments. Moreover, the N-terminal fragments of mutant Htt (HDx1) directly bind to and activate IKKβ. Thus, the IKKβ-dependent cleavage of full-length mutant Htt and the buildup of HDx1 could form a deleterious feed-forward loop. Elevated IKKβ activity is present throughout the CNS in a symptomatic mouse model of HD expressing HDx1, whereas in asymptomatic mice with full-length mutant Htt, it is confined to the striatum. IKKβ could also influence the phosphorylation of Htt at Ser13 and Ser16, which is linked to HD pathology. IKKβ inhibitors ameliorate the toxicity of mutant Htt in striatal neurons and prevent DNA damage-induced Htt cleavage. Inhibition of IKKβ in the CNS also reduces neuroinflammation and imparts neuroprotection in a chemical model of HD. These findings support an active role for IKKβ in HD pathogenesis and represent an example of how gene-environment (exemplified by DNA damage and inflammation) interactions can influence Htt neurotoxicity. We will summarize these findings and describe the therapeutic potentials of IKKβ for HD.

Hsa-miR-34b is a plasma-stable microRNA that is elevated in pre-manifest Huntington's disease

Huntington's disease (HD) is a devastating, neurodegenerative condition, which lacks effective treatment. Normal Huntingtin (HTT) and mutant Huntingtin (mHTT) are expressed in multiple tissues and can alter transcription of microRNAs (miRs). Importantly, miRs are present in a bio-stable form in human peripheral blood plasma and have recently been shown to be useful biomarkers in other diseases. We therefore sought to identify potential miR biomarkers of HD that are present in, and have functional consequences for, neuronal and non-neuronal tissues. In a cell line over-expressing mHTT-Exon-1, miR microarray analysis was used to identify candidate miRs. We then examined their presence and bio-stability in control and HD plasma. We found that miR-34b is significantly elevated in response to mHTT-Exon-1, and its blockade alters the toxicity of mHTT-Exon-1 in vitro. We also show that miR-34b is detectable in plasma from small input volumes and is insensitive to freeze-thaw-induced RNA degradation. Interestingly, miR-34b is significantly elevated in plasma from HD gene carriers prior to symptom onset. This is the first study suggesting that plasma miRs might be used as biomarkers for HD.

Nucleolar disruption in dopaminergic neurons leads to oxidative damage and parkinsonism through repression of mammalian target of rapamycin signaling

The nucleolus represents an essential stress sensor for the cell. However, the molecular consequences of nucleolar damage and their possible link with neurodegenerative diseases remain to be elucidated. Here, we show that nucleolar damage is present in both genders in Parkinson's disease (PD) and in the pharmacological PD model induced by the neurotoxin 1,2,3,6-tetrahydro-1-methyl-4-phenylpyridine hydrochloride (MPTP). Mouse mutants with nucleolar disruption restricted to dopaminergic (DA) neurons show phenotypic alterations that resemble PD, such as progressive and differential loss of DA neurons and locomotor abnormalities. At the molecular level, nucleolar disruption results in increased p53 levels and downregulation of mammalian target of rapamycin (mTOR) activity, leading to mitochondrial dysfunction and increased oxidative stress, similar to PD. In turn, increased oxidative stress induced by MPTP causes mTOR and ribosomal RNA synthesis inhibition. Collectively, these observations suggest that the interplay between nucleolar dysfunction and increased oxidative stress, involving p53 and mTOR signaling, may constitute a destructive axis in experimental and sporadic PD.

Transcription-induced DNA toxicity at trinucleotide repeats: double bubble is trouble

Trinucleotide repeats (TNR) are a blessing and a curse. In coding regions, where they are enriched, short repeats offer the potential for continuous, rapid length variation with linked incremental changes in the activity of the encoded protein, a valuable source of variation for evolution. But at the upper end of these benign and beneficial lengths, trinucleotide repeats become very unstable, with a dangerous bias toward continual expansion, which can lead to neurological diseases in humans. The mechanisms of expansion are varied and the links to disease are complex. Where they have been delineated, however, they have often revealed unexpected, fundamental aspects of the underlying cell biology. Nowhere is this more apparent than in recent studies, which indicate that expanded CAG repeats can form toxic sites in the genome, which can, upon interaction with normal components of DNA metabolism, trigger cell death. Here we discuss the phenomenon of TNR-induced DNA toxicity, with special emphasis on the role of transcription. Transcription-induced DNA toxicity may have profound biological consequences, with particular relevance to repeat-associated neurodegenerative diseases.

Telomere dysfunction induces metabolic and mitochondrial compromise

Telomere dysfunction activates p53-mediated cellular growth arrest, senescence and apoptosis to drive progressive atrophy and functional decline in high-turnover tissues. The broader adverse impact of telomere dysfunction across many tissues including more quiescent systems prompted transcriptomic network analyses to identify common mechanisms operative in haematopoietic stem cells, heart and liver. These unbiased studies revealed profound repression of peroxisome proliferator-activated receptor gamma, coactivator 1 alpha and beta (PGC-1α and PGC-1β, also known as Ppargc1a and Ppargc1b, respectively) and the downstream network in mice null for either telomerase reverse transcriptase (Tert) or telomerase RNA component (Terc) genes. Consistent with PGCs as master regulators of mitochondrial physiology and metabolism, telomere dysfunction is associated with impaired mitochondrial biogenesis and function, decreased gluconeogenesis, cardiomyopathy, and increased reactive oxygen species. In the setting of telomere dysfunction, enforced Tert or PGC-1α expression or germline deletion of p53 (also known as Trp53) substantially restores PGC network expression, mitochondrial respiration, cardiac function and gluconeogenesis. We demonstrate that telomere dysfunction activates p53 which in turn binds and represses PGC-1α and PGC-1β promoters, thereby forging a direct link between telomere and mitochondrial biology. We propose that this telomere-p53-PGC axis contributes to organ and metabolic failure and to diminishing organismal fitness in the setting of telomere dysfunction.

The importance of integrating basic and clinical research toward the development of new therapies for Huntington disease

Huntington disease (HD) is a dominantly inherited neurodegenerative disorder that results from expansion of the polyglutamine repeat in the huntingtin (HTT) gene. There are currently no effective treatments for this devastating disease. Given its monogenic nature, disease modification therapies for HD should be theoretically feasible. Currently, pharmacological therapies aimed at disease modification by altering levels of HTT protein are in late-stage preclinical development. Here, we review current efforts to develop new treatments for HD based on our current understanding of HTT function and the main pathological mechanisms. We emphasize the need to enhance translational efforts and highlight the importance of aligning the clinical and basic research communities to validate existing hypotheses in clinical studies. Human and animal therapeutic trials are presented with an emphasis on cellular and molecular mechanisms relevant to disease progression.

Energy deficit in Huntington disease: why it matters

Huntington disease (HD) is an autosomal dominant neurodegenerative disease with complete penetrance. Although the understanding of the cellular mechanisms that drive neurodegeneration in HD and account for the characteristic pattern of neuronal vulnerability is incomplete, defects in energy metabolism, particularly mitochondrial function, represent a common thread in studies of HD pathogenesis in humans and animal models. Here we review the clinical, biochemical, and molecular evidence of an energy deficit in HD and discuss the mechanisms underlying mitochondrial and related alterations.

Does Huntingtin play a role in selective macroautophagy?

The accumulation of protein aggregates in neurons appears to be a basic feature of neurodegenerative disease. In Huntington's Disease (HD), a progressive and ultimately fatal neurodegenerative disorder caused by an expansion of the polyglutamine repeat within the protein Huntingtin (Htt), the immediate proximal cause of disease is well understood. However, the cellular mechanisms which modulate the rate at which fragments of Htt containing polyglutamine accumulate in neurons is a central issue in the development of approaches to modulate the rate and extent of neuronal loss in this disease. We have recently found that Htt is phosphorylated by the kinase IKK on serine (S) 13, activating its phosphorylation on S16 and its acetylation and poly-SUMOylation, modifications that modulate its clearance by the proteasome and lysosome in cells. In the discussion here I suggest that Htt may have a normal function in the lysosomal mechanism of selective macroautophagy involved in its own degradation which may share some similarity with the yeast cytoplasm to vacuole targeting (Cvt) pathway. Pharmacologic activation of this pathway may be useful early in disease progression to treat HD and other neurodegenerative diseases characterized by the accumulation of disease proteins.

Glutamate-induced alterations in Ca2+ signaling are modulated by mitochondrial Ca2+ handling capacity in brain slices of R6/1 transgenic mice

Huntington's disease is a neurodegenerative disorder caused by an expansion of CAGs repeats and characterized by alterations in mitochondrial functions. Although changes in Ca(2+) handling have been suggested, the mechanisms involved are not completely understood. The aim of this study was to investigate the possible alterations in Ca(2+) handling capacity and the relationship with mitochondrial dysfunction evaluated by NAD(P)H fluorescence, reactive oxygen species levels, mitochondrial membrane potential (DeltaPsi(m)) measurements and respiration in whole brain slices from R6/1 mice of different ages, evaluated in situ by real-time real-space microscopy. We show that the cortex and striatum of the 9-month-old R6/1 transgenic mice present a significant sustained increase in cytosolic Ca(2+) induced by glutamate (Glu). This difference in Glu response was partially reduced in R6/1 when in the absence of extracellular Ca(2+), indicating that N-methyl-D-aspartate receptors participation in this response is more important in transgenic mice. In addition, Glu also lead to a decrease in NAD(P)H fluorescence, a loss in DeltaPsi(m) and a further increase in respiration, which may have evoked a decrease in mitochondrial Ca(2+) Ca(2+)(m) uptake capacity. Taken together, these results show that alterations in Ca(2+) homeostasis in transgenic mice are associated with a decrease in Ca(2+)(m) uptake mechanism with a diminished Ca(2+) handling ability that ultimately causes dysfunctions and worsening of the neurodegenerative and the disease processes.

Molecular biology of Huntington's disease

It has been more than 17 years since the causative mutation for Huntington's disease was discovered as the expansion of the triplet repeat in the N-terminal portion of the Huntingtin (HTT) gene. In the intervening time, researchers have discovered a great deal about Huntingtin's involvement in a number of cellular processes. However, the role of Huntingtin in the key pathogenic mechanism leading to neurodegeneration in the disease process has yet to be discovered. Here, we review the body of knowledge that has been uncovered since gene discovery and include discussions of the HTT gene, CAG triplet repeat expansion, HTT expression, protein features, posttranslational modifications, and many of its known protein functions and interactions. We also highlight potential pathogenic mechanisms that have come to light in recent years.

Pathogenic mechanisms in Huntington's disease

Huntington's disease (HD) is an autosomal dominant, progressive neurodegenerative disorder presenting in midlife. Multiple pathogenic mechanisms which hypothesise how the expanded CAG repeat causes manifest disease have been suggested since the mutation was first detected. These mechanisms include events that operate at both the gene and protein levels. It has been proposed that somatic instability of the CAG repeat could underlie the striatal-specific pathology observed in HD, although how this occurs and what consequences this has in the disease state remain unknown. The form in which the Htt protein exists within the cell has been extensively studied in terms of both its role in aggregate formation and its cellular processing. Protein-protein interactions, post-translational modifications and protein cleavage have all been suggested to contribute to HD pathogenesis. The potential downstream effects of the mutant Htt protein are also noted here. In particular, the adverse effect of the mutant Htt protein on cellular protein degradation, subcellular transport and transcription are explored, and its role in energy metabolism and excitotoxicity investigated. Elucidating the mechanisms at work in HD pathogenesis and determining when they occur in relation to disease is an important step in the pathway to therapeutic interventions.

Transgenic Animal Models of Huntington's Disease

Huntington's disease (HD) is a devastating neurodegenerative disorder that currently has no cure. In order to develop effective treatment, an understanding of HD pathogenesis and the evaluation of therapeutic efficacy of novel medications with the aid of animal models are critical steps. Transgenic animals sharing similar genetic defects that lead to HD have provided important discoveries in HD mechanisms that cell models are not able to replicate, which include psychiatric impairment, cognitive behavioral impact, and motor functions. Although transgenic HD rodent models have been widely used in HD research, it is clear that an animal model with comparable physiology to man, similar genetic defects that lead to HD, and the ability to develop similar cognitive and behavioral impairments is critical for explaining HD pathogenesis and the development of cures. Compared to HD rodents, HD transgenic nonhuman primates have not only developed comparable neuropathology but also present HD clinical features such as rigidity, seizure, dystonia, bradykinesia, and chorea that no other animal model has been able to replicate. Distinctive degenerating neurons and the accumulation of neuropil aggregates observed in HD monkey brain strongly support the hypothesis that the unique neuropathogenic events seen in HD monkey brain recapitulate HD in man. The latest development of transgenic HD primates has opened a new era of animal modeling that better represents human genetic disorders such as HD, which will accelerate the development of diagnostic tools and identifying novel biomarkers through longitudinal studies including gene expression and metabolite profiling, and noninvasive imaging. Furthermore, novel treatments with predictable efficacy in human patients can be developed using HD monkeys because of comparable neuropathology and clinical features.

Cancer and neurodegeneration: between the devil and the deep blue sea

Cancer and neurodegeneration are often thought of as disease mechanisms at opposite ends of a spectrum; one due to enhanced resistance to cell death and the other due to premature cell death. There is now accumulating evidence to link these two disparate processes. An increasing number of genetic studies add weight to epidemiological evidence suggesting that sufferers of a neurodegenerative disorder have a reduced incidence for most cancers, but an increased risk for other cancers. Many of the genes associated with either cancer and/or neurodegeneration play a central role in cell cycle control, DNA repair, and kinase signalling. However, the links between these two families of diseases remain to be proven. In this review, we discuss recent and sometimes as yet incomplete genetic discoveries that highlight the overlap of molecular pathways implicated in cancer and neurodegeneration.

Rescue of gene expression by modified REST decoy oligonucleotides in a cellular model of Huntington's disease

Transcriptional dysfunction is a prominent hallmark of Huntington's disease (HD). Several transcription factors have been implicated in the aetiology of HD progression and one of the most prominent is repressor element 1 (RE1) silencing transcription factor (REST). REST is a global repressor of neuronal gene expression and in the presence of mutant Huntingtin increased nuclear REST levels lead to elevated RE1 occupancy and a concomitant increase in target gene repression, including brain-derived neurotrophic factor. It is of great interest to devise strategies to reverse transcriptional dysregulation caused by increased nuclear REST and determine the consequences in HD. Thus far, such strategies have involved RNAi or mutant REST constructs. Decoys are double-stranded oligodeoxynucleotides corresponding to the DNA-binding element of a transcription factor and act to sequester it, thereby abrogating its transcriptional activity. Here, we report the use of a novel decoy strategy to rescue REST target gene expression in a cellular model of HD. We show that delivery of the decoy in cells expressing mutant Huntingtin leads to its specific interaction with REST, a reduction in REST occupancy of RE1s and rescue of target gene expression, including Bdnf. These data point to an alternative strategy for rebalancing the transcriptional dysregulation in HD.

p53-mediated apoptosis requires inositol hexakisphosphate kinase-2

Inositol pyrophosphates have been implicated in numerous biological processes. Inositol hexakisphosphate kinase-2 (IP6K2), which generates the inositol pyrophosphate, diphosphoinositol pentakisphosphate (IP7), influences apoptotic cell death. The tumor suppressor p53 responds to genotoxic stress by engaging a transcriptional program leading to cell-cycle arrest or apoptosis. We demonstrate that IP6K2 is required for p53-mediated apoptosis and modulates the outcome of the p53 response. Gene disruption of IP6K2 in colorectal cancer cells selectively impairs p53-mediated apoptosis, instead favoring cell-cycle arrest. IP6K2 acts by binding directly to p53 and decreasing expression of proarrest gene targets such as the cyclin-dependent kinase inhibitor p21.

Allele-selective inhibition of mutant huntingtin expression with antisense oligonucleotides targeting the expanded CAG repeat

Huntington's disease (HD) is a currently incurable neurodegenerative disease caused by the expansion of a CAG trinucleotide repeat within the huntingtin (HTT) gene. Therapeutic approaches include selectively inhibiting the expression of the mutated HTT allele while conserving function of the normal allele. We have evaluated a series of antisense oligonucleotides (ASOs) targeted to the expanded CAG repeat within HTT mRNA for their ability to selectively inhibit expression of mutant HTT protein. Several ASOs incorporating a variety of modifications, including bridged nucleic acids and phosphorothioate internucleotide linkages, exhibited allele-selective silencing in patient-derived fibroblasts. Allele-selective ASOs did not affect the expression of other CAG repeat-containing genes and selectivity was observed in cell lines containing minimal CAG repeat lengths representative of most HD patients. Allele-selective ASOs left HTT mRNA intact and did not support ribonuclease H activity in vitro. We observed cooperative binding of multiple ASO molecules to CAG repeat-containing HTT mRNA transcripts in vitro. These results are consistent with a mechanism involving inhibition at the level of translation. ASOs targeted to the CAG repeat of HTT provide a starting point for the development of oligonucleotide-based therapeutics that can inhibit gene expression with allelic discrimination in patients with HD.

Altered microRNA regulation in Huntington's disease models

Huntington's disease (HD) is a genetic neurodegenerative disease caused by abnormal CAG expansion. MicroRNAs (miRNAs) are short RNA molecules regulating gene expression, and are implicated in a variety of diseases including HD. However, the profiles and regulation of miRNAs in HD are not fully understood. Here, we analyzed the miRNA expression and miRNA regulators in two transgenic models of HD, YAC128 and R6/2 mice, and in a 3-nitropropionic acid (3NP)-induced striatal degeneration rat model. After characterizing the phenotypes by behavioral tests and histological analyses, we profiled striatal miRNAs using a miRNA microarray and we measured the key molecules involved in miRNA biogenesis and function. YAC128 mice showed upregulation-dominant miRNA expressions at 5 months and downregulation-dominant expressions at 12 months. Concomitantly, the expressions of Drosha-DGCR8, Exportin-5, and Dcp1 were increased at 5months, and the expression of Dicer was decreased at 12 months. In 10-week-old R6/2 mice, downregulation was dominant in the miRNA expressions and the level of Drosha decreased concomitantly. Nine miRNAs (miR-22, miR-29c, miR-128, miR-132, miR-138, miR-218, miR-222, miR-344, and miR-674*) were commonly down-regulated in both the 12-month-old YAC128 and 10-week-old R6/2 mice. Meanwhile, 3NP rats showed dynamic changes in the miRNA profiles during disease development and a few miRNAs with altered expression. Our results show that transgenic HD mice have abnormal miRNA biogenesis. This information should aid in future studies on therapeutic application of miRNAs in HD.

Hairless expression attenuates apoptosis in a mouse model and the COS cell line; involvement of p53

BACKGROUND

Neurons are more likely to die through apoptosis in the immature brain after injury whereas adult neurons in the mature brain die by necrosis. Several studies have suggested that this maturational change in the mechanism of cell death is regulated, in part, by thyroid hormone. We examined the involvement of the hairless (Hr) gene which has been suspected of having a role in cell cycle regulation and apoptosis in the hair follicle and is strongly regulated by the thyroid hormone in the brain.

METHODOLOGY

Forced expression of Hr by transfection decreased the number of apoptotic nuclei, levels of caspase-3 activity, and cytosolic cytochrome C in COS cells exposed to staurosporine and tunicamycin. Similarly, caspase-3 activity was lower and the decrease in mitochondrial membrane potential was smaller in cultures of adult cerebellar granule neurons from wild type mice compared to Hr knockout mice induced to undergo apoptosis. In vivo, apoptosis as detected by positive TUNEL labeling and caspase 3 activity was lower in wild-type mice compared to Hr knockouts after exposure to trimethyltin. Hr expression lowered levels of p53, p53 mediated reporter gene activity, and lower levels of the pro-apoptotic Bcl2 family member Bax in COS cells. Finally, Hr expression did not attenuate apoptosis in mouse embryonic fibroblasts from p53 knockout mice but was effective in mouse embryonic fibroblasts from wild type mice.

CONCLUSIONS/SIGNIFICANCE

Overall, our studies demonstrate that Hr evokes an anti-apoptotic response by repressing expression of p53 and pro-apoptotic events regulated by p53.