IKKalpha and IKKbeta regulation of DNA damage-induced cleavage of huntingtin

Therapeutic approaches targeting aging and cellular senescence in Huntington's disease

Huntington's disease (HD) is a devastating neurodegenerative disease that is manifested by a gradual loss of physical, cognitive, and mental abilities. As the disease advances, age has a major impact on the pathogenic signature of mutant huntingtin (mHTT) protein aggregation. This review aims to explore the intricate relationship between aging, mHTT toxicity, and cellular senescence in HD. Scientific data on the interplay between aging, mHTT, and cellular senescence in HD were collected from several academic databases, including PubMed, Google Scholar, Google, and ScienceDirect. The search terms employed were "AGING," "HUNTINGTON'S DISEASE," "MUTANT HUNTINGTIN," and "CELLULAR SENESCENCE." Additionally, to gather information on the molecular mechanisms and potential therapeutic targets, the search was extended to include relevant terms such as "DNA DAMAGE," "OXIDATIVE STRESS," and "AUTOPHAGY." According to research, aging leads to worsening HD pathophysiology through some processes. As a result of the mHTT accumulation, cellular senescence is promoted, which causes DNA damage, oxidative stress, decreased autophagy, and increased inflammatory responses. Pro-inflammatory cytokines and other substances are released by senescent cells, which may worsen the neuronal damage and the course of the disease. It has been shown that treatments directed at these pathways reduce some of the HD symptoms and enhance longevity in experimental animals, pointing to a new possibility of treating the condition. Through their amplification of the harmful effects of mHTT, aging and cellular senescence play crucial roles in the development of HD. Comprehending these interplays creates novel opportunities for therapeutic measures targeted at alleviating cellular aging and enhancing HD patients' quality of life.

IKBKB reduces huntingtin aggregation by phosphorylating serine 13 via a non-canonical IKK pathway

N-terminal phosphorylation at residues T3 and S13 is believed to have important beneficial implications for the biological and pathological properties of mutant huntingtin, where inhibitor of nuclear factor kappa B kinase subunit beta (IKBKB) was identified as a candidate regulator of huntingtin N-terminal phosphorylation. The paucity of mechanistic information on IKK pathways, together with the lack of sensitive methods to quantify endogenous huntingtin phosphorylation, prevented detailed study of the role of IKBKB in Huntington's disease. Using novel ultrasensitive assays, we demonstrate that IKBKB can regulate endogenous S13 huntingtin phosphorylation in a manner, dependent on its kinase activity and known regulators. We found that the ability of IKBKB to phosphorylate endogenous huntingtin S13 is mediated through a non-canonical interferon regulatory factor3-mediated IKK pathway, distinct from the established involvement of IKBKB in mutant huntingtin's pathological mechanisms mediated via the canonical pathway. Furthermore, increased huntingtin S13 phosphorylation by IKBKB resulted in decreased aggregation of mutant huntingtin in cells, again dependent on its kinase activity. These findings point to a non-canonical IKK pathway linking S13 huntingtin phosphorylation to the pathological properties of mutant huntingtin aggregation, thought to be significant to Huntington's disease.

Analysis of Huntington's Disease Modifiers Using the Hyperbolic Mapping of the Protein Interaction Network

Huntington's disease (HD) is caused by the production of a mutant huntingtin (HTT) with an abnormally long poly-glutamine (polyQ) tract, forming aggregates and inclusions in neurons. Previous work by us and others has shown that an increase or decrease in polyQ-triggered aggregates can be passive simply due to the interaction of proteins with the aggregates. To search for proteins with active (functional) effects, which might be more effective in finding therapies and mechanisms of HD, we selected among the proteins that interact with HTT a total of 49 pairs of proteins that, while being paralogous to each other (and thus expected to have similar passive interaction with HTT), are located in different regions of the protein interaction network (suggesting participation in different pathways or complexes). Three of these 49 pairs contained members with opposite effects on HD, according to the literature. The negative members of the three pairs, MID1, IKBKG, and IKBKB, interact with PPP2CA and TUBB, which are known negative factors in HD, as well as with HSP90AA1 and RPS3. The positive members of the three pairs interact with HSPA9. Our results provide potential HD modifiers of functional relevance and reveal the dynamic aspect of paralog evolution within the interaction network.

IKKβ signaling mediates metabolic changes in the hypothalamus of a Huntington disease mouse model

Huntington disease (HD) is a neurodegenerative disorder caused by a CAG repeat expansion in the huntingtin (HTT) gene. Metabolic changes are associated with HD progression, but underlying mechanisms are not fully known. As the IKKβ/NF-κB pathway is an essential regulator of metabolism, we investigated the involvement of IKKβ, the upstream activator of NF-κB in hypothalamus-specific HD metabolic changes. We expressed amyloidogenic N-terminal fragments of mutant HTT (mHTT) in the hypothalamus of mice with brain-specific ablation of IKKβ (Nestin/IKKβ^lox/lox) and control mice (IKKβ^lox/lox). We assessed effects on body weight, metabolic hormones, and hypothalamic neuropathology. Hypothalamic expression of mHTT led to an obese phenotype only in female mice. CNS-specific inactivation of IKKβ prohibited weight gain in females, which was independent of neuroprotection and microglial activation. Our study suggests that mHTT in the hypothalamus causes metabolic imbalance in a sex-specific fashion, and central inhibition of the IKKβ pathway attenuates the obese phenotype.

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Mutant huntingtin in the hypothalamus causes sex-specific metabolic imbalance

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CNS-specific inactivation of the IKKβ pathway prevents the obese phenotype

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IKKβ inactivation leads to an increased number of mutant huntingtin inclusions

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IKKβ inactivation does not prevent orexin or A13 TH neuron loss

Amplification of neurotoxic HTTex1 assemblies in human neurons

Huntington's disease (HD) is a genetically inherited neurodegenerative disorder caused by expansion of a polyglutamine (polyQ) repeat in the exon-1 of huntingtin protein (HTT). The expanded polyQ enhances the amyloidogenic propensity of HTT exon 1 (HTTex1), which forms a heterogeneous mixture of assemblies with a broad neurotoxicity spectrum. While predominantly intracellular, monomeric and aggregated mutant HTT species are also present in the cerebrospinal fluids of HD patients, however, their biological properties are not well understood. To explore the role of extracellular mutant HTT in aggregation and toxicity, we investigated the uptake and amplification of recombinant HTTex1 assemblies in cell culture models. We find that small HTTex1 fibrils preferentially enter human neurons and trigger the amplification of neurotoxic assemblies; astrocytes or epithelial cells are not permissive. The amplification of HTTex1 in neurons depletes endogenous HTT protein with non-pathogenic polyQ repeat, activates apoptotic caspase-3 pathway and induces nuclear fragmentation. Using a panel of novel monoclonal antibodies and genetic mutation, we identified epitopes within the N-terminal 17 amino acids and proline-rich domain of HTTex1 to be critical in neural uptake and amplification. Synaptosome preparations from the brain homogenates of HD mice also contain mutant HTT species, which enter neurons and behave similar to small recombinant HTTex1 fibrils. These studies suggest that amyloidogenic extracellular mutant HTTex1 assemblies may preferentially enter neurons, propagate and promote neurodegeneration.

Sub-lethal ultraviolet B irradiation and Poly I:C treatment synergistically induced apoptosis of HaCaT cells through NF-κB pathway

Ultraviolet B (UVB) irradiation exerts multiple effects on skin cells, inducing apoptosis, senescence and carcinogenesis. Toll-like receptor 3, a member of pattern recognition receptors, is reported to initiate inflammation by recognizing double-strand RNA (dsRNA) released from UVB-irradiated cells. It has not been studied, however, whether apoptosis induction in UVB irradiation is attributed to TLR3 activation. Here, we report on the pro-apoptotic role of TLR3 in UVB-irradiated epidermal cells. Poly I:C, an analogue of dsRNA that activates TLR3, was used in combination with sub-lethal UVB (4.8 mJ/cm²) irradiation for investigating the effects of TLR3 activation on human immortalized keratinocyte HaCaT cells. Although sub-lethal dose of either Poly I:C or UVB alone did not induce cell death, UVB-Poly I:C co-treatment synergistically induced cell death by activation of caspase-3 and cleavages of ICAD and PARP, with apoptotic features when stained with Annexin V/PI or Hoechst 33342. Treatment with pan-caspase inhibitor, Z-VAD, attenuated UVB-Poly I:C-induced cell death. Silencing TLR3 by siRNA rescued HaCaT cells from UVB-Poly I:C-induced apoptosis. NF-κB, a major downstream component of TLR3 pathway, that usually negatively regulates the classical TLR3 apoptotic pathway, was analyzed by western blotting and immunofluorescence confocal microscopy. The results indicate to our surprise that NF-κB is translocated to nucleus in the cells co-treated with UVB-Poly I:C. The nuclear translocation of NF-κB is attenuated by TLR3 silencing. Treatment with BAY, an inhibitor of NF-κB pathway, blocked UVB-Poly I:C-induced apoptosis. Therefore, we conclude that NF-κB pathway plays a cytotoxic role in UVB-Poly I:C-treated HaCaT cells, mediating TLR3-related apoptosis.

IKKβ and mutant huntingtin interactions regulate the expression of IL-34: implications for microglial-mediated neurodegeneration in HD

Neuronal interleukin-34 (IL-34) promotes the expansion of microglia in the central nervous system-microglial activation and expansion are in turn implicated in the pathogenesis of Huntington's disease (HD). We thus examined whether the accumulation of an amyloidogenic exon-1 fragment of mutant huntingtin (mHTTx1) modulates the expression of IL-34 in dopaminergic neurons derived from a human embryonic stem cell line. We found that mHTTx1 aggregates induce IL-34 production selectively in post-mitotic neurons. Exposure of neurons to DNA damaging agents or the excitotoxin NMDA elicited similar results suggesting that IL-34 induction may be a general response to neuronal stress including the accumulation of misfolded mHTTx1. We further determined that knockdown or blocking the activity of IκB kinase beta (IKKβ) prevented the aggregation of mHTTx1 and subsequent IL-34 production. While elevated IL-34 itself had no effect on the aggregation or the toxicity of mHTTx1 in neuronal culture, IL-34 expression in a rodent brain slice model with intact neuron-microglial networks exacerbated mHTTx1-induced degeneration of striatal medium-sized spiny neurons. Conversely, an inhibitor of the IL-34 receptor reduced microglial numbers and ameliorated mHTTx1-mediated neurodegeneration. Together, these findings uncover a novel function for IKKβ/mHTTx1 interactions in regulating IL-34 production, and implicate a role for IL-34 in non-cell-autonomous, microglial-dependent neurodegeneration in HD.

Regulation of Survival Motor Neuron Protein by the Nuclear Factor-Kappa B Pathway in Mouse Spinal Cord Motoneurons

Survival motor neuron (SMN) protein deficiency causes the genetic neuromuscular disorder spinal muscular atrophy (SMA), characterized by spinal cord motoneuron degeneration. Since SMN protein level is critical to disease onset and severity, analysis of the mechanisms involved in SMN stability is one of the central goals of SMA research. Here, we describe the role of several members of the NF-κB pathway in regulating SMN in motoneurons. NF-κB is one of the main regulators of motoneuron survival and pharmacological inhibition of NF-κB pathway activity also induces mouse survival motor neuron (Smn) protein decrease. Using a lentiviral-based shRNA approach to reduce the expression of several members of NF-κB pathway, we observed that IKK and RelA knockdown caused Smn reduction in mouse-cultured motoneurons whereas IKK or RelB knockdown did not. Moreover, isolated motoneurons obtained from the severe SMA mouse model showed reduced protein levels of several NF-κB members and RelA phosphorylation. We describe the alteration of NF-κB pathway in SMA cells. In the context of recent studies suggesting regulation of altered intracellular pathways as a future pharmacological treatment of SMA, we propose the NF-κB pathway as a candidate in this new therapeutic approach.

Complex inhibition of autophagy by mitochondrial aldehyde dehydrogenase shortens lifespan and exacerbates cardiac aging

Autophagy, a conservative degradation process for long-lived and damaged proteins, participates in a cascade of biological processes including aging. A number of autophagy regulators have been identified. Here we demonstrated that mitochondrial aldehyde dehydrogenase (ALDH2), an enzyme with the most common single point mutation in humans, governs cardiac aging through regulation of autophagy. Myocardial mechanical and autophagy properties were examined in young (4months) and old (26-28months) wild-type (WT) and global ALDH2 transgenic mice. ALDH2 overexpression shortened lifespan by 7.7% without affecting aging-associated changes in plasma metabolic profiles. Myocardial function was compromised with aging associated with cardiac hypertrophy, the effects were accentuated by ALDH2. Aging overtly suppressed autophagy and compromised autophagy flux, the effects were exacerbated by ALDH2. Aging dampened phosphorylation of JNK, Bcl-2, IKKβ, AMPK and TSC2 while promoting phosphorylation of mTOR, the effects of which were exaggerated by ALDH2. Co-immunoprecipitation revealed increased dissociation between Bcl-2 and Beclin-1 (result of decreased Bcl-2 phosphorylation) in aging, the effect of which was exacerbated with ALDH2. Chronic treatment of the autophagy inducer rapamycin alleviated aging-induced cardiac dysfunction in both WT and ALDH2 mice. Moreover, activation of JNK and inhibition of either Bcl-2 or IKKβ overtly attenuated ALDH2 activation-induced accentuation of cardiomyocyte aging. Examination of the otherwise elderly individuals revealed a positive correlation between cardiac function/geometry and ALDH2 gene mutation. Taken together, our data revealed that ALDH2 enzyme may suppress myocardial autophagy possibly through a complex JNK-Bcl-2 and IKKβ-AMPK-dependent mechanism en route to accentuation of myocardial remodeling and contractile dysfunction in aging. This article is part of a Special Issue entitled: Genetic and epigenetic control of heart failure - edited by Jun Ren & Megan Yingmei Zhang.

Inhibition of Radiation-Induced Oxidative Damage in the Lung Tissue: May Acetylsalicylic Acid Have a Positive Role?

The lung is relatively sensitive to irradiation. It is shown that acetylsalicylic acid (ASA) might reduce oxidative injury and that it has a place in protection from cancer. The aim of this study is to evaluate the potential radioprotective effects of ASA. Whole-body irradiation (6 Gy, single dose) was applied to the rats. Glutathione (GSH), malondialdehyde (MDA), myeloperoxidase (MPO), and nitric oxide (NO) levels in the lung tissue were measured. Control (C), Radiation (R), Radiation + ASA (R + ASA; received irradiation and 25 mg/kg of ASA intraperitoneally (i.p.)), and Radiation + Amifostine (R + WR-2721; received irradiation and 200 mg/kg of WR-2721 i.p.) groups were used. The MPO levels decreased statistically significantly in the group administered ASA. Histopathologically, a radioprotective effect of ASA was more evident in the R + ASA group. ASA is an agent which has not been used as a radioprotector in the clinic yet, and it is worth supporting with more advanced studies.

Targeting ATM ameliorates mutant Huntingtin toxicity in cell and animal models of Huntington's disease

Age-related neurodegenerative disorders including Alzheimer's disease and Huntington's disease (HD) consistently show elevated DNA damage, but the relevant molecular pathways in disease pathogenesis remain unclear. One attractive gene is that encoding the ataxia-telangiectasia mutated (ATM) protein, a kinase involved in the DNA damage response, apoptosis, and cellular homeostasis. Loss-of-function mutations in both alleles of ATM cause ataxia-telangiectasia in children, but heterozygous mutation carriers are disease-free. Persistently elevated ATM signaling has been demonstrated in Alzheimer's disease and in mouse models of other neurodegenerative diseases. We show that ATM signaling was consistently elevated in cells derived from HD mice and in brain tissue from HD mice and patients. ATM knockdown protected from toxicities induced by mutant Huntingtin (mHTT) fragments in mammalian cells and in transgenic Drosophila models. By crossing the murine Atm heterozygous null allele onto BACHD mice expressing full-length human mHTT, we show that genetic reduction of Atm gene dosage by one copy ameliorated multiple behavioral deficits and partially improved neuropathology. Small-molecule ATM inhibitors reduced mHTT-induced death of rat striatal neurons and induced pluripotent stem cells derived from HD patients. Our study provides converging genetic and pharmacological evidence that reduction of ATM signaling could ameliorate mHTT toxicity in cellular and animal models of HD, suggesting that ATM may be a useful therapeutic target for HD.

Chaperone-mediated autophagy and neurodegeneration: connections, mechanisms, and therapeutic implications

Lysosomes degrade dysfunctional intracellular components via three pathways: macroautophagy, microautophagy, and chaperone-mediated autophagy (CMA). Unlike the other two, CMA degrades cytosolic proteins with a recognized KFERQ-like motif in lysosomes and is important for cellular homeostasis. CMA activity declines with age and is altered in neurodegenerative diseases. Its impairment leads to the accumulation of aggregated proteins, some of which may be directly tied to the pathogenic processes of neurodegenerative diseases. Its induction may accelerate the clearance of pathogenic proteins and promote cell survival, representing a potential therapeutic approach for the treatment of neurodegenerative diseases. In this review, we summarize the current findings on how CMA is involved in neurodegenerative diseases, especially in Parkinson's disease.

The unfolded protein response and its potential role in Huntington's disease elucidated by a systems biology approach

Huntington ´s disease (HD) is a progressive, neurodegenerative disease with a fatal outcome. Although the disease-causing gene (huntingtin) has been known for over 20 years, the exact mechanisms leading to neuronal cell death are still controversial. One potential mechanism contributing to the massive loss of neurons observed in the brain of HD patients could be the unfolded protein response (UPR) activated by accumulation of misfolded proteins in the endoplasmic reticulum (ER). As an adaptive response to counter-balance accumulation of un- or misfolded proteins, the UPR upregulates transcription of chaperones, temporarily attenuates new translation, and activates protein degradation via the proteasome. However, persistent ER stress and an activated UPR can also cause apoptotic cell death. Although different studies have indicated a role for the UPR in HD, the evidence remains inconclusive. Here, we present extensive bioinformatic analyses that revealed UPR activation in different experimental HD models based on transcriptomic data. Accordingly, we have identified 53 genes, including RAB5A, HMGB1, CTNNB1, DNM1, TUBB, TSG101, EEF2, DYNC1H1, SLC12A5, ATG5, AKT1, CASP7 and SYVN1 that provide a potential link between UPR and HD. To further elucidate the potential role of UPR as a disease-relevant process, we examined its connection to apoptosis based on molecular interaction data, and identified a set of 40 genes including ADD1, HSP90B1, IKBKB, IKBKG, RPS3A and LMNB1, which seem to be at the crossroads between these two important cellular processes. Remarkably, we also found strong correlation of UPR gene expression with the length of the polyglutamine tract of Huntingtin, which is a critical determinant of age of disease onset in human HD patients pointing to the UPR as a promising target for therapeutic intervention. The study is complemented by a newly developed web-portal called UPR-HD (http://uprhd.sysbiolab.eu) that enables visualization and interactive analysis of UPR-associated gene expression across various HD models.

The unfolded protein response and its potential role in Huntington's disease elucidated by a systems biology approach

Huntington ´s disease (HD) is a progressive, neurodegenerative disease with a fatal outcome. Although the disease-causing gene (huntingtin) has been known for over 20 years, the exact mechanisms leading to neuronal cell death are still controversial. One potential mechanism contributing to the massive loss of neurons observed in the brain of HD patients could be the unfolded protein response (UPR) activated by accumulation of misfolded proteins in the endoplasmic reticulum (ER). As an adaptive response to counter-balance accumulation of un- or misfolded proteins, the UPR upregulates transcription of chaperones, temporarily attenuates new translation, and activates protein degradation via the proteasome. However, persistent ER stress and an activated UPR can also cause apoptotic cell death. Although different studies have indicated a role for the UPR in HD, the evidence remains inconclusive. Here, we present extensive bioinformatic analyses that revealed UPR activation in different experimental HD models based on transcriptomic data. Accordingly, we have identified 53 genes, including RAB5A, HMGB1, CTNNB1, DNM1, TUBB, TSG101, EEF2, DYNC1H1, SLC12A5, ATG5, AKT1, CASP7 and SYVN1 that provide a potential link between UPR and HD. To further elucidate the potential role of UPR as a disease-relevant process, we examined its connection to apoptosis based on molecular interaction data, and identified a set of 40 genes including ADD1, HSP90B1, IKBKB, IKBKG, RPS3A and LMNB1, which seem to be at the crossroads between these two important cellular processes. Remarkably, we also found strong correlation of UPR gene expression with the length of the polyglutamine tract of Huntingtin, which is a critical determinant of age of disease onset in human HD patients pointing to the UPR as a promising target for therapeutic intervention. The study is complemented by a newly developed web-portal called UPR-HD (http://uprhd.sysbiolab.eu) that enables visualization and interactive analysis of UPR-associated gene expression across various HD models.

Antioxidant catalase rescues against high fat diet-induced cardiac dysfunction via an IKKβ-AMPK-dependent regulation of autophagy

Autophagy, a conservative degradation process for long-lived and damaged proteins, participates in a variety of biological processes including obesity. However, the precise mechanism of action behind obesity-induced changes in autophagy still remains elusive. This study was designed to examine the role of the antioxidant catalase in high fat diet-induced changes in cardiac geometry and function as well as the underlying mechanism of action involved with a focus on autophagy. Wild-type (WT) and transgenic mice with cardiac overexpression of catalase were fed low or high fat diet for 20 weeks prior to assessment of myocardial geometry and function. High fat diet intake triggered obesity, hyperinsulinemia, and hypertriglyceridemia, the effects of which were unaffected by catalase transgene. Myocardial geometry and function were compromised with fat diet intake as manifested by cardiac hypertrophy, enlarged left ventricular end systolic and diastolic diameters, fractional shortening, cardiomyocyte contractile capacity and intracellular Ca²⁺ mishandling, the effects of which were ameliorated by catalase. High fat diet intake promoted reactive oxygen species production and suppressed autophagy in the heart, the effects of which were attenuated by catalase. High fat diet intake dampened phosphorylation of inhibitor kappa B kinase β(IKKβ), AMP-activated protein kinase (AMPK) and tuberous sclerosis 2 (TSC2) while promoting phosphorylation of mTOR, the effects of which were ablated by catalase. In vitro study revealed that palmitic acid compromised cardiomyocyte autophagy and contractile function in a manner reminiscent of fat diet intake, the effect of which was significantly alleviated by inhibition of IKKβ, activation of AMPK and induction of autophagy. Taken together, our data revealed that the antioxidant catalase counteracts against high fat diet-induced cardiac geometric and functional anomalies possibly via an IKKβ-AMPK-dependent restoration of myocardial autophagy. This article is part of a Special Issue entitled: Autophagy and protein quality control in cardiometabolic diseases.

Role of chaperone-mediated autophagy in degrading Huntington's disease-associated huntingtin protein

Mutant N-terminal huntingtin (Htt) protein resulting from Huntington's disease (HD) with expanded polyglutamine accumulates and forms aggregates in vulnerable neurons. Both ubiquitin proteasomal and autophagic pathways contribute to the degradation of mutant Htt. Here, we focus on the involvement of chaperone-mediated autophagy (CMA), a selective form of autophagy in the clearance of Htt. Selective catabolism in CMA is conferred by the presence of a KFERQ-like targeting motif in the substrates, by which molecular chaperones recognize the hydrophobic surfaces of the misfolded substrates, and transfer them to the lysosomal membrane protein type-2A, LAMP-2A. The substrates are taken into the lysosomes through LAMP-2A and are rapidly degraded by the lysosomal enzymes. Taken together, we summarize the recent evidence to elucidate that Htt is also a potential substrate of CMA. We propose that the manipulation of CMA could be a therapeutic strategy for HD.

Sustained, neuron-specific IKK/NF-κB activation generates a selective neuroinflammatory response promoting local neurodegeneration with aging

Background

Increasing evidence indicates that neuroinflammation is a critical factor contributing to the progression of various neurodegenerative diseases. The IKK/NF-κB signalling system is a central regulator of inflammation, but it also affects neuronal survival and differentiation. A complex interplay between different CNS resident cells and infiltrating immune cells, which produce and respond to various inflammatory mediators, determines whether neuroinflammation is beneficial or detrimental. The IKK/NF-κB system is involved in both production of and responses to these mediators, although the precise contribution depends on the cell type as well as the cellular context, and is only partially understood. Here we investigated the specific contribution of neuronal IKK/NF-κB signalling on the regulation of neuroinflammatory processes and its consequences. To address this issue, we established and analysed a conditional gain-of-function mouse model that expresses a constitutively active allele of IKK2 in principal forebrain neurons (IKK2^nCA). Proinflammatory gene and growth factor expression, histopathology, microgliosis, astrogliosis, immune cell infiltration and spatial learning were assessed at different timepoints after persistent canonical IKK2/NF-κB activation.

Results

In contrast to other cell types and organ systems, chronic IKK2/NF-κB signalling in forebrain neurons of adult IKK2^nCA animals did not cause a full-blown inflammatory response including infiltration of immune cells. Instead, we found a selective inflammatory response in the dentate gyrus characterized by astrogliosis, microgliosis and Tnf-α upregulation. Furthermore, downregulation of the neurotrophic factor Bdnf correlated with a selective and progressive atrophy of the dentate gyrus and a decline in hippocampus-dependent spatial learning. Neuronal degeneration was associated with increased Fluoro-jade staining, but lacked activation of apoptosis. Remarkably, neuronal loss could be partially reversed when chronic IKK2/NF-κB signalling was turned off and Bdnf expression was restored.

Conclusion

Our results demonstrate that persistent IKK2/NF-κB signalling in forebrain neurons does not induce overall neuroinflammation, but elicits a selective inflammatory response in the dentate gyrus accompanied by decreased neuronal survival and impaired learning and memory. Our findings further suggest that chronic activation of neuronal IKK2/NF-κB signalling, possibly as a consequence of neuroinflammatory conditions, is able to induce apoptosis-independent neurodegeneration via paracrine suppression of Bdnf synthesis.

Defining the role of the Bcl-2 family proteins in Huntington's disease

B-cell lymphoma 2 (Bcl-2) family proteins regulate survival, mitochondria morphology dynamics and metabolism in many cell types including neurons. Huntington's disease (HD) is a neurodegenerative disorder caused by an expanded CAG repeat tract in the IT15 gene that encodes for the protein huntingtin (htt). In vitro and in vivo models of HD and HD patients' tissues show abnormal mitochondrial function and increased cell death rates associated with alterations in Bcl-2 family protein expression and localization. This review aims to draw together the information related to Bcl-2 family protein alterations in HD to decipher their potential role in mutated htt-related cell death and mitochondrial dysfunction.

Sensitization of melanoma cells for TRAIL-induced apoptosis by BMS-345541 correlates with altered phosphorylation and activation of Bax

Resistance to TRAIL (TNF-related apoptosis-inducing ligand)- induced apoptosis limits its therapeutic use. Different strategies of TRAIL sensitization and a dependency on Bax have been reported, but common principles of TRAIL resistance and the way of Bax activation remained poorly understood. Applying a melanoma model of TRAIL-sensitive and -resistant cell lines, efficient sensitization for TRAIL-induced apoptosis is demonstrated by the kinase inhibitor BMS-345541 (N-(1,8-dimethylimidazo(1,2-a)quinoxalin-4-yl)-1,2-ethanediamine hydrochloride), which targets IκB (inhibitor of κB proteins) kinase β (IKKβ). This effect was completely abrogated by Bax knockout as well as by Bcl-2 overexpression, in accordance with a Bax dependency. Early loss of the mitochondrial membrane potential, release of cytochrome c and Smac (second mitochondria-derived activator of caspases) clearly indicated the activation of mitochondrial apoptosis pathways. Of note, BMS-345541 alone resulted in an early Bax activation, seen by conformational changes and by Bax translocation. The synergistic effects can be explained by Bid activation through TRAIL, which inhibits Bcl-2, and the activation of Bax through BMS-345541. The critical roles of XIAP (X-chromosome-linked inhibitor of apoptosis protein), Smac and Bid were clearly proven by overexpression and siRNA knockdown, respectively. The way of Bax activation by BMS-345541 was unraveled by establishing new assays for Bax activation. These showed reduction of the inactivating Bax phosphorylation at serine-184, while the activating Bax phosphorylation at threonine-167 was enhanced. Thus, modulation of Bax phosphorylation appeared as tightly related to TRAIL sensitivity/resistance in melanoma cells, and therapeutic strategies may be considered.

A genome-scale RNA-interference screen identifies RRAS signaling as a pathologic feature of Huntington's disease

A genome-scale RNAi screen was performed in a mammalian cell-based assay to identify modifiers of mutant huntingtin toxicity. Ontology analysis of suppressor data identified processes previously implicated in Huntington's disease, including proteolysis, glutamate excitotoxicity, and mitochondrial dysfunction. In addition to established mechanisms, the screen identified multiple components of the RRAS signaling pathway as loss-of-function suppressors of mutant huntingtin toxicity in human and mouse cell models. Loss-of-function in orthologous RRAS pathway members also suppressed motor dysfunction in a Drosophila model of Huntington's disease. Abnormal activation of RRAS and a down-stream effector, RAF1, was observed in cellular models and a mouse model of Huntington's disease. We also observe co-localization of RRAS and mutant huntingtin in cells and in mouse striatum, suggesting that activation of R-Ras may occur through protein interaction. These data indicate that mutant huntingtin exerts a pathogenic effect on this pathway that can be corrected at multiple intervention points including RRAS, FNTA/B, PIN1, and PLK1. Consistent with these results, chemical inhibition of farnesyltransferase can also suppress mutant huntingtin toxicity. These data suggest that pharmacological inhibition of RRAS signaling may confer therapeutic benefit in Huntington's disease.

Inactivation of Bcl-2 through IκB kinase (IKK)-dependent phosphorylation mediates apoptosis upon exposure to 4-hydroxynonenal (HNE)

Apoptosis of macrophage foam cells loaded with modified/oxidized lipids is implicated in destabilization of advanced atherosclerotic plaques in humans. Concentration of HNE, main aldehydic product of plasma LDL peroxidation, elevates in atherosclerotic lesions as well as in cultured cells under oxidative stress. Although this reactive aldehyde has been shown to promote apoptosis with the involvement of p38 MAPK and JNK in various mammalian cell lines, roles of B-cell lymphoma 2 (Bcl-2) family proteins remain to be deciphered. We demonstrated that HNE-induced apoptosis was accompanied by concurrent downregulations of antiapoptotic Bcl-x(L) and Mcl-1 as well as upregulation of proapoptotic Bak. Furthermore, phoshorylation of Bcl-2 at Thr56, Ser70, and probably more phosphorylation sites located on N-terminal loop domain associated with HNE-induced apoptosis in both U937 and HeLa cells while ectopic expression of a phospho-defective Bcl-2 mutant significantly attenuated apoptosis. In parallel to this, HNE treatment caused release of proapoptotic Bax from Bcl-2. Pharmacological inhbition of IKK inhibited HNE-induced Bcl-2 phosphorylation. Similarly, silencing IKKα and -β both ended up with abrogation of Bcl-2 phosphorylation along with attenuation of apoptosis. Moreover, both IKKα and -β coimmunoprecipitated with Bcl-2 and in vitro kinase assay proved the ability of IKK to phosphorylate Bcl-2. In view of these findings and considering HNE inhibits DNA-binding activity of nuclear factor-κB (NF-κB) through prevention of IκB phosphorylation/ubiquitination/proteolysis, IKK appears to directly interfere with Bcl-2 activity through phosphorylation in HNE-mediated apoptosis independent of NF-κB signaling.

Elevated IKKα accelerates the differentiation of human neuronal progenitor cells and induces MeCP2-dependent BDNF expression

The IκB kinase α (IKKα) is implicated in the differentiation of epithelial and immune cells. We examined whether IKKα also plays a role in the differentiation and maturation of embryonic human neuronal progenitor cells (NPCs). We find that expression of an extra copy of IKKα (IKKα+) blocks self-renewal and accelerates the differentiation of NPCs. This coincides with reduced expression of the Repressor Element Silencing Transcription Factor/Neuron-Restrictive Silencing Factor (REST/NRSF), which is a prominent inhibitor of neurogenesis, and subsequent induction of the pro-differentiation non-coding RNA, miR-124a. However, the effects of IKKα on REST/NRSF and miR-124a expression are likely to be indirect. IKKα+ neurons display extensive neurite outgrowth and accumulate protein markers of neuronal maturation such as SCG10/stathmin-2, postsynaptic density 95 (PSD95), syntaxin, and methyl-CpG binding protein 2 (MeCP2). Interestingly, IKKα associates with MeCP2 in the nuclei of human neurons and can phosphorylate MeCP2 in vitro. Using chromatin immunoprecipitation assays, we find that IKKα is recruited to the exon-IV brain-derived neurotrophic factor (BDNF) promoter, which is a well-characterized target of MeCP2 activity. Moreover, IKKα induces the transcription of BDNF and knockdown expression of MeCP2 interferes with this event. These studies highlight a role for IKKα in accelerating the differentiation of human NPCs and identify IKKα as a potential regulator of MeCP2 function and BDNF expression.

NF-κB signaling pathways: role in nervous system physiology and pathology

Intracellular pathways related to cell survival regulate neuronal physiology during development and neurodegenerative disorders. One of the pathways that have recently emerged with an important role in these processes is nuclear factor-κB (NF-κB). The activity of this pathway leads to the nuclear translocation of the NF-κB transcription factors and the regulation of anti-apoptotic gene expression. Different stimuli can activate the pathway through different intracellular cascades (canonical, non-canonical, and atypical), contributing to the translocation of specific dimers of the NF-κB transcription factors, and each of these dimers can regulate the transcription of different genes. Recent studies have shown that the activation of this pathway regulates opposite responses such as cell survival or neuronal degeneration. These apparent contradictory effects depend on conditions such as the pathway stimuli, the origin of the cells, or the cellular context. In the present review, the authors summarize these findings and discuss their significance with respect to survival or death in the nervous system.

Epigenetic treatment of neurological disease

Neurological disease, and in particular neurodegenerative diseases, cause significant burdens on both patient and healthcare costs. Despite extensive research, treatment options for patients with these conditions remain limited, and generally, only provide modest symptomatic relief. Aberrant epigenetic post-translational modifications of proteins are emerging as important elements in the pathogenesis of neurological disease. Using Alzheimer’s disease and Huntington’s disease as examples in the following article, some of latest data linking both the histone code and the various proteins that regulate this code to the pathogenesis of neurological disease are discussed. The current evidence suggesting that pharmacologically targeting one such family, the histone deacetylases, may be of potential benefit in the treatment of such diseases is also discussed. Finally, some of the potential mechanisms to specifically target these proteins within the neurological setting are discussed.

The canonical nuclear factor-κB pathway regulates cell survival in a developmental model of spinal cord motoneurons

In vivo and in vitro motoneuron survival depends on the support of neurotrophic factors. These factors activate signaling pathways related to cell survival or inactivate proteins involved in neuronal death. In the present work, we analyzed the involvement of the nuclear factor-κB (NF-κB) pathway in mediating mouse spinal cord motoneuron survival promoted by neurotrophic factors. This pathway comprises ubiquitously expressed transcription factors that could be activated by two different routes: the canonical pathway, associated with IKKα/IKKβ kinase phosphorylation and nuclear translocation RelA (p65)/p50 transcription factors; and the noncanonical pathway, related to IKKα kinase homodimer phosphorylation and RelB/p52 transcription factor activation. In our system, we show that neurotrophic factors treatment induced IKKα and IKKβ phosphorylation and RelA nuclear translocation, suggesting NF-κB pathway activation. Protein levels of different members of the canonical or noncanonical pathways were reduced in a primary culture of isolated embryonic motoneurons using an interference RNA approach. Even in the presence of neurotrophic factors, selective reduction of IKKα, IKKβ, or RelA proteins induced cell death. In contrast, RelB protein reduction did not have a negative effect on motoneuron survival. Together these results demonstrated that the canonical NF-κB pathway mediates motoneuron survival induced by neurotrophic factors, and the noncanonical pathway is not related to this survival effect. Canonical NF-κB blockade induced an increase of Bim protein level and apoptotic cell death. Bcl-x(L) overexpression or Bax reduction counteracted this apoptotic effect. Finally, RelA knockdown causes changes of CREB and Smn protein levels.

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.

Targeting histone deacetylases for the treatment of Huntington's disease

Huntington's disease is a debilitating neurodegenerative condition with significant burdens on both patient and healthcare costs. Despite the identification of the causative element, an expanded toxic polyglutamine tract in the mutant Huntingtin protein, treatment options for patients with this disease remain limited. In the following review I assess the current evidence suggesting that a family of important regulatory proteins known as histone deacetylases may be an important therapeutic target in the treatment of this disease.

Targeting Huntington's disease through histone deacetylases

Huntington’s disease (HD) is a debilitating neurodegenerative condition with significant burdens on both patient and healthcare costs. Despite extensive research, treatment options for patients with this condition remain limited. Aberrant post-translational modification (PTM) of proteins is emerging as an important element in the pathogenesis of HD. These PTMs include acetylation, phosphorylation, methylation, sumoylation and ubiquitination. Several families of proteins are involved with the regulation of these PTMs. In this review, I discuss the current evidence linking aberrant PTMs and/or aberrant regulation of the cellular machinery regulating these PTMs to HD pathogenesis. Finally, I discuss the evidence suggesting that pharmacologically targeting one of these protein families the histone deacetylases may be of potential therapeutic benefit in the treatment of HD.

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.

Downregulation of NF-kappaB signaling by mutant huntingtin proteins induces oxidative stress and cell death

Accumulation of abnormal proteins and endoplasmic reticulum stress accompany neurodegenerative diseases including Huntington's disease. We show that the expression of mutant huntingtin proteins with extended polyglutamine repeats differentially affected endoplasmic reticulum signaling cascades linked to the inositol-requiring enzyme-1 (IRE1) pathway. Thus, the p38 and c-Jun N-terminal kinase pathways were activated, while the levels of the nuclear factor-kappaB-p65 (NF-kappaB-p65) protein decreased. Downregulation of NF-kappaB signaling was linked to decreased antioxidant levels, increased oxidative stress, and enhanced cell death. Concomitantly, calpain was activated, and treatment with calpain inhibitors restored NF-kappaB-p65 levels and increased cell viability. The calpain regulator, calpastatin, was low in cells expressing mutant huntingtin, and overexpression of calpastatin counteracted the deleterious effects caused by N-terminal mutant huntingtin proteins. These results show that calpastatin and an altered NF-kappaB-p65 signaling are crucial factors involved in oxidative stress and cell death mediated by mutant huntingtin proteins.

IKK phosphorylates Huntingtin and targets it for degradation by the proteasome and lysosome

Expansion of the polyglutamine repeat within the protein Huntingtin (Htt) causes Huntington's disease, a neurodegenerative disease associated with aging and the accumulation of mutant Htt in diseased neurons. Understanding the mechanisms that influence Htt cellular degradation may target treatments designed to activate mutant Htt clearance pathways. We find that Htt is phosphorylated by the inflammatory kinase IKK, enhancing its normal clearance by the proteasome and lysosome. Phosphorylation of Htt regulates additional post-translational modifications, including Htt ubiquitination, SUMOylation, and acetylation, and increases Htt nuclear localization, cleavage, and clearance mediated by lysosomal-associated membrane protein 2A and Hsc70. We propose that IKK activates mutant Htt clearance until an age-related loss of proteasome/lysosome function promotes accumulation of toxic post-translationally modified mutant Htt. Thus, IKK activation may modulate mutant Htt neurotoxicity depending on the cell's ability to degrade the modified species.