Cytochrome C and caspase-9 expression in Huntington's disease

Did natural selection for increased cognitive ability in humans lead to an elevated risk of cancer?

Despite the overall genetic similarity that exists between humans and chimpanzees, the species are phenotypically distinct. Among the most notable distinctions are differences in brain size and cognitive abilities. Previous studies have shown that significant differences in gene expression exist between the human and chimpanzee brain. Integration of currently available gene expression data with known metabolic and signaling pathways indicates that the expression of genes involved in the programmed cell death of brain neurons is significantly different between humans and chimpanzees and predictive of a reduced level of neuron apoptosis in the human brain. This pattern of expression is generally maintained in other human organs suggesting that apoptosis is reduced in humans relative to chimpanzees. We propose that a decreased rate of programmed neuron death may have been a consequence of selection for increased cognitive ability in humans. Since reduced apoptotic function is associated with an increased risk of cancer and related diseases, we hypothesize that selection for increased cognitive ability in humans coincidently resulted in an increased risk of cancer and other diseases associated with reduced apoptotic function.

Neuronal cell death in neurodegenerative diseases: recurring themes around protein handling

Neuronal cell death plays a role in many chronic neurodegenerative diseases with the loss of particular subsets of neurons. The loss of the neurons occurs during a period of many years, which can make the mode(s) of cell death and the initiating factors difficult to determine. In vitro and in vivo models have proved invaluable in this regard, yielding insight into cell death pathways. This review describes the main mechanisms of neuronal cell death, particularly apoptosis, necrosis, excitotoxicity and autophagic cell death, and their role in neurodegenerative diseases such as ischaemia, Alzheimer's, Parkinson's and Huntington's diseases. Crosstalk between these death mechanisms is also discussed. The link between cell death and protein mishandling, including misfolded proteins, impairment of protein degradation, protein aggregation is described and finally, some pro-survival strategies are discussed.

Mitochondrial DNA damage and repair in neurodegenerative disorders

By producing ATP and regulating intracellular calcium levels, mitochondria are vital for the function and survival of neurons. Oxidative stress and damage to mitochondrial DNA during the aging process can impair mitochondrial energy metabolism and ion homeostasis in neurons, thereby rendering them vulnerable to degeneration. Mitochondrial abnormalities have been documented in all of the major neurodegenerative disorders-Alzheimer's, Parkinson's and Huntington's diseases, and amyotrophic lateral sclerosis. Mitochondrial DNA damage and dysfunction may be downstream of primary disease processes such as accumulation of pathogenic proteins. However, recent experimental evidence demonstrates that mitochondrial DNA damage responses play important roles in aging and in the pathogenesis of neurodegenerative diseases. Therapeutic interventions that target mitochondrial regulatory systems have been shown effective in cell culture and animal models, but their efficacy in humans remains to be established.

Apoptosis in meningoencephalitis of Angiostrongylus cantonensis-infected mice

A hallmark of eosinophilic meningoencephalitis is infiltration of leukocytes into brain parenchyma and subarachnoid space infected by Angiostrongylus cantonensis. Apoptosis, a process that eliminates useless cells and counterbalances tissue homeostasis, is important for homeostasis of the immune system. In this study, we investigated the characteristics of cell death induced in BABL/c mice infected with A. cantonensis. We observed increased expression of the apoptotic proteins, caspase-3, caspase-8, caspase-9, and cytochrome c, and decreased expression of anti-apoptotic proteins, B-cell leukemia 2 and inhibitor of apoptosis protein 1. On immunohistochemistry, apoptotic proteins were localized within the leukocytes infiltrate. A terminal deoxynucleotidyl transferase-mediated deoxyuridine 5-triphosphate nick-end labeling assay to detect DNA fragmentation confirmed these observations. The infiltration of leukocytes present in the brain parenchyma and subarachnoid space in vivo may also express these apoptotic regulatory molecules, which demonstrates the capacity of these cells to undergo apoptosis.

AAV-mediated expression of Bcl-xL or XIAP fails to induce neuronal resistance against quinolinic acid-induced striatal lesioning

Apoptotic mechanisms have been proposed to contribute to the selective loss of medium spiny striatal projection neurons in Huntington's disease (HD). This raises the question as to whether enhancing the expression of anti-apoptotic factors in vulnerable striatal projection neurons can reduce their susceptibility to neurotoxic processes occurring in the HD brain. In this study AAV 1/2 vectors encoding either the anti-apoptotic factor Bcl-xL or XIAP were used to transduce striatal neurons prior to an intrastriatal injection of the excitotoxic glutamate analogue quinolinic acid (QA). AAV 1/2 vector treated rats were observed in behavioural tests undertaken to assess whether anti-apoptotic factor expression provided amelioration of motor function impairment following unilateral QA-induced striatal lesioning. AAV-XIAP treated rats displayed complete amelioration of an ipsilateral forelimb use bias relative to control animals. However, neither AAV-XIAP nor AAV-Bcl-xL treated rats demonstrated an improvement in sensorimotor neglect compared to control animals. Furthermore, we did not observe a significant reduction of QA-induced pathology in assessed neuronal populations of the basal ganglia. These results indicate that sole enhancement of XIAP or Bcl-xL is not sufficient to counteract QA-induced excitotoxic insult of striatal neurons.

The neuroprotective role of creatine

Significant progress has been made in identifying neuroprotective agents and their translation to patients with neurological disorders. While the direct causative pathways of neurodegeneration remain unclear, they are under great clinical and experimental investigation. There are a number of interrelated pathogenic mechanisms triggering molecular events that lead to neuronal death. One putative mechanism reported to play a prominent role in the pathogenesis of neurological diseases is impaired energy metabolism. If reduced energy stores play a role in neuronal loss, then therapeutic strategies that buffer intracellular energy levels may prevent or impede the neurodegenerative process. Recent studies suggest that impaired energy production promotes neurological disease onset and progression. Sustained ATP levels are critical to cellular homeostasis and may have both direct and indirect influence on pathogenic mechanisms associated with neurological disorders. Creatine is a critical component in maintaining cellular energy homeostasis, and its administration has been reported to be neuroprotective in a wide number of both acute and chronic experimental models of neurological disease. In the context of this chapter, we will review the experimental evidence for creatine supplementation as a neurotherapeutic strategy in patients with neurological disorders, including Huntington's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Alzheimer's disease, as well as in ischemic stroke, brain and spinal cord trauma, and epilepsy.

Misfolded BRICHOS SP-C mutant proteins induce apoptosis via caspase-4- and cytochrome c-related mechanisms

Several mutations within the BRICHOS domain of surfactant protein C (SP-C) have been linked to interstitial lung disease. Recent studies have suggested that these mutations cause misfolding of the proprotein (proSP-C), which initiates the unfolded protein response to resolve improper folding or promote protein degradation. We have reported that in vitro expression of one of these proteins, the exon 4 deletion mutant (hSP-CΔexon4), causes endoplasmic reticulum (ER) stress, inhibits proteasome function, and activates caspase-3-mediated apoptosis. To further elucidate mechanisms and common pathways for cellular dysfunction, various assays were performed by transiently expressing two SP-C BRICHOS domain mutant (BRISPC) proteins (hSP-CΔexon4, hSP-CL188Q) and control proteins in lung epithelium-derived A549 and kidney epithelium-derived (HEK-293) GFPu-1 cell lines. Compared with controls, cells expressing either BRICHOS mutant protein consistently exhibited increased formation of insoluble aggregates, enhanced promotion of inositol-requiring enzyme 1-dependent splicing of X-box binding protein-1 (XBP-1), significant inhibition of proteasome activity, enhanced induction of mitochondrial cytochrome c release, and increased activations of caspase-4 and caspase-3, leading to apoptosis. These results suggest common cellular responses, including initiation of cell-death signaling pathways, to these lung disease-associated BRISPC proteins.

Neuronal apoptosis in neurodegeneration

Apoptosis mediates the precise and programmed natural death of neurons and is a physiologically important process in neurogenesis during maturation of the central nervous system. However, premature apoptosis and/or an aberration in apoptosis regulation is implicated in the pathogenesis of neurodegeneration, a multifaceted process that leads to various chronic disease states, such as Alzheimer's (AD), Parkinson's (PD), Huntington's (HD) diseases, amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), and diabetic encephalopathy. The current review focuses on two major areas (a) the fundamentals of apoptosis, which includes elements of the apoptotic machinery, apoptosis inducers, and emerging concepts in apoptosis research, and (b) apoptotic involvement in neurodegenerative disorders, neuroprotective treatment strategies/modalities, and the mechanisms of, and signaling in, neuronal apoptosis. Current and new experimental models for apoptosis research in neurodegenerative diseases are also discussed.

Type 2 transglutaminase differentially modulates striatal cell death in the presence of wild type or mutant huntingtin

Huntington's disease (HD), which is caused by an expanded polyglutamine tract in huntingtin (htt), is characterized by extensive loss of striatal neurons. The dysregulation of type 2 transglutaminase (TG2) has been proposed to contribute to the pathogenesis in HD as TG2 is up-regulated in HD brain and knocking out TG2 in mouse models of HD ameliorates the disease process. To understand the role of TG2 in the pathogenesis of HD, immortalized striatal cells established from mice in which mutant htt with a polyglutamine stretch of 111 Gln had been knocked-in and wild type (WT) littermates, were stably transfected with human TG2 in a tetracycline inducible vector. Overexpression of TG2 in the WT striatal cells resulted in significantly greater cell death under basal conditions as well as in response to thapsigargin treatment, which causes increased intracellular calcium concentrations. Furthermore, in WT striatal cells TG2 overexpression potentiated mitochondrial membrane depolarization, intracellular reactive oxygen species production, and apoptotic cell death in response to thapsigargin. In contrast, in mutant striatal cells, TG2 overexpression did not increase cell death, nor did it potentiate thapsigargin-induced mitochondrial membrane depolarization or intracellular reactive oxygen species production. Instead, TG2 overexpression in mutant striatal cells attenuated the thapsigargin-activated apoptosis. When in situ transglutaminase activity was quantitatively analyzed in these cell lines, we found that in response to thapsigargin treatment TG2 was activated in WT, but not mutant striatal cells. These data suggest that mutant htt alters the activation of TG2 in response to certain stimuli and therefore differentially modulates how TG2 contributes to cell death processes.

Systematic uncovering of multiple pathways underlying the pathology of Huntington disease by an acid-cleavable isotope-coded affinity tag approach

Huntington disease (HD) is an autosomal dominant neurodegenerative disease that results from a CAG (glutamine) trinucleotide expansion in exon 1 of huntingtin (Htt). The aggregation of mutant Htt has been implicated in the progression of HD. The earliest degeneration occurs in the striatum. To identify proteins critical for the progression of HD, we applied acid-cleavable ICAT technology to quantitatively determine changes in protein expressions in the striatum of a transgenic HD mouse model (R6/2). The cysteine residues of striatal proteins from HD and wild-type mice were labeled, respectively, with the heavy and light forms of the ICAT reagents. Samples were trypsinized, uncovered by avidin affinity chromatography, and analyzed by nano-LC-MS/MS. Western blot analyses were used to confirm and to calibrate the ICAT ratios. Linear regression was used to uncover a group of proteins that exhibited consistent changes. In two independent ICAT experiments, we identified 427 cysteine-containing striatal proteins among which approximately 66% (203 proteins) were detected in both ICAT experiments. Approximately two-thirds of proteins identified in each ICAT experiment were detected in both ICAT experiments. In total, 68 proteins with altered expressions in HD mice were identified. Elevated expressions of two down-regulated proteins (14-3-3sigma and FKBP12) effectively reduced Htt aggregates in a striatal cell line, supporting the functional relevance of the above findings. Collectively by using a well defined protocol for data analysis, large scale comparisons of protein expressions by ICAT can be reliable and can provide valuable clues for identifying proteins critical for pathophysiological functions.

Neuronal life-and-death signaling, apoptosis, and neurodegenerative disorders

When subjected to excessive oxidative stress, neurons may respond adaptively to overcome the stress, or they may activate a programmed cell death pathway called apoptosis. Apoptosis is characterized by alterations in mitochondria and the endoplasmic reticulum and activation of cysteine proteases called caspases. Increasing evidence suggests that apoptotic biochemical cascades are involved in the dysfunction and death of neurons in neurodegenerative disorders such as Alzheimer's, Parkinson, and Huntington's diseases. Studies of normal aging, of genetic mutations that cause disease, and of environmental factors that affect disease risk are revealing cellular and molecular alterations that may cause excessive oxidative stress and trigger neuronal apoptosis. Accumulation of self-aggregating proteins such as amyloid beta-peptide, tau, alpha-synuclein, and huntingtin may be involved in apoptosis both upstream and downstream of oxidative stress. Membrane-associated oxidative stress resulting in perturbed lipid metabolism and disruption of cellular calcium homeostasis may trigger apoptosis in several different neurodegenerative disorders. Counteracting neurodegenerative processes are an array of mechanisms including neurotrophic factor signaling, antioxidant enzymes, protein chaperones, antiapoptotic proteins, and ionostatic systems. Emerging findings suggest that the resistance of neurons to death during aging can be enhanced by modifications of diet and lifestyle.

Increased apoptosis, Huntingtin inclusions and altered differentiation in muscle cell cultures from Huntington's disease subjects

Mutated huntingtin (htt) is ubiquitously expressed in tissues of Huntington's disease (HD) patients. In the brain, the mutated protein leads to neuronal cell dysfunction and death, associated with formation of htt-positive inclusions. Given increasing evidence of abnormalities in HD skeletal muscle, we extensively analyzed primary muscle cell cultures from seven HD subjects (including two unaffected mutation carriers). Myoblasts from presymptomatic and symptomatic HD subjects showed cellular abnormalities in vitro, namely mitochondrial depolarization, cytochrome c release, increased caspase-3, -8, and -9 activities, and defective cell differentiation. Another notable feature was the formation of htt inclusions in differentiated myotubes. This study helps to advance current knowledge about the downstream effects of the htt mutation in human tissues. Further applications may include drug screening using this human cellular model.

Emerging chemotherapeutic strategies for Huntington's disease

Huntington's disease (HD) is a progressive and fatal neurological disorder caused by an expanded CAG repeat in the gene coding for the protein, huntingtin. There is no clinically proven treatment for HD. Although the exact cause of neuronal death in HD remains unknown, it has been postulated that the abnormal aggregation of the mutant huntingtin protein may cause toxic effects in neurons, leading to a cascade of pathogenic mechanisms associated with transcriptional dysfunction, oxidative stress, mitochondrial alterations, apoptosis, bioenergetic defects and subsequent excitotoxicity. Understanding how these processes interrelate has become important in identifying a pharmacotherapy in HD and in the design of clinical trials. A number of drug compounds that separately target these mechanisms have significantly improved the clinical and neuropathological phenotype of HD transgenic mice and, as such, are immediate candidates for human clinical trials in HD patients. These compounds are discussed herein.

Aging and neurodegeneration. Molecular mechanisms of neuronal loss in Huntington's disease

Huntington's disease (HD) is a fatal, genetically based late-onset neurodegenerative disorder in which a loss of neostriatal neurons is a main characteristic. The CAG trinucleotide repeat expansion encoding polyglutamine tract induces progressive deficits in intra- and inter-cellular signalling, and subsequent clinical signs developed with aging process. CAG-induced neurodegeneration and disease-onset shows aging-dependent pattern. Proposed mechanism of neurodegeneration includes intranuclear or intracellular protein aggregates, proteolytic cleavage of huntingtin (cf. caspase, calpain), altered transcription or other neurotransmitter signalling deficits. Recently, stem cell transplantation is of benefit to protect neurons against neurodegeneration and recover the functional deficit in the experimental HD model. This review focuses on current knowledge of molecular mechanisms in neurodegeneration and potential therapeutic targets in HD.

GENE–ENVIRONMENT INTERACTIONS, NEURONAL DYSFUNCTION AND PATHOLOGICAL PLASTICITY IN HUNTINGTON'S DISEASE

Huntington's disease (HD) is a fatal autosomal dominant disorder in which there is progressive neurodegeneration producing motor, cognitive and psychiatric symptoms. The dynamic mutation that causes the disease is common to numerous other brain disorders, which may share similar pathogenic mechanisms. Much progress has been made in the past decade in understanding how a trinucleotide (CAG) repeat expansion, encoding an expanded polyglutamine tract in the huntingtin protein, induces dysfunction at molecular and cellular levels. The present review integrates various lines of experimental evidence in an attempt to move towards a unifying mechanistic framework, which may explain the pathogenesis of HD, from molecular through to neuronal network and behavioural levels. Recent evidence, using transgenic mouse models, also suggests that environmental factors can modify the onset and progression of HD. The effects of specific environmental manipulations are discussed in the context of gene-environment interactions and experience-dependent plasticity in the healthy and diseased brain, particularly the cerebral cortex.

The use of the R6 transgenic mouse models of Huntington's disease in attempts to develop novel therapeutic strategies

Huntington's disease (HD) is a genetic neurodegenerative disorder. Since identification of the disease-causing gene in 1993, a number of genetically modified animal models of HD have been generated. The first transgenic mouse models, R6/1 and R6/2 lines, were established 8 years ago. The R6/2 mice have been the best characterized and the most widely used model to study pathogenesis of HD and therapeutic interventions. In the present review, we especially focus on the characteristics of R6 transgenic mouse models and, in greater detail, describe the different therapeutic strategies that have been tested in these mice. We also, at the end, critically assess the relevance of the HD mouse models compared with the human disease and discuss how they can be best used in the future.

Caspase-dependent and -independent cell death induced by 3-nitropropionic acid in rat cortical neurons

Mitochondria play a critical role in cell death by releasing apoptogenic factors, such as cytochrome c and apoptosis-inducing factor (AIF), from the intermembrane space into the cytoplasm. Because mitochondrial dysfunction has been shown to be involved in several neurodegenerative diseases, mitochondrial toxins are largely used to model these disorders. These include 3-nitropropionic acid (3-NP), an irreversible inhibitor of succinate dehydrogenase, which has been used to model Huntington's disease and was previously reported by us to induce apoptotic cell death through caspase activation. In the present study, we evaluated the involvement of caspase-independent neuronal cell death induced by 3-NP (1 mM) and the effect of z-VDVAD-fmk, an inhibitor of caspase-2, using cortical neurons in culture. Our results highly suggest that 3-NP induces both caspase-dependent and -independent cell death. We showed that z-VDVAD-fmk prevented both caspase-2 and -3-like activities evoked by 3-NP, but only partly prevented chromatin fragmentation/condensation. However, z-VDVAD-fmk did not avoid 3-NP-induced release of cytochrome c or AIF from mitochondria nor did it affect the levels of mitochondrial Bax. Furthermore, 3-NP-mediated decrease in plasma membrane integrity was not affected by z-VDVAD-fmk. Under these conditions, the inhibitor prevented the caspase-dependent cell death.

3-Nitropropionic acid: a mitochondrial toxin to uncover physiopathological mechanisms underlying striatal degeneration in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a mutation in the gene encoding Huntingtin. The mechanisms underlying the preferential degeneration of the striatum, the most striking neuropathological change in HD, are unknown. Of those probably involved, mitochondrial defects might play an important role. The behavioural and anatomical similarities found between HD and models using the mitochondrial toxin 3-nitropropionic acid (3NP) in rats and primates support this hypothesis. Here, we discuss the recently identified mechanisms of 3NP-induced striatal degeneration. Two types of important factor have been identified. The first are the 'executioner' components that have direct roles in cell death, such as c-Jun N-terminal kinase and Ca2+-activated protease calpains. The second are 'environmental' factors, such as glutamate, dopamine and adenosine, which modulate the striatal degeneration induced by 3NP. Interestingly, these recent studies support the hypothesis that 3NP and mutated Huntingtin have certain mechanisms of toxicity in common, suggesting that the use of 3NP might give new insights into the pathogenesis of HD. Research on 3NP provides additional proof that the neurochemical environment of a given neurone can determine its preferential vulnerability in neurodegenerative diseases.

The therapeutic role of creatine in Huntington's disease

Huntington's disease (HD) is an autosomal dominant and fatal neurological disorder characterized by a clinical triad of progressive choreiform movements, psychiatric symptoms, and cognitive decline. HD is caused by an expanded trinucleotide CAG repeat in the gene coding for the protein huntingtin. No proven treatment to prevent the onset or to delay the progression of HD currently exists. While a direct causative pathway from the gene mutation to the selective neostriatal neurodegeneration remains unclear, it has been hypothesized that interactions of the mutant huntingtin protein or its fragments may result in a number of interrelated pathogenic mechanisms triggering a cascade of molecular events that lead to the untimely neuronal death observed in HD. One putative pathological mechanism reported to play a prominent role in the pathogenesis of HD is mitochondrial dysfunction and the subsequent reduction of cellular energy. Indeed, if mitochondrial impairment and reduced energy stores play roles in the neuronal loss in HD, then a therapeutic strategy that buffers intracellular energy levels may ameliorate the neurodegenerative process. Sustained ATP levels may have both direct and indirect importance in ameliorating the severity of many of the pathogenic mechanisms associated with HD. Creatine, a guanidino compound produced endogenously and acquired exogenously through diet, is a critical component in maintaining much needed cellular energy. As such, creatine is one of a number of ergogens that may provide a relatively safe and immediately available therapeutic strategy to HD patients that may be the cornerstone of a combined treatment necessary to delay the relentless progression of HD.

The paradigm of Huntington's disease: therapeutic opportunities in neurodegeneration

Despite a relatively small number of affected patients, Huntington's disease (HD) has been a historically important disease, embodying many of the major themes in modern neuroscience, including molecular genetics, selective neuronal vulnerability, excitotoxicity, mitochondrial dysfunction, apoptosis, and transcriptional dysregulation. The discovery of the HD gene in 1993 opened the door to the mechanisms of HD pathogenesis. Multiple pathologic mechanisms have been discovered, each one serving as a potential therapeutic target. HD thus continues to serve as a paradigmatic disorder, with basic bench research generating clinically relevant insights and stimulating the development of therapeutic human trials.

Prevention of cytosolic IAPs degradation: a potential pharmacological target in Huntington's Disease

Huntington's Disease (HD) is a neurodegenerative disorder caused by an abnormally expanded polyglutamine trait in the amino-terminal region of huntingtin. Pathogenic mechanisms involve a gained toxicity of mutant huntingtin and a potentially reduced neuroprotective function of the wild-type allele. Among the molecular abnormalities reported, HD cells are characterized by the presence of aggregates, transcriptional dysregulation, altered mitochondrial membrane potential and aberrant Ca++ handling. In addition, upon exposure to toxic stimuli, increased mitochondrial release of cytochrome C and activation of caspase-9 and caspase-3 are found in HD cells and tissue. Here we report that HTRA2 and Smac/DIABLO, two additional mitochondrial pro-apoptotic factors, are aberrantly released from brain-derived cells expressing mutant huntingtin. This event causes a reduction in levels of the cytosolic IAP1 (Inhibitor of Apoptosis Protein-1) and XIAP (X-linked inhibitor apoptosis) antiapoptotic IAP family members. Reduced IAP levels are also found in post-mortem HD brain tissue. Treatment with ucf101, a serine protease HTRA2 specific inhibitor, counteracts IAPs degradation in HD cells and increases their survival. These results point to the IAPs as potential pharmacological targets in Huntington's Disease.

Sodium phenylbutyrate prolongs survival and regulates expression of anti-apoptotic genes in transgenic amyotrophic lateral sclerosis mice

Multiple molecular defects trigger cell death in amyotrophic lateral sclerosis (ALS). Among these, altered transcriptional activity may perturb many cellular functions, leading to a cascade of secondary pathological effects. We showed that pharmacological treatment, using the histone deacetylase inhibitor sodium phenylbutyrate, significantly extended survival and improved both the clinical and neuropathological phenotypes in G93A transgenic ALS mice. Phenylbutyrate administration ameliorated histone hypoacetylation observed in G93A mice and induced expression of nuclear factor-kappaB (NF-kappaB) p50, the phosphorylated inhibitory subunit of NF-kappaB (pIkappaB) and beta cell lymphoma 2 (bcl-2), but reduced cytochrome c and caspase expression. Curcumin, an NF-kappaB inhibitor, and mutation of the NF-kappaB responsive element in the bcl-2 promoter, blocked butyrate-induced bcl-2 promoter activity. We provide evidence that the pharmacological induction of NF-kappaB-dependent transcription and bcl-2 gene expression is neuroprotective in ALS mice by inhibiting programmed cell death. Phenylbutyrate acts to phosphorylate IkappaB, translocating NF-kappaB p50 to the nucleus, or to directly acetylate NF-kappaB p50. NF-kappaB p50 transactivates bcl-2 gene expression. Up-regulated bcl-2 blocks cytochrome c release and subsequent caspase activation, slowing motor neuron death. These transcriptional and post-translational pathways ultimately promote motor neuron survival and ameliorate disease progression in ALS mice. Phenylbutyrate may therefore provide a novel therapeutic approach for the treatment of patients with ALS.

Early apoptotic and late necrotic components associated with altered Ca2+ homeostasis in a peptide-delivery model of polyglutamine-induced neuronal death

The mechanisms by which polyglutamine expansion causes common features of neuronal death remain unclear. Here we describe an approach for delivering polyglutamine expansions directly into cultured sympathetic neurons. Glutamine (Q) residues (n = 10, 22, 30) were conjugated with a peptide possessing translocation properties across plasma membranes (PDP) and a nuclear localization signal (NLS). These peptides were rapidly incorporated into sympathetic neurons and showed neurotoxicity in a length- and dose-dependent manner. A robust induction of c-jun and cyclin D1 occurred following treatment with PDP-Q22-NLS. Enhanced c-Jun phosphorylation showed c-Jun N-terminal kinase (JNK) activation. Coincidentally, TrkA tyrosine phosphorylation was decreased in association with loss of phospho-Akt, the downstream target of PI-3 kinase. Despite such proapoptotic signals, neither release of cytochrome c from mitochondria nor caspase-3/7 activation was detected. TdT-mediated dUTP nick-end labeling-positive nuclear condensation, but no fragmentation, occurred. At 24 hr of treatment, cytoplasmic Ca2+ levels began to become elevated, and the cellular level of ATP was decreased. Cytoplasmic Ca2+ responses to KCl depolarization displayed a delayed recovery, providing evidence for lack of Ca2+ homeostasis. The neurons became committed to death at about 36 hr when mitochondrial Ca2+ uptake declined concurrently with loss of mitochondrial membrane potential. Collectively, these results show that, despite induction of early apoptotic signals, nonapoptotic neuronal cell death occurred via perturbed Ca2+ homeostasis and suggest that mitochondrial permeability transition may play important roles in this model of neuronal death.

Expression pattern of apoptosis-related markers in Huntington's disease

Inappropriate apoptosis has been implicated in the mechanism of neuronal death in Huntington's disease (HD). In this study, we report the expression of apoptotic markers in HD caudate nucleus (grades 1-4) and compare this with controls without neurological disease. Terminal transferase-mediated biotinylated-UTP nick end-labeling (TUNEL)-positive cells were detected in both control and HD brains. However, typical apoptotic cells were present only in HD, especially in grade 3 and 4 specimens. Expression of the pro-apoptotic protein Bax was increased in HD brains compared to controls, demonstrating a cytoplasmic expression pattern in predominantly shrunken and dark neurons, which were most frequently seen in grades 2 and 3. Control brains displayed weak perinuclear expression of the anti-apoptotic protein Bcl-2, whereas in HD brains Bcl-2 immunoreactivity was markedly enhanced, especially in severely affected grade 4 brains, and was observed in both healthy neurons and dark neurons. Caspase-3, an executioner protease, was only found in four HD brains of different grades and was not expressed in controls. A strong neuronal and glial expression of poly(ADP-ribose) polymerase (PARP)-immunoreactivity was observed in HD brains. These data strongly suggest the involvement of apoptosis in HD. The exact apoptotic pathway occurring in HD neurodegeneration remains yet unclear. However, the presence of late apoptotic events, such as enhanced PARP expression and many TUNEL-positive cells accompanied with weak caspase-3 immunoreactivity in severely affected HD brains, suggests that caspase-mediated neuronal death only plays a minor role in HD.

Development of novel therapies for Huntington's disease: hope and challenge

Huntington's disease (HD) is an autosomal dominant neurological disease. It is a fatal neurological disorder affecting 5-10 out of 10,000 people. While there are intensive research efforts focusing on uncovering molecular mechanisms of the pathogenesis of HD, a number of studies have begun to look for effective therapies for HD. There is a large body of encouraging news on novel therapeutic developments. The present paper reviews drugs used for symptomatic treatment of HD and experimental therapies targeting HD molecular pathology.

Minocycline in phenotypic models of Huntington's disease

Minocycline has been shown to be neuroprotective in various models of neurodegenerative diseases. However, its potential in Huntington's disease (HD) models characterized by calpain-dependent degeneration and inflammation has not been investigated. Here, we have tested minocycline in phenotypic models of HD using 3-nitropropionic acid (3NP) intoxication and quinolinic acid (QA) injections. In the 3NP rat model, where the development of striatal lesions involves calpain, we found that minocycline was not protective, although it attenuated the development of inflammation induced after the onset of striatal degeneration. The lack of minocycline activity on calpain-dependent cell death was also confirmed in vitro using primary striatal cells. Conversely, we found that minocycline reduced lesions and inflammation induced by QA. In cultured cells, minocycline protected against mutated huntingtin and staurosporine, stimulations known to promote caspase-dependent cell death. Altogether, these data suggested that, in HD, minocycline may counteract the development of caspase-dependent neurodegeneration, inflammation, but not calpain-dependent neuronal death.

Increased Alix (apoptosis-linked gene-2 interacting protein X) immunoreactivity in the degenerating striatum of rats chronically treated by 3-nitropropionic acid

Chronic intoxication by 3-nitropropionic acid in the Lewis rat reproduces many features reminiscent of Huntington's disease including behavioural alterations and cortico-striatal degeneration. In particular, in this model, striatal degeneration is accompanied by calpain activation as found in the human disease. The present study was undertaken to determine whether the expression of Alix (apoptosis linked gene-2 interacting protein), a widespread protein involved in neuronal death, would be modified in the striatum and cortex of 3NP-treated rats. The results clearly show that Alix immunoreactivity is increased in the neuronal cell bodies of the lateral striatum, where degeneration is massive. The medial striatum and the cortex that lack neurodegeneration remain only weakly labelled. This is further evidence suggesting an involvement of Alix in the events driving neuronal death.

Increased caspase activation in peripheral blood mononuclear cells of patients with Alzheimer's disease

In this study, we investigated whether alterations in the pattern of caspase activation could be found at the level of peripheral blood mononuclear cells (PBMCs) in patients with Alzheimer's disease (AD). The results showed that in experimental conditions resembling a physiological stimulation, there was a statistically significant increase in the enzymatic activity of caspase-3, caspase-8, and caspase-9 in PBMCs from a small, but well-characterized, cohort of sporadic AD patients compared to those from a comparable control group of aged adults (AA). This was accompanied by a parallel, early increase in the cleavage activity of the same caspases. The higher level of caspase activity in PBMCs from AD compared to AA was not associated with quantitative differences in cell subset profiles. Moreover, no increase in apoptosis level, in the same experimental conditions, was found in PBMCs from this cohort of AD patients compared to those from AA. Conversely, the higher proneness to caspase activation in PBMCs from AD patients in comparison with that from AA was associated with a higher proliferative response to PHA or CD3. These data show a new dysfunction in AD patients at the PBMCs level and suggest that increased proneness to caspase activation in lymphocytes could reflect an ongoing systemic response in neurodegenerative disease with pathogenetic implications.

A novel systemically active caspase inhibitor attenuates the toxicities of MPTP, malonate, and 3NP in vivo

Molecular machinery involved in apoptosis plays a role in neuronal death in neurodegenerative disorders such as Parkinson's disease (PD) and Huntington's disease (HD). Several caspase inhibitors, such as the well-known peptidyl inhibitor carbobenzoxy-Val-Ala-Asp-fluoromethylketone (zVADfmk), can protect neurons from apoptotic death caused by mitochondrial toxins. However, the poor penetrability of zVADfmk into brain and toxicity limits its use therapeutically. In the present study, a novel peptidyl broad-spectrum caspase inhibitor, Q-VD-OPH, which offers improvements in potency, stability, and toxicity over zVADfmk, showed significant protection against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 3-nitropropionic acid (3NP), and malonate toxicities. Q-VD-OPH significantly reduced dopamine depletion in striatum produced by MPTP administration and prevented MPTP-induced loss of dopaminergic neurons in the substantia nigra. It significantly reduced the size of striatal lesions produced by intrastriatal malonate injections and systemic administration of 3NP. Western blots performed on tissues from the midbrain following administration of MPTP or the striatum in 3NP-treated animals showed increases of the active forms of caspase-9 and caspase-8, as well as the caspase-8-mediated proapoptotic protein Bid, which were inhibited Q-VD-OPH treatment. These findings suggest that systematically active broad-spectrum caspase inhibitors maybe useful in the treatment of neurodegenerative diseases such as PD and HD.

Neuroprotective effects of phenylbutyrate in the N171-82Q transgenic mouse model of Huntington's disease

Huntington's disease (HD) is caused by an expansion of exonic CAG triplet repeats in the gene encoding the huntingtin protein (Htt), however, the means by which neurodegeneration occurs remains obscure. There is evidence that mutant Htt interacts with transcription factors leading to reduced histone acetylation. We report that administration of the histone deacetylase inhibitor phenylbutyrate after onset of symptoms in a transgenic mouse model of HD significantly extends survival and attenuates both gross brain and neuronal atrophy. Administration of phenylbutyrate increased brain histone acetylation and decreased histone methylation levels as assessed by both immunocytochemistry and Western blots. Phenylbutyrate increased mRNA for components of the ubiquitin-proteosomal pathway and down-regulated caspases implicated in apoptotic cell death, and active caspase 3 immunoreactivity in the striatum. These results show that administration of phenylbutyrate, at doses that are well tolerated in man, exerts significant neuroprotective effects in a transgenic mouse model of HD, and therefore represents a very promising therapeutic approach for HD.

Adenosine and glutamate extracellular concentrations and mitogen-activated protein kinases in the striatum of Huntington transgenic mice. Selective antagonism of adenosine A2A receptors reduces transmitter outflow

The basal ganglia and deep layers of cerebral cortex neurodegeneration typically characterize the postmortem brain of Huntington disease (HD) patients. In this study, we employed 10- to 11-week-old transgenic HD mice (R6/2 line), in which the striatal adenosine extracellular levels, measured using the microdialysis technique, are significantly increased in comparison to wild-type mice. An increase in striatal adenosine is probably a precocious index of mitochondrial dysfunction that is described in both the postmortem brain of HD patients and transgenic mice striatal cells. The adenosine increase is matched by activation of the p38 mitogen-activated protein kinase (MAPK) in the striatal neurons of R6/2 mouse but not in the cortex. This result indicates that p38 MAPK is a correlate of striatal damage and suggests a role for p38 in the striatal neuron suffering and apoptosis described in this disease. The selective adenosine A(2A) receptor antagonist SCH 58261, administered through microdialysis fiber into the striatum, significantly decreases the outflow of glutamate in R6/2 mice. Antagonism of adenosine A(2A) receptors might be regarded as potentially useful in the treatment of this disease to control striatal excitotoxicity.

Mitochondrial function in apoptotic neuronal cell death

Apoptosis can be defined as the regulated death of a cell and is conducted by conserved pathways. Apoptosis of neurons after injury or disease differs from programed cell death, in the sense that neurons in an adult brain are not "meant" to die and results in a loss of function. Thus apoptosis is an honorable process by a neuron, a cell with limited potential to replace itself, choosing instead to commit suicide to save neighboring cells from release of cellular components that cause injury directly or trigger secondary injury resulting from inflammatory reactions. The excess of apoptosis of neuronal cells underlies the progressive loss of neuronal populations in neurodegenerative disorders and thus is harmful. Mitochondria are the primary source for energy in neurons but are also poised, through the "mitochondrial apoptosis pathway," to signal the demise of cells. This duplicity of mitochondria is discussed, with particular attention given to the specialized case of pathological neuronal cell death.

Binding and Release of Cytochrome c in Brain Mitochondria Is Influenced by Membrane Potential and Hydrophobic Interactions with Cardiolipin

Factors influencing the release and anchorage of cytochrome c to the inner membrane of brain mitochondria have been investigated. Metabolic activity of mitochondria caused a decrease in the membrane potential delta psi(m), accompanied by detachment of the protein from the inner membrane. In a model system of cytochrome c reconstituted in cardiolipin (CL) liposomes, phosphate was used to breach the hydrophilic lipid-protein interactions. About 44% cytochrome c was removable when heart CL (80% 18:2n-6) was employed, whereas the remaining protein accounted for the tightly bound conformation characterized by hydrophobic lipid-protein interactions. Cytochrome c release from brain CL liposomes was higher compared to heart CL, consistent with lower polyunsaturated fatty acid content. The release was even higher with CL extracted from metabolically stressed mitochondria, exhibiting more saturated fatty acid profile compared to control (30% vs. 17%). Therefore, weakening of the hydrophobic interactions due to saturation of CL may account for the observed cytochrome c release from mitochondria following metabolic stress. Moreover, mitochondria enriched with polyunsaturated CL exhibited higher delta psi(m), compared to less unsaturated species, suggesting that CL fatty acid composition influences delta psi(m). Mitochondria incorporated exogenous cytochrome c without protease-sensitive factors or delta psi(m). The internalized protein anchored to the inner membrane without producing swelling, as monitored by forward and side light scattering, but produced delta psi(m) consumption, suggesting recovery of respiratory activity. The delta psi(m) decrease is ascribed to a selected mitochondrial population containing the incorporated cytochrome c.

Sequential activation of individual caspases, and of alterations in Bcl-2 proapoptotic signals in a mouse model of Huntington's disease

Caspases play an important role in neurodegeneration in Huntington's disease (HD). Members of the Bcl-2 family are critical modulators of terminal cell death pathways. However, alterations of Bcl-2 family members and their functional role in an in vivo model of HD have not been documented. With the goal of gaining mechanistic insight, we used a transgenic mouse model of HD (R6/2) to investigate the chronology of caspase activation and functional alterations in members of the Bcl-2 family. In R6/2 mice caspase activation precedes proapoptotic changes in Bcl-2 family members. Of the caspases that we screened, caspase-1-like activation was the first to be detected in the disease process (7 weeks). Proapoptotic changes in members of the Bcl-2 family were first detected at 9 weeks. To demonstrate a potential functional/therapeutic role of Bcl-2 in HD, we crossed R6/2 mice with mice overexpressing Bcl-2 in neurons. Transgenic expression of Bcl-2 in R6/2 mice resulted in slight prolonged survival. Understanding the chronology of apoptotic events provides important information for appropriate therapeutic targeting in this devastating and untreatable disease.

Mitochondrial dysfunction and reactive oxygen species in excitotoxicity and apoptosis: implications for the pathogenesis of neurodegenerative diseases

In recent years we have witnessed a major interest in the study of the role of mitochondria, not only as ATP producers through oxidative phosphorylation but also as regulators of intracellular Ca2+ homeostasis and endogenous producers of reactive oxygen species (ROS). Interestingly, the mitochondria have been also implicated as central executioners of cell death. Increased mitochondrial Ca2+ overload as a result of excitotoxicity has been associated with the generation of superoxide and may induce the release of proapoptotic mitochondrial proteins, proceeding through DNA fragmentation/condensation and culminating in cell demise by apoptosis and/or necrosis. In addition, these processes have been implicated in the pathogenesis of many neurodegenerative diseases, which share several features of cell death: selective brain areas undergo neurodegeneration, involving mitochondrial dysfunction (mitochondrial complexes are affected), loss of intracellular Ca2+ homeostasis, excitotoxicity, and the extracellular or intracellular accumulation of insoluble protein aggregates in the brain.

Minocycline inhibits caspase-independent and -dependent mitochondrial cell death pathways in models of Huntington's disease

Minocycline is broadly protective in neurologic disease models featuring cell death and is being evaluated in clinical trials. We previously demonstrated that minocycline-mediated protection against caspase-dependent cell death related to its ability to prevent mitochondrial cytochrome c release. These results do not explain whether or how minocycline protects against caspase-independent cell death. Furthermore, there is no information on whether Smac/Diablo or apoptosis-inducing factor might play a role in chronic neurodegeneration. In a striatal cell model of Huntington's disease and in R6/2 mice, we demonstrate the association of cell death/disease progression with the recruitment of mitochondrial caspase-independent (apoptosis-inducing factor) and caspase-dependent (Smac/Diablo and cytochrome c) triggers. We show that minocycline is a drug that directly inhibits both caspase-independent and -dependent mitochondrial cell death pathways. Furthermore, this report demonstrates recruitment of Smac/Diablo and apoptosis-inducing factor in chronic neurodegeneration. Our results further delineate the mechanism by which minocycline mediates its remarkably broad neuroprotective effects.