Apoptosis in Huntington's disease

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.

Neuronal macroautophagy: from development to degeneration

Macroautophagy, a lysosomal pathway responsible for the turnover of organelles and long-lived proteins, has been regarded mainly as an inducible process in neurons, which is mobilized in states of stress and injury. New studies show, however, that macroautophagy is also constitutively active in healthy neurons and is vital to cell survival. Neurons in the brain, unlike cells in the periphery, are protected from large-scale autophagy induction because they can use several different energy sources optimally, receive additional nutrients and neurotrophin support from glial cells, and benefit from hypothalamic regulation of peripheral nutrient supplies. Due to its exceptional efficiency, constitutive autophagy in healthy neurons proceeds in the absence of easily detectable autophagic vacuole intermediates. These intermediates can accumulate rapidly, however, when late steps in the autophagic process are blocked. Autophagic vacuoles also accumulate abnormally in affected neurons of several major neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease, where they have been linked to various aspects of disease pathogenesis including neuronal cell death. The build-up of autophagic vacuoles in these neurological disorders and others may reflect either heightened autophagy induction, impairment in later digestive steps in the autophagy pathway, or both. Determining the basis for AV accumulation is critical for understanding the pathogenic significance of autophagy in a given pathologic state and for designing possible therapies based on modulating autophagy. In this review, we discuss the special features of autophagy regulation in the brain, its suspected roles in neurodevelopment and plasticity, and recent progress toward understanding how dysfunctional autophagy contributes to neurodegenerative disease.

Apoptotic cascades as possible targets for inhibiting cell death in Huntington's disease

Huntington's disease (HD) is a devastating autosomal dominant disorder characterized by progressive motor and neuropsychological symptoms. Evidence implicating the apoptotic cascades as a possible cause for the neurodegeneration seen in HD has directed researchers toward investigating therapeutic treatments targeting caspases and other proapoptotic factors. Cellular and murine models, which have demonstrated that caspase-mediated cleavage could be the cause for the neurodegeneration seen in HD, have evoked more research investigating the possible inhibition of apoptosis in HD. In particular, minocycline, a tetracycline-derived antibiotic that has been shown to increase survival in transgenic mouse models of HD, exhibits a neuroprotective feature in HD and demonstrates an anti-inflammatory as well as an anti-microbial effect by inhibiting microglial activation known to cause apoptosis.

Genetic interaction between expanded murine Hdh alleles and p53 reveal deleterious effects of p53 on Huntington's disease pathogenesis

Huntingtin, the protein product of the Huntington's disease (HD) gene, is known to interact with the tumor suppressor p53. It has recently been shown that activation of p53 upregulates the level of huntingtin, both in vitro and in vivo, whereas p53 deficiency in HD-transgenic flies and mice has been found to be beneficial. To explore further the involvement of p53 in HD pathogenesis, we generated mice homozygous for a mutant allele of Hdh (HdhQ140) and with zero, one, or two functional alleles of p53. p53 deficiency resulted in a reduction of mutant huntingtin expression in brain and testis, an increase in proenkephalin mRNA expression and a significant increase in nuclear aggregate formation in the striatum. Because aggregation of mutant huntingtin is suggested to be a protective mechanism, both the increase in aggregate load and the restoration of proenkephalin expression suggest a functional rescue of at least several aspects of the HD phenotype by a deficiency in p53.

Recent clinical failures in Parkinson's disease with apoptosis inhibitors underline the need for a paradigm shift in drug discovery for neurodegenerative diseases

Understanding the mechanisms of neuronal death in concert with the identification of drugable molecular targets key to this process has held great promise for the development of novel chemical entities (NCEs) to halt neurodegenerative disease progression. Two key targets involved in the apoptotic process identified over the past decade include the mixed lineage kinase (MLK) family and glyceraldehyde phosphate dehydrogenase (GAPDH). Two NCEs, CEP-1347 and TCH346, directed against these respective targets have progressed to the clinic. For each, robust neuroprotective activity was demonstrated in multiple in vitro and in vivo models of neuronal cell death, but neither NCE proved effective Parkinson's disease (PD) patients. These recent clinical failures require a reassessment of both the relevance of apoptosis to neurodegenerative disease etiology and the available animal models used to prioritize NCEs for advancement to the clinic in this area.

Comparative Proteomics in Neurodegenerative and Non-neurodegenerative Diseases Suggest Nodal Point Proteins in Regulatory Networking

Neurodegenerative disorders (ND) encompass clinically and genetically heterogeneous diseases with considerable overlap of their clinical, neuropathological and molecular phenotype. Various causes of neurodegeneration in disease may affect eventually the same proteins within protein networks. To identify common changes in ND, we compared brain protein changes detected by 2-D electrophoresis in four mouse models for ND: (i) Parkinson's disease, (ii) Huntington's disease, (iii) prion disease Scrapie, and (iv) a model for impaired synaptic transmission. To determine specificity of these changes for ND, we extended the scope of our investigation to three neurological conditions that do not result in neurodegeneration (non-ND). We detected 12 to 216 consistent qualitative or quantitative protein changes in individual ND and non-ND models when compared to controls. Up to 36% of these proteins were found to be altered in multiple disease states (at least three) and were therefore termed nodal point proteins. Alterations in alpha B-Crystallin and splicing factor 3b (subunit 4) occurred in at least three ND but not in non-ND. In contrast, alterations in peroxiredoxin 1 and 3, astrocytic phosphoprotein PEA15, complexin 2 and aminoacylase 1 were common to both ND and non-ND. Finally, we investigated the expression pattern of the nodal point proteins in three inbred mouse strains and found different protein abundance (expression polymorphisms) in all cases. Nodal point proteins showing expression polymorphisms may be candidate proteins for disease associated modifiers.

Autophagy in neurodegenerative disease: friend, foe or turncoat?

Autophagy, a lysosomal pathway for degrading organelles and long-lived proteins, is becoming recognized as a key adaptive response that can preclude death in stressed or diseased cells. However, during development strong induction of autophagy in specific cell populations mediates a type of programmed cell death that has distinctive 'autophagic' morphology and a requirement for autophagy activity. The recent identification of autophagosomes in neurons in a growing number of neurodegenerative disorders has, therefore, sparked controversy about whether these structures are contributing to neuronal cell death or protecting against it. Emerging evidence supports the view that induction of autophagy is a neuroprotective response and that inadequate or defective autophagy, rather than excessive autophagy, promotes neuronal cell death in most of these disorders. In this review, we consider possible mechanisms underlying autophagy-associated cell death and their relationship to pathways mediating apoptosis and necrosis.

Methamphetamine-induced cell death: selective vulnerability in neuronal subpopulations of the striatum in mice

Methamphetamine (METH) is an illicit and potent psychostimulant, which acts as an indirect dopamine agonist. In the striatum, METH has been shown to cause long lasting neurotoxic damage to dopaminergic nerve terminals and recently, the degeneration and death of striatal cells. The present study was undertaken to identify the type of striatal neurons that undergo apoptosis after METH. Male mice received a single high dose of METH (30 mg/kg, i.p.) and were killed 24 h later. To demonstrate that METH induces apoptosis in neurons, we combined terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) staining with immunohistofluorescence for the neuronal marker neuron-specific nuclear protein (NeuN). Staining for TUNEL and NeuN was colocalized throughout the striatum. METH induces apoptosis in approximately 25% of striatal neurons. Cell counts of TUNEL-positive neurons in the dorsomedial, ventromedial, dorsolateral and ventrolateral quadrants of the striatum did not reveal anatomical preference. The type of striatal neuron undergoing cell death was determined by combining TUNEL with immunohistofluorescence for selective markers of striatal neurons: dopamine- and cAMP-regulated phosphoprotein, of apparent Mr 32,000, parvalbumin, choline acetyltransferase and somatostatin (SST). METH induces apoptosis in approximately 21% of dopamine- and cAMP-regulated phosphoprotein, of apparent Mr 32,000-positive neurons (projection neurons), 45% of GABA-parvalbumin-positive neurons in the dorsal striatum, and 29% of cholinergic neurons in the dorsal-medial striatum. In contrast, the SST-positive interneurons were refractory to METH-induced apoptosis. Finally, the amount of cell loss determined with Nissl staining correlated with the amount of TUNEL staining in the striatum of METH-treated animals. In conclusion, some of the striatal projection neurons and the GABA-parvalbumin and cholinergic interneurons were removed by apoptosis in the aftermath of METH. This imbalance in the populations of striatal neurons may lead to functional abnormalities in the output and processing of neural information in this part of the brain.

Induction of apoptosis in cells expressing exogenous Hippi, a molecular partner of huntingtin-interacting protein Hip1

To decipher the pathway of apoptosis induction downstream to caspase-8 activation by exogenous expression of Hippi, an interactor of huntingtin-interacting protein Hip1, we studied apoptosis in HeLa and Neuro2A cells expressing GFP-tagged Hippi. Nuclear fragmentation, caspase-1, caspase-8, caspase-9/caspase-6 and caspase-3 activation were increased significantly in Hippi expressing cells. Cleavage of Bid, release of cytochrome c and apoptosis inducing factor (AIF) from mitochondria were also increased in GFP-Hippi expressing cells. It was observed that caspase-1 and caspase-8 activation was earlier than caspase-3 activation and nuclear fragmentation. Expression of caspase-1, caspase-3 and caspase-7 was increased while anti-apoptotic gene Bcl-2 and mitochondrial genes ND1 and ND4 were reduced in Hippi expressing cells. Besides, the expression SDHA and SDHB, nuclear genes, subunits of mitochondrial complex II were decreased in GFP-Hippi expressing cells. Taken together, we concluded that Hippi expression induced apoptosis by releasing AIF and cytochrome c from mitochondria, activation of caspase-1 and caspase-3, and altering the expression of apoptotic genes and genes involved in mitochondrial complex I and II.

Neuronal cell death caused by inhibition of intracellular cholesterol trafficking is caspase dependent and associated with activation of the mitochondrial apoptosis pathway

An elevated level of cholesterol in mitochondrial membranes of Niemann-Pick disease type C1 (NPC1) mouse brains and neural cells has been found to cause mitochondrial dysfunction. In this study, we demonstrate that inhibition of intracellular cholesterol trafficking in primary neurons by class 2 amphiphiles, which mimics the major biochemical and cellular feature of NPC1, led to not only impaired mitochondrial function but also activation of the mitochondrial apoptosis pathway. In activation of this pathway both cytochrome c and Smac/Diablo were released but apoptosis-inducing factor (AIF) was not involved. Treatment of the neurons with taurine, a caspase 9-specific inhibitor, could prevent the amphiphile-induced apoptotic cell death, suggesting that formation of apoptosome, followed by caspase 9 and caspase 3 activation, might play a critical role in the neuronal death pathway. Taken together, the mitochondria-dependent death cascade induced by blocking intracellular cholesterol trafficking was caspase dependent. The findings provide clues for both understanding the molecular basis of neurodegeneration in NPC1 disease and developing therapeutic strategies for treatment of this disorder.

Induction of striatal pre- and postsynaptic damage by methamphetamine requires the dopamine receptors

Methamphetamine (METH) is a psychostimulant that induces excessive release of dopamine (DA) in the striatum. In this study we have assessed the role of DA D1 and D2 receptors (D1R and D2R) on striatal METH-induced apoptosis and depletion of DA-terminal markers. Male mice were given one i.p. injection of METH (30 mg/kg). Apoptosis was assessed at 24 h, and DA-terminal marker depletion 3 days, after METH. A single toxic dose of METH induced apoptosis in approximately 10-13% of striatal neurons. This was completely prevented by pretreatment (30 min before METH) with either the D1R antagonist SCH-23390 (0.1 mg/kg) or the D2R antagonist raclopride (1 mg/kg). The same dose of METH induced depletion of DA transporter sites up to 61, 56, 71, and 69% in dorsal-medial, ventral-medial, dorsal-lateral, and ventral-lateral striatum, respectively, relative to vehicle-injected controls. Similarly, METH induced depletion of TH protein levels up to 80, 72, 87, and 90% in those respective quadrants. METH induced the expression of glial fibrillary acidic protein throughout the striatum. All these neurochemical changes were significantly attenuated by pretreatment with SCH-23390 (0.1 mg/kg) or raclopride (1 mg/kg). However, pretreatment with either raclopride or SCH-23390 did not prevent METH-induced hyperthermia in mice. These data demonstrate that the induction by METH of both striatal apoptosis and DA-terminal damage requires the activity of the postsynaptic DA receptors in the mouse brain. Moreover, since blockade of either receptor subtype protected from METH, the activity of both DA receptor subtypes is required for the induction of toxicity by METH in the striatum.

Compounds blocking mutant huntingtin toxicity identified using a Huntington's disease neuronal cell model

Neuronal cell death in HD is believed to be largely a dominant cell-autonomous effect of the mutant huntingtin protein. We previously developed an inducible PC12 cell model which expresses an N-terminal huntingtin fragment with an expanded poly Q repeat (N63-148Q) under the control of the tet-off system. In order to evaluate the ability of compounds to protect against mutant huntingtin toxicity in our model, we measured LDH released by dead cells into the medium. We have now screened the library of 1040 compounds from the NINDS Custom Collection as part of a National Institute of Neurological Disorders and Stroke (NINDS) collaborative project. Each positive compound was tested at 3-8 concentrations. Five compounds significantly attenuated mutant huntingtin (htt)-induced LDH release without affecting the expression level of huntingtin and independent of effect on aggregates. We also tested a broad spectrum caspase inhibitor Z-VAD-fmk and previously proposed candidate compounds. This cell model can provide a method to screen potential therapeutic compounds for treating Huntington's disease.

Ethyl-EPA treatment improves motor dysfunction, but not neurodegeneration in the YAC128 mouse model of Huntington disease

Huntington disease (HD) is an adult-onset neurodegenerative disorder that is characterized by selective degeneration in the striatum. There are currently no treatments that can prevent the progressive decline of motor and cognitive function in HD. In parallel with a human clinical trial, we examined the efficacy of ethyl-EPA treatment in the YAC128 mouse model of HD. Oral delivery of ethyl-EPA to symptomatic YAC128 mice beginning at 7 months of age increased membrane EPA levels 3-fold (P < 0.001) and resulted in a modest but significant improvement in motor dysfunction by 12 months of age as measured by open-field activity (P = 0.01) and performance on the rotarod (P = 0.05). At this age, ethyl-EPA-treated YAC128 mice showed no improvement in striatal volume, striatal neuron counts, striatal neuronal cross-sectional area, or striatal DARPP-32 expression compared to untreated YAC128 mice, thereby indicating no reduction of striatal neuropathology. This result is congruent with modest motor benefits observed in HD patients treated with ethyl-EPA. Overall, this work demonstrates the feasibility of experimental therapeutics in the YAC128 mouse model and suggests that experiments in these mice may be predictive for future human clinical trials.

The neuroprotective action of JNK3 inhibitors based on the 6,7-dihydro-5H-pyrrolo[1,2-a]imidazole scaffold

Imidazole-based structures of p38 inhibitors served as a starting point for the design of JNK3 inhibitors. Construction of a 6,7-dihydro-5H-pyrrolo[1,2-a]imidazole scaffold led to the synthesis of the (S)-enantiomers, which exhibited p38/JNK3 IC50 ratio of up to 10 and were up to 20 times more potent inhibitors of JNK3 than the relevant (R)-enantiomers. The JNK3 inhibitory potency correlated well with inhibition of c-Jun phosphorylation and neuroprotective properties of the compounds in low K+-induced cell death of rat cerebellar granule neurones.

Evaluation of the neuronal apoptotic pathways involved in cytoskeletal disruption-induced apoptosis

The cytoskeleton is critical to neuronal functioning and survival. Cytoskeletal alterations are involved in several neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. We studied the possible pathways involved in colchicine-induced apoptosis in cerebellar granule neurons (CGNs). Although colchicine evoked an increase in caspase-3, caspase-6 and caspase-9 activation, selective caspase inhibitors did not attenuate apoptosis. Inhibitors of other cysteine proteases such as PD150606 (a calpain-specific inhibitor), Z-Phe-Ala fluoromethyl ketone (a cathepsins-inhibitors) and N(alpha)-p-tosyl-l-lysine chloromethyl ketone (serine-proteases inhibitor) also had no effect on cell death/apoptosis induced by colchicine. However, BAPTA-AM 10 microM (intracellular calcium chelator) prevented apoptosis mediated by cytoskeletal alteration. These data indicate that calcium modulates colchicine-induced apoptosis in CGNs. PARP-1 inhibitors did not prevent apoptosis mediated by colchicine. Finally, colchicine-induced apoptosis in CGNs was attenuated by kenpaullone, a cdk5 inhibitor. Kenpaullone and indirubin also prevented cdk5/p25 activation mediated by colchicine. These findings indicate that cytoskeletal alteration can compromise cdk5 activation, regulating p25 formation and suggest that cdk5 inhibitors attenuate apoptosis mediated by cytoskeletal alteration. The present data indicate the potential therapeutic value of drugs that prevent the formation of p25 for the treatment of neurodegenerative disorders.

Brain-derived neurotrophic factor modulates dopaminergic deficits in a transgenic mouse model of Huntington's disease

Dysfunction of dopaminergic neurons may contribute to motor impairment in Huntington's disease. Here, we study the role of brain-derived neurotrophic factor (BDNF) in alterations of the nigrostriatal system associated with transgenics carrying mutant huntingtin. Using huntingtin-BDNF+/- double-mutant mice, we analyzed the effects of reducing the levels of BDNF expression in a model of Huntington's disease (R6/1). When compared with R6/1 mice, these mice exhibit an increased number of aggregates in the substantia nigra pars compacta. In addition, reduction of BDNF expression exacerbates the dopaminergic neuronal dysfunction seen in mutant huntingtin mice, such as the decrease in retrograde labelling of dopaminergic neurons and striatal dopamine content. However, mutant huntingtin mice with normal or lowered BDNF expression show the same decrease in the anterograde transport, number of dopaminergic neurons and nigral volume. In addition, reduced BDNF expression causes decreased dopamine receptor expression in mutant huntingtin mice. Examination of changes in locomotor activity induced by dopamine receptor agonists revealed that, in comparison with R6/1 mice, the double mutant mice exhibit lower activity in response to amphetamine, but not to apomorphine. In conclusion, these findings demonstrate that the decreased BDNF expression observed in Huntington's disease exacerbates dopaminergic neuronal dysfunction, which may participate in the motor disturbances associated with this neurodegenerative disorder.

Glyceraldehyde-3-phosphate dehydrogenase, apoptosis, and neurodegenerative diseases

Increasing evidence supports the notion that glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a protein with multiple functions, including its surprising role in apoptosis. GAPDH is overexpressed and accumulates in the nucleus during apoptosis induced by a variety of insults in diverse cell types. Knockdown of GAPDH using an antisense strategy demonstrates its involvement in the apoptotic cascade in which GAPDH nuclear translocation appears essential. Knowledge concerning the mechanisms underlying GAPDH nuclear translocation and subsequent cell death is growing. Additional evidence suggests that GAPDH may be an intracellular sensor of oxidative stress during early apoptosis. Abnormal expression, nuclear accumulation, changes in physical properties, and loss of glycolytic activity of GAPDH have been found in cellular and transgenic models as well as postmortem tissues of several neurodegenerative diseases. The interaction of GAPDH with disease-related proteins as well as drugs used to treat these diseases suggests that it is a potential molecular target for drug development.

Heterogeneous cellular environments modulate one-hit neuronal death kinetics

We recently demonstrated that cell loss kinetics in diverse forms of neurodegeneration (ND) suggests a universal death switch mechanism in which each cell is at a constant risk to initiate apoptosis. We proposed that mutant and injured neurons exist in a viable state typified by an increased risk of initiating death processes [Clarke, Collins, Leavitt, Andrews, Hayden, Lumsden, McInnes, A one-hit model of cell death in inherited neuronal degenerations, Nature 406 (2000) 195-199]. To date, however, measurements of cell death risk have been available only as averages across the affected cell population. Here we develop and apply a method of death kinetic analysis in which the risk factors vary across the neuronal population, as for example due to regional heterogeneities in the cellular microenvironment. We find that most cases of ND for which cell loss data has been obtained are better explained by death risks that vary from cell to cell, compared to death risk that is constant across the neuronal population. Strikingly, a common form of the frequency distribution defining the death risk heterogeneity is shared across most of these cases. This first characterization of the kinetic heterogeneity in one-hit neuronal death, therefore, suggests that the wide variety of ND now known may share mechanisms through which regional differences in the cellular microenvironment modulate the kinetics of cell loss.

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.

Failure of caspase inhibition in the double-lesion rat model of striatonigral degeneration (multiple system atrophy)

In the present study we assessed the neuroprotective effects of the pan-caspase inhibitor z-VAD.fmk [N-benzyloxycarbony-valine-alanine-aspartate-(OMe)-fluoromethylketone], and the caspase-3 inhibitor Ac-DEVD.CHO (acetyl-aspartate-chloromethylketone) in the double-lesion rat model of striatonigral degeneration (SND), the core pathology underlying levodopa-unresponsive parkinsonism associated with multiple system atrophy (MSA). Male Wistar rats were divided into three groups, receiving either Ac-DEVD.CHO, z-VAD.fmk or normal saline before lesion surgery, comprising a sequential unilateral quinolinic acid (QA) lesion of the striatum followed by a 6-hydroxydopamine (6-OHDA) lesion of the ipsilateral medial forebrain bundle. At 2 weeks post lesion, all rats underwent testing of spontaneous nocturnal locomotor behavior in an automated Photobeam Activity System (FlexField). Immunohistochemistry was performed with tyrosine hydroxylase, dopamine and cyclic adenosine 3',5'-monophosphate-regulated phosphoprotein and glial fibrillary acidic protein antibodies. Morphometry was performed using computerized image analysis. Behavioral and morphological analysis failed to show striatal or nigral protection in caspase inhibitor-treated animals. Our findings suggest that anti-apoptotic strategies are unrewarding in the SND rat model and, therefore, alternative neuroprotective interventions such as anti-glutamatergic agents or inhibitors of microglial activation should be explored instead.

Fine-structural analysis and connexin expression in the retina of a transgenic model of Huntington's disease

Recent studies indicate that the visual system appears more frequently affected in polyglutamine diseases than expected previously. Here, we investigated retinal degenerations in adult transgenic R6/2 mice, a model for Huntington's disease (HD). Light microscopical analysis revealed retinal dystrophy all over the retina, with central areas showing major effects. Electron microscopical analysis showed strong degenerations of outer and inner photoreceptor segments, shrinkage of photoreceptor cell somata, and signs of degeneration in photoreceptor terminals in the outer plexiform layer. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling showed hints of apoptosis. Mutant huntingtin and ubiquitin were expressed in all classes of retinal neurons, the pigment epithelium, and to a minor extent in neuropil structures. For investigating possible links to functional impairments in the rod-cone pathway, expression levels of three connexins (Cx) were compared in R6/2 and wildtype mice retinae. In R6/2 mice, expression of Cx36, the major neuronal connexin in the retina, was slightly reduced in the outer plexiform layer, indicating affected photoreceptor terminals as detected at the electron microscopical level. In contrast, Cx45, a putative neuronal connexin in the retina, was remarkably reduced in the inner plexiform layer of R6/2 mice. This result corresponded to fainter signals of Cx45 mRNA as documented by in situ hybridization and to a lower level of mCx45 cDNA as obtained by polymerase chain reaction after reverse transcription, suggesting functional deficits in spatial processing of Cx45-mediated gap junction coupling due to transgene-induced retinal degenerations. Thus, it is important to clarify the meaning of visual involvement in HD.

Chronic antidepressant medication attenuates dexamethasone-induced neuronal death and sublethal neuronal damage in the hippocampus and striatum

Dexamethasone, a synthetic corticosteroid, which can induce a range of mood disorders including depression and affective psychosis, is toxic to specific hippocampal and striatal neuronal populations. Chronic administration of antidepressants can induce neuroprotective effects, potentially by raising cellular levels of brain-derived neurotrophic factor (BDNF). We accordingly tested the hypothesis that chronic pretreatment of rats (Sprague-Dawley, male) with antidepressants would attenuate dexamethasone-induced neuronal damage as revealed by reductions in the level of neuronal death and in sublethal neuronal damage shown by the increase in the number of MAP-2 immunoreactive neurons. In support of this hypothesis, we demonstrate that chronic treatment with a range of antidepressants prior to dexamethasone administration (0.7 mg/kg, i.p.) attenuated the levels of neuronal death and loss of MAP-2 immunoreactivity in both the hippocampus and striatum. The antidepressants used were: desipramine (8 mg/kg, i.p., tricyclic), fluoxetine (8 mg/kg, i.p., selective serotonin reuptake inhibitor) and tranylcypromine (10 mg/kg, i.p., monoamine oxidase inhibitor) with each drug being injected once per day for 10 days. In contrast, acute injection of none of the antidepressants exerted a protective effect from dexamethasone-associated neuronal damage. Similarly, injection of neither cocaine nor chlordiazepoxide (benzodiazepine) exerted protective effects when injected either chronically or acutely. The observed protection from dexamethasone-induced neuronal damage is in keeping with the potential of chronic antidepressant medication to increase BDNF levels. The potential for dexamethasone to induce disorders of mood by damaging specific neuronal populations in the hippocampus and dorsomedial striatum is discussed.

Targeting CREB-binding protein (CBP) loss of function as a therapeutic strategy in neurological disorders

Histone acetylation/deacetylation is a master regulation of gene expression. Among the enzymes involved in this process, the CREB-binding protein (CBP) displays important functions during central nervous system development. Increasing evidence shows that CBP function is altered during neurodegenerative processes. CBP loss of function has now been reported in several diseases characterized by neurological disorders such as the Rubinstein-Taybi syndrome or polyglutamine-related pathologies (Huntington's disease). Our recent work suggests that CBP loss of function could also be involved in Alzheimer's disease and amyotrophic lateral sclerosis. In a simplified apoptotic model of primary neurons, we described CBP as a substrate of apoptotic caspases, an alternative to its classical proteasomal degradation. In these neuronal death contexts, histone acetylation levels were decreased as well. Altogether, these data point to a central role of CBP loss of function during neurodegeneration. In order to restore proper acetylation levels, a proposed therapeutic strategy relies on HDAC inhibition. Nevertheless, this approach lacks of specificity. Therefore new drugs targeted at counteracting CBP loss of function could stand as a valid therapeutic approach in neurodegenerative disorders. The challenge will be to respect the fine-tuning between cellular HAT/HDAC activities.

A cell-based screen for drugs to treat Huntington's disease

We have developed a medium-throughput cell-based assay to screen drugs for Huntington's disease (HD). The assay measures the ability of drugs to protect cultured neuronal (PC12) cells from death caused by an expanded polyglutamine (poly Q) form of huntingtin exon 1. Using this assay, we have blindly screened a library of 1040 compounds compiled by the NINDS: the NIH Custom Collection (NCC). Each compound was tested at five concentrations for its ability to protect cells against huntingtin-induced cell death as well as for its toxicity. Of the compounds tested, 18 prevented cell death completely, and 51 partially. Some of these also exhibited toxicity at higher doses. The majority of drugs (81%) were ineffective. Caspase inhibitors and cannabinoids showed reproducible protection in our assay. We believe these compounds, and others in our hit list, are appealing candidates for further investigation. Additionally, this assay is amenable to scaling up to screen additional compounds for treating Huntington's disease.

Apoptosis: target for novel drugs

Targeting apoptotic cell death pathways provides wide-ranging opportunities for the discovery and development of novel drugs. Some targeted therapies that selectively induce apoptosis in cancer cells are already marketed, and numerous pro-apoptotic drugs for treating cancer are currently being developed. The anti-apoptotic drugs that are most advanced in development are targeting acute disease indications such as stroke, myocardial infarction and sepsis, in which the role of apoptosis has been best defined and inhibitors of the apoptotic pathway have shown activity in various animal models. In the future, novel drugs might also result from an understanding of apoptotic pathways in chronic disorders.

Genetic and environmental factors in the pathogenesis of Huntington's disease

Huntington's disease is a fatal inherited disorder in which there is progressive neurodegeneration in specific brain areas, mainly the striatum and cerebral cortex, producing motor, cognitive, and psychiatric symptoms. The trinucleotide repeat mutation involved is common to many other brain diseases, which may therefore involve similar mechanisms of pathogenesis. We are beginning to understand how a CAG trinucleotide repeat expansion in the disease gene, encoding an expanded polyglutamine tract, induces neuronal dysfunction and symptomatology in Huntington's disease. Recent evidence that environmental factors modify the onset and progression of neurodegeneration has shed new light on Huntington's disease and other devastating brain diseases. This review focuses on genetic mediators, environmental modulators, and associated gene-environment interactions in the pathogenesis of Huntington's disease.

Gene-environment interplay in neurogenesis and neurodegeneration

Factors associated with predisposition and vulnerability to neurodegenerative disorders may be described usefully within the context of gene-environment interplay. There are many identified genetic determinants for so-called genetic disorders, and it is possible to duplicate many elements of recognized human neurodegenerative disorders in either knock-in or knock-out mice. However, there are similarly, many identifiable environmental influences on outcomes of the genetic defects; and the course of a progressive neurodegenerative disorder can be greatly modified by environmental elements. Constituent cellular defense mechanisms responsive to the challenge of increased reactive oxygen species represent only one crossroad whereby environment can influence genetic predisposition. In this paper we highlight some of the major neurodegenerative disorders and discuss possible links of gene-environment interplay. The process of adult neurogenesis in brain is also presented as an additional element that influences gene-environment interplay. And the so-called priming processes (i.e., production of receptor supersensitization by repeated drug dosing), is introduced as yet another process that influences how genes and environment ultimately and co-dependently govern behavioral ontogeny and outcome. In studies attributing the influence of genetic alteration on behavioral phenotypy, it is essential to carefully control environmental influences.

Apoptotic signaling cascades

Apoptosis is a form of programmed cell death that results in the orderly and efficient removal of damaged or unnecessary cells, such as those resulting from DNA damage or during development. There are many factors that contribute to this process, each demonstrating specificity of function, regulation, and pathway involvement. The aim of this brief overview is to provide an introduction to a number of these factors as well as the various apoptotic pathways that have been identified.