Generation of neuronal intranuclear inclusions by polyglutamine-GFP: analysis of inclusion clearance and toxicity as a function of polyglutamine length

Polyglutamine Repeats in Viruses

This review explores the presence and functions of polyglutamine (polyQ) in viral proteins. In mammals, mutations in polyQ segments (and CAG repeats at the nucleotide level) have been linked to neural disorders and ataxias. PolyQ regions in normal human proteins have documented functional roles, in transcription factors and, more recently, in regulating autophagy. Despite the high frequency of polyQ repeats in eukaryotic genomes, little attention has been given to the presence or possible role of polyQ sequences in virus genomes. A survey described here revealed that polyQ repeats occur rarely in RNA viruses, suggesting that they have detrimental effects on virus replication at the nucleotide or protein level. However, there have been sporadic reports of polyQ segments in potyviruses and in reptilian nidoviruses (among the largest RNA viruses known). Conserved polyQ segments are found in the regulatory control proteins of many DNA viruses. Variable length polyQ tracts are found in proteins that contribute to transmissibility (cowpox A-type inclusion protein (ATI)) and control of latency (herpes viruses). New longer-read sequencing methods, using original biological samples, should reveal more details on the presence and functional role of polyQ in viruses, as well as the nucleotide regions that encode them. Given the known toxic effects of polyQ repeats, the role of these segments in neurovirulent and tumorigenic viruses should be further explored.

Degenerate codon mixing for PCR-based manipulation of highly repetitive sequences

Objective

Repeat expansion of polyglutamine tracks leads to a group of inherited human neurodegenerative disorders. Studying such repetitive sequences is required to gain insight into the pathophysiology of these diseases. PCR-based manipulation of repetitive sequences, however, is challenging due to the absence of unique primer binding sites or the generation of non-specific products.

Results

We have utilised the degeneracy of the genetic code to generate a polyglutamine sequence with low repeat similarity. This strategy allowed us to use conventional PCR to generate multiple constructs with approximately defined numbers of glutamine repeats. We then used these constructs to measure the in vivo variation in autophagic degradation activity related to the different numbers of glutamine repeats, providing an example of their applicability to study repeat expansion diseases. Our simple and easily generalised method of generating low repetition DNA sequences coding for uniform stretches of amino acid residues provides a strategy for generating particular lengths of polyglutamine tracts using standard PCR and cloning protocols.

Electronic supplementary material

The online version of this article (10.1186/s13104-018-3298-5) contains supplementary material, which is available to authorized users.

Fluorescence anisotropy uncovers changes in protein packing with inclusion growth in a cellular model of polyglutamine aggregation

The aggregation of polyglutamine-rich proteins is closely linked with numerous neurodegenerative disorders. In pathological and cellular models, the appearance of protein-rich inclusions in cells acts as a read out of protein aggregation. The precise organization of aggregated protein in these inclusions and their mode of growth are still poorly understood. Here, fluorescence anisotropy-based measurements have been used to probe protein packing across inclusions of varying brightness, formed by an monomeric enhanced green fluorescent protein (mEGFP)-tagged polyglutamine model peptide in cells. High-resolution, confocal-based steady-state anisotropy measurements report a large depolarization, consistent with extensive homo-Förster (fluorescence) resonance energy transfer (FRET) between the sequestered mEGFP-tagged protein molecules. An inverse correlation of fluorescence anisotropy with intensity is seen across inclusions, which becomes emphasized when the observed fluorescence anisotropy values of inclusions are corrected for the fluorescence contribution of the diffusible protein, present within and around smaller inclusions. Homo-FRET becomes enhanced as inclusion size increases. This enhancement is confirmed by two-photon excitation-based time-resolved fluorescence anisotropy decay measurements, which also suggest that the mEGFP-tagged protein molecules are arranged in multiple ways within inclusions. Bright inclusions display faster FRET rates with a larger number of mEGFP moieties participating in homo-FRET than faint inclusions do. These results are consistent with a model in which the protein is more closely packed in the brighter inclusions. In such a possible mechanism, the higher packing density of protein molecules in brighter inclusions would suggest that inclusion growth could involve an intermolecular compaction event within the inclusion, as more monomers and aggregates are recruited into the growing inclusion.

Expansion of the polyQ repeats in THAP11 forms intranuclear aggregation and causes cell G0/G1 arrest

Polyglutamine diseases are a group of neurodegenerative disorders caused by expansion of a CAG repeat that encodes polyglutamine in each respective disease gene. The transcription factor THAP11, a member of THAP family, is involved in cell growth, ES cell pluripotency and embryogenesis. Previous studies suggest that THAP11 protein contains a 29-residue repeat polyglutamine motif and the number of polyglutamine ranges from 20 to 41 in Indian population. We have investigated the CAG numbers at the THAP11 locus in normal individuals and neurodegenerative disease patients of Chinese Han population and a 38Q expansion (THAP11(38Q)) was found in patients. Using fluorescence confocal-based cell imaging, THAP11(38Q) protein formed intranuclear inclusions easier than THAP11(29Q) in PC12 cells. Enhanced toxicity was investigated in THAP11(38Q)-expressing cells by growth inhibition and G0/G1 arrest. CREB-mediated transcription activity was inhibited by THAP11(38Q). The transcription factor, TBP, coactivator CBP, and chaperon protein, HSP70, could be recruited to THAP11(38Q). These results indicate that expansion of the polyglutamine in THAP11 forms intracellular aggregation and is toxic in PC12 cells, suggesting a putative role of THAP11 in polyglutamine disease.

A network of genes connects polyglutamine toxicity to ploidy control in yeast

Neurodegeneration is linked to protein aggregation in several human disorders. In Huntington’s disease, the length of a polyglutamine stretch in Huntingtin is correlated to neuronal death. Here we utilize a model based on glutamine stretches of 0, 30 or 56 residues in Saccharomyces cerevisiae to understand how such toxic proteins interfere with cellular physiology. A toxicity-mimicking cytostatic effect is evident from compromised colony formation upon expression of polyglutamines. Interestingly, diploid cells are insensitive to polyglutamines and haploid cells can escape cytostasis by hyperploidization. Using a genome-wide screen for genes required to obtain the cytostatic effect, we identify a network related to the budding process and cellular division. We observe a striking mislocalization of the septins Cdc10 and Shs1 in cells arrested by polyglutamines, suggesting that the septin ring may be a pivotal structure connecting polyglutamine toxicity and ploidy.

Expansion of polyglutamines correlates with neuronal death in Huntington’s disease. Here the authors show that, in haploid yeast cells, the toxic effect of polyglutamine expression is associated with the disruption of the septin ring and cells may escape from toxicity by hyperploidization.

Characterization of the proteostasis roles of glycerol accumulation, protein degradation and protein synthesis during osmotic stress in C. elegans

Exposure of C. elegans to hypertonic stress-induced water loss causes rapid and widespread cellular protein damage. Survival in hypertonic environments depends critically on the ability of worm cells to detect and degrade misfolded and aggregated proteins. Acclimation of C. elegans to mild hypertonic stress suppresses protein damage and increases survival under more extreme hypertonic conditions. Suppression of protein damage in acclimated worms could be due to 1) accumulation of the chemical chaperone glycerol, 2) upregulation of protein degradation activity, and/or 3) increases in molecular chaperoning capacity of the cell. Glycerol and other chemical chaperones are widely thought to protect proteins from hypertonicity-induced damage. However, protein damage is unaffected by gene mutations that inhibit glycerol accumulation or that cause dramatic constitutive elevation of glycerol levels. Pharmacological or RNAi inhibition of proteasome and lyosome function and measurements of cellular protein degradation activity demonstrated that upregulation of protein degradation mechanisms plays no role in acclimation. Thus, changes in molecular chaperone capacity must be responsible for suppressing protein damage in acclimated worms. Transcriptional changes in chaperone expression have not been detected in C. elegans exposed to hypertonic stress. However, acclimation to mild hypertonicity inhibits protein synthesis 50–70%, which is expected to increase chaperone availability for coping with damage to existing proteins. Consistent with this idea, we found that RNAi silencing of essential translational components or acute exposure to cycloheximide results in a 50–80% suppression of hypertonicity-induced aggregation of polyglutamine-YFP (Q35::YFP). Dietary changes that increase protein production also increase Q35::YFP aggregation 70–180%. Our results demonstrate directly for the first time that inhibition of protein translation protects extant proteins from damage brought about by an environmental stressor, demonstrate important differences in aging- versus stress-induced protein damage, and challenge the widely held view that chemical chaperones are accumulated during hypertonic stress to protect protein structure/function.

Physical chemistry of polyglutamine: intriguing tales of a monotonous sequence

Polyglutamine (polyQ) sequences of unknown normal function are present in a significant number of proteins, and their repeat expansion is associated with a number of genetic neurodegenerative diseases. PolyQ solution structure and properties are important not only because of the normal and abnormal biology associated with these sequences but also because they represent an interesting case of a biologically relevant homopolymer. As the common thread in expanded polyQ repeat diseases, it is important to understand the structure and properties of simple polyQ sequences. At the same time, experience has shown that sequences attached to polyQ, whether in artificial constructs or in disease proteins, can influence structure and properties. The two major contenders for the molecular source of the neurotoxicity implicit in polyQ expansion within disease proteins are a populated toxic conformation in the monomer ensemble and a toxic aggregated species. This review summarizes experimental and computational studies on the solution structure and aggregation properties of both simple and complex polyQ sequences, and their repeat-length dependence. As a representative of complex polyQ proteins, the behavior of huntingtin N-terminal fragments, such as exon-1, receives special attention.

Humanins, the neuroprotective and cytoprotective peptides with antiapoptotic and anti-inflammatory properties

Humanin (HN) is a newly discovered 24-amino acid peptide, which may suppress neuronal cell death. HN cDNA includes an open reading frame (HN-ORF) of 75 bases located 950 bases downstream of the 5' end of the HN cDNA. It has been demonstrated that HN cDNA is 99% identical to the mitochondrial DNA (mtDNA) sequence. HN homologs have been identified as expressed sequence tags (ESTs) in both rats and nematodes. Certain regions that are homologous to the HN cDNA exist on human chromosomes. HN forms homodimers and multimers and this action seems to be essential for peptide function. HN acts as a ligand for formyl peptide receptor-like 1 (FPRL1) and 2 (FPRL2). It has been demonstrated that HN plays a protective role through its antiapoptotic activity that interferes with Bax activation, which suppresses Bax-dependent apoptosis. HN has also been shown to suppress the c-Jun N-terminal kinase (JNK) and ASK/JNK-mediated neuronal cell death. Several studies have also confirmed that HN could be important in the prevention of angiopathy-associated Alzheimer's disease dementia, diseases related to mitochondrial dysfunction (MELAS), and other types of β-amyloid accumulation-associated neurodegeneration. Avery recent study demonstrated a pluripotent cytoprotective effect and mechanisms of HNs in cells not from the CNS, such as germ cells or pancreatic β-cells, and the potent physiological consequences that result from HN interaction with IGFBP3 and STAT3. In vivo studies suggest that HN may also protect against cognitive impairment due to ischemia/reperfusion injury.

Electroconvulsive shock ameliorates disease processes and extends survival in huntingtin mutant mice

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by expanded polyglutamine repeats in the huntingtin (Htt) protein. Mutant Htt may damage and kill striatal neurons by a mechanism involving reduced production of brain-derived neurotrophic factor (BDNF) and increased oxidative and metabolic stress. Because electroconvulsive shock (ECS) can stimulate the production of BDNF and protect neurons against stress, we determined whether ECS treatment would modify the disease process and provide a therapeutic benefit in a mouse model of HD. ECS (50 mA for 0.2 s) or sham treatment was administered once weekly to male N171-82Q Htt mutant mice beginning at 2 months of age. Endpoints measured included motor function, striatal and cortical pathology, and levels of protein chaperones and BDNF. ECS treatment delayed the onset of motor symptoms and body weight loss and extended the survival of HD mice. Striatal neurodegeneration was attenuated and levels of protein chaperones (Hsp70 and Hsp40) and BDNF were elevated in striatal neurons of ECS-treated compared with sham-treated HD mice. Our findings demonstrate that ECS can increase the resistance of neurons to mutant Htt resulting in improved functional outcome and extended survival. The potential of ECS as an intervention in subjects that inherit the mutant Htt gene merits further consideration.

Spatial positions of homopolymeric repeats in the human proteome and their effect on cellular toxicity

Proteins with homopolymeric repeat tracts are very common in the human proteome. Intriguingly, some but not all repeat tracts show length variation in the population and, in a few, the expansion of repeat tract beyond the normal length is associated with neurodegenerative and developmental disorders. In this study we have addressed questions such as why some amino acid residues are favored in longer repeat tracts and why repeat tracts show terminal bias. Using cell biological assays for repeat tracts fused to green fluorescent protein we show here that homopolymeric repeats that are beyond their naturally occurring length in the proteome are cytotoxic in nature. This toxicity is further modulated by the length of the peptide that bears the repeat and the spatial location of the repeat within the peptide. Thus, the cellular toxicity appears to be one of the selective processes that regulate the evolution of homopolymeric repeats in the proteome.

The beta amyloid peptide can act as a modular aggregation domain

Although there is compelling evidence that the beta amyloid peptide (Abeta) can be centrally involved in Alzheimer's disease, the natural role (if any) of this peptide remains unclear. Here we use green fluorescent protein (GFP) fusions to demonstrate that the Abeta sequence, like prion domains, can act as a modular aggregation domain when terminally appended to a normally soluble protein. We find that a single amino acid substitution (Leu(17) to Pro) in the beta peptide sequence can abolish this cis capacity to induce aggregation. Introduction of this substitution into full-length APP (i.e., a Leu(613)Pro substitution in APP695) alters the processing of APP leading to the accumulation of the C99 C-terminal fragment (CTF). We suggest that in at least some aggregation disease-related proteins the presence of an aggregation domain is not "accidental", but reflects a selected role of these domains in modulating the trafficking or metabolism of the parental protein.

Nucleocytoplasmic trafficking and transcription effects of huntingtin in Huntington's disease

There are nine genetic neurodegenerative diseases caused by a similar genetic defect, a CAG DNA triplet-repeat expansion in the disease gene's open reading frame resulting in a polyglutamine expansion in the disease proteins. Despite the commonality of polyglutamine expansion, each of the polyglutamine diseases manifest as unique diseases, with some similarities, but important differences. These differences suggest that the context of the polyglutamine expansion is important to the mechanism of pathology of the disease proteins. Therefore, it is becoming increasingly paramount to understand the normal functions of these polyglutamine disease proteins, which include huntingtin, the polyglutamine-expanded protein in Huntington's disease (HD). Transcriptional dysregulation is seen in HD. Here we discuss the role of normal huntingtin in transcriptional regulation and misregulation in Huntington's disease in relation to potentially analogous model systems, and to other polyglutamine disease proteins. Huntingtin has functional roles in both the cytoplasm and the nucleus. One commonality of activity of polyglutamine disease proteins is at the level of protein dynamics and ability to import and export to and from the nucleus. Knowing the temporal location of huntingtin protein in response to signaling and neuronal communication could lead to valuable insights into an important trigger of HD pathology.

Soluble polyglutamine oligomers formed prior to inclusion body formation are cytotoxic

Expanded polyglutamine (polyQ) repeats cause neurodegenerative disorders, but their cytotoxic structures remain to be elucidated. Although soluble polyQ oligomers have been proposed as a cytotoxic structure, the cytotoxicity of soluble polyQ oligomers, not inclusion bodies (IBs), has not been proven in living cells. To clarify the cytotoxicity of soluble polyQ oligomers, we carried our fluorescence resonance energy transfer (FRET) confocal microscopy and distinguished oligomers from monomers and IBs in a single living cell. FRET signals were detected when donor and acceptor fluorescent proteins were attached to the same side, not the opposite side, of polyQ repeats, which agrees with a parallel beta-sheet or a head-to-tail cylindrical beta-sheet model. These FRET signals disappeared in semi-intact cells, indicating that these polyQ oligomers are soluble. PolyQ monomers assembled into soluble oligomers in a length-dependent manner, which was followed by the formation of IBs. Notably, survival assay of neuronally differentiated cells revealed that cells with soluble oligomers died faster than those with IBs or monomers. These results indicate that a length-dependent formation of oligomers is an essential mechanism underlying neurodegeneration in polyQ-mediated disorders.

Calcium and neurodegeneration

When properly controlled, Ca2+ fluxes across the plasma membrane and between intracellular compartments play critical roles in fundamental functions of neurons, including the regulation of neurite outgrowth and synaptogenesis, synaptic transmission and plasticity, and cell survival. During aging, and particularly in neurodegenerative disorders, cellular Ca2+-regulating systems are compromised resulting in synaptic dysfunction, impaired plasticity and neuronal degeneration. Oxidative stress, perturbed energy metabolism and aggregation of disease-related proteins (amyloid beta-peptide, alpha-synuclein, huntingtin, etc.) adversely affect Ca2+ homeostasis by mechanisms that have been elucidated recently. Alterations of Ca2+-regulating proteins in the plasma membrane (ligand- and voltage-gated Ca2+ channels, ion-motive ATPases, and glucose and glutamate transporters), endoplasmic reticulum (presenilin-1, Herp, and ryanodine and inositol triphosphate receptors), and mitochondria (electron transport chain proteins, Bcl-2 family members, and uncoupling proteins) are implicated in age-related neuronal dysfunction and disease. The adverse effects of aging on neuronal Ca2+ regulation are subject to modification by genetic (mutations in presenilins, alpha-synuclein, huntingtin, or Cu/Zn-superoxide dismutase; apolipoprotein E isotype, etc.) and environmental (dietary energy intake, exercise, exposure to toxins, etc.) factors that may cause or affect the risk of neurodegenerative disease. A better understanding of the cellular and molecular mechanisms that promote or prevent disturbances in cellular Ca2+ homeostasis during aging may lead to novel approaches for therapeutic intervention in neurological disorders such as Alzheimer's and Parkinson's diseases and stroke.

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.

C-termini of P/Q-type Ca2+ channel alpha1A subunits translocate to nuclei and promote polyglutamine-mediated toxicity

P/Q-type voltage-gated calcium channels are regulated, in part, through the cytoplasmic C-terminus of their alpha1A subunit. Genetic absence or alteration of the C-terminus leads to abnormal channel function and neurological disease. Here, we show that the terminal 60-75 kDa of the endogenous alpha1A C-terminus is cleaved from the full-length protein and is present in cell nuclei. Antiserum to the C-terminus (CT-2) labels both wild-type mouse and human Purkinje cell nuclei, but not leaner mouse cerebellum. Human embryonic kidney cells stably expressing beta3 and alpha2delta subunits and transiently transfected with full-length human alpha1A contain a 75 kDa CT-2 reactive peptide in their nuclear fraction. Primary granule cells transfected with C-terminally Green fluorescent protein (GFP)-tagged alpha1A exhibit GFP nuclear labeling. Nuclear translocation depends partly on the presence of three nuclear localization signals within the C-terminus. The C-terminal fragment bears a polyglutamine tract which, when expanded (Q33) as in spinocerebellar ataxia type 6 (SCA6), is toxic to cells. Moreover, polyglutamine-mediated toxicity is dependent on nuclear localization. Finally, in the absence of flanking sequence, the Q33 expansion alone does not kill cells. These results suggest a novel processing of the P/Q-type calcium channel and a potential mechanism for the pathogenesis of SCA6.

Polyhistidine tract expansions in HOXA1 result in intranuclear aggregation and increased cell death

HOXA1 gene is part of a cluster of homeotic selector genes that regulates the anteroposterior patterning of mammals during embryonic development. HOXA1 encodes two alternatively spliced mRNAs with two isoforms, A and B, the former contains the homeodomain and expressed in early embryonic development. HOXA1 contains a string of 10 histidine repeats. However, individuals heterozygous for 7, 9, 11, and 12 histidine repeat variants were present among the Japanese population, notably in some autism cases. To determine the biological implications of the different polyhistidine repeat lengths, we expressed these variants in COS-7 and a human neuroblastoma cell line (SK-N-SH). Expression of expanded variants of HOXA1 isoform A, containing 11 and 12 polyhistidine, resulted in early and great degree of protein aggregation in the nucleus. This aggregation resulted in accelerated cell death in cells expressing 11 and 12 expanded variants compared to those transfected with 7 and 10 polyhistidine variants. Furthermore, we showed that these aggregates were ubiquitinated and were inhibited by a histidine-modifying compound, DEPC. These data suggest that HOXA1 protein with polyhistidine tract expansions misfold, aggregate, and have a toxic effect on cell.

Transcriptional repression and cell death induced by nuclear aggregates of non-polyglutamine protein

Nuclear aggregates of polyglutamine (polyQ)-expanded proteins are associated with a number of neurodegenerative diseases including Huntington's disease (HD) and spinocerebellar ataxias (SCAs). The nuclear deposition of polyQ proteins correlates with rearrangements of nuclear matrix, transcriptional dysregulation, and cell death. To explore the requirement for polyQ tracks in educing such cellular responses, we examined whether a non-polyQ protein can deposit as nuclear aggregates and elicit similar responses. We report that a protein chimera (GFP170*) composed of the green fluorescent protein (GFP) fused to an internal fragment of the Golgi Complex Protein (GCP-170) forms nuclear aggregates analogous to those formed by polyQ proteins. Like the polyQ nuclear aggregates, GFP170* inclusions recruit molecular chaperones and proteasomal components, alter nuclear structures containing the promyelocytic leukemia protein (PML), recruit transcriptional factors such as CREB-binding protein (CBP) and p53, repress p53 transcriptional activity, and induce cell death. Our results indicate that nuclear aggregation and transcriptional effects are not unique to polyQ-containing proteins and may represent a general response to misfolded proteins in the nucleus.

Interference of CREB-dependent transcriptional activation by expanded polyglutamine stretches--augmentation of transcriptional activation as a potential therapeutic strategy for polyglutamine diseases

On the basis of the hypothesis that the interaction of mutant proteins with expanded polyglutamine stretches with transcriptional co-activator, TAFII130, leads to transcriptional dysregulation, the transcriptional activation of c-Fos and its suppression by expanded polyglutamine stretches was investigated. The phosphorylation of cAMP-responsive element binding protein (CREB) and induction of c-Fos in response to cAMP were strongly suppressed in Neuro2a cells expressing expanded polyglutamine. The suppression of CREB-dependent transcriptional activation was reversibly rescued by increasing the concentration of cAMP. Expanded polyglutamine-induced cytotoxicity was also substantially suppressed by augmenting CREB-dependent transcriptional activation with a high concentration of cAMP. FR901228, a histone deacetylase inhibitor, was also demonstrated as rescuing the expanded polyglutamine-induced suppression of CREB phosphorylation and c-Fos expression. Furthermore, nuclear fragmentation was significantly suppressed by FR901228. The co-expression of dominant-negative CREB vectors considerably abrogated the suppressive effect of cAMP and FR901228 on the expanded polyglutamine-induced nuclear fragmentation, suggesting that these compounds suppress polyglutamine-induced cytotoxicity, largely, via the enhancement of CREB-dependent transcriptional activation. These findings suggest that the interference of CREB-dependent transcriptional activation by expanded polyglutamine stretches is involved in the pathogenetic mechanisms underlying neurodegeneration, and that the augmentation of CREB-dependent transcriptional activation is a potential strategy in treating polyglutamine diseases.

Comparative analysis of the cytotoxicity of homopolymeric amino acids

Many human proteins have homopolymeric amino acid (HPAA) tracts, although the physiological significance or cellular effects of their presence is poorly understood. We previously reported that 20 kinds of HPAAs show characteristic intracellular localization and that among those, hydrophobic HPAAs aggregate strongly and form high molecular weight proteins when expressed in cultured cells. In this study, we investigated the cytotoxicity of 20 kinds of HPAAs. HPAA tracts of approximately 30 residues fused to the C-terminus of YFP were expressed in COS-7 cells. Cells expressing homopolymeric-Cys, -Ile, -Leu, and -Val showed low viability in Trypan Blue assay. Caspase-3 activity, which is usually upregulated in dying cells, was determined by measuring the cleavage of the peptide substrate Ac-DEVD-MCA and by detecting the cleaved active form of the caspase-3 by Western blotting. The activity of caspase-3 was drastically elevated in cells expressing those HPAAs which showed low viability in Trypan Blue assay. Interestingly, it was found that there is a correlation between the hydrophobicity of a single amino acid and the cytotoxicity of the corresponding HPAA as a homopolymer. These results indicate that the hydrophobicity of HPAAs may cause cytotoxicity.

Glial and neuronal expression of polyglutamine proteins induce behavioral changes and aggregate formation in Drosophila

Patients with polyglutamine expansion diseases, like Huntington's disease or several spinocerebellar ataxias, first present with neurological symptoms that can occur in the absence of neurodegeneration. Behavioral symptoms thus appear to be caused by neuronal dysfunction, rather than cell death. Pathogenesis in polyglutamine expansion diseases is largely viewed as a cell-autonomous process in neurons. It is likely, however, that this process is influenced by changes in glial physiology and, at least in the case of DRPLA glial inclusions and glial cell death, seems to be an important part in the pathogenesis. To investigate these aspects in a Drosophila model system, we expressed polyglutamine proteins in the adult nervous system. Glial-specific expression of a polyglutamine (Q)-expanded (n=78) and also a nonexpanded (n=27) truncated version of human ataxin-3 led to the formation of protein aggregates and glial cell death. Behavioral changes were observed prior to cell death. This reveals that glia is susceptible to the toxic action of polyglutamine proteins. Neuronal expression of the same constructs resulted in behavioral changes similar to those resulting from glial expression but did not cause neurodegeneration. Behavioral deficits were selective and affected two analyzed fly behaviors differently. Both glial and neuronal aggregates of Q78 and Q27 appeared early in pathogenesis and, at the electron microscopic resolution, had a fibrillary substructure. This shows that a nonexpanded stretch can cause similar histological and behavioral symptoms as the expanded stretch, however, with a significant delay.

Inactivation of Drosophila Apaf-1 related killer suppresses formation of polyglutamine aggregates and blocks polyglutamine pathogenesis

Huntington's disease (HD) is caused by expansion of a polyglutamine tract near the N-terminal of huntingtin. Mutant huntingtin forms aggregates in striatum and cortex, where extensive cell death occurs. We used a Drosophila polyglutamine peptide model to assess the role of specific cell death regulators in polyglutamine-induced cell death. Here, we report that polyglutamine-induced cell death was dramatically suppressed in flies lacking Dark, the fly homolog of human Apaf-1, a key regulator of apoptosis. Dark appeared to play a role in the accumulation of polyglutamine-containing aggregates. Suppression of cell death, caspase activation and aggregate formation were also observed when mutant huntingtin exon 1 was expressed in homozygous dark mutant animals. Expanded polyglutamine induced a marked increase in expression of Dark, and Dark was observed to colocalize with ubiquitinated protein aggregates. Apaf-1 also was found to colocalize with huntingtin-containing aggregates in a murine model and HD brain, suggesting a common role for Dark/Apaf-1 in polyglutamine pathogenesis in invertebrates, mice and man. These findings suggest that limiting Apaf-1 activity may alleviate both pathological protein aggregation and neuronal cell death in HD.

Inclusion body formation reduces levels of mutant huntingtin and the risk of neuronal death

Huntington's disease is caused by an abnormal polyglutamine expansion within the protein huntingtin and is characterized by microscopic inclusion bodies of aggregated huntingtin and by the death of selected types of neuron. Whether inclusion bodies are pathogenic, incidental or a beneficial coping response is controversial. To resolve this issue we have developed an automated microscope that returns to precisely the same neuron after arbitrary intervals, even after cells have been removed from the microscope stage. Here we show, by survival analysis, that neurons die in a time-independent fashion but one that is dependent on mutant huntingtin dose and polyglutamine expansion; many neurons die without forming an inclusion body. Rather, the amount of diffuse intracellular huntingtin predicts whether and when inclusion body formation or death will occur. Surprisingly, inclusion body formation predicts improved survival and leads to decreased levels of mutant huntingtin elsewhere in a neuron. Thus, inclusion body formation can function as a coping response to toxic mutant huntingtin.

Proteasome inhibitors suppress formation of polyglutamine-induced nuclear inclusions in cultured postmitotic neurons

At least nine neurodegenerative disorders are caused by expansion of polyglutamine repeats in various genes. This expansion induces the formation of nuclear inclusions (NI) within various cell types. In this study, we developed a model for polyglutamine diseases using primary cultures of sympathetic neurons from the superior cervical ganglia of prenatal rat pups. Transfection with a plasmid encoding 127 glutamine repeats causes NI to develop in approximately 70% of the sympathetic neurons within 6 days. In addition, it causes somatic atrophy and inhibits dendritic growth. The NIs contain ubiquitinated proteins and sequester the molecular chaperone heat shock protein 70 (Hsp70). We found that two specific proteasome inhibitors, lactacystin and CEP1612, suppress thezformation of polyglutamine-induced NI. In addition, lactacystin treatment induced the removal of preexisting NI. Western blotting and immunocytochemistry revealed that lactacystin and CEP1612 strongly induce the expression of Hsp70, whereas less specific proteasome inhibitor such as N-acetyl-Leu-Leu-Norleucinal does not. Coexpression of 127 glutamines with a plasmid encoding wild-type Hsp70 gene resulted in a marked reduction of the percentage of neurons containing NI. In addition, transfection with plasmids encoding mutant Hsp70 blocked the effects of lactacystin. These findings further implicate Hsp70 as a neuroprotective molecule and they suggest the potential utility of certain proteasome inhibitors in the treatment of polyglutamine diseases.

Tyrosine 981, a novel ret autophosphorylation site, binds c-Src to mediate neuronal survival

The glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) are neurotrophic factors that influence several aspects of the developing and injured nervous system. GFLs signal through a common receptor tyrosine kinase (Ret) and one of the four ligand-binding co-receptors (GFRalpha1 to 4). Ligand-induced translocation of Ret to lipid rafts, where it interacts with the nonreceptor tyrosine kinase Src, is a prerequisite for full biological activity of these neurotrophic factors. This interaction and subsequent activation of Src are required for GFL-mediated neuronal survival, neurite outgrowth, or cell proliferation. Here we show by multiple approaches that Ret tyrosine 981 constitutes the major binding site of the Src homology 2 domain of Src and therefore the primary residue responsible for Src activation upon Ret engagement. Other tyrosines such as 1015 and 1029 may contribute to the overall interaction between Ret and Src, as judged by overexpression experiments. By generating a phosphospecific antibody, we demonstrate that tyrosine 981 is a novel autophosphorylation site in Ret. Importantly, we also show that this tyrosine becomes phosphorylated in dissociated sympathetic neurons after ligand stimulation. Mutation of tyrosine 981 to phenylalanine reduces GDNF-mediated survival in a transfected cerebellar granule neuron paradigm.

Endoplasmic reticulum: a primary target in various acute disorders and degenerative diseases of the brain

Changes in neuronal calcium activity in the various subcellular compartments have divergent effects on affected cells. In the cytoplasm and mitochondria, where calcium activity is normally low, a prolonged excessive rise in free calcium levels is believed to be toxic, in the endoplasmic reticulum (ER), in contrast, calcium activity is relatively high and severe stress is caused by a depletion of ER calcium stores. Besides its role in cellular calcium signaling, the ER is the site where membrane and secretory proteins are folded and processed. These calcium-dependent processes are fundamental to normal cell functioning. Under conditions of ER dysfunction unfolded proteins accumulate in the ER lumen, a signal responsible for activation of the unfolded protein response (UPR) and the ER-associated degradation (ERAD). UPR is characterized by activation of two ER-resident kinases, PKR-like ER kinase (PERK) and IRE1. PERK induces phosphorylation of the eukaryotic initiation factor (eIF2alpha), resulting in a shut-down of translation at the initiation step. This stress response is needed to block new synthesis of proteins that cannot be correctly folded, and thus to protect cells from the effect of unfolded proteins which tend to form toxic aggregates. IRE1, on the other hand, is turned after activation into an endonuclease that cuts out a sequence of 26 bases from the coding region of xbp1 mRNA. Processed xbp1 mRNA is translated into the respective protein, an active transcription factor specific for ER stress genes such as grp78. In acute disorders and degenerative diseases, the ER calcium pool is a primary target of toxic metabolites or intermediates, such as oxygen free radicals, produced during the pathological process. Affected neurons need to activate the entire UPR to cope with the severe form of stress induced by ER dysfunction. This stress response is however hindered under conditions where protein synthesis is suppressed to such an extent that processed xbp1 mRNA is not translated into the processed XBP1 protein (XBP1(proc)). Furthermore, activation of ERAD is important for the degradation of unfolded proteins through the ubiquitin/proteasomal pathway, which is impaired in acute disorders and degenerative diseases, resulting in further ER stress. ER functioning is thus impaired in two different ways: first by the direct action of toxic intermediates, produced in the course of the pathological process, hindering vital ER reactions, and second by the inability of cells to fully activate UPR and ERAD, leaving them unable to withstand the severe form of stress induced by ER dysfunction.

Pathogenesis of polyglutamine disorders: aggregation revisited

Expansion of CAG trinucleotide repeats coding for polyglutamine in unrelated proteins causes at least nine late-onset progressive neurodegenerative disorders, including Huntington's disease and a number of spinocerebellar ataxias. Expanded polyglutamine provokes a dominant gain-of-function neurotoxicity, regardless of the specific protein context within which it resides. Nevertheless, the protein context does modulate polyglutamine toxicity, as evidenced by the distinct clinical and pathological features of the various disorders. Importantly, polyglutamine toxicity might derive from its ability to aggregate. Indeed, aggregation probably underlies some defining attributes of the polyglutamine disorders, such as their late onset, progressive nature, and the dependence of onset age on polyglutamine length. However, the central role of aggregation in polyglutamine pathogenesis has been challenged by several studies, which instead argued that the soluble form of the disease proteins is responsible for neuronal damage. Thus, the question whether polyglutamine aggregates are deleterious, harmless or protective remains the most passionately disputed issue in the study of these diseases. In this review, we attempt to reconcile some of these controversies.

JNK-mediated BIM phosphorylation potentiates BAX-dependent apoptosis

Trophic factor deprivation (TFD) activates c-Jun N-terminal kinases (JNKs), culminating in coordinate AP1-dependent transactivation of the BH3-only BCL-2 proteins BIM(EL) and HRK, which in turn are critical for BAX-dependent cytochrome c release, caspase activation, and apoptosis. Here, we report that TFD caused not only induction but also phosphorylation of BIM(EL). Mitochondrially localized JNKs but not upstream activators, like mixed-lineage kinases (MLKs) or mitogen-activated protein kinase kinases (MKKs), specifically phosphorylated BIM(EL) at Ser65, potentiating its proapoptotic activity. Inhibition of the JNK pathway attenuated BIM(EL) expression, prevented BIM(EL) phosphorylation, and abrogated TFD-induced apoptosis. Conversely, activation of this pathway promoted BIM(EL) expression and phosphorylation, causing BIM- and BAX-dependent cell death. Thus, JNKs regulate the proapoptotic activity of BIM(EL) during TFD, both transcriptionally and posttranslationally.

Molecular investigation of TBP allele length: a SCA17 cellular model and population study

Recently, an inherited spinocerebellar ataxia (SCA17) has been attributed to polyglutamine coding expansions within the gene coding for human TATA-box binding protein (TBP). The normal repeat range is 25-42 units with patients having as few as 46 repeats. We undertook a TBP repeat length population study showing its relative stability, skewed distribution, and substantial population specific differences. To investigate the mechanism of neurodegeneration in SCA17 we have developed a cellular model expressing full-length TBP with a range of polyQ expansions. As has been found with other polyQ cellular models, insoluble intracellular inclusions form in a repeat-length-dependent manner. In addition, we have shown that the expanded TBP polyQ tract is able to interact with other overexpressed polyQ-containing proteins. Importantly, overexpression of expanded TBP results in increased Cre-dependent transcriptional activity. As TBP is required for transcription by all RNA polymerases, this may indicate a mechanism for aberrant polyQ gain of function.

Proapoptotic protein glyceraldehyde-3-phosphate dehydrogenase: a possible site of action of antiapoptotic drugs

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) has long been recognized as a classical glycolytic protein and has been used as a "housekeeping" gene in studies of genetic expression and regulation. However, recent advances reveal that GAPDH displays diverse nonglycolytic functions depending on its subcellular localization. Among those functions, one of the most intriguing is likely to be the induction of apoptosis. Previous works by our group and others have demonstrated that the overexpression of GAPDH and its subsequent nuclear translocation appear to be implicated in the initiation of one or more apoptotic cascades and also in the etiology of some neurological diseases. This review addresses new data demonstrating that a protein, referred to as proapoptotic protein GAPDH, with a quite mundane function in healthy cells behaves very differently during cell suicide, and summarizes emphatically the importance of this protein as a putative molecular target in developing antiapoptotic therapeutic agents for the treatment of certain neurodegenerative disorders.

Pivotal role of oligomerization in expanded polyglutamine neurodegenerative disorders

The expansion of a CAG repeat coding for polyglutamine in otherwise unrelated gene products is central to eight neurodegenerative disorders including Huntington's disease. It has been well documented that expanded polyglutamine fragments, cleaved from their respective full-length proteins, form microscopically visible aggregates in affected individuals and in transgenic mice. The contribution of polyglutamine oligomers to neurodegeneration, however, is controversial. The azo-dye Congo red binds preferentially to beta-sheets containing amyloid fibrils and can specifically inhibit oligomerization and disrupt preformed oligomers. Here we show that inhibition of polyglutamine oligomerization by Congo red prevents ATP depletion and caspase activation, preserves normal cellular protein synthesis and degradation functions, and promotes the clearance of expanded polyglutamine repeats in vivo and in vitro. Infusion of Congo red into a transgenic mouse model of Huntington's disease, well after the onset of symptoms, promotes the clearance of expanded repeats in vivo and exerts marked protective effects on survival, weight loss and motor function. We conclude that oligomerization is a crucial determinant in the biochemical properties of expanded polyglutamine that are central to their chronic cytotoxicity.

Protein tyrosine phosphatases are up-regulated and participate in cell death induced by polyglutamine expansion

Polyglutamine expansion is the cause of several neurodegenerative diseases. An in vitro model of polyglutamine-induced neuronal cell death was developed using truncated mutant huntingtin (Htt) and PC12 cells. Cell death was specifically observed in cells expressing a truncated mutant huntingtin-green fluorescence protein (GFP) fusion protein with 118 glutamine repeats (Gln(118)), as demonstrated by the release of lactate dehydrogenase (LDH). To gain further insights into the mechanisms of polyglutamine expansion-induced cell death, the Affymetrix rat genome array U34A was used to investigate gene expression changes associated with polyglutamine-mediated protein aggregation and cell death. Among the up-regulated genes, the increase of four protein tyrosine phosphatases (PTPs) was further confirmed by real-time quantitative reverse transcription PCR. Protein expression of mitogen activated protein (MAP) kinase phosphatase 1 (MKP1) was also increased as demonstrated by Western blot. Furthermore, phosphorylation of MAP kinase extracellular signal-regulated kinase 1/2 (ERK1/2) was substantially reduced in association with protein aggregation, and two general PTP inhibitors, sodium orthovanadate and bpV(pic), dramatically rescued the cells from polyglutamine-induced cell death. These results suggest that one or more of the PTPs are involved in the polyglutamine-induced cell death.

siRNA-mediated gene silencing in vitro and in vivo

RNA interference is now established as an important biological strategy for gene silencing, but its application to mammalian cells has been limited by nonspecific inhibitory effects of long dsRNA on translation. Here, we describe a viral-mediated delivery mechanism that results in specific silencing of targeted genes through expression of small interfering RNA (siRNA). We establish proof of principle by markedly diminishing expression of exogenous and endogenous genes in vitro and in vivo in brain and liver, and further apply this strategy to a model system of a major class of neurodegenerative disorders, the polyglutamine diseases, to show reduced polyglutamine aggregation in cells. This viral-mediated strategy should prove generally useful in reducing expression of target genes to model biological processes or to provide therapy for dominant human diseases.

Androgen receptor with elongated polyglutamine tract forms aggregates that alter axonal trafficking and mitochondrial distribution in motor neuronal processes

The CAG/polyglutamine (polyGln)-related diseases include nine different members that together form the most common class of inherited neurodegenerative disorders; neurodegeneration is linked to the same type of mutation, found in unrelated genes, consisting of an abnormal expansion of a polyGln tract normally present in the wild-type proteins. Nuclear, cytoplasmic, or neuropil aggregates are detectable in CAG/polyGln-related diseases, but their role is still debated. Alteration of the androgen receptor (AR), one of these proteins, has been linked to spinal and bulbar muscular atrophy, an X-linked recessive disease characterized by motoneuronal death. By using immortalized motoneuronal cells (the neuroblastoma-spinal cord cell line NSC34), we analyzed neuropil aggregate formation and toxicity: green fluorescent protein-tagged wild-type or mutated ARs were cotransfected into NSC34 cells with a blue fluorescent protein tagged to mitochondria. Altered mitochondrial distribution was observed in neuronal processes containing aggregates; occasionally, neuropil aggregates and mitochondrial concentration corresponded to axonal swelling. Neuropil aggregates also impaired the distribution of the motor protein kinesin. These data suggest that neuropil aggregates may physically alter neurite transport and thus deprive neuronal processes of factors or components that are important for axonal and dendritic functions. The soma may then be affected, leading to neuronal dysfunctions and possibly to cell death.

Time course of polyglutamine aggregate body formation and cell death: enhanced growth in nucleus and an interval for cell death

Polyglutamine (polyQ) aggregate bodies are a hallmark of dentatorubral-pallidoluysian atrophy and related neurodegenerative disorders, although the relationship between aggregate body formation and cell death is not clear. We analyzed the kinetics of polyQ aggregate formation and the time intervals for cell death, tracking individual cells using fluorescence video microscopy, for the first time. Expanded polyQ tracts of atrophin-1 with or without nuclear localization signal (NLS) labeled with green fluorescent protein (GFP) were constructed, Q57NLS/GFP and Q56/GFP, respectively. All of the Q57NLS/GFP aggregate bodies were in nuclei, and all of the Q56/GFP aggregate bodies were in cytoplasm. Aggregates of Q56/GFP were larger than those of Q57NLS/GFP. Surprisingly, a kinetic analysis showed that the latter grew 5.37 times faster than the former. The time interval between transfection and cell death was shorter in Q57NLS/GFP, but the time between the end of the rapid growing phase of aggregation and the start of the cell death process did not show a significant difference. Aggregate growth was confirmed to correspond to the accumulated free polyQ by the time of starting aggregation. These findings suggest that aggregate body formation induced by expanded polyQ stretches is a self-limiting process and is enhanced by factor(s) in nuclei, whereas it is not tightly bound to the cell death process.

Polyglutamine expansion, protein aggregation, proteasome activity, and neural survival

Huntington's disease (HD) is one of eight established triplet repeat neurodegenerative disorders, which are collectively caused by the genetic expansion of polyglutamine repeats. While the mechanism(s) by which polyglutamine expansion causes neurodegeneration in each of these disorders is being intensely investigated, the underlying cause of polyglutamine toxicity has not been fully elucidated. A number of studies have focused on the potential role of protein aggregation and disruption of the proteasome proteolytic pathway in polyglutamine-mediated neurodegeneration. However, at present it is not clear whether polyglutamine-mediated protein aggregation is sufficient to induce cell death, nor has it been clearly determined whether proteasome inhibition precedes, coincides, or occurs as the result of the formation of polyglutamine-associated protein aggregation. To address these important components of polyglutamine toxicity, in the present study we utilized neural SH-SY5Y cells stably transfected with polyglutamine-green fluorescent protein constructs to examine the effects of polyglutamine expansion on protein aggregation, proteasome activity, and neural cell survival. Data from the present study demonstrate that polyglutamine expansion does not dramatically impair proteasome activity or elevate protein aggregate formation under basal conditions, but does significantly impair the ability of the proteasome to respond to stress, and increases stress-induced protein aggregation following stress, all in the absence of neural cell death.

Mutant huntingtin aggregates do not sensitize cells to apoptotic stressors

It has been postulated that neuronal inclusions composed of mutant huntingtin may play a causative role in the pathogenesis of Huntington's disease. To study the putative role of aggregates in modulating apoptotic vulnerability, SH-SY5Y cell lines stably expressing truncated huntingtin with 18 (wild-type) (N63-18Q) or 82 (mutant) (N63-82Q) glutamine repeats were established. Aggregates were observed in approximately 13% of the N63-82Q cells; no aggregates were observed in the N63-18Q cells. In response to apoptotic stimuli such as staurosporine or hyperosmotic stress, caspase-3 activity was significantly greater in the N63-82Q cells compared to the N63-18Q cells. However, double immunostaining for huntingtin and active caspase-3 revealed that the presence of aggregates did not correlate with the presence of active caspase-3, indicating that aggregates do not contribute to the increase in apoptosis in the N63-82Q cells.

Chaperone suppression of cellular toxicity of huntingtin is independent of polyglutamine aggregation

Polyglutamine protein aggregation is associated with eight inherited neurodegenerative disorders. In Huntington's disease, N-terminal fragments of mutant huntingtin form intracellular aggregates and mediate cellular toxicity. Recent studies have shown that chaperones inhibit polyglutamine-mediated aggregation and cellular toxicity. Because chaperones also inhibit caspase activation to protect cells from death, it remains unclear whether the protective effect of chaperones on polyglutamine-mediated cellular toxicity is dependent on their inhibition of protein aggregation. In this study, we show that several chaperones including HSP 40, HSP 70, and N-ethylmaleimide-sensitive factor can inhibit cellular toxicity caused by N-terminal mutant huntingtin fragments. However, only HSP 40 is able to inhibit huntingtin aggregation. Furthermore, time-course study suggests that the protection of chaperones against huntingtin toxicity is not the result of their suppression of huntingtin aggregation. Chaperones inhibit caspase-3 and caspase-9 activation mediated by mutant huntingtin, and this inhibition is independent of huntingtin aggregation. We propose that the inhibition of caspase activity by chaperones is involved in their suppression of polyglutamine toxicity.

Amino acid sequences flanking polyglutamine stretches influence their potential for aggregate formation

Expanded polyglutamine stretches have been shown to form aggregates and to be toxic to cells. In this study, we hypothesized that amino acid sequences flanking the polyglutamine stretches influence the aggregate formation potential of these stretches. Green fluorescent protein (GFP) fusion proteins containing glutamine repeats of various lengths and a fixed number of flanking amino acids of ataxin-2, huntingtin, dentatorubral-pallidoluysian atrophy protein (DRPLAP) or ataxin-3 were transiently expressed in COS-7 cells. The aggregate formation potential of ataxin-2 and DRPLAP increased in a CAG-repeat-length-dependent manner, with a threshold between 34 and 36. Truncated ataxin-2-Q56-GFP and truncated huntingtin-Q56-GFP showed a significantly higher aggregate formation potential than truncated DRPLAP-Q56-GFP or truncated ataxin-3-Q56-GFP. These results are in agreement with the clinical observation that ages of disease onset in patients with spinocerebellar ataxia type 2 or Huntington's disease are lower than those in patients with DRPLA or Machado-Joseph disease having expanded CAG repeats of the same length. Furthermore, mutagenesis of the flanking sequence of ataxin-2 markedly reduced its aggregate formation potential. These results indicate that the amino acid sequences flanking the polyglutamine stretches significantly influence their aggregate formation potential.

BH3-only Bcl-2 family members are coordinately regulated by the JNK pathway and require Bax to induce apoptosis in neurons

The Bcl-2 family of proteins are key regulators of programmed cell death. A distinct subfamily of BH3-only molecules has been identified, but their exact mechanism of action remains unclear. Here we show that the BH3-only Bcl-2 family members, Dp5/Hrk and Bim, are induced upstream of the Bax checkpoint in neuronal apoptosis in a manner that shows significant dependence on JNK signaling. We also show that Dp5 and other BH3-only proteins kill cerebellar granule neurons in a Bax-dependent manner. These studies demonstrate that BH3-only members do not act independently in their proapoptotic activities but rather require the action of multidomain proapoptotic Bcl-2 family members to produce cell death.

A population of PC12 cells that is initiating apoptosis can be rescued by nerve growth factor

Programmed cell death, or apoptosis, occurs asynchronously in neuronal cells. To overcome this asynchrony, rat pheochromocytoma (PC12) cells were separated at different stages of apoptosis on the basis of cell density. Live cells that exhibited no apoptotic features floated to the top of density gradients. The most dense cells showed extensive loss of cytochrome c from mitochondria, caspase activation, chromatin condensation, and DNA fragmentation. These cells were committed to apoptosis and could not be rescued by reculturing in with nerve growth factor (NGF). Cells of intermediate density displayed no DNA fragmentation, but had begun to show cytochrome c loss, caspase activation, and chromatin condensation. This population displayed upregulation of the prodeath factor, c-Jun, and downregulation of prosurvival kinase, Akt. Importantly, apoptosis was reversible by NGF in this population. These studies suggest that increased cell density correlates with an initial step in the apoptosis mechanism that precedes irreversible commitment to suicide.

Characterization of intracellular aggregates using fluorescently-tagged polyglutamine-expanded androgen receptor

Spinal bulbar muscular atrophy (SBMA) is a classic CAG-repeat neurodegenerative disease. It is caused by expansion of a polyglutamine (polyGln) tract in the androgen receptor (AR). Recent evidence has indicated a potential role for nuclear and cytoplasmic inclusions in the pathogenesis of these diseases. We have used blue and green fluorescently-tagged AR to show that both wild-type (WT) and poly-Gln-expanded full-length AR can form aggregates and that aggregation is not related to cytotoxicity. Twenty to thirty-five percent of all cell types transfected into COS cells showed aggregation containing both amino- and carboxy-terminal fluorescent tags. The aggregates reacted with (F39.4.1), an anti-AR antibody and with IC2, an expanded polyGln tract antibody. Western analysis of protein extracts revealed little evidence of proteolysis although some cleavage of the fusion proteins was seen. The general caspase inhibitor, Z-DEVD-FMK, did not affect aggregation in either wild type or polyGln-expanded GFP-AR transfected cells. Surprisingly, addition of Mibolerone a synthetic androgen significantly decreased inclusion formation in both WT and polyGln-expanded AR-transfected cells. Overall, we show that both WT and polyGln expanded full-length AR are found in aggregates and that proteolysis is not a requirement for aggregation. Our results also suggest that toxicity is not related to intracellular aggregation of polyGln expanded AR.

c-Src is required for glial cell line-derived neurotrophic factor (GDNF) family ligand-mediated neuronal survival via a phosphatidylinositol-3 kinase (PI-3K)-dependent pathway

The glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs), consisting of GDNF, neurturin, persephin, and artemin, signal via a multicomponent complex composed of Ret tyrosine kinase and the glycosyl-phosphatidylinositol (GPI)-anchored coreceptors GFRalpha1-alpha4. In previous work we have demonstrated that the localization of Ret to membrane microdomains known as lipid rafts is essential for GDNF-induced downstream signaling, differentiation, and neuronal survival. Moreover, we have found that Ret interacts with members of the Src family kinases (SFK) only when it is localized to these microdomains. In the present work we show by pharmacological and genetic approaches that Src activity was necessary to elicit optimal GDNF-mediated signaling, neurite outgrowth, and survival. In particular, p60Src, but not the other ubiquitous SFKs, Fyn and Yes, was responsible for the observed effects. Moreover, Src appeared to promote neuronal survival via a phosphatidylinositol-3 kinase (PI-3K)-dependent pathway because the PI-3K inhibitor LY294002 prevented GFL-mediated neuronal survival and prevented activated Src-mediated neuronal survival. In contrast, the inhibition of Src activity had no effects on NGF-mediated survival, indicating that the requirement for Src was selective for GFL-mediated neuronal survival. These data confirm the importance of protein-protein interactions between Ret and raft-associated proteins in the signaling pathways elicited by GDNF, and the data implicate Src as one of the major signaling molecules involved in GDNF-mediated bioactivity.

Early and progressive accumulation of reactive microglia in the Huntington disease brain

Microglia may contribute to cell death in neurodegenerative diseases. We studied the activation of microglia in affected regions of Huntington disease (HD) brain by localizing thymosin beta-4 (Tbeta4), which is increased in reactive microglia. Activated microglia appeared in the neostriatum, cortex, and globus pallidus and the adjoining white matter of the HD brain, but not in control brain. In the striatum and cortex, reactive microglia occurred in all grades of pathology, accumulated with increasing grade, and grew in density in relation to degree of neuronal loss. The predominant morphology of activated microglia differed in the striatum and cortex. Processes of reactive microglia were conspicuous in low-grade HD, suggesting an early microglia response to changes in neuropil and axons and in the grade 2 and grade 3 cortex, were aligned with the apical dendrites of pyramidal neurons. Some reactive microglia contacted pyramidal neurons with huntingtin-positive nuclear inclusions. The early and proximate association of activated microglia with degenerating neurons in the HD brain implicates a role for activated microglia in HD pathogenesis.

Glutaredoxin protects cerebellar granule neurons from dopamine-induced apoptosis by activating NF-kappa B via Ref-1

The neurotransmitter dopamine (DA) induces apoptosis via its oxidative metabolites. This study shows that glutaredoxin 2 (Grx2) from Escherichia coli and human glutaredoxin could protect cerebellar granule neurons from DA-induced apoptosis. E. coli Grx2, which catalyzes glutathione-disulfide oxidoreduction via its -Cys-Pro-Tyr-Cys- active site, penetrates into cerebellar granule neurons and exerts its activity via NF-kappaB activation. Analysis of single and double cysteine to serine substitutions in the active site of Grx2 showed that both cysteine residues were essential for activity. Although DA significantly reduced NF-kappaB binding activity, Grx2 could stimulate the binding of NF-kappaB to DNA by: (i) translocating NF-kappaB from the cytoplasm to the nucleus after promoting the phosphorylation and degradation of I-kappaBalpha, and (ii) activating the binding of pre existing nuclear NF-kappaB. The DNA binding activity of NF-kappaB itself was essential for neuronal survival. Overexpression of I-kappaB dominant negative gene (I-kappaB-DeltaN) in granule neurons significantly reduced their viability, irrespective of the presence of Grx2. Ref-1 expression was down-regulated by DA but up-regulated by Grx2, while treatment of neurons with Ref-1 antisense oligonucleotide reduced the ability of Grx2 to activate NF-kappaB binding activity. These results show that Grx2 exerts its anti apoptotic activity through the activation of Ref-1, which then activates NF-kappaB.