Expanded polyglutamine stretches form an 'aggresome'

Proteotoxic stresses stimulate dissociation of UBL4A from the tail-anchored protein recognition complex

Inclusion body formation is associated with cytotoxicity in a number of neurodegenerative diseases. However, the molecular basis of the toxicity caused by the accumulation of aggregation-prone proteins remains controversial. In this study, we found that disease-associated inclusions induced by elongated polyglutamine chains disrupt the complex formation of BAG6 with UBL4A, a mammalian homologue of yeast Get5. UBL4A also dissociated from BAG6 in response to proteotoxic stresses such as proteasomal inhibition and mitochondrial depolarization. These findings imply that the cytotoxicity of pathological protein aggregates might be attributed in part to disruption of the BAG6-UBL4A complex that is required for the biogenesis of tail-anchored proteins.

Recombinant Protein Production and Purification of Insoluble Proteins

Proteins are synthesized in heterologous systems because of the impossibility to obtain satisfactory yields from natural sources. The efficient production of soluble and functional recombinant proteins is among the main goals in the biotechnological field. In this context, it is important to point out that under stress conditions, protein folding machinery is saturated and this promotes protein misfolding and, consequently, protein aggregation. Thus, the selection of the optimal expression organism and its growth conditions to minimize the formation of insoluble protein aggregates should be done according to the protein characteristics and downstream requirements. Escherichia coli is the most popular recombinant protein expression system despite the great development achieved so far by eukaryotic expression systems. Besides, other prokaryotic expression systems, such as lactic acid bacteria and psychrophilic bacteria, are gaining interest in this field. However, it is worth mentioning that prokaryotic expression system poses, in many cases, severe restrictions for a successful heterologous protein production. Thus, eukaryotic systems such as mammalian cells, insect cells, yeast, filamentous fungus, and microalgae are an interesting alternative for the production of these difficult-to-express proteins.

The Molecular Physiopathogenesis of Islet Amyloidosis

Human islet amyloid polypeptide or amylin (hA) is a 37-amino acid peptide hormone produced and co-secreted with insulin by pancreatic β-cells. Under physiological conditions, hA regulates a broad range of biological processes including insulin release and slowing of gastric emptying, thereby maintaining glucose homeostasis. However, under the pathological conditions associated with type 2 diabetes mellitus (T2DM), hA undergoes a conformational transition from soluble random coil monomers to alpha-helical oligomers and insoluble β-sheet amyloid fibrils or amyloid plaques. There is a positive correlation between hA oligomerization/aggregation, hA toxicity, and diabetes progression. Because the homeostatic balance between hA synthesis, release, and uptake is lost in diabetics and hA aggregation is a hallmark of T2DM, this chapter focuses on the biophysical and cell biology studies investigating molecular mechanisms of hA uptake, trafficking, and degradation in pancreatic cells and its relevance to h's toxicity. We will also discuss the regulatory role of endocytosis and proteolytic pathways in clearance of toxic hA species. Finally, we will discuss potential pharmacological approaches for specific targeting of hA trafficking pathways and toxicity in islet β-cells as potential new avenues toward treatments of T2DM patients.

CAG Expansions Are Genetically Stable and Form Nontoxic Aggregates in Cells Lacking Endogenous Polyglutamine Proteins

Proteins containing polyglutamine (polyQ) regions are found in almost all eukaryotes, albeit with various frequencies. In humans, proteins such as huntingtin (Htt) with abnormally expanded polyQ regions cause neurodegenerative diseases such as Huntington's disease (HD). To study how the presence of endogenous polyQ aggregation modulates polyQ aggregation and toxicity, we expressed polyQ expanded Htt fragments (polyQ Htt) in Schizosaccharomyces pombe In stark contrast to other unicellular fungi, such as Saccharomyces cerevisiae, S. pombe is uniquely devoid of proteins with more than 10 Q repeats. We found that polyQ Htt forms aggregates within S. pombe cells only with exceedingly long polyQ expansions. Surprisingly, despite the presence of polyQ Htt aggregates in both the cytoplasm and nucleus, no significant growth defect was observed in S. pombe cells. Further, PCR analysis showed that the repetitive polyQ-encoding DNA region remained constant following transformation and after multiple divisions in S. pombe, in contrast to the genetic instability of polyQ DNA sequences in other organisms. These results demonstrate that cells with a low content of polyQ or other aggregation-prone proteins can show a striking resilience with respect to polyQ toxicity and that genetic instability of repetitive DNA sequences may have played an important role in the evolutionary emergence and exclusion of polyQ expansion proteins in different organisms.

IMPORTANCE

Polyglutamine (polyQ) proteins encoded by repetitive CAG DNA sequences serve a variety of normal biological functions. Yet some proteins with abnormally expanded polyQ regions cause neurodegeneration through unknown mechanisms. To study how distinct cellular environments modulate polyQ aggregation and toxicity, we expressed CAG-expanded huntingtin fragments in Schizosaccharomyces pombe In stark contrast to many other eukaryotes, S. pombe is uniquely devoid of proteins containing long polyQ tracts. Our results show that S. pombe cells, despite their low content of endogenous polyQ proteins, exhibit striking and unexpected resilience with respect to polyQ toxicity and that genetic instability of repetitive DNA sequences may have played an important role in the emergence and expansion of polyQ domains in eukaryotic evolution.

Proteasome regulates turnover of toxic human amylin in pancreatic cells

Toxic human amylin (hA) oligomers and aggregates are implicated in the pathogenesis of type 2 diabetes mellitus (T2DM). Although recent studies demonstrated a causal connection between hA uptake and toxicity in pancreatic cells, the mechanism of amylin's clearance following its internalization and its relationship to toxicity is yet to be determined, and hence was investigated here. Using pancreatic rat insulinoma β-cells and human islets as model systems, we show that hA, following its internalization, first accumulates in the cytosol followed by its translocation into nucleus, and to a lesser extent lysosomes, keeping the net cytosolic amylin content low. An increase in hA accumulation in the nucleus of pancreatic cells correlated with its cytotoxicity, suggesting that its excessive accumulation in the nucleus is detrimental. hA interacted with 20S core and 19S lid subunits of the β-cell proteasomal complex, as suggested by immunoprecipitation and confocal microscopy studies, which subsequently resulted in a decrease in the proteasome's proteolytic activity in these cells. In vitro binding and activity assays confirmed an intrinsic and potent ability of amylin to interact with the 20S core complex thereby modulating its proteolytic activity. Interestingly, less toxic and aggregation incapable rat amylin (rA) showed a comparable inhibitory effect on proteasome activity and protein ubiquitination, decoupling amylin aggregation/ toxicity and amylin-induced protein stress. In agreement with these studies, inhibition of proteasomal proteolytic activity significantly increased intracellular amylin content and toxicity. Taken together, our results suggest a pivotal role of proteasomes in amylin's turnover and detoxification in pancreatic cells.

Functional protein aggregates: just the tip of the iceberg

An increasing number of both prokaryotic and eukaryotic cell types are being adapted as platforms for recombinant protein production. The overproduction of proteins in such expression systems leads to the formation of insoluble protein-based aggregates. Although these protein clusters have been poorly studied in most of the eukaryotic systems, aggregates formed in E. coli, named inclusion bodies (IBs), have been deeply characterized in the last decades. Contrary to the general belief, an important fraction of the protein embedded in IB is functional, showing promise in biocatalysis, regenerative medicine and cell therapy. Thus, the exploration of all these functional protein clusters would largely expand their potential in both pharma and biotech industry.

A novel bio-functional material based on mammalian cell aggresomes

Aggresomes are protein aggregates found in mammalian cells when the intracellular protein degradation machinery is over-titered. Despite that they abound in cells producing recombinant proteins of biomedical and biotechnological interest, the physiological roles of these protein clusters and the functional status of the embedded proteins remain basically unexplored. In this work, we have determined for the first time that, like in bacterial inclusion bodies, deposition of recombinant proteins into aggresomes does not imply functional inactivation. As a model, human α-galactosidase A (GLA) has been expressed in mammalian cells as enzymatically active, mechanically stable aggresomes showing higher thermal stability than the soluble GLA version. Since aggresomes are easily produced and purified, we propose these particles as novel functional biomaterials with potential as carrier-free, self-immobilized catalyzers in biotechnology and biomedicine.

General introduction: recombinant protein production and purification of insoluble proteins

Proteins are synthesized in heterologous systems because of the impossibility to obtain satisfactory yields from natural sources. The production of soluble and functional recombinant proteins is among the main goals in the biotechnological field. In this context, it is important to point out that under stress conditions, protein folding machinery is saturated and this promotes protein misfolding and, consequently, protein aggregation. Thus, the selection of the optimal expression organism and the most appropriate growth conditions to minimize the formation of insoluble proteins should be done according to the protein characteristics and downstream requirements. Escherichia coli is the most popular recombinant protein expression system despite the great development achieved so far by eukaryotic expression systems. Besides, other prokaryotic expression systems, such as lactic acid bacteria and psychrophilic bacteria, are gaining interest in this field. However, it is worth mentioning that prokaryotic expression system poses, in many cases, severe restrictions for a successful heterologous protein production. Thus, eukaryotic systems such as mammalian cells, insect cells, yeast, filamentous fungus, and microalgae are an interesting alternative for the production of these difficult-to-express proteins.

ROCK-phosphorylated vimentin modifies mutant huntingtin aggregation via sequestration of IRBIT

BACKGROUND

Huntington's Disease (HD) is a fatal hereditary neurodegenerative disease caused by the accumulation of mutant huntingtin protein (Htt) containing an expanded polyglutamine (polyQ) tract. Activation of the channel responsible for the inositol-induced Ca²⁺ release from ensoplasmic reticulum (ER), was found to contribute substantially to neurodegeneration in HD. Importantly, chemical and genetic inhibition of inositol 1,4,5-trisphosphate (IP3) receptor type 1 (IP3R1) has been shown to reduce mutant Htt aggregation.

RESULTS

In this study, we propose a novel regulatory mechanism of IP3R1 activity by type III intermediate filament vimentin which sequesters the negative regulator of IP3R1, IRBIT, into perinuclear inclusions, and reduces its interaction with IP3R1 resulting in promotion of mutant Htt aggregation. Proteasome inhibitor MG132, which causes polyQ proteins accumulation and aggregation, enhanced the sequestration of IRBIT. Furthermore we found that IRBIT sequestration can be prevented by a rho kinase inhibitor, Y-27632.

CONCLUSIONS

Our results suggest that vimentin represents a novel and additional target for the therapy of polyQ diseases.

An aggregation sensing reporter identifies leflunomide and teriflunomide as polyglutamine aggregate inhibitors

Intracellular protein aggregation is a common pathologic feature in neurodegenerative diseases such as Huntington' disease, amyotrophic lateral sclerosis and Parkinson' disease. Although progress towards understanding protein aggregation in vitro has been made, little of this knowledge has translated to patient therapy. Moreover, mechanisms controlling aggregate formation and catabolism in cellulo remain poorly understood. One limitation is the lack of tools to quantitatively monitor protein aggregation and disaggregation. Here, we developed a protein-aggregation reporter that uses huntingtin exon 1 containing 72 glutamines fused to the N-terminal end of firefly luciferase (httQ72-Luc). httQ72-Luc fails to aggregate unless seeded by a non-luciferase-containing polyglutamine (polyQ) protein such as Q80-cfp. Upon co-aggregation, httQ72-luc becomes insoluble and loses its enzymatic activity. Using httQ72-Luc with Q80(CFP/YFP) as seeds, we screened the Johns Hopkins Clinical Compound Library and identified leflunomide, a dihydroorotate dehydrogenase inhibitor with immunosuppressive and anti-psoriatic activities, as a novel drug that prevents polyQ aggregation. Leflunomide and its active metabolite teriflunomide inhibited protein aggregation independently of their known role in pyrimidine biosynthesis, since neither uridine treatment nor other pyrimidine biosynthesis inhibitors affected polyQ aggregation. Inducible cell line and cycloheximide-chase experiments indicate that these drugs prevent incorporation of expanded polyQ into an aggregate. This study demonstrates the usefulness of luciferase-based protein aggregate reporters for high-throughput screening applications. As current trials are under-way for teriflunomide in the treatment of multiple sclerosis, we propose that this drug be considered a possible therapeutic agent for polyQ diseases.

Huntington's Disease

Huntington's disease (HD) is the most common inherited neurodegenerative disease and is characterized by uncontrolled excessive motor movements and cognitive and emotional deficits. The mutation responsible for HD leads to an abnormally long polyglutamine (polyQ) expansion in the huntingtin (Htt) protein, which confers one or more toxic functions to mutant Htt leading to neurodegeneration. The polyQ expansion makes Htt prone to aggregate and accumulate, and manipulations that mitigate protein misfolding or facilitate the clearance of misfolded proteins tend to slow disease progression in HD models. This article will focus on HD and the evidence that it is a conformational disease.

Recruitment of the oncoprotein v-ErbA to aggresomes

Aggresome formation, a cellular response to misfolded protein aggregates, is linked to cancer and neurodegenerative disorders. Previously we showed that Gag-v-ErbA (v-ErbA), a retroviral variant of the thyroid hormone receptor (TRα1), accumulates in and sequesters TRα1 into cytoplasmic foci. Here, we show that foci represent v-ErbA targeting to aggresomes. v-ErbA colocalizes with aggresomal markers, proteasomes, hsp70, HDAC6, and mitochondria. Foci have hallmark characteristics of aggresomes: formation is microtubule-dependent, accelerated by proteasome inhibitors, and they disrupt intermediate filaments. Proteasome-mediated degradation is critical for clearance of v-ErbA and T(3)-dependent TRα1 clearance. Our studies highlight v-ErbA's complex mode of action: the oncoprotein is highly mobile and trafficks between the nucleus, cytoplasm, and aggresome, carrying out distinct activities within each compartment. Dynamic trafficking to aggresomes contributes to the dominant negative activity of v-ErbA and may be enhanced by the viral Gag sequence. These studies provide insight into novel modes of oncogenesis across multiple cellular compartments.

Analyzing the birth and propagation of two distinct prions, [PSI+] and [Het-s](y), in yeast

Various proteins, like the infectious yeast prions and the noninfectious human Huntingtin protein (with expanded polyQ), depend on a Gln or Asn (QN)-rich region for amyloid formation. Other prions, e.g., mammalian PrP and the [Het-s] prion of Podospora anserina, although still able to form infectious amyloid aggregates, do not have QN-rich regions. Furthermore, [Het-s] and yeast prions appear to differ dramatically in their amyloid conformation. Despite these differences, a fusion of the Het-s prion domain to GFP (Het-sPrD-GFP) can propagate in yeast as a prion called [Het-s](y). We analyzed the properties of two divergent prions in yeast: [Het-s](y) and the native yeast prion [PSI(+)] (prion form of translational termination factor Sup35). Curiously, the induced appearance and transmission of [PSI(+)] and [Het-s](y) aggregates is remarkably similar. Overexpression of tagged prion protein (Sup35-GFP or Het-sPrD-GFP) in nonprion cells gives rise to peripheral, and later internal, ring/mesh-like aggregates. The cells with these ring-like aggregates give rise to daughters with one (perivacuolar) or two (perivacuolar and juxtanuclear) dot-like aggregates per cell. These line, ring, mesh, and dot aggregates are not really the transmissible prion species and should only be regarded as phenotypic markers of the presence of the prions. Both [PSI(+)] and [Het-s](y) first appear in daughters as numerous tiny dot-like aggregates, and both require the endocytic protein, Sla2, for ring formation, but not propagation.

Fluorescence lifetime dynamics of enhanced green fluorescent protein in protein aggregates with expanded polyglutamine

Protein aggregation is one of the characteristic steps in a number of neurodegenerative diseases eventually leading to neuronal death and thorough study of aggregation is required for the development of effective therapy. We apply fluorescence lifetime imaging for the characterization of the fluorescence dynamics of the enhanced green fluorescent protein (eGFP) in fusion with the polyQ-expanded polyglutamine stretch. At the expansion of polyQ above 39 residues, it has an inherent propensity to form amyloid-like fibrils and aggregates, and is responsible for Huntington's disease. The results of the experiments show that expression of the eGFP in fusion with the 97Q protein leads to the decrease of the eGFP fluorescence lifetime by approximately 300 ps. This phenomenon does not appear in Hsp104-deficient cells, where the aggregation in polyQ is prevented. We demonstrate that the lifetime decrease observed is related to the aggregation per se and discuss the possible role of refractive index and homo-FRET in these dynamics.

The heavy metal cadmium induces valosin-containing protein (VCP)-mediated aggresome formation

Cadmium (Cd2+) is a heavy metal ion known to have a long biological half-life in humans. Accumulating evidence shows that exposure to Cd2+ is associated with neurodegenerative diseases characterized by the retention of ubiquitinated and misfolded proteins in the lesions. Here, we report that Cd2+ directly induces the formation of protein inclusion bodies in cells. The protein inclusion body is an aggresome, a major organelle for collecting ubiquitinated or misfolded proteins. Our results show that aggresomes are enriched in the detergent-insoluble fraction of Cd2+-treated cell lysates. Proteomic analysis identified 145 proteins in the aggresome-enriched fractions. One of the proteins is the highly conserved valosin-containing protein (VCP), which has been shown to colocalize with aggresomes and bind ubiquitinated proteins through its N domain (#1-200). Our subsequent examination of VCP's role in the formation of aggresomes induced by Cd2+ indicates that the C-terminal tail (#780-806) of VCP interacts with histone deacetylase HDAC6, a mediator for aggresome formation, suggesting that VCP participates in transporting ubiquitinated proteins to aggresomes. This function of VCP is impaired by inhibition of the deacetylase activity of HDAC6 or by over-expression of VCP mutants that do not bind ubiquitinated proteins or HDAC6. Our results indicate that Cd2+ induces the formation of protein inclusion bodies by promoting the accumulation of ubiquitinated proteins in aggresomes through VCP and HDAC6. Our delineation of the role of VCP in regulating cell responses to ubiquitinated proteins has important implications for understanding Cd2+ toxicity and associated diseases.

Modulation of polyglutamine inclusion formation by the Hsp70 chaperone machine

Components of the Hsp70 chaperone machine have been implied in protection against polyglutamine (poly-Q) pathologies. Yet, little is known about specific mechanisms and the rate-limiting components that account for this protective effect. Here, we examined the effects of an Hsp70 chaperone family member (HspA1A) and its cofactors Hsp40 (DnaJB1), Bag-1 and CHIP on poly-Q protein inclusion formation and SDS-insolubilization. Overexpression of HspA1A alone did not suppress inclusion formation, while overexpression of DnaJB1 reduced poly-Q inclusion formation and insolubilization. The reducing effect of DnaJB1 on inclusion formation was enhanced by coexpressing HspA1A, and was dependent on the interaction of DnaJB1 with Hsp70/Hsc70 chaperones. Additionally, two factors connecting Hsp70 activity with protein degradation by the ubiquitin-proteasome system Bag-1 and CHIP slightly decreased the levels of soluble poly-Q protein, but the amount of aggregated protein and fraction of cells with inclusions remained unaltered. Our data suggest that the HspA1A chaperone machine can modulate poly-Q inclusion formation depending on the ratio of its components and that DnaJB1 is the rate-limiting step.

Protein aggregation and polyasparagine-mediated cellular toxicity in Saccharomyces cerevisiae

It is well established that protein aggregation is associated with many neurodegenerative disorders including polyglutamine diseases, but a mechanistic understanding of the role of protein aggregates in the disease pathogenesis remains elusive. Previously thought to be the cause of cellular toxicity such as cellular dysfunction and cell death, protein aggregation is now proposed to serve a protective role by sequestering toxic oligomers from interfering with essential physiological processes. To investigate the relationship between protein aggregation and cellular toxicity, we have characterized and compared the effects of two GFP-fusion proteins that form aggregates in Saccharomyces cerevisiae, one with a polyasparagine repeat (GFP(N104)) and one without (GFP(C)). Although both proteins can form microscopically visible GFP-positive aggregates, only the GFP(N104)-containing aggregates exhibit morphological and biochemical characteristics that resemble the aggregates formed by mutant huntingtin in yeast cells. Formation of both the GFP(C) and GFP(N104) aggregates depends on microtubules, while only the GFP(N104) aggregate requires the chaperone Hsp104 and the prion Rnq1 and is resistant to SDS. Although no microscopically visible GFP(N104) aggregates were observed in the hsp104Delta and rnq1Delta mutant cells, SDS-insoluble aggregates can still be detected by the filter trap assay. These observations argue that the GFP(N104)-containing aggregates can exist in at least two distinct states in vivo. We also show that a nucleus-targeted GFP(N104) interferes with transcription from two SAGA-dependant promoters and results in a decrease in cell viability. Overall, the results imply that the GFP(N104) protein behaves similarly to the mutant huntingtin in yeast cells and provides a new model for investigating the interplay between protein aggregates and the associated phenotypes.

Endocytosis machinery is involved in aggregation of proteins with expanded polyglutamine domains

The cell's failure to refold or break down abnormal polypeptides often leads to their aggregation, which could cause toxicity and various pathologies. Here we investigated cellular factors involved in protein aggregation in yeast and mammalian cells using model polypeptides containing polyglutamine domains. In yeast, a number of mutations affecting the complex responsible for formation of the endocytic vesicle reduced the aggregation. Components of the endocytic complex (EC) Sla1, Sla2, and Pan1 were seen as clusters in the polyglutamine aggregates. These proteins associate with EC at the later stages of its maturation. In contrast, Ede1 and Ent1, the elements of EC at the earlier stages, were not found in the aggregates, suggesting that late ECs are involved in polyglutamine aggregation. Indeed, stabilization of the late complexes by inhibition of actin polymerization enhanced aggregation of polypeptides with polyglutamine domains. Similarly, in mammalian cells, inhibitors of actin polymerization, as well as depletion of a mediator of actin polymerization, Arp2, strongly enhanced the aggregation. In contrast, destabilization of EC by depletion or inhibition of a scaffolding protein N-WASP effectively suppressed the aggregation. Therefore, EC appears to play a pivotal role in aggregation of cytosolic polypeptides with polyglutamine domains in both yeast and mammalian cells.

Mechanisms of ataxin-3 misfolding and fibril formation: kinetic analysis of a disease-associated polyglutamine protein

The polyglutamine diseases are a family of nine proteins where intracellular protein misfolding and amyloid-like fibril formation are intrinsically coupled to disease. Previously, we identified a complex two-step mechanism of fibril formation of pathologically expanded ataxin-3, the causative protein of spinocerebellar ataxia type-3 (Machado-Joseph disease). Strikingly, ataxin-3 lacking a polyglutamine tract also formed fibrils, although this occurred only via a single-step that was homologous to the first step of expanded ataxin-3 fibril formation. Here, we present the first kinetic analysis of a disease-associated polyglutamine repeat protein. We show that ataxin-3 forms amyloid-like fibrils by a nucleation-dependent polymerization mechanism. We kinetically model the nucleating event in ataxin-3 fibrillogenesis to the formation of a monomeric thermodynamic nucleus. Fibril elongation then proceeds by a mechanism of monomer addition. The presence of an expanded polyglutamine tract leads subsequently to rapid inter-fibril association and formation of large, highly stable amyloid-like fibrils. These results enhance our general understanding of polyglutamine fibrillogenesis and highlights the role of non-poly(Q) domains in modulating the kinetics of misfolding in this family.

Geldanamycin promotes nuclear localisation and clearance of PHOX2B misfolded proteins containing polyalanine expansions

Polyalanine expansions in the PHOX2B gene have been detected in the vast majority of patients affected with congenital central hypoventilation syndrome, a neurocristopathy characterized by absence of adequate control of breathing, especially during sleep, with decreased sensitivity to hypoxia and hypercapnia. The correlation between length of the alanine expanded tracts and severity of congenital central hypoventilation syndrome respiratory phenotype has been confirmed by length-dependent cytoplasmic PHOX2B retention with formation of aggregates. To deepen into the molecular mechanisms mediating the effects of PHOX2B polyalanine expansions, we have set up experiments aimed at assessing the fate of cells characterized by PHOX2B polyalanine aggregates. In particular, we have observed that activation of the heat shock response by the drug geldanamycin is efficient both in preventing formation and in inducing clearance of PHOX2B pre-formed polyalanine aggregates in COS-7 cells expressing PHOX2B-GFP fused proteins, and ultimately also in rescuing the PHOX2B ability to transactivate the Dopamine-beta-Hydroxilase promoter. In addition, we have demonstrated elimination of PHOX2B mutant proteins by the proteasome and autophagy, two cellular mechanisms already been involved in the clearance of proteins containing expanded polyglutamine and polyalanine tracts. Moreover, our data suggest that geldanamycin effects on PHOX2B aggregates may be also mediated by the proteasome pathway. Finally, analysis of cellular toxicity due to polyalanine aggregates has confirmed the occurrence of cell apoptosis consequent to expression of PHOX2B carrying the longest expanded alanine tract and shown that geldanamycin can delay cell progression toward the most advanced apoptotic stages.

Flanking sequences profoundly alter polyglutamine toxicity in yeast

Protein misfolding is the molecular basis for several human diseases. How the primary amino acid sequence triggers misfolding and determines the benign or toxic character of the misfolded protein remains largely obscure. Among proteins that misfold, polyglutamine (polyQ) expansion proteins provide an interesting case: Each causes a distinct neurodegenerative disease that selectively affects different neurons. However, all are broadly expressed and most become toxic when the glutamine expansion exceeds approximately 39 glutamine residues. The disease-causing polyQ expansion proteins differ profoundly in the amino acids flanking the polyQ region. We therefore hypothesized that these flanking sequences influence the specific toxic character of each polyQ expansion protein. Using a yeast model, we find that sequences flanking the polyQ region of human huntingtin exon I can convert a benign protein to a toxic species and vice versa. Further, we observe that flanking sequences can direct polyQ misfolding to at least two morphologically distinct types of polyQ aggregates. Very tight aggregates always are benign, whereas amorphous aggregates can be toxic. We thereby establish a previously undescribed systematic characterization of the influence of flanking amino acid sequences on polyQ toxicity.

Transgenic mice expressing mutant (N279K) human tau show mutation dependent cognitive deficits without neurofibrillary tangle formation

Mutations in the tau gene, which is located on chromosome 17, were found causative for autosomal dominantly inherited frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). To determine if cognitive deficits could be caused by tau mutations, two transgenic mouse lines were generated expressing a four-repeat isoform of human tau or its mutant, containing one of the FTDP-17 mutations (WILD mice and N279K mice). In open field test, N279K mice showed hyperactivity in locomotion and rearing. In prepulse inhibition test, N279K mice but not Wild mice showed significant deficits. Both transgenic mice, especially N279K mice, showed impairment in acquisition of spatial learning in Morris water maze. Although both N279K mice and Wild mice acquired passive avoidance as well as non-transgenic mice, N279K mice but not Wild mice showed severe deficits in acquisition of active avoidance. Histological analysis of the present mutant mice did not show any signs of neurofibrillary tangle formations in the brain, and cognitive dysfunction seemed to precede such neuropathological changes or occur independently from them. The behavioral phenotype of N279K mice mimics features of human FTDP-17 and provides a basic model for elucidating mechanisms underlying cognitive deficits in not only FTDP-17, but also diverse tauopathies.

Phospho-beta-catenin accumulation in Alzheimer's disease and in aggresomes attributable to proteasome dysfunction

Accumulation of cytoplasmic inclusion bodies in many neurodegenerative diseases, including Alzheimer's disease (AD), might result from dysfunction of the ubiquitin-proteasome system. This system degrades many cellular proteins, including beta-catenin, a member of the Wnt signaling pathway, and a presenilin-1-interacting protein. Phosphorylation of beta-catenin marks it for ubiquitination and rapid proteasomal degradation. We found phospho-beta-catenin accumulated as detergent-insoluble, punctate, cytoplasmic inclusions in hippocampal pyramidal neurons more abundantly in AD than in aged controls. In AD, beta-catenin was ubiquitin conjugated, thus suggesting impaired proteasome-dependent degradation. Phospho-beta-catenin was partially sequestered within granulovacuolar degeneration bodies but not in lysosomes, indicating sequestration within autophagosomes. Exposure of neuronal cultures to proteasome inhibitors induced formation of detergent-insoluble, phospho-beta-catenin-positive cytoplasmic inclusions that coalesced into aggresomes and colocalized with gamma-tubulin and vimentin. These aggregates were associated with apoptotic cell death and with activation of caspase-3, c-Jun-N-terminal kinases, and c-Jun. These findings suggest that phospho-beta-catenin accumulation in AD might result from impaired proteasome function.

Progressive phenotype and nuclear accumulation of an amino-terminal cleavage fragment in a transgenic mouse model with inducible expression of full-length mutant huntingtin

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized behaviorally by chorea, incoordination, and shortened lifespan and neuropathologically by huntingtin inclusions and neuronal degeneration. In order to facilitate studies of pathogenesis and therapeutics, we have generated a new inducible mouse model of HD expressing full-length huntingtin (Htt) using a tetracycline-regulated promoter. In double transgenic mice Htt was expressed widely in the brain under the control of the tet-transactivator (tTA) driven by the prion promoter PrP (in the absence of doxycycline). Mice expressing full-length mutant Htt, but not full-length normal Htt, displayed a progressive behavioral phenotype, consisting of slowed and irregular voluntary movements, gait ataxia, tremor and jerky movements, incoordination, and weight loss, with a shortened lifespan. Neuropathology included prominent intranuclear inclusions in cortex and striatum as well as cytoplasmic aggregates. This phenotype is very similar to the phenotypes of previous transgenic mice expressing N-terminal fragments of mutant Htt. The current HD-transgenic mice had nuclear accumulation of Htt, particularly an approximately 60-kDa fragment, which appears to represent an N-terminal cleavage product. This fragment is smaller than calpain or caspase-derived cleavage products of Htt, but it is comparable to a product, termed cp-A, which accumulates in nuclei of cells in a previously described cell model. This new mouse model may be useful in the future for pathogenic and preclinical therapeutic studies related to HD. The data suggest that proteolytic processing could be a part of the pathogenesis of HD, potentially representing an attractive therapeutic target.

Medical bioremediation: prospects for the application of microbial catabolic diversity to aging and several major age-related diseases

Several major diseases of old age, including atherosclerosis, macular degeneration and neurodegenerative diseases are associated with the intracellular accumulation of substances that impair cellular function and viability. Moreover, the accumulation of lipofuscin, a substance that may have similarly deleterious effects, is one of the most universal markers of aging in postmitotic cells. Reversing this accumulation may thus be valuable, but has proven challenging, doubtless because substances resistant to cellular catabolism are inherently hard to degrade. We suggest a radically new approach: augmenting humans' natural catabolic machinery with microbial enzymes. Many recalcitrant organic molecules are naturally degraded in the soil. Since the soil in certain environments - graveyards, for example - is enriched in human remains but does not accumulate these substances, it presumably harbours microbes that degrade them. The enzymes responsible could be identified and engineered to metabolise these substances in vivo. Here, we survey a range of such substances, their putative roles in age-related diseases and the possible benefits of their removal. We discuss how microbes capable of degrading them can be isolated, characterised and their relevant enzymes engineered for this purpose and ways to avoid potential side-effects.

Involvement of clusterin and the aggresome in abnormal protein deposits in myofibrillar myopathies and inclusion body myositis

Myofibrillar myopathies (MM) are characterized morphologically by the presence of non-hyaline structures corresponding to foci of dissolution of myofibrils, and hyaline lesions composed of aggregates of compacted and degraded myofibrillar elements. Inclusion body myositis (IBM) is characterized by the presence of rimmed vacuoles, eosinophilic inclusions in the cytoplasm, rare intranuclear inclusions, and by the accumulation of several abnormal proteins. Recent studies have demonstrated impaired proteasomal expression and activity in MM and IBM, thus accounting, in part, for the abnormal protein accumulation in these diseases. The present study examines other factors involved in protein aggregation in MM and IBM. Clusterin is a multiple-function protein which participates in Abeta-amyloid, PrP(res) and a-synuclein aggregation in Alzheimer disease, prionopathies and a-synucleinopathies, respectively. gamma-Tubulin is present in the centrosome and is an intracellular marker of the aggresome. Moderate or strong clusterin immunoreactivity has been found in association with abnormal protein deposits, as revealed by immunohistochemistry, single and double-labeling immunofluorescence and confocal microscopy, in MM and IBM, and in target structures in denervation atrophy. Gamma-Tubulin has also been observed in association with abnormal protein deposits in MM, IBM, and in target fibers in denervation atrophy. These morphological findings are accompanied by increased expression of clusterin and gamma-tubulin in muscle homogenates of MM and IBM cases, as revealed by gel electrophoresis and Western blots. Together, these observations demonstrate involvement of clusterin in protein aggregates, and increased expression of aggresome markers in association with abnormal protein inclusions in MM and IBM and in targets, as crucial events related to the pathogenesis of abnormal protein accumulation and degradation in these muscular diseases.

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.

Molecular genetics approaches in yeast to study amyloid diseases

The occurrence of protein aggregates in ordered fibrillar structures known as amyloid, found inside and outside of brain cells, is a feature shared by many neurodegenerative disorders, including Alzheimer's, Parkinson's, and Huntington's diseases. Although the molecular mechanisms that underlie neurodegeneration will ultimately have to be tested in neuronal and animal models, there are several distinct advantages in using model organisms to elucidate fundamental aspects of protein aggregation, amyloid formation, and toxicity. Here, we review recent studies indicating that amyloid formation by disease-causing proteins can be faithfully recapitulated in simple yeast-based models in Saccharomyces cerevisiae. These studies have already contributed to our basic understanding of molecular chaperone function/dysfunction in Huntington's disease, and functional genomics approaches being undertaken currently will likely bear novel insights into the genes and pathways that modulate neuronal cell dysfunction and death in these devastating diseases. A final advantage of using yeast to study amyloid formation and toxicity is the ease and rapidity with which large-scale drug-screening efforts can be conducted in this model organism.

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.