Yeast as a drug discovery platform in Huntington's and Parkinson's diseases

A yeast-based screening system identified bakkenolide B contained in Petasites japonicus as an inhibitor of interleukin-2 production in a human T cell line

Ca2+ signaling is related to various diseases such as allergies, diabetes, and cancer. We explored Ca2+ signaling inhibitors in natural resources using a yeast-based screening method and found bakkenolide B from the flower buds of edible wild plant, Petasites japonicus, using the YNS17 strain (zds1Δ erg3Δ pdr1/3Δ). Bakkenolide B exhibited growth-restoring activity against the YNS17 strain and induced Li+ sensitivity of wild-type yeast cells, suggesting that it inhibits the calcineurin pathway. Additionally, bakkenolide B inhibited interleukin-2 production at gene and protein levels in Jurkat cells, a human T cell line, but not the in vitro phosphatase activity of human recombinant calcineurin, an upstream regulator of interleukin-2 production. Furthermore, bakkenolide A showed weak activity in YNS17 and Jurkat cells compared with bakkenolide B. These findings revealed new biological effects and the structure-activity relationships of bakkenolides contained in P. japonicus as inhibitors of interleukin-2 production in human T cells.

Emerging Brain-Pathophysiology-Mimetic Platforms for Studying Neurodegenerative Diseases: Brain Organoids and Brains-on-a-Chip

Neurodegenerative diseases are a group of disorders characterized by progressive degeneration of the structural and functional integrity of the central and peripheral nervous systems. Millions of people suffer from degenerative brain diseases worldwide, and the mortality continues to increase every year, causing a growing demand for knowledge of the underlying mechanisms and development of therapeutic targets. Conventional 2D-based cell culture platforms and animal models cannot fully recapitulate the pathophysiology, and this has limited the capability for estimating drug efficacy. Recently, engineered platforms, including brain organoids and brain-on-a-chip, have emerged. They mimic the physiology of brain tissue and reflect the fundamental pathophysiological signatures of neurodegenerative diseases, such as the accumulation of neurotoxic proteins, structural abnormalities, and functional loss. In this paper, recent advances in brain-mimetic platforms and their potential for modeling features of neurodegenerative diseases in vitro are reviewed. The development of a physiologically relevant model should help overcome unresolved neurodegenerative diseases.

Targeting Cdc42 with the anticancer compound MBQ-167 inhibits cell polarity and growth in the budding yeast S. cerevisiae

The Rho GTPase Cdc42 is highly conserved in structure and function. Mechanical or chemical cues in the microenvironment stimulate the localized activation of Cdc42 to rearrange the actin cytoskeleton and establish cell polarity. A role for Cdc42 in cell polarization was first discovered in the budding yeast Saccharomyces cerevisiae, and subsequently shown to also regulate directional motility in animal cells. Accordingly, in cancer Cdc42 promotes migration, invasion, and spread of tumor cells. Therefore, we targeted Cdc42 as a therapeutic strategy to treat metastatic breast cancer and designed the small molecule MBQ-167 as a potent inhibitor against Cdc42 and the homolog Rac. MBQ-167 inhibited cancer cell proliferation and migration in-vitro, and tumor growth and spread in-vivo in a mouse xenograft model of metastatic breast cancer. Since haploid budding yeast express a single Cdc42 gene, and do not express Rac, we used this well characterized model of polarization to define the contribution of Cdc42 inhibition to the effects of MBQ-167 in eukaryotic cells. Growth, budding pattern, and Cdc42 activity was determined in wildtype yeast or cells expressing a conditional knockdown of Cdc42 in response to vehicle or MBQ-167 treatment. As expected, growth and budding polarity were reduced by knocking-down Cdc42, with a parallel effect observed with MBQ-167. Cdc42 activity assays confirmed that MBQ-167 inhibits Cdc42 activation in yeast, and thus, bud polarity. Hence, we have validated MBQ-167 as a Cdc42 inhibitor in another biological context and present a method to screen Cdc42 inhibitors with potential as anti-metastatic cancer drugs.

Sneaking Out for Happy Hour: Yeast-Based Approaches to Explore and Modulate Immune Response and Immune Evasion

Many pathogens (virus, bacteria, fungi, or parasites) have developed a wide variety of mechanisms to evade their host immune system. The budding yeast Saccharomyces cerevisiae has successfully been used to decipher some of these immune evasion strategies. This includes the cis-acting mechanism that limits the expression of the oncogenic Epstein–Barr virus (EBV)-encoded EBNA1 and thus of antigenic peptides derived from this essential but highly antigenic viral protein. Studies based on budding yeast have also revealed the molecular bases of epigenetic switching or recombination underlying the silencing of all except one members of extended families of genes that encode closely related and highly antigenic surface proteins. This mechanism is exploited by several parasites (that include pathogens such as Plasmodium, Trypanosoma, Candida, or Pneumocystis) to alternate their surface antigens, thereby evading the immune system. Yeast can itself be a pathogen, and pathogenic fungi such as Candida albicans, which is phylogenetically very close to S. cerevisiae, have developed stealthiness strategies that include changes in their cell wall composition, or epitope-masking, to control production or exposure of highly antigenic but essential polysaccharides in their cell wall. Finally, due to the high antigenicity of its cell wall, yeast has been opportunistically exploited to create adjuvants and vectors for vaccination.

Opportunities and Challenges in Phenotypic Screening for Neurodegenerative Disease Research

Toxic misfolded proteins potentially underly many neurodegenerative diseases, but individual targets which regulate these proteins and their downstream detrimental effects are often unknown. Phenotypic screening is an unbiased method to screen for novel targets and therapeutic molecules and span the range from primitive model organisms such as Sacchaomyces cerevisiae, which allow for high-throughput screening to patient-derived cell-lines that have a close connection to the disease biology but are limited in screening capacity. This perspective will review current phenotypic models, as well as the chemical screening strategies most often employed. Advances in in 3D cell cultures, high-content screens, robotic microscopy, CRISPR screening, and use of machine learning methods to process the enormous amount of data generated by these screens are certain to change the paradigm for phenotypic screening and will be discussed.

Studying Huntington's Disease in Yeast: From Mechanisms to Pharmacological Approaches

Huntington's disease (HD) is a neurodegenerative disorder that leads to progressive neuronal loss, provoking impaired motor control, cognitive decline, and dementia. So far, HD remains incurable, and available drugs are effective only for symptomatic management. HD is caused by a mutant form of the huntingtin protein, which harbors an elongated polyglutamine domain and is highly prone to aggregation. However, many aspects underlying the cytotoxicity of mutant huntingtin (mHTT) remain elusive, hindering the efficient development of applicable interventions to counteract HD. An important strategy to obtain molecular insights into human disorders in general is the use of eukaryotic model organisms, which are easy to genetically manipulate and display a high degree of conservation regarding disease-relevant cellular processes. The budding yeast Saccharomyces cerevisiae has a long-standing and successful history in modeling a plethora of human maladies and has recently emerged as an effective tool to study neurodegenerative disorders, including HD. Here, we summarize some of the most important contributions of yeast to HD research, specifically concerning the elucidation of mechanistic features of mHTT cytotoxicity and the potential of yeast as a platform to screen for pharmacological agents against HD.

RNA Aptamers Rescue Mitochondrial Dysfunction in a Yeast Model of Huntington's Disease

Huntington’s disease (HD) is associated with the misfolding and aggregation of mutant huntingtin harboring an elongated polyglutamine stretch at its N terminus. A distinguishing pathological hallmark of HD is mitochondrial dysfunction. Any strategy that can restore the integrity of the mitochondrial environment should have beneficial consequences for the disease. Specific RNA aptamers were selected that were able to inhibit aggregation of elongated polyglutamine stretch containing mutant huntingtin fragment (103Q-htt). They were successful in reducing the calcium overload, which leads to mitochondrial membrane depolarization in case of HD. In one case, the level of Ca²⁺ was restored to the level of cells not expressing 103Q-htt, suggesting complete recovery. The presence of aptamers was able to increase mitochondrial mass in cells expressing 103Q-htt, along with rescuing loss of mitochondrial genome. The oxidative damage to the proteome was prevented, which led to increased viability of cells, as monitored by flow cytometry. Thus, the presence of aptamers was able to inhibit aggregation of mutant huntingtin fragment and restore mitochondrial dysfunction in the HD cell model, confirming the advantage of the strategy in a disease-relevant parameter.

Induced Pluripotent HD Monkey Stem Cells Derived Neural Cells for Drug Discovery

Huntington's disease (HD) is a neurodegenerative disease caused by an expansion of CAG trinucleotide repeat (polyglutamine [polyQ]) in the huntingtin ( HTT) gene, which leads to the formation of mutant HTT (mHTT) protein aggregates. In the nervous system, an accumulation of mHTT protein results in glutamate-mediated excitotoxicity, proteosome instability, and apoptosis. Although HD pathogenesis has been extensively studied, effective treatment of HD has yet to be developed. Therapeutic discovery research in HD has been reported using yeast, cells derived from transgenic animal models and HD patients, and induced pluripotent stem cells from patients. A transgenic nonhuman primate model of HD (HD monkey) shows neuropathological, behavioral, and molecular changes similar to an HD patient. In addition, neural progenitor cells (NPCs) derived from HD monkeys can be maintained in culture and differentiated to neural cells with distinct HD cellular phenotypes including the formation of mHTT aggregates, intranuclear inclusions, and increased susceptibility to oxidative stress. Here, we evaluated the potential application of HD monkey NPCs and neural cells as an in vitro model for HD drug discovery research.

Attenuation of polyglutamine-induced toxicity by enhancement of mitochondrial OXPHOS in yeast and fly models of aging

Defects in mitochondrial biogenesis and function are common in many neurodegenerative disorders, including Huntington's disease (HD). We have previously shown that in yeast models of HD, enhancement of mitochondrial biogenesis through overexpression of Hap4, the catalytic subunit of the transcriptional complex that regulates mitochondrial gene expression, alleviates the growth arrest induced by expanded polyglutamine (polyQ) tract peptides in rapidly dividing cells. However, the mechanism through which HAP4 overexpression exerts this protection remains unclear. Furthermore, it remains unexplored whether HAP4 overexpression and increased respiratory function during growth can also protect against polyQ-induced toxicity during yeast chronological lifespan. Here, we show that in yeast, mitochondrial respiration and oxidative phosphorylation (OXPHOS) are essential for protection against the polyQ-induced growth defect by HAP4 overexpression. In addition, we show that not only increased HAP4 levels, but also alternative interventions, including calorie restriction, that result in enhanced mitochondrial biogenesis confer protection against polyQ toxicity during stationary phase. The data obtained in yeast models guided experiments in a fly model of HD, where we show that enhancement of mitochondrial biogenesis can also protect against neurodegeneration and behavioral deficits. Our results suggest that therapeutic interventions aiming at the enhancement of mitochondrial respiration and OXPHOS could reduce polyQ toxicity and delay disease onset.

Unraveling protein misfolding diseases using model systems

Experimental model systems have long been used to probe the causes, consequences and mechanisms of pathology leading to human disease. Ideally, such information can be exploited to inform the development of therapeutic strategies or treatments to combat disease progression. In the case of protein misfolding diseases, a wide range of model systems have been developed to investigate different aspects of disorders including Huntington's disease, Parkinson's disease, Alzheimer's disease as well as amyotrophic lateral sclerosis. Utility of these systems broadly correlates with evolutionary complexity: small animal models such as rodents and the fruit fly are appropriate for pharmacological modeling and cognitive/behavioral assessment, the roundworm Caenorhabditis elegans allows analysis of tissue-specific disease features, and unicellular organisms such as the yeast Saccharomyces cerevisiae and the bacterium Escherichia coli are ideal for molecular studies. In this chapter, we highlight key advances in our understanding of protein misfolding/unfolding disease provided by model systems.

From the baker to the bedside: yeast models of Parkinson's disease

The baker’s yeast Saccharomyces cerevisiae has been extensively explored for our understanding of fundamental cell biology processes highly conserved in the eukaryotic kingdom. In this context, they have proven invaluable in the study of complex mechanisms such as those involved in a variety of human disorders. Here, we first provide a brief historical perspective on the emergence of yeast as an experimental model and on how the field evolved to exploit the potential of the model for tackling the intricacies of various human diseases. In particular, we focus on existing yeast models of the molecular underpinnings of Parkinson’s disease (PD), focusing primarily on the central role of protein quality control systems. Finally, we compile and discuss the major discoveries derived from these studies, highlighting their far-reaching impact on the elucidation of PD-associated mechanisms as well as in the identification of candidate therapeutic targets and compounds with therapeutic potential.

Pluripotent hybrid stem cells from transgenic Huntington's disease monkey

Huntington's disease (HD) is a devastating disease that currently has no cure. Transgenic HD monkeys have developed key neuropathological and cognitive behavioral impairments similar to HD patients. Thus, pluripotent stem cells derived from transgenic HD monkeys could be a useful comparative model for clarifying HD pathogenesis and developing novel therapeutic approaches, which could be validated in HD monkeys. In order to create personal pluripotent stem cells from HD monkeys, here we present a tetraploid technique for deriving pluripotent hybrid HD monkey stem cells.

Using yeast models to probe the molecular basis of amyotrophic lateral sclerosis

ALS (amyotrophic lateral sclerosis) is a fatal neurodegenerative disease attributable to the death of motor neurons. Associated with ALS are mutations in the genes encoding SOD1 (superoxide dismutase 1), FUS (fused in Sarcoma) protein and TDP-43 (TAR DNA-binding protein-43) each of which leads to aggregation of the respective protein. For example, the ALS-associated mutations in the hSOD1 (human SOD1) gene typically destabilize the native SOD homodimer, leading to misfolding, aggregation and degradation of SOD1. The ALS-associated pathology is not a consequence of the functional inactivation of SOD1 itself, but is rather due to a toxic gain-of-function triggered by mutant SOD1. Recently, the molecular basis of a number of human neurodegenerative diseases resulting from protein misfolding and aggregation, including fALS (familial ALS), was probed by using the baker's yeast, Saccharomyces cerevisiae, as a highly tractable model. Such studies have, for example, identified novel mutant SOD1-specific interactions and demonstrated that mutant SOD1 disrupts mitochondrial homoeostasis. Features of ALS associated with TDP-43 aggregation have also been recapitulated in S. cerevisiae including the identification of modulators of the toxicity of TDP-43. In this paper, we review recent studies of ALS pathogenesis using S. cerevisiae as a model organism and summarize the potential mechanisms involved in ALS progression.

Neurotoxic 43-kDa TAR DNA-binding protein (TDP-43) triggers mitochondrion-dependent programmed cell death in yeast

Pathological neuronal inclusions of the 43-kDa TAR DNA-binding protein (TDP-43) are implicated in dementia and motor neuron disorders; however, the molecular mechanisms of the underlying cell loss remain poorly understood. Here we used a yeast model to elucidate cell death mechanisms upon expression of human TDP-43. TDP-43-expressing cells displayed markedly increased markers of oxidative stress, apoptosis, and necrosis. Cytotoxicity was dose- and age-dependent and was potentiated upon expression of disease-associated variants. TDP-43 was localized in perimitochondrial aggregate-like foci, which correlated with cytotoxicity. Although the deleterious effects of TDP-43 were significantly decreased in cells lacking functional mitochondria, cell death depended neither on the mitochondrial cell death proteins apoptosis-inducing factor, endonuclease G, and cytochrome c nor on the activity of cell death proteases like the yeast caspase 1. In contrast, impairment of the respiratory chain attenuated the lethality upon TDP-43 expression with a stringent correlation between cytotoxicity and the degree of respiratory capacity or mitochondrial DNA stability. Consistently, an increase in the respiratory capacity of yeast resulted in enhanced TDP-43-triggered cytotoxicity, oxidative stress, and cell death markers. These data demonstrate that mitochondria and oxidative stress are important to TDP-43-triggered cell death in yeast and may suggest a similar role in human TDP-43 pathologies.

Sir2 is induced by oxidative stress in a yeast model of Huntington disease and its activation reduces protein aggregation

Huntington disease (HD) is a neurodegenerative disorder caused by expansion of CAG trinucleotide repeats, leading to an elongated polyglutamine sequence (polyQ) in the huntingtin protein. Misfolding of mutant polyQ proteins with expanded tracts results in aggregation, causing cytotoxicity. Oxidative stress in HD has been documented in humans as important to disease progression. Using yeast cells as a model of HD, we report that when grown at high glucose concentration, cells expressing mutant polyQ do not show apparent oxidative stress. At higher cell densities, when glucose becomes limiting and cells are metabolically shifting from fermentation to respiration, protein oxidation and catalase activity increases in relation to the length of the polyQ tract. Oxidative stress, either endogenous as a result of mutant polyQ expression or exogenously generated, increases Sir2 levels. Δ sir2 cells expressing expanded polyQ lengths show signs of oxidative stress even at the early exponential phase. In a wild-type background, isonicotinamide, a Sir2 activator, decreases mutant polyQ aggregation and the stress generated by expanded polyQ. Taken together, these results describe mutant polyQ proteins as being more toxic in respiring cells, causing oxidative stress and an increase in Sir2 levels. Activation of Sir2 would play a protective role against this toxicity.

α-Synuclein: membrane interactions and toxicity in Parkinson's disease

In the late 1990s, mutations in the synaptic protein α-synuclein (α-syn) were identified in families with hereditary Parkinson's disease (PD). Rapidly, α-syn became the target of numerous investigations that have transformed our understanding of the pathogenesis underlying this disorder. α-Syn is the major component of Lewy bodies (LBs), cytoplasmic protein aggregates that form in the neurons of PD patients. α-Syn interacts with lipid membranes and adopts amyloid conformations that deposit within LBs. Work in yeast and other model systems has revealed that α-syn-associated toxicity might be the consequence of abnormal membrane interactions and alterations in vesicle trafficking. Here we review evidence regarding α-syn's normal interactions with membranes and regulation of synaptic vesicles as well as how overexpression of α-syn yields global cellular dysfunction. Finally, we present a model linking vesicle dynamics to toxicity with the sincere hope that understanding these disease mechanisms will lead to the development of novel, potent therapeutics.

Oxidative stress in yeast

The mechanisms of production and elimination of reactive oxygen species in the cells of the budding yeast Saccharomyces cerevisiae are analyzed. Coordinative role of special regulatory proteins including Yap1p, Msn2/4p, and Skn7p (Pos9p) in regulation of defense mechanisms in S. cerevisiae is described. A special section is devoted to two other well-studied species from the point of view of oxidative stress -- Schizosaccharomyces pombe and Candida albicans. Some examples demonstrating the use of yeast for investigation of apoptosis, aging, and some human diseases are given in the conclusion part.

Modelling neurodegeneration in Saccharomyces cerevisiae: why cook with baker's yeast?

In ageing populations, neurodegenerative diseases increase in prevalence, exacting an enormous toll on individuals and their communities. Multiple complementary experimental approaches are needed to elucidate the mechanisms underlying these complex diseases and to develop novel therapeutics. Here, we describe why the budding yeast Saccharomyces cerevisiae has a unique role in the neurodegeneration armamentarium. As the best-understood and most readily analysed eukaryotic organism, S. cerevisiae is delivering mechanistic insights into cell-autonomous mechanisms of neurodegeneration at an interactome-wide scale.

Monkey hybrid stem cells develop cellular features of Huntington's disease

Background

Pluripotent stem cells that are capable of differentiating into different cell types and develop robust hallmark cellular features are useful tools for clarifying the impact of developmental events on neurodegenerative diseases such as Huntington's disease. Additionally, a Huntington's cell model that develops robust pathological features of Huntington's disease would be valuable for drug discovery research.

Results

To test this hypothesis, a pluripotent Huntington's disease monkey hybrid cell line (TrES1) was established from a tetraploid Huntington's disease monkey blastocyst generated by the fusion of transgenic Huntington's monkey skin fibroblast and a wild-type non-transgenic monkey oocyte. The TrES1 developed key Huntington's disease cellular pathological features that paralleled neural development. It expressed mutant huntingtin and stem cell markers, was capable of differentiating to neural cells, and developed teratoma in severely compromised immune deficient (SCID) mice. Interestingly, the expression of mutant htt, the accumulation of oligomeric mutant htt and the formation of intranuclear inclusions paralleled neural development in vitro , and even mutant htt was ubiquitously expressed. This suggests the development of Huntington's disease cellular features is influenced by neural developmental events.

Conclusions

Huntington's disease cellular features is influenced by neural developmental events. These results are the first to demonstrate that a pluripotent stem cell line is able to mimic Huntington's disease progression that parallels neural development, which could be a useful cell model for investigating the developmental impact on Huntington's disease pathogenesis.

Probing the metabolic aberrations underlying mutant huntingtin toxicity in yeast and assessing their degree of preservation in humans and mice

Metabolomics is a powerful multiparameter tool for evaluating phenotypic traits associated with disease processes. We have used (1)H NMR metabolome profiling to characterize metabolic aberrations in a yeast model of Huntington's disease that are attributable to the mutant huntingtin protein's gain-of-toxic-function effects. A group of 11 metabolites (alanine, acetate, galactose, glutamine, glycerol, histidine, proline, succinate, threonine, trehalose, and valine) exhibited significant concentration changes in yeast expressing the N-terminal fragment of a mutant human huntingtin gene. Correspondence analysis was used to compare results from our yeast model to data reported from transgenic mice expressing a mutant huntingtin gene fragment and Huntington's disease patients. This technique enabled us to identify a variety of both model-specific (pertaining to a single species) and conserved (observed in multiple species) biomarkers related to mutant huntingtin's toxicity. Among the 59 metabolites identified, four compounds (alanine, glutamine, glycerol, and valine) changed significantly in concentration in all three Huntington's disease systems. We propose that alanine, glutamine, glycerol, and valine should be considered as promising biomarkers for evaluating new Huntington's disease therapies, as well as for providing unique insight into the mechanisms associated with mutant huntingtin toxicity.

Nutritional systems biology: definitions and approaches

Nutrigenetics and nutrigenomics are nascent areas that are evolving quickly and riding on the wave of "personalized medicine" that is providing opportunities in the discovery and development of nutraceutical compounds. The human genome sequence and sequences of model organisms provide the equivalent of comprehensive blueprints and parts lists that describe dynamic networks and the bases for understanding their responses to external and internal perturbations. Unfolding the interrelationships among genes, gene products, and dietary habits is fundamental for identifying individuals who will benefit most from, or be placed at risk by, intervention strategies. More accurate assessment of the inputs to human health and the consequences of those inputs measured as accurate transcriptomic, proteomic, and metabolomic analyses would bring personalized health/diet to practice far faster than would waiting for a predictive knowledge of genetic variation. It is widely recognized that systems and network biology has the potential to increase our understanding of how nutrition influences metabolic pathways and homeostasis, how this regulation is disturbed in a diet-related disease, and to what extent individual genotypes contribute to such diseases.

Protein aggregation and neurodegeneration: clues from a yeast model of Huntington's disease

A number of neurodegenerative diseases are accompanied by the appearance of intracellular protein aggregates. Huntington's disease (HD) is caused by a mutation in a gene encoding huntingtin. The mutation causes the expansion of the polyglutamine (polyQ) domain and consequently polyQ-containing aggregates accumulate and neurons in the striatum die. The role of the aggregates is still not clear: they may be the cause of cytotoxicity or a manifestation of the cellular attempt to remove the misfolded proteins. There is accumulating evidence that the main cause of HD is the interaction of the mutated huntingtin with other polyQ-containing proteins and molecular chaperones and most studies based on a yeast model of HD support this point of view. Data obtained using yeasts suggest pathological consequences of polyQ-proteasomal interaction: proteasomal overload by polyQs may interfere with functions of the cell cycle-regulating proteins.

Exploiting yeast genetics to inform therapeutic strategies for Huntington's disease

Huntington's disease (HD) is a devastating neurodegenerative disorder that is inherited in an autosomal dominant fashion and is caused by a polyglutamine expansion in the protein huntingtin (htt). In recent years, modeling of various aspects of HD in the yeast Saccharomyces cerevisiae has provided insight into the conserved mechanisms of mutant htt toxicity in eukaryotic cells. The high degree of conservation of cellular and molecular processes between yeast and mammalian cells have made it a valuable system for studying basic mechanisms underlying human disease. Yeast models of HD recapitulate conserved disease-relevant phenotypes and can be used for drug discovery efforts as well as to gain mechanistic and genetic insights into candidate drugs. Here we provide a detailed overview of yeast models of mutant htt misfolding and toxicity and the molecular and phenotypic characterization of these models. We also review how these models identified novel therapeutic targets and compounds for HD and discuss the benefits and limitations of this model genetic system. Finally, we discuss how yeast may be used to provide further insight into the molecular and cellular mechanisms underlying HD and treatment strategies for this devastating disorder.

The roles of galactitol, galactose-1-phosphate, and phosphoglucomutase in galactose-induced toxicity in Saccharomyces cerevisiae

The uptake and catabolism of galactose by the yeast Saccharomyces cerevisiae is much lower than for glucose and fructose, and in applications of this yeast for utilization of complex substrates that contain galactose, for example, lignocellulose and raffinose, this causes prolonged fermentations. Galactose is metabolized via the Leloir pathway, and besides the industrial interest in improving the flux through this pathway it is also of medical relevance to study the Leloir pathway. Thus, genetic disorders in the genes encoding galactose-1-phosphate uridylyltransferase or galactokinase result in galactose toxicity both in patients with galactosemia and in yeast. In order to elucidate galactose related toxicity, which may explain the low uptake and catabolic rates of S. cerevisiae, we have studied the physiological characteristics and intracellular metabolite profiles of recombinant S. cerevisiae strains with improved or impaired growth on galactose. Aerobic batch cultivations on galactose of strains with different combinations of overexpression of the genes GAL1, GAL2, GAL7, and GAL10, which encode proteins that together convert extracellular galactose into glucose-1-phosphate, revealed a decrease in the maximum specific growth rate when compared to the reference strain. The hypothesized toxic intermediate galactose-1-phosphate cannot be the sole cause of galactose related toxicity, but indications were found that galactose-1-phosphate might cause a negative effect through inhibition of phosphoglucomutase. Furthermore, we show that galactitol is formed in S. cerevisiae, and that the combination of elevated intracellular galactitol concentration, and the ratio between galactose-1-phosphate concentration and phosphoglucomutase activity seems to be important for galactose related toxicity causing decreased growth rates.

Drug Targeting of α-Synuclein Oligomerization in Synucleinopathies

The heterogeneity of symptoms and disease progression observed in synucleinopathies, of which Parkinson's disease (PD) is the most common representative, poses large problems for the discovery of novel therapeutics. The molecular basis for pathology is currently unclear, both in familial and in sporadic cases. While the therapeutic effects of L-DOPA and dopamine receptor agonists constitute good options for symptomatic treatment in PD, the development of neuroprotective and/or neurorestorative treatments for PD and other synucleinopathies faces significant challenges due to the poor knowledge of the putative targets. Recent experimental evidence strongly suggests a central role for neurotoxic α-synuclein oligomeric species in neurodegeneration. The events leading to protein oligomerization, as well as the oligomeric species themselves, are likely amenable to modulation by small molecules, which are beginning to emerge in high throughput compound screens in a variety of model organisms. The therapeutic potential of small molecule modulators of oligomer formation demands further exploration and validation in cellular and animal disease models in order to accelerate human drug development.

Yeast as a model for studying human neurodegenerative disorders

Protein misfolding and aggregation are central events in many disorders including several neurodegenerative diseases. This suggests that alterations in normal protein homeostasis may contribute to pathogenesis, but the exact molecular mechanisms involved are still poorly understood. The budding yeast Saccharomyces cerevisiae is one of the model systems of choice for studies in molecular medicine. Modeling human neurodegenerative diseases in this simple organism has already shown the incredible power of yeast to unravel the complex mechanisms and pathways underlying these pathologies. Indeed, this work has led to the identification of several potential therapeutic targets and drugs for many diseases, including the neurodegenerative diseases. Several features associated with these diseases, such as formation of protein aggregates, cellular toxicity mediated by misfolded proteins, oxidative stress and hallmarks of apoptosis have been faithfully recapitulated in yeast, enabling researchers to take advantage of this powerful model to rapidly perform genetic and compound screens with the aim of identifying novel candidate therapeutic targets and drugs. Here we review the work undertaken to model human brain disorders in yeast, and how these models provide insight into novel therapeutic approaches for these diseases.

Bioactivity profiling with parallel mass spectrometry reveals an assemblage of green tea metabolites affording protection against human huntingtin and alpha-synuclein toxicity

Aberrant protein aggregation and misfolding are key pathological features of many neurodegenerative disorders, including Huntington's and Parkinson's diseases. Compounds that offer protection from toxicity associated with aggregation-prone neurodegenerative proteins may have applications for the treatment of a multitude of disorders. A high-throughput bioassay system with parallel electrospray ionization mass spectrometry screening has been designed for critical evaluation of milligram quantities of natural product extracts, including dietary substances, for compounds of pharmacological relevance to the treatment of human neurodegenerative diseases. Using Saccharomyces cerevisiae strains engineered to express mutant human huntingtin and alpha-synuclein, we are able to identify extracts and compounds that protect cells from toxicity associated with these proteins. Applying this screening paradigm, we determined that a bioactive green tea extract contains an assemblage of catechins that were individually characterized for their respective protective effects against huntingtin and alpha-synuclein toxicity.

Genetic basis of individual differences in the response to small-molecule drugs in yeast

Individual response to small-molecule drugs is variable; a drug that provides a cure for some may confer no therapeutic benefit or trigger an adverse reaction in others. To begin to understand such differences systematically, we treated 104 genotyped segregants from a cross between two yeast strains with a collection of 100 diverse small molecules. We used linkage analysis to identify 124 distinct linkages between genetic markers and response to 83 compounds. The linked markers clustered at eight genomic locations, or quantitative-trait locus 'hotspots', that contain one or more polymorphisms that affect response to multiple small molecules. We also experimentally verified that a deficiency in leucine biosynthesis caused by a deletion of LEU2 underlies sensitivity to niguldipine, which is structurally related to therapeutic calcium channel blockers, and that a natural coding-region polymorphism in the inorganic phosphate transporter PHO84 underlies sensitivity to two polychlorinated phenols that uncouple oxidative phosphorylation. Our results provide a step toward a systematic understanding of small-molecule drug action in genetically distinct individuals.

Mechanisms of disease II: cellular protein quality control

Protein misfolding and aggregation are common to many disorders, including neurodegenerative diseases referred to as "conformational disorders," suggesting that alterations in the normal protein homeostasis might contribute to pathogenesis. Cells evolved 2 major components of the protein quality control system to deal with misfolded and/or aggregated proteins: molecular chaperones and the ubiquitin proteasome pathway. Recent studies have implicated components of both systems in neurodegenerative diseases such as Alzheimer's, Parkinson's, Huntington's, or the prion diseases. A detailed understanding of how the cellular quality control systems relate to neurodegeneration might lead to the development of novel therapeutic approaches for disorders associated with protein misfolding and aggregation.