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

The high degree of conservation of cellular and molecular processes between the budding yeast Saccharomyces cerevisiae and higher eukaryotes have made it a valuable system for numerous studies of the basic mechanisms behind devastating illnesses such as cancer, infectious disease, and neurodegenerative disorders. Several studies in yeast have already contributed to our basic understanding of cellular dysfunction in both Huntington's and Parkinson's disease. Functional genomics approaches currently being undertaken in yeast may lead to novel insights into the genes and pathways that modulate neuronal cell dysfunction and death in these diseases. In addition, the budding yeast constitutes a valuable system for identification of new drug targets, both via target-based and non-target-based drug screening. Importantly, yeast can be used as a cellular platform to analyze the cellular effects of candidate compounds, which is critical for the development of effective therapeutics. While the molecular mechanisms that underlie neurodegeneration will ultimately have to be tested in neuronal and animal models, there are several distinct advantages to using simple model organisms to elucidate fundamental aspects of protein aggregation, amyloid toxicity, and cellular dysfunction. Here, we review recent studies that have shown that amyloid formation by disease-causing proteins and many of the resulting cellular deficits can be faithfully recapitulated in yeast. In addition, we discuss new yeast-based techniques for screening candidate therapeutic compounds for Huntington's and Parkinson's diseases.     

Functional analyses of human DNA repair proteins important for aging and genomic stability using yeast genetics

Model systems have been extremely useful for studying various theories of aging. Studies of yeast have been particularly helpful to explore the molecular mechanisms and pathways that affect aging at the cellular level in the simple eukaryote. Although genetic analysis has been useful to interrogate the aging process, there has been both interest and debate over how functionally conserved the mechanisms of aging are between yeast and higher eukaryotes, especially mammalian cells. One area of interest has been the importance of genomic stability for age-related processes, and the potential conservation of proteins and pathways between yeast and human. Translational genetics have been employed to examine the functional roles of mammalian proteins using yeast as a pliable model system. In the current review recent advancements made in this area are discussed, highlighting work which shows that the cellular functions of human proteins in DNA repair and maintenance of genomic stability can be elucidated by genetic rescue experiments performed in yeast.     

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.     

Impact of diabetes on alpha-crystallins and other heat shock proteins in the eye

Diabetes and its related complications represent a major growing health concern and economic burden worldwide. Ocular manifestations of diabetes include cataractogenesis and retinopathy, the latter being the leading cause of blindness in the working-age population. Despite numerous studies and recent progress, the exact pathophysiology of the disease remains to be fully elucidated and development of new and improved therapeutic strategies for this chronic condition are greatly needed. Heat shock proteins (Hsps) are highly conserved families of proteins, which are generally regarded as protective molecules that play a wide variety of roles and can be expressed in response to different types of cellular stresses. In recent years, numerous studies have reported their implication in various ocular diseases including diabetic retinopathy. The present review focuses on the potential implication of Hsps in ocular diabetic complications and discusses their specific mechanisms of regulation with respect to their expression, functions and alteration during diabetes. The review will conclude by examining the potential of Hsps as therapeutic agents or targets for the treatment of diabetic retinopathy.     

Protein folding diseases and neurodegeneration: lessons learned from yeast

Budding yeast Saccharomyces cerevisiae has proven to be a valuable model organism for studying fundamental cellular processes across the eukaryotic kingdom including man. In this respect, complementation assays, in which the yeast protein is replaced by a homologous protein from another organism, have been very instructive. A newer trend is to use the yeast cell factory as a toolbox to understand cellular processes controlled by proteins for which the yeast lacks functional counterparts. An increasing number of studies have indicated that S. cerevisiae is a suitable model system to decipher molecular mechanisms involved in a variety of neurodegenerative disorders caused by aberrant protein folding. Here we review the current knowledge gained by the use of so-called humanized yeasts in the field of Huntington's, Parkinson's and Alzheimer's diseases.     

New insights into autophagy using a multiple knockout strain

Not only is autophagy the major intracellular pathway for degradation and recycling of long-lived proteins and organelles, it is also involved in both the pathogenesis and prevention of many human diseases. Much progress has been made on the identification and characterization of AuTophaGy-related (ATG) genes, in yeast and in mammals. However, our understanding of the molecular mechanisms of autophagy remains quite limited, far from enough to harness autophagy for therapeutic applications. To better understand the molecular mechanisms, we took a unique and novel approach to study autophagy in yeast. We generated a multiple knockout Saccharomyces cerevisiae strain with 24 ATG genes deleted, and determined the minimum requirements for different aspects of autophagy. Our data also provided us with new insights into autophagy, different from those obtained from in vitro analyses. In this addendum, we briefly discuss our findings and consider fields where this multiple knockout strain can be of potential use.     

Novel insights into the osmotic stress response of yeast

Response to hyperosmolarity in the baker's yeast Saccharomyces cerevisiae has attracted a great deal of attention of molecular and cellular biologists in recent years, from both the fundamental scientific and applied viewpoint. Indeed the underlying molecular mechanisms form a clear demonstration of the intricate interplay of (environmental) signalling events, regulation of gene expression and control of metabolism that is pivotal to any living cell. In this article we briefly review the cellular response to conditions of hyperosmolarity, with focus on the high-osmolarity glycerol mitogen-activated protein kinase pathway as the major signalling route governing cellular adaptations. Special attention will be paid to the recent finding that in the yeast cell also major structural changes occur in order to ensure maintenance of cell integrity. The intriguing role of glycerol in growth of yeast under (osmotic) stress conditions is highlighted.     

Cargo trafficking from the trans‐Golgi network towards the endosome

The trans‐Golgi network (TGN) is a major sorting, packing and delivering station of newly synthesised proteins and lipids to their final destination. These cargo molecules follow the secretory pathway, which is a vital part of cellular trafficking machinery in all eukaryotic cells. This secretory pathway is well conserved in all eukaryotes from low‐level eukaryotes, such as yeast, to higher level eukaryotes like mammals. The molecular mechanisms of protein sorting by adaptor proteins, membrane elongation and transport to the final destinations by motor proteins and the cytoskeleton, and membrane pinching‐off by scission proteins must be choreographically managed for efficient cargo delivery, and the understanding of these detailed processes is not yet completed. Functionally, defects in these mechanisms are associated with the pathology of prominent diseases such as acute myeloid leukaemia, Charcot–Marie–Tooth disease, I‐cell disease and Wiskott–Aldrich syndrome. The present review points out the recent advances in our knowledge of the molecular mechanisms involved in the transportation of the cargo from the TGN towards the endosome.     

Monitoring polyglutamine toxicity in yeast

Experiments in yeast have significantly contributed to our understanding of general aspects of biochemistry, genetics, and cell biology. Yeast models have also delivered deep insights in to the molecular mechanism underpinning human diseases, including neurodegenerative diseases. Many neurodegenerative diseases are associated with the conversion of a protein from a normal and benign conformation into a disease-associated and toxic conformation - a process called protein misfolding. The misfolding of proteins with abnormally expanded polyglutamine (polyQ) regions causes several neurodegenerative diseases, such as Huntington's disease and the Spinocerebellar Ataxias. Yeast cells expressing polyQ expansion proteins recapitulate polyQ length-dependent aggregation and toxicity, which are hallmarks of all polyQ-expansion diseases. The identification of modifiers of polyQ toxicity in yeast revealed molecular mechanisms and cellular pathways that contribute to polyQ toxicity. Notably, several of these findings in yeast were reproduced in other model organisms and in human patients, indicating the validity of the yeast polyQ model. Here, we describe different expression systems for polyQ-expansion proteins in yeast and we outline experimental protocols to reliably and quantitatively monitor polyQ toxicity in yeast.     

Beer and bread to brains and beyond: can yeast cells teach us about neurodegenerative disease?

For millennia, humans have harnessed the astonishing power of yeast, producing such culinary masterpieces as bread, beer and wine. Therefore, in this new millennium, is it very farfetched to ask if we can also use yeast to unlock some of the modern day mysteries of human disease? Remarkably, these seemingly simple cells possess most of the same basic cellular machinery as the neurons in the brain. We and others have been using the baker's yeast, Saccharomyces cerevisiae, as a model system to study the mechanisms of devastating neurodegenerative diseases such as Parkinson's, Huntington's, Alzheimer's and amyotrophic lateral sclerosis. While very different in their pathophysiology, they are collectively referred to as protein-misfolding disorders because of the presence of misfolded and aggregated forms of various proteins in the brains of affected individuals. Using yeast genetics and the latest high-throughput screening technologies, we have identified some of the potential causes underpinning these disorders and discovered conserved genes that have proven effective in preventing neuron loss in animal models. Thus, these genes represent new potential drug targets. In this review, I highlight recent work investigating mechanisms of cellular toxicity in a yeast Parkinson's disease model and discuss how similar approaches are being applied to additional neurodegenerative diseases.     

Harnessing the power of yeast to unravel the molecular basis of neurodegeneration

Several neurodegenerative diseases, such as Parkinson's disease (PD), Alzheimer's disease (AD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), or prion diseases, are known for their intimate association with protein misfolding and aggregation. These disorders are characterized by the loss of specific neuronal populations in the brain and are highly associated with aging, suggesting a decline in proteostasis capacity may contribute to pathogenesis. Nevertheless, the precise molecular mechanisms that lead to the selective demise of neurons remain poorly understood. As a consequence, appropriate therapeutic approaches and effective treatments are largely lacking. The development of cellular and animal models that faithfully reproduce central aspects of neurodegeneration has been crucial for advancing our understanding of these diseases. Approaches involving the sequential use of different model systems, starting with simpler cellular models and ending with validation in more complex animal models, resulted in the discovery of promising therapeutic targets and small molecules with therapeutic potential. Within this framework, the simple and well‐characterized eukaryote Saccharomyces cerevisiae, also known as budding yeast, is being increasingly used to study the molecular basis of several neurodegenerative disorders. Yeast provides an unprecedented toolbox for the dissection of complex biological processes and pathways. Here, we summarize how yeast models are adding to our current understanding of several neurodegenerative disorders.     

Yeast as a tool to study Bax/mitochondrial interactions in cell death

The budding yeast Saccharomyces cerevisiae has proven to be a powerful tool in investigations of the molecular aspects of the events involved in apoptosis, particularly the steps implicating mitochondria. Yeast does not have obvious homologs of the proteins involved in the regulation of apoptosis, and provides a simplified model system in which the function of these proteins can be unraveled. This review focuses on the interactions of two of the major pro-apoptotic Bcl-2 family members, Bax and Bid, with mitochondria. It is shown that yeast has allowed questioning of several crucial aspects of the function of these two proteins, namely the molecular mechanisms driving their insertion into the mitochondrial outer membrane and those leading to the permeabilization to cytochrome c. More recently, signaling pathways leading to Bax-induced cell death, as well as other forms of cell death, have been identified in yeast. Both 'apoptosis-like' and autophagy-related forms of cell degradation are involved, and mitochondria play a central role in these two signaling pathways.     

Autophagy,lipophagy and lysosomal lipid storage disorders

Autophagy is a catabolic process with an essential function in the maintenance of cellular and tissue homeostasis. It is primarily recognised for its role in the degradation of dysfunctional proteins and unwanted organelles, however in recent years the range of autophagy substrates has also been extended to lipids. Degradation of lipids via autophagy is termed lipophagy. The ability of autophagy to contribute to the maintenance of lipo-homeostasis becomes particularly relevant in the context of genetic lysosomal storage disorders where perturbations of autophagic flux have been suggested to contribute to the disease aetiology. Here we review recent discoveries of the molecular mechanisms mediating lipid turnover by the autophagy pathways. We further focus on the relevance of autophagy, and specifically lipophagy, to the disease mechanisms. Moreover, autophagy is also discussed as a potential therapeutic target in several key lysosomal storage disorders.     

Insights into the mechanism of prion propagation

Proteins with prion properties have been identified in both mammals and fungi. The tractability of yeast as a genetic model has contributed significantly to our understanding of prion formation and propagation. A number of molecular chaperones have been found to modulate the ability of yeast prion proteins to propagate. The results of recent genetic and in vitro studies have shed light on the mechanism of prion propagation, the physical and structural basis of different prion strains and the species barrier, as well as the function and mechanism of the chaperones that interact with the prion proteins. Whether aspects of the mechanisms of formation, maintenance and clearance of prions are conserved between fungi and mammals remains to be seen.     

Cellular responses to endoplasmic reticulum stress and apoptosis

The endoplasmic reticulum (ER) is the cell organelle where secretory and membrane proteins are synthesized and folded. Correctly folded proteins exit the ER and are transported to the Golgi and other destinations within the cell, but proteins that fail to fold properly—misfolded proteins—are retained in the ER and their accumulation may constitute a form of stress to the cell—ER stress. Several signaling pathways, collectively known as unfolded protein response (UPR), have evolved to detect the accumulation of misfolded proteins in the ER and activate a cellular response that attempts to maintain homeostasis and a normal flux of proteins in the ER. In certain severe situations of ER stress, however, the protective mechanisms activated by the UPR are not sufficient to restore normal ER function and cells die by apoptosis. Most research on the UPR used yeast or mammalian model systems and only recently Drosophila has emerged as a system to study the molecular and cellular mechanisms of the UPR. Here, we review recent advances in Drosophila UPR research, in the broad context of mammalian and yeast literature.     

Different 8-hydroxyquinolines protect models of TDP-43 protein, α-synuclein, and polyglutamine proteotoxicity through distinct mechanisms

No current therapies target the underlying cellular pathologies of age-related neurodegenerative diseases. Model organisms provide a platform for discovering compounds that protect against the toxic, misfolded proteins that initiate these diseases. One such protein, TDP-43, is implicated in multiple neurodegenerative diseases, including amyotrophic lateral sclerosis and frontotemporal lobar degeneration. In yeast, TDP-43 expression is toxic, and genetic modifiers first discovered in yeast have proven to modulate TDP-43 toxicity in both neurons and humans. Here, we describe a phenotypic screen for small molecules that reverse TDP-43 toxicity in yeast. One group of hit compounds was 8-hydroxyquinolines (8-OHQ), a class of clinically relevant bioactive metal chelators related to clioquinol. Surprisingly, in otherwise wild-type yeast cells, different 8-OHQs had selectivity for rescuing the distinct toxicities caused by the expression of TDP-43, α-synuclein, or polyglutamine proteins. In fact, each 8-OHQ synergized with the other, clearly establishing that they function in different ways. Comparative growth and molecular analyses also revealed that 8-OHQs have distinct metal chelation and ionophore activities. The diverse bioactivity of 8-OHQs indicates that altering different aspects of metal homeostasis and/or metalloprotein activity elicits distinct protective mechanisms against several neurotoxic proteins. Indeed, phase II clinical trials of an 8-OHQ has produced encouraging results in modifying Alzheimer disease. Our unbiased identification of 8-OHQs in a yeast TDP-43 toxicity model suggests that tailoring 8-OHQ activity to a particular neurodegenerative disease may be a viable therapeutic strategy.