Current Research and Scholarly Interests
Protein folding is critically important for all life, from microbes to man. A bafflingly diverse set of cellular mechanisms has evolved to coordinate this process. Not unexpectedly, problems in protein folding are the root cause of many of the most devastating diseases, which represent a major challenge to public health worldwide, especially as our population continues to age. Referred to collectively as protein-misfolding disorders, these truly disastrous neurodegenerative diseases include Alzheimer disease, Parkinson disease and ALS (Lou Gehrigs disease). Understanding at a mechanistic level the cellular consequences of protein misfolding will help to suggest potential strategies for therapeutic intervention. We use the bakers yeast, Saccharomyces cerevisiae, as a model system to study the cell biology underpinning protein-misfolding diseases. We do not limit ourselves to one model system or experimental approach. We start with yeast, perform genetic screens, and then move to other model systems (e.g. mammalian tissue culture, mouse, fly, zebrafish) and even work with human patient samples (tissue sections, patient-derived cells, including iPS cells, and next generation sequencing approaches to look for mutations in novel genes).
Parkinson's disease and alpha-synuclein
We have focused on the Parkinson disease (PD) linked protein, alpha-synuclein. By performing high-throughput genome-wide screens in yeast, we identified a set of genes, many with clear human homologs, which are able to antagonize cellular toxicity associated with the accumulation of misfolded alpha-synuclein. Remarkably, some genes are also able to rescue neuron loss in animal models of PD (Cooper et al., Science 2006). We recently found that one of the genes from our alpha-synuclein toxicity modifier screen is the yeast homolog of the human PARK9 gene and that yeast PARK9 functions to protect cells from manganese toxicity, an environmental risk factor for PD and PD-like syndromes (Gitler et al., Nature Genetics 2009).
New yeast models of neurodegenerative diseases
We recently developed a yeast model to study the ALS disease proteins TDP-43 and FUS (Johnson et al., PNAS 2008; Sun et al., 2011). We have used yeast and in vitro biochemistry (in collaboration with Jim Shorter at PENN) to analyze the effects of ALS-linked TDP-43 and FUS mutations on aggregation and toxicity (Johnson et al., JBC 2009; Sun et al., 2011). We are now using these models to perform high-throughput genetic screens to elucidate the molecular pathways affected by TDP-43 and FUS aggregation.
Ataxin-2 and ALS
Interestingly, one of the hits from our yeast TDP-43 genetic modifier screen is the homolog of a human neurodegenerative disease protein, ataxin 2. We have validated this genetic interaction in the fly nervous system (in collaboration with Nancy Bonini at PENN), and used biochemistry to show the proteins physically associate in an RNA-dependent manner.To extend our findings to human disease, we analyzed the ataxin 2 gene in a large number of ALS patients and healthy controls and found intermediate-length polyQ expansions in ataxin 2 significantly associated with increased risk for ALS (Elden et al., Nature 2010). We continue to characterize the role of ataxin 2 in ALS as well as other neurodegenerative disease situations (Hart et al., 2012; Hart and Gitler, 2012).
New ALS Disease Genes
We have begun a novel functional screen in yeast to identify new human ALS disease genes. From this seemingly simple yeast screen, we were able to predict a set of ALS candidate disease genes and, remarkably, have already identified mutations in two of them in human ALS patients (Couthouis et al., 2011; Couthouis et al., 2012). We continue to sequence more genes in a large cohort of sporadic and familial ALS patients using standard as well as next generation sequencing approaches.