Dr. Greene's primary research interest is in the formation and dissolution of normal and pathological protein complexes in the cell, with an emphasis on the role of molecular chaperones.
The translation of mRNA into its corresponding amino acid chain is only the first step in a protein's life cycle. Although some proteins spontaneously fold into their correct three-dimensional structures to execute their functions, many more require the assistance of molecular chaperones to achieve their correct conformational states in complexes with other proteins and cellular membranes. Dr. Greene studies the role of molecular chaperones and their co-factors in the formation of vesicular compartments from clathrin-coated pits in the cellular membrane during endocytosis. She has applied her wealth of experience in the cell biology of protein folding and membrane trafficking toward deciphering the mechanisms of prion formation and propagation.
Prions are a class of proteins that can adopt an altered conformational state that not only leads them to form aggregates known as amyloids, but also allows them to similarly corrupt their normally folded soluble counterparts in the cytoplasm. In yeast, there are approximately a dozen prions, and their propagation requires several molecular chaperones, including Hsp104. Dr. Greene's laboratory complements biochemical studies with fluorescence imaging to follow the process of prion propagation in real time. In tissue culture, she and her colleagues are studying the cellular prion protein PrPC and the propagation of its scrapie isoform PrPSc, the cause of bovine spongiform encephalopathy, also known as BSE or "mad cow" disease.
True prions are defined by the fact that they are genetically identical to their non-prion protein counterparts, i.e. they are inherited and propagated entirely through protein-protein interactions and are interconvertible. However, it is becoming increasingly clear that many neurodegenerative diseases—such as Huntington's disease, amyotrophic lateral sclerosis (ALS), and others that are associated with abnormal protein aggregation initiated through genetic mutations—have a prion-like component to their transmission. Once such proteins are misfolded, they may provide a template for other proteins to misfold. Moreover, these misfolded templates could be transmitted between cells. If correct, such a cumulative model of neurodegenerative transmission could partially account for the relatively late onset of these diseases.
Dr. Greene and her colleagues are delving into this new paradigm to question how cells transmit misfolded proteins. Why, for example, is such an abnormal and potentially harmful protein aggregate not simply shunted off to the lysosome for degradation? How in turn does this abnormal protein move between cells? These are cellular trafficking questions that Dr. Greene's laboratory is well poised to address. Aggregates are carried as cargo by membrane vesicles and can be microscopically visualized in different cellular compartments by association with known molecular markers. Their passage can also be studied by genetic and RNAi methods that disrupt different steps in vesicular trafficking. Dr. Greene will complement this research with genetic studies of both the normal and mutant forms of the huntingtin gene in yeast and mammalian cells. Answering these questions will shed insight into the cellular pathophysiology of Huntington's disease and other protein aggregation disorders, which could lead to better understanding of these disorders on a broader scale.