Ron Lab Research Program
"The cellular adaptation to proteotoxicity"
The folding state of polypeptides is easily perturbed by adverse conditions. Misfolded proteins are non-functional and lead to a measure of inefficiency in the cell's economy. Their presence has additional consequences that are unrelated to loss-of-function features as numerous genetic and biochemical observations suggest that structures elaborated by polypeptides that fail to attain their proper three dimensional structure have deleterious gain-of-function effects on cell function. This process, also referred to as "proteotoxicity", appears to be particularly important to the fate of non-renewable cells of long-lived organisms in which accumulating misfolded proteins can act over extended periods of time. The hypothesized contribution of such "proteotoxins" to cellular dysfunction fits our intuitive notions of aging as a time and use-dependent process. The progressive aging of the human population has led to an increase in the incidence of diseases hypothesized to be associated with various forms of proteotoxicity. These include not only the classic examples of the Amyloidoses, Prion disorders, Alzheimer's disease and various forms of Parkinsonism - in all of which the accumulation of abnormal proteins can be readily observed - but also other common diseases such as type-II diabetes mellitus in which low-levels protein misfolding in the secretory pathway, over time, may contribute to exhaustion of insulin-producing islet beta cells.
The pathophysiological effects of protein misfolding are likely to be influenced by the complex cellular adaptation to this form of stress. Some of the cellular responses to the presence and production of misfolded proteins are entirely adaptive whilst others may, in a physiological sense, over-shoot their mark and contribute to the morbid process. A detailed understanding of the signaling pathways involved in the response to misfolded proteins should distinguish between responses that are beneficial or adverse in any given context. Furthermore, by tracing these pathways upstream to the events involved in recognition of the misfolded protein signal(s) one may identify features that distinguish properly and improperly folded proteins and perhaps learn something about what renders the latter toxic to the cell.
Protein misfolding should be viewed in the context of the normal process by which polypeptides attain their proper three-dimensional structure. This is assisted by a dedicated cellular machinery whose functional state is responsive to the load of "client" proteins. Cells behaves as though they were fine-tuned to defend a certain reserve of protein folding capacity. This reserve can be challenged by increased synthesis of client proteins, by the synthesis of client proteins that are impaired in their capacity to fold (i.e. folding mutants) or by malfunction of the folding apparatus itself. A mismatch between capacity of the apparatus to fold proteins and the load imposed on it by the physiological state of the cell leads to misfolded protein stress.
In eukaryotic cells protein folding is compartmentalized with separate pathways for recognizing and responding to unfolded protein stress in the cytoplasm, secretory apparatus and mitochondria. Our lab has maintained a special interest in the adaptation of cells to the stress of unfolded proteins in the endoplasmic reticulum; so called "ER stress" to which cells respond with an "unfolded protein response" (UPR). We have employed biochemical and genetic tools to study the details of the UPR in complex (mammals) and simple metazoans (C. elegans). We explore biochemical events in the lumen of the ER, where malfolded proteins are recognized by the signal transducers of the UPR and the cytoplasm, through which the signal is transduced through to the nucleus, where changes in gene expression programs that are effected by the UPR take place. Signal transduction in the UPR has proven to be a fascinating business, employing unusual biochemical steps and integrating aspects of protein phosphorylation, mRNA processing, translational and transcriptional control.
As our understanding of the molecular details of UPR signaling matures, we uncover potential opportunities for external intervention, ultimately in the form of compounds that may affect specific biochemical steps required for UPR signaling. This, we believe, is an important area to emphasize in the coming years. Much also remains to be learned on the physiological ramifications of UPR signaling. The signaling pathways of the UPR are ancient and conserved and their activity appears to impact many aspects of cellular and organismal metabolism. This too is an area ripe for exploration, and we expect that reverse genetics in the mouse will figure heavily in these studies. Finally, our understanding of the biochemical nature of ER stress or the damage elicited by misfolded proteins is inferred in large part from the signals they elicit. In the future, we hope to use the insight garnered from studying the upstream-most events of the UPR to shed light on the (bio)physical nature of ER stress itself.
The long-term goal of our research is to identify new components of the cellular response to proteotoxic stress and to integrate these into an understanding of the pathophysiology of common human diseases. We expect that our basic research program into the biochemistry, cell biology and genetics of the cellular response to proteotoxins will contribute to the scaffold upon which translational research can later build to create pharmacological tools to manipulate the responses to favorable ends.
A list of publications from our lab, some in down-loadable pdf format are available here.