Protein Homeostasis as a Critical Mediator of Stem Cell Function
Experiments with mammalian embryonic stem cells (ESCs) have clearly demonstrated their capacity to replicate continuously. We hypothesized that the immortality of these cells depends in part upon an increased ability to protect themselves from the accumulation of damaged proteins normally seen with age. We found that stem cells had a six-fold higher proteasome activity than their differentiated counterparts. Notably, stem cell populations exhibit a proteasome activity that is correlated with increased levels of one proteasome subunit, PSMD11, and increased assembly of the proteasome. FOXO4, an insulin/IGF-1 responsive transcription factor associated with long lifespan in invertebrates, regulates proteasome activity by modulating the expression of PSMD11 in human embryonic stem cells (hESCs) and is necessary for hESC differentiation into neuronal lineages. Our results establish a novel regulation of proteostasis in hESCs that links longevity in invertebrates with hESCs function and identity. Manipulation of PSMD11 allows us to genetically induce proteasomal activity, providing a unique method for manipulating protein degradation in stem cells. We will take advantage of these findings to promote our understanding of exactly how stem cells are protected from damage. Moreover, we hypothesize that the activity and expression of the proteasome may be key determinants in our capacity to reprogram somatic cells and in mitigating the effects of age-onset protein misfolding diseases.
The number of aging Californians diagnosed with protein misfolding diseases, including Alzheimer’s disease, is currently undergoing exponential growth: within the next 20 years, well over a million Californians are expected to be diagnosed with Alzheimer’s. The cost of care and treatment for these individuals reaches into the 100’s of billions of dollars within California alone and could eventually undermine the economic and social stability of the state. Tragically, in such diseases, diagnosis usually occurs after wide spread neuronal death has already occurred. One of the more promising therapeutic options for patients with protein misfolding diseases is stem cell therapy, which hopes to replace lost neurons with ones generated from stem cells. However, we do not yet understand much of the basic biology of how stem cells maintain their health, including how they can protect themselves from the accumulation of damaged proteins -- especially in an older individual who is rapidly succumbing to proteotoxic disease in other cells. This research is designed to address a basic and often overlooked question about stem cell health: what machinery does the stem cell employ to guarantee the health of its proteome, and how is this related to the aging process? This research will provide fundamental insights into the mechanisms of protein homeostasis within the stem cell, findings that can be immediately applied by those searching for therapeutic options for these diseases.