Discovering and targeting the mechanisms that govern age-related decline

Abstract

Aging is recognized as a major risk factor for many diseases. Thus, the identification of a means to target aging would bear great potential to improve human health and quality of life. This thesis tries to help this identification by improving our understanding of how aging is regulated, and by developing and applying new strategies to identify aging-preventive pharmaceuticals.

The transcription factor (TF) FOXO, also known as DAF-16 in Caenorhabditis elegans (C. elegans), is considered a central nexus for aging regulation. It is located downstream of the insulin/insulin-like growth factor 1 signaling (IIS) pathway, and is activated by low IIS as well as many other stresses and aging-regulatory stimuli. Studies in many model organisms have shown that DAF-16/FOXO activation not only leads to longer lifespan, but also increased stress resistance. While the mechanisms leading to DAF-16 activation and its target genes have been extensively studied, the mechanism of how DAF-16 relays aging-regulatory stimuli to activate the transcriptional response remains unclear. In paper 1, we used C. elegans to determine that DAF-16 can form a complex with HLH-30 transcription factor, a master regulator in autophagy and lysosomal biogenesis, and known to be important for longevity in C. elegans. These two TFs require each other to mediate longevity in daf-2 (low IIS mutant) and glp-1 (a germline deficient mutant whose longevity also depends on DAF-16) animals. Moreover, they co-regulate many target genes and co-occupy many promoter regions, indicating that HLH-30 plays an important role in DAF-16-dependent transcriptional responses and longevity. Interestingly, even though both TFs are important for longevity and different types of stress resistance, we found that their genetic interaction is context-dependent. They function in the same genetic pathway to cope with oxidative stress, while they act through independent pathways to cope with heat stress. Finally, we observed that they have opposing roles for dauer formation (a developmental diapause state that enables worms to survive in harsh environments). By further mechanistic exploration, we provide a model whereby DAF-16 forms a transcriptional regulatory module with HLH-30 that relays diverse distress signals to stimulate the appropriate transcriptional responses, thereby ensuring the organism’s survival.

Despite a growing understanding of how aging is regulated, we are still lacking suitable methods of screening for pharmacological interventions which, by targeting these pathways, would lead to aging-preventive effects in humans. In paper 2, we developed and applied an in silico screening method to identify aging-preventive pharmaceuticals, also known as geroprotectors. We started by building machine-learning-based algorithms using age-stratified human tissue transcriptomic data from the Genotype-Tissue Expression (GTEx) project and applied them for in silico screen from more than 1300 compounds in the Connectivity Map (CMap) database. These compounds were ranked by the likelihood of their geroprotective ability, and the best candidates were further validated in C. elegans. We found that two heat shock protein 90 (HSP90) inhibitors, monorden and tanespimycin, can improve the worms’ lifespan and healthspan. We argue that HSP90 inhibitors activate the cytosolic unfolded protein response by elevating HSP expression through the TF HSF-1 (heat shock factor 1). This in turn improves protein homeostasis, leading to cytoprotection, and ultimately better health and a longer life.