?1-antitrypsin (AT) deficiency (ATD) is one of the most common genetic causes of liver disease in children and adults. The most common mutation causes the mutant protein (ATZ) to be retained in the endoplasmic reticulum (ER) of liver cells as misfolded toxic oligomers, polymers or aggregates. The only definitive treatment is liver transplantation. However, there is marked variation in the incidence, age-of-onset and severity of liver disease among homozygotes, which is attributed to genetic and environmental modifiers. Genetic modifiers are of significant clinical import as they are candidate drug targets for the treatment of cellular proteotoxicity associated with ATD liver disease. While a few genetic modifiers modulating ATD are under investigation, a non-biased, genome-wide search has not been conducted in metazoans. For this reason, we developed a model of ATZ-induced cellular damage in the nematode, C. elegans, one of the premier model systems for conducting comprehensive genome-wide genetic screens in higher eukaryotes. Remarkably, transgenic animals expressing ATZ recapitulated the ER-retention defect by acquiring intracellular inclusions similar to those characteristic of ATD hepatocytes. Animals show an easily scorable proteotoxic-stress-phenotype (slow growth, small brood sizes and decreased longevity). As in humans and mice, (macro) autophagy, is the main ATZ elimination pathway. We conducted a high-throughput genome-wide, systemic RNAi screen and identified 104 genetic modifiers of ATZ accumulation; including genes involved in insulin/insulin-like growth factor-signaling (IIS) pathway. Decreased IIS dramatically reduces ATZ accumulation and improves the viability of C. elegans, and has similar effects in mammalian cell lines and transgenic mice. Although the mechanism(s) is unknown, our preliminary data in C. elegans suggest that decreased IIS reduces ATZ accumulation by markedly diminishing the protein's half-life via enhanced degradation. However, this pathway is independent of the canonical autophagy pathway known to degrade ATZ. Interestingly, this elimination pathway, which is conserved in mammals, appears to engage membrane bound elements of the systemic RNAi machinery to increase the elimination of ATZ. We hypothesize that 1) decreased IIS activates a non-classical misfolded protein elimination pathway that dramatically reduces the half-life of ATZ, and 2) this pathway is linked to membrane bound elements of the systemic RNAi machinery, possibly by altering activity of the endomembrane/lysosomal system. The objective of this proposal is to capitalize upon the genetic tractability of C. elegans and identify the constituents of a protein elimination pathway regulated by IIS. Upon elucidating the mechanism in C. elegans, genetic elements will be tested for conservation of function in mammalian cell lines expressing ATZ. The discovery of potent proteostasis modifiers downstream of the IIS pathway provides an unprecedented opportunity for the future development of target-based therapeutics for ATZ-induced liver disease, while avoiding the untoward effects of directly altering insulin sensitivity.
Alpha-1-antitrypsin deficiency is a leading genetic cause of liver injury, but lacks effective treatments. The goal of this study is to model this disease in a simple organism, and use this tool identify new genes that can be manipulated to improve the outcome of the disease.
| Lanik, Wyatt E; Xu, Lily; Luke, Cliff J et al. (2018) Breast Milk Enhances Growth of Enteroids: An Ex Vivo Model of Cell Proliferation. J Vis Exp : |
