Reduced Insulin/IGF-1 Signaling Restores the Dynamic Properties of Key Stress Granule Proteins during Aging

摘要

class="head no_bottom_margin" id="sec1title">IntroductionYoung, healthy organisms strive to maintain their proteome in a functional state through the tight control of rates of protein synthesis, folding, and degradation. Extensive quality-control systems are set up throughout the cell to prevent and manage protein damage. As the organism ages, these control mechanisms become less efficient, leading to a disruption in protein homeostasis (, ). Aging is the main risk factor for a variety of neurodegenerative diseases where specific proteins accumulate as pathological aggregates. Recently, there has been considerable interest in investigating widespread protein aggregation in the absence of disease. Multiple studies have demonstrated that several hundred proteins become highly detergent-insoluble in aged animals (, , , , , , ). Computational analysis of the insoluble proteome indicates an overrepresentation of proteins with functional and structural similarities (). The examination of some of these proteins in vivo reveals their assembly into large “solid” aggregates with age similar to those formed in the context of disease. The discovery of endogenous age-dependent protein aggregation in model organisms gives us the unprecedented opportunity to dissect the intrinsic cellular machineries responsible for preventing protein aggregation without using ectopically expressed human disease-associated proteins. At this time, very little is known concerning the regulation of widespread protein insolubility with age and its consequences for the health of the organism. Interestingly, several studies show that protein insolubility is modified in long-lived animals with reduced insulin/insulin growth factor (IGF)-1 daf-2 signaling, but it remains unclear to which extent (, , ).A growing number of familial and sporadic forms of neurodegenerative diseases show pathological inclusions caused by abnormal aggregation of RNA-binding proteins (RBPs). The first RBPs identified in these inclusions were TAR DNA binding protein of 43 kDa (TDP-43) and fused in sarcoma (FUS), associated with amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) (, , href="#bib35" rid="bib35" class=" bibr popnode">Neumann et al., 2006). Since then additional RBPs such as TAF15, EWSR1, hnRNPA2B1, hnRNPA1, and hnRNPA3 have been associated with neurodegenerative diseases (href="#bib23" rid="bib23" class=" bibr popnode">Kim et al., 2013, href="#bib37" rid="bib37" class=" bibr popnode">Neumann et al., 2011). All of the known RBPs associated with dementia contain a low-complexity (LC) “prion-like” domain enriched in glycines and uncharged polar amino acids, and similar to the sequences driving yeast prion aggregation (href="#bib1" rid="bib1" class=" bibr popnode">Alberti et al., 2009, href="#bib25" rid="bib25" class=" bibr popnode">King et al., 2012). Mutations in this domain enhance pathology by accelerating aggregation (href="#bib19" rid="bib19" class=" bibr popnode">Johnson et al., 2009, href="#bib23" rid="bib23" class=" bibr popnode">Kim et al., 2013). LC prion-like domains are also present in key RBPs that mediate the assembly of RNA granules by liquid-liquid phase separation (href="#bib30" rid="bib30" class=" bibr popnode">Lin et al., 2015, href="#bib31" rid="bib31" class=" bibr popnode">Molliex et al., 2015, href="#bib34" rid="bib34" class=" bibr popnode">Murakami et al., 2015, href="#bib39" rid="bib39" class=" bibr popnode">Patel et al., 2015). Significantly, a small proportion of liquid droplets made by RBPs transform into solid aggregates over time in vitro (href="#bib30" rid="bib30" class=" bibr popnode">Lin et al., 2015, href="#bib31" rid="bib31" class=" bibr popnode">Molliex et al., 2015, href="#bib34" rid="bib34" class=" bibr popnode">Murakami et al., 2015, href="#bib39" rid="bib39" class=" bibr popnode">Patel et al., 2015). For clarity, we will use the term aggregation only when referring to the formation of non-dynamic RBP aggregates. An important question is whether the special assembly properties of RBPs puts them at risk of aggregating during aging in a multicellular organism and not just in the context of disease. Interestingly, several RBPs with LC prion-like domains were identified in the insoluble proteome of aged animals (href="#bib11" rid="bib11" class=" bibr popnode">David et al., 2010). Overall, it is imperative to know the causes and consequences of wild-type RBP aggregation during aging in order to fully understand RBP aggregation in neurodegenerative diseases. Furthermore, it is likely that the organism has evolved specific mechanisms to control liquid droplet protein aggregation.In the current study, we chose to focus on key RBPs responsible for stress granule formation. Stress granules are a specific type of RNA granule that protect the cell by sequestering mRNA from the translational machinery during periods of stress. Importantly, stress granule proteins are often found to co-localize with pathological inclusions of TDP-43 and FUS (href="#bib7" rid="bib7" class=" bibr popnode">Bentmann et al., 2013, href="#bib28" rid="bib28" class=" bibr popnode">Li et al., 2013). Whether these stress granule proteins are innocent bystanders transiently interacting with TDP-43 and FUS or whether they co-aggregate and accelerate disease-associated RBP aggregation remains intensely debated (href="#bib7" rid="bib7" class=" bibr popnode">Bentmann et al., 2013, href="#bib28" rid="bib28" class=" bibr popnode">Li et al., 2013).We show that key stress-granule-related RBPs (sgRBPs) accumulate in aberrant stress granule-like puncta and in large solid aggregates in aged C. elegans. Proteomic analysis revealed that long-lived animals with reduced daf-2 signaling preferentially abrogate the insolubility of RNA granule components. Importantly, sgRBP aggregates are associated with reduced animal size, motility, and lifespan. We show that sgRBP aggregation is triggered at an earlier age by their co-aggregation with other misfolded proteins, a process that is prevented by DAF-16 in daf-2 mutants. In addition, the proteostasis network established by heat shock transcription factor 1 (HSF-1) during development is required to maintain dynamic stress granule proteins throughout the animal’s life.