Quality control under stress: Ubiquilin 2 bridges protein and RNA homeostasis at stress granules in ALS/FTD pathology
Embargo until
2021-05-01
Date
2019-02-04
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Johns Hopkins University
Abstract
Both protein and RNA quality control are critical for cell survival during stress and are disrupted in the neurodegenerative diseases amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). ALS is the most common adult onset motor neuron disease, while FTD is the second most common type of dementia for individuals under 65. Both are characterized by the accumulation of proteins in intracellular inclusions and are thought to be part of a continuous clinical spectrum, but their causes are largely unknown. Stress granules (SGs), a type of RNA granule, have recently emerged as potential seed sites for patient protein inclusions that incorporate both protein quality control (PQC) factors and RNA binding proteins (RBPs). SGs transiently sequester stalled translation initiation complexes and apoptosis signaling molecules in response to stress. These dynamic particles form via a liquid-liquid phase separation mediated by oligomerization of the low complexity prion-like domains in RBPs bound to RNA in a process tuned by RNA length and structure. Stable interactions between these proteins in SGs leads to the formation of protein aggregates. The PQC factors that regulate the dynamic incorporation of RBPs into SGs to prevent aggregate formation remain largely unknown.
In 2011, missense mutations in the ubiquitin-like protein ubiquilin 2 (UBQLN2) were found to cause ALS with FTD in rare X-linked cases. In an effort to characterize UBQLN2’s normal cellular functions, I performed a proteomic screen for UBQLN2 interactors and found an enrichment of RBPs including SG components under non-stress conditions. Using an unconventional staining technique, I confirmed that UBQLN2 localizes to SGs under a variety of stress conditions. and found that this association is transient. In the time period that UBQLN2 localizes to SGs, it also suppresses large SG formation, suggesting that it modulates incorporation of SG components.
To characterize UBQLN2’s regulatory role in SG formation, I focused on its interaction with the RBP fused in sarcoma (FUS). FUS was not only the most enriched RBP among the UBQLN2 interactors, but also is genetically and pathologically linked to ALS/FTD. In a single molecule Förster resonance energy transfer (FRET) assay, UBQLN2 increased the dynamics of FUS-RNA interaction, increasing the fluidity of FUS-RNA complex formation. This effect translated to a decrease in FUS partitioning into liquid droplets in vitro and suppression of FUS-GFP recruitment into SGs in cells suggesting that UBQLN2 slows FUS recruitment into SGs by increasing the dynamics of its interaction with RNA. ALS/FTD-linked mutations in UBQLN2 partially impaired UBQLN2’s ability to increase the dynamics of FUS-RNA interaction and suppress SG formation, indicating a partial loss of UBQLN2 function. These results reveal a previously unrecognized role for UBQLN2 in regulating the early stages of liquid-liquid phase separation of RBPs in SG formation by affecting the dynamics of RNA-protein interaction. Because SGs potentially seed toxic ALS/FTD patient inclusions, these findings have implications for understanding ALS/FTD pathogenesis and designing new treatments for these diseases.
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Keywords
Ubiquilin 2, stress granules