Granulostasis: Protein Quality Control of RNP Granules

Abstract

Ribonucleoprotein (RNP) granules transport, store, or degrade messenger RNAs, thereby indirectly regulating protein synthesis. Normally, RNP granules are highly dynamic compartments. However, because of aging or severe environmental stress, RNP granules, in particular stress granules (SGs), convert into solid, aggregate-like inclusions. There is increasing evidence that such RNA-protein inclusions are associated with several age-related neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS), fronto-temporal dementia (FTD) and Alzheimer's disease (AD). Thus, understanding what triggers the conversion of RNP granules into aggregates and identifying the cellular players that control RNP granules will be critical to develop treatments for these diseases. In this review article, we discuss recent insight into RNP and SG formation. More specifically, we examine the evidence for liquid-liquid phase separation (LLPS) as an organizing principle of RNP granules and the role of aggregation-prone RNA-binding proteins (RBPs) in this process. We further discuss recent findings that liquid-like SGs can sequester misfolded proteins, which promote an aberrant conversion of liquid SGs into solid aggregates. Importantly, very recent studies show that a specific protein quality control (PQC) process prevents the accumulation of misfolding-prone proteins in SGs and, by doing so, maintains the dynamic state of SGs. This quality control process has been referred to as granulostasis and it relies on the specific action of the HSPB8-BAG3-HSP70 complex. Additional players such as p97/valosin containing protein (VCP) and other molecular chaperones (e.g., HSPB1) participate, directly or indirectly, in granulostasis, and ensure the timely elimination of defective ribosomal products and other misfolded proteins from SGs. Finally, we discuss recent findings that, in the stress recovery phase, SGs are preferentially disassembled with the assistance of chaperones, and we discuss evidence for a back-up system that targets aberrant SGs to the aggresome for autophagy-mediated clearance. Altogether the findings discussed here provide evidence for an intricate network of interactions between RNP granules and various components of the PQC machinery. Molecular chaperones in particular are emerging as key players that control the composition and dynamics of RNP granules, which may be important to protect against age-related diseases.

Keywords: RNA homeostasis; age-related neurodegenerative diseases; amyotrophic lateral sclerosis; molecular chaperone complexes; phase separation; protein homeostasis; stress granules.

Figure 1

The ribosome quality control (RQC), protein quality control (PQC) and cellular degradation systems. Defects at the level of the mRNA as well as defective ribosomal products (DRiPs) activate RQC mechanisms. This leads to the selective elimination of aberrant nascent polypeptides via the proteasome and/or autophagy. Key players of the RQC system include, e.g., LTN1, nuclear export mediator factor (NEMF), valosin containing protein (VCP) and co-factors Npl4 or Ufd1. Similarly, when misfolded proteins accumulate, for example because of proteotoxic stress, molecular chaperones, such as DNAJs and HSPBs, act in concert with HSP70 and specific co-chaperones to target misfolded proteins to the proteasome (BAG1) or autophagy (BAG3) for disposal. Autophagy receptors and adaptor molecules, such as p62 and HDAC6, participate in targeting (poly)ubiquitinated substrates to autophagosomes.

Figure 2

The continuum model of prion-like proteins. FUS and other prion-like proteins can adopt a range of different material states. When supersaturated, FUS demixes from a solution of FUS molecules and assembles into liquid-like droplets. These liquid droplets are unstable and will convert with time into gels and solid-like fibers. The liquid-like state is required for the formation of physiological compartments in the nucleus and cytoplasm of cells, whereas the gel and fiber states have been associated with pathological conditions.

Figure 3

Interplay between stress granules (SGs) and PQC. Polysomes disassemble when cells are exposed to stress, releasing mRNAs, ribosomal subunits and DRiPs (1). mRNAs are packaged into SGs (2). The concerted action of the HSPB8-BAG3-HSP70 complex limits the accumulation of misfolded proteins and DRiPs inside SGs (3). The surveillance function of HSPB8-BAG3-HSP70 ensures dynamic SG behavior and disassembly (granulostasis; 4), with subsequent translation restoration (5). Impairment of the HSPB8-BAG3-HSP70 complex and the presence of disease-linked mutations in RBPs leads to the formation of SGs that increasingly accumulate DRiPs (6) and, consequently, become less dynamic and more aggregate-like (7). Persisting SGs can be degraded via autophagy (8). SG persistence affects the ability of the cells to properly restore translation after stress, with potentially harmful effects and loss of cellular viability.