Stress granules (SGs) are ribonucleoprotein complexes induced by stress. They sequester mRNAs and disassemble when the stress subsides, allowing translation restoration. In amyotrophic lateral sclerosis (ALS), aberrant SGs cannot disassemble and therefore accumulate and are degraded by autophagy. However, the molecular events causing aberrant SG formation and the molecular players regulating this transition are largely unknown. We report that defective ribosomal products (DRiPs) accumulate in SGs and promote a transition into an aberrant state that renders SGs resistant to RNase. We show that only a minor fraction of aberrant SGs is targeted by autophagy, whereas the majority disassembles in a process that requires assistance by the HSPB8-BAG3-HSP70 chaperone complex. We further demonstrate that HSPB8-BAG3-HSP70 ensures the functionality of SGs and restores proteostasis by targeting DRiPs for degradation. We propose a system of chaperone-mediated SG surveillance, or granulostasis, which regulates SG composition and dynamics and thus may play an important role in ALS.

Targeting the PI3K/Akt/mTOR pathway may have pro-apoptotic and antiproliferative effects on hematological malignancies. Furthermore, modulation of miRNA can be used as a novel therapeutic approach to regulate the PI3K/Akt/mTOR pathway. However, both aspects require further clinical studies.

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.

The serine/threonine kinase Akt/PKB is a major signaling hub integrating metabolic, survival, growth, and cell cycle regulatory signals. The definition of the phospho-motif cipher driving phosphorylation by Akt led to the identification of hundreds of putative substrates, and it is therefore pivotal to identify those whose phosphorylation by Akt is of consequence to biological processes. The Lmna gene products lamin A/C and the lamin A precursor prelamin A are type V intermediate filament proteins forming a filamentous meshwork, the lamina, underneath the inner nuclear membrane, for nuclear envelope structures organization and interphase chromatin anchoring. In our previous work, we reported that A-type lamins are phosphorylated by Akt at S301 and S404 in physiological conditions and are therefore bona fide substrates of Akt. We report here that Akt phosphorylation at S404 targets the precursor prelamin A for degradation. We further demonstrate that Akt also regulates Lmna transcription. Our study unveils a previously unknown function of Akt in the control of prelamin A stability and expression. Moreover, given the large number of diseases related to prelamin A, our findings represent a further important step bridging basic A-type lamin physiology to therapeutic approaches for lamin A-linked disorders.

SummarySmall heat shock proteins (HSPBs) contain intrinsically disordered regions (IDRs), but the functions of these IDRs are still unknown. Here, we report that, in mammalian cells, HSPB2 phase separates to form nuclear compartments with liquid-like properties. We show that phase separation requires the disordered C-terminal domain of HSPB2. We further demonstrate that, in differentiating myoblasts, nuclear HSPB2 compartments sequester lamin A. Increasing the nuclear concentration of HSPB2 causes the formation of aberrant nuclear compartments that mislocalize lamin A and chromatin, with detrimental consequences for nuclear function and integrity. Importantly, phase separation of HSPB2 is regulated by HSPB3, but this ability is lost in two identified HSPB3 mutants that are associated with myopathy. Our results suggest that HSPB2 phase separation is involved in reorganizing the nucleoplasm during myoblast differentiation. Furthermore, these findings support the idea that aberrant HSPB2 phase separation, due to HSPB3 loss-of-function mutations, contributes to myopathy.