Actin remodelling controls proteasome homeostasis upon stress

Limiting cap-dependent translation increases 20S proteasomal degradation and protects the proteomic integrity in autophagy-deficient skeletal muscle

Postmitotic skeletal muscle critically depends on tightly regulated protein degradation to maintain proteomic stability. Impaired macroautophagy/autophagy-lysosomal or ubiquitin-proteasomal protein degradation causes the accumulation of damaged proteins, ultimately accelerating muscle dysfunction with age. While in vitro studies have demonstrated the complementary nature of these systems, their interplay at the organism levels remains poorly understood. Here, our study reveals novel insights into this complex relationship in autophagy-deficient skeletal muscle. We demonstrated that despite a compensatory increase in proteasome level in response to autophagy impairment, 26S proteasome activity was not proportionally enhanced in autophagy-deficient skeletal muscle. This functional deficit was partly attributed to reduced ATP levels to fuel the 26S proteasome. Remarkably, we found that activation of EIF4EBP1, a crucial inhibitor of cap-dependent translation, restored and even augmented proteasomal function through dual mechanisms. First, genetically activating EIF4EBP1 enhanced both ATP-dependent 26S proteasome and ATP-independent 20S proteasome activities, thereby expanding overall protein degradation capacity. Second, EIF4EBP1 activation caused muscle fiber transformation and increased mitochondrial biogenesis, thus replenishing ATP levels for 26S proteasome activation. Notably, the improved performance of the 20S proteasome in EIF4EBP1-activated skeletal muscle was attributed to an increased abundance of the immunoproteasome, a subtype specially adapted to function under oxidative stress conditions. This dual action of EIF4EBP1 activation preserved proteomic integrity in autophagy-deficient skeletal muscle. Our findings uncover a novel role of EIF4EBP1 in improving protein quality control, presenting a promising therapeutic strategy for autophagy-related muscular disorders and potentially other conditions characterized by proteostatic imbalance.Abbreviations: 3-MA: 3-methyladenine; ACAC/ACC: acetyl-Coenzyme A carboxylase; AMPK: AMP-activated protein kinase; ATG5: autophagy related 5; ATG7: autophagy related 7; ATP: adenosine triphosphate; ATP5F1A/ATP5A: ATP synthase F1 subunit alpha; CKM-Cre: creatine kinase, muscle-Cre; CMA: chaperone-mediated autophagy; CTSB: cathepsin B; CTSK: cathepsin K; CTSL: cathepsin L; CUL3: cullin 3; EDL: extensor digitorum longus; EIF4E: eukaryotic translation initiation factor 4E; EIF4EBP1: eukaryotic translation initiation factor 4E binding protein 1; EIF4F: eukaryotic translation initiation factor 4F complex; FBXO32/ATROGIN1/MAFbx: F-box protein 32; GFP: green fluorescent protein; IFNG/IFN-γ: interferon gamma; KEAP1: kelch-like ECH-associated protein 1; LAMP1: lysosomal-associated membrane protein 1; LAMP2: lysosomal-associated membrane protein 2; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MEF: mouse embryonic fibroblast; Myl1/Mlc1f-Cre: myosin, light polypeptide 1 (promoter driving Cre recombinase); mRFP: monomeric red fluorescent protein; MTOR: mechanistic target of rapamycin kinase; MTORC1: MTOR complex 1; NFE2L1/NRF1: nuclear factor, erythroid derived 2, like 1; NFE2L2/NRF2: nuclear factor, erythroid derived 2, like 2; NFKB1/NFκB1: nuclear factor of kappa light polypeptide gene enhancer in B cells 1, p105; OXPHOS: oxidative phosphorylation; PPARGC1A/PGC1α: peroxisome proliferator activated receptor, gamma, coactivator 1 alpha; PSMB5: proteasome (prosome, macropain) subunit, beta type 5; PSMB6: proteasome (prosome, macropain) subunit, beta type 6; PSMB7: proteasome (prosome, macropain) subunit, beta type 7; PSMB8: proteasome (prosome, macropain) subunit, beta type 8 (large multifunctional peptidase 7); PSMB9: proteasome (prosome, macropain) subunit, beta type 9 (large multifunctional peptidase 2); PSMB10: proteasome (prosome, macropain) subunit, beta type 10; PSME1: proteasome (prosome, macropain) activator subunit 1 (PA28 alpha); PSME2: proteasome (prosome, macropain) activator subunit 2 (PA28 beta); RBX1: ring-box 1; SQSTM1/p62: sequestosome 1; SREBF1/SREBP1: sterol regulatory element binding transcription factor 1; STAT3: signal transducer and activator of transcription 3; TRIM63/MURF1: tripartite motif-containing 63; ULK1: unc-51 like kinase 1; UPS: ubiquitin-proteasome system.

Translation regulation in response to stress

Cell stresses occur in a wide variety of settings: in disease, during industrial processes, and as part of normal day-to-day rhythms. Adaptation to these stresses requires cells to alter their proteome. Cells modify the proteins they synthesize to aid proteome adaptation. Changes in both mRNA transcription and translation contribute to altered protein synthesis. Here, we discuss the changes in translational mechanisms that occur following the onset of stress, and the impact these have on stress adaptation.

Actin-bundling protein fimbrin serves as a new auxin biosynthesis orchestrator in Arabidopsis root tips

Plants delicately regulate endogenous auxin levels through the coordination of transport, biosynthesis, and inactivation, which is crucial for growth and development. While it is well-established that the actin cytoskeleton can regulate auxin levels by affecting polar transport, its potential role in auxin biosynthesis has remained largely unexplored. Using LC-MS/MS-based methods combined with fluorescent auxin marker detection, we observed a significant increase in root auxin levels upon deletion of the actin bundling proteins AtFIM4 and AtFIM5. Fluorescent observation, immunoblotting analysis, and biochemical approaches revealed that AtFIM4 and AtFIM5 affect the protein abundance of the key auxin synthesis enzyme YUC8 in roots. AtFIM4 and AtFIM5 regulate the auxin synthesis enzyme YUC8 at the protein level, with its degradation mediated by the 26S proteasome. This regulation modulates auxin synthesis and endogenous auxin levels in roots, consequently impacting root development. Based on these findings, we propose a molecular pathway centered on the 'actin cytoskeleton-26S proteasome-YUC8-auxin' axis that controls auxin levels. Our findings shed light on a new pathway through which plants regulate auxin synthesis. Moreover, this study illuminates a newfound role of the actin cytoskeleton in regulating plant growth and development, particularly through its involvement in maintaining protein homeostasis via the 26S proteasome.

Deciphering ApoE Genotype-Driven Proteomic and Lipidomic Alterations in Alzheimer's Disease Across Distinct Brain Regions

Alzheimer's disease (AD) is a neurodegenerative disease with a complex etiology influenced by confounding factors such as genetic polymorphisms, age, sex, and race. Traditionally, AD research has not prioritized these influences, resulting in dramatically skewed cohorts such as three times the number of Apolipoprotein E (APOE) ε4-allele carriers in AD relative to healthy cohorts. Thus, the resulting molecular changes in AD have previously been complicated by the influence of apolipoprotein E disparities. To explore how apolipoprotein E polymorphism influences AD progression, 62 post-mortem patients consisting of 33 AD and 29 controls (Ctrl) were studied to balance the number of ε4-allele carriers and facilitate a molecular comparison of the apolipoprotein E genotype. Lipid and protein perturbations were assessed across AD diagnosed brains compared to Ctrl brains, ε4 allele carriers (APOE4+ for those carrying 1 or 2 ε4s and APOE4- for non-ε4 carriers), and differences in ε3ε3 and ε3ε4 Ctrl brains across two brain regions (frontal cortex (FCX) and cerebellum (CBM)). The region-specific influences of apolipoprotein E on AD mechanisms showcased mitochondrial dysfunction and cell proteostasis at the core of AD pathophysiology in the post-mortem brains, indicating these two processes may be influenced by genotypic differences and brain morphology.

Distinct TORC1 signalling branches regulate Adc17 proteasome assembly chaperone expression

When stressed, cells need to adapt their proteome to maintain protein homeostasis. This requires increased proteasome assembly. Increased proteasome assembly is dependent on increased production of proteasome assembly chaperones. In Saccharomyces cerevisiae, inhibition of the growth-promoting kinase complex TORC1 causes increased proteasome assembly chaperone translation, including that of Adc17. This is dependent upon activation of the mitogen-activated protein kinase (MAPK) Mpk1 and relocalisation of assembly chaperone mRNA to patches of dense actin. We show here that TORC1 inhibition alters cell wall properties to induce these changes by activating the cell wall integrity pathway through the Wsc1, Wsc3 and Wsc4 sensor proteins. We demonstrate that, in isolation, these signals are insufficient to drive protein expression. We identify that the TORC1-activated S6 kinase Sch9 must be inhibited as well. This work expands our knowledge on the signalling pathways that regulate proteasome assembly chaperone production.

Polarity-dependent expression and localization of secretory glucoamylase mRNA in filamentous fungal cells

In multinuclear and multicellular filamentous fungi little is known about how mRNAs encoding secreted enzymes are transcribed and localized spatiotemporally. To better understand this process we analyzed mRNA encoding GlaA, a glucoamylase secreted in large amounts by the industrial filamentous fungus Aspergillus oryzae, by the MS2 system, in which mRNA can be visualized in living cells. We found that glaA mRNA was significantly transcribed and localized near the hyphal tip and septum, which are the sites of protein secretion, in polarity-dependent expression and localization manners. We also revealed that glaA mRNA exhibits long-range dynamics in the vicinity of the endoplasmic reticulum (ER) in a manner that is dependent on the microtubule motor proteins kinesin-1 and kinesin-3, but independent of early endosomes. Moreover, we elucidated that although glaA mRNA localized to stress granules (SGs) and processing bodies (PBs) under high temperature, glaA mRNA was not seen under ER stress, suggesting that there are different regulatory mechanisms of glaA mRNA by SG and PB under high temperature and ER stress. Collectively, this study uncovers a dynamic regulatory mechanism of mRNA encoding a secretory enzyme in filamentous fungi.

Proteasome assembly chaperone translation upon stress requires Ede1 phase separation at the plasma membrane

Proteome adaptation is key to cells surviving stresses. Increased translation of proteasome assembly chaperones (PACs) is critical for increasing proteasome assembly and cell degradative capacity. The endocytic protein Ede1 recruits PAC mRNA to cortical actin patches in Saccharomyces cerevisiae for translation upon stress. We show, through genetic and pharmacological studies, that this is mediated by the capacity of Ede1 to phase separate. PAC expression is maintained when we exchange the phase separating domains from Ede1 for those of unrelated proteins. Without these phase separating regions, PAC expression is not induced upon stress, preventing increased proteasome assembly, causing cell death. This work identifies a mechanism underpinning Ede1-mediated increased translation of specific mRNAs at a time when general translation is repressed.

The ribosome-associated chaperone Zuo1 controls translation upon TORC1 inhibition

Protein requirements of eukaryotic cells are ensured by proteostasis, which is mediated by tight control of TORC1 activity. Upon TORC1 inhibition, protein degradation is increased and protein synthesis is reduced through inhibition of translation initiation to maintain cell viability. Here, we show that the ribosome-associated complex (RAC)/Ssb chaperone system, composed of the HSP70 chaperone Ssb and its HSP40 co-chaperone Zuo1, is required to maintain proteostasis and cell viability under TORC1 inhibition in Saccharomyces cerevisiae. In the absence of Zuo1, translation does not decrease in response to the loss of TORC1 activity. A functional interaction between Zuo1 and Ssb is required for proper translational control and proteostasis maintenance upon TORC1 inhibition. Furthermore, we have shown that the rapid degradation of eIF4G following TORC1 inhibition is mediated by autophagy and is prevented in zuo1Δ cells, contributing to decreased survival in these conditions. We found that autophagy is defective in zuo1Δ cells, which impedes eIF4G degradation upon TORC1 inhibition. Our findings identify an essential role for RAC/Ssb in regulating translation in response to changes in TORC1 signalling.

HIV-1 diverts cortical actin for particle assembly and release

Enveloped viruses assemble and bud from the host cell membranes. Any role of cortical actin in these processes have often been a source of debate. Here, we assessed if cortical actin was involved in HIV-1 assembly in infected CD4 T lymphocytes. Our results show that preventing actin branching not only increases HIV-1 particle release but also the number of individual HIV-1 Gag assembly clusters at the T cell plasma membrane. Indeed, in infected T lymphocytes and in in vitro quantitative model systems, we show that HIV-1 Gag protein prefers areas deficient in F-actin for assembling. Finally, we found that the host factor Arpin, an inhibitor of Arp2/3 branched actin, is recruited at the membrane of infected T cells and it can associate with the viral Gag protein. Altogether, our data show that, for virus assembly and particle release, HIV-1 prefers low density of cortical actin and may favor local actin debranching by subverting Arpin.

Immunoprecipitation of Reporter Nascent Chains from Active Ribosomes to Study Translation Efficiency

The study of translation is important to the understanding of gene expression. While genome-wide measurements of translation efficiency (TE) rely upon ribosome profiling, classical approaches to address translation of individual genes of interest rely on biochemical methods, such as polysome fractionation and immunoprecipitation (IP) of ribosomal components, or on reporter constructs, such as luciferase reporters. Methods to investigate translation have been developed that, however, require considerable research effort, including addition of numerous features to mRNA regions, genomic integration of reporters, and complex data analysis. Here, we describe a simple biochemical reporter assay to study TE of mRNAs expressed from a transiently transfected plasmid, which we term Nascent Chain Immunoprecipitation (NC IP). The assay is based on a plasmid expressing an N-terminally Flag-tagged protein and relies on the IP of Flag-tagged nascent chains from elongating ribosomes, followed by quantitative reverse transcription polymerase chain reaction (RT-qPCR) quantification of eluted mRNA. We report that elution of mRNA following IP can be achieved by treatment with puromycin, which releases ribosome-mRNA complexes, or with purified Flag peptide, which instead releases nascent chain-ribosome-mRNA complexes. In the example described in this protocol, untranslated regions (UTRs) of a gene of interest were used to flank a FlagVenus coding sequence, with the method allowing to infer UTR-dependent regulation of TE. Importantly, our method enables discrimination of translating from non-translating mRNAs. Additionally, it requires simple procedures and standard laboratory equipment. Our method can be used to test the effect of regulators, such as microRNAs or therapeutic drugs or of various genetic backgrounds, on translation of any user-selected mRNA. Key features • The novel NC IP protocol builds upon a previously published method for detection of mRNA-binding proteins (Williams et al., 2022). • The NC IP protocol is adapted for detecting mRNA actively undergoing translation. • The method uses mammalian cell culture but could be adapted to multiple organisms, including budding yeast (S. cerevisiae).

Codependencies of mTORC1 signaling and endolysosomal actin structures

The mechanistic target of rapamycin complex 1 (mTORC1) is part of the amino acid sensing machinery that becomes activated on the endolysosomal surface in response to nutrient cues. Branched actin generated by WASH and Arp2/3 complexes defines endolysosomal microdomains. Here, we find mTORC1 components in close proximity to endolysosomal actin microdomains. We investigated for interactors of the mTORC1 lysosomal tether, RAGC, by proteomics and identified multiple actin filament capping proteins and their modulators. Perturbation of RAGC function affected the size of endolysosomal actin, consistent with a regulation of actin filament capping by RAGC. Reciprocally, the pharmacological inhibition of actin polymerization or alteration of endolysosomal actin obtained upon silencing of WASH or Arp2/3 complexes impaired mTORC1 activity. Mechanistically, we show that actin is required for proper association of RAGC and mTOR with endolysosomes. This study reveals an unprecedented interplay between actin and mTORC1 signaling on the endolysosomal system.

The Yeast Protein Kinase Sch9 Functions as a Central Nutrient-Responsive Hub That Calibrates Metabolic and Stress-Related Responses

Yeast cells are equipped with different nutrient signaling pathways that enable them to sense the availability of various nutrients and adjust metabolism and growth accordingly. These pathways are part of an intricate network since most of them are cross-regulated and subject to feedback regulation at different levels. In yeast, a central role is played by Sch9, a protein kinase that functions as a proximal effector of the conserved growth-regulatory TORC1 complex to mediate information on the availability of free amino acids. However, recent studies established that Sch9 is more than a TORC1-effector as its activity is tuned by several other kinases. This allows Sch9 to function as an integrator that aligns different input signals to achieve accuracy in metabolic responses and stress-related molecular adaptations. In this review, we highlight the latest findings on the structure and regulation of Sch9, as well as its role as a nutrient-responsive hub that impacts on growth and longevity of yeast cells. Given that most key players impinging on Sch9 are well-conserved, we also discuss how studies on Sch9 can be instrumental to further elucidate mechanisms underpinning healthy aging in mammalians.

Differential translation of mRNA isoforms underlies oncogenic activation of cell cycle kinase Aurora A

Aurora Kinase A (AURKA) is an oncogenic kinase with major roles in mitosis, but also exerts cell cycle- and kinase-independent functions linked to cancer. Therefore, control of its expression, as well as its activity, is crucial. A short and a long 3'UTR isoform exist for AURKA mRNA, resulting from alternative polyadenylation (APA). We initially observed that in triple-negative breast cancer, where AURKA is typically overexpressed, the short isoform is predominant and this correlates with faster relapse times of patients. The short isoform is characterized by higher translational efficiency since translation and decay rate of the long isoform are targeted by hsa-let-7a tumor-suppressor miRNA. Additionally, hsa-let-7a regulates the cell cycle periodicity of translation of the long isoform, whereas the short isoform is translated highly and constantly throughout interphase. Finally, disrupted production of the long isoform led to an increase in proliferation and migration rates of cells. In summary, we uncovered a new mechanism dependent on the cooperation between APA and miRNA targeting likely to be a route of oncogenic activation of human AURKA.

Direct and Indirect Effects of Filamin A on Tau Pathology in Neuronal Cells

In Alzheimer disease (AD), Tau, an axonal microtubule-associated protein, becomes hyperphosphorylated, detaches from microtubules, accumulates, and self-aggregates in the somatodendritic (SD) compartment. The accumulation of hyperphosphorylated and aggregated Tau is also seen in other neurodegenerative diseases such as frontotemporal lobar degeneration (FTLD-Tau). Previous studies reported a link between filamin A (FLNA), an actin-binding protein found in the SD compartment, and Tau pathology. In the present study, we further explored this link. We confirmed the interaction of Tau with FLNA in neuroblastoma 2a (N2a) cells. This interaction was mediated by a domain located between the 157 and 383 amino acids (a.a.) of Tau. Our results also revealed that the overexpression of FLNA resulted in an intracellular accumulation of wild-type Tau and Tau mutants (P301L, V337M, and R406W) in N2a cells. Tau phosphorylation and cleavage by caspase-3 but not its aggregation were increased upon FLNA overexpression in N2a cells. In the parietal cortex of AD brain, insoluble FLNA was increased compared to control brain, but it did not correlate with Tau pathology. Interestingly, Tau binding to microtubules and F-actin was preserved upon FLNA overexpression in N2a cells. Lastly, our results revealed that FLNA also induced the accumulation of annexin A2, a Tau interacting partner involved in its axonal localization. Collectively, our data indicated that in Tauopathies, FLNA could contribute to Tau pathology by acting on Tau and annexin A2.

Actin dynamics in protein homeostasis

Cell homeostasis is maintained in all organisms by the constant adjustment of cell constituents and organisation to account for environmental context. Fine-tuning of the optimal balance of proteins for the conditions, or protein homeostasis, is critical to maintaining cell homeostasis. Actin, a major constituent of the cytoskeleton, forms many different structures which are acutely sensitive to the cell environment. Furthermore, actin structures interact with and are critically important for the function and regulation of multiple factors involved with mRNA and protein production and degradation, and protein regulation. Altogether, actin is a key, if often overlooked, regulator of protein homeostasis across eukaryotes. In this review, we highlight these roles and how they are altered following cell stress, from mRNA transcription to protein degradation.