bims-ershed Biomed News
on ER Stress in Health and Diseases
Issue of 2021‒08‒08
six papers selected by
Matías Eduardo González Quiroz
Worker’s Hospital

  1. Biomedicines. 2021 Jul 08. pii: 791. [Epub ahead of print]9(7):
      The spliced form of X-box binding protein 1 (XBP1s) is an active transcription factor that plays a vital role in the unfolded protein response (UPR). Under endoplasmic reticulum (ER) stress, unspliced Xbp1 mRNA is cleaved by the activated stress sensor IRE1α and converted to the mature form encoding spliced XBP1 (XBP1s). Translated XBP1s migrates to the nucleus and regulates the transcriptional programs of UPR target genes encoding ER molecular chaperones, folding enzymes, and ER-associated protein degradation (ERAD) components to decrease ER stress. Moreover, studies have shown that XBP1s regulates the transcription of diverse genes that are involved in lipid and glucose metabolism and immune responses. Therefore, XBP1s has been considered an important therapeutic target in studying various diseases, including cancer, diabetes, and autoimmune and inflammatory diseases. XBP1s is involved in several unique mechanisms to regulate the transcription of different target genes by interacting with other proteins to modulate their activity. Although recent studies discovered numerous target genes of XBP1s via genome-wide analyses, how XBP1s regulates their transcription remains unclear. This review discusses the roles of XBP1s in target genes transcriptional regulation. More in-depth knowledge of XBP1s target genes and transcriptional regulatory mechanisms in the future will help develop new therapeutic targets for each disease.
    Keywords:  ATF6; ER stress; IRE1; RIDD; UPR; XBP1s; unfolded protein response
  2. Front Plant Sci. 2021 ;12 707378
      The unfolded protein response (UPR) is an adaptive eukaryotic reaction that controls the protein folding capacities of the endoplasmic reticulum (ER). The most ancient and well-conserved component of the UPR is Inositol-Requiring Enzyme 1 (IRE1). Arabidopsis IRE1a (AtIRE1) is a transmembrane sensor of ER stress equipped with dual protein kinase and ribonuclease (RNase) activities, encoded by its C-terminal domain. In response to both physiological stresses and pathological perturbations, AtIRE1a directly cleaves bZIP60 (basic leucine zipper 60) mRNA. Here, we developed a quantitative in vitro cleavage assay that combines recombinant AtIRE1a protein that is expressed in Nicotiana benthamiana and total RNA isolated from Arabidopsis leaves. Wild-type AtIRE1a as well as its variants containing point mutations in the kinase or RNase domains that modify its cleavage activity were employed to demonstrate their contributions to cleavage activity levels. We show that, when exposed to total RNA in vitro, the AtIRE1a protein cleaves bZIP60 mRNA. Depletion of the bZIP60 transcript in the reaction mixture can be precisely quantified by a qRT-PCR-mediated assay. This method facilitates the functional studies of novel plant IRE1 variants by allowing to quickly and precisely assess the effects of protein mutations on the substrate mRNA cleavage activity before advancing to more laborious, stable transgenic approaches in planta. Moreover, this method is readily adaptable to other plant IRE1 paralogs and orthologs, and can also be employed to test additional novel mRNA substrates of plant IRE1, such as transcripts undergoing degradation through the process of regulated IRE1-dependent decay (RIDD). Finally, this method can also be modified and expanded to functional testing of IRE1 interactors and inhibitors, as well as for studies on the molecular evolution of IRE1 and its substrates, providing additional insights into the mechanistic underpinnings of IRE1-mediated ER stress homeostasis in plant tissues.
    Keywords:  Arabidopsis thaliana; ER stress; IRE1; Nicotiana benthamiana; RNase-dead; bZIP60; in vitro mRNA cleavage; unfolded protein response
  3. J Cell Sci. 2021 Aug 05. pii: jcs.258685. [Epub ahead of print]
      The phase separation of the non-membrane bound Sec bodies occurs in Drosophila S2 cells by coalescence of components of the ER exit sites under the stress of amino-acid starvation. Here we address which signaling pathways cause Sec body formation and find that two pathways are critical. The first is the activation of the salt inducible kinases (SIK) by Na+ stress, that when it is strong is sufficient. The second is activation of IRE1 and PERK downstream of ER stress induced by absence of amino- acids, which needs to be combined with moderate salt stress to induce Sec body formation. SIK and IRE1/PERK activation appear to potentiate each other through the stimulation of the unfolded protein response, a key parameter in Sec body formation. This work pioneers the role of SIK in phase transition and re-enforces the role of IRE1 and PERK as a metabolic sensor for the level of circulating amino-acids and salt.
    Keywords:  Amino-acid starvation; Drosophila S2 cells; Phase separation; Salt stress; Sec body; Unfolded Protein Response
  4. EMBO J. 2021 Aug 04. e107413
      DNA-protein crosslinks (DPCs) obstruct essential DNA transactions, posing a serious threat to genome stability and functionality. DPCs are proteolytically processed in a ubiquitin- and DNA replication-dependent manner by SPRTN and the proteasome but can also be resolved via targeted SUMOylation. However, the mechanistic basis of SUMO-mediated DPC resolution and its interplay with replication-coupled DPC repair remain unclear. Here, we show that the SUMO-targeted ubiquitin ligase RNF4 defines a major pathway for ubiquitylation and proteasomal clearance of SUMOylated DPCs in the absence of DNA replication. Importantly, SUMO modifications of DPCs neither stimulate nor inhibit their rapid DNA replication-coupled proteolysis. Instead, DPC SUMOylation provides a critical salvage mechanism to remove DPCs formed after DNA replication, as DPCs on duplex DNA do not activate interphase DNA damage checkpoints. Consequently, in the absence of the SUMO-RNF4 pathway cells are able to enter mitosis with a high load of unresolved DPCs, leading to defective chromosome segregation and cell death. Collectively, these findings provide mechanistic insights into SUMO-driven pathways underlying replication-independent DPC resolution and highlight their critical importance in maintaining chromosome stability and cellular fitness.
    Keywords:  DNA repair; DNA-protein crosslinks; SUMO; genome stability; ubiquitin
  5. Cancers (Basel). 2021 Jul 29. pii: 3819. [Epub ahead of print]13(15):
      Cells respond to genotoxic stress through a series of complex protein pathways called DNA damage response (DDR). These monitoring mechanisms ensure the maintenance and the transfer of a correct genome to daughter cells through a selection of DNA repair, cell cycle regulation, and programmed cell death processes. Canonical or non-canonical DDRs are highly organized and controlled to play crucial roles in genome stability and diversity. When altered or mutated, the proteins in these complex networks lead to many diseases that share common features, and to tumor formation. In recent years, technological advances have made it possible to benefit from the principles and mechanisms of DDR to target and eliminate cancer cells. These new types of treatments are adapted to the different types of tumor sensitivity and could benefit from a combination of therapies to ensure maximal efficiency.
    Keywords:  DDR inhibitors; DNA damage response; DNA damage therapy; DNA repair; cancers; cell cycle
  6. Cell Metab. 2021 Aug 03. pii: S1550-4131(21)00328-4. [Epub ahead of print]33(8): 1505-1506
      In a new study, Zhang et al. (2021) show that reducing iron levels in adipose tissue improves metabolic function. This occurs through an interorgan communication system where signals from the adipocyte reduce intestinal lipid absorption.