DDRGK1-mediated ER-phagy attenuates acute kidney injury through ER-stress and apoptosis

Acute kidney injury (AKI) constitutes a prevalent clinical syndrome characterized by elevated morbidity and mortality rates, emerging as a significant public health issue. This study investigates the interplay between endoplasmic reticulum (ER) stress, unfolded protein response (UPR), and ER-associated degradation (ER-phagy) in the pathogenesis of AKI. We employed four distinct murine models of AKI-induced by contrast media, ischemia-reperfusion injury, cisplatin, and folic acid-to elucidate the relationship between ER-phagy, ER stress, and apoptosis. Our findings reveal a marked decrease in ER-phagy coinciding with an accumulation of damaged ER, elevated ER stress, and increased apoptosis across all AKI models. Importantly, overexpression of DDRGK1 in HK-2 cells enhanced ER-phagy levels, ameliorating contrast-induced ER stress and apoptosis. These findings unveil a novel protective mechanism in AKI, wherein DDRGK1-UFL1-mediated ER-phagy mitigates ER stress and apoptosis in renal tubular epithelial cells. Our results thereby contribute to understanding the molecular underpinnings of AKI and offer potential therapeutic targets for its treatment.

Hypomorphic human SEL1L and HRD1 variants uncouple multilayered ER-associated degradation machinery

The suppressor of lin-12-like-HMG-CoA reductase degradation 1 (SEL1L-HRD1) complex of the endoplasmic reticulum-associated degradation (ERAD) machinery is a key cellular proteostasis pathway. Although previous studies have shown ERAD as promoting the development and maintenance of many cell types in mice, its importance to human physiology remained undetermined. In two articles in this issue of the JCI, Qi and colleagues describe four biallelic hypomorphic SEL1L and HRD1 variants that were associated with neurodevelopment disorders, locomotor dysfunction, impaired immunity, and premature death in patients. These pathogenic SEL1L-HRD1 variants shine a light on the critical importance of ERAD in humans and pave the way for future studies dissecting ERAD mechanisms in specific cell types.

Endoplasmic reticulum-associated SARS-CoV-2 ORF3a elicits heightened cytopathic effects despite robust ER-associated degradation

IMPORTANCE

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has tragically claimed millions of lives through coronavirus disease 2019 (COVID-19), and there remains a critical gap in our understanding of the precise molecular mechanisms responsible for the associated fatality. One key viral factor of interest is the SARS-CoV-2 ORF3a protein, which has been identified as a potent inducer of host cellular proinflammatory responses capable of triggering the catastrophic cytokine storm, a primary contributor to COVID-19-related deaths. Moreover, ORF3a, much like the spike protein, exhibits a propensity for frequent mutations, with certain variants linked to the severity of COVID-19. Our previous research unveiled two distinct types of ORF3a mutant proteins, categorized by their subcellular localizations, setting the stage for a comparative investigation into the functional and mechanistic disparities between these two types of ORF3a variants. Given the clinical significance and functional implications of the natural ORF3a mutations, the findings of this study promise to provide invaluable insights into the potential roles undertaken by these mutant ORF3a proteins in the pathogenesis of COVID-19.

Host IP3R channels are dispensable for rotavirus Ca2+ signaling but critical for intercellular Ca2+ waves that prime uninfected cells for rapid virus spread

IMPORTANCE

Many viruses exploit host Ca2+ signaling to facilitate their replication; however, little is known about how Ca2+ signals from different host and viral channels contribute to the overall dysregulation of Ca2+ signaling or promote virus replication. Using cells lacking IP3R, a host ER Ca2+ channel, we delineated intracellular Ca2+ signals within virus-infected cells and intercellular Ca2+ waves (ICWs), which increased Ca2+ signaling in neighboring, uninfected cells. In infected cells, IP3R was dispensable for rotavirus-induced Ca2+ signaling and replication, suggesting the rotavirus NSP4 viroporin supplies these signals. However, IP3R-mediated ICWs increase rotavirus replication kinetics and spread, indicating that the Ca2+ signals from the ICWs may prime nearby uninfected cells to better support virus replication upon eventual infection. This "pre-emptive priming" of uninfected cells by exploiting host intercellular pathways in the vicinity of virus-infected cells represents a novel mechanism for viral reprogramming of the host to gain a replication advantage.

Akt enhances the vulnerability of cancer cells to VCP/p97 inhibition-mediated paraptosis

Valosin-containing protein (VCP)/p97, an AAA+ ATPase critical for maintaining proteostasis, emerges as a promising target for cancer therapy. This study reveals that targeting VCP selectively eliminates breast cancer cells while sparing non-transformed cells by inducing paraptosis, a non-apoptotic cell death mechanism characterized by endoplasmic reticulum and mitochondria dilation. Intriguingly, oncogenic HRas sensitizes non-transformed cells to VCP inhibition-mediated paraptosis. The susceptibility of cancer cells to VCP inhibition is attributed to the non-attenuation and recovery of protein synthesis under proteotoxic stress. Mechanistically, mTORC2/Akt activation and eIF3d-dependent translation contribute to translational rebound and amplification of proteotoxic stress. Furthermore, the ATF4/DDIT4 axis augments VCP inhibition-mediated paraptosis by activating Akt. Given that hyperactive Akt counteracts chemotherapeutic-induced apoptosis, VCP inhibition presents a promising therapeutic avenue to exploit Akt-associated vulnerabilities in cancer cells by triggering paraptosis while safeguarding normal cells.

Hugan Qingzhi tablets attenuates endoplasmic reticulum stress in nonalcoholic fatty liver disease rats by regulating PERK and ATF6 pathways

BACKGROUND

Endoplasmic reticulum (ER) stress, promoting lipid metabolism disorders and steatohepatitis, contributes significantly to the pathogenesis of nonalcoholic fatty liver disease (NAFLD). Hugan Qingzhi tablets (HQT) has a definite effect in the clinical treatment of NAFLD patients, but its mechanism is still unclear. This study aims to investigate the effects of HQT on ER stress in the liver tissues of NAFLD rats and explore the underlying mechanism.

METHODS

The NAFLD rat model was managed with high-fat diet (HFD) for 12weeks. HQT was administrated in a daily basis to the HFD groups. Biochemical markers, pro-inflammatory cytokines, liver histology were assayed to evaluate HQT effects in HFD-induced NAFLD rats. Furthermore, the expression of ER stress-related signal molecules including glucose regulating protein 78 (GRP78), protein kinase RNA-like endoplasmic reticulum kinase (PERK), p-PERK, eukaryotic translation initiation factor 2α (EIF2α), p-EIF2α, activating transcription factor 4 (ATF4), acetyl-coenzyme A-carboxylase (ACC), activating transcription factor (ATF6), and nuclear factor-kappa B-p65 (NF-κB-p65) were detected by western blot and/or qRT-PCR.

RESULTS

The histopathological characteristics and biochemical data indicated that HQT exhibited protective effects on HFD-induced NAFLD rats. Furthermore, it caused significant reduction in the expression of ERS markers, such as GRP78, PERK, p-PERK, and ATF6, and subsequently downregulated the expression of EIF2α, p-EIF2α ATF4, ACC, and NF-κB-p65.

CONCLUSIONS

The results suggested that HQT has protective effect against hepatic steatosis and inflammation in NAFLD rats by attenuating ER stress, and the potential mechanism is through inhibition of PERK and ATF6 pathways.

Dioscin Activates Endoplasmic Reticulum Unfolded Protein Response for Defense Against Pathogenic Bacteria in Caenorhabditis elegans via IRE-1/XBP-1 Pathway

The unfolded protein response (UPR) is an evolutionarily conserved pathway that senses and responds to the accumulation of misfolded proteins in the endoplasmic reticulum (ER) lumen during bacterial infection. The IRE-1/XBP-1 pathway is a major branch of the UPRER that has been conserved from yeast to human. Dioscin, a steroidal saponin exhibits a broad spectrum of properties. However, whether dioscin influences the immune response and the underlying molecular mechanisms remain obscure. We find that dioscin increases resistance to Gram-negative pathogen Pseudomonas aeruginosa. Furthermore, dioscin also inhibits the growth of pathogenic bacteria. Meanwhile, dioscin enhances the resistance to pathogens by reducing bacterial burden in the intestine. Through genetic screening, we find that dioscin activates the UPRER to promote innate immunity via IRE-1/XBP-1 pathway. Intriguingly, dioscin requires the neural XBP-1 for immune response. Our findings suggest that dioscin may be a viable candidate for the treatment of infectious diseases.

Endoplasmic Reticulum-Targeting AIE Photosensitizers to Boost Immunogenic Cell Death for Immunotherapy of Bladder Carcinoma

Bladder cancer is characterized by high rates of recurrence and multifocality. Immunogenic cell death (ICD) of cancer cells has emerged as a promising strategy to improve the immunogenicity of tumor cells for enhanced cancer immunotherapy. Although photosensitizer-based photodynamic therapy (PDT) has been validated as capable of inducing ICD in cancer cells, the photosensitizers with a sufficient ICD induction ability are still rare, and there have been few reports on the development of advanced photosensitizers to strongly evoke the ICD of bladder cancer cells for eliciting potent antitumor immune responses and eradicating bladder carcinoma in situ. In this work, we have synthesized a new kind of endoplasmic reticulum (ER)-targeting aggregation-induced emission (AIE) photosensitizer (named DPASCP-Tos), which could effectively anchor to the cellular ER and trigger focused reactive oxygen species (ROS) production within the ER, thereby boosting ICD in bladder cancer cells. Furthermore, we have demonstrated that bladder cancer cells killed by ER-targeted PDT could serve as a therapeutic cancer vaccine to elicit a strong antitumor immunity. Prophylactic vaccination of the bladder cancer cells killed by DPASCP-Tos under light irradiation promoted the maturation of dendritic cells (DCs) and the expansion of tumor antigen-specific CD8+ T cells in vivo and protected mice from subsequent in situ bladder tumor rechallenge and extended animal survival. In summary, the ER-targeted AIEgens developed here significantly amplified the ICD of bladder cells through focused ROS-based ER oxidative stress and transformed bladder cancer cells into the therapeutic vaccine to enhance immunogenicity against orthotopic bladder cancer, providing valuable insights for bladder carcinoma treatment.

The UFM1 system: Working principles, cellular functions, and pathophysiology

Ubiquitin-fold modifier 1 (UFM1) is a ubiquitin-like protein covalently conjugated with intracellular proteins through UFMylation, a process similar to ubiquitylation. Growing lines of evidence regarding not only the structural basis of the components essential for UFMylation but also their biological properties shed light on crucial roles of the UFM1 system in the endoplasmic reticulum (ER), such as ER-phagy and ribosome-associated quality control at the ER, although there are some functions unrelated to the ER. Mouse genetics studies also revealed the indispensable roles of this system in hematopoiesis, liver development, neurogenesis, and chondrogenesis. Of critical importance, mutations of genes encoding core components of the UFM1 system in humans cause hereditary developmental epileptic encephalopathy and Schohat-type osteochondrodysplasia of the epiphysis. Here, we provide a multidisciplinary review of our current understanding of the mechanisms and cellular functions of the UFM1 system as well as its pathophysiological roles, and discuss issues that require resolution.

Structural Plasticity Allows Calumenin-1 to Moonlight as a Ca2+-Independent Chaperone: Pb2+ Enables Probing Alternate Inhibitory Conformation

Human calumenin-1 (HsCalu-1) is an endoplasmic reticulum (ER) and Golgi-resident Ca2+-binding protein of the hepta-EF-hand superfamily that plays a vital role in maintaining the cytoplasmic Ca2+ concentration below toxic levels by interacting with Sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) and ryanodine receptors (RyR), indicating its role in Ca2+ homeostasis in the ER. HsCalu-1 seems to be able to exhibit structural plasticity to achieve its plethora of functions. In this study, we demonstrate that HsCalu-1 acts as a chaperone in both its intrinsically disordered state (apo form) and the structured state (Ca2+-bound form). HsCalu-1 chaperone activity is independent of Ca2+ and Pb2+ binding attenuating its chaperone-like activity. Incidentally, Pb2+ binds to HsCalu-1 with lower affinity (KD = 38.46 μM) (compared to Ca2+-binding), leading to the formation of a less-stable conformation as observed by a sharp drop in its melting temperature Tm from 67 °C in the Ca2+-bound form to 43 °C in the presence of Pb2+. The binding site for Pb2+ was mapped as being in the EF-Hand-234 domain of HsCalu-1, a region that overlaps with the Ca2+-dependent initiator of its functional fold. A change in the secondary and tertiary structure, leading to a less-stable but compact conformation upon Pb2+ binding, is the mechanism by which the chaperone-like activity of HsCalu-1 is diminished. Our results not only demonstrate the chaperone activity by a protein in its disordered state but also explain, using Pb2+ as a probe, that the multiple functions of calumenin are due to its ability to adopt a quasi-stable conformation.

Mic19 depletion impairs endoplasmic reticulum-mitochondrial contacts and mitochondrial lipid metabolism and triggers liver disease

Endoplasmic reticulum (ER)-mitochondria contacts are critical for the regulation of lipid transport, synthesis, and metabolism. However, the molecular mechanism and physiological function of endoplasmic reticulum-mitochondrial contacts remain unclear. Here, we show that Mic19, a key subunit of MICOS (mitochondrial contact site and cristae organizing system) complex, regulates ER-mitochondria contacts by the EMC2-SLC25A46-Mic19 axis. Mic19 liver specific knockout (LKO) leads to the reduction of ER-mitochondrial contacts, mitochondrial lipid metabolism disorder, disorganization of mitochondrial cristae and mitochondrial unfolded protein stress response in mouse hepatocytes, impairing liver mitochondrial fatty acid β-oxidation and lipid metabolism, which may spontaneously trigger nonalcoholic steatohepatitis (NASH) and liver fibrosis in mice. Whereas, the re-expression of Mic19 in Mic19 LKO hepatocytes blocks the development of liver disease in mice. In addition, Mic19 overexpression suppresses MCD-induced fatty liver disease. Thus, our findings uncover the EMC2-SLC25A46-Mic19 axis as a pathway regulating ER-mitochondria contacts, and reveal that impairment of ER-mitochondria contacts may be a mechanism associated with the development of NASH and liver fibrosis.

A model for collagen secretion by intercompartmental continuities

Newly synthesized secretory proteins are exported from the endoplasmic reticulum (ER) at specialized subcompartments called exit sites (ERES). Cargoes like procollagen are too large for export by the standard COPII-coated vesicle of 60 nm average diameter. We have previously suggested that procollagen is transported from the ER to the next secretory organelle, the ER-Golgi intermediate compartment (ERGIC), in TANGO1-dependent interorganelle tunnels. In the theoretical model presented here, we suggest that intrinsically disordered domains of TANGO1 in the ER lumen induce an entropic contraction, which exerts a force that draws procollagen toward the ERES. Within this framework, molecular gradients of pH and/or HSP47 between the ER and ERGIC create a force in the order of tens of femto-Newtons. This force is substantial enough to propel procollagen from the ER at a speed of approximately 1 nm · s-1. This calculated speed and the quantities of collagen secreted are similar to its observed physiological secretion rate in fibroblasts, consistent with the proposal that ER export is the rate-limiting step for procollagen secretion. Hence, the mechanism we propose is theoretically adequate to explain how cells can utilize molecular gradients and export procollagens at a rate commensurate with physiological needs.

Dibromoacetonitrile induced autophagy by mediating the PERK signalling pathway and ROS interaction in HT22 cell

Dibromoacetonitrile (DBAN) is a high-risk haloacetonitrile (HAN) generated as a byproduct of chloramine disinfection in drinking water. DBAN-induced neurotoxicity in mouse hippocampal neuronal cells (HT22) and mammals was observed to be related to reactive oxygen species (ROS). ROS, endoplasmic reticulum stress (ERS) and autophagy play crucial roles in regulating a variety of cellular processes. However, whether ERS and autophagy are associated with HAN-responsive apoptosis remains unclear. This study indicated that DBAN (10 μM, 24 h) activated the ERS protein kinase like endoplasmic reticulum kinase (PERK) signaling pathway. The ERS inhibitor 4-phenylbutyric acid (4-PBA) reversed DBAN-inhibited cell viability and alleviated DBAN-induced apoptosis in HT22 cell, indicating that activation of the ERS PERK pathway mediates DBAN induced cytotoxicity. Moreover, DBAN activated autophagy. The autophagy inhibitor 3-methyladenine(3-MA) reversed DBAN-inhibited cell viability and alleviated DBAN-induced apoptosis in HT22 cell, suggesting that autophagy activation mediates DBAN-induced cell toxicity. Notably, the results showed that 4-PBA inhibited DBAN-activated autophagy, demonstrating that ERS-PERK promotes DBAN-induced cellular autophagy. Pretreatment with antioxidant N-acetylcysteine (NAC) inhibited the increase in ROS production and the activation of ERS, and protected cells from toxicity. Furthermore, 4-PBA pretreatment reduced the increase in ROS production, indicating that the ROS and PERK promote each other and form a positive feedback loop. ROS also promoted DBAN-induced autophagy. In summary, our findings indicate that DBAN induced autophagy by mediating the PERK signalling pathway and ROS interaction, leading to HT22 cell damage. Accordingly, targeting these pathogenic mechanisms may provide a potential target and theoretical basis for preventing and improving HAN-induced neurotoxicity.

The endoplasmic reticulum: Homeostasis and crosstalk in retinal health and disease

The endoplasmic reticulum (ER) is the largest intracellular organelle carrying out a broad range of important cellular functions including protein biosynthesis, folding, and trafficking, lipid and sterol biosynthesis, carbohydrate metabolism, and calcium storage and gated release. In addition, the ER makes close contact with multiple intracellular organelles such as mitochondria and the plasma membrane to actively regulate the biogenesis, remodeling, and function of these organelles. Therefore, maintaining a homeostatic and functional ER is critical for the survival and function of cells. This vital process is implemented through well-orchestrated signaling pathways of the unfolded protein response (UPR). The UPR is activated when misfolded or unfolded proteins accumulate in the ER, a condition known as ER stress, and functions to restore ER homeostasis thus promoting cell survival. However, prolonged activation or dysregulation of the UPR can lead to cell death and other detrimental events such as inflammation and oxidative stress; these processes are implicated in the pathogenesis of many human diseases including retinal disorders. In this review manuscript, we discuss the unique features of the ER and ER stress signaling in the retina and retinal neurons and describe recent advances in the research to uncover the role of ER stress signaling in neurodegenerative retinal diseases including age-related macular degeneration, inherited retinal degeneration, achromatopsia and cone diseases, and diabetic retinopathy. In some chapters, we highlight the complex interactions between the ER and other intracellular organelles focusing on mitochondria and illustrate how ER stress signaling regulates common cellular stress pathways such as autophagy. We also touch upon the integrated stress response in retinal degeneration and diabetic retinopathy. Finally, we provide an update on the current development of pharmacological agents targeting the UPR response and discuss some unresolved questions and knowledge gaps to be addressed by future research.

ER-mitochondria contact sites in mitochondrial DNA dynamics, maintenance, and distribution

Mitochondria are central cellular metabolic hubs. Their function requires proteins encoded by nuclear DNA, but also mitochondrial DNA (mtDNA) whose maintenance is essential for the proper function of the organelle. Defective mtDNA maintenance and distribution are associated with mitochondrial diseases. mtDNA is organized into nucleo-protein complexes called nucleoids that dynamically move along the mitochondrial network and interact with each other. mtDNA replication and nucleoid distribution is an active process regulated by the complex interplay of mitochondrial dynamics, endoplasmic reticulum (ER)-mitochondria contact sites, and cytoskeletal networks. For example, defects in mitochondrial fusion and fission or ER-mitochondria contact sites affect nucleoid maintenance and distribution. In this review, we discuss the process of nucleoid dynamics and the factors regulating nucleoid maintenance and distribution.

Peroxisome population control by phosphoinositide signaling at the endoplasmic reticulum-plasma membrane interface

Phosphoinositides are lipid signaling molecules acting at the interface of membranes and the cytosol to regulate membrane trafficking, lipid transport and responses to extracellular stimuli. Peroxisomes are multicopy organelles that are highly responsive to changes in metabolic and environmental conditions. In yeast, peroxisomes are tethered to the cell cortex at defined focal structures containing the peroxisome inheritance protein, Inp1p. We investigated the potential impact of changes in cortical phosphoinositide levels on the peroxisome compartment of the yeast cell. Here we show that the phosphoinositide, phosphatidylinositol-4-phosphate (PI4P), found at the junction of the cortical endoplasmic reticulum and plasma membrane (cER-PM) acts to regulate the cell's peroxisome population. In cells lacking a cER-PM tether or the enzymatic activity of the lipid phosphatase Sac1p, cortical PI4P is elevated, peroxisome numbers and motility are increased, and peroxisomes are no longer firmly tethered to Inp1p-containing foci. Reattachment of the cER to the PM through an artificial ER-PM "staple" in cells lacking the cER-PM tether does not restore peroxisome populations to the wild-type condition, demonstrating that integrity of PI4P signaling at the cell cortex is required for peroxisome homeostasis.

Mitofusin2 Ameliorated Endoplasmic Reticulum Stress and Mitochondrial Reactive Oxygen Species Through Maintaining Mitochondria-Associated Endoplasmic Reticulum Membrane Integrity in Cisplatin-Induced Acute Kidney Injury

Aims: This study investigated the regulatory effect of Mitofusin2 (Mfn2) on mitochondria-associated endoplasmic reticulum membrane (MAM) integrity and cellular injury in cisplatin-induced acute kidney injury (CP-AKI). Results: CP-AKI mice exhibited decreased expression of Mfn2, increased expression of phosphorylated adenosine monophosphate-activated protein kinase (p-AMPK), abnormal mitochondrial morphology, and reduced MAMs integrity, accompanied by the activation of mitochondrial reactive oxygen species (ROS) and endoplasmic reticulum (ER) stress (inositol-requiring enzyme 1 [IRE1] and PERK pathways). In in vitro studies, CP-induced mitochondrial ROS, ER-stress activation, and increased apoptosis were accompanied by the downregulation of Mfn2 and MAMs integrity reduction in Boston University mouse proximal tubular cells (BUMPT) and human proximal tubular epithelial cells (HK-2). Pretreatment of BUMPT cells with the Mfn2 plasmid partially restored the integrity of MAMs, negatively controlled IRE1 and PERK pathways, and inhibited cell apoptosis. In contrast, ER-stress and MAMs integrity violations were increased after Mfn2 small-interfering RNA (siRNA) treatment in HK-2 cells under CP treatment. Coimmunoprecipitation analysis demonstrated that Mfn2 interacted with PERK and IRE1. Furthermore, the adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK), acadesine (AICAR), had a similar effect to Mfn2 plasmid in the regulation of ER stress and MAMs. Conversely, the ER-stress inhibitor, 4-phenylbutyric acid (4-PBA), had no effect on the expression of Mfn2 and MAMs integrity. Innovation and Conclusion: This is the first study to explore the association between MAMs, ER stress, and Mfn2 in CP-AKI. Downregulation of Mfn2 expression abolished the MAMs integrity, and induced ER stress, mitochondrial ROS, and tubular cell apoptosis. This suggests that the Mfn2-MAMs pathway is a potential therapeutic target in CP-AKI. Antioxid. Redox Signal. 40, 16-39. The Ethical Registration number of animal experiment in this study was CSU-2022-01-0095.

Rescue of secretion of rare-disease-associated misfolded mutant glycoproteins in UGGT1 knock-out mammalian cells

Endoplasmic reticulum (ER) retention of misfolded glycoproteins is mediated by the ER-localized eukaryotic glycoprotein secretion checkpoint, UDP-glucose glycoprotein glucosyl-transferase (UGGT). The enzyme recognizes a misfolded glycoprotein and flags it for ER retention by re-glucosylating one of its N-linked glycans. In the background of a congenital mutation in a secreted glycoprotein gene, UGGT-mediated ER retention can cause rare disease, even if the mutant glycoprotein retains activity ("responsive mutant"). Using confocal laser scanning microscopy, we investigated here the subcellular localization of the human Trop-2-Q118E, E227K and L186P mutants, which cause gelatinous drop-like corneal dystrophy (GDLD). Compared with the wild-type Trop-2, which is correctly localized at the plasma membrane, these Trop-2 mutants are retained in the ER. We studied fluorescent chimeras of the Trop-2 Q118E, E227K and L186P mutants in mammalian cells harboring CRISPR/Cas9-mediated inhibition of the UGGT1 and/or UGGT2 genes. The membrane localization of the Trop-2 Q118E, E227K and L186P mutants was successfully rescued in UGGT1-/- cells. UGGT1 also efficiently reglucosylated Trop-2-Q118E-EYFP in cellula. The study supports the hypothesis that UGGT1 modulation would constitute a novel therapeutic strategy for the treatment of pathological conditions associated to misfolded membrane glycoproteins (whenever the mutation impairs but does not abrogate function), and it encourages the testing of modulators of ER glycoprotein folding quality control as broad-spectrum rescue-of-secretion drugs in rare diseases caused by responsive secreted glycoprotein mutants.

A Novel Endoplasmic Reticulum-Targeted Metal-Organic Framework-Confined Ruthenium (Ru) Nanozyme Regulation of Oxidative Stress for Central Post-Stroke Pain

Central post-stroke pain (CPSP) is a chronic neuropathic pain caused by cerebrovascular lesion or disfunction after stroke. Convincing evidence suggest that excessive reactive oxygen species (ROS), generated matrix metalloproteinase (MMPs) and neuroinflammation are largely involved in the development of pain. In this study, an effective strategy is reported for treating pain hypersensitivity using an endoplasmic reticulum (ER)-targeted metal-organic framework (MOF)-confined ruthenium (Ru) nanozyme. The Ru MOF is coated with a p-dodecylbenzene sulfonamide (p-DBSN) modified liposome with endoplasmic reticulum-targeted function. The experimental results reveals that ROS, Emmprin, MMP-2, and MMP-9 are upregulated in the brain of CPSP mice, along with the elevated expression of inflammation markers such as TNF-α and IL-6. Compared to vehicle, one-time intravenous administration of ER-Ru MOF significantly reduces mechanical hypersensitivity after CPSP for three days. Overall, ER-Ru MOF system can inhibit oxidative stress in the brain tissues of CPSP model, reduce MMPs expression, and suppress neuroinflammation response-induced injury, resulting in satisfactory prevention and effective treatment of CPSP during a hemorrhagic stroke. The ER-Ru MOF is expected to be useful for the treatment of neurological diseases associated with the vicious activation of ROS, based on the generality of the approach used in this study.

Overproduction of mycotoxin biosynthetic enzymes triggers Fusarium toxisome-shaped structure formation via endoplasmic reticulum remodeling

Mycotoxin deoxynivalenol (DON) produced by the Fusarium graminearum complex is highly toxic to animal and human health. During DON synthesis, the endoplasmic reticulum (ER) of F. graminearum is intensively reorganized, from thin reticular structure to thickened spherical and crescent structure, which was referred to as "DON toxisome". However, the underlying mechanism of how the ER is reorganized into toxisome remains unknown. In this study, we discovered that overproduction of ER-localized DON biosynthetic enzyme Tri4 or Tri1, or intrinsic ER-resident membrane proteins FgHmr1 and FgCnx was sufficient to induce toxisome-shaped structure (TSS) formation under non-toxin-inducing conditions. Moreover, heterologous overexpression of Tri1 and Tri4 proteins in non-DON-producing fungi F. oxysporum f. sp. lycopersici and F. fujikuroi also led to TSS formation. In addition, we found that the high osmolarity glycerol (HOG), but not the unfolded protein response (UPR) signaling pathway was involved in the assembly of ER into TSS. By using toxisome as a biomarker, we screened and identified a novel chemical which exhibited high inhibitory activity against toxisome formation and DON biosynthesis, and inhibited Fusarium growth species-specifically. Taken together, this study demonstrated that the essence of ER remodeling into toxisome structure is a response to the overproduction of ER-localized DON biosynthetic enzymes, providing a novel pathway for management of mycotoxin contamination.

The functions of triple gene block proteins and coat protein of apple stem pitting virus in viral cell-to-cell movement

Apple stem pitting virus is a species in the genus Foveavirus in the family Betaflexiviridae. Apple stem pitting virus (ASPV) commonly infects apple and pear plants grown worldwide. In this study, by integrating bimolecular fluorescence complementation, split-ubiquitin-based membrane yeast two-hybrid, and Agrobacterium-mediated expression assays, the interaction relationships and the subcellular locations of ASPV proteins TGBp1-3 and CP in Nicotiana benthamiana leaf cells were determined. Proteins CP, TGBp1, TGBp2, and TGBp3 were self-interactable, and TGBp2 played a role in the formation of perinuclear viroplasm and enhanced the colocalization of TGBp3 with CP and TGBp1. We found that the plant microfilament and endoplasmic reticulum structures were involved in the production of TGBp3 and TGBp2 vesicles, and their disruption decreased the virus accumulation level in the systemic leaves. The TGBp3 motile vesicles functioned in delivering the viral ribonucleoprotein complexes to the plasma membrane. Two cysteine residues at sites 35 and 49 of the TGBp3 sorting signal were necessary for the diffusion of TGBp3-marked vesicles. Furthermore, our results revealed that TGBp1, TGBp2, and CP could increase plasmodesmal permeability and move to the adjacent cells. This study demonstrates an interaction network and a subcellular location map of four ASPV proteins and for the first time provides insight into the functions of these proteins in the movement of a foveavirus.

System-wide analysis of RNA and protein subcellular localization dynamics

Although the subcellular dynamics of RNA and proteins are key determinants of cell homeostasis, their characterization is still challenging. Here we present an integrative framework to simultaneously interrogate the dynamics of the transcriptome and proteome at subcellular resolution by combining two methods: localization of RNA (LoRNA) and a streamlined density-based localization of proteins by isotope tagging (dLOPIT) to map RNA and protein to organelles (nucleus, endoplasmic reticulum and mitochondria) and membraneless compartments (cytosol, nucleolus and cytosolic granules). Interrogating all RNA subcellular locations at once enables system-wide quantification of the proportional distribution of RNA. We obtain a cell-wide overview of localization dynamics for 31,839 transcripts and 5,314 proteins during the unfolded protein response, revealing that endoplasmic reticulum-localized transcripts are more efficiently recruited to cytosolic granules than cytosolic RNAs, and that the translation initiation factor eIF3d is key to sustaining cytoskeletal function. Overall, we provide the most comprehensive overview so far of RNA and protein subcellular localization dynamics.

DNAJB12 and DNJB14 are non-redundant Hsp40 redox chaperones involved in endoplasmic reticulum protein reflux

BACKGROUND

The endoplasmic reticulum (ER) transmembrane chaperones DNAJB12(B12) and DNAJB14(B14) are cofactors that cooperate with cytosolic Heat Shock-70 protein (HSC70) facilitating folding/degradation of nascent membrane proteins and supporting the ER-membrane penetration of viral particles. Here, we assessed structural/functional features of B12/B14 with respect to their regulation by ER stress and their involvement in ER stress-mediated protein reflux.

METHODS

We investigated the effect of Unfolded Protein Response(UPR)-eliciting drugs on the expression/regulation of B12-B14 and their roles in ER-to-cytosol translocation of Protein Disulfide Isomerase-A1(PDI).

RESULTS

We show that B12 and B14 are similar but do not seem redundant. They share predicted structural features and show high homology of their cytosolic J-domains, while their ER-lumen DUF1977 domains are quite dissimilar. Interactome analysis suggested that B12/B14 associate with different biological processes. UPR activation did not significantly impact on B12 gene expression, while B14 transcripts were up-regulated. Meanwhile, B12 and B14 (33.4 kDa isoform) protein levels were degraded by the proteasome upon acute reductive challenge. Also, B12 degradation was impaired upon sulfenic-acid trapping by dimedone. We originally report that knockdown of B12/B14 and their cytosolic partner SGTA in ER-stressed cells significantly impaired the amount of the ER redox-chaperone PDI in a cytosolic-enriched fraction. Additionally, B12 but not B14 overexpression increased PDI relocalization in non-stressed cells.

CONCLUSIONS AND GENERAL SIGNIFICANCE

Our findings reveal that B12/B14 regulation involves thiol redox processes that may impact on their stability and possibly on physiological effects. Furthermore, we provide novel evidence that these proteins are involved in UPR-induced ER protein reflux.

Characterization of intracellular membrane structures derived from a massive expansion of endoplasmic reticulum (ER) membrane due to synthetic ER-membrane-resident polyproteins

The endoplasmic reticulum (ER) is a dynamic organelle that is amenable to major restructuring. Introduction of recombinant ER-membrane-resident proteins that form homo oligomers is a known method of inducing ER proliferation: interaction of the proteins with each other alters the local structure of the ER network, leading to the formation large aggregations of expanded ER, sometimes leading to the formation of organized smooth endoplasmic reticulum (OSER). However, these membrane structures formed by ER proliferation are poorly characterized and this hampers their potential development for plant synthetic biology. Here, we characterize a range of ER-derived membranous compartments in tobacco and show how the nature of the polyproteins introduced into the ER membrane affect the morphology of the final compartment. We show that a cytosol-facing oligomerization domain is an essential component for compartment formation. Using fluorescence recovery after photobleaching, we demonstrate that although the compartment retains a connection to the ER, a diffusional barrier exists to both the ER and the cytosol associated with the compartment. Using quantitative image analysis, we also show that the presence of the compartment does not disrupt the rest of the ER network. Moreover, we demonstrate that it is possible to recruit a heterologous, bacterial enzyme to the compartment, and for the enzyme to accumulate to high levels. Finally, transgenic Arabidopsis constitutively expressing the compartment-forming polyproteins grew and developed normally under standard conditions.

Photodynamic Modulation of Endoplasmic Reticulum and Mitochondria Network Boosted Cancer Immunotherapy

Immunogenic cell death (ICD) represents a promising approach for enhancing tumor therapy efficacy by inducing antitumor immune response. However, current ICD inducers often have insufficient endoplasmic reticulum (ER) enrichment and ineffectiveness in tumor immune escape caused by ER-mitochondria interaction. In this study, a kind of photoactivatable probe, THTTPy-PTSA, which enables sequential targeting of the ER and mitochondria is developed. THTTPy-PTSA incorporates p-Toluenesulfonamide (PTSA) for ER targeting, and upon light irradiation, the tetrahydropyridine group undergoes a photo oxidative dehydrogenation reaction, transforming into a pyridinium group that acts as a mitochondria-targeting moiety. The results demonstrate that THTTPy-PTSA exhibits exceptional subcellular translocation from the ER to mitochondria upon light irradiation treatment, subsequently triggers a stronger ER stress response through a cascade-amplification effect. Importantly, the augmented ER stress leads to substantial therapeutic efficacy in a 4T1 tumor model by eliciting the release of numerous damage-associated molecular patterns, thereby inducing evident and widespread ICD, consequently enhancing the antitumor immune efficacy. Collectively, the findings emphasize the pivotal role of photodynamic modulation of the ER-mitochondria network, facilitated by THTTPy-PTSA with precise spatial and temporal regulation, in effectively bolstering the antitumor immune response. This innovative approach presents a promising alternative for addressing the challenges associated with cancer immunotherapy.

Megalencephalic leukoencephalopathy with subcortical cysts protein-1: A new calcium-sensitive protein functionally activated by endoplasmic reticulum calcium release and calmodulin binding in astrocytes

BACKGROUND

MLC1 is a membrane protein highly expressed in brain perivascular astrocytes and whose mutations account for the rare leukodystrophy (LD) megalencephalic leukoencephalopathy with subcortical cysts disease (MLC). MLC is characterized by macrocephaly, brain edema and cysts, myelin vacuolation and astrocyte swelling which cause cognitive and motor dysfunctions and epilepsy. In cultured astrocytes, lack of functional MLC1 disturbs cell volume regulation by affecting anion channel (VRAC) currents and the consequent regulatory volume decrease (RVD) occurring in response to osmotic changes. Moreover, MLC1 represses intracellular signaling molecules (EGFR, ERK1/2, NF-kB) inducing astrocyte activation and swelling following brain insults. Nevertheless, to date, MLC1 proper function and MLC molecular pathogenesis are still elusive. We recently reported that in astrocytes MLC1 phosphorylation by the Ca2+/Calmodulin-dependent kinase II (CaMKII) in response to intracellular Ca2+ release potentiates MLC1 activation of VRAC. These results highlighted the importance of Ca2+ signaling in the regulation of MLC1 functions, prompting us to further investigate the relationships between intracellular Ca2+ and MLC1 properties.

METHODS

We used U251 astrocytoma cells stably expressing wild-type (WT) or mutated MLC1, primary mouse astrocytes and mouse brain tissue, and applied biochemistry, molecular biology, video imaging and electrophysiology techniques.

RESULTS

We revealed that WT but not mutant MLC1 oligomerization and trafficking to the astrocyte plasma membrane is favored by Ca2+ release from endoplasmic reticulum (ER) but not by capacitive Ca2+ entry in response to ER depletion. We also clarified the molecular events underlining MLC1 response to cytoplasmic Ca2+ increase, demonstrating that, following Ca2+ release, MLC1 binds the Ca2+ effector protein calmodulin (CaM) at the carboxyl terminal where a CaM binding sequence was identified. Using a CaM inhibitor and generating U251 cells expressing MLC1 with CaM binding site mutations, we found that CaM regulates MLC1 assembly, trafficking and function, being RVD and MLC-linked signaling molecules abnormally regulated in these latter cells.

CONCLUSION

Overall, we qualified MLC1 as a Ca2+ sensitive protein involved in the control of volume changes in response to ER Ca2+ release and astrocyte activation. These findings provide new insights for the comprehension of the molecular mechanisms responsible for the myelin degeneration occurring in MLC and other LD where astrocytes have a primary role in the pathological process.

Plastid Transient and Stable Interactions with Other Cell Compartments

Plastids are organelles delineated by two envelopes playing important roles in different cellular processes such as energy production or lipid biosynthesis. To regulate their biogenesis and their function, plastids have to communicate with other cellular compartments. This communication can be mediated by metabolites, signaling molecules, and by the establishment of direct contacts between the plastid envelope and other organelles such as the endoplasmic reticulum, mitochondria, peroxisomes, plasma membrane, and the nucleus. These interactions are highly dynamic and respond to different biotic and abiotic stresses. However, the mechanisms involved in the formation of plastid-organelle contact sites and their functions are still far from being understood. In this chapter, we summarize our current knowledge about plastid contact sites and their role in the regulation of plastid biogenesis and function.

Isolation of High-Quality Plastids from the Diatom Phaeodactylum tricornutum

Phaeodactylum tricornutum, a model pennate diatom, carries a secondary plastid surrounded by four membranes. Its biological function remains mysterious, supposed to combine features of the primary chloroplast and the endomembrane system. Isolation of high-quality plastid from the diatom enables a more conclusive understanding of the special structure and metabolic pathways in the plastid. Due to the direct continuity between the chloroplast endoplasmic reticulum membrane (cERM) and the outer nuclear envelope together with the integration of cERM into the cellular endoplasmic reticulum (ER) system, the plastid isolation is still challenging. In this study, highly purified P. tricornutum plastids with the four-layered membrane are obtained by Percoll density gradient centrifugation. The isolated plastids are unlikely to contain any residue of nuclear and coatomer compartments, and they might contain a relatively small contamination of mitochondrion and ER debris.

Towards a Unitary Hypothesis of Alzheimer's Disease Pathogenesis

The "amyloid cascade" hypothesis of Alzheimer's disease (AD) pathogenesis invokes the accumulation in the brain of plaques (containing the amyloid-β protein precursor [AβPP] cleavage product amyloid-β [Aβ]) and tangles (containing hyperphosphorylated tau) as drivers of pathogenesis. However, the poor track record of clinical trials based on this hypothesis suggests that the accumulation of these peptides is not the only cause of AD. Here, an alternative hypothesis is proposed in which the AβPP cleavage product C99, not Aβ, is the main culprit, via its role as a regulator of cholesterol metabolism. C99, which is a cholesterol sensor, promotes the formation of mitochondria-associated endoplasmic reticulum (ER) membranes (MAM), a cholesterol-rich lipid raft-like subdomain of the ER that communicates, both physically and biochemically, with mitochondria. We propose that in early-onset AD (EOAD), MAM-localized C99 is elevated above normal levels, resulting in increased transport of cholesterol from the plasma membrane to membranes of intracellular organelles, such as ER/endosomes, thereby upregulating MAM function and driving pathology. By the same token, late-onset AD (LOAD) is triggered by any genetic variant that increases the accumulation of intracellular cholesterol that, in turn, boosts the levels of C99 and again upregulates MAM function. Thus, the functional cause of AD is upregulated MAM function that, in turn, causes the hallmark disease phenotypes, including the plaques and tangles. Accordingly, the MAM hypothesis invokes two key interrelated elements, C99 and cholesterol, that converge at the MAM to drive AD pathogenesis. From this perspective, AD is, at bottom, a lipid disorder.

The spatiotemporal control of ER membrane fragmentation during reticulophagy

Reticulophagy is an evolutionarily conserved mechanism essential to maintain the endoplasmic reticulum (ER) homeostasis. A series of studies identified a panel of reticulophagy receptors. However, it remains unclear how these receptors sense upstream signals for spatiotemporal control of reticulophagy and how ER is fragmented into small pieces for sequestration into phagophores. Recently, we and others showed that the oligomerization of RETREG1/FAM134B (reticulophagy regulator 1), an reticulophagy receptor, triggers the scission of ER membrane to facilitate reticulophagy. Furthermore, we demonstrated that upstream signals are transduced by sequential phosphorylation and acetylation of RETREG1, which stimulate its oligomerization, ER fragmentation and reticulophagy. Our work provides further mechanistic insights into how reticulophagy receptor conveys cellular signals to fine-tune of ER homeostasis.Abbreviations: ER, endoplasmic reticulum; MAP1LC3, microtubule-associated protein light chain 3; RETREG1, reticulophagy regulator 1; RHD, reticulon-homology domain.

[Redox Biology Involved in the Toxicity of Enterohemorrhagic Escherichia coli Toxin SubAB]

AB5 toxins of pathogenic bacteria enter host cells and utilize the retrograde trafficking pathway to translocate to the cytoplasm and exert its pathogenesis. Cholera toxin and Shiga toxin reach the endoplasmic reticulum (ER), and the A subunit undergoes redox regulation by ER proteins to become active fragments, which pass through the ER membrane and translocate to the cytoplasm. By acting on molecular targets in the cytoplasm, the normal function of host cells are disrupted, causing diseases. ER chaperone proteins such as protein disulfide isomerase (PDI) and binding immunoglobulin protein (BiP) induce conformational changes triggered by the reduction of disulfide bonds in the A subunit. This is thought to be dependent on cysteine thiol-mediated redox regulation, but the detailed mechanism remains unclear. On the other hand, subtilase cytotoxin (SubAB), produced by enterohemorrhagic Escherichia coli (EHEC), localizes to the ER without translocating to the cytoplasm and cleaves BiP as a substrate. Therefore, it is thought that ER stress-based cytotoxicity and intestinal bleeding occur without translocating to the cytoplasm. We reported that PDI is involved in BiP cleavage through SubAB localization to the ER. Like other AB5 toxins, this indicates the involvement of redox regulation via chaperone proteins in the ER, but also suggests that SubAB does not translocate to the cytoplasm because it cleaves BiP. Although there are few reports on the redox state of ER protein thiols, it is suggested that polysulfidation, which is discussed in this symposium, may be involved.

Mitochondrial dysfunction abrogates dietary lipid processing in enterocytes

Digested dietary fats are taken up by enterocytes where they are assembled into pre-chylomicrons in the endoplasmic reticulum followed by transport to the Golgi for maturation and subsequent secretion to the circulation1. The role of mitochondria in dietary lipid processing is unclear. Here we show that mitochondrial dysfunction in enterocytes inhibits chylomicron production and the transport of dietary lipids to peripheral organs. Mice with specific ablation of the mitochondrial aspartyl-tRNA synthetase DARS2 (ref. 2), the respiratory chain subunit SDHA3 or the assembly factor COX10 (ref. 4) in intestinal epithelial cells showed accumulation of large lipid droplets (LDs) in enterocytes of the proximal small intestine and failed to thrive. Feeding a fat-free diet suppressed the build-up of LDs in DARS2-deficient enterocytes, which shows that the accumulating lipids derive mostly from digested fat. Furthermore, metabolic tracing studies revealed an impaired transport of dietary lipids to peripheral organs in mice lacking DARS2 in intestinal epithelial cells. DARS2 deficiency caused a distinct lack of mature chylomicrons concomitant with a progressive dispersal of the Golgi apparatus in proximal enterocytes. This finding suggests that mitochondrial dysfunction results in impaired trafficking of chylomicrons from the endoplasmic reticulum to the Golgi, which in turn leads to storage of dietary lipids in large cytoplasmic LDs. Taken together, these results reveal a role for mitochondria in dietary lipid transport in enterocytes, which might be relevant for understanding the intestinal defects observed in patients with mitochondrial disorders5.

ESYT1 tethers the ER to mitochondria and is required for mitochondrial lipid and calcium homeostasis

Mitochondria interact with the ER at structurally and functionally specialized membrane contact sites known as mitochondria-ER contact sites (MERCs). Combining proximity labelling (BioID), co-immunoprecipitation, confocal microscopy and subcellular fractionation, we found that the ER resident SMP-domain protein ESYT1 was enriched at MERCs, where it forms a complex with the outer mitochondrial membrane protein SYNJ2BP. BioID analyses using ER-targeted, outer mitochondrial membrane-targeted, and MERC-targeted baits, confirmed the presence of this complex at MERCs and the specificity of the interaction. Deletion of ESYT1 or SYNJ2BP reduced the number and length of MERCs. Loss of the ESYT1-SYNJ2BP complex impaired ER to mitochondria calcium flux and provoked a significant alteration of the mitochondrial lipidome, most prominently a reduction of cardiolipins and phosphatidylethanolamines. Both phenotypes were rescued by reexpression of WT ESYT1 and an artificial mitochondria-ER tether. Together, these results reveal a novel function for ESYT1 in mitochondrial and cellular homeostasis through its role in the regulation of MERCs.

Sarcolipin relates to fattening, but not sarco/endoplasmic reticulum Ca2+-ATPase uncoupling, in captive migratory gray catbirds

In order to complete their energetically demanding journeys, migratory birds undergo a suite of physiological changes to prepare for long-duration endurance flight, including hyperphagia, fat deposition, reliance on fat as a fuel source, and flight muscle hypertrophy. In mammalian muscle, SLN is a small regulatory protein which binds to sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) and uncouples Ca2+ transport from ATP hydrolysis, increasing energy consumption, heat production, and cytosolic Ca2+ transients that signal for mitochondrial biogenesis, fatigue resistance and a shift to fatty acid oxidation. Using a photoperiod manipulation of captive gray catbirds (Dumetella carolinensis), we investigated whether SLN may play a role in coordinating the development of the migratory phenotype. In response to long-day photostimulation, catbirds demonstrated migratory restlessness and significant body fat stores, alongside higher SLN transcription while SERCA2 remained constant. SLN transcription was strongly correlated with h-FABP and PGC1α transcription, as well as fat mass. However, SLN was not significantly correlated with HOAD or CD36 transcripts or measurements of SERCA activity, SR membrane Ca2+ leak, Ca2+ uptake rates, pumping efficiency or mitochondrial biogenesis. Therefore, SLN may be involved in the process of storing fat and shifting to fat as a fuel, but the mechanism of its involvement remains unclear.

Membrane contact site detection (MCS-DETECT) reveals dual control of rough mitochondria-ER contacts

Identification and morphological analysis of mitochondria-ER contacts (MERCs) by fluorescent microscopy is limited by subpixel resolution interorganelle distances. Here, the membrane contact site (MCS) detection algorithm, MCS-DETECT, reconstructs subpixel resolution MERCs from 3D super-resolution image volumes. MCS-DETECT shows that elongated ribosome-studded riboMERCs, present in HT-1080 but not COS-7 cells, are morphologically distinct from smaller smooth contacts and larger contacts induced by mitochondria-ER linker expression in COS-7 cells. RiboMERC formation is associated with increased mitochondrial potential, reduced in Gp78 knockout HT-1080 cells and induced by Gp78 ubiquitin ligase activity in COS-7 and HeLa cells. Knockdown of riboMERC tether RRBP1 eliminates riboMERCs in both wild-type and Gp78 knockout HT-1080 cells. By MCS-DETECT, Gp78-dependent riboMERCs present complex tubular shapes that intercalate between and contact multiple mitochondria. MCS-DETECT of 3D whole-cell super-resolution image volumes, therefore, identifies novel dual control of tubular riboMERCs, whose formation is dependent on RRBP1 and size modulated by Gp78 E3 ubiquitin ligase activity.

Activation of Ca2+ transport in cardiac microsomes enriches functional sets of ER and SR proteins

The importance of sarcoplasmic reticulum (SR) Ca2+-handling in heart has led to detailed understanding of Ca2+-release and re-uptake protein complexes, while less is known about other endoplasmic reticulum (ER) functions in the heart. To more fully understand cardiac SR and ER functions, we analyzed cardiac microsomes based on their increased density through the actions of the SR Ca2+-ATPase (SERCA) and the ryanodine receptor that are highly active in cardiomyocytes. Crude cardiac microsomal vesicles loaded with Ca oxalate produced two higher density subfractions, MedSR and HighSR. Proteins from 20.0 μg of MV, MedSR, and HighSR protein were fractionated using SDS-PAGE, then trypsinized from 20 separate gel pieces, and analyzed by LC-MS/MS to determine protein content. From 62,000 individual peptide spectra obtained, we identified 1105 different proteins, of which 354 were enriched ≥ 2.0-fold in SR fractions compared to the crude membrane preparation. Previously studied SR proteins were all enriched, as were proteins associated with canonical ER functions. Contractile, mitochondrial, and sarcolemmal proteins were not enriched. Comparing the levels of SERCA-positive SR proteins in MedSR versus HighSR vesicles produced a range of SR subfraction enrichments signifying differing levels of Ca2+ leak co-localized in the same membrane patch. All known junctional SR proteins were more enriched in MedSR, while canonical ER proteins were more enriched in HighSR membrane. Proteins constituting other putative ER/SR subdomains also exhibited average Esub enrichment values (mean ± S.D.) that spanned the range of possible Esub values, suggesting that functional sets of proteins are localized to the same areas of the ER/SR membrane. We conclude that active Ca2+ loading of cardiac microsomes, reflecting the combined activities of Ca2+ uptake by SERCA, and Ca2+ leak by RyR, permits evaluation of multiple functional ER/SR subdomains. Sets of proteins from these subdomains exhibited similar enrichment patterns across membrane subfractions, reflecting the relative levels of SERCA and RyR present within individual patches of cardiac ER and SR.

Association of the Arabidopsis oleoyl Δ12-desaturase FAD2 with pre-cis-Golgi stacks at endoplasmic reticulum-Golgi-exit sites

The unsaturation of phospholipids influences the function of membranes. In Arabidopsis thaliana, the oleoyl Δ12-desaturase FAD2 converts oleic (18:1Δ9 ) to linoleic acid (18:2Δ9,12 ) and influences phospholipid unsaturation in different cellular membranes. Despite its importance, the precise localization of Arabidopsis FAD2 has not been unambiguously described. As FAD2 is thought to modify phospholipid-associated fatty acids at the endoplasmic reticulum (ER), from where unsaturates are distributed to other cellular sites, we hypothesized that FAD2 locates to ER subdomains enabling trafficking of lipid intermediates through the secretory pathway. Fluorescent FAD2 fusions used to test this hypothesis were first assessed for functionality by heterologous expression in yeast (Saccharomyces cerevisiae), and in planta by Arabidopsis fad2 mutant rescue upon ectopic expression from an intrinsic FAD2 promoter fragment. Light sheet fluorescence, laser scanning confocal or spinning disc microscopy of roots, leaves, or mesophyll protoplasts showed the functional fluorescence-tagged FAD2 variants in flattened donut-shaped structures of ~0.5-1 μm diameter, in a pattern not resembling mere ER association. High-resolution imaging of coexpressed organellar markers showed fluorescence-tagged FAD2 in a ring-shaped pattern surrounding ER-proximal Golgi particles, colocalizing with pre-cis-Golgi markers. This localization required the unusual C-terminal retention signal of FAD2, and deletion or substitutions in this protein region resulted in relaxed distribution and diffuse association with the ER. The distinct association of FAD2 with pre-cis-Golgi stacks in Arabidopsis root and leaf tissue is consistent with a contribution of FAD2 to membrane lipid homeostasis through the secretory pathway, as verified by an increased plasma membrane liquid phase order in the fad2 mutant.

Endoglin mutants retained in the endoplasmic reticulum exacerbate loss of function in hereditary hemorrhagic telangiectasia type 1 (HHT1) by exerting dominant negative effects on the wild type allele

Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant disorder affecting 1 in 5000-8000 individuals. Hereditary hemorrhagic telangiectasia type 1 (HHT1) is the most common HHT and manifests as diverse vascular malformations ranging from mild symptoms such as epistaxis and mucosal and cutaneous telangiectases to severe arteriovenous malformations (AVMs) in the lungs, brain or liver. HHT1 is caused by heterozygous mutations in the ENG gene, which encodes endoglin, the TGFβ homodimeric co-receptor. It was previously shown that some endoglin HHT1-causing variants failed to traffic to the plasma membrane due to their retention in the endoplasmic reticulum (ER) and consequent degradation by ER-associated degradation (ERAD). Endoglin is a homodimer formed in the ER, and we therefore hypothesized that mixed heterodimers might form between ER-retained variants and WT protein, thus hampering its maturation and trafficking to the plasma membrane causing dominant negative effects. Indeed, HA-tagged ER-retained mutants formed heterodimers with Myc-tagged WT endoglin. Moreover, variants L32R, V105D, P165L, I271N and C363Y adversely affected the trafficking of WT endoglin by reducing its maturation and plasma membrane localization. These results strongly suggest dominant negative effects exerted by these ER-retained variants aggravating endoglin loss of function in patients expressing them in the heterozygous state with the WT allele. Moreover, this study may help explain some of the variability observed among HHT1 patients due to the additional loss of function exerted by the dominant negative effects in addition to that due to haploinsufficiency. These findings might also have implications for some of the many conditions impacted by ERAD.

Septins modulate the autophagy response after nutrient starvation

The pathways that induce macroautophagy (referred to as autophagy hereafter) in response to the stress of starvation are well conserved and essential under nutrient-limiting conditions. However, less is understood about the mechanisms that modulate the autophagy response. Here we present evidence that after induction of autophagy in budding yeast septin filaments rapidly assemble into discrete patches distributed along the cell cortex. These patches gradually mature over 12 h of nutrient deprivation to form extended structures around Atg9 membranes tethered at the cortical endoplasmic reticulum, a class of membranes that are limiting for autophagosome biogenesis. Loss of cortical septin structures alters the kinetics of autophagy activation and most dramatically extends the duration of the autophagy response. In wild-type cells, diffusion of Atg9 membranes at the cell cortex undergoes transient pauses that are dependent on septins, and septins at the bud neck block the diffusion of Atg9 membranes between mother and daughter cells. We conclude that septins reorganize at the cell cortex during autophagy to locally limit access of Atg9 membranes to autophagosome assembly sites, and thus modulate the autophagy response during nutrient deprivation.

EMC rectifies the topology of multipass membrane proteins

Most eukaryotic multipass membrane proteins are inserted into the membrane of the endoplasmic reticulum. Their transmembrane domains (TMDs) are thought to be inserted co-translationally as they emerge from a membrane-bound ribosome. Here we find that TMDs near the carboxyl terminus of mammalian multipass proteins are inserted post-translationally by the endoplasmic reticulum membrane protein complex (EMC). Site-specific crosslinking shows that the EMC's cytosol-facing hydrophilic vestibule is adjacent to a pre-translocated C-terminal tail. EMC-mediated insertion is mostly agnostic to TMD hydrophobicity, favored for short uncharged C-tails and stimulated by a preceding unassembled TMD bundle. Thus, multipass membrane proteins can be released by the ribosome-translocon complex in an incompletely inserted state, requiring a separate EMC-mediated post-translational insertion step to rectify their topology, complete biogenesis and evade quality control. This sequential co-translational and post-translational mechanism may apply to ~250 diverse multipass proteins, including subunits of the pentameric ion channel family that are crucial for neurotransmission.

The association between neuropsychiatric effects of substance use and occurrence of endoplasmic reticulum and unfolded protein response: A systematic review

INTRODUCTION

This systematic review aimed to assess the association between neuropsychiatric effects of substance use and occurrence of ER stress and unfolded protein response (UPR) through comprehensive electronic search of existing literature and review of their findings.

METHODS

A comprehensive electronic literature search was carried out on research articles published between 1950 to July 2023 through major databases, such as Scopus, Web of Science, Google Scholar, PubMed, PsycINFO, EMBASE, Medline and Cochrane Library.

RESULTS

A total of 21 research articles were selected for review, which were comprised of sixteen animal studies, four human studies and one study on postmortem human brain samples. The selected studies revealed that alcohol, methamphetamine, cocaine, opioid and kratom exposures contributed to neuropsychiatric effects: such as decline in learning and memory function, executive dysfunction, alcohol, methamphetamine, opioid, and kratom dependence. These effects were associated with activation and persistent of ER stress and UPR with elevation of BiP and CHOP expression and the direction of ER stress is progressing towards the PERK-eIF2α-ATF4-CHOP pathway and neuronal apoptosis and neurodegeneration at various regions of the brain. In addition, regular kratom use in humans also contributed to elevation of p-JNK expression, denoting progress of ER stress towards the IRE1-ASK1-JNK-p-JNK pathway which was linked to kratom use disorder. However, treatment with certain compounds or biological agents could reverse the activation of ER stress.

CONCLUSIONS

The neuropsychiatric effects of alcohol, methamphetamine, cocaine, opioid and kratom use may be associated with persistent ER stress and UPR.

Nanojunctions: Specificity of Ca2+ signaling requires nano-scale architecture of intracellular membrane contact sites

Spatio-temporal definition of Ca2+ signals involves the assembly of signaling complexes within the nano-architecture of contact sites between the sarco/endoplasmic reticulum (SR/ER) and the plasma membrane (PM). While the requirement of precise spatial assembly and positioning of the junctional signaling elements is well documented, the role of the nano-scale membrane architecture itself, as an ion-reflecting confinement of the signalling unit, remains as yet elusive. Utilizing the Na+/Ca2+ Exchanger-1 / SR/ER Ca2+ ATPase-2-mediated ER Ca2+ refilling process as a junctional signalling paradigm, we provide here the first evidence for an indispensable cellular function of the junctional membrane architecture. Our stochastic modeling approach demonstrates that junctional ER Ca2+ refilling operates exclusively at nano-scale membrane spacing, with a strong inverse relationship between junctional width and signaling efficiency. Our model predicts a breakdown of junctional Ca2+ signaling with loss of reflecting membrane confinement. In addition we consider interactions between Ca2+ and the phospholipid membrane surface, which may support interfacial Ca2+ transport and promote receptor targeting. Alterations in the molecular and nano-scale membrane organization at organelle-PM contacts are suggested as a new concept in pathophysiology.

Lipid exchange at ER-trans-Golgi contact sites governs polarized cargo sorting

Oxysterol binding protein (OSBP) extracts cholesterol from the ER to deliver it to the TGN via counter exchange and subsequent hydrolysis of the phosphoinositide PI(4)P. Here, we show that this pathway is essential in polarized epithelial cells where it contributes not only to the proper subcellular distribution of cholesterol but also to the trans-Golgi sorting and trafficking of numerous plasma membrane cargo proteins with apical or basolateral localization. Reducing the expression of OSBP, blocking its activity, or inhibiting a PI4Kinase that fuels OSBP with PI(4)P abolishes the epithelial phenotype. Waves of cargo enrichment in the TGN in phase with OSBP and PI(4)P dynamics suggest that OSBP promotes the formation of lipid gradients along the TGN, which helps cargo sorting. During their transient passage through the trans-Golgi, polarized plasma membrane proteins get close to OSBP but fail to be sorted when OSBP is silenced. Thus, OSBP lipid exchange activity is decisive for polarized cargo sorting and distribution in epithelial cells.

Endoplasmic reticulum regulation of glucose metabolism in glioma stem cells

Glioblastoma (GBM) treatment is extremely challenging due to the high complexity of the tumor. It is one of the tumors in which a subpopulation of highly resistant glioma initiating cells (GICs) has been clearly identified. Thus, understanding the differences between GICs and tumor bulk cells is therefore essential to move to less conventional but more efficient approaches. It was found that, unlike their differentiated progeny, GICs survival and maintenance of stem cell properties depend on mitochondrial metabolism. GICs present higher glucose uptake and mitochondrial membrane potential and reduced lactate dehydrogenase activity, being more sensitive to mitochondrial inhibition than their differentiated counterparts. Calcium flux to the mitochondria appears to play an essential role in the maintenance of this distinct metabolic phenotype with a decrease in the expression of voltage‑dependent anionic channel (VDAC) and Grp75, two of the proteins of the IP3R‑Grp75‑VDAC complex that transfers calcium from the endoplasmic reticulum (ER) to the mitochondria. Disruption of ER homeostasis using ER stress inducers or inhibition of ER‑mitochondrial contact sites using the Grp75 inhibitor MKT‑077 resulted in cytotoxicity of GICs and loss of stemness. Moreover, MKT‑077 also potentiates the effect of temozolomide, current treatment for glioblastoma. In summary, the present data indicated that ER‑mitochondrial homeostasis is essential for regulation of GICs glucose metabolism and survival.

Elucidation of molecular mechanism of the unfolded protein response

The endoplasmic reticulum (ER), where newly synthesized secretory and transmembrane proteins are folded and assembled, has the ability to discriminate folded proteins from unfolded proteins and controls the quality of synthesized proteins. Only correctly folded molecules are allowed to move along the secretory pathway, whereas unfolded proteins are retained in the ER.The ER contains a number of molecular chaperones and folding enzymes (ER chaperones hereafter), which assist productive folding of proteins, and therefore newly synthesized proteins usually gain correct tertiary and quaternary structures quite efficiently. Yet unfolded or misfolded proteins even after assistance of ER chaperones are retrotranslocated back to the cytosol, ubiquitinated and degraded by the proteasome. This disposal system is called ER-associated degradation (ERAD). Thus, the quality of proteins in the ER is ensured by two distinct mechanisms, productive folding and ERAD, which have opposite directions.Under a variety of conditions collectively termed ER stress, however, unfolded or misfolded proteins accumulate in the ER, which in turn activates ER stress response or Unfolded Protein Response (UPR). The UPR is mediated by transmembrane proteins in the ER, and three ER stress sensors/transducers, namely IRE1, PERK and ATF6, operates ubiquitously in mammals. Thanks to these signaling pathways, translation is generally attenuated to decrease the burden on the folding machinery; transcription of ER chaperones is induced to augment folding capacity; and transcription of components of ERAD machinery is induced to enhance degradation capacity, leading to maintenance of the homeostasis of the ER. If ER stress sustains, cells undergo to apoptosis.I will talk on the mechanism, evolution, and physiological importance of the UPR and ERAD as well as its involvement in development and progression of various diseases.

Endoplasmic Reticulum as a Therapeutic Target in Cancer: Is there a Role for Flavonoids?

Flavonoids are classified into subclasses of polyphenols, a multipurpose category of natural compounds which comprises secondary metabolites extracted from vascular plants and are plentiful in the human diet. Although the details of flavonoid mechanisms are still not realized correctly, they are generally regarded as antimicrobial, anti-fungal, anti-inflammatory, anti-oxidative; anti-mutagenic; anti-neoplastic; anti-aging; anti-diabetic, cardio-protective, etc. The anti-cancer properties of flavonoids are evident in functions such as prevention of proliferation, metastasis, invasion, inflammation and activation of cell death. Tumors growth and enlargement expose cells to acidosis, hypoxia, and lack of nutrients which result in endoplasmic reticulum (ER) stress; it triggers the unfolded protein response (UPR), which reclaims homeostasis or activates autophagy. Steady stimulation of ER stress can switch autophagy to apoptosis. The connection between ER stress and cancer, in association with UPR, has been explained. The signals provided by UPR can activate or inhibit anti-apoptotic or apoptotic pathways depending on the period and grade of ER stress. In this review, we will peruse the link between flavonoids and their impact on the endoplasmic reticulum in association with cancer therapy.

Advanced Imaging Techniques for the Characterization of Subcellular Organelle Structure in Pancreatic Islet β Cells

Type 2 diabetes (T2D) affects more than 32.3 million individuals in the United States, creating an economic burden of nearly $966 billion in 2021. T2D results from a combination of insulin resistance and inadequate insulin secretion from the pancreatic β cell. However, genetic and physiologic data indicate that defects in β cell function are the chief determinant of whether an individual with insulin resistance will progress to a diagnosis of T2D. The subcellular organelles of the insulin secretory pathway, including the endoplasmic reticulum, Golgi apparatus, and secretory granules, play a critical role in maintaining the heavy biosynthetic burden of insulin production, processing, and secretion. In addition, the mitochondria enable the process of insulin release by integrating the metabolism of nutrients into energy output. Advanced imaging techniques are needed to determine how changes in the structure and composition of these organelles contribute to the loss of insulin secretory capacity in the β cell during T2D. Several microscopy techniques, including electron microscopy, fluorescence microscopy, and soft X-ray tomography, have been utilized to investigate the structure-function relationship within the β cell. In this overview article, we will detail the methodology, strengths, and weaknesses of each approach. © 2024 American Physiological Society. Compr Physiol 14:5243-5267, 2024.

A signal recognition particle receptor gene from the sea cucumber, Apostichopus japonicas

The signal recognition particle (SRP) system delivers approximately 30% of the proteome to the endoplasmic reticulum (ER) membrane. SRP receptor alpha (SRα) binds to SRP for targeting nascent secreted proteins to the ER membrane in eukaryotic cells. In this study, the SRα homologous gene was identified in the sea cucumber, Apostichopus japonicus (AjSRα). AjSRα codes for 641 amino acids and has 54.94% identity with its mammalian homologs. Like mammalian SRα, it is expected to contain the SRP-alpha N domain, SRP54_N domain, and SRP54 domain. In addition, AjSRα is ubiquitously expressed in adult tissues and exhibits a sexually dimorphic expression pattern, with significantly higher expression in ovaries compared to testes. As a maternal factor, AjSRα can be continuously detected during embryonic development. Importantly, we first attempted to investigate its function by using lentiviral vectors for delivering SRα gene-specific shRNA, and we revealed that lentiviral vectors do not induce an upregulation of immune-related enzymes in sea cucumbers. However, compared to the dsRNA-based RNA interference (RNAi) method, lentivirus-mediated RNAi caused dynamic changes in gene expression at a later time. This study supplied the technical support for studying the functional mechanism of SRα in sea cucumbers.