Non-canonical function of IRE1α determines mitochondria-associated endoplasmic reticulum composition to control calcium transfer and bioenergetics

Remodeling the hepatic immune microenvironment and demolishing T cell traps to enhance immunotherapy efficacy in liver metastasis

Immune checkpoint inhibitors (ICIs) exhibit compromised therapeutic efficacy in many patients with advanced cancers, particularly those with liver metastases. Much of this incapability can be ascribed as an irresponsiveness resulting from the "cold" hepatic tumor microenvironment that acts as T cell "traps" for which there currently lack countermeasures. We report a novel nanomedicine that converts the hepatic immune microenvironment to a "hot" phenotype by targeting hepatic macrophage-centric T cell elimination. Using the nanomedicine, composed of KIRA6 (an endothelium reticulum stress inhibitor), α-Tocopherol nanoemulsions, and anti-PD1 antibodies, we found its potency in murine models of orthotopic colorectal tumors and hepatic metastases, restoring immune responses and enhancing anti-tumor effects. A post-treatment scrutiny of the immune microenvironment landscape in the liver reveals repolarization of immunosuppressive hepatic macrophages, upregulation of Th1-like effector CD4+ T cells, and rejuvenation of dendritic cells along with CD8+ T cells. These findings suggest adaptations of liver-centric immune milieu modulation strategies to improve the efficacy of ICIs for a variety of "cold" tumors and their liver metastases.

IP3R2 regulates apoptosis by Ca2+ transfer through mitochondria-ER contacts in hypoxic photoreceptor injury

Mitochondria-associated ER membranes (MAMs) are contact sites that enable bidirectional communication between the ER (endoplasmic reticulum) and mitochondria, including the transfer of Ca2+ signals. MAMs are essential for mitochondrial function and cellular energy metabolism. However, unrestrained Ca2+ transfer to the mitochondria can lead to mitochondria-dependent apoptosis. IP3R2 (Inositol 1,4,5-trisphosphate receptor 2) is an important intracellular Ca2+ channel. This study investigated the contribution of IP3R2-MAMs to hypoxia-induced apoptosis in photoreceptor cells. A photoreceptor hypoxia model was established by subretinal injection of hyaluronic acid (1%) in C57BL/6 mice and 1% O2 treatment in 661W cells. Transmission electron microscopy (TEM), ER-mitochondria colocalization, and the MAM reporter were utilized to evaluate MAM alterations. Cell apoptosis and mitochondrial homeostasis were evaluated using immunofluorescence (IF), flow cytometry, western blotting (WB), and ATP assays. SiRNA transfection was employed to silence IP3R2 in 661W cells. Upon hypoxia induction, MAMs were significantly increased in photoreceptors both in vivo and in vitro. This was accompanied by the activation of mitochondrial apoptosis and disruption of mitochondrial homeostasis. Elevated MAM-enriched IP3R2 protein levels induced by hypoxic injury led to mitochondrial calcium overload and subsequent photoreceptor apoptosis. Notably, IP3R2 knockdown not only improved mitochondrial morphology but also restored mitochondrial function in photoreceptors by limiting MAM formation and thereby attenuating mitochondrial calcium overload under hypoxia. Our results suggest that IP3R2-MAM-mediated mitochondrial calcium overload plays a critical role in mitochondrial dyshomeostasis, ultimately contributing to photoreceptor cell death. Targeting MAM constitutive proteins might provide an option for a therapeutic approach to mitigate photoreceptor death in retinal detachment.

Implications of endoplasmic reticulum stress and autophagy in aging and cardiovascular diseases

The average lifespan of humans has been increasing, resulting in a rapidly rising percentage of older individuals and high morbidity of aging-associated diseases, especially cardiovascular diseases (CVDs). Diverse intracellular and extracellular factors that interrupt homeostatic functions in the endoplasmic reticulum (ER) induce ER stress. Cells employ a dynamic signaling pathway of unfolded protein response (UPR) to buffer ER stress. Recent studies have demonstrated that ER stress triggers various cellular processes associated with aging and many aging-associated diseases, including CVDs. Autophagy is a conserved process involving lysosomal degradation and recycling of cytoplasmic components, proteins, organelles, and pathogens that invade the cytoplasm. Autophagy is vital for combating the adverse influence of aging on the heart. The present report summarizes recent studies on the mechanism of ER stress and autophagy and their overlap in aging and on CVD pathogenesis in the context of aging. It also discusses possible therapeutic interventions targeting ER stress and autophagy that might delay aging and prevent or treat CVDs.

Deciphering the mitochondria-inflammation axis: Insights and therapeutic strategies for heart failure

Heart failure (HF) is a clinical syndrome resulting from left ventricular systolic and diastolic dysfunction, leading to significant morbidity and mortality worldwide. Despite improvements in medical treatment, the prognosis of HF patients remains unsatisfactory, with high rehospitalization rates and substantial economic burdens. The heart, a high-energy-consuming organ, relies heavily on ATP production through oxidative phosphorylation in mitochondria. Mitochondrial dysfunction, characterized by impaired energy production, oxidative stress, and disrupted calcium homeostasis, plays a crucial role in HF pathogenesis. Additionally, inflammation contributes significantly to HF progression, with elevated levels of circulating inflammatory cytokines observed in patients. The interplay between mitochondrial dysfunction and inflammation involves shared risk factors, signaling pathways, and potential therapeutic targets. This review comprehensively explores the mechanisms linking mitochondrial dysfunction and inflammation in HF, including the roles of mitochondrial reactive oxygen species (ROS), calcium dysregulation, and mitochondrial DNA (mtDNA) release in triggering inflammatory responses. Understanding these complex interactions offers insights into novel therapeutic approaches for improving mitochondrial function and relieving oxidative stress and inflammation. Targeted interventions that address the mitochondria-inflammation axis hold promise for enhancing cardiac function and outcomes in HF patients.

Mitochondrial Structure and Function in Human Heart Failure

Despite clinical and scientific advancements, heart failure is the major cause of morbidity and mortality worldwide. Both mitochondrial dysfunction and inflammation contribute to the development and progression of heart failure. Although inflammation is crucial to reparative healing following acute cardiomyocyte injury, chronic inflammation damages the heart, impairs function, and decreases cardiac output. Mitochondria, which comprise one third of cardiomyocyte volume, may prove a potential therapeutic target for heart failure. Known primarily for energy production, mitochondria are also involved in other processes including calcium homeostasis and the regulation of cellular apoptosis. Mitochondrial function is closely related to morphology, which alters through mitochondrial dynamics, thus ensuring that the energy needs of the cell are met. However, in heart failure, changes in substrate use lead to mitochondrial dysfunction and impaired myocyte function. This review discusses mitochondrial and cristae dynamics, including the role of the mitochondria contact site and cristae organizing system complex in mitochondrial ultrastructure changes. Additionally, this review covers the role of mitochondria-endoplasmic reticulum contact sites, mitochondrial communication via nanotunnels, and altered metabolite production during heart failure. We highlight these often-neglected factors and promising clinical mitochondrial targets for heart failure.

Calcium signaling crosstalk between the endoplasmic reticulum and mitochondria, a new drug development strategies of kidney diseases

Calcium (Ca2+) acts as a second messenger and constitutes a complex and large information exchange system between the endoplasmic reticulum (ER) and mitochondria; this process is involved in various life activities, such as energy metabolism, cell proliferation and apoptosis. Increasing evidence has suggested that alterations in Ca2+ crosstalk between the ER and mitochondria, including alterations in ER and mitochondrial Ca2+ channels and related Ca2+ regulatory proteins, such as sarco/endoplasmic reticulum Ca2+-ATPase (SERCA), inositol 1,4,5-trisphosphate receptor (IP3R), and calnexin (CNX), are closely associated with the development of kidney disease. Therapies targeting intracellular Ca2+ signaling have emerged as an emerging field in the treatment of renal diseases. In this review, we focused on recent advances in Ca2+ signaling, ER and mitochondrial Ca2+ monitoring methods and Ca2+ homeostasis in the development of renal diseases and sought to identify new targets and insights for the treatment of renal diseases by targeting Ca2+ channels or related Ca2+ regulatory proteins.

The MICOS Complex Regulates Mitochondrial Structure and Oxidative Stress During Age-Dependent Structural Deficits in the Kidney

The kidney filters nutrient waste and bodily fluids from the bloodstream, in addition to secondary functions of metabolism and hormone secretion, requiring an astonishing amount of energy to maintain its functions. In kidney cells, mitochondria produce adenosine triphosphate (ATP) and help maintain kidney function. Due to aging, the efficiency of kidney functions begins to decrease. Dysfunction in mitochondria and cristae, the inner folds of mitochondria, is a hallmark of aging. Therefore, age-related kidney function decline could be due to changes in mitochondrial ultrastructure, increased reactive oxygen species (ROS), and subsequent alterations in metabolism and lipid composition. We sought to understand if there is altered mitochondrial ultrastructure, as marked by 3D morphological changes, across time in tubular kidney cells. Serial block facing-scanning electron microscope (SBF-SEM) and manual segmentation using the Amira software were used to visualize murine kidney samples during the aging process at 3 months (young) and 2 years (old). We found that 2-year mitochondria are more fragmented, compared to the 3-month, with many uniquely shaped mitochondria observed across aging, concomitant with shifts in ROS, metabolomics, and lipid homeostasis. Furthermore, we show that the mitochondrial contact site and cristae organizing system (MICOS) complex is impaired in the kidney due to aging. Disruption of the MICOS complex shows altered mitochondrial calcium uptake and calcium retention capacity, as well as generation of oxidative stress. We found significant, detrimental structural changes to aged kidney tubule mitochondria suggesting a potential mechanism underlying why kidney diseases occur more readily with age. We hypothesize that disruption in the MICOS complex further exacerbates mitochondrial dysfunction, creating a vicious cycle of mitochondrial degradation and oxidative stress, thus impacting kidney health.

Redox regulation of UPR signalling and mitochondrial ER contact sites

Mitochondria and the endoplasmic reticulum (ER) have a synergistic relationship and are key regulatory hubs in maintaining cell homeostasis. Communication between these organelles is mediated by mitochondria ER contact sites (MERCS), allowing the exchange of material and information, modulating calcium homeostasis, redox signalling, lipid transfer and the regulation of mitochondrial dynamics. MERCS are dynamic structures that allow cells to respond to changes in the intracellular environment under normal homeostatic conditions, while their assembly/disassembly are affected by pathophysiological conditions such as ageing and disease. Disruption of protein folding in the ER lumen can activate the Unfolded Protein Response (UPR), promoting the remodelling of ER membranes and MERCS formation. The UPR stress receptor kinases PERK and IRE1, are located at or close to MERCS. UPR signalling can be adaptive or maladaptive, depending on whether the disruption in protein folding or ER stress is transient or sustained. Adaptive UPR signalling via MERCS can increase mitochondrial calcium import, metabolism and dynamics, while maladaptive UPR signalling can result in excessive calcium import and activation of apoptotic pathways. Targeting UPR signalling and the assembly of MERCS is an attractive therapeutic approach for a range of age-related conditions such as neurodegeneration and sarcopenia. This review highlights the emerging evidence related to the role of redox mediated UPR activation in orchestrating inter-organelle communication between the ER and mitochondria, and ultimately the determination of cell function and fate.

Osteogenesis imperfecta type 10 and the cellular scaffolds underlying common immunological diseases

Osteogenesis imperfecta type 10 (OI10) is caused by loss of function codon variants in the gene SERPINH1 that encodes heat shock protein 47 (HSP47), rather than in a gene specifying bone formation. The HSP47 variants disrupt the folding of both collagen and the endonuclease IRE1α (inositol-requiring enzyme 1α) that splices X-Box Binding Protein 1 (XBP1) mRNA. Besides impairing bone development, variants likely affect osteoclast differentiation. Three distinct biochemical scaffold play key roles in the differentiation and regulated cell death of osteoclasts. These scaffolds consist of non-templated protein modifications, ordered lipid arrays, and protein filaments. The scaffold components are specified genetically, but assemble in response to extracellular perturbagens, pathogens, and left-handed Z-RNA helices encoded genomically by flipons. The outcomes depend on interactions between RIPK1, RIPK3, TRIF, and ZBP1 through short interaction motifs called RHIMs. The causal HSP47 nonsynonymous substitutions occur in a novel variant leucine repeat region (vLRR) that are distantly related to RHIMs. Other vLRR protein variants are causal for a variety of different mendelian diseases. The same scaffolds that drive mendelian pathology are associated with many other complex disease outcomes. Their assembly is triggered dynamically by flipons and other context-specific switches rather than by causal, mendelian, codon variants.

Non-canonical activation of IRE1α during Candida albicans infection enhances macrophage fungicidal activity

While the canonical function of IRE1α is to detect misfolded proteins and activate the unfolded protein response (UPR) to maintain cellular homeostasis, microbial pathogens can also activate IRE1α, which modulates innate immunity and infection outcomes. However, how infection activates IRE1α and its associated inflammatory functions have not been fully elucidated. Recognition of microbe-associated molecular patterns can activate IRE1α, but it is unclear whether this depends on protein misfolding. Here, we report that a common and deadly fungal pathogen, Candida albicans, activates macrophage IRE1α through C-type lectin receptor signaling, reinforcing a role for IRE1α as a central regulator of host responses to infection by a broad range of pathogens. This activation did not depend on protein misfolding in response to C. albicans infection. Moreover, lipopolysaccharide treatment was also able to activate IRE1α prior to protein misfolding, suggesting that pathogen-mediated activation of IRE1α occurs through non-canonical mechanisms. During C. albicans infection, we observed that IRE1α activity promotes phagolysosomal fusion that supports the fungicidal activity of macrophages. Consequently, macrophages lacking IRE1α activity displayed inefficient phagosome maturation, enabling C. albicans to lyse the phagosome, evade fungal killing, and drive aberrant inflammatory cytokine production. Mechanistically, we show that IRE1α activity supports phagosomal calcium flux after phagocytosis of C. albicans, which is crucial for phagosome maturation. Importantly, deletion of IRE1α activity decreased the fungicidal activity of phagocytes in vivo during systemic C. albicans infection. Together, these data provide mechanistic insight for the non-canonical activation of IRE1α during infection, and reveal central roles for IRE1α in macrophage antifungal responses.

ER stress signaling at the interphase between MASH and HCC

HCC is the most frequent primary liver cancer with an extremely poor prognosis and often develops on preset of chronic liver diseases. Major risk factors for HCC include metabolic dysfunction-associated steatohepatitis, a complex multifactorial condition associated with abnormal endoplasmic reticulum (ER) proteostasis. To cope with ER stress, the unfolded protein response engages adaptive reactions to restore the secretory capacity of the cell. Recent advances revealed that ER stress signaling plays a critical role in HCC progression. Here, we propose that chronic ER stress is a common transversal factor contributing to the transition from liver disease (risk factor) to HCC. Interventional strategies to target the unfolded protein response in HCC, such as cancer therapy, are also discussed.

By regulating the IP3R/GRP75/VDAC1 complex to restore mitochondrial dynamic balance, selenomethionine reduces lipopolysaccharide-induced neuronal apoptosis

Selenium (Se), as one of the essential trace elements, plays an anti-inflammatory, antioxidation, and immune-enhancing effect in the body. In addition, Se can also improve nervous system damage induced by various factors. Earlier studies have described the important role of mitochondrial dynamic imbalance in lipopolysaccharide (LPS)-induced nerve injury. The inositol 1,4,5-triphosphate receptor (IP3R)/glucose-regulated protein 75 (GRP75)/voltage-dependent anion channel 1 (VDAC1) complex is considered to be the key to regulating mitochondrial dynamics. However, it is not clear whether Selenomethionine (SeMet) has any influence on the IP3R/GRP75/VDAC1 complex. Therefore, the aim of this investigation was to determine whether SeMet can alleviate LPS-induced brain damage and to elucidate the function of the IP3R/GRP75/VDAC1 complex in it. We established SeMet and/or LPS exposure models in vivo and in vitro using laying hens and primary chicken nerve cells. We noticed that SeMet reversed endoplasmic reticulum stress (ERS) and the imbalance in mitochondrial dynamics and significantly prevented the occurrence of neuronal apoptosis. We made this finding by morphological observation of the brain tissue of laying hens and the detection of related genes such as ERS, the IP3R/GRP75/VDAC1 complex, calcium signal (Ca2+), mitochondrial dynamics, and apoptosis. Other than that, we also discovered that the IP3R/GRP75/VDAC1 complex was crucial in controlling Ca2+ transport between the endoplasmic reticulum and the mitochondrion when SeMet functions as a neuroprotective agent. In summary, our results revealed the specific mechanism by which SeMet alleviated LPS-induced neuronal apoptosis for the first time. As a consequence, SeMet has great potential in the treatment and prevention of neurological illnesses (like neurodegenerative diseases).

Endoplasmic reticular stress as an emerging therapeutic target for chronic pain: a narrative review

Chronic pain is a severely debilitating condition with enormous socioeconomic costs. Current treatment regimens with nonsteroidal anti-inflammatory drugs (NSAIDs), steroids, or opioids have been largely unsatisfactory with uncertain benefits or severe long-term side effects. This is mainly because chronic pain has a multifactorial aetiology. Although conventional pain medications can alleviate pain by keeping several dysfunctional pathways under control, they can mask other underlying pathological causes, ultimately worsening nerve pathologies and pain outcome. Recent preclinical studies have shown that endoplasmic reticulum (ER) stress could be a central hub for triggering multiple molecular cascades involved in the development of chronic pain. Several ER stress inhibitors and unfolded protein response modulators, which have been tested in randomised clinical trials or apprpoved by the US Food and Drug Administration for other chronic diseases, significantly alleviated hyperalgesia in multiple preclinical pain models. Although the role of ER stress in neurodegenerative disorders, metabolic disorders, and cancer has been well established, research on ER stress and chronic pain is still in its infancy. Here, we critically analyse preclinical studies and explore how ER stress can mechanistically act as a central node to drive development and progression of chronic pain. We also discuss therapeutic prospects, benefits, and pitfalls of using ER stress inhibitors and unfolded protein response modulators for managing intractable chronic pain. In the future, targeting ER stress to impact multiple molecular networks might be an attractive therapeutic strategy against chronic pain refractory to steroids, NSAIDs, or opioids. This novel therapeutic strategy could provide solutions for the opioid crisis and public health challenge.

Exploring the IRE1 interactome: From canonical signaling functions to unexpected roles

The unfolded protein response is a mechanism aiming at restoring endoplasmic reticulum (ER) homeostasis and is likely involved in other adaptive pathways. The unfolded protein response is transduced by three proteins acting as sensors and triggering downstream signaling pathways. Among them, inositol-requiring enzyme 1 alpha (IRE1α) (referred to as IRE1 hereafter), an endoplasmic reticulum-resident type I transmembrane protein, exerts its function through both kinase and endoribonuclease activities, resulting in both X-box binding protein 1 mRNA splicing and RNA degradation (regulated ire1 dependent decay). An increasing number of studies have reported protein-protein interactions as regulators of these signaling mechanisms, and additionally, driving other noncanonical functions. In this review, we deliver evolutive and structural insights on IRE1 and further describe how this protein interaction network (interactome) regulates IRE1 signaling abilities or mediates other cellular processes through catalytic-independent mechanisms. Moreover, we focus on newly discovered targets of IRE1 kinase activity and discuss potentially novel IRE1 functions based on the nature of the interactome, thereby identifying new fields to explore regarding this protein's biological roles.

ER stress promotes mitochondrial calcium overload and activates the ROS/NLRP3 axis to mediate fatty liver ischemic injury

BACKGROUND

Fatty livers are widely accepted as marginal donors for liver transplantation but are more susceptible to liver ischemia and reperfusion (IR) injury. Increased macrophage-related inflammation plays an important role in the aggravation of fatty liver IR injury. Here, we investigate the precise mechanism by which endoplasmic reticulum (ER) stress activates macrophage NOD-like receptor thermal protein domain-associated protein 3 (NLRP3) signaling by regulating mitochondrial calcium overload in fatty liver IR.

METHODS

Control- and high-fat diet-fed mice were subjected to a partial liver IR model. The ER stress, mitochondrial calcium levels, and NLRP3 signaling pathway in macrophages were analyzed.

RESULTS

Liver steatosis exacerbated liver inflammation and IR injury and enhanced NLRP3 activation in macrophages. Myeloid NLRP3 deficiency attenuated intrahepatic inflammation and fatty liver injury following IR. Mechanistically, increased ER stress and mitochondrial calcium overload were observed in macrophages obtained from mouse fatty livers after IR. Suppression of ER stress by tauroursodeoxycholic acid effectively downregulated mitochondrial calcium accumulation and suppressed NLRP3 activation in macrophages, leading to decreased inflammatory IR injury in fatty livers. Moreover, Xestospongin-C-mediated inhibition of mitochondrial calcium influx decreased reactive oxygen species (ROS) expression in macrophages after IR. Scavenging of mitochondrial ROS by mito-TEMPO suppressed macrophage NLRP3 activation and IR injury in fatty livers, indicating that excessive mitochondrial ROS production was responsible for macrophage NLRP3 activation induced by mitochondrial calcium overload. Patients with fatty liver also exhibited upregulated activation of NLRP3 and the ER stress signaling pathway after IR.

CONCLUSIONS

Our findings suggest that ER stress promotes mitochondrial calcium overload to activate ROS/NLRP3 signaling pathways within macrophages during IR-stimulated inflammatory responses associated with fatty livers.

Recent insights into the effect of endoplasmic reticulum stress in the pathophysiology of intestinal ischaemia‒reperfusion injury

Intestinal ischaemia‒reperfusion (I/R) injury is a surgical emergency. This condition is associated with a high mortality rate. At present, there are limited number of efficient therapeutic measures for this injury, and the prognosis is poor. Therefore, the pathophysiological mechanisms of intestinal I/R injury must be elucidated to develop a rapid and specific diagnostic and treatment protocol. Numerous studies have indicated the involvement of endoplasmic reticulum (ER) stress in the development of intestinal I/R injury. Specifically, the levels of unfolded and misfolded proteins in the ER lumen are increased due to unfolded protein response. However, persistent ER stress promotes apoptosis of intestinal mucosal epithelial cells through three signalling pathways in the ER, impairing intestinal mucosal barrier function and leading to the dysfunction of intestinal tissues and distant organ compartments. This review summarises the mechanisms of ER stress in intestinal I/R injury, diagnostic indicators, and related treatment strategies with the objective of providing novel insights into future therapies for this condition.

Protein-rich foods, sea foods, and gut microbiota amplify immune responses in chronic diseases and cancers - Targeting PERK as a novel therapeutic strategy for chronic inflammatory diseases, neurodegenerative disorders, and cancer

The endoplasmic reticulum (ER) is a cellular organelle that is physiologically responsible for protein folding, calcium homeostasis, and lipid biosynthesis. Pathological stimuli such as oxidative stress, ischemia, disruptions in calcium homeostasis, and increased production of normal and/or folding-defective proteins all contribute to the accumulation of misfolded proteins in the ER, causing ER stress. The adaptive response to ER stress is the activation of unfolded protein response (UPR), which affect a wide variety of cellular functions to maintain ER homeostasis or lead to apoptosis. Three different ER transmembrane sensors, including PKR-like ER kinase (PERK), activating transcription factor 6 (ATF6), and inositol-requiring enzyme-1 (IRE1), are responsible for initiating UPR. The UPR involves a variety of signal transduction pathways that reduce unfolded protein accumulation by boosting ER-resident chaperones, limiting protein translation, and accelerating unfolded protein degradation. ER is now acknowledged as a critical organelle in sensing dangers and determining cell life and death. On the other hand, UPR plays a critical role in the development and progression of several diseases such as cardiovascular diseases (CVD), metabolic disorders, chronic kidney diseases, neurological disorders, and cancer. Here, we critically analyze the most current knowledge of the master regulatory roles of ER stress particularly the PERK pathway as a conditional danger receptor, an organelle crosstalk regulator, and a regulator of protein translation. We highlighted that PERK is not only ER stress regulator by sensing UPR and ER stress but also a frontier sensor and direct senses for gut microbiota-generated metabolites. Our work also further highlighted the function of PERK as a central hub that leads to metabolic reprogramming and epigenetic modification which further enhanced inflammatory response and promoted trained immunity. Moreover, we highlighted the contribution of ER stress and PERK in the pathogenesis of several diseases such as cancer, CVD, kidney diseases, and neurodegenerative disorders. Finally, we discuss the therapeutic target of ER stress and PERK for cancer treatment and the potential novel therapeutic targets for CVD, metabolic disorders, and neurodegenerative disorders. Inhibition of ER stress, by the development of small molecules that target the PERK and UPR, represents a promising therapeutic strategy.

Endoplasmic reticulum stress and the unfolded protein response: emerging regulators in progression of traumatic brain injury

Traumatic brain injury (TBI) is a common trauma with high mortality and disability rates worldwide. However, the current management of this disease is still unsatisfactory. Therefore, it is necessary to investigate the pathophysiological mechanisms of TBI in depth to improve the treatment options. In recent decades, abundant evidence has highlighted the significance of endoplasmic reticulum stress (ERS) in advancing central nervous system (CNS) disorders, including TBI. ERS following TBI leads to the accumulation of unfolded proteins, initiating the unfolded protein response (UPR). Protein kinase RNA-like ER kinase (PERK), inositol-requiring protein 1 (IRE1), and activating transcription factor 6 (ATF6) are the three major pathways of UPR initiation that determine whether a cell survives or dies. This review focuses on the dual effects of ERS on TBI and discusses the underlying mechanisms. It is suggested that ERS may crosstalk with a series of molecular cascade responses, such as mitochondrial dysfunction, oxidative stress, neuroinflammation, autophagy, and cell death, and is thus involved in the progression of secondary injury after TBI. Hence, ERS is a promising candidate for the management of TBI.

The role of endoplasmic reticulum-mitochondria-associated membranes in diabetic kidney disease

Diabetic kidney disease (DKD) is the leading cause of end-stage renal disease worldwide. The pathomechanisms of DKD are multifactorial, yet haemodynamic and metabolic changes in the early stages of the disease appear to predispose towards irreversible functional loss and histopathological changes. Recent studies highlight the importance of endoplasmic reticulum-mitochondria-associated membranes (ER-MAMs), structures conveying important cellular homeostatic and metabolic effects, in the pathology of DKD. Disruption of ER-MAM integrity in diabetic kidneys is associated with DKD progression, but the regulation of ER-MAMs and their pathogenic contribution remain largely unknown. Exploring the cell-specific components and dynamic changes of ER-MAMs in diabetic kidneys may lead to the identification of new approaches to detect and stratify diabetic patients with DKD. In addition, these insights may lead to novel therapeutic approaches to target and/or reverse disease progression. In this review, we discuss the association of ER-MAMs with key pathomechanisms driving DKD such as insulin resistance, dyslipidaemia, ER stress, and inflammasome activation and the importance of further exploration of ER-MAMs as diagnostic and therapeutic targets in DKD.

Morin suppresses mTORc1/IRE-1α/JNK and IP3R-VDAC-1 pathways: Crucial mechanisms in apoptosis and mitophagy inhibition in experimental Huntington's disease, supported by in silico molecular docking simulations

AIMS

Endoplasmic reticulum stress (ERS) with aberrant mitochondrial-ER contact (MERC), mitophagy, and apoptosis are interconnected determinants in neurodegenerative diseases. Previously, we proved the potential of Morin hydrate (MH), a potent antioxidant flavonoid, to mitigate Huntington's disease (HD)-3-nitropropionic acid (3-NP) model by modulating glutamate/calpain/Kidins220/BDNF trajectory. Extending our work, we aimed to evaluate its impact on combating the ERS/MERC, mitophagy, and apoptosis.

METHODS

Rats were subjected to 3-NP for 14 days and post-treated with MH and/or the ERS inducer WAG-4S for 7 days. Disease progression was assessed by gross inspection and striatal biochemical, histopathological, immunohistochemical, and transmission electron microscopical (TEM) examinations. A molecular docking study was attained to explore MH binding to mTOR, JNK, the kinase domain of IRE1-α, and IP3R.

KEY FINDINGS

MH decreased weight loss and motor dysfunction using open field and rotarod tests. It halted HD degenerative striatal neurons and nucleus/mitochondria ultra-microscopic alterations reflecting neuroprotection. Mechanistically, MH deactivated striatal mTOR/IRE1-α/XBP1s&JNK/IP3R, PINK1/Ubiquitin/Mfn2, and cytochrome c/caspase-3 signaling pathways, besides enhancing p-PGC-1α and p-VDAC1. WAG-4S was able to ameliorate all effects initiated by MH to different extents. Molecular docking simulations revealed promising binding patterns of MH and hence its potential inhibition of the studied proteins, especially mTOR, IP3R, and JNK.

SIGNIFICANCE

MH alleviated HD-associated ERS, MERC, mitophagy, and apoptosis. This is mainly achieved by combating the mTOR/IRE1-α signaling, IP3R/VDAC hub, PINK1/Ubiquitin/Mfn2, and cytochrome c/caspase 3 axis to be worsened by WAG-4S. Molecular docking simulations showed the promising binding of MH to mTOR and JNK as novel identified targets.

Regulation of autophagy by perilysosomal calcium: a new player in β-cell lipotoxicity

Autophagy is an essential quality control mechanism for maintaining organellar functions in eukaryotic cells. Defective autophagy in pancreatic beta cells has been shown to be involved in the progression of diabetes through impaired insulin secretion under glucolipotoxic stress. The underlying mechanism reveals the pathologic role of the hyperactivation of mechanistic target of rapamycin (mTOR), which inhibits lysosomal biogenesis and autophagic processes. Moreover, accumulating evidence suggests that oxidative stress induces Ca2+ depletion in the endoplasmic reticulum (ER) and cytosolic Ca2+ overload, which may contribute to mTOR activation in perilysosomal microdomains, leading to autophagic defects and β-cell failure due to lipotoxicity. This review delineates the antagonistic regulation of autophagic flux by mTOR and AMP-dependent protein kinase (AMPK) at the lysosomal membrane, and both of these molecules could be activated by perilysosomal calcium signaling. However, aberrant and persistent Ca2+ elevation upon lipotoxic stress increases mTOR activity and suppresses autophagy. Therefore, normalization of autophagy is an attractive therapeutic strategy for patients with β-cell failure and diabetes.

Tripeptidyl peptidase II coordinates the homeostasis of calcium and lipids in the central nervous system and its depletion causes presenile dementia in female mice through calcium/lipid dyshomeostasis-induced autophagic degradation of CYP19A1

Rationale: Tripeptidyl peptidase II (TPP2) has been proven to be related to human immune and neurological diseases. It is generally considered as a cytosolic protein which forms the largest known protease complex in eukaryotic cells to operate mostly downstream of proteasomes for degradation of longer peptides. However, this canonical function of TPP2 cannot explain its role in a wide variety of biological and pathogenic processes. The mechanistic interrelationships and hierarchical order of these processes have yet to be clarified. Methods: Animals, cells, plasmids, and viruses established and/or used in this study include: TPP2 knockout mouse line, TPP2 conditional knockout mouse lines (different neural cell type oriented), TRE-TPP2 knockin mouse line on the C57BL/6 background; 293T cells with depletion of TPP2, ATF6, IRE1, PERK, SYVN1, UCHL1, ATG5, CEPT1, or CCTα, respectively; 293T cells stably expressing TPP2, TPP2 S449A, TPP2 S449T, or CCTα-KDEL proteins on the TPP2-depleted background; Plasmids for eukaryotic transient expression of rat CYP19A1-Flag, CYP19A1 S118A-Flag, CYP19A1 S118D-Flag, Sac I ML GFP Strand 11 Long, OMMGFP 1-10, G-CEPIA1er, GCAMP2, CEPIA3mt, ACC-GFP, or SERCA1-GFP; AAV2 carrying the expression cassette of mouse CYP19A1-3 X Flag-T2A-ZsGreen. Techniques used in this study include: Flow cytometry, Immunofluorescence (IF) staining, Immunohistochemical (IHC) staining, Luxol fast blue (LFB) staining, β-galactosidase staining, Lipid droplet (LD) staining, Calcium (Ca2+) staining, Stimulated emission depletion (STED) imaging, Transmission electron microscopic imaging, Two-photon imaging, Terminal deoxynucleotidyl transferase (TdT) dUTP nick-end Labeling (TUNEL) assay, Bromodeoxyuridine (BrdU) assay, Enzymatic activity assay, Proximity ligation assay (PLA), In vivo electrophysiological recording, Long-term potentiation (LTP) recording, Split-GFP-based mitochondria-associated membrane (MAM) detection, Immunoprecipitation (IP), Cellular fractionation, In situ hybridization, Semi-quantitative RT-PCR, Immunoblot, Mass spectrometry-based lipidomics, metabolomics, proteomics, Primary hippocampal neuron culture and Morris water maze (MWM) test. Results: We found that TPP2, independent of its enzymatic activity, plays a crucial role in maintaining the homeostasis of intracellular Ca2+ and phosphatidylcholine (PC) in the central nervous system (CNS) of mice. In consistence with the critical importance of Ca2+ and PC in the CNS, TPP2 gene ablation causes presenile dementia in female mice, which is closely associated with Ca2+/PC dysregulation-induced endoplasmic reticulum (ER) stress, abnormal autophagic degradation of CYP19A1 (aromatase), and estrogen depletion. This work therefore uncovers a new role of TPP2 in lipogenesis and neurosteroidogenesis which is tightly related to cognitive function of adult female mice. Conclusion: Our study reveals a crucial role of TPP2 in controlling homeostasis of Ca2+ and lipids in CNS, and its deficiency causes sexual dimorphism in dementia. Thus, this study is not only of great significance for elucidating the pathogenesis of dementia and its futural treatment, but also for interpreting the role of TPP2 in other systems and their related disorders.

Navigating the landscape of mitochondrial-ER communication in health and disease

Intracellular organelle communication enables the maintenance of tissue homeostasis and health through synchronized adaptive processes triggered by environmental cues. Mitochondrial-Endoplasmic Reticulum (ER) communication sustains cellular fitness by adjusting protein synthesis and degradation, and metabolite and protein trafficking through organelle membranes. Mitochondrial-ER communication is bidirectional and requires that the ER-components of the Integrated Stress Response signal to mitochondria upon activation and, likewise, mitochondria signal to the ER under conditions of metabolite and protein overload to maintain proper functionality and ensure cellular survival. Declines in the mitochondrial-ER communication occur upon ageing and correlate with the onset of a myriad of heterogeneous age-related diseases such as obesity, type 2 diabetes, cancer, or neurodegenerative pathologies. Thus, the exploration of the molecular mechanisms of mitochondrial-ER signaling and regulation will provide insights into the most fundamental cellular adaptive processes with important therapeutical opportunities. In this review, we will discuss the pathways and mechanisms of mitochondrial-ER communication at the mitochondrial-ER interface and their implications in health and disease.

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.

The ER stress sensor IRE1 interacts with STIM1 to promote store-operated calcium entry, T cell activation, and muscular differentiation

Store-operated Ca2+ entry (SOCE) mediated by stromal interacting molecule (STIM)-gated ORAI channels at endoplasmic reticulum (ER) and plasma membrane (PM) contact sites maintains adequate levels of Ca2+ within the ER lumen during Ca2+ signaling. Disruption of ER Ca2+ homeostasis activates the unfolded protein response (UPR) to restore proteostasis. Here, we report that the UPR transducer inositol-requiring enzyme 1 (IRE1) interacts with STIM1, promotes ER-PM contact sites, and enhances SOCE. IRE1 deficiency reduces T cell activation and human myoblast differentiation. In turn, STIM1 deficiency reduces IRE1 signaling after store depletion. Using a CaMPARI2-based Ca2+ genome-wide screen, we identify CAMKG2 and slc105a as SOCE enhancers during ER stress. Our findings unveil a direct crosstalk between SOCE and UPR via IRE1, acting as key regulator of ER Ca2+ and proteostasis in T cells and muscles. Under ER stress, this IRE1-STIM1 axis boosts SOCE to preserve immune cell functions, a pathway that could be targeted for cancer immunotherapy.

Call to action to properly utilize electron microscopy to measure organelles to monitor disease

This review provides an overview of the current methods for quantifying mitochondrial ultrastructure, including cristae morphology, mitochondrial contact sites, and recycling machinery and a guide to utilizing electron microscopy to effectively measure these organelles. Quantitative analysis of mitochondrial ultrastructure is essential for understanding mitochondrial biology and developing therapeutic strategies for mitochondrial-related diseases. Techniques such as transmission electron microscopy (TEM) and serial block face-scanning electron microscopy, as well as how they can be combined with other techniques including confocal microscopy, super-resolution microscopy, and correlative light and electron microscopy are discussed. Beyond their limitations and challenges, we also offer specific magnifications that may be best suited for TEM analysis of mitochondrial, endoplasmic reticulum, and recycling machinery. Finally, perspectives on future quantification methods are offered.

The Antioxidative Effects of Flavones in Hypertensive Disease

Hypertension is the leading remediable risk factor for cardiovascular morbidity and mortality in the United States. Excess dietary salt consumption, which is a catalyst of hypertension, initiates an inflammatory cascade via activation of antigen-presenting cells (APCs). This pro-inflammatory response is driven primarily by sodium ions (Na+) transporting into APCs by the epithelial sodium channel (ENaC) and subsequent NADPH oxidase activation, leading to high levels of oxidative stress. Oxidative stress, a well-known catalyst for hypertension-related illness development, disturbs redox homeostasis, which ultimately promotes lipid peroxidation, isolevuglandin production and an inflammatory response. Natural medicinal compounds derived from organic materials that are characterized by their anti-inflammatory, anti-oxidative, and anti-mutagenic properties have recently gained traction amongst the pharmacology community due to their therapeutic effects. Flavonoids, a natural phenolic compound, have these therapeutic benefits and can potentially serve as anti-hypertensives. Flavones are a type of flavonoid that have increased anti-inflammatory effects that may allow them to act as therapeutic agents for hypertension, including diosmetin, which is able to induce significant arterial vasodilation in several different animal models. This review will focus on the activity of flavones to illuminate potential preventative and potential therapeutic mechanisms against hypertension.

The unfolded protein response of the endoplasmic reticulum supports mitochondrial biogenesis by buffering nonimported proteins

Almost all mitochondrial proteins are synthesized in the cytosol and subsequently targeted to mitochondria. The accumulation of nonimported precursor proteins occurring upon mitochondrial dysfunction can challenge cellular protein homeostasis. Here we show that blocking protein translocation into mitochondria results in the accumulation of mitochondrial membrane proteins at the endoplasmic reticulum, thereby triggering the unfolded protein response (UPRER). Moreover, we find that mitochondrial membrane proteins are also routed to the ER under physiological conditions. The level of ER-resident mitochondrial precursors is enhanced by import defects as well as metabolic stimuli that increase the expression of mitochondrial proteins. Under such conditions, the UPRER is crucial to maintain protein homeostasis and cellular fitness. We propose the ER serves as a physiological buffer zone for those mitochondrial precursors that cannot be immediately imported into mitochondria while engaging the UPRER to adjust the ER proteostasis capacity to the extent of precursor accumulation.

Pharmacological Regulation of Endoplasmic Reticulum Structure and Calcium Dynamics: Importance for Neurodegenerative Diseases

The endoplasmic reticulum (ER) is the largest organelle of the cell, composed of a continuous network of sheets and tubules, and is involved in protein, calcium (Ca2+), and lipid homeostasis. In neurons, the ER extends throughout the cell, both somal and axodendritic compartments, and is highly important for neuronal functions. A third of the proteome of a cell, secreted and membrane-bound proteins, are processed within the ER lumen and most of these proteins are vital for neuronal activity. The brain itself is high in lipid content, and many structural lipids are produced, in part, by the ER. Cholesterol and steroid synthesis are strictly regulated in the ER of the blood-brain barrier protected brain cells. The high Ca2+ level in the ER lumen and low cytosolic concentration is needed for Ca2+-based intracellular signaling, for synaptic signaling and Ca2+ waves, and for preparing proteins for correct folding in the presence of high Ca2+ concentrations to cope with the high concentrations of extracellular milieu. Particularly, ER Ca2+ is controlled in axodendritic areas for proper neurito- and synaptogenesis and synaptic plasticity and remodeling. In this review, we cover the physiologic functions of the neuronal ER and discuss it in context of common neurodegenerative diseases, focusing on pharmacological regulation of ER Ca2+ Furthermore, we postulate that heterogeneity of the ER, its protein folding capacity, and ensuring Ca2+ regulation are crucial factors for the aging and selective vulnerability of neurons in various neurodegenerative diseases. SIGNIFICANCE STATEMENT: Endoplasmic reticulum (ER) Ca2+ regulators are promising therapeutic targets for degenerative diseases for which efficacious drug therapies do not exist. The use of pharmacological probes targeting maintenance and restoration of ER Ca2+ can provide restoration of protein homeostasis (e.g., folding of complex plasma membrane signaling receptors) and slow down the degeneration process of neurons.

Genome-wide kinase-MAM interactome screening reveals the role of CK2A1 in MAM Ca2+ dynamics linked to DEE66

The endoplasmic reticulum (ER) and mitochondria form a unique subcellular compartment called mitochondria-associated ER membranes (MAMs). Disruption of MAMs impairs Ca2+ homeostasis, triggering pleiotropic effects in the neuronal system. Genome-wide kinase-MAM interactome screening identifies casein kinase 2 alpha 1 (CK2A1) as a regulator of composition and Ca2+ transport of MAMs. CK2A1-mediated phosphorylation of PACS2 at Ser207/208/213 facilitates MAM localization of the CK2A1-PACS2-PKD2 complex, regulating PKD2-dependent mitochondrial Ca2+ influx. We further reveal that mutations of PACS2 (E209K and E211K) associated with developmental and epileptic encephalopathy-66 (DEE66) impair MAM integrity through the disturbance of PACS2 phosphorylation at Ser207/208/213. This, in turn, causes the reduction of mitochondrial Ca2+ uptake and the dramatic increase of the cytosolic Ca2+ level, thereby, inducing neurotransmitter release at the axon boutons of glutamatergic neurons. In conclusion, our findings suggest a molecular mechanism that MAM alterations induced by pathological PACS2 mutations modulate Ca2+-dependent neurotransmitter release.

Subcellular visualization: Organelle-specific targeted drug delivery and discovery

Organelles perform critical biological functions due to their distinct molecular composition and internal environment. Disorders in organelles or their interacting networks have been linked to the incidence of numerous diseases, and the research of pharmacological actions at the organelle level has sparked pharmacists' interest. Currently, cell imaging has evolved into a critical tool for drug delivery, drug discovery, and pharmacological research. The introduction of advanced imaging techniques in recent years has provided researchers with richer biological information for viewing and studying the ultrastructure of organelles, protein interactions, and gene transcription activities, leading to the design and delivery of precision-targeted drugs. Therefore, this reviews the research on organelles-targeted drugs based upon imaging technologies and development of fluorescent molecules for medicinal purposes. We also give a thorough analysis of a number of subcellular-level elements of drug development, including subcellular research instruments and methods, organelle biological event investigation, subcellular target and drug identification, and design of subcellular delivery systems. This review will make it possible to promote drug research from the individual/cellular level to the subcellular level, as well as give a new focus based on newly found organelle activities.

The Unfolded Protein Response and Its Implications for Novel Therapeutic Strategies in Inflammatory Bowel Disease

The endoplasmic reticulum (ER) is a multifunctional organelle playing a vital role in maintaining cell homeostasis, and disruptions to its functions can have detrimental effects on cells. Dysregulated ER stress and the unfolded protein response (UPR) have been linked to various human diseases. For example, ER stress and the activation of the UPR signaling pathways in intestinal epithelial cells can either exacerbate or alleviate the severity of inflammatory bowel disease (IBD), contingent on the degree and conditions of activation. Our recent studies have shown that EPICERTIN, a recombinant variant of the cholera toxin B subunit containing an ER retention motif, can induce a protective UPR in colon epithelial cells, subsequently promoting epithelial restitution and mucosal healing in IBD models. These findings support the idea that compounds modulating UPR may be promising pharmaceutical candidates for the treatment of the disease. In this review, we summarize our current understanding of the ER stress and UPR in IBD, focusing on their roles in maintaining cell homeostasis, dysregulation, and disease pathogenesis. Additionally, we discuss therapeutic strategies that promote the cytoprotection of colon epithelial cells and reduce inflammation via pharmacological manipulation of the UPR.

AMPK-endoplasmic reticulum stress axis contributes to lipopolysaccharide-caused mitochondrial dysfunction by regulating mitochondria-associated membrane function in bovine hepatocytes

Mitochondrial homeostasis is closely associated with cellular homeostasis process, whereas mitochondrial dysfunction contributes to apoptosis and mitophagy. Hence, analyzing the mechanism of lipopolysaccharide (LPS)-caused mitochondrial damage is necessary to understand how cellular homeostasis is maintained in bovine hepatocytes. Mitochondria-associated membranes (MAM), a connection between endoplasmic reticulum (ER) and mitochondria, is important to control mitochondrial function. To investigate the underlying mechanisms of the LPS-caused mitochondrial dysfunction, hepatocytes isolated from dairy cows at ∼160 d in milk (DIM) were pretreated with the specific inhibitors of adenosine 5'-monophosphate-activated protein kinase (AMPK), ER stress, RNA-activated protein kinase-like ER kinase (PERK), inositol-requiring enzyme 1α (IRE1α), c-Jun N-terminal kinase, and autophagy followed by a 12 μg/mL LPS treatment. The results showed that inhibiting ER stress with 4-phenylbutyric acid decreased the levels of autophagy and mitochondrial damage with AMPK inactivation in LPS-treated hepatocytes. The AMPK inhibitor compound C pretreatment alleviated LPS-induced ER stress, autophagy and mitochondrial dysfunction by regulating the expression of MAM-related genes, such as mitofusin 2 (MFN2), PERK, and IRE1α. Moreover, inhibiting PERK and IRE1α mitigated autophagy and mitochondrial dynamic disruption by regulating the MAM function. Additionally, blocking c-Jun N-terminal kinase, the downstream sensor of IRE1α, could reduce the levels of autophagy and apoptosis and restore the balance of mitochondrial fusion and fission by modulating the B cell leukemia 2 (BCL-2)/BCL-2 interacting protein 1 (BECLIN1) complex in the LPS-treated bovine hepatocytes. Furthermore, autophagy blockage with chloroquine could intervene in LPS-caused apoptosis to restore mitochondrial function. Collectively, these findings suggest that the AMPK-ER stress axis is involved in the LPS-caused mitochondrial dysfunction by mediating the MAM activity in bovine hepatocytes.

Interplay between calcium and endoplasmic reticulum stress

Cellular homeostasis is crucial for the healthy functioning of the organism. Disruption of cellular homeostasis activates endoplasmic reticulum (ER) stress coping responses including the unfolded protein response (UPR). There are three ER resident stress sensors responsible for UPR activation - IRE1α, PERK and ATF6. Ca²⁺ signaling plays an important role in stress responses including the UPR and the ER is the main Ca²⁺ storage organelle and a source of Ca²⁺ for cell signaling. The ER contains many proteins involved in Ca²⁺ import/export/ storage, Ca²⁺ movement between different cellular organelles and ER Ca²⁺ stores refilling. Here we focus on selected aspects of ER Ca²⁺ homeostasis and its role in activation of the ER stress coping responses.

The endoplasmic reticulum stress sensor IRE1 regulates collagen secretion through the enforcement of the proteostasis factor P4HB/PDIA1 contributing to liver damage and fibrosis

Collagen is one the most abundant proteins and the main cargo of the secretory pathway, contributing to hepatic fibrosis and cirrhosis due to excessive deposition of extracellular matrix. Here we investigated the possible contribution of the unfolded protein response, the main adaptive pathway that monitors and adjusts the protein production capacity at the endoplasmic reticulum, to collagen biogenesis and liver disease. Genetic ablation of the ER stress sensor IRE1 reduced liver damage and diminished collagen deposition in models of liver fibrosis triggered by carbon tetrachloride (CCl ₄ ) administration or by high fat diet. Proteomic and transcriptomic profiling identified the prolyl 4-hydroxylase (P4HB, also known as PDIA1), which is known to be critical for collagen maturation, as a major IRE1-induced gene. Cell culture studies demonstrated that IRE1 deficiency results in collagen retention at the ER and altered secretion, a phenotype rescued by P4HB overexpression. Taken together, our results collectively establish a role of the IRE1/P4HB axis in the regulation of collagen production and its significance in the pathogenesis of various disease states.

Organelle Communication: Joined in Sickness and in Health

Organelles are membrane-lined structures that compartmentalize subcellular biochemical functions. Therefore, interorganelle communication is crucial for cellular responses that require the coordination of such functions. Multiple principles govern interorganelle interactions, which arise from the complex nature of organelles: position, multilingualism, continuity, heterogeneity, proximity, and bidirectionality, among others. Given their importance, alterations in organelle communication have been linked to many diseases. Among the different types of contacts, endoplasmic reticulum mitochondria interactions are the best known; however, mounting evidence indicates that other organelles also have something to say in the pathophysiological conversation.

The ER-mitochondria interface, where Ca2+ and cell death meet

Endoplasmic reticulum (ER)-mitochondria contact sites are crucial to allow Ca²⁺ flux between them and a plethora of proteins participate in tethering both organelles together. Inositol 1,4,5-trisphosphate receptors (IP₃Rs) play a pivotal role at such contact sites, participating in both ER-mitochondria tethering and as Ca²⁺-transport system that delivers Ca²⁺ from the ER towards mitochondria. At the ER-mitochondria contact sites, the IP₃Rs function as a multi-protein complex linked to the voltage-dependent anion channel 1 (VDAC1) in the outer mitochondrial membrane, via the chaperone glucose-regulated protein 75 (GRP75). This IP₃R-GRP75-VDAC1 complex supports the efficient transfer of Ca²⁺ from the ER into the mitochondrial intermembrane space, from which the Ca²⁺ ions can reach the mitochondrial matrix through the mitochondrial calcium uniporter. Under physiological conditions, basal Ca²⁺ oscillations deliver Ca²⁺ to the mitochondrial matrix, thereby stimulating mitochondrial oxidative metabolism. However, when mitochondrial Ca²⁺ overload occurs, the increase in [Ca²⁺] will induce the opening of the mitochondrial permeability transition pore, thereby provoking cell death. The IP₃R-GRP75-VDAC1 complex forms a hub for several other proteins that stabilize the complex and/or regulate the complex's ability to channel Ca²⁺ into the mitochondria. These proteins and their mechanisms of action are discussed in the present review with special attention for their role in pathological conditions and potential implication for therapeutic strategies.

Mitochondria associated ER membrane and cerebral ischemia: molecular mechanisms and therapeutic strategies

Endoplasmic reticulum (ER) and mitochondria are two important organelles that are highly dynamic in mammalian cells. The physical connection between them is mitochondria associated ER membranes (MAM). In recent years, studies on endoplasmic reticulum and mitochondria have shifted from independent division to association and comparison, especially MAM has gradually become a research hotspot. MAM connects the two organelles, not only to maintain their independent structure and function, but also to promote metabolism and signal transduction between them. This paper reviews the morphological structure and protein localization of MAM, and briefly analyzes the functions of MAM in regulating Ca²⁺ transport, lipid synthesis, mitochondrial fusion and fission, endoplasmic reticulum stress and oxidative stress, autophagy and inflammation. Since ER stress and mitochondrial dysfunction are important pathological events in neurological diseases including ischemic stroke, MAM is likely to play an important role in cerebral ischemia by regulating the signaling of the two organelles and the crosstalk of the two pathological events.

Endoplasmic Reticulum Stress and Mitochondrial Stress in Drug-Induced Liver Injury

Drug-induced liver injury (DILI) is a widespread and harmful disease closely linked to mitochondrial and endoplasmic reticulum stress (ERS). Globally, severe drug-induced hepatitis, cirrhosis, and liver cancer are the primary causes of liver-related morbidity and mortality. A hallmark of DILI is ERS and changes in mitochondrial morphology and function, which increase the production of reactive oxygen species (ROS) in a vicious cycle of mutually reinforcing stress responses. Several pathways are maladapted to maintain homeostasis during DILI. Here, we discuss the processes of liver injury caused by several types of drugs that induce hepatocyte stress, focusing primarily on DILI by ERS and mitochondrial stress. Importantly, both ERS and mitochondrial stress are mediated by the overproduction of ROS, destruction of Ca2+ homeostasis, and unfolded protein response (UPR). Additionally, we review new pathways and potential pharmacological targets for DILI to highlight new possibilities for DILI treatment and mitigation.

Syntaxin17 contributes to obesity cardiomyopathy through promoting mitochondrial Ca2+ overload in a Parkin-MCUb-dependent manner

OBJECTIVE

Uncorrected obesity is accompanied by unfavorable structural and functional changes in the heart, known as obesity cardiomyopathy. Recent evidence has revealed a crucial role for mitochondria-associated endoplasmic reticulum membranes (MAMs) in obesity-induced cardiac complication. Syntaxin 17 (STX17) serves as a scaffolding molecule localized on MAMs although its role in obesity heart complication remains elusive.

METHODS AND MATERIALS

This study examined the role of STX17 in MAMs and mitochondrial Ca²⁺ homeostasis in HFD-induced obesity cardiomyopathy using tamoxifen-induced cardiac-specific STX17 knockout (STX17^cko) and STX17 overexpression mice using intravenously delivered recombinant adeno-associated virus serotype-9 (AAV9-cTNT-STX17).

RESULTS

STX17 levels were significantly elevated in plasma from obese patients and heart tissues of HFD-fed mice. Our data revealed that cardiac STX17 knockout alleviated cardiac remodeling and dysfunction in obese hearts without eliciting any notable effect itself, while STX17 overexpression aggravated cardiac dysfunction in obese mice. STX17 deletion and STX17 overexpression annihilated and aggravated, respectively, HFD-induced oxidative stress (O₂^- production) and mitochondrial injury in the heart. Furthermore, STX17 transfection facilitated obesity-induced MAMs formation in cardiomyocytes and evoked excess mitochondrial Ca²⁺ influx, dependent upon interaction with mitochondrial Ca²⁺ uniporter dominant negative β (MCUb) through Habc domain. Our data also suggested that STX17 promoted ubiquitination and degradation of MCUb through the E3 ligase parkin in the face of palmitate challenging.

CONCLUSION

Taken together, our results identified a novel role for STX17 in facilitating obesity-induced MAMs formation, and subsequently mitochondrial Ca²⁺ overload, mitochondrial O₂^- accumulation, lipid peroxidation, resulting in cardiac impairment. Our findings denoted therapeutic promises of targeting STX17 in obesity.

The endoplasmic reticulum stress and unfolded protein response in Alzheimer's disease: a calcium dyshomeostasis perspective

Protein misfolding is prominent in early cellular pathology of Alzheimer's disease (AD), implicating pathophysiological significance of endoplasmic reticulum stress/unfolded protein response (ER stress/UPR) and highlighting it as a target for drug development. Experimental data from animal AD models and observations on human specimens are, however, inconsistent. ER stress and associated UPR are readily observed in in vitro AD cellular models and in some AD model animals. In the human brain, components and markers of ER stress as well as UPR transducers are observed at Braak stages III-VI associated with severe neuropathology and neuronal death. The picture, however, is further complicated by the brain region- and cell type-specificity of the AD-related pathology. Terms 'disturbed' or 'non-canonical' ER stress/UPR were used to describe the discrepancies between experimental data and the classic ER stress/UPR cascade. Here we discuss possible 'disturbing' or 'interfering' factors which may modify ER stress/UPR in the early AD pathogenesis. We focus on the dysregulation of the ER Ca²⁺ homeostasis, store-operated Ca²⁺ entry, and the interaction between the ER and mitochondria. We suggest that a detailed study of the CNS cell type-specific alterations of Ca²⁺ homeostasis in early AD may deepen our understanding of AD-related dysproteostasis.

Determinants and outcomes of mitochondrial dynamics

Mitochondria are not only central organelles in metabolism and energy conversion but are also platforms for cellular signaling cascades. Classically, the shape and ultrastructure of mitochondria were depicted as static. The discovery of morphological transitions during cell death and of conserved genes controlling mitochondrial fusion and fission contributed to establishing the concept that mitochondrial morphology and ultrastructure are dynamically regulated by mitochondria-shaping proteins. These finely tuned, dynamic changes in mitochondrial shape can in turn control mitochondrial function, and their alterations in human diseases suggest that this space can be explored for drug discovery. Here, we review the basic tenets and molecular mechanisms of mitochondrial morphology and ultrastructure, describing how they can coordinately define mitochondrial function.

XBP1 modulates endoplasmic reticulum and mitochondria crosstalk via regulating NLRP3 in renal ischemia/reperfusion injury

The functional status of mitochondria and the endoplasmic reticulum are central to renal ischemia/reperfusion injury (IRI). X-box binding protein 1 (XBP1) is an important transcription factor in endoplasmic reticulum stress. NLR family pyrin domain containing-3 (NLRP3) inflammatory bodies are closely related to renal IRI. In vivo and in vitro, we examined the molecular mechanisms and functions of XBP1-NLRP3 signaling in renal IRI, which influences ER-mitochondrial crosstalk. In this study, mice were subjected to 45 min of unilateral renal warm ischemia, the other kidney resected, and reperfusion was performed for 24 h in vivo. In vitro, murine renal tubular epithelial cells (TCMK-1) were exposed to hypoxia for 24 h and reoxygenation for 2 h. Tissue or cell damage was evaluated by measuring blood urea nitrogen and creatinine levels, histological staining, flow cytometry, terminal deoxynucleotidyl transferase-mediated nick-end labeling, diethylene glycol staining, and transmission electron microscopy (TEM). Western blotting, immunofluorescence staining, and ELISA were used to analyze protein expression. Whether XBP1 regulates the NLRP3 promoter was evaluated using a luciferase reporter assay. Kidney damage was reduced with decreasing blood urea nitrogen, creatinine, interleukin-1β, and interleukin-18 levels. XBP1 deficiency reduced tissue damage and cell apoptosis, protecting the mitochondria. Disruption of XBP1 was associated with reduced NLRP3 and cleaved caspase-1 levels and markedly improved survival. In vitro in TCMK-1 cells, XBP1 interference inhibited caspase-1-dependent mitochondrial damage and reduced the production of mitochondrial reactive oxygen species. The luciferase assay showed that spliced XBP1 isoforms enhanced the activity of the NLRP3 promoter. These findings reveal that XBP1 downregulation suppresses the expression of NLRP3, a potential regulator of endoplasmic reticulum mitochondrial crosstalk in nephritic injury and a potential therapeutic target in XBP1-mediated aseptic nephritis.

Mitochondria-associated endoplasmic reticulum membranes: A promising toxicity regulation target

Mitochondria-associated endoplasmic reticulum membranes (MAMs) are dynamic suborganelle membranes that physically couple endoplasmic reticulum (ER) and mitochondria to provide a platform for exchange of intracellular molecules and crosstalk between the two organelles. Dysfunctions of mitochondria and ER and imbalance of intracellular homeostasis have been discovered in the research of toxics. Cellular activities such as oxidative stress, ER stress, Ca²⁺ transport, autophagy, mitochondrial fusion and fission, and apoptosis mediated by MAMs are closely related to the toxicological effects of various toxicants. These cellular activities mediated by MAMs crosstalk with each other. Regulating the structure and function of MAMs can alleviate the damage caused by toxicants to some extent. In this review, we discuss the relationships between MAMs and the mechanisms of toxicological effects, and highlight MAMs as a potential target for protection against toxicants.

The endoplasmic reticulum kinase PERK interacts with the oxidoreductase ERO1 to metabolically adapt mitochondria

Endoplasmic reticulum (ER) homeostasis requires molecular regulators that tailor mitochondrial bioenergetics to the needs of protein folding. For instance, calnexin maintains mitochondria metabolism and mitochondria-ER contacts (MERCs) through reactive oxygen species (ROS) from NADPH oxidase 4 (NOX4). However, induction of ER stress requires a quick molecular rewiring of mitochondria to adapt to new energy needs. This machinery is not characterized. We now show that the oxidoreductase ERO1⍺ covalently interacts with protein kinase RNA-like ER kinase (PERK) upon treatment with tunicamycin. The PERK-ERO1⍺ interaction requires the C-terminal active site of ERO1⍺ and cysteine 216 of PERK. Moreover, we show that the PERK-ERO1⍺ complex promotes oxidization of MERC proteins and controls mitochondrial dynamics. Using proteinaceous probes, we determined that these functions improve ER-mitochondria Ca²⁺ flux to maintain bioenergetics in both organelles, while limiting oxidative stress. Therefore, the PERK-ERO1⍺ complex is a key molecular machinery that allows quick metabolic adaptation to ER stress.

Therapeutic targeting of mitochondria–proteostasis axis by antioxidant polysaccharides in neurodegeneration

Aging is a major risk factor for many age-associated disorders, including neurodegenerative diseases. Both mitochondrial dysfunction and proteostatic decline are well-recognized hallmarks of aging and age-related neurodegeneration. Despite a lack of therapies for neurodegenerative diseases, a number of interventions promoting mitochondrial integrity and protein homeostasis (proteostasis) have been shown to delay aging-associated neurodegeneration. For example, many antioxidant polysaccharides are shown to have pharmacological potentials in Alzheimer's, Parkinson's and Huntington's diseases through regulation of mitochondrial and proteostatic pathways, including oxidative stress and heat shock responses. However, how mitochondrial and proteostatic mechanisms work together to exert the antineurodegenerative effect of the polysaccharides remains largely unexplored. Interestingly, recent studies have provided a growing body of evidence to support the crosstalk between mitostatic and proteostatic networks as well as the impact of the crosstalk on neurodegeneration. Here we summarize the recent progress of antineurodegenerative polysaccharides with particular attention in the mitochondrial and proteostatic context and provide perspectives on their implications in the crosstalk along the mitochondria-proteostasis axis.

IRE1α overexpression in malignant cells limits tumor progression by inducing an anti-cancer immune response

IRE1α is one of the three ER transmembrane transducers of the Unfolded Protein Response (UPR) activated under endoplasmic reticulum (ER) stress. IRE1α activation has a dual role in cancer as it may be either pro- or anti-tumoral depending on the studied models. Here, we describe the discovery that exogenous expression of IRE1α, resulting in IRE1α auto-activation, did not affect cancer cell proliferation in vitro but resulted in a tumor-suppressive phenotype in syngeneic immunocompetent mice. We found that exogenous expression of IRE1α in murine colorectal and Lewis lung carcinoma cells impaired tumor growth when syngeneic tumor cells were subcutaneously implanted in immunocompetent mice but not in immunodeficient mice. Mechanistically, the in vivo tumor-suppressive effect of overexpressing IRE1α in tumor cells was associated with IRE1α RNAse activity driving both XBP1 mRNA splicing and regulated IRE1-dependent decay of RNA (RIDD). We showed that the tumor-suppressive phenotype upon IRE1α overexpression was characterized by the induction of apoptosis in tumor cells along with an enhanced adaptive anti-cancer immunosurveillance. Hence, our work indicates that IRE1α overexpression and/or activation in tumor cells can limit tumor growth in immunocompetent mice. This finding might point toward the need of adjusting the use of IRE1α inhibitors in cancer treatments based on the predominant outcome of the RNAse activity of IRE1α.

Mitochondria-associated niches in health and disease

The appreciation of the importance of interorganelle contacts has steadily increased over the past decades. Advances in imaging, molecular biology and bioinformatic techniques allowed the discovery of new mechanisms involved in the interaction and communication between organelles, providing novel insights into the inner works of a cell. In this Review, with the mitochondria under the spotlight, we discuss the most recent findings on the mechanisms mediating the communication between organelles, focusing on Ca2+ signaling, lipid exchange, cell death and stress responses. Notably, we introduce a new integrative perspective to signaling networks that is regulated by interorganelle interactions – the mitochondria-associated niches – focusing on the link between the molecular determinants of contact sites and their functional outputs, rather than simply physical and structural communication. In addition, we highlight the neuropathological and metabolic implications of alterations in mitochondria-associated niches and outline how this concept might improve our understanding of multi-organelle interactions.

Mitochondria-endoplasmic reticulum contacts in sepsis-induced myocardial dysfunction

Mitochondrial and endoplasmic reticulum (ER) are important intracellular organelles. The sites that mitochondrial and ER are closely related in structure and function are called Mitochondria-ER contacts (MERCs). MERCs are involved in a variety of biological processes, including calcium signaling, lipid synthesis and transport, autophagy, mitochondrial dynamics, ER stress, and inflammation. Sepsis-induced myocardial dysfunction (SIMD) is a vital organ damage caused by sepsis, which is closely associated with mitochondrial and ER dysfunction. Growing evidence strongly supports the role of MERCs in the pathogenesis of SIMD. In this review, we summarize the biological functions of MERCs and the roles of MERCs proteins in SIMD.