Urban planning of the endoplasmic reticulum (ER): how diverse mechanisms segregate the many functions of the ER

A bird's-eye view of the endoplasmic reticulum in filamentous fungi

SUMMARYThe endoplasmic reticulum (ER) is one of the most extensive organelles in eukaryotic cells. It performs crucial roles in protein and lipid synthesis and Ca2+ homeostasis. Most information on ER types, functions, organization, and domains comes from studies in uninucleate animal, plant, and yeast cells. In contrast, there is limited information on the multinucleate cells of filamentous fungi, i.e., hyphae. We provide an analytical review of existing literature to categorize different types of ER described in filamentous fungi while emphasizing the research techniques and markers used. Additionally, we identify the knowledge gaps that need to be resolved better to understand the structure-function correlation of ER in filamentous fungi. Finally, advanced technologies that can provide breakthroughs in understanding the ER in filamentous fungi are discussed.

ER-trafficking triggers NRF1 ubiquitination to promote its proteolytic activation

The transcription factor NRF1 resides in the endoplasmic reticulum (ER) and is constantly transported to the cytosol for proteasomal degradation. However, when the proteasome is defective, NRF1 escapes degradation and undergoes proteolytic cleavage by the protease DDI2, generating a transcriptionally active form that restores proteostasis, including proteasome function. The mechanisms that regulate NRF1 proteolytic activation and transcriptional potential remain poorly understood. This study demonstrates that the ER is a crucial regulator of NRF1 function by orchestrating its ubiquitination through the E3 ubiquitin ligase HRD1. We show that HRD1-mediated NRF1 ubiquitination is necessary for DDI2-mediated processing in cells. Furthermore, we found that deficiency in both RAD23A and RAD23B impaired DDI2-mediated NRF1 processing, indicating that these genes are essential components of the DDI2 proteolytic machinery. Our findings highlight the intricate mechanism by which the ER activates NRF1 to coordinate the transcriptional activity of an adaptation response in cells.

Mitochondrial dysfunction in the pathogenesis of endothelial dysfunction

Cardiovascular diseases (CVDs) are a matter of concern worldwide, and mitochondrial dysfunction is one of the major contributing factors. Vascular endothelial dysfunction has a major role in the development of atherosclerosis because of the abnormal chemokine secretion, inflammatory mediators, enhancement of LDL oxidation, cytokine elevation, and smooth muscle cell proliferation. Endothelial cells transfer oxygen from the pulmonary circulatory system to the tissue surrounding the blood vessels, and a majority of oxygen is transferred to the myocardium by endothelial cells, which utilise a small amount of oxygen to generate ATP. Free radicals of oxide are produced by mitochondria, which are responsible for cellular oxygen uptake. Increased mitochondrial ROS generation and reduction in agonist-stimulated eNOS activation and nitric oxide bioavailability were directly linked to the observed change in mitochondrial dynamics, resulting in various CVDs and endothelial dysfunction. Presently, the manuscript mainly focuses on endothelial dysfunction, providing a deep understanding of the various features of mitochondrial mechanisms that are used to modulate endothelial dysfunction. We talk about recent findings and approaches that may make it possible to detect mitochondrial dysfunction as a potential biomarker for risk assessment and diagnosis of endothelial dysfunction. In the end, we cover several targets that may reduce mitochondrial dysfunction through both direct and indirect processes and assess the impact of several different classes of drugs in the context of endothelial dysfunction.

Dynamics of CLIMP-63 S-acylation control ER morphology

The complex architecture of the endoplasmic reticulum (ER) comprises distinct dynamic features, many at the nanoscale, that enable the coexistence of the nuclear envelope, regions of dense sheets and a branched tubular network that spans the cytoplasm. A key player in the formation of ER sheets is cytoskeleton-linking membrane protein 63 (CLIMP-63). The mechanisms by which CLIMP-63 coordinates ER structure remain elusive. Here, we address the impact of S-acylation, a reversible post-translational lipid modification, on CLIMP-63 cellular distribution and function. Combining native mass-spectrometry, with kinetic analysis of acylation and deacylation, and data-driven mathematical modelling, we obtain in-depth understanding of the CLIMP-63 life cycle. In the ER, it assembles into trimeric units. These occasionally exit the ER to reach the plasma membrane. However, the majority undergoes S-acylation by ZDHHC6 in the ER where they further assemble into highly stable super-complexes. Using super-resolution microscopy and focused ion beam electron microscopy, we show that CLIMP-63 acylation-deacylation controls the abundance and fenestration of ER sheets. Overall, this study uncovers a dynamic lipid post-translational regulation of ER architecture.

The 14-3-3γ isoform binds to and regulates the localization of endoplasmic reticulum (ER) membrane protein TMCC3 for the reticular network of the ER

The reticular network of the endoplasmic reticulum (ER) is formed by connecting ER tubules through three-way junctions and undergoes constant remodeling through formation and loss of the three-way junctions. Transmembrane and coiled-coil domain family 3 (TMCC3), an ER membrane protein localizing at three-way junctions, has been shown to positively regulate formation of the reticular ER network. However, elements that negatively regulate TMCC3 localization have not been characterized. In this study, we report that 14-3-3γ, a phospho-serine/phospho-threonine-binding protein involved in various signal transduction pathways, is a negative regulator of TMCC3. We demonstrate that overexpression of 14-3-3γ reduced localization of TMCC3 to three-way junctions and decreased the number of three-way junctions. TMCC3 bound to 14-3-3γ through the N terminus and had deduced 14-3-3 binding motifs. Additionally, we determined that a TMCC3 mutant substituting alanine for serine to be phosphorylated in the binding motif reduced binding to 14-3-3γ. The TMCC3 mutant was more prone than wildtype TMCC3 to localize at three-way junctions in the cells overexpressing 14-3-3γ. Furthermore, the TMCC3 mutant rescued the ER sheet expansion caused by TMCC3 knockdown less than wild-type TMCC3. Taken together, these results indicate that 14-3-3γ binding negatively regulates localization of TMCC3 to the three-way junctions for the proper reticular ER network, implying that the negative regulation of TMCC3 by 14-3-3γ would underlie remodeling of the reticular network of the ER.

UBXD8 mediates mitochondria-associated degradation to restrain apoptosis and mitophagy

The hexameric AAA-ATPase valosin-containing protein (VCP) is essential for mitochondrial protein quality control. How VCP is recruited to mammalian mitochondria remains obscure. Here we report that UBXD8, an ER- and lipid droplet-localized VCP adaptor, also localizes to mitochondria and locally recruits VCP. UBXD8 associates with mitochondrial and ER ubiquitin E3 ligases and targets their substrates for degradation. Remarkably, both mitochondria- and ER-localized UBXD8 can degrade mitochondrial and ER substrates in cis and in trans. UBXD8 also associates with the TOM complex but is dispensable for translocation-associated degradation. UBXD8 knockout impairs the degradation of the pro-survival protein Mcl1 but surprisingly sensitizes cells to apoptosis and mitochondrial stresses. UBXD8 knockout also hyperactivates mitophagy. We identify pro-apoptotic BH3-only proteins Noxa, Bik, and Bnip3 as novel UBXD8 substrates and determine that UBXD8 inhibits apoptosis via degrading Noxa and restrains mitophagy via degrading Bnip3. Collectively, our characterizations reveal UBXD8 as the major mitochondrial adaptor of VCP and unveil its role in apoptosis and mitophagy regulation.

Caveolin-1β promotes the production of active human microsomal glutathione S-transferase in induced intracellular vesicles inSpodoptera frugiperda21 insect cells

The heterologous expression in Spodoptera frugiperda 21 (Sf21) insect cells of the β isoform of canine caveolin-1 (caveolin-1β), using a baculovirus-based vector, resulted in intracellular vesicles enriched in caveolin-1β. We investigated whether these vesicles could act as membrane reservoirs, and promote the production of an active membrane protein (MP) when co-expressed with caveolin-1β. We chose hMGST1 (human microsomal glutathione S-transferase 1) as the co-expressed MP. It belongs to the membrane-associated proteins in eicosanoid and glutathione metabolism (MAPEG) family of integral MPs, and, as a phase II detoxification enzyme, it catalyzes glutathione conjugation of lipophilic drugs present in the lipid membranes. In addition to its pharmaceutical interest, its GST activity can be conveniently measured. The expression of both MPs were followed by Western blots and membrane fractionation on density gradient, and their cell localization by immunolabeling and transmission electron microscopy. We showed that caveolin-1β kept its capacity to induce intracellular vesicles in the host when co-expressed with hMGST1, and that hMGST1 is in part addressed to these vesicles. Remarkably, a fourfold increase in the amount of active hMGST1 was found in the most enriched membrane fraction, along with an increase of its specific activity by 60% when it was co-expressed with caveolin-1β. Thus, heterologously expressed caveolin-1β was able to induce cytoplasmic vesicles in which a co-expressed exogenous MP is diverted and sequestered, providing a favorable environment for this cargo.

Lunapark ubiquitinates atlastin-2 for the tubular network formation of the endoplasmic reticulum

Endoplasmic reticulum (ER) tubules are interconnected by three-way junctions, resulting in the formation of a tubular ER network. Lunapark (Lnp) localizes to and stabilizes the three-way junctions. The N-terminal cytoplasmic domain in Lnp has a ubiquitin ligase activity. However, the molecular mechanism of how the ubiquitin ligase activity of Lnp is involved in the formation of the tubular ER network remains unknown. In this study, we examined whether the ER membrane proteins responsible for the formation of the tubular ER network are ubiquitinated by Lnp. We found that atlastin-2 (ATL2), an isoform of the ATL family mediating the generation of the three-way junctions by connecting the ER tubules, is a novel substrate for ubiquitination by Lnp. The localization of Lnp at the three-way junctions is important for ubiquitination of ATL2. Lysine 56, 57, 282, and 302 are the potential ubiquitination sites by Lnp. Silencing ATL2 decreased the number of the three-way junctions, and the expression of the ATL2 mutant in which the lysine residues are substituted with arginine failed to rescue the decrease of the three-way junctions in the ATL2 knocked-down cells. These results suggest that Lnp ubiquitinates ATL2 at the three-way junctions for the proper tubular ER network formation.

The Puzzle of Hereditary Spastic Paraplegia: From Epidemiology to Treatment

Inherited neurodegenerative pathology characterized by lower muscle tone and increasing spasticity in the lower limbs is termed hereditary spastic paraplegia (HSP). HSP is associated with changes in about 80 genes and their products involved in various biochemical pathways, such as lipid droplet formation, endoplasmic reticulum shaping, axon transport, endosome trafficking, and mitochondrial function. With the inheritance patterns of autosomal dominant, autosomal recessive, X-linked recessive, and mitochondrial inheritance, HSP is prevalent around the globe at a rate of 1–5 cases in every 100,000 individuals. Recent technology and medical interventions somewhat aid in recognizing and managing the malaise. However, HSP still lacks an appropriate and adequate therapeutic approach. Current therapies are based on the clinical manifestations observed in the patients, for example, smoothing the relaxant spastic muscle and physiotherapies. The limited clinical trial studies contribute to the absence of specific pharmaceuticals for HSPs. Our current work briefly explains the causative genes, epidemiology, underlying mechanism, and the management approach undertaken to date. We have also mentioned the latest approved drugs to summarise the available knowledge on therapeutic strategies for HSP.

Regulation of liver subcellular architecture controls metabolic homeostasis

Cells display complex intracellular organization by compartmentalization of metabolic processes into organelles, yet the resolution of these structures in the native tissue context and their functional consequences are not well understood. Here we resolved the three-dimensional structural organization of organelles in large (more than 2.8 × 10⁵ µm³) volumes of intact liver tissue (15 partial or full hepatocytes per condition) at high resolution (8 nm isotropic pixel size) using enhanced focused ion beam scanning electron microscopy^1,2 imaging followed by deep-learning-based automated image segmentation and 3D reconstruction. We also performed a comparative analysis of subcellular structures in liver tissue of lean and obese mice and found substantial alterations, particularly in hepatic endoplasmic reticulum (ER), which undergoes massive structural reorganization characterized by marked disorganization of stacks of ER sheets³ and predominance of ER tubules. Finally, we demonstrated the functional importance of these structural changes by monitoring the effects of experimental recovery of the subcellular organization on cellular and systemic metabolism. We conclude that the hepatic subcellular organization of the ER architecture are highly dynamic, integrated with the metabolic state and critical for adaptive homeostasis and tissue health.

Dynamic changes in the association between maternal mRNAs and endoplasmic reticulum during ascidian early embryogenesis

Axis formation is one of the most important events occurring at the beginning of animal development. In the ascidian egg, the antero-posterior axis is established at this time owing to a dynamic cytoplasmic movement called cytoplasmic and cortical reorganisation. During this movement, mitochondria, endoplasmic reticulum (ER), and maternal mRNAs (postplasmic/PEM RNAs) are translocated to the future posterior side. Although accumulating evidence indicates the crucial roles played by the asymmetrical localisation of these organelles and the translational regulation of postplasmic/PEM RNAs, the organisation of ER has not been described in sufficient detail to date owing to technical difficulties. In this study, we developed three different multiple staining protocols for visualising the ER in combination with mitochondria, microtubules, or mRNAs in whole-mount specimens. We defined the internally expanded “dense ER” using these protocols and described cisterna-like structures of the dense ER using focused ion beam-scanning electron microscopy. Most importantly, we described the dynamic changes in the colocalisation of postplasmic/PEM mRNAs and dense ER; for example, macho-1 mRNA was detached and excluded from the dense ER during the second phase of ooplasmic movements. These detailed descriptions of the association between maternal mRNA and ER can provide clues for understanding the translational regulation mechanisms underlying axis determination during ascidian early embryogenesis.

Light-Induced Smooth Endoplasmic Reticulum Rearrangement in a Unique Interlaced Compartmental Pattern in Macaca mulatta RPE

Purpose

To investigate light-induced modifications of the smooth endoplasmic reticulum of the RPE in primates.

Methods

Eyes of three terminally anesthetized Rhesus monkeys were exposed to 5000 lux for 10 minutes or kept in the dark. Transmission electron microscopy and electron tomography were conducted on small fragments of retina sampled from different regions of the retina.

Results

RPE cells smooth endoplasmic reticulum shows a previously unknown arrangement characterized by an interlaced compartmental pattern (ICP). Electron tomograms and 3D-modelling demonstrated that the smooth endoplasmic reticulum with an ICP (ICPSER) consisted of four parallel, independent and interwoven networks of tubules arranged as interconnected coiled coils. Its architecture realized a compact labyrinthine structure of tightly packed tubules stabilized by intertubular filamentous tethers. On average, the ICPSER is present in about 14.6% of RPE cells. Although ICPSER was preferentially found in cells located in the peripheral and in the para/perifoveal retina, ICPSER cells significantly increased in number upon light exposure in the para/perifovea and in the fovea.

Conclusions

An ICPSER is apparently a unique feature to primate RPE. Its rapid appearance in the area centralis of the retina upon light exposure suggests a function related to the foveate structure of primate retina or to the diurnal habits of animals that may require additional protection from photo-oxidation or enhanced requests of visual pigments regeneration.

Mitochondrial Dynamics: A Key Role in Neurodegeneration and a Potential Target for Neurodegenerative Disease

In neurodegenerative diseases, neurodegeneration has been related to several mitochondrial dynamics imbalances such as excessive fragmentation of mitochondria, impaired mitophagy, and blocked mitochondria mitochondrial transport in axons. Mitochondria are dynamic organelles, and essential for energy conversion, neuron survival, and cell death. As mitochondrial dynamics have a significant influence on homeostasis, in this review, we mainly discuss the role of mitochondrial dynamics in several neurodegenerative diseases. There is evidence that several mitochondrial dynamics-associated proteins, as well as related pathways, have roles in the pathological process of neurodegenerative diseases with an impact on mitochondrial functions and metabolism. However, specific pathological mechanisms need to be better understood in order to propose new therapeutic strategies targeting mitochondrial dynamics that have shown promise in recent studies.

RTN4B interacting protein FAM134C promotes ER membrane curvature and has a functional role in autophagy

The endoplasmic reticulum (ER) is composed of a controlled ratio of sheets and tubules, which are maintained by several proteins with multiple functions. Reticulons (RTNs), especially RTN4, and DP1/Yop1p family members are known to induce ER membrane curvature. RTN4B is the main RTN4 isoform expressed in nonneuronal cells. In this study, we identified FAM134C as a RTN4B interacting protein in mammalian, nonneuronal cells. FAM134C localized specifically to the ER tubules and sheet edges. Ultrastructural analysis revealed that overexpression of FAM134C induced the formation of unbranched, long tubules or dense globular structures composed of heavily branched narrow tubules. In both cases, tubules were nonmotile. ER tubulation was dependent on the reticulon homology domain (RHD) close to the N-terminus. FAM134C plays a role in the autophagy pathway as its level elevated significantly upon amino acid starvation but not during ER stress. Moreover, FAM134C depletion reduced the number and size of autophagic structures and the amount of ER as a cargo within autophagic structures under starvation conditions. Dominant-negative expression of FAM134C forms with mutated RHD or LC3 interacting region also led to a reduced number of autophagic structures. Our results suggest that FAM134C provides a link between regulation of ER architecture and ER turnover by promoting ER tubulation required for subsequent ER fragmentation and engulfment into autophagosomes.

Endoplasmic reticulum stress, cell death and tumor: Association between endoplasmic reticulum stress and the apoptosis pathway in tumors (Review)

External and internal stimuli are often involved in the pathogenesis of tumors, and the deterioration of endoplasmic reticulum (ER) function within cells is also an important etiological factor of tumorigenesis resulting in the impairment of the endoplasmic reticulum, which is termed ER stress. The ER is an organelle that serves a crucial role in the process of protein synthesis and maturation, and also acts as a reservoir of calcium to maintain intracellular Ca2+ homeostasis. ER stress has been revealed to serve a critical role in tumorigenesis. In the present review, the association between ER stress‑related pathways and tumor cell apoptosis is examined. Primarily, the role of ER stress in tumor cell apoptosis is discussed, and it is stipulated that ER stress, induced by drugs both directly and indirectly, promotes tumor cell apoptosis.

TMEM147 interacts with lamin B receptor, regulates its localization and levels, and affects cholesterol homeostasis

The structurally and functionally complex endoplasmic reticulum (ER) hosts critical processes including lipid synthesis. Here, we focus on the functional characterization of transmembrane protein TMEM147, and report that it localizes at the ER and nuclear envelope in HeLa cells. Silencing of TMEM147 drastically reduces the level of lamin B receptor (LBR) at the inner nuclear membrane and results in mistargeting of LBR to the ER. LBR possesses a modular structure and corresponding bifunctionality, acting in heterochromatin organization via its N-terminus and in cholesterol biosynthesis via its sterol-reductase C-terminal domain. We show that TMEM147 physically interacts with LBR, and that the C-terminus of LBR is essential for their functional interaction. We find that TMEM147 also physically interacts with the key sterol reductase DHCR7, which is involved in cholesterol biosynthesis. Similar to what was seen for LBR, TMEM147 downregulation results in a sharp decline of DHCR protein levels and co-ordinate transcriptional decreases of LBR and DHCR7 expression. Consistent with this, lipidomic analysis upon TMEM147 silencing identified changes in cellular cholesterol levels, cholesteryl ester levels and profile, and in cellular cholesterol uptake, raising the possibility that TMEM147 is an important new regulator of cholesterol homeostasis in cells.This article has an associated First Person interview with the first author of the paper.

TMCC3 localizes at the three-way junctions for the proper tubular network of the endoplasmic reticulum

The tubular network of the endoplasmic reticulum (ER) is formed by connecting ER tubules through three-way junctions. Two classes of the conserved ER membrane proteins, atlastins and lunapark, have been shown to reside at the three-way junctions so far and be involved in the generation and stabilization of the three-way junctions. In this study, we report TMCC3 (transmembrane and coiled-coil domain family 3), a member of the TEX28 family, as another ER membrane protein that resides at the three-way junctions in mammalian cells. When the TEX28 family members were transfected into U2OS cells, TMCC3 specifically localized at the three-way junctions in the peripheral ER. TMCC3 bound to atlastins through the C-terminal transmembrane domains. A TMCC3 mutant lacking the N-terminal coiled-coil domain abolished localization to the three-way junctions, suggesting that TMCC3 localized independently of binding to atlastins. TMCC3 knockdown caused a decrease in the number of three-way junctions and expansion of ER sheets, leading to a reduction of the tubular ER network in U2OS cells. The TMCC3 knockdown phenotype was partially rescued by the overexpression of atlastin-2, suggesting that TMCC3 knockdown would decrease the activity of atlastins. These results indicate that TMCC3 localizes at the three-way junctions for the proper tubular ER network.

Targeting mitochondrial calcium pathways as a potential treatment against Parkinson's disease

Parkinson's disease (PD) is a major health problem worldwide affecting millions of people and is a result of neurodegeneration in a small part of the brain known as substantia nigra pars compacta. Aberration in mitochondrial Ca²⁺ homeostasis plays, among several other factors, an important role for the neuronal loss in PD. Mitochondria are vital for cellular physiology, e.g. for ATP generation, and mitochondrial Ca²⁺ is a key player in cell functioning and survival. Mitochondrial Ca²⁺ homeostasis is maintained by a fine balance between the activities of proteins mediating the influx and efflux of Ca²⁺ across mitochondrial membranes. Malfunctioning of these proteins leading to Ca²⁺ overload promotes ROS generation, which induces cell death by triggering the opening of mitochondrial permeability transition pore. Till now PD remains incurable and the "gold standard" drug which can only delays the disease progression is l-Dopa from the 1960s and therefore, the situation warrants the search for novel targets for the treatment of the PD patients. In this review, we summarize the current views that suggest mitochondrial Ca²⁺ regulatory pathways are good candidates for the treatment of PD.

Immunoglobulin expression and the humoral immune response is regulated by the non-canonical poly(A) polymerase TENT5C

TENT5C is a non-canonical cytoplasmic poly(A) polymerase highly expressed by activated B cells to suppress their proliferation. Here we measure the global distribution of poly(A) tail lengths in responsive B cells using a Nanopore direct RNA-sequencing approach, showing that TENT5C polyadenylates immunoglobulin mRNAs regulating their half-life and consequently steady-state levels. TENT5C is upregulated in differentiating plasma cells by innate signaling. Compared with wild-type, Tent5c^−/− mice produce fewer antibodies and have diminished T-cell-independent immune response despite having more CD138^high plasma cells as a consequence of accelerated differentiation. B cells from Tent5c^−/− mice also have impaired capacity of the secretory pathway, with reduced ER volume and unfolded protein response. Importantly, these functions of TENT5C are dependent on its enzymatic activity as catalytic mutation knock-in mice display the same defect as Tent5c^−/−. These findings define the role of the TENT5C enzyme in the humoral immune response.

Regulating polyadenylation is important for mRNA stability, which can in turn affect B cell maturation and humoral immune responses. Here the authors use Nanopore poly(A) sequencing to explore the importance of the cytoplasmic poly(A) polymerase TENT5C, particularly in the production of immunoglobulins.

ER-Phagy: Quality Control and Turnover of Endoplasmic Reticulum

The endoplasmic reticulum (ER) is the largest organelle in cells and has fundamental functions, such as folding, processing, and trafficking of proteins, cellular metabolism, and ion storage. To maintain its function, it is turned over constitutively, and even more actively under certain stress conditions. Quality control of the ER is mediated primarily by two pathways: the ubiquitin-proteasome system and autophagy (termed 'ER-phagy'). The identification of ER-phagy adaptor molecules has shed light on the mechanisms and physiological significance of ER-phagy. Here, we describe recent findings on various types of ER-phagy and present unanswered questions related to their mechanism and regulation.

Mitogen-Activated Protein Kinase (MAPK) and Obesity-Related Cancer

Obesity is a major public health concern worldwide. The increased risk of certain types of cancer is now an established deleterious consequence of obesity, although the molecular mechanisms of this are not completely understood. In this review, we aim to explore the links between MAPK signalling and obesity-related cancer. We focus mostly on p38 and JNK MAPK, as the role of ERK remains unclear. These links are seen through the implication of MAPK in obesity-related immune paralysis as well as through effects on the endoplasmic reticulum stress response and activation of aromatase. By way of example, we highlight areas of interest and possibilities for future research in endometrioid endometrial cancer and hepatocellular carcinoma associated with non-alcoholic fatty liver disease (NAFLD), non-alcoholic steatohepatitis (NASH) and MAPK.

Variants in Miro1 Cause Alterations of ER-Mitochondria Contact Sites in Fibroblasts from Parkinson's Disease Patients

Background: Although most cases of Parkinson´s disease (PD) are idiopathic with unknown cause, an increasing number of genes and genetic risk factors have been discovered that play a role in PD pathogenesis. Many of the PD-associated proteins are involved in mitochondrial quality control, e.g., PINK1, Parkin, and LRRK2, which were recently identified as regulators of mitochondrial-endoplasmic reticulum (ER) contact sites (MERCs) linking mitochondrial homeostasis to intracellular calcium handling. In this context, Miro1 is increasingly recognized to play a role in PD pathology. Recently, we identified the first PD patients carrying mutations in RHOT1, the gene coding for Miro1. Here, we describe two novel RHOT1 mutations identified in two PD patients and the characterization of the cellular phenotypes. Methods: Using whole exome sequencing we identified two PD patients carrying heterozygous mutations leading to the amino acid exchanges T351A and T610A in Miro1. We analyzed calcium homeostasis and MERCs in detail by live cell imaging and immunocytochemistry in patient-derived fibroblasts. Results: We show that fibroblasts expressing mutant T351A or T610A Miro1 display impaired calcium homeostasis and a reduced amount of MERCs. All fibroblast lines from patients with pathogenic variants in Miro1, revealed alterations of the structure of MERCs. Conclusion: Our data suggest that Miro1 is important for the regulation of the structure and function of MERCs. Moreover, our study supports the role of MERCs in the pathogenesis of PD and further establishes variants in RHOT1 as rare genetic risk factors for neurodegeneration.

Regulation of autophagy by canonical and non-canonical ER stress responses

Cancer cells encounter numerous stresses that pose a threat to their survival. Tumor microenviroment stresses that perturb protein homeostasis can produce endoplasmic reticulum (ER) stress, which can be counterbalanced by triggering the unfolded protein response (UPR) which is considered the canonical ER stress response. The UPR is characterized by three major proteins that lead to specific changes in transcriptional and translational programs in stressed cells. Activation of the UPR can induce apoptosis, but also can induce cytoprotective programs such as autophagy. There is increasing appreciation for the role that UPR-induced autophagy plays in supporting tumorigenesis and cancer therapy resistance. More recently several new pathways that connect cell stresses, components of the UPR and autophagy have been reported, which together can be viewed as non-canonical ER stress responses. Here we review recent findings on the molecular mechanisms by which canonical and non-canonical ER stress responses can activate cytoprotective autophagy and contribute to tumor growth and therapy resistance. Autophagy has been identified as a druggable pathway, however the components of autophagy (ATG genes) have proven difficult to drug. It may be the case that targeting the UPR or non-canonical ER stress programs can more effectively block cytoprotective autophagy to enhance cancer therapy. A deeper understanding of these pathways could provide new therapeutic targets in cancer.

ER morphology and endo-lysosomal crosstalk: Functions and disease implications

The endoplasmic reticulum (ER) is a continuous endomembrane system comprising the nuclear envelope, ribosome-studded sheets, dense peripheral matrices, and an extensive polygonal network of interconnected tubules. In addition to performing numerous critical cellular functions, the ER makes extensive contacts with other organelles, including endosomes and lysosomes. The molecular and functional characterization of these contacts has advanced significantly over the past several years. These contacts participate in key functions such as cholesterol transfer, endosome tubule fission, and Ca²⁺ exchange. Disruption of key proteins at these sites can result in often severe diseases, particularly those affecting the nervous system.

Mechanistic Connections between Endoplasmic Reticulum (ER) Redox Control and Mitochondrial Metabolism

The past decade has seen the emergence of endoplasmic reticulum (ER) chaperones as key determinants of contact formation between mitochondria and the ER on the mitochondria-associated membrane (MAM). Despite the known roles of ER–mitochondria tethering factors like PACS-2 and mitofusin-2, it is not yet entirely clear how they mechanistically interact with the ER environment to determine mitochondrial metabolism. In this article, we review the mechanisms used to communicate ER redox and folding conditions to the mitochondria, presumably with the goal of controlling mitochondrial metabolism at the Krebs cycle and at the electron transport chain, leading to oxidative phosphorylation (OXPHOS). To achieve this goal, redox nanodomains in the ER and the interorganellar cleft influence the activities of ER chaperones and Ca²⁺-handling proteins to signal to mitochondria. This mechanism, based on ER chaperones like calnexin and ER oxidoreductases like Ero1α, controls reactive oxygen production within the ER, which can chemically modify the proteins controlling ER–mitochondria tethering, or mitochondrial membrane dynamics. It can also lead to the expression of apoptotic or metabolic transcription factors. The link between mitochondrial metabolism and ER homeostasis is evident from the specific functions of mitochondria–ER contact site (MERC)-localized Ire1 and PERK. These functions allow these two transmembrane proteins to act as mitochondria-preserving guardians, a function that is apparently unrelated to their functions in the unfolded protein response (UPR). In scenarios where ER stress cannot be resolved via the activation of mitochondrial OXPHOS, MAM-localized autophagosome formation acts to remove defective portions of the ER. ER chaperones such as calnexin are again critical regulators of this MERC readout.

CAND1 regulates lunapark for the proper tubular network of the endoplasmic reticulum

Endoplasmic reticulum (ER) tubules connect each other by three-way junctions, resulting in a tubular ER network. Oligomerization of three-way junction protein lunapark (Lnp) is important for its localization and the three-way junction stability. On the other hand, Lnp has an N-terminal ubiquitin ligase activity domain, which is also important for the three-way junction localization. To understand the mode of action of Lnp, we isolated Cullin-associated and neddylation-dissociated 1 (CAND1), a regulator of Skp1-Cul1-F-box (SCF) ubiquitin ligase, as a Lnp-binding protein by affinity chromatography. CAND1 and Lnp form a higher-molecular-weight complex in vitro, while they do not co-localize at the three-way junctions. CAND1 reduces the auto-ubiquitination activity of Lnp. CAND1 knockdown enhances proteasomal degradation of Lnp and reduces the tubular ER network in mammalian cells. These results suggest that CAND1 has the potency to promote the formation of the higher-molecular-weight complex with Lnp and reduce the auto-ubiquitination activity of Lnp, thereby regulating the three-way junction stability of the tubular ER network.

Interorganellar calcium signaling in the regulation of cell metabolism: A cancer perspective

Organelles were originally considered to be individual cellular compartments with a defined organization and function. However, recent studies revealed that organelles deeply communicate within each other via Ca²⁺ exchange. This communication, mediated by specialized membrane regions in close apposition between two organelles, regulate cellular functions, including metabolism and cell fate decisions. Advances in microscopy techniques, molecular biology and biochemistry have increased our understanding of these interorganelle platforms. Research findings suggest that interorganellar Ca²⁺ signaling, which is altered in cancer, influences tumorigenesis and tumor progression by controlling cell death programs and metabolism. Here, we summarize the available data on the existence and composition of interorganelle platforms connecting the endoplasmic reticulum with mitochondria, the plasma membrane, or endolysosomes. Finally, we provide a timely overview of the potential function of interorganellar Ca²⁺ signaling in maintaining cellular homeostasis.

Compartmentalization and Selective Tagging for Disposal of Misfolded Glycoproteins

The ability of mammalian cells to correctly identify and degrade misfolded secretory proteins, most of them bearing N-glycans, is crucial for their correct function and survival. An inefficient disposal mechanism results in the accumulation of misfolded proteins and consequent endoplasmic reticulum (ER) stress. N-glycan processing creates a code that reveals the folding status of each molecule, enabling continued folding attempts or targeting of the doomed glycoprotein for disposal. We review here the main steps involved in the accurate processing of unfolded glycoproteins. We highlight recent data suggesting that the processing is not stochastic, but that there is selective accelerated glycan trimming on misfolded glycoprotein molecules.

TPR-containing proteins control protein organization and homeostasis for the endoplasmic reticulum

The endoplasmic reticulum (ER) is a complex, multifunctional organelle comprised of a continuous membrane and lumen that is organized into a number of functional regions. It plays various roles including protein translocation, folding, quality control, secretion, calcium signaling, and lipid biogenesis. Cellular protein homeostasis is maintained by a complicated chaperone network, and the largest functional family within this network consists of proteins containing tetratricopeptide repeats (TPRs). TPRs are well-studied structural motifs that mediate intermolecular protein-protein interactions, supporting interactions with a wide range of ligands or substrates. Seven TPR-containing proteins have thus far been shown to localize to the ER and control protein organization and homeostasis within this multifunctional organelle. Here, we discuss the roles of these proteins in controlling ER processes and organization. The crucial roles that TPR-containing proteins play in the ER are highlighted by diseases or defects associated with their mutation or disruption.

Heterogeneous translational landscape of the endoplasmic reticulum revealed by ribosome proximity labeling and transcriptome analysis

The endoplasmic reticulum (ER) is a nexus for mRNA localization and translation, and recent studies have demonstrated that ER-bound ribosomes also play a transcriptome-wide role in regulating proteome composition. The Sec61 translocon (SEC61) serves as the receptor for ribosomes that translate secretory/integral membrane protein-encoding mRNAs, but whether SEC61 also serves as a translation site for cytosolic protein-encoding mRNAs remains unknown. Here, using a BioID proximity-labeling approach in HEK293T Flp-In cell lines, we examined interactions between ER-resident proteins and ribosomes in vivo Using in vitro analyses, we further focused on bona fide ribosome interactors (i.e. SEC61) and ER proteins (ribophorin I, leucine-rich repeat-containing 59 (LRRC59), and SEC62) previously implicated in associating with ribosomes. We observed labeling of ER-bound ribosomes with the SEC61β and LRRC59 BioID reporters, comparatively modest labeling with the ribophorin I reporter, and no labeling with the SEC62 reporter. A biotin pulse-chase/subcellular fractionation approach to examine ribosome exchange at the SEC61β and LRRC59 sites revealed that, at steady state, ribosomes at these sites comprise both rapid- and slow-exchanging pools. Global translational initiation arrest elicited by the inhibitor harringtonine accelerated SEC61β reporter-labeled ribosome exchange. RNA-Seq analyses of the mRNAs associated with SEC61β- and LRRC59-labeled ribosomes revealed both site-enriched and shared mRNAs and further established that the ER has a transcriptome-wide role in regulating proteome composition. These results provide evidence that ribosomes interact with the ER membrane via multiple modes and suggest regulatory mechanisms that control global proteome composition via ER membrane-bound ribosomes.

Influenza A virus ribonucleoproteins form liquid organelles at endoplasmic reticulum exit sites

Influenza A virus has an eight-partite RNA genome that during viral assembly forms a complex containing one copy of each RNA. Genome assembly is a selective process driven by RNA-RNA interactions and is hypothesized to lead to discrete punctate structures scattered through the cytosol. Here, we show that contrary to the accepted view, formation of these structures precedes RNA-RNA interactions among distinct viral ribonucleoproteins (vRNPs), as they assemble in cells expressing only one vRNP type. We demonstrate that these viral inclusions display characteristics of liquid organelles, segregating from the cytosol without a delimitating membrane, dynamically exchanging material and adapting fast to environmental changes. We provide evidence that viral inclusions develop close to endoplasmic reticulum (ER) exit sites, depend on continuous ER-Golgi vesicular cycling and do not promote escape to interferon response. We propose that viral inclusions segregate vRNPs from the cytosol and facilitate selected RNA-RNA interactions in a liquid environment.

Influenza A virus forms cytosolic inclusions containing viral ribonucleoproteins. Here, the authors show that viral inclusions form juxtaposed the endoplasmic reticulum and have liquid properties, likely constituting sites of assembly of epidemic and pandemic influenza genomes.

The enigmatic ATP supply of the endoplasmic reticulum

The endoplasmic reticulum (ER) is a functionally and morphologically complex cellular organelle largely responsible for a variety of crucial functions, including protein folding, maturation and degradation. Furthermore, the ER plays an essential role in lipid biosynthesis, dynamic Ca²⁺ storage, and detoxification. Malfunctions in ER‐related processes are responsible for the genesis and progression of many diseases, such as heart failure, cancer, neurodegeneration and metabolic disorders. To fulfill many of its vital functions, the ER relies on a sufficient energy supply in the form of adenosine‐5′‐triphosphate (ATP), the main cellular energy source. Despite landmark discoveries and clarification of the functional principles of ER‐resident proteins and key ER‐related processes, the mechanism underlying ER ATP transport remains somewhat enigmatic. Here we summarize ER‐related ATP‐consuming processes and outline our knowledge about the nature and function of the ER energy supply.

Modeling the Geometry and Dynamics of the Endoplasmic Reticulum Network

The endoplasmic reticulum (ER) is an intricate network that pervades the entire cortex of plant cells and its geometric shape undergoes drastic changes. This paper proposes a mathematical model to reconstruct geometric network dynamics by combining the node movements within the network and topological changes engendered by these nodes. The network topology in the model is determined by a modified optimization procedure from the work (Lemarchand, et al. 2014) which minimizes the total length taking into account both degree and angle constraints, beyond the conditions of connectedness and planarity. A novel feature for solving our optimization problem is the use of "lifted" angle constraints, which allows one to considerably reduce the solution runtimes. Using this optimization technique and a Langevin approach for the branching node movement, the simulated network dynamics represent the ER network dynamics observed under latrunculin B treated condition and recaptures features such as the appearance/disappearance of loops within the ER under the native condition. The proposed modeling approach allows quantitative comparison of networks between the model and experimental data based on topological changes induced by node dynamics. An increased temporal resolution of experimental data will allow a more detailed comparison of network dynamics using this modeling approach.

Redox crosstalk at endoplasmic reticulum (ER) membrane contact sites (MCS) uses toxic waste to deliver messages

Many cellular redox reactions housed within mitochondria, peroxisomes and the endoplasmic reticulum (ER) generate hydrogen peroxide (H₂O₂) and other reactive oxygen species (ROS). The contribution of each organelle to the total cellular ROS production is considerable, but varies between cell types and also over time. Redox-regulatory enzymes are thought to assemble at a “redox triangle” formed by mitochondria, peroxisomes and the ER, assembling “redoxosomes” that sense ROS accumulations and redox imbalances. The redoxosome enzymes use ROS, potentially toxic by-products made by some redoxosome members themselves, to transmit inter-compartmental signals via chemical modifications of downstream proteins and lipids. Interestingly, important components of the redoxosome are ER chaperones and oxidoreductases, identifying ER oxidative protein folding as a key ROS producer and controller of the tri-organellar membrane contact sites (MCS) formed at the redox triangle. At these MCS, ROS accumulations could directly facilitate inter-organellar signal transmission, using ROS transporters. In addition, ROS influence the flux of Ca²⁺ ions, since many Ca²⁺ handling proteins, including inositol 1,4,5 trisphosphate receptors (IP₃Rs), SERCA pumps or regulators of the mitochondrial Ca²⁺ uniporter (MCU) are redox-sensitive. Fine-tuning of these redox and ion signaling pathways might be difficult in older organisms, suggesting a dysfunctional redox triangle may accompany the aging process.

Localization and Processing of the Amyloid-β Protein Precursor in Mitochondria-Associated Membranes

Alteration of mitochondria-associated membranes (MAMs) has been proposed to contribute to the pathogenesis of Alzheimer’s disease (AD). We studied herein the subcellular distribution, the processing, and the protein interactome of the amyloid-β protein precursor (AβPP) and its proteolytic products in MAMs. We reveal that AβPP and its catabolites are present in MAMs in cellular models overexpressing wild type AβPP or AβPP harboring the double Swedish or London familial AD mutations, and in brains of transgenic mice model of AD. Furthermore, we evidenced that both β- and γ-secretases are present and harbor AβPP processing activities in MAMs. Interestingly, cells overexpressing APP_swe show increased ER-mitochondria contact sites. We also document increased neutral lipid accumulation linked to Aβ production and reversed by inhibiting β- or γ-secretases. Using a proteomic approach, we show that AβPP and its catabolites interact with key proteins of MAMs controlling mitochondria and ER functions. These data highlight the role of AβPP processing and proteomic interactome in MAMs deregulation taking place in AD.

Defective STIM-mediated store operated Ca2+ entry in hepatocytes leads to metabolic dysfunction in obesity

Defective Ca²⁺ handling is a key mechanism underlying hepatic endoplasmic reticulum (ER) dysfunction in obesity. ER Ca²⁺ level is in part monitored by the store-operated Ca²⁺ entry (SOCE) system, an adaptive mechanism that senses ER luminal Ca²⁺ concentrations through the STIM proteins and facilitates import of the ion from the extracellular space. Here, we show that hepatocytes from obese mice displayed significantly diminished SOCE as a result of impaired STIM1 translocation, which was associated with aberrant STIM1 O-GlycNAcylation. Primary hepatocytes deficient in STIM1 exhibited elevated cellular stress as well as impaired insulin action, increased glucose production and lipid droplet accumulation. Additionally, mice with acute liver deletion of STIM1 displayed systemic glucose intolerance. Conversely, over-expression of STIM1 in obese mice led to increased SOCE, which was sufficient to improve systemic glucose tolerance. These findings demonstrate that SOCE is an important mechanism for healthy hepatic Ca²⁺ balance and systemic metabolic control.

Obesity is a chronic metabolic disorder. Some people’s genetics make them more vulnerable to the condition, and it is generally caused by eating too much and moving too little. The resulting surplus of nutrients affects the cells and organs of the body in several adverse ways. For example, excessive nutrients impair a compartment within cells called the endoplasmic reticulum. This compartment is where many proteins and fats are made and transported. It is also the site for a lot of metabolic processes, and the main place in the cell where calcium ions are stored. Many proteins need calcium ions to work properly, including metabolic enzymes. In obesity, the endoplasmic reticulum becomes less able to store calcium ions.

A protein called STIM1 senses and regulates the levels of calcium ions in the endoplasmic reticulum. When calcium levels drop, STIM1 moves along the endoplasmic reticulum membrane towards the part that is next to the cell surface. Here, STIM1 joins up with a calcium channel called Orai1. The STIM1-Orai1 complex allows calcium ions to enter the cell and replenish its levels in the endoplasmic reticulum.

Arruda, Pers et al. have now asked if STIM1 is altered in obesity and, if so, whether it contributes to the endoplasmic reticulum’s inability to maintain proper calcium levels. High-resolution microscopy and biochemical techniques confirmed that STIM1 is indeed compromised in liver cells from obese mice. In these cells, STIM1 was found in unusual small clusters. It also could not move along the endoplasmic reticulum membrane when calcium levels dropped. As a result of these navigational errors, STIM1 failed to couple with Orai1, meaning less calcium could enter the cell. Further work identified that a small sugar molecule that is added onto STIM1 in obesity is behind its reduced ability to move accurately.

Arruda, Pers et al. next created mice that lacked STIM1 in their liver. These mice showed signs of metabolic abnormalities. Notably, when STIM1 levels were experimentally increased in obese mice, it restored calcium levels in the endoplasmic reticulum closer to normal, and improved metabolism too.

Thus, regulating calcium levels in the endoplasmic reticulum via proteins such as STIM1 is essential for maintaining a healthy metabolism. Interventions to correct calcium levels may have therapeutic promise to combat metabolic diseases.

Protein Localization at Mitochondria-ER Contact Sites in Basal and Stress Conditions

Mitochondria-endoplasmic reticulum (ER) contacts (MERCs) are sites at which the outer mitochondria membrane and the Endoplasmic Reticulum surface run in parallel at a constant distance. The juxtaposition between these organelles determines several intracellular processes such as to name a few, Ca²⁺ and lipid homeostasis or autophagy. These specific tasks can be exploited thanks to the enrichment (or re-localization) of dedicated proteins at these interfaces. Recent proteomic studies highlight the tissue specific composition of MERCs, but the overall mechanisms that control MERCs plasticity remains unclear. Understanding how proteins are targeted at these sites seems pivotal to clarify such contextual function of MERCs. This review aims to summarize the current knowledge on protein localization at MERCs and the possible contribution of the mislocalization of MERCs components to human disorders.

The ins and outs of endoplasmic reticulum-controlled lipid biosynthesis

Endoplasmic reticulum (ER)-localized enzymes synthesize the vast majority of cellular lipids. The ER therefore has a major influence on cellular lipid biomass and balances the production of different lipid categories, classes, and species. Signals from outside and inside the cell are directed to ER-localized enzymes, and lipid enzyme activities are defined by the integration of internal, homeostatic, and external information. This allows ER-localized lipid synthesis to provide the cell with membrane lipids for growth, proliferation, and differentiation-based changes in morphology and structure, and to maintain membrane homeostasis across the cell. ER enzymes also respond to physiological signals to drive carbohydrates and nutritionally derived lipids into energy-storing triglycerides. In this review, we highlight some key regulatory mechanisms that control ER-localized enzyme activities in animal cells. We also discuss how they act in concert to maintain cellular lipid homeostasis, as well as how their dysregulation contributes to human disease.

Regulation of Calcium Fluxes by GPX8, a Type-II Transmembrane Peroxidase Enriched at the Mitochondria-Associated Endoplasmic Reticulum Membrane

Glutathione peroxidases (GPXs) are enzymes that are present in almost all organisms with the primary function of limiting peroxide accumulation. In mammals, two of the eight members (GPX7 and GPX8) reside in the endoplasmic reticulum (ER). A peculiar feature of GPX8 is the concomitant presence of a conserved N-terminal transmembrane domain (TMD) and a C-terminal KDEL-like motif for ER localization.

AIMS

Investigating whether and how GPX8 impacts Ca²⁺ homeostasis and signaling.

RESULTS

We show that GPX8 is enriched in mitochondria-associated membranes and regulates Ca²⁺ storage and fluxes. Its levels correlate with [Ca²⁺]_ER, and cytosolic and mitochondrial Ca²⁺ fluxes. GPX7, which lacks a TMD, does not share these properties. Deleting or replacing the GPX8 TMD with an unrelated N-terminal membrane integration sequence abolishes all effects on Ca²⁺ fluxes, whereas appending the GPX8 TMD to GPX7 transfers the Ca²⁺-regulating properties. Innovation and Conclusion: The notion that the TMD of GPX8, in addition to its enzymatic activity, is essential for regulating Ca²⁺ dynamics reveals a novel level of integration between redox-related proteins and Ca²⁺ signaling/homeostasis. Antioxid. Redox Signal. 27, 583-595.

Regulation of Endoplasmic Reticulum-Mitochondria Ca2+ Transfer and Its Importance for Anti-Cancer Therapies

Inter-organelle membrane contact sites are emerging as major sites for the regulation of intracellular Ca²⁺ concentration and distribution. Here, extracellular stimuli operate on a wide array of channels, pumps, and ion exchangers to redistribute intracellular Ca²⁺ among several compartments. The resulting highly defined spatial and temporal patterns of Ca²⁺ movement can be used to elicit specific cellular responses, including cell proliferation, migration, or death. Plasma membrane (PM) also can directly contact mitochondria and endoplasmic reticulum (ER) through caveolae, small invaginations of the PM that ensure inter-organelle contacts, and can contribute to the regulation of numerous cellular functions through scaffolding proteins such as caveolins. PM and ER organize specialized junctions. Here, many components of the receptor-dependent Ca²⁺ signals are clustered, including the ORAI1-stromal interaction molecule 1 complex. This complex constitutes a primary mechanism for Ca²⁺ entry into non-excitable cells, modulated by intracellular Ca²⁺. Several contact sites between the ER and mitochondria, termed mitochondria-associated membranes, show a very complex and specialized structure and host a wide number of proteins that regulate Ca²⁺ transfer. In this review, we summarize current knowledge of the particular action of several oncogenes and tumor suppressors at these specialized check points and analyze anti-cancer therapies that specifically target Ca²⁺ flow at the inter-organelle contacts to alter the metabolism and fate of the cancer cell.

Modulation of mitochondrial dysfunction and endoplasmic reticulum stress are key mechanisms for the wide-ranging actions of epoxy fatty acids and soluble epoxide hydrolase inhibitors

The arachidonic acid cascade is arguably the most widely known biologic regulatory pathway. Decades after the seminal discoveries involving its cyclooxygenase and lipoxygenase branches, studies of this cascade remain an active area of research. The third and less widely known branch, the cytochrome P450 pathway leads to highly active oxygenated lipid mediators, epoxy fatty acids (EpFAs) and hydroxyeicosatetraenoic acids (HETEs), which are of similar potency to prostanoids and leukotrienes. Unlike the COX and LOX branches, no pharmaceuticals currently are marketed targeting the P450 branch. However, data support therapeutic benefits from modulating these regulatory lipid mediators. This is being approached by stabilizing or mimicking the EpFAs or even by altering the diet. These approaches lead to predominantly beneficial effects on a wide range of apparently unrelated states resulting in an enigma of how this small group of natural chemical mediators can have such diverse effects. EpFAs are degraded by soluble epoxide hydrolase (sEH) and stabilized by inhibiting this enzyme. In this review, we focus on interconnected aspects of reported mechanisms of action of EpFAs and inhibitors of soluble epoxide hydrolase (sEHI). The sEHI and EpFAs are commonly reported to maintain homeostasis under pathological conditions while remaining neutral under normal physiological conditions. Here we provide a conceptual framework for the unique and broad range of biological activities ascribed to epoxy fatty acids. We argue that their mechanism of action pivots on their ability to prevent mitochondrial dysfunction, to reduce subsequent ROS formation and to block resulting cellular signaling cascades, primarily the endoplasmic reticulum stress. By stabilizing the mitochondrial - ROS - ER stress axis, the range of activity of EpFAs and sEHI display an overlap with the disease conditions including diabetes, fibrosis, chronic pain, cardiovascular and neurodegenerative diseases, for which the above outlined mechanisms play key roles.

Endoplasmic reticulum chaperones tweak the mitochondrial calcium rheostat to control metabolism and cell death

The folding of secretory proteins is a well-understood mechanism, based on decades of research on endoplasmic reticulum (ER) chaperones. These chaperones interact with newly imported polypeptides close to the ER translocon. Classic examples for these proteins include the immunoglobulin binding protein (BiP/GRP78), and the lectins calnexin and calreticulin. Although not considered chaperones per se, the ER oxidoreductases of the protein disulfide isomerase (PDI) family complete the folding job by catalyzing the formation of disulfide bonds through cysteine oxidation. Research from the past decade has demonstrated that ER chaperones are multifunctional proteins. The regulation of ER-mitochondria Ca²⁺ crosstalk is one of their additional functions, as shown for calnexin, BiP/GRP78 or the oxidoreductases Ero1α and TMX1. This function depends on interactions of this group of proteins with the ER Ca²⁺ handling machinery. This novel function makes perfect sense for two reasons: i. It allows ER chaperones to control mitochondrial apoptosis instantly without a lengthy bypass involving the upregulation of pro-apoptotic transcription factors via the unfolded protein response (UPR); and ii. It allows the ER protein folding machinery to fine-tune ATP import via controlling the speed of mitochondrial oxidative phosphorylation. Therefore, the role of ER chaperones in regulating ER-mitochondria Ca²⁺ flux identifies the progression of secretory protein folding as a central regulator of cell survival and death, at least in cell types that secrete large amount of proteins. In other cell types, ER protein folding might serve as a sentinel mechanism that monitors cellular well-being to control cell metabolism and apoptosis. The selenoprotein SEPN1 is a classic example for such a role. Through the control of ER-mitochondria Ca²⁺-flux, ER chaperones and folding assistants guide cellular apoptosis and mitochondrial metabolism.

Endoplasmic reticulum-mitochondria Ca2+ crosstalk in the control of the tumor cell fate

Mitochondria-associated membranes are juxtaposed between the endoplasmic reticulum and mitochondria and have been identified as a critical hub in the regulation of apoptosis and tumor growth. One key function of mitochondria-associated membranes is to provide asylum to a number of proteins with tumor suppressor and oncogenic properties. In this review, we discuss how Ca²⁺ flux manipulation represents the primary mechanism underlying the action of several oncogenes and tumor-suppressor genes and how these networks might be manipulated to provide novel therapies for cancer. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Alzheimer Disease

The most widely accepted hypothesis to explain the pathogenesis of Alzheimer disease (AD) is the amyloid cascade, in which the accumulation of extraneuritic plaques and intracellular tangles plays a key role in driving the course and progression of the disease. However, there are other biochemical and morphological features of AD, including altered calcium, phospholipid, and cholesterol metabolism and altered mitochondrial dynamics and function that often appear early in the course of the disease, prior to plaque and tangle accumulation. Interestingly, these other functions are associated with a subdomain of the endoplasmic reticulum (ER) called mitochondria-associated ER membranes (MAM). MAM, which is an intracellular lipid raft-like domain, is closely apposed to mitochondria, both physically and biochemically. These MAM-localized functions are, in fact, increased significantly in various cellular and animal models of AD and in cells from AD patients, which could help explain the biochemical and morphological alterations seen in the disease. Based on these and other observations, a strong argument can be made that increased ER-mitochondria connectivity and increased MAM function are fundamental to AD pathogenesis.

Cell Signaling and Stress Responses

Stress-signaling pathways are evolutionarily conserved and play an important role in the maintenance of homeostasis. These pathways are also critical for adaptation to new cellular environments. The endoplasmic reticulum (ER) unfolded protein response (UPR) is activated by biosynthetic stress and leads to a compensatory increase in ER function. The JNK and p38 MAPK signaling pathways control adaptive responses to intracellular and extracellular stresses, including environmental changes such as UV light, heat, and hyperosmotic conditions, and exposure to inflammatory cytokines. Metabolic stress caused by a high-fat diet represents an example of a stimulus that coordinately activates both the UPR and JNK/p38 signaling pathways. Chronic activation of these stress-response pathways ultimately causes metabolic changes associated with obesity and altered insulin sensitivity. Stress-signaling pathways, therefore, represent potential targets for therapeutic intervention in the metabolic stress response and other disease processes.

TMX1 determines cancer cell metabolism as a thiol-based modulator of ER-mitochondria Ca2+ flux

Cancer cells are critically dependent on ER–mitochondria Ca²⁺ flux that regulates their bioenergetics. Here, Raturi et al. identify the ER oxidoreductase TMX1 as a thiol-dependent regulator of this intracellular signaling mechanism within cancer cells.

The flux of Ca²⁺ from the endoplasmic reticulum (ER) to mitochondria regulates mitochondria metabolism. Within tumor tissue, mitochondria metabolism is frequently repressed, leading to chemotherapy resistance and increased growth of the tumor mass. Therefore, altered ER–mitochondria Ca²⁺ flux could be a cancer hallmark, but only a few regulatory proteins of this mechanism are currently known. One candidate is the redox-sensitive oxidoreductase TMX1 that is enriched on the mitochondria-associated membrane (MAM), the site of ER–mitochondria Ca²⁺ flux. Our findings demonstrate that cancer cells with low TMX1 exhibit increased ER Ca²⁺, accelerated cytosolic Ca²⁺ clearance, and reduced Ca²⁺ transfer to mitochondria. Thus, low levels of TMX1 reduce ER–mitochondria contacts, shift bioenergetics away from mitochondria, and accelerate tumor growth. For its role in intracellular ER–mitochondria Ca²⁺ flux, TMX1 requires its thioredoxin motif and palmitoylation to target to the MAM. As a thiol-based tumor suppressor, TMX1 increases mitochondrial ATP production and apoptosis progression.

Biological membranes

Biological membranes allow life as we know it to exist. They form cells and enable separation between the inside and outside of an organism, controlling by means of their selective permeability which substances enter and leave. By allowing gradients of ions to be created across them, membranes also enable living organisms to generate energy. In addition, they control the flow of messages between cells by sending, receiving and processing information in the form of chemical and electrical signals. This essay summarizes the structure and function of membranes and the proteins within them, and describes their role in trafficking and transport, and their involvement in health and disease. Techniques for studying membranes are also discussed.

Cargo Capture and Bulk Flow in the Early Secretory Pathway

Transport of newly synthesized proteins from the endoplasmic reticulum (ER) to the Golgi complex is highly selective. As a general rule, such transport is limited to soluble and membrane-associated secretory proteins that have reached properly folded and assembled conformations. To secure the efficiency, fidelity, and control of this crucial transport step, cells use a combination of mechanisms. The mechanisms are based on selective retention of proteins in the ER to prevent uptake into transport vesicles, on selective capture of proteins in COPII carrier vesicles, on inclusion of proteins in these vesicles by default as part of fluid and membrane bulk flow, and on selective retrieval of proteins from post-ER compartments by retrograde vesicle transport.