Reticulon 4 is necessary for endoplasmic reticulum tubulation, STIM1-Orai1 coupling, and store-operated calcium entry

Sculpting nuclear envelope identity from the endoplasmic reticulum during the cell cycle

The nuclear envelope (NE) regulates nuclear functions, including transcription, nucleocytoplasmic transport, and protein quality control. While the outer membrane of the NE is directly continuous with the endoplasmic reticulum (ER), the NE has an overall distinct protein composition from the ER, which is crucial for its functions. During open mitosis in higher eukaryotes, the NE disassembles during mitotic entry and then reforms as a functional territory at the end of mitosis to reestablish nucleocytoplasmic compartmentalization. In this review, we examine the known mechanisms by which the functional NE reconstitutes from the mitotic ER in the continuous ER-NE endomembrane system during open mitosis. Furthermore, based on recent findings indicating that the NE possesses unique lipid metabolism and quality control mechanisms distinct from those of the ER, we explore the maintenance of NE identity and homeostasis during interphase. We also highlight the potential significance of membrane junctions between the ER and NE.

Endoplasmic Reticulum Stress in Bronchopulmonary Dysplasia: Contributor or Consequence?

Bronchopulmonary dysplasia (BPD) is the most common complication of prematurity. Oxidative stress (OS) and inflammation are the major contributors to BPD. Despite aggressive treatments, BPD prevalence remains unchanged, which underscores the urgent need to explore more potential therapies. The endoplasmic reticulum (ER) plays crucial roles in surfactant and protein synthesis, assisting mitochondrial function, and maintaining metabolic homeostasis. Under OS, disturbed metabolism and protein folding transform the ER structure to refold proteins and help degrade non-essential proteins to resume cell homeostasis. When OS becomes excessive, the endogenous chaperone will leave the three ER stress sensors to allow subsequent changes, including cell death and senescence, impairing the growth potential of organs. The contributing role of ER stress in BPD is confirmed by reproducing the BPD phenotype in rat pups by ER stress inducers. Although chemical chaperones attenuate BPD, ER stress is still associated with cellular senescence. N-acetyl-lysyltyrosylcysteine amide (KYC) is a myeloperoxidase inhibitor that attenuates ER stress and senescence as a systems pharmacology agent. In this review, we describe the role of ER stress in BPD and discuss the therapeutic potentials of chemical chaperones and KYC, highlighting their promising role in future therapeutic interventions.

Secretory Nogo-B regulates Th2 differentiation in the lung cancer microenvironment

Nogo-B, a ubiquitously expressed member of the reticulon family, plays an important role in maintaining endoplasmic reticulum (ER) structure, regulating protein folding, and calcium homeostasis. In this study, we demonstrate that Nogo-B expression and secretion are upregulated in lung cancer and correlate to overall survival. Nogo-B is secreted by various cells, particularly lung cancer cells. ER stress and phosphorylation at serine 107 can induce Nogo-B secretion. Secretory Nogo-B suppresses the differentiation of Th2 cells and the release of type 2 cytokines, thus influencing the anti-tumor effects of Th2-related immune cells, including IgE+B cell class switching and eosinophil activation.

SigmaR1 shapes rough endoplasmic reticulum membrane sheets

Rough endoplasmic reticulum (ER) sheets are a fundamental domain of the ER and the gateway into the secretory pathway. Although reticulon proteins stabilize high-curvature ER tubules, it is unclear whether other proteins scaffold the flat membranes of rough ER sheets. Through a proteomics screen using ER sheet-localized RNA-binding proteins as bait, we identify the sigma-1 receptor (SigmaR1) as an ER sheet-shaping factor. High-resolution live cell imaging and electron tomography assign SigmaR1 as an ER sheet-localized factor whose levels determine the amount of rough ER sheets in cells. Structure-guided mutagenesis and in vitro reconstitution on giant unilamellar vesicles further support a mechanism whereby SigmaR1 oligomers use their extended arrays of amphipathic helices to bind and flatten the lumenal leaflet of ER membranes to oppose membrane curvature and stabilize rough ER sheets.

Dynamic Localization of Endoplasmic Reticulum during Tetrahymena Conjugation

Changes in lipid composition at membrane fusion sites in mating Tetrahymena are thought to involve the endoplasmic reticulum (ER), but its localization to these sites has not been observed. Here we show ER distribution during Tetrahymena mating using TtRET1-GFP and GFP-KDEL. We find that both markers localize to perinuclear membranes and tubular structures that connect perinuclear membrane to plasma membrane at fusion sites. Interestingly, both markers disappear from parental macronuclei after emergence of zygotic macronuclei. These similarities in localization of established ER marker, GFP-KDEL, and TtRET1-GFP reveal TtRET1-GFP as a useful new live cell marker for the ER in Tetrahymena.

Intestinal Nogo-B reduces GLP1 levels by binding to proglucagon on the endoplasmic reticulum to inhibit PCSK1 cleavage

Glucagon-like peptide 1 (GLP1), which is mainly processed and cleaved from proglucagon in enteroendocrine cells (EECs) of the intestinal tract, acts on the GLP1 receptor in pancreatic cells to stimulate insulin secretion and to inhibit glucagon secretion. However, GLP1 processing is not fully understood. Here, we show that reticulon 4B (Nogo-B), an endoplasmic reticulum (ER)-resident protein, interacts with the major proglucagon fragment of proglucagon to retain proglucagon on the ER, thereby inhibiting PCSK1-mediated cleavage of proglucagon in the Golgi. Intestinal Nogo-B knockout in male type 2 diabetes mellitus (T2DM) mice increases GLP1 and insulin levels and decreases glucagon levels, thereby alleviating pancreatic injury and insulin resistance. Finally, we identify aberrantly elevated Nogo-B expression and inhibited proglucagon cleavage in EECs from diabetic patients. Our study reveals the subcellular regulatory processes involving Nogo-B during GLP1 production and suggests intestinal Nogo-B as a potential therapeutic target for T2DM.

Endoplasmic reticulum morphology regulation by RTN4 modulates neuronal regeneration by curbing luminal transport

Cell functions rely on intracellular transport systems distributing bioactive molecules with high spatiotemporal accuracy. The endoplasmic reticulum (ER) tubular network constitutes a system for delivering luminal solutes, including Ca2+, across the cell periphery. How the ER structure enables this nanofluidic transport system is unclear. Here, we show that ER membrane-localized reticulon 4 (RTN4/Nogo) is sufficient to impose neurite outgrowth inhibition in human cortical neurons while acting as an ER morphoregulator. Improving ER transport visualization methodologies combined with optogenetic Ca2+ dynamics imaging and in silico modeling, we observed that ER luminal transport is modulated by ER tubule narrowing and dilation, proportional to the amount of RTN4. Excess RTN4 limited ER luminal transport and Ca2+ release, while RTN4 elimination reversed the effects. The described morphoregulatory effect of RTN4 defines the capacity of the ER for peripheral Ca2+ delivery for physiological releases and thus may constitute a mechanism for controlling the (re)generation of neurites.

Dysregulation of FLVCR1a-dependent mitochondrial calcium handling in neural progenitors causes congenital hydrocephalus

Congenital hydrocephalus (CH), occurring in approximately 1/1,000 live births, represents an important clinical challenge due to the limited knowledge of underlying molecular mechanisms. The discovery of novel CH genes is thus essential to shed light on the intricate processes responsible for ventricular dilatation in CH. Here, we identify FLVCR1 (feline leukemia virus subgroup C receptor 1) as a gene responsible for a severe form of CH in humans and mice. Mechanistically, our data reveal that the full-length isoform encoded by the FLVCR1 gene, FLVCR1a, interacts with the IP3R3-VDAC complex located on mitochondria-associated membranes (MAMs) that controls mitochondrial calcium handling. Loss of Flvcr1a in mouse neural progenitor cells (NPCs) affects mitochondrial calcium levels and energy metabolism, leading to defective cortical neurogenesis and brain ventricle enlargement. These data point to defective NPCs calcium handling and metabolic activity as one of the pathogenetic mechanisms driving CH.

Phenotypic characterization, plant growth and development, genome methylation, and mineral elements composition of neotetraploid lettuce (Lactuca sativa L.)

Stable neotetraploid lines of lettuce (Lactuca sativa L.) were produced from three phenotypically distinct cultivars (Annapolis, Eruption, Merlot) and an advanced breeding line (SM13-L2) using colchicine treatment of seeds or young seedlings. When tested under the greenhouse and field conditions, neotetraploids initially grew more rapidly than their diploid progenitors, however they reached their reproductive stage (bolting, flower bud formation, and flowering) substantially later. Seeds production on neotetraploids was delayed by more than 30 days compared to diploids. Tetraploid plants had fewer, but larger stomata and leaves, less chlorophyll per area, higher photosystem II photochemical efficiency, generally lighter root system, and produced less than 1% of seeds in comparison with diploids. Field-grown neotetraploids of all lines displayed a significant reduction in tipburn (1.8% vs. 22.2%, respectively), a highly undesirable physiological disorder. Changes in leaf and root mineral composition were detected in neotetraploids. Several elements were found in lower abundance than in diploids, most notably iron, calcium, and silicon. Whole genome bisulfite sequencing (WGBS) revealed 498 differentially methylated regions (DMR), with 106 of these regions having at least 50% difference in the level of methylation between neotetraploids and their diploid progenitors. At least 18 of the most prominent DMR were detected in proximity to genes predicted to be involved in plant development or reaction to biotic and abiotic stressors. Because neotetraploid lines have low seed production, they are not suitable for commercial cultivation. They can be used, however, in research to study the factors contributing to tipburn, traits affected by stomata size or density, and the effect of ploidy on resistance to environmental stressors.

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.

Ultrastructural analysis of prostate cancer tissue provides insights into androgen-dependent adaptations to membrane contact site establishment

Membrane trafficking and organelle contact sites are important for regulating cell metabolism and survival; processes often deregulated in cancer. Prostate cancer is the second leading cause of cancer-related death in men in the developed world. While early-stage disease is curable by surgery or radiotherapy there is an unmet need to identify prognostic biomarkers, markers to treatment response and new therapeutic targets in intermediate-late stage disease. This study explored the morphology of organelles and membrane contact sites in tumor tissue from normal, low and intermediate histological grade groups. The morphology of organelles in secretory prostate epithelial cells; including Golgi apparatus, ER, lysosomes; was similar in prostate tissue samples across a range of Gleason scores. Mitochondrial morphology was not dramatically altered, but the number of membrane contacts with the ER notably increased with disease progression. A three-fold increase of tight mitochondria-ER membrane contact sites was observed in the intermediate Gleason score group compared to normal tissue. To investigate whether these changes were concurrent with an increased androgen signaling in the tissue, we investigated whether an anti-androgen used in the clinic to treat advanced prostate cancer (enzalutamide) could reverse the phenotype. Patient-derived explant tissues with an intermediate Gleason score were cultured ex vivo in the presence or absence of enzalutamide and the number of ER-mitochondria contacts were quantified for each matched pair of tissues. Enzalutamide treated tissue showed a significant reduction in the number and length of mitochondria-ER contact sites, suggesting a novel androgen-dependent regulation of these membrane contact sites. This study provides evidence for the first time that prostate epithelial cells undergo adaptations in membrane contact sites between mitochondria and the ER during prostate cancer progression. These adaptations are androgen-dependent and provide evidence for a novel hormone-regulated mechanism that support establishment and extension of MAMs. Future studies will determine whether these changes are required to maintain pro-proliferative signaling and metabolic changes that support prostate cancer cell viability.

Nogo-A reduces ceramide de novo biosynthesis to protect from heart failure

AIMS

Growing evidence correlate the accrual of the sphingolipid ceramide in plasma and cardiac tissue with heart failure (HF). Regulation of sphingolipid metabolism in the heart and the pathological impact of its derangement remain poorly understood. Recently, we discovered that Nogo-B, a membrane protein of endoplasmic reticulum, abundant in the vascular wall, down-regulates the sphingolipid de novo biosynthesis via serine palmitoyltransferase (SPT), first and rate liming enzyme, to impact vascular functions and blood pressure. Nogo-A, a splice isoform of Nogo, is transiently expressed in cardiomyocyte (CM) following pressure overload. Cardiac Nogo is up-regulated in dilated and ischaemic cardiomyopathies in animals and humans. However, its biological function in the heart remains unknown.

METHODS AND RESULTS

We discovered that Nogo-A is a negative regulator of SPT activity and refrains ceramide de novo biosynthesis in CM exposed to haemodynamic stress, hence limiting ceramide accrual. At 7 days following transverse aortic constriction (TAC), SPT activity was significantly up-regulated in CM lacking Nogo-A and correlated with ceramide accrual, particularly very long-chain ceramides, which are the most abundant in CM, resulting in the suppression of 'beneficial' autophagy. At 3 months post-TAC, mice lacking Nogo-A in CM showed worse pathological cardiac hypertrophy and dysfunction, with ca. 50% mortality rate.

CONCLUSION

Mechanistically, Nogo-A refrains ceramides from accrual, therefore preserves the 'beneficial' autophagy, mitochondrial function, and metabolic gene expression, limiting the progression to HF under sustained stress.

Endoplasmic Reticulum Architecture and Inter-Organelle Communication in Metabolic Health and Disease

The endoplasmic reticulum (ER) is a key organelle involved in the regulation of lipid and glucose metabolism, proteostasis, Ca²⁺ signaling, and detoxification. The structural organization of the ER is very dynamic and complex, with distinct subdomains such as the nuclear envelope and the peripheral ER organized into ER sheets and tubules. ER also forms physical contact sites with all other cellular organelles and with the plasma membrane. Both form and function of the ER are highly adaptive, with a potent capacity to respond to transient changes in environmental cues such as nutritional fluctuations. However, under obesity-induced chronic stress, the ER fails to adapt, leading to ER dysfunction and the development of metabolic pathologies such as insulin resistance and fatty liver disease. Here, we discuss how the remodeling of ER structure and contact sites with other organelles results in diversification of metabolic function and how perturbations to this structural flexibility by chronic overnutrition contribute to ER dysfunction and metabolic pathologies in obesity.

Impact of Hypermannosylation on the Structure and Functionality of the ER and the Golgi Complex

Proteins of the secretory pathway undergo glycosylation in the endoplasmic reticulum (ER) and the Golgi apparatus. Altered protein glycosylation can manifest in serious, sometimes fatal malfunctions. We recently showed that mutations in GDP-mannose pyrophosphorylase A (GMPPA) can cause a syndrome characterized by alacrima, achalasia, mental retardation, and myopathic alterations (AAMR syndrome). GMPPA acts as a feedback inhibitor of GDP-mannose pyrophosphorylase B (GMPPB), which provides GDP-mannose as a substrate for protein glycosylation. Loss of GMPPA thus enhances the incorporation of mannose into glycochains of various proteins, including α-dystroglycan (α-DG), a protein that links the extracellular matrix with the cytoskeleton. Here, we further characterized the consequences of loss of GMPPA for the secretory pathway. This includes a fragmentation of the Golgi apparatus, which comes along with a regulation of the abundance of several ER- and Golgi-resident proteins. We further show that the activity of the Golgi-associated endoprotease furin is reduced. Moreover, the fraction of α-DG, which is retained in the ER, is increased. Notably, WT cells cultured at a high mannose concentration display similar changes with increased retention of α-DG, altered structure of the Golgi apparatus, and a decrease in furin activity. In summary, our data underline the importance of a balanced mannose homeostasis for the secretory pathway.

Site-Specific Activity-Based Protein Profiling Using Phosphonate Handles

Most drug molecules target proteins. Identification of the exact drug binding sites on these proteins is essential to understand and predict how drugs affect protein structure and function. To address this challenge, we developed a strategy that uses immobilized metal-affinity chromatography-enrichable phosphonate affinity tags, for efficient and selective enrichment of peptides bound to an activity-based probe, enabling the identification of the exact drug binding site. As a proof of concept, using this approach, termed PhosID-ABPP (activity-based protein profiling), over 500 unique binding sites were reproducibly identified of an alkynylated afatinib derivative (PF-06672131). As PhosID-ABPP is compatible with intact cell inhibitor treatment, we investigated the quantitative differences in approachable binding sites in intact cells and in lysates of the same cell line and observed and quantified substantial differences. Moreover, an alternative protease digestion approach was used to capture the previously reported binding site on the epidermal growth factor receptor, which turned out to remain elusive when using solely trypsin as protease. Overall, we find that PhosID-ABPP is highly complementary to biotin-based enrichment strategies in ABPP studies, with PhosID-ABPP providing the advantage of direct activity-based probe interaction site identification.

Decoding mechanism of action and sensitivity to drug candidates from integrated transcriptome and chromatin state

Omics-based technologies are driving major advances in precision medicine, but efforts are still required to consolidate their use in drug discovery. In this work, we exemplify the use of multi-omics to support the development of 3-chloropiperidines, a new class of candidate anticancer agents. Combined analyses of transcriptome and chromatin accessibility elucidated the mechanisms underlying sensitivity to test agents. Furthermore, we implemented a new versatile strategy for the integration of RNA- and ATAC-seq (Assay for Transposase-Accessible Chromatin) data, able to accelerate and extend the standalone analyses of distinct omic layers. This platform guided the construction of a perturbation-informed basal signature predicting cancer cell lines' sensitivity and to further direct compound development against specific tumor types. Overall, this approach offers a scalable pipeline to support the early phases of drug discovery, understanding of mechanisms, and potentially inform the positioning of therapeutics in the clinic.

Optimized Workflow for Enrichment and Identification of Biotinylated Peptides Using Tamavidin 2-REV for BioID and Cell Surface Proteomics

Chemical or enzymatic biotinylation of proteins is widely used in various studies, and proximity-dependent biotinylation coupled to mass spectrometry is a powerful approach for analyzing protein-protein interactions in living cells. We recently developed a simple method to enrich biotinylated peptides using Tamavidin 2-REV, an engineered avidin-like protein with reversible biotin-binding capability. However, the level of biotinylated proteins in cells is low; therefore, large amounts of cellular proteins were required to detect biotinylated peptides. In addition, the enriched biotinylated peptide solution contained many contaminant ions. Here, we optimized the workflow for efficient enrichment of biotinylated peptides and removal of contaminant ions. The efficient recovery of biotinylated peptides with fewer contaminant ions was achieved by heat inactivation of trypsin, prewashing Tamavidin 2-REV beads, clean-up of biotin solution, mock elution, and using optimal temperature and salt concentration for elution. The optimized workflow enabled identification of nearly 4-fold more biotinylated peptides with higher purity from RAW264.7 macrophages expressing TurboID-fused STING (stimulator of interferon genes). In addition, sequential digestion with Glu-C and trypsin revealed biotinylation sites that were not identified by trypsin digestion alone. Furthermore, the combination of this workflow with TMT labeling enabled large-scale quantification of cell surface proteome changes upon epidermal growth factor (EGF) stimulation. This workflow will be useful for BioID and cell surface proteomics and for various other applications based on protein biotinylation.

Store-operated calcium entry is reduced in spastin-linked hereditary spastic paraplegia

Pathogenic variants in SPAST, the gene coding for spastin, are the single most common cause of hereditary spastic paraplegia, a progressive motor neuron disease. Spastin regulates key cellular functions, including microtubule-severing and endoplasmic reticulum-morphogenesis. However, it remains unclear how alterations in these cellular functions due to SPAST pathogenic variants result in motor neuron dysfunction. Since spastin influences both microtubule network and endoplasmic reticulum structure, we hypothesized that spastin is necessary for the regulation of Ca2+ homeostasis via store-operated calcium entry. Here, we show that the lack of spastin enlarges the endoplasmic reticulum and reduces store-operated calcium entry. In addition, elevated levels of different spastin variants induced clustering of STIM1 within the endoplasmic reticulum, altered the transport of STIM1 to the plasma membrane and reduced store-operated calcium entry, which could be rescued by exogenous expression of STIM1. Importantly, store-operated calcium entry was strongly reduced in induced pluripotent stem cell-derived neurons from hereditary spastic paraplegia patients with pathogenic variants in SPAST resulting in spastin haploinsufficiency. These neurons developed axonal swellings in response to lack of spastin. We were able to rescue both store-operated calcium entry and axonal swellings in SPAST patient neurons by restoring spastin levels, using CRISPR/Cas9 to correct the pathogenic variants in SPAST. These findings demonstrate that proper amounts of spastin are a key regulatory component for store-operated calcium entry mediated Ca2+ homeostasis and suggest store-operated calcium entry as a disease relevant mechanism of spastin-linked motor neuron disease.

Shielding of actin by the endoplasmic reticulum impacts nuclear positioning

Nuclear position is central to cell polarization, and its disruption is associated with various pathologies. The nucleus is moved away from the leading edge of migrating cells through its connection to moving dorsal actin cables, and the absence of connections to immobile ventral stress fibers. It is unclear how these asymmetric nucleo-cytoskeleton connections are established. Here, using an in vitro wound assay, we find that remodeling of endoplasmic reticulum (ER) impacts nuclear positioning through the formation of a barrier that shields immobile ventral stress fibers. The remodeling of ER and perinuclear ER accumulation is mediated by the ER shaping protein Climp-63. Furthermore, ectopic recruitment of the ER to stress fibers restores nuclear positioning in the absence of Climp-63. Our findings suggest that the ER mediates asymmetric nucleo-cytoskeleton connections to position the nucleus.

Reticulon 2 promotes gastric cancer metastasis via activating endoplasmic reticulum Ca2+ efflux-mediated ERK signalling

Gastric cancer ranks fourth for mortality globally among various malignant tumours, and invasion and metastasis are the major reason leading to its poor prognosis. Recently, accumulating studies revealed the role of reticulon proteins in cell growth and transmigration. However, the expression and biological function of reticulon proteins in human gastric cancer remain largely unclear. Herein, we explored the potential role of reticulon 2 (RTN2) in the progression of gastric cancer. Tissue microarray was used to determine the expression levels of RTN2 in 267 gastric cancer patients by immunohistochemistry. Gastric cancer cell lines were utilised to examine the influences of RTN2 on cellular migration and invasion abilities, epithelial-to-mesenchymal transition (EMT) and signalling pathway. In vivo studies were also performed to detect the effect of RTN2 on tumour metastasis. We found that RTN2 expression was notably upregulated in tumour tissues compared to pericarcinomatous tissues. High RTN2 expression was positively correlated with patients’ age, vessel invasion, tumour invasion depth, lymph node metastasis and TNM stage. Besides, high RTN2 staining intensity was associated with adverse survival which was further identified as an independent prognostic factor for gastric cancer patients by multivariate analysis. And the predictive accuracy was also improved when incorporated RTN2 into the TNM-staging system. RTN2 could promote the proliferation, migration and invasion of gastric cancer cells in vitro and lung metastasis in vivo. Mechanistically, RTN2 interacted with IP3R, and activated ERK signalling pathway via facilitating Ca²⁺ release from the endoplasmic reticulum, and subsequently drove EMT in gastric cancer cells. These results proposed RTN2 as a novel promotor and potential molecular target for gastric cancer therapies.

Diagnostic and Prognostic Protein Biomarkers of β-Cell Function in Type 2 Diabetes and Their Modulation with Glucose Normalization

Development of type-2 diabetes(T2D) is preceded by β-cell dysfunction and loss. However, accurate measurement of β-cell function remains elusive. Biomarkers have been reported to predict β-cell functional decline but require validation. Therefore, we determined whether reported protein biomarkers could distinguish patients with T2D (onset < 10-years) from controls. A prospective, parallel study in T2D (n = 23) and controls (n = 23) was undertaken. In T2D subjects, insulin-induced blood glucose normalization from baseline 7.6 ± 0.4 mmol/L (136.8 ± 7.2 mg/dL) to 4.5 ± 0.07 mmol/L (81 ± 1.2 mg/dL) was maintained for 1-h. Controls were maintained at 4.9 ± 0.1 mmol/L (88.2 ± 1.8 mg/dL). Slow Off-rate Modified Aptamer (SOMA) -scan plasma protein measurement determined a 43-protein panel reported as diagnostic and/or prognostic for T2D. At baseline, 9 proteins were altered in T2D. Three of 13 prognostic/diagnostic proteins were lower in T2D: Adiponectin (p < 0.0001), Endocan (p < 0.05) and Mast/stem cell growth factor receptor-Kit (KIT) (p < 0.01). Two of 14 prognostic proteins [Cathepsin-D (p < 0.05) and Cadherin-E (p < 0.005)], and four of 16 diagnostic proteins [Kallikrein-4 (p = 0.001), Aminoacylase-1 (p = 0.001), Insulin-like growth factor-binding protein-4 (IGFBP4) (p < 0.05) and Reticulon-4 receptor (RTN4R) (p < 0.001)] were higher in T2D. Protein levels were unchanged following glucose normalization in T2D. Our results suggest that a focused biomarker panel may be useful for assessing β-cell dysfunction and may complement clinical decision-making on insulin therapy. Unchanged post-glucose normalization levels indicate these are not acute-phase proteins or affected by glucose variability.

Mapping the Proximity Interaction Network of STIM1 Reveals New Mechanisms of Cytoskeletal Regulation

Stromal interaction molecule 1 (STIM1) resides primarily in the sarco/endoplasmic reticulum, where it senses intraluminal Ca2+ levels and activates Orai channels on the plasma membrane to initiate Ca2+ influx. We have previously shown that STIM1 is involved in the dynamic remodeling of the actin cytoskeleton. However, the downstream effectors of STIM1 that lead to cytoskeletal remodeling are not known. The proximity-labeling technique (BioID) can capture weak and transient protein-protein interactions, including proteins that reside in the close vicinity of the bait, but that may not be direct binders. Hence, in the present study, we investigated the STIM1 interactome using the BioID technique. A promiscuous biotin ligase was fused to the cytoplasmic C-terminus of STIM1 and was stably expressed in a mouse embryonic fibroblast (MEF) cell line. Screening of biotinylated proteins identified several high confidence targets. Here, we report Gelsolin (GSN) as a new member of the STIM1 interactome. GSN is a Ca2+-dependent actin-severing protein that promotes actin filament assembly and disassembly. Results were validated using knockdown approaches and immunostaining. We tested our results in neonatal cardiomyocytes where STIM1 overexpression induced altered actin dynamics and cytoskeletal instability. This is the first time that BioID assay was used to investigate the STIM1 interactome. Our work highlights the role of STIM1/GSN in the structure and function of the cytoskeleton.

Nodal modulator (NOMO) is required to sustain endoplasmic reticulum morphology

The endoplasmic reticulum (ER) is a membrane-bound organelle responsible for protein folding, lipid synthesis, and calcium homeostasis. Maintenance of ER structural integrity is crucial for proper function, but much remains to be learned about the molecular players involved. To identify proteins that support the structure of the ER, we performed a proteomic screen and identified nodal modulator (NOMO), a widely conserved type I transmembrane protein of unknown function, with three nearly identical orthologs specified in the human genome. We found that overexpression of NOMO1 imposes a sheet morphology on the ER, whereas depletion of NOMO1 and its orthologs causes a collapse of ER morphology concomitant with the formation of membrane-delineated holes in the ER network positive for the lysosomal marker lysosomal-associated protein 1. In addition, the levels of key players of autophagy including microtubule-associated protein light chain 3 and autophagy cargo receptor p62/sequestosome 1 strongly increase upon NOMO depletion. In vitro reconstitution of NOMO1 revealed a "beads on a string" structure likely representing consecutive immunoglobulin-like domains. Extending NOMO1 by insertion of additional immunoglobulin folds results in a correlative increase in the ER intermembrane distance. Based on these observations and a genetic epistasis analysis including the known ER-shaping proteins Atlastin2 and Climp63, we propose a role for NOMO1 in the functional network of ER-shaping proteins.

Reticulon-3 Promotes Endosome Maturation at ER Membrane Contact Sites

ER tubules form and maintain membrane contact sites (MCSs) with endosomes. How and why these ER-endosome MCSs persist as endosomes traffic and mature is poorly understood. Here we find that a member of the reticulon protein family, Reticulon-3L (Rtn3L), enriches at ER-endosome MCSs as endosomes mature. We show that this localization is due to the long divergent N-terminal cytoplasmic domain of Rtn3L. We found that Rtn3L is recruited to ER-endosome MCSs by endosomal protein Rab9a, which marks a transition stage between early and late endosomes. Rab9a utilizes an FSV region to recruit Rtn3L via its six LC3-interacting region motifs. Consistent with our localization results, depletion or deletion of RTN3 from cells results in endosome maturation and cargo sorting defects, similar to RAB9A depletion. Together our data identify a tubular ER protein that promotes endosome maturation at ER MCSs.

SEC24A facilitates colocalization and Ca2 + flux between the endoplasmic reticulum and mitochondria

A genome-wide screen recently identified SEC24A as a novel mediator of thapsigargin-induced cell death in HAP1 cells. Here, we determined the cellular mechanism and specificity of SEC24A-mediated cytotoxicity. Measurement of Ca²⁺ levels using organelle-specific fluorescent indicator dyes showed that Ca²⁺ efflux from endoplasmic reticulum (ER) and influx into mitochondria were significantly impaired in SEC24A-knockout cells. Furthermore, SEC24A-knockout cells also showed ∼44% less colocalization of mitochondria and peripheral tubular ER. Knockout of SEC24A, but not its paralogs SEC24B, SEC24C or SEC24D, rescued HAP1 cells from cell death induced by three different inhibitors of sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPases (SERCA) but not from cell death induced by a topoisomerase inhibitor. Thapsigargin-treated SEC24A-knockout cells showed a ∼2.5-fold increase in autophagic flux and ∼10-fold reduction in apoptosis compared to wild-type cells. Taken together, our findings indicate that SEC24A plays a previously unrecognized role in regulating association and Ca²⁺ flux between the ER and mitochondria, thereby impacting processes dependent on mitochondrial Ca²⁺ levels, including autophagy and apoptosis.

Dictyostelium lacking the single atlastin homolog Sey1 shows aberrant ER architecture, proteolytic processes and expansion of the Legionella-containing vacuole

Dictyostelium discoideum Sey1 is the single ortholog of mammalian atlastin 1-3 (ATL1-3), which are large homodimeric GTPases mediating homotypic fusion of endoplasmic reticulum (ER) tubules. In this study, we generated a D. discoideum mutant strain lacking the sey1 gene and found that amoebae deleted for sey1 are enlarged, but grow and develop similarly to the parental strain. The ∆sey1 mutant amoebae showed an altered ER architecture, and the tubular ER network was partially disrupted without any major consequences for other organelles or the architecture of the secretory and endocytic pathways. Macropinocytic and phagocytic functions were preserved; however, the mutant amoebae exhibited cumulative defects in lysosomal enzymes exocytosis, intracellular proteolysis, and cell motility, resulting in impaired growth on bacterial lawns. Moreover, ∆sey1 mutant cells showed a constitutive activation of the unfolded protein response pathway (UPR), but they still readily adapted to moderate levels of ER stress, while unable to cope with prolonged stress. In D. discoideum ∆sey1 the formation of the ER-associated compartment harbouring the bacterial pathogen Legionella pneumophila was also impaired. In the mutant amoebae, the ER was less efficiently recruited to the "Legionella-containing vacuole" (LCV), the expansion of the pathogen vacuole was inhibited at early stages of infection and intracellular bacterial growth was reduced. In summary, our study establishes a role of D. discoideum Sey1 in ER architecture, proteolysis, cell motility and intracellular replication of L. pneumophila.

More Than Just Simple Interaction between STIM and Orai Proteins: CRAC Channel Function Enabled by a Network of Interactions with Regulatory Proteins

The calcium-release-activated calcium (CRAC) channel, activated by the release of Ca2+ from the endoplasmic reticulum (ER), is critical for Ca2+ homeostasis and active signal transduction in a plethora of cell types. Spurred by the long-sought decryption of the molecular nature of the CRAC channel, considerable scientific effort has been devoted to gaining insights into functional and structural mechanisms underlying this signalling cascade. Key players in CRAC channel function are the Stromal interaction molecule 1 (STIM1) and Orai1. STIM1 proteins span through the membrane of the ER, are competent in sensing luminal Ca2+ concentration, and in turn, are responsible for relaying the signal of Ca2+ store-depletion to pore-forming Orai1 proteins in the plasma membrane. A direct interaction of STIM1 and Orai1 allows for the re-entry of Ca2+ from the extracellular space. Although much is already known about the structure, function, and interaction of STIM1 and Orai1, there is growing evidence that CRAC under physiological conditions is dependent on additional proteins to function properly. Several auxiliary proteins have been shown to regulate CRAC channel activity by means of direct interactions with STIM1 and/or Orai1, promoting or hindering Ca2+ influx in a mechanistically diverse manner. Various proteins have also been identified to exert a modulatory role on the CRAC signalling cascade although inherently lacking an affinity for both STIM1 and Orai1. Apart from ubiquitously expressed representatives, a subset of such regulatory mechanisms seems to allow for a cell-type-specific control of CRAC channel function, considering the rather restricted expression patterns of the specific proteins. Given the high functional and clinical relevance of both generic and cell-type-specific interacting networks, the following review shall provide a comprehensive summary of regulators of the multilayered CRAC channel signalling cascade. It also includes proteins expressed in a narrow spectrum of cells and tissues that are often disregarded in other reviews of similar topics.

The plant endoplasmic reticulum: an organized chaos of tubules and sheets with multiple functions

The endoplasmic reticulum is a fascinating organelle at the core of the secretory pathway. It is responsible for the synthesis of one third of the cellular proteome and, in plant cells, it produces receptors and transporters of hormones as well as the proteins responsible for the biosynthesis of critical components of a cellulosic cell wall. The endoplasmic reticulum structure resembles a spider-web network of interconnected tubules and cisternae that pervades the cell. The study of the dynamics and interaction of this organelles with other cellular structures such as the plasma membrane, the Golgi apparatus and the cytoskeleton, have been permitted by the implementation of fluorescent protein and advanced confocal imaging. In this review, we report on the findings that contributed towards the understanding of the endoplasmic reticulum morphology and function with the aid of fluorescent proteins, focusing on the contributions provided by pioneering work from the lab of the late Professor Chris Hawes.

Research advances on neurite outgrowth inhibitor B receptor

Neurite outgrowth inhibitor‐B (Nogo‐B) is a membrane protein which is extensively expressed in multiple organs, especially in endothelial cells and vascular smooth muscle cells of blood vessels and belongs to the reticulon protein family. Notably, its specific receptor, Nogo‐B receptor (NgBR), encoded by NUS1, has been implicated in many crucial cellular processes, such as cholesterol trafficking, lipid metabolism, dolichol synthesis, protein N‐glycosylation, vascular remodelling, angiogenesis, tumorigenesis and neurodevelopment. In recent years, accumulating studies have demonstrated the statistically significant changes of NgBR expression levels in human diseases, including Niemann‐Pick type C disease, fatty liver, congenital disorders of glycosylation, persistent pulmonary hypertension of the newborn, invasive ductal breast carcinoma, malignant melanoma, non‐small cell lung carcinoma, paediatric epilepsy and Parkinson's disease. Besides, both the in vitro and in vivo studies have shown that NgBR overexpression or knockdown contribute to the alteration of various pathophysiological processes. Thus, there is a broad development potential in therapeutic strategies by modifying the expression levels of NgBR.

Loss of tumor susceptibility gene 101 (TSG101) perturbs endoplasmic reticulum structure and function

Tumor susceptibility gene 101 (TSG101), an ESCRT-I protein, is implicated in multiple cellular processes and its functional depletion can lead to blocked lysosomal degradation, cell cycle arrest, demyelination and neurodegeneration. Here, we show that loss of TSG101 results in endoplasmic reticulum (ER) stress and this causes ER membrane remodelling (EMR). This correlates with an expansion of ER, increased vacuolation, altered relative distribution of the rough and smooth ER and disruption of three-way junctions. Blocked lysosomal degradation due to TSG101 depletion leads to ER stress and Ca²⁺ leakage from ER stores, causing destabilization of actin cytoskeleton. Inhibiting Ca²⁺ release from the ER by blocking ryanodine receptors (RYRs) with Dantrolene partially rescues the ER stress phenotypes. Hence, in this study we have identified the involvement of TSG101 in modulating ER stress mediated remodelling by engaging the actin cytoskeleton. This is significant because functional depletion of TSG101 effectuates ER-stress, perturbs the structure, mobility and function of the ER, all aspects closely associated with neurodegenerative diseases. SUMMARY STATEMENT: We show that tumor susceptibility gene (TSG) 101 regulates endoplasmic reticulum (ER) stress and its membrane remodelling. Loss of TSG101 perturbs structure, mobility and function of the ER as a consequence of actin destabilization.

Hostile Takeover: Hijacking of Endoplasmic Reticulum Function by T4SS and T3SS Effectors Creates a Niche for Intracellular Pathogens

After entering a cell, intracellular pathogens must evade destruction and generate a niche for intracellular replication. A strategy shared by multiple intracellular pathogens is the deployment of type III secretion system (T3SS)- and type IV secretion system (T4SS)-injected proteins (effectors) that subvert cellular functions. A subset of these effectors targets activities of the host cell's endoplasmic reticulum (ER). Effectors are now appreciated to interfere with the ER in multiple ways, including capture of secretory vesicles, tethering of pathogen vacuoles to the ER, and manipulation of ER-based autophagy initiation and the unfolded-protein response. These strategies enable pathogens to generate a niche with access to cellular nutrients and to evade the host cell's defenses.

Type 2 inositol 1,4,5-trisphosphate receptor inhibits the progression of pulmonary arterial hypertension via calcium signaling and apoptosis

Pulmonary arterial hypertension (PAH) is a progressive disease associated with vasoconstriction and remodeling. Intracellular Ca²⁺ signaling regulates the contraction of pulmonary arteries and the proliferation of pulmonary arterial smooth muscle cells (PASMCs); however, it is not clear which molecules related to Ca²⁺ signaling contribute to the progression of PAH. In this study, we found the specific expression of type 2 inositol 1,4,5-trisphosphate receptor (IP₃R2), which is an intracellular Ca²⁺ release channel, on the sarco/endoplasmic reticulum in mouse PASMCs, and demonstrated its inhibitory role in the progression of PAH using a chronic hypoxia-induced PAH mouse model. After chronic hypoxia exposure, IP₃R2^-/- mice exhibited the significant aggravation of PAH, as determined by echocardiography and right ventricular hypertrophy, with significantly greater medial wall thickness by immunohistochemistry than that of wild-type mice. In IP₃R2^-/- murine PASMCs with chronic hypoxia, a TUNEL assay revealed the significant suppression of apoptosis, whereas there was no significant change in proliferation. Thapsigargin-induced store-operated Ca²⁺ entry (SOCE) was significantly enhanced in IP₃R2^-/- PASMCs in both normoxia and hypoxia based on in vitro fluorescent Ca²⁺ imaging. Furthermore, the enhancement of SOCE in IP₃R2^-/- PASMCs was remarkably suppressed by the addition of DPB162-AE, an inhibitor of the stromal-interacting molecule (STIM)-Orai complex which is about 100 times more potent than 2-APB. Our results indicate that IP₃R2 may inhibit the progression of PAH by promoting apoptosis and inhibiting SOCE via the STIM-Orai pathway in PASMCs. These findings suggest a previously undetermined role of IP₃R in the development of PAH and may contribute to the development of targeted therapies.

Platelet procoagulant phenotype is modulated by a p38-MK2 axis that regulates RTN4/Nogo proximal to the endoplasmic reticulum: utility of pathway analysis

Upon encountering physiological cues associated with damaged or inflamed endothelium, blood platelets set forth intracellular responses to ultimately support hemostatic plug formation and vascular repair. To gain insights into the molecular events underlying platelet function, we used a combination of interactome, pathway analysis, and other systems biology tools to analyze associations among proteins functionally modified by reversible phosphorylation upon platelet activation. While an interaction analysis mapped out a relative organization of intracellular mediators in platelet signaling, pathway analysis revealed directional signaling relations around protein kinase C (PKC) isoforms and mitogen-activated protein kinases (MAPKs) associated with platelet cytoskeletal dynamics, inflammatory responses, and hemostatic function. Pathway and causality analysis further suggested that platelets activate a specific p38-MK2 axis to phosphorylate RTN4 (reticulon-4, also known as Nogo), a Bcl-xl sequestration protein and critical regulator of endoplasmic reticulum (ER) physiology. In vitro, we find that platelets drive a p38-MK2-RTN4-Bcl-xl pathway associated with the regulation of the ER and platelet phosphatidylserine exposure. Together, our results support the use of pathway tools in the analysis of omics data sets as a means to help generate novel, mechanistic, and testable hypotheses for platelet studies while uncovering RTN4 as a putative regulator of platelet cell physiological responses.

The role of novel cytokines in inflammation: Defining peripheral artery disease among patients with coronary artery disease

Coronary artery disease (CAD) patients with concomitant peripheral artery disease (PAD) experience more extensive and calcified atherosclerosis, greater lesion progression and more common coronary events compared to patients with CAD only. To characterize the distinct features of this aggressive atherosclerotic disease, we studied novel cytokines that code different stages of atherogenesis. One hundred and eighty consecutive subjects (60 patients into each group of CAD+PAD, CAD and controls) were recruited among patients with stable angina pectoris scheduled for coronary angiography. An ankle-brachial index (ABI) ≤0.9 was determined as occlusive PAD. Fasting serum tumor necrosis factor (TNF)-like antigen 1A (TL1A) and its receptor death receptor 3 (DR3), NOGO-B (reticulon 4B) and its receptor NUS1, high-sensitivity C-reactive protein (hsCRP), A disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) 1, 4, 5 and interleukin (IL) 6 levels were determined. Serum hsCRP and DR3/TL1A concentrations were similar and higher than controls in the CAD and CAD+PAD groups. Levels of NOGO-B and its receptor NUS1 were increased and ADAMTS-5 was decreased in patients with CAD+PAD. Independent predictors of ABI in multivariate analysis were smoking (B = -0.13, p = 0.04), NUS1 (B = -0.88, p < 0.001), ADAMTS-5 (B = 0.63, p < 0.001) and SYNTAX score (B = -0.26, p < 0.001). Similarly, smoking (OR = 5.5, p = 0.019), SYNTAX score (OR = 1.2, p < 0.001), NUS1 (OR = 14.4, p < 0.001), ADAMTS-5 (OR = 1.1, p < 0.001) and age (OR = 1.1, p = 0.042) independently predicted the involvement of peripheral vasculature in logistic regression. The diagnostic performance of these cytokines to discriminate CAD+PAD were AUC 0.79 ( p < 0.001) for NUS1 and 0.37 ( p = 0.013) for ADAMTS-5. We report herein that circulating cytokines can give clues to the ongoing atherosclerotic process and the extent of vascular involvement in which distinct features of ADAMTS-5 and NUS1 make them promising cytokines for future research.

Chemoproteomics-enabled covalent ligand screen reveals a cysteine hotspot in reticulon 4 that impairs ER morphology and cancer pathogenicity

Chemical genetics has arisen as a powerful approach for identifying novel anti-cancer agents. However, a major bottleneck of this approach is identifying the targets of lead compounds that arise from screens. Here, we coupled the synthesis and screening of fragment-based cysteine-reactive covalent ligands with activity-based protein profiling (ABPP) chemoproteomic approaches to identify compounds that impair colorectal cancer pathogenicity and map the druggable hotspots targeted by these hits. Through this coupled approach, we discovered a cysteine-reactive acrylamide DKM 3-30 that significantly impaired colorectal cancer cell pathogenicity through targeting C1101 on reticulon 4 (RTN4). While little is known about the role of RTN4 in colorectal cancer, this protein has been established as a critical mediator of endoplasmic reticulum tubular network formation. We show here that covalent modification of C1101 on RTN4 by DKM 3-30 or genetic knockdown of RTN4 impairs endoplasmic reticulum and nuclear envelope morphology as well as colorectal cancer pathogenicity. We thus put forth RTN4 as a potential novel colorectal cancer therapeutic target and reveal a unique druggable hotspot within RTN4 that can be targeted by covalent ligands to impair colorectal cancer pathogenicity. Our results underscore the utility of coupling the screening of fragment-based covalent ligands with isoTOP-ABPP platforms for mining the proteome for novel druggable nodes that can be targeted for cancer therapy.

Reticulon 3-dependent ER-PM contact sites control EGFR nonclathrin endocytosis

The integration of endocytic routes is critical to regulate receptor signaling. A nonclathrin endocytic (NCE) pathway of the epidermal growth factor receptor (EGFR) is activated at high ligand concentrations and targets receptors to degradation, attenuating signaling. Here we performed an unbiased molecular characterization of EGFR-NCE. We identified NCE-specific regulators, including the endoplasmic reticulum (ER)-resident protein reticulon 3 (RTN3) and a specific cargo, CD147. RTN3 was critical for EGFR/CD147-NCE, promoting the creation of plasma membrane (PM)-ER contact sites that were required for the formation and/or maturation of NCE invaginations. Ca²⁺ release at these sites, triggered by inositol 1,4,5-trisphosphate (IP₃)-dependent activation of ER Ca²⁺ channels, was needed for the completion of EGFR internalization. Thus, we identified a mechanism of EGFR endocytosis that relies on ER-PM contact sites and local Ca²⁺ signaling.

The axonal endoplasmic reticulum: One organelle-many functions in development, maintenance, and plasticity

The endoplasmic reticulum (ER) is highly conserved in eukaryotes and neurons. Indeed, the localization of the organelle in axons has been known for nearly half a century. However, the relevance of the axonal ER is only beginning to emerge. In this review, we discuss the structure of the ER in axons, examining the role of ER-shaping proteins and highlighting reticulons. We analyze the multiple functions of the ER and their potential contribution to axonal physiology. First, we examine the emerging roles of the axonal ER in lipid synthesis, protein translation, processing, quality control, and secretory trafficking of transmembrane proteins. We also review the impact of the ER on calcium dynamics, focusing on intracellular mechanisms and functions. We describe the interactions between the ER and endosomes, mitochondria, and synaptic vesicles. Finally, we analyze available proteomic data of axonal preparations to reveal the dynamic functionality of the ER in axons during development. We suggest that the dynamic proteome and a validated axonal interactome, together with state-of-the-art methodologies, may provide interesting research avenues in axon physiology that may extend to pathology and regeneration. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 181-208, 2018.

Cortical actin contributes to spatial organization of ER-PM junctions

Endoplasmic reticulum-plasma membrane (ER-PM) junctions mediate crucial activities ranging from Ca2+ signaling to lipid metabolism. Spatial organization of ER-PM junctions may modulate the extent and location of these cellular activities. However, the morphology and distribution of ER-PM junctions are not well characterized. Using photoactivated localization microscopy, we reveal that the contact area of single ER-PM junctions is mainly oblong with the dimensions of ∼120 nm × ∼80 nm in HeLa cells. Using total internal reflection fluorescence microscopy and structure illumination microscopy, we show that cortical actin contributes to spatial distribution and stability of ER-PM junctions. Further functional assays suggest that intact F-actin architecture is required for phosphatidylinositol 4,5-bisphosphate homeostasis mediated by Nir2 at ER-PM junctions. Together, our study provides quantitative information on spatial organization of ER-PM junctions that is in part regulated by F-actin. We envision that functions of ER-PM junctions can be differentially regulated through dynamic actin remodeling during cellular processes.

Nogo-B Receptor Modulates Pulmonary Artery Smooth Muscle Cell Function in Developing Lungs

Nogo-B and its receptor (NgBR) are involved in blood vessel growth in developing lungs, but their role in pulmonary artery smooth muscle cell (PASMC) growth is unknown. We hypothesized that NgBR regulates growth of PASMCs by modulating the function of endoplasmic reticulum (ER) and formation of reactive oxygen species (ROS). In utero constriction of the ductus arteriosus created pulmonary hypertension in fetal lambs (hypertensive fetal lamb [HTFL]). PASMCs isolated 8 days after surgery were assessed for the alteration of protein levels by immunoblots and ROS formation by dihydroethidium and Cell ROX deep red fluorescence. NgBR small interfering RNA and plasmid DNA were used to manipulate NgBR levels. Proliferation and wound healing were assessed by cell counts and scratch recovery assay, respectively. Acute ER stress was induced by tunicamycin. Differences of mitogen-activated protein kinase and Akt pathway activation in HTFL versus control PASMCs were evaluated. Results showed that HTFL PASMCs had decreased NgBR levels and increased proliferation, wound healing, ER stress, and ROS formation compared with controls. Knockdown of NgBR in control PASMCs generated a phenotype similar to HTFL, and overexpression in HTFL restored the defective phenotype to control. Decreased NgBR levels were associated with increased ROS formation in HTFL PASMCs. Subsequently, scavenging ROS decreased proliferation and wound healing. Mechanistically, ROS formation decreases NgBR expression, which induces ER stress. This leads to extracellular signal-regulated kinase pathway activation and PASMC phenotype alteration. Our data suggest that decreased NgBR expression in pulmonary hypertension of the newborn contributes to increased PASMC proliferation and oxidative stress, which lead to the pathogenesis of lung injury.

ER-plasma membrane junctions: Why and how do we study them?

Endoplasmic reticulum (ER)-plasma membrane (PM) junctions are membrane microdomains important for communication between the ER and the PM. ER-PM junctions were first reported in muscle cells in 1957, but mostly ignored in non-excitable cells due to their scarcity and lack of functional significance. In 2005, the discovery of stromal interaction molecule 1 (STIM1) mediating a universal Ca²⁺ feedback mechanism at ER-PM junctions in mammalian cells led to a resurgence of research interests toward ER-PM junctions. In the past decade, several major advancements have been made in this emerging topic in cell biology, including the generation of tools for labeling ER-PM junctions and the unraveling of mechanisms underlying regulation and functions of ER-PM junctions. This review summarizes early studies, recently developed tools, and current advances in the characterization and understanding of ER-PM junctions. This article is part of a Special Issue entitled: Membrane Contact Sites edited by Christian Ungermann and Benoit Kornmann.

Cancer: Untethering Mitochondria from the Endoplasmic Reticulum?

Following the discovery of the mitochondria-associated membrane (MAM) as a hub for lipid metabolism in 1990 and its description as one of the first examples for membrane contact sites at the turn of the century, the past decade has seen the emergence of this structure as a potential regulator of cancer growth and metabolism. The mechanistic basis for this hypothesis is that the MAM accommodates flux of Ca²⁺ from the endoplasmic reticulum (ER) to mitochondria. This flux then determines mitochondrial ATP production, known to be low in many tumors as part of the Warburg effect. However, low mitochondrial Ca²⁺ flux also reduces the propensity of tumor cells to undergo apoptosis, another cancer hallmark. Numerous regulators of this flux have been recently identified as MAM proteins. Not surprisingly, many fall into the groups of tumor suppressors and oncogenes. Given the important role that the MAM could play in cancer, it is expected that proteins mediating its formation are particularly implicated in tumorigenesis. Examples for such proteins are mitofusin-2 and phosphofurin acidic cluster sorting protein 2 that likely act as tumor suppressors. This review discusses how these proteins that mediate or regulate ER–mitochondria tethering are (or are not) promoting or inhibiting tumorigenesis. The emerging picture of MAMs in cancer seems to indicate that in addition to the downregulation of mitochondrial Ca²⁺ import, MAM defects are but one way how cancer cells control mitochondria metabolism and apoptosis.

An endoplasmic reticulum protein, Nogo-B, facilitates alcoholic liver disease through regulation of kupffer cell polarization

Nogo-B (Reticulon 4B) is an endoplasmic reticulum (ER) resident protein that regulates ER structure and function. Because ER stress is known to induce M2 macrophage polarization, we examined whether Nogo-B regulates M1/M2 polarization of Kupffer cells and alters the pathogenesis of alcoholic liver disease (ALD). M1 and M2 phenotypes were assessed in relation to Nogo-B expression and disease severity in liver specimens from ALD patients (NCT01875211). Liver specimens from wild-type (WT) and Nogo-B knockout (KO) mice fed a control or Lieber-DeCarli ethanol liquid diet (5% ethanol) for 6 weeks were analyzed for liver injury and steatosis. Kupffer cells isolated from WT and Nogo-B KO mice were assessed for M1 and M2 activation. A significant positive correlation was observed between Nogo-B positive Kupffer cells and disease severity in ALD patients (n = 30, r = 0.66, P = 0.048). Furthermore, Nogo-B-positive Kupffer cells were correlated with M1 activation (inducible nitric oxide synthase) (r = 0.50, P = 0.05) and negatively with markers of M2 status (CD163) (r = -0.48, P = 0.07) in these patients. WT mice exhibited significantly increased liver injury (P < 0.05) and higher hepatic triglyceride levels (P < 0.01) compared with Nogo-B KO mice in response to chronic ethanol feeding. Nogo-B in Kupffer cells promoted M1 polarization, whereas absence of Nogo-B increased ER stress and M2 polarization in Kupffer cells.

CONCLUSION

Nogo-B is permissive of M1 polarization of Kupffer cells, thereby accentuating liver injury in ALD in humans and mice. Nogo-B in Kupffer cells may represent a new therapeutic target for ALD. (Hepatology 2017;65:1720-1734).

Atlastin regulates store-operated calcium entry for nerve growth factor-induced neurite outgrowth

Homotypic membrane fusion of the endoplasmic reticulum (ER) is mediated by a class of dynamin-like GTPases known as atlastin (ATL). Depletion of or mutations in ATL cause an unbranched ER morphology and hereditary spastic paraplegia (HSP), a neurodegenerative disease characterized by axon shortening in corticospinal motor neurons and progressive spasticity of the lower limbs. How ER shaping is linked to neuronal defects is poorly understood. Here, we show that dominant-negative mutants of ATL1 in PC-12 cells inhibit nerve growth factor (NGF)-induced neurite outgrowth. Overexpression of wild-type or mutant ATL1 or depletion of ATLs alters ER morphology and affects store-operated calcium entry (SOCE) by decreasing STIM1 puncta formation near the plasma membrane upon calcium depletion of the ER. In addition, blockage of the STIM1-Orai pathway effectively abolishes neurite outgrowth of PC-12 cells stimulated by NGF. These results suggest that SOCE plays an important role in neuronal regeneration, and mutations in ATL1 may cause HSP, partly by undermining SOCE.

Endoplasmic reticulum-mediated signalling in cellular microdomains

The endoplasmic reticulum (ER) is a prime mediator of cellular signalling due to its functions as an internal cellular store for calcium, as well as a site for synthesis of proteins and lipids. Its peripheral network of sheets and tubules facilitates calcium and lipid signalling, especially in areas of the cell that are more distant to the main cytoplasmic network. Specific membrane proteins shape the peripheral ER architecture and influence the network stability to project into restricted spaces. The signalling microdomains are anatomically separate from the cytoplasm as a whole and exhibit localized protein, ion channel and cytoskeletal element expression. Signalling can also occur between the ER and other organelles, such as the Golgi or mitochondria. Lipids made in the ER membrane can be sent to the Golgi via specialized transfer proteins and specific phospholipid synthases are enriched at ER-mitochondria junctions to more efficiently expedite phospholipid transfer. As a hub for protein and lipid synthesis, a store for intracellular calcium [Ca²⁺ ]_i and a mediator of cellular stress, the ER is an important cellular organelle. Its ability to organize into tubules and project into restricted spaces allows for discrete and temporal signalling, which is important for cellular physiology and organism homoeostasis.

NOGO-A/RTN4A and NOGO-B/RTN4B are simultaneously expressed in epithelial, fibroblast and neuronal cells and maintain ER morphology

Reticulons (RTNs) are a large family of membrane associated proteins with various functions. NOGO-A/RTN4A has a well-known function in limiting neurite outgrowth and restricting the plasticity of the mammalian central nervous system. On the other hand, Reticulon 4 proteins were shown to be involved in forming and maintaining endoplasmic reticulum (ER) tubules. Using comparative transcriptome analysis and qPCR, we show here that NOGO-B/RTN4B and NOGO-A/RTN4A are simultaneously expressed in cultured epithelial, fibroblast and neuronal cells. Electron tomography combined with immunolabelling reveal that both isoforms localize preferably to curved membranes on ER tubules and sheet edges. Morphological analysis of cells with manipulated levels of NOGO-B/RTN4B revealed that it is required for maintenance of normal ER shape; over-expression changes the sheet/tubule balance strongly towards tubules and causes the deformation of the cell shape while depletion of the protein induces formation of large peripheral ER sheets.

cis-Prenyltransferase: New Insights into Protein Glycosylation, Rubber Synthesis, and Human Diseases

cis-Prenyltransferases (cis-PTs) constitute a large family of enzymes conserved during evolution and present in all domains of life. cis-PTs catalyze consecutive condensation reactions of allylic diphosphate acceptor with isopentenyl diphosphate (IPP) in the cis (Z) configuration to generate linear polyprenyl diphosphate. The chain lengths of isoprenoid carbon skeletons vary widely from neryl pyrophosphate (C10) to natural rubber (C>10,000). The homo-dimeric bacterial enzyme, undecaprenyl diphosphate synthase (UPPS), has been structurally and mechanistically characterized in great detail and serves as a model for understanding the mode of action of eukaryotic cis-PTs. However, recent experiments have revealed that mammals, fungal, and long-chain plant cis-PTs are heteromeric enzymes composed of two distantly related subunits. In this review, the classification, function, and evolution of cis-PTs will be discussed with a special emphasis on the role of the newly described NgBR/Nus1 subunit and its plants' orthologs as essential, structural components of the cis-PTs activity.