On keeping the right ER size

Intracellular Citrate/acetyl-CoA flux and endoplasmic reticulum acetylation: Connectivity is the answer

BACKGROUND

Key cellular metabolites reflecting the immediate activity of metabolic enzymes as well as the functional metabolic state of intracellular organelles can act as powerful signal regulators to ensure the activation of homeostatic responses. The citrate/acetyl-CoA pathway, initially recognized for its role in intermediate metabolism, has emerged as a fundamental branch of this nutrient-sensing homeostatic response. Emerging studies indicate that fluctuations in acetyl-CoA availability within different cellular organelles and compartments provides substrate-level regulation of many biological functions. A fundamental aspect of these regulatory functions involves Nε-lysine acetylation.

SCOPE OF REVIEW

Here, we will examine the emerging regulatory functions of the citrate/acetyl-CoA pathway and the specific role of the endoplasmic reticulum (ER) acetylation machinery in the maintenance of intracellular crosstalk and homeostasis. These functions will be analyzed in the context of associated human diseases and specific mouse models of dysfunctional ER acetylation and citrate/acetyl-CoA flux. A primary objective of this review is to highlight the complex yet integrated response of compartment- and organelle-specific Nε-lysine acetylation to the intracellular availability and flux of acetyl-CoA, linking this important post-translational modification to cellular metabolism.

MAJOR CONCLUSIONS

The ER acetylation machinery regulates the proteostatic functions of the organelle as well as the metabolic crosstalk between different intracellular organelles and compartments. This crosstalk enables the cell to impart adaptive responses within the ER and the secretory pathway. However, it also enables the ER to impart adaptive responses within different cellular organelles and compartments. Defects in the homeostatic balance of acetyl-CoA flux and ER acetylation reflect different but converging disease states in humans as well as converging phenotypes in relevant mouse models. In conclusion, citrate and acetyl-CoA should not only be seen as metabolic substrates of intermediate metabolism but also as signaling molecules that direct functional adaptation of the cell to both intracellular and extracellular messages. Future discoveries in CoA biology and acetylation are likely to yield novel therapeutic approaches.

Sec62 promotes gastric cancer metastasis through mediating UPR-induced autophagy activation

BACKGROUND AND AIMS

Sec62 is a membrane protein of the endoplasmic reticulum that facilitates protein transport. Its role in cancer is increasingly recognised, but remains largely unknown. We investigated the functional role of Sec62 in gastric cancer (GC) and its underlying mechanism.

METHODS

Bioinformatics, tissue microarray, immunohistochemistry (IHC), western blotting (WB), quantitative polymerase chain reaction (qPCR), and immunofluorescence were used to examine the expression of target genes. Transwell, scratch healing assays, and xenograft models were used to evaluate cell migration and invasion. Transmission electron microscopy and mRFP-GFP-LC3 double-labeled adenoviruses were used to monitor autophagy. Co-immunoprecipitation (CO-IP) was performed to evaluate the binding activity between the proteins.

RESULTS

Sec62 expression was upregulated in GC, and Sec62 upregulation was an independent predictor of poor prognosis. Sec62 overexpression promoted GC cell migration and invasion both in vitro and in vivo. Sec62 promoted migration and invasion by affecting TIMP-1 and MMP2/9 balance. Moreover, Sec62 could activate autophagy by upregulating PERK/ATF4 expression and binding to LC3II with concomitant FIP200/Beclin-1/Atg5 activation. Furthermore, autophagy blockage impaired the promotive effects of Sec62 on GC cell migration and invasion, whereas autophagy activation rescued the inhibitory effect of Sec62 knockdown on GC metastasis. Notably, Sec62 inhibition combined with autophagy blockage exerted a synergetic anti-metastatic effect in vitro and in vivo.

CONCLUSION

Sec62 promotes GC metastasis by activating autophagy and subsequently regulating TIMP-1 and MMP2/9 balance. The activation of autophagy by Sec62 may involve the unfolded protein response (UPR)-related PERK/ATF4 pathway and binding of LC3II during UPR recovery involving FIP200/Beclin-1/Atg5 upregulation. Specifically, the dual inhibition of Sec62 and autophagy may provide a promising therapeutic strategy for GC metastasis.

ATG9A regulates proteostasis through reticulophagy receptors FAM134B and SEC62 and folding chaperones CALR and HSPB1

The acetylation of ATG9A within the endoplasmic reticulum (ER) lumen regulates the induction of reticulophagy. ER acetylation is ensured by AT-1/SLC33A1, a membrane transporter that maintains the cytosol-to-ER flux of acetyl-CoA. Defective AT-1 activity, as caused by heterozygous/homozygous mutations and gene duplication events, results in severe disease phenotypes. Here, we show that although the acetylation of ATG9A occurs in the ER lumen, the induction of reticulophagy requires ATG9A to engage FAM134B and SEC62 on the cytosolic side of the ER. To address this conundrum, we resolved the ATG9A interactome in two mouse models of AT-1 dysregulation: AT-1 sTg, a model of systemic AT-1 overexpression with hyperacetylation of ATG9A, and AT-1^S113R/+, a model of AT-1 haploinsufficiency with hypoacetylation of ATG9A. We identified CALR and HSPB1 as two ATG9A partners that regulate the induction of reticulophagy as a function of ATG9A acetylation and discovered that ATG9A associates with several proteins that maintain ER proteostasis.

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The ATG9A-FAM134B and ATG9A-SEC62 interaction requires specific structural features

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Opposite Ca⁺⁺-binding EF hands regulate ATG9A-FAM134B interaction

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HSBP1 and CALR regulate ATG9A-mediated induction of reticulophagy

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Many of the proteins that ensure ER proteostasis display spatial vicinity/cross talk

Exploiting strain diversity and rational engineering strategies to enhance recombinant cellulase secretion by Saccharomyces cerevisiae

Consolidated bioprocessing (CBP) of lignocellulosic material into bioethanol has progressed in the past decades; however, several challenges still exist which impede the industrial application of this technology. Identifying the challenges that exist in all unit operations is crucial and needs to be optimised, but only the barriers related to the secretion of recombinant cellulolytic enzymes in Saccharomyces cerevisiae will be addressed in this review. Fundamental principles surrounding CBP as a biomass conversion platform have been established through the successful expression of core cellulolytic enzymes, namely β-glucosidases, endoglucanases, and exoglucanases (cellobiohydrolases) in S. cerevisiae. This review will briefly address the challenges involved in the construction of an efficient cellulolytic yeast, with particular focus on the secretion efficiency of cellulases from this host. Additionally, strategies for studying enhanced cellulolytic enzyme secretion, which include both rational and reverse engineering approaches, will be discussed. One such technique includes bio-engineering within genetically diverse strains, combining the strengths of both natural strain diversity and rational strain development. Furthermore, with the advancement in next-generation sequencing, studies that utilise this method of exploiting intra-strain diversity for industrially relevant traits will be reviewed. Finally, future prospects are discussed for the creation of ideal CBP strains with high enzyme production levels.Key Points• Several challenges are involved in the construction of efficient cellulolytic yeast, in particular, the secretion efficiency of cellulases from the hosts.• Strategies for enhancing cellulolytic enzyme secretion, a core requirement for CBP host microorganism development, include both rational and reverse engineering approaches.• One such technique includes bio-engineering within genetically diverse strains, combining the strengths of both natural strain diversity and rational strain development.

Organelle size scaling over embryonic development

Cell division without growth results in progressive cell size reductions during early embryonic development. How do the sizes of intracellular structures and organelles scale with cell size and what are the functional implications of such scaling relationships? Model organisms, in particular Caenorhabditis elegans worms, Drosophila melanogaster flies, Xenopus laevis frogs, and Mus musculus mice, have provided insights into developmental size scaling of the nucleus, mitotic spindle, and chromosomes. Nuclear size is regulated by nucleocytoplasmic transport, nuclear envelope proteins, and the cytoskeleton. Regulators of microtubule dynamics and chromatin compaction modulate spindle and mitotic chromosome size scaling, respectively. Developmental scaling relationships for membrane-bound organelles, like the endoplasmic reticulum, Golgi, mitochondria, and lysosomes, have been less studied, although new imaging approaches promise to rectify this deficiency. While models that invoke limiting components and dynamic regulation of assembly and disassembly can account for some size scaling relationships in early embryos, it will be exciting to investigate the contribution of newer concepts in cell biology such as phase separation and interorganellar contacts. With a growing understanding of the underlying mechanisms of organelle size scaling, future studies promise to uncover the significance of proper scaling for cell function and embryonic development, as well as how aberrant scaling contributes to disease. This article is categorized under: Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing Early Embryonic Development > Fertilization to Gastrulation Comparative Development and Evolution > Model Systems.

The nucleoporin ELYS regulates nuclear size by controlling NPC number and nuclear import capacity

How intracellular organelles acquire their characteristic sizes is a fundamental question in cell biology. Given stereotypical changes in nuclear size in cancer, it is important to understand the mechanisms that control nuclear size in human cells. Using a high-throughput imaging RNAi screen, we identify and mechanistically characterize ELYS, a nucleoporin required for post-mitotic nuclear pore complex (NPC) assembly, as a determinant of nuclear size in mammalian cells. ELYS knockdown results in small nuclei, reduced nuclear lamin B2 localization, lower NPC density, and decreased nuclear import. Increasing nuclear import by importin α overexpression rescues nuclear size and lamin B2 import, while inhibiting importin α/β-mediated nuclear import decreases nuclear size. Conversely, ELYS overexpression increases nuclear size, enriches nuclear lamin B2 at the nuclear periphery, and elevates NPC density and nuclear import. Consistent with these observations, knockdown or inhibition of exportin 1 increases nuclear size. Thus, we identify ELYS as a novel positive effector of mammalian nuclear size and propose that nuclear size is sensitive to NPC density and nuclear import capacity.

Nε-lysine acetylation in the endoplasmic reticulum - a novel cellular mechanism that regulates proteostasis and autophagy

Protein post-translational modifications (PTMs) take many shapes, have many effects and are necessary for cellular homeostasis. One of these PTMs, Nε-lysine acetylation, was thought to occur only in the mitochondria, cytosol and nucleus, but this paradigm was challenged in the past decade with the discovery of lysine acetylation in the lumen of the endoplasmic reticulum (ER). This process is governed by the ER acetylation machinery: the cytosol:ER-lumen acetyl-CoA transporter AT-1 (also known as SLC33A1), and the ER-resident lysine acetyltransferases ATase1 and ATase2 (also known as NAT8B and NAT8, respectively). This Review summarizes the more recent biochemical, cellular and mouse model studies that underscore the importance of the ER acetylation process in maintaining protein homeostasis and autophagy within the secretory pathway, and its impact on developmental and age-associated diseases.

Increased transport of acetyl-CoA into the endoplasmic reticulum causes a progeria-like phenotype

The membrane transporter AT-1/SLC33A1 translocates cytosolic acetyl-CoA into the lumen of the endoplasmic reticulum (ER), participating in quality control mechanisms within the secretory pathway. Mutations and duplication events in AT-1/SLC33A1 are highly pleiotropic and have been linked to diseases such as spastic paraplegia, developmental delay, autism spectrum disorder, intellectual disability, propensity to seizures, and dysmorphism. Despite these known associations, the biology of this key transporter is only beginning to be uncovered. Here, we show that systemic overexpression of AT-1 in the mouse leads to a segmental form of progeria with dysmorphism and metabolic alterations. The phenotype includes delayed growth, short lifespan, alopecia, skin lesions, rectal prolapse, osteoporosis, cardiomegaly, muscle atrophy, reduced fertility, and anemia. In terms of homeostasis, the AT-1 overexpressing mouse displays hypocholesterolemia, altered glycemia, and increased indices of systemic inflammation. Mechanistically, the phenotype is caused by a block in Atg9a-Fam134b-LC3β and Atg9a-Sec62-LC3β interactions, and defective reticulophagy, the autophagic recycling of the ER. Inhibition of ATase1/ATase2 acetyltransferase enzymes downstream of AT-1 restores reticulophagy and rescues the phenotype of the animals. These data suggest that inappropriately elevated acetyl-CoA flux into the ER directly induces defects in autophagy and recycling of subcellular structures and that this diversion of acetyl-CoA from cytosol to ER is causal in the progeria phenotype. Collectively, these data establish the cytosol-to-ER flux of acetyl-CoA as a novel event that dictates the pace of aging phenotypes and identify intracellular acetyl-CoA-dependent homeostatic mechanisms linked to metabolism and inflammation.

Combining autophagy-inducing peptides and brefeldin A delivered by perinuclear-localized mesoporous silica nanoparticles: a manipulation strategy for ER-phagy

Autophagic degradation of the endoplasmic reticulum (ER-phagy) has been found to play a critical role in human sensory neuropathy. So far, however, specific and efficient intervention means for ER-phagy remain unexplored. Herein, brefeldin A (BFA), a blocking agent on protein transport between the ER and Golgi, was screened from ER stress inducers. BFA was then delivered to the perinuclear area co-localized with the ER by a mesoporous silica nanoparticle-based drug-carrier functionalized with autophagy-inducing peptides of TAT-beclin 1 (MSNs-BFA), to evoke a perturbation of ER-phagy. The molecular mechanism of ER-phagy regulated by BFA was explored by biochemical evaluation including time-lapse live-cell fluorescence imaging. We found that MSNs-BFA treatment caused a lower mRNA/protein expression level of FAM134b even under a compensation of autophagic flux in U2OS cells, and resulted in ER-expansion. The fragmentation of the ER was blocked as a response to ER stress mediated by inactivation of the AKT/TSC/mTOR pathway. Our work developed an efficient external manipulation strategy to regulate ER-phagy and may contribute to the therapeutic application of autophagy-related major human diseases.

Blocking variant surface glycoprotein synthesis alters endoplasmic reticulum exit sites/Golgi homeostasis in Trypanosoma brucei

The predominant secretory cargo of bloodstream form Trypanosoma brucei is variant surface glycoprotein (VSG), comprising ~10% total protein and forming a dense protective layer. Blocking VSG translation using Morpholino oligonucleotides triggered a precise pre‐cytokinesis arrest. We investigated the effect of blocking VSG synthesis on the secretory pathway. The number of Golgi decreased, particularly in post‐mitotic cells, from 3.5 ± 0.6 to 2.0 ± 0.04 per cell. Similarly, the number of endoplasmic reticulum exit sites (ERES) in post‐mitotic cells dropped from 3.9 ± 0.6 to 2.7 ± 0.1 eight hours after blocking VSG synthesis. The secretory pathway was still functional in these stalled cells, as monitored using Cathepsin L. Rates of phospholipid and glycosylphosphatidylinositol‐anchor biosynthesis remained relatively unaffected, except for the level of sphingomyelin which increased. However, both endoplasmic reticulum and Golgi morphology became distorted, with the Golgi cisternae becoming significantly dilated, particularly at the trans‐face. Membrane accumulation in these structures is possibly caused by reduced budding of nascent vesicles due to the drastic reduction in the total amount of secretory cargo, that is, VSG. These data argue that the total flux of secretory cargo impacts upon the biogenesis and maintenance of secretory structures and organelles in T. brucei, including the ERES and Golgi.