ER-to-lysosome-associated degradation acts as failsafe mechanism upon ERAD dysfunction

Glycosylation Regulation by TMEM230 in Aging and Autoimmunity

Aging is often a choice between developing cancer or autoimmune disorders, often due in part to loss of self-tolerance or loss of immunological recognition of rogue-acting tumor cells. Self-tolerance and cell recognition by the immune system are processes very much dependent on the specific signatures of glycans and glycosylated factors present on the cell plasma membrane or in the stromal components of tissue. Glycosylated factors are generated in nearly innumerable variations in nature, allowing for the immensely diverse role of these factors in aging and flexibility necessary for cellular interactions in tissue functionality. In previous studies, we showed that differential expression of TMEM230, an endoplasmic reticulum (ER) protein was associated with specific signatures of enzymes regulating glycan synthesis and processing and glycosylation in rheumatoid arthritis synovial tissue using single-cell transcript sequencing. In this current study, we characterize the genes and pathways co-modulated in all cell types of the synovial tissue with the enzymes regulating glycan synthesis and processing, as well as glycosylation. Genes and biological and molecular pathways associated with hallmarks of aging were in mitochondria-dependent oxidative phosphorylation and reactive oxygen species synthesis, ER-dependent stress and unfolded protein response, DNA repair (UV response and P53 signaling pathways), and senescence, glycolysis and apoptosis regulation through PI3K-AKT-mTOR signaling have been shown to play important roles in aging or neurodegeneration (such as Parkinson's and Alzheimer's disease). We propose that the downregulation of TMEM230 and RNASET2 may represent a paradigm for the study of age-dependent autoimmune disorders due to their role in regulating glycosylation, unfolded protein response, and PI3K-AKT-mTOR signaling.

Lysosomal Degradation of ER Client Proteins by ER-phagy and Related Pathways

The endoplasmic reticulum (ER) is a major site of cellular protein synthesis. Degradation of overabundant, misfolded, aggregating or unwanted proteins is required to maintain proteostasis and avoid the deleterious consequences of aberrant protein accumulation, at a cellular and organismal level. While extensive research has shown an important role for proteasomally-mediated, ER-associated degradation (ERAD) in maintaining proteostasis, it is becoming clear that there is a substantial role for lysosomal degradation of "client" proteins from the ER lumen or membrane (ER-to-lysosome degradation, ERLAD). Here we provide a brief overview of the broad categories of ERLAD - predominantly ER-phagy (ER autophagy) pathways and related processes. We collate the client proteins known to date, either individual species or categories of proteins. Where known, we summarise the molecular mechanisms by which they are selected for degradation, and the setting in which lysosomal degradation of the client(s) is important for correct cell or tissue function. Finally, we highlight the questions that remain open in this area.

Preliminary analysis of the role of small hepatitis B surface proteins mutations in the pathogenesis of occult hepatitis B infection via the endoplasmic reticulum stress-induced UPR-ERAD pathway

A growing body of evidence has shown that hepatitis B surface antigen (HBsAg) mutations can influence the occurrence of occult hepatitis B infection (OBI), particularly amino acid substitutions in small hepatitis B surface proteins (SHBs). The mechanistic basis for these results, however, remains unclear. This study was designed to explore the potential impact and mechanisms of OBI-related SHBs mutations on serum HBsAg. Huh7 and HepG2 cells were transfected with plasmids encoding wild-type (WT) or OBI-related SHB mutation-containing sequences, after which a chemiluminescence approach was used to detect HBsAg levels in cell culture supernatants. Western blotting was further used to assess HBsAg and endoplasmic reticulum stress (ERS)-related protein levels in lysates prepared from these cells, while the localization of HBsAg within cells was assessed via immunofluorescent staining. Cells transfected with OBI-related SHB mutation-encoding plasmids exhibited lower supernatant HBsAg levels than cells transfected with WT plasmids. Intracellular and extracellular HBsAg levels in these mutant plasmid-transfected cells were lower relative to those for WT plasmid-transfected cells, and HBsAg accumulation within the ER was detected via immunofluorescent staining in cells transfected with OBI-related SHB mutation-encoding plasmids, ERS-related protein content was also significantly increased in mutant plasmid-transfected cells as compared to those in the WT group. These results suggest that proteins harboring OBI-related mutations may tend to accumulate in the ER, thereby triggering an ERS response and impairing the transcription and translation of HBsAg via the activation of the unfolded protein response and ER-associated protein degradation pathway. These effects ultimately reduce the overall assembly of HBV virions in the ER and their associated secretion.

Liver CYP4A autophagic-lysosomal degradation (ALD): A major role for the autophagic receptor SQSTM1/p62 through an uncommon target interaction site

The hepatic P450 hemoproteins CYPs 4A are typical N-terminally anchored Type I endoplasmic reticulum (ER)-proteins, that are inducible by hypolipidemic drugs and other "peroxisome proliferators". They are engaged in the ω-/ω-1-oxidation of various fatty acids including arachidonic acid, prostaglandins and leukotrienes and in the biotransformation of some therapeutic drugs. Herein we report that of the mammalian liver CYPs 4A, human CYP4A11 and mouse Cyp4a12a are preferential targets of the ER-lysosome-associated degradation (ERLAD). Consequently, these proteins are stabilized both as 1%Triton X100-soluble and -insoluble species in mouse hepatocytes and HepG2-cells deficient in the autophagic initiation ATG5-gene. Although these proteins exhibit surface LC3-interacting regions (LIRs) that would target them directly to the autophagosome, they nevertheless interact intimately with the autophagic receptor SQSTM1/p62. Through structural deletion analyses and site-directed mutagenesis, we have identified the Cyp4A-interacting p62 subdomain to lie between residues 170 and 233, which include its Traf6-binding and LIM-binding subdomains. Mice carrying a liver-specific genetic deletion of p62 residues 69-251 (p62Mut) that includes the CYP4A-interacting subdomain also exhibit Cyp4a-protein stabilization both as Triton X100-soluble and -insoluble species. Consistently, p62Mut mouse liver microsomes exhibit enhanced ω- and ω-1-hydroxylation of arachidonic acid to its physiologically active metabolites 19- and 20-HETEs relative to the corresponding wild-type mouse liver microsomes. Collectively, our findings suggest that any disruption of CYP4A ERLAD results in functionally active P450 protein and consequent production of proinflammatory metabolites on one hand, and insoluble aggregates on the other, which may contribute to pathological aggregates i.e. Mallory-Denk bodies/inclusions, hallmarks of many liver diseases.

Fluorescent Reporters, Imaging, and Artificial Intelligence Toolkits to Monitor and Quantify Autophagy, Heterophagy, and Lysosomal Trafficking Fluxes

Lysosomal compartments control the clearance of cell-own material (autophagy) or of material that cells endocytose from the external environment (heterophagy) to warrant supply of nutrients, to eliminate macromolecules or parts of organelles present in excess, aged, or containing toxic material. Inherited or sporadic mutations in lysosomal proteins and enzymes may hamper their folding in the endoplasmic reticulum (ER) and their lysosomal transport via the Golgi compartment, resulting in lysosomal dysfunction and storage disorders. Defective cargo delivery to lysosomal compartments is harmful to cells and organs since it causes accumulation of toxic compounds and defective organellar homeostasis. Assessment of resident proteins and cargo fluxes to the lysosomal compartments is crucial for the mechanistic dissection of intracellular transport and catabolic events. It might be combined with high-throughput screenings to identify cellular, chemical, or pharmacological modulators of these events that may find therapeutic use for autophagy-related and lysosomal storage disorders. Here, discuss qualitative, quantitative and chronologic monitoring of autophagic, heterophagic and lysosomal protein trafficking in fixed and live cells, which relies on fluorescent single and tandem reporters used in combination with biochemical, flow cytometry, light and electron microscopy approaches implemented by artificial intelligence-based technology.