Eukaryotic cells maintain a highly organized endolysosomal system. This system regulates the protein and lipid content of the plasma membrane, it participates in the intracellular quality control ...machinery and is needed for the efficient removal of damaged organelles. This complex network comprises an endosomal membrane system that feeds into the lysosomes, yet also allows recycling of membrane proteins, and probably lipids. Moreover, lysosomal degradation provides the cell with macromolecules for further growth. In this review, we focus primarily on the role of the small Rab GTPases Rab5 and Rab7 as organelle markers and interactors of multiple effectors on endosomes and lysosomes and highlight their role in membrane dynamics, particularly fusion along the endolysosomal pathway.
Rab GTPases are key factors that determine organelle identity throughout the endomembrane system.
Endosomes change identity by replacing Rab5 with Rab7, which requires the specific and tightly regulated recruitment of the Rab7 guanine nucleotide exchange factor (GEF) to the endosome membrane.
Recent work on tethering proteins, including the lysosomal HOPS complex and the coiled-coil protein early endosome antigen 1 (EEA1), implies novel and conserved mechanisms of vesicle tethering and membrane fusion at organelles.
Lysosomes: fusion and function Luzio, J. Paul; Pryor, Paul R; Bright, Nicholas A
Nature reviews. Molecular cell biology,
200708, 2007-Aug, 2007-8-00, 20070801, Letnik:
8, Številka:
8
Journal Article
Recenzirano
Lysosomes are dynamic organelles that receive and degrade macromolecules from the secretory, endocytic, autophagic and phagocytic membrane-trafficking pathways. Live-cell imaging has shown that ...fusion with lysosomes occurs by both transient and full fusion events, and yeast genetics and mammalian cell-free systems have identified much of the protein machinery that coordinates these fusion events. Many pathogens that hijack the endocytic pathways to enter cells have evolved mechanisms to avoid being degraded by the lysosome. However, the function of lysosomes is not restricted to protein degradation: they also fuse with the plasma membrane during cell injury, as well as having more specialized secretory functions in some cell types.
Although lysosomes perform a number of essential cellular functions, damaged lysosomes represent a potential hazard to the cell. Such lysosomes are therefore engulfed by autophagic membranes in the ...process known as lysophagy, which is initiated by recognition of luminal glycoprotein domains by cytosolic lectins such as Galectin‐3. Here, we show that, under various conditions that cause injury to the lysosome membrane, components of the endosomal sorting complex required for transport (ESCRT)‐I, ESCRT‐II, and ESCRT‐III are recruited. This recruitment occurs before that of Galectin‐3 and the lysophagy machinery. Subunits of the ESCRT‐III complex show a particularly prominent recruitment, which depends on the ESCRT‐I component TSG101 and the TSG101‐ and ESCRT‐III‐binding protein ALIX. Interference with ESCRT recruitment abolishes lysosome repair and causes otherwise reversible lysosome damage to become cell lethal. Vacuoles containing the intracellular pathogen Coxiella burnetii show reversible ESCRT recruitment, and interference with this recruitment reduces intravacuolar bacterial replication. We conclude that the cell is equipped with an endogenous mechanism for lysosome repair which protects against lysosomal damage‐induced cell death but which also provides a potential advantage for intracellular pathogens.
Synopsis
Damaged lysosomes are lethal to cells and cleared by lysophagy. While pathways exist to protect lysosomes from damage, repair mechanisms for injured organelles are unknown. Here ESCRT proteins, originally identified as regulators of protein sorting, are shown to promote the repair of lysosomal membrane and bacteria‐containing vacuoles to protect cells against cell death and provide replicative advantage to pathogens, respectively.
ESCRT‐I, ‐II and ‐III proteins are recruited to damaged endolysosomal membranes.
The ESCRT machinery mediates repair of damaged lysosomes.
ESCRT recruitment to damaged lysosomes precedes lysophagy.
ESCRT‐mediated lysosome repair promotes cell viability after lysosome injury.
ESCRT recruitment to Coxiella burnetii vacuoles promotes bacterial replication.
ESCRT proteins, components of the protein sorting machinery, promote repair of damaged lysosomes and vacuoles. Lysosomal repair protects cells from cell death, while vacuolar repair provides a replicative advantage to certain pathogens.
COVID-19, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has become a global pandemic and has claimed over 2 million lives worldwide. Although the genetic sequences of ...SARS-CoV and SARS-CoV-2 have high homology, the clinical and pathological characteristics of COVID-19 differ significantly from those of SARS. How and whether SARS-CoV-2 evades (cellular) immune surveillance requires further elucidation. In this study, we show that SARS-CoV-2 infection leads to major histocompability complex class Ι (MHC-Ι) down-regulation both in vitro and in vivo. The viral protein encoded by open reading frame 8 (ORF8) of SARS-CoV-2, which shares the least homology with SARS-CoV among all viral proteins, directly interacts with MHC-Ι molecules and mediates their down-regulation. In ORF8-expressing cells, MHC-Ι molecules are selectively targeted for lysosomal degradation via autophagy. Thus, SARS-CoV-2-infected cells are much less sensitive to lysis by cytotoxic T lymphocytes. Because ORF8 protein impairs the antigen presentation system, inhibition of ORF8 could be a strategy to improve immune surveillance.
The MiT‐TFE family of basic helix‐loop‐helix leucine‐zipper transcription factors includes four members: TFEB, TFE3, TFEC, and MITF. Originally described as oncogenes, these factors play a major role ...as regulators of lysosome biogenesis, cellular energy homeostasis, and autophagy. An important mechanism by which these transcription factors are regulated involves their shuttling between the surface of lysosomes, the cytoplasm, and the nucleus. Such dynamic changes in subcellular localization occur in response to nutrient fluctuations and various forms of cell stress and are mediated by changes in the phosphorylation of multiple conserved amino acids. Major kinases responsible for MiT‐TFE protein phosphorylation include mTOR, ERK, GSK3, and AKT. In addition, calcineurin de‐phosphorylates MiT‐TFE proteins in response to lysosomal calcium release. Thus, through changes in the phosphorylation state of MiT‐TFE proteins, lysosome function is coordinated with the cellular metabolic state and cellular demands. This review summarizes the evidence supporting MiT‐TFE regulation by phosphorylation at multiple key sites. Elucidation of such regulatory mechanisms is of fundamental importance to understand how these transcription factors contribute to both health and disease.
This review discusses how MiT‐TFE transcription factors, key modulators of lysosome biogenesis and autophagy, are regulated upon nutrient fluctuations and cell stress.
Cancer stem cells (CSCs) represent a subset of cells within tumours that exhibit self-renewal properties and the capacity to seed tumours. CSCs are typically refractory to conventional treatments and ...have been associated to metastasis and relapse. Salinomycin operates as a selective agent against CSCs through mechanisms that remain elusive. Here, we provide evidence that a synthetic derivative of salinomycin, which we named ironomycin (AM5), exhibits a more potent and selective activity against breast CSCs in vitro and in vivo, by accumulating and sequestering iron in lysosomes. In response to the ensuing cytoplasmic depletion of iron, cells triggered the degradation of ferritin in lysosomes, leading to further iron loading in this organelle. Iron-mediated production of reactive oxygen species promoted lysosomal membrane permeabilization, activating a cell death pathway consistent with ferroptosis. These findings reveal the prevalence of iron homeostasis in breast CSCs, pointing towards iron and iron-mediated processes as potential targets against these cells.
Lysosomes are acidic organelles essential for degradation and cellular homoeostasis and recently lysosomes have been shown as signaling hub to respond to the intra and extracellular changes (e.g. ...amino acid availability). Compounds including pharmaceutical drugs that are basic and lipophilic will become sequestered inside lysosomes (lysosomotropic). How cells respond to the lysosomal stress associated with lysosomotropism is not well characterized. Our goal is to assess the lysosomal changes and identify the signaling pathways that involve in the lysosomal changes. Eight chemically diverse lysosomotropic drugs from different therapeutic areas were subjected to the evaluation using the human adult retinal pigmented epithelium cell line, ARPE-19. All lysosomotropic drugs tested triggered lysosomal activation demonstrated by increased lysosotracker red (LTR) and lysosensor green staining, increased cathepsin activity, and increased LAMP2 staining. However, tested lysosomotropic drugs also prompted lysosomal dysfunction exemplified by intracellular and extracellular substrate accumulation including phospholipid, SQSTM1/p62, GAPDH (Glyceraldehyde 3-phosphate dehydrogenase) and opsin. Lysosomal activation observed was likely attributed to lysosomal dysfunction, leading to compensatory responses including nuclear translocation of transcriptional factors TFEB, TFE3 and MITF. The adaptive changes are protective to the cells under lysosomal stress. Mechanistic studies implicate calcium and mTORC1 modulation involvement in the adaptive changes. These results indicate that lysosomotropic compounds could evoke a compensatory lysosomal biogenic response but with the ultimate consequence of lysosomal functional impairment. This work also highlights a pathway of response to lysosomal stress and evidences the role of TFEB, TFE3 and MITF in the stress response.
Exciting new discoveries have transformed the view of the lysosome from a static organelle dedicated to the disposal and recycling of cellular waste to a highly dynamic structure that mediates the ...adaptation of cell metabolism to environmental cues. Lysosome-mediated signalling pathways and transcription programmes are able to sense the status of cellular metabolism and control the switch between anabolism and catabolism by regulating lysosomal biogenesis and autophagy. The lysosome also extensively communicates with other cellular structures by exchanging content and information and by establishing membrane contact sites. It is now clear that lysosome positioning is a dynamically regulated process and a crucial determinant of lysosomal function. Finally, growing evidence indicates that the role of lysosomal dysfunction in human diseases goes beyond rare inherited diseases, such as lysosomal storage disorders, to include common neurodegenerative and metabolic diseases, as well as cancer. Together, these discoveries highlight the lysosome as a regulatory hub for cellular and organismal homeostasis, and an attractive therapeutic target for a broad variety of disease conditions.
Lysosomes play important roles in cellular degradation to maintain cell homeostasis. In order to understand whether and how lysosomes alter with age and contribute to lifespan regulation, we ...characterized multiple properties of lysosomes during the aging process in
. We uncovered age-dependent alterations in lysosomal morphology, motility, acidity and degradation activity, all of which indicate a decline in lysosome function with age. The age-associated lysosomal changes are suppressed in the long-lived mutants
,
and
, which extend lifespan by inhibiting insulin/IGF-1 signaling, reducing food intake and impairing mitochondrial function, respectively. We found that 43 lysosome genes exhibit reduced expression with age, including genes encoding subunits of the proton pump V-ATPase and cathepsin proteases. The expression of lysosome genes is upregulated in the long-lived mutants, and this upregulation requires the functions of DAF-16/FOXO and SKN-1/NRF2 transcription factors. Impairing lysosome function affects clearance of aggregate-prone proteins and disrupts lifespan extension in
,
and
worms. Our data indicate that lysosome function is modulated by multiple longevity pathways and is important for lifespan extension.
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