Lysosomes are acidic compartments filled with more than 60 different types of hydrolases. They mediate the degradation of extracellular particles from endocytosis and of intracellular components from ...autophagy. The digested products are transported out of the lysosome via specific catabolite exporters or via vesicular membrane trafficking. Lysosomes also contain more than 50 membrane proteins and are equipped with the machinery to sense nutrient availability, which determines the distribution, number, size, and activity of lysosomes to control the specificity of cargo flux and timing (the initiation and termination) of degradation. Defects in degradation, export, or trafficking result in lysosomal dysfunction and lysosomal storage diseases (LSDs). Lysosomal channels and transporters mediate ion flux across perimeter membranes to regulate lysosomal ion homeostasis, membrane potential, catabolite export, membrane trafficking, and nutrient sensing. Dysregulation of lysosomal channels underlies the pathogenesis of many LSDs and possibly that of metabolic and common neurodegenerative diseases.
Lysosomal exocytosis and lipid storage disorders Samie, Mohammad Ali; Xu, Haoxing
Journal of lipid research,
June 2014, 2014-06-00, 20140601, 2014-06-01, Letnik:
55, Številka:
6
Journal Article
Recenzirano
Odprti dostop
Lysosomes are acidic compartments in mammalian cells that are primarily responsible for the breakdown of endocytic and autophagic substrates such as membranes, proteins, and lipids into their basic ...building blocks. Lysosomal storage diseases (LSDs) are a group of metabolic disorders caused by genetic mutations in lysosomal hydrolases required for catabolic degradation, mutations in lysosomal membrane proteins important for catabolite export or membrane trafficking, or mutations in nonlysosomal proteins indirectly affecting these lysosomal functions. A hallmark feature of LSDs is the primary and secondary excessive accumulation of undigested lipids in the lysosome, which causes lysosomal dysfunction and cell death, and subsequently pathological symptoms in various tissues and organs. There are more than 60 types of LSDs, but an effective therapeutic strategy is still lacking for most of them. Several recent in vitro and in vivo studies suggest that induction of lysosomal exocytosis could effectively reduce the accumulation of the storage materials. Meanwhile, the molecular machinery and regulatory mechanisms for lysosomal exocytosis are beginning to be revealed. In this paper, we first discuss these recent developments with the focus on the functional interactions between lipid storage and lysosomal exocytosis. We then discuss whether lysosomal exocytosis can be manipulated to correct lysosomal and cellular dysfunction caused by excessive lipid storage, providing a potentially general therapeutic approach for LSDs.
The transient receptor potential (TRP) channel superfamily consists of a large group of non-selective cation channels that serve as cellular sensors for a wide spectrum of physical and environmental ...stimuli. The 28 mammalian TRPs, categorized into six subfamilies, including TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPA (ankyrin), TRPML (mucolipin) and TRPP (polycystin), are widely expressed in different cells and tissues. TRPs exhibit a variety of unique features that not only distinguish them from other superfamilies of ion channels, but also confer diverse physiological functions. Located at the plasma membrane or in the membranes of intracellular organelles, TRPs are the cellular safeguards that sense various cell stresses and environmental stimuli and translate this information into responses at the organismal level. Loss- or gain-of-function mutations of TRPs cause inherited diseases and pathologies in different physiological systems, whereas up- or down-regulation of TRPs is associated with acquired human disorders. In this Cell Science at a Glance article and the accompanying poster, we briefly summarize the history of the discovery of TRPs, their unique features, recent advances in the understanding of TRP activation mechanisms, the structural basis of TRP Ca2+ selectivity and ligand binding, as well as potential roles in mammalian physiology and pathology.
Sensing and clearance of dysfunctional lysosomes is critical for cellular homeostasis. Here we show that transcription factor EB (TFEB)-a master transcriptional regulator of lysosomal biogenesis and ...autophagy-is activated during the lysosomal damage response, and its activation is dependent on the function of the ATG conjugation system, which mediates LC3 lipidation. In addition, lysosomal damage triggers LC3 recruitment on lysosomes, where lipidated LC3 interacts with the lysosomal calcium channel TRPML1, facilitating calcium efflux essential for TFEB activation. Furthermore, we demonstrate the presence and importance of this TFEB activation mechanism in kidneys in a mouse model of oxalate nephropathy accompanying lysosomal damage. A proximal tubule-specific TFEB-knockout mouse exhibited progression of kidney injury induced by oxalate crystals. Together, our results reveal unexpected mechanisms of TFEB activation by LC3 lipidation and their physiological relevance during the lysosomal damage response.
To mediate the degradation of biomacromolecules, lysosomes must traffic towards cargo-carrying vesicles for subsequent membrane fusion or fission. Mutations of the lysosomal Ca(2+) channel TRPML1 ...cause lysosomal storage disease (LSD) characterized by disordered lysosomal membrane trafficking in cells. Here we show that TRPML1 activity is required to promote Ca(2+)-dependent centripetal movement of lysosomes towards the perinuclear region (where autophagosomes accumulate) following autophagy induction. ALG-2, an EF-hand-containing protein, serves as a lysosomal Ca(2+) sensor that associates physically with the minus-end-directed dynactin-dynein motor, while PtdIns(3,5)P(2), a lysosome-localized phosphoinositide, acts upstream of TRPML1. Furthermore, the PtdIns(3,5)P(2)-TRPML1-ALG-2-dynein signalling is necessary for lysosome tubulation and reformation. In contrast, the TRPML1 pathway is not required for the perinuclear accumulation of lysosomes observed in many LSDs, which is instead likely to be caused by secondary cholesterol accumulation that constitutively activates Rab7-RILP-dependent retrograde transport. Ca(2+) release from lysosomes thus provides an on-demand mechanism regulating lysosome motility, positioning and tubulation.
Phosphoinositides serve as address labels for recruiting peripheral cytoplasmic proteins to specific subcellular compartments, and as endogenous factors for modulating the activity of integral ...membrane proteins. Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P ₂) is a plasma-membrane (PM)–specific phosphoinositide and a positive cofactor required for the activity of most PM channels and transporters. This requirement for phosphoinositide cofactors has been proposed to prevent PM channel/transporter activity during passage through the biosynthetic/secretory and endocytic pathways. To determine whether intracellularly localized channels are similarly “inactivated” at the PM, we studied PIP ₂ modulation of intracellular TRPML1 channels. TRPML1 channels are primarily localized in lysosomes, but can also be detected temporarily in the PM upon lysosomal exocytosis. By directly patch-clamping isolated lysosomes, we previously found that lysosomal, but not PM-localized, TRPML1 is active with PI(3,5)P ₂, a lysosome-specific PIP ₂, as the underlying positive cofactor. Here we found that “silent” PM-localized TRPML1 could be activated by depleting PI(4,5)P ₂ levels and/or by adding PI(3,5)P ₂ to inside-out membrane patches. Unlike PM channels, surface-expressed TRPML1 underwent a unique and characteristic run-up upon patch excision, and was potently inhibited by a low micromolar concentration of PI(4,5)P ₂. Conversely, depletion of PI(4,5)P ₂ by either depolarization-induced activation or chemically induced translocation of 5′-phosphatase potentiated whole-cell TRPML1 currents. PI(3,5)P ₂ activation and PI(4,5)P ₂ inhibition of TRPML1 were mediated by distinct basic amino acid residues in a common PIP ₂-interacting domain. Thus, PI(4,5)P ₂ may serve as a negative cofactor for intracellular channels such as TRPML1. Based on these results, we propose that phosphoinositide regulation sets compartment-specific activity codes for membrane channels and transporters.
The mucolipin family of Transient Receptor Potential (TRPML) proteins is predicted to encode ion channels expressed in intracellular endosomes and lysosomes. Loss-of-function mutations of human
...TRPML1 cause type IV mucolipidosis (ML4), a childhood neurodegenerative disease. Meanwhile, gain-of-function mutations in the mouse
TRPML3 result in the
varitint-waddler (
Va) phenotype with hearing and pigmentation defects. The broad spectrum phenotypes of ML4 and
Va appear to result from certain aspects of endosomal/lysosomal dysfunction. Lysosomes, traditionally believed to be the terminal “recycling center” for biological “garbage”, are now known to play indispensable roles in intracellular signal transduction and membrane trafficking. Studies employing animal models and cell lines in which TRPML genes have been genetically disrupted or depleted have uncovered roles of TRPMLs in multiple cellular functions including membrane trafficking, signal transduction, and organellar ion homeostasis. Physiological assays of mammalian cell lines in which TRPMLs are heterologously overexpressed have revealed the channel properties of TRPMLs in mediating cation (Ca
2+/Fe
2+) efflux from endosomes and lysosomes in response to unidentified cellular cues. This review aims to summarize these recent advances in the TRPML field and to correlate the channel properties of endolysosomal TRPMLs with their biological functions. We will also discuss the potential cellular mechanisms by which TRPML deficiency leads to neurodegeneration.
Cellular stresses trigger autophagy to remove damaged macromolecules and organelles. Lysosomes 'host' multiple stress-sensing mechanisms that trigger the coordinated biogenesis of autophagosomes and ...lysosomes. For example, transcription factor (TF)EB, which regulates autophagy and lysosome biogenesis, is activated following the inhibition of mTOR, a lysosome-localized nutrient sensor. Here we show that reactive oxygen species (ROS) activate TFEB via a lysosomal Ca(2+)-dependent mechanism independent of mTOR. Exogenous oxidants or increasing mitochondrial ROS levels directly and specifically activate lysosomal TRPML1 channels, inducing lysosomal Ca(2+) release. This activation triggers calcineurin-dependent TFEB-nuclear translocation, autophagy induction and lysosome biogenesis. When TRPML1 is genetically inactivated or pharmacologically inhibited, clearance of damaged mitochondria and removal of excess ROS are blocked. Furthermore, TRPML1's ROS sensitivity is specifically required for lysosome adaptation to mitochondrial damage. Hence, TRPML1 is a ROS sensor localized on the lysosomal membrane that orchestrates an autophagy-dependent negative-feedback programme to mitigate oxidative stress in the cell.
Impaired homeostasis of lysosomal Ca(2+) causes lysosome dysfunction and lysosomal storage diseases (LSDs), but the mechanisms by which lysosomes acquire and refill Ca(2+) are not known. We developed ...a physiological assay to monitor lysosomal Ca(2+) store refilling using specific activators of lysosomal Ca(2+) channels to repeatedly induce lysosomal Ca(2+) release. In contrast to the prevailing view that lysosomal acidification drives Ca(2+) into the lysosome, inhibiting the V-ATPase H(+) pump did not prevent Ca(2+) refilling. Instead, pharmacological depletion or chelation of Endoplasmic Reticulum (ER) Ca(2+) prevented lysosomal Ca(2+) stores from refilling. More specifically, antagonists of ER IP3 receptors (IP3Rs) rapidly and completely blocked Ca(2+) refilling of lysosomes, but not in cells lacking IP3Rs. Furthermore, reducing ER Ca(2+) or blocking IP3Rs caused a dramatic LSD-like lysosome storage phenotype. By closely apposing each other, the ER may serve as a direct and primary source of Ca(2+)for the lysosome.
Transient receptor potential mucolipin 1 (TRPML1) is a cation channel located within endosomal and lysosomal membranes. Ubiquitously expressed in mammalian cells, its loss-of-function mutations are ...the direct cause of type IV mucolipidosis, an autosomal recessive lysosomal storage disease. Here we present the single-particle electron cryo-microscopy structure of the mouse TRPML1 channel embedded in nanodiscs. Combined with mutagenesis analysis, the TRPML1 structure reveals that phosphatidylinositol-3,5-bisphosphate (PtdIns(3,5)P
) binds to the N terminus of the channel-distal from the pore-and the helix-turn-helix extension between segments S2 and S3 probably couples ligand binding to pore opening. The tightly packed selectivity filter contains multiple ion-binding sites, and the conserved acidic residues form the luminal Ca
-blocking site that confers luminal pH and Ca
modulation on channel conductance. A luminal linker domain forms a fenestrated canopy atop the channel, providing several luminal ion passages to the pore and creating a negative electrostatic trap, with a preference for divalent cations, at the luminal entrance. The structure also reveals two equally distributed S4-S5 linker conformations in the closed channel, suggesting an S4-S5 linker-mediated PtdInsP
gating mechanism among TRPML channels.
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