Lipid storage disorders block lysosomal trafficking by inhibiting a TRP channel and lysosomal calcium release

Plasma Cathepsin D Activity Rather Than Levels Correlates With Metabolic Parameters of Type 2 Diabetes in Male Individuals

Objective

Type 2 diabetes mellitus is a metabolic disorder characterized by insulin resistance. Previous studies in patients demonstrated that plasma levels of cathepsin D (CTSD), which is optimally active in the acidic environment of lysosomes, correlate with insulin resistance. As plasma pH is slightly reduced in type 2 diabetic patients and we have previously shown that plasma CTSD activity is causally linked to insulin levels in vivo, it is likely that the activity of CTSD in plasma will be increased in type 2 diabetes compared to healthy individuals. However, so far the interaction between CTSD activity and levels to postprandial metabolic derangements in type 2 diabetes is not known.

Methods

Eighteen type 2 diabetes and 16 age-matched healthy males were given 2 consecutive standardized mixed meals, after which blood samples were collected. Plasma metabolic parameters as well as CTSD levels and activity were measured, and changes in plasma pH was assessed.

Results

In line with the elevation of plasma free fatty acids (FFA) levels in male type 2 diabetics patients, plasma pH in type 2 diabetic individuals was decreased compared to male healthy individuals. While plasma CTSD levels were similar, plasma CTSD activity was increased in male type 2 diabetic compared to male healthy individuals. Besides, plasma CTSD activity rather than levels significantly correlated with indicators of type 2 diabetes (HbA1c, HOMA-IR and glucose). Furthermore, FFA was also independently associated with plasma CTSD activity (standardized β = 0.493, p = 0.007).

Conclusions

Despite similar plasma CTSD levels, type 2 diabetic male individuals showed increased plasma CTSD activity compared to healthy males, which was independently linked to plasma FFA levels. Our data therefore point toward plasma CTSD as a metabolic regulator in male type 2 diabetes.

LC3 lipidation is essential for TFEB activation during the lysosomal damage response to kidney injury

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.

A DNA-Based FLIM Reporter for Simultaneous Quantification of Lysosomal pH and Ca2+ during Autophagy Regulation

pH and Ca²⁺ play important roles in regulating lysosomal activity and lysosome-mediated physiological and pathological processes. However, effective methods for simultaneous determination of pH and Ca²⁺ is the bottleneck. Herein, a single DNA-based FLIM reporter was developed for real-time imaging and simultaneous quantification of pH and Ca²⁺ in lysosomes with high affinity, in which a specific probe for recognition of Ca²⁺ was assembled onto a DNA nanostructure together with pH-responsive and lysosome-targeted molecules. The developed DNA reporter showed excellent biocompatibility and long-term stability up to ∼56 h in lysosomes. Using this powerful tool, it was discovered that pH was closely related to Ca²⁺ concentration in lysosome, whereas autophagy can be regulated by lysosomal pH and Ca²⁺. Furthermore, Aβ-induced neuronal death resulted from autophagy abnormal through lysosomal pH and Ca²⁺ changes. In addition, lysosomal pH and Ca²⁺ were found to regulate the transformation of NSCs, resulting in Rapamycin-induced antiaging.

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A DNA-based FLIM reporter was developed for tracking lysosomal pH and Ca²⁺

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It was found that autophagy could be induced by lysosomal pH and Ca²⁺

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Aβ-induced neuronal death was due to pH_ly- and [Ca²⁺]_ly-mediated autophagy abnormal

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Antiaging-related transformation of qNSCs can be regulated by pH_ly and [Ca²⁺]_ly

Mitochondria-lysosome contacts regulate mitochondrial Ca2+ dynamics via lysosomal TRPML1

Mitochondria and lysosomes are critical for cellular homeostasis, and dysfunction of both organelles has been implicated in numerous diseases. Recently, interorganelle contacts between mitochondria and lysosomes were identified and found to regulate mitochondrial dynamics. However, whether mitochondria-lysosome contacts serve additional functions by facilitating the direct transfer of metabolites or ions between the two organelles has not been elucidated. Here, using high spatial and temporal resolution live-cell microscopy, we identified a role for mitochondria-lysosome contacts in regulating mitochondrial calcium dynamics through the lysosomal calcium efflux channel, transient receptor potential mucolipin 1 (TRPML1). Lysosomal calcium release by TRPML1 promotes calcium transfer to mitochondria, which was mediated by tethering of mitochondria-lysosome contact sites. Moreover, mitochondrial calcium uptake at mitochondria-lysosome contact sites was modulated by the outer and inner mitochondrial membrane channels, voltage-dependent anion channel 1 and the mitochondrial calcium uniporter, respectively. Since loss of TRPML1 function results in the lysosomal storage disorder mucolipidosis type IV (MLIV), we examined MLIV patient fibroblasts and found both altered mitochondria-lysosome contact dynamics and defective contact-dependent mitochondrial calcium uptake. Thus, our work highlights mitochondria-lysosome contacts as key contributors to interorganelle calcium dynamics and their potential role in the pathophysiology of disorders characterized by dysfunctional mitochondria or lysosomes.

Contribution of NAADP to Glutamate-Evoked Changes in Ca2+ Homeostasis in Mouse Hippocampal Neurons

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a second messenger that evokes calcium release from intracellular organelles by the engagement of calcium release channels, including members of the Transient Receptor Potential (TRP) family, such as TRPML1, the (structurally) related Two Pore Channel type 1 (TPC1) and TPC2 channels as well as Ryanodine Receptors type 1 (RYR1; Guse, 2012). NAADP evokes calcium release from acidic calcium stores of many cell types (Guse, 2012), and NAADP-sensitive Ca²⁺ stores have been described in hippocampal neurons of the rat (Bak et al., 1999; McGuinness et al., 2007). Glutamate triggers Ca²⁺-mediated neuronal excitotoxicity in inflammation-induced neurodegenerative pathologies such as Multiple Sclerosis (MS; Friese et al., 2014), and when applied extracellularly to neurons glutamate can elevate NAADP levels in these cells. Accordingly, glutamate-evoked Ca²⁺ signals from intracellular organelles were inhibited by preventing organelle acidification (Pandey et al., 2009). Analysis of reported RNA sequencing experiments of cultured hippocampal neurons revealed the abundance of Mcoln1 (encoding TRPML1), Tpcn1, and Tpcn2 (encoding TPC1 and TPC2, respectively) as potential NAADP target channels in these cells. Transcripts encoding Ryr1 were not found in contrast to Ryr2 and Ryr3. To study the contribution of NAADP signaling to glutamate-evoked calcium transients in murine hippocampal neurons we used the NAADP antagonists Ned-19 (Naylor et al., 2009) and BZ194 (Dammermann et al., 2009). Our results show that both NAADP antagonists significantly reduce glutamate-evoked calcium transients. In addition to extracellular glutamate application, we studied synchronized calcium oscillations in the cells of the neuronal cultures evoked by addition of the GABA_A receptor antagonist bicuculline. Pretreatment with Ned-19 (50 μM) or BZ194 (100 μM) led to an increase in the frequency of bicuculline-induced calcium oscillations at the cost of calcium transient amplitudes. Interestingly, Ned-19 triggered a rise in intracellular calcium concentrations 25 min after bicuculline stimulation, leading to the question whether NAADP acts as a neuroprotective messenger in hippocampal neurons. Taken together, our results are in agreement with the concept that NAADP signaling significantly contributes to glutamate evoked Ca²⁺ rise in hippocampal neurons and to the amplitude and frequency of synchronized Ca²⁺ oscillations triggered by spontaneous glutamate release events.

TRPML1 channels initiate Ca2+ sparks in vascular smooth muscle cells

TRPML1 (transient receptor potential mucolipin 1) is a Ca²⁺-permeable, nonselective cation channel localized to the membranes of endosomes and lysosomes and is not present or functional on the plasma membrane. Ca²⁺ released from endosomes and lysosomes into the cytosol through TRPML1 channels is vital for trafficking, acidification, and other basic functions of these organelles. Here, we investigated the function of TRPML1 channels in fully differentiated contractile vascular smooth muscle cells (SMCs). In live-cell confocal imaging studies, we found that most endosomes and lysosomes in freshly isolated SMCs from cerebral arteries were essentially immobile. Using nanoscale super-resolution microscopy, we found that TRPML1 channels present in late endosomes and lysosomes formed stable complexes with type 2 ryanodine receptors (RyR2) on the sarcoplasmic reticulum (SR). Spontaneous Ca²⁺ signals resulting from the release of SR Ca²⁺ through RyR2s ("Ca²⁺ sparks") and corresponding Ca²⁺-activated K⁺ channel activity are critically important for balancing vasoconstriction. We found that these signals were essentially absent in SMCs from TRPML1-knockout (Mcoln1^-/- ) mice. Using ex vivo pressure myography, we found that loss of this critical signaling cascade exaggerated the vasoconstrictor responses of cerebral and mesenteric resistance arteries. In vivo radiotelemetry studies showed that Mcoln1^-/- mice were spontaneously hypertensive. We conclude that TRPML1 is crucial for the initiation of Ca²⁺ sparks in SMCs and the regulation of vascular contractility and blood pressure.

Lysosomal Ca2+ Homeostasis and Signaling in Health and Disease

Calcium (Ca²⁺) signaling is an essential process in all cells that is maintained by a plethora of channels, pumps, transporters, receptors, and intracellular Ca²⁺ sequestering stores. Changes in cytosolic Ca²⁺ concentration govern processes as far reaching as fertilization, cell growth, and motility through to cell death. In recent years, lysosomes have emerged as a major intracellular Ca²⁺ storage organelle with an increasing involvement in triggering or regulating cellular functions such as endocytosis, autophagy, and Ca²⁺ release from the endoplasmic reticulum. This review will summarize recent work in the area of lysosomal Ca²⁺ signaling and homeostasis, including newly identified functions, and the involvement of lysosome-derived Ca²⁺ signals in human disease. In addition, we explore recent controversies in the techniques used for measurement of lysosomal Ca²⁺ content.

The aging lysosome: An essential catalyst for late-onset neurodegenerative diseases

Lysosomes figure prominently in theories of aging as the proteolytic system most responsible for eliminating growing burdens of damaged proteins and organelles in aging neurons and other long lived cells. Newer evidence shows that diverse experimental measures known to extend lifespan in invertebrate aging models share the property of boosting lysosomal clearance of substrates through the autophagy pathway. Maintaining an optimal level of lysosome acidification is particularly crucial for these anti-aging effects. The exceptional dependence of neurons on fully functional lysosomes is reflected by the neurological phenotypes that develop in congenital lysosomal storage disorders, which commonly present as severe neurodevelopmental or neurodegenerative conditions even though the lysosomal deficit maybe systemic. Similar connections are now being appreciated between primary lysosomal deficit and the risk for late age-onset neurodegenerative disorders. In diseases such as Alzheimer's and Parkinson's, as in aging alone, primary lysosome dysfunction due to acidification impairment is emerging as a frequent theme, supported by the growing list of familial neurodegenerative disorders that involve primary vATPase dysfunction. The additional cellular roles played by intraluminal pH in sensing nutrient and stress and modulating cellular signaling have further expanded the possible ways that lysosomal pH dysregulation in aging and disease can disrupt neuronal function. Here, we consider the impact of cellular aging on lysosomes and how the changes during aging may create the tipping point for disease emergence in major late-age onset neurodegenerative disorders.

Lysosomal Biology and Function: Modern View of Cellular Debris Bin

Lysosomes are the main proteolytic compartments of mammalian cells comprising of a battery of hydrolases. Lysosomes dispose and recycle extracellular or intracellular macromolecules by fusing with endosomes or autophagosomes through specific waste clearance processes such as chaperone-mediated autophagy or microautophagy. The proteolytic end product is transported out of lysosomes via transporters or vesicular membrane trafficking. Recent studies have demonstrated lysosomes as a signaling node which sense, adapt and respond to changes in substrate metabolism to maintain cellular function. Lysosomal dysfunction not only influence pathways mediating membrane trafficking that culminate in the lysosome but also govern metabolic and signaling processes regulating protein sorting and targeting. In this review, we describe the current knowledge of lysosome in influencing sorting and nutrient signaling. We further present a mechanistic overview of intra-lysosomal processes, along with extra-lysosomal processes, governing lysosomal fusion and fission, exocytosis, positioning and membrane contact site formation. This review compiles existing knowledge in the field of lysosomal biology by describing various lysosomal events necessary to maintain cellular homeostasis facilitating development of therapies maintaining lysosomal function.

Involvement of the TRPML Mucolipin Channels in Viral Infections and Anti-viral Innate Immune Responses

The TRPML channels (TRPML1, TRPML2, and TRPML3), belonging to the mucolipin TRP subfamily, primary localize to a population of membrane-bonded vesicles along the endocytosis, and exocytosis pathways. Human viruses enter host cells by plasma membrane penetration or by receptor-mediated endocytosis. TRPML2 enhances the infectivity of a number of enveloped viruses by promoting virus vesicular trafficking and escape from endosomal compartment. TRPML2 expression is stimulated by interferon and by several toll like receptor (TLR) activators, suggesting a possible role in the activation of the innate immune response. Noteworthy, TRPML1 plays a major role in single strand RNA/DNA trafficking into lysosomes and the lack of TRPML1 impairs the TLR-7 and TLR-9 ligand transportation to lysosomes resulting in decreased dendritic cell maturation/activation and migration to the lymph nodes. TRPML channels are also expressed by natural killer (NK) cells, a subset of innate lymphocytes with an essential role during viral infections; recent findings have indicated a role of TRPML1-mediated modulation of secretory lysosomes in NK cells education. Moreover, as also NK cells express TLR recognizing viral pattern, an increased TLR-mediated activation of cytokine production can be envisaged, suggesting a dual role in the NK cell-mediated antiviral responses. Overall, TRPML channels might play a double-edged sword in resistance to viral infections: on one side they can promote virus cellular entry and infectivity; on the other side, by regulating TLR responses in the various immune cells, they contribute to enhance antiviral innate and possibly adaptive immune responses.

TRP Channels as Interior Designers: Remodeling the Endolysosomal Compartment in Natural Killer Cells

Cytotoxic lymphocytes, including natural killer (NK) cells and T cells are distinguished by their ability to eliminate target cells through release of secretory lysosomes. Conventional lysosomes and secretory lysosomes are part of the pleomorphic endolysosomal system and characterized by its highly dynamic nature. Several calcium-permeable TRP calcium channels play an essential role in endolysosomal calcium signaling to ensure proper function of these organelles. In NK cells, the expression of self MHC-specific inhibitory receptors dynamically tunes their secretory potential in a non-transcriptional, calcium-dependent manner. New insights suggest that TRPML1-mediated lysosomal calcium fluxes are tightly interconnected to NK cell functionality through modulation of granzyme B and perforin content of the secretory lysosome. Lysosomal TRP channels show a subset-specific expression pattern during NK differentiation, which is paralleled with gradually increased loading of effector molecules in secretory lysosomes. Methodological advances, including organellar patch-clamping, specific pharmacological modulators, and genetically-encoded calcium indicators open up new possibilities to investigate how TRP channels influence communication between intracellular organelles in immune cells. This review discusses our current understanding of lysosome biogenesis in NK cells with an emphasis on the TRP mucolipin family and the implications for NK cell functionality and cancer immunotherapy.

Lysosomal Exocytosis, Exosome Release and Secretory Autophagy: The Autophagic- and Endo-Lysosomal Systems Go Extracellular

Beyond the consolidated role in degrading and recycling cellular waste, the autophagic- and endo-lysosomal systems play a crucial role in extracellular release pathways. Lysosomal exocytosis is a process leading to the secretion of lysosomal content upon lysosome fusion with plasma membrane and is an important mechanism of cellular clearance, necessary to maintain cell fitness. Exosomes are a class of extracellular vesicles originating from the inward budding of the membrane of late endosomes, which may not fuse with lysosomes but be released extracellularly upon exocytosis. In addition to garbage disposal tools, they are now considered a cell-to-cell communication mechanism. Autophagy is a cellular process leading to sequestration of cytosolic cargoes for their degradation within lysosomes. However, the autophagic machinery is also involved in unconventional protein secretion and autophagy-dependent secretion, which are fundamental mechanisms for toxic protein disposal, immune signalling and pathogen surveillance. These cellular processes underline the crosstalk between the autophagic and the endosomal system and indicate an intersection between degradative and secretory functions. Further, they suggest that the molecular mechanisms underlying fusion, either with lysosomes or plasma membrane, are key determinants to maintain cell homeostasis upon stressing stimuli. When they fail, the accumulation of undigested substrates leads to pathological consequences, as indicated by the involvement of autophagic and lysosomal alteration in human diseases, namely lysosomal storage disorders, age-related neurodegenerative diseases and cancer. In this paper, we reviewed the current knowledge on the functional role of extracellular release pathways involving lysosomes and the autophagic- and endo-lysosomal systems, evaluating their implication in health and disease.

Mechanism of Secondary Ganglioside and Lipid Accumulation in Lysosomal Disease

Gangliosidoses are caused by monogenic defects of a specific hydrolase or an ancillary sphingolipid activator protein essential for a specific step in the catabolism of gangliosides. Such defects in lysosomal function cause a primary accumulation of multiple undegradable gangliosides and glycosphingolipids. In reality, however, predominantly small gangliosides also accumulate in many lysosomal diseases as secondary storage material without any known defect in their catabolic pathway. In recent reconstitution experiments, we identified primary storage materials like sphingomyelin, cholesterol, lysosphingolipids, and chondroitin sulfate as strong inhibitors of sphingolipid activator proteins (like GM2 activator protein, saposin A and B), essential for the catabolism of many gangliosides and glycosphingolipids, as well as inhibitors of specific catabolic steps in lysosomal ganglioside catabolism and cholesterol turnover. In particular, they trigger a secondary accumulation of ganglioside GM2, glucosylceramide and cholesterol in Niemann–Pick disease type A and B, and of GM2 and glucosylceramide in Niemann–Pick disease type C. Chondroitin sulfate effectively inhibits GM2 catabolism in mucopolysaccharidoses like Hurler, Hunter, Sanfilippo, and Sly syndrome and causes a secondary neuronal ganglioside GM2 accumulation, triggering neurodegeneration. Secondary ganglioside and lipid accumulation is furthermore known in many more lysosomal storage diseases, so far without known molecular basis.

Pathophysiological Role of Transient Receptor Potential Mucolipin Channel 1 in Calcium-Mediated Stress-Induced Neurodegenerative Diseases

Mucolipins (TRPML) are endosome/lysosome Ca²⁺ permeable channels belonging to the family of transient receptor potential channels. In mammals, there are three TRPML proteins, TRPML1, 2, and 3, encoded by MCOLN1-3 genes. Among these channels, TRPML1 is a reactive oxygen species sensor localized on the lysosomal membrane that is able to control intracellular oxidative stress due to the activation of the autophagic process. Moreover, genetic or pharmacological inhibition of the TRPML1 channel stimulates oxidative stress signaling pathways. Experimental data suggest that elevated levels of reactive species play a role in several neurological disorders. There is a need to gain better understanding of the molecular mechanisms behind these neurodegenerative diseases, considering that the main sources of free radicals are mitochondria, that mitochondria/endoplasmic reticulum and lysosomes are coupled, and that growing evidence links neurodegenerative diseases to the gain or loss of function of proteins related to lysosome homeostasis. This review examines the significant roles played by the TRPML1 channel in the alterations of calcium signaling responsible for stress-mediated neurodegenerative disorders and its potential as a new therapeutic target for ameliorating neurodegeneration in our ever-aging population.

Sulforaphane Activates a lysosome-dependent transcriptional program to mitigate oxidative stress

Oxidative stress underlies a number of pathological conditions, including cancer, neurodegeneration, and aging. Antioxidant-rich foods help maintain cellular redox homeostasis and mitigate oxidative stress, but the underlying mechanisms are not clear. For example, sulforaphane (SFN), an electrophilic compound that is enriched in cruciferous vegetables such as broccoli, is a potent inducer of cellular antioxidant responses. NFE2L2/NRF2 (nuclear factor, erythroid 2 like 2), a transcriptional factor that controls the expression of multiple detoxifying enzymes through antioxidant response elements (AREs), is a proposed target of SFN. NFE2L2/NRF2 is a target gene of TFEB (transcription factor EB), a master regulator of autophagic and lysosomal functions, which we show here to be potently activated by SFN. SFN induces TFEB nuclear translocation via a Ca²⁺-dependent but MTOR (mechanistic target of rapamycin kinase)-independent mechanism through a moderate increase in reactive oxygen species (ROS). Activated TFEB then boosts the expression of genes required for autophagosome and lysosome biogenesis, which are known to facilitate the clearance of damaged mitochondria. Notably, TFEB activity is required for SFN-induced protection against both acute oxidant bursts and chronic oxidative stress. Hence, by simultaneously activating macroautophagy/autophagy and detoxifying pathways, natural compound SFN may trigger a self-defense cellular mechanism that can effectively mitigate oxidative stress commonly associated with many metabolic and age-related diseases.Abbreviations: ANOVA: analyzes of variance; AREs: antioxidant response elements; Baf-A1: bafilomycin A₁; BHA: butylhydroxyanisole; CAT: catechin hydrate; CCCP: carbonyl cyanide m- chlorophenylhydrazone; CLEAR: coordinated lysosomal expression and regulation; DCFH-DA: 2',7'-dichlorofluorescin diacetate; FBS: fetal bovine serum; GFP: green fluorescent protein; HMOX1/HO-1: heme oxygenase 1; KD: knockdown; KEAP1: kelch like ECH associated protein 1; KO: knockout; LAMP1: lysosomal associated membrane protein 1; MCOLN1/TRPML1: mucolipin 1; ML-SA1: mucolipin-specific synthetic agonist 1; ML-SI3: mucolipin-specific synthetic inhibitor 3; MTOR: mechanistic target of rapamycin kinase; MTORC1: mechanistic target of rapamycin kinase complex 1; NAC: N-acetylcysteine; NFE2L2/NRF2: nuclear factor: erythroid 2 like 2; NPC: Niemann-Pick type C; PBS: phosphate-buffered saline; PPP2/PP2A: protein phosphatase 2; Q-PCR: real time polymerase chain reaction; ROS: reactive oxygen species; RPS6KB1/S6K1/p70S6K: ribosomal protein S6 kinase B1; SFN: sulforaphane; TFEB: transcription factor EB; WT, wild-type.

TMBIM6 (transmembrane BAX inhibitor motif containing 6) enhances autophagy through regulation of lysosomal calcium

Lysosomal Ca²⁺ contributes to macroautophagy/autophagy, an intracellular process for the degradation of cytoplasmic material and organelles in the lysosomes to protect cells against stress responses. TMBIM6 (transmembrane BAX inhibitor motif containing 6) is a Ca²⁺ channel-like protein known to regulate ER stress response and apoptosis. In this study, we examined the as yet unknown role of TMBIM6 in regulating lysosomal Ca²⁺ levels. The Ca²⁺ efflux from the ER through TMBIM6 was found to increase the resting lysosomal Ca²⁺ level, in which ITPR-independent regulation of Ca²⁺ status was observed. Further, TMBIM6 regulated the local release of Ca²⁺ through lysosomal MCOLN1/TRPML1 channels under nutrient starvation or MTOR inhibition. The local Ca²⁺ efflux through MCOLN1 channels was found to activate PPP3/calcineurin, triggering TFEB (transcription factor EB) nuclear translocation, autophagy induction, and lysosome biogenesis. Upon genetic inactivation of TMBIM6, lysosomal Ca²⁺ and the associated TFEB nuclear translocation were decreased. Furthermore, autophagy flux was significantly enhanced in the liver or kidney from starved Tmbim6^+/+ mice compared with that in the counter tmbim6^-/- mice. Together, our observations indicated that under stress conditions, TMBIM6 increases lysosomal Ca²⁺ release, leading to PPP3/calcineurin-mediated TFEB activation and subsequently enhanced autophagy. Thus, TMBIM6, an ER membrane protein, is suggested to be a lysosomal Ca²⁺ modulator that coordinates with autophagy to alleviate metabolism stress.Abbreviations: AVs: autophagic vacuoles; CEPIA: calcium-measuring organelle-entrapped protein indicator; ER: endoplasmic reticulum; GPN: glycyl-L-phenylalanine-beta-naphthylamide; ITPR/IP3R: inositol 1,4,5-trisphosphate receptor; LAMP1: lysosomal associated membrane protein 1; MCOLN/TRPML: mucolipin; MEF: mouse embryonic fibroblast; ML-SA1: mucolipin synthetic agonist 1; MTORC1: mechanistic target of rapamycin kinase complex 1; RPS6KB1: ribosomal protein S6 kinase B1; SQSTM1: sequestosome 1; TFEB: transcription factor EB; TKO: triple knockout; TMBIM6/BI-1: transmembrane BAX inhibitor motif containing 6.

Impairment of Lysosome Function and Autophagy in Rare Neurodegenerative Diseases

Rare genetic diseases affect a limited number of patients, but their etiology is often known, facilitating the development of reliable animal models and giving the opportunity to investigate physiopathology. Lysosomal storage disorders are a group of rare diseases due to primary alteration of lysosome function. These diseases are often associated with neurological symptoms, which highlighted the importance of lysosome in neurodegeneration. Likewise, other groups of rare neurodegenerative diseases also present lysosomal alteration. Lysosomes fuse with autophagosomes and endosomes to allow the degradation of their content thanks to hydrolytic enzymes. It has emerged that alteration of the autophagy–lysosome pathway could play a critical role in neuronal death in many neurodegenerative diseases. Using a repertoire of selected rare neurodegenerative diseases, we highlight that a variety of alterations of the autophagy–lysosome pathway are associated with neuronal death. Yet, in most cases, it is still unclear why alteration of this pathway can lead to neurodegeneration.

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Lysosome function is impaired in many rare neurodegenerative diseases, making it a convergent point for these diseases.

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Impaired lysosome function is associated with alteration of the autophagy pathway.

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Autophagy–lysosome pathway can be impaired at various steps in different rare neurodegenerative diseases.

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The mechanisms linking impaired autophagy–lysosome pathway to neurodegeneration are still not fully elucidated.

Nanoparticle-Mediated Therapeutic Application for Modulation of Lysosomal Ion Channels and Functions

Applications of nanoparticles in various fields have been addressed. Nanomaterials serve as carriers for transporting conventional drugs or proteins through lysosomes to various cellular targets. The basic function of lysosomes is to trigger degradation of proteins and lipids. Understanding of lysosomal functions is essential for enhancing the efficacy of nanoparticles-mediated therapy and reducing the malfunctions of cellular metabolism. The lysosomal function is modulated by the movement of ions through various ion channels. Thus, in this review, we have focused on the recruited ion channels for lysosomal function, to understand the lysosomal modulation through the nanoparticles and its applications. In the future, lysosomal channels-based targets will expand the therapeutic application of nanoparticles-associated drugs.

Acidifying Endolysosomes Prevented Low-Density Lipoprotein-Induced Amyloidogenesis

Cholesterol dyshomeostasis has been linked to the pathogenesis of sporadic Alzheimer's disease (AD). In furthering the understanding of mechanisms by which increased levels of circulating cholesterol augments the risk of developing sporadic AD, others and we have reported that low-density lipoprotein (LDL) enters brain parenchyma by disrupting the blood-brain barrier and that endolysosome de-acidification plays a role in LDL-induced amyloidogenesis in neurons. Here, we tested the hypothesis that endolysosome de-acidification was central to amyloid-β (Aβ) generation and that acidifying endolysosomes protects against LDL-induced increases in Aβ levels in neurons. We demonstrated that LDL, but not HDL, de-acidified endolysosomes and increased intraneuronal and secreted levels of Aβ. ML-SA1, an agonist of endolysosome-resident TRPML1 channels, acidified endolysosomes, and TRPML1 knockdown attenuated ML-SA1-induced endolysosome acidification. ML-SA1 blocked LDL-induced increases in intraneuronal and secreted levels of Aβ as well as Aβ accumulation in endolysosomes, prevented BACE1 accumulation in endolysosomes, and decreased BACE1 activity levels. LDL downregulated TRPML1 protein levels, and TRPML1 knockdown worsens LDL-induced increases in Aβ. Our findings suggest that endolysosome acidification by activating TRPML1 may represent a protective strategy against sporadic AD.

Podocyte Lysosome Dysfunction in Chronic Glomerular Diseases

Podocytes are visceral epithelial cells covering the outer surface of glomerular capillaries in the kidney. Blood is filtered through the slit diaphragm of podocytes to form urine. The functional and structural integrity of podocytes is essential for the normal function of the kidney. As a membrane-bound organelle, lysosomes are responsible for the degradation of molecules via hydrolytic enzymes. In addition to its degradative properties, recent studies have revealed that lysosomes may serve as a platform mediating cellular signaling in different types of cells. In the last decade, increasing evidence has revealed that the normal function of the lysosome is important for the maintenance of podocyte homeostasis. Podocytes have no ability to proliferate under most pathological conditions; therefore, lysosome-dependent autophagic flux is critical for podocyte survival. In addition, new insights into the pathogenic role of lysosome and associated signaling in podocyte injury and chronic kidney disease have recently emerged. Targeting lysosomal functions or signaling pathways are considered potential therapeutic strategies for some chronic glomerular diseases. This review briefly summarizes current evidence demonstrating the regulation of lysosomal function and signaling mechanisms as well as the canonical and noncanonical roles of podocyte lysosome dysfunction in the development of chronic glomerular diseases and associated therapeutic strategies.

Arterial Medial Calcification through Enhanced small Extracellular Vesicle Release in Smooth Muscle-Specific Asah1 Gene Knockout Mice

Arterial medial calcification (AMC) involves an increased small extracellular vesicle (sEV) secretion and apatite calcium precipitation in the arterial wall. The mechanisms mediating AMC remain poorly understood. In the present study, smooth muscle-specific acid ceramidase (Ac) gene knockout mice (Asah1^fl/fl/SM^Cre) were used to demonstrate the role of lysosomal ceramide signaling pathway in AMC. Asah1^fl/fl/SM^Cre mice were found to have more severe AMC in both aorta and coronary arteries compared to their littermates (Asah1^fl/fl/SM^wt and WT/WT mice) after receiving a high dose vitamin D. These mice also had pronounced upregulation of osteopontin and RUNX2 (osteogenic markers), CD63, AnX2 (sEV markers) and ALP expression (mineralization marker) in the arterial media. In cultured coronary arterial smooth muscle cells (CASMCs) from Asah1^fl/fl/SM^Cre mice, high dose of P_i led to a significantly increased calcium deposition, phenotypic change and sEV secretion compared to WT CASMCs, which was associated with reduced lysosome-multivesicular body (MVB) interaction. Also, GW4869, sEV release inhibitor decreased sEV secretion and calcification in these cells. Lysosomal transient receptor potential mucolipin 1 (TRPML1) channels regulating lysosome interaction with MVBs were found remarkably inhibited in Asah1^fl/fl/SM^Cre CASMCs as shown by GCaMP3 Ca²⁺ imaging and Port-a-Patch patch clamping of lysosomes. Lysosomal Ac in SMCs controls sEV release by regulating lysosomal TRPML1 channel activity and lysosome-MVB interaction, which importantly contributes to phenotypic transition and AMC.

Small-molecule activation of lysosomal TRP channels ameliorates Duchenne muscular dystrophy in mouse models

Duchenne muscular dystrophy (DMD) is a devastating disease caused by mutations in dystrophin that compromise sarcolemma integrity. Currently, there is no treatment for DMD. Mutations in transient receptor potential mucolipin 1 (ML1), a lysosomal Ca²⁺ channel required for lysosomal exocytosis, produce a DMD-like phenotype. Here, we show that transgenic overexpression or pharmacological activation of ML1 in vivo facilitates sarcolemma repair and alleviates the dystrophic phenotypes in both skeletal and cardiac muscles of mdx mice (a mouse model of DMD). Hallmark dystrophic features of DMD, including myofiber necrosis, central nucleation, fibrosis, elevated serum creatine kinase levels, reduced muscle force, impaired motor ability, and dilated cardiomyopathies, were all ameliorated by increasing ML1 activity. ML1-dependent activation of transcription factor EB (TFEB) corrects lysosomal insufficiency to diminish muscle damage. Hence, targeting lysosomal Ca²⁺ channels may represent a promising approach to treat DMD and related muscle diseases.

Role of endolysosomes and inter-organellar signaling in brain disease

Endosomes and lysosomes (endolysosomes) are membrane bounded organelles that play a key role in cell survival and cell death. These acidic intracellular organelles are the principal sites for intracellular hydrolytic activity required for the maintenance of cellular homeostasis. Endolysosomes are involved in the degradation of plasma membrane components, extracellular macromolecules as well as intracellular macromolecules and cellular fragments. Understanding the physiological significance and pathological relevance of endolysosomes is now complicated by relatively recent findings of physical and functional interactions between endolysosomes with other intracellular organelles including endoplasmic reticulum, mitochondria, plasma membranes, and peroxisomes. Indeed, evidence clearly indicates that endolysosome dysfunction and inter-organellar signaling occurs in different neurodegenerative diseases including Alzheimer's disease (AD), HIV-1 associated neurocognitive disease (HAND), Parkinson's disease (PD) as well as various forms of brain cancer such as glioblastoma multiforme (GBM). These findings open new areas of cell biology research focusing on understanding the physiological actions and pathophysiological consequences of inter-organellar communication. Here, we will review findings of others and us that endolysosome de-acidification and dysfunction coupled with impaired inter-organellar signaling is involved in the pathogenesis of AD, HAND, PD, and GBM. A more comprehensive appreciation of cell biology and inter-organellar signaling could lead to the development of new drugs to prevent or cure these diseases.

Readily Releasable Stores of Calcium in Neuronal Endolysosomes: Physiological and Pathophysiological Relevance

Neurons are long-lived post-mitotic cells that possess an elaborate system of endosomes and lysosomes (endolysosomes) for protein quality control. Relatively recently, endolysosomes were recognized to contain high concentrations (400-600 μM) of readily releasable calcium. The release of calcium from this acidic organelle store contributes to calcium-dependent processes of fundamental physiological importance to neurons including neurotransmitter release, membrane excitability, neurite outgrowth, synaptic remodeling, and cell viability. Pathologically, disturbances of endolysosome structure and/or function have been noted in a variety of neurodegenerative disorders including Alzheimer's disease (AD) and HIV-1 associated neurocognitive disorder (HAND). And, dysregulation of intracellular calcium has been implicated in the neuropathogenesis of these same neurological disorders. Thus, it is important to better understand mechanisms by which calcium is released from endolysosomes as well as the consequences of such release to inter-organellar signaling, physiological functions of neurons, and possible pathological consequences. In doing so, a path forward towards new therapeutic modalities might be facilitated.

Endo-Lysosomal Cation Channels and Infectious Diseases

Among the infectious diseases caused by pathogenic microorganisms such as bacteria, viruses, parasites, or fungi, the most prevalent ones today are malaria, tuberculosis, influenza, HIV/AIDS, Ebola, dengue fever, and methicillin-resistant Staphylococcus aureus (MRSA) infection, and most recently Covid-19 (SARS-CoV2). Others with a rather devastating history and high fatality rates such as plague, cholera, or typhus seem less threatening today but have not been eradicated, and with a declining efficacy of current antibiotics they ought to be watched carefully. Another emerging issue in this context is health-care associated infection. About 100,000 hospitalized patients in the USA ( www.cdc.gov ) and 33,000 in Europe ( https://www.ecdc.europa.eu ) die each year as a direct consequence of an infection caused by bacteria resistant to antibiotics. Among viral infections, influenza is responsible for about 3-5 million cases of severe illness, and about 250,000 to 500,000 deaths annually ( www.who.int ). About 37 million people are currently living with HIV infection and about one million die from it each year. Coronaviruses such as MERS-CoV, SARS-CoV, but in particular the recent outbreak of Covid-19 (caused by SARS-CoV2) have resulted in large numbers of infections worldwide with an estimated several hundred thousand deaths (anticipated fatality rate: <5%). With a comparatively low mortality rate dengue virus causes between 50 and 100 million infections every year, leading to 50,000 deaths. In contrast, Ebola virus is the causative agent for one of the deadliest viral diseases. The Ebola outbreak in West Africa in 2014 is considered the largest outbreak in history with more than 11,000 deaths. Many of the deadliest pathogens such as Ebola virus, influenza virus, mycobacterium tuberculosis, dengue virus, and cholera exploit the endo-lysosomal trafficking system of host cells for penetration into the cytosol and replication. Defects in endo-lysosomal maturation, trafficking, fusion, or pH homeostasis can efficiently reduce the cytotoxicity caused by these pathogens. Most of these functions critically depend on endo-lysosomal membrane proteins such as transporters and ion channels. In particular, cation channels such as the mucolipins (TRPMLs) or the two-pore channels (TPCs) are involved in all of these aspects of endo-lysosomal integrity. In this review we will discuss the correlations between pathogen toxicity and endo-lysosomal cation channel function, and their potential as drug targets for infectious disease therapy.

Agonist-specific voltage-dependent gating of lysosomal two-pore Na+ channels

Mammalian two-pore-channels (TPC1, 2; TPCN1, TPCN2) are ubiquitously- expressed, PI(3,5)P2-activated, Na+-selective channels in the endosomes and lysosomes that regulate luminal pH homeostasis, membrane trafficking, and Ebola viral infection. Whereas the channel activity of TPC1 is strongly dependent on membrane voltage, TPC2 lacks such voltage dependence despite the presence of the presumed 'S4 voltage-sensing' domains. By performing high-throughput screening followed by lysosomal electrophysiology, here we identified a class of tricyclic anti-depressants (TCAs) as small-molecule agonists of TPC channels. TCAs activate both TPC1 and TPC2 in a voltage-dependent manner, referred to as Lysosomal Na+ channel Voltage-dependent Activators (LyNa-VAs). We also identified another compound which, like PI(3,5)P2, activates TPC2 independent of voltage, suggesting the existence of agonist-specific gating mechanisms. Our identification of small-molecule TPC agonists should facilitate the studies of the cell biological roles of TPCs and can also readily explain the reported effects of TCAs in the modulation of autophagy and lysosomal functions.

TRPML1 links lysosomal calcium to autophagosome biogenesis through the activation of the CaMKKβ/VPS34 pathway

The lysosomal calcium channel TRPML1, whose mutations cause the lysosomal storage disorder (LSD) mucolipidosis type IV (MLIV), contributes to upregulate autophagic genes by inducing the nuclear translocation of the transcription factor EB (TFEB). Here we show that TRPML1 activation also induces autophagic vesicle (AV) biogenesis through the generation of phosphatidylinositol 3-phosphate (PI3P) and the recruitment of essential PI3P-binding proteins to the nascent phagophore in a TFEB-independent manner. Thus, TRPML1 activation of phagophore formation requires the calcium-dependent kinase CaMKKβ and AMPK, which increase the activation of ULK1 and VPS34 autophagic protein complexes. Consistently, cells from MLIV patients show a reduced recruitment of PI3P-binding proteins to the phagophore during autophagy induction, suggesting that altered AV biogenesis is part of the pathological features of this disease. Together, we show that TRPML1 is a multistep regulator of autophagy that may be targeted for therapeutic purposes to treat LSDs and other autophagic disorders.

It was known that prolonged TRMPL1 activation induces TFEB translocation and upregulates autophagic gene regulation. Here, the authors show that acute TRMPL1 activation also induces autophagy through VPS34 and by lysosomal calcium release independent of TFEB.

Lysosomal size matters

Lysosomes are key cellular catabolic centers that also perform fundamental metabolic, signaling and quality control functions. Lysosomes are not static and they respond dynamically to intra‐ and extracellular stimuli triggering changes in organelle numbers, size and position. Such physical changes have a strong impact on lysosomal activity ultimately influencing cellular homeostasis. In this review, we summarize the current knowledge on lysosomal size regulation, on its physiological role(s) and association to several disease conditions.

Disease-associated mutations in Niemann-Pick type C1 alter ER calcium signaling and neuronal plasticity

Niemann-Pick type C1 (NPC1) protein is essential for the transport of externally derived cholesterol from lysosomes to other organelles. Deficiency of NPC1 underlies the progressive NPC1 neurodegenerative disorder. Currently, there are no curative therapies for this fatal disease. Given the Ca2+ hypothesis of neurodegeneration, which posits that altered Ca2+ dynamics contribute to neuropathology, we tested if disease mutations in NPC1 alter Ca2+ signaling and neuronal plasticity. We determine that NPC1 inhibition or disease mutations potentiate store-operated Ca2+ entry (SOCE) due to a presenilin 1 (PSEN1)-dependent reduction in ER Ca2+ levels alongside elevated expression of the molecular SOCE components ORAI1 and STIM1. Associated with this dysfunctional Ca2+ signaling is destabilization of neuronal dendritic spines. Knockdown of PSEN1 or inhibition of the SREBP pathway restores Ca2+ homeostasis, corrects differential protein expression, reduces cholesterol accumulation, and rescues spine density. These findings highlight lysosomes as a crucial signaling platform responsible for tuning ER Ca2+ signaling, SOCE, and synaptic architecture in health and disease.

PSEN2 (presenilin 2) mutants linked to familial Alzheimer disease impair autophagy by altering Ca2+ homeostasis

PSEN2 (presenilin 2) is one of the 3 proteins that, when mutated, causes early onset familial Alzheimer disease (FAD) cases. In addition to its well-known role within the γ-secretase complex (the enzyme ultimately responsible for Aβ peptides formation), PSEN2 is endowed with some γ-secretase-independent functions in distinct cell signaling pathways, such as the modulation of intracellular Ca2+ homeostasis. Here, by using different FAD-PSEN2 cell models, we demonstrate that mutated PSEN2 impairs autophagy by causing a block in the degradative flux at the level of the autophagosome-lysosome fusion step. The defect does not depend on an altered lysosomal functionality but rather on a decreased recruitment of the small GTPase RAB7 to autophagosomes, a key event for normal autophagy progression. Importantly, FAD-PSEN2 action on autophagy is unrelated to its γ-secretase activity but depends on its previously reported ability to partially deplete ER Ca2+ content, thus reducing cytosolic Ca2+ response upon IP3-linked cell stimulations. Our data sustain the pivotal role for Ca2+ signaling in autophagy and reveal a novel mechanism by which FAD-linked presenilins alter the degradative process, reinforcing the view of a causative role for a dysfunctional quality control pathway in AD neurodegeneration.Abbreviations: Aβ: amyloid β; AD: Alzheimer disease; ACTB: actin beta; AMPK: AMP-activated protein kinase; APP: amyloid-beta precursor protein; BafA: bafilomycin A1; BAPTA-AM: 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester; CFP: cyan fluorescent protein; EGTA-AM: ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid acetoxymethyl ester; ER: endoplasmic reticulum; EGFP-HDQ74: enhanced GFP-huntingtin exon 1 containing 74 polyglutamine repeats; FAD: familial Alzheimer disease; FCS: fetal calf serum; FRET: fluorescence/Förster resonance energy transfer; GFP: green fluorescent protein; IP3: inositol trisphosphate; KD: knockdown; LAMP1: lysosomal associated membrane protein 1; MAP1LC3-II/LC3-II: lipidated microtubule-associated protein 1 light chain 3; MCU: mitochondrial calcium uniporter; MICU1: mitochondrial calcium uptake 1; MEFs: mouse embryonic fibroblasts; MFN2: mitofusin 2; MTOR: mechanistic target of rapamycin kinase; MTORC1: MTOR complex 1; SQSTM1/p62: sequestosome 1; PSEN1: presenilin 1; PSEN2: presenilin 2; RAB7: RAB7A: member RAS oncogene family; RFP: red fluorescent protein; ATP2A/SERCA: ATPase sarcoplasmic/endoplasmic reticulum Ca2+ transporting; siRNA: small interference RNA; V-ATPase: vacuolar-type H+-ATPase; WT: wild type.

Ion channels as potential redox sensors in lysosomes

Lysosomes are central organelles that recycle materials and energy to maintain intracellular homeostasis. Lysosomes are capable of sensing environmental cues such as nutrition to regulate their function accordingly. Whether lysosomes can sense redox signaling, however, was unclear. Here in this review, we summarized recent evidence of lysosomal ion channel as redox sensors for this organelle. We also discussed their roles in lysosomal diseases that features imbalanced redox.

The Interplay between Ca2+ Signaling Pathways and Neurodegeneration

Calcium (Ca²⁺) homeostasis is essential for cell maintenance since this ion participates in many physiological processes. For example, the spatial and temporal organization of Ca²⁺ signaling in the central nervous system is fundamental for neurotransmission, where local changes in cytosolic Ca²⁺ concentration are needed to transmit information from neuron to neuron, between neurons and glia, and even regulating local blood flow according to the required activity. However, under pathological conditions, Ca²⁺ homeostasis is altered, with increased cytoplasmic Ca²⁺ concentrations leading to the activation of proteases, lipases, and nucleases. This review aimed to highlight the role of Ca²⁺ signaling in neurodegenerative disease-related apoptosis, where the regulation of intracellular Ca²⁺ homeostasis depends on coordinated interactions between the endoplasmic reticulum, mitochondria, and lysosomes, as well as specific transport mechanisms. In neurodegenerative diseases, alterations-increased oxidative stress, energy metabolism alterations, and protein aggregation have been identified. The aggregation of α-synuclein, β-amyloid peptide (Aβ), and huntingtin all adversely affect Ca²⁺ homeostasis. Due to the mounting evidence for the relevance of Ca²⁺ signaling in neuroprotection, we would focus on the expression and function of Ca²⁺ signaling-related proteins, in terms of the effects on autophagy regulation and the onset and progression of neurodegenerative diseases.

Structural insights into group II TRP channels

The seven members of the TRP channel superfamily are divided into two main groups with five members comprising group I (TRPC/V/M/N/A) and TRPML (TRP MucoLipin) and TRPP (TRP Polycystin) making up group II. Group II channels share a high sequence homology on their transmembrane domains and are distinct from group I members as they contain a large luminal/extracellular domain between transmembrane helix 1 (S1) and S2. Since 2016, there are more than ten research papers reporting various structures of group II channels by either cryo-EM or X-ray crystallography. These studies along with recent functional analysis by the other groups have considerably strengthened our knowledge on TRPML and TRPP channels. In this review, we summarize and discuss these reports providing molecular insights into the group II TRP channel family.

Muscle-tendon weakness contributes to chronic fatigue syndrome in Gaucher's disease

Background

Chronic fatigue (CFg) is a prevalent symptom in Gaucher disease (GD) at diagnosis (79%) and remains in a quarter of patients after years of therapy. Bone abnormalities are present in over 70% and peripheral neuropathy in about 11% of the patients, which contributes to the disabling and debilitating complications. Our hypothesis is that other factors such as muscle-tendinous weakness could have influence in the development of CFg.

Methods

We have evaluated the fiber structure and elasticity of muscle-tendinous unit by strain-elastography (S-ELA) and analyzed their influence in the CFg. S-ELA study was performed in Achilles tendon in 25 type 1 and two type 3 GD patients, all of them with fatigue and were on enzymatic replacement therapy for mean 13 years; simultaneously, bone marrow burden by MRI and calcaneus ultrasound densitometry were evaluated. Blood cell counts, plasma biomarkers, GBA1 genotyping, and SF36 quality of life scale (QoL) were also performed. Statistical analysis: descriptive and comparative test.

Results

All patients showed a normal Achilles tendinous structure. Abnormal stiff grade 2–3 was found in 17/27 (62.9%); in 11/27 (40.7%) of patients, the alteration was bilateral. There were no correlations between the S-ELA results to other variables; nevertheless, a significant correlation between the degree of tendon hardness and the low score on the QoL scales (p = 0.0035) was found. The S-ELA is a sensitive painless, fast, and low cost method to detect muscle-tendinous subclinical dysfunction that could contribute to CFg in GD. The identification of subclinical tendon alteration would be a sign of alarm, focused on the risk of development of bone complications.

Conclusion

Intratendinous alteration in strain-elastography is an independent variable in GD patients with persistent fatigue.

Niemann-Pick type C disease: cellular pathology and pharmacotherapy

Niemann-Pick type C disease (NPCD) was first described in 1914 and affects approximately 1 in 150 000 live births. It is characterized clinically by diverse symptoms affecting liver, spleen, motor control, and brain; premature death invariably results. Its molecular origins were traced, as late as 1997, to a protein of late endosomes and lysosomes which was named NPC1. Mutation or absence of this protein leads to accumulation of cholesterol in these organelles. In this review, we focus on the intracellular events that drive the pathology of this disease. We first introduce endocytosis, a much-studied area of dysfunction in NPCD cells, and survey the various ways in which this process malfunctions. We briefly consider autophagy before attempting to map the more complex pathways by which lysosomal cholesterol storage leads to protein misregulation, mitochondrial dysfunction, and cell death. We then briefly introduce the metabolic pathways of sphingolipids (as these emerge as key species for treatment) and critically examine the various treatment approaches that have been attempted to date.

Sub-nanomolar sensitive GZnP3 reveals TRPML1-mediated neuronal Zn2+ signals

Although numerous fluorescent Zn²⁺ sensors have been reported, it is unclear whether and how Zn²⁺ can be released from the intracellular compartments into the cytosol due to a lack of probes that can detect physiological dynamics of cytosolic Zn²⁺. Here, we create a genetically encoded sensor, GZnP3, which demonstrates unprecedented sensitivity for Zn²⁺ at sub-nanomolar concentrations. Using GZnP3 as well as GZnP3-derived vesicular targeted probes, we provide the first direct evidence that Zn²⁺ can be released from endolysosomal vesicles to the cytosol in primary hippocampal neurons through the TRPML1 channel. Such TRPML1-mediated Zn²⁺ signals are distinct from Ca²⁺ in that they are selectively present in neurons, sustain longer, and are significantly higher in neurites as compared to the soma. Together, our work not only creates highly sensitive probes for investigating sub-nanomolar Zn²⁺ dynamics, but also reveals new pools of Zn²⁺ signals that can play critical roles in neuronal function.

Numerous fluorescent Zn²⁺ sensors are available but most are unsuitable to detect physiological dynamics of cytosolic Zn²⁺. In this study, the authors present a genetically encoded sensor with sub-nanomolar sensitivity and show that Zn^2 + is released from endolysosomal vesicles via TRPML1 in neurons.

Calcium Dyshomeostasis and Lysosomal Ca2+ Dysfunction in Amyotrophic Lateral Sclerosis

Recent findings in the understanding of amyotrophic lateral sclerosis (ALS) revealed that alteration in calcium (Ca²⁺) homeostasis may largely contribute to motor neuron demise. A large part of these alterations is due to dysfunctional Ca²⁺-storing organelles, including the endoplasmic reticulum (ER) and mitochondria. Very recently, lysosomal Ca²⁺ dysfunction has emerged as an important pathological change leading to neuronal loss in ALS. Remarkably, the Ca²⁺-storing organelles are interacting with each other at specialized domains controlling mitochondrial dynamics, ER/lysosomal function, and autophagy. This occurs as a result of interaction between specific ionic channels and Ca²⁺-dependent proteins located in each structure. Therefore, the dysregulation of these ionic mechanisms could be considered as a key element in the neurodegenerative process. This review will focus on the possible role of lysosomal Ca²⁺ dysfunction in the pathogenesis of several neurodegenerative diseases, including ALS and shed light on the possibility that specific lysosomal Ca²⁺ channels might represent new promising targets for preventing or at least delaying neurodegeneration in ALS.

Cell Death Induced by Cationic Amphiphilic Drugs Depends on Lysosomal Ca2+ Release and Cyclic AMP

Repurposing cationic amphiphilic drugs (CAD) for cancer treatment is emerging as an attractive means to enhance the efficacy of chemotherapy. Many commonly used CADs, including several cation amphiphilic antihistamines and antidepressants, induce cancer-specific, lysosome-dependent cell death and sensitize cancer cells to chemotherapy. CAD-induced inhibition of lysosomal acid sphingomyelinase is necessary, but not sufficient, for the subsequent lysosomal membrane permeabilization and cell death, while other pathways regulating this cell death pathway are largely unknown. Prompted by significant changes in the expression of genes involved in Ca2+ and cyclic AMP (cAMP) signaling pathways in CAD-resistant MCF7 breast cancer cells, we identified here an early lysosomal Ca2+ release through P2X purinergic receptor 4 (P2RX4) and subsequent Ca2+- and adenylyl cyclase 1 (ADCY1)-dependent synthesis of cAMP as a signaling route mediating CAD-induced lysosomal membrane permeabilization and cell death. Importantly, pharmacologic and genetic means to increase cellular cAMP levels either by activating cAMP-inducing G-protein-coupled receptors (GPR3 or β2 adrenergic receptor) or ADCY1, or by inhibiting cAMP-reducing guanine nucleotide-binding protein G(i) subunit α2, C-X-C motif chemokine receptor type 4, or cAMP phosphodiesterases, sensitized cancer cells to CADs. These data reveal a previously unrecognized lysosomal P2RX4- and ADCY1-dependent signaling cascade as a pathway essential for CAD-induced lysosome-dependent cell death and encourage further investigations to find the most potent combinations of CADs and cAMP-inducing drugs for cancer therapy.

Control of lysosomal TRPML1 channel activity and exosome release by acid ceramidase in mouse podocytes

The transient receptor potential mucolipin 1 (TRPML1) channel has been reported to mediate lysosomal Ca2+ release that is involved in Ca2+-dependent lysosome trafficking and autophagic flux. However, this regulatory mechanism of lysosomal TRPML1 channel activity in podocytes remains poorly understood. In the present study, we tested whether the TRPML1 channel in podocytes mediates lysosome trafficking, which is essential for multivesicular body (MVB) degradation by lysosomes. We first demonstrated the abundant expression of TRPML1 channel in podocytes. By GCaMP3 Ca2+ imaging, we characterized the lysosomal specificity of TRPML1 channel-mediated Ca2+ release in podocytes. Given the important role of acid ceramidase (AC) in lysosome function and podocyte injury, we tested whether AC regulates this TRPML1 channel-mediated Ca2+ release and consequent lysosome-dependent MVB degradation in podocytes. Pharmacologically, it was found that TRPML1 channel activity was remarkably attenuated by the AC inhibitor carmofur. Sphingosine, as an AC product, was demonstrated to induce TRPML1-mediated Ca2+ release, which was inhibited by a TRPML1 blocker, verapamil. Using a Port-a-Patch planar patch-clamp system, we found that AC-associated sphingolipids, sphingomyelin, ceramide, and sphingosine had different effects on TRPML1 channel activity in podocytes. Functionally, the inhibition of AC or blockade of TRPML1 channels was found to suppress the interaction of lysosomes and MVBs, leading to increased exosome release from podocytes. These results suggest that AC is critical for TRPML1 channel-mediated Ca2+ release, which controls lysosome-MVB interaction and exosome release in podocytes.

Transient Receptor Potential Channels and Metabolism

Transient receptor potential (TRP) channels are nonselective cationic channels, conserved among flies to humans. Most TRP channels have well known functions in chemosensation, thermosensation, and mechanosensation. In addition to being sensing environmental changes, many TRP channels are also internal sensors that help maintain homeostasis. Recent improvements to analytical methods for genomics and metabolomics allow us to investigate these channels in both mutant animals and humans. In this review, we discuss three aspects of TRP channels, which are their role in metabolism, their functional characteristics, and their role in metabolic syndrome. First, we introduce each TRP channel superfamily and their particular roles in metabolism. Second, we provide evidence for which metabolites TRP channels affect, such as lipids or glucose. Third, we discuss correlations between TRP channels and obesity, diabetes, and mucolipidosis. The cellular metabolism of TRP channels gives us possible therapeutic approaches for an effective prophylaxis of metabolic syndromes.

TRPML1 Promotes Protein Homeostasis in Melanoma Cells by Negatively Regulating MAPK and mTORC1 Signaling

We screen ion channels and transporters throughout the genome to identify those required by human melanoma cells but not by normal human melanocytes. We discover that Mucolipin-1 (MCOLN1), which encodes the lysosomal cation channel TRPML1, is preferentially required for the survival and proliferation of melanoma cells. Loss of MCOLN1/TRPML1 function impairs the growth of patient-derived melanomas in culture and in xenografts but does not affect the growth of human melanocytes. TRPML1 expression and macropinocytosis are elevated in melanoma cells relative to melanocytes. TRPML1 is required in melanoma cells to negatively regulate MAPK pathway and mTORC1 signaling. TRPML1-deficient melanoma cells exhibit decreased survival, proliferation, tumor growth, and macropinocytosis, as well as serine depletion and proteotoxic stress. All of these phenotypes are partially or completely rescued by mTORC1 inhibition. Melanoma cells thus increase TRPML1 expression relative to melanocytes to attenuate MAPK and mTORC1 signaling, to sustain macropinocytosis, and to avoid proteotoxic stress.

BK channels regulate extracellular Tat-mediated HIV-1 LTR transactivation

HIV-1 Tat is essential for HIV-1 replication and plays an important role in latent HIV-1 infection, HIV-1 associated neurological complication, and other HIV-1 comorbidities. Secreted from HIV-1 infected or transfected cells, Tat can be up-taken into cells by receptor-mediated endocytosis and internalized into endolysosomes. To reach nucleus where it can facilitate HIV-1 viral replication, exogenous Tat has to escape the degradation by endolysosomes. Because of findings that endolysosome de-acidification with, for example, the weak-base anti-malarial drug chloroquine prevents exogenous Tat degradation and enhances the amount of Tat available to activate HIV-1 LTR, we hypothesize that acidifying endolysosomes may enhance Tat degradation in endolysosomes and restrict LTR transactivation. Here, we determined the involvement of endolysosome-resident transient receptor potential mucolipin 1 channel (TRPML1) and the big conductance Ca²⁺-activated potassium (BK) channel in regulating endolysosome pH, as well as Tat-mediated HIV-1 LTR transactivation in U87MG cells stably integrated with HIV-1 LTR luciferase reporter. Activating TRPML1 channels with ML-SA1 acidified endolysosomes and restricted Tat-mediated HIV-1 LTR transactivation. These effects of ML-SA1 appeared to be mediated through activation of BK channels, because the effects of ML-SA1 on Tat-mediated HIV-1 LTR transactivation were blocked using pharmacological inhibitors or shRNA knock-down of BK channels. On the other hand, activating TRPML1 and BK channels enhanced cellular degradation of exogenous Tat. These results suggest that acidifying endolysosomes by activating TRPML1 or BK channels may provide therapeutic benefit against latent HIV-1 infection, HIV-1 associated neurocognitive disorders, and other HIV-1 comorbidities.

Involvement of organelles and inter-organellar signaling in the pathogenesis of HIV-1 associated neurocognitive disorder and Alzheimer's disease

Endolysosomes, mitochondria, peroxisomes, endoplasmic reticulum, and plasma membranes are now known to physically and functionally interact with each other. Such findings of inter-organellar signaling and communication has led to a resurgent interest in cell biology and an increased appreciation for the physiological actions and pathological consequences of the dynamic physical and chemical communications occurring between intracellular organelles. Others and we have shown that HIV-1 proteins implicated in the pathogenesis of neuroHIV and that Alzheimer's disease both affects the structure and function of intracellular organelles. Intracellular organelles are highly mobile, and their intracellular distribution almost certainly affects their ability to interact with other organelles and to regulate such important physiological functions as endolysosome acidification, cell motility, and nutrient homeostasis. Indeed, compounds that acidify endolysosomes cause endolysosomes to exhibit a mainly perinuclear pattern while compounds that de-acidify endolysosomes cause these organelles to exhibit a larger profile as well as movement towards plasma membranes. Endolysosome pH might be an early event in the pathogenesis of neuroHIV and Alzheimer's disease and in terms of organellar biology endolysosome changes might be upstream of HIV-1 protein-induced changes to other organelles. Thus, inter-organellar signaling mechanisms might be involved in the pathogenesis of neuroHIV and other neurological disorders, and a better understanding of inter-organellar signaling might lead to improved therapeutic strategies.

Morphine-Induced Modulation of Endolysosomal Iron Mediates Upregulation of Ferritin Heavy Chain in Cortical Neurons

HIV-associated neurocognitive disorders (HAND) remain prevalent and are aggravated by µ-opioid use. We have previously shown that morphine and other µ-opioids may contribute to HAND by inhibiting the homeostatic and neuroprotective chemokine receptor CXCR4 in cortical neurons, and this novel mechanism depends on upregulation of the protein ferritin heavy chain (FHC). Here, we examined the cellular events and potential mechanisms involved in morphine-mediated FHC upregulation using rat cortical neurons of either sex in vitro and in vivo. Morphine dose dependently increased FHC protein levels in primary neurons through µ-opioid receptor (µOR) and Gαi-protein signaling. Cytoplasmic FHC levels were significantly elevated, but nuclear FHC levels and FHC gene expression were unchanged. Morphine-treated rats also displayed increased FHC levels in layer 2/3 neurons of the prefrontal cortex. Importantly, both in vitro and in vivo FHC upregulation was accompanied by loss of mature dendritic spines, which was also dependent on µOR and Gαi-protein signaling. Moreover, morphine upregulated ferritin light chain (FLC), a component of the ferritin iron storage complex, suggesting that morphine altered neuronal iron metabolism. Indeed, prior to FHC upregulation, morphine increased cytoplasmic labile iron levels as a function of decreased endolysosomal iron. In line with this, chelation of endolysosomal iron (but not extracellular iron) blocked morphine-induced FHC upregulation and dendritic spine reduction, whereas iron overloading mimicked the effect of morphine on FHC and dendritic spines. Overall, these data demonstrate that iron mediates morphine-induced FHC upregulation and consequent dendritic spine deficits and implicate endolysosomal iron efflux to the cytoplasm in these effects.

The activation of Mucolipin TRP channel 1 (TRPML1) protects motor neurons from L-BMAA neurotoxicity by promoting autophagic clearance

Cellular clearance mechanisms including the autophagy-lysosome pathway are impaired in amyotrophic lateral sclerosis (ALS). One of the most important proteins involved in the regulation of autophagy is the lysosomal Ca²⁺ channel Mucolipin TRP channel 1 (TRPML1). Therefore, we investigated the role of TRPML1 in a neuronal model of ALS/Parkinson-dementia complex reproduced by the exposure of motor neurons to the cyanobacterial neurotoxin beta-methylamino-L-alanine (L-BMAA). Under these conditions, L-BMAA induces a dysfunction of the endoplasmic reticulum (ER) leading to ER stress and cell death. Therefore we hypothesized a dysfunctional coupling between lysosomes and ER in L-BMAA-treated motor neurons. Here, we showed that in motor neuronal cells TRPML1 as well as the lysosomal protein LAMP1 co-localized with ER. In addition, TRPML1 co-immunoprecipitated with the ER Ca²⁺ sensor STIM1. Functionally, the TRPML1 agonist ML-SA1 induced lysosomal Ca²⁺ release in a dose-dependent way in motor neuronal cells. The SERCA inhibitor thapsigargin increased the fluorescent signal associated with lysosomal Ca²⁺ efflux in the cells transfected with the genetically encoded Ca²⁺ indicator GCaMP3-ML1, thus suggesting an interplay between the two organelles. Moreover, chronic exposure to L-BMAA reduced TRPML1 protein expression and produced an impairment of both lysosomal and ER Ca²⁺ homeostasis in primary motor neurons. Interestingly, the preincubation of ML-SA1, by an early activation of AMPK and beclin 1, rescued motor neurons from L-BMAA-induced cell death and reduced the expression of the ER stress marker GRP78. Finally, ML-SA1 reduced the accumulation of the autophagy-related proteins p62/SQSTM1 and LC3-II in L-BMAA-treated motor neurons. Collectively, we propose that the pharmacological stimulation of TRPML1 can rescue motor neurons from L-BMAA-induced toxicity by boosting autophagy and reducing ER stress.

The protein interaction networks of mucolipins and two-pore channels

BACKGROUND

The endolysosomal, non-selective cation channels, two-pore channels (TPCs) and mucolipins (TRPMLs), regulate intracellular membrane dynamics and autophagy. While partially compensatory for each other, isoform-specific intracellular distribution, cell-type expression patterns, and regulatory mechanisms suggest different channel isoforms confer distinct properties to the cell.

SCOPE OF REVIEW

Briefly, established TPC/TRPML functions and interaction partners ('interactomes') are discussed. Novel TRPML3 interactors are shown, and a meta-analysis of experimentally obtained channel interactomes conducted. Accordingly, interactomes are compared and contrasted, and subsequently described in detail for TPC1, TPC2, TRPML1, and TRPML3.

MAJOR CONCLUSIONS

TPC interactomes are well-defined, encompassing intracellular membrane organisation proteins. TRPML interactomes are varied, encompassing cardiac contractility- and chaperone-mediated autophagy proteins, alongside regulators of intercellular signalling.

GENERAL SIGNIFICANCE

Comprising recently proposed targets to treat cancers, infections, metabolic disease and neurodegeneration, the advancement of TPC/TRPML understanding is of considerable importance. This review proposes novel directions elucidating TPC/TRPML relevance in health and disease. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

The pathogenesis of lysosomal storage disorders: beyond the engorgement of lysosomes to abnormal development and neuroinflammation

There is growing evidence that the complex clinical manifestations of lysosomal storage diseases (LSDs) are not fully explained by the engorgement of the endosomal-autophagic-lysosomal system. In this review, we explore current knowledge of common pathogenetic mechanisms responsible for the early onset of tissue abnormalities of two LSDs, Mucopolysaccharidosis type II (MPSII) and Niemann-Pick type C (NPC) diseases. In particular, perturbations of the homeostasis of glycosaminoglycans (GAGs) and cholesterol (Chol) in MPSII and NPC diseases, respectively, affect key biological processes, including morphogen signaling. Both GAGs and Chol finely regulate the release, reception and tissue distribution of Shh. Hence, not surprisingly, developmental processes depending on correct Shh signaling have been found altered in both diseases. Besides abnormal signaling, exaggerated activation of microglia and impairment of autophagy and mitophagy occur in both diseases, largely before the appearance of typical pathological signs.

Rapamycin directly activates lysosomal mucolipin TRP channels independent of mTOR

Rapamycin (Rap) and its derivatives, called rapalogs, are being explored in clinical trials targeting cancer and neurodegeneration. The underlying mechanisms of Rap actions, however, are not well understood. Mechanistic target of rapamycin (mTOR), a lysosome-localized protein kinase that acts as a critical regulator of cellular growth, is believed to mediate most Rap actions. Here, we identified mucolipin 1 (transient receptor potential channel mucolipin 1 [TRPML1], also known as MCOLN1), the principle Ca2+ release channel in the lysosome, as another direct target of Rap. Patch-clamping of isolated lysosomal membranes showed that micromolar concentrations of Rap and some rapalogs activated lysosomal TRPML1 directly and specifically. Pharmacological inhibition or genetic inactivation of mTOR failed to mimic the Rap effect. In vitro binding assays revealed that Rap bound directly to purified TRPML1 proteins with a micromolar affinity. In both healthy and disease human fibroblasts, Rap and rapalogs induced autophagic flux via nuclear translocation of transcription factor EB (TFEB). However, such effects were abolished in TRPML1-deficient cells or by TRPML1 inhibitors. Hence, Rap and rapalogs promote autophagy via a TRPML1-dependent mechanism. Given the demonstrated roles of TRPML1 and TFEB in cellular clearance, we propose that lysosomal TRPML1 may contribute a significant portion to the in vivo neuroprotective and anti-aging effects of Rap via an augmentation of autophagy and lysosomal biogenesis.