Transcriptional activation of lysosomal exocytosis promotes cellular clearance

Emerging role of transcription factor EB in mitochondrial quality control

Mitochondria are energy producers that play a vital role in cell survival. Mitochondrial dysfunction is involved in many diseases, including metabolic syndrome, neurodegenerative disorders, cardiomyopathies, cancer, obesity, and diabetic kidney disease, and challenges still remain in terms of treatments for these diseases. Mitochondrial quality control (MQC), which is defined as the maintenance of the quantity, morphology, and function of mitochondria, plays a pivotal role in maintaining cellular metabolic homeostasis and cell survival. Recently, growing evidence suggests that the transcription factor EB (TFEB) plays a pivotal role in MQC. Here, we systemically investigate the potential role and mechanisms of TFEB in MQC, which include the activation of mitophagy, regulation of mitochondrial biogenesis, reactive oxygen species (ROS) clearance, and the balance of mitochondria fission-fusion cycle. Importantly, we further discuss the therapeutic measures and effects aimed at TFEB on mitochondrial dysfunction-related diseases. Taken together, targeting TFEB to regulate MQC may represent an appealing therapeutic strategy for mitochondrial dysfunction related-diseases.

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.

TFEB Modulates p21/WAF1/CIP1 during the DNA Damage Response

The MiT/TFE family of transcription factors (MITF, TFE3, and TFEB), which control transcriptional programs for autophagy and lysosome biogenesis have emerged as regulators of energy metabolism in cancer. Thus, their activation increases lysosomal catabolic function to sustain cancer cell growth and survival in stress conditions. Here, we found that TFEB depletion dramatically reduces basal expression levels of the cyclin-dependent kinase (CDK) inhibitor p21/WAF1 in various cell types. Conversely, TFEB overexpression increases p21 in a p53-dependent manner. Furthermore, induction of DNA damage using doxorubicin induces TFEB-mediated activation of p21, delays G2/M phase arrest, and promotes cell survival. Pharmacological inhibition of p21, instead, abrogates TFEB-mediated protection during the DNA damage response. Together, our findings uncover a novel and direct role of TFEB in the regulation of p21 expression in both steady-state conditions and during the induction of DNA-damage response (DDR). Our observations might open novel therapeutic strategies to promote cancer cell death by targeting the TFEB-p21 pathway in the presence of genotoxic agents.

Regulation of TFEB activity and its potential as a therapeutic target against kidney diseases

The transcription factor EB (TFEB) regulates the expression of target genes bearing the Coordinated Lysosomal Expression and Regulation (CLEAR) motif, thereby modulating autophagy and lysosomal biogenesis. Furthermore, TFEB can bind to the promoter of autophagy-associated genes and induce the formation of autophagosomes, autophagosome-lysosome fusion, and lysosomal cargo degradation. An increasing number of studies have shown that TFEB stimulates the intracellular clearance of pathogenic factors by enhancing autophagy and lysosomal function in multiple kidney diseases, such as cystinosis, acute kidney injury, and diabetic nephropathy. Taken together, this highlights the importance of developing novel therapeutic strategies against kidney diseases based on TFEB regulation. In this review, we present an overview of the current data on TFEB and its implication in kidney disease.

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.

A Self-Assembled α-Synuclein Nanoscavenger for Parkinson's Disease

Although emerging evidence suggests that the pathogenesis of Parkinson's disease (PD) is closely related to the aggregation of alpha-synuclein (α-syn) in the midbrain, the clearance of α-syn remains an unmet clinical need. Here, we develop a simple and efficient strategy for fabricating the α-syn nanoscavenger for PD via a reprecipitation self-assembly procedure. The curcumin analogue-based nanoscavenger (NanoCA) is engineered to be capable of a controlled-release property to stimulate nuclear translocation of the major autophagy regulator, transcription factor EB (TFEB), triggering both autophagy and calcium-dependent exosome secretion for the clearance of α-syn. Pretreatment of NanoCA protects cell lines and primary neurons from MPP⁺-induced neurotoxicity. More importantly, a rapid arousal intranasal delivery system (RA-IDDS) was designed and applied for the brain-targeted delivery of NanoCA, which affords robust neuroprotection against behavioral deficits and promotes clearance of monomer, oligomer, and aggregates of α-syn in the midbrain of an MPTP mouse model of PD. Our findings provide a clinically translatable therapeutic strategy aimed at neuroprotection and disease modification in PD.

Cathepsin B-associated Activation of Amyloidogenic Pathway in Murine Mucopolysaccharidosis Type I Brain Cortex

Mucopolysaccharidosis type I (MPS I) is caused by genetic deficiency of α-l-iduronidase and impairment of lysosomal catabolism of heparan sulfate and dermatan sulfate. In the brain, these substrates accumulate in the lysosomes of neurons and glial cells, leading to neuroinflammation and neurodegeneration. Their storage also affects lysosomal homeostasis-inducing activity of several lysosomal proteases including cathepsin B (CATB). In the central nervous system, increased CATB activity has been associated with the deposition of amyloid plaques due to an alternative pro-amyloidogenic processing of the amyloid precursor protein (APP), suggesting a potential role of this enzyme in the neuropathology of MPS I. In this study, we report elevated levels of protein expression and activity of CATB in cortex tissues of 6-month-old MPS I (Idua -/- mice. Besides, increased CATB leakage from lysosomes to the cytoplasm of Idua -/- cortical pyramidal neurons was indicative of damaged lysosomal membranes. The increased CATB activity coincided with an elevated level of the 16-kDa C-terminal APP fragment, which together with unchanged levels of β-secretase 1 was suggestive for the role of this enzyme in the amyloidogenic APP processing. Neuronal accumulation of Thioflavin-S-positive misfolded protein aggregates and drastically increased levels of neuroinflammatory glial fibrillary acidic protein (GFAP)-positive astrocytes and CD11b-positive activated microglia were observed in Idua -/- cortex by confocal fluorescent microscopy. Together, our results point to the existence of a novel CATB-associated alternative amyloidogenic pathway in MPS I brain induced by lysosomal storage and potentially leading to neurodegeneration.

Key Roles of MiT Transcription Factors in Innate Immunity and Inflammation

Microphthalmia/TFE (MiT) transcription factors (TFs), such as transcription factor EB (TFEB) and transcription factor E3 (TFE3), are emerging as key regulators of innate immunity and inflammation. Rapid progress in the field requires a focused update on the latest advances. Recent studies show that TFEB and TFE3 function in innate immune cells to regulate antibacterial and antiviral responses downstream of phagocytosis, interferon (IFN)-γ, lipopolysaccharide (LPS), and adenosine receptors. Moreover, overexpression of TFEB or TFE3 can drive inflammation in vivo, such as in atherosclerosis, while in other scenarios they can perform anti-inflammatory functions. MiT factors may constitute potential therapeutic targets for a broad range of diseases; however, to harness their therapeutic potential, sophisticated ways to manipulate MiT factor activity safely and effectively must be developed.

Select autophagy genes maintain quiescence of tissue-resident macrophages and increase susceptibility to Listeria monocytogenes

Innate and adaptive immune responses that prime myeloid cells, such as macrophages, protect against pathogens^1,2. However, if left uncontrolled, these responses may lead to detrimental inflammation³. Macrophages, particularly those resident in tissues, must therefore remain quiescent between infections despite chronic stimulation by commensal microorganisms. The genes required for quiescence of tissue-resident macrophages are not well understood. Autophagy, an evolutionarily conserved cellular process by which cytoplasmic contents are targeted for lysosomal digestion, has homeostatic functions including maintenance of protein and organelle integrity and regulation of metabolism⁴. Recent research has shown that degradative autophagy, as well as various combinations of autophagy genes, regulate immunity and inflammation^5-12. Here, we delineate a function of the autophagy proteins Beclin 1 and FIP200-but not of other essential autophagy components ATG5, ATG16L1 or ATG7-in mediating quiescence of tissue-resident macrophages by limiting the effects of systemic interferon-γ. The perturbation of quiescence in mice that lack Beclin 1 or FIP200 in myeloid cells results in spontaneous immune activation and resistance to Listeria monocytogenes infection. While antibiotic-treated wild-type mice display diminished macrophage responses to inflammatory stimuli, this is not observed in mice that lack Beclin 1 in myeloid cells, establishing the dominance of this gene over effects of the bacterial microbiota. Thus, select autophagy genes, but not all genes essential for degradative autophagy, have a key function in maintaining immune quiescence of tissue-resident macrophages, resulting in genetically programmed susceptibility to bacterial infection.

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.

A lysosomal K+ channel regulates large particle phagocytosis by facilitating lysosome Ca2+ release

Macrophages are highly specialized in removing large particles including dead cells and cellular debris. When stimulated, delivery of the intracellular lysosomal membranes is required for the formation of plasmalemmal pseudopods and phagosomes. As a key lysosomal Ca²⁺ channel, Transient Receptor Potential Mucolipin-1 (TRPML1) regulates lysosomal exocytosis and subsequent phagosome biogenesis, thereby promoting phagocytosis of large extracellular particles. Recently, we have suggested that TRPML1-mediated lysosomal exocytosis is essentially dependent on lysosomal big conductance Ca²⁺-activated potassium (BK) channel. Therefore, we predict that lysosomal BK channels regulate large particle phagocytosis. In this study, by using RAW264.7 macrophage cell line and bone marrow-derived macrophages, we show that although BK is dispensable for small particle uptake, loss of BK significantly inhibits the ingestion of large particles whereas activating BK increases the uptake of large particles. BK facilitating effect on large particle ingestion is inhibited by either blocking TRPML1 or suppressing lysosomal exocytosis. Additionally, the increased uptake of large particles by activating TRPML1 is eliminated by inhibiting BK. These data suggest that BK and TRPML1 are functionally coupled to regulate large particle phagocytosis through modulating lysosomal exocytosis.

Promoting tau secretion and propagation by hyperactive p300/CBP via autophagy-lysosomal pathway in tauopathy

BACKGROUND

The trans-neuronal propagation of tau has been implicated in the progression of tau-mediated neurodegeneration. There is critical knowledge gap in understanding how tau is released and transmitted, and how that is dysregulated in diseases. Previously, we reported that lysine acetyltransferase p300/CBP acetylates tau and regulates its degradation and toxicity. However, whether p300/CBP is involved in regulation of tau secretion and propagation is unknown.

METHOD

We investigated the relationship between p300/CBP activity, the autophagy-lysosomal pathway (ALP) and tau secretion in mouse models of tauopathy and in cultured rodent and human neurons. Through a high-through-put compound screen, we identified a new p300 inhibitor that promotes autophagic flux and reduces tau secretion. Using fibril-induced tau spreading models in vitro and in vivo, we examined how p300/CBP regulates tau propagation.

RESULTS

Increased p300/CBP activity was associated with aberrant accumulation of ALP markers in a tau transgenic mouse model. p300/CBP hyperactivation blocked autophagic flux and increased tau secretion in neurons. Conversely, inhibiting p300/CBP promoted autophagic flux, reduced tau secretion, and reduced tau propagation in fibril-induced tau spreading models in vitro and in vivo.

CONCLUSIONS

We report that p300/CBP, a lysine acetyltransferase aberrantly activated in tauopathies, causes impairment in ALP, leading to excess tau secretion. This effect, together with increased intracellular tau accumulation, contributes to enhanced spreading of tau. Our findings suggest that inhibition of p300/CBP as a novel approach to correct ALP dysfunction and block disease progression in tauopathy.

Lysosomal Fusion: An Efficient Mechanism Increasing Their Sequestration Capacity for Weak Base Drugs without Apparent Lysosomal Biogenesis

Lysosomal sequestration of anticancer therapeutics lowers their cytotoxic potential, reduces drug availability at target sites, and contributes to cancer resistance. Only recently has it been shown that lysosomal sequestration of weak base drugs induces lysosomal biogenesis mediated by activation of transcription factor EB (TFEB) which, in turn, enhances their accumulation capacity, thereby increasing resistance to these drugs. Here, we addressed the question of whether lysosomal biogenesis is the only mechanism that increases lysosomal sequestration capacity. We found that lysosomal sequestration of some tyrosine kinase inhibitors (TKIs), gefitinib (GF) and imatinib (IM), induced expansion of the lysosomal compartment. However, an expression analysis of lysosomal genes, including lysosome-associated membrane proteins 1, 2 (LAMP1, LAMP2), vacuolar ATPase subunit B2 (ATP6V1B2), acid phosphatase (ACP), and galactosidase beta (GLB) controlled by TFEB, did not reveal increased expression. Instead, we found that both studied TKIs, GF and IM, induced lysosomal fusion which was dependent on nicotinic acid adenine dinucleotide phosphate (NAADP) mediated Ca²⁺signaling. A theoretical analysis revealed that lysosomal fusion is sufficient to explain the enlargement of lysosomal sequestration capacity. In conclusion, we demonstrated that extracellular TKIs, GF and IM, induced NAADP/Ca²⁺ mediated lysosomal fusion, leading to enlargement of the lysosomal compartment with significantly increased sequestration capacity for these drugs without apparent lysosomal biogenesis.

Stochastics of Degradation: The Autophagic-Lysosomal System of the Cell

Autophagy is a conservative and evolutionarily ancient process that enables the transfer of various cellular compounds, organelles, and potentially dangerous cellular components to the lysosome for their degradation. This process is crucial for the recycling of energy and substrates, which are required for cellular biosynthesis. Autophagy not only plays a major role in the survival of cells under stress conditions, but is also actively involved in maintaining cellular homeostasis. It has multiple effects on the immune system and cellular remodeling during organism development. The effectiveness of autophagy is ensured by a controlled interaction between two organelles – the autophagosome and the lysosome. Despite significant progress in the description of the molecular mechanisms underlying autophagic-lysosomal system (ALS) functioning, many fundamental questions remain. Namely, the specialized functions of lysosomes and the role of ALS in the pathogenesis of human diseases are still enigmatic. Understanding of the mechanisms that are triggered at all stages of autophagic- lysosomal degradation, from the initiation of autophagy to the terminal stage of substrate destruction in the lysosome, may result in new approaches that could help better uderstand ALS and, therefore, selectively control cellular proteostasis.

Targeting Cancer Lysosomes with Good Old Cationic Amphiphilic Drugs

Being originally discovered as cellular recycling bins, lysosomes are today recognized as versatile signaling organelles that control a wide range of cellular functions that are essential not only for the well-being of normal cells but also for malignant transformation and cancer progression. In addition to their core functions in waste disposal and recycling of macromolecules and energy, lysosomes serve as an indispensable support system for malignant phenotype by promoting cell growth, cytoprotective autophagy, drug resistance, pH homeostasis, invasion, metastasis, and genomic integrity. On the other hand, malignant transformation reduces the stability of lysosomal membranes rendering cancer cells sensitive to lysosome-dependent cell death. Notably, many clinically approved cationic amphiphilic drugs widely used for the treatment of other diseases accumulate in lysosomes, interfere with their cancer-promoting and cancer-supporting functions and destabilize their membranes thereby opening intriguing possibilities for cancer therapy. Here, we review the emerging evidence that supports the supplementation of current cancer therapies with lysosome-targeting cationic amphiphilic drugs.

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.

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.

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.

AMPK-ERK/CARM1 Signaling Pathways Affect Autophagy of Hepatic Cells in Samples of Liver Cancer Patients

Liver cancer is one of the most common malignant tumors, with the death rate ranking fourth among all types of cancer. Over the past few decades, several studies have reported that liver tumorigenesis is associated with dysfunction in autophagy. However, the detailed mechanism remains unclear. In this paper, we used tissue micro-array (TMA) of liver cancer to detect proteins associated with the regulation of autophagic signaling in non-cancerous and cancerous regions by immunohistochemical staining. Those proteins contained 4-HNE, p-AMPK, Erk1/2, p-Erk1/2, CARM1, TFEB, LAMP1, and p62. According to the degrees of tumor differentiation in patients (well differentiated group vs. moderately and poorly differentiated group), we analyzed each protein's expression in the ratio of the “cancerous region/non-cancerous region” in two groups. Current data showed that there were AMPK-ERK/CARM1 autophagic signaling pathways during the formation of liver cancer. The above-mentioned changes in signals indicated an upregulation of autophagy in cancerous regions, which means overactivated autophagy plays an important role in liver cancer.

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.

Neuromelanin, aging, and neuronal vulnerability in Parkinson's disease

Neuromelanin, a dark brown intracellular pigment, has long been associated with Parkinson's disease (PD). In PD, neuromelanin-containing neurons preferentially degenerate, tell-tale neuropathological inclusions form in close association with this pigment, and neuroinflammation is restricted to neuromelanin-containing areas. In humans, neuromelanin accumulates with age, which in turn is the main risk factor for PD. The potential contribution of neuromelanin to PD pathogenesis remains unknown because, in contrast to humans, common laboratory animals lack neuromelanin. The recent introduction of a rodent model exhibiting an age-dependent production of human-like neuromelanin has allowed, for the first time, for the consequences of progressive neuromelanin accumulation-up to levels reached in elderly human brains-to be assessed in vivo. In these animals, intracellular neuromelanin accumulation above a specific threshold compromises neuronal function and triggers a PD-like pathology. As neuromelanin levels reach this threshold in PD patients and presymptomatic PD patients, the modulation of neuromelanin accumulation could provide a therapeutic benefit for PD patients and delay brain aging. © 2019 The Author. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Pharmacological approaches to tackle NCLs

Neuronal ceroid lipofuscinoses, also collectively known as Batten disease, are a group of rare monogenic disorders caused by mutations in at least 13 different genes. They are characterized by the accumulation of lysosomal storage material and progressive neurological deterioration with dementia, epilepsy, retinopathy, motor disturbances, and early death [1]. Although the identification of disease-causing genes provides an important step for understanding the molecular mechanisms underlying neuronal ceroid lipofuscinoses, compared to other diseases, obstacles to the development of therapies for these rare diseases include less extensive physiopathology knowledge, limited number of patients to test treatments, and poor commercial interest from the industry. Current therapeutic strategies include enzyme replacement therapies, gene therapies targeting the brain and the eye, cell therapies, and pharmacological drugs that could modulate defective molecular pathways. In this review, we will focus in the emerging therapies based in the identification of small-molecules. Recent advances in high- throughput and high-content screening (HTS and HCS) using relevant cell-based assays and applying automation and imaging analysis algorithms, will allow the screening of a large number of compounds in lesser time. These approaches are particularly useful for drug repurposing for Batten disease, that takes the advantage to search for compounds that have already been tested in humans, thereby reducing significantly the resources needed for translation to clinics.

Biomolecular Functionalization of a Nanomaterial To Control Stability and Retention within Live Cells

Noncovalent hybrids of single-stranded DNA and single-walled carbon nanotubes (SWCNTs) have demonstrated applications in biomedical imaging and sensing due to their enhanced biocompatibility and photostable, environmentally responsive near-infrared (NIR) fluorescence. The fundamental properties of such DNA-SWCNTs have been studied to determine the correlative relationships between oligonucleotide sequence and length, SWCNT species, and the physical attributes of the resultant hybrids. However, intracellular environments introduce harsh conditions that can change the physical identities of the hybrid nanomaterials, thus altering their intrinsic optical properties. Here, through visible and NIR fluorescence imaging in addition to confocal Raman microscopy, we show that the oligonucleotide length controls the relative uptake, intracellular optical stability, and retention of DNA-SWCNTs in mammalian cells. Although the absolute NIR fluorescence intensity of DNA-SWCNTs in murine macrophages increases with increasing oligonucleotide length (from 12 to 60 nucleotides), we found that shorter oligonucleotide DNA-SWCNTs undergo a greater magnitude of spectral shift and are more rapidly internalized and expelled from the cell after 24 h. Furthermore, by labeling the DNA with a fluorophore that dequenches upon removal from the SWCNT surface, we found that shorter oligonucleotide strands are displaced from the SWCNT within the cell, altering the physical identity and changing the fate of the internalized nanomaterial. Finally, through a pharmacological inhibition study, we identified the mechanism of SWCNT expulsion from the cells as lysosomal exocytosis. These findings provide a fundamental understanding of the interactions between SWCNTs and live cells as well as evidence suggesting the ability to control the biological fate of the nanomaterials merely by varying the type of DNA wrapping.

mTORC1 inhibition attenuates necroptosis through RIP1 inhibition-mediated TFEB activation

Accumulating evidence indicates that necroptosis contributes to cardiovascular diseases. We recently reported suppression of autophagy by necroptotic signals in cardiomyocytes and protective action of rapamycin. Here we examined the mechanism by which mTORC1 inhibition protects cardiomyocytes from necroptosis. Necroptosis of H9c2 cells was induced by treatment with tumor necrotic factor-α (TNF) and z-VAD-fmk (zVAD), and the extent of necroptosis was determined as the level of LDH release (as % of total). TNF/zVAD increased RIP1-RIP3 interaction and LDH release from 3.4 ± 1.3% to 46.1 ± 2.3%. The effects of TNF/zVAD were suppressed by an mTORC1 inhibitor, rapamycin, and an mTORC1/2 inhibitor, Ku-0063794, but not by a p70s6K inhibitor, PF-4708671. Protection by rapamycin was not abolished by inhibitors of TAK1, IKKα/β, and cIAP, endogenous necroptosis suppressors upstream of RIP1. Rapamycin and Ku-0063794 suppressed TNF/zVAD-induced RIP1-Ser166 phosphorylation and increased phosphorylation of RIP1-Ser320, an inhibitory phosphorylation site, though such an effect on RIP1-Ser320 was not observed for PF-4708671. Protective effects of rapamycin on TNF/zVAD-induced RIP1-RIP3 binding and necroptosis were undetected in cells transfected with RIP1-S320A. In TNF/zVAD-treated cells, rapamycin and a RIP1 inhibitor, necrostatin-1, increased nuclear localization of transcriptional factor EB (TFEB) and promoted autolysosome formation from autophagosomes in a TFEB-dependent manner. Knockdown of TFEB expression attenuated rapamycin-induced protection from necroptosis in TNF/zVAD-treated cells. The results suggest that mTORC1 inhibition promotes autophagy and protects cardiomyocytes from necroptosis by a TFEB-dependent mechanism and that inhibition of RIP1 by increased phosphorylation at Ser320 is crucial in the cardiomyocyte protection afforded by mTORC1 inhibition.

Biomedical Implications of Autophagy in Macromolecule Storage Disorders

An imbalance between the production and clearance of macromolecules such as proteins, lipids and carbohydrates can lead to a category of diseases broadly known as macromolecule storage disorders. These include, but not limited to, neurodegenerative diseases such as Alzheimer’s, Parkinson’s and Huntington’s disease associated with accumulation of aggregation-prone proteins, Lafora and Pompe disease associated with glycogen accumulation, whilst lipid accumulation is characteristic to Niemann-Pick disease and Gaucher disease. One of the underlying factors contributing to the build-up of macromolecules in these storage disorders is the intracellular degradation pathway called autophagy. This process is the primary clearance route for unwanted macromolecules, either via bulk non-selective degradation, or selectively via aggrephagy, glycophagy and lipophagy. Since autophagy plays a vital role in maintaining cellular homeostasis, cell viability and human health, malfunction of this process could be detrimental. Indeed, defective autophagy has been reported in a number of macromolecule storage disorders where autophagy is impaired at distinct stages, such as at the level of autophagosome formation, autophagosome maturation or improper lysosomal degradation of the autophagic cargo. Of biomedical relevance, autophagy is regulated by multiple signaling pathways that are amenable to chemical perturbations by small molecules. Induction of autophagy has been shown to improve cell viability and exert beneficial effects in experimental models of various macromolecule storage disorders where the lysosomal functionality is not overtly compromised. In this review, we will discuss the role of autophagy in certain macromolecule storage disorders and highlight the potential therapeutic benefits of autophagy enhancers in these pathological conditions.

Therapeutic potential of BLT1 antagonist for COPD: involvement of inducing autophagy and ameliorating inflammation

Purpose

Leukotriene B4 (LTB4) is a major pro-inflammatory mediator that leads to the persistence of chronic inflammation in chronic obstructive pulmonary disease (COPD). The purpose of this study was to evaluate therapeutic potential of BLT1 antagonist for cigarette smoke (CS)-induced COPD and to explore the underlying mechanism.

Materials and methods

In vitro, autophagy proteins were determined by Western blotting in RAW264.7 macrophages treated with U75302 (BLT1 antagonist) or autophagy inhibitor in cigarette smoke extract-induced inflammation. In vivo, C57BL/6J mice were randomly divided into three groups: Control group, CS group and CS+U75302 group. After 12-week exposure, histological analysis and lung function tests were performed to evaluate the inflammatory infiltration and emphysema. The expression of inflammatory cytokines was measured by real-time PCR and enzyme-linked immunosorbent assay. Immunohistochemical analysis and Western blotting detected the expression of autophagy-related proteins. Transmission electron microscopy (TEM) showed the alterations of autophagosomes and lysosomes.

Results

Lower levels of inflammatory factors and autophagy markers were detected in U75302-treated cells and mice after CS exposure than control. In vitro, LC3 mRNA expression was elevated when treated with U75302. Autophagy inhibition resulted in augmented inflammatory response and autophagy proteins even with U75302 treatment. Furthermore, BLT1 antagonist decreased the number of lysosomes and autophagosomes in alveolar macrophages of mice and potentially enhanced the expression of transcriptional activation of transcription factor-EB (TFEB) in vitro and vivo.

Conclusion

Insufficient autophagy of macrophages was associated with LTB4-mediated inflammation in CS-exposure models. BLT1 antagonist ameliorated inflammatory response through inducing autophagy.

δ-Tocopherol Effect on Endocytosis and Its Combination with Enzyme Replacement Therapy for Lysosomal Disorders: A New Type of Drug Interaction?

Induction of lysosomal exocytosis alleviates lysosomal storage of undigested metabolites in cell models of lysosomal disorders (LDs). However, whether this strategy affects other vesicular compartments, e.g., those involved in endocytosis, is unknown. This is important both to predict side effects and to use this strategy in combination with therapies that require endocytosis for intracellular delivery, such as lysosomal enzyme replacement therapy (ERT). We investigated this using δ-tocopherol as a model previously shown to induce lysosomal exocytosis and cell models of type A Niemann-Pick disease, a LD characterized by acid sphingomyelinase (ASM) deficiency and sphingomyelin storage. δ-Tocopherol and derivative CF3-T reduced net accumulation of fluid phase, ligands, and polymer particles via phagocytic, caveolae-, clathrin-, and cell adhesion molecule (CAM)-mediated pathways, yet the latter route was less affected due to receptor overexpression. In agreement, δ-tocopherol lowered uptake of recombinant ASM by deficient cells (known to occur via the clathrin pathway) and via targeting intercellular adhesion molecule-1 (associated to the CAM pathway). However, the net enzyme activity delivered and lysosomal storage attenuation were greater via the latter route. Data suggest stimulation of exocytosis by tocopherols is not specific of lysosomes and affects endocytic cargo. However, this effect was transient and became unnoticeable several hours after tocopherol removal. Therefore, induction of exocytosis in combination with therapies requiring endocytic uptake, such as ERT, may represent a new type of drug interaction, yet this strategy could be valuable if properly timed for minimal interference.

What sustains the multidrug resistance phenotype beyond ABC efflux transporters? Looking beyond the tip of the iceberg

Identification of multidrug (MDR) efflux transporters that belong to the ATP-Binding Cassette (ABC) superfamily, represented an important breakthrough for understanding cancer multidrug resistance (MDR) and its possible overcoming. However, recent data indicate that drug resistant cells have a complex intracellular physiology that involves constant changes in energetic and oxidative-reductive metabolic pathways, as well as in the molecular circuitries connecting mitochondria, endoplasmic reticulum (ER) and lysosomes. The aim of this review is to discuss the key molecular mechanisms of cellular reprogramming that induce and maintain MDR, beyond the presence of MDR efflux transporters. We specifically highlight how cancer cells characterized by high metabolic plasticity - i.e. cells able to shift the energy metabolism between glycolysis and oxidative phosphorylation, to survive both the normoxic and hypoxic conditions, to modify the cytosolic and mitochondrial oxidative-reductive metabolism, are more prone to adapt to exogenous stressors such as anti-cancer drugs and acquire a MDR phenotype. Similarly, we discuss how changes in mitochondria dynamics and mitophagy rates, changes in proteome stability ensuring non-oncogenic proteostatic mechanisms, changes in ubiquitin/proteasome- and autophagy/lysosome-related pathways, promote the cellular survival under stress conditions, along with the acquisition or maintenance of MDR. After dissecting the complex intracellular crosstalk that takes place during the development of MDR, we suggest that mapping the specific adaptation pathways underlying cell survival in response to stress and targeting these pathways with potent pharmacologic agents may be a new approach to enhance therapeutic efficacy against MDR tumors.

Endoplasmic reticulum stress disrupts lysosomal homeostasis and induces blockade of autophagic flux in human trophoblasts

Pregnancy is a stress factor culminating into mild endoplasmic reticulum (ER) stress, which is necessary for placental development. However, excessive or chronic ER stress in pre-eclamptic placentas leads to placental dysfunction. The precise mechanisms through which excessive ER stress impacts trophoblasts are not well understood. Here, we showed that ER stress reduces the number of lysosomes, resulting in inhibition of autophagic flux in trophoblast cells. ER stress also disrupted the translocation of lysosomes to the surface of trophoblast cells, and inhibited lysosomal exocytosis, whereby the secretion of lysosomal-associated membrane protein 1 (LAMP1) into culture media was significantly attenuated. In addition, we found that serum LAMP1 and beta-galactosidase levels were significantly decreased in pre-eclampsia patients compared to normal pregnant women, potentially indicating lysosomal dysfunction through ER stress in pre-eclamptic placentas. Thus, we demonstrated that excessive ER stress essentially disrupts homeostasis in trophoblasts in conjunction with autophagy inhibition by lysosomal impairment.

Endothelial glycocalyx-the battleground for complications of sepsis and kidney injury

After briefly discussing endothelial glycocalyx and its role in vascular physiology and renal disease, this overview focuses on its degradation very early in the course of microbial sepsis. We describe our recently proposed mechanism for glycocalyx degradation induced by exocytosis of lysosome-related organelles and release of their cargo. Notably, an intermediate in nitric oxide synthesis, NG-hydroxy-l-arginine, shows efficacy in curtailing exocytosis of these organelles and improvement in animal survival. These data not only depict a novel mechanism responsible for very early glycocalyx degradation, but may also outline a potential preventive therapy. The second issue discussed in this article is related to the therapeutic acceleration of restoration of already degraded endothelial glycocalyx. Here, using as an example our recent findings obtained with sulodexide, we illustrate the importance of the expedited repair of degraded endothelial glycocalyx for the survival of animals with severe sepsis. These two focal points of the review on glycocalyx may not only have broader disease applicability, but they may also provide additional evidence to buttress the idea of the importance of endothelial glycocalyx and its maintenance and repair in the prevention and treatment of an array of renal and nonrenal diseases.

Association between Lysosomal Dysfunction and Obesity-Related Pathology: A Key Knowledge to Prevent Metabolic Syndrome

Obesity causes various health problems, such as type 2 diabetes, non-alcoholic fatty liver disease, and cardio- and cerebrovascular diseases. Metabolic organs, particularly white adipose tissue (WAT) and liver, are deeply involved in obesity. WAT contains many adipocytes with energy storage capacity and secretes adipokines depending on the obesity state, while liver plays pivotal roles in glucose and lipid metabolism. This review outlines and underscores the relationship between obesity and lysosomal functions, including lysosome biogenesis, maturation and activity of lysosomal proteases in WAT and liver. It has been revealed that obesity-induced abnormalities of lysosomal proteases contribute to inflammation and cellular senescence in adipocytes. Previous reports have demonstrated obesity-induced ectopic lipid accumulation in liver is associated with abnormality of lysosomal proteases as well as other lysosomal enzymes. These studies demonstrate that lysosomal dysfunction in WAT and liver underlies part of the obesity-related pathology, raising the possibility that strategies to modulate lysosomal function may be effective in preventing or treating the metabolic syndrome.

Reduction of Nanoparticle Load in Cells by Mitosis but Not Exocytosis

The long-term fate of biomedically relevant nanoparticles (NPs) at the single cell level after uptake is not fully understood yet. We report that lysosomal exocytosis of NPs is not a mechanism to reduce the particle load. Biopersistent NPs such as nonporous silica and gold remain in cells for a prolonged time. The only reduction of the intracellular NP number is observed via cell division, e.g., mitosis. Additionally, NP distribution after cell division is observed to be asymmetrical, likely due to the inhomogeneous location and distribution of the NP-loaded intracellular vesicles in the mother cells. These findings are important for biomedical and hazard studies as the NP load per cell can vary significantly. Furthermore, we highlight the possibility of biopersistent NP accumulation over time within the mononuclear phagocyte system.

Strategies to target bioactive molecules to subcellular compartments. Focus on natural compounds

Many potential pharmacological targets are present in multiple subcellular compartments and have different pathophysiological roles depending on location. In these cases, selective targeting of a drug to the relevant subcellular domain(s) may help to sharpen its impact by providing topological specificity, thus limiting side effects, and to concentrate the compound where needed, thus increasing its effectiveness. We review here the state of the art in precision subcellular delivery. The major approaches confer "homing" properties to the active principle via permanent or reversible (in pro-drug fashion) modifications, or through the use of special-design nanoparticles or liposomes to ferry a drug(s) cargo to its desired destination. An assortment of peptides, substituents with delocalized positive charges, custom-blended lipid mixtures, pH- or enzyme-sensitive groups provide the main tools of the trade. Mitochondria, lysosomes and the cell membrane may be mentioned as the fronts on which the most significant advances have been made. Most of the examples presented here have to do with targeting natural compounds - in particular polyphenols, known as pleiotropic agents - to one or the other subcellular compartment.

HIV-1 gp120 Promotes Lysosomal Exocytosis in Human Schwann Cells

Human immunodeficiency virus type 1 (HIV-1) associated neuropathy is the most common neurological complication of HIV-1, with debilitating pain affecting the quality of life. HIV-1 gp120 plays an important role in the pathogenesis of HIV neuropathy via direct neurotoxic effects or indirect pro-inflammatory responses. Studies have shown that gp120-induced release of mediators from Schwann cells induce CCR5-dependent DRG neurotoxicity, however, CCR5 antagonists failed to improve pain in HIV- infected individuals. Thus, there is an urgent need for a better understanding of neuropathic pain pathogenesis and developing effective therapeutic strategies. Because lysosomal exocytosis in Schwann cells is an indispensable process for regulating myelination and demyelination, we determined the extent to which gp120 affected lysosomal exocytosis in human Schwann cells. We demonstrated that gp120 promoted the movement of lysosomes toward plasma membranes, induced lysosomal exocytosis, and increased the release of ATP into the extracellular media. Mechanistically, we demonstrated lysosome de-acidification, and activation of P2X4 and VNUT to underlie gp120-induced lysosome exocytosis. Functionally, we demonstrated that gp120-induced lysosome exocytosis and release of ATP from Schwann cells leads to increases in intracellular calcium and generation of cytosolic reactive oxygen species in DRG neurons. Our results suggest that gp120-induced lysosome exocytosis and release of ATP from Schwann cells and DRG neurons contribute to the pathogenesis of HIV-1 associated neuropathy.

Neuronal XBP-1 Activates Intestinal Lysosomes to Improve Proteostasis in C. elegans

The unfolded protein response of the endoplasmic reticulum (UPR^ER) is a crucial mediator of secretory pathway homeostasis. Expression of the spliced and active form of the UPR^ER transcription factor XBP-1, XBP-1s, in the nervous system triggers activation of the UPR^ER in the intestine of Caenorhabditis elegans (C. elegans) through release of a secreted signal, leading to increased longevity. We find that expression of XBP-1s in the neurons or intestine of the worm strikingly improves proteostasis in multiple tissues, through increased clearance of toxic proteins. To identify the mechanisms behind this enhanced proteostasis, we conducted intestine-specific RNA-seq analysis to identify genes upregulated in the intestine when XBP-1s is expressed in neurons. This revealed that neuronal XBP-1s increases the expression of genes involved in lysosome function. Lysosomes in the intestine of animals expressing neuronal XBP-1s are more acidic, and lysosomal protease activity is higher. Moreover, intestinal lysosome function is necessary for enhanced lifespan and proteostasis. These findings suggest that activation of the UPR^ER in the intestine through neuronal signaling can increase the activity of lysosomes, leading to extended longevity and improved proteostasis across tissues.

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Xbp-1s expressed in the neurons or intestine of C. elegans improves proteostasis

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Neuronal xbp-1s drives expression of lysosomal genes in the intestine

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Intestinal lysosomes show enhanced acidity and activity upon xbp-1s expression

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Lysosome function is required for xbp-1s to increase proteostasis and longevity

Insight into the Role of Extracellular Vesicles in Lysosomal Storage Disorders

Extracellular vesicles (EVs) have received increasing attention over the last two decades. Initially, they were considered as just a garbage disposal tool; however, it has progressively become clear that their protein, nucleic acid (namely miRNA and mRNA), and lipid contents have signaling functions. Besides, it has been established that cells release different types of vesicular structures for which characterization is still in its infancy. Many stress conditions, such as hypoxia, senescence, and oncogene activation have been associated with the release of higher levels of EVs. Further, evidence has shown that autophagic–lysosomal pathway abnormalities also affect EV release. In fact, in neurodegenerative diseases characterized by the accumulation of toxic proteins, although it has not become clear to what extent the intracellular storage of undigested materials itself has beneficial/adverse effects, these proteins have also been shown to be released extracellularly via EVs. Lysosomal storage disorders (LSDs) are characterized by accumulation of undigested substrates within the endosomal–lysosomal system, due either to genetic mutations in lysosomal proteins or to treatment with pharmacological agents. Here, we review studies investigating the role of lysosomal and autophagic dysfunction on the release of EVs, with a focus on studies exploring the release of EVs in LSD models of both genetic and pharmacological origin. A better knowledge of EV-releasing pathways activated in lysosomal stress conditions will provide information on the role of EVs in both alleviating intracellular storage of undigested materials and spreading the pathology to the neighboring tissue.

Transcriptional Regulation of Autophagy: Mechanisms and Diseases

Macro (Autophagy) is a catabolic process that relies on the cooperative function of two organelles: the lysosome and the autophagosome. The recent discovery of a transcriptional gene network that co-regulates the biogenesis and function of these two organelles, and the identification of transcription factors, miRNAs and epigenetic regulators of autophagy, demonstrated that this catabolic process is controlled by both transcriptional and post-transcriptional mechanisms. In this review article, we discuss the nuclear events that control autophagy, focusing particularly on the role of the MiT/TFE transcription factor family. In addition, we will discuss evidence suggesting that the transcriptional regulation of autophagy could be targeted for the treatment of human genetic diseases, such as lysosomal storage disorders (LSDs) and neurodegeneration.

Oral Delivery of the P2Y12 Receptor Antagonist Ticagrelor Prevents Loss of Photoreceptors in an ABCA4-/- Mouse Model of Retinal Degeneration

Purpose

Accumulation of lysosomal waste is linked to neurodegeneration in multiple diseases, and pharmacologic enhancement of lysosomal activity is hypothesized to reduce pathology. An excessive accumulation of lysosomal-associated lipofuscin waste and an elevated lysosomal pH occur in retinal pigment epithelial cells of the ABCA4-/- mouse model of Stargardt's retinal degeneration. As treatment with the P2Y12 receptor antagonist ticagrelor was previously shown to lower lysosomal pH and lipofuscin-like autofluorescence in these cells, we asked whether oral delivery of ticagrelor also prevented photoreceptor loss.

Methods

Moderate light exposure was used to accelerate photoreceptor loss in albino ABCA4-/- mice as compared to BALB/c controls. Ticagrelor (0.1%-0.15%) was added to mouse chow for between 1 and 10 months. Photoreceptor function was determined with electroretinograms, while cell survival was determined using optical coherence tomography and histology.

Results

Protection by ticagrelor was demonstrated functionally by using the electroretinogram, as ticagrelor-treated ABCA4-/- mice had increased a- and b-waves compared to untreated mice. Mice receiving ticagrelor treatment had a thicker outer nuclear layer, as measured with both optical coherence tomography and histologic sections. Ticagrelor decreased expression of LAMP1, implicating enhanced lysosomal function. No signs of retinal bleeding were observed after prolonged treatment with ticagrelor.

Conclusions

Oral treatment with ticagrelor protected photoreceptors in the ABCA4-/- mouse, which is consistent with enhanced lysosomal function. As mouse ticagrelor exposure levels were clinically relevant, the drug may be of benefit in preventing the loss of photoreceptors in Stargardt's disease and other neurodegenerations associated with lysosomal dysfunction.

Pros and cons of different ways to address dysfunctional autophagy in Pompe disease

Autophagy is a major intracellular self-digestion process that brings cytoplasmic materials to the lysosome for degradation. Defective autophagy has been linked to a broad range of human disorders, including cancer, diabetes, neurodegeneration, autoimmunity, cardiovascular diseases, and myopathies. In Pompe disease, a severe neuromuscular disorder, disturbances in autophagic process manifest themselves as progressive accumulation of undegraded cellular debris in the diseased muscle cells. A growing body of evidence has connected this defect to the decline in muscle function and muscle resistance to the currently available treatment-enzyme replacement therapy (ERT). Both induction and inhibition of autophagy have been tested in pre-clinical studies in a mouse model of the disease. Here, we discuss strengths and weaknesses of different approaches to address autophagic dysfunction in the context of Pompe disease.

Ca2+-Dependent Regulation of TFEB and Lysosomal Function

Lysosomes are emerging as calcium store organelles that can modulate various intracellular processes such as the regulation of nutrient signaling through the activation of TFEB, a master gene for lysosomal function, or very specialized functions like lysosomal exocytosis. Here, we describe two different techniques that can be used to study these processes. In the case report, we described two studies where these methodologies allowed us to unravel the role of calcineurin in the dephosphorylation of TFEB as well as the involvement of TFEB in lysosomal exocytosis, respectively.

The Parkinson-associated human P5B-ATPase ATP13A2 modifies lipid homeostasis

Mutations in the ATP13A2 gene (PARK9, CLN12, OMIM 610513) were initially associated with a form of Parkinson's Disease (PD) known as Kufor Rakeb Syndrome (KRS). However, the genetic spectrum of ATP13A2-associated disorders was expanded in the last years, because it has been found to underlay variants of neuronal ceroid-lipofuscinoses (NCLs) and hereditary spastic paraplegia. As ATP13A2 seems to be a key component of the endo-lysosome pathway, the fact that these pathologies are commonly characterized by endo-lysosomal dysfunction is not surprising. Here we report that increasing the level of functional ATP13A2 in a stable SH-SY5Y cell line disrupts lipid homeostasis. ATP13A2 overexpression increases the fluorescence intensity of the fluorescent analog phosphatidylethanolamine (NBD-PE) and the formation of multilamellar bodies, resembling the so-called "drug-induced phospholipidosis". We also found that expression of ATP13A2 reduces the ceramide-fluorescence intensity and the content of bis(monoacylglyceryl)phosphate (BMP). BMP is required for lipid degradation and exosome biogenesis inside acidic compartments, so this result suggests that ATP13A2 may be modifying the lipid digestion capacity and/or the redistribution of lipids in these subcellular organelles. In addition, ATP13A2-overexpression decreased the total content of triglycerides (TGs), cholesterol and lipid droplets. As TGs are necessary for the synthesis of new membranes, this observation suggests that increasing the function of ATP13A2 switches the endo-lysosomal system towards vesicle secretion.

Increased Lysosomal Exocytosis Induced by Lysosomal Ca2+ Channel Agonists Protects Human Dopaminergic Neurons from α-Synuclein Toxicity

The accumulation of misfolded proteins is a common pathological feature of many neurodegenerative disorders, including synucleinopathies such as Parkinson's disease (PD), which is characterized by the presence of α-synuclein (α-syn)-containing Lewy bodies. However, although recent studies have investigated α-syn accumulation and propagation in neurons, the molecular mechanisms underlying α-syn transmission have been largely unexplored. Here, we examined a monogenic form of synucleinopathy caused by loss-of-function mutations in lysosomal ATP13A2/PARK9. These studies revealed that lysosomal exocytosis regulates intracellular levels of α-syn in human neurons. Loss of PARK9 function in patient-derived dopaminergic neurons disrupted lysosomal Ca²⁺ homeostasis, reduced lysosomal Ca²⁺ storage, increased cytosolic Ca²⁺, and impaired lysosomal exocytosis. Importantly, this dysfunction in lysosomal exocytosis impaired α-syn secretion from both axons and soma, promoting α-syn accumulation. However, activation of the lysosomal Ca²⁺ channel transient receptor potential mucolipin 1 (TRPML1) was sufficient to upregulate lysosomal exocytosis, rescue defective α-syn secretion, and prevent α-syn accumulation. Together, these results suggest that intracellular α-syn levels are regulated by lysosomal exocytosis in human dopaminergic neurons and may represent a potential therapeutic target for PD and other synucleinopathies.SIGNIFICANCE STATEMENT Parkinson's disease (PD) is the second most common neurodegenerative disease linked to the accumulation of α-synuclein (α-syn) in patient neurons. However, it is unclear what the mechanism might be. Here, we demonstrate a novel role for lysosomal exocytosis in clearing intracellular α-syn and show that impairment of this pathway by mutations in the PD-linked gene ATP13A2/PARK9 contributes to α-syn accumulation in human dopaminergic neurons. Importantly, upregulating lysosomal exocytosis by increasing lysosomal Ca²⁺ levels was sufficient to rescue defective α-syn secretion and accumulation in patient neurons. These studies identify lysosomal exocytosis as a potential therapeutic target in diseases characterized by the accumulation of α-syn, including PD.

Lysosomal Ca2+ release channel TRPML1 regulates lysosome size by promoting mTORC1 activity

Lysosomal Ca²⁺ release channel TRPML1 has been suggested to regulate lysosome size by activating calmodulin (CaM). To further understand how TRPML1 and CaM regulate lysosome size, in this study, we report that inhibiting mTORC1 causes enlarged lysosomes, and the recovery of enlarged lysosomes is suppressed by inhibiting mTORC1. We also show that lysosome vacuolation induced by inhibiting TRPML1 is corrected by mTORC1 upregulation, and the facilitating effect of TRPML1 on the recovery of enlarged lysosomes is suppressed by inhibiting mTORC1. In the meantime, lysosome vacuolation induced by inhibiting CaM is corrected by mTORC1 upregulation, and mTORC1 overexpression corrects the inhibitory effect of CaM antagonist on the recovery of enlarged lysosomes. Conversely, the vacuolation induced by suppressing mTORC1 is not corrected by upregulating CaM. These data suggest that mTORC1 functions downstream of TRPML1 and CaM to regulate lysosome size. Together with our recent finding showing that TRPML1, CaM and mTORC1 form a macromolecular complex to control mTORC1 activity, we suggest that TRPML1 and CaM control lysosome fission through regulating mTORC1, identifying an mTORC1-dependent molecular mechanism for lysosomal membrane fission.

Membrane Lipid Composition: Effect on Membrane and Organelle Structure, Function and Compartmentalization and Therapeutic Avenues

Biological membranes are key elements for the maintenance of cell architecture and physiology. Beyond a pure barrier separating the inner space of the cell from the outer, the plasma membrane is a scaffold and player in cell-to-cell communication and the initiation of intracellular signals among other functions. Critical to this function is the plasma membrane compartmentalization in lipid microdomains that control the localization and productive interactions of proteins involved in cell signal propagation. In addition, cells are divided into compartments limited by other membranes whose integrity and homeostasis are finely controlled, and which determine the identity and function of the different organelles. Here, we review current knowledge on membrane lipid composition in the plasma membrane and endomembrane compartments, emphasizing its role in sustaining organelle structure and function. The correct composition and structure of cell membranes define key pathophysiological aspects of cells. Therefore, we explore the therapeutic potential of manipulating membrane lipid composition with approaches like membrane lipid therapy, aiming to normalize cell functions through the modification of membrane lipid bilayers.

Modelling the neuropathology of lysosomal storage disorders through disease-specific human induced pluripotent stem cells

Mucopolysaccharidosis II (MPS II) is a lysosomal storage disorder (LSD), caused by iduronate 2-sulphatase (IDS) enzyme dysfunction. The neuropathology of the disease is not well understood, although the neural symptoms are currently incurable. MPS II-patient derived iPSC lines were established and differentiated to neuronal lineage. The disease phenotype was confirmed by IDS enzyme and glycosaminoglycan assay. MPS II neuronal precursor cells (NPCs) showed significantly decreased self-renewal capacity, while their cortical neuronal differentiation potential was not affected. Major structural alterations in the ER and Golgi complex, accumulation of storage vacuoles, and increased apoptosis were observed both at protein expression and ultrastructural level in the MPS II neuronal cells, which was more pronounced in GFAP + astrocytes, with increased LAMP2 expression but unchanged in their RAB7 compartment. Based on these finding we hypothesize that lysosomal membrane protein (LMP) carrier vesicles have an initiating role in the formation of storage vacuoles leading to impaired lysosomal function. In conclusion, a novel human MPS II disease model was established for the first time which recapitulates the in vitro neuropathology of the disorder, providing novel information on the disease mechanism which allows better understanding of further lysosomal storage disorders and facilitates drug testing and gene therapy approaches.