The type IV mucolipidosis-associated protein TRPML1 is an endolysosomal iron release channel

The role of endolysosomal trafficking in anticancer drug resistance

Multidrug resistance (MDR) remains a major obstacle towards curative treatment of cancer. Despite considerable progress in delineating the basis of intrinsic and acquired MDR, the underlying molecular mechanisms remain to be elucidated. Emerging evidences suggest that dysregulation in endolysosomal compartments is involved in mediating MDR through multiple mechanisms, such as alterations in endosomes, lysosomes and autophagosomes, that traffic and biodegrade the molecular cargo through macropinocytosis, autophagy and endocytosis. For example, altered lysosomal pH, in combination with transcription factor EB (TFEB)-mediated lysosomal biogenesis, increases the sequestration of hydrophobic anti-cancer drugs that are weak bases, thereby producing an insufficient and off-target accumulation of anti-cancer drugs in MDR cancer cells. Thus, the use of well-tolerated, alkalinizing compounds that selectively block Vacuolar H⁺-ATPase (V-ATPase) may be an important strategy to overcome MDR in cancer cells and increase chemotherapeutic efficacy. Other mechanisms of endolysosomal-mediated drug resistance include increases in the expression of lysosomal proteases and cathepsins that are involved in mediating carcinogenesis and chemoresistance. Therefore, blocking the trafficking and maturation of lysosomal proteases or direct inhibition of cathepsin activity in the cytosol may represent novel therapeutic modalities to overcome MDR. Furthermore, endolysosomal compartments involved in catabolic pathways, such as macropinocytosis and autophagy, are also shown to be involved in the development of MDR. Here, we review the role of endolysosomal trafficking in MDR development and discuss how targeting endolysosomal pathways could emerge as a new therapeutic strategy to overcome chemoresistance in cancer.

Emerging Applications for Quantitative Susceptibility Mapping in the Detection of Traumatic Brain Injury Pathology

Traumatic brain injury (TBI) is a common but heterogeneous injury underpinned by numerous complex and interrelated pathophysiological mechanisms. An essential trace element, iron is abundant within the brain and involved in many fundamental neurobiological processes, including oxygen transportation, oxidative phosphorylation, myelin production and maintenance, as well as neurotransmitter synthesis and metabolism. Excessive levels of iron are neurotoxic and thus iron homeostasis is tightly regulated in the brain, however, many details about the mechanisms by which this is achieved are yet to be elucidated. A key mediator of oxidative stress, mitochondrial dysfunction and neuroinflammatory response, iron dysregulation is an important contributor to secondary injury in TBI. Advances in neuroimaging that leverage magnetic susceptibility properties have enabled increasingly comprehensive investigations into the distribution and behaviour of iron in the brain amongst healthy individuals as well as disease states such as TBI. Quantitative Susceptibility Mapping (QSM) is an advanced neuroimaging technique that promises quantitative estimation of local magnetic susceptibility at the voxel level. In this review, we provide an overview of brain iron and its homeostasis, describe recent advances enabling applications of QSM within the context of TBI and summarise the current state of the literature. Although limited, the emergent research suggests that QSM is a promising neuroimaging technique that can be used to investigate a host of pathophysiological changes that are associated with TBI.

Mechanosensitive ion channels in cell migration

Cellular processes are initiated and regulated by different stimuli, including mechanical forces. Cell membrane mechanosensors represent the first step towards the conversion of mechanical stimuli to a biochemical or electrical response. Mechanosensitive (MS) ion channels form a growing family of ion gating channels that respond to direct physical force or plasma membrane deformations. A number of calcium (Ca2+) permeable MS channels are known to regulate the initiation, direction, and persistence of cell migration during development and tumour progression. While the evidence that links individual MS ion channels to cell migration is growing, a unified analysis of the molecular mechanisms regulated downstream of MS ion channel activation is lacking. In this review, we describe the MS ion channel families known to regulate cell migration. We discuss the molecular mechanisms that act downstream of MS ion channels with an emphasis on Ca2+ mediated processes. Finally, we propose the future directions and impact of MS ion channel activity in the field of cell migration.

Regulation of TRPML1 channel activity and inflammatory exosome release by endogenously produced reactive oxygen species in mouse podocytes

The nucleotide-binding oligomerization domain-like receptor containing pyrin domain 3 (NLRP3) inflammasome in podocytes has been implicated in the initiation of glomerular inflammation during hyperhomocysteinemia (hHcy). However, the mechanism by which NLRP3 inflammasome products are released from podocytes remains unknown. The present study tested whether exosome secretion from podocytes is enhanced by NADPH oxidase-produced reactive oxygen species (ROS), which may serve as a pathogenic mechanism mediating the release of inflammatory cytokines produced by the NLRP3 inflammasome in podocytes after Hcy stimulation. We first demonstrated the remarkable elevation of endogenously produced ROS in podocytes treated with Hcy compared with control podocytes, which was abolished by pre-treatment with the NADPH oxidase inhibitors, gp91 ds-tat peptide and diphenyleneiodonium (DPI). In addition, Hcy induced activation in podocytes of NLRP3 inflammasomes and the formation of multivesicular bodies (MVBs) containing inflammatory cytokines, which were prevented by treatment with gp91 ds-tat or the ROS scavenger, catalase. Given the importance of the transient receptor potential mucolipin 1 (TRPML1) channel in Ca²⁺-dependent lysosome trafficking and consequent lysosome-MVB interaction, we tested whether lysosomal Ca²⁺ release through TRPML1 channels is inhibited by endogenously produced ROS in podocytes after Hcy stimulation. By GCaMP3 Ca²⁺ imaging, we confirmed the inhibition of TRPML1 channel activity by Hcy which was remarkably ameliorated by catalase and gp91 ds-tat peptide. By structured illumination microscopy (SIM) and nanoparticle tracking analysis (NTA), we found that ML-SA1, a TRPML1 channel agonist, significantly enhanced lysosome-MVB interaction and reduced exosome release in podocytes, which were attenuated by Hcy. Pre-treatment of podocytes with catalase or gp91 ds-tat peptide restored ML-SA1-induced changes in lysosome-MVB interaction and exosome secretion. Moreover, we found that hydrogen peroxide (H₂O₂) mimicked the effect of Hcy on TRPML1 channel activity, lysosome-MVB interaction, and exosome secretion in podocytes. Based on these results, we conclude that endogenously produced ROS importantly contributes to inflammatory exosome secretion from podocytes through inhibition of TRPML1 channel activity, which may contribute to the initiation of glomerular inflammation during hHcy.

Endolysosomal Ca2+ signaling in cardiovascular health and disease

An increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) regulates a plethora of functions in the cardiovascular (CV) system, including contraction in cardiomyocytes and vascular smooth muscle cells (VSMCs), and angiogenesis in vascular endothelial cells and endothelial colony forming cells. The sarco/endoplasmic reticulum (SR/ER) represents the largest endogenous Ca²⁺ store, which releases Ca²⁺ through ryanodine receptors (RyRs) and/or inositol-1,4,5-trisphosphate receptors (InsP₃Rs) upon extracellular stimulation. The acidic vesicles of the endolysosomal (EL) compartment represent an additional endogenous Ca²⁺ store, which is targeted by several second messengers, including nicotinic acid adenine dinucleotide phosphate (NAADP) and phosphatidylinositol 3,5-bisphosphate [PI(3,5)P₂], and may release intraluminal Ca²⁺ through multiple Ca²⁺ permeable channels, including two-pore channels 1 and 2 (TPC1-2) and Transient Receptor Potential Mucolipin 1 (TRPML1). Herein, we discuss the emerging, pathophysiological role of EL Ca²⁺ signaling in the CV system. We describe the role of cardiac TPCs in β-adrenoceptor stimulation, arrhythmia, hypertrophy, and ischemia-reperfusion injury. We then illustrate the role of EL Ca²⁺ signaling in VSMCs, where TPCs promote vasoconstriction and contribute to pulmonary artery hypertension and atherosclerosis, whereas TRPML1 sustains vasodilation and is also involved in atherosclerosis. Subsequently, we describe the mechanisms whereby endothelial TPCs promote vasodilation, contribute to neurovascular coupling in the brain and stimulate angiogenesis and vasculogenesis. Finally, we discuss about the possibility to target TPCs, which are likely to mediate CV cell infection by the Severe Acute Respiratory Disease-Coronavirus-2, with Food and Drug Administration-approved drugs to alleviate the detrimental effects of Coronavirus Disease-19 on the CV system.

MCOLN1/TRPML1 finely controls oncogenic autophagy in cancer by mediating zinc influx

Macroautophagy/autophagy is elevated to ensure the high demand for nutrients for the growth of cancer cells. Here we demonstrated that MCOLN1/TRPML1 is a pharmaceutical target of oncogenic autophagy in cancers such as pancreatic cancer, breast cancer, gastric cancer, malignant melanoma, and glioma. First, we showed that activating MCOLN1, by increasing expression of the channel or using the MCOLN1 agonists, ML-SA5 or MK6-83, arrests autophagic flux by perturbing fusion between autophagosomes and lysosomes. Second, we demonstrated that MCOLN1 regulates autophagy by mediating the release of zinc from the lysosome to the cytosol. Third, we uncovered that zinc influx through MCOLN1 blocks the interaction between STX17 (syntaxin 17) in the autophagosome and VAMP8 in the lysosome and thereby disrupting the fusion process that is determined by the two SNARE proteins. Furthermore, we demonstrated that zinc influx originating from the extracellular fluid arrests autophagy by the same mechanism as lysosomal zinc, confirming the fundamental function of zinc as a participant in membrane trafficking. Last, we revealed that activating MCOLN1 with the agonists, ML-SA5 or MK6-83, triggers cell death of a number of cancer cells by evoking autophagic arrest and subsequent apoptotic response and cell cycle arrest, with little or no effect observed on normal cells. Consistent with the in vitro results, administration of ML-SA5 in Patu 8988 t xenograft mice profoundly suppresses tumor growth and improves survival. These results establish that a lysosomal cation channel, MCOLN1, finely controls oncogenic autophagy in cancer by mediating zinc influx into the cytosol.

Lysosomal calcium and autophagy

Lysosomal calcium is emerging as a modulator of autophagy and lysosomal compartment, an obligatory partner to complete the autophagic pathway. A variety of specific signals such as nutrient deprivation or oxidative stress can trigger lysosomal calcium-mediated nuclear translocation of the transcription factor EB (TFEB), a master regulator of global lysosomal function. Also, lysosomal calcium can promote the formation of autophagosome vesicles (AVs) by a mechanism that requires the production of the phosphoinositide PI3P by the VPS34 autophagic complex and the activation of the energy-sensing kinase AMPK. Additionally, lysosomal calcium plays a role in membrane fusion and fission events involved in cellular processes such as endocytic maturation, autophagosome-lysosome fusion, lysosomal exocytosis, and lysosomal reformation upon autophagy completion. Lysosomal calcium-dependent functions are defective in cellular and animal models of the non-selective cation channel TRPML1, whose mutations in humans cause the neurodegenerative lysosomal storage disease mucolipidosis type IV (MLIV). Lysosomal calcium is not only acting as a positive regulator of autophagy, but it is also responsible for turning-off this process through the reactivation of the mTOR kinase during prolonged starvation. More recently, it has been described the role of lysosomal calcium on an elegant sequence of intracellular signaling events such as membrane repair, lysophagy, and lysosomal biogenesis upon the induction of different grades of lysosomal membrane damage. Here, we will discuss these novel findings that re-define the importance of the lysosome and lysosomal calcium signaling at regulating cellular metabolism.

Knock-Down of Mucolipin 1 Channel Promotes Tumor Progression and Invasion in Human Glioblastoma Cell Lines

Among cancers that affect the central nervous system, glioblastoma is the most common. Given the negative prognostic significance of transient receptor potential mucolipin 1 (TRPML1) channel reduction in patients with glioblastoma, as discussed in previous publications, the aim of the current study was to investigate the biological advantage of TRPML1 loss for glioma cells. Human glioblastoma primary cancer cells (FSL and FCL) and glioblastoma cell lines (T98 and U251) were used for that purpose. TRPML1 silencing in T98 cells induces defective autophagy, nitric oxide (NO) production, and cathepsin B-dependent apoptosis in the first 48 h and then apoptotic-resistant cells proliferate with a high growth rate with respect to control cells. In U251 cells, knock-down of TRPML1 stimulates NO generation and protein oxidation, arrests cell cycle at G2/M phase, and induces autophagy leading to cathepsin B-dependent senescence. Finally, in both cell lines, the long-term effects of TRPML1 silencing promote survival and invasion capacity with respect to control cells. Silencing of TRPML1 also affects the phenotype of glioblastoma primary cells. FSL cells show increased proliferation ability, while FCL cells enter into senescence associated with an increased invasion ability. In conclusion, although the molecular heterogeneity among different glioblastoma cell lines mirrors the intercellular heterogeneity in cancer cells, our data support TRPML1 downregulation as a negative prognostic factor in glioblastoma.

MCOLN1 gene therapy corrects neurologic dysfunction in the mouse model of mucolipidosis IV

Mucolipidosis IV (MLIV) is an orphan disease leading to debilitating psychomotor deficits and vision loss. It is caused by loss-of-function mutations in the MCOLN1 gene that encodes the lysosomal transient receptor potential channel mucolipin1, or TRPML1. With no existing therapy, the unmet need in this disease is very high. Here, we showed that AAV-mediated CNS-targeted gene transfer of the human MCOLN1 gene rescued motor function and alleviated brain pathology in the MLIV mouse model. Using the AAV-PHP.b vector in symptomatic mice, we showed long-term reversal of declined motor function and significant delay of paralysis. Next, using self-complementary AAV9 clinical candidate vector, we showed that its intracerebroventricular administration in post-natal day 1 mice significantly improved motor function, myelination and reduced lysosomal storage load in the MLIV mouse brain. Based on our data and general advancements in the gene therapy field, we propose scAAV9-mediated CSF-targeted MCOLN1 gene transfer as a therapeutic strategy in MLIV.

Vascular Dysfunction in Diabetes and Obesity: Focus on TRP Channels

Transient receptor potential (TRP) superfamily consists of a diverse group of non-selective cation channels that has a wide tissue distribution and is involved in many physiological processes including sensory perception, secretion of hormones, vasoconstriction/vasorelaxation, and cell cycle modulation. In the blood vessels, TRP channels are present in endothelial cells, vascular smooth muscle cells, perivascular adipose tissue (PVAT) and perivascular sensory nerves, and these channels have been implicated in the regulation of vascular tone, vascular cell proliferation, vascular wall permeability and angiogenesis. Additionally, dysfunction of TRP channels is associated with cardiometabolic diseases, such as diabetes and obesity. Unfortunately, the prevalence of diabetes and obesity is rising worldwide, becoming an important public health problems. These conditions have been associated, highlighting that obesity is a risk factor for type 2 diabetes. As well, both cardiometabolic diseases have been linked to a common disorder, vascular dysfunction. In this review, we briefly consider general aspects of TRP channels, and we focus the attention on TRPC (canonical or classical), TRPV (vanilloid), TRPM (melastatin), and TRPML (mucolipin), which were shown to be involved in vascular alterations of diabetes and obesity or are potentially linked to vascular dysfunction. Therefore, elucidation of the functional and molecular mechanisms underlying the role of TRP channels in vascular dysfunction in diabetes and obesity is important for the prevention of vascular complications and end-organ damage, providing a further therapeutic target in the treatment of these metabolic diseases.

Iron Metabolism Disorders for Cognitive Dysfunction After Mild Traumatic Brain Injury

Traumatic brain injury (TBI) is one of the most harmful forms of acute brain injury and predicted to be one of the three major neurological diseases that cause neurological disabilities by 2030. A series of secondary injury cascades often cause cognitive dysfunction of TBI patients leading to poor prognosis. However, there are still no effective intervention measures, which drive us to explore new therapeutic targets. In this process, the most part of mild traumatic brain injury (mTBI) is ignored because its initial symptoms seemed not serious. Unfortunately, the ignored mTBI accounts for 80% of the total TBI, and a large part of the patients have long-term cognitive dysfunction. Iron deposition has been observed in mTBI patients and accompanies the whole pathological process. Iron accumulation may affect long-term cognitive dysfunction from three pathways: local injury, iron deposition induces tau phosphorylation, the formation of neurofibrillary tangles; neural cells death; and neural network damage, iron deposition leads to axonal injury by utilizing the iron sensibility of oligodendrocytes. Thus, iron overload and metabolism dysfunction was thought to play a pivotal role in mTBI pathophysiology. Cerebrospinal fluid-contacting neurons (CSF-cNs) located in the ependyma have bidirectional communication function between cerebral-spinal fluid and brain parenchyma, and may participate in the pathway of iron-induced cognitive dysfunction through projected nerve fibers and transmitted factor, such as 5-hydroxytryptamine, etc. The present review provides an overview of the metabolism and function of iron in mTBI, and to seek a potential new treatment target for mTBI with a novel perspective through combined iron and CSF-cNs.

Lysosomal ion channels involved in cellular entry and uncoating of enveloped viruses: Implications for therapeutic strategies against SARS-CoV-2

Ion channels are necessary for correct lysosomal function including degradation of cargoes originating from endocytosis. Almost all enveloped viruses, including coronaviruses (CoVs), enter host cells via endocytosis, and do not escape endosomal compartments into the cytoplasm (via fusion with the endolysosomal membrane) unless the virus-encoded envelope proteins are cleaved by lysosomal proteases. With the ongoing outbreak of severe acute respiratory syndrome (SARS)-CoV-2, endolysosomal two-pore channels represent an exciting and emerging target for antiviral therapies. This review focuses on the latest knowledge of the effects of lysosomal ion channels on the cellular entry and uncoating of enveloped viruses, which may aid in development of novel therapies against emerging infectious diseases such as SARS-CoV-2.

Ferroptosis and Its Modulation by Autophagy in Light of the Pathogenesis of Lysosomal Storage Diseases

Ferroptosis is one of the recently described types of cell death which is dependent on many factors, including the accumulation of iron and lipid peroxidation. Its induction requires various signaling pathways. Recent discovery of ferroptosis induction pathways stimulated by autophagy, so called autophagy-dependent ferroptosis, put our attention on the role of ferroptosis in lysosomal storage diseases (LSD). Lysosome dysfunction, observed in these diseases, may influence ferroptosis efficiency, with as yet unknown consequences for the function of cells, tissues, and organisms, due to the effects of ferroptosis on physiological and pathological metabolic processes. Modulation of levels of ferrous ions and enhanced oxidative stress, which are primary markers of ferroptosis, are often described as processes associated with the pathology of LSD. Inhibition of autophagy flux and resultant accumulation of autophagosomes in neuronopathic LSD may induce autophagy-dependent ferroptosis, indicating a considerable contribution of this process in neurodegeneration. In this review article, we describe molecular mechanisms of ferroptosis in light of LSD, underlining the modulation of levels of ferroptosis markers in these diseases. Furthermore, we propose a hypothesis about the possible involvement of autophagy-dependent ferroptosis in these disorders.

New Insights into the Role of Ferritin in Iron Homeostasis and Neurodegenerative Diseases

Growing evidence has indicated that iron deposition is one of the key factors leading to neuronal death in the neurodegenerative diseases. Ferritin is a hollow iron storage protein composed of 24 subunits of two types, ferritin heavy chain (FTH) and ferritin light chain (FTL), which plays an important role in maintaining iron homeostasis. Recently, the discovery of extracellular ferritin and ferritin in exosomes indicates that ferritin might be not only an iron storage protein within the cell, but might also be an important factor in the regulation of tissue and body iron homeostasis. In this review, we first described the structural characteristics, regulation and the physiological functions of ferritin. Secondly, we reviewed the current evidence concerning the mechanisms underlying the secretion of ferritin and the possible role of secreted ferritin in the brain. Then, we summarized the relationship between ferritin and the neurodegenerative diseases including Parkinson's disease (PD), Alzheimer's disease (AD) and neuroferritinopathy (NF). Given the importance and relationship between iron and neurodegenerative diseases, understanding the role of ferritin in the brain can be expected to contribute to our knowledge of iron dysfunction and neurodegenerative diseases.

Iron supplementation and iron-fortified foods: a review

About one-third of the world population is suffering from iron deficiency. Delivery of iron through diet is a practical, economical, and sustainable approach. Clinical studies have shown that the consumption of iron-fortified foods is one of the most effective methods for the prevention of iron deficiency. However, supplementing iron through diet can cause undesirable side-effects. Thus, it is essential to develop new iron-rich ingredients, iron-fortified products with high bioavailability, better stability, and lower cost. It is also essential to develop newer processing technologies for more effective fortification. This review compared the iron supplementation strategies used to treat the highly iron-deficient population and the general public. We also reviewed the efficacy of functional (iron-rich) ingredients that can be incorporated into food materials to produce iron-fortified foods. The most commonly available foods, such as cereals, bakery products, dairy products, beverages, and condiments are still the best vehicles for iron fortification and delivery.Scope of reviewThe manuscript aims at providing a comprehensive review of the latest publications that cover three aspects: administration routes for iron supplementation, iron-rich ingredients used for iron supplementation, and iron-fortified foods.

Nickel ions attenuate autophagy flux and induce transglutaminase 2 (TG2) mediated post-translational modification of SQSTM1/p62

Nickel, the most frequent contact allergy cause, is widely used for various metallic materials and medical devices. Autophagy is an intracellular protein degradation system and contributes to metal recycling. However, it is unclear the functions of nickel in autophagy. We here demonstrated that NiCl2 induced microtubule-associated protein 1 light chain 3 (LC3)-II and LC3 puncta, markers of autophagosomes. Bafilomycin A1 (BafA1) treatment did not enhance LC3 puncta under NiCl2 stimulation, suggesting that NiCl2 did not induce autophagic flux. In addition, NiCl2 promotes the accumulation of SQSTM1/p62 and increased SQSTM1/p62 colocalization with lysosomal-associated membrane protein 1 (LAMP1). These data indicated that NiCl2 attenuates autophagic flux. Interestingly, NiCl2 induced the expression of the high-molecular-weight (MW) form of SQSTM1/p62. Inhibition of NiCl2-induced reactive oxygen species (ROS) reduced the high-MW SQSTM1/p62. We also showed that NiCl2-induced ROS activate transglutaminase (TG) activity. We found that transglutaminase 2 (TG2) inhibition reduced high-MW SQSTM1/p62 and SQSTM1/p62 puncta under NiCl2 stimulation, indicating that TG2 regulates SQSTM1/p62 protein homeostasis under NiCl2 stimulation. Our study demonstrated that nickel ion regulates autophagy flux and TG2 restricted nickel-dependent proteostasis.

Endolysosomal TRPMLs in Cancer

Lysosomes, the degradative endpoints and sophisticated cellular signaling hubs, are emerging as intracellular Ca²⁺ stores that govern multiple cellular processes. Dys-homeostasis of lysosomal Ca²⁺ is intimately associated with a variety of human diseases including cancer. Recent studies have suggested that the Ca²⁺-permeable channels Transient Receptor Potential (TRP) Mucolipins (TRPMLs, TRPML1-3) integrate multiple processes of cell growth, division and metabolism. Dysregulation of TRPMLs activity has been implicated in cancer development. In this review, we provide a summary of the latest development of TRPMLs in cancer. The expression of TRPMLs in cancer, TRPMLs in cancer cell nutrient sensing, TRPMLs-mediated lysosomal exocytosis in cancer development, TRPMLs in TFEB-mediated gene transcription of cancer cells, TRPMLs in bacteria-related cancer development and TRPMLs-regulated antitumor immunity are discussed. We hope to guide readers toward a more in-depth discussion of the importance of lysosomal TRPMLs in cancer progression and other human diseases.

Lysosomal TRPML1 Channel: Implications in Cardiovascular and Kidney Diseases

Lysosomal ion channels mediate ion flux from lysosomes and regulate membrane potential across the lysosomal membrane, which are essential for lysosome biogenesis, nutrient sensing, lysosome trafficking, lysosome enzyme activity, and cell membrane repair. As a cation channel, the transient receptor potential mucolipin 1 (TRPML1) channel is mainly expressed on lysosomes and late endosomes. Recently, the normal function of TRPML1 channels has been demonstrated to be important for the maintenance of cardiovascular and renal glomerular homeostasis and thereby involved in the pathogenesis of some cardiovascular and kidney diseases. In arterial myocytes, it has been found that Nicotinic Acid Adenine Dinucleotide Phosphate (NAADP), an intracellular second messenger, can induce Ca²⁺ release through the lysosomal TRPML1 channel, leading to a global Ca²⁺ release response from the sarcoplasmic reticulum (SR). In podocytes, it has been demonstrated that lysosomal TRPML1 channels control lysosome trafficking and exosome release, which contribute to the maintenance of podocyte functional integrity. The defect or functional deficiency of lysosomal TRPML1 channels has been shown to critically contribute to the initiation and development of some chronic degeneration or diseases in the cardiovascular system or kidneys. Here we briefly summarize the current evidence demonstrating the regulation of lysosomal TRPML1 channel activity and related signaling mechanisms. We also provide some insights into the canonical and noncanonical roles of TRPML1 channel dysfunction as a potential pathogenic mechanism for certain cardiovascular and kidney diseases and associated therapeutic strategies.

A DNA-based voltmeter for organelles

The role of membrane potential in most intracellular organelles remains unexplored because of the lack of suitable tools. Here, we describe Voltair, a fluorescent DNA nanodevice that reports the absolute membrane potential and can be targeted to organelles in live cells. Voltair consists of a voltage-sensitive fluorophore and a reference fluorophore for ratiometry, and acts as an endocytic tracer. Using Voltair, we could measure the membrane potential of different organelles in situ in live cells. Voltair can potentially guide the rational design of biocompatible electronics and enhance our understanding of how membrane potential regulates organelle biology.

The journey of Ca2+ through the cell - pulsing through the network of ER membrane contact sites

Calcium is the third most abundant metal on earth, and the fundaments of its homeostasis date back to pre-eukaryotic life forms. In higher organisms, Ca²⁺ serves as a cofactor for a wide array of (enzymatic) interactions in diverse cellular contexts and constitutes the most important signaling entity in excitable cells. To enable responsive behavior, cytosolic Ca²⁺ concentrations are kept low through sequestration into organellar stores, particularly the endoplasmic reticulum (ER), but also mitochondria and lysosomes. Specific triggers are then used to instigate a local release of Ca²⁺ on demand. Here, communication between organelles comes into play, which is accomplished through intimate yet dynamic contacts, termed membrane contact sites (MCSs). The field of MCS biology in relation to cellular Ca²⁺ homeostasis has exploded in recent years. Taking advantage of this new wealth of knowledge, in this Review, we invite the reader on a journey of Ca²⁺ flux through the ER and its associated MCSs. New mechanistic insights and technological advances inform the narrative on Ca²⁺ acquisition and mobilization at these sites of communication between organelles, and guide the discussion of their consequences for cellular physiology.

Transient Receptor Potential Channels as an Emerging Target for the Treatment of Parkinson's Disease: An Insight Into Role of Pharmacological Interventions

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by the symptoms of motor deficits and cognitive decline. There are a number of therapeutics available for the treatment of PD, but most of them suffer from serious side effects such as bradykinesia, dyskinesia and on-off effect. Therefore, despite the availability of these pharmacological agents, PD patients continue to have an inferior quality of life. This has warranted a need to look for alternate strategies and molecular targets. Recent evidence suggests the Transient Receptor Potential (TRP) channels could be a potential target for the management of motor and non-motor symptoms of PD. Though still in the preclinical stages, agents targeting these channels have shown immense potential in the attenuation of behavioral deficits and signaling pathways. In addition, these channels are known to be involved in the regulation of ionic homeostasis, which is disrupted in PD. Moreover, activation or inhibition of many of the TRP channels by calcium and oxidative stress has also raised the possibility of their paramount involvement in affecting the other molecular mechanisms associated with PD pathology. However, due to the paucity of information available and lack of specificity, none of these agents have gone into clinical trials for PD treatment. Considering their interaction with oxidative stress, apoptosis and excitotoxicity, TRP channels could be considered as a potential future target for the treatment of PD.

Abnormal podocyte TRPML1 channel activity and exosome release in mice with podocyte-specific Asah1 gene deletion

Podocytopathy and associated nephrotic syndrome (NS) have been reported in a knockout mouse strain (Asah1^fl/fl/Podo^Cre) with a podocyte-specific deletion of α subunit (the main catalytic subunit) of acid ceramidase (Ac). However, the pathogenesis of podocytopathy of these mice remains unknown. The present study tested whether exosome release from podocytes is enhanced due to Asah1 gene knockout, which may serve as a pathogenic mechanism switching on podocytopathy and associated NS in Asah1^fl/fl/Podo^Cre mice. We first demonstrated the remarkable elevation of urinary exosome excretion in Asah1^fl/fl/Podo^Cre mice compared with WT/WT mice, which was accompanied by significant Annexin-II (an exosome marker) accumulation in glomeruli of Asah1^fl/fl/Podo^Cre mice, as detected by immunohistochemistry. In cell studies, we also confirmed that Asah1 gene knockout enhanced exosome release in the primary cultures of podocyte isolated from Asah1^fl/fl/Podo^Cre mice compared to WT/WT mice. In the podocytes from Asah1^fl/fl/Podo^Cre mice, the interactions of lysosome and multivesicular body (MVB) were demonstrated to be decreased in comparison with those from their control littermates, suggesting reduced MVB degradation that may lead to increase in exosome release. Given the critical role of transient receptor potential mucolipin 1 (TRPML1) channel in Ca²⁺-dependent lysosome trafficking and consequent lysosome-MVB interaction, we tested whether lysosomal Ca²⁺ release through TRPML1 channels is inhibited in the podocytes of Asah1^fl/fl/Podo^Cre mice. By GCaMP3 Ca²⁺ imaging, it was found that lysosomal Ca²⁺ release through TRPML1 channels was substantially suppressed in podocytes with Asah1 gene deletion. As an Ac product, sphingosine was found to rescue TRPML1 channel activity and thereby recover lysosome-MVB interaction and reduce exosome release of podocytes from Asah1^fl/fl/Podo^Cre mice. Combination of N, N-dimethylsphingosine (DMS), a potent sphingosine kinase inhibitor, and sphingosine significantly inhibited urinary exosome excretion of Asah1^fl/fl/Podo^Cre mice. Moreover, rescue of Aash1 gene expression in podocytes of Asah1^fl/fl/Podo^Cre mice showed normal ceramide metabolism and exosome secretion. Based on these results, we conclude that the normal expression of Ac importantly contributes to the control of TRPML1 channel activity, lysosome-MVB interaction, and consequent exosome release from podocytes. Asah1 gene defect inhibits TRPML1 channel activity and thereby enhances exosome release, which may contribute to the development of podocytopathy and associated NS.

The intracellular Ca2+ release channel TRPML1 regulates lower urinary tract smooth muscle contractility

TRPML1 (transient receptor potential mucolipin 1) is a Ca²⁺-permeable, nonselective cation channel that is localized to late endosomes and lysosomes. Here, we investigated the function of TRPML1 channels in regulating lower urinary tract (LUT) smooth muscle cell (SMC) contractility. We found that TRPML1 forms a stable signaling complex with ryanodine receptors (RyRs) in the sarcoplasmic reticulum (SR). We further showed that TRPML1 channels are important for initiating an essential Ca²⁺-signaling negative feedback mechanism between RyRs on SR membranes and K⁺ channels on the plasma membrane. Knockout of TRPML1 channels in mice impaired this pathway, resulting in LUT smooth muscle hypercontractility and symptoms of overactive bladder. Our findings demonstrate a critical role for TRPML1 in LUT function.

TRPML1 (transient receptor potential mucolipin 1) is a Ca²⁺-permeable, nonselective cation channel that is predominantly localized to the membranes of late endosomes and lysosomes (LELs). Intracellular release of Ca²⁺ through TRPML1 is thought to be pivotal for maintenance of intravesicular acidic pH as well as the maturation, fusion, and trafficking of LELs. Interestingly, genetic ablation of TRPML1 in mice (Mcoln1^−/−) induces a hyperdistended/hypertrophic bladder phenotype. Here, we investigated this phenomenon further by exploring an unconventional role for TRPML1 channels in the regulation of Ca²⁺-signaling activity and contractility in bladder and urethral smooth muscle cells (SMCs). Four-dimensional (4D) lattice light-sheet live-cell imaging showed that the majority of LELs in freshly isolated bladder SMCs were essentially immobile. Superresolution microscopy revealed distinct nanoscale colocalization of LEL-expressing TRPML1 channels with ryanodine type 2 receptors (RyR2) in bladder SMCs. Spontaneous intracellular release of Ca²⁺ from the sarcoplasmic reticulum (SR) through RyR2 generates localized elevations of Ca²⁺ (“Ca²⁺ sparks”) that activate plasmalemmal large-conductance Ca²⁺-activated K⁺ (BK) channels, a critical negative feedback mechanism that regulates smooth muscle contractility. This mechanism was impaired in Mcoln1^−/− mice, which showed diminished spontaneous Ca²⁺ sparks and BK channel activity in bladder and urethra SMCs. Additionally, ex vivo contractility experiments showed that loss of Ca²⁺ spark–BK channel signaling in Mcoln1^−/− mice rendered both bladder and urethra smooth muscle hypercontractile. Voiding activity analyses revealed bladder overactivity in Mcoln1^−/− mice. We conclude that TRPML1 is critically important for Ca²⁺ spark signaling, and thus regulation of contractility and function, in lower urinary tract SMCs.

TRP Channels Regulation of Rho GTPases in Brain Context and Diseases

Neurological and neuropsychiatric disorders are mediated by several pathophysiological mechanisms, including developmental and degenerative abnormalities caused primarily by disturbances in cell migration, structural plasticity of the synapse, and blood-vessel barrier function. In this context, critical pathways involved in the pathogenesis of these diseases are related to structural, scaffolding, and enzymatic activity-bearing proteins, which participate in Ca²⁺- and Ras Homologs (Rho) GTPases-mediated signaling. Rho GTPases are GDP/GTP binding proteins that regulate the cytoskeletal structure, cellular protrusion, and migration. These proteins cycle between GTP-bound (active) and GDP-bound (inactive) states due to their intrinsic GTPase activity and their dynamic regulation by GEFs, GAPs, and GDIs. One of the most important upstream inputs that modulate Rho GTPases activity is Ca²⁺ signaling, positioning ion channels as pivotal molecular entities for Rho GTPases regulation. Multiple non-selective cationic channels belonging to the Transient Receptor Potential (TRP) family participate in cytoskeletal-dependent processes through Ca²⁺-mediated modulation of Rho GTPases. Moreover, these ion channels have a role in several neuropathological events such as neuronal cell death, brain tumor progression and strokes. Although Rho GTPases-dependent pathways have been extensively studied, how they converge with TRP channels in the development or progression of neuropathologies is poorly understood. Herein, we review recent evidence and insights that link TRP channels activity to downstream Rho GTPase signaling or modulation. Moreover, using the TRIP database, we establish associations between possible mediators of Rho GTPase signaling with TRP ion channels. As such, we propose mechanisms that might explain the TRP-dependent modulation of Rho GTPases as possible pathways participating in the emergence or maintenance of neuropathological conditions.

The heteromeric PC-1/PC-2 polycystin complex is activated by the PC-1 N-terminus

Mutations in the polycystin proteins, PC-1 and PC-2, result in autosomal dominant polycystic kidney disease (ADPKD) and ultimately renal failure. PC-1 and PC-2 enrich on primary cilia, where they are thought to form a heteromeric ion channel complex. However, a functional understanding of the putative PC-1/PC-2 polycystin complex is lacking due to technical hurdles in reliably measuring its activity. Here we successfully reconstitute the PC-1/PC-2 complex in the plasma membrane of mammalian cells and show that it functions as an outwardly rectifying channel. Using both reconstituted and ciliary polycystin channels, we further show that a soluble fragment generated from the N-terminal extracellular domain of PC-1 functions as an intrinsic agonist that is necessary and sufficient for channel activation. We thus propose that autoproteolytic cleavage of the N-terminus of PC-1, a hotspot for ADPKD mutations, produces a soluble ligand in vivo. These findings establish a mechanistic framework for understanding the role of PC-1/PC-2 heteromers in ADPKD and suggest new therapeutic strategies that would expand upon the limited symptomatic treatments currently available for this progressive, terminal disease.

On the surface of most animal and other eukaryotic cells are small rod-like protrusions known as primary cilia. Each cilium is encased by a specialized membrane which is enriched in protein complexes that help the cell sense its local environment. Some of these complexes help transport ions in out of the cell, while others act as receptors that receive chemical signals called ligands. A unique ion channel known as the polycystin complex is able to perform both of these roles as it contains a receptor called PC-1 in addition to an ion channel called PC-2.

Various mutations in the genes that code for PC-1 and PC-2 can result in autosomal dominant polycystic kidney disease (ADPKD), which is the most common monogenetic disease in humans. However, due to the small size of primary cilia – which are less than a thousandth of a millimeter thick – little is known about how polycystin complexes are regulated and how mutations lead to ADPKD. To overcome this barrier, Ha et al. modified kidney cells grown in the lab so that PC-1 and PC-2 form a working channel in the plasma membrane which surrounds the entire cell. As the body of a cell is around 10,000 times bigger than the cilium, this allowed the movement of ions across the polycystin complex to be studied using conventional techniques.

Experiments using this newly developed assay revealed that a region at one of the ends of the PC-1 protein, named the C-type lectin domain, is essential for stimulating polycystin complexes. Ha et al. found that this domain of PC-1 is able to cut itself from the protein complex. Further experiments showed that when fragments of PC-1, which contain the C-type lectin domain, are no longer bound to the membrane, they can activate the polycystin channels in cilia as well as the plasma membrane. This suggests that this region of PC-1 may also act as a secreted ligand that can activate other polycystin channels.

Some of the genetic mutations that cause ADPKD likely disrupt the activity of the polycystin complex and reduce its ability to transport ions across the cilia membrane. Therefore, the cell assay created in this study could be used to screen for small molecules that can restore the activity of these ion channels in patients with ADPKD.

LRRC8 family proteins within lysosomes regulate cellular osmoregulation and enhance cell survival to multiple physiological stresses

LRRC8 family proteins on the plasma membrane play a critical role in cellular osmoregulation by forming volume-regulated anion channels (VRACs) necessary to prevent necrotic cell death. We demonstrate that intracellular LRRC8 proteins acting within lysosomes also play an essential role in cellular osmoregulation. LRRC8 proteins on lysosome membranes generate large lysosomal volume-regulated anion channel (Lyso-VRAC) currents in response to low cytoplasmic ionic strength conditions. When a double-leucine L⁷⁰⁶L⁷⁰⁷ motif at the C terminus of LRRC8A was mutated to alanines, normal plasma membrane VRAC currents were still observed, but Lyso-VRAC currents were absent. We used this targeting mutant, as well as pharmacological tools, to demonstrate that Lyso-VRAC currents are necessary for the formation of large lysosome-derived vacuoles, which store and then expel excess water to maintain cytosolic water homeostasis. Thus, Lyso-VRACs allow lysosomes of mammalian cells to act as the cell`s "bladder." When Lyso-VRAC current was selectively eliminated, the extent of necrotic cell death to sustained stress was greatly increased, not only in response to hypoosmotic stress, but also to hypoxic and hypothermic stresses. Thus Lyso-VRACs play an essential role in enabling cells to mount successful homeostatic responses to multiple stressors.

Two-pore and TRPML cation channels: Regulators of phagocytosis, autophagy and lysosomal exocytosis

The old Greek saying "Panta Rhei" ("everything flows") is true for all life and all living things in general. It also becomes nicely evident when looking closely into cells. There, material from the extracellular space is taken up by endocytic processes and transported to endosomes where it is sorted either for recycling or degradation. Cargo is also packaged for export through exocytosis involving the Golgi network, lysosomes and other organelles. Everything in this system is in constant motion and many proteins are necessary to coordinate transport along the different intracellular pathways to avoid chaos. Among these proteins are ion channels., in particular TRPML channels (mucolipins) and two-pore channels (TPCs) which reside on endosomal and lysosomal membranes to speed up movement between organelles, e.g. by regulating fusion and fission; they help readjust pH and osmolarity changes due to such processes, or they promote exocytosis of export material. Pathophysiologically, these channels are involved in neurodegenerative, metabolic, retinal and infectious diseases, cancer, pigmentation defects, and immune cell function, and thus have been proposed as novel pharmacological targets, e.g. for the treatment of lysosomal storage disorders, Duchenne muscular dystrophy, or different types of cancer. Here, we discuss the similarities but also differences of TPCs and TRPMLs in regulating phagocytosis, autophagy and lysosomal exocytosis, and we address the contradictions and open questions in the field relating to the roles TPCs and TRPMLs play in these different processes.

Extracellular SQSTM1 mediates bacterial septic death in mice through insulin receptor signalling

Sepsis is the most common cause of death for patients in intensive care worldwide due to a dysregulated host response to infection. Here, we investigate the role of sequestosome-1 (SQSTM1/p62), an autophagy receptor that functions as a regulator of innate immunity, in sepsis. We find that lipopolysaccharide elicits gasdermin D-dependent pyroptosis to enable passive SQSTM1 release from macrophages and monocytes, whereas transmembrane protein 173-dependent TANK-binding kinase 1 activation results in the phosphorylation of SQSTM1 at Ser403 and subsequent SQSTM1 secretion from macrophages and monocytes. Moreover, extracellular SQSTM1 binds to insulin receptor, which in turn activates a nuclear factor kappa B-dependent metabolic pathway, leading to aerobic glycolysis and polarization of macrophages. Intraperitoneal injection of anti-SQSTM1-neutralizing monoclonal antibodies or conditional depletion of Insr in myeloid cells using the Cre-loxP system protects mice from lethal sepsis (caecal ligation and puncture or infection by Escherichia coli or Streptococcus pneumoniae) and endotoxaemia. We also report that circulating SQSTM1 and the messenger RNA expression levels of SQSTM1 and INSR in peripheral blood mononuclear cells are related to the severity of sepsis in 40 patients. Thus, extracellular SQSTM1 has a pathological role in sepsis and could be targeted to develop therapies for sepsis.

Iron and manganese transport in mammalian systems

Studies in recent years have significantly expanded, refined, and redefined the repertoire of transporters and other proteins involved in iron and manganese (Mn) transport and homeostasis. In this review, we discuss highlights of the recent literature on iron and Mn transport, focusing on the roles of membrane transporters and related proteins. Studies are considered from the vantage point of main organs, tissues, and cell types that actively control whole-body iron or Mn homeostasis, with emphasis on studies in which in vivo metal transport was measured directly or implicated by using knockout mouse models. Overviews of whole-body and cellular iron and Mn homeostasis are also provided to give physiological context for key transporters and to highlight how they participate in the uptake, intracellular trafficking, and efflux of each metal. Important similarities and differences in iron and Mn transport are noted, and future research opportunities and challenges are identified.

Fe2+-Mediated Activation of BKCa Channels by Rapid Photolysis of CORM-S1 Releasing CO and Fe2

Heme catabolism by heme oxygenase (HO) with a decrease in intracellular heme concentration and a concomitant local release of CO and Fe²⁺ has the potential to regulate BK_Ca channels. Here, we show that the iron-based photolabile CO-releasing molecule CORM-S1 [dicarbonyl-bis(cysteamine)iron(II)] coreleases CO and Fe²⁺, making it a suitable light-triggered source of these downstream products of HO activity. To investigate the impact of CO, iron, and cysteamine on BK_Ca channel activation, human Slo1 (hSlo1) was expressed in HEK293T cells and studied with electrophysiological methods. Whereas hSlo1 channels are activated by CO and even more strongly by Fe²⁺, Fe³⁺ and cysteamine possess only marginal activating potency. Investigation of hSlo1 mutants revealed that Fe²⁺ modulates the channels mainly through the Mg²⁺-dependent activation mechanism. Flash photolysis of CORM-S1 suits for rapid and precise delivery of Fe²⁺ and CO in biological settings.

Early evidence of delayed oligodendrocyte maturation in the mouse model of mucolipidosis type IV

Mucolipidosis type IV (MLIV) is a lysosomal disease caused by mutations in the MCOLN1 gene that encodes the endolysosomal transient receptor potential channel mucolipin-1, or TRPML1. MLIV results in developmental delay, motor and cognitive impairments, and vision loss. Brain abnormalities include thinning and malformation of the corpus callosum, white-matter abnormalities, accumulation of undegraded intracellular 'storage' material and cerebellar atrophy in older patients. Identification of the early events in the MLIV course is key to understanding the disease and deploying therapies. The Mcoln1-/- mouse model reproduces all major aspects of the human disease. We have previously reported hypomyelination in the MLIV mouse brain. Here, we investigated the onset of hypomyelination and compared oligodendrocyte maturation between the cortex/forebrain and cerebellum. We found significant delays in expression of mature oligodendrocyte markers Mag, Mbp and Mobp in the Mcoln1-/- cortex, manifesting as early as 10 days after birth and persisting later in life. Such delays were less pronounced in the cerebellum. Despite our previous finding of diminished accumulation of the ferritin-bound iron in the Mcoln1-/- brain, we report no significant changes in expression of the cytosolic iron reporters, suggesting that iron-handling deficits in MLIV occur in the lysosomes and do not involve broad iron deficiency. These data demonstrate very early deficits of oligodendrocyte maturation and critical regional differences in myelination between the forebrain and cerebellum in the mouse model of MLIV. Furthermore, they establish quantitative readouts of the MLIV impact on early brain development, useful to gauge efficacy in pre-clinical trials.

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.

Molecular Mechanisms of Lysosome and Nucleus Communication

Lysosomes transcend the role of degradation stations, acting as key nodes for interorganelle crosstalk and signal transduction. Lysosomes communicate with the nucleus through physical proximity and functional interaction. In response to external and internal stimuli, lysosomes actively adjust their distribution between peripheral and perinuclear regions and modulate lysosome-nucleus signaling pathways; in turn, the nucleus fine-tunes lysosomal biogenesis and functions through transcriptional controls. Changes in coordination between these two essential organelles are associated with metabolic disorders, neurodegenerative diseases, and aging. In this review, we address recent advances in lysosome-nucleus communication by multi-tiered regulatory mechanisms and discuss how these regulations couple metabolic inputs with organellar motility, cellular signaling, and transcriptional network.

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.

Evolutionary Aspects of TRPMLs and TPCs

Transient receptor potential (TRP) or transient receptor potential channels are a highly diverse family of mostly non-selective cation channels. In the mammalian genome, 28 members can be identified, most of them being expressed predominantly in the plasma membrane with the exception of the mucolipins or TRPMLs which are expressed in the endo-lysosomal system. In mammalian organisms, TRPMLs have been associated with a number of critical endo-lysosomal functions such as autophagy, endo-lysosomal fusion/fission and trafficking, lysosomal exocytosis, pH regulation, or lysosomal motility and positioning. The related non-selective two-pore cation channels (TPCs), likewise expressed in endosomes and lysosomes, have also been found to be associated with endo-lysosomal trafficking, autophagy, pH regulation, or lysosomal exocytosis, raising the question why these two channel families have evolved independently. We followed TRP/TRPML channels and TPCs through evolution and describe here in which species TRP/TRPMLs and/or TPCs are found, which functions they have in different species, and how this compares to the functions of mammalian orthologs.

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.

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.

Role of Divalent Cations in HIV-1 Replication and Pathogenicity

Divalent cations are essential for life and are fundamentally important coordinators of cellular metabolism, cell growth, host-pathogen interactions, and cell death. Specifically, for human immunodeficiency virus type-1 (HIV-1), divalent cations are required for interactions between viral and host factors that govern HIV-1 replication and pathogenicity. Homeostatic regulation of divalent cations' levels and actions appear to change as HIV-1 infection progresses and as changes occur between HIV-1 and the host. In people living with HIV-1, dietary supplementation with divalent cations may increase HIV-1 replication, whereas cation chelation may suppress HIV-1 replication and decrease disease progression. Here, we review literature on the roles of zinc (Zn2+), iron (Fe2+), manganese (Mn2+), magnesium (Mg2+), selenium (Se2+), and copper (Cu2+) in HIV-1 replication and pathogenicity, as well as evidence that divalent cation levels and actions may be targeted therapeutically in people living with HIV-1.

Pre-clinical Mouse Models of Neurodegenerative Lysosomal Storage Diseases

There are over 50 lysosomal hydrolase deficiencies, many of which cause neurodegeneration, cognitive decline and death. In recent years, a number of broad innovative therapies have been proposed and investigated for lysosomal storage diseases (LSDs), such as enzyme replacement, substrate reduction, pharmacologic chaperones, stem cell transplantation, and various forms of gene therapy. Murine models that accurately reflect the phenotypes observed in human LSDs are critical for the development, assessment and implementation of novel translational therapies. The goal of this review is to summarize the neurodegenerative murine LSD models available that recapitulate human disease, and the pre-clinical studies previously conducted. We also describe some limitations and difficulties in working with mouse models of neurodegenerative LSDs.

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.

Abnormal Lysosomal Positioning and Small Extracellular Vesicle Secretion in Arterial Stiffening and Calcification of Mice Lacking Mucolipin 1 Gene

Recent studies have shown that arterial medial calcification is mediated by abnormal release of exosomes/small extracellular vesicles from vascular smooth muscle cells (VSMCs) and that small extracellular vesicle (sEV) secretion from cells is associated with lysosome activity. The present study was designed to investigate whether lysosomal expression of mucolipin-1, a product of the mouse Mcoln1 gene, contributes to lysosomal positioning and sEV secretion, thereby leading to arterial medial calcification (AMC) and stiffening. In Mcoln1-/- mice, we found that a high dose of vitamin D (Vit D; 500,000 IU/kg/day) resulted in increased AMC compared to their wild-type littermates, which was accompanied by significant downregulation of SM22-α and upregulation of RUNX2 and osteopontin in the arterial media, indicating a phenotypic switch to osteogenic. It was also shown that significantly decreased co-localization of lysosome marker (Lamp-1) with lysosome coupling marker (Rab 7 and ALG-2) in the aortic wall of Mcoln1-/- mice as compared to their wild-type littermates. Besides, Mcoln1-/- mice showed significant increase in the expression of exosome/ sEV markers, CD63, and annexin-II (AnX2) in the arterial medial wall, accompanied by significantly reduced co-localization of lysosome marker (Lamp-1) with multivesicular body (MVB) marker (VPS16), suggesting a reduction of the lysosome-MVB interactions. In the plasma of Mcoln1-/- mice, the number of sEVs significantly increased as compared to the wild-type littermates. Functionally, pulse wave velocity (PWV), an arterial stiffening indicator, was found significantly increased in Mcoln1-/- mice, and Vit D treatment further enhanced such stiffening. All these data indicate that the Mcoln1 gene deletion in mice leads to abnormal lysosome positioning and increased sEV secretion, which may contribute to the arterial stiffness during the development of AMC.

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.

Genome-wide scan identifies opioid overdose risk locus close to MCOLN1

The United States is experiencing the worst opioid overdose (OpOD) crisis in its history. We carried out a genome-wide association study on OpOD severity among 3 477 opioid-exposed individuals, 1 019 of whom experienced OpODs, including 2 032 European Americans (EAs) (653 overdose cases), and 1 445 African Americans (AAs) (366 overdose cases). Participants were scored 1 to 4 based on their reported overdose status and the number of times that medical treatment was required. Genome-wide association study (GWAS) of EAs and AAs separately resulted in two genome-wide significant (GWS) signals in AAs but none in EAs. The first signal was represented by three closely mapped variants (rs115208233, rs116181528, and rs114077267) located near mucolipin 1 (MCOLN1) and patatin-like phospholipase domain containing 6 (PNPLA6), and the other signal was represented by rs369098800 near dead-box helicase 18 (DDX18). There were no additional GWS signals in the trans-population meta-analysis, so that post-GWAS analysis focused on these loci. In network analysis, MCOLN1 was coexpressed with PNPLA6, but only MCOLN1-associated genes were enriched in functional categories relevant to OpOD, including calcium and cation channel activities; no enrichment was observed for PNPLA6-associated genes. Drug repositioning analysis was carried out in the connectivity map (CMap) database for MCOLN1 (PNPLA6 was not available in CMap) and showed that the opioid agonist drug-induced expression profile is similar to that of MCOLN1 overexpression and yielded the highest-ranked expression profile of 83 drug classes. Thus, MCOLN1 may be a risk gene for OpOD, but replication is needed. This knowledge could be helpful in the identification of drug targets for preventing OpOD.

TRPML Cation Channels in Inflammation and Immunity

Background: In 1883, Ilya Mechnikov discovered phagocytes and established the concept of phagocytosis by macrophages. In 1908, he was awarded the Nobel Prize in Physiology/Medicine for his findings, which laid the foundations for today's understanding of the innate immune response. Only in the 1960s, Max Cooper and Robert Good significantly advanced our understanding of the immune system by demonstrating that B- and T-cells cooperate to regulate the adaptive immune response. Both, innate and adaptive immune response are essential to effectively protect the individual against infectious agents, such as viruses, bacterial or insect toxins, or allergens. Innate immune responses occur rapidly upon exposure to noxious or infectious agents or organisms, in contrast to the adaptive immune system that needs days rather than hours to develop and acts primarily on the basis of antigen-specific receptors expressed on the surface of B- and T-lymphocytes. In recent years, it has become evident that endosomes and lysosomes are involved in many aspects of immune cell function, such as phagocytosis, antigen presentation and processing by antigen-presenting cells, release of proinflammatory mediators, e.g., by mast cells, or secretion of the pore-forming protein perforin by cytotoxic T lymphocytes. Several lysosomal storage disorders (LSDs) have been associated with defects in immune system function or immune system hyperactivity, such as Gaucher, Fabry, or Niemann-Pick type C1 disease, mucopolysaccharidoses (MPS), gangliosidosis, or juvenile neuronal ceroid lipofuscinosis (JNCL). Beside accumulating evidence on the importance of endolysosomes in immune cell function, recent results suggest direct roles of endolysosomal ion channels, such as the TRPML channels (mucolipins), which are members of the transient receptor potential (TRP) superfamily of non-selective cation channels, for different aspects of immune cell function. The aim of this review is to discuss the current knowledge about the roles of TRPML channels in inflammation and immunity, and to assess their potential as drug targets to influence immune cell functions.

Advances: Examples of recently established roles of TRPML channels in immune system function and immune response include the TRPML1-mediated modulation of secretory lysosomes, granzyme B content, and tuning of effector function in NK cells, TRPML1-dependent directional dendritic cell (DC) migration and DC chemotaxis, and the role of TRPML2 in chemokine release from LPS-stimulated macrophages.

Outlook: Although our understanding of the functional roles of TRPML channels in inflammation and immunity is still in its infancy, a few interesting findings have been made in the past years, encouraging further and more detailed work on the role of TRPMLs, e.g., in intracellular trafficking and release of chemokines, cytokines, or granzyme B, or in phagocytosis and bacterial toxin and virus trafficking through the endolysosomal machinery.

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.

Silver nanoparticles regulate autophagy through lysosome injury and cell hypoxia in prostate cancer cells

With the rapid development of nanotechnology, nanomaterials are now being used for cancer treatment. Although studies on the application of silver nanoparticles in cancer treatment are burgeoning, few studies have investigated the toxicology mechanisms of autophagy in cancer cells under exposure to sublethal silver nanoparticles. Here, we clarified the distinct mechanisms of silver nanoparticles for the regulation of autophagy in prostate cancer PC-3 cells under sublethal exposure. Silver nanoparticle treatment caused lysosome injury, including the decline of lysosomal membrane integrity, decrease of lysosomal quantity, and attenuation of lysosomal protease activity, which resulted in blockage of autophagic flux. In addition, sublethal silver nanoparticle exposure activated AMP-activated protein kinase/mammalian target of rapamycin-dependent signaling pathway to modulate autophagy, which resulted from silver nanoparticles-induced cell hypoxia and energy deficiency. Taken together, the results show that silver nanoparticles could regulate autophagy via lysosome injury and cell hypoxia in PC-3 cells under sublethal dose exposure. This study will provide an experimental basis for the cancer therapy of nanomaterials.