Mucolipin-1 is a lysosomal membrane protein required for intracellular lactosylceramide traffic

L-serine restored lysosomal failure in cells derived from patients with BPAN reducing iron accumulation with eliminating lipofuscin

Defective mitochondria and autophagy, as well as accumulation of lipid and iron in WDR45 mutant fibroblasts, is related to beta-propeller protein-associated neurodegeneration (BPAN). In this study, we found that enlarged lysosomes in cells derived from patients with BPAN had low enzyme activity, and most of the enlarged lysosomes had an accumulation of iron and oxidized lipid. Cryo-electron tomography revealed elongated lipid accumulation, and spectrometry-based elemental analysis showed that lysosomal iron and oxygen accumulation superimposed with lipid aggregates. Lysosomal lipid aggregates superimposed with autofluorescence as free radical generator, lipofuscin. To eliminate free radical stress by iron accumulation in cells derived from patients with BPAN, we investigated the effects of the iron chelator, 2,2'-bipyridine (bipyridyl, BIP). To study whether the defects in patient-derived cells can be rescued by an iron chelator BIP, we tested whether the level of iron and reactive oxygen species (ROS) in the cells and genes related to oxidative stress were rescued BIP treatment. Although BIP treatment decreased some iron accumulation in the cytoplasm and mitochondria, the accumulation of iron in the lysosomes and levels of cellular ROS were unaffected. In addition, the change of specific RNA levels related to free radical stress in patient fibroblasts was not rescued by BIP. To alleviate free radical stress, we investigated whether l-serine can regulate abnormal structures in cells derived from patients with BPAN through the regulation of free radical stress. l-serine treatment alleviated increase of enlarged lysosomes and iron accumulation and rescued impaired lysosomal activity by reducing oxidized lipid accumulation in the lysosomes of the cells. Lamellated lipids in the lysosomes of the cells were identified as lipofuscin through correlative light and electron microscopy, and l-serine treatment reduced the increase of lipofuscin. These data suggest that l-serine reduces oxidative stress-mediated lysosomal lipid oxidation and iron accumulation by rescuing lysosomal activity.

The ion channels of endomembranes

The endomembrane system consists of organellar membranes in the biosynthetic pathway [endoplasmic reticulum (ER), Golgi apparatus, and secretory vesicles] as well as those in the degradative pathway (early endosomes, macropinosomes, phagosomes, autophagosomes, late endosomes, and lysosomes). These endomembrane organelles/vesicles work together to synthesize, modify, package, transport, and degrade proteins, carbohydrates, and lipids, regulating the balance between cellular anabolism and catabolism. Large ion concentration gradients exist across endomembranes: Ca2+ gradients for most endomembrane organelles and H+ gradients for the acidic compartments. Ion (Na+, K+, H+, Ca2+, and Cl-) channels on the organellar membranes control ion flux in response to cellular cues, allowing rapid informational exchange between the cytosol and organelle lumen. Recent advances in organelle proteomics, organellar electrophysiology, and luminal and juxtaorganellar ion imaging have led to molecular identification and functional characterization of about two dozen endomembrane ion channels. For example, whereas IP3R1-3 channels mediate Ca2+ release from the ER in response to neurotransmitter and hormone stimulation, TRPML1-3 and TMEM175 channels mediate lysosomal Ca2+ and H+ release, respectively, in response to nutritional and trafficking cues. This review aims to summarize the current understanding of these endomembrane channels, with a focus on their subcellular localizations, ion permeation properties, gating mechanisms, cell biological functions, and disease relevance.

Endolysosomal transient receptor potential mucolipins and two-pore channels: implications for cancer immunity

Past research has identified that cancer cells sustain several cancer hallmarks by impairing function of the endolysosomal system (ES). Thus, maintaining the functional integrity of endolysosomes is crucial, which heavily relies on two key protein families: soluble hydrolases and endolysosomal membrane proteins. Particularly members of the TPC (two-pore channel) and TRPML (transient receptor potential mucolipins) families have emerged as essential regulators of ES function as a potential target in cancer therapy. Targeting TPCs and TRPMLs has demonstrated significant impact on multiple cancer hallmarks, including proliferation, growth, migration, and angiogenesis both in vitro and in vivo. Notably, endosomes and lysosomes also actively participate in various immune regulatory mechanisms, such as phagocytosis, antigen presentation, and the release of proinflammatory mediators. Yet, knowledge about the role of TPCs and TRPMLs in immunity is scarce. This prompts a discussion regarding the potential role of endolysosomal ion channels in aiding cancers to evade immune surveillance and destruction. Specifically, understanding the interplay between endolysosomal ion channels and cancer immunity becomes crucial. Our review aims to comprehensively explore the current knowledge surrounding the roles of TPCs and TRPMLs in immunity, whilst emphasizing the critical need to elucidate their specific contributions to cancer immunity by pointing out current research gaps that should be addressed.

Lisch Epithelial Corneal Dystrophy Is Caused by Heterozygous Loss-of-Function Variants in MCOLN1

PURPOSE

To report the genetic etiology of Lisch epithelial corneal dystrophy (LECD).

DESIGN

Multicenter cohort study.

METHODS

A discovery cohort of 27 individuals with LECD from 17 families, including 7 affected members from the original LECD family, 6 patients from 2 new families and 14 simplex cases, was recruited. A cohort of 6 individuals carrying a pathogenic MCOLN1 (mucolipin 1) variant was reviewed for signs of LECD. Next-generation sequencing or targeted Sanger sequencing were used in all patients to identify pathogenic or likely pathogenic variants and penetrance of variants.

RESULTS

Nine rare heterozygous MCOLN1 variants were identified in 23 of 27 affected individuals from 13 families. The truncating nature of 7 variants and functional testing of 1 missense variant indicated that they result in MCOLN1 haploinsufficiency. Importantly, in the homozygous and compound-heterozygous state, 4 of 9 LECD-associated variants cause the rare lysosomal storage disorder mucolipidosis IV (MLIV). Autosomal recessive MLIV is a systemic disease and comprises neurodegeneration as well as corneal opacity of infantile-onset with epithelial autofluorescent lysosomal inclusions. However, the 6 parents of 3 patients with MLIV confirmed to carry pathogenic MCOLN1 variants did not have the LECD phenotype, suggesting MCOLN1 haploinsufficiency may be associated with reduced penetrance and variable expressivity.

CONCLUSIONS

MCOLN1 haploinsufficiency is the major cause of LECD. Based on the overlapping clinical features of corneal epithelial cells with autofluorescent inclusions reported in both LECD and MLIV, it is concluded that some carriers of MCOLN1 haploinsufficiency-causing variants present with LECD.

The Biology of Lysosomes: From Order to Disorder

Since its discovery in 1955, the understanding of the lysosome has continuously increased. Once considered a mere waste removal system, the lysosome is now recognised as a highly crucial cellular component for signalling and energy metabolism. This notable evolution raises the need for a summarized review of the lysosome's biology. As such, throughout this article, we will be compiling the current knowledge regarding the lysosome's biogenesis and functions. The comprehension of this organelle's inner mechanisms is crucial to perceive how its impairment can give rise to lysosomal disease (LD). In this review, we highlight some examples of LD fine-tuned mechanisms that are already established, as well as others, which are still under investigation. Even though the understanding of the lysosome and its pathologies has expanded through the years, some of its intrinsic molecular aspects remain unknown. In order to illustrate the complexity of the lysosomal diseases we provide a few examples that have challenged the established single gene-single genetic disorder model. As such, we believe there is a strong need for further investigation of the exact abnormalities in the pathological pathways in lysosomal disease.

Impaired autophagic flux and dedifferentiation in podocytes lacking Asah1 gene: Role of lysosomal TRPML1 channel

Podocytopathy and associated nephrotic syndrome have been reported in a mouse strain (Asah1fl/fl/Podocre) with a podocyte-specific deletion of α subunit (the main catalytic subunit) of acid ceramidase (Ac). However, the pathogenesis of podocytopathy in these mice remains unclear. The present study tested whether Ac deficiency impairs autophagic flux in podocytes through blockade of transient receptor potential mucolipin 1 (TRPML1) channel as a potential pathogenic mechanism of podocytopathy in Asah1fl/fl/Podocre mice. We first demonstrated that impairment of autophagic flux occurred in podocytes lacking Asah1 gene, which was evidenced by autophagosome accumulation and reduced lysosome-autophagosome interaction. TRPML1 channel agonists recovered lysosome-autophagosome interaction and attenuated autophagosome accumulation in podocytes from Asah1fl/fl/Podocre mice, while TRPML1 channel inhibitors impaired autophagic flux in WT/WT podocytes and worsened autophagic deficiency in podocytes lacking Asah1 gene. The effects of TRPML1 channel agonist were blocked by dynein inhibitors, indicating a critical role of dynein activity in the control of lysosome movement due to TRPML1 channel-mediated Ca2+ release. It was also found that there is an enhanced phenotypic transition to dedifferentiation status in podocytes lacking Asah1 gene in vitro and in vivo. Such podocyte phenotypic transition was inhibited by TRPML1 channel agonists but enhanced by TRPML1 channel inhibitors. Moreover, we found that TRPML1 gene silencing induced autophagosome accumulation and dedifferentiation in podocytes. Based on these results, we conclude that Ac activity is essential for autophagic flux and maintenance of differentiated status of podocytes. Dysfunction or deficiency of Ac may impair autophagic flux and induce podocyte dedifferentiation, which may be an important pathogenic mechanism of podocytopathy and associated nephrotic syndrome.

TRPML1 and TFEB, an Intimate Affair

Ca²⁺ is a universal second messenger that plays a wide variety of fundamental roles in cellular physiology. Thus, to warrant selective responses and to allow rapid mobilization upon specific stimuli, Ca²⁺ is accumulated in organelles to keep it at very low levels in the cytoplasm during resting conditions. Major Ca²⁺ storage organelles include the endoplasmic reticulum (ER), the mitochondria, and as recently demonstrated, the lysosome (Xu and Ren, Annu Rev Physiol 77:57-80, 2015). The importance of Ca²⁺ signaling deregulation in human physiology is underscored by its involvement in several human diseases, including lysosomal storage disorders, neurodegenerative disease and cancer (Shen et al., Nat Commun 3:731, 2012; Bae et al., J Neurosci 34:11485-11503, 2014). Recent evidence strongly suggests that lysosomal Ca²⁺ plays a major role in the regulation of lysosomal adaptation to nutrient availability through a lysosomal signaling pathway involving the lysosomal Ca²⁺ channel TRPML1 and the transcription factor TFEB, a master regulator for lysosomal function and autophagy (Sardiello et al., Science 325:473-477, 2009; Settembre et al., Science 332:1429-1433, 2011; Medina et al., Nat Cell Biol 17:288-299, 2015; Di Paola et al., Cell Calcium 69:112-121, 2018). Due to the tight relationship of this lysosomal Ca²⁺ channel and TFEB, in this chapter, we will focus on the role of the TRPML1/TFEB pathway in the regulation of lysosomal function and autophagy.

Juvenile mucopolysaccharidosis plus disease caused by a missense mutation in VPS33A

A rare and fatal disease resembling mucopolysaccharidosis in infants, is caused by impaired intracellular endocytic trafficking due to deficiency of core components of the intracellular membrane-tethering protein complexes, HOPS, and CORVET. Whole exome sequencing identified a novel VPS33A mutation in a patient suffering from a variant form of mucopolysaccharidosis. Electron and confocal microscopy, immunoblotting, and glycosphingolipid trafficking experiments were undertaken to investigate the effects of the mutant VPS33A in patient-derived skin fibroblasts. We describe an attenuated juvenile form of VPS33A-related syndrome-mucopolysaccharidosis plus in a man who is homozygous for a hitherto unknown missense mutation (NM_022916.4: c.599 G>C; NP_075067.2:p. Arg200Pro) in a conserved region of the VPS33A gene. Urinary glycosaminoglycan (GAG) analysis revealed increased heparan, dermatan sulphates, and hyaluronic acid. We showed decreased abundance of VPS33A in patient derived fibroblasts and provided evidence that the p.Arg200Pro mutation leads to destablization of the protein and proteasomal degradation. As in the infantile form of mucopolysaccharidosis plus, the endocytic compartment in the fibroblasts also expanded-a phenomenon accompanied by increased endolysosomal acidification and impaired intracellular glycosphingolipid trafficking. Experimental treatment of the patient's cultured fibroblasts with the proteasome inhibitor, bortezomib, or exposure to an inhibitor of glucosylceramide synthesis, eliglustat, improved glycosphingolipid trafficking. To our knowledge this is the first report of an attenuated juvenile form of VPS33A insufficiency characterized by appreciable residual endosomal-lysosomal trafficking and a milder mucopolysaccharidosis plus than the disease in infants. Our findings expand the proof of concept of redeploying clinically approved drugs for therapeutic exploitation in patients with juvenile as well as infantile forms of mucopolysaccharidosis plus disease.

Neurodegenerative Lysosomal Storage Disorders: TPC2 Comes to the Rescue!

Lysosomal storage diseases (LSDs) resulting from inherited gene mutations constitute a family of disorders that disturb lysosomal degradative function leading to abnormal storage of macromolecular substrates. In most LSDs, central nervous system (CNS) involvement is common and leads to the progressive appearance of neurodegeneration and early death. A growing amount of evidence suggests that ion channels in the endolysosomal system play a crucial role in the pathology of neurodegenerative LSDs. One of the main basic mechanisms through which the endolysosomal ion channels regulate the function of the endolysosomal system is Ca²⁺ release, which is thought to be essential for intracellular compartment fusion, fission, trafficking and lysosomal exocytosis. The intracellular TRPML (transient receptor potential mucolipin) and TPC (two-pore channel) ion channel families constitute the main essential Ca²⁺-permeable channels expressed on endolysosomal membranes, and they are considered potential drug targets for the prevention and treatment of LSDs. Although TRPML1 activation has shown rescue effects on LSD phenotypes, its activity is pH dependent, and it is blocked by sphingomyelin accumulation, which is characteristic of some LSDs. In contrast, TPC2 activation is pH-independent and not blocked by sphingomyelin, potentially representing an advantage over TRPML1. Here, we discuss the rescue of cellular phenotypes associated with LSDs such as cholesterol and lactosylceramide (LacCer) accumulation or ultrastructural changes seen by electron microscopy, mediated by the small molecule agonist of TPC2, TPC2-A1-P, which promotes lysosomal exocytosis and autophagy. In summary, new data suggest that TPC2 is a promising target for the treatment of different types of LSDs such as MLIV, NPC1, and Batten disease, both in vitro and in vivo.

TPC2 rescues lysosomal storage in mucolipidosis type IV, Niemann-Pick type C1, and Batten disease

Lysosomes are cell organelles that degrade macromolecules to recycle their components. If lysosomal degradative function is impaired, e.g., due to mutations in lysosomal enzymes or membrane proteins, lysosomal storage diseases (LSDs) can develop. LSDs manifest often with neurodegenerative symptoms, typically starting in early childhood, and going along with a strongly reduced life expectancy and quality of life. We show here that small molecule activation of the Ca²⁺‐permeable endolysosomal two‐pore channel 2 (TPC2) results in an amelioration of cellular phenotypes associated with LSDs such as cholesterol or lipofuscin accumulation, or the formation of abnormal vacuoles seen by electron microscopy. Rescue effects by TPC2 activation, which promotes lysosomal exocytosis and autophagy, were assessed in mucolipidosis type IV (MLIV), Niemann–Pick type C1, and Batten disease patient fibroblasts, and in neurons derived from newly generated isogenic human iPSC models for MLIV and Batten disease. For in vivo proof of concept, we tested TPC2 activation in the MLIV mouse model. In sum, our data suggest that TPC2 is a promising target for the treatment of different types of LSDs, both in vitro and in‐vivo.

ESCRT-I fuels lysosomal degradation to restrict TFEB/TFE3 signaling via the Rag-mTORC1 pathway

ESCRT-I deficiency impairs lysosome membrane turnover and induces homeostatic responses to lysosomal nutrient starvation including activation of MiT-TFE signaling caused by inhibition of the substrate-specific mTORC1 pathway.

Within the endolysosomal pathway in mammalian cells, ESCRT complexes facilitate degradation of proteins residing in endosomal membranes. Here, we show that mammalian ESCRT-I restricts the size of lysosomes and promotes degradation of proteins from lysosomal membranes, including MCOLN1, a Ca²⁺ channel protein. The altered lysosome morphology upon ESCRT-I depletion coincided with elevated expression of genes annotated to biogenesis of lysosomes due to prolonged activation of TFEB/TFE3 transcription factors. Lack of ESCRT-I also induced transcription of cholesterol biosynthesis genes, in response to inefficient delivery of cholesterol from endolysosomal compartments. Among factors that could possibly activate TFEB/TFE3 signaling upon ESCRT-I deficiency, we excluded lysosomal cholesterol accumulation and Ca²⁺-mediated dephosphorylation of TFEB/TFE3. However, we discovered that this activation occurs due to the inhibition of Rag GTPase–dependent mTORC1 pathway that specifically reduced phosphorylation of TFEB at S122. Constitutive activation of the Rag GTPase complex in cells lacking ESCRT-I restored S122 phosphorylation and prevented TFEB/TFE3 activation. Our results indicate that ESCRT-I deficiency evokes a homeostatic response to counteract lysosomal nutrient starvation, that is, improper supply of nutrients derived from lysosomal degradation.

Progress in elucidating pathophysiology of mucolipidosis IV

Mucolipidosis IV (MLIV) is an autosomal-recessive disease caused by loss-of-function mutations in the MCOLN1 gene encoding the non-selective cationic lysosomal channel transient receptor potential mucolipin-1 (TRPML1). Patients with MLIV suffer from severe motor and cognitive deficits that manifest in early infancy and progressive loss of vision leading to blindness in the second decade of life. There are no therapies available for MLIV and the unmet medical need is extremely high. Here we review the spectrum of clinical presentations and the latest research in the MLIV pre-clinical model, with the aim of highlighting the progress in understanding the pathophysiology of the disease. These highlights include elucidation of the neurodevelopmental versus neurodegenerative features over the course of disease, hypomyelination as one of the major brain pathological disease hallmarks, and dysregulation of cytokines, with emerging evidence of IFN-gamma pathway upregulation in response to TRPML1 loss and pro-inflammatory activation of astrocytes and microglia. These scientific advances in the MLIV field provide a basis for future translational research, including biomarker and therapy development, that are desperately needed for this patient population.

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.

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.

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.

Proteomics analysis of a human brain sample from a mucolipidosis type IV patient reveals pathophysiological pathways

Background

Mucolipidosis type IV (MLIV), an ultra-rare neurodevelopmental and neurodegenerative disorder, is caused by mutations in the MCOLN1 gene, which encodes the late endosomal/lysosomal transient receptor potential channel TRPML1 (mucolipin 1). The precise pathophysiogical pathways that cause neurological disease in MLIV are poorly understood. Recently, the first post-mortem brain sample became available from a single MLIV patient, and in the current study we performed mass spectrometry (MS)-based proteomics on this tissue with a view to delineating pathological pathways, and to compare with previously-published data on MLIV, including studies using the Mcoln1^−/− mouse.

Results

A number of pathways were altered in two brain regions from the MLIV patient, including those related to the lysosome, lipid metabolism, myelination, cellular trafficking and autophagy, mTOR and calmodulin, the complement system and interferon signaling. Of these, levels of some proteins not known previously to be associated with MLIV were altered, including APOD, PLIN4, ATG and proteins related to interferon signaling. Moreover, when proteins detected by proteomics in the human brain were compared with their orthologs detected in the Mcoln1^−/− mouse by RNAseq, the results were remarkably similar. Finally, analysis of proteins in human and mouse CSF suggest that calbindin 1 and calbindin 2 might be useful as biomarkers to help chart the course of disease development.

Conclusions

Despite the sample size limitations, our findings are consistent with the relatively general changes in lysosomal function previously reported in MLIV, and shed light on new pathways of disease pathophysiology, which is required in order to understand the course of disease development and to determine the efficacy of therapies when they become available for this devastating disease.

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.

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.

Transient receptor potential channels: current perspectives on evolution, structure, function and nomenclature

The transient receptor potential superfamily of ion channels (TRP channels) is widely recognized for the roles its members play in sensory nervous systems. However, the incredible diversity within the TRP superfamily, and the wide range of sensory capacities found therein, has also allowed TRP channels to function beyond sensing an organism's external environment, and TRP channels have thus become broadly critical to (at least) animal life. TRP channels were originally discovered in Drosophila and have since been broadly studied in animals; however, thanks to a boom in genomic and transcriptomic data, we now know that TRP channels are present in the genomes of a variety of creatures, including green algae, fungi, choanoflagellates and a number of other eukaryotes. As a result, the organization of the TRP superfamily has changed radically from its original description. Moreover, modern comprehensive phylogenetic analyses have brought to light the vertebrate-centricity of much of the TRP literature; much of the nomenclature has been grounded in vertebrate TRP subfamilies, resulting in a glossing over of TRP channels in other taxa. Here, we provide a comprehensive review of the function, structure and evolutionary history of TRP channels, and put forth a more complete set of non-vertebrate-centric TRP family, subfamily and other subgroup nomenclature.

NAADP Receptors

Of the established Ca²⁺-mobilizing messengers, NAADP is arguably the most tantalizing. It is the most potent, often efficacious at low nanomolar concentrations, and its receptors undergo dramatic desensitization. Recent studies have identified a new class of calcium-release channel, the two-pore channels (TPCs), as the likely targets for NAADP regulation, even though the effect may be indirect. These channels localized at endolysosomes, where they mediate local Ca²⁺ release, and have highlighted a new role of acidic organelles as targets for messenger-evoked Ca²⁺ mobilization. Three distinct roles of TPCs have been identified. The first is to effect local Ca²⁺ release that may play a role in endolysosomal function including vesicular fusion and trafficking. The second is to trigger global calcium release by recruiting Ca²⁺-induced Ca²⁺-release (CICR) channels at lysosomal-endoplasmic reticulum (ER) junctions. The third is to regulate plasma membrane excitability by the targeting of Ca²⁺ release from appropriately positioned subplasma membrane stores to regulate plasma membrane Ca²⁺-activated channels. In this review, I discuss the role of nicotinic acid adenine nucleotide diphosphate (NAADP)-mediated Ca²⁺ release from endolysosomal stores as a widespread trigger for intracellular calcium signaling mechanisms, and how studies of TPCs are beginning to enhance our understanding of the central role of lysosomes in Ca²⁺ signaling.

Pyridine Nucleotide Metabolites and Calcium Release from Intracellular Stores

Ca²⁺ signals are probably the most common intracellular signaling cellular events, controlling an extensive range of responses in virtually all cells. Many cellular stimuli, often acting at cell surface receptors, evoke Ca²⁺ signals by mobilizing Ca²⁺ from intracellular stores. Inositol trisphosphate (IP₃) was the first messenger shown to link events at the plasma membrane to release Ca²⁺ from the endoplasmic reticulum (ER), through the activation of IP₃-gated Ca²⁺ release channels (IP₃ receptors). Subsequently, two additional Ca²⁺ mobilizing messengers were discovered, cADPR and NAADP. Both are metabolites of pyridine nucleotides, and may be produced by the same class of enzymes, ADP-ribosyl cyclases, such as CD38. Whilst cADPR mobilizes Ca²⁺ from the ER by activation of ryanodine receptors (RyRs), NAADP releases Ca²⁺ from acidic stores by a mechanism involving the activation of two pore channels (TPCs). In addition, other pyridine nucleotides have emerged as intracellular messengers. ADP-ribose and 2'-deoxy-ADPR both activate TRPM2 channels which are expressed at the plasma membrane and in lysosomes.

The lysosomal disease caused by mutant VPS33A

A rare lysosomal disease resembling a mucopolysaccharidosis with unusual systemic features, including renal disease and platelet dysfunction, caused by the defect in a conserved region of the VPS33A gene on human chromosome 12q24.31, occurs in Yakuts-a nomadic Turkic ethnic group of Southern Siberia. VPS33A is a core component of the class C core vacuole/endosome tethering (CORVET) and the homotypic fusion and protein sorting (HOPS) complexes, which have essential functions in the endocytic pathway. Here we show that cultured fibroblasts from patients with this disorder have morphological changes: vacuolation with disordered endosomal/lysosomal compartments and-common to sphingolipid diseases-abnormal endocytic trafficking of lactosylceramide. Urine glycosaminoglycan studies revealed a pathological excess of sialylated conjugates as well as dermatan and heparan sulphate. Lipidomic screening showed elevated β-D-galactosylsphingosine with unimpaired activity of cognate lysosomal hydrolases. The 3D crystal structure of human VPS33A predicts that replacement of arginine 498 by tryptophan will de-stabilize VPS33A folding. We observed that the missense mutation reduced the abundance of full-length VPS33A and other components of the HOPS and CORVET complexes. Treatment of HeLa cells stably expressing the mutant VPS33A with a proteasome inhibitor rescued the mutant protein from degradation. We propose that the disease is due to diminished intracellular abundance of intact VPS33A. Exposure of patient-derived fibroblasts to the clinically approved proteasome inhibitor, bortezomib, or inhibition of glucosylceramide synthesis with eliglustat, partially corrected the impaired lactosylceramide trafficking defect and immediately suggest therapeutic avenues to explore in this fatal orphan disease.

Transient Receptor Potential Mucolipin-1 Channels in Glioblastoma: Role in Patient's Survival

A link between mucolipin channels and tumors has been recently suggested. Herein, we aim to investigate the transient receptor potential mucolipin (TRPML)-1 relevance in glioblastoma. The expression of this channel was evaluated via qRT-PCR and immunohistochemistry in biopsies from 66 glioblastoma patients and two human glioblastoma cell lines and compared to normal human brain, astrocytes, and epileptic tissues. The subcellular distribution of TRPML-1 was examined via confocal microscopy in the glioma cell lines. Then, to assess the role of TRPML-1, cell viability assays have been conducted in T98 and U251 cell lines treated with the specific TRPML-1 agonist, MK6-83. We found that MK6-83 reduced cell viability and induced caspase-3-dependent apoptosis. Indeed, the TRPML-1 silencing or the blockage of TRPML-1 dependent [Ca²⁺]_i release abrogated these effects. In addition, exposure of glioma cells to the reactive oxygen species (ROS) inducer, carbonyl cyanide m-chlorophenylhydrazone (CCCP), stimulated a TRPML-1-dependent autophagic cell death, as demonstrated by the ability of the autophagic inhibitor bafilomycin A, the TRPML-1 inhibitor sphingomyelin, and the TRPML-1 silencing to completely inhibit the CCCP-mediated effects. To test a possible correlation with patient’s survival, Kaplan–Meier, univariate, and multivariate analysis have been performed. Data showed that the loss/reduction of TRPML-1 mRNA expression strongly correlates with short survival in glioblastoma (GBM) patients, suggesting that the reduction of TRPML-1 expression represents a negative prognostic factor in GBM patients.

Current concepts in the neuropathogenesis of mucolipidosis type IV

Mucolipidosis type IV (MLIV) is an autosomal recessive, lysosomal storage disorder causing progressively severe intellectual disability, motor and speech deficits, retinal degeneration often culminating in blindness, and systemic disease causing a shortened lifespan. MLIV results from mutations in the gene MCOLN1 encoding the transient receptor potential channel mucolipin-1. It is an ultra-rare disease and is currently known to affect just over 100 diagnosed individuals. The last decade has provided a wealth of research focused on understanding the role of the enigmatic mucolipin-1 protein in cell and brain function and how its absence causes disease. This review explores our current understanding of the mucolipin-1 protein in relation to neuropathogenesis in MLIV and describes recent findings implicating mucolipin-1's important role in mechanistic target of rapamycin and TFEB (transcription factor EB) signaling feedback loops as well as in the function of the greater endosomal/lysosomal system. In addition to addressing the vital role of mucolipin-1 in the brain, we also report new data on the question of whether haploinsufficiency as would be anticipated in MCOLN1 heterozygotes is associated with any evidence of neuron dysfunction or disease. Greater insights into the role of mucolipin-1 in the nervous system can be expected to shed light not only on MLIV disease but also on numerous processes governing normal brain function. This article is part of the Special Issue "Lysosomal Storage Disorders".

Autophagy in C. elegans development

Autophagy involves the sequestration of cytoplasmic contents in a double-membrane structure referred to as the autophagosome and the degradation of its contents upon delivery to lysosomes. Autophagy activity has a role in multiple biological processes during the development of the nematode Caenorhabditis elegans. Basal levels of autophagy are required to remove aggregate prone proteins, paternal mitochondria, and spermatid-specific membranous organelles. During larval development, autophagy is required for the remodeling that occurs during dauer development, and autophagy can selectively degrade components of the miRNA-induced silencing complex, and modulate miRNA-mediated silencing. Basal levels of autophagy are important in synapse formation and in the germ line, to promote the proliferation of proliferating stem cells. Autophagy activity is also required for the efficient removal of apoptotic cell corpses by promoting phagosome maturation. Finally, autophagy is also involved in lipid homeostasis and in the aging process. In this review, we first describe the molecular complexes involved in the process of autophagy, its regulation, and mechanisms for cargo recognition. In the second section, we discuss the developmental contexts where autophagy has been shown to be important. Studies in C. elegans provide valuable insights into the physiological relevance of this process during metazoan development.

Lipid⁻Protein Interactions in Niemann⁻Pick Type C Disease: Insights from Molecular Modeling

The accumulation of lipids in the late endosomes and lysosomes of Niemann⁻Pick type C disease (NPCD) cells is a consequence of the dysfunction of one protein (usually NPC1) but induces dysfunction in many proteins. We used molecular docking to propose (a) that NPC1 exports not just cholesterol, but also sphingosine, (b) that the cholesterol sensitivity of big potassium channel (BK) can be traced to a previously unappreciated site on the channel's voltage sensor, (c) that transient receptor potential mucolipin 1 (TRPML1) inhibition by sphingomyelin is likely an indirect effect, and (d) that phosphoinositides are responsible for both the mislocalization of annexin A2 (AnxA2) and a soluble NSF (N-ethylmaleimide Sensitive Fusion) protein attachment receptor (SNARE) recycling defect. These results are set in the context of existing knowledge of NPCD to sketch an account of the endolysosomal pathology key to this disease.

Remodeling of secretory lysosomes during education tunes functional potential in NK cells

Inhibitory signaling during natural killer (NK) cell education translates into increased responsiveness to activation; however, the intracellular mechanism for functional tuning by inhibitory receptors remains unclear. Secretory lysosomes are part of the acidic lysosomal compartment that mediates intracellular signalling in several cell types. Here we show that educated NK cells expressing self-MHC specific inhibitory killer cell immunoglobulin-like receptors (KIR) accumulate granzyme B in dense-core secretory lysosomes that converge close to the centrosome. This discrete morphological phenotype is independent of transcriptional programs that regulate effector function, metabolism and lysosomal biogenesis. Meanwhile, interference of signaling from acidic Ca²⁺ stores in primary NK cells reduces target-specific Ca²⁺-flux, degranulation and cytokine production. Furthermore, inhibition of PI(3,5)P₂ synthesis, or genetic silencing of the PI(3,5)P₂-regulated lysosomal Ca²⁺-channel TRPML1, leads to increased granzyme B and enhanced functional potential, thereby mimicking the educated state. These results indicate an intrinsic role for lysosomal remodeling in NK cell education.

Endolysosomal Ca2+ Signalling and Cancer Hallmarks: Two-Pore Channels on the Move, TRPML1 Lags Behind!

The acidic vesicles of the endolysosomal (EL) system are emerging as an intracellular Ca²⁺ store implicated in the regulation of multiple cellular functions. The EL Ca²⁺ store releases Ca²⁺ through a variety of Ca²⁺-permeable channels, including Transient Receptor Potential (TRP) Mucolipin 1-3 (TRPML1-3) and two-pore channels 1-2 (TPC1-2), whereas EL Ca²⁺ refilling is sustained by the proton gradient across the EL membrane and/or by the endoplasmic reticulum (ER). EL Ca²⁺ signals may be either spatially restricted to control vesicle trafficking, autophagy and membrane repair or may be amplified into a global Ca²⁺ signal through the Ca²⁺-dependent recruitment of ER-embedded channels. Emerging evidence suggested that nicotinic acid adenine dinucleotide phosphate (NAADP)-gated TPCs sustain multiple cancer hallmarks, such as migration, invasiveness and angiogenesis. Herein, we first survey the EL Ca²⁺ refilling and release mechanisms and then focus on the oncogenic role of EL Ca²⁺ signaling. While the evidence in favor of TRPML1 involvement in neoplastic transformation is yet to be clearly provided, TPCs are emerging as an alternative target for anticancer therapies.

Selective agonist of TRPML2 reveals direct role in chemokine release from innate immune cells

Cytokines and chemokines are produced and secreted by a broad range of immune cells including macrophages. Remarkably, little is known about how these inflammatory mediators are released from the various immune cells. Here, the endolysosomal cation channel TRPML2 is shown to play a direct role in chemokine trafficking and secretion from murine macrophages. To demonstrate acute and direct involvement of TRPML2 in these processes, the first isoform-selective TRPML2 channel agonist was generated, ML2-SA1. ML2-SA1 was not only found to directly stimulate release of the chemokine CCL2 from macrophages but also to stimulate macrophage migration, thus mimicking CCL2 function. Endogenous TRPML2 is expressed in early/recycling endosomes as demonstrated by endolysosomal patch-clamp experimentation and ML2-SA1 promotes trafficking through early/recycling endosomes, suggesting CCL2 being transported and secreted via this pathway. These data provide a direct link between TRPML2 activation, CCL2 release and stimulation of macrophage migration in the innate immune response.

Identification of a single aspartate residue critical for both fast and slow calcium-dependent inactivation of the human TRPML1 channel

Transient receptor potential mucolipin subfamily 1 (TRPML1) is a nonselective cation channel mainly located in late endosomes and lysosomes. Mutations of the gene encoding human TRPML1 can cause severe lysosomal diseases. The activity of TRPML1 is regulated by both Ca²⁺ and H⁺, which are important for its critical physiological functions in membrane trafficking, exocytosis, autophagy, and intracellular signal transduction. However, the molecular mechanism of its dual regulation by Ca²⁺ and H⁺ remains elusive. Here, using a mutant screening method in combination with a whole-cell patch clamp technique, we identified a key TRPML1 residue, Asp-472, responsible for both fast calcium-dependent inactivation (FCDI) and slow calcium-dependent inactivation (SCDI) as well as H⁺ regulation. We also found that, in acidic pH, H⁺ can significantly delay FCDI and abolish SCDI and thereby presumably facilitate the ion conductance of the human TRPML1 channel. In summary, we have identified a key residue critical for Ca²⁺-induced inhibition of TRPML1 channel currents and uncovered pH-dependent regulation of this channel, providing vital information regarding the detailed mechanism of action of human TRPML1.

Mucolipin-2 Cation Channel Increases Trafficking Efficiency of Endocytosed Viruses

Receptor-mediated endocytosis is a cellular process commonly hijacked by viruses to enter cells. The stages of entry are well described for certain viruses, but the host factors that mediate each step are less well characterized. We previously identified endosomal cation channel mucolipin-2 (MCOLN2) as a host factor that promotes viral infection. Here, we assign a role for MCOLN2 in modulating viral entry. We show that MCOLN2 specifically promotes viral vesicular trafficking and subsequent escape from endosomal compartments. This mechanism requires channel activity, occurs independently of antiviral signaling, and broadly applies to enveloped RNA viruses that require transport to late endosomes for infection, including influenza A virus, yellow fever virus, and Zika virus. We further identify a rare allelic variant of human MCOLN2 that has a loss-of-function phenotype with respect to viral enhancement. These findings establish a mechanistic link between an endosomal cation channel and late stages of viral entry.IMPORTANCE Viruses must co-opt cellular processes to complete their life cycle. To enter cells, viruses frequently take advantage of cellular receptor-mediated endocytosis pathways. A growing number of host proteins are implicated in these viral uptake pathways. Here, we describe a new role for the gated cation channel MCOLN2 in viral entry. This endosomal protein modulates viral entry by enhancing the efficiency of viral trafficking through the endosomal system. Thus, MCOLN2-mediated enhancement of infection may represent a key vulnerability in the viral life cycle that could be targeted for therapeutic intervention.

Inhibition of lysosomal Ca2+ signalling disrupts dendritic spine structure and impairs wound healing in neurons

A growing body of evidence suggests that lysosomes, which have traditionally been regarded as degradative organelles, can function as Ca²⁺ stores, regulated by the second messenger nicotinic acid adenine dinucleotide phosphate (NAADP). We previously demonstrated that in hippocampal pyramidal neurons, activity-dependent Ca²⁺ release from these stores triggers fusion of the lysosome with the plasma membrane. We found that the physiological role of this Ca²⁺-dependent fusion was to maintain the long-term structural enlargement of dendritic spines induced by synaptic activity. Here, we examined the pathophysiological consequences of lysosomal dysfunction in hippocampal pyramidal neurons by chronically inhibiting lysosomal Ca²⁺ signalling using the NAADP antagonist, NED-19. We found that within just 20 hours, inhibition of lysosomal function led to a profound intracellular accumulation of lysosomal membrane. This was accompanied by a significant change in dendritic spine structure, which included a lengthening of dendritic spines, an increase in the number of filipodia, and an overall decrease in spine number. Inhibition of lysosomal function also inhibited wound healing in neurons by preventing lysosomal fusion with the plasma membrane. Neurons were therefore more susceptible to injury. Our findings suggest that dysfunction in lysosomal Ca²⁺ signalling and lysosomal fusion with the plasma membrane may contribute to the loss of dendritic spines and neurons seen in neurological disorders, such as Niemann-Pick disease type C1, in which lysosomal function is impaired.

Regulation of Transporters and Channels by Membrane-Trafficking Complexes in Epithelial Cells

The vectorial secretion and absorption of fluid and solutes by epithelial cells is dependent on the polarized expression of membrane solute transporters and channels at the apical and basolateral membranes. The establishment and maintenance of this polarized expression of transporters and channels are affected by divers protein-trafficking complexes. Moreover, regulation of the magnitude of transport is often under control of physiological stimuli, again through the interaction of transporters and channels with protein-trafficking complexes. This review highlights the value in utilizing transporters and channels as cargo to characterize core trafficking machinery by which epithelial cells establish and maintain their polarized expression, and how this machinery regulates fluid and solute transport in response to physiological stimuli.

Vacuolar ATPase in phago(lyso)some biology

Many eukaryotic cells ingest extracellular particles in a process termed phagocytosis which entails the generation of a new intracellular compartment, the phagosome. Phagosomes change their composition over time and this maturation process culminates in their fusion with acidic, hydrolase-rich lysosomes. During the maturation process, degradation and, when applicable, killing of the cargo may ensue. Many of the events that are pathologically relevant depend on strong acidification of phagosomes by the 'vacuolar' ATPase (V-ATPase). This protein complex acidifies the lumen of some intracellular compartments at the expense of ATP hydrolysis. We discuss here the roles and importance of V-ATPase in intracellular trafficking, its distribution, inhibition and activities, its role in the defense against microorganisms and the counteractivities of pathogens.

TRPML1: The Ca(2+)retaker of the lysosome

Efficient functioning of lysosome is necessary to ensure the correct performance of a variety of intracellular processes such as degradation of cargoes coming from the endocytic and autophagic pathways, recycling of organelles, and signaling mechanisms involved in cellular adaptation to nutrient availability. Mutations in lysosomal genes lead to more than 50 lysosomal storage disorders (LSDs). Among them, mutations in the gene encoding TRPML1 (MCOLN1) cause Mucolipidosis type IV (MLIV), a recessive LSD characterized by neurodegeneration, psychomotor retardation, ophthalmologic defects and achlorhydria. At the cellular level, MLIV patient fibroblasts show enlargement and engulfment of the late endo-lysosomal compartment, autophagy impairment, and accumulation of lipids and glycosaminoglycans. TRPML1 is the most extensively studied member of a small family of genes that also includes TRPML2 and TRPML3, and it has been found to participate in vesicular trafficking, lipid and ion homeostasis, and autophagy. In this review we will provide an update on the latest and more novel findings related to the functions of TRPMLs, with particular focus on the emerging role of TRPML1 and lysosomal calcium signaling in autophagy. Moreover, we will also discuss new potential therapeutic approaches for MLIV and LSDs based on the modulation of TRPML1-mediated signaling.

The lysosomal Ca2+ release channel TRPML1 regulates lysosome size by activating calmodulin

Intracellular lysosomal membrane trafficking, including fusion and fission, is crucial for cellular homeostasis and normal cell function. Both fusion and fission of lysosomal membrane are accompanied by lysosomal Ca²⁺ release. We recently have demonstrated that the lysosomal Ca²⁺ release channel P2X4 regulates lysosome fusion through a calmodulin (CaM)-dependent mechanism. However, the molecular mechanism underlying lysosome fission remains uncertain. In this study, we report that enlarged lysosomes/vacuoles induced by either vacuolin-1 or P2X4 activation are suppressed by up-regulating the lysosomal Ca²⁺ release channel transient receptor potential mucolipin 1 (TRPML1) but not the lysosomal Na⁺ release channel two-pore channel 2 (TPC2). Activation of TRPML1 facilitated the recovery of enlarged lysosomes/vacuoles. Moreover, the effects of TRPML1 on lysosome/vacuole size regulation were eliminated by Ca²⁺ chelation, suggesting a requirement for TRPML1-mediated Ca²⁺ release. We further demonstrate that the prototypical Ca²⁺ sensor CaM is required for the regulation of lysosome/vacuole size by TRPML1, suggesting that TRPML1 may promote lysosome fission by activating CaM. Given that lysosome fission is implicated in both lysosome biogenesis and reformation, our findings suggest that TRPML1 may function as a key lysosomal Ca²⁺ channel controlling both lysosome biogenesis and reformation.

Structural basis of dual Ca2+/pH regulation of the endolysosomal TRPML1 channel

Organellar ion channels are essential for cell physiology. Their activities are often regulated by Ca²⁺ and H⁺, which are concentrated in many organelles. Here we report a novel structural element critical for Ca²⁺/pH dual regulation of TRPML1, a Ca²⁺ release channel crucial for endolysosomal functions. TRPML1 mutations cause mucolipidosis type IV (MLIV), a severe lysosomal storage disorder characterized by neurodegeneration, mental retardation and blindness. We obtained high-resolution crystal structures of a 213-amino acid luminal domain of human TRPML1 that harbors three missense MLIV-causing mutations. This domain forms a tetramer with a highly electronegative central pore formed by a novel luminal pore-loop. Cysteine crosslinking and cryo-EM confirm this structure in the full-length channel. Structure-function studies demonstrate that Ca²⁺ and H⁺ interact with the luminal pore to exert physiologically important regulation. The MLIV-causing mutations disrupt the luminal domain structure and cause TRPML1 mislocalization. Our study provides a structural underpinning for TRPML1's regulation, assembly and pathogenesis.

Deviant lysosomal Ca2+ signalling in neurodegeneration. An introduction

Lysosomes are key acidic Ca²⁺ stores. The principle Ca²⁺-permeable channels of the lysosome are TRP mucolipins (TRPMLs) and NAADP-regulated two-pore channels (TPCs). Recent studies, reviewed in this collection, have linked numerous neurodegenerative diseases to both gain and loss of function of TRPMLs/TPCs, as well as to defects in acidic Ca²⁺ store content. These diseases span rare lysosomal storage disorders such as Mucolipidosis Type IV and Niemann-Pick disease, type C, through to more common ones such as Alzheimer and Parkinson disease. Cellular phenotypes, underpinned by endo-lysosomal trafficking defects, are reversed by chemical or molecular targeting of TRPMLs and TPCs. Lysosomal Ca²⁺ channels therefore emerge as potential druggable targets in combatting neurodegeneration.

Two-pore channels at the intersection of endolysosomal membrane traffic

Two-pore channels (TPCs) are ancient members of the voltage-gated ion channel superfamily that localize to acidic organelles such as lysosomes. The TPC complex is the proposed target of the Ca2+-mobilizing messenger NAADP, which releases Ca2+ from these acidic Ca2+ stores. Whereas details of TPC activation and native ion permeation remain unclear, a consensus has emerged around their function in regulating endolysosomal trafficking. This role is supported by recent proteomic data showing that TPCs interact with proteins controlling membrane organization and dynamics, including Rab GTPases and components of the fusion apparatus. Regulation of TPCs by PtdIns(3,5)P2 and/or NAADP (nicotinic acid adenine dinucleotide phosphate) together with their functional and physical association with Rab proteins provides a mechanism for coupling phosphoinositide and trafficking protein cues to local ion fluxes. Therefore, TPCs work at the regulatory cross-roads of (patho)physiological cues to co-ordinate and potentially deregulate traffic flow through the endolysosomal network. This review focuses on the native role of TPCs in trafficking and their emerging contributions to endolysosomal trafficking dysfunction.

Impaired myelination and reduced brain ferric iron in the mouse model of mucolipidosis IV

Mucolipidosis type IV (MLIV) is a lysosomal storage disease caused by mutations in the MCOLN1 gene, which encodes the lysosomal transient receptor potential ion channel mucolipin-1 (TRPML1). MLIV causes impaired motor and cognitive development, progressive loss of vision and gastric achlorhydria. How loss of TRPML1 leads to severe psychomotor retardation is currently unknown, and there is no therapy for MLIV. White matter abnormalities and a hypoplastic corpus callosum are the major hallmarks of MLIV brain pathology. Here, we report that loss of TRPML1 in mice results in developmental aberrations of brain myelination as a result of deficient maturation and loss of oligodendrocytes. Defective myelination is evident in Mcoln1(-/-) mice at postnatal day 10, an active stage of postnatal myelination in the mouse brain. Expression of mature oligodendrocyte markers is reduced in Mcoln1(-/-) mice at postnatal day 10 and remains lower throughout the course of the disease. We observed reduced Perls' staining in Mcoln1(-/-) brain, indicating lower levels of ferric iron. Total iron content in unperfused brain is not significantly different between Mcoln1(-/-) and wild-type littermate mice, suggesting that the observed maturation delay or loss of oligodendrocytes might be caused by impaired iron handling, rather than by global iron deficiency. Overall, these data emphasize a developmental rather than a degenerative disease course in MLIV, and suggest that there should be a stronger focus on oligodendrocyte maturation and survival to better understand MLIV pathogenesis and aid treatment development.

Intracellular sphingosine releases calcium from lysosomes

To elucidate new functions of sphingosine (Sph), we demonstrate that the spontaneous elevation of intracellular Sph levels via caged Sph leads to a significant and transient calcium release from acidic stores that is independent of sphingosine 1-phosphate, extracellular and ER calcium levels. This photo-induced Sph-driven calcium release requires the two-pore channel 1 (TPC1) residing on endosomes and lysosomes. Further, uncaging of Sph leads to the translocation of the autophagy-relevant transcription factor EB (TFEB) to the nucleus specifically after lysosomal calcium release. We confirm that Sph accumulates in late endosomes and lysosomes of cells derived from Niemann-Pick disease type C (NPC) patients and demonstrate a greatly reduced calcium release upon Sph uncaging. We conclude that sphingosine is a positive regulator of calcium release from acidic stores and that understanding the interplay between Sph homeostasis, calcium signaling and autophagy will be crucial in developing new therapies for lipid storage disorders such as NPC.

Retinal Dystrophy and Optic Nerve Pathology in the Mouse Model of Mucolipidosis IV

Mucolipidosis IV is a debilitating developmental lysosomal storage disorder characterized by severe neuromotor retardation and progressive loss of vision, leading to blindness by the second decade of life. Mucolipidosis IV is caused by loss-of-function mutations in the MCOLN1 gene, which encodes the transient receptor potential channel protein mucolipin-1. Ophthalmic pathology in patients includes corneal haze and progressive retinal and optic nerve atrophy. Herein, we report ocular pathology in Mcoln1(-/-) mouse, a good phenotypic model of the disease. Early, but non-progressive, thinning of the photoreceptor layer, reduced levels of rhodopsin, disrupted rod outer segments, and widespread accumulation of the typical storage inclusion bodies were the major histological findings in the Mcoln1(-/-) retina. Electroretinograms showed significantly decreased functional response (scotopic a- and b-wave amplitudes) in the Mcoln1(-/-) mice. At the ultrastructural level, we observed formation of axonal spheroids and decreased density of axons in the optic nerve of the aged (6-month-old) Mcoln1(-/-) mice, which indicates progressive axonal degeneration. Our data suggest that mucolipin-1 plays a role in postnatal development of photoreceptors and provides a set of outcome measures that can be used for ocular therapy development for mucolipidosis IV.

Lysosomal physiology

Lysosomes are acidic compartments filled with more than 60 different types of hydrolases. They mediate the degradation of extracellular particles from endocytosis and of intracellular components from autophagy. The digested products are transported out of the lysosome via specific catabolite exporters or via vesicular membrane trafficking. Lysosomes also contain more than 50 membrane proteins and are equipped with the machinery to sense nutrient availability, which determines the distribution, number, size, and activity of lysosomes to control the specificity of cargo flux and timing (the initiation and termination) of degradation. Defects in degradation, export, or trafficking result in lysosomal dysfunction and lysosomal storage diseases (LSDs). Lysosomal channels and transporters mediate ion flux across perimeter membranes to regulate lysosomal ion homeostasis, membrane potential, catabolite export, membrane trafficking, and nutrient sensing. Dysregulation of lysosomal channels underlies the pathogenesis of many LSDs and possibly that of metabolic and common neurodegenerative diseases.

Novel Role of TRPML2 in the Regulation of the Innate Immune Response

TRPMLs (or mucolipins) constitute a family of endosomal cation channels with homology to the transient receptor potential superfamily. In mammals, the TRPML family includes three members: TRPML1-3. Although TRPML1 and TRPML3 have been well characterized, the cellular function of TRPML2 has remained elusive. To address TRPML2 function in a physiologically relevant cell type, we first analyzed TRPML2 expression in different mouse tissues and organs and found that it was predominantly expressed in lymphoid organs and kidney. Quantitative RT-PCR revealed tight regulation of TRPML2 at the transcriptional level. Although TRPML2 expression was negligible in resting macrophages, TRPML2 mRNA and protein levels dramatically increased in response to TLR activation both in vitro and in vivo. Conversely, TRPML1 and TRPML3 levels did not change upon TLR activation. Immunofluorescence analysis demonstrated that endogenous TRPML2 primarily localized to recycling endosomes both in culture and primary cells, in contrast with TRPML1 and TRPML3, which distribute to the late and early endosomal pathway, respectively. To better understand the in vivo function of TRPML2, we generated a TRPML2-knockout mouse. We found that the production of several chemokines, in particular CCL2, was severely reduced in TRPML2-knockout mice. Furthermore, TRPML2-knockout mice displayed impaired recruitment of peripheral macrophages in response to i.p. injections of LPS or live bacteria, suggesting a potential defect in the immune response. Overall, our study reveals interesting differences in the regulation and distribution of the members of the TRPML family and identifies a novel role for TRPML2 in the innate immune response.

The mucolipin-2 (TRPML2) ion channel: a tissue-specific protein crucial to normal cell function

The discovery of the TRPML subfamily of ion channels has created an exciting niche in the fields of membrane trafficking, signal transduction, autophagy, and metal homeostasis. The TRPML protein subfamily consists of three members, TRPML1, TRPML2, and TRPML3, which are encoded by MCOLN1, MCOLN2, and MCOLN3 genes, respectively. They are non-selective cation channels with six predicted transmembrane domains and intracellular amino- and carboxyl-terminus regions. They localize to the plasma membrane, endosomes, and lysosomes of cells. TRPML1 is associated with the human lysosomal storage disease known as mucolipidosis type IV (MLIV), but TRPML2 and TRPML3 have not been linked with a human disease. Although TRPML1 is expressed in many tissues, TRPML3 is expressed in a varied but limited set of tissues, while TRPML2 has a more limited expression pattern where it is mostly detected in lymphoid and myeloid tissues. This review focuses on TRPML2 because it appears to play an important, yet unrecognized role in the immune system. While the evidence has been mostly indirect, we present and discuss relevant data that strengthen the connection of TRPML2 with cellular immunity. We also discuss the functional redundancy between the TRPML proteins, and how such features could be exploited as a potential therapeutic strategy for MLIV disease. We present evidence that TRPML2 expression may complement certain phenotypic alterations in MLIV cells and briefly examine the challenges of functional complementation. In conclusion, the function of TRPML2 still remains obscure, but emerging data show that it may serve a critical role in immune cell development and inflammatory responses.