Identification of the gene causing mucolipidosis type IV

TRPML1 gating modulation by allosteric mutations and lipids

Transient Receptor Potential Mucolipin 1 (TRPML1) is a lysosomal cation channel whose loss-of-function mutations directly cause the lysosomal storage disorder mucolipidosis type IV (MLIV). TRPML1 can be allosterically regulated by various ligands including natural lipids and small synthetic molecules and the channel undergoes a global movement propagated from ligand-induced local conformational changes upon activation. In this study, we identified a functionally critical residue, Tyr404, at the C-terminus of the S4 helix, whose mutations to tryptophan and alanine yield gain- and loss-of-function channels, respectively. These allosteric mutations mimic the ligand activation or inhibition of the TRPML1 channel without interfering with ligand binding and both mutant channels are susceptible to agonist or antagonist modulation, making them better targets for screening potent TRPML1 activators and inhibitors. We also determined the high-resolution structure of TRPML1 in complex with the PI(4,5)P2 inhibitor, revealing the structural basis underlying this lipid inhibition. In addition, an endogenous phospholipid likely from sphingomyelin is identified in the PI(4,5)P2-bound TRPML1 structure at the same hotspot for agonists and antagonists, providing a plausible structural explanation for the inhibitory effect of sphingomyelin on agonist activation.

Quantitative Chemical Imaging of Organelles

ConspectusDNA nanodevices are nanoscale assemblies, formed from a collection of synthetic DNA strands, that may perform artificial functions. The pioneering developments of a DNA cube by Nadrian Seeman in 1991 and a DNA nanomachine by Turberfield and Yurke in 2000 spawned an entire generation of DNA nanodevices ranging from minimalist to rococo architectures. Since our first demonstration in 2009 that a DNA nanodevice can function autonomously inside a living cell, it became clear that this molecular scaffold was well-placed to probe living systems. Its water solubility, biocompatibility, and engineerability to yield molecularly identical assemblies predisposed it to probe and program biology.Since DNA is a modular scaffold, one can integrate independent or interdependent functionalities onto a single assembly. Work from our group has established a new class of organelle-targeted, DNA-based fluorescent reporters. These reporters comprise three to four oligonucleotides that each display a specific motif or module with a specific function. Given the 1:1 stoichiometry of Watson-Crick-Franklin base pairing, all modules are present in a fixed ratio in every DNA nanodevice. These modules include an ion-sensitive dye or a detection module and a normalizing dye for ratiometry that along with detection module forms a "measuring module". The third module is an organelle-targeting module that engages a cognate protein so that the whole assembly is trafficked to the lumen of a target organelle. Together, these modules allow us to measure free ion concentrations with accuracies that were previously unattainable, in subcellular locations that were previously inaccessible, and at single organelle resolution. By revealing that organelles exist in different chemical states, DNA nanodevices are providing new insights into organelle biology. Further, the ability to deliver molecules with cell-type and organelle level precision in animal models is leading to biomedical applications.This Account outlines the development of DNA nanodevices as fluorescent reporters for chemically mapping or modulating organelle function in real time in living systems. We discuss the technical challenges of measuring ions within endomembrane organelles and show how the unique properties of DNA nanodevices enable organelle targeting and chemical mapping. Starting from the pioneering finding that an autonomous DNA nanodevice could map endolysosomal pH in cells, we chart the development of strategies to target organelles beyond the endolysosomal pathway and expanding chemical maps to include all the major ions in physiology, reactive species, enzyme activity, and voltage. We present a series of vignettes highlighting the new biology unlocked with each development, from the discovery of chemical heterogeneity in lysosomes to identifying the first protein importer of Ca2+ into lysosomes. Finally, we discuss the broader applicability of targeting DNA nanodevices organelle-specifically beyond just reporting ions, namely using DNA nanodevices to modulate organelle state, and thereby cell state, with potential therapeutic applications.

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.

Multimodal Imaging in Mucolipidosis Type IV: Siblings With Novel Genetic Variant

PURPOSE

Utilization of multimodal imaging techniques to diagnose cases of mucolipidosis type IV (ML-IV) and report a new genetic variant.

METHODS

This study is a case report.

RESULTS

Case 1 involves a 4-year-old boy with corneal haziness and global developmental delay who showed an increased reflectivity of the corneal epithelium on anterior segment optical coherence tomography (AS-OCT). In addition, neurologic evaluation was suggestive of ML-IV. Further genetics evaluation confirmed ML-IV. Histology of the button revealed a thickened epithelial basement membrane. Case 2, the younger sibling, showed a milder corneal haze with similar changes on AS-OCT prompting us to further evaluate for ML-IV by genetics (positive MCOLN1 gene mutation). Both instances highlighted varied ML-IV presentations, but a persistent feature was hyperreflective epithelium.

CONCLUSIONS

Our study emphasizes AS-OCT's role in screening ML-IV and advocates the role of genetic counseling of affected parents. We present 2 South-Asian siblings with ML-IV with a new genetic variant, emphasizing the utility of detailed ophthalmic and neurologic assessments using multimodal imaging.

A blood-brain barrier-penetrant AAV gene therapy improves neurological function in symptomatic mucolipidosis IV mice

Mucolipidosis IV (MLIV) is a rare, autosomal recessive, lysosomal disease characterized by intellectual disability, motor deficits, and progressive vision loss. Using adeno-associated vector 9 (AAV9) and AAV-PHP.B as delivery vectors, we previously demonstrated the feasibility of modifying disease course in a mouse model of MLIV by the human MCOLN1 gene transfer. Here, using a primate-enabling capsid AAV.CPP.16 (CPP16), we constructed a new, clinic-oriented MCOLN1 gene expression vector and demonstrated its efficacy in the preclinical model of MLIV. Systemic administration of CPP16-MCOLN1 in adult symptomatic Mcoln1 -/- mice at a dose of 1e12 vg per mouse resulted in MCOLN1 expression in the brain and peripheral tissues, alleviated brain pathology, rescued neuromotor function, and completely prevented paralysis. Notable expression of MCOLN1 transcripts was also detected in the retina of the mouse, which had exhibited significant degeneration at the time of the treatment. However, no increase in retinal thickness was observed after gene therapy treatment. Our results suggest a new AAV-based systemic gene replacement therapy for the treatment of MLIV that could be translated into clinical studies.

Biogenic selenium nanoparticles alleviate intestinal barrier injury in mice through TBC1D15/Fis1/Rab7 pathway

Intestinal diseases often stem from a compromised intestinal barrier. This barrier relies on a functional epithelium and proper turnover of intestinal cells, supported by mitochondrial health. Mitochondria and lysosomes play key roles in cellular balance. Our previous researches indicate that biogenic selenium nanoparticles (SeNPs) can alleviate intestinal epithelial barrier damage by enhancing mitochondria-lysosome crosstalk, though the detailed mechanism is unclear. This study aimed to investigate the role of mitochondria-lysosome crosstalk in the protective effect of SeNPs on intestinal barrier function in mice exposed to lipopolysaccharide (LPS). The results showed that LPS exposure increased intestinal permeability in mice, leding to structural and functional damage to mitochondrial and lysosomal. Oral administration of SeNPs significantly upregulated the expression levels of TBC1D15 and Fis1, downregulated the expression levels of Rab7, Caspase-3, Cathepsin B, and MCOLN2, effectively alleviated LPS-induced mitochondrial and lysosomal dysfunction and maintained the intestinal barrier integrity in mice. Furthermore, SeNPs notably inhibited mitophagy caused by adenovirus-associated virus (AAV)-mediated RNA interference the expression of TBC1D15 in the intestine of mice, maintained mitochondrial and lysosomal homeostasis, and effectively alleviated intestinal barrier damage. These results suggested that SeNPs can regulate mitochondria-lysosome crosstalk and inhibit its damage by regulating the TBC1D15/Fis1/Rab7- signaling pathway. thereby alleviating intestinal barrier damage. It lays a theoretical foundation for elucidating the mechanism of mitochondria-lysosome crosstalk in regulating intestinal barrier damage and repair, and provides new ideas and new ways to establish safe and efficient nutritional regulation strategies to prevent and treat intestinal diseases caused by inflammation.

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.

Coordinating activation of endo-lysosomal two-pore channels and TRP mucolipins

Two-pore channels and TRP mucolipins are ubiquitous endo-lysosomal cation channels of pathophysiological relevance. Both are Ca2+-permeable and regulated by phosphoinositides, principally PI(3,5)P2. Accumulating evidence has uncovered synergistic channel activation by PI(3,5)P2 and endogenous metabolites such as the Ca2+ mobilizing messenger NAADP, synthetic agonists including approved drugs and physical cues such as voltage and osmotic pressure. Here, we provide an overview of this coordination.

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.

Aza Analogs of the TRPML1 Inhibitor Estradiol Methyl Ether (EDME)

Estradiol methyl ether (EDME) has recently been described by us as a very potent and subtype-specific inhibitor of the lysosomal cation channel TRPML1. Following the principle of bioisosteres, we worked out efficient synthetic approaches to ring-A aza-analogs of EDME, namely a methoxypyridine and a methoxypyrimidine analog. Both target compounds were obtained in good overall yields in six and eight steps starting from 19-nortestosterone via the oxidative cleavage of ring A followed over several intermediates and with the use of well-selected protective groups by re-cyclization to provide the desired hetero-analogs. The methoxypyridine analog largely retained its TRPML1-inhibitory activity, whereas the methoxypyrimidine analog significantly lost activity.

TRP (transient receptor potential) ion channel family: structures, biological functions and therapeutic interventions for diseases

Transient receptor potential (TRP) channels are sensors for a variety of cellular and environmental signals. Mammals express a total of 28 different TRP channel proteins, which can be divided into seven subfamilies based on amino acid sequence homology: TRPA (Ankyrin), TRPC (Canonical), TRPM (Melastatin), TRPML (Mucolipin), TRPN (NO-mechano-potential, NOMP), TRPP (Polycystin), TRPV (Vanilloid). They are a class of ion channels found in numerous tissues and cell types and are permeable to a wide range of cations such as Ca²⁺, Mg²⁺, Na⁺, K⁺, and others. TRP channels are responsible for various sensory responses including heat, cold, pain, stress, vision and taste and can be activated by a number of stimuli. Their predominantly location on the cell surface, their interaction with numerous physiological signaling pathways, and the unique crystal structure of TRP channels make TRPs attractive drug targets and implicate them in the treatment of a wide range of diseases. Here, we review the history of TRP channel discovery, summarize the structures and functions of the TRP ion channel family, and highlight the current understanding of the role of TRP channels in the pathogenesis of human disease. Most importantly, we describe TRP channel-related drug discovery, therapeutic interventions for diseases and the limitations of targeting TRP channels in potential clinical applications.

Gene expression of TRPMLs and its regulation by pathogen stimulation

The transient receptor potential mucolipin (TRPML) subfamily in mammalian has three members, namely TRPML1, TRPML2, and TRPML3, who play key roles in regulating intracellular Ca²⁺ homeostasis, endosomal pH, membrane trafficking and autophagy. Previous studies had shown that three TRPMLs are closely related to the occurrence of pathogen invasion and immune regulation in some immune tissues or cells, but the relationship between TRPMLs expression and pathogen invasion in lung tissue or cell remains elusive. Here, we investigated the expression distribution of three TRPML channels in mouse different tissues by qRT-PCR, and then found that all three TRPMLs were highly expressed in the mouse lung tissue, as well as mouse spleen and kidney tissues. The expression of TRPML1 or TRPML3 in all three mouse tissues had a significant down-regulation after the treatment of Salmonella or LPS, but TRPML2 expression showed a remarkable increase. Consistently, TRPML1 or TRPML3 but not TRPML2 in A549 cells also displayed a decreased expression induced by LPS stimulation, which shared a similar regulation pattern in the mouse lung tissue. Furthermore, the treatment of the TRPML1 or TRPML3 specific activator induced a dose-dependent up-regulation of inflammatory factors IL-1β, IL-6 and TNFα, suggesting that TRPML1 and TRPML3 are likely to play an important role in immune and inflammatory regulation. Together, our study identified the gene expression of TRPMLs induced by pathogen stimulation in vivo and in vitro, which may provide novel targets for innate immunity or pathogen regulation.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

The second PI(3,5)P2 binding site in the S0 helix of KCNQ1 stabilizes PIP2-at the primary PI1 site with potential consequences on intermediate-to-open state transition

The Phosphatidylinositol 3-phosphate 5-kinase Type III PIKfyve is the main source for selectively generated phosphatidylinositol 3,5-bisphosphate (PI(3,5)P₂), a known regulator of membrane protein trafficking. PI(3,5)P₂ facilitates the cardiac KCNQ1/KCNE1 channel plasma membrane abundance and therewith increases the macroscopic current amplitude. Functional-physical interaction of PI(3,5)P₂ with membrane proteins and its structural impact is not sufficiently understood. This study aimed to identify molecular interaction sites and stimulatory mechanisms of the KCNQ1/KCNE1 channel via the PIKfyve-PI(3,5)P₂ axis. Mutational scanning at the intracellular membrane leaflet and nuclear magnetic resonance (NMR) spectroscopy identified two PI(3,5)P₂ binding sites, the known PIP₂ site PS1 and the newly identified N-terminal α-helix S0 as relevant for functional PIKfyve effects. Cd²⁺ coordination to engineered cysteines and molecular modeling suggest that repositioning of S0 stabilizes the channel s open state, an effect strictly dependent on parallel binding of PI(3,5)P₂ to both sites.

Lysosomal Ion Channels: What Are They Good For and Are They Druggable Targets?

Lysosomes play fundamental roles in material digestion, cellular clearance, recycling, exocytosis, wound repair, Ca²⁺ signaling, nutrient signaling, and gene expression regulation. The organelle also serves as a hub for important signaling networks involving the mTOR and AKT kinases. Electrophysiological recording and molecular and structural studies in the past decade have uncovered several unique lysosomal ion channels and transporters, including TPCs, TMEM175, TRPMLs, CLN7, and CLC-7. They underlie the organelle's permeability to major ions, including K⁺, Na⁺, H⁺, Ca²⁺, and Cl^-. The channels are regulated by numerous cellular factors, ranging from H⁺ in the lumen and voltage across the lysosomal membrane to ATP in the cytosol to growth factors outside the cell. Genetic variations in the channel/transporter genes are associated with diseases that include lysosomal storage diseases and neurodegenerative diseases. Recent studies with human genetics and channel activators suggest that lysosomal channels may be attractive targets for the development of therapeutics for the prevention of and intervention in human diseases.

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.

Expanding the Toolbox: Novel Modulators of Endolysosomal Cation Channels

Functional characterization of endolysosomal ion channels is challenging due to their intracellular location. With recent advances in endolysosomal patch clamp technology, it has become possible to directly measure ion channel currents across endolysosomal membranes. Members of the transient receptor potential (TRP) cation channel family, namely the endolysosomal TRPML channels (TRPML1-3), also called mucolipins, as well as the distantly related two-pore channels (TPCs) have recently been characterized in more detail with endolysosomal patch clamp techniques. However, answers to many physiological questions require work in intact cells or animal models. One major obstacle thereby is that the known endogenous ligands of TRPMLs and TPCs are anionic in nature and thus impermeable for cell membranes. Microinjection, on the other hand, is technically demanding. There is also a risk of losing essential co-factors for channel activation or inhibition in isolated preparations. Therefore, lipophilic, membrane-permeable small-molecule activators and inhibitors for TRPMLs and TPCs are urgently needed. Here, we describe and discuss the currently available small-molecule modulators of TRPMLs and TPCs.

A Structural Overview of TRPML1 and the TRPML Family

This chapter explores the existing structural and functional studies on the endo-lysosomal channel TRPML1 and its analogs TRPML2, TRPML3. These channels represent the mucolipin subfamily of the TRP channel superfamily comprising important roles in sensory physiology, ion homeostasis, and signal transduction. Since 2016, numerous structures have been determined for all three members using either cryo-EM or X-ray crystallography. These studies along with recent functional analysis have considerably strengthened our knowledge on TRPML channels and its related endo-lysosomal function. This chapter, together with relevant reports in other chapters from this handbook, provides an informative and detailed tool to study the endo-lysosomal cation channels.

Lysosomal positioning diseases: beyond substrate storage

Lysosomal storage diseases (LSDs) comprise a group of inherited monogenic disorders characterized by lysosomal dysfunctions due to undegraded substrate accumulation. They are caused by a deficiency in specific lysosomal hydrolases involved in cellular catabolism, or non-enzymatic proteins essential for normal lysosomal functions. In LSDs, the lack of degradation of the accumulated substrate and its lysosomal storage impairs lysosome functions resulting in the perturbation of cellular homeostasis and, in turn, the damage of multiple organ systems. A substantial number of studies on the pathogenesis of LSDs has highlighted how the accumulation of lysosomal substrates is only the first event of a cascade of processes including the accumulation of secondary metabolites and the impairment of cellular trafficking, cell signalling, autophagic flux, mitochondria functionality and calcium homeostasis, that significantly contribute to the onset and progression of these diseases. Emerging studies on lysosomal biology have described the fundamental roles of these organelles in a variety of physiological functions and pathological conditions beyond their canonical activity in cellular waste clearance. Here, we discuss recent advances in the knowledge of cellular and molecular mechanisms linking lysosomal positioning and trafficking to LSDs.

Transmembrane 163 (TMEM163) protein interacts with specific mammalian SLC30 zinc efflux transporter family members

Recently, we reported that TMEM163 is a zinc efflux transporter that likely belongs to the mammalian solute carrier 30 (Slc30/ZnT) subfamily of the cation diffusion facilitator (CDF) protein superfamily. We hypothesized that human TMEM163 forms functional heterodimers with certain ZNT proteins based on their overlapping subcellular localization with TMEM163 and previous reports that some ZNT monomers interact with each other. In this study, we heterologously expressed individual constructs with a unique peptide tag containing TMEM163, ZNT1, ZNT2, ZNT3, and ZNT4 (negative control) or co-expressed TMEM163 with each ZNT in cultured cells for co-immunoprecipitation (co-IP) experiments. We also co-expressed TMEM163 with two different peptide tags as a positive co-IP control. Western blot analyses revealed that TMEM163 dimerizes with itself but that it also heterodimerizes with ZNT1, ZNT2, ZNT3, and ZNT4 proteins. Confocal microscopy revealed that TMEM163 and ZNT proteins partially co-localize in cells, suggesting that they exist as homodimers and heterodimers in their respective subcellular sites. Functional zinc flux assays using Fluozin-3 and Newport Green dyes show that TMEM163/ZNT heterodimers exhibit similar efflux function as TMEM163 homodimers. Cell surface biotinylation revealed that the plasma membrane localization of TMEM163 is not markedly influenced by ZNT co-expression. Overall, our results show that the interaction between TMEM163 and distinct ZNT proteins is physiologically relevant and that their heterodimerization may serve to increase the functional diversity of zinc effluxers within specific tissues or cell types.

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TMEM163 protein heterodimerizes with ZNT1, ZNT2, ZNT3 and ZNT4 zinc efflux transporters.

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Partial co-localization of TMEM163 and ZNT proteins in cells suggests distinct roles as homodimers and heterodimers.

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Zinc efflux activity of TMEM163 or ZNT protein homodimers did not differ from their TMEM163/ZNT heterodimer counterparts.

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TMEM163/ZNT heterodimerization attests to the role of TMEM163 as a bona fide SLC30 protein family member.

Activated Endolysosomal Cation Channel TRPML1 Facilitates Maturation of α-Synuclein-Containing Autophagosomes

Background: Protein aggregates are degraded via the autophagy-lysosome pathway and alterations in the lysosomal system leading to the accumulation of pathogenic proteins, including aggregates of α-synuclein in Parkinson’s disease (PD). The importance of the endolysosomal transient receptor potential cation channel, mucolipin subfamily 1 (TRPML1) for the lysosomal function is highlighted by the fact that TRPML1 mutations cause the lysosomal storage disease mucolipidosis type IV. In this study, we investigated the mechanism by which activation of TRPML1 affects the degradation of α-synuclein.

Methods: As a model of α-synuclein pathology, we expressed the pathogenic A53Tα-synuclein mutant in HEK293T cells. These cells were treated with the synthetic TRPML1 agonist ML-SA1. The amount of α-synuclein protein was determined by immunoblots. The abundance of aggregates and autolysosomal vesicles was determined by fluorescence microscopy and immunocytochemistry. Findings were confirmed by life-cell imaging and by application of ML-SA1 and the TRPML1 antagonist ML-SI3 to human dopaminergic neurons and human stem cell-derived neurons.

Results: ML-SA1 reduced the percentage of HEK293T cells with α-synuclein aggregates and the amount of α-synuclein protein. The effect of ML-SA1 was blocked by pharmacological and genetic inhibition of autophagy. Consistent with TRPML function, it required the membrane lipid PI(3,5)P_2, and cytosolic calcium. ML-SA1 shifted the composition of autophagosomes towards a higher fraction of mature autolysosomes, also in presence of α-synuclein. In neurons, inhibition of TRPML1 by its antagonist ML-SI3 blocked autophagosomal clearance, whereas the agonist ML-SA1 shifted the composition of a-synuclein particles towards a higher fraction of acidified particles. ML-SA1 was able to override the effect of Bafilomycin A1, which blocks the fusion of the autophagosome and lysosome and its acidification.

Conclusion: These findings suggest, that activating TRPML1 with ML-SA1 facilitates clearance of α-synuclein aggregates primarily by affecting the late steps of the autophagy, i.e., by promoting autophagosome maturation. In agreement with recent work by others, our findings indicate that TRPML1 might constitute a plausible therapeutic target for PD, that warrants further validation in rodent models of α-synuclein pathology.

Regulation of Aging and Longevity by Ion Channels and Transporters

Despite significant advances in our understanding of the mechanisms that underlie age-related physiological decline, our ability to translate these insights into actionable strategies to extend human healthspan has been limited. One of the major reasons for the existence of this barrier is that with a few important exceptions, many of the proteins that mediate aging have proven to be undruggable. The argument put forth here is that the amenability of ion channels and transporters to pharmacological manipulation could be leveraged to develop novel therapeutic strategies to combat aging. This review delves into the established roles for ion channels and transporters in the regulation of aging and longevity via their influence on membrane excitability, Ca2+ homeostasis, mitochondrial and endolysosomal function, and the transduction of sensory stimuli. The goal is to provide the reader with an understanding of emergent themes, and prompt further investigation into how the activities of ion channels and transporters sculpt the trajectories of cellular and organismal aging.

Cross-sectional Observations on the Natural History of Mucolipidosis Type IV

Background and Objectives

Mucolipidosis type IV (MLIV) is an ultra-rare lysosomal disorder initially described as a static neurodevelopmental condition. However, patient caregivers frequently report progressive muscular hypertonicity and functional decline. We evaluated a cohort of patients with MLIV to determine whether neurologic disability correlates with age.

Methods

We performed a cross-sectional, observational study of 26 patients with MLIV in the United States and Israel ranging in age from 2 to 40 years. Medical history was obtained from caregivers, and patients underwent a full neurologic examination. The Brief Assessment of Motor Function (BAMF), Gross Motor Function Classification System, and modified Ashworth scales were applied. Caregivers identified developmental skills on the Oregon Project for Visually Impaired and Blind Children checklist that their child had lost the ability to perform.

Results

Three patients were clinically classified as mildly affected and the remaining 23 patients as typical, severely affected cases. Timing of first symptom onset ranged from 1.5 months to 8 years of age (median 7.25 months). Across typical patients, modified Ashworth scores demonstrated a positive age dependence illustrating worsening spasticity across the lifespan. Signs of extrapyramidal motor dysfunction were also qualitatively observed. In parallel, gross and fine motor function assessed with the BAMF and Gross Motor Function Classification System scales declined across age. All typical patients had restricted tongue mobility and lacked rotary jaw movement when chewing, but BAMF scores for deglutition declined only in the oldest patients. In contrast, scores for articulation were low in all patients and did not correlate with age. Finally, loss of developmental skills frequently occurred in early adolescence.

Discussion

This cross-sectional natural history study of MLIV demonstrates worse motor function in older patients. These data support a neurodegenerative component of MLIV that manifests as developmental regression in the second decade of life. Whether the emergence of functional decline results from the cumulative, nonlinear interactions of steadily progressive neurodegenerative processes or reflects an inflection from impaired CNS development to degeneration is uncertain. However, understanding the relationship between CNS pathology and clinical course of disease will be imperative to guiding future interventional trials and optimizing patient care.

Structural mechanism of allosteric activation of TRPML1 by PI(3,5)P2 and rapamycin

Rapamycin is a specific inhibitor of mammalian target of rapamycin (mTOR). Rapamycin can also activate transient receptor potential mucolipin 1 (TRPML1), a phosphatidylinositol 3,5-bisphosphate [PI(3,5)P₂]–gated lysosomal cation channel whose loss-of-function mutations directly cause mucolipidosis type IV disease. We determined the high-resolution cryoelectron microscopy structures of TRPML1 in various ligand-bound states, including the open TRPML1 in complex with PI(3,5)P₂ and a rapamycin analog at 2.1 Å. These structures reveal how rapamycin and PI(3,5)P₂ bind at two distinct sites and allosterically activate the channel. Considering the high potency of TRPML1 activation by rapamycin and PI(3,5)P₂, it is conceivable that some pharmacological effects from the therapeutic use of rapamycin may come from the TRPML1-dependent mechanism rather than mTOR inhibition.

Transient receptor potential mucolipin 1 (TRPML1) is a Ca²⁺-permeable, nonselective cation channel ubiquitously expressed in the endolysosomes of mammalian cells and its loss-of-function mutations are the direct cause of type IV mucolipidosis (MLIV), an autosomal recessive lysosomal storage disease. TRPML1 is a ligand-gated channel that can be activated by phosphatidylinositol 3,5-bisphosphate [PI(3,5)P₂] as well as some synthetic small-molecule agonists. Recently, rapamycin has also been shown to directly bind and activate TRPML1. Interestingly, both PI(3,5)P₂ and rapamycin have low efficacy in channel activation individually but together they work cooperatively and activate the channel with high potency. To reveal the structural basis underlying the synergistic activation of TRPML1 by PI(3,5)P₂ and rapamycin, we determined the high-resolution cryoelectron microscopy (cryo-EM) structures of the mouse TRPML1 channel in various states, including apo closed, PI(3,5)P₂-bound closed, and PI(3,5)P₂/temsirolimus (a rapamycin analog)-bound open states. These structures, combined with electrophysiology, elucidate the molecular details of ligand binding and provide structural insight into how the TRPML1 channel integrates two distantly bound ligand stimuli and facilitates channel opening.

Roles of Intramolecular Interactions in the Regulation of TRP Channels

The transient receptor potential (TRP) channels, classified into six (-A, -V, -P, -C, -M, -ML, -N and -Y) subfamilies, are important membrane sensors and mediators of diverse stimuli including pH, light, mechano-force, temperature, pain, taste, and smell. The mammalian TRP superfamily of 28 members share similar membrane topology with six membrane-spanning helices (S1-S6) and cytosolic N-/C-terminus. Abnormal function or expression of TRP channels is associated with cancer, skeletal dysplasia, immunodeficiency, and cardiac, renal, and neuronal diseases. The majority of TRP members share common functional regulators such as phospholipid PIP2, 2-aminoethoxydiphenyl borate (2-APB), and cannabinoid, while other ligands are more specific, such as allyl isothiocyanate (TRPA1), vanilloids (TRPV1), menthol (TRPM8), ADP-ribose (TRPM2), and ML-SA1 (TRPML1). The mechanisms underlying the gating and regulation of TRP channels remain largely unclear. Recent advances in cryogenic electron microscopy provided structural insights into 19 different TRP channels which all revealed close proximity of the C-terminus with the N-terminus and intracellular S4-S5 linker. Further studies found that some highly conserved residues in these regions of TRPV, -P, -C and -M members mediate functionally critical intramolecular interactions (i.e., within one subunit) between these regions. This review provides an overview on (1) intramolecular interactions in TRP channels and their effect on channel function; (2) functional roles of interplays between PIP2 (and other ligands) and TRP intramolecular interactions; and (3) relevance of the ligand-induced modulation of intramolecular interaction to diseases.

Structural biology of cation channels important for lysosomal calcium release

Calcium is one of the most important second messengers in cells. The uptake and release of calcium ions are conducted by channels and transporters. Inside a eukaryotic cell, calcium is stored in intracellular organelles including the endoplasmic reticulum (ER), mitochondrion, and lysosome. Lysosomes are acid membrane-bounded organelles serving as the crucial degradation and recycling center of the cell. Lysosomes involve in multiple important signaling events, including nutrient sensing, lipid metabolism, and trafficking. Hitherto, two lysosomal cation channel families have been suggested to function as calcium release channels, namely the Two-pore Channel (TPC) family, and the Transient Receptor Potential Channel Mucolipin (TRPML) family. Additionally, a few plasma membrane calcium channels have also been found in the lysosomal membrane under certain circumstances. In this review, we will discuss the structural mechanism of the cation channels that may be important for lysosomal calcium release, primarily focusing on the TPCs and TRPMLs.

Cryo-EM structure of mouse TRPML2 in lipid nanodiscs

In mammalians, transient receptor potential mucolipin ion channels (TRPMLs) exhibit variable permeability to cations such as Ca²⁺, Fe²⁺, Zn²⁺, and Na⁺, and can be activated by the phosphoinositide PI(3,5)P2 in the endolysosomal system. Loss or dysfunction of TRPMLs has been implicated in lysosomal storage disorders, infectious diseases, and metabolic diseases. TRPML2 has recently been identified as a mechanosensitive and hypotonicity-sensitive channel in endolysosomal organelles, which distinguishes it from TRPML1 and TRPML3. However, the molecular and gating mechanism of TRPML2 remains elusive. Here, we present the cryo-EM structure of the full-length mouse TRPML2 in lipid nanodiscs at 3.14 Å resolution. The TRPML2 homo-tetramer structure at pH 7.4 in the apo state reveals an inactive conformation and some unique features of the extracytosolic/luminal domain and voltage sensor-like domain that have implications for the ion-conducting pathway. This structure enables new comparisons between the different subgroups of TRPML channels with available structures and provides structural insights into the conservation and diversity of TRPML channels. These comparisons have broad implications for understanding a variety of molecular mechanisms of TRPMLs in different pH conditions, including with and without bound agonists and antagonists.

Effective chiral pool synthesis of both enantiomers of the TRPML inhibitor trans-ML-SI3

Two independent chiral pool syntheses of both enantiomers of the TRPML inhibitor, trans-ML-SI3, were developed, starting from commercially available (1S,2R)- and (1R,2S)-configured cis-2-aminocyclohexanols. Both routes lead to the target compounds in excellent enantiomeric purity and good overall yields. For the most attractive (-)-trans-enantiomer, the R,R-configuration was identified by these unambiguous syntheses, and the results were confirmed by single-crystal X-ray structure analysis. These effective synthetic approaches further allow flexible variation of prominent residues in ML-SI3 for future in-depth analysis of structure-activity relationships as both the piperazine and the N-sulfonyl residues are introduced into the molecule at late stages of the synthesis.

White matter abnormalities and iron deposition in prenatal mucolipidosis IV- fetal imaging and pathology

Mucolipidosis type IV (MLIV; OMIM 252,650) is an autosomal recessive lysosomal disorder caused by mutations in MCOLN1. MLIV causes psychomotor impairment and progressive vision loss. The major hallmarks of postnatal brain MRI are hypomyelination and thin corpus callosum. Human brain pathology data is scarce and demonstrates storage of various inclusion bodies in all neuronal cell types. The current study describes novel fetal brain MRI and neuropathology findings in a fetus with MLIV. Fetal MRI was performed at 32 and 35 weeks of gestation due to an older sibling with spastic quadriparesis, visual impairment and hypomyelination. Following abnormal fetal MRI results, the parents requested termination of pregnancy according to Israeli regulations. Fetal autopsy was performed after approval of the high committee for pregnancy termination. A genetic diagnosis of MLIV was established in the fetus and sibling. Sequential fetal brain MRI showed progressive curvilinear hypointensities on T2-weighted images in the frontal deep white matter and a thin corpus callosum. Fetal brain pathology exhibited a thin corpus callosum and hypercellular white matter composed of reactive astrocytes and microglia, multifocal white matter abnormalities with mineralized deposits, and numerous aggregates of microglia with focal intracellular iron accumulation most prominent in the frontal lobes. This is the first description in the literature of brain MRI and neuropathology in a fetus with MLIV. The findings demonstrate prenatal white matter involvement with significant activation of microglia and astrocytes and impaired iron metabolism.

Transient Receptor Potential (TRP) Ion Channels Involved in Malignant Glioma Cell Death and Therapeutic Perspectives

Among the most biologically, thus clinically, aggressive primary brain tumors are found malignant gliomas. Despite recent advances in adjuvant therapies, which include targeted and immunotherapies, after surgery and radio/chemotherapy, the tumor is recurrent and always lethal. Malignant gliomas also contain a pool of initiating stem cells that are highly invasive and resistant to conventional treatment. Ion channels and transporters are markedly involved in cancer cell biology, including glioma cell biology. Transient receptor potential (TRP) ion channels are calcium-permeable channels implicated in Ca²⁺ changes in multiple cellular compartments by modulating the driving force for Ca²⁺ entry. Recent scientific reports have shown that these channels contribute to the increase in glioblastoma aggressiveness, with glioblastoma representing the ultimate level of glioma malignancy. The current review focuses on each type of TRP ion channel potentially involved in malignant glioma cell death, with the ultimate goal of identifying new therapeutic targets to clinically combat malignant gliomas. It thus appears that cannabidiol targeting the TRPV2 type could be such a potential target.

Structural Pharmacology of TRP Channels

Transient receptor potential (TRP) ion channels are a super-family of ion channels that mediate transmembrane cation flux with polymodal activation, ranging from chemical to physical stimuli. Furthermore, due to their ubiquitous expression and role in human diseases, they serve as potential pharmacological targets. Advances in cryo-EM TRP channel structural biology has revealed general, as well as diverse, architectural elements and regulatory sites among TRP channel subfamilies. Here, we review the endogenous and pharmacological ligand-binding sites of TRP channels and their regulatory mechanisms.

Berbamine inhibits Japanese encephalitis virus (JEV) infection by compromising TPRMLs-mediated endolysosomal trafficking of low-density lipoprotein receptor (LDLR)

Japanese encephalitis virus (JEV), a member of the Flavivirus genus, is an important pathogen that causes human and animal infectious diseases in Asia. So far, no effective antiviral agents are available to treat JEV infection. Here, we found that LDLR is a host factor required for JEV entry. Berbamine significantly decreases the level of LDLR at the plasma membrane by inducing the secretion of LDLR via extracellular vesicles (EVs), thereby inhibiting JEV infection. Mechanistically, berbamine blocks TRPMLs (Ca²⁺ permeable non-selective cation channels in endosomes and lysosomes) to compromise the endolysosomal trafficking of LDLR. This leads to the increased secretion of LDLR via EVs and the concomitant decrease in its level at the plasma membrane, thereby rendering cells resistant to JEV infection. Berbamine also protects mice from the lethal challenge of JEV. In summary, these results indicate that berbamine is an effective anti-JEV agent by preventing JEV entry.

Cyclotriphosphazene-Based "Butterfly" Fluorescence Probe for Lysosome Targeting

A cyclotriphosphazene-based "butterfly" fluorescence probe HCCP-MNI bearing two naphthalimide and morpholine units were developed for lysosome targeting. The synthesized HCCP-MNI exhibited stable fluorescence signals and was cytocompatible in the given concentration range. Co-localization experimental results showed that cells treated with the HCCP-MNI and a commercial dye (Lyso-Tracker Red DND-99) had overlapped fluorescence signals, demonstrating its targeting specificity to lysosomes. The developed HCCP-MNI may be used for cell tracking applications associated with the functionalities of lysosomes.

Synthesis, X-ray Structure Determination, and Comprehensive Photochemical Characterization of (Trifluoromethyl)diazirine-Containing TRPML1 Ligands

Potential (trifluoromethyl)diazirine-based TRPML1 ion channel ligands were designed and synthesized, and their structures were determined by single-crystal X-ray diffraction analysis. Photoactivation studies via ¹⁹F NMR spectroscopy and HPLC-MS analysis revealed distinct kinetical characteristics in selected solvents and favorable photochemical properties in an aqueous buffer. These photoactivatable TRPML activators represent useful and valuable tools for TRPML photoaffinity labeling combined with mass spectrometry.

Estradiol analogs attenuate autophagy, cell migration and invasion by direct and selective inhibition of TRPML1, independent of estrogen receptors

The cation channel TRPML1 is an important regulator of lysosomal function and autophagy. Loss of TRPML1 is associated with neurodegeneration and lysosomal storage disease, while temporary inhibition of this ion channel has been proposed to be beneficial in cancer therapy. Currently available TRPML1 channel inhibitors are not TRPML isoform selective and block at least two of the three human isoforms. We have now identified the first highly potent and isoform-selective TRPML1 antagonist, the steroid 17β-estradiol methyl ether (EDME). Two analogs of EDME, PRU-10 and PRU-12, characterized by their reduced activity at the estrogen receptor, have been identified through systematic chemical modification of the lead structure. EDME and its analogs, besides being promising new small molecule tool compounds for the investigation of TRPML1, selectively affect key features of TRPML1 function: autophagy induction and transcription factor EB (TFEB) translocation. In addition, they act as inhibitors of triple-negative breast cancer cell migration and invasion.

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.

Ca2+ dysregulation in the pathogenesis of amyotrophic lateral sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a devastating neurodegenerative disease without appropriate cure. One of the main reasons for the lack of a proper pharmacotherapy in ALS is the narrow knowledge on the molecular causes of the disease. In this respect, the identification of dysfunctional pathways in ALS is now considered a critical medical need. Among the causative factors involved in ALS, Ca²⁺ dysregulation is one of the most important pathogenetic mechanisms of the disease. Of note, Ca²⁺ dysfunction may induce, directly or indirectly, motor neuron degeneration and loss. Interestingly, both familial (fALS) and sporadic ALS (sALS) share the progressive dysregulation of Ca²⁺ homeostasis as a common noxious mechanism. Mechanicistically, Ca²⁺ dysfunction involves both plasma membrane and intracellular mechanisms, including AMPA receptor (AMPAR)-mediated excitotoxicity, voltage-gated Ca²⁺ channels (VGCCs) and Ca²⁺ transporter dysregulation, endoplasmic reticulum (ER) Ca²⁺ deregulation, mitochondria-associated ER membranes (MAMs) dysfunction, lysosomal Ca²⁺ leak, etc. Here, a comprehensive analysis of the main pathways involved in the dysregulation of Ca²⁺ homeostasis has been reported with the aim to focus the attention on new putative druggable targets.

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.

Exploiting the Potential of Drosophila Models in Lysosomal Storage Disorders: Pathological Mechanisms and Drug Discovery

Lysosomal storage disorders (LSDs) represent a complex and heterogeneous group of rare genetic diseases due to mutations in genes coding for lysosomal enzymes, membrane proteins or transporters. This leads to the accumulation of undegraded materials within lysosomes and a broad range of severe clinical features, often including the impairment of central nervous system (CNS). When available, enzyme replacement therapy slows the disease progression although it is not curative; also, most recombinant enzymes cannot cross the blood-brain barrier, leaving the CNS untreated. The inefficient degradative capability of the lysosomes has a negative impact on the flux through the endolysosomal and autophagic pathways; therefore, dysregulation of these pathways is increasingly emerging as a relevant disease mechanism in LSDs. In the last twenty years, different LSD Drosophila models have been generated, mainly for diseases presenting with neurological involvement. The fruit fly provides a large selection of tools to investigate lysosomes, autophagy and endocytic pathways in vivo, as well as to analyse neuronal and glial cells. The possibility to use Drosophila in drug repurposing and discovery makes it an attractive model for LSDs lacking effective therapies. Here, ee describe the major cellular pathways implicated in LSDs pathogenesis, the approaches available for their study and the Drosophila models developed for these diseases. Finally, we highlight a possible use of LSDs Drosophila models for drug screening studies.

Membrane Transport Proteins in Osteoclasts: The Ins and Outs

During bone resorption, the osteoclast must sustain an extraordinarily low pH environment, withstand immense ionic pressures, and coordinate nutrient and waste exchange across its membrane to sustain its unique structural and functional polarity. To achieve this, osteoclasts are equipped with an elaborate set of membrane transport proteins (pumps, transporters and channels) that serve as molecular ‘gatekeepers’ to regulate the bilateral exchange of ions, amino acids, metabolites and macromolecules across the ruffled border and basolateral domains. Whereas the importance of the vacuolar-ATPase proton pump and chloride voltage-gated channel 7 in osteoclasts has long been established, comparatively little is known about the contributions of other membrane transport proteins, including those categorized as secondary active transporters. In this Special Issue review, we provide a contemporary update on the ‘ins and outs’ of membrane transport proteins implicated in osteoclast differentiation, function and bone homeostasis and discuss their therapeutic potential for the treatment of metabolic bone diseases.

Lysosome function in glomerular health and disease

The lysosome represents an important regulatory platform within numerous vesicle trafficking pathways including the endocytic, phagocytic, and autophagic pathways. Its ability to fuse with endosomes, phagosomes, and autophagosomes enables the lysosome to break down a wide range of both endogenous and exogenous cargo, including macromolecules, certain pathogens, and old or damaged organelles. Due to its center position in an intricate network of trafficking events, the lysosome has emerged as a central signaling node for sensing and orchestrating the cells metabolism and immune response, for inter-organelle and inter-cellular signaling and in membrane repair. This review highlights the current knowledge of general lysosome function and discusses these findings in their implication for renal glomerular cell types in health and disease including the involvement of glomerular cells in lysosomal storage diseases and the role of lysosomes in nongenetic glomerular injuries.

Persistently elevated CK and lysosomal storage myopathy associated with mucolipin 1 defects

Mucolipidosis type IV is a rare autosomal recessive lysosomal storage disorder caused by bi-allelic pathogenic variants in the gene MCOLN1. This encodes for mucolipin-1 (ML1), an endo-lysosomal transmembrane Ca⁺⁺ channel involved in vesicular trafficking. Although experimental models suggest that defects in mucolipin-1 can cause muscular dystrophy, putatively due to defective lysosomal-mediated sarcolemma repair, the role of mucolipin-1 in human muscle is still poorly deciphered. Elevation of creatine kinase (CK) had been reported in a few cases in the past but comprehensive descriptions of muscle pathology are lacking. Here we report a 7-year-old boy who underwent muscle biopsy due to persistently elevated CK levels (780-15,000 UI/L). Muscle pathology revealed features of a lysosomal storage myopathy with mild regenerative changes. Next generation sequencing confirmed homozygous nonsense variants in MCOLN1. This is a comprehensive pathological description of ML1-related myopathy, supporting the role of mucolipin-1 in muscle homoeostasis.

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.

Inherited disorders of lysosomal membrane transporters

Disorders caused by defects in lysosomal membrane transporters form a distinct subgroup of lysosomal storage disorders (LSDs). To date, defects in only 10 lysosomal membrane transporters have been associated with inherited disorders. The clinical presentations of these diseases resemble the phenotypes of other LSDs; they are heterogeneous and often present in children with neurodegenerative manifestations. However, for pathomechanistic and therapeutic studies, lysosomal membrane transport defects should be distinguished from LSDs caused by defective hydrolytic enzymes. The involved proteins differ in function, localization, and lysosomal targeting, and the diseases themselves differ in their stored material and therapeutic approaches. We provide an overview of the small group of disorders of lysosomal membrane transporters, emphasizing discovery, pathomechanism, clinical features, diagnostic methods and therapeutic aspects. We discuss common aspects of lysosomal membrane transporter defects that can provide the basis for preclinical research into these disorders.

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.

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.

Chemical and pharmacological characterization of the TRPML calcium channel blockers ML-SI1 and ML-SI3

The members of the TRPML subfamily of non-selective cation channels (TRPML1-3) are involved in the regulation of important lysosomal and endosomal functions, and mutations in TRPML1 are associated with the neurodegenerative lysosomal storage disorder mucolipidosis type IV. For in-depth investigation of functions and (patho)physiological roles of TRPMLs, membrane-permeable chemical tools are urgently needed. But hitherto only two TRPML inhibitors, ML-SI1 and ML-SI3, have been published, albeit without clear information about stereochemical details. In this investigation we developed total syntheses of both inhibitors. ML-SI1 was only obtained as a racemic mixture of inseparable diastereomers and showed activator-dependent inhibitory activity. The more promising tool is ML-SI3, hence ML-SI1 was not further investigated. For ML-SI3 we confirmed by stereoselective synthesis that the trans-isomer is significantly more active than the cis-isomer. Separation of the enantiomers of trans-ML-SI3 further revealed that the (-)-isomer is a potent inhibitor of TRPML1 and TRPML2 (IC₅₀ values 1.6 and 2.3 μM) and a weak inhibitor (IC₅₀ 12.5 μM) of TRPML3, whereas the (+)-enantiomer is an inhibitor on TRPML1 (IC₅₀ 5.9 μM), but an activator on TRPML 2 and 3. This renders the pure (-)-trans-ML-SI3 more suitable as a chemical tool for the investigation of TRPML1 and 2 than the racemate. The analysis of 12 analogues of ML-SI3 gave first insights into structure-activity relationships in this chemotype, and showed that a broad variety of modifications in both the N-arylpiperazine and the sulfonamide moiety is tolerated. An aromatic analogue of ML-SI3 showed an interesting alternative selectivity profile (strong inhibitor of TRPML1 and strong activator of TRPML2).

The genetic cause of intellectual deficiency and/or congenital malformations in two parental reciprocal translocation carriers and implications for assisted reproduction

PURPOSE

To elucidate the genetic cause of intellectual deficiency and/or congenital malformations in two parental reciprocal translocation carriers and provide appropriate strategies of assisted reproductive therapy (ART).

MATERIALS AND METHODS

Two similar couples having a child with global developmental delay/intellectual disability symptoms attended the Reproductive and Genetic Hospital of CITIC-Xiangya (Changsha, China) in 2017 and 2019, respectively, in order to determine the cause(s) of the conditions affecting their child and to seek ART to have a healthy baby. Both of the healthy couples were not of consanguineous marriage, denied exposure to toxicants, and had no adverse life history. This study was approved by the Institutional Ethics Committee of the Reproductive & Genetic Hospital of CITIC-Xiangya, and written informed consent was obtained from the parents. Genetic diagnoses were performed by karyotype analysis, breakpoint mapping analysis of chromosomal translocation(s), single-nucleotide polymorphism (SNP) microarray analysis, and whole-exome sequencing (WES) for the two children and different appropriate reproductive strategies were performed in the two families.

RESULTS

Karyotype analysis revealed that both patients carried parental reciprocal translocations [46,XY,t(7;16)(p13;q24)pat and 46,XY,t(13;17)(q12.3;p11.2)pat, respectively]. Follow-up breakpoint mapping analysis showed no interruption of associated genes, and SNP microarray analysis identified no significant copy number variations (CNVs) in the two patients. Moreover, WES results revealed that patients 1 and 2 harbored candidate compound heterozygous mutations of MCOLN1 [c.195G>C (p.K65N) and c.1061G>A (p.W354*)] and MCPH1 [c.877A>G (p.S293G) and c.1869_1870delAT (p.C624*)], respectively, that were inherited from their parents and not previously reported. Furthermore, the parents of patient 1 obtained 10 embryos during ART cycle, and an embryo of normal karyotype and non-carrier of observed MCOLN1 mutations according to preimplantation genetic testing for structural rearrangement and monogenic defect was successfully transferred, resulting in the birth of a healthy boy. The parents of patient 2 chose to undergo ART with donor sperm to reduce the risk of recurrence.

CONCLUSIONS

Systematic genetic diagnosis of two carriers of inherited chromosomal translocations accompanied by clinical phenotypes revealed their cause of disease, which was critical for genetic counseling and further ART for these families.