Mucolipidosis type IV: abnormal transport of lipids to lysosomes

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.

The Dictyostelium Model for Mucolipidosis Type IV

Mucolipidosis type IV, a devastating neurological lysosomal disease linked to mutations in the transient receptor potential channel mucolipin 1, TRPML1, a calcium permeable channel in the membranes of vesicles in endolysosomal system. TRPML1 function is still being elucidated and a better understanding of the molecular pathogenesis of Mucolipidosis type IV, may facilitate development of potential treatments. We have created a model to study mucolipin function in the eukaryotic slime mould Dictyostelium discoideum by altering expression of its single mucolipin homologue, mcln. We show that in Dictyostelium mucolipin overexpression contributes significantly to global chemotactic calcium responses in vegetative and differentiated cells. Knockdown of mucolipin also enhances calcium responses in vegetative cells but does not affect responses in 6–7 h developed cells, suggesting that in developed cells mucolipin may help regulate local calcium signals rather than global calcium waves. We found that both knocking down and overexpressing mucolipin often, but not always, presented the same phenotypes. Altering mucolipin expression levels caused an accumulation or increased acidification of Lysosensor Blue stained vesicles in vegetative cells. Nutrient uptake by phagocytosis and macropinocytosis were increased but growth rates were not, suggesting defects in catabolism. Both increasing and decreasing mucolipin expression caused the formation of smaller slugs and larger numbers of fruiting bodies during multicellular development, suggesting that mucolipin is involved in initiation of aggregation centers. The fruiting bodies that formed from these smaller aggregates had proportionately larger basal discs and thickened stalks, consistent with a regulatory role for mucolipin-dependent Ca²⁺ signalling in the autophagic cell death pathways involved in stalk and basal disk differentiation in Dictyostelium. Thus, we have provided evidence that mucolipin contributes to chemotactic calcium signalling and that Dictyostelium is a useful model to study the molecular mechanisms involved in the cytopathogenesis of Mucolipidosis type IV.

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.

Neuropathophysiology, Genetic Profile, and Clinical Manifestation of Mucolipidosis IV-A Review and Case Series

Mucolipidosis type IV (MLIV) is an ultra-rare lysosomal storage disorder caused by biallelic mutations in MCOLN1 gene encoding the transient receptor potential channel mucolipin-1. So far, 35 pathogenic or likely pathogenic MLIV-related variants have been described. Clinical manifestations include severe intellectual disability, speech deficit, progressive visual impairment leading to blindness, and myopathy. The severity of the condition may vary, including less severe psychomotor delay and/or ocular findings. As no striking recognizable facial dysmorphism, skeletal anomalies, organomegaly, or lysosomal enzyme abnormalities in serum are common features of MLIV, the clinical diagnosis may be significantly improved because of characteristic ophthalmological anomalies. This review aims to outline the pathophysiology and genetic defects of this condition with a focus on the genotype–phenotype correlation amongst cases published in the literature. The authors will present their own clinical observations and long-term outcomes in adult MLIV cases.

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

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

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

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

Novel compound heterozygous MCOLN1 mutations identified in a Japanese girl with severe developmental delay and thin corpus callosum

Mucolipidosis type IV (MLIV) is a rare lysosomal storage disorder causing severe psychomotor developmental delay and progressive visual impairment. MLIV is an autosomal recessive disease caused by mutations in MCOLN1, which encodes for mucolipin-1. Here, we report a case of a 4-year-old Japanese girl with severe intellectual disability and motor deficits. Brain magnetic resonance imaging showed signal abnormalities in the white matter and thinning of the corpus callosum. Whole-exome sequencing was performed on the proband and her parents, and novel compound heterozygous mutations at c.936_938del (p.Phe313del) and c.1503dupC (p.Ile502Hisfs*106) in MCOLN1 (NM_020533.2) were identified in the proband. Additional biochemical examinations revealed elevated serum gastrin level and iron deficiency anemia, leading to the diagnosis of MLIV. More reports of such pathogenic mutations are expected to broaden the understanding of the channel function of mucolipin-1 and genotype-phenotype correlations.

Beyond the CRAC: Diversification of ion signaling in B cells

Although calcium signaling and the important role of calcium release–activated calcium channels is well recognized in the context of immune cell signaling, there is a vast diversity of ion channels and transporters that regulate the entry of ions beyond calcium, including magnesium, zinc, potassium, sodium, and chloride. These ions play a critical role in numerous metabolic and cellular processes. The importance of ions in human health and disease is illustrated by the identification of primary immunodeficiencies in patients with mutations in genes encoding ion channels and transporters, as well as the immunological defects observed in individuals with nutritional ion deficiencies. Despite progress in identifying the important role of ions in immune cell development and activation, we are still in the early stages of exploring the diversity of ion channels and transporters and mechanistically understanding the role of these ions in immune cell biology. Here, we review the biology of ion signaling in B cells and the identification of critical ion channels and transporters in B‐cell development, activation, and differentiation into effector cells. Elucidating the role of ion channels and transporters in immune cell signaling is critical for expanding the repertoire of potential therapeutics for the treatment of immune disorders. Moreover, increased understanding of the role of ions in immune cell function will enhance our understanding of the potentially serious consequences of ion deficiencies in human health and disease.

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.

Lysosomal Ca2+ Signaling is Essential for Osteoclastogenesis and Bone Remodeling

Lysosomal Ca²⁺ emerges as a critical component of receptor-evoked Ca²⁺ signaling and plays a crucial role in many lysosomal and physiological functions. Lysosomal Ca²⁺ release is mediated by the transient receptor potential (TRP) family member TRPML1, mutations that cause the lysosomal storage disease mucolipidosis type 4. Lysosomes play a key role in osteoclast function. However, nothing is known about the role of lysosomal Ca²⁺ signaling in osteoclastogenesis and bone metabolism. In this study, we addressed this knowledge gap by studying the role of lysosomal Ca²⁺ signaling in osteoclastogenesis, osteoclast and osteoblast functions, and bone homeostasis in vivo. We manipulated lysosomal Ca²⁺ signaling by acute knockdown of TRPML1, deletion of TRPML1 in mice, pharmacological inhibition of lysosomal Ca²⁺ influx, and depletion of lysosomal Ca²⁺ storage using the TRPML agonist ML-SA1. We found that knockdown and deletion of TRPML1, although it did not have an apparent effect on osteoblast differentiation and bone formation, markedly attenuated osteoclast function, RANKL-induced cytosolic Ca²⁺ oscillations, inhibited activation of NFATc1 and osteoclastogenesis-controlling genes, suppressed the formation of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells (MNCs), and markedly reduced the differentiation of bone marrow-derived macrophages into osteoclasts. Moreover, deletion of TRPML1 resulted in enlarged lysosomes, inhibition of lysosomal secretion, and attenuated the resorptive activity of mature osteoclasts. Notably, depletion of lysosomal Ca²⁺ with ML-SA1 similarly abrogated RANKL-induced Ca²⁺ oscillations and MNC formation. Deletion of TRPML1 in mice reduced the TRAP-positive bone surfaces and impaired bone remodeling, resulting in prominent osteopetrosis. These findings demonstrate the essential role of lysosomal Ca²⁺ signaling in osteoclast differentiation and mature osteoclast function, which play key roles in bone homeostasis. © 2016 American Society for Bone and Mineral Research.

The mucolipidosis IV Ca2+ channel TRPML1 (MCOLN1) is regulated by the TOR kinase

The exact mechanisms underlying the lysosomal storage disorder (LSD) mucolipidosis type IV (MLIV) are unclear. In the present study, we provide evidence that mTOR regulates the opening and closing of the lysosomal channel responsible for MLIV through phosphorylation.

Autophagy is a complex pathway regulated by numerous signalling events that recycles macromolecules and may be perturbed in lysosomal storage disorders (LSDs). During autophagy, aberrant regulation of the lysosomal Ca²⁺ efflux channel TRPML1 [transient receptor potential mucolipin 1 (MCOLN1)], also known as MCOLN1, is solely responsible for the human LSD mucolipidosis type IV (MLIV); however, the exact mechanisms involved in the development of the pathology of this LSD are unknown. In the present study, we provide evidence that the target of rapamycin (TOR), a nutrient-sensitive protein kinase that negatively regulates autophagy, directly targets and inactivates the TRPML1 channel and thereby functional autophagy, through phosphorylation. Further, mutating these phosphorylation sites to unphosphorylatable residues proved to block TOR regulation of the TRPML1 channel. These findings suggest a mechanism for how TOR activity may regulate the TRPML1 channel.

Mucolipidosis type IV protein TRPML1-dependent lysosome formation

Lysosomes are dynamic organelles that undergo cycles of fusion and fission with themselves and with other organelles. Following fusion with late endosomes to form hybrid organelles, lysosomes are reformed as discrete organelles. This lysosome reformation or formation is a poorly understood process that has not been systematically analyzed and that lacks known regulators. In this study, we quantitatively define the multiple steps of lysosome formation and identify the first regulator of this process.

Mucolipin co-deficiency causes accelerated endolysosomal vacuolation of enterocytes and failure-to-thrive from birth to weaning

During the suckling period, intestinal enterocytes are richly endowed with endosomes and lysosomes, which they presumably utilize for the uptake and intracellular digestion of milk proteins. By weaning, mature intestinal enterocytes replace those rich in lysosomes. We found that mouse enterocytes before weaning express high levels of two endolysosomal cation channels, mucolipins 3 and 1 -products of Trpml3 and Trpml1 genes; moreover neonatal enterocytes of mice lacking both mucolipins (Trpml3-/-;Trpml1-/-) vacuolated pathologically within hours of birth and remained so until weaning. Ultrastructurally and chemically these fast-forming vacuoles resembled those that systemically appear in epithelial cells of mucolipidosis type IV (MLIV) patients, which bear mutations in Trpml1. Hence, lack of both mucolipins 1 and 3 causes an accelerated MLIV-type of vacuolation in enterocytes. The vacuoles were aberrant hybrid organelles with both endosomal and lysosomal components, and were not generated by alterations in endocytosis or exocytosis, but likely by an imbalance between fusion of lysosomes and endosomes and their subsequent scission. However, upon extensive vacuolation enterocytes displayed reduced endocytosis from the intestinal lumen, a defect expected to compromise nutrient uptake. Mice lacking both mucolipins suffered a growth delay that began after birth and continued through the suckling period but recovered after weaning, coinciding with the developmental period of enterocyte vacuolation. Our results demonstrate genetic redundancy between lysosomal mucolipins 3 and 1 in neonatal enterocytes. Furthermore, our Trpml3-/-;Trpml1-/- mice represent a polygenic animal model of the poorly-understood, and often intractable, neonatal failure-to-thrive with intestinal pathology. Our results implicate lysosomes in neonatal intestinal pathologies, a major cause of infant mortality worldwide, and suggest transient intestinal dysfunction might affect newborns with lysosomal storage disorders. Finally, we conclude that mucolipin-endowed lysosomes in the young play an evolutionarily-conserved role in the intracellular digestion of maternally-provided nutrients, whether milk in mammals or yolk in oviparous species.

The role of TRPMLs in endolysosomal trafficking and function

Members of the Transient Receptor Potential-Mucolipin (TRPML) constitute a family of evolutionarily conserved cation channels that function predominantly in endolysosomal vesicles. Whereas loss-of-function mutations in human TRPML1 were first identified as being causative for the lysosomal storage disease, Mucolipidosis type IV, most mammals also express two other TRPML isoforms called TRPML2 and TRPML3. All three mammalian TRPMLs as well as TRPML related genes in other species including Caenorhabditis elegans and Drosophila exhibit overlapping functional and biophysical properties. The functions of TRPML proteins include roles in vesicular trafficking and biogenesis, maintenance of neuronal development, function, and viability, and regulation of intracellular and organellar ionic homeostasis. Biophysically, TRPML channels are non-selective cation channels exhibiting variable permeability to a host of cations including Na(+), Ca(2+), Fe(2+), and Zn(2+), and are activated by a phosphoinositide species, PI(3,5)P2, that is mostly found in endolysosomal membranes. Here, we review the functional and biophysical properties of these enigmatic cation channels, which represent the most ancient and archetypical TRP channels.

Small regulators, major consequences - Ca²⁺ and cholesterol at the endosome-ER interface

The ER is the largest cellular compartment and a major storage site for lipids and ions. In recent years, much attention has focused on contacts between the ER and other organelles, and one particularly intimate relationship is that between the ER and the endosomal system. ER-endosome contacts intensify when endosomes mature, and the ER participates in endosomal processes, such as the termination of surface receptor signaling, multi-vesicular body formation, and transport and fusion events. Cholesterol and Ca(2+) are transferred between the ER and endosomes, possibly acting as messengers for ER-endosome crosstalk. Here, we summarize different types of ER-endosomal communication and discuss membrane contact sites that might facilitate this crosstalk. We review the protein pairs that interact at the ER-endosome interface and find that many of these have a role in cholesterol exchange. We also summarize Ca(2+) exchange between the ER and endosomes, and hypothesize that ER-endosome contacts integrate several cellular functions to guide endosomal maturation. We post the hypothesis that failure in ER-endosome contacts is an unrecognized but important contributor to diseases, such as Niemann-Pick type C disease, Alzheimer's disease and amyotrophic lateral sclerosis.

Systematic screens for proteins that interact with the mucolipidosis type IV protein TRPML1

Mucolipidosis type IV is a lysosomal storage disorder resulting from mutations in the MCOLN1 gene, which encodes the endosomal/lysosomal Transient Receptor Potential channel protein mucolipin-1/TRPML1. Cells isolated from Mucolipidosis type IV patients and grown in vitro and in in vivo models of this disease both show several lysosome-associated defects. However, it is still unclear how TRPML1 regulates the transport steps implicated by these defects. Identifying proteins that associate with TRPML1 will facilitate the elucidation of its cellular and biochemical functions. We report here two saturation screens for proteins that interact with TRPML1: one that is based on immunoprecipitation/mass spectrometry and the other using a genetic yeast two-hybrid approach. From these screens, we identified largely non-overlapping proteins, which represent potential TRPML1-interactors., Using additional interaction assays on some of the potential interactors from each screen, we validated some proteins as candidate TRPML1 interactors In addition, our analysis indicates that each of the two screens not only identified some false-positive interactors, as expected from any screen, but also failed to uncover potential TRPML1 interactors. Future studies on the true interactors, first identified in these screens, will help elucidate the structure and function of protein complexes containing TRPML1.

Multilevel regulation of autophagosome content by ethanol oxidation in HepG2 cells

Acute and chronic ethanol administration increase autophagic vacuole (i.e., autophagosome; AV) content in liver cells. This enhancement depends on ethanol oxidation. Here, we used parental (nonmetabolizing) and recombinant (ethanol-metabolizing) Hep G2 cells to identify the ethanol metabolite that causes AV enhancement by quantifying AVs or their marker protein, microtubule-associated protein 1 light chain 3-II (LC3-II). The ethanol-elicited rise in LC3-II was dependent on ethanol dose, was seen only in cells that expressed alcohol dehydrogenase (ADH) and was augmented in cells that coexpressed cytochrome CYP2E1 (P450 2E1). Furthermore, the rise in LC3-II was inversely related to a decline in proteasome activity. AV flux measurements and colocalization of AVs with lysosomes or their marker protein Lysosomal-Associated Membrane Protein 1 (LAMP1) in ethanol-metabolizing VL-17A cells (ADH (+) /CYP2E1 (+) ) revealed that ethanol exposure not only enhanced LC3-II synthesis but also decreased its degradation. Ethanol-induced accumulation of LC3-II in these cells was similar to that induced by the microtubule inhibitor, nocodazole. After we treated cells with either 4-methylpyrazole to block ethanol oxidation or GSH-EE to scavenge reactive species, there was no enhancement of LC3-II by ethanol. Furthermore, regardless of their ethanol-metabolizing capacity, direct exposure of cells to acetaldehyde enhanced LC3-II content. We conclude that both ADH-generated acetaldehyde and CYP2E1-generated primary and secondary oxidants caused LC3-II accumulation, which rose not only from enhanced AV biogenesis, but also from decreased LC3 degradation by the proteasome and by lysosomes.

Reconstitution of lysosomal NAADP-TRP-ML1 signaling pathway and its function in TRP-ML1(-/-) cells

It is well known that the mutation of TRP-ML1 (transient receptor potential-mucolipin-1) causes mucolipidosis IV, a lysosomal storage disease. Given that lysosomal nicotinic acid adenine dinucleotide phosphate (NAADP)-Ca(2+) release channel activity is associated with TRP-ML1, the present study was designed to test the hypothesis that NAADP regulates lysosome function via activation of TRP-ML1 channel activity. Using lysosomal preparations from wild-type (TRP-ML1(+/+)) human fibroblasts, channel reconstitution experiments demonstrated that NAADP (0.01-1.0 μM) produced a concentration-dependent increase in TRP-ML1 channel activity. This NAADP-induced activation of TRP-ML1 channels could not be observed in lysosomes from TRP-ML1(-/-) cells, but was restored by introducing a TRP-ML1 transgene into these cells. Microscopic Ca(2+) fluorescence imaging showed that NAADP significantly increased intracellular Ca(2+) concentration to 302.4 ± 74.28 nM (vs. 180 ± 44.13 nM of the basal) in TRP-ML1(+/+) cells, but it had no effect in TRP-ML1(-/-) cells. If a TRP-ML1 gene was transfected into TRP-ML1(-/-) cells, the Ca(2+) response to NAADP was restored to the level comparable to TRP-ML1(+/+) cells. Functionally, confocal microscopy revealed that NAADP significantly enhanced the dynamic interaction of endosomes and lysosomes and the lipid delivery to lysosomes in TRP-ML1(+/+) cells. This functional action of NAADP was abolished in TRP-ML1(-/-) cells, but restored after TRP-ML1 gene was rescued in these cells. Our results suggest that NAADP increases lysosomal TRP-ML1 channel activity to release Ca(2+), which promotes the interaction of endosomes and lysosomes and thereby regulates lipid transport to lysosomes. Failure of NAADP-TRP-ML1 signaling may be one of the important mechanisms resulting in intracellular lipid trafficking disorder and consequent mucolipidosis.

The cation channel mucolipin-1 is a bifunctional protein that facilitates membrane remodeling via its serine lipase domain

Phospholipase modulators have been shown to affect the topology of lipid bilayers and the formation of tubulo-vesicular structures, but the specific endogenous phospholipases involved have yet to be identified. Here we show that TRPML1 (MLN1), a Ca(2+)-permeable channel, contributes to membrane remodeling through a serine lipase consensus domain, and thus represents a novel type of bifunctional protein. Remarkably, this serine lipase active site determines the ability of MLN1 to generate tubulo-vesicular extensions in mucolipin-1-expressing oocytes, human fibroblasts and model membrane vesicles. Our demonstration that MLN1 is involved in membrane remodeling and the formation of extensions suggests that it may play a role in the formation of cellular processes linked to the late endosome/lysosome (LE/L) pathway. MLN1 is absent or mutated in patients with mucolipidosis IV (MLIV), a lysosomal disorder with devastating neurological and other consequences. This study provides potential insight into the pathophysiology of MLIV.

International Union of Basic and Clinical Pharmacology. LXXVI. Current progress in the mammalian TRP ion channel family

Transient receptor potential (TRP) channels are a large family of ion channel proteins, surpassed in number in mammals only by voltage-gated potassium channels. TRP channels are activated and regulated through strikingly diverse mechanisms, making them suitable candidates for cellular sensors. They respond to environmental stimuli such as temperature, pH, osmolarity, pheromones, taste, and plant compounds, and intracellular stimuli such as Ca(2+) and phosphatidylinositol signal transduction pathways. However, it is still largely unknown how TRP channels are activated in vivo. Despite the uncertainties, emerging evidence using TRP channel knockout mice indicates that these channels have broad function in physiology. Here we review the recent progress on the physiology, pharmacology and pathophysiological function of mammalian TRP channels.

Mucolipidosis type IV and the mucolipins

MLIV (mucolipidosis type IV) is a neurodegenerative lysosomal storage disorder caused by mutations in MCOLN1, a gene that encodes TRPML1 (mucolipin-1), a member of the TRPML (transient receptor potential mucolipin) cation channels. Two additional homologues are TRPML2 and TRPML3 comprising the TRPML subgroup in the TRP superfamily. The three proteins play apparently key roles along the endocytosis process, and thus their cellular localization varies among the different group members. Thus TRPML1 is localized exclusively to late endosomes and lysosomes, TRPML2 is primarily located in the recycling clathrin-independent GPI (glycosylphosphatidylinositol)-anchored proteins and early endosomes, and TRPML3 is primarily located in early endosomes. Apparently, all three proteins' main physiological function underlies Ca2+ channelling, regulating the endocytosis process. Recent findings also indicate that the three TRPML proteins form heteromeric complexes at least in some of their cellular content. The physiological role of these complexes in lysosomal function remains to be elucidated, as well as their effect on the pathophysiology of MLIV. Another open question is whether any one of the TRPMLs bears additional function in channel activity

Mucolipidosis type IV: the effect of increased lysosomal pH on the abnormal lysosomal storage

Mucolipidosis type IV (MLIV) is a neurodegenerative channelopathy that is caused by the deficiency of TRPML1 activity, a nonselective cation channel. TRPML1 is a lysosomal membrane protein, and thus, MLIV is a lysosomal storage disorder. The basic, specific function of TRPML1 has not been yet clarified. A recent report (Soyombo AA, Tjon-Kon-Sang S, Rbaibi Y, Bashllari E, Bisceglia J, Muallem S, Kiselyov K: J Biol Chem 281:7294-7301, 2006) indicated that TRPML1 functions as an outwardly proton channel whose function is the prevention of overacidification of these organelles. Thus, in MLIV the lysosomal pH is lower than normal. Furthermore, attempts by these investigators to increase slightly the lysososmal pH with either Nigericin or Chloroquine suggested corrective effect of the abnormal storage in MLIV cells. We investigated this approach using these agents with cultured fibroblasts from severely affected and milder patients. Our data indicated that there was no reduction in the total number of storage vesicles by either agent, although Nigericin resulted in a change in the nature of the storage materials, reducing the presence of lamellated substances (lipids) so that the storage vesicles contained predominantly granulated substances. On the other hand, transfection with the normal MCOLN1 cDNA (the gene coding for TRPML1) resulted in the removal of almost all the storage materials.

Neuropathology of the Mcoln1(-/-) knockout mouse model of mucolipidosis type IV

The recently developed Mcoln1(-/-) knockout mouse provides a novel model for analyzing mucolipin 1 function and mucolipidosis type IV disease. Here we characterize the neuropathology of Mcoln1(-/-) mouse at the end stage. Evidence of ganglioside accumulation, including increases in GM2, GM3, and GD3 and redistribution of GM1, was found throughout the central nervous system (CNS) independent of significant cholesterol accumulation. Unexpectedly, colocalization studies using immunofluorescence confocal microscopy revealed that GM1 and GM2 were present in separate vesicles within individual neurons. While GM2 was significantly colocalized with LAMP2, consistent with late-endosomal/lysosomal processing, some GM2-immunoreactivity occurred in LAMP2-negative sites, suggesting involvement of other vesicular systems. P62/Sequestosome 1 (P62/SQSTM1) inclusions were also identified in the CNS of the Mcoln1(-/-) mouse, suggesting deficiencies in protein degradation. Glial cell activation was increased in brain, and there was evidence of reduced myelination in cerebral and cerebellar white matter tracts. Autofluorescent material accumulated throughout the brains of the knockout mice. Finally, axonal spheroids were prevalent in white matter tracts and Purkinje cell axons. This neuropathological characterization of the Mcoln1(-/-) mouse provides an important step in understanding how mucolipin 1 loss of function affects the CNS and contributes to mucolipidosis type IV disease.

Membrane traffic and turnover in TRP-ML1-deficient cells: a revised model for mucolipidosis type IV pathogenesis

The lysosomal storage disorder mucolipidosis type IV (MLIV) is caused by mutations in the transient receptor potential-mucolipin-1 (TRP-ML1) ion channel. The "biogenesis" model for MLIV pathogenesis suggests that TRP-ML1 modulates postendocytic delivery to lysosomes by regulating interactions between late endosomes and lysosomes. This model is based on observed lipid trafficking delays in MLIV patient fibroblasts. Because membrane traffic aberrations may be secondary to lipid buildup in chronically TRP-ML1-deficient cells, we depleted TRP-ML1 in HeLa cells using small interfering RNA and examined the effects on cell morphology and postendocytic traffic. TRP-ML1 knockdown induced gradual accumulation of membranous inclusions and, thus, represents a good model in which to examine the direct effects of acute TRP-ML1 deficiency on membrane traffic. Ratiometric imaging revealed decreased lysosomal pH in TRP-ML1-deficient cells, suggesting a disruption in lysosomal function. Nevertheless, we found no effect of TRP-ML1 knockdown on the kinetics of protein or lipid delivery to lysosomes. In contrast, by comparing degradation kinetics of low density lipoprotein constituents, we confirmed a selective defect in cholesterol but not apolipoprotein B hydrolysis in MLIV fibroblasts. We hypothesize that the effects of TRP-ML1 loss on hydrolytic activity have a cumulative effect on lysosome function, resulting in a lag between TRP-ML1 loss and full manifestation of MLIV.

Gene expression profiling of mucolipidosis type IV fibroblasts reveals deregulation of genes with relevant functions in lysosome physiology

Mucolipidosis type IV (MLIV, MIM 252650) is an autosomal recessive lysosomal storage disorder that causes mental and motor retardation as well as visual impairment. The lysosomal storage defect in MLIV is consistent with abnormalities of membrane traffic and organelle dynamics in the late endocytic pathway. MLIV is caused by mutations in the MCOLN1 gene, which codes for mucolipin-1 (MLN1), a member of the large family of transient receptor potential (TRP) cation channels. Although a number of studies have been performed on mucolipin-1, the pathological mechanisms underlying MLIV are not fully understood. To identify genes that characterize pathogenic changes in mucolipidosis type IV, we compared the expression profiles of three MLIV and three normal skin fibroblasts cell lines using oligonucleotide microarrays. Genes that were differentially expressed in patients' cells were identified. 231 genes were up-regulated, and 116 down-regulated. Real-Time RT-PCR performed on selected genes in six independent MLIV fibroblasts cell lines was generally consistent with the microarray findings. This study allowed to evidence the modulation at the transcriptional level of a discrete number of genes relevant in biological processes which are altered in the disease such as endosome/lysosome trafficking, lysosome biogenesis, organelle acidification and lipid metabolism.

Lysosomal trafficking functions of mucolipin-1 in murine macrophages

BACKGROUND

Mucolipidosis Type IV is currently characterized as a lysosomal storage disorder with defects that include corneal clouding, achlorhydria and psychomotor retardation. MCOLN1, the gene responsible for this disease, encodes the protein mucolipin-1 that belongs to the "Transient Receptor Potential" family of proteins and has been shown to function as a non-selective cation channel whose activity is modulated by pH. Two cell biological defects that have been described in MLIV fibroblasts are a hyperacidification of lysosomes and a delay in the exit of lipids from lysosomes.

RESULTS

We show that mucolipin-1 localizes to lysosomal compartments in RAW264.7 mouse macrophages that show subcompartmental accumulations of endocytosed molecules. Using stable RNAi clones, we show that mucolipin-1 is required for the exit of lipids from these compartments, for the transport of endocytosed molecules to terminal lysosomes, and for the transport of the Major Histocompatibility Complex II to the plasma membrane.

CONCLUSION

Mucolipin-1 functions in the efficient exit of molecules, destined for various cellular organelles, from lysosomal compartments.

Neurologic, gastric, and opthalmologic pathologies in a murine model of mucolipidosis type IV

Mucolipidosis type IV (MLIV) is an autosomal recessive lysosomal storage disorder caused by mutations in the MCOLN1 gene, which encodes the 65-kDa protein mucolipin-1. The most common clinical features of patients with MLIV include severe mental retardation, delayed motor milestones, ophthalmologic abnormalities, constitutive achlorhydria, and elevated plasma gastrin levels. Here, we describe the first murine model for MLIV, which accurately replicates the phenotype of patients with MLIV. The Mcoln1(-/-) mice present with numerous dense inclusion bodies in all cell types in brain and particularly in neurons, elevated plasma gastrin, vacuolization in parietal cells, and retinal degeneration. Neurobehavioral assessments, including analysis of gait and clasping, confirm the presence of a neurological defect. Gait deficits progress to complete hind-limb paralysis and death at age ~8 mo. The Mcoln1(-/-) mice are born in Mendelian ratios, and both male and female Mcoln1(-/-) mice are fertile and can breed to produce progeny. The creation of the first murine model for human MLIV provides an excellent system for elucidating disease pathogenesis. In addition, this model provides an invaluable resource for testing treatment strategies and potential therapies aimed at preventing or ameliorating the abnormal lysosomal storage in this devastating neurological disorder.

TRPML and lysosomal function

Mucolipin 1 (MLN1), also known as TRPML1, is a member of the mucolipin family. The mucolipins are the only lysosomal proteins within the TRP superfamily. Mutations in the gene coding for TRPML1 result in a lysosomal storage disorder (LSD). This review summarizes the current knowledge related to this protein and the rest of the mucolipin family.

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

Mucolipin-1 is a membrane protein encoded by the gene MCOLN1, mutations in which result in the lysosomal storage disorder mucolipidosis type IV (MLIV). Efficient lysosomal targeting of mucolipin-1 requires di-leucine motifs in both the N-terminal and the C-terminal cytosolic tails. We have shown that aberrant lactosylceramide trafficking in MLIV cells may be rescued by wild-type mucolipin-1 expression but not by mucolipin-1 mistargeted to the plasma membrane or by lysosome-localized mucolipin-1 mutated in its predicted ion pore-selectivity region. Our data demonstrate that the correct localization of mucolipin-1 and the integrity of its ion pore are essential for its physiological function in the late endocytic pathway.

Mucolipidosis IV: report of a case with ocular restricted phenotype caused by leaky splice mutation

PURPOSE

To confirm and define a molecular basis for a case of mucolipidosis type IV (ML IV) with an extremely atypical phenotype pattern.

DESIGN

Observational case report of a patient with ML IV with disease progression restricted to ocular symptoms.

METHODS

Complete ophthalmologic and neurologic examination. Ultrastructural examination of white blood cells, skin, conjunctiva, and corneal epithelium. The MCOLN1 gene was sequenced from cDNA and the proportion of splicing variants were assessed by quantitative allele-specific polymerase chain reaction.

RESULTS

Absence of any neurological abnormalities. Retinal pathologic features were the main cause of visual disability: low visual acuity and cloudy corneas since 2 years of age, progressive decrease in visual acuity since the age of 9 years. Ultrastructural examination showed storage lysosomes filled with either concentric membranes or lucent precipitate in corneal and conjunctive epithelia and in vascular endothelium. Cultured fibroblasts were free of any autofluorescence. Sequencing of the MCOLN1 gene identified compound heterozygosity for D362Y and A-->T transition leading to the creation of a novel donor splicing site and a 4-bp deletion from exon 13 at the mRNA level. Both normal and pathologic splice forms were detected in skin fibroblasts and leukocytes, with the normal form being more abundant.

CONCLUSIONS

The case of this patient with ML IV is unique and is characterized by a curious lack of generalized symptoms. In this patient, the disorder was limited to the eyes and appeared without the usual psychomotor deterioration. The resulting phenotype is the mildest seen to date.

Lysosomal exocytosis is impaired in mucolipidosis type IV

Mucolipidosis type IV (MLIV) is an autosomal recessive disease characterized by severe neurological impairment, ophthalmologic defects, and gastric dysfunction. MLIV cells have a deficiency in the late endosomal/lysosomal (LEL) pathway that results in the buildup of lysosomal inclusions. Using a Xenopus oocyte expression system, we previously showed that mucolipin-1 (MLN1), the protein encoded by the MCOLN1 gene is a Ca2+ -permeable non-selective cation channel that is transiently modulated by elevations in intracellular Ca2+. We further showed that MLN1 is translocated to the plasma membrane during lysosomal exocytosis. In this study we show that lysosomal exocytosis is impaired in fibroblasts from MLIV patients, indicating that MLN1 plays an active role in this process. Further, we show that transfection with wild type MLN1 cDNA rescues exocytosis, suggesting the possibility of treatments based on the restoration of this crucial cellular function.

TRPML cation channels regulate the specialized lysosomal compartment of vertebrate B-lymphocytes

B-lymphocytes possess a specialized lysosomal compartment, the regulated transformation of which has been implicated in B-cell antigen presentation. Members of the mucolipin (TRPML) family of cation channels have been implicated in regulated vesicular transport in several tissues, but a role for TRPML function in lymphocyte vesicular transport physiology has not been previously described. To address the role of TRPML proteins in lymphocyte vesicular transport, we analyzed the lysosomal compartment in cultured B-lymphocytes engineered to lack TRPML1 or after expression of N- or C-terminal GFP fusion proteins of TRPML1 or TRPML2. Consistent with previous analyses of lymphocytes derived from human patients with mutations in TRPML1, we were not able to detect abnormalities in the lysosomes of TRPML1-deficient DT40 B-lymphocytes. However, while N-terminal GFP fusions of TRPML2 localized to normal appearing lysosomes, C-terminal GFP fusions of either TRPML1 or TRPML2 acted to antagonize endogenous TRPML function, localizing to large vesicular structures, the histological properties of which were indistinguishable from the enlarged lysosomes observed in affected tissues of TRPML1-deficient humans. Endocytosed B-cell receptors were delivered to these enlarged lysosomes, demonstrating that a TRPML-dependent process is required for normal regulation of the specialized lysosome compartment of vertebrate B-lymphocytes.

Suppression of thecup-5mucolipidosis type IV-related lysosomal dysfunction by the inactivation of an ABC transporter inC. elegans

Mutations in MCOLN1, which encodes the protein mucolipin 1, result in the lysosomal storage disease mucolipidosis Type IV. Studies on human mucolipin 1 and on CUP-5, the Caenorhabditis elegans ortholog of mucolipin 1, have shown that these proteins are required for lysosome biogenesis/function. Loss of CUP-5 results in a defect in lysosomal degradation, leading to embryonic lethality. We have identified a mutation in the ABC transporter MRP-4 that rescues the degradation defect and the corresponding lethality, owing to the absence of CUP-5. MRP-4 localizes to endocytic compartments and its levels are elevated in the absence of CUP-5. These results indicate that the lysosomal degradation defect is exacerbated in some cells because of the accumulation of MRP-4 in lysosomes rather than the loss of CUP-5 per se. We also show that under some conditions, loss of MRP-4 rescues the embryonic lethality caused by the loss of the cathepsin L protease, indicating that the accumulation of ABC transporters may be a more general mechanism whereby an initial lysosomal dysfunction is more severely compromised.

Posttranslational cleavage and adaptor protein complex-dependent trafficking of mucolipin-1

Mucolipin-1 (ML1) is a member of the transient receptor potential ion channel superfamily that is thought to function in the biogenesis of lysosomes. Mutations in ML1 result in mucolipidosis type IV, a lysosomal storage disease characterized by the intracellular accumulation of enlarged vacuolar structures containing phospholipids, sphingolipids, and mucopolysaccharides. Little is known about how ML1 trafficking or activity is regulated. Here we have examined the processing and trafficking of ML1 in a variety of cell types. We find that a significant fraction of ML1 undergoes cell type-independent cleavage within the first extracellular loop of the protein during a late step in its biosynthetic delivery. To determine the trafficking route of ML1, we systematically examined the effect of ablating adaptor protein complexes on the localization of this protein. Whereas ML1 trafficking was not apparently affected in fibroblasts from mocha mice that lack functional adaptor protein complex (AP)-3, small interfering RNA-mediated knockdown revealed a requirement for AP-1 in Golgi export of ML1. Knockdown of functional AP-2 had no effect on ML1 localization. Interestingly, cleavage of ML1 was not compromised in AP-1-deficient cells, suggesting that proteolysis occurs in a prelysosomal compartment, possibly the trans-Golgi network. Our results suggest that posttranslational processing of ML1 is more complex than previously described and that this protein is delivered to lysosomes primarily via an AP-1-dependent route that does not involve passage via the cell surface.

TRP-ML1 regulates lysosomal pH and acidic lysosomal lipid hydrolytic activity

Mucolipidosis type IV (MLIV) is caused by mutations in the ion channel mucolipin 1 (TRP-ML1). MLIV is typified by accumulation of lipids and membranous materials in intracellular organelles, which was hypothesized to be caused by the altered membrane fusion and fission events. How mutations in TRP-ML1 lead to aberrant lipolysis is not known. Here we present evidence that MLIV is a metabolic disorder that is not associated with aberrant membrane fusion/fission events. Thus, measurement of lysosomal pH revealed that the lysosomes in TRP-ML1(-/-) cells obtained from the patients with MLIV are over-acidified. TRP-ML1 can function as a H(+) channel, and the increased lysosomal acidification in TRP-ML1(-/-) cells is likely caused by the loss of TRP-ML1-mediated H(+) leak. Measurement of lipase activity using several substrates revealed a marked reduction in lipid hydrolysis in TRP-ML1(-/-) cells, which was rescued by the expression of TRP-ML1. Cell fractionation indicated specific loss of acidic lipase activity in TRP-ML1(-/-) cells. Furthermore, dissipation of the acidic lysosomal pH of TRP-ML1(-/-) cells by nigericin or chloroquine reversed the lysosomal storage disease phenotype. These findings provide a new mechanism to account for the pathogenesis of MLIV.

Cation channel activity of mucolipin-1: the effect of calcium

Mucolipidosis type IV (MLIV) is a rare, neurogenetic disorder characterized by developmental abnormalities of the brain, and impaired neurological, ophthalmological, and gastric function. Considered a lysosomal disease, MLIV is characterized by the accumulation of large vacuoles in various cell types. Recent evidence indicates that MLIV is caused by mutations in MCOLN1, the gene that encodes mucolipin-1 (ML1), a 65-kDa protein showing sequence homology and topological similarities with polycystin-2 and other transient receptor potential (TRP) channels. In this report, our observations on the channel properties of ML1, and molecular pathophysiology of MLIV are reviewed and expanded. Our studies have shown that ML1 is a multiple sub-conductance, non-selective cation channel. MLIV-causing mutations result in functional differences in the channel protein. In particular, the V446L and DeltaF408 mutations retain channel function but have interesting functional differences with regards to pH dependence and Ca(2+) transport. While the wild-type protein is inhibited by Ca(2+) transport, mutant ML1 is not. Atomic force microscopy imaging of ML1 channels shows that changes in pH modify the aggregation and size of the ML1 channels, which has an impact on vesicular fusogenesis. The new evidence provides support for a novel role of ML1 cation channels in vesicular acidification and normal endosomal function.

Mucolipin 1: endocytosis and cation channel--a review

Mucolipidosis type IV (MLIV) is a neurodegenerative, recessive, lysosomal storage disorder characterized by psychomotor retardation and visual impairment due to various ophthalmologic abnormalities. MLIV is found in relatively high frequency in the Ashkenazi Jewish population. The disease is caused by mutations in the gene MCOLN1, which encodes the protein mucolipin 1 (MLN1), a member of the mucolipins family. MLN1 is a non-specific cation channel, and its putative structure attributes it to the TRP superfamily; thus, the gene is also referred as TRPML1. Over 16 MLIV-causing mutations, including two founder mutations in the Ashkenazi population, have been identified hitherto. Atypical increased lysosomal storage in MLIV is present in the cells of all patients. This accumulation is caused by an abnormal endocytosis process of the membrane components to late endosomes to the lysosomes, resulting in an apparent block in the traffic process in pre-lysosomal vacuoles with intraluminal pH of >5.0. MLN1 was localized in cultured cells to late endosomes and lysosomes. The exact function of this cation channel in the late stages of lysosomal maintenance is currently under study.

Functional links between mucolipin-1 and Ca2+-dependent membrane trafficking in mucolipidosis IV

Most of the membrane trafficking phenomena including those involving the interactions between endosomes and lysosomes are regulated by changes in intracellular Ca2+ (Cai). These processes are disturbed in some types of mucolipidoses and other lysosomal storage disorders, such as mucolipidosis IV (MLIV), a neurological disorder that usually presents during the first year of life with blindness, cognitive impairment, and psychomotor delays. It is caused by mutations in MCOLN1, the gene encoding mucolipin-1 (MLN1), which we have recently established to represent a Ca2+-permeable cation channel that is transiently modulated by changes in Cai. The cells of MLIV patients contain enlarged lysosomes that are likely associated with abnormal sorting and trafficking of these and related organelles. We studied fibroblasts from MLIV patients and found disturbed Ca2+ signaling and large acidic organelles such as late endosomes and lysosomes (LEL) with altered cellular localization in these cells. The fusion between LEL vesicles in these cells was defective. This is a Ca2+-dependent process related to signaling pathways involved in regulation of Ca2+ homeostasis and trafficking. The MLN1 channels could play a key role in Ca2+ release from LEL vesicles, which triggers the fusion and trafficking of these organelles. The characterization of this MLN1-mediated Ca2+-dependent process should provide new insights into the pathophysiological mechanisms that lead to the development of MLIV and other mucolipidoses associated with similar disturbances in membrane trafficking.

Mucolipidosis type IV a rare genetic disorder: new addition to the Ashkenazi Jewish panel

Mucolipidosis type IV (MLIV) is a rare genetic disorder that primarily affects persons of Ashkenazi Jewish descent. Current information available about testing options and the Ashkenazi Jewish Screening panel, including the addition of screening for MLIV, is presented. The importance of genetic screening and counseling is emphasized.

Overexpression of wild-type and mutant mucolipin proteins in mammalian cells: effects on the late endocytic compartment organization

Mucolipin-1 is a 65-kDa membrane protein encoded by the MCOLN1 gene, which is mutated in patients with mucolipidosis type IV (MLIV), a rare neurodegenerative lysosomal storage disorder. We studied the subcellular localization of wild-type and three different mutant forms (T232P, F408del and F465L) of mucolipin by expressing Myc-tagged proteins in HeLa cells. The overexpressed wild-type mucolipin colocalizes to late endocytic structures and induces an aberrant distribution of these compartments. F408del and F465L MLIV mutant proteins show a distribution similar to the wild-type protein, whereas T232P is retained in the endoplasmic reticulum. Among the mutants, only F408del induces a redistribution of the late endocytic compartment. These findings suggest that the overexpression of the mucolipin cation channel influences the dynamic equilibrium of late endocytic compartments.

Caenorhabditis elegans functional orthologue of human protein h-mucolipin-1 is required for lysosome biogenesis

Mucolipidosis type IV (MLIV) is an autosomal recessive lysosomal storage disease characterized by severe psychomotor retardation, achlorhydria, and ophthalmological abnormalities. Cells from several tissues in MLIV patients accumulate large vacuoles that are presumed to be lysosomes, but whose exact nature remains to be determined. Other defects include the deterioration of neuronal integrity in the retina and the cerebellum. MCOLN1, the gene mutated in MLIV patients, encodes a protein called h-mucolipin-1 that has six predicted transmembrane domains and functions as a Ca(2+)-permeable channel that is modulated by changes in Ca2+ concentration. CUP-5 is the Caenorhabditis elegans functional orthologue of h-mucolipin-1. Mutations in cup-5 result in the accumulation of large vacuoles in several cells, in increased cell death, and in embryonic lethality. We demonstrate here that CUP-5 functions in the biogenesis of lysosomes originating from hybrid organelles. We also show that at least two h-mucolipin family members rescue cup-5 mutant endocytic defects, indicating that there may be functional redundancy among the human proteins. Finally, we propose a model that relates the lysosome biogenesis defect in the absence of CUP-5/h-mucolipin-1 to cellular phenotypes in worms and in humans.

Identification and characterization of the single channel function of human mucolipin-1 implicated in mucolipidosis type IV, a disorder affecting the lysosomal pathway

Mucolipin-1 (MLN1) is a membrane protein with homology to the transient receptor potential channels and other non-selective cation channels. It is encoded by the MCOLN1 gene, which is mutated in patients with mucolipidosis type IV (MLIV), an autosomal recessive disease that is characterized by severe abnormalities in neurological development as well as by ophthalmologic defects. At the cellular level, MLIV is associated with abnormal lysosomal sorting and trafficking. Here we identify the channel function of human MLN1 and characterize its properties. MLN1 represents a novel Ca(2+)-permeable channel that is transiently modulated by changes in [Ca(2+)]. It is also permeable to Na(+) and K(+). Large unitary conductances were measured in the presence of these cations. With its Ca(2+) permeability and modulation by [Ca(2+)], MLN1 could play a major role in Ca(2+) transport regulating lysosomal exocytosis and potentially other phenomena related to the trafficking of late endosomes and lysosomes.

Mutations in Mcoln3 associated with deafness and pigmentation defects in varitint-waddler (Va) mice

Deafness in spontaneously occurring mouse mutants is often associated with defects in cochlea sensory hair cells, opening an avenue to systematically identify genes critical for hair cell structure and function. The classical semidominant mouse mutant varitint-waddler (Va) exhibits early-onset hearing loss, vestibular defects, pigmentation abnormalities, and perinatal lethality. A second allele, Va(J), which arose in a cross segregating for Va, shows a less severe phenotype. By using a positional cloning strategy, we identify two additional members of the mucolipin gene family (Mcoln2 and Mcoln3) in the 350-kb Va(J) minimal interval and provide evidence for Mcoln3 as the gene mutated in varitint-waddler. Mcoln3 encodes a putative six-transmembrane-domain protein with sequence and motif similarities to the family of nonselective transient-receptor-potential (TRP) ion channels. In the Va allele an Ala419Pro substitution occurs in the fifth transmembrane domain of Mcoln3, and in Va(J), a second sequence alteration (Ile362Thr) occurring in cis partially rescues the Va allele. Mcoln3 localizes to cytoplasmic compartments of hair cells and plasma membrane of stereocilia. Hair cell defects are apparent by embryonic day 17.5, assigning Mcoln3 an essential role during early hair cell maturation. Our data suggest that Mcoln3 is involved in ion homeostasis and acts cell-autonomously. Hence, we identify a molecular link between hair cell physiology and melanocyte function. Last, MCOLN2 and MCOLN3 are candidate genes for hereditary and/or sporadic forms of neurosensory disorders in humans.

The Caenorhabditis elegans mucolipin-like gene cup-5 is essential for viability and regulates lysosomes in multiple cell types

The misregulation of programmed cell death, or apoptosis, contributes to the pathogenesis of many diseases. We used Nomarski microscopy to screen for mutants containing refractile cell corpses in a C. elegans strain in which all programmed cell death is blocked and such corpses are absent. We isolated a mutant strain that accumulates refractile bodies resembling irregular cell corpses. We rescued this mutant phenotype with the C. elegans mucolipidosis type IV (ML-IV) homolog, the recently identified cup-5 (coelomocyte-uptake defective) gene. ML-IV is a human autosomal recessive lysosomal storage disease characterized by psychomotor retardation and ophthalmological abnormalities. Our null mutations in cup-5 cause maternal-effect lethality. In addition, cup-5 mutants contain excess lysosomes in many and possibly all cell types and contain lamellar structures similar to those observed in ML-IV cell lines. The human ML-IV gene is capable of rescuing both the maternal-effect lethality and the lysosome-accumulation abnormality of cup-5 mutants. cup-5 mutants seem to contain excess apoptotic cells as detected by staining with terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling. We suggest that the increased apoptosis seen in cup-5 mutants is a secondary consequence of the lysosomal defect, and that abnormalities in apoptosis may be associated with human lysosomal storage disorders.

Carrier screening for mucolipidosis type IV in the American Ashkenazi Jewish population

Mutations in the MCOLN1 gene cause mucolipidosis type IV (MLIV), a severely debilitating, autosomal recessive, lysosomal storage disorder. Approximately 80% of patients with MLIV are of Ashkenazi Jewish (AJ) descent, and two mutations, IVS3-2A-->G and 511del6434, account for >95% of the mutant alleles in this population. To determine the carrier frequencies of these two mutations, 2,029 anonymous, unrelated, unaffected AJ individuals from the greater New York metropolitan area were screened. A multiplex PCR method coupled with allele-specific oligonucleotide hybridization was developed, to enable large-scale screening. The frequencies of the IVS3-2A-->G and 511del6434 mutations were 0.54% and 0.25%, respectively, for a combined carrier frequency of 0.79%, or 1 in 127 individuals (95% CI 0.40%-1.17%). The addition of both AJ mutations causing this neurodegenerative disorder should be considered for prenatal carrier screening in this population.

Rapid detection of the two common mutations in Ashkenazi Jewish patients with mucolipidosis type IV

Among Ashkenazi Jewish individuals with mucolipidosis IV (ML IV), two mutations in the ML IV gene, IVS3-1A --> G and delEX1-EX7, account for more than 95% of disease alleles. The reported method of genotyping for the delEX1-EX7 mutation involves a cumbersome multistep procedure. In the present study, a new simplified one-step procedure is described that detects this mutation in both patients and carriers. An improved procedure is also described for detection of the IVS3-1A --> G mutation. Using these improved procedures, we have characterized the ML IV mutant alleles in 27 patients and 95 of their relatives from 22 families, and in 123 unrelated and unaffected Ashkenazi Jewish controls. Of the 27 ML IV patients, 16 patients (59.3%) were found to be homozygous for the IVS3-1A --> G mutation and 1 patient (3.7%) homozygous for the delEX1-EX7 mutation. Additionally, 9 patients (33.3%) were compound heterozygotes for IVS3-1A --> G/delEX1-EX7. Among the 123 Ashkenazi Jewish controls, two individuals were identified as heteroallelic with one IVS3-1A --> G mutation (carrier frequency: approximately 1 in 61); none showed the delEX1-EX7 mutation. The modifications described here provide a more facile means of genotyping patients and carriers and expand the possibilities for screening at-risk populations.