A helix-breaking mutation in TRPML3 leads to constitutive activity underlying deafness in the varitint-waddler mouse

Progressive degeneration of dopaminergic neurons through TRP channel-induced cell death

Progressive neurodegenerative diseases are among the most frequently occurring aging-associated human pathologies. In a screen for Caenorhabditis elegans mutant animals that lack their normal complement of dopaminergic neurons, we identified two strains with progressive loss of dopaminergic neurons during postembryonic life. Through whole-genome sequencing we show that both strains harbor dominant (d), gain-of-function mutations in the Transient Receptor Potential (TRP) mechanosensory channel trp-4, a member of the invertebrate and vertebrate TRPN-type of the TRP family channels. Gain-of-function mutations in TRP channels have not been previously implicated in dopaminergic neuronal degeneration. We show that trp-4(d) induces cell death in dopamine neurons through a defined, calcium-related downstream pathway.

Ca²⁺-sensor proteins in the autophagic and endocytic traffic

Autophagy and endocytosis are two evolutionarily conserved catabolic processes that comprise vesicle trafficking events for the clearance of the sequestered intracellular and extracellular cargo. Both start differently but end in the same compartment, the lysosome. Mounting evidences from the last years have established the involvement of proteins sensitive to intracellular Ca²⁺ in the control of the early autophagic steps and in the traffic of autophagic, endocytic and lysosomal vesicles. However, this knowledge is based on dispersed outcomes that do not set up a consensus model of the Ca²⁺-dependent control of autophagy and endocytosis. Here, we will provide a critical synopsis of insights from the last decade on the involvement of Ca²⁺-sensor proteins in the activation of autophagy and in fusion events of endocytic vesicles, autophagosomes and lysosomes.

TRPV5: a Ca(2+) channel for the fine-tuning of Ca(2+) reabsorption

TRPV5 is one of the two channels in the TRPV family that exhibit high selectivity to Ca(2+) ions. TRPV5 mediates Ca(2+) influx into cells as the first step to transport Ca(2+) across epithelia. The specialized distribution in the distal tubule of the kidney positions TRPV5 as a key player in Ca(2+) reabsorption. The responsiveness in expression and/or activity of TRPV5 to hormones such as 1,25-dihydroxyvitamin D3, parathyroid hormone, estrogen, and testosterone makes TRPV5 suitable for its role in the fine-tuning of Ca(2+) reabsorption. This role is further optimized by the modulation of TRPV5 trafficking and activity via its binding partners; co-expressed proteins; tubular factors such as calbindin-D28k, calmodulin, klotho, uromodulin, and plasmin; extracellular and intracellular factors such as proton, Mg(2+), Ca(2+), and phosphatidylinositol-4,5-bisphosphate; and fluid flow. These regulations allow TRPV5 to adjust its overall activity in response to the body's demand for Ca(2+) and to prevent kidney stone formation. A point mutation in mouse Trpv5 gene leads to hypercalciuria similar to Trpv5 knockout mice, suggesting a possible role of TRPV5 in hypercalciuric disorders in humans. In addition, the single nucleotide polymorphisms in Trpv5 gene prevalently present in African descents may contribute to the efficient renal Ca(2+) reabsorption among African descendants. TRPV5 represents a potential therapeutic target for disorders with altered Ca(2+) homeostasis.

TRPs in hearing

Hearing is a particularly sensitive form of mechanosensation that relies on dedicated ion channels transducing sound-induced vibrations that hardly exceed Brownian motion. Attempts to molecularly identify these auditory transduction channels have put the focus on TRPs in ears. In Drosophila, hearing has been shown to involve TRPA, TRPC, TRPN, and TRPV subfamily members, with candidate auditory transduction channels including NOMPC (=TRPN1) and the TRPVs Nan and Iav. In vertebrates, TRPs are unlikely to form auditory transduction channels, yet most TRPs are expressed in inner ear tissues, and mutations in TRPN1, TRPVA1, TRPML3, TRPV4, and TRPC3/TRPC6 have been implicated in inner ear function. Starting with a brief introduction of fly and vertebrate auditory anatomies and transduction mechanisms, this review summarizes our current understanding of the auditory roles of TRPs.

TRPML3

TRPML3 belongs to the MCOLN (TRPML) subfamily of transient receptor potential (TRP) channels comprising three genes in mammals. Since the discovery of the pain sensing, capsaicin- and heat-activated vanilloid receptor (TRPV1), TRP channels have been found to be involved in regulating almost all kinds of our sensory modalities. Thus, TRP channel members are sensitive to heat or cold; they are involved in pain or osmosensation, vision, hearing, or taste sensation. Loss or mutation of TRPML1 can cause retina degeneration and eventually blindness in mice and men (mucolipidosis type IV). Gain-of-function mutations in TRPML3 cause deafness and circling behavior in mice. A special feature of TRPML channels is their intracellular expression. They mostly reside in membranes of organelles of the endolysosomal system such as early and late endosomes, recycling endosomes, lysosomes, or lysosome-related organelles. Although the physiological roles of TRPML channels within the endolysosomal system are far from being fully understood, it is speculated that they are involved in the regulation of endolysosomal pH, fusion/fission processes, trafficking, autophagy, and/or (hormone) secretion and exocytosis.

TRPML2 and mucolipin evolution

The TRPML2 protein, encoded by the Mcoln2 gene, is one of the three mucolipins (TRPML1-3), a subset of the TRP superfamily of ion channels. Although there are no thorough studies on the cellular distribution of TRPML2, its mRNA appears to be largely restricted to lymphocytes and other immune cells. This contrasts with the ubiquitous expression of TRPML1 and the limited but diverse expression of TRPML3 and clearly suggests a specialized role for TRPML2 in immunity. Localization studies indicate that TRPML2 is present in lysosomes (including the specialized lysosome-related organelle that B-lymphocytes use for processing of the antigen-bound B-cell receptor), late endosomes, recycling endosomes, and, at a much lower level, the plasma membrane. Heterologously expressed TRPML2, like TRPML1 and/or TRPML3, forms ion channels that can be activated by a gain-of-function mutation (alanine to proline in the fifth transmembrane domain, close to the pore) that favors the open state, by a transient reduction of extracellular sodium followed by sodium replenishment, by small chemicals related to sulfonamides, and by PI(3,5)P2, a rare phosphoinositide that naturally accumulates in the membranes of endosomes and lysosomes and thus could act as a physiologically relevant agonist. TRPML2 channels are inwardly rectifying and permeable to Ca(2+), Na(+), and Fe(2+). When heterologously co-expressed, TRPML2 can form heteromultimers with TRPML1 and TRPML3. In B-lymphocytes, TRPML2 and TRPML1 may play redundant roles in the function of their specialized lysosome. Although the specific subcellular function of TRPML2 is unknown, distribution and channel properties suggest roles in calcium release from endolysosomes, perhaps to regulate vesicle fusion and/or subsequent scission or to release calcium from intracellular acidic stores for signaling in the cytosol. Alternatively, TRPML2 could function in the plasma membrane, and its abundance in vesicles of the endocytic pathway could simply be due to regulation by endocytosis and exocytosis. The Mcoln2 gene is closely downstream from and in the same orientation as Mcoln3 in the genomes of most jawed vertebrates (from humans to sharks) with the exception of pigs, Xenopus tropicalis, and ray-finned fishes. The close homology of TRPML2 and 3 (closer to each other than to TRPML1) suggests that Mcoln2 and Mcoln3 arose from unequal crossing over that duplicated a common ancestor and placed both gene copies in tandem. These genes would have come apart subsequently in pigs, Xenopus, and the ancestor to ray-finned fishes. All jawed vertebrates for which we have thorough genomic knowledge have distinct Mcoln1, 2, and 3 genes (except ray-finned fishes which, probably due to the whole-genome duplication in their common ancestor, have two Mcoln1-like genes and two Mcoln3-like genes, although only one Mcoln2 gene). However, the available genomes of invertebrate deuterostomes (a sea urchin, lancelet, and two tunicates) contain a single mucolipin gene that is equally distant from the three vertebrate mucolipins. Hence, vertebrate mucolipins arose through two rounds of gene duplication (the first one likely producing Mcoln1 and the ancestor to Mcoln2 and 3) at some time between the onset of craniates and that of jawed vertebrates. This is also the evolutionary period during which adaptive immunity appeared. Given the restricted expression of TRPML2 in immune cells, this evolutionary history suggests a functional role in the adaptive immunity characteristic of vertebrates.

Drosophila TRPML forms PI(3,5)P2-activated cation channels in both endolysosomes and plasma membrane

Transient Receptor Potential mucolipin (TRPML) channels are implicated in endolysosomal trafficking, lysosomal Ca(2+) and Fe(2+) release, lysosomal biogenesis, and autophagy. Mutations in human TRPML1 cause the lysosome storage disease, mucolipidosis type IV (MLIV). Unlike vertebrates, which express three TRPML genes, TRPML1-3, the Drosophila genome encodes a single trpml gene. Although the trpml-deficient flies exhibit cellular defects similar to those in mammalian TRPML1 mutants, the biophysical properties of Drosophila TRPML channel remained uncharacterized. Here, we show that transgenic expression of human TRPML1 in the neurons of Drosophila trpml mutants partially suppressed the pupal lethality phenotype. When expressed in HEK293 cells, Drosophila TRPML was localized in both endolysosomes and plasma membrane and was activated by phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2) applied to the cytoplasmic side in whole lysosomes and inside-out patches excised from plasma membrane. The PI(3,5)P2-evoked currents were blocked by phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2), but not other phosphoinositides. Using TRPML A487P, which mimics the varitint-waddler (Va) mutant of mouse TRPML3 with constitutive whole-cell currents, we show that TRPML is biphasically regulated by extracytosolic pH, with an optimal pH about 0.6 pH unit higher than that of human TRPML1. In addition to monovalent cations, TRPML exhibits high permeability to Ca(2+), Mn(2+), and Fe(2+), but not Fe(3+). The TRPML currents were inhibited by trivalent cations Fe(3+), La(3+), and Gd(3+). These features resemble more closely to mammalian TRPML1 than TRPML2 and TRPML3, but with some obvious differences. Together, our data support the use of Drosophila for assessing functional significance of TRPML1 in cell physiology.

The Concise Guide to PHARMACOLOGY 2013/14: ion channels

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Ion channels are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

TRP channels

TRP channels constitute a large superfamily of cation channel forming proteins, all related to the gene product of the transient receptor potential (trp) locus in Drosophila. In mammals, 28 different TRP channel genes have been identified, which exhibit a large variety of functional properties and play diverse cellular and physiological roles. In this article, we provide a brief and systematic summary of expression, function, and (patho)physiological role of the mammalian TRP channels.

TRPC3 and TRPC6 are essential for normal mechanotransduction in subsets of sensory neurons and cochlear hair cells

Transient receptor potential (TRP) channels TRPC3 and TRPC6 are expressed in both sensory neurons and cochlear hair cells. Deletion of TRPC3 or TRPC6 in mice caused no behavioural phenotype, although loss of TRPC3 caused a shift of rapidly adapting (RA) mechanosensitive currents to intermediate-adapting currents in dorsal root ganglion sensory neurons. Deletion of both TRPC3 and TRPC6 caused deficits in light touch and silenced half of small-diameter sensory neurons expressing mechanically activated RA currents. Double TRPC3/TRPC6 knock-out mice also showed hearing impairment, vestibular deficits and defective auditory brain stem responses to high-frequency sounds. Basal, but not apical, cochlear outer hair cells lost more than 75 per cent of their responses to mechanical stimulation. FM1-43-sensitive mechanically gated currents were induced when TRPC3 and TRPC6 were co-expressed in sensory neuron cell lines. TRPC3 and TRPC6 are thus required for the normal function of cells involved in touch and hearing, and are potential components of mechanotransducing complexes.

Conserved gating elements in TRPC4 and TRPC5 channels

TRPC4 and TRPC5 proteins share 65% amino acid sequence identity and form Ca(2+)-permeable nonselective cation channels. They are activated by stimulation of receptors coupled to the phosphoinositide signaling cascade. Replacing a conserved glycine residue within the cytosolic S4-S5 linker of both proteins by a serine residue forces the channels into an open conformation. Expression of the TRPC4G503S and TRPC5G504S mutants causes cell death, which could be prevented by buffering the Ca(2+) of the culture medium. Current-voltage relationships of the TRPC4G503S and TRPC5G504S mutant ion channels resemble that of fully activated TRPC4 and TRPC5 wild-type channels, respectively. Modeling the structure of the transmembrane domains and the pore region (S4-S6) of TRPC4 predicts a conserved serine residue within the C-terminal sequence of the predicted S6 helix as a potential interaction site. Introduction of a second mutation (S623A) into TRPC4G503S suppressed the constitutive activation and partially rescued its function. These results indicate that the S4-S5 linker is a critical constituent of TRPC4/C5 channel gating and that disturbance of its sequence allows channel opening independent of any sensor domain.

A novel ion channel formed by interaction of TRPML3 with TRPV5

TRPML3 and TRPV5 are members of the mucolipin (TRPML) and TRPV subfamilies of transient receptor potential (TRP) cation channels. Based on sequence similarities of the pore forming regions and on structure-function evidence, we hypothesized that the pore forming domains of TRPML and TRPV5/TRPV6 channels have similarities that indicate possible functional interactions between these TRP channel subfamilies. Here we show that TRPML3 and TRPV5 associate to form a novel heteromeric ion channel. This novel conductance is detectable under conditions that do not activate either TRPML3 or TRPV5. It has pharmacological similarity with TRPML3 and requires functional TRPML3 as well as functional TRPV5. Single channel analyses revealed that TRPML3 and TRPV5 heteromers have different features than the respective homomers, and furthermore, that they occur in potentially distinct stoichiometric configurations. Based on overlapping expression of TRPML3 and TRPV5 in the kidney and the inner ear, we propose that TRPML3 and TRPV5 heteromers could have a biological function in these organs.

Cross-talk between TRPML1 channel, lipids and lysosomal storage diseases

Described by the Belgian cytologist Christian De Duve in 1949,¹ lysosomes (from the Greek “digestive bodies”) are ubiquitous specialized intracellular organelles that ensure the degradation/recycling of macromolecules (proteins, lipids, membranes) through the activity of specific enzymes (i.e., acid hydrolases). They receive their substrates through different internalization pathways (i.e., endocytosis, phagocytosis and autophagy) and are involved in a wide range of physiological functions from cell death and signaling to cholesterol homeostasis and plasma membrane repair.² In Mammals, 50 soluble lysosomal hydrolases have been described, each targeting specific substrates. They are confined in the lumen of the lysosome and require an optimum pH (i.e., pH 4.5) to work. This acidic pH compared with the slightly alkaline pH of the cytosol (i.e., ~pH 7.2) is maintained by the activity of integral lysosomal membrane proteins (LMPs, that represent the second class of lysosomal proteins), including the V-type proton (H⁺)-ATPase³ and the chloride ion channel CLC7⁴ that pumps protons from the cytosol across the lysosomal membrane.

Constitutive activity of TRPML2 and TRPML3 channels versus activation by low extracellular sodium and small molecules

The transient receptor potential channels TRPML2 and TRPML3 (MCOLN2 and MCOLN3) are nonselective cation channels. They are widely expressed in mammals. However, little is known about their physiological function(s) and activation mechanism(s). TRPML3 can be activated or rather de-inhibited by exposing it first to sodium-free extracellular solution and subsequently to high extracellular sodium. TRPML3 can also be activated by a variety of small chemical compounds identified in a high throughput screen and is inhibited by low pH. Furthermore, it was found that TRPML3 is constitutively active in low or no sodium-containing extracellular solution. This constitutive activity is independent of the intracellular presence of sodium, and whole-cell current densities are similar with pipette solutions containing cesium, potassium, or sodium. Here, we present mutagenesis data generated based on the hypothesis that negatively charged amino acids in the extracellular loops of TRPML3 may interfere with the observed sodium inhibition. We systematically mutated negatively charged amino acids in the first and second extracellular loops and found that mutating Glu-361 in the second loop has a significant impact on the sodium-mediated block of TRPML3. We further demonstrate that the TRPML3-related cation channel TRPML2 is also activated by lowering the extracellular sodium concentration as well as by a subset of small chemical compounds that were previously identified as activators of TRPML3, thus confirming the functional activity of TRPML2 at the plasma membrane and suggesting similar gating mechanisms for both TRPML channels.

The intracellular Ca²⁺ channels of membrane traffic

Regulation of organellar fusion and fission by Ca ( 2+) has emerged as a central paradigm in intracellular membrane traffic. Originally formulated for Ca ( 2+) -driven SNARE-mediated exocytosis in the presynaptic terminals, it was later expanded to explain membrane traffic in other exocytic events within the endo-lysosomal system. The list of processes and conditions that depend on the intracellular membrane traffic includes aging, antigen and lipid processing, growth factor signaling and enzyme secretion. Characterization of the ion channels that regulate intracellular membrane fusion and fission promises novel pharmacological approaches in these processes when their function becomes aberrant. The recent identification of Ca ( 2+) permeability through the intracellular ion channels comprising the mucolipin (TRPMLs) and the two-pore channels (TPCs) families pinpoints the candidates for the Ca ( 2+) channel that drive intracellular membrane traffic. The present review summarizes the recent developments and the current questions relevant to this topic.

Role of TRPML and two-pore channels in endolysosomal cation homeostasis

The transient receptor potential (TRP) channels TRPML1, TRPML2, and TRPML3 (also called mucolipins 1-3 or MCOLN1-3) are nonselective cation channels. Mutations in the Trpml1 gene cause mucolipidosis type IV in humans with clinical features including psychomotor retardation, corneal clouding, and retinal degeneration, whereas mutations in the Trpml3 gene cause deafness, circling behavior, and coat color dilution in mice. No disease-causing mutations are reported for the Trpml2 gene. Like TRPML channels, which are expressed in the endolysosomal pathway, two-pore channels (TPCs), namely TPC1, TPC2, and TPC3, are found in intracellular organelles, in particular in endosomes and lysosomes. Both TRPML channels and TPCs may function as calcium/cation release channels in endosomes, lysosomes, and lysosome-related organelles with TRPMLs being activated by phosphatidylinositol 3,5-bisphosphate and regulated by pH and TPCs being activated by nicotinic acid adenine dinucleotide phosphate in a calcium- and pH-dependent manner. They may also be involved in endolysosomal transport and fusion processes, e.g., as intracellular calcium sources. Currently, however, the exact physiological roles of TRPML channels and TPCs remain quite elusive, and whether TRPML channels are purely endolysosomal ion channels or whether they may also be functionally active at the plasma membrane in vivo remains to be determined.

Lipid storage disorders block lysosomal trafficking by inhibiting a TRP channel and lysosomal calcium release

Lysosomal lipid accumulation, defects in membrane trafficking and altered Ca(2+) homoeostasis are common features in many lysosomal storage diseases. Mucolipin transient receptor potential channel 1 (TRPML1) is the principle Ca(2+) channel in the lysosome. Here we show that TRPML1-mediated lysosomal Ca(2+) release, measured using a genetically encoded Ca(2+) indicator (GCaMP3) attached directly to TRPML1 and elicited by a potent membrane-permeable synthetic agonist, is dramatically reduced in Niemann-Pick (NP) disease cells. Sphingomyelins (SMs) are plasma membrane lipids that undergo sphingomyelinase (SMase)-mediated hydrolysis in the lysosomes of normal cells, but accumulate distinctively in lysosomes of NP cells. Patch-clamp analyses revealed that TRPML1 channel activity is inhibited by SMs, but potentiated by SMases. In NP-type C cells, increasing TRPML1's expression or activity was sufficient to correct the trafficking defects and reduce lysosome storage and cholesterol accumulation. We propose that abnormal accumulation of luminal lipids causes secondary lysosome storage by blocking TRPML1- and Ca(2+)-dependent lysosomal trafficking.

Permeation properties of the hair cell mechanotransducer channel provide insight into its molecular structure

Mechanoelectric transducer (MET) channels, located near stereocilia tips, are opened by deflecting the hair bundle of sensory hair cells. Defects in this process result in deafness. Despite this critical function, the molecular identity of MET channels remains a mystery. Inherent channel properties, particularly those associated with permeation, provide the backbone for the molecular identification of ion channels. Here, a novel channel rectification mechanism is identified, resulting in a reduced pore size at positive potentials. The apparent difference in pore dimensions results from Ca(2+) binding within the pore, occluding permeation. Driving force for permeation at hyperpolarized potentials is increased because Ca(2+) can more easily be removed from binding within the pore due to the presence of an electronegative external vestibule that dehydrates and concentrates permeating ions. Alterations in Ca(2+) binding may underlie tonotopic and Ca(2+)-dependent variations in channel conductance. This Ca(2+)-dependent rectification provides targets for identifying the molecular components of the MET channel.

TRPML2 and the evolution of mucolipins

TRPML2, the polypeptide product of the gene Trpml2 (aka Mcoln2), is a member of the TRPML or mucolipin branch of the TRP super family of ion channels. Although no known agonists have been discovered, the wild type channel gives basal currents when heterologously expressed in Drosophila (S2) cells and is constitutively active in mammalian cells when bearing a cell degeneration-causing, proline to alanine substitution in the fifth trans-membrane domain. TRPML2 forms channels that are inwardly rectifying and permeable to Ca(+2), Na(+), and Fe(+2). Localization studies indicate TRPML2 is present in lysosomes, late endosomes, recycling endosomes and, at a lower level, the plasma membrane. Tissue and organ distribution of TRPML2 is solely reported through RT-PCR and it is uncertain which cell types express this channel. However, various studies suggest that lymphoid cells express TRPML2. Although the function of TRPML2 is not known, distribution and channel properties suggest it could play roles in calcium release from endolysosomes, perhaps to mediate calcium-dependent events such as vesicle fusion, or to release calcium from intracellular acidic stores. However, TRPML2 may also function in the plasma membrane and its abundance in vesicles of the endocytic pathaway might occur because its presence in the cell surface is regulated by endocytosis and exocytosis. An evolutionary analysis of Trpml2 and its relatives reveals that vertebrate and invertebrate chordates have only one Trpml gene, that Trpml1 and Trpml2 are common to vertebrates, and that Trpml3 is only found in tetrapods. Ray-finned fishes contain another isoform, which we term Trpml4 or Mcoln4 (and its product TRPML4). Trpml2 is next to Trpml3 in all tetrapod genomes except that of the frog Xenopus tropicalis and of the domesticated pig, which seems to lack most of the Trpml3 gene. This close linkage across species implies that it is maintained by selective pressure and suggests that the regulation of both genes is interdependent.

TRPML: transporters of metals in lysosomes essential for cell survival?

Key aspects of lysosomal function are affected by the ionic content of the lysosomal lumen and, therefore, by the ion permeability in the lysosomal membrane. Such functions include regulation of lysosomal acidification, a critical process in delivery and activation of the lysosomal enzymes, release of metals from lysosomes into the cytoplasm and the Ca(2+)-dependent component of membrane fusion events in the endocytic pathway. While the basic mechanisms of lysosomal acidification have been largely defined, the lysosomal metal transport system is not well understood. TRPML1 is a lysosomal ion channel whose malfunction is implicated in the lysosomal storage disease Mucolipidosis Type IV. Recent evidence suggests that TRPML1 is involved in Fe(2+), Ca(2+) and Zn(2+) transport across the lysosomal membrane, ascribing novel physiological roles to this ion channel, and perhaps to its relatives TRPML2 and TRPML3 and illuminating poorly understood aspects of lysosomal function. Further, alterations in metal transport by the TRPMLs due to mutations or environmental factors may contribute to their role in the disease phenotype and cell death.

Increased basal activity is a key determinant in the severity of human skeletal dysplasia caused by TRPV4 mutations

TRPV4 is a mechanically activated Ca²⁺-passing channel implicated in the sensing of forces, including those acting on bones. To date, 33 mutations are known to affect human bone development to different extents. The spectrum of these skeletal dysplasias (SD) ranges from dominantly inherited mild brachylomia (BO) to neonatal lethal forms of metatropic dysplasia (MD). Complexities of the results from fluorescence and electrophysiological studies have led to questions on whether channel activity is a good predictor of disease severity. Here we report on a systematic examination of 14 TRPV4 mutant alleles covering the entire SD spectrum. Expressed in Xenopus oocyte and without any stimulation, the wild-type channel had a ∼1% open probability (Po) while those of most of the lethal MD channels approached 100%. All mutant channels had higher basal open probabilities, which limited their further increase by agonist or hypotonicity. The magnitude of this limitation revealed a clear correlation between the degree of over-activity (the molecular phenotype) and the severity of the disease over the entire spectrum (the biological phenotype). Thus, while other factors are at play, our results are consistent with the increased TRPV4 basal activity being a critical determinant of the severity of skeletal dysplasia. We discuss how the channel over-activity may lead to the “gain-of-function” phenotype and speculate that the function of wild-type TRPV4 may be secondary in normal bone development but crucial in an acute process such as fracture repair in the adult.

Transient receptor potential mucolipin 1 (TRPML1) and two-pore channels are functionally independent organellar ion channels

NAADP is a potent second messenger that mobilizes Ca(2+) from acidic organelles such as endosomes and lysosomes. The molecular basis for Ca(2+) release by NAADP, however, is uncertain. TRP mucolipins (TRPMLs) and two-pore channels (TPCs) are Ca(2+)-permeable ion channels present within the endolysosomal system. Both have been proposed as targets for NAADP. In the present study, we probed possible physical and functional association of these ion channels. Exogenously expressed TRPML1 showed near complete colocalization with TPC2 and partial colocalization with TPC1. TRPML3 overlap with TPC2 was more modest. TRPML1 and to some extent TRPML3 co-immunoprecipitated with TPC2 but less so with TPC1. Current recording, however, showed that TPC1 and TPC2 did not affect the activity of wild-type TRPML1 or constitutively active TRPML1(V432P). N-terminally truncated TPC2 (TPC2delN), which is targeted to the plasma membrane, also failed to affect TRPML1 and TRPML1(V432P) channel function or TRPML1(V432P)-mediated Ca(2+) influx. Whereas overexpression of TPCs enhanced NAADP-mediated Ca(2+) signals, overexpression of TRPML1 did not, and the dominant negative TRPML1(D471K) was without affect on endogenous NAADP-mediated Ca(2+) signals. Furthermore, the single channel properties of NAADP-activated TPC2delN were not affected by TRPML1. Finally, NAADP-evoked Ca(2+) oscillations in pancreatic acinar cells were identical in wild-type and TRPML1(-/-) cells. We conclude that although TRPML1 and TPCs are present in the same complex, they function as two independent organellar ion channels and that TPCs, not TRPMLs, are the targets for NAADP.

Expression and vesicular localization of mouse Trpml3 in stria vascularis, hair cells, and vomeronasal and olfactory receptor neurons

TRPML3 is a member of the mucolipin branch of the transient receptor potential cation channel family. A dominant missense mutation in Trpml3 (also known as Mcoln3) causes deafness and vestibular impairment characterized by stereocilia disorganization, hair cell loss, and endocochlear potential reduction. Both marginal cells of the stria vascularis and hair cells express Trpml3 mRNA. Here we used in situ hybridization, quantitative RT-qPCR, and immunohistochemistry with several antisera raised against TRPML3 to determine the expression and subcellular distribution of TRPML3 in the inner ear as well as in other sensory organs. We also use Trpml3 knockout tissues to distinguish TRPML3-specific from nonspecific immunoreactivities. We find that TRPML3 localizes to vesicles of hair cells and strial marginal cells but not to stereociliary ankle links or pillar cells, which nonspecifically react with two antisera raised against TRPML3. Upon cochlear maturation, TRPML3 protein is redistributed to perinuclear vesicles of strial marginal cells and is augmented in inner hair cells vs. outer hair cells. Mouse somatosensory neurons, retinal neurons, and taste receptor cells do not appear to express physiologically relevant levels of TRPML3. Finally, we found that vomeronasal and olfactory sensory receptor cells do express TRPML3 mRNA and protein, which localizes to vesicles in their somas and dendrites as well as at apical dendritic knobs.

Effect of capsaicin on potassium conductance and electromotility of the guinea pig outer hair cell

Capsaicin, the classic activator of TRPV-1 channels in primary sensory neurons, evokes nociception. Interestingly, auditory reception is also modulated by this chemical, possibly by direct actions on outer hair cells (OHCs). Surprisingly, we find two novel actions of capsaicin unrelated to TRPV-1 channels, which likely contribute to its auditory effects in vivo. First, capsaicin is a potent blocker of OHC K conductances (I(K) and I(K,n)). Second, capsaicin substantially alters OHC nonlinear capacitance, the signature of electromotility - a basis of cochlear amplification. These new findings of capsaicin have ramifications for our understanding of the pharmacological properties of OHC I(K), I(K,n) and electromotility and for interpretation of capsaicin pharmacological actions.

Mucolipin-3 regulates luminal calcium, acidification, and membrane fusion in the endosomal pathway

Mucolipin-3 (MCOLN3) is a pH-regulated Ca(2+) channel that localizes to the endosomal pathway. Gain-of-function mutation in MCOLN3 causes the varitint-waddler (Va) phenotype in mice, which is characterized by hearing loss, vestibular dysfunction, and coat color dilution. The Va phenotype results from a punctual mutation (A419P) in the pore region of MCOLN3 that locks the channel in an open conformation causing massive entry of Ca(2+) inside cells and inducing cell death by apoptosis. Overexpression of wild-type MCOLN3 produces severe alterations of the endosomal pathway, including enlargement and clustering of endosomes, delayed EGF receptor degradation, and impaired autophagosome maturation, thus suggesting that MCOLN3 plays an important role in the regulation of endosomal function. To understand better the physiological role of MCOLN3, we inhibited MCOLN3 function by expression of a channel-dead dominant negative mutant (458DD/KK) or by knockdown of endogenous MCOLN3. Remarkably, we found that impairment of MCOLN3 activity caused a significant accumulation of luminal Ca(2+) in endosomes. This accumulation led to severe defects in endosomal acidification as well as to increased endosomal fusion. Our findings reveal a prominent role for MCOLN3 in regulating Ca(2+) homeostasis at the endosomal pathway and confirm the importance of luminal Ca(2+) for proper acidification and membrane fusion.

Genetic inactivation of Trpml3 does not lead to hearing and vestibular impairment in mice

TRPML3, a member of the transient receptor potential (TRP) family, is an inwardly rectifying, non-selective Ca²⁺-permeable cation channel that is regulated by extracytosolic Na⁺ and H⁺ and can be activated by a variety of small molecules. The severe auditory and vestibular phenotype of the TRPML3(A419P) varitint-waddler mutation made this protein particularly interesting for inner ear biology. To elucidate the physiological role of murine TRPML3, we conditionally inactivated Trpml3 in mice. Surprisingly, lack of functional TRPML3 did not lead to circling behavior, balance impairment or hearing loss.

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

A helix-breaking mutation in the epithelial Ca(2+) channel TRPV5 leads to reduced Ca(2+)-dependent inactivation

TRPV5, a member of transient receptor potential (TRP) superfamily of ion channels, plays a crucial role in epithelial calcium transport in the kidney. This channel has a high selectivity for Ca(2+) and is tightly regulated by intracellular Ca(2+) concentrations. Recently it was shown that the molecular basis of deafness in varitint-waddler mouse is the result of hair cell death caused by the constitutive activity of transient receptor potential mucolipin 3 (TRPML3) channel carrying a helix breaking mutation, A419P, at the intracellular proximity of the fifth transmembrane domain (TM5). This mutation significantly elevates intracellular Ca(2+) concentration and causes rapid cell death. Here we show that substituting the equivalent location in TRPV5, the M490, to proline significantly modulates Ca(2+)-dependent inactivation of TRPV5. The single channel conductance, time constant of inactivation (τ) and half maximal inhibition constant (IC(50)) of TRPV5(M490P) were increased compared to TRPV5(WT). Moreover TRPV5(M490P) showed lower Ca(2+) permeability. Out of different point mutations created to characterize the importance of M490 in Ca(2+)-dependent inactivation, only TRPV5(M490P)-expressing cells showed apoptosis and extremely altered Ca(2+)-dependent inactivation. In conclusion, the TRPV5 channel is susceptible for helix breaking mutations and the proximal intracellular region of TM5 of this channel plays an important role in Ca(2+)-dependent inactivation.

Zinc dyshomeostasis is linked with the loss of mucolipidosis IV-associated TRPML1 ion channel

Chelatable zinc is important in brain function, and its homeostasis is maintained to prevent cytotoxic overload. However, certain pathologic events result in intracellular zinc accumulation in lysosomes and mitochondria. Abnormal lysosomes and mitochondria are common features of the human lysosomal storage disorder known as mucolipidosis IV (MLIV). MLIV is caused by the loss of TRPML1 ion channel function. MLIV cells develop large hyperacidic lysosomes, membranous vacuoles, mitochondrial fragmentation, and autophagic dysfunction. Here, we observed that RNA interference of mucolipin-1 gene (TRPML1) in HEK-293 cells mimics the MLIV cell phenotype consisting of large lysosomes and membranous vacuoles that accumulate chelatable zinc. To show that abnormal chelatable zinc levels are indeed correlated with MLIV pathology, we quantified its concentration in cultured MLIV patient fibroblast and control cells with a spectrofluorometer using N-(6-methoxy-8-quinolyl)-p-toluene sulfonamide fluorochrome. We found a significant increase of chelatable zinc levels in MLIV cells but not in control cells. Furthermore, we quantified various metal isotopes in whole brain tissue of TRPML1(-/-) null mice and wild-type littermates using inductively coupled plasma mass spectrometry and observed that the zinc-66 isotope is markedly elevated in the brain of TRPML1(-/-) mice when compared with controls. In conclusion, we show for the first time that the loss of TRPML1 function results in intracellular chelatable zinc dyshomeostasis. We propose that chelatable zinc accumulation in large lysosomes and membranous vacuoles may contribute to the pathogenesis of the disease and progressive cell degeneration in MLIV patients.

Heteromultimeric TRPML channel assemblies play a crucial role in the regulation of cell viability models and starvation-induced autophagy

The mucolipin (TRPML) subfamily of transient receptor potential (TRP) cation channels consists of three members that play various roles in the regulation of membrane and protein sorting along endo-lysosomal pathways. Loss-of-function mutations in TRPML1 cause the neurodegenerative lysosomal storage disorder, mucolipidosis type IV (MLIV), whereas a gain-of-function mutation in TRPML3 is principally implicated in the hearing-impaired and abnormally pigmented varitint-waddler mouse. Currently, TRPML2 is not implicated in any pathological disorder, but we have recently shown that it is a functional cation channel that physically interacts with TRPML1 and TRPML3 to potentially regulate lysosomal integrity. Here, we show that mutant TRPMLs heteromultimerize with other mutant and wild-type TRPMLs to regulate cell viability and starvation-induced autophagy, a process that mediates macromolecular and organellar turnover under cell starvation conditions. Heteromultimerization of dominant-negative TRPMLs with constitutively active TRPMLs rescues cells from the cytotoxic effects of TRPML constitutive activity. Moreover, dominant-negative TRPML1 channels, including a mutant channel directly implicated in MLIV pathology, also inhibit starvation-induced autophagy by interacting with and affecting native TRPML channel function. Collectively, our results indicate that heteromultimerization of TRPML channels plays a role in various TRPML-regulated mechanisms.

PI(3,5)P(2) controls membrane trafficking by direct activation of mucolipin Ca(2+) release channels in the endolysosome

Membrane fusion and fission events in intracellular trafficking are controlled by both intraluminal Ca(2+) release and phosphoinositide (PIP) signalling. However, the molecular identities of the Ca(2+) release channels and the target proteins of PIPs are elusive. In this paper, by direct patch-clamping of the endolysosomal membrane, we report that PI(3,5)P(2), an endolysosome-specific PIP, binds and activates endolysosome-localized mucolipin transient receptor potential (TRPML) channels with specificity and potency. Both PI(3,5)P(2)-deficient cells and cells that lack TRPML1 exhibited enlarged endolysosomes/vacuoles and trafficking defects in the late endocytic pathway. We find that the enlarged vacuole phenotype observed in PI(3,5)P(2)-deficient mouse fibroblasts is suppressed by overexpression of TRPML1. Notably, this PI(3,5)P(2)-dependent regulation of TRPML1 is evolutionarily conserved. In budding yeast, hyperosmotic stress induces Ca(2+) release from the vacuole. In this study, we show that this release requires both PI(3,5)P(2) production and a yeast functional TRPML homologue. We propose that TRPMLs regulate membrane trafficking by transducing information regarding PI(3,5)P(2) levels into changes in juxtaorganellar Ca(2+), thereby triggering membrane fusion/fission events.

Opening the TRPML gates

The intracellular TRPML channels have multiple biological roles, but the physiological stimuli that open them remained unknown. In a previous issue of Chemistry & Biology, Grimm et al. report a high-throughput chemical screen that identified a plethora of selective activators of TRPML3 that should open the way to fully characterize these channels and their physiological roles.

Small molecule activators of TRPML3

We conducted a high-throughput screen for small molecule activators of the TRPML3 ion channel, which, when mutated, causes deafness and pigmentation defects. Cheminformatics analyses of the 53 identified and confirmed compounds revealed nine different chemical scaffolds and 20 singletons. We found that agonists strongly potentiated TRPML3 activation with low extracytosolic [Na(+)]. This synergism revealed the existence of distinct and cooperative activation mechanisms and a wide dynamic range of TRPML3 activity. Testing compounds on TRPML3-expressing sensory hair cells revealed the absence of activator-responsive channels. Epidermal melanocytes showed only weak or no responses to the compounds. These results suggest that TRPML3 in native cells might be absent from the plasma membrane or that the protein is a subunit of heteromeric channels that are nonresponsive to the activators identified in this screen.

Properties of the TRPML3 channel pore and its stable expansion by the Varitint-Waddler-causing mutation

TRPML3 is a H(+)-regulated Ca(2+) channel that shuttles between intracellular compartments and the plasma membrane. The A419P mutation causes the varitint-waddler phenotype as a result of gain-of-function (GOF). The mechanism by which A419P leads to GOF is not known. Here, we show that the TRPML3 pore is dynamic when conducting Ca(2+) to change its conductance and permeability, which appears to be mediated by trapping Ca(2+) within the pore. The pore properties can be restored by strong depolarization or by conducting Na(+) through the pore. The A419P mutation results in expanded channel pore with altered permeability that limits modulation of the pore by Ca(2+). This effect is specific for the A419P mutation and is not reproduced by other GOF mutations, including A419G, H283A, and proline mutations in the fifth transmembrane domain. These findings describe a novel mode of a transient receptor potential channel behavior and suggest that pore expansion by the A419P mutation may contribute to the varitint-waddler phenotype.

Transient receptor potential channelopathies

In the past years, several hereditary diseases caused by defects in transient receptor potential channels (TRP) genes have been described. This review summarizes our current knowledge about TRP channelopathies and their possible pathomechanisms. Based on available genetic indications, we will also describe several putative pathological conditions in which (mal)function of TRP channels could be anticipated.

TRP channels of intracellular membranes

Ion channels are classically understood to regulate the flux of ions across the plasma membrane in response to a variety of environmental and intracellular cues. Ion channels serve a number of functions in intracellular membranes as well. These channels may be temporarily localized to intracellular membranes as a function of their biosynthetic or secretory pathways, i.e., en route to their destination location. Intracellular membrane ion channels may also be located in the endocytic pathways, either being recycled back to the plasma membrane or targeted to the lysosome for degradation. Several channels do participate in intracellular signal transduction; the most well known example is the inositol 1,4,5-trisphosphate receptor (IP(3)R) in the endoplasmic reticulum. Some organellar intracellular membrane channels are required for the ionic homeostasis of their residing organelles. Several newly-discovered intracellular membrane Ca(2+) channels actually play active roles in membrane trafficking. Transient receptor potential (TRP) proteins are a superfamily (28 members in mammal) of Ca(2+)-permeable channels with diverse tissue distribution, subcellular localization, and physiological functions. Almost all mammalian TRP channels studied thus far, like their ancestor yeast TRP channel (TRPY1) that localizes to the vacuole compartment, are also (in addition to their plasma membrane localization) found to be localized to intracellular membranes. Accumulated evidence suggests that intracellularly-localized TRP channels actively participate in regulating membrane traffic, signal transduction, and vesicular ion homeostasis. This review aims to provide a summary of these recent works. The discussion will also be extended to the basic membrane and electrical properties of the TRP-residing compartments.

Constitutive activity of the human TRPML2 channel induces cell degeneration

The mucolipin (TRPML) ion channel proteins represent a distinct subfamily of channel proteins within the transient receptor potential (TRP) superfamily of cation channels. Mucolipin 1, 2, and 3 (TRPML1, -2, and -3, respectively) are channel proteins that share high sequence homology with each other and homology in the transmembrane domain with other TRPs. Mutations in the TRPML1 protein are implicated in mucolipidosis type IV, whereas mutations in TRPML3 are found in the varitint-waddler mouse. The properties of the wild type TRPML2 channel are not well known. Here we show functional expression of the wild type human TRPML2 channel (h-TRPML2). The channel is functional at the plasma membrane and characterized by a significant inward rectification similar to other constitutively active TRPML mutant isoforms. The h-TRPML2 channel displays nonselective cation permeability, which is Ca(2+)-permeable and inhibited by low extracytosolic pH but not Ca(2+) regulated. In addition, constitutively active h-TRPML2 leads to cell death by causing Ca(2+) overload. Furthermore, we demonstrate by functional mutation analysis that h-TRPML2 shares similar characteristics and structural similarities with other TRPML channels that regulate the channel in a similar manner. Hence, in addition to overall structure, all three TRPML channels also share common modes of regulation.

Mucolipins: Intracellular TRPML1-3 channels

The mucolipin family of Transient Receptor Potential (TRPML) proteins is predicted to encode ion channels expressed in intracellular endosomes and lysosomes. Loss-of-function mutations of human TRPML1 cause type IV mucolipidosis (ML4), a childhood neurodegenerative disease. Meanwhile, gain-of-function mutations in the mouse TRPML3 result in the varitint-waddler (Va) phenotype with hearing and pigmentation defects. The broad spectrum phenotypes of ML4 and Va appear to result from certain aspects of endosomal/lysosomal dysfunction. Lysosomes, traditionally believed to be the terminal "recycling center" for biological "garbage", are now known to play indispensable roles in intracellular signal transduction and membrane trafficking. Studies employing animal models and cell lines in which TRPML genes have been genetically disrupted or depleted have uncovered roles of TRPMLs in multiple cellular functions including membrane trafficking, signal transduction, and organellar ion homeostasis. Physiological assays of mammalian cell lines in which TRPMLs are heterologously overexpressed have revealed the channel properties of TRPMLs in mediating cation (Ca(2+)/Fe(2+)) efflux from endosomes and lysosomes in response to unidentified cellular cues. This review aims to summarize these recent advances in the TRPML field and to correlate the channel properties of endolysosomal TRPMLs with their biological functions. We will also discuss the potential cellular mechanisms by which TRPML deficiency leads to neurodegeneration.

The tissue-specific expression of TRPML2 (MCOLN-2) gene is influenced by the presence of TRPML1

Mucolipidosis type IV is a lysosomal storage disorder caused by the loss or dysfunction of the mucolipin-1 (TRPML1) protein. It has been suggested that TRPML2 could genetically compensate (i.e., become upregulated) for the loss of TRPML1. We thus investigated this possibility by first studying the expression pattern of mouse TRPML2 and its basic channel properties using the varitint-waddler (Va) model. Here, we confirmed the presence of long variant TRPML2 (TRPML2lv) and short variant (TRPML2sv) isoforms. We showed for the first time that, heterologously expressed, TRPML2lv-Va is an active, inwardly rectifying channel. Secondly, we quantitatively measured TRPML2 and TRPML3 mRNA expressions in TRPML1-/- null and wild-type (Wt) mice. In wild-type mice, the TRPML2lv transcripts were very low while TRPML2sv and TRPML3 transcripts have predominant expressions in lymphoid and kidney organs. Significant reductions of TRPML2sv, but not TRPML2lv or TRPML3 transcripts, were observed in lymphoid and kidney organs of TRPML1-/- mice. RNA interference of endogenous human TRPML1 in HEK-293 cells produced a comparable decrease of human TRPML2 transcript levels that can be restored by overexpression of human TRPML1. Conversely, significant upregulation of TRPML2sv transcripts was observed when primary mouse lymphoid cells were treated with nicotinic acid adenine dinucleotide phosphate, or N-(2-[p-bromocinnamylamino]ethyl)-5-isoquinoline sulfonamide, both known activators of TRPML1. In conclusion, our results indicate that TRPML2 is unlikely to compensate for the loss of TRPML1 in lymphoid or kidney organs and that TRPML1 appears to play a novel role in the tissue-specific transcriptional regulation of TRPML2.

Aberrant Ca2+ handling in lysosomal storage disorders

Lysosomal storage diseases (LSDs) are caused by inability of cells to process the material captured during endocytosis. While they are essentially diseases of cellular "indigestion", LSDs affect large number of cellular activities and, as such, they teach us about the integrative function of the cell, as well as about the gaps in our knowledge of the endocytic pathway and membrane transport. The present review summarizes recent findings on Ca2+ handling in LSDs and attempts to identify the key questions on alterations in Ca2+ signaling and membrane transport in this group of diseases, answers to which may lie in delineating the cellular pathogeneses of LSDs.

Constitutive activity of TRP channels methods for measuring the activity and its outcome

TRP channels participate in many cellular processes including cell death. These channels mediate these effects mainly by changing the cellular concentration of Ca(2+), a prominent cellular second messenger. Measuring the current-voltage relationship and state of activation of TRP channels is of utmost importance for evaluating their contribution to a cellular process within a spatial and temporal context. The study of TRP channels and characterization of their mode of activation will benefit and progress our understanding of each channel's role in specific cellular mechanisms. Many TRP channels exhibit constitutive activity, which is mostly observed in cell-based expression systems. This constitutive activity can lead, in many cases, to cellular degeneration, which can be readily observed morphologically and by biochemical assays. This chapter describes in brief different modes of TRP channel activity and their current-voltage relationships. The chapter outlines methods for visualizing this activity and methods to correlate between TRP channel activity and cell death, and it illustrates mechanisms that prevent cell death in spite of constitutive activity. Finally, it describes methods for qualitatively and quantitatively measuring the accompanied cellular degeneration.

Functional multimerization of mucolipin channel proteins

MCOLN1 encodes mucolipin-1 (TRPML1), a member of the transient receptor potential TRPML subfamily of channel proteins. Mutations in MCOLN1 cause mucolipidosis-type IV (MLIV), a lysosomal storage disorder characterized by severe neurologic, ophthalmologic, and gastrointestinal abnormalities. Along with TRPML1, there are two other TRPML family members, mucolipin-2 (TRPML2) and mucolipin-3 (TRPML3). In this study, we used immunocytochemical analysis to determine that TRPML1, TRPML2, and TRPML3 co-localize in cells. The multimerization of TRPML proteins was confirmed by co-immunoprecipitation and Western blot analysis, which demonstrated that TRPML1 homo-multimerizes as well as hetero-multimerizes with TRPML2 and TRPML3. MLIV-causing mutants of TRPML1 also interacted with wild-type TRPML1. Lipid bilayer re-constitution of in vitro translated TRPML2 and TRPML3 confirmed their cation channel properties with lower single channel conductance and higher partial permeability to anions as compared to TRPML1. We further analyzed the electrophysiological properties of single channel TRPML hetero-multimers, which displayed functional differences when compared to individual TRPMLs. Our data shows for the first time that TRPMLs form distinct functional channel complexes. Homo- and hetero-multimerization of TRPMLs may modulate channel function and biophysical properties, thereby increasing TRPML functional diversity.

How is the highly positive endocochlear potential formed? The specific architecture of the stria vascularis and the roles of the ion-transport apparatus

Cochlear endolymph, an extracellular solution containing 150 mM K(+), exhibits a positive potential of +80 mV. This is called the endocochlear potential (EP) and is essential for audition. The mechanism responsible for formation of the EP has been an enigma for the half century since its first measurement. A key element is the stria vascularis, which displays a characteristic tissue structure and expresses multiple ion-transport apparatus. The stria comprises two epithelial layers: a layer of marginal cells and one composed of intermediate and basal cells. Between the two layers lies an extracellular space termed the intrastrial space (IS), which is thus surrounded by the apical membranes of intermediate cells and the basolateral membranes of marginal cells. The fluid in the IS exhibits a low concentration of K(+) and a positive potential similar to the EP. We have demonstrated that the IS is electrically isolated from the neighboring extracellular fluids, perilymph, and endolymph, which allows the IS to sustain its positive potential. This IS potential is generated by K(+) diffusion across the apical membranes of intermediate cells, where inwardly rectifying Kir4.1 channels are localized. The low K(+) concentration in the IS, which is mandatory for the large K(+)-diffusion potential, is maintained by Na(+),K(+)-ATPases and Na(+),K(+),2Cl(-)-cotransporters expressed at the basolateral membranes of marginal cells. An additional K(+)-diffusion potential formed by KCNQ1/KCNE1-K(+) channels at the apical membranes of marginal cells also contributes to the EP. Therefore, the EP depends on an electrically isolated space and two K(+)-diffusion potentials in the stria vascularis.

Activating mutations of the TRPML1 channel revealed by proline-scanning mutagenesis

The mucolipin TRP (TRPML) proteins are a family of endolysosomal cation channels with genetically established importance in humans and rodent. Mutations of human TRPML1 cause type IV mucolipidosis, a devastating pediatric neurodegenerative disease. Our recent electrophysiological studies revealed that, although a TRPML1-mediated current can only be recorded in late endosome and lysosome (LEL) using the lysosome patch clamp technique, a proline substitution in TRPML1 (TRPML1(V432P)) results in a large whole cell current. Thus, it remains unknown whether the large TRPML1(V432P)-mediated current results from an increased surface expression (trafficking), elevated channel activity (gating), or both. Here we performed systemic Pro substitutions in a region previously implicated in the gating of various 6 transmembrane cation channels. We found that several Pro substitutions displayed gain-of-function (GOF) constitutive activities at both the plasma membrane (PM) and endolysosomal membranes. Although wild-type TRPML1 and non-GOF Pro substitutions localized exclusively in LEL and were barely detectable in the PM, the GOF mutations with high constitutive activities were not restricted to LEL compartments, and most significantly, exhibited significant surface expression. Because lysosomal exocytosis is Ca(2+)-dependent, constitutive Ca(2+) permeability due to Pro substitutions may have resulted in stimulus-independent intralysosomal Ca(2+) release, hence the surface expression and whole cell current of TRPML1. Indeed, surface staining of lysosome-associated membrane protein-1 (Lamp-1) was dramatically increased in cells expressing GOF TRPML1 channels. We conclude that TRPML1 is an inwardly rectifying, proton-impermeable, Ca(2+) and Fe(2+)/Mn(2+) dually permeable cation channel that may be gated by unidentified cellular mechanisms through a conformational change in the cytoplasmic face of the transmembrane 5 (TM5). Furthermore, activation of TRPML1 in LEL may lead to the appearance of TRPML1 proteins at the PM.