TRPML3 mutations cause impaired mechano-electrical transduction and depolarization by an inward-rectifier cation current in auditory hair cells of varitint-waddler mice

Transient Receptor Potential Channels and Auditory Functions

Significance: Transient receptor potential (TRP) channels are cation-gated channels that serve as detectors of various sensory modalities, such as pain, heat, cold, and taste. These channels are expressed in the inner ear, suggesting that they could also contribute to the perception of sound. This review provides more details on the different types of TRP channels that have been identified in the cochlea to date, focusing on their cochlear distribution, regulation, and potential contributions to auditory functions. Recent Advances: To date, the effect of TRP channels on normal cochlear physiology in mammals is still unclear. These channels contribute, to a limited extent, to normal cochlear physiology such as the hair cell mechanoelectrical transduction channel and strial functions. More detailed information on a number of these channels in the cochlea awaits future studies. Several laboratories focusing on TRPV1 channels have shown that they are responsive to cochlear stressors, such as ototoxic drugs and noise, and regulate cytoprotective and/or cell death pathways. TRPV1 expression in the cochlea is under control of oxidative stress (produced primarily by NOX3 NADPH oxidase) as well as STAT1 and STAT3 transcription factors, which differentially modulate inflammatory and apoptotic signals in the cochlea. Inhibition of oxidative stress or inflammation reduces the expression of TRPV1 channels and protects against cochlear damage and hearing loss. Critical Issues: TRPV1 channels are activated by both capsaicin and cisplatin, which produce differential effects on the inner ear. How these differential actions are produced is yet to be determined. It is clear that TRPV1 is an essential component of cisplatin ototoxicity as knockdown of these channels protects against hearing loss. In contrast, activation of TRPV1 by capsaicin protected against subsequent hearing loss induced by cisplatin. The cellular targets that are influenced by these two drugs to account for their differential profiles need to be fully elucidated. Furthermore, the potential involvement of different TRP channels present in the cochlea in regulating cisplatin ototoxicity needs to be determined. Future Directions: TRPV1 has been shown to mediate the entry of aminoglycosides into the hair cells. Thus, novel otoprotective strategies could involve designing drugs to inhibit entry of aminoglycosides and possibly other ototoxins into cochlear hair cells. TRP channels, including TRPV1, are expressed on circulating and resident immune cells. These receptors modulate immune cell functions. However, whether they are activated by cochlear stressors to initiate cochlear inflammation and ototoxicity needs to be determined. A better understanding of the function and regulation of these TRP channels in the cochlea could enable development of novel treatments for treating hearing loss. Antioxid. Redox Signal. 36, 1158-1170.

Molecular Structure of the Hair Cell Mechanoelectrical Transduction Complex

Cochlear hair cells employ mechanically gated ion channels located in stereocilia that open in response to sound wave-induced motion of the basilar membrane, converting mechanical stimulation to graded changes in hair cell membrane potential. Membrane potential changes in hair cells cause neurotransmitter release from hair cells that initiate electrical signals in the nerve terminals of afferent fibers from spiral ganglion neurons. These signals are then propagated within the central nervous system (CNS) to mediate the sensation of hearing. Recent studies show that the mechanoelectrical transduction (MET) machinery of hair cells is formed by an ensemble of proteins. Candidate components forming the MET channel have been identified, but none alone fulfills all criteria necessary to define them as pore-forming subunits of the MET channel. We will review here recent findings on the identification and function of proteins that are components of the MET machinery in hair cells and consider remaining open questions.

Role of TRP Channels in Dinoflagellate Mechanotransduction

Transient receptor potential (TRP) ion channels are common components of mechanosensing pathways, mainly described in mammals and other multicellular organisms. To gain insight into the evolutionary origins of eukaryotic mechanosensory proteins, we investigated the involvement of TRP channels in mechanosensing in a unicellular eukaryotic protist, the dinoflagellate Lingulodinium polyedra. BLASTP analysis of the protein sequences predicted from the L. polyedra transcriptome revealed six sequences with high similarity to human TRPM2, TRPM8, TRPML2, TRPP1, and TRPP2; and characteristic TRP domains were identified in all sequences. In a phylogenetic tree including all mammalian TRP subfamilies and TRP channel sequences from unicellular and multicellular organisms, the L. polyedra sequences grouped with the TRPM, TPPML, and TRPP clades. In pharmacological experiments, we used the intrinsic bioluminescence of L. polyedra as a reporter of mechanoresponsivity. Capsaicin and RN1734, agonists of mammalian TRPV, and arachidonic acid, an agonist of mammalian TRPV, TRPA, TRPM, and Drosophila TRP, all stimulated bioluminescence in L. polyedra. Mechanical stimulation of bioluminescence, but not capsaicin-stimulated bioluminescence, was inhibited by gadolinium (Gd³⁺), a general inhibitor of mechanosensitive ion channels, and the phospholipase C (PLC) inhibitor U73122. These pharmacological results are consistent with the involvement of TRP-like channels in mechanosensing by L. polyedra. The TRP channels do not appear to be mechanoreceptors but rather are components of the mechanotransduction signaling pathway and may be activated via a PLC-dependent mechanism. The presence and function of TRP channels in a dinoflagellate emphasize the evolutionary conservation of both the channel structures and their functions.

Are TMCs the Mechanotransduction Channels of Vertebrate Hair Cells?

Sensory transduction in vertebrate hair cells and the molecules that mediate it have long been of great interest. Some components of the mechanotransduction apparatus have been identified, most as deafness gene products. Although prior candidates for the mechanotransduction channel have been proposed, each has faded with new evidence. Now, two strong candidates, TMC1 and TMC2 (transmembrane channel-like), have emerged from discovery of deafness genes in humans and mice. They are expressed at the right time during development: exactly at the onset of mechanosensitivity. They are expressed in the right place: in hair cells but not surrounding cells. Fluorescently tagged TMCs localize to the tips of stereocilia, the site of the transduction channels. TMCs bind other proteins essential for mechanosensation, suggesting a larger transduction complex. Although TMC1 and TMC2 can substitute for each other, genetic deletion of both renders mouse hair cells mechanically insensitive. Finally, the conductance and Ca²⁺ selectivity of the transduction channels depend on the TMC proteins, differing when hair cells express one or the other TMC, and differing if TMC1 harbors a point mutation. Some contrary evidence has emerged: a current activated in hair cells by negative pressure, with some similarity to the transduction current, persists in TMC knock-outs. But it is not clear that this anomalous current is carried by the same proteins. Further evidence is desired, such as production of a mechanically gated conductance by pure TMCs. But the great majority of evidence is consistent with these TMCs as pore-forming subunits of the long-sought hair-cell transduction channel.

Hair-Cell Mechanotransduction Persists in TRP Channel Knockout Mice

Members of the TRP superfamily of ion channels mediate mechanosensation in some organisms, and have been suggested as candidates for the mechanotransduction channel in vertebrate hair cells. Some TRP channels can be ruled out based on lack of an inner ear phenotype in knockout animals or pore properties not similar to the hair-cell channel. Such studies have excluded Trpv4, Trpa1, Trpml3, Trpm1, Trpm3, Trpc1, Trpc3, Trpc5, and Trpc6. However, others remain reasonable candidates. We used data from an RNA-seq analysis of gene expression in hair cells as well as data on TRP channel conductance to narrow the candidate group. We then characterized mice lacking functional Trpm2, Pkd2, Pkd2l1, Pkd2l2 and Pkd1l3, using scanning electron microscopy, auditory brainstem response, permeant dye accumulation, and single-cell electrophysiology. In all of these TRP-deficient mice, and in double and triple knockouts, mechanotransduction persisted. Together with published studies, these results argue against the participation of any of the 33 mouse TRP channels in hair cell transduction.

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.

The how and why of identifying the hair cell mechano-electrical transduction channel

Identification of the auditory hair cell mechano-electrical transduction (hcMET) channel has been a major focus in the hearing research field since the 1980s when direct mechanical gating of a transduction channel was proposed (Corey and Hudspeth J Neurosci 3:962-976, 1983). To this day, the molecular identity of this channel remains controversial. However, many of the hcMET channel's properties have been characterized, including pore properties, calcium-dependent ion permeability, rectification, and single channel conductance. At this point, elucidating the molecular identity of the hcMET channel will provide new tools for understanding the mechanotransduction process. This review discusses the significance of identifying the hcMET channel, the difficulties associated with that task, as well as the establishment of clear criteria for this identification. Finally, we discuss potential candidate channels in light of these criteria.

The physiology of mechanoelectrical transduction channels in hearing

Much is known about the mechanotransducer (MT) channels mediating transduction in hair cells of the vertrbrate inner ear. With the use of isolated preparations, it is experimentally feasible to deliver precise mechanical stimuli to individual cells and record the ensuing transducer currents. This approach has shown that small (1-100 nm) deflections of the hair-cell stereociliary bundle are transmitted via interciliary tip links to open MT channels at the tops of the stereocilia. These channels are cation-permeable with a high selectivity for Ca(2+); two channels are thought to be localized at the lower end of the tip link, each with a large single-channel conductance that increases from the low- to high-frequency end of the cochlea. Ca(2+) influx through open channels regulates their resting open probability, which may contribute to setting the hair cell resting potential in vivo. Ca(2+) also controls transducer fast adaptation and force generation by the hair bundle, the two coupled processes increasing in speed from cochlear apex to base. The molecular intricacy of the stereocilary bundle and the transduction apparatus is reflected by the large number of single-gene mutations that are linked to sensorineural deafness, especially those in Usher syndrome. Studies of such mutants have led to the discovery of many of the molecules of the transduction complex, including the tip link and its attachments to the stereociliary core. However, the MT channel protein is still not firmly identified, nor is it known whether the channel is activated by force delivered through accessory proteins or by deformation of the lipid bilayer.

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.

TRPs in mechanosensing and volume regulation

Mechanosensitive channels allow cells to respond to changes in membrane stretch that occur due to external stimuli like pressure or flow or that occur because of osmotically induced cell swelling or shrinkage. Ion fluxes through the channels change the membrane potential and ion concentrations and link the stretch to cellular signalling. Changes in cellular activity evoked by mechanical stimuli can be used to elicit local tissue responses or can be transmitted further to generate more widespread responses. Channels can respond directly to membrane stress, can be conferred mechanosensitive by interaction with structural proteins, or can be activated by mechanosensitive signalling pathways. Because mechanosensitive channels are often nonselective cation channels, and invertebrate TRP isoforms are involved in mechanosensation, many of the mammalian TRP isoforms have been investigated with regard to their mechanosensitivity. There is evidence that members of the TRPC, TRPV, TRPM, TRPA and TRPP subfamilies could be in some way mechanosensitive, and each of the activation mechanisms described above is used by a TRP channel. TRP channels may be involved in mechanosensitive processes ranging from flow and pressure sensing in the vasculature and other organs to mechanosensation in sensory neurones and sensory organs. There is also evidence for a role of mechano- or osmosensitive TRP isoforms in osmosensing and the regulation of cell volume. Often, a number of different TRP isoforms have been implicated in a single type of mechanosensitive response. In many cases, the involvement of the isoforms needs to be confirmed, and their exact role in the signalling process determined.

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.

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.

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.

HCN1 and HCN2 proteins are expressed in cochlear hair cells: HCN1 can form a ternary complex with protocadherin 15 CD3 and F-actin-binding filamin A or can interact with HCN2

A unique coupling between HCN1 and stereociliary tip-link protein protocadherin 15 has been described for a teleost vestibular hair-cell model and mammalian organ of Corti (OC) (Ramakrishnan, N. A., Drescher, M. J., Barretto, R. L., Beisel, K. W., Hatfield, J. S., and Drescher, D. G. (2009) J. Biol. Chem. 284, 3227-3238). We now show that Ca(2+)-dependent interaction of the organ of Corti HCN1 and protocadherin 15 CD3 is mediated by amino-terminal sequence specific to HCN1 and is not replicated by analogous specific peptides for HCN2 or HCN4 nor by amino-terminal sequence conserved across HCN isoforms utilized in channel formation. Furthermore, the HCN1-specific peptide binds both phosphatidylinositol (3,4,5)-trisphosphate and phosphatidylinositol (4,5)-bisphosphate but not phosphatidylinositol 4-phosphate. Singly isolated cochlear inner and outer hair cells express HCN1 transcript, and HCN1 and HCN2 protein is immunolocalized to hair-cell stereocilia by both z-stack confocal and pre-embedding EM immunogold microscopy, with stereociliary tip-link and subcuticular plate sites. Quantitative PCR indicates HCN1/HCN2/HCN3/HCN4 = 9:9:1:89 in OC of the wild-type mouse, with HCN4 protein primarily attributable to inner sulcus cells. A mutant form of HCN1 mRNA and protein is expressed in the OC of an HCN1 mutant, corresponding to a full-length sequence with the in-frame deletion of pore-S6 domains, predicted by construct. The mutant transcript of HCN1 is ∼9-fold elevated relative to wild-type levels, possibly representing molecular compensation, with unsubstantial changes in HCN2, HCN3, and HCN4. Immunoprecipitation protocols indicate alternate interactions of full-length proteins; HCN1 can interact with protocadherin 15 CD3 and F-actin-binding filamin A forming a complex that does not include HCN2, or HCN1 can interact with HCN2 forming a complex without protocadherin 15 CD3 but including F-actin-binding fascin-2.

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.

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.

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.

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.

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

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.

Developmental regulation of TRPC3 ion channel expression in the mouse cochlea

Canonical transient receptor potential type 3 (TRPC3) ion channels assemble from TRPC3 subunits and exhibit multiple activation mechanisms. TRPC3 has been proposed to contribute to Ca(2+) entry supporting Ca(2+) homeostasis in cochlear hair cells and to be activated by G protein-coupled receptor (GPCR) signaling in spiral ganglion neurons. The present study was designed to determine the spatiotemporal profile of TRPC3 expression during mouse cochlear ontogeny. TRPC3 immunofluorescence of cryosectioned cochleae was performed using E16-adult tissue. We found that prior to birth, TRPC3 expression was strongest in epithelial cells that form the cochlear partition. In the early postnatal period, to the onset of hearing (~P12), immunofluorescence was strongest in the hair cells, with increased expression in stria vascularis and Reissner's membrane. Afferent neurite labeling in inner spiral plexus and outer spiral bundles developed transiently in the perinatal period, corresponding to the critical period of synaptic consolidation, while signal in the spiral ganglion soma increased from the perinatal period through to adulthood. Compared with the late embryonic/early postnatal levels, hair cell expression was relatively weaker from the third postnatal week, whereas spiral ganglion soma labeling was stronger. In the adult, TRPC3 expression was primarily in the soma of spiral ganglion neurons, the hair cells, and the inner and outer sulcus regions. This spatiotemporal profile of TRPC3 expression was consistent with this ion channel contributing to development of sensory, neural and epithelial cochlear tissues, as well as hair cell Ca(2+) homeostasis and regulation of auditory neurotransmission via GPCR signaling.

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.

Harmonin-b, an actin-binding scaffold protein, is involved in the adaptation of mechanoelectrical transduction by sensory hair cells

We assessed the involvement of harmonin-b, a submembranous protein containing PDZ domains, in the mechanoelectrical transduction machinery of inner ear hair cells. Harmonin-b is located in the region of the upper insertion point of the tip link that joins adjacent stereocilia from different rows and that is believed to gate transducer channel(s) located in the region of the tip link's lower insertion point. In Ush1c^{dfcr-2J/dfcr-2J} mutant mice defective for harmonin-b, step deflections of the hair bundle evoked transduction currents with altered speed and extent of adaptation. In utricular hair cells, hair bundle morphology and maximal transduction currents were similar to those observed in wild-type mice, but adaptation was faster and more complete. Cochlear outer hair cells displayed reduced maximal transduction currents, which may be the consequence of moderate structural anomalies of their hair bundles. Their adaptation was slower and displayed a variable extent. The latter was positively correlated with the magnitude of the maximal transduction current, but the cells that showed the largest currents could be either hyperadaptive or hypoadaptive. To interpret our observations, we used a theoretical description of mechanoelectrical transduction based on the gating spring theory and a motor model of adaptation. Simulations could account for the characteristics of transduction currents in wild-type and mutant hair cells, both vestibular and cochlear. They led us to conclude that harmonin-b operates as an intracellular link that limits adaptation and engages adaptation motors, a dual role consistent with the scaffolding property of the protein and its binding to both actin filaments and the tip link component cadherin-23.

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.

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.

TRPC3 ion channel subunit immunolocalization in the cochlea

Canonical transient receptor potential (TRPC) subunits assemble as tetramers to form ion channels with high calcium (Ca(2+)) permeability. Here, we investigated the possibility that TRPC3 ion channels are broadly expressed in the adult guinea pig and mouse cochleae. Using immunofluorescence, pronounced labeling occurred in the spiral ganglion (SG) neurons, inner hair cells (IHC), outer hair cells (OHC) and epithelial cells lining scala media. TRPC3 expression was homogeneous in the SG throughout the cochlea. In contrast, there was marked spatial variation in the immunolabeling in the cochlear hair cells with respect to location. This likely relates to the tonotopy of these cells. TRPC3 immunolabeling was more pronounced in the IHC than OHC. Both basal region IHC and OHC had higher TRPC3 expression levels than the corresponding cells from the apical region of the cochlea. These data suggest that TRPC3 ion channels contribute to Ca(2+) homeostasis associated with the hair cells, with higher ion fluxes in more basal regions of the cochlea, and may also be a significant pathway for Ca(2+) entry associated with auditory neurotransmission via the SG neurons. TRPC3 expression was also identified within the spiral limbus region, inner and outer sulcus, but without evidence for spatial variation in expression level. Expression in these gap junction-coupled epithelial cells lining scala media is indicative of a contribution of TRPC3 channels to cochlear electrochemical homeostasis.

A quantitative analysis of the spatiotemporal pattern of transient receptor potential gene expression in the developing mouse cochlea

TRP genes encode a diverse family of ion channels which have been implicated in many sensory functions. Because several TRP channels have similar properties to the elusive hair cell transduction channel, recent attention has focused on TRP gene expression in the inner ear. At least four TRP genes are known to be expressed in hair cells: TRPC3, TRPV4, TRPA1, and TRPML3. However, there is little evidence supporting any of these as a component of the transduction complex. Other less well-characterized TRP channels are expressed in the inner ear, in particular, within the organ of Corti. Because of their potential role in sensory function, we investigated the developmental expression of RNA that encodes all 33 TRP subunits as well as several splice variants. We designed a quantitative PCR screen using cochlear samples acquired before, during, and after the time when mechanotransduction is acquired in sensory hair cells (embryonic day 17 to postnatal day 8). Cochleas, which included the organ of Corti, stria vascularis, and Reissner's membrane, were subdivided into four equal quadrants which allowed for regional comparison during development. Expression of RNA transcripts that encoded 33 TRP subunits plus several splice forms and beta-actin were quantified in 28 samples for a total of 1,092 individual measurements, each done in triplicate. We detected RNA that encoded all TRP channels except two: TRPC7 and TRPM8. The largest changes in RNA expression were for TRPA1 (>100-fold), TRPP3 (>50-fold), and TRPC5.2 (>20-fold) which suggested that these subunits may contribute to normal cochlear function. Furthermore, the screen revealed TRPP3 and PKD1L3 RNA expression patterns that were correlated with the acquisition of sensory transduction in outer hair cells (Lelli et al., J Neurophysiol. 101:2961-2973, 2009). Numerous spatiotemporal expression gradients were identified many of which may contribute to the normal functional development of the mouse cochlea.

A potentially dynamic lysosomal role for the endogenous TRPML proteins

Lysosomal storage disorders (LSDs) constitute a diverse group of inherited diseases that result from lysosomal storage of compounds occurring in direct consequence to deficiencies of proteins implicated in proper lysosomal function. Pathology in the LSD mucolipidosis type IV (MLIV), is characterized by lysosomal storage of lipids together with water-soluble materials in cells from every tissue and organ of affected patients. Mutations in the mucolipin 1 (TRPML1) protein cause MLIV and TRPML1 has also been shown to interact with two of its paralogous proteins, mucolipin 2 (TRPML2) and mucolipin 3 (TRPML3), in heterologous expression systems. Heterogeneous lysosomal storage is readily identified in electron micrographs of MLIV patient cells, suggesting that proper TRPML1 function is essential for the maintenance of lysosomal integrity. In order to investigate whether TRPML2 and TRPML3 also play a role in the maintenance of lysosomal integrity, we conducted gene-specific knockdown assays against these protein targets. Ultrastructural analysis revealed lysosomal inclusions in both TRPML2 and TRPML3 knockdown cells, suggestive of a common mechanism for these proteins, in parallel with TRPML1, in the regulation of lysosomal integrity. However, co-immunoprecipitation assays revealed that physical interactions between each of the endogenous TRPML proteins are quite limited. In addition, we found that all three endogenous proteins only partially co-localize with each other in lysosomal as well as extra-lysosomal compartments. This suggests that native TRPML2 and TRPML3 might participate with native TRPML1 in a dynamic form of lysosomal regulation. Given that depletion of TRPML2/3 led to lysosomal storage typical to an LSD, we propose that depletion of these proteins might also underlie novel LSD pathologies not described hitherto.

The Ca(2+) channel TRPML3 regulates membrane trafficking and autophagy

TRPML3 is an inward rectifying Ca(2+) channel that is regulated by extracytosolic H(+). Although gain-of-function mutation in TRPML3 causes the varitint-waddler phenotype, the role of TRPML3 in cellular physiology is not known. In this study, we report that TRPML3 is a prominent regulator of endocytosis, membrane trafficking and autophagy. Gradient fractionation and confocal localization reveal that TRPML3 is expressed in the plasma membrane and multiple intracellular compartments. However, expression of TRPML3 is dynamic, with accumulation of TRPML3 in the plasma membrane upon inhibition of endocytosis, and recruitment of TRPML3 to autophagosomes upon induction of autophagy. Accordingly, overexpression of TRPML3 leads to reduced constitutive and regulated endocytosis, increased autophagy and marked exacerbation of autophagy evoked by various cell stressors with nearly complete recruitment of TRPML3 into the autophagosomes. Importantly, both knockdown of TRPML3 by siRNA and expression of the channel-dead dominant negative TRPML3(D458K) have a reciprocal effect, reducing endocytosis and autophagy. These findings reveal a prominent role for TRPML3 in regulating endocytosis, membrane trafficking and autophagy, perhaps by controlling the Ca(2+) in the vicinity of cellular organelles that is necessary to regulate these cellular events.

Defining features of the hair cell mechanoelectrical transducer channel

This review summarizes current knowledge of the hair cell mechanotransducer channel, the ion channel responsible for detecting mechanical stimuli in the inner ear and one of the few channels whose molecular structure is still unknown. Several candidate proteins have been proposed, especially members of the transient receptor potential (TRP) channel family, but all have so far failed in one test or another. Furthermore, none has biophysical properties exactly matching the native channel. The defining features of the native mechanotransducer channel are documented, including ionic permeability, channel structure inferred from blocking agents, diversity in channel conductance, and regulation by Ca(2+), which are compared with a potential candidate, TRP channels of the polycystin family. The strengths and weaknesses of a TRP channel contender are discussed.

Linking genes underlying deafness to hair-bundle development and function

The identification of genes underlying monogenic, early-onset forms of deafness in humans has provided unprecedented insight into the molecular mechanisms of hearing in the peripheral auditory system. The molecules involved in the development and function of the cochlea eluded characterization until recently due to the paucity of the principle cell types present in cochlear hair cells, yet a genetic approach has circumvented this problem and succeeded in identifying proteins and deciphering some of the molecular complexes that operate in these cells . In combination with mouse models, the genetic approach is now revealing some of the principles underlying the development and physiology of the cochlea. The review centers on this facet of the genetics of hearing. Focusing on the hair bundle, the mechanosensory device of the sensory hair cell, we highlight recent advances in understanding the way in which the hair bundle is formed, how it operates as a mechanotransducer and how it processes sound. In particular, we discuss how this work highlights the roles played by various hair-bundle link types.

Differential regulation of TRP channels in a rat model of neuropathic pain

Neuropathic pain is a chronic disease resulting from dysfunction of the nervous system often due to peripheral nerve injury. Hypersensitivity to sensory stimuli (mechanical, thermal or chemical) is a common source of pain in patients and ion channels involved in detecting these stimuli are possible candidates for inducing and/or maintaining the pain. Transient receptor potential (TRP) channels expressed on nociceptors respond to different sensory stimuli and a few of them have been studied previously in the models of neuropathic pain. Using real-time PCR for quantification of all known TRP channels we identified several TRP channels, which have not been associated with nociception or neuropathic pain before, to be expressed in the DRG and to be differentially regulated after spared nerve injury (SNI). Of all TRP channel members, TRPML3 showed the most dramatic change in animals exhibiting neuropathic pain behaviour compared to control animals. In situ hybridisation showed a widespread increase of expression in neurons of small, medium and large cell sizes, indicating expression in multiple subtypes. Co-localisation of TRPML3 with CGRP, NF200 and IB4 staining confirmed a broad subtype distribution. Expression studies during development showed that TRPML3 is an embryonic channel that is induced upon nerve injury in three different nerve injury models investigated. Thus, the current results link for the first time a re-expression of TRPML3 with the development of neuropathic pain conditions. In addition, decreased mRNA levels after SNI were seen for TRPM6, TRPM8, TRPV1, TRPA1, TRPC3, TRPC4 and TRPC5.

Fast adaptation and Ca2+ sensitivity of the mechanotransducer require myosin-XVa in inner but not outer cochlear hair cells

In inner ear hair cells, activation of mechanotransduction channels is followed by extremely rapid deactivation that depends on the influx of Ca(2+) through these channels. Although the molecular mechanisms of this "fast" adaptation are largely unknown, the predominant models assume Ca(2+) sensitivity as an intrinsic property of yet unidentified mechanotransduction channels. Here, we examined mechanotransduction in the hair cells of young postnatal shaker 2 mice (Myo15(sh2/sh2)). These mice have no functional myosin-XVa, which is critical for normal growth of mechanosensory stereocilia of hair cells. Although stereocilia of both inner and outer hair cells of Myo15(sh2/sh2) mice lack myosin-XVa and are abnormally short, these cells have dramatically different hair bundle morphology. Myo15(sh2/sh2) outer hair cells retain a staircase arrangement of the abnormally short stereocilia and prominent tip links. Myo15(sh2/sh2) inner hair cells do not have obliquely oriented tip links, and their mechanosensitivity is mediated exclusively by "top-to-top" links between equally short stereocilia. In both inner and outer hair cells of Myo15(sh2/sh2) mice, we found mechanotransduction responses with a normal "wild-type" amplitude and speed of activation. Surprisingly, only outer hair cells exhibit fast adaptation and sensitivity to extracellular Ca(2+). In Myo15(sh2/sh2) inner hair cells, fast adaptation is disrupted and the transduction current is insensitive to extracellular Ca(2+). We conclude that the Ca(2+) sensitivity of the mechanotransduction channels and the fast adaptation require a structural environment that is dependent on myosin-XVa and is disrupted in Myo15(sh2/sh2) inner hair cells, but not in Myo15(sh2/sh2) outer hair cells.

Life and death of sensory hair cells expressing constitutively active TRPML3

The varitint-waddler mutation A419P renders TRPML3 constitutively active, resulting in cationic overload, particularly in sustained influx of Ca(2+). TRPML3 is expressed by inner ear sensory hair cells, and we were intrigued by the fact that hair cells are able to cope with expressing the TRPML3(A419P) isoform for weeks before they ultimately die. We hypothesized that the survival of varitint-waddler hair cells is linked to their ability to deal with Ca(2+) loads due to the abundance of plasma membrane calcium ATPases (PMCAs). Here, we show that PMCA2 significantly reduced [Ca(2+)](i) increase and apoptosis in HEK293 cells expressing TRPML3(A419P). The deaf-waddler isoform of PMCA2, operating at 30% efficacy, showed a significantly decreased ability to rescue the Ca(2+) loading of cells expressing TRPML3(A419P). When we combined mice heterozygous for the varitint-waddler mutant allele with mice heterozygous for the deaf-waddler mutant allele, we found severe hair bundle defects as well as increased hair cell loss compared with mice heterozygous for each mutant allele alone. Furthermore, 3-week-old double mutant mice lacked auditory brainstem responses, which were present in their respective littermates containing single mutant alleles. Likewise, heterozygous double mutant mice exhibited severe circling behavior, which was not observed in mice heterozygous for TRPML3(A419P) or PMCA2(G283S) alone. Our results provide a molecular rationale for the delayed hair cell loss in varitint-waddler mice. They also show that hair cells are able to survive for weeks with sustained Ca(2+) loading, which implies that Ca(2+) loading is an unlikely primary cause of hair cell death in ototoxic stress situations.

TRPMLs: in sickness and in health

TRPML1, TRPML2 and TRPML3 belong to the mucolipin family of the TRP superfamily of ion channels. The founding member of this family, TRPML1, was cloned during the search for the genetic determinants of the lysosomal storage disease mucolipidosis type IV (MLIV). Mucolipins are predominantly expressed within the endocytic pathway, where they appear to regulate membrane traffic and/or degradation. The physiology of mucolipins raises some of the most interesting questions of modern cell biology. Their traffic and localization is a multistep process involving a system of adaptor proteins, while their ion channel activity possibly exemplifies the rare cases of regulation of endocytic traffic and hydrolysis by ion channels. Finally, dysregulation of mucolipins results in cell death leading to neurodegenerative phenotypes of MLIV and of the varitint-waddler mouse model of familial deafness. The present review discusses current knowledge and questions regarding this novel family of disease-relevant ion channels with a specific focus on mucolipin regulation and their role in membrane traffic and cell death. Since mucolipins are ubiquitously expressed, this review may be useful for a wide audience of basic biologists and clinicians.