Endolysosomal Cation Channels As Therapeutic Targets—Pharmacology of TRPML Channels

Review

Endolysosomal Cation Channels as Therapeutic Targets—Pharmacology of TRPML Channels

Christian Grimm

Munich Center for Integrated Protein Science CIPSM and Department of Pharmacy-Center for Drug Research, Ludwig-Maximilians-Universität München, 81377, Germany

In recent years it has become more and more accepted that the endolysosomal system (ES) plays a key role for human health. Dysfunction of the ES has been found to be implicated in a range of human diseases ranging

MESSENGER from infectious and metabolic to lysosomal storage, retinal and neurodegenerative diseases. Results obtained from animal models and human mutations have also spurred the interest in endolysosomal membrane proteins. In particular, the study of endolysosomal ion channels as potential novel drug targets for the treatment of various diseases has gained momentum with recently established endolysosomal patch-clamp techniques. These techniques now allow functional characterization of these organellar membrane proteins in more detail. Another key development was the discovery of small molecule agonists and antagonists to pharmacologically interfere with these endolysosomal ion channels in vitro and in vivo. This review gives an overview of the currently available small molecule agonists and antagonists of one major group of endolysosomal cation channels, the mucolipins or TRPML channels and how they have helped to speed up research in the ﬁeld. Keywords: Calcium, TRPML, MCOLN, TRPML1, Mucolipin.

MESSENGER MESSENGER IP: 192.168.39.151 On: Thu, 30 Sep 2021 22:36:36 Copyright: American Scientific Publishers CONTENTS Delivered byand Ingenta neurodegenerative diseases, retinal and pigmentation disorders, trace metal dishomeostasis and infectious dis- Introduction ...... 30 eases (Fig. 1(A)). Besides soluble proteins in the matrix Discovery of Small Molecule Agonists and Antagonists of TRPMLChannels ...... 32 of endolysosomal organelles, endolysosomal membrane SmallMoleculesLinkTRPMLFunctionwithPhysiology...... 32 proteins, in particular endolysosomal ion channels and Species-DependentEffectsofTRPMLChannelOpeners...... 34 transport proteins are increasingly recognized as impor- TRPMLChannelsasTherapeuticTargets ...... 35 tant regulators of endolysosomal function. Thus, they are Conclusion...... 35 highly critical for the regulation of ionic concentrations Acknowledgment...... 35 References...... 35 within the endolysosomal vesicle lumen and they partic- MESSENGER ipate in the regulation of the numerous endolysosomal vesicle trafﬁcking processes. They also participate in the INTRODUCTION regulation of phagocytosis, lysosomal exocytosis, lysosomal vesicle motility, metal homeostasis, lysosomal pH, Endosomes and lysosomes are cell organelles known autophagosome-lysosome fusion and lysosome reforma- to be involved in transport, breakdown and secretion from autolysosomes (Dong et al., 2010; Samie et al., tion of lipids, proteins, and other macromolecules. The 2013; Grimm et al., 2014; Bae et al., 2014; Sakurai et al., endolysosomal system (ES) as it is found in almost 2015; Patel et al., 2015; Li et al., 2016; Park et al., 2016). every cell of the human body consists in particular of Very recently also a link between oxidative stress, ROS early endosomes (EE), recycling endosomes (RE), late (reactive oxygen species) and TRPML channels has been endosomes (LE), and lysosomes (LY). Endolysosomal

MESSENGER established (Zhang et al., 2016; Kiselyov and Muallem, dysfunction can cause multiple storage disorders such as 2016). mucolipidoses, sphingolipidoses or neuronal ceroid lipo- The endolysosomal channelome/transportome which fuscinoses (NCL), which typically go along with fatal comprises at least 70 different proteins (Schwake et al., neurodegenerative processes. Dysfunction of the ES has 2013; Chapel et al., 2013) is however, with few exceptions also been implicated in the development of metabolic still “terra incognita” (Fig. 1(B)). With recent advances in endolysosomal patch-clamp technology, it has now E-mail: [email protected] become possible to directly investigate the biophysical

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Figure 1. The endolysosmal system and the endolysosomal transportome. (A) The endolysosomal system (ES) as it is found in almost every cell of the human body, typically consists of early endosomes (EE), late endosomes (LE), and lysosomes (LY). In addition, recycling endosomes, autophagosomes, and multivesicular bodies are also part of the ES (not shown here). The ES is required for the intracellular transport, recycling and degradation of receptors and other macromolecules as well as trace metals such as iron or zinc. Other important cell-dependent functional roles of the ES are lysosomal exocytosis and phagocytosis. (B) Endolysosomal ion channels such as TRPML channels (TRPML1, 2 and 3) or two-pore channels (TPC1 and TPC2) have recently emerged as important regulators of trafﬁcking and fusion processes within the ES and appear to be essential for a proper communication between the various endolysosomal organelles. Their biophysical properties and ion channel characteristics have been well described while the vast majority of the proteins of the endolysosomal transportome is not characterized yet. MESSENGER properties and thus the physiological roles of endolysoso- channel family, namely the endolysosomal TRPML chan- mal ion channels with important implications for diseases nels (TRPML1-3), also called mucolipins and the two- which until now have been classiﬁed as non-treatable. pore channels (TPCs) have recently been characterized Members of the transient receptor potential (TRP) cation in more detail with these novel patch-clamp techniques

Christian Grimm (degree in Pharmaceutical Sciences from the University of Würzburg, Germany (2000) and Ph.D. in Pharmacology (2004), Freie Universität (FU) Berlin, Germany). After several years as Postdoctoral Research Fellow at Harvard and Stanford Universities, USA (2004–2009) and time as a Principal Scientist at Pfizer R&D in the UK (2009–2011) he is now a Senior Lecturer (Privatdozent) at the Department of Chemistry and Pharmacy, Section of Pharmacology at the University of München (LMU), Germany. His main scientific interest is on TRP cation channels and in recent years he focused on members of the TRP family that are expressed in the endolysosomal system: TRPML channels and two-pore channels (TPCs). The results of his activity have been published in more than 40 peer reviewed scientific articles, reviews and book chapters.

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(Dong et al., 2008; Dong et al., 2010; Shen et al., 2012; (Grimm et al., 2010) and against a plethora of other targets Wang et al., 2012; Zhang et al., 2012; Cang et al., 2013; (generally more than 500, each; for further information see Cheng et al., 2014; Grimm et al., 2014; Chen et al., 2014; http://pubchem.ncbi.nlm.nih.gov; AID: 1448, 1525, 1526, Jha et al., 2014; Sakurai et al., 2015; Ruas et al., 2015; Li 1562, 2719, 1809, and 2694), cross-reactivity within the et al., 2016; Zhang et al., 2016). TRPML subfamily as well as differences between human Apart from newly developed endolysosomal patch- and murine channel isoforms remained an issue. Thus, clamp techniques, it was the discovery of small molecule ML-SA1 (SF-51-type) for example is non-selective and activators (Grimm et al., 2010) that has significantly fos- potently activates all three TRPML channel isoforms while tered and facilitated TRPML channel and thus endolyso- other compounds including MK6-83 are more selective somal ion channel research in general. The availability (Grimm et al., 2010; Fig. 2(D)). In addition, some com- of small molecule TRPML channel agonists has helped pounds do show off-target effects, e.g., ML-SA1 (but to establish and to validate endolysosomal patch-clamp not SF-51) inhibits TRPC4 with an IC50 of 4.2 M techniques and to better comprehend the physiological (confirmation dose-response assay) and SF-21 (EC50 for and biophysical properties of TRPML channels includ- TRPML3 = 860 nM) activates the CRF-binding protein, ing TRPML1 and the pathophysiological consequences of albeit with a much higher EC50 (13.7 M; confirma- TRPML1 mutations in humans suffering from mucolipido- tion dose-response assay) (for further information see http://pubchem.ncbi.nlm.nih.gov; AID: 1448, 1525, 1526,

MESSENGER sis type IV (Shen et al., 2012; Samie et al., 2013; Cheng et al., 2014; Wang et al., 2014; Bae et al., 2014; Chen 1562, 2719, 1809, 2694 and Grimm and Cuajungco, 2014). et al., 2014; Li et al., 2016; Zhang et al., 2016). TRPML Pharmacological inhibition of TRPML channel activity channel activators are also considered to serve as poten- has also been shown recently. Three compounds have been tial therapeutic agents for the treatment of other lysosomal developed so far, namely ML-SI1, ML-SI2, and ML-SI3 storage disorders (LSDs) and neurodegenerative patholo- (Samie et al., 2013, Cheng et al., 2014; Wang et al., 2015; gies (Shen et al., 2012; Bae et al., 2014). In the following, Zhang et al., 2016; Kilpatrick et al., 2016). Only for two of currently available pharmacological tools to interfere with the three compounds (ML-SI1 and ML-SI3) the chemical TRPML channel function will be discussed in more detail structures have been revealed (Wang et al., 2015). as well as potential therapeutic applications. SMALL MOLECULES LINK TRPML MESSENGER MESSENGER IP: 192.168.39.151 On: Thu,FUNCTION 30 Sep 2021 WITH 22:36:36 PHYSIOLOGY DISCOVERY OF SMALL MOLECULECopyright: American Scientific Publishers AGONISTS AND ANTAGONISTS OF Delivered byCurrently Ingenta available compounds have been proven to be TRPML CHANNELS very useful tools to investigate and to advance our under- standing of TRPML channel function and physiology. Screening procedures and results of the first TRPML chan- Thus, Shen et al. (2012) showed that TRPML1-mediated nel high-throughput-screen (HTS; with more than 217000 lysosomal Ca2+-release, measured using SF-51 or ML- compounds being screened) have been discussed exten- SA1 in combination with GCaMP3 directly attached to sively in previous publications and reviews (Grimm et al., TRPML1, is reduced in Niemann-Pick disease type C1 2010; Yamaguchi and Muallem, 2010; Saldanha et al., (NPC1) cells. They further demonstrated in patch-clamp 2011; Grimm et al., 2012a; Grimm and Cuajungco, 2014; analyses using SF-51 or ML-SA1 that TRPML1 channel MESSENGER Grimm et al., 2014). Briefly, the TRPML channel activa- activity is inhibited by sphingomyelins accumulating in tors identified in this HTS belong to different chemotypes NPC1 cells. It was therefore proposed that abnormal accu- including (aryl)sulfonamides (e.g., SN-1-, SF-11-, SF-21-, mulation of luminal lipids causes secondary lysosome stor- or SF-22-type including MK6-83; Fig. 2(A)), aminoac- age by blocking TRPML1- and Ca2+-dependent lysosomal etamides (SF-51-type including ML-SA1; Fig. 2(B)), isox- trafficking. It should be mentioned however that despite azolines (e.g., SN-2-type) and others (Grimm et al., these apparent similarities between MLIV and NPC1 cells 2010; Saldanha et al., 2011; Grimm et al., 2012a; Shen converging on TRPML1 function, there are also intrigu- et al., 2012; Saldanha et al., 2013; Chen et al., 2014; ing differences found between MLIV and NPC1 cells. For Grimm and Cuajungco, 2014; Grimm et al., 2014). Sev- example, Park et al. recently showed that excessive unreg- eral lead structures were identified which were later further ulated exocytosis is something that occurs only in MLIV

MESSENGER improved by chemical modification, in particular MK6- cells but not in NPC1 cells, suggesting that a major role 83 was developed from compound SF-22 (Chen et al., of TRPML1 is to guard against unintended, pathologi- 2014; Fig. 2(C)) and ML-SA1 is a modified version of cal fusion of lysosomes with other intracellular organelles SF-51 with strongly increased efficacy on TRPML1 com- including secretory organelles (Park et al., 2016). pared to SF-51 (Shen et al., 2012; Chen et al., 2014). Bae and colleagues (2014) found that ML-SA1 cleared While all candidate compounds were found to be inac- calcium, sphingomyelin and A from lysosomal com- tive against other TRP channels, including members of partments in neurons with evidence of lysosome storage the TRPC, TRPV, TRPM, TRPA, and TRPN subfamilies induced by HIV gp120. These findings suggested that

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Figure 2. Synthesis and selectivity profiles of small molecule activators of TRPML channels: SF-22, SF-51, ML-SA1, and MK6-83. (A–B) Shown are the chemical structures of TRPML channel activators. (A) The group of (aryl)sulfonamides comprises SF-22 and its derivative MK6-83 as well as most other known lead structures (SF-11, SF-21, SF-22, SF-23, SF-24, SF-31, SF-32, SF-33, SF-41, and SN-1; Grimm et al., 2010; Grimm et al., 2012a; Grimm et al., 2014). (B) In contrast, SF-51 and its derivative ML-SA1 as well as SF-61, SF-71, SF-81, and SN-2 belong to other, unique scaffolds (Grimm et al., 2010; Grimm et al., 2012a; Grimm et al., 2014). (C) Strategy used for the chemical synthesis of MK6-83. (D) Shown are the effects of SF-22, MK6-83, and ML-SA1 on intracellular calcium levels (fura-2 ratios F340/F380) in HEK293 cells transiently transfected with the human TRPML1 plasma membrane variant (hTRPML1(NC)-YFP), human TRPML2-YFP, or human TRPML3-YFP. The latter two variants also reach the PM when overexpressed (shown are mean values ± SEM of at least 3 independent experiments with n = 5–10 cells, each). (E) Dose-response curves showing effects of SF-22 and MK6-83 on hTRPML1 WT MESSENGER expressing lysosomes isolated from HEK293 cells and measured using the endolysosomal planar patch-clamp technique. EC50 values were calculated as 0.11 ± 0.01 M for MK6-83 versus 0.51 ± 0.05 M for SF-22. therapeutics designed to restore lysosome function may molecule inhibitor ML-SI3. ML-SI3 used in this study as protect the CNS in HIV infected patients. Because deficits well as additional inhibitors, ML-SI1 and ML-SI2 were in lysosome function are common to a number of neurode- also the first reported inhibitors for TRPML channels. generative conditions, further development and testing of They were initially described by Samie et al. (2013) who TRPML1 agonists was suggested to possibly reveal neuro- found that phagocytosis of large particles in macrophages protective effects with application for a variety of neurode- is inhibited by these compounds. generative conditions. Cheng et al. (2014) reported that Chen et al. (2014) used SF-22 and MK6-83 to demon- TRPML1-null mice develop a primary, early-onset mus- strate that the effect of specific MLIV causing mutations cular dystrophy independent of neural degeneration. They on TRPML1 channel activity and its functions in endolyso- found that membrane resealing was defective in TRPML1- somal trafficking and heavy metal homeostasis can be null muscle fibers and also upon acute and pharmacolog- restored. While the synthetic ligands strongly activate ical inhibition of TRPML1 channel activity by the small WT and the mutant isoforms F408 and F465L, channel

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activation by its endogenous ligand PI(3,5)P2 was found to required to promote calcium dependent centripetal move- be remarkably reduced in these mutants. Endolysosomal ment of lysosomes towards the perinuclear region follow- patch-clamp measurements with MK6-83 further revealed ing autophagy induction. that F465L has lost-in contrast to WT TRPML1-its pH Recently, Kilpatrick and colleagues (2016) made use of sensitivity (Chen et al., 2014). Leucine instead of pheny- MK6-83 and ML-SA1 to show that activation of endoge- lalanine at this position was found to abrogate the proton- nous and recombinant TRPMLs with these agonists can mediated increase in channel activity observed in WT evoke global Ca2+ signals in human cells. These signals TRPML1. were blocked by a dominant negative TRPML1 construct Wang et al. (2015) postulated a role of TRPML1 in and the TRPML channel inhibitor ML-SI3. The authors starvation-induced enhancement of lysosomal function. further showed that despite a predominant lysosomal local- Nutrient deprivation results in an inhibition of mTOR isation, TRPML1 supports both Ca2+ release and Ca2+ 2+ 2+ kinase activity and a decrease in lysosomal PI(3,5)P2 entry. Ca release required lysosomal and ER Ca stores, levels, both of which promote TFEB nuclear transloca- suggesting that TRPMLs like other endolysosomal Ca2+ tion. Activated TFEB (transcription factor EB) induces channels are capable of “chatter” with ER Ca2+ channels. the expression of genes needed for autophagosome biogenesis and lysosome biogenesis. Through transcrip- SPECIES-DEPENDENT EFFECTS OF TRPML tional, translational, and posttranslational mechanisms, MESSENGER CHANNEL OPENERS lysosomal TRPML1 channel activity is up-regulated to increase lysosomal Ca2+ release, thereby promot- Compared with TRPML1 which is ubiquitously expressed, ing both autophagosome-lysosome fusion and lysosome the expression of the related channels TRPML2 and reformation from autolysosomes. The starvation effect TRPML3 is much more limited to certain tissues and also enhanced clearance of lysosomal storage, including within those tissues to certain cell types. To unequivo- cholesterol accumulation in NPC1 cells. Pharmacological cally identify endogenous TRPML2 and TRPML3 cur- inhibition or genetic deletion of TRPML1 completely abol- rents it is desired to generate more potent and selective ished the effects of nutrient deprivation on boosting the tools for both murine and human TRPML2 and TRPML3. degradation capability of lysosomes (Wang et al., 2015). Isoform-selective and more potent small molecule tools Onyenwoke and colleagues (2015) reported that mTOR in combination with knockout cells will eventually help

MESSENGERdirectly targets and inactivates MESSENGER IP: TRPML1. 192.168.39.151 They identi- On: Thu,to 30 clearly Sep 2021 discriminate 22:36:36 between endogenous TRPML1 fied a TRPML1 mutant, TRPML1 (S572E/S576E)Copyright: American which Scientificand other Publishers TRPML channel currents. Several of the pub- was non-responsive to ML-SA1 and the mTORDelivered inhibitor bylished Ingenta TRPML3 channel activators have been shown to rapamycin while TRPML1 wild-type was activated by be already relatively isoform-selective depending on the rapamycin-mediated mTOR inhibition or by ML-SA1. species (Grimm et al., 2010). For example, compounds However, Li et al., (2016) stated that, based on their patch- SN-2 and SF-11 have little effect on human TRPML1 clamp results mTOR inhibition does not activate TRPML1. and mouse TRPML2 but show significantly more effect Zhang et al., (2016) suggested that TRPML1 is a reac- on mouse TRPML1 and human TRPML2, once again tive oxygen species (ROS) sensor localized on the lyso- demonstrating that selectivity profiles can vary consider- somal membrane orchestrating an autophagy-dependent ably between species (Grimm et al., 2010 and unpub- negative-feedback program to mitigate oxidative stress in lished data). In this context, the study by Feng and MESSENGER the cell. They showed that ROS activate TFEB via a mech- colleagues (2014) on the effects of TRPML channel acti- anism that is dependent on lysosomal Ca2+. Exogenous vators on Drosophila TRPML is of interest. Feng et al. application of oxidants or pharmacologically-increasing found that ML-SA1 failed to activate Drosophila TRPML mitochondrial ROS levels resulted in direct, specific, and unless exogenous phosphatidylinositol 3,5-bisphosphate 2+ sustained activation of TRPML1, inducing Ca release (PI(3,5)P2 was applied. At 1 MML-SA1,thesensi- from the lysosomal lumen. This activation was found to tivity of TRPML to PI(3,5)P2 increased approximately be required for ROS-induced TFEB nuclear translocation, by 10-fold and at 10 M ML-SA1, the deactivation of

autophagy induction, and lysosomal biogenesis. In this PI(3,5)P2-evoked TRPML currents was markedly slowed. study different TRPML-activators were used as reference The authors also conﬁrmed that, different from the insect compounds for the ROS effect on TRPML1 and TRPML TRPML, human TRPML1 was readily activated by ML-

MESSENGER channel inhibitors were used to demonstrate speciﬁcity of SA1 or SF-22 independent of PI(3,5)P2. Thus, their data the effects. Thus, when TRPML1 was pharmacologically reveal that while ML-SA1 and SF-22 act as true agonists inhibited, clearance of damaged mitochondria and removal at human TRPML1, they behave as allosteric activators of excess ROS could be blocked. of the Drosophila TRPML channel. In contrast to SF-22 Li et al., (2016) examined the role of TRPML1 in and ML-SA1, SF-21 and SF-41 did not activate human regulating lysosome motility by manipulating TRPML1 TRPML1 or Drosophila TRPML. Both, SF-21 and SF-41 activity acutely through TRPML1 agonists or antagonists. had been shown before not to activate TRPML1 (human) Thus, they could demonstrate that TRPML1 activity is while TRPML3 (human and mouse) as well as TRPML2

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(human and mouse) were found to be potently activated The cross-talk between lysosomal Ca2+ release, (Grimm et al., 2010; Grimm et al., 2012b). TRPML1 and TFEB (Wang et al., 2015; Zhang et al., 2016), the master gene of lysosomal biogenesis, is another TRPML CHANNELS AS interesting finding as this further suggests that TRPML1 THERAPEUTIC TARGETS may be a promising target for treating lysosome-related diseases (Li et al., 2016), including lysosomal storage, TRPML1 is a ubiquitously expressed protein and it metabolic, neurological, and immune disorders. In this appears to regulate a plethora of cellular processes, includ- context, it should also be noted that dysregulation of ing endocytic membrane traffic, phagocytosis, lysosomal the mTOR pathway and TFEB leads to different forms MESSENGER exocytosis, lysosomal biogenesis, metal distribution, and of human cancer (Karar and Maity, 2011; Raben and autophagy. Mutation or loss of TRPML1 like in mucolipi- Puertollano, 2016), opening up a link between TRPML1 dosis type IV (MLIV), leads to severe neurologic deficits and oncogenic diseases. Interestingly, a link between cal- in human patients. The most profound clinical manifesta- cium release from acidic stores, TFEB and the two- tions of MLIV are severe psychomotor retardation during pore channel TPC1 has also recently been discovered the first year of life and slowly progressing vision loss (Höglinger et al., 2015). due to corneal clouding and retinal degeneration result- Future studies will have to demonstrate whether ing in blindness by the second decade of life. Neuromotor TRPML1 targeting is sensitive enough to result in signifi- abnormalities include delayed attainment of motor mile- cant effects, e.g., on lysosomal storage in LSDs or whether

stones, spasticity, hypotonia, inability to walk indepen- it is too sensitive and thus resulting in severe side-effects. MESSENGER dently, ptosis, myopathic facies, drooling, difficulties in chewing and swallowing, and severely impaired fine-motor function. Most patients reach a level of motor, speech CONCLUSION and cognitive development of about 15 months (Grishchuk Endolysosomal cation channels, in particular TRPML et al., 2014). Furthermore, patients show a severe lack of channels and TPCs, undoubtedly play fundamental roles myelin in the brain, resulting in a distinct loss of the cor- for the function of the endolysosomal system and thus for pus callosum and a small cerebellum. Patients also have human health and disease. The availability of highly spe- severe iron deficiency anemia secondary to hypochlorhy- cific and potent TRPML channel activators and inhibitors dria (Altarescu et al., 2002). ThisIP: characteristic192.168.39.151 lack On: of Thu,is 30 unprecedented Sep 2021 22:36:36 in the TRP channel field. As outlined gastric acid also causes a dramatic increaseCopyright: in the American level Scientifichere, these Publishers tools are well suited to investigate TRPML Delivered by Ingenta MESSENGER MESSENGER of blood gastrin, a hormone that stimulates gastric acid channels both in-vitro and in-vivo andtheyhavegiven secretion (Altarescu et al., 2002). The role of TRPML1 major insights into the function of these proteins. The in gastric acid release has recently been further investi- development of selective TRPML1 channel agonists for gated in mice lacking TRPML1 (Chandra et al., 2011). −/− therapeutic use will be the next step to corroborate the Thus, Trpml1 mice had significant impairments in validity of TRPML1 as a potentially powerful novel target basal and histamine-stimulated gastric acid secretion and to treat LSDs, lysosome-, and autophagy-related diseases. markedly reduced levels of the gastric proton pump. Histo- Trpml1−/− logic and ultrastructural analyses revealed that Acknowledgment: This work was supported by fund- parietal cells were enlarged, had multivesicular and multi- ing of the German Research Foundation (SFB/TRR152 lamellated lysosomes, and maintained an abnormal intra- TP04 to C.G.). cellular canalicular membrane (Chandra et al., 2011). MESSENGER Most MLIV cells are highly vacuolated and mitochondria seem to be fragmented. Cells further show defects REFERENCES in autophagy, intracellular trafficking and intracellular Altarescu, G., Sun, M., Moore, D. F., Smith, J. A., Wiggs, E. A., metabolism. Solomon, B. I., Patronas, N. J., Frei, K. P., Gupta, S., Kaneski, C. R., Loss of TRPML1 (function) in MLIV patients will Quarrell, O. W., Slaugenhaupt, S. A., Goldin, E., and Schiffmann, likely require a molecular surrogate. However, targeting R. (2002). The neurogenetics of mucolipidosis type IV. Neurology TRPML1 seems an appealing strategy in other lysosome- 59, 306–313. related diseases. Shen et al. (2012) as well as Bae et al. Bae, M., Patel, N., Xu, H., Lee, M., Tominaga-Yamanaka, K., Nath, A., (2014) have already demonstrated that stimulation of Geiger, J., Gorospe, M., Mattson, M. P., and Haughey, N. J. (2014). Activation of TRPML1 clears intraneuronal A in preclinical models of TRPML1 has positive effects on the pathology of other HIV infection. J. Neurosci. 34, 11485–11503. lysosomal storage disorders such as NPC1 or neurode- Chandra, M., Zhou, H., Li, Q., Muallem, S., Hofmann, S. L., and generative diseases such as the progressive brain-aging Soyombo, A. A. (2011). A role for the Ca2+ channel TRPML1 in gas- observed in a high number of HIV patients which show tric acid secretion, based on analysis of knockout mice. Gastroenterology deficits in attention, executive functioning, memory and 140, 857–867. processing speed that are reminiscent of advanced age, and Cang, C., Zhou, Y., Navarro, B., Seo, Y. J., Aranda, K., Shi, L., Battaglia- overlap with Alzheimer’s disease. Hsu, S., Nissim, I., Clapham, D. E., and Ren, D. (2013). mTOR regulates

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Received: 8 August 2016. Accepted: 23 December 2016.

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