Summary Mammalian Two-Pore Channels (TPC1, 2; TPCN1, TPCN2) encode ion channels in intracellular endosomes and lysosomes and were proposed to mediate endolysosomal calcium release triggered by the second messenger, nicotinic acid adenine dinucleotide phosphate (NAADP). By directly recording TPCs in endolysosomes from wild-type and TPC double knockout mice, here we show that, in contrast to previous conclusions, TPCs are in fact sodium-selective channels activated by PI(3,5)P2, and are not activated by NAADP. Moreover, the primary endolysosomal ion is Na+, not K+, as had been previously assumed. These findings suggest that the organellar membrane potential may undergo large regulatory changes, and may explain the specificity of PI(3,5)P2 in regulating the fusogenic potential of intracellular organelles.

Membrane fusion and fi ssion events in intracellular traffi cking 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-specifi c PIP, binds and activates endolysosome-localized mucolipin transient receptor potential (TRPML) channels with specifi city and potency. Both PI(3,5)P 2 -defi cient cells and cells that lack TRPML1 exhibited enlarged endolysosomes / vacuoles and traffi cking defects in the late endocytic pathway. We fi nd that the enlarged vacuole phenotype observed in PI(3,5)P 2 -defi cient mouse fi broblasts 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 traffi cking by transducing information regarding PI(3,5)P 2 levels into changes in juxtaorganellar Ca 2 + , thereby triggering membrane fusion / fi ssion events.

Lysosomal lipid accumulation, defects in membrane trafficking, and altered Ca2+ homeostasis are common features in many lysosomal storage diseases. Mucolipin TRP channel 1 (TRPML1) is the principle Ca2+ channel in the lysosome. Here we show that TRPML1-mediated lysosomal Ca2+ release, measured using a genetically-encoded Ca2+ 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 NP cell lysosomes. Patch-clamp analyses revealed that TRPML1 channel activity is inhibited by SMs, but potentiated by SMases. In NP type C (NPC) cells, increasing TRPML1’s expression/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 Ca2+-dependent lysosomal trafficking.

TRPML1 (mucolipin-1/MCOLN1) is predicted to be an intracellular late endosomal and lysosomal ion channel protein belonging to the mucolipin subfamily of Transient Receptor Potential (TRP) proteins 1–3. Mutations in the human TRPML1 gene cause mucolipidosis type IV disease (ML4) 4, 5. ML4 patients exhibit motor impairment, mental retardation, retinal degeneration, and iron-deficiency anemia. Since aberrant iron metabolism may cause neural and retinal degeneration 6, 7, it may be a primary cause of ML4 phenotypes. In most mammalian cells, release of iron from endosomes and lysosomes following iron uptake via endocytosis of Fe3+-bound transferrin receptors 6, or following lysosomal degradation of ferritin-Fe complexes and autophagic ingestion of iron-containing macromolecules 6, 8, is the major source of cellular iron. The Divalent Metal Transporter protein (DMT1) is the only endosomal Fe2+ transporter currently known and is highly expressed in erythroid precursors 6, 9, but genetic studies suggest the existence of a DMT1-independent endosomal/lysosomal Fe2+ transport protein 9. Here, by measuring radiolabeled iron uptake, monitoring the levels of cytosolic and intra-lysosomal iron and directly patch-clamping the late endosomal/lysosomal membrane, we show that TRPML1 functions as a Fe2+ permeable channel in late endosomes and lysosomes. ML4 mutations are shown to impair TRPML1’s ability to permeate Fe2+ at varying degrees, which correlate well with the disease severity. A comparison of TRPML1−/− ML4 and control skin fibroblasts showed a reduction of cytosolic Fe2+ levels, an increase of intra-lysosomal Fe2+ levels, and an accumulation of lipofuscin-like molecules in TRPML1−/− cells. We propose that TRPML1 mediates a mechanism by which Fe2+ is released from late endosomes/lysosomes. Our results suggest that impaired iron transport may contribute to both hematological and degenerative symptoms of ML4 patients.

Neuregulin-1 (NRG1), a regulator of neural development, has been shown to regulate neurotransmission at excitatory synapses. Although ErbB4, a key NRG1 receptor, is expressed in glutamic acid decarboxylase (GAD)-positive neurons, little is known about its role in GABAergic transmission. We show that ErbB4 is localized at GABAergic terminals of the prefrontal cortex. Our data indicate a role of NRG1, both endogenous and exogenous, in regulation of GABAergic transmission. This effect was blocked by inhibition or mutation of ErbB4, suggesting the involvement of ErbB4. Together, these results indicate that NRG1 regulates GABAergic transmission via presynaptic ErbB4 receptors, identifying a novel function of NRG1. Because both NRG1 and ErbB4 have emerged as susceptibility genes of schizophrenia, these observations may suggest a mechanism for abnormal GABAergic neurotransmission in this disorder.