Mucolipins: Intracellular TRPML1-3 channels

Abstract

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.

Figure 1. TRPMLs in the endocytic pathway

Intracellular compartments undergo cargo-dependent maturation (indicated by black arrows), membrane fusion (white arrows), and fission/budding (red arrows). The molecular identities of intracellular compartments are defined by specific recruitment of small G proteins (Rab and Arf GTPases) and the composition of phosphoinositides (PIPs). Endolysosomes are Ca²⁺ stores with luminal Ca²⁺ concentration estimated to be approximately 0.5 mM. The pH of each organelle is indicated. In endolysosomes, TRPML-mediated intra-endosomal Ca²⁺ release may activate Ca²⁺ sensor proteins such as Synaptotagmin (Syt) and calmodulin (CaM) to trigger homotypic and heterotypic fusion. TRPML3 is present in early endosomes (EEs; pH 6.0; PI(3)P; Rab5), which are derived from the primary endocytic vesicles after endocytosis. In addition to the late endocytic pathway, contents in the EE can also be sorted into recycling endosomes (RE; pH 6.4), which are subsequently recycled back to the plasma membrane. TRPML2 is present in Arf6 – positive REs. The channel activity of TRPML2 (in RE) may regulate the activation of small GTPase Arf6, an important regulator of the recycling pathway. EEs can undergo maturation (membrane trafficking) to become late endosomes (LEs; pH 5.5; PI(3)P+PI(3,5)P2; Rab7). Membrane proteins enter the degradation pathway following membrane invagination to form multi-vesicular bodies (MVB) in LEs. LEs can fuse with LYs (LYs; pH 4.5; PI(3)P+PI(3,5)P2; Rab7) to form LE-LY hybrids. LYs can then be reformed from LE-LY hybrids. Other than fusion with LEs, LYs can also undergo fusion with autophagosomes (APs) to form autolysosomes (ALs) or with the plasma membrane during lysosomal exocytosis. TRPML1-3 channels are predominantly localized in LEs and LYs. Activation of TRPML channels by unidentified cellular cues may induce intralysosomal Ca²⁺ release. LEs, LYs, or hybrids of LEs and LYs will then undergo CaM- or Syt- dependent membrane fusion or fission/budding. Retrograde transport vesicles, derived from EEs, LEs, or LYs upon membrane fission transport lipids and proteins in a retrograde direction to the trans-Golgi Network (TGN).

Figure 2. TRPMLs in the lysosome

A) While the ionic compositions of the extracellular space and cytosol have been well established, ion concentrations in the lumen of lysosomes are not clear. The luminal pH is between 4 and 5 and is established by a V-type ATPase. The [Ca²⁺]_LY is approximately 0.5 mM and maintained by an unidentified H⁺-Ca²⁺ exchanger. The resting membrane potential (Δφ) of the cell is approximately −70 mV (cytoplasmic-side negative). Based on studies of phagosomes, the membrane potential across the lysosomal membrane is estimated to be approximately + 30 mV (luminal-side positive). TRPMLs are permeable to Fe²⁺ (except for TRPML3) and Ca²⁺; none of the TRPML isoforms are permeable to H⁺. Although wild-type TRPMLs are predominantly expressed in the lysosome, TRPML^Vaproteins are present at the plasma membrane. B) Current-voltage (I–V) relationship of TRPML channels. All TRPMLs exhibit a strong inward rectification.