Loss of TRPML1 promotes production of reactive oxygen species: is oxidative damage a factor in mucolipidosis type IV?

TRPML1 (transient receptor potential mucolipin 1) is a lysosomal ion channel permeable to cations, including Fe2+. Mutations in MCOLN1, the gene coding for TRPML1, cause the LSD (lysosomal storage disease) MLIV (mucolipidosis type IV). The role of TRPML1 in the cell is disputed and the mechanisms of cell deterioration in MLIV are unclear. The demonstration of Fe2+ buildup in MLIV cells raised the possibility that TRPML1 dissipates lysosomal Fe2+ and prevents its accumulation. Since Fe2+ catalyses the production of ROS (reactive oxygen species), we set out to test whether or not the loss of TRPML1 promotes ROS production by Fe2+ trapped in lysosomes. Our data show that RPE1 (retinal pigmented epithelial 1) cells develop a punctate mitochondrial phenotype within 48 h of siRNA-induced TRPML1-KD (knockdown). This mitochondrial fragmentation was aggravated by Fe2+ exposure, but was reversed by incubation with the ROS chelator α-Toc (α-tocopherol). The exposure of TRPML1-KD cells to Fe2+ led to loss of ΔΨm (mitochondrial membrane potential), ROS buildup, lipid peroxidation and increased transcription of genes responsive to cytotoxic oxidative stress in TRPML1-KD cells. These data suggest that TRPML1 redistributes Fe2+ between the lysosomes and the cytoplasm. Fe2+ buildup caused by TRPML1 loss potentiates ROS production and leads to mitochondrial deterioration. Beyond suggesting a new model for MLIV pathogenesis, these data show that TRPML1's role in the cell extends outside lysosomes.

Zinc-dependent lysosomal enlargement in TRPML1-deficient cells involves MTF-1 transcription factor and ZnT4 (Slc30a4) transporter.

Zinc is critical for a multitude of cellular processes, including gene expression, secretion and enzymatic activities. Cellular zinc is controlled by zinc-chelating proteins and by zinc transporters. The recent identification of zinc permeability of the lysosomal ion channel TRPML1 (transient receptor potential mucolipin 1), and the evidence of abnormal zinc levels in cells deficient in TRPML1, suggested a role for TRPML1 in zinc transport. In the present study we provide new evidence for such a role and identify additional cellular components responsible for it. In agreement with the previously published data, an acute siRNA (small interfering RNA)-driven TRPML1 KD (knockdown) leads to the build-up of large cytoplasmic vesicles positive for LysoTracker™ and zinc staining, when cells are exposed to high concentrations of zinc. We now show that lysosomal enlargement and zinc build-up in TRPML1-KD cells exposed to zinc are ameliorated by KD of the zinc-sensitive transcription factor MTF-1 (metal-regulatory-element-binding transcription factor-1) or the zinc transporter ZnT4. TRPML1 KD is associated with a build-up of cytoplasmic zinc and with enhanced transcriptional response of mRNA for MT2a (metallothionein 2a). TRPML1 KD did not suppress lysosomal secretion, but it did delay zinc leak from the lysosomes into the cytoplasm. These results underscore a role for TRPML1 in zinc metabolism. Furthermore, they suggest that TRPML1 works in concert with ZnT4 to regulate zinc translocation between the cytoplasm and lysosomes.

Membrane potential regulates nicotinic acid adenine dinucleotide phosphate (NAADP) dependence of the pH- and Ca2+-sensitive organellar two-pore channel TPC1.

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent second messenger that mobilizes Ca(2+) from the acidic endolysosomes by activation of the two-pore channels TPC1 and TPC2. The channel properties of human TPC1 have not been studied before, and its cellular function is not known. In the present study, we characterized TPC1 incorporated into lipid bilayers. The native and recombinant TPC1 channels are activated by NAADP. TPC1 activity requires acidic luminal pH and high luminal Ca(2+). With Ba(2+) as the permeable ion, luminal Ca(2+) activates TPC1 with an apparent K(m) of 180 µm. TPC1 operates in two tightly coupled conductance states of 47 ± 8 and 200 ± 9 picosiemens. Importantly, opening of the large conductance markedly increases the small conductance mean open time. Changes in membrane potential from 0 to -60 mV increased linearly both the small and the large conductances and NP(o), indicating that TPC1 is regulated by voltage. Intriguingly, the apparent affinity for activation of TPC1 by its ligand NAADP is not constant. Rather, hyperpolarization increases the apparent affinity of TPC1 for NAADP by 10 nm/mV. The concerted regulation of TPC1 activity by luminal Ca(2+) and by membrane potential thus provides a potential mechanism to explain NAADP-induced Ca(2+) oscillations. These findings reveal unique properties of TPC1 to explain its role in Ca(2+) oscillations and cell function.

Impaired myelination and reduced brain ferric iron in the mouse model of mucolipidosis IV.

Mucolipidosis type IV (MLIV) is a lysosomal storage disease caused by mutations in the MCOLN1 gene, which encodes the lysosomal transient receptor potential ion channel mucolipin-1 (TRPML1). MLIV causes impaired motor and cognitive development, progressive loss of vision and gastric achlorhydria. How loss of TRPML1 leads to severe psychomotor retardation is currently unknown, and there is no therapy for MLIV. White matter abnormalities and a hypoplastic corpus callosum are the major hallmarks of MLIV brain pathology. Here, we report that loss of TRPML1 in mice results in developmental aberrations of brain myelination as a result of deficient maturation and loss of oligodendrocytes. Defective myelination is evident in Mcoln1(-/-) mice at postnatal day 10, an active stage of postnatal myelination in the mouse brain. Expression of mature oligodendrocyte markers is reduced in Mcoln1(-/-) mice at postnatal day 10 and remains lower throughout the course of the disease. We observed reduced Perls' staining in Mcoln1(-/-) brain, indicating lower levels of ferric iron. Total iron content in unperfused brain is not significantly different between Mcoln1(-/-) and wild-type littermate mice, suggesting that the observed maturation delay or loss of oligodendrocytes might be caused by impaired iron handling, rather than by global iron deficiency. Overall, these data emphasize a developmental rather than a degenerative disease course in MLIV, and suggest that there should be a stronger focus on oligodendrocyte maturation and survival to better understand MLIV pathogenesis and aid treatment development.