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Reactivation of Lysosomal Ca2+ Efflux Rescues Abnormal Lysosomal Storage in FIG4-Deficient Cells
Jianlong Zou1, Bo Hu2, Sezgi Arpag2
1Department of Histology and Embryology, Zhongshan School of Medicine, Sun Yat-sen University, Guangzhou 510085, China, Department of Neurology, Center for Human Genetics Research, Vanderbilt Brain Institute, Vanderbilt University School of Medicine, Nashville, Tennessee 37232.
Abstract:
Loss of function of FIG4 leads to Charcot-Marie-Tooth disease Type 4J, Yunis-Varon syndrome, or an epilepsy syndrome. FIG4 is a phosphatase with its catalytic specificity toward 5'-phosphate of phosphatidylinositol-3,5-diphosphate (PI3,5P2). However, the loss of FIG4 decreases PI3,5P2 levels likely due to FIG4's dominant effect in scaffolding a PI3,5P2 synthetic protein complex. At the cellular level, all these diseases share similar pathology with abnormal lysosomal storage and neuronal degeneration. Mice with no FIG4 expression (Fig4(-/-)) recapitulate the pathology in humans with FIG4 deficiency. Using a flow cytometry technique that rapidly quantifies lysosome sizes, we detected an impaired lysosomal fission, but normal fusion, in Fig4(-/-) cells. The fission defect was associated with a robust increase of intralysosomal Ca(2+) in Fig4(-/-) cells, including FIG4-deficient neurons. This finding was consistent with a suppressed Ca(2+) efflux of lysosomes because the endogenous ligand of lysosomal Ca(2+) channel TRPML1 is PI3,5P2 that is deficient in Fig4(-/-) cells. We reactivated the TRPML1 channels by application of TRPML1 synthetic ligand, ML-SA1. This treatment reduced the intralysosomal Ca(2+) level and rescued abnormal lysosomal storage in Fig4(-/-) culture cells and ex vivo DRGs. Furthermore, we found that the suppressed Ca(2+) efflux in Fig4(-/-) culture cells and Fig4(-/-) mouse brains profoundly downregulated the expression/activity of dynamin-1, a GTPase known to scissor organelle membranes during fission. This downregulation made dynamin-1 unavailable for lysosomal fission. Together, our study revealed a novel mechanism explaining abnormal lysosomal storage in FIG4 deficiency. Synthetic ligands of the TRPML1 may become a potential therapy against diseases with FIG4 deficiency.
Insights
Loss of FIG4 function causes lysosomal storage and neuronal issues. Restoring lysosomal calcium (Ca2+) efflux via TRPML1 activation rescues these defects, offering potential therapies for FIG4 deficiency diseases.
Area of Science:
- Cell Biology
- Neuroscience
- Genetics
Background:
- Loss of function mutations in FIG4 are linked to neurodegenerative disorders like Charcot-Marie-Tooth disease Type 4J.
- FIG4 deficiency results in abnormal lysosomal storage and neuronal degeneration, sharing common pathology across related diseases.
- FIG4 plays a critical role in maintaining phosphatidylinositol-3,5-diphosphate (PI3,5P2) levels, essential for cellular homeostasis.
Purpose of the Study:
- To elucidate the cellular mechanisms underlying lysosomal abnormalities in FIG4 deficiency.
- To investigate the role of lysosomal calcium homeostasis in FIG4-related pathology.
- To explore potential therapeutic strategies targeting lysosomal dysfunction.
Main Methods:
- Utilized flow cytometry to quantify lysosome size and assess lysosomal fission/fusion dynamics in FIG4-deficient cells.
- Measured intralysosomal calcium levels and lysosomal Ca2+ efflux in wild-type and FIG4-deficient cells and tissues.
- Administered the TRPML1 synthetic ligand ML-SA1 to rescue lysosomal defects and analyzed dynamin-1 expression and activity.
Main Results:
- FIG4-deficient cells exhibit impaired lysosomal fission, normal fusion, and increased intralysosomal Ca2+ due to suppressed Ca2+ efflux.
- TRPML1 channel activation with ML-SA1 reduced intralysosomal Ca2+ and rescued lysosomal storage defects in cellular and ex vivo models.
- Suppressed Ca2+ efflux in FIG4 deficiency led to downregulated dynamin-1, hindering membrane scission required for lysosomal fission.
Conclusions:
- FIG4 deficiency impairs lysosomal fission through a mechanism involving disrupted Ca2+ homeostasis and reduced dynamin-1 activity.
- TRPML1 channel activation represents a promising therapeutic avenue for treating lysosomal storage and neurodegeneration in FIG4 deficiency disorders.
- This study reveals a novel pathway linking FIG4, PI3,5P2, lysosomal Ca2+, and dynamin-1 in regulating lysosomal dynamics.
