TRPML1 in Autophagy and Neuromuscular Diseases

Abstract

Autophagy is a process that maintains cellular homeostasis and monitors nutrient requirements. During autophagy, cytoplasmic components are engulfed by autophagosomes. These autophagosomes fuse with lysosomes, where lysosomal hydrolases degrade engulfed material. Autophagic flux is regulated by a series of events, including autophagy initiation, autophagosome-lysosome fusion, autolysosome/lysosome degradation, and autophagic lysosome reformation. Disruption of any of them could lead to compromised autophagy and subsequent abnormalities.

X-linked myotubular myopathy (XLMTM) is a severe form of centronuclear myopathy that is caused by the loss of myotubularin 1 (MTM1), a member of the family of phosphoinositide phosphatases that dephosphorylate PI3P and PI3,5P2. MTM1-deficient cells are characterized by PI3P and PI3,5P2 accumulation and autophagic defects.

Amyotrophic lateral sclerosis (ALS) is a late-onset fatal neurodegenerative disorder characterized by the loss of both upper and lower motoneurons in the central nervous system (CNS). ALS is traditionally classified into two categories, i.e., familial ALS (fALS) and sporadic ALS (sALS). Within all familial cases of ALS, approximately 20% have point mutations in the gene encoding superoxide dismutase type 1 (SOD1), a redox enzyme. While it is believed that the disease onset occurs inside motoneurons, different cell types, such as glial cells in CNS, may control the disease progression. Microglia are resident immune cells of the CNS activated by infection, neuronal injury, and inflammation. Since glial-mediated neuroinflammation is one of the most striking hallmarks of ALS, much effort has been devoted to understanding the role of microglia in ALS progression.

Transient receptor potential cation channel 1 (TRPML1, also known as mucolipin 1) serves as an ion channel on the lysosomal membrane, activated by PI3,5P2 and reactive oxygen species (ROS). Mutations in the human TRPML1 gene cause Mucolipidosis type IV (ML-IV), a genetic disease characterized by neurodegeneration and movement disorders. TRPML1 mutant cells are characterized by abnormal autophagosome accumulation. Intriguingly, activation of TRPML1 results in autophagosome accumulation due to increased autophagosome biogenesis or impaired autophagic degradation.

In this study, we focused on understanding the role of TRPML1 in XLMTM and ALS, two neuromuscular diseases associated with muscle and motoneuron, respectively. We reported that in XLMTM, MTM1 deficiency hyperactivated TRPML1 due to PI3,5P2 elevation, and this suppressed autophagosome-lysosome fusion and myogenesis. The defective autophagosome-lysosome fusion and myogenesis in MTM1 deficient cells were corrected by TRPML1 suppression. Mechanistically, TRPML1 overactivation in MTM1 deficient cells led to a dynamin 2 increase due to the activation of transcription factor EB (TFEB), which further caused muscle defects. In ALS, we reported that SOD1G93A mutation activated TRPML1 in a ROS-dependent manner in microglia. Activated TRPML1 in SOD1G93A microglia impaired autophagic flux, potentially by inhibiting autophagosome-lysosome fusion. Activated TRPML1 also increased proinflammatory cytokine release from SOD1G93A microglia. Downregulating TRPML1 rescued the impaired autophagic flux and proinflammatory cytokine release in SOD1G93A microglia.

Taken together, our studies suggest that hyperactivation of TRPML1, either by PI3,5P2 in XLMTM muscle or by ROS in ALS microglia, disrupts autophagic flux, thereby affecting disease progression. Inhibiting TRPML1 could be an approach to mitigate the progression of the disease.