Autophagy is affected in patients with hypokalemic periodic paralysis: an involvement in vacuolar myopathy?

Hypokalemic periodic paralysis is an autosomal dominant, rare disorder caused by variants in the genes for voltage-gated calcium channel CaV1.1 (CACNA1S) and NaV1.4 (SCN4A). Patients with hypokalemic periodic paralysis may suffer from periodic paralysis alone, periodic paralysis co-existing with permanent weakness or permanent weakness alone. Hypokalemic periodic paralysis has been known to be associated with vacuolar myopathy for decades, and that vacuoles are a universal feature regardless of phenotype. Hence, we wanted to investigate the nature and cause of the vacuoles. Fourteen patients with the p.R528H variation in the CACNA1S gene was included in the study. Histology, immunohistochemistry and transmission electron microscopy was used to assess general histopathology, ultrastructure and pattern of expression of proteins related to muscle fibres and autophagy. Western blotting and real-time PCR was used to determine the expression levels of proteins and mRNA of the proteins investigated in immunohistochemistry. Histology and transmission electron microscopy revealed heterogenous vacuoles containing glycogen, fibrils and autophagosomes. Immunohistochemistry demonstrated autophagosomes and endosomes arrested at the pre-lysosome fusion stage. Expression analysis showed a significant decrease in levels of proteins an mRNA involved in autophagy in patients, suggesting a systemic effect. However, activation level of the master regulator of autophagy gene transcription, TFEB, did not differ between patients and controls, suggesting competing control over autophagy gene transcription by nutritional status and calcium concentration, both controlling TFEB activity. The findings suggest that patients with hypokalemic periodic paralysis have disrupted autophagic processing that contribute to the vacuoles seen in these patients.

Direct uptake mechanism in lysosome required for neuromuscular homeostasis

Regulated degradation of cellular components plays an essential role in homeostasis. Accumulating evidence indicates the importance of lysosomal degradation of cellular proteins1: Dysfunctions in multiple pathways to deliver cytosolic substrates into lysosomes are related to various diseases, including cancers, neurodegenerative diseases, and myopathies2. However, much of the effort at understanding such pathways has been devoted to studies on macroautophagy, which entails vast and dynamic rearrangement of membrane structure, and knowledge on other delivery systems and functions of lysosomes per se remains scant. Here, we show that cytosolic proteins are directly imported into lysosomes by a mechanism distinct from any known pathways and degraded. We find that a lysosomal membrane protein, SIDT2, which was previously reported as a putative nucleic acid transporter, is involved in the translocation of substrate proteins in this system. Gain- and loss-of-function analyses reveal that SIDT2 contributes conspicuously to the lysosomal degradation of a wide range of cytosolic proteins in cells at the constitutive level. Furthermore, a dominant-negative type of mutation in SIDT2 causes familial rimmed vacuolar myopathy in humans. Sidt2 knockout mice recapitulated typical features of rimmed vacuolar myopathy, including atrophy and accumulation of cytoplasmic inclusions in skeletal muscles. These results reveal a previously unknown pathway of proteolysis in lysosomes and highlight the importance of noncanonical types of autophagy in human physiology and pathophysiology.