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.

New family with HSPB8-associated autosomal dominant rimmed vacuolar myopathy

ObjectiveWe clinically and molecularly characterize a new family with autosomal dominant rimmed vacuolar myopathy (RVM) caused by mutations in the HSPB8 gene.MethodsWe performed whole-exome and whole-genome sequencing in the family. Western blot and immunocytochemistry were used to analyze 3 patient fibroblasts, and findings were compared with their age- and sex-matched controls.ResultsAffected patients have distal and proximal myopathy, with muscle biopsy showing rimmed vacuoles, muscle fiber atrophy, and endomysial fibrosis typical of RVM. Muscle MRI showed severe relatively symmetric multifocal fatty degenerative changes of the lower extremities. We identified a duplication of C at position 515 of the HSPB8 gene (c.515dupC) by whole-genome sequencing, which caused a frameshift with a predicted alternate stop codon p.P173SFS*43 in all affected individuals, resulting in an elongated protein product. Western blot and immunocytochemistry studies revealed reduced expression of heat shock protein beta 8 in patient fibroblasts compared with control fibroblasts, in addition to disrupted autophagy pathology.ConclusionsWe report a novel family with autosomal dominant RVM caused by the c.515dupC mutation of the HSPB8 gene, causing a translational frameshift that results in an elongated protein. Understanding the mechanism for the RVM pathology caused by mutated chaperone will permit novel targeted strategies to alter the natural history progression. As next-generation sequencing becomes more available, additional myopathic families will be identified with HSPB8 mutations.