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Mucolipidosis IV (MLIV) is a lysosomal storage disorder characterized by severe neurological and ophthalmologic abnormalities. In contrast with most lysosomal storage disorders, which are attributed to the absence of specific lysosomal hydrolases, accumulation of material in MLIV results from defects in membrane transport along the late endocytic pathway. Mutations in MCOLN1 are the cause of MLIV; however, how lack of MCOLN1 function ultimately leads to neurodegeneration remains largely unknown. We found that MCOLN1 is required for efficient fusion of both late endosomes and autophagosomes with lysosomes. Impaired autophagosome degradation results in accumulation of autophagosomes in MLIV fibroblasts. In addition, we found increased levels and aggregation of p62, suggesting that abnormal accumulation of ubiquitinated protein inclusions may contribute to the neurodegenerative phenotype observed in MLIV patients. These findings corroborate recent evidence indicating that defects in autophagy may be a common feature of many neurodegenerative disorders.
Recent studies reveal an interesting connection between lysosomal function, autophagy, and neurodegeneration. Autophagy is a crucial clearance mechanism that protects against the accumulation of toxic protein aggregates and damaged organelles. The last step of autophagy requires fusion of autophagosomes with late endosomes/lysosomes to ensure degradation of the autophagosome’s content. In some lysosomal storage disorders (LSDs) characterized by severe neurodegeneration, such as Multiple Sulfatase Deficiency and Mucopolysaccharidosis type III, accumulation of undegraded substrates in lysosomes due to the lack of specific lysosomal hydrolases impairs autophagosome degradation and leads to defective organelle turnover and accumulation of autophagosomes and protein deposits.1 In addition, recent evidence shows that the ESCRT machinery that regulates sorting of proteins into late endosomes is required for efficient autophagosome degradation;2 and that mutations in the ESCRT-III subunit CHMP2B are associated with amyotrophic lateral sclerosis (ALS)3 and a rare form of frontotemporal dementia (FTD),4 two neurodegenerative disorders characterized by the accumulation of abnormal ubiquitinated protein inclusions in neurons. Therefore, is suggested that defects in autophagy may lead to neurodegeneration.
Mucolipidosis type IV (MLIV) is a lysosomal storage disorder characterized by severe neurological and ophthalmologic abnormalities due to defective transport of membrane components in the late endosomal-lysosomal pathway.5-7 MLIV is caused by mutations in mucolipin 1 (MCOLN1), a late endosomal/lysosomal ion channel belonging to the transient receptor potential superfamily.8-11 Cells from patients with MLIV accumulate enlarged vacuolar structures containing phospholipids, sphingolipids, mucopolysaccharides, and gangliosides.8,12-14 It is therefore plausible that disruption of lysosomal function due to lipid buildup has important repercussions on the autophagic pathway. Using multiple approaches, including LC3 localization, LC3-II/LC3-I westernblot, electron microscopy, and LC3/CD63 co-localization analysis, we find increased basal autophagy in MLIV fibroblasts15 (Fig.1). Autophagosome accumulation in MCOLN1-deficient cells is due to decreased autophagosome degradation and increased autophagosome formation. We also find increased levels of p62, a protein commonly found in protein inclusions associated with neurodegenerative disorders16 (Fig. 1), as well as accumulation of ubiquitinated aggregates. Importantly, the majority of p62 accumulated in MLIV cells is resistant to extraction with detergents, an indication that p62 is part of aggregates or protein inclusions.
Based on these data, we propose a model in which, analogous to other LSDs, defective lysosomal function caused by mutations in MCOLN1, results in impaired fusion of autophagosomes with lysosomes and subsequent degradation (Fig. 2). Defective autophagy leads then to unproductive removal of protein aggregates and damaged organelles. Cells may try to respond by increasing autophagosome formation (accordingly, we observed elevated levels of Beclin 1 in lysates from MLIV fibroblasts); however, the system cannot be balanced and deposits of ubiquitinated aggregates and protein inclusions build up in the cytosol. Accumulation of misfolded proteins may not be very deleterious in fibroblasts, in which the rapid rate of division may help to split the burden of protein inclusions between daughter cells. In contrast, neurons do not divide, making the accumulation of protein inclusions progressively more harmful. In addition, neurons are particularly sensitive to the accumulation of toxic products. Therefore, it is understandable that defects in autophagy often translate into neuronal death and neurodegeneration.
Autophagic dysfunction may also cause accumulation of damaged organelles. The connection between defective autophagy and accumulation of aberrant mitochondria is well established in MLIV and other LSDs.1,17 These abnormal mitochondria are deficient in ATP production, produce increased amounts of reactive oxygen species, and increase susceptibility of cells to pro-apoptotic stimuli.17,18 However, it is important to keep in mind that autophagy also plays a fundamental role in the turnover of other organelles such as peroxisomes19 and ribosomes,20 as well as in diverse cellular processes including degradation of parts of the nucleolus,21 delivery of certain proteins to the vacuole,22 removal of viruses and other pathogens,23 and processing of intracellular proteins for presentation on MHC-II molecules.24 Although to date, the role of autophagy in some of these processes has only been described in yeast, it seems clear that a general deficiency in autophagic function may have many important and diverse consequences for cellular function. Particularly interesting is a recent report showing autophagy activation favors fusion of multivesicular bodies (MVBs) with autophagosomes while blocking exosome secretion.25 This diversion of MVBs to the autophagic pathway might have an impact on the delivery of cargo from the plasma membrane to lysosomes. In addition, Rab5 and PI3KIII, two components of the sorting machinery at early endosomes, are also required for autophagosome formation.26,27 Therefore, increased autophagosome formation and/or autophagosome accumulation might “sequester” these proteins at the autophagic pathway to the detriment of protein sorting along the endocytic pathway. In support of this idea, we find that delivery of platelet-derived growth factor receptor (PDGFR) from the plasma membrane to lysosomes is delayed in MLIV fibroblasts15 (Fig. 2). Further studies will be required to clarify the multiple interconnections between the autophagic and endosomal pathways.
Our results also revealed a robust increase in both the number and the size of autophagosomes after autophagy induction (e.g., by starvation), indicating that the autophagic machinery remains functional in MLIV fibroblasts and that cells can respond to stress conditions. Therefore, it is tempting to speculate that situations that promote autophagy induction, such as caloric restriction or treatment with rapamycin, could potentially favor the elimination of protein inclusions and ubiquitinated aggregates.28,29 However, it is important to be cautious in this regard, as the nature of the protein inclusions might determine the susceptibility to clearance by autophagy. Cuervo’s laboratory has recently described that AIMP2 (p38) and desmin-positive inclusions are resistant to autophagy clearance, whereas aggresomes containing huntingtin, mutant tau or a combination of synphilin-1 and α-synuclein are eliminated after autophagy induction.30 Therefore, an approach that combines autophagy induction with activation of other cellular degradative systems, such as the ubiquitin-proteasome system or chaperone-mediated autophagy, or promotes simultaneously autophagy induction and enhanced lysosomal degradation, may be necessary for effective patient treatment.
This project was supported by the Intramural Research Program of the NIH, National Heart, Lung, and Blood Institute (NHLBI).