Mosaic PKHD1 in Polycystic Kidneys Caused Aberrant Protein Expression in the Mitochondria and Lysosomes

Autosomal recessive polycystic kidney disease (ARPKD) is a severe renal cystic disease caused mainly by the polycystic kidney and hepatic disease 1 (PKHD1). However, the genetic cause, pathologic features, and mechanism of action of ARPKD are not well known. Here, we identified a family with ARPKD. Two siblings harbored biallelic variants in PKHD1 (c.7205G>A, c.7973T>A). We determined that the “de novo” variant, c.7205G>A, arose from the mosaicism of the father and had a 7.4% level. Pathologic characterization, using biopsy analysis, was evidenced with predominant cystic dilation in proximal tubules, slight ectasia of collecting ducts, defective ciliogenesis, and impaired cell-cell junctions in renal tubules and collecting ducts. Exosome proteomics in the urine from patients with ARPKD were markedly different from those of controls, with the most significant alterations occurring in mitochondrial and lysosomal proteins. Expression of the proteins of OXPHOS was downregulated sharply, in parallel with upregulated expression of the proteins involved in glycolysis in patients with ARPKD. Several lysosomal proteins associated with renal lesions were more abundant in the exosome of the patient than in controls. Moreover, the lysosomal enzyme sulfamidase, which is produced by the SGSH gene, was abrupt uniquely in the exosome of the patient. Consistently, swollen mitochondria and abundant lysosomes were visualized in the mutant tubular epithelial cells of patients with mutant PKHD1. Collectively, these findings provide new insights on the pathophysiology of the polycystic kidney due to PKHD1 deficiency. PKHD1 mosaicism should be considered in genetic testing of ARPKD patients.

Estrogen-related receptor alpha and Rplp1 ribosome protein-dependent translation coordinately regulate starvation response and decrease NASH progression

Although general translation declines during fasting, maintaining the translation of a subset or proteins is necessary for metabolic homeostasis and cell viability. Using unbiased proteome analysis of hepatic cells during starvation, we identified a novel pathway in which Esrra-mediated transcription of Rplp1-dependent translation of lysosomal proteins declined during early starvation and recovered after prolonged starvation to restore autophagy-lysosome function. Interestingly, hepatic Esrra-Rplp1-dependent translation rate of lysosomal proteins also was impaired in patients and mice with non-alcoholic steatohepatitis (NASH), and translational response to starvation was dysregulated in mice with NASH. Remarkably, activation of Esrra pharmacologically, genetically, or by alternate day fasting restored protein translation, increased expression of lysosomal proteins, induced autophagy, and reduced lipotoxicity, inflammation, and fibrosis in cell culture and in vivo models of NASH. Thus, hepatic Esrra is essential for ribosome-dependent translation of lysosomal proteins during starvation, and prevention of lipotoxicity and progression in NASH.

Quantitative proteomics reveals the selectivity of ubiquitin-binding autophagy receptors in the turnover of damaged lysosomes by lysophagy

Removal of damaged organelles via the process of selective autophagy constitutes a major form of cellular quality control. Damaged organelles are recognized by a dedicated surveillance machinery, leading to the assembly of an autophagosome around the damaged organelle, prior to fusion with the degradative lysosomal compartment. Lysosomes themselves are also prone to damage and are degraded through the process of lysophagy. While early steps involve recognition of ruptured lysosomal membranes by glycan-binding galectins and ubiquitylation of transmembrane lysosomal proteins, many steps in the process, and their interrelationships, remain poorly understood, including the role and identity of cargo receptors required for completion of lysophagy. Here, we employ quantitative organelle capture and proximity biotinylation proteomics of autophagy adaptors, cargo receptors, and galectins in response to acute lysosomal damage, thereby revealing the landscape of lysosome-associated proteome remodeling during lysophagy. Among the proteins dynamically recruited to damaged lysosomes were ubiquitin-binding autophagic cargo receptors. Using newly developed lysophagic flux reporters including Lyso-Keima, we demonstrate that TAX1BP1, together with its associated kinase TBK1, are both necessary and sufficient to promote lysophagic flux in both HeLa cells and induced neurons (iNeurons). While the related receptor Optineurin (OPTN) can drive damage-dependent lysophagy when overexpressed, cells lacking either OPTN or CALCOCO2 still maintain significant lysophagic flux in HeLa cells. Mechanistically, TAX1BP1-driven lysophagy requires its N-terminal SKICH domain, which binds both TBK1 and the autophagy regulatory factor RB1CC1, and requires upstream ubiquitylation events for efficient recruitment and lysophagic flux. These results identify TAX1BP1 as a central component in the lysophagy pathway and provide a proteomic resource for future studies of the lysophagy process.

Immunofluorescence of Galectin-3 Puncta after lysosomal damage with LLoMe v2

Lysophagy-the selective elimination of damaged lysosomes by the autophagy pathway-is a critical housekeeping mechanism in cells. This pathway surveils lysosomes and selectively demarcates terminally damaged lysosomes for elimination. Among the most upstream signaling proteins in this pathway are the glycan binding proteins-Galectins-which recognize N and O linked glycan chains on the luminal side of transmembrane lysosomal proteins. These glycosyl modifications are only accessible to galectin proteins upon extensive lysosomal membrane rupture and serve as a sensitive measure of lysosomal damage and eventual clearance by selective autophagy. Indeed, prior work has shown that immunofluorescence of Galectin-3 serves as a convenient proxy for lysophagic flux in tissue culture cells (Aits et al., 2015; Maejima et al., 2013). Here we describe our method for monitoring galectin-3 puncta clearance as a proxy for turnover of damaged lysosomes via immunofluorescence and confocal imaging.

Analysis of Lysophagic Flux in Cultured Induced Neurons using RFP-GFP-galectin3 v1

Lysophagy-the selective elimination of damaged lysosomes by the autophagy pathway-is a critical housekeeping mechanism in cells. This pathway surveils lysosomes and selectively demarcates terminally damaged lysosomes for elimination. Among the most upstream signaling proteins in this pathway are the glycan binding proteins-Galectins-which recognize N and O linked glycan chains on the luminal side of transmembrane lysosomal proteins. These glycosyl modifications are only accessible to galectin proteins upon extensive lysosomal membrane rupture and serve as a sensitive measure of lysosomal damage and eventual clearance by selective autophagy. Indeed, prior work has shown that immunofluorescence of Galectin-3 serves as a convenient proxy for lysophagic flux in tissue culture cells (Aits et al., 2015; Maejima et al., 2013). Here we describe our method for monitoring GFP positive RFP-GFP-galectin-3 GFP positive puncta clearance as a proxy for turnover of damaged lysosomes via immunofluorescence and confocal imaging.

Proteomic Analysis of Niemann-Pick Type C Hepatocytes Reveals Potential Therapeutic Targets for Liver Damage

Niemann-Pick type C disease (NPCD) is a lysosomal storage disorder caused by mutations in the NPC1 gene. The most affected tissues are the central nervous system and liver, and while significant efforts have been made to understand its neurological component, the pathophysiology of the liver damage remains unclear. In this study, hepatocytes derived from wild type and Npc1−/− mice were analyzed by mass spectrometry (MS)-based proteomics in conjunction with bioinformatic analysis. We identified 3832 proteins: 416 proteins had a p-value smaller than 0.05, of which 37% (n = 155) were considered differentially expressed proteins (DEPs), 149 of them were considered upregulated, and 6 were considered downregulated. We focused the analysis on pathways related to NPC pathogenic mechanisms, finding that the most significant changes in expression levels occur in proteins that function in the pathways of liver damage, lipid metabolism, and inflammation. Moreover, in the group of DEPs, 30% (n = 47) were identified as lysosomal proteins and 7% (n = 10) were identified as mitochondrial proteins. Importantly, we found that lysosomal DEPs, including CTSB/D/Z, LIPA, DPP7 and GLMP, and mitocondrial DEPs, AKR1B10, and VAT1 had been connected with liver fibrosis, damage, and steatosis in previous studies, validiting our dataset. Our study found potential therapeutic targets for the treatment of liver damage in NPCD.

Quantitative Phosphoproteomic Analyses Identify STK11IP as a Lysosome-Specific Substrate of mTORC1 that Regulates Lysosomal Acidification

The evolutionarily conserved serine/threonine kinase mTORC1 is a central regulator of cell growth and proliferation. mTORC1 is activated on the lysosome surface. However, once mTORC1 is activated, it is unclear whether mTORC1 phosphorylates local lysosomal proteins to regulate specific aspects of lysosomal biology. Through cross-reference analyses of lysosome proteomic with mTORC1-regulated phosphoproteomic, we identified STK11IP as a novel lysosome-specific substrate of mTORC1. mTORC1 directly phosphorylates STK11IP at S404. Knockout of STK11IP led to a robust increase of autophagosome-lysosome fusion and autophagy flux. Dephosphorylation of STK11IP at S404 represses the role of STK11IP as an autophagy inhibitor. Mechanistically, STK11IP binds to V-ATPase, and regulates the activity of V-ATPase. Knockout of STK11IP protects mice from fasting and Methionine-Choline-Deficient Diet (MCD) diet induced fatty liver. Thus, our study demonstrates that STK11IP phosphorylation represents a novel mechanism for mTORC1 to regulate lysosomal acidification, and points to STK11IP as a promising therapeutic target for the amelioration of diseases with aberrant autophagy signaling.

Immunofluorescence of Galectin-3 Puncta after lysosomal damage with LLoMe v1

Lysophagy-the selective elimination of damaged lysosomes by the autophagy pathway-is a critical housekeeping mechanism in cells. This pathway surveils lysosomes and selectively demarcates terminally damaged lysosomes for elimination. Among the most upstream signaling proteins in this pathway are the glycan binding proteins-Galectins-which recognize N and O linked glycan chains on the luminal side of transmembrane lysosomal proteins. These glycosyl modifications are only accessible to galectin proteins upon extensive lysosomal membrane rupture and serve as a sensitive measure of lysosomal damage and eventual clearance by selective autophagy. Indeed, prior work has shown that immunofluorescence of Galectin-3 serves as a convenient proxy for lysophagic flux in tissue culture cells (Aits et al., 2015; Maejima et al., 2013). Here we describe our method for monitoring galectin-3 puncta clearance as a proxy for turnover of damaged lysosomes via immunofluorescence and confocal imaging.

Immunofluorescence of autophagic cargo receptors and p-TBK1 at LAMP1 lysosomes during lysophagy v1

Lysophagy-the selective elimination of damaged lysosomes by the autophagy pathway-is a critical housekeeping mechanism in cells. This pathway surveils lysosomes and selectively demarcates terminally damaged lysosomes for elimination. Among the most upstream signaling proteins in this pathway are the glycan binding proteins-Galectins-which recognize N and O linked glycan chains on the luminal side of transmembrane lysosomal proteins. These glycosyl modifications are only accessible to galectin proteins upon extensive lysosomal membrane rupture and serve as a sensitive measure of lysosomal damage and eventual clearance by selective autophagy. Indeed, prior work has shown that immunofluorescence of Galectin-3 serves as a convenient proxy for lysophagic flux in tissue culture cells (Aits et al., 2015; Maejima et al., 2013). Here we describe a method for monitoring protein recruitment to damaged lysosomes via immunofluorescence and confocal imaging.