Ming Li

Lysosomes and peroxisomes are essential for cellular homeostasis, yet how their activities are coordinated remains poorly understood. Here, we identify peroxisome-derived ether lipids as key regulators of lysosomal function. A genome-wide CRISPR/Cas9 screen in LYSET-deficient mucolipidosis V cells revealed that disruption of ether lipid synthesis genes or peroxins markedly reduces lysosome accumulation and restores degradative capacity. Genetic or pharmacological inhibition of ether lipid synthesis enhanced lysosomal exocytosis and promoted the clearance of undigested material independently of mannose-6-phosphate trafficking. Conversely, supplementation with the ether lipid precursor hexadecylglycerol increased lysosome abundance, while reducing their degradative capacity. These findings uncover a peroxisome-lysosome metabolic axis, in which ether lipids act as bidirectional regulators of lysosomal number and function independently of the lysosomal master regulator TFEB. Our findings reveal how peroxisome-localized lipid metabolism modulates lysosomal homeostasis, and suggest potential new strategies to combat lysosomal and peroxisomal disorders.

In vertebrates, newly synthesized lysosomal enzymes traffic to lysosomes through the mannose-6-phosphate (M6P) pathway. The Golgi membrane protein LYSET was recently discovered to regulate lysosome biogenesis by controlling the level of GlcNAc-1-phosphotransferase (GNPT). However, its working mechanism remained unclear. In this study, we demonstrate that LYSET is a two-transmembrane protein essential for GNPT stability, cleavage by Site-1 Protease (S1P), and enzymatic activity. We reconcile conflicting models by showing that LYSET enhances GNPT cleavage and prevents its mislocalization to lysosomes for degradation. We further establish that LYSET achieves this by interacting with GOLPH3 and retromer complexes to anchor the LYSET-GNPT complex at the Golgi. Alanine mutagenesis identified an F ⁴ XXR ⁷ motif in LYSET’s N-tail for GOLPH3 binding. The retromer further promotes Golgi retention by binding to the C-terminal of LYSET and recycling it from endolysosomes. Together, our findings reveal LYSET’s multifaceted role in stabilizing GNPT, retaining it at the Golgi, and ensuring the fidelity of the M6P pathway, thereby providing insights into its molecular function.

Healthy cells need functional lysosomes to degrade cargo delivered by autophagy and endocytosis. Defective lysosomes can lead to severe conditions such as lysosomal storage diseases (LSDs) and neurodegeneration. To maintain lysosome integrity and functionality, cells have evolved multiple quality control pathways corresponding to different types of stress and damage. These can be divided into five levels: regulation, reformation, repair, removal, and replacement. The different levels of lysosome quality control often work together to maintain the integrity of the lysosomal network. This review summarizes the different quality control pathways and discusses the less-studied area of lysosome membrane protein regulation and degradation, highlighting key unanswered questions in the field.

In vertebrates, newly synthesized lysosomal enzymes traffick to lysosomes through the mannose-6-phosphate (M6P) pathway. The Golgi membrane protein TMEM251 was recently discovered to regulate lysosome biogenesis by controlling the level of GlcNAc-1-phosphotransferase (GNPT). However, its precise function remained unclear. In this study, we demonstrate that TMEM251 is a two-transmembrane protein indispensable for GNPT stability, cleavage by Site-1-Protease (S1P), and enzymatic activity. We reconcile conflicting models by showing that TMEM251 enhances GNPT cleavage and prevents its mislocalization to lysosomes for degradation. We further establish that TMEM251 achieves this by interacting with GOLPH3 and retromer complexes to anchor the TMEM251-GNPT complex at the Golgi. Alanine mutagenesis identified F ⁴ XXR ⁷ motif in TMEM251’s N-tail for GOLPH3 binding. Together, our findings uncover TMEM251’s multi-faceted role in stabilizing GNPT, retaining it at the Golgi, and ensuring the fidelity of the M6P pathway, thereby providing insights into its molecular function.

Cystinosis is a lysosomal storage disease that is characterized by the accumulation of di-peptide cystine within the lumen. It is caused by mutations in the cystine exporter, cystinosin. Most of the clinically reported mutations are due to the loss of transporter function. In this study, we identified a rapidly degrading disease variant, referred to as cystinosin(7∆). We demonstrated that this mutant is retained in the endoplasmic reticulum (ER) and degraded via the ER-associated degradation (ERAD) pathway. Using genetic and chemical inhibition methods, we elucidated the role of HRD1, p97, EDEMs, and the proteasome complex in cystinosin(7∆) degradation pathway. Having understood the degradation mechanisms, we tested some chemical chaperones, previously used for treating CFTR F508∆, and demonstrated that they could facilitate the folding and trafficking of cystinosin(7∆). Strikingly, chemical chaperone treatment can reduce the lumenal cystine level by ~70%. Conclusively, our study establishes the connection between ERAD and cystinosis pathogenesis and demonstrates the possibility of using chemical chaperones to treat cystinosin(7∆).

Lysosomes achieve their function through numerous transporters that import or export nutrients across their membrane. However, technical challenges in membrane protein overexpression, purification, and reconstitution hinder our understanding of lysosome transporter function. Here, we developed a platform to overexpress and purify the putative lysine transporter Ypq1 using a constitutive overexpression system in protease- and ubiquitination-deficient yeast vacuoles. Using this method, we purified and reconstituted Ypq1 into proteoliposomes and showed lysine transport function, supporting its role as a basic amino acid transporter on the vacuole membrane. We also found that the absence of lysine destabilizes purified Ypq1 and causes it to aggregate, consistent with its propensity to be downregulated in vivo upon lysine starvation. Our approach may be useful for the biochemical characterization of many transporters and membrane proteins to understand organellar transport and regulation.

Vertebrate cells rely on mannose-6-phosphate (M6P) modifications to deliver most lumenal hydrolases to the lysosome. As a critical trafficking signal for lysosomal enzymes, the M6P biosynthetic pathway has been thoroughly investigated. However, its regulatory mechanism is largely unknown. Here, we summarize three recent studies that independently discovered LYSET/TMEM251/GCAF as a key regulator of the M6P pathway. LYSET/TMEM251 directly interacts with GNPT, the enzyme that catalyzes the transfer of M6P, and is critical for its activity and stability. Deleting LYSET/TMEM251 impairs the GNPT function and M6P modifications. Consequently, lysosomal enzymes are mistargeted for secretion. Defective lysosomes fail to degrade cargoes such as endocytic vesicles and autophagosomes, leading to a newly identified lysosomal storage disease in humans. These discoveries open up a new direction in the regulation of the M6P biosynthetic pathway.

The mannose-6-phosphate (M6P) biosynthetic pathway for lysosome biogenesis has been studied for decades and is considered a well-understood topic. However, whether this pathway is regulated remains an open question. In a genome-wide CRISPR/Cas9 knockout screen, we discover TMEM251 as the first regulator of the M6P modification. Deleting TMEM251 causes mistargeting of most lysosomal enzymes due to their loss of M6P modification and accumulation of numerous undigested materials. We further demonstrate that TMEM251 localizes to the Golgi and is required for the cleavage and activity of GNPT, the enzyme that catalyzes M6P modification. In zebrafish, TMEM251 deletion leads to severe developmental defects including heart edema and skeletal dysplasia, which phenocopies Mucolipidosis Type II. Our discovery provides a mechanism for the newly discovered human disease caused by TMEM251 mutations. We name TMEM251 as GNPTAB cleavage and activity factor (GCAF) and its related disease as Mucolipidosis Type V.

The mannose-6-phosphate (M6P) biosynthetic pathway for lysosome biogenesis has been studied for decades and is considered a well-understood topic. However, whether this pathway is regulated remains an open question. In a genome-wide CRISPR/Cas9 knockout screen, we discovered TMEM251 as the first regulator of the M6P modification. Deleting TMEM251 caused mistargeting of most lysosomal enzymes due to their loss of M6P modification and accumulation of numerous undigested materials. We further demonstrated that TMEM251 localizes to the Golgi and is required for the cleavage and activation of GNPT, the enzyme that catalyzes M6P modification. In zebrafish, TMEM251 deletion leads to severe developmental defects including heart edema and skeletal dysplasia, which phenocopies Mucolipidosis Type II. Our discovery provides a mechanism for the newly discovered human disease caused by TMEM251 mutations. We name TMEM251 as GNPTAB cleavage and activation factor (GCAF) and its related disease as Mucolipidosis Type V.