Structural insights into how GlcNAc-1-phosphotransferase directs lysosomal protein transport

Molecular Insights into the Regulation of GNPTAB by TMEM251

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 F4XXR7 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.

Structure of a truncated human GlcNAc-1-phosphotransferase variant reveals the basis for its hyperactivity

Mutations that cause loss of function of GlcNAc-1-phosphotransferase (PTase) lead to the lysosomal storage disorder mucolipidosis II. PTase is the key enzyme of the mannose 6-phosphate (M6P) targeting system that is responsible for tagging lysosomal hydrolases with the M6P moiety for their delivery to the lysosome. We had previously generated a truncated hyperactive form of PTase termed S1S3 which was shown to notably increase the phosphorylation level of secreted lysosomal enzymes and enhance their uptake by cells. Here, we report the 3.4 Å cryo-EM structure of soluble S1S3 lacking both transmembrane domains and cytosolic tails. The structure reveals a high degree of conservation of the catalytic core to full-length PTase. In this dimeric structure, the EF-hand of one protomer is observed interacting with the conserved region four of the other. In addition, we present a high-quality EM 3D map of the UDP-GlcNAc bound form of the full-length soluble protein showing the key molecular interactions between the nucleotide sugar donor and side chain amino acids of the protein. Finally, although the domain organization of S1S3 is very similar to that of the Drosophila melanogaster (fruit fly) PTase homolog, we establish that the latter does not act on lysosomal hydrolases.

A review of the current state of natural biomaterials in wound healing applications

Skin, the largest biological organ, consists of three main parts: the epidermis, dermis, and subcutaneous tissue. Wounds are abnormal wounds in various forms, such as lacerations, burns, chronic wounds, diabetic wounds, acute wounds, and fractures. The wound healing process is dynamic, complex, and lengthy in four stages involving cells, macrophages, and growth factors. Wound dressing refers to a substance that covers the surface of a wound to prevent infection and secondary damage. Biomaterials applied in wound management have advanced significantly. Natural biomaterials are increasingly used due to their advantages including biomimicry of ECM, convenient accessibility, and involvement in native wound healing. However, there are still limitations such as low mechanical properties and expensive extraction methods. Therefore, their combination with synthetic biomaterials and/or adding bioactive agents has become an option for researchers in this field. In the present study, the stages of natural wound healing and the effect of biomaterials on its direction, type, and level will be investigated. Then, different types of polysaccharides and proteins were selected as desirable natural biomaterials, polymers as synthetic biomaterials with variable and suitable properties, and bioactive agents as effective additives. In the following, the structure of selected biomaterials, their extraction and production methods, their participation in wound healing, and quality control techniques of biomaterials-based wound dressings will be discussed.

Novel synthetic biological study on intracellular distribution of human GlcNAc-1-phosphotransferase expressed in insect cells

Many lysosomal enzymes contain N-glycans carrying mannose 6-phosphate (M6P) residues. Modifying lysosomal enzymes by M6P residues requires a two-step process in the Golgi apparatus. Then the lysosomal enzymes with M6P residues are transported from the trans-Golgi network to endosomes and lysosomes by M6P receptors. In insect cells, M6P residues are not added to N-glycans. Therefore, many insect lysosomal enzymes are transported to lysosomes by the M6P-independent pathway. The expression and subcellular distribution of M6P-modifying enzymes were examined by amplifying DNA fragments of M6P-modifying enzymes, generating the corresponding plasmid constructs, and transfection each construct into Sf9 cells, an insect cell line. The human GlcNac-1-phosphotransferase α/β subunit, one of the M6P-modifying enzymes, was found to differ in maturation and localization between mammalian and insect cells. In mammalian cells, newly biosynthesized α/β subunit localized in the cis-Golgi. In Sf9 cells, most of the α/β subunit was localized in the endoplasmic reticulum, and few mature forms of α/β subunit were observed. However, by the co-expression of the human site-1 protease, the mature forms were observed significantly and co-localization with each protein. Our study indicates new insights into regulating the intracellular distribution of the human GlcNac-1-phosphotransferase α/β subunit in insect cells.

Genome Editing Tools for Lysosomal Storage Disorders

Genome editing has multiple applications in the biomedical field. They can be used to modify genomes at specific locations, being able to either delete, reduce, or even enhance gene transcription and protein expression. Here, we summarize applications of genome editing used in the field of lysosomal disorders. We focus on the development of cell lines for study of disease pathogenesis, drug discovery, and pathogenicity of specific variants. Furthermore, we highlight the main studies that use gene editing as a gene therapy platform for these disorders, both in preclinical and clinical studies. We conclude that gene editing has been able to change quickly the scenario of these disorders, allowing the development of new therapies and improving the knowledge on disease pathogenesis. Should they confirm their hype, the first gene editing-based products for lysosomal disorders could be available in the next years.

Structures of the mannose-6-phosphate pathway enzyme, GlcNAc-1-phosphotransferase

The mannose-6-phosphate (M6P) pathway is responsible for the transport of hydrolytic enzymes to lysosomes. N-acetylglucosamine-1-phosphotransferase (GNPT) catalyzes the first step of tagging these hydrolases with M6P, which when recognized by receptors in the Golgi diverts them to lysosomes. Genetic defects in the GNPT subunits, GNPTAB and GNPTG, cause the lysosomal storage diseases mucolipidosis types II and III. To better understand its function, we determined partial three-dimensional structures of the GNPT complex. The catalytic domain contains a deep cavity for binding of uridine diphosphate-N-acetylglucosamine, and the surrounding residues point to a one-step transfer mechanism. An isolated structure of the gamma subunit of GNPT reveals that it can bind to mannose-containing glycans in different configurations, suggesting that it may play a role in directing glycans into the active site. These findings may facilitate the development of therapies for lysosomal storage diseases.