Rescue of secretion of rare-disease-associated misfolded mutant glycoproteins in UGGT1 knock-out mammalian cells

Endoplasmic reticulum (ER) retention of misfolded glycoproteins is mediated by the ER-localized eukaryotic glycoprotein secretion checkpoint, UDP-glucose glycoprotein glucosyl-transferase (UGGT). The enzyme recognizes a misfolded glycoprotein and flags it for ER retention by re-glucosylating one of its N-linked glycans. In the background of a congenital mutation in a secreted glycoprotein gene, UGGT-mediated ER retention can cause rare disease, even if the mutant glycoprotein retains activity ("responsive mutant"). Using confocal laser scanning microscopy, we investigated here the subcellular localization of the human Trop-2-Q118E, E227K and L186P mutants, which cause gelatinous drop-like corneal dystrophy (GDLD). Compared with the wild-type Trop-2, which is correctly localized at the plasma membrane, these Trop-2 mutants are retained in the ER. We studied fluorescent chimeras of the Trop-2 Q118E, E227K and L186P mutants in mammalian cells harboring CRISPR/Cas9-mediated inhibition of the UGGT1 and/or UGGT2 genes. The membrane localization of the Trop-2 Q118E, E227K and L186P mutants was successfully rescued in UGGT1-/- cells. UGGT1 also efficiently reglucosylated Trop-2-Q118E-EYFP in cellula. The study supports the hypothesis that UGGT1 modulation would constitute a novel therapeutic strategy for the treatment of pathological conditions associated to misfolded membrane glycoproteins (whenever the mutation impairs but does not abrogate function), and it encourages the testing of modulators of ER glycoprotein folding quality control as broad-spectrum rescue-of-secretion drugs in rare diseases caused by responsive secreted glycoprotein mutants.

Validation of a highly sensitive HaloTag-based assay to evaluate the potency of a novel class of allosteric β-Galactosidase correctors

Site-directed Enzyme Enhancement Therapy (SEE-Tx®) technology is a disease-agnostic drug discovery tool that can be applied to any protein target of interest with a known three-dimensional structure. We used this proprietary technology to identify and characterize the therapeutic potential of structurally targeted allosteric regulators (STARs) of the lysosomal hydrolase β-galactosidase (β-Gal), which is deficient due to gene mutations in galactosidase beta 1 (GLB1)-related lysosomal storage disorders (LSDs). The biochemical HaloTag cleavage assay was used to monitor the delivery of wildtype (WT) β-Gal and four disease-related β-Gal variants (p.Ile51Thr, p.Arg59His, p.Arg201Cys and p.Trp273Leu) in the presence and absence of two identified STAR compounds. In addition, the ability of STARs to reduce toxic substrate was assessed in a canine fibroblast cell model. In contrast to the competitive pharmacological chaperone N-nonyl-deoxygalactonojirimycin (NN-DGJ), the two identified STAR compounds stabilized and substantially enhanced the lysosomal transport of wildtype enzyme and disease-causing β-Gal variants. In addition, the two STAR compounds reduced the intracellular accumulation of exogenous GM1 ganglioside, an effect not observed with the competitive chaperone NN-DGJ. This proof-of-concept study demonstrates that the SEE-Tx® platform is a rapid and cost-effective drug discovery tool for identifying STARs for the treatment of LSDs. In addition, the HaloTag assay developed in our lab has proved valuable in investigating the effect of STARs in promoting enzyme transport and lysosomal delivery. Automatization and upscaling of this assay would be beneficial for screening STARs as part of the drug discovery process.

TMX4-driven LINC complex disassembly and asymmetric autophagy of the nuclear envelope upon acute ER stress

The endoplasmic reticulum (ER) is an organelle of nucleated cells that produces proteins, lipids and oligosaccharides. ER volume and activity are increased upon induction of unfolded protein responses (UPR) and are reduced upon activation of ER-phagy programs. A specialized domain of the ER, the nuclear envelope (NE), protects the cell genome with two juxtaposed lipid bilayers, the inner and outer nuclear membranes (INM and ONM) separated by the perinuclear space (PNS). Here we report that expansion of the mammalian ER upon homeostatic perturbations results in TMX4 reductase-driven disassembly of the LINC complexes connecting INM and ONM and in ONM swelling. The physiologic distance between ONM and INM is restored, upon resolution of the ER stress, by asymmetric autophagy of the NE, which involves the LC3 lipidation machinery, the autophagy receptor SEC62 and the direct capture of ONM-derived vesicles by degradative LAMP1/RAB7-positive endolysosomes in a catabolic pathway mechanistically defined as micro-ONM-phagy.

Rescue of secretion of a rare-disease associated mis-folded mutant glycoprotein in UGGT1 knock-out mammalian cells

Endoplasmic reticulum (ER) retention of mis-folded glycoproteins is mediated by the ERlocalised eukaryotic glycoprotein secretion checkpoint, UDP-glucose glycoprotein glucosyl-transferase (UGGT). The enzyme recognises a mis-folded glycoprotein and flags it for ER retention by reglucosylating one of its N-linked glycans. In the background of a congenital mutation in a secreted glycoprotein gene, UGGT-mediated ER retention can cause rare disease even if the mutant glycoprotein retains activity ("responsive mutant"). Here, we investigated the subcellular localisation of the human Trop-2 Q118E variant, which causes gelatinous droplike corneal dystrophy (GDLD). Compared with the wild type Trop-2, which is correctly localised at the plasma membrane, the Trop-2-Q118E variant is found to be heavily retained in the ER. Using Trop-2-Q118E, we tested UGGT modulation as a rescue-of-secretion therapeutic strategy for congenital rare disease caused by responsive mutations in genes encoding secreted glycoproteins. We investigated secretion of a EYFP-fusion of Trop-2-Q118E by confocal laser scanning microscopy. As a limiting case of UGGT inhibition, mammalian cells harbouring CRISPR/Cas9-mediated inhibition of the UGGT1 and/or UGGT2 gene expressions were used. The membrane localisation of the Trop-2-Q118E-EYFP mutant was successfully rescued in UGGT1-/- and UGGT1/2-/- cells. UGGT1 also efficiently reglucosylated Trop-2-Q118E-EYFP in cellula. The study supports the hypothesis that UGGT1 modulation constitutes a novel therapeutic strategy for the treatment of Trop-2-Q118E associated GDLD, and it encourages the testing of modulators of ER glycoprotein folding Quality Control (ERQC) as broad-spectrum rescueof-secretion drugs in rare diseases caused by responsive secreted glycoprotein mutants.

N-glycan processing selects ERAD-resistant misfolded proteins for ER-to-lysosome-associated degradation

Efficient degradation of by‐products of protein biogenesis maintains cellular fitness. Strikingly, the major biosynthetic compartment in eukaryotic cells, the endoplasmic reticulum (ER), lacks degradative machineries. Misfolded proteins in the ER are translocated to the cytosol for proteasomal degradation via ER‐associated degradation (ERAD). Alternatively, they are segregated in ER subdomains that are shed from the biosynthetic compartment and are delivered to endolysosomes under control of ER‐phagy receptors for ER‐to‐lysosome‐associated degradation (ERLAD). Demannosylation of N‐linked oligosaccharides targets terminally misfolded proteins for ERAD. How misfolded proteins are eventually marked for ERLAD is not known. Here, we show for ATZ and mutant Pro‐collagen that cycles of de‐/re‐glucosylation of selected N‐glycans and persistent association with Calnexin (CNX) are required and sufficient to mark ERAD‐resistant misfolded proteins for FAM134B‐driven lysosomal delivery. In summary, we show that mannose and glucose processing of N‐glycans are triggering events that target misfolded proteins in the ER to proteasomal (ERAD) and lysosomal (ERLAD) clearance, respectively, regulating protein quality control in eukaryotic cells.

ER-to-lysosome-associated degradation of proteasome-resistant ATZ polymers occurs via receptor-mediated vesicular transport

Maintenance of cellular proteostasis relies on efficient clearance of defective gene products. For misfolded secretory proteins, this involves dislocation from the endoplasmic reticulum (ER) into the cytosol followed by proteasomal degradation. However, polypeptide aggregation prevents cytosolic dislocation and instead activates ill-defined lysosomal catabolic pathways. Here, we describe an ER-to-lysosome-associated degradation pathway (ERLAD) for proteasome-resistant polymers of alpha1-antitrypsin Z (ATZ). ERLAD involves the ER-chaperone calnexin (CNX) and the engagement of the LC3 lipidation machinery by the ER-resident ER-phagy receptor FAM134B, echoing the initiation of starvation-induced, receptor-mediated ER-phagy. However, in striking contrast to ER-phagy, ATZ polymer delivery from the ER lumen to LAMP1/RAB7-positive endolysosomes for clearance does not require ER capture within autophagosomes. Rather, it relies on vesicular transport where single-membrane, ER-derived, ATZ-containing vesicles release their luminal content within endolysosomes upon membrane:membrane fusion events mediated by the ER-resident SNARE STX17 and the endolysosomal SNARE VAMP8. These results may help explain the lack of benefits of pharmacologic macroautophagy enhancement that has been reported for some luminal aggregopathies.

A novel UGGT1 and p97-dependent checkpoint for native ectodomains with ionizable intramembrane residue

There is unexpected collaboration of the cytosolic AAA-ATPase p97 and the luminal quality control factor UGGT1 in a novel, BiP- and CNX-independent protein quality checkpoint. This prevents Golgi transport of a chimera with a native ectodomain that passes the luminal quality control scrutiny but displays an intramembrane defect.

Only native polypeptides are released from the endoplasmic reticulum (ER) to be transported at the site of activity. Persistently misfolded proteins are retained and eventually selected for ER-associated degradation (ERAD). The paradox of a structure-based protein quality control is that functional polypeptides may be destroyed if they are architecturally unfit. This has health-threatening implications, as shown by the numerous “loss-of-function” proteopathies, but also offers chances to intervene pharmacologically to promote bypassing of the quality control inspection and export of the mutant, yet functional protein. Here we challenged the ER of human cells with four modular glycopolypeptides designed to alert luminal and membrane protein quality checkpoints. Our analysis reveals the unexpected collaboration of the cytosolic AAA-ATPase p97 and the luminal quality control factor UDP-glucose:glycoprotein glucosyltransferase (UGGT1) in a novel, BiP- and CNX-independent checkpoint. This prevents Golgi transport of a chimera with a native ectodomain that passes the luminal quality control scrutiny but displays an intramembrane defect. Given that human proteopathies may result from impaired transport of functional polypeptides with minor structural defects, identification of quality checkpoints and treatments to bypass them as shown here upon silencing or pharmacologic inhibition of UGGT1 or p97 may have important clinical implications.

Malectin participates in a backup glycoprotein quality control pathway in the mammalian ER

Malectin is a conserved, endoplasmic reticulum (ER)-resident lectin that recognizes high mannose oligosaccharides displaying terminal glucose residues. Here we show that Malectin is an ER stress-induced protein that selectively associates with glycopolypeptides without affecting their entry and their retention in the Calnexin chaperone system. Analysis of the obligate Calnexin client influenza virus hemagglutinin (HA) revealed that Calnexin and Malectin associated with different timing to different HA conformers and that Malectin associated with misfolded HA. Analysis of the facultative Calnexin clients NHK and α1-antitrypsin (α1AT) revealed that induction of Malectin expression to simulate conditions of ER stress resulted in persistent association between the ER lectin and the model cargo glycoproteins, interfered with processing of cargo-linked oligosaccharides and reduced cargo secretion. We propose that Malectin intervention is activated upon ER stress to inhibit secretion of defective gene products that might be generated under conditions of aberrant functioning of the ER quality control machinery.

Substrate-specific requirements for UGT1-dependent release from calnexin

Newly synthesized glycoproteins displaying monoglucosylated N-glycans bind to the endoplasmic reticulum (ER) chaperone calnexin, and their maturation is catalyzed by the calnexin-associated oxidoreductase ERp57. Folding substrates are eventually released from calnexin, and terminal glucoses are removed from N-glycans. The UDP-glucose:glycoprotein glucosyltransferase (UGT1, UGGT, GT) monitors the folding state of polypeptides released from calnexin and adds back a glucose residue on N-glycans of nonnative polypeptides, thereby prolonging retention in the calnexin chaperone system for additional folding attempts. Here we show that for certain newly synthesized glycoproteins UGT1 deletion has no effect on binding to calnexin. These proteins must normally complete their folding program in one binding event. Other proteins normally undergo multiple binding events, and UGT1 deletion results in their premature release from calnexin. For other proteins, UGT1 deletion substantially delays release from calnexin, unexpectedly showing that UGT1 activity might be required for a structural maturation needed for substrate dissociation from calnexin and export from the ER.

Saporin as a novel suicide gene in anticancer gene therapy

We used a non-viral gene delivery approach to explore the potential of the plant saporin (SAP) gene as an alternative to the currently employed suicide genes in cancer therapy. Plasmids expressing cytosolic SAP were generated by placing the region encoding the mature plant ribosome-inactivating protein under the control of cytomegalovirus (CMV) or simian virus 40 (SV40) promoters. Their ability to inhibit protein synthesis was first tested in cultured tumor cells co-transfected with a luciferase reporter gene. In particular, SAP expression driven by CMV promoter (pCI-SAP) demonstrated that only 10 ng of plasmid per 1.6 x 10(4) B16 cells drastically reduced luciferase activity to 18% of that in control cells. Direct intratumoral injection of pCI-SAP complexed with either lipofectamine or N-(2,3-dioleoyloxy-1-propyl) trimethylammonium methyl sulfate (DOTAP) in B16 melanoma-bearing mice resulted in a noteworthy attenuation of tumor growth. This antitumor effect was increased in mice that received repeated intratumoral injections. A SAP catalytic inactive mutant (SAP-KQ) failed to exert any antitumor effect demonstrating that this was specifically owing to the SAP N-glycosidase activity. Our overall data strongly suggest that the gene encoding SAP, owing to its rapid and effective action and its independence from the proliferative state of target cells might become a suitable candidate suicide gene for oncologic applications.

Consequences of ERp57 Deletion on Oxidative Folding of Obligate and Facultative Clients of the Calnexin Cycle

Members of the protein-disulfide isomerase superfamily catalyze the formation of intra- and intermolecular disulfide bonds, a rate-limiting step of protein folding in the endoplasmic reticulum (ER). Here we compared maturation of one obligate and two facultative calnexin substrates in cells with and without ERp57, the calnexin-associated, glycoprotein-specific oxidoreductase. ERp57 deletion did not prevent the formation of disulfide bonds during co-translational translocation of nascent glycopolypeptides in the ER. It affected, however, the post-translational phases of oxidative influenza virus hemagglutinin (HA) folding, resulting in significant loss of folding efficiency for this obligate calnexin substrate. Without ERp57, HA also showed reduced capacity to recover from an artificially induced aberrant conformation, thus revealing a crucial role of ERp57 during post-translational reshuffling to the native set of HA disulfides. ERp57 deletion did not affect maturation of the model facultative calnexin substrates E1 and p62 (and of most cellular proteins, as shown by lack of induction of ER stress). ERp72 was identified as one of the ER-resident oxidoreductases associating with the orphan ERp57 substrates to maintain their folding competence.