Synthetic Site-Specific Antibody-Ligand Conjugates Promote Asialoglycoprotein Receptor-Mediated Degradation of Extracellular Human PCSK9.

Targeted degradation using cell-specific lysosome targeting receptors is emerging as a new therapeutic strategy for the elimination of disease-associated proteins. The liver-specific human asialoglycoprotein receptor (ASGPR) is a particularly attractive lysosome targeting receptor leveraged for targeted protein degradation (TPD). However, the efficiency of different glycan ligands for ASGPR-mediated lysosomal delivery remains to be further characterized. In this study, we applied a chemoenzymatic Fc glycan remodeling method to construct an array of site-specific antibody-ligand conjugates carrying natural bi- and tri-antennary N-glycans as well as synthetic tri-GalNAc ligands. Alirocumab, an anti-PCSK9 (proprotein convertase subtilisin/kexin type 9) antibody, and cetuximab (an anti-EGFR antibody) were chosen to demonstrate the ASGPR-mediated degradation of extracellular and membrane-associated proteins, respectively. It was found that the nature of the glycan ligands and the length of the spacer in the conjugates are critical for the receptor binding and the receptor-mediated degradation of PCSK9, which blocks low-density lipoprotein receptor (LDLR) function and adversely affects clearance of low-density lipoprotein cholesterol. Interestingly, the antibody-tri-GalNAc conjugates showed a clear hook effect for its binding to ASGPR, while antibody conjugates carrying the natural N-glycans did not. Both the antibody-tri-antennary N-glycan conjugate and the antibody-tri-GalNAc conjugate could significantly decrease extracellular PCSK9, as shown in the cell-based assays. However, the tri-GalNAc conjugate showed a clear hook effect in the receptor-mediated degradation of PCSK9, while the antibody conjugate carrying the natural N-glycans did not. The cetuximab-tri-GalNAc conjugates also showed a similar hook effect on degradation of the membrane-associated protein, epidermal growth factor receptor (EGFR). These results suggest that the two types of ligands may involve a distinct mode of interactions in the receptor binding and target-degradation processes. Interestingly, the alirocumab-tri-GalNAc conjugate was also found to upregulate LDLR levels in comparison with the antibody alone. This study showcases the potential of the targeted degradation strategy against PCSK9 for reducing low-density lipoprotein cholesterol, a risk factor for heart disease and stroke.

Catanionic Vesicles as a Facile Scaffold to Display Natural N-Glycan Ligands for Probing Multivalent Carbohydrate-Lectin Interactions.

Multivalent interactions are a key characteristic of protein-carbohydrate recognition. Phospholipid-based liposomes have been explored as a popular platform for multivalent presentation of glycans, but this platform has been plagued by the instability of typical liposomal formulations in biological media. We report here the exploitation of catanionic vesicles as a stable lipid-based nanoparticle scaffold for displaying large natural N-glycans as multivalent ligands. Hydrophobic insertion of lipidated N-glycans into the catanionic vesicle bilayer was optimized to allow for high-density display of structurally diverse N-glycans on the outer membrane leaflet. In an enzyme-linked competitive lectin-binding assay, the N-glycan-coated vesicles demonstrated a clear clustering glycoside effect, with significantly enhanced affinity for the corresponding lectins including Sambucus nigra agglutinin (SNA), concanavalin A (ConA), and human galectin-3, in comparison with their respective natural N-glycan ligands. Our results showed that relatively low density of high-mannose and sialylated complex type N-glycans gave the maximal clustering effect for binding to ConA and SNA, respectively, while relatively high-density display of the asialylated complex type N-glycan provided maximal clustering effects for binding to human galectin 3. Moreover, we also observed a macromolecular crowding effect on the binding of ConA to high-mannose N-glycans when catanionic vesicles bearing mixed high-mannose and complex-type N-glycans were used. The N-glycan-coated catanionic vesicles are stable and easy to formulate with varied density of ligands, which could serve as a feasible vehicle for drug delivery and as potent inhibitors for intervening protein-carbohydrate interactions implicated in disease.

Synthesis and Immunological Study of N-Glycan-Bacteriophage Qß Conjugates Reveal Dominant Antibody Responses to the Conserved Chitobiose Core.

N-Glycosylation plays an important role in many biological recognition processes. However, very few N-glycan-specific antibodies are available for functional studies and potentially for therapeutic development. In this study, we sought to synthesize bacteriophage Qß conjugates with representative N-glycans and investigate their immunogenicity for raising N-glycan-specific antibodies. An array of Qß glycoconjugates bearing five different human N-glycans and two different chemical linkers were synthesized, and the immunization of the N-glycan-Qß conjugates was performed in mice. We found that the N-glycan-Qß conjugates raised significant IgG antibodies that recognize N-glycans, but, surprisingly, most of the glycan-dependent antibodies were directed to the shared chitobiose core and were nonspecific for respective N-glycan structures. The linker chemistry was found to affect antibody specificity with adipic acid-linked N-glycan-Qß immunogens raising antibodies capable of recognizing both the N-acetylglucosamine (GlcNAc) moieties of the chitobiose core. In contrast, antibodies raised by N-glycan-Qß immunogens with a triazole linker preferentially recognized the innermost N-acetylglucosamine moiety at the reducing end. We also found that sialylation of the N-glycans significantly suppressed the immune response. Furthermore, the N-glycan-Qß immunogens with an adipic acid linker elicited higher glycan-specific antibody titers than the N-glycan-triazole-Qß immunogens. These findings delineate several challenges in eliciting mammalian N-glycan-specific antibodies through the conventional glycoconjugate vaccine design and immunization.

Site-Specific Chemoenzymatic Conjugation of High-Affinity M6P Glycan Ligands to Antibodies for Targeted Protein Degradation.

Lysosome-targeting chimeras (LYTACs) offer an opportunity for the degradation of extracellular and membrane-associated proteins of interest. Here, we report an efficient chemoenzymatic method that enables a single-step and site-specific conjugation of high-affinity mannose-6-phosphate (M6P) glycan ligands to antibodies without the need of protein engineering and conventional click reactions that would introduce "unnatural" moieties, yielding homogeneous antibody-M6P glycan conjugates for targeted degradation of membrane-associated proteins. Using trastuzumab and cetuximab as model antibodies, we showed that the wild-type endoglycosidase S (Endo-S) could efficiently perform the antibody deglycosylation and simultaneous transfer of an M6P-glycan from a synthetic M6P-glycan oxazoline to the deglycosylated antibody in a one-pot manner, giving structurally well-defined antibody-M6P glycan conjugates. A two-step procedure, using wild-type Endo-S2 for deglycosylation followed by transglycosylation with an Endo-S2 mutant (D184M), was also efficient to provide M6P glycan-antibody conjugates. The chemoenzymatic approach was highly specific for Fc glycan remodeling when both Fc and Fab domains were glycosylated, as exemplified by the selective Fc-glycan remodeling of cetuximab. SPR binding analysis indicated that the M6P conjugates possessed a nanomolar range of binding affinities for the cation-independent mannose-6-phosphate receptor (CI-MPR). Preliminary cell-based assays showed that the M6P-trastuzumab and M6P-cetuximab conjugates were able to selectively degrade the membrane-associated HER2 and EGFR, respectively. This modular glycan-remodeling strategy is expected to find wide applications for antibody-based lysosome-targeted degradation of extracellular and membrane proteins.

GH18 endo-ß-N-acetylglucosaminidases use distinct mechanisms to process hybrid-type N-linked glycans.

N-glycosylation is one of the most abundant posttranslational modifications of proteins, essential for many physiological processes, including protein folding, protein stability, oligomerization and aggregation, and molecular recognition events. Defects in the N-glycosylation pathway cause diseases that are classified as congenital disorders of glycosylation. The ability to manipulate protein N-glycosylation is critical not only to our fundamental understanding of biology but also for the development of new drugs for a wide range of human diseases. Chemoenzymatic synthesis using engineered endo-ß-N-acetylglucosaminidases (ENGases) has been used extensively to modulate the chemistry of N-glycosylated proteins. However, defining the molecular mechanisms by which ENGases specifically recognize and process N-glycans remains a major challenge. Here we present the X-ray crystal structure of the ENGase EndoBT-3987 from Bacteroides thetaiotaomicron in complex with a hybrid-type glycan product. In combination with alanine scanning mutagenesis, molecular docking calculations and enzymatic activity measurements conducted on a chemically engineered monoclonal antibody substrate unveil two mechanisms for hybrid-type recognition and processing by paradigmatic ENGases. Altogether, the experimental data provide pivotal insight into the molecular mechanism of substrate recognition and specificity for GH18 ENGases and further advance our understanding of chemoenzymatic synthesis and remodeling of homogeneous N-glycan glycoproteins.

Symptomatic expansile vertebral hemangioma causing conus medullaris compression.

OBJECTIVE: To present a case of symptomatic, expansile L1 vertebral hemangioma. CLINICAL FEATURES: A 46-year-old man presented with progressive neurologic changes and insidious onset of low back pain. INTERVENTION AND OUTCOME: After a trial of 3 visits of conservative chiropractic care, no improvement was noted. Magnetic resonance imaging was obtained, revealing an expansile hemangioma with extra-osseous component compromising the conus medullaris at the level of the L1 lumbar vertebra. Neurosurgical intervention resulted in clinical improvement. CONCLUSION: Primary care physicians treating patients with low back pain should be aware of neurologic red flags requiring prompt attention. Magnetic resonance imaging is the imaging modality of choice when evaluating a neurologic abnormality presumably related to a space-occupying lesion. Although a disk herniation is the most common cause of these symptoms, clues in the history and examination must prompt physicians to expand their differential diagnosis to include a variety of other extradural masses.