O-GlcNAc forces an α-synuclein amyloid strain with notably diminished seeding and pathology.

Amyloid-forming proteins such α-synuclein and tau, which are implicated in Alzheimer's and Parkinson's disease, can form different fibril structures or strains with distinct toxic properties, seeding activities and pathology. Understanding the determinants contributing to the formation of different amyloid features could open new avenues for developing disease-specific diagnostics and therapies. Here we report that O-GlcNAc modification of α-synuclein monomers results in the formation of amyloid fibril with distinct core structure, as revealed by cryogenic electron microscopy, and diminished seeding activity in seeding-based neuronal and rodent models of Parkinson's disease. Although the mechanisms underpinning the seeding neutralization activity of the O-GlcNAc-modified fibrils remain unclear, our in vitro mechanistic studies indicate that heat shock proteins interactions with O-GlcNAc fibril inhibit their seeding activity, suggesting that the O-GlcNAc modification may alter the interactome of the α-synuclein fibrils in ways that lead to reduce seeding activity in vivo. Our results show that posttranslational modifications, such as O-GlcNAc modification, of α-synuclein are key determinants of α-synuclein amyloid strains and pathogenicity.

Backbone Modification Provides a Long-Acting Inverse Agonist of Pathogenic, Constitutively Active PTH1R Variants.

Parathyroid hormone 1 receptor (PTH1R) plays a key role in mediating calcium homeostasis and bone development, and aberrant PTH1R activity underlies several human diseases. Peptidic PTH1R antagonists and inverse agonists have therapeutic potential in treating these diseases, but their poor pharmacokinetics and pharmacodynamics undermine their in vivo efficacy. Herein, we report the use of a backbone-modification strategy to design a peptidic PTH1R inhibitor that displays prolonged activity as an antagonist of wild-type PTH1R and an inverse agonist of the constitutively active PTH1R-H223R mutant both in vitro and in vivo. This peptide may be of interest for the future development of therapeutic agents that ameliorate PTH1R malfunction.

Top/Middle-Down Characterization of α-Synuclein Glycoforms.

α-Synuclein is an intrinsically disordered protein that plays a critical role in the pathogenesis of neurodegenerative disorders, such as Parkinson's disease. Proteomics studies of human brain samples have associated the modification of the O-linked N-acetyl-glucosamine (O-GlcNAc) to several synucleinopathies; in particular, the position of the O-GlcNAc can regulate protein aggregation and subsequent cell toxicity. There is a need for site specific O-GlcNAc α-synuclein screening tools to direct better therapeutic strategies. In the present work, for the first time, the potential of fast, high-resolution trapped ion mobility spectrometry (TIMS) preseparation in tandem with mass spectrometry assisted by an electromagnetostatic (EMS) cell, capable of electron capture dissociation (ECD), and ultraviolet photodissociation (213 nm UVPD) is illustrated for the characterization of α-synuclein positional glycoforms: T72, T75, T81, and S87 modified with a single O-GlcNAc. Top-down 213 nm UVPD and ECD MS/MS experiments of the intact proteoforms showed specific product ions for each α-synuclein glycoforms associated with the O-GlcNAc position with a sequence coverage of ∼68 and ∼82%, respectively. TIMS-MS profiles of α-synuclein and the four glycoforms exhibited large structural heterogeneity and signature patterns across the 8+-15+ charge state distribution; however, while the α-synuclein positional glycoforms showed signature mobility profiles, they were only partially separated in the mobility domain. Moreover, a middle-down approach based on the Val40-Phe94 (55 residues) chymotrypsin proteolytic product using tandem TIMS-q-ECD-TOF MS/MS permitted the separation of the parent positional isomeric glycoforms. The ECD fragmentation of the ion mobility and m/z separated isomeric Val40-Phe94 proteolytic peptides with single O-GlcNAc in the T72, T75, T81, and S87 positions provided the O-GlcNAc confirmation and positional assignment with a sequence coverage of ∼80%. This method enables the high-throughput screening of positional glycoforms and further enhances the structural mass spectrometry toolbox with fast, high-resolution mobility separations and 213 nm UVPD and ECD fragmentation capabilities.

Mechanistic roles for altered O-GlcNAcylation in neurodegenerative disorders.

Neurodegenerative diseases such as Alzheimer's and Parkinson's remain highly prevalent and incurable disorders. A major challenge in fully understanding and combating the progression of these diseases is the complexity of the network of processes that lead to progressive neuronal dysfunction and death. An ideal therapeutic avenue is conceivably one that could address many if not all of these multiple misregulated mechanisms. Over the years, chemical intervention for the up-regulation of the endogenous posttranslational modification (PTM) O-GlcNAc has been proposed as a potential strategy to slow down the progression of neurodegeneration. Through the development and application of tools that allow dissection of the mechanistic roles of this PTM, there is now a growing body of evidence that O-GlcNAc influences a variety of important neurodegeneration-pertinent mechanisms, with an overall protective effect. As a PTM that is appended onto numerous proteins that participate in protein quality control and homeostasis, metabolism, bioenergetics, neuronal communication, inflammation, and programmed death, O-GlcNAc has demonstrated beneficence in animal models of neurodegenerative diseases, and its up-regulation is now being pursued in multiple clinical studies.

α-Synuclein O-GlcNAcylation alters aggregation and toxicity, revealing certain residues as potential inhibitors of Parkinson's disease.

A compelling link is emerging between the posttranslational modification O-GlcNAc and protein aggregation. A prime example is α-synuclein, which forms toxic aggregates that are associated with neurodegeneration in Parkinson's and related diseases. α-Synuclein has been shown to be O-GlcNAcylated at nine different positions in in vivo proteomics experiments from mouse and human tissues. This raises the possibility that O-GlcNAc may alter the aggregation of this protein and could be both an important biological mediator of neurodegeneration and also a therapeutic target. Here, we expand upon our previous research in this area through the chemical synthesis of six site-specifically O-GlcNAcylated variants of α-synuclein. We then use a variety of biochemical experiments to show that O-GlcNAc in general inhibits the aggregation of α-synuclein but can also alter the structure of α-synuclein aggregates in site-specific ways. Additionally, an α-synuclein protein bearing three O-GlcNAc modifications can inhibit the aggregation of unmodified protein. Primary cell culture experiments also show that several of the O-GlcNAc sites inhibit the toxicity of extracellular α-synuclein fibers that are likely culprits in the spread of Parkinson's disease. We also demonstrate that O-GlcNAcylation can inhibit the aggregation of an aggressive mutant of α-synuclein, indicating that therapies currently in development that increase this modification might be applied in animal models that rely on this mutant. Finally, we also show that the pan-selective antibody for O-GlcNAc does not generally recognize this modification on α-synuclein, potentially explaining why it remains understudied. These results support further development of O-GlcNAcylation tools and therapeutic strategies in neurodegenerative diseases.

O-GlcNAcylation of High Mobility Group Box 1 (HMGB1) Alters Its DNA Binding and DNA Damage Processing Activities.

Protein O-GlcNAcylation is an essential and dynamic regulator of myriad cellular processes, including DNA replication and repair. Proteomic studies have identified the multifunctional nuclear protein HMGB1 as O-GlcNAcylated, providing a potential link between this modification and DNA damage responses. Here, we verify the protein's endogenous modification at S100 and S107 and found that the major modification site is S100, a residue that can potentially influence HMGB1-DNA interactions. Using synthetic protein chemistry, we generated site-specifically O-GlcNAc-modified HMGB1 at S100 and characterized biochemically the effect of the sugar modification on its DNA binding activity. We found that O-GlcNAc alters HMGB1 binding to linear, nucleosomal, supercoiled, cruciform, and interstrand cross-linked damaged DNA, generally resulting in enhanced oligomerization on these DNA structures. Using cell-free extracts, we also found that O-GlcNAc reduces the ability of HMGB1 to facilitate DNA repair, resulting in error-prone processing of damaged DNA. Our results expand our understanding of the molecular consequences of O-GlcNAc and how it affects protein-DNA interfaces. Importantly, our work may also support a link between upregulated O-GlcNAc levels and increased rates of mutations in certain cancer states.

Ubiquitination Can Change the Structure of the α-Synuclein Amyloid Fiber in a Site Selective Fashion.

Toxic amyloid aggregates are a feature of many neurodegenerative diseases. A number of biochemical and structural studies have demonstrated that not all amyloids of a given protein are equivalent but rather that an aggregating protein can form different amyloid structures or polymorphisms. Different polymorphisms can also induce different amounts of pathology and toxicity in cells and in mice, suggesting that the structural differences may play important roles in disease. However, the features that cause the formation of polymorphisms in vivo are still being uncovered. Posttranslational modifications on several amyloid forming proteins, including the Parkinson's disease causing protein α-synuclein, may be one such cause. Here, we explore whether ubiquitination can induce structural changes in α-synuclein aggregates in vitro. We used protein chemistry to first synthesize ubiquitinated analogues at three different positions using disulfide linkages. After aggregation, these linkages can be reversed, allowing us to make relative comparisons between the structures using a proteinase K assay. We find that, while ubiquitination at residue 6, 23, or 96 inhibits α-synuclein aggregation, only modification at residue 96 causes an alteration in the aggregate structure, providing further evidence that posttranslational modifications may be an important feature in amyloid polymorphism formation.

O-GlcNAc Engineering of GPCR Peptide-Agonists Improves Their Stability and in Vivo Activity.

Peptide agonists of GPCRs and other receptors are powerful signaling molecules with high potential as biological tools and therapeutics, but they are typically plagued by instability and short half-lives in vivo. Nature uses protein glycosylation to increase the serum stability of secreted proteins. However, these extracellular modifications are complex and heterogeneous in structure, making them an impractical solution. In contrast, intracellular proteins are subjected to a simple version of glycosylation termed O-GlcNAc modification. In our studies of this modification, we found that O-GlcNAcylation inhibits proteolysis, and strikingly, this stabilization occurs despite large distances in primary sequence (10-15 amino acids) between the O-GlcNAc and the site of cleavage. We therefore hypothesized that this "remote stabilization" concept could be useful to engineer the stability and potentially additional properties of peptide or protein therapeutics. Here, we describe the application of O-GlcNAcylation to two clinically important peptides: glucagon-like peptide-1 (GLP-1) and the parathyroid hormone (PTH), which respectively help control glucose and calcium levels in the blood. For both peptides, we found O-GlcNAcylated analogs that are equipotent to unmodified peptide in cell-based activation assays, while several GLP-1 analogs were biased agonists relative to GLP-1. As we predicted, O-GlcNAcylation can improve the stability of both GLP-1 and PTH in serum despite the fact that the O-GlcNAc can be quite remote from characterized sites of peptide cleavage. The O-GlcNAcylated GLP-1 and PTH also displayed significantly improved in vivo activity. Finally, we employed structure-based molecular modeling and receptor mutagenesis to predict how O-GlcNAcylation can be accommodated by the receptors and the potential interactions that contribute to peptide activity. This approach demonstrates the potential of O-GlcNAcylation for generating analogs of therapeutic peptides with enhanced proteolytic stability.

O-GlcNAc modification forces the formation of an α-Synuclein amyloid-strain with notably diminished seeding activity and pathology.

The process of amyloid fibril formation remains one of the primary targets for developing diagnostics and treatments for several neurodegenerative diseases (NDDs). Amyloid-forming proteins such α-Synuclein and Tau, which are implicated in the pathogenesis of Alzheimer's and Parkinson's disease, can form different types of fibril structure, or strains, that exhibit distinct structures, toxic properties, seeding activities, and pathology spreading patterns in the brain. Therefore, understanding the molecular and structural determinants contributing to the formation of different amyloid strains or their distinct features could open new avenues for developing disease-specific diagnostics and therapies. In this work, we report that O-GlcNAc modification of α-Synuclein monomers results in the formation of amyloid fibril with distinct core structure, as revealed by Cryo-EM, and diminished seeding activity in seeding-based neuronal and rodent models of Parkinson's disease. Although the mechanisms underpinning the seeding neutralization activity of the O-GlcNAc modified fibrils remain unclear, our in vitro mechanistic studies indicate that heat shock proteins interactions with O-GlcNAc fibril inhibit their seeding activity, suggesting that the O-GlcNAc modification may alter the interactome of the α-Synuclein fibrils in ways that lead to reduce seeding activity in vivo. Our results show that post-translational modifications, such as O-GlcNAc modification, of α-Synuclein are key determinants of α-Synuclein amyloid strains and pathogenicity. These findings have significant implications for how we investigate and target amyloids in the brain and could possibly explain the lack of correlation between amyloid burden and neurodegeneration or cognitive decline in some subtypes of NDDs.

Multifaceted Regulation of Akt by Diverse C-Terminal Post-translational Modifications.

Akt is a Ser/Thr protein kinase that regulates cell growth and metabolism and is considered a therapeutic target for cancer. Regulation of Akt by membrane recruitment and post-translational modifications (PTMs) has been extensively studied. The most well-established mechanism for cellular Akt activation involves phosphorylation on its activation loop on Thr308 by PDK1 and on its C-terminal tail on Ser473 by mTORC2. In addition, dual phosphorylation on Ser477 and Thr479 has been shown to activate Akt. Other C-terminal tail PTMs have been identified, but their functional impacts have not been well-characterized. Here, we investigate the regulatory effects of phosphorylation of Tyr474 and O-GlcNAcylation of Ser473 on Akt. We use expressed protein ligation as a tool to produce semisynthetic Akt proteins containing phosphoTyr474 and O-GlcNAcSer473 to dissect the enzymatic functions of these PTMs. We find that O-GlcNAcylation at Ser473 and phosphorylation at Tyr474 can also partially increase Akt's kinase activity toward both peptide and protein substrates. Additionally, we performed kinase assays employing human protein microarrays to investigate global substrate specificity of Akt, comparing phosphorylated versus O-GlcNAcylated Ser473 forms. We observed a high similarity in the protein substrates phosphorylated by phosphoSer473 Akt and O-GlcNAcSer473 Akt. Two Akt substrates identified using microarrays, PPM1H, a protein phosphatase, and NEDD4L, an E3 ubiquitin ligase, were validated in solution-phase assays and cell transfection experiments.

Generation of Potent and Stable GLP-1 Analogues Via "Serine Ligation".

Peptide and protein bioconjugation technologies have revolutionized our ability to site-specifically or chemoselectively install a variety of functional groups for applications in chemical biology and medicine, including the enhancement of bioavailability. Here, we introduce a site-specific bioconjugation strategy inspired by chemical ligation at serine that relies on a noncanonical amino acid containing a 1-amino-2-hydroxy functional group and a salicylaldehyde ester. More specifically, we harness this technology to generate analogues of glucagon-like peptide-1 that resemble Semaglutide, a long-lasting blockbuster drug currently used in the clinic to regulate glucose levels in the blood. We identify peptides that are more potent than unmodified peptide and equipotent to Semaglutide in a cell-based activation assay, improve the stability in human serum, and increase glucose disposal efficiency in vivo. This approach demonstrates the potential of "serine ligation" for various applications in chemical biology, with a particular focus on generating stabilized peptide therapeutics.

O-GlcNAcylated peptides and proteins for structural and functional studies.

O-GlcNAcylation is an enzymatic post-translational modification occurring in hundreds of protein substrates. This modification occurs through the addition of the monosaccharide N-acetylglucosamine to serine and threonine residues on intracellular proteins in the cytosol, nucleus, and mitochondria. As a highly dynamic form of modification, changes in O-GlcNAc levels coincide with alterations in metabolic state, the presence of stressors, and cellular health. At the protein level, the consequences of the sugar modification can vary, thus necessitating biochemical investigations on protein-specific and site-specific effects. To this end, enzymatic and chemical methods to 'encode' the modification have been developed and the utilization of these synthetic glycopeptides and glycoproteins has since been instrumental in the discovery of the mechanisms by which O-GlcNAcylation can affect a diverse array of biological processes.

Consequences of post-translational modifications on amyloid proteins as revealed by protein semisynthesis.

Alterations to the global levels of certain types of post-translational modifications (PTMs) are commonly observed in neurodegenerative diseases. The net influence of these PTM changes to the progression of these diseases can be deduced from cellular and animal studies. However, at the molecular level, how one PTM influences a given protein is not uniform and cannot be easily generalized from systemic observations, thus requiring protein-specific interrogations. Given that protein aggregation is a shared pathological hallmark in neurodegeneration, it is important to understand how these PTMs affect the behavior of amyloid-forming proteins. For this purpose, protein semisynthesis techniques, largely via native chemical and expressed protein ligation, have been widely used. These approaches have thus far led to our increased understanding of the site-specific consequences of certain PTMs to amyloidogenic proteins' endogenous function, their propensity for aggregation, and the structural variations these PTMs induce toward the aggregates formed.

Comparison of N-Acetyl-Glucosamine to Other Monosaccharides Reveals Structural Differences for the Inhibition of α-Synuclein Aggregation.

O-GlcNAc modification of the microtubule associated protein tau and α-synuclein can directly inhibit the formation of the associated amyloid fibers associated with major classes of neurodegenerative diseases. However, the mechanism(s) by which this posttranslational modification (PTM) inhibit amyloid aggregation are still murky. One hypothesis is that O-GlcNAc simply acts as a polyhydroxylated steric impediment to the formation of amyloid oligomers and fibers. Here, we begin to test this hypothesis by comparing the effects of O-GlcNAc to other similar monosaccharides-glucose, N-acetyl-galactosamine (GalNAc), or mannose-on α-synuclein amyloid formation. Interestingly, we find that this quite reasonable hypothesis is not entirely correct. More specifically, we used four types of biochemical and biophysical assays to discover that the different sugars display different effects on the inhibition of amyloid formation, despite only small differences between the structures of the monosaccharides. These results further support a more detailed investigation into the mechanism of amyloid inhibition by O-GlcNAc and has potential implications for the evolution of N-acetyl-glucosamine as the monosaccharide of choice for widespread intracellular glycosylation.

O-GlcNAc modification of small heat shock proteins enhances their anti-amyloid chaperone activity.

A major role for the intracellular post-translational modification O-GlcNAc appears to be the inhibition of protein aggregation. Most of the previous studies in this area focused on O-GlcNAc modification of the amyloid-forming proteins themselves. Here we used synthetic protein chemistry to discover that O-GlcNAc also activates the anti-amyloid activity of certain small heat shock proteins (sHSPs), a potentially more important modification event that can act broadly and substoichiometrically. More specifically, we found that O-GlcNAc increases the ability of sHSPs to block the amyloid formation of both α-synuclein and Aß(1-42). Mechanistically, we show that O-GlcNAc near the sHSP IXI-domain prevents its ability to intramolecularly compete with substrate binding. Finally, we found that, although O-GlcNAc levels are globally reduced in Alzheimer's disease brains, the modification of relevant sHSPs is either maintained or increased, which suggests a mechanism to maintain these potentially protective O-GlcNAc modifications. Our results have important implications for neurodegenerative diseases associated with amyloid formation and potentially other areas of sHSP biology.