Galectin-4 Antimicrobial Activity Primarily Occurs Through its C-Terminal Domain.

Although immune tolerance evolved to reduce reactivity with self, it creates a gap in the adaptive immune response against microbes that decorate themselves in self-like antigens. This is particularly apparent with carbohydrate-based blood group antigens, wherein microbes can envelope themselves in blood group structures similar to human cells. In this study, we demonstrate that the innate immune lectin, galectin-4 (Gal-4), exhibits strain-specific binding and killing behavior towards microbes that display blood group-like antigens. Examination of binding preferences using a combination of microarrays populated with ABO(H) glycans and a variety of microbial strains, including those that express blood group-like antigens, demonstrated that Gal-4 binds mammalian and microbial antigens that have features of blood group and mammalian-like structures. Although Gal-4 was thought to exist as a monomer that achieves functional bivalency through its two linked carbohydrate recognition domains, our data demonstrate that Gal-4 forms dimers and that differences in the intrinsic ability of each domain to dimerize likely influences binding affinity. While each Gal-4 domain exhibited blood group-binding activity, the C-terminal domain (Gal-4C) exhibited dimeric properties, while the N-terminal domain (Gal-4N) failed to similarly display dimeric activity. Gal-4C not only exhibited the ability to dimerize but also possessed higher affinity toward ABO(H) blood group antigens and microbes expressing glycans with blood group-like features. Furthermore, when compared to Gal-4N, Gal-4C exhibited more potent antimicrobial activity. Even in the context of the full-length protein, where Gal-4N is functionally bivalent by virtue of Gal-4C dimerization, Gal-4C continued to display higher antimicrobial activity. These results demonstrate that Gal-4 exists as a dimer and exhibits its antimicrobial activity primarily through its C-terminal domain. In doing so, these data provide important insight into key features of Gal-4 responsible for its innate immune activity against molecular mimicry.

Glycoproteomic landscape and structural dynamics of TIM family immune checkpoints enabled by mucinase SmE.

Mucin-domain glycoproteins are densely O-glycosylated and play critical roles in a host of biological functions. In particular, the T cell immunoglobulin and mucin-domain containing family of proteins (TIM-1, -3, -4) decorate immune cells and act as key regulators in cellular immunity. However, their dense O-glycosylation remains enigmatic, primarily due to the challenges associated with studying mucin domains. Here, we demonstrate that the mucinase SmE has a unique ability to cleave at residues bearing very complex glycans. SmE enables improved mass spectrometric analysis of several mucins, including the entire TIM family. With this information in-hand, we perform molecular dynamics (MD) simulations of TIM-3 and -4 to understand how glycosylation affects structural features of these proteins. Finally, we use these models to investigate the functional relevance of glycosylation for TIM-3 function and ligand binding. Overall, we present a powerful workflow to better understand the detailed molecular structures and functions of the mucinome.

New insights into the glycobiology of immune thrombocytopenia.

PURPOSE OF REVIEW: The platelet surface harbors a lush forest of glycans (carbohydrate polymers) attached to membrane proteins and lipids. Accumulating evidence suggests that these glycans may be relevant to the pathophysiology of immune thrombocytopenia (ITP). Here, we critically evaluate data that point to a possible role for loss of sialic acid in driving platelet clearance in ITP, comment on the potential use of neuraminidase inhibitors for treatment of ITP, and highlight open questions in this area. RECENT FINDINGS: Multiple lines of evidence suggest a role for loss of platelet sialic acid in the pathophysiology of thrombocytopenia. Recent work has tested the hypothesis that neuraminidase-mediated cleavage of platelet sialic acid may trigger clearance of platelets in ITP. Some clinical evidence supports efficacy of the viral neuraminidase inhibitor oseltamivir in ITP, which is surprising given its lack of activity against human neuraminidases. SUMMARY: Further study of platelet glycobiology in ITP is necessary to fill key knowledge gaps. A deeper understanding of the roles of platelet glycans in ITP pathophysiology will help to guide development of novel therapies.

Comprehensive analysis of platelet glycoprotein Ibα ectodomain glycosylation.

BACKGROUND: Platelet glycoprotein (GP) Ibα is the major ligand-binding subunit of the GPIb-IX-V complex that binds von Willebrand factor. GPIbα is heavily glycosylated, and its glycans have been proposed to play key roles in platelet clearance, von Willebrand factor binding, and as target antigens in immune thrombocytopenia syndromes. Despite its importance in platelet biology, the glycosylation profile of GPIbα is not well characterized. OBJECTIVES: The aim of this study was to comprehensively analyze GPIbα amino acid sites of glycosylation (glycosites) and glycan structures. METHODS: GPIbα ectodomain that was recombinantly expressed or that was purified from human platelets was analyzed by Western blot, mass spectrometry glycomics, and mass spectrometry glycopeptide analysis to define glycosites and the structures of the attached glycans. RESULTS: We identified a diverse repertoire of N- and O-glycans, including sialoglycans, Tn antigen, T antigen, and ABO(H) blood group antigens. In the analysis of the recombinant protein, we identified 62 unique O-glycosites. In the analysis of the endogenous protein purified from platelets, we identified 48 unique O-glycosites and 1 N-glycosite. The GPIbα mucin domain is densely O-glycosylated. Glycosites are also located within the macroglycopeptide domain and mechanosensory domain. CONCLUSIONS: This comprehensive analysis of GPIbα glycosylation lays the foundation for further studies to determine the functional and structural roles of GPIbα glycans.

Glycoproteomic landscape and structural dynamics of TIM family immune checkpoints enabled by mucinase SmE.

Mucin-domain glycoproteins are densely O-glycosylated and play critical roles in a host of biological functions. In particular, the T cell immunoglobulin and mucin-domain containing family of proteins (TIM-1, -3, -4) decorate immune cells and act as key checkpoint inhibitors in cancer. However, their dense O-glycosylation remains enigmatic both in terms of glycoproteomic landscape and structural dynamics, primarily due to the challenges associated with studying mucin domains. Here, we present a mucinase (SmE) and demonstrate its ability to selectively cleave along the mucin glycoprotein backbone, similar to others of its kind. Unlike other mucinases, though, SmE harbors the unique ability to cleave at residues bearing extremely complex glycans which enabled improved mass spectrometric analysis of several mucins, including the entire TIM family. With this information in-hand, we performed molecular dynamics (MD) simulations of TIM-3 and -4 to demonstrate how glycosylation affects structural features of these proteins. Overall, we present a powerful workflow to better understand the detailed molecular structures of the mucinome.

ABO blood group antigens and differential glycan expression: Perspective on the evolution of common human enzyme deficiencies.

Enzymes catalyze biochemical reactions and play critical roles in human health and disease. Enzyme variants and deficiencies can lead to variable expression of glycans, which can affect physiology, influence predilection for disease, and/or directly contribute to disease pathogenesis. Although certain well-characterized enzyme deficiencies result in overt disease, some of the most common enzyme deficiencies in humans form the basis of blood groups. These carbohydrate blood groups impact fundamental areas of clinical medicine, including the risk of infection and severity of infectious disease, bleeding risk, transfusion medicine, and tissue/organ transplantation. In this review, we examine the enzymes responsible for carbohydrate-based blood group antigen biosynthesis and their expression within the human population. We also consider the evolutionary selective pressures, e.g. malaria, that may account for the variation in carbohydrate structures and the implications of this biology for human disease.

Bridging the Divide: Student Grand Rounds at the Interface of Basic Science and Clinical Medicine.

PROBLEM: As biomedical research and clinical medicine become increasingly complex, physician-scientists and clinically oriented biomedical researchers play important roles in bridging the gap between disciplines. A lack of educational programming that addresses the unique needs of students preparing for careers at the interface of basic science and clinical medicine may contribute to trainee attrition. APPROACH: The MD-PhD/LHB Grand Rounds was introduced in 2008 as a trainee-driven collaborative effort of the Harvard/Massachusetts Institute of Technology MD-PhD program at Harvard Medical School (HMS MD-PhD program), Harvard's Leder Human Biology and Translational Medicine (LHB) program, and the Brigham and Women's Hospital (BWH) Internal Medicine Department. Each of the program's approximately 4 sessions per year begins with dinner, followed by a clinical case presentation led by a BWH MD-PhD resident with a master clinician faculty discussant, then a research presentation by an LHB PhD student or an MD-PhD student on a basic science topic related to the clinical case, and time for socialization. OUTCOMES: In a July 2017 survey of participating students and residents, respondents reported being highly satisfied with the program. Mean satisfaction ratings were 4.3 (SD 0.5) for 12 MD-PhD students, 4.2 (SD 0.7) for 31 LHB students, and 4.4 (SD 0.9) for 5 residents on a 5-point scale (5 = very satisfied). Free-text responses suggested MD-PhD students valued opportunities for active engagement with the resident presenter and faculty discussant. LHB students appreciated the absence of medical jargon in the clinical presentations. Residents' reported reasons for participating included enjoyment of teaching and interaction with students. NEXT STEPS: The Harvard MD-PhD/LHB Grand Rounds can serve as a template for developing similar programs at other institutions. Research is needed to determine whether such grand rounds programs can help fix the leaky pipeline in the training of future physician-scientists and clinically oriented biomedical researchers.

Clinical decision support and improved blood use in patient blood management.

Despite many years of published medical society guidelines for red blood cell (RBC) transfusion therapy, along with clinical trials that provide Level 1 evidence that restrictive transfusion practices can be used safely and are equivalent to transfusions given more liberally, annualized blood transfusion activity did not begin to decline in the United States until 2010. Adoption of electronic medical records has subsequently allowed implementation of clinical decision support (CDS): best practice alerts that can be initiated to improve the use of blood components. We describe our own institutional experience using a targeted CDS to promote restrictive blood transfusion practice and to improve RBC use. A 42% reduction in RBC transfusions was demonstrated at our institution from a baseline in 2008 through 2015, and the rate remained stable through 2018. Although the data cannot be used to infer causality, this decreased RBC use was accompanied by improved clinical outcomes.

Markers of autoimmunity in immune thrombocytopenia: prevalence and prognostic significance.

Prior studies have demonstrated an increased prevalence of autoimmune markers in patients with immune thrombocytopenia (ITP). Clinical experience has suggested that there may be an association between autoimmune markers and poor outcomes in ITP, but current guidelines do not encourage routine testing in these patients. We retrospectively assessed the prevalence of autoimmune markers in adult patients with ITP from our institutional database and used multiple logistic regression analyses to test for an association between autoimmune marker positivity and thrombotic events or clinical remission. We also assessed whether positivity for common autoimmune markers was associated with positivity for platelet autoantibodies. There was a high rate of autoimmune marker positivity in this population, with antinuclear antibody (65%), antithyroid peroxidase antibody (31%), and direct antiglobulin (29%) the most commonly found. Antithyroid peroxidase antibody positivity was associated with a lower probability of remission (odds ratio [OR], 0.26; 95% confidence interval [CI], 0.09-0.79; P = .017). Lupus anticoagulant positivity was associated with a higher rate of thrombosis (OR, 8.92; 95% CI, 1.94-40.95; P = .005), and antinuclear antibody was strongly associated with thrombosis (P = .001). There was no relation between platelet autoantibody positivity and the presence of autoimmune markers. These results suggest that many patients with ITP have a state of immune dysregulation that extends beyond platelet autoantibodies and that certain autoimmune markers may be prognostically useful in this disorder.

Thrombosis, Hypercoagulable States, and Anticoagulants.

Patients with derangements of secondary hemostasis resulting from inherited or acquired thrombophilias are at increased risk of venous thromboemboli (VTE). Evaluation of a patient with suspected VTE proceeds via evidence-based algorithms that involve computing a pretest probability based on the history and physical examination; this guides subsequent work-up, which can include D dimer and/or imaging. Testing for hypercoagulable disorders should be pursued only in patients with VTE with an increased risk for an underlying thrombophilia. Direct oral anticoagulants are first-line VTE therapies, but they should be avoided in patients who are pregnant, have active cancer, antiphospholipid antibody syndrome, severe renal insufficiency, or prosthetic heart valves.

A head-to-head comparison of eneamide and epoxyamide inhibitors of glucosamine-6-phosphate synthase from the dapdiamide biosynthetic pathway.

The dapdiamides make up a family of antibiotics that have been presumed to be cleaved in the target cell to enzyme-inhibitory N-acyl-2,3-diaminopropionate (DAP) warheads containing two alternative electrophilic moieties. Our prior biosynthetic studies revealed that an eneamide warhead is made first and converted to an epoxyamide via a three-enzyme branch pathway. Here we provide a rationale for this logic. We report that the R,R-epoxyamide warhead is a more efficient covalent inactivator of glucosamine-6-phosphate synthase by 1 order of magnitude versus the eneamide, and this difference correlates with a >10-fold difference in antibiotic activity for the corresponding acyl-DAP dipeptides.

The nonribosomal peptide synthetase enzyme DdaD tethers N(ß)-fumaramoyl-l-2,3-diaminopropionate for Fe(II)/α-ketoglutarate-dependent epoxidation by DdaC during dapdiamide antibiotic biosynthesis.

The gene cluster from Pantoea agglomerans responsible for biosynthesis of the dapdiamide antibiotics encodes an adenylation-thiolation didomain protein, DdaD, and an Fe(II)/α-ketoglutarate-dependent dioxygenase homologue, DdaC. Here we show that DdaD, a nonribosomal peptide synthetase module, activates and sequesters N(ß)-fumaramoyl-l-2,3-diaminopropionate as a covalently tethered thioester for subsequent oxidative modification of the fumaramoyl group. DdaC catalyzes Fe(II)- and α-ketoglutarate-dependent epoxidation of the covalently bound N(ß)-fumaramoyl-l-2,3-diaminopropionyl-S-DdaD species to generate N(ß)-epoxysuccinamoyl-DAP (DAP = 2,3-diaminopropionate) in thioester linkage to DdaD. After hydrolytic release, N(ß)-epoxysuccinamoyl-DAP can be ligated to l-valine by the ATP-dependent ligase DdaF to form the natural antibiotic N(ß)-epoxysuccinamoyl-DAP-Val.

The ATP-dependent amide ligases DdaG and DdaF assemble the fumaramoyl-dipeptide scaffold of the dapdiamide antibiotics.

The enzymes DdaG and DdaF, encoded in the Pantoea agglomerans dapdiamide antibiotic biosynthetic gene cluster, when expressed in Escherichia coli, form the tandem amide bonds of the dapdiamide scaffold at the expense of ATP cleavage. DdaG uses fumarate, 2,3-diaminopropionate (DAP), and ATP to make fumaroyl-AMP transiently on the way to the N(beta)-fumaroyl-DAP regioisomer. Then DdaF acts as a second ATP-dependent amide ligase, but this enzyme cleaves ATP to ADP and P(i) during amide bond formation. However, DdaF will not accept N(beta)-fumaroyl-DAP; the enzyme requires the fumaroyl moiety to be first converted to the fumaramoyl half-amide in N(beta)-fumaramoyl-DAP. DdaF adds Val, Ile, or Leu to the carboxylate of fumaramoyl-DAP to make dapdiamide A, B, or C, respectively. Thus, to build the dapdiamide antibiotic scaffold, amidation must occur on the fumaroyl-DAP scaffold, after DdaG action but before DdaF catalysis. This is an unusual instance of two ligases acting sequentially in untemplated amide bond formations using attack of substrate carboxylates at P(alpha) (AMP-forming) and then at P(gamma) (ADP-forming) of ATP cosubstrates.