Biallelic variants in FLII cause pediatric cardiomyopathy by disrupting cardiomyocyte cell adhesion and myofibril organization.

Pediatric cardiomyopathy (CM) represents a group of rare, severe disorders that affect the myocardium. To date, the etiology and mechanisms underlying pediatric CM are incompletely understood, hampering accurate diagnosis and individualized therapy development. Here, we identified biallelic variants in the highly conserved flightless-I (FLII) gene in 3 families with idiopathic, early-onset dilated CM. We demonstrated that patient-specific FLII variants, when brought into the zebrafish genome using CRISPR/Cas9 genome editing, resulted in the manifestation of key aspects of morphological and functional abnormalities of the heart, as observed in our patients. Importantly, using these genetic animal models, complemented with in-depth loss-of-function studies, we provided insights into the function of Flii during ventricular chamber morphogenesis in vivo, including myofibril organization and cardiomyocyte cell adhesion, as well as trabeculation. In addition, we identified Flii function to be important for the regulation of Notch and Hippo signaling, crucial pathways associated with cardiac morphogenesis and function. Taken together, our data provide experimental evidence for a role for FLII in the pathogenesis of pediatric CM and report biallelic variants as a genetic cause of pediatric CM.

Broad betacoronavirus neutralization by a stem helix-specific human antibody.

The spillovers of betacoronaviruses in humans and the emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants highlight the need for broad coronavirus countermeasures. We describe five monoclonal antibodies (mAbs) cross-reacting with the stem helix of multiple betacoronavirus spike glycoproteins isolated from COVID-19 convalescent individuals. Using structural and functional studies, we show that the mAb with the greatest breadth (S2P6) neutralizes pseudotyped viruses from three different subgenera through the inhibition of membrane fusion, and we delineate the molecular basis for its cross-reactivity. S2P6 reduces viral burden in hamsters challenged with SARS-CoV-2 through viral neutralization and Fc-mediated effector functions. Stem helix antibodies are rare, oftentimes of narrow specificity, and can acquire neutralization breadth through somatic mutations. These data provide a framework for structure-guided design of pan-betacoronavirus vaccines eliciting broad protection.

Broad sarbecovirus neutralization by a human monoclonal antibody.

The recent emergence of SARS-CoV-2 variants of concern1-10 and the recurrent spillovers of coronaviruses11,12 into the human population highlight the need for broadly neutralizing antibodies that are not affected by the ongoing antigenic drift and that can prevent or treat future zoonotic infections. Here we describe a human monoclonal antibody designated S2X259, which recognizes a highly conserved cryptic epitope of the receptor-binding domain and cross-reacts with spikes from all clades of sarbecovirus. S2X259 broadly neutralizes spike-mediated cell entry of SARS-CoV-2, including variants of concern (B.1.1.7, B.1.351, P.1, and B.1.427/B.1.429), as well as a wide spectrum of human and potentially zoonotic sarbecoviruses through inhibition of angiotensin-converting enzyme 2 (ACE2) binding to the receptor-binding domain. Furthermore, deep-mutational scanning and in vitro escape selection experiments demonstrate that S2X259 possesses an escape profile that is limited to a single substitution, G504D. We show that prophylactic and therapeutic administration of S2X259 protects Syrian hamsters (Mesocricetus auratus) against challenge with the prototypic SARS-CoV-2 and the B.1.351 variant of concern, which suggests that this monoclonal antibody is a promising candidate for the prevention and treatment of emergent variants and zoonotic infections. Our data reveal a key antigenic site that is targeted by broadly neutralizing antibodies and will guide the design of vaccines that are effective against all sarbecoviruses.

SARS-CoV-2 RBD antibodies that maximize breadth and resistance to escape.

An ideal therapeutic anti-SARS-CoV-2 antibody would resist viral escape1-3, have activity against diverse sarbecoviruses4-7, and be highly protective through viral neutralization8-11 and effector functions12,13. Understanding how these properties relate to each other and vary across epitopes would aid the development of therapeutic antibodies and guide vaccine design. Here we comprehensively characterize escape, breadth and potency across a panel of SARS-CoV-2 antibodies targeting the receptor-binding domain (RBD). Despite a trade-off between in vitro neutralization potency and breadth of sarbecovirus binding, we identify neutralizing antibodies with exceptional sarbecovirus breadth and a corresponding resistance to SARS-CoV-2 escape. One of these antibodies, S2H97, binds with high affinity across all sarbecovirus clades to a cryptic epitope and prophylactically protects hamsters from viral challenge. Antibodies that target the angiotensin-converting enzyme 2 (ACE2) receptor-binding motif (RBM) typically have poor breadth and are readily escaped by mutations despite high neutralization potency. Nevertheless, we also characterize a potent RBM antibody (S2E128) with breadth across sarbecoviruses related to SARS-CoV-2 and a high barrier to viral escape. These data highlight principles underlying variation in escape, breadth and potency among antibodies that target the RBD, and identify epitopes and features to prioritize for therapeutic development against the current and potential future pandemics.

Antibodies to the SARS-CoV-2 receptor-binding domain that maximize breadth and resistance to viral escape.

An ideal anti-SARS-CoV-2 antibody would resist viral escape 1-3 , have activity against diverse SARS-related coronaviruses 4-7 , and be highly protective through viral neutralization 8-11 and effector functions 12,13 . Understanding how these properties relate to each other and vary across epitopes would aid development of antibody therapeutics and guide vaccine design. Here, we comprehensively characterize escape, breadth, and potency across a panel of SARS-CoV-2 antibodies targeting the receptor-binding domain (RBD), including S309 4 , the parental antibody of the late-stage clinical antibody VIR-7831. We observe a tradeoff between SARS-CoV-2 in vitro neutralization potency and breadth of binding across SARS-related coronaviruses. Nevertheless, we identify several neutralizing antibodies with exceptional breadth and resistance to escape, including a new antibody (S2H97) that binds with high affinity to all SARS-related coronavirus clades via a unique RBD epitope centered on residue E516. S2H97 and other escape-resistant antibodies have high binding affinity and target functionally constrained RBD residues. We find that antibodies targeting the ACE2 receptor binding motif (RBM) typically have poor breadth and are readily escaped by mutations despite high neutralization potency, but we identify one potent RBM antibody (S2E12) with breadth across sarbecoviruses closely related to SARS-CoV-2 and with a high barrier to viral escape. These data highlight functional diversity among antibodies targeting the RBD and identify epitopes and features to prioritize for antibody and vaccine development against the current and potential future pandemics.

Structural basis for broad sarbecovirus neutralization by a human monoclonal antibody.

The recent emergence of SARS-CoV-2 variants of concern (VOC) and the recurrent spillovers of coronaviruses in the human population highlight the need for broadly neutralizing antibodies that are not affected by the ongoing antigenic drift and that can prevent or treat future zoonotic infections. Here, we describe a human monoclonal antibody (mAb), designated S2X259, recognizing a highly conserved cryptic receptor-binding domain (RBD) epitope and cross-reacting with spikes from all sarbecovirus clades. S2X259 broadly neutralizes spike-mediated entry of SARS-CoV-2 including the B.1.1.7, B.1.351, P.1 and B.1.427/B.1.429 VOC, as well as a wide spectrum of human and zoonotic sarbecoviruses through inhibition of ACE2 binding to the RBD. Furthermore, deep-mutational scanning and in vitro escape selection experiments demonstrate that S2X259 possesses a remarkably high barrier to the emergence of resistance mutants. We show that prophylactic administration of S2X259 protects Syrian hamsters against challenges with the prototypic SARS-CoV-2 and the B.1.351 variant, suggesting this mAb is a promising candidate for the prevention and treatment of emergent VOC and zoonotic infections. Our data unveil a key antigenic site targeted by broadly-neutralizing antibodies and will guide the design of pan-sarbecovirus vaccines.

Cavin4b/Murcb Is Required for Skeletal Muscle Development and Function in Zebrafish.

Skeletal muscles provide metazoans with the ability to feed, reproduce and avoid predators. In humans, a heterogeneous group of genetic diseases, termed muscular dystrophies (MD), lead to skeletal muscle dysfunction. Mutations in the gene encoding Caveolin-3, a principal component of the membrane micro-domains known as caveolae, cause defects in muscle maintenance and function; however it remains unclear how caveolae dysfunction underlies MD pathology. The Cavin family of caveolar proteins can form membrane remodeling oligomers and thus may also impact skeletal muscle function. Changes in the distribution and function of Cavin4/Murc, which is predominantly expressed in striated muscles, have been reported to alter caveolae structure through interaction with Caveolin-3. Here, we report the generation and phenotypic analysis of murcb mutant zebrafish, which display impaired swimming capacity, skeletal muscle fibrosis and T-tubule abnormalities during development. To understand the mechanistic importance of Murc loss of function, we assessed Caveolin-1 and 3 localization and found it to be abnormal. We further identified an in vivo function for Murc in Erk signaling. These data link Murc with developmental defects in T-tubule formation and progressive muscle dysfunction, thereby providing a new candidate for the etiology of muscular dystrophy.

Dynamics of zebrafish fin regeneration using a pulsed SILAC approach.

The zebrafish owns remarkable regenerative capacities allowing regeneration of several tissues, including the heart, liver, and brain. To identify protein dynamics during fin regeneration we used a pulsed SILAC approach that enabled us to detect the incorporation of (13) C6 -lysine (Lys6) into newly synthesized proteins. Samples were taken at four different time points from noninjured and regrowing fins and incorporation rates were monitored using a combination of single-shot 4-h gradients and high-resolution tandem MS. We identified more than 5000 labeled proteins during the first 3 weeks of fin regeneration and were able to monitor proteins that are responsible for initializing and restoring the shape of these appendages. The comparison of Lys6 incorporation rates between noninjured and regrowing fins enabled us to identify proteins that are directly involved in regeneration. For example, we observed increased incorporation rates of two actinodin family members at the actinotrichia, which is a hairlike fiber structure at the tip of regrowing fins. Moreover, we used quantitative real-time RNA measurements of several candidate genes, including osteoglycin, si:ch211-288h17.3, and prostaglandin reductase 1 to correlate the mRNA expression to Lys6 incorporation data. This novel pulsed SILAC methodology in fish can be used as a versatile tool to monitor newly synthesized proteins and will help to characterize protein dynamics during regenerative processes in zebrafish beyond fin regeneration.

Translational profiling through biotinylation of tagged ribosomes in zebrafish.

Heterogeneity within a population of cells of the same type is a common theme in metazoan biology. Dissecting complex developmental and physiological processes crucially relies on our ability to probe the expression profile of these cell subpopulations. Current strategies rely on cell enrichment based on sequential or simultaneous use of multiple intersecting markers starting from a heterogeneous cell suspension. The extensive tissue manipulations required to generate single-cell suspensions, as well as the complexity of the required equipment, inherently complicate these approaches. Here, we propose an alternative methodology based on a genetically encoded system in the model organism Danio rerio (zebrafish). In transgenic fish, we take advantage of the combinatorial biotin transfer system, where polysome-associated mRNAs are selectively recovered from cells expressing both a tagged ribosomal subunit, Rpl10a, and the bacterial biotin ligase BirA. We have applied this technique to skeletal muscle development and identified new genes with interesting temporal expression patterns. Through this work we have thus developed additional tools for highly specific gene expression profiling.

Nodal signaling regulates endodermal cell motility and actin dynamics via Rac1 and Prex1.

Embryo morphogenesis is driven by dynamic cell behaviors, including migration, that are coordinated with fate specification and differentiation, but how such coordination is achieved remains poorly understood. During zebrafish gastrulation, endodermal cells sequentially exhibit first random, nonpersistent migration followed by oriented, persistent migration and finally collective migration. Using a novel transgenic line that labels the endodermal actin cytoskeleton, we found that these stage-dependent changes in migratory behavior correlated with changes in actin dynamics. The dynamic actin and random motility exhibited during early gastrulation were dependent on both Nodal and Rac1 signaling. We further identified the Rac-specific guanine nucleotide exchange factor Prex1 as a Nodal target and showed that it mediated Nodal-dependent random motility. Reducing Rac1 activity in endodermal cells caused them to bypass the random migration phase and aberrantly contribute to mesodermal tissues. Together, our results reveal a novel role for Nodal signaling in regulating actin dynamics and migration behavior, which are crucial for endodermal morphogenesis and cell fate decisions.

A PGC-1alpha-O-GlcNAc transferase complex regulates FoxO transcription factor activity in response to glucose.

Metabolic and stress response gene regulation is crucial for the survival of an organism to a changing environment. Three key molecules that sense nutrients and broadly affect gene expression are the FoxO transcription factors, the transcriptional co-activator PGC-1alpha, and the dynamic post-translational modification, O-linked beta-N-acetylglucosamine (O-GlcNAc). Here we identify novel post-translational modifications of PGC-1alpha, including O-GlcNAc, and describe a novel mechanism for how PGC-1alpha co-activates transcription by FoxOs. In liver, in cultured cells, and in vitro with recombinant proteins, PGC-1alpha binds to O-GlcNAc transferase and targets the enzyme to FoxOs, resulting in their increased GlcNAcylation and increased transcriptional activity. Furthermore, glucose-enhanced activation of FoxO1 occurs via this PGC-1alpha-O-GlcNAc transferase-mediated GlcNAcylation. Therefore, one mechanism by which PGC-1alpha can serve as a co-activator of transcription is by targeting the O-GlcNAc transferase to increase GlcNAcylation of specific transcription factors important to nutrient/stress sensing and energy metabolism.

O-linked beta-N-acetylglucosaminyltransferase substrate specificity is regulated by myosin phosphatase targeting and other interacting proteins.

O-GlcNAc-transferase (OGT) substrate specificity is regulated by transiently interacting proteins. To further examine the regulation of OGT, we have identified 27 putative OGT-interacting proteins through a yeast two-hybrid screen. Two of these proteins, Trak1 (OIP106) and O-GlcNAcase, have been shown previously to interact with and regulate OGT. We demonstrate here that MYPT1 and CARM1 also interact with and target OGT. MYPT1 and CARM1 are substrates of OGT in vitro and in vivo. MYPT1 and CARM1 also function to alter OGT substrate specificity in vitro. Furthermore depletion of MYPT1 in Neuro-2a neuroblastoma cells alters GlcNAcylation of several proteins under basal conditions, suggesting that MYPT1 regulates OGT substrate specificity in vivo.

O-GlcNAc regulates FoxO activation in response to glucose.

FoxO proteins are key transcriptional regulators of nutrient homeostasis and stress response. The transcription factor FoxO1 activates expression of gluconeogenic, including phosphoenolpyruvate carboxykinase and glucose-6-phosphatase, and also activates the expression of the oxidative stress response enzymes catalase and manganese superoxide dismutase. Hormonal and stress-dependent regulation of FoxO1 via acetylation, ubiquitination, and phosphorylation, are well established, but FoxOs have not been studied in the context of the glucose-derived O-linked beta-N-acetylglucosamine (O-GlcNAc) modification. Here we show that O-GlcNAc on hepatic FoxO1 is increased in diabetes. Furthermore, O-GlcNAc regulates FoxO1 activation in response to glucose, resulting in the paradoxically increased expression of gluconeogenic genes while concomitantly inducing expression of genes encoding enzymes that detoxify reactive oxygen species. GlcNAcylation of FoxO provides a new mechanism for direct nutrient control of transcription to regulate metabolism and stress response through control of FoxO1 activity.

Cycling of O-linked beta-N-acetylglucosamine on nucleocytoplasmic proteins.

All animals and plants dynamically attach and remove O-linked beta-N-acetylglucosamine (O-GlcNAc) at serine and threonine residues on myriad nuclear and cytoplasmic proteins. O-GlcNAc cycling, which is tightly regulated by the concerted actions of two highly conserved enzymes, serves as a nutrient and stress sensor. On some proteins, O-GlcNAc competes directly with phosphate for serine/threonine residues. Glycosylation with O-GlcNAc modulates signalling, and influences protein expression, degradation and trafficking. Emerging data indicate that O-GlcNAc glycosylation has a role in the aetiology of diabetes and neurodegeneration.

Polymeric immunoglobulin receptor in intestinal immune defense against the lumen-dwelling protozoan parasite Giardia.

The polymeric Ig receptor (pIgR) is conserved in mammals and has an avian homologue, suggesting evolutionarily important functions in vertebrates. It transports multimeric IgA and IgM across polarized epithelia and is highly expressed in the intestine, yet little direct evidence exists for its importance in defense against common enteric pathogens. In this study, we demonstrate that pIgR can play a critical role in intestinal defense against the lumen-dwelling protozoan parasite Giardia, a leading cause of diarrheal disease. The receptor was essential for the eradication of Giardia when high luminal IgA levels were required. Clearance of Giardia muris, in which IgA plays a dominant role, was severely compromised in pIgR-deficient mice despite significant fecal IgA output at 10% of normal levels. In contrast, eradication of the human strain Giardia lamblia GS/M, for which adaptive immunity is less IgA dependent in mice, was unaffected by pIgR deficiency, indicating that pIgR had no physiologic role when lower luminal IgA levels were sufficient for parasite elimination. Immune IgA was greatly increased in the serum of pIgR-deficient mice, conferred passive protection against Giardia, and recognized several conserved giardial Ags, including ornithine carbamoyltransferase, arginine deiminase, alpha-enolase, and alpha- and beta-giardins, that are also detected in human giardiasis. Corroborative observations were made in mice lacking the J chain, which is required for pIgR-dependent transepithelial IgA transport. These results, together with prior data on pIgR-mediated immune neutralization of luminal cholera toxin, suggest that pIgR is essential in intestinal defense against pathogenic microbes with high-level and persistent luminal presence.

O-GlcNAc cycling: how a single sugar post-translational modification is changing the way we think about signaling networks.

O-GlcNAc is an ubiquitous post-translational protein modification consisting of a single N-acetlyglucosamine moiety linked to serine or threonine residues on nuclear and cytoplasmic proteins. Recent work has begun to uncover the functional roles of O-GlcNAc in cellular processes. O-GlcNAc modified proteins are involved in sensing the nutrient status of the surrounding cellular environment and adjusting the activity of cellular proteins accordingly. O-GlcNAc regulates cellular responses to hormones such as insulin, initiates a protective response to stress, modulates a cell's capacity to grow and divide, and regulates gene transcription. This review will focus on recent work involving O-GlcNAc in sensing the environment and regulating signaling cascades.

Clearance of Citrobacter rodentium requires B cells but not secretory immunoglobulin A (IgA) or IgM antibodies.

Citrobacter rodentium, a murine model pathogen for human enteropathogenic Escherichia coli, predominantly colonizes the lumen and mucosal surface of the colon and cecum and causes crypt hyperplasia and mucosal inflammation. Mice infected with C. rodentium develop a secretory immunoglobulin A (IgA) response, but the role of B cells or secretory antibodies in host defense is unknown. To address this question, we conducted oral C. rodentium infections in mice lacking B cells, IgA, secreted IgM, polymeric Ig receptor (pIgR), or J chain. Normal mice showed peak bacterial numbers in colon and feces at 1 week and bacterial eradication after 3 to 4 weeks. B-cell-deficient mice were equally susceptible initially but could not control infection subsequently. Tissue responses showed marked differences, as infection of normal mice was accompanied by transient crypt hyperplasia and mucosal inflammation in the colon and cecum at 2 but not 6 weeks, whereas B-cell-deficient mice had few mucosal changes at 2 weeks but severe epithelial hyperplasia with ulcerations and mucosal inflammation at 6 weeks. The functions of B cells were not mediated by secretory antibodies, since mice lacking IgA or secreted IgM or proteins required for their transport into the lumen, pIgR or J chain, cleared C. rodentium normally. Nonetheless, systemic administration of immune sera reduced bacterial numbers significantly in normal and pIgR-deficient mice, and depletion of IgG abrogated this effect. These results indicate that host defense against C. rodentium depends on B cells and IgG antibodies but does not require production or transepithelial transport of IgA or secreted IgM.

Central importance of immunoglobulin A in host defense against Giardia spp.

The protozoan pathogen Giardia is an important cause of parasitic diarrheal disease worldwide. It colonizes the lumen of the small intestine, suggesting that effective host defenses must act luminally. Immunoglobulin A (IgA) antibodies are presumed to be important for controlling Giardia infection, but direct evidence for this function is lacking. B-cell-independent effector mechanisms also exist and may be equally important for antigiardial host defense. To determine the importance of the immunoglobulin isotypes that are transported into the intestinal lumen, IgA and IgM, for antigiardial host defense, we infected gene-targeted mice lacking IgA-expressing B-cells, IgM-secreting B-cells, or all B-cells as controls with Giardia muris or Giardia lamblia GS/M-83-H7. We found that IgA-deficient mice could not eradicate either G. muris or G. lamblia infection, demonstrating that IgA is required for their clearance. Furthermore, although neither B-cell-deficient nor IgA-deficient mice could clear G. muris infections, IgA-deficient mice controlled infection significantly better than B-cell-deficient mice, suggesting the existence of B-cell-dependent but IgA-independent antigiardial defenses. In contrast, mice deficient for secreted IgM antibodies cleared G. muris infection normally, indicating that they have no unique functions in antigiardial host defense. These data, together with the finding that B-cell-deficient mice have some, albeit limited, residual capacity to control G. muris infection, show that IgA-dependent host defenses are central for eradicating Giardia spp. Moreover, B-cell-dependent but IgA-independent and B-cell-independent antigiardial host defenses exist but are less important for controlling infection.