Despite the odds: formation of the SARS-CoV-2 methylation complex.

Coronaviruses modify their single-stranded RNA genome with a methylated cap during replication to mimic the eukaryotic mRNAs. The capping process is initiated by several nonstructural proteins (nsp) encoded in the viral genome. The methylation is performed by two methyltransferases, nsp14 and nsp16, while nsp10 acts as a co-factor to both. Additionally, nsp14 carries an exonuclease domain which operates in the proofreading system during RNA replication of the viral genome. Both nsp14 and nsp16 were reported to independently bind nsp10, but the available structural information suggests that the concomitant interaction between these three proteins would be impossible due to steric clashes. Here, we show that nsp14, nsp10, and nsp16 can form a heterotrimer complex upon significant allosteric change. This interaction is expected to encourage the formation of mature capped viral mRNA, modulating nsp14's exonuclease activity, and protecting the viral RNA. Our findings show that nsp14 is amenable to allosteric regulation and may serve as a novel target for therapeutic approaches.

Myocyte repolarization modulates myocardial function in aging dogs.

Studies of myocardial aging are complex and the mechanisms involved in the deterioration of ventricular performance and decreased functional reserve of the old heart remain to be properly defined. We have studied a colony of beagle dogs from 3 to 14 yr of age kept under a highly regulated environment to define the effects of aging on the myocardium. Ventricular, myocardial, and myocyte function, together with anatomical and structural properties of the organ and cardiomyocytes, were evaluated. Ventricular hypertrophy was not observed with aging and the structural composition of the myocardium was modestly affected. Alterations in the myocyte compartment were identified in aged dogs, and these factors negatively interfere with the contractile reserve typical of the young heart. The duration of the action potential is prolonged in old cardiomyocytes contributing to the slower electrical recovery of the myocardium. Also, the remodeled repolarization of cardiomyocytes with aging provides inotropic support to the senescent muscle but compromises its contractile reserve, rendering the old heart ineffective under conditions of high hemodynamic demand. The defects in the electrical and mechanical properties of cardiomyocytes with aging suggest that this cell population is an important determinant of the cardiac senescent phenotype. Collectively, the delayed electrical repolarization of aging cardiomyocytes may be viewed as a critical variable of the aging myopathy and its propensity to evolve into ventricular decompensation under stressful conditions.

Evidence for human lung stem cells.

BACKGROUND: Although progenitor cells have been described in distinct anatomical regions of the lung, description of resident stem cells has remained elusive. METHODS: Surgical lung-tissue specimens were studied in situ to identify and characterize human lung stem cells. We defined their phenotype and functional properties in vitro and in vivo. RESULTS: Human lungs contain undifferentiated human lung stem cells nested in niches in the distal airways. These cells are self-renewing, clonogenic, and multipotent in vitro. After injection into damaged mouse lung in vivo, human lung stem cells form human bronchioles, alveoli, and pulmonary vessels integrated structurally and functionally with the damaged organ. The formation of a chimeric lung was confirmed by detection of human transcripts for epithelial and vascular genes. In addition, the self-renewal and long-term proliferation of human lung stem cells was shown in serial-transplantation assays. CONCLUSIONS: Human lungs contain identifiable stem cells. In animal models, these cells participate in tissue homeostasis and regeneration. They have the undemonstrated potential to promote tissue restoration in patients with lung disease. (Funded by the National Institutes of Health.).

Cardiomyogenesis in the developing heart is regulated by c-kit-positive cardiac stem cells.

RATIONALE: Embryonic and fetal myocardial growth is characterized by a dramatic increase in myocyte number, but whether the expansion of the myocyte compartment is dictated by activation and commitment of resident cardiac stem cells (CSCs), division of immature myocytes or both is currently unknown. OBJECTIVE: In this study, we tested whether prenatal cardiac development is controlled by activation and differentiation of CSCs and whether division of c-kit-positive CSCs in the mouse heart is triggered by spontaneous Ca(2+) oscillations. METHODS AND RESULTS: We report that embryonic-fetal c-kit-positive CSCs are self-renewing, clonogenic and multipotent in vitro and in vivo. The growth and commitment of c-kit-positive CSCs is responsible for the generation of the myocyte progeny of the developing heart. The close correspondence between values computed by mathematical modeling and direct measurements of myocyte number at E9, E14, E19 and 1 day after birth strongly suggests that the organogenesis of the embryonic heart is dependent on a hierarchical model of cell differentiation regulated by resident CSCs. The growth promoting effects of c-kit-positive CSCs are triggered by spontaneous oscillations in intracellular Ca(2+), mediated by IP3 receptor activation, which condition asymmetrical stem cell division and myocyte lineage specification. CONCLUSIONS: Myocyte formation derived from CSC differentiation is the major determinant of cardiac growth during development. Division of c-kit-positive CSCs in the mouse is promoted by spontaneous Ca(2+) spikes, which dictate the pattern of stem cell replication and the generation of a myocyte progeny at all phases of prenatal life and up to one day after birth.

Tracking chromatid segregation to identify human cardiac stem cells that regenerate extensively the infarcted myocardium.

RATIONALE: According to the immortal DNA strand hypothesis, dividing stem cells selectively segregate chromosomes carrying the old template DNA, opposing accumulation of mutations resulting from nonrepaired replication errors and attenuating telomere shortening. OBJECTIVE: Based on the premise of the immortal DNA strand hypothesis, we propose that stem cells retaining the old DNA would represent the most powerful cells for myocardial regeneration. METHODS AND RESULTS: Division of human cardiac stem cells (hCSCs) by nonrandom and random segregation of chromatids was documented by clonal assay of bromodeoxyuridine-tagged hCSCs. Additionally, their growth properties were determined by a series of in vitro and in vivo studies. We report that a small class of hCSCs retain during replication the mother DNA and generate 2 daughter cells, which carry the old and new DNA, respectively. hCSCs with immortal DNA form a pool of nonsenescent cells with longer telomeres and higher proliferative capacity. The self-renewal and long-term repopulating ability of these cells was shown in serial-transplantation assays in the infarcted heart; these cells created a chimeric organ, composed of spared rat and regenerated human cardiomyocytes and coronary vessels, leading to a remarkable restoration of cardiac structure and function. The documentation that hCSCs divide by asymmetrical and symmetrical chromatid segregation supports the view that the human heart is a self-renewing organ regulated by a compartment of resident hCSCs. CONCLUSIONS: The impressive recovery in ventricular hemodynamics and anatomy mediated by clonal hCSCs carrying the "mother" DNA underscores the clinical relevance of this stem cell class for the management of heart failure in humans.

Cardiomyogenesis in the aging and failing human heart.

BACKGROUND: Two opposite views of cardiac growth are currently held; one views the heart as a static organ characterized by a large number of cardiomyocytes that are present at birth and live as long as the organism, and the other views the heart a highly plastic organ in which the myocyte compartment is restored several times during the course of life. METHODS AND RESULTS: The average age of cardiomyocytes, vascular endothelial cells (ECs), and fibroblasts and their turnover rates were measured by retrospective (14)C birth dating of cells in 19 normal hearts 2 to 78 years of age and in 17 explanted failing hearts 22 to 70 years of age. We report that the human heart is characterized by a significant turnover of ventricular myocytes, ECs, and fibroblasts, physiologically and pathologically. Myocyte, EC, and fibroblast renewal is very high shortly after birth, decreases during postnatal maturation, remains relatively constant in the adult organ, and increases dramatically with age. From 20 to 78 years of age, the adult human heart entirely replaces its myocyte, EC, and fibroblast compartment ≈8, ≈6, and ≈8 times, respectively. Myocyte, EC, and fibroblast regeneration is further enhanced with chronic heart failure. CONCLUSIONS: The human heart is a highly dynamic organ that retains a remarkable degree of plasticity throughout life and in the presence of chronic heart failure. However, the ability to regenerate cardiomyocytes, vascular ECs, and fibroblasts cannot prevent the manifestations of myocardial aging or oppose the negative effects of ischemic and idiopathic dilated cardiomyopathy.

Binding mechanism and biological effects of flavone DYRK1A inhibitors for the design of new antidiabetics.

The selective inhibition of kinases from the diabetic kinome is known to promote the regeneration of beta cells and provide an opportunity for the curative treatment of diabetes. The effect can be achieved by carefully tailoring the selectivity of inhibitor toward a particular kinase, especially DYRK1A, previously associated with Down syndrome and Alzheimer's disease. Recently DYRK1A inhibition has been shown to promote both insulin secretion and beta cells proliferation. Here, we show that commonly available flavones are effective inhibitors of DYRK1A. The observed biochemical activity of flavone compounds is confirmed by crystal structures solved at 2.06 Å and 2.32 Å resolution, deciphering the way inhibitors bind in the ATP-binding pocket of the kinase, which is driven by the arrangement of hydroxyl moieties. We also demonstrate antidiabetic properties of these biomolecules and prove that they could be further improved by therapy combined with TGF-ß inhibitors. Our data will allow future structure-based optimization of the presented scaffolds toward potent, bioavailable and selective anti-diabetic drugs.

Genome-wide patterns of genetic distances reveal candidate Loci contributing to human population-specific traits.

Modern humans originated in Africa before migrating across the world with founder effects and adaptations to new environments contributing to their present phenotypic diversity. Determining the genetic basis of differences between populations may provide clues about our evolutionary history and may have clinical implications. Herein, we develop a method to detect genes and biological processes in which populations most differ by calculating the genetic distance between modern populations and a hypothetical ancestral population. We apply our method to large-scale single nucleotide polymorphism (SNP) data from human populations of African, European and Asian origin. As expected, ancestral alleles were more conserved in the African populations and we found evidence of high divergence in genes previously suggested as targets of selection related to skin pigmentation, immune response, senses and dietary adaptations. Our genome-wide scan also reveals novel candidates for contributing to population-specific traits. These include genes related to neuronal development and behavior that may have been influenced by cultural processes. Moreover, in the African populations, we found a high divergence in genes related to UV protection and to the male reproductive system. Taken together, these results confirm and expand previous findings, providing new clues about the evolution and genetics of human phenotypic diversity.

Characterization of SARS-CoV-2 replication complex elongation and proofreading activity.

The replication complex (RC) of SARS-CoV-2 was recently shown to be one of the fastest RNA-dependent RNA polymerases of any known coronavirus. With this rapid elongation, the RC is more prone to incorporate mismatches during elongation, resulting in a highly variable genomic sequence. Such mutations render the design of viral protein targets difficult, as drugs optimized for a given viral protein sequence can quickly become inefficient as the genomic sequence evolves. Here, we use biochemical experiments to characterize features of RNA template recognition and elongation fidelity of the SARS-CoV-2 RdRp, and the role of the exonuclease, nsp14. Our study highlights the 2'OH group of the RNA ribose as a critical component for RdRp template recognition and elongation. We show that RdRp fidelity is reduced in the presence of the 3' deoxy-terminator nucleotide 3'dATP, which promotes the incorporation of mismatched nucleotides (leading to U:C, U:G, U:U, C:U, and A:C base pairs). We find that the nsp10-nsp14 heterodimer is unable to degrade RNA products lacking free 2'OH or 3'OH ribose groups. Our results suggest the potential use of 3' deoxy-terminator nucleotides in RNA-derived oligonucleotide inhibitors as antivirals against SARS-CoV-2.

Refolding of lid subdomain of SARS-CoV-2 nsp14 upon nsp10 interaction releases exonuclease activity.

During RNA replication, coronaviruses require proofreading to maintain the integrity of their large genomes. Nsp14 associates with viral polymerase complex to excise the mismatched nucleotides. Aside from the exonuclease activity, nsp14 methyltransferase domain mediates cap methylation, facilitating translation initiation and protecting viral RNA from recognition by the innate immune sensors. The nsp14 exonuclease activity is modulated by a protein co-factor nsp10. While the nsp10/nsp14 complex structure is available, the mechanistic basis for nsp10-mediated modulation remains unclear in the absence of the nsp14 structure. Here, we provide a crystal structure of nsp14 in an apo-form. Comparative analysis of the apo- and nsp10-bound structures explain the modulatory role of the co-factor protein and reveal the allosteric nsp14 control mechanism essential for drug discovery. Further, the flexibility of the N-terminal lid of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nsp14 structure presented in this study rationalizes the recently proposed idea of nsp14/nsp10/nsp16 ternary complex.

Acriflavine, a clinically approved drug, inhibits SARS-CoV-2 and other betacoronaviruses.

The COVID-19 pandemic caused by SARS-CoV-2 has been socially and economically devastating. Despite an unprecedented research effort and available vaccines, effective therapeutics are still missing to limit severe disease and mortality. Using high-throughput screening, we identify acriflavine (ACF) as a potent papain-like protease (PLpro) inhibitor. NMR titrations and a co-crystal structure confirm that acriflavine blocks the PLpro catalytic pocket in an unexpected binding mode. We show that the drug inhibits viral replication at nanomolar concentration in cellular models, in vivo in mice and ex vivo in human airway epithelia, with broad range activity against SARS-CoV-2 and other betacoronaviruses. Considering that acriflavine is an inexpensive drug approved in some countries, it may be immediately tested in clinical trials and play an important role during the current pandemic and future outbreaks.

DYRK1A Kinase Inhibitors Promote ß-Cell Survival and Insulin Homeostasis.

The rising prevalence of diabetes is threatening global health. It is known not only for the occurrence of severe complications but also for the SARS-Cov-2 pandemic, which shows that it exacerbates susceptibility to infections. Current therapies focus on artificially maintaining insulin homeostasis, and a durable cure has not yet been achieved. We demonstrate that our set of small molecule inhibitors of DYRK1A kinase potently promotes ß-cell proliferation, enhances long-term insulin secretion, and balances glucagon level in the organoid model of the human islets. Comparable activity is seen in INS-1E and MIN6 cells, in isolated mice islets, and human iPSC-derived ß-cells. Our compounds exert a significantly more pronounced effect compared to harmine, the best-documented molecule enhancing ß-cell proliferation. Using a body-like environment of the organoid, we provide a proof-of-concept that small-molecule-induced human ß-cell proliferation via DYRK1A inhibition is achievable, which lends a considerable promise for regenerative medicine in T1DM and T2DM treatment.

p53 Modulates the Fate of Cardiac Progenitor Cells Ex Vivo and in the Diabetic Heart In Vivo.

p53 is an important modulator of stem cell fate, but its role in cardiac progenitor cells (CPCs) is unknown. Here, we tested the effects of a single extra-copy of p53 on the function of CPCs in the presence of oxidative stress mediated by doxorubicin in vitro and type-1 diabetes in vivo. CPCs were obtained from super-p53 transgenic mice (p53-tg), in which the additional allele is regulated in a manner similar to the endogenous protein. Old CPCs with increased p53 dosage showed a superior ability to sustain oxidative stress, repair DNA damage and restore cell division. With doxorubicin, a larger fraction of CPCs carrying an extra-copy of the p53 allele recruited γH2A.X reestablishing DNA integrity. Enhanced p53 expression resulted in a superior tolerance to oxidative stress in vivo by providing CPCs with defense mechanisms necessary to survive in the milieu of the diabetic heart; they engrafted in regions of tissue injury and in three days acquired the cardiomyocyte phenotype. The biological advantage provided by the increased dosage of p53 in CPCs suggests that this genetic strategy may be translated to humans to increase cellular engraftment and growth, critical determinants of successful cell therapy for the failing heart.

Single-cell analysis of the fate of c-kit-positive bone marrow cells.

The plasticity of c-kit-positive bone marrow cells (c-kit-BMCs) in tissues different from their organ of origin remains unclear. We tested the hypothesis that c-kit-BMCs are functionally heterogeneous and only a subgroup of these cells possesses cardiomyogenic potential. Population-based assays fall short of identifying the properties of individual stem cells, imposing on us the introduction of single cell-based approaches to track the fate of c-kit-BMCs in the injured heart; they included viral gene-tagging, multicolor clonal-marking and transcriptional profiling. Based on these strategies, we report that single mouse c-kit-BMCs expand clonally within the infarcted myocardium and differentiate into specialized cardiac cells. Newly-formed cardiomyocytes, endothelial cells, fibroblasts and c-kit-BMCs showed in their genome common sites of viral integration, providing strong evidence in favor of the plasticity of a subset of BMCs expressing the c-kit receptor. Similarly, individual c-kit-BMCs, which were infected with multicolor reporters and injected in infarcted hearts, formed cardiomyocytes and vascular cells organized in clusters of similarly colored cells. The uniform distribution of fluorescent proteins in groups of specialized cells documented the polyclonal nature of myocardial regeneration. The transcriptional profile of myogenic c-kit-BMCs and whole c-kit-BMCs was defined by RNA sequencing. Genes relevant for engraftment, survival, migration, and differentiation were enriched in myogenic c-kit-BMCs, a cell subtype which could not be assigned to a specific hematopoietic lineage. Collectively, our findings demonstrate that the bone marrow comprises a category of cardiomyogenic, vasculogenic and/or fibrogenic c-kit-positive cells and a category of c-kit-positive cells that retains an undifferentiated state within the damaged heart.

Reduction in Kv Current Enhances the Temporal Dispersion of the Action Potential in Diabetic Myocytes: Insights From a Novel Repolarization Algorithm.

BACKGROUND: Diabetes is associated with prolongation of the QT interval of the electrocardiogram and enhanced dispersion of ventricular repolarization, factors that, together with atherosclerosis and myocardial ischemia, may promote the occurrence of electrical disorders. Thus, we tested the possibility that alterations in transmembrane ionic currents reduce the repolarization reserve of myocytes, leading to action potential (AP) prolongation and enhanced beat-to-beat variability of repolarization. METHODS AND RESULTS: Diabetes was induced in mice with streptozotocin (STZ), and effects of hyperglycemia on electrical properties of whole heart and myocytes were studied with respect to an untreated control group (Ctrl) using electrocardiographic recordings in vivo, ex vivo perfused hearts, and single-cell patch-clamp analysis. Additionally, a newly developed algorithm was introduced to obtain detailed information of the impact of high glucose on AP profile. Compared to Ctrl, hyperglycemia in STZ-treated animals was coupled with prolongation of the QT interval, enhanced temporal dispersion of electrical recovery, and susceptibility to ventricular arrhythmias, defects observed, in part, in the Akita mutant mouse model of type I diabetes. AP was prolonged and beat-to-beat variability of repolarization was enhanced in diabetic myocytes, with respect to Ctrl cells. Density of Kv K(+) and L-type Ca(2+) currents were decreased in STZ myocytes, in comparison to cells from normoglycemic mice. Pharmacological reduction of Kv currents in Ctrl cells lengthened AP duration and increased temporal dispersion of repolarization, reiterating features identified in diabetic myocytes. CONCLUSIONS: Reductions in the repolarizing K(+) currents may contribute to electrical disturbances of the diabetic heart.

Expression profile of microRNAs regulating proliferation and differentiation in mouse adult cardiac stem cells.

The identification of cardiac cells with stem cell properties changed the paradigm of the heart as a post mitotic organ. These cells proliferate and differentiate into cardiomyocytes, endothelial and vascular smooth muscle cells, providing for cardiac cell homeostasis and regeneration. microRNAs are master switches controlling proliferation and differentiation, in particular regulating stem cell biology and cardiac development. Modulation of microRNAs -regulated gene expression networks holds the potential to control cell fate and proliferation, with predictable biotechnologic and therapeutic applications. To obtain insights into the regulatory networks active in cardiac stem cells, we characterized the expression profile of 95 microRNAs with reported functions in stem cell and tissue differentiation in mouse cardiac stem cells, and compared it to that of mouse embryonic heart and mesenchymal stem cells. The most highly expressed microRNAs identified in cardiac stem cells are known to target key genes involved in the control of cell proliferation and adhesion, vascular function and cardiomyocyte differentiation. We report a subset of differentially expressed microRNAs that are proposed to act as regulators of differentiation and proliferation of adult cardiac stem cells, providing novel insights into active gene expression networks regulating their biological properties.