Retroviruses have been invading mammalian germlines for millions of years, accumulating in the form of endogenous retroviruses (ERVs) that account for nearly one-tenth of the mouse and human genomes. ERVs are epigenetically silenced during development, yet the cellular factors recognizing ERVs in a sequence-specific manner remain elusive. Here we demonstrate that ZFP809, a member of the Kr€ uppel-associated box zinc finger protein (KRAB-ZFP) family, initiates the silencing of ERVs in a sequence-specific manner via recruitment of heterochromatin-inducing complexes. ZFP809 knockout mice display highly elevated levels of ZFP809-targeted ERVs in somatic tissues. ERV reactivation is accompanied by an epigenetic shift from repressive to active histone modifications but only slight destabilization of DNA methylation. Importantly, using conditional alleles and rescue experiments, we demonstrate that ZFP809 is required to initiate ERV silencing during embryonic development but becomes largely dispensable in somatic tissues. Finally, we show that the DNA-binding specificity of ZFP809 is evolutionarily conserved in the Muroidea superfamily of rodents and predates the endogenization of retroviruses presently targeted by ZFP809 in Mus musculus. In sum, these data provide compelling evidence that ZFP809 evolved to recognize foreign DNA and establish histone modification-based epigenetic silencing of ERVs.

The transcriptional enhancers of retroviruses that lack an oncogene are important determinants of their oncogenicity. However, no specific cellular transcriptional activator has yet been found to determine the oncogenicity for any of these viruses. The SL3-3 enhancer factor 1 (SEF1) cellular transcriptional activators are expressed preferentially in T lymplhocytes. In the SL3-3 murine leukemia virus enhancer, two different sequences can bind SEF1 activators. We show that mutation of the SEF1 binding sites disrupts the disease potential of SL3-3 murine leukemia virus, implying that SEF1 transcriptional activators are required for tumor induction by SL3-3. The SEF1 site mutations did not appear to affect the pathogenicity of SL3-3 by impairment of virus multiplication, but rather by a specific defect in the ability of neoplastic transformation.

Abstract-Although the biophysical fingerprints (ion selectivity, voltage-dependence, kinetics, etc) of Ca 2ϩ -activated Cl Ϫ currents are well established, their molecular identity is still controversial. Several molecular candidates have been suggested; however, none of them has been fully accepted. We have recently characterized a cGMP-dependent Ca 2ϩ -activated Cl Ϫ current with unique characteristics in smooth muscle cells. This novel current has been shown to coexist with a "classic" (cGMP-independent) Ca 2ϩ -activated Cl Ϫ current and to have characteristics distinct from those previously known for Ca 2ϩ -activated Cl Ϫ currents. Here, we suggest that a bestrophin, a product of the Best gene family, is responsible for the cGMP-dependent Ca 2ϩ -activated Cl Ϫ current based on similarities between the membrane currents produced by heterologous expressions of bestrophins and the cGMP-dependent Ca 2ϩ -activated Cl Ϫ current. This is supported by similarities in the distribution pattern of the cGMP-dependent Ca 2ϩ -activated Cl Ϫ current and bestrophin-3 (the product of Best-3 gene) expression in different smooth muscle. Furthermore, downregulation of Best-3 gene expression with small interfering RNA both in cultured cells and in vascular smooth muscle cells in vivo was associated with a significant reduction of the cGMP-dependent Ca 2ϩ -activated Cl Ϫ current, whereas the magnitude of the classic Ca 2ϩ -activated Cl Ϫ current was not affected. Ϫ channel, which results in depolarization in vascular smooth muscle. Furthermore, the current is of similar magnitude as "classic" Ca 2ϩ -activated Cl Ϫ currents in most vascular beds and even larger in some vascular smooth muscles. 3 It is, therefore, highly desirable to know the molecular structure of the channel responsible for this current because it is likely to play an important role in smooth muscle function.Although their biophysical fingerprints (ion selectivity, voltage-dependence, kinetics, etc) are well established, 4 -6 the molecular identity of Ca 2ϩ -sensitive Cl Ϫ channels is still controversial. 7 Recently, the gene responsible for vitelliform macular dystrophy 8 and its homologs that code for bestrophin proteins have been suggested as candidates. 9,10 Four bestrophin family members in the mammalian genome and many homologues in genomes of invertebrates and even prokaryotes have been identified. 11-13 Two different nomenclatures for mammalian bestrophins were previously devel- The majority of suggestions that bestrophins function as Cl Ϫ channels are based on the findings that expression of the gene in different cell types leads to the appearance of a Cl Ϫ conductance 9,10 and that mutations or chemical modifications of the predicted channel pore change this conductance. [15][16][17][18] Although downregulation by small interfering (si)RNA in recent studies demonstrated a direct association between the endogenous Cl Ϫ current in epithelial cell culture and Best-1 expression, 19 -21 the exact role of the bestrophins in native tissues remains questionable...

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The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has created an urgent need for new technologies to treat COVID-19. Here we report a 2′-fluoro protected RNA aptamer that binds with high affinity to the receptor binding domain (RBD) of SARS-CoV-2 spike protein, thereby preventing its interaction with the host receptor ACE2. A trimerized version of the RNA aptamer matching the three RBDs in each spike complex enhances binding affinity down to the low picomolar range. Binding mode and specificity for the aptamer–spike interaction is supported by biolayer interferometry, single-molecule fluorescence microscopy, and flow-induced dispersion analysis in vitro. Cell culture experiments using virus-like particles and live SARS-CoV-2 show that the aptamer and, to a larger extent, the trimeric aptamer can efficiently block viral infection at low concentration. Finally, the aptamer maintains its high binding affinity to spike from other circulating SARS-CoV-2 strains, suggesting that it could find widespread use for the detection and treatment of SARS-CoV-2 and emerging variants.