COVID-19 is a disease with unique characteristics that include lung thrombosis 1 , frequent diarrhoea 2 , abnormal activation of the inflammatory response 3 and rapid deterioration of lung function consistent with alveolar oedema 4 . The pathological substrate for these findings remains unknown. Here we show that the lungs of patients with COVID-19 contain infected pneumocytes with abnormal morphology and frequent multinucleation. The generation of these syncytia results from activation of the SARS-CoV-2 spike protein at the cell plasma membrane level. On the basis of these observations, we performed two high-content microscopy-based screenings with more than 3,000 approved drugs to search for inhibitors of spike-driven syncytia. We converged on the identification of 83 drugs that inhibited spike-mediated cell fusion, several of which belonged to defined pharmacological classes. We focused our attention on effective drugs that also protected against virus replication and associated cytopathicity. One of the most effective molecules was the antihelminthic drug niclosamide, which markedly blunted calcium oscillations and membrane conductance in spike-expressing cells by suppressing the activity of TMEM16F (also known as anoctamin 6), a calcium-activated ion channel and scramblase that is responsible for exposure of phosphatidylserine on the cell surface. These findings suggest a potential mechanism for COVID-19 disease pathogenesis and support the repurposing of niclosamide for therapy.One of the defining features of coronavirus biology is the coordinated process by which the virus binds and enters the host cell, which involves both docking to receptors at the cell surface (ACE2 for SARS-CoV2 5 ), and proteolytic activation of the spike protein by host encoded proteases at two distinct sites 6 . One activation step is spike cleavage at the S1-S2 boundary, which can occur either before or after receptor binding. A second proteolytic activation exposes the S2 portion, and primes S2 for fusion of virus and cellular membranes. The protease priming event at this S2′ site and subsequent fusion can occur after endocytosis, in which cleavage is carried out by endosomal low pH-activated proteases such as cathepsin B and cathepsin L 7 , or at the plasma membrane, where cleavage can be mediated by TMPRSS2 [8][9][10] . The spike proteins of MERS-CoV and SARS-CoV-2 possess a multibasic amino acid sequence at the S1-S2 interface, which is not present in SARS-CoV 11 , that also allows cleavage by the ubiquitously expressed serine protease furin [12][13][14] . As a consequence, cells that express MERS-CoV and SARS-CoV-2 spike protein at the plasma membrane can fuse with other cells that express the respective receptors and form syncytia.

Prompt coronary catheterization and revascularization have dramatically improved the outcome of myocardial infarction, but also have resulted in a growing number of survived patients with permanent structural damage of the heart, which frequently leads to heart failure. Finding new treatments for this condition is a largely unmet clinical need 1, especially because of the incapacity of cardiomyocytes to replicate after birth and thus achieve regeneration of the lost contractile tissue 2. Here we show that expression of human microRNA-199a in infarcted pig hearts is capable of stimulating cardiac repair. One month after myocardial infarction and delivery of this microRNA through an adeno-associated viral vector, the treated animals showed marked improvements in both global and regional contractility, increased muscle mass and reduced scar size. These functional and morphological findings correlated with cardiomyocyte de-differentiation and proliferation. At longer follow-up, however, persistent and uncontrolled expression of the microRNA resulted in sudden arrhythmic death of most of the treated pigs. Such events were concurrent with myocardial infiltration of proliferating cells displaying a poorly differentiated myoblastic phenotype. These results show that achieving cardiac repair through the stimulation of endogenous cardiomyocyte proliferation is attainable in large mammals, however this therapy needs to be tightly dosed.