Growth differentiation factor 15 (GDF15; also known as MIC-1) is a divergent member of the TGF-β superfamily and is associated with body-weight regulation in humans and rodents. However, the cognate receptor of GDF15 is unknown. Here we show that GDF15 binds specifically to GDNF family receptor α-like (GFRAL) with high affinity, and that GFRAL requires association with the coreceptor RET to elicit intracellular signaling in response to GDF15 stimulation. We also found that GDF15-mediated reductions in food intake and body weight of mice with obesity were abolished in GFRAL-knockout mice. We further found that GFRAL expression was limited to hindbrain neurons and not present in peripheral tissues, which suggests that GDF15-GFRAL-mediated regulation of food intake is by a central mechanism. Lastly, given that GDF15 did not increase energy expenditure in treated mice with obesity, the anti-obesity actions of the cytokine are likely driven primarily by a reduction in food intake.

Substitution for aspartic acid by glycine at position 614 in the spike (S) protein of severe acute respiratory syndrome coronavirus 2 appears to facilitate rapid viral spread. The G614 strain and its recent variants are now the dominant circulating forms. We report here cryo-EM structures of a full-length G614 S trimer, which adopts three distinct prefusion conformations differing primarily by the position of one receptor-binding domain. A loop disordered in the D614 S trimer wedges between domains within a protomer in the G614 spike. This added interaction appears to prevent premature dissociation of the G614 trimer, effectively increasing the number of functional spikes and enhancing infectivity, and to modulate structural rearrangements for membrane fusion. These findings extend our understanding of viral entry and suggest an improved immunogen for vaccine development.

The COVID-19 pandemic caused by the SARS-CoV-2 virus continually poses serious threats to global public health. The main protease (Mpro) of SARS-CoV-2 plays a central role in viral replication. We designed and synthesized 32 new bicycloproline-containing Mpro inhibitors derived from either Boceprevir or Telaprevir, both of which are approved antivirals. All compounds inhibited SARS-CoV-2 Mpro activity in vitro with IC50 values ranging from 7.6 to 748.5 nM. The co-crystal structure of Mpro in complex with MI-23, one of the most potent compounds, revealed its interaction mode. Two compounds (MI-09 and MI-30) showed excellent antiviral activity in cell-based assays. In a SARS-CoV-2 infection transgenic mouse model, oral or intraperitoneal treatment with MI-09 or MI-30 significantly reduced lung viral loads and lung lesions. Both also displayed good pharmacokinetic properties and safety in rats.

A tight junction (TJ) protein, claudin-1 (CLDN1), was identified recently as a key factor for hepatitis C virus (HCV) entry. Here, we show that another TJ protein, occludin, is also required for HCV entry. Mutational study of CLDN1 revealed that its tight junctional distribution plays an important role in mediating viral entry. Together, these data support the model in which HCV enters liver cells from the TJ. Interestingly, HCV infection of Huh-7 hepatoma cells downregulated the expression of CLDN1 and occludin, preventing superinfection. The altered TJ protein expression may contribute to the morphological and functional changes observed in HCV-infected hepatocytes.Recently, considerable progress has been made in elucidating the molecular mechanisms by which hepatitis C virus (HCV) infects human liver cells. The current accepted model of HCV infection is that virus particles associated with lipoproteins, found circulating in the bloodstream, use glycosaminoglycans and/or the LDL receptor on host cells as initial attachment factors. After binding, the HCV particle interacts with SR-BI and CD81 and is subsequently relocalized to the tight junction (TJ) protein claudin-1 (CLDN1) (6). Next, the HCV particle becomes internalized via clathrin-mediated endocytosis, followed by viral fusion, which likely occurs in early endosomes. Some critical information, however, is missing in such a model with regard to the role of CLDN1. (i) The interaction between CLDN1 and incoming HCV virions has yet to be verified experimentally; (ii) the precise site of viral entry needs to be determined given that CLDN1 predominantly localizes to TJs in polarized cells; and (iii) the potential involvement of other TJ proteins in HCV entry remains untested. We have shown previously that the TJ-like CLDN1 distribution correlates with cellular permissiveness to HCV infection (19). In the current study, we intend to define the importance of junctional CLDN1 and other TJ proteins in HCV entry.TJ protein OCLN is required for HCV entry. As hepatocytes are highly polarized in vivo, we first sought to investigate whether HCV entry mimics the major group B coxsackievirus (CVB) entry, in which CVB enters polarized epithelial cells through TJs by a complex mechanism requiring attachment to occludin (OCLN) and the induction of caveolar endocytosis (3). To this end, we utilized synthetic interference RNA (siRNA) or packaged retroviruses to deliver short-hairpinbased RNA (shRNA) to knock down the expression of TJ proteins CLDN1, OCLN, ZO-1, JAM-1, and CAR (CVB receptor) to examine the roles of each of the proteins during HCV infection. Targeted sequences of the siRNAs and shRNAs are presented in the supplemental material. As shown in Fig. 1A and B, depletion of OCLN affected neither the expression level nor the localization of CLDN1; however, depletion of ZO-1 by siRNA modestly reduced the CLDN1 level (Fig. 1A). We then performed the infection assay according to a previously established procedure (19). Reduction of CLDN1, OCLN, and ZO-1 expression inhibi...

Several fast-spreading variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have become the dominant circulating strains in the COVID-19 pandemic. We report here cryo-EM structures of the full-length spike (S) trimers of the B.1.1.7 and B.1.351 variants, as well as their biochemical and antigenic properties. Amino acid substitutions in the B.1.1.7 protein increase the accessibility of its receptor binding domain and also the binding affinity for receptor angiotensin-converting enzyme 2 (ACE2). The enhanced receptor engagement may account for the increased transmissibility. The B.1.351 variant has evolved to reshape antigenic surfaces of the major neutralizing sites on the S protein, making it resistant to some potent neutralizing antibodies. These findings provide structural details on how SARS-CoV-2 has evolved to enhance viral fitness and immune evasion.