A viral block on host protein synthesis: As the coronavirus disease 2019 (COVID-19) pandemic continues to cause devastation, scientists race to increase their understanding of the disease-causing severe acute respiratory syndrome coronavirus 2. Once inside host cells, not only does the virus hijack the cells' translational machinery to make viral proteins, but the virulence factor nonstructural protein 1 (Nsp1) also shuts down translation of host messenger RNA. Thoms et al. determined a 2.6-angstrom resolution cryo–electron microscopy structure of a reconstituted complex of Nsp1 bound to the human 40 S ribosomal subunit and showed that Nsp1 blocks the messenger RNA entry tunnel. A structural inventory of native Nsp1-ribosome complexes from human cells confirms this mechanism. Cellular studies show that the translational shutdown almost completely inhibits the innate immune response. The binding pocket on the ribosome may be a target for drugs to treat COVID-19. Science , this issue p. 1249

Background: Many genetically multi‐modified donor lines for xenotransplantation have a background of domestic pigs with rapid body and organ growth. The intrinsic growth potential of porcine xeno‐organs may impair their long‐term function after orthotopic transplantation in non‐human primate models. Since growth hormone is a major stimulator of postnatal growth, we deleted its receptor (GHR‐KO) to reduce the size of donor pigs in one step.Methods: Heart weight and proteome profile of myocardium were investigated in GHR‐KO and control pigs. GHR‐KO mutations were introduced using CRISPR/Cas9 in an α1,3‐galactosyltransferase (GGTA1)‐deficient background expressing the human cluster of differentiation (hCD46) and human thrombomodulin (hTHBD) to generate quadruple‐modified (4GM) pigs.Results: At age 6 months, GHR‐KO pigs had a 61% reduced body weight and a 63% reduced heart weight compared with controls. The mean minimal diameter of cardiomyocytes was 28% reduced. A holistic proteome study of myocardium samples from the two groups did not reveal prominent differences. Two 4GM founder sows had low serum insulin‐like growth factor 1 (IGF1) levels (24 ± 1 ng/mL) and reached body weights of 70.3 and 73.4 kg at 9 months. Control pigs with IGF1 levels of 228 ± 24 ng/mL reached this weight range three months earlier. The 4GM sows showed normal sexual development and were mated with genetically multi‐modified boars. Offspring revealed the expected Mendelian transmission of the genetic modifications and consistent expression of the transgenes.Conclusion: GHR‐KO donor pigs can be used at an age beyond the steepest phase of their growth curve, potentially reducing the problem of xeno‐organ overgrowth in preclinical studies.

Bacterial cell wall components provide various unique molecular structures that are detected by pattern recognition receptors (PRRs) of the innate immune system as non-self. Most bacterial species form a cell wall that consists of peptidoglycan (PGN), a polymeric structure comprising alternating amino sugars that form strands cross-linked by short peptides. Muramyl dipeptide (MDP) has been well documented as a minimal immunogenic component of peptidoglycan^1–3. MDP is sensed by the cytosolic nucleotide-binding oligomerization domain-containing protein 2⁴ (NOD2). Upon engagement, it triggers pro-inflammatory gene expression, and this functionality is of critical importance in maintaining a healthy intestinal barrier function⁵. Here, using a forward genetic screen to identify factors required for MDP detection, we identified N-acetylglucosamine kinase (NAGK) as being essential for the immunostimulatory activity of MDP. NAGK is broadly expressed in immune cells and has previously been described to contribute to the hexosamine biosynthetic salvage pathway⁶. Mechanistically, NAGK functions upstream of NOD2 by directly phosphorylating the N-acetylmuramic acid moiety of MDP at the hydroxyl group of its C6 position, yielding 6-O-phospho-MDP. NAGK-phosphorylated MDP—but not unmodified MDP—constitutes an agonist for NOD2. Macrophages from mice deficient in NAGK are completely deficient in MDP sensing. These results reveal a link between amino sugar metabolism and innate immunity to bacterial cell walls.

Toll-like receptor 7 (TLR7) is essential for recognition of RNA viruses and initiation of antiviral immunity. TLR7 contains two ligand-binding pockets that recognize different RNA degradation products: pocket 1 recognizes guanosine, while pocket 2 coordinates pyrimidine-rich RNA fragments. We found that the endonuclease RNase T2, along with 5' exonucleases PLD3 and PLD4, collaboratively generate the ligands for TLR7. Specifically, RNase T2 generated guanosine 2',3'-cyclic monophosphate-terminated RNA fragments. PLD exonuclease activity further released the terminal 2',3'-cyclic guanosine monophosphate (2',3'-cGMP) to engage pocket 1 and was also needed to generate RNA fragments for pocket 2. Loss-of-function studies in cell lines and primary cells confirmed the critical requirement for PLD activity. Biochemical and structural studies showed that PLD enzymes form homodimers with two ligand-binding sites important for activity. Previously identified disease-associated PLD mutants failed to form stable dimers. Together, our data provide a mechanistic basis for the detection of RNA fragments by TLR7.

Melanoma arising from pigment-producing melanocytes is the deadliest form of skin cancer. Extensive ultraviolet light exposure is a major cause of melanoma and individuals with low levels of melanin are at particular risk. Humans carrying gain-of-function polymorphisms in the melanosomal/endolysosomal two-pore cation channel TPC2 present with hypopigmentation, blond hair, and albinism. Loss of TPC2 is associated with decreased cancer/melanoma proliferation, migration, invasion, tumor growth and metastasis formation, and TPC2 depleted melanoma cells show increased levels of melanin. How TPC2 activity is controlled in melanoma and the downstream molecular effects of TPC2 activation on melanoma development remain largely elusive. Here we show that the small GTPase Rab7a strongly enhances the activity of TPC2 and that effects of TPC2 on melanoma hallmarks, in vitro and in vivo strongly depend on the presence of Rab7a, which controls TPC2 activity to modulate GSK3β, β-Catenin, and MITF, a major regulator of melanoma development and progression.