The Myxobacterial Antibiotic Myxovalargin: Biosynthesis, Structural Revision, Total Synthesis, and Molecular Characterization of Ribosomal Inhibition.

Resistance of bacterial pathogens against antibiotics is declared by WHO as a major global health threat. As novel antibacterial agents are urgently needed, we re-assessed the broad-spectrum myxobacterial antibiotic myxovalargin and found it to be extremely potent against Mycobacterium tuberculosis. To ensure compound supply for further development, we studied myxovalargin biosynthesis in detail enabling production via fermentation of a native producer. Feeding experiments as well as functional genomics analysis suggested a structural revision, which was eventually corroborated by the development of a concise total synthesis. The ribosome was identified as the molecular target based on resistant mutant sequencing, and a cryo-EM structure revealed that myxovalargin binds within and completely occludes the exit tunnel, consistent with a mode of action to arrest translation during a late stage of translation initiation. These studies open avenues for structure-based scaffold improvement toward development as an antibacterial agent.

Conformational tuning improves the stability of spirocyclic nitroxides with long paramagnetic relaxation times.

Nitroxides are widely used as probes and polarization transfer agents in spectroscopy and imaging. These applications require high stability towards reducing biological environments, as well as beneficial relaxation properties. While the latter is provided by spirocyclic groups on the nitroxide scaffold, such systems are not in themselves robust under reducing conditions. In this work, we introduce a strategy for stability enhancement through conformational tuning, where incorporating additional substituents on the nitroxide ring effects a shift towards highly stable closed spirocyclic conformations, as indicated by X-ray crystallography and density functional theory (DFT) calculations. Closed spirocyclohexyl nitroxides exhibit dramatically improved stability towards reduction by ascorbate, while maintaining long relaxation times in electron paramagnetic resonance (EPR) spectroscopy. These findings have important implications for the future design of new nitroxide-based spin labels and imaging agents.

Vps37a regulates hepatic glucose production by controlling glucagon receptor localization to endosomes.

During mammalian energy homeostasis, the glucagon receptor (Gcgr) plays a key role in regulating both glucose and lipid metabolisms. However, the mechanisms by which these distinct signaling arms are differentially regulated remain poorly understood. Using a Cy5-glucagon agonist, we show that the endosomal protein Vps37a uncouples glucose production from lipid usage downstream of Gcgr signaling by altering intracellular receptor localization. Hepatocyte-specific knockdown of Vps37a causes an accumulation of Gcgr in endosomes, resulting in overactivation of the cAMP/PKA/p-Creb signaling pathway to gluconeogenesis without affecting ß-oxidation. Shifting the receptor back to the plasma membrane rescues the differential signaling and highlights the importance of the spatiotemporal localization of Gcgr for its metabolic effects. Importantly, since Vps37a knockdown in animals fed with a high-fat diet leads to hyperglycemia, although its overexpression reduces blood glucose levels, these data reveal a contribution of endosomal signaling to metabolic diseases that could be exploited for treatments of type 2 diabetes.