Cryo-EM helps engineer enhanced IL-10: Interleukin-10 (IL-10) binds to the IL-10 receptor (IL-10R), comprising two high-affinity α chains and two low-affinity β chains. Depending on the cellular context of its recognition, IL-10 can either suppress or activate immune responses. This pleiotropic behavior has complicated efforts to use IL-10 as an anti-inflammatory agent. Saxton et al. generated a high-affinity version of IL-10 (super-10), which allowed them to visualize IL-10 in complex with both IL-10Rα and IL-10Rβ by cryo–electron microscopy (cryo-EM). This enabled them to engineer additional variants of IL-10 with variable IL-10Rβ–binding affinities. Some of the partial agonists exhibited biased actions on myeloid cells, which exhibited anti-inflammatory properties without concomitant CD8 T cell activation. These findings may serve as a blueprint for future enhanced cytokine–based therapeutics. Science , this issue p. eabc8433

Frizzleds (Fzd) are the primary receptors for Wnt morphogens, which are essential regulators of stem cell biology, yet the structural basis of Wnt signaling through Fzd remains poorly understood. Here we report the structure of an unliganded human Fzd5 determined by single-particle cryo-EM at 3.7 Å resolution, with the aid of an antibody chaperone acting as a fiducial marker. We also analyzed the topology of low-resolution XWnt8/Fzd5 complex particles, which revealed extreme flexibility between the Wnt/Fzd-CRD and the Fzd-TM regions. Analysis of Wnt/β-catenin signaling in response to Wnt3a versus a ‘surrogate agonist’ that cross-links Fzd to LRP6, revealed identical structure-activity relationships. Thus, canonical Wnt/β-catenin signaling appears to be principally reliant on ligand-induced Fzd/LRP6 heterodimerization, versus the allosteric mechanisms seen in structurally analogous class A G protein-coupled receptors, and Smoothened. These findings deepen our mechanistic understanding of Wnt signal transduction, and have implications for harnessing Wnt agonism in regenerative medicine.

Metabotropic γ-aminobutyric acid receptors (GABA_B) are involved in the modulation of synaptic responses in the central nervous system and have been implicated in neuropsychological conditions that range from addiction to psychosis¹. GABA_B belongs to class C of the G-protein-coupled receptors, and its functional entity comprises an obligate heterodimer that is composed of the GB1 and GB2 subunits². Each subunit possesses an extracellular Venus flytrap domain, which is connected to a canonical seven-transmembrane domain. Here we present four cryo-electron microscopy structures of the human full-length GB1–GB2 heterodimer: one structure of its inactive apo state, two intermediate agonist-bound forms and an active form in which the heterodimer is bound to an agonist and a positive allosteric modulator. The structures reveal substantial differences, which shed light on the complex motions that underlie the unique activation mechanism of GABA_B. Our results show that agonist binding leads to the closure of the Venus flytrap domain of GB1, triggering a series of transitions, first rearranging and bringing the two transmembrane domains into close contact along transmembrane helix 6 and ultimately inducing conformational rearrangements in the GB2 transmembrane domain via a lever-like mechanism to initiate downstream signalling. This active state is stabilized by a positive allosteric modulator binding at the transmembrane dimerization interface.

This review outlines the current understanding of the structure and activation mechanism of the metabotropic GABA _B receptors.