Elena Farah Lehmann

Volume-regulated anion channels, which are activated in response to hypotonicity, are heteromers composed of LRRC8 family members. They consist of the obligatory subunit LRRC8A and at least another paralog that determines their substrate preference. Here we investigate heteromeric assemblies composed of LRRC8A and D subunits, the latter of which confer permeability to amino acids, osmolytes and anti-cancer drugs. Using patch-clamp electrophysiology, we show that LRRC8A/D channels are reversibly activated by cell swelling and that their response is modulated by sybodies targeting the A subunits, which either potentiate or repress channel activity. Structures of the LRRC8A/D channel in complex with the inhibitory sybody Sb1 define the channel stoichiometry of four A and two D subunits, with Sb1 stabilizing the A subunits in a similar closed channel conformation as found in homomeric LRRC8A assemblies. In LRRC8A/D heteromers, the two D subunits weaken the arrangement of A subunits and thus enhance the activation properties of the channel. This channel conformation is strongly perturbed upon binding of the potentiating sybody Sb4, which disrupts the packing of the cytoplasmic domains linking their mobility to pore opening.

Volume-regulated anion channels (VRACs) are multimeric proteins composed of different paralogs of the LRRC8 family. They are activated in response to hypotonic swelling, but little is known about their specific functions. We studied two human individuals with the same congenital syndrome affecting blood vessels, brain, eyes, and bones. The LRRC8C gene harbored de novo variants in both patients, located in a region of the gene encoding the boundary between the pore and a cytoplasmic domain, which is depleted of sequence variations in control subjects. When studied by cryo-EM, both LRRC8C mutant proteins assembled as their wild-type counterparts, but showed increased flexibility, suggesting a destabilization of subunit interactions. When co-expressed with the obligatory LRRC8A subunit, the mutants exhibited enhanced activation, resulting in channel activity even at isotonic conditions in which wild-type channels are closed. We conclude that structural perturbations of LRRC8C impair channel gating and constitute the mechanistic basis of the dominant gain-of-function effect of these pathogenic variants. The pleiotropic phenotype of this novel clinical entity associated with monoallelic LRRC8C variants indicates the fundamental roles of VRACs in different tissues and organs.

A central regulatory mechanism of iron homeostasis in humans involves ferroportin (FPN), the sole cellular iron exporter, and the peptide hormone hepcidin, which inhibits Fe²⁺ transport and induces internalization and degradation of FPN. Dysregulation of the FPN/hepcidin axis leads to diverse pathological conditions, and consequently, pharmacological compounds that inhibit FPN-mediated iron transport are of high clinical interest. Here, we describe the cryo-electron microscopy structures of human FPN in complex with synthetic nanobodies and vamifeport (VIT-2763), the first clinical-stage oral FPN inhibitor. Vamifeport competes with hepcidin for FPN binding and is currently in clinical development for β-thalassemia and sickle cell disease. The structures display two distinct conformations of FPN, representing outward-facing and occluded states of the transporter. The vamifeport site is located in the center of the protein, where the overlap with hepcidin interactions underlies the competitive relationship between the two molecules. The introduction of point mutations in the binding pocket of vamifeport reduces its affinity to FPN, emphasizing the relevance of the structural data. Together, our study reveals conformational rearrangements of FPN that are of potential relevance for transport, and it provides initial insight into the pharmacological targeting of this unique iron efflux transporter.