Christine S. Huang

Some antibodies exhibit elevated viscosity at high concentrations, making them poorly suited for therapeutic applications requiring administration by injection such as subcutaneous or ocular delivery. Here we studied an anti-IL-13/IL-17 bispecific IgG₄ antibody, which has anomalously high viscosity compared to its parent monospecific antibodies. The viscosity of the bispecific IgG₄ in solution was decreased by only ~30% in the presence of NaCl, suggesting electrostatic interactions are insufficient to fully explain the drivers of viscosity. Intriguingly, addition of arginine-HCl reduced the viscosity of the bispecific IgG₄ by ~50% to its parent IgG level. These data suggest that beyond electrostatics, additional types of interactions such as cation-π and/or π-π may contribute to high viscosity more significantly than previously understood. Molecular dynamics simulations of antibody fragments in the mixed solution of free arginine and explicit water were conducted to identify hotspots involved in self-interactions. Exposed surface aromatic amino acids displayed an increased number of contacts with arginine. Mutagenesis of the majority of aromatic residues pinpointed by molecular dynamics simulations effectively decreased the solution’s viscosity when tested experimentally. This mutational method to reduce the viscosity of a bispecific antibody was extended to a monospecific anti-GCGR IgG₁ antibody with elevated viscosity. In all cases, point mutants were readily identified that both reduced viscosity and retained antigen-binding affinity. These studies demonstrate a new approach to mitigate high viscosity of some antibodies by mutagenesis of surface-exposed aromatic residues on complementarity-determining regions that may facilitate some clinical applications.

The integrity of the mammalian epidermis depends on a balance of proliferation and differentiation in the resident population of stem cells¹. The kinase RIPK4 and the transcription factor IRF6 are mutated in severe developmental syndromes in humans, and mice lacking these genes display epidermal hyperproliferation and soft-tissue fusions that result in neonatal lethality^2,3,4,5. Our understanding of how these genes control epidermal differentiation is incomplete. Here we show that the role of RIPK4 in mouse development requires its kinase activity; that RIPK4 and IRF6 expressed in the epidermis regulate the same biological processes; and that the phosphorylation of IRF6 at Ser413 and Ser424 primes IRF6 for activation. Using RNA sequencing (RNA-seq), histone chromatin immunoprecipitation followed by sequencing (ChIP–seq) and assay for transposase-accessible chromatin using sequencing (ATAC-seq) of skin in wild-type and IRF6-deficient mouse embryos, we define the transcriptional programs that are regulated by IRF6 during epidermal differentiation. IRF6 was enriched at bivalent promoters, and IRF6 deficiency caused defective expression of genes that are involved in the metabolism of lipids and the formation of tight junctions. Accordingly, the lipid composition of the stratum corneum of Irf6^−/− skin was abnormal, culminating in a severe defect in the function of the epidermal barrier. Collectively, our results explain how RIPK4 and IRF6 function to ensure the integrity of the epidermis and provide mechanistic insights into why developmental syndromes that are characterized by orofacial, skin and genital abnormalities result when this axis goes awry.

Geranyl-CoA carboxylase (GCC) is essential for the growth of Pseudomonas organisms with geranic acid as the sole carbon source. GCC has the same domain organization and shares strong sequence conservation with the related biotin-dependent carboxylases 3-methylcrotonyl-CoA carboxylase (MCC) and propionyl-CoA carboxylase (PCC). Here we report the crystal structure of the 750-kDa α₆β₆ holoenzyme of GCC, which is similar to MCC but strikingly different from PCC. The structures provide evidence in support of two distinct lineages of biotin-dependent acyl-CoA carboxylases, one carboxylating the α carbon of a saturated organic acid and the other carboxylating the γ carbon of an α-β unsaturated acid. Structural differences in the active site region of GCC and MCC explain their distinct substrate preferences. Especially, a glycine residue in GCC is replaced by phenylalanine in MCC, which blocks access by the larger geranyl-CoA substrate. Mutation of this residue in the two enzymes can change their substrate preferences.

Cholesteryl ester transfer protein (CETP) inhibitors raise HDL-C in animals and humans and may be antiatherosclerotic by enhancing reverse cholesterol transport (RCT). In this article, we describe the lead optimization efforts resulting in the discovery of a series of triphenylethanamine (TPE) ureas and amides as potent and orally available CETP inhibitors. Compound 10g is a potent CETP inhibitor that maximally inhibited cholesteryl ester (CE) transfer activity at an oral dose of 1 mg/kg in human CETP/apoB-100 dual transgenic mice and increased HDL cholesterol content and size comparable to torcetrapib (1) in moderately-fat fed hamsters. In contrast to the off-target liabilities with 1, no blood pressure increase was observed with 10g in rat telemetry studies and no increase of aldosterone synthase (CYP11B2) was detected in H295R cells. On the basis of its preclinical profile, compound 10g was advanced into preclinical safety studies.

The crystal structure of Pseudomonas aeruginosa 3-methylcrotonyl-CoA carboxylase is determined and found to be markedly different from that of propionyl-CoA carboxylase. The biotin-dependent enzyme 3-methylcrotonyl-CoA carboxylase (MCC), essential for the metabolism of leucine, is highly homologous to propionyl-CoA carboxylase (PCC). The structures of MCC and PCC were therefore expected to be very similar. But Liang Tong et al. find that the architecture of MCC differs considerably from that of PCC. The structure of MCC also reveals why known disease-causing mutations lead to enzyme dysfunction, as they are at amino-acid positions critical to substrate binding and/or catalysis. The extensive structural differences between MCC and PCC also have general implications for the relationship between sequence conservation and structural similarity. 3-Methylcrotonyl-CoA carboxylase (MCC), a member of the biotin-dependent carboxylase superfamily, is essential for the metabolism of leucine, and deficient mutations in this enzyme are linked to methylcrotonylglycinuria (MCG) and other serious diseases in humans1,2,3,4,5,6,7,8. MCC has strong sequence conservation with propionyl-CoA carboxylase (PCC), and their holoenzymes are both 750-kilodalton (kDa) α6β6 dodecamers. Therefore the architecture of the MCC holoenzyme is expected to be highly similar to that of PCC9. Here we report the crystal structures of the Pseudomonas aeruginosa MCC (PaMCC) holoenzyme, alone and in complex with coenzyme A. Surprisingly, the structures show that the architecture and overall shape of PaMCC are markedly different when compared to PCC. The α-subunits show trimeric association in the PaMCC holoenzyme, whereas they have no contacts with each other in PCC. Moreover, the positions of the two domains in the β-subunit of PaMCC are swapped relative to those in PCC. This structural information establishes a foundation for understanding the disease-causing mutations of MCC and provides new insights into the catalytic mechanism and evolution of biotin-dependent carboxylases. The large structural differences between MCC and PCC also have general implications for the relationship between sequence conservation and structural similarity.

The mitochondrial biotin-dependent enzyme propionyl-coenzyme A carboxylase (PCC) is essential for the catabolism of several amino acids, cholesterol and some fatty acids. The X-ray crystal and cryo-electron microscopy structures of the α6β6 dodecamer of PCC have now been determined. They reveal that the α-subunit contains the biotin carboxylase (BC) and biotin carboxyl carrier protein (BCCP) domains; the β-subunit is responsible for the carboxyltransferase activity. The BC domain and the carboxyltransferase domain are 55 ångströms apart, indicating that the BCCP domain must move a large distance during the catalytic cycle. Propionyl-coenzyme A carboxylase (PCC) is a biotin-dependent enzyme that is essential for the catabolism of several amino acids, cholesterol and some fatty acids. Here, the crystal structure of a bacterial PCC is presented, along with a cryo-electron microscopy reconstruction showing a similar structure for human PCC. The structural information establishes a molecular basis for understanding the known disease-causing mutations in PCC, and is relevant to the holoenzymes of other biotin-dependent carboxylases. Propionyl-coenzyme A carboxylase (PCC), a mitochondrial biotin-dependent enzyme, is essential for the catabolism of the amino acids Thr, Val, Ile and Met, cholesterol and fatty acids with an odd number of carbon atoms. Deficiencies in PCC activity in humans are linked to the disease propionic acidaemia, an autosomal recessive disorder that can be fatal in infants1,2,3,4. The holoenzyme of PCC is an α6β6 dodecamer, with a molecular mass of 750 kDa. The α-subunit contains the biotin carboxylase (BC) and biotin carboxyl carrier protein (BCCP) domains, whereas the β-subunit supplies the carboxyltransferase (CT) activity. Here we report the crystal structure at 3.2-Å resolution of a bacterial PCC α6β6 holoenzyme as well as cryo-electron microscopy (cryo-EM) reconstruction at 15-Å resolution demonstrating a similar structure for human PCC. The structure defines the overall architecture of PCC and reveals unexpectedly that the α-subunits are arranged as monomers in the holoenzyme, decorating a central β6 hexamer. A hitherto unrecognized domain in the α-subunit, formed by residues between the BC and BCCP domains, is crucial for interactions with the β-subunit. We have named it the BT domain. The structure reveals for the first time the relative positions of the BC and CT active sites in the holoenzyme. They are separated by approximately 55 Å, indicating that the entire BCCP domain must translocate during catalysis. The BCCP domain is located in the active site of the β-subunit in the current structure, providing insight for its involvement in the CT reaction. The structural information establishes a molecular basis for understanding the large collection of disease-causing mutations in PCC and is relevant for the holoenzymes of other biotin-dependent carboxylases, including 3-methylcrotonyl-CoA carboxylase (MCC)5,6,7 and eukaryotic acetyl-CoA carboxylase (ACC)8,9.