Peptide Inhibitors of Vascular Endothelial Growth Factor A: Current Situation and Perspectives

Vascular endothelial growth factors (VEGFs) are the family of extracellular signaling proteins involved in the processes of angiogenesis. VEGFA overexpression and altered regulation of VEGFA signaling pathways lead to pathological angiogenesis, which contributes to the progression of various diseases, such as age-related macular degeneration and cancer. Monoclonal antibodies and decoy receptors have been extensively used in the anti-angiogenic therapies for the neutralization of VEGFA. However, multiple side effects, solubility and aggregation issues, and the involvement of compensatory VEGFA-independent pro-angiogenic mechanisms limit the use of the existing VEGFA inhibitors. Short chemically synthesized VEGFA binding peptides are a promising alternative to these full-length proteins. In this review, we summarize anti-VEGFA peptides identified so far and discuss the molecular basis of their inhibitory activity to highlight their pharmacological potential as anti-angiogenic drugs.

TNF Decoy Receptors Encoded by Poxviruses

Tumour necrosis factor (TNF) is an inflammatory cytokine produced in response to viral infections that promotes the recruitment and activation of leukocytes to sites of infection. This TNF-based host response is essential to limit virus spreading, thus poxviruses have evolutionarily adopted diverse molecular mechanisms to counteract TNF antiviral action. These include the expression of poxvirus-encoded soluble receptors or proteins able to bind and neutralize TNF and other members of the TNF ligand superfamily, acting as decoy receptors. This article reviews in detail the various TNF decoy receptors identified to date in the genomes from different poxvirus species, with a special focus on their impact on poxvirus pathogenesis and their potential use as therapeutic molecules.

Analysis of the Evolution of Pandemic Influenza A(H1N1) Virus Neuraminidase Reveals Entanglement of Different Phenotypic Characteristics

ABSTRACT The influenza A virus (IAV) neuraminidase (NA) is essential for virion release from cells and decoy receptors and an important target of antiviral drugs and antibodies. Adaptation to a new host sialome and escape from the host immune system are forces driving the selection of mutations in the NA gene. Phylogenetic analysis shows that until 2015, 16 amino acid substitutions in NA became fixed in the virus population after introduction in the human population of the pandemic IAV H1N1 (H1N1pdm09) in 2009. The accumulative effect of these substitutions, in the order in which they appeared, was analyzed using recombinant proteins and viruses in combination with different functional assays. The results indicate that NA activity did not evolve to a single optimum but rather fluctuated within a certain bandwidth. Furthermore, antigenic and enzymatic properties of NA were intertwined, with several residues affecting multiple properties. For example, the substitution K432E in the second sialic acid binding site, next to the catalytic site, was shown to affect catalytic activity, substrate specificity, and the pH optimum for maximum activity. This substitution also altered antigenicity of NA, which may explain its selection. We propose that the entanglement of NA phenotypes may be an important determining factor in the evolution of NA. IMPORTANCE Since its emergence in 2009, the pandemic H1N1 influenza A virus (IAV) has caused significant disease and mortality in humans. IAVs contain two envelope glycoproteins, the receptor-binding hemagglutinin (HA) and the receptor-destroying neuraminidase (NA). NA is essential for virion release from cells and decoy receptors, is an important target of antiviral drugs, and is increasingly being recognized as an important vaccine antigen. Not much is known, however, about the evolution of this protein upon the emergence of the novel pandemic H1N1 virus, with respect to its enzymatic activity and antigenicity. By reconstructing the evolutionary path of NA, we show that antigenic and enzymatic properties of NA are intertwined, with several residues affecting multiple properties. Understanding the entanglement of NA phenotypes will lead to better comprehension of IAV evolution and may help the development of NA-based vaccines.

Deep Mutational Scanning of Viral Glycoproteins and Their Host Receptors

Deep mutational scanning or deep mutagenesis is a powerful tool for understanding the sequence diversity available to viruses for adaptation in a laboratory setting. It generally involves tracking an in vitro selection of protein sequence variants with deep sequencing to map mutational effects based on changes in sequence abundance. Coupled with any of a number of selection strategies, deep mutagenesis can explore the mutational diversity available to viral glycoproteins, which mediate critical roles in cell entry and are exposed to the humoral arm of the host immune response. Mutational landscapes of viral glycoproteins for host cell attachment and membrane fusion reveal extensive epistasis and potential escape mutations to neutralizing antibodies or other therapeutics, as well as aiding in the design of optimized immunogens for eliciting broadly protective immunity. While less explored, deep mutational scans of host receptors further assist in understanding virus-host protein interactions. Critical residues on the host receptors for engaging with viral spikes are readily identified and may help with structural modeling. Furthermore, mutations may be found for engineering soluble decoy receptors as neutralizing agents that specifically bind viral targets with tight affinity and limited potential for viral escape. By untangling the complexities of how sequence contributes to viral glycoprotein and host receptor interactions, deep mutational scanning is impacting ideas and strategies at multiple levels for combatting circulating and emergent virus strains.

Receptor Oligomerization and Its Relevance for Signaling by Receptors of the Tumor Necrosis Factor Receptor Superfamily

With the exception of a few signaling incompetent decoy receptors, the receptors of the tumor necrosis factor receptor superfamily (TNFRSF) are signaling competent and engage in signaling pathways resulting in inflammation, proliferation, differentiation, and cell migration and also in cell death induction. TNFRSF receptors (TNFRs) become activated by ligands of the TNF superfamily (TNFSF). TNFSF ligands (TNFLs) occur as trimeric type II transmembrane proteins but often also as soluble ligand trimers released from the membrane-bound form by proteolysis. The signaling competent TNFRs are efficiently activated by the membrane-bound TNFLs. The latter recruit three TNFR molecules, but there is growing evidence that this is not sufficient to trigger all aspects of TNFR signaling; rather, the formed trimeric TNFL–TNFR complexes have to cluster secondarily in the cell-to-cell contact zone for full TNFR activation. With respect to their response to soluble ligand trimers, the signaling competent TNFRs can be subdivided into two groups. TNFRs of one group, designated as category I TNFRs, are robustly activated by soluble ligand trimers. The receptors of a second group (category II TNFRs), however, failed to become properly activated by soluble ligand trimers despite high affinity binding. The limited responsiveness of category II TNFRs to soluble TNFLs can be overcome by physical linkage of two or more soluble ligand trimers or, alternatively, by anchoring the soluble ligand molecules to the cell surface or extracellular matrix. This suggests that category II TNFRs have a limited ability to promote clustering of trimeric TNFL–TNFR complexes outside the context of cell–cell contacts. In this review, we will focus on three aspects on the relevance of receptor oligomerization for TNFR signaling: (i) the structural factors which promote clustering of free and liganded TNFRs, (ii) the signaling pathway specificity of the receptor oligomerization requirement, and (iii) the consequences for the design and development of TNFR agonists.

An engineered decoy receptor for SARS-CoV-2 broadly binds protein S sequence variants

The spike S of SARS-CoV-2 recognizes ACE2 on the host cell membrane to initiate entry. Soluble decoy receptors, in which the ACE2 ectodomain is engineered to block S with high affinity, potently neutralize infection and, because of close similarity with the natural receptor, hold out the promise of being broadly active against virus variants without opportunity for escape. Here, we directly test this hypothesis. We find that an engineered decoy receptor, sACE22.v2.4, tightly binds S of SARS-associated viruses from humans and bats, despite the ACE2-binding surface being a region of high diversity. Saturation mutagenesis of the receptor-binding domain followed by in vitro selection, with wild-type ACE2 and the engineered decoy competing for binding sites, failed to find S mutants that discriminate in favor of the wild-type receptor. We conclude that resistance to engineered decoys will be rare and that decoys may be active against future outbreaks of SARS-associated betacoronaviruses.

Soluble ACE2 as a potential therapy for COVID-19

Soluble angiotensin converting enzyme 2 (sACE2) could be a therapeutic option to treat coronavirus disease 2019 (COVID-19) infection. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV2) utilizes ACE2 receptors on cell surfaces to gain intracellular entry making them an ideal target for therapy. High-affinity variants of sACE2, engineered using high-throughput mutagenesis, are capable of neutralizing COVID-19 infection as decoy receptors. These variants compete with native ACE2 present on cells by binding with spike (S) protein of SARS-CoV2, making native ACE2 on cell surfaces available to convert angiotensin-II to angiotensin-1,7, thus alleviating the exaggerated inflammatory response associated with COVID-19 infection. This article explores the use of sACE2 as potential therapy for COVID-19 infection.

Soluble Receptors Affecting Stroke Outcomes: Potential Biomarkers and Therapeutic Tools

Soluble receptors are widely understood to be freestanding moieties formed via cleavage from their membrane-bound counterparts. They have unique structures, are found among various receptor families, and have intriguing mechanisms of generation and release. Soluble receptors’ ability to exhibit pleiotropic action by receptor modulation or by exhibiting a dual role in cytoprotection and neuroinflammation is concentration dependent and has continually mystified researchers. Here, we have compiled findings from preclinical and clinical studies to provide insights into the role of soluble/decoy receptors, focusing on the soluble cluster of differentiation 36, the soluble cluster of differentiation 163, and soluble lipoprotein-related protein 1 (sCD36, sCD163, and sLRP1, respectively) and the functions they could likely serve in the management of stroke, as they would notably regulate the bioavailability of the hemoglobin and heme after red blood cell lysis. The key roles that these soluble receptors play in inflammation, oxidative stress, and the related pharmacotherapeutic potential in improving stroke outcomes are described. The precise pleiotropic physiological functions of soluble receptors remain unclear, and further scientific investigation/validation is required to establish their respective role in diagnosis and therapy.