Structure of papain-like protease from SARS-CoV-2 and its complexes with non-covalent inhibitors

SARS-CoV-2 Papain-Like Protease Potential Inhibitors-In Silico Quantitative Assessment

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) encodes the papain-like protease (PLpro). The protein not only plays an essential role in viral replication but also cleaves ubiquitin and ubiquitin-like interferon-stimulated gene 15 protein (ISG15) from host proteins, making it an important target for developing new antiviral drugs. In this study, we searched for novel, noncovalent potential PLpro inhibitors by employing a multistep in silico screening of a 15 million compound library. The selectivity of the best-scored compounds was evaluated by checking their binding affinity to the human ubiquitin carboxy-terminal hydrolase L1 (UCH-L1), which, as a deubiquitylating enzyme, exhibits structural and functional similarities to the PLpro. As a result, we identified 387 potential, selective PLpro inhibitors, from which we retrieved the 20 best compounds according to their IC50 values toward PLpro estimated by a multiple linear regression model. The selected candidates display potential activity against the protein with IC50 values in the nanomolar range from approximately 159 to 505 nM and mostly adopt a similar binding mode to the known, noncovalent SARS-CoV-2 PLpro inhibitors. We further propose the six most promising compounds for future in vitro evaluation. The results for the top potential PLpro inhibitors are deposited in the database prepared to facilitate research on anti-SARS-CoV-2 drugs.

Common low complexity regions for SARS-CoV-2 and human proteomes as potential multidirectional risk factor in vaccine development

BACKGROUND

The rapid spread of the COVID-19 demands immediate response from the scientific communities. Appropriate countermeasures mean thoughtful and educated choice of viral targets (epitopes). There are several articles that discuss such choices in the SARS-CoV-2 proteome, other focus on phylogenetic traits and history of the Coronaviridae genome/proteome. However none consider viral protein low complexity regions (LCRs). Recently we created the first methods that are able to compare such fragments.

RESULTS

We show that five low complexity regions (LCRs) in three proteins (nsp3, S and N) encoded by the SARS-CoV-2 genome are highly similar to regions from human proteome. As many as 21 predicted T-cell epitopes and 27 predicted B-cell epitopes overlap with the five SARS-CoV-2 LCRs similar to human proteins. Interestingly, replication proteins encoded in the central part of viral RNA are devoid of LCRs.

CONCLUSIONS

Similarity of SARS-CoV-2 LCRs to human proteins may have implications on the ability of the virus to counteract immune defenses. The vaccine targeted LCRs may potentially be ineffective or alternatively lead to autoimmune diseases development. These findings are crucial to the process of selection of new epitopes for drugs or vaccines which should omit such regions.

Knowing and combating the enemy: a brief review on SARS-CoV-2 and computational approaches applied to the discovery of drug candidates

Since the emergence of the new severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) at the end of December 2019 in China, and with the urge of the coronavirus disease 2019 (COVID-19) pandemic, there have been huge efforts of many research teams and governmental institutions worldwide to mitigate the current scenario. Reaching more than 1,377,000 deaths in the world and still with a growing number of infections, SARS-CoV-2 remains a critical issue for global health and economic systems, with an urgency for available therapeutic options. In this scenario, as drug repurposing and discovery remains a challenge, computer-aided drug design (CADD) approaches, including machine learning (ML) techniques, can be useful tools to the design and discovery of novel potential antiviral inhibitors against SARS-CoV-2. In this work, we describe and review the current knowledge on this virus and the pandemic, the latest strategies and computational approaches applied to search for treatment options, as well as the challenges to overcome COVID-19.

Discovery of SARS-CoV-2 papain-like protease inhibitors through a combination of high-throughput screening and FlipGFP-based reporter assay

The papain-like protease (PL pro ) of SARS-CoV-2 is a validated antiviral drug target. PL pro is involved in the cleavage of viral polyproteins and antagonizing host innate immune response through its deubiquitinating and deISG15ylating activities, rendering it a high profile antiviral drug target. Through a FRET-based high-throughput screening, several hits were identified as PL pro inhibitors with IC 50 values at the single-digit micromolar range. Subsequent lead optimization led to potent inhibitors with IC 50 values ranging from 0.56 to 0.90 µM. To help prioritize lead compounds for the cellular antiviral assay against SARS-CoV-2, we developed the cell-based FlipGFP assay that is suitable for quantifying the intracellular enzymatic inhibition potency of PL pro inhibitors in the BSL-2 setting. Two compounds selected from the FlipGFP-PL pro assay, Jun9-53-2 and Jun9-72-2, inhibited SARS-CoV-2 replication in Caco-2 hACE2 cells with EC 50 values of 8.89 and 8.32 µM, respectively, which were 3-fold more potent than GRL0617 (EC 50 = 25.1 µM). The X-ray crystal structures of PL pro in complex with GRL0617 showed that binding of GRL0617 to SARS-CoV-2 induced a conformational change in the BL2 loop to the more closed conformation. Overall, the PL pro inhibitors identified in this study represent promising starting points for further development as SARS-CoV-2 antivirals, and FlipGFP-PL pro assay might be a suitable surrogate for screening PL pro inhibitors in the BSL-2 setting.

Cannabidiol Inhibits SARS-CoV-2 Replication and Promotes the Host Innate Immune Response

The rapid spread of COVID-19 underscores the need for new treatments. Here we report that cannabidiol (CBD), a compound produced by the cannabis plant, inhibits SARS-CoV-2 infection. CBD and its metabolite, 7-OH-CBD, but not congeneric cannabinoids, potently block SARS-CoV-2 replication in lung epithelial cells. CBD acts after cellular infection, inhibiting viral gene expression and reversing many effects of SARS-CoV-2 on host gene transcription. CBD induces interferon expression and up-regulates its antiviral signaling pathway. A cohort of human patients previously taking CBD had significantly lower SARS-CoV-2 infection incidence of up to an order of magnitude relative to matched pairs or the general population. This study highlights CBD, and its active metabolite, 7-OH-CBD, as potential preventative agents and therapeutic treatments for SARS-CoV-2 at early stages of infection.

Identification of evolutionarily stable functional and immunogenic sites across the SARS-CoV-2 proteome and the greater coronavirus family

Since the first recognized case of COVID-19, more than 100 million people have been infected worldwide. Global efforts in drug and vaccine development to fight the disease have yielded vaccines and drug candidates to cure COVID-19. However, the spread of SARS-CoV-2 variants threatens the continued efficacy of these treatments. In order to address this, we interrogate the evolutionary history of the entire SARS-CoV-2 proteome to identify evolutionarily conserved functional sites that can inform the search for treatments with broader coverage across the coronavirus family. Combining this information with the mutations observed in the current COVID-19 outbreak, we systematically and comprehensively define evolutionarily stable sites that may provide useful drug and vaccine targets and which are less likely to be compromised by the emergence of new virus strains. Several experimentally-validated effective drugs interact with these proposed target sites. In addition, the same evolutionary information can prioritize cross reactive antigens that are useful in directing multi-epitope vaccine strategies to illicit broadly neutralizing immune responses to the betacoronavirus family. Although the results are focused on SARS-CoV-2, these approaches stem from evolutionary principles that are agnostic to the organism or infective agent.

Potent, Novel SARS-CoV-2 PLpro Inhibitors Block Viral Replication in Monkey and Human Cell Cultures

Antiviral agents blocking SARS-CoV-2 viral replication are desperately needed to complement vaccination to end the COVID-19 pandemic. Viral replication and assembly are entirely dependent on two viral cysteine proteases: 3C-like protease (3CLpro) and the papain-like protease (PLpro). PLpro also has deubiquitinase (DUB) activity, removing ubiquitin (Ub) and Ub-like modifications from host proteins, disrupting the host immune response. 3CLpro is inhibited by many known cysteine protease inhibitors, whereas PLpro is a relatively unusual cysteine protease, being resistant to blockade by such inhibitors. A high-throughput screen of biased and unbiased libraries gave a low hit rate, identifying only CPI-169 and the positive control, GRL0617, as inhibitors with good potency (IC50 < 10 lower case Greek μM). Analogues of both inhibitors were designed to develop structure-activity relationships; however, without a co-crystal structure of the CPI-169 series, we focused on GRL0617 as a starting point for structure-based drug design, obtaining several co-crystal structures to guide optimization. A series of novel 2-phenylthiophene-based non-covalent SARS-CoV-2 PLpro inhibitors were obtained, culminating in low nanomolar potency. The high potency and slow inhibitor off-rate were rationalized by newly identified ligand interactions with a 'BL2 groove' that is distal from the active site cysteine. Trapping of the conformationally flexible BL2 loop by these inhibitors blocks binding of viral and host protein substrates; however, until now it has not been demonstrated that this mechanism can induce potent and efficacious antiviral activity. In this study, we report that novel PLpro inhibitors have excellent antiviral efficacy and potency against infectious SARS-CoV-2 replication in cell cultures. Together, our data provide structural insights into the design of potent PLpro inhibitors and the first validation that non-covalent inhibitors of SARS-CoV-2 PLpro can block infection of human cells with low micromolar potency.

In silico analysis of SARS-CoV-2 papain-like protease potential inhibitors

The emergent outbreak caused by severe acute respiratory syndrome coronavirus 2 continues spreading and causing huge social and economic disruption. Papain-like protease (PLpro) has a crucial role in the cleavage of viral polyproteins, and disruption of host responses. PLpro is considered an important goal for the development of SARS-CoV-2 inhibitors. ZINC101291108 (lead 1) and ZINC16449029 (lead 2) were identified as potent SARS-CoV-2 PLpro inhibitors with IC₅₀ values of 0.085 μM and 0.063 μM, respectively. Molecular dynamics simulations (MD) were carried out for lead 1, 2 and several reported SARS-CoV-2 inhibitors. Analysis results of the simulations confirmed the stability of both compounds and showed that they adopted two confirmations along the simulation period. The per-residue decomposition results revealed that the key residues involved in inhibitor binding were E167, P247, P248, Y264, Y268 and Q269. H-bond analyses showed H-bonds with G266 and N267 and salt bridges with G209 and Y273, which are essential for strengthening the substrate-binding pocket. Both inhibitors showed hydrophobic interactions with the S4 site and BL2 loop residues. The RMSD of the BL2 loop with the two inhibitors was investigated, and the results showed that the Y268 and Q269 BL2 loop residues moved outward to accommodate the large size of lead 2. The van der Waals interaction was the main energy contribution that stabilized lead 2, while van der Waals and electrostatic interactions were the main energy contributions stabilizing lead 1. Rational design strategies were suggested to replace the 2-(2-hydroxybenzylidene) hydrazine moiety with naphthalene or nitrobenzene at the P4 position of lead 2 and introduce polar substituents as aniline and benzoate groups at position P1 to enhance hydrophobic interactions and H-bonds, respectively.

The emergent outbreak caused by severe acute respiratory syndrome coronavirus 2 continues spreading and causing huge social and economic disruption.

Structural biology of SARS-CoV-2 and implications for therapeutic development

The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is an unprecedented global health crisis. However, therapeutic options for treatment are still very limited. The development of drugs that target vital proteins in the viral life cycle is a feasible approach for treating COVID-19. Belonging to the subfamily Orthocoronavirinae with the largest RNA genome, SARS-CoV-2 encodes a total of 29 proteins. These non-structural, structural and accessory proteins participate in entry into host cells, genome replication and transcription, and viral assembly and release. SARS-CoV-2 proteins can individually perform essential physiological roles, be components of the viral replication machinery or interact with numerous host cellular factors. In this Review, we delineate the structural features of SARS-CoV-2 from the whole viral particle to the individual viral proteins and discuss their functions as well as their potential as targets for therapeutic interventions.

Proposing high-affinity inhibitors from Glycyrrhiza glabra L. against SARS-CoV-2 infection: virtual screening and computational analysis

Licorice as a traditional medicine introduces promising antiviral phytochemicals against SARS-CoV-2.