Development of a Cost-Effective Process for the Heterologous Production of SARS-CoV-2 Spike Receptor Binding Domain (RBD) Using Pichia pastoris in Stirred-Tank Bioreactor (preprint)

SARS-CoV-2 was identified as the pathogenic agent causing the COVID-19 pandemic. Among the proteins codified by this virus, the Spike is one of the most external and exposed. A fragment of the Spike protein, named the Receptor Binding Protein (RBD) interacts with the ACE2 receptors of human cells, allowing the entrance of the viruses. RBD has been proposed as an interesting protein for the development of diagnosis tools, treatment and prevention of the disease. In this work, a method for recombinant RBD production using Pichia pastoris as a cell factory in a Stirred Tank Bioreactor (SRTB) up to 7 L was developed. Using a basal saline medium with glycerol, methanol and compressed air in a four stages procedure, around 500 mg/L of raw yRBD (RBD produced by yeasts) and 206 mg/L of purified (>95%) RBD were obtained. Thereby, the proposed method represents a feasible, simple, scalable and cheap procedure for the obtention of RBD for diagnosis kits and vaccines formulation

Structural and Functional Comparison of SARS-CoV-2-Spike Receptor Binding Domain Produced in Pichia pastoris and Mammalian Cells (preprint)

The yeast Pichia pastoris is a cost-effective and easily scalable system for recombinant protein production. In this work we compared the conformation of the receptor binding domain (RBD) from SARS-CoV-2 Spike protein expressed in P. pastoris and in the well established HEK-293T mammalian cell system. RBD obtained from both yeast and mammalian cells was properly folded, as indicated by UV-absorption, circular dichroism and tryptophan fluorescence. They also had similar stability, as indicated by temperature-induced unfolding (observed Tm were 50 {degrees}C and 52 {degrees}C for RBD produced in P. pastoris and HEK-293T cells, respectively). Moreover, the stability of both variants was similarly reduced when the ionic strength was increased, in agreement with a computational analysis predicting that a set of ionic interactions may stabilize RBD structure. Further characterization by HPLC, size-exclusion chromatography and mass spectrometry revealed a higher heterogeneity of RBD expressed in P. pastoris relative to that produced in HEK-293T cells, which disappeared after enzymatic removal of glycans. The production of RBD in P. pastoris was scaled-up in a bioreactor, with yields above 45 mg/L of 90% pure protein, thus potentially allowing large scale immunizations to produce neutralizing antibodies, as well as the large scale production of serological tests for SARS-CoV-2.

Fixed single-cell RNA sequencing for understanding virus infection and host response (preprint)

Single-cell RNA sequencing studies requiring intracellular protein staining, rare-cell sorting, or pathogen inactivation are severely limited because current high-throughput methods are incompatible with paraformaldehyde treatment, a very common and simple tissue/cell fixation and preservation technique. Here we present FD-seq, a high-throughput method for droplet-based RNA sequencing of paraformaldehyde-fixed, stained and sorted single-cells. We used FD-seq to address two important questions in virology. First, by analyzing a rare population of cells supporting lytic reactivation of the human tumor virus KSHV, we identified TMEM119 as a host factor that mediates reactivation. Second, we studied the transcriptome of lung cells infected with the 2 coronavirus OC43, which causes the common cold and also serves as a safer model pathogen for SARS-CoV-2. We found that pro-inflammatory pathways are primarily upregulated in abortively-infected or uninfected bystander cells, which are exposed to the virus but fail to express high level of viral genes. FD-seq is suitable for characterizing rare cell populations of interest, for studying high-containment biological samples after inactivation, and for integrating intracellular phenotypic with transcriptomic information.

Mutational signatures in countries affected by SARS-CoV-2: Implications in host-pathogen interactome (preprint)

We are in the midst of the third severe coronavirus outbreak caused by SARS-CoV-2 with unprecedented health and socio-economic consequences due to the COVID-19. Globally, the major thrust of scientific efforts has shifted to the design of potent vaccine and anti-viral candidates. Earlier genome analyses have shown global dominance of some mutations purportedly indicative of similar infectivity and transmissibility of SARS-CoV-2 worldwide. Using high-quality large dataset of 25k whole-genome sequences, we show emergence of new cluster of mutations as result of geographic evolution of SARS-CoV-2 in local population ({greater than or equal to}10%) of different nations. Using statistical analysis, we observe that these mutations have either significantly co-occurred in globally dominant strains or have shown mutual exclusivity in other cases. These mutations potentially modulate structural stability of proteins, some of which forms part of SARS-CoV-2-human interactome. The high confidence druggable host proteins are also up-regulated during SARS-CoV-2 infection. Mutations occurring in potential hot-spot regions within likely T-cell and B-cell epitopes or in proteins as part of host-viral interactome, could hamper vaccine or drug efficacy in local population. Overall, our study provides comprehensive view of emerging geo-clonal mutations which would aid researchers to understand and develop effective countermeasures in the current crisis.

A structural model for the Coronavirus Nucleocapsid (preprint)

We propose a mesoscale model structure for the coronavirus nucleocapsid, assembled from the high resolution structures of the basic building blocks of the N-protein, CryoEM imaging and mathematical constraints for an overall quasi-spherical particle. The structure is a truncated octahedron that accommodates two layers: an outer shell composed of triangular and quadrangular lattices of the N-terminal domain and an inner shell of equivalent lattices of coiled parallel helices of the C-terminal domain. The model is consistent with the dimensions expected for packaging large viral genomes and provides a rationale to interpret the apparent pleomorphic nature of coronaviruses.