VIEWPOINT Forty years with coronaviruses Susan R. Weiss I have been researching coronaviruses for more than forty years. This viewpoint summarizes some of the major findings in coronavirus research made before the SARS epidemic and how they inform current research on the newly emerged SARS-CoV-2. A virulent new coronavirus is currently didn’t want to continue working in that including infectious bronchitis virus and bo- holding hostage much of the human popu- field. In reading the literature, I came upon vine coronavirus. There were a handful of Downloaded from https://rupress.org/jem/article-pdf/217/5/e20200537/1041300/jem_20200537.pdf by guest on 30 March 2020 lation worldwide. This virus, SARS-CoV-2, coronaviruses as an attractive topic, with presentations on human coronavirus 229E, a which causes the COVID-19 disease, so much possible. The model coronavirus, poorly understood agent of the common cold. emerged in China from bats into a presumed mouse hepatitis virus (MHV), was easy to Leaving that meeting, and with the en- intermediate species and then into humans. grow in tissue culture in the laboratory and couragement and mentorship of Neal It then spread around the globe with ongo- also provided compelling mouse models for Nathanson, my chair, and Don Gilden, a ing devastating effects. This round of human human disease, especially those of the liver professor in the neurology department, I coronavirus disease follows the appearance and the central nervous system. Julian Lei- was excited to expand my research to of the related lethal coronaviruses, SARS- bowitz, then at the University of California, studies utilizing the MHV animal models of CoV and MERS-CoV, in 2002 and 2012 re- San Diego, working on MHV, very gener- both encephalitis/chronic demyelinating spectively. The emergence of SARS-CoV was ously shared his viruses and cells with me. disease and hepatitis. While far from my the first time the general public, as well as Julian and I worked together at that time comfort zone of molecular biology, this was many scientists, became aware of this group and have been friends and collaborators for a new and exciting direction to explore. of viruses and its potential to cause lethal the past 40 yr. It was not until much later During the 1980s and 1990s, we made fun- infections in humans. Indeed, each one of that I appreciated the great gift of time Mike damental discoveries using the animal these events has instigated an influx of re- Bishop gave me to start my own future re- model, which was greatly enhanced in later searchers into this field. However, there is a search while still in his laboratory. years by the use of genetically modified long history of coronavirus research, start- Indeed, the first international coronavi- viral strains by reverse genetics. Indeed, ing as early as the 1930s, that has built a rus conference, organized by Volker ter my career evolved in parallel to coronavi- large knowledge base as well as technical Meulen, met in Wurzburg, Germany in the rus research. It was also during this period tools for investigating these human patho- fall of 1980, the same year I started my that I was promoted to Associate Professor gens. As someone who has worked with this laboratory as an assistant professor in the (1986) and Professor (1992). group of viruses since the end of my post- Department of Microbiology at the Univer- During the 1980s, several important doctoral period in the late 1970s until the sity of Pennsylvania, now the Perelman findings were reported. The coronavirus present, I want to review some of the major School of Medicine in September 1980. Dr. genomic RNA is transcribed into a set of findings over the last 40 yr in the field, ter Meulen was passing through Philadel- subgenomic mRNAs that encode viral pro- summarized in Fig. 1, and point out how they phia, and he invited me to this meeting. teins. Using the model MHV, these sub- are relevant to the current coronavirus That meeting was attended by ∼60 people, genomic RNAs were shown to each contain outbreak. which was virtually the entire coronavirus a leader sequence derived from the 59 end of Coronavirus researchers encompassed a group at that time. This was the era during the genome (Lai et al., 1982), initially by small community in the late 1970s and early which the RNA genome and replication RNA fingerprinting methods. These nested 1980s. As I was finishing my postdoctoral strategy used by this group of viruses subgenomic RNAs came to define a larger training with J. Michael Bishop and Harold was discovered. While many talks focused superfamily of Nidoviruses (“nidus” means Varmus at the University of California, San on the model coronavirus, MHV, others nest) including several other virus families. Francisco, working on the RNAs of avian were presented on the replication of impor- Later it was found that subgenomic mRNAs sarcoma virus (Weiss et al., 1977), I realized I tant coronaviruses of domesticated animals, are transcribed from negative strand RNAs ............................................................................................................................................................................. Department of Microbiology, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA. Susan R. Weiss: [email protected]. © 2020 Weiss. This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://www.rupress.org/terms/). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 4.0 International license, as described at https://creativecommons.org/licenses/by-nc-sa/4.0/). Rockefeller University Press https://doi.org/10.1084/jem.20200537 1of4 J. Exp. Med. 2020 Vol. 217 No. 5 e20200537 Downloaded from https://rupress.org/jem/article-pdf/217/5/e20200537/1041300/jem_20200537.pdf by guest on 30 March 2020 Figure 1. Timeline for coronavirus research. Major findings in coronavirus research (gray boxes) as well as identification of human coronaviruses (red boxes) are indicated along the timeline. that were synthesized from the full-length them out. These seemingly nuanced studies receptor or even the cells with the highest genomic RNA (Wu and Brian, 2010). This is demonstrated that these details impact en- expression are the major targets. This is a unique mechanism involving noncontig- try routes, cellular and organ tropism, and exemplified by MHV studies in which the uous transcription of the genome, presumed possibly antiviral targets (Millet and receptor, MHVR, is expressed highly in the to occur by a process involving viral poly- Whittaker, 2015). The sequence of SARS- liver but at barely detectable levels in neu- merase jumping from one part of the ge- CoV-2 was released within weeks of the rons. In contrast, during infection, the MHV nome template to another. This leads to a outbreak, and it was clear that the SARS- strain JHM.SD is highly neurovirulent and high rate of recombination for coronavi- COV-2 spike protein has a furin cleavage cannot replicate in the liver (Bender et al., ruses, which may play a role in viral evo- site in the S1/S2 junction, different from 2010). We devoted considerable time and lution and interspecies infections. However, SARS-CoV and other closely related bat vi- energy to mapping viral tropism in vivo and the ancestral bat virus of SARS-CoV-2, the ruses (Coutard et al., 2020). This has im- virulence factors that contributed to patho- species from which it was transferred to plications for viral entry routes and possible genesis. Perhaps surprisingly, our studies humans, and the sequence changes leading protease inhibitors as antivirals, and has led showed it was not only the spike protein to the evolution of this highly pathogenic to speculation about virulence and patho- that impacted tissue tropism; other “back- virus have yet to be identified. genesis. Interestingly, this new furin site ground genes,” including nucleocapsid and Virions contain the genetic material and may serve as a marker to identify a possible replicase, as well as accessory genes, were the structural proteins necessary for inva- precursor virus. also important determinants of tropism sion. Early studies on the viral structural In addition to viral glycoprotein cleav- (reviewed in Weiss and Leibowitz, 2011). proteins by Kathryn Holmes and Larry ages impacting tropism, defining the cellu- Therefore, one cannot infer pathogenesis Sturman (Frana et al., 1985) showed that the lar receptor used by each coronavirus has from knowledge of the spike protein and MHV spike protein (S, initially called E2) also informed our understanding of infec- receptor alone. Future studies on SARS- was synthesized as a precursor, and during tion. In 1991, Kathryn Holmes identified the CoV-2 will define tissue tropism and intracellular processing, is proteolytically MHV receptor as CEACAM1a (MHVR), a whether it parallels SARS-CoV or not. cleaved by the cellular enzyme furin into S1 carcinoembryonic antigen family member Sequencing RNA genomes was difficult and S2 subunits. Spike protein cleavage was (Williams et al., 1991), one of the earliest in the 1980s, and cDNA cloning was just required for cell-to-cell fusion during in- identified viral receptors. Subsequently, emerging. The first complete genome se- fection and was both cell type dependent ACE2 for SARS-CoV and DPP4 for MERS quence of a coronavirus was for infectious and virus strain dependent. Much later, it were identified as receptors, and within bronchitis virus in 1987 (Boursnell et al., was found that a second cleavage S29 ex- only a short period of time, it was found that 1987) and a few years later it was com- poses the fusion peptide and is thus neces- ACE2 is also used by SARS-CoV-2 (Zhou pleted for MHV (Lee et al., 1991). These ge- sary for viral entry.