Coronavirus infections of multiple origins have spread to date worldwide, causing severe respiratory diseases. Seven coronaviruses that infect humans have been identified: HCoV-229E, HCoV-OC43, HCoV-NL63, HCoV-HKU1, SARS-CoV, MERS-CoV, and SARS-CoV-2. Among them, SARS-CoV and MERS-CoV caused outbreaks in 2002 and 2012, respectively. SARS-CoV-2 (COVID-19) is the most recently discovered. It has created a severe worldwide outbreak beginning in late 2019, leading to date to over 4 million cases globally. Viruses are genetically simple, yet highly diverse. However, the recent outbreaks of SARS-CoV and MERS-CoV, and the ongoing outbreak of SARS-CoV-2, indicate that there remains a long way to go to identify and develop specific therapeutic treatments. Only after gaining a better understanding of their pathogenic mechanisms can we minimize viral pandemics. This paper mainly focuses on SARS-CoV, MERS-CoV, and SARS-CoV-2. Here, recent studies are summarized and reviewed, with a focus on virus-host interactions, vaccine-based and drug-targeted therapies, and the development of new approaches for clinical diagnosis and treatment.

In this special issue of the Glycoconjugate Journal focusing on glycosciences and development, we summarize recent advances in our understanding of the role of mucintype O-glycans in development and disease. The presence of this widespread protein modification has been known for decades, yet identification of its biological functions has been hampered by the redundancy and complexity of the enzyme family controlling the initiation of O-glycosylation, as well as the diversity of extensions of the core sugar. Recent studies in organisms as diverse as mammals and Drosophila have yielded insights into the function of this highly abundant and evolutionarily-conserved protein modification. Gaining an understanding of mucin-type O-glycans in these diverse systems will elucidate crucial conserved processes underlying many aspects of development and homeostasis.

The SARS-CoV-2 coronavirus responsible for the global pandemic contains a novel furin cleavage site in the spike protein (S) that increases viral infectivity and syncytia formation in cells. Here, we show that O-glycosylation near the furin cleavage site is mediated by members of the GALNT enzyme family, resulting in decreased furin cleavage and decreased syncytia formation. Moreover, we show that O-glycosylation is dependent on the novel proline at position 681 (P681). Mutations of P681 seen in the highly transmissible alpha and delta variants abrogate O-glycosylation, increase furin cleavage, and increase syncytia formation. Finally, we show that GALNT family members capable of glycosylating S are expressed in human respiratory cells that are targets for SARS-CoV-2 infection. Our results suggest that host O-glycosylation may influence viral infectivity/tropism by modulating furin cleavage of S and provide mechanistic insight into the role of the P681 mutations found in the highly transmissible alpha and delta variants.

The UDP-GalNAc : polypeptide N-acetylgalactosaminyltransferase (ppGaNTase or ppGalNAcT or pgant) enzyme family is responsible for the first committed step in the synthesis of mucin-type O-glycans on protein substrates. Previous work from our group has demonstrated both sequence and functional conservation between members of this family in mammals and the fruit fly, Drosophila melanogaster. One member of this family in Drosophila has been shown to be essential for viability and development. In an effort to understand the developmental stages and processes in which O-glycosylation is involved, we have determined the expression pattern of each functional family member as well as putative isoforms during Drosophila development. Our studies indicate that isoforms are expressed in discrete spatial and temporal fashions during development, with some isoforms being found uniquely in restricted areas of the developing embryo (brain, trachea, pharynx, esophagus, proventriculus, and amnioserosa), whereas others are found in multiple regions and overlap with the expression of other isoforms (salivary glands, posterior midgut, anterior midgut, and the fore-/hindgut) during embryogenesis. Additionally, we examined expression patterns in imaginal discs from third instar larvae, which will become the adult structures. Most isoforms are also expressed in the imaginal discs, with some showing unique transcript localization and spatial regulatory control. Thus, this report provides insight into the specific regions during Drosophila development that may require O-linked glycosylation in vivo as well as which isoforms may act cooperatively in certain tissues and which may be uniquely responsible for glycosylation in others.