SUMMARY Many RNA viruses remodel intracellular membranes to generate specialized sites for RNA replication. How membranes are remodeled and what properties make them conducive for replication are unknown. Here we show how RNA viruses can manipulate multiple components of the cellular secretory pathway to generate organelles specialized for replication that are distinct in protein and lipid composition from the host cell. Specific viral proteins modulate effector recruitment by Arf1 GTPase and its guanine nucleotide exchange factor GBF1, promoting preferential recruitment of phosphatidylinositol-4-kinase IIIβ (PI4KIIIβ) to membranes over coat proteins, yielding uncoated phosphatidylinositol-4-phosphate (PI4P) lipid-enriched organelles. The PI4P-rich lipid micro-environment is essential for both enteroviral and flaviviral RNA replication; PI4KIIIβ inhibition interferes with this process; and enteroviral RNA polymerases specifically bind PI4P. These findings reveal how RNA viruses can selectively exploit specific elements of the host to form specialized organelles where cellular phosphoinositide lipids are key to regulating viral RNA replication.

In enteric viral infections, such as those with rotavirus and norovirus, individual viral particles shed in stool are considered the optimal units of fecal-oral transmission. We reveal that rotaviruses and noroviruses are also shed in stool as viral clusters enclosed within vesicles that deliver a high inoculum to the receiving host. Cultured cells non-lytically release rotaviruses and noroviruses inside extracellular vesicles. In addition, stools of infected hosts contain norovirus and rotavirus within vesicles of exosomal or plasma membrane origin. These vesicles remain intact during fecal-oral transmission and thereby transport multiple viral particles collectively to the next host, enhancing both the MOI and disease severity. Vesicle-cloaked viruses are non-negligible populations in stool and have a disproportionately larger contribution to infectivity than free viruses. Our findings indicate that vesicle-cloaked viruses are highly virulent units of fecal-oral transmission and highlight a need for antivirals targeting vesicles and virus clustering.

Cholesterol is a critical component of cellular membranes, regulating assembly and function of membrane-based protein/lipid complexes. Many RNA viruses, including enteroviruses, remodel host membranes to generate organelles with unique lipid blueprints on which they assemble replication complexes and synthesize viral RNA. Here we find that clathrin-mediated endocytosis (CME) is harnessed by enteroviruses to traffic cholesterol from the plasma membrane (PM) and extracellular medium to replication organelles where cholesterol then regulates viral polyprotein processing and facilitates genome synthesis. When CME is disrupted, cellular cholesterol pools are instead stored in lipid droplets; cholesterol cannot be trafficked to replication organelles; and replication is inhibited. In contrast, replication is stimulated in cholesterol-elevated cells like those lacking caveolins or those from Niemann-Pick disease patients. Our findings indicate cholesterol as a critical determinant for enteroviral replication and outline roles for the endocytic machinery in both the enteroviral lifecycle and host cell cholesterol homeostasis.

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Enteric viruses like norovirus, rotavirus and astrovirus have long been accepted as spreading in the population through fecal–oral transmission: viruses are shed into feces from one host and enter the oral cavity of another, bypassing salivary glands (SGs) and reaching the intestines to replicate, be shed in feces and repeat the transmission cycle 1 . Yet there are viruses (for example, rabies) that infect the SGs 2 , 3 , making the oral cavity one site of replication and saliva one conduit of transmission. Here we report that enteric viruses productively and persistently infect SGs, reaching titres comparable to those in the intestines. We demonstrate that enteric viruses get released into the saliva, identifying a second route of viral transmission. This is particularly significant for infected infants, whose saliva directly transmits enteric viruses to their mothers’ mammary glands through backflow during suckling. This sidesteps the conventional gut–mammary axis route 4 and leads to a rapid surge in maternal milk secretory IgA antibodies 5 , 6 . Lastly, we show that SG-derived spheroids 7 and cell lines 8 can replicate and propagate enteric viruses, generating a scalable and manageable system of production. Collectively, our research uncovers a new transmission route for enteric viruses with implications for therapeutics, diagnostics and importantly sanitation measures to prevent spread through saliva.

An individual virion was long believed to act as an independent infectious unit in virology, until the recent discovery of vesicle-cloaked virus clusters which has greatly challenged this central paradigm. Vesicle-cloaked virus clusters (also known as viral vesicles) are phospholipid-bilayer encapsulated fluid sacs that contain multiple virions or multiple copies of viral genomes. Norovirus is a global leading causative agent of gastroenteritis, and the reported prevalence of vesicle-cloaked norovirus clusters in stool has raised concerns whether the current disinfection, sanitation, and hygiene practices can effectively control environmental pollution by these pathogenic units. In this study, we have demonstrated that vesicle-cloaked murine norovirus (MNV-1) clusters were highly persistent under temperature variation (i.e., freeze–thaw) and they were partially resistant to detergent decomposition. MNV-1 vesicles were 1.89–3.17-fold more infectious in vitro than their free virus counterparts. Most importantly, MNV-1 vesicles were up to 2.16-times more resistant to UV254 disinfection than free MNV-1 at a low viral load in vitro. Interestingly, with the increase of the viral load, free MNV-1 and MNV-1 vesicles showed equivalent resistance to UV254 disinfection. We show that the increased multiplicity of infection provided by vesicles is in part responsible for these attributes. Our study, for the first time, sheds light on the environmental behavior of vesicle-cloaked virus clusters as unique emerging pathogenic units. Our study highlights the need to revisit current paradigms of disinfection, sanitation, and hygiene practices for protecting public health.

Extracellular vesicles (EVs) are major vehicles for transporting viruses en bloc among hosts. While RNA viruses make up the great majority of transmission by EVs, in a recent article in mBio (mBio 10:e00379-19, 2019, https://mbio.asm.org/content/10/2/e00379-19.long), Morris-Love and colleagues revealed that a double-stranded DNA (dsDNA) virus, JC polyomavirus (JCPyV), a major cause of progressive multifocal leukoencephalopathy (PML), can be released from and transmitted to other glia in EVs. This mode of transmission appears to be highly infectious, independent of the free virus attachment and entry receptors LSTc and 5-HT2, and protected from neutralizing antibodies. This novel form of JCPyV transmission may potentially explain its dissemination into the central nervous system (CNS) and its increased virulence.

Human induced pluripotent stem cells (iPSCs) are a powerful tool for studying development and disease. However, different iPSC lines show considerable phenotypic variation. The lack of common well-characterized cell lines that are used widely frustrates efforts to integrate data across research groups or replicate key findings. Inspired by model organism communities who addressed this issue by establishing a limited number of widely accepted strains, we characterised candidate iPSC lines in unprecedented detail to select a well-performing line to underpin collaborative studies. Specifically, we characterised the morphology, growth rates, and single-cell transcriptomes of iPSC lines in the pluripotent state and assessed their genomic integrity using karyotyping, DNA microarrays, whole genome sequencing, and functional assays for p53 activity. We further tested their ability to be edited by CRISPR/Cas9 and used single-cell RNA sequencing to compare the efficiency with which they could be differentiated into multiple lineages. We found that there was significant variability in the performance of lines across the tested assays that enabled the rational selection of a lead line, KOLF2.1J, which is a gene-corrected derivative of a publicly available line from the Human Induced Pluripotent Stem Cells Initiative (HipSci) resource. We are now using this line in an initiative from the NIH Center for Alzheimer’s and Related Dementias to derive hundreds of gene-edited and functionalized sub-clones to be distributed widely throughout the research community along with associated datasets, with the aim of promoting the standardisation required for large-scale collaborative science in the stem cell field.SummaryThe authors of this collaborative science study describe a deep characterization of widely available induced pluripotent stem cell (iPSC) lines to rationally select a line that performs well in multiple experimental approaches. Analysis of transcriptional patterns in the pluripotent state, whole genome sequencing, genomic stability after highly efficient CRISPR-mediated gene editing, integrity of the p53 pathway, and differentiation efficiency towards multiple lineages identified KOLF2.1J as a well-performing cell line. The widespread distribution and use of this line makes it an attractive cell line for comparative and collaborative efforts in the stem cell field.HighlightsDeep genotyping and phenotyping reveals KOLF2.1J as well-performing cell line that is readily distributed and could serve as common reference lineDespite rare copy-neutral loss of heterozygosity (CN-LOH) events, iPSC lines retain genomic fidelity after CRISPR/Cas9-based gene editingOur multifactorial pipeline serves as a blueprint for future efforts to identify other lead iPSC linesGraphical abstract