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The SARS-CoV nucleocapsid (N) protein serves multiple functions in viral replication, transcription, and assembly of the viral genome complex. Coronaviruses specifically package genomic RNA into assembled virions, and in SARS-CoV, it is reported that this process is driven by an interaction between the N-protein and a packaging signal encoded within the viral RNA. While recent studies have uncovered the sequence of this packaging signal, little is known about the specific interaction between the N-protein and the packaging signal sequence, and the mechanisms by which this interaction drives viral genome packaging. In this study, we developed a novel in vivo cell-based assay for examining this interaction between the N-protein and packaging signal RNA for SARS-CoV, as well as other viruses within the coronaviridae family. Our results demonstrate that the N-protein specifically recognizes the SARS-CoV packaging signal with greater affinity compared to signals from other coronaviruses or non-coronavirus species. We also use deletion mapping to identify a 151-nt region within the packaging signal sequence that is critical for N-protein-RNA binding, and conversely, we show that both the N-terminal and C-terminal domains of the N protein are necessary for recognizing the packaging RNA. These results describe, for the first time, in vivo evidence for an interaction between the SARS-CoV N-protein and its packaging signal RNA, and demonstrate the feasibility of using this cell-based assay to further probe viral RNA-protein interactions in future studies., Highlights • Development of a novel assay to examine interactions between the SARS-CoV N-protein and viral RNA in cells in vivo. • N-protein alone is able specifically recognize the CoV packaging signal RNA in vivo. • Both the N-terminal and C-terminal are required for the N-protein to interact with the viral RNA.

Recent developments in tissue clearing methods have significantly advanced the three-dimensional analysis of biological structures in whole, intact tissue, providing a greater understanding of spatial relationships and biological circuits. Nonetheless, studies have reported issues with maintaining structural integrity and preventing tissue disintegration, limiting the wide application of these techniques to fragile tissues such as developing embryos. Here, we present an optimized passive tissue clearing technique (PACT)-based embryo clearing method, initial embedding PACT (IMPACT)-Basic, that improves tissue rigidity without compromising optical transparency. We also present IMPACT-Advance, which is specifically optimized for thin slices of mouse embryos past E13.5. We demonstrate proof-of-concept by investigating the expression of two relatively understudied PR domain (PRDM) proteins, PRDM10 and PRDM13, in intact cleared mouse embryos at various stages of development. We observed strong PRDM10 and PRDM13 expression in the developing nervous system and skeletal cartilage, suggesting a functional role for these proteins in these tissues throughout embryogenesis.

Recent developments in tissue clearing methods such as CLARITY (Clear Lipid-exchanged Acrylamide-hybridized Rigid Imaging/Immunostaining/In situ hybridization-compatible Tissue hYdrogel) have allowed for the three-dimensional analysis of biological structures in whole, intact tissue, providing greater understanding of spatial relationships and biological circuits. Nonetheless, studies have reported issues with maintaining structural integrity and preventing tissue disintegration, preventing the wide application of these techniques to fragile tissues such as developing embryos. Here, we present optimized passive clearing techniques, mPACT-A, that improve tissue rigidity without the expense of optical transparency. We also present a further modified mPACT-A protocol that is specifically optimized for handling mouse embryos, which are small and fragile, such that they easily dismantle when processed via established tissue clearing methods. We demonstrate proof-of-concept by investigating the expression of two relatively understudied PRDM proteins, PRDM7 and PRDM12, in intact cleared mouse embryos at various stages of development. We observed strong PRDM7 and PRDM12 expression in the developing mouse nervous system, suggestive of potential roles in neural development that will be tested in future functional studies.

Since the development of CLARITY, a bioelectrochemical clearing technique that allows for three-dimensional phenotype mapping within transparent tissues, a multitude of novel clearing methodologies including CUBIC (clear, unobstructed brain imaging cocktails and computational analysis), SWITCH (system-wide control of interaction time and kinetics of chemicals), MAP (magnified analysis of the proteome), and PACT (passive clarity technique), have been established to further expand the existing toolkit for the microscopic analysis of biological tissues. The present study aims to improve upon and optimize the original PACT procedure for an array of intact rodent tissues, including the whole central nervous system (CNS), kidneys, spleen, and whole mouse embryos. Termed psPACT (process-separate PACT) and mPACT (modified PACT), these novel techniques provide highly efficacious means of mapping cell circuitry and visualizing subcellular structures in intact normal and pathological tissues. In the following protocol, we provide a detailed, step-by-step outline on how to achieve maximal tissue clearance with minimal invasion of their structural integrity via psPACT and mPACT.

Although cis-acting packaging signal RNA sequences for the influenza virus NP encoding vRNA have been identified recently though genetic studies, little is known about the interaction between NP and the vRNA packaging signals either in vivo or in vitro. Here, we provide evidence that NP is able to interact specifically with the vRNA packaging sequence RNA within living cells and that the specific RNA binding activity of NP in vivo requires both the N-terminal and central region of the protein. This assay established would be a valuable tool for further detailed studies of the NP-packaging signal RNA interaction in living cells.