A virally encoded high-resolution screen of cytomegalovirus dependencies.

Genetic screens have transformed our ability to interrogate cellular factor requirements for viral infections1,2, but most current approaches are limited in their sensitivity, biased towards early stages of infection and provide only simplistic phenotypic information that is often based on survival of infected cells2-4. Here, by engineering human cytomegalovirus to express single guide RNA libraries directly from the viral genome, we developed virus-encoded CRISPR-based direct readout screening (VECOS), a sensitive, versatile, viral-centric approach that enables profiling of different stages of viral infection in a pooled format. Using this approach, we identified hundreds of host dependency and restriction factors and quantified their direct effects on viral genome replication, viral particle secretion and infectiousness of secreted particles, providing a multi-dimensional perspective on virus-host interactions. These high-resolution measurements reveal that perturbations altering late stages in the life cycle of human cytomegalovirus (HCMV) mostly regulate viral particle quality rather than quantity, establishing correct virion assembly as a critical stage that is heavily reliant on virus-host interactions. Overall, VECOS facilitates systematic high-resolution dissection of the role of human proteins during the infection cycle, providing a roadmap for in-depth study of host-herpesvirus interactions.

Molecular characterization of human cytomegalovirus infection with single-cell transcriptomics.

Human cytomegalovirus (HCMV) can result in either productive or non-productive infection, with the latter potentially leading to viral latency. The molecular factors dictating these outcomes are poorly understood. Here we used single-cell transcriptomics to analyse HCMV infection progression in monocytes, which are latently infected, and macrophages, considered to be permissive for productive infection. We show that early viral gene expression levels, specifically of those encoding immediate early proteins IE1 and IE2, are a major factor dictating productive infection. We also revealed that intrinsic, not induced, host cell interferon-stimulated gene expression level is a main determinant of infection outcome. Intrinsic interferon-stimulated gene expression is downregulated with monocyte to macrophage differentiation, partially explaining increased macrophage susceptibility to productive HCMV infection. Furthermore, non-productive macrophages could reactivate, making them potential latent virus reservoirs. Overall, we decipher molecular features underlying HCMV infection outcomes and propose macrophages as a potential HCMV reservoir.

Different rates of flux through the biosynthetic pathway for long-chain versus very-long-chain sphingolipids.

The backbone of all sphingolipids (SLs) is a sphingoid long-chain base (LCB) to which a fatty acid is N-acylated. Considerable variability exists in the chain length and degree of saturation of both of these hydrophobic chains, and recent work has implicated ceramides with different LCBs and N-acyl chains in distinct biological processes; moreover, they may play different roles in disease states and possibly even act as prognostic markers. We now demonstrate that the half-life, or turnover rate, of ceramides containing diverse N-acyl chains is different. By means of a pulse-labeling protocol using stable-isotope, deuterated free fatty acids, and following their incorporation into ceramide and downstream SLs, we show that very-long-chain (VLC) ceramides containing C24:0 or C24:1 fatty acids turn over much more rapidly than long-chain (LC) ceramides containing C16:0 or C18:0 fatty acids due to the more rapid metabolism of the former into VLC sphingomyelin and VLC hexosylceramide. In contrast, d16:1 and d18:1 ceramides show similar rates of turnover, indicating that the length of the sphingoid LCB does not influence the flux of ceramides through the biosynthetic pathway. Together, these data demonstrate that the N-acyl chain length of SLs may not only affect membrane biophysical properties but also influence the rate of metabolism of SLs so as to regulate their levels and perhaps their biological functions.