CRISPR-Cas9 screens are powerful tools for high-throughput interrogation of genome function, but can be confounded by nuclease-induced toxicity at both on-and off-target sites, likely due to DNA damage. Here, to test potential solutions to this issue, we design and analyse a CRISPR-Cas9 library with 10 variable-length guides per gene and thousands of negative controls targeting non-functional, non-genic regions (termed safe-targeting guides), in addition to non-targeting controls. We find this library has excellent performance in identifying genes affecting growth and sensitivity to the ricin toxin. The safe-targeting guides allow for proper control of toxicity from on-target DNA damage. Using this toxicity as a proxy to measure off-target cutting, we demonstrate with tens of thousands of guides both the nucleotide position-dependent sensitivity to single mismatches and the reduction of off-target cutting using truncated guides. Our results demonstrate a simple strategy for high-throughput evaluation of target specificity and nuclease toxicity in Cas9 screens.

Summary The study of synaptic specificity and plasticity in the Central Nervous System (CNS) is limited by the inability to efficiently visualize synapses in identified neurons using light microscopy. Here we describe Synaptic Tagging with Recombination (STaR), a method for labeling endogenous presynaptic and postsynaptic proteins in a cell-type specific fashion. We modified genomic loci encoding synaptic proteins within Bacterial Artificial Chromosomes such that these proteins, expressed at endogenous levels and with normal spatiotemporal patterns, were labeled in an inducible fashion in specific neurons through targeted expression of site-specific recombinases. Within the Drosophila visual system, the number and distribution of synapses correlate with Electron Microscopy studies. Using two different recombination systems, presynaptic and postsynaptic specializations of synaptic pairs can be co-labeled. STaR also allows synapses within the CNS to be studied in live animals non-invasively. In principle, STaR can be adapted to the mammalian nervous system.

Hexanucleotide repeat expansions in the C9orf72 gene are the most common cause of amyotrophic lateral sclerosis and frontotemporal dementia (c9FTD/ALS). The nucleotide repeat expansions are translated into dipeptide repeat (DPR) proteins, which are aggregation-prone and may contribute to neurodegeneration. We used the CRISPR-Cas9 system to perform genome-wide gene knockout screens for suppressors and enhancers of C9orf72 DPR toxicity in human cells. We validated hits by performing secondary CRISPR-Cas9 screens in primary mouse neurons. We uncovered potent modifiers of DPR toxicity whose gene products function in nucleocytoplasmic transport, the endoplasmic reticulum (ER), proteasome, RNA processing pathways, and in chromatin modification. One modifier, TMX2, modulated the ER-stress signature elicited by C9orf72 DPRs in neurons, and improved survival of human induced motor neurons from C9orf72 ALS patients. Together, this work demonstrates the promise of CRISPR-Cas9 screens to define mechanisms of neurodegenerative diseases.

Summary Phagocytosis is required for a broad range of physiological functions, from pathogen defense to tissue homeostasis, but mechanisms required for phagocytosis of diverse substrates remain incompletely understood. Here, we develop a rapid magnet-based phenotypic screening strategy, and perform eight genome-wide CRISPR screens in human cells to identify genes regulating phagocytosis of distinct substrates. After validating select hits in focused mini-screens, orthogonal assays and primary human macrophages, we demonstrate that 1) the previously-uncharacterized gene NHLRC2 is a central player in phagocytosis, regulating RhoA-Rac1 signaling cascades that control actin polymerization and filopodia formation, 2) very long chain fatty acids are essential for efficient phagocytosis of certain substrates, and 3) the previously-uncharacterized Alzheimer’s disease-associated gene TM2D3 can preferentially influence uptake of amyloid-β aggregates. These findings illuminate new regulators and core principles of phagocytosis, and more generally establish an efficient method for unbiased identification of cellular uptake mechanisms across diverse physiological and pathological contexts.

SummaryB cells constitute an essential line of defense from pathogenic infections through the generation of class-switched antibody-secreting cells (ASCs) in germinal centers. Although this process is known to be regulated by follicular helper T (TfH) cells, the mechanism by which B cells initially seed germinal center reactions remains elusive. We found that NKT cells, a population of innate-like T lymphocytes, are critical for the induction of B cell immunity upon viral infection. The positioning of NKT cells at the interfollicular areas of lymph nodes facilitates both their direct priming by resident macrophages and the localized delivery of innate signals to antigen-experienced B cells. Indeed, NKT cells secrete an early wave of IL-4 and constitute up to 70% of the total IL-4-producing cells during the initial stages of infection. Importantly, the requirement of this innate immunity arm appears to be evolutionarily conserved because early NKT and IL-4 gene signatures also positively correlate with the levels of neutralizing antibodies in Zika-virus-infected macaques. In conclusion, our data support a model wherein a pre-TfH wave of IL-4 secreted by interfollicular NKT cells triggers the seeding of germinal center cells and serves as an innate link between viral infection and B cell immunity.