Defining Essential Enhancers for Pluripotent Stem Cells Using a Features-Oriented CRISPR-Cas9 Screen.

cis-regulatory elements (CREs) regulate the expression of genes in their genomic neighborhoods and influence cellular processes such as cell-fate maintenance and differentiation. To date, there remain major gaps in the functional characterization of CREs and the identification of their target genes in the cellular native environment. In this study, we perform a features-oriented CRISPR-utilized systematic (FOCUS) screen of OCT4-bound CREs using CRISPR-Cas9 to identify functional enhancers important for pluripotency maintenance in mESCs. From the initial 235 candidates tested, 16 CREs are identified to be essential stem cell enhancers. Using RNA-seq and genomic 4C-seq, we further uncover a complex network of candidate CREs and their downstream target genes, which supports the growth and self-renewal of mESCs. Notably, an essential enhancer, CRE111, and its target, Lrrc31, form the important switch to modulate the LIF-JAK1-STAT3 signaling pathway.

Unraveling Heterogeneity in Transcriptome and Its Regulation Through Single-Cell Multi-Omics Technologies.

Cellular heterogeneity plays a pivotal role in tissue homeostasis and the disease development of multicellular organisms. To deconstruct the heterogeneity, a multitude of single-cell toolkits measuring various cellular contents, including genome, transcriptome, epigenome, and proteome, have been developed. More recently, multi-omics single-cell techniques enable the capture of molecular footprints with a higher resolution by simultaneously profiling various cellular contents within an individual cell. Integrative analysis of multi-omics datasets unravels the relationships between cellular modalities, builds sophisticated regulatory networks, and provides a holistic view of the cell state. In this review, we summarize the major developments in the single-cell field and review the current state-of-the-art single-cell multi-omic techniques and the bioinformatic tools for integrative analysis.

Parallel bimodal single-cell sequencing of transcriptome and chromatin accessibility.

Joint profiling of transcriptome and chromatin accessibility within single cells allows for the deconstruction of the complex relationship between transcriptional states and upstream regulatory programs determining different cell fates. Here, we developed an automated method with high sensitivity, assay for single-cell transcriptome and accessibility regions (ASTAR-seq), for simultaneous measurement of whole-cell transcriptome and chromatin accessibility within the same single cell. To show the utility of ASTAR-seq, we profiled 384 mESCs under naive and primed pluripotent states as well as a two-cell like state, 424 human cells of various lineage origins (BJ, K562, JK1, and Jurkat), and 480 primary cord blood cells undergoing erythroblast differentiation. With the joint profiles, we configured the transcriptional and chromatin accessibility landscapes of discrete cell states, uncovered linked sets of cis-regulatory elements and target genes unique to each state, and constructed interactome and transcription factor (TF)-centered upstream regulatory networks for various cell states.

Global H3.3 dynamic deposition defines its bimodal role in cell fate transition.

H3.3 is a histone variant, which is deposited on genebodies and regulatory elements, by Hira, marking active transcription. Moreover, H3.3 is deposited on heterochromatin by Atrx/Daxx complex. The exact role of H3.3 in cell fate transition remains elusive. Here, we investigate the dynamic changes in the deposition of the histone variant H3.3 during cellular reprogramming. H3.3 maintains the identities of the parental cells during reprogramming as its removal at early time-point enhances the efficiency of the process. We find that H3.3 plays a similar role in transdifferentiation to hematopoietic progenitors and neuronal differentiation from embryonic stem cells. Contrastingly, H3.3 deposition on genes associated with the newly reprogrammed lineage is essential as its depletion at the later phase abolishes the process. Mechanistically, H3.3 deposition by Hira, and its K4 and K36 modifications are central to the role of H3.3 in cell fate conversion. Finally, H3.3 safeguards fibroblast lineage by regulating Mapk cascade and collagen synthesis.

PRDM15 safeguards naive pluripotency by transcriptionally regulating WNT and MAPK-ERK signaling.

The transcriptional network acting downstream of LIF, WNT and MAPK-ERK to stabilize mouse embryonic stem cells (ESCs) in their naive state has been extensively characterized. However, the upstream factors regulating these three signaling pathways remain largely uncharted. PR-domain-containing proteins (PRDMs) are zinc-finger sequence-specific chromatin factors that have essential roles in embryonic development and cell fate decisions. Here we characterize the transcriptional regulator PRDM15, which acts independently of PRDM14 to regulate the naive state of mouse ESCs. Mechanistically, PRDM15 modulates WNT and MAPK-ERK signaling by directly promoting the expression of Rspo1 (R-spondin1) and Spry1 (Sprouty1). Consistent with these findings, CRISPR-Cas9-mediated disruption of PRDM15-binding sites in the Rspo1 and Spry1 promoters recapitulates PRDM15 depletion, both in terms of local chromatin organization and the transcriptional modulation of these genes. Collectively, our findings uncover an essential role for PRDM15 as a chromatin factor that modulates the transcription of upstream regulators of WNT and MAPK-ERK signaling to safeguard naive pluripotency.