Highlights d High-resolution transcriptome map of 40,000 cells from developing human brain d Cell-type-specific transcription factor (TF) expression and TF-gene networks d Defines intermediate cell transition states during early neurogenesis d Implicates specific cell types in neuropsychiatric disorders

Summary Naïve human embryonic stem cells (hESCs) can be derived from primed hESCs or directly from blastocysts, but their X-chromosome state has remained unresolved. Here, we show that the inactive X-chromosome (Xi) of primed hESCs was reactivated in naïve culture conditions. Like cells of the blastocyst, the resulting naive cells contained two active X-chromosomes with XIST expression and chromosome-wide transcriptional dampening, and initiated XIST-mediated X-inactivation upon differentiation. Both establishment of and exit from the naïve state (differentiation) happened via an XIST-negative XaXa intermediate. Together, these findings identify a cell culture system for functionally exploring the two X-chromosome dosage compensation processes in early human development: X-dampening and X-inactivation. However, remaining differences between naïve hESCs and embryonic cells related to monoallelic XIST expression and non-random X-inactivation highlight the need for further culture improvement. As the naïve state reset Xi abnormalities seen in primed hESCs it may provide cells better suited for downstream applications.

SUMMARY Applications of ESCs require faithful chromatin changes during differentiation but the fate of each X-chromosome-state in differentiating ESCs is unclear. Female human ESC-lines either carry two active X-chromosomes (XaXa), an Xa and inactive-X-chromosome with or without XIST-RNA-coating (XiXIST+Xa;XiXa), or an Xa and an eroded-Xi (XeXa) where the Xi no longer expresses XIST-RNA and has partially reactivated. Here, we established XiXa, XeXa, and XaXa ESC-lines and followed their X-chromosome-state during differentiation. Surprisingly, we found that the X-state pre-existing in primed ESCs is maintained in differentiated cells. Consequently, differentiated XeXa and XaXa cells lacked XIST, did not initiate X-inactivation, and displayed higher X-linked gene-expression than XiXa cells. These results demonstrate that X-chromosome-dosage-compensation is not required for ESC differentiation. Our data imply that XiXIST+Xa ESCs are most suited for downstream applications and show that all other X-states are abnormal byproducts of our ESC-derivation and propagation methods.

Highlights d Human atlas of limb skeletal muscle in embryonic, fetal, and adult tissues d Human limb skeletal muscle populations and supportive cells vary across development d PAX7 muscle progenitor and stem cells are not identical across developmental states d hPSC-PAX7 cells align to the embryonic-to-fetal transition in human development

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Viruses hijack host cell metabolism to acquire the building blocks required for replication. Understanding how SARS-CoV-2 alters host cell metabolism may lead to potential treatments for COVID-19. Here we profile metabolic changes conferred by SARS-CoV-2 infection in kidney epithelial cells and lung air-liquid interface (ALI) cultures, and show that SARS-CoV-2 infection increases glucose carbon entry into the TCA cycle via increased pyruvate carboxylase expression. SARS-CoV-2 also reduces oxidative glutamine metabolism while maintaining reductive carboxylation. Consistent with these changes, SARS-CoV-2 infection increases the activity of mTORC1 in cell lines and lung ALI cultures. Lastly, we show evidence of mTORC1 activation in COVID-19 patient lung tissue, and that mTORC1 inhibitors reduce viral replication in kidney epithelial cells and lung ALI cultures. Our results suggest that targeting mTORC1 may be a feasible treatment strategy for COVID-19 patients, although further studies are required to determine the mechanism of inhibition and potential efficacy in patients.

Current smoking is associated with increased risk of severe COVID-19 but it is not clear how cigarette smoke (CS) exposure affects SARS-CoV-2 airway cell infection. We directly exposed air-liquid interface (ALI) cultures derived from primary human nonsmoker airway basal stem cells (ABSCs) to short term CS and then infected them with SARS-CoV-2. We found an increase in the number of infected airway cells after CS exposure with a lack of ABSC proliferation. Single cell profiling of the cultures showed that the normal interferon response was reduced after CS exposure with infection. Treatment of CS-exposed ALI cultures with Interferon β-1 abrogated the viral infection, suggesting one potential mechanism for more severe viral infection. Our data show that acute CS exposure allows for more severe airway epithelial disease from SARS-CoV-2 by reducing the innate immune response and ABSC proliferation and has implications for disease spread and severity in people exposed to CS.

SummaryA dramatic difference in global DNA methylation between male and female cells characterizes mouse embryonic stem cells (ESCs), unlike somatic cells. We analyzed DNA methylation changes during reprogramming of male and female somatic cells and in resulting induced pluripotent stem cells (iPSCs). At an intermediate reprogramming stage, somatic and pluripotency enhancers are targeted for partial methylation and demethylation. Demethylation within pluripotency enhancers often occurs at ESC binding sites of pluripotency transcription factors. Late in reprogramming, global hypomethylation is induced in a female-specific manner. Genome-wide hypomethylation in female cells affects many genomic landmarks, including enhancers and imprint control regions, and accompanies the reactivation of the inactive X chromosome. The loss of one of the two X chromosomes in propagating female iPSCs is associated with genome-wide methylation gain. Collectively, our findings highlight the dynamic regulation of DNA methylation at enhancers during reprogramming and reveal that X chromosome dosage dictates global DNA methylation levels in iPSCs.