Single-cell analysis of cardiogenesis reveals basis for organ-level developmental defects.

Organogenesis involves integration of diverse cell types; dysregulation of cell-type-specific gene networks results in birth defects, which affect 5% of live births. Congenital heart defects are the most common malformations, and result from disruption of discrete subsets of cardiac progenitor cells1, but the transcriptional changes in individual progenitors that lead to organ-level defects remain unknown. Here we used single-cell RNA sequencing to interrogate early cardiac progenitor cells as they become specified during normal and abnormal cardiogenesis, revealing how dysregulation of specific cellular subpopulations has catastrophic consequences. A network-based computational method for single-cell RNA-sequencing analysis that predicts lineage-specifying transcription factors2,3 identified Hand2 as a specifier of outflow tract cells but not right ventricular cells, despite the failure of right ventricular formation in Hand2-null mice4. Temporal single-cell-transcriptome analysis of Hand2-null embryos revealed failure of outflow tract myocardium specification, whereas right ventricular myocardium was specified but failed to properly differentiate and migrate. Loss of Hand2 also led to dysregulation of retinoic acid signalling and disruption of anterior-posterior patterning of cardiac progenitors. This work reveals transcriptional determinants that specify fate and differentiation in individual cardiac progenitor cells, and exposes mechanisms of disrupted cardiac development at single-cell resolution, providing a framework for investigating congenital heart defects

Investigating the role of SCAR-Arp2/3 dependent cortical remodelling in asymmetrically dividing Drosophila neuroblasts

Division is one of the most important events in the life of a cell, which ensures that the genetic material and the entire set of cellular components segregate in the correct way between the two daughter cells. To divide, cells have to undergo profound shape changes in a timely manner. While the actin cortex is known to control the changes in cell shape that accompany division, much remains to be discovered about the molecular and cellular mechanisms that control cortical remodelling and that coordinate asymmetric stem cell divisions. In this work, I identify the SCAR-Arp2/3 pathway as a potential new regulator of the polarised shape changes in anaphase that help drive asymmetric stem cell divisions in Drosophila melanogaster. I show that filopodia-like membrane protrusions are found at the apical cortex in metaphase and their organization is dependent on Arp2/3. Interestingly, SCAR localizes preferentially at the apical cortex of neural stem cells, and both SCAR and the membrane protrusions disappear from the apical cortex as cells undergo cortical expansion when they enter anaphase. Finally, cells depleted of SCAR or the Arp2/3 complex show a disorganized microtubule spindle and cortical defects at the end of mitosis, suggesting a role for Arp2/3 in stabilizing cortical shape and tension at the metaphase-anaphase transition. This is surprising as the branched actin network nucleated by Arp2/3 is known for its role in trafficking, motility of organelles and cell migration, rather than in cell division, which depends on Formin-based actin nucleation. Through this work I propose a role for the SCAR-Arp2/3 pathway in maintaining proper cortical organization in the dividing neuroblasts to aid proper asymmetric division, hence suggesting a new cellular mechanism that contributes to asymmetric cell division in the Drosophila neural stem cells