Abstract
Cohesin is a multi-subunit protein with roles in both cell division and gene expression. It comprises three core subunits: Smc1a, Smc3 and Rad21, which form a ring-shaped complex that incorporates either Stag1 or Stag2. Mutations in genes encoding smc1a, smc3 and rad21 are homozygous lethal because they disrupt the formation of the cohesin ring and block cell division. Unlike in mice and humans, homozygous mutations in stag genes are viable in zebrafish, likely because the paralogues can functionally substitute for each other in the cohesin complex. In this study, the predominant Stag1 (Stag1b) and Stag2 (Stag2b) proteins in zebrafish were identified by investigating their combined loss, which resulted in lethality and resembled embryos with mutations in core cohesin subunits like rad21.
The zebrafish embryonic tailbud represents an ideal model to study cell fate commitment from multipotent progenitors. Neuromesodermal progenitors (NMPs) are bipotent tailbud cells that differentiate into neuroectoderm or mesoderm under the control of Wnt signalling. Additionally, bipotent midline progenitor cells (MPCs) contribute to the formation of the floor plate, notochord, and hypochord under the control of Wnt and Notch signalling pathways.
Zebrafish carrying mutations in rad21 and stag2b were used to delineate multiple roles of cohesin in development using an embryonic tailbud model. I performed RNA-sequencing of wild type, stag2b and rad21 mutant tailbuds to investigate the consequences of cohesin mutations for cell fate decisions in tailbud progenitors. Transcription dysregulation in rad21 mutants reflects disruption of mesoderm formation that could be explained by tbxta and sox2 dysregulation in NMPs. In contrast, stag2b mutants showed potential compensation for tbxta dysregulation through upregulation of Wnt signalling. Alternatively, MPCs might be differentiating into paraxial mesoderm in stag2b mutants as apparent from narrow notochords and downregulation of midline progenitor markers, including noto.
CUT&RUN revealed that homozygous loss of stag2b leads to a decrease in Rad21 cohesin binding but does not affect the distribution of the H3K27me3 mark deposited by the Polycomb Repressor Complex. Decreased cohesin occupancy could explain why loss of Stag2b affects the regulation of gene expression in tailbud cells.
Overall, my results show that developmental outcomes depend on which cohesin subunit is mutated, implying that individual cohesin subunits modulate how cohesin functions in development.