ABSTRACT Somitogenesis is controlled by the segmentation clock, a molecular oscillator that controls periodic gene expression in unsegmented mesoderm and is regulated by a negative feedback loop driven by Hes/Her transcriptional repressors. In zebrafish, Pnrc2 is required for decay of her1 transcripts and additional oscillatory gene transcripts. Despite accumulation of numerous mRNAs, including those encoding developmental regulators, overt embryonic phenotypes are absent in pnrc2 mutants. Our previous work suggested that accumulated mRNAs are not translated in pnrc2 mutants, although the underlying mechanism(s) was unknown. We show here that many overexpressed transcripts in pnrc2 mutants have shortened poly(A) tails and are disengaged from ribosomes, and that deadenylation inhibition leads to somite defects in pnrc2 mutants. Transcripts encoding the P-body-associated protein Ddx61 are overexpressed and engaged with ribosomes in pnrc2 mutants, leading to an increase in Ddx61 protein that may compensate for the negative effects of transcript accumulation. Co-depletion of Ddx61 and Ddx6 in pnrc2 mutants increases her1 accumulation and leads to pleiotropic defects. Together, our results show that multiple post-transcriptional mechanisms ensure proper translation when mRNA decay is inhibited.
The binary cell fate decision between neuroendocrine (NE) and club cell lineages is fundamental to lung development and epithelial homeostasis, yet the mechanisms that maintain these lineages remain incompletely understood. Here, we show that Hes1, a Notch target gene, maintains club cell identity by suppressing neuroendocrine differentiation. Lineage-tracing analyses revealed that Scgb1a1-expressing club cells give rise to NE cells following Hes1 loss during development and adulthood. Single-cell RNA sequencing of endogenous NE (eNE) cells and induced NE (iNE) cells generated by Hes1 loss uncovered substantial heterogeneity within the iNE populations and transitional cells expressing Notch-associated genes and keratins. Lineage tracing further showed that iNE cells integrate into neuroepithelial bodies (NEBs), and iNE cells reconstructed cell-cell communication networks that closely resemble those of eNE cells. These findings indicate that club cells, including Scgb1a1-positive club cell progenitors and mature club cells, retain latent potential to adopt an NE fate, which is normally suppressed by Hes1. Our results identify club cells as a previously unrecognized cellular source of NE cells and reveal a key mechanism that preserves epithelial lineage identity in the lung.
Cell fate determination is fundamental to development. In the Drosophila ovary, a 16-cell germline cyst forms in which two pro-oocytes arise; one of the pro-oocytes is selected as the oocyte, while the remaining 15 cells differentiate into nurse cells. Here, we identify Mini spindles (Msps), a Drosophila XMAP215-family microtubule polymerase, as a key regulator of oocyte fate specification. msps mRNA and Msps protein are enriched in the pro-oocytes prior to oocyte specification. Elimination of Msps prevents oocyte specification and disrupts the accumulation of the oocyte marker Orb, producing egg chambers with 16 nurse cells. Conversely, optogenetic recruitment of Msps increases microtubule polymerization and promotes Orb accumulation in nurse cells, suggesting that Msps activity is sufficient to initiate key molecular events associated with oocyte specification. Furthermore, Msps associates with the germline-specific organelles, the spectrosome and fusome, and becomes asymmetrically distributed, biasing the two pro-oocytes toward oocyte fate. We propose that an Msps–dynein feedback loop amplifies small initial differences between the two pro-oocytes to specify a single oocyte.
ABSTRACT During epithelial remodeling, tissues exhibit solid- or fluid-like properties depending on developmental constraints. In soft matter systems like foams and colloidal glasses, component size influences material mechanics. Here, we investigated how cell and nuclear size affect tissue mechanics. mh mutant embryos have epithelia with doubled cell densities, creating cells with altered 3D aspect ratios. Nuclear volumes scale non-linearly with cell volumes, adopting rounder shapes and reduced apical-basal heights, suggesting that stiff nuclei deform less than cell cortices. These compacted nuclei resemble a jammed state, reducing tissue fluidity and impairing extension movements. Interestingly, oscillatory amplitudes and frequencies in mh cells are comparable to control despite their reduced size, demonstrating an uncoupling between biochemical oscillation machinery and cell dimensions. Additionally, though planar-polarized magnitudes are dampened, the angular assignment of interfacial behaviors is preserved, suggesting that this planar informational system still operates with high fidelity despite reduced cell dimensions. Thus, these results indicate that development has fine-tuned epithelial dimensions to work at time-scales compatible with embryonic requirements.
ABSTRACT The cloaca serves as the only opening for the digestive, urinary and reproductive tracts in multiple species. Apoptosis is crucial for shaping the cloaca, yet the underlying mechanisms remain largely unclarified. Here, we find that Wnt2bb-triggered neighbor cell proliferation is essential for zebrafish cloaca formation by inducing apoptosis in the cloaca, likely through mechanical cell competition. wnt2bb is expressed robustly in the cloaca throughout its formation. wnt2bb inactivation downregulates nuclear accumulation of β-catenin and expression of the downstream target cyclin D1, severely suppressing peri-cloacal cell proliferation. The resulting decline in local cell density is associated with a biosensor-indicated reduction in mechanical compression on the cloacal cells, which prevents their competitive elimination and eventually causes cloacal obstruction. Moreover, palbociclib, a CDK4/6 inhibitor, not only inhibits peri-cloacal cell proliferation, but also abolishes apoptosis in the cloaca, generating similar defects in wild-type embryos. Overexpression of constitutively active β-catenin exclusively in the cells surrounding the cloaca, but not within the cloaca itself, is sufficient to rescue the cloacal obstruction. Thus, we uncover an important role of Wnt2bb-regulated cell competition during cloaca development.
