Functional consequences of somatic polyploidy in development

Chronic replication stress-mediated genomic instability disrupts placenta development in mice

Abnormal placentation drives many pregnancy-related pathologies and poor fetal outcomes, but the underlying molecular causes are understudied. Here, we show that persistent replication stress due to mutations in the MCM2-7 replicative helicase disrupts placentation and reduces embryo viability in mice. MCM-deficient embryos exhibited normal morphology but their placentae had a drastically diminished junctional zone (JZ). Whereas cell proliferation in the labyrinth zone (LZ) remained unaffected, JZ cell proliferation was reduced, independent of cell death during early development. Mouse trophoblast stem cells (TSCs) with high genomic instability failed to maintain stemness, suggesting that replication stress affects the initial trophoblast progenitor pool in a manner that preferentially impacts the developing JZ. Genetically increasing chromatin-bound MCM levels in the mutants rescued placental defects and embryo viability. Developing female mice deficient for FANCM, a protein involved in replication-associated DNA repair, also had placentae with a diminished JZ. These findings indicate that replication stress-induced genomic instability compromises embryo outcomes by impairing placentation.

Somatic polyploidy supports biosynthesis and tissue function by increasing transcriptional output

Cell size and biosynthetic capacity generally increase with increased DNA content. Somatic polyploidy has therefore been proposed to be an adaptive strategy to increase cell size in specialized tissues with high biosynthetic demands. However, if and how DNA concentration limits cellular biosynthesis in vivo is not well understood. Here, we show that polyploidy in the Caenorhabditis elegans intestine is critical for cell growth and yolk biosynthesis, a central role of this organ. Artificially lowering the DNA/cytoplasm ratio by reducing polyploidization in the intestine gave rise to smaller cells with dilute mRNA. Highly expressed transcripts were more sensitive to this mRNA dilution, whereas lowly expressed genes were partially compensated-in part by loading more RNA Polymerase II on the remaining genomes. Polyploidy-deficient animals produced fewer and slower-growing offspring, consistent with reduced synthesis of highly expressed yolk proteins. DNA-dilute cells had normal total protein concentration, which we propose is achieved by increasing the expression of translational machinery at the expense of specialized, cell-type-specific proteins.

Acute inflammation induces acute megakaryopoiesis with impaired platelet production during fetal hematopoiesis

Hematopoietic development is tightly regulated by various factors. The role of RNA m6A modification during fetal hematopoiesis, particularly in megakaryopoiesis, remains unclear. Here, we demonstrate that loss of m6A methyltransferase METTL3 induces formation of double-stranded RNAs (dsRNAs) and activates acute inflammation during fetal hematopoiesis in mouse. This dsRNA-mediated inflammation leads to acute megakaryopoiesis, which facilitates the generation of megakaryocyte progenitors but disrupts megakaryocyte maturation and platelet production. The inflammation and immune response activate the phosphorylation of STAT1 and IRF3, and upregulate downstream interferon-stimulated genes (ISGs). Inflammation inhibits the proliferation rate of hematopoietic progenitors and further skews the cell fate determination toward megakaryocytes rather than toward erythroid from megakaryocyte-erythroid progenitors (MEPs). Transcriptional-wide gene expression analysis identifies IGF1 as a major factor whose reduction is responsible for the inhibition of megakaryopoiesis and thrombopoiesis. Restoration of IGF1 with METTL3-deficient hematopoietic cells significantly increases megakaryocyte maturation. In summary, we elucidate that the loss of RNA m6A modification-induced acute inflammation activates acute megakaryopoiesis, but impairs its final maturation through the inhibition of IGF1 expression during fetal hematopoiesis.

Unusual modes of cell and nuclear divisions characterise Drosophila development

The growth and development of metazoan organisms is dependent upon a co-ordinated programme of cellular proliferation and differentiation, from the initial formation of the zygote through to maintenance of mature organs in adult organisms. Early studies of proliferation of ex vivo cultures and unicellular eukaryotes described a cyclic nature of cell division characterised by periods of DNA synthesis (S-phase) and segregation of newly synthesized chromosomes (M-phase) interspersed by seeming inactivity, the gap phases, G1 and G2. We now know that G1 and G2 play critical roles in regulating the cell cycle, including monitoring of favourable environmental conditions to facilitate cell division, and ensuring genomic integrity prior to DNA replication and nuclear division. M-phase is usually followed by the physical separation of nascent daughters, termed cytokinesis. These phases where G1 leads to S phase, followed by G2 prior to M phase and the subsequent cytokinesis to produce two daughters, both identical in genomic composition and cellular morphology are what might be termed an archetypal cell division. Studies of development of many different organs in different species have demonstrated that this stereotypical cell cycle is often subverted to produce specific developmental outcomes, and examples from over 100 years of analysis of the development of Drosophila melanogaster have uncovered many different modes of cell division within this one species.

Tissue-Level Integration Overrides Gradations of Differentiating Cell Identity in Beetle Extraembryonic Tissue

During animal embryogenesis, one of the earliest specification events distinguishes extraembryonic (EE) from embryonic tissue fates: the serosa in the case of the insects. While it is well established that the homeodomain transcription factor Zen1 is the critical determinant of the serosa, the subsequent realization of this tissue's identity has not been investigated. Here, we examine serosal differentiation in the beetle Tribolium castaneum based on the quantification of morphological and morphogenetic features, comparing embryos from a Tc-zen1 RNAi dilution series, where complete knockdown results in amnion-only EE tissue identity. We assess features including cell density, tissue boundary morphology, and nuclear size as dynamic readouts for progressive tissue maturation. While some features exhibit an all-or-nothing outcome, other key features show dose-dependent phenotypic responses with trait-specific thresholds. Collectively, these findings provide nuance beyond the known status of Tc-Zen1 as a selector gene for serosal tissue patterning. Overall, our approach illustrates how the analysis of tissue maturation dynamics from live imaging extends but also challenges interpretations based on gene expression data, refining our understanding of tissue identity and when it is achieved.

Somatic polyploidy supports biosynthesis and tissue function by increasing transcriptional output

Cell size and biosynthetic capacity generally increase with increased DNA content. Polyploidy has therefore been proposed to be an adaptive strategy to increase cell size in specialized tissues with high biosynthetic demands. However, if and how DNA concentration limits cellular biosynthesis in vivo is not well understood, and the impacts of polyploidy in non-disease states is not well studied. Here, we show that polyploidy in the C. elegans intestine is critical for cell growth and yolk biosynthesis, a central role of this organ. Artificially lowering the DNA/cytoplasm ratio by reducing polyploidization in the intestine gave rise to smaller cells with more dilute mRNA. Highly-expressed transcripts were more sensitive to this mRNA dilution, whereas lowly-expressed genes were partially compensated - in part by loading more RNA Polymerase II on the remaining genomes. DNA-dilute cells had normal total protein concentration, which we propose is achieved by increasing production of translational machinery at the expense of specialized, cell-type specific proteins.