In vitro models of the segmentation clock reveal metabolic regulation of developmental rate

Abstract

How species-specific rates of development are set represents a major unanswered question in developmental biology. This gap in our knowledge stems in large part from a lack of simple assays to precisely measure developmental rate in different species. To address this fundamental question, we first established an in vitro system that recapitulates the two-fold difference in developmental rate between early mouse and human embryos. This system relies on the differentiation of pluripotent stem cells towards presomitic mesoderm, an embryonic cell type that harbors a molecular oscillator known as the segmentation clock. This clock underlies the rhythmic production of vertebral precursors (somites) and its period provides a high-resolution, quantitative proxy for developmental rate. Here we show that in vitro-derived presomitic mesoderm cells undergo segmentation clock oscillations with species-specific periods corresponding to 2.5 hours in mouse cells and 5 hours in human cells. Importantly, this work represented the first direct visualization of the human segmentation clock.

Using this in vitro segmentation clock system, we directly compared mouse and human presomitic mesoderm cells to identify factors that control developmental speed. This analysis revealed that mass-specific metabolic rates scale with developmental rate and are therefore elevated two-fold in mouse cells compared to human cells. Partially reducing metabolic rates by inhibiting the electron transport chain slowed down the segmentation clock. This effect was mediated by the cellular NAD+/NADH redox balance. Further downstream, metabolism modulated the global rate of protein synthesis, which was found to operate approximately twice as fast in mouse cells as in human cells. These studies therefore revealed that metabolism works upstream of translation speed to control pace of the segmentation clock. Our findings represent a starting point for the understanding and manipulation of developmental rate, which would find multiple translational applications including the acceleration of human pluripotent stem cell differentiation for disease modeling and cell-based therapies.