Academic journal article Genetics

Establishment of Signaling Interactions with Cellular Resolution for Every Cell Cycle of Embryogenesis

Academic journal article Genetics

Establishment of Signaling Interactions with Cellular Resolution for Every Cell Cycle of Embryogenesis

Article excerpt

SYMMETRY breaking in cell division timing and cell fate specification has long been a focus of developmental biology. Intercellular signaling plays a key role in breaking these symmetries (Yochem et al. 1988; Clevers and Nusse 2012; Sawa 2012; Greenwald 2013; Zacharias et al. 2015), although maternal control is critical for establishing polarity during early development (Rose and Gonczy 2014). For example, a Notch signaling interaction is necessary for fate asymmetry between cells ABa and ABp (Mickey et al. 1996; Priess 2005), whereas a Wnt interaction is required for both fate asymmetry and division asynchrony between cells EMS and P2 in a four-cell Caenorhabditis elegans embryo (Rocheleau et al. 1997). The Notch interaction is achieved by a contact between the P2 cell, which expresses a Notch ligand called apx-1, and the ABp but not the ABa cell, although both the latter cells express the Notch receptor glp-1 (Mickey et al. 1996). This demonstrates that a contact between cells is essential for triggering a signaling interaction to drive differential fate specification (Good et al. 2004). A similar scenario is observed for the Wnt interaction between the EMS and P2 cells, which is necessary for asymmetric division of the former into MS and E cells during C. elegans embryogenesis (Goldstein 1992; Rocheleau et al. 1997). Notably, the two pathways are used repeatedly throughout development in a cellular context-dependent fashion to establish further asymmetries in fate specification or division timing (Huang etal. 2007; Zacharias etal. 2015). For example, in a 12-cell C. elegans embryo, the four great-granddaughters of AB express the Notch receptor GLP-1, but only two of them, i.e., ABalp and ABara, are in contact with a Notch ligandexpressing cell, MS, leading to their differential differentiation into mesodermal and ectodermal fates, respectively (Hutter and Schnabel 1994; Shelton and Bowerman 1996). Importantly, signaling interactions from the same pathway may have an opposite consequence depending on their timing or cellular context. For example, the first Notch interaction inactivates its targets, tbx-37/38 (Good et al. 2004); whereas the second one activates its targets including PHA-4, a FoxA transcription factor required for pharynx organogenesis (Priess 2005). These time-dependent signaling events indicate that dissecting signaling interactions with precise spatial and temporal resolution would be essential for a thorough understanding of symmetry breaking during metazoan development.

One of the biggest challenges in defining a signaling interaction during embryogenesis is the establishment of cell identity, especially in an embryo with a large number of cells (Keller et al. 2008; Zacharias and Murray 2016). Another challenge is that one must have access to the cellular expression patterns of signaling molecules for each cell cycle. These requirements inhibit the functional characterization of cellular signaling during rapid development. This is because defining a signaling interaction requires knowledge of the identities of cell pairs that are in contact with each other, with one expressing a ligand and the other a receptor.

The development of cell-tracking techniques using timelapse three-dimensional (3D) [hereafter referred to as fourdimensional (4D)] microscopy has greatly facilitated cell lineage analysis (Schnabel et al. 1997, 2006, Zhao et al. 2008, 2010a; Muzzey and van Oudenaarden 2009). In particular, a recently developed automated lineaging technique allows routine tracing of cell division and single-cell expression profiling in a C. elegans embryo with up to 350 cells within approximately half an hour, and up to the last round of cell division of embryogenesis in ~1 day (Bao et al. 2006; Murray et al. 2008; Richards et al. 2013; Du et al. 2014; Shah et al. 2017). This technique makes it possible to infer signaling interactions at cellular resolution for every cell cycle (Figure 1) because the output of automated lineaging contains quantitative positional information for nuclei of all cells for every minute during embryogenesis, thus allowing systematic modeling of cell contacts with exceptional spatial and temporal resolution. …

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