Progenitor Cell Behaviors That Influence the Synchronization of Cycling Gene Expression

Mesodermal progenitors display several cell biological features that may be responsible for providing robustness to the segmentation process. Each individual presomitic cell functions as an autonomous oscillating unit, with cycling her/Hes gene expression that is part of the somite clock (Jiang et al., 2000). For proper segmentation to occur, neighboring presomitic cells must have synchronized oscillations. These oscillations are synchronized by Notch/Delta signaling, as her/Hes genes are targets of notch signaling (Jiang et al., 2000). The Notch receptor and Delta ligand are both transmembrane proteins, such that cells must be juxtaposed to each other for signaling to occur (called juxtacrine signaling) (Kopan and Hagan, 2009). As previously mentioned, when cells enter the partial EMT state as they transition from NMP to mesodermal progenitor, they undergo a rapid mixing process with nondirectional movement (Goto et al., 2017; Lawton et al., 2013; Manning and Kimelman, 2015; Row et al., 2011). The reason for this random mixing is unknown, but it is hypothesized to rapidly synchronize gene expression oscillations in a group of cells through neighbor exchange-mediated Notch/Delta signaling (Uriu and Morelli, 2017). Disruption of Notch/Delta signaling causes asynchronous oscillations of clock gene expression and the subsequent failure of segment formation (Jiang et al., 2000). When Notch signaling is inhibited with small molecule drugs, somitogenesis fails to occur properly. Segmentation can recover after the drug is washed out and signaling is restored (Riedel-Kruse et al., 2007). In prior modeling simulations, recovery of segment formation after a transient Notch signaling inhibition did not occur until after tens to hundreds of oscillations, whereas in real embryos, segmentation recovers after just ten oscillations. When rapid random cell mixing of mesodermal progenitors is incorporated into the model, recovery of segmentation occurs with the same timeframe as real embryonic recovery (Uriu et al., 20Ю). Further modeling of cell mixing in the zebrafish tailbud using experimentally observed rates of movement reveals that the movement is fast enough to enhance the synchronization of neighboring cells and affect the coherence of oscillating gene expression (Uriu et al., 2017). This suggests that the rapid mixing of NMP-derived mesoderm as it leaves the tailbud is important in synchronizing clock gene oscillations in neighboring cells, allowing for proper somitogenesis (Figure 5.5A).

Another possible mechanism of clock oscillation synchrony relates to the cell cycle behavior of mesodermal progenitors. In zebrafish, post-gastrulation NMPs are arrested in the G2 phase of the cell cycle, and after being induced to become mesoderm, they undergo a synchronous mitotic event (Bouldin et al., 2014) (Figure 5.5B). A transgenic zebrafish line using the herl promoter driving the expression of a destabilized fluorescent protein, which is useful when rapid reporter turnover is necessary, allows for the visualization of endogenous segmentation clock oscillations in vivo (Delaune et al., 2012). Live imaging using this line revealed that oscillations of clock gene expression in daughter cells that have just finished dividing exhibit

Progenitor cell behaviors help synchronize oscillating gene expression

FIGURE 5.5 Progenitor cell behaviors help synchronize oscillating gene expression. (A) Mesodermal progenitors exhibit a period of random mixing, which helps synchronize the oscillatory expression of the her/Hes genes that are part of the segmentation clock. (B) Mesodermal progenitors have also been observed to undergo synchronized mitosis after exiting the G2 arrested NMP state. This may also help to synchronize the oscillatory expression of her/Hes genes between cells through Notch signaling dependent and independent mechanisms.

more highly synchronized clock oscillations relative to neighbor cells. This is true even in embryos where Notch signaling is disrupted, suggesting that this cell-cycle based entrainment is independent of Notch signaling (Delaune et ah, 2012). Further effects of the cell cycle state relate to Notch signaling itself. When the Notch receptor is activated, the intracellular domain (NICD) of the transmembrane receptor is cleaved and enters the nucleus, where it functions as a transcription factor (Kopan and Ilagan, 2009). A recent report showed that the NICD is phosphorylated by the cyclin-dependent kinases CDK1 and CDK2, which facilitates SCF-mediated degradation of NICD (Carrieri et ah, 2019). When cells are in the G1 phase of the cell cycle, where CDK1 and CDK2 activity are low, NICD will be stable which in turn will facilitate signaling. The dependence of NICD activity on cell cycle phase was shown to influence the rate of somitogenesis through control of clock oscillations (Carrieri et ah, 2019). Thus, the enhanced stabilization of NICD during G1 of recently divided daughter cells may help entrain their oscillations. Additionally, this suggests that the prolonged G2 arrest of NMPs in zebrafish could prevent activation of the Notch pathway and premature cycling of the somite clock genes. In agreement with these cell cycle-related phenomena, experimental disruptions of the cell cycle impact the somitogenesis process. Zebrafish emil mutants, which fail to progress from G2 to M phase throughout the embryo after gastrulation, have asynchronous herl expression and disorganized somite border formation (Zhang et al., 2008). Together these studies indicate that the synchronous division of mesodermal progenitors before they join the presomitic mesoderm may help to synchronize clock oscillations in the newly induced group of mesodermal progenitors.


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