Differential Axial Requirements for Lunatic Fringe and Hes7 Transcription during Mouse Somitogenesis

Abstract

Vertebrate segmentation is regulated by the “segmentation clock”, which drives cyclic expression of several genes in the caudal presomitic mesoderm (PSM). One such gene is Lunatic fringe (Lfng), which encodes a modifier of Notch signalling, and which is also expressed in a stripe at the cranial end of the PSM, adjacent to the newly forming somite border. We have investigated the functional requirements for these modes of Lfng expression during somitogenesis by generating mice in which Lfng is expressed in the cranial stripe but strongly reduced in the caudal PSM, and find that requirements for Lfng activity alter during axial growth. Formation of cervical, thoracic and lumbar somites/vertebrae, but not sacral and adjacent tail somites/vertebrae, depends on caudal, cyclic Lfng expression. Indeed, the sacral region segments normally in the complete absence of Lfng and shows a reduced requirement for another oscillating gene, Hes7, indicating that the architecture of the clock alters as segmentation progresses. We present evidence that Lfng controls dorsal-ventral axis specification in the tail, and also suggest that Lfng controls the expression or activity of a long-range signal that regulates axial extension.

The oscillation of Notch activation, but not its boundary, is required for somite border formation and rostral-caudal patterning within a somite

“…The authors of these reports produced transgenic mice harboring Lfng expression without oscillation. Their data indicate that cNICD oscillation is disrupted, but that normal segmented somites form, after E10.5 and they concluded that oscillating Lfng expression is required only for early stage, but not later stage, somitogenesis (Shifley et al, 2008;Stauber et al, 2009). We wish therefore to discuss some possible explanations for the discrepancies between our current findings and these previous experimental results.…”

“…It is reasonable to assume that the requirement of Notch clock oscillation by Lfng changes during somitogenesis and is lesser at later stages, as now suggested by a number of studies (Shifley et al, 2008;Stauber et al, 2009). …”

“…Another possibility is that the slightly oscillating expression reported previously might be responsible for the rescue event. Previous studies have shown that two distinct enhancers are involved in the oscillatory expression of Lfng, one of which is disrupted in the mouse reported by Shifley et al (Shifley et al, 2008), and a slight cyclic expression of Lfng exists in the mouse generated by Stauber et al (Stauber et al, 2009). Hence, one possible interpretation for these discrepancies is that the slight cyclic expression of Lfng might be sufficient for normal development in the enhancer-specific…”

“…However, this result will probably be viewed somewhat controversially given the recent findings that have underscored the significance of Lfng expression in the anterior PSM during this process, at least after E10.5 (Shifley et al, 2008;Stauber et al, 2009). The authors of these reports produced transgenic mice harboring Lfng expression without oscillation.…”

“…However, a recent study has shown that mouse embryos expressing Notch activity throughout the PSM still show the ability to form more than ten somites, indicating that the Notch on/off state is dispensable for boundary formation (Feller et al, 2008). By contrast, however, other studies have reported that transgenic mice expressing Lfng only in the anterior PSM show normal segmental border formation after embryonic day 10.5 (E10.5), suggesting that the Notch on-off state generated in the anterior PSM is sufficient to create a somite boundary at least in the later stage embryos (Shifley et al, 2008;Stauber et al, 2009).…”