Development of the vertebral morphogenetic field in the mouse: Interactions between Crossveinless-2 and Twisted Gastrulation

Abstract

Crossveinless-2 (Cv2), Twisted Gastrulation (Tsg) and Chordin (Chd) are components of an extracellular biochemical pathway that regulates Bone Morphogenetic Protein (BMP) activity during dorso-ventral patterning of Drosophila and Xenopus embryos, the formation of the fly wing, and mouse skeletogenesis. Because the nature of their genetic interactions remained untested in the mouse, we generated a null allele for Cv2 which was crossed to Tsg and Chd mutants to obtain Cv2;Tsg and Cv2;Chd compound mutants. We found that Cv2 is essential for skeletogenesis as its mutation caused the loss of multiple bone structures and posterior homeotic transformation of the last thoracic vertebra. During early vertebral development, Smad1 phosphorylation in the intervertebral region was decreased in the Cv2 mutant, even though CV2 protein is normally located in the future vertebral bodies. Because Cv2 mutation affects BMP signaling at a distance, this suggested that CV2 is involved in the localization of the BMP morphogenetic signal. Cv2 and Chd mutations did not interact significantly. However, mutation of Tsg was epistatic to all CV2 phenotypes. We propose a model in which CV2 and Tsg participate in the generation of a BMP signaling morphogenetic field during vertebral formation in which CV2 serves to concentrate diffusible Tsg/BMP4 complexes in the vertebral body cartilage.

Position matters: Variability in the spatial pattern of BMP modulators generates functional diversity

“…Thus, a shuttle mechanism directs BMPs to the pcv, while Cv-2 acts to concentrate BMP activity to a restricted domain far from the source of dpp expression. A similar role has been suggested in the mouse, where Cv-2 concentrates high levels of BMP activity during vertebrae formation (Zakin et al, 2008).…”

The extracellular regulation of bone morphogenetic protein signaling

“…The sink hypothesis might also explain some otherwise confounding results from mouse vertebral development. The levels of BMPs, Chordin, Tsg and BMP signaling are all initially highest in the condensed mesenchyme, the precursor of the intervertebral disc, whereas Cv2 levels are high in the complementary uncondensed mesenchyme, the precursor to the vertebral body (Zakin et al, 2008). Oddly, Cv2 knockouts reduced BMP signaling in the condensed mesenchyme, where Cv2 is not normally detected.…”

“…This leads to exchange of BMPs between Cv2 and the receptor; depending on the Cv2 concentration and binding affinities, this exchange either sequesters BMPs from the receptors, or provides BMPs for the receptors. Another model, while agreeing that Cv2 inhibits signaling by sequestering BMPs, suggests that Cv2 increases signaling by a mechanism that is independent of BMP binding , perhaps via interactions with Chordin/Sog or Tsg (Ambrosio et al, 2008;Zakin et al, 2008). It is entirely possible that both models are correct.…”

“…But Cv2 proteins are not simple co-receptors, as they can also inhibit signaling. Cv2 regulates BMP signaling in early axis formation in Xenopus (Ambrosio et al, 2008), as well as axis formation, hematopoiesis and vascular development in zebrafish (Moser et al, 2007;Rentzsch et al, 2006), neural crest formation in chick (Coles et al, 2004) and skeletogenesis in mice (Ikeya et al, 2006;Ikeya et al, 2008;Zakin et al, 2008). But the direction of that modulation varies, even in contexts that appear homologous (e.g.…”

“…The binding of Cv2 and Chordin/Sog cannot mediate all of the effects of Cv2 because, in zebrafish, Cvl2 can still promote signaling in the absence of Chordin (Rentzsch et al, 2006). Nonetheless, it is possible that some aspects of Cv2 activity are mediated by changes in the levels or activity of Chordin/Sog or Tsg in the extracellular space (see Ambrosio et al, 2008;Serpe et al, 2008;Zakin et al, 2008).…”