Noggin-mediated antagonism of BMP signaling is required for growth and patterning of the neural tube and somite

Next Section Abstract Embryonic patterning in vertebrates is dependent upon the balance of inductive signals and their specific antagonists. We show thatNoggin, which encodes a bone morphogenetic protein (BMP) antagonist expressed in the node, notochord, and dorsal somite, is required for normal mouse development. Although Noggin has been implicated in neural induction, examination of null mutants in the mouse indicates that Noggin is not essential for this process. However, Noggin is required for subsequent growth and patterning of the neural tube. Early BMP-dependent dorsal cell fates, the roof plate and neural crest, form in the absence of Noggin. However, there is a progressive loss of early, Sonic hedgehog(Shh)-dependent ventral cell fates despite the normal expression of Shh in the notochord. Further, somite differentiation is deficient in both muscle and sclerotomal precursors. Addition of BMP2 or BMP4 to paraxial mesoderm explants blocks Shh-mediated induction of Pax-1, a sclerotomal marker, whereas addition of Noggin is sufficient to induce Pax-1. Noggin and Shh induce Pax-1 synergistically. Use of protein kinase A stimulators blocks Shh-mediated induction of Pax-1, but not induction by Noggin, suggesting that induction is mediated by different pathways. Together these data demonstrate that inhibition of BMP signaling by axially secreted Noggin is an important requirement for normal patterning of the vertebrate neural tube and somite. Keywords Noggin

somite

neural tube

patterning

Patterning of the vertebrate body axes is dependent upon signals produced by discrete organizing centers. Perhaps the best studied of these is Spemann’s organizer, which encompasses the dorsal lip of the blastopore in the gastrulating amphibian embryo. Organizer signaling is implicated in dorsalization of both mesodermal and ectodermal derivatives. Dorsalization of the mesoderm leads to notochord and somite formation, whereas the dorsalized ectoderm forms neural tissue (for review, see Kessler and Melton 1994; De Robertis and Sasai 1996; Harland and Gerhart 1997). Four signals have been described that are expressed within the organizer and that have dorsalizing activity: Noggin (Smith and Harland 1992), Follistatin (Hemmati-Brivanlou et al. 1994), Chordin (Sasai et al. 1994), and Frzb (Leyns et al. 1997; Wang et al. 1997). None of these share identifiable sequence similarity, but there is evidence to suggest that the first three may act by blocking bone morphogenetic protein (BMP) signaling.

Noggin binds several BMPs with very high (picomolar) affinities, with a marked preference for BMP2 and BMP4 over BMP7. By binding tightly to BMPs, Noggin prevents BMPs from binding their receptors (Zimmerman et al. 1996). Chordin also antagonizes BMP signaling by directly binding BMP proteins, thereby preventing receptor activation (Piccolo et al. 1996). Follistatin binds to Activins, thereby preventing Activin signaling (Nakamura et al. 1990). This antagonism extends to the more distantly related BMPs with a preference for BMP7 over BMP4 (Yamashita et al. 1995; Liem et al. 1997). Thus, a key function of these peptides is to antagonize signaling by distinct members of the TGF-β superfamily.

Examination of postgastrulation stage Xenopus embryos indicates that some of these signaling factors are expressed at later stages. For example, Noggin is expressed in the notochord and dorsal neural tube, suggesting a possible role in the central nervous system (CNS) and somite patterning (Smith and Harland 1992); and in the chick, Noggin expression in the dorsal lip of the somite has been implicated in the control of myogenesis (Marcelle et al. 1997;Reshef et al. 1998). We have addressed the role of Noggin in mouse development. Noggin is not essential for neural induction but is required for normal growth and patterning of the neural tube and somite. Thus, inhibition of endogenous BMP signaling by Noggin is essential for elaboration of the vertebrate body plan.

Previous Section Next Section Acknowledgments We thank the staff in our animal facility, Audrey Huang for assistance in isolating the Noggin genomic clone, Marty Shea for initial in situ hybridization analysis of Nogginexpression, and Scott Lee for generously jumping in at the last minute, Lisa Brunet for the Southern analysis in Fig. 2B, José de Jesus for Noggin protein, Phil Beachy for Shh protein, Genetics Institute for BMP2 and BMP4, and Bianca Klumpar for histology. We thank the following people for gifts of probes: B. Hermann, J. Johnson, M. Goulding, T. Jessell, A. Dudley, B. Hogan, V. Pachnis, A. Joyner, H. Weintraub, E. Olson, C. Wright, P. Koopman, and G. Fischbach. Work in A.P.M’s laboratory was supported by grants from the American Cancer Society (DB 88) and National Institutes of Health (NIH). Work in R.M.H.’s laboratory was supported by NIH grant GM49346. S.T. was supported by a long-term fellowship from the Human Frontier Science Program (HFSP) and L.B.Z. by a National Research Service Award (NRSA) fellowship from NIH. C.M.F. is supported by the Arnold and Mabel Beckman Foundation and the Alfred P. Sloan Foundation. The publication costs of this article were defrayed in part by payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 USC section 1734 solely to indicate this fact.