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LSSS 2014-2015

2014LSSS2015

Life Sciences Seminar Series

 

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Rolf Zeller

Developmental Genetics, Department of Biomedicine, University of Basel Medical Faculty, Basel

Signaling networks and genomic landscapes in vertebrate limb development and evolution

Selected Publications

Attenuated sensing of SHH by Ptch1 underlies evolution of bovine limbs.Lopez-Rios J, Duchesne A, Speziale D, Andrey G, Peterson KA, Germann P, Unal E, Liu J, Floriot S, Barbey S, Gallard Y, Müller-Gerbl M, Courtney AD, Klopp C, Rodriguez S, Ivanek R, Beisel C, Wicking C, Iber D, Robert B, McMahon AP, Duboule D, Zeller R
Nature 2014 Jul 3; 511(7507):46-51

Abstract

The large spectrum of limb morphologies reflects the wide evolutionary diversification of the basic pentadactyl pattern in tetrapods. In even-toed ungulates (artiodactyls, including cattle), limbs are adapted for running as a consequence of progressive reduction of their distal skeleton to symmetrical and elongated middle digits with hoofed phalanges. Here we analyse bovine embryos to establish that polarized gene expression is progressively lost during limb development in comparison to the mouse. Notably, the transcriptional upregulation of the Ptch1 gene, which encodes a Sonic hedgehog (SHH) receptor, is disrupted specifically in the bovine limb bud mesenchyme. This is due to evolutionary alteration of a Ptch1 cis-regulatory module, which no longer responds to graded SHH signalling during bovine handplate development. Our study provides a molecular explanation for the loss of digit asymmetry in bovine limb buds and suggests that modifications affecting the Ptch1 cis-regulatory landscape have contributed to evolutionary diversification of artiodactyl limbs.

Smad4 is required to induce digit ray primordia and to initiate the aggregation and differentiation of chondrogenic progenitors in mouse limb buds.Bénazet JD, Pignatti E, Nugent A, Unal E, Laurent F, Zeller R
Development 2012 Nov; 139(22):4250-60

Abstract

SMAD4 is an essential mediator of canonical TGFβ/BMP signal transduction and we inactivated Smad4 in mouse limb buds from early stages onward to study its functions in the mesenchyme. While this Smad4 inactivation did not alter the early Sox9 distribution, prefiguring the chondrogenic primordia of the stylopod and zeugopod, it disrupted formation of all Sox9-positive digit ray primordia. Specific inactivation of Smad4 during handplate development pointed to its differential requirement for posterior and anterior digit ray primordia. At the cellular level, Smad4 deficiency blocked the aggregation of Sox9-positive progenitors, thereby preventing chondrogenic differentiation as revealed by absence of collagen type II. The progressive loss of SOX9 due to disrupting digit ray primordia and chondrogenesis was paralleled by alterations in genes marking other lineages. This pointed to a general loss of tissue organization and diversion of mutant cells toward non-specific connective tissue. Conditional inactivation of Bmp2 and Bmp4 indicated that the loss of digit ray primordia and increase in connective tissue were predominantly a consequence of disrupting SMAD4-mediated BMP signal transduction. In summary, our analysis reveals that SMAD4 is required to initiate: (1) formation of the Sox9-positive digit ray primordia; and (2) aggregation and chondrogenic differentiation of all limb skeletal elements.

GLI3 constrains digit number by controlling both progenitor proliferation and BMP-dependent exit to chondrogenesis.Lopez-Rios J, Speziale D, Robay D, Scotti M, Osterwalder M, Nusspaumer G, Galli A, Holländer GA, Kmita M, Zeller R
Dev. Cell 2012 Apr 17; 22(4):837-48

Abstract

Inactivation of Gli3, a key component of Hedgehog signaling in vertebrates, results in formation of additional digits (polydactyly) during limb bud development. The analysis of mouse embryos constitutively lacking Gli3 has revealed the essential GLI3 functions in specifying the anteroposterior (AP) limb axis and digit identities. We conditionally inactivated Gli3 during mouse hand plate development, which uncoupled the resulting preaxial polydactyly from known GLI3 functions in establishing AP and digit identities. Our analysis revealed that GLI3 directly restricts the expression of regulators of the G(1)-S cell-cycle transition such as Cdk6 and constrains S phase entry of digit progenitors in the anterior hand plate. Furthermore, GLI3 promotes the exit of proliferating progenitors toward BMP-dependent chondrogenic differentiation by spatiotemporally restricting and terminating the expression of the BMP antagonist Gremlin1. Thus, Gli3 is a negative regulator of the proliferative expansion of digit progenitors and acts as a gatekeeper for the exit to chondrogenic differentiation.

A self-regulatory system of interlinked signaling feedback loops controls mouse limb patterning.Bénazet JD, Bischofberger M, Tiecke E, Gonçalves A, Martin JF, Zuniga A, Naef F, Zeller R
Science 2009 Feb 20; 323(5917):1050-3

Abstract

Embryogenesis depends on self-regulatory interactions between spatially separated signaling centers, but few of these are well understood. Limb development is regulated by epithelial-mesenchymal (e-m) feedback loops between sonic hedgehog (SHH) and fibroblast growth factor (FGF) signaling involving the bone morphogenetic protein (BMP) antagonist Gremlin1 (GREM1). By combining mouse molecular genetics with mathematical modeling, we showed that BMP4 first initiates and SHH then propagates e-m feedback signaling through differential transcriptional regulation of Grem1 to control digit specification. This switch occurs by linking a fast BMP4/GREM1 module to the slower SHH/GREM1/FGF e-m feedback loop. This self-regulatory signaling network results in robust regulation of distal limb development that is able to compensate for variations by interconnectivity among the three signaling pathways.