title: Smad1/Smad5 signaling in limb ectoderm functions redundantly and is required for interdigital programmed cell death.
abstract: Bone morphogenetic proteins (BMPs) are secreted signals that regulate apical ectodermal ridge (AER) functions and interdigital programmed cell death (PCD) of developing limb. However the identities of the intracellular mediators of these signals are unknown. To investigate the role of Smad proteins in BMP-regulated AER functions in limb development, we inactivated Smad1 and Smad5 selectively in AER and ventral ectoderm of developing limb, using Smad1 or/and Smad5 floxed alleles and an En1(Cre/+) knock-in allele. Single inactivation of either Smad1 or Smad5 did not result in limb abnormalities. However, the Smad1/Smad5 double mutants exhibited syndactyly due to a reduction in interdigital PCD and an increase in interdigital cell proliferation. Cell tracing experiments in the Smad1/Smad5 double mutants showed that ventral ectoderm became thicker and the descendents of ventral En1(Cre/+) expressing ectodermal cells were located at dorsal interdigital regions. At the molecular level, Fgf8 expression was prolonged in the interdigital ectoderm of embryonic day (E) 13 Smad1/Smad5 double mutants, suggesting that the ectopic Fgf8 expression may serve as a survival signal for interdigital epithelial and mesenchymal cells. Our result suggests that Smad1 and Smad5 are required and function redundantly as intracellular mediators for BMP signaling in the AER and ventral ectoderm. Smad1/Smad5 signaling in the AER and ventral ectoderm regulates interdigital tissue regression of developing limb. Our mutants with defects in interdigital PCD could also serve as a valuable model for investigation of PCD regulation machinery.
authors.affiliation: School of Life Sciences, The Chinese University of Hong Kong, Hong Kong, PR China.
authors.forename: Yuk Lau
authors.initials: YL
authors.lastname: Wong
authors.forename: Richard R
authors.initials: RR
authors.lastname: Behringer
authors.forename: Kin Ming
authors.initials: KM
authors.lastname: Kwan
mesh_terms: D000818:Animals; D017209:Apoptosis; D019521:Body Patterning / Q000235:genetics; D055396:Bone Morphogenetic Protein 2 / Q000235:genetics / Q000378:metabolism; D055415:Bone Morphogenetic Protein 4 / Q000235:genetics / Q000378:metabolism; D049109:Cell Proliferation; D004475:Ectoderm / Q000196:embryology / Q000378:metabolism* / Q000502:physiology; D004622:Embryo, Mammalian / Q000196:embryology / Q000378:metabolism; D005121:Extremities / Q000196:embryology; D051524:Fibroblast Growth Factor 8 / Q000235:genetics / Q000378:metabolism; D005455:Fluorescent Antibody Technique; D018507:Gene Expression Regulation, Developmental; D053823:Hedgehog Proteins / Q000235:genetics / Q000378:metabolism; D018398:Homeodomain Proteins / Q000235:genetics / Q000378:metabolism; D017403:In Situ Hybridization; D017880:Limb Deformities, Congenital / Q000235:genetics / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D008822:Mice, Transgenic; D008957:Models, Genetic; D015398:Signal Transduction / Q000235:genetics* / Q000502:physiology; D051898:Smad1 Protein / Q000235:genetics* / Q000378:metabolism; D051902:Smad5 Protein / Q000235:genetics* / Q000378:metabolism
chemical_list: C525719:Bmp2 protein, mouse; C525810:Bmp4 protein, mouse; D055396:Bone Morphogenetic Protein 2; D055415:Bone Morphogenetic Protein 4; C080330:En1 protein, mouse; C496457:Fgf8 protein, mouse; D053823:Hedgehog Proteins; D018398:Homeodomain Proteins; C465570:Shh protein, mouse; D051898:Smad1 Protein; C497401:Smad1 protein, mouse; D051902:Smad5 Protein; C497798:Smad5 protein, mouse; D051524:Fibroblast Growth Factor 8