title: BMPs are direct triggers of interdigital programmed cell death.
abstract: During vertebrate embryogenesis the interdigital mesenchyme is removed by programmed cell death (PCD), except in species with webbed limbs. Although bone morphogenetic proteins (BMPs) have long been known to be players in this process, it is unclear if they play a direct role in the interdigital mesenchyme or if they only act indirectly, by affecting fibroblast growth factor (FGF) signaling. A series of genetic studies have shown that BMPs act indirectly by regulating the withdrawal of FGF activity from the apical ectodermal ridge (AER); this FGF activity acts as a cell survival factor for the underlying mesenchyme. Other studies using exogenous factors to inhibit BMP activity in explanted mouse limbs suggest that BMPs do not act directly in the mesenchyme. To address the question of whether BMPs act directly, we used an interdigit-specific Cre line to inactivate several genes that encode components of the BMP signaling pathway, without perturbing the normal downregulation of AER-FGF activity. Of three Bmps expressed in the interdigital mesenchyme, Bmp7 is necessary for PCD, but Bmp2 and Bmp4 both have redundant roles, with Bmp2 being the more prominent player. Removing BMP signals to the interdigit by deleting the receptor gene, Bmpr1a, causes a loss of PCD and syndactyly, thereby unequivocally proving that BMPs are direct triggers of PCD in this tissue. We present a model in which two events must occur for normal interdigital PCD: the presence of a BMP death trigger and the absence of an FGF survival activity. We demonstrate that neither event is required for formation of the interdigital vasculature, which is necessary for PCD. However, both events converge on the production of reactive oxygen species that activate PCD.
authors.affiliation: Genetics of Vertebrate Development Section, Cancer and Developmental Biology Lab, National Cancer Institute, National Institutes of Health, Frederick, MD 21702, USA.
authors.forename: Maria M
authors.initials: MM
authors.lastname: Kaltcheva
authors.affiliation: Genetics of Vertebrate Development Section, Cancer and Developmental Biology Lab, National Cancer Institute, National Institutes of Health, Frederick, MD 21702, USA.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Anderson
authors.affiliation: Department of Molecular Genetics and Microbiology, The Genetics Institute, University of Florida, Gainesville, FL 32610, USA.
authors.forename: Brian D
authors.initials: BD
authors.lastname: Harfe
authors.affiliation: Genetics of Vertebrate Development Section, Cancer and Developmental Biology Lab, National Cancer Institute, National Institutes of Health, Frederick, MD 21702, USA. Electronic address: Lewandom@mail.nih.gov.
authors.forename: Mark
authors.initials: M
authors.lastname: Lewandoski
mesh_terms: D000818:Animals; D017209:Apoptosis; D055396:Bone Morphogenetic Protein 2 / Q000502:physiology*; D055415:Bone Morphogenetic Protein 4 / Q000502:physiology*; D055419:Bone Morphogenetic Protein 7 / Q000502:physiology*; D052005:Bone Morphogenetic Protein Receptors, Type I / Q000235:genetics*; D003433:Crosses, Genetic; D005121:Extremities / Q000196:embryology; D005260:Female; D005346:Fibroblast Growth Factors / Q000378:metabolism; D005552:Forelimb / Q000473:pathology; D018507:Gene Expression Regulation, Developmental; D019426:Integrases / Q000378:metabolism; D008297:Male; D008648:Mesoderm / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D018613:Microscopy, Confocal; D017382:Reactive Oxygen Species / Q000378:metabolism; D015398:Signal Transduction; D013576:Syndactyly / Q000235:genetics; D013997:Time Factors; D014034:Toes / Q000473:pathology; D001616:beta-Galactosidase / Q000378:metabolism
chemical_list: C525719:Bmp2 protein, mouse; C525810:Bmp4 protein, mouse; D055396:Bone Morphogenetic Protein 2; D055415:Bone Morphogenetic Protein 4; D055419:Bone Morphogenetic Protein 7; D017382:Reactive Oxygen Species; C525820:bmp7 protein, mouse; D005346:Fibroblast Growth Factors; C498212:Bmpr1a protein, mouse; D052005:Bone Morphogenetic Protein Receptors, Type I; C045073:Cre recombinase; D019426:Integrases; D001616:beta-Galactosidase
keywords: BMP; FGF; Limb development; Programmed cell death; Reactive oxygen species; Syndactyly
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