Human Molecular Genetics Advance Access originally published online on May 12, 2009
Human Molecular Genetics 2009 18(15):2899-2911; doi:10.1093/hmg/ddp227
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Loss of connexin43-mediated gap junctional coupling in the mesenchyme of limb buds leads to altered expression of morphogens in mice
1 Division of Molecular Genetics, Institute of Genetics, Berlin, Germany 2 Program Unit Development, Genetic and Molecular Physiology, Life and Medical Sciences (LIMES) Institute, Bonn, Germany 3 Department of Developmental Pathology, Bonn Medical School, Institute for Pathology, University of Bonn, Bonn, Germany
* To whom correspondence should be addressed at: Roemerstr. 164, 53117 Bonn, Germany. Tel: +49 228734791; Fax: +49 228734263; Email: k.willecke{at}uni-bonn.de
Received January 26, 2009; Revised March 26, 2009; Accepted May 8, 2009
Mutations in the GJA1 gene coding for connexin43 (Cx43) cause oculodentodigital dysplasia (ODDD), a pleiotropic human disorder with characteristic morphologic anomalies of face, teeth, bones and digits. Interdigital webbings, also called syndactylies, are a characteristic phenotype of this disease showing high intra- and interfamilial penetrance. Therefore, we decided to study the molecular basis of syndactylies caused by Cx43 mutations. In order to reveal the impact of Cx43-mediated gap junctional coupling, we used mice expressing the human point mutation Cx43G138R and, in addition, ‘knock-out’ mice lacking Cx43. Both conditional mouse models developed syndactylies as a consequence of disturbed interdigital apoptosis, which we show to be due to reduced expression of two key morphogens: sonic hedgehog (Shh) and bone morphogenic protein 2 (Bmp2). Diminished levels of Bmp2 and subsequent up-regulation of fibroblast growth factors (Fgfs) lead to an insufficient induction of interdigital apoptosis. Interestingly, the reduction of Shh expression in Cx43 mutants begins on embryonic day 10.5 indicating a disturbance of the Fgf/Shh regulatory feedback loop, and confirming the recently published observation that gap junctions can relay Fgf signals to neighboring cells. Thus, Cx43-mediated gap junctional coupling in the mesenchyme of limb buds after ED11 is essential to maintain Shh expression, which regulates the downstream signaling of Bmp2. Besides diminished interdigital apoptosis, the decreased expression of Bmp2 in Cx43 mutants may also be involved in other morphological alterations in patients suffering from ODDD.
Present address: Howard Hughes Medical Institute, University of California, Los Angeles, CA, USA.