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First published online March 20, 2009
doi: 10.1242/10.1242/dev.026476

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LIM homeobox transcription factors integrate signaling events that control three-dimensional limb patterning and growth

Itai Tzchori^1,*, Timothy F. Day^2,*, Peter J. Carolan^2,, Yangu Zhao¹, Christopher A. Wassif³, LiQi Li⁴, Mark Lewandoski⁵, Marat Gorivodsky², Paul E. Love⁴, Forbes D. Porter³, Heiner Westphal^1, and Yingzi Yang^2,

¹ Section on Mammalian Molecular Genetics, Laboratory of Mammalian Genes and Development, Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, MD 20892, USA.
² Section on Developmental Genetics, Genetic Disease Research Branch, National Human Genome Research Institute, Bethesda, MD 20892, USA.
³ Section on Molecular Dysmorphology, Heritable Disorders Branch, Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, MD 20892, USA.
⁴ Section on Cellular and Developmental Biology, Laboratory of Mammalian Genes and Development, Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, MD 20892, USA.
⁵ Laboratory of Cancer and Developmental Biology, NCI-Frederick, National Institutes of Health, Frederick, MD 21702, USA.

Authors for correspondence (e-mails: hw{at}mail.nih.gov; yingzi{at}mail.nih.gov)

Accepted 12 February 2009

Vertebrate limb development is controlled by three signaling centers that regulate limb patterning and growth along the proximodistal (PD), anteroposterior (AP) and dorsoventral (DV) limb axes. Coordination of limb development along these three axes is achieved by interactions and feedback loops involving the secreted signaling molecules that mediate the activities of these signaling centers. However, it is unknown how these signaling interactions are processed in the responding cells. We have found that distinct LIM homeodomain transcription factors, encoded by the LIM homeobox (LIM-HD) genes Lhx2, Lhx9 and Lmx1b integrate the signaling events that link limb patterning and outgrowth along all three axes. Simultaneous loss of Lhx2 and Lhx9 function resulted in patterning and growth defects along the AP and the PD limb axes. Similar, but more severe, phenotypes were observed when the activities of all three factors, Lmx1b, Lhx2 and Lhx9, were significantly reduced by removing their obligatory co-factor Ldb1. This reveals that the dorsal limb-specific factor Lmx1b can partially compensate for the function of Lhx2 and Lhx9 in regulating AP and PD limb patterning and outgrowth. We further showed that Lhx2 and Lhx9 can fully substitute for each other, and that Lmx1b is partially redundant, in controlling the production of output signals in mesenchymal cells in response to Fgf8 and Shh signaling. Our results indicate that several distinct LIM-HD transcription factors in conjunction with their Ldb1 co-factor serve as common central integrators of distinct signaling interactions and feedback loops to coordinate limb patterning and outgrowth along the PD, AP and DV axes after limb bud formation.

Key words: Ldb1, Lhx9, Lhx2, Limb development, Lmx1b, Signaling, Mouse

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