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A CRITICAL ROLE FOR THE SHH‐FGF LOOP DURING THE INITIATION OF LIMB REGENERATION
Author(s) -
Mishima Noboru,
Halverson Brian E,
Liu Jesse J,
Pira Charmaine U,
Oberg Kerby C
Publication year - 2007
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.21.5.a200
Subject(s) - zone of polarizing activity , apical ectodermal ridge , ectoderm , limb development , limb bud , mesoderm , fibroblast growth factor , ectopic expression , regeneration (biology) , anatomy , biology , microbiology and biotechnology , sonic hedgehog , embryo , embryonic stem cell , embryogenesis , receptor , signal transduction , genetics , cell culture , gene
Limb outgrowth is initiated and maintained by the secretion of FGFs in the apical ectodermal ridge (AER), and patterning regulated by SHH emanating from the posteriorly positioned zone of polarizing activity (ZPA). These factors are tightly linked during progressive limb formation by the SHH‐FGF loop. Distal limb bud amputation (removing AER and ZPA) followed by application of FGF to the posterior (but not anterior) stump reactivates SHH expression and regeneration of the distal limb structures. We wondered whether SHH was sufficient to induce regenerative capacity and FGF‐responsiveness in limb mesoderm. To determine the role of SHH in limb regeneration, we amputated developing wings (HH 23) and ectopically expressed SHH by confined microeclectroporation (CMEP) in the anterior and posterior stump margins in the presence or absence of FGF soaked beads. Also we ectopically expressed SHH in the anterior margin at HH 19 by CMEP, then applied FGF soaked beads to the anterior margin 24 h later (HH 23‐24). Chicks were harvested 24–40 h after amputation/FGF application, and analyzed by whole mount in situ hybridization. Although ectopic expression of SHH alone in the posterior stump could not re‐establish segmented limb regeneration, the expression of ectopic SHH in the anterior stump induced segmented regeneration in the absence of applied FGF. Ectopic FGF4/9 expressions were identified in the ectoderm overlying anterior ectopic SHH expression. Our data suggests that SHH induces both positive and negative regulators of FGF expression. Initial SHH expression in naïve limb mesoderm (anterior) upregulates FGFs in the overlying ectoderm, while cells with prior exposure to SHH (posterior mesoderm) no longer have the ability to upregulate FGFs in response to SHH.