Joseph J Lancman1, Sean M Hasso2, Takayuki Suzuki3, Yacine Kherdjemil4,5, Marie Kmita4,5,6, Andrea Ferris7, P Duc S Dong1,8, Marian A Ros9,10, John F Fallon11. 1. Human Genetics Program, Sanford Burnham Prebys Medical Discovery Institute, La Jolla, California, USA. 2. Heat Biologics, Morrisville, North Carolina, USA. 3. Department of Animal Sciences, Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, Japan. 4. Genetics and Development Research Unit, Institut de Recherches Cliniques de Montréal, Montréal, Québec, Canada. 5. Département de Médecine, Université de Montréal, Montréal, Québec, Canada. 6. Department of Experimental Medicine, McGill University, Montreal, Quebec, Canada. 7. Center for Cancer Research, National Cancer Institute, Frederick, Maryland, USA. 8. Graduate School of Biomedical Sciences, Sanford Burnham Prebys Medical Discovery Institute, La Jolla, California, USA. 9. Instituto de Biomedicina y Biotecnología de Cantabria, Consejo Superior de Investigaciones Científicas-Universidad de Cantabria-Sociedad para al Desarrollo Cantabria, Santander, Spain. 10. Departamento de Anatomía y Biología Celular, Facultad de Medicina, Universidad de Cantabria, Santander, Spain. 11. Department of Anatomy, University of Wisconsin Madison, Madison, Wisconsin, USA.
Abstract
BACKGROUND: The phalanges are the final skeletal elements to form in the vertebrate limb and their identity is regulated by signaling at the phalanx forming region (PFR) located at the tip of the developing digit ray. Here, we seek to explore the relationship between PFR activity and phalanx morphogenesis, which define the most distal limb skeletal elements, and signals associated with termination of limb outgrowth. RESULTS: As Grem1 is extinguished in the distal chick limb mesoderm, the chondrogenesis marker Aggrecan is up-regulated in the metatarsals and phalanges. Fate mapping confirms that subridge mesoderm cells contribute to the metatarsal and phalanges when subridge Grem1 is down-regulated. Grem1 overexpression specifically blocks chick phalanx development by inhibiting PFR activity. PFR activity and digit development are also disrupted following overexpression of a Gli3 repressor, which results in Grem1 expression in the distal limb and downregulation of Bmpr1b. CONCLUSIONS: Based on expression and fate mapping studies, we propose that downregulation of Grem1 in the distal limb marks the transition from metatarsal to phalanx development. This suggests that downregulation of Grem1 in the distal limb mesoderm is necessary for phalanx development. Grem1 downregulation allows for full PFR activity and phalanx progenitor cell commitment to digit fate.
BACKGROUND: The phalanges are the final skeletal elements to form in the vertebrate limb and their identity is regulated by signaling at the phalanx forming region (PFR) located at the tip of the developing digit ray. Here, we seek to explore the relationship between PFR activity and phalanx morphogenesis, which define the most distal limb skeletal elements, and signals associated with termination of limb outgrowth. RESULTS: As Grem1 is extinguished in the distal chick limb mesoderm, the chondrogenesis marker Aggrecan is up-regulated in the metatarsals and phalanges. Fate mapping confirms that subridge mesoderm cells contribute to the metatarsal and phalanges when subridge Grem1 is down-regulated. Grem1 overexpression specifically blocks chick phalanx development by inhibiting PFR activity. PFR activity and digit development are also disrupted following overexpression of a Gli3 repressor, which results in Grem1 expression in the distal limb and downregulation of Bmpr1b. CONCLUSIONS: Based on expression and fate mapping studies, we propose that downregulation of Grem1 in the distal limb marks the transition from metatarsal to phalanx development. This suggests that downregulation of Grem1 in the distal limb mesoderm is necessary for phalanx development. Grem1 downregulation allows for full PFR activity and phalanx progenitor cell commitment to digit fate.