Limb patterning in mouse
The normal development of both limbs and epidermal appendages (nails, hairs and sweat glands) requires the correct execution of complex signaling cascades between mesoderm and ectoderm. We are interested in delineating the genes critical for regulating these processes and defining the mechanisms by which they function in man using mouse as a model system. In our analyses, we combine standard loss-of-function (knock-out) and gain-of function (transgenic) strategies; we also utilize a novel approach that permits us to deliver retroviral vectors to mid-gestation mouse tissues, thereby allowing us to temporally and spatially restrict the impact of our genetic manipulations.
Genetic and tissue transplantation studies have demonstrated that three distinct signaling regions direct patterning and growth of the vertebrate limb. Proximal to distal outgrowth depends on fibroblast growth factor signaling from the apical ectodermal ridge (AER), a thickened rim of ectoderm along the distal limb margin. The zone of polarizing activity (ZPA), a specialized cluster of mesenchymal cells at the posterior limb margin, directs anterior-posterior patterning via sonic hedgehog signaling. We and others have shown that dorsal-ventral limb patterning relies on choreographed interactions of regulatory molecules that include the signaling molecule Wnt7a in the dorsal limb ectoderm, the LIM-homeodomain protein Lmx1b in the dorsal limb mesenchyme and the homeobox transcription factor Engrailed-1 (En1) in the ventral limb ectoderm. Formation and maintenance of these distinct signaling zones is mutually interdependent. Our current work focuses on delineating the pathways important in directing early dorsal-ventral limb patterning as well as AER morphogenesis. Specifically, we are examining the mechanisms by which En1 and Shh regulate AER formation and maintenance.
As a pediatric dermatologist, I am also interested in studying the functions
of these regulatory genes in the development of skin appendages. Many
of the genes critical in regulating outgrowth of the limbs are re-used
during later embryogenesis to direct downgrowth and differentiation of
epidermal appendages. For example, our recent studies indicate that ectodermally
expressed En1 is critical for promoting eccrine gland morphogenesis
and for repressing nail development on palmar/plantar skin both pre- and
postnatally. In general, skin appendages are ideal mammalian model systems
for investigating developmental regulatory cascades because of their multiplicity
and easy accessibility. Moreover, since each type of skin appendage undergoes
similar, though unique, morphogenetic events, appendages provide an excellent
opportunity for defining the impact of subtle differences in combinatorial
genetic regulation on tissue morphogenesis. In addition, these studies
will also aid in the identification of candidate genes that might be mutated
in the over 150 different forms of ectodermal dysplasias, clinical syndromes
characterized by defects in the development of ectodermal appendages,
as well as in congenital disorders that display both skin and limb anomalies.