Roles for Fgf signaling during zebrafish fin regeneration

Cell proliferation and movement during early fin regeneration in zebrafish

Cell proliferation and cell movement during early regeneration of zebrafish caudal fins were examined by injecting BrdU and Di-I, respectively. In normal fins of adult fish, a small number of proliferating cells are observed in the epidermis only. Shortly following amputation, epithelial cells covered the wound to form the epidermal cap but did not proliferate. However, by 24 hr, epithelial cells proximal to the level of amputation were strongly labeled with BrdU. Label incorporation was also detected in a few mesenchymal cells. Proliferating cells in the basal epithelial layer were first observed at 48 hr at the level of the newly formed lepidotrichia. At 72 hr, proliferating mesenchymal cells were found distal to the plane of amputation whereas more proximal labeled cells included mainly those located between the lepidotrichia and the basal membrane. When BrdU-injected fins were allowed to regenerate for longer periods, labeled cells were observed in the apical epidermal cap, a location where cells are not thought to proliferate. This result is suggestive of cell migration. Epithelial cells, peripheral to the rays or in the tissue between adjacent rays, were labeled with Di-I and were shown to quickly migrate towards the site of amputation, the cells closer to the wound migrating faster. Amputation also triggered migration of cells of the connective tissue located between the hemirays. Although cell movement was induced up to seven segments proximal from the level of amputation, cells located within two segments from the wound provided the main contribution to the blastema. Thus, cell proliferation and migration contribute to the early regeneration of zebrafish fins.

sprouty4 acts in vivo as a feedback-induced antagonist of FGF signaling in zebrafish

In looking for novel factors involved in the regulation of the fibroblast growth factor (FGF) signaling pathway, we have isolated a zebrafish sprouty4 gene, based on its extensive similarities with the expression patterns of both fgf8 and fgf3. Through gain- and loss-of-function experiments, we demonstrate that Fgf8 and Fgf3 act in vivo to induce the expression of Spry4, which in turn can inhibit activity of these growth factors. When overexpressed at low doses, Spry4 induces loss of cerebellum and reduction in size of the otic vesicle, thereby mimicking the fgf8/acerebellar mutant phenotype. Injections of high doses of Spry4 cause ventralization of the embryo, an opposite phenotype to the dorsalisation induced by overexpression of Fgf8 or Fgf3. Conversely we have shown that inhibition of Spry4 function through injection of antisense morpholino oligonucleotide leads to a weak dorsalization of the embryo, the phenotype expected for an upregulation of Fgf8 or Fgf3 signaling pathway. Finally, we show that Spry4 interferes with FGF signaling downstream of the FGF receptor 1 (FGFR1). In addition, our analysis reveals that signaling through FGFR1/Ras/mitogen-activated protein kinase pathway is involved, not in mesoderm induction, but in the control of the dorsoventral patterning via the regulation of bone morphogenetic protein (BMP) expression.

Muscle development: reversal of the differentiated state

Cell fate selection and cell cycle exit are fundamental features of differentiation during animal development. Accumulating data suggest that these processes are more readily reversible than previously supposed and are beginning to point at the underlying molecular mechanisms.

Dedifferentiation of mammalian myotubes induced by msx1

The process of cellular differentiation culminating in terminally differentiated mammalian cells is thought to be irreversible. Here, we present evidence that terminally differentiated murine myotubes can be induced to dedifferentiate. Ectopic expression of msx1 in C2C12 myotubes reduced the nuclear muscle proteins MyoD, myogenin, MRF4, and p21 to undetectable levels in 20%-50% of the myotubes. Approximately 9% of the myotubes cleave to produce either smaller multinucleated myotubes or proliferating, mononucleated cells. Finally, clonal populations of the myotube-derived mononucleated cells can be induced to redifferentiate into cells expressing chondrogenic, adipogenic, myogenic, and osteogenic markers. These results suggest that terminally differentiated mammalian myotubes can dedifferentiate when stimulated with the appropriate signals and that msx1 can contribute to the dedifferentiation process.