Wnt/Dkk negative feedback regulates sensory organ size in zebrafish

Development of the lateral line canal system through a bone remodeling process in zebrafish

The lateral line system of teleost fish is composed of mechanosensory receptors (neuromasts), comprising superficial receptors and others embedded in canals running under the skin. Canal diameter and size of the canal neuromasts are correlated with increasing body size, thus providing a very simple system to investigate mechanisms underlying the coordination between organ growth and body size. Here, we examine the development of the trunk lateral line canal system in zebrafish. We demonstrated that trunk canals originate from scales through a bone remodeling process, which we suggest is essential for the normal growth of canals and canal neuromasts. Moreover, we found that lateral line cells are required for the formation of canals, suggesting the existence of mutual interactions between the sensory system and surrounding connective tissues.

Dynamic gene expression by putative hair-cell progenitors during regeneration in the zebrafish lateral line

Hearing loss is most commonly caused by the destruction of mechanosensory hair cells in the ear. This condition is usually permanent: Despite the presence of putative hair-cell progenitors in the cochlea, hair cells are not naturally replenished in adult mammals. Unlike those of the mammalian ear, the progenitor cells of nonmammalian vertebrates can regenerate hair cells throughout life. The basis of this difference remains largely unexplored but may lie in molecular dissimilarities that affect how progenitors respond to hair-cell death. To approach this issue, we analyzed gene expression in hair-cell progenitors of the lateral-line system. We developed a transgenic line of zebrafish that expresses a red fluorescent protein in the presumptive hair-cell progenitors known as mantle cells. Fluorescence-activated cell sorting from the skins of transgenic larvae, followed by microarray-based expression analysis, revealed a constellation of transcripts that are specifically enriched in these cells. Gene expression analysis after hair-cell ablation uncovered a cohort of genes that are differentially regulated early in regeneration, suggesting possible roles in the response of progenitors to hair-cell death. These results provide a resource for studying hair-cell regeneration and the biology of sensory progenitor cells.

Gene-expression analysis of hair cell regeneration in the zebrafish lateral line

Deafness caused by the terminal loss of inner ear hair cells is one of the most common sensory diseases. However, nonmammalian animals (e.g., birds, amphibians, and fish) regenerate damaged hair cells. To understand better the reasons underpinning such disparities in regeneration among vertebrates, we set out to define at high resolution the changes in gene expression associated with the regeneration of hair cells in the zebrafish lateral line. We performed RNA-Seq analyses on regenerating support cells purified by FACS. The resulting expression data were subjected to pathway enrichment analyses, and the differentially expressed genes were validated in vivo via whole-mount in situ hybridizations. We discovered that cell cycle regulators are expressed hours before the activation of Wnt/β-catenin signaling following hair cell death. We propose that Wnt/β-catenin signaling is not involved in regulating the onset of proliferation but governs proliferation at later stages of regeneration. In addition, and in marked contrast to mammals, our data clearly indicate that the Notch pathway is significantly down-regulated shortly after injury, thus uncovering a key difference between the zebrafish and mammalian responses to hair cell injury. Taken together, our findings lay the foundation for identifying differences in signaling pathway regulation that could be exploited as potential therapeutic targets to promote either sensory epithelium or hair cell regeneration in mammals.

ErbB expressing Schwann cells control lateral line progenitor cells via non-cell-autonomous regulation of Wnt/β-catenin

Proper orchestration of quiescence and activation of progenitor cells is crucial during embryonic development and adult homeostasis. We took advantage of the zebrafish sensory lateral line to define niche-progenitor interactions to understand how integration of diverse signaling pathways spatially and temporally regulates the coordination of these processes. Our previous studies demonstrated that Schwann cells play a crucial role in negatively regulating lateral line progenitor proliferation. Here we demonstrate that ErbB/Neuregulin signaling is not only required for Schwann cell migration but that it plays a continued role in postmigratory Schwann cells. ErbB expressing Schwann cells inhibit lateral line progenitor proliferation and differentiation through non-cell-autonomous inhibition of Wnt/β-catenin signaling. Subsequent activation of Fgf signaling controls sensory organ differentiation, but not progenitor proliferation. In addition to the lateral line, these findings have important implications for understanding how niche-progenitor cells segregate interactions during development, and how they may go wrong in disease states. DOI: http://dx.doi.org/10.7554/eLife.01832.001.