Building the Zebrafish Lateral Line System
- Ajay Chitnis, MBBS, PhD, Head, Section on Neural Developmental Dynamics
- Gregory Palardy, BS, Research Technician
- Chongmin Wang, MS, Research Technician
- Pritesh Krishnakumar, PhD, Postdoctoral Fellow
- Sana Fatma, PhD, Postdoctoral Visiting Fellow
- Abhishek Mukherjee, PhD, Postdoctoral Visiting Fellow
- Megan Schupp, BS, Postbaccalaureate Intramural Research Training Award Fellow
Cells divide, move, adhere, and interact with their neighbors and their environment to determine the formation of multicellular organ systems with unique fates, morphologies, function, and behavior. Our goal is to understand how such interactions determine the self-organization of cell communities in the nervous system of the zebrafish (Danio rerio) embryo. The lateral line is a mechano-sensory system that helps sense the pattern of water flow over the fish and amphibian body; it consists of sensory organs called neuromasts, which are distributed in a stereotypic pattern over the body surface. Each neuromast has sensory hair cells at its center, surrounded by support cells that serve as progenitors for the production of more hair cells during growth and for the regeneration of neuromasts. The development of this superficial sensory system in zebrafish is spearheaded by the posterior Lateral Line primordia (pLLp), groups of about 150 cells formed on either side of a day-old embryo near the ear. Cells in the primordia migrate collectively under the skin to the tip of the tail, as they divide and reorganize to form nascent neuromasts, which are deposited sequentially from the lateral line's trailing end. Their journey is easily observed in live transgenic embryos with fluorescent primordium cells. Furthermore, a range of genetic and cellular manipulations can be used to investigate gene function and morphogenesis in the system. Understanding the self-organization of this relatively simple and accessible system in zebrafish will help elucidate the broader principles that determine cell-fate specification, morphogenesis, and collective cell migration in the developing vertebrate nervous system.
Signaling and mechanics influence the number and size of epithelial rosettes in the migrating zebrafish posterior Lateral Line primordium.
Protoneuromasts are formed within the migrating primordium, starting from its trailing end as clusters of cells apically constrict and form epithelial rosettes. Their formation is promoted by fibroblast growth factor (Fgf)–signaling centers that form periodically in the wake of a shrinking Wnt–active domain that inhibits epithelial rosette formation and progressively shrinks toward the leading end of the primordium (Wnt and Fgf pathways are signaling pathways). However, the precise number and size of epithelial rosettes is not strictly dependent on a prepattern of Fgf–signaling activity, as it is broadly influenced by the balance of mechanical interactions that promote or oppose formation of epithelial rosettes. When chemokine-dependent migration of leading cells is compromised, the resulting slowing of the primordium is accompanied by the fusion of epithelial rosettes to form fewer larger rosettes. However, such fusion is not observed when Fgf signaling, responsible for migration of trailing cells, is inhibited to slow primordium migration. These observations can be accounted for by a mechanics-based model, in which local interactions associated with apical constriction and cell adhesion promote aggregation, while tension along the length of the primordium, influenced by the relative efficacy of leading and trailing cell migration, opposes such aggregation. We described the development of a computational Cellular Potts model, which allowed us to explore how the relative speed of leading versus trailing cells, as well as changes in cell adhesion and mechanical coupling, differentially regulated by Wnt and Fgf signaling, can influence the pattern of neuromast formation and deposition by the migrating primordium. Our studies illustrate how signaling and mechanics cooperate to coordinate self-organization of morphogenesis in the migrating primordium.
Sox2 stabilizes maturing epithelial rosettes in the zebrafish posterior Lateral Line primordium in part by inhibiting destabilizing Wnt–signaling activity.
Protoneuromasts are formed within the migrating primordium, starting from its trailing end, as clusters of cells sequentially reorganize to form epithelial rosettes, each around a central Atoh1a–expressing cell specified as a sensory hair cell progenitor. The rosettes' formation is initiated in Fgf–signaling domains that are periodically established in response to Fgfs produced by cells in an adjacent leading Wnt–active zone, where Wnt signaling also inhibits these leading cells from responding to Fgfs and forming protoneuromasts. Fgf signaling–dependent expression of the diffusible Wnt antagonist Dkk1b (dickkopf Wnt–signaling pathway inhibitor 1b) facilitates establishment of stable Fgf–signaling centers in nascent protoneuromasts by preventing potentially destabilizing inhibition from Wnt signaling. Dkk1b also contributes to progressive restriction of the initially broad Wnt–signaling domain to a smaller leading zone, as new Fgf signaling–dependent protoneuromasts form in the wake of the shrinking Wnt system. As the leading Wnt system shrinks, Atoh1a expression (a transcription factor that enables chromatin-binding activity) and epithelial rosette morphogenesis in maturing neuromasts formed earlier in more trailing parts of the primordium, become self-sustaining and independent of the Fgfs signals produced by leading Wnt active cells that initiated protoneuromast formation. However, Dkk1b is not expressed in these maturing neuromasts, raising a question about what inhibits potentially destabilizing Wnt signaling in the trailing neuromasts. We showed that Sox2 (a member of the Sox family of transcription factors that regulate cell-fate decisions during development and is essential for maintaining self-renewal, or pluripotency, of undifferentiated embryonic stem cells) is expressed in nascent and maturing protoneuromasts in a pattern that is complementary to domains with Wnt signaling activity. Furthermore, Sox2 functions in a partially redundant manner with Sox1a and Sox3, to inhibit Wnt signaling. This helps keep Wnt activity restricted to a leading zone, which we suggest is essential for effective stabilization of maturing protoneuromasts in the trailing zone. Together our observations show how patterning events that initiate protoneuromast formation are followed by changes in regulation requiring SoxB1 family factors that help consolidate neuromast morphogenesis, prior to their deposition by the migrating primordium.
Collaborators
- Harshad Vishwasrao, PhD, Advanced Imaging and Microscopy Resource, NIBIB, Bethesda, MD
Contact
For more information, email chitnisa@mail.nih.gov or visit https://chitnislab.nichd.nih.gov.
