Macrophages Break Interneuromast Cell Quiescence by Intervening in the Inhibition of Schwann Cells in the Zebrafish Lateral Line

Abstract

In the zebrafish lateral line system, interneuromast cells (INCs) between neuromasts are kept quiescent by underlying Schwann cells (SWCs). Upon severe injuries that cause the complete loss of an entire neuromast, INCs can occasionally differentiate into neuromasts but how they escape from the inhibition by SWCs is still unclear. Using a genetic/chemical method to ablate a neuromast precisely, we found that a small portion of larvae can regenerate a new neuromast. However, the residual regeneration capacity was hindered by inhibiting macrophages. Using in toto imaging, we further discovered heterogeneities in macrophage behavior and distribution along the lateral line. We witnessed the crawling of macrophages between the injured lateral line and SWCs during regeneration and between the second primordium and the first mature lateral line during development. It implies that macrophages may physically alleviate the nerve inhibition to break the dormancy of INCs during regeneration and development in the zebrafish lateral line.

Macrophages enhance regeneration of lateral line neuromast derived from interneuromast cells through TGF‐β in zebrafish

“…This hypothesis could be tested through observations of nerve degradation/regeneration. We have previously observed the upregulation of Wnt activity in regenerating lateral line cells with infiltrating macrophages and the necessity of Wnt signal for lateral line regeneration (Lin et al, 2022).…”

“…The rate of neuromast regeneration after tail amputation was much higher than when using the previously reported nitroreductase (NTR) ablation approach, which resulted in a regeneration rate of $20% (Lin et al, 2022). This provides more room for chemical manipulation.…”

“…We have previously demonstrated the retraction and possibly damage of nerves in tail-amputated larvae (Lin et al, 2022). This might affect the associated Schwann cells, allowing proliferation of interneuromast cells.…”

“…Larvae were anesthetized at 3 dpf in 0.16 g/L tricaine (ethyl 3-aminobenzoate methanesulfonate salt; Sigma-Aldrich, St. Louis, MO, USA) for 5 min. We then microinjected 1-2 nL of Encapsome ® or Clodrosome ® (SKU# CLD-8901, Encapsula NanoSciences, Brentwood, TN, USA) into the circulation system via the posterior cardinal vein (Lin et al, 2022) to ablate macrophages. Finally, the larvae were subjected to tail amputation as described in Section 2.2.…”

“…TGF-β can be released not only from macrophages, but also from wounded cells, particularly from large wounds such as those caused by tail amputation. Therefore, we conducted similar experiments using the chemical-genetic neuromast-ablated larvae as described previously (Lin et al, 2022). Embryos from the cross of Et(HG7L) and Tg (À8.0cldnb:NTR-hkiKGRkKikGR) were treated with 2 mM Mtz.…”