Influence of systemic immune responses in the brain after wound injury of tail amputation in zebrafish

Abstract

Background: Increasing lines of evidence demonstrate that systemic immune responses triggered by either sterile or infectious inflammation can activate neuroimmune responses, resulting in developing Alzheimer’s pathology, such as phosphorylation of tau protein. Systemic inflammation can be a risk factor for developing cognitive dysfunctions and even Alzheimer’s disease. We have been using a laboratory murine model, laparotomy, to demonstrate that systemic immune responses stimulate production of cytokines and activation of microglia in the brain. However, it is still unclear how systemic monocytes/macrophages responses after sterile inflammation. As zebrafish embryo has transparent body that suitable for imaging monocytes/macrophages, we aim to use zebrafish and tail amputation for investigating which factor can affect the influence of systemic inflammation in the brain. Method: Zebrafish was used as experimental model. Tg(coro1a:DsRed) transgenic fish was used to monitor microglia and macrophages. Tg(mpx:mCherry) was used for monitoring neutrophils, Tg(mepg1:GFP) was used for monitoring macrophages. We have used TALEN method to make knockout or mutation of IL-1b in zebrafish. Quantitative PCR and immunohistochemical staining were used for examination. Acridine orange and TUNEL were employed for labeling apoptotic cells. Tail amputation was performed at day 3 of embryos under anesthetization with tricaine. Result: Tail amputation recruited accumulation of monocytes/macrophages into the wound site in zebrafish tail. Surprisingly, increased number of macrophages and microglia were also found in the brain, which was tissue specific as no other organs showed similar phenomenon. Expression of mRNA for different cytokines was examined in the brain. Among which, IL-1b was significantly increased. Therefore, we prepared IL-1b knockout and mutated zebrafish. After amputation, the number of macrophages in the brain did not show any significant increase. To further understand why monocytes/macrophages migrate into the brain after peripheral wound injury, we found increased number of apoptotic cells in the brain after wound injury. IL-1b had a key role in regulating migration of monocytes/macrophages. Conclusion: Our study demonstrated that zebrafish is a good animal model to investigate how peripheral wound injury triggers neuroimmune responses. IL-1b is a key player to regulate migration of monocytes/macrophages into the brain.