Colony stimulating factor 1 signaling regulates myeloid fates in zebrafish via distinct action of its receptors and ligands

Macrophage colony-stimulating factor receptor (M-CSFR/CSF1R) signaling is crucial for the differentiation, proliferation, and survival of myeloid cells. Therapeutic targeting of the CSF1R pathway is a promising strategy in many human diseases, including neurological disorders or cancer. Zebrafish are commonly used for human disease modeling and preclinical therapeutic screening. Therefore, it is necessary to understand the proper function of cytokine signaling in zebrafish to reliably model human-related diseases. Here, we investigate the roles of zebrafish Csf1rs and their ligands - Csf1a, Csf1b and Il34, in embryonic and adult myelopoiesis. The proliferative effect of exogenous Csf1a on embryonic macrophages is connected to both receptors as it is diminished in both csf1ra Δ5bp and csf1rb Δ4bp mutants. There is no evident effect of Csf1b in zebrafish embryonic myelopoiesis. Further, we uncover an unknown role of Csf1rb in zebrafish granulopoiesis. Deregulation of Csf1rb signaling leads to failure in myeloid differentiation resulting in neutropenia throughout the whole lifespan. Surprisingly, Il34 signaling through Csf1rb seems to be of high importance as both csf1rb Δ4bp and il34 Δ5bp deficient zebrafish larvae lack granulocytes. Our single-cell RNA sequencing analysis of adult whole kidney marrow (WKM) hematopoietic cells suggests that csf1rb is expressed mainly by blood and myeloid progenitors and that the expression of csf1ra and csf1rb is non-overlapping. We point out differentially expressed genes important in hematopoietic cell differentiation and immune response in selected WKM populations. Our findings could improve the understanding of myeloid cell function and lead to the further study of CSF1R pathway deregulation in disease, mostly in cancerogenesis. Key points: ### Competing Interest Statement The authors have declared no competing interest.