Role of Granulin in Lineage Differentiation during Primitive and Definitive Hematopoiesis

Mature blood cells have a limited lifespan and have to be replenished continuously through a group of pluripotent precursors known as hematopoietic stem cells (HSCs). HSCs undergo proliferation and differentiation to achieve this goal. During differentiation, a group of multipotent progenitor cells with lineage-differentiation potential are generated from HSCs before becoming mature blood cells. The fate of these progenitor cells is regulated by many molecular factors during hematopoiesis. The zebrafish provides a great model to study hematopoiesis during embryonic development. Progranulin a (Grna) was found to be expressed during embryonic development by quantitative PCR (qPCR) and at the sites of hematopoiesis by whole mount in situ hybridization (WISH). Using live imaging and flow cytometry, we detected a reduction of macrophages and neutrophils in both Grna morphants and homozygous mutants. This phenotype could be partially rescued through overexpression of grna. Further analysis of grna transcripts in Pu.1 morphants suggested that Pu.1 acts upstream of Grna, regulating its expression. In addition, we found that Grna downregulated gata1 expression in Grna morphants. Together, our data suggest that Grna is essential for proper myeloid lineage differentiation

A zebrafish model of granulin deficiency reveals essential roles in myeloid cell differentiation

Granulin is a pleiotropic protein involved in inflammation, wound healing, neurodegenerative disease, and tumorigenesis. These roles in human health have prompted research efforts to use granulin to treat rheumatoid arthritis and frontotemporal dementia and to enhance wound healing. But how granulin contributes to each of these diverse biological functions remains largely unknown. Here, we have uncovered a new role for granulin during myeloid cell differentiation. We have taken advantage of the tissue-specific segregation of the zebrafish granulin paralogues to assess the functional role of granulin in hematopoiesis without perturbing other tissues. By using our zebrafish model of granulin deficiency, we revealed that during normal and emergency myelopoiesis, myeloid progenitors are unable to terminally differentiate into neutrophils and macrophages in the absence of granulin a (grna), failing to express the myeloid-specific genes cebpa, rgs2, lyz, mpx, mpeg1, mfap4, and apoeb. Functionally, macrophages fail to recruit to the wound, resulting in abnormal healing. Our CUT&RUN experiments identify Pu.1, which together with Irf8, positively regulates grna expression. In vivo imaging and RNA sequencing experiments show that grna inhibits the expression of gata1, leading to the repression of the erythroid program. Importantly, we demonstrated functional conservation between the mammalian granulin and the zebrafish ortholog grna. Our findings uncover a previously unrecognized role for granulin during myeloid cell differentiation, which opens a new field of study that can potentially have an impact on different aspects of human health and expand the therapeutic options for treating myeloid disorders such as neutropenia or myeloid leukemia