Characterization of isthmin-1 (ISM-1) in vertebrate hematopoietic proliferation and differentiation

Hematopoiesis is a highly regulated biological process that begins with a singular adult stem cell called the hematopoietic stem cell (HSC). HSCs are responsible for generation of trillions of cells a day, yet the pathways that instruct an HSC to self-renew and differentiate into mature blood cells are not fully known. To understand these molecular pathways, we investigated novel genes expressed in hematopoietic-supportive cell lines. We sequenced the transcriptome of three stromal cell lines derived from developing and mature zebrafish and identified 100 highly expressed transcripts. For our studies, we focused on ism-1 due to its shared synteny with humans. To characterize ism-1, we performed loss-of-function experiments to identify if mature blood cell production was disrupted. Myeloid, erythroid and lymphoid lineages were visualized and scored with transgenic zebrafish expressing lineage-specific markers. ism-1 knockdown led to reduced proliferation of neutrophils, macrophages, erythrocytes, and T cells. Analysis of methylcellulose plating with ism-1 morphants also showed a reduction in total hematopoietic stem and progenitor cell (HSPC) proliferation. Overall, we identified that ism-1 is required for normal proliferation and differentiation of HSPCs and their downstream progeny in zebrafish hematopoiesis. Further investigation into ism-1 and its associated signaling pathways may lead to the discovery of novel drug therapies for blood-based diseases and help expand HSPCs in vitro.