Smyd5 plays pivotal roles in both primitive and definitive hematopoiesis during zebrafish embryogenesis

Histone modification constitutes one epigenetic mechanism that plays a critical role in the dynamic regulation of chromatin structure and gene expression, and several enzymes that catalyse histone modifications have been identified1. Histone lysine residue methylation contributes both positively and negatively to gene transcription, and a family of histone lysine methyltransferase containing the evolutionally conserved catalytic SET domain has been reported2. More than 60 SET domain-containing proteins have been identified in the mammalian genome; among them, the SMYD family, which is comprised of five members in humans, SMYD1–5, has been described3,4. Members of the SMYD family have been implicated in diverse biological functions in skeletal and cardiac muscle development as well as in cancer progression5,6,7,8,9. SMYD1, SMYD2, and SMYD3 show histone H3K4 methyltransferase activity7,10,11, as SMYD2 and SMYD3 methylate histones H3K36me2 and K5me1, respectively12,13. In addition, SMYD2 mediates the methylation of lysine residues of non-histone proteins such as tumour suppressor p53, retinoblastoma (RB), heat shock protein 90 (HSP90), and poly ADP-ribose polymerase (PARP1)14,15,16,17. Moreover, SMYD3 also catalyses non-histone proteins, such as vascular endothelial growth factor receptor (VEGFR) and mitogen-activated protein kinase 3/2 (MAPK3/K2)18,19. Unlike other family members, SMYD5 does not contain a C-terminal tetratricopeptide repeat (TPR) domain20. SMYD5 trimethylates H4K20 and negatively regulates inflammatory response genes21. However, the physiological function of SMYD5 remains largely unknown. Zebrafish (Danio rerio) provide an excellent model system with which to study the biological processes of vertebrates. Similar to mammalian models, zebrafish hematopoiesis consists of both primitive and definitive waves22. The primitive hematopoiesis wave occurs in the intermediate cell mass (ICM). Blood cell circulation begins around 24 hours post-fertilisation (hpf), at which time, hematopoiesis shifts from ICM to the posterior blood island (PBI)22. The definitive wave occurs in the aorta-gonadmesonephros (AGM) around 30 hpf 23. There are three hematopoietic stem cell (HSC) migration and colonisation events beginning around 48 hpf. AGM progenitor cells migrate to the caudal hematopoietic tissue (CHT), an intermediate site of hematopoiesis. Next, lymphocyte differentiation occurs in the thymus. Finally, kidney marrow produces all hematopoietic cell types, which corresponds to bone marrow hematopoiesis in mammals24. Five members of the Smyd family have been identified in zebrafish25. In the work described herein, we aimed to determine the physiological function of Smyd5 in zebrafish embryogenesis. Using a morpholino oligonucleotide (MO)-mediated knockdown and CRISPR/Cas9 knockout approach to smyd5 during zebrafish embryonic development, we found that Smyd5 plays a crucial role in hematopoiesis. These results indicate that Smyd5 represents an epigenetic regulator of hematopoiesis during zebrafish embryogenesis.