“Plod2” a Lysyl Hydroxylase Required for HSC Specification

Hematopoietic stem cells (HSCs) are self-renewing precursors that continuously renew hematopoietic lineages during life and form the basis of transplant therapies for hematologic and autoimmune diseases. Across vertebrate phyla, HSCs are developmentally specified from a transient population of specialized endothelial cells known as hemogenic endothelium (HE) located in the ventral floor of the dorsal aorta. Under the influence of intrinsic determinants and extrinsic differentiation cues, HE cells transition to a hematopoietic identity through an endothelial-to-hematopoietic transition (EHT). A complete understanding of the key signals required for HSC specification remains elusive. We have identified a novel regulator of HSC specification: Plod2 (Lysyl Hydroxylase 2) an endoplasmic reticulum-resident enzyme that catalyzes hydroxylation of key collagen lysines as a precondition to fibril crosslinking and maturation. Plod2 regulates extracellular matrix (ECM) stability, permeability, and stiffness, and has established roles described in malignant epithelial-to-mesenchymal transition (EMT), fibrosis and metastasis. We have examined in vivo expression and performed functional analyses in mouse and zebrafish. Our data show that zebrafish plod2 is expressed in tissues adjacent to the dorsal aorta at the time of EHT, while plod2 loss-of-function by targeted deletion or antisense morpholino abrogates emergence of HSCs in the AGM. Plod2 germline deletion in mice results in embryonic lethality by E12.5. Conditional deletion studies are currently being employed to identify the timing and tissue requirements for Plod2 in mammalian hematopoietic development. Hence, our studies identify Plod2 as a critical regulator of HSC specification.