Abstract
Although the hepatic and hematopoietic progenitors of the liver are well characterized, the interactions between these two lineages remain mostly elusive. Hepatoblasts express delta-like noncanonical Notch ligand 1 (Dlk1), whose cleaved extracellular domain can become a soluble protein. We assessed the effects of DLK1 gene expression knockdown in cultures of total fetal liver cells. Furthermore, we separated Dlk1+ hepatoblasts from the total liver cell fraction and investigated effects of direct cell contact. Dlk1− cells were cultured either without Dlk1+ hepatoblasts, in direct contact with hepatoblasts, or separated from hepatoblasts by a porous membrane in inserts to inhibit cell contact but allow free exchange of molecules. Expression of the hepatic and hematopoietic genes, colony forming unit potential of various hematopoietic progenitors, and cell numbers and types were investigated. We found that DLK1 knockdown in total fetal liver cell cultures decreased total cell numbers. The expression of hepatic progenitor genes and mature hematopoietic genes was affected. Hematopoietic BFU-E and CFU-GM colony numbers were reduced significantly. The depletion of Dlk1+ hepatoblasts in culture decreased the potential of all hematopoietic progenitors to form colonies of all types and reduced the percentage of mature hematopoietic cells. The addition of hepatoblasts in inserts to Dlk1− cells further decreased the potential to form the CFU-GM and CFU-GEMM colonies and the percentage of mature hematopoietic cells but increased total cell numbers. Conclusively, direct contact of Dlk1 supports hematopoietic progenitor expansion and functionality that cannot be reconstituted in coculture without direct cell contact.
Highlights
During fetal liver development, hepatic stem cells give rise to transient hepatic progenitors, hepatoblasts [1, 2]
When total fetal liver cells were cultured with DLK1-targeting siRNA, the expression of the various hepatic
Our findings indicate that direct contact of delta-like noncanonical Notch ligand 1 (Dlk1) supported the expansion and functionality of hematopoietic progenitors; this effect could not be recapitulated in coculture without direct cell contact
Summary
Hepatic stem cells give rise to transient hepatic progenitors, hepatoblasts [1, 2]. Whereas hepatic stem cells are negative for the delta-like noncanonical Notch ligand 1 (Dlk1), fetal hepatoblasts are strongly Dlk1-positive [3]. The extracellular domain can be cleaved by ADAM17 (disintegrin and metalloproteinase domain-containing protein 17) or TACE (tumor necrosis factor-α-converting enzyme), giving rise to a soluble fragment. In adipogenesis, this soluble fragment has been shown to inhibit the differentiation of preadipocytes into adipocytes, while the membrane-bound Dlk, represses preadipocyte proliferation [5, 6]
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