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Cell-cell and cell-extracellular matrix interactions between hematopoietic stem cells (HSCs) and their niches are critical for the maintenance of stem cell properties. Here, it is demonstrated that a cell adhesion molecule, N-cadherin, is expressed in hematopoietic stem/progenitor cells (HSPCs) and plays a critical role in the regulation of HSPC engraftment. Furthermore, overexpression of N-cadherin in HSCs promoted quiescence and preserved HSC activity during serial bone marrow (BM) transplantation (BMT). Inhibition of N-cadherin by the transduction of N-cadherin short hairpin (sh) RNA (shN-cad) reduced the lodgment of donor HSCs to the endosteal surface, resulting in a significant reduction in long-term engraftment. shN-cad-transduced cells were maintained in the spleen for six months after BMT, indicating that N-cadherin expression in HSCs is specifically required in the BM. These findings suggest that N-cadherin-mediated cell adhesion is functionally essential for the regulation of HSPC activities in the BM niche.

Key Points• PGE 2 signaling positively regulates hematopoietic stem cells both directly and via activation of a nonhematopoietic cell population.• EP4 is a major receptor for the PGE 2 -mediated regulation of hematopoietic stem and progenitor cells.Prostaglandin E 2 (PGE 2 ) regulates hematopoietic stem/progenitor cell (HSPC) activity. However, the receptor(s) responsible for PGE 2 signaling remains unclear. Here, we identified EP4 as a receptor activated by PGE 2 to regulate HSPCs. Knockdown of Ep4 in HSPCs reduced long-term reconstitution capacity, whereas an EP4-selective agonist induced phosphorylation of GSK3b and b-catenin and enhanced long-term reconstitution capacity. Next, we analyzed the niche-mediated effect of PGE 2 in HSPC regulation. Bone marrow mesenchymal progenitor cells (MPCs) expressed EP receptors, and stimulation of MPCs with PGE 2 significantly increased their ability to support HSPC colony formation. Among the EP receptor agonists, only an EP4 agonist facilitated the formation of HSPC colonies after the coculture with MPCs. PGE 2 up-regulated the expression of cytokine-, cell adhesion-, extracellular matrix-, and protease-related genes in MPCs. We also examined the function of PGE 2 /EP4 signaling in the recovery of the HSPCs after myelosuppression. The administration of PGE 2 or an EP4 agonist facilitated the recovery of HSPCs from 5-fluorouracil (5-FU)-induced myelosuppression, indicating a role for PGE 2 /EP4 signaling in this process. Altogether, these data suggest that EP4 is a key receptor for PGE 2 -mediated direct and indirect regulation of HSPCs. (Blood.

Folliculin (FLCN) is an autosomal dominant tumor suppressor gene that modulates diverse signaling pathways required for growth, proliferation, metabolism, survival, motility and adhesion. FLCN is an essential protein required for murine embryonic development, embryonic stem cell (ESC) commitment, and Drosophila germline stem cell maintenance, suggesting that Flcn may be required for adult stem cell homeostasis. Conditional inactivation of Flcn in adult hematopoietic stem/progenitor cells (HSPCs) drives HSC into proliferative exhaustion resulting in the rapid depletion of HSPC, loss of all hematopoietic cell lineages, acute bone marrow failure, and mortality after 40 days. HSC that lack Flcn fail to reconstitute the hematopoietic compartment in recipient mice, demonstrating a cell-autonomous requirement for Flcn in HSC maintenance. Bone marrow cells (BMC) showed increased phosphorylation of Akt and mTorc1, and extramedullary hematopoiesis was significantly reduced by treating mice with rapamycin in vivo, suggesting that the mTorc1 pathway was activated by loss of Flcn expression in hematopoietic cells in vivo. Tfe3 was activated and preferentially localized to the nucleus of Flcn knockout (KO) HSPCs. Tfe3 overexpression in HSPCs impaired long term hematopoietic reconstitution in vivo, recapitulating the Flcn KO phenotype, and supporting the notion that abnormal activation of Tfe3 contributes to the Flcn KO phenotype. Flcn KO mice develop an acute histiocytic hyperplasia in multiple organs, suggesting a novel function for Flcn in macrophage development. Thus, Flcn is intrinsically required to maintain adult HSC quiescence and homeostasis, and Flcn loss leads to bone marrow failure and mortality in mice.