Regulation of hematopoiesis in normal and stressed conditions

Over decades the research on pathophysiology and therapeutic solutions for β-thalassemia (BT) has been mostly focused on erythropoiesis, directly involved in the genetic defect. However, ineffective erythropoiesis and secondary alterations associated to the disease, as abnormal regulation of bone metabolism, iron overload and hormonal factors, induce changes in the BM homeostasis, thus having an impact on the BT bone marrow (BM) microenvironment. As hematopoietic stem cells (HSC) are regulated by signals from the BM niche, we hypothesized that an altered BM milieu might affect BT HSC-niche crosstalk and potentially the outcome of therapeutic HSC transplantation.  
We have recently demonstrated an altered BT HSC function in Hbbth3/+ mutant mice and in particular a lower quiescence and reduced reconstituting potential, as compared to normal controls (Aprile et al., Blood 2020). BT HSC have impaired self-renewal, which is rescued upon transplantation in a normal recipient niche, proving an active role of the BM niche environment. Guided by the clinical evidence of osteoporosis in a significant proportion of patients affected by BT and by recent results showing altered mesenchymal stromal cells in patients (Crippa et al., JCI 2019), we focused on the role of bone and mesenchymal cells in supporting HSC. We highlighted a reduced expression of key molecules of HSC-stromal niche interactions, such as Jagged-1 and Osteopontin, associated to the decreased levels of parathyroid hormone (PTH). Administration of PTH is sufficient to restore the stromal niche and reestablish the quiescent HSC pool.  
Starting from these findings, we are now investigating the potential mechanisms underlying the decreased PTH and bone defect in BT and we are focusing on molecules at the crossroads between erythropoiesis and bone metabolism, such as the fibroblast growth factor-23 (FGF-23). In vivo studies and molecular analyses will allow to evaluate the negative role of this hormone on bone mineralization and HSC-stromal niche interactions in BT, thus suggesting FGF-23 inhibition as a potential target to ameliorate bone and BM microenvironment defects. Moreover, since we found in BT additional abnormalities in the hematopoietic component of the BM, as defective maturation of megakaryocytes (Mk) and reduced levels of thrombopoietin (TPO), we hypothesized the contribution of other factors to the impaired HSC function. As both Mk and TPO play a key role in regulating the fate of HSC, we are currently investigating the molecular causes of BT dysmegakaryopoiesis and Mk-HSC crosstalk. These and other ongoing studies, such as the analysis of HSC metabolic profile and of the role of iron and inflammation on BT HSC activity, will unravel the multiple molecular mechanisms that affect in trans BT hematopoiesis in the complexity of the stressed BM microenvironment. Our results uncover a defect of HSC in BT, induced by an altered BM microenvironment and unveil underexplored mechanisms in the pathophysiology of the disease, with a potential impact on improving transplantation and gene therapy approaches.