Impact of osteoblast maturation on their paracrine growth enhancing activity on cord blood progenitors

Angiopoietin-like 2 is essential to aortic valve development in mice

Aortic valve (AoV) abnormalities during embryogenesis are a major risk for the development of aortic valve stenosis (AVS) and cardiac events later in life. Here, we identify an unexpected role for Angiopoietin-like 2 (ANGPTL2), a pro-inflammatory protein secreted by senescent cells, in valvulogenesis. At late embryonic stage, mice knocked-down for Angptl2 (Angptl2-KD) exhibit a premature thickening of AoV leaflets associated with a dysregulation of the fine balance between cell apoptosis, senescence and proliferation during AoV remodeling and a decrease in the crucial Notch signalling. These structural and molecular abnormalities lead toward spontaneous AVS with elevated trans-aortic gradient in adult mice of both sexes. Consistently, ANGPTL2 expression is detected in human fetal semilunar valves and associated with pathways involved in cell cycle and senescence. Altogether, these findings suggest that Angptl2 is essential for valvulogenesis, and identify Angptl2-KD mice as an animal model to study spontaneous AVS, a disease with unmet medical need.

Role of ex vivo Expanded Mesenchymal Stromal Cells in Determining Hematopoietic Stem Cell Transplantation Outcome

Overall, the human organism requires the production of ∼1 trillion new blood cells per day. Such goal is achieved via hematopoiesis occurring within the bone marrow (BM) under the tight regulation of hematopoietic stem and progenitor cell (HSPC) homeostasis made by the BM microenvironment. The BM niche is defined by the close interactions of HSPCs and non-hematopoietic cells of different origin, which control the maintenance of HSPCs and orchestrate hematopoiesis in response to the body’s requirements. The activity of the BM niche is regulated by specific signaling pathways in physiological conditions and in case of stress, including the one induced by the HSPC transplantation (HSCT) procedures. HSCT is the curative option for several hematological and non-hematological diseases, despite being associated with early and late complications, mainly due to a low level of HSPC engraftment, impaired hematopoietic recovery, immune-mediated graft rejection, and graft-versus-host disease (GvHD) in case of allogenic transplant. Mesenchymal stromal cells (MSCs) are key elements of the BM niche, regulating HSPC homeostasis by direct contact and secreting several paracrine factors. In this review, we will explore the several mechanisms through which MSCs impact on the supportive activity of the BM niche and regulate HSPC homeostasis. We will further discuss how the growing understanding of such mechanisms have impacted, under a clinical point of view, on the transplantation field. In more recent years, these results have instructed the design of clinical trials to ameliorate the outcome of HSCT, especially in the allogenic setting, and when low doses of HSPCs were available for transplantation.

Insights into the Hematopoietic Regulatory Activities of Osteoblast by Secretomics

Osteoblasts are a key component of the endosteal hematopoietic stem cell (HSC) niche and are recognized with strong hematopoietic supporting activity. Similarly, mesenchymal stromal cells (MSC)-derived osteoblast (M-OST) conditioned media (OCM) enhances the growth of hematopoietic progenitors in culture and modulate their engraftment activity. We aimed to characterize the hematopoietic supporting activity of OCM by comparing the secretome of M-OST to that of their precursor. Over 300 proteins were quantified by mass spectroscopy in media conditioned with MSC or M-OST, with 47 being differentially expressed. Included were growth factors, extracellular matrix (ECM) proteins and proteins from the complement pathways. The functional contribution of selected proteins on the growth and differentiation of cord blood (CB) progenitors was tested. Secreted Protein Acidic and Rich in Cysteine (SPARC) and Galectin 3 (Gal3) had little impact on the growth of CB cells in serum-free medium (SFM). In contrast, inhibition of the complement 3 A receptor (C3a-R) present on CB progenitors significantly reduced the growth of CD34+ cells in OCM cultures but not in SFM. These results provide new insights into changes in factors released by MSC undergoing osteoblast differentiation, and on paracrine factors that are partially responsible for the hematopoietic supporting activity of osteoblasts. This article is protected by copyright. All rights reserved.

The effects of mechanically loaded osteocytes and inflammation on bone remodeling in a bisphosphonate-induced environment

Bisphosphonate-related osteonecrosis of the jaw is a disease appearing after tooth removal in patients undergoing bisphosphonate treatment for metastasizing cancers and osteoporosis. The complexity of the condition requires a multicellular model to address the net effects of two key risk factors: mechanical trauma (pathologic overload) and inflammation. In this work, a system comprised of a polydimethylsiloxane chip and mechanical loading device is used to expose bisphosphonate-treated osteocytes to mechanical trauma. Specifically, osteocytes are treated with the potent nitrogen-containing bisphosphonate, zoledronic acid, and exposed to short-term pathologic overload via substrate stretch. During bone remodeling, osteocyte apoptosis plays a role in attracting pre-osteoclasts to sites of damage; as such, lactate dehydrogenase activity, cell death and protein expression are evaluated as functions of load. Additionally, the effects of osteocyte soluble factors on osteoclast and osteoblast functional activity are quantified. Osteoclast activity and bone resorption are quantified in the presence and absence of inflammatory components, lipopolysaccharide and interferon gamma. Results suggest that inflammation associated with bacterial infection may hinder bone resorption by osteoclasts. In addition, osteocytes may respond to overload by altering expression of soluble signals that act on osteoblasts to attenuate bone formation. These findings give insight into the multicellular interactions implicated in bisphosphonate-related osteonecrosis of the jaw.

Paracrine Factors Released by Osteoblasts Provide Strong Platelet Engraftment Properties

Ex vivo expansion of hematopoietic stem cell (HSCs) and progenitors may one day overcome the slow platelet engraftment kinetics associated with umbilical cord blood transplantation. Serum-free medium conditioned with osteoblasts (i.e., osteoblast-conditioned medium [OCM]) derived from mesenchymal stromal cells (MSC) was previously shown to increase cell growth and raise the levels of human platelets in mice transplanted with OCM-expanded progenitors. Herein, we characterized the cellular and molecular mechanisms responsible for these osteoblast-derived properties. Limiting dilution transplantation assays revealed that osteoblasts secrete soluble factors that synergize with exogenously added cytokines to promote the production of progenitors with short-term platelet engraftment activities, and to a lesser extent with long-term platelet engraftment activities. OCM also modulated the expression repertoire of cell-surface receptors implicated in the trafficking of HSC and progenitors to the bone marrow. Furthermore, OCM contains growth factors with prosurvival and proliferation activities that synergized with stem cell factor. Insulin-like growth factor (IGF)-2 was found to be present at higher levels in OCM than in control medium conditioned with MSC. Inhibition of the IGF-1 receptor, which conveys IGF-2' intracellular signaling, largely abolished the growth-promoting activity of OCM on immature CD34⁺ subsets and progenitors in OCM cultures. Finally, IGF-1R effects appear to be mediated in part by the coactivator β-catenin. In summary, these results provide new insights into the paracrine regulatory activities of osteoblasts on HSC, and how these can be used to modulate the engraftment properties of human HSC and progenitors expanded in culture. Stem Cells 2019;37:345-356.