Osteosclerosis and inhibition of human hematopoiesis in NOG mice expressing human Delta-like 1 in osteoblasts

Notch signaling pathway in osteogenesis, bone development, metabolism, and diseases

The skeletal system provides vital importance to support organ development and functions. The Notch signaling pathway possesses well-established functions in organ development and cellular homeostasis. The Notch signaling pathway comprises five typical ligands (JAG1, JAG2, DLL1, DLL3, and DLL4), four receptors (Notch1-4), and four intracellular domains (NICD1-4). Each component of the Notch signaling pathway has been demonstrated to be fundamental in osteoblast differentiation and bone formation. The dysregulation in the Notch signaling pathway is highly linked with skeletal disorders or diseases at the developmental and postnatal stages. Recent studies have highlighted the importance of the elements of the Notch signaling pathway in the skeletal system, as well as its interaction with signaling, such as Wnt/β-catenin, BMP, TGF-β, FGF, autophagy, and hedgehog (Hh) to construct a potential gene regulatory network to orchestrate osteogenesis and ossification. Our review has provided a comprehensive summary of the Notch signaling pathway in the skeletal system, as well as the insights targeting Notch signaling for innovative potential drug discovery targets or therapeutic interventions to treat bone disorders, such as osteoporosis and osteoarthritis. An in-depth molecular mechanistic strategy to modulate the Notch signaling pathway and its associated signaling pathway will be encouraged for consideration to trigger enhanced therapeutic approaches for bone disorders by defining Notch-regulating drugs for clinical use.

Progression of Notch signaling regulation of B cells under radiation exposure

With the continuous development of nuclear technology, the radiation exposure caused by radiation therapy is a serious health hazard. It is of great significance to further develop effective radiation countermeasures. B cells easily succumb to irradiation exposure along with immunosuppressive response. The approach to ameliorate radiation-induced B cell damage is rarely studied, implying that the underlying mechanisms of B cell damage after exposure are eager to be revealed. Recent studies suggest that Notch signaling plays an important role in B cell-mediated immune response. Notch signaling is a critical regulator for B cells to maintain immune function. Although accumulating studies reported that Notch signaling contributes to the functionality of hematopoietic stem cells and T cells, its role in B cells is scarcely appreciated. Presently, we discussed the regulation of Notch signaling on B cells under radiation exposure to provide a scientific basis to prevent radiation-induced B cell damage.

Transcriptomics Analysis Reveals New Insights into the Roles of Notch1 Signaling on Macrophage Polarization

Naïve macrophages (Mφ) polarize in response to various environmental cues to a spectrum of cells that have distinct biological functions. The extreme ends of the spectrum are classified as M1 and M2 macrophages. Previously, we demonstrated that Notch1 deficiency promotes Tgf-β2 dependent M2-polarization in a mouse model of abdominal aortic aneurysm. The present studies aimed to characterize the unique set of genes regulated by Notch1 signaling in macrophage polarization. Bone marrow derived macrophages isolated from WT or Notch1+/- mice (n = 12) were differentiated to Mφ, M1 or M2-phenotypes by 24 h exposure to vehicle, LPS/IFN-γ or IL4/IL13 respectively and total RNA was subjected to RNA-Sequencing (n = 3). Bioinformatics analyses demonstrated that Notch1 haploinsufficiency downregulated the expression of 262 genes at baseline level, 307 genes with LPS/IFN-γ and 254 genes with IL4/IL13 treatment. Among these, the most unique genes downregulated by Notch1 haploinsufficiency included fibromodulin (Fmod), caspase-4, Has1, Col1a1, Alpl and Igf. Pathway analysis demonstrated that extracellular matrix, macrophage polarization and osteogenesis were the major pathways affected by Notch1 haploinsufficiency. Gain and loss-of-function studies established a strong correlation between Notch1 haploinsufficiency and Fmod in regulating Tgf-β signaling. Collectively, our studies suggest that Notch1 haploinsufficiency increases M2 polarization through these newly identified genes.

Notch Signaling in the Regulation of Hematopoietic Stem Cell

Purpose of Review

Understanding the signaling pathways that govern hematopoietic stem and progenitor cells (HSPCs) is fundamental to uncover their regulation and how this is skewed in hematological malignancies. Whether Notch is necessary for the regulation of mammalian HSPCs is still unclear. We therefore critically review the current literature on the role of Notch in HSPCs.

Recent Findings

HSPCs have shown different requirements for Notch signals in vitro and in vivo and at different stages of differentiation. Additionally, bone marrow niche cells activate Notch signaling in HSPCs enhancing their regenerative and self-renewal capacity.

Summary

Despite the controversy, adequate levels of Notch signaling appear necessary to avoid the development of hematological malignancies. Contrary to early studies, recent research suggests that Notch signaling may play a role in homeostatic and regenerative hematopoiesis but further investigation is necessary to understand how it is regulated by the different ligand/receptor pairings and the molecular mechanisms that are triggered.

Maintenance of Bone Homeostasis by DLL1-Mediated Notch Signaling

Adult bone mass is maintained through a balance of the activities of osteoblasts and osteoclasts. Although Notch signaling has been shown to maintain bone homeostasis by controlling the commitment, differentiation, and function of cells in both the osteoblast and osteoclast lineages, the precise mechanisms by which Notch performs such diverse and complex roles in bone physiology remain unclear. By using a transgenic approach that modified the expression of delta‐like 1 (DLL1) or Jagged1 (JAG1) in an osteoblast‐specific manner, we investigated the ligand‐specific effects of Notch signaling in bone homeostasis. This study demonstrated for the first time that the proper regulation of DLL1 expression, but not JAG1 expression, in osteoblasts is essential for the maintenance of bone remodeling. DLL1‐induced Notch signaling was responsible for the expansion of the bone‐forming cell pool by promoting the proliferation of committed but immature osteoblasts. However, DLL1‐Notch signaling inhibited further differentiation of the expanded osteoblasts to become fully matured functional osteoblasts, thereby substantially decreasing bone formation. Osteoblast‐specific expression of DLL1 did not alter the intrinsic differentiation ability of cells of the osteoclast lineage. However, maturational arrest of osteoblasts caused by the DLL1 transgene impaired the maturation and function of osteoclasts due to a failed osteoblast‐osteoclast coupling, resulting in severe suppression of bone metabolic turnover. Taken together, DLL1‐mediated Notch signaling is critical for proper bone remodeling as it regulates the differentiation and function of both osteoblasts and osteoclasts. Our study elucidates the importance of ligand‐specific activation of Notch signaling in the maintenance of bone homeostasis. J. Cell. Physiol. 232: 2569–2580, 2017. © 2016 The Authors. Journal of Cellular Physiology Published by Wiley Periodicals Inc.

Myelosuppressive therapies significantly increase pro-inflammatory cytokines and directly cause bone loss

Skeletal-related events resulting from accelerated bone loss are common complications in patients treated for a range of cancers. However, the mechanisms and rate of bone loss after myelosuppression are unclear. We, therefore, investigated this in mice and humans. We treated mice with different myelosuppressive therapies (chemotherapy or irradiation with or without transplantation) and studied their effects on bone structure. Myelosuppression of mice rapidly caused an increase in bone resorption that was not matched by bone formation. The resultant significant and persistent bone loss early after therapy was associated with increased inflammatory cytokines, in particular, monocyte chemoattractant protein 1 (MCP1). Therapy-induced bone loss was prevented with a single dose of the bisphosphonate zoledronic acid (ZA), administered before myelosuppression. Importantly, ZA treatment of mice did not impair hematopoiesis, including hematopoietic stem cell function. Furthermore, examination of serum from patients before and after autologous or allogeneic stem cell transplantion (SCT) revealed altered levels of bone turnover markers and elevated inflammatory cytokines. MCP1 levels in serum obtained between days 7 and 14 post-SCT positively correlated with bone loss observed at 100 days after allogeneic SCT. Similar to that observed in our studies in mice, the bone loss was long term, persisting at 12 months post-SCT. Furthermore, patients who received chemotherapy less than 100 days before SCT had significantly more bone loss at the hip. In these patients, serum levels of MCP1, but not routine biomarkers of bone turnover, including C-terminal cross-linking telopeptide of type-1 collagen (β-CTx), positively correlated with their bone loss. Hence, myelosuppressive therapies increase inflammation and directly contribute to bone loss. Administration of an osteoclast inhibitor before the initiation of cancer therapy is likely to have the best outcome in preventing bone loss in patients with cancer.

Effective expansion of engrafted human hematopoietic stem cells in bone marrow of mice expressing human Jagged1

The human immune system can be reconstituted in experimental animals by transplanting human hematopoietic stem cells (hHSCs) into immunodeficient mice. To generate such humanized mice, further improvements are required, particularly to ensure that transplanted hHSCs are maintained in mice and proliferate long enough to follow prolonged immune responses to chronic diseases or monitor therapeutic effects. To prepare the relatively human bone marrow environment in mice, we generated nonobese diabetic/severe combined immunodeficiency/interleukin-2 receptor gamma chain null (NOG) mice expressing human Jagged1 (hJ1) in an osteoblast-specific manner (hJ1-NOG mice) to examine whether Notch signaling induced by hJ1 mediates hHSC proliferation and/or maintenance in mice. The established hJ1-NOG mice possess relatively larger bone marrow space and thinner cortical bone compared with nontransgenic littermates, but the number of c-kit(+) Sca-1(+) lineage(-) cells was not significantly different between hJ1-NOG and nontransgenic littermates. In the transplantation experiments of CD34(+) cells obtained from human cord blood, CD34(+)CD38(-) cells (hHSCs) were more increased in hJ1-NOG recipient mice than in nontransgenic littermates in mouse bone marrow environment. In contrast, the transplanted mouse c-kit(+) Sca-1(+) lineage(-) cells did not show significant increase in the same hJ1-NOG mice. These results suggest that hJ1-NOG mice could contribute to the growth of transplanted human CD34(+) cells in a human-specific manner and be useful to study the in vivo behavior and/or development of human stem cells, including cancer stem cells and immune cells.