The role and mechanism of cell surface proteins in normal hematopoietic system and malignant hematological diseases
Authors:   , Wang Chen 
Published: November 2024

[EMBARGOED UNTIL 08/01/2025] Hematopoiesis, the production of all blood cells, is a critical and complex biological process essential for life. Governed by the intricate interplay of various signaling pathways, hematopoietic stem cells (HSCs) at the helm ensure a continual supply of blood cells throughout an organism's lifespan. Among these pathways, the Transforming Growth Factor-beta (TGF-[beta]) pathway is paramount, known for its pivotal role in modulating stem cell quiescence and activation. TGF-[beta] is a multifunctional cytokine that regulates a wide range of cellular behaviors, including proliferation, differentiation, and apoptosis. In the bone marrow niche, it particularly mediates HSC quiescence, preserving stem cell longevity and preventing premature exhaustion by maintaining these cells in a dormant state. This balance is crucial for hematopoietic health and regeneration, enabling the mobilization of stem cells for repair and recovery when needed. Src homology region 2 domain-containing phosphatase-1 (SHP-1) is a non-receptor protein tyrosine phosphatase initially recognized for its regulatory role in immune cell signaling. Recent discoveries have extended its significance to HSC biology, where it acts as a critical modulator of signaling pathways, particularly TGF-[beta]. SHP-1 dephosphorylates key tyrosine residues, thereby influencing the TGF-[beta] signaling cascade and subsequently, stem cell fate decisions. The interaction between SHP-1 and the TGF-[beta] pathway within HSCs underscores a sophisticated regulatory mechanism essential for the fine-tuning of hematopoietic stem cell behavior. Our research has explored this interaction extensively. The first study investigates how SHP-1 modulates TGF-[beta]1 production by megakaryocytes in the bone marrow, affecting HSC quiescence. This study unveils that SHP-1 interacts directly with the immunoreceptor tyrosine-based inhibition motif (ITIM) of the TGF-[beta] receptor 1, playing a crucial role in activating TGF-[beta] signaling in HSCs and maintaining their quiescence. Building on this foundational knowledge, the following study extends the understanding of SHP-1's function beyond mere maintenance of quiescence to its role in facilitating tissue repair post-ischemia. This research highlights SHP-1's mediation of HSC and progenitor cell responses to ischemic stress, regulating TGF-[beta]1 signaling to promote angiogenesis and effective tissue regeneration. These studies collectively enhance our understanding of the dual functionality of SHP-1 in both maintaining HSC quiescence and promoting tissue repair. The insights provided not only advance our knowledge of stem cell biology but also open new avenues for therapeutic interventions in treating hematological disorders. The progression from a normal hematopoietic system to malignant hematological diseases such as acute myeloid leukemia (AML) involves uncontrolled proliferation and accumulation of immature leukemic blasts. These cells exhibit genetic and epigenetic abnormalities that disrupt blood cell production. Recent research has focused on understanding the genetic and molecular drivers of leukemia, identifying mutations in key genes like FLT3 and TP53, and exploring targeted therapy approaches. Another significant focus is on leukemia stem cells (LSCs), aiming to understand their resistance to therapies and targeting them to prevent relapse. Despite advances, challenges remain. Resistance to targeted therapies, heterogeneity of the disease, and difficulties in detecting and eradicating minimal residual disease (MRD) are major unresolved issues. Traditional chemotherapy, while effective, often causes severe side effects due to its non-specific action against rapidly dividing cells, highlighting a critical need for more specific and less toxic therapeutic options. Leukemic-stem-cell-specific targeting may improve the survival of patients with AML by avoiding the ablative effects of standard regimens on normal hematopoiesis. Herein, we performed an unbiased screening of compounds targeting cell surface proteins and identify clinically used DPP4 inhibitors as strong suppressors of AML development in both murine AML models and primary human AML cells xenograft model. We found in retrovirus-induced AML mouse models that DPP4-deficient AML cell-transplanted mice exhibit delay and reversal of AML development, whereas deletion of DPP4 has no significant effect on normal hematopoiesis. DPP4 activates and sustains survival of AML stem cells that are critical for AML development in both human and animal models via binding with Src kinase and activation of nuclear factor kB (NF-kB) signaling. Thus, inhibition of DPP4 is a potential therapeutic strategy against AML development through suppression of survival and stemness of AML cells. The retention of minimal residual leukemic stem cells (LSCs) within the BM niche significantly influences therapeutic outcomes and the risk of leukemia relapse. While Cxcl12 is a well-established niche factor for LSCs, our following study uncovers a novel mechanism whereby alterations in the Cxcl12 gradient dictate LSC localization between the metaphysis (characterized by N-cadherin-expressing mesenchymal stromal/stem cells, N-cad+ MSCs) and the central marrow (CM) region (marked by sinusoid endothelial cells and associated stromal cells). This gradient modulation impacts distinct LSC properties--either quiescence/survival or proliferation/apoptosis. We demonstrate that the Cxcl12 levels within the BM microenvironment are finely regulated by the interaction between DPP4 on AML cells and glypican-3 (GPC3) on N-cad+ MSCs, thereby establishing the Cxcl12 gradient across the BM. Depletion of DPP4 in LSCs or Cxcl12 deficiency in N-cad+ MSCs shifts LSC localization from the metaphysis to the CM, resulting in impaired AML progression. These findings suggest a potential therapeutic strategy by targeting this gradient. Our results provide critical insights into the role of Cxcl12 in LSC localization, emphasizing the niche's influence on LSC behavior and AML progression. This novel understanding presents a promising avenue for targeted therapeutic interventions against minimal residual LSCs, potentially paving the way for achieving a lasting cure for AML.