CD226 is involved in megakaryocyte activation and early-stage differentiation

Megakaryocyte Secreted Factors Regulate Bone Marrow Niche Cells During Skeletal Homeostasis, Aging, and Disease

The bone marrow microenvironment contains a diverse array of cell types under extensive regulatory control and provides for a novel and complex mechanism for bone regulation. Megakaryocytes (MKs) are one such cell type that potentially acts as a master regulator of the bone marrow microenvironment due to its effects on hematopoiesis, osteoblastogenesis, and osteoclastogenesis. While several of these processes are induced/inhibited through MK secreted factors, others are primarily regulated by direct cell-cell contact. Notably, the regulatory effects that MKs exert on these different cell populations has been found to change with aging and disease states. Overall, MKs are a critical component of the bone marrow that should be considered when examining regulation of the skeletal microenvironment. An increased understanding of the role of MKs in these physiological processes may provide insight into novel therapies that can be used to target specific pathways important in hematopoietic and skeletal disorders.

The Effect of CD226 on the Balance between Inflammatory Monocytes and Small Peritoneal Macrophages in Mouse Ulcerative Colitis

Ulcerative colitis (UC) is a refractory and recurring inflammatory bowel disease (IBD). Monocytes and macrophages are major components of the mononuclear phagocyte system (MPS), and the balance between inflammatory monocytes and small peritoneal macrophages plays important roles in UC. However, the mechanisms governing the balance between inflammatory monocytes and small peritoneal macrophages in UC need to be clarified further. Here, we found that the expression levels of CD226 on different subsets of monocytes/macrophages are varied in UC mice. The expression levels of CD226 on patrolling monocytes (pMos) and small peritoneal macrophages (SPMs) were markedly increased, while the expression levels of CD226 on inflammatory monocytes (iMos) were decreased in UC mice. Significantly, the percentage of iMos was enhanced while the percentage of SPMs were decreased in CD226 knockout UC mice compared with that in wildtype UC mice. Moreover, CD226 deficiency suppressed the migration capacity of macrophages. Therefore, our data suggest that CD226 plays critical roles in regulating the function and balance of monocytes/macrophages in mouse UC and targeting CD226 in MPS may be developed as a potent therapy for UC.

CD226 Is Required to Maintain Megakaryocytes/Platelets Homeostasis in the Treatment of Knee Osteoarthritis With Platelet-Rich Plasma in Mice

Platelet-rich plasma (PRP) is a platelet-based application used to treat osteoarthritis (OA) clinically. The co-stimulatory molecule CD226 is expressed in T cells, NK cells, and also platelets. However, exact effects of CD226 on platelets and whether its expression level influences PRP efficacy are largely unknown. Here, CD226^fl/flPF4-Cre mice were obtained from mating CD226 ^fl/fl mice with PF4-Cre mice. Blood samples and washed platelets were collected from the mice eyeballs to undergo routine blood tests and transmission electron microscopy. Differentially expressed proteins were detected by iTRAQ-based proteomics analysis. Animal OA models were established through surgical destabilization of the medial meniscus (DMM) for C57BL/6 wildtype mice, followed by PRP injection to evaluate the effects of platelet CD226 on PRP efficacy. The results showed that deletion of platelet CD226 increased the number of megakaryocytes (MKs) in bone marrow (BM) but reduced MKs in spleen, combined with significantly decreased platelet amounts, α-granule secretion, and reduced immature platelets; indicating that absence of platelet CD226 may disrupt MK/platelet homeostasis and arrested platelet release from MKs. Sequencing analysis showed abnormal ribosomal functions and much downregulated proteins in the absence of platelet CD226. Autophagy-related proteins were also reduced in the CD226-absent MKs/platelets. Moreover, deletion of platelet CD226 diminished the protective effects of PRP on DMM-induced cartilage lesions in mice, and PDGF restored it. Therefore, deficiency of platelet CD226 inhibited platelet maturation, secretion, and normal ribosomal functions, which may lead to depressed PRP efficacy on OA, suggesting that CD226 is required to regulate platelet growth, functions, and its application.

Thrombopoietic effects of CCAAT/enhancer-binding protein β on the early-stage differentiation of megakaryocytes

CCAAT/enhancer-binding protein β (C/EBPβ) is a transcription factor that is involved in adipocytic and monocytic differentiation. However, the physiological role of C/EBPβ in megakaryocytes (MKs) is not clear. In this study, we investigated the effects of C/EBPβ on the early-stage differentiation of MKs, and explored the potential mechanisms of action. We established a cytosine arabinoside-induced thrombocytopenia mouse model using C57BL/6 mice. In the thrombocytopenia mice, the platelet count was found to be decreased, and the mRNA and protein expression levels of C/EBPβ in MKs were also reduced. Furthermore, the maturation of Dami (MKs cell line) cells was induced by phorbol 12-myristate 13-acetate. When C/EBPβ was silenced in Dami cells by transfection using C/EBPβ-small interfering RNA, the expression of MKs-specific markers CD41 and CD62P, was dramatically decreased, resulting in morphological changes and differentiation retardation in low ploidy, which were evaluated using flow cytometry, real-time polymerase chain reaction, western blot, and confocal microscopy. The mitogen activated protein kinase-extracellular signal-regulated kinase signaling pathway was found to be required for the differentiation of MKs; knockdown of C/EBPβ in MEK/ERK1/2 pathway attenuated MKs differentiation. Overexpression of C/EBPβ in MEK/ERK1/2 pathway inhibited by U0126 did not promote MKs differentiation. To the best of our knowledge, C/EBPβ plays an important role in MKs differentiation and polyploidy cell cycle control. Taken together, C/EBPβ may have thrombopoietic effects in the differentiation of MKs, and may assist in the development of treatments for various disorders.

CD226 deficiency attenuates the homeostasis and suppressive capacity of Tr1 cells

T regulatory type 1 (Tr1) cells act as a key regulator in maintaining peripheral immune tolerance. Several costimulatory molecules for T cells have been identified in Tr1 cells, but their intrinsic functions are still unclear. Here we showed CD226 was highly expressed in Tr1 cells. CD226-deficient Tr1 cells were defective in proliferation and sensitive to apoptosis. In addition, CD226-deficient Tr1 cells showed lower inhibitory capacity of T cell proliferation and reduced IL-10 production. CD226 deficiency also inhibited Tr1 cell differentiation in vitro. When stimulated with IL-2, CD226-deficient Tr1 cells showed impaired STAT5 signaling. Therefore, our data suggest CD226 might play an important role in Tr1 cell homeostasis, function and differentiation. This study facilitates further biological characterization of this regulatory T cell subset.

Deficiency of platelet adhesion molecule CD226 causes megakaryocyte development and platelet hyperactivity

This study used constitutive CD226 gene knockout (KO) mice as a model to investigate the functions and mechanisms of CD226 in megakaryocyte (MK) maturation and platelet activation. Although CD226 deficiency did not cause MK polyploidization or platelet granule abnormalities, increased MK counts were detected in the femora bone marrow (BM) and spleen of CD226 KO mice. Particularly, CD226 KO mice have a more extensive membrane system in MKs and platelets than wild-type (WT) mice. We also demonstrated that CD226 KO mice displayed increased platelet counts, shortened bleeding time, and enhanced platelet aggregation. CD226 KO platelets had an increased mature platelet ratio compared to the control platelets. In addition, the observed reduction in bleeding time may be due to decreased nitric oxide (NO) production in the platelets. Platelet-specific CD226-deficient mice showed similar increased MK counts, shortened bleeding time, enhanced platelet aggregation, and decreased NO production in platelets. Furthermore, we performed middle cerebral artery occlusion-reperfusion surgery on WT and CD226 KO mice to explore the potential effect of CD226 on acute ischemia-reperfusion injury; the results revealed that CD226 deficiency led to significantly increased infarct area. Thus, CD226 is a promising candidate for the treatment of thrombotic disorders.