Role of let-7b/Fzd4 axis in mitochondrial biogenesis through wnt signaling: In neonatal and adult megakaryocytes

Epigenetic regulation of megakaryopoiesis and platelet formation

Platelets, produced by megakaryocytes, play unique roles in physiological processes, such as hemostasis, coagulation, and immune regulation, while also contributing to various clinical diseases. During megakaryocyte differentiation, the morphology and function of cells undergo significant changes due to the programmed expression of a series of genes. Epigenetic changes modify gene expression without altering the DNA base sequence, effectively affecting the inner workings of the cell at different stages of growth, proliferation, differentiation, and apoptosis. These modifications also play important roles in megakaryocyte development and platelet biogenesis. However, the specific mechanisms underlying epigenetic processes and the vast epigenetic regulatory network formed by their interactions remain unclear. In this review, we systematically summarize the key roles played by epigenetics in megakaryocyte development and platelet formation, including DNA methylation, histone modification, and non-coding RNA regulation. We expect our review to provide a deeper understanding of the biological processes underlying megakaryocyte development and platelet formation and to inform the development of new clinical interventions aimed at addressing platelet-related diseases and improving patients' prognoses.

Role of microRNAs and their downstream target transcription factors in zebrafish thrombopoiesis

Previous studies have shown that human platelets and megakaryocytes carry microRNAs suggesting their role in platelet function and megakaryocyte development, respectively. However, a comprehensive study on the microRNAs and their targets has not been undertaken. Zebrafish thrombocytes could be used as a model to study their role in megakaryocyte maturation and platelet function because thrombocytes have both megakaryocyte features and platelet properties. In our laboratory, we identified 15 microRNAs in thrombocytes using single-cell RNA sequencing. We knocked down each of these 15 microRNAs by the piggyback method and found knockdown of three microRNAs, mir-7148, let-7b, and mir-223 in adult zebrafish led to an increase in the percentage of thrombocytes. Functional thrombocyte analysis using plate tilt assay showed no modulatory effect of the three microRNAs on thrombocyte aggregation/agglutination. We also found enhanced thrombosis using arterial laser thrombosis assay in a group of zebrafish larvae after mir-7148, let-7b, and mir-223 knockdowns. These results suggested mir-7148, let-7b, and mir-223 are repressors for thrombocyte production. We then explored miRWalk database for let-7b downstream targets and then selected those that are expressed in thrombocytes, and from this list based on their role in differentiation selected 14 genes, rorca, tgif1, rfx1a, deaf1, zbtb18, mafba, cebpa, spi1a, spi1b, fhl3b, ikzf1, irf5, irf8, and lbx1b that encode transcriptional regulators. The qRT-PCR analysis of expression levels of the above genes following let-7b knockdown showed changes in the expression of 13 targets. We then studied the effect of the 13 targets on thrombocyte production and identified 5 genes, irf5, tgif1, irf8, cebpa, and rorca that showed thrombocytosis and one gene, ikzf1 that showed thrombocytopenia. Furthermore, we tested whether mir-223 regulates any of the above 13 transcription factors after mir-223 knockdown using qRT-PCR. Six of the 13 genes showed similar gene expression as observed with let-7b knockdown and 7 genes showed opposing results. Thus, our results suggested a possible regulatory network in common with both let-7b and mir-223. We also identified that tgif1, cebpa, ikzf1, irf5, irf8, and ikzf1 play a role in thrombopoiesis. Since the ikzf1 gene showed a differential expression profile in let-7b and mir-223 knockdowns but resulted in thrombocytopenia in ikzf1 knockdown in both adults and larvae we also studied an ikzf1 mutant and showed the mutant had thrombocytopenia. Taken together, these studies showed that thrombopoiesis is controlled by a network of transcription regulators that are regulated by multiple microRNAs in both positive and negative manner resulting in overall inhibition of thrombopoiesis.

Role of MicroRNAs and their Downstream Target Transcription Factors in Zebrafish Thrombopoiesis

Previous studies have shown that human platelets and megakaryocytes carry microRNAs suggesting their role in platelet function and megakaryocyte development, respectively. However, a comprehensive study on the microRNAs and their targets has not been undertaken. Zebrafish thrombocytes could be used as a model to study their role in megakaryocyte maturation and platelet function because thrombocytes have both megakaryocyte features and platelet properties. In our laboratory, we identified 15 microRNAs in thrombocytes using single-cell RNA sequencing. We knocked down each of these 15 microRNAs by the piggyback method and found knockdown of three microRNAs, mir-7148, let-7b , and mir-223 in adult zebrafish led to an increase in the percentage of thrombocytes. Functional thrombocyte analysis using plate tilt assay showed no modulatory effect of the three microRNAs on thrombocyte aggregation/agglutination. We also found enhanced thrombosis using arterial laser thrombosis assay in a group of zebrafish larvae after mir-7148, let-7b , and mir-223 knockdowns. These results suggested mir-7148, let-7b , and mir-223 are repressors for thrombocyte production. We then explored miRWalk database for let-7b downstream targets and then selected those that are expressed in thrombocytes, and from this list based on their role in differentiation selected 14 genes, rorca, tgif1, rfx1a, deaf1, zbtb18, mafba, cebpa, spi1a, spi1b, fhl3b, ikzf1, irf5, irf8 , and lbx1b that encode transcriptional regulators. The qRT-PCR analysis of expression levels of the above genes following let-7b knockdown showed changes in the expression of 13 targets. We then studied the effect of the 13 targets on thrombocyte production and identified 5 genes, irf5, tgif1, irf8, cebpa , and rorca that showed thrombocytosis and one gene, ikzf1 that showed thrombocytopenia. Furthermore, we tested whether mir-223 regulates any of the above 13 transcription factors after mir-223 knockdown using qRT-PCR. Six of the 13 genes showed similar gene expression as observed with let-7b knockdown and 7 genes showed opposing results. Thus, our results suggested a possible regulatory network in common with both let-7b and mir-223 . We also identified that tgif1, cebpa, ikzf1, irf5 , irf8 , and ikzf1 play a role in thrombopoiesis. Since the ikzf1 gene showed a differential expression profile in let-7b and mir-223 knockdowns but resulted in thrombocytopenia in ikzf1 knockdown in both adults and larvae we also studied an ikzf1 mutant and showed the mutant had thrombocytopenia. Taken together, these studies showed that thrombopoiesis is controlled by a network of transcription regulators that are regulated by multiple microRNAs in both positive and negative manner resulting in overall inhibition of thrombopoiesis.

Megakaryocyte ontogeny: Clinical and molecular significance

Fetal megakaryocytes (Mks) differ from adult Mks in key parameters that affect their capacity for platelet production. However, despite being smaller, more proliferative, and less polyploid, fetal Mks generally mature in the same manner as adult Mks. The phenotypic features unique to fetal Mks predispose patients to several disease conditions, including infantile thrombocytopenia, infantile megakaryoblastic leukemias, and poor platelet recovery after umbilical cord blood stem cell transplantations. Ontogenic Mk differences also affect new strategies being developed to address global shortages of platelet transfusion units. These donor-independent, ex vivo production platforms are hampered by the limited proliferative capacity of adult-type Mks and the inferior platelet production by fetal-type Mks. Understanding the molecular programs that distinguish fetal versus adult megakaryopoiesis will help in improving approaches to these clinical problems. This review summarizes the phenotypic differences between fetal and adult Mks, the disease states associated with fetal megakaryopoiesis, and recent advances in the understanding of mechanisms that determine ontogenic Mk transitions.

Bcl-xL/Bak interaction and regulation by miRNA let-7b in the intrinsic apoptotic pathway of stored platelets

Bcl-2 family proteins play key roles in the intrinsic apoptosis pathway in platelets, with both pro- and antiapoptotic protein expressions regulating survival during ex vivo storage. We detected a significant decrease in antiapoptotic Bcl-x_L and increase in proapoptotic Bak expression on the third day of storage and as a result the ratio of Bak:Bcl-x_L also decreased. Moreover, we identified an interaction between Bcl-x_L and Bak. These shifts corresponded with activation of the apoptotic pathway, suggesting these proteins might play an important role in platelet survival. We then performed bioinformatic analysis to gain insight into protein expression regulation during storage. This identified a potential binding site of the microRNA (miRNA) let-7b in the 3'-UTR of the Bcl-x_L gene, which we confirmed by a dual-luciferase reporter assay. We also determined that let-7b was upregulated during platelet storage, and let-7b transfection influenced Bcl-x_L and Bak protein, but not mRNA, expression. Together, these data suggest that only posttranscriptional mechanisms are available for regulating gene expression in anucleate platelets.

Let7b modulates the Wnt/β-catenin pathway in liver cancer cells via downregulated Frizzled4

Let7 microRNA implicated in many cellular processes and participated in the progress of various tumors. Similarly, Wnt signaling pathway plays an important role in morphogenesis, differentiation, cell survival, and proliferation. However, there is little research focusing on the relevance between Let7b and Wnt/β-catenin signaling pathway, especially in liver cancer cell. To study this, human liver cancer cells HUH7 and MHCC97H were cultured, enhanced, or inhibited the expression of Let7b in two cell lines. Western blotting was used to measure the expression of Wnt signaling-related protein β-catenin and Frizzled family receptor. CD24⁺133⁺ was used as a cancer stem cell marker, and the proportion of CD24⁺133⁺ in liver cancer cell lines was observed by flow cytometry. The proliferation, invasiveness, and migration of liver cancer cells were assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, transwell, and wound healing assays. The research revealed that enhanced expression of Let7b decreased the expression of Frizzled4, while inhibited Let7b expression increased Frizzled4 expression. Enhanced Let7b expression reduced the proportion of cancer stem cell in liver cancer cell; meanwhile, Let7b inhibition increased the proportion of cancer stem cell. Upregulated Let7b expression repressed the proliferation, invasion, and migration of liver cancer cell. This study showed that Let7b modulates the proliferation, invasiveness, and migration of liver cancer cell and reduces the proportion of cancer stem cells in liver cancer cell by inhibiting Wnt/β-catenin signaling pathway via downregulated Frizzled4.