Protein tyrosine phosphatase PTPN9 regulates erythroid cell development through STAT3 dephosphorylation in zebrafish

Using the Zebrafish as a Genetic Model to Study Erythropoiesis

Vertebrates generate mature red blood cells (RBCs) via a highly regulated, multistep process called erythropoiesis. Erythropoiesis involves synthesis of heme and hemoglobin, clearance of the nuclei and other organelles, and remodeling of the plasma membrane, and these processes are exquisitely coordinated by specific regulatory factors including transcriptional factors and signaling molecules. Defects in erythropoiesis can lead to blood disorders such as congenital dyserythropoietic anemias, Diamond–Blackfan anemias, sideroblastic anemias, myelodysplastic syndrome, and porphyria. The molecular mechanisms of erythropoiesis are highly conserved between fish and mammals, and the zebrafish (Danio rerio) has provided a powerful genetic model for studying erythropoiesis. Studies in zebrafish have yielded important insights into RBC development and established a number of models for human blood diseases. Here, we focus on latest discoveries of the molecular processes and mechanisms regulating zebrafish erythropoiesis and summarize newly established zebrafish models of human anemias.

Tyrosine-Protein Phosphatase Non-receptor Type 9 (PTPN9) Negatively Regulates the Paracrine Vasoprotective Activity of Bone-Marrow Derived Pro-angiogenic Cells: Impact on Vascular Degeneration in Oxygen-Induced Retinopathy

Background and Aim

Insufficient post-ischemic neovascularization is an initial key step in the pathogenesis of Oxygen-Induced Retinopathy (OIR). During neovascularization, pro-angiogenic cells (PACs) are mobilized from the bone marrow and integrate into ischemic tissues to promote angiogenesis. However, the modulation of PAC paracrine activity during OIR and the specific mechanisms involved remain to be explored. Because Tyrosine-protein phosphatase non-receptor type 9 (PTPN9) is reported to be a negative regulator of stem cell differentiation and angiogenesis signaling, we investigated its effect on PAC activity in the context of OIR.

Methods and Results

In a rat model of OIR, higher levels of PTPN9 in the retina and in bone marrow derived PACs are associated with retinal avascular areas, lower levels of the mobilization factor SDF-1 and decreased number of CD34⁺/CD117⁺/CD133⁺ PACs. PACs exposed ex vivo to hyperoxia display increased PTPN9 expression, which is associated with impaired ability of PAC secretome to promote angiogenesis ex vivo (choroidal vascular sprouting) and in vitro (endothelial cell tubule formation) compared to the secretome of PACs maintained in normoxia. Suppression of PTPN9 (using siRNA) increases VEGF and SDF-1 expression to normalize PAC secretome during hyperoxia, leading to restored angiogenic ability of PAC secretome. Moreover, endothelial cells exposed to the secretome of siPTPN9-treated PACs expressed increased levels of activated form of VEGF receptor 2 (VEGFR2). In the rat model of OIR, intravitreal injection of secretome from siPTPN9-treated PACs significantly reduced retinal vaso-obliteration; this was associated with higher retinal levels of VEGF/SDF-1, and increased recruitment of PACs (CD34⁺ cells) to the retinal and choroidal vessels.

Conclusion

Our results suggest that hyperoxia alters the paracrine proangiogenic activity of BM-PACs by inducing PTPN9, which can contribute to impair post-ischemic revascularization in the context of OIR. Targeting PTPN9 restores PAC angiogenic properties, and provide a new target for vessel integrity in ischemic retinopathies.

Splicing variation of BMP2K balances abundance of COPII assemblies and autophagic degradation in erythroid cells

Intracellular transport undergoes remodeling upon cell differentiation, which involves cell type-specific regulators. Bone morphogenetic protein 2-inducible kinase (BMP2K) has been potentially implicated in endocytosis and cell differentiation but its molecular functions remained unknown. We discovered that its longer (L) and shorter (S) splicing variants regulate erythroid differentiation in a manner unexplainable by their involvement in AP-2 adaptor phosphorylation and endocytosis. However, both variants interact with SEC16A and could localize to the juxtanuclear secretory compartment. Variant-specific depletion approach showed that BMP2K isoforms constitute a BMP2K-L/S regulatory system that controls the distribution of SEC16A and SEC24B as well as SEC31A abundance at COPII assemblies. Finally, we found L to promote and S to restrict autophagic degradation and erythroid differentiation. Hence, we propose that BMP2K-L and BMP2K-S differentially regulate abundance and distribution of COPII assemblies as well as autophagy, possibly thereby fine-tuning erythroid differentiation.

PTPN9 induces cell apoptosis by mitigating the activation of Stat3 and acts as a tumor suppressor in colorectal cancer

Background

Accumulating evidence has shown that protein tyrosine phosphatases (PTPs) are involved in regulating the transduction of many signaling pathways and play important roles in modulating the progression of some cancers, but the functions of PTPs in cancers have not been well elucidated until now. Here, we aimed to identify the roles of protein tyrosine phosphatase nonreceptor type 9 (PTPN9), a cytoplasmic PTP, in the development of colorectal cancer and elucidate the regulatory mechanism involved.

Materials and methods

Cell viability assessment, colony formation assay, caspase-3 and caspase-9 activity assay, real-time PCR, and Western blot analysis were applied.

Results

Our results showed that PTPN9 expression was frequently downregulated in colorectal cancer tissues compared with adjacent normal tissues. Overexpression of PTPN9 mitigated cell growth and colony formation and induced cell apoptosis in colorectal cancer. Conversely, PTPN9 knockdown promoted cell growth and survival. Moreover, PTPN9 negatively regulated the activation of Stat3 and depressed its nuclear translocation in colorectal cancer. The effects of PTPN9 knockdown on cell apoptosis were attenuated by inhibition of the Stat3 pathway.

Conclusion

These results indicate that PTPN9 inhibits cell growth and survival by repressing the activation of Stat3 in colorectal cancer, which suggests an important underlying mechanism of regulating cell growth and provides a novel candidate therapeutic target for colorectal cancer.

The Roles of Protein Tyrosine Phosphatases in Hepatocellular Carcinoma

The protein tyrosine phosphatase (PTP) family is involved in multiple cellular functions and plays an important role in various pathological and physiological processes. In many chronic diseases, for example cancer, PTP is a potential therapeutic target for cancer treatment. In the last two decades, dozens of PTP inhibitors which specifically target individual PTP molecules were developed as therapeutic agents. Hepatocellular carcinoma (HCC) is one of the most common malignant tumors and is the second most lethal cancer worldwide due to a lack of effective therapies. Recent studies have unveiled both oncogenic and tumor suppressive functions of PTP in HCC. Here, we review the current knowledge on the involvement of PTP in HCC and further discuss the possibility of targeting PTP in HCC.

Spliceosomal protein eftud2 mutation leads to p53-dependent apoptosis in zebrafish neural progenitors

Haploinsufficiency of EFTUD2 (Elongation Factor Tu GTP Binding Domain Containing 2) is linked to human mandibulofacial dysostosis, Guion-Almeida type (MFDGA), but the underlying cellular and molecular mechanisms remain to be addressed. We report here the isolation, cloning and functional analysis of the mutated eftud2 (snu114) in a novel neuronal mutant fn10a in zebrafish. This mutant displayed abnormal brain development with evident neuronal apoptosis while the development of other organs appeared less affected. Positional cloning revealed a nonsense mutation such that the mutant eftud2 mRNA encoded a truncated Eftud2 protein and was subjected to nonsense-mediated decay. Disruption of eftud2 led to increased apoptosis and mitosis of neural progenitors while it had little effect on differentiated neurons. Further RNA-seq and functional analyses revealed a transcriptome-wide RNA splicing deficiency and a large amount of intron-retaining and exon-skipping transcripts, which resulted in inadequate nonsense-mediated RNA decay and activation of the p53 pathway in fn10a mutants. Therefore, our study has established that eftud2 functions in RNA splicing during neural development and provides a suitable zebrafish model for studying the molecular pathology of the neurological disease MFDGA.

Chromatin-remodelling factor Brg1 regulates myocardial proliferation and regeneration in zebrafish

The zebrafish possesses a remarkable capacity of adult heart regeneration, but the underlying mechanisms are not well understood. Here we report that chromatin remodelling factor Brg1 is essential for adult heart regeneration. Brg1 mRNA and protein are induced during heart regeneration. Transgenic over-expression of dominant-negative Xenopus Brg1 inhibits the formation of BrdU⁺/Mef2C⁺ and Tg(gata4:EGFP) cardiomyocytes, leading to severe cardiac fibrosis and compromised myocardial regeneration. RNA-seq and RNAscope analyses reveal that inhibition of Brg1 increases the expression of cyclin-dependent kinase inhibitors such as cdkn1a and cdkn1c in the myocardium after ventricular resection; and accordingly, myocardial-specific expression of dn-xBrg1 blunts myocardial proliferation and regeneration. Mechanistically, injury-induced Brg1, via its interaction with Dnmt3ab, suppresses the expression of cdkn1c by increasing the methylation level of CpG sites at the cdkn1c promoter. Taken together, our results suggest that Brg1 promotes heart regeneration by repressing cyclin-dependent kinase inhibitors partly through Dnmt3ab-dependent DNA methylation.

Role of STAT3 in Genesis and Progression of Human Malignant Gliomas

Signal transducer and activator of transcription 3 (STAT3) is aberrantly activated in glioblastoma and has been identified as a relevant therapeutic target in this disease and many other human cancers. After two decades of intensive research, there is not yet any approved STAT3-based glioma therapy. In addition to the canonical activation by tyrosine 705 phosphorylation, concordant reports described a potential therapeutic relevance of other post-translational modifications including mainly serine 727 phosphorylation. Such reports reinforce the need to refine the strategy of targeting STAT3 in each concerned disease. This review focuses on the role of serine 727 and tyrosine 705 phosphorylation of STAT3 in glioma. It explores their contribution to glial cell transformation and to the mechanisms that make glioma escape to both immune control and standard treatment.

A Drosophila-centric view of protein tyrosine phosphatases

Most of our knowledge on protein tyrosine phosphatases (PTPs) is derived from human pathologies and mouse knockout models. These models largely correlate well with human disease phenotypes, but can be ambiguous due to compensatory mechanisms introduced by paralogous genes. Here we present the analysis of the PTP complement of the fruit fly and the complementary view that PTP studies in Drosophila will accelerate our understanding of PTPs in physiological and pathological conditions. With only 44 PTP genes, Drosophila represents a streamlined version of the human complement. Our integrated analysis places the Drosophila PTPs into evolutionary and functional contexts, thereby providing a platform for the exploitation of the fly for PTP research and the transfer of knowledge onto other model systems.

Antimalarial drug artemisinin depletes erythrocytes by activating apoptotic pathways in zebrafish

Despite its extraordinary efficacy, administration of the major antimalarial drug artemisinin leads to anemia, and the underlying cellular and molecular mechanisms are not well understood. Here, we report the effects of artemisinin on erythroid development and apoptosis in zebrafish and human cells. By performing a small-molecule screen with zebrafish embryos, we found that artemisinin treatment depleted red blood cells and slightly decreased definitive hematopoietic stem cells, but had no effect on primitive hematopoietic progenitors. RNA-Seq revealed that artemisinin suppressed a cluster of genes in the heme biosynthesis and globin synthesis pathways. Furthermore, artemisinin induced apoptosis in erythrocytes in zebrafish embryos, as assessed by terminal deoxynucleotidyl transferase dUTP nick end labeling assay, and preferentially acted on differentiated erythrocytes by elevating caspase 8 and caspase 9 activity in differentiated human K562 cells. Consistently, artemisinin suppressed the ectopic expression of erythroid genes in jak2aV581F-injected embryos, a zebrafish model for human polycythemia vera in which the bone marrow makes too many red blood cells. Taken together, our data suggested that artemisinin, in addition to killing parasites, has a direct action on differentiated erythrocytes other than definitive hematopoietic stem cells and causes erythroid apoptosis by interfering with the heme biosynthesis pathway in zebrafish and human cells.