The role of Activin beta E in receptor mediated signaling

Activin beta E gehört zur TGF beta Familie von Wachstums- und Differenzierungsfaktoren. Wie seine nahen Verwandten in Säugetieren, Activin beta A, Activin beta B und Activin beta C, wird es als inaktive, monomerische, Proform synthetisiert. Diese kann in Folge Homo- und Heterodimere bilden. Ein Homodimer aus zwei beta E Untereinheiten wird Activin E, ein Heterodimer etwa aus einer beta E und einer beta A Untereinheit Activin AE genannt. Nach der Prozessierung durch Endoproteasen wird das mature Dimer sezerniert. In Säugetieren wird Activin beta E hauptsächlich in der Leber exprimiert. In Lebertumoren ist die Expression jedoch reduziert. Selbiges geschieht im Zuge der Leberregeneration nach einer Schädigung. Künstliche Überexpression führte zu einer Hemmung der DNA-Synthese und verminderter Zellproliferation. Die physiologische Funktion des Proteins ist jedoch großteils unbekannt, da bisher keine Interaktionspartner nachgewiesen werden konnten, und Knock-out Mäuse keine Veränderungen bezüglich Entwicklung oder Leberregeneration zeigten. Zur Ergründung der Rolle des Proteins in Säugetieren wurden von uns Expressionssysteme für epitopmarkiere Formen des Proteins etabliert. Behandlung der Leberzelllinien HepG2 und Hep3B mit Überständen von Activin beta E exprimierenden Zellen zeigte keine deutliche Beeinflussung des Wachstumsverhaltens der Zellen. Die überexprimierenden Zelllinien selbst jedoch wuchsen langsamer und hatten einen deutlich größeren Zelldurchmesser als Kontrolllinien. Die Leukäme-Zelllinie K562 reagierte mit Differenzierung auf die Zugabe von Activin A zum Medium, nicht jedoch auf die Zugabe von Activin E. Ebenso wenig hemmte Activin E die Wirkung von Activin A. Behandlung von HepG2 Zellen mit den Überständen von Activin beta A exprimierenden Zellen, nicht jedoch mit denen von Activin beta E exprimierenden Zellen, führte zur Phosphorylierung von Smad 2. Die gemeinsame Behandlung der HepG2 Zellen mit Activin A und Activin E führte zu keiner deutlichen Verminderung der durch Activin A hervorgerufenen Phosphorylierung im Vergleich zu Kontrollen. Obwohl die Effekte von Activin E auf andere Signalwege, welche durch Activin A aktiviert werden, noch nicht untersucht werden konnten, deuten unsere Ergebnisse darauf hin, dass Activin E deutlich anders wirkt als Activin A und dieses auch nicht in seiner Wirkung inhibiert.Activin beta E is a member of the TGF beta family of growth and differentiation factors. It is closely related to the other mammalian activin subunits activin beta A, beta B and beta C. Like them, it is synthesized as a biologically inactive monomeric proform which forms homo and heterodimers and is processed by endoproteases, producing the mature peptide. The mature homodimer (activin E), or heterodimers containing the beta E subunit (e.g. activin AE) are subsequently secreted by the cell. Activin beta E has been found to be predominantly expressed in the liver, but to be downregulated in hepatocellular carcinoma. Overexpression of the protein in liver cells inhibited DNA synthesis and increased apoptosis. Upon liver damage, expression of INHBE, the gene coding for activin beta E, was downregulated. The role of the protein in mammalian physiology remains unknown, as interaction partners of the protein have not been identified so far and knock out mice did not show an abnormal phenotype in liver regeneration or development. In order to investigate the biological functions of activin beta E, we established mammalian and insect cell expression systems, producing various epitope tagged and untagged variants of the protein. While exogenous addition of the prospective signaling molecule did not significantly alter the proliferation of the liver cell lines HepG2 and Hep3B, the producer cell lines themselves showed strongly decreased proliferation and increased cell size. Treatment of the erythroleukemia cell line K562 with activin A, but not activin E, induced differentiation of the cells towards the erythroid lineage. Also cotreatment with activin A and activin E did not significantly alter the effects of activin A on the cells. Similarly, incubation of HepG2 cells with supernatants of activin beta A but not activin beta E expressing cells caused phosphorylation of smad 2. Cotreatment with low amounts of activin A and supernatants from activin beta E expressing cells did not noticeably affect the smad 2 phosphorylation due to activin A in comparison to mock supernatants. While effects on MAP kinase signaling and on other pathways activated by activin A remain to be elucidated, our results indicate that activin E does not signal via similar pathways as activin A, nor does it inhibit the action of its more famous relative at this level

Cohesin acetyltransferase Esco2 is a cell viability factor and is required for cohesion in pericentric heterochromatin

Genetic deletion of murine Esco2 (an acetylase known to establish cohesin during S-phase and genetically linked to Roberts syndrome) results in embryonic lethality that can be molecularly linked to novel and specific functions of Esco2 at pericentric heterochromatin

Activins and follistatins: Emerging roles in liver physiology and cancer

Activins are secreted proteins belonging to the TGF-β family of signaling molecules. Activin signals are crucial for differentiation and regulation of cell proliferation and apoptosis in multiple tissues. Signal transduction by activins relies mainly on the Smad pathway, although the importance of crosstalk with additional pathways is increasingly being recognized. Activin signals are kept in balance by antagonists at multiple levels of the signaling cascade. Among these, follistatin and FLRG, two members of the emerging family of follistatin-like proteins, can bind secreted activins with high affinity, thereby blocking their access to cell surface-anchored activin receptors. In the liver, activin A is a major negative regulator of hepatocyte proliferation and can induce apoptosis. The functions of other activins expressed by hepatocytes have yet to be more clearly defined. Deregulated expression of activins and follistatin has been implicated in hepatic diseases including inflammation, fibrosis, liver failure and primary cancer. In particular, increased follistatin levels have been found in the circulation and in the tumor tissue of patients suffering from hepatocellular carcinoma as well as in animal models of liver cancer. It has been argued that up-regulation of follistatin protects neoplastic hepatocytes from activin-mediated growth inhibition and apoptosis. The use of follistatin as biomarker for liver tumor development is impeded, however, due to the presence of elevated follistatin levels already during preceding stages of liver disease. The current article summarizes our evolving understanding of the multi-faceted activities of activins and follistatins in liver physiology and cancer

Activins and activin antagonists in hepatocellular carcinoma

In many parts of the world hepatocellular carcinoma (HCC) is among the leading causes of cancer-related mortality but the underlying molecular pathology is still insufficiently understood. There is increasing evidence that activins, which are members of the transforming growth factor β (TGFβ) superfamily of growth and differentiation factors, could play important roles in liver carcinogenesis. Activins are disulphide-linked homo- or heterodimers formed from four different β subunits termed βA, βB, βC, and βE, respectively. Activin A, the dimer of two βA subunits, is critically involved in the regulation of cell growth, apoptosis, and tissue architecture in the liver, while the hepatic function of other activins is largely unexplored so far. Negative regulators of activin signals include antagonists in the extracellular space like the binding proteins follistatin and FLRG, and at the cell membrane antagonistic co-receptors like Cripto or BAMBI. Additionally, in the intracellular space inhibitory Smads can modulate and control activin activity. Accumulating data suggest that deregulation of activin signals contributes to pathologic conditions such as chronic inflammation, fibrosis and development of cancer. The current article reviews the alterations in components of the activin signaling pathway that have been observed in HCC and discusses their potential significance for liver tumorigenesis

Biallelic PAX5 mutations cause hypogammaglobulinemia, sensorimotor deficits, and autism spectrum disorder

The genetic causes of primary antibody deficiencies and autism spectrum disorder (ASD) are largely unknown. Here, we report a patient with hypogammaglobulinemia and ASD who carries biallelic mutations in the transcription factor PAX5. A patient-specific Pax5 mutant mouse revealed an early B cell developmental block and impaired immune responses as the cause of hypogammaglobulinemia. Pax5 mutant mice displayed behavioral deficits in all ASD domains. The patient and the mouse model showed aberrant cerebellar foliation and severely impaired sensorimotor learning. PAX5 deficiency also caused profound hypoplasia of the substantia nigra and ventral tegmental area due to loss of GABAergic neurons, thus affecting two midbrain hubs, controlling motor function and reward processing, respectively. Heterozygous Pax5 mutant mice exhibited similar anatomic and behavioral abnormalities. Lineage tracing identified Pax5 as a crucial regulator of cerebellar morphogenesis and midbrain GABAergic neurogenesis. These findings reveal new roles of Pax5 in brain development and unravel the underlying mechanism of a novel immunological and neurodevelopmental syndrome