Muriel Lizé

E2F1-inducible microRNA 449a/b suppresses cell proliferation and promotes apoptosis.

E2F1 is a positive regulator of cell cycle progression and also a potent inducer of apoptosis, especially when activated by DNA damage. We identified E2F1-inducible microRNAs (miRNAs) by microarray hybridization and found that the levels of miRNAs 449a and 449b, as well as their host gene CDC20B, are strongly upregulated by E2F1. High miR-449 levels were found in testes, lung, and trachea, but not in testicular and other cancer cells. MiR-449a/b structurally resemble the p53-inducible miRNA 34 family. In agreement with a putative tumor-suppressive role, miR-449a as well as miR-34a reduced proliferation and strongly promoted apoptosis by at least partially p53-independent mechanisms. Both miRNAs reduced the levels of CDK6, implying miR-449 in a negative feedback mechanism for E2F1. Moreover, miR-449a and miR-34a diminished the deacetylase Sirt1 and augmented p53 acetylation. We propose that both miRNAs provide a twofold safety mechanism to avoid excessive E2F1-induced proliferation by cell cycle arrest and by apoptosis. While responding to different transactivators, miRNAs 449 and 34 each repress E2F1, but promote p53 activity, allowing efficient cross-talk between two major DNA damage-responsive gene regulators.

MicroRNA-449 in cell fate determination

The microRNAs 449a, b, and c (miR-449) are potent inducers of cell death, cell cycle arrest, and/or cell differentiation. They belong to the same family as the p53-responsive microRNAs miR-34. Instead of p53, however, the cell cycle regulatory transcription factor E2F1 induces miR-449. All members of this microRNA family are capable of mediating cell cycle arrest and apoptosis and might thereby contribute to tumor suppression. Underlying mechanisms include the downregulation of histone acetyl transferases and consecutive activation of p53, but also the targeting of cyclin dependent kinases and their association partners. Thus, miR-34 and miR-449 provide an asymmetric feedback loop to balance E2F and p53 activities. More recently, it was discovered that miR-449 displays strong tissue specificity, with high levels in lung and testes. Two model systems (Xenopus embryos and cultured human cells) revealed that miR-449 is essential for the development of ciliated epithelia, and this appears to depend on miR-449-mediated modulation of the Notch signaling pathway. Here we summarize our current knowledge on cell fate determination by miR-449, and we propose future directions to explore the function of miR-449 in cell regulation and organismal development. MiR-449 helps to ensure proper cell function but also to avoid cancer, marking a close link between cell differentiation and tumor suppression.

miR-34/449 miRNAs are required for motile ciliogenesis by repressing cp110

The mir-34/449 family consists of six homologous miRNAs at three genomic loci. Redundancy of miR-34/449 miRNAs and their dominant expression in multiciliated epithelia suggest a functional significance in ciliogenesis. Here we report that mice deficient for all miR-34/449 miRNAs exhibited postnatal mortality, infertility and strong respiratory dysfunction caused by defective mucociliary clearance. In both mouse and Xenopus, miR-34/449-deficient multiciliated cells (MCCs) exhibited a significant decrease in cilia length and number, due to defective basal body maturation and apical docking. The effect of miR-34/449 on ciliogenesis was mediated, at least in part, by post-transcriptional repression of Cp110, a centriolar protein suppressing cilia assembly. Consistent with this, cp110 knockdown in miR-34/449-deficient MCCs restored ciliogenesis by rescuing basal body maturation and docking. Altogether, our findings elucidate conserved cellular and molecular mechanisms through which miR-34/449 regulate motile ciliogenesis.

MicroRNA-449a levels increase by several orders of magnitude during mucociliary differentiation of airway epithelia.

MicroRNAs of the miR-34/449 family mediate cell cycle arrest and tumor suppression. Here we show that the expression of microRNA miR-449a, unlike its paralogue miR-34a, is highly tissue specific and largely restricted to pulmonary and testicular tissue. MiR-449a levels in the murine lung are particularly high shortly before and after birth, coinciding with terminal differentiation of lung epithelia. Strikingly, miR-449a is upregulated by more than 1000-fold when epithelial cells from human airways are lifted from a liquid environment to air, allowing them to undergo mucociliary differentiation. The induction of miR-449a occurs in parallel to its host gene CDC20B and the transcription factor FoxJ1. Exposure to tobacco smoke induces a moderate further increase in the levels of miR-449a, and also miR-34a, in differentiated airway epithelia. We propose that miR-449a can serve as an exquisitely sensitive and specific biomarker for the differentiation of bronchial epithelia. Moreover, miR-449a may actively promote mucociliary differentiation through its ability to block cell cycle progression, and it may conribute to a first line of defence against genotoxic stress by its proapoptotic functions.

TAp73 is a central transcriptional regulator of airway multiciliogenesis

Motile multiciliated cells (MCCs) have critical roles in respiratory health and disease and are essential for cleaning inhaled pollutants and pathogens from airways. Despite their significance for human disease, the transcriptional control that governs multiciliogenesis remains poorly understood. Here we identify TP73, a p53 homolog, as governing the program for airway multiciliogenesis. Mice with TP73 deficiency suffer from chronic respiratory tract infections due to profound defects in ciliogenesis and complete loss of mucociliary clearance. Organotypic airway cultures pinpoint TAp73 as necessary and sufficient for basal body docking, axonemal extension, and motility during the differentiation of MCC progenitors. Mechanistically, cross-species genomic analyses and complete ciliary rescue of knockout MCCs identify TAp73 as the conserved central transcriptional integrator of multiciliogenesis. TAp73 directly activates the key regulators FoxJ1, Rfx2, Rfx3, and miR34bc plus nearly 50 structural and functional ciliary genes, some of which are associated with human ciliopathies. Our results position TAp73 as a novel central regulator of MCC differentiation.

EndoNet: an information resource about regulatory networks of cell-to-cell communication

EndoNet is an information resource about intercellular regulatory communication. It provides information about hormones, hormone receptors, the sources (i.e. cells, tissues and organs) where the hormones are synthesized and secreted, and where the respective receptors are expressed. The database focuses on the regulatory relations between them. An elementary communication is displayed as a causal link from a cell that secretes a particular hormone to those cells which express the corresponding hormone receptor and respond to the hormone. Whenever expression, synthesis and/or secretion of another hormone are part of this response, it renders the corresponding cell an internal node of the resulting network. This intercellular communication network coordinates the function of different organs. Therefore, the database covers the hierarchy of cellular organization of tissues and organs as it has been modeled in the Cytomer ontology, which has now been directly embedded into EndoNet. The user can query the database; the results can be used to visualize the intercellular information flow. A newly implemented hormone classification enables to browse the database and may be used as alternative entry point. EndoNet is accessible at: http://endonet.bioinf.med.uni-goettingen.de/

MIR449A (microRNA 449a)

Review on MIR449A (microRNA 449a), with data on DNA, on the protein encoded, and where the gene is implicated.

MicroRNA-449 sustains cilia-related networks in the absence of transcription factor TAp73

Motile cilia serve vital functions in development, homeostasis and regeneration. We recently demonstrated that TAp73 is an essential transcriptional regulator of respiratory motile multiciliogenesis. Here, we show that TAp73 is expressed in multiciliated cells (MCCs) of diverse tissues. Analysis of TAp73-/- animals revealed that TAp73 regulates Foxj1, Rfx2, Rfx3, axonemal dyneins Dnali1 and Dnai1, plays a pivotal role in the generation of MCCs in reproductive ducts, and contributes to fertility. However, in the brain the function of MCCs appears to be preserved upon loss of TAp73, and robust activity from cilia-related networks is maintained in TAp73-/-. Consistent with TAp73-/-, its target miR34bc was reduced, whereas strong and specific induction of miR449 was observed along with an increase in E2f4, that induced transcriptional response from miR449 genomic regions. Depletion of both TAp73 and miR449 resulted in defective multiciliogenesis in the brain and hydrocephalus, indicating that miR449 and potentially additional pro-ciliogenic factors cooperate with TAp73 to ensure brain multiciliogenesis and CP development.Motile cilia serve vital functions in development, homeostasis and regeneration. We recently demonstrated that TAp73 is an essential transcriptional regulator of respiratory motile multiciliogenesis. Here we show that TAp73 is expressed in multiciliated cells (MCCs) of diverse tissues. Analysis of TAp73-/- animals revealed that TAp73 regulates Foxj1, Rfx2/3, axonemal dyneins Dnali1 and Dnai1, plays a pivotal role in the generation of MCCs in reproductive ducts, and contributes to fertility. However, TAp73 is dispensable for brain MCCs, and robust activity from cilia-related networks is maintained in TAp73-/-. Consistent with TAp73-/-, its target miR34bc was reduced, whereas strong and specific induction of miR449 was observed, along with an increase in E2f4 that induced transcriptional response from miR449 genomic regions. Depletion of both TAp73 and miR449 resulted in defective multiciliogenesis in the brain and hydrocephalus, indicating that miR449 and potentially additional pro-ciliogenic factors cooperate with TAp73 to ensure brain multiciliogenesis and CP development.