Maud Martin

Class IIa histone deacetylases: regulating the regulators

In the last decade, the identification of enzymes that regulate acetylation of histones and nonhistone proteins has revealed the key role of dynamic acetylation and deacetylation in various cellular processes. Mammalian histone deacetylases (HDACs), which catalyse the removal of acetyl groups from lysine residues, are grouped into three classes, on the basis of similarity to yeast counterparts. An abundance of experimental evidence has established class IIa HDACs as crucial transcriptional regulators of various developmental and differentiation processes. In the past 5 years, a tremendous effort has been dedicated to characterizing the regulation of these enzymes. In this review, we summarize the latest discoveries in the field and discuss the molecular and structural determinants of class IIa HDACs regulation. Finally, we emphasize that comprehension of the mechanisms underlying class IIa HDAC functions is essential for potential therapeutic applications.

Protein phosphatase 2A controls the activity of histone deacetylase 7 during T cell apoptosis and angiogenesis.

Class IIa histone deacetylases (HDACs) act as key transcriptional regulators in several important developmental programs. Their activities are controlled via phosphorylation-dependent nucleocytoplasmic shuttling. Phosphorylation of conserved serine residues triggers association with 14-3-3 proteins and cytoplasmic relocalization of class IIa HDACs, which leads to the derepression of their target genes. Although a lot of effort has been made toward the identification of the inactivating kinases that phosphorylate class IIa HDAC 14-3-3 motifs, the existence of an antagonistic protein phosphatase remains elusive. Here we identify PP2A as a phosphatase responsible for dephosphorylating the 14-3-3 binding sites in class IIa HDACs. Interestingly, dephosphorylation of class IIa HDACs by PP2A is prevented by competitive association of 14-3-3 proteins. Using both okadaic acid treatment and RNA interference, we demonstrate that PP2A constitutively dephosphorylates the class IIa member HDAC7 to control its biological functions as a regulator of T cell apoptosis and endothelial cell functions. This study unravels a dynamic interplay among 14-3-3s, protein kinases, and PP2A and provides a model for the regulation of class IIa HDACs.

New role for hPar-1 kinases EMK and C-TAK1 in regulating localization and activity of class IIa histone deacetylases.

ABSTRACT Class IIa histone deacetylases (HDACs) are found both in the cytoplasm and in the nucleus where they repress genes involved in several major developmental programs. In response to specific signals, the repressive activity of class IIa HDACs is neutralized through their phosphorylation on multiple N-terminal serine residues and 14-3-3-mediated nuclear exclusion. Here, we demonstrate that class IIa HDACs are subjected to signal-independent nuclear export that relies on their constitutive phosphorylation. We identify EMK and C-TAK1, two members of the microtubule affinity-regulating kinase (MARK)/Par-1 family, as regulators of this process. We further show that EMK and C-TAK1 phosphorylate class IIa HDACs on one of their multiple 14-3-3 binding sites and alter their subcellular localization and repressive function. Using HDAC7 as a paradigm, we extend these findings by demonstrating that signal-independent phosphorylation of the most N-terminal serine residue by the MARK/Par-1 kinases, i.e., Ser155, is a prerequisite for the phosphorylation of the nearby 14-3-3 site, Ser181. We propose that this multisite hierarchical phosphorylation by a variety of kinases allows for sophisticated regulation of class IIa HDACs function.

Host-Pathogen Interactome Mapping for HTLV-1 and -2 Retroviruses

BackgroundHuman T-cell leukemia virus type 1 (HTLV-1) and type 2 both target T lymphocytes, yet induce radically different phenotypic outcomes. HTLV-1 is a causative agent of Adult T-cell leukemia (ATL), whereas HTLV-2, highly similar to HTLV-1, causes no known overt disease. HTLV gene products are engaged in a dynamic struggle of activating and antagonistic interactions with host cells. Investigations focused on one or a few genes have identified several human factors interacting with HTLV viral proteins. Most of the available interaction data concern the highly investigated HTLV-1 Tax protein. Identifying shared and distinct host-pathogen protein interaction profiles for these two viruses would enlighten how they exploit distinctive or common strategies to subvert cellular pathways toward disease progression.ResultsWe employ a scalable methodology for the systematic mapping and comparison of pathogen-host protein interactions that includes stringent yeast two-hybrid screening and systematic retest, as well as two independent validations through an additional protein interaction detection method and a functional transactivation assay. The final data set contained 166 interactions between 10 viral proteins and 122 human proteins. Among the 166 interactions identified, 87 and 79 involved HTLV-1 and HTLV-2 -encoded proteins, respectively. Targets for HTLV-1 and HTLV-2 proteins implicate a diverse set of cellular processes including the ubiquitin-proteasome system, the apoptosis, different cancer pathways and the Notch signaling pathway.ConclusionsThis study constitutes a first pass, with homogeneous data, at comparative analysis of host targets for HTLV-1 and -2 retroviruses, complements currently existing data for formulation of systems biology models of retroviral induced diseases and presents new insights on biological pathways involved in retroviral infection.

Class IIa histone deacetylases: conducting development and differentiation

The emergence of specialized cell types and their organisation into organs and tissues involve the temporal modulation of many genes that are essential for coordinating the correct timing of instructive signals. These transcriptional changes are orchestrated with a precision that reminds that of a classical symphony. Extracellular signals are transmitted to key integrators, which then orchestrate activation or repression of specific genes. In the last decade, class IIa HDACs have emerged as crucial regulators in various developmental and differentiation processes. This review focuses on the latest studies that have provided new insights into the biological functions of class IIa HDACs and discusses important aspects of their regulation. Elucidating cellular and molecular mechanisms by which functions of class IIa HDACs are modulated could potentially lead to new therapeutic opportunities for various diseases.

Control of apico–basal epithelial polarity by the microtubule minus-end-binding protein CAMSAP3 and spectraplakin ACF7

ABSTRACT The microtubule cytoskeleton regulates cell polarity by spatially organizing membrane trafficking and signaling processes. In epithelial cells, microtubules form parallel arrays aligned along the apico–basal axis, and recent work has demonstrated that the members of CAMSAP/Patronin family control apical tethering of microtubule minus ends. Here, we show that in mammalian intestinal epithelial cells, the spectraplakin ACF7 (also known as MACF1) specifically binds to CAMSAP3 and is required for the apical localization of CAMSAP3-decorated microtubule minus ends. Loss of ACF7 but not of CAMSAP3 or its homolog CAMSAP2 affected the formation of polarized epithelial cysts in three-dimensional cultures. In short-term epithelial polarization assays, knockout of CAMSAP3, but not of CAMSAP2, caused microtubule re-organization into a more radial centrosomal array, redistribution of Rab11-positive (also known as Rab11A) endosomes from the apical cell surface to the pericentrosomal region and inhibition of actin brush border formation at the apical side of the cell. We conclude that ACF7 is an important regulator of apico–basal polarity in mammalian intestinal cells and that a radial centrosome-centered microtubule organization can act as an inhibitor of epithelial polarity. Highlighted Article: The spectraplakin ACF7 and microtubule minus-end-stabilizing protein CAMSAP3 cooperate in organizing non-centrosomal microtubule arrays in intestinal epithelial cells.

Molecular Pathway of Microtubule Organization at the Golgi Apparatus

The Golgi apparatus controls the formation of non-centrosomal microtubule arrays important for Golgi organization, polarized transport, cell motility, and cell differentiation. Here, we show that CAMSAP2 stabilizes and attaches microtubule minus ends to the Golgi through a complex of AKAP450 and myomegalin. CLASPs stabilize CAMSAP2-decorated microtubules but are not required for their Golgi tethering. AKAP450 is also essential for Golgi microtubule nucleation, and myomegalin and CDK5RAP2 but not CAMSAP2 contribute to this function. In the absence of centrosomes, AKAP450- and CAMSAP2-dependent pathways of microtubule minus-end organization become dominant, and the presence of at least one of them is needed to maintain microtubule density. Strikingly, a compact Golgi can be assembled in the absence of both centrosomal and Golgi microtubules. However, CAMSAP2- and AKAP450-dependent Golgi microtubules facilitate Golgi reorientation and cell invasion in a 3D matrix. We propose that Golgi-anchored microtubules are important for polarized cell movement but not for coalescence of Golgi membranes.

PP2A regulatory subunit Bα controls endothelial contractility and vessel lumen integrity via regulation of HDAC7

To supply tissues with nutrients and oxygen, the cardiovascular system forms a seamless, hierarchically branched, network of lumenized tubes. Here, we show that maintenance of patent vessel lumens requires the Bα regulatory subunit of protein phosphatase 2A (PP2A). Deficiency of Bα in zebrafish precludes vascular lumen stabilization resulting in perfusion defects. Similarly, inactivation of PP2A‐Bα in cultured ECs induces tubulogenesis failure due to alteration of cytoskeleton dynamics, actomyosin contractility and maturation of cell–extracellular matrix (ECM) contacts. Mechanistically, we show that PP2A‐Bα controls the activity of HDAC7, an essential transcriptional regulator of vascular stability. In the absence of PP2A‐Bα, transcriptional repression by HDAC7 is abrogated leading to enhanced expression of the cytoskeleton adaptor protein ArgBP2. ArgBP2 hyperactivates RhoA causing inadequate rearrangements of the EC actomyosin cytoskeleton. This study unravels the first specific role for a PP2A holoenzyme in development: the PP2A‐Bα/HDAC7/ArgBP2 axis maintains vascular lumens by balancing endothelial cytoskeletal dynamics and cell–matrix adhesion.

DUSP3/VHR is a pro-angiogenic atypical dual-specificity phosphatase

BackgroundDUSP3 phosphatase, also known as Vaccinia-H 1 R elated (VHR) phosphatase, encoded by DUSP3/Dusp3 gene, is a relatively small member of the dual-specificity protein phosphatases. In vitro studies showed that DUSP3 is a negative regulator of ERK and JNK pathways in several cell lines. On the other hand, DUSP3 is implicated in human cancer. It has been alternatively described as having tumor suppressive and oncogenic properties. Thus, the available data suggest that DUSP3 plays complex and contradictory roles in tumorigenesis that could be cell type-dependent. Since most of these studies were performed using recombinant proteins or in cell-transfection based assays, the physiological function of DUSP3 has remained elusive.ResultsUsing immunohistochemistry on human cervical sections, we observed a strong expression of DUSP3 in endothelial cells (EC) suggesting a contribution for this phosphatase to EC functions. DUSP3 downregulation, using RNA interference, in human EC reduced significantly in vitro tube formation on Matrigel and spheroid angiogenic sprouting. However, this defect was not associated with an altered phosphorylation of the documented in vitro DUSP3 substrates, ERK1/2, JNK1/2 and EGFR but was associated with an increased PKC phosphorylation. To investigate the physiological function of DUSP3, we generated Dusp3-deficient mice by homologous recombination. The obtained DUSP3−/− mice were healthy, fertile, with no spontaneous phenotype and no vascular defect. However, DUSP3 deficiency prevented neo-vascularization of transplanted b-FGF containing Matrigel and LLC xenograft tumors as evidenced by hemoglobin (Hb) and FITC-dextran quantifications. Furthermore, we found that DUSP3 is required for b-FGF-induced microvessel outgrowth in the aortic ring assay.ConclusionsAll together, our data identify DUSP3 as a new important player in angiogenesis.

Predicting interactome network perturbations in human cancer: application to gene fusions in acute lymphoblastic leukemia

Genomic variations such as gene fusions are directly or indirectly associated with human diseases. A method is presented combining gene expression and interactome data analyses to identify specific targets in leukemia. The Myc network and the mRNA export machinery are perturbed in ETV6-RUNX1 and TCF3-PBX1 subtypes of leukemia.