Characterization of the Pseudomonas aeruginosa Glycoside Hydrolase PslG Reveals that its Levels are Critical for Psl Polysaccharide Biosynthesis and Biofilm Formation. [Enzymology]

September 30th, 2015 by

A key component of colonization, biofilm formation, and protection of the opportunistic human pathogen Pseudomonas aeruginosa is the biosynthesis of the exopolysaccharide Psl. Composed of a pentameric repeating unit of mannose, glucose and rhamnose, the biosynthesis of Psl is proposed to occur via a Wzx/Wzy-dependent mechanism. Previous genetic studies have shown that the putative glycoside hydrolase PslG is essential for Psl biosynthesis. To understand the function of this protein, the apo structure of the periplasmic domain of PslG (PslG31-442) and its complex with mannose were determined to 2.0 and 1.9 Å resolution, respectively. Despite similar domain architecture and positioning of catalytic residues to other family 39 glycoside hydrolases (GH39), PslG31-442 exhibits a unique 32 Å long active site groove that is distinct from other structurally characterized family members. PslG formed a complex with two mannose monosaccharides in this groove, consistent with binding data obtained from intrinsic tryptophan fluorescence. PslG was able to catalyze the hydrolysis of surface-associated Psl and this activity was abolished in a E165Q/E276Q double catalytic variant. Surprisingly, P. aeruginosa variants with these chromosomal mutations as well as a pslG deletion mutant were still capable of forming Psl biofilms. However, overexpression of PslG in a pslG deletion background impaired biofilm formation and resulted in less surface-associated Psl, suggesting that regulation of this enzyme is important during polysaccharide biosynthesis.
  • Posted in Journal of Biological Chemistry, Publications
  • Comments Off on Characterization of the Pseudomonas aeruginosa Glycoside Hydrolase PslG Reveals that its Levels are Critical for Psl Polysaccharide Biosynthesis and Biofilm Formation. [Enzymology]

Retinal Degeneration Slow (RDS) Glycosylation Plays a Role in Cone Function and the Regulation of RDS/ROM-1 Complex Formation [Cell Biology]

September 29th, 2015 by Stuck, M. W., Conley, S. M., Naash, M. I.

The photoreceptor-specific glycoprotein retinal degeneration slow (RDS, also called PRPH2) is necessary for the formation of rod and cone outer segments. Mutations in RDS cause rod and cone dominant retinal disease, and it is well established that both cell types have different requirements for RDS. However, the molecular mechanisms for this difference remain unclear. Although RDS glycosylation is highly conserved, previous study revealed no apparent function for the glycan in rods. In light of the highly conserved nature of RDS glycosylation we hypothesized that it is important for RDS function in cones, and could underlie part of the differential requirement for RDS in the two photoreceptor subtypes. We generated a knockin mouse expressing RDS without the N-glycosylation site (N229S). Normal levels of RDS and the unglycosylated RDS binding partner rod outer segment membrane protein-1 (ROM-1) were found in N229S retinas however cone electroretinogram responses were decreased by 40% at 6 months of age. Since cones make up only 3-5% of photoreceptors in the wild-type background, N229S mice were crossed into the nrl-/- background (in which all rods are converted to cone-like cells) for biochemical analysis. In N229S/nrl-/- retinas, RDS and ROM-1 levels were decreased by ~60% each. These data suggest that glycosylation of RDS is required for RDS function or stability in cones, a difference that may be due to extracellular vs. intradiscal localization of the RDS glycan in cones vs. rods.

The Kaposi’s sarcoma herpesvirus latency-associated nuclear antigen DNA binding domain dorsal positive electrostatic patch facilitates DNA replication and episome persistence [Microbiology]

September 29th, 2015 by

Kaposi's sarcoma-associated herpesvirus (KSHV) has a causative role in several human malignancies. KSHV latency-associated nuclear antigen (LANA) mediates persistence of viral episomes in latently infected cells. LANA mediates KSHV DNA replication and segregates episomes to progeny nuclei. The structure of the LANA DNA binding domain was recently solved, revealing a positive electrostatic patch opposite the DNA binding surface, which is the site of BET protein binding. Here, we investigate the functional role of the positive patch in LANA mediated episome persistence. As expected, LANA mutants with alanine or glutamate substitutions in the central, peripheral, or lateral portions of the positive patch maintained the ability to bind DNA by EMSA. However, all of the substitution mutants were deficient for LANA DNA replication and episome maintenance. Mutation of the peripheral region generated the largest deficiencies. Despite these deficiencies, all positive patch mutants concentrated to dots along mitotic chromosomes in cells containing episomes, similar to LANA. The central and peripheral mutants, but not the lateral mutants, were reduced for BET protein interaction as assessed by co-immunoprecipitation. However, defects in BET protein binding were independent of episome maintenance function. Overall, the reductions in episome maintenance closely correlated with DNA replication deficiencies, suggesting that the replication defects account for the reduced episome persistence. Therefore, the electrostatic patch exerts a key role in LANA mediated DNA replication and episome persistence, and may act through a host cell partner(s) other than a BET protein or by inducing specific structures or complexes.
  • Posted in Journal of Biological Chemistry, Publications
  • Comments Off on The Kaposi’s sarcoma herpesvirus latency-associated nuclear antigen DNA binding domain dorsal positive electrostatic patch facilitates DNA replication and episome persistence [Microbiology]

Structural Basis for Ligand Recognition and Functional Selectivity at Angiotensin Receptor [Signal Transduction]

September 29th, 2015 by

Angiotensin II type 1 receptor (AT1R) is the primary blood pressure regulator. AT1R blockers (ARBs) have been widely used in clinical settings as anti-hypertensive drugs, and share a similar chemical scaffold, although even minor variations can lead to distinct therapeutic efficacies towards cardiovascular etiologies. The structural basis for AT1R modulation by different peptide and non-peptide ligands has remained elusive. Here we report the crystal structure of the human AT1R in complex with an inverse agonist olmesartan (BenicarTM), a highly potent anti-hypertensive drug. Olmesartan is anchored to the receptor primarily by the residues Tyr351.39, Trp842.60, and Arg167ECL2, similar to the antagonist ZD7155, corroborating a common binding mode of different ARBs. Using docking simulations and site-directed mutagenesis we identified specific interactions between AT1R and different ARBs, including olmesartan derivatives with inverse agonist, neutral antagonist or agonist activities. We further observed that the mutation Asn1113.35Ala in the putative sodium-binding site affects binding of the endogenous peptide agonist Angiotensin II, but not the β-arrestin-biased peptide TRV120027.

Ethosuximide induces hippocampal neurogenesis and reverses cognitive deficits in amyloid-{beta} toxin induced Alzheimer’s rat model via PI3K/Akt/Wnt/{beta}-catenin pathway [Molecular Bases of Disease]

September 29th, 2015 by

Neurogenesis involves generation of new neurons through finely tuned multistep processes such as neural stem cell's (NSC) proliferation, migration, differentiation, and integration into existing neuronal circuitry in the dentate gyrus of the hippocampus and sub-ventricular zone (SVZ). Adult hippocampal neurogenesis is involved in cognitive functions and altered in various neurodegenerative disorders including Alzheimer's disease (AD). Ethosuximide (ETH), an anticonvulsant drug is used for the treatment of epileptic seizure. However, the effects of ETH on adult hippocampal neurogenesis and underlying cellular and molecular mechanism(s) are still elusive. Herein, we studied the effects of ETH on rat multipotent NSC proliferation and neuronal differentiation, and adult hippocampal neurogenesis in an amyloid beta (Aβ) toxin induced rat model of AD like phenotypes. ETH potently induced NSC proliferation and neuronal differentiation in the hippocampal derived NSC in vitro. ETH enhanced NSC proliferation and neuronal differentiation, and reduced Aβ toxin mediated toxicity and neurodegeneration, leading to behavioral recovery in rat AD model. ETH inhibited Aβ mediated suppression of neurogenic and Akt-Wnt/β-catenin pathway gene's expression in the hippocampus. ETH activated the PI3K/Akt and Wnt/β-catenin transduction pathways that are known to be involved in the regulation of neurogenesis. Inhibition of the PI3K/Akt and Wnt/β-catenin pathways effectively blocked the mitogenic and neurogenic effects of ETH. In silico molecular target prediction docking studies suggest that ETH interacts with Akt, Dkk-1 and GSK-3β. Our findings suggest that ETH stimulates NSC proliferation and differentiation in vitro and adult hippocampal neurogenesis via the PI3K/Akt and Wnt/β-catenin signaling.
  • Posted in Journal of Biological Chemistry, Publications
  • Comments Off on Ethosuximide induces hippocampal neurogenesis and reverses cognitive deficits in amyloid-{beta} toxin induced Alzheimer’s rat model via PI3K/Akt/Wnt/{beta}-catenin pathway [Molecular Bases of Disease]

Histone deacetylase 3 coordinates deacetylase-independent epigenetic silencing of TGF{beta}1 to orchestrate second heart field development [Molecular Bases of Disease]

September 29th, 2015 by Lewandowski, S. L., Janardhan, H. P., Trivedi, C. M.

About two-thirds of human congenital heart disease (CHD) involves second heart field (SHF) derived structures. Histone-modifying enzymes, histone deacetylases (HDACs), regulate the epigenome; however, their functions within the second heart field remain elusive. Here we demonstrate that histone deacetylase 3 (Hdac3) orchestrates epigenetic silencing of Tgfβ1, a causative factor in CHD pathogenesis, in a deacetylase-independent manner to regulate development of SHF-derived structures. In murine embryos lacking Hdac3 in the SHF, increased Tgfβ1 bioavailability is associated with ascending aortic dilatation, outflow tract malrotation, overriding aorta, double outlet right ventricle, aberrant semilunar valve development, bicuspid aortic valve, ventricular septal defects, and embryonic lethality. Activation of Tgfβ signaling causes aberrant endothelial-to-mesenchymal transition (EndMT) and altered extracellular matrix homeostasis in Hdac3-null outflow tracts and semilunar valves and pharmacological inhibition of Tgfβ rescues these defects. Hdac3 recruits components of PRC2 complex, methyltransferase Ezh2, Eed, and Suz12 to the Ncor complex to enrich trimethylation of lys27 on histone H3 at the Tgfβ1 regulatory region and thereby maintains epigenetic silencing of Tgfβ1 specifically within the SHF-derived mesenchyme. Wild-type Hdac3 or catalytically-inactive Hdac3 expression rescue aberrant EndMT and epigenetic silencing of Tgfβ1 in Hdac3-null outflow tracts and semilunar valves. These findings reveal that epigenetic dysregulation within the SHF is a predisposing factor for CHD.
  • Posted in Journal of Biological Chemistry, Publications
  • Comments Off on Histone deacetylase 3 coordinates deacetylase-independent epigenetic silencing of TGF{beta}1 to orchestrate second heart field development [Molecular Bases of Disease]

ATM-dependent phosphorylation of the Fanconi anemia protein PALB2 promotes the DNA Damage Response [DNA and Chromosomes]

September 29th, 2015 by Guo, Y., Feng, W., Sy, S. M. H., Huen, M. S. Y.

The Fanconi anemia protein PALB2, also known as FANCN, protects genome integrity by regulating DNA repair and cell cycle checkpoints. Exactly how PALB2 functions may be temporally coupled with detection and signaling of DNA damage is not known. Intriguingly, we found that PALB2 is transformed into a hyper-phosphorylated state in response to ionizing radiation (IR). IR treatment specifically triggered PALB2 phosphorylation at Ser157 and Ser376 in manners that required the master DNA Damage Response (DDR) kinase Ataxia telangiectasia mutated (ATM), revealing potential mechanistic links between PALB2 and the ATM-dependent DDRs. Consistently, dysregulated PALB2 phosphorylation resulted in sustained activation of DDRs. Full-blown PALB2 phosphorylation also required the breast and ovarian susceptible gene product BRCA1, highlighting important roles of the BRCA1-PALB2 interaction in orchestrating cellular responses to genotoxic stress. In summary, our phosphorylation analysis of tumour suppressor protein PALB2 uncovers new layers of regulatory mechanisms in the maintenance of genome stability and tumor suppression.

Structural mechanism of the interaction of Alzheimer’s disease A{beta} fibrils with the NSAID sulindac sulfide [Molecular Biophysics]

September 28th, 2015 by

Alzheimer's disease is the most severe neurodegenerative disease worldwide. In the past years, a plethora of small molecules interfering with amyloid-β (Aβ) aggregation have been suggested. However, their mode of interaction with amyloid fibers is not understood. Non-steroidal anti-inflammatory drugs (NSAIDs) are known γ-secretase modulators (GSMs). It has been suggested that NSAIDs are pleiotrophic and can interact with more than one pathomechanism. We present here a magic angle spinning (MAS) solid-state NMR study that shows that the NSAID sulindac sulfide interacts specifically with Alzheimer's disease Aβ fibrils. We find that sulindac sulfide does not induce drastic architectural changes in the fibrillar structure, but intercalates between the two β-strands of the amyloid fibril and binds to hydrophobic cavities, which are found consistently in all analyzed structures. The characteristic D23-K28 salt bridge is not affected upon interacting with sulindac sulfide. The primary binding site is located in the vicinity of residue G33, a residue involved in M35 oxidation. The results presented here could be useful in the search for pharmacologically active molecules which can potentially be employed as lead structures to guide the design of small molecules for the treatment of Alzheimer's disease.

Affinity Purification and Structural Features of the Yeast Vacuolar ATPase Vo Membrane Sector [Membrane Biology]

September 28th, 2015 by Couoh-Cardel, S., Milgrom, E., Wilkens, S.

The membrane sector (Vo) of the proton pumping vacuolar ATPase (V-ATPase; V1Vo-ATPase) from Saccharomyces cerevisiae was purified to homogeneity and its structure was characterized by electron microscopy (EM) of single molecules and two-dimensional (2-D) crystals. Projection images of negatively stained Vo 2-D crystals showed a ring like structure with a large asymmetric mass at the periphery of the ring. A cryo EM reconstruction of Vo from single particle images showed subunits a and d in close contact on the cytoplasmic side of the proton channel. A comparison of 3-D reconstructions of free Vo and Vo as part of holo V1Vo revealed that the cytoplasmic N-terminal domain of subunit a (aNT) must undergo a large conformational change upon enzyme disassembly or (re)assembly from Vo, V1 and subunit C. Isothermal titration calorimetry using recombinant subunit d and aNT revealed that the two proteins bind each other with a Kd of ~ 5 μM. Treatment of purified Vo sector with lyso 1-palmitoyl phosphatidylglycerol (LPPG) resulted in selective release of subunit d, allowing purification of a VoΔd complex. Passive proton translocation assays revealed that both Vo and VoΔd are impermeable to protons. We speculate that the structural change in subunit a upon release of V1 from Vo during reversible enzyme dissociation plays a role in blocking passive proton translocation across free Vo and that the interaction between aNT and d seen in free Vo functions to stabilize the Vo sector for efficient reassembly of V1Vo.

Retinal attachment instability is diversified among mammalian melanopsins [Molecular Biophysics]

September 28th, 2015 by

Melanopsins play a key role in non-visual photoreception in mammals. Their close phylogenetic relationship to the photopigments in invertebrate visual cells suggests they have evolved to acquire molecular characteristics that are more suited for their non-visual functions. Here we set out to identify such characteristics, by comparing the molecular properties of mammalian melanopsin to those of invertebrate melanopsin and visual pigment. Our data show that the Schiff base linking the chromophore retinal to the protein is more susceptive to spontaneous cleavage in mammalian melanopsins. We also find this stability is highly diversified between mammalian species, being particularly unstable for human melanopsin. Through mutagenesis analyses, we find that this diversified stability is mainly due to parallel amino acid substitutions in extra-cellular regions. We propose that the different stability of the retinal attachment in melanopsins may contribute to functional tuning of non-visual photoreception in mammals.