Calyculins and related marine natural products as serine-threonine protein phosphatase PP1 and PP2A inhibitors and total syntheses of calyculin A, B, and C

Phosphatases Decrease Water and Urea Permeability in Rat Inner Medullary Collecting Ducts

We previously showed that the phosphatases PP1/PP2A and PP2B dephosphorylate the water channel, AQP2, suggesting their role in water reabsorption. In this study, we investigated whether protein phosphatase 2A (PP2A) and protein phosphatase 2B (PP2B or calcineurin), which are present in the inner medullary collecting duct (IMCD), are regulators of urea and water permeability. Inhibition of calcineurin by tacrolimus increased both basal and vasopressin-stimulated osmotic water permeability in perfused rat IMCDs. However, tacrolimus did not affect osmotic water permeability in the presence of aldosterone. Inhibition of PP2A by calyculin increased both basal and vasopressin-stimulated osmotic water permeability, and aldosterone reversed the increase by calyculin. Previous studies showed that adrenomedullin (ADM) activates PP2A and decreases osmotic water permeability. Inhibition of PP2A by calyculin prevented the ADM-induced decrease in water reabsorption. ADM reduced the phosphorylation of AQP2 at serine 269 (pSer269 AQP2). Urea is linked to water reabsorption by building up hyperosmolality in the inner medullary interstitium. Calyculin increased urea permeability and phosphorylated UT-A1. Our results indicate that phosphatases regulate water reabsorption. Aldosterone and adrenomedullin decrease urea or osmotic water permeability by acting through calcineurin and PP2A, respectively. PP2A may regulate water reabsorption by dephosphorylating pSer269, AQP2, and UT-A1.

In Silico Prediction of Secondary Metabolites and Biosynthetic Gene Clusters Analysis of Streptomyces thinghirensis HM3 Isolated from Arid Soil

Natural products produced by microorganisms are considered an important resource of bioactive secondary metabolites, such as anticancer, antifungal, antibiotic, and immunosuppressive molecules. Streptomyces are the richest source of bioactive natural products via possessing a wide number of secondary metabolite biosynthetic gene clusters (SM-BGCs). Based on rapid development in sequencing technologies with advances in genome mining, exploring the newly isolated Streptomyces species for possible new secondary metabolites is mandatory to find novel natural products. The isolated Streptomyces thinghirensis strain HM3 from arid and sandy texture soil in Qassim, SA, exerted inhibition activity against tested animal pathogenic Gram-positive bacteria and pathogenic fungal species. In this study, we report the draft genome of S. thinghirensis strain HM3, which consists of 7,139,324 base pairs (bp), with an average G+C content of 71.49%, predicting 7949 open reading frames, 12 rRNA operons (5S, 16S, 23S) and 60 tRNAs. An in silico analysis of strain HM3 genome by the antiSMASH and PRISM 4 online software for SM-BGCs predicted 16 clusters, including four terpene, one lantipeptide, one siderophore, two polyketide synthase (PKS), two non-ribosomal peptide synthetase (NRPS) cluster)/NRPS-like fragment, two RiPP/RiPP-like (ribosomally synthesised and post-translationally modified peptide product), two butyrolactone, one CDPS (tRNA-dependent cyclodipeptide synthases), and one other (cluster containing a secondary metabolite-related protein that does not fit into any other category) BGC. The presented BGCs inside the genome, along with antibacterial and antifungal activity, indicate that HM3 may represent an invaluable source for new secondary metabolites.

Differential roles and regulation of the protein kinases PAK4, PAK5 and PAK6 in melanoma cells

The protein kinases PAK4, PAK5 and PAK6 comprise a family of ohnologues. In multiple cancers including melanomas PAK5 most frequently carries non-synonymous mutations; PAK6 and PAK4 have fewer; and PAK4 is often amplified. To help interpret these genomic data, initially we compared the cellular regulation of the sister kinases and their roles in melanoma cells. In common with many ohnologue protein kinases, PAK4, PAK5 and PAK6 each have two 14-3-3-binding phosphosites of which phosphoSer99 is conserved. PAK4 localises to the leading edge of cells in response to phorbol ester-stimulated binding of 14-3-3 to phosphoSer99 and phosphoSer181, which are phosphorylated by two different PKCs or PKDs. These phosphorylations of PAK4 are essential for its phorbol ester-stimulated phosphorylation of downstream substrates. In contrast, 14-3-3 interacts with PAK5 in response to phorbol ester-stimulated phosphorylation of Ser99 and epidermal growth factor-stimulated phosphorylation of Ser288; whereas PAK6 docks onto 14-3-3 and is prevented from localising to cell–cell junctions when Ser133 is phosphorylated in response to cAMP-elevating agents via PKA and insulin-like growth factor 1 via PKB/Akt. Silencing of PAK4 impairs viability, migration and invasive behaviour of melanoma cells carrying BRAF^V600E or NRAS^Q61K mutations. These defects are rescued by ectopic expression of PAK4, more so by a 14-3-3-binding deficient PAK4, and barely by PAK5 or PAK6. Together these genomic, biochemical and cellular data suggest that the oncogenic properties of PAK4 are regulated by PKC–PKD signalling in melanoma, while PAK5 and PAK6 are dispensable in this cancer.

Natural Products Containing the Nitrile Functional Group and Their Biological Activities

The importance of nitriles as a key class of chemicals with applications across the sciences is widely appreciated. The natural world is an underappreciated source of chemically diverse nitriles. With this in mind, this review describes novel nitrile-containing molecules isolated from natural sources from 1998 to 2021, as well as a discussion of the biological activity of these compounds. This study gathers 192 molecules from varied origins across the plant, animal, and microbial worlds. Their biological activity is extremely diverse, with many potential medicinal applications.

The phosphate ester group in secondary metabolites

Covering: 2000 to mid-2021The phosphate ester is a versatile, widespread functional group involved in a plethora of biological activities. Its presence in secondary metabolites, however, is relatively rare compared to other functionalities and thus is part of a rather unexplored chemical space. Herein, the chemistry of secondary metabolites containing the phosphate ester group is discussed. The text emphasizes their structural diversity, biological and pharmacological profiles, and synthetic approaches employed in the phosphorylation step during total synthesis campaigns, covering the literature from 2000 to mid-2021.

Renal AT2 Receptors Mediate Natriuresis via Protein Phosphatase PP2A

BACKGROUND

How signals from activated angiotensin type-2 receptors (AT₂R) mediate inhibition of sodium ion (Na⁺) reabsorption in renal proximal tubule cells is currently unknown. Protein phosphatases including PP2A (protein phosphatase 2A) have been implicated in AT₂R signaling in tissues other than kidney. We investigated whether inhibition of protein phosphatase PP2A reduced AT₂R-mediated natriuresis and evaluated changes in PP2A activity and localization after renal AT₂R activation in normal 4- and 10-week-old control Wistar-Kyoto rats and 4-week-old prehypertensive and 10-week-old hypertensive spontaneously hypertensive rats.

METHODS AND RESULTS

In Wistar-Kyoto rats, direct renal interstitial administration of selective AT₂R nonpeptide agonist Compound-21 (C-21) increased renal interstitial cyclic GMP (cGMP) levels, urine Na⁺ excretion, and simultaneously increased PP2A activity ≈2-fold in homogenates of renal cortical tubules. The cyclic GMP and natriuretic responses were abolished by concurrent renal interstitial administration of protein phosphatase inhibitor calyculin A. In renal proximal tubule cells in response to C-21, PP2A subunits A, B55α and C, but not B56γ, were recruited to apical plasma membranes together with AT₂Rs. Calyculin A treatment abolished C-21-induced translocation of both AT₂R and PP2A regulatory subunit B55α to apical plasma membranes. Immunoprecipitation of AT₂R solubilized from renal cortical homogenates demonstrated physical association of AT₂R with PP2A A, B55α, and C but not B56γ subunits. In contrast, in spontaneously hypertensive rats, administration of C-21 did not alter urine Na⁺ excretion or PP2A activity and failed to translocate AT₂Rs and PP2A subunits to apical plasma membranes.

CONCLUSIONS

In renal proximal tubule cells of Wistar-Kyoto rats, PP2A is activated and PP2A subunits AB55αC are recruited to C-21-activated AT₂Rs during induction of natriuresis. This response is defective in prehypertensive and hypertensive spontaneously hypertensive rats, presenting a potential novel therapeutic target for treating renal Na⁺ retention and hypertension.

The isolation of water-soluble natural products - challenges, strategies and perspectives

Covering period: up to 2019Water-soluble natural products constitute a relevant group of secondary metabolites notably known for presenting potent biological activities. Examples are aminoglycosides, β-lactam antibiotics, saponins of both terrestrial and marine origin, and marine toxins. Although extensively investigated in the past, particularly during the golden age of antibiotics, hydrophilic fractions have been less scrutinized during the last few decades. This review addresses the possible reasons on why water-soluble metabolites are now under investigated and describes approaches and strategies for the isolation of these natural compounds. It presents examples of several classes of hydrosoluble natural products and how they have been isolated. Novel stationary phases and chromatography techniques are also reviewed, providing a perspective towards a renaissance in the investigation of water-soluble natural products.

Identification of trans-AT polyketide clusters in two marine bacteria reveals cryptic similarities between distinct symbiosis factors

Glutarimide-containing polyketides are known as potent antitumoral and antimetastatic agents. The associated gene clusters have only been identified in a few Streptomyces producers and Burkholderia gladioli symbiont. The new glutarimide-family polyketides, denominated sesbanimides D, E and F along with the previously known sesbanimide A and C, were isolated from two marine alphaproteobacteria Stappia indica PHM037 and Labrenzia aggregata PHM038. Structures of the isolated compounds were elucidated based on 1D and 2D homo and heteronuclear NMR analyses and ESI-MS spectrometry. All compounds exhibited strong antitumor activity in lung, breast and colorectal cancer cell lines. Subsequent whole genome sequencing and genome mining revealed the presence of the trans-AT PKS gene cluster responsible for the sesbanimide biosynthesis, described as sbn cluster. Strikingly, the modular architecture of downstream mixed type PKS/NRPS, SbnQ, revealed high similarity to PedH in pederin and Lab13 in labrenzin gene clusters, although those clusters are responsible for the production of structurally completely different molecules. The unexpected presence of SbnQ homologues in unrelated polyketide gene clusters across phylogenetically distant bacteria, raises intriguing questions about the evolutionary relationship between glutarimide-like and pederin-like pathways, as well as the functionality of their synthetic products.

Autophosphorylation-induced self-assembly and STIL-dependent reinforcement underlie Plk4's ring-to-dot localization conversion around a human centriole

Polo-like kinase 4 (Plk4) is a key regulator of centriole biogenesis. Studies have shown that Plk4 undergoes dynamic relocalization from a ring-like pattern around a centriole to a dot-like morphology at the procentriole assembly site and this event is central for inducing centriole biogenesis. However, the detailed mechanisms underlying Plk4's capacity to drive its symmetry-breaking ring-to-dot relocalization remain largely unknown. Here, we showed that Plk4 self-initiates this process in an autophosphorylation-dependent manner and that STIL, its downstream target, is not required for this event. Time-dependent analyses with mEOS-fused photoconvertible Plk4 revealed that a portion of ring-state Plk4 acquires a capacity, presumably through autophosphorylation, to linger around a centriole, ultimately generating a dot-state morphology. Interestingly, Plk4 WT, but not its catalytically inactive mutant, showed the ability to form a nanoscale spherical assembly in the cytosol of human cells or heterologous E. coli, demonstrating its autophosphorylation-dependent self-organizing capacity. At the biochemical level, Plk4 - unlike its N-terminal βTrCP degron motif - robustly autophosphorylated the PC3 SSTT motif within its C-terminal cryptic polo-box, an event critical for inducing its physical clustering. Additional in vivo experiments showed that although STIL was not required for Plk4's initial ring-to-dot conversion, coexpressed STIL greatly enhanced Plk4's ability to generate a spherical condensate and recruit Sas6, a major component of the centriolar cartwheel structure. We propose that Plk4's autophosphorylation-induced clustering is sufficient to induce its ring-to-dot localization conversion and that subsequently recruited STIL potentiates this process to generate a procentriole assembly body critical for Plk4-dependent centriole biogenesis.

Stereoselective Synthesis of 1,6,9-Tri-oxaspiro[4.5]decanes From d-Glucose: Novel Structural Motifs of Spiroacetal Natural Products

Spiroacetals are the central structural core element of numerous natural products and are essential for their biological activity. A typical structural representative of a spiroacetal is the bicyclic 1,6-dioxaspiro[4.5]decane ring system. It represents the complete or partial structure of many biologically potent natural products such as the Paravespula pheromone 1, the antibiotic (+)-monensin A 2, the anticancer agent (−)-berkelic acid 3, the antimitotic ingredient spirastrellolide F, characterized after methylation as (+)-methyl ester 4, and the marine toxin (−)-calyculin A 5. In these compounds, the 1,6-dioxaspiro[4.5]decane ring system is found in either spiro ( R)-6 or ( S) - 6 configuration. The corresponding 1,6,9-trioxaspiro[4.5]decane framework ( S)-7 and ( R)-7 with opposite chirality at the spiro center due to an additional oxygen atom at position 9 in the pyran portion has so far not been found in living organisms or been synthesized. To close this gap and enable structure–activity relationship studies, potentially leading to novel antibiotics and selective anticancer agents, we have developed an efficient and stereocontrolled route to the ( R)- and ( S)-configurated 1,6,9-trioxaspiro[4.5]decane ring system leading to oxa analog motifs of the above natural products.

Measurement of Microcystin and Nodularin Activity in Human Urine by Immunocapture-Protein Phosphatase 2A Assay

Microcystins (MC) and nodularin (NOD) are toxins released by cyanobacteria during harmful algal blooms. They are potent inhibitors of protein phosphatases 1 and 2A (PP1 and PP2A) and cause a variety of adverse symptoms in humans and animals if ingested. More than 250 chemically diverse congeners of MCs have been identified, but certified reference materials are only available for a few. A diagnostic test that does not require each reference material for detection is necessary to identify human exposures. To address this need, our lab has developed a method that uses an antibody to specifically isolate MCs and NOD from urine prior to detection via a commercially available PP2A kit. This assay quantitates the summed inhibitory activity of nearly all MCs and NOD on PP2A relative to a common MC congener, microcystin-LR (MC-LR). The quantitation range for MC-LR using this method is from 0.050–0.500 ng/mL. No background responses were detected in a convenience set of 50 individual urines. Interday and intraday % accuracies ranged from 94%–118% and relative standard deviations were 15% or less, meeting FDA guidelines for receptor binding assays. The assay detected low levels of MCs in urines from three individuals living in close proximity to harmful algal blooms (HABs) in Florida.

Inactivated AMPK-α2 promotes the progression of diabetic brain damage by Cdk5 phosphorylation at Thr485 site

Changes in brain energy metabolism in diabetes mellitus, including increased insulin resistance and mitochondrial dysfunction, are critically involved in diabetes-related neurodegeneration, and associate with early cognitive impairment as well. The aim of this study is to detect the specific phosphorylated-Thr485- AMP-activated protein kinase (AMPK-α2), regulated by cyclin-dependent kinase 5 (Cdk5) paly the inhibitory functional role of AMPK-α2, Which is maybe the link to the accelerated diabetic brain damage progression. Here, we used GK rats, the type 2 diabetic animal model for in vivo studies and performed In vitro kinase assay, high glucose treatment, -phosphorylated mutation and protein expression in both HEK-293T and HT-22 cell lines. In vitro, the results show that murine wild-type AMPK-α2 was phosphorylated by Cdk5 at a (S/T)PX(K/H/R) phosphorylation consensus sequence, which was associated with decreased AMPK-α2 activity. Surprisingly, mutation of Thr485 to alanine in AMPK-α2 results in the abolished Cdk5 effects, demonstrating that Thr485-phosphorylation is critical to AMPK-α2 inhibition by Cdk5. In addition, these alterations in AMPK-α2-phosphorylation and -activity induced by Cdk5 is specific at Thr485. Furthermore, in GK rats, the increased phosphorylated- Thr 485 of AMPK-α2 results in the decreased AMPK-α2 activity, which is correlated with the apoptosis of neurons in hippocamps. After high glucose treatment, the decreased survival showed in AMPK-α2^T485A HT-22 cells compared to AMPK-α2^WT. The down-regulated of p-CREB, SNAP25, synaptophysin as well as synapsin-1were shown in both GK rats and HT-22 cell line. Meanwhile, pre-treated with either the specific Cdk5-inhibitor (roscovitine) or the antidiabetic AMPK-α2^-inhibitor (metformin) could restore the alterations in neuronal protein expression. Our results suggest that Cdk5-mediated phosphorylated- Thr485 in AMPK-α2 may be involved in the pathogenesis of diabetic brain damage.

N-(1-Phenylethyl)aziridine-2-carboxylate esters in the synthesis of biologically relevant compounds

Since Garner’s aldehyde has several drawbacks, first of all is prone to racemization, alternative three-carbon chirons would be of great value in enantioselective syntheses of natural compounds and/or drugs. This review article summarizes applications of N-(1-phenylethyl)aziridine-2-carboxylates, -carbaldehydes and -methanols in syntheses of approved drugs and potential medications as well as of natural products mostly alkaloids but also sphingoids and ceramides and their 1- and 3-deoxy analogues and several hydroxy amino acids and their precursors. Designed strategies provided new procedures to several drugs and alternative approaches to natural products and proved efficiency of a 2-substituted N-(1-phenylethyl)aziridine framework as chiron bearing a chiral auxiliary.

The Role of Serine-Threonine Protein Phosphatase PP2A in Plant Oxidative Stress Signaling-Facts and Hypotheses

Abiotic and biotic factors induce oxidative stress involving the production and scavenging of reactive oxygen species (ROS). This review is a survey of well-known and possible roles of serine-threonine protein phosphatases in plant oxidative stress signaling, with special emphasis on PP2A. ROS mediated signaling involves three interrelated pathways: (i) perception of extracellular ROS triggers signal transduction pathways, leading to DNA damage and/or the production of antioxidants; (ii) external signals induce intracellular ROS generation that triggers the relevant signaling pathways and (iii) external signals mediate protein phosphorylation dependent signaling pathway(s), leading to the expression of ROS producing enzymes like NADPH oxidases. All pathways involve inactivation of serine-threonine protein phosphatases. The metal dependent phosphatase PP2C has a negative regulatory function during ABA mediated ROS signaling. PP2A is the most abundant protein phosphatase in eukaryotic cells. Inhibitors of PP2A exert a ROS inducing activity as well and we suggest that there is a direct relationship between these two effects of drugs. We present current findings and hypotheses regarding PP2A-ROS signaling connections related to all three ROS signaling pathways and anticipate future research directions for this field. These mechanisms have implications in the understanding of stress tolerance of vascular plants, having applications regarding crop improvement.

The Phosphatase Inhibitor Calyculin-A Impairs Clot Retraction, Platelet Activation, and Thrombin Generation

The aim of this study was to investigate the effect of the serine/threonine protein phosphatase inhibitor, calyculin-A (CLA), on clot formation and on the procoagulant activity of human platelets. Platelet-rich plasma (PRP) samples were preincubated with buffer or CLA and subsequently platelets were activated by the protease-activated receptor 1 (PAR-1) activator, thrombin receptor activating peptide (TRAP). Clot retraction was detected by observing clot morphology up to 1 hour, phosphatidylserine- (PS-) expression was studied by flow cytometry, and thrombin generation was measured by a fluorimetric assay. For the intracellular Ca²⁺ assay, platelets were loaded with calcium-indicator dyes and the measurements were carried out using a ratiometric method with real-time confocal microscopy. CLA preincubation inhibited clot retraction, PS-expression, and thrombin formation. TRAP activation elicited Ca²⁺ response and PS-expression in a subset of platelets. The activated PRP displayed significantly faster and enhanced thrombin generation compared to nonactivated samples. CLA pretreatment abrogated PS-exposure and clot retraction also in TRAP-activated samples. As a consequence of the inhibitory effect on calcium elevation and PS-expression, CLA significantly downregulated thrombin generation in PRP. Our results show that CLA pretreatment may be a useful tool to investigate platelet activation mechanisms that contribute to clot formation and thrombin generation.

Calyculin: Nature's way of making the sponge-derived cytotoxin

Covering: up to 2015.Calyculin A is a major cytotoxic compound isolated from the Japanese marine sponge Discodermia calyx. Its potent cytotoxicity is attributable to the specific inhibition of protein phosphatases 1 and 2A, as in the case of okadaic acid and the microcystins. Its chemical structure is well-designed not only for enzyme inhibition but also for higher membrane permeability in order to impart its potent cytotoxicity. The biosynthetic gene cluster of this densely functionalized polyketide and nonribosomal peptide hybrid molecule was recently identified from the sponge-microbe association. The producer organism and the dynamic bioconversion process were also revealed. In this highlight, we focus on the recent studies addressing nature's design and biogenesis of the sponge-derived cytotoxin, calyculin A.

Phosphatase inhibition increases AQP2 accumulation in the rat IMCD apical plasma membrane

Vasopressin triggers the phosphorylation and apical plasma membrane accumulation of aquaporin 2 (AQP2), and it plays an essential role in urine concentration. Vasopressin, acting through protein kinase A, phosphorylates AQP2. However, the phosphorylation state of AQP2 could also be affected by the action of protein phosphatases (PPs). Rat inner medullas (IM) were incubated with calyculin (PP1 and PP2A inhibitor, 50 nM) or tacrolimus (PP2B inhibitor, 100 nM). Calyculin did not affect total AQP2 protein abundance (by Western blot) but did significantly increase the abundances of pS256-AQP2 and pS264-AQP2. It did not change pS261-AQP2 or pS269-AQP2. Calyculin significantly enhanced the membrane accumulation (by biotinylation) of total AQP2, pS256-AQP2, and pS264-AQP2. Likewise, immunohistochemistry showed an increase in the apical plasma membrane association of pS256-AQP2 and pS264-AQP2 in calyculin-treated rat IM. Tacrolimus also did not change total AQP2 abundance but significantly increased the abundances of pS261-AQP2 and pS264-AQP2. In contrast to calyculin, tacrolimus did not change the amount of total AQP2 in the plasma membrane (by biotinylation and immunohistochemistry). Tacrolimus did increase the expression of pS264-AQP2 in the apical plasma membrane (by immunohistochemistry). In conclusion, PP1/PP2A regulates the phosphorylation and apical plasma membrane accumulation of AQP2 differently than PP2B. Serine-264 of AQP2 is a phosphorylation site that is regulated by both PP1/PP2A and PP2B. This dual regulatory pathway may suggest a previously unappreciated role for multiple phosphatases in the regulation of urine concentration.

Synthesis of Highly Selective Submicromolar Microcystin-Based Inhibitors of Protein Phosphatase (PP)2A over PP1

Research and therapeutic targeting of the phosphoserine/threonine phosphatases PP1 and PP2A is hindered by the lack of selective inhibitors. The microcystin (MC) natural toxins target both phosphatases with equal potency, and their complex synthesis has complicated structure–activity relationship studies in the past. We report herein the synthesis and biochemical evaluation of 11 MC analogues, which was accomplished through an efficient strategy combining solid‐ and solution‐phase approaches. Our approach led to the first MC analogue with submicromolar inhibitory potency that is strongly selective for PP2A over PP1 and does not require the complex lipophilic Adda group. Through mutational and structural analyses, we identified a new key element for binding, as well as reasons for the selectivity. This work gives unprecedented insight into how selectivity between these phosphatases can be achieved with MC analogues.

Structure and Configuration of Phosphoeleganin, a Protein Tyrosine Phosphatase 1B Inhibitor from the Mediterranean Ascidian Sidnyum elegans

A new phosphorylated polyketide, phosphoeleganin (1), has been isolated from the Mediterranean ascidian Sidnyum elegans. Its structure and configuration have been determined by extensive use of 2D NMR and microscale chemical degradation and/or derivatization. Phosphoeleganin (1) inhibited the protein tyrosine phosphatase 1B (PTP1B) activity.

Condensin I and II behaviour in interphase nuclei and cells undergoing premature chromosome condensation

Condensin is an integral component of the mitotic chromosome condensation machinery, which ensures orderly segregation of chromosomes during cell division. In metazoans, condensin exists as two complexes, condensin I and II. It is not yet clear what roles these complexes may play outside mitosis, and so we have examined their behaviour both in normal interphase and in premature chromosome condensation (PCC). We find that a small fraction of condensin I is retained in interphase nuclei, and our data suggests that this interphase nuclear condensin I is active in both gene regulation and chromosome condensation. Furthermore, live cell imaging demonstrates condensin II dramatically increases on G1 nuclei following completion of mitosis. Our PCC studies show condensins I and II and topoisomerase II localise to the chromosome axis in G1-PCC and G2/M-PCC, while KIF4 binding is altered. Individually, condensins I and II are dispensable for PCC. However, when both are knocked out, G1-PCC chromatids are less well structured. Our results define new roles for the condensins during interphase and provide new information about the mechanism of PCC.

Structure of Saccharomyces cerevisiae Rtr1 reveals an active site for an atypical phosphatase

Changes in the phosphorylation status of the carboxyl-terminal domain (CTD) of RNA polymerase II (RNAPII) correlate with the process of eukaryotic transcription. The yeast protein regulator of transcription 1 (Rtr1) and the human homolog RNAPII-associated protein 2 (RPAP2) may function as CTD phosphatases; however, crystal structures of Kluyveromyces lactis Rtr1 lack a consensus active site. We identified a phosphoryl transfer domain in Saccharomyces cerevisiae Rtr1 by obtaining and characterizing a 2.6 Å resolution crystal structure. We identified a putative substrate-binding pocket in a deep groove between the zinc finger domain and a pair of helices that contained a trapped sulfate ion. Because sulfate mimics the chemistry of a phosphate group, this structural data suggested that this groove represents the phosphoryl transfer active site. Mutagenesis of the residues lining this groove disrupted catalytic activity of the enzyme assayed in vitro with a fluorescent chemical substrate, and expression of the mutated Rtr1 failed to rescue growth of yeast lacking Rtr1. Characterization of the phosphatase activity of RPAP2 and a mutant of the conserved putative catalytic site in the same chemical assay indicated a conserved reaction mechanism. Our data indicated that the structure of the phosphoryl transfer domain and reaction mechanism for the phosphoryl transfer activity of Rtr1 is distinct from those of other phosphatase families.

Biosynthesis of polyketides by trans-AT polyketide synthases

This review discusses the biosynthesis of natural products that are generated bytrans-AT polyketide synthases, a family of catalytically versatile enzymes that represents one of the major group of proteins involved in the production of bioactive polyketides.

Counteracting Protein Kinase Activity in the Heart: The Multiple Roles of Protein Phosphatases

Decades of cardiovascular research have shown that variable and flexible levels of protein phosphorylation are necessary to maintain cardiac function. A delicate balance between phosphorylated and dephosphorylated states of proteins is guaranteed by a complex interplay of protein kinases (PKs) and phosphatases. Serine/threonine phosphatases, in particular members of the protein phosphatase (PP) family govern dephosphorylation of the majority of these cardiac proteins. Recent findings have however shown that PPs do not only dephosphorylate previously phosphorylated proteins as a passive control mechanism but are capable to actively control PK activity via different direct and indirect signaling pathways. These control mechanisms can take place on (epi-)genetic, (post-)transcriptional, and (post-)translational levels. In addition PPs themselves are targets of a plethora of proteinaceous interaction partner regulating their endogenous activity, thus adding another level of complexity and feedback control toward this system. Finally, novel approaches are underway to achieve spatiotemporal pharmacologic control of PPs which in turn can be used to fine-tune misleaded PK activity in heart disease. Taken together, this review comprehensively summarizes the major aspects of PP-mediated PK regulation and discusses the subsequent consequences of deregulated PP activity for cardiovascular diseases in depth.

Protein kinase Cδ-mediated phosphorylation of Connexin43 gap junction channels causes movement within gap junctions followed by vesicle internalization and protein degradation

Phosphorylation of gap junction proteins, connexins, plays a role in global signaling events involving kinases. Connexin43 (Cx43), a ubiquitous and important connexin, has several phosphorylation sites for specific kinases. We appended an imaging reporter tag for the activity of the δ isoform of protein kinase C (PKCδ) to the carboxyl terminus of Cx43. The FRET signal of this reporter is inversely related to the phosphorylation of serine 368 of Cx43. By activating PKC with the phorbol ester phorbol 12,13-dibutyrate (PDBu) or a natural stimulant, UTP, time lapse live cell imaging movies indicated phosphorylated Ser-368 Cx43 separated into discrete domains within gap junctions and was internalized in small vesicles, after which it was degraded by lysosomes and proteasomes. Mutation of Ser-368 to an Ala eliminated the response to PDBu and changes in phosphorylation of the reporter. A phosphatase inhibitor, calyculin A, does not change this pattern, indicating PKC phosphorylation causes degradation of Cx43 without dephosphorylation, which is in accordance with current hypotheses that cells control their intercellular communication by a fast and constant turnover of connexins, using phosphorylation as part of this mechanism.

Marine pharmacology in 2009-2011: marine compounds with antibacterial, antidiabetic, antifungal, anti-inflammatory, antiprotozoal, antituberculosis, and antiviral activities; affecting the immune and nervous systems, and other miscellaneous mechanisms of action

The peer-reviewed marine pharmacology literature from 2009 to 2011 is presented in this review, following the format used in the 1998–2008 reviews of this series. The pharmacology of structurally-characterized compounds isolated from marine animals, algae, fungi and bacteria is discussed in a comprehensive manner. Antibacterial, antifungal, antiprotozoal, antituberculosis, and antiviral pharmacological activities were reported for 102 marine natural products. Additionally, 60 marine compounds were observed to affect the immune and nervous system as well as possess antidiabetic and anti-inflammatory effects. Finally, 68 marine metabolites were shown to interact with a variety of receptors and molecular targets, and thus will probably contribute to multiple pharmacological classes upon further mechanism of action studies. Marine pharmacology during 2009–2011 remained a global enterprise, with researchers from 35 countries, and the United States, contributing to the preclinical pharmacology of 262 marine compounds which are part of the preclinical pharmaceutical pipeline. Continued pharmacological research with marine natural products will contribute to enhance the marine pharmaceutical clinical pipeline, which in 2013 consisted of 17 marine natural products, analogs or derivatives targeting a limited number of disease categories.

Targeting the untargetable: recent advances in the selective chemical modulation of protein phosphatase-1 activity

Protein phosphatase-1 (PP1) has long been neglected as a potential drug target owing to its misinterpreted unselective nature. However, growing evidence demonstrates that PP1 is highly selective in complex with regulatory proteins at the holoenzyme level, each of which is involved in different essential cellular signaling events. Here we summarize promising approaches to specifically activate or inhibit PP1 activity, and discuss remaining challenges and potential solutions. The summarized chemical tools pave the way for a better understanding of PP1's role in signaling networks, and the effects resulting from their application suggest their potential as future therapeutic candidates.

siRNA screen identifies the phosphatase acting on the G protein-coupled thyrotropin-releasing hormone receptor

G protein-coupled receptors (GPCRs) are an ubiquitously expressed class of transmembrane proteins involved in the signal transduction of neurotransmitters, hormones and various other ligands. Their signaling output is desensitized by mechanisms involving phosphorylation, internalization, and dissociation from G proteins and resensitized by mechanisms involving dephosphorylation, but details about the phosphatases responsible are generally lacking. We describe here the use of an siRNA-based library to knock down expression of specific phosphatase subunits to identify protein phosphatase 1-α (PP1α) as important for the thyrotropin-releasing hormone (TRH) receptor. Inhibition of PP1α synthesis and overexpression of dominant negative PP1α preserved receptor phosphorylation under conditions favoring dephosphorylation, whereas overexpression of PP1α accelerated dephosphorylation. Knockdown of all three PP1 catalytic subunits inhibited TRH receptor phosphorylation much more powerfully than knockdown of PP1α alone, suggesting that different PP1 isoforms function redundantly. Knockdown of a structural subunit of PP2A, a second potential hit in the library screen, was ineffective. Calyculin A, a potent inhibitor of PP1 family phosphatases, strongly inhibited dephosphorylation of transfected TRH receptors and endogenous receptors in pituitary cells, but fostriecin, which is selective for PP2A family phosphatases, did not. We conclude that the PP1 class of phosphatases is essential for TRH receptor dephosphorylation.

Marine natural products

Covering: 2010. Previous review: Nat. Prod. Rep., 2011, 28, 196. This review covers the literature published in 2010 for marine natural products, with 895 citations (590 for the period January to December 2010) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1003 for 2010), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Muscarinic-activated potassium current mediates the negative chronotropic effect of pilocarpine on the rabbit sinoatrial node

Pilocarpine is a nonspecific agonist of muscarinic receptors which was recently found to activate the M(2) receptor subtype in a voltage-dependent manner. The purpose of our study was to investigate the role of the acetylcholine (muscarinic)-activated K(+) current (I (KACh)) on the negative chronotropic effect of pilocarpine in rabbit sinoatrial node. In multicellular preparations, we studied the effect of pilocarpine on spontaneous action potentials. In isolated myocytes, using the patch clamp technique, we studied the effects of pilocarpine on I (KACh). Pilocarpine produced a decrease in spontaneous frequency, hyperpolarization of the maximum diastolic potential, and a decrease in the diastolic depolarization rate. These effects were partially antagonized by tertiapin Q. Cesium and calyculin A in the presence of tertiapin Q partially prevented the effects of pilocarpine. In isolated myocytes, pilocarpine activated the muscarinic potassium current, I (KACh) in a voltage-dependent manner. In conclusion, the negative chronotropic effects of pilocarpine on the sinatrial node could be mainly explained by activation of I (KACh).

The Lithistida: important sources of compounds useful in biomedical research

Lithistid sponges have been an important source of structurally complex natural products with potent biological activities. Examples of compounds marketed as biological markers along with recent advances in defining the modes of action and biomedical potential of lithistid-derived compounds are presented.

Calcium/calmodulin-dependent protein kinase II activity regulates the proliferative potential of growth plate chondrocytes

For tissues that develop throughout embryogenesis and into postnatal life, the generation of differentiated cells to promote tissue growth is at odds with the requirement to maintain the stem cell/progenitor cell population to preserve future growth potential. In the growth plate cartilage, this balance is achieved in part by establishing a proliferative phase that amplifies the number of progenitor cells prior to terminal differentiation into hypertrophic chondrocytes. Here, we show that endogenous calcium/calmodulin-dependent protein kinase II (CamkII, also known as Camk2) activity is upregulated prior to hypertrophy and that loss of CamkII function substantially blocks the transition from proliferation to hypertrophy. Wnt signaling and Pthrp-induced phosphatase activity negatively regulate CamkII activity. Release of this repression results in activation of multiple effector pathways, including Runx2- and β-catenin-dependent pathways. We present an integrated model for the regulation of proliferation potential by CamkII activity that has important implications for studies of growth control and adult progenitor/stem cell populations.

Crosstalk of the EphA2 receptor with a serine/threonine phosphatase suppresses the Akt-mTORC1 pathway in cancer cells

Receptor tyrosine kinases of the Eph family play multiple roles in the physiological regulation of tissue homeostasis and in the pathogenesis of various diseases, including cancer. The EphA2 receptor is highly expressed in most cancer cell types, where it has disparate activities that are not well understood. It has been reported that interplay of EphA2 with oncogenic signaling pathways promotes cancer cell malignancy independently of ephrin ligand binding and receptor kinase activity. In contrast, stimulation of EphA2 signaling with ephrin-A ligands can suppress malignancy by inhibiting the Ras-MAP kinase pathway, integrin-mediated adhesion, and epithelial to mesenchymal transition. Here we show that ephrin-A1 ligand-dependent activation of EphA2 decreases the growth of PC3 prostate cancer cells and profoundly inhibits the Akt-mTORC1 pathway, which is hyperactivated due to loss of the PTEN tumor suppressor. Our results do not implicate changes in the activity of Akt upstream regulators (such as Ras family GTPases, PI3 kinase, integrins, or the Ship2 lipid phosphatase) in the observed loss of Akt T308 and S473 phosphorylation downstream of EphA2. Indeed, EphA2 can inhibit Akt phosphorylation induced by oncogenic mutations of not only PTEN but also PI3 kinase. Furthermore, it can decrease the hyperphosphorylation induced by constitutive membrane-targeting of Akt. Our data suggest a novel signaling mechanism whereby EphA2 inactivates the Akt-mTORC1 oncogenic pathway through Akt dephosphorylation mediated by a serine/threonine phosphatase. Ephrin-A1-induced Akt dephosphorylation was observed not only in PC3 prostate cancer cells but also in other cancer cell types. Thus, activation of EphA2 signaling represents a possible new avenue for anti-cancer therapies that exploit the remarkable ability of this receptor to counteract multiple oncogenic signaling pathways.

Biological activities of aqueous and organic extracts from tropical marine sponges

We report on screening tests of 66 extracts obtained from 35 marine sponge species from the Caribbean Sea (Curaçao) and from eight species from the Great Barrier Reef (Lizard Island). Extracts were prepared in aqueous and organic solvents and were tested for hemolytic, hemagglutinating, antibacterial and anti-acetylcholinesterase (AChE) activities, as well as their ability to inhibit or activate cell protein phosphatase 1 (PP1). The most interesting activities were obtained from extracts of Ircinia felix, Pandaros acanthifolium, Topsentia ophiraphidites, Verongula rigida and Neofibularia nolitangere. Aqueous and organic extracts of I. felix and V. rigida showed strong antibacterial activity. Topsentia aqueous and some organic extracts were strongly hemolytic, as were all organic extracts from I. felix. The strongest hemolytic activity was observed in aqueous extracts from P. acanthifolium. Organic extracts of N. nolitangere and I. felix inhibited PP1. The aqueous extract from Myrmekioderma styx possessed the strongest hemagglutinating activity, whilst AChE inhibiting activity was found only in a few sponges and was generally weak, except in the methanolic extract of T. ophiraphidites.

Multiple pathways for protein phosphatase 1 (PP1) regulation of Na-K-2Cl cotransporter (NKCC1) function: the N-terminal tail of the Na-K-2Cl cotransporter serves as a regulatory scaffold for Ste20-related proline/alanine-rich kinase (SPAK) AND PP1

The Na-K-2Cl cotransporter (NKCC1) participates in epithelial transport and in cell volume maintenance by mediating the movement of ions and water across plasma membranes. Functional studies have previously demonstrated that NKCC1 activity is stimulated by protein phosphatase 1 (PP1) inhibitors. In this study, we utilized both in vivo (heterologous cRNA expression in Xenopus laevis oocytes) and in vitro ((32)P-phosphorylation assays with glutathione S-transferase fusion proteins) experiments to determine whether PP1 exerts its inhibitory effect directly on the cotransporter, or indirectly by affecting the activating kinase. We found that PP1 reduced NKCC1 activity in oocytes under both isotonic and hypertonic conditions to the same level as in water-injected controls. Interestingly, mutation of key residues in the PP1 binding motif located in the N-terminal tail of NKCC1 significantly reduced the inhibitory effect of PP1. In vitro experiments performed with recombinant PP1, SPAK (Ste20-related proline/alanine-rich kinase, which activates NKCC1), and the N terminus of NKCC1 fused to glutathione S-transferase demonstrated that PP1 dephosphorylated both the kinase and the cotransporter in a time-dependent manner. More importantly, PP1 dephosphorylation of SPAK was significantly greater when protein-protein interaction between the kinase and the N-terminal tail of NKCC1 was present in the reaction, indicating the necessity of scaffolding the phosphatase and kinase in proximity to one another. Taken together, our data are consistent with PP1 inhibiting NKCC1 activity directly by dephosphorylating the cotransporter and indirectly by dephosphorylating SPAK.