Discordant population structure among rhizobium divided genomes and their legume hosts

Symbiosis often occurs between partners with distinct life history characteristics and dispersal mechanisms. Many bacterial symbionts have genomes comprising multiple replicons with distinct rates of evolution and horizontal transmission. Such differences might drive differences in population structure between hosts and symbionts and among the elements of the divided genomes of bacterial symbionts. These differences might, in turn, shape the evolution of symbiotic interactions and bacterial evolution. Here we use whole genome resequencing of a hierarchically structured sample of 191 strains of Sinorhizobium meliloti collected from 21 locations in southern Europe to characterize population structures of this bacterial symbiont, which forms a root nodule symbiosis with the host plant Medicago truncatula. S. meliloti genomes showed high local (within-site) variation and little isolation by distance. This was particularly true for the two symbiosis elements, pSymA and pSymB, which have population structures that are similar to each other, but distinct from both the bacterial chromosome and the host plant. Given limited recombination on the chromosome, compared to the symbiosis elements, distinct population structures may result from differences in effective gene flow. Alternatively, positive or purifying selection, with little recombination, may explain distinct geographical patterns at the chromosome. Discordant population structure between hosts and symbionts indicates that geographically and genetically distinct host populations in different parts of the range might interact with genetically similar symbionts, potentially minimizing local specialization.

Effects of Domestication on Plant-Microbiome Interactions

Through the process of domestication, selection is targeted on a limited number of plant traits that are typically associated with yield. As an unintended consequence, domesticated plants often perform poorly compared to their wild progenitors for a multitude of traits that were not under selection during domestication, including abiotic and biotic stress tolerance. Over the past decade, advances in sequencing technology have allowed for the rigorous characterization of host-associated microbial communities, termed the microbiome. It is now clear that nearly every conceivable plant interaction with the environment is mediated by interactions with the microbiome. For this reason, plant-microbiome interactions are an area of great promise for plant breeding and crop improvement. Here, we review the literature to assess the potential impact that domestication has had on plant-microbiome interactions and the current understanding of the genetic basis of microbiome variation to inform plant breeding efforts. Overall, we find limited evidence that domestication impacts the diversity of microbiomes, but domestication is often associated with shifts in the abundance and composition of microbial communities, including taxa of known functional significance. Moreover, genome-wide association studies and mutant analysis have not revealed a consistent set of core candidate genes or genetic pathways that confer variation in microbiomes across systems. However, such studies do implicate a consistent role for plant immunity, root traits, root and leaf exudates and cell wall integrity as key traits that control microbiome colonization and assembly. Therefore, selection on these key traits may pose the most immediate promise for enhancing plant-microbiome interactions through breeding.

Combining GWAS and population genomic analyses to characterize coevolution in a legume-rhizobia symbiosis

The mutualism between legumes and rhizobia is clearly the product of past coevolution. However, the nature of ongoing evolution between these partners is less clear. To characterize the nature of recent coevolution between legumes and rhizobia, we used population genomic analysis to characterize selection on functionally annotated symbiosis genes as well as on symbiosis gene candidates identified through a two-species association analysis. For the association analysis, we inoculated each of 202 accessions of the legume host Medicago truncatula with a community of 88 Sinorhizobia (Ensifer) meliloti strains. Multistrain inoculation, which better reflects the ecological reality of rhizobial selection in nature than single-strain inoculation, allows strains to compete for nodulation opportunities and host resources and for hosts to preferentially form nodules and provide resources to some strains. We found extensive host by symbiont, that is, genotype-by-genotype, effects on rhizobial fitness and some annotated rhizobial genes bear signatures of recent positive selection. However, neither genes responsible for this variation nor annotated host symbiosis genes are enriched for signatures of either positive or balancing selection. This result suggests that stabilizing selection dominates selection acting on symbiotic traits and that variation in these traits is under mutation-selection balance. Consistent with the lack of positive selection acting on host genes, we found that among-host variation in growth was similar whether plants were grown with rhizobia or N-fertilizer, suggesting that the symbiosis may not be a major driver of variation in plant growth in multistrain contexts.

Cofilactin filaments regulate filopodial structure and dynamics in neuronal growth cones

Cofilin is best known for its ability to sever actin filaments and facilitate cytoskeletal recycling inside of cells, but at higher concentrations in vitro, cofilin stabilizes a more flexible, hyper-twisted state of actin known as “cofilactin”. While this filament state is well studied, a structural role for cofilactin in dynamic cellular processes has not been observed. With a combination of cryo-electron tomography and fluorescence imaging in neuronal growth cones, we observe that filopodial actin filaments switch between a fascin-linked and a cofilin-decorated state, and that cofilactin is associated with a variety of dynamic events within filopodia. The switch to cofilactin filaments occurs in a graded fashion and correlates with a decline in fascin cross-linking within the filopodia, which is associated with curvature in the bundle. Our tomographic data reveal that the hyper-twisting of actin from cofilin binding leads to a rearrangement of filament packing, which largely excludes fascin from the base of filopodia. Our results provide mechanistic insight into the fundamentals of cytoskeletal remodeling inside of confined cellular spaces, and how the interplay between fascin and cofilin regulates the dynamics of searching filopodia.

In this manuscript the authors show that Filopodia switch between bundles of fascin-crosslinked actin and cofilin-decorated filaments, which exclude fascin binding due to altered structure and packing, as well as affect filopodial searching dynamics.

Soil origin and plant genotype structure distinct microbiome compartments in the model legume Medicago truncatula

BACKGROUND

Understanding the genetic and environmental factors that structure plant microbiomes is necessary for leveraging these interactions to address critical needs in agriculture, conservation, and sustainability. Legumes, which form root nodule symbioses with nitrogen-fixing rhizobia, have served as model plants for understanding the genetics and evolution of beneficial plant-microbe interactions for decades, and thus have added value as models of plant-microbiome interactions. Here we use a common garden experiment with 16S rRNA gene amplicon and shotgun metagenomic sequencing to study the drivers of microbiome diversity and composition in three genotypes of the model legume Medicago truncatula grown in two native soil communities.

RESULTS

Bacterial diversity decreased between external (rhizosphere) and internal plant compartments (root endosphere, nodule endosphere, and leaf endosphere). Community composition was shaped by strong compartment × soil origin and compartment × plant genotype interactions, driven by significant soil origin effects in the rhizosphere and significant plant genotype effects in the root endosphere. Nevertheless, all compartments were dominated by Ensifer, the genus of rhizobia that forms root nodule symbiosis with M. truncatula, and additional shotgun metagenomic sequencing suggests that the nodulating Ensifer were not genetically distinguishable from those elsewhere in the plant. We also identify a handful of OTUs that are common in nodule tissues, which are likely colonized from the root endosphere.

CONCLUSIONS

Our results demonstrate strong host filtering effects, with rhizospheres driven by soil origin and internal plant compartments driven by host genetics, and identify several key nodule-inhabiting taxa that coexist with rhizobia in the native range. Our results set the stage for future functional genetic experiments aimed at expanding our pairwise understanding of legume-rhizobium symbiosis toward a more mechanistic understanding of plant microbiomes. Video Abstract.

Live Imaging of Type I Collagen Assembly Dynamics in Osteoblasts Stably Expressing GFP and mCherry-Tagged Collagen Constructs

Type I collagen is the most abundant extracellular matrix protein in bone and other connective tissues and plays key roles in normal and pathological bone formation as well as in connective tissue disorders and fibrosis. Although much is known about the collagen biosynthetic pathway and its regulatory steps, the mechanisms by which it is assembled extracellularly are less clear. We have generated GFPtpz and mCherry-tagged collagen fusion constructs for live imaging of type I collagen assembly by replacing the α2(I)-procollagen N-terminal propeptide with GFPtpz or mCherry. These novel imaging probes were stably transfected into MLO-A5 osteoblast-like cells and fibronectin-null mouse embryonic fibroblasts (FN-null-MEFs) and used for imaging type I collagen assembly dynamics and its dependence on fibronectin. Both fusion proteins co-precipitated with α1(I)-collagen and remained intracellular without ascorbate but were assembled into α1(I) collagen-containing extracellular fibrils in the presence of ascorbate. Immunogold-EM confirmed their ultrastuctural localization in banded collagen fibrils. Live cell imaging in stably transfected MLO-A5 cells revealed the highly dynamic nature of collagen assembly and showed that during assembly the fibril networks are continually stretched and contracted due to the underlying cell motion. We also observed that cell-generated forces can physically reshape the collagen fibrils. Using co-cultures of mCherry- and GFPtpz-collagen expressing cells, we show that multiple cells contribute collagen to form collagen fiber bundles. Immuno-EM further showed that individual collagen fibrils can receive contributions of collagen from more than one cell. Live cell imaging in FN-null-MEFs expressing GFPtpz-collagen showed that collagen assembly was both dependent upon and dynamically integrated with fibronectin assembly. These GFP-collagen fusion constructs provide a powerful tool for imaging collagen in living cells and have revealed novel and fundamental insights into the dynamic mechanisms for the extracellular assembly of collagen. © 2018 American Society for Bone and Mineral Research.

Control of Neuronal Ryanodine Receptor-Mediated Calcium Signaling by Calsenilin

Calsenilin is a calcium ion (Ca²⁺)-binding protein involved in regulating the intracellular concentration of Ca²⁺, a second messenger that controls multiple cellular signaling pathways. The ryanodine receptor (RyR) amplifies Ca²⁺ signals entering the cytoplasm by releasing Ca²⁺ from endoplasmic reticulum (ER) stores, a process termed calcium-induced calcium release (CICR). Here, we describe a novel mechanism, in which calsenilin controls the activity of neuronal RyRs. We show calsenilin co-localized with RyR2 and 3 in the ER of mouse hippocampal and cortical neurons using immunocytochemistry. The underlying protein-protein interaction between calsenilin and the RyR was determined in mouse central nervous system (CNS) neurons using immunoprecipitation studies. The functional relevance of this interaction was assayed with single-channel electrophysiology. At low physiological Ca²⁺ concentrations, calsenilin binding to the cytoplasmic face of neuronal RyRs decreased the RyR's open probability, while calsenilin increased the open probability at high physiological Ca²⁺ concentrations. This novel molecular mechanism was studied further at the cellular level, where faster release kinetics of caffeine-induced Ca²⁺ release were measured in SH-SY5Y neuroblastoma cells overexpressing calsenilin. The interaction between calsenilin and neuronal RyRs reveals a new regulatory mechanism and possibly a novel pharmacological target for the control of Ca²⁺ release from intracellular stores.

Intrapopulation genomics in a model mutualist: Population structure and candidate symbiosis genes under selection in Medicago truncatula

Bottom-up evolutionary approaches, including geographically explicit population genomic analyses, have the power to reveal the mechanistic basis of adaptation. Here, we conduct a population genomic analysis in the model legume, Medicago truncatula, to characterize population genetic structure and identify symbiosis-related genes showing evidence of spatially variable selection. Using RAD-seq, we generated over 26,000 SNPs from 191 accessions from within three regions of the native range in Europe. Results from STRUCTURE analysis identify five distinct genetic clusters with divisions that separate east and west regions in the Mediterranean basin. Much of the genetic variation is maintained within sampling sites, and there is evidence for isolation by distance. Extensive linkage disequilibrium was identified, particularly within populations. We conducted genetic outlier analysis with F_ST -based genome scans and a Bayesian modeling approach (PCAdapt). There were 70 core outlier loci shared between these distinct methods with one clear candidate symbiosis related gene, DMI1. This work sets that stage for functional experiments to determine the important phenotypes that selection has acted upon and complementary efforts in rhizobium populations.

Nitrogen addition does not influence pre-infection partner choice in the legume-rhizobium symbiosis

PREMISE OF THE STUDY

Resource mutualisms such as the symbiosis between legumes and nitrogen-fixing rhizobia are context dependent and are sensitive to various aspects of the environment, including nitrogen (N) addition. Mutualist hosts such as legumes are also thought to use mechanisms such as partner choice to discriminate among potential symbionts that vary in partner quality (fitness benefits conferred to hosts) and thus impose selection on rhizobium populations. Together, context dependency and partner choice might help explain why the legume-rhizobium mutualism responds evolutionarily to N addition, since plant-mediated selection that shifts in response to N might be expected to favor different rhizobium strains in different N environments.

METHODS

We test for the influence of context dependency on partner choice in the model legume, Medicago truncatula, using a factorial experiments with three plant families across three N levels with a mixed inoculation of three rhizobia strains.

KEY RESULTS

Neither the relative frequencies of rhizobium strains occupying host nodules, nor the size of those nodules, differed in response to N level.

CONCLUSIONS

Despite the lack of context dependence, plant genotypes respond very differently to mixed populations of rhizobia, suggesting that these traits are genetically variable and thus could evolve in response to longer-term increases in N.

N-Palmitoylethanolamine depot injection increased its tissue levels and those of other acylethanolamide lipids

N-Palmitoylethanolamine (NAE 16:0) is an endogenous lipid signaling molecule that has limited water solubility, and its action is short-lived due to its rapid metabolism. This poses a problem for use in vivo as oral administration requires a high concentration for significant levels to reach target tissues, and injection of the compound in a dimethyl sulfoxide- or ethanol-based vehicle is usually not desirable during long-term treatment. A depot injection of NAE 16:0 was successfully emulsified in sterile corn oil (10 mg/kg) and administered in young DBA/2 mice in order to elevate baseline levels of NAE 16:0 in target tissues. NAE 16:0 levels were increased in various tissues, particularly in the retina, 24 and 48 hours following injections. Increases ranged between 22% and 215% (above basal levels) in blood serum, heart, brain, and retina and induced an entourage effect by increasing levels of other 18 carbon N-Acylethanolamines (NAEs), which ranged between 31% and 117% above baseline. These results indicate that NAE 16:0 can be used as a depot preparation, avoiding the use of inadequate vehicles, and can provide the basis for designing tissue-specific dosing regimens for therapies involving NAEs and related compounds.

Genetic architecture of flowering time differentiation between locally adapted populations of Arabidopsis thaliana

To gain an understanding of the genetic basis of adaptation, we conducted quantitative trait locus (QTL) mapping for flowering time variation between two winter annual populations of Arabidopsis thaliana that are locally adapted and display distinct flowering times. QTL mapping was performed with large (n = 384) F(2) populations with and without vernalization, in order to reveal both the genetic basis of a vernalization requirement and that of variation in flowering time given vernalization. In the nonvernalization treatment, none of the Sweden parents flowered, whereas all of the Italy parents and 42% of the F(2)s flowered. We identified three QTLs for flowering without vernalization, with much of the variation being attributed to a QTL co-localizing with FLOWERING LOCUS C (FLC). In the vernalization treatment, all parents and F(2)s flowered, and six QTLs of small to moderate effect were revealed, with underlying candidate genes that are members of the vernalization pathway. We found no evidence for a role of FRIGIDA in the regulation of flowering times. These results contribute to a growing body of evidence aimed at the identification of ecologically relevant genetic variation for flowering time in Arabidopsis, and set the stage for functional studies to determine the link between flowering time loci and fitness.

Distribution and function of polycystin-2 in mouse retinal ganglion cells

The polycystin family of transient receptor potential (TRP) channels form Ca(2+) regulated cation channels with distinct subcellullar localizations and functions. As part of heteromultimeric channels and multi-protein complexes, polycystins control intracellular Ca(2+) signals and more generally the translation of extracellular signals and stimuli to intracellular responses. Polycystin-2 channels have been cloned from retina, but their distribution and function in retinal ganglion cells (RGCs) have not yet been established. In the present study, we determined cellular and subcellular localization as well as functional properties of polycystin-2 channels in RGCs. Polycystin-2 expression and distribution in RGCs was assessed by immunohistochemistry on vertical cryostat section of mouse retina as well as primary cultured mouse RGCs, using fluorescence microscopy. Biophysical and pharmacological properties of polycystin-2 channels isolated from primary cultured RGCs were determined using planar lipid bilayer electrophysiology. We detected polycystin-2 immunoreactivity both in the ganglion cell layer as well as in primary cultured RGCs. Subcellular analysis revealed strong cytosolic localization pattern of polycystin-2. Polycystin-2 channel current was Ca(2+) activated, had a maximum slope conductance of 114 pS, and could be blocked in a dose-dependent manner by increasing concentrations of Mg(2+). The cytosolic localization of polycystin-2 in RGCs is in accordance with its function as intracellular Ca(2+) release channel. We conclude that polycystin-2 forms functional channels in RGCs, of which biophysical and pharmacological properties are similar to polycystin-2 channels reported for other tissues and organisms. Our data suggest a potential role for polycystin-2 in RGC Ca(2+) signaling.

The unliganded long isoform of estrogen receptor beta stimulates brain ryanodine receptor single channel activity alongside with cytosolic Ca2+

Ca(2+) release from intracellular stores mediated by endoplasmic reticulum membrane ryanodine receptors (RyR) plays a key role in activating and synchronizing downstream Ca(2+)-dependent mechanisms, in different cells varying from apoptosis to nuclear transcription and development of defensive responses. Recently discovered, atypical "nongenomic" effects mediated by estrogen receptors (ER) include rapid Ca(2+) release upon estrogen exposure in conditions implicitly suggesting involvement of RyRs. In the present study, we report various levels of colocalization between RyR type 2 (RyR2) and ER type beta (ER beta) in the neuronal cell line HT-22, indicating a possible functional interaction. Electrophysiological analyses revealed a significant increase in single-channel ionic currents generated by mouse brain RyRs after application of the soluble monomer of the long form ER beta (ER beta 1). The effect was due to a strong increase in open probability of RyR higher open channel sublevels at cytosolic [Ca(2+)] concentrations of 100 nM, suggesting a synergistic action of ER beta 1 and Ca(2+) in RyR activation, and a potential contribution to Ca(2+)-induced Ca(2+) release rather than to basal intracellular Ca(2+) concentration level at rest. This RyR/ER beta interaction has potential effects on cellular physiology, including roles of shorter ER beta isoforms and modulation of the RyR/ER beta complexes by exogenous estrogens.

Transport of amino acids through the placenta and their role

Amino acids are transported across the human placenta mediated by transporter proteins that differ in structure, mechanism and substrate specificity. Some of them are Na+-dependent systems, whereas others are Na+-independent. Among these there are transporters composed of a heavy chain, a glycoprotein, and a light chain. Moreover, they can be differently distributed in the two membranes forming the syncytiotrophoblast. The transport mechanisms involved and their regulation are only partially known. In the placenta itself, part of the amino acids is metabolized to form other compounds important for the fetus. This occurs for instance for arginine, which gives rise to polyamines and to NO. Interconversion occurs among few other amino acids Transport is altered in pregnancy complications, such as restricted fetal growth.

Potassium channel blockers quinidine and caesium halt cell proliferation in C6 glioma cells via a polyamine-dependent mechanism

Potassium channels are ubiquitous in cells and serve essential functions in physiology and pathophysiology. Potassium channel blockers have been shown to block tumour growth by arresting cells at the G(0)/G(1) checkpoint of the cell cycle. We investigated the effect of quinidine and caesium (Cs(+)) on cell proliferation, LDH (lactate dehydrogenase) release, free internal calcium, membrane potential, polyamine concentration, ODC (ornithine decarboxylase) activity and polyamine uptake in C6 glioma cells. The EC(50) for reducing cell proliferation was 112 microM for quinidine, whereas Cs(+) was less effective with an EC(50) of 4.75 mM. KCl or sucrose did not affect proliferation. LDH release was augmented by quinidine. Quinidine caused a transient increase in free internal calcium but decreased calcium after a 48 h incubation period. Further 300 microM quinidine depolarized the cell membrane in a similar range as did 30 mM KCl. Quinidine decreased cellular putrescine beyond detection levels while spermidine and spermine remained unaffected. ODC activity was reduced. Addition of putrescine could not override the antiproliferative effect owing to a reduced activity of the polyamine transporter. Our study indicates that the antiproliferative effect of quinidine is not due to a simple membrane depolarization but is caused by a block of ODC activity.

Inhibition of agmatine transport in liver mitochondria by new charge-deficient agmatine analogues

The charge of the agmatine analogues AO-Agm [N-(3-aminooxypropyl)guanidine], GAPA [N-(3-aminopropoxy)guanidine] and NGPG [N-(3-guanidinopropoxy)guanidine] is deficient as compared with that of agmatine and they are thus able to inhibit agmatine transport in liver mitochondria. The presence of the guanidine group is essential for an optimal effect, since AO-Agm and NGPG display competitive inhibition, whereas that of GAPA is non-competitive. NGPG is the most effective inhibitor (Ki=0.86 mM). The sequence in the inhibitory efficacy is not directly dependent on the degree of protonation of the molecules; in fact NGPG has almost the same charge as GAPA. When the importance of the guanidine group for agmatine uptake is taken into account, this observation suggests that the agmatine transporter is a single-binding, centre-gated pore rather than a channel.

S-adenosylmethionine and its products

S-adenosylmethionine is involved in many processes, mainly methylation, polyamine synthesis and radical-based catalysis. It is synthesised through the catalysis of differently regulated enzyme forms. When it is used, the compounds formed are reutilized in different ways: in case of methylation, its end product is homocysteine, which can be remethylated to methionine, give rise to cysteine in the so-called transsulphuration pathway, or be released; in the case of polyamine synthesis, the methylthioadenosine formed is cleaved and gives rise to compounds which can be reutilized; during radical-based catalysis, 5-deoxyadenosine is formed and this, too, is cleaved and reutilized.

Different behavior of agmatine in liver mitochondria: inducer of oxidative stress or scavenger of reactive oxygen species?

Agmatine, at concentrations of 10 microM or 100 microM, is able to induce oxidative stress in rat liver mitochondria (RLM), as evidenced by increased oxygen uptake, H(2)O(2) generation, and oxidation of sulfhydryl groups and glutathione. One proposal for the production of H(2)O(2) and, most probably, other reactive oxygen species (ROS), is that they are the reaction products of agmatine oxidation by an unknown mitochondrial amine oxidase. Alternatively, by interacting with an iron-sulfur center of the respiratory chain, agmatine can produce an imino radical and subsequently the superoxide anion and other ROS. The observed oxidative stress causes a drop in ATP synthesis and amplification of the mitochondrial permeability transition (MPT) induced by Ca(2+). Instead, 1 mM agmatine generates larger amounts of H(2)O(2) than the lower concentrations, but does not affect RLM respiration or redox levels of thiols and glutathione. Indeed, it maintains the normal level of ATP synthesis and prevents Ca(2+)-induced MPT in the presence of phosphate. The self-scavenging effect against ROS production by agmatine at higher concentrations is also proposed.

S-adenosylmethionine and radical-based catalysis

S-adenosylmethionine is the major methyl donor in all living organisms, but it is also involved in many other reactions occurring through radical-based catalysis. The structure and function of some of these enzymes, including those involved in the synthesis of the molybdenum cofactors, biotin, lipoate, will be discussed.

S-adenosylmethionine and protein methylation

The enzymes responsible for protein methylation by S-adenosylmethionine, both at the carboxyl groups and at the nitrogen groups, are reviewed. The possibility that the reactions involved may be reversible is also considered.

Arginine revisited: Minireview article

Arginine is a precursor of proteins and employed in urea synthesis. It is also the precursor of many other compounds, such as creatine, nitric oxide, polyamines, agmatine, proline. In this review, its transport and that of other basic amino acids are examined, along with its transformation into nitric oxide, agmatine and proline, and the mutual regulation of the individual pathways.

Metabolism and function in animal tissues of agmatine, a biogenic amine formed from arginine

Recently agmatine, decarboxylated arginine, has been shown to be an important biological compound in several animal tissues. This paper summarizes the known information regarding the transport of arginine, its decarboxylation and the effects of the agmatine formed mainly on NO and polyamine synthesis.

Agmatine induces apoptosis in rat hepatocyte cultures

BACKGROUND/AIMS

Agmatine, the compound formed by decarboxylation of arginine, is believed to be an endogenous neurotransmitter through interaction with the imidazoline receptors. However, it also appears to regulate rat hepatocyte polyamines by modifying both their synthesis and their catabolism. As the decrease in polyamine content has been correlated with apoptosis, we examined the possibility that agmatine has an effect on this phenomenon.

METHODS

Apoptotic cells were detected by visualizing nuclear shrinkage/fragmentation in hepatocytes cultured at 21 and 5% oxygen tension. Caspase-3 activity, cleavage of PARP, release of cytochrome c and mitochondrial swelling were therefore measured in the two conditions and in the presence or not of agmatine.

RESULTS

In rat hepatocytes agmatine promoted apoptosis, procaspase 3 processing and increase of caspase-3 like activity. This occurred through mitochondria swelling and release of cytochrome c. Cyclosporin A and catalase blocked the swelling.

CONCLUSIONS

Our experiments show that agmatine, besides all the known biological effects, has also part, at least in hepatocytes, in the modulation of programmed cell death.

Transport and metabolism of agmatine in rat hepatocyte cultures

Rat hepatocytes in culture take up [14C]-agmatine by both a high-affinity transport system [KM = 0.03 mM; Vmax = 30 pmol x min x (mg protein)-1] and a low-affinity system. The high-affinity system also transports putrescine, but not cationic amino acids such as arginine, and the polyamines spermidine and spermine. The rate of agmatine uptake is increased in cells deprived of polyamines with difluoromethylornithine. Of the agmatine taken up, 10% is transformed into polyamines and 50% is transformed into 4-guanidinobutyrate, as demonstrated by HPLC and MS. Inhibition by aminoguanidine and pargyline shows that this is due to diamine oxidase and an aldehyde dehydrogenase. 14C-4-aminobutyrate is also accumulated in the presence of an inhibitor of 4-aminobutyrate transaminase.

Oxygen regulation of rat hepatocyte iNOS gene expression

BACKGROUND/AIMS

The human iNOS promoter contains a consensus sequence for binding the hypoxia inducible factor. The aim of this study was to see whether iNOS gene expression is triggered by oxygen tension in rat hepatocytes exposed in vivo to high (periportal) and low (perivenous) oxygen tension.

METHODS

Hepatocytes transfected or not with a plasmid containing rat iNOS promoter linked to chloramphenicol acetyltransferase were cultured at 21% and 5% oxygen tension. In normal hepatocytes, iNOS protein, mRNA and activity were detected. In transfected cells, chloramphenicol acetyltransferase activity was measured.

RESULTS

In cells cultured in a hypoxic environment, both iNOS protein and mRNA increased, whereas the nitrite level in the medium decreased. However, electron paramagnetic resonance analysis and in vitro iNOS activity indicated that iNOS was active. Transfection experiments showed that the expression of chloramphenicol acetyltransferase driven by iNOS promoter was increased in cells maintained at low oxygen tension.

CONCLUSIONS

Our experiments show that in rat hepatocytes: 1) iNOS is induced by low oxygen tension; 2) the modification occurs at the transcriptional level; 3) the enzyme at 5% oxygen is able to catalyze the synthesis of NO, although no nitrites are accumulated in the medium. These findings could have physiopathological relevance, e.g. in determining the resistance of perivenous hepatocytes to ischemia injury.

Long chain diamines inhibit growth of C6 glioma cells according to their hydrophobicity. An in vitro and molecular modeling study

A series of diamines with the general structure NH2(CH2)xNH2, x=2-12, was tested for their potential effects on cell proliferation of cultured rat C6 glioma cells in comparison to natural polyamines. Long chain diamines reduced cell number after 48 h in culture with a sequence of 1,12-diaminododecane (1,12-DD) >1,10-diaminodecane >1,9-diaminononane. Polyamines (putrescine, spermidine and spermine) as well as diamines up to a CH2-chain length of x=8 were found to be ineffective. The spermine analogue 1,12-DD was the most effective molecule in reducing cell number in an irreversible, dose-dependent manner (EC50=3 microM under serum-free conditions). In further experiments we investigated the mechanisms of action of 1,12-DD. The compound had only a minor effect on cell cycle and did not affect free internal calcium concentration. Under physiological conditions 1,12-DD interacts with triplex DNA but not with duplex DNA. Ornithine decarboxylase activity as well as the concentration of internal polyamines were found to be reduced by 1,12-DD. Polyamine application, however, was not able to reverse the effect of 1,12-DD, indicating a polyamine-independent or non-competitive mechanism of action. 1,12-DD reduced cell number by induction of apoptosis as well as necrosis. In molecular modeling studies it was found that a minimal hydrophobic intersegment of at least 4 A was required to make a diamine an effective drug in respect to cellular growth. A hydrophobic gap of this size fits the minimum requirement expected from molecular modeling to provide space for hydrophobic interactions with parts of proteins like a CH3-group. Our results show that 1,12-DD acts as a potent drug, reducing the number of C6 glioma cells, and suggest that its spatial and hydrophobic properties are responsible for its mechanism of action.

Agmatine modulates polyamine content in hepatocytes by inducing spermidine/spermine acetyltransferase

Agmatine has been proposed as the physiological ligand for the imidazoline receptors. It is not known whether it is also involved in the homoeostasis of intracellular polyamine content. To show whether this is the case, we have studied the effect of agmatine on rat liver cells, under both periportal and perivenous conditions. It is shown that agmatine modulates intracellular polyamine content through its effect on the synthesis of the limiting enzyme of the interconversion pathway, spermidine/spermine acetyltransferase (SSAT). Increased SSAT activity is accompanied by depletion of spermidine and spermine, and accumulation of putrescine and N1-acetylspermidine. Immunoblotting with a specific polyclonal antiserum confirms the induction. At the same time S-adenosylmethionine decarboxylase activity is significantly increased, while ornithine decarboxylase (ODC) activity and the rate of spermidine uptake are reduced. This is not due to an effect on ODC antizyme, which is not significantly changed. All these modifications are observed in HTC cells also, where they are accompanied by a decrease in proliferation rate. SSAT is also induced by low oxygen tension which mimics perivenous conditions. The effect is synergic with that promoted by agmatine.

Casein kinase 2 phosphorylates recombinant human spermidine/spermine N1-acetyltransferase on both serine and threonine residues

Casein kinase 2 purified from human erythrocyte cytosol has been found to phosphorylate human spermidine/spermine N1-acetyltransferase (SSAT) expressed as a fusion protein in E. coli and purified to homogeneity with a specific activity similar to that reported for pure human SSAT. The amino acid sequence of the protein revealed not less than four phosphorylable residues, optimal target for protein kinase 2 phosphorylation being flanked by acid residues in position +1 and +3. Our results indicate that most 32P-phosphate is taken up by Ser residues, as evidenced by HCl hydrolysis and electrophoresis and that the phosphorylation extent is modulated by the physiological polyamine concentration. Partial digestion with trypsin at a low concentration for less than one hour preferentially hydrolyzes Lys-Arg-Arg in position 141-143 of the SSAT suggesting that the Ser-phosphorylated residues are located in the C-terminus of the protein, probably Ser 146 and 149.

Spermidine acetyltransferase in rat hepatocytes cultured at different oxygen tensions

To understand the mechanism involved in the liver zonation of polyamines, we have studied the possible role of oxygen tension. When hepatocytes were cultured at 21% and at 5% oxygen in atmosphere to mimic periportal and perivenous conditions, polyamine content was modified. The observed modifications suggested an effect on the interconversion pathway. Spermidine acetyltransferase (SAT) activity and N1-acetylspermidine were therefore measured in the same conditions. SAT activity was markedly increased after 6 hours and N1-acetylspermidine was accumulated in the cells. This was caused by new enzyme synthesis. The higher expression of SAT was accompanied by an increase in the content of the specific messenger RNA (mRNA). When liver cells were depleted of polyamines, SAT activity and the specific mRNA content were not enhanced by oxygen deprivation, but they increased when polyamines were added again. Polyamines therefore appear to be necessary to promote the increase in SAT mRNA.

JAR human placental choriocarcinoma cells actively synthesize, take up and release polyamines

Uptake of polyamines has been investigated extensively in many cells, but not in placenta, where the polyamine-polyamine oxidase system is supposed to have an immunoregulatory function in pregnancy. Due to the importance of the transfer in this tissue, we have started this study. JAR human placental choriocarcinoma cells in monolayer at confluency were used as a model for measuring the key enzymes of polyamine synthesis and interconversion, rate of uptake and efflux, and the polyamine content. Polyamines were taken up by JAR cells and released by an independent mechanism. Ornithine decarboxylase and spermidine acetyltransferase activities and the rate of transport in and out of the cell were much higher than in other cells, such as L1210 cells. However the systems used for uptake and release appear in many respects to be similar to those observed in L1210 cells, but different from others. The uptake appears to be regulated by an inhibitory protein. Moreover, protein kinase C appears to be involved in the process. The efflux also is regulated as in L1210 cells, through control of H+ and Ca2+ concentration. In conclusion, this study shows that, in JAR cells, ornithine decarboxylase and spermidine acetyltransferase activities were much higher than in other cells, and so was the rate of transport in and out of the cells. As a result, a much higher polyamine content was observed.

Modulation of ornithine aminotransferase activity by oxygen in rat hepatocyte cultures

In hepatocytes in culture, ornithine aminotransferase activity remained higher when the cells were cultured at low oxygen tension (5%) than at high tension (21%), that is, it was higher in hepatovenous conditions. Northern blot analysis showed that the amount of the specific mRNA for the enzyme was also higher. Results of experiments performed in the presence of CoCl2, to replace the central Fe2+ in heme, or succinylacetone, to inhibit heme synthesis, support the view that a heme protein participates in the regulation of ornithine aminotransferase activity by oxygen. The oxygen sensor does not appear to act through phosphorylation by kinase C, as TPA has no significant effect on the process, but a phosphorylation dependent on cAMP might be involved.

Pharmacodynamic effects of cotinine in abstinent cigarette smokers

The nicotine metabolite cotinine was administered to abstinent cigarette smokers to determine whether it has pharmacologic activity as assessed by various physiologic and subjective measurements. By means of a randomized, double-blind, placebo-controlled counterbalanced-order design, subjects received cotinine base (30 mg) intravenously after 48 hours of abstinence from cigarette smoking. Serum cotinine concentrations increased to levels commonly achieved during daily cigarette smoking, whereas no change in serum nicotine concentration was observed. Cotinine compared with placebo produced subjective differences in self-reported ratings of restlessness, anxiety and tension, insomnia, sedation, and pleasantness. Cotinine had minimal effects on cardiovascular measurements. These findings indicate that cotinine is behaviorally active in the setting of cigarette abstinence at blood concentrations similar to those commonly achieved through daily cigarette smoking.

Regulation of spermidine transport in L1210 cells

1. 1 mM 2-amino isobutyric acid (AIB), glutamine or asparagine when preincubated for 3 hr with L1210 cells promoted a marked increase in the rate of spermidine uptake. 2. Cycloheximide also increased the transport rate and completely prevented the increase due to AIB. 3. Trifluoperazine and iso-H7 inhibited the uptake of spermidine, much less the uptake of AIB. 4. Adenosine promoted an increase in the uptake of AIB, a decrease in that of spermidine. 5. Hypotonic stress also increased the rate of spermidine transport. This modification was only partially prevented by cycloheximide. 6. Okadaic acid had no effect on this increase, whereas it prevented the increase of ODC activity.

In vivo effect of berenil on rat liver polyamine metabolism

1. Berenil, administered to rats in vivo, promoted a decrease in liver SAMDC activity, but an increase in ODC and SAT activity. 2. Its effect on ODC was completely prevented by cycloheximide, that on SAT only partially. 3. Berenil had no effect on ODC activity in adrenalectomized rats. Adrenergic antagonists counteracted the effect of Berenil on ODC activity. 4. Polyamine content was increased. The maximum modification was observed for putrescine and N1-acetylspermidine.

Effect of Berenil on polyamine metabolism in primary cultured rat hepatocytes

1. Putrescine and spermidine content increased in hepatocytes during culture. In the presence of 10 microM Berenil, putrescine content was further increased, while the increase of spermidine was prevented. 2. Ornithine decarboxylase activity was markedly reduced, and to a lesser extent also S-adenosylmethionine decarboxylase activity. 3. Berenil appears to promote an increase in the transformation of spermidine into putrescine, and to inhibit the polyamine efflux.

Okadaic acid inhibits insulin stimulation of both ornithine decarboxylase and spermidine transport in hepatocyte cultures

1. Okadaic acid inhibited basal ODC activity in rat hepatocytes in culture and prevented any increase in ODC activity and in the rate of spermidine uptake promoted by both insulin and hypotonicity. 2. The increase promoted by AIB was not counteracted by okadaic acid.

Efflux of polyamines from human lymphocytes and from L 1210 cells

1. In human lymphocytes alkalinization of the cytoplasm with monensin or NH4Cl promotes release of polyamines. The effect of NH4Cl is abolished by EGTA and diltiazem. 2. Concanavalin A also promotes an increase of the efflux, counteracted again by EGTA and diltiazem. 3. By effect of TPA, polyamine efflux is decreased in the first 90 min, and later increased. The activation is partially prevented by H7 and by sphingosine. 4. In contrast with human lymphocytes, L 1210 cells release actively endogenous polyamines, but slowly radioactive polyamines. 5. Concanavalin A does not activate the latter process; A 23187 and NH4Cl on the contrary promote a much higher increase in the efflux rate than in normal lymphocytes. EGTA and diltiazem partially counteract the effect of NH4Cl on the release of radioactivity.

Polyamines in rat hepatocyte cultures

In rat hepatocytes cultured for 120 h polyamine content was markedly modified. Putrescine concentration reached a maximum at 48 h, spermidine increased for 48 h and then remained constant, spermine after a decrease returned to its initial values. Total polyamine amount was increased by 75%. Both ornithine decarboxylase and the retroconversion pathway were responsible for these modifications. The possible correlation between polyamine metabolism and retrodifferentiation process was investigated by studying them in conditions which are known to preserve differentiated functions.

Difference in polyamine transport in human B and T lymphocytes

Preparations enriched in human blood B lymphocytes are able to take up polyamines efficiently. Uptake by T cells is barely detectable. Human non-circulating B cells (from tonsils) have a much lower ability to take up polyamines, as do mixed populations of bovine lymph nodes. B cells contain a higher amount of endogenous polyamines and show higher ornithine decarboxylase activity than T cells.

Enhancement of ouabain and calcium ionophore A23187 of outward transport of polyamines from lymphocytes

Human lymphocytes in culture loaded with radioactive polyamines slowly release radioactivity into the medium. N1-Acetylspermidine is mostly released from spermidine and spermine. Both ouabain and calcium ionophore A23187 increase the outward transport, but by different mechanisms. Ouabain inhibits the acetylation of spermidine, and free spermidine is released, whereas A23187 increases both acetylation of spermidine and the efflux of N1-acetylspermidine.

Transport and metabolism of polyamines in human lymphocytes

1. Polyamines are taken up by human peripheral lymphocytes in a concentration, time and pH dependent manner, with an energy-dependent transport system. 2. Each polyamine inhibits the uptake of the others, with the exception of putrescine. Spermine appears to have the highest affinity for the transporter/s. 3. Inhibition by ouabain, amiloride and vanadate suggests that the transport is dependent on Na+. 4. Polyamine content inside the cells increases by ca 6 and 3 times respectively during incubation with spermidine or spermine. 5. The incorporated polyamines are partially transformed into each other.