A new gene for Tourette's syndrome: a window into causal mechanisms?

Gilles de la Tourette syndrome (GTS) is a neurodevelopmental disorder characterized by impairing motor-vocal tics. Locating genetic loci by associating the phenotype with DNA translocations, inversions, gain or losses, State et al. identified SLITRK1 as a candidate gene in an individual with GTS and inv(13) (q31.1; q33.1). This gene was also associated with abnormal axonal-dendritic development in embryonic mouse cells. Although SLITRK1 is not a major causal gene for GTS, it can shed light on our understanding of the gene-based neural correlates of this disease.     

ROS signaling in the hypersensitive response: When,where and what for?

Plants generally react to the attack of non-host and incompatible host microorganisms by inducing pathogenesis-related (PR) genes and localised cell death (LCD) at the site of infection, a process collectively known as the hypersensitive response (HR). Reactive oxygen species (ROS) are generated in various sub-cellular compartments shortly after pathogen recognition, and proposed to cue subsequent orchestration of the HR. Although apoplast-associated ROS production by plasma membrane NADPH oxidases have been most thoroughly studied, recent observations suggest that ROS are generated in chloroplasts earlier in the response and play a key role in execution of LCD. A model is presented in which the initial outcome of successful pathogen detection is ROS accumulation in plastids, likely mediated by mitogen-activated protein kinases and caused by dysfunction of the photosynthetic electron transport chain. ROS signaling is proposed to spread from plastids to the apoplast, through the activation of NADPH oxidases, and from there to adjacent cells, leading to suicidal death in the region of attempted infection.Key words: biotic stress, chloroplasts, flavodoxin, hypersensitive response (HR), reactive oxygen species (ROS), ROS signaling     

Toxoplasma-induced autophagy: A window into nutritional futile cycles in mammalian cells?

The regulation and function of autophagy in response to metabolic signals is not yet well understood. A recent study from our laboratory indicates than an intracellular parasite, Toxoplasma gondii, derives nutritive benefit from the upregulation of host cell autophagy. We discuss these and related findings suggesting that autophagy in infected cells functions as part of a metabolic futile cycle. The hypothesis is presented that endogenous autophagy-based futile cycles may operate in normal mammalian cells, providing a substrate for manipulation by pathogens.     

Resistance to oomycetes: a general role for the hypersensitive response?

Oomycete plant pathogens, such as Phytophthora, downy mildews and Pythium, have devastating disease effects on numerous crop and ornamental plants. Various types of genetic resistance to oomycetes occur in plants, and can be determined at the subspecific or varietal level (race or cultivar-specific resistance), or at the species or genus level (nonhost resistance). In addition, resistance might be a quantitative phenotype (partial resistance). Resistance reactions are often associated with the hypersensitive response – a programed cell death pathway. Recent advances in the genetic, biochemical and cytological characterization of disease resistance suggests that the hypersensitive response is associated with all forms of resistance to Phytophthora and downy mildews. Identification of the resistance genes involved in nonhost and partial resistance to oomycetes remains an important challenge.     

A metagenomic window into carbon metabolism at 3?km depth in Precambrian continental crust

Subsurface microbial communities comprise a significant fraction of the global prokaryotic biomass; however, the carbon metabolisms that support the deep biosphere have been relatively unexplored. In order to determine the predominant carbon metabolisms within a 3-km deep fracture fluid system accessed via the Tau Tona gold mine (Witwatersrand Basin, South Africa), metagenomic and thermodynamic analyses were combined. Within our system of study, the energy-conserving reductive acetyl-CoA (Wood-Ljungdahl) pathway was found to be the most abundant carbon fixation pathway identified in the metagenome. Carbon monoxide dehydrogenase genes that have the potential to participate in (1) both autotrophic and heterotrophic metabolisms through the reversible oxidization of CO and subsequent transfer of electrons for sulfate reduction, (2) direct utilization of H₂ and (3) methanogenesis were identified. The most abundant members of the metagenome belonged to Euryarchaeota (22%) and Firmicutes (57%)—by far, the highest relative abundance of Euryarchaeota yet reported from deep fracture fluids in South Africa and one of only five Firmicutes-dominated deep fracture fluids identified in the region. Importantly, by combining the metagenomics data and thermodynamic modeling of this study with previously published isotopic and community composition data from the South African subsurface, we are able to demonstrate that Firmicutes-dominated communities are associated with a particular hydrogeologic environment, specifically the older, more saline and more reducing waters.     

Induction of phlorotannin production in a brown alga: defense or resource dynamics?

Increase of phenolic secondary metabolites, phlorotannins, in brown algae due to gastropod grazing has been interpreted as an anti-herbivore adaptation. Here we tested whether such a response could be due to changes in truly available resources for the alga, not by the grazing activity of snails as such. We allowed two species of snails, Theodoxus fluviatilis and Physa fontinalis to graze on Fucus vesiculosus . These species feed on epibiota and particulate matter on the thallus but do not eat the thallus of F. vesiculosus . We further simulated snail grazing by nutrient enhancement, removal of epibiota and by a combination of the two. Manipulations of nutrient and light availability revealed the crucial role of epibiota in mediating resource availability for F. vesiculosus . Nutrient enhancement alone increased epibiota and decreased phlorotannins. Cleaning the thallus resulted in increased growth, and together with nutrient enhancement also in a trade-off with phlorotannins. Presence of T. fluviatilis on the thallus induced phlorotannin production, a response differing from the simulations of snail grazing. However, we suggest that the increase in phlorotannins may not be an induced defense but rather a consequence of a specific way of resource manipulation by this snail species. T. fluviatilis removes hyaline hairs that facilitate nutrient uptake. P. fontinalis did not remove hyaline hairs and the response of the alga to its grazing was similar to the treatment where we mechanically removed epibiota suggesting that cleaning of the thallus is the major mechanism how this snail species affects F. vesiculosus . Genetic variation in phlorotannin concentrations highly exceeded the induced responses of simulated or real snail grazing. This casts doubt for the efficiency of induced phlorotannin production to act as a defense, but is not contradictory with the interpretation of phlorotannins responding to variation in resource availability.     

The Arabidopsis MAP kinase kinase 7: A crosstalk point between auxin signaling and defense responses?

Plant-pathogen interaction induces a complex host response that coordinates various signaling pathways through multiple signal molecules. Besides the well-documented signal molecules salicylic acid (SA), ethylene and jasmonic acid, auxin is emerging as an important player in this response. We recently characterized an Arabidopsis activation-tagged mutant, bud1, in which the expression of the MAP kinase kinase 7 (AtMKK7) gene is increased. The bud1 mutant plants accumulate elevated levels of SA and display constitutive pathogenesis-related (PR) gene expression and enhanced resistance to pathogens. Additionally, increased expression of AtMKK7 in the bud1 mutant causes deficiency in polar auxin transport, indicating that AtMKK7 negatively regulates auxin signaling. Based on these results, we hypothesized that AtMKK7 may serve as a crosstalk point between auxin signaling and defense responses. Here we show that increased expression of AtMKK7 in bud1 results in a significant reduction in free auxin (indole-3-acetic acid) levels in the mutant plants. We propose three possible mechanisms to explain how AtMKK7 coordinates the growth hormone auxin and the defense signal molecule SA in the bud1 mutant plants. We suggest that AtMKK7 may play a role in cell death and propose that AtMPK3 and AtMPK6 may function downstream of AtMKK7.Key words: Arabidopsis, MAP kinase kinase 7, auxin signaling, defense responses, crosstalkPathogen invasion of a plant induces multiple physiological changes at the site of infection, including the accumulation of reactive oxygen species, nitric oxide and salicylic acid (SA).^1–6 Jasmonic acid (JA) and ethylene (ET) are also produced in response to pathogen infection.^7–11 Numerous reports have documented that SA, JA and ET work synergistically or antagonistically to fine-tune plant defense responses, based on a multitude of environmental, host and pathogen genetic factors that vary depending on the pathogen-host combinations.^4,12The growth hormone auxin may also play an important role in plant defense responses. Many plant-pathogenic microorganisms have the ability to produce indole-3-acetic acid (IAA),¹³ which is important for the pathogenicity for some pathogens.^14–16 In the Arabidopsis-Xanthomonas campestris pv. campestris (Xcc) compatible interaction, Xcc triggers IAA synthesis in the host plants.17 Exogenous treatment of plants with the auxin analogs, NAA and 2,4-D, leads to disease susceptibility.¹⁸ A flagellin-derived-peptid e-induced microRNA (miRNA) was found to negatively regulate messenger RNAs for the F-box auxin receptors TIR1, AFB2 and AFB3, to repress auxin signaling, resulting in significantly enhanced host resistance.¹⁸ These results suggest that auxin likely functions as a virulence factor to suppress host defense.We previously identified an Arabidopsis activation-tagged mutant bud1 from a transgenic population generated by a sense/antisense RNA expression system.¹⁹ Further characterization indicated that bud1 is a semidominant mutant, in which the expression of the Arabidopsis MAP kinase kinase 7 (AtMKK7) gene is increased.²⁰ The increased expression of AtMKK7 in bud1 causes deficiency in auxin transport, whereas reducing mRNA levels of AtMKK7 by antisense RNA expression leads to enhancement of auxin transport, indicating that AtMKK7 negatively regulates polar auxin transport (PAT).²⁰ Recently, we have shown that the bud1 mutant plants accumulate elevated levels of SA and exhibit constitutive pathogenesis-related (PR) gene expression and enhanced resistance to both the bacterial pathogen Pseudomonas syringae pv. maculicola (Psm) ES4326 and the oomycete pathogen Hyaloperonospora parasitica Noco2.²¹ Reducing mRNA levels of AtMKK7 by antisense RNA expression not only compromises basal resistance but also blocks the induction of systemic acquired resistance (SAR), demonstrating that AtMKK7 is a positive regulator required for both basal resistance and SAR.²¹ Furthermore, we found that the free IAA levels in the bud1 mutant plants were significantly reduced, compared to those in wild-type plants (Fig. 1A). All these results taken together suggest that AtMKK7 may positively regulate SA signaling and negatively regulate auxin signaling.Open in a separate windowFigure 1(A) Free IAA levels in wild type (WT) and bud1 mutant plants. Thirty-day-old soil grown plants were used for free IAA measurement. (B) A schematic representation of three possible mechanisms through which MKK7 regulates host responses after pathogen invasion.Given that SA is a positive regulator of defense responses, whereas auxin is likely a negative regulator of defense responses, we propose three possible mechanisms through which AtMKK7 coordinates the growth hormone auxin and the defense signal molecule SA in the bud1 mutant plants (Fig. 1B): (1) AtMKK7 induces SA accumulation, which suppresses auxin signaling, leading to increased defense responses; (2) AtMKK7 independently induces SA accumulation and suppresses auxin signaling; (3) AtMKK7 suppresses auxin signaling, which relieves the repression of SA signaling by auxin, resulting in SA accumulation.We could test the hypotheses using different approaches. We can examine whether the expression of YUC1, YUC2, YUC4 and YUC6, genes that have been suggested to play essential roles in auxin biosynthesis,²² is altered in the bud1 mutant. We can also analyze the expression of YUC1, YUC2, YUC4 and YUC6, as well as the levels of free IAA in the double mutant bud1sid2 (sid2 is a SA deficient mutant) to test whether IAA biosynthesis is derepressed in the double mutant. Furthermore, polar auxin transport in the bud1sid2 plants should be determined. Finally, we can test whether exogenous application of auxin is able to suppress AtMKK7-induced constitutive defense responses in the bud1 mutant, including elevated levels of SA, constitutive PR gene expression and enhanced resistance to Psm ES4326 and H. parasitica Noco2.AtMKK7 belongs to the Group D of plant MAPKKs.²³ Functions of two other members of this group, LeMKK4 and NbMKK1, have been described.^24,25 LeMKK4 and NbMKK1 are orthologs of AtMKK7 in tomato and Nicotiana benthamiana, respectively. When overexpressed in leaves, wild-type LeMKK4 elicits cell death in both tomato and N. benthamiana.²⁴ Overexpression of wild-type NbMKK1 also causes cell death on N. benthamiana leaves.²⁵ We expected that overexpression of AtMKK7 would also result in cell death. However, neither increased expression of AtMKK7 in the bud1 mutant plants, nor overexpression of wild-type AtMKK7 from the dexamethasone-inducible promoter causes cell death.²¹ This is probably because the expression levels of AtMKK7 in these plants were below the threshold to induce cell death. Consistently, ectopic and constitutive expression of AtMKK7 driven by the cauliflower mosaic virus (CaMV) 35S promoter in wild-type plants leads to lethality of the transgenic plants.²⁰ Therefore, to characterize the function of AtMKK7 in cell death, transgenic plants expressing a constitutively active form of AtMKK7 (AtMKK7^S193A/S199D) from the dexamethasone-inducible promoter will be useful.What MAPK(s) acts downstream of AtMKK7? LeMKK4 directly phosphorylates LeMPK1, LeMPK2 and LeMPK3 in vitro, and activates LeMPK2 and LeMPK3 when expressed in tomato leaves,²⁴ whereas NbMKK1 activates NbSIPK when expressed in N. benthamiana leaves.²⁵ LeMPK2 and LeMPK3 are tomato orthologs of the well-studied tobacco proteins SIPK (salicylic acid-induced protein kinase) and WIPK (wound-induced protein kinase),^26,27 respectively. The Arabidopsis orthologs of SIPK and WIPK are AtMPK6 and AtMPK3, respectively. Based on previous in-gel kinase assay results,²¹ we predict that both AtMPK3 and AtMPK6 may function downstream of AtMKK7. Characterization of double mutants bud1atmpk3 and bud1atmpk6, as well as atmpk3 and atmpk6 mutant plants expressing the constitutively active form of AtMKK7 from the dexamethasone-inducible promoter will shed light on this question.     

Nitric oxide production in tobacco leaf cells: a generalized stress response?

The function of nitric oxide (NO), a gaseous free radical emitted by many plants, is incompletely understood. In the present study the hypothesis that NO generation, like that of the reactive oxygen species, occurs as a general response to different environmental cues was tested. Leaf peels and mesophyll cell suspensions of Nicotiana tabacum cv. Xanthi were loaded with the NO‐specific fluorophore, diaminofluorescein, and subjected to an abiotic stressor. Light stress and mechanical injury had no apparent effect on NO production. In contrast, high temperatures, hyperosmotic stress, salinity and epi‐illumination in a microscope all led to rapid surges in NO‐induced fluorescence. The fluorescence originated from cells of the palisade mesophyll and across all epidermal cell types, including guard cells, subsidiary cells, and long and short trichomes. Fluorescence was evident first in the plastids, then in the nucleus and finally throughout the cytosol. Nicotiana plumbaginifolia cell suspensions expressing the calcium reporter aequorin provided evidence that, under hyperosmotic stress, NO participates in the elevation of free Ca²⁺ in the cytoplasm. The physiological significance of NO production in response to abiotic stressors is discussed.     

Pathophysiology of hypertension in response to placental ischemia during pregnancy: A central role for endothelin?

Background: Preeclampsia is new-onset hypertension with proteinuria during pregnancy. The initiating event in preeclampsia has been postulated to involve reduced placental perfusion, which leads to widespread dysfunction of the maternal vascular endothelium.Objective: The main objective of this brief review was to highlight some of the recent advances in our understanding of the mechanisms whereby the endothelin (ET) system, via ET type A (ET_A) receptor activation, modulates blood pressure in preeclamptic women and in animal models of pregnancy-related hypertension.Methods: This review focused on the role of ET and tumor necrosis factor-α (TNF-α) in preeclampsia, with emphasis on the pathophysiology of hypertension in response to placental ischemia in animal models of pregnancy. Relevant published data were identified by searching PubMed and supplemented with contributions from our laboratory.Results: Studies in preeclamptic women indicate that their hypertension is associated with increases in ET synthesis. Recent studies in pregnant rats indicate that the ET system is activated in response to reductions in uterine perfusion pressure and to chronic elevations in serum TNF-α concentrations. In these 2 animal models, the findings also suggest that ET A receptor activation may play a role in mediating hypertension.Conclusions: Although recent studies in animal models implicate an important role for the ET system in preeclampsia, the usefulness of selective ET A receptor antagonists for the treatment of hypertension in women with preeclampsia remains unclear. This important question will not be answered until well-controlled clinical studies using specific ET A receptor antagonists are conducted for women with preeclampsia.     

A window to the heart: can zebrafish mutants help us understand heart disease in humans?

Heart disease is a leading cause of death in the developed world. Abnormalities of heart muscle (cardiomyopathies) and/or electrical conduction (arrhythmias) are frequent causes of heart failure and sudden death. During the past twelve years, identification of genetic mutations that cause familial cardiomyopathies and arrhythmias has fueled a massive increase in molecular investigation into these diseases. Today, studies of zebrafish mutants with defective heart function are providing insight into the genes required to generate a normal heartbeat.     

Ethylene production in habituated and auxin-requiring tobacco callus cultures. Does ethylene play a role in the habituation?

Ethylene production of habituated and auxin-requiring tobacco ( Nicotiana tabacum L. cv. Xanthi) callus cultures were compared. More ethylene was produced by auxinrequiring i.e. auxin-heterotrophic cultures than by habituated ones. Treatment with 2,4-dichlorophenoxyacetic acid increased the ethylene evolution of habituated cultures over the range 10⁻⁷ to 10⁻⁴ M , which suggests that the higher ethylene production of auxin-dependent callus is caused by the 2,4-D in the medium. The IAA levels depended on the age of both types of callus cultures.     

Control mechanisms of the behavior ‘secondary defense’ in the grasshopper Gomphocerus rufus L. (Gomphocerinae: Orthoptera)

In the female grasshopper Gomphocerus rufus mating elicits secondary defense which makes remating impossible. The behavioral change is caused by the liquid white secretions, proteins of less than 90 kD, which are produced by the white tubuli of the male's accessory glands. Experimental injection of the white secretions directly into the spermathecal duct of receptive virgins provokes secondary defense instantly whereas sperm transfer had no such effect. Secondary defense is also released by eggs entering the oviducts and excerting pressure against the oviductal walls on their way to oviposition.