A p90rsk Mutant Constitutively Interacting with MAP Kinase Uncouples MAP Kinase from p34cdc2/Cyclin B Activation in Xenopus Oocytes

The efficient activation of p90^rsk by MAP kinase requires their interaction through a docking site located at the C-terminal end of p90^rsk. The MAP kinase p42^mpk1 can associate with p90^rsk in G₂-arrested but not in mature Xenopus oocytes. In contrast, an N-terminally truncated p90^rsk mutant named D2 constitutively interacts with p42^mpk1. In this report we show that expression of D2 inhibits Xenopus oocyte maturation. The inhibition requires the p42^mpk1 docking site. D2 expression uncouples the activation of p42^mpk1 and p34^cdc2/cyclin B in response to progesterone but does not prevent signaling through p90^rsk. Instead, D2 interferes with a p42^mpk1-triggered pathway, which regulates the phosphorylation and activation of Plx1, a potential activator of the Cdc25 phosphatase. This new pathway that links the activation of p42^mpk1 and Plx1 during oocyte maturation is independent of p34^cdc2/cyclin B activity but requires protein synthesis. Using D2, we also provide evidence that the sustained activation of p42^mpk1 can trigger nuclear migration in oocytes. Our results indicate that D2 is a useful tool to study MAP kinase function(s) during oocyte maturation. Truncated substrates such as D2, which constitutively interact with MAP kinases, may also be helpful to study signal transduction by MAP kinases in other cellular processes.     

Histidine button engineered into cardiac troponin I protects the ischemic and failing heart

The myofilament protein troponin I (TnI) has a key isoform-dependent role in the development of contractile failure during acidosis and ischemia. Here we show that cardiac performance in vitro and in vivo is enhanced when a single histidine residue present in the fetal cardiac TnI isoform is substituted into the adult cardiac TnI isoform at codon 164. The most marked effects are observed under the acute challenges of acidosis, hypoxia, ischemia and ischemia-reperfusion, in chronic heart failure in transgenic mice and in myocytes from failing human hearts. In the isolated heart, histidine-modified TnI improves systolic and diastolic function and mitigates reperfusion-associated ventricular arrhythmias. Cardiac performance is markedly enhanced in transgenic hearts during reperfusion despite a high-energy phosphate content similar to that in nontransgenic hearts, providing evidence for greater energetic economy. This pH-sensitive 'histidine button' engineered in TnI produces a titratable molecular switch that 'senses' changes in the intracellular milieu of the cardiac myocyte and responds by preferentially augmenting acute and long-term function under pathophysiological conditions. Myofilament-based inotropy may represent a therapeutic avenue to improve myocardial performance in the ischemic and failing heart.     

Net carbon exchange and evapotranspiration in postfire and intact sagebrush communities in the Great Basin

Invasion of non-native annuals across the Intermountain West is causing a widespread transition from perennial sagebrush communities to fire-prone annual herbaceous communities and grasslands. To determine how this invasion affects ecosystem function, carbon and water fluxes were quantified in three, paired sagebrush and adjacent postfire communities in the northern Great Basin using a 1-m³ gas exchange chamber. Most of the plant cover in the postfire communities was invasive species including Bromus tectorum L., Agropyron cristatum (L.) Gaertn and Sisymbrium altissimum L. Instantaneous morning net carbon exchange (NCE) and evapotranspiration (ET) in native shrub plots were greater than either intershrub or postfire plots. Native sagebrush communities were net carbon sinks (mean NCE 0.2–4.3 μmol m⁻² s⁻¹) throughout the growing season. The magnitude and seasonal variation of NCE in the postfire communities were controlled by the dominant species and availability of soil moisture. Net C exchange in postfire communities dominated by perennial bunchgrasses was similar to sagebrush. However, communities dominated by annuals (cheatgrass and mustard) had significantly lower NCE than sagebrush and became net sources of carbon to the atmosphere (NCE declined to −0.5 μmol m⁻² s⁻¹) with increased severity of the summer drought. Differences in the patterns of ET led to lower surface soil moisture content and increased soil temperatures during summer in the cheatgrass-dominated community compared to the adjacent sagebrush community. Intensive measurements at one site revealed that temporal and spatial patterns of NCE and ET were correlated most closely with changes in leaf area in each community. By altering the patterns of carbon and water exchange, conversion of native sagebrush to postfire invasive communities may disrupt surface-atmosphere exchange and degrade the carbon storage capacity of these systems.     

Arachidonic acid release from mammalian cells transfected with human groups IIA and X secreted phospholipase A(2) occurs predominantly during the secretory process and with the involvement of cytosolic phospholipase A(2)-alpha

Stable expression of human groups IIA and X secreted phospholipases A(2) (hGIIA and hGX) in CHO-K1 and HEK293 cells leads to serum- and interleukin-1beta-promoted arachidonate release. Using mutant CHO-K1 cell lines, it is shown that this arachidonate release does not require heparan sulfate proteoglycan- or glycosylphosphatidylinositol-anchored proteins. It is shown that the potent secreted phospholipase A(2) inhibitor Me-Indoxam is cell-impermeable. By use of Me-Indoxam and the cell-impermeable, secreted phospholipase A(2) trapping agent heparin, it is shown that hGIIA liberates free arachidonate prior to secretion from the cell. With hGX-transfected CHO-K1 cells, arachidonate release occurs before and after enzyme secretion, whereas all of the arachidonate release from HEK293 cells occurs prior to enzyme secretion. Immunocytochemical studies by confocal laser and electron microscopies show localization of hGIIA to the cell surface and Golgi compartment. Additional results show that the interleukin-1beta-dependent release of arachidonate is promoted by secreted phospholipase A(2) expression and is completely dependent on cytosolic (group IVA) phospholipase A(2). These results along with additional data resolve the paradox that efficient arachidonic acid release occurs with hGIIA-transfected cells, and yet exogenously added hGIIA is poorly able to liberate arachidonic acid from mammalian cells.     

Selenoproteins and human health: Insights from epidemiological data

Knowledge of the plasma selenium levels associated with optimised concentration or activity of specific selenoproteins can provide considerable insights from epidemiological data on the possible involvement of those selenoproteins in health, most notably with respect to cancer. For cohort studies, if selenoproteins such as glutathione peroxidase and selenoprotein P are relevant to cancer, one might only expect to see an effect on risk when the concentrations in the cohort range from below, to above, the level needed to optimise the activity or concentration of these enzymes. Similarly, trials would only show a beneficial effect of supplementation if selenium status were raised from below, to above, the optimal concentration for the selenoproteins likely to be implicated in cancer risk, as occurred in the NPC trial but not in SELECT. The most powerful evidence for the involvement of selenoproteins in human health comes from epidemiological studies that have related single nucleotide polymorphisms in selenoproteins to disease risk. The totality of the evidence currently implicates GPx1, GPx4, SEPS1, Sep15, SEPP1 and TXNRD1 in conditions such as cardiovascular disease, pre-eclampsia and cancer. Future studies therefore need to determine not only selenium status, but genotype, both in selenoproteins and related pathways, when investigating the relationship of selenium with disease risk.     

Ephrin‐A5 regulates the formation of the ascending midbrain dopaminergic pathways

Dopaminergic neurons from the substantia nigra and the ventral tegmental area of the midbrain project to the caudate/putamen and nucleus accumbens, respectively, establishing the mesostriatal and the mesolimbic pathways. However, the mechanisms underlying the development of these pathways are not well understood. In the current study, the EphA5 receptor and its corresponding ligand, ephrin‐A5, were shown to regulate dopaminergic axon outgrowth and influence the formation of the midbrain dopaminergic pathways. Using a strain of mutant mice in which the EphA5 cytoplasmic domain was replaced with β‐galactosidase, EphA5 protein expression was detected in both the ventral tegmental area and the substantia nigra of the midbrain. Ephrin‐A5 was found in both the dorsolateral and the ventromedial regions of the striatum, suggesting a role in mediating dopaminergic axon‐target interactions. In the presence of ephrin‐A5, dopaminergic neurons extended longer neurites in in vitro coculture assays. Furthermore, in mice lacking ephrin‐A5, retrograde tracing studies revealed that fewer neurons sent axons to the striatum. These observations indicate that the interactions between ephrin‐A ligands and EphA receptors promote growth and targeting of the midbrain dopaminergic axons to the striatum. © 2008 Wiley Periodicals, Inc. Develop Neurobiol, 2009     

Deletion 17q12 is a recurrent copy number variant that confers high risk of autism and schizophrenia

Autism spectrum disorders (ASD) and schizophrenia are neurodevelopmental disorders for which recent evidence indicates an important etiologic role for rare copy number variants (CNVs) and suggests common genetic mechanisms. We performed cytogenomic array analysis in a discovery sample of patients with neurodevelopmental disorders referred for clinical testing. We detected a recurrent 1.4 Mb deletion at 17q12, which harbors HNF1B, the gene responsible for renal cysts and diabetes syndrome (RCAD), in 18/15,749 patients, including several with ASD, but 0/4,519 controls. We identified additional shared phenotypic features among nine patients available for clinical assessment, including macrocephaly, characteristic facial features, renal anomalies, and neurocognitive impairments. In a large follow-up sample, the same deletion was identified in 2/1,182 ASD/neurocognitive impairment and in 4/6,340 schizophrenia patients, but in 0/47,929 controls (corrected p = 7.37 × 10⁻⁵). These data demonstrate that deletion 17q12 is a recurrent, pathogenic CNV that confers a very high risk for ASD and schizophrenia and show that one or more of the 15 genes in the deleted interval is dosage sensitive and essential for normal brain development and function. In addition, the phenotypic features of patients with this CNV are consistent with a contiguous gene syndrome that extends beyond RCAD, which is caused by HNF1B mutations only.     

Climate change and multitrophic interactions in soil: the primacy of plants and functional domains

Soil multitrophic interactions transfer energy from plants as the predominant primary producer to communities of organisms that occupy different positions in the food chain and are linked by multiple ecological networks, which is the soil food web. Soil food web sequesters carbon, cycles nutrients, maintains soil health to suppress pathogens, helps plants tolerate abiotic and biotic stress, and maintains ecosystem resilience and sustainability. Understanding the influence of climate change on soil multitrophic interactions is necessary to maintain these essential ecosystem services. But summarising this influence is a daunting task due to a paucity of knowledge and a lack of clarity on the ecological networks that constitute these interactions. The scant literature is fragmented along disciplinary lines, often reporting inconsistent findings that are context and scale‐dependent. We argue for the differentiation of soil multitrophic interactions along functional and spatial domains to capture cross‐disciplinary knowledge and mechanistically link all ecological networks to reproduce full functionalities of the soil food web. Distinct from litter mediated interactions in detritosphere or elsewhere in the soil, the proposed ‘pathogen suppression’ and ‘stress tolerance’ interactions operate in the rhizosphere. A review of the literature suggests that climate change will influence the relative importance, frequency and composition of functional groups, their trophic interactions and processes controlling these interactions. Specific climate change factors generally have a beneficial influence on pathogen suppression and stress tolerance, but findings on the overall soil food web are inconsistent due to a high level of uncertainty. In addition to an overall improvement in the understanding of soil multitrophic interactions using empirical and modelling approaches, we recommend linking biodiversity to function, understanding influence of combinations of climatic factors on multitrophic interactions and the evolutionary ecology of multitrophic interactions in a changing climate as areas that deserve most attention.     

Microbial carbon limitation: The need for integrating microorganisms into our understanding of ecosystem carbon cycling

Numerous studies have demonstrated that fertilization with nutrients such as nitrogen, phosphorus, and potassium increases plant productivity in both natural and managed ecosystems, demonstrating that primary productivity is nutrient limited in most terrestrial ecosystems. In contrast, it has been demonstrated that heterotrophic microbial communities in soil are primarily limited by organic carbon or energy. While this concept of contrasting limitations, that is, microbial carbon and plant nutrient limitation, is based on strong evidence that we review in this paper, it is often ignored in discussions of ecosystem response to global environment changes. The plant‐centric perspective has equated plant nutrient limitations with those of whole ecosystems, thereby ignoring the important role of the heterotrophs responsible for soil decomposition in driving ecosystem carbon storage. To truly integrate carbon and nutrient cycles in ecosystem science, we must account for the fact that while plant productivity may be nutrient limited, the secondary productivity by heterotrophic communities is inherently carbon limited. Ecosystem carbon cycling integrates the independent physiological responses of its individual components, as well as tightly coupled exchanges between autotrophs and heterotrophs. To the extent that the interacting autotrophic and heterotrophic processes are controlled by organisms that are limited by nutrient versus carbon accessibility, respectively, we propose that ecosystems by definition cannot be ‘limited’ by nutrients or carbon alone. Here, we outline how models aimed at predicting non‐steady state ecosystem responses over time can benefit from dissecting ecosystems into the organismal components and their inherent limitations to better represent plant–microbe interactions in coupled carbon and nutrient models.