The “High Irradiance Responses” of plant photomorphogenesis

Plants growing in the natural environment are exposed daily to prolonged periods of high intensity irradiation. Many plant photomorphogenic responses are fully expressed only under prolonged exposures to high irradiances of light. The intensive study of these responses, the “High Irradiance Responses” (HIR) of plant photomorphogenesis, which started about 20 years ago, has been essentially directed—so far—toward the identification of the HIR photoreceptor, or photoreceptors, a problem that has not been satisfactorily and definitively solved, as yet. There is a great deal of evidence in support of the hypothesis that phytochrome, the pigment mediating the redfar red reversible plant photo-responses to low fluences of light, is involved in the photocontrol of the HIR. It seems likely that phytochrome may be the only photomorphogenic receptor responsible for the photocontrol of HIR responses brought about by irradiation at wavelengths longer than 600 nm. Phytochrome is probably also involved in the photocontrol of the HIR effects brought about by irradiation in the 350 to 500 nm region of the spectrum, but it cannot be excluded that other photochemical systems may also be involved. From a theoretical point of view, it does not seem unreasonable that the final expression of an HIR response may involve an interaction between phytochrome and other photochemical systems, with phytochrome probably playing the primary role and being responsible for the control of the activity of the other systems. Numerous “phytochrome only” interpretations (models) of the HIR have been proposed. Some of them have been developed to a fairly high degree of elaboration and have allowed the prediction of at least some of the features of the HIR. These “models,” although not rigorously and completely tested yet, seem to provide a reasonable interpretation for the HIR effects displayed under prolonged far red irradiation and for those HIR responses for which far red is the most effective spectral region. However, they do not provide a satisfactory explanation for the HIR responses for which blue is the most or the only effective spectral region, nor for the high effectiveness of white light. But, in spite of these problems, the “phytochrome only” interpretations of the HIR can be considered more satisfactory than those based on an interaction between phytochrome and other photochemical systems, especially in relation to the fact that the identity of these other photochemical systems has not been defined yet.     

Mice expressing T4826I-RYR1 are viable but exhibit sex- and genotype-dependent susceptibility to malignant hyperthermia and muscle damage

Mutation T4825I in the type 1 ryanodine receptor (RYR1(T4825I/+)) confers human malignant hyperthermia susceptibility (MHS). We report a knock-in mouse line that expresses the isogenetic mutation T4826I. Heterozygous RYR1(T4826I/+) (Het) or homozygous RYR1(T4826I/T4826I) (Hom) mice are fully viable under typical rearing conditions but exhibit genotype- and sex-dependent susceptibility to environmental conditions that trigger MH. Hom mice maintain higher core temperatures than WT in the home cage, have chronically elevated myoplasmic[Ca(2+)](rest), and present muscle damage in soleus with a strong sex bias. Mice subjected to heat stress in an enclosed 37°C chamber fail to trigger MH regardless of genotype, whereas heat stress at 41°C invariably triggers fulminant MH in Hom, but not Het, mice within 20 min. WT and Het female mice fail to maintain euthermic body temperature when placed atop a bed whose surface is 37°C during halothane anesthesia (1.75%) and have no hyperthermic response, whereas 100% Hom mice of either sex and 17% of the Het males develop fulminant MH. WT mice placed on a 41°C bed maintain body temperature while being administered halothane, and 40% of the Het females and 100% of the Het males develop fulminant MH within 40 min. Myopathic alterations in soleus were apparent by 12 mo, including abnormally distributed and enlarged mitochondria, deeply infolded sarcolemma, and frequent Z-line streaming regions, which were more severe in males. These data demonstrate that an MHS mutation within the S4-S5 cytoplasmic linker of RYR1 confers genotype- and sex-dependent susceptibility to pharmacological and environmental stressors that trigger fulminant MH and promote myopathy.     

Rhenium(III) and rhenium(V) complexes stabilized by the potentially tetradentate ligand tris(2-diphenylphosphinoethyl)amine

The reaction of the rhenium(V) nitrido complex [Re(N)Cl₂(PPh₃)₂] with the tripodal ligand N(CH₂CH₂PPh₂)₃ (NP₃) in THF gave [Re(N)Cl₂(η²-P,P-NP₃)] (1) in which NP₃ acts as a tridentate ligand using the nitrogen and two phosphorus donors for coordination. Refluxing 1 in a polar solvent such as ethanol produced [(η⁴-NP₃)Re(N)Cl]Cl (2) in which NP₃ acts as a tetradentate ligand. Treatment of complex [Re(O)Cl₃(AsPh₃)₂] containing the [ReO]³⁺ core with NP₃ in THF yielded [ReCl₃{η³-N,P,P-(N{CH₂CH₂Ph₂}₂{CH₂CH₂P(O)Ph₂})}] (3). Complexes 1 and 3 have been characterized by single-crystal X-ray analyses.     

Engineering the nucleotide coenzyme specificity and sulfhydryl redox sensitivity of two stress-responsive aldehyde dehydrogenase isoenzymes of Arabidopsis thaliana

Lipid peroxidation is one of the consequences of environmental stress in plants and leads to the accumulation of highly toxic, reactive aldehydes. One of the processes to detoxify these aldehydes is their oxidation into carboxylic acids catalyzed by NAD(P)+-dependent ALDHs (aldehyde dehydrogenases). We investigated kinetic parameters of two Arabidopsis thaliana family 3 ALDHs, the cytosolic ALDH3H1 and the chloroplastic isoform ALDH3I1. Both enzymes had similar substrate specificity and oxidized saturated aliphatic aldehydes. Catalytic efficiencies improved with the increase of carbon chain length. Both enzymes were also able to oxidize α,β-unsaturated aldehydes, but not aromatic aldehydes. Activity of ALDH3H1 was NAD+-dependent, whereas ALDH3I1 was able to use NAD+ and NADP+. An unusual isoleucine residue within the coenzyme-binding cleft was responsible for the NAD+-dependence of ALDH3H1. Engineering the coenzyme-binding environment of ALDH3I1 elucidated the influence of the surrounding amino acids. Enzyme activities of both ALDHs were redox-sensitive. Inhibition was correlated with oxidation of both catalytic and non-catalytic cysteine residues in addition to homodimer formation. Dimerization and inactivation could be reversed by reducing agents. Mutant analysis showed that cysteine residues mediating homodimerization are located in the N-terminal region. Modelling of the protein structures revealed that the redox-sensitive cysteine residues are located at the surfaces of the subunits.     

Melatonin prevents glutamate-induced oxytosis in the HT22 mouse hippocampal cell line through an antioxidant effect specifically targeting mitochondria

The pineal hormone melatonin has neuroprotective effects in a large number of models of neurodegeneration. Melatonin crosses the blood-brain barrier, shows a decrease in its nocturnal peaks in blood with age that has been associated with the development of neurodegenerative disorders, and has been shown to be harmless at high concentrations. These properties make melatonin a potential therapeutic agent against neurodegenerative disorders but the pathways involved in such neuroprotective effects remain unknown. In the present report we study the intracellular pathways implicated in the complete neuroprotection provided by melatonin against glutamate-induced oxytosis in the HT22 mouse hippocampal cell line. Our results strongly suggest that melatonin prevents oxytosis through a direct antioxidant effect specifically targeted at the mitochondria. Firstly, none of the described transducers of melatonin signalling seems to be implicated in the neuroprotection provided by this indole. Secondly, melatonin does not prevent cytosolic GSH depletion-dependent increase in reactive oxygen species (ROS), but it totally prevents mitochondrial ROS production despite the fact that the latter is much higher than the former. And finally, there is a high correlation between the concentration at which melatonin and closely related indoles exert a direct antioxidant effect in vitro and a neuroprotective effect against glutamate-induced oxytosis.     

Uracil recognition by replicative DNA polymerases is limited to the archaea, not occurring with bacteria and eukarya

Family B DNA polymerases from archaea such as Pyrococcus furiosus, which live at temperatures ~100°C, specifically recognize uracil in DNA templates and stall replication in response to this base. Here it is demonstrated that interaction with uracil is not restricted to hyperthermophilic archaea and that the polymerase from mesophilic Methanosarcina acetivorans shows identical behaviour. The family B DNA polymerases replicate the genomes of archaea, one of the three fundamental domains of life. This publication further shows that the DNA replicating polymerases from the other two domains, bacteria (polymerase III) and eukaryotes (polymerases δ and ε for nuclear DNA and polymerase γ for mitochondrial) are also unable to recognize uracil. Uracil occurs in DNA as a result of deamination of cytosine, either in G:C base-pairs or, more rapidly, in single stranded regions produced, for example, during replication. The resulting G:U mis-pairs/single stranded uracils are promutagenic and, unless repaired, give rise to G:C to A:T transitions in 50% of the progeny. The confinement of uracil recognition to polymerases of the archaeal domain is discussed in terms of the DNA repair pathways necessary for the elimination of uracil.     

Mitochondrial-targeted antioxidants represent a promising approach for prevention of cisplatin-induced nephropathy

Cisplatin is a widely used antineoplastic agent; however, its major limitation is the development of dose-dependent nephrotoxicity whose precise mechanisms are poorly understood. Here we show not only that mitochondrial dysfunction is a feature of cisplatin nephrotoxicity, but also that targeted delivery of superoxide dismutase mimetics to mitochondria largely prevents the renal effects of cisplatin. Cisplatin induced renal oxidative stress, deterioration of mitochondrial structure and function, an intense inflammatory response, histopathological injury, and renal dysfunction. A single systemic dose of mitochondrially targeted antioxidants, MitoQ or Mito-CP, dose-dependently prevented cisplatin-induced renal dysfunction. Mito-CP also prevented mitochondrial injury and dysfunction, renal inflammation, and tubular injury and apoptosis. Despite being broadly renoprotective against cisplatin, Mito-CP did not diminish cisplatin's antineoplastic effect in a human bladder cancer cell line. Our results highlight the central role of mitochondrially generated oxidants in the pathogenesis of cisplatin nephrotoxicity. Because similar compounds seem to be safe in humans, mitochondrially targeted antioxidants may represent a novel therapeutic approach against cisplatin nephrotoxicity.