Hydroximic acid derivatives: pleiotropic HSP co-inducers restoring homeostasis and robustness.

According to the "membrane sensor" hypothesis, the membrane's physical properties and microdomain organization play an initiating role in the heat shock response. Clinical conditions such as cancer, diabetes and neurodegenerative diseases are all coupled with specific changes in the physical state and lipid composition of cellular membranes and characterized by altered heat shock protein levels in cells suggesting that these "membrane defects" can cause suboptimal hsp-gene expression. Such observations provide a new rationale for the introduction of novel, heat shock protein modulating drug candidates. Intercalating compounds can be used to alter membrane properties and by doing so normalize dysregulated expression of heat shock proteins, resulting in a beneficial therapeutic effect for reversing the pathological impact of disease. The membrane (and lipid) interacting hydroximic acid (HA) derivatives discussed in this review physiologically restore the heat shock protein stress response, creating a new class of "membrane-lipid therapy" pharmaceuticals. The diseases that HA derivatives potentially target are diverse and include, among others, insulin resistance and diabetes, neuropathy, atrial fibrillation, and amyotrophic lateral sclerosis. At a molecular level HA derivatives are broad spectrum, multi-target compounds as they fluidize yet stabilize membranes and remodel their lipid rafts while otherwise acting as PARP inhibitors. The HA derivatives have the potential to ameliorate disparate conditions, whether of acute or chronic nature. Many of these diseases presently are either untreatable or inadequately treated with currently available pharmaceuticals. Ultimately, the HA derivatives promise to play a major role in future pharmacotherapy.

The novel hydroxylamine derivative NG-094 suppresses polyglutamine protein toxicity in Caenorhabditis elegans.

Aggregation-prone polyglutamine (polyQ) expansion proteins cause several neurodegenerative disorders, including Huntington disease. The pharmacological activation of cellular stress responses could be a new strategy to combat protein conformational diseases. Hydroxylamine derivatives act as co-inducers of heat-shock proteins (HSPs) and can enhance HSP expression in diseased cells, without significant adverse effects. Here, we used Caenorhabditis elegans expressing polyQ expansions with 35 glutamines fused to the yellow fluorescent protein (Q35-YFP) in body wall muscle cells as a model system to investigate the effects of treatment with a novel hydroxylamine derivative, NG-094, on the progression of polyQ diseases. NG-094 significantly ameliorated polyQ-mediated animal paralysis, reduced the number of Q35-YFP aggregates and delayed polyQ-dependent acceleration of aging. Micromolar concentrations of NG-094 in animal tissues with only marginal effects on the nematode fitness sufficed to confer protection against polyQ proteotoxicity, even when the drug was administered after disease onset. NG-094 did not reduce insulin/insulin-like growth factor 1-like signaling, but conferred cytoprotection by a mechanism involving the heat-shock transcription factor HSF-1 that potentiated the expression of stress-inducible HSPs. NG-094 is thus a promising candidate for tests on mammalian models of polyQ and other protein conformational diseases.

Photosynthetic performance and water relations in young pubescent oak (Quercus pubescens) trees during drought stress and recovery.

The capability to withstand and to recover from severe summer droughts is becoming an important issue for tree species in central Europe, as dry periods are predicted to occur more frequently over the coming decades. Changes in leaf gas exchange, chlorophyll a fluorescence and leaf compounds related to photoprotection were analysed in young Quercus pubescens trees under field conditions during two summers (2004 and 2005) of progressive drought and subsequent rewatering. Photochemistry was reversibly down-regulated and dissipation of excess energy was enhanced during the stress phase, while contents of leaf pigments and antioxidants were almost unaltered. Plant water status was restored immediately after rewatering. Net photosynthesis (P(n)) measured at ambient CO2 recovered from inhibition by drought within 4 wk. P(n) measured at elevated CO2--to overcome stomatal limitations--was restored after a few days. A network of photoprotective mechanisms acted in preserving the potential functionality of the photosynthetic apparatus during severe drought, leading to a rapid recovery of photosynthetic activity after rewatering. Thus, Q. pubescens seems to be capable of withstanding and surviving extreme drought events.

Non-photochemical quenching of chlorophyll a fluorescence by oxidised plastoquinone: new evidences based on modulation of the redox state of the endogenous plastoquinone pool in broken spinach chloroplasts.

Twenty-five years ago, non-photochemical quenching of chlorophyll fluorescence by oxidised plastoquinone (PQ) was proposed to be responsible for the lowering of the maximum fluorescence yield reported to occur when leaves or chloroplasts were treated in the dark with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of electron flow beyond the primary quinone electron acceptor (Q(A)) of photosystem (PS) II. Since then, the notion of PQ-quenching has received support but has also been put in doubt, due to inconsistent experimental findings. In the present study, the possible role of the native PQ-pool as a non-photochemical quencher was reinvestigated, employing measurements of the fast chlorophyll a fluorescence kinetics (from 50 micros to 5 s). The about 20% lowering of the maximum fluorescence yield F(M), observed in osmotically broken spinach chloroplasts treated with DCMU, was eliminated when the oxidised PQ-pool was non-photochemically reduced to PQH(2) by dark incubation of the samples in the presence of NAD(P)H, both under anaerobic and aerobic conditions. Incubation under anaerobic conditions in the absence of NAD(P)H had comparatively minor effects. In DCMU-treated samples incubated in the presence of NAD(P)H fluorescence quenching started to develop again after 20-30 ms of illumination, i.e., the time when PQH(2) starts getting reoxidized by PS I activity. NAD(P)H-dependent restoration of F(M) was largely, if not completely, eliminated when the samples were briefly (5 s) pre-illuminated with red or far-red light. Addition to the incubation medium of HgCl(2) that inhibits dark reduction of PQ by NAD(P)H also abolished NAD(P)H-dependent restoration of F(M). Collectively, our results provide strong new evidence for the occurrence of PQ-quenching. The finding that DCMU alone did not affect the minimum fluorescence yield F(0) allowed us to calculate, for different redox states of the native PQ-pool, the fractional quenching at the F(0) level (Q(0)) and to compare it with the fractional quenching at the F(M) level (Q(M)). The experimentally determined Q(0)/Q(M) ratios were found to be equal to the corresponding F(0)/F(M) ratios, demonstrating that PQ-quenching is solely exerted on the excited state of antenna chlorophylls.

Plastidial alpha-glucan phosphorylase is not required for starch degradation in Arabidopsis leaves but has a role in the tolerance of abiotic stress.

To study the role of the plastidial alpha-glucan phosphorylase in starch metabolism in the leaves of Arabidopsis, two independent mutant lines containing T-DNA insertions within the phosphorylase gene were identified. Both insertions eliminate the activity of the plastidial alpha-glucan phosphorylase. Measurement of other enzymes of starch metabolism reveals only minor changes compared with the wild type. The loss of plastidial alpha-glucan phosphorylase does not cause a significant change in the total accumulation of starch during the day or its remobilization at night. Starch structure and composition are unaltered. However, mutant plants display lesions on their leaves that are not seen on wild-type plants, and mesophyll cells bordering the lesions accumulate high levels of starch. Lesion formation is abolished by growing plants under 100% humidity in still air, but subsequent transfer to circulating air with lower humidity causes extensive wilting in the mutant leaves. Wilted sectors die, causing large lesions that are bordered by starch-accumulating cells. Similar lesions are caused by the application of acute salt stress to mature plants. We conclude that plastidial phosphorylase is not required for the degradation of starch, but that it plays a role in the capacity of the leaf lamina to endure a transient water deficit.

Mercury inhibits the non-photochemical reduction of plastoquinone by exogenous NADPH and NADH: evidence from measurements of the polyphasic chlorophyll a fluorescence rise in spinach chloroplasts.

Chlorophyll a fluorescence rise kinetics (from 50 mus to 1 s) were used to investigate the non-photochemical reduction of the plastoquinone (PQ) pool in osmotically broken spinach chloroplasts (Spinacia oleracea L.). Incubation of the chloroplasts in the presence of exogenous NADPH or NADH resulted in significant changes in the shape of the fluorescence transient reflecting an NAD(P)H-dependent accumulation of reduced PQ in the dark, with an extent depending on the concentration of NAD(P)H and the availability of oxygen; the dark reduction of the PQ pool was saturated at lower NAD(P)H concentrations and reached a higher level when the incubation took place under anaerobic conditions than when it occurred under aerobic conditions. Under both conditions NADPH was more effective than NADH in reducing PQ, however only at sub-saturating concentrations. Neither antimycin A nor rotenone were found to alter the effect of NAD(P)H. The addition of mercury chloride to the chloroplast suspension decreased the NAD(P)H-dependent dark reduction of the PQ pool, with the full inhibition requiring higher mercury concentrations under anaerobic than under aerobic conditions. This is the first time that this inhibitory role of mercury is reported for higher plants. The results demonstrate that in the dark the redox state of the PQ pool is regulated by the reduction of PQ via a mercury-sensitive NAD(P)H-PQ oxidoreductase and the reoxidation of reduced PQ by an O(2)-dependent pathway, thus providing additional evidence for the existence of a chlororespiratory electron transport chain in higher plant chloroplasts.