Activation of violaxanthin cycle in darkness is a common response to different abiotic stresses : a case study in Pelvetia canaliculata

Background: In the violaxanthin (V) cycle, V is de-epoxidized to zeaxanthin (Z) when strong light or light combined with other stressors lead to an overexcitation of photosystems. However, plants can also suffer stress in darkness and recent reports have shown that dehydration triggers V-de-epoxidation in the absence of light. In this study, we used the highly stress-tolerant brown alga Pelvetia canaliculata as a model organism, due to its lack of lutein and its non-photochemical quenching independent of the transthylakoidal-ΔpH, to study the triggering of the V-cycle in darkness induced by abiotic stressors. Results: We have shown that besides desiccation, other factors such as immersion, anoxia and high temperature also induced V-de-epoxidation in darkness. This process was reversible once the treatments had ceased (with the exception of heat, which caused lethal damage). Irrespective of the stressor applied, the resulting de-epoxidised xanthophylls correlated with a decrease in Fv/Fm, suggesting a common function in the down-regulation of photosynthetical efficiency. The implication of the redox-state of the plastoquinone-pool and of the differential activity of V-cycle enzymes on V-de-epoxidation in darkness was also examined. Current results suggest that both violaxanthin de-epoxidase (VDE) and zeaxanthin-epoxidase (ZE) have a basal constitutive activity even in darkness, being ZE inhibited under stress. This inhibition leads to Z accumulation. Conclusion: This study demonstrates that V-cycle activity is triggered by several abiotic stressors even when they occur in an absolute absence of light, leading to a decrease in Fv/Fm. This finding provides new insights into an understanding of the regulation mechanism of the V-cycle and of its ecophysiological roles.

The Oxidative State of Chylomicron Remnants Influences Their Modulation of Human Monocyte Activation

8 p.

A Model of Ischemia-Induced Neuroblast Activation in the Adult Subventricular Zone

12 p.

NMDA modulates oligodendrocyte differentiation of subventricular zone cells through PKC activation

7 p.

Increase in the density of resting microglia precedes neuritic plaque formation and microglial activation in a transgenic model of Alzheimer's disease

The formation of cerebral senile plaques composed of amyloid beta peptide (A beta) is a fundamental feature of Alzheimer's disease (AD). Glial cells and more specifically microglia become reactive in the presence of A beta. In a triple transgenic model of AD (3 x Tg-AD), we found a significant increase in activated microglia at 12 (by 111%) and 18 (by 88%) months of age when compared with non-transgenic (non-Tg) controls. This microglial activation correlated with A beta plaque formation, and the activation in microglia was closely associated with A beta plaques and smaller A beta deposits. We also found a significant increase in the area density of resting microglia in 3 x Tg-AD animals both at plaque-free stage (at 9 months by 105%) and after the development of A plaques (at 12 months by 54% and at 18 months by 131%). Our results show for the first time that the increase in the density of resting microglia precedes both plaque formation and activation of microglia by extracellular A beta accumulation. We suggest that AD pathology triggers a complex microglial reaction: at the initial stages of the disease the number of resting microglia increases, as if in preparation for the ensuing activation in an attempt to fight the extracellular A beta load that is characteristic of the terminal stages of the disease. Cell Death and Disease (2010) 1, e1; doi:10.1038/cddis.2009.2; published online 14 January 2010

Analysing the contribution of ATM/ATR pathway activation in establishing the premature senescence of E2F1/E2F2-/- bone-marrow-derived macrophages

E2F1 and E2F2 transcription factors have an important role during the regulation of cell cycle. In experiments done with E2F1/E2F2 knockout mice, it has been described that bone-marrow-derived macrophages (BMDM) undergo an early rapid proliferation event related to DNA hyper-replication. As a consequence, DNA damage response (DDR) pathway is triggered and E2F1/E2F2 knockout macrophages enter premature senescence related to G2/M phase arrest. The exact mechanism trough which DNA hyper-replication leads to DDR in absence of E2F1 and E2F2 remains undiscovered. To determine whether the ATR/ATM pathway, the master regulator of G2/M checkpoint, might be the surveillance mechanism in order to regulate uncontrolled proliferation in the DKO model, we monitored and analysis biochemical properties of BMDM cultures in the presence of caffeine, a potent inhibitor of ATM/ATR activity. Our results show that the addition of caffeine abolishes premature senescence in DKO BMDM, stimulates γ-H2AX accumulation and decreases Mcm2 expression.

Specific Interaction with Cardiolipin Triggers Functional Activation of Dynamin-Related Protein 1

Dynamin-Related Protein 1 (Drp1), a large GTPase of the dynamin superfamily, is required for mitochondrial fission in healthy and apoptotic cells. Drp1 activation is a complex process that involves translocation from the cytosol to the mitochondrial outer membrane (MOM) and assembly into rings/spirals at the MOM, leading to membrane constriction/division. Similar to dynamins, Drp1 contains GTPase (G), bundle signaling element (BSE) and stalk domains. However, instead of the lipid-interacting Pleckstrin Homology (PH) domain present in the dynamins, Drp1 contains the so-called B insert or variable domain that has been suggested to play an important role in Drp1 regulation. Different proteins have been implicated in Drp1 recruitment to the MOM, although how MOM-localized Drp1 acquires its fully functional status remains poorly understood. We found that Drp1 can interact with pure lipid bilayers enriched in the mitochondrion-specific phospholipid cardiolipin (CL). Building on our previous study, we now explore the specificity and functional consequences of this interaction. We show that a four lysine module located within the B insert of Drp1 interacts preferentially with CL over other anionic lipids. This interaction dramatically enhances Drp1 oligomerization and assembly-stimulated GTP hydrolysis. Our results add significantly to a growing body of evidence indicating that CL is an important regulator of many essential mitochondrial functions.

Study on the activation mechanism of BCL-2 related ovarian killer (BOK)

BCL-2 family proteins are key regulators of the mitochondrial apoptotic machinery, controlling the mitochondrial outer membrane (MOM) permeabilization (MOMP). BCL-2 related Ovarian Killer (BOK) is a poorly understood pro-apoptotic member of this protein family. It has been reported that BOK localizes predominantly (although not exclusively) at membranes of the endoplasmic reticulum and of the Golgi apparatus. However, it is unclear whether BOK also operates at the MOM to promote apoptosis, as other pro-apoptotic BCL-2 family members do. Basing on the fact that the other two BAX-like pro-apoptotic members have been reported to oligomerize in order to induce MOMP, site-directed mutagenesis was used to generate two point mutations that predictably eliminated BOK’s oligomerization capacity. Then, the effect of such mutations on BOK’s membrane activity was examined using fluorescence spectroscopy.