α-Synuclein Fibrils as Penrose Machines: A Chameleon in the Gear

In 1957, Lionel Penrose built the first man-made self-replicating mechanical device and illustrated its function in a series of machine prototypes, prefiguring our current view of the genesis and the proliferation of amyloid fibrils. He invented and demonstrated, with the help of his son Roger, the concepts that decades later, would become the fundamentals of prion and prion-like neurobiology: nucleation, seeding and conformational templating of monomers, linear polymer elongation, fragmentation, and spread. He published his premonitory discovery in a movie he publicly presented at only two conferences in 1958, a movie we thus reproduce here. By making a 30-year-jump in the early 90’s, we evoke the studies performed by Peter Lansbury and his group in which α-Synuclein (α-Syn) was for the first time (i) compared to a prion; (ii) shown to contain a fibrillization-prone domain capable of seeding its own assembly into fibrils; (iii) identified as an intrinsically disordered protein (IDP), and which, in the early 2000s, (iv) was described by one of us as a protein chameleon. We use these temporally distant breakthroughs to propose that the combination of the chameleon nature of α-Syn with the rigid gear of the Penrose machine is sufficient to account for a phenomenon that is of current interest: the emergence and the spread of a variety of α-Syn fibril strains in α-Synucleinopathies.

Similar neuronal imprint and no cross-seeded fibrils in α-synuclein aggregates from MSA and Parkinson's disease

Aggregated alpha-synuclein (α-syn) is a principal constituent of Lewy bodies (LBs) and glial cytoplasmic inclusions (GCIs) observed respectively inside neurons in Parkinson’s disease (PD) and oligodendrocytes in multiple system atrophy (MSA). Yet, the cellular origin, the pathophysiological role, and the mechanism of formation of these inclusions bodies (IBs) remain to be elucidated. It has recently been proposed that α-syn IBs eventually cause the demise of the host cell by virtue of the cumulative sequestration of partner proteins and organelles. In particular, the hypothesis of a local cross-seeding of other fibrillization-prone proteins like tau or TDP-43 has also been put forward. We submitted sarkosyl-insoluble extracts of post-mortem brain tissue from PD, MSA and control subjects to a comparative proteomic analysis to address these points. Our studies indicate that: (i) α-syn is by far the most enriched protein in PD and MSA extracts compared to controls; (ii) PD and MSA extracts share a striking overlap of their sarkosyl-insoluble proteomes, consisting of a vast majority of mitochondrial and neuronal synaptic proteins, and (iii) other fibrillization-prone protein candidates possibly cross-seeded by α-syn are neither found in PD nor MSA extracts. Thus, our results (i) support the idea that pre-assembled building blocks originating in neurons serve to the formation of GCIs in MSA, (ii) show no sign of amyloid cross-seeding in either synucleinopathy, and (iii) point to the sequestration of mitochondria and of neuronal synaptic components in both LBs and GCIs.

Reduced oligodendrocyte exosome secretion in multiple system atrophy involves SNARE dysfunction

Transportation of key proteins via extracellular vesicles has been recently implicated in various neurodegenerative disorders, including Parkinson's disease, as a new mechanism of disease spreading and a new source of biomarkers. Extracellular vesicles likely to be derived from the brain can be isolated from peripheral blood and have been reported to contain higher levels of α-synuclein (α-syn) in Parkinson's disease patients. However, very little is known about extracellular vesicles in multiple system atrophy, a disease that, like Parkinson's disease, involves pathological α-syn aggregation, though the process is centred around oligodendrocytes in multiple system atrophy. In this study, a novel immunocapture technology was developed to isolate blood CNPase-positive, oligodendrocyte-derived enriched microvesicles (OEMVs), followed by fluorescent nanoparticle tracking analysis and assessment of α-syn levels contained within the OEMVs. The results demonstrated that the concentrations of OEMVs were significantly lower in multiple system atrophy patients, compared to Parkinson's disease patients and healthy control subjects. It is also noted that the population of OEMVs involved was mainly in the size range closer to that of exosomes, and that the average α-syn concentrations (per vesicle) contained in these OEMVs were not significantly different among the three groups. The phenomenon of reduced OEMVs was again observed in a transgenic mouse model of multiple system atrophy and in primary oligodendrocyte cultures, and the mechanism involved was likely related, at least in part, to an α-syn-mediated interference in the interaction between syntaxin 4 and VAMP2, leading to the dysfunction of the SNARE complex. These results suggest that reduced OEMVs could be an important mechanism related to pathological α-syn aggregation in oligodendrocytes, and the OEMVs found in peripheral blood could be further explored for their potential as multiple system atrophy biomarkers.

Overexpression of α-Synuclein by Oligodendrocytes in Transgenic Mice Does Not Recapitulate the Fibrillar Aggregation Seen in Multiple System Atrophy

The synucleinopathy underlying multiple system atrophy (MSA) is characterized by the presence of abundant amyloid inclusions containing fibrillar α-synuclein (α-syn) aggregates in the brains of the patients and is associated with an extensive neurodegeneration. In contrast to Parkinson's disease (PD) where the pathological α-syn aggregates are almost exclusively neuronal, the α-syn inclusions in MSA are principally observed in oligodendrocytes (OLs) where they form glial cytoplasmic inclusions (GCIs). This is intriguing because differentiated OLs express low levels of α-syn, yet pathogenic amyloid α-syn seeds require significant amounts of α-syn monomers to feed their fibrillar growth and to eventually cause the buildup of cytopathological inclusions. One of the transgenic mouse models of this disease is based on the targeted overexpression of human α-syn in OLs using the PLP promoter. In these mice, the histopathological images showing a rapid emergence of S129-phosphorylated α-syn inside OLs are considered as equivalent to GCIs. Instead, we report here that they correspond to the accumulation of phosphorylated α-syn monomers/oligomers and not to the appearance of the distinctive fibrillar α-syn aggregates that are present in the brains of MSA or PD patients. In spite of a propensity to co-sediment with myelin sheath contaminants, the phosphorylated forms found in the brains of the transgenic animals are soluble (>80%). In clear contrast, the phosphorylated species present in the brains of MSA and PD patients are insoluble fibrils (>95%). Using primary cultures of OLs from PLP-αSyn mice we observed a variable association of S129-phosphorylated α-syn with the cytoplasmic compartment, the nucleus and with membrane domains suggesting that OLs functionally accommodate the phospho-α-syn deriving from experimental overexpression. Yet and while not taking place spontaneously, fibrillization can be seeded in these primary cultures by challenging the OLs with α-syn preformed fibrils (PFFs). This indicates that a targeted overexpression of α-syn does not model GCIs in mice but that it can provide a basis for seeding aggregation using PFFs. This approach could help establishing a link between α-syn aggregation and the development of a clinical phenotype in these transgenic animals.

Novel self-replicating α-synuclein polymorphs that escape ThT monitoring can spontaneously emerge and acutely spread in neurons

The conformational strain diversity characterizing α-synuclein (α-syn) amyloid fibrils is thought to determine the different clinical presentations of neurodegenerative diseases underpinned by a synucleinopathy. Experimentally, various α-syn fibril polymorphs have been obtained from distinct fibrillization conditions by altering the medium constituents and were selected by amyloid monitoring using the probe thioflavin T (ThT). We report that, concurrent with classical ThT-positive products, fibrillization in saline also gives rise to polymorphs invisible to ThT (τ^-). The generation of τ^- fibril polymorphs is stochastic and can skew the apparent fibrillization kinetics revealed by ThT. Their emergence has thus been ignored so far or mistaken for fibrillization inhibitions/failures. They present a yet undescribed atomic organization and show an exacerbated propensity toward self-replication in cortical neurons, and in living mice, their injection into the substantia nigra pars compacta triggers a synucleinopathy that spreads toward the dorsal striatum, the nucleus accumbens, and the insular cortex.

NRF-1 and HIF-1α contribute to modulation of human VDAC1 gene promoter during starvation and hypoxia in HeLa cells

VDAC (Voltage Dependent Anion Channel) is a family of pore forming protein located in the outer mitochondrial membrane. Its channel property ensures metabolites exchange between mitochondria and the rest of the cell resulting in metabolism and bioenergetics regulation, and in cell death and life switch. VDAC1 is the best characterized and most abundant isoform, and is involved in many pathologies, as cancer or neurodegenerative diseases. However, little information is available about its gene expression regulation in normal and/or pathological conditions. In this work, we explored VDAC1 gene expression regulation in normal conditions and in the contest of some metabolic and energetic mitochondrial dysfunction and cell stress as example. The core of the putative promoter region was characterized in terms of transcription factors responsive elements both by bioinformatic studies and promoter activity experiments. In particular, we found an abundant presence of NRF-1 sites, together with other transcription factors binding sites involved in cell growth, proliferation, development, and we studied their prevalence in gene activity. Furthermore, upon depletion of nutrients or controlled hypoxia, as detected in various pathologies, we found that VDAC1 transcripts levels were significantly increased in a time related manner. VDAC1 promoter activity was also validated by gene reporter assays. According to PCR real-time experiments, it was confirmed that VDAC1 promoter activity is further stimulated when cells are exposed to stress. A bioinformatic survey suggested HIF-1α, besides NRF-1, as a most active TFBS. Their validation was obtained by TFBS mutagenesis and TF overexpression experiments. In conclusion, we experimentally demonstrated the involvement of both NRF-1 and HIF-1α in the regulation of VDAC1 promoter activation at basal level and in some peculiar cell stress conditions.

In vitro α-synuclein neurotoxicity and spreading among neurons and astrocytes using Lewy body extracts from Parkinson disease brains

Synucleinopathies are a group of diseases characterized by the presence of intracellular protein aggregates containing α-synuclein (α-syn). While α-syn aggregates have been shown to induce multimodal cellular dysfunctions, uptake and transport mechanisms remain unclear. Using high-content imaging on cortical neurons and astrocytes, we here define the kinetics of neuronal and astrocytic abnormalities induced by human-derived α-syn aggregates grounding the use of such system to identify and test putative therapeutic compounds. We then aimed at characterizing uptake and transport mechanisms using primary cultures of cortical neurons and astrocytes either in single well or in microfluidic chambers allowing connection between cells and cell-types. We report that astrocytes take up α-syn-aggregates far more efficiently than neurons through an endocytic event. We also highlight that active α-syn transport occurs between cells and any cell-types. Of special interest regarding the disease, we also show that uptake and spreading of α-syn from astrocytes to neurons can lead to neuronal death. Altogether, we here show that patients-derived α-synuclein aggregates, which are taken up by neurons and astrocytes, induce a differential endogenous response in the two cell types including a peculiar astrocytic toxic gain-of-function that leads to neuronal death.

A phase II trial evaluating the efficacy and safety of efavirenz in metastatic castration-resistant prostate cancer

BACKGROUND

Preclinical studies demonstrated that non-nucleoside reverse transcriptase inhibitors used for the treatment of HIV could antagonize tumor development. This led us to assess the efficacy of efavirenz in patients with metastatic castration-resistant prostate cancer (mCRPC) in a multicenter phase II study.

METHODS

We used a Simon two-stage design and a 3-month prostate-specific antigen (PSA) nonprogression rate of 40% as a primary objective. Patients received 600 mg efavirenz daily with the possibility of a dose increase in case of PSA progression. Exploratory analyses included pharmacokinetics of efavirenz plasma concentrations and correlations with clinical outcomes.

RESULTS

Among 53 assessable patients, we observed 15 instances of PSA nonprogression at 3 months, corresponding to a nonprogression rate of 28.3% (95% confidence interval: 16.8%-42.3%). The exploratory analysis revealed that for the 7 patients in whom optimal plasma concentration of efavirenz was achieved, PSA progression was observed in only 28.6% compared with 81.8% of patients with suboptimal plasma concentrations of efavirenz.

CONCLUSION

Although 600 mg efavirenz did not statistically improve the PSA nonprogression rate, our exploratory analysis suggests that higher plasma concentrations of this drug (i.e., use of increased dosages) may be of potential benefit for the treatment of mCRPC.

Identification and super-resolution imaging of ligand-activated receptor dimers in live cells

Molecular interactions are key to many chemical and biological processes like protein function. In many signaling processes they occur in sub-cellular areas displaying nanoscale organizations and involving molecular assemblies. The nanometric dimensions and the dynamic nature of the interactions make their investigations complex in live cells. While super-resolution fluorescence microscopies offer live-cell molecular imaging with sub-wavelength resolutions, they lack specificity for distinguishing interacting molecule populations. Here we combine super-resolution microscopy and single-molecule Förster Resonance Energy Transfer (FRET) to identify dimers of receptors induced by ligand binding and provide super-resolved images of their membrane distribution in live cells. By developing a two-color universal-Point-Accumulation-In-the-Nanoscale-Topography (uPAINT) method, dimers of epidermal growth factor receptors (EGFR) activated by EGF are studied at ultra-high densities, revealing preferential cell-edge sub-localization. This methodology which is specifically devoted to the study of molecules in interaction, may find other applications in biological systems where understanding of molecular organization is crucial.

HIPK1 drives p53 activation to limit colorectal cancer cell growth

HIPK1 (homeodomain interacting protein kinase 1) is a serine/threonine kinase that belongs to the CMGC superfamily. Emerging data point to the role of HIPK1 in cancer, but it is still not clear whether it acts as a tumor suppressor or promoter. Here we identified HIPK1 as a kinase that is significantly overexpressed in colorectal cancer (CRC) and whose expression is stage-dependent. Being abundantly expressed at the onset of the disease, the HIPK1 level gradually decreased as tumor stage progressed. To further uncover how this factor regulates tumorigenesis and establish whether it constitutes an early factor necessary for neoplastic transformation or for cellular defense, we studied the effect of its overexpression in vitro by investigating various cancer-related signaling cascades. We found that HIPK1 mostly regulates the p53 signaling pathway both in HCT116 and HeLa cells. By phosphorylating p53 on its serine-15, HIPK1 favored its transactivation potential, which led to a rise in p21 protein level and a decline in cell proliferation. Assuming that HIPK1 could impede CRC growth by turning on the p53/p21 pathway, we then checked p21 mRNA levels in patients. Interestingly, p21 transcripts were only increased in a subset of patients expressing high levels of HIPK1. Unlike the rest of the cohort, the majority of these patients hosted a native p53 protein, meaning that such a pro-survival pathway (HIPK1+ > p53 > p21) is active in patients, and that HIPK1 acts rather as a tumor suppressor.

The polyphenolic ellagitannin vescalagin acts as a preferential catalytic inhibitor of the α isoform of human DNA topoisomerase II

Polyphenolic ellagitannins are natural compounds that are often associated with the therapeutic activity of plant extracts used in traditional medicine. They display cancer-preventing activity in animal models by a mechanism that remains unclear. Potential targets have been proposed, including DNA topoisomerases II (Top2). Top2α and Top2β, the two isoforms of the human Top2, play a crucial role in the regulation of replication, transcription, and chromosome segregation. They are the target of anticancer agents used in the clinic such as anthracyclines (e.g., doxorubicin) or the epipodophyllotoxin etoposide. It was recently shown that the antitumor activity of etoposide was due primarily to the inhibition of Top2α, whereas inhibition of Top2β was responsible for the development of secondary malignancies, pointing to the need for more selective Top2α inhibitors. Here, we show that the polyphenolic ellagitannin vescalagin preferentially inhibits the decatenation activity of Top2α in vitro, by a redox-independent mechanism. In CEM cells, we also show that transient small interfering RNA-mediated down-regulation of Top2α but not of Top2β conferred a resistance to vescalagin, indicating that the α isoform is a preferential target. We further confirmed that Top2α inhibition was due to a catalytic inhibition of the enzyme because it did not induce DNA double-strand breaks in CEM-treated cells but prevented the formation of Top2α- rather than Top2β-DNA covalent complexes induced by etoposide. To our knowledge, vescalagin is the first example of a catalytic inhibitor for which cytotoxicity is due, at least in part, to the preferential inhibition of Top2α.

Outer membrane VDAC1 controls permeability transition of the inner mitochondrial membrane in cellulo during stress-induced apoptosis

Voltage-dependent anion channel (VDAC)1 is the main channel of the mitochondrial outer membrane (MOM) and it has been proposed to be part of the permeability transition pore (PTP), a putative multiprotein complex candidate agent of the mitochondrial permeability transition (MPT). Working at the single live cell level, we found that overexpression of VDAC1 triggers MPT at the mitochondrial inner membrane (MIM). Conversely, silencing VDAC1 expression results in the inhibition of MPT caused by selenite-induced oxidative stress. This MOM-MIM crosstalk was modulated by Cyclosporin A and mitochondrial Cyclophilin D, but not by Bcl-2 and Bcl-X(L), indicative of PTP operation. VDAC1-dependent MPT engages a positive feedback loop involving reactive oxygen species and p38-MAPK, and secondarily triggers a canonical apoptotic response including Bax activation, cytochrome c release and caspase 3 activation. Our data thus support a model of the PTP complex involving VDAC1 at the MOM, and indicate that VDAC1-dependent MPT is an upstream mechanism playing a causal role in oxidative stress-induced apoptosis.

Anticancer drugs exert differential apoptotic and cytotoxic effects on glioblastoma primary cultures with various EGFR and bcl-2 profiles

The aim of this study was to determine the apoptotic and cytotoxic effects induced on glioblastoma cells by various anticancer agents that possess different mechanisms of action (alkylating drugs, anti-EGFR (Epidermal Growth Factor receptor), proteasome inhibitor). Primary cell cultures were obtained from patients who underwent surgery for their glioblastoma. The cytotoxic effects of drugs were determined by MTT (dimethylthiazolyl diphenyl tetrazolium bromide) assay and apoptosis was evaluated by measuring mitochondrial potential by flow cytometry. Biological markers (EGFR, bcl-2) were studied by a immunoblotting technique to find out predictive markers of response. We found a large interindividual sensitivity, thus confirming the interest of the primary cultures. New proteasome inhibitor bortezomib had considerable cytotoxic and apoptotic potential in glioblastoma, even at very low concentrations. Moreover, the characterization of patients' cells for EGFR and bcl-2 status could constitute an interest, with the evaluation of other markers, in the study of expected chemotherapy response.

An intracellular wave of cytochrome c propagates and precedes Bax redistribution during apoptosis

Bax is considered to be pivotal in inducing cytochrome c release (CCR) from mitochondria during apoptosis. Indeed, Bax redistributes to the mitochondrial outer membrane (MOM) upon activation and forms homo-multimers that are capable of permeabilizing the MOM. Our attempts to image this sequence of events in single live cells resulted in unexpected observations. Bax redistribution exhibited two distinct components: an early minor redistribution that was silent in terms of homo-multimerization and a major late redistribution that was synchronous with the formation of Bax multimers, but that proceeded belatedly, i.e. only after caspase 3/7 (C3/7) had already been activated. Intriguingly, neither of these two components of redistribution correlated with CCR, which turned out to be spatially organized, propagating as a traveling wave at constant velocity. Strikingly, propagation of the CCR wave (1) preceded signs of in situ Bax conformational activation; (2) appeared to be independent of autocatalytic loops involving a positive feedback of either C3/7, Ca(2+) mobilization or mitochondrial permeability transition; and (3) was triggered by diffuse stimulation with the synthetic Bak activator BH3I-1 but then proceeded independently of Bak activation. Thus, the CCR wave not only questions the exact role of Bax redistribution in cell death, but also indicates the existence of yet unidentified positive-feedback loops that ensure a spatiotemporal control of apoptosis at the subcellular scale.

Localization of Fas/CD95 into the lipid rafts on down-modulation of the phosphatidylinositol 3-kinase signaling pathway

Activation of the phosphatidylinositol 3-kinase (PI3K) signaling pathway is known to protect tumor cells from apoptosis and more specifically from the Fas-mediated apoptotic signal. The antitumoral agent edelfosine sensitizes leukemic cells to death by inducing the redistribution of the apoptotic receptor Fas into plasma membrane subdomains called lipid rafts. Herein, we show that inhibition of the PI3K signal by edelfosine triggers a Fas-mediated apoptotic signal independently of the Fas/FasL interaction. Furthermore, similarly to edelfosine, blockade of the PI3K activity, using specific inhibitors LY294002 and wortmannin, leads to the clustering of Fas whose supramolecular complex is colocalized within the lipid rafts. These findings indicate that the antitumoral agent edelfosine down-modulates the PI3K signal to sensitize tumor cells to death through the redistribution of Fas into large platform of membrane rafts.

Spatial relational memory requires hippocampal adult neurogenesis

The dentate gyrus of the hippocampus is one of the few regions of the mammalian brain where new neurons are generated throughout adulthood. This adult neurogenesis has been proposed as a novel mechanism that mediates spatial memory. However, data showing a causal relationship between neurogenesis and spatial memory are controversial. Here, we developed an inducible transgenic strategy allowing specific ablation of adult-born hippocampal neurons. This resulted in an impairment of spatial relational memory, which supports a capacity for flexible, inferential memory expression. In contrast, less complex forms of spatial knowledge were unaltered. These findings demonstrate that adult-born neurons are necessary for complex forms of hippocampus-mediated learning.

Protein arginine (N)-methyl transferase 7 (PRMT7) as a potential target for the sensitization of tumor cells to camptothecins

PRMT7 belongs to the protein arginine methyl-transferases family. We show that downregulation of PRMT7alpha and beta isoforms in DC-3F hamster cells was associated with increased sensitivity to the Top1 inhibitor camptothecin (CPT). This effect was not due to a change in Top1 contents or catalytic activity, or to a difference in the reversal of DNA breaks. Overexpression of PRMT7alpha and beta in DC-3F cells had no effect on CPT sensitivity, whereas it conferred a resistance to DC-3F/9-OH-E cells for which both isoforms are reduced by two- to three-fold as compared to DC-3F parental cells. Finally, downregulation of the human PRMT7 could also sensitize HeLa cells to CPT, suggesting that it could be used as a target to potentiate CPT derivatives.

Cytotoxic and apoptotic effects of bortezomib and gefitinib compared to alkylating agents on human glioblastoma cells

Glioblastoma is a malignant astrocytic tumor with a median survival of about 12 months for which new therapeutic strategies are required. We therefore examined the cytotoxicity of anticancer drugs with different mechanisms of action on two human glioblastoma cell lines expressing various levels of EGFR (epidermal growth factor receptor). Apoptosis induced by these anticancer agents was evaluated by flow cytometry. The cytotoxicity of alkylating drugs followed a dose-effect curve and cytotoxicity index values were lower with carboplatin than with BCNU and temozolomide. Anti-EGFR gefitinib (10 microM) cytotoxicity on DBTRG.05-MG expressing high levels of EGFR was significantly higher than on U87-MG expressing low levels of EGFR. Carboplatin and temozolomide cytotoxicity was potentiated with the addition of gefitinib on DBTRG.05-MG. Among the anticancer agents tested, the proteasome inhibitor bortezomib was the most cytotoxic with very low IC50 on the two cell lines. Moreover, all anticancer drugs tested induced apoptosis in a concentration-dependent manner. Bortezomib proved to be a more potent inductor of apoptosis than gefitinib and alkylating agents. These results show the efficacy of bortezomib and of the association between conventional chemotherapy and gefitinib on glioblastoma cells and therefore suggest the interest of these molecules in the treatment of glioblastoma.

Label-free optical imaging of mitochondria in live cells

The far-field optical imaging of mitochondria of live cells without the use of any label is demonstrated. It uses a highly sensitive photothermal method and has a resolution comparable to confocal fluorescence setups. The morphological states of mitochondria were followed under different physiological treatments, and the role of cytochrome c was ruled out as the main origin of the photothermal signals. This label free optical method provides a high contrast imaging of live mitochondria and should find many applications in biosciences.

Tyrosine hydroxylase and dopamine transporter expression in lactotrophs from postlactating rats: involvement in dopamine-induced apoptosis

Cessation of lactation causes a massive loss of surplus lactotrophs in the rat pituitary gland. The factors and mechanisms involved in this phenomenon have not yet been elucidated. Besides its inhibitory control on prolactin secretion and lactotroph proliferation, evidence suggests that dopamine (DA) may be a proapoptotic factor for lactotrophs. We therefore tested the proapoptotic effect of DA on pituitary glands from virgin, lactating, and postlactating rats. By measuring mitochondrial membrane potential loss, caspase-3 activation, and nuclear fragmentation, we show that DA induces apoptosis specifically in lactotrophs from postlactating rats. We then determined that this effect was partly mediated by the DA transporter (DAT) rather than the D(2) receptor, as corroborated by the detection of DAT expression exclusively in lactotrophs from postlactating rats. We also observed tyrosine hydroxylase (TH) expression in postlactating lactotrophs that was accompanied by an increase in DA content in the anterior pituitary gland of postlactating compared with virgin rats. Finally, we observed that cells expressing TH coexpressed DAT and cleaved caspase-3. These findings show that DA may play a role in lactotroph regression during the postlactation period by inducing apoptosis. The fact that this process requires DAT and TH expression by lactotrophs themselves suggests that it may be "autocrine" in nature.

Signaling pathway involved in the pro-apoptotic effect of dopamine in the GH3 pituitary cell line

Besides its physiological role as a neurotransmitter, dopamine (DA) induces apoptosis in the central nervous system. This effect is mediated partly by the DA transporter (DAT) and involves reactive oxygen species (ROS) formation as well as oxidative stress. In the pituitary, the inhibitory control by DA of prolactin release and synthesis and lactotrope cell proliferation is well known, while the pro-apoptotic effect of DA remains unclear. Our aim was to study the pro-apoptotic effect of DA in the GH3 mammosomatotrope cell line and determine the DA mechanism that leads to apoptosis in these cells. Using flow cytometry, Western blot, and confocal microscopy, we showed for the first time that DA induced: (1) loss of mitochondrial potential; (2) relocation of Bax to the mitochondria; (3) cytochrome c release; (4) caspase-3 activation, and (5) nuclear fragmentation, resulting in apoptosis. We observed that DAT was expressed in GH3 cells and participated in the DA effect, as apoptosis was significantly reversed in the presence of DAT inhibitors. Direct measurement showed that DA rapidly increased the formation of intracellular ROS. The antioxidant N-acetyl-L-cysteine (NAC) effectively blocked DA-induced ROS formation and apoptosis. Neither JNK nor p38 were involved in this process, so we suggest that the mitochondrial pore of transition is the likely target of the ROS generated by DA. These data provide the first evidence that DA triggers apoptosis in pituitary cells via a mechanism involving DAT and oxidative stress. These findings may be particularly relevant in understanding lactotrope apoptosis during postnatal life.

Amplification of Fas-mediated apoptosis in type II cells via microdomain recruitment

Fas triggers apoptosis via the caspase cascade when bound to its ligand FasL. In type I cells, Fas is concentrated into the plasma membrane lipid rafts, and these domains are required for the apoptotic signal to occur. In contrast, Fas is excluded from the microdomains in type II cells. We report that the coligation with Fas of the membrane receptor CD28 strongly increases Fas-induced apoptosis in type II T lymphocytes, whereas it has no effect in a type I cell line. The effect of CD28 is independent of its intracellular region and requires the recruitment of the microdomains. Indeed, upon CD28 costimulation, Fas is redistributed in the lipid rafts, and their disruption with a cholesterol chelator abrogates the effect of CD28. The microdomain-mediated cell death amplification does not alter death-induced signaling complex formation and is mediated by the enhancement of the mitochondrial apoptotic pathway. These findings indicate that the sensitivity to Fas-induced apoptosis of type II cells can be amplified in vivo by the recruitment of lipid rafts following interactions between nonapoptotic ligand/receptor pairs during cell-to-cell contacts.

Hepatitis C virus core triggers apoptosis in liver cells by inducing ER stress and ER calcium depletion

Hepatitis C virus (HCV) core, known to be involved in liver carcinogenesis, is processed in the endoplasmic reticulum (ER). We thus investigated the impact of three HCV core isolates on ER stress, ER calcium signalling and apoptosis. We show that HCV core constructs trigger hyperexpression of Grp78/BiP, Grp 94, calreticulin and sarco/endoplasmic reticulum calcium ATPase, inducing ER stress. By using the ER-targeted aequorin calcium probe, we found that ER calcium depletion follows ER stress in core-expressing cells. HCV core induces apoptosis through overexpression of the CHOP/GADD153 proapoptotic factor, Bax translocation to mitochondria, mitochondrial membrane depolarization, cytochrome c release, caspase-3 and PARP cleavage. Furthermore, reversion of HCV core-induced ER calcium depletion (by transfection of SERCA2) completely abolished mitochondrial membrane depolarization, suggesting that both ER stress (through CHOP overexpression) and calcium signalling play a major role in the HCV core-mediated control of apoptosis. ER stress and apoptosis were also found in a proportion of HCV-full-length replicon-expressing cells and in the liver of HCV core transgenic mice. In conclusion, our data demonstrate that HCV core deregulates the control of apoptosis by inducing ER stress and ER calcium depletion providing new elements to understand the mechanisms involved in HCV-related liver chronic diseases.

Oligomeric Bax is a component of the putative cytochrome c release channel MAC, mitochondrial apoptosis-induced channel

Bcl-2 family proteins regulate apoptosis, in part, by controlling formation of the mitochondrial apoptosis-induced channel (MAC), which is a putative cytochrome c release channel induced early in the intrinsic apoptotic pathway. This channel activity was never observed in Bcl-2-overexpressing cells. Furthermore, MAC appears when Bax translocates to mitochondria and cytochrome c is released in cells dying by intrinsic apoptosis. Bax is a component of MAC of staurosporine-treated HeLa cells because MAC activity is immunodepleted by Bax antibodies. MAC is preferentially associated with oligomeric, not monomeric, Bax. The single channel behavior of recombinant oligomeric Bax and MAC is similar. Both channel activities are modified by cytochrome c, consistent with entrance of this protein into the pore. The mean conductance of patches of mitochondria isolated after green fluorescent protein-Bax translocation is significantly higher than those from untreated cells, consistent with onset of MAC activity. In contrast, the mean conductance of patches of mitochondria indicates MAC activity is present in apoptotic cells deficient in Bax but absent in apoptotic cells deficient in both Bax and Bak. These findings indicate Bax is a component of MAC in staurosporine-treated HeLa cells and suggest Bax and Bak are functionally redundant as components of MAC.

New functions of an old protein: the eukaryotic porin or voltage dependent anion selective channel (VDAC)

Mitochondrial porin or VDAC (Voltage Dependent Anion selective Channels) was identified for the first time in 1976, on the basis of the evolutionary similarity between the gram negative and mitochondrial outer membranes. Since this achievement VDAC has been extensively investigated: its functional features have been sharply defined upon reconstitution in artificial membranes and its sequence has been determined in many genomes. Unfortunately the tertiary structure has not yet been solved, mainly because it proved to be very difficult to get suitable crystals. Despite this established knowledge, in the last few years this protein has attracted renewed interest. There are two main reasons for this interest: the discovery, in most eukaryotes, of a family of genes encoding VDAC isoforms and the claims of VDAC involvement in the intrinsic pathway of apoptosis and in particular in the mechanism of cytochrome c release from mitochondria. We can affirm that nowadays the eukaryotic porin (or VDAC) is studied in a more general cellular contest, looking at the interactions and integration with other molecules, since VDAC is in a crucial position in the cell, forming the main interface between the mitochondrial and the cellular metabolisms. In this minireview we will briefly focus our attention onto the following topics: 1) recent advances about the structure of VDAC; 2) the VDAC-related multigene families; 3) the presence, targeting and function of VDAC in various cell membranes.

RGD-functionalized spherulites as targeted vectors captured by adherent cultured cells

Spherulites are multilamellar vesicles consisting of concentric shells that can encapsulate small organic molecules or macromolecules. We investigate the possibility of targeting neutral spherulites to adherent culture cells by functionalizing their surface with RGD-containing ligands. The strength and specificity of association of RGD spherulites with several cell lines (EAhy 926 endothelial cell line, human umbilical vein endothelial cell (HUVEC) and human osteoprogenitor (HOP) primary cells) was studied, and the molecular interaction of RGD spherulites with the EAhy 926 cell surface was investigated. We show that, after binding to cells, spherulites are internalized.

Selective targeting of synthetic antioxidants to mitochondria: towards a mitochondrial medicine for neurodegenerative diseases?

Mitochondria are the major source of superoxide, and are responsible for activating apoptosis and oxidative damage during acute neuronal cell death and neurodegenerative disorders like Alzheimer and Parkinson diseases. While the molecular mechanisms by which mitochondrial oxidative stress triggers apoptosis are still investigated, attempts to achieve neuroprotection using antioxidant molecules have already been successful in several models of neuronal cell death. To increase the availability of antioxidant drugs at the mitochondrial level within cells, Michael P. Murphy recently proposed to covalently couple antioxidant molecules to a membrane-permeable lipophilic cation serving as carrier. Since mitochondria maintain at rest a potential of -180 mV, the diffusible cationic moiety drives the accumulation of the complex inside the matrix towards a diffusion equilibrium: for a monovalent cationic carrier, a thousand-fold accumulation of the complex is theoretically achievable; for a divalent cation, a million-fold accumulation is expected. Such mitochondria-targeted versions of natural antioxidants have successfully been synthesized and were found to counteract the pro-apoptotic effects of exogenous oxidative insults, while having no effects in models mimicking physiological apoptosis. Based on these observations, we carried out the synthesis of targeted variants of the artificial free radical scavengers 4-hydroxy-2,2,6,6-tetramethylpiperidin-N-oxide (TEMPOL) and Salen-Mn(III) complex of o-vanillin (EUK-134). Our preliminary results indicate that these targeted compounds, while delaying apoptosis after an exogenous oxidative insult, are not more active than their untargeted variants. This questions the general efficiency of the targeting procedure used and/or suggests that the main pro-apoptotic effector targets of exogenous oxidative insults are not located within mitochondria.

The permeability transition pore signals apoptosis by directing Bax translocation and multimerization

Mitochondria are key players of apoptosis and can irreversibly commit the cell to death by releasing cytochrome c (Cyt.c) to the cytosol, where caspases 9 and 3 subsequently get activated. Under conditions of oxidative stress, opening of the mitochondrial permeability transition pore (PTP) represents an early trigger and is crucial in causing Cyt.c release. To account for the latter, current models propose that PTP gating would result, as is the case in vitro, in the rupture of the outer mitochondrial membrane caused by mitochondrial matrix swelling. Using live cell imaging and recombinant fluorescent probes based on the green fluorescent protein (GFP) and its mutants, we report that directed repetitive gating of the PTP triggers a delayed Cyt.c efflux, which is not associated with mitochondrial swelling. Instead, subcellular imaging shows that PTP opening signals the redistribution of the cytosolic protein Bax to the mitochondria, where it secondarily forms clusters that appear to be a prerequisite for Cyt.c release. Fluorescence resonance energy transfer imaging further reveals that Bax clustering coincides with the formation of Bax multimers. We conclude that the PTP is not itself a component of the Cyt.c release machinery, but that it acts indirectly by signaling Bax translocation and multimerization.

Electrical coupling and plasticity of the mitochondrial network

Kinetic fluorescence imaging and the potentiometric probe tetramethylrhodamine methyl ester (TMRM) were used to evoke and detect changes in membrane potential (delta Psi(m)) of individual mitochondria in living cells. As a combined effect of preferential TMRM accumulation in mitochondria, and of TMRM photoactivation, individual organelles displayed sharp transient depolarizations caused by local reactive oxygen species (ROS)-mediated gatings of the mitochondrial permeability transition pore (PTP). In COS-7 cells, such directed repetitive gatings of the PTP gave rise to stochastic delta Psi(m)flickering at the level of individual organelles, but also to prominent synchronous delta Psi(m)transitions in whole subgroups of the mitochondrial population, indicative of the existence of an underlying electrically coupled mitochondrial network. In single cells, this network could comprise as much as 65% of the total mitochondrial population, a nd exhibited a high plasticity with mitochondrial units spontaneously connecting to and disconnecting from the coupled structure within seconds. These results indicate that in resting cells, the mitochondrial network is a dynamic proton-conducting structure capable to commute and coordinate electrical signals generated by the PTP.

A ubiquinone-binding site regulates the mitochondrial permeability transition pore

We have investigated the regulation of the mitochondrial permeability transition pore (PTP) by ubiquinone analogues. We found that the Ca2+-dependent PTP opening was inhibited by ubiquinone 0 and decylubiquinone, whereas all other tested quinones (ubiquinone 5, 1,4-benzoquinone, 2-methoxy-1,4-benzoquinone, 2,3-dimethoxy-1, 4-benzoquinone, and 2,3-dimethoxy-5,6-dimethyl-1,4-benzoquinone) were ineffective. Pore inhibition was observed irrespective of the method used to induce the permeability transition (addition of Pi or atractylate, membrane depolarization, or dithiol cross-linking). Inhibition of PTP opening by decylubiquinone was comparable with that exerted by cyclosporin A, whereas ubiquinone 0 was more potent. Ubiquinone 5, which did not inhibit the PTP per se, specifically counteracted the inhibitory effect of ubiquinone 0 or decylubiquinone but not that of cyclosporin A. These findings define a ubiquinone-binding site directly involved in PTP regulation and indicate that different quinone structural features are required for binding and for stabilizing the pore in the closed conformation. At variance from all other quinones tested, decylubiquinone did not inhibit respiration. Our results define a new structural class of pore inhibitors and may open new perspectives for the pharmacological modulation of the PTP in vivo.

From calcium signaling to cell death: two conformations for the mitochondrial permeability transition pore. Switching from low- to high-conductance state

The permeability transition pore (PTP) is a channel of the inner mitochondrial membrane that appears to operate at the crossroads of two distinct physiological pathways, i.e., the Ca2+ signaling network during the life of the cell, and the effector phase of the apoptotic cascade during Ca2+-dependent cell death. Correspondingly, two open conformations of the PTP can also be observed in isolated organelles. A low-conductance state, that allows the diffusion of small ions like Ca2+, is pH-operated, promoting spontaneous closure of the channel. A high-conductance state, that allows the unselective diffusion of big molecules, stabilizes the channel in the open conformation, disrupting in turn the mitochondrial structure and causing the release of proapoptotic factors. Our current results indicate that switching from low- to high-conductance state is an irreversible process that is strictly dependent on the saturation of the internal Ca2+-binding sites of the PTP. Thus, the high-conductance state of the PTP, which was shown to play a pivotal role in the course of excitotoxic and thapsigargin-induced cell death, might result from a Ca2+-dependent conformational shift of the low-conductance state, normally participating in the regulation of cellular Ca2+ homeostasis as a pH-operated channel. These observations lead us to propose a simple biophysical model of the transition between Ca2+ signaling and Ca2+-dependent apoptosis.

Modulation of cell calcium signals by mitochondria

It is now clearer and clearer that mitochondria play a role, and perhaps an active role, in cell calcium signalling. The fact that mitochondria can exhibit a Ca2+-induced Ca2+ release (mCICR, Ichas et al. [37]) reinforces this concept and makes the mitochondria an essential element in the relay of Ca2+ wave propagation. It must be emphasized that the modulation of cell Ca2+ signals by mitochondria depends upon their energetic status, thus making mitochondria an essential link between energy metabolism and calcium signalling inside the cell.

Regulation of the permeability transition pore in skeletal muscle mitochondria. Modulation By electron flow through the respiratory chain complex i

We have investigated the regulation of the permeability transition pore (PTP), a cyclosporin A-sensitive channel, in rat skeletal muscle mitochondria. As is the case with mitochondria isolated from a variety of sources, skeletal muscle mitochondria can undergo a permeability transition following Ca2+ uptake in the presence of Pi. We find that the PTP opening is dramatically affected by the substrates used for energization, in that much lower Ca2+ loads are required when electrons are provided to complex I rather than to complex II or IV. This increased sensitivity of PTP opening does not depend on differences in membrane potential, matrix pH, Ca2+ uptake, oxidation-reduction status of pyridine nucleotides, or production of H2O2, but is directly related to the rate of electron flow through complex I. Indeed, and with complex I substrates only, pore opening can be observed when depolarization is induced with uncoupler (increased electron flow) but not with cyanide (decreased electron flow). Consistent with pore regulation by electron flow, we find that PTP opening is inhibited by ubiquinone 0 at concentrations that partially inhibit respiration and do not depolarize the inner membrane. These data allow identification of a novel site of regulation of the PTP, suggest that complex I may be part of the pore complex, and open new perspectives for its pharmacological modulation in living cells.

A model of mitochondrial Ca(2+)-induced Ca2+ release simulating the Ca2+ oscillations and spikes generated by mitochondria

Recent evidence underlines a key role of mitochondrial Ca2+ fluxes in cell Ca2+ signalling. We present here a kinetic model simulating the Ca2+ fluxes generated by mitochondria during mitochondrial Ca(2+)-induced Ca2+ release (mCICR) resulting from the operation of the permeability transition pore (PTP). Our model connects the Ca2+ fluxes through the ruthenium redsensitive Ca2+ uniporter, the respiration-dependent and passive H+ fluxes, the rate of oxygen consumption, the movements of weak acids across the mitochondrial membrane, the electrical transmembrane potential (delta psi), and operation of the PTP. We find that two factors are crucial to account for the various mCICR profiles that can be observed experimentally: (i) the dependence of PTP opening and closure on matrix pH (pHi), and (ii) the relative inhibition of the respiratory rate consecutive to PTP opening. The resulting model can simulate irreversible Ca2+ efflux from mitochondria, as well as the genesis of damped or sustained Ca2+ oscillations, and of single Ca2+ spikes. The model also simulates the main features of mCICR, i.e. the threshold-dependence of mCICR triggering, and the all-or-nothing nature of mCICR operation. Our model should appear useful to further mathematically address the consequences of mCICR on the spatiotemporal organisation of Ca2+ signals, as a 'plug-in' module for the existing models of cell Ca2+ signalling.

The mitochondrial permeability transition

This review summarizes recent work on the regulation of the permeability transition pore, a cyclosporin A-sensitive mitochondrial channel that may play a role in intracellular calcium homeostasis and in a variety of forms of cell death. The basic bioenergetics aspects of pore modulation are discussed, with some emphasis on the links between oxidative stress and pore dysregulation as a potential cause of mitochondrial dysfunction that may be relevant to cell injury.

Mitochondria are excitable organelles capable of generating and conveying electrical and calcium signals

We report Ca2(+)-induced release of Ca2+ from mitochondria (mCICR) dependent on transitory opening of the permeability transition pore (PTP) operating in a low conductance mode. The Ca2+ fluxes taking place during mCICR are a direct consequence of the mitochondrial depolarization spike (mDPS) caused by PTP opening. Both mDPS and mCICR can propagate from one mitochondrion to another in vitro, generating traveling depolarization and Ca2+ waves. Mitochondria thus appear to be excitable organelles capable of generating and conveying electrical and Ca2+ signals. In living cells, mDPS/mCICR is triggered during IP3-induced Ca2+ mobilization and results in the amplification of the Ca2+ signals primarily emitted from the endoplasmic reticulum.

Microtubule-active drugs suppress the closure of the permeability transition pore in tumour mitochondria

We report the effects of anticancer drugs, inhibitors of microtubule organisation, on the mitochondrial permeability transition pore (PTP) in Ehrlich ascites tumour cells. Taxol (5-20 microM) and colchicine (100-500 microM) prevented closing of the cyclosporin A-sensitive PTP. No taxol or colchicine effects on oxidative phosphorylation were observed in the range of concentrations used. We suggest that either membrane-bound tubulin per se can be part of PTP and/or the attachment of mitochondria to the microtubular network is essential for PTP regulation. The taxol inhibition of PTP closure, mediated through interaction with the cytoskeleton, sheds new light on the cytotoxic properties of this anticancer drug.

Synchronization of calcium waves by mitochondrial substrates in Xenopus laevis oocytes

In Xenopus oocytes, as well as other cells, inositol-1,4,5-trisphosphate (Ins(1,4,5)P3)-induced Ca2+ release is an excitable process that generates propagating Ca2+ waves that annihilate upon collision. The fundamental property responsible for excitability appears to be the Ca2+ dependency of the Ins(1,4,5)P3 receptor. Here we report that Ins(1,4,5)P3-induced Ca2+ wave activity is strengthened by oxidizable substrates that energize mitochondria, increasing Ca2+ wave amplitude, velocity and interwave period. The effects of pyruvate/malate are blocked by ruthenium red at the Ca2+ uniporter, by rotenone at complex I, and by antimycin A at complex III, and are subsequently rescued at complex IV by ascorbate tetramethylphenylenediamine (TMPD). Our data reveal that potential-driven mitochondrial Ca2+ uptake is a major factor in the regulation of Ins(1,4,5)P3-induced Ca2+ release and clearly demonstrate a physiological role of mitochondria in intracellular Ca2+ signalling.

Differential effects of verapamil and quinine on the reversal of doxorubicin resistance in a human leukemia cell line

We studied the restoration of doxorubicin accumulation and sensitivity by verapamil and quinine in a variant of the human erythroleukemia cell line K562 selected for resistance to doxorubicin and presenting a multidrug-resistance (MDR) phenotype. Verapamil was able to completely restore doxorubicin accumulation in the resistant cells to the level obtained in sensitive cells, but only partially reversed doxorubicin resistance. Quinine, in contrast, had a relatively weak effect on doxorubicin accumulation but was able to completely restore doxorubicin sensitivity in the resistant cells. In addition, verapamil was able to decrease azidopine binding to P-glycoprotein, whereas quinine was not. Quinine also modified the intracellular tolerance to doxorubicin, which suggests that it is able to modify drug distribution within the cells. Confocal microscopy revealed that verapamil and quinine were able to restore nuclear fluorescence staining of doxorubicin in resistant cells; since this was obtained for quinine without significant increase of doxorubicin accumulation, this observation confirms that quinine acts principally on doxorubicin redistribution within the cells, allowing the drug to reach its nuclear targets. When used in association, verapamil and quinine reversed doxorubicin resistance in a synergistic fashion. We conclude that verapamil and quinine do not share the same targets for reversal of MDR in this cell line; whereas verapamil directly interferes with P-glycoprotein and mainly governs drug accumulation, quinine has essentially intracellular targets involved in drug redistribution from sequestration compartments.

Effects of the anaesthetic propofol on the calcium-induced permeability transition of rat heart mitochondria: direct pore inhibition and shift of the gating potential

Mitochondrial calcium exchanges are involved in intracellular calcium homeostasis and in the contraction-relaxation process in myocytes. The calcium-induced permeability transition of the heart mitochondria inner membrane appears to be an important calcium efflux mechanism involved in some physiological and pathological situations. The negative inotropic effect of the anaesthetic propofol results in part from a decrease in intracellular calcium availability. Thus, this study evaluates the effects of propofol on calcium transport and permeability transition of heart mitochondria. The propofol-inhibition of the permeability transition of liver mitochondria was previously investigated [Eriksson, O. (1991) FEBS Lett. 279, 45-48] in such conditions that its uncoupling effect was not taken into account. We show here that propofol uncoupling results in a decrease in calcium uptake rate which could in part explain the decreased permeability transition rate. However, comparison of equipotent uncoupling concentrations of propofol and carbonylcyanide m-chlorophenylhydrazone reveals that beyond this uncoupling effect, propofol has a direct inhibitory action on the permeability transition pore, concomittant with a shift of its gating potential.

Action of testosterone on the estradiol-induced feminization of the male chick embryo

The early treatment of male chick embryos with estradiol induces the feminization of their sex tract, i.e. both their gonads and müllerian tract exhibit female features. The additional treatment of estrogenized male embryos with testosterone propionate antagonizes the effects of estradiol on both gonads and müllerian ducts. Our data give further support to the view that testosterone and estrogens act respectively as agonist and antagonist modulators of the secretion of the anti-müllerian hormone.