Photoaffinity labelling with fluorescence detection. Dye accumulation at four mitochondrial proteins in HeLa and LM cells

Visualizing Mitochondrial FoF1-ATP Synthase as the Target of the Immunomodulatory Drug Bz-423

Targeting the mitochondrial enzyme F_oF₁-ATP synthase and modulating its catalytic activities with small molecules is a promising new approach for treatment of autoimmune diseases. The immunomodulatory compound Bz-423 is such a drug that binds to subunit OSCP of the mitochondrial F_oF₁-ATP synthase and induces apoptosis via increased reactive oxygen production in coupled, actively respiring mitochondria. Here, we review the experimental progress to reveal the binding of Bz-423 to the mitochondrial target and discuss how subunit rotation of F_oF₁-ATP synthase is affected by Bz-423. Briefly, we report how Förster resonance energy transfer can be employed to colocalize the enzyme and the fluorescently tagged Bz-423 within the mitochondria of living cells with nanometer resolution.

Selective photoaffinity labelling of one mitochondrial protein in living cells of Saccharomyces cerevisiae with the fluorescent probe APMC. Identification of the target protein as subunit I of cytochrome c oxidase

The lipophilic, cationic fluorochrome azopentylmethylindocarbocyanine (APMC) specifically stains the mitochondria in living yeast cells (Saccharomyces cerevisiae WT X 2180). It contains a photosensitive diazirine ring and is suitable for photoaffinity labelling. By combining photoaffinity labelling, micro-gel electrophoresis (SDS-PAGE), and detection of the APMC fluorescence with a microfluorimeter, we established a highly sensitive procedure for determining the apparent molecular weight of the APMC-labelled proteins in yeast cells. On vital staining at 0.1 microM APMC for 30 min, only one mitochondrial protein with an apparent molecular weight of 40 kDa is labelled with high intensity. At increased dye concentrations proteins of 47 and 49 kDa are labelled too, however not until all binding sites of the 40 kDa protein are occupied. Obviously, the APMC cations have a pronounced affinity for this protein. It was shown by fractional centrifugation that the labelled 40 kDa protein is a constituent of the inner mitochondrial membrane. One driving force for the accumulation of the APMC cations is the trans-membrane potential (TMP) across the inner mitochondrial membrane. Consequently, uncouplers like dinitrophenol (DNP) and carbonylcyanidechlorophenyl-hydrazone (CCCP), ionophores (valinomycin, gramicidin), and inhibitors of the respiratory chain (myxothiazol, KCN), which decrease the TMP, also diminish the APMC accumulation and labelling. And conversely, drugs, which hyperpolarize the inner membrane (nigericin, atractyloside), favour APMC labelling. Another driving force of APMC accumulation is the dye's lipophilicity, which facilitates dye accumulation by hydrophobic interaction with the very lipophilic proteins of the inner mitochondrial membranes. This was shown by competitive double staining experiments. Thiamine strongly inhibits APMC labelling. Obviously, the transport of the APMC cations is facilitated by the thiamine carrier, and thiamine competes for the same binding sites, which are occupied by the dye cations. Chloramphenicol is an inhibitor of the mitochondrial protein synthesis without affecting the TMP. On preincubation, chloramphenicol completely quenches the signal of the 40 kDa protein. Therefore, this protein must be encoded on the mtDNA. The only 40 kDa protein with adequate properties is the subunit I of cytochrome c oxidase. Obviously, it is the preferred target of the APMC cations on photoaffinity labelling. This assignment agrees with the strong hydrophobicity of the labelled 40 kDa protein, which was tested with various detergents. It also agrees with the solvatochromism of the protein-bound APMC label, and finally with the paralellism of the labelled protein with cytochrome c oxidase on fractional ammonium sulfate precipitation.

Selective photosensitization of mitochondria in HeLa cells by cationic Zn (II) phthalocyanines with lipophilic side-chains

In order to optimize the photosensitization of mitochondria, we prepared the new cationic zinc (II) phthalocyanines with lipophilic side-chains (2,9,16,23-tetrakis (trimethylammonio) zinc (II) phthalocyanine tetramethylsulphate (ZnPcA1) and 2,9,16,23-tetrakis(hexyldimethylammonio) zinc (II) phthalocyanine tetramethylsulphate (ZnPcA6)) and compared them with the known cationic lipophilic sensitizers 2,9,16,23-tetrakis[2-(trimethylammonio) ethoxy]zinc (II) phthalocyanine tetraiodide (ZnPcE1) and 2,9,16,23-tetrakis[2-(hexyldimethylammonio) ethoxy] zinc (II) phthalocyanine tetrabromide (ZnPcE6) and the anionic hydrophilic sulphonated sensitizer ZnPcS. The absorption and fluorescence spectra, fluorescence quantum yields, partition coefficients Po/w between octanol and water, dimerization of ZnPcA1 in water and photosensitized generation of singlet oxygen were examined. Moreover, the phototoxic and cytotoxic effects of the phthalocyanines on HeLa cells, their photodynamic action on the proliferation rate and the influence of irradiation on the respiratory activity were studied and compared with electron and fluorescence microscopic observations of the ultrastructure of incubated and irradiated HeLa cells. From our experiments, we conclude that cationic lipophilic sensitizers, such as ZnPcA6, etc., specifically accumulate in the inner mitochondrial membranes. Therefore they can be used for the selective photosensitization of mitochondria. On irradiation of the incubated HeLa cells, the cristae are affected and finally completely destroyed. The respiration stops and the energy metabolism breaks down. Our experiments indicate that the phototoxic efficacy of the sensitizers decreases in the following order: ZnPcA6 > ZnPcA1 > ZnPcE6 > ZnPcE1 > ZnPcS. The cationic lipophilic ZnPcA6 has the strongest phototoxic activity. It acts on incubation at very low sensitizer concentrations and short irradiation periods. The dark toxicity is completely negligible. The phototoxic activity follows the type II mechanism.

Selective photosensitization of mitochondria by the lipophilic cationic porphyrin POR10

The lipophilic cationic porphyrin derivative 5.10.15.20-tetrakis(1-decylpyridinium-4-yl)-21H.23H-++ +porphin tetrabromide (POR10) binds to the mitochondria of living HeLa cells and can be used for the selective photosensitization of mitochondria. Cytotoxic and phototoxic effects were studied by means of electron microscopy and respiratory activity and compared with the effects of the hydrophilic cationic 5.10.15.20-tetrakis(1-methylpyridinium-4-yl)-21H.23H-porp hin tetraiodide (POR1) and the anionic 5.10.15.20-tetrakis(1-sulphophen-4-yl)-21H,23H-porp hin (TPPS). Lipophilic and hydrophilic molecules behave quite differently. The mitochondria are much more attacked by the lipophilic POR10 than by the hydrophilic POR1 and TPPS. At low POR10 concentrations and short incubation periods (C(D) < 0.1 microM; tI < 1 h), the mitochondrial ultrastructure scarcely alters, but, on increasing concentrations and prolonged incubation periods, in particular the cristae were affected. Irradiation (lambda > 425 nm) of incubated HeLa cells also strongly attacks the cristae. For example, after incubation under mild conditions (0.1 microM; 1 h) and subsequent irradiation (12 min at 65 mW cm(-2)) the cristae were partially destroyed. After incubation under intensified conditions (1 microM; 1 h) and subsequent irradiation (15 min at 25 mW cm(-2)) the cristae disappeared completely and the mitochondria were transformed into globular vesicles surrounded by a double membrane. Surprisingly, respiratory activity increased (115%) after POR10 treatment under mild conditions (0.1 microM; 30 min). However, subsequent irradiation for only 2 min (65 mW cm(-2)) decreased the respiratory activity to 18% relative to the untreated control (100%). Much smaller effects were observed with POR1-treated cells. We assume that the lipophilic POR10 cations, like other lipophilic dye cations, bind to the very lipophilic enzymes of the respiratory chain, which are constituents of the cristae. In contrast with POR10 the hydrophilic porphyrin derivatives are spread over several cellular compartments and are less phototoxic.

Time-resolved microfluorometric study of the binding sites of lipophilic cationic pyrene probes in mitochondria of living HeLa cells

Lipophilic dye cations specifically bind to the mitochondria of living cells. Using fluorescent dyes, the mitochondria can easily be observed with a fluorescence microscope. Electron microscopy has shown that the dyes are bound to the inner mitochondrial membranes and the cristae. Using time-resolved fluorescence microscopy we have investigated, whether the dye molecules are preferentially accumulated at the strongly hydrophobic protein complexes of energy metabolism or at the lipids of the inner membrane system. In order to use our nanosecond-pulsed laser fluorometer we synthesized specially designed lipophilic pyrene cations with S1 lifetimes in the nanosecond domain, which specifically stain mitochondria in living HeLa cells. Model experiments with artificial membranes such as liposomes, proteoliposomes and also protein complexes have shown that the fluorescence is strongly quenched by oxygen if the pyrene probes are bound to lipids. Binding to proteins causes a much smaller quenching effect. In artificial systems, all decays were single exponential. This is in contrast with incubated HeLa cells, which showed double-exponential fluorescence decays. Comparing these with the artificial systems we came to the conclusion that in HeLa cells the long-lived species 1 are pyrene probes preferentially bound to the proteins of the inner mitochondrial membranes. The short-lived species 2 is caused by fluorescence resonance energy transfer from the pyrene probes as donors to cytochromes of the inner membranes as acceptors. From our decay data we estimated a mean distance between donor and acceptor of about 40 A. This is the same order of magnitude as the mean diameters of several mitochondrial protein complexes. Therefore we assumed that species 2 are pyrene probes bound either to mitochondrial proteins with cytochromes as constituents or to the interface between these proteins and the phospholipids of the membranes. Thus both species 1 and species 2 are spatially related to mitochrondrial proteins. This agrees with the observation that respiration of HeLa cells as well as cytochrome c oxidase vesicles (COVs) are inhibited with increasing concentration of pyrene probes. Finally, we studied the photodynamic effect on irradiation of HeLa cells and of COVs after incubation with lipophilic pyrene and porphyrine cations.

Alteration of mitochondrial gene expression and disruption of respiratory function by the lipophilic antifolate pyrimethamine in mammalian cells

To clone the mammalian gene(s) associated with a novel lipophilic antifolate resistance provoked by the antiparasitic drug pyrimethamine (Assaraf, Y. G., and Slotky, J. I. (1993) J. Biol. Chem. 268, 4556-4566), differential screening of a cDNA library from pyrimethamine-resistant (PyrR100) cells was used. This library was screened with total cDNA from wild-type and PyrR100 cells. Surprisingly, several differentially overexpressed cDNA clones were isolated from PyrR100 cells, many of which mapped to the mitochondrial genome. Several lines of evidence establish mitochondria as a new target for the cytotoxic activity of pyrimethamine. (a) At > or = 10 microM, pyrimethamine inhibited mitochondrial respiration in viable wild-type cells. (b) Electron microscopy revealed degenerated mitochondrial membrane cristae in PyrR100 cells. (c) Some mitochondrially encoded transcripts were prominently elevated, whereas the normally stable 12 S/16 S rRNA was decreased in PyrR100 cells. (d) Metabolic pulse-chase labeling suggested an increased turnover rate of mitochondrially synthesized proteins in PyrR100 cells. (e) The specific activity of the key respiratory enzymatic complex cytochrome c oxidase was reduced by 6-fold in PyrR100 cells. (f) Consequently, the rate of respiration in intact PyrR100 cells was reduced by 3-fold. We conclude that pyrimethamine and possibly lipophilic analogues of methotrexate possess a folinic acid nonrescuable toxicity involving disruption of mitochondrial inner membrane structure and respiratory function, thereby establishing a new organellar target for the cytotoxic effect elicited by lipid-soluble antifolates.

Relaxation technique for kinetic measurements of dye accumulation at the mitochondria of HeLa cells in situ

The lipophilic cationic fluorescent dye ESI4 specifically stains the mitochondria in living cells. It was used to determine the kinetics of vital staining of mitochondria in single selected HeLa cells by quantitative microfluorometry in the concentration range CD = 6 x 10(-8) to 4 x 10(-6) M. The experiments were performed using the concentration jump method. They were evaluated with a quantitative staining model which considers dye binding as well as dye release at mitochondrial binding sites. The kinetic equations of the model are discussed in terms of the relaxation technique. The predictions of the theory agree well with the experimental results. The staining model was used to determine the rate constant k1 = 7.6 x 10(2)s-1 M-1 and k2 = 1.6 x 10(-3) s-1 for dye binding and dye release respectively, and the binding constant K = 4.8 x 10(5) M-1 of dye molecules at the mitochondria in HeLa cells (values according to relaxation experiments). The constants are discussed in detail. In addition, some experiments were performed on dye accumulation under the influence of antimycin, nigericin and oligomycin.

Influence of trans-membrane potential and of hydrophobic interactions on dye accumulation in mitochondria of living cells. Photoaffinity labelling of mitochondrial proteins, action of potential dissipating drugs, and competitive staining

The lipophilic cationic fluorescent dye azopentylmethylindocarbocyanine (APMC) specifically stains the mitochondria in living cells. The dye contains a photosensitive diazirine ring and is suitable for photoaffinity labelling of mitochondrial proteins. By a combination of photoaffinity labelling cell cultures of mouse fibroblasts (LM) with APMC, lysis of the labelled cells, subsequent micro-gel electrophoresis and detection of the fluorescence of the labelled proteins in the gel lanes with a sensitive microfluorimeter, we determined the number, apparent molecular masses, and relative intensity of the labelled proteins. In LM cells, three proteins with apparent molecular masses of 31, 40, and 74 kDa were labelled with high intensity, and proteins of 28, 29, 44, 48, 49, 66, and 105 kDa with low intensity. Two effects mainly determine the binding of lipophilic dye cations to mitochondrial proteins in living cells: (1) interaction of the trans-membrane potential of the inner mitochondrial membrane with the dye cations; and (2) hydrophobic interactions between the strongly lipophilic proteins of the inner membrane and the lipophilic dye molecules. Preincubation of the cell cultures with drugs that dissipate the trans-membrane potential, such as valinomycin, 2,4-dinitrophenol (DNP) and 3-chlorcarbonyl-cyanide-phenylhydrazone (CCCP), strongly reduces or even prevents APMC labelling of mitochondrial proteins. The influence of hydrophobic interactions was investigated by competitive staining experiments using dyes with very different lipophilic properties. The lipophilicity of the dyes was characterized by their Rm values in reversed phase thin-layer chromatography.(ABSTRACT TRUNCATED AT 250 WORDS)