Genetic engineering and molecular characterization of yeast strain expressing hybrid human-yeast squalene synthase as a tool for anti-cholesterol drug assessment

AIMS: The main objective of the study is molecular and biological characterization of the human-yeast hybrid squalene synthase (SQS), as a promising target for treatment of hypercholesterolaemia. METHODS AND RESULTS: The human-yeast hybrid SQS, with 67% amino acids, including the catalytic site derived from human enzyme, was expressed in Saccharomyces cerevisiae strain deleted of its own SQS gene. The constructed strain has a decreased level of sterols compared to the control strain. The mevalonate pathway and sterol biosynthesis genes are induced and the level of triacylglycerols is increased. Treatment of the strain with rosuvastatin or zaragozic acid, two mevalonate pathway inhibitors, decreased the amounts of squalene, lanosterol and ergosterol, and up-regulated expression of several genes encoding enzymes responsible for biosynthesis of ergosterol precursors. Conversely, expression of the majority genes implicated in the biosynthesis of other mevalonate pathway end products, ubiquinone and dolichol, was down-regulated. CONCLUSIONS: The S. cerevisiae strain constructed in this study enables to investigate the physiological and molecular effects of inhibitors on cell functioning. SIGNIFICANCE AND IMPACT OF THE STUDY: The yeast strain expressing hybrid SQS with the catalytic core of human enzyme is a convenient tool for efficient screening for novel inhibitors of cholesterol-lowering properties

Investigating the Effects of Statins on Cellular Lipid Metabolism Using a Yeast Expression System

In humans, defects in lipid metabolism are associated with a number of severe diseases such as atherosclerosis, obesity and type II diabetes. Hypercholesterolemia is a primary risk factor for coronary artery disease, the major cause of premature deaths in developed countries. Statins are inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), the key enzyme of the sterol synthesis pathway. Since yeast Saccharomyces cerevisiae harbours many counterparts of mammalian enzymes involved in lipid-synthesizing pathways, conclusions drawn from research with this single cell eukaryotic organism can be readily applied to higher eukaryotes. Using a yeast strain with deletions of both HMG1 and HMG2 genes (i.e. completely devoid of HMGR activity) with introduced wild-type or mutant form of human HMGR (hHMGR) gene we investigated the effects of statins on the lipid metabolism of the cell. The relative quantification of mRNA demonstrated a different effect of simvastatin on the expression of the wild-type and mutated hHMGR gene. GC/MS analyses showed a significant decrease of sterols and enhanced conversion of squalene and sterol precursors into ergosterol. This was accompanied by the mobilization of ergosterol precursors localized in lipid particles in the form of steryl esters visualized by confocal microscopy. Changes in the level of ergosterol and its precursors in cells treated with simvastatin depend on the mutation in the hHMGR gene. HPLC/MS analyses indicated a reduced level of phospholipids not connected with the mevalonic acid pathway. We detected two significant phenomena. First, cells treated with simvastatin develop an adaptive response compensating the lower activity of HMGR. This includes enhanced conversion of sterol precursors into ergosterol, mobilization of steryl esters and increased expression of the hHMGR gene. Second, statins cause a substantial drop in the level of glycerophospholipids

The Suppressor of AAC2 Lethality SAL1 Modulates Sensitivity of Heterologously Expressed Artemia ADP/ATP Carrier to Bongkrekate in Yeast

The ADP/ATP carrier protein (AAC) expressed in Artemia franciscana is refractory to bongkrekate. We generated two strains of Saccharomyces cerevisiae where AAC1 and AAC3 were inactivated and the AAC2 isoform was replaced with Artemia AAC containing a hemagglutinin tag (ArAAC-HA). In one of the strains the suppressor of ΔAAC2 lethality, SAL1, was also inactivated but a plasmid coding for yeast AAC2 was included, because the ArAACΔsal1Δ strain was lethal. In both strains ArAAC-HA was expressed and correctly localized to the mitochondria. Peptide sequencing of ArAAC expressed in Artemia and that expressed in the modified yeasts revealed identical amino acid sequences. The isolated mitochondria from both modified strains developed 85% of the membrane potential attained by mitochondria of control strains, and addition of ADP yielded bongkrekate-sensitive depolarizations implying acquired sensitivity of ArAAC-mediated adenine nucleotide exchange to this poison, independent from SAL1. However, growth of ArAAC-expressing yeasts in glycerol-containing media was arrested by bongkrekate only in the presence of SAL1. We conclude that the mitochondrial environment of yeasts relying on respiratory growth conferred sensitivity of ArAAC to bongkrekate in a SAL1-dependent manner. © 2013 Wysocka-Kapcinska et al

Perturbation of the yeast mitochondrial lipidome and associated membrane proteins following heterologous expression of Artemia-ANT

Heterologous expression is a landmark technique for studying a protein itself or its effect on the expression host, in which membrane-embedded proteins are a common choice. Yet, the impact of inserting a foreign protein to the lipid environment of host membranes, has never been addressed. Here we demonstrated that heterologous expression of the Artemia franciscana adenine nucleotide translocase (ANT) in yeasts altered lipidomic composition of their inner mitochondrial membranes. Along with this, activities of complex II, IV and ATP synthase, all membrane-embedded components, were significantly decreased while their expression levels remained unaffected. Although the results represent an individual case of expressing a crustacean protein in yeast inner mitochondrial membranes, it cannot be excluded that host lipidome alterations is a more widespread epiphenomenon, potentially biasing heterologous expression experiments. Finally, our results raise the possibility that not only lipids modulate protein function, but also membrane-embedded proteins modulate lipid composition, thus revealing a reciprocal mode of regulation for these two biomolecular entities

Alterations in voltage-sensing of the mitochondrial permeability transition pore in ANT1-deficient cells

The probability of mitochondrial permeability transition (mPT) pore opening is inversely related to the magnitude of the proton electrochemical gradient. The module conferring sensitivity of the pore to this gradient has not been identified. We investigated mPT's voltage-sensing properties elicited by calcimycin or H2O2 in human fibroblasts exhibiting partial or complete lack of ANT1 and in C2C12 myotubes with knocked-down ANT1 expression. mPT onset was assessed by measuring in situ mitochondrial volume using the 'thinness ratio' and the 'cobalt-calcein' technique. De-energization hastened calcimycin-induced swelling in control and partially-expressing ANT1 fibroblasts, but not in cells lacking ANT1, despite greater losses of mitochondrial membrane potential. Matrix Ca(2+) levels measured by X-rhod-1 or mitochondrially-targeted ratiometric biosensor 4mtD3cpv, or ADP-ATP exchange rates did not differ among cell types. ANT1-null fibroblasts were also resistant to H2O2-induced mitochondrial swelling. Permeabilized C2C12 myotubes with knocked-down ANT1 exhibited higher calcium uptake capacity and voltage-thresholds of mPT opening inferred from cytochrome c release, but intact cells showed no differences in calcimycin-induced onset of mPT, irrespective of energization and ANT1 expression, albeit the number of cells undergoing mPT increased less significantly upon chemically-induced hypoxia than control cells. We conclude that ANT1 confers sensitivity of the pore to the electrochemical gradient

Alterations in voltage-sensing of the mitochondrial permeability transition pore in ANT1-deficient cells

The probability of mitochondrial permeability transition (mPT) pore opening is inversely related to the magnitude of the proton electrochemical gradient. The module conferring sensitivity of the pore to this gradient has not been identified. We investigated mPT's voltage-sensing properties elicited by calcimycin or H2O2 in human fibroblasts exhibiting partial or complete lack of ANT1 and in C2C12 myotubes with knocked-down ANT1 expression. mPT onset was assessed by measuring in situ mitochondrial volume using the 'thinness ratio' and the 'cobalt-calcein' technique. De-energization hastened calcimycin-induced swelling in control and partially-expressing ANT1 fibroblasts, but not in cells lacking ANT1, despite greater losses of mitochondrial membrane potential. Matrix Ca(2+) levels measured by X-rhod-1 or mitochondrially-targeted ratiometric biosensor 4mtD3cpv, or ADP-ATP exchange rates did not differ among cell types. ANT1-null fibroblasts were also resistant to H2O2-induced mitochondrial swelling. Permeabilized C2C12 myotubes with knocked-down ANT1 exhibited higher calcium uptake capacity and voltage-thresholds of mPT opening inferred from cytochrome c release, but intact cells showed no differences in calcimycin-induced onset of mPT, irrespective of energization and ANT1 expression, albeit the number of cells undergoing mPT increased less significantly upon chemically-induced hypoxia than control cells. We conclude that ANT1 confers sensitivity of the pore to the electrochemical gradient

Strains used in this work.

<p>Strains used in this work.</p

Primary sequence coverage of <i>ArAAC</i> expressed in yeast and <i>ArAAC</i> expressed in <i>Artemia franciscana</i> as determined from all mass spectrometry experiments described in ‘Materials and Methods’.

<p>Sequence coverage was based on searching peptide tandem spectra against the <i>Artemia franciscana</i> protein from NCBI, as described in the text. A: Primary sequence coverage of <i>Artemia franciscana</i> adenine nucleotide translocator protein (gi|308390607) expressed in <i>Artemia franciscana</i>. 70% Sequence coverage was obtained by identifying 210/301 residues (green) in the protein. B: Primary sequence coverage of <i>Artemia franciscana</i> adenine nucleotide translocator protein (gi|308390607) expressed in <i>Saccharomyces cerevisiae</i> (strain MWY79/15). 62% Sequence coverage was obtained by identifying 188/301 residues (green).</p

Viability of modified and control yeast strains in response to BKA (1 M for the glucose-containing plates, and 20 nM for the glycerol-containing plates).

<p>Yeast strains were grown on media indicated in the panels in the absence or presence of BKA at 28°C. Each lane contains spots with initially 10⁶, 10⁵, 10⁴, 10³ and 10² cells (top to bottom). The genotypes of the strains are given in the table shown on the top of the figure: ‘no’ signifies absence of the gene indicated in the left-most column; ‘Yes’ signifies the presence of the gene indicated in the left-most column. 1 (MWY80) - <i>aac1Δ aac2Δ aac3Δ</i>, 2 (MWY85/9) - <i>sal1Δ</i>, 3 (MR6) – <i>WT</i>, 4 (RKY67-1C), 5 (RKY67-1D) – two independent clones <i>aac1Δ aac3Δ AAC2</i>, 6 (MWY79/15), 7 (MWY79/17) - two independent clones <i>aac1Δ aac3Δ ArAAC</i>, 8 (MWY84/4) - <i>aac1Δ aac3Δ AAC2 sal1Δ</i>, 9 (MWY83/5), 10 (MWY83/1) - two independent clones <i>aac1Δ aac3Δ ArAAC sal1Δ [AAC2].</i> Plates were scanned after 4 (glucose) or 6 (glycerol) days of incubation.</p

The ArAac protein expressed in yeasts is sensitive to BKA.

<p>In the table on top the yeast strains and their expression of adenine nucleotide carriers and <i>SAL1</i> is indicated; ‘no’ signifies absence of the gene indicated in the left-most column; ‘<b>Yes</b>’ signifies the presence of the gene indicated in the left-most column. In the panels, reconstructed time courses of rhodamine 123 fluorescence (expressed as % of m) as functions of time are shown. In the top three panels mitochondria from strains RKY67-1C (black and grey traces) and MWY79/15 (green and magenta traces) express <i>SAL1</i>. In the bottom three panels, mitochondria from strains MWY84/4 (black traces) and MWY83/5 (green traces) do not express <i>SAL1</i>. The pH of the media is indicated on the top of each panel. EtOH signifies ethanol.</p