Intracellular Proprotein convertase subtilisin/kexin type 9: Recruitment and regulatory role in mitochondrial architecture and bioenergetic

Objective

Circulating Proprotein convertase subtilisin/kexin type 9 (PCSK9) captured the scientific community's attention as powerful target to fight against high serum cholesterol and its associated adverse consequences. Despite the well-described role of PCSK9 in impairing the recycling of low-density lipoprotein receptor (LDLR) on hepatocytes, its intracellular importance in metabolism modulations remain largely unexplored. Here, we describe a novel intracellular function of PCSK9 in hepatocyte metabolism and in liver regeneration.

Methods

A surgical model of partial hepatectomy (PH) was used to induce liver cell proliferation and serological and histological lipo- and glyco-metabolic perturbations were investigated in PCSK9 knockout and wild-type mice before and after PH. Mitochondrial bioenergetic level, compartmental architecture as well as intracellular localization and function of PCSK9 were further analyzed in vitro on murine primary isolated hepatocytes and PCSK9 lentiviral knockdown human liver cancer cell lines.

Results

Loss of PCSK9 expression led to a basal higher mitochondrial bioenergetic status in hepatocytes, characterized by a profound remodeling of mitochondrial architecture. Livers from PCSK9 knockout mice displayed a diffuse oxidative stress and a significant upregulation of cellular detoxification and superoxide radicals removal genes induced by the activation of c-Jun N-terminal kinase (JNK) pathway before PH. PCSK9 deficient mice showed a primed compensatory hepatic hyperplasia after PH, accompanied by an earlier transient regeneration-associated steatosis due to a higher LDLR expression and cholesterol uptake, with an increased β-oxidation, gluconeogenesis and glycolysis rate. Tetramethylrhodamine ethyl ester staining of isolated hepatocytes and liver cancer PCSK9 knockdown clones revealed a significantly reduced mitochondrial membrane potential when compared to controls. Confocal live-cell imaging and proteins immunoprecipitation showed respectively a mitochondrial recruitment of PCSK9 in proliferating cells and its interaction with mitochondrial membrane carriers and chaperons.

Conclusion

Despite the role played by circulating PCSK9 in regulating systemic cholesterol levels, our data shed light on its intracellular impact on mitochondrial architecture, membrane polarization and related metabolic perturbations in compartmental bioenergetics.

Colonization pattern of the biocontrol strain Pseudomonas chlororaphis MA 342 on barley seeds visualized by using green fluorescent protein

Pseudomonas chlororaphis MA 342 is a potent biocontrol agent that can be used against several seed-borne diseases of cereal crops, including net blotch of barley caused by the fungus Drechslera teres. In this study, strain MA 342 was tagged with the gfp gene (encoding the green fluorescent protein) in order to study the fate of cells after seed inoculation. The gfp-tagged strain, MA 342G2, had the same biocontrol efficacy as the wild type when it was applied at high cell concentrations to seeds but was less effective at lower cell concentrations. By comparing cell counts determined by microscopy to the number of CFU, we found that the number of culturable cells was significantly lower than the total number of bacteria on seeds which were inoculated and dried for 20 h. Confocal microscopy and epifluorescence stereomicroscopy were used to determine the pattern of MA 342G2 colonization and cell aggregation on barley seeds. Immediately after inoculation of seeds, bacteria were found mainly under the seed glume, and there was no particular aggregation pattern. However, after the seeds were sown, irregularly distributed areas of bacterial aggregation were found, which reflected epiphytic colonization of glume cells. There was a trend towards bacterial aggregation near the embryo but never within the embryo. Bacterial aggregates were regularly found in the groove of each seed formed by the base of the coleoptile and the scutellum. Based on these results, we suggest that MA 342 colocalizes with the pathogen D. teres, which facilitates the action of the fungistatic compound(s) produced by this strain.

Simultaneous monitoring of cell number and metabolic activity of specific bacterial populations with a dual gfp-luxAB marker system

A dual marker system was developed for simultaneous quantification of bacterial cell numbers and their activity with the luxAB and gfp genes, encoding bacterial luciferase and green fluorescent protein (GFP), respectively. The bioluminescence phenotype of the luxAB biomarker is dependent on cellular energy status. Since cellular metabolism requires energy, bioluminescence output is directly related to the metabolic activity of the cells. By contrast, GFP fluorescence has no energy requirement. Therefore, by combining these two biomarkers, total cell number and metabolic activity of a specific marked cell population could be monitored simultaneously. Two different bacterial strains, Escherichia coli DH5alpha and Pseudomonas fluorescens SBW25, were chromosomally tagged with the dual marker cassette, and the cells were monitored under different conditions by flow cytometry, plate counting, and luminometry. During log-phase growth, the luciferase activity was proportional to the number of GFP-fluorescent cells and culturable cells. Upon entrance into stationary phase or during starvation, luciferase activity decreased due to a decrease in cellular metabolic activity of the population, but the number of GFP-fluorescing cells and culturable cells remained relatively stable. In addition, we optimized a procedure for extraction of bacterial cells from soil, allowing GFP-tagged bacteria in soil samples to be quantitated by flow cytometry. After 30 days of incubation of P. fluorescens SBW25::gfp/lux in soil, the cells were still maintained at high population densities, as determined by GFP fluorescence, but there was a slow decline in luciferase activity, implicating nutrient limitation. In conclusion, the dual marker system allowed simultaneous monitoring of the metabolic activity and cell number of a specific bacterial population and is a promising tool for monitoring of specific bacteria in situ in environmental samples.

Poly-beta-hydroxybutyrate (PHB) biosynthetic genes in Rhizobium meliloti 41

Genes encoding beta-ketothiolase (phaA), acetoacetyl-CoA reductase (phaB) and PHB-synthase (phaC) from R. meliloti 41, together with a fourth gene, referred to as ORF1, presumed to be involved in PHB biosynthesis, have been cloned and sequenced. phaA, phaB and ORF1 were identified by heterologous hybridization on a cosmid library, while phaC was isolated by cloning the transposon-tagged fragment from a R. meliloti PHB- Tn5 mutant. phaA and phaB were functionally expressed in Escherichia coli while phaC was able to complement a PHB- strain of R. meliloti 41. The three genes were sufficient to direct the production of polyhydroxyalkanoate in E. coli. The homology of ORF1 with an ORF located near the PHB genes in two phototrophic bacteria suggests its involvement in PHB synthesis.