Localization of mitochondrial DNA encoded cytochrome c oxidase subunits I and II in rat pancreatic zymogen granules and pituitary growth hormone granules

Androgen receptor signaling-mitochondrial DNA-oxidative phosphorylation: A critical triangle in early prostate cancer

Mitochondria are more than just the cellular powerhouse. They also play key roles in vital functions such as apoptosis, metabolism regulation, and other intracellular interactions. The mitochondrial DNA (mtDNA) encodes for 12 subunits of the oxidative phosphorylation (OXPHOS) system. Depletion of mtDNA in androgen-dependent prostate cancer (PCa) cell lines renders them androgen-independent and more aggressive. Paradoxically, pharmaceutical inhibition of OXPHOS is lethal for subsets of PCa cells, whereas others become dependent on androgen receptor (AR) signaling for survival. Given that the AR-mitochondria interaction is critical for early PCa, it is crucial to understand the details of this interaction. Technical hurdles have made mitochondria traditionally difficult to study, with many techniques used for isolation masking the properties of given individual mitochondria. Although the isolation of mitochondria enables us to study OXPHOS, we miss the context in which mitochondria interact with the rest of the cell. Both AR signaling and mtDNA affect apoptosis, metabolism regulation, cellular calcium storage and homeostasis, intracellular calcium signaling, and redox homeostasis. In this review, we will attempt to understand how the crosstalk between AR-mtDNA-OXPHOS is responsible for "life or death" decisions inside the cells. Our aim is to point toward potential vulnerabilities that can lead to the discovery of novel therapeutic targets.

Brain metabolic DNA: recent evidence for a mitochondrial connection

This review highlights recent data concerning the synthesis of brain metabolic DNA (BMD) by cytoplasmic reverse transcription and the prompt acquisition of the double-stranded configuration that allows its partial transfer to nuclei. BMD prevails in the mitochondrial fraction and is present in presynaptic regions and astroglial processes where it undergoes a turnover lasting a few weeks. Additional data demonstrate that BMD sequences are modified by learning, thus indicating that the modified synaptic activity allowing proper brain responses is encoded in learning BMD. In addition, several converging observations regarding the origin of BMD strongly suggest that BMD is reverse transcribed by mitochondrial telomerase.

Two types of rat pituitary somatotrophs secrete growth hormone with different biological and immunological profiles

OBJECTIVE

Two stable subpopulations of somatotrophs reside in the rat pituitary gland. We tested the hypothesis that one produced growth hormone (GH) with greater activity when tested in the tibial line bioassay (BGH) than the other, while differences in the activities between the two groups would be less dramatic when measured by immunoassay (IGH).

DESIGN

A series of studies using hypophysectomized rats, hollow fibers, treatments and culture models were used to differentiate differences in Type I and Type II anterior pituitary somatotrophs in both function and production of immunoactive and bioactive growth hormone.

RESULTS

We found that dense, Type II somatotrophs (>1.070g·cm^-3) differed markedly in their secretion patterns of IGH vs BGH in different In vitro and in vivo tests. In culture, Type II cells secreted five times as much BGH, and three fourths as much IGH as the less dense Type I cells. Production (storage and secretion) of BGH was 7-fold greater by Type II cells whereas IGH production was identical for the two cell types. Implantation of Type II cells into hypophysectomized rats significantly increased body weight, epiphyseal cartilage thickness, and muscle weight of the recipients; in contrast, Type I cells elicited only a small increase in body weight. Type I somatotrophs isolated from rats which had been previously fasted or insulin-treated subsequently showed only small, inconsistent changes in release relative to that from cells in the unfractionated cell population. However, release of BGH from the Type II cells was markedly decreased.

CONCLUSION

Both IGH and BGH should be considered in the elucidation of GH physiology.

Isolation of Endoplasmic Reticulum, Mitochondria, and Mitochondria-Associated Membrane and Detergent Resistant Membrane Fractions from Transfected Cells and from Human Cytomegalovirus-Infected Primary Fibroblasts

Increasingly mechanistic virology studies require dependable and sensitive methods for isolating purified organelles containing functional cellular sub-domains. The mitochondrial network is, in part, closely apposed to the endoplasmic reticulum (ER). The mitochondria-associated membrane (MAM) fraction provides direct physical contact between the ER and mitochondria. Characterization of the dual localization and trafficking of human cytomegalovirus (HCMV) UL37 proteins required establishing protocols in which the ER and mitochondria could be reliably separated. Because of its documented role in lipid and ceramide transfer from the ER to mitochondria, a method to purify MAM from infected cells was also developed. Two robust procedures were developed to efficiently isolate mitochondria, ER, and MAM fractions while providing substantial protein yields from HCMV-infected primary fibroblasts and from transfected HeLa cells. Furthermore, this unit includes protocols for isolation of detergent resistant membranes from subcellular fractions as well as techniques that allow visualization of the mitochondrial network disruption that occurs in permissively infected cells by their optimal resolution in Percoll gradients.

Polymorphisms in mitochondrial genes encoding complex I subunits are maternal factors of voluntary alcohol consumption in the rat

OBJECTIVE

Alcohol is detoxified in the liver by oxidizing enzymes that require nicotinamide adenine dinucleotide (NAD+) such that, in the rat, the availability of NAD+ contributes to control voluntary ethanol intake. The UChA and UChB lines of Wistar rats drink low and high amounts of ethanol respectively and differ in the capacity of their mitochondria to oxidize NADH into NAD+. This function resides in complex I of the respiratory chain and its variation is linked to genes transmitted through the maternal line. The aim of this study was to identify the genetic basis for the difference in the reoxidation of NADH in these nondrinker (UChA) and drinker (UChB) rats.

METHODS

Seven mitochondrial genes and two chromosome X genes encoding complex I subunits from rats of both lineages were amplified from liver DNA and sequenced.

RESULTS

The UChA and UChB rat lines differ in their Nd2, Nd4, Nd5 and Nd6 mitochondrial genes and in the encoded proteins. Most noteworthy are ND2 and ND4 whose amino acid variations lead to changes in three-dimensional structure models. The ND2 proteins also differ in the number of predicted transmembrane domains. The Nd1 and Nd3 genes have silent substitutions, whereas Nd4L and the exonic sequences of the nuclear genes Ndufa1 and Ndufb11 show no differences between the UChA and UChB lines.

CONCLUSION

Amino acid variations in four complex I subunits encoded in the mitochondrial genome may contribute to explain the differences between UChA and UChB rats in their capacity to reoxidize NADH and in their alcohol intake, suggesting that mitochondrial genes may constitute maternal factors of alcoholism.

Modulation of rat pituitary growth hormone by 670 nm light

In rat pituitary somatotrophs, cytochrome oxidase is co-packaged with growth hormone (GH) in some storage granules. Because this enzyme is thought to be the molecular photoacceptor of red-near infrared light, and because exposure of diverse tissue systems to 670 nm visible light affects their biological responses (e.g., wound healing), we tested the idea that exposure of rat pituitary cells, rat hemi-pituitary glands and rat pituitary homogenates to 670 nm light in vitro might alter GH storage and/or release. In this report we offer evidence to show that light treatment (670 nm, 80s, intensity 50 mW/cm(2), energy density 4 J/cm(2)) up-regulates GH release, in part by breakdown of intracellular, oligomeric GH as determined by gel filtration chromatography.

Isolation of endoplasmic reticulum, mitochondria, and mitochondria-associated membrane fractions from transfected cells and from human cytomegalovirus-infected primary fibroblasts

Increasingly mechanistic virology studies require dependable and sensitive methods for isolating purified organelles containing functional cellular sub-domains. The mitochondrial network is, in part, closely apposed to the endoplasmic reticulum (ER). The mitochondria-associated membrane (MAM) fraction provides direct physical contact between the ER and mitochondria. Characterization of the dual localization and trafficking of human cytomegalovirus (HCMV) UL37 proteins required establishing protocols in which the ER and mitochondria could be reliably separated. Because of its documented role in lipid and ceramide transfer from the ER to mitochondria, a method to purify MAM from infected cells was also developed. Two robust procedures were developed to efficiently isolate mitochondria, ER, and MAM fractions while providing the substantial protein yields from HCMV-infected primary fibroblasts and from transfected HeLa cells. Moreover, this unit includes a protocol that allows visualization of the mitochondria network disruption that occurs in permissively infected cells by their optimal resolution in Percoll gradients.

Mitochondrial DNA heteroplasmy in ovine fetuses and sheep cloned by somatic cell nuclear transfer

Background

The mitochondrial DNA (mtDNA) of the cloned sheep "Dolly" and nine other ovine clones produced by somatic cell nuclear transfer (SCNT) was reported to consist only of recipient oocyte mtDNA without any detectable mtDNA contribution from the nucleus donor cell. In cattle, mouse and pig several or most of the clones showed transmission of nuclear donor mtDNA resulting in mitochondrial heteroplasmy. To clarify the discrepant transmission pattern of donor mtDNA in sheep clones we analysed the mtDNA composition of seven fetuses and five lambs cloned from fetal fibroblasts.

Results

The three fetal fibroblast donor cells used for SCNT harboured low mtDNA copy numbers per cell (A: 753 ± 54, B: 292 ± 33 and C: 561 ± 88). The ratio of donor to recipient oocyte mtDNAs was determined using a quantitative amplification refractory mutation system (ARMS) PCR (i.e. ARMS-qPCR). For quantification of SNP variants with frequencies below 0.1% we developed a restriction endonuclease-mediated selective quantitative PCR (REMS-qPCR). We report the first cases (n = 4 fetuses, n = 3 lambs) of recipient oocyte/nuclear donor mtDNA heteroplasmy in SCNT-derived ovine clones demonstrating that there is no species-effect hindering ovine nucleus-donor mtDNA from being transmitted to the somatic clonal offspring. Most of the heteroplasmic clones exhibited low-level heteroplasmy (0.1% to 0.9%, n = 6) indicating neutral transmission of parental mtDNAs. High-level heteroplasmy (6.8% to 46.5%) was observed in one case. This clone possessed a divergent recipient oocyte-derived mtDNA genotype with three rare amino acid changes compared to the donor including one substitution at an evolutionary conserved site.

Conclusion

Our study using state-of-the-art techniques for mtDNA quantification, like ARMS-qPCR and the novel REMS-qPCR, documents for the first time the transmission of donor mtDNA into somatic sheep clones. MtDNA heteroplasmy was detected in seven of 12 clones tested, whereby all but one case revealed less than 1% mtDNA contribution from the nuclear donor cell suggesting neutral segregation.

Reproductive function for a C-terminus extended, male-transmitted cytochrome c oxidase subunit II protein expressed in both spermatozoa and eggs

Our previous study documented expression of a male-transmitted cytochrome c oxidase subunit II protein (MCOX2), with a C-terminus extension (MCOX2e), in unionoidean bivalve testes and sperm mitochondria. Here, we present evidence demonstrating that MCOX2 is seasonally expressed in testis, with a peak shortly before fertilization that is independent of sperm density. MCOX2 is localized to the inner and outer sperm mitochondrial membranes and the MCOX2 antibody's epitope is conserved across >65 million years of evolution. We also demonstrate the presence of male-transmitted mtDNA and season-specific MCOX2 spatial variation in ovaries. We hypothesize that MCOX2 plays a role in reproduction through gamete maturation, fertilization and/or embryogenesis.

Subcellular localization of fumarase in mammalian cells and tissues

Fumarase, a mitochondrial matrix protein, is previously indicated to be present in substantial amounts in the cytosol as well. However, recent studies show that newly synthesized human fumarase is efficiently imported into mitochondria with no detectable amount in the cytosol. To clarify its subcellular localization, the subcellular distribution of fumarase in mammalian cells/tissues was examined by a number of different methods. Cell fractionation using either a mitochondria fraction kit or extraction with low concentrations of digitonin, detected no fumarase in a 100,000 g supernatant fraction. Immunofluorescence labeling with an affinity-purified antibody to fumarase and an antibody to the mitochondrial Hsp60 protein showed identical labeling pattern with labeling seen mainly in mitochondria. Detailed studies were performed using high-resolution immunogold electron microscopy to determine the subcellular localization of fumarase in rat tissues, embedded in LR White resin. In thin sections from kidney, liver, heart, adrenal gland and anterior pituitary, strong and specific labeling due to fumarase antibody was only detected in mitochondria. However, in the pancreatic acinar cells, in addition to mitochondria, highly significant labeling was also observed in the zymogen granules and endoplasmic reticulum. The observed labeling in all cases was completely abolished upon omission of the primary antibody indicating that it was specific. In a western blot of purified zymogen granules, a fumarase-antibody cross-reactive protein of the same molecular mass as seen in the mitochondria was present. These results provide evidence that fumarase in mammalian cells/tissues is mainly localized in mitochondria and significant amounts of this protein are not present in the cytosol. However, these studies also reveal that in certain tissues, in addition to mitochondria, this protein is also present at specific extramitochondrial sites. Although the cellular function of fumarase at these extramitochondrial locations is not known, the appearance/localization of fumarase outside mitochondria may help explain how mutations in this mitochondrial protein can give rise to a number of different types of cancers.

Mitochondrial import of human and yeast fumarase in live mammalian cells: Retrograde translocation of the yeast enzyme is mainly caused by its poor targeting sequence

Studies on yeast fumarase provide the main evidence for dual localization of a protein in mitochondria and cytosol by means of retrograde translocation. We have examined the subcellular targeting of yeast and human fumarase in live cells to identify factors responsible for this. The cDNAs for mature yeast or human fumarase were fused to the gene for enhanced green fluorescent protein (eGFP) and they contained, at their N-terminus, a mitochondrial targeting sequence (MTS) derived from either yeast fumarase, human fumarase, or cytochrome c oxidase subunit VIII (COX) protein. Two nuclear localization sequences (2x NLS) were also added to these constructs to facilitate detection of any cytosolic protein by its targeting to nucleus. In Cos-1 cells transfected with these constructs, human fumarase with either the native or COX MTSs was detected exclusively in mitochondria in >98% of the cells, while the remainder 1-2% of the cells showed varying amounts of nuclear labeling. In contrast, when human fumarase was fused to the yeast MTS, >50% of the cells showed nuclear labeling. Similar studies with yeast fumarase showed that with its native MTS, nuclear labeling was seen in 80-85% of the cells, but upon fusion to either human or COX MTS, nuclear labeling was observed in only 10-15% of the cells. These results provide evidence that extramitochondrial presence of yeast fumarase is mainly caused by the poor mitochondrial targeting characteristics of its MTS (but also affected by its primary sequence), and that the retrograde translocation mechanism does not play a significant role in the extramitochondrial presence of mammalian fumarase.