Pierre U. Blier

Natural selection and the evolution of mtDNA-encoded peptides: evidence for intergenomic co-adaptation

Mitochondrial DNA (mtDNA) variation is an important tool for the investigation of the population genetics of animal species. Recently, recognition of the role of mtDNA mutations in human disease has spurred increasing interest in the function and evolution of mtDNA and the 13 polypeptides it encodes. These proteins interact with a large number of peptides encoded in the nucleus to form the mitochondrial electron transport system (ETS). As the ETS is the primary energy generation system in aerobic metazoans, natural selection would be expected to favor mutations that enhance ETS function. Such mutations could occur in either the mitochondrial or nuclear genes encoding ETS proteins and would lead to positive intergenomic interactions, or co-adaptation. Direct evidence for intergenomic co-adaptation comes from functional studies of systems where nuclear-mitochondrial DNA combinations vary naturally or can be manipulated experimentally.

Comparative Analysis of Gender-Associated Complete Mitochondrial Genomes in Marine Mussels (Mytilus spp.)

Marine mussels of the genus Mytilus have an unusual mode of mitochondrial DNA (mtDNA) transmission termed doubly uniparental inheritance (DUI). Female mussels are homoplasmic for the F mitotype, which is inherited maternally, while males are usually heteroplasmic, carrying a mixture of the maternal F mitotype and the paternally inherited M genome. Two classes of M genomes have been observed: “standard” M genomes and “recently masculinized” M genomes. The latter are more similar to F genomes at the sequence level but are transmitted paternally like standard M genomes. In this study we report the complete sequences of two standard male M. edulis and one recently masculinized male M. trossulus mitochondrial genome. A comparative analysis, including the previously sequenced M. edulis F and M. galloprovincialis F and M mtDNAs, reveals that these genomes are identical in gene order, but highly divergent in nucleotide and amino acid sequence. The large amount (>20%) of nucleotide substitutions that fall in coding regions implies that there are several amino acid replacements between the F and M genomes, which likely have an impact on the structural and functional properties of the mitochondrial proteome. Correlation of the divergence rate of different protein-coding genes indicates that mtDNA-encoded proteins of the M genome are still under selective constraints, although less highly than genes of the F genome. The mosaic F/M control region of the masculinized F genome provides evidence for lineage-specific sequences that may be responsible for the different mode of transmission genetics. This analysis shows the value of comparative genomics to better understand the mechanisms of maintenance and segregation of mtDNA sequence variants in mytilid mussels.

Pharmacological Blockade of 5-HT7 Receptors as a Putative Fast Acting Antidepressant Strategy

Current antidepressants still display unsatisfactory efficacy and a delayed onset of therapeutic action. Here we show that the pharmacological blockade of serotonin 7 (5-HT7) receptors produced a faster antidepressant-like response than the commonly prescribed antidepressant fluoxetine. In the rat, the selective 5-HT7 receptor antagonist SB-269970 counteracted the anxiogenic-like effect of fluoxetine in the open field and exerted an antidepressant-like effect in the forced swim test. In vivo, 5-HT7 receptors negatively regulate the firing activity of dorsal raphe 5-HT neurons and become desensitized after long-term administration of fluoxetine. In contrast with fluoxetine, a 1-week treatment with SB-269970 did not alter 5-HT firing activity but desensitized cell body 5-HT autoreceptors, enhanced the hippocampal cell proliferation, and counteracted the depressive-like behavior in olfactory bulbectomized rats. Finally, unlike fluoxetine, early-life administration of SB-269970, did not induce anxious/depressive-like behaviors in adulthood. Together, these findings indicate that the 5-HT7 receptor antagonists may represent a new class of antidepressants with faster therapeutic action.

Comparative Mitochondrial Genomics of Freshwater Mussels (Bivalvia: Unionoida) With Doubly Uniparental Inheritance of mtDNA: Gender-Specific Open Reading Frames and Putative Origins of Replication

Doubly uniparental inheritance (DUI) of mitochondrial DNA in marine mussels (Mytiloida), freshwater mussels (Unionoida), and marine clams (Veneroida) is the only known exception to the general rule of strict maternal transmission of mtDNA in animals. DUI is characterized by the presence of gender-associated mitochondrial DNA lineages that are inherited through males (male-transmitted or M types) or females (female-transmitted or F types), respectively. This unusual system constitutes an excellent model for studying basic aspects of mitochondrial DNA inheritance and the evolution of mtDNA genomes in general. Here we compare published mitochondrial genomes of unionoid bivalve species with DUI, with an emphasis on characterizing unassigned regions, to identify regions of the F and M mtDNA genomes that could (i) play a role in replication or transcription of the mtDNA molecule and/or (ii) determine whether a genome will be transmitted via the female or the male gamete. Our results reveal the presence of one F-specific and one M-specific open reading frames (ORFs), and we hypothesize that they play a role in the transmission and/or gender-specific adaptive functions of the M and F mtDNA genomes in unionoid bivalves. Three major unassigned regions shared among all F and M unionoid genomes have also been identified, and our results indicate that (i) two of them are potential heavy-strand control regions (OH) for regulating replication and/or transcription and that (ii) multiple and potentially bidirectional light-strand origins of replication (OL) are present in unionoid F and M mitochondrial genomes. We propose that unassigned regions are the most promising candidate sequences in which to find regulatory and/or gender-specific sequences that could determine whether a mitochondrial genome will be maternally or paternally transmitted.

Bioenergetic and genetic parameters in relation to susceptibility of blue mussels, Mytilus edulis (L.) to summer mortality

Abstract Our study examined whether the differences in susceptibility to summer mass mortality of two stocks of mussels from the Magdalen Islands (Quebec, Canada) are related to bioenergetic and/or genetic factors. The relative importance of maintenance and maximal metabolic rates, scope for growth (SFG) and the O:N ratio were followed over time to assess whether the increased incidence of mortality in late summer reflects a decrease in bioenergetic status at this period. The stock of mussels which was more susceptible to summer mortality had higher values of V O 2 . Furthermore this stock had a more negative scope for growth and lower O/N ratio in early August. These parameters are likely to reflect unfavourable environmental conditions, which led the mussels to rely upon protein catabolism. We also observed a negative correlation between multiple-locus heterozygosity and standard V O 2 . The more susceptible stock of mussels had a lower degree of multiple-locus heterozygosity. Thus, we suggest that the periodic, but irregular, outbreaks of summer mortality are the result of a synergistic interaction involving dietary deficiencies, temperature, a possible post-spawning stress and the genetic characteristics of the stock. The higher metabolic demand associated with a reduced degree of heterozygosity will impose a supplementary stress and render such stocks more vulnerable to summer mortality. The results are in agreement with the hypothesis that high levels of heterozygosity are related with lower costs of maintenance.

Sustained administration of pramipexole modifies the spontaneous firing of dopamine, norepinephrine, and serotonin neurons in the rat brain.

Pramipexole (PPX) is a D2/D3 receptor agonist that has been shown to be effective in the treatment of depression. Serotonin (5-HT), norepinephrine (NE) and dopamine (DA) systems are known to be involved in the pathophysiology and treatment of depression. Due to reciprocal interactions between these neuronal systems, drugs selectively targeting one system-specific receptor can indirectly modify the firing activity of neurons that contribute to firing patterns in systems that operate via different neurotransmitters. It was thus hypothesized that PPX would alter the firing rate of DA, NE and 5-HT neurons. To test this hypothesis, electrophysiological experiments were carried out in anesthetized rats. Subcutaneously implanted osmotic minipumps delivered PPX at a dose of 1 mg/kg per day for 2 or 14 days. After a 2-day treatment with PPX the spontaneous neuronal firing of DA neurons was decreased by 40%, NE neuronal firing by 33% and the firing rate of 5-HT neurons remained unaltered. After 14 days of PPX treatment, the firing rate of DA had recovered as well as that of NE, whereas the firing rate of 5-HT neurons was increased by 38%. It was also observed that sustained PPX administration produced desensitization of D2/D3 and 5-HT1A cell body autoreceptors, as well as a decrease in sensitivity of α2-adrenergic cell body autoreceptors. These adaptive changes are implicated in long-term firing rate adaptations of DA, NE and 5-HT neurons after prolonged PPX administration. In conclusion, the therapeutic action of PPX in depression might be attributed to increased DA and 5-HT neurotransmission.

Mitochondrial whims: metabolic rate, longevity and the rate of molecular evolution

The evolutionary rate of mitochondrial DNA (mtDNA) is highly variable across lineages in animals, and particularly in mammals. This variation has been interpreted as reflecting variations in metabolic rate: mitochondrial respiratory activity would tend to generate mutagenic agents, thus increasing the mutation rate. Here we review recent evidence suggesting that a direct, mechanical effect of species metabolic rate on mtDNA evolutionary rate is unlikely. We suggest that natural selection could act to reduce the (somatic) mtDNA mutation rate in long-lived species, in agreement with the mitochondrial theory of ageing.

Is the growth rate of fish set by digestive enzymes or metabolic capacity of the tissues? Insight from transgenic coho salmon

The aim of this study was to compare metabolic and digestive enzyme activities in fish with different growth capacities using growth hormone transgenic and nontransgenic coho salmon (Oncorhynchus kisutch) as a model system. The following enzyme activities were measured: trypsin and chymotrypsin in pyloric caeca, alkaline phosphatase in intestine, pyruvate kinase (PK), lactate dehydrogenase (LDH) and citrate synthase (CS) in gills and white muscle. Transgenic salmon showed higher LDH activity in muscle and PK activities in muscle and gills, whereas chymotrypsin activity in pyloric caeca was higher in the control group. No significant differences were observed between transgenic and control groups for CS, trypsin and alkaline phosphatase activities. The small or no differences observed in this study suggests that the enzyme activities measured were not related to the high growth rate of the transgenic coho salmon.

NATURALLY OCCURRING MITOCHONDRIAL DNA HAPLOTYPES EXHIBIT METABOLIC DIFFERENCES: INSIGHT INTO FUNCTIONAL PROPERTIES OF MITOCHONDRIA

Linking the mitochondrial genotype and the organismal phenotype is of paramount importance in evolution of mitochondria. In this study, we determined the differences in catalytic properties of mitochondria dictated by divergences in the siII and siIII haplogroups of Drosophila simulans using introgressions of siII mtDNA type into the siIII nuclear background. We used a novel in situ method (permeabilized fibers) that allowed us to accurately measure the consumption of oxygen by mitochondria in constructed siII‐introgressed flies and in siIII‐control flies. Our results showed that the catalytic capacity of the electron transport system is not impaired by introgressions, suggesting that the functional properties of mitochondria are tightly related to the mtDNA haplogroup and not to the nuclear DNA or to the mito‐nuclear interactions. This is the first study, to our knowledge, that demonstrates a naturally occurring haplogroup can confer specific functional differences in aspects of mitochondrial metabolism. This study illustrates the importance of mtDNA changes on organelle evolution and highlights the potential bioenergetic and metabolic impacts that divergent mitochondrial haplogroups may have upon a wide variety of species including humans.

The impact of the thermal sensitivity of cytochrome c oxidase on the respiration rate of Arctic charr red muscle mitochondria

Abstract. To assess if cytochrome c oxidase could determine the response of mitochondrial respiration to changes in environmental temperature in ectotherms, we performed KCN titration of the respiration rate and cytochrome c oxidase activity in mitochondria from Arctic charr (Salvelinus fontinalis) muscle at four different temperatures (1°C, 6°C, 12°C, and 18°C). Our data showed an excess of cytochrome c oxidase activity over the mitochondrial state 3 respiration rate. Mitochondrial oxygen consumption rates reached approximately 12% of the cytochrome c oxidase maximal capacity at every temperature. Also, following titration, the mitochondrial respiration rate significantly decreased when KCN reached concentrations that inhibit almost 90% of the cytochrome c oxidase activity. This strongly supports the idea that the thermal sensitivity of the maximal mitochondrial respiration rate cannot be dictated by the effect of temperature on cytochrome c oxidase catalytic capacity. Furthermore, the strong similarity of the Q10s of mitochondrial respiration and cytochrome c oxidase activity suggests a functional or structural link between the two. The functional link could be coevolution of parts of the mitochondrial system to maintain optimal functions in most of the temperature range encountered by organisms.