Molecular characterisation and expression of Atlantic cod (Gadus morhua) myoglobin from two populations held at two different acclimation temperatures

Integrated studies of organismal plasticity through physiological and transcriptomic approaches: examples from marine polar regions

Transcriptomic methods are now widely used in functional genomic research. The vast amount of information received from these studies comes along with the challenge of developing a precise picture of the functional consequences and the characteristic regulatory mechanisms. Here we assess recent studies in marine species and their adaptation to polar (and seasonal) cold and explore how they have been able to draw reliable conclusions from transcriptomic patterns on functional consequences in the organisms. Our analysis indicates that the interpretation of transcriptomic data suffers from insufficient understanding of the consequences for whole organism performance and fitness and comes with the risk of supporting only preliminary and superficial statements.We propose that the functional understanding of transcriptomic data may be improved by their tighter integration into overarching physiological concepts that support the more specific interpretation of the 'omics' data and, at the same time, can be developed further through embedding the transcriptomic phenomena observed. Such possibilities have not been fully exploited.In the context of thermal adaptation and limitation, we explore preliminary evidence that the concept of oxygen and capacity limited thermal tolerance (OCLTT) may provide sufficient complexity to guide the integration of such data and the development of associated functional hypotheses. At the same time, we identify a lack of methodological approaches linking genes and function to higher levels of integration, in terms of organism and ecosystem functioning, at temporal and geographical scales, to support more reliable conclusions and be predictive with respect to the effects of global changes.

Nucleotide variation and balancing selection at the Ckma gene in Atlantic cod: analysis with multiple merger coalescent models

High-fecundity organisms, such as Atlantic cod, can withstand substantial natural selection and the entailing genetic load of replacing alleles at a number of loci due to their excess reproductive capacity. High-fecundity organisms may reproduce by sweepstakes leading to highly skewed heavy-tailed offspring distribution. Under such reproduction the Kingman coalescent of binary mergers breaks down and models of multiple merger coalescent are more appropriate. Here we study nucleotide variation at the Ckma (Creatine Kinase Muscle type A) gene in Atlantic cod. The gene shows extreme differentiation between the North (Canada, Greenland, Iceland, Norway, Barents Sea) and the South (Faroe Islands, North-, Baltic-, Celtic-, and Irish Seas) with F_ST > 0.8 between regions whereas neutral loci show no differentiation. This is evidence of natural selection. The protein sequence is conserved by purifying selection whereas silent and non-coding sites show extreme differentiation. The unfolded site-frequency spectrum has three modes, a mode at singleton sites and two high frequency modes at opposite frequencies representing divergent branches of the gene genealogy that is evidence for balancing selection. Analysis with multiple-merger coalescent models can account for the high frequency of singleton sites and indicate reproductive sweepstakes. Coalescent time scales vary with population size and with the inverse of variance in offspring number. Parameter estimates using multiple-merger coalescent models show that times scales are faster than under the Kingman coalescent.

Molecular characterization, phylogenetic analysis and expression profiling of myoglobin and cytoglobin genes in response to heat stress in channel catfish Ictalurus punctatus

To understand the function of myoglobin (Mb) and cytoglobin (Cygb) in channel catfish Ictalurus punctatus in response to heat stress, mb and cygb genes were identified and characterized in this study. These genes were widely expressed in all the tested tissues, but strong tissue preferences were observed, with the mb gene being expressed most highly in the heart, cygb1 most highly expressed in the intestine and cygb2 most highly expressed in the brain. After heat-stress challenge, mb and cygb genes were up-regulated in almost all tested tissues. In general, such up-regulation was more dramatic in the tolerant group than in the intolerant group, suggesting that higher expression of mb and cygb genes contributed to greater tolerance of I. punctatus to heat stress.

Expression patterns and adaptive functional diversity of vertebrate myoglobins

Recent years have witnessed a new round of research on one of the most studied proteins - myoglobin (Mb), the oxygen (O2) carrier of skeletal and heart muscle. Two major discoveries have stimulated research in this field: 1) that Mb has additional protecting functions, such as the regulation of in vivo levels of the signaling molecule nitric oxide (NO) by scavenging and generating NO during normoxia and hypoxia, respectively; and 2) that Mb in vertebrates (particularly fish) is expressed as tissue-specific isoforms in other tissues than heart and skeletal muscle, such as vessel endothelium, liver and brain, as found in cyprinid fish. Furthermore, Mb has also been found to protect against oxidative stress after hypoxia and reoxygenation and to undergo allosteric, O2-linked S-nitrosation, as in rainbow trout. Overall, the emerging evidence, particularly from fish species, indicates that Mb fulfills a broader array of physiological functions in a wider range of different tissues than hitherto appreciated. This new knowledge helps to better understand how variations in Mb structure and function may correlate with differences in animals' lifestyles and hypoxia-tolerance. This review integrates old and new results on Mb expression patterns and functional properties amongst vertebrates and discusses how these may relate to adaptive variations in different species. This article is part of a special issue entitled: Oxygen Binding and Sensing Proteins.

Site E14 in hemoglobins and myoglobins: a key residue that affects hemin loss and lipid oxidation capacity

Fish hemoglobins (Hbs) frequently contain glycine at site E14 while mammalian Hbs contain larger residues (e.g., alanine and serine). These differences were examined by creating structural variants at E14 using recombinant bovine myoglobin (Mb) as a model heme protein that contains alanine at E14. The Ala(E14)Gly mutation increased k(ox) and hemin loss 3-fold and 45-fold, respectively. Glycine at E14 creates a channel for solvent to enter the heme crevice, which enhances autoxidation and hemin loss rates. Hydration of the proximal heme pocket facilitates hemin loss because protonation of the proximal histidine weakens the linkage of the imidazole group to the iron atom of the hemin moiety. Ala(E14)Gly promoted lipid oxidation in washed fish muscle more rapidly during iced storage compared to wild type Mb at pH 5.7. This suggested that the rapid hemin loss from Ala(E14)Gly accelerated lipid oxidation. Ala(E14)Ser and Ala(E14)Val had little effect on k(ox) but somewhat accelerated net hemin loss. These studies suggest that enhanced access of solvent to the heme crevice of many fish Hbs at site E14 facilitates rapid hemin loss and moderately accelerates autoxidation. This likely is part of the reason fish Hbs promote lipid oxidation much more effectively compared to mammalian Hbs.

Thermal physiology of warm-spring colonists: variation among lake chub (Cyprinidae: Couesius plumbeus) populations

In northern Canada, lake chub (Cyprinidae: Couesius plumbeus) have colonized a variety of thermal springs that differ substantially from the ancestral environment in both mean temperature and thermal variation. To examine whether this environmental change is associated with differences in physiological traits, we compared the thermal breadth, capacity for acclimation of thermal tolerance, and metabolic enzymes in populations of lake chub from three habitats: a warm but variable hot spring, a thermally constant warm spring, and a seasonally variable temperate lake. Thermal breadth was generally lowest in fish from the constant environment, and this difference was statistically significant in fish acclimated at 10° and 25°C. Critical thermal maximum (CT(max)) increased with increasing acclimation temperature in all populations. CT(max) was similar among populations when acclimated at high temperatures but greater in the variable-spring population acclimated to low temperature (10°C). Critical thermal minimum was also dependent on acclimation temperature in all populations but differed among populations such that fish from the stable-spring habitat were not as tolerant to cold temperature when acclimated to 25°C. Temperate- and variable-spring populations showed an increase in mitochondrial enzyme activities (citrate synthase and cytochrome c oxidase) with decreasing acclimation temperature, but this response was absent in the stable-temperature population. Protein content did not change with acclimation temperature in the stable-temperature population, while it increased with decreasing acclimation temperature in both variable thermal habitat populations. Our study suggests that interpopulation variation in thermal physiology is associated with habitat thermal variability.

Different molecular and structural adaptations with eccentric and conventional strength training in elderly men and women

Reprogramming of gene expression contributes to structural and functional adaptation of muscle tissue in response to altered use. The aim of this study was to investigate mechanisms for observed improvements in leg extension strength, gain in relative thigh muscle mass and loss of body and thigh fat content in response to eccentric and conventional strength training in elderly men (n = 14) and women (n = 14; average age of the men and women: 80.1 ± 3.7 years) by means of structural and molecular analyses. Biopsies were collected from m. vastus lateralis in the resting state before and after 12 weeks of training with two weekly resistance exercise sessions (RET) or eccentric ergometer sessions (EET). Gene expression was analyzed using custom-designed low-density PCR arrays. Muscle ultrastructure was evaluated using EM morphometry. Gain in thigh muscle mass was paralleled by an increase in muscle fiber cross-sectional area (hypertrophy) with RET but not with EET, where muscle growth is likely occurring by the addition of sarcomeres in series or by hyperplasia. The expression of transcripts encoding factors involved in muscle growth, repair and remodeling (e.g., IGF-1, HGF, MYOG, MYH3) was increased to a larger extent after EET than RET. MicroRNA 1 expression was decreased independent of the training modality, and was paralleled by an increased expression of IGF-1 representing a potential target. IGF-1 is a potent promoter of muscle growth, and its regulation by microRNA 1 may have contributed to the gain of muscle mass observed in our subjects. EET depressed genes encoding mitochondrial and metabolic transcripts. The changes of several metabolic and mitochondrial transcripts correlated significantly with changes in mitochondrial volume density. Intramyocellular lipid content was decreased after EET concomitantly with total body fat. Changes in intramyocellular lipid content correlated with changes in body fat content with both RET and EET. In the elderly, RET and EET lead to distinct molecular and structural adaptations which might contribute to the observed small quantitative differences in functional tests and body composition parameters. EET seems to be particularly convenient for the elderly with regard to improvements in body composition and strength but at the expense of reducing muscular oxidative capacity.

Structural and autooxidation profiles of myoglobins from three species and one hybrid of tilapia (Cichlidae, Perciformes)

cDNAs encoding myoglobin were cloned from the slow skeletal muscles of three representative species of tilapia, namely, Nile tilapia Oreochromis niloticus, blue tilapia O. aureus, Mozambique tilapia O. mossambicus and one hybrid O. niloticus female symbol x O. aureus male symbol, and the primary structures were deduced. All cDNAs contained an open reading frame of 444 base pairs, encoding 147 amino acids. The amino acid sequences of Mb were completely conserved among these species, though species variations in the nucleotide sequences were recognized both in coding and non-coding regions. The amino acid sequence identity was around 70-80% compared to other teleostean Mbs. In comparison of each alpha-helical segment (A through H) and the intersegment regions to the counterparts of tuna myoglobin, the alpha-helical segments C and F as well as the intersegment regions F-G and G-H were identical. The identities of alpha-helical segments B and H and the intersegment region F-G were relatively low. Differences were also recognized in the hydropathy plot and the tertiary structures obtained by homology modeling. The autooxidation rates at 25 degrees C of myoglobin fraction from the slow skeletal muscle were essentially the same among the above tilapia species, as expected from the conserved amino acid sequences.

Haemoglobin polymorphisms affect the oxygen-binding properties in Atlantic cod populations

A major challenge in evolutionary biology is to identify the genes underlying adaptation. The oxygen-transporting haemoglobins directly link external conditions with metabolic needs and therefore represent a unique system for studying environmental effects on molecular evolution. We have discovered two haemoglobin polymorphisms in Atlantic cod populations inhabiting varying temperature and oxygen regimes in the North Atlantic. Three-dimensional modelling of the tetrameric haemoglobin structure demonstrated that the two amino acid replacements Met55beta1Val and Lys62beta1Ala are located at crucial positions of the alpha1beta1 subunit interface and haem pocket, respectively. The replacements are proposed to affect the oxygen-binding properties by modifying the haemoglobin quaternary structure and electrostatic feature. Intriguingly, the same molecular mechanism for facilitating oxygen binding is found in avian species adapted to high altitudes, illustrating convergent evolution in water- and air-breathing vertebrates to reduction in environmental oxygen availability. Cod populations inhabiting the cold Arctic waters and the low-oxygen Baltic Sea seem well adapted to these conditions by possessing the high oxygen affinity Val55-Ala62 haplotype, while the temperature-insensitive Met55-Lys62 haplotype predominates in the southern populations. The distinct distributions of the functionally different haemoglobin variants indicate that the present biogeography of this ecologically and economically important species might be seriously affected by global warming.