Molecular ecophysiology of Antarctic notothenioid fishes

The notothenioid fishes of the Southern Ocean surrounding Antarctica are remarkable examples of organismal adaptation to extreme cold. Their evolution since the mid-Miocene in geographical isolation and a chronically cold marine environment has resulted in extreme stenothermality of the extant species. Given the unique thermal history of the notothenioids, one may ask what traits have been gained, and conversely, what characters have been lost through change in the information content of their genomes. Two dramatic changes that epitomize such evolutionary transformations are the gain of novel antifreeze proteins, which are obligatory for survival in icy seawater, by most notothenioids and the paradoxical loss of respiratory haemoproteins and red blood cells, normally deemed indispensable for vertebrate life, by the species of a highly derived notothenioid family, the icefishes. Here, we review recent advances in our understanding of these traits and their evolution and suggest future avenues of investigation. The formerly coherent paradigm of notothenioid freeze avoidance, developed from three decades of study of antifreeze glycoprotein (AFGP) based cold adaptation, now faces challenges stemming from the recent discovery of antifreeze-deficient, yet freeze-resistant, early notothenioid life stages and from definitive evidence that the liver is not the physiological source of AFGPs in notothenioid blood. The resolution of these intriguing observations is likely to reveal new physiological traits that are unique to the notothenioids. Similarly, the model of AFGP gene evolution from a notothenioid pancreatic trypsinogen-like gene precursor is being expanded and refined based on genome-level analyses of the linked AFGP loci and their ancestral precursors. Finally, the application of comparative genomics to study evolutionary change in the AFGP genotypes of cool-temperate notothenioids from sub-Antarctic habitats, where these genes are not necessary, will contribute to the mechanistic understanding of the dynamics of AFGP gene gain and loss. In humans and most vertebrates, mutations in the alpha- or beta-globin genes or defects in globin chain synthesis are causes of severe genetic disease. Thus, the 16 species of haemoglobinless, erythrocyte-null icefishes are surprising anomalies -- in fact, they could only have evolved and thrived due to relaxed selection pressure for oxygen-binding proteins in the cold, oxygen-rich waters of the Southern Ocean. Fifteen of the sixteen icefish species have lost most of the adult alphabeta-globin locus and retain only a small 3' fragment of the alpha-globin gene. The only exception to this pattern occurs in Neopagetopsis ionah, which possesses a disrupted alphabeta-globin gene complex that probably represents a non-functional intermediate on the evolutionary pathway to near total globin gene extinction. By contrast, six of the icefish species fail to express myoglobin. The absence of myoglobin expression has occurred by several independent mutations and distinct mechanisms. Haemoprotein loss is correlated with dramatic increases in cellular mitochondrial density, heart size, blood volume and capillary bed volume. Evolution of these compensatory traits was probably facilitated by the homeostatic activity of nitric oxide, a key modulator of angiogenesis and mitochondrial biogenesis. These natural knockouts of the red blood cell lineage are an excellent genomic resource for erythroid gene discovery by comparative genomics, as illustrated for the newly described gene, bloodthirsty.

Protein metabolism in marine animals: the underlying mechanism of growth

Growth is a fundamental process within all marine organisms. In soft tissues, growth is primarily achieved by the synthesis and retention of proteins as protein growth. The protein pool (all the protein within the organism) is highly dynamic, with proteins constantly entering the pool via protein synthesis or being removed from the pool via protein degradation. Any net change in the size of the protein pool, positive or negative, is termed protein growth. The three inter-related processes of protein synthesis, degradation and growth are together termed protein metabolism. Measurement of protein metabolism is vital in helping us understand how biotic and abiotic factors affect growth and growth efficiency in marine animals. Recently, the developing fields of transcriptomics and proteomics have started to offer us a means of greatly increasing our knowledge of the underlying molecular control of protein metabolism. Transcriptomics may also allow us to detect subtle changes in gene expression associated with protein synthesis and degradation, which cannot be detected using classical methods. A large literature exists on protein metabolism in animals; however, this chapter concentrates on what we know of marine ectotherms; data from non-marine ectotherms and endotherms are only discussed when the data are of particular relevance. We first consider the techniques available to measure protein metabolism, their problems and what validation is required. Protein metabolism in marine organisms is highly sensitive to a wide variety of factors, including temperature, pollution, seasonality, nutrition, developmental stage, genetics, sexual maturation and moulting. We examine how these abiotic and biotic factors affect protein metabolism at the level of whole-animal (adult and larval), tissue and cellular protein metabolism. Available gene expression data, which help us understand the underlying control of protein metabolism, are also discussed. As protein metabolism appears to comprise a significant proportion of overall metabolic costs in marine organisms, accurate estimates of the energetic cost per unit of synthesised protein are important. Measured costs of protein metabolism are reviewed, and the very high variability in reported costs highlighted. Two major determinants of protein synthesis rates are the tissue concentration of RNA, often expressed as the RNA to protein ratio, and the RNA activity (k(RNA)). The effects of temperature, nutrition and developmental stage on RNA concentration and activity are considered. This chapter highlights our complete lack of knowledge of protein metabolism in many groups of marine organisms, and the fact we currently have only limited data for animals held under a narrow range of experimental conditions. The potential assistance that genomic methods may provide in increasing our understanding of protein metabolism is described.

Antifreeze proteins and their potential use in frozen foods

Antifreeze proteins (AFPs) are proteins that have the ability to modify the growth of ice, resulting in the stabilization of ice crystals over a defined temperature range and in the inhibition of the recrystallization of ice. AFPs are found in a wide range of organisms, including bacteria, fungi, plants, invertebrates and fish. Moreover, multiple forms of AFPs are synthesized within each organism. As a result, it should be possible to select an AFP with appropriate characteristics and a suitable level of activity for a particular food product. Antifreeze proteins may improve the quality of foods that are eaten while frozen by inhibiting recrystallization and maintaining a smooth texture. In foods that are frozen only for preservation, AFPs may inhibit recrystallization during freezing, storage, transport and thawing, thus preserving food texture by reducing cellular damage and also minimizing the loss of nutrients by reducing drip. Antifreeze proteins are naturally present in many foods consumed as part of the human diet. However, AFPs may be introduced into other food products either by physical processes, such as mixing and soaking, or by gene transfer.

A comparative study of protein synthesis in nototheniids and icefish at Palmer Station, Antarctica

Protein synthetic rates were measured in tissues of Notothenia corriceps, N. gibberifrons and Chaenocephalus aceratus in vivo at 2 degrees C by a method in which high doses of 14C-phenylalanine are used for stabilization of specific radioactivity. Rates in N. coriiceps, as per cent of tissue protein synthesized per day, were: liver 10.4, head kidney 3.5, testis 2.6, spleen 2.1, kidney 1.9, gill 1.6, heart 1.4, pectoral muscle 1.0, epaxial muscle 0.37, brain 0.42. With the exception of liver and head kidney (9.8 and 3.4, respectively) all rates in the icefish C. aceratus were significantly reduced compared to the nototheniids, consistent with the dependence of protein synthesis on oxidative metabolism. Icefish lack hemoglobin in the blood. The effects of two-week starvation were tissue-specific. Rates declined markedly in pectoral and epaxial muscle, were unchanged in liver, kidney, brain, heart and testis, and were increased in gill and head kidney. The results are discussed in relation to cold adaptation of Antarctic fishes and to the adaptation of metabolism required during non-feeding periods and for species which lack an oxygen-binding pigment in their blood.

Antifreeze glycopeptides of antarctic fishes

A method is described for analysis of amino acid composition and concentration of antifreeze glycopeptides of Antarctic fish by automatic amino acid analysis of hydrolyzed samples purified by trichloroacetic acid fractionation. Preparations were 98% pure and showed characteristic alanine, threonine and proline compositions. Analyses were carried out in 17 known species of Ross Sea, Antarctica, fishes and in 11 undescribed species tentatively placed in genera, where possible, or families. Collections have been preserved for classification. The results provide evidence of species specificity in amino acid composition and plasma concentration of these substances. Closely related species showed nearly identical Pro/Ala ratios, but differing concentrations. The results do not support reclassification of Trematomus and Notothenia into a single genus or the inclusion of the genus Pleuragramma in the family Nototheniidae. Data obtained for developing ovaries of 6 species showed concentrations of these substances (per unit wet wt) about one-half those of plasma and similar, but not identical, amino acid compositions to those of plasma. The total amount in eggs was 4 times that of plasma in pre-spawning individuals. Calculations are presented to indicate the role of these substances in the protein metabolic economy of the animal for Nototheniidae and Channichthyidae.

Characterization of glycoprotein antifreeze biosynthesis in isolated hepatocytes from Pagothenia borchgrevinki

Incorporation of 14C-leucine and 3H-alanine into TCA-precipitable protein. TCA-soluble protein, and antifreeze glycoproteins (AFGP) was measured in isolated hepatocytes from Pagothenia borchgrevinki Boulenger following acclimation to -1.5 degrees C and +4 degrees C. the rate of 3H-alanine incorporation into AFGP followed Michaelis-Menten kinetics with a Vmax of 4.8 nM X mg protein-1 X h-1 at -1.5 degrees C and 7.5 nM X mg protein-1 X h-1 at +4 degrees C. Km values were 27.9 microM and 41.7 microM at -1.5 degrees C and +4 degrees C, respectively. Incorporation of 14C-leucine into TCA-precipitable protein also showed Michaelis-Menten kinetics with a Vmax of 20 nM X mg protein-1 X hr-1 at 1.5 degrees C and 32.3 nM X mg protein-1 X hr-1 at +4 degrees C. Km values were 83.3 microM at -1.5 degrees C and 125 microM at +4 degrees C. AFGP synthesis was monitored over a 120-h period by radioimmunoassay in cultures of hepatocytes from cold acclimated fish (-1.5 degrees C) incubated at both -1.5 degrees C and +4 degrees C. The estimated Q10 for AFGP from these data is 3.23. Polyacrylamide gel electrophoresis of antifreeze glycoproteins produced by isolated hepatocytes showed that all four antifreeze fractions normally present in the serum of P. borchgrevinki are also synthesized by isolated hepatocytes. The two major conclusions from these experiments were that 1) P. brochgrevinki, unlike many northern fishes, does not show thermal acclimation, and 2) environmental factors responsible for modification of peptide antifreeze synthesis in northern fishes do not elicit changes in AFGP synthesis in P. borchgrevinki.