Polymerization of Antarctic fish tubulins at low temperatures: energetic aspects

Monte Carlo simulation and experimental validation of plant microtubules cathode in biodegradable battery

For the first time, electrochemical methods are utilized to study the response of tubulin monomers (extracted from plant source such as Green Peas: Arachis Hypogea) towards charge perturbations in the form of conductivity, conformational changes via self-assembly and adsorption on Au surface. The obtained dimerization and surface adsorption energetics of the tubulins from Cyclic Voltammetry agree well with the literature value of 6.9 and 14.9 kCal/mol for lateral and longitudinal bond formation energy respectively. In addition to the effects of charge perturbations on change in structure, ionic and electronic conductivity of tubulin with increasing load are investigated and found to be 1.25 Sm^-1 and 2.89 mSm^-1 respectively. The electronic conductivity is 1.93 times higher than the literature value of 1.5 mSm^-1, demonstrating the fact that the microtubules (dimer of tubulins, MTs) from plant source can be used as a semiconductor electrode material in energy conversion and storage applications. Thus, motivated by the Monte Carlo simulation and electrochemical results the MTs extracted from plant source are used as cathode material for energy storage device such as Bio-battery and the Galvanostatic Charge/Discharge studies are carried out in coin cell configuration. The configuration of the bio-battery cell is as follows: Al/CB//PP-1M KCl//MTs/SS; where SS and Al are used as current collectors for cathode and anode respectively, Polypropylene (PP) membrane soaked in 1M KCl as electrolyte and Carbon Black (CB) is the anode material. Another configuration of the cell would be replacement of CB by biopolymer such as ethyl cellulose anode (Al/EC/PP-1M KCl/MTs/SS).

Reconstituting Microtubules: A Decades-Long Effort From Building Block Identification to the Generation of Recombinant α/β-Tubulin

Microtubules are cytoskeletal filaments underlying the morphology and functions of all eukaryotic cells. In higher eukaryotes, the basic building blocks of these non-covalent polymers, ɑ- and β-tubulins, are encoded by expanded tubulin family genes (i.e., isotypes) at distinct loci in the genome. While ɑ/β-tubulin heterodimers have been isolated and examined for more than 50 years, how tubulin isotypes contribute to the microtubule organization and functions that support diverse cellular architectures remains a fundamental question. To address this knowledge gap, in vitro reconstitution of microtubules with purified ɑ/β-tubulin proteins has been employed for biochemical and biophysical characterization. These in vitro assays have provided mechanistic insights into the regulation of microtubule dynamics, stability, and interactions with other associated proteins. Here we survey the evolving strategies of generating purified ɑ/β-tubulin heterodimers and highlight the advances in tubulin protein biochemistry that shed light on the roles of tubulin isotypes in determining microtubule structures and properties.

The macronuclear genome of the Antarctic psychrophilic marine ciliate Euplotes focardii reveals new insights on molecular cold adaptation

The macronuclear (MAC) genomes of ciliates belonging to the genus Euplotes species are comprised of numerous small DNA molecules, nanochromosomes, each typically encoding a single gene. These genomes are responsible for all gene expression during vegetative cell growth. Here, we report the analysis of the MAC genome from the Antarctic psychrophile Euplotes focardii. Nanochromosomes containing bacterial sequences were not found, suggesting that phenomena of horizontal gene transfer did not occur recently, even though this ciliate species has a substantial associated bacterial consortium. As in other euplotid species, E. focardii MAC genes are characterized by a high frequency of translational frameshifting. Furthermore, in order to characterize differences that may be consequent to cold adaptation and defense to oxidative stress, the main constraints of the Antarctic marine microorganisms, we compared E. focardii MAC genome with those available from mesophilic Euplotes species. We focussed mainly on the comparison of tubulin, antioxidant enzymes and heat shock protein (HSP) 70 families, molecules which possess peculiar characteristic correlated with cold adaptation in E. focardii. We found that α-tubulin genes and those encoding SODs and CATs antioxidant enzymes are more numerous than in the mesophilic Euplotes species. Furthermore, the phylogenetic trees showed that these molecules are divergent in the Antarctic species. In contrast, there are fewer hsp70 genes in E. focardii compared to mesophilic Euplotes and these genes do not respond to thermal stress but only to oxidative stress. Our results suggest that molecular adaptation to cold and oxidative stress in the Antarctic environment may not only be due to particular amino acid substitutions but also due to duplication and divergence of paralogous genes.

Adaptation of Proteins to the Cold in Antarctic Fish: A Role for Methionine?

The evolution of antifreeze glycoproteins has enabled notothenioid fish to flourish in the freezing waters of the Southern Ocean. Whereas successful at the biodiversity level to life in the cold, paradoxically at the cellular level these stenothermal animals have problems producing, folding, and degrading proteins at their ambient temperatures of -1.86 °C. In this first multi-species transcriptome comparison of the amino acid composition of notothenioid proteins with temperate teleost proteins, we show that, unlike psychrophilic bacteria, Antarctic fish provide little evidence for the mass alteration of protein amino acid composition to enhance protein folding and reduce protein denaturation in the cold. The exception was the significant overrepresentation of positions where leucine in temperate fish proteins was replaced by methionine in the notothenioid orthologues. We hypothesize that these extra methionines have been preferentially assimilated into the genome to act as redox sensors in the highly oxygenated waters of the Southern Ocean. This redox hypothesis is supported by analyses of notothenioids showing enrichment of genes associated with responses to environmental stress, particularly reactive oxygen species. So overall, although notothenioid fish show cold-associated problems with protein homeostasis, they may have modified only a selected number of biochemical pathways to work efficiently below 0 °C. Even a slight warming of the Southern Ocean might disrupt the critical functions of this handful of key pathways with considerable impacts for the functioning of this ecosystem in the future.

Embryogenesis and early skeletogenesis in the antarctic bullhead notothen, Notothenia coriiceps

BACKGROUND

Environmental temperature influences rates of embryonic development, but a detailed staging series for vertebrate embryos developing in the subzero cold of Antarctic waters is not yet available from fertilization to hatching. Given projected warming of the Southern Ocean, it is imperative to establish a baseline to evaluate potential effects of changing climate on fish developmental dynamics.

RESULTS

We studied the Bullhead notothen (Notothenia coriiceps), a notothenioid fish inhabiting waters between -1.9 and +2 °C. In vitro fertilization produced embryos that progressed through cleavage, epiboly, gastrulation, segmentation, organogenesis, and hatching. We compared morphogenesis spatially and temporally to Zebrafish and medaka. Experimental animals hatched after about 6 months to early larval stages. To help understand skeletogenesis, we analyzed late embryos for expression of sox9 and runx2, which regulate chondrogenesis, osteogenesis, and eye development. Results revealed that, despite their prolonged developmental time course, N. coriiceps embryos developed similarly to those of other teleosts with large yolk cells.

CONCLUSIONS

Our studies set the stage for future molecular analyses of development in these extremophile fish. Results provide a foundation for understanding the impact of ocean warming on embryonic development and larval recruitment of notothenioid fish, which are key factors in the marine trophic system. Developmental Dynamics 245:1066-1080, 2016. © 2016 Wiley Periodicals, Inc.

Antarctic notothenioid fish: what are the future consequences of 'losses' and 'gains' acquired during long-term evolution at cold and stable temperatures?

Antarctic notothenioids dominate the fish fauna of the Southern Ocean. Evolution for millions of years at cold and stable temperatures has led to the acquisition of numerous biochemical traits that allow these fishes to thrive in sub-zero waters. The gain of antifreeze glycoproteins has afforded notothenioids the ability to avert freezing and survive at temperatures often hovering near the freezing point of seawater. Additionally, possession of cold-adapted proteins and membranes permits them to sustain appropriate metabolic rates at exceptionally low body temperatures. The notothenioid genome is also distinguished by the disappearance of traits in some species, losses that might prove costly in a warmer environment. Perhaps the best-illustrated example is the lack of expression of hemoglobin in white-blooded icefishes from the family Channichthyidae. Loss of key elements of the cellular stress response, notably the heat shock response, has also been observed. Along with their attainment of cold tolerance, notothenioids have developed an extreme stenothermy and many species perish at temperatures only a few degrees above their habitat temperatures. Thus, in light of today's rapidly changing climate, it is critical to evaluate how these extreme stenotherms will respond to rising ocean temperatures. It is conceivable that the remarkable cold specialization of notothenioids may ultimately leave them vulnerable to future thermal increases and threaten their fitness and survival. Within this context, our review provides a current summary of the biochemical losses and gains that are known for notothenioids and examines these cold-adapted traits with a focus on processes underlying thermal tolerance and acclimation capacity.

The genome sequence of the Antarctic bullhead notothen reveals evolutionary adaptations to a cold environment

Background

Antarctic fish have adapted to the freezing waters of the Southern Ocean. Representative adaptations to this harsh environment include a constitutive heat shock response and the evolution of an antifreeze protein in the blood. Despite their adaptations to the cold, genome-wide studies have not yet been performed on these fish due to the lack of a sequenced genome. Notothenia coriiceps, the Antarctic bullhead notothen, is an endemic teleost fish with a circumpolar distribution and makes a good model to understand the genomic adaptations to constant sub-zero temperatures.

Results

We provide the draft genome sequence and annotation for N. coriiceps. Comparative genome-wide analysis with other fish genomes shows that mitochondrial proteins and hemoglobin evolved rapidly. Transcriptome analysis of thermal stress responses find alternative response mechanisms for evolution strategies in a cold environment. Loss of the phosphorylation-dependent sumoylation motif in heat shock factor 1 suggests that the heat shock response evolved into a simple and rapid phosphorylation-independent regulatory mechanism. Rapidly evolved hemoglobin and the induction of a heat shock response in the blood may support the efficient supply of oxygen to cold-adapted mitochondria.

Conclusions

Our data and analysis suggest that evolutionary strategies in efficient aerobic cellular respiration are controlled by hemoglobin and mitochondrial proteins, which may be important for the adaptation of Antarctic fish to their environment. The use of genome data from the Antarctic endemic fish provides an invaluable resource providing evidence of evolutionary adaptation and can be applied to other studies of Antarctic fish.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-014-0468-1) contains supplementary material, which is available to authorized users.

Assisted protein folding at low temperature: evolutionary adaptation of the Antarctic fish chaperonin CCT and its client proteins

Eukaryotic ectotherms of the Southern Ocean face energetic challenges to protein folding assisted by the cytosolic chaperonin CCT. We hypothesize that CCT and its client proteins (CPs) have co-evolved molecular adaptations that facilitate CCT–CP interaction and the ATP-driven folding cycle at low temperature. To test this hypothesis, we compared the functional and structural properties of CCT–CP systems from testis tissues of an Antarctic fish, Gobionotothen gibberifrons (Lönnberg) (habitat/body T = −1.9 to +2°C), and of the cow (body T = 37°C). We examined the temperature dependence of the binding of denatured CPs (β-actin, β-tubulin) by fish and bovine CCTs, both in homologous and heterologous combinations and at temperatures between −4°C and 20°C, in a buffer conducive to binding of the denatured CP to the open conformation of CCT. In homologous combination, the percentage of G. gibberifrons CCT bound to CP declined linearly with increasing temperature, whereas the converse was true for bovine CCT. Binding of CCT to heterologous CPs was low, irrespective of temperature. When reactions were supplemented with ATP, G. gibberifrons CCT catalyzed the folding and release of actin at 2°C. The ATPase activity of apo-CCT from G. gibberifrons at 4°C was ∼2.5-fold greater than that of apo-bovine CCT, whereas equivalent activities were observed at 20°C. Based on these results, we conclude that the catalytic folding cycle of CCT from Antarctic fishes is partially compensated at their habitat temperature, probably by means of enhanced CP-binding affinity and increased flexibility of the CCT subunits.

Combined transcriptomic and metabolomic approach uncovers molecular mechanisms of cold tolerance in a temperate flesh fly

The ability to respond rapidly to changes in temperature is critical for insects and other ectotherms living in variable environments. In a physiological process termed rapid cold-hardening (RCH), exposure to nonlethal low temperature allows many insects to significantly increase their cold tolerance in a matter of minutes to hours. Additionally, there are rapid changes in gene expression and cell physiology during recovery from cold injury, and we hypothesize that RCH may modulate some of these processes during recovery. In this study, we used a combination of transcriptomics and metabolomics to examine the molecular mechanisms of RCH and cold shock recovery in the flesh fly, Sarcophaga bullata. Surprisingly, out of ∼15,000 expressed sequence tags (ESTs) measured, no transcripts were upregulated during RCH, and likewise RCH had a minimal effect on the transcript signature during recovery from cold shock. However, during recovery from cold shock, we observed differential expression of ∼1,400 ESTs, including a number of heat shock proteins, cytoskeletal components, and genes from several cell signaling pathways. In the metabolome, RCH had a slight yet significant effect on several metabolic pathways, while cold shock resulted in dramatic increases in gluconeogenesis, amino acid synthesis, and cryoprotective polyol synthesis. Several biochemical pathways showed congruence at both the transcript and metabolite levels, indicating that coordinated changes in gene expression and metabolism contribute to recovery from cold shock. Thus, while RCH had very minor effects on gene expression, recovery from cold shock elicits sweeping changes in gene expression and metabolism along numerous cell signaling and biochemical pathways.

Molecular adaptations in Antarctic fish and marine microorganisms

The Antarctic marine environment is one of the most extreme on Earth due to its stably low temperature and high oxygen content. Here we discuss various aspects of the molecular adaptations evolved by Antarctic fish and marine microorganisms living in this environment. This review will in particular focus on: (i) the genetic/genomic bases of adaptation in Antarctic notothenioid fish; (ii) the role of neuroglobin recently identified in the brain of Antarctic icefish; (iii) the structural and functional features of globins of the Antarctic marine bacterium Pseudoalteromonas haloplanktis TAC125.

Genome enablement of the notothenioidei: genome size estimates from 11 species and BAC libraries from 2 representative taxa

The perciform suborder Notothenoidei provides a compelling opportunity to study the adaptive radiation of a marine species flock in the cold Southern Ocean surrounding Antarctica. To enable genome-level studies of these psychrophilic fishes, we estimated the sizes of the genomes of 11 Antarctic species and generated high-quality BAC libraries for 2, the notothen Notothenia coriiceps and the icefish Chaenocephalus aceratus. Our results indicate that evolution of phylogenetically derived notothenioid families, [e.g., the icefishes (Channichthyidae)], was accompanied by genome expansion. Species (n=6) of the basal family Nototheniidae had C values that ranged between 0.98 and 1.20 pg, whereas those of the icefishes, the notothenioid crown group, were 1.66-1.83 pg (n=4 species). The BAC libraries VMRC-19 (N. coriiceps) and VMRC-21 (C. aceratus) comprised 12X and 10X coverage of the respective genomes and had average insert sizes of 138 and 168 kb. Greater than 60% of paired BAC ends sampled from each library ( approximately 0.1% of each genome) contained repetitive sequences, and the repetitive element landscapes of the 2 genomes (13.4% of the N. coriiceps genome and 14.5% for C. aceratus) were similar. The representation and depth of coverage of the libraries were verified by identification of multiple Hox gene contigs: six discrete Hox clusters were found in N. coriiceps and at least five Hox clusters were found in C. aceratus. Given the unusual anatomical and physiological adaptations of the notothenioids, the availability of these BAC libraries sets the stage for expanded analysis of the psychrophilic mode of life.

Antarctic notothenioid fishes: genomic resources and strategies for analyzing an adaptive radiation

The perciform suborder Notothenoidei provides a compelling opportunity to study the adaptive radiation of a marine species-flock in the cold Southern Ocean that surrounds Antarctica. To facilitate genome-level studies of the diversification of these fishes, we present estimates of the genome sizes of 11 Antarctic species and describe the production of high-quality bacterial artificial chromosome (BAC) libraries for two, the red-blooded notothen Notothenia coriiceps and the white-blooded icefish Chaenocephalus aceratus. Our results indicate that evolution of phylogenetically derived notothenioid families (e.g., the crown group Channichthyidae [icefishes]), was accompanied by genome expansion. Six species from the basal family Nototheniidae had C-values between 0.98 and 1.20 pg, a range that is consistent with the genome sizes of proposed outgroups (e.g., percids) of the notothenioid suborder. In contrast, four icefishes had C-values in the range 1.66-1.83 pg. The BAC libraries VMRC-19 (N. coriiceps) and VMRC-21 (C. aceratus) comprise 12× and 10× coverage of the respective genomes and have average insert sizes of 138 and 168 kb. Paired BAC-end reads representing ∼0.1% of each genome showed that the repetitive element landscapes of the two genomes (13.4% of the N. coriiceps genome and 14.5% for C. aceratus) were similar. The availability of these high-quality and well-characterized BAC libraries sets the stage for targeted genomic analyses of the unusual anatomical and physiological adaptations of the notothenioids, some of which mimic human diseases. Here we consider the evolution of secondary pelagicism by various taxa of the group and illustrate the utility of Antarctic icefishes as an evolutionary-mutant model of human osteopenia (low-mineral density of bones).

Molecular adaptations in haemoglobins of notothenioid fishes

Since haemoglobins of all animal species have the same haem group, differences in their properties, including oxygen affinity, electrophoretic mobility and pH sensitivity, must result from the interaction of the prosthetic group with specific amino-acid residues in the primary structure. For this reason, fish globins have been the subject of extensive studies in recent years, not only for their structural characteristics, but also because they offer the possibility to investigate the evolutionary history of these ancient molecules in marine and freshwater species living in a great variety of environmental conditions. This review summarizes the current knowledge on the structure, function and phylogeny of haemoglobins of notothenioid fishes. On the basis of crystallographic analysis, the evolution of the Root effect is analysed. Adaptation of the oxygen transport system in notothenioids seems to be based on evolutionary changes, involving levels of biological organization higher than the structure of haemoglobin. These include changes in the rate of haemoglobin synthesis or in regulation by allosteric effectors, which affect the amount of oxygen transported in blood. These factors are thought to be more important for short-term response to environmental challenges than previously believed.

Mass spectrometry analysis of C-terminal posttranslational modifications of tubulins

In mammalian brain and ciliary axonemes from ciliates, alpha- and beta-tubulins exhibit an extraordinary heterogeneity due to a combination of multigene family expression and numerous posttranslational modifications (PTMs). The combination of several PTMs located in the C-terminal tail of tubulins plays a major role in this important polymorphism of tubulin: polyglutamylation, polyglycylation, detyrosination, tyrosination, removal of the penultimate glutamate residue, and phosphorylation. In order to document the relationship and functions of these PTMs, we have developed a tubulin C-terminal Peptide Mass Fingerprinting (PMF) method. Using simplified microtubule proteins and tubulin C-terminal peptides purifications, direct matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS) analysis can generate a complete picture of all tubulin isotype-specific C-terminal peptides together with their respective PTMs. This chapter will illustrate the capability of this approach to compare tubulin isoform compositions and document the changes in PTMs between samples with different tubulin assembly properties or consecutively to inactivation of modification sites or modification enzymes. Complementary MS-based approaches useful to document the structure of the highly heterogeneous posttranslational polymodifications will also be presented.

Decrease in expression of beta-tubulin and microtubule abundance in flight muscles during diapause in adults of Culex pipiens

The cDNA encoding beta-tubulin in the mosquito Culex pipiens has high similarity with the beta-tubulins reported in other insects. In this study, we examine expression of this gene and microtubule abundance in relation to diapause and low temperature. While non-diapausing mosquitoes express beta-tubulin highly in their thoracic muscles, expression is quite low during adult diapause. The abundance of microtubules was also much lower in flight muscles of diapausing adults than in flight muscles from non-diapausing individuals, as confirmed by laser confocal microscopy of tubulins stained using indirect immunofluorescence. Low temperatures decreased microtubule abundance in midguts of non-diapausing mosquitoes, but microtubule abundance in diapausing mosquitoes was already low and remained unchanged by low temperature exposure. Overall, pixel intensity averages were higher in the flight muscles than in the midguts, and again low temperatures decreased microtubule abundance in the flight muscles of non-diapausing females, while levels remained consistently low in diapausing females. These results clearly indicate that a decrease in microtubule abundance is evoked both by the programming of diapause and, in non-diapausing females, by exposure to low temperatures. Quite possibly the reduced microtubule abundance in the flight muscles and reduced expression of beta-tubulin are functionally correlated to the reduction in flight activity that is associated with low temperature and diapause.

Molecular cold-adaptation of protein function and gene regulation: The case for comparative genomic analyses in marine ciliated protozoa

Euplotes focardii is a marine ciliated protozoan discovered in the Ross Sea near Terra Nova Bay, Antarctica. This organism is strictly psychrophilic, survives and reproduces optimally at 4-5 °C, and has a genome rich in A/T base pairs. Like other ciliated protozoans, Euplotes spp. are characterized by nuclear dimorphism: 1) the germline micronucleus contains the entire genome as large chromosomes; and 2) the somatic macronucleus (∼50 megabases, or 5% of the micronuclear genome) contains small linear DNA nanochromosomes [1-12 kilobases], each of which constitutes a single genetic unit. These characteristics make E. focardii an ideal model for genome-level analysis to understand the evolutionary mechanisms that determine the adaptation of organisms to cold environments. Here we describe two examples that are controlled by phylogenetically appropriate comparison with mesophilic and psychrotolerant Euplotes species: 1) the genes and encoded proteins of the E. focardii tubulin superfamily, including α-, β-, and γ-tubulins; and 2) the genes of the heat-shock protein (Hsp) 70 family. The tubulins provide particular insight into protein-level structural changes that are likely to facilitate microtubule nucleation and polymerization in an energy poor environment. By contrast, the hsp70 genes of E. focardii and of its psychrotolerant relative E. nobilii reveal adaptive alterations in the regulation of gene expression in the cold. The unique characteristics of the E. focardii genome and the results that we present here argue strongly for a concerted effort to characterize the relatively low complexity macronuclear genome of this psychrophilic organism.

Different roles of two gamma-tubulin isotypes in the cytoskeleton of the Antarctic ciliate Euplotes focardii: remodelling of interaction surfaces may enhance microtubule nucleation at low temperature

Gamma-tubulin belongs to the tubulin superfamily and plays an essential role in the nucleation of cellular microtubules. In the present study, we report the characterization of gamma-tubulin from the psychrophilic Antarctic ciliate Euplotes focardii. In this organism, gamma-tubulin is encoded by two genes, gamma-T1 and gamma-T2, that produce distinct isotypes. Comparison of the gamma-T1 and gamma-T2 primary sequences to a Euplotesgamma-tubulin consensus, derived from mesophilic (i.e. temperate) congeneric species, revealed the presence of numerous unique amino acid substitutions, particularly in gamma-T2. Structural models of gamma-T1 and gamma-T2, obtained using the 3D structure of human gamma-tubulin as a template, suggest that these substitutions are responsible for conformational and/or polarity differences located: (a) in the regions involved in longitudinal 'plus end' contacts; (b) in the T3 loop that participates in binding GTP; and (c) in the M loop that forms lateral interactions. Relative to gamma-T1, the gamma-T2 gene is amplified by approximately 18-fold in the macronuclear genome and is very strongly transcribed. Using confocal immunofluorescence microscopy, we found that the gamma-tubulins of E. focardii associate throughout the cell cycle with basal bodies of the non-motile dorsal cilia and of all of the cirri of the ventral surface (i.e. adoral membranelles, paraoral membrane, and frontoventral transverse, caudal and marginal cirri). By contrast, only gamma-T2 interacts with the centrosomes of the spindle during micronuclear mitosis. We also established that the gamma-T1 isotype associates only with basal bodies. Our results suggest that gamma-T1 and gamma-T2 perform different functions in the organization of the microtubule cytoskeleton of this protist and are consistent with the hypothesis that gamma-T1 and gamma-T2 have evolved sequence-based structural alterations that facilitate template nucleation of microtubules by the gamma-tubulin ring complex at cold temperatures.

Induction of hypothermic conditions associated with increased micronuclei formation in sigma-1 receptor knockout mice after administration of the antipsychotic compound E-5842

The antipsychotic sigma-1 (sigma(1)) receptor ligand E-5842 has been shown to increase micronucleated polychromatic erythrocyte (MNPCE) frequency in mouse bone marrow secondary to compound-induced hypothermia. Interaction with sigma(1) receptor has been considered a plausible contributing factor for E-5842-induced hypothermia, raising concern for a possible class effect of sigma receptor ligands in the mouse micronucleus (MN) test. We assessed the potential of E-5842 (200 mg/kg, oral) to produce hypothermic conditions associated with increased micronuclei formation in sigma(1) receptor knockout (sigma(1)R-KO) and wild type (WT) mice. After administration, animal's rectal temperature was recorded and peripheral blood and bone marrow samples were obtained (48 hr) and assessed for induction of micronucleated reticulocytes (MNRET) and MNPCE, respectively. E-5842 administration produced marked hypothermia both in sigma(1)R-KO and WT mice. Maximum decreases from preadministration temperature were 12.2 and 13.5 degrees C in sigma(1)R-KO and WT mice, respectively. Temperature returned to normal approximately 32 hr after administration. Bone marrow examination revealed a statistical significant increase (P < 0.05) in MNPCE frequency both in sigma(1)R-KO and WT animals. Examination of peripheral blood samples showed a slight, although nonstatistical significant, increase in MNRET frequency in sigma(1)R-KO mice. No similar effect was observed among WT animals. The results obtained after E-5842 administration to sigma(1)R-KO mice indicate that induction of hypothermic conditions associated with increased MNPCE formation is not mediated by compound interaction with sigma(1) receptor, ruling out concern for a possible class effect of similar high affinity sigma(1) receptor ligands in the mouse MN test.

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.

Up-regulation of heat shock proteins is essential for cold survival during insect diapause

Diapause, the dormancy common to overwintering insects, evokes a unique pattern of gene expression. In the flesh fly, most, but not all, of the fly's heat shock proteins (Hsps) are up-regulated. The diapause up-regulated Hsps include two members of the Hsp70 family, one member of the Hsp60 family (TCP-1), at least four members of the small Hsp family, and a small Hsp pseudogene. Expression of an Hsp70 cognate, Hsc70, is uninfluenced by diapause, and Hsp90 is actually down-regulated during diapause, thus diapause differs from common stress responses that elicit synchronous up-regulation of all Hsps. Up-regulation of the Hsps begins at the onset of diapause, persists throughout the overwintering period, and ceases within hours after the fly receives the signal to reinitiate development. The up-regulation of Hsps appears to be common to diapause in species representing diverse insect orders including Diptera, Lepidoptera, Coleoptera, and Hymenoptera as well as in diapauses that occur in different developmental stages (embryo, larva, pupa, adult). Suppressing expression of Hsp23 and Hsp70 in flies by using RNAi did not alter the decision to enter diapause or the duration of diapause, but it had a profound effect on the pupa's ability to survive low temperatures. We thus propose that up-regulation of Hsps during diapause is a major factor contributing to cold-hardiness of overwintering insects.

Upregulation of two actin genes and redistribution of actin during diapause and cold stress in the northern house mosquito, Culex pipiens

Two actin genes cloned from Culex pipiens L. are upregulated during adult diapause. Though actins 1 and 2 were expressed throughout diapause, both genes were most highly expressed early in diapause. These changes in gene expression were accompanied by a conspicuous redistribution of polymerized actin that was most pronounced in the midguts of diapausing mosquitoes that were exposed to low temperature. In nondiapausing mosquitoes reared at 25 degrees C and in diapausing mosquitoes reared at 18 degrees C, polymerized actin was clustered at high concentrations at the intersections of the muscle fibers that form the midgut musculature. When adults 7-10 days post-eclosion were exposed to low temperature (-5 degrees C for 12 h), the polymerized actin was evenly distributed along the muscle fibers in both nondiapausing and diapausing mosquitoes. Exposure of older adults (1 month post-eclosion) to low temperature (-5 degrees C for 12 h) elicited an even greater distribution of polymerized actin, an effect that was especially pronounced in diapausing mosquitoes. These changes in gene expression and actin distribution suggest a role for actins in enhancing survival of diapausing adults during the low temperatures of winter by fortification of the cytoskeleton.

Environmental constraints on life histories in Antarctic ecosystems: tempos, timings and predictability

Knowledge of Antarctic biotas and environments has increased dramatically in recent years. There has also been a rapid increase in the use of novel technologies. Despite this, some fundamental aspects of environmental control that structure physiological, ecological and life-history traits in Antarctic organisms have received little attention. Possibly the most important of these is the timing and availability of resources, and the way in which this dictates the tempo or pace of life. The clearest view of this effect comes from comparisons of species living in different habitats. Here, we (i) show that the timing and extent of resource availability, from nutrients to colonisable space, differ across Antarctic marine, intertidal and terrestrial habitats, and (ii) illustrate that these differences affect the rate at which organisms function. Consequently, there are many dramatic biological differences between organisms that live as little as 10 m apart, but have gaping voids between them ecologically. Identifying the effects of environmental timing and predictability requires detailed analysis in a wide context, where Antarctic terrestrial and marine ecosystems are at one extreme of the continuum of available environments for many characteristics including temperature, ice cover and seasonality. Anthropocentrically, Antarctica is harsh and as might be expected terrestrial animal and plant diversity and biomass are restricted. By contrast, Antarctic marine biotas are rich and diverse, and several phyla are represented at levels greater than global averages. There has been much debate on the relative importance of various physical factors that structure the characteristics of Antarctic biotas. This is especially so for temperature and seasonality, and their effects on physiology, life history and biodiversity. More recently, habitat age and persistence through previous ice maxima have been identified as key factors dictating biodiversity and endemism. Modern molecular methods have also recently been incorporated into many traditional areas of polar biology. Environmental predictability dictates many of the biological characters seen in all of these areas of Antarctic research.

Induction of micronuclei in mouse bone-marrow erythrocytes in association with hypothermia after administration of the sigma receptor ligand E-5842

Oral administration of E-5842, a new sigma1 receptor ligand being developed as an antipsychotic drug, to male mice at single doses of 50, 100, 200 and 400 mg/kg produced marked and sustained decreases in rectal temperature. Both the intensity and the duration of the hypothermic effect increased with dose. Maximum decreases from the mean pre-administration temperature (36.2 degrees C) ranged from 7.5 to 12.9 degrees C for animals receiving 50 and 400 mg/kg doses, respectively. Examination of bone-marrow smears obtained 24, 48 and 72 h after administration revealed a slight but statistically significant (p < 0.05) increase in the frequency of micronucleated polychromatic erythrocytes (MNPCE) at the 48 h sampling for animals receiving the 200 mg/kg dose. These animals showed decreases from pre-administration temperature of approximately 12 degrees C, with recovery being observed 24 h after administration. When the hypothermic effect of E-5842 administration was avoided by housing treated animals under conditions of increased environmental temperature (30 degrees C) for 24 h, MNPCE frequency reverted to vehicle control values. Further, in E-5842-treated animals with an increased MNPCE frequency there was a shift in the distribution of the relative areas of micronuclei in MNPCE towards higher values. In addition, there was a statistically significant increase (p < 0.001) in the number of relatively large micronuclei (micronucleus diameter > or = 1/4 cytoplasm diameter) similar to that produced by administration of the mitotic spindle inhibitor colchicine (1 mg/kg), suggesting disturbance of mitotic apparatus as the possible underlying mechanism. The results suggest that the slight increase in MNPCE frequency observed 48 h after administration of a 200 mg/kg dose of E-5842 is due to a hypothermic effect and not to a direct effect of E-5842 on DNA.

Meiosis-specific failure of cell cycle progression in fission yeast by mutation of a conserved beta-tubulin residue

The microtubule cytoskeleton is involved in regulation of cell morphology, differentiation, and cell cycle progression. Precisely controlled dynamic properties are required for these microtubule functions. To better understand how tubulin's dynamics are embedded in its primary sequence, we investigated in vivo the consequences of altering a single, highly conserved residue in beta-tubulin that lies at the interface between two structural domains. The residue differs between the cold-adapted Antarctic fish and temperate animals in a manner that suggests a role in microtubule stability. Fungi, like the Antarctic fish, have a phenylalanine in this position, whereas essentially all other animals have tyrosine. We mutated the corresponding residue in fission yeast to tyrosine. Temperature effects were subtle, but time-lapse microscopy of microtubule dynamics revealed reduced depolymerization rates and increased stability. Mitotic exit signaled by breakdown of the mitotic spindle was delayed. In meiosis, microtubules displayed prolonged contact to the cell cortex during horsetail movement, followed by completion of meiosis I but frequent asymmetric failure of meiosis II spindle formation. Our results indicate that depolymerization dynamics modulated through interdomain motion may be important for regulating a subset of plus-end microtubule complexes in Schizosaccharomyces pombe.

Low-temperature protein metabolism: seasonal changes in protein synthesis and RNA dynamics in the Antarctic limpet Nacella concinna Strebel 1908

Protein synthesis is a fundamental and energetically expensive physiological process in all living organisms. Very few studies have examined the specific challenges of manufacturing proteins at low ambient temperatures. At high southern latitudes, water temperatures are continually below or near freezing and are highly stable, while food availability is very seasonal. To examine the effects of low temperature and a highly seasonal food supply on protein metabolism, we have measured wholebody protein synthesis, RNA concentrations, RNA:protein ratios and RNA translational efficiencies in the Antarctic limpet Nacella concinna at four times of the year. From summer to winter, protein synthesis rates decreased by 52%, RNA concentrations decreased by 55% and RNA:protein ratios decreased by 68%, while RNA translational efficiencies were low and very variable. Protein synthesis rates in N. concinna approached those measured in temperate mussels, while RNA:protein ratios were considerably higher than in temperate species. Interspecific comparisons show that species living at low temperatures have elevated RNA:protein ratios, which are probably needed to counteract a thermally induced reduction in RNA translational efficiency. Calculations using theoretical energetic costs of protein synthesis suggest that Antarctic species may allocate a larger proportion of their metabolic budget to protein synthesis than do temperate or tropical species.

Effects of temperature on the early stages of rat spinal motoneurone development in vitro

Effects of temperature on rat spinal motoneurone morphogenesis during the early stages of development were investigated in vitro through a statistical morphometric analysis, and examined in the frame of a basic theoretical aggregation growth model. Morphological measurements in the 31.0-39.4 degrees C range revealed that: (1) primary neurite initiation was promoted by increased temperatures; (2) collateral branches formation was particularly enhanced over 37 degrees C; and (3) the elongation properties of all processes were not significantly altered. In parallel, an Arrhenius analysis proved that: (4) an activation energy of about 23 kcal/mol was required for primary neurites to emerge from a soma. These results suggest that the very first molecular events underlying neuritogenesis are rather sensitive to temperature and could imply both the transport and assembly properties of microtubules.

Cold adaptation of microtubule assembly and dynamics. Structural interpretation of primary sequence changes present in the alpha- and beta-tubulins of Antarctic fishes

The microtubules of Antarctic fishes, unlike those of homeotherms, assemble at very low temperatures (-1.8 degrees C). The adaptations that enhance assembly of these microtubules are intrinsic to the tubulin dimer and reduce its critical concentration for polymerization at 0 degrees C to approximately 0.9 mg/ml (Williams, R. C., Jr., Correia, J. J., and DeVries, A. L. (1985) Biochemistry 24, 2790-2798). Here we demonstrate that microtubules formed by pure brain tubulins of Antarctic fishes exhibit slow dynamics at both low (5 degrees C) and high (25 degrees C) temperatures; the rates of polymer growth and shortening and the frequencies of interconversion between these states are small relative to those observed for mammalian microtubules (37 degrees C). To investigate the contribution of tubulin primary sequence variation to the functional properties of the microtubules of Antarctic fishes, we have sequenced brain cDNAs that encode 9 alpha-tubulins and 4 beta-tubulins from the yellowbelly rockcod Notothenia coriiceps and 4 alpha-tubulins and 2 beta-tubulins from the ocellated icefish Chionodraco rastrospinosus. The tubulins of these fishes were found to contain small sets of unique or rare residue substitutions that mapped to the lateral, interprotofilament surfaces or to the interiors of the alpha- and beta-polypeptides; longitudinal interaction surfaces are not altered in the fish tubulins. Four changes (A278T and S287T in alpha; S280G and A285S in beta) were present in the S7-H9 interprotofilament "M" loops of some monomers and would be expected to increase the flexibility of these regions. A fifth lateral substitution specific to the alpha-chain (M302L or M302F) may increase the hydrophobicity of the interprotofilament interaction. Two hydrophobic substitutions (alpha:S187A in helix H5 and beta:Y202F in sheet S6) may act to stabilize the monomers in conformations favorable to polymerization. We propose that cold adaptation of microtubule assembly in Antarctic fishes has occurred in part by evolutionary restructuring of the lateral surfaces and the cores of the tubulin monomers.

Cold-adapted microtubules: characterization of tubulin posttranslational modifications in the Antarctic ciliate Euplotes focardii

In cold poikilotherm organisms, microtubule assembly is promoted at temperatures below 4 degrees C and cold-induced depolymerization is prevented. On the basis of the results of investigations on cold-adapted fishes, the property of cold adaptation is ascribed to intrinsic characteristics of the tubulins. To fully understand cold adaptation, we studied the tubulins of Euplotes focardii, an Antarctic ciliated protozoan adapted to temperatures ranging from -2 to +4 degrees C. In this organism, we had previously sequenced one beta-tubulin gene and, then identified three other genes (denoted as beta-T1, beta-T2, beta-T3 and beta-T4). Here we report that the amino acid sequence of the carboxy-terminal domain predicted from the beta-T3 gene (apparently the most expressed of the gene family) contains six modifications (five substitutions and one insertion) of conserved residues, unique with respect to all the other known beta-tubulin sequences. These modifications can change the structural conformation of the carboxy-terminal domain. Furthermore, in the variable terminal end of that domain, a consensus sequence for a phosphorylation site is present, and the residue Glu-438, the most frequent site for polyglutamylation in beta-tubulin, is substituted by Asp. Starting from these observations, we showed that in E. focardii only alpha-tubulin is polyglutamylated, while beta-tubulin undergoes phosphorylation. Polyglutamylated microtubules appear to colocalize with cilia and microtubular bundles, all structures in which microtubules undergo a sliding process. This finding supports the idea that alpha-tubulin polyglutamylation is involved in the interaction between tubulin and motor microtubule-associated proteins. Phosphorylation, usually a rare posttranslational modification of beta-tubulin, which is found extensively distributed in the beta-tubulin of this cold-adapted organism, may play a determinant role in the dynamic of polymerization and depolymerization at low temperatures.

Identification of betaIII- and betaIV-tubulin isotypes in cold-adapted microtubules from Atlantic cod (Gadus morhua): antibody mapping and cDNA sequencing

Isolated microtubule proteins from the Atlantic cod (Gadus morhua) assemble at temperatures between 8 and 30 degrees C. The cold-adaptation is an intrinsic property of the tubulin molecules, but the reason for it is unknown. To increase our knowledge of tubulin diversity and its role in cold-adaptation we have further characterized cod tubulins using alpha- and beta-tubulin site-directed antibodies and antibodies towards posttranslationally modified tubulin. In addition, one cod brain beta-tubulin isotype has been sequenced. In mammals there are five beta-tubulins (betaI, betaII, betaIII, betaIVa and betaIVb) expressed in brain. A cod betaIII-tubulin was identified by its electrophoretic mobility after reduction and carboxymethylation. The betaIII-like tubulin accounted for more than 30% of total brain beta-tubulins, the highest yield yet observed in any animal. This tubulin corresponds most probably with an additional band, designated beta(x), which was found between alpha- and beta-tubulins on SDS-polyacrylamide gels. It was found to be phosphorylated and neurospecific, and constituted about 30% of total cod beta-tubulin isoforms. The sequenced cod tubulin was identified as a betaIV-tubulin, and a betaIV-isotype was stained by a C-terminal specific antibody. The amount of staining indicates that this isotype, as in mammals, only accounts for a minor part of the total brain beta-tubulin. Based on the estimated amounts of betaIII- and betaIV-tubulins in cod brain, our results indicate that cod has at least one additional beta-tubulin isotype and that beta-tubulin diversity evolved early during fish evolution. The sequenced cod betaIV-tubulin had four unique amino acid substitutions when compared to beta-tubulin sequences from other animals, while one substitution was in common with Antarctic rockcod beta-tubulin. Residues 221, Thr to Ser, and 283, Ala to Ser, correspond in the bovine tubulin dimer structure to loops that most probably interact with other tubulin molecules within the microtubule, and might contribute to cold-adaptation of microtubules.

MAP 0, a 400-kDa microtubule-associated protein unique to teleost fish

Microtubules from neural tissues of the Atlantic cod, Gadus morhua, and of several species of Antarctic teleosts are composed of tubulin and several microtubule-associated proteins (MAPs), one of which has an apparent molecular weight of approximately 400-430 kDa. Because its apparent molecular weight exceeds those of the MAP 1 proteins, we designate this high molecular weight teleost protein MAP 0. Cod MAP 0 failed to cross-react with antibodies specific for MAPs 1A, 1B and 2 of mammalian brain, for MAP H1 of squid optic lobe, and for chicken erythrocyte syncolin, which suggests that it has a novel structure. Similarly, MAP 0 from the Antarctic fish was not recognized by an antibody specific for bovine MAP 2. Together, these observations suggest that MAP 0 is a novel MAP that may be unique to fish. To determine the tissue specificity and phylogenetic distribution of this protein, we generated a rabbit polyclonal antibody against cod MAP 0. Using this antibody, we found that MAP 0 was present in microtubule proteins isolated from cod brain tissues and spinal cord but was absent in microtubules from heart, liver, and spleen. At the subcellular level, MAP 0 was distributed in cod brain cells in a punctate pattern coincident with microtubules but was absent in skin cells. MAP 0 was also detected in cells of the peripheral nervous system. A survey of microtubule proteins from chordates and invertebrates showed that anti-MAP 0-reactive homologs were present in five teleost species but not in more primitive fish and invertebrates or in higher vertebrates. MAP 0 bound to cod microtubules by ionic interaction at a site recognized competitively by bovine MAP 2. Although its function is unknown, MAP 0 does not share the microtubule-binding properties of the motor proteins kinesin and dynein. We propose that MAP 0 is a unique, teleost-specific MAP.

The myth of metabolic cold adaptation: oxygen consumption in stenothermal Antarctic bivalves

Antarctic marine ectotherms are often described as only being capable of living in a restricted temperature range, i.e. they are stenothermal. However, few data exist demonstrating that for a given group this is the case. The Antarctic bivalve molluscs Laternula elliptica and Limopsis marionensis are similar to other Antarctic invertebrates and can only exist within a temperature window of 6–12°C. This is two to six times smaller than the range for temperate and tropical bivalves, thus demonstrating their stenothermal nature. The possibility of elevated metabolic rates of cold-water ectotherms has been a topic of debate over many years. Recently, the suggestion that metabolic rates must be elevated at low temperatures to overcome constraints has been supported by findings that mitochondrial contents of muscles in ectotherms are higher at low temperatures. Data, presented here for standard or routine metabolic rates of 41 species of bivalve mollusc from polar, temperate and tropical sites, indicate that oxygen consumption is not elevated at low temperatures. Indeed, analysis of Q10 coefficients between 0 and 25°C suggests that metabolic rates of polar species may be lower than would be expected by comparison with temperate bivalves.

Evolution, organization, and expression of alpha-tubulin genes in the antarctic fish Notothenia coriiceps. Adaptive expansion of a gene family by recent gene duplication, inversion, and divergence

To assess the organization and expression of tubulin genes in ectothermic vertebrates, we have chosen the Antarctic yellowbelly rockcod, Notothenia coriiceps, as a model system. The genome of N. coriiceps contains approximately 15 distinct DNA fragments complementary to alpha-tubulin cDNA probes, which suggests that the alpha-tubulins of this cold-adapted fish are encoded by a substantial multigene family. From an N. coriiceps testicular DNA library, we isolated a 13.8-kilobase pair genomic clone that contains a tightly linked cluster of three alpha-tubulin genes, designated NcGTbalphaa, NcGTbalphab, and NcGTbalphac. Two of these genes, NcGTbalphaa and NcGTbalphab, are linked in head-to-head (5' to 5') orientation with approximately 500 bp separating their start codons, whereas NcGTbalphaa and NcGTbalphac are linked tail-to-tail (3' to 3') with approximately 2.5 kilobase pairs between their stop codons. The exons, introns, and untranslated regions of the three alpha-tubulin genes are strikingly similar in sequence, and the intergenic region between the alphaa and alphab genes is significantly palindromic. Thus, this cluster probably evolved by duplication, inversion, and divergence of a common ancestral alpha-tubulin gene. Expression of the NcGTbalphac gene is cosmopolitan, with its mRNA most abundant in hematopoietic, neural, and testicular tissues, whereas NcGTbalphaa and NcGTbalphab transcripts accumulate primarily in brain. The differential expression of the three genes is consistent with distinct suites of putative promoter and enhancer elements. We propose that cold adaptation of the microtubule system of Antarctic fishes is based in part on expansion of the alpha- and beta-tubulin gene families to ensure efficient synthesis of tubulin polypeptides.

Microtubule assembly in cold-adapted organisms: functional properties and structural adaptations of tubulins from antarctic fishes

Fishes native to the coastal waters of the Antarctic have adapted to habitat and body temperatures in the range -1.8 to +2 degrees C. Their cytoplasmic microtubules, unlike those of mammals and temperate poikilotherms, have evolved to assemble efficiently at these low temperatures. To learn about the underlying molecular adaptations, my laboratory is studying microtubule proteins [tubulin alpha beta dimers and microtubule-associated proteins (MAPs)] and tubulin genes from several Antarctic fishes, including the rockcods Notothenia coriiceps and Gobionotothen gibberifrons. We find that the assembly-enhancing adaptations of the fish microtubule proteins are intrinsic to the tubulin subunits themselves. Furthermore, microtubule formation by Antarctic fish tubulins is strongly entropy driven, due in part to an increased reliance, relative to tubulins from other species, on hydrophobic interactions. Based on analyses of tubulin polypeptides and cDNAs, we suggest that the structural adaptations of Antarctic fish tubulins most likely involve alterations in the primary sequences of tubulin isotypes. With respect to neural beta tubulins from other vertebrates, for example, the class II beta-tubulin isotype of N. coriiceps brain contains seven unique amino acid substitutions and one novel insertion in its 446-residue primary sequence. Most of these changes are located in a structural domain that forms contacts between tubulin dimers during microtubule assembly and would be expected to enhance polypeptide flexibility, thereby facilitating addition of tubulin to microtubule ends. The acidic carboxy-terminal tails of the alpha and beta tubulins, by contrast, appear not to be sites of cold adaptation of polymerization. We have also found that brain and egg tubulins from Antarctic fishes differ strikingly in their polymerization efficiencies, which demonstrates, in agreement with the multitubulin hypothesis, that tissue-specific tubulin isoforms can possess distinct functional properties. Thus, study of microtubule proteins from organisms, such as the Antarctic fishes, that have adapted to extreme thermal regimes should contribute significantly to an understanding of the quaternary interactions that control microtubule assembly in all eukaryotes.

Psychrophilic enzymes: a thermodynamic challenge

Psychrophilic microorganisms, hosts of permanently cold habitats, produce enzymes which are adapted to work at low temperatures. When compared to their mesophilic counterparts, these enzymes display a higher catalytic efficiency over a temperature range of roughly 0-30 degrees C and a high thermosensitivity. The molecular characteristics of cold enzymes originating from Antarctic bacteria have been approached through protein modelling and X-ray crystallography. The deduced three-dimensional structures of cold alpha-amylase, beta-lactamase, lipase and subtilisin have been compared to their mesophilic homologs. It appears that the molecular adaptation resides in a weakening of the intramolecular interactions, and in some cases in an increase of the interaction with the solvent, leading to more flexible molecular edifices capable of performing catalysis at a lower energy cost.

Hypothermia induces micronuclei in mouse bone marrow cells

We investigated the effect of hypothermia on micronucleus induction in mouse bone marrow cells. Reserpine, which was negative in an in vitro chromosome aberration test, was administered intraperitoneally at 1, 5, 10, 100, and 1000 mg/kg to mice to induce hypothermia. Doses of 10-1000 mg/kg decreased rectal temperature to less than 33.3 degrees C from 24 h to 96 h after dosing and produced a statistically significant (p < 0.01) increase in micronucleated polychromatic erythrocyte frequencies (4.0-12.0/1000). When mice that were administered reserpine at 50, 100, or 200 mg/kg were exposed to an environmental temperature of 30 degrees C for 40 h to keep their body temperature within normal range, the frequency of micronucleated erythrocytes did not increase, while it did without increased environmental temperature. In addition, relatively large micronuclei (diameter of micronucleus > or = 1/4 diameter of cytoplasm) accounted for approximately 50% of the induced micronuclei. The results suggest that the low body temperature of less than 33 degrees C for 40 h induced micronuclei in bone marrow cells, and one possible mechanism was disturbance of the mitotic apparatus.

Cold-stable and cold-adapted microtubules

Most mammalian microtubules disassemble at low temperature, but some are cold stable. This probably has little to do with a need for cold-stable microtubules, but reflects that certain populations of microtubules must be stabilized for specific functions. There are several routes by which to achieve cold stability. Factors that interact with microtubules, such as microtubule-associated proteins, STOPs (stable tubule only polypeptides), histones, and possibly capping factors, are involved. Specific tubulin isotypes and posttranslational modifications might also be of importance. More permanent stable microtubules can be achieved by bundling factors, associations to membranes, as well as by assembly of microtubule doublets and triplets. This is, however, not the explanation for cold adaptation of microtubules from poikilothermic animals, that is, animals that must have all their microtubules adapted to low temperatures. All evidence so far suggests that cold adaptation is intrinsic to the tubulins, but it is unknown whether it depends on different amino acid sequences or posttranslational modifications.

Identification of the tubulin gene family and sequence determination of one beta-tubulin gene in a cold-poikilotherm protozoan, the antarctic ciliate Euplotes focardii

Four different tubulin genes were identified in the somatic nucleus (macronucleus) of Euplotes focardii, a strictly cold-adapted, Antarctic ciliate: one of 1,800 bp for alpha-tubulin and three of 2,150, 1,900, and 1,600 bp, respectively, for beta-tubulin. Preliminarily analysed for restriction fragment length polymorphisms, these genes showed remarkable differences in organisation from tubulin genes of other ciliates which live in temperate areas and were analysed in parallel with E. focardii. The complete coding sequence of the 1,600 bp beta-tubulin gene was then determined and shown to contain unique structural features of potential importance for E. focardii microtubule organization and activity. Of eight unique substitutions detected, seven were concentrated in the large amino terminal domain of the molecule that directly interacts with the carboxy terminal region of alpha-tubulin for heterodimer formation. Sequence analysis of the cloned gene revealed, in addition, a potential new exception in the use of the genetic code by ciliates. A TAG codon was aligned in correspondence with Trp-21 which is strictly conserved in every tubulin sequence so far determined.

Dynamic instability of microtubules from cold-living fishes

The dynamic instability of microtubules free of microtubule-associated proteins from two genera of cold-living fishes was measured, by means of video-enhanced differential-interference-contrast microscopy, at temperatures near those of their habitats. Brain microtubules were isolated from the boreal Atlantic cod (Gadus morhua; habitat temperature approximately 2-15 degrees C) and from two austral Antarctic rockcods (Notothenia gibberifrons and N. coriiceps neglecta; habitat temperature approximately -1.8 to + 2 degrees C). Critical concentrations for polymerization of the fish tubulins were in the neighborhood of 1 mg/ml, consistent with high interdimer affinities. Rates of elongation and frequencies of growth-to-shortening transitions ("catastrophes") for fish microtubules were significantly smaller than those for mammalian microtubules. Slow dynamics is therefore an intrinsic property of these fish tubulins, presumably reflecting their adaptation to low temperatures. Two-dimensional electrophoresis showed striking differences between the isoform compositions of the cod and the rockcod tubulins, which suggests that the cold-adapted microtubule phenotypes of northern and southern fishes may have arisen independently.

Divergent neural beta tubulin from the Antarctic fish Notothenia coriiceps neglecta: potential sequence contributions to cold adaptation of microtubule assembly

The cytoplasmic microtubules of the cold-adapted Antarctic fishes, unlike those of homeotherms and temperate poikilotherms, assemble and function at body temperatures in the range -1.8 to +2 degrees C. To determine whether alterations to the primary sequence of beta tubulin may contribute to enhancement of microtubule assembly at cold temperatures, we have cloned and sequenced a 1.8-kilobase neural beta-chain cDNA, Ncn beta 1, from an Antarctic rockcod, Notothenia coriiceps neglecta. Based on nucleotide sequence homology, Ncn beta 1 probably corresponds to a class-II beta-tubulin gene. The 446-residue beta chain encoded by Ncn beta 1 is closely related (sequence homology approximately 95%) both to the neural class-I/II isotypes and to the neural/testicular class-IV variants of higher vertebrates, but the sequence of its carboxy-terminal isotype-defining region (residues 431-446) has diverged markedly (> or = 25% change relative to the I/II/IV referents). Furthermore, the Ncn beta 1 polypeptide contains six unique amino-acid substitutions (five conservative, one nonconservative) not found in other vertebrate brain isotypes, and the carboxy-terminal region possesses a unique tyrosine inserted at position 442. We conclude that Ncn beta 1 encodes a class-II beta tubulin that contains sequence modifications, located largely in its interdimer contact domain, that may contribute to cold adaptation of microtubule assembly.

Colchicine-binding sites of brain tubulins from an antarctic fish and from a mammal are functionally similar, but not identical: implications for microtubule assembly at low temperature

The tubulins of Antarctic fishes possess adaptations that favor microtubule formation at low body temperatures (Detrich et al.: Biochemistry 28:10085-10093, 1989). To determine whether some of these adaptations may be present in a domain of tubulin that participates directly or indirectly in lateral contact between microtubule protofilaments, we have examined the energetics of the binding of colchicine, a drug thought to bind to such a site, to pure brain tubulins from an Antarctic fish (Notothenia gibberifrons) and from a mammal (the cow, Bos taurus). At temperatures between 0 and 20 degrees C, the affinity constants for colchicine binding to the fish tubulin were slightly smaller (1.5-2.6-fold) than those for bovine tubulin. van't Hoff analysis showed that the standard enthalpy changes for colchicine binding to the two tubulins were comparable (delta H degrees = +10.6 and +7.4 kcal mol-1 for piscine and bovine tubulins, respectively), as were the standard entropy changes (delta S degrees = +61.3 eu for N. gibberifrons tubulin, +51.2 eu for bovine tubulin). At saturating concentrations of the ligand, the maximal binding stoichiometry for each tubulin was approximately 1 mol colchicine/mol tubulin dimer. The data indicate that the colchicine-binding sites of the two tubulins are similar, but probably not identical, in structure. The apparent absence of major structural modifications at the colchicine site suggests that this region of tubulin is not involved in functional adaptation for low-temperature polymerization. Rather, the colchicine site of tubulin may have been conserved evolutionarily to serve in vivo as a receptor for endogenous molecules (i.e., "colchicine-like" molecules or MAPs) that regulate microtubule assembly.

Different assembly properties of cod, bovine, and rat brain microtubules

Assembly properties of cod, bovine, and rat brain microtubules were compared. Estramustine phosphate, heparin, poly-L-aspartic acid, as well as NaCl, inhibited the assembly and disassembled both bovine and rat microtubules by inhibition of the binding between tubulin and MAPs. The assembly of cod brain microtubules was in contrast only marginally affected by these agents, in spite of a release of the MAPs. The results suggest that cod tubulin has a high intrinsic ability to assemble. This was confirmed by studies on phosphocellulose-purified cod tubulin, since the critical concentration for assembly was independent of the presence or absence of MAPs. The results show therefore that cod brain tubulin has, in contrast to bovine and rat brain tubulins, a high propensity to assembly under conditions which normally require the presence of MAPs. Even if cod MAPs, which have an unusual protein composition, were not needed for the assembly of cod microtubules, they were able to induce assembly of bovine brain tubulin. Both cod and bovine MAPs bound to cod microtubules, and bovine MAP1 and MAP2 bound to, and substituted at least the 400 kDa cod protein. This suggests that the tubulin-binding sites and the assembly-stimulatory ability of MAPs are common properties of MAPs from different species, independent of the tubulin assembly propensity.

Thermodynamics of reversible monomer-dimer association of tubulin

The equilibrium between the rat brain tubulin alpha beta dimer and the dissociated alpha and beta monomers has been studied by analytical ultracentrifugation with use of a new method employing short solution columns, allowing rapid equilibration and hence short runs, minimizing tubulin decay. Simultaneous analysis of the equilibrium concentration distributions of three different initial concentrations of tubulin provides clear evidence of a single equilibrium characterized by an association constant, Ka, of 4.9 X 10(6) M-1 (Kd = 2 X 10(-7) M) at 5 degrees, corresponding to a standard free energy change on association delta G degrees = -8.5 kcal mol-1. Colchicine and GDP both stabilize the dimer against dissociation, increasing the Ka values (at 4.5 degrees C) to 20 X 10(6) and 16 X 10(6) M-1, respectively. Temperature dependence of association was examined with multiple three-concentration runs at temperatures from 2 to 30 degrees C. The van't Hoff plot was linear, yielding positive values for the enthalpy and entropy changes on association, delta S degrees = 38.1 +/- 2.4 cal deg-1 mol-1 and delta H degrees = 2.1 +/- 0.7 kcal mol-1, and a small or zero value for the heat capacity change on association, delta C p degrees. The entropically driven association of tubulin monomers is discussed in terms of the suggested importance of hydrophobic interactions to the stability of the monomer association and is compared to the thermodynamics of dimer polymerization.

Functions of tubulin isoforms

The biological significance of tubulin isotypes lies in their ability to function in different chemical and physical environments. Recent papers document the origin and distribution of several new tubulin isotypes and suggest new ways for studying their assembly and function in specialized cells.