Seasonal variation in the level of antifreeze protein mRNA from the winter flounder

Origin of an antifreeze protein gene in response to Cenozoic climate change

Antifreeze proteins (AFPs) inhibit ice growth within fish and protect them from freezing in icy seawater. Alanine-rich, alpha-helical AFPs (type I) have independently (convergently) evolved in four branches of fishes, one of which is a subsection of the righteye flounders. The origin of this gene family has been elucidated by sequencing two loci from a starry flounder, Platichthys stellatus, collected off Vancouver Island, British Columbia. The first locus had two alleles that demonstrated the plasticity of the AFP gene family, one encoding 33 AFPs and the other allele only four. In the closely related Pacific halibut, this locus encodes multiple Gig2 (antiviral) proteins, but in the starry flounder, the Gig2 genes were found at a second locus due to a lineage-specific duplication event. An ancestral Gig2 gave rise to a 3-kDa "skin" AFP isoform, encoding three Ala-rich 11-a.a. repeats, that is expressed in skin and other peripheral tissues. Subsequent gene duplications, followed by internal duplications of the 11 a.a. repeat and the gain of a signal sequence, gave rise to circulating AFP isoforms. One of these, the "hyperactive" 32-kDa Maxi likely underwent a contraction to a shorter 3.3-kDa "liver" isoform. Present day starry flounders found in Pacific Rim coastal waters from California to Alaska show a positive correlation between latitude and AFP gene dosage, with the shorter allele being more prevalent at lower latitudes. This study conclusively demonstrates that the flounder AFP arose from the Gig2 gene, so it is evolutionarily unrelated to the three other classes of type I AFPs from non-flounders. Additionally, this gene arose and underwent amplification coincident with the onset of ocean cooling during the Cenozoic ice ages.

Reflections on antifreeze proteins and their evolution

The discovery of radically different antifreeze proteins (AFPs) in fishes during the 1970s and 1980s suggested that these proteins had recently and independently evolved to protect teleosts from freezing in icy seawater. Early forays into the isolation and characterization of AFP genes in these fish showed they were massively amplified, often in long tandem repeats. The work of many labs in the 1980s onward led to the discovery and characterization of AFPs in other kingdoms, such as insects, plants, and many different microorganisms. The distinct ice-binding property that these ice-binding proteins (IBPs) share has facilitated their purification through adsorption to ice, and the ability to produce recombinant versions of IBPs has enabled their structural characterization and the mapping of their ice-binding sites (IBSs) using site-directed mutagenesis. One hypothesis for their ice affinity is that the IBS organizes surface waters into an ice-like pattern that freezes the protein onto ice. With access now to a rapidly expanding database of genomic sequences, it has been possible to trace the origins of some fish AFPs through the process of gene duplication and divergence, and to even show the horizontal transfer of an AFP gene from one species to another.

Protein evolution revisited

Antifreeze proteins (AFPs) protect marine fishes from freezing in icy seawater. They evolved relatively recently, most likely in response to the formation of sea ice and Cenozoic glaciations that occurred less than 50 million years ago, following a greenhouse Earth event. Based on their diversity, AFPs have independently evolved on many occasions to serve the same function, with some remarkable examples of convergent evolution at the structural level, and even instances of lateral gene transfer. For some AFPs, the progenitor gene is recognizable. The intense selection pressure exerted by icy seawater, which can rapidly kill unprotected fish, has led to massive AFP gene amplification, as well as some partial gene duplications that have increased the size and activity of the antifreeze. The many protein evolutionary processes described in Gordon H. Dixon's Essays in Biochemistry article will be illustrated here by examples from studies on AFPs. Abbreviations: AFGP: antifreeze glycoproteins; AFP: antifreeze proteins; GHD: Gordon H. Dixon; SAS: sialic acid synthase; TH: thermal hysteresis.

Transcription of antifreeze protein genes in Choristoneura fumiferana

Antifreeze proteins (AFPs) are encoded by approximately 17 genes in the spruce budworm, Choristoneura fumiferana. Northern analysis using 6 different cDNA probes showed isoform-specific patterns that varied during development. Transcripts for the majority of isoforms were most abundant in the second instar overwintering stage, but some were also detected in first instar and even in egg stages. In situ hybridization using riboprobes corresponding to two 9 kDa protein isoforms showed differential AFP expression even in second instars; CfAFP10 RNA was detected in all tissues, but CfAFP337 RNA distribution was more limited. Two genomic regions encoding three AFP genes have been isolated. Presumptive regulatory regions conferred transcriptional activity when placed upstream of a luciferase reporter sequence and transfected into a C. fumiferana cell line. The CfAFP2.26 core promoter is an 87 bp sequence containing a TATA box, whereas the CfAFP2.7 core promoter is a 76 bp sequence with both a TATA box and CAAT box, which directed higher reporter activities when tested in vitro. Reporter activity was not enhanced with five different hormones, although lower activities were observed with all intron-containing constructs. AFP message half-life, as assessed using reporter assays, was not appreciably influenced by isoform-specific-3'UTRs. These studies successfully demonstrate the temporal and spatial diversity of AFP expression encoded by this small gene family, and underscore the complexity of their regulation.

Antifreeze proteins of teleost fishes

Marine teleosts at high latitudes can encounter ice-laden seawater that is approximately 1 degrees C colder than the colligative freezing point of their body fluids. They avoid freezing by producing small antifreeze proteins (AFPs) that adsorb to ice and halt its growth, thereby producing an additional non-colligative lowering of the freezing point. AFPs are typically secreted by the liver into the blood. Recently, however, it has become clear that AFP isoforms are produced in the epidermis (skin, scales, fin, and gills) and may serve as a first line of defense against ice propagation into the fish. The basis for the adsorption of AFPs to ice is something of a mystery and is complicated by the extreme structural diversity of the five antifreeze types. Despite the recent acquisition of several AFP three-dimensional structures and the definition of their ice-binding sites by mutagenesis, no common ice-binding motif or even theme is apparent except that surface-surface complementarity is important for binding. The remarkable diversity of antifreeze types and their seemingly haphazard phylogenetic distribution suggest that these proteins might have evolved recently in response to sea level glaciation occurring just 1-2 million years ago in the northern hemisphere and 10-30 million years ago around Antarctica. Not surprisingly, the expression of AFP genes from different origins can also be quite dissimilar. The most intensively studied system is that of the winter flounder, which has a built-in annual cycle of antifreeze expression controlled by growth hormone (GH) release from the pituitary in tune with seasonal cues. The signal transduction pathway, transcription factors, and promoter elements involved in this process are just beginning to be characterized.

The rat ortholog of the presumptive flounder antifreeze enhancer-binding protein is a helicase domain-containing protein

The expression of winter flounder liver-type antifreeze protein (wflAFP) genes is tissue-specific and under seasonal and hormonal regulation. The only intron of the major wflAFP gene was demonstrated to be a liver-specific enhancer in both mammalian cell lines and flounder hepatocytes. Element B, the core enhancer sequence, was shown to interact specifically with a liver-enriched transcription factor, CCAAT/enhancer-binding protein alpha (C/EBPalpha), as well as a presumptive antifreeze enhancer-binding protein (AEP). In this study, the identity of the rat AEP ortholog was revealed via its DNA-protein interaction with element B. It is a helicase-domain-containing protein, 988 amino acids in length, and is homologous to mouse Smubp-2, hamster Rip-1 and human Smubp-2. The specific binding between element B and AEP was confirmed by South-Western analysis and gel retardation assays. Residues in element B important to this interaction were identified by methylation interference assays. Mutation on one of the residues disrupted the binding between element B and AEP and its enhancer activity was significantly reduced, suggesting that AEP is essential for the transactivation of the wflAFP gene intron. The rat AEP is ubiquitously expressed in various tissues, and the flounder homolog is present as shown by genomic Southern analysis. The potential role of AEP in regulating the flounder AFP gene expression is discussed.

Isolation and characterization of an antifreeze protein from the longhorn sculpin, Myoxocephalus octodecimspinosis

A new type of antifreeze protein was isolated from the serum of the longhorn sculpin, Myoxocephalus octodecimspinosis, by gel filtration and high-performance liquid chromatography. This protein (LS-12) exhibits freezing point depression activity (thermal hysteresis) and ice crystal modification properties similar to those seen for other types of fish antifreeze polypeptide, except that ice crystals grow as hexagonal trapezohedra in the presence of LS-12, rather than hexagonal bipyramids usually seen. Ice crystal etching studies demonstrate that LS-12 does not bind to the hexagonal bipyramidal or secondary prism surfaces reported for the antifreeze polypeptides from winter flounder and shorthorn sculpin, respectively. Circular dichroism studies indicate that LS-12 has an alpha-helix content of about 60% at 1 degreesC, which is in good agreement with a value of about 70% predicted from the amino acid sequence. Limited proteolysis studies and further analysis of the amino acid sequence suggest that LS-12 consists of four amphipathic alpha-helices of similar length which are folded into a four-helix bundle. Based on its size (Mr=12299) and predicted tertiary structure, LS-12 can be regarded as the first example of a new class (type IV) of fish antifreeze protein.

Cloning and sequencing of cDNA encoding the LS-12 antifreeze protein in the longhorn sculpin, Myoxocephalus octodecimspinosis

cDNA coding for an antifreeze protein (LS-12) in the longhorn sculpin, Myoxocephalus octodecimspinosis, was prepared from liver mRNA using reverse transcriptase-polymerase chain reaction coupled with 3' and 5' RACE procedures. This cDNA contains 609 base pairs, including a 384-bp open reading frame which codes for a 128-residue LS-12 precursor protein. The predicted amino acid sequence of the mature LS-12 corresponds exactly to the amino acid sequence obtained from Edman degradation [G. Deng, D.W. Andrews, R.A. Laursen, FEBS Lett., 402, 1997, pp. 17-20]. The 20 residues preceding mature LS-12 are predicted to be a signal sequence, which is presumably cleaved off before the mature, 108-residue protein is secreted into the circulatory system. This is the first report of a cDNA sequence from M. octodecimspinosis.

Skin antifreeze protein genes of the winter flounder, Pleuronectes americanus, encode distinct and active polypeptides without the secretory signal and prosequences

Distinct antifreeze polypeptides (AFP) were isolated from the skin of the winter flounder, Pleuronectes americanus, by gel filtration and reverse phase high performance liquid chromatography. In parallel, several cDNA clones were isolated from a skin cDNA library using a liver AFP cDNA probe. Both protein and DNA sequence analyses indicate that flounder skin contains several distinct but homologous alanine-rich AFPs. Although the skin type AFPs contain 11 similar amino acid repeats found in the secretory liver type AFPs, the skin type AFPs are mature polypeptides lacking both the signal and prosequences, indicating that they may function intracellularly. The skin type AFP is significantly less active in thermal hysteretic activity than the liver type AFP. Genomic Southern analysis indicates that like the liver type AFP genes, there are multiple copies (30-40 copies) of skin type AFP. Although the liver type AFP genes are specifically expressed in the liver and to a lesser extent in intestine, the skin type AFP genes are expressed in all tissues examined including the liver and abundantly in exterior tissues, i.e. skin, scales, fin, and gills, suggesting an important protecting role in these exterior tissues.

Low temperature persistence of type I antifreeze protein is mediated by cold-specific mRNA stability

In winter flounder, the levels of type I antifreeze protein (AFP) and its mRNA vary seasonally by as much as 1000-fold. Elevated levels in the fall are prompted by the loss of long day-lengths, while higher spring temperatures correlate with AFP clearance. We have investigated the role of temperature on AFP accumulation using transgenic Drosophila melanogaster by expressing multiple AFP genes under control of the heat-inducible hsp70 promoter. AFP and AFP mRNA persisted far longer in flies reared at 10 degrees C compared to 22 degrees C. This difference appears to be mediated by cold-specific mRNA stability since no such temperature effect was observed with either an endogenous heat-inducible mRNA or a constitutively expressed mRNA.

Antifreeze protein pseudogenes

Three members, 11-3, F2 and 5a, of the type-I antifreeze protein (AFP) multigene family in winter flounder were sequenced. All three belong to the subset of AFP genes that are linked, but irregularly spaced, and show significant differences from the functional genes in tandem repeats. 11-3 and F2 appear to be pseudogenes. Their intron, 3'-exon and 3'-flanking DNAs are similar to those of other AFP genes, but their 5'-exon is either missing or extensively modified, and has stop codons present in all three reading frames. Based on a comparison of intron sequences of family members, 11-3/F2 may represent a residual progenitor AFP gene which was duplicated after reaching pseudogene status. The third gene, 5a, is remarkable in having a 3'-exon that encodes an exceptionally long, Ala-rich sequence that lacks any semblance of the 11-amino acid repeats found in 11-3, F2 and functional AFP genes. 5a might also be a pseudogene, because its presumed TATA box appears to have mutated.

Conservation of antifreeze protein-encoding genes in tandem repeats

The antifreeze protein (AFP) multigene family in winter flounder is made up of genes in tandem repeats and others that are linked, but irregularly spaced. The close spacing of the tandemly repeated genes has provided an opportunity to determine how well nearest-neighbour AFP genes from the tandem repeats resemble each other, the genes elsewhere in the tandem repeats, and the genes outside the tandem repeats. Four pairs of tandemly repeated genes were sequenced. Two pairs were identical and probably represent independent clones of the same chromosomal region. The six unique genes coded for one or other of the two major AFPs, HPLC-6 and HPLC-8. Their comparison over a region of 900 bp. from just after the CAAT box to just before the polyadenylation signal, showed a maximum of 26 single-bp changes and no major insertions or deletions. Nearest-neighbour genes had almost as many changes as genes elsewhere in the tandem repeats. However, these six genes were much more homogeneous than five AFP genes from outside the tandem repeats, none of which encode a major AFP component, and all but one had large insertions, deletions, or rearrangements. Despite the similarity of the genes in tandem repeats, three variants due to short insertions or deletions in the 3'-flanking DNA were interspersed, and genes coding for HPLC-6 and HPLC-8 were nearest neighbours. It is suggested that enhanced opportunities for recombination between repeats have helped keep these genes more uniform than those outside the tandem repeats.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential translatability of antifreeze protein mRNAs in a transgenic host

The expression of fusion gene constructs containing Drosophila regulatory sequences and the structural portions of fish antifreeze protein genes have been examined by transfer into Drosophila melanogaster using P elements. A fusion gene, containing the enhancer, promoter, and cap site of the yolk polypeptide 1 gene, joined in the 5'-untranslated region to the structural portion of the winter flounder type I antifreeze gene, was transcribed in mature female transformants to give an mRNA of the predicted size, but no antifreeze protein was detected by Western blotting. When the same antifreeze protein gene was fused to a Drosophila hsp 70 gene regulatory region and placed downstream of the yolk polypeptide gene enhancer, appropriate expression of mRNA was directed by both gene regulatory elements. However, a translation product from this mRNA was only observed under heat shock conditions and was present at low levels. It is suggested that type I antifreeze mRNA, with its high content of alanine codons and their grouping into clusters of up to seven in a row, is poorly translated when in competition with other host mRNAs. In agreement with this hypothesis, a fusion gene construct between the yolk protein gene regulatory region and two type III antifreeze protein genes produced sub-mmolar concentrations of antifreeze protein in mature females from each of several transgenic lines analysed. The type III antifreeze protein does not have an imbalanced amino acid composition or sequence irregularities, and may be an appropriate choice for conferring freeze protection to frost-susceptible hosts by gene transfer.

Seasonal cycle and regulation by temperature of antifreeze protein mRNA in a Long Island population of winter flounder

The seasonal cycle and regulation by temperature of antifreeze protein mRNA (AF mRNA) were investigated in a Long Island population of winter flounder (Pseudopleuronectes americanus) by Northern blot hybridization and by in vitro translation of liver RNA. AF mRNA was expressed at high levels in the fall and winter (Nov.-Feb.) and at low or undetectable levels in the summer. The time of accumulation of AF mRNA coincides with the time during which water temperature and photoperiod decrease to 4°C and 9 h of light per day, respectively. A temperature and photoperiod decrease in the laboratory during this time also resulted in high levels of AF mRNA. The levels of other mRNAs, as assayed by in vitro translation, were relatively constant during both seasonal acclimation and laboratory acclimation. The seasonal cycle of AF mRNA in Long Island winter flounder is similar to that of a more northern, Newfoundland population of winter flounder and different from that of an intermediate, New Brunswick population. These similarities and dissimilarities are discussed in light of potentially different exogenous and endogenous regulatory cues in the different populations.

Genetic influences on reproductive system development and function: A review

This paper presents a current view of the genomic and neuroendocrine interaction based on our studies of the reproductive system in the platyfish (Xiphophorus maculatus). It also presents observations from basic research and applied biologists on natural and artificially reared fishes and indicates that there is a direct genetic involvement in the control of spawning, growth rates, size and age at maturation and final body size, similar to that described in platyfish. The past, present and future association of aquaculture and basic science, especially DNA technology, is discussed and potential directions for future research are presented.

Regulation of antifreeze protein production in winter flounder: a unique function for growth hormone

Salmon pituitary extract and the protein fraction unabsorbed on concanavalin A-Sepharose, the carbohydrate-poor fraction, depressed plasma levels of antifreeze proteins (AFP) when the pituitary fractions were administered to flounder in late fall or winter. The active pituitary protein occurred in the fraction with a mean molecular weight of 25,000. The two major isohormones of growth hormone (GH) were the only biologically active proteins identified from the pituitary. Hypophysectomized flounder synthesize AFP in the spring and the two isohormones of GH suppress the synthesis. The fraction of flounder pituitaries containing putative GH depressed flounder plasma levels of AFP in late fall.

Hormonal regulation of antifreeze protein gene expression in winter flounder

The essential features of experiments carried out over the past fifteen years are brought together with new data to formulate a model describing the hormonal regulation of the annual plasma antifreeze polypeptide (AFP) cycle in winter flounder (Pseudopleuronectes americanus). The precise time of onset of antifreeze synthesis in the fall appears to be regulated by photoperiod acting through the central nervous system (CNS) on the pituitary gland. During the summer, growth hormone (GH) blocks transcription of the AFP genes. With the loss of long daylengths in the fall, the CNS inhibits the release of GH allowing AFP gene transcription in the liver to proceed. In the spring GH is again released from the pituitary and AFP gene transcription is blocked.

Flounder antifreeze protein synthesis under heat shock control in transgenic Drosophila melanogaster

The gene coding for the most abundant antifreeze protein (AFP) in the winter flounder was placed downstream of the Drosophila melanogaster hsp70 promoter and introduced into the D. melanogaster germ line by P-element-mediated transformation. In each of six transgenic strains tested, heat shock treatment induced the expression of two major AFP gene transcripts and one minor one. All three transcripts were spliced despite the lack of an obvious D. melanogaster internal intron-splicing sequence. The variation in transcript length was caused by selection of different polyadenylation sites. Western blots showed the presence of immunoreactive AFP in hemolymph from heat-shocked transformants. The immunoreactive material had a molecular weight of 6,200, which is consistent with the loss of the signal sequence from the primary translation product and the retention of the pro sequence. Thus, all the signals for flounder pre-mRNA and preprotein processing were recognized in D. melanogaster.

Flounder antifreeze protein synthesis under heat shock control in transgenic Drosophila melanogaster

The gene coding for the most abundant antifreeze protein (AFP) in the winter flounder was placed downstream of the Drosophila melanogaster hsp70 promoter and introduced into the D. melanogaster germ line by P-element-mediated transformation. In each of six transgenic strains tested, heat shock treatment induced the expression of two major AFP gene transcripts and one minor one. All three transcripts were spliced despite the lack of an obvious D. melanogaster internal intron-splicing sequence. The variation in transcript length was caused by selection of different polyadenylation sites. Western blots showed the presence of immunoreactive AFP in hemolymph from heat-shocked transformants. The immunoreactive material had a molecular weight of 6,200, which is consistent with the loss of the signal sequence from the primary translation product and the retention of the pro sequence. Thus, all the signals for flounder pre-mRNA and preprotein processing were recognized in D. melanogaster.

Biosynthesis of antifreeze polypeptides in the winter flounder. Characterization and seasonal occurrence of precursor polypeptides

The precursor proteins for winter flounder antifreeze polypeptide (AFP) were isolated from liver using gel filtration chromatography and reverse-phase high-performance liquid chromatography. Two major pro-antifreezes (Mr 5000), corresponding to the precursors for AFP-6 and AFP-8, were characterized by amino acid analyses and automated Edman degradation. These precursors showed significant antifreeze activity. The pro-antifreezes were synthesized in the liver seasonally as demonstrated by immunoblotting and in vitro liver incorporation studies. No mature AFP were detected in liver, thus indicating that the processing of pro-antifreezes, including amidation of the C-termini, occurred mainly in the serum. The function(s) of the prosequences, if any, remain unclear.

Antifreeze protein genes are tandemly linked and clustered in the genome of the winter flounder

We have used genomic Southern blots and restriction maps of genomic clones to examine the organization of the antifreeze protein multigene family in the winter flounder. The majority of the approximately equal to 40 antifreeze protein (AFP) genes in this fish are present in 7- to 8-kilobase-pair (kbp) elements of DNA, which are iterated as tandem direct repeats. Each repeat contains a single antifreeze protein gene that is 1 kbp long, and all of these genes have the same transcriptional orientation. Although the repeated elements are highly homologous, they do show some restriction site and restriction length polymorphisms. When flounder genomic DNA is digested with restriction endonucleases that do not cut within the repeats, most of the antifreeze protein genes reside in fragments that are at least 40 kbp long, representing clusters of five or more repeats in tandem. After genomic DNA is digested with Xba I or Xho I, these genes are present in fragments of exceptionally high molecular weight, suggesting that the clusters themselves are grouped together in the genome. The AFP gene locus may have evolved by gene amplification as recently as 10(6) years ago in response to the onset of the Cenozoic ice age.

Accumulation of winter flounder antifreeze messenger RNA after hypophysectomy

The effect of hypophysectomy (hypex) on winter flounder antifreeze mRNA accumulation in the liver was examined. Hypophysectomy resulted in a significant decrease in serum freezing temperature, and increases in liver weights, total liver poly(A)+ RNA and antifreeze polypeptides (AFP) mRNA accumulation. The identity of the AFP mRNA in hypex animals was confirmed by gel electrophoresis, cell-free translation, and Northern blot hybridization techniques. Cytoplasmic dot hybridization analysis indicated that the AFP mRNA level in hypex fish approximated that observed in winter animals actively synthesizing AFP. An increase in AFP mRNA was detectable as early as the first day after hypophysectomy and by Day 7 reached 25% of the level found in fish actively synthesizing AFP mRNA during the winter months. Since AFP mRNA is found at very low levels in the control flounder, we suggest that its accumulation after hypophysectomy depends on accelerated transcription. The pituitary gland appears to regulate the liver AFP mRNA level by a negative transcriptional control mechanism.