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.

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.

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.

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.