Assignment 2

Hormonal regulation of antifreeze protein gene expression in winter flounder

MUN Library Link: https://link-springer-com.qe2a-proxy.mun.ca/content/pdf/10.1007/BF00004733.pdf

PubMed Link: https://pubmed.ncbi.nlm.nih.gov/24221798/ 

Summary

    This paper discusses antifreeze proteins (AFP) or glycopeptide antifreeze proteins (AFGP) in teleost fish, and in particular, how these proteins are regulated by environmental conditions stimulating or suppressing growth hormone (GH) in Pseudopleuronectes americanus. These proteins are important since they are distributed throughout the body in the blood plasma and prevent ice crystals from penetrating the epidermis causing trauma. This allows the fish to survive in sub freezing temperatures which occur in their habitat during winter months. The production of these proteins are cyclic and relate to the length of the days, with protein synthesis increasing in fall as days become shorter, since GH is suppressed in the pituitary which allows AFP genes transcription to occur in the liver. GH production in the pituitary increases in the spring and summer, as they days are longer, and the GH blocks the transcription of AFP genes during the summer. This hormonal regulation is stimulated by the CNS. 

    The experiment indicates that an intact pituitary is necessary for the changes in AFP expression during the spring, however, in fish with the pituitary removed (hypophysectomized), the annual increase in AFP in blood plasma still occurred, meaning the levels stayed high throughout the year which indicates that the fish either cannot break down the proteins or the proteins continue to be synthesized in the liver. 

    The specific hormonal regulation of this cyclic process was not entirely understood at the time of publication, while the researchers note that it may be one hormone regulating the turning off of AFP gene transcription, there may be more hormones involved in the process, the paper details experiments with salmon in which GH plays a role in mRNA transcription in the liver in the production of AFP. It is also noted that the hormonal control of AFP production occurs at the level of AFP mRNA transcription, and that GH suppresses the transcription of AFP genes. These hormonal changes and protein synthesis are also related to the fishes food intake and growth cycle. The link between photoreceptors and the CNS is evident in that in experiments where day length is increased, the AFP production does not cease. In fish with pituitaries implanted into muscle tissue, when compared to intact fish, AFP production is also suppressed, as it would normally be, suggesting that the CNS inhibits GH from being released in winter when days are shorter. As days grow shorter in the fall, the hypothalamus inhibits GH release from the pituitary, which then stimulates the transcription of AFP proteins in the liver. AFP mRNA is then translated into precursor proteins which are translated into proAFP sequences. These become mature proteins and are secreted into the blood and transported to the epidermis where they can keep the epidermis from freezing in the colder temperatures in winter. 

Critique

    This study was quite interesting to me, at first since it was research carried out at Memorial, but upon further reading the hormonal control of protein synthesis became a topic that I enjoyed learning more about. This study was a culmination of 15 years of prior research into the topic of antifreeze proteins, and provided further insights into how GH affects AFP synthesis, and may also regulate the suppression of protein transcription. The paper was concise and provided clear background information on the research that had already been carried out, and provided further questions about what might be done in future experiments to learn more about this process. The exact mechanisms of gene activation, mRNA transcription and translation, and hormone synthesis were not explained by the researchers, however most of the individuals reading or citing this paper would presumably have an understanding of these processes.The figures and data presented were clear and concise, particularly those illustrating the  pathway in which photoperiod relates to mature AFP in the epidermis. While there were several questions posed by the researchers that were not answered at the time of publishing, this provides an opportunity to carry out more experiments in the field, and can help expand the understanding of the complex mechanisms behind protein expression. 

Further Study

    One particular area to pursue further research would be to first determine the role of the pituitary in the protein degradation process and clearance rate of AFP plasma levels, as noted in the paper. Determining the pituitary hormone that blocks the synthesis of AFP would also provide crucial insights into how this cycle is regulated, and studying both hypophysectomized fish and intact fish. 

    As well, the 3D structure of GH in P. americanus has not been ascertained, therefore this modelling may be useful to determine its role in regulation of the degradation process of AFP. As suggested in the paper, the precise biochemical pathways which regulate AFP gene transcription by growth hormone need to be determined in order to ascertain the mechanisms in which the AFP protein synthesis is stimulated and suppressed.