Organisms respond to changes in temperature in a variety of ways, showing both acute and longer term changes in physiology and molecular-level activity. With work done by various researchers a response has been identified that is undergone by Gillichthys mirabilis in response to “heat shock” or in some instances a stress response that allows the organism to acclimate its body in order to tolerate changes in temperature. These heat shock proteins cover a very broad class of proteins that help reduce protein degradation and therefore conserve energy.
Besides the ability to allow acclimation, these proteins also share another important function which is to initiate the proper actions of proteins such as folding, oligomerization and activation of cellular proteins. The review of work done with G. mirabilis by various researchers shows that a greater understanding of the role that proteins play in this process is becoming clear. These studies have all discovered a piece of the puzzle which in the end will give light to the final picture.
To review these articles I feel that is it only right that I go in chronological order seeing that future research that is done after initial studies should have more conclusive or deeper findings in their results. The work done by Dietz (1994) was an exploration into the basis of acclimation in G. mirabilis. In the Dietz study he identified two stress proteins that changed their threshold induction temperature in a way significant enough to compensate for much of the needed acclimation.
These proteins were able to accomplish these changes at a translational, transcriptional or even post- translational level, or at a combination of these. HSP70 and HSP90 are the proteins that were identified that were able to alter their temperature thresholds allowing them to acclimate to a greater or lower temperature. One last discovery that Dietz made that I found to be interesting was that in the G. mirabilis the brain was lacking HSP70 and therefore had no ability to synthesize this protein in the brain.
This protein has been identified in other fish of similar family therefore the question was raised how this species keeps its brain from overheating (Dietz, 1994). Work done by Kultz (1996) again had a focus on heat stress; however, he expanded his study to see if osmotic stress also induced a similar effect in order to have the species acclimate to its environment. Kultz also looked more at gill cells to see if the effect of heat stress or osmotic stress was the same as in other cells in the body that had been studied previously.
It was found when dealing with hyposmotic and hyperosmotic stimuli the response, while similar to that of heat stress, involved different proteins. In the case of gill cells, the proteins involved in acclimation to heat stress included eight different isoforms of HSP70; however, this was not the case with osmotic stress. It seemed that the stress response by the gill cells is specific based on the stressor that is acting on them. Finally there were two observations that were made at the cellular level with protein metabolism.
Kultz recorded that osmotic stress had a short term effect on protein metabolism that was strong but lasted anywhere from a few minutes to hours. It was also noticed that when the cells were dealing with osmotic stress the responses were interdependent with single cells that were already somewhat acclimated (Kultz, 1996). The next two studies that I reviewed were done by the same researchers over about four or five years. Buckley and Hofmann (2002) published their first study with G. irabilis in which they looked at the activation of HSF1 and how the concentration of this protein was affected by varying temperatures in fish acclimated to different temperatures. Buckley and Hofmann (2002) also expanded their research to include both the eurythermal as well as the poikilothermic properties of the organism. The research showed that there was a potential for the activation temperature of HSF1 to vary based on acclimation.
This supports the hypotheses coupling plasticity in promoter binding events to downstream variations in the temperature at which Hsp levels vary in stressed cells (Buckley and Hofmann, 2002). While continuing their work Buckley and Hofmann (2004) examined the effect of season change on activation of HSF1 in the liver. They wanted to see if the hsp gene that was induced in the earlier laboratory acclimation study (Buckley & Hofmann 2002) was going to be affected by environmental conditions, or whether because of the gradual nature of the change in conditions it remained relatively stable.
Surprisingly, the change of season had no real effect on the HSF1 activation level, meaning that while things still got activated there was no significant amount of increase or decrease of the hsp gene. However, Buckley and Hofmann did find that HSP70 and HSF1 did vary based on season; however I would say that, based on previous findings, this should be expected since these organisms were going to acclimate to temperature and temperature seems to change based on season.
One discovery they did make in this study was that there was a difference in higher and lower heat stress temperatures and that is that HSF1 and HSP70, coupled with mRNA synthesis, is activated more quickly in higher temperatures then in lower temperatures. This could be significant when it comes to figuring out what affects these proteins have differing and quickly fluctuating temperatures which many poikilotherms deal with and how the plasticity of the hsp gene works in the organism (Buckely & Hofmann, 2004). To further their research, Buckely and Hofmann (2006) used microarray in order to see how genes actually responded to heat stress.
They also looked on the molecular level to see how cells dealt with the stress and it was found that molecular chaperones helped a great deal in order for the transcription of genes to take place so that the organism could acclimate to the change in the environment. They found when looking at the genes that there were a great deal more genes that responded to cooling stress then those that responded to heat stress; however, it was noted that this could have been because when studied, the acclimation time for cooling was done in a time frame of 22 days.
The heat stress was done in only hours and therefore allowed more genes to be implemented in cooling then there were in heating. The most important finding of this study was to show the real possibility of using microarray and that further research would be sure to follow after seeing what was found with this work (Buckely & Hofmann, 2006). Finally the last research paper I looked over was done by Lund et. al. (2006) which involved the closer inspection of the Hsp gene in regards to the mechanisms on the molecular level in the G. mirabilis species.
After month-long acclimation in three environments in the laboratory, Lund and others sought to figure out the kinetics involved in the activation of the HSF1, and the corresponding transcription of Hsp gene and HSP70 mRNA that are involved with regulatory expression of Hsp. They found that thermal history influenced the activity of HSP70 mRNA and because of this the exposure time to the temperature to be acclimated was changed. It was also observed that, no matter what temperature history or environment the fish was placed in, the level of regulation was always conserved and this could provide a greater range of thermotolerance.
Lund et. al. came to the conclusion that the kinetics of HSF1 DNA-binding activity was being changed by the fact that the DNA-binding activity had come from a different environment. The fact that some were acclimated in a warmer environment changed the rate at which they could acclimate to another one. Their findings lead them to believe that if an organism is acclimated at a colder temperature then it will be able to have more HSF1 activity, therefore allowing it to acclimate more quickly to another environment, as opposed to an organism that was originally acclimated to a warmer environment.
So again, the ability of these animals to acclimate more quickly is based on where they were prior to exposure to this new environment (Lund etc. al. , 2006). As I have reviewed these six studies it is clear to see that these organisms and many just like them have a fantastic ability to be able to change their body and prepare it for various environments.
Just looking at these six studies which I know is only a few out of the many that have been done it is clear that our knowledge of the inter-workings of these systems is growing everyday. While these researchers have come up with some great ideas of how all these processes take place but while they have done this with various angles it seems clear that no definitive answer as been made and that hopefully with future work maybe soon we will have more of an answer.