Wednesday, March 11, 2009
Cold Resistant Microtubules
"In mammals, microtubules are unstable at temperatures below 50 degrees F" (25). Although Carrol states that icefish DNA have evolved to code for proteins that construct microtubules capable of thriving at temperatures much lower than this, he does not explain how a micrtubule can funtion at these lower temperatures. What about the microtubule proteins in icefish are different from the microtubule proteins in all other mammals? Also, how does the new microtubule structure affect cell shape and function? Finally, are there any current applications for microtubules that can survive low temperatures in fields such as medicine or technology?
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Microtubules are a part of the cytoskeleton and are involved in many cellular processes including mitosis and cytokinesis and they are made up of tubulin. They function both to determine cell shape, by stretching it out enough for processes to have room to occur, and in a variety of cell movements, including the intracellular transport of organelles and the separation of chromosomes during mitosis. Microtubules are dynamically unstable by nature and that is why they are frequently disassembled in the cell.
ReplyDeleteAccording to an article called, “The bloodless icefishes” by Peter Rejcek, these fish have a complex called CCT chaperonin that assists other proteins in the folding process, especially their role in the folding of tubulins that form microtubules.
The genes that code for tubulin in the icefish have been modified over time to be stable below 50 degrees Fahrenheit. They have Anti-freeze glycoproteins proteins (AFGPs) which help keep the microtubules stable in the cold habitat of the icefish. They have evolved to accommodate for their extremely cold environment.
In the article, “The suppression of brain cold-stable microtubules in mice induces synaptic defects associated with neuroleptic-sensitive behavioral disorders” the cold stability of microtubules in mice was discussed. Neuronal microtubules, specifically, are stabilized by microtubule-associated proteins or MAPs. Only a few certain MAPs have the ability to stabilize the microtubules against the cold. In this article, the scientists experimented eliminating MAPs and other proteins from mice from mice which then inhibited neuronal differentiation, an important process in a mouse that can be fatal if it doesn’t occur correctly. The scientists wrote that, “Stable microtubules are thought to be essential for neuronal development, maintenance, and function.” Microtubules are very necessary for regulation of many cellular processes so the icefish have evolved to keep their microtubules functioning correctly.
The Antarctic icefish probably could not survive anywhere else after adapting to the freezing water for millions of years. The water reaches as low as negative 1.8 degrees centigrade. Even Bill Detrich, a professor biochemistry and marine biology at Northeastern University in Boston is "interested in how the fish are able to fold their proteins in a cold, energy-poor environment." Detrich and a team of crewmen spent two months studying in Antarctica to get their answers. Proteins do act as enzymes that do most of the biological processes that our body requires to survive. Proteins also function as antibodies that recognizes intruders and starts the immune system to fight off the invaders. However, each protein must fold into a different three dimensional shape in order to carry out its function. CCT chaperonin does help in this process of folding tubulines to form microtubules especially in icefish. Microtubules maintains the cell shape and motility.
ReplyDeleteThe icefish were first brought to the freezing Antarctic waters after the drake Passage opened 40 million years ago that created a current of cold water and turned Antarctica into an icehouse. Almost all of the fish went extinct except for the Notothenoids, which later became the icefish. The Notothenoids were first benthic fish that lived at the bottom of the sea floor and they didn't have swim bladders so they couldn't move up and down in the water column. Because they didn't have a swim bladder, they evolved a demineralized skeleton so they became less dense and had now had the ability to float and find prey. They didn't require a lot of energy to float so this was very advantageous for them. In addition, icefish don't make red blood cells that transport oxygen to the cells and their blood is transparent because of this. Detrich believed that this occurred because they did live in cold, stormy waters which dissolves oxygen more than tropical waters. Therefore, the icefish received oxygen from the water through their gills.
The microtubules of the icefish have actually evolved to assemble efficiently at the low temperatures, unlike those of other mammals. These proteins have assembly-enhancing adaptations, and the ice fish's tubulins are strongly entropy driven, which is a thermodynamic quality representing the unavailability of a system's thermal energy for conversion into mechanical work. Through lab work it has been discovered from the polypeptides and cDNAs of the tubulin that the structural adaptations are due to the primary sequences of tubulin isotypes. For example, the class II B-tubulin siotype of N. coriiceps brain contains seven unique amino acid substitutions and one insertion of its 446-residuc primary sequence. These changes enhance polypeptide flexibility so the tubulin can easily add to the microtubule ends. The quaternary structure allows for such interactions to occur so the assembly of the microtubules can function in such cold temperatures.
For humans there are no (that I could find) applications for microtubules that can survive in freezing temperatures because as humans we don't need such an advantage due to the fact that we give in relatively stable environments. Most organisms who do require these types of microtubules have already adapted and received them through evolution.
http://books.google.com/books?id=dTPeqe-bO6AC&pg=PA301&lpg=PA301&dq=applications+for+microtubules+that+can+s
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T2P-3TGJRCW-22&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=ad2daeff1a70730f0b128537ef7480b6
http://antarcticsun.usap.gov/science/contentHandler.cfm?id=1541
Microtubules are components of the cytoskeleton of the cell. Made up of polymers of α- and β-tubulin dimers, they are the “highways” of the cell. They provide a pathway for vesicles and other organelles to travel along. Two motor proteins, kinesin and dynein, move along the microtubule structure. In organisms native to warmer areas, microtubules break down into their simpler subunits at low temperatures. Icefish microtubules, however, do not. This is because of alterations in the structures of tubulin subunits a microtubules associated proteins. Researchers are particularly interested in how a complex called CCT chaperonin assists other proteins in the folding process, especially their role in the folding of tubulins that form microtubules.
ReplyDeleteResearchers studying the icefish have discovered, as Jackie mentioned, that the very adaptation in icefish that allows them to remain on the bottom of the ocean to feed on krill and small crustaceans is a disease in humans. They hope to understand how icefish can prevent their skeletons from mineralizing and thus be able to understand why the elderly lose calcium from their bones. In addition to studying the bone composition of the icefish, researchers are also looking at the icefish’s lack of blood hemoglobin. Using gene subtraction, scientists discovered genes that were previously unknown to be involved in hemoglobin formation. Potential applications of this research include new therapies for anemics. Because the icefish has virtually no heat shock protection, it is very unlikely that the icefish can survive increases in water temperature.
http://en.wikipedia.org/wiki/Microtubule
http://antarcticsun.usap.gov/science/contenthandler.cfm?id=1540
Although my original question asked for the function of cold resistant microtubules in icefish, I had wished that your responses would contain information pertaining to the other mechanisms that icefish implement to survive the harsh environments that they are subject to. Jackie talked about the lack of red blood cells in the icefish, and how the oxygen simply traveled in the fluid circulating through the fish. She also addressed their demineralized skeleton that allowed them to stay at the bottom of the ocean floor, which would keep them at the warmest area possible in their immediate location. Melissa also touched on the icefish’s lack of hemoglobin. However, none of the responses addressed the Antifreeze Proteins (AFPs) that are present in icefish bodies and keep their insides from freezing up. It is these proteins that I was referring to that could have potential applications in society. First off let me explain how these AFPs work. Normally, in a freezing solution, when ice touches water that is just above the freezing temperature, the water molecules bind to the ice lattice and expand the ice front. Any particles in the solution are just pushed ahead by the “front”, thus allowing the ice to continue its expansion (if the temperature is low enough). However, the structure of the AFPs is unique in that instead of being pushed ahead by the ice, it actually binds to the growing front. This fusion of multiple AFPs to the ice front stops its growth, until enough AFPs have surrounded the areas of frozen water to contain it permanently. This is why AFPs are responsible for lowering the freezing point of the liquid they are in (1). These unique molecules could have many useful applications in society, and I had wished that you all would have touched upon some because it would have been interesting to hear about what your ideas were. I have come up with a few AFP applications myself. The first one that I could think of is in meat packing and other cold food storage systems. The idea behind freezing meats and other organic foods is to prevent them from denaturing and rotting so that they can be preserved for long periods of time. However, with some foods such as steak or chicken or even some vegetables, keeping them in cold storage for extended periods of time would cause them to freeze over completely to the point where they get “freezer burn”, which ruins all the flavor in the effected food. Genetically modifying the organisms that this food comes from to contain AFPs might prolong this effect from happening, if not prevent it all together. Even though it is too late, I truly would have liked to hear more ideas about what could be done with the unique traits that the icefish has developed, and how society might have utilized such a fascinating creature.
ReplyDelete1. Peter L. Davies, Jason Baardsnes, Michael J. Kuiper and Virginia K. Walker.
Structure and Function of Antifreeze Proteins. Philosophical Transactions: Biological Sciences, Vol. 357, No. 1423, Coping with the Cold: The Molecular and Structural . Biology of Cold Stress Survivors (Jul. 29, 2002), pp. 927-935. The Royal Society. April 14, 2009. http://www.jstor.org/stable/3066911