This web page was produced as an assignment for an undergraduate course at Davidson College
Can it make me smarter?: GAP-43
On June 19, 2000, BBC News Online reported, "'Smart Gene'
makes for brainy mice." The article went on to describe a study at Northwestern
University that caused mice to overexpress a protein called GAP-43 in their
brains. GAP-43 stimulates nerve fiber growth, and higher levels caused more
nerve fibers to grow. Mice overexpressing GAP-43 apparently learned to find
food in a maze much faster than their non-enhanced counterparts, and were
also able to remember how to find the food for an abnormally long period of
time. The author goes on to give a little bit of background information concerning
research of genes that aid in learning, describing the discovery of NR2B,
another protein that, when overexpressed, helped maintain the 'youthfulness'
of mice brains. It is suggested that GAP-43 could aid humans, possibly those
with Alzheimer's or mental retardation. Briefly touching on the sticky ethics
of gene therapy in humans, the author quotes the main GAP-43 researcher, Aryeh
Routtenberg, as saying that he is against using GAP-43 to create drugs for
people who want a magic antidote to become smarter. Although the article gives
little scientific detail, the scientific points are all straightforward and
true. In fact, the only sensationalized aspect of this article is its title,
which, with its quotation marks around 'smart gene', could even be interpreted
as mocking people who hailed this as discovery of the 'smart gene'. Although
not very in-depth, the article did a good job of giving a quick summary that
any average person could understand and appreciate.
Note: All information attributed to Routtenberg et al., 2000 unless otherwise noted.
After reading the rather sparse BBC News Online article, I wanted
to know more. Fortunately, the author of the study and the journal of publication
were given in the online article, making the more in-depth study easy to locate.
First, I used the PNAS article as well as a few others to learn some basics
about GAP-43, finding that it is an essential growth associated protein; mice
who lack it do not survive long after birth (Strittmatter et al.,
1995). GAP-43 is phospyorylated by protein kinase C (PKC), and plays a large
role in morphological changes at the synapse. It is located in nerve tissue
and is attached to the presynaptic membrane (OMIM, 2003). Chiefly, GAP-43
helps axons grow and serves to create new neurological connections; its mechanism
of action at synapses may lie in its ability to bind both actin and fodrin.
Homologs are present in mice, rats, and chicks, suggesting that GAP-43 plays
a necessary role in many organisms. GAP-43, because of its possible regenerative
abilities, is and will continue to be a useful protein to study in patients
with disorders such as Alzheimer's and schizophrenia (Halim et al.,
2003, Regland et al., 2001).
Before Routtenberg et al. undertook their study, it had been shown that GAP–43 might play a critical role in memory storage. The purpose of Routtenberg’s research was to support this idea with concrete proof. Researchers created transgenic mice that overexpressed either phospyorylatable (G-phos) or non-phosphorylatable (G-nonP) GAP-43 (Ser-Ala at position 42). Additionally, they created a mouse stain overexpressing a permanently phospyorylated GAP-43 (Gperm). Wild type mice from the same genetic background as transgenic mice were used as control subjects. During each trial, researchers put a food-deprived mouse in the center of an 8-arm Olton radial maze with food in a cup at the end of each arm. Mice were tested for their ability to learn and remember where the food was located.
In the maze trials, G-Phos mice made fewer mistakes remembering where the food was located than wild-type or G-nonP mice. Also, wild-type mice and Gperm mice performed better than the G-nonP mice. When the interval between trials was increased, the abilities of G-Phos mice were outstanding, as indicated by figure 1B.
Figure 1. Number of maze errors in transgenic and wild-type mice A. Mice were placed in center of maze and allowed to roam. After finding four food rewards, mice were removed from the maze for one minute. The maze was re-oriented randomly, but remaining food was placed in the same spatial location as it had been when the mice had been removed from the maze. Mice were again placed in the maze (after one minute was up) to seek out the remaining four food rewards. Errors were recorded when mice entered arms of the maze they had already entered during the first trial. B. Same as A, except mice were removed from the maze for a total of twenty minutes as opposed to one minute. Figure Credit: Figure 1 from Routtenberg, A. et al., 2000. Permission requested.
Routtenberg et al. studied LTP in the second part of their research for this paper. LTP, which stands for long term potentiation, involves a high-frequency stimulation followed by an extended increase in the efficacy of the synapse. LTP is of particular interest in the hippocampus of many mammals, where it is thought to be involved in learning and formation of memories (Shors & Matzel, 1997). Routtenberg et al. hoped to correlate increased levles of LTP with enhanced learning abilities. After carrying out LTP (using a complex electrophysiological method) on each of the four mouse strains, researchers saw that GPhos as well as GPerm mice showed stronger LTP than their wild type or GNonP counterparts.
By using transgenic mice with only the PKC phosphorylation site mutated, the researchers in this study were able to decisively show that phosphorylation regulates the action of GAP-43. Phosphorylation of GAP-43 is a critical step in learning and in determining how long information is retained; mice with phosphorylatable GAP-43 showed superior learning, while mice with unphosphorylatable GAP-43 showed no enhanced learning. Routtenberg et al., in their discussion, go on to hypothesize mechanisms by which GAP-43 overexpression could have enhanced the learning and long term potentiation of mice in the study. For one, synapses are altered slightly during development based on inputs received at the synaptic terminal, and elevated levels of GAP-43 at the pre-synaptic terminal would allow for the receipt of stronger inputs, which could lead to a higher density of synapses formed during development. Additionally, more GAP-43 in the presynaptic terminal at the time of the study could have aided in the release of increased amounts of neurotransmitters as well as in additional growth of the presynaptic terminal. These changes would support an environment for increased learning.
In addition to the GAP-43, NR2B has also been implicated in aiding superior learning ability and increased LTP. NR2B is a subunit of the NMDA receptor, which is present on the postsynaptic terminal. NR2B was dubbed as the initial 'Smart Gene' in some popular press articles, including one from Time magazine with the title, "Smart Genes?" (Lemonick, 1999).
After examining both a popular press and a scientific article on the same topic, I've realized the benefits and drawbacks of each type of publication. The Routtenberg et al. article from PNAS provided me with a wealth of useful information. Because I am a biology student, I was able to deduce important conclusions from the article. However, were I trained specifically in neuroscience, I would have had a much easier time extracting relevant information from the article. I can see how a lay person with no biology training would benefit little from the scientific article; complex words and ideas would prevent them from taking away any useful conclusions. Articles from the popular press, if written without sensationalizing data, provide relevant information to the public in a concise format. They also give people information they might not otherwise learn (When was the last time your English teacher picked up a copy of Cell or Proceedings of the National Academy of Sciences?). These articles are beneficial for citizens who want to be up on the latest news, but not on every detail of a study. The BBC News Online article was useful in that it provided information about the scientific article that made it easy to access. In this fashion, if a person wanted to know more about learning enhancement, the citation was close at hand.
Click here to view human GAP-43's amino acid sequence.
Click here to view the sequence of human GAP-43 mRNA.
Click here for GAP-43's OMIM (Online Mendelian Inheritance in Man) Citation
To view conserved domains in GAP-43, go to NCBI's CD-search at this link and enter: I52638
Want to buy GAP-43 antibodies?
Halim, N.D., et al. 2003. Presynaptic proteins in the prefrontal cortex of patients with schizophrenia and rats with abnormal prefrontal development. Mol. Psychiatry. 8(9):797-810.
Lemonick, Michael. 1999. Smart Genes? Time: 13 September; 54-58.
NCBI Conserved Domain Search. <http://www.ncbi.nlm.nih.gov/Structure/cdd/wrpsb.cgi>. Accessed 2003 Sept 14 .
[OMIM] Online Mendelian Inheritance in Man. 2003 Aug 15. Growth Associated Protein 43 Citation. <http://www.ncbi.nlm.nih.gov/htbin-post/Omim/dispmim?162060>. Accessed 2003 Sept. 7.
Regland, B. et al. 2001. Treatment of Alzheimer's disease with clioquinol. Dement Geriatr Cogn Disord. 12(6):408-414.
Routtenberg, A., et al. 2000. Enhanced learning after genetic overexpression of a brain growth protein. Proc. Nat. Acad. Sci. 97(13); 7657-7662. http://www.pnas.org/cgi/content/full/97/13/7657
Shors, T.J., Matzel, L.D. 1997. Long-term potentiation: What's learning got to do with it? Behavioral and Brain Sciences 20 (4): 597-655.
Strittmatter, S.M. et al. 1995. Neuronal pathfinding is abnormal in mice lacking the neuronal growth cone protein GAP-43. Cell. 80;445-452.
Whitehouse, D. 2000. 'Smart Gene' makes for brainy mice. BBC News Online.<http://news.bbc.co.uk/1/hi/sci/tech/797471.stm> Accessed 2003 Sept 7.
Questions, Comments? E-mail Sara