Lecture 17

th BIOL 300 October 17 2012 Lecture 17 Dr. Shock When looking at neurons in the brain, we can see that the composition of DSCAM isoforms in a cell will give the neuron its identity when compared to other neurons. • This can be important during development, when a neuron’s cell body must begin to branch out to form axons and dendrites. If all of these branches have a “self” identity given by a specific DSCAM isoform composition, this would prevent the branches from crossing one another. The spreading out of dendrites without touching is very important for proper brain function; what DSCAM may do is: • In dendrites from the same cell, make sure that when homophilic interactions occur between the branches, a cell signal which induces repulsion of one of the branches occur • In dendrites from different cells, nothing would happen because this homophilic reaction will not be able to occur. The first mutants of the DSCAM gene showed that the neurons axons or dendrites lost their ability to spread properly when compared to the wild-type neurons. • The simplest example is the axon (A), in which these is only one branch (the axon iself) whose growth is not always mediated by DSCAM. In fly brains, there are structures known as mushroom bodies (MB) in which the mushroom “top” s formed by a cluster of cell bodies, and the tail is formed by axons which split into two branches: one going to the dorsal lobe and the other to the medial lobe. The lobes are formed by the axons themselves; without an axon, there is no lobe. • Somehow, these axons have to be told to split into two branches. DSCAM mutants in one neuron in the mushroom body caused both of its axons’ branches to go to only one lobe, so we can see the repulsion mechanism between branches no longer works. • Therefore, DSCAM plays a role in segregation of sister branches of an axon, but we don’t yet know whether or not isoform diversity is necessary for this mechanism. An experiment was developed to be able to answer this question: the actual picture shows the path of the axon in the MB, followed by pictorial representations of what’s happening. • The hypothesis of this experiment was that DSCAM diversity is important for mediation of segregation of axons in the MB. • The experiment replaced all the isoforms of DSCAM with one single cDNA, thus eliminating any form of variability; they did this with 3 randomly selected cDNAs for 3 different DSCAM isoforms, using one at a time. 1 th BIOL 300 October 17 2012 Lecture 17 Dr. Shock • In the first example, DSCAMFRT (wild-type/null heterozygote, i.e. +/-) is fully functional, showing that DSCAM is dominant. We can see that both lobes are receiving axons, shown by the 2 red lines. • This is because, at the developmental stage where they are just beginning to split, homophilic interactions between identical DSCAM isoforms induces a form of repulsive signalling which tells the sister branches to stay away from one another. • At the bottom, we have a null DSCAM mutant on one chromosome, and on the other chromosome a single isoform of DSCAM cDNA. • In this example, we see there is still a medial lobe, but there is no dorsal lobe; this is because the axons were no longer able to segregate. • This is odd, because the presence of only one isoform should mean that all interactions will be homophilic and therefore segregation should always occur. • What we think may happen is that all of this repulsion due to the presence of only one isoform may overwhelm the cell and they will cancel each other out. • Another scenario could be that, since cells normally have 10-20 different isoforms, maybe only having 1 isoform is not enough to mediate the repulsion mechanism (this experiment would be much too complicated to carry out) • Another problem with having only one isoform is that you could be missing another important isoform of DSCAM necessary for function; this was solved by another experiment in which the cell has one chromosome expressed a wild type DSCAM, while other only expre