Bret Pearson
bpearson@darkwing.uoregon.edu

University of Oregon
Institute of Neuroscience
1254 University of Oregon
Eugene, OR 97403-1254 USA

Advisors:
Dr. Chris Q. Doe, Institute of Neuroscience
Dr. Monte Westerfield, Institute of Neuroscience

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Research Interests

Perhaps the most impressive organ in cell number and function is the metazoan central nervous system (CNS). The Drosophila embryonic nervous system develops from a relatively small set of neural precursor cells (neuroblasts, NB's) which are chosen out of the neuroectoderm by lateral inhibition. Once chosen, NB's delaminate out of the neuroectoderm into the embryo in 5 different waves (S1-S5). The final NB pattern after delamination is invariant and forms orthogonal rows (A-P) and columns (D-V) of about 30 NB's per hemisegment. Interestingly, this "NB Map" is conserved throughout the insects. Furthermore, every Drosophila NB lineage is described, and many NB's generate complex lineages of 2 or more cell types. For instance, neuroblast 7-4 begins its life generating interneurons, then generates 6 glial cells, and then resumes generating interneurons. How neuroblasts keep track of where they are in their lineages, and how this temporal information is transferred to daughter cells is a current focus of the field.

Using Drosophila, we have identified a molecular mechanism for specifying birth-order from a neuroblast, which is independent of cell type. We find that the zinc-finger transcription factors hunchback (hb) and Kruppel (Kr) are necessary and sufficient to specify first and second born cell identities from a neuroblast, respectively. In embryos which are mutant for hb, neuroblast lineages lose their first-born (normally hb+) progeny, yet the rest of the neuroblast lineage is unaffected. When neuroblasts are provided with ectopic hb, they are stuck reiterating first-born fate, at the expense of later born fates. Furthermore, the temporal specification of cell identity by hb is independent of the actual cell type generated (i.e. Hb+ cells can be motoneurons, interneurons or glia).

Since Hb and Kr don't control cell type per se, but control temporal identity, what are their direct transcriptional targets? To answer this question, we are using several techniques. 

1. Computational. High-affinity binding sites for Hb and Kr are known, so we can use them to search stepwise through the Drosophila genome looking for statistically high clusters of binding sites for each transcription factor. Potential positives can be tested for regulation by Hb in vivo. 

2. Microarray. Expressing the gene mCD8 in neuroblasts allows us to specifically purify all neuroblasts for use in microarray experiments. To find temporally regulated genes, we can purify neuroblasts from different time points, and can run the microarrray competition against samples from several different genetic backgrounds. 

3. Chromatin-IP chips. Spots on these chips which light up are fragments of the Drosophila genome where Hb is binding in vivo. The current resolution of our chips is about 7KB.

Common potential targets found using all 3 techniques are the most likely to be real targets, and will be tested in vivo. It is known that in higher insects, neuroblast lineages are generally longer then in lower arthropods, while the number of neuroblasts has not been increased. Thus, it is of interest to see what hb/kr expression looks like in these lower arthropods, and to see if real targets of hb/kr in Drosophila are the ancestral condition. If early lineage patterning in more basal arthropods involves hb/kr, then it is likely that the factors which pattern later parts of lineages have changed to accommodate the longer lineages in higher insects.

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(click on picture to enlarge)

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Publications

• Grosskortenhaus, R., Pearson, BJ., Marusich, A., and CQ Doe “Regulation of temporal identity transitions in Drosophila neuroblast lineages” In preparation.
• Pearson, B. J. and C. Q. Doe. “Specification of temporal identity in the developing nervous system” Ann. Rev. Cell Dev. Bio. Vol. 20, 2004, in press.
• Pearson, BJ., and CQ. Doe “Regulation of neuroblast competence in Drosophila” Nature 425:624-628, Oct. 2003.
• Isshiki, T., Pearson, B., Holbrook, S., and Doe, C.Q. (2001) Drosophila neuroblasts sequentially express transcription factors which specify the temporal identity of their neuronal progeny. Cell 106(4):511-521.
• Pierce-Shimomura, J.T., Faumont, S., Gaston, M.R., Pearson, B.J., and Lockery, S.R. (2001) The homeobox gene lim-6 is required for distinct chemosensory representations in C. elegans. Nature 410(6829):694-698.