Chris Doe and his lab group study the assembly of the nervous system in the fruit fly Drosophila. This developmental process begins with neural stem cells, called neuroblasts, each of which has the potential to make multiple types of neurons. Doe’s team is looking at how neuroblasts produce these different neurons, and how the neurons “wire up” to form circuits that generate the earliest movements in animals.
Although neuroblasts are destined to produce the full adult nervous system, initially they are naive, and know only their position within the embryo (such as head or body). Soon after forming, however, neuroblasts produce a series of daughter cells that generate different types of neurons. Work by the Doe lab has shown that neuroblasts accomplish this feat by producing a series of transcription factors (proteins that turn genes on or off); each successive daughter neuron inherits a different transcription factor, which gives the cell a unique identity. In this way, a small pool of neuroblasts contributes hundreds of different neurons to the growing brain.
The production of different types of neurons is an essential first step, but it must be followed by the “wiring up” of neurons into neural circuits – much like circuit wiring in a computer. Doe’s group is learning more about this process by using genetic tools to turn the activity of single neurons on or off, and then measuring the response of surrounding neurons. In this way, the scientists can map out the circuitry within the entire fly brain. Ultimately, Doe and his team hope to understand the developmental rules that underlie the assembly of neural circuits, which may someday help clinicians direct human stem cells to form the precise types of neurons needed to repair injured or diseased brains.
Grants from the National Institutes of Health provided support for portions of this work.
Figure 1: Asymmetric cell division of Drosphila neural stem cells (neuroblasts) in the larval brain. The atypical protein kinase C (aPKC; blue) is localized to the apical cortex of the mitotic neuroblast and partitioned into the self-renewing neuroblast. Miranda (red) is localized to the basal cortex of the mitotic neuroblast and partitioned into the differentiating daughter cell. Microtubules are shown in green.
Photo credit: Sarah Siegrist, Doe lab.
Figure 2: Three types of neural stem cells (neuroblasts) in the Drosophila larval brain. There are 100 “type I” neuroblasts with simple lineages in each lobe of the central brain (large magenta cells) plus ~800 “type I” optic lobe neuroblasts that make the visual system (small magenta cells). In addition, there are 8 “type II” neuroblasts in the medial part of the brain that contribute large numbers of neurons to the brain due to the production of intermediate neural progenitors and their neuronal progeny (green).
Photo credit: Omer Bayraktar, Doe lab.
Figure 3: Schematic of a larval type II neuroblast lineage, showing that the neuroblast changes molecular markers over time (large cells; black to grey shading) whereas the intermediate neuronal progenitors (INPs, medium sized cells) also change molecular markers over time (Dichaete, red; Grainy head, aqua; Eyeless, blue). The combination of neuroblast temporal patterning and INP temporal patterning generates more neuronal diversity than the more common type I neuroblast lineage.
Photo credit: Omer Bayraktar, Doe lab.
Figure 4: Sequential expression of temporal transcription factors (Hb, Kr, Pdm, Cas; left panel) are necessary and sufficient to specify temporal identity within most embryonic neuroblast lineages (middle panel). The precise type of neuron produced in each temporal window depends on the spatial identity of the neuroblast (right panel).
Photo credit: Takako Isshiki, Doe lab.
Figure 5: Second instar larvae stained using the multi color flip out (MCFO) method of Nern and Rubin to reveal a stochastic subset of sensory neurons, interneurons, and motor neurons. Whole larva staining protocol developed by Laurina Manning.
Photo credit: Laurina Manning, Doe lab.
Figure 6: The evolutionarily-conserved Even-skipped (Eve)+ interneurons are required to maintain left/right symmetric motor output during larval locomotion (see Heckscher, E.S. et al. 2015 Neuron, 88:314).
Photo credit: Aref Zarin, Doe lab.