Specification of Neuronal Identities in the Drosophila nervous system

The nervous system contains vast numbers of cells and great diversity in cell types; in humans it is estimated that the nervous system comprises 10^11 neurons belonging to at least 10,000 different classes. Neurons differ from each other in many ways, including in the morphology of their axons and dendrites and in the type of neurotransmitters they express. These and other properties will together govern each neuron’s unique role within the nervous system. The molecular genetic mechanisms controlling neuronal specification have been intensely studied during the last two decades. These studies have revealed that neurons do not appear to be specified by the action of any one regulatory gene alone, but rather by the sequential and combinatorial action of many regulators and their unique interplay with key signaling pathways. However, many issues are left un-resolved. How do regulatory genes intersect with the cell cycle and cell death machinery to ensure precise numbers of each neuronal cell type? What is the input of early regulators upon post-mitotic events in early born neurons? How does regulatory information match neurotransmitter identity with proper axon and dendrite morphology? The importance of resolving these fundamental issues is all the more apparent in light of the heightened interest in stem cell biology.

We are interested in understanding nervous system development and are using the developing Drosophila melanogaster (Drosophila) CNS as our model system. The Drosophila CNS can be subdivided into the brain and the ventral nerve cord (VNC). These tissues are in essence functionally equivalent to the mammalian brain and spinal cord, respectively. The VNC is segmentally organized into three thoracic and 8 abdominal segments, and contains a total of ~10,000 cells, the majority of which are neurons. In the VNC, only 90 cells express the LIM-homeodomain transcription factor Apterous (Ap), but these neurons, the Ap neurons, are remarkably diverse. They differ in axon pathfinding; most Ap neurons extend their axons in the common fascicle, whereas the Tv Ap neurons instead innervates a peripheral secretory gland, the dorsal neurohemal organ (DNH) a specialized glial-derived structure present in the three thoracic VNC segments. They also differ in neurotransmitter expression and approximately half are peptidergic. Of that peptidergic subclass, the Tv cell selectively expresses the neuropeptide gene FMRFamide, whereas dAp and Tvb cells selectively express the neuropeptide gene Nplp1, as well as a dopamine type I receptor gene, Dop-R. Importantly, for these terminal differentiation genes, their expression within the VNC is confined to subsets of ap-neurons; FMRFa is only expressed in 6 and Nplp1/Dop-R in 28 out of 10,000 cells, respectively. How is this remarkably specific gene expression controlled?

Recent studies have led to the identification of a number of regulatory genes that act postmitotically in the specification and differentiation of the ap-neurons. These include the transcription factors ap, the Col/Olf-1/EBF family member collier (col), the zinc-finger gene squeeze (sqz) and the bHLH gene dimmed (dimm) as well as the transcriptional co-factors eyes absent (eya) and dachshund (dac). In addition, FMRFa expression in the Tv neuron is critically dependent upon a target-derived BMP signal. These regulatory genes act in a combinatorial manner such that a combinatorial code of ap/sqz/dac/eya/dimm and target-derived BMP signaling dictates Tv neuron identity and activates FMRFa expression. Similarly, a combinatorial code of ap/eya/dimm/col dictates Tvb/dAp identity and activates Nplp1/Dop-R expression. Together, these studies are emerging as one of the most elaborate examples of how combinatorial codes act within postmitotic cells to specify unique neuronal identities. We are now pursuing these issues in several directions, trying to address questions like: How do these postmitotic combinatorial codes act at the molecular level? What exactly is the role of the BMP pathway in the Tv neurons? Which genes are involved in directly carrying out the unique axon pathfinding of the different Ap neurons? What are the molecular genetic mechanisms that act to generate Ap neurons in such precise numbers and at precisely the correct position? We hope that by answering these and other questions using this model system, we will provide important information that aids in the understanding of human biology as well.

References

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Figure 1

The expression of neuropeptides define unique subsets of neurons in the Drosophila CNS. Often just one cell per hemisegment.

Figure 2

Neuropeptide genes are typically expressed at high levels.