We are interested in the basic mechanisms of neural development. Our strategy is to use the relatively simple Drosophila peripheral nervous system (PNS) to discover the genetic program that controls its development. In doing so, we hope to uncover evolutionarily conserved core programs that control different steps of neural development in animals. We started with the earliest steps in neural development (neurogenesis and neuronal cell fate specification) and gradually worked our way toward later steps (neuronal morphogenesis and the assembly of a functional neuronal circuit).
Some highlights of our earlier efforts include the discoveries of atonal and numb. Atonal is a basic helix-loop-helix (bHLH) protein, which is the founding member of an important family of proneural genes that initiate the development of two major types of sensory neurons used in vision and hearing. Its mammalian homologs include Neurogenin, Math1, and Math5. During asymmetric cell division, numb functions as a cell fate determinant. Numb provided a starting point for the study of asymmetric cell division in Drosophila and vertebrates, which led to insights into the molecular basis of asymmetric cell division. In recent years, the major focus of our lab has shifted to the study of dendrite development and neuronal circuit assembly.
Mechanisms Controlling Dendrite Development in Drosophila
Dendrite arborization patterns are critical determinants of neural circuit formation and influence the type of synaptic or sensory inputs a neuron is able to receive. Moreover, dendrite defects are associated with a variety of human mental disorders such as autism. Relatively little is known about the molecular mechanisms that control dendrite development. We use the fly transgenic technique to express green fluorescent protein (GFP) in the dendritic arborization (da) neurons, a group of sensory neurons with a stereotyped dendritic branching pattern. This allows us to visualize the development of the dendrites of da neurons in living fly embryos and to use them as an assay system for a genetic dissection of neuronal polarity and dendrite development. We have gained insights about the mechanisms underlying (1) how axons and dendrites are made differently, (2) how a neuron acquires its neuronal type-specific morphology, (3) how the dendrites of different neurons are organized, (4) how the size of a dendritic arbor is controlled, and (5) how the pruning and remodeling of dendrites are regulated during development.
1. Dendrite-specific developmental regulators. Similar to the majority of vertebrate neurons, the Drosophila da neurons show clear dendrite versus axon polarity, including the orientations of microtubules (MTs). It is well known from numerous studies in various organisms that the MT cytoskeleton plays a major role in the proper establishment and maintenance of neuronal architecture. For example, the differences in MT polarity in dendrites and axons have substantial impact on the structural and functional differences of axons and dendrites. An important unanswered question is how MTs are nucleated in neurons. Many recent studies point to the existence of acentrosomal MT-nucleating sites. We recently found that Golgi outposts serve this role in the dendrites of Drosophila da neurons.
From our mutant screen, we identified a group of dar (dendritic arborization reduction) genes. Mutations of any of the dar genes lead to defective dendritic arbors but normal axonal projections. Thus, studies of dar genes should reveal how axons and dendrites are made differently. We estimate that there may be a total of about 20 dar genes in Drosophila. All five of the dar genes that we have cloned so far have mammalian homologs. Remarkably, three encode components of the secretory pathway. These results reveal the preferential role of endoplasmic reticulum–Golgi trafficking and Golgi outposts in dendrite arborization.
2. Transcription factors regulate dendritic field size and complexity. Transcription factors are important regulators of the size and complexity of dendritic fields, and the logic of their usage is beginning to emerge. For example, in the Drosophila PNS, the zinc finger–containing protein Hamlet functions as a binary switch between the elaborate multiple-dendrite morphology of da neurons and the single, unbranched dendrite morphology of external sensory neurons. In most cases, however, the dendritic morphology is determined by the combined action of multiple transcription factors, such as Cut and Spineless.
3. The molecular mechanism for dendritic self-avoidance and tiling. Dendrite-dendrite repulsion can have a profound influence on the size and shape of the dendritic field, as well as the spatial relationship between different dendritic fields. The dendrites of each da neuron show self-avoidance and tend to spread out without crossing over. Class III and class IV da neurons show tiling: there is little overlap between the dendritic fields of adjacent neurons of the same class because their dendrites show homotypic repulsion. We found that Dscam (Down syndrome cell adhesion molecules), originally identified as axon guidance receptors by Lawrence Zipursky (HHMI, University of California, Los Angeles) and his colleagues, are needed for self-avoidance and contribute to the spreading of dendrites. Without Dscam, the dendrites of each da neuron would bundle together or cross over. Parallel and complementary works were done in the labs of Wesley Grueber (Columbia University Medical Center), Zipursky, and Dietmar Schmucker (Vesalius Research Center, VIB, Leuven, Belgium). In contrast, tiling requires some cell surface recognition molecules other than Dscam to mediate the homotypic repulsion. Although the signals that mediate tiling behavior remain elusive, we recently discovered that integrins regulate repulsion-mediated dendritic patterning of sensory neurons by restricting dendrites to a two-dimensional space, as defined by the extracellular matrix (ECM). The previously identified tiling mutants of the Tricornered (Trc)/Furry pathway affect dendritic tethering to the ECM and thus allow the dendrites to cross over in different planes.
4. The maintenance of dendritic fields. Our genetic screen revealed specific mechanisms that ensure maintenance of dendritic arbors. We found that components of the Hippo pathway, including the tumor-suppressor Warts (Wts), as well as the Polycomb group of genes, are required for the maintenance of the class IV da dendrites. Loss-of-function mutations of any of those genes cause a progressive defect in the maintenance of dendritic tiling, resulting in large gaps in the receptive field. We are continuing our study of how the establishment and maintenance of dendritic fields are coordinated.
5. The remodeling of dendritic fields. Drosophila class IV da neurons undergo dramatic remodeling during metamorphosis. Early in the pupal stage, those neurons prune all their dendrites. Later each neuron grows a completely new dendrite for adult function. While the dendrites are being remodeled, the axons stay largely intact. We have continued to identify the molecular mechanisms that control this large-scale, dendrite-specific remodeling.
Functional Implication of Dendrite Morphogenesis and the Study of Mechanosensation
To understand dendrite morphogenesis, we felt that we need to study it in the context of function. The fly uses a variety of mechanisms, such as self-avoidance and tiling, to form a beautiful array of class IV da neuronal dendrites to tile the body wall. Why? Based on the work of several labs, including ours, we know that class IV da neurons are polymodel nociception receptors: they can sense high temperature or noxious mechanical stimuli. Recently we discovered that class IV da neurons also function as photoreceptors. Drosophila larvae have two different types of photosensors. The previously known larval photoreceptors, the Bolwig organ, function primarily in avoidance of low light, whereas class IV da neurons constitute a second photoreceptor system using a novel phototransduction machinery to sense harmful short-wavelength light. This regular array of photosensors enables the larva to sense light exposure over its entire body and to move out of danger. This may explain why the organization of class IV da neurons is so similar to that of tiled neurons in the vertebrate retina.
We have also begun to use da neurons to study mechanosensation, which, among our senses, is less well understood than vision, olfaction, and taste. Little is known about the molecular mechanisms underlying mechanosensation, partly because of the difficulties in identifying mechanical transducing molecules. The Drosophila PNS has emerged as an excellent system to study mechanosensation. At least three of the four classes of da neurons are involved in different modes of mechanosensation. Ardem Patapoutian's lab (Scripps Research Institute) recently discovered that Drosophila Piezo is a mechanosensitive channel that mediates mechanical nociception by class IV da neurons, whereas our lab found that NompC is a mechanosensitive channel that mediates gentle touch by class III da neurons and sound sensing by chordotonal neurons. The Drosophila PNS promises to provide important insights about mechanosensation.
Degeneration and Regeneration of Dendrites and Axons
Recently we found that after it is severed, the class IV da neuron's axon regenerates in a manner strikingly similar to that of mammalian dorsal root ganglion neurons after injury: it regenerates well in the periphery but poorly in the CNS. Furthermore, as in mammals, activating the Akt pathway enhances axon regeneration of da neurons in the Drosophila CNS. Those results strongly suggest that class IV da neurons can serve as an excellent model system for uncovering and studying the evolutionarily conserved axon regeneration mechanisms. We are performing a candidate-based screen, as well as an unbiased screen, to identify potential inhibitors for CNS axon regeneration. This strategy offers an opportunity to gain insights into the repertoire of regulators for axon regeneration, which may inspire novel treatments for nerve injury and neurodegenerative diseases. Furthermore, class IV da neurons are not only capable of axon regeneration but also display dendrite regeneration. We have begun to compare the mechanisms for dendrite versus axon regeneration, a subject that has hardly been studied in any system.
Dendrite Development of Mammalian Central Neurons and Implications in Neurological Disorders
We have been using the knowledge gained from our Drosophila studies of dendrite development to extend our studies to mammalian cortical neurons. Our previous work suggests that the Hippo kinase signaling pathway has a central role in controlling dendrite morphogenesis in Drosophila. Recently we demonstrated that NDR (nuclear Dbf2-related) kinases, the mammalian homologs of Trc kinase in the Hippo pathway of Drosophila, have an evolutionarily conserved role in controlling mammalian dendrite morphogenesis. Furthermore, in collaboration with Kevan Shokat (HHMI, University of California, San Francisco), we have succeeded in applying the chemical genetic approach his lab has developed to identify substrates of NDR kinases, which turn out to have important functions in dendrite morphogenesis. We have continued to apply this technique to identify the substrates of the mammalian homologs of other important kinases in the Hippo pathway and to study their roles in dendrite morphogenesis. Some of the substrates have been implicated in neurological disorders such as autism and schizophrenia.
Drosophila Behavior and Underlying Neuronal Circuitry
Our lab has also contributed to the characterization of adult behaviors in Drosophila, including egg-laying site selection, female postmating switch from receptive behavior to rejection of male courtship, rival-induced prolongation of mating duration in the male, and social influence of male aggression. We have made progress in identifying components of neuronal circuitry underlying these behaviors.
Grants from the National Institutes of Health provided partial support for these studies.
As of February 19, 2014