The conserved RNA-binding protein Imp is required for the specification and function of olfactory navigation circuitry in Drosophila

Loss of Neuronal Imp Contributes to Seizure Behavior through Syndecan Function

Seizures affect a large proportion of the global population and occur due to abnormal neuronal activity in the brain. Unfortunately, widespread genetic and phenotypic heterogeneity contributes to insufficient treatment options. It is critical to identify the genetic underpinnings of how seizures occur to better understand seizure disorders and improve therapeutic development. We used the Drosophila melanogaster model to identify that IGF-II mRNA-binding protein (Imp) is linked to the onset of this phenotype. Specific reduction of Imp in neurons causes seizures after mechanical stimulation. Importantly, gross motor behavior is unaffected, showing Imp loss does not affect general neuronal activity. Developmental loss of Imp is sufficient to cause seizures in adults; thus, Imp-modulated neuron development affects mature neuronal function. Since Imp is an RNA-binding protein, we sought to identify the mRNA target that Imp regulates in neurons to ensure proper neuronal activity after mechanical stress. We find that the Imp protein binds Syndecan (Sdc) mRNA, and the reduction of Sdc also causes mechanically induced seizures. Expression of Sdc in Imp-deficient neurons rescues seizure defects, showing that Sdc is sufficient to restore normal behavior after mechanical stress. We suggest that the Imp protein binds Sdc mRNA in neurons, and this functional interaction is important for normal neuronal biology and animal behavior in a mechanically induced seizure model. Since Imp and Sdc are conserved, our work highlights a neuronal-specific pathway that might contribute to seizure disorder when mutated in humans.

Cell type-specific driver lines targeting the Drosophila central complex and their use to investigate neuropeptide expression and sleep regulation

The central complex (CX) plays a key role in many higher-order functions of the insect brain including navigation and activity regulation. Genetic tools for manipulating individual cell types, and knowledge of what neurotransmitters and neuromodulators they express, will be required to gain mechanistic understanding of how these functions are implemented. We generated and characterized split-GAL4 driver lines that express in individual or small subsets of about half of CX cell types. We surveyed neuropeptide and neuropeptide receptor expression in the central brain using fluorescent in situ hybridization. About half of the neuropeptides we examined were expressed in only a few cells, while the rest were expressed in dozens to hundreds of cells. Neuropeptide receptors were expressed more broadly and at lower levels. Using our GAL4 drivers to mark individual cell types, we found that 51 of the 85 CX cell types we examined expressed at least one neuropeptide and 21 expressed multiple neuropeptides. Surprisingly, all co-expressed a small molecule neurotransmitter. Finally, we used our driver lines to identify CX cell types whose activation affects sleep, and identified other central brain cell types that link the circadian clock to the CX. The well-characterized genetic tools and information on neuropeptide and neurotransmitter expression we provide should enhance studies of the CX.

Developmental Genetic and Molecular Analysis of Drosophila Central Complex Lineages

Complex behaviors are mediated by a diverse class of neurons and glia produced during development. Both neural stem cell-intrinsic and -extrinsic temporal cues regulate the appropriate number, molecular identity, and circuit assembly of neurons. The Drosophila central complex (CX) is a higher-order brain structure regulating various behaviors, including sensory-motor integration, celestial navigation, and sleep. Most neurons and glia in the adult CX are formed during larval development by 16 Type II neural stem cells (NSCs). Unlike Type I NSCs, which directly give rise to the ganglion mother cells (GMCs), Type II NSCs give rise to multiple intermediate neural progenitors (INPs), and each INP in turn generates multiple GMCs, hence fostering the generation of longer and more diverse lineages. This makes Type II NSCs a suitable model to unravel the molecular mechanisms regulating neural diversity in more complex lineages. In this review, we elaborate on the classification and identification of NSCs based on the types of division adopted and the molecular markers expressed in each type. In the end, we discuss genetic methods for lineage analysis and birthdating. We also explain the temporal expression of stem cell factors and genetic techniques to study how stem cell factors may regulate neural fate specification.

Lineage Analysis and Birthdating of Drosophila Central Complex Lineages

From insects to humans, the nervous system generates complex behaviors mediated by distinct neural circuits that are composed of diverse cell types. During development, the spatiotemporal gene expression of the neural progenitors expands the diversity of neuronal and glial subtypes. Various neural stem cell-intrinsic and -extrinsic gene programs have been identified that are thought to play a major role in generating diverse neuronal and glial cell types. Drosophila has served as an excellent model system for discovering the fundamental principles of nervous system development and function. The sophisticated genetic tools allow us to link the origin and birth timing (the time when a particular neuron is born during development) of neuron types to unique neural stem cells (NSCs) and to a developmental time. In Drosophila, a special class of NSCs called Type II NSCs has adopted a more advanced division mode to generate lineages for the higher-order brain center, the central complex, which is an evolutionarily conserved brain region found in all insects. Type II NSCs, similar to the human outer radial glia, generate intermediate neural progenitors (INPs), which divide many times to produce about eight to 10 progeny. Both Type II NSCs and INPs express distinct transcription factors and RNA-binding proteins that have been proposed to regulate the specification of cell types populating the adult central complex. Here, we describe the recently invented lineage filtering system, called cell class-lineage intersection (CLIn), which enables the tracking and birthdating of the Type II NSC lineages. Using CLIn, one can easily generate clones of different Type II NSCs and identify not only the origins of neurons of interest but also their birth time.

Imp/IGF2BP and Syp/SYNCRIP temporal RNA interactomes uncover combinatorial networks of regulators of Drosophila brain development

Temporal patterning of neural progenitors is an evolutionarily conserved mechanism generating neural diversity. In Drosophila, postembryonic neurogenesis requires the RNA binding proteins (RBPs) Imp/IGF2BP and Syp/SYNCRIP. However, how they coachieve their function is not well understood. Here, we elucidate the in vivo temporal RNA interactome landscapes of Imp and Syp during larval brain development. Imp and Syp bind a highly overlapping set of conserved mRNAs encoding proteins involved in neurodevelopment. We identify transcripts differentially occupied by Imp/Syp over time, featuring a network of known and previously unknown candidate temporal regulators that are post-transcriptionally regulated by Imp/Syp. Furthermore, the physical and coevolutionary relationships between Imp and Syp binding sites reveal a combinatorial, rather than competitive, mode of molecular interplay. Our study establishes an in vivo framework for dissecting the temporal coregulation of RBP networks as well as providing a resource for understanding neural fate specification.

Cell type-specific driver lines targeting the Drosophila central complex and their use to investigate neuropeptide expression and sleep regulation

The central complex (CX) plays a key role in many higher-order functions of the insect brain including navigation and activity regulation. Genetic tools for manipulating individual cell types, and knowledge of what neurotransmitters and neuromodulators they express, will be required to gain mechanistic understanding of how these functions are implemented. We generated and characterized split-GAL4 driver lines that express in individual or small subsets of about half of CX cell types. We surveyed neuropeptide and neuropeptide receptor expression in the central brain using fluorescent in situ hybridization. About half of the neuropeptides we examined were expressed in only a few cells, while the rest were expressed in dozens to hundreds of cells. Neuropeptide receptors were expressed more broadly and at lower levels. Using our GAL4 drivers to mark individual cell types, we found that 51 of the 85 CX cell types we examined expressed at least one neuropeptide and 21 expressed multiple neuropeptides. Surprisingly, all co-expressed a small neurotransmitter. Finally, we used our driver lines to identify CX cell types whose activation affects sleep, and identified other central brain cell types that link the circadian clock to the CX. The well-characterized genetic tools and information on neuropeptide and neurotransmitter expression we provide should enhance studies of the CX.

Castor is a temporal transcription factor that specifies early born central complex neuron identity

The generation of neuronal diversity is important for brain function, but how diversity is generated is incompletely understood. We used the development of the Drosophila central complex (CX) to address this question. The CX develops from eight bilateral Type 2 neuroblasts (T2NBs), which generate hundreds of different neuronal types. T2NBs express broad opposing temporal gradients of RNA-binding proteins. It remains unknown whether these protein gradients are sufficient to directly generate all known neuronal diversity, or whether there are temporal transcription factors (TTFs) with narrow expression windows that each specify a small subset of CX neuron identities. Multiple candidate TTFs have been identified, but their function remains uncharacterized. Here, we show that: (1) the adult E-PG neurons are born from early larval T2NBs; (2) the candidate TTF Castor is expressed transiently in early larval T2NBs when E-PG and P-EN neurons are born; and (3) Castor is required to specify early born E-PG and P-EN neuron identities. We conclude that Castor is a TTF in larval T2NB lineages that specifies multiple, early born CX neuron identities.

Neuronal circuit mechanisms of competitive interaction between action-based and coincidence learning

How information is integrated across different forms of learning is crucial to understanding higher cognitive functions. Animals form classic or operant associations between cues and their outcomes. It is believed that a prerequisite for operant conditioning is the formation of a classical association. Thus, both memories coexist and are additive. However, the two memories can result in opposing behavioral responses, which can be disadvantageous. We show that Drosophila classical and operant olfactory conditioning rely on distinct neuronal pathways leading to different behavioral responses. Plasticity in both pathways cannot be formed simultaneously. If plasticity occurs at both pathways, interference between them occurs and learning is disrupted. Activity of the navigation center is required to prevent plasticity in the classical pathway and enable it in the operant pathway. These findings fundamentally challenge hierarchical views of operant and classical learning and show that active processes prevent coexistence of the two memories.

Loss of neuronal Imp induces seizure behavior through Syndecan function

Seizures affect a large proportion of the global population and occur due to abnormal neuronal activity in the brain. Unfortunately, widespread genetic and phenotypic heterogeneity contribute to insufficient treatment options. It is critical to identify the genetic underpinnings of how seizures occur to better understand seizure disorders and improve therapeutic development. We used the Drosophila model to identify that IGF-II mRNA Binding Protein (Imp) is linked to the onset of this phenotype. Specific reduction of Imp in neurons causes seizures after mechanical stimulation. Importantly, gross motor behavior is unaffected, showing Imp loss does not affect general neuronal activity. Developmental loss of Imp is sufficient to cause seizures in adults, thus Imp-modulated neuron development affects mature neuronal function. Since Imp is an RNA-binding protein, we sought to identify the mRNA target that Imp regulates in neurons to ensure proper neuronal activity after mechanical stress. We find that Imp protein binds Syndecan ( Sdc ) mRNA, and reduction of Sdc also causes mechanically-induced seizures. Expression of Sdc in Imp deficient neurons rescues seizure defects, showing that Sdc is sufficient to restore normal behavior after mechanical stress. We suggest that Imp protein binds Sdc mRNA in neurons, and this functional interaction is important for normal neuronal biology and animal behavior in a mechanically-induced seizure model. Since Imp and Sdc are conserved, our work highlights a neuronal specific pathway that might contribute to seizure disorder when mutated in humans.

Stem cell-specific ecdysone signaling regulates the development of dorsal fan-shaped body neurons and sleep homeostasis

Complex behaviors arise from neural circuits that assemble from diverse cell types. Sleep is a conserved behavior essential for survival, yet little is known about how the nervous system generates neuron types of a sleep-wake circuit. Here, we focus on the specification of Drosophila 23E10-labeled dorsal fan-shaped body (dFB) long-field tangential input neurons that project to the dorsal layers of the fan-shaped body neuropil in the central complex. We use lineage analysis and genetic birth dating to identify two bilateral type II neural stem cells (NSCs) that generate 23E10 dFB neurons. We show that adult 23E10 dFB neurons express ecdysone-induced protein 93 (E93) and that loss of ecdysone signaling or E93 in type II NSCs results in their misspecification. Finally, we show that E93 knockdown in type II NSCs impairs adult sleep behavior. Our results provide insight into how extrinsic hormonal signaling acts on NSCs to generate the neuronal diversity required for adult sleep behavior. These findings suggest that some adult sleep disorders might derive from defects in stem cell-specific temporal neurodevelopmental programs.