Spatio-temporal relays control layer identity of direction-selective neuron subtypes in Drosophila

Toward a probabilistic definition of neural cell types

A classical view of cell type relies on a definite set of stable properties that are critical for brain functions. Single-cell technologies led to an extensive multimodal characterization of nervous systems and perhaps achieved one of Santiago Ramón y Cajal's dreams: to unveil a comprehensive view of the brain composition. While global analyses of brain structures highlight a degree of mesoscale stereotypy, a finer-scale resolution of brain composition shows significant variance in essential neural cellular phenotypes, including morphology, gene expression, electrophysiology, and connectivity. This highlights the need for novel conceptualization of the definition of a neural "cell type." The challenge of modern neural classification is thus to integrate various distinct cellular properties into a unifying descriptor.

Mechanistic insights into Wnt-β-catenin pathway activation and signal transduction

In multicellular organisms, Wnt proteins govern stem and progenitor cell renewal and differentiation to regulate embryonic development, adult tissue homeostasis and tissue regeneration. Defects in canonical Wnt signalling, which is transduced intracellularly by β-catenin, have been associated with developmental disorders, degenerative diseases and cancers. Although a simple model describing Wnt-β-catenin signalling is widely used to introduce this pathway and has largely remained unchanged over the past 30 years, in this Review we discuss recent studies that have provided important new insights into the mechanisms of Wnt production, receptor activation and intracellular signalling that advance our understanding of the molecular mechanisms that underlie this important cell-cell communication system. In addition, we review the recent development of molecules capable of activating the Wnt-β-catenin pathway with selectivity in vitro and in vivo that is enabling new lines of study to pave the way for the development of Wnt therapies for the treatment of human diseases.

Columnar cholinergic neurotransmission onto T5 cells of Drosophila

Several nicotinic and muscarinic acetylcholine receptors (AChRs) are expressed in the brain of Drosophila melanogaster. However, the contribution of different AChRs to visual information processing remains poorly understood. T5 cells are the primary motion-sensing neurons in the OFF pathway and receive input from four different columnar cholinergic neurons, Tm1, Tm2, Tm4, and Tm9. We reasoned that different AChRs in T5 postsynaptic sites might contribute to direction selectivity, a central feature of motion detection. We show that the nicotinic nAChRα1, nAChRα3, nAChRα4, nAChRα5, nAChRα7, and nAChβ1 subunits localize on T5 dendrites. By targeting synaptic markers specifically to each cholinergic input neuron, we find a prevalence of the nAChRα5 in Tm1, Tm2, and Tm4-to-T5 synapses and of nAChRα7 in Tm9-to-T5 synapses. Knockdown of nAChRα4, nAChRα5, nAChRα7, or mAChR-B individually in T5 cells alters the optomotor response and reduces T5 directional selectivity. Our findings indicate the contribution of a consortium of postsynaptic receptors to input visual processing and, thus, to the computation of motion direction in T5 cells.

Depletion of Gtsf1L impairs development of eupyrene sperm and ovary in Gtsf1L

Sperm delivers genetic information from the male to the ovum, playing vital roles in sexual reproduction. Like other Lepidoptera, Bombyx mori exhibits dimorphic spermatogenesis, generating coexisting nucleated eupyrene sperm and anucleated apyrene sperm. The mechanism of dimorphic spermatogenesis is still to be clarified. In a previous study, we demonstrated that Gametocyte-specific factor1 (Gtsf1) is essential for female sex determination and PIWI-interacting RNA (piRNA) mediated transposon silencing in B. mori. Here, we performed functional analysis of the Gtsf1 paralog BmGtsf1L using a binary transgenic CRISPR/Cas9 system. BmGtsf1L is dispensable for sex determination but critical for fertility in both males and females. We separated different types of sperm and found that BmGtsf1L is highly expressed in both types of sperm. BmGtsf1L deficiency (△BmGtsf1L) impaired the formation and migration of eupyrene sperm, whereas the development and movement of apyrene sperm were normal. Furthermore, through a sperm culture experiment, we confirmed that eupyrene spermatogenesis defects appeared before the elongation stage. Double copulations of a female with △BmGtsf1L and Sex-lethal mutant males can rescue infertility phenotypes, revealing that the apyrene sperm of BmGtsf1L mutants is functional. We also found that the depletion of BmGtsf1L impacted proper oogenesis. This study provided the first functional analysis of Gtsf1 paralogs on physiology, demonstrating the critical role of BmGtsf1L in the development of eupyrene sperm and the ovary.

Establishment of terminal selector combinations in optic lobe neurons

The medulla is the part of the Drosophila optic lobe with the greatest neuronal diversity, in which the identity of each neuronal type is specified in progenitors and newborn neurons via the integration of temporal, spatial, and Notch-driven patterning mechanisms. This identity is maintained in differentiating and adult neurons by the expression of neuronal type-specific combinations of terminal selectors, which are transcription factors expressed continuously during development and in the adult that are thought to control all neuronal type-specific gene expression. However, how the patterning mechanisms establish terminal selector expression is unknown. We have previously characterized the temporal and Notch origin of medulla neurons. Here we have used single-cell mRNA-sequencing to characterize their spatial origins and identified two new spatial subdomains. Together, this makes the medulla the first complex brain structure for which the patterning mechanisms specifying the identity of each neuronal type are known. This knowledge allowed us to identify correlations between patterning information, terminal selector expression and neuronal features. Our results suggest that different subsets of the patterning information accessible to a given neuronal type control the expression of each of its terminal selectors and of modules of terminal features, including neurotransmitter identity. Therefore, the evolution of new neuronal types could rely on the acquisition of modules of neuronal features predetermined by their developmental origin.

Morphological and functional convergence of visual projection neurons from diverse neurogenic origins in Drosophila

The Drosophila visual system is a powerful model to study the development of neural circuits. Lobula columnar neurons-LCNs are visual output neurons that encode visual features relevant to natural behavior. There are ~20 classes of LCNs forming non-overlapping synaptic optic glomeruli in the brain. To address their origin, we used single-cell mRNA sequencing to define the transcriptome of LCN subtypes and identified lines that are expressed throughout their development. We show that LCNs originate from stem cells in four distinct brain regions exhibiting different modes of neurogenesis, including the ventral and dorsal tips of the outer proliferation center, the ventral superficial inner proliferation center and the central brain. We show that this convergence of similar neurons illustrates the complexity of generating neuronal diversity, and likely reflects the evolutionary origin of each subtype that detects a specific visual feature and might influence behaviors specific to each species.

Temporal and Notch identity determine layer targeting and synapse location of medulla neurons

How specification mechanisms that generate neural diversity translate into specific neuronal targeting, connectivity, and function in the adult brain is not understood. In the medulla region of the Drosophila optic lobe, neural progenitors generate different neurons in a fixed order by sequentially expressing a series of temporal transcription factors as they age. Then, Notch signaling in intermediate progenitors further diversifies neuronal progeny. By establishing the birth order of medulla neurons, we found that their temporal identity correlates with the depth of neuropil targeting in the adult brain, for both local interneurons and projection neurons. We show that this temporal identity-dependent targeting of projection neurons unfolds early in development and is genetically determined. By leveraging the Electron Microscopy reconstruction of the adult fly brain, we determined the synapse location of medulla neurons in the different optic lobe neuropils and find that it is significantly associated with both their temporal identity and Notch status. Moreover, we show that all the putative medulla neurons with the same predicted function share similar neuropil synapse location, indicating that ensembles of neuropil layers encode specific visual functions. In conclusion, we show that temporal identity and Notch status of medulla neurons can predict their neuropil synapse location and visual function, linking their developmental patterning with their specific connectivity and functional features in the adult brain.

Spatial patterning controls neuron numbers in the Drosophila visual system

Neurons must be made in the correct proportions to communicate with the appropriate synaptic partners and form functional circuits. In the Drosophila visual system, multiple subtypes of distal medulla (Dm) inhibitory interneurons are made in distinct, reproducible numbers-from 5 to 800 per optic lobe. These neurons are born from a crescent-shaped neuroepithelium called the outer proliferation center (OPC), which can be subdivided into specific domains based on transcription factor and growth factor expression. We fate mapped Dm neurons and found that more abundant neural types are born from larger neuroepithelial subdomains, while less abundant subtypes are born from smaller ones. Additionally, morphogenetic Dpp/BMP signaling provides a second layer of patterning that subdivides the neuroepithelium into smaller domains to provide more granular control of cell proportions. Apoptosis appears to play a minor role in regulating Dm neuron abundance. This work describes an underappreciated mechanism for the regulation of neuronal stoichiometry.

Development of the Drosophila Optic Lobe

The Drosophila visual system has been a great model to study fundamental questions in neurobiology, such as neural fate specification, axon guidance, circuit formation, and information processing. The Drosophila visual system is composed of the compound eye and the optic lobe. The optic lobe is divided into four neuropils-namely, the lamina, medulla, lobula, and lobula plate. There are around 200 types of optic lobe neurons, which wire together to form a complex neural structure to processes visual information. These neurons are derived from two neuroepithelial structures-namely, the outer proliferation center (OPC) and the inner proliferation center (IPC), in the larval brain. Recent work on the Drosophila optic lobe has revealed basic principles underlying the development of this complex neural structure, and immunostaining has been a key tool in these studies. Here, we provide a brief overview of the Drosophila optic lobe structure and development, as revealed by immunostaining. First, we introduce the structure of the adult optic lobe. Then, we summarize recent advances in the study of neural fate specification during development of different parts of the optic lobe. Last, we briefly summarize general aspects of axon guidance and neuropil assembly in the optic lobe. With this review, we aim to familiarize readers with this complex neural structure and highlight the power of this great model to study neural development to facilitate further developmental and functional studies using this system.

The USP46 deubiquitylase complex increases Wingless/Wnt signaling strength by stabilizing Arrow/LRP6

The control of Wnt receptor abundance is critical for animal development and to prevent tumorigenesis, but the mechanisms that mediate receptor stabilization remain uncertain. We demonstrate that stabilization of the essential Wingless/Wnt receptor Arrow/LRP6 by the evolutionarily conserved Usp46-Uaf1-Wdr20 deubiquitylase complex controls signaling strength in Drosophila. By reducing Arrow ubiquitylation and turnover, the Usp46 complex increases cell surface levels of Arrow and enhances the sensitivity of target cells to stimulation by the Wingless morphogen, thereby increasing the amplitude and spatial range of signaling responses. Usp46 inactivation in Wingless-responding cells destabilizes Arrow, reduces cytoplasmic accumulation of the transcriptional coactivator Armadillo/β-catenin, and attenuates or abolishes Wingless target gene activation, which prevents the concentration-dependent regulation of signaling strength. Consequently, Wingless-dependent developmental patterning and tissue homeostasis are disrupted. These results reveal an evolutionarily conserved mechanism that mediates Wnt/Wingless receptor stabilization and underlies the precise activation of signaling throughout the spatial range of the morphogen gradient.

How Flies See Motion

How neurons detect the direction of motion is a prime example of neural computation: Motion vision is found in the visual systems of virtually all sighted animals, it is important for survival, and it requires interesting computations with well-defined linear and nonlinear processing steps-yet the whole process is of moderate complexity. The genetic methods available in the fruit fly Drosophila and the charting of a connectome of its visual system have led to rapid progress and unprecedented detail in our understanding of how neurons compute the direction of motion in this organism. The picture that emerged incorporates not only the identity, morphology, and synaptic connectivity of each neuron involved but also its neurotransmitters, its receptors, and their subcellular localization. Together with the neurons' membrane potential responses to visual stimulation, this information provides the basis for a biophysically realistic model of the circuit that computes the direction of visual motion.

From temporal patterning to neuronal connectivity in Drosophila type I neuroblast lineages

The development of the central nervous system (CNS) in flies and mammals requires the production of distinct neurons in different locations and times. Here we review progress on how Drosophila stem cells (neuroblasts; NBs) generate distinct neurons over time. There are two types of NBs: type I and type II NBs (defined below); here we focus on type I NBs; type II NBs are reviewed elsewhere in this issue. Type I NBs generate neural diversity via the cascading expression of specific temporal transcription factors (TTFs). TTFs are sequentially expressed in neuroblasts and required for the identity of neurons born during each TTF expression window. In this way TTFs specify the "temporal identity" or birth-order dependent identity of neurons. Recent studies have shown that TTF expression in neuroblasts alter the identity of their progeny, including directing motor neurons to form proper connectivity to the proper muscle targets, independent of their birth-order. Similarly, optic lobe (OL) type I NBs express a series of TTFs that promote proper neuron morphology and targeting to the four OL neuropils. Together, these studies demonstrate how temporal identity is crucial in promoting proper circuit assembly within the Drosophila CNS. In addition, TTF orthologs in mouse are good candidates for specifying neuron types in the neocortex and retina. In this review we highlight the recent advances in understanding the role of TTFs in CNS circuit assembly in Drosophila and reflect on the conservation of these mechanisms in mammalian CNS development.

Generating neural diversity through spatial and temporal patterning

The nervous system consists of a vast diversity of neurons and glia that are accurately assembled into functional circuits. What are the mechanisms that generate these diverse cell types? During development, an epithelial sheet with neurogenic potential is initially regionalised into spatially restricted domains of gene expression. From this, pools of neural stem cells (NSCs) with distinct molecular profiles and the potential to generate different neuron types, are specified. These NSCs then divide asymmetrically to self-renew and generate post-mitotic neurons or glia. As NSCs age, they experience transitions in gene expression, which further allows them to generate different neurons or glia over time. Versions of this general template of spatial and temporal patterning operate during the development of different parts of different nervous systems. Here, I cover our current knowledge of Drosophila brain and optic lobe development as well as the development of the vertebrate cortex and spinal cord within the framework of this above template. I highlight where our knowledge is lacking, where mechanisms beyond these might operate, and how the emergence of new technologies might help address unanswered questions.

The logistics of Wnt production and delivery

Wnts are secreted proteins that control stem cell maintenance, cell fate decisions, and growth during development and adult homeostasis. Wnts carry a post-translational modification not seen in any other secreted protein: during biosynthesis, they are appended with a palmitoleoyl moiety that is required for signaling but also impairs solubility and hence diffusion in the extracellular space. In some contexts, Wnts act only in a juxtacrine manner but there are also instances of long range action. Several proteins and processes ensure that active Wnts reach the appropriate target cells. Some, like Porcupine, Wntless, and Notum are dedicated to Wnt function; we describe their activities in molecular detail. We also outline how the cell infrastructure (secretory, endocytic, and retromer pathways) contribute to the progression of Wnts from production to delivery. We then address how Wnts spread in the extracellular space and form a signaling gradient despite carrying a hydrophobic moiety. We highlight particularly the role of lipid-binding Wnt interactors and heparan sulfate proteoglycans. Finally, we briefly discuss how evolution might have led to the emergence of this unusual signaling pathway.

Asymmetric requirement of Dpp/BMP morphogen dispersal in the Drosophila wing disc

How morphogen gradients control patterning and growth in developing tissues remains largely unknown due to lack of tools manipulating morphogen gradients. Here, we generate two membrane-tethered protein binders that manipulate different aspects of Decapentaplegic (Dpp), a morphogen required for overall patterning and growth of the Drosophila wing. One is "HA trap" based on a single-chain variable fragment (scFv) against the HA tag that traps HA-Dpp to mainly block its dispersal, the other is "Dpp trap" based on a Designed Ankyrin Repeat Protein (DARPin) against Dpp that traps Dpp to block both its dispersal and signaling. Using these tools, we found that, while posterior patterning and growth require Dpp dispersal, anterior patterning and growth largely proceed without Dpp dispersal. We show that dpp transcriptional refinement from an initially uniform to a localized expression and persistent signaling in transient dpp source cells render the anterior compartment robust against the absence of Dpp dispersal. Furthermore, despite a critical requirement of dpp for the overall wing growth, neither Dpp dispersal nor direct signaling is critical for lateral wing growth after wing pouch specification. These results challenge the long-standing dogma that Dpp dispersal is strictly required to control and coordinate overall wing patterning and growth.

Pegasus, a small extracellular peptide enhancing short-range diffusion of Wingless

Small Open Reading Frames (smORFs) coding for peptides of less than 100 amino-acids are an enigmatic and pervasive gene class, found in the tens of thousands in metazoan genomes. Here we reveal a short 80 amino-acid peptide (Pegasus) which enhances Wingless/Wnt1 protein short-range diffusion and signalling. During Drosophila wing development, Wingless has sequential functions, including late induction of proneural gene expression and wing margin development. Pegasus mutants produce wing margin defects and proneural expression loss similar to those of Wingless. Pegasus is secreted, and co-localizes and co-immunoprecipitates with Wingless, suggesting their physical interaction. Finally, measurements of fixed and in-vivo Wingless gradients support that Pegasus increases Wingless diffusion in order to enhance its signalling. Our results unveil a new element in Wingless signalling and clarify the patterning role of Wingless diffusion, while corroborating the link between small open reading frame peptides, and regulation of known proteins with membrane-related functions.

Single-cell transcriptomics in the Drosophila visual system: Advances and perspectives on cell identity regulation, connectivity, and neuronal diversity evolution

The Drosophila visual system supports complex behaviors and shares many of its anatomical and molecular features with the vertebrate brain. Yet, it contains a much more manageable number of neurons and neuronal types. In addition to the extensive Drosophila genetic toolbox, this relative simplicity has allowed decades of work to yield a detailed account of its neuronal type diversity, morphology, connectivity and specification mechanisms. In the past three years, numerous studies have applied large scale single-cell transcriptomic approaches to the Drosophila visual system and have provided access to the complete gene expression profile of most neuronal types throughout development. This makes the fly visual system particularly well suited to perform detailed studies of the genetic mechanisms underlying the evolution and development of neuronal systems. Here, we highlight how these transcriptomic resources allow exploring long-standing biological questions under a new light. We first present the efforts made to characterize neuronal diversity in the Drosophila visual system and suggest ways to further improve this description. We then discuss current advances allowed by the single-cell datasets, and envisage how these datasets can be further leveraged to address fundamental questions regarding the regulation of neuronal identity, neuronal circuit development and the evolution of neuronal diversity.

Neural specification, targeting, and circuit formation during visual system assembly

Like other sensory systems, the visual system is topographically organized: Its sensory neurons, the photoreceptors, and their targets maintain point-to-point correspondence in physical space, forming a retinotopic map. The iterative wiring of circuits in the visual system conveniently facilitates the study of its development. Over the past few decades, experiments in Drosophila have shed light on the principles that guide the specification and connectivity of visual system neurons. In this review, we describe the main findings unearthed by the study of the Drosophila visual system and compare them with similar events in mammals. We focus on how temporal and spatial patterning generates diverse cell types, how guidance molecules distribute the axons and dendrites of neurons within the correct target regions, how vertebrates and invertebrates generate their retinotopic map, and the molecules and mechanisms required for neuronal migration. We suggest that basic principles used to wire the fly visual system are broadly applicable to other systems and highlight its importance as a model to study nervous system development.

Generation of excitatory and inhibitory neurons from common progenitors via Notch signaling in the cerebellum

Brain neurons arise from relatively few progenitors generating an enormous diversity of neuronal types. Nonetheless, a cardinal feature of mammalian brain neurogenesis is thought to be that excitatory and inhibitory neurons derive from separate, spatially segregated progenitors. Whether bi-potential progenitors with an intrinsic capacity to generate both lineages exist and how such a fate decision may be regulated are unknown. Using cerebellar development as a model, we discover that individual progenitors can give rise to both inhibitory and excitatory lineages. Gradations of Notch activity determine the fates of the progenitors and their daughters. Daughters with the highest levels of Notch activity retain the progenitor fate, while intermediate levels of Notch activity generate inhibitory neurons, and daughters with very low levels of Notch signaling adopt the excitatory fate. Therefore, Notch-mediated binary cell fate choice is a mechanism for regulating the ratio of excitatory to inhibitory neurons from common progenitors.

Transcription factor encoding of neuron subtype: Strategies that specify arbor pattern

Dendrite and axon arbors form scaffolds that connect a neuron to its partners; they are patterned to support the specific connectivity and computational requirements of each neuron subtype. Transcription factor networks control the specification of neuron subtypes, and the consequent diversification of their stereotyped arbor patterns during differentiation. We outline how the differentiation trajectories of stereotyped arbors are shaped by hierarchical deployment of precursor cell and postmitotic transcription factors. These transcription factors exert modular control over the dendrite and axon features of a single neuron, create spatial and functional compartmentalization of an arbor, instruct implementation of developmental patterning rules, and exert operational control over the cell biological processes that construct an arbor.

Slit/Robo Signaling Regulates Multiple Stages of the Development of the Drosophila Motion Detection System

Neurogenesis is achieved through a sequence of steps that include specification and differentiation of progenitors into mature neurons. Frequently, precursors migrate to distinct positions before terminal differentiation. The Slit-Robo pathway, formed by the secreted ligand Slit and its membrane bound receptor Robo, was first discovered as a regulator of axonal growth. However, today, it is accepted that this pathway can regulate different cellular processes even outside the nervous system. Since most of the studies performed in the nervous system have been focused on axonal and dendritic growth, it is less clear how versatile is this signaling pathway in the developing nervous system. Here we describe the participation of the Slit-Robo pathway in the development of motion sensitive neurons of the Drosophila visual system. We show that Slit and Robo receptors are expressed in different stages during the neurogenesis of motion sensitive neurons. Furthermore, we find that Slit and Robo regulate multiple aspects of their development including neuronal precursor migration, cell segregation between neural stem cells and daughter cells and formation of their connectivity pattern. Specifically, loss of function of slit or robo receptors in differentiated motion sensitive neurons impairs dendritic targeting, while knocking down robo receptors in migratory progenitors or neural stem cells leads to structural defects in the adult optic lobe neuropil, caused by migration and cell segregation defects during larval development. Thus, our work reveals the co-option of the Slit-Robo signaling pathway in distinct developmental stages of a neural lineage.

Inhibitory Interactions and Columnar Inputs to an Object Motion Detector in Drosophila

The direction-selective T4/T5 cells innervate optic-flow processing projection neurons in the lobula plate of the fly that mediate the visual control of locomotion. In the lobula, visual projection neurons coordinate complex behavioral responses to visual features, however, the input circuitry and computations that bestow their feature-detecting properties are less clear. Here, we study a highly specialized small object motion detector, LC11, and demonstrate that its responses are suppressed by local background motion. We show that LC11 expresses GABA-A receptors that serve to sculpt responses to small objects but are not responsible for the rejection of background motion. Instead, LC11 is innervated by columnar T2 and T3 neurons that are themselves highly sensitive to small static or moving objects, insensitive to wide-field motion and, unlike T4/T5, respond to both ON and OFF luminance steps.

The development and function of neuronal subtypes processing color and skylight polarization in the optic lobes of Drosophila melanogaster

The retinal mosaics of many insects contain different ommatidial subtypes harboring photoreceptors that are both molecularly and morphologically specialized for comparing between different wavelengths versus detecting the orientation of skylight polarization. The neural circuits underlying these different inputs and the characterization of their specific cellular elements are the subject of intense research. Here we review recent progress on the description of both assembly and function of color and skylight polarization circuitry, by focusing on two cell types located in the distal portion of the medulla neuropil of the fruit fly Drosophila melanogaster's optic lobes, called Dm8 and Dm9. In the main part of the retina, Dm8 cells fall into two molecularly distinct subtypes whose center becomes specifically connected to either one of randomly distributed 'pale' or 'yellow' R7 photoreceptor fates during development. Only in the 'dorsal rim area' (DRA), both polarization-sensitive R7 and R8 photoreceptors are connected to different Dm8-like cell types, called Dm-DRA1 and Dm-DRA2, respectively. An additional layer of interommatidial integration is introduced by Dm9 cells, which receive input from multiple neighboring R7 and R8 cells, as well as providing feedback synapses back into these photoreceptors. As a result, the response properties of color-sensitive photoreceptor terminals are sculpted towards being both maximally decorrelated, as well as harboring several levels of opponency (both columnar as well as intercolumnar). In the DRA, individual Dm9 cells appear to mix both polarization and color signals, thereby potentially serving as the first level of integration of different celestial stimuli. The molecular mechanisms underlying the establishment of these synaptic connections are beginning to be revealed, by using a combination of live imaging, developmental genetic studies, and cell type-specific transcriptomics.

Integrated Patterning Programs During Drosophila Development Generate the Diversity of Neurons and Control Their Mature Properties

During the approximately 5 days of Drosophila neurogenesis (late embryogenesis to the beginning of pupation), a limited number of neural stem cells produce approximately 200,000 neurons comprising hundreds of cell types. To build a functional nervous system, neuronal types need to be produced in the proper places, appropriate numbers, and correct times. We discuss how neural stem cells (neuroblasts) obtain so-called area codes for their positions in the nervous system (spatial patterning) and how they keep time to sequentially produce neurons with unique fates (temporal patterning). We focus on specific examples that demonstrate how a relatively simple patterning system (Notch) can be used reiteratively to generate different neuronal types. We also speculate on how different modes of temporal patterning that operate over short versus long time periods might be linked. We end by discussing how specification programs are integrated and lead to the terminal features of different neuronal types.

Neuronal diversity and convergence in a visual system developmental atlas

Deciphering how neuronal diversity is established and maintained requires a detailed knowledge of neuronal gene expression throughout development. In contrast to mammalian brains^1,2, the large neuronal diversity of the Drosophila optic lobes³ and its connectome^4–6 are almost completely characterized. However, a molecular characterization of this diversity, particularly during development, has been lacking. We present novel insights into brain development through a nearly exhaustive description of the transcriptomic diversity of the optic lobes. We acquired the transcriptome of 275,000 single-cells at adult and five pupal stages, and developed a machine learning framework to assign them to almost 200 cell-types at all timepoints. We discovered two large neuronal populations that wrap neuropils during development but die just before adulthood, as well as neuronal subtypes that partition dorsal and ventral visual circuits by differential Wnt signaling throughout development. Moreover, we showed that neurons of the same type but produced days apart synchronize their transcriptomes shortly after being produced. We also resolved during synaptogenesis neuronal subtypes that converge to indistinguishable transcriptomic profiles in adults while greatly differing in morphology and connectivity. Our datasets almost completely account for the known neuronal diversity of the optic lobes and serve as a paradigm to understand brain development across species.

Wg secreted by conventional Golgi transport diffuses and forms Wg gradient whereas Wg tethered to extracellular vesicles do not diffuse

Wingless (Wg)/Wnt family proteins are essential for animal development and adult homeostasis. Drosophila Wg secreted from the dorsal-ventral (DV) midline in wing discs forms a concentration gradient that is shaped by diffusion rate and stability of Wg. To understand how the gradient of extracellular Wg is generated, we compared the secretion route of NRT-Wg, an artificial membrane-tethered form of Wg that is supposedly not secreted but still supports fly development, to that of wild-type Wg. We found that wild-type Wg is secreted by both conventional Golgi transport and via extracellular vesicles (EVs), and NRT-Wg can be also secreted via EVs. Furthermore, wild-type Wg secreted by Golgi transport diffused and formed Wg gradient but Wg-containing EVs did not diffuse at all. In case of Wg stability, Sol narae (Sona), a metalloprotease that cleaves Wg, contributes to generate a steep Wg gradient. Interestingly, Wg was also produced in the presumptive wing blade region, which indicates that NRT-Wg on EVs expressed in the blade allows the blade cells to proliferate and differentiate without Wg diffused from the DV midline. We propose that EV-associated Wg induces Wg signaling in autocrine and juxtaposed manners whereas Wg secreted by Golgi transport forms gradient and acts in the long-range signaling, and different organs differentially utilize these two types of Wg signaling for their own development.

Neurogenesis From Embryo to Adult - Lessons From Flies and Mice

The human brain is composed of billions of cells, including neurons and glia, with an undetermined number of subtypes. During the embryonic and early postnatal stages, the vast majority of these cells are generated from neural progenitors and stem cells located in all regions of the neural tube. A smaller number of neurons will continue to be generated throughout our lives, in localized neurogenic zones, mainly confined at least in rodents to the subependymal zone of the lateral ventricles and the subgranular zone of the hippocampal dentate gyrus. During neurogenesis, a combination of extrinsic cues interacting with temporal and regional intrinsic programs are thought to be critical for increasing neuronal diversity, but their underlying mechanisms need further elucidation. In this review, we discuss the recent findings in Drosophila and mammals on the types of cell division and cell interactions used by neural progenitors and stem cells to sustain neurogenesis, and how they are influenced by glia.

A combinatorial code of transcription factors specifies subtypes of visual motion-sensing neurons in Drosophila

Direction-selective T4/T5 neurons exist in four subtypes, each tuned to visual motion along one of the four cardinal directions. Along with their directional tuning, neurons of each T4/T5 subtype orient their dendrites and project their axons in a subtype-specific manner. Directional tuning, thus, appears strictly linked to morphology in T4/T5 neurons. How the four T4/T5 subtypes acquire their distinct morphologies during development remains largely unknown. Here, we investigated when and how the dendrites of the four T4/T5 subtypes acquire their specific orientations, and profiled the transcriptomes of all T4/T5 neurons during this process. This revealed a simple and stable combinatorial code of transcription factors defining the four T4/T5 subtypes during their development. Changing the combination of transcription factors of specific T4/T5 subtypes resulted in predictable and complete conversions of subtype-specific properties, i.e. dendrite orientation and matching axon projection pattern. Therefore, a combinatorial code of transcription factors coordinates the development of dendrite and axon morphologies to generate anatomical specializations that differentiate subtypes of T4/T5 motion-sensing neurons.

Summary: Morphological and transcriptomic analyses allowed the identification of a combinatorial code of transcription factors that controls the development of subtype-specific morphologies in motion-detecting neurons of the Drosophila visual system.

Gene regulatory networks during the development of the Drosophila visual system

The Drosophila visual system integrates input from 800 ommatidia and extracts different features in stereotypically connected optic ganglia. The development of the Drosophila visual system is controlled by gene regulatory networks that control the number of precursor cells, generate neuronal diversity by integrating spatial and temporal information, coordinate the timing of retinal and optic lobe cell differentiation, and determine distinct synaptic targets of each cell type. In this chapter, we describe the known gene regulatory networks involved in the development of the different parts of the visual system and explore general components in these gene networks. Finally, we discuss the advantages of the fly visual system as a model for gene regulatory network discovery in the era of single-cell transcriptomics.

Neuronal strategies for meeting the right partner during brain wiring

Two neurons can only form a synapse if their axonal and dendritic projections meet at the same time and place. While spatiotemporal proximity is necessary for synapse formation, it remains unclear to what extent the underlying positional strategies are sufficient to ensure synapse formation between the right partners. Many neurons readily form synapses with wrong partners if they find themselves at the wrong place or time. Minimally, restricting spatiotemporal proximity can prevent incorrect synapses. Maximally, restricting encounters in time and space could be sufficient to ensure correct partnerships between neurons that can form synapses promiscuously. In this review we explore recent findings on positional strategies during developmental growth that contribute to precise outcomes in brain wiring.

Slit neuronal secretion coordinates optic lobe morphogenesis in Drosophila

The complexity of the nervous system requires the coordination of multiple cellular processes during development. Among them, we find boundary formation, axon guidance, cell migration and cell segregation. Understanding how different cell populations such as glial cells, developing neurons and neural stem cells contribute to the formation of boundaries and morphogenesis in the nervous system is a critical question in neurobiology. Slit is an evolutionary conserved protein essential for the development of the nervous system. For signaling, Slit has to bind to its cognate receptor Robo, a single-pass transmembrane protein. Although the Slit/Robo signaling pathway is well known for its involvement in axon guidance, it has also been associated to boundary formation in the Drosophila visual system. In the optic lobe, Slit is expressed in glial cells, positioned at the boundaries between developing neuropils, and in neurons of the medulla ganglia. Although it has been assumed that glial cells provide Slit to the system, the contribution of the neuronal expression has not been tested. Here, we show that, contrary to what was previously thought, Slit protein provided by medulla neurons is also required for boundary formation and morphogenesis of the optic lobe. Furthermore, tissue specific rescue using modified versions of Slit demonstrates that this protein acts at long range and does not require processing by extracellular proteases. Our data shed new light on our understanding of the cellular mechanisms involved in Slit function in the fly visual system morphogenesis.

A genetic, genomic, and computational resource for exploring neural circuit function

The anatomy of many neural circuits is being characterized with increasing resolution, but their molecular properties remain mostly unknown. Here, we characterize gene expression patterns in distinct neural cell types of the Drosophila visual system using genetic lines to access individual cell types, the TAPIN-seq method to measure their transcriptomes, and a probabilistic method to interpret these measurements. We used these tools to build a resource of high-resolution transcriptomes for 100 driver lines covering 67 cell types, available at http://www.opticlobe.com. Combining these transcriptomes with recently reported connectomes helps characterize how information is transmitted and processed across a range of scales, from individual synapses to circuit pathways. We describe examples that include identifying neurotransmitters, including cases of apparent co-release, generating functional hypotheses based on receptor expression, as well as identifying strong commonalities between different cell types.

Modular transcriptional programs separately define axon and dendrite connectivity

Patterns of synaptic connectivity are remarkably precise and complex. Single-cell RNA sequencing has revealed a vast transcriptional diversity of neurons. Nevertheless, a clear logic underlying the transcriptional control of neuronal connectivity has yet to emerge. Here, we focused on Drosophila T4/T5 neurons, a class of closely related neuronal subtypes with different wiring patterns. Eight subtypes of T4/T5 neurons are defined by combinations of two patterns of dendritic inputs and four patterns of axonal outputs. Single-cell profiling during development revealed distinct transcriptional programs defining each dendrite and axon wiring pattern. These programs were defined by the expression of a few transcription factors and different combinations of cell surface proteins. Gain and loss of function studies provide evidence for independent control of different wiring features. We propose that modular transcriptional programs for distinct wiring features are assembled in different combinations to generate diverse patterns of neuronal connectivity.

The Organization of the Second Optic Chiasm of the Drosophila Optic Lobe

Visual pathways from the compound eye of an insect relay to four neuropils, successively the lamina, medulla, lobula, and lobula plate in the underlying optic lobe. Among these neuropils, the medulla, lobula, and lobula plate are interconnected by the complex second optic chiasm, through which the anteroposterior axis undergoes an inversion between the medulla and lobula. Given their complex structure, the projection patterns through the second optic chiasm have so far lacked critical analysis. By densely reconstructing axon trajectories using a volumetric scanning electron microscopy (SEM) technique, we reveal the three-dimensional structure of the second optic chiasm of Drosophila melanogaster, which comprises interleaving bundles and sheets of axons insulated from each other by glial sheaths. These axon bundles invert their horizontal sequence in passing between the medulla and lobula. Axons connecting the medulla and lobula plate are also bundled together with them but do not decussate the sequence of their horizontal positions. They interleave with sheets of projection neuron axons between the lobula and lobula plate, which also lack decussations. We estimate that approximately 19,500 cells per hemisphere, about two thirds of the optic lobe neurons, contribute to the second chiasm, most being Tm cells, with an estimated additional 2,780 T4 and T5 cells each. The chiasm mostly comprises axons and cell body fibers, but also a few synaptic elements. Based on our anatomical findings, we propose that a chiasmal structure between the neuropils is potentially advantageous for processing complex visual information in parallel. The EM reconstruction shows not only the structure of the chiasm in the adult brain, the previously unreported main topic of our study, but also suggest that the projection patterns of the neurons comprising the chiasm may be determined by the proliferation centers from which the neurons develop. Such a complex wiring pattern could, we suggest, only have arisen in several evolutionary steps.

Flybow to Dissect Circuit Assembly in the Drosophila Brain: An Update

Visualization of single neurons and glia, as well as neural lineages within their complex environment is a pivotal step towards uncovering the mechanisms that control neural circuit development and function. This chapter provides detailed technical information on how to use Drosophila variants of the mouse Brainbow-2 system, called Flybow, for stochastic labeling of individual cells or lineages with different fluorescent proteins in one sample. We describe the genetic strategies and the heat shock regime required for induction of recombination events. Furthermore, we explain how Flybow and the mosaic analysis with a repressible cell marker (MARCM) approach can be combined to generate wild-type or homozygous mutant clones that are positively labeled in multiple colors. This is followed by a detailed protocol as to how to prepare samples for imaging. Finally, we provide specifications to facilitate multichannel image acquisition using confocal microscopy.

Essential long-range action of Wingless/Wnt in adult intestinal compartmentalization

Signal transduction activated by Wingless/Wnt ligands directs cell proliferation and fate specification in metazoans, and its overactivation underlies the development of the vast majority of colorectal cancers. In the conventional model, the secretion and movement of Wingless to cells distant from its source of synthesis are essential for long-range signaling in tissue patterning. However, this model was upended recently by an unanticipated finding: replacement of wild-type Drosophila Wingless with a membrane-tethered form produced viable adults with largely normal external morphology, which suggested that Wingless secretion and movement are dispensable for tissue patterning. Herein, we tested this foundational principle in the adult intestine, where Wingless signaling gradients coincide with all major boundaries between compartments. We find that the critical roles of Wingless during adult intestinal development, which include regulation of target gene activation, boundary formation, stem cell proliferation, epithelial cell fate specification, muscle differentiation, gut folding, and signaling crosstalk with the Decapentaplegic pathway, are all disrupted by Wingless tethering. These findings provide new evidence that supports the requirement for the direct, long-range action of Wingless in tissue patterning, with relevance for animal development, tissue homeostasis and Wnt-driven disease.

Novel Strategies for the Generation of Neuronal Diversity: Lessons From the Fly Visual System

Among all organs of an adult animal, the central nervous system stands out because of its vast complexity and morphological diversity. During early development, the entire central nervous system develops from an apparently homogenous group of progenitors that differentiate into all neural cell types. Therefore, understanding the molecular and genetic mechanisms that give rise to the cellular and anatomical diversity of the brain is a key goal of the developmental neurobiology field. With this aim in mind, the development of the central nervous system of model organisms has been extensively studied. From more than a century, the mechanisms of neurogenesis have been studied in the fruit fly Drosophila melanogaster. The visual system comprises one of the major structures of the Drosophila brain. The visual information is collected by the eye-retina photoreceptors and then processed by the four optic lobe ganglia: the lamina, medulla, lobula and lobula plate. The molecular mechanisms that originate neuronal diversity in the optic lobe have been unveiled in the past decade. In this article, we describe the early development and differentiation of the lobula plate ganglion, from the formation of the optic placode and the inner proliferation center to the specification of motion detection neurons. We focused specifically on how the precise combination of signaling pathways and cell-specific transcription factors patterns the pool of neural stem cells that generates the different neurons of the motion detection system.

Coordination of neural patterning in the Drosophila visual system

Precise formation of neuronal circuits requires the coordinated development of the different components of the circuit. Here, we review examples of coordination at multiples scales of development in one of the best-studied systems for neural patterning and circuit assembly, the Drosophila visual system, from coordination of gene expression in photoreceptors to the coordinated patterning of the different neuropiles of the optic lobe.

Transcriptional control of morphological properties of direction-selective T4/T5 neurons in Drosophila

In the Drosophila visual system, T4/T5 neurons represent the first stage of computation of the direction of visual motion. T4 and T5 neurons exist in four subtypes, each responding to motion in one of the four cardinal directions and projecting axons into one of the four lobula plate layers. However, all T4/T5 neurons share properties essential for sensing motion. How T4/T5 neurons acquire their properties during development is poorly understood. We reveal that the transcription factors SoxN and Sox102F control the acquisition of properties common to all T4/T5 neuron subtypes, i.e. the layer specificity of dendrites and axons. Accordingly, adult flies are motion blind after disruption of SoxN or Sox102F in maturing T4/T5 neurons. We further find that the transcription factors Ato and Dac are redundantly required in T4/T5 neuron progenitors for SoxN and Sox102F expression in T4/T5 neurons, linking the transcriptional programmes specifying progenitor identity to those regulating the acquisition of morphological properties in neurons. Our work will help to link structure, function and development in a neuronal type performing a computation that is conserved across vertebrate and invertebrate visual systems.

From Early to Late Neurogenesis: Neural Progenitors and the Glial Niche from a Fly's Point of View

Drosophila melanogaster is an important model organism used to study the brain development of organisms ranging from insects to mammals. The central nervous system in fruit flies is formed primarily in two waves of neurogenesis, one of which occurs in the embryo and one of which occurs during larval stages. In order to understand neurogenesis, it is important to research the behavior of progenitor cells that give rise to the neural networks which make up the adult nervous system. This behavior has been shown to be influenced by different factors including interactions with other cells within the progenitor niche, or local tissue microenvironment. Glial cells form a crucial part of this niche and play an active role in the development of the brain. Although in the early years of neuroscience it was believed that glia were simply scaffolding for neurons and passive components of the nervous system, their importance is nowadays recognized. Recent discoveries in progenitors and niche cells have led to new understandings of how the developing brain shapes its diverse regions. In this review, we attempt to summarize the distinct neural progenitors and glia in the Drosophila melanogaster central nervous system, from embryo to late larval stages, and make note of homologous features in mammals. We also outline the recent advances in this field in order to define the impact that glial cells have on progenitor cell niches, and we finally emphasize the importance of communication between glia and progenitor cells for proper brain formation.

Temporal patterning of neurogenesis and neural wiring in the fly visual system

During neural development, a wide variety of neurons are produced in a highly coordinated manner and form complex and highly coordinated neural circuits. Temporal patterning of neuron type specification plays very important roles in orchestrating the production and wiring of neurons. The fly visual system, which is composed of the retina and the optic lobe of the brain, is an outstanding model system to study temporal patterning and wiring of the nervous system. All of the components of the fly visual system are topographically connected, and each ommatidial unit in the retina corresponds to a columnar unit in the optic lobe. In the retina, the wave of differentiation follows the morphogenetic furrow, which progresses in a posterior-to-anterior direction. At the same time, differentiation of the optic lobe also accompanies the wave of differentiation or temporally coordinated neurogenesis. Thus, temporal patterning plays important roles in establishing topographic connections throughout the fly visual system. In this article, we review how neuronal differentiation and connectivity are orchestrated in the fly visual system by temporal patterning mechanisms.