Modulators of hormonal response regulate temporal fate specification in the Drosophila brain

The Drivers of Diversity: Integrated genetic and hormonal cues regulate neural diversity

Proper functioning of the nervous system relies not only on the generation of a vast repertoire of distinct neural cell types but also on the precise neural circuitry within them. How the generation of highly diverse neural populations is regulated during development remains a topic of interest. Landmark studies in Drosophila have identified the genetic and temporal cues regulating neural diversity and thus have provided valuable insights into our understanding of temporal patterning of the central nervous system. The development of the Drosophila central complex, which is mostly derived from type II neural stem cell (NSC) lineages, showcases how a small pool of NSCs can give rise to vast and distinct progeny. Similar to the human outer subventricular zone (OSVZ) neural progenitors, type II NSCs generate intermediate neural progenitors (INPs) to expand and diversify lineages that populate higher brain centers. Each type II NSC has a distinct spatial identity and timely regulated expression of many transcription factors and mRNA binding proteins. Additionally, INPs derived from them show differential expression of genes depending on their birth order. Together type II NSCs and INPs display a combinatorial temporal patterning that expands neural diversity of the central brain lineages. We cover advances in current understanding of type II NSC temporal patterning and discuss similarities and differences in temporal patterning mechanisms of various NSCs with a focus on how cell-intrinsic and extrinsic hormonal cues regulate temporal transitions in NSCs during larval development. Cell extrinsic ligands activate conserved signaling pathways and extrinsic hormonal cues act as a temporal switch that regulate temporal progression of the NSCs. We conclude by elaborating on how a progenitor's temporal code regulates the fate specification and identity of distinct neural types. At the end, we also discuss open questions in linking developmental cues to neural identity, circuits, and underlying behaviors in the adult fly.

Temporal regulation of neural diversity in Drosophila and vertebrates

The generation of neuronal diversity involves temporal patterning mechanisms by which a given progenitor sequentially produces multiple cell types. Several parallels are evident between the brain development programs of Drosophila and vertebrates, such as the successive emergence of specific cell types and the use of combinations of transcription factors to specify cell fates. Furthermore, cell-extrinsic cues such as hormones and signaling pathways have also been shown to be regulatory modules of temporal patterning. Recently, transcriptomic and epigenomic studies using large single-cell sequencing datasets have provided insights into the transcriptional dynamics of neurogenesis in the Drosophila and mammalian central nervous systems. We review these commonalities in the specification of neuronal identity and highlight the conserved or convergent strategies of brain development by discussing temporal patterning mechanisms found in flies and vertebrates.

The making of the Drosophila mushroom body

The mushroom body (MB) is a computational center in the Drosophila brain. The intricate neural circuits of the mushroom body enable it to store associative memories and process sensory and internal state information. The mushroom body is composed of diverse types of neurons that are precisely assembled during development. Tremendous efforts have been made to unravel the molecular and cellular mechanisms that build the mushroom body. However, we are still at the beginning of this challenging quest, with many key aspects of mushroom body assembly remaining unexplored. In this review, I provide an in-depth overview of our current understanding of mushroom body development and pertinent knowledge gaps.

Imp and Syp mediated temporal patterning of neural stem cells in the developing Drosophila CNS

The assembly of complex neural circuits requires that stem cells generate diverse types of neurons in the correct temporal order. Pioneering work in the Drosophila embryonic ventral nerve cord has shown that neural stem cells are temporally patterned by the sequential expression of rapidly changing transcription factors to generate diversity in their progeny. In recent years, a second temporal patterning mechanism, driven by the opposing gradients of the Imp and Syp RNA-binding proteins, has emerged as a powerful way to generate neural diversity. This long-range temporal patterning mechanism is utilized in the extended neural stem cell lineages of the postembryonic fly brain. Here, we review the role played by Imp and Syp gradients in several neural stem cell lineages, focusing on how they specify sequential neural fates through the post-transcriptional regulation of target genes, including the Chinmo and Mamo transcription factors. We further discuss how upstream inputs, including hormonal signals, modify the output of these gradients to couple neurogenesis with the development of the organism. Finally, we review the roles that the Imp and Syp gradients play beyond the generation of diversity, including the regulation of stem cell proliferation, the timing of neural stem cell lineage termination, and the coupling of neuronal birth order to circuit assembly.

Kr-h1, a Cornerstone Gene in Insect Life History

Insect life cycle is coordinated by hormones and their downstream effectors. Krüppel homolog1 (Kr-h1) is one of the crucial effectors which mediates the actions of the two critical hormones of insects, the juvenile hormone (JH) and 20-hydroxyecdysone (20E). It is a transcription factor with a DNA-binding motif of eight C₂H₂ zinc fingers which is found to be conserved among insect orders. The expression of Kr-h1 is fluctuant during insect development with high abundance in juvenile instars and lower levels in the final instar and pupal stage, and reappearance in adults, which is governed by the coordination of JH, 20E, and miRNAs. The dynamic expression pattern of Kr-h1 is closely linked to its function in the entire life of insects. Over the past several years, accumulating studies have advanced our understanding of the role of Kr-h1 during insect development. It acts as a universal antimetamorphic factor in both hemimetabolous and holometabolous species by directly inhibiting the transcription of 20E signaling genes Broad-Complex (Br-C) and Ecdysone induced protein 93F (E93), and steroidogenic enzyme genes involved in ecdysone biosynthesis. Meanwhile, it promotes vitellogenesis and ovarian development in the majority of studied insects. In addition, Kr-h1 regulates insect behavioral plasticity and caste identity, neuronal morphogenesis, maturation of sexual behavior, as well as embryogenesis and metabolic homeostasis. Hence, Kr-h1 acts as a cornerstone regulator in insect life.

Hormone-controlled changes in the differentiation state of post-mitotic neurons

While we think of neurons as having a fixed identity, many show spectacular plasticity.^1-10 Metamorphosis drives massive changes in the fly brain;^11,12 neurons that persist into adulthood often change in response to the steroid hormone ecdysone.^13,14 Besides driving remodeling,^11-14 ecdysone signaling can also alter the differentiation status of neurons.^7,15 The three sequentially born subtypes of mushroom body (MB) Kenyon cells (γ, followed by α'/β', and finally α/β)¹⁶ serve as a model of temporal fating.^17-21 γ neurons are also used as a model of remodeling during metamorphosis. As γ neurons are the only functional Kenyon cells in the larval brain, they serve the function of all three adult subtypes. Correspondingly, larval γ neurons have a similar morphology to α'/β' and α/β neurons-their axons project dorsally and medially. During metamorphosis, γ neurons remodel to form a single medial projection. Both temporal fate changes and defects in remodeling therefore alter γ-neuron morphology in similar ways. Mamo, a broad-complex, tramtrack, and bric-à-brac/poxvirus and zinc finger (BTB/POZ) transcription factor critical for temporal specification of α'/β' neurons,^18,19 was recently described as essential for γ remodeling.²² In a previous study, we noticed a change in the number of adult Kenyon cells expressing γ-specific markers when mamo was manipulated.¹⁸ These data implied a role for Mamo in γ-neuron fate specification, yet mamo is not expressed in γ neurons until pupariation,^18,22 well past γ specification. This indicates that mamo has a later role in ensuring that γ neurons express the correct Kenyon cell subtype-specific genes in the adult brain.

Transcriptional and epigenetic regulation of temporal patterning in neural progenitors

During development, neural progenitors undergo temporal patterning as they age to sequentially generate differently fated progeny. Temporal patterning of neural progenitors is relatively well-studied in Drosophila. Temporal cascades of transcription factors or opposing temporal gradients of RNA-binding proteins are expressed in neural progenitors as they age to control the fates of the progeny. The temporal progression is mostly driven by intrinsic mechanisms including cross-regulations between temporal genes, but environmental cues also play important roles in certain transitions. Vertebrate neural progenitors demonstrate greater plasticity in response to extrinsic cues. Recent studies suggest that vertebrate neural progenitors are also temporally patterned by a combination of transcriptional and post-transcriptional mechanisms in response to extracellular signaling to regulate neural fate specification. In this review, we summarize recent advances in the study of temporal patterning of neural progenitors in Drosophila and vertebrates. We also discuss the involvement of epigenetic mechanisms, specifically the Polycomb group complexes and ATP-dependent chromatin remodeling complexes, in the temporal patterning of neural progenitors.

Glia-derived temporal signals orchestrate neurogenesis in the Drosophila mushroom body

Intrinsic mechanisms such as temporal series of transcription factors orchestrate neurogenesis from a limited number of neural progenitors in the brain. Extrinsic regulations, however, remain largely unexplored. Here we describe a two-step glia-derived signal that regulates neurogenesis in the Drosophila mushroom body (MB). In a temporal manner, glial-specific ubiquitin ligase dSmurf activates non-cell-autonomous Hedgehog signaling propagation by targeting the receptor Patched to suppress and promote the exit of MB neuroblast (NB) proliferation, thereby specifying the correct α/β cell number without affecting differentiation. Independent of NB proliferation, dSmurf also stabilizes the expression of the cell-adhesion molecule Fasciclin II (FasII) via its WW domains and regulates FasII homophilic interaction between glia and MB axons to refine α/β-lobe integrity. Our findings provide insights into how extrinsic glia-to-neuron communication coordinates with NB proliferation capacity to regulate MB neurogenesis; glial proteostasis is likely a generalized mechanism in orchestrating neurogenesis.

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.

Steroid hormones, dietary nutrients, and temporal progression of neurogenesis

Temporal patterning of neural progenitors, in which different factors are sequentially expressed, is an evolutionarily conserved strategy for generating neuronal diversity during development. In the Drosophila embryo, mechanisms that mediate temporal patterning of neural stem cells (neuroblasts) are largely cell-intrinsic. However, after embryogenesis, neuroblast temporal patterning relies on extrinsic cues as well, as freshly hatched larvae seek out nutrients and other key resources in varying natural environments. We recap current understanding of neuroblast-intrinsic temporal programs and discuss how neuroblast extrinsic cues integrate and coordinate with neuroblast intrinsic programs to control numbers and types of neurons produced. One key emerging extrinsic factor that impacts temporal patterning of neuroblasts and their daughters as well as termination of neurogenesis is the steroid hormone, ecdysone, a known regulator of large-scale developmental transitions in insects and arthropods. Lastly, we consider evolutionary conservation and discuss recent work on thyroid hormone signaling in early vertebrate brain development.

Extrinsic activin signaling cooperates with an intrinsic temporal program to increase mushroom body neuronal diversity

Temporal patterning of neural progenitors leads to the sequential production of diverse neurons. To understand how extrinsic cues influence intrinsic temporal programs, we studied Drosophila mushroom body progenitors (neuroblasts) that sequentially produce only three neuronal types: γ, then α’β’, followed by αβ. Opposing gradients of two RNA-binding proteins Imp and Syp comprise the intrinsic temporal program. Extrinsic activin signaling regulates the production of α’β’ neurons but whether it affects the intrinsic temporal program was not known. We show that the activin ligand Myoglianin from glia regulates the temporal factor Imp in mushroom body neuroblasts. Neuroblasts missing the activin receptor Baboon have a delayed intrinsic program as Imp is higher than normal during the α’β’ temporal window, causing the loss of α’β’ neurons, a decrease in αβ neurons, and a likely increase in γ neurons, without affecting the overall number of neurons produced. Our results illustrate that an extrinsic cue modifies an intrinsic temporal program to increase neuronal diversity.