Homemade: building the structure of the neurogenic niche

Neural stem/progenitor cells live in an intricate cellular environment, the neurogenic niche, which supports their function and enables neurogenesis. The niche is made of a diversity of cell types, including neurons, glia and the vasculature, which are able to signal to and are structurally organised around neural stem/progenitor cells. While the focus has been on how individual cell types signal to and influence the behaviour of neural stem/progenitor cells, very little is actually known on how the niche is assembled during development from multiple cellular origins, and on the role of the resulting topology on these cells. This review proposes to draw a state-of-the art picture of this emerging field of research, with the aim to expose our knowledge on niche architecture and formation from different animal models (mouse, zebrafish and fruit fly). We will span its multiple aspects, from the existence and importance of local, adhesive interactions to the potential emergence of larger-scale topological properties through the careful assembly of diverse cellular and acellular components.

Differential adhesion during development establishes individual neural stem cell niches and shapes adult behaviour in Drosophila

Neural stem cells (NSCs) reside in a defined cellular microenvironment, the niche, which supports the generation and integration of newborn neurons. The mechanisms building a sophisticated niche structure around NSCs and their functional relevance for neurogenesis are yet to be understood. In the Drosophila larval brain, the cortex glia (CG) encase individual NSC lineages in membranous chambers, organising the stem cell population and newborn neurons into a stereotypic structure. We first found that CG wrap around lineage-related cells regardless of their identity, showing that lineage information builds CG architecture. We then discovered that a mechanism of temporally controlled differential adhesion using conserved complexes supports the individual encasing of NSC lineages. An intralineage adhesion through homophilic Neuroglian interactions provides strong binding between cells of a same lineage, while a weaker interaction through Neurexin-IV and Wrapper exists between NSC lineages and CG. Loss of Neuroglian results in NSC lineages clumped together and in an altered CG network, while loss of Neurexin-IV/Wrapper generates larger yet defined CG chamber grouping several lineages together. Axonal projections of newborn neurons are also altered in these conditions. Further, we link the loss of these 2 adhesion complexes specifically during development to locomotor hyperactivity in the resulting adults. Altogether, our findings identify a belt of adhesions building a neurogenic niche at the scale of individual stem cell and provide the proof of concept that niche properties during development shape adult behaviour.

Infection of the Developing Central Nervous System of Drosophila by Mammalian Eukaryotic and Prokaryotic Pathogens

Pathogen invasion of the central nervous system (CNS) is an important cause of infection-related mortality worldwide and can lead to severe neurological sequelae. To gain access to the CNS cells, pathogens have to overcome the blood-brain barrier (BBB), a protective fence from blood-borne factors. To study host-pathogen interactions, a number of cell culture and animal models were developed. However, in vitro models do not recapitulate the 3D architecture of the BBB and CNS tissue, and in vivo mammalian models present cellular and technical complexities as well as ethical issues, rendering systematic and genetic approaches difficult. Here, we present a two-pronged methodology allowing and validating the use of Drosophila larvae as a model system to decipher the mechanisms of infection in a developing CNS. First, an ex vivo protocol based on whole CNS explants serves as a fast and versatile screening platform, permitting the investigation of molecular and cellular mechanisms contributing to the crossing of the BBB and consequences of infection on the CNS. Then, an in vivo CNS infection protocol through direct pathogen microinjection into the fly circulatory system evaluates the impact of systemic parameters, including the contribution of circulating immune cells to CNS infection, and assesses infection pathogenicity at the whole host level. These combined complementary approaches identify mechanisms of BBB crossing and responses of a diversity of CNS cells contributing to infection, as well as novel virulence factors of the pathogen. This protocol was validated in: Nat Commun (2020), DOI: 10.1038/s41467-020-19826-2 Graphical abstract Procedures flowchart. Mammalian neurotropic pathogens could be tested in two Drosophila central nervous system (CNS) infection setups (ex vivo and in vivo) for their ability to: (1) invade the CNS (pathogen quantifications), (2) disturb blood-brain barrier permeability, (3) affect CNS host cell behaviour (gene expression), and (4) alter host viability.

Brain inflammation triggers macrophage invasion across the blood-brain barrier in Drosophila during pupal stages

The nervous system is shielded from circulating immune cells by the blood-brain barrier (BBB). During infections and autoimmune diseases, macrophages can enter the brain where they participate in pathogen elimination but can also cause tissue damage. Here, we establish a Drosophila model to study macrophage invasion into the inflamed brain. We show that the immune deficiency (Imd) pathway, but not the Toll pathway, is responsible for attraction and invasion of hemolymph-borne macrophages across the BBB during pupal stages. Macrophage recruitment is mediated by glial, but not neuronal, induction of the Imd pathway through expression of Pvf2. Within the brain, macrophages can phagocytose synaptic material and reduce locomotor abilities and longevity. Similarly, we show that central nervous system infection by group B Streptococcus elicits macrophage recruitment in an Imd-dependent manner. This suggests that evolutionarily conserved inflammatory responses require a delicate balance between beneficial and detrimental activities.

Breaking down barriers: Tumors make a leaky brain

The causes of death among cancer patients are multifactorial, and the mechanisms that drive pathological conditions that are associated with, but take place outside of, the tumor are still poorly characterized. In this issue of Developmental Cell, Kim et al. (2021) identify a paraneoplastic syndrome that affects blood-brain barrier permeability and host survival.

An original infection model identifies host lipoprotein import as a route for blood-brain barrier crossing

Pathogens able to cross the blood-brain barrier (BBB) induce long-term neurological sequelae and death. Understanding how neurotropic pathogens bypass this strong physiological barrier is a prerequisite to devise therapeutic strategies. Here we propose an innovative model of infection in the developing Drosophila brain, combining whole brain explants with in vivo systemic infection. We find that several mammalian pathogens are able to cross the Drosophila BBB, including Group B Streptococcus (GBS). Amongst GBS surface components, lipoproteins, and in particular the B leucine-rich Blr, are important for BBB crossing and virulence in Drosophila. Further, we identify (V)LDL receptor LpR2, expressed in the BBB, as a host receptor for Blr, allowing GBS translocation through endocytosis. Finally, we show that Blr is required for BBB crossing and pathogenicity in a murine model of infection. Our results demonstrate the potential of Drosophila for studying BBB crossing by pathogens and identify a new mechanism by which pathogens exploit the machinery of host barriers to generate brain infection.

Molecular to organismal chirality is induced by the conserved myosin 1D

The emergence of asymmetry from an initially symmetrical state is a universal transition in nature. Living organisms show asymmetries at the molecular, cellular, tissular, and organismal level. However, whether and how multilevel asymmetries are related remains unclear. In this study, we show that Drosophila myosin 1D (Myo1D) and myosin 1C (Myo1C) are sufficient to generate de novo directional twisting of cells, single organs, or the whole body in opposite directions. Directionality lies in the myosins' motor domain and is swappable between Myo1D and Myo1C. In addition, Myo1D drives gliding of actin filaments in circular, counterclockwise paths in vitro. Altogether, our results reveal the molecular motor Myo1D as a chiral determinant that is sufficient to break symmetry at all biological scales through chiral interaction with the actin cytoskeleton.

Systemic and local cues drive neural stem cell niche remodelling during neurogenesis in Drosophila

Successful neurogenesis requires adequate proliferation of neural stem cells (NSCs) and their progeny, followed by neuronal differentiation, maturation and survival. NSCs inhabit a complex cellular microenvironment, the niche, which influences their behaviour. To ensure sustained neurogenesis, niche cells must respond to extrinsic, environmental changes whilst fulfilling the intrinsic requirements of the neurogenic program and adapting their roles accordingly. However, very little is known about how different niche cells adjust their properties to such inputs. Here, we show that nutritional and NSC-derived signals induce the remodelling of Drosophila cortex glia, adapting this glial niche to the evolving needs of NSCs. First, nutrition-induced activation of PI3K/Akt drives the cortex glia to expand their membrane processes. Second, when NSCs emerge from quiescence to resume proliferation, they signal to glia to promote membrane remodelling and the formation of a bespoke structure around each NSC lineage. The remodelled glial niche is essential for newborn neuron survival.

Control of brain development and homeostasis by local and systemic insulin signalling

Insulin and insulin-like growth factors (IGFs) are important regulators of growth and metabolism. In both vertebrates and invertebrates, insulin/IGFs are made available to various organs, including the brain, through two routes: the circulating systemic insulin/IGFs act on distant organs via endocrine signalling, whereas insulin/IGF ligands released by local tissues act in a paracrine or autocrine fashion. Although the mechanisms governing the secretion and action of systemic insulin/IGF have been the focus of extensive investigation, the significance of locally derived insulin/IGF has only more recently come to the fore. Local insulin/IGF signalling is particularly important for the development and homeostasis of the central nervous system, which is insulated from the systemic environment by the blood-brain barrier. Local insulin/IGF signalling from glial cells, the blood-brain barrier and the cerebrospinal fluid has emerged as a potent regulator of neurogenesis. This review will address the main sources of local insulin/IGF and how they affect neurogenesis during development. In addition, we describe how local insulin/IGF signalling couples neural stem cell proliferation with systemic energy state in Drosophila and in mammals.

Gap junction proteins in the blood-brain barrier control nutrient-dependent reactivation of Drosophila neural stem cells

Neural stem cells in the adult brain exist primarily in a quiescent state but are reactivated in response to changing physiological conditions. How do stem cells sense and respond to metabolic changes? In the Drosophila CNS, quiescent neural stem cells are reactivated synchronously in response to a nutritional stimulus. Feeding triggers insulin production by blood-brain barrier glial cells, activating the insulin/insulin-like growth factor pathway in underlying neural stem cells and stimulating their growth and proliferation. Here we show that gap junctions in the blood-brain barrier glia mediate the influence of metabolic changes on stem cell behavior, enabling glia to respond to nutritional signals and reactivate quiescent stem cells. We propose that gap junctions in the blood-brain barrier are required to translate metabolic signals into synchronized calcium pulses and insulin secretion.

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Blood-brain barrier gap junctions are required for neural stem cell reactivation

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Gap junctions control both insulin transcription and secretion

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Calcium oscillations in the blood-brain barrier depend on gap junctions and nutrition

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Blood-brain barrier membrane polarization links calcium to insulin secretion

DE-Cadherin regulates unconventional Myosin ID and Myosin IC in Drosophila left-right asymmetry establishment

In bilateria, positioning and looping of visceral organs requires proper left-right (L/R) asymmetry establishment. Recent work in Drosophila has identified a novel situs inversus gene encoding the unconventional type ID myosin (MyoID). In myoID mutant flies, the L/R axis is inverted, causing reversed looping of organs, such as the gut, spermiduct and genitalia. We have previously shown that MyoID interacts physically with β-Catenin, suggesting a role of the adherens junction in Drosophila L/R asymmetry. Here, we show that DE-Cadherin co-immunoprecipitates with MyoID and is required for MyoID L/R activity. We further demonstrate that MyoIC, a closely related unconventional type I myosin, can antagonize MyoID L/R activity by preventing its binding to adherens junction components, both in vitro and in vivo. Interestingly, DE-Cadherin inhibits MyoIC, providing a protective mechanism to MyoID function. Conditional genetic experiments indicate that DE-Cadherin, MyoIC and MyoID show temporal synchronicity for their function in L/R asymmetry. These data suggest that following MyoID recruitment by β-Catenin at the adherens junction, DE-Cadherin has a twofold effect on Drosophila L/R asymmetry by promoting MyoID activity and repressing that of MyoIC. Interestingly, the product of the vertebrate situs inversus gene inversin also physically interacts with β-Catenin, suggesting that the adherens junction might serve as a conserved platform for determinants to establish L/R asymmetry both in vertebrates and invertebrates.

Coupling of apoptosis and L/R patterning controls stepwise organ looping

Handed asymmetry in organ shape and positioning is a common feature among bilateria, yet little is known about the morphogenetic mechanisms underlying left-right (LR) organogenesis. We utilize the directional 360° clockwise rotation of genitalia in Drosophila to study LR-dependent organ looping. Using time-lapse imaging, we show that rotation of genitalia by 360° results from an additive process involving two ring-shaped domains, each undergoing 180° rotation. Our results show that the direction of rotation for each ring is autonomous and strictly depends on the LR determinant myosin ID (MyoID). Specific inactivation of MyoID in one domain causes rings to rotate in opposite directions and thereby cancels out the overall movement. We further reveal a specific pattern of apoptosis at the ring boundaries and show that local cell death is required for the movement of each domain, acting as a brake-releaser. These data indicate that organ looping can proceed through an incremental mechanism coupling LR determination and apoptosis. Furthermore, they suggest a model for the stepwise evolution of genitalia posture in Diptera, through the emergence and duplication of a 180° LR module.

The Drosophila serine protease homologue Scarface regulates JNK signalling in a negative-feedback loop during epithelial morphogenesis

In Drosophila melanogaster, dorsal closure is a model of tissue morphogenesis leading to the dorsal migration and sealing of the embryonic ectoderm. The activation of the JNK signal transduction pathway, specifically in the leading edge cells, is essential to this process. In a genome-wide microarray screen, we identified new JNK target genes during dorsal closure. One of them is the gene scarface (scaf), which belongs to the large family of trypsin-like serine proteases. Some proteins of this family, like Scaf, bear an inactive catalytic site, representing a subgroup of serine protease homologues (SPH) whose functions are poorly understood. Here, we show that scaf is a general transcriptional target of the JNK pathway coding for a secreted SPH. scaf loss-of-function induces defects in JNK-controlled morphogenetic events such as embryonic dorsal closure and adult male terminalia rotation. Live imaging of the latter process reveals that, like for dorsal closure, JNK directs the dorsal fusion of two epithelial layers in the pupal genital disc. Genetic data show that scaf loss-of-function mimics JNK over-activity. Moreover, scaf ectopic expression aggravates the effect of the JNK negative regulator puc on male genitalia rotation. We finally demonstrate that scaf acts as an antagonist by negatively regulating JNK activity. Overall, our results identify the SPH-encoding gene scaf as a new transcriptional target of JNK signalling and reveal the first secreted regulator of the JNK pathway acting in a negative-feedback loop during epithelial morphogenesis.

Left-right asymmetry in Drosophila

Seminal studies of left-right (L/R) patterning in vertebrate models have led to the discovery of roles for the nodal pathway, ion flows and cilia in this process. Although the molecular mechanisms underlying L/R asymmetries seen in protostomes are less well understood, recent work using Drosophila melanogaster as a novel genetic model system to study this process has identified a number of mutations affecting directional organ looping. The genetic analysis of this, the most evolutionary conserved feature of L/R patterning, revealed the existence of a L/R pathway that involves the actin cytoskeleton and an associated type I myosin. In this review, we describe this work in the context of Drosophila development, and discuss the implications of these results for our understanding of L/R patterning in general.

Strategies to establish left/right asymmetry in vertebrates and invertebrates

Left/right (L/R) asymmetry is essential during embryonic development for organ positioning, looping and handed morphogenesis. A major goal in the field is to understand how embryos initially determine their left and right hand sides, a process known as symmetry breaking. A number of recent studies on several vertebrate and invertebrate model organisms have provided a more complex view on how L/R asymmetry is established, revealing an apparent partition between deuterostomes and protostomes. In deuterostomes, nodal cilia represent a conserved symmetry-breaking process; nevertheless, growing evidence shows the existence of pre-cilia L/R asymmetries involving active ion flows. In protostomes like snails and Drosophila, symmetry breaking relies on different mechanisms, involving, in particular, the actin cytoskeleton and associated molecular motors.

Left-right asymmetry: class I myosins show the direction

Myosins are actin-based molecular motors that are found in almost all eukaryotes. Phylogenetic analysis allows the discrimination of 37 different types of myosins, most with unknown functions. Recent work in Drosophila has revealed a crucial role for type ID unconventional myosin in left-right asymmetry. Mutations in Myosin ID completely reverse the left-right axis (situs inversus), a phenotype that is dependent on an intact actin cytoskeleton. How this myosin might orient the left-right axis has began to be elucidated by showing that it interacts directly with beta-catenin, suggesting that myosin ID interacts with the adherens junction to control the direction of organ looping. This is the first demonstration of a role of a myosin in body patterning.

Type ID unconventional myosin controls left-right asymmetry in Drosophila

Breaking left-right symmetry in Bilateria embryos is a major event in body plan organization that leads to polarized adult morphology, directional organ looping, and heart and brain function. However, the molecular nature of the determinant(s) responsible for the invariant orientation of the left-right axis (situs choice) remains largely unknown. Mutations producing a complete reversal of left-right asymmetry (situs inversus) are instrumental for identifying mechanisms controlling handedness, yet only one such mutation has been found in mice (inversin) and snails. Here we identify the conserved type ID unconventional myosin 31DF gene (Myo31DF) as a unique situs inversus locus in Drosophila. Myo31DF mutations reverse the dextral looping of genitalia, a prominent left-right marker in adult flies. Genetic mosaic analysis pinpoints the A8 segment of the genital disc as a left-right organizer and reveals an anterior-posterior compartmentalization of Myo31DF function that directs dextral development and represses a sinistral default state. As expected of a determinant, Myo31DF has a trigger-like function and is expressed symmetrically in the organizer, and its symmetrical overexpression does not impair left-right asymmetry. Thus Myo31DF is a dextral gene with actin-based motor activity controlling situs choice. Like mouse inversin, Myo31DF interacts and colocalizes with beta-catenin, suggesting that situs inversus genes can direct left-right development through the adherens junction.

An unconventional myosin in Drosophila reverses the default handedness in visceral organs

The internal organs of animals often have left-right asymmetry. Although the formation of the anterior-posterior and dorsal-ventral axes in Drosophila is well understood, left-right asymmetry has not been extensively studied. Here we find that the handedness of the embryonic gut and the adult gut and testes is reversed (not randomized) in viable and fertile homozygous Myo31DF mutants. Myo31DF encodes an unconventional myosin, Drosophila MyoIA (also referred to as MyoID in mammals; refs 3, 4), and is the first actin-based motor protein to be implicated in left-right patterning. We find that Myo31DF is required in the hindgut epithelium for normal embryonic handedness. Disruption of actin filaments in the hindgut epithelium randomizes the handedness of the embryonic gut, suggesting that Myo31DF function requires the actin cytoskeleton. Consistent with this, we find that Myo31DF colocalizes with the cytoskeleton. Overexpression of Myo61F, another myosin I (ref. 4), reverses the handedness of the embryonic gut, and its knockdown also causes a left-right patterning defect. These two unconventional myosin I proteins may have antagonistic functions in left-right patterning. We suggest that the actin cytoskeleton and myosin I proteins may be crucial for generating left-right asymmetry in invertebrates.