Ambient effects on neural stem cells: Insights from Drosophila

Abstract

Neural stem cells (NSCs) play a fundamental role in generating diverse neuronal populations that contribute to the formation of intricate neural circuitry. Disturbances arising from intrinsic or extrinsic factors can alter the developmental behavior of NSCs and disrupt nervous system homeostasis. While intrinsic regulatory mechanisms have been elucidated extensively in invertebrate or vertebrate models, the regulatory mechanisms underlying extrinsic cues from the cellular environment remain poorly understood. This review synthesized recent research on cellular ambient effects, including the microenvironment, systemic environment and external factors, on NSCs in Drosophila. Key topics include spatial cues, NSC-glia interactions, long-distance regulation by tissues such as the fat body, and the external environmental stressors like irradiation or viral infection. By integrating these findings, this review provides new insights into how extrinsic signals shape NSCs and bridges gaps between foundational research and clinical translation.

Key Words: Neural stem cells; Drosophila; Ambient effects; Spatial factors; Microenvironment; Systemic environment; External environment

Core Tip: Neural stem cells (NSCs) are fundamental to the formation of neuronal circuitry, and their dysfunction can lead to systemic neural impairments. While intrinsic regulatory mechanisms of NSCs have been studied extensively, emerging evidence highlights the critical contributions of extrinsic factors, such as glial interactions, intertissue signaling, oxygen tension, and irradiation, to NSC development. This mini-review synthesizes current insights into these extrinsic regulators, leveraging the Drosophila model to elucidate their role in NSC dynamics and neural system integrity.

INTRODUCTION

Understanding how to construct an integral neural system precisely remains a fundamental question in neuroscience. Neural stem cells (NSCs) or neural progenitors play a pivotal role in this process by dividing continuously to generate the diverse daughter cells required for forming intricate neural circuitry[1-5]. Critical functions of NCSs govern neural development under both physiological conditions and during responses to neurodegeneration or injury[6,7]. Elucidating the mechanisms that regulate NSC development spatiotemporally will deepen our understanding of the neural ontogeny and enhance therapeutic strategies for neurological disorders.

Regulatory mechanisms driving NSC development are broadly categorized into either intrinsic factors (inherited from progenitor cells) and extrinsic cues (derived from the cellular environment)[3,8,9]. Over the past decades, intrinsic regulatory networks-determining NSC identity, division patterns and spatiotemporal dynamics-have been characterized extensively in such model organisms as Caenorhabditis elegans, Drosophila, and mice[2,10-12]. Recent studies, however, have highlighted the indispensable role of extrinsic ambient signals in maintaining NSC identities, division modes and developmental timing[13-15]. These extrinsic causes synergize with intrinsic programs to ensure precise neural system assembly. For example, the steroid hormone ecdysone regulates neural temporal identities through Seven-up, the orphan nuclear receptor of ecdysone[16,17].

The extrinsic ambient regulatory landscape comprises three layers. The first is the external environment and includes such factors as irradiation, oxygen tension and viral infections[18-21]. The second is the systemic environment, especially intertissue interactions involving organs like the fat body, trachea, gut and heart[20,22-24]. The last is the microenvironment (niche), which encompasses local signals from the adjacent glia, sibling cells and niche component[15,25,26].

Model organisms, particularly Drosophila, play a pivotal role in elucidating the mechanisms underlying neurogenesis[27]. Studies on NSC behaviors, such as asymmetric division, temporal regulation, termination, and the reactivation of quiescent NSCs, have significantly advanced our understanding of their developmental processes in vertebrates[10,16,22,28]. Recent research in Drosophila has also provided critical insights into how extrinsic cues regulate NSC development.

This review synthesizes recent advances in understanding environmental systemic and niche-mediated regulation of NSCs in Drosophila. Such findings bridge foundational discoveries to clinical applications, offering novel perspectives for neural regeneration and disease intervention.

DROSOPHILA NSCs

Drosophila NSCs, known as neuroblasts, generate neuronal progeny during development[28,29]. These NSCs arise during embryogenesis and cease activity by the early pupal stage (Figure 1). Their lifecycle comprises two proliferative phases separated by a mitotically quiescent stage[30,31]. Upon delamination from the neuroepithelium, embryonic NSCs initiate a phase of asymmetric divisions to produce differentiated cells while retaining stem cell identity[29]. By late embryogenesis, they enter a transient quiescent stage[25,32]. Reactivation occurs during the early second larval stage, triggering a second wave of asymmetric divisions reminiscent of embryonic proliferation[22,25,33,34]. NSCs ultimately exit the cell cycle and undergo decommissioning in the early pupal stage[35,36].

Figure 1 Developmental timeline of the Drosophila central nervous system and neural stem cell activity. The Drosophila central nervous system (CNS) arises from neural stem cells (NSCs) across developmental stages (embryonic, larval, and adult). NSC development progresses through five phases: Initiation, proliferation, quiescence/reactivation, secondary proliferation and termination. During embryogenesis, CNS NSCs delaminate from the neuroepithelium and undergo asymmetric divisions. Quiescent NSCs (smaller in size) reactivate during the larval stage, leading to glial niche remodeling. Terminating NSCs shift to symmetric divisions. GMC: Ganglion mother cell; NB: Neuroblast.

The asymmetric division process ensures self-renewal of the NSC pool while generating diverse neuronal progeny for circuit assembly[10,37]. Embryonic NSCs contribute primarily to larval behavior circuits, whereas larval and pupal division establish adult-specific neural networks[38].

AMBIENT REGULATION OF DROSOPHILA NSCs

While asymmetric cell division and intrinsic temporal patterning confer autonomous properties to NSCs, accumulating evidence has underscored the critical role of ambient factors of NSCs in shaping their development[39-41]. For example, steroid hormones and dietary nutrients influence NSC temporal identities and development states[42]. These extrinsic ambient regulators of NSCs are broadly categorized into four groups (Figure 2). The first is the spatial patterning cues, especially body segregation, mediated by the gap genes, establishes regional identities of NSCs[43-45], leading to distinct lifespan and termination mechanism in brain, thoracic and abdominal NSCs[46,47]. Most of the ventral nerve cord NSCs undergo apoptosis, while brain NSCs are reactivated and undergo proliferation during the larval stage[32,48]. The second is the interaction between NSCs and their neighboring cells. The physical positioning of daughter cells relative to the niche influences NSC division orientation[49]. Additionally, surrounding glia cells modulate NSC reactivation and proliferation via nutrients and irons[15,50]. The third is intertissue communication. Systemic signals from organs such as the fat body, trachea and gut regulate NSC behavior[20,22,51]. For instance, the fat body integrates nutrient-derived energy status and relays growth-promoting signals to NSCs[16,22]. The last is external environmental inputs. External environmental factors, including irradiation, oxygen levels, virus infection, and dietary composition, directly or indirectly influence NSC dynamics[18-21,52,53]. Irradiation can induce NSC premature decommissioning in the third instar larval stage[21]. The interplay between these extrinsic ambient factors and intrinsic genetic programs ensures the precise spatiotemporal control of NSC development. Below, we detail mechanistic insights into how environmental cues orchestrate NSC behavior.

Figure 2 Ambient cues regulating neural stem cells in Drosophila. Four major extrinsic regulatory categories influence neural stem cell development: Spatial patterning, niche interactions, intertissue signaling, and external environmental inputs. GMCs: Ganglion mother cells; VNC: Ventral nerve cord.

INFLUENCES OF AMBIENT CUES ON NSC DEVELOPEMNT

Intrinsic program and extrinsic cues jointly underpin NSC development. Ambient signals collaborate with intrinsic factors to regulate NSC division models, identity maintenance, and lifespan. These ambient signals originate from various sources, including the external environment, systemic environment, neighboring cells and spatial positioning cues (Figure 3). Recent studies have elucidated molecular mechanisms underlying these regulatory interactions. Key factors among these ambient cues are summarized in Table 1.

Figure 3 Ambient factors regulating neural stem cells in Drosophila. Key extrinsic regulators include external environment (e.g., diet, virus infections, irradiation and oxygen tension), systemic environment (e.g., gut, fat body, trachea), microenvironment (e.g., neighboring cells, glia and lipid droplets), and spatial factors (e.g., anteroposterior and abdomen-basal positioning). aa: Amino acid; Abd: Abdominal; Antp: Antennapedia; CLC: Chloride channel; Dap: Dapaco; DE-Cad: Drosophila E-cadherin; Dpp: Decapentaplegic; EGFr: Epidermal growth factor receptor; EMT: Epithelial-mesenchymal transition; ER: Ecdysone receptor; Hox: Homeobox gene; LD: Lipid droplet; Msh: Muscle segment homeobox; PcG: Polycomb group; PI3K: Phosphoinositide 3-kinase; PRC2: Polycomb-repressive complex 2; ROS: Reactive oxygen species; SLC: Solute carrier transporter; TOR: Target of rapamycin; Ubx: Ubiquitination regulatory x; VGSC: Voltage-gated sodium channel; Vnd: Ventral nervous system defective.

Table 1 Environmental factors influencing neural stem cell development.

Source	Factors	Observed effects	Ref.
Microenvironment	Snail, Notch, Sox, EMT	Altered cell number/division defect	[54,67,69,70]
	Dap, Msh	Quiescent regulation (G0 or G2)	[44]
	PcG and Hox homeotic network	Anteroposterior stemness gradient	[47]
	DE-cad	Proliferation control	[71]
	ROS	Proliferation modulation	[65,81]
	GMC positioning	Division orientation	[49]
	Ecdysone	NCS termination	[16]
	Hh signaling	Proliferation promotion	[50]
	Insulin/IGF-like peptides	NSC reactivation	[25,40]
	Scaf protease	Proliferation regulation	[78]
	CLC, VGSC	Proliferation modulation	[39,76]
Systemic environment	Fat body	NSC reactivation	[22]
	Trachea	Proliferation regulation	[20]
	Gut	Proliferation modulation	[23]
External environment	Irradiation	Premature differentiation	[18]
	Oxygen	Proliferation suppression	[20]
	Virus infection	Differentiation defects	[19,80]
	High-sugar diet	Proliferation enhancement	[83]
	High-fat diet	Proliferation modulation	[84]
	Iron, silver ions	Nuclear deformation	[53]

CLC: Chloride channel; Dap: Dapaco; DE-cad: Drosophila E-cadherin; EMT: Epithelial-mesenchymal transition; GMC: Ganglion mother cell; Hh: Hedgehog; Hox: Homeobox gene; IGF: Insulin like growth factor; Msh: Muscle segment homeobox; NSC: Neural stem cell; PcG: Polycomb group; ROS: Reactive oxygen species; Scaf: Scarface; VGSC: Voltage-gated sodium channel.

SPATIAL CUES DETERMINE NSC ORIGINATION AND POSITIONING

Localization critically determines NSC progeny positioning within the neural hierarchy. NSCs originate among the embryonic midline, guided by homeobox and proneural genes[54,55]. During segmentation, NSCs adopt segment-specific fates[47,56]. The NSCs along each segmentation present a specific pattern[57]. At the end of the embryonic stage, brain NSCs predominantly enter quiescence, while most of ventral nerve cord NSCs undergo apoptosis[33,58]. Post-embryonically, region-specific NSC behavior persists[59], basal and dorsal NSCs exhibit distinct developmental trajectories[44,60], and the polycomb group > Hox gene expression gradient controls brain expansion[47]. These spatial cues ensure regionally specialized neuronal outputs.

NEIGHBORING CELLS SERVE AS A NICHE TO REGULATE NSC DEVELOPMENT STATES

The neighboring cells of NSC serve as a microenvironment (niche) to govern NSC behavior. While early studies emphasized the intrinsic regulation via asymmetric division and temporal regulation[33,61,62], recent work has highlighted the microenvironment contributions[63-65].

Embryonic niche governs NSC birth and division orientation

Lateral inhibition is essential for the ‘birth’ of NSCs. Delta-expressing NSC precursors suppress pro-neural genes in neighboring cells via Notch[66,67]. Pro-neural genes and pan-neural genes work together to decide the central nervous system (CNS) fates[54]. Then, the CNS NSCs undergo delamination. During the first round of asymmetric division, the delaminating NSCs retain the apical stalk that is linked to the neuroepithelium, anchoring polarity[68]. After each round of division in the CNS, the newly ‘born’ NSCs adject to the neuroepithelia. Tight junctions, between the NSCs and epithelia, are essential for division orientation[69-71].

Larval niche maintains NSC development status

The larval brain NSC polarity axis lacks embryonic alignment from the epithelia to the inside[72,73] but orients division based on differentiated daughter positioning[49]. Neighboring glia regulate NSC metabolism via phosphoinositide 3-kinase/Akt/mammalian target of rapamycin signaling[74], secrete insulin-like peptides for reactivation[26,31,75], provide ions for proliferation[39,76], and buffer reactive oxygen species via unsaturated fatty acids[65,77]. Glia also form the protective blood-brain barrier[78,79].

TISSUE INTERACTIONS COORDINATE NSC DEVELOPMENT

During development, intertissue communication coordinates NSC activity. The fat body, the human liver homolog, integrates nutrient status to release insulin-like peptides, activating quiescent NSCs during the first larval stage[22,25]. The evidence indicates that dysfunctional trachea or gut impairs brain development[20,23,51,62], although the direct mechanistic links that underlie NSC development remain underexplored.

EXTERNAL ENVIRONMENTAL CUES PLAY KEY ROLES IN NSC DEVELOPMENT

Global environmental inputs exert localized NSC effects. Irradiation induces NSC differentiation prematurely[18,21]. Virus infection impacts NSC development; for instance, Zika virus receptor mutants have been shown to disrupt NSC division[19,80]. Oxygen tension elevates reactive oxygen species, which in turn suppresses NSC proliferation[81]. Metabolic shifts, from glycolysis to oxidative phosphorylation, correlate with NSC termination[82]. Finally, dietary factors, such as high glucose, fats, or ions, modulate both NSC proliferation and differentiation[53,65,83,84]. However, how these external cues crosstalk within the NSC milieu remains unknown.

DROSOPHILA ADULT BRAIN SERVES AS A NOVEL ENVIRONMENT FOR NEURAL REGENERATION

Adult brain neurogenesis represents a compelling area of study; however, in Drosophila, NSCs terminate activity during the early pupal stage and do not persist into adulthood[85]. Intriguingly, evidence from NSC-derived tumors or injury models has suggested that limited neurogenesis may occur in the adult-stage Drosophila brain[86].

Survival of NSCs in the adult Drosophila brain

No endogenous NSCs have been detected in the Drosophila adult brain under normal conditions. However, NSC-derived tumors retaining stem cell-like properties can persist in adult brains[72,87]. Additionally, the svp- or E93-mutant NSCs, which exhibit extended proliferation without tumorigenic traits, have been observed in adult brains[35,88]. These findings indirectly indicate that the adult Drosophila brain environment may support NSC survival under specific genetic perturbations.

Neurogenesis in adult Drosophila brain

While neuronal division is undetectable in the adult Drosophila brain under physiological conditions, injury-induced neurogenesis has been documented[85,89]. Advances in methodologies may further establish the adult Drosophila brain as a model for studying NSC regeneration and neurogenic plasticity.

CONCLUSION

The division patterns, temporal dynamics, and functional attributes of Drosophila NSCs are shaped by extrinsic environmental cues-encompassing niche signals, and external factors. These cues collectively safeguard NSCs from damage, deliver growth signals, and regulate division orientation and proliferation. For example, oxygen from the external environment, ecdysone from the systemic environment, nutrients from the neighboring glia cells and Hox protein from among the segmental factors collectively establish temporal regulation and facilitate NSC development during the larval stage[42]. Such precise coordination ensures the generation of specialized progeny and the maintenance of NSC identity, ultimately enabling the construction of refined neural circuitry.

However, several questions remain unanswered. How do these extrinsic ambient cues crosstalk? How do these extrinsic signals interact with intrinsic genetic factors? What are the responses of NSCs to the same extrinsic cue at different developmental stages? For instance, the Notch signaling pathway inhibits NSC birth at early stages[67] but promotes NSC proliferation at later stages[54]. Furthermore, how do other environmental inputs such as circadian rhythms, temperature, and microbial symbionts influence NSC behavior? Addressing these questions and validating these mechanisms functionally in vertebrate models (e.g., mouse/human NSCs) could inform therapeutic approaches directly, such as in mitigating irradiation-induced NSC damage or enhancing neurogenesis.

In summary, while the research in this field remains nascent, the insights reviewed here illuminate novel directions in further understanding NSC regulation and underscore the translational potential for clinical therapies targeting neural regeneration.

ACKNOWLEDGEMENTS

We acknowledge our collective global colleagues whose work could not be cited due to space constraints.