Trends in Genetics
ReviewPlaying Well with Others: Extrinsic Cues Regulate Neural Progenitor Temporal Identity to Generate Neuronal Diversity
Section snippets
Temporal Patterning Generates Neuronal Diversity
Neural diversity is essential for proper brain function including sensory perception, motor control, and consciousness. Neural diversity is generated by both spatial and temporal cues acting in combination on neural progenitors. Spatial cues assign progenitor regional identity, whereas temporal cues or temporal patterning (see Glossary) mechanisms allow single progenitors to make a sequence of different neurons and glia over time. Vertebrate neural progenitors have long been known to respond to
Temporal Patterning in Mammalian and Drosophila Neural Progenitors
In mammals, most neural progenitors throughout the central nervous system (CNS) (cortex, retina, and spinal cord) can generate multiple neuronal subtypes over time, followed by a later phase of gliogenesis 5, 7, 8, 9, 10, 11, 12. Unlike early findings in Drosophila, temporal patterning mechanisms characterized in mammals primarily involve extrinsic signals – either feedback cues from previously generated neurons or cues from unknown sources – although some evidence from in vitro culture
Glial-Derived Cues Regulate the Timing of Neuroblast Quiescence
All Drosophila neuroblasts undergo quiescence in the late embryo/early larvae, with the exception of five central brain neuroblasts (four mushroom body neuroblasts and one ventrolateral neuroblast) [33]. Entry and exit from quiescence occurs in a stereotyped sequence: embryonic neuroblast proliferation, neuroblast size reduction, neuroblast quiescence, neuroblast enlargement and proliferation in the young larva, and finally neuroblast size reduction and terminal differentiation in the early
Hormonal Cues Regulate Larval Neuroblast Temporal Identity
Embryonic neuroblasts use an intrinsic TTF cascade to generate neuronal diversity – this mechanism is ideally suited for rapid, invariant, short cell lineages of just 3–10 progenitor divisions [4]. In contrast, larval neuroblasts can divide >50 times over 120 h to generate hundreds of neurons and glia [47] – this likely requires a completely different temporal patterning mechanism, particularly to coordinate the timing of neuron production between different lineages, which might be important for
Hormonal Cues Regulate Neuronal Temporal Identity
Drosophila mushroom body neuroblasts display the longest phase of neurogenesis, beginning their lineage during embryogenesis and continuing until late pupal stages. They sequentially produce three types of neurons; γ, α′/β′, and α/β neurons, each with a unique projection pattern 61, 62. Early studies showed that the transcription factor Chinmo is detected in a high to low gradient in early-born to late-born neurons, and is required for specification of early-born γ and α′/β′ neuron identity [63]
Concluding Remarks
Interestingly, all known extrinsic cues regulating temporal transitions in proliferation or neuronal identity occur in larvae, and not embryos. The short length of embryogenesis, less than 1 day, may require neuroblast-intrinsic ‘hard-wired’ temporal patterning mechanisms to produce the correct number and type of neurons in a short time interval. In contrast, the relatively long length of larval stages (5 days) and complexity of developmental events (larval molts, initiation of metamorphosis)
Glossary
- Ecdysone signaling pathway
- the steroid hormone ecdysone, made in the prothoracic gland, binds the ecdysone receptor (A, B1, and B2 isoforms) which dimerizes with the common co-receptor Ultraspiracle to regulate gene expression in many or all embryonic, larval, and pupal tissues. Pulses of ecdysone trigger a diverse spectrum of developmental events and coordinate development between tissues.
- Temporal identity
- a cell fate that is specified by a temporal patterning mechanism. Temporal identity
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The Drivers of Diversity: Integrated genetic and hormonal cues regulate neural diversity
2023, Seminars in Cell and Developmental BiologyTemporal regulation of neural diversity in Drosophila and vertebrates
2023, Seminars in Cell and Developmental BiologyCitation Excerpt :However, changes in neural progenitor competence are also susceptible to cell-extrinsic cues. Several reviews [50–53] have extensively discussed the role of cell-extrinsic factors such as nutritional sensing, hormones, neurogenic niches and glia-derived signals in regulating neural progenitor fate and identity. We, therefore, briefly highlight a few classical studies and discuss only the recent findings.
Steroid hormones, dietary nutrients, and temporal progression of neurogenesis
2021, Current Opinion in Insect ScienceCitation Excerpt :Temporal transitions are a common theme in animal development and include larval molts of many invertebrate phyla, insect and amphibian metamorphosis, and the mammalian pre-to post-natal transition and puberty. Less overt but equally important are the temporal transitions of neural stem cells and progenitors that allow for the sequential generation of different neuron types and glia over time [1–5]. Many of the mechanisms that govern metamorphic and neural stem cell transitions are shared and, in both cases, the cellular and genetic programs governing these transitions are somehow coordinated with underlying animal physiology and attuned to cues received from the outside environment.
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