Reprogramming of neural circuits in the hippocampus of the adult brain induced by a single transcriptional regulator Id2 in PNAS

Responsible EditorThe Enzymatic Neural Circuit is the basis of our perception of the world, thinking and behavior
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During development, neurons are connected to each other under the control of complex molecular programs, and gradually form a complex and precise network
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In the adult central nervous system, the regeneration capacity of neural circuits is extremely limited due to the lack of molecular basis during development
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Therefore, damage to mature neural circuits due to injury or disease is irreversible
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Studies on developing and damaged neurons have revealed many molecules that contribute to the formation and regeneration of adult brain circuits [1, 2], but can successfully induce the growth of mature neuron axons and synapse formation in the adult brain It is still an unsolved mystery
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On October 5, 2021, the Csaba Földy team from the Institute of Brain Science of the University of Zurich in Switzerland published an online research paper entitled Recurrent rewiring of the adult hippocampal mossy fiber system by a single transcriptional regulator, Id2 in PNAS.
The granulosa cells in the hippocampus of healthy mice and rats express the transcriptional regulator Id2, which successfully drives specific molecular programs, induces the growth of mature neuron axons, and establishes synaptic connections with target neurons.
On the basis of this, a new functional neural circuit is established
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This study reveals the molecular mechanism of adult neural circuit reorganization, and brings new ideas for brain repair and neurological disease treatment after injury
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In order to explore the mechanism of reorganization of mature neural circuits in the adult brain, the author studied mossy fiber sprouting in the hippocampus
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In the brain of a healthy person, the axons (mossy fiber) of granular cells in the hippocampus dentate gyrus extend to the hippocampal hilus area and CA3 area, forming synaptic connections with different types of cells there
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In the brains of patients with temporal lobe epilepsy, the axons of granular cells will rewire.
They form synaptic connections with other granular cells in the dentate gyrus to establish abnormal excitatory circuits.
This pathological phenomenon is called mossy fiber sprouting (mossy fiber sprouting).
fiber sprouting) [3]
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The sprouting of mossy fibers in the epileptic brain involves all the key stages of adult neural circuit reprogramming, including axon growth, targeted neuron projection and synapse formation.
Therefore, it becomes an excellent model for understanding the reprogramming mechanism of adult brain neural circuits
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The author used a drug (kainic acid) in the hippocampus of adult mice to induce sprouting of mossy fibers
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Three days after drug injection, the sprouting of mossy fibers can be clearly observed through histological staining, and it becomes denser after 14 days.
At this time, the abnormal excitatory circuit in the hippocampal dentate gyrus has been fully established (Figure 1)
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Using single-cell RNA sequencing, the authors analyzed the transcriptome changes in mature granulosa cells 1 day after drug injection (representing the induced acute cellular response) and 14 days later
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One day after drug induction, the transcriptome detected the up-regulation of transcription factors or regulatory factors related to axon growth
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Among the up-regulated factors include Id2, a transcription factor inhibitor
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Using immunostaining, the authors observed that Id2 was expressed in the nucleus of a few granulosa cells after 1 day of chemical induction, and was enriched in most granulosa cells after 3 days (Figure 1A).
This high expression continued until 14 days after induction.
(Figure 1B)
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Figure 1.
Id2 is highly expressed in the dentate gyrus of the hippocampus of a drug-induced mossy fiber sprouting model.
Next, the authors used adenovirus and transgenic mice to specifically express Id2 in granulosa cells
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Histological analysis showed that 1-3 months after gene induction, mossy fiber sprouting gradually formed and became dense with time (Figure 2C-E)
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Using the same method, the author successfully induced mossy fiber sprouting on the ventral and dorsal hippocampus of mice and the hippocampus of healthy adult rats
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Figure 2.
Id2 expression induces sprouting of mossy fibers.
Next, the author used ZnT3 immunostaining to mark the axon synaptic apex of granulosa cells and observed it with an electron microscope
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The labeled axon ends are filled with synaptic vesicles and form synaptic connections with the dendritic shafts and dendritic spines of granular cells
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Later, optogenetic techniques were used to verify whether this synaptic connection is functional
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The author used adenovirus to express Id2 in some granular cells to induce mossy fiber sprouting and channel-rhodopsin (ChR) to control neural activity
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Three months later, the author performed a patch clamp recording on the granular cells that did not express ChR
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Compared with the control group, the authors detected larger and more frequent excitatory postsynaptic currents (Figure 3C-D) caused by blue light stimulation (ie activation of ChR-expressing cells) in brain slices expressing Id2
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The above results indicate that a single transcription regulator Id2 can induce the axon growth of mature neurons and induce the formation of new functional neural circuits
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In order to understand the molecular mechanism of the formation of adult neural circuits, they analyzed the changes in the transcriptome of granulosa cells induced by the Id2 gene
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Id2 inhibits their transcriptional activity by directly binding transcription factors and inhibiting their binding to DNA
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Therefore, the increase in Id2 expression may lead to the up-regulation and down-regulation of gene expression, and the changes depend on the transcription factors suppressed by Id2
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Single-cell RNA sequencing of granulosa cells one month after Id2 induction showed that the changes in the transcriptome were mainly related to the members of the wiring-related JAK-STAT, Wnt, cAMP and Slit/Robo signaling pathways
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In addition, since Id2 binds to transcription factors without changing their expression, transcriptome changes only represent downstream effects and do not indicate which transcription factors Id2 directly acts on
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Therefore, the authors conducted a transcription factor-target enrichment analysis with the purpose of identifying transcription factors that are directly or indirectly inhibited or indirectly de-suppressed by Id2
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Using Enrichr, the author found 26 such transcription factors (Figure 4D)
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The above results reveal the transcriptome model of Id2 inducing mossy fiber sprouting
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So far, the author has replicated mossy fiber sprouting in healthy adult brains through gene induction, providing an excellent model for studying whether it is the cause of epilepsy
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Because there are other pathological changes in the brain of patients with epilepsy (for example, apoptosis, granular cell layer dispersion, abnormal changes in the dendritic structure of granular cells and changes in cell excitability), because it is impossible to distinguish one disease from other diseases Separation and independent research have made the causal relationship between them and epilepsy still controversial [3]
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The authors analyzed the brain dynamics of mice with mossy fiber sprouting by recording multi-channel silicon probes in the hippocampus of mice
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In the local field potential range of 1 to 400 Hz, no pathological oscillations or epileptiform activities were recorded
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This result shows that mossy fiber sprouting is not the cause of epilepsy
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Finally, the author evaluated the processing of objects and space-related information in mice with mossy fiber sprouting through a series of animal behavior tests
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Mice with mossy fiber sprouting have the same normal learning ability and memory as the control group, and can solve spatial problems well, but they have different problem-solving strategies.
They seem to rely more on local rather than global spatial cues
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In summary, repairing damaged neural circuits in the adult brain has always been the focus of scientists.
Most studies usually look for key factors that promote the formation of adult brain neural circuits in the context of pathology or injury
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In this study, a single gene induced axon growth and circuit formation in healthy and mature neurons without development and damage signals
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In the future, the analysis of the role of related molecules in this process will help us to further understand the molecular mechanism of adult brain circuit reorganization and realize the directed reorganization of adult brain circuit engineering
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Original link: https:// Plate maker: Eleven References 1.
DL Moore, JL Goldberg (2011).
Multiple transcription factor families regulate axon growth and regeneration.
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M.
Mahar, V.
Cavalli (2018).
Intrinsic mechanisms of neuronal axon regeneration.
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19, 323–337 .
3.
PS Buckmaster (2014), Does mossy fiber sprouting give rise to the epileptic state? Adv.
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813, 161–168
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