Fine-tuned rest: unveiling the regulatory landscape of adult quiescent neural stem cells

The Critical Balance Between Quiescence and Reactivation of Neural Stem Cells

Neural stem cells (NSC) are multipotent, self-renewing cells that give rise to all neural cell types within the central nervous system. During adulthood, most NSCs exist in a quiescent state which can be reactivated in response to metabolic and signalling changes, allowing for long-term continuous neurogenesis and response to injury. Ensuring a critical balance between quiescence and reactivation is required to maintain the limited NSC reservoir and neural replenishment throughout lifetime. The precise mechanisms and signalling pathways behind this balance are at the focus of current research. In this review, we highlight and discuss recent studies using Drosophila, mammalian and zebrafish models contributing to the understanding of molecular mechanisms underlying quiescence and reactivation of NSCs.

Sox9 and nuclear factor I transcription factors regulate the timing of neurogenesis and ependymal maturation in dopamine progenitors

Correct timing of neurogenesis is crucial for generating the correct number and subtypes of glia and neurons in the embryo, and for preventing tumours and stem cell depletion in the adults. Here, we analyse how the midbrain dopamine (mDA) neuron progenitors transition into cell cycle arrest (G0) and begin to mature into ependymal cells. Comparison of mDA progenitors from different embryonic stages revealed upregulation of the genes encoding Sox9 and nuclear factor I transcription factors during development. Their conditional inactivation in the early embryonic midbrain led to delayed G0 entry and ependymal maturation in the entire midbrain ventricular zone, reduced gliogenesis and increased generation of neurons, including mDA neurons. In contrast, their inactivation in late embryogenesis did not result in mitotic re-entry, suggesting that these factors are necessary for G0 induction, but not for its maintenance. Our characterisation of adult ependymal cells by single-cell RNA sequencing and histology show that mDA-progenitor-derived cells retain several progenitor features but also secrete neuropeptides and contact neighbouring cells and blood vessels, indicating that these cells may form part of the circumventricular organ system.

Lateral Ventricular Neural Stem Cells Provide Negative Feedback to Circuit Activation Through GABAergic Signaling

Recent studies have demonstrated that circuit activation in vivo can regulate proliferation of lateral ventricular neural stem cells (LV NSCs), although the underlying molecular and cellular mechanisms are not yet fully understood. Here, we investigated the role of GABAergic signaling in the interaction between LV NSCs and the anterior cingulate cortex-subependymal-choline acetyltransferase+ (ChAT+) neuron (ACC-subep-ChAT+) circuit. We found that monoamine oxidase B (MAOB), a key enzyme involved in gamma-aminobutyric acid (GABA) synthesis, is expressed in LV NSCs, and that activation of the ACC-subep-ChAT+ circuit can modulate MAOB activity. Additionally, LV NSCs express LRRC8D, a core component of volume-regulated anion channels, and GABA transporter-1 (GAT-1, SLC6A1). We show evidence that, through GABA signaling, LRRC8D and GAT-1 can provide a negative feedback signal to ChAT+ neurons, a key component of the ACC-subep-ChAT+ circuit that regulate proliferation of LV NSCs. These findings suggest that MAOB-driven GABA synthesis, LRRC8D-regulated chloride and GABA transport, and GAT-1-facilitated GABA reuptake can regulate neural circuit activation and influence NSC proliferation dynamics in the LV.

Flow Cytometry-Based Protocols for the Mouse Subependymal Neurogenic Niche Phenotyping

The cellular compartment of the adult rodent subependymal zone (SEZ)-neurogenic niche is the most active regenerative area of the brain and of great interest to the regenerative medicine field. It is complex and highly heterogeneous, including neural stem cells (NSCs) in different states of activation, rapid-amplifying progenitors, immature neuroblasts (NBs), mature neurons and other non-neurogenic populations. This chapter provides a step-by-step overview of a versatile flow cytometry-based protocol, which has been molecularly and functionally validated to classify and isolate the complete neurogenic lineage, including three NSC fractions (quiescent, primed, and activated), without the need for reporter mice. The panel is adaptable to diverse fluorescence needs and different cell targets, including niche differentiated cells such as endothelial cells, oligodendrocytes, or microglia, enabling the identification and isolation of the vast majority of cell types present in the SEZ. Additionally, it allows the study of cell cycling dynamics by means of 5-ethynyl-20-deoxyuridine (EdU) incorporation. The method enables the isolation of the different SEZ fractions and the functional assay of their cycling heterogeneity, including quiescence-activation transitions of NSC.

Development of a Bmi1+ Cardiac Mouse Progenitor Immortalized Model to Unravel the Relationship with Its Protective Vascular Endothelial Niche

The adult mammalian heart has been demonstrated to be endowed with low but real turnover capacity, especially for cardiomyocytes, the key functional cell type. The source, however, of that turnover capacity remains controversial. In this regard, we have defined and characterized a resident multipotent cardiac mouse progenitor population, Bmi1+DR (for Bmi1+ Damage-Responsive cells). Bmi1+DR is one of the cell types with the lowest ROS (Reactive Oxygen Species) levels in the adult heart, being particularly characterized by their close relationship with cardiac vessels, most probably involved in the regulation of proliferation/maintenance of Bmi1+DR. This was proposed to work as their endothelial niche. Due to the scarcity of Bmi1+DR cells in the adult mouse heart, we have generated an immortalization/dis-immortalization model using Simian Vacuolating Virus 40-Large Antigen T (SV40-T) to facilitate their in vitro characterization. We have obtained a heterogeneous population of immortalized Bmi1+DR cells (Bmi1+DRIMM) that was validated attending to different criteria, also showing a comparable sensitivity to strong oxidative damage. Then, we concluded that the Bmi1-DRIMM population is an appropriate model for primary Bmi1+DR in vitro studies. The co-culture of Bmi1+DRIMM cells with endothelial cells protects them against oxidative damage, showing a moderate depletion in non-canonical autophagy and also contributing with a modest metabolic regulation.

Epigenetic maintenance of adult neural stem cell quiescence in the mouse hippocampus via Setd1a

Quiescence, a hallmark of adult neural stem cells (NSCs), is required for maintaining the NSC pool to support life-long continuous neurogenesis in the adult dentate gyrus (DG). Whether long-lasting epigenetic modifications maintain NSC quiescence over the long term in the adult DG is not well-understood. Here we show that mice with haploinsufficiency of Setd1a, a schizophrenia risk gene encoding a histone H3K4 methyltransferase, develop an enlarged DG with more dentate granule cells after young adulthood. Deletion of Setd1a specifically in quiescent NSCs in the adult DG promotes their activation and neurogenesis, which is countered by inhibition of the histone demethylase LSD1. Mechanistically, RNA-sequencing and CUT & RUN analyses of cultured quiescent adult NSCs reveal Setd1a deletion-induced transcriptional changes and many Setd1a targets, among which down-regulation of Bhlhe40 promotes quiescent NSC activation in the adult DG in vivo. Together, our study reveals a Setd1a-dependent epigenetic mechanism that sustains NSC quiescence in the adult DG.