Helena Mira

Astrocyte diversity is currently expanding both between and within specific brain regions. Here, we assessed the spatial distribution and transcriptomic profile of two hippocampal astrocyte subpopulations, defined by combinatorial expression of the cell surface astrocyte markers ACSA‐1 or GLAST/SLC1A3, and ACSA‐2 or ATP1B2. Fluorescence activated cell sorting and genome‐wide transcriptomics by bulk RNAseq uncovered distinct transcriptional signatures of the two astrocyte subsets and highlighted heterogeneous responses during aging. The most abundant ATP1B2/GLAST double‐positive astrocytes corresponded to functionally specialized glial cells with increased protein glycosylation and stable gene expression patterns. Signatures related to mitochondrial respiration and cholesterol metabolism were induced during aging in ATP1B2 single‐positive astrocytes, while cell adhesion genes from the γ‐protocadherin cluster were repressed in double‐positive astrocytes. Heterochronic co‐culture assays with primary neurons show the loss of synaptogenic function of old ATP1B2/GLAST astrocytes. Our results complement previous studies demonstrating the presence of morphological and molecular astrocyte heterogeneity within the hippocampus, and uncover differences among astrocyte subsets in their transcriptomic response to aging.

Cellular senescence is an irreversible state linked to aging that involves molecular and functional alterations. The mammalian hippocampus, a key brain region for learning and memory, is highly vulnerable to damage in age‐related neurodegenerative diseases, yet the role of cellular senescence in hippocampal aging remains underexplored. Here, we report an early onset of senescence signatures in hippocampal astrocytes of the accelerated aging and frailty mouse model SAMP8. We examine how astrocyte senescence affects excitatory synapse formation, focusing on soluble signals released by astrocytes. Astrocytes isolated from SAMP8 brain and those differentiated from SAMP8 neural stem cells show senescence hallmarks (SA‐β‐gal, p16 ^INK4a , Lamin B1 loss), alongside a significant reduction in synaptogenic function. While astrocyte‐conditioned medium (ACM) from control mice promotes excitatory synaptogenesis through thrombospondin‐1/α2δ‐1 neuronal receptor signaling, ACM from senescent SAMP8 astrocytes lacks this capacity. Supplementing senescent ACM with thrombospondin‐1 protein or overexpressing thrombospondin‐1 gene in senescent astrocytes reinstates synaptogenesis. At the hippocampal level, thrombospondin‐1 and synaptic puncta are reduced in SAMP8 mice. Our findings reveal that senescent astrocytes exhibit reduced synaptogenic capacity due to thrombospondin‐1 loss, highlighting their contribution to synaptic dysfunction during aging. Preventing senescence in hippocampal astrocytes may thus restore astrocyte‐mediated synaptogenesis in the aged brain.

Cellular senescence is an irreversible state linked to aging that involves molecular and functional alterations. The mammalian hippocampus, a key brain region for learning and memory, is highly vulnerable to damage in age-related neurodegenerative diseases, yet the role of cellular senescence in hippocampal aging remains underexplored. Here, we report an early onset of senescence signatures in hippocampal astrocytes of the accelerated aging and frailty mouse model SAMP8. We examine how astrocyte senescence affects excitatory synapse formation, focusing on soluble signals released by astrocytes. Astrocytes isolated from SAMP8 brain and those differentiated from SAMP8 neural stem cells show senescence hallmarks (SA-β-gal, p16 ^INK4a , Lamin B1 loss), alongside a significant reduction in synaptogenic function. While astrocyte-conditioned medium (ACM) from control mice promotes excitatory synaptogenesis through thrombospondin-1 / α2δ-1 neuronal receptor signalling, ACM from senescent SAMP8 astrocytes lacks this capacity. Supplementing senescent ACM with thrombospondin-1 protein, or overexpressing thrombospondin-1 gene in senescent astrocytes, reinstates synaptogenesis. At the hippocampal level, thrombospondin-1 and synaptic puncta are reduced in SAMP8 mice. Our findings reveal that senescent astrocytes exhibit reduced synaptogenic capacity due to thrombospondin-1 loss, highlighting their contribution to synaptic dysfunction during aging. Preventing senescence in hippocampal astrocytes may thus restore astrocyte-mediated synaptogenesis in the aged brain.

Astrocyte diversity is currently expanding both between and within specific brain regions. Here, we assessed the spatial distribution and transcriptomic profile of two hippocampal astrocyte subpopulations, defined by combinatorial expression of the cell surface astrocyte markers ACSA-1 or GLAST/SLC1A3, and ACSA-2 or ATP1B2. Fluorescence activated cell sorting and genome-wide transcriptomics by bulk RNAseq uncovered distinct transcriptional signatures of the two astrocyte subsets and highlighted heterogeneous responses during aging. The most abundant ATP1B2/GLAST double-positive astrocytes corresponded to mature glial cells with increased protein glycosylation and stable gene expression patterns. Signatures related to mitochondrial respiration and cholesterol metabolism were induced during aging in ATP1B2 single-positive astrocytes, while cell adhesion genes from the γ-protocadherin cluster were repressed in double-positive astrocytes. Heterochronic co-culture assays with primary neurons show the loss of synaptogenic function of old ATP1B2/GLAST astrocytes. Our results complement previous studies demonstrating the presence of morphological and molecular astrocyte heterogeneity within the hippocampus, and uncover differences among astrocyte subsets in their transcriptomic response to aging.

Neurogenesis in the adult mammalian brain relies on the lifelong persistence of quiescent neural stem cell (NSC) reservoirs. Little is known about the mechanisms that lead to the initial establishment of quiescence, the main hallmark of adult stem cells, during development. Here we show that protein aggregates and autophagy machinery components accumulate in developmental radial glia-like NSCs as they enter quiescence and that pharmacological or genetic blockade of autophagy disrupts quiescence acquisition and maintenance. Conversely, increasing autophagy through AMPK/ULK1 activation instructs the acquisition of the quiescent state without affecting BMP signaling, a gatekeeper of NSC quiescence during adulthood. Selective ablation of Atg7, a critical gene for autophagosome formation, in radial glia-like NSCs at early and late postnatal stages compromises the initial acquisition and maintenance of quiescence during the formation of the hippocampal dentate gyrus NSC niche. Therefore, we demonstrate that autophagy is cell-intrinsically required to establish NSC quiescence during hippocampal development. Our results uncover an important role of autophagy in the transition of developmental NSCs into their dormant adult form, paving the way for studies directed at further understanding the mechanisms of stem cell niche formation and maintenance in the mammalian brain.

Neurogenesis in the adult mammalian brain relies on the lifelong persistence of quiescent neural stem cell (NSC) reservoirs. Little is known about the mechanisms that lead to the initial establishment of NSC quiescence during development. Here, we show that protein aggregates and autophagy machinery components accumulate in quiescent NSCs and that pharmacological blockade of autophagy disrupts quiescence. Conversely, increasing autophagy through AMPK/ULK1 activation instructs the acquisition of the quiescent state. Selective ablation of Atg7 , a critical gene for autophagosome formation, in hippocampal radial-glia like NSCs at early and late postnatal stages compromises the initial acquisition and maintenance of quiescence during the formation of the dentate gyrus SGZ niche. Therefore, we demonstrate that autophagy is cell-intrinsically required to establish radial glia-like NSC quiescence during hippocampal development. Our results uncover a fundamental role of autophagy in the transition of developmental NSCs into their dormant adult form, paving the way for studies directed at further understanding the mechanisms of stem cell niche formation and maintenance in the mammalian brain.

Stem cells in adult mammalian tissues are held in a reversible resting state, known as quiescence, for prolonged periods of time. Recent studies have greatly increased our understanding of the epigenetic and transcriptional landscapes that underlie stem cell quiescence. However, the transcription factor code that actively maintains the quiescence program remains poorly defined. Similarly, alternative splicing events affecting transcription factors in stem cell quiescence have been overlooked. Here we show that the transcription factor T-cell factor/lymphoid enhancer factor LEF1, a central player in canonical β-catenin-dependent Wnt signalling, undergoes alternative splicing and switches isoforms in quiescent neural stem cells. We found that active β-catenin and its partner LEF1 accumulated in quiescent hippocampal neural stem and progenitor cell (Q-NSPC) cultures. Accordingly, Q-NSPCs showed enhanced TCF/LEF1-driven transcription and a basal Wnt activity that conferred a functional advantage to the cultured cells in a Wnt-dependent assay. At a mechanistic level, we found a fine regulation of Lef1 gene expression. The coordinate upregulation of Lef1 transcription and retention of alternative spliced exon 6 (E6) led to the accumulation of a full-length protein isoform (LEF1-FL) that displayed increased stability in the quiescent state. Prospectively isolated GLAST + cells from the postnatal hippocampus also underwent E6 retention at the time quiescence is established in vivo. Interestingly, LEF1 motif was enriched in quiescence-associated enhancers of genes upregulated in Q-NSPCs and quiescence-related NFIX transcription factor motifs flanked the LEF1 binding sites. We further show that LEF1 interacts with NFIX and identify putative LEF1/NFIX targets. Together, our results uncover an unexpected role for LEF1 in gene regulation in quiescent NSPCs, and highlight alternative splicing as a post-transcriptional regulatory mechanism in the transition from stem cell activation to quiescence.

Corrigendum: Neurogenesis From Embryo to Adult – Lessons From Flies and Mice

Editorial: Generation of Neurons and Their Integration in Pre-existing Circuits in the Postnatal Brain: Signalling in Physiological and Regenerative Contexts