CNS synaptogenesis promoted by glia-derived cholesterol

The molecular mechanisms controlling synaptogenesis in the central nervous system (CNS) are poorly understood. Previous reports showed that a glia-derived factor strongly promotes synapse development in cultures of purified CNS neurons. Here, we identify this factor as cholesterol complexed to apolipoprotein E-containing lipoproteins. CNS neurons produce enough cholesterol to survive and grow, but the formation of numerous mature synapses demands additional amounts that must be provided by glia. Thus, the availability of cholesterol appears to limit synapse development. This may explain the delayed onset of CNS synaptogenesis after glia differentiation and neurobehavioral manifestations of defects in cholesterol or lipoprotein homeostasis.

A pericyte origin of spinal cord scar tissue

There is limited regeneration of lost tissue after central nervous system injury, and the lesion is sealed with a scar. The role of the scar, which often is referred to as the glial scar because of its abundance of astrocytes, is complex and has been discussed for more than a century. Here we show that a specific pericyte subtype gives rise to scar-forming stromal cells, which outnumber astrocytes, in the injured spinal cord. Blocking the generation of progeny by this pericyte subtype results in failure to seal the injured tissue. The formation of connective tissue is common to many injuries and pathologies, and here we demonstrate a cellular origin of fibrosis.

Origin of new glial cells in intact and injured adult spinal cord

Several distinct cell types in the adult central nervous system have been suggested to act as stem or progenitor cells generating new cells under physiological or pathological conditions. We have assessed the origin of new cells in the adult mouse spinal cord by genetic fate mapping. Oligodendrocyte progenitors self-renew, give rise to new mature oligodendrocytes, and constitute the dominating proliferating cell population in the intact adult spinal cord. In contrast, astrocytes and ependymal cells, which are restricted to limited self-duplication in the intact spinal cord, generate the largest number of cells after spinal cord injury. Only ependymal cells generate progeny of multiple fates, and neural stem cell activity in the intact and injured adult spinal cord is confined to this cell population. We provide an integrated view of how several distinct cell types contribute in complementary ways to cell maintenance and the reaction to injury.

Forebrain ependymal cells are Notch-dependent and generate neuroblasts and astrocytes after stroke

Neurons are continuously generated from stem cells in discrete regions in the adult mammalian brain. We found that ependymal cells lining the lateral ventricles were quiescent and did not contribute to adult neurogenesis under normal conditions in mice but instead gave rise to neuroblasts and astrocytes in response to stroke. Ependymal cell quiescence was actively maintained by canonical Notch signaling. Inhibition of this pathway in uninjured animals allowed ependymal cells to enter the cell cycle and produce olfactory bulb neurons, whereas forced Notch signaling was sufficient to block the ependymal cell response to stroke. Ependymal cells were depleted by stroke and failed to self-renew sufficiently to maintain their own population. Thus, although ependymal cells act as primary cells in the neural lineage to produce neurons and glial cells after stroke, they do not fulfill defining criteria for stem cells under these conditions and instead serve as a reservoir that is recruited by injury.

Reducing Pericyte-Derived Scarring Promotes Recovery after Spinal Cord Injury

CNS injury often severs axons. Scar tissue that forms locally at the lesion site is thought to block axonal regeneration, resulting in permanent functional deficits. We report that inhibiting the generation of progeny by a subclass of pericytes led to decreased fibrosis and extracellular matrix deposition after spinal cord injury in mice. Regeneration of raphespinal and corticospinal tract axons was enhanced and sensorimotor function recovery improved following spinal cord injury in animals with attenuated pericyte-derived scarring. Using optogenetic stimulation, we demonstrate that regenerated corticospinal tract axons integrated into the local spinal cord circuitry below the lesion site. The number of regenerated axons correlated with improved sensorimotor function recovery. In conclusion, attenuation of pericyte-derived fibrosis represents a promising therapeutic approach to facilitate recovery following CNS injury.

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Inhibition of pericyte proliferation reduces fibrotic scar tissue following injury

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Attenuated pericyte-derived scarring facilitates motor axon regeneration

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Regenerated axons functionally re-integrate into the local spinal circuitry

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Attenuated pericyte-derived scarring improves sensorimotor recovery

Pericyte-derived fibrotic scarring is conserved across diverse central nervous system lesions

Fibrotic scar tissue limits central nervous system regeneration in adult mammals. The extent of fibrotic tissue generation and distribution of stromal cells across different lesions in the brain and spinal cord has not been systematically investigated in mice and humans. Furthermore, it is unknown whether scar-forming stromal cells have the same origin throughout the central nervous system and in different types of lesions. In the current study, we compared fibrotic scarring in human pathological tissue and corresponding mouse models of penetrating and non-penetrating spinal cord injury, traumatic brain injury, ischemic stroke, multiple sclerosis and glioblastoma. We show that the extent and distribution of stromal cells are specific to the type of lesion and, in most cases, similar between mice and humans. Employing in vivo lineage tracing, we report that in all mouse models that develop fibrotic tissue, the primary source of scar-forming fibroblasts is a discrete subset of perivascular cells, termed type A pericytes. Perivascular cells with a type A pericyte marker profile also exist in the human brain and spinal cord. We uncover type A pericyte-derived fibrosis as a conserved mechanism that may be explored as a therapeutic target to improve recovery after central nervous system lesions.

Transgenic mice for conditional gene manipulation in astroglial cells

Astrocytes are thought to exert diverse functions in the brain, but it has been difficult to prove this in vivo because of a scarcity of tools to manipulate these cells. Here, we report the generation of new transgenic mouse lines that allow for conditional gene ablation in astrocytes using the tamoxifen- (TAM-) inducible CreER(T2)/loxP system and bacterial artificial chromosome (BAC)-based transgenesis. In adult transgenic mice, where CreER(T2) expression is driven by the promoter of the sodium-dependent glutamate/aspartate transporter (Glast/Slc1a3) or of connexin 30 (Cx30/Gjb6), intraperitoneal TAM-injection induced Cre-mediated recombination in astroglial cells throughout the brain. Targeting efficacies varied in a region-specific manner from 20 to 90% as indicated by enzyme-based reporter lines and immunohistochemical staining. In addition, the Glast-line allowed to target retinal Müller cells and adult neural stem/progenitor cells in neurogenic regions of the adult brain. Transgenic mice expressing CreER(T2) under the control of the apolipoprotein e (ApoE) or aquaporin 4 (Aqp4) promoter showed inducible recombination in different areas of the central nervous system (CNS) albeit at low levels. Transgenic lines showed TAM-induced recombination in specific peripheral organs. These new mouse lines should help to further explore the relevance of astrocytes for brain function, as well as their contribution to pathological conditions because of aging, disease or injury.

Fibrotic scarring following lesions to the central nervous system

Following lesions to the central nervous system, scar tissue forms at the lesion site. Injury often severs axons and scar tissue is thought to block axonal regeneration, resulting in permanent functional deficits. While scar-forming astrocytes have been extensively studied, much less attention has been given to the fibrotic, non-glial component of the scar. We here review recent progress in understanding fibrotic scar formation following different lesions to the brain and spinal cord. We specifically highlight recent evidence for pericyte-derived fibrotic scar tissue formation, discussing the origin, recruitment, function and therapeutic relevance of fibrotic scarring.

Role of glia-derived cholesterol in synaptogenesis: new revelations in the synapse-glia affair

Brain development and function relies on the exchange of signals between neurons and glial cells. Here we review a series of recent studies on cultures of purified retinal ganglion cells (RGCs) that point to a new role of glial cells in the formation and plasticity of synaptic connections. The results suggest that neurons must import glia-derived cholesterol via lipoproteins to form numerous and efficient synaptic connections. This finding may explain why throughout the central nervous system (CNS) the main phase of synaptogenesis starts synchronously after glia differentiation and why astrocytes produce apolipoprotein E (apoE) and cholesterol-containing lipoproteins. Experimental tests of these hypotheses may further our understanding of the cholesterol metabolism in the brain and may help to explain neurologic symptoms resulting from defective cholesterol and lipoprotein metabolism.

EphB signaling controls lineage plasticity of adult neural stem cell niche cells

Stem cells remain in specialized niches over the lifespan of the organism in many organs to ensure tissue homeostasis and enable regeneration. How the niche is maintained is not understood, but is probably as important as intrinsic stem cell self-renewal capacity for tissue integrity. We here demonstrate a high degree of phenotypic plasticity of the two main niche cell types, ependymal cells and astrocytes, in the neurogenic lateral ventricle walls in the adult mouse brain. In response to a lesion, astrocytes give rise to ependymal cells and ependymal cells give rise to niche astrocytes. We identify EphB2 forward signaling as a key pathway regulating niche cell plasticity. EphB2 acts downstream of Notch and is required for the maintenance of ependymal cell characteristics, thereby inhibiting the transition from ependymal cell to astrocyte. Our results show that niche cell identity is actively maintained and that niche cells retain a high level of plasticity.

In cultured oligodendrocytes the A/B-type hnRNP CBF-A accompanies MBP mRNA bound to mRNA trafficking sequences

Heterogeneous ribonucleoproteins (hnRNPs) have key roles in RNA biogenesis, including pre-mRNP assembly, transport and cytoplasmic localization. Here we show by biochemical fractionation of nuclear extracts and protein-protein interaction assays that the A/B-type hnRNP CBF-A is in a multiprotein complex with hnRNP A2 and A3 and hnRNP U. Using RNA affinity chromatography and gel retardation assays, CBF-A was found to bind directly to RNA trafficking sequences in the 3'-UTR of the myelin basic protein (MBP) mRNA. In primary oligodendrocytes, astrocytes, neurons, and mouse forebrain sections, CBF-A revealed a characteristic granular cytoplasmic distribution. In mouse forebrain CBF-A-positive granules were preferentially found in regions with loosely bundled myelin fibers. In cultured oligodendrocytes, CBF-A was found to be specifically associated with endogenous MBP mRNA and CBF-A gene silencing resulted in the retention of MBP granules in the cell body. Finally, immunoelectron microscopy in differentiating oligodendrocytes showed that CBF-A is located in cytoplasmic granules that are often associated with the cytoskeleton. The results suggest that CBF-A is a novel transacting factor required for cytoplasmic mRNA transport and localization.

In neurons, activity-dependent association of dendritically transported mRNA transcripts with the transacting factor CBF-A is mediated by A2RE/RTS elements

We report that the transacting factor CArG Box binding Factor A (CBF-A) binds the RNA trafficking sequences found in activity-regulated cytoskeleton-associated protein, calmodulin-dependent protein kinase II, and brain-derived neurotrophic factor mRNAs in an activity-dependent manner and accompanies the transcripts from gene to dendrites. CBF-A gene silencing impaired dendritic mRNA localization. We propose that CBF-A is important for trafficking of RNA trafficking sequence–containing neuronal mRNAs.

In neurons certain mRNA transcripts are transported to synapses through mechanisms that are not fully understood. Here we report that the heterogeneous nuclear ribonucleoprotein CBF-A (CArG Box binding Factor A) facilitates dendritic transport and localization of activity-regulated cytoskeleton-associated protein (Arc), brain-derived neurotrophic factor (BDNF), and calmodulin-dependent protein kinase II (CaMKIIα) mRNAs. We discovered that, in the adult mouse brain, CBF-A has a broad distribution. In the nucleus, CBF-A was found at active transcription sites and interchromosomal spaces and close to nuclear pores. In the cytoplasm, CBF-A localized to dendrites as well as pre- and postsynaptic sites. CBF-A was found in synaptosomal fractions, associated with Arc, BDNF, and CaMKIIα mRNAs. Electrophoretic mobility shift assays demonstrated a direct interaction mediated via their hnRNP A2 response element (A2RE)/RNA trafficking sequence (RTS) elements located in the 3′ untranslated regions. In situ hybridization and microscopy on live hippocampal neurons showed that CBF-A is in dynamic granules containing Arc, BDNF, and CaMKIIα mRNAs. N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) postsynaptic receptor stimulation led to CBF-A accumulation in dendrites; increased Arc, BDNF, and CaMKIIα mRNA levels; and increased amounts of transcripts coprecipitating with CBF-A. Finally, CBF-A gene knockdown led to decreased mRNA levels. We propose that CBF-A cotranscriptionally binds RTSs in Arc, BDNF, and CaMKIIα mRNAs and follows the transcripts from genes to dendrites, promoting activity-dependent nuclear sorting of transport-competent mRNAs.

Glia-induced neuronal differentiation by transcriptional regulation

There is increasing evidence that different phases of brain development depend on neuron-glia interactions including postnatal key events like synaptogenesis. To address how glial cells influence synapse development, we analyzed whether and how glia-derived factors affect gene expression in primary cultures of immunoisolated rat retinal ganglion cells (RGCs) by oligonucleotide microarrays. Our results show that the transcript pattern matched the developmental stage and characteristic properties of RGCs in vitro. Glia-conditioned medium (GCM) and cholesterol up- and downregulated a limited number of genes that influence the development of dendrites and synapses and regulate cholesterol and fatty acid metabolism. The oligonucleotide microarrays detected the transcriptional regulation of neuronal cholesterol homeostasis in response to GCM and cholesterol treatment. Surprisingly, our study revealed neuronal expression and glial regulation of matrix gla protein (Mgp). Together, our results suggest that glial cells promote different aspects of neuronal differentiation by regulating transcription of distinct classes of genes.

Glutamate transporter Slc1a3 mediates inter-niche stem cell activation during skin growth

Tissues contain distinct stem cell niches, but whether cell turnover is coordinated between niches during growth is unknown. Here, we report that in mouse skin, hair growth is accompanied by sebaceous gland and interfollicular epidermis expansion. During hair growth, cells in the bulge and outer root sheath temporarily upregulate the glutamate transporter SLC1A3, and the number of SLC1A3⁺ basal cells in interfollicular epidermis and sebaceous gland increases. Fate mapping of SLC1A3⁺ cells in mice revealed transient expression in proliferating stem/progenitor cells in all three niches. Deletion of slc1a3 delays hair follicle anagen entry, uncouples interfollicular epidermis and sebaceous gland expansion from the hair cycle, and leads to reduced fur density in aged mice, indicating a role of SLC1A3 in stem/progenitor cell activation. Modulation of metabotropic glutamate receptor 5 activity mimics the effects of SLC1A3 deletion or inhibition. These data reveal that stem/progenitor cell activation is synchronized over distinct niches during growth and identify SLC1A3 as a general marker and effector of activated epithelial stem/progenitor cells throughout the skin.

Identification of a discrete subpopulation of spinal cord ependymal cells with neural stem cell properties

Spinal cord ependymal cells display neural stem cell properties in vitro and generate scar-forming astrocytes and remyelinating oligodendrocytes after injury. We report that ependymal cells are functionally heterogeneous and identify a small subpopulation (8% of ependymal cells and 0.1% of all cells in a spinal cord segment), which we denote ependymal A (EpA) cells, that accounts for the in vitro stem cell potential in the adult spinal cord. After spinal cord injury, EpA cells undergo self-renewing cell division as they give rise to differentiated progeny. Single-cell transcriptome analysis revealed a loss of ependymal cell gene expression programs as EpA cells gained signaling entropy and dedifferentiated to a stem-cell-like transcriptional state after an injury. We conclude that EpA cells are highly differentiated cells that can revert to a stem cell state and constitute a therapeutic target for spinal cord repair.

Circadian vulnerability of cisplatin-induced ototoxicity in the cochlea

The chemotherapeutic agent cisplatin is renowned for its ototoxic effects. While hair cells in the cochlea are established targets of cisplatin, less is known regarding the afferent synapse, which is an essential component in the faithful temporal transmission of sound. The glutamate aspartate transporter (GLAST) shields the auditory synapse from excessive glutamate release, and its loss of function increases the vulnerability to noise, salicylate, and aminoglycosides. Until now, the involvement of GLAST in cisplatin-mediated ototoxicity remains unknown. Here, we test in mice lacking GLAST the effects of a low-dose cisplatin known not to cause any detectable change in hearing thresholds. When administered at nighttime, a mild hearing loss in GLAST KO mice was found but not at daytime, revealing a potential circadian regulation of the vulnerability to cisplatin-mediated ototoxicity. We show that the auditory synapse of GLAST KO mice is more vulnerable to cisplatin administration during the active phase (nighttime) when compared to WT mice and treatment during the inactive phase (daytime). This effect was not related to the abundance of platinum compounds in the cochlea, rather cisplatin had a dose-dependent impact on cochlear clock rhythms only after treatment at nighttime suggesting that cisplatin can modulate the molecular clock. Our findings suggest that the current protocols of cisplatin administration in humans during daytime may cause a yet undetectable damage to the auditory synapse, more so in already damaged ears, and severely impact auditory sensitivity in cancer survivors.

Distinct origin and region-dependent contribution of stromal fibroblasts to fibrosis following traumatic injury in mice

Fibrotic scar tissue formation occurs in humans and mice. The fibrotic scar impairs tissue regeneration and functional recovery. However, the origin of scar-forming fibroblasts is unclear. Here, we show that stromal fibroblasts forming the fibrotic scar derive from two populations of perivascular cells after spinal cord injury (SCI) in adult mice of both sexes. We anatomically and transcriptionally identify the two cell populations as pericytes and perivascular fibroblasts. Fibroblasts and pericytes are enriched in the white and gray matter regions of the spinal cord, respectively. Both cell populations are recruited in response to SCI and inflammation. However, their contribution to fibrotic scar tissue depends on the location of the lesion. Upon injury, pericytes and perivascular fibroblasts become activated and transcriptionally converge on the generation of stromal myofibroblasts. Our results show that pericytes and perivascular fibroblasts contribute to the fibrotic scar in a region-dependent manner.

Decoding fibrosis in the human central nervous system

Recent advancements in human tissue analyses and animal models have revealed that fibrotic scarring is a common response to various lesions in the central nervous system (CNS). Perivascular cells within the brain or spinal cord give rise to stromal fibroblasts that form fibrotic scar tissue. In this review, we summarize the current understanding of fibrotic scar formation in different CNS lesions and evaluate published human single-cell gene expression datasets to gather information on perivascular cells. Specifically, we highlight the classification of pericytes and fibroblast subtypes and compare the marker expression of perivascular cells across different datasets.

CD163+ macrophages monitor enhanced permeability at the blood-dorsal root ganglion barrier

In dorsal root ganglia (DRG), macrophages reside close to sensory neurons and have largely been explored in the context of pain, nerve injury, and repair. However, we discovered that most DRG macrophages interact with and monitor the vasculature by sampling macromolecules from the blood. Characterization of the DRG vasculature revealed a specialized endothelial bed that transformed in molecular, structural, and permeability properties along the arteriovenous axis and was covered by macrophage-interacting pericytes and fibroblasts. Macrophage phagocytosis spatially aligned with peak endothelial permeability, a process regulated by enhanced caveolar transcytosis in endothelial cells. Profiling the DRG immune landscape revealed two subsets of perivascular macrophages with distinct transcriptome, turnover, and function. CD163+ macrophages self-maintained locally, specifically participated in vasculature monitoring, displayed distinct responses during peripheral inflammation, and were conserved in mouse and man. Our work provides a molecular explanation for the permeability of the blood-DRG barrier and identifies an unappreciated role of macrophages as integral components of the DRG-neurovascular unit.

Macrophages upregulate mural cell-like markers and support healing of ischemic injury by adopting functions important for vascular support

Sterile inflammation after injury is important for tissue restoration. In injured human and mouse tissues, macrophages were recently found to accumulate perivascularly. This study investigates if macrophages adopt a mural cell phenotype important for restoration after ischemic injury. Single-cell RNA sequencing of fate-mapped macrophages from ischemic mouse muscles demonstrates a macrophage-toward-mural cell switch of a subpopulation of macrophages with downregulated myeloid cell genes and upregulated mural cell genes, including PDGFRβ. This observation was further strengthened when including unspliced transcripts in the analysis. The macrophage switch was proven functionally relevant, as induction of macrophage-specific PDGFRβ deficiency prevented their perivascular macrophage phenotype, impaired vessel maturation and increased vessel leakiness, which ultimately reduced limb function. In conclusion, macrophages in adult ischemic tissue were demonstrated to undergo a cellular program to morphologically, transcriptomically and functionally resemble mural cells while weakening their macrophage identity. The macrophage-to-mural cell-like phenotypic switch is crucial for restoring tissue function and warrants further exploration as a potential target for immunotherapies to enhance healing.

Multimodal analyses of immune cells during bone repair identify macrophages as a therapeutic target in musculoskeletal trauma

Musculoskeletal traumatic injuries (MTI) involve soft tissue lesions adjacent to a bone fracture leading to fibrous nonunion. The impact of MTI on the inflammatory response to fracture and on the immunomodulation of skeletal stem/progenitor cells (SSPCs) remains unknown. Here, we used single cell transcriptomic analyses to describe the immune cell dynamics after bone fracture and identified distinct macrophage subsets with successive pro-inflammatory, pro-repair and anti-inflammatory profiles. Concurrently, SSPCs transition via a pro- and anti-inflammatory fibrogenic phase of differentiation prior to osteochondrogenic differentiation. In a preclinical MTI mouse model, the injury response of immune cells and SSPCs is disrupted leading to a prolonged pro-inflammatory phase and delayed resolution of inflammation. Macrophage depletion improves bone regeneration in MTI demonstrating macrophage involvement in fibrous nonunion. Finally, pharmacological inhibition of macrophages using the CSF1R inhibitor Pexidartinib ameliorates healing. These findings reveal the coordinated immune response of macrophages and skeletal stem/progenitor cells as driver of bone healing and as a primary target for the treatment of trauma-associated fibrosis.

Summary

Hachemi et al. report the immune cell atlas of bone repair revealing macrophages as pro-fibrotic regulators and a therapeutic target for musculoskeletal regeneration. Genetic depletion or pharmacological inhibition of macrophages improves bone healing in musculoskeletal trauma.

Regionalized cell and gene signatures govern esophageal epithelial homeostasis

Regionalized disease prevalence is a common feature of the gastrointestinal tract. Herein, we employed regionally resolved Smart-seq3 single-cell sequencing, generating a comprehensive cell atlas of the adult mouse esophagus. Characterizing the esophageal axis, we identify non-uniform distribution of epithelial basal cells, fibroblasts, and immune cells. In addition, we demonstrate a position-dependent, but cell subpopulation-independent, transcriptional signature, collectively generating a regionalized esophageal landscape. Combining in vivo models with organoid co-cultures, we demonstrate that proximal and distal basal progenitor cell states are functionally distinct. We find that proximal fibroblasts are more permissive for organoid growth compared with distal fibroblasts and that the immune cell profile is regionalized in two dimensions, where proximal-distal and epithelial-stromal gradients impact epithelial maintenance. Finally, we predict and verify how WNT, BMP, insulin growth factor (IGF), and neuregulin (NRG) signaling are differentially engaged along the esophageal axis. We establish a cellular and transcriptional framework for understanding esophageal regionalization, providing a functional basis for epithelial disease susceptibility.

Multimodal analyses of immune cells during bone repair identify macrophages as a therapeutic target in musculoskeletal trauma

Musculoskeletal traumatic injuries (MTI) involve soft tissue lesions adjacent to a bone fracture leading to fibrous nonunion. The impact of MTI on the inflammatory response to fracture and on the immunomodulation of skeletal stem/progenitor cells (SSPCs) remains unknown. Here, we used single-nucleus transcriptomic analyses to describe the immune cell dynamics after bone fracture and identified distinct macrophage subsets with successive pro-inflammatory, pro-repair and anti-inflammatory profiles. Concurrently, SSPCs transition via a pro- and anti-inflammatory fibrogenic phase of differentiation prior to osteochondrogenic differentiation. In a preclinical MTI mouse model, the injury response of immune cells and SSPCs is disrupted leading to a prolonged pro-inflammatory phase and delayed resolution of inflammation. Macrophage depletion improves bone regeneration in MTI demonstrating macrophage involvement in fibrous nonunion. Finally, pharmacological inhibition of macrophages using the CSF1R inhibitor Pexidartinib ameliorates healing. These findings reveal the coordinated immune response of macrophages and skeletal stem/progenitor cells as a driver of bone healing and as a primary target for the treatment of trauma-associated fibrosis.

Corpora cavernosa fibroblasts mediate penile erection

Penile erection is mediated by the corpora cavernosa, a trabecular-like vascular bed that enlarges upon vasodilation, but its regulation is not completely understood. Here, we show that perivascular fibroblasts in the corpora cavernosa support vasodilation by reducing norepinephrine availability. The effect on penile blood flow depends on the number of fibroblasts, which is regulated by erectile activity. Erection dynamically alters the positional arrangement of fibroblasts, temporarily down-regulating Notch signaling. Inhibition of Notch increases fibroblast numbers and consequently raises penile blood flow. Continuous Notch activation lowers fibroblast numbers and reduces penile blood perfusion. Recurrent erections stimulate fibroblast proliferation and limit vasoconstriction, whereas aging reduces the number of fibroblasts and lowers penile blood flow. Our findings reveal adaptive, erectile activity-dependent modulation of penile blood flow by fibroblasts.