Summary Blood-brain barrier (BBB) disruption alters the composition of the brain microenvironment by allowing blood proteins into the CNS. However, whether blood-derived molecules serve as extrinsic inhibitors of remyelination is unknown. Here we show that the coagulation factor fibrinogen activates the bone morphogenetic protein (BMP) signaling pathway in oligodendrocyte progenitor cells (OPCs) and suppresses remyelination. Fibrinogen induces phosphorylation of Smad 1/5/8 and inhibits OPC differentiation into myelinating oligodendrocytes (OLs), while promoting an astrocytic fate in vitro. Fibrinogen effects are rescued by BMP type I receptor inhibition using dorsomorphin homologue 1 (DMH1) or CRISPR/Cas9 activin A receptor type I (ACVR1) knockout in OPCs. Fibrinogen and the BMP target Id2 are increased in demyelinated multiple sclerosis (MS) lesions. Therapeutic depletion of fibrinogen decreases BMP signaling and enhances remyelination in vivo. Targeting fibrinogen may be an upstream therapeutic strategy to promote the regenerative potential of CNS progenitors in diseases with remyelination failure.

Neural stem/progenitor cells (NSPCs) originating from the subventricular zone (SVZ) contribute to brain repair during CNS disease. The microenvironment within the SVZ stem cell niche controls NSPC fate. However, extracellular factors within the niche that trigger astrogliogenesis over neurogenesis during CNS disease are unclear. Here, we show that blood-derived fibrinogen is enriched in the SVZ niche following distant cortical brain injury in mice. Fibrinogen inhibited neuronal differentiation in SVZ and hippocampal NSPCs while promoting astrogenesis via activation of the BMP receptor signaling pathway. Genetic and pharmacologic depletion of fibrinogen reduced astrocyte formation within the SVZ after cortical injury, reducing the contribution of SVZ-derived reactive astrocytes to lesion scar formation. We propose that fibrinogen is a regulator of NSPC-derived astrogenesis from the SVZ niche via BMP receptor signaling pathway following injury.

Adult neural stem/precursor cells (NSPCs) of the subventricular zone (SVZ) are an endogenous source for neuronal replacement in CNS disease. However, adult neurogenesis is compromised after brain injury in favor of a glial cell fate, which is mainly attributed to changes in the NSPC environment. Yet, it is unknown how this unfavorable extracellular environment translates into a transcriptional program altering NSPC differentiation. Here, we show that genetic depletion of the transcriptional regulator Id3 decreased the number of astrocytes generated from SVZ-derived adult NSPCs in the cortical lesion area after traumatic brain injury. Cortical brain injury resulted in rapid BMP-2 and Id3 up-regulation in the SVZ stem cell niche. Id3 À/À adult NSPCs failed to differentiate into BMP-2-induced astrocytes, while NSPCs deficient for the Id3-controlled transcription factor E47 readily differentiated into astrocytes in the absence of BMP-2. Mechanistically, E47 repressed the expression of several astrocyte-specific genes in adult NSPCs. These results identify Id3 as the BMP-2-induced transcriptional regulator, promoting adult NSPC differentiation into astrocytes upon CNS injury and reveal a molecular link between environmental changes and NSPC differentiation in the CNS after injury.

Reactive astrocytes at the border of damaged neuronal tissue organize into a barrier surrounding the fibrotic lesion core, separating this central region of inflammation and fibrosis from healthy tissue. Astrocytes are essential to form the border and for wound repair but interfere with neuronal regeneration. However, the mechanisms driving these astrocytes during central nervous system (CNS) disease are unknown. Here we show that blood‐derived fibrinogen is enriched at the interface of lesion border‐forming elongated astrocytes after cortical brain injury. Anticoagulant treatment depleting fibrinogen reduces astrocyte reactivity, extracellular matrix deposition and inflammation with no change in the spread of inflammation, whereas inhibiting fibrinogen conversion into fibrin did not significantly alter astrocyte reactivity, but changed the deposition of astrocyte extracellular matrix. RNA sequencing of fluorescence‐activated cell sorting‐isolated astrocytes of fibrinogen‐depleted mice after cortical injury revealed repressed gene expression signatures associated with astrocyte reactivity, extracellular matrix deposition and immune‐response regulation, as well as increased gene expression signatures associated with astrocyte metabolism and astrocyte‐neuron communication. Systemic pharmacologic depletion of fibrinogen resulted in the absence of elongated, border‐forming astrocytes and increased the survival of neurons in the lesion core after cortical injury. These results identify fibrinogen as a critical trigger for lesion border‐forming astrocyte properties in CNS disease.