Scar formation in the nervous system begins within hours after traumatic injury and is characterized primarily by reactive astrocytes depositing proteoglycans that inhibit regeneration. A fundamental question in CNS repair has been the identity of the initial molecular mediator that triggers glial scar formation. Here we show that the blood protein fibrinogen, which leaks into the CNS immediately after blood-brain barrier (BBB) disruption or vascular damage, serves as an early signal for the induction of glial scar formation via the TGF-␤/Smad signaling pathway. Our studies revealed that fibrinogen is a carrier of latent TGF-␤ and induces phosphorylation of Smad2 in astrocytes that leads to inhibition of neurite outgrowth. Consistent with these findings, genetic or pharmacologic depletion of fibrinogen in mice reduces active TGF-␤, Smad2 phosphorylation, glial cell activation, and neurocan deposition after cortical injury. Furthermore, stereotactic injection of fibrinogen into the mouse cortex is sufficient to induce astrogliosis. Inhibition of the TGF-␤ receptor pathway abolishes the fibrinogen-induced effects on glial scar formation in vivo and in vitro. These results identify fibrinogen as a primary astrocyte activation signal, provide evidence that deposition of inhibitory proteoglycans is induced by a blood protein that leaks in the CNS after vasculature rupture, and point to TGF-␤ as a molecular link between vascular permeability and scar formation.

The blood protein fibrinogen as a ligand for integrin and non-integrin receptors functions as the molecular nexus of coagulation, inflammation and immunity. Studies in animal models and in human disease have demonstrated that extravascular fibrinogen that is deposited in tissues upon vascular rupture is not merely a marker, but a mediator of diseases with an inflammatory component, such as rheumatoid arthritis, multiple sclerosis, sepsis, myocardial infarction and bacterial infection. The present article focuses on the recent discoveries of specific cellular targets and receptors for fibrinogen within tissues that have extended the role of fibrinogen from a coagulation factor to a regulator of inflammation and immunity. Fibrinogen has the potential for selective drug targeting that would target its proinflammatory properties without affecting its beneficial effects in hemostasis, since it interacts with different receptors to mediate blood coagulation and inflammation. Strategies to target receptors for fibrinogen and fibrin within the tissue microenvironment could reveal selective and disease-specific agents for therapeutic intervention in a variety of human diseases associated with fibrin deposition.

Changes in the molecular and cellular composition of the CNS after injury or disease result in the formation of an inhibitory environment that inhibits the regeneration of adult mammalian CNS neurons. Although a dramatic change in the CNS environment after traumatic injury or disease is hemorrhage because of vascular rupture or leakage of the blood-brain barrier, the potential role for blood proteins in repair processes remains unknown. Here we show that the blood protein fibrinogen is an inhibitor of neurite outgrowth that is massively deposited in the spinal cord after injury. We show that fibrinogen acts as a ligand for ␤3 integrin and induces the transactivation of EGF receptor (EGFR) in neurons. Fibrinogen-mediated inhibition of neurite outgrowth is reversed by blocking either ␤3 integrin or phoshorylation of EGFR. Inhibition of Src family kinases that mediate the cross-talk between integrin and growth factor receptors rescue the fibrinogen-induced phosphorylation of EGFR. These results identify fibrinogen as the first blood-derived inhibitor of neurite outgrowth and suggest fibrinogen-induced EGFR transactivation on neuronal cells as a molecular link between vascular and neuronal damage in the CNS after injury.blood-brain barrier ͉ regeneration ͉ spinal cord injury ͉ transactivation ͉ scar T he identification of common molecular mechanisms that regulate vascular and neural development has expanded the role of the CNS vasculature from nutrition to regulation of axonal guidance, synaptic activity, metabolic trafficking, and adult neurogenesis (1). Although there is a causal interaction between the nervous system and the vasculature, a physical and metabolic barrier between the brain and the systemic circulation, namely the blood-brain barrier (BBB), prohibits the entry of blood proteins from the vasculature into the nervous tissue (2). Leakage of blood components in the CNS parenchyma is a common denominator of several CNS diseases characterized by edema formation and neuronal damage, such as stroke, HIV encephalitis, Alzheimer's disease (AD), multiple sclerosis (MS), glioblastoma, and bacterial meningitis (2). After traumatic injury, such as spinal cord injury (SCI), the primary mechanical injury results in pronounced hemorrhage into the spinal cord and disruption of the blood vessel walls (3). Evidence in AD that correlates microhemorrhages with amyloid plaque formation (4) and in MS that identifies leakage of blood components in the brain as one of the earliest histopathologic abnormalities (5) has postulated a role for blood components in the onset and progression of neurodegeneration. However, the cellular and molecular mechanisms of action of blood proteins within the CNS microenvironment and their contribution to disease pathogenesis remain poorly characterized. Given that inhibition of regeneration in the CNS results in part from the presence of inhibitory factors in the neuronal environment (6), investigating the role of blood proteins as modulators of neuronal functions could be crucial for the ide...

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.