PROJECT SUMMARY/ABSTRACT Preterm infants are at risk for central nervous system (CNS) hemorrhage which can disrupt cerebellar maturation and lead to permanent neurodevelopmental impairment. The molecular signals in the disrupted neurovascular niche that block cerebellar development are not known. Thus, no therapeutics are available to prevent the developmental disabilities associated with preterm brain hemorrhage. Fibrinogen, a blood coagulation protein, crosses a leaky blood-brain barrier (BBB) and is a key driver of neuroinflammation, oxidative stress, neurodegeneration, glial scar formation, and inhibition of repair. We hypothesize that fibrinogen is a critical component of the neurovascular niche after BBB disruption that blocks cerebellar development in preterm infants. Our preliminary studies show: 1) Lipopolysaccharide (LPS)-induced systemic inflammation in neonatal mice increases vascular activation, fibrinogen deposition, and neuroinflammation in the cerebellum; 2) Fibrinogen depletion rescues cerebellar growth in systemic neonatal inflammation and plasma injection models of BBB disruption; 3) Fibrinogen inhibits neurogenesis from cerebellar granule neuronal progenitors (CGNPs) and is sufficient to disrupt cerebellar growth in vivo; 4) Fibrinogen activates the bone morphogenetic protein (BMP) receptor activin A receptor type I (ACVR1) in CNS progenitor cells to inhibit remyelination and neurogenesis; 5) Fibrin binds the CD11b/CD18 integrin receptor on microglia/macrophages to induce pro- inflammatory and pro-oxidant pathways that are toxic to CNS progenitor cells and impair regeneration; 6) fggγ390-396A knock-in mice, in which the binding site of fibrin to the CD11b integrin is mutated, have improved cerebellar growth during systemic neonatal inflammation. Our specific aims will test our working model, whereby fibrinogen deposition after BBB disruption alters cerebellar development through: 1) direct inhibitory effects on CGNPs via ACVR1 signaling, and 2) activation of innate immune responses via CD11b. In Aim 1, we will define the contribution of aberrant ACVR1 signaling to fibrinogen-induced cerebellar injury using CGNP-specific ACVR1 mutant mice and clinically relevant ACVR1 small molecule inhibitors. In Aim 2, we will determine the role of fibrin- CD11b-induced innate immune activation to cerebellar injury using fibrinogen mutant mice and a novel monoclonal antibody that blocks the interaction of fibrin with CD11b. In Aim 3, we will define how fibrinogen- ACVR1 signaling alters human cerebellar progenitor cell fate in induced pluripotent stem cell-derived cerebellar organoids using single cell transcriptomics. These studies will reveal molecular mechanisms at the neurovascular interface that link BBB disruption to CNS progenitor cell dysfunction in preterm infant brain injury. Thus, results from this proposal may open new treatment strategies to overcome the inhibitory neurovascular niche in preterm infant brain injury as well as other neurologic diseases with BBB disruption and fibrinogen deposition, such as multiple sclerosis, Alzheimer disease, and traumatic injury.

PROJECT NARRATIVE Preterm infants are at risk of bleeding in the brain which can cause permanent damage and long-term disabilities for which no treatment exists. We discovered that fibrinogen, a blood clotting protein, leaks into the injured brain and blocks stem cells from forming the new neurons and myelin needed for normal brain development. The goal of this project is to determine if we can overcome fibrinogen’s inhibitory effects in the brain as a new treatment strategy for preterm infant brain injury.