Katerina Akassoglou

Cerebrovascular pathology and neuronal network dysfunction are early features of Alzheimers disease (AD) associated with neuroinflammation and cognitive decline, but the vascular and immune triggers of neuronal hyperactivity remain largely unknown. Here, we show that the blood coagulation protein fibrin disrupts microglia-neuron interactions, promoting neuronal hyperactivity in an AD mouse model. Genetic elimination of the fibrin inflammatory domain reduced neuronal hyperactivity, restored dynamic microglial interactions with active neurons and protected from high-risk decision making in 5XFAD mice. Leveraging the transcriptional signatures of microglia and inhibitory and excitatory neurons, a ligand-receptor atlas revealed fibrin-dependent disruption of innate immune and glutamatergic signaling between microglia and neurons in AD mice. Patients with AD also showed a correlation of cerebrospinal fluid (CSF) fibrinogen levels with biomarkers of inflammation, vascular and synaptic dysfunction. Thus, resilience to neuronal hyperactivity and restoration of the neuroimmune interactome by targeting fibrin may have therapeutic implications for Alzheimers disease and related conditions. There is a companion manuscript cited as Ref #62 submitted to bioRxiv (Lauderdale et al., 2026)

Background: Cerebrovascular alterations and innate immune activation are key features of Alzheimer’s disease (AD). However, the mechanisms that link blood‐brain barrier disruption to neurodegeneration are poorly understood and well‐defined druggable targets at the neurovascular interface are limited.Method: By developing a multiomic and genetic loss‐of‐function pipeline, we reported the transcriptomic and global phosphoproteomic landscape of blood‐induced microglia activation and the causal role for fibrin in induction of neurodegenerative genes and oxidative stress pathways in innate immune cells¹. Volume imaging in cleared mouse and human AD brains combined with repetitive in vivo two‐photon imaging showed focal fibrinogen deposits associated with loss of dendritic spines independent of amyloid plaques. We developed a first‐in‐class fibrin‐targeting immunotherapy to selectively inhibit fibrin‐induced inflammation without interfering with its beneficial coagulation effects².Result: We identified the blood coagulation factor fibrinogen as necessary and sufficient for the induction of pathogenic neuroinflammation in neurologic diseases³. Fibrinogen induces spine elimination and promotes cognitive deficits in AD mouse models mediated by oxidative stress induction in microglia⁴. Fibrin‐targeting immunotherapy entered the CNS, bound to fibrin, and inhibited amyloid‐driven neurotoxicity and neurotoxic inflammatory gene programs in AD mice².Conclusion: Thus, fibrinogen links cerebrovascular damage with immune‐mediated neurodegeneration and fibrin therapeutics may have important therapeutic implications inhibiting vascular‐driven neurodegeneration in AD and related conditions.References1. Mendiola et al. Nat Immunol 2023, 24:1173‐11872. Ryu et al. Nat Immunol 2018, 19:1212‐12233. Petersen et al., Nat Rev Neurosci. 2018, 19:283‐3014. Merlini et al. Neuron 2019, 101:1099‐1108

Life-threatening thrombotic events and neurological symptoms are prevalent in COVID-19 and are persistent in patients with long COVID experiencing post-acute sequelae of SARS-CoV-2 infection^1–4. Despite the clinical evidence^1,5–7, the underlying mechanisms of coagulopathy in COVID-19 and its consequences in inflammation and neuropathology remain poorly understood and treatment options are insufficient. Fibrinogen, the central structural component of blood clots, is abundantly deposited in the lungs and brains of patients with COVID-19, correlates with disease severity and is a predictive biomarker for post-COVID-19 cognitive deficits^1,5,8–10. Here we show that fibrin binds to the SARS-CoV-2 spike protein, forming proinflammatory blood clots that drive systemic thromboinflammation and neuropathology in COVID-19. Fibrin, acting through its inflammatory domain, is required for oxidative stress and macrophage activation in the lungs, whereas it suppresses natural killer cells, after SARS-CoV-2 infection. Fibrin promotes neuroinflammation and neuronal loss after infection, as well as innate immune activation in the brain and lungs independently of active infection. A monoclonal antibody targeting the inflammatory fibrin domain provides protection from microglial activation and neuronal injury, as well as from thromboinflammation in the lung after infection. Thus, fibrin drives inflammation and neuropathology in SARS-CoV-2 infection, and fibrin-targeting immunotherapy may represent a therapeutic intervention for patients with acute COVID-19 and long COVID.

Cerebellar injury in preterm infants with central nervous system (CNS) hemorrhage results in lasting neurological deficits and an increased risk of autism. The impact of blood-induced pathways on cerebellar development remains largely unknown, so no specific treatments have been developed to counteract the harmful effects of blood after neurovascular damage in preterm infants. Here, we show that fibrinogen, a blood-clotting protein, plays a central role in impairing neonatal cerebellar development. Longitudinal MRI of preterm infants revealed that cerebellar bleeds were the most critical factor associated with poor cerebellar growth. Using inflammatory and hemorrhagic mouse models of neonatal cerebellar injury, we found that fibrinogen increased innate immune activation and impeded neurogenesis in the developing cerebellum. Fibrinogen inhibited sonic hedgehog (SHH) signaling, the main mitogenic pathway in cerebellar granule neuron progenitors (CGNPs), and was sufficient to disrupt cerebellar growth. Genetic fibrinogen depletion attenuated neuroinflammation, promoted CGNP proliferation, and preserved normal cerebellar development after neurovascular damage. Our findings suggest that fibrinogen alters the balance of SHH signaling in the neurovascular niche and may serve as a therapeutic target to mitigate developmental brain injury after CNS hemorrhage.

Cerebrovascular alterations and innate immune activation are key features of Alzheimer’s disease (AD). However, the mechanisms that link blood‐brain barrier disruption to neurodegeneration are poorly understood and well‐defined druggable targets at the neurovascular interface are limited. We identified the blood coagulation factor fibrinogen as necessary and sufficient for the induction of pathogenic neuroinflammation in neurologic diseases¹. Fibrinogen induces spine elimination and promotes cognitive deficits in AD mouse models mediated by oxidative stress induction in microglia^{2, 3}. Volume imaging in cleared mouse and human AD brains combined with repetitive in vivo two‐photon imaging showed focal fibrinogen deposits associated with loss of dendritic spines independent of amyloid plaques. We developed a first‐in‐class fibrin‐targeting immunotherapy to selectively inhibit fibrin‐induced inflammation without interfering with its beneficial coagulation effects⁴. Fibrin‐targeting immunotherapy entered the CNS, bound to fibrin, and inhibited amyloid‐driven neurotoxicity and neurotoxic inflammatory gene programs in AD mice. Thus, fibrinogen links cerebrovascular damage with immune‐mediated neurodegeneration and fibrin therapeutics may have important therapeutic implications inhibiting vascular‐driven neurodegeneration in AD and related conditions.1. Petersen et al., Nat Rev Neurosci. 2018, 19:283‐3012. Merlini et al. Neuron 2019, 101:1099‐11083. Mendiola et al. Nat Immunol 2020, 21:513‐5244. Ryu et al. Nat Immunol 2018, 19:1212‐1223

Objective: We sought to create and characterize a mouse model of the inflammatory, cerebral demyelinating phenotype of X-linked adrenoleukodystrophy (ALD) that would facilitate the study of disease pathogenesis and therapy development. We also sought to cross-validate potential therapeutic targets such as fibrin, oxidative stress, and the NLRP3 inflammasome, in post-mortem human and murine brain tissues. Background: ALD is caused by mutations in the gene ABCD1 encoding a peroxisomal transporter. More than half of males with an ABCD1 mutation develop the cerebral phenotype (cALD). Incomplete penetrance and absence of a genotype-phenotype correlation imply a role for environmental triggers. Mechanistic studies have been limited by the absence of a cALD phenotype in the Abcd1- null mouse. Methods: We generated a cALD phenotype in 8-week-old, male Abcd1- null mice by deploying a two-hit method that combines cuprizone (CPZ) and experimental autoimmune encephalomyelitis (EAE) models. We employed in vivo MRI and post-mortem immunohistochemistry to evaluate myelin loss, astrogliosis, blood-brain barrier (BBB) disruption, immune cell infiltration, fibrin deposition, oxidative stress, and Nlrp3 inflammasome activation in mice. We used bead-based immunoassay and immunohistochemistry to evaluate IL-18 in CSF and post-mortem human cALD brain tissue. Results: MRI studies revealed T2 hyperintensities and post-gadolinium enhancement in the medial corpus callosum of cALD mice, similar to human cALD lesions. Both human and mouse cALD lesions shared common histologic features of myelin phagocytosis, myelin loss, abundant microglial activation, T and B-cell infiltration, and astrogliosis. Compared to wild-type controls, Abcd1 -null mice had more severe cerebral inflammation, demyelination, fibrin deposition, oxidative stress, and IL-18 activation. IL-18 immunoreactivity co-localized with macrophages/microglia in the perivascular region of both human and mouse brain tissue. Interpretation: This novel mouse model of cALD suggests loss of Abcd1 function predisposes to more severe cerebral inflammation, oxidative stress, fibrin deposition, and Nlrp3 pathway activation, which parallels the findings seen in humans with cALD. We expect this model to enable long-sought investigations into cALD mechanisms and accelerate development of candidate therapies for lesion prevention, cessation, and remyelination. Graphical

Blood protein extravasation through a disrupted blood–brain barrier and innate immune activation are hallmarks of neurological diseases and emerging therapeutic targets. However, how blood proteins polarize innate immune cells remains largely unknown. Here, we established an unbiased blood-innate immunity multiomic and genetic loss-of-function pipeline to define the transcriptome and global phosphoproteome of blood-induced innate immune polarization and its role in microglia neurotoxicity. Blood induced widespread microglial transcriptional changes, including changes involving oxidative stress and neurodegenerative genes. Comparative functional multiomics showed that blood proteins induce distinct receptor-mediated transcriptional programs in microglia and macrophages, such as redox, type I interferon and lymphocyte recruitment. Deletion of the blood coagulation factor fibrinogen largely reversed blood-induced microglia neurodegenerative signatures. Genetic elimination of the fibrinogen-binding motif to CD11b in Alzheimer’s disease mice reduced microglial lipid metabolism and neurodegenerative signatures that were shared with autoimmune-driven neuroinflammation in multiple sclerosis mice. Our data provide an interactive resource for investigation of the immunology of blood proteins that could support therapeutic targeting of microglia activation by immune and vascular signals.

Systemic sclerosis (SSc) is an autoimmune disease characterized by progressive multiorgan fibrosis. While the cause of SSc remains unknown, a perturbed vasculature is considered a critical early step in the pathogenesis. Using fibrinogen as a marker of vascular leakage, we found extensive extravascular fibrinogen deposition in the dermis of both limited and diffuse systemic sclerosis disease, and it was present in both early and late-stage patients. Based on a timed series of excision wounds, retention on the fibrin deposit of the splice variant domain, fibrinogen α _E C, indicated a recent event, while fibrin networks lacking the α _E C domain were older. Application of this timing tool to SSc revealed considerable heterogeneity in α _E C domain distribution providing unique insight into disease activity. Intriguingly, the fibrinogen-α _E C domain also accumulated in macrophages. These observations indicate that systemic sclerosis is characterized by ongoing vascular leakage resulting in extensive interstitial fibrin deposition that is either continually replenished and/or there is impaired fibrin clearance. Unresolved fibrin deposition might then incite chronic tissue remodeling.

Cerebrovascular dysfunction is a hallmark of Alzheimer’s disease (AD) that is linked to cognitive decline. However, blood-brain barrier (BBB) disruption in AD is focal and requires sensitive methods to detect extravasated blood proteins and vasculature in large brain volumes. Fibrinogen, a blood coagulation factor, is deposited in AD brains at sites of BBB disruption and cerebrovascular damage. This chapter presents the methodology of fibrinogen immunolabeling-enabled three-dimensional (3D) imaging of solvent-cleared organs (iDISCO) which, when combined with immunolabeling of amyloid β (Aβ) and vasculature, enables sensitive detection of focal BBB vascular abnormalities, and reveals the spatial distribution of Aβ plaques and fibrin deposits, in large tissue volumes from cleared human brains. Overall, fibrinogen iDISCO enables the investigation of neurovascular and neuroimmune mechanisms driving neurodegeneration in disease.