Hyung Jin Ahn

INTRODUCTION: Cerebral amyloid angiopathy (CAA), marked by amyloid beta (Aβ) accumulation in perivascular spaces (PVSs), contributes to vascular injury and inflammation in Alzheimer's disease. The mechanisms underlying CAA‐related vascular pathology remain unclear. Increasing evidence indicates fibrinogen, the main component in blood clots, interacts with Aβ and exacerbates inflammation and co‐deposits in PVS of CAA‐positive vessels, yet its role in cerebrovascular dysfunction remains poorly defined. METHODS: Using TgSwDI transgenic mice, which develop robust CAA, we examined fibrin(ogen) deposition, aquaporin‐4 (AQP4) polarization, and caveolin‐1 (Cav‐1) expression. We further assessed the effects of fibrinogen depletion via small interfering RNA. RESULTS: TgSwDI mice showed increased fibrin(ogen) extravasation, colocalization with Aβ in PVS, AQP4 depolarization, and elevated Cav‐1 expression. Fibrinogen depletion reduced CAA, restored AQP4 polarization, decreased Cav‐1 levels, attenuated microglial activation, and improved spatial memory. DISCUSSION: These findings suggest that modulating fibrinogen‐related pathways could be a promising strategy for mitigating CAA pathology and its associated cerebrovascular pathology. Highlights: Fibrin(ogen)‐Aβ colocalization aggravates CAA. AQP4 depolarization links fibrinogen‐Aβ deposits to impaired clearance in CAA. Cav‐1 increase drives fibrinogen leakage and neuroinflammation. Fibrinogen depletion reduces CAA, restores AQP4 polarity, and improves memory.

Familial Danish Dementia (FDD) is a rare autosomal dominant neurodegenerative disorder caused by a mutation in the integral membrane protein 2B (ITM2b) gen

Familial Danish Dementia (FDD) is a rare autosomal dominant neurodegenerative disorder caused by a mutation in the integral membrane protein 2B ( ITM2b ) gene. Clinically, FDD is characterized by cerebral amyloid angiopathy (CAA), cerebellar ataxia, and dementia. Notably, FDD shares several neuropathological features with Alzheimer’s disease (AD), including CAA, neuroinflammation, and neurofibrillary tangles. In this study, we investigate the pathological mechanisms linking CAA, white matter damage, and motor dysfunction using a recently developed FDD knock-in (FDD-KI) rat model. This model harbors the Danish mutation in the endogenous rat Itm2b gene, along with an App gene encoding humanized amyloid-β (Aβ). Our analysis revealed substantial vascular Danish amyloid (ADan) deposition in the cerebellar subpial and leptomeningeal vessels of FDD-KI rats, showing an age-related increase comparable to that observed in human FDD patients. Additionally, vascular Aβ deposits (Aβ-CAA) were present in FDD-KI rats, but Aβ-CAA patterns showed some differences between species. Motor function assessments in FDD-KI rats demonstrated age-accelerated motor deficits and gait abnormalities, mirroring the clinical characteristics of FDD patients. To further explore the mechanisms underlying these deficits, we examined cerebellar pathology and found age-related myelin disruption and axonal fiber loss, consistent with postmortem human FDD pathology. Cerebellar demyelination appeared to be driven by neuroinflammation, marked by increased microglial/macrophage activation in response to vascular amyloid deposition. Additionally, we observed extravascular fibrinogen leakage, indicating widespread vascular permeability in both white and gray matter, with fibrinogen deposits surrounding amyloid-positive vessels in aged FDD-KI rats and postmortem FDD cerebellum. These findings suggest that this FDD-KI rat model is the first animal model to recapitulate key neuropathological features of human FDD patients, including both ADan- and Aβ-type CAA, neuroinflammation, and white matter lesions—pathologies that may underlie the motor and gait impairments seen in the disease.

Cerebral Amyloid Angiopathy (CAA), characterized by amyloid-β (Aβ) accumulation within perivascular spaces (PVS), contributes to vascular damage and inflammation in Alzheimer’s disease (AD). Despite its significance, the mechanisms driving Aβ deposition in PVS and the resulting vascular pathology remain poorly understood. Growing evidence suggests that fibrinogen, the main component in blood clots, interacts with Aβ and exacerbates inflammation in AD. Fibrinogen also co-deposits with Aβ in the PVS of CAA-positive vessels in the brains of hereditary CAA patients. However, the mechanisms by which fibrinogen contributes to cerebrovascular impairment remain poorly understood. To investigate this, we used TgSwDI transgenic mice, which develop robust CAA pathology, and observed a significant increase in fibrin(ogen) extravasation and colocalization with Aβ in the PVS. Moreover, we observed a significant aquaporin-4 (AQP4) depolarization in CAA-laden blood vessels of TgSwDI mice, which correlated with fibrin(ogen)-Aβ colocalization. Given AQP4 crucial role in Aβ clearance through glymphatic pathway, its depolarization may disrupt critical Aβ clearance, thereby exacerbating CAA pathology. Additionally, Caveolin-1, a protein involved in non-specific transcytosis across the endothelium, significantly increased with age in TgSwDI mice and correlated with fibrin(ogen) extravasation. To further explore the relationship between fibrin(ogen) and these cerebrovascular alterations, we depleted fibrinogen in TgSwDI mice using siRNA approach. This intervention resulted in decreased CAA, restored polarized expression of AQP4, reduced caveolin-1 levels, attenuated microglial activation, and improved spatial memory in fibrinogen-depleted TgSwDI mice. These findings suggest that targeting fibrinogen could be a promising strategy for mitigating CAA pathology and its associated cerebrovascular pathology. Significance Statement: Our study uncovers the mechanism by which fibrin(ogen)-Aβ colocalization exacerbates CAA pathology. Our findings highlight the potential link between fibrinogen/ fibrin(ogen)-Aβ colocalization and AQP4 depolarization thereby exacerbating CAA pathology. The age-dependent increase of endothelial caveolin-1 could facilitate fibrin(ogen) extravasation, assisting the later to binds to Aβ in the perivascular space which ultimately induce microglial neuroinflammation and AQP4 depolarization, thus exacerbating CAA pathology. Furthermore, fibrinogen depletion could mitigate CAA severity, reduce microglial activation, restore AQP4 polarization and memory impairment. These results suggest that targeting fibrinogen and caveolin-1-mediated transcytosis may offer new strategies to address CAA-associated cerebrovascular pathology.

Many Alzheimer’s disease (AD) patients suffer from altered cerebral blood flow and damaged cerebral vasculature. Cerebrovascular dysfunction could play an important role in this disease. However, the mechanism underlying a vascular contribution in AD is still unclear. Cerebrovascular reactivity (CVR) is a critical mechanism that maintains cerebral blood flow and brain homeostasis. Most current methods to analyze CVR require anesthesia which is known to hamper the investigation of molecular mechanisms underlying CVR. We therefore combined spectroscopy, spectral analysis software, and an implantable device to measure cerebral blood volume fraction (CBVF) and oxygen saturation (S_O2) in unanesthetized, freely-moving mice. Then, we analyzed basal CBVF and S_O2, and CVR of 5-month-old C57BL/6 mice during hypercapnia as well as during basic behavior such as grooming, walking and running. Moreover, we analyzed the CVR of freely-moving AD mice and their wildtype (WT) littermates during hypercapnia and could find impaired CVR in AD mice compared to WT littermates. Our results suggest that this optomechanical approach to reproducibly getting light into the brain enabled us to successfully measure CVR in unanesthetized freely-moving mice and to find impaired CVR in a mouse model of AD.

Introduction: Alzheimer’s Disease (AD) patients exhibit signs of motor dysfunction, including gait, locomotion, and balance deficits. Changes in motor function often precede other symptoms of AD as well as correlate with increased severity and mortality. Despite the frequent occurrence of motor dysfunction in AD patients, little is known about the mechanisms by which this behavior is altered.Methods and Results: In the present study, we investigated the relationship between cerebrovascular impairment and motor dysfunction in a mouse model of AD (Tg6799). We found an age-dependent increase of extravasated fibrinogen deposits in the cortex and striatum of AD mice. Interestingly, there was significantly decreased cerebrovascular density in the striatum of the 15-month-old as compared to 7-month-old AD mice. We also found significant demyelination and axonal damage in the striatum of aged AD mice. We analyzed striatum-related motor function and anxiety levels of AD mice at both ages and found that aged AD mice exhibited significant impairment of motor function but not in the younger AD mice.Discussion: Our finding suggests an enticing correlation between extravasated fibrinogen, cerebrovascular damage of the striatum, and motor dysfunction in an AD mouse model, suggesting a possible mechanism underlying motor dysfunction in AD.

Significance: Cerebral vascular reactivity is critical parameters of brain homeostasis in health and disease, but the investigational value of brain oxymetry is diminished by anesthesia and mechanical fixation of the mouse scull. Aim: We needed to reduce the physical restrictivity of hemodynamic spectroscopy to enable Alzheimer’s disease (AD) studies in freely-moving mice. Approach: We combined spectroscopy, spectral analysis software and a magnetic, implantable device to measure vascular reactivity in unanesthetized, freely-moving mice. We measured cerebral blood volume fraction ( CBVF ) and oxygen saturation (S _O2 ). Results: We validated that our system could detect delayed cerebrovascular recovery from hypoxia in an orthotopic xenograft glioma model under anesthetized condition and we also found increased CBVF and impaired vascular reactivity during hypercapnia in a freely-moving mouse model of AD compared to wild-type littermates. Conclusions: Our optomechanical approach to reproducibly getting light into and out of the brain enabled us to successfully measure CBVF and S _O2 during hypercapnia in unanesthetized freely-moving mice. We present hardware and software enabling oximetric analysis of metabolic activity, which provides a safe and reliable method for rapid assessment of vascular reactivity in murine disease models as well as CBVF and S _O2 .

Significance: We found an increased interaction between CAA-linked mutant Aβ and fibrin(ogen) that may not only result in severely altered fibrin structure and function but may also lead to vast amounts of fibrin(ogen)/Aβ codeposition, as well as fibrin deposits in HCAA patients. This finding provides a molecular mechanism for how CAA-linked mutations may lead to severe cerebrovascular pathology in HCAA patients by enhancing the protein–protein interaction.