Abstract:
Amyloids occur as localized or systemic deposits composed of compact β-sheet fibrils of abnormally folded proteins that may disrupt the physiological function of the afflicted tissue by loss-of-function or gain-of-toxicity. The best characterized amyloid, amyloid-β (Aβ) is a known trigger of the molecular and cellular cascade leading to Alzheimer’s disease (AD). Aβ aggregation can amplify via a prion-like seeding mechanism and rapidly spread in the brain connectome. Several downstream processes potentiate the toxic cascade, ultimately resulting in brain damage and memory loss. Interestingly, vascular dysfunction is described as an early symptom of AD in humans. Recently, a study identified another protein that deposits in the vasculature of the AD brain – Medin amyloid (an internal fragment of MFG-E8). Medin is the most common human vascular amyloid known to date, as it can be detected within the thoracic aorta and the upper body arteries of nearly everyone above 50 years of age. As the name Medin suggests, it predominantly deposits along the elastic fibers of the tunica media. Because of its vascular localization and potential role in arterial aging it has been suggested as a possible risk factor of AD. However, despite its high prevalence in the aging population, no causal role in human pathogenesis was previously reported. Nevertheless, in vitro studies have described common structural, physicochemical and cytotoxic amyloid properties for Medin, and analyses of human tissue derived from autopsy or surgery implicate a role for Medin in vascular diseases. However, the question of a mechanistic link between the presence of Medin, cerebrovascular dysfunction and AD pathology remains elusive due to lack of appropriate experimental in vivo models, which would enable mechanistic studies. In this dissertation, a suitable animal model to investigate the influence of Medin aggregates on cerebrovascular function was established. In particular, we found that with advancing age wild-type mice develop endogenous Medin aggregates in the aorta and cerebral arteries that resemble the sequence, biochemical properties, and morphology of human Medin amyloid. We also showed in our first study that age-associated decline in cerebral vascular function (evident by impaired artery elasticity) in living mice was rescued by prevention of vascular Medin aggregate formation through a genetic knockout of the Medin-containing C2 domain of MFG-E8 (Mfge8 C2 KO). These observations implicate Medin as a causal factor in age-associated cerebrovascular dysfunction for the first time. Given the fact that Medin was recently shown to template aggregation of serum amyloid A, our second study addressed whether potential co-aggregation of Medin and Aβ may exacerbate AD pathology. Indeed, our study provided first evidence for a direct amyloid-amyloid interaction of Medin and Aβ in two mouse models for cerebral β-amyloidosis. Medin did not only co-localize substantially with Aβ deposits in the brain, but the genetic deletion of Medin also altered plaque and fibril structure in vitro and in vivo, prevented MFG-E8/Medin accumulation within plaques and vasculature, and slowed the onset of pathology in cerebral β-amyloidosis. It is further important to note that in line with its vascular localization in humans, genetic Medin deficiency reduced cerebral β-amyloid angiopathy (CAA) and related microhemorrhages even at end-stage amyloid pathology. Additionally, aorta-derived Medin 'seeds' promoted pre-mature Aβ aggregation in the brain, suggesting that Medin promotes Aβ by a heterologous seeding mechanism. To analyze the translational relevance of a potential heterologous seeding mechanism between Medin and Aβ, we also examined data from the dorsolateral prefrontal cortex of >500 patients from the ROS/MAP cohorts, two longitudinal clinical studies on aging, cognitive decline and AD. Importantly, AD patients showed significantly increased MFGE8 expression compared to non-demented patients. Moreover, MFGE8 gene expression levels predicted cognitive decline independent of amyloid neuropathology, namely Aβ plaques and neurofibrillary tangles. Notably, Medin was previously reported to be increased within cerebral arteries of AD patients in comparison to non-demented controls, and levels of arteriole Medin predicted risk for AD independent of amyloid pathology, age, sex and ApoE status. In summary, this thesis demonstrates the importance to develop new technologies and models to study amyloid pathobiology and cross-talk, as amyloids contribute to common age-related damage and dysfunction, but also affect progression, heterogeneity and co-morbidity of other diseases. Our studies demonstrate a new mechanism that may drive age-associated vascular disease and cerebral β-amyloid angiopathy, highlighting Medin as a potential therapeutic target to maintain vascular health and cognitive function with age.
