Antifungal activity of amyloid beta as a driver of dementia and AD pathogenesis.

University Of California At Davis, Davis CA

Investigators

Abstract

PROJECT ABSTRACT The blood-brain barrier (BBB) is a highly-restrictive structure that protects the central nervous system (CNS). Few fungal pathogens, such as Cryptococcus neoformans (Cn), can breach the BBB and invade the CNS. Fungal spores are prevalent in our environment and we become infected following inhalation of aerosolized spores. Once inhaled, spores of Cn proliferate in lung tissue and disseminate to the CNS due to their neurotropic nature. Cn is the leading cause of fungal meningoencephalitis in adults and it is often misdiagnosed as Alzheimer’s Disease (AD) due to similar neurological symptoms. Indeed, cognitive impairment was reported in 61% of subjects at least one year following Cn infection diagnosis. In AD, inflammatory and infectious processes promote BBB dysfunction underscoring the potentially critical role of the amyloid precursor protein (APP) in endothelial inflammation of the BBB and its subsequent dysfunction. We resolved the transcriptional signature of human brain microvascular endothelial cells (BMECs) infected with Cn. BMECs challenged with Cn showed significant dysregulation of several genes essential to the proper function of the brain endothelium (i.e. BBB), including EphA2-receptor tyrosine kinase, a key mediator of BBB dysfunction during Cn challenge (based on studies from our parent grant). We also identified APBB3 (amyloid beta precursor binding protein 3) as a potential mediator of BBB dysfunction. APBB3 is an uncharacterized protein whose role in APP processing/regulation has never been examined. Our results are consistent with similar changes reported in the abnormal BBB in AD pathogenesis. Reduced BBB integrity is associated with neuroinflammation, neuronal injury, oxidative stress and faulty clearance of amyloid beta (A). Based on our data we propose that fungal brain infection causes BBB dysfunction in part by dysregulating APP and reducing A clearance which would further damage the BBB. Our data are consistent with recent reports that found various brain regions from AD patients at autopsy infected with different species of fungi. Fungal DNA, proteins and structures were identified in frozen brain tissue from AD patients, but not from control patient tissue. Moreover fungal material was detected intra- and extracellularly in neurons from AD patients. Chitin polysaccharides, a key component of fungal cell walls, were also identified and human AD cerebral spinal fluid samples further confirmed the presence of fungi, including Cryptococcus and Candida. Indeed, following these recent discoveries, and the proposed antimicrobial properties of A a new AD hypothesis referred to as “antimicrobial protection hypothesis” has emerged. This hypothesis suggests that deposition of A plaque in brain can initiate early innate immune responses, where A entraps and neutralizes the invading fungal pathogen under normal conditions. The proposed supplement research project will address 2 specific aims that are an extension of the parent project and are fundamentally relevant to the pathogenesis of AD. The following specific aims will test the hypothesis that brain fungal infection dysregulates APP via APBB3 and the resulting antifungal immunity activity of amyloid beta (A) is mediated by EphA2- signaling. Specific aim 1 will examine the role of APP in BBB dysfunction in a mouse model of fungal brain infection. APP and APBB3 activity and their effects on BBB integrity will be examined in wild type and ephA2-/- mice infected with Cn. The physical interaction of A peptides with fungal cells will be examined in vivo and direct effects of A peptides on BBB integrity will be assessed in BBB spheroids, a highly-relevant in vitro model of the BBB. Specific aim 2 will determine whether A exhibits antifungal activity or induces antifungal immunity activity via EphA2-mediated signaling. We have performed several animal studies to resolve mechanisms of fungal brain pathogenesis by examining EphA2 activity in BBB dysfunction, however we have not examined neurological markers of AD in the context of cryptococcal brain infection. Given our expertise in animal models of fungal infection and experience working with in vitro models of the BBB, the studies proposed below are a natural segue to exploring an exciting and potentially ground breaking area of AD pathogenesis and therapeutic development.