Exploring Iron Deposition Patterns Using Light and Electron Microscopy in the Mouse Brain Across Aging and...

Alzheimer's disease (AD) involves cognitive decline, possibly via multiple concurrent pathologies associated with iron accumulation. To investigate if iron accumulation in AD is more likely due to pathological iron-rich compartments, or a compensatory response of iron within oligodendrocytes to disease progression, we sought to quantify iron-rich staining (via Perl's diaminobenzidine; DAB). Healthy wild-type (WT) and APP^Swe-PS1Δe9 (APP-PS1; amyloid-beta overexpressing) male mice were examined during middle age, at 14 months. The frontal cortex, a brain region affected over the course of dementia progression, was investigated. Iron-rich compartments were found across genotypes, including oligodendrocytes and immune cells at the blood-brain barrier, and exclusively amyloid plaques in the APP-PS1 genotype. A semi-automated approach was employed to quantify the staining intensity of iron-rich compartments with light microscopy. Mouse frontal cortex of each genotype was also assessed qualitatively and ultrastructurally with scanning electron microscopy, to novelly discern and confirm iron-rich staining (via Perl's DAB). We found parenchymal iron staining corresponding to oligodendrocytes, pericytes, astrocytes, microglia and/or infiltrating macrophages, and amyloid plaques; increased iron deposition and clustering were detected in middle-aged male APP-PS1 versus WT frontal cortex, supporting that AD pathology may involve greater brain iron levels and local clustering. Unexpectedly, iron-rich cells were enriched at the central nervous system (CNS) interface and perivascular space in control and APP-PS1 mouse models, with ultrastructural examination revealing examples of these cells loaded with many secretory granules containing iron. Together, our results provide novel exploration and confirmation of iron-rich cells/compartments in scanning electron microscopy and reinforce literature that iron deposition is relatively increased in AD over healthy cognitive aging and involves greater local clusters of iron burden. Increased iron burden along the aging trajectory, regardless of cognitive status, may also be attributed to novelly discovered iron-rich cells secreting granules along the CNS border.