INTEGRATIVE DEFENSE SYSTEMS IN THE AGING BRAIN: NEUROPROTECTIVE ROLES OF NEUROPLASTICITY, ANTIOXIDANT CASCADES, NEUROTROPHIC SIGNALING, AND GLYMPHATIC WASTE CLEARANCE

Brain aging is a multi-systemic process characterized by a coordinated decline in structural integrity, proteostasis, metabolic clearance, and cognitive flexibility. Rather than viewing these deficits as isolated pathologies, current research conceptualizes them as interconnected nodes within a consolidated homeostatic defense system. This review provides a comprehensive synthesis of the integrated defense systems of the aging brain, detailing the complex crosstalk between morphological neuroplasticity, enzymatic antioxidant networks, neurotrophic factors, and the glymphatic system. Morphologically, senescence is defined by the regression of dendritic arbors and a selective loss of stable mushroom spines, a process mediated by the downregulation of membrane-deforming Bin/Amphiphysin/Rvs (BAR) domain proteins. Synaptically, cognitive decline correlates with region-specific losses of vesicular glutamate transporter-1 (VGluT1) in the dorsal hippocampus, while cognitive resilience is associated with the preservation of vesicular GABA transporter (VGAT). Concurrently, transcription factor dysregulation and cofactor depletion impair the antioxidant triad of superoxide dismutase, catalase, and glutathione peroxidase. Crucially, unchecked oxidative stress triggers caveolin-1 phosphorylation, driving the depolarization of astrocytic aquaporin-4 (AQP4) water channels. This disruption of AQP4 polarity collapses convective cerebrospinal fluid-interstitial fluid (CSF-ISF) exchange, severely limiting the clearance of amyloid-beta, hyperphosphorylated tau, and metabolic waste. This clearance failure is further compounded by age-related vascular remodeling and immunosenescence. Downregulated neurotrophin signaling compromises synaptic consolidation while promoting pro-inflammatory glial phenotypes. We explore these interactions through the lens of cognitive reserve, highlighting non-invasive diffusion tensor imaging along the perivascular space (DTI-ALPS) as a key structural and functional biomarker of brain resilience.