TDP-43 Coincides with Noradrenergic Neurons in Alzheimer’s Disease (AD) and Primary Age-Related Tauopathy (PART)

TAR DNA-binding protein 43 (TDP-43) pathology is a major contributor to cognitive decline in Alzheimer’s disease (AD), yet the cellular substrates and circuit-level consequences of its limbic accumulation remain poorly defined. Here, we combined multiplex immunofluorescence with 3D Artificial Intelligence (AI)-based object detection and quantitative colocalization to map phosphorylated TDP-43 (pTDP-43) across neuronal subtypes in the amygdala, hippocampal subfields, and entorhinal cortex (ERC) in human AD, AD with Lewy body disease (AD-LBD), and primary age-related tauopathy (PART). Across 15 deeply phenotyped cases, pTDP-43 showed preferential accumulation in neuromodulatory noradrenergic neurons, with more limited involvement of glutamatergic terminals and cholinergic populations. Noradrenergic neurons marked by dopamine β-hydroxylase (DβH) demonstrated extensive colocalization with pTDP-43 across regions, correlating with APOE genotype, amyloid burden, and tau staging in this cross-sectional cohort, suggesting that neuromodulatory circuits might represent a major convergence point for AD-related molecular stressors. Dopaminergic (tyrosine hydroxylase (TH)) and cholinergic (choline acetyltransferase (ChAT)) neurons showed more restricted but pathology-dependent involvement, with LBD consistently showing lower pTDP-43 burden across multiple transmitter systems. Glutamatergic vesicular glutamate transporter-1 (VGLUT1)-positive neurons and terminals exhibited minimal colocalization except in PART. Principal component (PCA) and hierarchical clustering analyses (HCA) integrating pTDP-43 colocalization with neuropathologic staging metrics (A, B, C, Thal, Braak, LBD) reliably separated PART, AD-TDP, and AD-TDP-LBD groups, with LBD emerging as a dominant modifier of cellular vulnerability. Together, these findings point out that TDP-43 pathology in AD preferentially targets neuromodulatory circuits, is strongly shaped by coexisting synucleinopathy, and provides a cell-type-resolved framework for understanding how TDP-43 contributes to limbic network dysfunction in neurodegeneration.