Multiscale characterization and distribution of the thyroid hormone transporter MCT8 in the adult and aged human brain.

The local availability of thyroid hormones (TH) depends on TH transporters such as monocarboxylate transporter 8 (MCT8, encoded by SLC16A2). Mutations of SLC16A2 cause Allan-Herndon-Dudley syndrome (AHDS), a disorder characterized by severe neurodevelopmental and motor deficits. While MCT8's importance in foetal brain development is well-established, its precise topographical, cellular, and subcellular distribution in the adult human brain has remained largely uncharacterized, limiting our understanding of which structures depend on MCT8-mediated TH transport throughout life and whether neurodegenerative processes such as Alzheimer's disease (AD) affect local transporter availability. In this study, we systematically mapped MCT8 expression across eight functionally distinct brain regions from 14 aged human body donors. Our multiscale approach combined immunostaining, automated whole-slide image analysis of over 13 million cells, multiplex RNAscope in situ hybridization, stimulated emission depletion (STED) microscopy, and single-nucleus RNA sequencing data analysis. We additionally applied standardized Braak staging to investigate whether AD progression correlates with alterations in MCT8 expression. We demonstrate robust MCT8 expression across brain barriers, glial cells, and in distinct neuronal populations. RNAscope confirmed a strong concordance between SLC16A2 mRNA presence and MCT8 immunoreactivity. STED and confocal microscopy localized MCT8 to plasma membranes, perinuclear regions, and axonal/dendritic compartments, indicating roles in both trans- and intracellular TH transport. MCT8 expression was preserved across Braak stages, with no significant difference between donors with varying degrees of neurofibrillary pathology. Our findings establish a comprehensive cellular and regional framework for MCT8-mediated transport in the human brain, mapping its specific anatomical distribution across regions crucial for motor control and cognitive processing. Stable MCT8 expression across Braak stages suggests that cerebral TH dysregulation in AD likely reflects altered hormone metabolism rather than transporter loss.