Understanding why two diseases built on the same misfolded protein — α-synuclein — produce such different clinical outcomes has been one of the central puzzles of neurodegeneration. A high-resolution structural study now offers a partial answer, and it lies not in the inclusions themselves, but in what surrounds them.
Using large-scale correlative light and electron microscopy (CLEM) applied to postmortem brain tissue from clinically confirmed Parkinson's disease (PD) and dementia with Lewy bodies (DLB) donors, researchers systematically mapped α-synuclein pathology across four distinct brain regions: the entorhinal cortex, anterior cingulate cortex, hippocampal CA2 region, and substantia nigra. Cortical Lewy inclusions displayed striking heterogeneity in maturation stage — from loosely organized fibrillar networks interspersed with intact organelles to densely compacted fibrillar masses — suggesting that cortical pathology represents a dynamic, ongoing process rather than a uniform endpoint. Critically, when PD and DLB were directly compared, no significant ultrastructural difference in Lewy body architecture was detected in either cortical or nigral tissue. The diseases appear to converge on the same protein aggregation machinery. However, a quantitative analysis of more than 10,000 individual mitochondria revealed disease- and region-specific signatures: PD was associated with increased mitochondrial density and enlargement in the substantia nigra, while DLB showed a contrasting pattern. The study also identified a previously unreported population of electron-dense, degenerating, α-synuclein-positive cortical neurons in DLB.
These findings carry important implications for biomarker development and therapeutic targeting. The observation that Lewy body ultrastructure does not distinguish PD from DLB challenges the assumption that inclusion morphology drives differential neurodegeneration. Instead, mitochondrial homeostasis — a well-established vulnerability in dopaminergic neurons — may represent the diverging variable. This is an observational, postmortem study with inherent limitations around causality and donor number, but the methodological rigor of CLEM applied at this scale is notable. The discovery of a novel degenerating neuronal subpopulation in DLB cortex warrants independent replication and could eventually inform staging frameworks for Lewy body dementias.