Understanding why two diseases shaped by the same misfolded protein — α-synuclein — produce such different clinical pictures has long frustrated neurologists. New high-resolution mapping of brain tissue from confirmed Parkinson's disease (PD) and dementia with Lewy bodies (DLB) donors offers a structural answer: the differences may lie not in the inclusions themselves, but in where they accumulate and how mitochondria respond around them.
Using large-scale correlative light and electron microscopy (CLEM), researchers systematically mapped α-synuclein pathology across four brain regions — the entorhinal cortex, anterior cingulate cortex, hippocampal CA2, and substantia nigra — in clinically and pathologically confirmed PD and DLB donors. Cortical inclusions displayed a wide spectrum of maturation stages, from loosely organized fibrillar networks interspersed with intact organelles to densely compacted fibrillar masses. Strikingly, Lewy body ultrastructure was statistically indistinguishable between PD and DLB in both cortical and nigral tissue, challenging the assumption that the two diseases are structurally distinct at the inclusion level. A previously unreported population of electron-dense, degenerating α-synuclein-positive cortical neurons was also identified in DLB. A quantitative analysis of more than 10,000 mitochondria revealed disease- and region-specific signatures: PD showed increased mitochondrial density and enlargement in the substantia nigra, while DLB exhibited distinct cortical mitochondrial alterations.
This work is methodologically significant. CLEM at this scale — spanning multiple donors and regions simultaneously — is rare in human neuropathology and elevates the findings above typical single-region studies. The convergence of Lewy body ultrastructure between PD and DLB suggests that differential clinical severity may be driven more by the spatial burden of pathology and secondary organelle stress than by intrinsic differences in α-synuclein aggregate architecture. The mitochondrial findings are particularly notable: mitochondrial dysfunction has long been implicated in PD through environmental and genetic evidence, but documenting region-specific mitochondrial remodeling at the ultrastructural level in human tissue adds meaningful resolution. A key limitation is the cross-sectional, post-mortem nature of the data — temporal progression of these changes cannot be inferred. Nevertheless, for researchers developing α-synuclein-targeting therapies, the implication that inclusion structure may not be the primary distinguishing variable between these two conditions is potentially paradigm-shifting.