This study investigates the molecular signatures of tau and alpha-synuclein (aSyn) in Alzheimer's disease (AD) cases with and without Lewy bodies (AD-LB), pure AD, and Parkinson's disease with dementia (PDD). The researchers used a range of tau and aSyn antibodies targeting different epitopes and post-translational modifications (PTMs) to analyze brain tissue from the middle temporal gyrus (MTG) and amygdala (AMY) of clinically diagnosed and pathologically confirmed cases. Key findings include: 1. **Phospho-tau (pTau) Species**: All investigated pTau species, except pT181, were upregulated in AD-LB and AD cases compared to PDD and control cases. However, no significant differences were observed between AD-LB and AD subjects. 2. **PTM-Specific Antibodies**: Tau antibodies targeting proline-rich regions and the C-terminus showed preferential binding to AD-LB and AD brain homogenates. C-terminal aSyn epitopes and pS129 aSyn showed stronger binding to AD-LB and PDD cases compared to AD and control cases. 3. **Specific pTau Species**: Two pTau species (pS198 and pS396) were specifically detected in the soluble protein fractions of AD-LB and AD subjects, indicating early involvement of these PTMs in tau multimerization. 4. **aSyn Load**: Higher aSyn load was observed in the AMY of AD-LB cases compared to PDD cases, suggesting aggravated aSyn pathology under the presence of AD pathology. Tau load was similar between AD-LB and AD cases. 5. **Co-localization**: Co-localization of pTau and aSyn was observed within astrocytes of AD-LB cases in the MTG. These findings highlight a unique pathological signature for AD-LB cases compared to pure AD and PDD cases, emphasizing the distinct roles of tau and aSyn in these conditions.This study investigates the molecular signatures of tau and alpha-synuclein (aSyn) in Alzheimer's disease (AD) cases with and without Lewy bodies (AD-LB), pure AD, and Parkinson's disease with dementia (PDD). The researchers used a range of tau and aSyn antibodies targeting different epitopes and post-translational modifications (PTMs) to analyze brain tissue from the middle temporal gyrus (MTG) and amygdala (AMY) of clinically diagnosed and pathologically confirmed cases. Key findings include: 1. **Phospho-tau (pTau) Species**: All investigated pTau species, except pT181, were upregulated in AD-LB and AD cases compared to PDD and control cases. However, no significant differences were observed between AD-LB and AD subjects. 2. **PTM-Specific Antibodies**: Tau antibodies targeting proline-rich regions and the C-terminus showed preferential binding to AD-LB and AD brain homogenates. C-terminal aSyn epitopes and pS129 aSyn showed stronger binding to AD-LB and PDD cases compared to AD and control cases. 3. **Specific pTau Species**: Two pTau species (pS198 and pS396) were specifically detected in the soluble protein fractions of AD-LB and AD subjects, indicating early involvement of these PTMs in tau multimerization. 4. **aSyn Load**: Higher aSyn load was observed in the AMY of AD-LB cases compared to PDD cases, suggesting aggravated aSyn pathology under the presence of AD pathology. Tau load was similar between AD-LB and AD cases. 5. **Co-localization**: Co-localization of pTau and aSyn was observed within astrocytes of AD-LB cases in the MTG. These findings highlight a unique pathological signature for AD-LB cases compared to pure AD and PDD cases, emphasizing the distinct roles of tau and aSyn in these conditions.