Co-aggregation of amyloidogenic proteins in age-related neurodegenerative diseases

Age-related neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), and related dementias, are increasingly understood as multifactorial proteinopathies involving co-aggregation of amyloidogenic proteins such as microtubule-associated protein-Tubulin-associated unit protein (Tau), α-synuclein (α-syn), amyloid-β (Aβ), and TAR DNA-binding protein 43 (TDP-43). Rather than acting independently, these proteins often cross-seed, co-localize, and modulate each other's aggregation dynamics and toxicity. This review critically examines the mechanistic and pathological underpinnings of heterotypic protein co-aggregation, integrating biophysical, cellular, animal, and human data. This review further proposes a conceptual framework that views neurodegeneration as a network of interacting misfolded proteins shaped by age-related changes in lipid membranes, redox balance, proteostasis, and genetic factors. Emphasis is placed on translational opportunities: co-aggregation-specific biomarkers in cerebrospinal fluid and extracellular vesicles, and emerging multi-targeted therapies including immunotherapy, proteostasis modulators, and autophagy-inducing chimeras. This review also discusses the clinical implications of co-pathology in mixed dementias and overlapping disorders. It is therefore time to move beyond the classical one protein-one disease paradigm and embrace models that explicitly incorporate heterotypic co-aggregation, mixed pathologies, and shared vulnerability pathways across age-related disorders. By reframing co-aggregation as a central pathogenic mechanism, this review highlights the need for diagnostics and therapeutics that address the interconnectivity of protein misfolding in the ageing brains.