Alpha-synuclein (aSyn) post-translational modifications (PTM), especially phosphorylation at serine 129 and C-terminal truncations, are highly enriched in Lewy bodies (LB), Lewy neurites, and other pathological aggregates in Parkinson's disease and synucleinopathies. However, the precise role of these PTM in pathology formation, neurodegeneration, and pathology spreading remains unclear. Here, we systematically investigated the role of post-fibrillization C-terminal aSyn truncations in regulating uptake, processing, seeding, and LB-like inclusion formation using a neuronal seeding model that recapitulates LB formation and neurodegeneration. We show that C-terminal cleavage of aSyn fibrils occurs rapidly post exogenous fibril internalization and during intracellular LB-like inclusion formation. Blocking cleavage of internalized fibrils does not affect seeding, but inhibiting enzymes such as calpains 1 and 2 alters LB-like inclusion formation. We show that C-terminal truncations, along with other PTMs, regulate fibril interactome remodeling, shortening, lateral association, and packing. These findings reveal distinct roles of C-terminal truncations at different aggregation stages on the pathway to LB formation, highlighting the need for consideration of stage‑specific strategies to target aSyn proteolytic cleavages.