Homocysteine binds copper and induces COX deficiency and apoptosis in different neuron-like cell culture models preventable by copper supplementation

Objective: Severe hyperhomocysteinemia in children with hereditary homocystinuria often presents with a variety of neurological symptoms like mental retardation and seizures, and mild hyperhomocysteinemia has been associated with neurological impairment of the elderly. The causative link of hyperhomocysteinemia and neurological dysfunction is unknown.

Methods: We investigated the mechanisms of homocysteine (HCys) neurotoxicity in rat dopaminergic pheochromocytoma (PC12) cells, human neuroblastoma (SHSY) cells and primary rat cerebellar granule neurons (CGN).

Results: HCys dose-dependently induced apoptotic cell death. During 24h of incubation with HCys in LD50 (10mmol/L), cytochrome c oxidase (COX) activity rapidly decreased, whereas reactive oxygen species (ROS) initially decreased and began to increase, after COX deficiency and apoptosis had occurred (figure 1). Spectrum analysis revealed that HCys incubation made COX unstable. We suggest that HCys binds the COX cofactor copper (Cu2+) and thereby leads to a functional lack of available Cu2+ with subsequent COX dysfunction. Accordingly, Cu2+ supplementation prior to HCys treatment prevented both the decrease of COX activity and cell death (figure 2).

Conclusions: Cu2+ binding to HCys and loss of COX activity are key mechanisms of HCys neurotoxicity, whereas oxidative stress does not seem to be causatively involved.