Cadmium-cardiolipin disruption of respirasome assembly and redox balance through mitochondrial membrane rigidification

The environmental pollutant cadmium (Cd) poses a threat to human health through the consumption of contaminated foodstuffs culminating in chronic nephrotoxicity. Mitochondrial dysfunction and excessive reactive oxygen species (ROS) are key to Cd cellular toxicity. Cd-lipid interactions have been less considered. We hypothesized Cd binding to the inner mitochondrial membrane (IMM) phospholipid cardiolipin (CL) and membrane rigidification underlies defective electron transfer by disrupted respiratory supercomplexes (SCs). In Cd-treated rat kidney cortex (rKC) mitoplasts, laurdan (lipid-water interface), and diphenylhexatriene (hydrophobic core) revealed increased and decreased membrane fluidity, respectively. Laurdan-loaded pure CL or IMM biomimetic (40 mol % POPC, 35 mol % DOPE, 20 mol % TOCL, 5 mol % SAPI) nanoliposomes were rigidified by 25 μM Cd, which was confirmed in live-cell imaging of laurdan or di-4-ANEPPDHQ loaded human proximal convoluted tubule (HPCT) cells. Blue native gel electrophoresis evidenced ∼30% loss of I+III₂+IV_n SC formation after 5 μM Cd for 6 h in HPCTs, which was reversed by CL-binding drug MTP-131/SS-31/elamipretide (0.1 μM), yet α-tocopherol-insensitive. Moreover, MTP-131 attenuated Cd-induced H₂O₂ (∼30%) and cytochrome c release (∼25%), but not osmotic swelling, in rKC mitochondria as well as Cd-induced ROS (∼25%) in HPCTs. MTP-131 binding to IMM biomimetic nanoliposomes decreased zeta potential, prevented Cd-induced liposome size increase, and membrane rigidification reported by laurdan. Heterologous CRLS1 expression reversed Cd (5 μM, 24 h) cytotoxicity (∼25%) by MTT assay, Cd (5 μM, 3 h)-induced ROS and mitochondrial membrane rigidification by Cd (1 μM, 1 h) in HPCT cells. In summary, we report a novel mechanism for Cd toxicity in which Cd-CL interactions cause IMM rigidification, thereby disrupting correct SC assembly and increasing ROS.