Specific vulnerability of mouse spinal cord motoneurons to membrane depolarization

Intracellular calcium (Ca²⁺) concentration determines neuronal dependence on neurotrophic factors (NTFs) and susceptibility to cell death. Ca²⁺ overload induces neuronal death and the consequences are thought to be a probable cause of motoneuron (MN) degeneration in neurodegenerative diseases. In the present study, we show that membrane depolarization with elevated extracellular potassium (K⁺) was toxic to cultured embryonic mouse spinal cord MNs even in the presence of NTFs. Membrane depolarization induced an intracellular Ca²⁺ increase. Depolarization-induced toxicity and increased intracellular Ca²⁺ were blocked by treatment with antagonists to some of the voltage-gated Ca²⁺ channels (VGCCs), indicating that Ca²⁺ influx through these channels contributed to the toxic effect of depolarization. Ca²⁺ activates the calpains, cysteine proteases that degrade a variety of substrates, causing cell death. We investigated the functional involvement of calpain using a calpain inhibitor and calpain gene silencing. Pre-treatment of MNs with calpeptin (a cell-permeable calpain inhibitor) rescued MNs survival; calpain RNA interference had the same protective effect, indicating that endogenous calpain contributes to the cell death caused by membrane depolarization. These findings suggest that MNs are especially vulnerable to extracellular K⁺ concentration, which induces cell death by causing both intracellular Ca²⁺ increase and calpain activation.