Protective Effect of Parvalbumin on Excitotoxic Motor Neuron Death

The mechanism responsible for the selective vulnerability of motor neurons in amyotrophic lateral sclerosis (ALS) is poorly understood. Several lines of evidence indicate that susceptibility of motor neurons to Ca2+ overload induced by excitotoxic stimuli is involved. In this study, we investigated whether the high density of Ca2+-permeable AMPA receptors on motor neurons gives rise to higher Ca2+ transients in motor neurons compared to dorsal horn neurons. Dorsal horn neurons were chosen as controls as these cells do not degenerate in ALS. In cultured spinal motor neurons, the rise of the cytosolic Ca2+ concentration induced by kainic acid (KA) and mediated by the AMPA receptor was almost twice as high as in spinal neurons from the dorsal horn. Furthermore, we investigated whether increasing the motor neuron's cytosolic Ca2+-buffering capacity protects them from excitotoxic death. To obtain motor neurons with increased Ca2+ buffering capacity, we generated transgenic mice overexpressing parvalbumin (PV). These mice have no apparent phenotype. PV overexpression was present in the central nervous system, kidney, thymus, and spleen. Motor neurons from these transgenic mice expressed PV in culture and were partially protected from KA-induced death as compared to those isolated from nontransgenic littermates. PV overexpression also attenuated KA-induced Ca2+ transients, but not those induced by depolarization. We conclude that the high density of Ca2+-permeable AMPA receptors on the motor neuron's surface results in high Ca2+ transients upon stimulation and that the low cytosolic Ca2+-buffering capacity of motor neurons may contribute to the selective vulnerability of these cells in ALS. Overexpression of a high-affinity Ca2+ buffer such as PV protects the motor neuron from excitotoxicity and this protective effect depends upon the mode of Ca2+ entry into the cell.