Suppression of Death Receptor Signaling in Cerebellar Purkinje Neurons Protects Neighboring Granule Neurons from Apoptosis via an Insulin-like Growth Factor I-dependent Mechanism

Neuronal apoptosis contributes to the progression of neurodegenerative disease. Primary cerebellar granule neurons are an established in vitro model for investigating neuronal death. After removal of serum and depolarizing potassium, granule neurons undergo apoptosis via a mechanism that requires intrinsic (mitochondrial) death signals; however, the role of extrinsic (death receptor-mediated) signals is presently unclear. Here, we investigate involvement of death receptor signaling in granule neuron apoptosis by expressing adenoviral, AU1-tagged, dominant-negative Fas-associated death domain (Ad-AU1-ΔFADD). Ad-AU1-ΔFADD decreased apoptosis of granule neurons from 65 ± 5 to 27 ± 2% (n= 7, p < 0.01). Unexpectedly, immunocytochemical staining for AU1 revealed that 95% of calbindin-positive Purkinje neurons (∼2% of the cerebellar culture). Granule neurons in proximity to ΔFADD-expressing Purkinje cells demonstrated markedly increased survival. Both granule and Purkinje neurons expressed insulin-like growth factor-I (IGF-I) receptors, and ΔFADD-mediated survival of granule neurons was inhibited by an IGF-I receptor blocking antibody. These results demonstrate that the selective suppression of death receptor signaling in Purkinje neurons is sufficient to rescue neighboring granule neurons that depend on Purkinje cell-derived IGF-I. Thus, the extrinsic death pathway has a profound but indirect effect on the survival of cerebellar granule neurons.