Regulation of neuronal apoptosis in physiological and pathological conditions

Apoptosis is essential during development of the nervous system. However, inappropriate activation of apoptosis may contribute to neuronal loss during neurodegeneration. Therefore, it is crucial to understand the regulation of apoptosis in physiological and pathological conditions.A second aspect of my studies aims to understand the mechanisms involved in neuronal apoptosis and axonal degeneration induced by beta-amyloid peptide (Abeta), a pivotal player in Alzheimer's disease. Degeneration of basal forebrain cholinergic neurons correlates well with the degree of dementia. Basal forebrain neurons extend their axons to areas of the brain in which accumulation of Abeta is exacerbated. Hence, axons of basal forebrain neurons are preferentially exposed to insults linked to neuronal death in Alzheimer's disease. To mimic this situation in vitro I cultured neurons in compartmented dishes, which allow the treatment of axons independently of cell bodies. I demonstrate that Abeta1-42 causes rapid axonal degeneration, which leads to nuclear apoptosis. Abeta-induced axonal degeneration does not involve activation of caspases in axons, but caspases are activated in cell bodies in response to exposure of distal axons to Abeta. The calpain inhibitor calpastatin provided to distal axons protects from Abeta-induced axonal degeneration and consequently prevents apoptosis. I also demonstrate that Abeta-induced nuclear apoptosis is indistinguishable of apoptosis due to NGF deprivation. Therefore, NGF prevents apoptosis when given to cell bodies of neurons treated with Abeta in distal axons. This result supports the notion that impairment of retrograde transport of NGF or NGF signaling is involved in the death program activated by Abeta.As a paradigm of physiological apoptosis I used sympathetic neurons deprived of nerve growth factor (NGF). Apoptosis activated by NGF withdrawal can be prevented by several mechanisms. I investigated the role of the sphingolipid ceramide. I found that the ceramide analogue, C6-ceramide, as well as endogenous long-chain ceramides support survival of NGF-deprived sympathetic neurons. C6-ceramide induces activation of the NGF receptor TrkA and selective activation of the PI3-kinase/Akt pathway but not the MAPK/ERK pathway. Furthermore, addition of C6-ceramide exclusively to distal axons is sufficient to inhibit nuclear apoptosis. In several cell types, activation of the neurotrophin receptor p75NTR leads to ceramide accumulation. However, I demonstrate that, in sympathetic neurons, p75NTR activation does not generate sufficient ceramide to block apoptosis.