Challenges in neuronal apoptosis

There are myriads of reasons and ways for a neuron to die, among which apoptosis is a specific form that is processed in two major signaling pathways, the TNF-receptor-mediated (extrinsic) and the mitochondria-based (intrinsic) cell death pathway with several avenues of crosstalk between them. The molecular key players of apoptosis, the importance of the Csp cascade via interaction with different death effector domains and the role of the effector Csp-3 driving the execution of the cell death program are reviewed. Recent data suggest that caspases converge amyloid and tau Alzheimer pathologies: beta amyloid peptide activates caspases which on turn cleave tau and via phosphorylation of tau initiate tangle pathology in both Alzheimer disease and other tauopathies. Several mediators show a bifunctional regulation of apoptosis, with both pro- and anti-apoptotic activities. The latter modify the cell death pathway due to inhibition of Csp activation or other protective mechanisms and may delay it or, via abortive apoptosis ("abortosis") lead to prolonged survival of nerve cells. While the role of apoptosis in neurodegeneration is well documented in tissue culture and transgenic animal models, in human postmortem AD brain its occurrence and role are discussed controversially. Given the short duration required for the completion of apoptosis and the chronic progressive course of neurodegeneration in Alzheimer disease and related disorders, the detection of rare neurons displaying morphological signs of apoptosis and expression of the activated key-executing enzyme Csp-3 is realistic, although there is significantly increased incidence of cells with DNA fragmentation, mainly glia, and markers for a "proapoptotic" environment in the aged human brain indicate increased susceptibility of neurons to metabolic and other noxious factors. Postmortem analysis can bridge some but not all of our knowledge gaps, but the results are still controversial, and we need a better understanding of the molecular basis and pathways that drive the yin-yang between neuronal survival and death.