In subsequent
studies, low-molecular weight caspase inhibitors have also been examined
for their effect on neurotrophin withdrawal-induced PCD. Milligan et al.
demonstrated that trophic factor withdrawal-induced death of motor neurons
in vitro was delayed by Ac-YVAD-aldehyde, but not by leupeptin
(another aldehyde protease inhibitor) or E64 (a calpain inhibitor).168 Interestingly,
administration of Ac-YVAD-aldehyde to chick embryos inhibited apoptosis
occurring spontaneously in spinal cord motoneurons in vivo, but not
apoptosis provoked by extirpation of limb buds.168 In a similar fashion, ZVAD-
fluoromethylketone delayed the occurrence of apoptosis in PC12 cells
after NGF withdrawal.169,170 Interestingly, activation of c-Jun N-terminal
kinase (JNK) occurred normally in these PC12 cells,171 suggesting that protease
activation is downstream of JNK activation. In a similar vein, studies
by Deshmukh et al.138 indicated that Boc-Aspartyl(OMe)-fluoromethylketone
inhibited apoptotic morphological changes in rat sympathetic neurons
deprived of NGF in vitro, even though this caspase inhibitor had no effect
on the expression of immediate early genes.