Caspase-dependent cleavage of the retinoblastoma protein is an early step in neuronal apoptosis

Rb-deficient embryos (Rb-/-) show abnormal degeneration of neurons and die at mid-gestation, suggesting that RE may protect against apoptosis. Having previously shown that cyclin D1 accumulates during K+-induced apoptosis of granule neurons, we chose to investigate the role of RB under these conditi ons. We show that RB is cleaved in its C-terminus during the onset of neuro nal apoptosis. Caspase 3-like activity increases following K+ deprivation a nd the time course correlates with RB cleavage and apoptosis, Although the use of a specific caspase 3-like inhibitor (z-DEBD.fmk) delays RB cleavage and reduces DNA fragmentation, data implicate other caspases in these proce sses. However K+ deprivation induces a gradual production of the active p20 subunit of caspase 3 (CPP32) that coincides with RB disappearance at the c ellular level. Nuclear detection of a transfected HA-tagged caspase cleavag e-resistant RB mutant (DEAG/D to DEAA/D) revealed a significant decrease in apoptosis of neurons expressing the RB mutant (less than 5%) relative to t he wild type form of RB (40%) during K+ deprivation. Taken together, these data show that caspase-dependent cleavage of RB is an early permissive step of the apoptosis-inducing signaling pathway in neurons, They indicate a ma jor role of RB in neuronal protection.