The induction of multiple cell cycle events precedes target-related neuronal death

Unexpected nerve cell death has been reported in several experimental situations where neurons have been forced to re-enter the cell cycle after leaving the ventricular zone and entering the G0, non-mitotic stage. To determine whether an association between cell death and unscheduled cell cycling might be found in conjunction with any naturally occurring developmental events, we have examined target-related cell death in two neuronal populations, the granule cells of the cerebellar cortex and the neurons of the inferior olive. Both of these cell populations have a demonstrated developmental dependency on their synaptic target, the cerebellar Purkinje cell. Two mouse neurological mutants, staggerer (sg/sg) and lurcher (+/Lc), are characterized by intrinsic Purkinje cell deficiencies and, in both mutants, substantial numbers of cerebellar granule cells and inferior olive neurons die due to the absence of trophic support from their main postsynaptic target. We report here that the levels of three independent cell cycle markers--cyclin D, proliferating cell nuclear antigen and bromodeoxyuridine incorporation--are elevated in the granule cells before they die. Although lurcher Purkinje cells die during a similar developmental period, no compelling evidence for any cell cycle involvement in this instance of pre-programmed cell death could be found. While application of the TUNEL technique (in situ terminal transferase end-labeling of fragmented DNA) failed to label dying granule cells in either mutant, light and electron microscopic observations are consistent with the interpretation that the death of these cells is apoptotic in nature. Together, the data indicate that target-related cell death in the developing central nervous system is associated with a mechanism of cell death that involves an apparent loss of cell cycle control.