Increased cerebellar Purkinje cell numbers in mice overexpressing a human bcl-2 transgene

The Purkinje cell is a primary organizer in the development of the cerebellum. Purkinje cells may provide positional information cues that regulate afferent innervation, and Purkinje cell target size controls the adult number of afferent olivary neurons and granule cells. While Purkinje cells are necessary for the survival of olivary neurons and granule cells during periods of programmed cell death, little is known about the survival requirements of Purkinje cells in vivo. To determine if Purkinje cells are subject to programmed cell death during development we have analyzed Purkinje cell numbers in two lines of transgenic mice that overexpress a human gene for bcl-2 (Hu-bcl-2). Bcl-2 is a protooncogene that inhibits apoptosis in many cell types. Overexpression of bcl-2 in vitro and in vivo rescues neurons from trophic factor deprivation or naturally occurring cell death. In the mice analyzed in this study, transgene expression is driven by the neuron-specific enolase promoter that is first expressed embryonically in most regions of the brain in one line and postnatally in the second line. We have counted Purkinje cells in three adult control mice, five early overexpressing transgenics, and three late expressing transgenics. The number of Purkinje cells in the Hu-bcl-2 transgenic mice is significantly increased above control numbers, with an increase of 43% in the embryonically overexpressing line and an increase of 27% in the postnatally overexpressing line. Because bcl-2 overexpression has been shown to rescue other neurons from programmed cell death, the increase in Purkinje cell numbers in overexpressing bcl-2 transgenics suggests that Purkinje cells undergo a period of cell death during normal development.