Activity-dependent regulation of the BAX/BCL-2 pathway protects cortical neurons from apoptotic death during early development

During early brain development, homeostatic removal of cortical neurons is crucial and requires multiple control mechanisms. We investigated in the cerebral cortex of mice whether the BAX/BCL-2 pathway, an important regulator of apoptosis, is part of this machinery and how electrical activity might serve as a set point of regulation. Activity is known to be a pro-survival factor; however, how this effect is translated into enhanced survival chances on a neuronal level is not fully understood. In this study, we show that caspase activity is highest at the neonatal stage, while developmental cell death peaks at the end of the first postnatal week. During the first postnatal week, upregulation of BAX is accompanied by downregulation of BCL-2 protein, resulting in a high BAX/BCL-2 ratio when neuronal death rates are high. In cultured neurons, pharmacological blockade of activity leads to an acute upregulation of Bax, while elevated activity results in a lasting increase of BCL-2 expression. Spontaneously active neurons not only exhibit lower Bax levels than inactive neurons but also show almost exclusively BCL-2 expression. Disinhibition of network activity prevents the death of neurons overexpressing activated CASP3. This neuroprotective effect is not the result of reduced caspase activity but is associated with a downregulation of the BAX/BCL-2 ratio. Notably, increasing neuronal activity has a similar, non-additive effect as the blockade of BAX. Conclusively, high electrical activity modulates BAX/BCL-2 expression and leads to higher tolerance to CASP3 activity, increases survival, and presumably promotes non-apoptotic CASP3 functions in developing neurons.