A Critical Temporal Requirement for the Retinoblastoma Protein Family During Neuronal Determination

Abstract

In this report, we have examined the requirement for the retinoblastoma (Rb) gene family in neuronal determination with a focus on the developing neocortex. To determine whether pRb is required for neuronal determination in vivo, we crossed the Rb−/− mice with transgenic mice expressing β-galactosidase from the early, panneuronal Tα1 α-tubulin promoter (Tα1:nlacZ). In E12.5 Rb−/− embryos, the Tα1:nlacZ transgene was robustly expressed throughout the developing nervous system. However, by E14.5, there were perturbations in Tα1:nlacZ expression throughout the nervous system, including deficits in the forebrain and retina. To more precisely define the temporal requirement for pRb in neuronal determination, we functionally ablated the pRb family in wild-type cortical progenitor cells that undergo the transition to postmitotic neurons in vitro by expression of a mutant adenovirus E1A protein. These studies revealed that induction of Tα1:nlacZ did not require proteins of the pRb family. However, in their absence, determined, Tα1:nlacZ-positive cortical neurons underwent apoptosis, presumably as a consequence of “mixed signals” deriving from their inability to undergo terminal mitosis. In contrast, when the pRb family was ablated in postmitotic cortical neurons, there was no effect on neuronal survival, nor did it cause the postmitotic neurons to reenter the cell cycle. Together, these studies define a critical temporal window of requirement for the pRb family; these proteins are not required for induction of neuronal gene expression or for the maintenance of postmitotic neurons, but are essential for determined neurons to exit the cell cycle and survive.

E2F2 converts reversibly differentiated PC12 cells to an irreversible, neurotrophin-dependent state

“…Previous work has established a central role for Rb and E2F family members in cell cycle progression and exit by neuronal progenitors (Callaghan et al, 1999;Kastner et al, 1998;Slack et al, 1998). The functional results presented here, along with the demonstration that E2F2 is expressed in postmitotic neurons such as CGNs, suggest that E2F2 has an important role in maintaining the terminally arrested and di erentiated state of postmitotic neurons.…”

Id proteins at the cross-road of development and cancer

“…This model is illustrated by the control of Rb upon Id2, a critical mechanism to prevent inappropriate Id2 signaling in di erentiating neurons (Lasorella et al, 2000). In the absence of Rb, neurons undergo ectopic proliferation and apoptosis (Lee et al, 1994;Slack et al, 1998). Interestingly, these defects are rescued by ablation of Id2 in vivo and are recapitulated in vitro by overexpression of Id2 in normal cortical progenitors (Lasorella et al, 2000;Toma et al, 2000).…”

Rb regulates proliferation and rod photoreceptor development in the mouse retina

“…There is also evidence that Rb has a role in cell fate specification [9][10][11] . In support of this notion, Rb can bind more than 110 different proteins, several of which are tissue-restricted transcription factors 12 .…”