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Loss of Rb activates both p53-dependent and independent cell death pathways in the developing mouse nervous system
1Center for Cancer Research, Department of Biology, Massachusetts Institute of Technology, Cambridge 02139, USA.
Abstract:
Extensive apoptosis occurs in the nervous system of mouse embryos homozygous mutant for a targeted disruption of the retinoblastoma (Rb) gene. This cell death is present in both the central (CNS) and peripheral nervous systems (PNS) and is associated with abnormal S phase entry of normally post-mitotic neurons. Aberrant proliferation in the CNS correlates with increased free E2F DNA binding activity and increased expression of cyclin E, an E2F target gene and critical cell cycle regulator. Cell death in the CNS is accompanied by increased levels of the p53 tumor suppressor gene product and increased expression of the p53 target gene, p21Waf-1/Cip-1. However, induction of p53 is not observed in the PNS of Rb-mutant embryos, nor does loss of p53 function inhibit cell death in the PNS. Surprisingly, p21Waf-1/Cip-1 is induced in the sensory ganglia of Rb-mutant embryos in a p53-independent manner. Although loss of p53 gene function prevents cell death in the CNS of Rb-mutant embryos, it does not restore normal proliferative control.
Insights
Retinoblastoma (Rb) gene disruption in mouse embryos causes extensive nervous system apoptosis. p53-independent mechanisms contribute to cell death in the peripheral nervous system, despite p53’s role in the central nervous system.
Area of Science:
- Developmental Biology
- Molecular Biology
- Neuroscience
Background:
- The retinoblastoma (Rb) protein is a critical regulator of the cell cycle.
- Loss of Rb function can lead to uncontrolled cell proliferation and tumor formation.
- Apoptosis, or programmed cell death, is essential for proper development and tissue homeostasis.
Purpose of the Study:
- To investigate the role of the retinoblastoma (Rb) gene in embryonic nervous system development.
- To elucidate the mechanisms of cell death and aberrant proliferation in Rb-mutant mouse embryos.
- To determine the involvement of p53 and its downstream targets in Rb-deficient neurodevelopment.
Main Methods:
- Generation of mouse embryos with a targeted disruption of the retinoblastoma (Rb) gene.
- Analysis of apoptosis, cell cycle progression (S phase entry), and gene expression in the central nervous system (CNS) and peripheral nervous system (PNS).
- Assessment of E2F DNA binding activity, cyclin E expression, and p53 and p21Waf-1/Cip-1 levels.
- Evaluation of the effects of p53 loss-of-function on cell death and proliferation in Rb-mutant embryos.
Main Results:
- Extensive apoptosis was observed in both the CNS and PNS of Rb-mutant mouse embryos.
- Abnormal S phase entry of post-mitotic neurons and aberrant CNS proliferation correlated with increased E2F activity and cyclin E expression.
- Cell death in the CNS was associated with increased p53 and p21Waf-1/Cip-1, while PNS apoptosis occurred independently of p53 induction.
- p21Waf-1/Cip-1 was induced in the PNS in a p53-independent manner.
- Loss of p53 function prevented CNS cell death but did not restore normal proliferative control.
Conclusions:
- Loss of Rb function in mouse embryos leads to widespread apoptosis in the developing nervous system.
- Aberrant cell cycle re-entry and proliferation contribute to neurodevelopmental defects in Rb mutants.
- Distinct p53-dependent and p53-independent pathways mediate apoptosis in the CNS and PNS, respectively, in the absence of Rb.