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Genetic evidence for the interactions of cyclin D1 and p27(Kip1) in mice
1Department of Developmental and Molecular Biology, Center for the Study of Reproductive Biology and Women's Health, Albert Einstein College of Medicine, New York, New York 10461, USA.
Abstract:
The cell cycle of cultured cells appears to be regulated by opposing actions of the cyclins together with their partners, the cyclin-dependent kinases (Cdk), and their inhibitors (Cki). Consistent with this situation null mutations in the genes for cyclin D1 and Cki p27(Kip1) in mice give opposite phenotypes of dwarfism and gigantism. To test their genetic interactions, we generated mice nullizygous for both genes. Correction of cyclin D1 or p27 null to wild-type phenotypes was observed for many but not all traits. These included, for cyclin D1(-/-) mice, body weight, early lethality, retinal hypoplasia, and male aggressiveness and, for p27(-/-) mice, body weight, retinal hyperplasia, and embryo implantation. p27(-/-) traits that were not corrected were the aberrant estrus cycles, luteal cell proliferation, and susceptibility to pituitary tumors. This mutual correction of these phenotypes is the first genetic demonstration of the interaction of these inhibitory and stimulatory cell cycle-regulatory molecules in vivo. The molecular basis for the correction was analyzed in the neonatal retina. Retinal cellularity was rescued in the cyclin D1 null mouse by loss of p27 with only a partial restoration of phosphorylation of retinoblastoma protein (Rb) and Cdk4 activity but with a dramatic elevation of Cdk2 activity. Our data provide in vivo genetic validation of cell culture experiments that indicated that p27 acts as a negative regulator of cyclin E-Cdk2 activity and that it can be titrated away by cyclin D-Cdk4 complexes. It also supports the suggestion that the cyclin E/Cdk2 pathway can largely bypass Rb in regulating the cell cycle in vivo.
Insights
This study demonstrates how cyclin D1 and Cki p27(Kip1) interact in vivo to regulate cell cycle. Genetic analysis in mice reveals their opposing roles and mutual correction of phenotypes, validating cell culture findings.
Area of Science:
- Cell Biology
- Genetics
- Developmental Biology
Background:
- Cell cycle regulation involves opposing actions of cyclins, cyclin-dependent kinases (Cdk), and cyclin-dependent kinase inhibitors (Cki).
- Null mutations in cyclin D1 and Cki p27(Kip1) genes in mice result in opposing phenotypes: dwarfism and gigantism, respectively.
Purpose of the Study:
- To investigate the in vivo genetic interactions between cyclin D1 and Cki p27(Kip1).
- To determine if genetic inactivation of one component can rescue phenotypes caused by the absence of the other.
Main Methods:
- Generation of mice nullizygous for both cyclin D1 and p27(Kip1) genes.
- Phenotypic analysis of single and double knockout mice, including body weight, lethality, retinal development, reproductive traits, and tumor susceptibility.
- Molecular analysis of the neonatal retina to understand the basis of phenotypic correction.
Main Results:
- Mutual correction of many phenotypes was observed in double knockout mice, indicating genetic interaction.
- Specific traits like body weight, retinal hypoplasia/hyperplasia, and embryo implantation were corrected.
- Some p27(-/-) traits, including aberrant estrus cycles and pituitary tumor susceptibility, remained uncorrected.
- Molecular analysis revealed that loss of p27 rescued retinal cellularity in cyclin D1 null mice by modulating Cdk2 activity.
Conclusions:
- This study provides the first in vivo genetic evidence for the interaction between stimulatory (cyclin D1) and inhibitory (p27) cell cycle regulators.
- The findings validate cell culture models showing p27 inhibits cyclin E-Cdk2 activity and can be titrated by cyclin D-Cdk4 complexes.
- The results suggest the cyclin E/Cdk2 pathway can bypass retinoblastoma protein (Rb) in regulating the cell cycle in vivo.