Molecular control of centrosome copy number

Centrosomes are major microtubule organizing centers that play an important role in organizing the two poles of the bipolar microtubule spindle apparatus upon which chromosomes are segregated. Centrosomes are one of only two structures in mammalian cells whose copy number is precisely controlled (the other structure is the chromosome). At the beginning of the cycle, cells contain exactly one centrosome that duplicates once, and only once, before the next cell division to ensure that cells possess only two centrosomes when they divide. Cancer cells often acquire extra copies of the centrosome which leads to frequent chromosome segregation errors, creating an abnormal and evolving chromosome content.

 

Despite the fundamental role that centrosomes play in ensuring the fidelity of genome transmission during mitosis, we have a limited understanding of the molecular pathways that allow cells to create a single centrosome per cycle. The weight of evidence demonstrates that centrosome duplication is under the control of Polo-like kinase 4 (Plk4): loss of Plk4 results in a failure of centrosome duplication while increase levels of the kinase drives the assembly of excessive new centrosomes. This defines Plk4 as a dose-dependent regulator of centrosome duplication. Determining how the levels and activity of Plk4 are regulated is important for understanding how centrosome duplication is tightly controlled to maintain genome stability.

 

We discovered that Plk4 is a self-regulated kinase whose stability is directly linked to its own activity. Active Plk4 was shown to phosphorylate itself, thereby targeting itself for capture by the SCF-TrCP E3 ubiquitin ligase and promoting ubiquitination and destruction of the kinase (Holland et al., JCB 2010). To directly test whether the autoregulated instability of endogenous Plk4 acts to limit normal centriole duplication to once per cell cycle, we used Adeno-Associated Virus (AAV)-mediated gene targeting to create human somatic cells with conditional, reactivatable alleles of Plk4 that are deficient in Plk4 autoregulation. Using these genetically engineered cells, we demonstrated that the autoregulated destruction of Plk4 acts to control Plk4 abundance and limit centrosome duplication to once per cell cycle (Holland et al., Genes and Dev, 2012). We also showed that centrosome amplification leads to activation of p53 and loss of cell proliferation. Importantly, removal of p53 overcomes this proliferative block, providing a mechanism for how loss of p53 could lay the foundation for centrosome amplification and further genomic instability.