In order to divide, cells must faithfully duplicate their entire contents, and then segregate the duplicated contents equally into two daughter cells.  Duplication events and segregation events must be triggered in a strict temporal order to ensure that each daughter cell receives one complete genome.  My lab's research is focused on how a conserved family of cell cycle regulatory protein complexes, called cyclin-dependent kinases (Cdks), coordinate duplication and segregation events.  We employ powerful genetic, molecular and cellular approaches in the budding yeast, Saccharomyces cerevisia.

The yeast analog of the centrosome, called a spindle pole body (SPB), directs the formation of a bipolar spindle that is essential for the faithful segregation of chromosomes at mitosis.  We have found that the disruption of specific Cdk activities leads to the re-duplication of SPBs in the absence of mitosis.  We have also shown that the amplification of SPBs can lead to the formation of multi-polar spindles and the mis-segregation of DNA, a phenotype commonly observed in tumor cells.  One focus of the lab is to understand how Cdk activities insure that the SPB duplicates only once each cell cycle.  Similarly, we have observed that cells disrupted for specific cyclin activities undergo multiple rounds of DNA replication in the absence of mitosis, giving rise to polyploid cells.   As we have seen for SPB duplication, Cdk activities are essential for preventing the re-initiation of replication until the completion of mitosis. While both SPB duplication and DNA replication are regulated by Cdk activities, our findings suggest that they are regulated by different mechanisms.

Our work with cells disrupted for cyclin genes has suggested the existence of an independent cell cycle oscillator.  In somatic cells and yeast, checkpoint controls insure that the initiation of cell cycle events is dependent on the completion of the preceding events.  However, in several embryonic systems, cell cycle events can be initiated independently of the completion of preceding events, suggesting that cell cycle events may be triggered by a free-running oscillator.  We have observed the periodic activation of early cell cycle events in yeast cells lacking mitotic cyclin activities that are required for mitotic progression.  Periodic activation of events was also observed in cells lacking all Cdk activities, suggesting that a Cdk-independent oscillator activity may trigger the initiation of early cell cycle events.  One goal of the lab will be to identify the components and characterize the functions of this independent oscillator.

It is well established that transcriptional control of important regulators is fundamental to the proper regulation of cell cycle events. Cdk complexes are thought to play an essential role in regulating the temporal activation of transcription during the cell cycle. We are currently investigating how specific cyclin/Cdk complexes regulate the cell cycle transcription program at a systems level.  These studies combine traditional cell and molecular biology with statistical and computational approaches. 

Microscope images showing a yeast cell as it buds and divides. DNA is stained blue and spindle pole bodies (SPBs) are stained in green.

Disruption of specific CDK activities leads to reduplication of SPBs. Above: An electron micrographs of a cell nucleus. Duplicated pairs of SPBs are numbered 1,2,3,4. Below: A light micrograph of a similar cell. DNA is stained in blue and SPBs are stained in green.