Thomas J. Maresca
University of Massachusetts at Amherst
United States Minor Outlying Islands
We study the vital process of cell division. The goal of cell division is to ensure equal segregation of the genome, which is packaged into a defined number of chromosomes, between two daughter cells. Mistakes in this process lead to cells receiving an incorrect number of chromosomes – a state called aneuploidy. Since aneuploidy is the cause of Down syndrome and is linked to metastatic tumor progression, understanding cell division at its most basic levels has significant relevance to human disease and cancer. In order to succeed at division, cells must coordinate a dizzying array of highly complex cellular events that include chromosome condensation, spindle assembly, and chromosome congression/alignment - to name a few (see Movie 1). On top of this, cells have evolved a biochemical pathway called the spindle assembly checkpoint (SAC) that acts as a surveillance mechanism to ensure that chromosome segregation does not occur prematurely. All of these processes are fair game for research in the Maresca lab but we are particularly focused on combining molecular and biochemical approaches with innovative high-resolution microscopy towards understanding SAC function and error correction mechanisms during mitosis. The “brains” behind the SAC pathway reside on each chromosome at macro-molecular protein assemblies called kinetochores. Each kinetochore complex consists of at least 100 known proteins, which are present in multiple copy numbers and spatially organized into a specific and conserved molecular architecture. Kinetochores carry out two critical functions. First, they mediate the attachment of chromosomes to dynamic microtubules so they can be aligned and segregated by the spindle. When there are problems with this process, kinetochores fulfill their second essential function of producing an inhibitory signal that delays chromosome segregation when chromosomes are not properly aligned. We have previously discovered that a physical reorganization within each kinetochore structure, deemed intrakinetochore stretch, is associated with whether a “wait-anaphase” signal is generated (see Figure 1). We hypothesize that the kinetochore acts as a mechanical switch that functions upstream of checkpoint signaling proteins to determine whether a wait-anaphase signal is generated. We are using newly-emerging microscopy-based approaches to test this hypothesis. Accurate segregation of the genetic material generally requires that every chromosome becomes bioriented with each of its sister kinetochores attached to microtubules emanating from opposite poles of the mitotic spindle. However, no cell is perfect, and during cell division chromosomes can become improperly attached. We are particularly interested in how cells detect and correct syntelic attachments - an aberrant attachment state in which both sister kinetochores are attached to the same pole (see Figure 2 ). Syntelic attachments trigger generation of the wait-anaphase signal in order to give the cell time to correct the error and biorient the chromosome before entering anaphase (see Movie 2 ). Understanding how the cell carries out detection and correction of syntelic attachments is of central importance because failure to do so leads to chromosome mis-segregation and aneuploidy (see Movie 3 ). We postulate that intrakinetochore stretch lies at the molecular intersection of both error detection and error correction mechanisms and plan to further investigate this theory experimentally.
Cell Division, The Spindle Assembly Checkpoint, Detecting and Correcting Improper Chromosome Attachments
Maresca, T.J., Freedman, B., Heald, R. 2005. Histone H1 is essential for mitotic chromosome architecture and segregation in Xenopus laevis egg extracts. The Journal of Cell Biology, 169(6): 859-69.
Wignall, S.M., Deehan, R., Maresca, T.J., Heald, R. 2003. The condensin complex is required for proper spindle assembly and chromosome segregation in Xenopus egg extracts. The Journal of Cell Biology, 161(6): 1041-51.
Nachury, M.V., Maresca, T.J., Salmon, W.C., Waterman-Storer, C.M., Heald R., Weis, K. 2001. Importin beta is a mitotic target of the small GTPase Ran in spindle assembly. Cell, 104(1): 95-106.