PgmNr W4167: Coordinating microtubule organization with cell cycle state.

Authors:
Maria Sallee; Jessica Feldman


Institutes
Stanford Univ., Palo Alto, CA.


Keyword: Microtubules

Abstract:

Microtubule organizing centers (MTOCs) generate specific arrangements of microtubules that are essential for many cellular functions, including division, polarization, and transport. Different subcellular sites serve as the MTOC to accommodate these processes; dividing animal cells establish an MTOC at the centrosome, while many differentiated cells designate a non-centrosomal site as the MTOC. A conflict arises when differentiated cells divide — the centrosome must be re-established as the MTOC for mitosis to occur. Regulating this switch in the subcellular site of the MTOC (the “MTOC switch”) is important for controlling proliferation in epithelial cells, and a dysregulated switch may contribute to disease states such as cancer. We are using the developing C. elegans intestine as a model to understand the mechanistic nature of this MTOC switch. In polarized embryonic intestinal cells, the site of the MTOC switches very stereotypically from the membrane to the centrosome as they divide, and then back to the membrane as they exit mitosis. This binary MTOC switch offers a simple and elegant system to study how the MTOC location is regulated by and coordinated with the cell cycle. We hypothesize that cell cycle genes directly control MTOC location, and that MTOC location is a critical determinant in regulating a cell’s potential to divide. Using fluorescently-tagged markers, we can observe the MTOC switch as the cell cycle progresses in real time, and have determined that the membrane switch occurs in G1, while the centrosome switch occurs in G2/M phase. Our hypothesis predicts that cell cycle regulators must interface with MTOC components at some level to coordinate the MTOC location with cell cycle state.  We are now testing several candidate regulators of the MTOC switch, including a surprising candidate, the microtubule-severing enzyme mei-1/Katanin. In mei-1(0) embryos, intestinal cells initially undergo the membrane switch, but soon after, they aberrantly activate the centrosome switch and divide. Intestinal cell number is normal prior to the E16 intestinal stage, suggesting that the additional intestinal cells are not due to early cell fate specification defects. The centrosome switch and division defects are rescued by GFP-MEI-1, and also observed in mutants for the other subunit of the Katanin complex mei-2, suggesting that the defects are caused specifically by loss of the Katanin complex.  MEI-1 localizes to the membrane MTOC, suggesting a potential role at the membrane in maintaining the MTOC state.  In addition to mei-1, we are currently investigating several candidates and carrying out an unbiased genetic screen to identify additional MTOC regulators.  This work will help reveal the molecular mechanism underlying the coordination of MTOC location with cell cycle state.



Wormbase Genetic Index
1. mei-1
2. mei-2
3. gip-1
4. tbg-1