Mechanisms controlling microtubule organization during cell differentiation

Stanford University, Stanford CA

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Abstract

? DESCRIPTION (provided by applicant): The microtubule cytoskeleton is a critical regulator of cell differentiation, and must be spatially organized in order to fulfill its cellular functions. Although the concept that microtubules are organized by specific sites called microtubule organizing centers (MTOCs) has been appreciated for half a century, the vast majority of research on MTOCs has focused on the centrosome. While all animal cells use centrosomes as MTOCs during mitosis, MTOC function is reassigned to non-centrosomal sites during cell differentiation. For example, non-centrosomal MTOCs (ncMTOCs) form at the apical membrane of epithelial cells, down the length of axons and dendrites in neurons, and at the nuclear envelope in myotubes. Therefore, ncMTOCs are critical for neurogenesis, muscle function, and in morphogenesis and polarization of most tissues including the heart, brain, and intestine. Despite the importance of ncMTOCs in differentiated cells, little is known about the mechanisms that govern MTOC reassignment following mitosis in part due to the lack of a good genetic model. Using the model organism C. elegans, we are taking innovative approaches to study ncMTOC formation within a developing organism. We are using laser-induced cell fusion, high-throughput forward and reverse genetic screening techniques, in vivo force measurements, and manipulation of embryonic membranes to determine the mechanisms controlling ncMTOC formation. Our preliminary work suggests three key events in ncMTOC formation that we will study mechanistically, 1) The mitotic centrosome is inactivated as an MTOC in differentiated cells (Question 1: How does the cell choose its MTOC state), 2) Microtubule nucleators emanating from the centrosome are captured by a complex at the lateral membrane, thereby establishing a nascent ncMTOC (Question 2: What factors link microtubule minus ends to the membrane?), 3) Nascent ncMTOCs move to specific sites to establish an ncMTOC (Question 3: How are nascent ncMTOCs targeted to the proper cellular location?). Proper microtubule organization is essential for normal development and cell function and hyperactive MTOC function at the centrosome is a hallmark of some cancers. Thus, the molecules uncovered in these studies could provide potential therapeutic targets as well as shed light on an important, but understudied topic in cell and developmental biology.