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Stephen J. Doxsey, PhD |
A discovery by scientists at UMass Medical School could provide insight into the origin and progression of epithelial disorders such as human tumors, microcephaly and primordial dwarfism. They have identified a function of the subdistal appendage protein cenexin. The study, published in Current Biology, shows that cenexin is necessary for microtubule anchoring at the centrosome and ensures proper microtubule organization and microtubule stability during cell division and cell migration.
Animals begin as a single cell such as a fertilized oocyte. This cell undergoes the first cell division to generate two cells. However, trillions of cell divisions are required to build a remarkably diverse set of organs, tissues and body parts. During the cell division process, it is important that cells divide in an orderly and specific fashion. For example, epithelial cells, the building blocks of many organs, must divide parallel to the cell substrate to establish tubular/luminal structures in many different tissues. Misorientation of cell division can lead to defects in tissue architecture and function such as aberrant formation of multiple tubules/lumina.
“Our lab is interested in how centrosomes control the orientation of cell division. Our research is focused on the mitotic spindle, which not only orchestrates equal separation of chromosomes into the two daughter cells during cell division, but also directs the orientation of the cell division axis,” said Stephen J. Doxsey, PhD, professor of molecular medicine and lead author of the study. “A better understanding of the mechanisms involved in spindle orientation, and, therefore, cell division orientation, may provide insight into the origin and progression of epithelial disorders such as human tumors, microcephaly and primordial dwarfism.”
An important contributor to proper cell division orientation is the centrosome. The centrosome/spindle pole is a cellular organelle that functions as a microtubule-organizing center to ensure orderly cell division, among other things. During mitosis, the centrosome replicates, migrates to opposite ends of the cell, and forms a microtubule-based mitotic spindle between the centrosomes. This spindle is responsible for guiding chromosomes into two daughter cells and orienting the axis of cell division. Each spindle pole comprises a pair of barrel-like centrioles. The mother centriole has two sets of appendages (distal and subdistal) that are not present on the daughter centriole. The spindle poles anchor and organize microtubules in the cytoplasm.
Dr. Doxsey and colleagues were interested in learning what happens when microtubule anchoring is compromised. It is known that the subdistal appendages are involved in microtubule anchoring, but their function is unclear. To better understand the role of subdistal appendages and anchored microtubules, they generated cell lines depleted of the protein cenexin, which eliminated subdistal appendages, and cells depleted of the protein CEP164, which eliminated the distal appendages. Each cell line was then tested for important biological functions, such as cell division and lumen formation.
Hui-Fang Hung, a doctoral student in the Graduate School of Biomedical Sciences and first author of the study, found that microtubules in cells lacking cenexin and its associated appendages were unstable and lacked organization. These microtubule defects lead to misalignment of the mitotic spindle during mitosis, which caused multi-lumen formation. Importantly, the CEP164-depleted cell lines retained a single lumen, confirming the previous observation that distal appendages do not play a role in this organizational function and that subdistal appendages were acting independently. Moreover, the effects of cenexin depletion could be reversed when microtubule stability was restored by adding the drug Paclitaxel, which is a microtubule stabilizer. This result again shows cenexin and subdistal appendages are important for microtubule stability.
Similar results were obtained in a different biological process. Depletion of the subdistal appendage protein cenexin in interphase cells disrupted the stereotypic positioning of the centrosome to the centroid of the cell during cell migration in response to injury. The mechanism underlying centrosome misorientaiton was found to be disruption of anchored microtubules.
Taken together, these results demonstrate that cenexin is necessary for both centrosome positioning during cell migration and spindle orientation during cell division.
