Few processes are as fundamental to life as that of one cell dividing into two. It’s an intricately choreographed dance of cellular structures, organelles and genetic material. Everything that the new daughter cells need to function must be precisely duplicated, organized and ushered to mirroring areas of the parent cell, before the membrane pinches in and the whole thing splits into two. Despite more than a century of study, however, there are many remaining mysteries about how cells perform this remarkable feat.
Michael Glotzer, PhD, professor of molecular genetics and cell biology, and his colleagues have investigated this question for years. They’ve previously shown that the protein complex centralspindlin is key for this process, but how it did so was unclear. Especially odd was the observation that centralspindlin clusters in the center of a cell. How could this protein organize an intricate set of events far away (by molecular standards) in the membrane?
In a study published in Developmental Cell in April, Glotzer and his team, including graduate student Angika Basant, shed light on this problem by revealing the precise mechanism by which centralspindlin coordinates initiation of cell division – through a series of carefully timed interactions with other proteins throughout the cell.
“Our study uncovers a mechanism by which clusters of a protein complex, centralspindlin, that are generated in a spatially and temporally controlled manner, can localize to the presumptive site of division on the plasma membrane to locally activate the cell division machinery,” said Glotzer, who is senior author on the study.
One of the key steps in cell division is the formation of a contractile ring containing filaments of the cytoskeletal protein actin as well as myosin motor proteins that, together, drive constriction of the cell membrane. This ring contracts, forming a cleavage furrow in the membrane which pinches in until the two cells can completely separate. Correctly positioning this plane of division is critical, as errors can cause the new cells to inherit unequal numbers of chromosomes or organelles.
The primary molecule involved in this process is a switch-like protein called RhoA, which accumulates at the membrane and induces the assembly and constriction of the contractile ring. However, the mechanism by which RhoA becomes activated is unclear. Another protein, ECT-2, is known to be required, as are interactions between ECT-2 and centralspindlin.
To unravel this complex puzzle, Glotzer and his team studied the single-celled embryos of nematode worms, a commonly-used model organism in biological research. They began by artificially turning off a protein, PAR-5. Without PAR-5, nematode embryos cannot determine where to begin dividing, resulting in multiple cleavage furrows.The team observed centralspindlin on the membrane in these embryos, an exciting finding as the protein cannot usually be readily detected there. It turns out that centralspindlin does in fact get to the membrane, where it interacts with ECT-2 to turn on RhoA and initiate contraction. To do so, it needs to form large complexes, a process that is inhibited by PAR-5.
“PAR-5 appears to prevent centralspindlin from forming large complexes, and as a result keeps it off the plasma membrane, thereby preventing RhoA activation,” said Basant, who is first author on the study. “When we generated a mutation in one of the centralspindlin proteins that was predicted to abolish PAR-5 binding, we found that the cells behave as though PAR-5 were absent – specifically, they are unable to decide exactly where to make the cut.”
How then, does PAR-5 get turned off? The team discovered that the protein Aurora B kinase is responsible for modifying centralspindlin, thereby inhibiting PAR-5, and enabling centralspindlin to form complexes and get to the membrane. Importantly, Aurora B kinase concentrates on the cell membrane around the equator of the cell. Because of this highly specific localization, the formation of a cleavage furrow and ultimately the division only occurs along this plane, and nowhere else in the cell.
The team verified this by artificially depleting embryos of Aurora B kinase, which resulted in cells that never formed cleavage furrows. When PAR-5 was also artificially depleted, multiple furrows formed.
“Our data shows that a very small amount of centralspindlin, has the capacity to initiate furrow formation,” Glotzer said. “It was exciting to find that this specific RhoA activator, which we’ve been studying all along, could perform this role on the membrane – a completely different location from where it was previously known to function.”
Dissection of fundamental processes such as cell division are vital for progress in understanding the incredible complexities involved in biology. The description of this mechanism now gives scientists around the world a solid framework in which to study cell division. These regulatory principles may be utilized in other cellular events. As for Glotzer and his team, they are now investigating other unanswered questions such as how Aurora B kinase gets to the right site on the membrane at the right time, the precise nature of the interaction between centralspindlin and RhoA and how other additional regulatory mechanisms affect cell division.
The study, “Aurora B Kinase Promotes Cytokinesis by Inducing Centralspindlin Oligomers that Associate with the Plasma Membrane,” was supported by the National Institutes of Health and American Heart Association Midwest Affiliate Predoctoral Fellowship. Additional authors include Sergey Lekomtsev, Yu Chung Tse, Donglei Zhang, Katrina M. Longhini and Mark Petronczki.