Cell Division Pathway in Animals- Biologists Observed Branching Microtubule Nucleation For the First Time in Animals
A cell biologist, Thomas Maresca and a senior research fellow Vikash Verma, University of Massachusetts Amherst, said that for the first time they have directly observed and also recorded in animal cells a pathway called branching microtubule nucleation which is a mechanism in cell division that had been imaged only in cellular extracts and plant cells but not directly observed in cell division pathway in animals. The details of the study appeared this month in the Journal of Cell Biology.
In this study supported by NIH’s National Institute of General Medical Sciences, the scientists set out to explore specific mechanisms of cell division, what Verma calls “the rules of faithful and complete cell division,” in fruit fly cells. In particular, these researchers wanted to understand how structures called microtubules help to define where the cell splits in half during the cell division process.
Maresca explained that cell division has been studied for a long time since microscopy made it possible to see the cells and how actually it divides, but very intensely for about 40 or 50 years. Now, what are the cues that tell a cell where to divide? And how does the cell know where to put the division plane? It is the ultimate conclusion of mitosis i.e, the actual division of the cell into two.
In a normal cell division, chromosomes line up near the center of the cell. Here is where a structure called the spindle aligns copies of each chromosome by interacting with a bridge-like structure called the kinetochore. Once chromosomes have been aligned, microtubules pull the chromosome copies apart just like a zipper. The cell physically divides at a location positioned between the segregated chromosomes. They produce 2 daughter cells each with a complete copy of the genome.
In imaging the microtubules often described as nanoscale highways, the scientists noticed that the spatial cue for locating division plane requires microtubules, Maresca explained that they grow out to touch the edges inside the cell membrane. Verma found that the growing tips of the tubes i.e, the plus ends that touch the membrane, say to the cell, ‘This is where to divide.’ The regulatory proteins get recruited to the sites contacted by the plus ends kicking into gear and a whole new pathway assemble a ring that will constrict like a purse string to split one large cell into 2 smaller ones.
The scientists found that timing plays a role in the cell division process. Maresca added that it seems that all the microtubule tips have a special ability to trigger the purse-string pathway. He further explained that over time something changes and only the microtubule tips in the middle of the cell retain that ability. Referring to the study published in eLife in February, he added that the team found what they think is a very important spatial cue for how the cell positions its division plane.
Visualization of the behavior of microtubules during the cell division process in detail is typically hampered by facts that so many microtubules are growing and shrinking at the same time throughout the cell, Verma said that it is like many highways converging at the same place and time in the spindle. This looks like the Los Angeles freeway map. Using a powerful technique called the total internal reflection fluorescence (TIRF) microscopy, the team could more easily visualize the properties of individual microtubules. Maresca added that they went from a stressful L.A. traffic jam into a Sunday drive on a country road view.
This is when they witnessed the microtubule branching. Using a multi-color TIRF microscopy, the scientists could now clearly see and also quantitatively define the branching microtubule nucleation process. This had never been visualized before in real-time in animal cells. Verma recalled that it was very exciting!
Maresca said when you see such beautiful thing right before your eyes, you just have to follow it. This study started out as an investigation on how cells define where they divide, but they could see this branching phenomenon so often and so clearly that the team realized they had to look at it more closely. He added that they do not think you could have seen the microtubule branching process as well in other model systems as you can in the fruit fly cells. This highlights the fact that every model system has its own strengths and weaknesses and, in this case, the cells and the phase at which the team was imaging them just offered a uniquely beautiful, birds-eye view of branching. The team could actually see all of this happening in real-time before their eyes, he explained.
Once the team could visualize the entire process of branching nucleation in a cell, he added that they knew that they could next ‘tag’ proteins that regulate the process with different colors to further quantify the fundamental parameters of the phenomenon. All of a sudden the team realized that this is the first time one could see this happening in a living animals cell.
Branching microtubule nucleation is fundamental and conserved which is one of the essential parts of mitosis, but it has been difficult to directly visualize in other model systems, Maresca points out. He added that the course of this study was a reminder that some of the most exciting work the team does as scientists are unplanned and especially for microscopists it begins with seeing something in the cell unfold right in front your eyes!