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Deciphering the molecular mechanisms of mitotic spindle behaviour

In a multicellular organism when a life form begins at the one-cell stage embryo, how do different cell types come into existence during development? Part of the answer to this puzzles lies in the process of asymmetric cell division. During asymmetric cell division a cell undergoes division in such a way that it produces two daughter cells with distinct fates and thus having different developmental potential.

The proper positioning of the mitotic spindle is one of the essential features by which asymmetric cell division is achieved. In animal cells, the mitotic spindle is a diamond-shaped microtubules-based structure that plays a crucial role for the segregation of the genetic material and proper distribution of the cellular fates. Prior to cell division, the chromosomes (which contain the genetic material, DNA) must also duplicate and divide. The mitotic spindle attaches to specific places on these chromosomes and “pulls” them apart to the opposite end of the cell. The other function of the mitotic spindle is to determine the positioning of the cell division in a three-dimensional cellular space. The cell divides in a plane that is perpendicular to that of the mitotic spindle. If the spindle is located in the center, the cell divides into two halves of similar size. But, if the spindle is located closer to one end of the cell, then the cell divides unequally, giving rise to one larger and one smaller cell and thus causes unequal cell division.

Thus, accurate positioning of the mitotic spindle guarantees that besides genetic material, cellular fate determinants are also properly segregated in the newly formed daughter cells during asymmetric cell divisions. Moreover, these events guide proper tissue architecture and morphogenesis and thus regulate shape/structure of an organ. What are the mechanisms that control spindle behaviour? Dr Sachin Kotak hopes to find some of the answers to these fundamental questions through his recently established research group in the Department of Microbiology and Cell Biology (MCB) at the Indian Institute of Science (IISc).

We have now evidence that the positioning of the mitotic spindle during the process of development of the fertilized egg plays an essential role in the correct developmental program. Incorrect positioning of the mitotic spindle is also associated with neurological diseases such as microcephaly (small brain) and cancer. The various proteins that participate in this process work in a similar manner across a wide-range of life forms. Dr Kotak’s lab works on both, human cells and Caenorrhabditis elegans, transparent worms that are as small as 1 mm.

In C. elegans, one such protein is dynein. This protein is anchored to specific places on the inner leaflet of the cell membrane. Dynein is a microtubule-dependent motor protein complex, it provides the force to pull the two ends to the spindle to opposite sides. InC. elegans, the fertilized egg divides asymmetrically to form two unequal cells. The two cells then follow distinct fates. While one cell gives rise to mostly neuronal lineages, the other cell is mainly responsible for the formation of the gut, muscle and germline lineages. However, if the membrane located dynein is lost in the one-cell stage, it gives rise to two equal cells, which divides to form an irregular mass of cells and which is embryonically lethal.

Dr. Kotak and his team hope to unravel some of the mysteries surrounding how dynein and its associated proteins are distributed within the three dimensional space of the cell, and how dynein amounts and localization vary with the mitotic progression.

Interestingly, Dr. Kotak started his research career by working not on animals, but plants. Dr Kotak obtained his PhD in plant science from University of Frankfurt, where he worked on the plant Arabidopsis thaliana. It was during this period that he grew interested in the mechanism of asymmetric cell divisions. He then went on to EPFL Switzerland to further delve into this subject. When asked if the shift from plant cells to human cells has been a challenging one, he referred to his area of research, stating, “In Biology the question is more important”, and one should choose a system based on the question that need to be answered.

His lab uses a combination of microscopy techniques like confocal microscopy, and time lapse differential interference contrast (DIC) microscopy, live cell imaging along with proteomics studies to track how the different proteins involved in positioning the mitotic spindle move and interact. With a major section of the lab up and running in a relatively short span of ten months, his enthusiasm and optimism are effusive. While he says it is too early for him to be giving advice, he thinks that students aspiring to develop careers in science would benefit from developing independent thinking and problem solving skills in early stages of their careers.

About the scientist:

Dr. Sachin Kotak is an Assistant Professor at the Department of Microbiology and Cell Biology at IISc, Bangalore. He can be contacted on +91 80 22932292 or sachin.kotak@mcbl.iisc.ernet.in