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348 || AWSAR Awarded Popular Science Stories - 2019
grow it would require coordination between all these various parts.
I was curious about how a cell expands its boundaries or grows its cell membrane? I observed that rather than growing uniformly in all directions, epidermal cells, which are shaped like a covered bowl, were selectively adding to only the top surface (apical), which faces the external environment. Such anisometric or non-uniform growth stretches the cell rendering it flatter (from a bowl shaped cell to a saucer-shaped cell of a bigger diameter) and covers larger surfaces to encapsulate the whole animal using lesser number of cells. This seems like a more energy efficient way for the cells to grow in the time of distress. What this also suggests is that the cells must have a specialized mechanism to grow specific domains, which would require specialized transport system towards the apical surface. To study such mechanisms, I looked at the transport, or trafficking machinery, of the cells and eventually uncovered the
role of a molecular motor of
the Myosin family of proteins,
which are tiny biological motors
carrying the allotted cargo onto
them and walking/driving those
to the site of delivery. I showed
that upon depleting the motor,
which specifically interacts with
the membrane bound cargo;
the cells lost the ability to grow
in the apical direction and
hence, could not withstand the
stress.
Further, cell membranes
are mainly composed of
membrane lipids, which are
synthesized by a series of metabolic processes and are stored in the Golgi from where they are packed and docked to the motor protein for further transportation. Hence, when demand of lipids in the membrane increases, it is expected of the lipid synthesis and storage
capacity of the cell to keep up with it. Although intuitive, such cross-talk between these various departments of a cell is not well established. Hence, to understand how a cell keeps up with such increased demand, I checked how lipid synthesis and storage is affected in a growing cell. First, I showed that both lipid productions as well storage capacity of the Golgi increases when the cells face a need to grow in size. Next, to understand whether there is a cross-talk between the trafficking arm and the production arm of cell growth, I disrupted trafficking by genetic depletion of Myosin motor specifically carrying the lipids to the membrane and checked its effect on lipid synthesis and storage. To my surprise, rather than reducing the production when the transport was blocked, the cells increased lipid synthesis and Golgi capacity. This might suggest a very interesting phenomenon of intracellular compensation, which could be understood by imagining a deficit of particular
goods in the market because of a blockage on a major road. Under such circumstances, the city can recognize the problem and respond by producing even more goods, so that the supply can be maintained by alternate, although less efficient roads. Though wasteful, such mechanisms are crucial to maintain supply of the essential goods.
To conclude, my work brings out how growing a cell and, in turn, a tissue is not just simple scaling. It is rather a sophisticated and well-
orchestrated interaction of cellular metabolism, cell organelles and the tissue dynamics. Finally, this also emphasizes essentiality and robustness of cell size regulation for maintaining form and function of an organ.
If one kidney fails, to take up the extra work load, the other kidney grows by increasing cell size rather than adding new cells. Similarly in partial liver damage, where a substantial part of the liver is rendered useless, the rest of the liver tissue first grows by increasing cell size and subsequently, by addition of new cells.