How do cells develop; how do tissues form? For example, what guides cells to form a heart? These are crucial questions for molecular biology and an area with decades of research that still feels like it’s just getting started. Case in point: Researchers at the Boyce Thompson Institute for Plant Research (BTI, New York, USA) in collaboration with Duke University (USA) and the Universities of Helsinki (Finland) and Uppsala (Sweden) have published a paper in Nature describing the discovery that microRNA moves between cells, acting as a signaling pathway that helps determine how cells develop and differentiate. It’s an important piece of information.
It’s been known for some time that microRNA (or miRNA), one of the many forms of ribonucleic acid used to transmit the instructions of the DNA code, is produced in the ribosomes of cells and is used within the cell to regulate genetic expression – mostly by turning genes off. The human genome may encode thousands of microRNAs and they are abundant in many human cell types. Their role in the cell is itself open to much more exploration, but until the work outlined in this paper, it was not known that microRNAs also perform an important role in communication between cells.
“Many organisms are made up of multiple types of cells, and we do not yet fully understand how these cells are put in the right places, although we believe cells communicate with each other ,” said Ji-Young Lee, assistant scientist at BTI and a lead author of the article. “This is the first time anyone has clearly demonstrated cells are communicating through the movement of microRNA. It’s likely that this kind of communication process is generally happening in many cell types in many organisms.”
This research is based on plants, specifically Arabidopsis, a small relative of the cabbage. The point in question was how plant cells (in the procambium) differentiated first into protoxylem, and then into metaxylem on the way to becoming xylem, which forms the cellular systems by which most plants move water and mineral nutrients from the roots to higher locations.
Investigation of these cells at the molecular level requires a special combination of high-resolution imaging and molecular tagging. By watching the development of cell types through the actions of protein molecules they were able to discover that a protein called SHORTROOT migrates to the inside of the root skin where it activates another protein, SCARECROW. In combination, these molecules trigger the production of microRNA 165/6. This microRNA is able to cross the cell membrane and penetrate the membranes of neighboring cells.
The microRNA apparently carries a message, or put another way, it has a controlling effect: High levels of microRNA 165/6 create protoxylem; low levels turn cells into metaxylem. It has a signaling effect that helps in the process of cell differentiation and is in some way important to the formation of specific tissue – in this case, the construction of the woody stems that transport water throughout the plant.
This discovery opens for exploration the role of microRNA in all kinds of cells (including animals), since the ability to pass between cells and share genetic control information is essential to the growth and development of almost all living things. Notice, however, that this observation is like looking at microbiology from the 10,000 meter level – it’s still a long way from describing the detail of chemical/molecular activity responsible for the development.