On a daily basis, multitudes of molecules enter each cell in our body. These can be nutrients or signal molecules or pathogenic microorganisms. An organelle in the cell directs these molecules to other stations for further processing. This organelle is called the endosome.
If the pathways by which this sorting occurs fails at any stage, several diseases such as neurodegenerative diseases and certain cancers can occur. Thus, a better understanding of the steps in these pathways is of utmost importance.
In a recent study published in Communications Biology, a group of scientists from Japan and Austria, led by Prof Jiro Toshima from the Tokyo University of Science, reports a new finding regarding the maintenance and functioning of the endosome.
Conventional knowledge is that two processes are necessary for the upkeep of endosomes: a) sacs of molecules constantly form at the cell membrane, are transported to the endosome, and fuse into it; b) protein-containing vesicles transported from the Golgi (another cell organelle) fuse with the endosome.
The researchers of this study claim that this is not the case.
They introduce genetic mutations and drugs into yeast cells to inhibit each of these transport processes at a time.
When transport from the Golgi does not occur, a protein essential to the upkeep of the endosome, Rab5, is not activated, and endosome formation is affected. When cell transport from the membrane is inhibited, there is no effect on the endosome.
Thus, essentially, transport from the Golgi is necessary and that from the cell membrane is dispensable, or not as crucial. "Our results provide a different view of endosome formation and identify the Golgi as critical for the optimal maintenance and functioning of endosomes," Prof Toshima says.
This study clarifies only a fraction of the molecule-sorting pathway in cells. But, this is certainly one giant step in the research in this field. Perhaps, the insights from this study will soon appear on the pages of cell biology textbooks.
Nagano, M. et al. (2019) Rab5-mediated endosome formation is regulated at the trans-Golgi network. Communications Biology. doi.org/10.1038/s42003-019-0670-5.