Lysosomes are involved in the dismantling and re-cycling of various substrates including some polysaccharides, lipids, DNA and RNA, however, the view of lysosomes as just ‘garbage disposal units’ of the cell, is no longer tenable.
Recent research has shown lysosomes are not ‘stand alone’ organelles but part of a dynamic lysosomal system in which lysosomes link up with other organelles to combine their chemical cargoes. This is often to activate chemicals that are normally kept inactive by being carried in separate organelles.
As the animations and video clip in this presentation show, this can be done by encounters called ‘fusion’ and ‘kiss and run’. In ‘kiss and run’ the linking is brief giving time only for the combining of chemicals. In ‘fusion’, as the name implies, two organelles, an endosome and a lysosome fuse to form a ‘hybrid organelle’. Currently this is the more favoured model.
The lysosomal system is complex and is itself part of an even larger trafficking and signalling network within the cell.
The first animation illustrates ‘fusion’ between an endosome (green) and a lysosome (red) to form a hybrid organelle (orange). The fusion brings together two sets of chemicals. Uncombined they are inactive. Combined they are very active. After the substrates are dismantled and the break down products recycled, the hybrid organelle is reformed into a true lysosome. This is illustrated by the colour changing from orange back to red. The lysosome is then ready to fuse with another endosome.
Kiss and Run
The second animation models a ‘kiss and run’ encounter where the endosome (green) briefly links with the lysosome (red). During this brief encounter (link) chemical cargoes are combined and the lysosme gradually becomes a hybrid organelle (orange).
Fusion occuring in a living cell
The video shows real organelles in a cell fusing to form a ‘hybrid organelle’ .The left hand window showing merged images is the main one to watch. In this window lysosomes are red and endosomes are green, the hybrid structure becomes orange as the contents mix. The false colour image (right) shows the lysosomal content only, green representing a low concentration and red representing a high concentration. You can observe the colour of the hybrid structure change from pale green to bright red as it fills up with lysosomal content.
There can be as many as several hundred lysosomes in an animal cell.
This live cell imaging video clip shows very clearly how new imaging technology, together with informed observation, is helping us to see directly how cells really work. This is important work since there are about 30 disorders in humans that are attributable to problems with endolysosomal function. These disorders are fairly rare but are of course very distressing to those who suffer.