Image of the week - The tiny but mighty centrosome

Figure 1. Staining of MKKS (green) with DNA (blue) and microtubules (red) in U-2 OS cells.

Let's have a look at another fascinating compartment of the cell, the centrosome! Located in close proximity to the nucleus, the centrosome is so small that it sometimes suffers from being overlooked. However, despite its humble size it is a very important organelle with great impact on cellular function.

The centrosome was first described in 1888 and has been a very popular organelle to study among biology researchers ever since (Conduit P.T. 2015). The most well characterized function of the centrosome is to serve as the organizing center for the microtubules that build up the internal architecture of the cell, the so-called cytoskeleton. This function is of extra importance during cell division, where the centrosomes make sure that chromosomes are segregated in the correct manner into daughter cells as discussed in a previous blog.

More recent research has expanded our view of the centrosome and today we know that the function of this organelle extends well beyond just serving as the organizing center for the microtubules during cell division. In fact, in some cells the centrosome is not even required for division to occur (Arquint C. et al. 2014).

Occupying a volume of just 1-2 µm3, the centrosome is organized into two barrel shaped structures called the centrioles. These centrioles are further organized into a nine-fold sub-structure that is symmetrically ordered to maintain both the stability and functional activity of the centrosome. It is not only the proteins residing in the dense area of the centrioles that are important for the functions of this organelle however, surrounding the centrioles is a matrix referred to as the pericentriolar material (PCM) that contains hundreds of proteins that are associated to the centrosomal machinery and also take part in its molecular activities. While some of these will appear very close to the centrosome, others may be slightly more spread referred to as the microtubule organizing center (MTOC). In the confocal images one can commonly see the two centrioles resolved as two small and distinct dots that overlap well with the center of the microtubules shown in red in our images.

In Figure 1, the gene called McKusick-Kaufman syndrome (MKKS) is stained in U-2 OS cells, a human osteosarcoma cell line of epithelial morphology. The antibody stains centrosomes in all cell types tested in the subcellular atlas and the localization of MKKS around the two centrioles is visible in every single cell. As the name suggests, disruption of this protein leads to McKusick-Kaufman syndrome, a disease that manifests with impaired development, especially of hands and feet, as well as heart and genital defects.

MKKS is a centrosome shuttling protein that localizes to a tube-like structure around the centrioles in the PCM and is important for cell division. In normally functioning cells, MKKS shuttles between the centrosome and the cytosol throughout the cell cycle but when mutated it fails to localize to the centrosome, leading to McKusick-Kaufman syndrome (Hirayama S. et al. 2007). Mutations in MKKS have also been linked to causing Bardet-Biedl syndrome, a disorder whos symptoms include obesity, diabetes, retinal and kidney deficiencies (Kim J.K. et al. 2005, Katsanis N. et al. 2000).

We would like to thank all the members of the Subcellular Human Protein Atlas who generate these images and especially to Frida Danielsson for contributing this article about the tiny but mighty centrosome!