Figure 1. Staining of TOMM5 (green) with DNA (blue) in HeLa cells.

Welcome back to Image of The Week! We will be periodically highlighting an organelle in the coming image of the week posts, written by members of the Subcellular Human Protein Atlas project. This week we kick things off with a post by HPA_Illuminator, and what better way to start than with the mitochondria, the (true) powerhouse of the cell!!!

Mitochondria are found in almost all human cells, in varying numbers. They are known as the powerhouse of the cell as they are responsible for producing the majority of the energy in your body (in the form of ATP, adenosine triphosphate). However, the mitochondria have been shown to be important for a range of other functions in the cell, including intracellular signaling and programmed cell death known as apoptosis (Frezza C. et al 2006 ). One protein involved in this process was briefly highlighted in last week's blog. Apoptosis is a commonly occurring event in the body that enables the shutdown of malfunctioning cells without damaging the surrounding tissue.

The mitochondria have a very distinct structure, consisting of an outer and inner membrane, where the inner one is folded, forming the easily recognized cristae. The protein stained in Fig 1. is an image of mitochondrial import receptor subunit TOM5 homolog (TOMM5) found in the mitochondria of the cell. Proteins need to be actively imported into the mitochondria, and this protein is a subunit of the larger TOM (translocase of outer membrane) complex which provides one mechanism of transport into the mitochondria (Ryan K.R. & Jensen R.E. 1995). The cells stained here are HeLa cells, the first continuous human cancer cell line used in research, stemming from a cervical cancer sample (Masters J.R. 2002).

Notably, the mitochondrion is the only organelle containing a DNA of its own. The mitochondrial DNA (mtDNA) only consists of 37 genes (to be compared with the close to 20 000 genes in the nuclear DNA), but the proteins they code for are vital for the functions of the mitochondria.

Mutations in genes coding for mitochondrial proteins (both mtDNA and nuclear DNA) can cause severe diseases. Recently, it was discovered that pathogenic mutations in some mitochondrial proteins may cause epileptic seizures (Zsurka G. & Kunz W.S. 2015). In order to choose the correct treatment, it is necessary to understand the link between dysfunctional mitochondria and disease such as epilepsy.

Although mitochondrial dysfunction is implicated in several diseases, it seems that mitochondria are less likely to have pathogenic mutations in human cancer samples. This could indicate that fully functional mitochondria are important for cancer tumorigenesis (the production of tumors), and as a result, disruption of the mitochondria could be a possible target for therapeutic strategies (Zong W.X. et al. 2016).

To learn even more about the mitochondria, and see more examples checkout the HPA dictionary.

A big thanks to all the members of the Subcellular Atlas team working to generate these images, and especially to Mikaela Wiking (HPA_Illuminator, credit to MizhirStarsurge) for contributing this great article!