Lay summary: “Rab18 and a Rab18 GEF complex are required for normal ER structure” Gerondopoulos et el., 2014
Warburg Micro Syndrome is a rare genetic disorder characterised by eye, nervous system and endocrine abnormalities. Genetic alterations in four different proteins have so far been found to cause Warburg Micro Syndrome: Rab18, Rab3GAP1, Rab3GAP2 and TBC1D20. Researchers want to understand the key pathway or pathways affected by all four proteins, so that we might better understand what is causing the disorder, and importantly, how we might develop therapies to target these pathways. This paper focuses on examining the function of Rab3GAP1 and Rab3GAP2 in cells, and in particular, it examines how genetic alterations that have been found to cause Warburg Micro Syndrome disrupt this function. Overall this research provides convincing evidence that Warburg Micro Syndrome-causing genetic alterations in Rab3GAP1 and Rab3GAP2 give rise to disease by disrupting the function of Rab18 (Figure 1). The significance of this work is, because of these findings, researchers knew to focus their efforts on understanding the role of the Rab18 pathway to better understand this disorder.
Genetic alterations in Rab3GAP1 and Rab3GAP2 were the first discovered to cause Warburg Micro Syndrome. These proteins are key regulators of the Rab3 pathway whereby they switch OFF Rab3. As a result this initially implicated the Rab3 pathway in Warburg Micro Syndrome. However, it was later found that genetic alterations in another Rab protein, Rab18, also cause Warburg Micro Syndrome. The researchers that published this paper firstly set out to investigate the relationship between Rab18, Rab3GAP1 and Rab3GAP2. They showed that both Rab3GAP1 and Rab3GAP2 are required to switch ON Rab18. By contrast, they showed very little activity towards Rab3. Interestingly, Warburg Micro Syndrome-associated variants in either subunit of Rab3GAP1 or Rab3GAP2 were shown to prevent Rab18 from being turned ON. Therefore, Warburg Micro Syndrome can be caused by loss of Rab18 activation by Rab3GAP1 or Rab3GAP2.
Identifying the location of Rab18 inside cells can help to better understand the precise role that Rab18 plays. To do this the researchers labelled Rab18 with a green tag and found Rab18 to be located on a large structure within cells called the endoplasmic reticulum (ER for short). The ER has a complicated structure made up of a network of sheets and tubes. The researchers showed that Rab3GAP1 and Rab3GAP2 work together to direct Rab18 to its normal location on the ER. This was shown by reducing the amount of Rab3GAP1 and Rab3GAP2 proteins in the cells and visualising that this disrupted the localisation of the green-labelled Rab18.
Of particular interest, the researchers created cells in which Rab3GAP1 and Rab3GAP2 had been edited to carry genetic alterations that are known to cause Warburg Micro Syndrome in children. In these cells they found that Rab18 was not properly activated or localised within the cells to the ER. It was also found in these cells that the distinctive tube and sheet structure of the ER was lost. By contrast, the activity of Rab3 was not changed in these cells. These findings provide evidence that genetic alterations in Rab3GAP1 and Rab3GAP2 give rise to Warburg Micro Syndrome by causing a loss of Rab18 function.
There are numerous proteins known to be involved in the formation of the ER in cells. A number of these proteins involved in shaping the ER are known to be mutated in another group of inherited neurologic disorders called hereditary spastic paraplegias. Individuals with hereditary spastic paraplegias experience stiffness and weakness in the leg muscles that progressively gets worse over time. Whilst, Rab18, Rab3GAP1 and Rab3GAP2 which have been shown in this paper to be necessary for a normal ER structure are known to be mutated in WMS. There is clearly an interesting link between disrupted structure of the ER and inherited neurological disorders. Further work is required to better understand the requirement for a normal ER structure in complex cellular processes.
Figure 1: Summary of the role of the Rab3GAP1 and Rab3GAP2 protein complex in regulating Rab18.