Cavernous malformations and CCM genes




Cerebral cavernous malformations (CCMs) are vascular lesions located in the Central Nervous System which are characterized by enlarged vessels (caverns) lined by endothelium, with no underlying smooth muscle or elastic tissue. CCMs affect over 0.5% of the population, and frequently give rise to important clinical problems, such as intense headaches, seizures, and bleeding, which can be life threatening1, 2. The treatment of choice is usually neurosurgical resection3 but, depending on the number and location of the lesions, this approach may not be feasible.
The study of cavernomas is important. Of course, anything we learn on their pathobiology may result in a better quality of life for patients with this condition. However, its interest goes even beyond that. The existence of the disease implies that there is a cellular process that is important for the biology of the endothelial cell. Given the central role of the endothelium in many clinically important diseases, the relevance of the knowledge we gain on cavernoma biology will be applicable in many other fields of biomedicine.




A cerebral cavernous malformation as seen in an MRI scan. The lesion is in the red area. From Clatterbuck et al, 2004.





Some CCMs have a genetic basis, and three different CCM loci have been identified: CCM1/KRIT14, CCM2/Malcavernin/OSM5, and CCM3/PDCD106, and in all cases the malformations are the consequence of the inactivation of the relevant gene7, 8. CCM gene products have been proposed to act as adaptor proteins. Also, all three CCM genes are important in endothelial cell biology and vascular development9-13. However, not all CCM gene products seem to perform the same functions. While CCM1 and CCM2 can regulate the cytoskeleton by small G proteins10, 14, the importance of CCM3 in this regulation is not clear. Moreover, CCM1 and CCM2 form a complex with the endothelial specific orphan receptor HEG1, but CCM3 can only be identified in that complex when overexpressed15. On its part, CCM3 has recently been found to bind to VEGF receptor VEGFR2 and to be important for its signal transduction13, and to bind to the focal adhesion protein paxillin when overexpressed16, two functions that have not been reported for CCM1 or CCM2. Further, CCM3 interacts with all three members of the GCKIII proteins, a family of protein kinases involved in the response to cellular stress, Golgi biogenesis, and cytoskeletal regulation17. Importantly, it is this interaction, rather than its binding to CCM1 and CCM2, what may be important for its role in endothelial cell biology18.
The GCKIII proteins are a group of three serine-threonine kinases (Mst3/STK24, Mst4/MASK, and SOK1/YSK1/STK25) that belong to the wider family of Ste20 kinases. They share an N-terminal kinase domain that comprises more than half of their coding region and is well conserved with other Ste20 kinases, and a GCKIII-specific C-terminal domain, through which they bind to CCM319. GCKIII proteins have been related to several important cellular processes. All three of them are activated by oxidative stress, and while Mst3 and SOK1 are proapoptotic, Mst4 has a prosurvival function20, 24. On the other hand, in unstressed cells at least Mst4 and SOK1 are important for Golgi morphology and directed cell migration21. Mst3 and Mst4 influence cytoskeletal regulation: Mst3 phosphorylates the kinase NDR22, and Mst4 phosphorylates and activates the adaptor proteins Ezrin/Radixin/Moesin under specific circumstances23, 24.
Our group has been studying the biology of CCM3 and GCKIII proteins in the last few years. We have shown that endogenous CCM3 can interact with all three endogenous GCKIII proteins (Mst3, Mst4, and SOK1), and have also shown that at least part of CCM3 is on the cis face of the Golgi apparatus, and that it influences Golgi morphology and cell polarity through GCKIII-dependent phosphorylation of the adaptor protein 14-3-3ξ17.







Left: Confocal microscopy image showing CCM3 (red) co-localizing with the cis-Golgi protein GM130 (green) on the Goli apparatus. Nucleus are stained in blue.

Right: Confocal microscopy image of a CCM3-depleted cell. The Golgi apparatus has been visualized with GM130 (green) and TGN46 (red) antibodies. Its morphology is altered, as it loses its compact appearance.

 More recently, we have shown that CCM3 is also important for cell survival after oxidative stress, and that this function is dependent on phosphorylation of the cytoskeletal adaptor proteins Ezrin/Radixin/Moesin by the GCKIII protein Mst424.


Schematic of the CCM3-Mst4-ERM pathway. Mst4 is at least partially located on the cis Golgi forming a complex with GM130 in unstressed cells. After oxidative stress, it is activated and relocated to the cell periphery in a CCM3-dependent manner. There, it phosphorylates the activating residues of ERM proteins. As a result, the cell is protected from death. From Fidalgo et al, 2012.






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