CCM is a unique example of a genetic disorder that results from the dysfunction of three non-catalytic signaling proteins. Krit1-OSM-PDCD10 form a complex in the cell and CCM lesions appear to result from a loss of integrity of this complex. While Krit1 and OSM proteins both have known structural domains and functionally link to several important signaling pathways, including integrin signaling, the p38 MAP kinase pathway, and RhoA-rho kinase regulation, PDCD10 lacks known structural domains. PDCD10 was thought to relate to apoptosis; however, there is no clear apoptotic pathway known. Defining a functional domain of PDCD10 is therefore important in learning how this protein functions and contributes to CCM development. Combining molecular modeling and site-directed mutagenesis, it appears that PDCD10 is a six helical bundle protein composed of a two heptad repeat, and α5 is an important amphipathic helix housing five lysine residues essential for PtdIns(3,4,5)P3 binding. Unlike phosphatidylinositol specific domains, for instance C1, PH, PX and FYVE, amphipathic helices often only demonstrate high specificity to certain phosphatidylinositol molecules under specific circumstances, such as when the membrane area possesses a highly curved region 
. We speculate that this may be the case for PDCD10 since there seem to be multiple lysine residues beyond the amphipathic helix similar to some phosphatidylinositol binding proteins that use these outside amphipathic helix lysine residues to interact with the curved membrane region 
. We further show that the Δ5KA, a mutant lacking these five important K residues, does not bind to PtdIns(3,4,5)P3 in vitro
. While the WT and Δ5KA both appear to have the same helical secondary/tertiary structure, and are dimeric in solution, the Δ5KA does not bind OSM. In addition, when WT PDCD10 and Δ5KA are co-expressed with p110-CAAX, a membrane-bound/constitutively active PI3K, WT is co-localized with p110-CAAX at the plasma membrane. However, Δ5KA stays in the cytoplasm. The results suggest that PDCD10 may function with PI3K.
A recent study shows that PDCD10 functions in VEGF-dependent translocation of vascular endothelial growth factor receptor 2 (VEGFR2) and further, suggests that the C-terminal portion of PDCD10 is important in PDCD10-VEGFR2 interaction (66). VEGF is an upstream regulator of PI3K. It is therefore plausible that PDCD10 may play a role in VEGF-PI3K signaling. Interestingly, colocalization of PDCD10 and PI3K in our study is almost identical to the colocalization of PDCD10 and VEGFR2 
. It is possible that this colocalization is a result of PDCD10 and VEGFR2 interactions. It was shown that PDCD10 interacts with VEGFR2 through its C-terminal region 
. This region overlaps with the PtdIns(3,4,5)P3
binding where we located the five PtdIns(3,4,5)P3
binding lysines. These lysines are also important in PDCD10-OSM interaction. Furthermore, genetic studies demonstrate that this region is predisposed to frameshift mutations that often cause early termination of the protein resulting in CCM 
. We therefore speculate that PDCD10-OSM and PDCD10-VEGFR2 interactions may be regulated by the availability of PtdIns(3,4,5)P3
generated by PI3K.
Based on our findings and recent studies, we composed a signaling model for PDCD10 (). We propose that PDCD10 functions closely with VEGFR2 and PI3K. Upon activation of VEGFR2 by VEGF, VEGFR2 binds to dimeric PDCD10, translocates to the membrane, and becomes activated. As a result of VEGFR2 activation, PI3K is activated and favors the catalysis of PtdIns(3,4)P2 or PIP2 to PtdIns(3,4,5)P3 or PIP3 (). PtdIns(3,4,5)P3 goes on to bind PDCD10 and AKT. There may be an equilibrium between PDCD10-PtdIns(3,4,5)P3 and PDCD10-OSM/Krit1, as well as a equilibrium between PDCD10-PtdIns(3,4,5)P3 and AKT-PtdIns(3,4,5)P3. PtdIns(3,4,5)P3 and OSM seem to have the same interactive site on the PDCD10 dimer, and it is possible that VEGFR2 may also share the same binding site. It is therefore plausible that PDCD10 may regulate the function of these three important signaling molecules at the same time using simple chemical equilbrium.
Proposed PDCD10 signaling model.
We propose that CCM development may result from the dysregulation of the VEGF/PI3K signaling pathway through PDCD10- PtdIns(3,4,5)P3
interaction. Future experiments will need to be completed to define the role of PDCD10 in PI3K signaling. PI3K can be activated by several growth factors, including VEGF. Characterizing the role of PDCD10 during PI3K activation by VEGF, as well as downstream effectors such as AKT1, will be important in understanding CCM development and how we may treat this condition by modulating the activity of these kinases. Linking the role of PDCD10 in the VEGF/PI3K pathway to other CCM proteins will elucidate the individual roles of CCM proteins, as well as the CCM protein complex . We recently showed that OSM plays a role in regulating the degradation of the small GTPase RhoA and that the CCM endothelial phenotype can be rescued with knockdown or inhibitors of ROCK-RhoA effectors 
. Interestingly, activation of PI3K can also increase RhoA activation 
. Further experiments will also be needed to determine if PDCD10 functions in concert with OSM and Krit1 in signaling the regulation of RhoA and PI3K.