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Page 14 of 23 Padarti et al. Vessel Plus 2018;2:21 I http://dx.doi.org/10.20517/2574-1209.2018.34

suggesting that the KLF4/KLF2 signaling is upstream of the Rho activation in CCM lesions . Increased
[90]
KLF4/KLF2 expression can lead to increased ADAMTS that functions to cleave proteoglycan matrix such
as versican. Therefore, the up-regulation of ADAMTS can lead to disruption of intervening parenchyma
around the blood vessel resulting in the formation of a cavernoma [12,13] .

CCM3 is a regulator of cell apoptosis
CCM3 has been linked to both apoptosis and cell survival pathways. Initially, CCM3 was discovered as a
protein for granulocyte apoptosis. One proposed mechanism is that CCM3 binds with VEGFR2 resulting
in increased stabilization of the receptor. Decreased CCM3 results in increased degradation of VEGFR2
[135]
leading to decreased VEGF stimulation . Furthermore, CCM3 is implicated in translocation of MST4 to
the periphery of the cell where it activates ERM proteins. These ERM proteins are anti-oxidative and thereby
prolong cell survival .
[159]

CCM3 plays a role in exocytosis
CCM3 is known to interact with STK24 and UNC13D, a known vesicle fusion regulator, in neutrophils.
STK24 is an inhibitor of neutrophil vesicle exocytosis. STK24 deficient neutrophils release larger amounts
of enzymes through exocytosis. STK24 localizes to neutrophil granules. There are two pools of granules
in neutrophils: readily available and reserved. STK24 is associated with increased release of the reserved
pool. UNC13D is a protein that binds to vesicles to promote their exocytosis. STK24 inhibits UNC13D [160,161] .
CCM3 has a dual effect on neutrophil exocytosis. CCM3 binds to STK24 and stabilizes it to increase
neutrophil exocytosis. However, CCM3 also increases the binding of UNC13D to liposomes through a
calcium mediated mechanism, which is only seen in high intracellular concentrations. This inactivates excess
UNC13D, resulting in decreased exocytosis. Simply stated, CCM3 is important for maintaining equilibrium
of neutrophil exocytosis. Loss of CCM3 increases exocytosis of granules in neutrophils. This has been shown
in renal ischemia-reperfusion injury model where reperfusion resulted in increased damage, suggesting that
CCM3 deficiency results in an increased oxidative damage due to neutrophil exocytosis .
[162]

CCM3 controls EC proliferation. Weibel-Palade bodies are granules in EC cells that contain angiopoietin-2
(ANGPT2) [163,164] . ANGPT2 binds to a tyrosine kinase receptor, TIE-2, and regulates the formation of EC
cell-cell junction in angiogenesis. Ccm3 knockout mice were shown to have increased Angpt2 expression.
Furthermore, TIE-2 showed more phosphorylation in areas such as cerebellum and retina, areas classically
known to form CCM lesions. As explained previously, CCM3 is a mediator of exocytosis in neutrophils
through UNC13B. It was seen to mediate exocytosis in EC as well. EC cells were shown to have increased
exocytosis of granules which can be rescued by the suppression of UNK13B. ANGPT2 transcription or
translation was not affected by CCM3 silencing, suggesting that this is not regulated at the transcriptional
level. This is consistent with the theory that CCM3 blocks of exocytosis of ANGPT2. A decreased CCM lesion
burden was observed in Ccm3 knockout mice with the introduction of ANGPT2 antibodies, reaffirming
the involvement of TIE2 signaling in the CCM lesion formation. This finding provides another venue for
potential pharmacotherapy .
[90]

CCM3 might have multiple cellular functions through its partners
CCM3 interacts with STK25 or MST4 to form the STRIPAK complex which localizes to the cis-face
[165]
of the Golgi complex . At this location, it plays a role in appropriate positioning of the Golgi. GCKIII
[61]
kinases are activated by homodimerization and resulting in its autophosphorylation, but activation is
tightly regulated by CCM3 . Dysfunction of CCM3 results in the malposition of Golgi complex and
[95]
centrosome . Migration is essential for proper placement of EC cells during angiogenesis. Increased
[166]
expression of CCM3 causes over migration of EC cell . Therefore, dysfunction of this process could be
[162]
involved in the formation of CCM lesions.