Cerebral Cavernous Malformation 2 Protein Promotes Smad Ubiquitin Regulatory Factor 1-mediated RhoA Degradation in Endothelial Cells

Abstract

Mutation of CCM2 predisposes individuals to cerebral cavernous malformations, vascular abnormalities that cause seizures and hemorrhagic stroke. CCM2 has been proposed to regulate the activity of RhoA for maintenance of vascular integrity. Herein, we define a novel mechanism where the CCM2 phosphotyrosine binding (PTB) domain binds the ubiquitin ligase (E3) Smurf1, controlling RhoA degradation. Brain endothelial cells with knockdown of CCM2 have increased RhoA protein and display impaired directed cell migration. CCM2 binding of Smurf1 increases Smurf1-mediated degradation of RhoA. CCM2 does not significantly alter the catalytic activity of Smurf1, nor is CCM2 a Smurf1 substrate. Rather the CCM2-Smurf1 interaction functions to localize Smurf1 for RhoA degradation. These findings provide a molecular mechanism for the pathogenesis of cerebral cavernous malformations (CCM) resulting from loss of CCM2-mediated localization of Smurf1, which controls RhoA degradation required for maintenance of normal endothelial cell physiology.We previously characterized a scaffold-like protein named osmosensing scaffold for MEKK3 (OSM) for its ability to bind actin and localize to Rac-containing membrane ruffles and its obligate requirement for p38 activation in response to hyperosmotic stress (1). Subsequently, the gene encoding OSM, CCM2, was found to be mutated in the human disease cerebral cavernous malformations (CCM) 2 (2). Cerebral cavernous malformations are vascular lesions of the central nervous system characterized as clusters of dilated, thin walled blood vessels. CCM lesions are fragile and prone to vascular leakiness and rupture, leading to hemorrhages that cause seizure and stroke (3, 4).Recently, CCM2 knockdown endothelial cells were shown to have increased activation of RhoA (5), although the mechanism was not defined. Herein, we demonstrate a molecular mechanism for activation of this pathway. Through a novel CCM2 PTB domain interaction with the Smurf1 homologous to the E6-AP C terminus (HECT) domain, we now show that CCM2 binds the E3 ligase Smurf1 for the control of RhoA degradation. EXPERIMENTAL PROCEDURESRNAi-bEND.3 cells were purchased from ATCC. pLKO.1 and pLKO.1 CCM2 shRNA plasmids were purchased from Open Biosystems. Lentiviral infection was done according to the RNAi consortium protocol.In Vitro Ubiquitination Assay-The in vitro ubiquitination assay was done as described (6).In Vitro Wound Healing Assay-bEND.3 cells stably expressing pLKO.1 or CCM2 shRNAs were transduced with pLL5.0 for expression of green fluorescent protein. Cells were grown in 24-well dishes to a monolayer and imaged on the BD Biosciences Pathway high content imager. Wounds were imaged every 15 min for 16 h on a ϫ10 objective. The percentage of wound healing was determined as the area of the wound remaining after 16 h divided by the initial area of the wound (ImageJ). RESULTS AND DISCUSSIONCCM2 Binds Smurf1-We have previously demonstrated that MEKK3, a Ste11-like MAP3K, regulates the p38 pathway in mammalian cells in response to hypero...

ICAP-1 monoubiquitylation coordinates matrix density and rigidity sensing for cell migration through ROCK2–MRCKα balance

“…In addition to its ability to ubiquitylate talin (Huang et al, 2009), Smurf1 was a good candidate for ICAP-1 monoubiquitylation because Smurf1 associates with the cerebral cavernous malformations (CCM) complex (Crose et al, 2009), which interacts with ICAP-1 (Hilder et al, 2007). Smurf1 also possesses an NPxY motif that might be able to interact with ICAP-1 phosphotyrosine-binding (PTB) domain.…”

Differential angiogenesis function of CCM2 and CCM3 in cerebral cavernous malformations

“…17 Although this proposal remains speculative, one could imagine that the endothelium in CCM lesions harbors certain unique properties because the lesions have been exposed to this abnormal, yet undefined, microenvironment. Within this context and considering that the functional study of CCM genes in culture has been performed exclusively in cell lines 3,5,6,14 or in commercially purchased normal ECs (HUVECs and HBMECs), 6,25 silencing CCM genes in CCM-ECs established in the present work may simulate some aspects of the diseased endothelium in the CCM lesions, and thus may serve as a unique and valuable model for investigating the function of CCM genes in the pathogenesis of CCMs.…”

“…8,24 The role of CCM2 in maintenance of vascular integrity has been proposed through regulating the activity of Ras homolog gene family member A. The mechanism of this regulation has been recently addressed by Crose et al 6 The CCM3 gene is suggested to be involved in apoptosis 5,14 and tumor signaling. 14 We found that loss of CCM3 led to a significant resistance of all 3 tested types of ECs to apoptotic stimuli (Fig.…”

Cerebral cavernous malformation proteins at a glance

“…The first indication that RhoA dysregulation might contribute to CCM pathology came from the observation of increased stress fiber formation (a sign of activated RhoA) after RNAinterference-mediated knockdown of any of the CCM proteins (Crose et al, 2009;Glading et al, 2007;Stockton et al, 2010;Whitehead et al, 2009;Zheng et al, 2010). Consistent with this, activated (GTP-bound) RhoA is increased in KRIT1-, CCM2-or PDCD10-deficient endothelial cells.…”

“…In addition, CCM2 appears to direct the degradation of RhoA through a CCM2 PTB-domain-mediated interaction with the E3 ubiquitin ligase Smad ubiquitin regulatory factor 1 (SMURF1) (Crose et al, 2009). Notably, loss of CCM2 leads to increases in RhoA, but not of other SMURF1 substrates, suggesting that CCM2 might selectively promote SMURF1-mediated RhoA degradation.…”