Rap1 Deficiency Research Articles

Rap1 in the Context of PCSK9, Atherosclerosis, and Diabetes.

The focus of this article is to highlight the importance of the small GTPase, Ras-associated protein 1 (Rap1), in proprotein convertase subtilisin/kexin type 9 (PCSK9) regulation and atherosclerosis and type 2 diabetes etiology and discuss the potential therapeutic implications of targeting Rap1 in these disease areas. Cardiometabolic disease characterized by obesity, glucose intolerance, dyslipidemia, and atherosclerotic cardiovascular disease remain an important cause of mortality. Evidence using mouse models of obesity and insulin resistance indicates that Rap1 deficiency increases proatherogenic PCSK9 and low-density lipoprotein cholesterol levels and predisposes these mice to develop obesity- and statin-induced hyperglycemia, which highlights Rap1's role in cardiometabolic dysfunction. Rap1 may also contribute to cardiovascular disease through its effects on vascular wall cells involved in the atherosclerosis progression. Rap1 activation, specifically in the liver, could be beneficial in the prevention of cardiometabolic perturbations, including type 2 diabetes, hypercholesterolemia, and atherosclerosis.

Rasa3 deficiency minimally affects thrombopoiesis but promotes severe thrombocytopenia due to integrin-dependent platelet clearance.

Platelet homeostasis is dependent on a tight regulation of both platelet production and clearance. The small GTPase Rap1 mediates platelet adhesion and hemostatic plug formation. However, Rap1 signaling is also critical for platelet homeostasis as both Rap1 deficiency and uninhibited Rap1 signaling lead to marked thrombocytopenia in mice. Here, we investigated the mechanism by which deficiency in Rasa3, a critical negative regulator of Rap1, causes macrothrombocytopenia in mice. Despite marked morphological and ultrastructural abnormalities, megakaryocytes in hypomorphic Rasa3hlb/hlb (R3hlb/hlb) or Rasa3–/– mice demonstrated robust proplatelet formation in vivo, suggesting that defective thrombopoiesis is not the main cause of thrombocytopenia. Rather, we observed that R3hlb/hlb platelets became trapped in the spleen marginal zone/red pulp interface, with evidence of platelet phagocytosis by macrophages. Clearance of mutant platelets was also observed in the liver, especially in splenectomized mice. Platelet count and platelet life span in Rasa3-mutant mice were restored by genetic or pharmacological approaches to inhibit the Rap1/talin1/αIIbβ3 integrin axis. A similar pattern of splenic clearance was observed in mice injected with anti-αIIbβ3 but not anti–glycoprotein Ibα platelet-depleting antibodies. In summary, we describe a potentially novel, integrin-based mechanism of platelet clearance that could be critical for our understanding of select inherited and acquired thrombocytopenias.

Rap1 prevents colitogenic Th17 cell expansion and facilitates Treg cell differentiation and distal TCR signaling

T-cell-specific Rap1 deletion causes spontaneous colitis in mice. In the present study, we revealed that Rap1 deficiency in T cells impaired the preceding induction of intestinal RORγt+ Treg cells. In the large intestinal lamina propria (LILP) of T-cell-specific Rap1-knockout mice (Rap1KO mice), Th17 cells were found to increase in a microbiota-dependent manner, and the inhibition of IL-17A production prevented the development of colitis. In the LILP of Rap1KO mice, RORγt+ Treg cells were scarcely induced by 4 weeks of age. The expression of CTLA-4 on Rap1-deficient Treg cells was reduced and the expression of CD80 and CD86 on dendritic cells was consequently elevated in Rap1KO mice. When cultured under each polarizing condition, Rap1-deficient naïve CD4+ T cells did not show biased differentiation into Th17 cells; their differentiation into Treg cells as well as Th1 and Th2 cells was lesser than that of wild-type cells. Rap1-deficient naïve CD4+ T cells were found to exhibit the defective nuclear translocation of NFAT and formation of actin foci in response to TCR engagement. These data suggest that Rap1 amplifies the TCR signaling required for Treg-mediated control of intestinal colitogenic Th17 responses.

Rap1 Is Essential for B-Cell Locomotion, Germinal Center Formation and Normal B-1a Cell Population.

Integrin regulation by Rap1 is indispensable for lymphocyte recirculation. In mice having B-cell-specific Rap1a/b double knockouts (DKO), the number of B cells in lymph nodes decreased to approximately 4% of that of control mice, and B cells were present in the spleen and blood. Upon the immunization with NP-CGG, DKO mice demonstrated the defective GC formation in the spleen, and the reduced NP-specific antibody production. In vitro, Rap1 deficiency impaired the movement of activated B cells along the gradients of chemoattractants known to be critical for their localization in the follicles. Furthermore, B-1a cells were almost completely absent in the peritoneal cavity, spleen and blood of adult DKO mice, and the number of B-cell progenitor/precursor (B-p) were reduced in neonatal and fetal livers. However, DKO B-ps normally proliferated, and differentiated into IgM+ cells in the presence of IL-7. CXCL12-dependent migration of B-ps on the VCAM-1 was severely impaired by Rap1 deficiency. Immunostaining study of fetal livers revealed defects in the co-localization of DKO B-ps and IL-7-producing stromal cells. This study proposes that the profound effects of Rap1-deficiency on humoral responses and B-1a cell generation may be due to or in part caused by impairments of the chemoattractant-dependent positioning and the contact with stromal cells.

Rap1 in the VMH regulates glucose homeostasis.

The hypothalamus is a critical regulator of glucose metabolism and is capable of correcting diabetes conditions independently of an effect on energy balance. The small GTPase Rap1 in the forebrain is implicated in high-fat diet–induced (HFD-induced) obesity and glucose imbalance. Here, we report that increasing Rap1 activity selectively in the medial hypothalamus elevated blood glucose without increasing the body weight of HFD-fed mice. In contrast, decreasing hypothalamic Rap1 activity protected mice from diet-induced hyperglycemia but did not prevent weight gain. The remarkable glycemic effect of Rap1 was reproduced when Rap1 was specifically deleted in steroidogenic factor-1–positive (SF-1–positive) neurons in the ventromedial hypothalamic nucleus (VMH) known to regulate glucose metabolism. While having no effect on body weight regardless of sex, diet, and age, Rap1 deficiency in the VMH SF1 neurons markedly lowered blood glucose and insulin levels, improved glucose and insulin tolerance, and protected mice against HFD-induced neural leptin resistance and peripheral insulin resistance at the cellular and whole-body levels. Last, acute pharmacological inhibition of brain exchange protein directly activated by cAMP 2, a direct activator of Rap1, corrected glucose imbalance in obese mouse models. Our findings uncover the primary role of VMH Rap1 in glycemic control and implicate Rap1 signaling as a potential target for therapeutic intervention in diabetes.

Endothelial Rap1 (Ras-Association Proximate 1) Restricts Inflammatory Signaling to Protect From the Progression of Atherosclerosis.

Small GTPase Rap1 (Ras-association proximate 1) is a novel, positive regulator of NO release and endothelial function with a potentially key role in mechanosensing of atheroprotective, laminar flow. Our objective was to delineate the role of Rap1 in the progression of atherosclerosis and its specific functions in the presence and absence of laminar flow, to better define its role in endothelial mechanisms contributing to plaque formation and atherogenesis. Approach and Results: In a mouse atherosclerosis model, endothelial Rap1B deletion exacerbates atherosclerotic plaque formation. In the thoracic aorta, where laminar shear stress-induced NO is otherwise atheroprotective, plaque area is increased in Athero-Rap1BiΔEC (atherogenic endothelial cell-specific, tamoxifen-inducible Rap1A+Rap1B knockout) mice. Endothelial Rap1 deficiency also leads to increased plaque size, leukocyte accumulation, and increased CAM (cell adhesion molecule) expression in atheroprone areas, whereas vascular permeability is unchanged. In endothelial cells, in the absence of protective laminar flow, Rap1 deficiency leads to an increased proinflammatory TNF-α (tumor necrosis factor alpha) signaling and increased NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) activation and elevated inflammatory receptor expression. Interestingly, this increased signaling to NF-κB activation is corrected by AKTVIII-an inhibitor of Akt (protein kinase B) translocation to the membrane. Together, these data implicate Rap1 in restricting Akt-dependent signaling, preventing excessive cytokine receptor signaling and proinflammatory NF-κB activation. Via 2 distinct mechanisms, endothelial Rap1 protects from the atherosclerosis progression in the presence and absence of laminar flow; Rap1-stimulated NO release predominates in laminar flow, and restriction of proinflammatory signaling predominates in the absence of laminar flow. Our studies provide novel insights into the mechanisms underlying endothelial homeostasis and reveal the importance of Rap1 signaling in cardiovascular disease.

Dissection of \u03b14\u03b27 integrin regulation by Rap1 using novel conformation-specific monoclonal anti-\u03b27 antibodies

Integrin activation is associated with conformational regulation. In this study, we developed a system to evaluate conformational changes in α4β7 integrin. We first inserted the PA tag into the plexin-semaphorin-integrin (PSI) domain of β7 chain, which reacted with an anti-PA tag antibody (NZ-1) in an Mn2+-dependent manner. The small GTPase Rap1 deficiency, as well as chemokine stimulation and the introduction of the active form of Rap1, Rap1V12, enhanced the binding of NZ-1 to the PA-tagged mutant integrin, and increased the binding affinity to mucosal addressing cell adhesion molecule-1 (MAdCAM-1). Furthermore, we generated two kinds of hybridomas producing monoclonal antibodies (mAbs) that recognized Mn2+-dependent epitopes of β7. Both epitopes were exposed to bind to mAbs on the cells by the introduction of Rap1V12. Although one epitope in the PSI domain of β7 was exposed on Rap1-deficienct cells, the other epitope in the hybrid domain of β7 was not. These data indicate that the conversion of Rap1-GDP to GTP exerts two distinct effects stepwise on the conformation of α4β7. The induction of colitis by Rap1-deficient CD4+ effector/memory T cells suggests that the removal of constraining effect by Rap1-GDP on α4β7 is sufficient for homing of these pathogenic T cells into colon lamina propria (LP).

Repressor Activator Protein 1 Worsens Cardiomyopathy in Diet‐induced Type 2 Diabetic Mice

BackgroundDiabetic cardiomyopathy is defined by the structural and functional abnormalities of the myocardium in the absence of other cardiac risk factors in diabetic patients. In diabetes, the increment of fatty acid metabolism and subsequently cardiac inefficiency and myocardial lipid accumulation may underlie the pathology of diabetic cardiomyopathy. The telomeric protein repressor activator protein 1 (Rap1) is an identified regulator that regulates gene transcription by binding to non‐telomeric sites. In particular, Rap1 can bind to nuclear PPARα (an important transcriptional regulator of lipid metabolism) loci, and regulate transcription of PPARα and its target genes, including CD36 (fatty acid transporter). However, the effects of Rap1 on diabetic cardiomyopathy are largely unknown. Thus, the present study investigated if Rap1 deficiency protects against diabetic cardiomyopathy and explored the underlying mechanisms.Methods and ResultsHigh fat diet feeding is a widely used protocol to induce type 2 diabetes in mice. Upon high fat feeding (60% fat) for 12 weeks, male Rap1 knockout (KO) mice displayed significantly reduced body weight gain compared with their wild type (WT) littermates. The glucose tolerance test and insulin tolerance test showed that Rap1 deficiency in the high fat diet fed‐mice led to improved glucose intolerance and insulin action. Furthermore, Rap1 deficiency attenuates high fat diet‐induced cardiac structural remodeling and dysfunction in mice, as evidenced by significantly reduction in left ventricular internal dimension in systole (LVIDs), left ventricular mass, increment in ejection fraction (EF) and fractional shortening (FS), as well as improving of myocardial performance index (MPI). These changes were associated with remarkable reductions in the mRNA and protein expression of PPARα and CD36 in the myocardium of high fat fed‐Rap1 KO mice, indicating that Rap1 deficiency may protect against diabetic cardiomyopathy via the suppression of PPARα/CD36 signaling.ConclusionIn summary, these results suggest that Rap1 impairment has cardioprotective consequences in the high fat diet‐induced diabetes model, identifying a new dimension in the roles of endogenous Rap1.Support or Funding InformationThe authors’ work was supported by The General Research Fund (17117217M, 17123718M, Research Grants Council of Hong Kong) and The National Natural Science Foundation of China (No. 81800245).

Loss of Repressor Activator Protein 1 Precipitates Cardiac Aging in Mice via p53/PPARα Signaling

BackgroundTelomere dysfunction has been linked to many aspects of the aging process. Increasing evidence indicates that telomere dysfunction leads to cardiac mitochondrial defects. Mammalian repressor activator protein 1 (Rap1) is one of the components of shelterin complex and essential for the maintenance of telomere length and structural integrity. Our preliminary work showed that aged Rap1 knockout (Rap1−/−) mice exhibited more pronounced aging‐associated phenotypes (including earlier hair graying, hair loss, reduced body weight). However, the effects of Rap1 on mitochondrial function and its contribution to cardiac aging are largely unknown. Thus, the present study investigated whether or not loss of Rap1 in mice precipitates cardiac aging and explored the underlying mechanisms.Methods and ResultsMice cardiac function was assessed with transthoracic echocardiography performed noninvasively using a Vevo 2100 high‐resolution imaging system. Deletion of Rap1 in mice aggravated aging‐related cardiac structural changes and dysfunction, as evidenced by increased left ventricular (LV) posterior wall end‐diastole and LV mass, reductions in ejection fraction and fractional shortening, as well as significantly reduced myocardial performance index (all p < 0.05, Rap1−/− vs. age‐matched counterparts). These changes were associated with greater cardiac senescence (enhanced SA‐β‐gal staining), cardiac hypertrophy (enhanced wheat germ agglutinin staining and increased the ratio of heart weight with body weight), abnormalities in the mitochondrial ultrastructure (cristae fragmentation, vacuolization). Mechanistically, Rap1 deficiency led to shorter telomere (T/S ratio), enhanced DNA damage (γH2AX) and increased nuclear p53 level in vivo in the myocardium and in primary cardiomyocytes when compared to age‐matched counterparts (one year old). Chromatin immunoprecipitation revealed that p53 directly binded to the promote site of PPARα [a central regulator of cardiac fatty acid metabolism (FAM)] and repressed its expression. Rap1 deficiency impaired FAM (reduced CD36, CPT1α, ACADL level and palmitate‐induced oxygen consumption), while PFTα (a p53 inhibitor, i.p. 1.1 mg/kg/day) treatment in Rap1−/− mice significantly alleviated cardiac aging and enhanced FAM by restoration of PPARα.ConclusionsWith aging, Rap1 deficiency may lead to shorter telomere length, increase DNA damage and thereby activate p53, which in turn suppresses PPARα, leading to impaired FAM and mitochondrial defects with abnormal cardiac structure and dysfunction. These findings may identify a new cardiac dimension in the physiological role played by telomere‐Rap1.Support or Funding InformationThe authors’ work was supported by the General Research Fund (17123718M, Research Grants Council of Hong Kong).

Rap1 exacerbates myocardial ischemia/reperfusion injury through activation of NFκB signaling pathway and NLRP3 inflammasome

BackgroundIschemic heart disease is a leading cause of morbidity and mortality worldwide. Activation of nuclear factor kappa B (NFκB)‐mediated inflammation has been demonstrated in various models of experimental myocardial ischemia/reperfusion injury (I/RI). Repressor activator protein 1 (Rap1), an established telomere‐associated protein, is a novel modulator of NFκB‐mediated inflammatory response. However, it is unknown whether Rap1 modulates NFκB signaling in cardiomyocytes to contribute to myocardial I/RI. Therefore, the present study aimed to explore the potential pro‐inflammatory role of Rap1 in myocardial I/RI and to determine the underlying molecular mechanism.Methods and ResultsIn a mouse model of myocardial I/RI (30 minutes of left descending coronary artery ligation followed by 24 hours reperfusion), Rap1 deficiency significantly reduced myocardial infarct size and improved cardiac systolic/diastolic function (all p < 0.05, Rap1−/− I/R vs. Rap1+/+ I/R). This was associated with a reduction in NFκB activity [reduced phosphorylation, degradation of IκBα and phosphorylation of p65] and suppressed production of NFκB‐mediated pro‐inflammatory cytokines, including tumor necrosis factor α (TNFα), interleukin (IL)6 and IL1β in the post‐ischemic myocardium (all p < 0.05, Rap1−/− I/R vs. Rap1+/+ I/R). In cultured cardiac origin H9C2 cardiomyocytes, Rap1 knockdown did not significantly influence the pro‐inflammatory response in hypoxia/reoxygenation or in lipopolysaccharide‐stimulated H9C2 cells, but deletion of Rap1 significantly attenuated macrophage infiltration into the post‐ischemic myocardium of mice (p < 0.05, Rap1−/− I/R vs. Rap1+/+ I/R). Also, knockdown of Rap1 significantly suppressed NLRP3 inflammasome formation in THP‐1 macrophages in response to LPS and ATP (p < 0.05, Rap1KD vs. Rap1WT). When post‐hypoxic H9C2 cardiomyocytes were co‐cultured with mock‐ or Rap1‐siRNA transfected THP‐1 macrophages, knockdown of Rap1 in macrophages significantly attenuated macrophage‐mediated/exacerbated cardiomyocyte injury (p < 0.05, Rap1KD vs. Rap1WT).ConclusionsThese data collectively suggest that Rap1 may exacerbate myocardial I/RI through enhancing activation of NFκB signaling pathway and NLRP3 inflammasome formation, which may yield a novel therapeutic target for myocardial I/RI.Support or Funding InformationThe work was supported by the National Natural Science Foundation of China (No. 81800245) and General Research Fund (17117217M, 17123718M Research Grants Council of Hong Kong).

Deletion of Rap1 protects against myocardial ischemia/reperfusion injury through suppressing cell apoptosis via activation of STAT3 signaling.

Ischemic heart disease is a leading cause of morbidity and mortality. Repressor activator protein 1 (Rap1), an established telomere-associated protein, is a novel modulator of hypoxia-induced apoptosis. This study aimed to explore the potential direct role of Rap1 in myocardial ischemia/reperfusion injury (I/RI) and to determine the underlying molecular mechanism. In a mouse model of myocardial I/RI (30-min of left descending coronary artery ligation followed by 2-h reperfusion), Rap1 deficiency significantly reduced myocardial infarct size (IS) and improved cardiac systolic/diastolic function. This was associated with a reduction in apoptosis in the post-ischemic myocardium. In H9C2 and primary cardiomyocytes, Rap1 knockdown or knockout significantly suppressed hypoxia/reoxygenation (H/R)-induced cell injury and apoptosis through increasing the phosphorylation/activation of STAT3 at site Ser727 and translocation of STAT3 to the nucleus. We surmise this since Stattic (selective STAT3 inhibitor) pretreatment canceled the abovementioned protective effect. Furthermore, co-immunoprecipitation assay revealed a direct interaction between Rap1 and STAT3, but not JAK2, suggesting that the association of Rap1 with STAT3 may contribute to the reduced activity of STAT3 (Ser727 ) upon H/R stimulation. In conclusion, Rap1 deficiency protects the heart from ischemic damage through STAT3-dependent reduction of cardiomyocyte apoptosis, which may yield viable target for pharmacological intervention in ischemic heart disease.

Deletion of Telomere‐Rap1 aggravates cardiac aging by impairing fatty acid oxidation via PPARα signaling

The heart undergoes multiple functional and structural declining with aging, including impaired fatty acid metabolism, cardiac dysfunction and hypertrophy. Repressor activator protein 1 (Rap1) as a component of shelterin complex is essential for the maintenance of telomere function. Moreover, Rap1 also regulates gene transcription by binding to non‐telomeric sites such as the nuclear hormone receptor ‐ peroxisome proliferator‐activated receptor α (PPARα) which plays a central role in the cardiac fatty acid metabolism and mitochondrial function. However, it's still not clear whether or not the heart of Rap1−/− mice also suffers more aging associated disorders. Thus, the present study aims at the role of Rap1 in cardiac aging and its possible mechanism. Echocardiography showed enlarged left ventricular internal dimension (LVID) and LV mass, which suggested dilated hypertrophy, and elevated myocardial performance index (MPI) in Rap1−/− mice at age of 85weeks, indicating systolic and diastolic dysfunction (all P<0.05 vs. Rap1+/+). The greater adverse cardiac remodeling in aged Rap1−/− mice was paralleled by elevated cardiomyocyte area showed by Wheat germ agglutinin (WGA) staining (P<0.05 vs. Rap1+/+). p53 as a senescence marker was also increased indicating more severe cellular senescence in cardiomyocyte of Rap1−/− mice (P<0.05 vs. Rap1+/+). From apex cordis of Rap1 wildtype and knockout mice, electron microscopy showed more enlarged pleomorphic mitochondria containing cristae fragmentation, vacuolization and disrupted external membranes in aged Rap1 knockout mice compared to age‐matched wildtype counterparts. Decreased Acetyl‐CoA carboxylase (ACC) (catalyzes the production of malonyl‐CoA) and Elevated Carnitine palmitoyl transferase I (CPT‐1) (transports fatty acyl‐CoA into mitochondria) and Acyl‐CoA dehydrogenase long chain (ACADL, catalyzes most of fatty acid β‐oxidation) were detected in aged Rap1 knockout mice(all P<0.05 vs. Rap1+/+), indicating Rap1 deficiency may lead to reduced fatty acid oxidation during aging. In the heart of aged Rap1−/− mice, PPARα was reduced compared with age‐matched wildtype counterparts (P<0.05 vs. Rap1+/+), suggesting that with aging, Rap1 may down‐regulate PPARα signaling in the heart and control fatty acid metabolism resulting in cardiac dysfunction and hypertrophy. In conclusion, Rap1 deficiency may impair myocardial fatty acid oxidation via PPARα signaling, leading to more severe cardiac aging as cardiac dysfunction and hypertrophy. These findings may identify a new extratelomeric role of Rap1 in cardiac metabolism during aging.Support or Funding InformationThe authors' work was supported by the Health and Medical Research Fund (04151816, Hong Kong) and General Research Fund (17121315M, 17123915M and 17117217M, Research Grants Council of Hong Kong).This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Telomere-dependent and telomere-independent roles of RAP1 in regulating human stem cell homeostasis

RAP1 is a well-known telomere-binding protein, but its functions in human stem cells have remained unclear. Here we generated RAP1-deficient human embryonic stem cells (hESCs) by using CRISPR/Cas9 technique and obtained RAP1-deficient human mesenchymal stem cells (hMSCs) and neural stem cells (hNSCs) via directed differentiation. In both hMSCs and hNSCs, RAP1 not only negatively regulated telomere length but also acted as a transcriptional regulator of RELN by tuning the methylation status of its gene promoter. RAP1 deficiency enhanced self-renewal and delayed senescence in hMSCs, but not in hNSCs, suggesting complicated lineage-specific effects of RAP1 in adult stem cells. Altogether, these results demonstrate for the first time that RAP1 plays both telomeric and nontelomeric roles in regulating human stem cell homeostasis.

Mice lacking RAP1 show early onset and higher rates of DEN-induced hepatocellular carcinomas in female mice.

RAP1, a component of the telomere-protective shelterin complex, has been shown to have both telomeric and non-telomeric roles. In the liver, RAP1 is involved in the regulation of metabolic transcriptional programs. RAP1-deficient mice develop obesity and hepatic steatosis, these phenotypes being more severe in females than in males. As hepatic steatosis and obesity have been related to increased liver cancer in mice and humans, we set out to address whether RAP1 deficiency resulted in increased liver cancer upon chemical liver carcinogenesis. We found that Rap1-/- females were more susceptible to DEN-induced liver damage and hepatocellular carcinoma (HCC). DEN-treated Rap1-/- female livers showed an earlier onset of both premalignant and malignant liver lesions, which were characterized by increased abundance of γH2AX-positive cells, increased proliferation and shorter telomeres. These findings highlight an important role for RAP1 in protection from liver damage and liver cancer.

Abstract 229: Small GTPase Rap1 deficiency Accelerates Development of Atherosclerosis in ApoE Deficient Mice

Rap1, a ubiquitously expressed small GTPase, integrates signals from multiple receptors and promotes activation and signaling by cell adhesion receptors. In vivo , endothelial (EC) Rap1 is required for normal vessel formation, angiogenesis and dynamic regulation of EC barrier. Two Rap1 isoforms, Rap1A and Rap1B, share 95% identity, and EC deletion of both isoforms leads to embryonic lethality due to cardiovascular defects. We have recently demonstrated that Rap1 is a novel regulator of shear sensing in endothelium and that it is essential for nitric oxide release, and that Rap1 deficiency leads to EC dysfunction. These findings demonstrated that Rap1 plays an essential role in vasoprotective response of endothelium in response to shear stress. The objective of this study is to determine the specific role of Rap1 response to laminar, vasoprotective flow and pro-inflammatory, disturbed flow by examining the effect of EC deletion of Rap1B, the more prominent Rap1 isoform, on progression of atherosclerosis. To investigate the role of Rap1 in progression of atherosclerosis, we crossed apolipoprotein E-deficient (ApoE -/- ) mice with tamoxifen-inducible, EC specific Rap1B-knockout (Cadh5-CreERT +/0 ; Rap1B f/f ) to generate atherogenic EC-Rap1B KO (Athero-Rap1B KO) mice. We hypothesized that Rap1B deficiency will exacerbate progression of atherosclerosis. Athero-Rap1B KO and littermate controls (Cadh5-Cre-negative mice or mice injected with carrier oil only) were fed high fat, western diet (21.2% fat, 0.2% cholesterol) for 16 weeks. No difference in total cholesterol levels were observed between Athero- Rap1B KO and control mice. Atherosclerotic lesion formation was visualized with oil O red and plaque area was quantified in en-face preparations. (n=5-9). We observed increased lesion area in the descending aorta of Athero-Rap1B KO mice (0.7351 ± 0.1065 vs 0.4866 ± 0.09812; p =0.0584, one-tailed t-test, Welch corrected) and a strong trend towards larger lesion area in the total aorta (2.142 ± 0.5058 vs 1.267 ± 0.1718; p=0.0679), and the arch of the aorta (1.489 ± 0.4343 vs 0.7807 ± 0.1152; p=0.0747) compared with control mice. These findings demonstrate that Rap1B deficiency accelerates progression of atherosclerosis and show that Rap1 plays a vasoprotective role in both laminar flow and disturbed flow areas.

Deletion of Rap1 disrupts redox balance and impairs endothelium-dependent relaxations

AimsRepressor activator protein 1 (Rap1) is conventionally known as a static structural component of the telomere, but recent evidence indicates that it exerts functions within and outside the nucleus taking part in metabolic regulation and promoting inflammatory responses. The present study investigated whether or not Rap1 deletion affects oxidative stress and nitric oxide (NO) bioavailability in the vascular wall, thus modulating endothelial function. Methods and resultsVascular responsiveness was studied in wire myographs in aortae from Rap1 wildtype and knockout mice. Deletion of Rap1 impaired endothelium-dependent relaxations elicited by acetylcholine. Rap1 deficiency did not affect the activation of endothelial NO synthase or the sensitivity of vascular smooth muscle to NO donors. The blunted acetylcholine-mediated relaxations in Rap1 deficient aortae were restored with nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitors, apocynin or VAS2870. Rap1 deletion lowered cellular thiol-redox status and diminished activities of thiol-redox enzymes, thioredoxin 1 and glutaredoxin 1. ConclusionsThe capacity of thioredoxin 1 and glutaredoxin 1 to reduce intra-protein disulfide bridges is weakened in Rap1 deficient mice, resulting in hyper-activation of NADPH oxidase and greater reactive oxygen species generation. The high oxidative stress in Rap1 deficient mice is implicated with greater oxidative breakdown of NO, explaining the blunted acetylcholine-mediated relaxations in this animal. These findings imply that Rap1 plays an unanticipated role in regulating the fate of NO (a pivotal determinant of vascular homeostasis) and thus identify a new physiological importance of the telomere-associated protein.

Rap1GAP inhibits tumor progression in endometrial cancer

ObjectiveEndometrioid adenocarcinoma (EAC) is a common endometrial cancer with recent dramatic increases in incidence. Previous findings indicate that Rap1GAP acts as a tumor suppressor inhibiting Ras superfamily protein Rap1 in multiple aggressive carcinomas; however, Rap1GAP expression in EAC has not been investigated. In this study, the tumor suppressing activity of Rap1GAP in EAC was explored. MethodsEAC cell lines were used to examine Rap1GAP levels by real-time RT-PCR and western blotting and the effects of Rap1GAP on cancer cell invasion and migration. Rap1GAP expression was analyzed by immunohistochemical staining for Rap1GAP, E-cadherin in surgically resected tumors of 114 EAC patients scored according to EAC differentiation grade. Prognostic variables such as age, stage, grade, tumor size, and immunostaining for Rap1GAP, E-cadherin were evaluated using Cox regression multivariate analysis. ResultsLow Rap1GAP expression was detected in poorly differentiated EAC cells. Rap1GAP deficiency significantly accelerated while Rap1 deficiency decreased cancer cell migration and invasion. Patients with higher Rap1GAP, E-cadherin, and especially combined Rap1GAP/E-cadherin levels had better overall survival than EAC patients with no or weak expression. In addition, Rap1GAP expression was an independent prognostic factor in EAC. ConclusionsInhibition of Rap1GAP expression increases EAC cell migration and invasion through upregulation of Rap1. Low expression of Rap1GAP correlates with poor EAC differentiation. Our findings suggest that Rap1GAP is an important tumor suppressor with high prognostic value in EAC.

A genetic interaction between RAP1 and telomerase reveals an unanticipated role for RAP1 in telomere maintenance.

Summary RAP1 is one of the components of shelterin, the capping complex at chromosome ends or telomeres, although its role in telomere length maintenance and protection has remained elusive. RAP1 also binds subtelomeric repeats and along chromosome arms, where it regulates gene expression and has been shown to function in metabolism control. Telomerase is the enzyme that elongates telomeres, and its deficiency causes a premature aging in humans and mice. We describe an unanticipated genetic interaction between RAP1 and telomerase. While RAP1 deficiency alone does not impact on mouse survival, mice lacking both RAP1 and telomerase show a progressively decreased survival with increasing mouse generations compared to telomerase single mutants. Telomere shortening is more pronounced in Rap1 −/− Terc −/− doubly deficient mice than in the single‐mutant Terc −/− counterparts, leading to an earlier onset of telomere‐induced DNA damage and degenerative pathologies. Telomerase deficiency abolishes obesity and liver steatohepatitis provoked by RAP1 deficiency. Using genomewide ChIP sequencing, we find that progressive telomere shortening owing to telomerase deficiency leads to re‐localization of RAP1 from telomeres and subtelomeric regions to extratelomeric sites in a genomewide manner. These findings suggest that although in the presence of sufficient telomere reserve RAP1 is not a key factor for telomere maintenance and protection, it plays a crucial role in the context of telomerase deficiency, thus in agreement with its evolutionary conservation as a telomere component from yeast to humans.

Dual functions of Rap1 are crucial for T-cell homeostasis and prevention of spontaneous colitis.

Rap1-GTP activates leukocyte function-associated antigen-1 (LFA-1) to induce arrest on the high endothelial venule (HEV). Here we show that Rap1-GDP restrains rolling behaviours of T cells on the peripheral lymph node addressin (PNAd), P-selectin and mucosal addressin cell adhesion molecule-1 (MadCAM-1) by inhibiting tether formation. Consequently, Rap1 deficiency impairs homing of naive T cells to peripheral lymph nodes, but accelerates homing of TH17 and TH1 cells to the colon, resulting in spontaneous colitis with tumours. Rap1-GDP associates with and activates lymphocyte-oriented kinase, which phosphorylates ERM (ezrin, radixin and moesin) in resting T cells. Phosphomimetic ezrin reduces the rolling of Rap1-deficient cells, and thereby decreases their homing into the colon. On the other hand, chemokines activate Rap1 at the plasma membrane within seconds, and Rap1-GTP binds to filamins, which diminishes its association with the β2 chain of LFA-1 and results in LFA-1 activation. This Rap1-dependent regulation of T-cell circulation prevents the onset of colitis.

Rap1 promotes endothelial mechanosensing complex formation, NO release and normal endothelial function.

Decreased nitric oxide (NO) bioavailability underlies a number of cardiovascular pathologies, including hypertension. The shear stress exerted by flowing blood is the main determinant of NO release. Rap1 promotes integrin- and cadherin-mediated signaling. Here, we show that Rap1 is a critical regulator of NO production and endothelial function. Rap1 deficiency in murine endothelium attenuates NO production and diminishes NO-dependent vasodilation, leading to endothelial dysfunction and hypertension, without deleterious effects on vessel integrity. Mechanistically, Rap1 is activated by shear stress, promotes the formation of the endothelial mechanosensing complex-comprised of PECAM-1, VE-cadherin and VEGFR2- and downstream signaling to NO production. Our study establishes a novel paradigm for Rap1 as a regulator of mechanotransduction.