Regulation of the mTOR-Rac1 axis in platelet function

Co-expression analysis to identify key modules and hub genes associated with COVID-19 in platelets

Corona virus disease 2019 (COVID-19) increases the risk of cardiovascular occlusive/thrombotic events and is linked to poor outcomes. The underlying pathophysiological processes are complex, and remain poorly understood. To this end, platelets play important roles in regulating the cardiovascular system, including via contributions to coagulation and inflammation. There is ample evidence that circulating platelets are activated in COVID-19 patients, which is a primary driver of the observed thrombotic outcome. However, the comprehensive molecular basis of platelet activation in COVID-19 disease remains elusive, which warrants more investigation. Hence, we employed gene co-expression network analysis combined with pathways enrichment analysis to further investigate the aforementioned issues. Our study revealed three important gene clusters/modules that were closely related to COVID-19. These cluster of genes successfully identify COVID-19 cases, relative to healthy in a separate validation data set using machine learning, thereby validating our findings. Furthermore, enrichment analysis showed that these three modules were mostly related to platelet metabolism, protein translation, mitochondrial activity, and oxidative phosphorylation, as well as regulation of megakaryocyte differentiation, and apoptosis, suggesting a hyperactivation status of platelets in COVID-19. We identified the three hub genes from each of three key modules according to their intramodular connectivity value ranking, namely: COPE, CDC37, CAPNS1, AURKAIP1, LAMTOR2, GABARAP MT-ND1, MT-ND5, and MTRNR2L12. Collectively, our results offer a new and interesting insight into platelet involvement in COVID-19 disease at the molecular level, which might aid in defining new targets for treatment of COVID-19–induced thrombosis.

mTOR regulates GPVI-mediated platelet activation

Background

Due to mTOR (mammalian/mechanistic target of rapamycin) gene-loss mice die during embryonic development, the role of mTOR in platelets has not been evaluated using gene knockout technology.

Methods

A mouse model with megakaryocyte/platelet-specific deletion of mTOR was established, and be used to evaluate the role of mTOR in platelet activation and thrombus formation.

Results

mTOR^−/− platelets were deficient in thrombus formation when grown on low-concentration collagen-coated surfaces; however, no deficiency in thrombus formation was observed when mTOR^−/− platelets were perfused on higher concentration collagen-coated surfaces. In FeCl₃-induced mouse mesenteric arteriole thrombosis models, wild-type (WT) and mTOR^−/− mice displayed significantly different responses to low-extent injury with respect to the ratio of occluded mice, especially within the first 40 min. Additionally, mTOR^−/− platelets displayed reduced aggregation and dense granule secretion (ATP release) in response to low doses of the glycoprotein VI (GPVI) agonist collagen related peptide (CRP) and the protease-activated receptor-4 (PAR4) agonist GYPGKF-NH₂; these deficiencies were overcame by stimulation with higher concentration agonists, suggesting dose dependence of the response. At low doses of GPVI or PAR agonist, the activation of α_IIbβ₃ in mTOR^−/− platelets was reduced. Moreover, stimulation of mTOR^−/− platelets with low-dose CRP attenuated the phosphorylation of S6K1, S6 and Akt Ser473, and increased the phosphorylation of PKCδ Thr505 and PKCε Ser729. Using isoform-specific inhibitors of PKCs (δ, ɛ, and α/β), we established that PKCδ/ɛ, and especially PKCδ but not PKCα/β or PKCθ, may be involved in low-dose GPVI-mediated/mTOR-dependent signaling.

Conclusion

These observations indicate that mTOR plays an important role in GPVI-dependent platelet activation and thrombus formation.

Rapamycin restrains platelet procoagulant responses via FKBP-mediated protection of mitochondrial integrity

BACKGROUND AND PURPOSE

Rapamycin is a potent immunosuppressant and anti-proliferative agent used clinically to prevent organ transplant rejection and for coating coronary stents to counteract restenosis. Rapamycin complexes with the immunophilin FKBP12, which subsequently binds and inhibits mTORC1. Despite several reports demonstrating that rapamycin affects platelet-mediated responses, the underlying mechanism of how it alters platelet function is poorly characterised. This study aimed to elucidate the effect of rapamycin on platelet procoagulant responses.

EXPERIMENTAL APPROACH

The effect of rapamycin on platelet activation and signalling was investigated alongside the catalytic mTOR inhibitors KU0063794 and WYE-687, and the FKBP12-binding macrolide FK506.

KEY RESULTS

Rapamycin affects platelet procoagulant responses by reducing externalisation of the procoagulant phospholipid phosphatidylserine, formation of balloon-like structures and local generation of thrombin. Catalytic mTOR kinase inhibitors did not alter platelet procoagulant processes, despite having a similar effect as rapamycin on Ca²⁺ signalling, demonstrating that the effect of rapamycin on procoagulant responses is independent of mTORC1 inhibition and not linked to a reduction in Ca²⁺ signalling. FK506, which also forms a complex with FKBP12 but does not target mTOR, reduced platelet procoagulant responses to a similar extent as rapamycin. Both rapamycin and FK506 prevented the loss of mitochondria integrity induced by platelet activation, one of the central regulatory events leading to PS externalisation.

CONCLUSIONS AND IMPLICATIONS

Rapamycin suppresses platelet procoagulant responses by protecting mitochondrial integrity in a manner independent of mTORC1 inhibition. Rapamycin and other drugs targeting FKBP immunophilins could aid the development of novel complementary anti-platelet therapies.

Rac1 regulates platelet microparticles formation and rheumatoid arthritis deterioration

Platelets play important roles in blood clotting, hemostasis and wound repair, while more and more research show that platelets also have significant contributions in the process of inflammation. Rheumatoid arthritis is a chronic systemic inflammatory autoimmune disease. Platelet microparticles, which are membrane vesicles shed by activated platelets, are reported to amplify inflammation in Rheumatoid arthritis. Here we show that either platelet-specific deletion of Rac1 (Rac1^-/-) or Rac1-specific inhibitor NSC23766 dramatically inhibit platelet-derived microparticles formation. As we all know, collagen-induced arthritis (CIA) mouse model is the most common autoimmune model of rheumatoid arthritis. Interestingly, NSC23766 alleviated the process of collagen-induced arthritis of DBA mice in vivo, including the reduced hind paw thickness and ankle stiffness, the reduction of arthritic scores and incidence of arthritis. Our work also found that NSC23766-treated CIA mouse spleen is less swollen and contains less enlarged white pulp than PBS control. The histological analysis shows that NSC23766-treated but not solvent control improve the cartilage erosion symptom in the joint of CIA mouse. Interestingly, platelet microparticles in the peripheral blood of NSC23766-treated CIA mice were decreased significantly compared with PBS-treated CIA mice. In conclusion, our work demonstrated that Rac1 inhibition alleviates collagen-induced arthritis through the decrease of platelet microparticles' release. In short, Rac1 aggravate the rheumatoid arthritis deterioration through the regulation of platelet microparticles formation.

Rapamycin in ischemic stroke: Old drug, new tricks?

The significant morbidity that accompanies stroke makes it one of the world's most devastating neurological disorders. Currently, proven effective therapies have been limited to thrombolysis and thrombectomy. The window for the administration of these therapies is narrow, hampered by the necessity of rapidly imaging patients. A therapy that could extend this window by protecting neurons may improve outcome. Endogenous neuroprotection has been shown to be, in part, due to changes in mTOR signalling pathways and the instigation of productive autophagy. Inducing this effect pharmacologically could improve clinical outcomes. One such therapy already in use in transplant medicine is the mTOR inhibitor rapamycin. Recent evidence suggests that rapamycin is neuroprotective, not only via neuronal autophagy but also through its broader effects on other cells of the neurovascular unit. This review highlights the potential use of rapamycin as a multimodal therapy, acting on the blood-brain barrier, cerebral blood flow and inflammation, as well as directly on neurons. There is significant potential in applying this old drug in new ways to improve functional outcomes for patients after stroke.

Rapamycin inhibits epithelial-to-mesenchymal transition of peritoneal mesothelium cells through regulation of Rho GTPases

Epithelial-mesenchymal transition (EMT) of peritoneal mesothelial cells (PMCs) is a key process of peritoneal fibrosis. Rapamycin has been previously shown to inhibit EMT of PMCs and prevent peritoneal fibrosis. In this study, we investigated the undefined molecular mechanisms by which rapamycin inhibits EMT of PMCs. To define the protective effect of rapamycin, we initially used a rat PD model which was daily infused with 20 mL of 4.25% high glucose (HG) dialysis solution for 6 weeks to induce fibrosis. The HG rats showed decreased ultrafiltration volume and obvious fibroproliferative response, with markedly increased peritoneal thickness and higher expression of α-smooth muscle actin (α-SMA) and transforming growth factor-β1. Rapamycin significantly ameliorated those pathological changes. Next, we treated rat PMCs with HG to induce EMT and/or rapamycin for indicated time. Rapamycin significantly inhibited HG-induced EMT, which manifests as increased expression of α-SMA, fibronectin, and collagen I, decreased expression of E-cadherin, and increased mobility. HG increased the phosphorylation of PI3K, Akt, and mTOR. Importantly, rapamycin inhibits the RhoA, Rac1, and Cdc42 activated by HG. Moreover, rapamycin repaired the pattern of F-actin distribution induced by HG, reducing the formation of stress fiber, focal adhesion, lamellipodia, and filopodia. Thus, rapamycin shows an obvious protective effect on HG-induced EMT, by inhibiting the activation of Rho GTPases (RhoA, Rac1, and Cdc42).

Protein kinase C and P2Y12 take center stage in thrombin-mediated activation of mammalian target of rapamycin complex 1 in human platelets

BACKGROUND

Rapamycin, an inhibitor of mammalian target of rapamycin complex-1 (mTORC1), reduces platelet spreading, thrombus stability, and clot retraction. Despite an important role of mTORC1 in platelet function, little is known about how it is regulated. The objective of this study was to determine the signaling pathways that regulate mTORC1 in human platelets.

METHODS

Mammalian target of rapamycin complex-1 activation was assessed by measuring the phosphorylation of its downstream substrate ribosomal S6 kinase 1 (p70S6K).

RESULTS

Thrombin or the protein kinase C (PKC) activator phorbal 12-myristate 13-acetate stimulated activation of mTORC1 in a PKC-dependent, Akt-independent manner that correlated with phosphorylation of tuberin/tuberous sclerosis 2 (TSC2) (Ser939 and Thr1462). In contrast, insulin-like growth factor 1 (IGF-1)-stimulated TSC2 phosphorylation was completely dependent on phosphoinositide 3 kinase (PI3 kinase)/Akt but did not result in any detectable mTORC1 activation. Early (Ser939 and Thr1462) and late (Thr1462) TSC2 phosphorylation in response to thrombin were directly PKC dependent, whereas later TSC2 (Ser939) and p70S6K phosphorylation were largely dependent on paracrine signaling through P2Y(12). PKC-mediated adenosine diphosphate (ADP) secretion was essential for thrombin-stimulated mTORC1 activation, as (i) ADP rescued p70S6K phosphorylation in the presence of a PKC inhibitor and (ii) P2Y(12) antagonism prevented thrombin-mediated mTORC1 activation. Rescue of mTORC1 activation with exogenous ADP was completely dependent on the Src family kinases but independent of PI3 kinase/Akt. Interestingly, although inhibition of Src blocked the ADP rescue, it had little effect on thrombin-stimulated p70S6K phosphorylation under conditions where PKC was not inhibited.

CONCLUSION

These results demonstrate that thrombin activates the mTORC1 pathway in human platelets through PKC-mediated ADP secretion and subsequent activation of P2Y(12), in a manner largely independent of the canonical PI3 kinase/Akt pathway.

mRNA encoding WAVE-Arp2/3-associated proteins is co-localized with foci of active protein synthesis at the leading edge of MRC5 fibroblasts during cell migration

During cell spreading, mammalian cells migrate using lamellipodia formed from a large dense branched actin network which produces the protrusive force required for leading edge advancement. The formation of lamellipodia is a dynamic process and is dependent on a variety of protein cofactors that mediate their local regulation, structural characteristics and dynamics. In the present study, we show that mRNAs encoding some structural and regulatory components of the WAVE [WASP (Wiskott-Aldrich syndrome protein) verprolin homologous] complex are localized to the leading edge of the cell and associated with sites of active translation. Furthermore, we demonstrate that steady-state levels of ArpC2 and Rac1 proteins increase at the leading edge during cell spreading, suggesting that localized protein synthesis has a pivotal role in controlling cell spreading and migration.

p21 activated kinase signaling coordinates glycoprotein receptor VI-mediated platelet aggregation, lamellipodia formation, and aggregate stability under shear

OBJECTIVE

Rho GTPase proteins play a central role in regulating the dynamics of the platelet actin cytoskeleton. Yet, little is known regarding how Rho GTPase activation coordinates platelet activation and function. In this study, we aimed to characterize the role of the Rho GTPase effector, p21 activated kinase (PAK), in platelet activation, lamellipodia formation, and aggregate formation under shear.

APPROACH AND RESULTS

Stimulation of platelets with the glycoprotein receptor VI agonist, collagen-related peptide, rapidly activated PAK in a time course preceding phosphorylation of PAK substrates, LIM domain kinase LIMK1 and the MAPK/ERK kinase MEK, and the subsequent activation of MAPKs and Akt. Pharmacological inhibitors of PAK blocked signaling events downstream of PAK and prevented platelet secretion as well as platelet aggregation in response to collagen-related peptide. PAK inhibitors also prevented PAK activation and platelet spreading on collagen surfaces. PAK was also required for the formation of platelet aggregates and to maintain aggregate stability under physiological shear flow conditions.

CONCLUSIONS

These results suggest that PAK serves as an orchestrator of platelet functional responses after activation downstream of the platelet collagen receptor, glycoprotein receptor VI.

The Rac1 hypervariable region in targeting and signaling: a tail of many stories

Cellular signaling by small GTPases is critically dependent on proper spatio-temporal orchestration of activation and output. In addition to their core G (guanine nucleotide binding)-domain, small GTPases comprise a hypervariable region (HVR) and a lipid anchor that are generally accepted to control subcellullar localization. The HVR encodes in many small GTPases a polybasic region (PBR) that permits charge-mediated association to the inner leaflet of the plasma membrane or to intracellular organelles. Over the past 15-20 years, evidence has accumulated for specific protein-protein interactions, mediated by the HVR, that control both targeting and signaling specificity of small GTPases. Using the RhoGTPase Rac1 as a paradigm we here review a series of protein partners that require the Rac1 HVR for association and that control various aspects of localized Rac1 signaling. Some of these proteins represent Rac1 activators, whereas others mediate Rac1 inactivation and degradation and yet others potentiate Rac1 downstream signaling. Finally, evidence is discussed which shows that the HVR of Rac1 also contributes to effector interactions, co-operating with the N-terminal effector domain. The complexity of localized Rac1 signaling, reviewed here, is most likely exemplary for many other small GTPases as well, representing a challenge to identify and define similar mechanisms controlling the specific signaling induced by small GTPases.

Rho GTPases in platelet function

The Rho family of GTP binding proteins, also commonly referred to as the Rho GTPases, are master regulators of the platelet cytoskeleton and platelet function. These low-molecular-weight or 'small' GTPases act as signaling switches in the spatial and temporal transduction, and amplification of signals from platelet cell surface receptors to the intracellular signaling pathways that drive platelet function. The Rho GTPase family members RhoA, Cdc42 and Rac1 have emerged as key regulators in the dynamics of the actin cytoskeleton in platelets and play key roles in platelet aggregation, secretion, spreading and thrombus formation. Rho GTPase regulators, including GEFs and GAPs and downstream effectors, such as the WASPs, formins and PAKs, may also regulate platelet activation and function. In this review, we provide an overview of Rho GTPase signaling in platelet physiology. Previous studies of Rho GTPases and platelets have had a shared history, as platelets have served as an ideal, non-transformed cellular model to characterize Rho function. Likewise, recent studies of the cell biology of Rho GTPase family members have helped to build an understanding of the molecular regulation of platelet function and will continue to do so through the further characterization of Rho GTPases as well as Rho GAPs, GEFs, RhoGDIs and Rho effectors in actin reorganization and other Rho-driven cellular processes.