SEVTAR-A multicenter randomized controlled trial to investigate the impact of prophylactic endoluminal placed vacuum sponge for prevention of anastomotic leakage after low rectal resections

Background

Low anterior resection for rectal cancer is commonly associated with a diverting stoma. In general, the stoma is closed 3 months after the initial operation. The diverting stoma reduces the rate of anastomotic leakage as well as the severeness of a potential leakage itself. Nevertheless, anastomotic leakage is still a life-threatening complication and might reduce the quality of life in the short and long term. In case of leakage, the construction can be converted into a Hartmann situation or it could be treated by endoscopic vacuum therapy or by leaving the drains. In recent years, endoscopic vacuum therapy has become the treatment of choice in many institutions. In this study, the hypothesis is to be evaluated, if a prophylactic endoscopic vacuum therapy reduces the rate of anastomotic leakage after rectal resections.

Methods

A multicenter parallel group randomized controlled trial is planned in as many as possible centers in Europe. The study aims to recruit 362 analyzable patients with a resection of the rectum combined with a diverting ileostoma. The anastomosis has to be between 2 and 8 cm off the anal verge. Half of these patients receive a sponge for 5 days, and the control group is treated as usual in the participating hospitals. There will be a check for anastomotic leakage after 30 days. Primary end point is the rate of anastomotic leakages. The study will have 60% power to detect a difference of 10%, at a one-sided alpha significance level of 5%, assuming an anastomosis leakage rate of 10%-15%.

Discussion

If the hypothesis proves to be true, anastomosis leakage could be reduced significantly by placing a vacuum sponge over the anastomosis for 5 days.

Trial registration

The trial is registered at DRKS: DRKS00023436. It has been accredited by Onkocert of the German Society of Cancer: ST-D483. The leading Ethics Committee is the Ethics Committee of Rostock University with the registration ID A 2019-0203.

Defective diacylglycerol-induced Ca2+ entry but normal agonist-induced activation responses in TRPC6-deficient mouse platelets

BACKGROUND

Platelet adhesion, activation and aggregation at sites of vascular injury are essential processes for primary hemostasis. Elevation of the intracellular Ca(2+) concentration is a central event in platelet activation but the underlying mechanisms are not fully understood. Store-operated calcium entry (SOCE) through Orai1 was shown to be the main Ca(2+) influx pathway in murine platelets, but there are additional non-store-operated Ca(2+) (non-SOC) and receptor operated Ca(2+) (ROC) channels expressed in the platelet plasma membrane.

OBJECTIVE

Canonical transient receptor potential (TRPC) channel 6 is found both in human and murine platelets and has been proposed to mediate diacylglycerol (DAG) activated ROCE but also a role in the regulation of SOCE has been suggested.

METHODS

To investigate the function of TRPC6 in platelet Ca(2+) signaling and activation, we analyzed platelets from mice deficient in TRPC6 using a wide range of in vitro and in vivo assays.

RESULTS

In the mutant platelets, DAG activated Ca(2+) influx was found to be abolished. However, this did not significantly affect SOCE or agonist induced Ca(2+) responses. Platelet function in vitro and in vivo was also unaltered in the absence of TRPC6.

CONCLUSION

Our results indicate that DAG activated ROCE is mediated exclusively by TRPC6 in murine platelets, but this Ca(2+) influx has no major functional relevance for hemostasis and thrombosis. Further, in contrast to previous suggestions, based on studies with human platelets, TRPC6 appears to play an insignificant role in the regulation of SOCE in murine platelets.

Roles of platelet STIM1 and Orai1 in glycoprotein VI- and thrombin-dependent procoagulant activity and thrombus formation

In platelets, STIM1 has been recognized as the key regulatory protein in store-operated Ca(2+) entry (SOCE) with Orai1 as principal Ca(2+) entry channel. Both proteins contribute to collagen-dependent arterial thrombosis in mice in vivo. It is unclear whether STIM2 is involved. A key platelet response relying on Ca(2+) entry is the surface exposure of phosphatidylserine (PS), which accomplishes platelet procoagulant activity. We studied this response in mouse platelets deficient in STIM1, STIM2, or Orai1. Upon high shear flow of blood over collagen, Stim1(-/-) and Orai1(-/-) platelets had greatly impaired glycoprotein (GP) VI-dependent Ca(2+) signals, and they were deficient in PS exposure and thrombus formation. In contrast, Stim2(-/-) platelets reacted normally. Upon blood flow in the presence of thrombin generation and coagulation, Ca(2+) signals of Stim1(-/-) and Orai1(-/-) platelets were partly reduced, whereas the PS exposure and formation of fibrin-rich thrombi were normalized. Washed Stim1(-/-) and Orai1(-/-) platelets were deficient in GPVI-induced PS exposure and prothrombinase activity, but not when thrombin was present as co-agonist. Markedly, SKF96365, a blocker of (receptor-operated) Ca(2+) entry, inhibited Ca(2+) and procoagulant responses even in Stim1(-/-) and Orai1(-/-) platelets. These data show for the first time that: (i) STIM1 and Orai1 jointly contribute to GPVI-induced SOCE, procoagulant activity, and thrombus formation; (ii) a compensating Ca(2+) entry pathway is effective in the additional presence of thrombin; (iii) platelets contain two mechanisms of Ca(2+) entry and PS exposure, only one relying on STIM1-Orai1 interaction.

Integrins in platelet activation

Heterodimeric receptors of the beta1 and beta3 integrin families mediate platelet adhesion and aggregation in hemostasis and thrombosis. In resting platelets, integrins are expressed in a low-affinity state but they shift to a high-affinity state and efficiently bind their ligands in response to cellular activation. This review summarizes recent advances in understanding the functional regulation and (patho-) physiological significance of individual platelet integrins with a special focus on studies in genetically modified mice. It is now recognized that beta1 and beta3 integrins have partially redundant roles in the adhesion process and that their activation is regulated by similar mechanisms, involving Ca2+-dependent and -independent signaling events and essential functions of talin-1 and kindlin-3 in the terminal activation step.

Calcium signaling in platelets

Agonist-induced elevation in cytosolic Ca2+ concentrations is essential for platelet activation in hemostasis and thrombosis. It occurs through Ca2+ release from intracellular stores and Ca2+ entry through the plasma membrane (PM). Ca2+ store release is a well-established process involving phospholipase (PL)C-mediated production of inositol-1,4,5-trisphosphate (IP3), which in turn releases Ca2+ from the intracellular stores through IP3 receptor channels. In contrast, the mechanisms controlling Ca2+ entry and the significance of this process for platelet activation have been elucidated only very recently. In platelets, as in other non-excitable cells, the major way of Ca2+ entry involves the agonist-induced release of cytosolic sequestered Ca2+ followed by Ca2+ influx through the PM, a process referred to as store-operated calcium entry (SOCE). It is now clear that stromal interaction molecule 1 (STIM1), a Ca2+ sensor molecule in intracellular stores, and the four transmembrane channel protein Orai1 are the key players in platelet SOCE. The other major Ca2+ entry mechanism is mediated by the direct receptor-operated calcium (ROC) channel, P2X1. Besides these, canonical transient receptor potential channel (TRPC) 6 mediates Ca2+ entry through the PM. This review summarizes the current knowledge of platelet Ca2+ homeostasis with a focus on the newly identified Ca2+ entry mechanisms.

STIM1-independent T cell development and effector function in vivo

Store-operated Ca(2+) entry (SOCE) is believed to be of pivotal importance in T cell physiology. To test this hypothesis, we generated mice constitutively lacking the SOCE-regulating Ca(2+) sensor stromal interaction molecule 1 (STIM1). In vitro analyses showed that SOCE and Ag receptor complex-triggered Ca(2+) flux into STIM1-deficient T cells is virtually abolished. In vivo, STIM1-deficient mice developed a lymphoproliferative disease despite normal thymic T cell maturation and normal frequencies of CD4(+)Foxp3(+) regulatory T cells. Unexpectedly, STIM1-deficient bone marrow chimeric mice mounted humoral immune responses after vaccination and STIM1-deficient T cells were capable of inducing acute graft-versus-host disease following adoptive transfer into allogeneic hosts. These results demonstrate that STIM1-dependent SOCE is crucial for homeostatic T cell proliferation, but of much lesser importance for thymic T cell differentiation or T cell effector functions.

The calcium sensor STIM1 is an essential mediator of arterial thrombosis and ischemic brain infarction

Platelet activation and aggregation are essential to limit posttraumatic blood loss at sites of vascular injury but also contributes to arterial thrombosis, leading to myocardial infarction and stroke. Agonist-induced elevation of [Ca²⁺]_i is a central step in platelet activation, but the underlying mechanisms are not fully understood. A major pathway for Ca²⁺ entry in nonexcitable cells involves receptor-mediated release of intracellular Ca²⁺ stores, followed by activation of store-operated calcium (SOC) channels in the plasma membrane. Stromal interaction molecule 1 (STIM1) has been identified as the Ca²⁺ sensor in the endoplasmic reticulum (ER) that activates Ca²⁺ release–activated channels in T cells, but its role in mammalian physiology is unknown. Platelets express high levels of STIM1, but its exact function has been elusive, because these cells lack a normal ER and Ca²⁺ is stored in a tubular system referred to as the sarcoplasmatic reticulum. We report that mice lacking STIM1 display early postnatal lethality and growth retardation. STIM1-deficient platelets have a marked defect in agonist-induced Ca²⁺ responses, and impaired activation and thrombus formation under flow in vitro. Importantly, mice with STIM1-deficient platelets are significantly protected from arterial thrombosis and ischemic brain infarction but have only a mild bleeding time prolongation. These results establish STIM1 as an important mediator in the pathogenesis of ischemic cardio- and cerebrovascular events.

An EF hand mutation in Stim1 causes premature platelet activation and bleeding in mice

Changes in cytoplasmic Ca2+ levels regulate a variety of fundamental cellular functions in virtually all cells. In nonexcitable cells, a major pathway of Ca2+ entry involves receptor-mediated depletion of intracellular Ca2+ stores followed by the activation of store-operated calcium channels in the plasma membrane. We have established a mouse line expressing an activating EF hand motif mutant of stromal interaction molecule 1 (Stim1), an ER receptor recently identified as the Ca2+ sensor responsible for activation of Ca2+ release-activated (CRAC) channels in T cells, whose function in mammalian physiology is not well understood. Mice expressing mutant Stim1 had macrothrombocytopenia and an associated bleeding disorder. Basal intracellular Ca2+ levels were increased in platelets, which resulted in a preactivation state, a selective unresponsiveness to immunoreceptor tyrosine activation motif-coupled agonists, and increased platelet consumption. In contrast, basal Ca2+ levels, but not receptor-mediated responses, were affected in mutant T cells. These findings identify Stim1 as a central regulator of platelet function and suggest a cell type-specific activation or composition of the CRAC complex.

Loss of talin1 in platelets abrogates integrin activation, platelet aggregation, and thrombus formation in vitro and in vivo

Platelet adhesion and aggregation at sites of vascular injury are essential for normal hemostasis but may also lead to pathological thrombus formation, causing diseases such as myocardial infarction or stroke. Heterodimeric receptors of the integrin family play a central role in the adhesion and aggregation of platelets. In resting platelets, integrins exhibit a low affinity state for their ligands, and they shift to a high affinity state at sites of vascular injury. It has been proposed that direct binding of the cytoskeletal protein talin1 to the cytoplasmic domain of the integrin β subunits is necessary and sufficient to trigger the activation of integrins to this high affinity state, but direct in vivo evidence in support of this hypothesis is still lacking. Here, we show that platelets from mice lacking talin1 are unable to activate integrins in response to all known major platelet agonists while other cellular functions are still preserved. As a consequence, mice with talin-deficient platelets display a severe hemostatic defect and are completely resistant to arterial thrombosis. Collectively, these experiments demonstrate that talin is required for inside-out activation of platelet integrins in hemostasis and thrombosis.

Diverging signaling events control the pathway of GPVI down-regulation in vivo

Coronary artery thrombosis is often initiated by platelet activation on collagen-rich subendothelial layers in the disrupted atherosclerotic plaque. The activating platelet collagen receptor glycoprotein VI (GPVI) noncovalently associates with the Fc receptor γ-chain (FcRγ), which signals through its immunoreceptor-tyrosine–based activation motif (ITAM) via the adaptor LAT leading to the activation of phospholipase Cγ2 (PLCγ2). GPVI is a promising antithrombotic target as anti-GPVI antibodies induce the irreversible loss of the receptor from circulating platelets by yet undefined mechanisms in humans and mice and long-term antithrombotic protection in the latter. However, the treatment is associated with transient but severe thrombocytopenia and reduced platelet reactivity to thrombin questioning its clinical usefulness. Here we show that GPVI down-regulation occurs through 2 distinct pathways, namely ectodomain shedding or internalization/intracellular clearing, and that both processes are abrogated in mice carrying a point mutation in the FcRγ-associated ITAM. In mice lacking LAT or PLCγ2, GPVI shedding is abolished, but the receptor is irreversibly down-regulated through internalization/intracellular clearing. This route of GPVI loss is not associated with thrombocytopenia or altered thrombin responses. These results reveal the existence of 2 distinct signaling pathways downstream of the FcRγ-ITAM and show that it is possible to uncouple GPVI down-regulation from undesired side effects with obvious therapeutic implications.