A novel histochemical method for the visualization of thrombin activity in the nervous system

Effects of Thrombin on the Neurovascular Unit in Cerebral Ischemia

Cerebral ischemia is a cerebrovascular disease with high morbidity and mortality that poses a significant burden on society and the economy. About 60% of cerebral ischemia is caused by thrombus, and the formation of thrombus proceeds from insoluble fibrin, following its transformation from liquid fibrinogen. In thrombus-induced ischemia, increased permeability of the blood-brain barrier (BBB), followed by the extravasation of blood components into the brain results in an altered brain microenvironment. Changes in the brain microenvironment affect brain function and the neurovascular unit (NVU), the working unit of the brain. Recent studies have reported that coagulation factors interact with the NVU and its components, but the specific function of this interaction is highly speculative and warrants further investigations. In this article, we reviewed the role of coagulation factors in cerebral ischemia and the role of coagulation factors in thrombosis. Additionally, the influence of thrombin on the NVU is introduced, as well as in the function of NVU, which may help to explore part of brain injury mechanism during ischemia. Lastly, we propose some novel therapeutic approaches on ischemic stroke by reducing the risk of coagulation.

Intrinsic Expression of Coagulation Factors and Protease Activated Receptor 1 (PAR1) in Photoreceptors and Inner Retinal Layers

The aim of this study was to characterize the distribution of the thrombin receptor, protease activated receptor 1 (PAR1), in the neuroretina. Neuroretina samples of wild-type C57BL/6J and PAR1^−/− mice were processed for indirect immunofluorescence and Western blot analysis. Reverse transcription quantitative real-time PCR (RT-qPCR) was used to determine mRNA expression of coagulation Factor X (FX), prothrombin (PT), and PAR1 in the isolated neuroretina. Thrombin activity following KCl depolarization was assessed in mouse neuroretinas ex vivo. PAR1 staining was observed in the retinal ganglion cells, inner nuclear layer cells, and photoreceptors in mouse retinal cross sections by indirect immunofluorescence. PAR1 co-localized with rhodopsin in rod outer segments but was not expressed in cone outer segments. Western blot analysis confirmed PAR1 expression in the neuroretina. Factor X, prothrombin, and PAR1 mRNA expression was detected in isolated neuroretinas. Thrombin activity was elevated by nearly four-fold in mouse neuroretinas following KCl depolarization (0.012 vs. 0.044 mu/mL, p = 0.0497). The intrinsic expression of coagulation factors in the isolated neuroretina together with a functional increase in thrombin activity following KCl depolarization may suggest a role for the PAR1/thrombin pathway in retinal function.

Protective role of serpina3c as a novel thrombin inhibitor against atherosclerosis in mice

Abnormal vascular smooth muscle cell (VSMC) proliferation is a critical step in the development of atherosclerosis. Serpina3c is a serine protease inhibitor (serpin) that plays a key role in metabolic diseases. The present study aimed to investigate the role of serpina3c in atherosclerosis and regulation of VSMC proliferation and possible mechanisms. Serpina3c is down-regulated during high-fat diet (HFD)-induced atherosclerosis. An Apoe-/-/serpina3c-/--double-knockout mouse model was used to determine the role of serpina3c in atherosclerosis after HFD for 12 weeks. Compared with Apoe-/- mice, the Apoe-/-/serpina3c-/- mice developed more severe atherosclerosis, and the number of VSMCs and macrophages in aortic plaques was significantly increased. The present study revealed serpina3c as a novel thrombin inhibitor that suppressed thrombin activity. In circulating plasma, thrombin activity was high in the Apoe-/-/serpina3c-/- mice, compared with Apoe-/- mice. Immunofluorescence staining showed thrombin and serpina3c colocalization in the liver and aortic cusp. In addition, inhibition of thrombin by dabigatran in serpina3c-/- mice reduced neointima lesion formation due to partial carotid artery ligation. Moreover, an in vitro study confirmed that thrombin activity was also decreased by serpina3c protein, supernatant and cell lysate that overexpressed serpina3c. The results of experiments showed that serpina3c negatively regulated VSMC proliferation in culture. The possible mechanism may involve serpina3c inhibition of ERK1/2 and JNK signaling in thrombin/PAR-1 system-mediated VSMC proliferation. Our results highlight a protective role for serpina3c as a novel thrombin inhibitor in the development of atherosclerosis, with serpina3c conferring protection through the thrombin/PAR-1 system to negatively regulate VSMC proliferation through ERK1/2 and JNK signaling.

The Effect of Neuronal Activity on Glial Thrombin Generation

Thrombin through its receptor PAR-1 plays an important role in the peripheral nervous system. PAR-1 is located at the microvilli of Schwann cells at the node of Ranvier, and thrombin is generated by the coagulation system on these glial structures. In the present study, we examined the link between neuronal activity and modulation of thrombin generation by glial Schwann cells. Thrombin activity was assessed in sciatic nerves in reaction to high KCl as a model of neuronal activity. We demonstrated a significant transient effect of high KCL on thrombin activity (F(5, 20) = 42.65, p < 0.0001, by ANOVA) compared to normal KCl levels. Since the sciatic nerve includes components of axons and Schwann cell myelin sheath, we continued to investigate the effect of high KCl on a Schwannoma cell line as a model for nodal Schwann cell microvilli. We demonstrated a transient decrease in thrombin activity in response to high extracellular KCl (F(1, 18) = 9.56, p = 0.0063). The major neuronal inhibitor of thrombin is PN-1, and we therefore measured the effect of high KCL on PN-1 immunofluorescence intensity. We found significantly higher PN-1 staining intensity 3 min after the application of high KCL in comparison to cells exposed to high KCL for 7 min and to cells in regular KCL (F(2, 102) = 8.4737, p < 0.0004), and this effect may explain the changes in thrombin activity. The present results support an interaction between neuronal activity and the coagulation pathway as a novel mechanism for neuron-glia crosstalk at the node of Ranvier.

Apixaban decreases brain thrombin activity in a male mouse model of acute ischemic stroke

Factor Xa (FXa) plays a critical role in the coagulation cascade by generation of thrombin. During focal ischemia thrombin levels increase in the brain tissue and cause neural damage. This study examined the hypothesis that administration of the FXa inhibitor, apixaban, following focal ischemic stroke may have therapeutic potential by decreasing brain thrombin activity and infarct volume. Male mice were divided into a treated groups that received different doses of apixaban (2, 20, 100 mg/kg administered I.P.) or saline (controls) immediately after blocking the middle cerebral artery (MCA). Thrombin activity was measured by a fluorescence assay on fresh coronal slices taken from the mice brains 24 hr following the MCA occlusion. Infarct volume was assessed using triphenyltetrazolium chloride staining. A high dose of apixaban (100 mg/kg) significantly decreased thrombin activity levels in the ipsilateral hemisphere compared to the control group (Slice#5, p = .016; Slice#6, p = .016; Slice#7, p = .016; Slice#8, p = .036; by the nonparametric Mann-Whitney test). In addition, treatment with apixaban doses of both 100 mg/kg (32 ± 8% vs. 76 ± 7% in the treatment vs. control groups respectively; p = .005 by the nonparametric Mann-Whitney test) and 20 mg/kg (43 ± 7% vs. 76 ± 7% in the treatment vs. control groups respectively; p = .019 by the nonparametric Mann-Whitney test) decreased infarct volumes in areas surrounding the ischemic core (Slices #3 and #8). No brain hemorrhages were observed either in the treated or control groups. In summary, I.P. administration of high dose of apixaban immediately after MCA occlusion decreases brain thrombin activity and reduces infarct size.

Local Regulation of Thrombin Activity by Factor Xa in Peripheral Nerve Schwann Cells

Thrombin through its receptor plays an important role in the peripheral nervous system (PNS) but the pathways leading to its generation there are not known. In the blood, activated factor X (FXa) which is formed from factor X (FX) by tissue factor (TF) and factor VII (FVII), cleaves prothrombin into thrombin. We here studied these factors in vivo in mouse sciatic nerve and in vitro in a Schwannoma cell line and provide mRNA, immunoblot and immunohistochemistry evidence that FX and FXa are expressed in the normal and injured peripheral nerve and in Schwannoma cells. Furthermore, TF and FVII were localized histologically to the node of Ranvier in the sciatic nerve. Adding exogenous FXa increased the thrombin levels in sciatic nerve (11.6 ± 1.6 mU/ml compared to 35.2 ± 6 mU/ml p = 0.02) and in Schwannoma cell line (4.5 ± 0.2 mU/ml compared to 18.1 ± 0.5 mU/ml p < 0.001), indicating a large reserve of prothrombin. In the injured nerve, FX mRNA was upregulated 1 day after injury compared to normal nerve (103 ± 38 versus 1 ± 0.3 FOI p < 0.001). FXa protein levels increased 1 h after the injury and then decreased significantly at 1 and 2 days following injury despite an increase in its precursor, FX. Injecting the selective FXa inhibitor apixaban immediately upon injury decreased thrombin activation and improved motor function after nerve injury. The results localize the extrinsic coagulation pathway and FXa to the PNS, suggesting a critical role for FXa in PNS thrombin formation and the possible therapeutic use of selective FXa inhibitors in nerve injuries.

A Linear Temporal Increase in Thrombin Activity and Loss of Its Receptor in Mouse Brain following Ischemic Stroke

BACKGROUND

Brain thrombin activity is increased following acute ischemic stroke and may play a pathogenic role through the protease-activated receptor 1 (PAR1). In order to better assess these factors, we obtained a novel detailed temporal and spatial profile of thrombin activity in a mouse model of permanent middle cerebral artery occlusion (pMCAo).

METHODS

Thrombin activity was measured by fluorescence spectroscopy on coronal slices taken from the ipsilateral and contralateral hemispheres 2, 5, and 24 h following pMCAo (n = 5, 6, 5 mice, respectively). Its spatial distribution was determined by punch samples taken from the ischemic core and penumbra and further confirmed using an enzyme histochemistry technique (n = 4). Levels of PAR1 were determined using western blot.

RESULTS

Two hours following pMCAo, thrombin activity in the stroke core was already significantly higher than the contralateral area (11 ± 5 vs. 2 ± 1 mU/ml). At 5 and 24 h, thrombin activity continued to rise linearly (r = 0.998, p = 0.001) and to expand in the ischemic hemisphere beyond the ischemic core reaching deleterious levels of 271 ± 117 and 123 ± 14 mU/ml (mean ± SEM) in the basal ganglia and ischemic cortex, respectively. The peak elevation of thrombin activity in the ischemic core that was confirmed by fluorescence histochemistry was in good correlation with the infarcts areas. PAR1 levels in the ischemic core decreased as stroke progressed and thrombin activity increased.

CONCLUSION

In conclusion, there is a time- and space-related increase in brain thrombin activity in acute ischemic stroke that is closely related to the progression of brain damage. These results may be useful in the development of therapeutic strategies for ischemic stroke that involve the thrombin-PAR1 pathway in order to prevent secondary thrombin related brain damage.