Thrombin and the Protease-Activated Receptor-1 in Organophosphate-Induced Status Epilepticus

Cognitive and Cellular Effects of Combined Organophosphate Toxicity and Mild Traumatic Brain Injury

Traumatic brain injury (TBI) is considered the most common neurological disorder among people under the age of 50. In modern combat zones, a combination of TBI and organophosphates (OP) can cause both fatal and long-term effects on the brain. We utilized a mouse closed-head TBI model induced by a weight drop device, along with OP exposure to paraoxon. Spatial and visual memory as well as neuron loss and reactive astrocytosis were measured 30 days after exposure to mild TBI (mTBI) and/or paraoxon. Molecular and cellular changes were assessed in the temporal cortex and hippocampus. Cognitive and behavioral deficits were most pronounced in animals that received a combination of paraoxon exposure and mTBI, suggesting an additive effect of the insults. Neuron survival was reduced in proximity to the injury site after exposure to paraoxon with or without mTBI, whereas in the dentate gyrus hilus, cell survival was only reduced in mice exposed to paraoxon prior to sustaining a mTBI. Neuroinflammation was increased in the dentate gyrus in all groups exposed to mTBI and/or to paraoxon. Astrocyte morphology was significantly changed in mice exposed to paraoxon prior to sustaining an mTBI. These results provide further support for assumptions concerning the effects of OP exposure following the Gulf War. This study reveals additional insights into the potentially additive effects of OP exposure and mTBI, which may result in more severe brain damage on the modern battlefield.

The role of thrombin in early-onset seizures

A variety of clinical observations and studies in animal models of temporal lobe epilepsy (TLE) reveal dysfunction of blood-brain barrier (BBB) during seizures. It is accompanied by shifts in ionic composition, imbalance in transmitters and metabolic products, extravasation of blood plasma proteins in the interstitial fluid, causing further abnormal neuronal activity. A significant amount of blood components capable of causing seizures get through the BBB due to its disruption. And only thrombin has been demonstrated to generate early-onset seizures. Using the whole-cell recordings from the single hippocampal neurons we recently showed the induction of epileptiform firing activity immediately after the addition of thrombin to the blood plasma ionic media. In the present work, we mimic some effects of BBB disruption in vitro to examine the effect of modified blood plasma artificial cerebrospinal fluid (ACSF) on the excitability of hippocampal neurons and the role of serum protein thrombin in seizure susceptibility. Comparative analysis of model conditions simulating BBB dysfunction was performed using the lithium-pilocarpine model of TLE, which most clearly reflects the BBB disruption in the acute stage. Our results demonstrate the particular role of thrombin in seizure-onset in conditions of BBB disruption.

Modulation of the Thrombin Pathway Restores LTP in a Pilocarpine Mice Model of Status Epilepticus

Background

Status epilepticus (SE) leads to memory impairment following a seizure, attributed to long-term potentiation (LTP) reduction. Thrombin, a coagulation factor that activates protease-activated receptor 1 (PAR1) is involved in cognitive impairment following traumatic brain injury by reducing hippocampal LTP and in seizures as seen in a SE pilocarpine-induced mice model. Thrombin pathway inhibition prevents this cognitive impairment. We evaluated the effect of thrombin pathway inhibition in the pilocarpine-induced SE mice model, on LTP, hippocampal, and serum markers for inflammation, the PAR1 pathway, and neuronal cell damage.

Methods

SE was induced by injecting C57BL/6J mice with pilocarpine. Before pilocarpine injection, mice were injected with either the specific thrombin inhibitor α-NAPAP [Nα-(2-naphthalene-sulfonylglycyl)-4-amidino-DL-phenylalaninepiperidide], the PAR1 antagonist SCH79797, or vehicle-only solution. Recordings of excitatory postsynaptic potentials (EPSP) were conducted from hippocampal slices 24 h following pilocarpine injection. Hippocampal real-time PCR for the quantification of the PAR1, prothrombin, and tumor necrosis factor α (TNF-α) mRNA expression levels was conducted. Serum levels of neurofilament light chain (NfL) and TNF-α were measured by a single molecule array assay.

Results

The EPSP was reduced in the pilocarpine-induced SE mice (p < 0.001). This reduction was prevented by both NAPAP and SCH79797 treatments (p < 0.001 for both treatments). Hippocampal expression of TNF-α was elevated in the pilocarpine-induced SE group compared to the control (p < 0.01), however, serum levels of TNF-α were not changed. NfL levels were elevated in the pilocarpine-induced SE group (p = 0.04) but not in the treated groups.

Conclusions

Pilocarpine-induced SE reduces LTP, in a thrombin PAR1-related mechanism. Elevation of serum NfL supports neuronal damage accompanying this functional abnormality which may be prevented by PAR1 pathway modulation.

Technology-based approaches toward a better understanding of neuro-coagulation in brain homeostasis

Blood coagulation factors can enter the brain under pathological conditions that affect the blood-brain interface. Besides their contribution to pathological brain states, such as neural hyperexcitability, neurodegeneration, and scar formation, coagulation factors have been linked to several physiological brain functions. It is for example well established that the coagulation factor thrombin modulates synaptic plasticity; it affects neural excitability and induces epileptic seizures via activation of protease-activated receptors in the brain. However, major limitations of current experimental and clinical approaches have prevented us from obtaining a profound mechanistic understanding of "neuro-coagulation" in health and disease. Here, we present how novel human relevant models, i.e., Organ-on-Chips equipped with advanced sensors, can help overcoming some of the limitations in the field, thus providing a perspective toward a better understanding of neuro-coagulation in brain homeostasis.

Persistent neuropathology and behavioral deficits in a mouse model of status epilepticus induced by acute intoxication with diisopropylfluorophosphate

Organophosphate (OP) nerve agents and pesticides are a class of neurotoxic compounds that can cause status epilepticus (SE), and death following acute high-dose exposures. While the standard of care for acute OP intoxication (atropine, oxime, and high-dose benzodiazepine) can prevent mortality, survivors of OP poisoning often experience long-term brain damage and cognitive deficits. Preclinical studies of acute OP intoxication have primarily used rat models to identify candidate medical countermeasures. However, the mouse offers the advantage of readily available knockout strains for mechanistic studies of acute and chronic consequences of OP-induced SE. Therefore, the main objective of this study was to determine whether a mouse model of acute diisopropylfluorophosphate (DFP) intoxication would produce acute and chronic neurotoxicity similar to that observed in rat models and humans following acute OP intoxication. Adult male C57BL/6J mice injected with DFP (9.5 mg/kg, s.c.) followed 1 min later with atropine sulfate (0.1 mg/kg, i.m.) and 2-pralidoxime (25 mg/kg, i.m.) developed behavioral and electrographic signs of SE within minutes that continued for at least 4 h. Acetylcholinesterase inhibition persisted for at least 3 d in the blood and 14 d in the brain of DFP mice relative to vehicle (VEH) controls. Immunohistochemical analyses revealed significant neurodegeneration and neuroinflammation in multiple brain regions at 1, 7, and 28 d post-exposure in the brains of DFP mice relative to VEH controls. Deficits in locomotor and home-cage behavior were observed in DFP mice at 28 d post-exposure. These findings demonstrate that this mouse model replicates many of the outcomes observed in rats and humans acutely intoxicated with OPs, suggesting the feasibility of using this model for mechanistic studies and therapeutic screening.

Systemic thrombin inhibition ameliorates seizures in a mouse model of pilocarpine-induced status epilepticus

Status epilepticus (SE) is a life-threatening condition characterized by ongoing seizure activity which can lead to severe brain damage and death if not treated properly. Recent work suggests that alterations in blood-brain barrier (BBB) function and subsequent cortical exposure to coagulation factors may initiate, promote, and/or sustain SE. This suggestion is based on the observation that the serine protease thrombin, which plays a fundamental role in the blood coagulation cascade, increases neural excitability through the activation of protease-activated receptor 1 (PAR1). However, it remains unclear whether systemic inhibition of thrombin asserts "anti-epileptic" effects in vivo. We here used the pilocarpine model of SE in adult 3-month-old male mice to address the question whether intraperitoneal injection of the thrombin inhibitor α-NAPAP (0.75 mg/kg) counters SE. Indeed, pharmacological inhibition of thrombin ameliorates the behavioral outcome of pilocarpine-induced SE. Similar results are obtained when the thrombin receptor PAR1 is pharmacologically blocked using intraperitoneal injection of SCH79797 (25 μg/kg) prior to SE induction. Consistent with these results, an increase in thrombin immunofluorescence is detected in the hippocampus of pilocarpine-treated animals. Moreover, increased hippocampal serine protease activity is detected 90 min after SE induction, which is not observed in animals treated with α-NAPAP prior to SE induction. Together, these results corroborate and extend recent studies suggesting that novel oral anticoagulants which target thrombin (and PAR1) may assert anti-epileptic effects in vivo. KEY MESSAGES: Systemic thrombin/PAR1-inhibition ameliorates anticoagulants behavioral seizures. Status epilepticus increases thrombin levels in the hippocampus. Increased serine protease activity in the hippocampus after status epileptic. Anti-epileptic potential of clinically used anticoagulants must be evaluated.

The neuro-glial coagulonome: the thrombin receptor and coagulation pathways as major players in neurological diseases

The neuro-glial interface extends far beyond mechanical support alone and includes interactions through coagulation cascade proteins. Here, we systematically review the evidence indicating that synaptic and node of Ranvier glia cell components modulate synaptic transmission and axonal conduction by a coagulation cascade protein system, leading us to propose the concept of the neuro-glial coagulonome. In the peripheral nervous system, the main thrombin receptor protease activated receptor 1 (PAR1) is located on the Schwann microvilli at the node of Ranvier and at the neuromuscular junction. PAR1 activation effects can be both neuroprotective or harmful, depending on thrombin activity levels. Low physiological levels of thrombin induce neuroprotective effects in the Schwann cells which are mediated by the endothelial protein C receptor. High levels of thrombin induce conduction deficits, as found in experimental autoimmune neuritis, the animal model for Guillaine-Barre syndrome. In the central nervous system, PAR1 is located on the peri-synaptic astrocyte end-feet. Its activation by high thrombin levels is involved in the pathology of primary inflammatory brain diseases such as multiple sclerosis, as well as in other central nervous system insults, including trauma, neoplasms, epilepsy and vascular injury. Following activation of PAR1 by high thrombin levels the seizure threshold is lowered. On the other hand, PAR1 activation by lower levels of thrombin in the central nervous system protects against a future ischemic insult. This review presents the known structure and function of the neuro-glial coagulonome, focusing on coagulation, thrombin and PAR1 in a pathway which may be either physiological (neuroprotective) or detrimental in peripheral nervous system and central nervous system diseases. Understanding the neuro-glial coagulonome may open opportunities for novel pharmacological interventions in neurological diseases.