Thrombin, a serine protease involved in the blood coagulation cascade has been shown to affect neural function following blood-brain barrier breakdown. However, several lines of evidence exist that thrombin is also expressed in the brain under physiological conditions, suggesting an involvement of thrombin in the regulation of normal brain functions. Here, we review ours’ as well as others’ recent work on the role of thrombin in synaptic transmission and plasticity through direct or indirect activation of Protease-Activated Receptor-1 (PAR1). These studies propose a novel role of thrombin in synaptic plasticity, both in physiology as well as in neurological diseases associated with increased brain thrombin/PAR1 levels.

Thrombin, a key player in thrombogenesis, affects cells in the brain through activation of its receptors. Low levels of thrombin activity are protective while high levels are toxic. We sought to quantify thrombin activity levels and their spatial distribution in brains of mice following reperfusion after ischemic stroke focusing on infarct, peri-infarct and contralateral areas. In order to find out the contribution of brain-derived thrombin, mRNA levels of both prothrombin and factor X were determined. Furthermore, we assessed the effect of thrombin levels that were measured in the ischemic brain on synaptic transmission. We found that in the brains of mice following transient middle cerebral artery occlusion, thrombin activity is elevated throughout the ischemic hemisphere, including in peri-infarct areas (90 AE 33 and 60 AE 18 mU/mL, in the infarct and peri-infarct areas, respectively, compared to 11 AE 3 and 12 AE 5 mU/mL, in the corresponding contralateral areas; mean AE SE; p < 0.05). Brain mRNA levels of prothrombin and, in particular, factor X are up-regulated in the ischemic core. Hippocampal slices treated with thrombin concentrations as found in the ischemic hemisphere show altered synaptic responses. We conclude that high thrombin activity following reperfusion after ischemic stroke may cause synaptic dysfunction.

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A small but significant proportion of COVID‐19 patients develop life‐threatening cytokine storm. We have developed a new anti‐inflammatory drug, EXO‐CD24, a combination of an immune checkpoint (CD24) and a delivery platform (exosomes). CD24 inhibits the NF‐kB pathway and the production of cytokines/chemokines. EXO‐CD24 discriminates damage‐from pathogen‐associated molecular patterns (DAMPs and PAMPs) therefore does not interfere with viral clearance. EXO‐CD24 was produced and purified from CD24‐expressing 293‐TREx™ cells. Exosomes displaying murine CD24 (mCD24) were also created. EXO‐CD24/mCD24 were characterized and examined, for safety and efficacy, in vitro and in vivo . In a phase Ib/IIa study, 35 patients with moderate–high severity COVID‐19 were recruited and given escalating doses, 10 8 –10 10 , of EXO‐CD24 by inhalation, QD, for 5 days. No adverse events related to the drug were observed up to 443–575 days. EXO‐CD24 effectively reduced inflammatory markers and cytokine/chemokine, although randomized studies are required. EXO‐CD24 may be a treatment strategy to suppress the hyper‐inflammatory response in the lungs of COVID‐19 patients and further serve as a therapeutic platform for other pulmonary and systemic diseases characterized by cytokine storm.

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Systemic inflammation and brain pathologies are known to be linked. In the periphery, the inflammation and coagulation systems are simultaneously activated upon diseases and infections. Whether this well-established interrelation also counts for neuroinflammation and coagulation factor expression in the brain is still an open question. Our aim was to study whether the interrelationship between coagulation and inflammation factors may occur in the brain in the setting of systemic inflammation. The results indicate that systemic injections of lipopolysaccharide (LPS) upregulate the expression of both inflammatory and coagulation factors in the brain. The activity of the central coagulation factor thrombin was tested by a fluorescent method and found to be significantly elevated in the hippocampus following systemic LPS injection (0.5 ± 0.15 mU/mg versus 0.2 ± 0.03 mU/mg in the control). A panel of coagulation factors and effectors (such as thrombin, FX, PAR1, EPCR, and PC) was tested in the hippocampus, isolated microglia, and N9 microglia cell by Western blot and real-time PCR and found to be modulated by LPS. One central finding is a significant increase in FX expression level following LPS induction both in vivo in the hippocampus and in vitro in N9 microglia cell line (5.5 ± 0.6- and 2.3 ± 0.1-fold of increase, resp.). Surprisingly, inhibition of thrombin activity (by a specific inhibitor NAPAP) immediately after LPS injection results in a reduction of both the inflammatory (TNFα, CXL9, and CCL1; p < 0.006) and coagulation responses (FX and PAR1; p < 0.004) in the brain. We believe that these results may have a profound clinical impact as they might indicate that reducing coagulation activity in the setting of neurological diseases involving neuroinflammation may improve disease outcome and survival.

Epilepsy is a complex neurological disorder which can severely affect neuronal function. Some patients may experience status epilepticus, a life-threatening state of ongoing seizure activity associated with postictal cognitive dysfunction. However, the molecular mechanisms by which status epilepticus influences brain function beyond seizure activity remain not well understood. Here, we addressed the question of whether pilocarpine-induced status epilepticus affects synaptopodin (SP), an actin-binding protein, which regulates the ability of neurons to express synaptic plasticity. This makes SP an interesting marker for epilepsy-associated alterations in synaptic function. Indeed, single dose intraperitoneal pilocarpine injection (250 mg/kg) in three-month-old male C57BL/6J mice leads to a rapid reduction in hippocampal SP-cluster sizes and numbers (in CA1 stratum radiatum of the dorsal hippocampus; 90 min after injection). In line with this observation (and previous work using SP-deficient mice), a defect in the ability to induce long-term potentiation (LTP) of Schaffer collateral-CA1 synapses is observed. Based on these findings we propose that status epilepticus could exert its aftereffects on cognition at least in part by perturbing SP-dependent mechanisms of synaptic plasticity.

Transient amnesia is a common consequence of minimal traumatic brain injury (mTBI). However, while recent findings have addressed the mechanisms involved in its onset, the processes contributing to its recovery have not yet been addressed. Recently, we have found that thrombin is detected at high concentrations in the brain of mice after exposure to mTBI and that in such settings amnesia is rescued by either inhibiting thrombin activity or by blockade of PAR1. Here, we report that mice spontaneously recover from amnesia after two weeks from mTBI exposure. At this time point, long term potentiation was equally evoked in injured vs. control animals with thrombin concentration in the brain being normalized at this stage. These findings, which refer to the specific aspect of memory retrieval upon mTBI, together with our previous work, hint to a strong correlation between cognitive defects in the context of mTBI and thrombin concentrations in the brain. This may suggest that a possible scavenging of thrombin in the brain at early phases following mTBI may improve memory function.