Thrombin and Protease-Activated Receptors
Thrombin is a serine protease that plays a critical role in coagulation in distal microvessels. In addition, thrombin has pleiotropic extravascular effects. It can induce protection at low doses but act as a neurotoxin at high doses, killing cells via the protease-activated receptors (PAR). Using a protease activatable cell-penetrating probe, the Lyden Laboratory has shown directly that thrombin protease activity participates in damaging the neurovascular unit. The lab did this by comparing quantified vascular leakage and cell death markers with the quantity of thrombin activation (probe fluorescence) in the parenchyma after middle cerebral artery occlusion (MCAo). The Lyden Lab blocked thrombin with a direct thrombin inhibitor, argatroban, and greatly reduced ischemic edema and tissue damage at realistic therapeutic time window (delay dose three hours after reperfusion).
|Thrombin activity associated with neurons. Photomicrograph showing thrombin activatable cell-penetrating peptides (ACPP) in parenchymal tissue with severe vascular disruption (FITC-dextran leakage). ACPP-positive cells colocalize with neuronal cellular marker (NeuN). Scale Bar = 3mm.|
The Lyden Laboratory also exacerbated tissue damage by infusing thrombin during ischemia. The results suggested a critical role for thrombin in mediating brain injury in focal ischemia. The lab hypothesizes that thrombin partially mediates edema and cell death during stroke via activity at the PAR-1 receptor. The Lyden Laboratory has made significant progress in studying this mechanism of neuroprotection post-MCAo with different activated protein C (APC) mutants selective for PAR-1 receptors (3K3A-APC mutant in Phase I clinical trial). The lab has made significant progress in understanding the role of PAR-1 by using lentivirus-mediated shRNA knockdown of PAR-1 in rats and by studying the effect of stroke in aged PAR-1, PAR-3 and PAR-4 knockout animals. In addition, we have developed conditional knockout animal models to further investigate the function of thrombin and thrombin receptors in the central nervous system.
|A. Stereotaxic injection of PAR-1 shRNA lentivirus transfects neurons and GFAP-positive cells. Lentivirus was injected into rat striatum, and animals were subjected to MCAo after one week. Post-MCAo, they survived for 24 hours. Large numbers of GFAP-positive cells can be detected following injection. Confocal analysis of images shows GFP-labeled cells colocalize with NeuN (neuronal marker), GFAP (astrocytes) and Nissl body (neurons). GFAP-positive cells do not colocalize with IBA1 (microglia marker). Scale bars (B) 100μm; (C, D, E) 50μm; (F) 20μm.|
Therapeutic hypothermia is the most powerful neuroprotectant ever documented in stroke models. The Lyden Laboratory showed the protective effect of a single degree Celsius (C) in our quantal bioassay model. However, this preclinical benefit has failed to translate into clinical trial success. Based on our parallel observations of astrocyte protection of neurons, members of the Lyden Lab asked whether cooling could disrupt the astrocyte-mediated protection of neurons. In fact, we showed that hypothermia interferes with the astrocytes, in a graded, temperature-dependent manner (Lyden et al. JCBFM 2018). We then showed that ultra-fast cooling to 33°C for a short time was more powerful in the middle cerebral artery occlusion model than were longer cooling periods. These exciting and novel data, if confirmed, suggest a reason for the failures of therapeutic hypothermia in some clinical trials. Moreover, our data suggests testable hypotheses about the effects of temperature in modulating neurovascular unit protection during ischemia.
Proposed method for optimized therapeutic hypothermia (TH). The critical time epochs driving TH include the time from onset of ischemia to the time of reperfusion (recanalization after stroke or ROSC after cardiac arrest), labelled T0; and the time from reperfusion to the onset of TH, labelled TR. We propose dividing TH into segments, starting with T1, the initial, deepest target temperature. After an optimum duration of TH at the first target temperature, at time C1, the target temperature changes, perhaps to an intermediate temperature target, or perhaps to normothermia, for another epoch labeled T2. Another change could occur at C2, followed by a third TH epoch, T3. In selected patients who develop cerebral edema, related to endothelial barrier disruption, at time C3 another target temperature change occurs and cooling for cerebral edema begins in epoch T4. Finally, at C4, final warming to normothermia occurs; this could be a ramp rewarm if indicated. The dashed red lines represent the actual transitions that may require time to move from one core target temperature to another.