Neuron‐generated thrombin induces a protective astrocyte response via protease activated receptors

Abstract

Astrocytes protect neurons during cerebral injury through several postulated mechanisms. Recent therapeutic attention has focused on enhancing or augmenting the neuroprotective actions of astrocytes but in some instances astrocytes can assume a neurotoxic phenotype. The signaling mechanisms that drive astrocytes toward a protective versus toxic phenotype are not fully known but cell–cell signaling via proteases acting on cell‐specific receptors underlies critical mechanistic steps in neurodevelopment and disease. The protease activated receptor (PAR), resides in multiple brain cell types, and most PARs are found on astrocytes. We asked whether neuron‐generated thrombin constituted an important astrocyte activation signal because our previous studies have shown that neurons contain prothrombin gene and transcribed protein. We used neuron and astrocyte mono‐cell cultures exposed to oxygen‐glucose deprivation and a model of middle cerebral artery occlusion. We found that ischemic neurons secrete thrombin into culture media, which leads to astrocyte activation; such astrocyte activation can be reproduced with low doses of thrombin. Media from prothrombin‐deficient neurons failed to activate astrocytes and adding thrombin to such media restored activation. Astrocytes lacking PAR1 did not respond to neuron‐generated thrombin. Induced astrocyte activation was antagonized dose‐dependently with thrombin inhibitors or PAR1 antagonists. Ischemia‐induced astrocyte activation in vivo was inhibited after neuronal prothrombin knockout, resulting in larger strokes. Restoring prothrombin to neurons with a lentiviral gene vector restored astrocyte activation and reduced stroke damage. We conclude that neuron‐generated thrombin, released during ischemia, acts via PAR1 and may cause astrocyte activation and paracrine neuroprotection.

Neurocoagulation from a Mechanistic Point of View in the Central Nervous System

“…This was demonstrated recently by showing that neuron-generated thrombin induced a protective astrocyte response during ischemia via PAR1. 49 The PAR1 pathway was found to negatively affect the myelination process potential of oligodendrocyte progenitor cells. This occurs by suppressing the expression of proteolipid protein RNA in an Erk1/2-mediated signaling and decreasing myelin basic protein.…”

The Role of Thrombin in Brain Injury After Hemorrhagic and Ischemic Stroke

“…Those protective effects were attenuated by thrombin inhibitors, hirudin, and protease nexin-1 [52], as well as with PAR-1 knockout [15]. In addition, during ischemic stroke, neuronally generated thrombin can promote astrocyte activation towards a neuroprotective phenotype via PAR-1 [7].…”

“…HIF-1α and its target genes may be involved in thrombin-induced brain protection [56] and promote angiogenesis after ICH [57]. An upregulation of endogenous thrombin inhibitors [58,59] and heat shock proteins (HSPs) especially HSP27and HSP32 [14], along with the astrocyte-neuron interaction [7,60], may also contribute to neuroprotection. In contrast, high concentration of thrombin causes neuron death by inducing neuronal edema, enhancing excitotoxicity, and neuroinflammatory response.…”

“…Interestingly, the neuroprotective effect of thrombin also seems to be related to PAR-1 [15,180]. A recent study demonstrated that neuron-generated thrombin, released during ischemia, acts via PAR-1 activating astrocytes and paracrine neuroprotection [7]. The pleiotropic effects of thrombin in ischemic brain disease are likely mediated by distinct signaling pathways dependent on thrombin levels, the cell types involved, and, potentially, timing.…”

“…However, prothrombin can also be produced by neurons and astrocytes [2,3]. The physiological functions of this brain-derived thrombin are mostly unknown, although there is evidence of its roles in brain development [4] and synaptic transmission and plasticity [5,6] as well as neuronal protection after ischemic brain injury [7]. The amount of thrombin in brain is relatively low in pathological conditions like neurodegenerative diseases [8].…”