The protease thrombin is an endogenous mediator of hippocampal neuroprotection against ischemia at low concentrations but causes degeneration at high concentrations

Abstract

We have considered the extracellular serine protease thrombin and its receptor as endogenous mediators of neuronal protection against brain ischemia. Exposure of gerbils to prior mild ischemic insults, here two relatively short-lasting occlusions (2 min) of both common carotid arteries applied at 1-day intervals 2 days before a severe occlusion (6 min), caused a robust ischemic tolerance of hippocampal CA1 neurons. This resistance was impaired if the specific thrombin inhibitor hirudin was injected intracerebroventricularly before each short-lasting insult. Thus, efficient native neuroprotective mechanisms exist and endogenous thrombin seems to be involved therein. In vitro experiments using organotypic slice cultures of rat hippocampus revealed that thrombin can have protective but also deleterious effects on hippocampal CA1 neurons. Low concentrations of thrombin (50 pM, 0.01 unit͞ml) or of a synthetic thrombin receptor agonist (10 M) induced significant neuroprotection against experimental ischemia. In contrast, 50 nM (10 units͞ml) thrombin decreased further the reduced neuronal survival that follows the deprivation of oxygen and glucose, and 500 nM even caused neuronal cell death by itself. Degenerative thrombin actions also might be relevant in vivo, because hirudin increased the number of surviving neurons when applied before a 6-min occlusion. Among the thrombin concentrations tested, 50 pM induced intracellular Ca 2؉ spikes in fura-2-loaded CA1 neurons whereas higher concentrations caused a sustained Ca 2؉ elevation. Thus, distinct Ca 2؉ signals may define whether or not thrombin initiates protection. Taken together, in vivo and in vitro data suggest that thrombin can determine neuronal cell death or survival after brain ischemia. T he extracellular serine protease thrombin, a well known, key player in blood coagulation and platelet activation, has been found to be expressed in different brain regions (1, 2). Its physiological importance in the central nervous system is emphasized further by the parallel expression of the highly specific thrombin inhibitor protease nexin-1 (3, 4) and PAR-1, the classical thrombin receptor (2, 5-7). Some recent evidence indicates that thrombin and its receptor might be involved in neurodegenerative processes observed after different insults such as stroke, traumatic brain injuries, and heart arrest or as a frequent consequence of bypass surgeries (8-11). Normal brain function depends critically on a permanent supply of glucose and oxygen. Depending on its source, a disruption of the cerebrospinal blood flow leads to global or focal ischemia (hypoxia͞ hypoglycemia) and irreversible neuronal damage. Prothrombin as well as the classical thrombin receptor are expressed in brain regions that are particularly vulnerable to ischemia, e.g., neocortex, cortex, striatum, hypothalamus, hippocampus, and cerebellum (2). Furthermore, studies performed on isolated cells (neurons, astrocytes) have demonstrated that nanomolar concentrations of thrombin exert cytotoxic effects (12-14). How...

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