Kallikrein 6 signals through PAR1 and PAR2 to promote neuron injury and exacerbate glutamate neurotoxicity

Abstract CNS trauma generates a proteolytic imbalance contributing to secondary injury, including axonopathy and neuron degeneration. Kallikrein 6 (Klk6) is a serine protease implicated in neurodegeneration, and here we investigate the role of protease‐activated receptors 1 ( PAR 1) and PAR 2 in mediating these effects. First, we demonstrate Klk6 and the prototypical activator of PAR 1, thrombin, as well as PAR 1 and PAR 2, are each elevated in murine experimental traumatic spinal cord injury ( SCI ) at acute or subacute time points. Recombinant Klk6 triggered extracellular signal‐regulated kinase ( ERK 1/2) signaling in cerebellar granule neurons and in the NSC 34 spinal cord motoneuron cell line, in a phosphoinositide 3‐kinase and MEK ‐dependent fashion. Importantly, lipopeptide inhibitors of PAR 1 or PAR 2, and PAR 1 genetic deletion, each reduced Klk6‐ ERK 1/2 activation. In addition, Klk6 and thrombin promoted degeneration of cerebellar neurons and exacerbated glutamate neurotoxicity. Moreover, genetic deletion of PAR 1 blocked thrombin‐mediated cerebellar neurotoxicity and reduced the neurotoxic effects of Klk6. Klk6 also increased glutamate‐mediated Bim signaling, poly‐ADP‐ribose polymerase cleavage and lactate dehydrogenase release in NSC 34 motoneurons and these effects were blocked by PAR 1 and PAR 2 lipopeptide inhibitors. Taken together, these data point to a novel Klk6‐signaling axis in CNS neurons that is mediated by PAR 1 and PAR 2 and is positioned to contribute to neurodegeneration. image Here, we show kallikrein 6 (Klk6) and thrombin contribute to the proteolytic imbalance that occurs in CNS injury. Activation of the G‐protein coupled receptor PAR1 was sufficient to mediate the neurotoxic effects of thrombin while KLK6 neurotoxicity involved activation of PAR1 and PAR2. In addition, both proteases exacerbated glutamate neurotoxicity. These data suggest Klk6, thrombin and PARs each represent new targets for the development of neuroprotective therapies.