A novel function of the endothelial thrombomodulin-protein C system for cellular function in the CNS

Loss of endothelial thrombomodulin (TM) function is associated with macrovascular, microvascular, and inflammatory diseases. Loss of TM function impairs the generation of the anticoagulant protease activated protein C (aPC), which conveys important neuroprotective effects. Yet, the physiological relevance and the mechanism through which the TM-aPC system maintains nerve function remain unknown. Using mice expressing a mutant TM (TM) resulting in markedly reduced aPC generation we uncover a previously unknown function of TM-mediated aPC-generation for physiological nociception. While conductivity of isolated peripheral nerves is normal in TM mice, loss of TM-dependent aPC generation impairs myelination, increases ROS generation, and disrupts mitochondrial function within the CNS in unchallenged (non-diseased) mice. Substitution of aPC in TM mice normalises nociception as well as myelination, reduces ROS-generation, and ameliorates mitochondrial function within the CNS. To evaluate whether functional impairment of the TM-PC system is of pathophysiological relevance we evaluated the impact of impaired aPC-generation on peripheral neuropathy and on demyelinating disease models within the CNS. In experimental diabetic neuropathy, a disease of the peripheral nerve system, impaired TM-dependent aPC-generation does not aggravate the disease progression, consistent with a primary myelination defect within the CNS. Contrary, in a murine model of experimental autoimmune encephalitis (EAE) impaired TM-dependent PC-generation aggravates the disease progression. Impaired endogenous aPC generation enhances ROS generation and reduces markers reflecting mitochondrial mass (porin) and biogenesis (PGC1α). This establishes that endogenous aPC generation protects from neuronal demyelination and mitochondrial dysfunction in EAE. Restoring endogenous aPC levels or genetically restraining mitochondrial ROS generation partially reverses the disease severity in TM mice. Intriguingly, therapeutic application of soluble TM (solulin) conveys pronounced neuroprotection, delaying the disease onset and diminishing the disease severity in TM mice. The effect of soluble TM is superior to that of aPC or ROS-inhibition. A similar effect of soluble TM is apparent in TM wild-type mice, corroborating the therapeutic potential of soluble TM. Of note, soluble TM improves myelination even in a non-immunological mediated demyelination model (cuprizone-induced demyelination), establishing that soluble TM protects myelin independent of its known immune-modulatory function. Our results identify TM-dependent aPC activation as a new physiological pathway regulating neuronal function and as an important pharmacological target to alleviate impaired nociception and demyelinating diseases of the CNS. 5 Dipl. biol. Wolter Juliane Title: A novel function of the endothelial thrombomodulin-protein C system for cellular function in the CNS