tPA mediated activation of PDGF-C/PDGFRα signaling in the CNS

Abstract

The platelet derived growth factors (PDGFs) and their tyrosine kinase receptors play an essential role during development, in adult tissue homeostasis and in several pathological events. PDGF-CC, a ligand for PDGFRα, is secreted as an inactive dimer. Proteolytic activation of PDGF-CC is mediated by the serine protease tissue plasminogen activator (tPA) and subsequently this allows binding of the active PDGF-CC dimer to and signaling through PDGFRα. In the blood tPA is involved in fibrinolysis and recombinant tPA is the only FDA approved treatment of acute ischemic stroke. However, thrombolytic therapy with tPA is highly limited due to an increased risk of intracerebral hemorrhage and it has been hypothesized that this is caused by unique activities of tPA in the neurovascular unit (NVU).

To better understand the biological function of tPA and PDGF-CC signaling in the central nervous system (CNS) we first characterized the effect of tPA ablation on brain development in mice (Paper I). We found that tPA deficient mice presented with rearrangements in the cerebrovascular tree, including a shift towards small diameter vessels as compared to wild-type mice. Additionally, we found ventricular malformations, including asymmetry of the lateral ventricles and a distorted ependymal lining. Since PDGF-C deficient mice have previously been described with asymmetric lateral ventricles, this potentially provides a first in vivo link between tPA and PDGF signaling in CNS development. In Paper II we aimed to identify the mechanism underlying abnormal ventricular development in PDGF-C deficient mice. Our findings suggest that PDGF-CC/PDGFRα signaling controls radial glia migration and differentiation, subsequently affecting ependymal development and maturation. This in turn might explain the ventricular expansion and asymmetry associated with PDGF-C ablation. tPA-mediated activation of PDGF-CC and subsequent PDGFRα signaling has been associated with increased cerebrovascular permeability in CNS disorders. However, activation of PDGF-CC by tPA in vitro has been described as inefficient, thus we investigated potential co-factors needed for tPA mediated activation of PDGF-CC in the NVU in Paper III. We found that Mac-1 on microglia is required to facilitate efficient activation of PDGF-CC by tPA. Subsequently this enhanced PDGFRα phosphorylation in the NVU, resulting in loss of BBB integrity and intracerebral hemorrhage. In support of Mac-1 being a co-factor for PDGF-CC/PDGFRα activation, we found that Mac-1 deficient mice showed decreased BBB permeability and smaller infarct size in an experimental model of ischemic stroke. Previous studies have shown that the tyrosine kinase inhibitor imatinib improves neurological and functional outcome after ischemic stroke via inhibition of PDGFRα signaling in both mice and patients. In Paper IV we described the mechanism how imatinib ameliorates stroke pathology. In the acute phase we found that imatinib preserved BBB integrity and reduced reactive gliosis, whereas in the chronic phase blocking PDGFRα signaling in the NVU in ischemic stroke moderated scar formation.

Collectively these findings will help us to better understand the role of tPA in the NVU and provide new insights regarding the role of PDGF-CC/PDGFRα signaling in CNS development and in ischemic stroke. Ultimately this might lead to novel treatment strategies to improve outcome following ischemic stroke.