Edited by Lawrence Steinman, Stanford University School of Medicine, Stanford, CA, and approved January 15, 2019 (received for review September 6, 2018)
This article demonstrates a role for podocalyxin in blood–brain barrier (BBB) function. Podocalyxin expression on endothelial cells promotes the formation of adhesion interactions and barrier formation. Furthermore, it describes a neuroelectric phenotype elicited by a selective protease-activated receptor-1 (PAR-1) agonist. PAR-1 activation by serum proteases (e.g., thrombin) leaking into the central nervous system during inflammation has the potential to impact the course of brain injury and recovery. However, the role of PAR-1 in the pathophysiology of neuroinflammation is poorly understood. In summary, this study offers unique insight into the impact of PAR-1 activation in neuroinflammation and implicates podocalyxin as a therapeutic target for the manipulation of BBB permeability and the treatment of neurodegenerative disease.
Podocalyxin (Podxl) is broadly expressed on the luminal face of most blood vessels in adult vertebrates, yet its function on these cells is poorly defined. In the present study, we identified specific functions for Podxl in maintaining endothelial barrier function. Using electrical cell substrate impedance sensing and live imaging, we found that, in the absence of Podxl, human umbilical vein endothelial cells fail to form an efficient barrier when plated on several extracellular matrix substrates. In addition, these monolayers lack adherens junctions and focal adhesions and display a disorganized cortical actin cytoskeleton. Thus, Podxl has a key role in promoting the appropriate endothelial morphogenesis required to form functional barriers. This conclusion is further supported by analyses of mutant mice in which we conditionally deleted a floxed allele of Podxl in vascular endothelial cells (vECs) using Tie2Cre mice (PodxlΔTie2Cre). Although we did not detect substantially altered permeability in naïve mice, systemic priming with lipopolysaccharide (LPS) selectively disrupted the blood–brain barrier (BBB) in PodxlΔTie2Cre mice. To study the potential consequence of this BBB breach, we used a selective agonist (TFLLR-NH2) of the protease-activated receptor-1 (PAR-1), a thrombin receptor expressed by vECs, neuronal cells, and glial cells. In response to systemic administration of TFLLR-NH2, LPS-primed PodxlΔTie2Cre mice become completely immobilized for a 5-min period, coinciding with severely dampened neuroelectric activity. We conclude that Podxl expression by CNS tissue vECs is essential for BBB maintenance under inflammatory conditions.
Author contributions: J.C., M.R.H., A.W.C., T.H.M., C.D.R., and K.M.M. designed research; J.C., M.R.H., M.R.Z., A.W.C., S.O., R.W.S., D.C.H., A.C., and A.W.V. performed research; P.B., T.M.U., D.J.G., and T.H.M. contributed new reagents/analytic tools; J.C., M.R.H., M.R.Z., and A.W.C. analyzed data; and J.C., M.R.H., and K.M.M. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1814766116/-/DCSupplemental.
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