Altered Expression of Angiopoietins During Blood-Brain Barrier Breakdown and Angiogenesis

Abstract

SUMMARY:Angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2) belong to a novel family of endothelial growth factors that function as ligands for the endothelial-specific receptor tyrosine kinase, Tie-2. Ang-1 reduces endothelial permeability of noncerebral vessels and has a major role in vascular stabilization and maturation, whereas Ang-2 is thought to be an endogenous antagonist of the action of Ang-1 at Tie-2. Expression of these ligands at the mRNA and protein level were studied during both blood-brain barrier (BBB) breakdown and cerebral angiogenesis occurring in the rat cortical cold-injury model by RT-PCR analysis and immunohistochemistry respectively, during a time course of 6 hours to 6 days. In addition, immunohistochemical detection of fibronectin was used to detect BBB breakdown at the lesion site and dual labeling was used to determine whether the vessels demonstrating BBB breakdown expressed endothelial Ang-1 or Ang-2. Endothelial Ang-1 and Tie-2 proteins were present in all cerebral vessels of normal brain including those of the choroid plexuses, whereas both these proteins as well as Ang-2 were present in choroid plexus epithelium and in ependymal cells, suggesting that angiopoietins have an autocrine effect on these cell types as well. In contrast, in the early phase after injury during the known period of BBB breakdown, increased Ang-2 mRNA and protein and decreased endothelial Ang-1 and Tie-2 proteins were observed. Two to 6 days after injury, the progressive increase in Ang-1 mRNA and protein and the decrease in Ang-2 coincided with cerebrovascular angiogenesis. Confocal microscopy showed colocalization of both Ang-1 and Ang-2 in endothelium of lesion vessels, and our observation of colocalization of Ang-1 and Ang-2 in polymorphonuclear leukocytes and macrophages has not been reported previously. This study demonstrates that Ang-1 is an important factor in maintaining normal homeostasis in the brain. Thus Ang-1 therapy may have therapeutic potential in reducing BBB breakdown and the ensuing edema after massive brain injury. (Lab Invest 2003, 83:1211-1222.

Morphogens and blood-brain barrier function in health and disease

“…In the days after the injury, Ang-2 levels decreased while Ang-1 levels increased and new blood vessels formed. 97 Many more studies have been performed showing similar results. The permeability-lowering effects of Ang-1 on the BBB are due to upregulation of various junctional proteins, including ZO-1, occludin, and VE-cadherin 98 and counteraction of the permeability enhancing VEGF.…”

EphrinB2 Activation Enhances Vascular Repair Mechanisms and Reduces Brain Swelling After Mild Cerebral Ischemia

“…Angiopoietins/Tie2 also play an important role in development and stability of the blood vessels, 44,45 and it has been shown that their expression is altered during BBB breakdown. 46 Here, we show that ephrinB2 activation leads to an increased expression of Ang-1 mainly in PCs after ischemia. Parallel to this, expression of receptor Tie2 is also enhanced in the ECs.…”

The Blood–Brain Barrier

“…This occurs through the up-regulation of junctional protein expression . In contrast, Ang-2 is known to participate in the early phases of BBB breakdown during injury and disease (Nourhaghighi et al 2003). Interestingly, when factors known to compromise BBB function, such as VEGF, are coexpressed with Ang1, the barrier integrity is enhanced and neuroprotective properties are induced (Shen et al 2011).…”