Signaling Network Map of Endothelial TEK Tyrosine Kinase

TEK tyrosine kinase is primarily expressed on endothelial cells and is most commonly referred to as TIE2. TIE2 is a receptor tyrosine kinase modulated by its ligands, angiopoietins, to regulate the development and remodeling of vascular system. It is also one of the critical pathways associated with tumor angiogenesis and familial venous malformations. Apart from the vascular system, TIE2 signaling is also associated with postnatal hematopoiesis. Despite the involvement of TIE2-angiopoietin system in several diseases, the downstream molecular events of TIE2-angiopoietin signaling are not reported in any pathway repository. Therefore, carrying out a detailed review of published literature, we have documented molecular signaling events mediated by TIE2 in response to angiopoietins and developed a network map of TIE2 signaling. The pathway information is freely available to the scientific community through NetPath, a manually curated resource of signaling pathways. We hope that this pathway resource will provide an in-depth view of TIE2-angiopoietin signaling and will lead to identification of potential therapeutic targets for TIE2-angiopoietin associated disorders.


Introduction
Angiopoietin-TIE2 is one of the major signaling systems that regulates development and remodeling of vascular system [1,2]. TIE2 is a member of the TIE receptor tyrosine kinase family that is preferentially expressed in endothelial cells [3]. Among the angiopoietins (angiopoietin-1, angiopoietin-2, and angiopoietin-4 in humans), angiopoietin-1 (ANGPT1) is known as a constitutive agonist of TIE2. ANGPT1/TIE2 signaling promotes endothelial cell survival, endothelium integrity, and anti-inflammatory/antiapoptotic responses supporting reduced vascular permeability [4,5]. ANGPT2 is generally considered as antagonist as it competes with ANGPT1 for binding to TIE2, reduces vessel stability, and enhances vascular remodeling [6]. However, under specific experimental conditions, ANGPT2 has been shown to promote endothelial-cell survival, sprouting, and migration in a temporal and concentration-dependent manner [7][8][9]. Therefore, angiopoietin-2 (ANGPT2) is currently considered as a context dependant agonist or antagonist of TIE2 [6,10]. Angiopoietin-4 (ANGPT4) is also known to be an agonist of TIE2 while angiopoietin-3 (ANGPT3), the mouse ortholog of angiopoietin-4, is reported to be antagonistic to TIE2 [11]. The other member of the TIE family is the orphan receptor TIE1. It heterodimerizes with TIE2 and modulates TIE2 signaling induced by ANGPT1 and ANGPT2 [12]. ANGPT1 binding to TIE2 induces dissociation of the TIE1-TIE2 complex [12]. This suggests that TIE2 signaling is regulated by the molecular balance between ANGPT1 and ANGPT2 [6,13] and TIE1 and TIE2, with another one being the ectodomain cleavage of TIE receptors [14]. The activation of TIE2 is achieved by the assembly with tetrameric or higher order multimeric angiopoietins, clearly differentiating TIE2 from other tyrosine kinase receptors [15]. ANGPT1 induces the translocation of TIE2 to cell-cell junctions and transassociation in the form of homomeric complexes to activate the downstream signaling of TIE2 [16].
Besides the defects in vascular system and angiogenesis [42][43][44], TIE2 signaling has also been associated with rheumatoid arthritis [45] and asthma [46]. Considering the importance of TIE2 signaling, here we provide a manually curated enhanced network map of angiopoietin(s)-induced TIE2-mediated signaling events as a reference platform for further biomedical investigations.

Methods
We screened published research articles related to TIE2 signaling. NetPath criteria described earlier [47,48] were followed for the annotation of protein-protein interactions (PPIs), enzyme-substrate relationships, and posttranslational modifications (PTMs) (catalytic events). Activation/inhibition status of proteins, alterations in protein localization, and also genes regulated at mRNA level by TIE2 signaling were also documented. PathBuilder, an in-house pathway annotation tool, was used for the curation of these reactions [49]. Each curated reaction was internally reviewed by trained biocurators followed by an external review by a Pathway Authority, an expert in the field (FMG, coauthor of this paper).

Results and Discussion
Our analysis resulted in the cataloging of 140 unique molecules that are reported in TIE2 signaling. These molecules were part of 43 PPIs and 102 catalytic events, 23 activation/inhibition events, and 11 protein translocation events. We have also documented 124 and 65 genes that were reported to be upregulated and downregulated, respectively, by TIE2 signaling in response to angiopoietin(s) in human cells. The curated data for TIE2 signaling pathway is freely available to the scientific community for visualization and download in different community standard data exchange formats through NetPath   For effective visualization, we have graphically represented the reactions and various cellular processes that those reactions mediate in the context of specific studies on TIE2 signaling ( Figure 1). PathVisio, an open visualization tool was used to manually depict this information [50]. The pathway map can also be accessed through Net-Slim (http://www.netpath.org/netslim/TIE2 pathway.html), a resource that provides a smaller version of the pathway by filtering data based on predefined confidence threshold criteria [51]. At NetSlim, a "map with citation" is also provided in which each reaction is linked to the corresponding literature through PubMed. Users can download these maps in customizable formats such as GenMAPP and gpml.

Conclusions
This open-access pathway data enables better analysis of high-throughput experimental data and hypothesis-driven approaches to study the dynamics of TIE2 signaling for therapeutic interventions. Information on TIE2 pathway in NetPath will be periodically updated to reflect novel findings relevant to TIE2 signaling. We intend to provide information pertaining to cross-talks of other ligand/receptor systems such as VEGF, TNF-alpha, and integrins with TIE2 and vice versa, in the subsequent versions in NetPath. We encourage scientific community to help us maintain this resource up-to-date and error-free through http://www.netpath.org/comments.