Research & Publications
We are interested in extracellular control of receptor tyrosine kinases (RTKs) in both normal and neoplastic environments. The 58 RTKs in the human proteome fall into 20 classes based on the domains of their extracellular regions (ECRs). Ligand-induced dimerization is a central component in activation of most RTKs, but it is increasingly clear that there is great diversity in the mechanisms of regulation of receptor activation across the RTK superfamily. RTKs can be subject to complex allosteric regulation by their ligands, by co-receptors and by other modulators. We seek molecular understanding of these diverse mechanisms, and of how receptor activity can be modulated by disease linked mutations, or regulated by therapeutic agents. We are particularly interested in the regulatory mechanisms of RTKs that appear to rely on higher order oligomerization or clustering for their biological activity, such as the Tie receptor family that plays key roles in angiogenesis, and the TAM family that are important in immune system homeostasis and regulation of inflammation. To investigate questions about extracellular control of RTKs, we use a combination of biophysical, structural, biochemical and cellular approaches.
Extensive Research Description
1. Regulation of Tie2 activation by homo- and hetero-oligomerization:
The Tie family of RTKs are involved in both vascular homeostasis and in angiogenesis. Both the receptors and their angiopoietin (Ang) ligands are attractive targets for pharmacologic intervention in cancer, inflammation and other disease states. Impeding development of therapeutic agents is the current incomplete understanding of the mechanisms of activation of Tie receptors. Whereas RTKs such as EGFR, Kit and FGFR are well known to be regulated by growth factor-induced dimerization, for the Tie receptors, studies to date have failed to reveal the mechanism of ligand-induced receptor activation. We have shown that the extracellular region (ECR) of Tie2 forms a ligand-independent dimer that is mediated by its membrane-proximal fibronectin type III (FNIII) domains, and is essential for Tie2 activation in cells. The oligomeric Ang ligands all bind to the membrane distal domains of Tie2 via the Ang fibrinogen-related domain (FReD). Tie2 differs from most RTKs in that an oligomeric ligand regulates an already oligomeric receptor. We are investigating whether signaling arises through allosteric changes in a receptor dimer or by the promotion of receptor crosslinking or clustering. Activation of Tie2 is clearly influenced but other interacting partners such as the orphan family member Tie1, and co-receptors such as RPTP-beta and integrins. Exactly how these co-receptors modulate ligand induced Tie2 responses is unclear, and we are actively investigating this question.
2. TAM receptor activation
We are also interested in the mechanism of activation of a second family of RTKs with membrane proximal FNIII domains, the TAM family (named for its members Tyro3, Axl and Mer), which are involved in immune homeostasis and regulation of inflammation. Like the Tie receptors, TAM ligands bind to a membrane distal region of the extracellular region, and there is limited understanding of the role of the FNIII domains. We are investigating whether these FNIII domains may mediate homo- or hetero-interactions of TAMs, as we see for Tie2. The molecular mechanism of ligand induced receptor activation of TAM receptors is not well understood. To elicit the full complement of biological TAM receptor responses, engagement of ligand alone is not sufficient. The ligand must also interact with phosphatidylserine on the outer leaflet of an adjacent cell membrane. We are investigating the molecular changes that occur in TAM receptors upon ligand engagement that lead to receptor activation.
3. Inhibition of RTKs by therapeutic antibodies
Our laboratory has a long-standing interest in the mechanisms of inhibition of receptor tyrosine kinases (RTKs) by therapeutic antibodies, most notably of those that bind the epidermal growth factor receptor (EGFR) - one of the first targets of antibody-based drugs to treat cancer. We combine X-ray crystallography, biochemistry, computational analysis and cellular studies to evaluate and compare existing antibody drugs, and to learn about mechanisms of resistance to these therapeutic antibodies. Through a number of collaborations, we are also exploiting mechanism driven selection strategies to identify antibodies that can modulate the activity of other RTKs, such as Tie2, which may have therapeutic potential in cancer and other diseases.