Biology of Matrix-Activated Receptors

Dr Birgit Leitinger, Group Leader

My group's principal research interests are in cell adhesion, the interactions of cells with the extracellular matrix, and the mechanism of activation of cellular receptors.

Our current interest is to characterise two unusual receptor tyrosine kinases, the human discoidin domain receptors, DDR1 and DDR2. These receptors are widely expressed, and, uniquely among receptor tyrosine kinases, activated by the extracellular matrix protein collagen. The DDRs play important roles in developmental processes and disease. DDR1 is essential for mammary gland development in the mouse and plays important roles in fibrotic lung disease and cancer cell migration/invasion. DDR2 is important for the growth of long bones and skin wound healing and is associated with liver fibrosis and osteoarthritis. Both receptors are associated with collagen turnover in obstructive diseases of blood vessels. In addition, the DDRs show enhanced expression in a number of fast growing invasive tumours, suggesting an important role of these matrix-activated receptor tyrosine kinases in the proliferation and stromal invasion of tumours.

I have been interested in the molecular details of DDR activation by collagen and have shown that this activation is due to a direct, high affinity interaction of collagen with the DDR discoidin domain. The collagen binding site was mapped to three spatially close surface loops within the DDR2 discoidin domain.

The DDRs recognise a number of different collagen types, with the recognition being specific for the triple-helical conformation in fibrillar collagens. Currently, there is little information on the distribution of matrix receptor binding sites within the collagen triple helix. In collaboration with Dr A. Fertala (Thomas Jefferson University, Philadelphia, PA) we localised a specific DDR2 binding site to the D2 period of collagen II. This study was the first to demonstrate a site-specific interaction of DDR2 with collagen.

The DDRs seem to have a unique mechanism of receptor activation, with very slow and sustained kinetics of autophosphorylation, the first step in transmembrane signalling. We could show that unlike conventional receptor tyrosine kinases, which are dimerised upon ligand binding, the DDRs are predimerised in the absence of ligand. How collagen binding to dimeric DDRs is translated into tyrosine kinase activation in the cytoplasmic domain is unknown, but is likely to involve substanital conformational changes along the entire molecule. Interestingly, we identified a leucine zipper motif in the DDR1 transmembrane domain that is essential for transmembrane signalling. This suggests the possibility that the conformational change upon collagen binding could involve rotation of the transmembrane domains within the dimer.

In collaboration with Dr R Farndale (University of Cambridge) we recently showed that DDR2 recognises discrete amino acid sequences in triple-helical collagen. We made use of the Collagen II Toolkit, a set of overlapping triple-helical peptides, to comprehensively screen for DDR2 binding sites on the COLL II domain. We characterised the amino acid specificities of an important, high-affinity DDR2 binding site that encompasses the sequence GVMGFO (O is hydroxyproline) as the minimum collagen sequence required for DDR2 binding. Interestingly, triple helical peptides comprising the DDR2 binding motif could activate DDR2 transmembrane signalling in a specific manner, indicating that DDR2 activation may be effected by single triple helices rather than fibrillar collagen.

Selected Publications

Konitsiotis, A.D., Raynal, N., Bihan, D., Hohenester, E., Farndale, R.W. & Leitinger, B. (2008). Characerzation of high affinity binding motifs for the discoidin domain receptor DDR2 in collagen. J. Biol. Chem., 283, 6861-6868. Selected as Paper of the Week

Leitinger, B. & Hohenester E. (2007). Mammalian collagen receptors. Matrix Biol., 25, 355-364.

Noordeen, N.A., Carafoli, F., Hohenester, E., Horton, M.A. & Leitinger, B. (2006). A transmembrane leucine zipper is required for activation of the dimeric receptor tyrosine kinase DDR1. J. Biol. Chem., 281, 22744-22751.

Leitinger, B., & Kwan, A.P.L. (2006). The discoidin domain receptor DDR2 is a receptor for type X collagen. Matrix Biol., 25, 355-364.

Leitinger, B., Steplewski, A. & Fertala, A. (2004). The D2 period of collagen II contains a specific binding site for the human discoidin domain receptor, DDR2. J. Mol. Biol., 344, 993-1003.

Leitinger, B. (2003). Molecular analysis of collagen binding by the human discoidin domain receptors, DDR1 and DDR2: Identification of collagen binding sites in DDR2. J. Biol. Chem., 278, 16761-16769.

Additional Collaborator:

Pallavi Bhatta (PhD student)