Discoidin domain receptors: Micro insights into macro assemblies

Abstract

Assembly of cell-surface receptors into specific oligomeric states and/or clusters before and after ligand binding is an important feature governing their biological function. Receptor oligomerization can be mediated by specific domains of the receptor, ligand binding, configurational changes or other interacting molecules. In this review we summarize our understanding of the oligomeric state of discoidin domain receptors (DDR1 and DDR2), which belong to the receptor tyrosine kinase family (RTK). DDRs form an interesting system from an oligomerization perspective as their ligand collagen(s) can also undergo supramolecular assembly to form fibrils. Even though DDR1 and DDR2 differ in the domains responsible to form ligand-free dimers they share similarities in binding to soluble, monomeric collagen. However, only DDR1b forms globular clusters in response to monomeric collagen and not DDR2. Interestingly, both DDR1 and DDR2 are assembled into linear clusters by the collagen fibril. Formation of these clusters is important for receptor phosphorylation and is mediated in part by other membrane components. We summarize how the oligomeric status of DDRs shares similarities with other members of the RTK family and with collagen receptors. Unraveling the multiple macro-molecular configurations adopted by this receptor-ligand pair can provide novel insights into the intricacies of cell-matrix interactions.

Keywords: Cluster; Collagen; Dimer; Discoidin; Oligomer; Receptor tyrosine kinases.

Figure 1

Members of the DDR family known to exist as (a) monomers, (b) homodimers and (c) heterodimers before ligand stimulation. (a) Besides naturally occurring isoforms DDR1a to DDR1e, DDR1 is also known to shed its ECD resulting in a membrane tethered C-terminal fragment lacking the DDR1 ECD (DDR1-ΔECD) and a soluble DDR1 ECD. Only one naturally occurring isoform of DDR2 has been reported. (b) Homodimers and (c) heterodimers reported to exist for recombinantly expressed full-length or truncated receptors. Other possible combinations of homo- and heterodimers may exist but have not been reported. Black dots and dashes represent regions mediating dimer contacts. Red cross indicates glycosylation in DS-like domain which inhibits dimer formation in DDR1. DS: discoidin; DSL: discoidin like; EJXM: extracellular juxtamembrane; TMD: transmembrane domain; IJXM: intracellular juxtamembrane; KD: kinase domain; CT: C-terminal.

Figure 2

Possible mechanisms explaining enhanced binding of dimeric (or oligomeric DDR ECDs (shown in blue) to collagen (shown in green). As compared to monomers (a) dimers of DDR ECD (b) may bind to a high-affinity binding site as well as an adjacent low-affinity site thus enhancing the overall affinity. This could lead to reduced dissociation of the bound DDR ECD thus increasing the number of binding events on collagen molecules present. In addition dimers may bind to additional sites on the collagen triple helix (b) or to an end-to-end overlap of collagen molecules (c), which may not happen with DDR monomers. Clustering of DDR1 ECD can also increase the amount of bound protein present on collagen (d). While dimeric DDR1 ECD has been shown to cluster, clustering of monomeric DDR1 is not known.

Figure 3

(a): Clustering of DDR1b in response to monomeric collagen. Upon exposure to soluble, monomeric collagen, high-affinity DDR1b dimers bind to collagen (as in 1) and rapidly undergo clustering (as in 2). Thus, while clustering of DDR1b sequesters the dimer population, it remains to be examined if monomers are also incorporated in clusters. DDR1b clusters also recruit other membrane components like the insulin receptor (IR), insulin-like growth factor receptor (IGF1R) and tetraspanins. The DDR1b clusters thus formed are endocytosed to early endosomes, likely via a clathrin mediated endocytosis pathway. The clusters recycle back to the cell membrane after several hours and exhibit DDR1b phosphorylation at Y513 in its IJXM region (as in 3). Since tyrosine phosphorylation in KD does not occur in these clusters the ensuing signaling due to DDR1b clusters is termed non-canonical signaling. While this process is likely to be present in DDR1c due to NPXY motif in its IJXM as in DDR1b, it remains to be investigated if DDR1a also undergoes clustering and endocytosis. (b) Unlike DDR1b, DDR2 does not undergo clustering or endocytosis in response to monomeric collagen. Binding of high-affinity dimers to monomeric collagen inhibits collagen fibril formation on the cell surface.

Figure 4

Formation of linear clusters of DDR1b and DDR2 in response to collagen fibrils. The collagen fibrils serves as a multivalent ligand and likely recruits both monomers and dimers of DDRs. The DDRs thus assemble together along the fibril length to form filamentous structures or linear clusters. These assemblies can be homo-oligomers or hetero-oligomers. Receptor phosphorylation (at Y513) in IJXM for DDR1b and (at Y793 for DDR1 and Y740 for DDR2) in KD occurs upon formation of linear clusters due to trans-phosphorylation leading to canonical DDR signaling. Formation of these linear clusters is likely mediated by several domains including the ECD, TMD, IJXM and KD. All DDR1 isoforms as well as DDR1-ΔECD could putatively participate in formation of linear clusters.