Integrins are dimeric receptors composed of an α and a β subunit and are well known for their ability to interact with the extracellular matrix (ECM), through which integrins contribute to cell adhesion and migration and cell survival. There are 18 α and 8 β subunits in mammals, and at least 24 heterodimers have been reported. Given this complexity, it may not be surprising that integrins have also been reported to interact with molecules other than those in the extracellular matrix. For example, α9β1, a widely distributed integrin, interacts with several classes of ligands, including tenascin, thrombospondin 1, and osteopontin (ECM constituents), VCAM1 (a cell adhesion molecule), ADAM12 and 15 (metalloproteinases), and VEGFs (growth factors). Staniszewska et al. now provide evidence that this promiscuous integrin α9β1 also binds to the neurotrophins NGF, NT3, and BDNF. Most of their experiments were performed with cells transfected to express α9β1 (α9SW480 cells). In a cell adhesion assay, α9SW480 cells adhered to microplate wells coated with purified mouse NGF, human recombinant NGF, BDNF, or NT3 at the same efficiency as those cells adhered to VCAM1, a known ligand for α9β1. Untransfected cells exhibited little adherence. Adherence to NGF-coated wells was blocked in the presence of an antibody specific for α9β1 (Y9A2), snake venom protein selective for α9β1 or α4β1 (VLO5), or an antibody selective for β1 (Lia1/2) but was not affected by antibodies or venom proteins specific for other integrins. Using an enzyme-linked immunosorbent assay (ELISA) with α9β1 coating the plate, human recombinant NGF bound in a dose-dependent manner with a Kd of ~4.5 nM. Although the α9SW480 cells did appear to have some high-affinity neurotrophin receptors (likely composed of TrkA and p75NTR), the responses detected appeared to be mediated by the integrin, because VLO5 or Y9A2 blocked the responses. The responses detected when the α9SW480 cells bound to NGF included proliferation (likely involving extracellular signal-regulated kinases 1 and 2, which were phosphorylated) and chemotaxis and migration (likely involving paxillin, which was phosphorylated). Neutrophils were used in a transmigration assay to show that a cell that endogenously expresses α9β1 could be stimulated to migrate by NGF and this migration was blocked by Y9A2.
I. Staniszewska, I. K. Sariyer, S. Lecht, M. C. Brown, E. M. Walsh, G. P. Tuszynski, M. Safak, P. Lazarovici, C. Marcinkiewicz, Integrin α9β1 is a receptor for nerve growth factor and other neurotrophins. J. Cell Sci. 121, 504-513 (2008). [Abstract] [Full Text]