α2MACROGLOBULIN DIFFERENTIALLY REGULATES LIGAND-DEPENDENT P75 AND TRKA RECEPTOR ACTIVATION IN VIVO AND IN VITRO

BARCELONA PF; SARAGOVI HU

Newport

Conferencia; Gordon Research Conference; 2015

Many growth factors are bound by proteins that form [growth factor?cofactor] complexes which have distinctive receptor-binding or signaling properties. Previously we and others showed that the soluble protein α2Macroglobulin (α2M) binds to the growth factor NGF making [α2M ?NGF] complexes, and inhibiting mature NGF actions1. However, whether α2M binds pro-neurotrophins is unknown, and the potential neurotoxic mechanism of α2M in pathological states is unknown. These questions are relevant because in neurodegenerative diseases (human glaucoma and human diabetic retinopathy, as well as in mouse models of these pathologies) α2M is up-regulated in the retina and vitreous1-4. Furthermore, intravitreal injection of α2M in a normal eye produces neuronal death, whereas neutralization of α2M during glaucoma delays neuronal death progression3. Here, we report that α2M binds to proNGF making stable complexes. The complex is resistant to proteolytic activity that normally processes proNGF to mature NGF. Hence proNGF is stable and its activity is potentiated. α2M enhances the proNGF neurotoxicity that is mediated by the p75NTR receptor. These results were validated ex vivo in protein-protein binding, protein-receptor binding, biochemical, and biological assays. Moreover, the same mechanisms were also validated in vivo. In vivo α2M induces neuronal death by potentiation of proNGF:p75NTR activity. Neutralization of proNGF or antagonism of the p75NTR prevents α2M-induced neuronal death. Additionally, α2M reduces NGF:TrkA activity. In summary, α2M is a protein that binds to and differentially regulates the function of proNGF and mature NGF. The combination of lack of trophic support by mature NGF and the enhanced neurotoxic signals through proNGF:p75 result in neurodegeneration. α2M is up-regulated in neurodegenerative diseases and could be exploited as a therapeutic target or as a tool to develop modifiers of neurotrophin signals.