Integrin signaling cascades are operational in adult hippocampal synapses and modulate NMDA receptor physiology

Abstract

Integrin class adhesion proteins are concentrated at adult brain synapses. Whether synaptic integrins engage kinase signaling cascades has not been determined, but is a question of importance to ideas about integrin involvement in functional synaptic plasticity. Accordingly, synaptoneurosomes from adult rat brain were used to test if matrix ligands activate integrin-associated tyrosine kinases, and if integrin signaling targets include NMDA-class glutamate neurotransmitter receptors. The integrin ligand peptide Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP) induced rapid (within 5 min) and robust increases in tyrosine phosphorylation of focal adhesion kinase, prolinerich tyrosine kinase 2 and Src family kinases. Increases were similarly induced by the native ligand fibronectin, blocked with neutralizing antibodies to b1 integrin, and not obtained with control peptides, indicating that kinase activation was integrinmediated. Both GRGDSP and fibronectin caused rapid Src kinase-dependent increases in tyrosine phosphorylation of NMDA receptor subunits NR2A and NR2B in synaptoneurosomes and acute hippocampal slices. Tests of the physiological significance of the latter result showed that ligand treatment caused a rapid and b1 integrin-dependent increase in NMDA receptor-mediated synaptic responses. These results provide the first evidence that, in adult brain, synaptic integrins activate local kinase cascades with potent effects on the operation of nearby neurotransmitter receptors implicated in synaptic plasticity.

Synaptic plasticity in early aging

“…This is not wholly unexpected because cytoskeletal regulation is a primary function of integrins across all types of cells (Zhu and Assoian, 1995;Geiger et al, 2002;DeMali et al, 2003) and there is compelling evidence that ß1 integrins regulate synaptic kinase signaling, the activities of synaptic NMDA receptors (Bernard-Trifilo et al, 2005), and processes critical for the stabilization of LTP (Chun et al, 2001;Kramár et al, 2002Kramár et al, , 2006Chan et al, 2003Chan et al, , 2006Kramár and Lynch, 2003). Nevertheless, the new results obtained with the F-actin labeling method provide the first evidence that theta bursts engage the integrin-actin system in the adult nervous system.…”

Integrin regulation of cytoplasmic calcium in excitatory neurons depends upon glutamate receptors and release from intracellular stores

“…These subunits define the two major families of integrins expressed in brain (Milner and Campbell, 2002); ß1-containing integrins are of particular interest because they modulate glutamatergic synaptic transmission Bernard-Trifilo et al, 2005) and plasticity (Kramár et al, 2002;Chan et al, 2003;Chan et al, 2006;Huang et al, 2006;Kramár et al, 2006). The effects of the integrin ligands and antisera on perikaryal [Ca 2+ ]i as assessed over time and at the initial peak RGD-induced increase for three experimental sets are summarized in figure 2.…”

“…Integrin ligand induced increases in [Ca 2+ ]i are blocked by the broad spectrum tyrosine kinase inhibitor genistein (Xie et al, 1998;Schottelndreier et al, 1999;Wu et al, 2001) as well as by the Src family kinase inhibitor PP2 (Schottelndreier et al, 2001;Wu et al, 2001) in non-neural cells. Other work has shown that integrin signaling activates kinases (CamKII, Src, MAPK p42/44) that modulate glutamate neurotransmitter receptor and VSCC function in neurons Bernard-Trifilo et al, 2005;Watson et al, 2007) and receptor tyrosine kinases in other systems (Miranti and Brugge, 2002). Moreover, there is evidence that CamKII-mediated phosphorylation of ryanodine receptors potentiates calciuminduced release from intracellular stores (Zhang et al, 2004).…”

“…Although integrins lack direct kinase activity, ligand binding initiates conformational changes and signaling through tyrosine kinase and Rho GTPase initiated cascades (Miranti and Brugge, 2002;Schwartz and Ginsberg, 2002;DeMali et al, 2003;Bernard-Trifilo et al, 2005). Recent evidence indicates that brain integrins are concentrated at synapses, modulate excitatory transmission , and play a critical role in the stabilization of Long-Term Potentiation (LTP) (Staubli et al, 1990;Staubli et al, 1998;Chun et al, 2001;Chan et al, 2006;Huang et al, 2006;Kramár et al, 2006;Nagy et al, 2006;Meighan et al, 2007;Wang et al, 2007), a form of synaptic plasticity widely thought to underlie learning (Morris, 2003;Lynch et al, 2007).…”

“…Most of the integrins expressed in brain recognize an Arg-Gly-Asp (RGD) consensus sequence in their native ligands (e.g., fibronectin, vitronectin, others) (Pinkstaff et al, 1999;Plow et al, 2000). Treatment of mature hippocampal slices or synaptoneurosomes with ligand mimetic RGDpeptides or native ligands activates LTP-related kinases including calcium calmodulin protein kinase II , focal adhesion kinase (FAK), proline-rich tyrosine kinase 2 (Pyk2) and Src family kinases (Bernard-Trifilo et al, 2005). Signaling cascades in which these kinases participate mediate integrin-dependent increases in the phosphorylation and synaptic responses of AMPA-and NMDA-type glutamate receptors BernardTrifilo et al, 2005).…”