AMPA Receptor Signaling through BRAG2 and Arf6 Critical for Long-Term Synaptic Depression

Abstract

Central nervous system synapses undergo activity-dependent alterations to support learning and memory. Long-term depression (LTD) reflects a sustained reduction of the synaptic AMPA receptor content based on targeted clathrin-mediated endocytosis. Here we report a current-independent form of AMPA receptor signaling, fundamental for LTD. We found that AMPA receptors directly interact via the GluA2 subunit with the synaptic protein BRAG2, which functions as a guanine-nucleotide exchange factor (GEF) for the coat-recruitment GTPase Arf6. BRAG2-mediated catalysis, controlled by ligand-binding and tyrosine phosphorylation of GluA2, activates Arf6 to internalize synaptic AMPA receptors upon LTD induction. Furthermore, acute blockade of the GluA2-BRAG2 interaction and targeted deletion of BRAG2 in mature hippocampal CA1 pyramidal neurons prevents LTD in CA3-to-CA1 cell synapses, irrespective of the induction pathway. We conclude that BRAG2-mediated Arf6 activation triggered by AMPA receptors is the convergent step of different forms of LTD, thus providing an essential mechanism for the control of vesicle formation by endocytic cargo.

The BRAG/IQSec family of Arf GEFs

“…Moreover, BRAG2 catalytic activity was stimulated by the cytoplasmic domain of GluA2 or a peptide derived from it. 8 Together these findings suggest that BRAG2 activity can be regulated by interactions with both lipids and proteins, perhaps simultaneously.…”

“…7 In contrast, Kornau and colleagues demonstrated a direct interaction between BRAG2 and a non-phosphorylated tyrosine (Y876) in the cytoplasmic domain of the GluA2 subunit of neuronal AMPA receptors that required the PH domain. 8 In this case, phosphorylation of Y876 actually inhibited BRAG2 binding. Moreover, BRAG2 catalytic activity was stimulated by the cytoplasmic domain of GluA2 or a peptide derived from it.…”

“…Importantly, this site is actively de-phosphorylated in response to activation of metabotropic glutamate receptors (mGluRs), which induces AMPA-R internalization. 8 A more recent study from the same laboratory found that BRAG1 and BRAG2 also interact with NMDA receptor subunits, in a calcium-dependent and isoformspecific manner. In this context, BRAG1 binds to the Cterminal cytoplasmic tail of GluN2B, while BRAG2 binds to a similar region of GluN2A.…”

“…8 These authors found that BRAG2 binds directly to the GluA2, GluA3 and GluA4(short) subunits of AMPA-R in a manner that enhances BRAG2 catalytic activity. Interestingly, binding to GluA2 was demonstrated for constructs containing both the Sec7 and PH domains of BRAG2, but not either domain alone, raising the possibility that the linker connecting the 2 domains could mediate this interaction.…”

Brag2 differentially regulates β1- and β3-integrin-dependent adhesion in endothelial cells and is involved in developmental and pathological angiogenesis

“…Brag2 was found to promote the invasive activity of breast cancer cells [34] and myoblast fusion [11,37]. Furthermore, Brag2 was shown to be involved in trafficking processes such as the endocytosis of synaptic AMPA-receptors, E-Cadherin recycling, and phagocytosis of monocytes [21,46,47]. Silencing of Brag2 in HeLa cells increased the surface expression of b1-integrins [15] and a later study indicated that Brag2 might contribute to b1-integrin endocytosis in HeLa cells [33].…”