Subunit-selective N-Methyl-d-aspartate (NMDA) Receptor Signaling through Brefeldin A-resistant Arf Guanine Nucleotide Exchange Factors BRAG1 and BRAG2 during Synapse Maturation

Abstract

The maturation of glutamatergic synapses in the CNS is regulated by NMDA receptors (NMDARs) that gradually change from a GluN2B-to a GluN2A-dominated subunit composition during postnatal development. Here we show that NMDARs control the activity of the small GTPase ADP-ribosylation factor 6 (Arf6) by consecutively recruiting two related brefeldin A-resistant Arf guanine nucleotide exchange factors, BRAG1 and BRAG2, in a GluN2 subunit-dependent manner. In young cortical cultures, GluN2B and BRAG1 tonically activated Arf6. In mature cultures, Arf6 was activated through GluN2A and BRAG2 upon NMDA treatment, whereas the tonic Arf6 activation was not detectable any longer. This shift in Arf6 regulation and the associated drop in Arf6 activity were reversed by a knockdown of BRAG2. Given their sequential recruitment during development, we examined whether BRAG1 and BRAG2 influence synaptic currents in hippocampal CA1 pyramidal neurons using patch clamp recordings in acute slices from mice at different ages. The number of AMPA receptor (AMPAR) miniature events was reduced by depletion of BRAG1 but not by depletion of BRAG2 during the first 2 weeks after birth. In contrast, depletion of BRAG2 during postnatal weeks 4 and 5 reduced the number of AMPAR miniature events and compromised the quantal sizes of both AMPAR and NMDAR currents evoked at Schaffer collateral synapses. We conclude that both Arf6 activation through GluN2B-BRAG1 during early development and the transition from BRAG1-to BRAG2-dependent Arf6 signaling induced by the GluN2 subunit switch are critical for the development of mature glutamatergic synapses.

Changes in the Synaptic Proteome in Tauopathy and Rescue of Tau-Induced Synapse Loss by C1q Antibodies

“…Loss of these GTPase regulators could provide a molecular explanation for why actin is compromised in spines of AD patients (Kommaddi et al, 2018). Because these proteins normally regulate dendritic spine integrity and synapse density as well as synaptic signaling, the PSD-specific depletion of Iqgap1, Iqgap2, Iqsec2, Kalirin, and Trio could contribute to synaptic dysfunction and cognitive impairment in AD, as evidenced by synaptic and/or cognitive deficits upon deletion of these genes (Cahill et al, 2009;Elagabani et al, 2016;Gao et al, 2011;Herring and Nicoll, 2016). It is intriguing that this set of GTPase regulators, as well as a disproportionate number of the other downregulated PSD proteins in tauopathy, are genetically linked to neurodevelopmental disorders such as autism spectrum disorder and schizophrenia (Fromer et al, 2014;Li et al, 2016;Noebels, 2015;Shoubridge et al, 2010;Zoghbi and Bear, 2012).…”

The BRAG/IQSec family of Arf GEFs

“…In this context, BRAG1 binds to the Cterminal cytoplasmic tail of GluN2B, while BRAG2 binds to a similar region of GluN2A. 42 Importantly, expression of these subunits is developmentally regulated, such that GluN2B is more abundant during embryogenesis and GluN2A predominates postnatally. In agreement with this, BRAG1-mediated Arf6 activation predominates in the developing brain, while BRAG2 activity increases with age after birth.…”

BRAG2a, a Guanine Nucleotide Exchange Factor for Arf6, Is a Component of the Dystrophin-Associated Glycoprotein Complex at the Photoreceptor Terminal

“…Concerning the targeting mechanism for BRAG2a to the photoreceptor terminal, accumulating evidence for the existence of multiple BRAG2-interacting proteins 19,29,[35][36][37][38][39][40][41] (Table) suggested that BRAG2a can be recruited to various subcellular locations by partner proteins, thereby regulating the precise spatiotemporal activation of Arf6. In the present study, we demonstrated the dystrophin WW motif can interact specifically with BRAG2a through its proline-rich domain, whereas the b-dystroglycan intracellular domain can interact with both BRAG2a and BRAG2b.…”