BRAG2a Mediates mGluR-Dependent AMPA Receptor Internalization at Excitatory Postsynapses through the Interaction with PSD-95 and Endophilin 3

Brefeldin A-resistant ArfGEF 2 (BRAG2) [or Iqsec1 (IQ motif and Sec7 domain-containing protein 1)] is a guanine nucleotide exchange factor for ADP ribosylation factor 6 (Arf6), a small GTPase implicated in the membrane trafficking between the plasma membrane and endosomes. BRAG2 regulates Arf6-dependent endocytosis of AMPA receptors (AMPARs) through the direct interaction during the hippocampal long-term depression. However, the molecular mechanism by which the BRAG2-Arf6 pathway links AMPARs to the endocytic machinery remains elusive. Herein, using mouse brains of both sexes, we demonstrated that BRAG2a, an alternative isoform with a long C-terminal insert containing a proline-rich domain and type I PDZ-binding motif, was selectively localized to the excitatory postsynaptic density (PSD). Using yeast two-hybrid screening, we identified PSD-95 and endophilin 1/3 as BRAG2a-binding partners in the brain. The interaction with PSD-95 was required for synaptic targeting of BRAG2a. In cultured hippocampal neurons, stimulation of group I metabotropic glutamate receptors (mGluRs) increased the interaction of BRAG2a with endophilin 3 and concomitant Arf6 activation in a time-dependent manner. Knockdown of BRAG2 in cultured hippocampal neurons blocked the mGluR-dependent decrease in surface AMPAR levels, which was rescued by introducing wild-type BRAG2a, but not wild-type BRAG2b or BRAG2a mutants lacking the ability to activate Arf6 or to interact with endophilin 3 or PSD-95. Further postembedding immunoelectron microscopic analysis revealed the preorganized lateral distribution of BRAG2a, Arf6, and endophilin 3 for efficient endocytosis at the postsynaptic membrane. Together, the present findings unveiled a novel molecular mechanism by which BRAG2a links AMPARs to the clathrin-dependent endocytic pathway through its interaction with PSD-95 and endophilin 3.SIGNIFICANCE STATEMENT BRAG2/Iqsec1 is a GDP/GTP exchange factor for ADP ribosylation factor 6 (Arf6), a small GTPase implicated in the membrane trafficking between the plasma membrane and endosomes, and regulates Arf6-dependent endocytosis of AMPARs through direct interaction during hippocampal long-term depression, one of the mechanisms of synaptic plasticity related to learning and memory. However, the molecular mechanism by which the BRAG2-Arf6 pathway links AMPARs to the endocytic machinery remains elusive. Here, we identified isoform-specific mechanisms of BRAG2-mediated AMPAR internalization. We demonstrated that the interaction of BRAG2a isoform with PSD-95 and endophilin 3 was required for the mGluR-dependent decrease in surface AMPARs in hippocampal neurons. These results unveiled a novel molecular mechanism by which BRAG2 links AMPARs to the clathrin-mediated endocytic machinery at postsynaptic sites.

Discrete localization patterns of Arf6, and its activators EFA6A and BRAG2, and its effector PIP5kinaseγ on myofibrils of myotubes and plasma membranes of myoblasts in developing skeletal muscles of mice

Arf6 (ADP ribosylation factor 6), activated by Arf-GEF (guanine nucleoside exchange factor), is involved in the membrane trafficking and actin-remodeling which are critical for maintenance of cell organization and activity and for fusion of myoblasts to form myotubes/myofibers. EFA6A (exchange factor for Arf6 type A) and BRAG2 (brefeldin A-resistant Arf-GEF 2) represent members of discrete subfamilies of Arf-GEF, while PIP5Kγ (phosphatidylinositol4-phosphate5-kinase γ) produces PI 4,5-bisphosphate (PIP2) and it is target for Arf6. In the present study, immunoreactive bands for Arf6, EFA6A, BRAG2 and PIP5Kγ were detected in immunoblots of skeletal muscle homogenates of mice at E18D (embryonic day 18), while the bands for Arf6, EFA6A and PIP5Kγ were reduced in density and no significant bands for BRAG2 were discerned at P1D (postnatal 1 day). No immunoblot bands for any of the molecules were eventually detected in skeletal fibers of adult mice. Immunoreactivities for endogenous Arf6, EFA6A and PIP5Kγ were visualized using immuno-light microscopy localized as periodic striations running perpendicular to the longitudinal axes of skeletal muscle fibers of mice at E18D and P1D. All the striations were co-immunoreactive for β-actin in double immunofluorescence microscopy, and the immunoreactivities were confined to thin myofilaments at sarcomeric I-domains in immuno-electron microscopy. On the other hand, immunoreactivities for Arf6, BRAG2 and PIP5Kγ were conspicuous on plasmalemma of myoblasts at E14D, while immunoreactivity for EFA6A was already distinct in striations perpendicular to myofibrils in myotubes at E14D. The present findings suggest three possibilities: involvement of EFA6A-activated Arf6 together with PIP5Kγ in maturation of myofibrils, movement of Arf6 and PIP5Kγ from the plasmalemma of myoblasts to myofibrils of myotubes, and that of BRAG2 to the cytoplasm of myotubes; and further a function of EFA6A independent of the activation of Arf6 in immature myofibrils. In addition, the involvement of Arf6, BRAG2 and PIP5Kγ in the fusion of myoblasts into myotubes was supported by the present finding.

Bi-allelic Variants in IQSEC1 Cause Intellectual Disability, Developmental Delay, and Short Stature

We report two consanguineous families with probands that exhibit intellectual disability, developmental delay, short stature, aphasia, and hypotonia in which homozygous non-synonymous variants were identified in IQSEC1 (GenBank: NM_001134382.3). In a Pakistani family, the IQSEC1 segregating variant is c.1028C>T (p.Thr343Met), while in a Saudi Arabian family the variant is c.962G>A (p.Arg321Gln). IQSEC1-3 encode guanine nucleotide exchange factors for the small GTPase ARF6 and their loss affects a variety of actin-dependent cellular processes, including AMPA receptor trafficking at synapses. The ortholog of IQSECs in the fly is schizo and its loss affects growth cone guidance at the midline in the CNS, also an actin-dependent process. Overexpression of the reference IQSEC1 cDNA in wild-type flies is lethal, but overexpression of the two variant IQSEC1 cDNAs did not affect viability. Loss of schizo caused embryonic lethality that could be rescued to 2^nd instar larvae by moderate expression of the human reference cDNA. However, the p.Arg321Gln and p.Thr343Met variants failed to rescue embryonic lethality. These data indicate that the variants behave as loss-of-function mutations. We also show that schizo in photoreceptors is required for phototransduction. Finally, mice with a conditional Iqsec1 deletion in cortical neurons exhibited an increased density of dendritic spines with an immature morphology. The phenotypic similarity of the affecteds and the functional experiments in flies and mice indicate that IQSEC1 variants are the cause of a recessive disease with intellectual disability, developmental delay, and short stature, and that axonal guidance and dendritic projection defects as well as dendritic spine dysgenesis may underlie disease pathogenesis.