Molecular mechanisms underlying activity-dependent AMPA receptor cycling in retinal ganglion cells

Abstract

On retinal ganglion cells (RGCs) transmit light encoded information to the brain and receive excitatory input from On cone bipolar cells (CBPs). The synaptic CBP input onto On RGCs is mediated by AMPA-type glutamate receptors (AMPARs) that include both those lacking a GluA2 subunit, and are therefore permeable to Ca2+, and those that possess at least one GluA2 subunit and are Ca2+-impermeable. We have previously demonstrated in electrophysiological studies that periods of low synaptic activity, brought about by housing animals in darkness, enhances the proportion of GluA2-lacking AMPARs at the On CBP-On RGC synapse by mobilizing surface GluA2 containing receptors into a receptor pool that rapidly cycles in and out of the membrane. AMPAR cycling induction by reduced synaptic activity takes several hours. This delay suggests that changes in expression of proteins which regulate AMPAR trafficking may mediate the altered mobility of GluA2 AMPARs in RGCs. In this study, we test the hypothesis that AMPAR trafficking proteins couple synaptic activity to AMPAR cycling in RGCs. Immunocytochemical and biochemical analysis confirmed that darkness decreases surface GluA2 in RGCs and changed the expression levels of three proteins associated with GluA2 trafficking. GRIP was decreased, while PICK1 and Arc were increased. Knockdown of GRIP with siRNA elevated constitutive AMPAR cycling, mimicking effects of reduced synaptic activity, while knockdown of PICK1 and ARC blocked increases in constitutive GluA2 trafficking. Our results support a role for correlated, activity-driven changes in multiple AMPAR trafficking proteins that modulate GluA2 cycling which can in turn affect synaptic AMPAR composition in RGCs.

Elevated Pressure Increases Ca2+ Influx Through AMPA Receptors in Select Populations of Retinal Ganglion Cells

“…We have previously shown that ON, but not OFF type RGCs mobilize internal CP-AMPARs for insertion to the surface via both homeostatic and NMDAR dependent mechanisms (Xia et al, 2007 ; Jones et al, 2012 ; Casimiro et al, 2013 ), and we wondered whether ON alpha RGCs labeled in the Opn4 line might similarly maintain a constant number of surface CP-AMPAR by removal and sequestration of excess receptors into an intracellular pool. To test this, AMPA currents were first recorded in the presence and absence of 100 μM PhTX to estimate the size of the surface pool of CP-AMPARs.…”

Impaired AMPA receptor trafficking by a double knockout of zebrafish olfactomedin1a/b

“…Changes in the composition of AMPARs and trafficking of their subunits may lead to changes in synaptic transmission in the retina (Casimiro et al . ; Henley and Wilkinson ) and changes in escape responses (smaller tail velocities and aberrant turning directions) (Roy et al . ).…”

Postsynaptic Plasticity Triggered by Ca2+-Permeable AMPA Receptor Activation in Retinal Amacrine Cells

“…Several downstream mechanisms might produce this plasticity. First, exocytosis of AMPARs onto dendritic membranes and/or movement of AMPARs from extra-synaptic to synaptic sites may be involved (Casimiro et al, 2013). Second, Ca 2+ influx through CP-AMPARs can trigger a Ca 2+ -dependent phosphorylation of CP-AMPARs (Guire et al, 2008; Kristensen et al, 2011).…”

“…In mammalian ON-type ganglion cells an activity-dependent process of “constitutive cycling” of AMPARs has been described (Casimiro et al, 2013). This process involved the cycling of extrasynaptic GluA2-containing (Ca 2+ -impermeable) AMPARs, but not synaptic AMPARs.…”