Abstract
The predominate type of AMPA receptor expressed in the CNS is impermeable to Ca2+ (CI-AMPAR). However, some AMPA receptors are permeable to Ca2+ (CP-AMPAR) and play important roles in development, plasticity and disease. In the retina, ganglion cells (RGCs) are targets of disease including glaucoma and diabetic retinopathy, but there are many types of RGCs and not all types are targeted equally. In the present study, we sought to determine if there are differences in expression of AMPARs amongst RGC subtypes, and if these differences might contribute to differential vulnerability in a model of stress. Using cultured RGCs we first show that acute exposure to elevated pressure increased expression of Ca2+-permeable AMPA receptors (CP-AMPARs) in some, but not all classes of RGCs. When RGCs were sampled without regard to subtype, AMPA currents, measured using patch clamp recording, were blocked by the CP-AMPAR blocker PhTX-74 to a greater extent in pressure-treated RGCs vs. control. Furthermore, imaging experiments revealed an increase in Ca2+ influx during AMPA application in pressure-treated RGCs. However, examination of specific RGC subtypes using reporter lines revealed striking differences in both baseline AMPAR composition and modulation of this baseline composition by stress. Notably, ON alpha RGCs identified using the Opn4 mouse line and immunohistochemistry, had low expression of CP-AMPARs. Conversely, an ON-OFF direction selective RGC and putative OFF alpha RGC each expressed high levels of CP-AMPARs. These differences between RGC subtypes were also observed in RGCs from whole retina. Elevated pressure further lowered expression of CP-AMPARs in ON alpha RGCs, but raised expression in ON-OFF and OFF RGCs. Changes in CP-AMPAR expression following challenge with elevated pressure were correlated with RGC survival: ON alpha RGCs were unaffected by application of pressure, while the number of putative OFF alpha RGCs declined by approximately 50% following challenge with pressure. Differences in expression of CP-AMPARs between RGC subtypes may form the underpinnings for subtype-specific synaptic plasticity. Furthermore, the differential responses of these RGC subtypes to elevated pressure may contribute to the reported resistance of ON alpha, and susceptibility of OFF and ON-OFF RGCs to injury in models of glaucoma.
Highlights
AMPA receptors are tetramers, composed of combinations of four subunits, termed GluA1-A4
Retinal ganglion cells (RGCs) were identified by use of the Thy1-YFP-16 line developed in the Sanes lab (Feng et al, 2000; Figure 1A)
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 maintain a constant number of surface CP-AMPAR by removal and sequestration of excess receptors into an intracellular pool
Summary
AMPA receptors are tetramers, composed of combinations of four subunits, termed GluA1-A4. Ca2+ permeability through a population of AMPARs can be augmented by reducing the number of edited GluA2 subunits, by removing the GluA2 subunit from the receptor completely, or by a combination of both. The vast majority of AMPARs expressed in the brain express the edited form of GluA2 and are impermeable to Ca2+ (Isaac et al, 2007; Lu et al, 2009; Hanley, 2014). Expression of CP-AMPARs may be more common (Diamond, 2011) They are known to be expressed by OFF bipolar cells (Gilbertson et al, 1991), multiple types of amacrine cells (Morkve et al, 2002; Singer and Diamond, 2003; Chávez et al, 2006) and ganglion cells (Jones et al, 2012, 2014)
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