1. AMPA receptors in the healthy and diseased retina
In collaboration with Dr. Reed Carroll we have been studying synaptic plasticity of a subclass of glutamate receptor called AMPA receptors (Jones et al, 2012, 2014). We found that the visual experience of ganglion cells changes the composition of AMPA receptors from those that contain a GluA2 subunit and are impermeable to Ca2+ to a type of receptor that lacks the GluA2 subunit and is Ca2+ permeable. This kind of synaptic plasticity was previously believed to occur only in brain regions associated with information storage. What we want to know next is why this boost in Ca2+ influx is important to the synapse. To accomplish this, we use patch clamp recording and calcium imaging of ganglion cells in the mouse retina.
AMPA receptors in ganglion cells are also modified by disease. Glaucoma is a multifactorial disease which culminates in the death of retinal ganglion cells (RGCs). In a mouse model of glaucoma, almost all of the AMPA receptors that are expressed by RGCs become permeable to Ca2+. This is because high intraocular pressure seems to lower expression of the enzyme adenosine deaminase acting on RNA 2 (ADAR2), which converts Ca2+-permeable AMPA receptors (CP-AMPAR) to a Ca2+-impermeable form (CI-AMPAR). Under normal conditions, post-translational mRNA editing of AMPARs by ADAR2 is nearly 100% efficient, resulting in the absence of Ca2+ permeability in most types of AMPARs. However, when intraocular pressure in the mouse eye is raised, the loss of ADAR2 lowers this efficiency. We are currently exploring the relationship between glaucoma and Ca2+-permeable AMPA receptors.
2. Targeting ON bipolar cells.
The synapse between photoreceptors and an interneuron called the On bipolar cell is a critically important synapse in vision because all visual information flows through it. It has long been appreciated that this is a very different kind of synapse because it is inhibitory despite the fact that glutamate is the transmitter. The On bipolar cell expresses a metabotropic glutamate receptor (mGluR6) that negatively couples to the synaptic channel, recently identified by our group as the novel channel Trpm1 (Shen et al, 2009,2012). In the dark, binding of glutamate to mGluR6 activates a G protein, and one or more G protein subunits then closes the synaptic channel. In the light, glutamate is not released by photoreceptors, and so the synaptic channel (Trpm1) now opens and the cell depolarizes. Thus glutamate mimic an inhibitory transmitter by closing an excitatory synaptic channel. Mutations in Trpm1 have been discovered in a number of families, and is associated with congenital stationary night blindness (Peachey et al, 2012).
There are at least 6 different subclasses of On bipolar cell. One connects exclusively to rods, the others to cones. Why so many? How is information from all of these cells processed downstream to form a coherent representation of the world? A goal of the lab is to acquire a complete functional wiring diagram of On bipolar cell pathways, including not only the cell types, but also the kind of transmitter receptors that each pathway uses. This is important not only for understanding the retina as a microprocessor, but to better target specific cells for insertion of light sensitive molecules for vision restoration.