ZO-1 and the Spatial Organization of Gap Junctions and Glutamate Receptors in the Outer Plexiform Layer of the Mammalian Retina

Abstract

Information processing in the retina starts at the first synaptic layer, where photoreceptors and second-order neurons exhibit a complex architecture of glutamatergic and electrical synapses. To investigate the composition of this highly organized synaptic network, we determined the spatial relationship of zonula occludens-1 (ZO-1) with different connexins (Cx) and glutamate receptor (GluR) subunits in the outer plexiform layer (OPL) of rabbit, mouse, and monkey retinas. ZO-1 is well known as an intracellular component of tight and adherens junctions, but also interacts with various connexins at gap junctions. We found ZO-1 closely associated with Cx50 on dendrites of A-type horizontal cells in rabbit, and with Cx57 at dendro-dendritic gap junctions of mouse horizontal cells. The spatial arrangement of ZO-1 at the giant gap-junctional plaques in rabbit was particularly striking. ZO-1 formed a clear margin around the large Cx50 plaques instead of being colocalized with the connexin staining. Our finding suggests the involvement of ZO-1 in the composition of tight or adherens junctions around gap-junctional plaques instead of interacting with connexins directly. Furthermore, gap junctions were found to be clustered in close proximity to GluRs at the level of desmosome-like junctions, where horizontal cell dendrites converge before invaginating the cone pedicle. Based on this distinct spatial organization of gap junctions and GluRs, it is tempting to speculate that glutamate released from the photoreceptors may play a role in modulating the conductance of electrical synapses in the OPL.

Connexin 57 is expressed by the axon terminal network of B‐type horizontal cells in the rabbit retina

“…There was no obvious association between cone pedicles and the Cx57 labeling. Furthermore, most Cx57 clusters occurred distinctly below the level of rod spherules, yet occasionally above the cone pedicles, as previously reported (Puller et al, 2009). This suggests the Cx57 plaques may be associated with the AT plexus, which lies high in the OPL (Janssen-Bienhold et al, 2009).…”

“…This antibody stained large Cx50 gap-junction plaques on A-type HCs in the rabbit retina as previously reported (O’Brien et al, 2006; Puller et al, 2009). …”

“…These Cx57 channels had a unitary conductance of 57pS and were strongly modulated by pH (Palacios-Prado et al, 2009). Previous results suggest that Cx57 is also expressed in the rabbit OPL (Puller et al, 2009). …”

Distinct Expression and Localization of Diacylglycerol Kinase Isozymes in Rat Retina

“…Because our in situ hybridization histochemistry shows that DGKε mRNA signal is detected in photoreceptors (Sato et al 2011), the DGKε-immunoreactive structure should be a component of photoreceptor cells. Previous reports have described that the OLM is immunoreactive for adherens junctionrelated molecules, including zonula occludens-1 (Puller et al 2009), N-cadherin (Koike et al 2005), and membraneassociated palmitoylated proteins (Kantardzhieva et al 2006). These molecules, which are well known as intracellular components of tight and adherens junctions, are involved in the maintenance of photoreceptor-Müller glial cell adhesion (Kantardzhieva et al 2006).…”

Specialized synaptic pathway for chromatic signals beneath S-cone photoreceptors is common to human, Old and New World primates

“…It was clustered at the invaginating dendritic and axonal tips of these cells, postsynaptic to cones and rods, respectively. Beneath cone pedicles in more proximal regions of the OPL, syntaxin-4 was clustered in horizontal cells forming another dense immunoreactive layer per pedicle approximately at the level of desmosome-like junctions [48], where horizontal cells are known to express further synaptic proteins, including glutamate receptors and connexins [25, 43, 49]. …”