The University of Texas MD Anderson Cancer Center UTHealth Graduate School of Biomedical Sciences Dissertations and Theses (Open Access)
MAGUK SCAFFOLDS ORGANIZE A KEY SYNAPTIC COMPLEX IN HORIZONTAL CELL PROCESSES CONTACTING PHOTORECEPTORS
Date of Graduation
Doctor of Philosophy (PhD)
Jeffrey A. Frost
Synaptic processes and plasticity of synapses are mediated by large suites of proteins. In most cases, many of these proteins are tethered together by synaptic scaffold proteins. Scaffold proteins have a large number and typically a variety of protein interaction domains that allow many different proteins to be assembled into functional complexes. As each scaffold protein has a different set of protein interaction domains and a unique set of interacting partners, the presence of synaptic scaffolds can provide insight into the molecular mechanisms that regulate synaptic processes. In studies of rabbit retina, we found SAP102 and Chapsyn110 selectively localized in the tips of B-type horizontal cell processes where they contact cone and rod photoreceptors. We further identified some known SAP102 binding partners, kainate receptor GluR6/7 and inward rectifier potassium channel Kir2.1, closely associated with SAP102 in the processes of invaginating HCs. In contrast, in the mouse retina we identified Chapsyn110 as the major scaffold in the tips of horizontal cells contacting photoreceptors. Kir2.1 was found to be assembled with SAP102 into a complex with GluR6/7 in photoreceptor invaginations in Rabbit. GluR6/7 and Kir2.1 presumably are involved in synaptic processes that govern cell-to-cell communication, and could both contribute in different ways to synaptic currents that mediate feedback signaling. Notably, we failed to find evidence for the presence of Cx57 or Cx59, but Pannexin1 immunolabeling was positive in the OPL of mouse retina suggesting that it could play a role in ephaptic and pH mediated signaling. Polyamines regulate many ion channels including Kir2.1. During the day polyamine immunolabeling was unexpectedly high in photoreceptor terminals compared to other areas of the retina. If polyamines are released, they may regulate the activity of Kir2.1 channels located in the tips of HCs. Alternatively, the presence of polyamines may potentiate GluR6/7 by reducing the transition to desensitized state causing an increase in channel conductance. The presence of SAP102 and Chapsyn110 and their binding partners in both cone and rod invaginating synapses suggests that whatever mechanism is supported by this protein complex is present in both types of photoreceptors.
Dlg3; Dlg2; Kcnj2; Grik2; synaptic signaling; proximity ligation assays
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