Dynamic assembly of ribbon synapses and circuit maintenance in a vertebrate sensory system

Abstract

Ribbon synapses transmit information in sensory systems, but their development is not well understood. To test the hypothesis that ribbon assembly stabilizes nascent synapses, we performed simultaneous time-lapse imaging of fluorescently-tagged ribbons in retinal cone bipolar cells (BCs) and postsynaptic densities (PSD95-FP) of retinal ganglion cells (RGCs). Ribbons and PSD95-FP clusters were more stable when these components colocalized at synapses. However, synapse density on ON-alpha RGCs was unchanged in mice lacking ribbons (ribeye knockout). Wildtype BCs make both ribbon-containing and ribbon-free synapses with these GCs even at maturity. Ribbon assembly and cone BC-RGC synapse maintenance are thus regulated independently. Despite the absence of synaptic ribbons, RGCs continued to respond robustly to light stimuli, although quantitative examination of the responses revealed reduced frequency and contrast sensitivity.

Ribbon synapses in our sensory nervous system are central to hearing and sight, yet little is known about how these synapses are assembled and maintained during development. In this study, authors use live imaging techniques to monitor ribbon appearance, loss and maintenance in a retinal circuit during development to show that nascent synapses comprising of both ribbons and PSD95 are more stable over time compared to contacts without ribbons.