Grabner, C. | bioRxiv Science

bioRxiv2022

Design of the mammalian cone photoreceptor to Off bipolar cell synapse

Graded synapses in sensory systems reliably transmit small signals in the presence of continuous quantal noise. To understand how signaling is optimized during graded transmission, we counted the number of vesicles released by a mammalian cone terminal and compared it to the simultaneous responses in each Off bipolar cell type. Off bipolar cells contacting the terminal base comprised two groups depending on how they sampled transmitter release. In both groups, responses initially grew non-linearly with the number of released vesicles implicating a role for cooperativity during sparse release. One group sampled release from most of a cones [~]20 ribbons and can exploit averaging to improve signal reliability. The other, less-sensitive group made 1-3 contacts at the terminal center and responded to pooled transmitter, a consequence of membrane depolarization, using an insensitive kainate receptor. Off bipolar cells use different strategies to minimize transmission noise and encode cone output over different ranges.

neuroscience↗

bioRxiv2021

Resolving the molecular architecture of the photoreceptor active zone by MINFLUX nanoscopy

Cells assemble macromolecular complexes into scaffoldings that serve as substrates for catalytic processes. Years of molecular neurobiology indicate that neurotransmission depends on such optimization strategies, yet the molecular topography of the presynaptic Active Zone (AZ) where transmitter is released upon synaptic vesicle (SV) fusion remains to be visualized. Therefore, we implemented MINFLUX optical nanoscopy to resolve the AZ of rod photoreceptors. To facilitate MINFLUX nanoscopy of the AZ, we developed and verified an immobilization technique, we name Heat Assisted Rapid Dehydration (HARD). Here fresh retinal slices are directly stamped onto glass coverslips yielding a single layer of rod AZs. These AZs exhibited excellent labeling efficiency and minimal signal redundancy in the Z-direction. Our data indicate that the SV release site is a molecular complex of bassoon-Rab3-binding molecule 2 (RIM2)-ubMunc13-2-CAST. The complexes are serially duplicated longitudinally, and reflected in register along the axis of symmetry of the synaptic ribbon. One sentence summaryStructural motifs formed by active zone proteins at the photoreceptor synapse.

neuroscience↗