Synaptotagmins 1 and 7 in vesicle release from rods and cones of mouse retina

Abstract

Purpose : Synaptotagmins are the primary Ca²⁺ sensors for synaptic exocytosis. Previous evidence suggested synaptotagmin-1 (Syt1) mediates evoked vesicle release from cones, but release from rods involves Syt1 and another sensor.

Methods : We performed immunohistochemistry, electroretinograms (ERG) and single-cell recordings using mice in which Syt1 and Syt7 were conditionally removed from rods (Rho-Cre) and/or cones (HRGP-Cre). Glutamate release was measured in rods from anion currents activated during glutamate re-uptake.

Results : Deletion of Syt1 from rods abolished fast glutamate release evoked by short depolarizations but not slower release evoked by long steps. Immunohistochemical labeling showed syntaptotagmin-7 (Syt7) in rod terminals and deletion of Syt7 reduced slow but not fast glutamate release. Deleting both sensors from rods fully abolished depolarization-evoked glutamate release. Using Ca²⁺ buffers, we found that fast release involves vesicles close to ribbon-associated Ca²⁺ channels whereas slow release involves more distant sites. Confirming that Syt1 is the sole sensor in cones, eliminating Syt1 from cones abolished photopic ERG b-waves, matching responses of GNAT2KO mice that lack functional cones. Eliminating Syt1 from rods reduced scotopic ERG b-waves. However, selective elimination of Syt7 from rods, as well as global knockout of Syt7, did not significantly reduce scotopic or photopic b-waves. Furthermore, mice lacking both Syt1 and Syt7 in rods showed the same responses as mice only lacking Syt1 in rods. Eliminating Syt1 from both rods and cones abolished photopic b-waves and left almost no scotopic b-waves. These b-waves did not differ significantly from those of mice lacking both Syt1 and Syt7 in rods and cones.

Conclusions : Syt1 is the principal sensor shaping rod and cone inputs to bipolar cells during light flashes. Syt7 contributes to slow non-ribbon release from rods, but has little impact on ERG b-waves suggesting it plays a modulatory role (e.g., shaping synaptic cleft glutamate levels).

This is a 2021 ARVO Annual Meeting abstract.

 

A. Photopic b-wave amplitude in C56Bl6, GNAT2KO and HRGPCre mice lacking Syt1 in cones. B. Scotopic b-waves after eliminating Syt1 and/or Syt7 from rods (RhoCre) and/or cones (HRGPCre).