Syntaxin 3B is essential for the exocytosis of synaptic vesicles in ribbon synapses of the retina

Abstract

Ribbon synapses of the vertebrate retina are specialized synapses that release neurotransmitter by synaptic vesicle exocytosis in a manner that is proportional to the level of depolarization of the cell. This release property is different from conventional neurons, in which the release of neurotransmitter occurs as a short-lived burst triggered by an action potential. Synaptic vesicle exocytosis is a calcium regulated process that is dependent on a set of interacting synaptic proteins that form the so-called SNARE (soluble N-ethylmaleimide sensitive factor attachment protein receptor) complex. Syntaxin 3B has been identified as a specialized SNARE molecule in ribbon synapses of the rodent retina. However, the best physiologically-characterized neuron that forms ribbon-style synapses is the rod-dominant or Mb1 bipolar cell of the goldfish retina. We report here the molecular characterization of syntaxin 3B from the goldfish retina. Using a combination of reverse transcription (RT) PCR and immunostaining with a specific antibody, we show that syntaxin 3B is highly enriched in the plasma membrane of bipolar cell synaptic terminals of the goldfish retina. Using membrane capacitance measurements we demonstrate that a peptide derived from goldfish syntaxin 3B inhibits synaptic vesicle exocytosis. These experiments demonstrate that syntaxin 3B is an important factor for synaptic vesicle exocytosis in ribbon synapses of the vertebrate retina.

Phosphorylation of syntaxin 3B by CaMKII regulates the formation of t-SNARE complexes

“…Syntaxin 3B has a lower binding affinity to SNAP-25 than syntaxin1A and is less efficient in catalyzing SNARE mediated in vitro fusion of vesicles (Curtis et al 2008), but seems to efficiently catalyze vesicle fusion in ribbon synapses in vivo (Curtis et al 2010). This discrepancy between in vitro and in vivo activity suggests that extra factors are required that allow syntaxin 3B to function efficiently in ribbon synapses.…”

“…In contrast syntaxin 3B has been shown to be important for synaptic vesicle exocytosis in retinal bipolar cells indicating a specific role for syntaxin 3B in ribbon synapses (Curtis et al, 2010). Furthermore, Syntaxin 3B, SNAP-25 and synaptobrevin can catalyze SNARE mediated vesicle fusion in an in vitro fusion assay (Curtis et al, 2008) but in comparison to syntaxin 1A, syntaxin 3B has a lower affinity to bind SNAP-25 and is less efficient in catalyzing vesicle fusion.…”

The diverse roles of ribbon synapses in sensory neurotransmission

“…Among those identified so far are Rab3 interacting protein 1 (Rim1)85 (involved in priming vesicles for exocytosis), Kinesin-like protein KIF3A86 (a component of kinesin motor complexes), Piccolo87 (a component of the cytomatrix at the active zone), CtBP180 (a transcriptional regulator that might bind to the B-domain of RIBEYE), and guanylyl cyclase-activating protein 2 (GCAP2)88 (a calcium-binding protein). In addition to proteins associated with the ribbon, ribbon synapses express specific isoforms of the SNARE protein Syntaxin (Syntaxin3b89,90) and SNARE-associated complexins (Complexin3/491). In cochlear hair cells a C2-domain transmembrane protein, otoferlin, is required for normal transmitter release, and is mutated in a form of human deafness92.…”

Presynaptic [Ca2+] and GCAPs: aspects on the structure and function of photoreceptor ribbon synapses

“…In photoreceptor ribbon synapses, syntaxin 1 is replaced by syntaxin 3b (Curtis et al, 2008, 2010); complexin 1/2 by complexin 3/4 (Reim et al, 2005, 2009); Munc13-1 by ubMunc13-2 (Schmitz et al, 2001; Cooper et al, 2012); piccolo by piccolino (Regus-Leidig et al, 2013). But the key players of exocytosis are remarkably conserved.…”