Differential effects of SNAP-25 deletion on Ca2+ -dependent and Ca2+ -independent neurotransmission.
At the synapse, SNAP-25, along with syntaxin/HPC-1 and synaptobrevin/VAMP, forms SNARE N-ethylmaleimide-sensitive factor [soluble (NSF) attachment protein receptor] complexes that are thought to catalyze membrane fusion. Results from neuronal cultures of synaptobrevin-2 knockout (KO) mice showed that loss of synaptobrevin has a more severe effect on calcium-evoked release than on spontaneous release or on release evoked by hypertonicity. In this study, we recorded neurotransmitter release from neuronal cultures of SNAP-25 KO mice to determine whether they share this property. In neurons lacking SNAP-25, as those deficient in synaptobrevin-2, we found that approximately 10-12% of calcium-independent excitatory and inhibitory neurotransmitter release persisted. However, in contrast to synaptobrevin-2 knockouts, this remaining readily releasable pool in SNAP-25-deficient synapses was virtually insensitive to calcium-dependent-evoked stimulation. Although field stimulation reliably evoked neurotransmitter release in synaptobrevin-2 KO neurons, responses were rare in neurons lacking SNAP-25, and unlike synaptobrevin-2-deficient synapses, SNAP-25-deficient synapses did not exhibit facilitation of release during high-frequency stimulation. This severe loss of evoked exocytosis was matched by a reduction, but not a complete loss, of endocytosis during evoked stimulation. Moreover, synaptic vesicle turnover probed by FM-dye uptake and release during hypertonic stimulation was relatively unaffected by the absence of SNAP-25. This last difference indicates that in contrast to synaptobrevin, SNAP-25 does not directly function in endocytosis. Together, these results suggest that SNAP-25 has a more significant role in calcium-secretion coupling than synaptobrevin-2.
Bronk
Peter
P
Department of Neuroscience, The University of Texas Southwestern Medical Center, Dallas, Texas 75390-9111, USA.
Deák
Ferenc
F
Wilson
Michael C
MC
Liu
Xinran
X
Südhof
Thomas C
TC
Kavalali
Ege T
ET
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002478:Cells, Cultured; D004307:Dose-Response Relationship, Radiation; D004558:Electric Stimulation; D004622:Embryo, Mammalian; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D006624:Hippocampus / Q000166:cytology; D015759:Ionomycin / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000235:genetics; D051379:Mice; D018345:Mice, Knockout; D016253:Microscopy, Immunoelectron / Q000379:methods; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000494:pharmacology; D017966:Pyramidal Cells / Q000187:drug effects / Q000502:physiology*; D011726:Pyridinium Compounds / Q000493:pharmacokinetics; D000644:Quaternary Ammonium Compounds / Q000493:pharmacokinetics; D013569:Synapses / Q000187:drug effects / Q000502:physiology* / Q000648:ultrastructure; D016704:Synapsins / Q000378:metabolism; D009435:Synaptic Transmission / Q000187:drug effects / Q000235:genetics*; D050825:Synaptosomal-Associated Protein 25 / Q000172:deficiency*; D013997:Time Factors
D018691:Excitatory Amino Acid Antagonists; C086686:FM2 10; D011726:Pyridinium Compounds; D000644:Quaternary Ammonium Compounds; D016704:Synapsins; D050825:Synaptosomal-Associated Protein 25; D015759:Ionomycin; D011188:Potassium; D002118:Calcium