Modulation of L-Type Ca2+ Channels by Distinct Domains Within SNAP-25
Bibliographic record
Abstract
Cognate soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins are now known to associate the secretory vesicle with both the target plasma membrane and Ca(2+) channels in order to mediate the sequence of events leading to exocytosis in neurons and neuroendocrine cells. Neuroendocrine cells, particularly insulin-secreting islet beta-cells, t-SNARE proteins, 25-kDa synaptosomal-associated protein (SNAP-25), and syntaxin 1A, independently inhibit the L-type Ca(2+) channel (L(Ca)). However, when both are present, they actually exhibit stimulatory actions on the L(Ca). This suggests that the positive regulation of the L(Ca) is conferred by a multi-SNARE protein complex. We hypothesized an alternate explanation, which is that each of these SNARE proteins possess distinct inhibitory and stimulatory domains that act on the L(Ca). These SNARE proteins were recently shown to bind the Lc(753-893) domain corresponding to the II and III intracellular loop of the alpha1C subunit of the L(Ca). In this study, using patch-clamp methods on primary pancreatic beta-cells and insulinoma HIT-T15 cells, we examined the functional interactions of the botulinum neurotoxin A (BoNT/A) cleavage products of SNAP-25, including NH(2)-terminal (1-197 amino acids) and COOH-terminal (amino acid 198-206) domains, on the L(Ca), particularly at the Lc(753-893) domain. Intracellular application of SNAP-25(1-206) in primary beta-cells decreased L(Ca) currents by approximately 15%. The reduction in L(Ca) currents was counteracted by coapplication of Lc(753-893). Overexpression or injection of wild-type SNAP-25 in HIT cells reduced L(Ca) currents by approximately 30%, and this inhibition was also blocked by the recombinant Lc(753-893) peptide. Expression of BoNT/A surprisingly caused an even greater reduction of L(Ca) currents (by 41%), suggesting that the BoNT/A cleavage products of SNAP-25 might possess distinct inhibitory and positive regulatory domains. Indeed, expression of SNAP-25(1-197) increased L(Ca) currents (by 19% at 10 mV), and these effects were blocked by the Lc(753-893) peptide. In contrast, injection of SNAP-25(198-206) peptide into untransfected cells inhibited L(Ca) currents (by 47%), and more remarkably, these inhibitory effects dominated over the stimulatory effects of SNAP-25(1-197) overexpression (by 34%). Therefore, the SNARE protein SNAP-25 possesses distinct inhibitory and stimulatory domains that act on the L(Ca). The COOH-terminal 197-206 domain of SNAP-25, whose inhibitory actions dominate over the opposing stimulatory NH(2)-terminal domain, likely confers the inhibitory actions of SNAP-25 on the L(Ca). We postulate that the eventual accelerated proteolysis of SNAP-25 brought about by BoNT/A cleavage allows the relatively intact NH(2)-terminal SNAP-25 domain to assert its stimulatory action on the L(Ca) to increase Ca(2+) influx, and this could in part explain the observed weak or inconsistent inhibitory effects of BoNT/A on insulin secretion. The present study suggests that distinct domains within SNAP-25 modulate L(C) subtype Ca(2+) channel activity in both primary beta-cells and insulinoma HIT-T15 cells.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".