Fluorescence Investigations of the Rate‐Limiting Step in the HCN Ion Channel Deactivation Pathway
Bibliographic record
Abstract
Hyperpolarization‐ and cyclic nucleotide‐activated (HCN) ion channels support electrical oscillations of neuronal networks and help prevent absence seizures. The S4 helix of the voltage‐sensor (VS) region in HCN channels moves inwards upon hyperpolarization and this movement is coupled to pore opening. An open HCN channel can further be stabilized by cAMP binding to its C‐terminal region. Modifying the speed of the rate‐limiting step for HCN channel gating might help treat neuropathies, but the steps in the gating pathway are unclear. Specifically, it has been proposed that the depolarization‐dependent deactivation pathway of HCN channels includes a voltage‐independent pore closure step that becomes rate‐limiting at strong depolarizations. To clarify which step is rate‐limiting at different depolarizations, we performed voltage clamp fluorometry on mouse HCN2 channel derivatives expressed in Xenopus oocytes using a fluorophore attached to the N‐terminal region of the S4 helix. These results represent the first direct measurement of VS movements of an HCN2 channel. The fluorophore tracked depolarization‐dependent VS movements during deactivation that preceded pore closure significantly at strong depolarizations; the current decay rate was approximately 4‐fold slower than the fluorescence decay rate at +20 mV. We call this ratio of decay rates the “transmembrane coupling quotient”. At weaker depolarizations, VS movements still preceded pore closure, but less dramatically (transmembrane coupling quotient approximately 1.5‐fold at ‐60 mV). This voltage dependence of the transmembrane coupling quotient was cAMP‐independent, i.e., it remained similar even when cAMP binding was prevented by a mutation in the C‐terminal region. The voltage dependence of the transmembrane coupling quotient is consistent with a model of mammalian HCN channel deactivation where the VS undergoes movements that have a greater voltage dependence compared to pore closure movements. VS movements may serve as the rate‐limiting step for deactivation at weak depolarizations, but another step – perhaps pore closure – is the rate‐limiting step at strong depolarizations. The temporal association between VS movements and pore movements would thus be loose at strong depolarizations, and tight at weak depolarizations. Support or Funding Information Natural Sciences and Engineering Research Council (NSERC) Postgraduate Scholarship to K.E.A.M; Natural Sciences and Engineering Research Council (NSERC) Discovery Grant to E.C.Y
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| 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 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 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 source (direct Gemma or distilled Codex), 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".