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Pathophysiological Context of AMPA Receptors and Epilepsy

Global Burden and Characterization of Epilepsy:
- Epilepsy is defined as a chronic neurological brain disease affecting over $50 \times 10^6$ individuals across the globe.
- The primary clinical hallmark consists of recurrent, unprovoked epileptic seizures.
- Pathophysiologically, seizures arise from aberrant excitation across neuronal circuits, prominently within the hippocampus, creating an acute, powerful imbalance between excitatory and inhibitory neurotransmission.
Glutamatergic Neurotransmission:
- Glutamate serves as the principal excitatory neurotransmitter throughout the central nervous system (CNS).
- Glutamate orchestrates fast excitatory synaptic transmission by activating ionotropic glutamate receptors and modulates baseline neuronal excitability via metabotropic glutamate receptor cascades.
Role of Calcium-Permeable AMPA Receptors (CP-AMPARs) in Excitotoxicity:
- The $\alpha$ -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor family represents the primary mediator of fast excitatory synaptic responses.
- AMPA receptors lacking the $\text{GluA2}$ subunit, or those harboring the unedited $\text{GluA2}$ form at the glutamine/arginine ( $\text{Q/R}$ ) site ( $\text{GluA2(Q)}$ ), conduct significant calcium ( $\text{Ca}^{2+}$ ) influx upon activation.
- Enhanced $\text{Ca}^{2+}$ entry directly amplifies neuronal excitability and reinforces excitatory signaling pathways.
- Sustained seizure activity promotes the pathological upregulation of $\text{GluA2}$ -lacking, calcium-permeable AMPA receptors at the synapse, driving $\text{Ca}^{2+}$ -dependent excitotoxic neuronal damage.

Structural Architecture and Functional Kinetics of AMPA Receptors

Quaternary Assembly and Domain Organization:
- AMPA receptors exist as modular tetrameric ion channels arranged in a dimer-of-dimers configuration.
- Subunit combinations comprise variable assemblies of four principal core proteins: $\text{GluA1}$ , $\text{GluA2}$ , $\text{GluA3}$ , and $\text{GluA4}$ .
- Each receptor subunit possesses dedicated structural domains:
  - An amino-terminal domain (ATD).
  - A ligand-binding domain (LBD) characterized by a dynamic clamshell-like structure.
  - A transmembrane domain (TMD) comprising three membrane-spanning helices ( $\text{M1}$ , $\text{M3}$ , $\text{M4}$ ) and a re-entrant pore loop ( $\text{M2}$ ).
- Dimer interfaces situated at the ATD and LBD regions coordinate receptor assembly, gating transitions, and desensitization pathways.
- Agonist (glutamate) binding within the LBD clamshell promotes domain closure, transmitting mechanical tension through linker segments to pull open the $\text{M3}$ bundle crossing, depolarizing the postsynaptic membrane via cation influx.
Gating Kinetic Millisecond Frameworks:
- Deactivation:
  - Deactivation defines the rate of current decay following immediate agonist removal.
  - This rapid process reflects channel closure from agonist-bound open states and operates on a sub-millisecond to millisecond timescale ( $\approx 0.5\text{--}5\,\text{ms}$ ).
- Desensitization:
  - Desensitization defines the decline of macroscopic current during prolonged, continuous agonist exposure.
  - This structural transition involves the rupture of the LBD dimer interface while agonist remains bound in the clamshell, allowing the channel gate to shut.
  - Desensitization typically develops over several milliseconds to tens of milliseconds ( $\approx 2\text{--}30\,\text{ms}$ ) under physiological conditions.

Transmembrane AMPA Receptor Regulatory Protein Auxiliary Subunits

TARP Classification and Distribution:
- Native AMPA receptor complexes in the mammalian brain are constitutively assembled with transmembrane AMPA receptor regulatory proteins (TARPs) that govern trafficking, channel gating, pharmacology, and synaptic localization.
- $\text{TARP}\gamma8$ (encoded by the CACNG8 gene) represents the predominant auxiliary subunit expressed throughout hippocampal excitatory neurons, specifically localized to both synaptic and extrasynaptic membranes of CA1 pyramidal cell dendrites and spines.
- Targeted genetic deletion of $\text{TARP}\gamma8$ produces a selective, pronounced reduction in AMPA receptor density in the hippocampal CA1 stratum radiatum.
Mechanisms of Kinetic Modulation by TARP $\gamma$ 8:
- Cryo-electron microscopy (cryo-EM) and functional profiling show that $\text{TARP}\gamma8$ interacts with AMPA receptor cores through extended extracellular loops that make direct contact with the LBD lower lobes, as well as transmembrane segments that pack against the channel pore helices.
- The extracellular loop interactions stabilize the active, open-channel, and pre-open conformations of the receptor, significantly retarding both desensitization and deactivation kinetics.
- The transmembrane helices of $\text{TARP}\gamma8$ alter pore vestibule electrostatics, dampening intracellular polyamine block (such as spermine block) and partially linearizing current–voltage ( $\text{I–V}$ ) relationships.
- Due to its localized enrichment in seizure-susceptible hippocampal networks, $\text{TARP}\gamma8$ represents a primary target for selective antiepileptic drug design.

Chemical Classification and Structural Features of Dibenzobarrelene Derivatives

The Dibenzobarrelene Scaffold:
- A set of eight synthetic compounds, designated $\text{ND-1}$ through $\text{ND-8}$ , are built upon a rigid polycyclic aromatic dibenzobarrelene skeleton.
- The three-dimensional geometry provides a highly constrained, spatially rigid architecture that facilitates interaction with shallow or interfacial protein cavities rather than narrow orthosteric clefts.
- The dibenzobarrelene core displays an extended $\pi$ -system capable of hydrophobic and $\pi\text{--}\pi$ stacking interactions with aromatic residues embedded within the receptor's linker and transmembrane segments.
Classification by Heterocyclic Modification:
- Symmetrical Bis-Imidazoline Amides: Compounds $\text{ND-1}$ , $\text{ND-2}$ , $\text{ND-7}$ , and $\text{ND-8}$ contain dual imidazoline rings linked via amide bonds.
- Di-Imidazolidine Dicarboxamides: Compound $\text{ND-3}$ features two saturated imidazolidine rings flanking the dicarboxamide bridge.
- Fused Diazepinedione Derivatives: Compounds $\text{ND-4}$ and $\text{ND-5}$ display a rigid, fused seven-membered diazepinedione heterocyclic system.
- Fused Imidazodiazepinedione Derivative: Compound $\text{ND-6}$ contains an imidazo-fused diazepinedione heterocycle.
Hydrogen-Bonding and Polar Interfacial Motifs:
- The combination of amide carbonyl oxygen atoms, basic nitrogen atoms ( $\text{N3}$ of the imidazoline ring), and auxiliary aromatic substituents (such as thiophene rings or thiol groups) introduces precise hydrogen-bond donor and acceptor arrays.
- These polar groups enable specific interactions with the peptide backbone and polar side chains located at the junction of the ligand-binding and transmembrane domains (LBD-TMD interface).

Molecular structures of dibenzobarrelene derivatives ND-1 to ND-8

Heterologous Expression System and Molecular Cloning Methodology

cDNA Constructs and Engineering:
- Wild-type rat AMPA receptor cDNAs encoding $\text{GluA1}$ and $\text{GluA2}$ were utilized in the flip splice variant form.
- The $\text{GluA2}$ construct harbored the unedited glutamine residue at the site of RNA editing ( $\text{Q-form}$ ), preserving calcium permeability and polyamine sensitivity in both homomeric and heteromeric configurations.
- Receptor cDNAs were originally provided by S.F. Heinemann (Salk Institute, La Jolla, CA, USA) and cloned into the mammalian expression vector $\text{pRK5}$ provided by P.H. Seeburg (Max Planck Institute for Medical Research, Heidelberg, Germany).
- The complete coding sequence of human $\text{TARP}\gamma8$ (CACNG8) was amplified using high-fidelity DNA polymerase (New England Biolabs, Ipswich, MA, USA) to suppress point mutations and ligated downstream of a cytomegalovirus (CMV) promoter in $\text{pRK5}$ .
- To qualitatively verify expression in living cells, an enhanced green fluorescent protein ( $\text{EGFP}$ ) sequence was fused via a flexible linker to the N-terminus of $\text{TARP}\gamma8$ .
- To ensure fixed, invariant stoichiometry ( $1:1$ ) between receptor subunits and auxiliary proteins, tandem constructs were synthesized joining the open reading frame of $\text{TARP}\gamma8$ directly to the C-terminus of the AMPA receptor subunit via an uncharged, flexible peptide linker.
- All expression plasmids were transformed into chemically competent Escherichia coli, verified by restriction endonuclease mapping, and confirmed across their entire open reading frames by Sanger sequencing.
Plasmid Isolation and Purification Procedure:
- Transformed E. coli cells were plated on selective Luria-Bertani (LB) agar supplemented with ampicillin and grown overnight at $37\,^\circ\text{C}$ .
- Single isolated colonies were expanded in selective LB liquid medium for an initial $8\,\text{h}$ starter culture at $37\,^\circ\text{C}$ with vigorous agitation.
- The starter culture was diluted into fresh selective medium and cultivated for an additional $12\text{--}16\,\text{h}$ at $37\,^\circ\text{C}$ .
- Bacterial cells were harvested by centrifugation, resuspended, and subjected to alkaline lysis using the QIAGEN Plasmid Mini Kit (Catalog No. 12123, QIAGEN, Hilden, Germany).
- Cellular lysates were cleared of debris by centrifugation following acidic neutralization.
- Supernatants were passed through silica-membrane columns by gravity flow, washed to eliminate contaminants, and eluted with basic buffer.
- Plasmid DNA was precipitated using isopropanol, washed with ice-cold ethanol, air-dried, and reconstituted in sterile nuclease-free buffer.
- Purity and concentration were validated via ultraviolet (UV) spectrophotometry by measuring $A_{260} / A_{280}$ absorbance ratios ( $\approx 260\,\text{nm}$ ) and size-verified by agarose gel electrophoresis, achieving yields up to $20\,\mu\text{g}$ per preparation.
Cell Culture and jetPRIME Transfection Protocol:
- Human embryonic kidney 293 cells expressing the SV40 large T antigen (HEK293T cells, Catalog No. 85120602, Sigma-Aldrich, Darmstadt, Germany) were maintained in Dulbecco's Modified Eagle Medium (DMEM; Sigma-Aldrich) supplemented with $10\%$ fetal bovine serum (FBS; Sigma-Aldrich) and $0.1\,\text{mg/mL}$ penicillin–streptomycin (Biological Industries, Beit-Haemek, Israel).
- Cultures were maintained at $37\,^\circ\text{C}$ under a humidified atmosphere containing $5\%\,\text{CO}_2$ and passaged twice weekly.
- To prevent drift in receptor expression efficiency and gating physiology, all patch-clamp recordings were restricted to cell passages $\le 20$ .
- Transient transfections were carried out using the jetPRIME nanoparticle transfection reagent (Polyplus, New York, NY, USA).
- The fluorescent reporter plasmid ( $\text{pEGFP-C1}$ ; Clontech, Palo Alto, CA, USA) was co-transfected with the AMPA receptor subunit expression plasmid at a fixed $1:9$ DNA mass ratio.
- This $1:9$ stoichiometry guarantees sufficient EGFP fluorescence to visually identify successfully transfected cells under epifluorescence microscopy while minimizing cellular stress and avoiding non-specific disruptions in receptor subunit assembly and trafficking.
- Cells remained in 12-well culture plates for $36\,\text{h}$ post-transfection, after which they were enzymatically detached and replated at low density onto glass coverslips coated with $1\,\mu\text{g/mL}$ laminin (Sigma-Aldrich) for electrophysiological interrogation.

Patch-Clamp Electrophysiological Recording Configurations and Solution Exchange Protocols

Perfusion and Piezoelectric Fast-Solution Exchange Mechanics:
- Whole-cell configuration recordings were established using an Integrated Patch Amplifier system (IPA, Sutter Instruments, Novato, CA, USA) coupled with an automated piezoelectric translation device (Automate Scientific, Berkeley, CA, USA).
- Microsolutions were delivered through a double-barrel (theta) borosilicate glass pipette mounted on the piezoelectric translator.
- Solution switching speed was calibrated before each experimental run by recording liquid junction open-tip potentials between normal and diluted ( $10\%$ ) bath solutions, yielding $10\text{--}90\%$ rise times ranging strictly between $200\,\mu\text{s}$ and $300\,\mu\text{s}$ .
- Cells yielding peak inward current amplitudes of $< 20\,\text{pA}$ were excluded from data collection due to insufficient signal-to-noise ratios.
Electrolyte Compositions:
- Extracellular (Bath) Solution:
  - $150\,\text{mM}\,\text{NaCl}$
  - $2.8\,\text{mM}\,\text{KCl}$
  - $0.5\,\text{mM}\,\text{MgCl}_2$
  - $2\,\text{mM}\,\text{CaCl}_2$
  - $10\,\text{mM}\,\text{HEPES}$
  - Adjusted to $\text{pH } 7.4$ with $\text{NaOH}$ .
- Intracellular (Pipette) Solution:
  - $110\,\text{mM}\,\text{CsF}$
  - $30\,\text{mM}\,\text{CsCl}$
  - $4\,\text{mM}\,\text{NaCl}$
  - $0.5\,\text{mM}\,\text{CaCl}_2$
  - $10\,\text{mM}\,\text{EGTA}$
  - $10\,\text{mM}\,\text{HEPES}$
  - $100\,\mu\text{M}$ spermine (polyamine included to assess inward rectification properties)
  - Adjusted to $\text{pH } 7.2$ with $\text{CsOH}$ .
- Recording pipettes were pulled from thick-walled borosilicate glass capillaries to tip resistances of $2\text{--}4\,\text{M}\Omega$ .
Kinetic Application Protocols:
- All recordings were maintained at a command holding potential of $-60\,\text{mV}$ , ambient temperature ( $20\text{--}23\,^\circ\text{C}$ , nominally $22\,^\circ\text{C}$ ), and $\text{pH } 7.4$ .
- Desensitization Protocol: Rapid application of $10\,\text{mM}$ L-glutamate for a prolonged duration of $500\,\text{ms}$ .
- Deactivation Protocol: Rapid application of $10\,\text{mM}$ L-glutamate for a brief pulse of $1\,\text{ms}$ .
- Compounds $\text{ND-1}$ through $\text{ND-8}$ were dissolved in dimethyl sulfoxide (DMSO) and diluted into the extracellular recording solution to a working concentration of $14\,\mu\text{M}$ , maintaining residual DMSO at $\le 0.1\%\,(\text{v/v})$ .
- The maximal concentration of $14\,\mu\text{M}$ was chosen based on concentration-inhibition curves that showed an onset of current suppression starting at $2\,\mu\text{M}$ and plateauing at $14\,\mu\text{M}$ , avoiding cellular detachment and cytotoxicity observed at concentrations above this threshold.

Quantitative Data Processing, Rectification Profiling, and Statistical Evaluation

Fitting of Weighted Kinetic Time Constants:
- Current decay trajectories for desensitization ( $500\,\text{ms}$ pulse) and deactivation ( $1\,\text{ms}$ pulse) were mathematically resolved by fitting a standard double-exponential function from $95\%$ of the peak inward amplitude back to the steady-state or baseline current:

$I(t) = a_f \cdot e^{-\frac{t}{\tau_f}} + a_s \cdot e^{-\frac{t}{\tau_s}}$

The weighted decay time constant ( $\tau_w$ ) for desensitization ( $\tau_{w\,\text{des}}$ ) and deactivation ( $\tau_{w\,\text{deact}}$ ) was calculated according to the formula:

$\tau_w = (\tau_f \times a_f) + (\tau_s \times a_s)$

*   Where  $\tau_f$  and  $\tau_s$  represent the fast and slow decay time constants, and  $a_f$  and  $a_s$  denote the relative fractional amplitudes of each respective component ( $a_f + a_s = 1$ ).

Within-Cell Paired Pharmacological Design and Fold Inhibition:
- Assays implemented a sequential within-cell paired protocol wherein each individual cell was first pulsed with agonist alone to determine control amplitude ( $A$ ), followed by pulse application of agonist in the continuous presence of $14\,\mu\text{M}$ of the respective ND compound to yield the inhibited amplitude ( $A_I$ ).
- Modulation magnitude was quantified as fold inhibition:

$\text{Fold Inhibition} = \frac{A}{A_I}$

*   Larger values reflect a greater fold reduction in channel opening.

Current–Voltage ( $\text{I–V}$ ) Rectification Analysis:
- To verify functional $\text{GluA2(Q)}$ assembly and physical coupling of $\text{TARP}\gamma8$ , a $1\,\text{s}$ linear voltage ramp from $-70\,\text{mV}$ to $+40\,\text{mV}$ was imposed during the sustained plateau phase of the glutamate-evoked current.
- Net receptor-mediated currents were isolated by digitally subtracting baseline ramp currents (recorded in the absence of glutamate) from ramp currents recorded during glutamate application.
- All current values were normalized to the inward peak current observed at $-70\,\text{mV}$ .
- Homomeric $\text{GluA1}$ and heteromeric $\text{GluA1/2(Q)}$ receptors exhibited inward rectification due to voltage-dependent block by intracellular spermine ( $100\,\mu\text{M}$ ).
- Co-expression of $\text{TARP}\gamma8$ attenuated this intracellular polyamine blockade, generating partially linearized $\text{I–V}$ curves that functionally confirmed the formation of receptor–auxiliary subunit complexes.
Statistical Evaluation:
- Data processing was performed using GraphPad Prism version 9.5.0.730.
- Paired two-tailed Student's t-tests evaluated direct comparisons between control and drug conditions within the same cell.
- Repeated-measures one-way analysis of variance (ANOVA) followed by Tukey's post-hoc multiple comparison tests were applied when assessing three conditions in the same recorded cell.
- Data are expressed as the mean $\pm$ standard deviation ( $\text{SD}$ ).
- Sample sizes were fixed at $n = 10$ individual cells per receptor construct, sourced across four independent biological transfections for homomeric $\text{GluA1}$ and six independent transfections for heteromeric $\text{GluA1/2}$ .
- Statistical significance was established at $p < 0.05$ , designated as:
  - $*\,p < 0.05$
  - $**\,p < 0.01$
  - $***\,p < 0.001$
  - $\text{ns}$ (not significant)

Inhibitory Effects of Dibenzobarrelene Heterocycles on Peak Whole-Cell Currents

Suppression of Glutamate-Induced Inward Currents:
- All eight dibenzobarrelene derivatives ($$\