Neurosteroids as positive and negative allosteric modulators of ligand-gated ion channels: P2X receptor perspective

Progress on functions of intracellular domain trimeric ligand-gated ion channels

Ligand-gated ion channels are a large category of essential ion channels, modulating their state by binding to specific ligands to allow ions to pass through the cell membrane. Purinergic ligand-gated ion channel receptors (P2XRs) and acid-sensitive ion channels (ASICs) are representative members of trimeric ligand-gated ion channel. Recent studies have shown that structural differences in the intracellular domain of P2XRs may determine the desensitization process. The lateral fenestrations of P2XRs potentially serve as a pathway for ion conductance and play a decisive role in ion selectivity. Phosphorylation of numerous amino acid residues in the P2XRs are involved in regulating the activity of ion channels. Additionally, the P2XRs interact with other ligand-gated ion channels including N-methyl-D-aspartate receptors, γ-aminobutyric acid receptors, 5-hydroxytryptamin receptors and nicotinic acetylcholine receptors, mediating physiological processes such as synaptic plasticity. Conformational changes in the intracellular domain of the ASICs expose binding sites of intracellular signal partners, facilitating metabolic signal transduction. Amino acids such as Val16, Ser17, Ile18, Gln19 and Ala20 in the ASICs participate in channel opening and membrane expression. ASICs can also bind to intracellular proteins, such as CIPP and p11, to regulate channel function. Many phosphorylation sites at the C-terminus and N-terminus of ASICs are involved in the regulation of receptors. Furthermore, ASICs are involved in various physiological and pathophysiological processes, which include pain, ischemic stroke, psychiatric disorders, and neurodegenerative disease. In this article, we review the roles of the intracellular domains of these trimeric ligand-gated ion channels in channel gating as well as their physiological and pathological functions, in order to provide new insights into the discovery of related drugs.

Is there a biochemical basis for purinergic P2X3 and P2X4 receptor antagonists to be considered as anti-seizure medications?

Patients with epilepsy require improved medications. Purinergic receptors were identified as late as 1976 and are slowly emerging as potential drug targets for the discovery of antiseizure medications. While compounds interacting with these receptors have been approved for use as medicines (e.g., gefapixant for cough) and continue to be explored for a number of diseases (e.g., pain, cancer), there have been no purinergic receptor antagonists that have been advanced for epilepsy. There are very few studies on the channel conducting receptors, P2X3 and P2X4, that suggest their possible role in seizure generation or control. However, the limited data available provides some compelling reasons to believe that they could be valuable antiseizure medication drug targets. The data implicating P2X3 and P2X4 receptors in epilepsy includes the role played by ATP in neuronal excitability and seizures, receptor localization, increased receptor expression in epileptic brain, the involvement of these receptors in seizure-associated inflammation, crosstalk between these purinergic receptors and neuronal processes involved in seizures (GABAergic and glutamatergic neurotransmission), and the significant attenuation of seizures and seizure-like activity with P2X receptor blockade. The discovery of new and selective antagonists for P2X3 and P2X4 receptors is ongoing, armed with new structural data to guide rational design. The availability of safe, brain-penetrant compounds will likely encourage the clinical exploration of epilepsy as a disease entity.

Editorial - Purinergic signalling: 50 years

The function of almost all cells of the human and animal body is synchronized by purinergic/pyrimidinergic extracellular signalling molecules. This network activity is especially efficient in the central and peripheral nervous systems, driven by secretion of the (co)transmitter ATP (including its enzymatic degradation products ADP, AMP, and adenosine), as well as ATP/UTP (including UDP) released from the cytoplasm by either Ca2+-dependent vesicular exocytosis or by non-exocytotic pathways via a family of diverse channels. It must be pointed out that neural cells (neurons, astrocytes, and oligodendrocytes) are equal sources of nucleotides/nucleosides, as non-neural cells (e.g. the endothelium of small blood vessels). A whole plethora of purinergic receptors responding to the endogenously released purine and pyrimidine nucleotides as well as to adenosine, are instrumental in providing the structural basis for cell stimulation. The present collection of papers summarizes current knowledge and recent findings in the medicinal chemistry, electrophysiology, neuropharmacology and neurobiology of purinergic transmission. Accruing evidence supports the key role of extracellular nucleotides and nucleosides in neuroinflammation, neurodegeneration, and in neuropsychiatric diseases, thus paving the way for pharmacological intervention thanks to the development of novel brain-permeant, drug-like, purinergic ligands. We are confident that these therapies will open a new avenue for the treatment of so far uncurable diseases of the central and peripheral nervous systems.

Epilepsy: Mitochondrial connections to the 'Sacred' disease

Over 65 million people suffer from recurrent, unprovoked seizures. The lack of validated biomarkers specific for myriad forms of epilepsy makes diagnosis challenging. Diagnosis and monitoring of childhood epilepsy add to the need for non-invasive biomarkers, especially when evaluating antiseizure medications. Although underlying mechanisms of epileptogenesis are not fully understood, evidence for mitochondrial involvement is substantial. Seizures affect 35%-60% of patients diagnosed with mitochondrial diseases. Mitochondrial dysfunction is pathophysiological in various epilepsies, including those of non-mitochondrial origin. Decreased ATP production caused by malfunctioning brain cell mitochondria leads to altered neuronal bioenergetics, metabolism and neurological complications, including seizures. Iron-dependent lipid peroxidation initiates ferroptosis, a cell death pathway that aligns with altered mitochondrial bioenergetics, metabolism and morphology found in neurodegenerative diseases (NDDs). Studies in mouse genetic models with seizure phenotypes where the function of an essential selenoprotein (GPX4) is targeted suggest roles for ferroptosis in epilepsy. GPX4 is pivotal in NDDs, where selenium protects interneurons from ferroptosis. Selenium is an essential central nervous system micronutrient and trace element. Low serum concentrations of selenium and other trace elements and minerals, including iron, are noted in diagnosing childhood epilepsy. Selenium supplements alleviate intractable seizures in children with reduced GPX activity. Copper and cuproptosis, like iron and ferroptosis, link to mitochondria and NDDs. Connecting these mechanistic pathways to selenoproteins provides new insights into treating seizures, pointing to using medicines including prodrugs of lipoic acid to treat epilepsy and to potential alternative therapeutic approaches including transcranial magnetic stimulation (transcranial), photobiomodulation and vagus nerve stimulation.