Ion channels are protein molecules that span the lipid bilayer of the cell membrane (Hille,1992) which allows the passage of ions across the membrane and establish the small voltage gradient. It plays a role in the excitability of neurons and muscles. Ion channel signaling starts at the plasma membrane in response to changes in membrane potential, mechanical stress, chemical compounds. Ion channels are transmembrane protein pores, hetero-oligomeric structures composed of noncovalently bonded subunits. The conformational change in the protein structure causes the ion channel to open which then allows the ions to pass through the aqueous pore . Ion channels have two main signaling functions: they can generate second messengers, or they can function as effectors by responding to such messengers. Its role in signal generation is based on the Ca2+ signaling pathway which has many Ca2+ entry and release channels which is involved in the generation of Ca2+ signals. Ion channel also acts as effector which is involved in intracellular signaling pathways .
Ion channels are divided into two main types: voltage-gated ion channels that are opened in response to change in membrane potential and ligand -gated ion channels which are opened by the binding of a ligand, such as neurotransmitter or hormones .
Voltage-gated ion channels (VGICs) are transmembrane proteins that play important roles in the electrical signaling of cells. The activity of VGICs is regulated by the membrane potential of a cell, and open channels allow the movement of ions along an electrochemical gradient across cellular membranes. Depending on the ions conducted, VGICs can be classified as voltage-gated sodium, potassium, calcium, or chloride channels. Many ion channels such as sodium and potassium ion channels are responsive to changes in membrane potential. These are known as voltage-gated ion channels. These channels typically exist in three states- closed, open and inactivated.
Voltage-gated sodium channel consists of four domains (I-IV) formed from a single polypeptide chain. Voltage-gated sodium channels play roles in action potential initiation and propagation of nerve, muscle, and neuroendocrine cell types. A single domain contains six α-helical transmembrane proteins. There are positively charged arginine residues in the fourth transmembrane segment of each domain which is important in voltage sensing and forms the pore domain through the sodium ions flow . The β subunits co-assembles with the sodium α-subunit. Voltage-gated potassium channels and hyperpolarization-activated cyclic-nucleotide-gated (HCN) cation channels have four similar or identical α-subunits, each with a single domain. Voltage-gated calcium (Cav) channels have a similar structure to sodium channels in their α-subunits. Calcium channels can have up to four associated auxiliary subunits: a disulfide-linked α2δ-complex, an intracellular β-subunit, and an occasional γ-subunit with four transmembrane segments. The voltage-gated potassium and sodium channels reside in the axons whereas Kv2, Kv4 and the cyclic nucleotide-gated channels are present in the dendrites. Cyclic-nucleotide-gated ion channels are intracellularly activated which includes the cAMP / cGMP-gated ion channels found in olfactory and retinal neurons .
Voltage-gated ion channels, extracellularly activated, open in response to neuronal transmembrane voltage changes. These types of ion channels are involved in neuronal action potentials, electrical impulses which stops after pore closure. Neuronal membrane resting potential is around -70 mV. Depolarization of neural tissue is induced in response to stimuli such as voltage or neurotransmitter which causes the potential to drop to 0 mV. Membrane depolarization activates voltage-gated sodium channels which causes and influx of sodium ions down an electrochemical gradient. This depolarization creates electrical excitation which triggers the nerve impulses, called action potentials. Transmembrane voltage changes cause an inward flux of calcium ions when action potential reach the presynaptic terminal. This causes the membrane fusion of exocytic vesicles. Neurotransmitter diffusion across the synaptic cleft is followed by activation of postsynaptic receptors. This leads to increased membrane depolarization and activation of voltage-gated sodium channel. Na+ channel inactivation terminates the resultant action potential which increases potassium efflux leading to membrane repolarization.
Ligand-gated ion channels are transmembrane protein complexes that allows the passage of selective ions across the plasma membrane . The channels get opened or gated by the binding of ligands such as neurotransmitters and neuromodulators which causes the conformational change which results in changes in membrane potential. The changes in membrane potential generates action potential, release hormones, muscle contraction or activate a lymphocyte. Ligand-gated ion channel consists of glutamate (AMPA, NMDA), γ-aminobutyric acid (GABA) and acetylcholine receptors. Many ion channels respond to the binding of specific stimulatory molecules, ligands.
Glutamate receptors are important neurotransmitters. The ionotropic receptors are tetramers of subunits which possess four hydrophobic regions within the central region of the sequence (TMI -IV). Glutamate receptors mediate fast excitatory neurotransmission in the brain. The action potential arrives at the nerve terminal which opens the calcium channel and Ca2+ ions cause synaptic vessel to be pulled against the cell membrane. Glutamate molecules undergo exocytosis into the synaptic cleft. Binding of transmitter to the AMPA-K receptor opens the ion channel which allows large influx of sodium and small efflux of potassium. This results in excitatory postsynaptic potential producing 20 mV of depolarization which permits the glutamate ligand to activate the NMDA receptor by removal of its Mg2+. Excitatory postsynaptic potential is induced by NMDA is enough to trigger action potential to have an extended repolarization period [6,7].
Ligand-gated ion channels such as NMDA glutamate receptor subtype, are the second different extracellularly gated channel. This ion channel changes the calcium and potassium channel conductance. The nicotinic acetylcholine-gated receptor (nAChR) and the inhibitory glycine receptor function via this mechanism. Glycine receptor selectivity is anionic, Cl– linked whereas the nicotinic acetylcholine-gated receptor selectivity is cationic, Na+or Ca2+ linked.
One of the most well studied ligand-gated channels is the acetylcholine receptor found on postsynaptic cells. Acetylcholine receptors are ion channels that respond to the binding of neurotransmitters called acetylcholine. When an action potential arrives at the terminal end of the pre-synaptic nerve cell, it stimulates the release of hundreds if vesicles that contain acetylcholine. Acetylcholine then diffuses across the synaptic cleft and binds onto target acetylcholine receptors. The acetylcholine receptor is a pentamer that consists of four types of subunits- the α, β, γ and δ chains. When acetylcholine is not bound to the channel, the channel exists in its closed state. In this state, the cavity of the channel contains large and hydrophobic amino acids. These repel the polar ions and prevent them from passing across. Acetylcholine can bind at the α-δ and α-γ boundary regions. Once it binds, it stimulates a conformational change that rotates the membrane-spanning α-helices. This hides the large, hydrophobic residues and express small, polar ones inside the cavity. This widens the internal cavity and allows the ions to make their way across. This is called the open state .
Recent research has suggested a mechanism how EGF activates the expression of HVA channels and the secretory behavior of pituitary cells by activating Ras/Raf/MEK/ERK/ELK-1 pathway. TRPV4 is a signaling effector for multiple PAR family members which supports the role of GPCR-TRP channel functional interactions in inflammatory associated changes to vascular function .
In conclusion, ion channel signaling study is a multidimensional research grasping different areas of science, pharmacology, biophysical methods, structural and cell biology, chemical and computational design approaches, which is giving much better understanding of the physiopathology of ion channel-related diseases and ways to cure it. It provides a new insight in ion channel signaling research and drug discovery.
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