G protein-gated ion channel

right|thumb|300px|Generalized diagram of G protein-gated ion channel: (A) Typically, the activated effector protein begins a signaling cascade which leads to the eventual opening of the ion channel. (B) The GTP-bound α-subunit in some cases can directly activate the ion channel. (C) In other cases, the activated βγ-complex of the G protein may interact with the ion channel.

G protein-gated ion channels are a family of transmembrane ion channels in neurons and atrial myocytes that are directly gated by G proteins.

Overview of mechanisms and function

Generally, G protein-gated ion channels are specific ion channels located in the plasma membrane of cells that are directly activated by a family of associated proteins. Ion channels allow for the selective movement of certain ions across the plasma membrane in cells. More specifically, in nerve cells, along with ion transporters, they are responsible for maintaining the electrochemical gradient across the cell.

G proteins are a family of intracellular proteins capable of mediating signal transduction pathways. Each G protein is a heterotrimer of three subunits: α-, β-, and γ- subunits. The α-subunit (G_α) typically binds the G protein to a transmembrane receptor protein known as a G protein-coupled receptor, or GPCR. This receptor protein has a large, extracellular binding domain which will bind its respective ligands (e.g. neurotransmitters and hormones). Once the ligand is bound to its receptor, a conformational change occurs. This conformational change in the G protein allows G_α to bind GTP. This leads to yet another conformational change in the G protein, resulting in the separation of the βγ-complex (G_βγ) from G_α. A class known as metabotropic glutamate receptors play a large role in indirect ion channel activation by G proteins. These pathways are activated by second messengers which initiate signal cascades involving various proteins which are important to the cell's response.

G protein-gated ion channels are associated with a specific type of G protein-coupled receptor. These ion channels are transmembrane ion channels with selectivity filters and a G protein binding site. The GPCRs associated with G protein-gated ion channels are not involved in signal transduction pathways. They only directly activate these ion channels using effector proteins or the G protein subunits themselves (see picture). Unlike most effectors, not all G protein-gated ion channels have their activity mediated by G_α of their corresponding G proteins. For instance, the opening of inwardly rectifying K⁺ (GIRK) channels is mediated by the binding of G_βγ.

G protein-gated ion channels are primarily found in CNS neurons and atrial myocytes, and affect the flow of potassium (K⁺), calcium (Ca²⁺), sodium (Na⁺), and chloride (Cl⁻) across the plasma membrane. These domains on the N-and C-terminal ends which interact with the G proteins contain certain residues which are critical for the proper activation of the GIRK channel. In GIRK4, the N-terminal residue is His-64 and the C-terminal residue is Leu-268; in GIRK1 they are His-57 and Leu-262, respectively. Mutations in these domains lead to the channel's desensitivity to the βγ-complex and therefore reduce the activation of the GIRK channel. Because of their similarity, the GIRK channel subunits can come together easily to form heteromultimers (a protein with two or more different polypeptide chains). GIRK1, GIRK2, and GIRK3 show abundant and overlapping distribution in the central nervous system (CNS) while GIRK1 and GIRK4 are found primarily in the heart. which contributes to the regulation of heart rate. These channels are almost entirely dependent on G protein activation, making them unique when compared to other G protein-gated channels. Activation of the IKACh channels begins with release of acetylcholine (ACh) from the vagus nerve IKACh is composed of two homologous GIRK channel subunits: GIRK1 and GIRK4. The G_βγ-complex binds directly and specifically to the IKACh channel through interactions with both the GIRK1 and GIRK4 subunits. Once the ion channel is activated, K⁺ ions flow out of the cell and cause it to hyperpolarize. In its hyperpolarized state, the neuron cannot fire action potentials as quickly, which slows the heartbeat.

• GIRKs found in the brain

The G protein inward rectifying K⁺ channel found in the CNS is a heterotetramer composed of GIRK1 and GIRK2 subunits The 5-HT1A receptor, a serotonin receptor and type of GPCR, has been shown to be coupled directly with the α-subunit of a G protein, while the βγ-complex activates GIRK without use of a second messenger. The subsequent activation of the GIRK channel mediates hyperpolarization of orexin neurons, which regulate the release of many other neurotransmitters including noradrenaline and acetylcholine. Such bypassing of the second-messenger pathways is observed in mammalian cardiac myocytes and associated sarcolemmal vesicles in which Ca²⁺ channels are able to survive and function in the absence of cAMP, ATP or protein kinase C when in the presence of the activated α-subunit of the G protein. For example, G_α, which is stimulatory to adenylyl cyclase, acts on the Ca²⁺ channel directly as an effector. This short circuit is membrane-delimiting, allowing direct gating of calcium channels by G proteins to produce effects more quickly than the cAMP cascade could.

Function

Several high-threshold, slowly inactivating calcium channels in neurons are regulated by G proteins. This inhibition of voltage-gated Calcium channels by G protein-coupled receptors has been demonstrated in the dorsal root ganglion of a chick among other cell lines. ASIC1a currents have also been shown to be inhibited in the presence of oxidizing agents and potentiated in the presence of reducing agents. A decrease and increase in acid-induced intracellular Ca²⁺ accumulation were found, respectively.

Sodium channels

Patch clamp measurements suggest a direct role for G_α in the inhibition of fast Na⁺ current within cardiac cells. Other studies have found evidence for a second-messenger pathway which may indirectly control these channels. Whether G proteins indirectly or directly activate Na⁺ ion channels not been defined with complete certainty.

Chloride channels

Chloride channel activity in epithelial and cardiac cells has been found to be G protein-dependent. However, the cardiac channel that has been shown to be directly gated by the G_α subunit has not yet been identified. As with Na⁺ channel inhibition, second-messenger pathways cannot be discounted in Cl⁻ channel activation.

Studies done on specific Cl⁻ channels show differing roles of G protein activation. It has been shown that G proteins directly activate one type of Cl⁻ channel in skeletal muscle.

Epilepsy, chronic pain, and addictive drugs such as cocaine, opioids, cannabinoids, and ethanol all affect neuronal excitability and heart rate. GIRK channels have been shown to be involved in seizure susceptibility, cocaine addiction, and increased tolerance for pain by opioids, cannabinoids, and ethanol. This connection suggests that GIRK channel modulators may be useful therapeutic agents in the treatment of these conditions. GIRK channel inhibitors may serve to treat addictions to cocaine, opioids, cannabinoids, and ethanol while GIRK channel activators may serve to treat withdrawal symptoms. When ethanol acts as an agonist, GIRK channels in the brain experience prolonged opening. This causes decreased neuronal activity, the result of which manifests as the symptoms of alcohol intoxication. The discovery of the hydrophobic pocket capable of binding ethanol is significant in the field of clinical pharmacology. Agents that can act as agonists to this binding site can be potentially useful in the creation of drugs for the treatment of neurological disorders such as epilepsy in which neuronal firing exceeds normal levels. Treatment of breast cancer tissue with alcohol has been shown to trigger increased growth of the cancer cells. The mechanism of this activity is still a subject of research. People with Down Syndrome have three copies of chromosome 21, resulting in an overexpression of the GIRK2 subunit. Studies have found that recombinant mice overexpressing GIRK2 subunits show altered responses to drugs that activate G protein-gated K⁺ channels. These altered responses were limited to the sino-atrial node and atria, both areas which contain many G protein-gated K⁺ channels. The I_K,ACh channel, when activated by G proteins, allows for the flow of K⁺ across the plasma membrane and out of the cell. This current hyperpolarizes the cell, thus terminating the action potential. It has been shown that in chronic atrial fibrillation there an increase in this inwardly rectifying current because of constantly activated I_K,ACh channels. These specific channels have been the target of recent studies dealing with genetic variance and sensitivity to opioid analgesics due to their role in opioid-induced analgesia. Several studies have shown that when opioids are prescribed to treat chronic pain, GIRK channels are activated by certain GPCRs, namely opioid receptors, which leads to the inhibition of nociceptive transmission, thus functioning in pain relief. Furthermore, studies have shown that G proteins, specifically the Gi alpha subunit, directly activate GIRKs which were found to participate in propagation of morphine-induced analgesia in inflamed spines of mice. Research pertaining to chronic pain management continues to be performed in this field.

See also

• G protein
• G protein-coupled receptor
• Metabotropic receptor

References