Cyclic nucleotide-gated ion channel
Encyclopedia
Cyclic nucleotide-gated ion channels are ion channel
s that function in response to the binding of cyclic nucleotide
s. CNG channels are nonselective cation channels that are found in the membranes of various types of cells.
. In 1985, it was discovered that cGMP
was able to directly activate the light-dependent response of rod
channels by studying light-adapted retina of frogs. Soon, CNG channels were found in cone photoreceptors, chemo sensitive cilia of olfactory sensory neurons, and the pineal gland
. Cloning and functional expression of CNG channels followed, after the identification of amino acids from purified proteins. Molecular cloning
allowed for the discovery of similar channels in many other tissues.
or hyperpolarization
. CGN channels can be activated by cAMP
or cGMP
, or sometimes both. Some channels are more selective while other channels are not. The main role of GNC is sensory transduction
in various tissues. Many studies have shown CNG channels in rod and cone receptors. CNG channels also have been found in brain, heart, kidney, and gonads.
line the channel. These also form a 6 helix bundle that signifies the entrance. In order to open the pore, a conformation change must occur in the inner 6 helix bundle.
is an intracellular domain located in the C-terminal and has a similar sequence to other cyclic nucleotide-binding proteins. The domain is believed to have a ß-pleated sheet
and two α-helices. The ß-pleated sheet is antiparallel and 8 stranded. The α helices are named B and C helices. A ligand
initially binds to the ß-pleated sheet by following an opening allosteric transition involving the movement to an α-helix toward the ß-pleated sheet. The α-helix is flexible in closed channels. When an α-helix of a CNGA1 subunit is in close proximity to another α-helix, they form intersubunit disulfide bonds. This occurs mainly in closed channels, inhibiting movement of the α-helix towards the ß-pleated sheet. When a ligand binds to the ß-pleated sheet, the now bind ligand stabilizes the movement of the α-helix toward the ß-pleated sheet of each subunit, pulling the α-helices away from each other.
, zinc
, copper
, and magnesium
. The C-linker region is involved in the coupling of ligand binding to the opening of the pore. The C linker region forms disulfide bonds with N-terminal regions. Disulfide bonds alter the channel function therefore they most likely lie close to the tertiary structure. Disulfide bonds decrease the free energy of the open state compared to the closed state.
The specific cysteine residue C481 on the C-linker region is located only a few amino acids away from the binding domain. In the closed state C481 is nonreactive; C481 must undergo a conformational change so that it is accessible for the opening of the channel. Disulfide bonds form between neighboring subunits and C481. Simultaneously there is a C35 cysteine
residue at the N-terminal of the C-linker region that can reach two C481 residues, making a favorable disulfide bond compared to a C481-C481 bond.
formation is state-dependent, implying that the conformational change
in the helix bundle is affiliated with channel gating.
When the cyclic nucleotide-gated ion channels are closed, the cytoplasmic ends of the S6 helices are in close proximity to each other. Small cations are able to move through an opening, which implies that the gate is beyond the helix bundle
and that S6 helices are in conjunction with conformational changes in the selectivity filter.
. When the retina of the eye detects light, a reaction known as a phototransduction cascade occurs. It is a signal transduction
pathway that leads to the activation of the enzyme phosphodiesterase
, which hydrolyzes cGMP into 5’-GMP, decreasing the concentration of cGMP. In the absence of cGMP, the CNGCs in the photoreceptors close preventing the flow of the aforementioned dark current. This in turn causes a hyperpolarization
of the outer segment of the photoreceptor , preventing the propagation of an action potential
and the release of glutamate.
. This causes a downstream reaction activating the enzyme adenylyl cyclase. This enzyme is responsible for an increase of cAMP concentration within the OSN. cAMP binds to the CNGCs in the OSN membrane opening them, making the cell highly permeable to Ca2+. Calcium ions flow into the cell causing a depolarization
. As in all other cell types, CNGCs in OSNs also allow Na+ to flow into the cell. Additionally, the increased Ca2+ concentration inside the cell activates Ca2+-dependent Cl- channels, which causes intracellular Cl- ions to also flow out of the cell augmenting the depolarization event. In addition to cAMP gated ion channels, a small subset of OSNs also has cGMP-selective CNGCs.
(cAMP) and cyclic guanosine monophosphate
(cGMP) mediate several cellular responses for instance swimming behavior, acrosomal exocytosis, and chemoattraction. In the sea urchin
species, Strongylocentrotus purpuratus
, speract, a short peptide
, was studied. Speract activates a receptor-type GC and stimulates a rise of intracellular
cGMP concentrations. Speract also increases the concentration of calcium
. Although there has yet to be any establishment of a direct causal relationship, the previously mentioned observations suggest that cGMAP activates calcium conductance
. CNG channels are prime candidates for the calcium-entry pathway, due to their high calcium permeability. CNG channels have yet to be detected by homology screening.
In mammals, testicular CNG channel subunits expressed are A3, B1, and B3. Heterologous
expression of the A3 subunit was cloned from testis and produced channels that are cGMP sensitive and selective. It is possible that these channels are involved in a cGMP-stimulates calcium influx into the sperm
. However a more extensive characterization of the channel has not been accomplished due to low success rate of detecting channel activity. Since A3 subunit knockout mice are fertile
, CNG channels could be involved in some form of motility control and even in chemotactic swimming behavior or in the acrosomal exocytosis. However, a receptor-type GC in mammalian sperm has yet to be identified. Mouse sperm express other channels such as CatSper1
. Male sterility can be accomplished by disrupting the CatSper1 gene; additionally, the cAMP-induced calcium influx is abolished in mutant
mice. CNG channels and CatSper are unrealted because CatSper lacks a cAMP/cGMP-binding site but does need additional subunits to become functional. It is possible that CNG and CatSper subunits assemble to form calcium-permeable and cyclic nucleotide-sensitive ion channels.
and fluid balance
. CNG channel activity is controlled by the interaction between cGMP-dependent protein kinase
and G1 protein. In the cells from an inner medullary collecting duct, the channel exhibits cation selectivity unit conductance, calcium permeability, and pharmacology
very similar to cyclic nucleotide-gated ion channels.
(GnRH). The three subunits make up the CNG channels on chemosensitive cilia of OSNs. In high extracellular calcium, the unit conductance of CNG channels in rods and OSNs are significantly smaller than those measured in the neuronal line. It seems doubtful that CNG channels would create large unit conductance.
and Drosophila melanogaster
. A subunit of a CNG channel CNGA1, previously called the rod α subunit, was expressed in rod photoreceptors and produced functional channels that were gated by cGMP (cyclic guanosine monophosphate
) when expressed externally in either Xenopus oocytes or in a human embryonic kindney cell line (HEK293). In humans, mutated CNGA1 genes result in an autosomal recessive form of retinitis pigmentosa
, a degenerative form of blindness. CNGB1, previously called the rod β subunit, is a second subunit of the rod channel. Unlike CNGA1, CNGB1 subunits expressed alone do not produce functional CNG channels, but coexpression of CNGA1 and CNGB1 subunits produces heteromeric channels with modulation, permeation, pharmacology, and cyclic-nucleotide specificity comparable to that of native channels. CNG channels form tetramers, and recent studies indicate that native rod channels consist of three CNGA1 subunits and one CNGB1 subunit. CNGA3 subunits, previously called the cone α subunits, form functional channels when its expression occurs exogenously. On the other hand, CNGB3, previously called the cone β subunit, does not. Mutations in human CNGA3 and CNGB3 are involved in complete achromatopsia
, which is a rare, autosomal recessive inherited and congenital disorder characterized by the complete failure in color distinction. CNGA2, previously called the olfactory α subunit and CNGA4, previously called the olfactory β subunit, are involved in transduction of odorant signals in olfactory neurons for which the subunit stoichiometry and arrangement are unknown.
In invertebrates, a CNG channel subunit called CNG-P1 has been cloned from D. melanogaster and is expressed in antennae and the visual system, an indication that CNG channels may be linked to the transduction of light in invertebrates. A second putative CNG-like subunit called CNGL, cloned from D. melanogaster, is found to be expressed in the brain. Two CNG channel subunits, Tax-2 and Tax-4, have been cloned in C. elegans and are responsible for chemosensation, thermosensation, and normal axon
outgrowth of some sensory neurons in C. elegans.
concentrations, the ligand sensitivity and selectivity of both native and exogenously expressed CNG channels have been studied. All native CNG channels react to both cAMP and cGMP, but smaller concentrations of cGMP than those of cAMP are needed to activate and open the channels. CNG channels are sharply selective between cGMP and cAMP in rods and cones, whereas in OSNs, the channels respond equally well to both ligands. CNG channels found in OSNs are much more sensitive to both cGMP and cAMP than photoreceptor CNG channels. Studies of dose response relations
showed that channel activation is greatly dependent on cGMP concentration; several cGMP molecules bind to the channel in a cooperative manner. Since each subunit contains a single cNMP-binding site, and homo- and heteromeric channels most likely form a tetrameric complex, a maxiumum of four ligand molecules can bind to the channel.
Selectivity can be achieved by differential control of the affinity for binding of the ligand, efficacy of gating, or a combination of both. Binding affinity means how tightly cyclic nucleotides bind to the channel. Efficacy refers to the ability of ligand to activate and open the channel once it is bound. Although these processes are useful in understanding selectivity, they are inextricably coupled to each other that it is very difficult to experimentally separate one from another.
CNG channels play a large role in plant immunity and response to pathogens. They have also been implicated in apoptosis
in plants. Cyclic nucleotide-gated ion channels are also thought to be involved in pollen development in plants, however its exact role in this mechanism is still not known.
Unlike animal CNG channels, plant CNG channels have not been extensively analyzed biochemically with respect to their structure.
. In other physiological areas, the role of CNG channels is less defined. With technological growth, there now exists more possibilities for understanding these mechanisms.
Because nitrous oxide
(NO) is involved in the stimulation of the synthesis of cGMP, further research is being conducted to understand the physiological interaction of NO with CNG channels, particularly in the covalent modification of CNG channels in OSNs.
Ion channel
Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of cells by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells...
s that function in response to the binding of cyclic nucleotide
Cyclic nucleotide
A cyclic nucleotide is any nucleotide in which the phosphate group is bonded to two of the sugar's hydroxyl groups, forming a cyclical or ring structure.These include:* cyclic AMP* cyclic GMP* cyclic ADP-ribose...
s. CNG channels are nonselective cation channels that are found in the membranes of various types of cells.
Discovery
The discovery of CGN channels was related to the discovery of intracellular messengers responsible for the mediation of responses in retinal photoreceptors. Before their discovery, it was thought that cyclic nucleotides played a role in phosphorylationPhosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
. In 1985, it was discovered that cGMP
CGMP
CGMP is an initialism. It can refer to:*cyclic guanosine monophosphate *current good manufacturing practice *CGMP, the Cisco version of the Internet Group Management Protocol snooping...
was able to directly activate the light-dependent response of rod
Rod cell
Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...
channels by studying light-adapted retina of frogs. Soon, CNG channels were found in cone photoreceptors, chemo sensitive cilia of olfactory sensory neurons, and the pineal gland
Pineal gland
The pineal gland is a small endocrine gland in the vertebrate brain. It produces the serotonin derivative melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions...
. Cloning and functional expression of CNG channels followed, after the identification of amino acids from purified proteins. Molecular cloning
Molecular cloning
Molecular cloning refers to a set of experimental methods in molecular biology that are used to assemble recombinant DNA molecules and to direct their replication within host organisms...
allowed for the discovery of similar channels in many other tissues.
Function
CNG channels are directly activated by cyclic nucleotides. Around 4 cyclic nucleotides are needed to activate a channel. CNG channels are nonselective and allow many alkali ions to flow into or out of the cell expressing the channels on it membrane. This results in either depolarizationDepolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
or hyperpolarization
Hyperpolarization
Hyperpolarization has several meanings:* Hyperpolarization occurs when the strength of the electric field across the width of a cell membrane increases...
. CGN channels can be activated by cAMP
CAMP
CAMP may stand for:* Cyclic adenosine monophosphate * Cathelicidin, or Cathelicidin antimicrobial peptide* Campaign Against Marijuana Planting* Central Atlantic Magmatic Province* CAMP , an Italian manufacturer of climbing equipment...
or cGMP
CGMP
CGMP is an initialism. It can refer to:*cyclic guanosine monophosphate *current good manufacturing practice *CGMP, the Cisco version of the Internet Group Management Protocol snooping...
, or sometimes both. Some channels are more selective while other channels are not. The main role of GNC is sensory transduction
Transduction
Transduction is a mechanism whereby genetic material may be transferred from the genes of a bacterium to another bacterium. This may include the actual covalent-bonding of new genetic markers...
in various tissues. Many studies have shown CNG channels in rod and cone receptors. CNG channels also have been found in brain, heart, kidney, and gonads.
Structure
A NCG channel consists of four subunits around a central pore. Each subunit consists 6 transmembrane segments (S1-S6), a P-loop, intracellular amino terminal region, and carboxy terminal region. The P-loop and S6 segments around the pore, which plays a role in ion conduction. There is a cyclic nucleotide binding domain (CNBD) and connection region to the S6 segment in the carboxy terminal. There is a post-CNDB region in the amino terminal.Alpha subunits
Cyclic nucleotide gated channel alpha subunits include- Cyclic nucleotide-gated channel alpha 1Cyclic nucleotide-gated channel alpha 1Cyclic nucleotide-gated channel alpha 1, also known as CNGA1, is a human gene encoding an ion channel protein.-External links:*...
- Cyclic nucleotide-gated channel alpha 2Cyclic nucleotide-gated channel alpha 2Cyclic nucleotide gated channel alpha 2, also known as CNGA2, is a human gene encoding an ion channel protein....
- Cyclic nucleotide-gated channel alpha 3Cyclic nucleotide-gated channel alpha 3Cyclic nucleotide-gated cation channel alpha-3 is a protein that in humans is encoded by the CNGA3 gene.-Clinical relevance:Variants in this gene have been shown to cause achromatopsia and color blindness.- External Links :* *...
- Cyclic nucleotide-gated channel alpha 4Cyclic nucleotide-gated channel alpha 4Cyclic nucleotide-gated cation channel alpha-4 is a protein that in humans is encoded by the CNGA4 gene....
Beta subunits
Cyclic nucleotide gated channel beta-subunits include:- Cyclic nucleotide-gated channel beta 1
- Cyclic nucleotide-gated channel beta 3
Pore
The structure of the pore is similar to other ion channels that contain P-loops. The P-loop enters the membrane of the pore from the extracelluar side and exists to the extracelluar side. The P loop enters as an alpha helix and exists as an uncoiled strand. Helices that cover the inner membraneInner membrane
The inner membrane is the biological membrane of an organelle or Gram-negative bacteria that is within an outer membrane....
line the channel. These also form a 6 helix bundle that signifies the entrance. In order to open the pore, a conformation change must occur in the inner 6 helix bundle.
Cyclic nucleotide binding domain
A cyclic nucleotide binding domainBinding domain
A binding domain is a protein domain which binds to a specific atom or molecule, such as calcium or DNA. Upon binding, proteins may undergo a conformational change. Binding domains are essential for the function of many proteins....
is an intracellular domain located in the C-terminal and has a similar sequence to other cyclic nucleotide-binding proteins. The domain is believed to have a ß-pleated sheet
Beta sheet
The β sheet is the second form of regular secondary structure in proteins, only somewhat less common than the alpha helix. Beta sheets consist of beta strands connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet...
and two α-helices. The ß-pleated sheet is antiparallel and 8 stranded. The α helices are named B and C helices. A ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
initially binds to the ß-pleated sheet by following an opening allosteric transition involving the movement to an α-helix toward the ß-pleated sheet. The α-helix is flexible in closed channels. When an α-helix of a CNGA1 subunit is in close proximity to another α-helix, they form intersubunit disulfide bonds. This occurs mainly in closed channels, inhibiting movement of the α-helix towards the ß-pleated sheet. When a ligand binds to the ß-pleated sheet, the now bind ligand stabilizes the movement of the α-helix toward the ß-pleated sheet of each subunit, pulling the α-helices away from each other.
C-Linker
The C-Linker is a region that connects the CNBD to the S6 segment. There are many residues that play a role in modulation of CNG channels. This process uses metals such as nickelNickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
, copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, and magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
. The C-linker region is involved in the coupling of ligand binding to the opening of the pore. The C linker region forms disulfide bonds with N-terminal regions. Disulfide bonds alter the channel function therefore they most likely lie close to the tertiary structure. Disulfide bonds decrease the free energy of the open state compared to the closed state.
The specific cysteine residue C481 on the C-linker region is located only a few amino acids away from the binding domain. In the closed state C481 is nonreactive; C481 must undergo a conformational change so that it is accessible for the opening of the channel. Disulfide bonds form between neighboring subunits and C481. Simultaneously there is a C35 cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
residue at the N-terminal of the C-linker region that can reach two C481 residues, making a favorable disulfide bond compared to a C481-C481 bond.
S6 region
Spontaneous disulfide bondDisulfide bond
In chemistry, a disulfide bond is a covalent bond, usually derived by the coupling of two thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry...
formation is state-dependent, implying that the conformational change
Conformational change
A macromolecule is usually flexible and dynamic. It can change its shape in response to changes in its environment or other factors; each possible shape is called a conformation, and a transition between them is called a conformational change...
in the helix bundle is affiliated with channel gating.
When the cyclic nucleotide-gated ion channels are closed, the cytoplasmic ends of the S6 helices are in close proximity to each other. Small cations are able to move through an opening, which implies that the gate is beyond the helix bundle
Helix bundle
A helix bundle is a small protein fold composed of several alpha helices that are usually nearly parallel or antiparallel to each other.-Three-helix bundles:Three-helix bundles are among the smallest and fastest known cooperatively folding structural domains...
and that S6 helices are in conjunction with conformational changes in the selectivity filter.
P region
The P region forms a loop connecting the S5 and S6 regions which extend to the central axis of the channel. Ionic properties are determined by the residues in the loop between S5 and S6 transmembrane segments. The P region dictates the ion selectivity of the cyclic-nucleotide gated ion channel, which also determine the pore diameter of CNG channels. The P region functions as a channel gate since it prevents ion permeation in the closed state. The pore may be hindered by small conformational changes in this region. The P region acts as a ion selectivity filter that changes structure in the open conformation. The four identical subunits sacrifice a single P region loop from selectivity filter in the open state.Photoreceptors
In the absence of light, the cyclic nucleotide guanosine 3’-5’-cyclic monophosphate (cGMP) binds to CNGCs in photoreceptors. This binding causes the channels to open, which allows sodium (Na+) and calcium (Ca2+) ions to flow into the cell causing the outer segment of the photoreceptor to depolarize. This depolarizing flow of ions is known as the dark currentDark current
Dark current is the constant response exhibited by a receptor of radiation during periods when it is not actively being exposed to light. It may refer to:...
. When the retina of the eye detects light, a reaction known as a phototransduction cascade occurs. It is a signal transduction
Signal transduction
Signal transduction occurs when an extracellular signaling molecule activates a cell surface receptor. In turn, this receptor alters intracellular molecules creating a response...
pathway that leads to the activation of the enzyme phosphodiesterase
Phosphodiesterase
A phosphodiesterase is any enzyme that breaks a phosphodiester bond. Usually, people speaking of phosphodiesterase are referring to cyclic nucleotide phosphodiesterases, which have great clinical significance and are described below...
, which hydrolyzes cGMP into 5’-GMP, decreasing the concentration of cGMP. In the absence of cGMP, the CNGCs in the photoreceptors close preventing the flow of the aforementioned dark current. This in turn causes a hyperpolarization
Hyperpolarization (biology)
Hyperpolarization is a change in a cell's membrane potential that makes it more negative. It is the opposite of a depolarization.Hyperpolarization is often caused by efflux of K+ through K+ channels, or influx of Cl– through Cl– channels. On the other hand, influx of cations, e.g...
of the outer segment of the photoreceptor , preventing the propagation of an action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
and the release of glutamate.
Olfactory sensory neurons
Almost all responses to odorants in olfactory sensory neurons (OSNs) are facilitated by CNGCs. When an odorant binds to its specific receptor in the chemosenstive cilia membrane, it activated a G proteinG protein
G proteins are a family of proteins involved in transmitting chemical signals outside the cell, and causing changes inside the cell. They communicate signals from many hormones, neurotransmitters, and other signaling factors. G protein-coupled receptors are transmembrane receptors...
. This causes a downstream reaction activating the enzyme adenylyl cyclase. This enzyme is responsible for an increase of cAMP concentration within the OSN. cAMP binds to the CNGCs in the OSN membrane opening them, making the cell highly permeable to Ca2+. Calcium ions flow into the cell causing a depolarization
Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
. As in all other cell types, CNGCs in OSNs also allow Na+ to flow into the cell. Additionally, the increased Ca2+ concentration inside the cell activates Ca2+-dependent Cl- channels, which causes intracellular Cl- ions to also flow out of the cell augmenting the depolarization event. In addition to cAMP gated ion channels, a small subset of OSNs also has cGMP-selective CNGCs.
In spermatozoa
Cyclic adenosine monophosphateCyclic adenosine monophosphate
Cyclic adenosine monophosphate is a second messenger important in many biological processes...
(cAMP) and cyclic guanosine monophosphate
Cyclic guanosine monophosphate
Cyclic guanosine monophosphate is a cyclic nucleotide derived from guanosine triphosphate . cGMP acts as a second messenger much like cyclic AMP...
(cGMP) mediate several cellular responses for instance swimming behavior, acrosomal exocytosis, and chemoattraction. In the sea urchin
Sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...
species, Strongylocentrotus purpuratus
Strongylocentrotus purpuratus
The purple sea urchin, Strongylocentrotus purpuratus, lives along the eastern edge of the Pacific Ocean extending from Ensenada, Mexico to British Columbia, Canada. This sea urchin species is deep purple in color and lives in lower intertidal and nearshore subtidal communities...
, speract, a short peptide
Peptide
Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...
, was studied. Speract activates a receptor-type GC and stimulates a rise of intracellular
Intracellular
Not to be confused with intercellular, meaning "between cells".In cell biology, molecular biology and related fields, the word intracellular means "inside the cell".It is used in contrast to extracellular...
cGMP concentrations. Speract also increases the concentration of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
. Although there has yet to be any establishment of a direct causal relationship, the previously mentioned observations suggest that cGMAP activates calcium conductance
Conductance
Conductance may refer to:* Electrical conductance, the ability for electricity to flow a certain path* Fluid conductance, the ability for fluid to transmit through materials* Thermal conductivity, the ability for temperatures to transmit through materials...
. CNG channels are prime candidates for the calcium-entry pathway, due to their high calcium permeability. CNG channels have yet to be detected by homology screening.
In mammals, testicular CNG channel subunits expressed are A3, B1, and B3. Heterologous
Heterologous
In medicine a heterologous transplant means 'between species' or 'from one species to another'.In cell biology and protein biochemistry, heterologous expression means that a protein is experimentally put into a cell that does not normally make that protein...
expression of the A3 subunit was cloned from testis and produced channels that are cGMP sensitive and selective. It is possible that these channels are involved in a cGMP-stimulates calcium influx into the sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
. However a more extensive characterization of the channel has not been accomplished due to low success rate of detecting channel activity. Since A3 subunit knockout mice are fertile
Fertile
The term fertile describes a condition whereby organisms are able to produce physically healthy offspring.Fertile may also refer to:...
, CNG channels could be involved in some form of motility control and even in chemotactic swimming behavior or in the acrosomal exocytosis. However, a receptor-type GC in mammalian sperm has yet to be identified. Mouse sperm express other channels such as CatSper1
CatSper1
CatSper1, is a protein which in humans is encoded by the CATSPER1 gene. CatSper1 is a member of the cation channels of sperm family of protein. The four proteins in this family together form a Ca2+-permeant ion channel specific essential for the correct function of sperm cells.- Function :Calcium...
. Male sterility can be accomplished by disrupting the CatSper1 gene; additionally, the cAMP-induced calcium influx is abolished in mutant
Mutant
In biology and especially genetics, a mutant is an individual, organism, or new genetic character, arising or resulting from an instance of mutation, which is a base-pair sequence change within the DNA of a gene or chromosome of an organism resulting in the creation of a new character or trait not...
mice. CNG channels and CatSper are unrealted because CatSper lacks a cAMP/cGMP-binding site but does need additional subunits to become functional. It is possible that CNG and CatSper subunits assemble to form calcium-permeable and cyclic nucleotide-sensitive ion channels.
In kidney
cGMP-sensitive channels have been analyzed in renal inner medullary collecting duct cells, which influence the body’s electrolyteElectrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
and fluid balance
Fluid balance
Fluid balance is the concept of human homeostasis that the amount of fluid lost from the body is equal to the amount of fluid taken in. Euvolemia is the state of normal body fluid volume. Water is necessary for all life on Earth...
. CNG channel activity is controlled by the interaction between cGMP-dependent protein kinase
CGMP-dependent protein kinase
cGMP-dependent protein kinase or Protein Kinase G is a serine/threonine-specific protein kinase that is activated by cGMP. It phosphorylates a number of biologically important targets and is implicated in the regulation of smooth muscle relaxation, platelet function, sperm metabolism, cell...
and G1 protein. In the cells from an inner medullary collecting duct, the channel exhibits cation selectivity unit conductance, calcium permeability, and pharmacology
Pharmacology
Pharmacology is the branch of medicine and biology concerned with the study of drug action. More specifically, it is the study of the interactions that occur between a living organism and chemicals that affect normal or abnormal biochemical function...
very similar to cyclic nucleotide-gated ion channels.
In gonads
There has been identification of cyclic nucleotide-gated ion channel subunits A2, A4, and B1 in a neuronal cell line that secrets gonadotropin-releasing hormoneGonadotropin-releasing hormone
Gonadotropin-releasing hormone , also known as Luteinizing-hormone-releasing hormone and luliberin, is a tropic peptide hormone responsible for the release of follicle-stimulating hormone and luteinizing hormone from the anterior pituitary. GnRH is synthesized and released from neurons within...
(GnRH). The three subunits make up the CNG channels on chemosensitive cilia of OSNs. In high extracellular calcium, the unit conductance of CNG channels in rods and OSNs are significantly smaller than those measured in the neuronal line. It seems doubtful that CNG channels would create large unit conductance.
CNG channel family
In vertebrates, the CNG channel gene family consists of six members. These genes are divided based on sequence similarity into two subtypes CNGA and CNGB . Additional genes that code for CNG channels have been cloned from Caenorhabditis elegansCaenorhabditis elegans
Caenorhabditis elegans is a free-living, transparent nematode , about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner and it has since been used extensively as a model...
and Drosophila melanogaster
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
. A subunit of a CNG channel CNGA1, previously called the rod α subunit, was expressed in rod photoreceptors and produced functional channels that were gated by cGMP (cyclic guanosine monophosphate
Cyclic guanosine monophosphate
Cyclic guanosine monophosphate is a cyclic nucleotide derived from guanosine triphosphate . cGMP acts as a second messenger much like cyclic AMP...
) when expressed externally in either Xenopus oocytes or in a human embryonic kindney cell line (HEK293). In humans, mutated CNGA1 genes result in an autosomal recessive form of retinitis pigmentosa
Retinitis pigmentosa
Retinitis pigmentosa is a group of genetic eye conditions that leads to incurable blindness. In the progression of symptoms for RP, night blindness generally precedes tunnel vision by years or even decades. Many people with RP do not become legally blind until their 40s or 50s and retain some...
, a degenerative form of blindness. CNGB1, previously called the rod β subunit, is a second subunit of the rod channel. Unlike CNGA1, CNGB1 subunits expressed alone do not produce functional CNG channels, but coexpression of CNGA1 and CNGB1 subunits produces heteromeric channels with modulation, permeation, pharmacology, and cyclic-nucleotide specificity comparable to that of native channels. CNG channels form tetramers, and recent studies indicate that native rod channels consist of three CNGA1 subunits and one CNGB1 subunit. CNGA3 subunits, previously called the cone α subunits, form functional channels when its expression occurs exogenously. On the other hand, CNGB3, previously called the cone β subunit, does not. Mutations in human CNGA3 and CNGB3 are involved in complete achromatopsia
Achromatopsia
Achromatopsia , is a medical syndrome that exhibits symptoms relating to at least five separate individual disorders. Although the term may refer to acquired disorders such as color agnosia and cerebral achromatopsia, it typically refers to an autosomal recessive congenital color vision disorder,...
, which is a rare, autosomal recessive inherited and congenital disorder characterized by the complete failure in color distinction. CNGA2, previously called the olfactory α subunit and CNGA4, previously called the olfactory β subunit, are involved in transduction of odorant signals in olfactory neurons for which the subunit stoichiometry and arrangement are unknown.
In invertebrates, a CNG channel subunit called CNG-P1 has been cloned from D. melanogaster and is expressed in antennae and the visual system, an indication that CNG channels may be linked to the transduction of light in invertebrates. A second putative CNG-like subunit called CNGL, cloned from D. melanogaster, is found to be expressed in the brain. Two CNG channel subunits, Tax-2 and Tax-4, have been cloned in C. elegans and are responsible for chemosensation, thermosensation, and normal axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
outgrowth of some sensory neurons in C. elegans.
Ligand selectivity
By measuring the currents activated in excised inside-out membrane patches upon superfusion with varying ligandLigand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
concentrations, the ligand sensitivity and selectivity of both native and exogenously expressed CNG channels have been studied. All native CNG channels react to both cAMP and cGMP, but smaller concentrations of cGMP than those of cAMP are needed to activate and open the channels. CNG channels are sharply selective between cGMP and cAMP in rods and cones, whereas in OSNs, the channels respond equally well to both ligands. CNG channels found in OSNs are much more sensitive to both cGMP and cAMP than photoreceptor CNG channels. Studies of dose response relations
Dose-response relationship
The dose-response relationship, or exposure-response relationship, describes the change in effect on an organism caused by differing levels of exposure to a stressor after a certain exposure time...
showed that channel activation is greatly dependent on cGMP concentration; several cGMP molecules bind to the channel in a cooperative manner. Since each subunit contains a single cNMP-binding site, and homo- and heteromeric channels most likely form a tetrameric complex, a maxiumum of four ligand molecules can bind to the channel.
Selectivity can be achieved by differential control of the affinity for binding of the ligand, efficacy of gating, or a combination of both. Binding affinity means how tightly cyclic nucleotides bind to the channel. Efficacy refers to the ability of ligand to activate and open the channel once it is bound. Although these processes are useful in understanding selectivity, they are inextricably coupled to each other that it is very difficult to experimentally separate one from another.
Significance in plants
Cyclic nucleotide-gated ions channels in plants are similar in amino acid sequence and structure to non-selective cation CNG channels in animals, as well as trans-membrane-domain K+-selective shaker family channels. However, there are drastic differences that are seen exclusively in plant CNG channels. The amino acid sequence of the pore sequence in plant CNG channels lacks the selectivity filter found in animal CNG channels as well as lacks a GYGD K+-selectivity filter sequence. Other sequence differences are seen in plant CNG channels s, particularly in the cyclic nucleotide binding domain (CNBD). In plants, the CaM binding domain (CaMBD) is found to overlap α-helix C in the CNBD of CNG channels. In animals the CaMBDs are located far away from the CNBD.CNG channels play a large role in plant immunity and response to pathogens. They have also been implicated in apoptosis
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
in plants. Cyclic nucleotide-gated ion channels are also thought to be involved in pollen development in plants, however its exact role in this mechanism is still not known.
Unlike animal CNG channels, plant CNG channels have not been extensively analyzed biochemically with respect to their structure.
Future research
Researchers have answered many important questions regarding cyclic nucleotide gated ion channels functions in vision and olfactionOlfaction
Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...
. In other physiological areas, the role of CNG channels is less defined. With technological growth, there now exists more possibilities for understanding these mechanisms.
Because nitrous oxide
Nitrous oxide
Nitrous oxide, commonly known as laughing gas or sweet air, is a chemical compound with the formula . It is an oxide of nitrogen. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...
(NO) is involved in the stimulation of the synthesis of cGMP, further research is being conducted to understand the physiological interaction of NO with CNG channels, particularly in the covalent modification of CNG channels in OSNs.