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Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of ions across the cell membrane, controlling the flow of ions across secretory and epithelial cells, and regulating cell volume. Ion channels are present in the membranes of all cells. Ion channels are one of the two classes of ionophoric proteins, the other being ion transporters.
Transient receptor potential channels are a group of ion channels located mostly on the plasma membrane of numerous animal cell types. Most of these are grouped into two broad groups: Group 1 includes TRPC, TRPV, TRPVL, TRPM, TRPS, TRPN, and TRPA. Group 2 consists of TRPP and TRPML. Other less-well categorized TRP channels exist, including yeast channels and a number of Group 1 and Group 2 channels present in non-animals. Many of these channels mediate a variety of sensations such as pain, temperature, different kinds of tastes, pressure, and vision. In the body, some TRP channels are thought to behave like microscopic thermometers and used in animals to sense hot or cold. Some TRP channels are activated by molecules found in spices like garlic (allicin), chili pepper (capsaicin), wasabi ; others are activated by menthol, camphor, peppermint, and cooling agents; yet others are activated by molecules found in cannabis or stevia. Some act as sensors of osmotic pressure, volume, stretch, and vibration. Most of the channels are activated or inhibited by signaling lipids and contribute to a family of lipid-gated ion channels.
Voltage-gated ion channels are a class of transmembrane proteins that form ion channels that are activated by changes in the electrical membrane potential near the channel. The membrane potential alters the conformation of the channel proteins, regulating their opening and closing. Cell membranes are generally impermeable to ions, thus they must diffuse through the membrane through transmembrane protein channels. They have a crucial role in excitable cells such as neuronal and muscle tissues, allowing a rapid and co-ordinated depolarization in response to triggering voltage change. Found along the axon and at the synapse, voltage-gated ion channels directionally propagate electrical signals. Voltage-gated ion-channels are usually ion-specific, and channels specific to sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl−) ions have been identified. The opening and closing of the channels are triggered by changing ion concentration, and hence charge gradient, between the sides of the cell membrane.
Voltage-gated potassium channels (VGKCs) are transmembrane channels specific for potassium and sensitive to voltage changes in the cell's membrane potential. During action potentials, they play a crucial role in returning the depolarized cell to a resting state.
TRPC is a family of transient receptor potential cation channels in animals.
TRPV is a family of transient receptor potential cation channels in animals. All TRPVs are highly calcium selective.
TRPM is a family of transient receptor potential ion channels. Functional TRPM channels are believed to form tetramers. The TRPM family consists of eight different channels, TRPM1–TRPM8.
TRPML comprises a group of three evolutionarily related proteins that belongs to the large family of transient receptor potential ion channels. The three proteins TRPML1, TRPML2 and TRPML3 are encoded by the mucolipin-1 (MCOLN1), mucolipin-2 (MCOLN2) and mucolipin-3 (MCOLN3) genes, respectively.
Polycystin 1 is a protein that in humans is encoded by the PKD1 gene. Mutations of PKD1 are associated with most cases of autosomal dominant polycystic kidney disease, a severe hereditary disorder of the kidneys characterised by the development of renal cysts and severe kidney dysfunction.
Transient receptor potential canonical 1 (TRPC1) is a protein that in humans is encoded by the TRPC1 gene.
Transient receptor potential cation channel subfamily V member 2 is a protein that in humans is encoded by the TRPV2 gene. TRPV2 is a nonspecific cation channel that is a part of the TRP channel family. This channel allows the cell to communicate with its extracellular environment through the transfer of ions, and responds to noxious temperatures greater than 52 °C. It has a structure similar to that of potassium channels, and has similar functions throughout multiple species; recent research has also shown multiple interactions in the human body.
Polycystin-2 is a protein that in humans is encoded by the PKD2 gene.
Polycystic kidney disease 2-like 1 protein also known as transient receptor potential polycystic 2 is a protein that in humans is encoded by the PKD2L1 gene.
Polycystic kidney disease is a genetic disorder in which the renal tubules become structurally abnormal, resulting in the development and growth of multiple cysts within the kidney. These cysts may begin to develop in utero, in infancy, in childhood, or in adulthood. Cysts are non-functioning tubules filled with fluid pumped into them, which range in size from microscopic to enormous, crushing adjacent normal tubules and eventually rendering them non-functional as well.
Polycystic kidney disease 2-like 2 protein (PKD2L2) also known as transient receptor potential polycystic 5 (TRPP5) is a protein that in humans is encoded by the PKD2L2 gene.
Cell surface receptors are receptors that are embedded in the plasma membrane of cells. They act in cell signaling by receiving extracellular molecules. They are specialized integral membrane proteins that allow communication between the cell and the extracellular space. The extracellular molecules may be hormones, neurotransmitters, cytokines, growth factors, cell adhesion molecules, or nutrients; they react with the receptor to induce changes in the metabolism and activity of a cell. In the process of signal transduction, ligand binding affects a cascading chemical change through the cell membrane.
PKD domain was first identified in the polycystic kidney disease protein, polycystin-1, and contains an Ig-like fold consisting of a beta-sandwich of seven strands in two sheets with a Greek key topology, although some members have additional strands. Polycystin-1 is a large cell-surface glycoprotein involved in adhesive protein–protein and protein–carbohydrate interactions; however it is not clear if the PKD domain mediates any of these interactions.
The Polycystin Cation Channel (PCC) Family consists of several transporters ranging in size from 500 to over 4000 amino acyl residues (aas) in length and exhibiting between 5 and 18 transmembrane segments (TMSs). This family is a constituent of the Voltage-Gated Ion Channel (VIC) Superfamily. These transporters generally catalyze the export of cations. A representative list of proteins belonging to the PCC family can be found in the Transporter Classification Database.
The transient receptor potential Ca2+ channel (TRP-CC) family (TC# 1.A.4) is a member of the voltage-gated ion channel (VIC) superfamily and consists of cation channels conserved from worms to humans. The TRP-CC family also consists of seven subfamilies (TRPC, TRPV, TRPM, TRPN, TRPA, TRPP, and TRPML) based on their amino acid sequence homology:
- the canonical or classic TRPs,
- the vanilloid receptor TRPs,
- the melastatin or long TRPs,
- ankyrin (whose only member is the transmembrane protein 1 [TRPA1])
- TRPN after the nonmechanoreceptor potential C (nonpC), and the more distant cousins,
- the polycystins
- and mucolipins.
Polycystic kidney disease 3 is a protein that in humans is encoded by the PKD3 gene.
References
- ↑ Islam, M. S. Transient Receptor Potential Channels. Advances in Experimental Medicine and Biology. Vol 704. ISBN 978-94-007-0264-6
- 1 2 Nilius B, Owsianik G, Voets T, Peters JA (2007). "Transient receptor potential cation channels in disease" (PDF). Physiol. Rev. 87 (1): 165–217. doi:10.1152/physrev.00021.2006. PMID 17237345.
- ↑
- "Transient Receptor Potential Channels". IUPHAR Database of Receptors and Ion Channels. International Union of Basic and Clinical Pharmacology.