Ryanodine receptor

Ryanodine receptors (RyR) are classified as high-conductance, intracellular calcium release channels, typically present in the membrane of the sarcoplasmic and endoplasmic reticulum. The major role these channels play are to regulate cell signaling and muscle contraction by releasing calcium ions. RyR1 is responsible for skeletal muscle contraction, RyR2 is responsible for cardiac muscle contraction, and RyR3 is responsible for maintaining calcium homeostasis in the brain and throughout other tissues in the body.

thumb|Cytoplasmic face of phosphorylated RyR2 in open conformation.

Origin

The ryanodine receptors were originally identified in the 1980s and named after the plant alkaloid ryanodine, found in Central and South America. When ryanodine was first found it was investigated to be a potential insecticide due to its ability to induce paralysis amongst insects.These receptors have weights exceeding 2 megadaltons and their structural complexity enables a wide variety of allosteric regulation mechanisms. Due to these receptors size and structure they are able to integrate regulatory signals and readily control the rate of calcium release.

Studies involving cryo-electron microscopy (cryo-EM) have revealed 3-D structures of Ryanodine receptors. It has been shown that RyR's have fourfold symmetry and adopt a mushroom-like structure: a large cytosolic assembly that forms the "cap" and smaller transmembrane regions that form the "stalk", which is embedded into the sarco/endoplasmatic reticulum's membrane.

Each major ryanodine receptor isoform (RyR1, RyR2, and RyR3) share a similar structure with minor differences that depend on where they are located and how they function, i.e. heart or skeletal muscle.

Isoforms

There are multiple isoforms of ryanodine receptors:

• RyR1 is primarily expressed in skeletal muscle
• It is essential for excitation-contraction coupling
• RyR2 is primarily expressed in myocardium (heart muscle)
• the major cellular mediator of calcium-induced calcium release (CICR) in animal cells.
• RyR3 is expressed more widely, but especially in the brain.
• It is involved in neuroprotection, memory, pain modulation, and social behavior.

Non-mammalian vertebrates typically express two RyR isoforms, referred to as RyR-alpha and RyR-beta. Many invertebrates, including the model organisms Drosophila melanogaster (fruitfly) and Caenorhabditis elegans, only have a single isoform. In non-metazoan species, calcium-release channels with sequence homology to RyRs can be found, but they are shorter than the mammalian ones and may be closer to inositol trisphosphate (IP₃) receptors.

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Physiology

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Ryanodine receptors mediate the release of calcium ions from the sarcoplasmic reticulum and endoplasmic reticulum, an essential step in muscle contraction.

It has been shown that calcium release from a number of ryanodine receptors in a RyR cluster results in a spatiotemporally-restricted rise in cytosolic calcium that can be visualized as a calcium spark. Calcium release from RyR has been shown to regulate ATP production in heart and pancreas cells.

Ryanodine receptors are similar to the inositol trisphosphate (IP₃ or InsP₃) receptor, and stimulated to transport Ca²⁺ into the cytosol by recognizing Ca²⁺ on its cytosolic side, thus establishing a positive feedback mechanism; a small amount of Ca²⁺ in the cytosol near the receptor will cause it to release even more Ca²⁺ (calcium-induced calcium release/CICR). Furthermore, RyR can sense the Ca²⁺ concentration inside the ER/SR and spontaneously open in a process known as store overload-induced calcium release (SOICR).

RyRs are especially important in neurons and muscle cells. In heart and pancreas cells, another second messenger (cyclic ADP-ribose) takes part in the receptor activation.

The localized and time-limited activity of Ca²⁺ in the cytosol is also called a Ca²⁺ wave. The propagation of the wave is accomplished by the feedback mechanism of the ryanodine receptor. The activation of phospholipase C by GPCR or RTK triggers the production of inositol trisphosphate, which activates of the InsP₃ receptor.

Pharmacology

Ryanodine receptors are ligand-gated calcium channels, which are highly sensitive to modulation by endogenous ions, small molecules, pharmacological agents. The receptors located along the sarcoplasmic reticulum, are responsible for storing calcium. When calcium is triggered to released, via action potentials down the cell membrane, contractile proteins are activated, this is essential for excitation-contraction coupling of striated muscle. Some of these disorders related to RyR mutations have high mortality rates so having pharmacological agents to treat these disorders or reverse these mutations would be a great step in future medicine.

The plant alkaloid ryanodine, for which this receptor was named, has become an invaluable investigative tool. It can block the phasic release of calcium, but at low doses may not block the tonic cumulative calcium release. The binding of ryanodine to RyRs is use-dependent, that is the channels have to be in the activated state. At low (<10 micromolar, works even at nanomolar) concentrations, ryanodine binding locks the RyRs into a long-lived subconductance (half-open) state and eventually depletes the store, while higher (~100 micromolar) concentrations irreversibly inhibit channel-opening.

• Activators:
• Direct Agonist: 4-chloro-m-cresol (4CmC)
• Induces calcium release from ryanodine receptors and is used to investigate physiological and molecular functions in tissue, cells, and membranes. 4CmC has also shown potential for being a diagnostic tool to recognize muscles that are susceptible to MH.
• Xanthines like caffeine and pentifylline activate it by potentiating sensitivity to native ligand Ca.
• Physiological agonist: Cyclic ADP-ribose can act as a physiological gating agent. It has been suggested that it may act by making FKBP12.6 (12.6 kilodalton FK506 binding protein, as opposed to 12 kDa FKBP12 which binds to RyR1) which normally bind (and blocks) RyR2 channel tetramer in an average stoichiometry of 3.6, to fall off RyR2 (which is the predominant RyR in pancreatic beta cells, cardiomyocytes and smooth muscles).

• Antagonists:
• Ryanodine
• High concentration ryanodine (micromolar, > 10 ℳM), completely closes the RyR channels.
• Low concentration ryanodine (nanomolar, < 10 ℳM), locks the RyR channels at a half-open state.
• Specifically targets RyR1 and RyR3.
• Azumolene
• Analog of Dantrolene, used to relax skeletal muscle by inhibiting the release of calcium from the SR.
• Ruthenium red
• Experimental antagonist that binds to the channel pore and blocks RyR1 medicated calcium release.
• Procaine, tetracaine, etc. (local anesthetics)

A variety of other molecules may interact with and regulate ryanodine receptor. For example: dimerized Homer physical tether linking inositol trisphosphate receptors (IP3R) and ryanodine receptors on the intracellular calcium stores with cell surface group 1 metabotropic glutamate receptors and the Alpha-1D adrenergic receptor

As Potential Drug Targets

The expression, distribution, and gating of RyRs are modified by cellular proteins, presenting an opportunity to develop new drugs that target RyR channel complexes by manipulating these proteins. Several drugs, such as FK506, rapamycin, and K201, can modify interactions between RyRs and their accessory proteins. Calsequestrin (CASQ) has multiple Ca²⁺ binding sites that bind with very low affinity, allowing easy ion release. It acts as RyR gate modulators by signaling when Ca²⁺ stores are full. Triadin and Junctin are sarcoplasmic reticulum (SR) membrane proteins that link RyRs to CASQ and also respond to Ca²⁺ store levels.

Calmodulin (CaM) and S100A1 both bind the same site on RyRs (especially RyR1 and RyR2), but exert opposite effects: Ca²⁺-bound CaM inhibits RyRs while S100A1 enhances its opening. Expression levels and competition between these proteins tune RyR responses to Ca²⁺ signals. Mutant-type RyR1 receptors exposed to volatile anesthetics or other triggering agents can display an increased affinity for cytoplasmic Ca²⁺ at activating sites as well as a decreased cytoplasmic Ca²⁺ affinity at inhibitory sites. The breakdown of this feedback mechanism causes uncontrolled release of Ca²⁺ into the cytoplasm, and increased ATP hydrolysis resulting from ATPase enzymes shuttling Ca²⁺ back into the sarcoplasmic reticulum leads to excessive heat generation.

RyR2 mutations play a role in stress-induced polymorphic ventricular tachycardia (a form of cardiac arrhythmia) and ARVD. and thus may play a role in the pathogenesis of neurodegenerative diseases, like Alzheimer's disease.

The presence of antibodies against ryanodine receptors in blood serum has also been associated with myasthenia gravis (i.e., MG).

Sudden cardiac death in several young individuals in the Amish community (four of which were from the same family) was traced to homozygous duplication of a mutant RyR2 (Ryanodine Receptor) gene. Normal (wild type) ryanodine receptors are involved in CICR in heart and other muscles, and RyR2 functions primarily in the myocardium (heart muscle).

See also

• Diamide, a class of insecticide that act through ryanodine receptors
• The diamides, an important class of insecticide making up 13% of the insecticide market, work by activating insect RyRs.
• Neuromuscular junction
• Dihydropyridine receptor

References

External links