Ryanodine receptor

Etymology

The ryanodine receptors are named after the plant alkaloid ryanodine, to which they show a high affinity:

Isoforms

There are multiple isoforms of ryanodine receptors:

Physiology

Ryanodine receptors mediate the release of calcium ions from the sarcoplasmic reticulum and endoplasmic reticulum, an essential step in muscle contraction. In skeletal muscle, activation of ryanodine receptors occurs via a physical coupling to the dihydropyridine receptor (a voltage dependent L-type calcium channel), whereas, in cardiac muscle, the primary mechanism of activation is calcium-induced calcium release, which causes calcium outflow from the sarcoplasmic reticulum.

It has been shown that calcium release from a number of ryanodine receptors in a ryanodine receptor cluster results in a spatiotemporally restricted rise in cytosolic calcium that can be visualised as a calcium spark. Ryanodine receptors are very close to mitochondria and 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) 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).

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 building of the wave is done by

Associated proteins

RyRs form docking platforms for a multitude of proteins and small molecule ligands. The cardiac-specific isoform of the receptor (RyR2) is known to form a quaternary complex with luminal calsequestrin, junctin, and triadin. Calsequestrin has multiple Ca²⁺ binding sites and binds Ca²⁺ ions with very low affinity so they can be easily released.

Pharmacology

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

Ryanodine

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.

Caffeine

RyRs are activated by millimolar caffeine concentrations. High (greater than 5 mmol/L) caffeine concentrations cause a pronounced increase (from micromolar to picomolar) in the sensitivity of RyRs to Ca²⁺ in the presence of caffeine, such that basal Ca²⁺ concentrations become activatory. At low millimolar caffeine concentrations, the receptor opens in a quantal way, but has complicated behavior in terms of repeated use of caffeine or dependence on cytosolic or luminal calcium concentrations.

Role in disease

RyR1 mutations are associated with malignant hyperthermia and central core disease. RyR2 mutations play a role in stress-induced polymorphic ventricular tachycardia (a form of cardiac arrhythmia) and ARVD. It has also been shown that levels of type RyR3 are greatly increased in PC12 cells overexpressing mutant human Presenilin 1, and in brain tissue in knockin mice that express mutant Presenilin 1 at normal levels, 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.

Structure

RyR1 cryo-EM structure revealed a large cytosolic assembly built on an extended α-solenoid scaffold connecting key regulatory domains to the pore. The RyR1 pore architecture shares the general structure of the six-transmembrane ion channel superfamily. A unique domain inserted between the second and third transmembrane helices interacts intimately with paired EF-hands originating from the α-solenoid scaffold, suggesting a mechanism for channel gating by Ca²⁺.