Introduction

A ryanodine receptor is a homotetrameric channel with a molecular mass of more than 2.2 megadaltons.[1] It is the largest known ion channel and gets its name from one of its exogenous ligands, ryanodine, an alkaloid plant toxin from Ryania speciosa.[2] Ryanodine receptor ion channels are embedded in the internal side of the sarcoplasmic reticulum that stores calcium, which is fundamental to striated muscle.[3] They are high-conductance, monovalent, or divalent conducting channels regulated by multiple factors, including calcium, magnesium, adenosine triphosphate, calmodulin, protein kinases and phosphatases, and redox-active species.[4] Ryanodine receptors are essential for excitation-contraction coupling, linking action potentials and contraction of the striated muscle by releasing calcium ions required to activate the contractile proteins.[3] These proteins are studied to develop therapeutic advances in diseases associated with striated muscle.

Issues of Concern

One issue of concern when studying these proteins is their size. Ryanodine receptors are the largest known ion channels capable of creating a rapid and transient increase in the cytosolic calcium levels within the cell.[5] The analysis of the primary structure of these proteins demonstrates many functional motifs that are present in other proteins. Still, the role of ryanodine receptors has not been fully elucidated due to the size of the gene.[5] The massive size of the gene, the multitude of modulators, and the dynamic nature of the channel make the structural analysis very difficult.[5] The many advances in electron microscopy are beginning to decode many vital receptor features.[5]

Cellular Level

A ryanodine receptor comprises 4 polypeptides, each consisting of around 5000 amino acids, and four FK506-binding proteins, each consisting of approximately 110 amino acids.[4] Moreover, the type 1 ryanodine receptor is the major isoform expressed in skeletal muscle, and the type 2 ryanodine receptor is the major isoform expressed in cardiac muscle.[3] 

In skeletal muscle, calcium release units arrange themselves in triads composed of three elements: 2 terminal expansions of the sarcoplasmic reticulum on either side that form a junction with the third component, the t-tubule.[3] Within these junctions, the ryanodine receptors form a double row of channels that form "couplons" with the dihydropyridine receptors in the t-tubule membrane.[3] The t-tubules enter the muscle fibers via caveolae, and these align with the A. I band and overlap on either side of the Z-line within the sarcomere, which forms a double band in each sarcomere that repeats at spaced intervals along the length of a muscle fiber.[3]

In cardiac muscle, calcium release units arrange themselves in dyads, containing only 2 elements: a single terminal sarcoplasmic reticulum expansion forming a junction with the surface of the t-tubule membrane.[3] The cardiac sarcoplasmic reticulum expansions are relatively narrow, and these dyads align with the Z-line, forming a single band in each cardiac muscle sarcomere.[3]

Development

The functional ryanodine receptor ion channel is formed by 4 monomers, each with more than 5000 residues, constituting the largest ion channel protein identified so far.[3] In humans, the gene that encodes the type 1 ryanodine receptor channel protein that commonly presents in skeletal muscle has its location on chromosome 19q13.2 and spans 104 exons.[5] The gene encodes the type 2 ryanodine receptor in cardiac muscle on chromosome 1q43 and spans 102 exons.[5] The type 3 ryanodine receptor gene is located in chromosome 15q13.3-14 and spans 103 exons.[5] 

Organ Systems Involved

There are 3 different isoforms of ryanodine receptors found in mammals.[2] They share around 65% of the gene sequence identity and have subtle functional differences.[2]

The type 1 ryanodine receptor is primarily found in skeletal muscles and is at the junctional region of the terminal sarcoplasmic reticulum.[5] The type 1 channel may also express very low levels in cardiac muscle, smooth muscle, stomach, kidney, thymus, cerebellum, Purkinje cells, adrenal glands, ovaries, and the testes.[5] New studies have shown that type 1 receptors may even be expressed in B-lymphocytes.[5] The predominant form of the ryanodine receptor in cardiac muscle is the type 2 receptor.[5] The type 2 ryanodine receptor also expresses at high levels in the Purkinje cells of the cerebellum and cerebral cortex and low levels in the stomach, kidneys, adrenal glands, ovaries, thymus, and lungs.[5] The type 3 ryanodine receptors get expressed in the hippocampal neurons, thalamus, Purkinje cells, corpus striatum, and in the skeletal muscles predominantly located in the diaphragm, the smooth muscle cells of the coronary vasculature, lungs, kidneys, ileum, jejunum, spleen, stomach, aorta, uterus, ureters, urinary bladder, and esophagus.[5] 

Function

The ryanodine receptor is a conduit for calcium to flow through and produce contractions in striated muscles.

In skeletal muscle, the depolarization from the action potential leads to a protein-to-protein interaction across the junctional cleft between the dihydropyridine receptor and the ryanodine receptor type 1 to release calcium from the sarcoplasmic reticulum to produce a muscle contraction.[6]

In cardiac muscle during systole, calcium is released from the sarcoplasmic reticulum by directly activating the type 2 ryanodine receptors by an inward current of calcium via the L-type calcium channels during excitation produced from the action potential.[7] This calcium-induced calcium release occurs at specialized microdomains where the sarcolemma T- tubule closely approaches the sarcoplasmic reticulum's junction, forming the dyad structure.[7] The spatiotemporal summation of this calcium-induced calcium release generates contraction within the cardiac muscle.[7]

Mechanism

Historically, it has been shown that 3 properties are essential for muscle contraction: sensitivity level to ryanodine, the rapidity of calcium release from the sarcoplasmic reticulum, and the ryanodine receptor.[2] The ryanodine receptors are calcium-gated cationic ion channels weakly selective for calcium over other cations.[2] The mechanism via which the channel activates is excitation-coupling and is tissue-specific. Excitation-contraction coupling encompasses the many complex processes linking surface membrane action potentials to the shortening of the muscle fiber, which has evolved over the years to incorporate only the mechanism linking the depolarization-dependent response of the dihydropyridine receptor voltage sensor with the release of the calcium from the sarcoplasmic reticulum by the ryanodine receptors.[3] 

The skeletal muscle excitation-contraction coupling appears to depend on the conformational change denoted through the protein-to-protein interactions that link dihydropyridine subunits to the type 1 ryanodine receptors.[3] The voltage sensor response to the depolarization is measured as a "charge movement" across the surface and the t-tubule membrane.[3]

In cardiac excitation-contraction coupling, calcium ions enter the dyadic junction through the dihydropyridine receptor and activate the type 2 ryanodine receptors via calcium-induced calcium release.[3] The dihydropyridine calcium influx is sufficient to activate several type 2 ryanodine receptors, and there is only one dihydropyridine receptor for every 4 to 10 type 2 receptor tetramer.[3] This cardiac mechanism is dependent upon the extracellular level of calcium.[3]

Related Testing

One area of testing that has proven beneficial has been the use of fluorescence resonance energy transfer.[8] This method allows the user to discover compounds that modulate an intracellular calcium concentration of the ryanodine receptor.[8] The N-terminus intersubunit of the ryanodine receptor self-associates and has recently emerged as a critical structure-function parameter in regulating the channel.[9] Empirical evidence from a combination of testing has indicated that type 2 ryanodine receptor N-terminus, also conserved in type 1 and type 3 ryanodine receptors, is intricately involved in the canal's closure.[9] These methods of testing have helped develop therapeutic applications in medicine.[8]

Pathophysiology

Mutations in ryanodine receptors primarily correlate with a range of myopathies and cardiac arrhythmia disorders. The Human Gene Mutation Database contains 563 mutations in the type 1 ryanodine receptors and 287 mutations in the type 2 ryanodine receptors known to cause inherited disease.[2]

For example, in malignant hyperthermia, findings indicate that disruption of the N-terminus and central domain interaction within the ryanodine receptor becomes disrupted, leading to increased channel activity.[9] In catecholaminergic polymorphic ventricular tachycardia, findings indicate that a single amino acid substitution in the ryanodine receptor gene leads to the N-terminus and central domain unzipping that underlies the channel dysfunction and diastolic sarcoplasmic reticulum calcium leak.[9]

Clinical Significance

More than 100 type-1 ryanodine receptor channel mutations are potentially present in skeletal muscle, including malignant hyperthermia, central core disease, and multi/minicore disease.[4] As for type 2 ryanodine receptors, more than 150 mutations lead to inherited pathology, including catecholaminergic polymorphic ventricular tachycardia, arrhythmogenic right ventricular dysplasia type 2, and idiopathic ventricular fibrillation.[4]

Malignant hyperthermia is a hypermetabolic syndrome that appears in susceptible patients after exposure to specific pharmacologic agents that induce abnormal regulation of the ryanodine receptors, producing a massive calcium release from the sarcoplasmic reticulum in striated muscle.[10] The symptoms include high fever, tachycardia, and muscle rigidity. Laboratory workup will show high CK and high potassium levels. It most commonly results from using volatile anesthetic agents (sevoflurane, halothane, isoflurane, and enflurane) and rarely by succinylcholine. Inheritance to the susceptibility is often autosomal dominant. Very infrequently, the disease may occur as a new mutation. Dantrolene sodium is the therapy used to counteract malignant hyperthermia because it inhibits calcium release from the sarcoplasmic reticulum by antagonizing the type 1 ryanodine receptors.[10]

Catecholaminergic polymorphic ventricular tachycardia is characterized by a polymorphic ventricular tachycardia in a high adrenergic tone, as in exercise.[11] The mainstay of therapy is to avoid strenuous activity, and the first-line pharmacological treatment is beta-blockers to control the heart rate.[11]