Use of substituted and tandem-repeated peptides to probe the relevance of the highly conserved RGD tripeptide in the immune response against foot-and-mouth disease virus

Identification of a conformational neutralizing epitope on the VP1 protein of type A foot-and-mouth disease virus

Foot-and-mouth disease (FMD) caused by foot-and-mouth disease virus (FMDV), is a highly contagious infectious disease that affects domestic and wild cloven-hoofed animals worldwide. In recent years, outbreaks of serotype A FMD have occurred in many countries. High-affinity neutralizing antibodies against a conserved epitope could provide protective immunity against diverse subtypes of FMDV serotype A and protect against future pandemics. In this study, we generated a serotype A FMDV-specific potent neutralizing monoclonal antibody (MAb), 6C9, which recognizes a conformation-dependent epitope. MAb 6C9 potently neutralized FMDV A/XJBC/CHA/2010 with a 50% neutralization titer (NT₅₀) of 4096. Screening of a phage-displayed random 12-mer peptide library revealed that MAb 6C9 bound to phages displaying the consensus motif YxxPxGDLG, which is highly homologous to the ¹³⁵YxxPxxxxxGDLG¹⁴⁷ motif found in the serotype A FMDV virus-encoded structural protein VP1. To further verify the authentic epitope recognized by MAb 6C9, two FMDV A/XJBC/CHA/2010 mutant viruses, P138A and G144A, were generated using a reverse genetic system. Subsequent micro-neutralization assays and double-antibody sandwich (DAS) ELISA analyses revealed that the Pro¹³⁸ and Gly¹⁴⁴ residues of the conformational epitope that are recognized by 6C9 are important for MAb 6C9 binding. Importantly, the epitope ¹³⁵YxxPxxxxxGDLG¹⁴⁷ was highly conserved among different topotypes of serotype A FMDV strains in a sequence alignment analysis. Thus, the results of this study could have potential applications in the development of novel epitope-based vaccines and suitable a MAb-based diagnostic method for the detection of serotype A FMDV and the quantitation of antibodies against this serotype.

Delivery of synthetic RNA can enhance the immunogenicity of vaccines against foot-and-mouth disease virus (FMDV) in mice

We have recently described the antiviral effect in mice of in vitro-transcribed RNAs mimicking structural domains in the non-coding regions of the foot-and-mouth disease virus (FMDV) genome RNA. These small, synthetic and non-infectious RNA molecules (ncRNAs) are potent type-I interferon (IFN) inducers in vivo. In this work, the immunomodulatory effect of the ncRNA corresponding to the internal ribosome entry site (IRES) on immunization with two different FMD vaccine formulations, both based on inactivated virus, including or not a commercial adjuvant, was analyzed in the mice model. The effect of the time interval between RNA inoculation and immunization was also studied. RNA delivery consistently increased the titers of specific anti-FMDV antibodies, including neutralizing antibodies, elicited after vaccination. Moreover, at day 2 after immunization, significant differences in mean antibody titers could be detected between the groups of mice receiving either vaccine co-administered with the RNA and the control group, unlike those immunized with the vaccine alone. When vaccinated mice were challenged with FMDV, the mean values of viral load were lower in the groups receiving the RNA together with the vaccine. Our results show the enhancing effect of the IRES RNA on the immune response elicited after vaccination and suggest the potential of this molecule as an adjuvant for new FMD vaccine design.

Viral quasispecies evolution

Evolution of RNA viruses occurs through disequilibria of collections of closely related mutant spectra or mutant clouds termed viral quasispecies. Here we review the origin of the quasispecies concept and some biological implications of quasispecies dynamics. Two main aspects are addressed: (i) mutant clouds as reservoirs of phenotypic variants for virus adaptability and (ii) the internal interactions that are established within mutant spectra that render a virus ensemble the unit of selection. The understanding of viruses as quasispecies has led to new antiviral designs, such as lethal mutagenesis, whose aim is to drive viruses toward low fitness values with limited chances of fitness recovery. The impact of quasispecies for three salient human pathogens, human immunodeficiency virus and the hepatitis B and C viruses, is reviewed, with emphasis on antiviral treatment strategies. Finally, extensions of quasispecies to nonviral systems are briefly mentioned to emphasize the broad applicability of quasispecies theory.

Characterization of antilytic peptide antibody: application for the detection of lytic-based hybrid peptide in serum samples

We previously reported that a novel targeted drug termed hybrid epidermal growth factor receptor (EGFR)-lytic peptide, made by chemical conjugation of targeted binding peptide and cell-killing, lytic-peptide components, has selective cytotoxic activity that allows it to discriminate between normal and cancer cells. In addition, in vivo analysis revealed that this hybrid peptide displays significant antitumor activity in a xenograft model of human breast and pancreatic cancer in mice. Here, we characterized antilytic peptide antibody, which was raised from rabbit serum using the antigen of lytic peptide conjugated with keyhole limpet hemocyanin. It was found that antilytic peptide antibody is specific to the lytic peptide as assessed by both ELISA and surface plasmon resonance analysis and can also bind to EGFR-lytic peptide. Epitope mapping analysis using Biacore showed that two successive lysine regions in the lytic-peptide sequence are significant for recognition by this antibody. In addition, it was shown that this antibody can detect lytic-based hybrid peptide in serum samples from mouse blood and also in cultured breast cancer MDA-MB-231 cell samples by immunocytochemical staining experiments. It was found that the maximum concentrations of this peptide in serum were reached within 15-30 min of i.v. administration of EGFR-lytic peptide to mice. These results indicate that this antibody will be a useful tool for the detection of lytic-based peptides to investigate their in vivo stability and pharmacokinetics.

Foot-and-mouth disease virus (FMDV) causes an acute disease that can be lethal for adult laboratory mice

Foot-and-mouth disease virus (FMDV) is a picornavirus that causes an acute vesicular disease of cloven-hoofed animals. This virus continues to be threat to livestock worldwide with outbreaks causing severe economic losses. However, very little is known about FMDV pathogenesis, partially due to the inconveniences of working with cattle and swine, the main natural hosts of the virus. Here we demonstrate that C57BL/6 and BALB/C adult mice are highly susceptible to FMDV infection when the virus is administered subcutaneously or intraperitoneally. The first clinical signs are ruffled fur, apathy, humped posture, and wasting, which are followed by neurological signs such as hind-limb paralysis. Within 2-3 days of disease onset, the animals die. Virus is found in all major organs, indicating a systemic infection. Mice developed microvesicles near the basal layer of the epithelium, event that precedes the vesiculation characteristics of FMD. In addition, a lymphoid depletion in spleen and thymus and severe lymphopenia is observed in the infected mice. When these mice were immunized with conventional inactivated FMDV vaccine, they were protected (100% of vaccinated animals) against challenge with a lethal dose of FMDV. The data indicate that this mouse model may facilitate the study of FMDV pathogenesis, and the development of new effective vaccines for FMD.

Phenotypic mixing and hiding may contribute to memory in viral quasispecies

BACKGROUND

In a number of recent experiments with food-and-mouth disease virus, a deleterious mutant, RED, was found to avoid extinction and remain in the population for long periods of time. Since RED characterizes the past evolutionary history of the population, this observation was called quasispecies memory. While the quasispecies theory predicts the existence of these memory genomes, there is a disagreement between the expected and observed mutant frequency values. Therefore, the origin of quasispecies memory is not fully understood.

RESULTS

We propose and analyze a simple model of complementation between the wild type virus and a mutant that has an impaired ability of cell entry, the likely cause of fitness differences between wild type and RED mutants. The mutant will go extinct unless it is recreated from the wild type through mutations. However, under phenotypic mixing-and-hiding as a mechanism of complementation, the time to extinction in the absence of mutations increases with increasing multiplicity of infection (m.o.i.). If the RED mutant is constantly recreated by mutations, then its frequency at equilibrium under selection-mutation balance also increases with increasing m.o.i. At high m.o.i., a large fraction of mutant genomes are encapsidated with wild-type protein, which enables them to infect cells as efficiently as the wild type virions, and thus increases their fitness to the wild-type level. Moreover, even at low m.o.i. the equilibrium frequency of the mutant is higher than predicted by the standard quasispecies model, because a fraction of mutant virions generated from wild-type parents will also be encapsidated by wild-type protein.

CONCLUSIONS

Our model predicts that phenotypic hiding will strongly influence the population dynamics of viruses, particularly at high m.o.i., and will also have important effects on the mutation-selection balance at low m.o.i. The delay in mutant extinction and increase in mutant frequencies at equilibrium may, at least in part, explain memory in quasispecies populations.

The dipeptide, gamma-glutamylcysteine, is recognized by the anti-glutathione antibody single chain Fv fragment 20C9

The anti-glutathione antibody scFv 20C9, which we previously isolated from a human synthetic phage antibody scFv library [Hirose, M., Hayano, T., Shirai, H., Nakamura, H., and Kikuchi, M. (1998) Protein Eng. 11, 243-248], was expressed in the E. coli pET system and purified by sequential chromatography on Ni and glutathione-conjugated affinity resins. The purified scFv 20C9 antibody was characterized for its binding affinity for several glutathione derivatives by the BIACORE system. Although GSH, GSSG, and gamma-Glu-Cys could bind to the immobilized antibody, this was not the case for Cys-Gly, l-Glu, l-Cys, l-Gly, or several other glutathione derivatives such as gamma-Glu-Ser-Gly. The results suggest that a gamma-glutamic acid and sulfur atom are important for scFv 20C9 antibody recognition of glutathione. This is the first report to indicate that an scFv antibody can recognize a region as small as a dipeptide.

A multiply substituted G-H loop from foot-and-mouth disease virus in complex with a neutralizing antibody: a role for water molecules

The crystal structure of a 15 amino acid synthetic peptide, corresponding to the sequence of the major antigenic site A (G-H loop of VP1) from a multiple variant of foot-and-mouth disease virus (FMDV), has been determined at 2.3 A resolution. The variant peptide includes four amino acid substitutions in the loop relative to the previously studied peptide representing FMDV C-S8c1 and corresponds to the loop of a natural FMDV isolate of subtype C(1). The peptide was complexed with the Fab fragment of the neutralizing monoclonal antibody 4C4. The peptide adopts a compact fold with a nearly cyclic conformation and a disposition of the receptor-recognition motif Arg-Gly-Asp that is closely related to the previously determined structure for the viral loop, as part of the virion, and for unsubstituted synthetic peptide antigen bound to neutralizing antibodies. New structural findings include the observation that well-defined solvent molecules appear to play a major role in stabilizing the conformation of the peptide and its interactions with the antibody. Structural results are supported by molecular-dynamic simulations. The multiply substituted peptide developed compensatory mechanisms to bind the antibody with a conformation very similar to that of its unsubstituted counterpart. One water molecule, which for steric reasons could not occupy the same position in the unsubstituted antigen, establishes hydrogen bonds with three peptide amino acids. The constancy of the structure of an antigenic domain despite multiple amino acid substitutions has implications for vaccine design.

Native-like cyclic peptide models of a viral antigenic site: finding a balance between rigidity and flexibility

Antigenic site A of foot-and-mouth disease virus (serotype C) has been reproduced by means of cyclic versions of peptide A15, YTASARGDLAHLTTT, corresponding to residues 136-150 of envelope protein VP1. A structural basis for the design of the cyclic peptides is provided by crystallographic data from complexes between the Fab fragments of anti-site A monoclonal antibodies and A15, in which the bound peptide is folded into a quasi-cyclic pattern. Head-to-tail cyclizations of A15 do not provide peptides of superior antigenicity. Internal disulfide cyclization, however, leads to analogs which are recognized as one to two orders of magnitude better than linear A15 in both ELISA and biosensor experiments. CD and NMR studies show that the best antigen, CTASARGDLAHLTT-Ahx-C (disulfide), is very insensitive to environment-induced conformational change, suggesting that cyclization helps to stabilize a bioactive-like structure.

Antigenic properties and population stability of a foot-and-mouth disease virus with an altered Arg-Gly-Asp receptor-recognition motif

The antigenic properties and genetic stability of a multiply passaged foot-and-mouth disease virus (FMDV) clone C-S8c1 with an Arg-Gly-Gly triplet (RGG) instead of the Arg-Gly-Asp (RGD) integrin-recognition motif at positions 141 to 143 of capsid protein VP1 are described. Clear antigenic differences between FMDV RGG and clone C-S8c1 have been documented in ELISA, enzyme-linked immunoelectrotransfer (Western) blot and neutralization assays using site A-specific monoclonal antibodies and anti-FMDV polyclonal antibodies from swine and guinea pigs. The results validate with a live virus the role of the RGD (in particular Asp-143) in recognition of (and neutralization by) antibodies, a role previously suggested by immunochemical and structural studies with synthetic peptides. The FMDV RGG was genetically stable in a large proportion of serial infections of BHK-21 cells. However, a revertant virus with RGD was generated in one out of six passage series. Interestingly, this revertant FMDV did not reach dominance but established an equilibrium with its parental FMDV RGG, accompanied by an increase of quasispecies complexity at the sequences around the RGG triplet. FMDV RGG exhibited a selective disadvantage relative to other RGD-containing clones isolated from the same parental FMDV population. The results suggest that large antigenic variations can be prompted by replacements at critical capsid sites, including those involved in receptor recognition. These critical replacements may yield viruses whose stability allows them to replicate efficiently and to expand the sequence repertoire of an antigenic site.

Conformational flexibility in a highly mobile protein loop of foot-and-mouth disease virus: distinct structural requirements for integrin and antibody binding

The G-H loop of foot-and-mouth disease virus VP1 protein is a highly mobile peptide, that extends from the capsid surface and that in native virions is invisible by X-ray crystallography. In serotype C, this segment contains a hypervariable region with several continuous, overlapping, B-cell epitopes that embrace the conserved Arg-Gly-Asp (RGD) cell attachment motif. The solvent-exposed positioning of this peptide by selective insertion into different structural frameworks of E. coli beta-galactosidase, generates a spectrum of antigenic variants which react distinctively with a panel of anti-VP1 monoclonal antibodies and exhibit different efficiencies as cell ligands. The cell attachment efficiency is much less restricted by the different positioning of the viral segment at the insertion sites. A molecular model of an inserted stretch reveals a highest flexibility of the RGD tripeptide segment compared with the flanking sequences, that could allow a proper accommodation to integrin receptors even in poorly antigenic conformations. The non-converging structural requirements for RGD-mediated integrin binding and antibody recognition, explains the dynamism of the generation of neutralisation-resistant antigenic variants in the viral quasi-species, arising from a conformational space of integrin-binding competent peptides. This might be of special relevance for foot-and-moth disease virus evolution, since unlike in other picornaviruses, the cell binding motif and the major neutralising B-cell epitopes overlap in a solvent-exposed peptide accessible to the host immune system, in a virion lacking canyons and similar hiding structures.

A similar pattern of interaction for different antibodies with a major antigenic site of foot-and-mouth disease virus: implications for intratypic antigenic variation

The three-dimensional structures of the Fab fragment of a neutralizing antibody raised against a foot-and-mouth disease virus (FMDV) of serotype C1, alone and complexed to an antigenic peptide representing the major antigenic site A (G-H loop of VP1), have been determined. As previously seen in a complex of the same antigen with another antibody which recognizes a different epitope within antigenic site A, the receptor recognition motif Arg-Gly-Asp and some residues from an adjacent helix participate directly in the interaction with the complementarity-determining regions of the antibody. Remarkably, the structures of the two antibodies become more similar upon binding the peptide, and both undergo considerable induced fit to accommodate the peptide with a similar array of interactions. Furthermore, the pattern of reactivities of five additional antibodies with versions of the antigenic peptide bearing amino acid replacements suggests a similar pattern of interaction of antibodies raised against widely different antigens of serotype C. The results reinforce the occurrence of a defined antigenic structure at this mobile, exposed antigenic site and imply that intratypic antigenic variation of FMDV of serotype C is due to subtle structural differences that affect antibody recognition while preserving a functional structure for the receptor binding site.

Mutational analysis of discontinuous epitopes of foot-and-mouth disease virus using an unprocessed capsid protomer precursor

An unprocessed capsid precursor (P1) of foot-and-mouth disease virus (FMDV) has been expressed in mammalian cells to study discontinuous epitopes involved in viral neutralization. Amino acid replacements found in virus-escape mutants were engineered in the P1 precursor by site-directed mutagenesis of the plasmid. In all cases the replacements abolished recognition of unprocessed P1 by the relevant monoclonal antibodies (MAbs), paralleling the effects of the corresponding substitutions in neutralization of infectious FMDV. Five capsid surface residues within the same discontinuous antigenic area that were never found replaced in escape mutants were also engineered in P1. None of the substitutions affected antibody recognition, suggesting that these residues were not directly involved in the interaction with the antibodies tested. The results validate site-directed mutagenesis of constructs encoding capsid precursors as an approach to probe the structure of viral discontinuous epitopes not amenable to analysis with synthetic peptides.

Efficient neutralization of foot-and-mouth disease virus by monovalent antibody binding

Neutralization of an aphthovirus by monovalent binding of an antibody is reported. Foot-and-mouth disease virus (FMDV) clone C-S8c1 was neutralized by monoclonal antibody (MAb) SD6, which was directed to a continuous epitope within a major antigenic site of the G-H loop of capsid protein VP1. On a molar basis, the Fab fragment was at most fivefold less active in neutralization than the intact antibody, and both blocked virus attachment to cells. Neither the antibody nor the Fab fragment caused aggregation of virions, as evidenced by sucrose gradient sedimentation studies of the antibody-virus complex formed at antibody to virion ratios of 1:50 to 1:10,000. The results of neutralization of infectivity and of ultracentrifugation are fully consistent with structural data based on X-ray crystallographic and cryoelectron microscopy studies, which showed monovalent interaction of the antibody with a critical receptor binding motif Arg-Gly-Asp. The conclusions of these neutralization studies are that (i) bivalent binding of antibody is not a requisite for strong neutralization of aphthoviruses and (ii) aggregation of viral particles, which has been proposed to be the dominant neutralization mechanism of antibodies that bind monovalently to virions, is not necessary for the neutralization of FMDV C-S8c1 by MAb SD6.

Evolution subverting essentiality: dispensability of the cell attachment Arg-Gly-Asp motif in multiply passaged foot-and-mouth disease virus

Aphthoviruses use a conserved Arg-Gly-Asp triplet for attachment to host cells and this motif is believed to be essential for virus viability. Here we report that this triplet-which is also a widespread motif involved in cell-to-cell adhesion-can become dispensable upon short-term evolution of the virus harboring it. Foot-and-mouth disease virus (FMDV), which was multiply passaged in cell culture, showed an altered repertoire of antigenic variants resistant to a neutralizing monoclonal antibody. The altered repertoire includes variants with substitutions at the Arg-Gly-Asp motif. Mutants lacking this sequence replicated normally in cell culture and were indistinguishable from the parental virus. Studies with individual FMDV clones indicate that amino acid replacements on the capsid surface located around the loop harboring the Arg-Gly-Asp triplet may mediate in the dispensability of this motif. The results show that FMDV quasispecies evolving in a constant biological environment have the capability of rendering totally dispensable a receptor recognition motif previously invariant, and to ensure an alternative pathway for normal viral replication. Thus, variability of highly conserved motifs, even those that viruses have adapted from functional cellular motifs, can contribute to phenotypic flexibility of RNA viruses in nature.

Systematic replacement of amino acid residues within an Arg-Gly-Asp-containing loop of foot-and-mouth disease virus and effect on cell recognition

The conserved Arg-Gly-Asp (RGD) motif found in a hypervariable, mobile antigenic loop of foot-and-mouth disease virus (FMDV) is critically involved in virus attachment to cells by binding to an integrin, probably related to alphavbeta3. Here we describe (i) the synthesis of 241 15-mer peptides, which represent this loop of FMDV (isolate C-S8c1) and single variants in which each amino acid residue was replaced by 16 others and (ii) the inhibitory activity of these peptides on the ability of FMDV C-S8c1 to recognize and infect susceptible cells. This approach has allowed a first detailed evaluation of the specificity of each residue within a RGD-containing protein loop on cell recognition. The results indicate that, in addition to the exquisitely specific RGD triplet, two highly conserved Leu residues located at positions +1 and +4 downstream of the RGD and, to a lesser extent, the residue at position +2 are the only critical and specific determinants within the loop in promoting cell recognition of a viral ligand. The results support the proposal that, in spite of their involvement in antibody recognition, RGD and other FMDV loop residues are remarkably conserved because of their essential role in cell recognition.

Emerging foot-and-mouth disease virus variants with antigenically critical amino acid substitutions predicted by model studies using reference viruses

One of the major obstacles to the design of effective antiviral vaccines is the frequent generation of antigenic viral variants in the field. The types of variants that will become dominant during disease outbreaks is often unpredictable. However, here we report the genetic and antigenic characterization of emerging foot-and-mouth disease virus (FMDV) variants with antigenically critical amino acid substitutions predicted by model studies using reference viruses and monoclonal antibodies. The new variants belong to serotype C and have caused a number of recent disease outbreaks in Argentina. The variants harbor antigenically drastic amino acid substitutions in each of the antigenic sites identified in FMDV. In particular, a substitution found at a major antigenic site (site A, the G-H loop of VP1) had been repeatedly selected in viruses resistant to neutralization by monoclonal and polyclonal antibodies. The association of critical amino acid replacements at predicted positions with new FMD outbreaks has a number of implications for FMD epidemiology and for the design of vaccines intended to control diseases caused by highly variable RNA viruses.

Antibody recognition of picornaviruses and escape from neutralization: a structural view

Escape of picornaviruses from neutralization by monoclonal antibodies is mediated by substitutions of very few, defined amino acid residues of the capsid, generally located on the tip of some surface-exposed loops. Substitutions at the same positions are possibly of major relevance to antigenic variation of picornaviruses in the field. Such residues tend to cluster in discrete areas, termed antigenic sites. The structure of virus-antibody and peptide-antibody complexes, determined by cryoelectron microscopy and X-ray crystallography, combined with studies using site-directed mutagenesis, are beginning to reveal new features of picornavirus epitopes. This information complements and expands the view on picornavirus antigenicity previously provided by analyses of antibody-escape mutants. In addition to amino acids found replaced in escape mutants, other surface residues which remain invariant in spite of immune pressure also participate in contacts with the antibody molecule. Some invariant residues are even critical for the antigen-antibody interaction. Escape mutations occur at the subset of antigenically critical residues which are tolerant to change because they are not essentially involved in capsid structure or function. Restrictions to variation differ among epitopes; this may contribute to explain the different number of serotypes among picornaviruses, and the frequency at which antigenically highly divergent variants occur in the field.

Improved mimicry of a foot-and-mouth disease virus antigenic site by a viral peptide displayed on beta-galactosidase surface

A major antigenic site (site A) of foot-and-mouth disease virus includes multiple overlapping epitopes located within the flexible G-H loop of capsid protein VP1. We have studied the antigenicity of several recombinant E. coli beta-galactosidases displaying the site A from a serotype C virus in different surface regions of the bacterial enzyme. In each one of the explored insertion sites, the recombinant peptide shows different specificity with a set of anti-virus monoclonal antibodies directed to site A. In some of them, the inserted stretch mimics better than free or haemocyanin-coupled peptide the antigenicity of site A in the intact virus. In particular, an insertion within an exposed loop involved in the activating interface of beta-galactosidase (amino acids 272 to 287) led to a significant improvement of the overall reactivity. Since insertions at this site renders proteins enzymatically active, the activating interface could be an adequate place for the presentation of foreign antigens in correctly assembled beta-galactosidase tetramers. These results also suggest that anti-virus antibodies directed against the major antigenic site of FMDV recognize different conformations of the G-H loop, which are better reproduced in some of the recombinant proteins because of the dissimilar restrictions imposed by each particular insertion site.

Molecular evolution of aphthoviruses

Aphthoviruses are an important group of animal pathogens. A combination of genetic and structural studies has revealed one of the main principles governing their evolution: severe limitations to variation imposed by functional and structural constraints, in conjunction with high mutation and recombination rates operating during genome replication. Evolution occurs by positive selection and random drift acting on complex quasispecies distributions. The mutant composition of a quasi-species (or mutant spectrum) is largely dictated by tolerance to nucleotide and amino acid substitutions in viral RNAs and proteins, which must remain functionally competent. We review recent evidence to support this proposal, and we suggest that similar concepts may apply to other RNA viruses as well.

Role of ryanodine receptors

Recent findings on the ryanodine receptor of vertebrates, a Ca-release channel protein for the caffeine- and ryanodine-sensitive Ca pools, are reviewed in this article. Three distinct genes, i.e., ryr1, ryr2, and ryr3, express different isoforms in specific locations: Ryr1 in skeletal muscle and Purkinje cells of cerebellum; Ryr2 in cardiac muscle and brain, especially cerebellum; Ryr3 in skeletal muscle of nonmammalian vertebrates, the corpus striatum, and limbic cortex of brain, smooth muscles, and the other cells in vertebrates. While only one isoform (Ryr1) is expressed in mammalian skeletal muscles, two isoforms (alpha- and beta-isoforms expressed by ryr1 and ryr3, respectively) are found in nonmammalian vertebrate skeletal muscles. Although the coexistence of two isoforms may merely be related to differentiation and specialization, the biological significance remains to be clarified. Ryanodine receptors in vertebrate skeletal muscles are believed to mediate two different modes of Ca release: Ca(2+)-induced Ca release and action potential-induced Ca release. All results obtained so far with any isoform of ryanodine receptor are related to Ca(2+)-induced Ca release and show very similar characteristics. Ca(2+)-induced Ca release, however, cannot be the underlying mechanism of Ca release on skeletal muscle activation. Susceptibility of the ryanodine receptor's ryanodine-binding activity to modification by physical factors, such as osmolality of the medium, might be related to action potential-induced Ca release. A hypothesis of molecular interaction in view of the plunger model of action potential-induced Ca release is discussed, suggesting that the model could be compatible with Ryr1 and Ryr3, but incompatible with Ryr2. The functional relevance of ryanodine receptor isoforms, especially Ryr3, in brain also remains to be clarified. Among ryr1 gene-related diseases, malignant hyperthermia was the first to be identified; however, there is still the possibility of involvement of the other genes. Central core disease has been added to the list recently. A molecular approach for the diagnosis and treatment of diseases is now in progress.