Regulation of tissue-specific expression of the skeletal muscle ryanodine receptor gene

Subcellular localization and transcriptional regulation of brain ryanodine receptors. Functional implications

Ryanodine receptors (RyR) are intracellular Ca2+ channels localized in the endoplasmic reticulum, where they act as critical mediators of Ca2+-induced Ca2+ calcium release (CICR). In the brain, mammals express in both neurons, and non-neuronal cells, a combination of the three RyR-isoforms (RyR1-3). Pharmacological approaches, which do not distinguish between isoforms, have indicated that RyR-isoforms contribute to brain function. However, isoform-specific manipulations have revealed that RyR-isoforms display different subcellular localizations and are differentially associated with neuronal function. These findings raise the need to understand RyR-isoform specific transcriptional regulation, as this knowledge will help to elucidate the causes of neuronal dysfunction for a growing list of brain disorders that show altered RyR channel expression and function.

Ryanodine receptor 1-mediated Ca2+ signaling and mitochondrial reprogramming modulate uterine serous cancer malignant phenotypes

Background

Uterine serous cancer (USC) is the most common non-endometrioid subtype of uterine cancer, and is also the most aggressive. Most patients will die of progressively chemotherapy-resistant disease, and the development of new therapies that can target USC remains a major unmet clinical need. This study sought to determine the molecular mechanism by which a novel unfavorable prognostic biomarker ryanodine receptor 1 (RYR1) identified in advanced USC confers their malignant phenotypes, and demonstrated the efficacy of targeting RYR1 by repositioned FDA-approved compounds in USC treatment.

Methods

TCGA USC dataset was analyzed to identify top genes that are associated with patient survival or disease stage, and can be targeted by FDA-approved compounds. The top gene RYR1 was selected and the functional role of RYR1 in USC progression was determined by silencing and over-expressing RYR1 in USC cells in vitro and in vivo. The molecular mechanism and signaling networks associated with the functional role of RYR1 in USC progression were determined by reverse phase protein arrays (RPPA), Western blot, and transcriptomic profiling analyses. The efficacy of the repositioned compound dantrolene on USC progression was determined using both in vitro and in vivo models.

Results

High expression level of RYR1 in the tumors is associated with advanced stage of the disease. Inhibition of RYR1 suppressed proliferation, migration and enhanced apoptosis through Ca²⁺-dependent activation of AKT/CREB/PGC-1α and AKT/HK1/2 signaling pathways, which modulate mitochondrial bioenergetics properties, including oxidative phosphorylation, ATP production, mitochondrial membrane potential, ROS production and TCA metabolites, and glycolytic activities in USC cells. Repositioned compound dantrolene suppressed USC progression and survival in mouse models.

Conclusions

These findings provided insight into the mechanism by which RYR1 modulates the malignant phenotypes of USC and could aid in the development of dantrolene as a repurposed therapeutic agent for the treatment of USC to improve patient survival.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-022-02419-w.

Ca2+ handling abnormalities in early-onset muscle diseases: Novel concepts and perspectives

The physiological process by which Ca²⁺ is released from the sarcoplasmic reticulum is called excitation-contraction coupling; it is initiated by an action potential which travels deep into the muscle fiber where it is sensed by the dihydropyridine receptor, a voltage sensing L-type Ca²⁺channel localized on the transverse tubules. Voltage-induced conformational changes in the dihydropyridine receptor activate the ryanodine receptor Ca²⁺ release channel of the sarcoplasmic reticulum. The released Ca²⁺ binds to troponin C, enabling contractile thick-thin filament interactions. The Ca²⁺ is subsequently transported back into the sarcoplasmic reticulum by specialized Ca²⁺ pumps (SERCA), preparing the muscle for a new cycle of contraction. Although other proteins are involved in excitation-contraction coupling, the mechanism described above emphasizes the unique role played by the two Ca²⁺ channels (the dihydropyridine receptor and the ryanodine receptor), the SERCA Ca²⁺ pumps and the exquisite spatial organization of the membrane compartments endowed with the proteins responsible for this mechanism to function rapidly and efficiently. Research over the past two decades has uncovered the fine details of excitation-contraction coupling under normal conditions while advances in genomics have helped to identify mutations in novel genes in patients with neuromuscular disorders. While it is now clear that many patients with congenital muscle diseases carry mutations in genes encoding proteins directly involved in Ca²⁺ homeostasis, it has become apparent that mutations are also present in genes encoding for proteins not thought to be directly involved in Ca²⁺ regulation. Ongoing research in the field now focuses on understanding the functional effect of individual mutations, as well as understanding the role of proteins not specifically located in the sarcoplasmic reticulum which nevertheless are involved in Ca²⁺ regulation or excitation-contraction coupling. The principal challenge for the future is the identification of drug targets that can be pharmacologically manipulated by small molecules, with the ultimate aim to improve muscle function and quality of life of patients with congenital muscle disorders. The aim of this review is to give an overview of the most recent findings concerning Ca²⁺ dysregulation and its impact on muscle function in patients with congenital muscle disorders due to mutations in proteins involved in excitation-contraction coupling and more broadly on Ca²⁺ homeostasis.

Epigenetic allele silencing unveils recessive RYR1 mutations in core myopathies

Epigenetic regulation of gene expression is a source of genetic variation, which can mimic recessive mutations by creating transcriptional haploinsufficiency. Germline epimutations and genomic imprinting are typical examples, although their existence can be difficult to reveal. Genomic imprinting can be tissue specific, with biallelic expression in some tissues and monoallelic expression in others or with polymorphic expression in the general population. Mutations in the skeletal-muscle ryanodine-receptor gene (RYR1) are associated with malignant hyperthermia susceptibility and the congenital myopathies central core disease and multiminicore disease. RYR1 has never been thought to be affected by epigenetic regulation. However, during the RYR1-mutation analysis of a cohort of patients with recessive core myopathies, we discovered that 6 (55%) of 11 patients had monoallelic RYR1 transcription in skeletal muscle, despite being heterozygous at the genomic level. In families for which parental DNA was available, segregation studies showed that the nonexpressed allele was maternally inherited. Transcription analysis in patients' fibroblasts and lymphoblastoid cell lines indicated biallelic expression, which suggests tissue-specific silencing. Transcription analysis of normal human fetal tissues showed that RYR1 was monoallelically expressed in skeletal and smooth muscles, brain, and eye in 10% of cases. In contrast, 25 normal adult human skeletal-muscle samples displayed only biallelic expression. Finally, the administration of the DNA methyltransferase inhibitor 5-aza-deoxycytidine to cultured patient skeletal-muscle myoblasts reactivated the transcription of the silenced allele, which suggests hypermethylation as a mechanism for RYR1 silencing. Our data indicate that RYR1 undergoes polymorphic, tissue-specific, and developmentally regulated allele silencing and that this unveils recessive mutations in patients with core myopathies. Furthermore, our data suggest that imprinting is a likely mechanism for this phenomenon and that similar mechanisms could play a role in human phenotypic heterogeneity.

HRC is a direct transcriptional target of MEF2 during cardiac, skeletal, and arterial smooth muscle development in vivo

The HRC gene encodes the histidine-rich calcium-binding protein, which is found in the lumen of the junctional sarcoplasmic reticulum (SR) of cardiac and skeletal muscle and within calciosomes of arterial smooth muscle. The expression of HRC in cardiac, skeletal, and smooth muscle raises the possibility of a common transcriptional mechanism governing its expression in all three muscle cell types. In this study, we identified a transcriptional enhancer from the HRC gene that is sufficient to direct the expression of lacZ in the expression pattern of endogenous HRC in transgenic mice. The HRC enhancer contains a small, highly conserved sequence that is required for expression in all three muscle lineages. Within this conserved region is a consensus site for myocyte enhancer factor 2 (MEF2) proteins that we show is bound efficiently by MEF2 and is required for transgene expression in all three muscle lineages in vivo. Furthermore, the entire HRC enhancer sequence lacks any discernible CArG motifs, the binding site for serum response factor (SRF), and we show that the enhancer is not activated by SRF. Thus, these studies identify the HRC enhancer as the first MEF2-dependent, CArG-independent transcriptional target in smooth muscle and represent the first analysis of the transcriptional regulation of an SR gene in vivo.

Binding of kidney nuclear proteins to the 5'-flanking region of the rat gene for Ca2+-binding protein regucalcin: involvement of Ca2+/calmodulin signaling

Ca2+-binding protein regucalcin is expressed in the kidney cortex of rats, as assayed by Northern blot analysis. The existence of kidney nuclear factor which binds to the 5'-flanking region of the rat regucalcin gene was investigated. When nuclear extracts obtained from the kidney cortex of rats were used in gel mobility-shift assays, two protein-DNA complexes were uniquely formed with the DNA fragment containing the 5'-flanking region of the rat regucalcin gene. Competition gel shift experiments indicated the specific binding region of kidney cortex nuclear proteins in the 5'-flanking region of the rat regucalcin gene. The two nuclear protein-DNA complexes were formed with the same mobility in rat kidney cortex and liver, which possess detectable amounts of regucalcin mRNA in Northern blot analysis. The binding activities of nuclear factors from kidney cortex to the 5'-flanking region of the rat regucalcin gene were inhibited by a single intraperitoneal administration of trifluoperazine, an antagonist of calmodulin, to rats. The present study demonstrates that kidney cortex nuclear proteins specifically bind to the 5'-flanking region of the rat regucalcin gene, and that the binding activity may be partly mediated through the Ca2+/calmodulin-dependent process.

The structure, function, and cellular regulation of ryanodine-sensitive Ca2+ release channels

The fundamental biological process of Ca2+ signaling is known to be important in most eukaryotic cells, and inositol 1,2,5-trisphosphate and ryanodine receptors, intracellular Ca2+ release channels encoded by two distantly related gene families, are central to this phenomenon. Ryanodine receptors in the sarcoplasmic reticulum of skeletal and cardiac muscle have a predominant role in excitation-contraction coupling, but the channels are also present in the endoplasmic reticulum of noncontractile tissues including the central nervous system and the immune system. In all, three highly homologous ryanodine receptor isoforms have been identified, all very large proteins which assemble as (homo)tetramers of approximately 2 MDa. They contain large cytoplasmically disposed regulatory domains and are always associated with other structural or regulatory proteins, including calmodulin and immunophilins, which can have marked effects on channel function. The type 1 isoform in skeletal muscle is electromechanically coupled to surface membrane voltage sensors, whereas the remaining isoforms appear to be activated solely by endogenous cytoplasmic second messengers or other ligands, including Ca2+ itself ("Ca(2+)-induced Ca2+ release"). This review concentrates on ryanodine receptor structure-function relationships as probed by a variety of methods and on the molecular mechanisms of channel modulation at the cellular level (including evidence for the regulation of gene expression and transcription). It also touches on the relevance of ryanodine receptors to complex cellular functions and disease.

Tissue-specific binding of nuclear factors to the 5'-flanking region of the rat gene for calcium-binding protein regucalcin

The existence of nuclear factors which bind to the 5'-flanking region of calcium-binding protein regucalcin gene in rats was investigated. We previously reported that rat regucalcin mRNA is expressed in a highly tissue-specific manner; the mRNA was mainly present in the liver but only slightly in the kidney. When the nuclear proteins extracted from the liver and kidney of rats were used in the gel mobility shift assays, a protein-DNA complex was uniquely formed with the DNA fragment containing the upstream region from the first exon of rat regucalcin gene. On the other hand, this complex was not found by using the nuclear extracts from rat brain, spleen, and heart. The nuclear proteins of these extracts, however, could specifically bind to the DNA fragment containing the first exon region of rat regucalcin gene, although Northern blot analysis did not show detectable amount of regucalcin mRNA levels in rat brain, spleen, and heart. The present study demonstrates that the existence of nuclear protein components which bind to the regucalcin gene. These identified components may be involved in the tissue-specific regulation of regucalcin gene expression.

Cloning and characterization of the 5'-upstream regulatory region of the Ca(2+)-release channel gene of cardiac sarcoplasmic reticulum

To elucidate the transcriptional regulation mechanism for the Ca(2+)-release channel gene of the cardiac sarcoplasmic reticulum (RYR2), we isolated and analyzed the 5'-upstream flanking region of the gene. Sequence analysis indicated that the core promoter region lacks canonical TATA and CAAT boxes, but contains three overlapping GC boxes. A gel shift assay indicated that Sp1 binds to the region containing the GC boxes. Different 5'-deletion constructs in the 5'-flanking region of the RYR2 gene were fused to the luciferase gene, and their promoter activity in rat neonatal cardiac myocytes was subsequently determined. The results revealed the presence of a region containing positive regulatory elements in the 5'-flanking region. Analyses of substitutional mutations introduced into the GC boxes and the regulatory region indicated that in addition to the GC box located at -56 to -51, two regulatory elements (RYR2P1 and RYR2P2) are essential for the promoter activity. These results indicated that Sp1 and transcription factors that bind to RYR2P1 and RYR2P2 cooperatively enhance the expression of the RYR2 gene. In a transient transfection experiment involving promoter-luciferase gene constructs in skeletal muscle cells, we identified a negative regulatory region between positions -209 and -90 that represses the expression of the RYR2 gene in skeletal muscle cells.