GO:0014801 longitudinal sarcoplasmic reticulum: Components, Assembly and Research Methods, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0014801 longitudinal sarcoplasmic reticulum is the portion of the free sarcoplasmic reticulum consisting of longitudinal tubules that connect terminal cisternae.
• It is a cellular_component term in the Gene Ontology and is distinct from junctional sarcoplasmic reticulum, which faces the T-tubule and contains ryanodine receptors.
• Longitudinal sarcoplasmic reticulum is enriched in SERCA pumps and calcium buffer proteins such as calsequestrin and sarcalumenin, supporting Ca2+ uptake and storage.
• Structural and biochemical studies have characterized longitudinal tubules from fast skeletal muscle and heart, showing they can be separated from junctional membranes.
• Altered longitudinal sarcoplasmic reticulum structure is linked to cylindrical spirals in skeletal muscle pathology and to mitochondria-SR tethering in cardiac remodeling.
• Research on this compartment uses electron microscopy, subcellular fractionation, Ca2+ kinetics, and CRISPR-based models of SR proteins.
Description
The longitudinal sarcoplasmic reticulum (GO:0014801) is a specialized subdomain of the sarcoplasmic reticulum (SR) in muscle cells, defined as the portion of the free SR consisting of longitudinal tubules that connect terminal cisternae. This compartment forms a network of tubules that run along the myofibrils and is distinct from the junctional SR, which is docked at the T-tubule and enriched in ryanodine receptors. Understanding the longitudinal SR is essential because it is the primary site of calcium reuptake and storage, processes that determine muscle relaxation and the refilling of intracellular Ca2+ stores. Biochemical fractionation of muscle homogenates has allowed researchers to separate longitudinal tubules from junctional SR, revealing that the longitudinal SR is enriched in the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) and in luminal Ca2+ buffer proteins such as calsequestrin and sarcalumenin. Rapid kinetic studies of vesicular fragments have further shown that active transport and passive release of calcium are functionally partitioned between longitudinal and junctional SR membranes. These findings established the longitudinal SR as a distinct functional entity rather than a simple extension of the junctional SR. In addition to its role in Ca2+ handling, the longitudinal SR is increasingly recognized as a dynamic structure that can undergo remodeling in disease. For example, cylindrical spirals in skeletal muscle have been shown to originate from the longitudinal SR, and enhanced mitochondria-SR tethering triggers adaptive cardiac muscle remodeling. This article summarizes the definition, composition, molecular mechanisms, disease links, and research methods relevant to GO:0014801, with a focus on how CRISPR-based models can be used to study its components.
longitudinal sarcoplasmic reticulum At A Glance
| GO ID | GO:0014801 |
|---|---|
| GO term | longitudinal sarcoplasmic reticulum |
| Ontology | cellular_component |
| Synonym | none listed |
| Definition | The portion of the free sarcoplasmic reticulum consisting of longitudinal tubules that connect terminal cisternae. |
| Major function | Calcium uptake and storage via SERCA pumps and luminal Ca2+ buffer proteins. |
| Distinct from | Junctional sarcoplasmic reticulum, which faces the T-tubule and contains ryanodine receptors. |
| Tissue context | Skeletal muscle and cardiac muscle. |
| Key proteins | SERCA, calsequestrin, sarcalumenin, and other SR Ca2+ handling proteins. |
What Is GO:0014801?
GO:0014801 longitudinal sarcoplasmic reticulum is defined in the Gene Ontology as the portion of the free sarcoplasmic reticulum consisting of longitudinal tubules that connect terminal cisternae. In other words, it is the tubular network of the SR that lies between the junctional SR cisternae, forming the main body of the free SR. It is a cellular_component term and has no listed synonyms in QuickGO. Functionally, it is the compartment where SERCA-mediated Ca2+ uptake and luminal Ca2+ buffering occur, and it is structurally and biochemically distinct from the junctional SR.
Why Is longitudinal sarcoplasmic reticulum Important in Cell Biology?
The longitudinal sarcoplasmic reticulum is important because it is the main site of calcium reuptake and storage in muscle cells, and its function directly determines the speed and completeness of muscle relaxation. Defects in its protein composition or structural integrity can impair Ca2+ homeostasis, contributing to muscle weakness, arrhythmias, and other pathologies. Because it is a distinct subdomain, it also provides a tractable system for studying how membrane subdomains are assembled and maintained in excitable cells.
• It is the primary site of SERCA-mediated Ca2+ uptake, which is required for muscle relaxation.
• It stores Ca2+ via luminal buffer proteins such as calsequestrin and sarcalumenin.
• It is structurally distinct from junctional SR, allowing functional partitioning of Ca2+ release and uptake.
• Alterations in its structure are associated with cylindrical spirals in skeletal muscle pathology.
• Enhanced mitochondria-SR tethering involving this compartment can trigger adaptive cardiac remodeling.
• It is a model system for studying membrane subdomain assembly in muscle.
• Its protein composition can be analyzed by subcellular fractionation and proteomics.
• It is relevant to diseases of skeletal and cardiac muscle, including myopathies and arrhythmias.
• CRISPR-based models of SR proteins can be used to dissect its function.
• It provides a target for therapeutic strategies aimed at improving Ca2+ handling.
What Happens During longitudinal sarcoplasmic reticulum?
Calcium uptake by SERCA pumps
In simple terms: The longitudinal SR uses pumps to pull calcium back into the SR, which helps the muscle relax.
The longitudinal sarcoplasmic reticulum is enriched in sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) pumps, which actively transport Ca2+ from the cytosol into the SR lumen. Rapid kinetic studies of vesicular fragments have shown that active transport of calcium is a key function of longitudinal SR membranes. This uptake is essential for terminating the Ca2+ signal and allowing muscle relaxation.
Calcium storage and buffering
In simple terms: Inside the longitudinal SR, proteins bind calcium so it can be stored and released later.
The lumen of the longitudinal SR contains Ca2+ buffer proteins such as calsequestrin and sarcalumenin, which bind Ca2+ and facilitate its storage. Sarcalumenin is a luminal glycoprotein that has been proposed to buffer Ca2+ and modulate SERCA activity in skeletal muscle. These buffers help maintain a high Ca2+ concentration in the SR lumen, which is necessary for rapid release during contraction.
Connection to terminal cisternae
In simple terms: The longitudinal tubules connect to the terminal cisternae, forming a continuous SR network.
By definition, the longitudinal sarcoplasmic reticulum consists of tubules that connect terminal cisternae. This continuity allows Ca2+ taken up by the longitudinal SR to diffuse to the junctional SR, where it can be released through ryanodine receptors. The structural link between longitudinal and junctional SR is therefore critical for efficient Ca2+ cycling.
Structural remodeling in disease
In simple terms: The shape of the longitudinal SR can change in disease, forming abnormal structures.
In pathological conditions, the longitudinal SR can undergo structural remodeling. For example, cylindrical spirals in skeletal muscle have been shown to originate from the longitudinal sarcoplasmic reticulum. In cardiac muscle, enhanced mitochondria-SR tethering can trigger adaptive remodeling. These observations indicate that the longitudinal SR is not a static structure but can respond to cellular stress.
Key Genes Involved in GO:0014801 longitudinal sarcoplasmic reticulum
The following genes and proteins are key components or regulators of the longitudinal sarcoplasmic reticulum and its functions.
| Gene | Major Role | Research Relevance |
|---|---|---|
| ATP2A1 | SERCA1 Ca2+-ATPase; fast-twitch skeletal muscle | Mediates Ca2+ uptake into longitudinal SR |
| ATP2A2 | SERCA2 Ca2+-ATPase; cardiac and slow-twitch muscle | Mediates Ca2+ uptake into longitudinal SR |
| CASQ1 | Calsequestrin 1; luminal Ca2+ buffer in skeletal muscle | Stores Ca2+ in SR lumen |
| CASQ2 | Calsequestrin 2; luminal Ca2+ buffer in cardiac muscle | Stores Ca2+ in SR lumen |
| SAR | Sarcalumenin; luminal Ca2+ buffer glycoprotein | Modulates Ca2+ storage and SERCA activity |
| RYR1 | Ryanodine receptor 1; junctional SR Ca2+ release channel | Connects junctional SR to longitudinal SR function |
| RYR2 | Ryanodine receptor 2; cardiac junctional SR Ca2+ release channel | Connects junctional SR to longitudinal SR function |
| CALR | Calreticulin; Ca2+-binding chaperone | May influence SR Ca2+ handling |
| TRDN | Triadin; junctional SR protein | Links junctional SR proteins to Ca2+ release |
| JPH1 | Junctophilin 1; skeletal muscle | Maintains junctional SR-T-tubule contact |
| JPH2 | Junctophilin 2; cardiac muscle | Maintains junctional SR-T-tubule contact |
| ASPH | Aspartyl beta-hydroxylase; junctional SR protein | Organizes junctional SR proteins |
| HRC | Histidine-rich calcium-binding protein; luminal SR | Modulates Ca2+ storage and release |
| ATP2A3 | SERCA3; ubiquitous Ca2+-ATPase | May contribute to Ca2+ uptake in some tissues |
| PLN | Phospholamban; SERCA regulator | Regulates SERCA activity in cardiac muscle |
| SLN | Sarcolipin; SERCA regulator | Regulates SERCA activity in skeletal muscle |
| FKBP1A | FKBP12; ryanodine receptor modulator | Modulates Ca2+ release from junctional SR |
How Is longitudinal sarcoplasmic reticulum Regulated?
The longitudinal sarcoplasmic reticulum is regulated at multiple levels. SERCA activity is modulated by phospholamban and sarcolipin, which can inhibit or uncouple the pump depending on phosphorylation status. Luminal Ca2+ buffer proteins such as calsequestrin and sarcalumenin influence the free Ca2+ concentration inside the SR and can feedback on SERCA function. In addition, structural interactions with mitochondria can regulate SR Ca2+ handling; enhanced mitochondria-SR tethering triggers adaptive cardiac muscle remodeling. These regulatory mechanisms ensure that Ca2+ uptake and storage are matched to cellular demand.
longitudinal sarcoplasmic reticulum and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| CASQ1 | Skeletal muscle myopathy; Ca2+ storage defect | Casq1 knockout mouse or CRISPR KO in C2C12 cells |
| SAR | Skeletal muscle disease; impaired Ca2+ buffering | Sar knockout mouse or CRISPR KO in myotubes |
| ATP2A1 | Brody myopathy; impaired muscle relaxation | Atp2a1 knockout or point mutation in muscle cells |
| ATP2A2 | Cardiac dysfunction; arrhythmia | Atp2a2 knockout or knock-in in cardiomyocytes |
| RYR1 | Malignant hyperthermia; central core disease | RYR1 point mutation knock-in in muscle cells |
Skeletal muscle myopathies
Alterations in the longitudinal sarcoplasmic reticulum have been observed in skeletal muscle pathology. Cylindrical spirals, which are abnormal structures in muscle fibers, have been shown to originate from the longitudinal sarcoplasmic reticulum. Mutations in SR proteins such as calsequestrin 1 and sarcalumenin can impair Ca2+ handling and contribute to myopathy. These findings link the structural integrity of the longitudinal SR to muscle health.
Cardiac arrhythmias and remodeling
In cardiac muscle, the longitudinal SR is part of a continuous network with junctional SR, and its function is critical for normal excitation-contraction coupling. Enhanced mitochondria-SR tethering can trigger adaptive cardiac muscle remodeling, suggesting that changes in SR structure contribute to cardiac adaptation. Dysregulation of Ca2+ uptake or storage in the longitudinal SR can promote arrhythmias and heart failure.
Calcium-handling disorders
Because the longitudinal SR is the primary site of Ca2+ storage, defects in its buffer proteins or pumps can lead to calcium-handling disorders. For example, loss of sarcalumenin function has been proposed to affect skeletal muscle health and disease. Similarly, altered expression of SERCA pumps can impair muscle relaxation and contribute to pathological conditions.
From longitudinal sarcoplasmic reticulum-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| What is the role of SERCA1 in longitudinal SR Ca2+ uptake? | ATP2A1 knockout in skeletal muscle cells |
| How does calsequestrin 1 affect Ca2+ storage? | CASQ1 knockout or overexpression in myotubes |
| Does sarcalumenin modulate SERCA activity? | SAR knockout or tagged knock-in in skeletal muscle cells |
| How does mitochondria-SR tethering affect cardiac remodeling? | Knock-in of tethering proteins in cardiomyocytes |
| What is the structural origin of cylindrical spirals? | Knockout of SR structural proteins in muscle cells |
| How do junctional SR proteins organize the longitudinal SR? | Knockout of TRDN, JPH1, or ASPH in muscle cells |
How to Study the longitudinal sarcoplasmic reticulum Process
| Method | What It Measures | Typical Application |
|---|---|---|
| Electron microscopy | Ultrastructure of longitudinal SR tubules | Visualizing SR network and cylindrical spirals |
| Subcellular fractionation | Separation of longitudinal and junctional SR | Biochemical characterization of SR domains |
| Ca2+ kinetics assays | Active transport and passive release of Ca2+ | Functional analysis of SERCA and buffers |
| Western blotting | Protein enrichment in SR fractions | Validation of fraction purity and protein levels |
| Immunofluorescence | Localization of SR proteins | Mapping proteins to longitudinal vs junctional SR |
| Proteomics | Protein composition of SR fractions | Identifying novel longitudinal SR components |
| CRISPR knockout | Loss-of-function effects on SR function | Testing causal roles of SR proteins |
| Live-cell Ca2+ imaging | Dynamic Ca2+ changes in intact cells | Monitoring SR uptake and release |
Electron microscopy and imaging
Electron microscopy is a classic method for visualizing the longitudinal sarcoplasmic reticulum and its connection to terminal cisternae. Immunoelectron microscopy can localize specific proteins such as SERCA and calsequestrin to longitudinal tubules. Live-cell imaging with fluorescent Ca2+ indicators can monitor Ca2+ uptake and release in real time.
Subcellular fractionation and proteomics
Subcellular fractionation allows separation of longitudinal SR from junctional SR, enabling biochemical characterization. Proteomic analysis of these fractions can identify enriched proteins and post-translational modifications. Western blotting with domain-specific markers confirms the purity of the fractions.
Calcium kinetics assays
Rapid kinetic assays using vesicular fragments of longitudinal SR can measure active Ca2+ transport and passive release. These assays provide quantitative parameters such as uptake rates and leak rates. They are useful for comparing wild-type and mutant SR proteins.
CRISPR-based genetic models
CRISPR-Cas9 knockout, point mutation, knock-in, and overexpression models can be used to study the function of longitudinal SR proteins in muscle cells. These models allow causal testing of candidate genes in Ca2+ handling and SR structure. Combined with imaging and Ca2+ assays, they provide a powerful platform for dissecting longitudinal SR biology.
How CRISPR Can Be Used to Study GO:0014801 longitudinal sarcoplasmic reticulum
Knockout
CRISPR knockout of genes encoding longitudinal SR proteins such as ATP2A1, CASQ1, or SAR can reveal their roles in Ca2+ uptake and storage. Knockout models are useful for testing whether a protein is required for normal SR structure and function. These models can be generated in muscle cell lines or primary myotubes.
Point Mutation
CRISPR point mutation can introduce disease-associated mutations into SR protein genes to model their effects on longitudinal SR function. For example, mutations in RYR1 or ATP2A1 can be recapitulated to study Ca2+ handling defects. Point mutation models allow precise structure-function analysis.
Knock-in
CRISPR knock-in can be used to tag endogenous SR proteins with fluorescent or affinity tags, enabling live-cell imaging and proteomics. Knock-in of tethering proteins can also be used to study mitochondria-SR interactions. These models preserve endogenous regulation and are valuable for studying longitudinal SR dynamics.
Overexpression
CRISPR-mediated overexpression or cDNA overexpression of SR proteins such as calsequestrin or sarcalumenin can test gain-of-function effects on Ca2+ storage. Overexpression models can reveal whether increased buffer capacity alters SR function. They complement knockout studies to provide a full picture of protein function.
How EDITGENE Supports longitudinal sarcoplasmic reticulum Research
Researchers studying longitudinal sarcoplasmic reticulum-related genes often need to determine whether a candidate gene is causally involved in SR function, Ca2+ handling, or disease. EDITGENE provides a comprehensive suite of CRISPR-based services to generate precisely engineered cell models for such studies.
Contact EDITGENE today to design your custom CRISPR model for longitudinal sarcoplasmic reticulum research.
Frequently Asked Questions About longitudinal sarcoplasmic reticulum
What is the longitudinal sarcoplasmic reticulum?
The longitudinal sarcoplasmic reticulum (GO:0014801) is the portion of the free sarcoplasmic reticulum consisting of longitudinal tubules that connect terminal cisternae.
What genes are involved in the longitudinal sarcoplasmic reticulum?
Key genes include ATP2A1, ATP2A2, CASQ1, CASQ2, SAR, RYR1, RYR2, and PLN, among others.
How is the longitudinal sarcoplasmic reticulum different from junctional SR?
The longitudinal SR consists of tubules that connect terminal cisternae and is enriched in SERCA pumps, while junctional SR faces the T-tubule and contains ryanodine receptors.
What is the function of the longitudinal sarcoplasmic reticulum?
It is the primary site of Ca2+ uptake and storage, mediated by SERCA pumps and luminal buffer proteins such as calsequestrin and sarcalumenin.
Which proteins are enriched in the longitudinal sarcoplasmic reticulum?
SERCA pumps, calsequestrin, sarcalumenin, and other Ca2+ handling proteins are enriched in the longitudinal SR.
How can I study the longitudinal sarcoplasmic reticulum in the lab?
Common methods include electron microscopy, subcellular fractionation, Ca2+ kinetics assays, and CRISPR-based genetic models.
What diseases are linked to the longitudinal sarcoplasmic reticulum?
Skeletal muscle myopathies, cardiac arrhythmias, and calcium-handling disorders have been linked to alterations in this compartment.
Can CRISPR be used to study longitudinal sarcoplasmic reticulum genes?
Yes, CRISPR knockout, point mutation, knock-in, and overexpression models can be used to dissect the function of SR proteins.
What is the role of sarcalumenin in the longitudinal sarcoplasmic reticulum?
Sarcalumenin is a luminal Ca2+ buffer glycoprotein that may modulate SERCA activity and Ca2+ storage in skeletal muscle.
Where is the longitudinal sarcoplasmic reticulum located?
It is located in muscle cells, forming a network of tubules that connect terminal cisternae and run along myofibrils.
Conclusion
The longitudinal sarcoplasmic reticulum (GO:0014801) is a distinct subdomain of the SR that is essential for Ca2+ uptake and storage in muscle cells. Its protein composition, including SERCA pumps and luminal buffers, supports efficient Ca2+ cycling, and its structural integrity is linked to muscle health and disease. Continued research using advanced imaging, proteomics, and CRISPR-based models will further clarify its roles in physiology and pathology.
References
- 1. Okura GC et al.. 2024. Calreticulin-Enigmatic Discovery.. Biomolecules 14(7) PMID: 39062580
- 2. Nichtová Z et al.. 2023. Enhanced Mitochondria-SR Tethering Triggers Adaptive Cardiac Muscle Remodeling.. Circ Res 132(11):e171-e187 PMID: 37057625
- 3. Inesi G. 1988. Rapid kinetic characterization of active transport and passive release of calcium in vesicular fragments of longitudinal and junctional sarcoplasmic reticulum.. Braz J Med Biol Res 21(6):1241-9 PMID: 2977952
- 4. Conte E et al.. 2023. Sarcoplasmic Reticulum Ca(2+) Buffer Proteins: A Focus on the Yet-To-Be-Explored Role of Sarcalumenin in Skeletal Muscle Health and Disease.. Cells 12(5) PMID: 36899851
- 5. Barone V et al.. 2015. Organization of junctional sarcoplasmic reticulum proteins in skeletal muscle fibers.. J Muscle Res Cell Motil 36(6):501-15 PMID: 26374336
- 6. Chu A et al.. 1987. Preparation and characterization of longitudinal tubules of sarcoplasmic reticulum from fast skeletal muscle.. Arch Biochem Biophys 258(1):13-23 PMID: 2444161
- 7. Xu JW et al.. 2016. Cylindrical Spirals in Skeletal Muscles Originate From the Longitudinal Sarcoplasmic Reticulum.. J Neuropathol Exp Neurol 75(2):148-55 PMID: 26733584
- 8. Inui M et al.. 1988. Characterization of junctional and longitudinal sarcoplasmic reticulum from heart muscle.. J Biol Chem 263(22):10843-50 PMID: 2968985
