GO:0097540 axonemal central pair: Components, Assembly and Research Methods, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0097540 axonemal central pair is the inner two microtubule doublets of the 9+2 axoneme in most motile cilia.
• The central pair is a structural and regulatory hub that patterns outer doublet microtubules and coordinates ciliary beating.
• Mutations in central pair genes such as HYDIN, DNHD1, CFAP221, CFAP54, and SPEF2 cause primary ciliary dyskinesia and asthenoteratozoospermia [2,3,6,7].
• Central pair proteins including CCDC176 and SPEF1 stabilize specific microtubule doublets and the microtubule seam [5,8].
• CRISPR knockout, point mutation, knock-in, and overexpression models are essential to dissect central pair gene function in cilia and flagella [2,5,7].
• EDITGENE provides end-to-end CRISPR cell model and library screening services to accelerate central pair research.
Description
The axonemal central pair (GO:0097540) is a conserved structural module of the 9+2 axoneme, consisting of two singlet microtubules that lie at the core of motile cilia and flagella. This central pair is not merely a passive scaffold; it is a key regulator of dynein-driven motility and of the precise patterning of the nine outer doublet microtubules. Because the central pair is essential for normal ciliary and flagellar beating, its dysfunction is increasingly linked to human disease, including primary ciliary dyskinesia (PCD) and male infertility [2,3,6]. Understanding the molecular composition and assembly of the central pair is therefore a high-priority goal in cilia biology and translational medicine. Recent structural and genetic studies have begun to reveal how central pair proteins such as HYDIN, DNHD1, CCDC176, and SPEF1 contribute to axoneme stability and function [2,3,5,8]. This article integrates authoritative GO annotation with verified PubMed literature to provide a research-grade overview of the axonemal central pair, its genes, disease relevance, and the CRISPR-based methods used to study it.
axonemal central pair At A Glance
| GO ID | GO:0097540 |
|---|---|
| GO term | axonemal central pair |
| Ontology | cellular_component |
| Synonym | axonemal microtubule central pair; axoneme central pair; axoneme microtubule central pair; central pair; central-pair microtubules |
| Major function | Structural core of the 9+2 axoneme that patterns outer doublet microtubules and regulates ciliary/flagellar motility |
| Cellular location | Axoneme of motile cilia and flagella |
| Key components | Central pair microtubules and associated proteins including HYDIN, DNHD1, CCDC176, SPEF1, CFAP221, CFAP54, SPEF2 [2,3,5,7,8] |
| Disease relevance | Primary ciliary dyskinesia, asthenoteratozoospermia, and other ciliopathies [2,3,6] |
What Is GO:0097540?
According to the Gene Ontology, GO:0097540 axonemal central pair is defined as the part of the axoneme consisting of the inner two microtubule doublets of the 9+2 axoneme occurring in most motile cilia. In simpler terms, it is the central structural pair of microtubules that runs down the middle of a motile cilium or flagellum, distinct from the nine outer doublets that surround it.
Why Is axonemal central pair Important in Cell Biology?
The axonemal central pair is important because it is required for the normal beating of motile cilia and flagella, which are essential for mucociliary clearance in the airways, sperm motility, and embryonic left-right patterning [1,4]. Disruption of central pair components leads to primary ciliary dyskinesia, a multisystem disorder characterized by chronic respiratory infections, situs inversus, and male infertility [1,3,6]. In addition, central pair defects are a recognized cause of asthenoteratozoospermia in humans and mice. Studying the central pair therefore provides mechanistic insight into ciliary motility and offers direct clinical relevance for diagnosing and potentially treating ciliopathies.
• The central pair is essential for the 9+2 axoneme architecture and for coordinated ciliary beating.
• Mutations in central pair genes such as HYDIN cause primary ciliary dyskinesia [3,6].
• Bi-allelic variants in DNHD1 cause flagellar axoneme defects and asthenoteratozoospermia.
• Central pair proteins CFAP221, CFAP54, and SPEF2 genetically interact in mouse models of PCD.
• CCDC176 stabilizes microtubule doublets 1 and 9 to ensure proper sperm movement.
• SPEF1 binds the microtubule seam and has a unique role at the ciliary tip central pair.
• Central pair dysfunction is linked to male infertility and chronic respiratory disease [1,2].
• The central pair is a target for structural biology, live imaging, and CRISPR-based functional genomics [4,5,8].
Core Biology of the axonemal central pair
Assembly and positioning of the central pair
In simple terms: The central pair is built and placed in the middle of the axoneme during cilia formation.
The axonemal central pair forms as two singlet microtubules that are positioned at the center of the 9+2 axoneme. Structural studies show that the central pair apparatus is a distinct module that patterns the surrounding outer doublet microtubules, ensuring the characteristic 9+2 arrangement. Proper assembly requires central pair-associated proteins, and defects in these proteins can lead to abnormal axoneme structure [2,7].
Structural organization of the central pair
In simple terms: The central pair has a precise shape with specific proteins that hold it together.
Cryo-electron microscopy has revealed that the ciliary central apparatus has a defined architecture with repeating structural elements that interact with the outer doublets. The central pair microtubules are associated with accessory proteins that form projections and bridges, contributing to the overall stability of the axoneme [4,8]. The microtubule seam of the central pair is bound by SPEF1, which plays a unique role at the ciliary tip.
Molecular components and interactions
In simple terms: Many proteins work together to build and maintain the central pair.
Key central pair proteins include HYDIN, DNHD1, CCDC176, SPEF1, CFAP221, CFAP54, and SPEF2 [2,3,5,7,8]. HYDIN is a large central pair protein, and heterozygous cis HYDIN mutations cause primary ciliary dyskinesia [3,6]. DNHD1 is required for flagellar axoneme integrity, and its bi-allelic variants cause asthenoteratozoospermia. CCDC176 stabilizes microtubule doublets 1 and 9, which is necessary for proper sperm movement. CFAP221, CFAP54, and SPEF2 show genetic interactions in mouse models of PCD.
Regulation of central pair function
In simple terms: The central pair is controlled by signals that adjust ciliary beating.
The central pair is thought to regulate dynein arm activity and thus ciliary beat frequency and waveform. Although the precise regulatory mechanisms are still being elucidated, structural and genetic evidence indicates that central pair proteins modulate the mechanical output of the axoneme [4,7]. Mutations in central pair genes disrupt this regulation, leading to ciliary dyskinesia and flagellar defects [2,3,6].
Role in ciliary motility and disease
In simple terms: When the central pair is broken, cilia and flagella do not beat properly, causing disease.
Defects in the central pair cause primary ciliary dyskinesia, characterized by impaired mucociliary clearance, chronic respiratory infections, and situs inversus [1,3,6]. In sperm, central pair abnormalities lead to asthenoteratozoospermia and male infertility [2,5]. These clinical phenotypes underscore the importance of the central pair for human health [1,2].
Key Genes Involved in GO:0097540 axonemal central pair
The following genes encode proteins that localize to or are functionally associated with the axonemal central pair, based on verified PubMed literature.
| Gene | Major Role | Research Relevance |
|---|---|---|
| HYDIN | Central pair protein; mutations cause PCD [3,6] | Model for PCD and ciliary dyskinesia [3,6] |
| DNHD1 | Flagellar axoneme integrity; bi-allelic variants cause asthenoteratozoospermia | Model for male infertility and flagellar defects |
| CCDC176 | Stabilizes microtubule doublets 1 and 9 for proper sperm movement | Target for sperm motility studies |
| SPEF1 | Microtubule-seam binding protein at the ciliary tip central pair | Structural studies of central pair tip |
| CFAP221 | Central pair apparatus gene; interacts with CFAP54 and SPEF2 | Mouse models of PCD |
| CFAP54 | Central pair apparatus gene; interacts with CFAP221 and SPEF2 | Mouse models of PCD |
| SPEF2 | Central pair apparatus gene; interacts with CFAP221 and CFAP54 | Mouse models of PCD |
| DNAH5 | Outer dynein arm component; commonly mutated in PCD | Diagnostic marker for PCD |
| DNAI1 | Outer dynein arm component; PCD-associated | Genetic testing for PCD |
| DNAI2 | Outer dynein arm component; PCD-associated | Genetic testing for PCD |
| RSPH1 | Radial spoke head component; PCD-associated | PCD diagnostics |
| RSPH4A | Radial spoke head component; PCD-associated | PCD diagnostics |
| CCDC39 | Dynein regulatory complex; PCD-associated | PCD diagnostics |
| CCDC40 | Dynein regulatory complex; PCD-associated | PCD diagnostics |
| GAS8 | Dynein regulatory complex; PCD-associated | PCD diagnostics |
| OFD1 | Centriole/basal body protein; ciliopathy-associated | Model for oral-facial-digital syndrome |
| HYDIN2 | Central pair protein; related to HYDIN | Comparative studies of central pair |
| SPAG6 | Central pair protein; involved in ciliary motility | Structural and functional studies |
How Is axonemal central pair Regulated?
The axonemal central pair is regulated at multiple levels, including transcriptional control of central pair genes, post-translational modifications, and interactions with other axonemal components [4,7]. Genetic interactions between CFAP221, CFAP54, and SPEF2 suggest a coordinated regulatory network that ensures proper central pair assembly and function. Structural studies indicate that the central pair communicates with outer doublet microtubules to modulate dynein activity and ciliary beating. However, the precise signaling pathways that regulate central pair function remain an active area of research [4,8].
axonemal central pair and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| HYDIN | Primary ciliary dyskinesia [3,6] | Knockout mouse or human airway epithelial cells [3,6] |
| DNHD1 | Asthenoteratozoospermia | Knockout mouse and human sperm analysis |
| CCDC176 | Sperm motility defects | Knockout mouse and flagellar imaging |
| CFAP221 | Primary ciliary dyskinesia | Knockout mouse |
| SPEF2 | Primary ciliary dyskinesia | Knockout mouse |
Primary ciliary dyskinesia (PCD)
Primary ciliary dyskinesia is a genetically heterogeneous disorder caused by defects in motile cilia, including central pair abnormalities [1,3,6]. Heterozygous cis HYDIN mutations have been identified in PCD patients, linking central pair dysfunction to chronic respiratory disease and situs inversus [3,6]. Mouse models with mutations in central pair genes CFAP221, CFAP54, and SPEF2 recapitulate PCD phenotypes, providing functional evidence for their role in disease.
Asthenoteratozoospermia and male infertility
Bi-allelic variants in DNHD1 cause flagellar axoneme defects and asthenoteratozoospermia in humans and mice. CCDC176 is required for stabilizing microtubule doublets 1 and 9, and its loss leads to abnormal sperm movement. These findings establish the central pair as a critical determinant of sperm motility and male fertility [2,5].
Other ciliopathies
Central pair defects may contribute to a broader spectrum of ciliopathies, including those affecting left-right asymmetry and embryonic development [1,4]. The structural and functional conservation of the central pair across species makes it a valuable model for understanding ciliary disease mechanisms [4,8].
From axonemal central pair-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| Does loss of a central pair gene cause ciliary dyskinesia? | CRISPR knockout in human airway epithelial cells or mouse [2,7] |
| Does a specific point mutation in HYDIN cause PCD? | CRISPR point mutation knock-in in cell lines or mouse |
| How does a central pair protein localize within the axoneme? | Tagged knock-in with fluorescent protein [4,8] |
| Does overexpression of a central pair gene rescue motility defects? | CRISPR overexpression in mutant cells |
| What are the genetic interactions among central pair genes? | Double knockout models |
| Can central pair gene variants be validated for clinical relevance? | Patient-derived cells and organoids [1,2] |
How to Study the axonemal central pair Process
| Method | What It Measures | Typical Application |
|---|---|---|
| CRISPR knockout screen | Gene essentiality for ciliary function | Discovery of central pair genes [2,7] |
| Cryo-electron microscopy | High-resolution structure of central pair | Structural biology [4,8] |
| Fluorescence microscopy | Protein localization and ciliary beating | Functional validation |
| RNA-seq | Transcriptional changes in mutants | Pathway analysis |
| Proteomics | Protein interactions and abundance | Network discovery |
| High-speed video microscopy | Ciliary beat frequency and pattern | PCD diagnostics |
| Sperm motility analysis | Flagellar function | Male infertility studies [2,5] |
| Genotyping/PCR | Mutation validation | Clinical genetics [3,6] |
CRISPR knockout screens
CRISPR knockout screens can identify genes required for central pair assembly and ciliary motility. Pooled sgRNA libraries targeting cilia-related genes, followed by high-content imaging or sequencing, enable unbiased discovery of central pair components [2,7].
High-resolution imaging
Cryo-electron microscopy and fluorescence microscopy are used to visualize the central pair structure and its interactions with outer doublets [4,8]. Live imaging of ciliary beating in mutant cells provides functional readouts.
Transcriptomics and proteomics
RNA-seq and proteomics can reveal expression changes and protein interactions in central pair mutants. These approaches help define the molecular network centered on the central pair.
Functional assays for ciliary motility
High-speed video microscopy and sperm motility analysis are standard assays to assess the functional consequences of central pair gene mutations [2,5].
How CRISPR Can Be Used to Study GO:0097540 axonemal central pair
Knockout
CRISPR knockout of central pair genes such as DNHD1 or CFAP221 in cell lines or mouse models recapitulates axoneme defects and ciliary dyskinesia, providing causal evidence for gene function [2,7].
Point Mutation
CRISPR point mutation knock-in can model patient-specific variants, such as HYDIN mutations found in PCD, to assess their impact on central pair assembly and function [3,6].
Knock-in
Tagged knock-in of central pair proteins with fluorescent or affinity tags enables live imaging and proteomic analysis of the central pair apparatus [4,8].
Overexpression
Overexpression of central pair genes can rescue loss-of-function phenotypes or reveal dominant-negative effects, helping to dissect gene dosage requirements.
How EDITGENE Supports axonemal central pair Research
Researchers studying axonemal central pair-related genes often need to determine whether a candidate gene is causally involved in ciliary or flagellar dysfunction. EDITGENE provides a comprehensive suite of CRISPR services to generate precisely engineered cell and animal models, enabling rigorous functional validation of central pair genes.
Contact EDITGENE today to design your custom CRISPR model for axonemal central pair research.
Frequently Asked Questions About axonemal central pair
What is the axonemal central pair?
The axonemal central pair (GO:0097540) is the inner two microtubule doublets of the 9+2 axoneme in most motile cilia, essential for ciliary beating.
What genes are involved in the axonemal central pair?
Key genes include HYDIN, DNHD1, CCDC176, SPEF1, CFAP221, CFAP54, and SPEF2 [2,3,5,7,8].
What diseases are associated with axonemal central pair defects?
Primary ciliary dyskinesia and asthenoteratozoospermia are linked to central pair gene mutations [2,3,6].
How is the axonemal central pair structured?
It consists of two singlet microtubules with associated proteins that form a defined architecture, as revealed by cryo-electron microscopy [4,8].
What is the role of HYDIN in the central pair?
HYDIN is a central pair protein; heterozygous cis HYDIN mutations cause primary ciliary dyskinesia [3,6].
How does DNHD1 affect sperm motility?
Bi-allelic variants in DNHD1 cause flagellar axoneme defects and asthenoteratozoospermia.
What is the function of CCDC176 in the central pair?
CCDC176 stabilizes microtubule doublets 1 and 9 to ensure proper sperm movement.
How can CRISPR be used to study central pair genes?
CRISPR knockout, point mutation, knock-in, and overexpression models allow functional dissection of central pair genes in cilia and flagella [2,5,7].
What model systems are used for central pair research?
Human airway epithelial cells, mouse models, and sperm cells are commonly used [1,2,7].
What methods are used to study the central pair?
Cryo-electron microscopy, fluorescence imaging, CRISPR screens, RNA-seq, and proteomics are key methods [4,7,8].
Conclusion
The axonemal central pair (GO:0097540) is a fundamental component of motile cilia and flagella, with critical roles in ciliary beating and human health. Mutations in central pair genes such as HYDIN, DNHD1, and CCDC176 lead to primary ciliary dyskinesia and male infertility, underscoring the clinical importance of this structure [2,3,5,6]. Continued research using CRISPR-based models and advanced imaging will further elucidate the molecular mechanisms of central pair assembly and function, potentially leading to new therapeutic strategies for ciliopathies.
References
- 1. Adam MP et al.. 1993. Primary Ciliary Dyskinesia.. PMID: 20301301
- 2. Tan C et al.. 2022. Bi-allelic variants in DNHD1 cause flagellar axoneme defects and asthenoteratozoospermia in humans and mice.. Am J Hum Genet 109(1):157-171 PMID: 34932939
- 3. Suryadinata R et al.. 2025. Heterozygous cis HYDIN mutations cause primary ciliary dyskinesia.. Med 6(1):100508 PMID: 39317196
- 4. Gui M et al.. 2022. Ciliary central apparatus structure reveals mechanisms of microtubule patterning.. Nat Struct Mol Biol 29(5):483-492 PMID: 35578023
- 5. Liu C et al.. 2023. CCDC176 stabilizes microtubule doublets 1 and 9 to ensure proper sperm movement.. Curr Biol 33(16):3371-3388.e7 PMID: 37494937
- 6. Dutcher SK et al.. 2020. HY-DIN' in the Cilia: Discovery of Central Pair-related Mutations in Primary Ciliary Dyskinesia.. Am J Respir Cell Mol Biol 62(3):281-282 PMID: 31604022
- 7. McKenzie CW et al.. 2020. Genetic interaction between central pair apparatus genes CFAP221, CFAP54, and SPEF2 in mouse models of primary ciliary dyskinesia.. Sci Rep 10(1):12337 PMID: 32704025
- 8. Legal T et al.. 2025. Structure of the ciliary tip central pair reveals the unique role of the microtubule-seam binding protein SPEF1.. Curr Biol 35(14):3404-3417.e6 PMID: 40651469
