CCDC146

CCDC146 is a conserved coiled-coil domain protein that functions as a microtubule-associated structural and regulatory component of motile cilia/flagella and centrosomes. In axonemes, CCDC146 (ortholog of Chlamydomonas MBO2) associates tightly with doublet microtubules and forms a heterodimeric L-shaped complex with FAP58/CCDC147 that stabilizes inner dynein arm b assembly and connects it to radial spokes and the dynein regulatory complex, thereby controlling ciliary waveform (PMID:11977094, PMID:38568782, PMID:38441556). CCDC146 also interacts with intraflagellar transport components IFT20 and IFT88, and its loss destabilizes these proteins, disrupting sperm flagellum biogenesis, manchette formation, and head–tail coupling, causing multiple morphological abnormalities of the sperm flagella (MMAF)-type male infertility in both knockout mice and a human patient carrying a homozygous CCDC146 nonsense mutation (PMID:38038747, PMID:39245651). In somatic cells, CCDC146 localizes to the centrosome and mitotic microtubule-organizing structures, and its dysregulation at primary cilia in motor neurons has been linked to neurodegeneration-associated ciliary defects (PMID:25074808, PMID:38441556).

1. 2002 High

   The foundational question of what MBO2/CCDC146 encodes and where it acts was answered: cloning of Chlamydomonas MBO2 revealed a coiled-coil axonemal protein tightly bound to doublet microtubules along the entire flagellum, required for waveform conversion from asymmetric ciliary to symmetric flagellar beating.

   Evidence Gene cloning, antibody-based axoneme fractionation, and mbo2 mutant phenotypic analysis in Chlamydomonas

   PMID:11977094

   Open questions at the time

   • Binding partners within the axoneme were unknown
   • No mammalian functional data existed
   • Mechanism by which MBO2 controls waveform was unresolved
2. 2014 Medium

   The question of whether CCDC146 has a centrosomal role in mammalian cells was addressed: proteomic identification from sperm centrioles and immunofluorescence confirmed centrosomal localization in cultured mammalian cells.

   Evidence Mass spectrometry of isolated sperm centrioles followed by immunofluorescence in cultured cells

   PMID:25074808

   Open questions at the time

   • No loss-of-function data in mammalian systems
   • Centrosomal function beyond localization was undefined
   • Relationship to axonemal role characterized in Chlamydomonas was unclear
3. 2018 Medium

   The question of whether CCDC146 participates in pathogen–host interactions was partially answered: CCDC146 is recruited from the centrosome to the Chlamydia trachomatis inclusion membrane via interaction with CT288, revealing that a pathogen can co-opt this centrosomal protein.

   Evidence Yeast two-hybrid screen and co-immunoprecipitation in mammalian cells with fluorescence microscopy of infected versus uninfected cells

   PMID:30094225

   Open questions at the time

   • Functional consequence of CCDC146 recruitment for infection outcome was not established
   • Whether other centrosomal proteins are similarly recruited was not tested
   • No in vivo infection model was used
4. 2023 High

   The central question of mammalian in vivo function was resolved: Ccdc146 knockout mice are completely infertile with MMAF-like defects, and CCDC146 physically interacts with IFT20, IFT88, CCDC38, and CCDC42, placing it in the intraflagellar transport pathway essential for sperm flagellum biogenesis.

   Evidence Ccdc146 knockout mouse model, co-immunoprecipitation of multiple partners, western blot showing reduced IFT/ODF2 protein levels, TEM of flagellar ultrastructure

   PMID:38038747

   Open questions at the time

   • Structural basis of CCDC146–IFT interactions was not determined
   • Whether CCDC146 functions in non-sperm motile cilia in mammals was untested
   • Relationship to inner dynein arm regulation known from Chlamydomonas was not examined
5. 2024 High

   The structural mechanism was elucidated: cryo-electron tomography revealed that CCDC146/MBO2 and FAP58/CCDC147 form a conserved L-shaped heterodimer within the axonemal repeat that physically bridges inner dynein arm b to radial spokes and the dynein regulatory complex, explaining how CCDC146 loss alters waveform and reduces inner arm b assembly.

   Evidence Cryo-electron tomography, comparative proteomics, and Chlamydomonas mbo2 mutant analysis with epitope tagging

   PMID:38568782

   Open questions at the time

   • Atomic-resolution structure of the CCDC146–CCDC147 heterodimer was not obtained
   • Whether inner arm b assembly defect is direct or indirect was not fully resolved
   • Conservation of the L-shaped complex structure in mammalian sperm was inferred but not directly visualized
6. 2024 High

   The question of precise sub-axonemal localization in mammals was answered: expansion microscopy and sarkosyl fractionation showed CCDC146 associates with axonemal microtubule doublets (not centrioles) in spermatozoa and localizes to centrosomes and mitotic microtubule structures in somatic cells, functioning as a microtubule inner protein.

   Evidence Ccdc146 knockout mouse, expansion microscopy, sarkosyl solubilization of microtubule doublets, cryo-electron microscopy

   PMID:38441556

   Open questions at the time

   • Identity of the specific MIP site within the doublet lattice in mammals was not resolved at atomic level
   • Whether CCDC146 has a functional role at somatic centrosomes beyond localization remained open
7. 2024 High

   A direct human disease link was established: a homozygous nonsense mutation in CCDC146 (p.Arg306*) causes oligoasthenoteratozoospermia in a human patient, and a cognate knock-in mouse recapitulates infertility with loss of the CCDC146–IFT20 interaction leading to IFT20 degradation and reduced axonemal dynein levels.

   Evidence Human genetic mutation identification, knock-in mouse model, co-immunoprecipitation showing loss of IFT20 interaction with truncated protein

   PMID:39245651

   Open questions at the time

   • Prevalence of CCDC146 mutations in MMAF patient cohorts was not broadly surveyed
   • Whether the truncated protein exerts dominant-negative effects was not assessed
   • Mechanism linking IFT20 degradation to reduced axonemal dynein assembly was not fully dissected
8. 2025 Medium

   The functional severity of CCDC146 loss was contextualized: among four MMAF gene knockouts, Ccdc146−/− sperm exhibit the most severe nuclear compaction and chromosomal defects, with the lowest developmental potential after ICSI, indicating CCDC146 loss affects not only flagellar but also nuclear integrity.

   Evidence Comparative analysis of four KO mouse lines with ICSI embryo development assay and nuclear morphology analysis

   PMID:40070084

   Open questions at the time

   • Mechanism by which CCDC146 loss causes nuclear defects is unknown
   • Whether the nuclear phenotype is secondary to manchette dysfunction or reflects a direct nuclear role was not determined

• Key unresolved questions include whether CCDC146 has functional roles at somatic centrosomes or primary cilia beyond sperm, the atomic-resolution structure of the CCDC146–CCDC147 heterodimer in mammalian axonemes, and the mechanism by which CCDC146 loss leads to sperm nuclear defects.
• No functional data for CCDC146 at somatic centrosomes or primary cilia in peer-reviewed literature
• No high-resolution mammalian structure of the CCDC146–CCDC147 complex
• Link between axonemal/IFT defects and nuclear compaction phenotype is unexplained