ARMC2

ARMC2 is an armadillo repeat-containing protein that functions as an obligate cargo adapter for intraflagellar transport (IFT), specifically mediating anterograde delivery of radial spoke complexes along the axoneme; after cargo unloading at the flagellar tip, ARMC2 recycles to the cell body by diffusion (PMID:34982025). Loss of ARMC2 abolishes radial spoke IFT and causes selective absence of the axonemal central pair complex (CPC) in sperm flagella and respiratory cilia, resulting in multiple morphological abnormalities of the flagella (MMAF), male infertility, and primary ciliary dyskinesia phenotypes (PMID:30686508, PMID:40158138). Armc2 deficiency additionally impairs sperm nuclear integrity, including DNA compaction and chromosomal architecture, compromising embryo developmental competence (PMID:40070084). In vivo delivery of Armc2 mRNA to spermatogenic cells in knockout mice restores normal sperm morphology, motility, and fertilization capacity, confirming that ARMC2 is the causative gene for the observed infertility (PMID:41773826).

1. 2019 High

   Establishing ARMC2 as a disease gene: bi-allelic ARMC2 mutations were shown to cause MMAF by specifically disrupting CPC assembly/stability in sperm flagella, resolving the question of whether ARMC2 has a ciliary structural role.

   Evidence CRISPR-Cas9 knockout mice and human patient sperm immunostaining plus TEM

   PMID:30686508

   Open questions at the time

   • Mechanism by which ARMC2 supports CPC assembly was unknown
   • Whether ARMC2 acts in transport or structural scaffolding was unresolved
   • Role in non-sperm cilia not addressed
2. 2021 Medium

   Independent patient studies confirmed the CPC-specific defect, reinforcing that ARMC2 loss-of-function selectively destroys the central pair rather than globally disrupting the axoneme.

   Evidence Whole-exome sequencing and TEM of patient spermatozoa carrying a homozygous stop-gain ARMC2 mutation

   PMID:34493464

   Open questions at the time

   • No biochemical reconstitution of ARMC2–CPC interaction
   • Genotype–phenotype relationship across mutation types not systematically characterized
3. 2022 High

   The mechanistic basis was resolved: ARMC2 functions as an obligate IFT cargo adapter for radial spokes, co-migrating with RSP3 on anterograde IFT trains and recycling after unloading, explaining how CPC assembly depends on ARMC2-mediated transport.

   Evidence Live fluorescence imaging of tagged ARMC2 and RSP3 in Chlamydomonas, with armc2/pf27 mutant analysis

   PMID:34982025

   Open questions at the time

   • Structural basis of ARMC2–radial spoke or ARMC2–IFT train interaction unknown
   • Whether ARMC2 transports additional cargo beyond radial spokes not determined
   • Mammalian live-imaging confirmation of IFT adapter function lacking
4. 2025 Medium

   ARMC2 function was extended beyond sperm flagella: patient variants linked to pulmonary primary ciliary dyskinesia phenotypes, establishing ARMC2 as a general motile cilia gene.

   Evidence TEM, immunofluorescence, and immunoblotting of patient spermatozoa combined with respiratory ciliary phenotyping

   PMID:40158138

   Open questions at the time

   • Respiratory cilia ultrastructural analysis not shown in detail
   • Whether the same CPC-specific defect underlies the respiratory phenotype not directly demonstrated
5. 2025 Medium

   ARMC2 deficiency was shown to impair not only flagellar structure but also sperm nuclear integrity and embryo developmental competence, broadening understanding of its spermatogenic role.

   Evidence Armc2-/- knockout mice with nuclear morphology, DNA compaction, ploidy, and ICSI embryo development assays

   PMID:40070084

   Open questions at the time

   • Whether nuclear defects are a direct consequence of ARMC2 loss or secondary to flagellar disorganization is unknown
   • Molecular mechanism linking ARMC2 to nuclear compaction not identified
6. 2026 High

   Functional rescue by in vivo mRNA delivery proved that ARMC2 is both necessary and sufficient for restoring spermatogenesis in the knockout, providing proof-of-concept for therapeutic correction.

   Evidence Testicular Armc2 mRNA electroporation in KO mice with sperm morphology, motility, IVF, and ICSI readouts

   PMID:41773826

   Open questions at the time

   • Duration and efficiency of mRNA-based rescue in long-term spermatogenesis not characterized
   • Applicability of electroporation-based delivery to clinical settings unknown

• The structural basis of ARMC2's interaction with IFT trains and radial spoke cargo, and whether ARMC2 transports additional axonemal subcomplexes in mammalian cilia, remain unresolved.
• No cryo-EM or crystal structure of ARMC2 or its complexes
• Mammalian live-imaging of ARMC2 IFT behavior not performed
• Whether ARMC2 nuclear effects are direct or indirect is unresolved