ARMC2

ARMC2 is an armadillo repeat protein that functions as an obligate cargo adapter for intraflagellar transport (IFT) of radial spokes during ciliary and flagellar assembly (PMID:34982025). Live imaging of its Chlamydomonas ortholog (PF27) showed that tagged ARMC2 co-migrates with the radial spoke protein RSP3 on anterograde IFT trains, then diffuses back to the cell body after RSP3 is unloaded and incorporated at the flagellar tip; in armc2/pf27 mutants radial spoke transport is abolished and spokes are confined to the proximal flagellum (PMID:34982025). Loss of ARMC2 in humans and mice causes multiple morphological abnormalities of the flagella (MMAF) and male infertility, with specific absence of the axonemal central pair complex proteins (e.g. SPAG6, SPEF2) while other axonemal components persist, establishing a selective requirement in central pair complex assembly and/or stability (PMID:30686508, PMID:34493464). ARMC2 deficiency also produces sperm nuclear and chromatin defects and reduced embryo developmental potential after ICSI (PMID:40070084), and bi-allelic ARMC2 mutations extend to primary ciliary dyskinesia with pulmonary manifestations (PMID:40158138). Testicular mRNA replacement in Armc2-deficient mice restores normal, motile sperm competent for fertilization, demonstrating that loss of ARMC2 protein is the direct cause of the phenotype (PMID:41773826).

1. 2019 High

   Established that ARMC2 is genetically required for sperm flagellar formation and, specifically, for assembly or stability of the axonemal central pair complex, defining its first biological role.

   Evidence Whole-exome sequencing of MMAF patients, CRISPR-Cas9 Armc2 knockout mice, and immunostaining for central pair proteins SPAG6/SPEF2

   PMID:30686508

   Open questions at the time

   • Did not reveal the molecular activity through which ARMC2 supports central pair assembly
   • No direct interaction partners or transport mechanism identified
   • Restricted to sperm flagella; broader ciliary role untested
2. 2021 Medium

   Confirmed in an additional family that a null ARMC2 allele produces MMAF with complete absence of the central pair complex, reinforcing the genotype-phenotype link.

   Evidence Whole-exome and Sanger sequencing plus transmission electron microscopy of patient sperm ultrastructure

   PMID:34493464

   Open questions at the time

   • Single family, no mechanistic follow-up
   • Does not address how ARMC2 acts at the molecular level
3. 2022 High

   Resolved the molecular mechanism by showing ARMC2 is an obligate IFT cargo adapter that ferries radial spokes into the flagellum, explaining why its loss disrupts axonemal architecture.

   Evidence Live fluorescence imaging of tagged ARMC2 and RSP3, IFT motility assays, and genetic mutant analysis in Chlamydomonas

   PMID:34982025

   Open questions at the time

   • Direct biochemical interaction between ARMC2 and radial spoke proteins not reconstituted
   • How ARMC2 links cargo to IFT trains and triggers unloading at the tip unknown
   • Relationship between radial spoke adapter role and central pair complex loss not mechanistically connected
4. 2022 Low

   Attempted to define the ARMC2 interactome in sperm, nominating candidate partners involved in spermatogenesis.

   Evidence Co-immunoprecipitation and mass spectrometry from patient sperm identifying CEP78, PGAM5, RHOA, FXR1, SKIV2L2

   PMID:35543806

   Open questions at the time

   • Single co-IP/MS without reciprocal validation or functional follow-up
   • Interactions may be indirect or non-specific
   • No link established between these candidates and the IFT/central pair role
5. 2024 Medium

   Demonstrated that missense ARMC2 variants act as partial loss-of-function alleles yet still abolish the central pair complex, refining the allelic spectrum of disease.

   Evidence Whole-exome sequencing, TEM and immunostaining of patient sperm, and in vitro expression of variants in HEK293T cells

   PMID:38492154

   Open questions at the time

   • Expression-level effects measured but functional consequence on transport not assayed
   • Single lab, limited families
6. 2025 Medium

   Extended the ciliary role of ARMC2 beyond sperm by linking its mutations to primary ciliary dyskinesia pulmonary phenotypes.

   Evidence TEM, immunoblotting and immunofluorescence of patient spermatozoa plus whole-exome sequencing

   PMID:40158138

   Open questions at the time

   • Direct evidence of ARMC2 function in respiratory cilia not shown
   • Mechanism of pulmonary manifestation inferred from sperm data
7. 2025 Medium

   Showed that ARMC2 loss compromises sperm nuclear integrity and post-fertilization embryo development, broadening its phenotypic impact beyond flagellar structure.

   Evidence Comparative knockout mouse study with sperm nuclear morphology, DNA compaction, chromosomal architecture, ploidy assays and ICSI embryo tracking

   PMID:40070084

   Open questions at the time

   • Mechanistic link between an IFT cargo adapter and nuclear/chromatin defects unclear
   • Single lab comparative study
8. 2026 High

   Provided causal proof that absence of ARMC2 protein directly produces the infertility phenotype by rescuing it with mRNA replacement.

   Evidence In vivo testicular electroporation of Armc2 mRNA in knockout mice with sperm morphology/motility analysis and IVF/ICSI embryo production

   PMID:41773826

   Open questions at the time

   • Durability and developmental window of rescue not fully defined
   • Does not resolve the molecular steps between ARMC2 restoration and central pair assembly

• How ARMC2 biochemically couples radial spoke cargo to IFT trains, and how this adapter role mechanistically governs central pair complex assembly and sperm nuclear integrity, remains unresolved.
• No structural model of ARMC2-cargo or ARMC2-IFT engagement
• No reconstituted biochemistry of the adapter function
• Connection between radial spoke transport and central pair loss not directly demonstrated