ANKRD55 interacts with an IFT-B-like complex in microglia

IntroductionSNPs associated with genome-wide risk for multiple sclerosis (MS) modulate expression of ankyrin repeat domain protein 55 (ANKRD55). The function of ANKRD55 is not well understood. A role for ANKRD55 in ciliar transport in multiciliated cells has been reported. To gain deeper insight in how ANKRD55 may modulate neuro-inflammatory parameters, we identified the ANKRD55 interactomes from human neuroblastoma, astrocytic, microglial and monocytic cell lines. MethodsCell lines were transfected with synthetic ANKRD55 RNA in conjunction with nanoparticles. ANKRD55 interactomes were determined by affinity purification coupled to mass spectrometry (AP-MS) and analyzed bioinformatically. Results were validated and interpreted using confocal immunofluorescence microscopy, RNAseq transcriptomics, and a visible immunoprecipitation assay (VIP). ResultsShared among the interactomes were the 14-3-3 isoforms 14-3-3{eta} and 14-3-3{beta}{eta}. Unique to the microglial interactome were eight proteins belonging to the intraflagellar transport complex B (IFT-B). The IFT-B complex is known to mediate anterograde protein trafficking from the base to the tip of cilia. The dimer IFT46-IFT56 was identified as the minimum entity of IFT-B needed to support interaction with ANKRD55. To verify whether ANKRD55 is a ciliar transport protein, we induced ciliogenesis by serum starvation. Primary ARL13B+ cilia could be induced in the astrocytic and neuroblastoma, but not microglial, cell lines. By confocal microscopy, ANKRD55 was not detectable in these cilia but was enriched at the basal body. In the microglial cell line, ANKRD55 and IFT-B components were enriched at the centrosome. In two human primary myeloid cell models, monocyte-derived microglia (MoMG) and monocyte-derived dendritic cells (MoDC), we were able to recapitulate the co-localization of ANKRD55 and IFT81 at the centrosome. DiscussionOur work shows that an ANKRD55 - IFT-B-like complex is assembled in microglial cells. Together with the finding that ANKRD55 was not detected in primary cilia, the results suggest that ANKRD55 is associated with an IFT-B pathway that can operate independent of ciliogenesis.