A microprotein encoded by FERMT3 modulates endothelial cell protein catabolism and induces cell cycle arrest and...

Endothelial cells express numerous microproteins (miPs) encoded by small open reading frames (smORFs), yet the biological function of most remains unknown. This study set out to characterize a novel 69 amino acid miP encoded within the FERM domain containing kindlin-3 transcript (miP-FERMT3), which is upregulated under inflammatory conditions.

Confocal microscopy was used to determine miP-FERMT3 localization, and its interaction partners were determined by mass spectrometry and immunoblotting. RNA sequencing and quantitative mass spectrometry were performed to assess transcriptional and proteomic alterations. Cell proliferation and cell cycle progression were examined by live cell imaging, EdU incorporation and flow cytometry, while senescence was determined by β-galactosidase staining, live cell imaging and RT-qPCR-based analysis of telomere length.

In endothelial cells, miP-FERMT3 localized mainly to centriole subdistal appendages, where it colocalized with ninein and CEP170 and induced centrosome amplification. The expression of miP-FERMT3 caused cell cycle arrest and DNA damage, evidenced by γ-H2AX foci and nuclear p53 accumulation. Consistent with this, miP-FERMT3-expressing endothelial cells exhibited downregulation of genes required for cell-cycle progression and upregulation of genes involved in cell cycle inhibition and senescence. However, canonical p53 target genes were not induced and cell cycle arrest occurred independently of p53. Mechanistically, miP-FERMT3 interacted with proteins involved in ubiquitin/proteasome-dependent protein catabolism, including PSMD9, CUL2 and TRIM8, and its expression increased protein ubiquitination, centrosomal neddylation and proteasomal activity. Notably, enhanced proteasomal turnover of p21 in miP-FERMT3-expressing endothelial cells resulted in replication stress, as evidenced by increased CHK1 phosphorylation. These alterations culminated in rapid induction of cellular senescence, characterized by enlarged cell size, β-galactosidase activity, telomere shortening and a paracrine pro-inflammatory activation of naïve endothelial cells. Analyses of independent murine and human transcriptomic and proteomic aging datasets further revealed that FERMT3 expression and protein abundance increase with age.

miP-FERMT3 is a novel regulator of protein catabolism that promotes p21 degradation, replication stress and p53-independent cell cycle arrest and senescence in endothelial cells. Given the aging-associated upregulation of FERMT3 in mouse and human endothelial cells, increased miP-FERMT3 expression may contribute to the onset of vascular senescence as a hallmark of aging.