Soft matrix promotes ciliogenesis in human retinal pigment epithelial cells

The primary cilium (PC) is a microtubule-based mechanosensory organelle involved in signal transduction. Although the signaling functions of PC have been well studied, its interaction with the cell´s mechanical environment remains unclear. Here, we used polyacrylamide hydrogels to investigate how extracellular matrix (ECM) stiffness influences ciliogenesis in human retinal pigment epithelial (RPE1) cells. We found that a soft hydrogel (1 kPa) induces cilia formation and elongation in RPE1 cells independent of serum starvation, a classical in vitro method to promote ciliogenesis. Transcriptome analysis of serum-fed RPE1 cells on soft matrix revealed cell cycle exit and alterations in ECM and cytoskeletal gene expression that favor ciliogenesis. Moreover, we observed that despite being very long, cilia formed on a soft substrate are functional, as they transduce Sonic hedgehog signal normally. Transcriptome and microscopy analysis of two cilia-inducing conditions (soft substrate and serum starvation) revealed upregulation of cilia-related genes and downregulation of proliferation markers on both conditions. Autophagy genes were more enriched in serum-starved cells, whereas upregulation of ECM genes and downregulation of actin-related genes were more pronounced on soft matrix. Our study demonstrates that PC biogenesis can be mechanically induced in RPE1 cells on soft substrates independent of external biochemical cues.